A layered organic cathode for high-energy, fast-charging, and long-lasting Li-ion batteries

Abstract

Eliminating the use of critical metals in cathode materials can accelerate global adoption of rechargeable Lithium-ion batteries. Organic cathode materials, derived entirely from earth abundant elements, are in principle ideal alternatives, but have not yet challenged inorganic cathodes due to poor conductivity, low practical storage capacity, or poor cyclability. Here, we describe a layered organic electrode material whose high electrical conductivity, high storage capacity, and complete insolubility enable reversible intercalation of Li+ ions, allowing it to compete at the electrode level, in all relevant metrics, with inorganic-based lithium-ion battery cathodes. Our optimized cathode stores 306 mAh g–1cathode, delivers an energy density of 765 Wh kg–1cathode, higher than most cobalt-based cathodes, and can charge-discharge in as little as six minutes. These results demonstrate operational competitiveness of sustainable organic electrode materials in practical batteries.

Full text

A layered organic cathode for high-energy, fast-charging, and long-lasting Li-
ion batteries
Tianyang Chen1†, Harish Banda1†, Jiande Wang1, Julius J. Oppenheim1, Alessandro Franceschi2,
Mircea Dincă1*
1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139,
United States
2Department of Industrial Engineering, University of Bologna, Bologna 40136, Italy
*Corresponding author. Email: mdinca@mit.edu
†These authors contributed equally to this work
Abstract:
Eliminating the use of critical metals in cathode materials can accelerate global adoption of
rechargeable Lithium-ion batteries. Organic cathode materials, derived entirely from earth
abundant elements, are in principle ideal alternatives, but have not yet challenged inorganic
cathodes due to poor conductivity, low practical storage capacity, or poor cyclability. Here, we
describe a layered organic electrode material whose high electrical conductivity, high storage
capacity, and complete insolubility enable reversible intercalation of Li+ ions, allowing it to
compete at the electrode level, in all relevant metrics, with inorganic-based lithium-ion battery
cathodes. Our optimized cathode stores 306 mAh g–1cathode, delivers an energy density of 765 Wh
kg–1cathode, higher than most cobalt-based cathodes, and can charge-discharge in as little as six
minutes. These results demonstrate operational competitiveness of sustainable organic electrode
materials in practical batteries.
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Main Text:
Lithium-ion batteries (LIBs) are dominant energy storage solutions for electrifying the
transportation sector and are becoming increasingly important for decarbonizing the grid.
Traditional cathodes for LIBs are made from inorganic oxides, especially those of Co, Ni, and Mn
(e.g., LiCoO2 (LCO) and LiNi1–x–yMnxCoyO2 (NMC)) (1). Of these, cobalt poses severe limitations
due to its scarcity and high social cost (e.g. child labor) (2, 3). Complete removal of cobalt has
proven difficult, as oxide cathodes that are Co-free suffer from poor cyclability or capacity (4, 5).
Indeed, electric vehicles today overwhelmingly use Co-based batteries. However, expansion of the
global EV fleet is essentially impossible without the development of cobalt-free technologies (6).
This has led to significant efforts in developing LIBs using more abundant and cost-effective
lithium iron phosphate (LFP) as a cathode, (7, 8) despite LFP’s known inferior energy density
relative to oxide-based cathodes (9) and phosphate’s critical role in agriculture notwithstanding
(10, 11). Clearly, LIBs would benefit from the development of sustainable cathode technologies
based on inexpensive, abundant precursors that can be sourced and scaled globally through more
environmentally benign processes.
Redox-active organic materials, derived entirely from earth abundant elements, offer just such
an opportunity (12, 13). They benefit from excellent compositional diversity and structural
tunability while offering requisite synthetic control for targeted designs as cathode materials for
LIBs. Although the merits of replacing inorganic cathodes with organic electrode materials
(OEMs) have long been appreciated in the literature, (14–16) material candidates in this class that
deliver comprehensive performance along all metrics relevant for practical batteries have remained
elusive. From a design perspective, small organic molecules offer high specific capacities by virtue
of a dense arrangement of redox sites and their low molar masses relative to redox-active polymers
or framework materials. However, discrete molecules typically have low bulk conductivity and
often dissolve in battery electrolytes, which lead to poor utilization of redox sites, low charge-
discharge rates, and poor cycling stability (17). These issues are routinely managed by adding
electrically conducting and/or stabilizing polymeric additives typically exceeding 50 wt.%, which
greatly reduce the effective capacity, rendering the electrodes impractical (18–25). Alternative
strategies to polymerize redox-active OEM candidates or to immobilize them into host frameworks
often require compromise in at least one of the critical practical metrics (15,16). As such, there
continues to be a strong interest in designing intrinsically insoluble and electrically conducting
OEMs that exhibit high specific capacity at appropriate cathodic voltages (> 2.0 V) for LIBs (Fig.
1A). To our knowledge, OEMs that fulfill all these criteria so as to rival inorganic cathodes are not
known (14–17).
Here, we demonstrate that bis-tetraaminobenzoquinone (TAQ), a fused conjugated molecule
with a layered solid-state structure, functions as a fast-charging, high-energy, and long-lasting
OEM for LIB cathodes. As reported recently (26), TAQ is characterized by a dense arrangement
of redox-active carbonyl (C=O) and imine (C=N) groups on a conjugated backbone. Two 2e– redox
couples give TAQ a high theoretical specific capacity of 356 mAh g–1. Strong intermolecular
hydrogen bonding and donor-acceptor (D-A) π-π interactions in TAQ render it insoluble in
common battery electrolytes and impart extended electronic delocalization that leads to high bulk
electrical conductivity. These features allow optimized electrodes that comprise at least 90 wt.%
of TAQ to reversibly store charge and cycle safely for over 2000 cycles, strongly contrasting with
the issues of electrode dissolution chronically experienced in known OEMs (Fig. 1A). The two-
dimensional (2D) layered arrangement of TAQ molecules enables facile insertion/extraction of Li+
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between the layers and delivers excellent rate capabilities even at full charging in as little as 3
minutes. Optimized electrodes deliver excellent performance even at high areal mass loadings up
to 16 mg cm–2 with areal capacity up to 3.25 mAh cm–2, which is on par with commercial lithium-
ion batteries (27), comprehensively demonstrating the viability of TAQ in practical LIBs..
Crystal structure, electronic structure and electrical conductivity of TAQ
TAQ is obtained in gram quantities by Michael condensation of tetraamino-p-benzoquinone
(see Materials and Methods) as highly crystalline micro-rods, whose identity was verified by wide-
angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and cryogenic electron
microscopy (Cryo-EM) (fig. S1A-C). TAQ exhibits significant keto-enol tautomerization (Fig.
1B) through the conjugation of carbonyl and amino groups within the two diaminobenzoquinone
moieties that are connected by a dihydropyrazine core (Table S1), leading to both quinone and
imine forms. The contribution of the imine tautomer is evidenced by the C=N signal at 140.8 ppm
in its 13C solid-state NMR (ssNMR) spectrum (Fig. 1C, S2) and from the partial double bond
character of the two C–NH2 bonds evidenced in the single crystal structure (fig. S1D). The
dihydropyrazine linkage is crucial for enabling significant electronic delocalization between the
two neighboring diaminobenzoquinone moieties. This distinguishes it from the related molecule
tetraamino-phenazine-1,4,6,9-tetrone (fig. S3) (28), whose calculated HOMO-LUMO gap, 2.212
eV, is nearly 1 eV higher than that of TAQ, 1.242 eV (Fig. 1B, Table S2).
Planar TAQ molecules are surrounded by six neighbors and closely pack into two-dimensional
layers through pervasive intermolecular hydrogen bonding between carbonyl and amine/imine
functional groups (Fig. 1D). These layers stack through strong donor-acceptor π–π interactions
with a short interlayer distance, 3.14 Å (Fig. 1E). High-resolution Cryo-EM images of TAQ (Fig.
1F,G, S4-S6) and the corresponding fast Fourier transform (FFT) further confirmed its in-plane
dense molecular packing and the out-of-plane close stacking of 2D layers. Owing to its compact
solid-state packing, TAQ exhibits very low solubility in common organic solvents and battery
electrolytes (fig. S1E). Notably, heating TAQ in deuterated N,N-dimethylformamide at 120 ℃
overnight leads to little dissolution, as verified by the absence of TAQ signals in the 1H and 13C
spectra of the supernatant (Fig. 1H). The unusually low solubility of TAQ, even at elevated
temperature, stands in stark contrast to other OEMs reported for LIBs (fig. S7) and is key for long
cycling lifetime.
Owing to a combination of intramolecular extended conjugation, intermolecular hydrogen
bonding, and interlayer π–π stacking, TAQ also exhibits broadband electronic absorption from 200
to nearly 1600 nm (Fig. 1G), indicating significant electronic delocalization. This again contrasts
with any other molecular OEMs, and some prototypical charge-transfer complexes (29),
conjugated polymers (30), and organic radical polymers (31), which show absorption only below
800 nm (Fig. 1I, S8) and thus have poor electronic delocalization. TAQ also exhibits an optical
gap of ~0.8 eV, which is comparable with doped poly(pyrrole) (32). The electron paramagnetic
resonance (EPR) spectrum of TAQ (fig. S9A) revealed the presence of delocalized organic spins,
as verified by the Dysonian lineshape of the signal and the corresponding lineshape asymmetry
indicator (i.e., the ratio of the positive to the negative part of the EPR signal), 1.32, which is similar
to some single-walled carbon nanotubes (33). The organic spins, which likely originate from the
partial oxidation of the dihydropyrazine moiety, have a Curie spin density of 0.032 per TAQ
molecule, as indicated by the variable temperature direct current susceptibility measurement of
TAQ (fig. S9B), and delocalize through extended conjugation. Because of these features, TAQ
exhibits semiconducting behavior with a charge transport activation energy of 319 meV and room-
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temperature electrical conductivity of up to 2.1×10–4 S cm–1 (fig. S9C,D). This is substantially
higher than most molecular OEMs, which are either poor conductors or insulators (fig. S10, Table
S3), and is also higher than or comparable with charge-transfer complexes, organosulfur
compounds, conjugated polymers, and organic radical polymers (29–31) Remarkably, the
electrical conductivity of TAQ is on par with that of LCO (34) and state-of-the-art NMC (35), and
is approximately five orders of magnitude higher than that of LFP (36). These favorable properties
of TAQ allow fabrication of battery electrodes with little to no additives. In contrast, most, if not
all, OEMs routinely need at least 50 wt.% additives (Fig. 1J, S9E) (16).
Fig. 1. Characterization of TAQ. (A) Common organic cathodes with low active material
content, and TAQ-based cathodes with high and practical-relevant active material content. (B) The
keto-enol tautomerism is represented by both quinone and imine forms with different energy
levels. (C) Solid state 13C-NMR spectrum confirms both quinone and imine forms. (D) A 2D layer
of TAQ molecules formed by intermolecular hydrogen bonding (dashed lines). (E) π-π stacking
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of 2D layers with an interlayer spacing of 3.14 Å. (F, G) In-plane and out-of-plane molecular
packing of TAQ observed in Cryo-EM images. (H) 1H and 13C spectra of the supernatant obtained
after heating TAQ in deuterated N,N-dimethylformamide (DMF-d7) at 120 ℃ for 12 hours.
Asterisks indicate solvent peaks. (I) DRUV-Vis spectra of TAQ and other prototypical OEMs.
Among these, only TAQ shows significant near-IR absorption. (J) Electrical conductivities of
different classes of OEMs, TAQ, and state-of-the-art inorganic electrode materials versus typical
amounts of additives used for electrode fabrication. Poly(acetylene), poly(pyrrole),
poly(thiophene), and poly(aniline) are excluded because they operate through anion insertion
instead of Li+ insertion. Asterisks indicate rapid capacity decay during the first few cycles mainly
due to dissolution. The details of OEMs in I and J are summarized in fig. S7 and S9.
Neat TAQ cathodes
Due to its high conductivity and poor solubility, neat TAQ can be directly used as a cathode
(see Methods) in Li-ion half cells using lithium anodes and commercial LP30 electrolyte (1.0 M
LiPF6 in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC)). Galvanostatic charge-discharge
(GCD) voltage profiles (Fig. 2A) recorded at 25 mA g–1 (0.125C) between 1.6 V and 3.2 V (all
potentials are referenced to the Li+/Li couple unless otherwise noted) exhibit initial discharge and
charge capacities of 297 mAh g–1 and 258 mAh g–1 based on the cathode mass, respectively. The
first-cycle Coulombic efficiency (CEfirst) is 87%, which is comparable to the values of 80%–86%
in commercial NMC cathodes (37). Whereas three broad plateaus centered around 2.3 V, 2.7 V,
and 3.0 V were observed during charging, two distinct plateaus between 2.9 V–2.6 V and 2.3 V–
2.0 V were observed during discharge. Both of these discharge plateaus store nearly equal amounts
of charge, ~130 mAh g–1, resulting in a nominal discharge voltage of 2.5 V. Replacing lithium
anodes with pre-lithiated graphite anodes (GrLi, see Methods and fig. S11A) gives similar voltage
profiles and capacity. Increasing the areal mass loading of neat TAQ to 10 mg cm–2, a loading
rarely reached with organic cathodes even when these are mixed with 50 wt.% of carbon,
maintained a capacity as high as 181 mAh g–1, compared to 210 mAh g–1 at a loading of 2.5 mg
cm–2 (Fig. 2A). Faster charging rates of 0.4C and 2.5C delivered discharge capacities of 207 and
106 mAh g–1 (Fig. 2B), respectively. Furthermore, 10-minute and 6-minute CCCV (constant-
current constant-voltage) charging delivered discharge capacities of 125 and 105 mAh g–1,
respectively (Fig. 2C). Diffusion coefficients of Li+ in neat TAQ electrodes obtained using
Galvanostatic intermittent titration techniques (GITT) revealed values of ~10–10 cm2 s–1 throughout
the discharging/charging processes (fig. S11B,C). These coefficients are similar to those of state-
of-the-art optimized inorganic cathodes (35), and highlight facile Li+ diffusion within TAQ crystals
and in bulk neat TAQ.
Cycling studies at low charge/discharge rates are generally employed to evaluate the ability of
OEMs to withstand dissolution into the electrolyte under operating conditions. Neat TAQ
electrodes are stable to at least 50 cycles at 0.2CCCV/0.125C, at 100% depth of discharge (DOD),
with a capacity retention of 75% and an average CE greater than 98% (Fig. 2D, S11D). No
electrode dissolution was observed after cycling, but TAQ rods did show fracturing into flakes, as
verified by ex-situ SEM images (fig. S12). Cycling at higher rates of 1CCCV/1C and 4CCCV/0.5C
maintains near 100% CE and with a capacity retention of 88% over 100 cycles and 80% over 200
cycles (Fig. 2E). This cycling performance of neat OEM electrodes is unprecedented and serves
as testament to facile ion diffusion and charge transport ability of TAQ, as well as its virtual
insolubility in common battery electrolytes.
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Fig. 2. Characterization of neat TAQ electrodes. (A) GCD voltage profiles of three neat TAQ||Li
cells at 25 mA g–1. Increasing the electrode mass loadings from 1.5 to 10 mg cm–2 leads to 70%
retention of reversible capacity. (B) Power capability of neat TAQ electrodes recorded from 40
mA g–1 (0.2C) to 750 mA g–1 (3.75C). (C) Power capability recorded at various CCCV charging
rates and a discharge rate of 0.5 C. (D) Slow cycling of a TAQ/GrLi half-cell. Inset shows the
photo of a disassembled coin cell after cycling. (E) Cycling studies of neat TAQ half cells at higher
rates: 1CCCV/1C and 4CCCV/0.5C. (F) Ex-situ 13C ssNMR spectrum of neat TAQ electrode
discharged to 2.0 V shows disappearance of both C=N (purple) and C=O (red) signals. (G) Ex-situ
FTIR spectra of neat TAQ electrodes at various stages of a discharge-charge cycle show reversible
changes in chemical signatures. (H) Ex-situ DRUV-Vis spectra of TAQ at various states of
discharge. (I) Schematic representation of the redox mechanism of both quinone- and imine-form
of TAQ.
Employing neat TAQ as the cathode also enabled direct spectroscopic analysis of the redox
processes without interference from electrode additives. An ex-situ 13C ssNMR spectrum of TAQ
discharged to 2.0 V (Fig. 2F) revealed the disappearance of both C=N (140.8 ppm) and C=O (169.2
ppm) signals, indicating that both functional groups are reduced during discharge despite the
overall spectrum broadening. An EPR spectrum of discharged TAQ (fig. S13A) revealed
significantly increased radical content, corresponding to approximately 0.85 free radicals per TAQ,
close to the theoretical value of 1 radical per TAQ for a sample with a discharge capacity of 268.3
mAh g–1 (~75% of the theoretical capacity). A fit of the EPR spectrum attributed the signal to a
TAQ biradical with spin density on both oxygen and nitrogen atoms (fig. S13B). Ex-situ FTIR
spectra of TAQ measured at different potentials (Fig. 2G) exhibit a gradual decrease and recovery
of the C=O and C=N stretching bands at 1618 cm–1 and 1531 cm–1, respectively, reflecting the
discharge and recharge processes. Interestingly, the O–H (3464 cm–1) and imine N–H (3346 cm–
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1) stretching bands, the O–H scissoring mode, and the dihydropyrazine ring modes (1460 cm–1),
all of which stem from the imine tautomer (fig. S14), almost completely disappear when TAQ is
discharged to the first plateau at ~2.5 V, suggesting that the reduced imine readily transforms to
the more stable reduced quinone form, likely through the hydrogen bonding network (Fig. 2I).
Given that the quinone form has a lower LUMO energy (Fig. 1B), which in principle translates to
a higher reduction potential relative to the imine form, it is more likely to accept electrons initially
during discharge, which simultaneously shifts the tautomerization equilibrium from imine to
quinone. TAQ discharged at 2.5 V is proposed to contain diradicals, which is supported by its
DRUV-Vis spectrum revealing a significant polaronic band centered around 1200 nm (Fig. 2H).
Subsequent two-electron reduction gives the fully reduced TAQ, corresponding to the second
plateau centered around 2.2 V. Ex-situ DRUV-Vis spectra of TAQ discharged to both 2.5 V and
2.0 V (fig. S13C) also reveal less intramolecular electronic delocalization relative to charged TAQ
due to the lack of tautomerization (Fig. 2I), as verified by the blue-shifted absorption at 2.67 eV
(2.0 V) and 2.56 eV (2.5 V) relative to 2.41 eV for charged TAQ. Surprisingly, discharge promotes
intermolecular electronic delocalization, indicated by the significantly enhanced polaronic
absorption in the near-IR (Fig. 2H). The result is that the electrical conductivity of TAQ discharged
to 2.0 V remains essentially unchanged compared to its charged state (fig. S13D). The redox
behavior of TAQ upon prolonged cycling does not affect its molecular structure: FTIR spectra of
pristine neat electrodes are nearly indistinguishable from those cycled for over 100 cycles at low
rate. (fig. S15). Overall, these features establish the redox cycling of TAQ as a two-step, four-
electron process corresponding to a high theoretical capacity of 356 mAh g–1.
Optimized TAQ cathodes
Mixing TAQ with as little as 10 wt.% additives further enhances its performance to reach near
theoretical capacity. Specifically, carboxymethyl cellulose (CMC) and/or styrene butadiene rubber
(SBR) allow the formulation of TAQ slurries in water (see Methods), more environmentally
friendly than N-methylpyrrolidone (38), and improve performance without sacrificing electrode-
level metrics.
Compared to neat TAQ electrodes, TAQ/CMC composite electrodes delivered greater
reversible capacity of 286 mAh g–1TAQ (Li anode) or 299 mAh g–1TAQ (GrLi anode) at 25 mA g–1
in LP30, enhanced CEfirst of 92%~94% (Fig. 3A, S16A,B), significantly improved rate capability,
and greater cycling stability (fig. S16C-F). A proof-of-concept TAQ/CMC||GrLi full cell with a
nearly balanced negative/positive electrode capacity ratio (N/P) of 1.1 exhibited a cathode capacity
of 180 mAh g–1 (fig. S17).
Despite their superior performance relative to neat TAQ, TAQ/CMC electrodes suffered from
poor adhesion to the current collectors, which prompted us to use increments of SBR additive for
optimized electrode formulations. Although increasing SBR content decreases the overall
electrode capacity (Fig. 3A), the CMC/SBR combination, common in commercial graphite
anodes, substantially enhances the mechanical integrity and adhesion of TAQ to the stainless steel
current collector (fig. S18). A CMC:SBR ratio of 4:1 provided a good balance of capacity and
mechanical properties, leading to optimized electrodes with uniform and robust coatings (Fig. 3A
insets) and a reversible capacity of 275 mAh g–1TAQ. The CMC/SBR composite further increases
the stability of already insoluble TAQ, such that TAQ/CMC/SBR electrodes show very limited
dissolution in LP30 even at 100 °C after 24 hours (fig. S19A).
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Fig. 3. Performance of composite TAQ electrodes with 90 wt.% active material. (A) GCD
voltage profiles for various additive compositions in LP30 electrolyte. Insets are SEM images of
composite electrodes using CMC:SBR = 4:1 (optimized electrode). (B) GCD voltage profiles of
optimized electrodes in different electrolytes. (C) Power capability studies at constant current
discharge of 0.75C and charge from 0.5 to 20CCCV. Inset shows continuous switching between
cycling at 0.5C and 30C. (D) Cycling of optimized electrode at constant charge and discharge
current density of 25 mA g–1 at 100% DOD in LiTFSI/DOL/DME. (E) Average capacity of
cathodes over 600 hour-cycling at current densities ranging from 25 mA g–1 to 2000 mA g–1. (F)
Cycling at 0.4 A g–1 (2C/2C for charge|discharge) and 1 A g–1 (5C/5C for charge|discharge) at
100% DOD in LiTFSI/DOL/DME. (G) Rct and Rdiffusion of TAQ/CMC/SBR||Li cells in LP30 and
LiTFSI/DOL/DME.
Further increase in capacity is possible by addition of 5% vinylene carbonate (VC) to the LP30
electrolyte (LP30VC), a common strategy in commercial devices (39). This led to an initial
discharge capacity of 356 mAh g–1TAQ, equal to TAQ’s theoretical capacity, and an improved
reversible capacity of 324 mAh g–1TAQ (Fig. 3B). Rate capability studies in LP30VC revealed a
cathode capacity of 192 mAh g–1 at 10CCCV and 166 mAh g–1 at 20CCCV (Fig. 3C), which
correspond to total charging times of 6 and 3 minutes, with capacity retention of 80% and 70%,
respectively, relative to 240 mAh g–1 at 0.5CCCV (i.e., a total charging time of 2 hours). Stable
fast-switching between 239 mAh g–1 at 0.5C and 90 mAh g–1 at 30C further highlights the high
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power capability of TAQ (Fig. 3C inset). Replacing LP30VC with LP40VC (1.0 M LiPF6 in 1:1
EC/diethyl carbonate (DEC) with 5% VC) enhanced the cycling stability (fig. S20) and the
performance in cells with GrLi anodes (fig. S21).
Compared with carbonate electrolytes, ether-based electrolytes such as 1.0 M lithium
bis(trifluoromethanesulfonyl)imide in 1:1 1,3-dioxolane/dimethoxyethane (LiTFSI/DOL/DME)
are known to deliver better CE with lithium anodes (40, 41). TAQ composite electrodes delivered
reversible capacities of up to 340 mAh g–1TAQ (Fig. 3B) with enhanced CEfirst of 95%~100% (fig.
S22) based on five cells, and stable cycling at 25 mA g–1 and 100% DOD in LiTFSI/DOL/DME,
exhibiting cathode capacity of 254 mAh g–1 after 100 cycles (Fig. 3D). Such capacity retention
(92%) outperforms the slow cycling in carbonate electrolytes (fig. S20). Cycling studies at higher
current densities (fig. S19B,C) ranging from 40 mA g–1 (0.2C) to 2000 mA g–1 (10C) revealed
limited decrease of the average cathode capacity over 600-hour cycling from 256 to 157 mAh g–1
(Fig. 3E), suggesting both outstanding power performance and cycling stability. Specifically,
cycling studies at 0.4 A g–1 (2C) and 1 A g–1 (5C) revealed cathode capacity of 213 mAh g–1 after
1000 cycles and 159 mAh g–1 after 2000 cycles (Fig. 3F), corresponding to capacity retention of
88% and 70%, respectively. More importantly, TAQ’s molecular structure remains unchanged
upon prolonged cycling (fig. S23). GITT studies revealed slightly higher diffusion coefficients of
Li+ in LiTFSI/DOL/DME (~10–9 cm2 s–1) relative to LP30 (~10–10 cm2 s–1) (fig. S19D,E).
Importantly, significantly lower charge transfer resistances (Rct), ranging from 20 Ω to 40 Ω during
the whole charge/discharge process, were observed in LiTFSI/DOL/DME relative to LP30 (Fig.
3G, S24, S25). Ion diffusion resistances (Rdiffusion) remained below 40 Ω from 2.1 V to 3.2 V in
LiTFSI/DOL/DME, superior to the step-like increase of Rdiffusion at higher degree of discharge in
LP30. Nevertheless, the Rct and Rdiffusion values observed in both LP30 and LiTFSI/DOL/DME are
among the lowest values observed for any cathode materials at similar active content levels, and
lower even than organic cathodes mixed with significant amounts of conducting additives (42).
Overall, these metrics are indicative of rapid and reversible Li ion intercalation in TAQ, which
enable its function as a cathode against metallic Li and GrLi anodes.
Structural and morphological evolution
TAQ’s crystallinity and extreme insolubility enabled structural studies of Li+ intercalation and
deintercalation through a combination of in-operando powder X-ray diffraction (PXRD; fig. S26)
and ex-situ electron microscopy. Most tellingly, the position of the reflection corresponding to the
interlayer distance, d102, which appears at 3.14 Å in pristine TAQ, fluctuates smoothly between
3.19 Å and 3.32 Å upon repeated slow charging and discharging at 20 mA g–1 in
LiTFSI/DOL/DME (fig. S27A, S28). The first two discharge/charge cycles starting from pristine
TAQ revealed more pronounced fluctuations within the same range, as may be expected for first
intercalation of Li+ into the non-lithiated phase, which upon discharge needs to accommodate both
four Li+ ions and a greater electrostatic repulsion between tetranionic TAQ4– molecules. Notably,
fully recharged TAQ has slightly expanded lattice dimensions (575.8 Å3; see Cryo-EM analysis in
Supplementary Text and fig. S27B,C) relative to pristine TAQ (540.3 Å3), suggesting that initial
lithiation of TAQ and delithiation of Li4TAQ induce a slight rearrangement of individual
TAQ/TAQ4– molecules, which subsequently support continuous cycling without further structural
changes up to at least 31 cycles (fig. S29). A similar structural evolution is observed in LP30 (fig.
S30, S31).
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Fig. 4. Structural evolution of TAQ during charge/discharge. In-operando PXRD patterns in
the region of the (102) reflection and the corresponding voltage profile of a TAQ cell in
LiTFSI/DOL/DME cycled five times at 200 mA g–1. A diagram representing the phase
transformation mechanism of TAQ during cycling at 1C is shown to the right.
Cycling at higher current density of 200 mA g–1 (1C; Fig. 4, S32, S33) diminishes the range
observed for the interlayer distance, which now fluctuates between 3.24 Å for TAQ and 3.32 Å
for Li4TAQ. The structural progression between the limiting compositions, charged TAQ and
discharged Li4TAQ, is smooth and is therefore indicative of a Li+ insertion/deinsertion mechanism
that occurs in a single-phase solid solution LixTAQ (x = 0 - 4) at steady state (43). Importantly,
differences in the interlayer spacing between TAQ and Li4TAQ at a rate of 1C suggest a maximal
volume change of only 2.5% during cycling, an important consideration for potential practical use.
The volume change is even smaller at higher rates: cycling TAQ/CMC/SBR at 1,000 mA g–1 (5C)
reveals minimal oscillation of the interlayer spacing at steady state of only 0.9%, between 3.27 Å
and 3.30 Å (fig. S27D). The coherent phase transformations through the formation of extended
solid solutions during charge/discharge, likely enabled by the flexible layered structure and the
strong in-plane molecular packing of TAQ, account for the good rate capability of TAQ cathodes.
A similar smooth transition between charged and discharged phases is also key for enabling the
high-rate performance of nanosized olivine phosphate cathodes (44).
Benchmarking TAQ in practically relevant metrics against other cathode material classes
One of the major challenges for OEMs is the difficulty of achieving high areal mass loadings
(45). OEMs have relatively low densities and thus require fabrication into thicker electrodes in
order to achieve mass loadings that are comparable with inorganic cathodes. However, thicker
electrodes made from intrinsically insulating common OEMs compound the problem of high
ohmic resistances at practical-level mass loadings. As exposed above, TAQ is electrically
conductive and virtually insoluble, but it also has high crystallographic density (for an OEM) of
1.9 g cm–3, which enables high mass loadings and areal capacities. Indeed, TAQ/CMC/SBR||Li
cells with cathode mass loadings up to 15 mg cm–2 deliver cathode capacities ~230 mAh g–1 in
both LP40VC and LiTFSI/DOL/DME (Fig. 5A, S34A). Cycling electrodes with a mass loading of
12 mg cm–2 at a current density of 0.1 A g–1 (0.5C) and 100% DOD delivered a cathode capacity
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of 166 mAh g–1 after 150 cycles, with 87% capacity retention (Fig. 5B). Moreover, increasing the
rate from 25 to 125 mA g–1 delivered a consistent average discharge voltage of 2.5 V and a 75%
capacity retention (Fig. 5B, S34B). Most relevantly, full TAQ/CMC/SBR||GrLi cells using
LP40VC with TAQ mass loadings as high as 16 mg cm–2 and N/P ratios as low as 1.1 reached
areal cathode capacities of 3.25 mAh cm–2 at 25 mA g–1 (Fig. 5A; S34C,D), on par with the highest
areal capacities reported previously for OEMs employing advanced electrode engineering (16, 46).
Fig. 5. Benchmarking TAQ performance at high mass loading and against state-of-the-art
cathodes. (A) GCD voltage profiles of TAQ/CMC/SBR composite electrodes at TAQ mass
loadings over 10 mg cm–2. Li anodes were used except for the purple trace, collected with a GrLi
anode. (B) Cycling of a TAQ/CMC/SBR composite electrode with a TAQ mass loading of 12 mg
cm–2 at 0.1 A g–1. Inset shows the rate capability study of a TAQ/CMC/SBR composite electrode
with a TAQ mass loading of 11 mg cm–2. (C) A comparison of active material-based and electrode-
based gravimetric specific capacities for various reported OEMs, TAQ, and inorganic cathodes.
(D) Comparison of electrode-based gravimetric energy densities of various LIB cathode materials.
Materials chosen in this comparison have an average discharge voltage greater than 2 V vs. Li+/Li.
For each inorganic cathode material, data is selected from reports with the highest level of material
optimization, either through electrode coatings, doping, or control on the crystalline domain size
(47–50). For organic cathodes, best performing materials are chosen from comprehensive recent
reviews (14–17).
The electrode-level specific capacities and energies of TAQ are compared with state-of-the-art
inorganic and organic cathode materials in Fig. 5C and 5D (material details and source data in fig.
S35, Tables S4 and S5). As discussed briefly earlier, typical OEMs are insulating and require large
amounts (30~70 wt.%) of conducting additives and binders, significantly above the commercial
standard of 5~10 wt.%. Thus, even though many OEMs exhibit high material-level metrics, their
more practically relevant electrode-level metrics are modest. Here, because TAQ cells function
with 90 wt.% active material loading, TAQ-based cathodes store up to 306 mAh g–1 at 2~4 mg
cm–2 and 240 mAh g–1 at >10 mg cm–2 active-material mass loadings (Fig. 5C). In fact, electrode-
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level specific energies for TAQ cathodes over a range of C rates outperform even optimized
inorganic cathodes (Fig. 5D). For instance, TAQ cathodes deliver at least 20%-30% higher energy
density, at the electrode level, than optimized composite cathodes based on single-crystalline
NMC811 (NMC811-sc) or commercial polycrystalline NMC811 (NMC811-pc), at rates from
~0.1C to 10C (47). Notably, TAQ also delivers higher energy density than graphite-coated
LiFePO4 (LFP-GC) cathodes at charging rates that are at least 10 times faster (48). TAQ thus
presents measurable advantages relative to leading contemporary LIB cathode technologies.
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Acknowledgments: This work was supported by Automobili Lamborghini S.p.A. Part of this
work made use of the MRSEC Shared Experimental Facilities at MIT. This research used resources
of the Center for Functional Nanomaterials, and the SMI beamline (12-ID) of the National
Synchrotron Light Source II at Brookhaven National Laboratory. Cryo-EM specimens were
prepared and imaged at the Automated Cryogenic Electron Microscopy Facility in MIT.nano on a
Talos Arctica microscope, a gift from the Arnold and Mabel Beckman Foundation. We thank Dr.
Yugang Zhang for the help with WAXS measurements, and Bowen Tan for the help with EPR
measurements.
Funding:
Automobili Lamborghini S.p.A.
Author contributions: T.C., H.B. and M.D. conceived the idea. T.C. synthesized and
characterized TAQ. T.C. and H.B. fabricated and tested battery cells, performed in-operando and
ex-situ characterizations, and analyzed data. J.W. conducted the GITT measurements and helped
with battery tests and in-operando PXRD measurements. J.J.O. conducted computational studies.
A.F. helped with EIS studies. T.C., H.B., and M.D. wrote the manuscript. All authors contributed
to the preparation of manuscript.
Competing interests: M.D., H.B., and T.C. have filed a U.S./International Patent (application
number PCT/US2022/047839).
Data and materials availability: All data are available in the main text and the supplementary
materials, or available from the corresponding author (mdinca@mit.edu).
https://doi.org/10.26434/chemrxiv-2023-j91zf ORCID: https://orcid.org/0000-0002-1262-1264 Content not peer-reviewed by ChemRxiv. License: CC BY-NC-ND 4.0