Exploring 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as a high voltage electrolyte solvent for 5-V Li2CoPO4F cathode

Abstract

1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) is investigated as a cosolvent for high voltage electrolytes of Li2CoPO4F. Compared with conventional carbonate-based electrolyte (1-M LiPF6 ethylene carbonate [EC]/dimethyl carbonate [DMC] [1:1, wt:wt]), 1 M LiPF6 F-EPE/DMC (1:2, wt:wt) exhibits significantly improved antioxidant ability in high voltage, thus greatly enhances the electrochemical performance of 5.0-V Li2CoPO4F/Li cells. Linear sweep voltammetry (LSV) and charging/discharging tests demonstrate that the F-EPE/DMC electrolyte possesses both a high oxidation voltage up to 6.2 V vs. Li+/Li on Pt electrode and superior oxidation stability on Li2CoPO4F cathode. Benefiting from its high antioxidant ability, the capacity retention of Li2CoPO4F cathode increases from 15% in EC/DMC electrolyte to 51% in F-EPE/DMC electrolyte after 100 cycles at 1 C between 3.0 and 5.4 V. Moreover, differential capacity (dQ/dV) analysis, electrochemical impedance spectroscopy, ex situ X-ray diffraction, and X-ray photoelectron spectroscopy are used to analyze the effects of F-EPE/DMC electrolyte on the improved electrochemical performance. It is illustrated that the high stability of F-EPE/DMC electrolyte effectively inhibits the oxidative decomposition of the electrolyte on Li2CoPO4F electrode above 5.0 V and suppresses the damage to the surface of Li2CoPO4F, thus alleviate the increase in electrode polarization and cell impedance.

Introduction

The rapid evolution of human society toward electrification, informatization, and intelligence has put forward higher requirements on the performance of rechargeable batteries. Lithium ion battery has been widely used to portable electronic devices, electric vehicle, and grid-scale energy storage since its commercialization in 1990s, while the energy density of currently available commercial lithium ion batteries is hard to satisfy the increasing demands for higher energy density. So, the development and research of high energy density lithium ion battery has attracted extensive attention from both industry and academia. Usually the energy density of lithium ion batteries can be improved from two aspects, to improve the battery discharge capacity or increase the operating voltage plateau. Li2CoPO4F as the cathode material for high voltage lithium ion battery has attracted much attention because of its high operating voltage plateau of about 5.0 V versus Li+/Li, which is the highest among state-of-the-art cathode materials [1,2,3,4,5,6,7,8]. Its high working voltage is attributed to the cooperative inductive effects of the PO43− polyanion group and the extremely electronegative F anion [9, 10]. Moreover, the lower affinity of lithium ions toward fluoride than oxide anions is expected to improve the ionic conductivity of Li2CoPO4F [11], therefore significantly enhancing the rate capability. Li2CoPO4F also exhibits both good chemical stability, structural stability, and thermal stability owing to its three-dimensional polyanion frame structure [3, 8].

However, the high working voltage of ~5.0 V far exceeds the electrochemical stability windows of conventional organic carbonate-based liquid electrolytes (~4.5 V vs. Li+/Li), which results in rapid capacity decay [2, 7, 12, 13]. Quang et al. [6] synthesized Li2CoPO4F material using a two-step method (the combination of sol–gel method and solid state calcination). It exhibited a first discharge capacity of 91 mAh∙g−1 when cycled in 1-M LiPF6 ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1:3, v:v:v) at 0.1 C with the voltage range of 3–5.5 V. However, the discharge capacity rapidly declines to 68 mAh∙g−1 after 20 cycles. Many efforts have been undertaken to address this challenge, including surface coating of the active material and/or applying functional electrolyte additives, which facilitates the formation of a passivation film on the cathode surface and stabilizes the electrode/electrolyte interface. Surface coating techniques have been widely applied to modify the cathode materials for suppressing the parasitic reactions at the electrode/electrode interface, thus enhance the cycling stability. The discharge capacity and cycling performance of Li2CoPO4F were effectively improved by coating it with ZrO2 [14], Al2O3 [15], Li3PO4 [16], and SiO2 [17]. In addition to surface modification, the exploitation of new electrolytes with broader electrochemical stability windows is another effective strategy to address this challenge [2]. Improved capacity and cycling behaviors were obtained with the use of sulfone-based electrolyte. A high capacity around 109 mAh∙g−1 and stable cycling for 20 cycles were obtained for Li2CoPO4F with 1-M LiPF6 dimethyl sulfone (DMS) and ethyl methyl sulfone (EMS) (85:15, wt:wt) upon cycling between 2.0 and 5.5 V.

In the past decade, a lot of efforts have been dedicated to the exploitation of more stable solvents and salts for electrolyte with a broad electrochemical stability window for high voltage Li-ion batteries [12, 18, 19]. The fluorinated compounds (carbonates, esters, and ethers) have received extensive attention as cosolvents for high voltage electrolytes because of their higher oxidation stability and lower flammability compared with their nonfluorinated counterparts. According to the calculated results of Takashi et al. [20], the highest occupied molecular orbital (HOMO) energy of fluorinated compounds is lower than their nonfluorine counterparts, resulting in higher oxidation stability. As cosolvents, fluorinated compounds have been found to inhibit the parasitic reactions, thus suppressing impedance growth and improving cycling stability when cycling under high voltage (>4.5 V) [21,22,23]. With the using of fluorinated electrolytes, such as 1.0-M LiPF6 in FEC/F-EMC/F-EPE at volume ratio of 3:5:2, Hu et al. [24] reported a significantly enhanced cycling stability on the 4.8-V LiNi0.5Mn1.5O4/graphite cells. Amine [25, 26] and Xie et al. [27] also founded that the fluorinated electrolytes containing 3-(1,1,2,2-tetrafluoroethoxy)-1,1,2,2-tetrafluoropropane exhibited a superior oxidative stability, which significantly enhanced the cycling performance of 4.8-V spinel LiNi0.5Mn1.5O4 at both room temperature (RT) and elevated temperature (55°C). Lavi et al. [28] showed that the cycling performance of Li1.2Mn0.56Co0.08Ni0.16O2/Li cells charged up to 4.8 V can be greatly enhanced via replacing DEC by fluorinated solvents.

Herein, the effects of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) as fluorinated cosolvent on 5.0-V Li2CoPO4F cathode were investigated. It demonstrated that as a cosolvent, F-EPE is promising for improving the high voltage stability of electrolytes, which significantly enhance the electrochemical performance of the 5.0-V Li2CoPO4F/Li cells. The effects of F-EPE on the electrochemical performance of high voltage Li2CoPO4F cathode were analyzed using electrochemical and spectroscopic characterization.

Experiment

Li2CoPO4F material was synthesized using solid state reaction method. Briefly, 0.02-mol LiF, 0.01-mol Co(CH3COO)4H2O, and 0.01-mol NH4H2PO4 were mixed by ball milling at 500 rpm for 10 h with acetone as a dispersant. After evaporating the solvent, the mixtures were pressed into pellet. The pellet was sintered at 650°C for 6 h under Ar streams, then quenching to RT.

Battery grade DMC solvent and lithium hexafluorophosphate (LiPF6) salt were purchased from Dongguan Shanshan Battery Materials Co., Ltd. (China) and used as received. 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) (99.90%) was purchased from Tianhe Chemicals (China) and dried with 4-Å molecular sieves for 7 days before use. A commercial electrolyte of 1.0-M LiPF6 in EC/DMC (1:1, by weight) purchased from Dongguan Shanshan Battery Materials Co., Ltd. (China) was used as baseline electrolyte. Fluorinated electrolyte was prepared by dissolving 1.0-M LiPF6 in the mixed solvent of F-EPE and DMC at weight ratio of 1:2 in an Ar-filled glove box (H2O < 5 ppm, O2 < 5 ppm).

To evaluate the electrochemical performance of Li2CoPO4F cathode with different electrolytes, CR2025 type coin cells were assembled. The Li2CoPO4F electrodes were prepared via slurry casting. The active material Li2CoPO4F, conductive additive (acetylene black), and binder (polyvinylidene difluoride, PVDF) with a weight ratio of 8:1:1 were dispersed in N-methyl pyrrolidione (NMP) solvent to form a uniform slurry. The slurry was coated on aluminum foils to obtain the electrodes, then the electrodes were dried at 80°C for 20 h under vacuum and pressed at 20 MPa. The area loading of Li2CoPO4F cathode is around 3 ~ 4 mg cm−2. The coin cell was assembled with Li2CoPO4F as the cathode, lithium metal as the anode, and tri-layer polypropylene-polyethylene-polypropylene film (Celgard 2325) as separator in an Ar-filled glove box.

Ionic conductivities of the two electrolytes were measured with a conductivity meter at RT. Linear sweep voltammetry (LSV) studies were performed to evaluate the electrochemical stability windows of the electrolytes. The three electrodes electrochemical cell with smooth platinum sheet as the working electrode and lithium metal foil as the counter and reference electrodes are used for LSV measurements. LSV tests were performed from the open circuit voltage (OCV) to 7.0 V at a scan rate of 0.1 mV s−1 at RT. Electrochemical impedance spectroscopy (EIS) test was carried out with a frequency range from 100,000 to 0.01 Hz under a signal amplitude of 10 mV. The electrochemical tests were performed by using a CHI 660 Electrochemical Workstation (Chenhua Instruments, Shanghai, China). The electrochemical performance was tested by galvanostatic cycling under various current rates (1 C = 143 mA g−1) between 3.0 and 5.4 V using Land CT2001A battery testing system at 30°C. The cycled Li2CoPO4F coin cells were carefully disassembled in an Ar-filled glove box. The collected cathodes were rinsed with DMC to remove the residual electrolyte and then were dried and stored under vacuum environment before X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The XRD measurement was carried out on Rigaku Ultima IV X-ray Diffractometer (Rigaku Corporation, Japan) equipped with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 4° min−1. The surface chemical components of the cycled Li2CoPO4F cathodes were analyzed by XPS on an ESCA spectrometer (Quantum 2000, Physical Electronics, USA) with monochromatic Al Kα 1,486.6-eV radiation operated at 23.2 W in a vacuum of <10−8 Torr.

Results and discussion

Fig. 1 shows the XRD pattern of as prepared Li2CoPO4F. It shows that all the diffraction peaks can be indexed to an orthorhombic structure with space group Pnma, which indicates that pure phase Li2CoPO4F is synthesized.

Fig. 1
figure1

Powder XRD pattern of the as obtained Li2CoPO4F material

The ionic conductivities of 1.0-M LiPF6 in EC/DMC (1:1, by weight) and 1.0-M LiPF6 in F-EPE/DMC (1:2, by weight) electrolytes were determined to be 12.4 and 7.2 mS cm−1 at RT, respectively. The EC/DMC-based electrolyte exhibits a high ionic conductivity of 12.4 mS cm−1, which is consistent with the results reported in the literature [29]. The conductivity of F-EPE/DMC-based electrolyte slightly lower than EC/DMC electrolyte, owing to the poorer ability of F-EPE to dissociate lithium salt LiPF6 [30].

Fig. 2 shows the LSV curves for EC/DMC and F-EPE/DMC electrolytes. It shows that the conventional EC/DMC electrolyte begins to decompose over 5.5 V accompanying with a sharp increase in the oxidation current. On the contrary, the F-EPE/DMC electrolyte can remain stable up to 6.2 V. At the same time, the oxidation current is obvious lower compared with the EC/DMC electrolyte. LSV results demonstrate that F-EPE/DMC electrolyte possesses much higher oxidization stability than EC/DMC electrolyte on platinum electrode. It is expected that the electrochemical performances of Li2CoPO4F/Li cells will be enhanced with the application of F-EPE/DMC electrolyte. The electrochemical floating tests were performed with Li2CoPO4F/Li cells to further investigate the oxidative stability of the EC/DMC and F-EPE/DMC electrolytes. Fig. 3 shows the current-time curves at 5.2 V for 10 h. It can be seen that the F-EPE/DMC electrolyte exhibits much smaller leakage current, indicating that the degree of decomposition of the F-EPE/DMC electrolyte on Li2CoPO4F electrode is lower than EC/DMC electrolyte. This result suggests that F-EPE/DMC electrolyte has higher oxidation stability than EC/DMC electrolyte in practical Li-ion batteries.

Fig. 2
figure2

LSV curves of platinum electrode in the two electrolytes from OCV to 7.0 V at the scanning rate of 0.1 mV s−1

Fig. 3
figure3

The current-time curves of electrochemical floating tests of Li2CoPO4F/Li cells with EC/DMC and F-EPE/DMC electrolytes at 5.2 V for 10 h

The electrochemical performances of the 5-V Li2CoPO4F cathode in EC/DMC and F-EPE/DMC electrolytes were studied. Fig. 4a and b show the first charge/discharge profiles of the Li2CoPO4F/Li batteries with EC/DMC and F-EPE/DMC-based electrolytes cycled at 0.2 and 1 C. The Li2CoPO4F cathodes deliver typical charge/discharge behaviors in both electrolytes. Compared with the cells with traditional EC/DMC electrolyte, the cells with F-EPE/DMC electrolyte show higher initial discharge capacity at both 0.2 and 1 C. More importantly, the initial charge capacities of Li2CoPO4F cathode in EC/DMC electrolyte are much higher than that of in F-EPE/DMC electrolyte, especially at 0.2 C. And the initial Coulombic efficiencies (CEs) of the cells applying F-EPE/DMC electrolyte are significantly higher than that of applying EC/DMC electrolyte. The charge capacities of Li2CoPO4F cathode in EC/DMC electrolyte are much higher than the theoretical value (143 mAh∙g−1) of Li2CoPO4F, which can be attributed to the severe oxidizing decomposition of the electrolyte at high voltage, since the charge cutoff voltage up to 5.4 V is far beyond the electrochemical stability window of traditional carbonate-based liquid electrolyte. Although LSV tests show that the oxidizing decomposition potential of 1.0-M LiPF6 in EC/DMC is higher than 5.5 V on platinum electrode. In practical lithium ion batteries, the electrode composed of micro- or nano-size cathode materials possesses high surface areas. Also, the transition metal ions, especially cobalt and nickel ions, show high catalytic activity toward electrolyte decomposition. These will significantly aggravate the oxidization of electrolyte at high potential [16, 31]. The cycling performances of the Li2CoPO4F/Li cells with the two electrolytes are presented in Fig. 4c and d. It can be observed that the cycling performance of the batteries with F-EPE/DMC electrolyte is greatly enhanced compared with that of with EC/DMC electrolyte at both 0.2 and 1 C. The capacity retentions of Li2CoPO4F/Li cells increased from 10% to 70% after 50 cycles at 0.2 C and 18% to 51% after 100 cycles at 1 C, respectively. Moreover, the CEs of cells with F-EPE/DMC electrolyte are obviously higher than that of with EC/DMC electrolyte throughout the entire cycling (Fig. 4e and f).

Fig. 4
figure4

The first charge/discharge profiles (a, b), cycling performance (c, d), and Coulombic efficiencies (e, f) of the Li2CoPO4F/Li batteries with the two electrolytes at 0.2 C (a, c, e) and 1 C (b, d, f)

To analyze the electrochemical behavior in more detail, the representative differential capacity (dQ/dV) curves of Li2CoPO4F/Li cells applying EC/DMC and F-EPE/DMC electrolytes at 0.2 C are presented in Fig. 5. It shows that two pairs of distinguishable redox peaks can be obviously observed during the first cycle. The irreversible anodic peak at around 5.4 V can be assigned to electrolyte oxidation. It is worth noting that the intensity of electrolyte oxidation peak in F-EPE/DMC electrolyte is much lower than that of in EC/DMC electrolyte, indicating the higher antioxidant capability of the F-EPE/DMC electrolyte. After the initial cycle, the cell with EC/DMC electrolyte developed huge polarization after 25 cycles, indicating that parasitic side reactions may occur and lead to polarization resistance. And the dQ/dV curve was significantly deformed indicating the degradation of the electrode materials. On the contrary, Li2CoPO4F cathode in F-EPE/DMC electrolyte only shows moderate polarization even after 50 cycles. The electrochemical test results indicate that F-EPE as a cosolvent can significant enhance antioxidation ability of the electrolyte, thus greatly improve the electrochemical performance of 5-V Li2CoPO4F cathode, resulting in both higher capacity retention and CEs.

Fig. 5
figure5

The dQ/dV curves of Li2CoPO4F/Li cells with (a) EC/DMC and (b) F-EPE/DMC electrolytes during cycling at 0.2 C

Fig. 6 shows the EIS spectra of the Li2CoPO4F/Li batteries applying EC/DMC and F-EPE/DMC electrolytes after different cycles. The Nyquist plots exhibit two semicircles at the high and medium frequencies region with a slope tail at the low frequency region. The leftmost semicircle can be assigned to the interfacial resistance (RSEI) corresponding to lithium ion migration through the solid electrolyte interface (SEI). The semicircle in the middle frequency range is related to the charge transfer

Fig. 6
figure6

EIS spectra of Li2CoPO4F/Li cells with (a) EC/DMC- and (b) F-EPE/DMC-based electrolytes after different cycles at 1 C and resistances (c, d) obtained from fitting the Nyquist profiles with the equivalent circuit (inset in Fig. 6c)

resistance (Rct), and the slope tail can be assigned to lithium ion diffusion processes inside electrode materials (Warburg impedance, Zw). The EIS spectra were fitted with the equivalent circuit shown in inset of Fig. 6c, and the obtained values of RSEI and Rct are presented in Fig. 6c and d. It can be seen that the cell with EC/DMC electrolyte shows a similar resistance values to that of with F-EPE/DMC electrolyte after the 5 cycles. The resistances of the Li2CoPO4F/Li cells with both electrolytes increase with increasing cycle number. However, the resistances' increase rate of the cell with F-EPE/DMC is significantly slower than that of with EC/DMC electrolyte. As the cycle number increases, the RSEI and Rct of the cell with EC/DMC electrolyte raised drastically, which is highly related to the thickening of the interphase film owing to the severe oxidizing decomposition of the electrolyte under high voltage and the degradation of the cathode/electrolyte interface.

To further analyze the mechanism of capacity fading, the structure stability of Li2CoPO4F upon cycling was investigated using XRD. Fig. 7 shows XRD patterns of Li2CoPO4F electrode after different cycles in EC/DMC and F-EPE/DMC electrolytes. After the first cycle, an obvious change in the relative intensity of (002) and (200) reflections can be observed, corresponding to the structural transformation and forming a new “modified” framework during the initial lithium extraction/insertion processes. After 100 cycles, the width of the diffraction peaks becomes broadened, indicating the slight structural degradation of the Li2CoPO4F electrode during cycling. However, the Li2CoPO4F electrode cycled in F-EPE/DMC electrolyte exhibits higher structural stability compared with that of cycled in EC/DMC electrolyte. The improved structural stability of the Li2CoPO4F electrode cycled in F-EPE/DMC electrolyte is closely linked to the high oxidation stability of F-EPE/DMC electrolyte, which suppresses the damage to the surface of Li2CoPO4F by mitigating from attack by the electrolyte.

Fig. 7
figure7

XRD patterns of the as synthesized Li2CoPO4F material and Li2CoPO4F electrodes after the first and 100 cycles in EC/DMC- and F-EPE/DMC-based electrolytes at 1 C

To further clarify the effects of the F-EPE on the components of decomposition products, XPS is applied to determine the surface composition of the cycled electrodes. Fig. 8 shows the C 1s, F 1s, and P 2p XPS spectra of the Li2CoPO4F electrodes after 100 cycles in EC/DMC and F-EPE/DMC electrolytes. For the C 1s spectra, five peaks can be observed, corresponding to carbon species such as C–C (284.8 eV), C–H (285.8 eV), C–O (286.9 eV), O–C〓O (288.6 eV), and Li2CO3/−CF2 (290.1 eV) [32, 33]. The peaks at 285.8 and 284.8 eV are generated from PVDF binder and conductive carbon. The peaks at 290.1 eV related to Li2CO3 is overlapped with the peak of PVDF (−CF2−). The C–O (286.9 eV) and O–C〓O (288.6 eV) related to lithium alkoxide (ROLi) and lithium alkyl carbonate are generated from electrolyte decomposition. Compared with EC/DMC electrolyte, the intensity of the peaks of C–O and O–C〓O is significantly lower in F-EPE/DMC electrolyte. As for F 1s spectra, the peak at 684.5 eV comes from PVDF binder. It shows that the electrode cycled in F-EPE/EMC electrolyte exhibits much higher PVDF content than in EC/DMC electrolyte. While the peak intensity of LixPOyFz (686.0 eV) [27, 34] related to the decomposition product of the LiPF6 in F-EPE/DMC electrolyte is much lower than that in EC/DMC electrolyte. The peaks at 684.4 eV related to LiF is overlapped with the peak of fluorine in fluorophosphate group (PO4F4−) [15], which makes it difficult to quantify the amount of LiF by the F 1s spectra. The P 2p spectra mainly contain three peaks; a peak at 133.4 eV belongs to PO43− group from Li2CoPO4F [15]; the other two peaks at 134.5 and 135.7 eV are originated from LixPOyFz and LixPFy, which are the decomposition products of LiPF6 [34]. The peak intensity of LixPOyFz and LixPFy in F-EPE/DMC electrolyte is lower than in EC/DMC electrolyte. While the peak intensity of PO43− is higher in F-EPE/DMC electrolyte. The results of P 2p spectra are consistent with the F 1s spectra. XPS results indicate that the F-EPE as a cosolvent can prevent the serious oxidative decomposition of the electrolyte at high voltage and promote the formation of a thin and stable cathode/electrolyte interphase (CEI) film, thus enhance the cycling stability of Li2CoPO4F cathode.

Fig. 8
figure8

The C 1s, F 1s, and P 2p XPS patterns of Li2CoPO4F electrodes after 100 cycles in EC/DMC and F-EPE/DMC electrolytes

Conclusions

F-EPE was evaluated as a cosolvent for high voltage electrolytes of 5-V Li2CoPO4F cathode. The LSV test results show that F-EPE/DMC-based electrolyte possesses high oxidation stability up to 6.2 V on Pt electrode, which suppresses the oxidation decomposition of electrolyte on the surface of Li2CoPO4F cathode. Compared with conventional carbonate-based EC/DMC electrolyte, the Li2CoPO4F/Li cells with F-EPE/DMC electrolyte exhibit lower increase in cell impedance and promote electrochemical performance with higher reversible capacities and cycling stability cycled between 3.0 and 5.4 V. Li2CoPO4F/Li cells with EC/DMC electrolyte show poor electrochemical performance with very low CEs and fast capacity decay due to the violent oxidizing decomposition of electrolyte in high voltage. The electrochemical performance was significant improved with the use of F-EPE/DMC electrolyte, which delivers a high initial capacity of 104 mAh∙g−1 with a capacity retention of 51% after 100 cycles at 1 C. dQ/dV analysis and EIS reveal that Li2CoPO4F/Li cells with F-EPE/DMC electrolyte show less increase in polarization resistance compared with that of with EC/DMC electrolyte, owing to its higher antioxidant ability. Moreover, XRD results demonstrate that the use of F-EPE/DMC electrolyte can help to preserve the structural stability of Li2CoPO4F cathode during cycling via suppressing the erosion damage to the surface of Li2CoPO4F from decomposition products of electrolyte.

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Acknowledgments

We acknowledge the support from the open fund of Fujian Provincial Key Laboratory of Functional Materials and Applications (Xiamen University of Technology, Grant No. fma2018008), the National Natural Science Foundation of China (Grants No. 21875196 and 21573184), and the Science and Technology Planning Projects of Fujian Province, China (Grant 2019H0003).

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Correspondence to Mi Lu or Zhengliang Gong.

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Wang, Z., Zhuang, S., Lu, M. et al. Exploring 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as a high voltage electrolyte solvent for 5-V Li2CoPO4F cathode. J Solid State Electrochem (2021). https://doi.org/10.1007/s10008-021-04915-z

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Keywords

  • Lithium ion batteries
  • High voltage electrolytes
  • Li2CoPO4F
  • 3-(1,1,2,2-Tetrafluoroethoxy)-1,1,2,2-tetra-fluoropropane
  • Fluorinated solvents