A new ionic liquid-based electrolyte with both high conductivity and electrochemical stability

Recent development of ionic liquid-based electrolytes in lithium-ion batteries

Lithium metal batteries (LMBs) are the promising battery system to push energy density to high level at low temperatures. In this work, the linear carboxylic esters methyl propionate (MP)-based electrolyte with 4 wt% fluorinated ethylene carbonate (FEC) is reported. The low melting point and viscosity of MP makes it the candidate solvent for the low temperature field. Assisted with FEC, the optimized electrolyte shows quite high ionic conductivity and better compatibility with separator and lithium metal anode, resulting in stable cycling performance at room temperature. Besides, it keeps liquid state at −70 °C and exhibits lower binding energy with lithium ion, enabling the Li/LiCoO2 batteries to cycle at −40 °C for 60 cycles. Apart from that, this battery can retain 88.6% discharge capacity at −70 °C of that at room temperature, reaching the highest discharge capacity retention at this ultra-low temperature to the best of our knowledge. This work demonstrates a simple but effective way to design the electrolytes with excellent low temperature performance.

Enabling all-solid-state Li-ion batteries requires solid electrolytes with high Li ionic conductivity and good electrochemical stability. Following recent experimental reports of Li3 YCl6 and Li3 YBr6 as promising new solid electrolytes, we used first principles computation to investigate the Li-ion diffusion, electrochemical stability, and interface stability of chloride and bromide materials and elucidated the origin of their high ionic conductivities and good electrochemical stabilities. Chloride and bromide chemistries intrinsically exhibit low migration energy barriers, wide electrochemical windows, and are not constrained to previous design principles for sulfide and oxide Li-ion conductors, allowing for much greater freedom in structure, chemistry, composition, and Li sublattice for developing fast Li-ion conductors. Our study highlights chloride and bromide chemistries as a promising new research direction for solid electrolytes with high ionic conductivity and good stability.

Enabling high-performance all-solid-state lithium batteries with high ionic conductive sulfide-based composite solid electrolyte and ex-situ artificial SEI film

The sulfide-type solid electrolyte (SE) is a promising candidate for realizing all-solid-state batteries (ASSBs) due to its good mechanical properties and high ionic conductivity. However, challenges such as limited electrochemical stability, inadequate solid-solid contact with the electrode, and reactivity with lithium often lead to dendrite growth and electrolyte reduction. A simple, solvent-free method was developed to form a robust artificial interphase between lithium metal and the solid electrolyte to address these issues. This was achieved by incorporating a composite electrolyte composed of Li6PS5Cl, polyethylene glycol (PEG), and lithium bis(fluorosulfonyl)imide (LiFSI), leading to the in-situ formation of a LiF-rich interfacial layer. This interphase effectively suppresses electrolyte reduction and enhances lithium-ion diffusion. Notably, the PEG additive improves the mechanical strength of the electrolyte by enhancing adhesion between sulfide particles. It also increases the flexibility of the Li6PS5Cl solid electrolyte, improving physical contact with the lithium anode and acting as a protective barrier to prevent direct SE-Li interaction, thus reducing electrolyte decomposition. As a result, Li/Li symmetric cells exhibited excellent cycling stability, maintaining performance for over 3000 hours at a current density of 0.1 mA cm-2, while the composite solid electrolyte achieved a critical current density of 4.75 mA cm-2. This study underscores the synergistic benefits of combining an artificial solid electrolyte interphase (SEI) with composite solid electrolytes, paving the way for high-performance all-solid-state lithium metal batteries (ASSLBs).

This study investigated the influence of variations in the mixing ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) on the composition and effectiveness of the solid electrolyte interphase (SEI) in lithium-metal batteries. The SEI is crucial for battery performance, as it prevents continuous electrolyte decomposition and inhibits the growth of lithium dendrites, which can cause internal short circuits leading to battery failure. Although the properties of the SEI largely depend on the electrolyte solvent, the influence of the EC:EMC ratio on SEI properties has not yet been elucidated. Through electrochemical testing, ionic conductivity measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, the formation of Li2CO3, LiF, and organolithium compounds on lithium surfaces was systematically analyzed. This study demonstrated that the EC:EMC ratio significantly affected the SEI structure, primarily owing to the promotion of the formation of a denser SEI layer. Specifically, the ratios of 1:1 and 1:3 facilitated a uniform distribution and prevalence of Li2CO3 and LiF throughout the SEI, thereby affecting cell performance. Thus, precise control of the EC:EMC ratio is essential for enhancing the mechanical robustness and electrochemical stability of the SEI, thereby providing valuable insights into the factors that either enhance or impede effective SEI formation.

Ionic liquids that are non-volatile, non-flammable, and stable with Li metal and exhibit wide electrochemical stability window are promising for application to lithium batteries, including Li/air cells. 1-Methyl-3-Propylpyrrolidinium bis (trifluoromethanesulfonyl)imide (P13TFSI) is identified to be such an ionic liquid. Dimensionally stable, elastic polymer electrolyte membranes with high thermal and electrochemical stability, high ionic conductivity and other desirable properties are synthesized by blending P13TFSI with lithium imide salt dissolved in it and the PVDF-HFP co-polymer. Preliminary solid-state, thin-film Li-oxygen cells are constructed using the polymer electrolyte membranes and their performance analyzed.

Potassium-ion batteries (K-ion batteries; KIBs) are attractive as high-voltage, low-cost energy storage devices. Ionic liquids (ILs) are potential candidates as safe and high-performance electrolytes for large-scale devices. Imidazolium-based ILs are known to possess high ionic conductivities and moderate electrochemical stabilities. In this study, we report the physicochemical and electrochemical properties of K[FSA]–[C2C1im][FSA] (FSA = bis(fluorosulfonyl)amide; C2C1im = 1-ethyl-3-methylimidazolium) ILs as new electrolytes for KIBs. A phase diagram constructed from differential scanning calorimetry results indicates that the melting point of this IL is below 273 K at compositions of x(K[FSA]) = 0–0.20 (x(K[FSA]) = molar fraction of K[FSA]). The viscosity, ionic conductivity, and density of the IL were measured for x(K[FSA]) = 0–0.20. The ionic conductivity at x(K[FSA]) = 0.20 is 10.1 mS cm–1 at 298 K, which is comparable to that of typical organic solvent-based KIB electrolytes and higher than that of other ionic liquid electrolytes for KIBs. The electrochemical stability of M[FSA]–[C2C1im][FSA] (x(M[FSA]) = 0.20; M = K, Na, and Li) ILs was determined by cyclic voltammetry measurements. The K-based IL exhibits the widest electrochemical window of 5.19 V due to the negative redox potential of the K+/K couple. Thus, the K[FSA]–[C2C1im][FSA] ionic liquid is expected to be a highly conductive KIB electrolyte.

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Li metal is considered as one of optimal candidates for high-energy anode material because it has the highest theoretical specific capacity (3,860 mAh g-1) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode). However, safety concerns and low coulombic efficiency issues, which are caused by the inhomogeneous Li deposition/dissolution and continuous corrosion by electrolytes during battery cycling, have prohibited the use of metallic Li as anode in practical Li metal batteries. Lithium metal batteries (LMBs), which replace graphite anode with Li metal from conventional lithium ion batteries (LIBs), are considered as the most realistic approach to increase energy density of rechargeable battery. Furthermore, an anode-free LMB which features the only use of current collector in the anode compartment is the design to maximize the energy density of LMB. Because the current collector could affect the nucleation behaviors at the initial state of Li plating and the morphological development of the subsequently plated Li, it is a key component for achieving cycling stability of anode-free LMBs. Cu current collector is the most widely adopted current collector for the anode of LIBs and LMBs due to its high conductivity, mechanical property, and electrochemical stability, but is known to have a large Li nucleation overpotential in comparison with Au or Ag. That is because the binding energies of Li atom on bulk Li is much larger than that of Li atom on bulk Cu (which are around 24~30 and 2.5~2.7 kcal mol-1, respectively). Therefore, the surficial modulation of Cu current collector is needed to improve the affinity between Li metal and Cu and induce uniform initial Li deposition on Cu current collector. Cu collectors used for lithium ion battery, which are fabricated by electrodeposition method, have various crystalline facets on their surfaces. On the basis of the principle of nucleation and growth, the degree of crystalline misfit between adsorbent and substrate can vary depending on the surface facet of substrate. We believe that it can be a key principle for manipulating an initial Li nucleation mode on Cu substrate. In detail, we explore a facet selective Li nucleation and growth phenomenon on Cu and demonstrate that controlling the facet structure can improve the uniformity in Li deposition and the cycling stability. Preferential Li deposition on the Cu(100) plane is demonstrated by electrochemical analysis of the Cu single crystal surfaces and by EBSD analysis of the Li-deposited Cu surfaces. DFT calculations show that a difference in the Li adsorption energy during the initial Li deposition process among the Cu facets is responsible for the facet selectivity. A majorly (100) plane-orientated Cu foil fabricated with a simple annealing method has a more uniform Li nucleation with a 6-times higher nuclei density and a two-fold enhancement in the Li cycling stability compared with a conventional Cu foil with randomly oriented surface facets. The control of the surface facet provides a new design principle for the current collector of lithium metal batteries.

Given the urgent need to improve energy resilience, our interest in exploring alternative electrolyte chemistries is driven by the limitations of lithium resources and the safety risks associated with organic electrolytes. Sodium-ion batteries, paired with ionic liquid (IL) electrolytes, offer a promising solution due to the abundance of sodium and inherent safety advantages of ILs. ILs—molten salts at room temperature—exhibit exceptional properties such as high electrochemical stability, non-volatility, and non-flammability. However, key aspects of IL-based electrolytes, including the structure of the electrical double layer (EDL), electrochemical behavior, and solid electrolyte interphase (SEI) formation, remain poorly understood. This study investigates the electrochemical and interfacial properties of a salt-in-ionic liquid (SIL) electrolyte, where sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) is dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI]). We employ a multi-technique approach to gain comprehensive insights into the fundamental behavior of this SIL at the interface with a gold surface as the working electrode. Cyclic voltammetry is utilized to determine the electrochemical stability window (ESW), with both anodic and cathodic scans revealing the influence of initial SEI formation on ESW. Electrochemical impedance spectroscopy is used to characterize electrochemical properties of the SEI. To probe interfacial structures at the molecular level, atomic force microscopy (AFM) is employed to study EDL layering and SEI formation in situ, while surface-enhanced infrared absorption spectroscopy (SEIRAS) is used to analyze the enrichment of specific species near the electrode. Furthermore, electrochemical quartz crystal microbalance (EQCM) measurements track mass changes on the electrode, shedding light on SEI formation kinetics. By integrating these techniques, this work advances the understanding of EDL structure and SEI evolution in IL-based electrolytes at the nanoscale, contributing to the design of next-generation batteries beyond lithium-based chemistries.

Anion modification for stable solid electrolyte interphase in anode-free lithium metal batteries

Lithium batteries are the most attractive candidate to replace the traditional energy resources due to their high working voltage, high specific energy and long cycle life [1,2]. However, the widely use have been limited by their safety issues, especially under the condition of abusing or overcharging. In order to solve this issue, it is significant to screen a safe material as lithium batteries electrolyte.Room temperature ionic liquids (RTILs) have aroused much attention for use in lithium batteries, owing to their low volatility, inflammability, wide electrochemical window and high ionic conductivity [3]. Particularly, their excellent thermal stability makes them key to solve the safety issues of lithium batteries at high temperatures and their physicochemical properties can be tuned by introducing functionalized groups onto the cation or anion of RTILs. The functionalized task-special ionic liquids can reduce the viscosity and enhance their ionic conductivity of RTILs. In this study, a piperidinium-based ionic liquid, N-methyl-N-methoxycarbonylpiperidinium bis(trifluoromethanesulfonyl)imide ([MMOCPip][TFSI]), was synthesized by anion exchange reaction and used as an additive to electrolyte of Li-ion battery. The electrochemical performance of the electrolytes had been evaluated by electrochemical tests, such as the line sweep voltammetry, cyclic voltammetry and impedance spectroscopy.The N-methyl-N-methoxycarbonylpiperidinium bis(trifluoromethanesulfonyl)imide ([MMOCPip][TFSI]) ionic liquid was prepared by mixing equimolar N-methyl-N-methoxycarbonylpiperidinium bromide ([MMOCPip][Br]) and bis(trifluoromethanesulfonyl)imide lithium at room temperature in deionized water and by stirring for several hours until a hydrophobic [MMOCPip][TFSI] layer was formed below the water phase [4]. The ionic conductivity of [MMOCPip][TFSI] was 0.77 mS·cm-1 (25℃) and 2.41 mS·cm-1 (60℃), however, after adding 0.1 M LiTFSI, the ionic conductivity decreased to 0.51 mS·cm-1 (25℃) and 1.85 mS·cm-1 (60℃), respectively. The electrochemical window was 0.5 V~5.8 V (vs. Li+/Li), which was enough widely that could be used as common electrode material. The Li-ion transference number of ionic Liquid electrolyte containing 0.1 M LiTFSI was 0.56. The battery tests indicated that the [MMOCPip][TFSI] obviously improved the cyclability of the LiFePO4 cell. For the Li/ LiFePO4 half-cells, after 20 cycles at room temperature at 0.1 C, the discharge capacity was 109.7 mAh·g-1 with 98.7% capacity retention in the [MMOCPip][TFSI]/0.1 M LiTFSI electrolyte. The good electrochemical performance demonstrated that the [MMOCPip][TFSI] could be used as electrolyte additive for lithium batteries. Keywords: Ionic liquid; Li-ion battery; Ionic conductivity; Transference number Reference: [1] Wongittharoma N, Wang CH, Wang YC, Fey GTK, Li HY, Wu TY, Lee TC, Chang JK (2014). Charge-storage performance of Li/LiFePO4 cells with additive incorporated ionic liquid electrolytes at various temperatures. J Power Sources. 260: 268–275.[2] Yang J, Zhou XY, Zou YL, Tang JJ (2011). A hierarchical porous carbon material for high power, lithium ion batteries. Electrochim Acta. 56: 8576–8581.[3] Pandian S, Raju SG., Hariharan KS, Kolake SM, Park DH, Lee MJ (2015). Functionalized ionic liquids as electrolytes for lithium-ion batteries. J Power Sources. 286: 204-209. [4] Yang PX, Cui WY, Li LB, Liu L, An MZ (2012). Characterization and properties of ternary P(VdF-HFP)-LiTFSI-EMITFSI ionic liquid polymer electrolytes. Solid State Sciences. 14(5), 598–606. Figure 1

Alkali and alkaline earth metal anodes are gaining increasing research attention because they could provide high energy densities and theoretical specific capacities. Among them, lithium metal is the most investigated anode candidate for next generation batteries. However, due to some challenges, the commercialization of lithium metal anodes is still delayed. Reactions between lithium metal and the electrolyte lead to the formation of an inhomogeneous solid electrolyte interphase (SEI). Consequently, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, resulting in high surface area lithium and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also causes safety issues due to an increased risk of short-circuits and thermal runaway.1,2 To enable increased safety in high energy batteries with lithium metal anodes an effective SEI is required to limit lithium dendrite growth. There is a variety of approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods and chemical modification.3-5 Recently, alkaline earth metal anodes, such as magnesium or calcium, are also of considerable interest for next generation anodes. Their main advantage is the significantly lower susceptibility to dendrite formation, while still offering high theoretical specific capacities. However, their oxide passivation layer does not allow significant mobility for their divalent ions, leading to large overvoltage and slow reaction kinetics which hinders the cycling. Therefore, electrolytes which enable a non-passivating SEI have to be applied and additives are investigated on their ability to remove the oxide layers.6,7 Herein, we present a novel mechanochemical approach to form an effective SEI on the lithium metal surface by combining mechanical and chemical modification utilizing ionic liquids (ILs) prior to cell assembly. This method suppresses dendrite growth even at high current densities of 10 mA cm-2 in lithium metal batteries with liquid electrolytes. To further highlight the importance of surface treatment and SEI formation, we investigate electrolytes for calcium metal batteries as well as mechanical modification of calcium metal anodes. Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145).

Solvate ionic liquid-based ionogels for lithium metal batteries over a wide temperature range