Sacrificial Additives Stabilize Interfaces for Durable 5 V LiCoO2 Operation in Zr-Based Halide Solid-State Batteries

High-voltage all-solid-state batteries (ASSBs) promise high-energy-density storage, but oxidative instability of solid-state electrolytes at cathode interfaces remains a major obstacle. Here, we report a sacrificial-additive strategy for durable, high-voltage operation of low-cost Zr-based halide ASSBs. Introducing LiPO2F2 (LPOF) into Li2ZrCl4O (LZCO) markedly enhances interfacial and oxidative stability. Paired with uncoated LiCoO2, the ASSBs deliver over 80% capacity after 700 cycles at 2 C (4.6 V cutoff) and cycle stably under ultrahigh cathode loadings of 35.7 mg cm-2. Notably, the cells sustain 5 V operation with over 80% capacity after 380 cycles at 2 C and over 70% retention after 3500 cycles at 7 C. Experimental characterizations reveal that LPOF preferentially forms a LiF- and Li3PO4-rich layer that stabilizes the high-voltage cathode/electrolyte interface, which is further supported by theoretical calculations. This work establishes sacrificial-additive engineering as an effective route toward durable high-voltage halide-based ASSBs.