Nonflammable single-solvent electrolyte towards highly stable Li-rich Mn-based cathode materials

Abstract

Li-rich Mn-based cathode materials (LRMs) with high specific capacity are considered as one of the most promising cathodes in the next-generation lithium-ion batteries. However, the mismatch between high-voltage LRM cathodes and conventional carbonate electrolytes causes a series of problems such as cathode structure degradation, transition metal dissolution, and continuous electrolyte decomposition. Herein, we demonstrate a nonflammable and highly oxidation-resistant electrolyte by introducing methyl (2,2,2-trifluoroethyl) carbonate (FEMC) as the single solvent. Theoretical calculations show that the low binding energy between FEMC and Li⁺ promotes the desolvation process of Li⁺ while reducing the competition with anions, which is beneficial for more anions to enter the first solvation shell structure of Li⁺. This regulation in solvation structure optimizes the cathode–electrolyte interphase (CEI) components and forms a durable and inorganic-rich CEI layer on the cathode interface. Meanwhile, the low viscosity and flame-retardant properties of the FEMC molecule also improve the wettability and safety of the electrolyte. Therefore, the assembled Li‖LRM batteries with the single-solvent electrolyte exhibit a high capacity retention of 83.9% after 400 cycles in the voltage range of 2–4.8 V at 1C. Even at the high cut-off voltage of 5 V, a high capacity retention of 91.9% is achieved after 150 cycles. This work provides new insights into the development of high-safety and high-voltage electrolytes for high-energy-density lithium-ion batteries.

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Qingshui Xie

Qingshui Xie is currently a professor and doctoral supervisor at the College of Materials, Xiamen University. He got his B.S. and M.S. degrees from Lanzhou University in 2009 and 2012, respectively. After that, he moved to Xiamen University as a PhD candidate and received his PhD degree in Materials Physics and Chemistry in 2015. His research interests concentrate on advanced electrode materials for high-performance lithium-ion batteries.

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Introduction

In recent decades, lithium-ion batteries (LIBs) have developed rapidly as efficient and convenient energy storage devices in every scene of daily lives.¹ Continuously increasing the energy density of LIBs is an important prerequisite for meeting the social development. In this regard, it is particularly important to develop cathode materials with high specific capacity. Among numerous cathode materials, Li-rich Mn-based cathode materials (LRMs) exhibit ultra-high specific capacity (>300 mA h g⁻¹) due to the simultaneous participation of transition metal (TM) ions and oxygen anions in the redox reactions,^2,3 and are considered as a promising candidate for the next-generation cathode materials. However, LRMs require high voltage (>4.5 V) to activate the lattice oxygen to contribute to extra capacity, which poses a significant challenge to the high-voltage tolerance of conventional carbonate electrolytes.

Currently, the widely used electrolytes with low oxidation resistance mainly consist of cyclic ethylene carbonate (EC) and some linear carbonate solvents. Among them, EC has a high dielectric constant to dissolve lithium salts and can also effectively passivate the electrode interface to achieve a protective effect. However, the EC-derived passivation layer is predominantly composed of organic compounds, containing a large amount of C, H, O, etc., which is unstable and cannot support the rapid migration of Li⁺ at the interface. At the same time, the poor oxidation resistance makes it susceptible to produce many undesired by-products in high-voltage environments, and leads to an increase in impedance, especially in the presence of oxygen release.⁴ In addition, the high melting point and viscosity of EC are also detrimental to the low-temperature performance of LIBs.⁵ Therefore, the EC-free electrolyte has attracted tremendous attention. Currently, these studies typically employ some solvents such as propylene carbonate (PC) or fluoroethylene carbonate (FEC) as the substitutes for EC, or incorporate some organic or lithium salt additives into the EC-free electrolyte to maintain cycling stability, while most of them are used in low-voltage LIBs below 4.6 V.^5–9 This is because these EC-free electrolytes still contain conventional linear carbonate solvents, which have poor oxidation resistance and will also undergo severe oxidative decomposition at high voltage.^10–13 Then the undesired decomposition process results in the formation of an unstable cathode interface layer (CEI) with large amounts of organic species. This inferior CEI finds it difficult to suppress electron tunneling at the cathode interface and leads to continuous consumption of electrolyte and active lithium, ultimately causing the rapid voltage and capacity fading of the batteries.^14,15 In this regard, it is necessary to propose a new strategy for EC-free high-voltage electrolytes to enhance the long-term electrochemical performance of the LRM cathode.

In this work, methyl (2,2,2-trifluoroethyl) carbonate (FEMC) with a linear structure is selected as the single solvent to create a high-voltage electrolyte for LRM cathodes. Compared to the conventional carbonate electrolytes, the incorporation of a single FEMC solvent simplifies the electrolyte composition and alters the solvation environment of Li⁺, reducing the desolvation energy and increasing the coordination number of the anions. This modification decreases the proportion of organic compounds in the interfacial layer and promotes the formation of beneficial substances such as LiF, Li₂CO₃, and PO-containing species, which contributes to the high mechanical stability of CEI and reduces the overall impedance of Li⁺ migration. As a result, the LRM using the single-solvent electrolyte exhibits an enhanced cycling stability with a high specific capacity of 215 mA h g⁻¹ and high retention of 83.9% at 1C after 400 cycles between 2 and 4.8 V. When the upper cut-off voltage is raised to 5 V, the retention can still be up to 91.9% after 150 cycles at 1C, which is significantly better than that with conventional carbonate electrolytes. In addition, the nonflammable property of the electrolyte also enhances the safety of LIBs. This single-solvent electrolyte strategy provides a feasible method for developing advanced cathodes and electrolytes at high operating voltages.

Experimental section

Preparation of LRM cathodes and electrolytes

The LRM cathodes were prepared by a co-precipitation method. A chelating agent dissolved in deionized water was first added into a continuously stirred tank reactor (CSTR). Subsequently, a certain stoichiometric ratio of TM salt solution (Mn : Co : Ni = 4 : 1 : 1 mol%) and precipitant solution were dropped into the CSTR, where the co-precipitation reaction occurred under water bath conditions at 60 °C with N₂ protection. The collected reaction product was then washed, dried, and heated at 500 °C to yield the TM oxide. Finally, the TM oxide was uniformly mixed with LiOH·H₂O and calcined at 800 °C to obtain the active materials (LRMs).

The base electrolyte used in the experiments was 1 M LiPF₆ dissolved in a mixture of EC and EMC (3 : 7 by volume), denoted as EM. An appropriate amount of LiPF₆ was added into the methyl (2,2,2-trifluoroethyl) carbonate (FEMC) solvent to obtain a single-solvent electrolyte with a lithium salt concentration of 1 M, named FM. The EM electrolyte and LiPF₆ were purchased from Guangdong Canrd New Energy Technology Co., Ltd (China), while FEMC was obtained from Aladdin and dried with molecular sieves before use. All the procedures were performed in an argon-filled glove box with water and oxygen content controlled to be less than 1 ppm.

Characterization

The surface morphologies of the cycled electrodes were detected by scanning electron microscopy (SEM, Hitachi SU-70) and transmission electron microscopy (TEM, Talos F200). Structural characterization was carried out on an X-ray diffractometer with Cu-Kα radiation (XRD, Bruker Axs, λ = 1.5418 Å). The Raman spectrum (Xplora) was used to assess the structural change of cathodes. The proportion of TM dissolved from LRM cathodes and deposited onto the lithium anodes was confirmed by inductively coupled plasma mass spectrometry (ICP-MS, iCAP7400). The chemical compositions of the cathode surface were tested using X-ray photoelectron spectroscopy (XPS, PHI Quantum 2000) and corrected with the C 1s peak. Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was used to record the fragment information separated from the surface layer. The structural evolution of the LRM cathodes during the first two cycles was monitored by in situ XRD investigation, and the cell for the in situ XRD test was assembled in a customized mold and tested in the voltage range of 2–4.8 V at 0.33C. The CO₂ and O₂ generated during the initial cycle were recorded by online continuous flow differential electrochemical mass spectrometry (DEMS). Nuclear Magnetic Resonance spectroscopy (NMR, Avance II 400 M) was carried out to detect the ⁷Li chemical environment. The contact angles were measured using an automatic measuring instrument (OCA20, Dataphysics Instruments, Germany).

Electrochemical measurements

The cathode slurry consisted of 80 wt% of LRM, 10 wt% of polyvinylidene fluoride (PVDF), 10 wt% of acetylene black, and N-methyl-2-pyrrolidone (NMP) as the solvent. The above slurry was coated on aluminum foil and dried at 90 °C in vacuum overnight. Then the electrode was cut into round pieces of 1.2 cm in diameter. The coin cells (CR2025) were assembled in a glove box filled with argon. The Celgard 2500 microporous membrane and lithium metal were used as the separator and counter electrode, respectively. The Neware battery testing system (CT-4008T, Neware Electronics Co., Ltd, China) was applied to perform the electrochemical performance tests with a voltage range of 2–4.8 V. A higher voltage range of 2–5 V was also measured, and the first cycle was activated at 0.2C. A PARSTAT 3000A-DX electrochemical workstation (AMETEK Instrument Corp., USA) was used to record the electrochemical impedance spectroscopy (EIS) with the frequency of 100 000–0.001 Hz. Linear sweep voltammetry (LSV) was carried out on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Corp.) at a scan rate of 1 mV s⁻¹. A stainless steel sheet was employed as the working electrode, and lithium foil was utilized as the counter and reference electrodes. Self-discharge and floating charge tests (chronoamperometry) were conducted to evaluate the high-voltage stability of the electrolyte and CEI after 50 and 100 cycles. For the high and low temperature tests, the cells were first activated at normal temperature, and then cycled at −10 °C and 50 °C, respectively.

Theoretical calculations

Molecular dynamic (MD) simulations were performed using the GROMACS package with the General Amber Force Fields (GAFF). The relevant parameters were obtained using Autoff and Multiwfn 3.8 programs, and the initial periodic systems were constructed using the Packmol software.^16,17 During the equilibrium stage of the system, the conjugate gradient method was used to minimize the energy of the simulated system. An equilibrium simulation of 10 ns was conducted using the isothermal isobaric ensemble (NPT), followed by a continued simulation of 20 ns with the canonical ensemble (NVT). The last 10 ns of the trajectory was recorded for the subsequent data analysis.

Density functional theory (DFT) calculations were conducted with the Gaussian software package. All the molecules were optimized at the level of B3LYP/6-311G++ (d, p). The dispersion correction was also taken into account with GD3BJ. The universal solvation model of SMD was adopted for optimizing the solvent–solute interaction.¹⁸ The binding energy and desolvation energy were obtained from the difference between the reactant energy and complex energy.

Results and discussion

Solvation structure of the electrolytes

The solvents used in the EM electrolyte are conventional cyclic (EC) and linear (EMC) carbonates, in which the lithium salts can well dissolve to form a uniform and stable solution due to the high dielectric constant of EC. In contrast, the solvent in the FM electrolyte is only FEMC, which is obtained by the fluorination of EMC. The strong electron-withdrawing property of the F-group in FEMC reduces the negative charge density around O atoms, thereby affecting the ability of the solvent to dissolve lithium salts.¹⁹ According to the prepared FM electrolyte in Fig. S1,† lithium salts with a concentration of 1 M can be completely dissociated into the solvent. To gain a deep understanding of the solvation structures, molecular dynamics (MD) calculations are performed for the EM and FM electrolytes (Fig. 1a–d and S2†) firstly. Fig. 1c and d provide a more intuitive view of the microscopic differences caused by different solvents in the two electrolytes. It is evident that in the EM electrolyte, EC and EMC solvents dominate the structure of the first solvation shell of Li⁺. The corresponding coordination numbers of Li⁺–EC and Li⁺–EMC are 2.29 and 2.86, respectively. However, the coordination number between Li⁺ and anions is only 0.61, indicating that more anions are in the free states. In contrast, the solvation shell of Li⁺ contains only FEMC and anions in the FM electrolyte, which is also verified by the Raman spectra (Fig. 1e).^20,21 The coordination number between Li⁺ and anions is significantly increased to 2.07 due to the regulation of the single fluorinated solvent. As confirmed by the NMR tests in Fig. 1f, the upward shift of the ⁷Li peak indicates a stronger electronegative environment around Li⁺.²²

Fig. 1 (a and b) The radial distribution functions g(r) and coordination numbers n(r) in different electrolytes. (c and d) Corresponding configurations of the solvation structures for (c) EM and (d) FM electrolytes. (e) Raman spectra and (f) ⁷Li NMR spectra of the two electrolytes. (g) Binding energies between Li⁺ and different solvents.

In addition, it is found that the binding energies of Li⁺ with EC, EMC, and FEMC are −2.28, −2.21, and −1.74 eV, respectively (Fig. 1g). The lower binding energy of Li⁺–FEMC in the FM electrolyte promotes entry of anions into the solvation shell structure, which is consistent with the calculation results of MD.¹⁹ Based on the differences of solvent coordination numbers in the two electrolytes, the corresponding desolvation energies are also calculated, and the value between Li⁺ and FEMC is the lowest, which is beneficial for enhancing the reaction kinetics (Fig. S3†). Besides, the FEMC molecule has a lower HOMO energy level (−8.53 eV) compared to the solvents in the EM electrolyte, indicating stronger oxidation resistance at high voltage (Fig. 2a), which is also confirmed by the LSV tests in Fig. 2b. Moreover, further investigation of the physicochemical properties reveals that the FM electrolyte has better wetting ability with a contact angle of 20.9°, which is twice less than that of the EM electrolyte (Fig. 2c–f). Compared to the highly flammable EM electrolyte, the FM electrolyte with a single fluorinated solvent is nonflammable, ensuring the safety during cell operation (Fig. 2g and h).

Fig. 2 (a) HOMO energy level of different solvents. (b) Oxidation stability of electrolytes tested by LSV. (c and d) Optical images of the two electrolytes dropped onto the separators and (e and f) corresponding contact angles. (g and h) Flammability tests of the EM and FM electrolytes.

Electrochemical performance of Li‖LRM cells

The Li‖LRM cells were assembled to evaluate the stability of the single-solvent electrolyte at high voltage. Clearly, the cells using different electrolytes show different long-term cycling performances at 1C as presented in Fig. 3a. The discharge capacity of the cell with the EM electrolyte (EM cell) is about 199 mA h g⁻¹ after 100 cycles, and rapidly decreases to 181 mA h g⁻¹ after 150 cycles, equaling to a capacity retention of only 75.6%. In comparison, a high discharge capacity of 215 mA h g⁻¹ and 83.9% of capacity retention are obtained for the cell with the FM electrolyte (FM cell) even after 400 cycles. Moreover, the average coulombic efficiency (CE) of the FM cell remains over 99% during the 400 cycles and is higher than that of the EM cell in the 150 cycles. This indicates that using the single-solvent electrolyte can significantly enhance the long-term cycling stability of LRM cathodes at 2–4.8 V. As shown in Fig. 3b and c, the cells with both EM and FM electrolytes present the two typical voltage plateaus of LRM cathodes during the initial activation process.^23,24 Besides the severe capacity decay, the EM cell also exhibits larger voltage polarization (Fig. 3b–d). In particular, the polarizing voltage of the FM cell is only 0.47 V after 100 cycles, which is lower than that of the EM cell (0.62 V) as shown in Fig. 3d. These impressive results can be ascribed to the positive effects of the higher oxidation stability of the FEMC solvent and the stable CEI protective layer formed on the cathode surface.

Fig. 3 (a) Cycling performance of Li‖LRM cells with the voltage range of 2–4.8 V at 1C. (b and c) Charge and discharge curves of cells with (b) EM and (c) FM electrolytes. (d) Average charge and discharge voltage. (e and f) The floating charge test of cells at 4.8 V in different electrolytes after (e) 50 and (f) 100 cycles. (g) Self-discharge testing curves with EM and FM electrolytes at 4.8 V after 100 cycles.

To further verify the stability of the two electrolytes at high voltage, the self-discharge and leakage current of Li‖LRM cells after 50 and 100 cycles are tested as shown in Fig. 3e and f. The current value of the FM cell decreases rapidly in the initially standing stage, reflecting its lower polarization level, and the leakage current intensity also remains smaller subsequently. According to the self-discharge results after 50 cycles as shown in Fig. S4,† the voltage of the EM cell drops to nearly 4 V after 30 h and to 3.5 V after about 52 h. After 100 cycles, this downward trend becomes more apparent (Fig. 3g). In contrast, the self-discharge rate of the FM cell is largely suppressed. Upon the 100 h resting after 100 cycles, the open circuit voltage can still be maintained above 4.0 V. This significant difference is closely related to the formed CEI layers modulated by the two electrolytes. The inferior CEI derived from the EM electrolyte cannot hinder the sustained adverse reactions on the cathode interface. In contrast, the interfacial layer generated from the single-solvent electrolyte has sufficient stability and integrity to resist unfavorable side reactions at high voltage, thereby improving the electrochemical performance of the LRM cathode.

Fig. S5† illustrates the rate performance of the cells with different electrolytes. The FM cell exhibits higher discharge capacities of 285.8, 259.0, 240.9, 215.7, 194.8, and 161.3 mA h g⁻¹ at the current densities from 0.2C to 5C, which is correlated with its lower impedance. As shown in Fig. S6,† the charge transfer impedance (R_ct) of the FM cell is significantly smaller than that of the EM cell after 5 and 100 cycles. The slightly enhanced film impedance (R_f) of the FM cell is attributed to the component difference of the CEI derived from the FM electrolyte.^25,26

Cycling tests at high voltage are important to verify the oxidation resistance of the electrolytes as shown in Fig. 4a and S7.† A poor capacity retention of 74% is observed for the EM cell after 150 cycles when the upper cut-off voltage rises to 5.0 V. In contrast, the FM cell provides a high capacity retention of 91.9% and a low decrease rate of 0.054% per cycle under the same testing conditions. Besides, the average CE of the FM cell is 98.7%, while that the EM cell is only 97.8%. The specific energy retention with the FM electrolyte is also higher than that of the EM electrolyte (74.1% vs. 55.9%). The high and low temperature tests were also conducted to explore the application potential of the FM electrolyte. As displayed in Fig. 4b and S8,† the FM cell could achieve 99.8% retention of its initial capacity after 100 cycles at 1C and 50 °C, as well as 93.2% retention after 200 cycles, while that of the EM cell decreases to 86.5% and 65.3% after 100 and 200 cycles, respectively. Moreover, the EM cell presents obvious CE fluctuations after 80 cycles, which is attributed to the aggravated interfacial side reactions at high temperature. On one hand, the mechanical stability of the EM-derived CEI layer is inadequate and prone to breakage due to the larger changes of cathode structure at high temperature.^23,27 On the other hand, the oxidation stability of the EM electrolyte is also insufficient. These two reasons combine to exacerbate the occurrence of side reactions and lead to serious reduction in capacity and voltage. In addition, at the low temperature of −10 °C, the FM cell exerts a superior discharge capacity of 129.8 mA h g⁻¹ compared to the EM cell at 0.5C after 100 cycles as shown in Fig. 4c. After 200 cycles at 50 °C and 100 cycles at −10 °C, the specific energy retentions of FM cells are also 24.1% and 9.9% higher than those of EM cells, respectively. These results indicate that the single-solvent electrolyte is promising for improving the electrochemical performance of LRM cathodes under harsh conditions.

Fig. 4 Discharge specific capacity and discharge specific energy of Li‖LRM cells with EM and FM electrolytes under different test conditions: (a) at 1C and upper cut-off voltage of 5 V; (b) at 1C and 50 °C; (c) at 0.5C and −10 °C.

Morphological and structural changes of LRM cathodes during cycling

To investigate the influence of different electrolytes on the structural evolution of LRM cathodes, in situ XRD tests were conducted and are shown in Fig. 5a and b. The (003) diffraction peaks of the LRM cathodes with EM and FM electrolytes have similar changing trend in the first two cycles. During the initial charging process, the extraction of Li⁺ leads to the increased electrostatic repulsion between the oxygen layers, thus resulting in the increase of lithium layer spacings and the shift of the (003) peak towards a lower angle. When charging to near 4.5 V, the (003) peak moves back to the higher angle due to the increased valences of the oxygen anions during the activation process of the Li₂MnO₃ phase. In the subsequent discharge process, the (003) peak first shifts to a lower angle and then to a higher angle, which is associated with Li⁺ insertion into the TM and Li layers. Compared to the initial position of the (003) peak, the average offset angle of the LRM cathode with the FM electrolyte is 0.134°, which is smaller than that with the EM electrolyte (0.155°) as displayed in Fig. 5c. Furthermore, ex situ XRD tests are also conducted on the LRM cathodes after 100 cycles as shown in Fig. S9.† The smaller angle shift of the (003) peak is still obtained for the LRM cathode with the FM electrolyte. In situ and ex situ XRD testing results indicate that using the FM electrolyte can effectively alleviate the structural changes of LRM cathodes, which is beneficial to improve the electrochemical performance. In the Raman spectra (Fig. 5d), the E_g peak near 480 cm⁻¹ and the A_1g peak near 590 cm⁻¹ belong to the bending vibration of the O–TM–O bond and stretching vibration of the TM–O bond in the layered structure, respectively.²⁸ After 100 cycles, the A_1g peaks of the LRM cathodes with both electrolytes display a blue shift, which is caused by the transition from the layered to the spinel structure.²⁹ However, the larger blue shift of the cycled cathode with the EM electrolyte suggests that it has suffered more severe structural degradation, which is related to the inferior CEI layer formed on the cathode surface (Fig. S10†). It is difficult for the uneven or ruptured CEI layer to effectively isolate the electrolyte from the cathode. Then the high-valent TM ions lower their own valences by oxidizing the electrolyte, which further accelerates the structural transition of the LRM towards the spinel phase.^30,31 Besides, the side reactions that occur upon cycling also produce some harmful substances, which can corrode the LRM surface and aggravate the TM dissolution. The dissolved TM ions migrate to the anode under the influence of the electric field and get deposited on the surface of lithium metal. In this regard, inductively coupled plasma mass spectrometry (ICP-MS) is carried out to confirm the ratio of TM dissolved from the LRM cathode. As shown in Fig. 5e, less TM is detected for the FM cell, further proving the important role of the FM electrolyte in stabilizing the LRM cathode. The CEI formed from the EM electrolyte has poor uniformity and integrity and cannot effectively avoid the direct contact between the cathode and electrolyte. In contrast, the high oxidation resistance of the FEMC solvent reduces substantial decomposition of the FM electrolyte. And the superior CEI derived from the FM electrolyte can also isolate the electrolyte and further suppress interfacial side reactions, thereby decreasing TM dissolution and stabilizing the cathode structure.

Fig. 5 (a and b) In situ evolution of the (003) peak and charge–discharge curves of LRM cathodes during the first two cycles with (a) EM and (b) FM electrolytes. (c) Corresponding variation of (003) peak position from in situ XRD tests. (d) Raman spectra of LRM cathodes before and after 100 cycles. (e) ICP-MS tests of dissolved TM from LRM cathodes after 100 cycles with different electrolytes. (f and g) DEMS tests for O₂ and CO₂ release during the initial charging process with (f) EM and (g) FM electrolytes.

The oxygen release of LRM at high voltage will accelerate the electrolyte decomposition, in which the release of O₂ and CO₂ is recorded using online continuous flow differential electrochemical mass spectrometry (DEMS) as shown in Fig. 5f and g. Obviously, the gas release of O₂ and CO₂ of the FM cell is monitored to be 8.22 and 3.16 μmol g⁻¹, which is 24.5% and 45.8% lower than that of the EM cell, respectively. This notable discrepancy is attributed to the role of the FM electrolyte in stabilizing the cathode interface and mitigating interfacial side reactions.³²

The interfacial side reactions during cycling have direct impact on the surface morphology of LRM particles, which are analyzed by scanning electron microscopy (SEM). As displayed in Fig. S11,† the cycled cathode with the EM electrolyte exhibits a rougher surface with locally uneven coverage of by-products compared to the relatively smooth surface of the cathode with the FM electrolyte. This is due to the fact that the poor uniformity and continuity of the EM-derived CEI cannot provide sufficient protection for the cathode as discussed above (Fig. S10†). Meanwhile, X-ray photoelectron spectroscopy (XPS) was used to analyze the CEI composition formed on the cathode surface as presented in Fig. 6. In O 1s XPS spectra, the peaks located at 530, 532, and 533.4 eV belong to TM–O, C=O, and C–O, respectively. The presence of more C=O species on the EM-derived CEI reflects the occurrence of more interfacial side reactions. In the F 1s XPS spectra, the higher LiF content with the FM electrolyte implies that the FEMC solvent participates in the interfacial reaction. Because of its large bandgap, LiF has low electronic conductivity, which can prevent electron tunneling and thus reduce undesirable side reactions under high voltage.^33–35 The XPS spectra of P 2p presents two peaks, corresponding to Li_xPO_yF_z and PO₄³⁻ species, respectively, wherein Li_xPO_yF_z is derived from the decomposition of lithium salts, while PO₄³⁻ is generated by further reactions between organic solvents and the decomposition products of lithium salts.³⁶ In addition, it can be seen that the CEI with the FM electrode has more PO₄³⁻ and less Li_xPO_yF_z species compared to that with the EM electrode, which may be related to the different reaction processes of various species at the cathode interface. Li_xPO_yF_z plays an important role in promoting Li⁺ transport, and the strong interaction between PO₄³⁻ polyanion and TM ions is also beneficial for protecting the LRM cathode interface and inhibiting TM dissolution.^37,38 In addition, more LiF and Li₂CO₃ species are observed in the Li 1s peaks with the FM electrolyte. Li₂CO₃ can promote fast charge transfer but has poor electronic insulation, while this disadvantage can be mitigated by LiF.^26,39Fig. 6i shows that the CEI derived from the FM electrolyte contains less C and O species but more products containing F, P, and Li, where C and O are the primary components of organic compounds. This property enhances the durability of the CEI and helps to inhibit the electrolyte oxidation and TM dissolution, thereby benefitting the stable cathode structure during cycling. In addition, the fragment information of the cathode interface after cycling was probed by the time-of-flight secondary-ion mass spectrometry (TOF-SIMS), revealing that the CEI derived from the FM electrolyte has larger amounts of PO⁻ and LiF₂⁻ species (Fig. 7a–d and S12†). These results further suggest that the FM electrolyte is beneficial for the formation of P and F-containing species on the cathode surface, which is consistent with the XPS results.

Fig. 6 XPS spectra of the LRM cathode after 100 cycles (a–c and g) in the EM electrolyte and (d–f and h) in the FM electrolyte. (i) Contents of various elements in the CEI layer on the LRM surface with different electrolytes.

Fig. 7 (a and d) The 2D and 3D plots of PO⁻ and LiF₂⁻ fragments for LRM cathodes after 100 cycles in (a and b) EM and (c and d) FM electrolytes by TOF-SIMS. (e and f) Schematic illustrations of the influence mechanism of the two electrolytes on the performance of the LRM cathode.

According to the above theoretical calculations and a series of experimental results, the improvement mechanism of the electrochemical performance of the LRM cathode using the single-solvent electrolyte is summarized in Fig. 7e and f. Firstly, the weak binding energy between FEMC solvent and Li⁺ in the FM electrolyte increases the coordination number of anions in the Li⁺ solvation structure, which is beneficial for optimizing the components of CEI and then improving its durability. Secondly, the presence of the F-group in the molecular structure of FEMC decreases its HOMO energy and thus enhances the intrinsic oxidation resistance of the FM electrolyte. The optimized CEI effectively prevents direct contact between the electrolyte and LRM cathode, mitigating the dissolution of TM on the cathode surface, and the highly oxidation-resistant FEMC enhances the high-voltage stability of the electrolyte, further reducing the harmful interfacial side reactions. Moreover, the low desolvation energy between FEMC and Li⁺ is conducive to elevating the kinetic performance. Therefore, the comprehensive electrochemical performance of the LRM cathode with the FM electrolyte has been significantly enhanced. Fig. S13† compares the capacity retentions of LRM cathodes with different electrolytes in some reported literature studies, further highlighting the significant advantage of the single-solvent electrolyte in improving the cycling performance. In addition, the FM electrolyte also has better safety due to the nonflammability of the FEMC solvent.

Conclusions

In summary, a single-solvent electrolyte is constructed for a high-voltage LRM cathode by using a nonflammable fluorinated solvent. The introduction of the single linear fluorinated solvent enhances the intrinsic oxidation resistance of the electrolyte, while the weak interaction between FEMC and Li⁺ can introduce more anions into the first solvation shell of Li⁺. Under the dual effects, the CEI derived from the FM electrolyte contains more inorganic components, such as LiF, Li₂CO₃, and PO⁻ species, which not only improve the mechanical stability and durability of CEI, but also lower the overall impedance and alleviate electrolyte decomposition during long-term cycling. Therefore, the assembled FM battery can achieve a high specific capacity of 215 mA h g⁻¹ and excellent capacity retention up to 83.9% after 400 cycles at 1C. Even at a higher cut-off voltage of 5.0 V, the FM cell also delivers a superior capacity retention of 91.9% after 150 cycles, which is much superior to the EM cell. This flame-retardant and single-solvent electrolyte provides a feasible method for improving the cycling stability of LRM cathodes, and offers valuable insights for the research of high-voltage electrolytes.

Data availability

The data supporting this article can be found in the ESI.†

Author contributions

Dongwei Zhou: conceptualization, methodology, formal analysis, investigation, data curation, writing – original draft. Shihao Wang, Jie Mei, Guiyang Gao, Saichao Li, Guoying Wei: investigation, formal analysis, validation, writing – review & editing. Jiansen Wen, Baisheng Sa: resources, software, visualization, writing – review & editing. Jie Lin, Laisen Wang, Dong-Liang Peng, Qingshui Xie: funding acquisition, project administration, resources, supervision, writing – review & editing.

Conflicts of interest

The authors have no conflicts to declare.

Acknowledgements

The work received financial support from the National Natural Science Foundation of China (Grant No. 52272240, U22A20118, 52431009 and 52101273), Science and Technology Planning Projects of Fujian Province of China (Grant No. 2023H0003), the Natural Science Foundation of Fujian Province of China (No. 2022J01042), the Fundamental Research Funds for the Central Universities of China (Xiamen University: No. 20720220074 and 20720240053), and the “Double-First Class” Foundation of Materials Intelligent Manufacturing Discipline of Xiamen University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02568k

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