Revisiting Cationic Doping Impacts in Ni-Rich Cathodes

As a promising cathode material for high-energydensity Li-ion batteries, Ni-rich layered oxide cathode active chemical performance degrades rapidly upon charge/discharge cycles probably due to electrochemical/thermochemical instabilities. While cationic doping in the transition-metal site has been regarded as an effective strategy to enhance the electrochemical performance, the true impact of cation doping is not well understood. To quantitatively assess the impact of cationic doping, in this work, the electrochemical performance and lattice oxygen stability of LiNi0.82Co0.18O2, isovalent Al3+-doped LiNi0.82Co0.15Al0.03O2, and high-valent Ti4+-doped LiNi0.82Co0.15Ti0.03O2 were investigated. Despite significant improvements in electrochemical performance by Al3+ and Ti4+ doping, it was revealed that these cation dopings had no discernible effect on the lattice oxygen stability. Such information suggests that the electrochemical enhancement by Al3+/Ti4+ doping is not attributed to the stabilization of lattice oxygen. This work highlights the importance of independent and quantitative experimental evaluations on kinetic electrochemical properties and thermodynamic stability of lattice oxygen to establish rational guidelines for doping strategy toward high-energy-density and reliable cathode-active materials.