Ion Speciation and Mobility in Solid Polymer Electrolytes: Insights from Molecular Dynamics Simulations

As demand for high energy density and safety in rechargeable batteries intensifies, lithium metal batteries based on solid polymer electrolytes have emerged as promising alternatives. However, challenges such as low Li-ion mobility and limited cation transference numbers restrict their wider application. This study uses molecular dynamics simulations to investigate the effects of various salts (LiTFSI, LiPF₆, and LiClO₄) in PEO systems on ion speciation (solvent-separated ion pairs, contact ion pairs, and aggregates) and their impacts on ion mobility and transference numbers under varying field strengths. Notably, even ions that exhibit similar speciation distributions can demonstrate distinct mobility and transference behaviors, suggesting the influence of additional factors. We assess quantitatively the ion mobility contributions from each speciation type, clarifying how each influences the overall mobility. Despite similar ion speciation distributions in systems with LiTFSI and LiPF₆, the primary contributors to ion mobility differ significantly. Moreover, cation transference numbers correlate strongly with the solvation radius ratio of cations to anions, emphasizing its pivotal role in ion transport. These findings offer critical insights for designing advanced solid polymer electrolytes to enhance the efficiency of lithium metal batteries.