Understanding improved stability of Co-free Ni-rich single crystal cathode materials by combined bulk and surface modifications

Co-free Ni-rich single crystal LiNixMn1-xO2 (x ≥ 0.6) (denoted as SCNM) have been actively studied as a candidate cathode for high energy density and low-cost lithium-ion batteries (LIBs). However, their practical use in LIBs is significantly hindered by their poor chemo-mechanical stability and short cycle life. Herein, we show much improved structural stability and cycle life by bulk and surface modifications of SCNM. We show strong evidence that bulk Ti-doping and gradient Nb-doping are effective in stabilizing lattice oxygen and suppressing detrimental phase transformations, while LiTi2-xNbx(PO4)3 (LTNP) surface coating protects SCNM from reacting with liquid electrolyte and promotes formation of thin and robust interphase layers. As a result, LTNP-modified SCNM achieves an excellent electrochemical performance with 87.2% capacity retention after 100 cycles at high voltage of 4.4 V. This work demonstrates a new approach to stabilize lattice oxygen of Co-free and Ni-rich cathodes and their interfaces with liquid electrolytes, thus contributing to the development of high-energy–density LIBs.