Comparative Investigation of Three Mg Doping Strategies in NMC811 Cathodes for High‐Energy Density Lithium‐Ion Batteries

Ni‐rich layered oxides, such as LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811), offer high capacity but suffer from structural degradation and rapid fading under extended cycling. Herein, Mg doping as a structural stabilization strategy, directly comparing incorporation via precursor coprecipitation, solid‐state lithiation, and a novel two‐step route is introduced. All Mg doped NMC811 compositions show reduced cation mixing and enhanced cycling stability, with 0.1% Mg yielding the most favorable lattice evolution as revealed by operando X‐ray diffraction. For the first time, operando electrochemical dilatometry is utilized to investigate the doped NMC811, revealing direct correlations between electrode volume changes and electrochemical behaviour. While Mg primarily suppresses structural degradation, the solid‐state lithiation route delivers the greatest performance gains, outperforming both coprecipitation and the combined approach. This work demonstrates that targeted Mg incorporation can noticeably extend the lifetime of Ni‐rich cathodes and establishes operando dilatometry as a powerful tool for linking atomic‐scale stabilization strategies with macroscopic electrode mechanics.