LiPF₆ Induces Phosphorization of Garnet-Type Solid-State Electrolyte for Stable Lithium Metal Batteries

Garnet solid electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) is an excellent inorganic ceramic-type solid electrolyte; however, the presence of Li₂CO₃ impurities on its surface hinders Li-ion transport and increases the interface impedance. In contrast to traditional methods of mechanical polishing, acid corrosion, and high-temperature reduction for removing Li₂CO₃, herein, a straightforward “waste-to-treasure” strategy is proposed to transform Li₂CO₃ into Li₃PO₄ and LiF in LiPF₆ solution under 60 °C. It is found that the formation of Li₃PO₄ during LLZTO pretreatment facilitates rapid Li-ion transport and enhances ionic conductivity, and the LLZTO/PAN composite polymer electrolyte shows the highest Li-ion transference number of 0.63. Additionally, the dense LiF layer serves to safeguard the internal garnet solid electrolyte against solvent decomposition-induced chemical adsorption. Symmetric Li/Li cells assembled with treated LLZTO/PAN composite electrolyte exhibit a critical current density of 1.1 mA cm⁻² and a long lifespan of up to 700 h at a current density of 0.2 mA cm⁻². The Li/LiFePO₄ solid-state cells demonstrate stable cycling performances for 141 mAh g⁻¹ at 0.5 C, with capacity retention of 93.6% after 190 cycles. This work presents a novel approach to converting waste into valuable resources, offering the advantages of simple processes, and minimal side reactions.