Aluminium batteries have garnered significant research interest in recent years owing to the high theoretical capacity, natural abundance and safety of an Al metal electrode. Non-aqueous electrolytes such as chloroaluminate ionic liquids are widely used in Al batteries due to their relatively high room-temperature ionic conductivity and their ability to reversibly electrodeposit Al at coulombic efficiencies close to 100% ¹. The development of polymer electrolytes based on these ionic liquids has been explored as a means of enhancing battery performance by widening the electrochemical stability window ^{2, 3}, aiming to allow for higher discharge potentials to boost specific capacity and power. This increase in potential stability can significantly reduce the electrolyte decomposition that typically occurs beyond 2.4 V in EMImCl-AlCl₃ ionic liquids and improve the resulting coulombic efficiency of battery cycling.

Here, solid polymer electrolytes were synthesised with 1-ethyl-3-methylimidazolium chloride-aluminium chloride (EMImCl-AlCl₃, AlCl₃:EMImCl = 1.5:1), polyethylene oxide (PEO) and varying amounts of fumed SiO₂ via a solvent-free process. The electrolytes were characterised electrochemically to determine their potential stability range, ionic conductivities and Al₂Cl₇⁻ diffusion coefficients. The polymer electrolytes, both with and without the addition of fumed SiO₂, exhibited markedly wider potential stability windows in comparison with the neat ionic liquid. Incorporating fumed SiO₂ in the polymer electrolytes was found to promote the pseudocapacitive behaviour of AlCl₄⁻ intercalation in Al-natural graphite cells, which is key to creating fast-charging and high-power batteries. Cells assembled with the EMImCl-AlCl₃ polymer electrolytes containing 6 wt.% PEO and 0.5 wt.% SiO₂ demonstrated stable cycling of up to 2.8 V with 97% coulombic efficiency for over 100 cycles. This significant increase in the cut-off potential yields specific capacities close to 190 mA h g^-1 at 66 mA g^-1, 46% higher than the specific capacity of the same cell when cycled to only 2.4 V.

We explore the reasons behind these findings and the influence of fumed SiO₂ on ion mobility by both electrochemical and spectroscopic methods. The solid polymer electrolytes were characterised up from the molecular level by solid-state ¹H, ¹³C, ²⁷Al, ²⁹Si nuclear magnetic resonance spectroscopy, revealing insights into the local environments and dynamics of the polymer chains and constituent ions. The faradaic diffusion-limited and pseudocapacitive current contributions of intercalation were decoupled through variable-rate cyclic voltammetry to provide a deeper understanding of the effects of mass transport on the charge storage mechanisms in solid polymer electrolytes compared to ionic liquids. The results of this study may offer useful insights into the strategies available to tune the electrochemical properties of ionic liquid-based electrolytes for high-performance energy storage applications.

References

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