Lithium-Ion Capacitors: A Review of Design and Active Materials

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 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.

Full text

energies
Review
Lithium-Ion Capacitors: A Review of Design and Active Materials
Jacob J. Lamb and Odne S. Burheim *


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
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Copyright: © 2021 by the authors.
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Attribution (CC BY) license (https://
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4.0/).
Department of Energy and Process Engineering and ENERSENSE, Faculty of Engineering, Norwegian University
of Science and Technology (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
significant requirement for electrical energy storage technologies. Energy storage offers the
ability to moderate the variability of electrical energy [
2
]. This represents a rapidly emerging
market for energy storage that is currently underutilised. The characteristics of the energy
storage needs, in general, are electro-compatibility and will relate more specifically 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 production from
fossil-based energy to renewables to reduce the embedded emissions of energy storage
systems. Moreover, the materials required in the production of energy storage 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 environmental, 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 capac-
itor, but it must be noted that there are many similarities between LICs, NICs and KICs.
LIBs normally have high energy density (>150 W h kg
−1
) and have no memory impact
as in conventional 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 comparison, LICs can provide high power density (>10 kW kg
−1
) and
long cycle life (usually >5000 cycles); however, their comparatively low energy density
Energies 2021,14, 979. https://doi.org/10.3390/en14040979 https://www.mdpi.com/journal/energies

Energies 2021,14, 979 2 of 27
(5–10 W h kg
−1
) restricts their applications in certain fields (LIC applications have been
thoroughly discussed in [
8
–
10
]). To close the performance gap between LIBs and tradi-
tional capacitors, LICs have been developed to incorporate the strengths of both LIBs and
traditional capacitors [6,11–14].
By practice, LIBs consist of a metal oxide cathode, separator, electrolyte, and a Lithium-
based anode. In contrast, non-aqueous liquid electrolyte LICs with high power densities
(>10 kW kg
−1
) 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 elec-
trode and a high-rate capacitor-type electrode [
5
,
16
]. During the charge-discharge cycle,
charges are deposited concurrently and asymmetrically in the LIC by surface ion adsorp-
tion/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-capacitors
(PCs) and electric double-layer capacitors (EDLCs) depending on the charging storage
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 densities 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-carbon 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 electro-
chemical 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 vol-
ume variability and strong polarisation of these active materials [
5
]. Carbonaceous materi-
als 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 specific surface re-
gion, strong conductivity and excellent usability to electrolytes, carbon materials were also
inserted into the electrodes to solve such issues [
26
,
27
]. Furthermore, carbon 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
electrodes [
30
,
31
]. Their capacitances rely primarily on the surface of carbon-based elec-
trodes [
32
] for the adsorption/desorption of ions. Therefore, porous structures with
sufficient distribution of pore size play an important role in the electrochemical produc-
tion 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.
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

Energies 2021,14, 979 3 of 27
associated 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 industries,
electrical energy-storage devices play a key function. There are several types of reversible
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 capacitors
have average power density as high as 10 kW kg
−1
, and with gravimetric energy of
up to
7 Wh kg−1[2]
. Figure 1gives an overview of the power and energy densities of
ultracapacitors 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.
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
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
of a battery-type electrode with the insertion/extraction of lithium ions and a pseudo-
capacitance 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
previously separated into two types [41]:

Energies 2021,14, 979 4 of 27
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 re-
quires some redox reaction at the cathode (e.g., cathode oxidation: LiCoO
2→
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 increasingly
light-duty vehicles represents a major potential opportunity that is not completely explored
due to the shortcomings of current secondary battery and supercapacitor technologies
(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 implementation.
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 technologies.
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
certain 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
significant by increasing the open-circuit voltage (efficiency = (E
ocp
-rj)/E
ocp
). Nevertheless,
a phase transition frequently follows the faradaic reaction of the battery-type electrode,
which results in a weak rate efficiency, lower cycle life and slow dynamics [
46
–
49
]. To over-
come these issues, the incorporation of highly conductive carbonaceous additives such as
graphene, carbon nanotubes (CNTs) and AC are important in addition to the production of
nanoscale-structured electrodes to obtain improved electronic conductivity [50–52].
Unlike the pure battery-type electrode, reversible ion adsorption or rapid redox
reactions 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
implementation 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.

Energies 2021,14, 979 5 of 27
The charges for an asymmetric cell should be distributed at both electrodes (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]:
mcath
manode
=
Canode ×∆Eanode
Ccath ×∆Ec ath
(1)
where the electrode mass, the basic capacitance and the voltage range in the load/discharge
phase for the anode and the cathode are
m
,
C
and
∆E
, respectively. Nonetheless, the ca-
pacitance 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 prod-
ucts (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 described
above, carbon products, either as active products or as conductive additives, perform
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 TiO
2
. 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 allotropes, ex-
tremely deficient or heteroatomic carbons do not fall into the classic taxonomy, but instead
reflect the similarity of structure and chemistry between pure graphite/graphene and
completely amorphous activated carbon. Lithium deposition is typically poorly known in
nongraphic carbons, except at fairly high concentrations typical of a battery. 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 g
−1
below 0.2 V and will guarantee a
secure 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.
The coexistence of graphical structures and amorphous structures allows graphised carbon
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.

Energies 2021,14, 979 6 of 27
Table 1. Carbonaceous material-based lithium-ion capacitor (LIC) summary.
Configuration (Anode//Cathode) Voltage 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 exhib-
ited optimum electrochemical efficiency with a high energy density of up to
85.7 W h kg−1
and a power density of up to 7.6 kW kg
−1
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 mi-
crosphere (CPIMS) [
79
], can also be a feasible solution. Despite this, the voltage hysteresis
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 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-
lithiated graphene nanosheets in LICs as an anode, providing a cumulative power density
of
220 W kg−1
at an energy density of 62 W h kg
−1
with a capacity retention of 74 % at
400 mA g−1after 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 diffusivity
of the lithium-ion than the AC/graphitized carbon LICs. As a result, they obtained large
energy densities of 232 at 57 W kg
−1
and 131.9 W h kg
−1
at 2.8 kW kg
−1
. 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 specific
energy of 170.28 W h kg
−1
in the voltage range of 2.0–4.0 V, and the average power density
exceeded 25.75 kW kg−1after 2000 cycles with almost no decay in efficiency.

Energies 2021,14, 979 7 of 27
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 Li
4
Ti
5
O
12
, TiO
2
, Nb
2
O
5
, Fe
2
O
3
and SnO
2
,
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 plateaus
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 composition
of TiO
2
and Li
4
Ti
5
O
12
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 2gives an overview of
transition metals used in LIC electrode design.
Table 2. Transition metal-based LIC summary.
Configuration (Anode//Cathode) Voltage Max Energy (Wh/kg)
at Power (W/kg)
Energy (Wh/kg) at
Max Power (W/kg) Cyclability
TiO2hollow spheres at graphene//graphene [93] 0–3 V 72 at 303 10 at 2000 65% over 1000 cycles
TiO2at mesoporous carbon//AC [94] 0–3 V 67.4 at 75 27.5 at 5000 80.5% over 10,000 cycles
TiO2nanobelt arrays//graphene hydrogels [95] 0–3.8 V 82 at 570 21 at 19,000 73% over 600 cycles
TiO2at 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
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 Li
4
Ti
5
O
12
//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 Li
3
V
2
(PO
4
)
3
, developed with VO
6
octahedra and PO
4
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 Li
3
V
2
(PO
4
)
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 (1.0–3.0 V) and high (3.0–4.3 V) voltage applications for Li
3
V
2
(PO
4
)
3
has been
studied [
105
]. These were observed to provide maximum energy densities between 27 and
25 W h kg
−1
, respectively. Unlike Li
3
V
2
(PO
4
)
3
, LiTi
2
(PO
4
)
3
has a NASICON-type frame
structure, which consists of PO
4
tetrahedra bound by octahedral unit corners of TiO
6
.
Each of the PO
4
tetrahedrons are connected to four octahedral TiO
6
units, and in effect a
TiO
6
unit is connected to six PO
4
tetrahedrons, allowing for multiple ionic replacements
at various 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,

Energies 2021,14, 979 8 of 27
the LICs based on LiTi
2
(PO
4
)
3
carbon-coated anodes display ultra-high energy and power
densities of 14 W h kg−1and 180 W kg−1, respectively [106].
For LIC anodes, TiNb
2
O
7
will serve as an alternate nominee for Li
4
Ti
5
O
12
. TiNb
2
O
7
’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 kg
−1
at 99.58 W kg
−1
[
107
]. Importantly, the TiNb
2
O
7
on C electrode’s Li
injection activity was analysed in detail and the pseudo-capacitive reaction mechanism for
intercalation 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 g
−1
), 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 [111–113].
Composite processing is an efficient way to combine multiple forms of active materials
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
provide surplus lithium to slow the rate of active lithium consumption in long-term cycling
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
contradictory to how LIBs are produced [
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., a carbon–carbon LIC).
Sn has a greater electrical conductivity relative to Si, which will contribute to Sn
anodes 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 [
116
–
118
]. The construction of tiny Sn nanoparticles with a binding
carbon 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 because
it shows a large surface area (33,000 m
2
g
−1
), excellent conductivity (almost
60 S m−1
) and
strong chemical stability [
41
]. An AC cathode’s energy storage capability also provides a
power of approximately 50 mAh g
−1
[
41
]. Compared with anode electrode performance,
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 capacitances [
41
].
Amatucci et al. [
4
] produced a LIC system using AC as the cathode, and nanostructured
Li
4
Ti
5
O
12
(LTO) as the anode, the first use of AC in LICs. The voltage window 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].

Energies 2021,14, 979 9 of 27
Additionally, carbon derived from metal-organic frameworks (MOF) with various
architectures has also been widely researched in LICs. For example, large surface area
(2714 m
2
g
−1
) carbon cuboids were synthesised by pyrolysing the zinc-based MOF-5,
which exhibits a peculiar crumpled-sheet porous morphology assembled with the required
micro and mesoporosity values [
119
]. The MOF-dependent LIC provides a maximum
effective energy density of 65 W h kg
−1
with excellent power capacity, dependent on the
advanced structure. Likewise, polyhedral hollow carbon derived from MOF was also
produced 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-performance
3D architectures are strong candidates for use as the cathodes of LICs. Various tech-
niques for constructing graphene-based architectures for LICs have been comprehensively
documented in recent years [
13
,
41
,
63
,
98
]. Importantly, integrating microporous carbon
with surface connectivity into graphene structures is an important technique for further
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 efficiency and
cyclic stability. An ideal working capacity can usually be accomplished by intercalating Li
+
into the interlayer of graphite. In this way, it is possible to achieve reduced electrode resis-
tance, 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 electrolyte (e.g., from LiPF
6
),
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. During cycling, one will need to hold
the SEI steady and prevent Li plating that involves careful regulation of the negative elec-
trode 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 material
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 perma-
nent active lithium loss during cycles induced by the creation of solid electrolyte interface
films (SEI) on the anode. The following can be performed using distinctive pre-lithiation
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 intro-
duction 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].

Energies 2021,14, 979 10 of 27
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 LiPF
6
or NaClO
4
in carbonate solvents, permanent ion loss results in ionic conductivity degradation [
63
].
During prolonged cycling, devices focused on non-lithiated electrodes have been shown
to have decreased lithium ion content in the electrolyte, significantly deteriorating the
overall LIC potential [
81
,
121
]. Besides SEI, Li can be trapped in the majority of electrode
materials (e.g., hard carbons) [
125
]. Pre-charging at least one electrode is a key element
in evaluating 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 forming
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
,
128
–
130
], 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 unit
ends up being assembled twice. While scientifically useful and to some extent convenient,
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
presence of an electrolyte [
132
–
134
]. This would occur through the direct reaction between
the electrode substrate and lithium. The main advantage of this approach is its sleek
simplicity 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-reassembly
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 materials
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 example,
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-
dihydroxybenzonitrile 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

Energies 2021,14, 979 11 of 27
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 being
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 microbeads.
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 reassembly phase
of the pre-lithiated anode is a time-consuming procedure unfeasible currently 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
electrode and a targeting electrode (e.g., graphite) are separated using a porous polymer
separator in a non-aqueous electrolyte and are connected to wire to naturally facilitate
the penetration of lithium ions into the electrode [
81
,
141
]. Kim et al. [
142
] compared the
different 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
powder) 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
lithium-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.
Li
6
CoO
4
, Li
5
FeO
4
, Li
2
CuO
2
, Li
5
ReO
6
and Li
2
RuO
3
have been developed to be used as
pre-lithiation agents [
115
,
143
–
146
]. 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
Je˙
zowski et al. [
56
] published a pre-lithiation process for sacrificial organic lithium
salt, using 3,4-dihydroxybenzonitrile dilithium salt (Li
2
DHBN) as a sacrificial salt in 1 M
LiPF
6
/EC–DMC. The insoluble Li
2
DHBN 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 ap-
proaches, stable metallic lithium powder and the inclusion of permanent lithium transition
metal oxides could have the most promising prospects for industrial production; however,
monitoring the lithiation degree and health issues are still relevant factors. Recently, Li
3
N

Energies 2021,14, 979 12 of 27
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
cyclability 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 resis-
tance, 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 instability,
good thermal, chemical and electrochemical resilience, low flammability and superior
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 improvement has been
made, their ionic conductivity needs to be significantly enhanced for room temperature use.
Organic electrolytes may have the best prospects for commercially available LICs.
They have a larger voltage window than aqueous electrolytes, and their operating con-
dition 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
glycol-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,
diethyl 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 connected 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 improving ion conductivity; improving device properties (e.g., the SEI); improving
low-temperature efficiency and thermal stability; preventing overloading; and, reducing
electrolyte 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 single
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 accurate
hybridisation of the LIB components with the traditional capacitor components. Using
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
thorough theoretical analysis that aids the design of high-performance LICs, and also

Energies 2021,14, 979 13 of 27
developed 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 supercapacitor
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 introduced
a new model that was specific to LICs and includes accurate pore sizes and distributions
of the cathode. This allowed accurate modelling of impedance values at all frequencies
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 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 performance.
5. Knowledge and Research Gaps
Low Coulombic-related decreases in available charge in batteries are usually correlated
with the irreversible and continuous creation of an SEI and certain cathode electrolyte
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 carbons
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 operate
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
performance [
166
,
167
]. Such reduction films may make the anode surface unreactive,
protecting 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
,
168
–
170
], 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
,
171
–
173
]. Apart
from solvent-reduction products such as Li2CO3and alkyl carbonates, the SEI anode also
partially consists of LiF, which is a decomposition product of the LiPF
6
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 [176–179].
For a variety of carbon-supported nanomaterials (e.g., oxides and sulphides), an addi-
tional 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 increase
in the performance induced by cycling is not uncommon for Li-based anodes, especially for

Energies 2021,14, 979 14 of 27
oxides [
180
–
184
]. 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 com-
pound 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 reversible 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 [186–188].
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
electrode surface [
191
–
194
]. 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 [
193
–
196
]; however,
the CEI chemistry, composition or voltage impact on its development are yet to be analysed.
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 catalysed
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 [199–206].
Nanostructured lithium titanate could greatly reduce the magnitude of lithium metal
plating [
121
]; however, Li
4
Ti
5
O
12
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-capacity ratio
in a true two-electrode cell. This causes more problems in specifically regulating 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 3gives 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 materials
emerge as a significant subset of materials for LICs, together with higher gravimetric and
volumetric efficiency relative to true EDLC electrodes [
5
]. Augustyn et al. [
54
] demonstrated

Energies 2021,14, 979 15 of 27
that the Li
+
intercalation into the orderly channels of bulk orthorhombic T-Nb
2
O
5
was
straight-forward, making the charging behaviour capacitor-like. These materials were
referred to as pseudocapacitive intercalation compounds since, although the Li storage
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 maximum
of 76 Wh kg
−1
rendering their total effective energy around a factor of 2 lower than that for
carbon-based electrodes [207].
Vanadium oxide V
2
O
5
undergoes a bulk ion intercalation reaction during reversible
charging, also producing a sloping profile similar to a capacitor when used as an an-
ode. Bulk V
2
O
5
can accommodate electrolyte cations (e.g., H
+
, Li
+
and K
+
) in aqueous
systems [
208
–
210
]. They also mixed elemental analysis and X-ray diffraction to analyse
the charging-storage process of V
2
O
5
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 kg
−1
can be achieved with a loss of <5% over 10,000 cycles [
208
].
A V
2
O
5
-based hybrid solution can also operate in a far wider voltage window in Li+
organic systems. The authors tested the same materials V
2
O
5
on CNT in lithium struc-
tures [
210
]. The LIC cell energy value was in the region of 40 Wh kg
−1
, 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
+
, Mg
2+
and Al
3+
) can inter-
calate reversibly through the bulk of exfoliated multilayer T
3
C
2
T
x
MXene in an aqueous
electrolyte. 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 [
211
–
213
]. In addition, the electrochem-
ical analysis revealed that storage of Li was a reaction rather than a diffusion-controlled
Faradic cycle, similar to the other pseudocapacitive materials.
An LIC system based on Ti
2
C coupled with Kuraray YP17 activated carbon has also
been proposed [
214
]. Here, the Ti
2
C 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 kg
−1
. 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 kg
−1
of energy and
23.4 Wh kg
−1
energy at an intense strength of 67,500 W kg
−1
. Additionally, Luo et al. [
215
]
developed a CTAB-Sn pillared Ti
3
C
2
MXene-based LIC system that provided energy of
105.6 Wh kg
−1
. 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 performance
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 electrodes.
A conversion electrode is defined initially in LIB literature as the crystalline or amorphous
A
x
B
y
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, Fe
2
O
3
/Fe
3
O
4
, NiCo
2
O
4
and various alloys. Although MoS
2
undergoes a Lithium
intercalation reaction down to 1.1 V, most reversible capability stems from reversible Mo
and Li
2
S conversion reaction down to 0 V [
217
]. The reversible capability of MoS
2
is
stated to be as high as 1000 mAhg
−1
, 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 MoS
2
-graphene composites in LICs, using

Energies 2021,14, 979 16 of 27
sloping charge and discharge profiles, with the LIC delivering energy density as high
as 188 Wh kg
−1
at 200 W kg
−1
and 45.3 Wh kg
−1
at 40,000 W kg
−1
. MoS
2
’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 possibly 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 profile.
Aravindan et al. [
220
] provided an excellent description of intercalation-type materials 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 cathodes used in
LICs. Olivine LiFePO
4
is a well-established LIB commercial cathode which has also seen
applications in LIC systems. During lithium intercalation/deintercalation it undergoes a
two-phase reaction and shows a flat plateau at 3.4 V. Ping et al. [
221
] constructed a hybrid
activated carbon + LiFePO
4
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 kg−1and 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 Li
3
V
2
(PO
4
)
3
(LVP) and
Na
3
V
2
(PO
4
)
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, obtaining
25 Wh kg
−1
. For an activated carbon/LVP configuration an energy density of 28 Wh kg
−1
was observed [
222
]; however, the energy efficiency becomes greatly increased (
125 Wh kg−1
at 300 W kg−1) where activated carbon is used as a cathode when LVP is the anode.
Table 3. Next-generation electrode materials for LICs.
Configuration (Anode//Cathode) Voltage 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
V2O5on 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 Ti3C2MXene//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-Nb2O5on 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
LiNbO3on 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
FexO on graphene//porous graphene [237] 0–3.5 V 129.6 at 19 45 at 3500 75% over 3000 cycles

Energies 2021,14, 979 17 of 27
Table 3. Cont.
Configuration (Anode//Cathode) Voltage Max Energy (Wh/kg)
at Power (W/kg)
Energy (Wh/kg) at
Max Power (W/kg) Cyclability
Fe2O3//activated carbon [238] 0–3.5 V 90 Not reported 55% over 2500 cycles
Fe3O4in 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
LiNi0.5Mn1.5 O4//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 existing
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 maintaining
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 evaluate 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 carbon-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 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 active 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 electrochemically available
for the electrolyte, restricting their electrochemical efficiency. Furthermore, the physical
properties of some materials provide additional areas of concern through their deformation
during charge and discharge cycles. Although ambitious, attention should be given to the
design of electrodes with precisely controllable microstructures.

Energies 2021,14, 979 18 of 27
Author Contributions:
Conceptualization, J.J.L. and O.S.B.; methodology, J.J.L. and O.S.B.; software,
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.
Both 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 conflict of interest.
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