Regulating the internal structure by magnesium doping to enhance cycle stability of full-concentration-gradient Ni-rich layered cathodes

Ni-rich layered LiNixCoyMn1−x−yO2 (x ≥ 0.6) cathodes suffer from the extreme structural instability of their surface and interior owing to high Ni content. Although the structural design of full-concentration-gradient (FCG) better solved their surface instability, the deterioration of internal structure caused by the higher Ni content in the interior of particles became more prominent, and seriously limiting the cycle performance of FCG Ni-rich layered cathode materials. In this study, a strategy for the internal structure regulation by magnesium doping was proposed to overcome the internal structure instability of FCG Ni-rich layered LiNi0.80Co0.05Mn0.15O2 (FNCM) cathode materials. The experimental results and DFT calculations revealed that Mg2+ doping not only suppresses the layer structure collapse and H2–H3 phase transition but also improves the electronic conductivity by enhancing the free electron number near the Fermi level. The enhancement of the internal structure stability and dynamic migration process of ions and electrons contributed to improving cycle stability. As a result, the 0.02 mol% Mg2+ doped sample delivers an outstanding initial discharge specific capacity of 182.6 mAh g−1 at 1C with a satisfactory capacity retention of 90.7% after 200 cycles, which is higher than that of the original sample (capacity retention of only 81.3%). Most importantly, it still exhibits excellent capacity retention of 82.5% and 83.3% at a high cutoff voltage (4.5 V) and a high temperature (50 °C) after 200 cycles, respectively. These results indicated that magnesium doping is an attractive strategy to further improve the cycle stability of FCG Ni-rich cathode materials.