Microstructure and Ion Transport in Mixed-Anion Poly(ionic liquid) Electrolytes

Polymeric ionic liquids (polyILs) with high-salt concentrations are promising polymer electrolytes for lithium and sodium metal batteries. While prior studies have focused on single-anion systems, this work investigates mixed-anion high-salt polyILs at a 1:2 polyIL-to-salt ratio by using combined computational and experimental approaches. Clear phase separations are observed both computationally and experimentally, related to distinct anion structures, large disparities in Na+-anion binding strength, and differences in ion-packing stability, which collectively affect anion coordination tendency and electrolyte microstructure. In the molten salt subphase, molecular dynamics simulations reveal highly correlated Na+-anion diffusion, with markedly higher diffusivities for Na+ and BF4– within tens of nanoseconds. However, diffusivity decreases substantially after ∼200 ns of equilibration due to NaBF4 crystallization, consistent with experimental observations. Although crystallization phenomena are experimentally observed across all mixed-anion systems, they remain difficult to fully capture within 300 ns of MD simulation, highlighting the need for advanced modeling techniques. Overall, this study elucidates the behavior of polyILs with mixed anions, revealing the fundamental challenges of the mixed-anion designs and the importance of iterative modeling–experiment–modeling approaches for rational computational electrolyte development.