Dual-Functional Additive for Solid Polymer Electrolytes for Enabling Highly Safe and Long-Life All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) are a promising alternative to carbonate-based liquid electrolytes for realizing flexible lithium batteries with high energy density and safety owing to their advantages such as lightweight, thinness, no leakage of electrolytes, excellent flexibility and processability, and good compatibility with Li-metal electrodes. However, SPEs present new challenges such as poor ionic conductivity and electrochemical stability as well as flammability, which are still a concern even though they are less flammable than liquid electrolytes. Herein, we demonstrate the dual functionalities of dimethyl methylphosphonate (DMMP) in facilitating Li ion migration and improving the flame retardancy of a poly(ethylene oxide) (PEO)-based polymer electrolyte. It acts as a plasticizer that aids the dissociation of Li salts and alleviates the binding energy between ethylene oxide (EO) groups and Li ions by counterbalancing the binding force between EOs and Li ions through the formation of binding interactions of DMMP molecules and Li ions. This significantly facilitates Li ion migration within the polymer electrolyte. Consequently, the prepared SPE exhibited improved ionic conductivity (1.29 × 10^–5 S cm^–1 at 25 °C), Li transference number (0.46), and oxidative stability (>4.3 V). The fabricated Li/Li symmetric cell maintained stable cycling performance over 500 cycles with low overpotential (41 mV) without short circuit. Importantly, the LiFePO₄(LFP)/Li battery exhibited a high discharge capacity of 134.1 mAh g^–1 with outstanding capacity retention of 95.4% after 400 cycles at 1C and excellent rate capability (123.3 mAh g^–1 at 2C). Furthermore, stable cycling was confirmed to be possible at an extended voltage range (2.5–4.1 V) and low operating temperature (45 °C). Moreover, DMMP effectively suppressed combustion of the polymer electrolyte owing to its strong flame retardancy arising from the propensity to capture active radicals.