Sustainable, Dendrite Free Lithium-Metal Electrode Cycling Achieved with Polymer Composite Electrolytes Based on a Poly(Ionic Liquid) Host

Polymer composite solid-state electrolyte materials based on ionic liquids stand out as viable alternatives to flammable liquid electrolytes for solid state lithium-metal batteries. They offer a compromise between favourable mechanical properties and stability against Li-metal, coupled with favourable ion transport. However, insufficient Li⁺ transport properties for practical battery operation may result from the higher mobility of other ionic species from the ionic liquid (IL). Here, this issue was addressed by confining a highly concentrated IL electrolyte in a poly(ionic liquid) matrix with the addition of 5 wt % of alumina nanoparticles; these superconcentrated IL electrolytes favour Li⁺ ion transport. The composites are based on a poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA NTf₂) matrix, and an electrolyte solution (ES) of high lithium concentration phosphonium IL, trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P_111i4FSI), with 3.8 mol kg⁻¹ (3.8 m) LiFSI. The impact of ES content within the composite on Li⁺ transference number and electrochemical stability against Li-metal is reported. For the 50 wt % ES and 50 wt % of PDADMA NTf₂ composite, up to 0.5 mAh cm⁻² of Li-metal plating/stripping for over 20 days at 50 °C is shown. Scanning electron microscopy (SEM) confirmed that no Li dendrite formation was visible at the Li-metal/polymer composite interface. Competitive performance of LiFePO₄ electrodes (1.2 mAh cm⁻²) is also reported.