LiBF₄-Derived Coating on LiCoO₂ for 4.5 V Operation of Li₆PS₅Cl-Based Solid-State Batteries

Solid-state batteries are attracting considerable attention for their high-energy density and improved safety over conventional lithium-ion batteries. Among solid-state electrolytes, sulfide-based options like Li₆PS₅Cl are especially promising due to their superior ionic conductivity. However, interfacial degradation between sulfide electrolytes and high-voltage cathodes, such as LiCoO₂, limits long-term performance. This study demonstrates that a LiBF₄-derived F-rich coating on LiCoO₂, applied by immersing LiCoO₂ particles in a LiBF₄ solution followed by annealing, can significantly enhance performance in Li₆PS₅Cl-based solid-state batteries. This coating enables stable high-voltage (4.5 V vs Li⁺/Li) operation, achieving an initial specific capacity of 153.82 mAh g⁻¹ and 87.1% capacity retention over 300 cycles at 0.5C. The enhanced performance stems from the F-rich coating, composed of multiple phases including LiF, CoF₂, Li_xBF_yO_z, and Li_xBO_y, which effectively suppresses side reactions at the LiCoO₂|Li₆PS₅Cl interface and improves lithium-ion diffusivity, thereby enabling greater Li capacity utilization. Our findings provide a practical pathway for advancing solid-state batteries with high-voltage LiCoO₂ cathodes, offering substantial promise for next-generation energy storage systems.