High-concentrated Electrolyte Design Enables Lithium-ion Pouch Batteries to Stably Operate at Extremely High Temperatures

Abstract

Due to the poor thermal stability of the lithium hexafluorophosphate (LiPF₆) electrolyte system, commercial lithium-ion batteries (LIBs) are difficult for normal operation at high temperatures above 55 °C. The limitation of the LiPF₆ electrolyte severely limits the practical application of LIBs under extremely high temperatures conditions. Here, a high-concentration electrolyte based on lithium bis(fluorosufonyl)imide (LiFSI) as electrolyte salt and ethyl methyl carbonate (EMC) as solvent is proposed, which possesses superior electrochemical stability and thermal stability. The LiCoO₂/graphite (Gr) pouch battery with the LiFSI high-concentration electrolyte (5.0 mol L⁻¹ (M)) has been shown excellent cycling performance even at 100 °C, an impressive capacity retention of 87.7 % can be still maintained after 100 cycles at 1.0 C-rate. The superior high temperatures performance is mainly attributed to the unique solvated structure, along with the robust solid electrolyte interphase (SEI) rich in anions. This work presents an effective strategy for promoting the development of high-temperature lithium-ion batteries.

1. Introduction

Since invented in 1986, lithium ion batteries (LIBs) have rapidly taken over the power supply market.^1–4 As the most widely applied battery technology, LIBs are highly praised for high energy density, high output voltage, long cycle life, and low self-discharge, which always maintain strong competitiveness.⁵ Although the production technology of LIBs has been developed and the capacity density has been continuously improved, they still cannot meet the application of some high temperatures scenarios, such as solar energy storage and oil exploration.⁶ The inherent drawbacks of the current state-of-the-art lithium-ion electrolytes used commercially impose limitations on the widespread application of LIBs at high temperatures.⁷ Electrolytes of LIBs typically consist of 1.0 M LiPF₆ salts dissolved in a solvent mixture of ethylene carbonate (EC) and straight-chain carbonates. These organic carbonate solvents exhibit high volatility and flammability, and LiPF₆ is thermally unstable when exposed to temperatures exceeding 55 °C.^8,9 Another essential factor is SEI. Conventional SEI also shows thermal instability, decomposing at 80 °C and initiating successive reductive decomposition of the electrolyte, generating heat and gaseous byproducts.^10,11 Furthermore, a cascade of physical and chemical chain reactions occurs within the battery, including increased pressure inside the battery, mechanical deformation of the battery configuration, and accelerated heat propagation toward thermal runaway.¹² Developing new electrolytes to replace conventional ones for LIBs is imperative to realize the normal operation of LIBs at high temperatures.

The research and development of high temperatures-resistant battery systems mainly focus on optimizing and improving the electrolyte from the perspective of lithium salt, solvent, and additives. Zheng et al.¹³ developed an amide-based deep eutectic electrolyte (AEEs-5) consisting of N-methyltrifluoroacetamide (NMTFA) and lithium difluoro-oxalate borate (LiDFOB), which realizes stable cycling of lithium-metal batteries in the temperature range of 25 °C–100 °C with excellent performance of 20 C-rate fast charging. Xia’s group,¹⁴ employed a hybrid lithium salt system of LiTFSI and LiDFOB in the mixed solvent of adiponitrile and EC as electrolytes. The electrolyte was applied to Li/Li₄Ti₅O₁₂ (LTO) batteries, in which the LTO electrodes could cycle for 1000 cycles at 100 °C, and capacity was almost non-degradable at 5.0 C-rate. Qian,¹⁵ reported that tripropargyl phosphate (TPP) could be an affected additive for high temperatures electrolytes. Under high voltage conditions, TPP could induce the formation of a denser CEI film on the surface of a Ni-rich (NMC₅₃₂) cathode and a SEI on lithium metal and artificial graphite (AG) anode. Adding 1 wt% TPP can significantly improve the high temperatures (45 °C) cycling performance of the NMC₅₃₂/AG battery. However, the operating temperature of LIBs improved by adding additives is limited, and there are fewer related studies.

In recent years, it demonstrated that high-concentrated electrolytes (usually >3.0 M),¹⁶ are significantly different from conventional dilute electrolytes in physicochemical and electrochemical properties, such as low flammability and volatility, fast charge/discharge rates, robust interfacial compatibility, and excellent lithium dendrite inhibition.^17–19 These outstanding properties of high-concentrated electrolytes can support application in higher operating temperatures, and there is reported that 2.0 M LiDFOB/EC/DMC concentrated electrolytes can enable LiCoO₂/Gr pouch batteries to stably cycle at 90 °C for 160 cycles.²⁰ However, most current reports are based on batteries with lithium metal as an anode, while operating temperature rarely exceed 100 °C with few studies on graphite as anode in LIBs. To further extent the application range of high-temperature LIBs to graphite-based anode, and further increase the operating temperature to 100 °C, it is demonstrated the LiFSI/EMC concentrated electrolyte strategy could improve the cycling stability of LiCoO₂/Gr pouch batteries at high-temperature. Compared to other common solvents, EMC has a better overall physical properties with a lower vapor pressure and a higer boiling point that meets requirement of 100 °C. At the same time, lithium salts is more soluable in EMC, and the price of EMC is more reasonable (Table S1).²¹ On the other hand, the thermal decomposition temperature of LiFSI can reach 200 °C, which ensures the normal operation of LIBs at high temperatures. As everyone knows, LiFSI will cause corrosion to the aluminum (Al),²² and the high-concentration electrolyte can significantly moderate this phenomenon.²³ In the design of the electrolyte, we want LiFSI to participate in the formation of inorganic compound-rich SEI more easily than solvents, which can make the SEI more stable. Results have shown that the high-concentrated electrolyte can enalble LiCoO₂/Gr pouch batteries stably cycling 100 cycles at 100 °C without obvious capacity deterioration.

2. Experimental

2.1 Preparation of electrolytes and electrodes

EMC and the conventional electrolyte (1.0 M LiPF₆ in EC/DEC (1/1, by vol.) were obtained from Shanghai Songjing New Energy Technology Co., Ltd., (Shanghai, China). LiFSI was purchased from Shanghai Dermo Pharmaceutical Technology Co., Ltd., (Shanghai, China). The EMC and LiFSI were used to prepare electrolyte by adding a certain amount of LiFSI in an Ar-filled glove box with oxygen and water below 0.1 ppm. The as-obtained electrolytes were 2.0, 3.0, 4.0 and 5.0 M LiFSI in EMC with different proportion. For conveniently describing, the commercial electrolyte (1.0 M LiPF₆ in EC/DEC, 1/1 by vol.) was labeled as LiPF₆ electrolyte, the 2.0 M LiFSI in EMC was abbreviated as 2.0 M, and 3.0 M LiFSI in EMC was shown as 3.0 M, 4.0 M LiFSI in EMC was shown as 4.0 M, and 5.0 M LiFSI in EMC was shown as 5.0 M, respectively.

The CR2032 coin cells were assembled in a glove box in a high-purity argon atmosphere with water oxygen content all less than 0.1 ppm. The coin cells use stainless steel tabs as both the anode and cathode, and the separator is Polyimide (PI) separator. LiPF₆ electrolyte and LiFSI electrolyte with different concentrations were used as electrolytes for the coin cells. The above coin cells are used for Linear Sweep Voltammetry (LSV) and ionic conductivity testing.

LiCoO₂/Gr jelly-rolls (650 mAh) were obtained from Dongguan HaiLiang Energy Technology Co. (Guangdong, China). Before electrolyte injection, the jelly-rolls were dried at 80 °C under vacuum for 12 h to remove any residual water. Then the jelly-rolls were transferred immediately to an argon-filled glove box for electrolyte injection (2.5 g per pouch cells) and vacuum sealing (with a compact vacuum sealer (JYHC). The formation process is described as follows. First, pouch cells were placed in an incubator at 40 °C and held for 24 h, to allow for the complete wetting. Then, pouch cells were charged at C/10-rate (65 mA) to 3.8 V. After the first charging, cells were transferred and moved into the glove box, cut open to release gas and then vacuum sealed again. This process is known as degassing. After the degassing step, the formation process was completed.

2.2 Electrochemical measurements

The capacity and cycling performance were carried out using a Neware (Shenzhen, China) charger system between 2.7 V–4.2 V at test ambient temperatures of 80 °C, 90 °C, 100 °C, respectively. Electrochemical impedance spectra (EIS) were measured at an electrochemical workstation (DH7000 Ametek, America) in the frequency range of 10000 Hz to 0.01 Hz and with an amplitude of 10 mV, the voltage of the battery tested was 3.0 V. LSV testing of electrolytes using CR2032 coin cell batteries, the scan rate of 1 mV s⁻¹ in the voltage range of 1.0 V–5.0 V at 100 °C. The Tafel polarization test was performed at a CR2032 coin cell with Li/PI/Al, and the voltage range of the test was 2.5 V–5.0 V, with a scan rate of 0.5 mV s⁻¹.

2.3 Material characterization

The ionic conductivity is calculated using the following equation.

  

\begin{equation} \sigma = \frac{L}{R \cdot S} \end{equation} (1)

Where σ is the ionic conductivity, L is the thickness of the seperator, R is the resistance and S is the area of the electrode. R was obtained by EIS test using a CR2032 coin cells with stainless steel/PI/stainless steel at testing temperatures of 25 °C, 60 °C and 100 °C, respectively.

Thermogravimetric analysis tests (TGA, Rigaku, Japan) and Differential Scanning Calorimetry (DSC, Netzsch, Germany) tests were carried out on different electrolytes using a thermogravimetric differential thermal analyzer (Nitrogen filled, test range from room temperature to 350 °C, scan rate at 10 °C min⁻¹). Raman spectroscopy (Horiba LabRAM HR Evolution, Japan) was used to analyze the coordination structures of various electrolytes (a solid-state laser with a wavelength of 532 nm was adopted). The morphology of electrodes was obtained by Hitachi apparatus Scanning Electron Microscope (SEM, S-4800) at 8 kV and 7 mA. The morphology of SEI was characterized using Transmission Electron Microscope (TEM, Tecnai F20, FEI, America). The SEI of Gr was investigated by X-Ray Photoelectron Spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi, America) with monochromatized 72 W Al Ka radiation. All the XPS data were calibrated by C standard peak at 284.6 eV. All the electrodes used for XPS characterization were cycled for 30 cycles.

2.4 Computational details

Density Functional Theory (DFT) calculation was conducted via Gaussian 16,²⁴ sofware package, the structure was optimized with a basis set of 6-311+G (d) and confirmed as true local minimum by vibrational frequency analyzes.²⁵

3. Results and Discussion

In order to investigate the feasibility of LiFSI/EMC electrolyte systems application under high-temperature conditions, the theoretical analysis is carried out through DFT calculations firstly. The results are shown in Fig. 1a, LiFSI possesses a significantly lower lowest unoccupied molecular orbital (LUMO) energy level of −2.46 eV compared to EMC of 0.07 eV, which endows the LiFSI prefer to be reduced to participate in the formation of the SEI according to frontier molecular orbital theory.²⁶ Notably, SEI enriched with fluorine (F) ions, formed through the preferential decomposition of LiFSI lithium salts, are projected to considerably enhance the electrochemical and thermal stability of batteries.²⁷ Based on the comprehensive information provided, we put forward a hypothesis proposing that electrolytes composed of LiFSI and EMC may serve as a promising option for the practical implementation at high temperatures.

Figure 1.

(a) The molecular orbital, HOMO and LUMO of EMC and LiFSI. (b) Ionic conductivities of different electrolytes at different temperatures. (c) DSC curves and (d) TGA curves of different electrolytes.

Firstly, the various concentrations of the LiFSI electrolytes were prepared to evaluate their suitability. The LiFSI was dissolved in the EMC solvent with various concentrations, and the solubility of each electrolyte are illustrated in Fig. S1. Notably, the 6.0 M LiFSI/EMC electrolyte exhibited turbidity and opacity, with incomplete dissolution of LiFSI. Consequently, we narrowed down the concentrations of the LiFSI electrolyte to range from 2.0 M to 5.0 M. The 1.0 M LiPF₆/EC/DEC (v : v = 1 : 1) electrolyte was selected as comparison sample.

Ionic conductivity is pivotal in determining batteries’ internal resistance and high-rate performance. The ionic conductivity of electrolyte with various concentrations at different temperatures are depicted in Fig. 1b. At room temperature, the ionic conductivity of the LiPF₆ electrolyte is 10.2 mS cm⁻¹, surpassing that of the LiFSI electrolyte due to the higher viscosity of the LiFSI electrolyte at room temperature. The ionic conductivity of all electrolytes exhibits an upward trend with rising temperatures. Beyond 60 °C, measuring the ionic conductivity of LiPF₆ electrolyte becomes impractical due to their heightened volatility. On the other hand, the ionic conductivity of all LiFSI-based electrolytes rises continuously with increasing temperature. Moreover, the thermal stability of all the electrolytes was evaluated by DSC. The DSC curves in Fig. 1c reveal that the exothermic peak of the LiPF₆ electrolyte occurs around 76 °C, which according to the decomposition of LiPF₆. In contrast, the exothermic peak of LiFSI electrolytes is approximately at 120 °C, highlighting the LiFSI has superior thermal stability than LiPF₆ in solvent. As illustrated in Fig. 1d, the TGA curves of all electrolytes exhibit notable weight loss as temperature increases. The weight loss ratio of LiFSI electrolyte varies with concentration, the 5.0 M LiFSI electrolyte displaying a roughly 10 % weight loss when heated to 100 °C—considerably lower than the 23 % weight loss observed for LiPF₆ electrolyte. DSC and TGA results suggest that high-concentration LiFSI electrolytes is expected to be applied to high temperatures LIBs.

There are several explanations for the instability of LIBs at high temperatures. The most crucial factor is the solvation structure of the electrolyte, which, in addition to the electrochemical stability of the cathode and anode materials.^28,29 It is critical to control the solvation structure of the electrolyte to enhance the high temperatures cycling stability of LIBs, as it significantly impacts the SEI/CEI characteristics, charge transfer behaviors, and desolation energy barriers. The Raman spectra of the LiPF₆ electrolyte present in Fig. 2a, where the characteristic peaks at 715 cm⁻¹ and 730 cm⁻¹ correspond to EC and free PF₆⁻, respectively. In the 2.0 M LiFSI electrolyte, the peak present at 725 cm⁻¹ attributed to free FSI⁻. With an increase in electrolyte concentration, the 730 cm⁻¹ peak representing contact ion pairs (CIP) formed by FSI⁻ and Li⁺ appears in the 3.0 M electrolyte, and the aggregate (AGG) at 750 cm⁻¹ emerges in the 4.0 M electrolyte.³⁰ In 5.0 M electrolyte, the solvation structure mainly exists in the form of AGG, characterized by strong Coulomb interactions with multiple Li⁺. The solvation structures of different electrolytes can be summarized as shown in Fig. 2b. In the LiPF₆ electrolyte, the predominant species are free solvent molecules EC and solvent-separated ion pairs (SSIP), where SSIP is formed by one Li⁺ from the electrolyte and four solvent molecules dissolved around it.³¹ In LiFSI high-concentration electrolytes, solvated structures mainly exists in the form of CIPs and AGGs. CIP comprises one Li⁺, one FSI⁻, and EMC solvent molecules, while AGG comprises two Li⁺, one FSI⁻, and EMC solvent molecules.

Figure 2.

(a) Raman spectra of different electrolytes. (b) Schematic diagrams of solvation structures of LiPF₆ electrolytes and high-concentration electrolytes. (c) LSV curves of different electrolytes at 100 °C. (d) Difference between SEI formed by LiPF₆ electrolyte and LiFSI high-concentration electrolyte.

Based on the above data, we can understand that the SEI in LiPF₆ electrolyte and 2.0 M LiFSI electrolyte mainly originates from solvent decomposition, which leads to poor stability. However, the SEI formed in high concentration LiFSI electrolytes are mainly from the decomposition of FSI⁻ and the SEI are mainly composed of inorganic compounds rich in fluorine (Fig. 2d).^32–34

The ideal high temperatures electrolyte should have a wide electrochemical operate window. Figure 2c shows the linear scanning voltammetry (LSV) curves of different electrolyte with potential window range of 1.0 V–5.0 V at a scan rate of 0.1 mV s⁻¹. It can be seen that the LiPF₆ electrolyte would decompose at 3.6 V, the lower decomposition voltage of LiPF₆ electrolyte will deteriorate the electrochemical performance of LIBs. In LFSI electrolyte system, the decomposition voltage of the electrolyte tends to grow as the concentration continues to increase. At low concentrations of 2.0 M and 3.0 M LFSI electrolytes, the decomposition voltage of the electrolyte is also as low as 3.72 V and 3.82 V, respectively. The initial decomposition voltage of the 4.0 M electrolyte was around 4.3 V, while the 5.0 M LiFSI electrolyte did not show a obvious decomposition phenomenon until to 5.0 V. According to the above data, the 4.0 M and 5.0 M LiFSI electrolytes show a wider electrochemical operation window, which can meet the normal charging/discharging operation of battery in the voltage range of 2.7 V–4.2 V. The improvement of electrochemical window stability in high-concentration electrolytes can be attributed to the substantial formation of CIPs and AGGs. The increase in the number of CIPs and AGGs in the electrolyte reinforcing the passivation of the electrolyte at the electrodes. Consequently, the initial decomposition potential of the electrolyte be elevated significantly.³⁵

The stable existence of electrolytes in chemical systems is an essential prerequisite to ensure the normal operation of LIBs at high temperatures. For this reason, the stability of the electrolyte itself (Fig. S2), the stability of the electrolyte in the battery with a discharging state (Fig. S3), and the stability of the electrolyte in the battery with a high charging state (Fig. S4) were further evaluated at 100 °C. The photos show that the LiPF₆ electrolyte causes the battery to bulge in all three cases, indicating that the LiPF₆ electrolyte is volatile at high temperatures, the reason is mainly attributed to the occurrence of decomposition reactions leading to the emission of gases such as HF. When 5.0 M electrolyte was used, no bulging was observed in all the batteries, indicating that 5.0 M electrolyte has good thermal stability and chemical stability at high operation temperatures. The results of electrolyte stability show that the high-concentration electrolyte (5.0 M) can obviously inhibit the occurrence of decomposition reactions and reduce the gas production, thus ensuring the normal operation of the pouch batteries at 100 °C.

The different electrolytes were used to assemble LiCoO₂/Gr batteries with a rated capacity of 650 mAh and electrochemical performance tests also were performed. As shown in Fig. 3a, the first discharge capacity of the battery with 5.0 M electrolyte can exceed 620 mAh at a rate of 1.0 C-rate. It can also be seen from Figs. S5–S8 that the polarization change of the battery with the 5.0 M electrolyte is smaller, and the cycling stability of the battery was improved with the increase of electrolyte concentration. This is due to the fact that the higher concentration electrolyte has a more stable solvation structure, which ensuring the stability of electrolyte at high operation temperatures.³⁶

Figure 3.

(a) First cycle charge/discharge curves for different electrolytes (1.0 C-rate). (b) Cycle performance of LiCoO₂/Gr pouch batteries with different electrolytes at 100 °C (1.0 C-rate, 2.7 V–4.2 V). (c) Rate capability of LiCoO₂/Gr batteries at 100 °C. (d) EIS of LiCoO₂/Gr pouch batteries with 1.0 M LiPF₆. (e) EIS of LiCoO₂/Gr pouch batteries with 5.0 M LiFSI. (f) EIS before and after cycling with different electrolytes.

Figure 3b shows the cycling performance of LIBs with different concentrations over the potetial windor of 2.7 V–4.2 V at a rate of 1.0 C-rate at 100 °C. When the battery after 100 charging-discharging cycles, the 2.0 M and 3.0 M electrolytes have capacity retention of 57.6 % and 63.4 %, respectively. As mentioned, in the 2.0 M electrolyte, SEI is formed mainly by solvent preferentially decomposition. The SEI formation by solvent decomposition is unstable, which continue deteriorate the cycling stability of the battery, particularly under high temperatures conditions. The cycling stability of the battery was obviously improved with the increase of electrolyte concentration. For example, the capacity retention of the battery in 4.0 M electrolyte is 82.3 %, while the 5.0 M electrolyte can increase the capacity retention of the battery to 87.7 % after 100 cycles. The free solvent molecules were significantly reduced in the high concentration electrolyte, the SEI composition predominantly derives from decomposition of FSI⁻, the F-rich SEI contributing to enhanced the cycling stability of battery. Additionally, at elevated temperatures of 80 °C and 90 °C, the high-concentration LiFSI electrolyte achieves remarkable capacity retentions of 84.9 % and 87.6 % after 200 cycles (refer to Figs. S9 and S10).

Figure 3c illustrates the cycle curves of batteries at different current densities ranging from 0.2 C-rate to 5.0 C-rate. Notably, the discharge capacity of the high concentration electrolyte is better than that of the low concentration electrolyte at each current density. At a rate of 5.0 C-rate, the discharge capacities of 2.0 M, 3.0 M and 4.0 M electrolytes are 360 mAh, 378 mAh and 407 mAh, respectively. Conversely, the discharge capacity of the 5.0 M electrolyte can reaches to 428 mAh at 5.0 C-rate. The high-concentration electrolyte exhibits robust thermal and electrochemical stability under high temperatures conditions, which ensuring the batteries can maintain a high discharge capacity after continuous charging-discharging.

The charge transfer performance of batteries with different electrolytes before and after cycling were evaluated by electrochemical impedance spectroscopy (EIS) technique with equivalent circuit fitting and the results are shown in Figs. 3d–3f. Before cycling, the charge transfer resistance (R_ct) of the battery with LiPF₆ electrolyte is lower than others, and the R_ct of the LiFSI electrolyte increases with increasing concentration (Figs. S11–S13). This is due to the charge transfer resistance mainly originates from the desolvation process of Li⁺ at the electrode/electrolyte interface for conventional graphite-based LIBs, and the low-concentration electrolyte is more easily desolvated than the high-concentration electrolyte.³⁷ After cycling at 100 °C, the R_ct of the battery in 1.0 M LiPF₆ electrolyte increased from 0.064 Ω to 0.340 Ω, At the same time, the R_SEI increases which is due to the poor thermal stability of both the LiPF₆ electrolyte and the SEI. In addition, many decomposition products were deposited on the electrode surface, increasing the electrode thickness and impedance.³⁸ On the other hand, Highly concentrated LiFSI electrolyte system showed a decreasing trend of R_ct and R_SEI after cycling compared with that fresh one, which was mainly related to the formation of more stable F-rich and denser SEI than in the LiPF₆ electrolyte (We will discuss it later). For example, the R_ct of the 2.0 M electrolyte decreased from 0.127 Ω to 0.112 Ω and the R_ct of 5.0 M LiFSI electrolyte decreased from 0.489 Ω to 0.218 Ω after 100 cycling, respectively.

In order to evaluate the changes of surface morphology on the electrode before and after cycling, SEM analysis was conducted on both fresh and cycled electrodes and the results presented in Fig. 4. As depicted in Fig. 4a, the fresh Gr electrode has a smooth surface without any particle accumulation. However, the surface of the electrode exhibits irregular and flaky exposed to the LiPF₆ electrolyte after cycling, as illustrated in Fig. 4b. The SEM image at high-magnification (insert) showed that the morphology of the graphite electrode surface become to agglomerated particles, which is mainly due to the attack of HF and deposition of other by-products produced by the decomposition of LiPF₆ at high temperatures. Concurrently, the SEI undergoes continuous reconstruction, leading to the exfoliation of the graphite material. These substances accumulate on the electrode surface, forming irregular particles.³⁹ In contrast, the surface of the Gr electrode in the LiFSI electrolyte exhibited a highly flat appearance. However, the high-magnification SEM images in the 2.0 M and 3.0 M electrolytes (insert in Figs. 4c and 4d) have some lumpy particles were observed on the surface of the graphite particles. On the other hand, the Gr particles appeared more uniform and intact in the 4.0 M and 5.0 M electrolytes (insert in Figs. 4e and 4f). These results indicate that high-concentration of LiFSI electrolyte has the ability to inhibit the side reaction between electrolyte and Gr electrode, thereby preserving the surface integrity of the Gr electrode.

Figure 4.

SEM of Gr electrodes after 100 °C cycling in different electrolytes: (a) fresh electrode; (b) 1.0 M LiPF₆; (c) 2.0 M LiFSI; (d) 3.0 M LiFSI; (e) 4.0 M LiFSI; (f) 5.0 M LiFSI. SEM of Al foil in different electrolytes: (g) Al foil in 2.0 M LiFSI electrolyte; (h) fresh Al; (i) Al foil in 5.0 M LiFSI electrolyte.

Some studies have shown that use LiFSI as electrolyte salt will cause corrosion of aluminum collector. Hereby, the corrosion degree of LiFSI on aluminum was determined by SEM technology take Al foil from cycling cathode (in both 2.0 M and 5.0 M electrolyte) and fresh cathode. Figure 4h shows that the surface of fresh Al foil present a very flat and smooth. However, some obvious pits appeared on the Al foil surface after cycling in the 2.0 M electrolyte, which indicating that the Al foil was corroded obviously (Fig. 4g). Through the Tafel curve, it can be found that the passivation potential of Al increases with the increase concentration of LiFSI electrolyte, and the corrosion current density decreases (Fig. S14). The above phenomenon indicates that the increase of electrolyte concentration can effectively inhibit Al corrosion. The Al³⁺ being solvated by free-EMC and free-FSI⁻ in the low-concentrations of LiFSI/EMC electrolytes, which facilitating their diffusion from the Al surface into the electrolyte and thus allowing Al corrosion to continues.⁴⁰ Once the electrolyte concentration was increased to 5.0 M, the corrosion degree of the Al foil was significantly alleviated (Fig. 4i). In highly concentrated electrolytes, the solvation of Al³⁺ is decreased due to the lack of free solvent molecules. At the same time, increasing the concentration of electrolyte can form a protective layer (such as LiF) and inhibit the diffusion and movement of Al-containing species, thus inhibiting the corrosion of Al foil.^41,42

The composition and morphology of SEI are crucial related to the performance of LIBs. On the other hand, the composition and concentration of the electrolyte determine the formation of SEI to a large extent. In order to determine the effect of electrolyte differences on the formation of SEI, the details of SEI were analyzed by TEM and XPS technology. The TEM results suggest that the SEI formed in LiPF₆ electrolytes was fuzzy and irregular, it attributed to the continuous generation and destruction of SEI at high temperatures due to the unstable SEI (shown in Fig. 5a). Interestingly, once the electrolyte salts changed from LiPF₆ to LiFSI, the SEI morphology changed significantly. For example, the SEI in 2.0 M and 3.0 M electrolytes was thick and inhomogeneous (Figs. 5b and 5c), while a uniform and continuous amorphous SEI were formed in 4.0 M and 5.0 M electrolytes (Figs. 5d and 5e). It indicates that the high-concentration LiFSI electrolyte facilitates the formation of uniform and stable SEI during high temperatures cycling and can improves the cycling performance of the battery. The XPS results are shown in Fig. 5f. The C-F bonds in the C 1s were mainly from the binder of PVDF, the C-C bond belonged to the conductive carbon black,^43,44 the C-O and the C=O was from the decomposition of the carbonate solvent.⁴⁵ The content of C=O bond in LiPF₆ electrolyte is significantly higher than that in LiFSI electrolyte. Additionally, the intensity of all characteristic peaks related to carbon decreases significantly as the concentration of LiFSI electrolyte increases. The XPS results of F 1s spectra show that the characteristic peak of F element in LiPF₆ electrolyte is mainly attributed to Li_xPO_yF_z and LiF, the Li_xPO_yF_z is a byproduct of the decomposition of LiPF₆ salts at high temperatures.⁴⁶ In contrast, the S-F peaks in addition to LiF peaks was observed in LiFSI electrolytes, and the intensity of S-F peaks increased with the increase of electrolyte concentration. Similarly, O 1s XPS spectra in the LiPF₆ electrolyte indicated that characteristics are mainly composed of carbonate compounds such as Li₂CO₃ and ROCO₂Li. The XPS spectra results show that the SEI formation in the LiPF₆ electrolyte mainly originate from the decomposition of LiPF₆ salts and carbonate solvent.⁴⁷ SEI based on carbonate compounds such as Li₂CO₃ and ROCO₂Li are extremely unstable at high temperatures, which can continuously consume electrolyte and eventually lead to rapid deterioration of battery performance. XPS spectral of F 1s, O 1s and Li 1s show SEI formed in LFSI electrolyte that the SEI formed in LiFSI electrolyte is mainly composed of inorganic substances such as SO₂F, LiF and Li₃N. Meanwhile, the peak intensity of these inorganic substances increased with the increase of electrolyte concentration. As shown in Fig. S15, the N 1s spectrum of LiFSI electrolyte is dominated by C-N and Li₃N, and the S 2p spectrum is dominated by SO₂F and other sulfides, and the peak intensities of Li₃N and SO₂F are greater in the highly concentrated electrolyte, suggests that more LiFSI is involved in the formation of the SEI. The SEI with more rich-F/S/N inorganic components can further improve the cycling stability of the LIBs. Moreover, Li₃N has a high ionic conductivity, which facilitate Li⁺ transport in the SEI bulk.⁴⁸ Above results show that SEI formed in high concentration electrolyte has high ionic conductivity and is more stable, all of this ensures that LIBs can operate stably at high temperatures and have high rate performance simultaneously.

Figure 5.

TEM of Gr electrodes after 100 cycling in different electrolytes: (a) 1.0 M LiPF₆; (b) 2.0 M LiFSI; (c) 3.0 M LiFSI; (d) 4.0 M LiFSI; (e) 5.0 M LiFSI. (f) XPS spectra of SEI after 30 cycles at 100 °C with different electrolytes.

4. Conclusions

In order to expand the application of LIBs at high temperatures, a high concentration electrolyte approach is proposed in this work. Compared to conventional LiPF₆ electrolytes, the LiFSI electrolyte demonstrates significantly enhanced thermal and electrochemical stability. The concentrated LiFSI electrolyte can effectively mitigates Al corrosion and suppresses gas generation at high temperatures. 5.0 M LiFSI/EMC electrolyte enables 87.7 % capacity retention of LiCoO₂/Gr battery after 100 cycles at 100 °C, and the discharge capacity at 5.0 C-rate can reaches 72.6 % of normal capacity. The excellent performance of the LiCoO₂/Gr battery at high temperatures is mainly attributed to the unique solvation structure of the LiFSI high-concentration electrolyte and the formation of a uniform and dense SEI composed of rich-F/S/N inorganic substances, which effectively inhibits continuous decomposition and consume of electrolyte. This work is an important step forward in promoting the practical application of lithium-ion batteries at high temperatures.

Acknowledgments

This work was supported by the Project for Full-time High-end Talents Introduction of Hebei of China (2020HBQZYC017), the Hebei Province Postdoctoral Research Funding Project (B2021003023), the Natural Science Foundation of Hebei Province of China (E2022202181), Science and technology research project of colleges and universities in Hebei Province (BJK2024123, JZX2023003), and the Basic Research Fund of Hebei University of Technology.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.25679931.

• 1) Due to the poor thermal stability of the lithium hexafluorophosphate (LiPF₆) electrolyte system, commercial lithium-ion batteries (LIBs) are difficult for normal operation at high temperatures above 55 °C. The limitation of the LiPF₆ electrolyte severely limits the practical application of LIBs under extremely high temperatures conditions. Here, a high-concentration electrolyte based on lithium bis(fluorosufonyl)imide (LiFSI) as electrolyte salt and ethyl methyl carbonate (EMC) as solvent is proposed, which possesses superior electrochemical stability and thermal stability. The LiCoO₂/graphite (Gr) pouch battery with the LiFSI high-concentration electrolyte (5.0 mol L⁻¹ (M)) has been shown excellent cycling performance even at 100 °C, an impressive capacity retention of 87.7 % can be still maintained after 100 cycles at 1.0 C-rate. The superior high temperatures performance is mainly attributed to the unique solvated structure, along with the robust solid electrolyte interphase (SEI) rich in anions. This work presents an effective strategy for promoting the development of high-temperature lithium-ion batteries.

CRediT Authorship Contribution Statement

Liyuan Yao: Data curation (Lead), Investigation (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Xihua Wang: Data curation (Supporting), Investigation (Supporting)

Dongze Li: Methodology (Equal)

Xingai Wang: Writing – review & editing (Supporting)

Haichang Zhang: Writing – review & editing (Lead)

Ning Wang: Data curation (Equal)

Chunsheng Shi: Computation (Lead)

Fei Ding: Project administration (Lead), Supervision (Lead), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Project for Full-time High-end Talents Introduction of Hebei of China: 2020HBQZYC017

Hebei Provincial Postdoctoral Science Foundation: B2021003023

Natural Science Foundation of Hebei Province: E2022202181

Science and technology research project of colleges and universities in Hebei Province: BJK2024123

Science and technology research project of colleges and universities in Hebei Province: JZX2023003

the Basic Research Fund of Hebei University of Technology

References

• 1)  J. M. Tarascon and M. Armand, Nature, 414, 359 (2001).
• 2)  R. Koksbang, J. Barker, H. Shi, and M. Y. Saïdi, Solid State Ionics, 84, 1 (1996).
• 3)  G. Crabtree, Science, 366, 422 (2019).
• 4)  L. Chen, X. Fan, E. Hu, X. Ji, J. Chen, S. Hou, T. Deng, J. Li, D. Su, X. Yang, and C. Wang, Chem, 5, 896 (2019).
• 5)  H. Wang, S. Chen, C. Fu, Y. Ding, G. Liu, Y. Cao, and Z. Chen, ACS Mater. Lett., 3, 956 (2021).
• 6)  R. R. Kohlmeyer, G. A. Horrocks, A. J. Blake, Z. Yu, B. Maruyama, H. Huang, and M. F. Durstock, Nano Energy, 64, 103927 (2019).
• 7)  K. Xu, Chem. Rev., 104, 4303 (2004).
• 8)  J. Han, K. Kim, Y. Lee, and N. Choi, Adv. Mater., 31, 1804822 (2019).
• 9)  S. E. Sloop, J. B. Kerr, and K. Kinoshita, J. Power Sources, 119–121, 330 (2003).
• 10)  M. Gauthier, T. J. Carney, A. Grimaud, L. Giordano, N. Pour, H. H. Chang, D. P. Fenning, S. F. Lux, O. Paschos, C. Bauer, F. Maglia, S. Lupart, P. Lamp, and Y. Shao-Horn, J. Phys. Chem. Lett., 6, 4653 (2015).
• 11)  R. Spotnitz and J. Franklin, J. Power Sources, 113, 81 (2003).
• 12)  D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, and G. Hinds, Nat. Commun., 6, 6924 (2015).
• 13)  M. Fang, X. Yue, Y. Dong, Y. Chen, and Z. Liang, Joule, 8, 91 (2024).
• 14)  T. Zheng, J. Xiong, B. Zhu, X. Shi, Y. J. Cheng, H. Zhao, and Y. Xia, J. Mater. Chem. A, 9, 9307 (2021).
• 15)  Y. Qian, Y. Kang, S. Hu, Q. Shi, Q. Chen, X. Tang, Y. Xiao, H. Zhao, G. Luo, K. Xu, and Y. Deng, ACS Appl. Mater. Interfaces, 12, 10443 (2020).
• 16)  J. Zheng, J. A. Lochala, A. Kwok, Z. D. Deng, and J. Xiao, Adv. Sci., 4, 1700032 (2017).
• 17)  Z. Zeng, V. Murugesan, K. S. Han, X. Jiang, Y. Cao, L. Xiao, X. Ai, H. Yang, J. G. Zhang, and M. L. Sushko, Nat. Energy, 3, 674 (2018).
• 18)  J. Wang, Y. Yamada, K. Sodeyama, E. Watanabe, K. Takada, Y. Tateyama, and A. Yamada, Nat. Energy, 3, 22 (2018).
• 19)  Y. Yamada, J. Wang, S. Ko, E. Watanabe, and A. Yamada, Nat. Energy, 4, 269 (2019).
• 20)  M. Zhang, J. Zhang, J. Yang, J. Yao, Z. Chen, C. Lu, X. Du, Z. Zhang, H. Zhang, and G. Cui, Chem. Commun., 55, 9785 (2019).
• 21)  X. Dong, Y. G. Wang, and Y. Xia, Acc. Chem. Res., 54, 3883 (2021).
• 22)  S. S. Zhang and T. R. Jow, J. Power Sources, 109, 458 (2002).
• 23)  D. W. McOwen, D. M. Seo, O. Borodin, J. Vatamanu, P. D. Boyle, and W. A. Henderson, Energy Environ. Sci., 7, 416 (2014).
• 24)  M. ea Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, and H. Nakatsuji, Gaussian, Inc., Wallingford, CT.
• 25)  H. Zhang, R. Zhang, X. Liu, F. Ding, C. Shi, Z. Zhou, and N. Zhao, J. Mater. Chem. A, 9, 24915 (2021).
• 26)  L. Qiao, Z. Cui, B. Chen, G. Xu, Z. Zhang, J. Ma, H. Du, X. Liu, S. Huang, and K. Tang, Chem. Sci., 9, 3451 (2018).
• 27)  X. B. Cheng, S. J. Yang, Z. Liu, J. X. Guo, F. N. Jiang, F. Jiang, X. Xiong, W. B. Tang, H. Yuan, and J. Q. Huang, Adv. Mater., 36, 2307370 (2024).
• 28)  D. Yu, X. Pan, J. E. Bostwick, C. J. Zanelotti, L. Mu, R. H. Colby, F. Lin, and L. A. Madsen, Adv. Energy Mater., 11, 2003559 (2021).
• 29)  Y. Xiao, R. Xu, L. Xu, J. F. Ding, and J. Q. Huang, Energy Mater., 1, 100013 (2022).
• 30)  S. Weng, X. Zhang, G. Yang, S. Zhang, B. Ma, Q. Liu, Y. Liu, C. Peng, H. Chen, H. Yu, X. Fan, T. Cheng, L. Chen, Y. Li, Z. Wang, and X. Wang, Nat. Commun., 14, 4474 (2023).
• 31)  A. von Wald Cresce, O. Borodin, and K. Xu, J. Phys. Chem. C, 116, 26111 (2012).
• 32)  Z. Wang, Z. Sun, Y. Shi, F. Qi, X. Gao, H. Yang, H. M. Cheng, and F. Li, Adv. Energy Mater., 11, 2100935 (2021).
• 33)  Z. Wang, F. Qi, L. Yin, Y. Shi, C. Sun, B. An, H. M. Cheng, and F. Li, Adv. Energy Mater., 10, 1903843 (2020).
• 34)  H. Jia, L. Zou, P. Gao, X. Cao, W. Zhao, Y. He, M. H. Engelhard, S. D. Burton, H. Wang, and X. Ren, Adv. Energy Mater., 9, 1900784 (2019).
• 35)  T. D. Pham, A. Bin Faheem, J. Kim, H. M. Oh, and K. K. Lee, Small, 18, 2107492 (2022).
• 36)  H. Zhou, K. Xiao, and J. Li, J. Power Sources, 302, 274 (2016).
• 37)  Z. Li, Y. X. Yao, S. Sun, C. B. Jin, N. Yao, C. Yan, and Q. Zhang, Angew. Chem. Int. Ed., 62, e202303888 (2023).
• 38)  J. Kim, J. G. Lee, H. Kim, T. J. Lee, H. Park, J. H. Ryu, and S. M. Oh, J. Electrochem. Soc., 164, A2418 (2017).
• 39)  B. Philippe, R. Dedryvère, M. Gorgoi, H. Rensmo, D. Gonbeau, and K. Edström, J. Am. Chem. Soc., 135, 9829 (2013).
• 40)  Y. Yamada, C. H. Chiang, K. Sodeyama, J. Wang, Y. Tateyama, and A. Yamada, ChemElectroChem, 2, 1687 (2015).
• 41)  E. Yoon, J. Lee, S. Byun, D. Kim, and T. Yoon, Adv. Funct. Mater., 32, 2200026 (2022).
• 42)  C. Han, G. Chen, Y. Ma, J. Ma, X. Shui, S. Dong, G. Xu, X. Zhou, Z. Cui, L. Qiao, and G. Cui, Energy Mater., 3, 300052 (2023).
• 43)  Y. Dong, J. Demeaux, Y. Zhang, M. Xu, L. Zhou, A. D. MacIntosh, and B. L. Lucht, J. Electrochem. Soc., 164, A128 (2017).
• 44)  L. Dong, F. Liang, D. Wang, C. Zhu, J. Liu, D. Gui, and C. Li, Electrochim. Acta, 270, 426 (2018).
• 45)  Y. Jeoun, K. Kim, S. Y. Kim, S. H. Lee, S. H. Huh, S. H. Kim, X. Huang, Y. E. Sung, H. D. Abruna, and S. H. Yu, ACS Energy Lett., 7, 2219 (2022).
• 46)  S. Klein, S. Van Wickeren, S. Röser, P. Bärmann, K. Borzutzki, B. Heidrich, M. Börner, M. Winter, T. Placke, and J. Kasnatscheew, Adv. Energy Mater., 11, 2003738 (2021).
• 47)  L. Wang, A. Menakath, F. Han, Y. Wang, P. Y. Zavalij, K. J. Gaskell, O. Borodin, D. Iuga, S. P. Brown, and C. Wang, Nat. Chem., 11, 789 (2019).
• 48)  X. Ji, S. Hou, P. Wang, X. He, N. Piao, J. Chen, X. Fan, and C. Wang, Adv. Mater., 32, 2002741 (2020).