Journal of Materials Chemistry A, 6 (2018) 16111-16120.
Although lithium-mixed transition metal oxides (LiTM) have promising properties suitable for practical applications, unavoidable cation disorder in their structure during their synthesis or their operation leads to complex effects on their electrochemical performance. The microscopic mechanism of the cation disorder remains elusive owing to the lack of information on the atomic structures with specific chemical identities. In this study, Li-content-dependent cation disorder phenomenon near the grain boundary of LiNi0.5Co0.2Mn0.3O2 particles is uncovered by atom-resolved chemical and valence mapping techniques. LiTM with 1% excess Li (LiTM101) shows outstanding electrical conductivity at the grain boundary, whereas no enhancement in the electrical conductivity is manifested in LiTM with 7% excess Li. Remarkably, this superior property of LiTM101 is coupled to the combined cation disorder of Ni and Co in the Li layer with their increased valences, while the Mn ions in both samples are not labile to migrate. This work highlights the hitherto hidden role of highly oxidized Co ions in the Li layer as a key agent for enhancing the electrochemical performance, together with Ni ions acting as pillars to stabilize the layered structure, thus providing a new insight for engineering stable and durable cathode materials with high performance.