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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 4.3. Ionic liquid electrolytes S Sen1,2 and R G Palgrave1,2 1 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom 2 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom Status NIBs have attracted attention in parallel to Li-ion technology due to their cost-effectiveness and the high earth abundance of sodium [71, 183, 184]. Despite the commercialisation of NIBs by several companies (Faradion [185], Tiamat [185, 186], SHARP labs [185, 186]), thermal runaway with volatile electrolytes and insufficient cyclability at room temperature demand more serious electrolyte research. Ionic liquids (ILs) are promising alternative electrolytes due to their non-flammability and wide operating electrochemical window. They are glass-forming molten salts, consisting of versatile combinations of aromatic cations and weakly coordinating anions. The structures of the ions and their interactions (hydrogen bond, electrostatic, and Van der Waals) [187] govern their physicochemical properties, (phase transitions, viscosities, thermal stability, and HOMO-LUMO (lowest unoccupied molecular orbital) gap) [187] and electrochemistry (electrochemical window, ionicity, ionic conductivity, ionic association/dissociation, and the double-layer structure at the electrode–electrolyte interface) [187, 188]. In the realm of NIBs, ILs offer specific additional advantages, such as reduced corrosion of the Al current collector, a wide range of operating temperatures, and suitable electrode–electrolyte interface processes (homogeneous SEI formation, a conductive interface, and a dendrite-free anode–electrolyte interface) [185] (figure 21). The wider electrochemical window of ILs permits a wide variety of cathode materials for NIBs. Only anion decomposition defines oxidative stability and SEI composition. ILs offer a less resistive, more uniform, and consistent SEI throughout the battery cycle with hard carbon and alloy-based anodes [186]. The composition of an IL-derived SEI (with an HC anode) (SEIs formed with polyolefin, SCO) is distinctly different from those formed in organic electrolytes (SEIs formed with organic and inorganic sodium carbonates) [186]. Reduced aluminium corrosion in ILs is notable, even with salts that corrode the Al current collector in organic solvents [186]. Several imidazolium-, pyrrolidinium-, phosphonium-, and ammonium-cation-based ionic liquids have been explored in NIBs (e.g., [C2C1Im][FSA], [C2C1Im][BF4], [C3C1Pyrr][FSA], [C3C1Pyrr][TFSI], N2(20201)3][FSA], [P111i4][FSA], [C4C1pyrr][DCN], [C4C1Im][SO3CF3]. CnC1im+: 1-alkyl-3-methylimidazolium, CnC1pyrr+: N-alkyl-N-methylpyrrolidinium, CnC1pip+: N-alkyl-N-methylpiperidinium, and Nnnnn+: tetraalkylammonium) [186]. Current and future challenges Despite several advantages, NIB research is falling behind the current state of the art of LIB technology. Safety and the battery cycle life are significantly influenced by the electrolyte formulation and various interfacial issues, including uncontrolled dendrite formation due to the use of sodium metal, unstable and increasingly resistive electrolyte interface SEI formation due to electrolyte decomposition, cracked and thicker SEIs, pronounced dissolution of SEIs, and the significant reactivity of sodium metal in the majority of volatile organic electrolytes. Ionic liquids serve as green solvents. They mitigate safety issues and offer wider oxidative stability and relatively lower solvent degradation at the electrodes. However, the major challenges of ionic liquids are their relatively high cost, tedious purification methods, and the hygroscopic nature of widely used imidazolium- and pyrrolidinium-based ionic liquids. This is despite their excellent stability and wider temperature operability. Their relatively high viscosity and inferior ionic conductivity limit their battery cycling performance at room temperature, as compared to conventional organic electrolytes. The ionic liquids offer larger interfacial resistance and slower Na+ transfer at the sodium metal anode, compared to organic electrolytes. In NIBs, the advantages of ionic liquids are only fully utilised at elevated temperatures (∼90 ◦C) in half cells. Another important challenge with ionic liquids is to achieve a unity sodium transference number and reduce concentration polarisation. The majority of ionic liquid electrolytes are made at a maximum of 1 M concentration, a concentration at which unity transference numbers are difficult to achieve. Recently, super-concentrated ionic liquid-salt systems have been shown to offer relatively stable SEI properties and near-unity transference numbers. However, the availability of a wider range of IL structures with high salt dissolution capability is still limited. Designing electrolytes to achieve high transference numbers and reasonably good ionic conductivity is an unavoidable challenge for ILs in NIBs. Although minimal, the current-collector corrosion is not fully mitigated by the widely used fluorinated anion-based ionic liquids. 44PDF Image | 2021 roadmap for sodium-ion batteries
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