Dual Lithium Salt Derived Favorable Interface Layer Enables High-Performance Polycarbonate-Based Composite Electrolytes for Stable and Safe Solid Lithium Metal Batteries

Developing solid electrolytes with high ionic conductivity, a high voltage window, low flammability, and excellent interface compatibilities with both the anode and cathode for lithium-metal batteries is still a great challenge but highly desirable. Herein, we achieve this target through an in situ copolymerization of vinyl ethylene carbonate (VEC) together with acrylonitrile (AN) under fitting ratios inside a porous polyacrylonitrile (PAN) fiber membrane doped with flame-retardant decabromodiphenyl ethane (DBDPE) molecules. The received fiber-reinforced polycarbonate-based composite electrolyte with an ultrathin thickness of 13 μm exhibits good internal interfacial compatibility because of the same AN structure and superior flame-retardant performance due to the doped DBDPE molecules. The dual lithium salt strategy facilitates the formation of a stable interface layer on the lithium anode surface. The electrolyte also exhibits high thermal stability, a high ion transference number (0.75), a wide voltage window (up to 4.9 V), and high ionic conductivity (6.3 × 10^-4 S cm^-1 at room temperature). As a result, its Li//Li symmetric cells could maintain stable cycling for over 2500 h, and its LiFePO₄//Li full cells also exhibit an initial capacity of 135 mAh g^-1 and a retention of 82.9% after 300 cycles at 2 C and 25 °C. In addition, the full cells with either high voltage or high loading cathodes both deliver excellent rate performance and cycling stability. All these results confirm its superior properties, excellent stability, and high safety for potential application in high-energy-density solid lithium metal batteries.