A practical 4.8-V Li||LiCoO₂ battery

Abstract

Raising the charging voltage of a lithium||lithium cobalt oxide (Li||LiCoO₂) battery is a shortcut to realize high energy density in portable electronics, while the fragile interface of highly delithiated LiCoO₂ (>4.55 volts) can trigger the lattice oxygen release, thus leading to severe interfacial degradation and structural collapse. Here, using lithium pentadecafluorooctanoate as a fluorine source to build robust lithium fluoride–rich electrode-electrolyte interfaces, stable Li||LiCoO₂ batteries at high voltage have been realized, capable of cycling 1500, 600, and 188 times at 4.6, 4.7, and 4.8 volts, respectively. Furthermore, the practicality of Li||LiCoO₂ batteries at an unprecedented cutoff voltage of 4.8 volts has been validated by a 2.7–ampere hour pouch cell, which shows a superior energy density of 544 watt-hours per kilogram and can operate more than 50 cycles. Our exploration of 4.8-volt LiCoO₂ may pave the way to ceaselessly approach its theoretical capacity.

Benefiting from the stabilization of both anode and cathode interfaces, a practical 4.8-V Li||LiCoO₂ battery has been realized.

INTRODUCTION

The application of rechargeable lithium-ion batteries has experienced a few decades since their commercialization in 1991 by Sony Corporation, and the demand for portable electronics with prolonged life span has become more and more urgent. Although lots of cathodes have been explored and used in lithium-ion batteries, LiCoO₂ (LCO) is still the dominant cathode material for portable 3C (computer, communication, and consumer) electronics within a limited space due to its high theoretical specific capacity (274 mA·hours g⁻¹) and exceptionally high density (5.05 g cm⁻³) (1, 2). Typically, the output energy of LCO mainly depends on its charging cutoff voltage, and the practical discharge capacity is only limited to about 185 mA·hours g⁻¹ at 4.5 V (3, 4). Once the charging voltage approaches 4.6 V, LCO undergoes a harmful phase transition from the hexagonal O3 (H3) phase to the H1-3 phase (a hybrid of O3 and O1 phase). Further elevating the charging voltage to 4.7 V for extracting the remaining Li⁺, the discharge capacity of LCO is able to exceed 230 mA·hours g⁻¹. Meanwhile, the most destructive O1 phase emerges in the highly delithiated LCO cathode. The H1-3 phase consists of two kinds of CoO₂ layers: one is the O3 type with some Li atoms stacked in adjacent CoO₂ layers, and the other is the O1 type (Li_1–xCoO₂; x = 1) without any Li atoms, and the interlayer distances of O3 and O1 phases are 4.8 and 4.2 Å, respectively (5, 6). Therefore, the c axis of highly delithiated LCO notably decreases with the formation of H1-3 phase from O3 phase, especially the appearance of a more harmful O1 phase alongside the rapid shrinkage of interlayer distance, thus leading to the sliding of O-Co-O slabs and the collapse of the crystal structure. Meanwhile, the oxidized lattice oxygen (O^n–2; 0 < n < 2) will form and be released from the accumulated microcracks, inducing the irreversible transition of layered structure (R-3m) into ionic-insulating spinel structure (Fd-3m). Besides, the highly oxidative nature of O^n–2 and Co⁴⁺ at the cathode surface can trigger parasitic reactions with electrolytes, leading to additional O loss and Co dissolution and the formation of a thick cathode electrolyte interface (CEI). Compared with the bulk degradation of LCO, the deterioration from the surface is more severe, which can accelerate the contagion of structural damage into the whole crystal (7, 8). Consequently, maintaining the structural integrality of LCO by stabilizing the electrode surface is indispensable to realize a stable high-voltage Li||LCO battery.

To achieve this goal, successes have been achieved in stabilizing such metastable H1-3 and O1 phases and preventing the resultant structure damage using various methods, such as cation doping (3, 4, 9, 10), surface coating (11, 12), electrolyte optimization (13–17), and some integrated methods (18, 19). Electrolyte optimization, which is aimed at forming an ideal CEI, is regarded as the most economical and facile approach to building a protective armor for inhibiting the parasitic reactions between surficial O^n–2 and Co⁴⁺ and blocking the escape of oxygen from the inner crystal. In addition, LiF, an important component of CEI with high electrochemical stability and high modulus (20), has been demonstrated to be able to retain structural integrity and avoid electrolyte penetration, but the LiF from the oxidation of solvents or additives is also accompanied by the formation of organic components, which can sacrifice the protective efficacy of the CEI (21, 22). Up to now, despite so many electrolyte formulations that have been proposed and claimed decent battery performance in LCO with a moderate cutoff voltage (≤4.6 V), realizing a stable ultrahigh-voltage (>4.6 V) LCO is quite difficult, and further raising the charging voltages beyond 4.7 V nearly remains a no-go area.

Li metal anode has been placed with great expectations to achieve advanced batteries with high energy density due to its lowest redox potential (−3.04 V versus standard hydrogen electrode) and highest specific capacity (3861 mA·hours g⁻¹) (23, 24). Although promising, uncontrollable lithium dendrite growth and low coulombic efficiency (CE) in traditional electrolytes restrict the replacement of traditional graphite anode with Li anode. Electrolyte optimization to build a stable solid electrolyte interphase (SEI) on the Li anode is facile and efficient in avoiding the formation of cracks, and LiF is considered to play a vital role in uniform lithium deposition. Although some progress has been achieved in engineering a LiF-rich SEI (24–26), tailoring a LiF-dominated CEI and SEI simultaneously for a stable and ultrahigh-voltage Li||LCO battery remains a challenge.

In this work, lithium pentadecafluorooctanoate (LiPFOA), which has low lowest unoccupied molecular orbital (LUMO) and high highest occupied molecular orbital (HOMO) energy levels, has been proposed as the fluorinated film-forming agent to preferentially generate LiF-rich SEI and CEI for stabilizing the interfaces of the Li anode and the LCO cathode. Notably, LiPFOA-enabled LiF-rich CEI can not only maintain the structural integrity but also suspend the phase transition from O3 to H1-3/O1 phases to some extent. Consequently, the Li||LCO batteries with LiPFOA exhibit high capacity retention of 80% after 1500 cycles at 4.6 V. Even when the charging voltages are elevated to 4.7 and 4.8 V, the batteries can still retain 70% capacity after 600 and 188 cycles, respectively. Benefiting from the LiPFOA-constructed sturdy surface on the Li anode and the LCO cathode, a 2.7–A·hour Li||LCO pouch cell at 4.8 V achieves an exceptional energy density of 544 watt-hours (Wh) kg⁻¹.

RESULTS

Design principle of additive

The lithium dendrite growth and cathode cracking are big concerns for a high-voltage lithium battery in conventional electrolyte, in which the organic solvent will be decomposed into organic-rich SEI and CEI with poor ability to smooth lithium deposition and remain cathode integrity, respectively (Fig. 1A). Because LiF has been regarded as an efficient component of SEI and CEI for stabilizing the Li anode and the LCO cathode (Fig. 1B) (21–24), screening fluorine-rich components as a film-forming agent to inhibit the decomposition of organic solvent becomes the prerequisite for achieving this goal. Unfortunately, the simple fluorination of organic molecules will decrease the electron density of the whole molecule because F is the most electronegative element, which simultaneously lowers both HOMO and LUMO energy levels, thus making it easy to be reduced and difficult to be oxidized (27). As illustrated in Fig. 1C with a long-chain alkane [octane (C₈H₁₈)] as the precursor, the sole fluorination of C₈H₁₈ into electron-deficient octadecafluorooctane (C₈F₁₈) only grants it a reduction active site, whereas the carboxylation of C₈H₁₈ into electron-rich octanoate (C₇H₁₅COO⁻) can only render it an oxidation active site. To address this paradox, incorporating an electron-rich carboxyl group (–COO⁻) into this electron-deficient perfluorinated molecule is a good choice, and we choose LiPFOA (C₇F₁₅COO⁻Li⁺) as the representative additive because it simultaneously has electron-deficient and electron-rich fragments (Fig. 1C). More specifically, the –C₇F₁₅ fragment has abundant F atoms, which can grab the nearby electrons on the carbon skeleton and notably decrease the electron density on the –C₇F₁₅ fragment, making it easy to gain some electrons to be reduced. Meanwhile, the electron-rich –COO⁻ fragment incorporates the oxidation active site into this electron-deficient molecule, thus making it possible to realize the coexistence of high reduction and oxidation tendency (Fig. 1C).

Fig. 1. Design principle and verification.

Illustration of (A) Li dendrite growth and cathode cracking in conventional electrolyte, and (B) smooth Li deposition and intact cathode enabled by fluorinated additive. (C) Design principle of fluorine-rich additive with dual active sites for reduction and oxidation. (D) Frontier molecular orbital energy levels of different molecules. (E) Verification of electrolyte stabilities with linear sweep voltammetry (LSV) curves.

To verify the above speculation, we used density functional theory (DFT) calculations to investigate the frontier molecular orbital energy levels. As expected, the fluorination of precursor (C₈H₁₈) into C₈F₁₈ makes the LUMO and HOMO energy levels synchronously decrease, and the octanoate (C₇H₁₆COO⁻) with sole incorporation of electron-rich –COO⁻ fragment can only increase the HOMO energy level, and little difference is observable on the LUMO energy level (Fig. 1B and table S1). Only when fluorine atoms and –COO⁻ were simultaneously introduced, the molecule (PFOA⁻) can deliver the lowest LUMO and highest HOMO energy levels, even compared with other electrolyte components, such as fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and hexafluorophosphate anion (PF₆⁻). Then, the electrochemical windows of the electrolytes were tested by anodic and cathodic polarization on Al and Cu foil, respectively. As can be seen from Fig. 1E, with the addition of a trace amount (0.5 wt %) of LiPFOA, the electrolyte shows a much higher reduction peak (~1.9 V) and a low oxidation onset potential (~4.0 V), which means LiPFOA can be preferentially reduced and oxidized on the electrodes to form LiF-rich SEI and CEI, following the DFT calculation results and theoretical analyses.

Electrochemical performance of Li||LCO batteries

Here, we used 1 M lithium hexafluorophosphate (LiPF₆) FEC/EMC [3:7 (v/v)] as the base electrolyte, as carbonate electrolytes exhibit good electrochemical stability under high voltages (>4.5 V). FEC is used instead of traditional ethylene carbonate (EC) because EC can be oxidized on a layered cathode at a low voltage of about 3.8 V (28). LiPF₆ FEC/EMC (1 M) is abbreviated as base electrolyte, while LiPFOA electrolyte denotes the base electrolyte with 0.5 wt % LiPFOA added. With the potential to establish LiF-rich interfaces on the cathode and anode of Li||LCO batteries by LiPFOA, we tentatively evaluated the discharge specific capacity influenced by the charging cutoff voltage on the same cell, and the cell was tested for 5 cycles at each cutoff voltage to get a more stable and reliable value. As shown in Fig. 2 (A and B), the LCO cathode delivers only 135 mA·hours g⁻¹ at 4.2 V and 0.2C (1C = 200 mA·hours g⁻¹), which is approximately half of the theoretical specific capacity. When the cutoff voltages are elevated to 4.3, 4.4, and 4.5 V, the discharge capacities increase to 155, 169, and 185 mA·hours g⁻¹, respectively, in line with previously reported values (3, 4). Because the parasitic reactions between the electrolytes and the highly delithiated LCO cathode worsen with increasing voltage, a slight voltage drop and minor capacity decay can be observed at high voltage (>4.5 V) after dozens of cycles. Despite that, when the cutoff voltages are further raised to 4.6, 4.7, and even 4.8 V, the discharge capacities climb to 205, 230, and 244 mA·hours g⁻¹, respectively. Meanwhile, as the average discharge voltages increase with the cutoff voltage, when the cutoff voltages increase from 4.2 V to 4.5 and 4.8 V, the energy densities surge 40.7 and 91.6%, respectively. The details of the relationship between output energy and cutoff voltage are shown in Fig. 2C.

Fig. 2. Cutoff voltage–dependent capacity of LCO cathode.

(A) 0.2C charge-discharge profiles of the LCO batteries with LiPFOA at different cutoff voltages and (B) the corresponding gravimetric and volumetric energy density. (C) Summarization of the output energy of LCO at different cutoff voltages.

Inspired by the superiority of elevating the LCO cutoff voltage to increase the output energy, the practical cycling stability of Li||LCO batteries was assessed at a high-voltage range (4.6 to 4.8 V) with a high LCO loading of about 9.3 mg cm⁻². Before that, we thoroughly demonstrated LiPFOA’s ability to stabilize the Li anode (figs. S1 to S15 and tables S2 and S3) and also optimized the electrolyte components (figs. S16 and S17). After two formation cycles at 0.1C, the batteries were charged at 0.5C and discharged at 1C. At 4.6 V, the Li||LCO batteries deliver a capacity of 2.1 mA·hours cm⁻², and the battery with base electrolyte can only cycle 117 times with a capacity retention of 70% (fig. S18A). Owing to the LiPFOA-endowed LiF-rich SEI and CEI on the Li anode and the LCO cathode, respectively, the Li||LCO battery with LiPFOA electrolyte shows superior stability at 4.6 V, which can retain 80% capacity with a high average CE of 99.92% after 1500 cycles (Fig. 3, A and B). When we attempt to elevate the cutoff voltages to ultrahigh values of 4.7 and 4.8 V (Fig. 3, C to E, and fig. S18, B and C), more reactive oxidative O^n–2 and Co⁴⁺ will form at the highly delithiated LCO cathode surface; therefore, the batteries with base electrolyte quickly attenuate to their 70% initial capacity retention after 46 and 28 cycles, respectively, and their CEs drop to below 99% (Fig. 3E). In contrast, the LiPFOA-rendered LiF-rich SEI and CEI empower the 4.7-V Li||LCO battery to operate 600 cycles with a capacity retention of 70% and CE of 99.95% (Fig. 3, C and E). Even when charging to 4.8 V, the battery with only 0.5 wt % LiPFOA can still operate 188 cycles with a capacity retention of 70% and CE of 99.71% (Fig. 3, D and E, and figs. S18 and S19). Although perfluorinated alkyl substances (PFAS) are known to pose environmental and health risks, including persistence, bioaccumulation, and toxicity, the 0.5 wt % LiPFOA additive used in this work offers benefits for stabilizing high-voltage (4.6 to 4.8 V) LCO batteries, greatly extending their service life. This extended life span, coupled with the environmental impact of battery production and recycling, suggests that the benefits of using this trace amount of PFAS may outweigh its potential harm.

Fig. 3. Electrochemical performance of the Li||LCO batteries.

(A) Charge-discharge profiles and (B) cycling stabilities of the Li||LCO batteries with LiPFOA electrolyte at cutoff voltages of 4.6 V. Charge-discharge profiles of the Li||LCO batteries with LiPFOA electrolyte at cutoff voltages of (C) 4.7 V and (D) 4.8 V, and (E) the corresponding cycling stabilities at 4.7 and 4.8 V. The data were collected after two activation cycles at 0.1C, and charge and discharge were conducted at 0.5C and 1C, respectively. (F) Summaries of the cycling performance of Li||LCO batteries at 4.6 and 4.7 V, and those at 4.8 V with rare reports were not mentioned.

The stabilization of Li||LCO batteries by LiPFOA can also be confirmed by the floating tests, which are often used to evaluate the stability of the electrolyte toward the charged cathode. During the potentiostatic polarization at 4.7 and 4.8 V, the leakage current of charged batteries in the LiPFOA electrolyte is much lower than that in the base electrolyte, particularly at 4.8 V (fig. S20). The LiPFOA-bestowed stabilities in the high-voltage Li||LCO batteries (4.6 to 4.8 V) also show much higher superiority than those in the reported literature (Fig. 3F and table S4) (2, 8, 9, 11, 17, 29–35). Hence, we hope that our explorations of ultrahigh-voltage Li||LCO batteries, especially at 4.8 V (36, 37), can bring the hopes to approach the theoretical capacity of cathodes for fulfilling much higher energy density.

LCO structure evolution

To elucidate the phase transition during the charging and discharging process, differential capacity (dQ/dV) analyses have been used to visualize the peaks of different phases. During the charging process, the initial hexagonal H1 phase of pristine LCO turns into the H2 phase; when charging to about 4.2 V, half of the lithium ions of the LCO are extracted to form Li_0.5CoO₂ as a monoclinic phase (M1). Further delithiation leads to the formation of hexagonal O3 (H3) phase and H1-3 phase (hybrid of O3 and O1 phase) sequentially, and lastly, the Li_1–xCoO₂ turns into O1 phase composed only of CoO₂ layers (5, 6). In the initial dQ/dV plots of two Li||LCO batteries, the M1 phases appear to have nearly identical voltage of about 4.2 V (Fig. 4, A and B). However, a pair of strong peaks indicative of the phase transition between H1-3 and O1 phases is evident in the battery with base electrolyte but not distinguishable in the battery with LiPFOA electrolyte.

Fig. 4. Characterization of the cathode stability.

Differential capacity (dQ/dV) profiles of 4.8-V Li||LCO batteries with base (A) and LiPFOA (B) electrolytes at the first cycle. (C to H) Contour plots [(C) and (F)] and stacked plots [(D) and (G)] of LCO (003) peak from in situ x-ray diffraction (XRD) tests at 0.2C in base [(C) and (D)] and LiPFOA [(F) and (G)] electrolytes, and the corresponding phase separation [(E) and (H)]. In situ differential electrochemical mass spectrometry (DEMS) profiles of the Li||LCO batteries with base (I) and LiPFOA (J) electrolytes at 0.2C charge. (K) Electron paramagnetic resonance (EPR) spectra of the LCO cathode at different states. a.u., arbitrary units.

Then, in situ x-ray diffraction (XRD) tests were used to track the phase evolutions during the charging and discharging process (Fig. 4, C to H), and we focused on the (003) peak because it represents the interlayer distance of CoO₂ slabs and will suffer from marked shrinkage and expansion. As shown in Fig. 4 (C and D), the signals of H1-3 and O1 phases in the base electrolyte emerge when the voltages surpass 4.55 and 4.7 V, respectively, and increase with the state of charge. From the peak fitting analyses, the portions of H1-3, H1-3/O1, and O1 phases at 4.8 V can be estimated to be 6.4, 19.2, and 19.2%, respectively (Fig. 4E), whereas for the LCO cathode in LiPFOA electrolyte, no isolated O1 phase can be observed, less H1-3/O1 phase appears at 4.8 V, and the ratios of H1-3 and H1-3/O1 phases can be decoupled to be 64.4 and 35.6% (Fig. 4H), respectively, which are much lower than those in base electrolyte. Unlike the (003) peak of LCO in base electrolyte, which suffers a larger amplitude of 1.85°, the peak variation in LiPFOA electrolyte is only 1.33° (Fig. 4, C and F), which means that the LCO structure in base electrolyte withstands a larger volume change. Besides, for the LCO in LiPFOA electrolyte, the H2a peak centered at ~18.75°, an intermediate of the H2 phase (38), shows a stronger signal, and the H2 and H3 peaks last a much longer time before vanishing. However, for the LCO in the base electrolyte, the H3 peak broadens into a wide peak at relatively smaller 2θ angles and then decouples with the formation of the H1-3 phase. As the initial (003) peaks of LCO have been normalized to the same height and the two Li||LCO batteries used for in situ XRD tests delivered identical discharge capacities (Fig. 4, C and F; see more discussions in fig. S21), the less shift of (003) peak could be attributed to the sluggish phase transition from H2 and H3 phases to the H1-3 phase. With regard to the H1-3 and, especially, O1 phases, they are very detrimental to the cycling stability of the LCO battery; the severe shrinkage of the CoO₂ layer distance will ruin the LCO structure with the formation of spinel phase and O^n–2 (0 < n < 2). Assuming there is equilibrium (H3 $\rightleftharpoons$ H1-3 $\rightleftharpoons$ O1) for the phase transition between H3, H1-3, and O1 phases, once the H1-3 or O1 phase collapses into the spinel phase or the lattice oxygen is released, the equilibrium will shift to the right with the formation of more harmful H1-3 and O1 phases. Because of the poor ability to build a sturdy CEI on the LCO surface with base electrolyte, O^n–2 would disproportionate into O₂ to accelerate the phase collapse and formation of more H1-3 and O1 phases. Therefore, the slightly higher charge capacity in base electrolyte can be attributed to the irreversible phase transition and severe parasitic reactions with the highly delithiated LCO cathode, leading to a low CE of 94.72%, compared to 98.45% in the LiPFOA electrolyte.

The liberation of O─O dimer (O₂) from oxidized oxygen can be demonstrated by the in situ differential electrochemical mass spectrometry (DEMS). As seen in Fig. 4 (I and J), great amounts of O₂ and CO₂ evolve during the charging process of the Li||LCO battery with base electrolyte. The initial CO₂ and O₂ evolutions in base electrolyte have been imputed to the decomposition of carbonate solvents and trace residual Li₂CO₃ on the LCO surface, while the release of lattice oxygen can also contribute to the CO₂ and O₂ evolutions at the later stage. At the same time, there is nearly no observable gas evolution in the battery with LiPFOA electrolyte, indicating the LCO interface can be efficiently stabilized by LiPFOA-derived LiF-rich CEI. Then, we also use electron paramagnetic resonance (EPR) spectra to determine the existence of oxidized oxygen (O^α–2; 0 < α < 2) and oxygen vacancy in the charged LCO cathode, and the pristine LCO cathode shows no visible adsorption in the sweep region of 3480 to 3560 G (Fig. 4K). Because the O 2p and Co 3d orbitals of LCO overlap partially, the O²⁻ in LCO will be oxidized at a high charging cutoff voltage (39). Upon charging to 4.8 V after 20 cycles, strong adsorption of unpaired electron with g = 2.00 emerges on the LCO cathode cycled in the base electrolyte, which preserves a pair of high peaks after discharge (Fig. 4K and fig. S22), indicating the irreversible formation of oxygen vacancy or radical, while negligible signals can be detected on the LCO cathode operated in LiPFOA electrolyte, despite some lattice oxygen being oxidized during charging (Fig. 4K and fig. S22). Therefore, we can speculate that the LiPFOA-enabled robust LiF-rich CEI can retain the surface integrity of the LCO cathode, thus preventing the release of lattice oxygen and structural collapse.

CEI and structure characterizations

Subsequently, the CEI components on the LCO cathodes were unveiled by x-ray photoelectron spectroscopy (XPS) analyses. As presented in Fig. 5A, fig. S23, and table S5, the decomposition of fluorine-rich PFOA⁻ can be validated by –CF_x intermediate, which produces larger amounts of LiF. The high F/C (46.2%) and low P/C (1.5%) atomic ratios in LiPFOA electrolyte validly prove that LiPFOA-derived LiF-rich CEI plays a vital role in suppressing the decompositions of organic solvents and LiPF₆ (figs. S24 and S25). As PF₆⁻ has the highest LUMO energy level (Fig. 1D), the decompositions of FEC and EMC are much easier than that of PF₆⁻. Although the decomposition of LiPF₆ can proffer some LiF for CEI, the preferential decomposition of organic solvents (FEC and EMC) cannot establish a robust interface to stabilize the LCO cathode, leading to the formation of massive organic solvent–derived components, such as C─O and C═O (fig. S23). Therefore, the codecomposition of organic solvent and PF₆⁻ in base electrolyte leads to a much lower F/C and higher P/C atomic ratios in CEI (figs. S24 and S25), which makes the signal of polyvinylidene difluoride on the LCO surface nearly shrouded by the thick organic-rich CEI (Fig. 5A). Afterward, we further used time-of-flight–secondary ion mass spectrometry (TOF-SIMS) to visualize the depth distribution of CEI components. As shown in Fig. 5 (B and C) and fig. S26, tremendous signals of F⁻ and LiF⁻ assigned to LiF can be observed on the LiPFOA-derived CEI and dominate in the whole spectra, and less organic solvent–derived components (C₂HO⁻, CHO₂⁻, etc.) exist during the sputtering process. Besides, the appearance of some not fully decomposed –CF_x fragments (CF⁻, CF₃⁻, C₃F⁻, etc.) from PFOA⁻ further corroborates that the PFOA⁻ can be dedicated to LiF-rich CEI and inhibit the severe decomposition of electrolyte (figs. S27 and S28). Considering the high modulus of LiF (20), thus we can deduce that the LiPFOA-enabled LiF-rich CEI can retain the structural integrity of the LCO cathode and avoid the lattice oxygen release.

Fig. 5. Analyses of cathode surface and structure.

(A) F 1s XPS analysis of the LCO cathodes in base and LiPFOA electrolytes. TOF-SIMS depth profiles (B) of the LCO cathodes in LiPFOA electrolyte and (C) the spatial distributions of F⁻, CF⁻, PO₂F₂⁻, Li₂CO₃⁻, CHO₂⁻, and C₂HO⁻ secondary ions. Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images of the cathodes after 50 cycles in base (D to F) and LiPFOA (G to I) electrolytes. (J) X-ray adsorption spectra of Co L_2,3-edge of the LCO cathodes after 10 and 20 cycles.

The microstructures of the charged LCO cathodes were confirmed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM). After 50 cycles in base electrolyte, the CEI’s inability to prevent the lattice oxygen release leads to the structural collapse, thus obvious microcracks can be found in SEM and TEM images (Fig. 5, D and E, and figs. S29A and S30A), forming a ~15-nm-thick CEI (Fig. 5E). Besides, the fast Fourier transform image indicates that the layered structure (R-3m) has collapsed into spinel phase (Fd-3m), and the lattice fringe spacing of 4.59 Å can be indexed to its (111) plane (Fig. 5F). In a sharp contrast, the 4.8-V LCO charged in LiPFOA electrolyte still remains intact (Fig. 5G, and figs. S29B and S30B), and the thickness of CEI is measured to be only 5 to 6 nm (Fig. 5H). Moreover, the delithiated LCO maintains layered structure (R-3m), and the lattice fringe with a spacing of 2.39 Å belongs to its unique (101) plane (Fig. 5I). The formation of spinel phase can also be evidenced by x-ray adsorption spectra, in which a shoulder on the left of Co-L₃ peak assigned to the formed Co²⁺ can be observed and strengthened during cycling in the base electrolyte (Fig. 5J) (40). Last, we can draw a conclusion that LiPFOA-endowed LiF-rich CEI can avoid the lattice oxygen loss, thus inhibiting the formation of spinel phase and delaying the phase transition from H3 to H1-3 and O1 phases to some extent.

4.8-V pouch cell

Taking advantage of the ultrahigh specific capacity of the 4.8-V LCO cathode and the LiPFOA’s capability of stabilizing the interfaces on the LCO cathode and the Li anode, materializing an ultrahigh energy density in a 4.8-V Li||LCO pouch cell seems to be possible. To verify the above hypothesis and highlight the practicality of LiPFOA electrolyte, we pushed the test to harsh conditions, including a high mass-loading cathode (~23.5 mg cm⁻²; LCO: 96.4 wt %), low negative/positive (N/P) ratio (50 μm; N/P = 1.9), and lean electrolyte (2 g A·hour⁻¹); the brief parameters of the Li||LCO pouch cell can be observed in Fig. 6A and are detailed in table S6. After equipping with LiPFOA electrolyte, the Li||LCO pouch cell charged at 4.8 V delivers an initial capacity of 2.73 A·hours at 0.1C (movie S1), and the specific capacity of LCO still remains 241 mA·hours g⁻¹ at such a high areal capacity of about 5.5 mA·hours cm⁻², well accords with the test result in the coin cell (figs. S31 and S32). Therefore, the 4.8-V Li||LCO pouch cell achieves an ultrahigh energy density of 544 Wh kg⁻¹ and can retain 64% capacity after 50 cycles at 0.2C charge and 0.4C discharge (Fig. 6, B and C). Eventually, we can confidently proclaim that LiPFOA-endowed LiF-rich electrode-electrolyte interfaces can stabilize the interfaces of both the Li anode and the LCO cathode, especially for the LCO cathode, thus enabling the Li||LCO cell to steadily operate at a high-voltage range of 4.6–4.8 V. In addition, we hope that our unprecedented realization of a stable 4.8-V Li||LCO pouch cell will inspire us to target a high-voltage LCO battery to approach its theoretical specific capacity.

Fig. 6. Demonstration of 4.8-V Li||LCO pouch cell.

(A) Parameters of 4.8-V Li||LCO pouch cell. Charge-discharge voltage profiles (B) and cycling stability (C) of the 4.8-V Li||LCO pouch cell. E/C, Electrolyte/Capacity.

DISCUSSION

To conclude, we designed and introduced LiPFOA as a fluorine-rich additive to the battery electrolyte, which is gifted with both low LUMO and high HOMO energy levels, thus building robust LiF-rich SEI and CEI on both the Li anode and the LCO cathode, respectively. With the help of LiF-rich CEI, the LCO cathode can retain the structural integrity during the delithiation and lithiation; thus, stable Li||LCO batteries at 4.6 to 4.8 V have been realized. The Li||LCO batteries exhibit high capacity retention of 80% after 1500 cycles and 70% after 600 cycles at 4.6 and 4.7 V, respectively. Even by pushing the charging cutoff voltage to 4.8 V, the battery’s life span can still reach 188 times, with 70% capacity retention. The practicality of the LiPFOA-enabled 4.8-V Li||LCO battery has been unprecedentedly verified by a 2.7–A·hour Li||LCO pouch cell, which delivers an ultrahigh energy density of 544 Wh kg⁻¹ and can cycle more than 50 times. Therefore, we can thoroughly believe the LiPFOA’s powerful efficacies in constructing robust LiF-rich SEI and CEI to stabilize the Li anode and the LCO cathode. Last but not least, we trust that our exploration and demonstration of a 4.8-V Li||LCO pouch cell can behave as a forerunner to energize more research on ultrahigh-voltage LCO to harvest as much energy as possible.

MATERIALS AND METHODS

Materials

Pentadecafluorooctanoic acid (>98%) and lithium carbonate (Li₂CO₃; 99.9%) were purchased from Shanghai Macklin Biochemical Co. Ltd. (Mackin). Anhydrous and analytical grade ethanol and acetone were purchased from Sinopharm Chemical Reagent Co. Ltd. Anhydrous LiPF₆ (99.99%), FEC (>99%), and EMC (99.9%) were purchased from Aldrich Chemical Company Inc. LCO single crystal was purchased from Rongbai Technology, the ratio of the active materials for preparing the LCO cathode is 96.4 wt %, and the areal loading masses of about 10 and 23.5 mg cm⁻² were chosen for the cathodes in the coin cell and pouch cell, respectively. Fifty-micrometer Li (≥99.9%) was purchased from China Energy Lithium Co. Ltd., and polypropylene membrane (Celgard 2500; 25 μm) and polyethylene membrane (12 μm) were purchased from Guangdong Canrd New Energy Technology Co. Ltd.

Preparation of LiPFOA

Pentadecafluorooctanoic acid (8.28 g) was first dissolved in ethanol to form a uniform solution, and then 1.5 g of Li₂CO₃ was added to neutralize the above acidic solution with violent stirring. After stirring for about 2 hours, the titrated solution was filtered and dried under 80°C. Next, the dried white powder was dissolved in acetone to remove the trace insoluble Li₂CO₃, and the solution was further filtered and dried subsequently. Last, we obtained the LiPFOA.

Electrochemical measurements

To prepare the base electrolyte at 1 M LiPF₆, 1.52 g of LiPF₆ was added to 2.80 ml of FEC and 6.93 ml of EMC. In addition, for the LiPFOA electrolyte, 65 mg of LiPFOA was added to the base electrolyte to get a weight percentage of 0.5 wt %. CR2032 was used for the tests in coin cells. Li||Cu and Li||Al cells were used to evaluate the redox behaviors of the electrolytes. Leakage current tests were performed on Li||LCO cells after the 50th charge at 4.7 or 4.8 V, and then the potentials of the cells were held at 4.7 or 4.8 V for 10 hours to record the leakage currents. Galvanostatic intermittent titration technique (GITT) experiments were performed at 0.2C for 20 min with a relaxation time of 2 hours. CEs were evaluated by Li-Cu cells at 0.5 mA cm⁻² and 0.5 mA·hours cm⁻², and the Aurbach protocol was also taken as a valuation criterion to validate the cycling efficiency of the Li anode (41).

Characterizations

Galvanostatic charge/discharge and GITT tests were conducted on LAND CT2001A. Cyclic voltammetry, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy, and chronoamperometry tests were performed on the CHI760 electrochemical analyzer. XPS was collected on the Thermo Fisher Scientific K-Alpha. XRD experiments were recorded on the D2 PHASER XE-T X-ray Diffractometer System. The Quattro S Scanning Electron Microscope was used to visualize the surface morphologies of the LCO cathode and Li anode. TOF-SIMS analysis was used to visualize the distribution of CEI components, which was performed on ToF.SIMS 5-100 instrument (IONTOF GmbH) with high mass resolution mode; 30-keV Bi⁺ ion beam and 1-keV Cs⁺ ion beam were used for depth profiling and sputtering the cathodes, respectively, and the detecting area was fixed at 200 μm by 200 μm with a sputtering time of 20 min. QMG220 (Linglu Instrument Co. Ltd.) was used to record the in situ DEMS, and high-purity (99.999%) Ar was used as the carrier gas with a flow rate of 0.8 ml/min. After purging the gas system of the homemade electrochemical cell for 6 hours, the in situ electrochemical test began. The charging cutoff voltage and charging current were 4.8 V and 0.25C, respectively. Bruker EMXplus-6/1 was used to collect the EPR spectra at room temperature for checking the existence of oxygen vacancies and unpaired electron in the LCO cathode. X-ray adsorption spectra were collected from Beamline BL02B02 of Shanghai Synchrotron Radiation Facility in total electron yield mode.

Molecular simulations

Gaussian 16A software (42) was used to perform DFT calculations, including structure optimizations and calculations of single-point energies and RESP2_0.5 charges. More specifically, B3LYP-D3(BJ)/6-311g(d,p) was used to optimize the structure first, then B2PLYP-D3(BJ)/ma-def2-TZVP was used to obtain the single point energies, and B3LYP-D3(BJ)/def2-TZVP was used to calculate the RESP2_0.5 charges. Here, the solvation effect was included under the Solvation Model based on Density (SMD) (43), in which methyl ethanoate was chosen as the solvent and 22.7 was adopted as the dielectric constant (44). As for the classic molecular dynamics simulations, they were calculated on the GROMACS package, version 2018.8 (45). For the base electrolyte, it includes 330 Li⁺, 330 PF₆⁻, 1280 FEC, and 2100 EMC. For the 0.5 wt % LiPFOA electrolyte, five Li⁺ and five PFOA⁻ were added to the simulation system of base electrolyte (table S2). Multiwfn was used to find the minima of molecular electrostatic potential, and Sobtop was used to generate the force field parameters and GROMACS topology file (46). General Amber force field was adopted for FEC, EMC, PF₆⁻, and PFOA⁻, while optimized potentials for liquid simulation all-atom force field was used for Li⁺. The initial energy minimization was performed at 298.15 K under 1 atm., 1000 ps with a time step of 1 fs was taken for equilibrating the simulation system, and production runs were generated during 1000 ps with a time step of 1 fs. Cutoff radii of 1.0 nm were used to account for the short-range van der Waals interactions and long-range electrostatic interactions with particle mesh Ewald summation method. Visual Molecular Dynamics software was used to integrate the radial distribution functions with a step size of 0.02 Å.

Supplementary Materials

The PDF file includes:

Supplementary Text

Figs. S1 to S32

Tables S1 to S6

References

Legend for movie S1

Other Supplementary Material for this manuscript includes the following:

Movie S1