Alleviating Structure Collapse of Polycrystalline LiNi<sub><i>x</i></sub>Co<sub><i>y</i></sub>Mn<sub>1–<i>x</i>–<i>y</i></sub>O<sub>2</sub> via Surface Co Enrichment
Mingjie Shang, Hengyu Ren, Wenguang Zhao, Zijian Li, Jianjun Fang, Hui Chen, Wenguang Fan, Feng Pan, Qinghe Zhao
Abstract
The structure collapse issues have long restricted the application of polycrystalline LiNi x Co y Mn 1– x – y O 2 (NCM) at high voltages beyond 4.4 V vs Li/Li + . Herein, for LiNi 0.55 Co 0.12 Mn 0.33 O 2 (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.