Scalable Slurry-Casting Fabrication of Ultrathin, Flexible, and High-Voltage Halide-based Composite Solid-State Electrolytes for Lithium Metal Batteries

Flexible composite polymer electrolytes with high ionic conductivity, high voltage, and small thickness are critical for achieving scalable fabrication of high-energy-density solid-state lithium metal batteries (SSLMBs). Owing to the intrinsically lower density (2.5–3.0 g cm⁻³) than that of oxides (>4.0 g cm⁻³), high ionic conductivity (∼10⁻³ S cm⁻¹), high modulus, and high voltage, halides can be used as effective functional Li-ion-conductive fillers to construct thin, lightweight, and high-performance composite polymer electrolytes while achieving high-energy-density of SSLMBs. Nevertheless, the chemical vulnerability of halide solid electrolyte materials to common polar solvents restricts the scalable slurry-casting fabrication of halide-based composite polymer electrolytes for practical SSLMBs. To this end, a bi-functional low-polarity solvent, dimethyl carbonate, is screened to render halides, which are usually slurry-incompatible, amenable to scalable slurry fabrication. As a result, an ultrathin (10 µm) and flexible halide-incorporated composite electrolyte with a high electrochemical window up to 4.8 V vs. Li⁺/Li, high thermal stability, and desirable self-extinguishing ability is developed. Benefiting from the multiple Li-ion transport mechanisms enabled by the interaction between fillers, salts, and polymers, the obtained composite polymer electrolyte can achieve a high ionic conductivity of 0.325 mS cm^–1 at 25 °C. The assembled solid-state Li|LiFePO₄ cell based on the halide-based composite electrolyte achieves a high capacity of 153 mAh g⁻¹ at 0.2 C with a capacity retention of 98% after 175 cycles, and the Li|LiNi_0.6Co_0.2Mn_0.2O₂ cell can stably cycle at a cut-off voltage of 4.3 V and achieve a high capacity of 160 mAh g⁻¹ at 0.2 C with a capacity retention of 89% after 170 cycles. This work provides an effective strategy for large-scale manufacturing of ultrathin and flexible halide-based composite electrolytes for high-performance SSLMBs.