Difference in initial capacity loss between single-crystal and polycrystalline Ni-rich layered cathodes

Abstract

Ni-rich layered cathodes are promising candidates for high-energy-density lithium-ion batteries (LIBs). However, they experience substantial initial capacity loss (ICL) of 10–20 % during the first cycle. Single-crystal materials particularly exhibit even greater ICL than polycrystalline materials. This increased ICL poses significant challenges as it directly reduces overall capacity and efficiency. Despite its importance, the pronounced ICL in single-crystal Ni-rich cathodes remains underexplored, as most studies have focused on polycrystalline materials. Here, we elucidate the difference in ICL between single-crystal LiNi_0·90Co_0·08Al_0·02O₂ (S-NCA) and polycrystalline LiNi_0·90Co_0·08Al_0·02O₂ (P-NCA). Electrochemical analyses reveal that S-NCA exhibits lower discharge capacity due to the absence of a kinetic plateau near the 3.5 V region. Furthermore, it presents higher proportions of both recoverable and irrecoverable ICL. X-ray analyses further demonstrate that S-NCA contains more residual lithium compounds and NiO-like rock-salt phases on its surface. It also has longer Li⁺ diffusion pathways due to its larger particle size. These features hinder lithium insertion and increase recoverable ICL. Additionally, greater cation mixing in the bulk of S-NCA induces irreversible structural changes, contributing to both irrecoverable and recoverable ICL. This comprehensive understanding of mechanisms underlying the intensified ICL in S-NCA provides valuable insights for designing high-capacity, stable Ni-rich cathodes.