Investigations of ligand hole of Ni-rich layered cathodes: a new organic coverage to battery performance enhancement

Abstract

Ni-rich (Ni content > 60%) layered cathodes are currently the most promising materials for lithium-ion batteries due to their high capacity and elevated voltage plateau compared to LiFePO4. However, Ni-rich cathodes face significant challenges, such as Jahn-Teller distortion, cation mixing, electrolyte deprotonation, and the presence of residual lithium compounds. These issues present barriers to the widespread use of high-energy-density lithium-ion batteries employing Ni-rich cathodes. Ni-rich cathodes, containing a high concentration of Ni3+, encounter another problem known as the oxygen ligand hole effect, which affects the hybridization of O 2p and Ni 3d orbitals. Anionic redox occurs at the oxygen site with Ni3+, leading to a decrease in electron density, making the formation of Ni4+ at high states of charge (SOC) difficult. Consequently, battery capacity is primarily derived from anionic redox reactions. This study presents an organic coverage (OC) designed to enhance the stability of the oxygen ligand hole, enabling greater capacity through enhanced Li+ interaction. Additionally, Ni-rich cathodes often suffer from gas evolution when charged to a high SOC, primarily due to the instability of the Ni-O bond. The OC is hypothesized to support the chemical reduction of Ni 2p53dn+2L back to Ni 2p63dn+1L, the L represents O 1s12px+1 → O 1s22px on the surface of NMC811, thereby strengthening the oxygen ligand hole and stabilizing covalent Ni3+. This improvement results in the OC-modified NMC811 exhibiting outstanding cycle performance under high-rate tests and excellent stability at high temperatures.