Increasing the Lithium Ion Mobility in Poly(Phosphazene)-Based Solid Polymer Electrolytes through Tailored Cation Doping

Abstract

Solid polymer electrolytes (SPEs) are promising candidates for usage in rechargeable lithium metal batteries (LMBs) as they possess high mechanical, thermal, and chemical stability. However, the poor ionic conductivity of SPEs in comparison to liquid electrolytes hampers the commercialization of SPE-based LMBs. In the case of poly[bis(methoxy-ethoxy-ethoxy-)phosphazene] (MEEP), one explanation for the low ionic conductivity is the trapping of lithium cations in backbone coordination sites, hindering lithium ion movement through the electrolyte membrane. Herein, modelling the ion coordination in MEEP using DFT calculations reveals that, compared to lithium, heavier alkali cations are more likely to be complexed at the backbone coordination sites. With other alkali cations masking these coordination sites, enhanced lithium ion mobility through the SPE is expected. Experimental data proves these expectations: doping MEEP-based LiBOB-containing SPE membranes with small amounts of in-house synthesized potassium bis(oxalato)borate (KBOB) increases the lithium ion transference number from 0.08 to 0.18. Also, the partial lithium ion conductivity of the salt-in-MEEP electrolyte is boosted to outstanding 0.08 mS cm−1, far exceeding state-of-the-art literature values for this material. A cross-check using SPEs based on the structurally similar poly(ethylene oxide) (PEO) validates the proposed cation displacement model. The obtained insights may aid the development of highly effective poly(phosphazene)-based SPEs.