Microstructure Evolution of Concentration Gradient Li[Ni0.75Co0.10Mn0.15]O2 Cathode for Lithium-Ion Batteries

Abstract

Detailed analysis of the microstructural changes during lithiation of a full-concentration-gradient (FCG) cathode with an average composition of Li[Ni0.75Co0.10Mn0.15]O2 is performed starting from its hydroxide precursor, FCG [Ni0.75Co0.10Mn0.15](OH)2 prior to lithiation. Transmission electron microscopy (TEM) reveals that a unique rod-shaped primary particle morphology and radial crystallographic texture are present in the prelithiation stage. In addition, TEM detected a two-phase structure consisting of MnOOH and Ni(OH)2, and crystallographic twins of MnOOH on the Mn-rich precursor surface. The formation of numerous twins is driven by the lattice mismatch between MnOOH and Ni(OH)2. Furthermore, the twins persist in the lithiated cathode; however, their density decrease with increasing lithiation temperature. Cation disordering, which influences cathode performance, is observed to continuously decrease with increasing lithiation temperature with a minimum observed at 790 °C. Consequently, lithiation at 790 °C (for 10 h) produced optimal discharge capacity and cycling stability. Above 790 °C, an increase in cation disordering and excessive coarsening of the primary particles lead to the deterioration of electrochemical properties. The twins in the FCG cathode precursor may promote the optimal primary particle morphology by retarding the random coalescence of primary particles during lithiation, effectively preserving both the morphology and crystallographic texture of the precursor.