Discovery of a surface protective layer: A new insight into countering capacity and voltage degradation for high-energy lithium-ion batteries

Highlights: • Li_1.14Mn_0.48Ni_0.19Co_0.19O₂ nanoplate exposed (001) is prepared. • A surface protective layer (SPL) is discovered at the outer edge of nanoplate. • The direction and order of TM ions migration during cycling have been demonstrated. • Capacity and voltage degradation of Li-rich layered cathodes is countered. Capacity and voltage degradation is a crucial factor that restricts the commercialization of Li-rich layered cathode materials. Recently, it has been demonstrated that the degradation results from the structural evolution from layered to spinel-like phase, caused by the migration of transition metal (TM) ions. However, the direction and order of TM ions migration is hard to identify. In this study, a surface protective layer (SPL) is discovered for the first time at the outer edge of Li_1.14Mn_0.48Ni_0.19Co_0.19O₂ nanoplate by the research on the surface condition of nanoplates after charge–discharge cycles. More importantly, by the detailed analysis for the SPL, we further reveal that the formation of SPL is active facet dependent, TM ions migrate toward the active facets, and Ni and Co ions hop more preferentially than Mn ions during the cycling. These discoveries can help to understand the fading mechanism of Li-rich layered cathode materials. In addition, electrochemical test indicates that the SPL greatly delays the corrosion of active surfaces in electrolyte and improves the cycling stability of Li-rich layered cathode materials. This insight provides a new thought for preparing long-life cathodes of high energy Li-ion batteries.