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 (P111i4FSI), 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.