Phosphonium outperforms ammonium poly(ionic liquids) for fast ion transport polymer electrolytes

Cationic poly(ionic liquids) (polyILs) are promising solid polymer electrolytes, yet the role of cationic charge-centre chemistry in regulating alkali-metal transport and interfacial behaviour remains poorly understood. Here, we computationally investigated a phosphonium-based polyIL at high lithium or sodium concentrations and compared these systems with previously reported ammonium-based analogues. The phosphonium systems demonstrated superior alkali-metal ion diffusion, which was consistent with their high ionic conductivities. Molecular-level analyses show that phosphonium cation chemistry does not necessarily lead to weaker isolated interactions with the anion, as demonstrated here. Rather, it is associated with weaker collective ionic interactions in the condensed phase, contributing to lower T _g, enhanced temperature-dependent free-volume growth, and faster ion dynamics. PolyIL-electrode interface modelling investigation further reveals different ion adsorbed behaviour under increasingly negative electrode polarization. Compared with the ammonium analogue, the phosphonium system exhibits significantly higher populations of Na⁺ and FSI^- near the highest negatively charged electrode surface, which may influence the subsequent SEI formation. Collectively, these molecular insights highlight the phosphonium-based polyILs for achieving enhanced ionic conductivity and motivate future interface design strategies for tuning interfacial performance.