Doping of group IVB elements for nickel-rich cobalt-free cathodes

Hetero-element doping is a promising strategy to improve the cycling stability of nickel-rich cobalt-free cathodes for the next-generation high energy-density Li ion batteries. To make doping effective, it is important to understand the mechanism of how the dopants regulate the electronic band, lattice parameter adjusting, or hetero-phase formation to achieve high stability. In this study, we investigate LiNi0.9Mn0.1O2 cathodes doped with IVB grouping elements via multiple characterization techniques. By utilizing in situ XRD and TEM methods, we found that the stronger Ti–O bond effectively improves the cathode stability via a dual protection mechanism. Specifically, the bulk lattice of cathode is well-preserved during cycling as a result of the suppressed H2-H3 phase transition, while a in situ formed Ti-rich surface layer can prevent continuous surface degradation. As a result, the 5% Ti doped LiNi0.9Mn0.1O2 cathode exhibits a high capacity retention of 96% after 100 cycles. Whereas, despite IVB group elements Zr and Hf have stronger bonding energy with oxygen, their larger ionic radii actually impede their diffusion into the cathode, thereby they can not improve the cycling stability. Our findings uncover the functional origin of doped elements with their dynamic modification on cathode structure, providing mechanistic insights into the design of nickel-rich cobalt-free cathodes.