Alleviating Structure Collapse of Polycrystalline LiNiCoMnO via Surface Co Enrichment.

The structure collapse issues have long restricted the application of polycrystalline LiNiCoMnO (NCM) at high voltages beyond 4.4 V vs Li/Li. Herein, for LiNiCoMnO (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 at 0.2 C and a high capacity retention of 86% after 500 cycles at 1 C (1 C = 200 mA g), which is among the best reported cell performances. This work provides a different path for alleviating particle fragmentation of NCM cathodes.