Mixed ionic-electronic conductivity of high-nickel, single-crystal cathodes influencing the cycling stability of all-solid-state lithium-ion batteries

Abstract

High-nickel, cobalt-free layered oxides are emerging as promising cathode materials for traditional lithium-ion batteries (LIBs), but their application in all-solid-state batteries (ASSBs) remains largely unexplored. This study benchmarks the electrochemical properties of single-crystal LiNi_0.8Co_0.2O₂ (SC-NC80) and cobalt-free LiNi_0.8Mn_0.2O₂ (SC-NM80) in both conventional LIBs with liquid electrolytes and in ASSBs. While SC-NM80 displays inferior electronic conductivity and lithium diffusion kinetics, its cycling stability in conventional LIBs is improved relative to SC-NC80, owing to the stability provided by redox-inactive Mn⁴⁺ in the layered structure. Despite such benefits, the kinetic limitations of SC-NM80 become problematic for the cycling stability of ASSBs with a poor electronic percolating network in the cathode composite. Conductivity measurements demonstrate the negative impact of reduced cathode mixed ionic-electronic conductivity on the effective transport properties within the cathode composite for ASSBs. Insufficient electronic conductivity in the cathode composite induces lithiation inhomogeneity at a moderate C/3 rate, leading to severe active material (AM) loss at low states-of-charge (SOC). This study reveals the intrinsic challenges of incorporating Co-free cathodes in ASSBs and underscores the necessity for strategic electrode engineering to enhance the effective electronic conductivity of high-Ni, Co-free cathode composites.