An Evaluation of a Systematic Series of Cobalt-Free Ni-Rich Core-Shell Materials as Positive Electrode Materials for Li-Ion Batteries

Abstract

A wide range of Co-free Ni-rich core–shell precursors with different core/shell compositions, core sizes and shell thicknesses heated at various temperatures were evaluated in this work. In all cases, the shell phase had a larger Mn content than the core phase. The heating temperature of the precursor/lithium hydroxide mixture during synthesis must be carefully selected. Core–shell materials made at insufficient temperature have low specific capacity due to low crystallinity and a high percentage of Ni atoms in the Li layer. Materials made at excessive temperature show interdiffusion between the shell and core compositions leading to poor capacity retention and poor safety performance. When the heating temperature is selected appropriately, materials which retain the core–shell structure after synthesis can be made. Two of these materials show excellent specific capacity, capacity retention and safety performance. Accelerating rate calorimetry experiments show that charged core–shell materials with a thick and Mn-rich shell have the least reactivity with electrolyte at elevated temperatures, suggesting such materials will lead to the safest Li-ion cells.