Solvent-Free Single-Ion Conducting Polymer Electrolytes for Lithium Metal Batteries under Harsh Environments

Rechargeable lithium-ion batteries are widely used in consumer applications. However, these batteries are limited in their ability to function under extreme thermal environments, such as high temperatures, which are required for medical devices requiring sterilization and various industries like subsurface exploration and thermal reactors. However, the conventional lithium-ion batteries have limited high-temperature performance due to material properties that lead to safety concerns or low power usage. Many efforts have focused on developing solvent-free single-ion conducting (SIC) polymer electrolytes to solve these issues by covalently immobilizing anionic groups onto a polymer backbone, which only allows Li+ cations to be mobile through the polymer matrix, leading to minimal dendrite growth during charging/discharging process. While the SIC polymer electrolytes hold much promise for next-generation batteries possessing high safety and high energy density, this approach typically yields a significant drop of ionic conductivity (<10⁻⁵ S cm⁻¹ at ambient temperature), compared to liquid electrolytes. Quasi-solid or gel polymer electrolytes incorporating organic plasticizers have been employed to enhance the ionic conductivities, but their practical implementation is still constrained by their vulnerability to thermal runaway under high-temperature circumstances. Herein, we present a novel approach for the one-step synthesis of a solvent-free single-ion perfluorinated-tetraphenylborate-anions membrane via click reaction. The resulting flexible membrane displays exceptional cyclability performance at high temperatures, high lithium selectivity (~0.932), and a wide working window (up to 5 V). The one-step synthesis of the membrane exhibits a high ionic conductivity (~3 *10^-5 S cm^-1) at 88°C due to its consistent structure. Notably, the membrane demonstrates superior non-flammability properties. Furthermore, the membrane can operate under large temperature ranges from 60 to 120°C, even under negative pressure, making it suitable for a variety of applications. Remarkably, stable long-term cycling of LiFePO₄ cathodes can be achieved at 100°C with a coulombic efficiency of approximately 100% over 300 cycles at 0.5C.