Mg,Ti-base surface integrated layer and bulk doping to suppress lattice oxygen evolution of Ni-rich cathode material at a high cut-off voltage

Abstract

The Nickel-rich layered cathode materials charged to 4.5 V can obtain a specific capacity of more than 200 mAh g−1. However, the nickel-rich layered cathode materials suffer from the severe capacity fade during high-voltage cycling, which is related to the phase transformation and the surface sides reactions caused by the lattice oxygen evolution. Here, the simultaneous construction of a Mg, Ti-based surface integrated layer and bulk doping through Mg, Ti surface treatment could suppress the lattice oxygen evolution of Ni-rich material at deep charging. More importantly, Mg and Ti are co-doped into the particles surface to form an Mg2TiO4 and Mg0.5–xTi2–y(PO4)3 outer layer with Mg and Ti vacancies. In the constructed surface integrated layer, the reverse electric field in the Mg2TiO4 effectively suppressed the outward migration of the lattice oxygen anions, while Mg0.5–xTi2–y(PO4)3 outer layer with high electronic conductivity and good lithium ion conductor could effectively maintained the stability of the reaction interface during high-voltage cycling. Meanwhile, bulk Mg and Ti co-doping can mitigate the migration of Ni ions in the bulk to keep the stability of transition metal–oxygen (M−O) bond at deep charging. As a result, the NCM@MTP cathode shows excellent long cycle stability at high-voltage charging, which keep high capacity retention of 89.3% and 84.3% at 1C after 200 and 100 cycles under room and elevated temperature of 25 and 55 °C, respectively. This work provides new insights for manipulating the surface chemistry of electrode materials to suppress the lattice oxygen evolution at high charging voltage.