Composite Cathode Design for High-Energy All-Solid-State Lithium Batteries with Long Cycle Life

All-solid-state batteries (ASSBs) consisting of a 4 V class layered oxide cathode active material (CAM), an inorganic solid-state electrolyte (SE), and a lithium metal anode are considered the future of energy storage technologies. To date, aside from the known dendrite issues at the anode, cathode instabilities due to oxidative degradation of the SE and reactivities between the SE and CAM as well as loss of mechanical integrity are considered to be the most significant barriers in ASSB development. In the present study, we address these challenges by developing composite cathode structures featuring two key design elements: (1) a halide SE with high oxidative stability to enable direct use of an uncoated 4 V class CAM and (2) a single-crystal (SC) CAM to eliminate intergranular cracking associated with volume changes and mechanical instability. We demonstrate exceptional performance achieved on such ASSB cells incorporating an uncoated SC-LiNi_0.8Co_0.1Mn_0.1O₂ (NMC811) CAM, a Li₃YCl₆ (LYC) SE, and a Li–In alloy anode, delivering a high discharge capacity of 170 mAh/g at C/5 and an impressive capacity retention of nearly 90% after 1000 cycles. Through comparative studies on polycrystalline and single-crystal NMC811 composite cathodes, we reveal the working mechanism that enables such stable cycling in the latter cell design. The study highlights the importance of proper cathode composite design and provides key insights in the future development of better-performing ASSB cells.