Alleviating Structure Collapse of Polycrystalline LiNiCoMn_1-O₂ via Surface Co Enrichment

The structure collapse issues have long restricted the application of polycrystalline LiNiCoMn_1-O₂ (NCM) at high voltages beyond 4.4 V vs Li/Li⁺. Herein, for LiNi_0.55Co_0.12Mn_0.33O₂ (P-NCM), rapid surface degradation is observed upon the first charge, along with serious particle fragmentation upon repeated cycles. To alleviate these issues, a surface Co enrichment strategy is proposed [i.e., Co-enriched NCM (C-NCM)], which promotes the in situ formation of a robust surface rock-salt (RS) layer upon charge, serving as a highly stable interface for effective Li⁺ migration. Benefiting from this stabilized surface RS layer, Li⁺ extraction occurs mainly through this surface RS layer, rather than along the grain boundaries (GBs), thus reducing the risk of GBs' cracking and even particle fragmentation upon cycles. Besides, O loss and TM (TM = Ni, Co, and Mn) dissolution are also effectively reduced with fewer side reactions. The C-NCM/graphite cell presents a highly reversible capacity of 205.1 mA h g^-1 at 0.2 C and a high capacity retention of 86% after 500 cycles at 1 C (1 C = 200 mA g^-1), which is among the best reported cell performances. This work provides a different path for alleviating particle fragmentation of NCM cathodes.