The effect of cation chemistry on physicochemical behaviour of superconcentrated NaFSI based ionic liquid electrolytes and the implications for Na battery performance

Fluoroborate ionic liquids as sodium battery electrolytes

High-voltage sodium batteries are an appealing solution for economical energy storage applications. Currently available electrolyte materials have seen limited success in such applications therefore the identification of high-performing and safer alternatives is urgently required. Herein we synthesise six novel ionic liquids derived from two fluoroborate anions which have shown great promise in recent battery literature. This study reports for the first time the electrochemically applicable room-temperature ionic liquid (RTIL) N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium (tetrakis)hexafluoroisopropoxy borate ([N2(2O2O1)3][B(hfip)4]). The RTIL shows promising physical properties with a very low glass-transition at -73 °C and low viscosity. The RTIL exhibits an electrochemical window of 5.3 V on a glassy carbon substrate which enables high stability electrochemical cycling of sodium in a 3-electrode system. Of particular note is the strong passivation behaviour of [N2(2O2O1)3][B(hfip)4] on aluminium current-collector foil at potentials as high as 7 V (vs. Na+/Na) which is further improved with the addition of 50 mol% Na[FSI]. This study shows [B(hfip)4]- ionic liquids have the desired physical and electrochemical properties for high-voltage sodium electrolytes.

Stability of FeF3-Based Sodium-Ion Batteries in Nonflammable Ionic Liquid Electrolytes at Room and Elevated Temperatures

Iron trifluoride (FeF3), a conversion-type cathode for sodium-ion batteries (SIBs), is based on cheap and abundant Fe and provides high theoretical capacity. However, the applications of FeF3-based SIBs have been hindered by their low-capacity utilization and poor cycling stability. Herein, we report greatly enhanced performance of FeF3 in multiple types of ionic liquid (IL) electrolytes at both room temperature (RT) and elevated temperatures. The Pyr1,4FSI electrolyte demonstrated the best cycling stability with an unprecedented decay rate of only ∼0.023% per cycle after the initial stabilization and an average coulombic efficiency of ∼99.5% for over 1000 cycles at RT. The Pyr1,3FSI electrolyte demonstrated the best cycling stability with a capacity decay rate of only ∼0.25% per cycle at 60 °C. Cells using Pyr1,3FSI and EMIMFSI electrolytes also showed promising cycling stability with capacity decay rates of ∼0.039% and ∼0.030% per cycle over 1000 cycles, respectively. A protective and ionically conductive cathode electrolyte interphase (CEI) layer is formed during cycling in ILs, diminishing side reactions that commonly lead to gassing and excessive CEI growth in organic electrolytes, especially at elevated temperatures. Furthermore, the increased ionic conductivity and decreased viscosity of ILs at elevated temperatures help attain higher accessible capacity. The application of ILs sheds light on designing a protective CEI for its use in stable SIBs.

High-Performance Cycling of Na Metal Anodes in Phosphonium and Pyrrolidinium Fluoro(sulfonyl)imide Based Ionic Liquid Electrolytes

We have investigated the sodium electrochemistry and the evolution and chemistry of the solid-electrolyte interphase (SEI) upon cycling Na metal electrodes in two ionic liquid (IL) electrolytes. The effect of the IL cation chemistry was determined by examining the behavior of a phosphonium IL (P_111i4FSI) in comparison to its pyrrolidinium-based counterpart (C₃mpyrFSI) at near-saturated NaFSI salt concentrations (superconcentrated ILs) in their dry state and with water additive. The differences in their physical properties are reported, with the P_111i4FSI system having a lower viscosity, higher conductivity, and higher ionicity in comparison to the C₃mpyrFSI-based electrolyte, although the addition of 1000 ppm (0.1 wt %) of water had a more dramatic effect on these properties in the latter case. Despite these differences, there was little effect in the ability to sustain stable cycling at moderate current densities and capacities (being nearly identical at 1 mA cm^-2 and 1 mAh cm^-2). However, the IL based on the phosphonium cation is shown to support more demanding cycling with high stability (up to 4 mAh cm^-2 at 1, 2, and 4 mA cm^-2 current density), whereas C₃mpyrFSI rapidly failed (at 1 mA cm^-2 /4 mAh cm^-2). The SEI was characterized ex situ using solid-state ²³Na NMR, XPS, and SEM and showed that the presence of a Na complex, identified in our previous work on C₃mpyrFSI to correlate with stable, dendrite-free Na metal cycling, was also more prominent and coexisted with a NaF-rich surface. The results here represent a significant breakthrough in the development of high-capacity Na metal anodes, clearly demonstrating the superior performance and stability of the P_111i4FSI electrolyte, even after the addition of water (up to 1000 ppm (0.1 wt %)), and show great promise to enable future higher-temperature (50 °C) Na-metal-based batteries.

Polyoxyethylene (PEO)|PEO-Perovskite|PEO Composite Electrolyte for All-Solid-State Lithium Metal Batteries

Composite solid electrolytes (CSEs) are regarded as one of the most promising candidates for all-solid-state lithium metal batteries (ASSLMBs) due to inherited desirable features from both ceramic and polymer materials. However, poor interfacial contact/compatibility between the electrodes and solid electrolytes remains a critical challenge. In this work, we prepare a flexible CSE composed of polyoxyethylene (PEO)-perovskite composite with a layer of PEO on either side. This PEO|PEO-perovskite|PEO structure prevents direct contact between the perovskite and lithium metal at the anode side, avoiding the undesired reaction between the two materials (Ti⁴⁺ + Li → Ti³⁺ + Li⁺). Moreover, the design incorporating the PEO surface on either side enables superb contact between the electrolyte and the electrodes and buffers the change in electrolyte volume from the cathode and lithium metal during repeated cycling, resulting in low interfacial resistances and excellent cycling stability. Meanwhile, perovskite inorganic electrolyte Li_0.33La_0.557TiO₃ (LLTO) 3D nanofiber networks formed by electrospinning enable the CSE to achieve enhanced mechanical strength and high ionic conductivity of 0.16 mS cm^-1 at 24 °C. As a result, a Li|PEO-LiTFSI-LLTO|Li symmetric cell remains stable after 400 h of operation without short-circuiting. Most notably, a Li|PEO-LiTFSI-LLTO|LiFePO₄ full battery is capable of delivering a high capacity of 135.0 mAh g^-1 even at 2 C with a retention rate of 79.0% after 300 cycles at 60 °C. These results demonstrate that the integrated sandwich structure proposed in this work is effective in developing high-performance composite solid electrolytes for ASSLMBs.

Computational Investigation of Mixed Anion Effect on Lithium Coordination and Transport in Salt Concentrated Ionic Liquid Electrolytes

The use of high concentrations of alkali metal ion salts in ionic liquids (ILs) has been demonstrated to significantly improve electrolyte performance, increase alkali metal ion transference numbers, and promote the formation of favorable SEI structures enabling long-term stable cycling. One challenge in using this material is the overall low ionic conductivity, which is a common effect of increased salt concentration. This simulation work first investigated the strategy of using mixed anions to tune the ionic conductivity in a concentrated IL (or "ionic liquid-in-salt") system having 50 mol % lithium salt. The effects of binding strength, size, and mobility of selected anions on coordination and dynamics of lithium ions were discussed. The results confirm its feasibility and provide general guidance for the selection of anions to improve the ionic conductivity of salt-concentrated electrolyte systems based on ionic liquids and other solvent systems.

Tuning Sodium Interfacial Chemistry with Mixed-Anion Ionic Liquid Electrolytes

The interphase layer that forms on either the anode or the cathode is considered to be one of the critical components of a high performing battery. This solid-electrolyte interphase (SEI) layer determines the stability of the electrode in the presence of a given electrolyte as well as the internal resistance of a battery, and hence the overpotential of a cell. In the case of lithium ion batteries where carbonate based electrolytes are used, additives including hexafluorophosphate (PF₆), bis-trifluoromethylsulfonimide (TFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), and fluorosulfonimde (FSI) are used to obtain favorable SEI layers. Ionic liquids and salts based on anions containing nitrile groups, including dicyanamide (DCA), offer a less expensive alternative to a fluorinated anion and have also been shown to support stable electrochemistry in lithium and sodium systems. However, longer term cycling leads to the eventual passivation of the electrode, presumed to be due to the instability of the DCA anion. We herein consider the use of a fluorinated anion to control the interfacial electrochemistry and provide a more stable SEI in DCA ILs. We investigate the addition of NaDCA, NaFSI, NaTFSI, and NaFTFSI to the methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) ionic liquid. NaFSI was found to generate a more stable SEI layer, as evidenced by extended symmetric cell cycling, while the TFSI and FTFSI salts both lead to thicker, highly passivating surfaces. We use molecular dynamics, infrared spectroscopy and X-ray photoelectron spectroscopy to interrogate and discuss the influence of the anion on the bulk electrolyte, the interfacial electrolyte structure, and the formation of the SEI layer, in order to rationalize the contrasting electrochemical observations.

Toward Stable Electrode/Electrolyte Interface of P2-Layered Oxide for Rechargeable Na-Ion Batteries

The electrochemical properties of P2-Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ layered oxide, which is a promising cathode material for rechargeable Na-ion batteries (NIBs), are evaluated with an optimized in-house ionic liquid (IL)-based electrolyte, and its performance is compared with that using carbonate-based electrolyte. The IL-based system reveals better electrochemical performance at room temperature than the carbonate electrolyte-based one at 0.1C and 1C, especially in terms of cycling stability, with a 97% capacity retention after 100 deep cycles (0.1C). The electrode/electrolyte interface is thoroughly studied in both systems by means of X-ray photoelectron spectroscopy and scanning electron microscopy so as proof that the formed interface is crucial to optimizing the electrochemical performance of NIBs. The carbonate-based system shows a thin, inhomogeneous, and unstable interface layer, while the IL-based one exhibits an even thinner but homogeneous and more stable interface, which may result in safer and longer-lasting NIBs.