Dual‐site Cation Co‐doping Strategies Mitigating H2‐H3 Phase Transition for Cycle Stable Nickel‐rich Layered Oxide Cathodes in Lithium Batteries

Layered Ni‐rich transition metal oxide cathodes (LiNi 0.65 Mn 0.1 Co 0.25 O 2 ‐NMC) continue to dominate the market owing to the demand for high‐energy‐density lithium‐ion batteries. However, these cathodes suffer from rapid capacity and voltage decay during high voltage operation (≤4.5 V) due to unavoidable cation disorder (Li + /Ni 2+ ) and H2 → H3 phase transformation, inflicting structural instability. Herein, dual‐site cation doping using Mg 2+ in Li 3 a sites and Zr 4+ in transition metal‐T M (preferably Ni sites) 3 b sites is shown to preserve structure at high voltage, improving capacity retention and voltage fade. The Rietveld refinement of diffraction data shows that cation disorder reduced from ≈11% (NMC) to ≈2.9% (ZrMg‐NMC) with a low defect formation energy (Δ E ) of NMC (≈202.33 meV), favoring the formation of antisite defects. Cyclic voltammogram confirms that codoping prevents the H2 → H3 transformation at high voltage (≤4.5 V), reflected in better capacity retention (ZrMg‐NMC: ≈85%, vs. NMC: ≈51% at 200 cycles during cycling at 0.1C) and sluggish average voltage fading rate (ZrMg‐NMC: ≈0.12 mV/cycle, vs. NMC: ≈0.22 mV/cycle). The remarkable performance improvement by Zr–Mg codoping in NMC stems from preventing oxygen migration to the surface, dissuading rock salt phase formation, and stronger NiO and NiT M bond lengths, immensely helping to retain structural stability during charging up to 4.5 V.