Improved electrochemical performance of Nb5+ doped NCM cathode upon 4.5 V application beneficial from the suppressed H2-H3 phase transition and promoted Li+ migration kinetics

Abstract

As a commercial cathode material, Li(Ni,Co,Mn)O2 cathode (NCM) suffers from rapid performance degradation due to structural instability. In this paper, Nb5+ is introduced to NCM layered structure upon the application with a cut-off voltage up to 4.5 V. After the doping of Nb5+, XRD investigation suggests that the average bond length of LiO increases while that of TM-O reduces, due to the changes in charge density around the transition metal cations as confirmed from DFT calculation. The shortened TM-O bond length enhances stability of the lattice that suppresses H2-H3 phase conversion so the capacity retention is promoted. On the other hand, GITT investigation indicates that the increased LiO bond length offers enhanced diffusion coefficient which accelerates the migration of the Li cations and therefore improves the rate behavior. As a whole, the incorporation of Nb5+ leads to reversible cycling of NCM cathode upon 4.5 V, with specific capacity of 205 mA h g−1 at 0.1C, capacity retention of 89.12 % after 100 cycles and rate capacity of 158 mA h g−1 at 5C. Such performance exceeds those of other NCM cathode materials and suggests high potential of Nb5+ doped NCM cathode for application in high energy batteries.