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Lithium-ion capacitors (LICs) have gained significant attention in recent years for their increased energy density without altering their power density. LICs achieve higher capacitance than traditional supercapacitors due to their hybrid battery electrode and subsequent higher voltage. This is due to the asymmetric action of LICs, which serves as an enhancer of traditional supercapacitors. This culminates in the potential for pollution-free, long-lasting, and efficient energy-storing that is required to realise a renewable energy future. This review article offers an analysis of recent progress in the production of LIC electrode active materials, requirements and performance. In-situ hybridisation and ex-situ recombination of composite materials comprising a wide variety of active constituents is also addressed. The possible challenges and opportunities for future research based on LICs in energy applications are also discussed.
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Energies 2021, 14, 979. https://doi.org/10.3390/en14040979 www.mdpi.com/journal/energies
Review
Lithium-Ion Capacitors: A Review of Design and Active
Materials
Jacob J. Lamb and Odne S. Burheim *
Department of Energy and Process Engineering and ENERSENSE, Faculty of Engineering, NTNU,
7491 Trondheim, Norway; jacob.j.lamb@ntnu.no
* Correspondence: odne.s.burheim@ntnu.no
Abstract: Lithium-ion capacitors (LICs) have gained significant attention in recent years for their
increased energy density without altering their power density. LICs achieve higher capacitance than
traditional supercapacitors due to their hybrid battery electrode and subsequent higher voltage.
This is due to the asymmetric action of LICs, which serves as an enhancer of traditional supercapac-
itors. This culminates in the potential for pollution-free, long-lasting, and efficient energy-storing
that is required to realise a renewable energy future. This review article offers an analysis of recent
progress in the production of LIC electrode active materials, requirements and performance. In-situ
hybridisation and ex-situ recombination of composite materials comprising a wide variety of active
constituents is also addressed. The possible challenges and opportunities for future research based
on LICs in energy applications are also discussed.
Keywords: lithium-ion capacitors; nodes; cathodes; oxides; carbon; silicon
1. Introduction
Due to the rapid growth of renewable energy production recently [1], there is a sig-
nificant requirement for electrical energy storage technologies. Energy storage offers the
ability to moderate the variability of electrical energy [2]. This represents a rapidly emerg-
ing market for energy storage that is currently underutilised. The characteristics of the
energy storage needs, in general, are electro-compatibility and will relate more specifi-
cally to cheap and highly efficient storage solutions for stationary purposes, and energy
and power intensity for the transport and transmission sector (to which lithium-based
batteries are the answer). In addition, there is a need for shifting the current battery pro-
duction from fossil-based energy to renewables to reduce the embedded emissions of en-
ergy storage systems. Moreover, the materials required in the production of energy stor-
age system should ideally originate from areas free of geopolitical conflicts, child labour,
corruption and environment unfriendly excavation and extraction methods [3]. In this
light, lithium-ion batteries (LIBs) utilising ethically mined materials and energy produced
by renewables have huge international market advantages when considering environ-
mental, social and corporate governance (ESG) aspects.
Lithium-ion capacitors (LICs) were first produced in 2001 by Amatucci et al. [4]. LICs
are considered one of the most effective devices for storing energy and are often seen as
an offspring from LIBs for several reasons. In addition, Sodium-ion and Potassium-ion
capacitors (SIC and KIC, respectively), have also become of commercial interest as they
similarly are a hybrid device combining an ion battery with a traditional capacitor. This
review will focus on the LIC developments as the main example of a hybrid capacitor, but
it must be noted that there are many similarities between LICs, NICs and KICs. LIBs nor-
mally have high energy density (>150 W h kg
1
) and have no memory impact as in con-
ventional Ni-Cd/Ni-MH batteries [5,6]. Despite this, their low power density (< 1 kW kg
1
),
and lower cycle life and capacity loss [7] hinders their utilisation in some applications. By
Citation: Lamb, J.J.; Burheim, O.S.
Lithium-Ion Capacitors: A Review of
Design and Active Materials.
Energies 2021, 14, 979.
https://doi.org/10.3390/en14040979
Academic Editor: Woojin Choi
Received: 5 January 2021
Accepted: 11 February 2021
Published: 12 February 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and insti-
tutional affiliations.
Copyright: © 2021 by the author. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://cre-
ativecommons.org/licenses/by/4.0/).
Energies 2021, 14, 979 2 of 28
comparison, LICs can provide high power density (>10 kW kg1) and long cycle life (usu-
ally > 5000 cycles); however, their comparatively low energy density (5–10 W h kg1) re-
stricts their applications in certain fields (LIC applications have been thoroughly dis-
cussed in [8–10]). To close the performance gap between LIBs and traditional capacitors,
LICs have been developed to incorporate the strengths of both LIBs and traditional capac-
itors [6,11–14].
By practice, LIBs consist of a metal oxide cathode, separator, electrolyte, and a Lith-
ium-based anode. In contrast, non-aqueous liquid electrolyte LICs with high power den-
sities (>10 kW kg1) and long-term cyclic durability (10,000–100,000 cycles), are ideal for
large-scale energy storage [5,15]. Typical LIC designs use a high-capacity battery-type
electrode and a high-rate capacitor-type electrode [5,16]. During the charge-discharge cy-
cle, charges are deposited concurrently and asymmetrically in the LIC by surface ion ad-
sorption/desorption on the capacitor-type electrode and Li+ intercalation/de-intercalation
on the battery-type electrode, respectively [17,18]. It is worth noting that the battery- and
capacitor-type electrodes of the LIC system work in various potential windows, which
may expand the range of operating voltage of LICs and contribute to high energy density.
Supercapacitors can be classified into asymmetric supercapacitors, pseudo-capaci-
tors (PCs) and electric double-layer capacitors (EDLCs) depending on the charging stor-
age mechanisms [19]. PCs store energy using the fast redox reaction on the surface of the
electrode components, such as metal oxides, metal sulphides and polymer conductors.
Whereas energy storage on the surface of the porous carbon with a large specific surface
area (SSA) is accomplished through strong ion absorption and desorption for EDLCs.
LIBs operate through Li+ that travels back and forth between the electrodes followed
by the electrode materials’ bulk redox reaction. The combination of the positive EDLC
electrode and the negative LIB electrode forms a LIC, reducing the deficiencies in both
LICs and LIBs. LICs may typically be categorised as dual-carbon, non-carbon, and mixed
forms, distinguished by whether or not the electrodes contain carbon materials. The den-
sities of energy and power of LICs depend primarily on the configuration of electrode
materials in the devices [17,20]. The most promising industrial prospects are for dual-car-
bon LICs (DC-LICs), in which both electrodes are composed of carbon materials. Owing
to the surplus and low carbon levels, the average expense of DC-LICs may be effectively
popular.
Until now, owing to their strong gravimetric functional potential and excellent elec-
trochemical operation, a number of components, such as metal compounds, polyanions
and metalloid/metal compounds, have been produced in LICs for battery-type electrodes
[21]. However, their further production in LICs is constrained by the poor conductivity,
wide volume variability and strong polarisation of these active materials [5]. Carbona-
ceous materials have been extensively researched and used in energy storage fields like
LICs [22–25], benefiting from low expense, sufficient supplies, plenty of allotropes and
transformations as well as superior physical/chemical stability. Because of their large spe-
cific surface region, strong conductivity and excellent usability to electrolytes, carbon ma-
terials were also inserted into the electrodes to solve such issues [26,27]. Furthermore, car-
bon compounds may also specifically function as the active compounds of battery-type
electrodes in LICs, since they have active Li+ intercalation / de-intercalation sites [28,29].
Unlike battery-type electrodes, LICs counter-electrodes are capacitor-type. Conse-
quently, numerous porous carbon materials with a wide specific surface region, such as
activated carbon (AC) and graphene are potential candidates for LICs capacitor-type elec-
trodes [30,31]. Their capacitances rely primarily on the surface of carbon-based electrodes
[32] for the adsorption/desorption of ions. Therefore, porous structures with sufficient dis-
tribution of pore size play an important role in the electrochemical production of carbon
materials in LICs [33,34]. Specific materials are typically inserted into the carbonaceous
materials to have pseudo-capacity to further improve the capacitance of the capacitor-
type electrode.
Energies 2021, 14, 979 3 of 28
In this review, we will identify the fundamental active materials used for LICs and
analyse electrode/capacitor-type carbon-based battery-based electrode materials over
many decades. By concentrating on the most common cases with industrial potential, we
demonstrate the main components of LICs. The knowledge gaps and future trends asso-
ciated with LICs are also addressed, which provides a detailed insight into the potential
areas of further research of LIC active materials.
2. Lithium-Ion Capacitor Fundamentals
Throughout the consumer electronics, automobile, aerospace and stationary indus-
tries, electrical energy-storage devices play a key function. There are several types of re-
versible electrochemical energy storage systems in the format of accumulators, flow cell
systems, secondary batteries (rechargeable) and primary batteries (single-use). In terms of
secondary and primary batteries, the former has a lower energy intensity while the latter
offers better power; however, the cyclable nature of secondary batteries makes them ideal
for use as an energy storage solution. The current prevailing technology is LIBs. Large
LIBs have a gravimetric energy upwards of 200 Wh kg
1
, and with an overall effective
power density of up to 350 W kg
1
. In comparison, most industrial electrochemical capac-
itors have average power density as high as 10 kW kg
1
, and with gravimetric energy of
up to 7 Wh kg
1
[2]. Figure 1 gives an overview of the power and energy densities of ul-
tracapacitors and LIBs compared to other energy storage technologies. A common target
for advanced electrical energy storage systems is to provide high energy as well as high
power in a single system [35–38]. A LIC is a comparatively modern system, intermediate
in energy between batteries and supercapacitors, though giving supercapacitor-like
power and cyclability properties.
Figure 1. Ragone plot of current LIB, hydrogen, gasoline, ammonia and jet fuel—energy storage solutions in comparison
to traditional industrial ultracapacitors.
It is important to define the components of LICs before recognising the basic energy
storage function of the LICs. As a kind of asymmetric supercapacitor, LICs usually consist
Energies 2021, 14, 979 4 of 28
of a battery-type electrode with the insertion/extraction of lithium ions and a pseudo-ca-
pacitance or ion adsorption/desorption capacitor-type electrode [39,40]. As the battery-
type electrode does not only serve as an anode but also as a cathode, LICs have been pre-
viously separated into two types [41]:
1. The battery-like electrode acts as the anode, and the capacitor-like electrode acts as
the cathode. Anions are usually absorbed on the porous surface or defects that may be
apparent on the cathode during the charge cycle, whereas Li+ ions are intercalated (as Li)
in the active anode.
2. The capacitor-type electrode functions as the anode and the battery-type electrode
functions as the cathode. Li+ de-intercalates from the cathode during the charging phase.
Li+ immediately migrates in the electrolyte and adsorbs on the anode. This requires some
redox reaction at the cathode (e.g., cathode oxidation: LiCoO2 Li+ + e + Li-CoO2; and,
anode reduction: Li++C6-graphite + e LiC6-graphite).
Regenerative braking and grid stabilisers are significant possible end-uses of the
LICs. Regenerative braking energy recovery from cars, heavy-duty engines, and increas-
ingly light-duty vehicles represents a major potential opportunity that is not completely
explored due to the shortcomings of current secondary battery and supercapacitor tech-
nologies (electrochemical capacitor and ultracapacitor). On-board regenerative braking is
a possible mechanism to restore a significant fraction of the energy [42]. LICs do not have
the inherent density of energy-storage needed for economic and spatially effective imple-
mentation. In comparison, LIBs do not have enough capacity, which needs a charge rate
of up to 200 C (1 C is 1 h charge, 200 C is 18 s charge), or around 100 times larger size than
what is required for just breaking adsorption.
LICs form a new class of devices capable of bridging the output of commercial EDLC
supercapacitors and traditional LIBs [5,15,20]. As previously discussed, LICs are capable
of producing 4 to 5 times higher energy values than EDLC ultracapacitors. This superior
capacity is obtained by depending on a carbonate-based battery electrolyte that generates
a higher voltage of the unit than acetonitrile (4.2 V vs. 2.7 V). Although 5000 cycles is a
substantial lifetime for a LIB, they cannot be implemented in place of an ultracapacitor.
This is because ultracapacitors run continuously up to thousands of charges per day in
some cases. Therefore, LICs appear ideal to replace ultracapacitor electrochemical tech-
nologies.
Typically, while the battery-type electrode acts as an anode (case 1 above), it needs
to be pre-lithiated during production before being implemented in a LICs, which may
bring the anode’s capacity closer to that of Li [43,44]. Pre-lithiation is a method used for
LIBs and LICs to compensate for the potential loss of active lithium so higher reversible
potentials and higher gravimetric energies are obtained. It is achieved by storage of a cer-
tain amount of active lithium in the anode prior to the cells first charge/discharge cycle.
There are many methods available for pre-lithiation and can be performed to the active
material or the electrode as a whole [45]. As a consequence, a LICs working voltage can
raise and even exceed 4.5 V. The energy density is compared by their capacitance and the
operating voltage [39,46], as typically chosen comparable specific energy units are V,
kJ/mol and kWh/kg. The energy efficiency of LICs can be significantly enhanced due to
the large working voltage of the battery-type electrodes. That is, with constant operating
currents and resistances (rj-loss), this efficiency reduction becomes relatively less signifi-
cant by increasing the open-circuit voltage (efficiency = (Eocp-rj)/Eocp). Nevertheless, a phase
transition frequently follows the faradaic reaction of the battery-type electrode, which re-
sults in a weak rate efficiency, lower cycle life and slow dynamics [46–49]. To overcome
these issues, the incorporation of highly conductive carbonaceous additives such as gra-
phene, carbon nanotubes (CNTs) and AC are important in addition to the production of
nanoscale-structured electrodes to obtain improved electronic conductivity [50–52].
Energies 2021, 14, 979 5 of 28
Unlike the pure battery-type electrode, reversible ion adsorption or rapid redox re-
actions occurs on the capacitor-type electrode sheet, which provides the possibility that
LICs have comparable power density to that of ultracapacitors by integrating an effective
battery-type electrode with strong kinetics [53]. Also essential to their functional imple-
mentation is the cycling efficiency of LICs. Besides the inherent behaviour of the electrode
of the battery type and the electrode of the capacitor-type, the mass ratio between the
electrode of the battery type and the electrode of the capacitor-type often plays a crucial
role in the cycle life of the LIC.
Th e charg es for an a symme tri c cell s hould be d istrib uted a t bo th elec trodes (i.e. , Qanode
= Qcath). The charges deposited are aligned with the electrode's basic potential (C) and mass
(m) (Q C × ∆𝐸 × m) [54,55]. The optimal mass ratio between the battery-type electrode
and the capacitor-type electrode can therefore be determined by the following equation
[56]:
𝑚
𝑚
=
𝐶 ×∆𝐸
𝐶 ×∆𝐸
(1)
Where the electrode mass, the basic capacitance and the voltage range in the load/dis-
charge phase for the anode and the cathode are 𝑚, 𝐶 and ∆𝐸, respectively. Nonetheless,
the capacitance of the capacitor-type electrode is significantly smaller compared to that of
the battery-type electrode [23,31]. Therefore, the manufacturing of high-density carbon
products (e.g., micro- and macro-carbon composites), is the primary aim in the production
of advanced LICs.
Two techniques are typically used to improve the natural capacitance of capacitor-
type electrodes: construction of capacitor-type electrodes with broad natural surface area
and incorporation of pseudocapacitive or heteroatom doping materials [57,58]. As de-
scribed above, carbon products, either as active products or as conductive additives, per-
form irreplaceable roles in the applications of LICs due to the excellent intra- or inter-
particle conductivity and the outstanding electrolyte accessibility [59–62].
3. Electrode Materials
Nanostructured carbons are significant LIC materials used either independently or
in conjunction with a second active step of Li such as TiO2. Yao et al [59] published a
review of the carbons used in LICs (Table 1). The carbons are often rich in heteroatoms
(especially oxygen) and contain varying degrees of graphene ordering. For carbon allo-
tropes, extremely deficient or heteroatomic carbons do not fall into the classic taxonomy,
but instead reflect the similarity of structure and chemistry between pure graphite/gra-
phene and completely amorphous activated carbon. Lithium deposition is typically
poorly known in nongraphic carbons, except at fairly high concentrations typical of a bat-
tery. The charge levels expected by hybrid devices are far less known and require further
research.
3.1. Anodes
3.1.1. Carbon Materials
Graphite, with a hexagonal mesh structure made from carbon atoms, has been used
in consumer LIBs goods as a negative electrode material since 1991 [63]. Graphite will
provide a large plateau power around 300 mAh g1 below 0.2 V and will guarantee a se-
cure charge-discharge plateau. Alternatively, graphite suffers from a low rate capability
induced by the strong crystallinity and anisotropy that further limits the power density
[6]. To increase the capacity of graphite, ball milling, extremely conductive carbon surface
grinding, graphisation degree regulation, and the application of defects and additives are
all used [64–66].
Graphitised carbon, for which the space and microstructures of the interlayer can be
conveniently modified, has the most interesting perspectives for use in anodes of LICs.
Energies 2021, 14, 979 6 of 28
The coexistence of graphical structures and amorphous structures allows graphised car-
bon to successfully combine a high plateau potential and superior rate efficiency. Catalytic
graphisation with the aid of a metal-ions is an important process for preparing graphised
carbon under fairly mild conditions (<1200 °C) [67]. The degree of graphitisation can be
regulated by adjusting the temperature, precursor and catalyst structures.
Table 1. Carbonaceous material-based lithium-ion capacitor (LIC) summary.
Configuration (Anode//Cathode) Voltag
e
Max Energy
(Wh/kg) at Power
(W/kg)
Energy (Wh/kg) at
Max Power (W/kg) Cyclability
N-doped carbon nanopipes//reduced
graphene oxides [68] 0–4 V 262 at 450 78 at 9000 91% over
4000 cycles
graphene//armored graphene [69] 0–4.3 V 160 at 900 59 at 19,000 89% over
1000 cycles
microcrystalline
graphite//mesoporous carbon
nanospheres/graphene [70]
2.2–4.2
V 80 at 152 32 at 11,600 93% over
4000 cycles
reduced GO//resin-derived carbon
combined with GO [71] 0–4 V 148.3 at 150 45 at 6500 79% over
3000 cycles
B&N-doped carbon nanofiber//B&N-
doped carbon nanofiber [32] 0–4.5 V 220 at 225 104 at 22,500 81% over
5000 cycles
graphite//activated graphene [72] 2–4 V 147.8 Not reported Not reported
graphite//functionalised graphene
[73] 2–4 V 106 at 84 85 at 4200 100% over
1000 cycles
hard carbon//activated carbon [74] 1.4–4.3
V 80 at 150 65 at 2350 82% over
10,000 cycles
hard carbon//bioderived
mesoporous carbon [75,76]
1.7–4.2
V 121 at 300 50 at 9000 81% over
8000 cycles
graphite//activated carbon [72] 1.5–5 V 145.8 at 65 18 at 18,000 65% over
10,000 cycles
hard carbon//activated carbon [77] 2–4 V 82 at 100 14 at 20,000 97% over 600
cycles
graphite//graphene [78] 2–4 V 135 at 50 105 at 1500 97% over
3500 cycles
N-doped hard carbon// activated
carbon [79] 2–4 V 28.5 at 348 13.1 at 6940 97% over
5000 cycles
soft carbon//activated carbon [76] 0–4.4 V 115 at 25 16 at 15,000 63% over
15,000 cycles
graphene//activated carbon [80] 2–4 V 95 at 27 61.7 at 222.2 74% over 300
cycles
graphite//activated carbon [81] 2–4 V 103 Not reported 77% over 100
cycles
Hard carbon is a suitable choice for high power LICs [64,77,82], with better kinetics
and a greater space distance between the carbon layers than graphite. Zhang et al. [83]
contrasted the impact of LIC output on two specific hard carbon materials (spherical and
irregular hard carbon) with an AC cathode, observing that the irregular hard carbon ex-
hibited optimum electrochemical efficiency with a high energy density of up to 85.7 W h
kg1 and a power density of up to 7.6 kW kg1 centred on the active material mass of two
electrodes in the voltage range of 2–4 V. Furthermore, preparation of a hard carbon anode
extracted from a carbonaceous source, such as a nitrogen-doped carbonised polyimide
microsphere (CPIMS) [79], can also be a feasible solution. Despite this, the voltage hyste-
resis and sloping characteristics during hard carbon charging/discharge are unfavourable
to the cycling stability of LICs. Sun et al. [84] extensively examined the electrochemical
efficiency and power fading behaviours of hard carbon LICs and noticed that power
Energies 2021, 14, 979 7 of 28
fading of LICs during cycling was induced by a rise in internal resistance and a depletion
of lithium deposited on the anode.
Benefiting from the superior mobility of electron carriers and strong lithium-ion
transport kinetics, graphene is also a promising candidate as an anode material, rather
than being used exclusively as a cathode material [15]. Ren et al. [80] fabricated pre-lithi-
ated graphene nanosheets in LICs as an anode, providing a cumulative power density of
220 W kg1 at an energy density of 62 W h kg1 with a capacity retention of 74 % at 400 mA
g1 after 300 cycles.
Ahn et al. [85] provided a highly oriented graphene sponge (HOG) with an ultra-
high energy density as an anode. The AC/HOG LICs demonstrated 3.6 times greater dif-
fusivity of the lithium-ion than the AC/graphitized carbon LICs. As a result, they obtained
large energy densities of 232 at 57 W kg1 and 131.9 W h kg1 at 2.8 kW kg1. In a DC-LICs
device with AC as the cathode, Phattharasupakun et al. [86] documented a nitrogen-
doped reduced graphene oxide (N-rGO) aerogel anode. They exhibited a maximum spe-
cific energy of 170.28 W h kg1 in the voltage range of 2.0–4.0 V, and the average power
density exceeded 25.75 kW kg1 after 2000 cycles with almost no decay in efficiency.
Composites processing is an efficient way to combine multiple forms of carbon ma-
terials (e.g., graphite, hard carbon and graphene.) as an active substrate for producing
high-performance LIC materials. Lim et al. [66] recorded high energy intensity and high
power intensity DC-LICs, using natural graphite-coated hard carbon as anode materials.
The DC-LIC achieved improved densities in energy and strength, as well as enhanced
cycling efficiency.
3.1.2. Transition Metal Materials
Standard transition metal oxides, including Li4Ti5O12, TiO2, Nb2O5, Fe2O3 and SnO2,
have been extensively studied as anodes of LICs due to their large abundance and strong
basic gravimetric potential [87–91]. Despite this, most transition metal oxides typically
experience a phase change during the charge/discharge cycle, which is seen by the plat-
eaus in the galvanostatic charge–discharge curves and strong peaks in the CV profiles.
These phase transitions can result in broad volume growth, resulting in a negative impact
on the integrity of electrodes, resulting in low cycling efficiency. Nevertheless, the com-
position of TiO2 and Li4Ti5O12 during reversible lithium intercalation/extraction is more
robust. The integration of such transition metal oxides into porous carbon materials would
solve or mitigate the above issues to achieve hybrid architectures. A recent review by Liu
et al. [92] gives a comprehensive overview of transition metal LICs. Table 2 gives an over-
view of transition metals used in LIC electrode design.
Table 2. Transition metal-based LIC summary.
Configuration
(Anode//Cathode)
Voltag
e
Max Energy (Wh/kg)
at Power (W/kg)
Energy (Wh/kg) at
Max power (W/kg) Cyclability
TiO2 hollow spheres at
graphene//graphene [93] 0–3 V 72 at 303 10 at 2000 65% over 1000
cycles
TiO2 at mesoporous
carbon//AC [94] 0–3 V 67.4 at 75 27.5 at 5000 80.5% over
10,000 cycles
TiO2 nanobelt
arrays//graphene hydrogels
[95]
0–3.8 V 82 at 570 21 at 19,000 73% over 600
cycles
TiO2 at rGO//AC [87] 1–3 V 42 at 800 8.9 at 8000 Not reported
TiO2–CNT//active carbon [96] 1–3 V 59.6 at 120 31.2 at 13,900 Not reported
Li4Ti5O12–CNT//graphene
foam [97] 1–3.6 V 101.8 at 436.1 12.7 at 12,300 84.8% over
5000 cycles
Li4Ti5O12//reduced graphene
oxide [98] 1–3 V 45 at 400 30 at 3300 100% over 5000
cycles
Energies 2021, 14, 979 8 of 28
nanocrystalline
Li4Ti5O12//active carbon [99] 1.5–3 V 55 at 64.6 28.8 at 10,300 Not reported
TiO2-coated Li4Ti5O12//active
carbon [100]
0.5–2.5
V 74.85 at 300 36 at 7500 83.3% over
5000 cycles
Li4Ti5O12//N-doped porous
carbon [101] 1–3 V 63 at 200 16 at 5000 Not reported
graphene-
Li4Ti5O12//graphene-sucrose
[102]
0–3 V 95 at 45 32 at 3000 94% over 500
cycles
spheres Li4Ti5O12//active
carbon [103] 1–3.5 V 74.3 at 156.26 41.7 at 468.7 93% over 500
cycles
graphene-wrapped
Li4Ti5O12//active carbon [104] 1–2.5 V 50 at 16 15 at 2500 75% over 1000
cycles
3.1.3. Polyanion and Carbon Composites
Typically, the Li3V2(PO4)3, developed with VO6 octahedra and PO4 tetrahedra corner-
sharing, crystallises in a monoclinical configuration. The comparatively broad gap space
of these crystals allows for the quick diffusion and reaction kinetics of different lithium
sites. Because Li3V2(PO4)3 content is an amphoteric powder that can be either decreased
by lithium injection or oxidised by lithium elimination, the electrochemical efficiency of
both low (1.03.0 V) and high (3.04.3 V) voltage applications for Li3V2(PO4)3 has been
studied [105]. These were observed to provide maximum energy densities between 27 and
25 W h kg1, respectively. Unlike Li3V2(PO4)3, LiTi2(PO4)3 has a NASICON-type frame
structure, which consists of PO4 tetrahedra bound by octahedral unit corners of TiO6. Each
of the PO4 tetrahedrons are connected to four octahedral TiO6 units, and in effect a TiO6
unit is connected to six PO4 tetrahedrons, allowing for multiple ionic replacements at var-
ious lattices [41]. Cyclic voltammetry calculation was observed with a two-phase reaction
process at 2.38 V during Li-insertion and an extraction at 2.60 V. As a result, the LICs
based on LiTi2(PO4)3 carbon-coated anodes display ultra-high energy and power densities
of 14 W h kg1 and 180 W kg1, respectively [106].
For LIC anodes, TiNb2O7 will serve as an alternate nominee for Li4Ti5O12. TiNb2O7's
monoclinical crystal structure comprises of disordered Nb and Ti atoms, which may have
two-dimensional interstitial spaces for rapid Li-ion injection and show an energy density
of 110.4 W h kg1 at 99.58 W kg1 [107]. Importantly, the TiNb2O7 on C electrode's Li injec-
tion activity was analysed in detail and the pseudo-capacitive reaction mechanism for in-
tercalation was achieved [41].
3.1.4. Metalloid/carbon and metal materials
The production and materials used for sustainable LICs manufacturing is becoming
increasingly important. The use of silicon within carbon electrodes provide a promising
route for sustainable LIC production in the future. Silicon is particularly of interest due to
it being the second most abundant element in the Earth’s crust. Due to its low lithiation
ability (<0.5 V) and strong real theoretical efficiency (4200 mAh g1), Si is a promising
material for high-performance LIC anodes [108,109]. Despite its excellent load-discharge
platforms and extremely high specific capacity, its cycling and rate performance may be
poor due to its severe volume expansion and low electronic and ionic conductivity [110].
Furthermore, due to its inherent semi-conductive design, the low conductivity may also
restrict its performance in charge/discharge at high current density [111113].
Composite processing is an efficient way to combine multiple forms of active mate-
rials as an active substrate for producing high-performance LIC materials. Soft carbon is
a frequently used commercial anode material with high conductivity, fast lithium-ion
transport and long cycling performance, but its specific capacity and operating platform
still require improvement. Therefore, using a small amount of silicon-carbon composite
in a soft carbon anode could ameliorate the anode's charge/discharge kinetics and also
Energies 2021, 14, 979 9 of 28
provide surplus lithium to slow the rate of active lithium consumption in long-term cy-
cling after anode pre-lithiation. Using this approach, it has been observed that such a LIC
has over 95% capacitance retention after 10,000 cycles at 20 °C [110].
Based on 3-electrode hybrid configuration [77], other types of lithium, such as lithium
silicide, can be used for the anodes [114]. Pre-lithiation has traditionally been done on the
anode; however, pre-lithiation on the cathode has been observed, which in turn is contra-
dictory to how LIBs are produced [115]. That would be potentially better because the sys-
tem is instead constructed in a thermodynamically stable manner (which may not be the
case with a symmetric unit, e.g., a carbon–carbon LIC).
Sn has a greater electrical conductivity relative to Si, which will contribute to Sn an-
odes showing a higher rate capacity. Nevertheless, during the charge/discharge phase, Sn
also suffers from a significant volume shift which can result in extreme polarisation of the
electrode material [116118]. The construction of tiny Sn nanoparticles with a binding car-
bon substratum is currently a successful method for overcoming these problems.
3.2. Cathodes
In the early research into LICs, activated carbon was dominantly used as the cathode
material with a focus on the energy-storage process of surface adsorption. This was be-
cause it shows a large surface area (33,000 m2 g1), excellent conductivity (almost 60 S m1)
and strong chemical stability [41]. An AC cathode's energy storage capability also pro-
vides a power of approximately 50 mAh g1 [41]. Compared with anode electrode perfor-
mance, it is comparatively smaller. For load-balance, the cathode's mass load is typically
two to four times that of the anode, depending on the cathode and anode's different ca-
pacitances [41]. Amatucci et al. [4] produced a LIC system using AC as the cathode, and
nanostructured Li4Ti5O12 (LTO) as the anode, the first use of AC in LICs. The voltage win-
dow of LICs in an organic electrolyte dependent on AC cathodes has since been designed
in the range of 1.5–4.5 V to ensure high energy density and preserve the long cycle stability
of LICs [72].
Additionally, carbon derived from metal-organic frameworks (MOF) with various
architectures has also been widely researched in LICs. For example, large surface area
(2714 m2 g1) carbon cuboids were synthesised by pyrolysing the zinc-based MOF-5, which
exhibits a peculiar crumpled-sheet porous morphology assembled with the required mi-
cro and mesoporosity values [119]. The MOF-dependent LIC provides a maximum effec-
tive energy density of 65 W h kg1 with excellent power capacity, dependent on the ad-
vanced structure. Likewise, polyhedral hollow carbon derived from MOF was also pro-
duced and used in LICs [120]. It is worth noting that the removal of metal ions during the
preparation of MOF-derived carbon is a required step due to the presence of metal ions
in the MOF precursor.
Graphene's unique 2D structure and physical properties and its derivatives make
them distinctive building blocks for producing various porous 3D architectures. Thanks
to the combination of porous structures and the excellent intrinsic properties of graphene
and its derivatives, these 3D architectures exhibit excellent chemical stability and strong
specific surface area as well as fast electron transport kinetics. Therefore, these high-per-
formance 3D architectures are strong candidates for use as the cathodes of LICs. Various
techniques for constructing graphene-based architectures for LICs have been comprehen-
sively documented in recent years [13,41,63,98]. Importantly, integrating microporous car-
bon with surface connectivity into graphene structures is an important technique for fur-
ther increasing the LIC's strength and energy densities.
4. Design of Lithium-Ion Capacitors
In terms of LIC design, the process of pre-lithiation, the working voltage and the
mass ratio of the cathode to the anode allow a difference in energy capacity, power effi-
ciency and cyclic stability. An ideal working capacity can usually be accomplished by in-
tercalating Li+ into the interlayer of graphite. In this way, it is possible to achieve reduced
Energies 2021, 14, 979 10 of 28
electrode resistance, increased energy density and stable cycling efficiency [63]. A very
critical element in maintaining power equilibrium is the mass ratio of the cathode to the
anode (m+/m). Taking into account the aforementioned criteria, finding a compromise
between the correct degree of pre-lithiation, an effective functioning voltage window and
the optimum m+/m is of considerable importance. Although some researchers perform
pre-lithiation on one or both of the electrodes, others depend entirely on Li in the electro-
lyte (e.g., from LiPF6), resulting in reduced performance due to the loss of active Li
[59,121,122]. The need for a solid electrolyte interface (SEI), its creation, continuous
growth and cracking is a major problem correlated with the usage of carbon anodes. Dur-
ing cycling, one will need to hold the SEI steady and prevent Li plating that involves care-
ful regulation of the negative electrode voltage, temperature and effective current density.
Therefore, a carbon–carbon design requires an asymmetric mass loading with the weight
of the lower-capacity adsorption carbon cathode being up to 5 times higher than that for
the lithium-containing anode.
4.1. Pre-Lithiation Strategy
During the production of LICs, a pre-lithiation cycle is performed to the active mate-
rial or whole electrode of the anode guarantee that it operates at fairly low potential and
to supply lithium ions for the anode-side insertion/extraction reaction. In turn, the added
lithium through pre-lithiation reduces the electrode resistance and accounts for the per-
manent active lithium loss during cycles induced by the creation of solid electrolyte inter-
face films (SEI) on the anode. The following can be performed using distinctive pre-lithi-
ation approaches [13,30,45,63]: the electrochemical process (ECP); the short-circuit pre-
lithiation through external short-circuit (ESC) and internal short-circuit (ISC) methods; the
introduction of permanent lithium transition metal oxides (LTMOs) on the cathode side;
and, the application of sacrificial lithium chloride to the electrolyte. Pre-lithiation methods
can also be adapted between different capacitor chemistries (e.g., LIC, NIC and KIC) [123].
Jin et al. (2020) give a thorough review of pre-lithiation technologies in LICs, outlining the
current progress and perspectives [124].
An important step in creating high-performance LIC systems is the pre-lithiation of
the electrodes. According to the restricted solubility of salts such as LiPF6 or NaClO4 in
carbonate solvents, permanent ion loss results in ionic conductivity degradation [63]. Dur-
ing prolonged cycling, devices focused on non-lithiated electrodes have been shown to
have decreased lithium ion content in the electrolyte, significantly deteriorating the over-
all LIC potential [81,121]. Besides SEI, Li can be trapped in the majority of electrode mate-
rials (e.g., hard carbons) [125]. Pre-charging at least one electrode is a key element in eval-
uating a hybrid device's output in terms of initial energy, power conservation and cycling
conservation [63]. A study of Ragone properties typically shows that, in addition to the
electrolyte [80,81,126,127], systems at the peak of the energy–power range are those that
utilise a secondary supply of ions.
This training compensates for the depletion of the Li ion induced by initial SEI form-
ing and permanent bulk trapping. As long as there is no substantial increase in SEI and
bulk trapping during cycling, the concentration of electrolytes stays relatively constant.
Pre-lithiation has also been successfully used to expand the voltage window and extend
LIC device cyclability by regulating the anode’s voltage oscillation, thereby reducing the
load depth and related SEI production, volumetric expansion and ion trapping [72,81,121].
The most prevalent pre-lithiation procedure is electrochemical deposition. A closed
electrical circuit develops bonding between the anode and the lithium metal [128]. This
uses an electrically insulating and ionically conducting separator, similar to what is found
in LIBs. Depending on the pre-doping process controllability [72,128130], the external
circuit can contain an electrical capacitor or resistor, or even a short circuit. The charging
cycle can be operated with a programmable system added to the terminals [74,131]. The
main drawback to this approach is extra cell assembly/disassembly procedures, where the
Energies 2021, 14, 979 11 of 28
unit ends up being assembled twice. While scientifically useful and to some extent con-
venient, it is not obvious how the industry can make such an approach cost-effective.
A theoretically more efficient solution could be based on chemical doping in the pres-
ence of an electrolyte [132134]. This would occur through the direct reaction between the
electrode substrate and lithium. The main advantage of this approach is its sleek simplic-
ity and scale-up potential for industries; however, lithiation is in effect unobservable,
meaning the state of charge (i.e., lithium content in the electrode), cannot be controlled or
monitored until after the doping is complete. Despite this, research has been performed
to alleviate this by constructing LICs with multielectrode structures to escape cell-reas-
sembly and track the electrical signals during doping [74,135,136].
Cao and Zheng [77] demonstrated a three-electrode hybrid configuration that used
stable lithium energy. Based on this theory, other types of lithium, such as lithium silicide,
lithium-rich transition metal oxides, were used for different anodes (e.g., steel, silicone,
etc.) [114,127,132,137]. Pre-lithiation has traditionally been done on the anode, which in
turn is contradictory to how LIBs are produced; however, pre-lithiation on the cathode
has been observed [115]. That would be potentially better because the system is instead
constructed in a thermodynamically stable manner (which may not be the case with a
symmetric unit, e.g., carbon–carbon).
During LIB production, lithium-rich ion-donating materials show a strong hysteresis
during charging and discharge. This in turn causes a cycle of permanent de-lithiation dur-
ing the LIBs first charge. The ions are then inserted between the main storage processes in
the electrolyte and transfer back and forth during subsequent cycling. In this way materi-
als such as Li3N [129,138] and Li6CoO4 [128,139] were able to be used as cathode additives
to minimise failure. It may be possible to directly transfer this method to LICs, for exam-
ple, Lukatskaya et al. [56] reported an example of the concept of a chemical additive ion
source that could advance LIC technology due to its efficacy and scalability [56].
In a LIC, the activated carbon can contain sacrificial synthetic lithium salt (3,4-dihy-
droxybenzonitrile dilithium) [13]. This compound is irreversibly decomposed after the
first cycle, with the residues dissolving in the electrolyte. It acts as a lithium-ion source
inside the graphite anode without adversely impacting the electrochemical output of the
LIC. Due to the ease of converting this principle into industrial production of various LIC
electrodes, the sacrificial additive method seems to have the greatest potential for practical
implementations among all pre-charging approaches. In addition, the recent development
of a cascade-based methodology for pre-lithiation may lead to advances in the stability of
pre-lithiation of anodes.
4.1.1. Electrochemical Pre-Lithiation
Lithium metal is used as the counter-electrode for the EC method, with graphite be-
ing used as a functional electrode. Lithium ions migrate from the metal to the graphite by
cycling at a fairly small current, during which the lithium electrode and graphite electrode
become physically separated. Lee et al. [73] obtained pre-lithiated graphite by cycling
graphite half-cells 2 times at 0.1 °C followed by a discharge of the LIC to 0.05 V during the
third cycle. Similarly, by combining a mesocarbon microbead working electrode with a
lithium reference electrode, Zhang et al. [140] formed pre-lithiated mesocarbon mi-
crobeads. The merit of this method is that, by setting the cut-off capacity and producing a
stable SEI layer, the lithiated content can be precisely controlled. Nevertheless, the reas-
sembly phase of the pre-lithiated anode is a time-consuming procedure unfeasible cur-
rently for use in industrial production.
4.1.2. Short-Circuit Pre-Lithiation
Often pre-lithiation of short circuits may be split into ESC (external short-circuit) and
ISC (internal short-circuit) techniques. For the ESC process, a sacrificial lithium metal elec-
trode and a targeting electrode (e.g., graphite) are separated using a porous polymer sep-
arator in a non-aqueous electrolyte and are connected to wire to naturally facilitate the
Energies 2021, 14, 979 12 of 28
penetration of lithium ions into the electrode [81,141]. Kim et al. [142] compared the dif-
ferent pre-lithiation approaches and showed that the ISC method was straight-forward
and effective for obtaining high energy intensity DC-LICs. In the presence of an electrolyte
media, the ISC technique is performed by solid metallic lithium (e.g., Li metal or Li pow-
der) touching a carbon anode directly. Cao and Zheng [77] described a network of hard
carbon/activated carbon DC-LICs by utilising a mixture of stable lithium metal powder
(SLMP). Once the hard carbon—SLMP mass ratio reached between 5:1–8:1, a healthy life
cycle was obtained. The extra Li powder-covered hydrogen fluoride (HF) against etching
of the SEI layers. The biggest benefit of short circuit pre-lithiation is its straight-forward
usability and scale-up. Despite this, the key disadvantage is that lithium utilisation cannot
be precisely regulated [132].
4.1.3. Irreversible Transition Metal Oxides at the Cathode
Compared to the usage of metallic lithium, the use of permanent electrochemical lith-
ium-transition metal oxides as a pre-lithiation agent will significantly enhance the health
of LICs [63]. Usually, the chosen lithium transition metal oxides only operate on the first
charge cycle to facilitate the incorporation of lithium cations into the graphite anode.
Li6CoO4, Li5FeO4, Li2CuO2, Li5ReO6 and Li2RuO3 have been developed to be used as pre-
lithiation agents [115,143146]. Through this method, the degree of pre-lithiation can be
managed by regulating the volume of lithium metal oxides in the cathode.
4.1.4. Adding Sacrificial Organic Lithium Salt
Jeżowski et al. [56] published a pre-lithiation process for sacrificial organic lithium
salt, using 3,4-dihydroxybenzonitrile dilithium salt (Li2DHBN) as a sacrificial salt in 1 M
LiPF6/EC – DMC. The insoluble Li2DHBN was combined with activated carbon and re-
leased during the first charge cycle to form the soluble 3,4-dioxobenzonitrile (DOBN). The
Lithium ions intercalated into the graphite to achieve pre-lithiation. Of such approaches,
stable metallic lithium powder and the inclusion of permanent lithium transition metal
oxides could have the most promising prospects for industrial production; however, mon-
itoring the lithiation degree and health issues are still relevant factors. Recently, Li3N was
employed as a sacrificial lithium compound for metal-free pre-lithiation by adding it to
the electrode mixture during active material synthesis. Using this additive, stable cycla-
bility of 91% after 10,000 cycles was observed [147].
4.2. Electrolytes
The electrolyte often serves as an integral part of the pairing LIC electrodes, greatly
influencing the energy capacity, power efficiency, and cycling stability. To obtain suffi-
cient energy and power efficiency, multiple electrolyte specifications should be met [6]:
outstanding ionic conductivity; strong electrical insulation; large voltage operating win-
dow; exceptional stability; and, ideally low toxicity. Electrolytes can be classified into 4
types: aqueous electrolyte; organic electrolyte; ionic liquid electrolyte; and inorganic
solid-state electrolyte. Aqueous electrolytes have high ionic conductivity and low re-
sistance, favourable for ion transfer; however, the working voltage range is generally less
than 1.2 V and is constrained by water decomposition [148]. Due to their marginal insta-
bility, good thermal, chemical and electrochemical resilience, low flammability and supe-
rior conductivity, ionic liquid-based electrolytes have been examined [149,150]; however,
their use is not ideal industrially due to their high economic costs. A solid-state polymer
may be an acceptable alternative electrolyte because the ionic conducting medium has
electronic separators that preserve the LICs health through high cycle numbers. While in
recent years solid-state polymer electrolytes have flourished and considerable improve-
ment has been made, their ionic conductivity needs to be significantly enhanced for room
temperature use.
Energies 2021, 14, 979 13 of 28
Organic electrolytes may have the best prospects for commercially available LICs.
They have a larger voltage window than aqueous electrolytes, and their operating condi-
tion is milder than that of solid-state polymer electrolytes and ionic liquid electrolytes.
Organic electrolytes are commonly used in LIBs and LICs; however, their high economic
cost, low potential power, low conductivity and health concerns linked to flammability,
variability and toxicity are problematic [149,150]. Despite this, a modern polyethylene gly-
col-functionalised polysilsesquioxane has been developed for the manufacture of hybrid
ionogel electrolytes for LICs [151]. This gave the electrolyte excellent thermal stability and
superior ionic conductivity.
The most significant or distinct required components in electrolytes are lithium salts,
solvents and additives. Solvents such as polycarbonates, ethers, ethylene carbonates, di-
ethyl carbonates, dimethyl carbonates, propylmethyl carbonates and ethyl methyl car-
bonates can dissolve lithium salts and transport lithium ions [72,81,152,153]; however, the
SEI shape and structure can be highly affected by the use of solvents. Additives are con-
nected to other compounds that are applied to the electrolyte and can effectively boost
efficiency. According to the different effects of the additives, they can be classified as im-
proving ion conductivity; improving device properties (e.g., the SEI); improving low-tem-
perature efficiency and thermal stability; preventing overloading; and, reducing electro-
lyte acid and water content [6,154,155].
4.3. Modelling and Simulation of LICs
LICs have a clear advantage in offering high power and energy density within a sin-
gle energy storage system while maintaining a longer cycle life. Although there has been
substantial research into materials and chemistries for the production of LICs, there is a
requirement for research on the effect of electrode balancing and pre-lithiation on the LICs
usable energy. Computational approaches have been able to provide aid in determining
these effects using physics-based models based on experimental data.
When designing an LIC, many factors need to be taken into account to ensure accu-
rate hybridisation of the LIB components with the traditional capacitor components. Us-
ing theoretical and experimental data, computational modelling has been able to be used
to assist this process. Moreover, such modelling has allowed theoretical guidelines to aid
the design of LICs to obtain optimal operation. Choi and Park (2014) developed a thor-
ough theoretical analysis that aids the design of high-performance LICs, and also devel-
oped guidelines to improve the development phase of new LICs [156]. In summary, these
guidelines state that:
battery and capacitor components must be homogeneously hybridised to operate as
a single electrode (see [157] for in-depth hybridisation approaches and principles);
the capacitor component must be highly electronically-conductive and able to store
electrochemical energy in the organic electrolyte by electrostatic force, and,
electrode material must have a high surface area to achieve high energy density and
an open porous structure to achieve optimal ionic conduction.
Ghossein et al. (2018) noted that there were limitations in using LIB and supercapac-
itor models based on electrochemical impedance spectroscopy for LICs [158]. They stated
that these models were not suitable for accurately describing the impedance of LICs at
low frequencies. They concluded that this was due to the size of the pores in the cathode
being identical to the anode in the LIC and supercapacitor models. Therefore, they intro-
duced a new model that was specific to LICs and includes accurate pore sizes and distri-
butions of the cathode. This allowed accurate modelling of impedance values at all fre-
quencies when compared to experimental data. Madabattula el al. (2019) developed a
model to study the relationship between usable energy at a variety of effective C rates and
mass ratios of the electrodes in a LIC [159]. By extending the model to analyse the pre-
lithiation requirement, they were able to determine the limits of pre-lithiation in Lithium
Titanium oxide anodes, and how negative polarisation of the activated carbon in the
Energies 2021, 14, 979 14 of 28
cathode can improve cell capacity. In addition, in a LIC cell with a higher mass ratio, the
authors were able to relate the effects of electrolyte depletion with poor power perfor-
mance.
5. Knowledge and Research Gaps
Low Coulombic-related decreases in available charge in batteries are usually corre-
lated with the irreversible and continuous creation of an SEI and certain cathode electro-
lyte interlay (CEI) [160,161]. This can also be the case with LICs. LICs usually have a
smaller Lithium pool than traditional LIBs, have more strict criteria for reducing internal
resistance losses attributable to high voltage and undergo more cycling. All three of these
aspects render SEI forming a substantial challenge for LICs, and due to the wide voltage
window, CEI formation can result in plugging of surface pores and, therefore, a loss of
active surface area [13].
SEI products are expected to be similar to what is observed with other types of car-
bons used in LIBs, due to carbonate electrolyte reduction in LICs [162,163]. The electrolyte
solutions solvents and salts are both thermodynamically unstable and undergo reduction
on the anode during charging (when the anode is temporarily a cathode), which may op-
erate at a potential near (or below) that of metallic lithium [164,165]. Therefore, a substance
with a wide surface-to-volume ratio can induce higher irreversible losses in terms of per-
formance [166,167]. Such reduction films may make the anode surface unreactive, protect-
ing the electrolyte solution from further decomposition; however, any volumetric changes
experienced during electrochemical cycling may weaken and fracture the SEI layer, with
each cycle exposing fresh material to the electrolyte [109,168170], creating a new layer of
the SEI.
The SEI comprises primarily of electrolyte reducing materials. The instability of the
SEI will inevitably trigger the LIC unit to lose overall power and fail [168,171173]. Apart
from solvent-reduction products such as Li2CO3 and alkyl carbonates, the SEI anode also
partially consists of LiF, which is a decomposition product of the LiPF6 salt but can also
be produced by reaction with trace amounts of water to HF and eventually LiF [174,175].
Research on this points to radial compositional and structural gradients inside the SEI
sheet, as well as complex growth-shrinking characteristics with cycling [176179].
For a variety of carbon-supported nanomaterials (e.g., oxides and sulphides), an ad-
ditional aspect relating to SEI formation in hybrid ion capacitor anode materials is linked
to the cycling-induced capability gain sometimes mentioned in the literature [13]. An in-
crease in the performance induced by cycling is not uncommon for Li-based anodes, es-
pecially for oxides [180184]. This phenomenon has been attributed to a contribution to
load-storage through surface adsorption as a result of the extra surface area created by
conversion compound cycling [184,185]; however, recent discussions have suggested this
is not the case as when a carbonate electrolyte is used, the surface area of both electrodes
during cycling decreases considerably [13]. They further claim that the increase in power
is due to the internally parallel nanostructured anodes rather is the cycling-induced re-
versible forming of a polymer gel on the surface of the nanostructured electrode during
lithium charging. This Faradaic cycle can be mechanistically identical to reversible and
high Coulombic quality redox reactions in polymer-based electrochemical capacitors
[186188].
In LIC devices a parasitic oxidation substance (i.e., CEI), is most likely formed on the
cathode. The most intuitive evidence for this oxidation layer is the commonly observed
loss of the first-cycle capacity for carbon cathodes in carbonate electrolytes [98,189,190]. A
secure passivation sheet could prevent any further oxidation of electrolytes on the elec-
trode surface [191194]. The chemical structure of the passivation substrate (e.g., size or
type) on the cathodes has been observed to differ greatly from material to material [193].
Surface characterisation methods (e.g., X-ray photoelectron spectroscopy and atomic force
microscopy), have been used to study passivation layers in cathodes [193196]; however,
Energies 2021, 14, 979 15 of 28
the CEI chemistry, composition or voltage impact on its development are yet to be ana-
lysed.
In terms of its chemistry and composition, the CEI in LICs can be somewhat different
from that developed on the classic LIB intercalation cathodes [13]. CEI creation is cata-
lysed by a faradic mechanism. The sum of irreversible ability due to CEI should be directly
linked to the surface area and the functionality of the surface heteroatoms, as well as the
electrolyte carbonate types. Electrochemical impedance analysis has been used to observe
the development and evolution of the passivation layer on a carbon cathode in an indirect
way [77,197,198]; however, more in-depth analysis of the CEI in LIC devices is required,
because its growth may be important to performance.
Besides the SEI and CEI materials used in LICs, carbon-based electrodes can suffer
from Li plating to a greater degree than as observed in LIBs. This specifically applies to
graphite, which continues to plate Lithium at various charging rates [199206].
Nanostructured lithium titanate could greatly reduce the magnitude of lithium metal
plating [121]; however, Li4Ti5O12 alone is not adequately electrically conductive to reach
the high levels required in LICs. The ideal method to prevent metal plating is to ensure
that the anode stays far enough from 0 V vs. Li / Li+, especially at high charging levels;
however, this would require the quantification of a three-to-four-electrode cell (including
reference electrodes), something that is rarely done. It should also be pointed out that the
voltage swing of the individual electrodes can only be estimated from their mass-to-ca-
pacity ratio in a true two-electrode cell. This causes more problems in specifically regulat-
ing the composition of both the metal and SEI.
6. Next-Generation Lithium-Ion Capacitors
Much research in recent years has revolved around developing the electrodes used
in LICs. As a result, many types have been observed to have varied performance in LIC
designs. Table 3 gives an overview of the different materials researched recently.
6.1. Pseudocapacitive Oxides
Apart from carbons and titanium compounds, a variety of new anode materials is
available that are either specifically designed for LICs or are added to specific systems
that have original uses in LIBs. Those nanomaterials are primarily used for anodes, not
for cathodes. Conversely, even the best carbon-based adsorption cathodes offer a fraction
of the capacity of existing LIC anodes. Cathode work in the field is even more important,
as LIB cathodes are not specifically transferable.
Because of their fast charging and discharging kinetics, the pseudocapacitive materi-
als emerge as a significant subset of materials for LICs, together with higher gravimetric
and v olumetr ic efficien cy re lativ e to true EDLC e lectrod es [5]. Augu styn et al. [ 54] d emon-
strated that the Li+ intercalation into the orderly channels of bulk orthorhombic T-Nb2O5
was straight-forward, making the charging behaviour capacitor-like. These materials
were referred to as pseudocapacitive intercalation compounds since, although the Li stor-
age mechanism was not considered EDLC, EDLC-like triangular galvanostatic curves and
box-like CVs were shown. Despite this, these LICs produce at fairly low power a maxi-
mum of 76 Wh kg1 rendering their total effective energy around a factor of 2 lower than
that for carbon-based electrodes [207].
Vanadium oxide V2O5 undergoes a bulk ion intercalation reaction during reversible
charging, also producing a sloping profile similar to a capacitor when used as an anode.
Bulk V2O5 can accommodate electrolyte cations (e.g., H+, Li+ and K+) in aqueous systems
[208210]. They also mixed elemental analysis and X-ray diffraction to analyse the charg-
ing-storage process of V2O5 in an aqueous environment. They also discovered that K+ ions
inject gaps between the (00l) Miller index planes into the interlayer, and an average energy
of 42 Wh kg1 can be achieved with a loss of < 5 % over 10,000 cycles [208]. A V2O5-based
hybrid solution can also operate in a far wider voltage window in Li+ organic systems.
Energies 2021, 14, 979 16 of 28
The authors tested the same materials V2O5 on CNT in lithium structures [210]. The LIC
cell energy value was in the region of 40 Wh kg1, similar to aqueous electrolyte systems.
6.2. MXenes
MXene is a large family of metal carbides and carbonitrides in the two-dimensional
form [211]. It has been observed that several cations (Na+, K+, Mg2+ and Al3+) can intercalate
reversibly through the bulk of exfoliated multilayer T3C2Tx MXene in an aqueous electro-
lyte. It was also stated that Li+ ions would intercalate reversibly between layers of MXene
in the organic electrolytes [212]. Because there is a significant intercalation reaction, the
MXene shows pseudocapacitor CVs and galvanostatic profiles, and could therefore be
grouped into the pseudocapacitive substance family [211213]. In addition, the electro-
chemical analysis revealed that storage of Li was a reaction rather than a diffusion-con-
trolled Faradic cycle, similar to the other pseudocapacitive materials.
An LIC system based on Ti2C coupled with Kuraray YP17 activated carbon has also
been proposed [214]. Here, the Ti2C anode showed moderate working voltage with the
LIC operating at an upper cut-off voltage of 3.5 V and delivering maximum energy of 50
Wh kg1. Wang et al. [214] also synthesised 3D TiC nanoparticle chains operating in a
lower voltage (0.7 V vs. Li / Li+) area. The resulting LIC system, together with a high-
capacity, nitrogen-doped porous carbon cathode, provided 101.5 Wh kg1 of energy and
23.4 Wh kg1 energy at an intense strength of 67,500 W kg1. Additionally, Luo et al. [215]
developed a CTAB-Sn pillared Ti3C2 MXene-based LIC system that provided energy of
105.6 Wh kg1. An important issue worth pursuing is the cyclability of these materials.
Whether the existing intercalation compounds can survive such extended service and, if
not, whether their structure and/or electrolyte chemistry can be tuned to enhance perfor-
mance is still unknown.
6.3. Conversion Compounds
Reversible materials dependent on reaction conversion are another type that was
considered pseudocapacitive and has gained popularity as a substrate for the LIC elec-
trodes. A conversion electrode is defined initially in LIB literature as the crystalline or
amorphous AxBy material, which then decomposes reversibly into one or two Lithium
compounds after charging [216]. Some examples of the structures used for LICs are MoS2,
NbN, VN, MnO, Fe2O3/Fe3O4, NiCo2O4 and various alloys. Although MoS2 undergoes a
Lithium intercalation reaction down to 1.1 V, most reversible capability stems from re-
versible Mo and Li2S conversion reaction down to 0 V [217]. The reversible capability of
MoS2 is stated to be as high as 1000 mAhg1, which actually is higher than the theoretical
figure. The discrepancy may be due to a capacitive contribution and a reversible SEI
growth contribution [218]. Wang et al. [219] used MoS2-graphene composites in LICs, us-
ing sloping charge and discharge profiles, with the LIC delivering energy density as high
as 188 Wh kg1 at 200 W kg1 and 45.3 Wh kg1 at 40,000 W kg1. MoS2's functional efficiency
was around 600 mAh per gram and helps balance the sloping voltage plateau with fast
energy delivery. The outstanding rate capability and the retention of cycling power pos-
sibly emerged in the anode from the engineered secondary carbon-based process.
6.4. Battery-Related Intercalation Ceramics
Research has been performed using ceramic LIB cathode materials coupled against
carbon counter electrodes, developing a high-power LIB with a more sloping tension pro-
file. Aravindan et al. [220] provided an excellent description of intercalation-type materi-
als for configurations of LICs. The overwhelming majority of such architectures exhibit
cyclability like batteries, lasting many thousands of cycles. For example, layered oxides,
spinel oxide, olivine, NASICON and silicates are specific forms of ceramic battery cath-
odes used in LICs. Olivine LiFePO4 is a well-established LIB commercial cathode which
has also seen applications in LIC systems. During lithium intercalation/deintercalation it
Energies 2021, 14, 979 17 of 28
undergoes a two-phase reaction and shows a flat plateau at 3.4 V. Ping et al. [221] con-
structed a hybrid activated carbon + LiFePO4 composite as the cathode and mesocarbon
microbeads as the anode. The unit was cycled between 2 and 4 V with an average energy
density of 69 Wh kg1 and a lifespan of 100 cycles with marginal decay.
NASICON cathodes have also recently gained recognition for applications in hybrid
systems. The most frequently studied NASICONs are the phases Li3V2(PO4)3 (LVP) and
Na3V2(PO4)3 (NVP), with vanadium as the active metal transfer part. Satish et al. [105]
incorporated an LVP-C cathode with activated carbon as an anode in a LIC device, ob-
taining 25 Wh kg1. For an activated carbon / LVP configuration an energy density of 28
Wh kg1 was observed [222]; however, the energy efficiency becomes greatly increased
(125 Wh kg1 at 300 W kg1) where activated carbon is used as a cathode when LVP is the
anode.
Table 3. Next-generation electrode materials for LICs.
Configuration (anode//cathode) Voltag
e
Max Energy
(Wh/kg) at Power
(W/kg)
Energy (Wh/kg) at
Max Power (W/kg) Cyclability
T-Nb2O5-graphene//activated carbon
[223] 0.8–3 V 47 at 393 15 at 18,000 93% over 2000
cycles
mesoporous Nb2O5–C//activated
carbon [224] 1–3.5 V 74 at 120 20 at 12,137 Not reported
V2O5 on CNT//activated carbon
[210] 0–2.7 V 40 at 210 6.9 at 6300 78% over
10,000 cycles
γ-LixV2O5-BM50//activated carbon
[225] 0–4.5 V 54.59 at 230 Not reported 100% over 400
cycles
CTAB-Sn on Ti3C2 MXene//activated
carbon [215] 1–4 V 105.6 at 495 45.3 at 10,800 70% over 4000
cycles
Ti2C MXene//YP17 active carbon
[212] 1–3.5 V 50 at 190 15 at 600 85% over 1000
cycles
TiC MXene//N-doped porous
carbon [214] 0–4.5 V 101.5 at 450 23.4 at 67,500 82% over 5000
cycles
T
-Nb2O5 on C//MSP-20 activated
carbon [226] 1–3.5 V 63 at 70 10 at 6500 75% over 1000
cycles
Nb2O5-carbide-derived carbon//YP-
50F AC [227] 1–2.8 V 30 at 220 18 at 5000 Not reported
Nb2O5–CNT//activated carbon [228] 0.5–3 V 33.5 at 82 4 at 4000 Not reported
LiNbO3 on graphene aerogel//boron
carbonitride nanotube [229] 1–4 V 148 at 200 69.4 at 9900 82% over 7000
cycles
MoS2–C-RGO//PANI-derived
porous carbon [219] 0–4 V 188 at 200 45.3 at 40,000 80% over
10,000 cycles
NbN//activated PANI-derived
carbon [230] 0–4 V 149 at 200 5 at 45,000 95% over
15,000 cycles
VN-rGO//activated carbon [231] 0–4 V 162 at 200 64 at 10,000 83% over 1000
cycles
MnO cubes//activated carbon [232] 0–4 V 227 at 55 21 at 2952 93% over 3000
cycles
3D MnO array//activated carbon
nanosheets [233] 1–4 V 184 at 83 83 at 18,000 83% over 5000
cycles
MnO nanoparticles//activated
carbon [234] 0–4 V 220 at 100 35 at 2608 95.3% over
3600 cycles
MnO on C//trisodium citrate-
derived carbon [235] 0–3.9 V 235 at 120 61 at 25,000 85.69% over
10,000 cycles
MnNCN//activated carbon [236] 0.1–4 V 103 at 150 22 at 4500 100% over
5000 cycles
Energies 2021, 14, 979 18 of 28
FexO on graphene//porous graphene
[237] 0–3.5 V 129.6 at 19 45 at 3500 75% over 3000
cycles
Fe2O3//activated carbon [238] 0–3.5 V 90 Not reported 55% over 2500
cycles
Fe3O4 in graphene//3D graphene
[239] 1–4 V 204 at 55 85 at 2650 70% over 1000
cycles
NiCo2O4//activated carbon [240] 0–4.5 V 39.4 at 120 10 at 554 100% over
2000 cycles
activated
carbon//LiMn1/3Ni1/3Fe1/3O2–PANI
[241]
0–3 V 49 at 900 19 at 3000 100% over
3000 cycles
activated carbon//LiMn2O4 [242] 0.7–3 V 45 at 60 10 at 800 Not reported
LiNi0.5Mn1.5O4//activated carbon
[243]
1.5–
3.25 V 19 at 120 8 at 3500 81% over 1000
cycles
mesocarbon microbeads//LiFePO4
[221] 2–4 V 69 Not reported 100% over 100
cycles
Li3V2(PO4)3//activated carbon [222] 0–4 V 125 at 300 65 at 6000 80% over 200
cycles
activated carbon//Li3V2(PO4)3 [222] 0–2.5 V 28 at 35 14 at 1500 87% over 1000
cycle
activated carbon//Li3V2(PO4)3 [105] 0.5–2.7
V 25 at 88 13 at 320 Not reported
Li2MnSiO4//activated carbon [244] 0–3 V 54 at 150 37 at 1500 85% over 1000
cycles
LiMnBO3//PANI [245] 0–3 V 42 at 1500 15 at 5350 91% over 1000
cycles
CoNiP2O7//activated carbon [246] 0–4 V 116.3 at 200 66.7 at 6486.5 86.5 over 500
cycles
7. Concluding Remarks
The final power and energy of LIC systems are based primarily on design, charging-
storage structure and materials used in electrodes. Overall, designs with the smoothest
anode and cathode voltage, and the highest overall voltage gap are ideal. Although exist-
ing LIC designs are reasonably straightforward in material terms, the associated high-rate
charging and storage mechanisms in the electrodes (especially in the anode) remain
poorly understood and require further research and development. Key unresolved issues
for LICs include electrode design, energy-to-power limitations, fast charging mechanisms
in anodes that vary from LIB activity, SEI formation, and LIC cycling (although superior
to LIBs, not yet at traditional EDLC level).
Because of SEI development, the risk of low-voltage metal plating and volume ex-
pansion, it would seem as though the anode is the bottle-neck for cycling life. Although a
high-surface-area cathode may function at a voltage where a CEI is formed, the overall
chemistry would be less harmful due to reduced volume expansion (CEI does not accu-
mulate through cycling). Despite this, the creation of the CEI for LICs is almost entirely
unexplored. In terms of energy density, there currently does not appear to be an ideal
cathode material that could operate at comparable capacity as the anode while maintain-
ing cycling stability. Activated carbon is stable but offers up to one-fifth of a hard carbon
anode reversible capacity. More specifically, many features of carbon-based electrodes in
LICs still need further investigation. One is cycling stability during high current density
testing. As long-term cycling stability is essential for LICs, it appears important to evalu-
ate the advantages of using carbon-based electrodes in LICs.
The developments in LICs are mainly due to the production of advanced carbon-
based electrodes. Although significant strides have been made in the manufacture of car-
bon-based electrodes, more research remains to be done. The inherent electrochemical
performance of the electrode materials and the growth of active materials on the electrode
Energies 2021, 14, 979 19 of 28
depends on the porous structure and surface properties of the materials used. Although
many materials have been designed to either serve as the electrodes or to assist other ac-
tive materials, their microstructures are disordered and random in most situations, their
surface properties are not well regulated and some pores of these materials are not elec-
trochemically available for the electrolyte, restricting their electrochemical efficiency. Fur-
thermore, the physical properties of some materials provide additional areas of concern
through their deformation during charge and discharge cycles. Although ambitious, at-
tention should be given to the design of electrodes with precisely controllable microstruc-
tures.
Author Contributions: Conceptualization, J.J.L. and O.S.B.; methodology, J.J.L. and O.S.B.; soft-
ware, J.J.L. and O.S.B.; validation, J.J.L. and O.S.B.; formal analysis, J.J.L. and O.S.B.; investigation,
J.J.L. and O.S.B.; resources, J.J.L. and O.S.B.; data curation, J.J.L. and O.S.B.; writing—original draft
preparation, J.J.L. and O.S.B.; writing—review and editing, J.J.L. and O.S.B.; visualization, J.J.L and
O.S.B.; supervision, J.J.L. and O.S.B.; project administration, O.S.B.; funding acquisition, O.S.B. All
authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding. The APC was funded by ENERSENSE, NTNU.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.
Acknowledgments: Jacob J. Lamb and Odne S. Burheim acknowledge the support from the EN-
ERSENSE research initiative at NTNU. We would also like to extend our thanks to Svein
Kvernstuen (CEO, Beyonder), Kristin Skofteland (CCO, Beyonder) Fengliu Lou (R & D manager,
Beyonder) and Dmytro Drobnyi (senior engineer, Beyonder).
Conflicts of Interest: The authors declare no conflicts of interest.
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... The prelithiation process is vital in LIC cell assemblies because it has a large impact on various cell performance characteristics, such as cell capacitance, internal resistance, energy and power density, rate capability, and cycle stability [4,5]. In this study, a Si-based anode that was fully lithiated by lowering the potential to 0 V vs. Li/Li + in a half-cell configuration was incorporated into a three-electrode full cell with an AC cathode (hereafter defined as 100% prelithiation cell), which provided variations in cathode and anode potentials during the rate and cycle tests. ...
Article
Full-text available
The impact of full prelithiation on the rate and cycle performance of a Si-based Li-ion capacitor (LIC) was investigated. Full prelithiation of the anode was achieved by assembling a half cell with a 2 µm-sized Si anode (0 V vs. Li/Li+) and Li metal. A three-electrode full cell (100% prelithiation) was assembled using an activated carbon (AC) cathode with a high specific surface area (3041 m2/g), fully prelithiated Si anode, and Li metal reference electrode. A three-electrode full cell (87% prelithiation) using a Si anode prelithiated with 87% Li ions was also assembled. Both cells displayed similar energy density levels at a lower power density (200 Wh/kg at ≤100 W/kg; based on the total mass of AC and Si). However, at a higher power density (1 kW/kg), the 100% prelithiation cell maintained a high energy density (180 Wh/kg), whereas that of the 87% prelithiation cell was significantly reduced (80 Wh/kg). During charge/discharge cycling at ~1 kW/kg, the energy density retention of the 100% prelithiation cell was higher than that of the 87% prelithiation cell. The larger irreversibility of the Si anode during the initial Li-ion uptake/release cycles confirmed that the simple full prelithiation process is essential for Si-based LIC cells.
... Increasing the effective specific surface area reduces the ESR resulting in power density enhancement. Moreover, increasing the electrode material's conductivity and employing an aqueous electrolyte would also increase the power density [30]. ...
Article
Full-text available
This review paper aims to provide the background and literature review of a hybrid energy storage system (ESS) called a lithium-ion capacitor (LiC). Since the LiC structure is formed based on the anode of lithium-ion batteries (LiB) and cathode of electric double-layer capacitors (EDLCs), a short overview of LiBs and EDLCs is presented following the motivation of hybrid ESSs. Then, the used materials in LiC technology are elaborated. Later, a discussion regarding the current knowledge and recent development related to electro-thermal and lifetime modeling for the LiCs is given. As the performance and lifetime of LiCs highly depends on the operating temperature, heat transfer modeling and heat generation mechanisms of the LiC technology have been introduced, and the published papers considering the thermal management of LiCs have been listed and discussed. In the last section, the applications of LiCs have been elaborated.
... Increasing the effective specific surface area reduces the ESR that resulting in enhancing the power density. Moreover, increasing the electrode material's conductivity and employing an aqueous electrolyte would also increase the power density [43]. ...
Thesis
Lithium-ion capacitors (LiCs) are hybrid energy storage systems that combine the advantages of lithium-ion batteries (LiB) and electric double-layer capacitors (EDLC). Therefore, LiCs have higher power capability and longer lifetime compared to LiBs. LiCs have also higher energy density and higher voltage range than EDLCs. Based on the mentioned advantages, LiCs are perfect solutions for high power applications where high charge and discharge currents are applied. Nevertheless, LiCs’ performance highly depends on temperature. Therefore, a robust thermal management system (TMS) is indispensable to ensure reliability. Such a system-level management is linked to robust modeling tools, which are called electro-thermal models. The need for a validated electro-thermal model is trivial in high power applications where LiCs are subjected to high current rate of 150 A, which is typical for fast acceleration of electric vehicles. In this PhD dissertation, the target cell is a commercial prismatic 2300 F LiC with 1 Ah capacity. A holistic methodology has been considered for electrical, thermal, and lifetime models that are developed in MATLAB/SIMULINK® environment for high dynamic current rates under a wide range of temperatures from -30 °C to +60 °C. The electrical model is validated against the experiments considering the voltage measurements. The thermal model is validated against the experiments by temperature verification. Additionally, the lifetime model is validated against the experiments considering the capacity degradation. A critical parameter from the developed 1D model is the LiCs’ power loss that is an input for the 3D thermal analysis of the proposed active, passive, and hybrid thermal management systems (TMS). The calculated power loss enables us to investigate the heat dissipation and temperature pattern inside the LiC cell. Such a coupled 1D/3D model for the LiC technology was not found during the comprehensive literature review. The proposed TMSs in this PhD thesis are active, passive, and hybrid cooling methods including air cooled TMS (ACTMS), liquid cooled TMS (LCTMS), heat sink cooling system (HSCS), phase change materials (PCM), and heat pipe cooling system (HPCS). Test benches for all of the proposed TMSs are made experimentally, and analyzed and verified numerically employing COMSOL Multiphysics® 5.5 software package. The main case study that all the proposed TMSs are being compared with, is natural convection (NC), in which the thermal behavior of the cell is studied under a 150 A continuous current rate without any rest that is considered as super-fast charging/discharging that is unique. After investigation of the NC case study, the cooling performance of the proposed active and passive TMSs have been investigated under the super-fast driving profile. Then, hybrid solutions comprising different combinations of the proposed active/passive TMSs are developed and tested experimentally and numerically. Finally, the best thermal solutions are chosen and compared to be implemented for a module of LiC cells to study their performance.
... The maximum power efficiency of the WSTx unit can be calculated as in (18). Fortunately, the current revolution in the Lithium-ion capacitor may significantly improve the supercapacitor electrical characteristics [54] by reducing the selfdischarging rate. ...
Preprint
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The current revolution in communication and information technology is facilitating the Internet of Things (IoT) infrastructure. Wireless Sensor Networks (WSN) are a broad category of IoT applications. However, power management in WSN poses a significant challenge when the WSN is required to operate for a long duration without the presence of a consistent power source. In this paper, we develop a batteryless, ultra-low-power Wireless Sensor Transmission Unit (WSTx) depending on the solar-energy harvester and LoRa technology. We investigate the feasibility of harvesting ambient indoor light using polycrystalline photovoltaic (PV) cells with a maximum power of 1.4mW. The study provides comprehensive power management design details and a description of the anticipated challenges. The power consumption of the developed WSTx was 21.09µW during the sleep mode and 11.1mW during the operation mode. The harvesting system can harvest energy up to 1.2mW per second, where the harvested energy can power the WSTx for six hours with a maximum power efficiency of 85.714%.
... At present, researchers are studying more on the fundamentals, electrode materials (anodes, cathodes), electrolytes, pre-lithiation, modelling and simulation performance characteristics for LICs [25][26][27][28][29][30][31], and less on consistent screening methods, working power control, fast-charge and application conditions experiment for LIC packs. On the one hand, in order to meet AGV power and driving requirements in practical applications, several LIC cells are required to form pack in series and parallel, and the LIC pack take on high frequency and high power in both charge and / or discharge. ...
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Automated guided vehicle (AGV) plays an important role in the context of industry 4.0. The power supply is the key to ensure reliable and efficient AGV. Lithium-ion capacitor (LIC) is an innovative hybrid energy storage device, possessing the advantages of high energy density, high power density, long cycle life and wide working temperature range. LIC can be used with Opportunity (OP) charging for a vehicle during the operation phase, using predefined fast charging stations and avoiding full and long-time charges. In this work, the consistent screening and grouping techniques of LIC pack, equivalent circuit model, maximum working power matching method, fast-charge and working condition measurement for AGV have been studied. By the fast-charge test of constant current and constant voltage for LIC pack at various constant current (60−360 A), the experimental results show that LIC pack can complete 100% department of defense charging within 2 min when the constant current is higher than 300 A. According to the constant power (200−1200 W) and working condition measurement for LIC pack, this work provides technical support for the maintenance-free and uninterrupted operation of AGVs.
Chapter
Capacitors are fundamental electronic passive components and there are nearly everywhere. There are many different capacitors technologies, with different dielectric materials, form factors and terminals and housings available. This short encyclopedic article discuss the main capacitor types which are relevant for power electronic applications. The main types are Aluminum Electrolytic Capacitors, Metallized Film Capacitor, Ceramic Capacitors and Supercapacitors. The principal construction, materials and properties and technological limitations are discussed. Further new upcoming trends of new materials and designs are presented.
Article
Co-intercalation, intercalation of solvated ions received significant research interest in the last decade mainly due to faster charge-discharge kinetics with enhanced diffusion and more excellent stability. Moreover, for the assembly of alkali metal-ion hybrid supercapacitors, with battery type anode and capacitive cathode, a co-intercalation-based anode is a suitable option to avoid the kinetic mismatch between the two electrodes and hence can guarantee better performance. In the present work, we considered pencil graphite (PG B), a cheap and readily available graphite silica composite, as a battery-type anode and commercial activated carbon (AC) as cathode for the assembly of glyme solvated Na and Li-ion capacitors ((PG B/1 M NaCF3SO3 in diglyme/AC) gs−NIC & ((PG B/1 M LiPF6 in tetraglyme/AC) gs−LIC). Such device prototypes could exhibit maximum energy-power storage capability of 78.7 Wh kg⁻¹ and 3.73 kW kg⁻¹ for gs−NIC and 47 Wh kg⁻¹ and 3.13 kW kg⁻¹ for gs−LIC. Besides, the gs−NIC system with the minimum capacity and kinetic imbalance between the two electrodes displayed brilliant cyclic stability of >97% capacity retention after 6000 charge-discharge cycles at a current density of 1 A g⁻¹. However, the co-intercalation electrolyte system (salt and solvent) plays a vital role in the device's overall performance.
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Full-text available
The current revolution in communication and information technology is facilitating the Internet of Things (IoT) infrastructure. Wireless Sensor Networks (WSN) are a broad category of IoT applications. However, power management in WSN poses a significant challenge when the WSN is required to operate for a long duration without the presence of a consistent power source. In this paper, we develop a batteryless, ultra-low-power Wireless Sensor Transmission Unit (WSTx) depending on the solar-energy harvester and LoRa technology. We investigate the feasibility of harvesting ambient indoor light using polycrystalline photovoltaic (PV) cells with a maximum power of 1.4 mW. The study provides comprehensive power management design details and a description of the anticipated challenges. The measured power consumption of the developed WSTx was 0.02109 mW during the sleep mode and 11.1 mW during the operation mode. The harvesting system can harvest energy up to 1.2 mW per second, where the harvested energy can power the WSTx for six hours with a maximum power efficiency of 85.714%.
Article
The next generation of IoT, IoMT, and wearable bioelectronics demands the development of a novel form of thin‐film and flexible energy storage devices that offer high energy and power densities, mechanical reliability, and biocompatibility. Hydrogels are a class of materials that can be engineered with a range of desired properties, including stretchability, ionic conductivity, biocompatibility, adhesiveness, and mechanical match with organs, as they can be designed with Young´s modulus in the range of the human skin. This review covers different types of hydrogels, including biopolymer hydrogels, carbon‐based hydrogels, and self‐healing hydrogels, and presents their use in the components of supercapacitors and batteries such as electrolyte; electrode, binder, separating membrane, and current collector. We also explain how these hydrogels contribute to improved properties of the energy storage devices and include cases in which the hydrogel is used for several functions in the same device. The contribution of hydrogels in the development of flexible energy storage devices and their impact on electrochemical performance are also discussed.
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Herein, a binder‐free graphene nanostructure anode formulated by drawing on Cu current collector with a commercial 10B pencil is reported. The prepared pencil‐trace electrode (10B pencil graphite, 10B PG) shows an excellent cycling profile with ≈87% capacity retention characteristics after 1000 cycles. Further, a dual‐carbon lithium‐ion capacitor (LIC) is fabricated using the pencil‐trace electrode as battery‐type electrode and commercial activated carbon (AC) as capacitor electrode in aprotic organic solutions. The 10B PG is pre‐lithiated prior to LIC fabrication to ensure an ample supply of Li‐ions during cycling. The developed LIC, 10B PG/AC assembly, displays a remarkable energy density of 109.9 Wh kg⁻¹ irrespective of the applied current density, that is, power.
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n the second part of the review on electrochemical energy storage, the devolvement of batteries is explored. First, fundamental aspects of battery operation will be given, then, different materials and chemistry of rechargeable batteries will be explored, including each component of the cell. In negative electrodes, metallic, intercalation and transformation materials will be addressed. Examples are Li or Na metal batteries, graphite and other carbonaceous materials (such as graphene) for intercalation of metal-ions and transition metal oxides and silicon for transformation. In the positive electrode section, materials for intercalation and transformation will be reviewed. The state-of-the-art on intercalation as lithium cobalt oxide and nickel containing oxides will be approached for intercalation materials, whereas sulfur and metal-air will also be explored for transformation. Alongside, the role of electrolyte will be discussed concerning performance and safety, with examples for the next generation devices. Finally, a general future perspective will address both electrochemical capacitors and batteries.
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The Nobel Prize in Chemistry 2019 recognized the importance of Li-ion batteries and the revolution they allowed to happen during the past three decades. They are part of a broader class of electrochemical energy storage devices, which are employed where electrical energy is needed on demand and so, the electrochemical energy is converted into electrical energy as required by the application. This opens a variety of possibilities on the utilization of energy storage devices, beyond the well-known mobile applications, assisting on the decarbonization of energy production and distribution. In this series of reviews in two parts, two main types of energy storage devices will be explored: electrochemical capacitors (part I) and rechargeable batteries (part II). More specifically, we will discuss about the materials used in each type of device, their main role in the energy storage process, their advantages and drawbacks and, especially, strategies to improve their performance. In the present part, electrochemical capacitors will be addressed. Their fundamental difference to batteries is explained considering the process at the electrode/electrolyte surface and the impact in performance. Materials used in electrochemical capacitors, including double layer capacitors and pseudocapacitive materials will be reviewed, highlighting the importance of electrolytes. As an important part of these strategies, synthetic routes for the production of nanoparticles will also be approached (part I).
Article
The lithium-ion capacitor is a promising energy storage system with a higher energy density than traditional supercapacitors. However, its cycling and rate performances, which depend on the electrochemical properties of the anode, are still required to be improved. In this work, soft carbon anodes reinforced using carbon–Si composites of various compositions were fabricated to investigate their beneficial influences on the performance of lithium-ion capacitors. The results showed that the specific capacities of the anodes increased significantly by 16.6 mAh·g⁻¹ with 1.0 wt% carbon–Si composite, while the initial discharge efficiency barely changed. The specific capacity of the anode with a 10.0 wt% carbon–Si composite reached 513.1 mAh·g⁻¹, and the initial discharge efficiency was 83.79%. Furthermore, the anodes with 7.5 wt% or lower amounts of carbon–Si composite demonstrated reduced charge transfer resistances, which caused an improvement in the rate performance of the lithium-ion capacitors. Moreover, the use of the optimized amount (7.5 wt%) of carbon–Si composite in the anode could significantly improve the cycling performance of the lithium-ion capacitor by compensating the consumption of active lithium. The capacity retention of the lithium-ion capacitor reached 95.14% at 20 C after 10,000 cycles, while the anode potential remained below 0.412 V, which is much lower than that of a soft carbon anode.
Article
Lithium ion capacitors (LICs) can generally deliver higher energy density than supercapacitors (SCs) and have much larger power density and longer cycle life than lithium ion batteries (LIBs). Due to their great potential to bridge the gap between SCs and LIBs, LICs are becoming important electrochemical energy storage systems in the field of energy storage and conversion. Although it is generally accepted that pre-lithiation technologies are indispensable for the operation of LICs, no comprehensive overview of the existing pre-lithiation technologies has been conducted. In this progress report, we first classify LICs according to their energy storage mechanisms and discuss the multiple roles that the pre-lithiation technologies play for improving the performance of LICs. Then, we present the existing pre-lithiation methods used in LICs in detail and the current research progress is summarized. Finally, we provide a comprehensive comparison of the current pre-lithiation methods and propose the prospects and challenges of these methods from both a fundamental and a practical point of view. The broader impact of pre-lithiation technologies on next-generation LIBs is also discussed. This progress report aims at providing the fundamental knowledge necessary to researchers who are new to study LICs and also serving as a guideline to senior researchers in the fields of LICs and LIBs for future research directions.
Chapter
Li-ion batteries have become the cornerstone of electrical energy storage in recent decades, resulting in a significant transition to hybrid and fully electric cars. Furthermore, the energy density of batteries, in general, has developed significantly from around 30 Wh kg⁻¹ for lead-based batteries, up to over 200 Wh kg⁻¹ for Li-ion batteries [1]. Because of these significant increases in specific energy (as well as reductions in cost and improvements in durability), Li-ion-based batteries have already been implemented into small transport vehicles. Presently Li-ion batteries are being implemented into large-scale hybrid and electric vehicles [2], such as electric buses, hybrid electric buses and hybrid-powered ships [3], as bigger cells have become cost-effective. Because bigger cars use electricity, there is a need for bigger battery packs that can withstand more severe usage. To realise the full potential of Li-ion batteries, thermal management of their internal and external environments is required. To achieve this, small sensors (e.g. 10 μm thick), stable and inert are required. In this chapter, thermal management with regard to the structure of Li-ion batteries will be discussed, and how micro-optical sensors may facilitate improvements of the thermal management.