Flexible and Scalable Magnesium Replenishment in NCM Cathode Enabled by Mobile Mg ²⁺ Enriched MgV ₂ O ₄

A small amount of Mg²⁺ doping can show a significant effect on the surface protection and structural stability of the nickel-rich layered oxide cathode, but the traditional doping process involves completely changing the initial raw material proportions with subsequent trial and error adjustments. Herein, a concept of Mg²⁺ release film is proposed, in which Mg²⁺ can easily permeate into various layered oxide cathodes during cycling. Meanwhile, to realize this concept, MgV₂O₄ with mobile Mg²⁺ in the structures and then fabricated a self-supporting MgV₂O₄ membrane are synthesized. As a protective layer for cathode, the MgV₂O₄ membrane release Mg²⁺ in situ during the electrochemical process, providing structural reinforcement to the cathode surface as a "pillar" within the lattice. Thanks to the MgV₂O₄ membrane, the cycle life of LiNi_o.8Co_0.1Mn_0.1O₂(NCM811) coupled with the MgV₂O₄ interlayer at 1.0 C is increased by 1.9 times compared to bare NCM811. Furthermore, this novel Mg²⁺ releasing film demonstrates excellent versatility, enabling other nickel-based layered oxide to achieve a high-capacity retention of 86.4% after 800 cycles at 1.0 C. This approach provides scalable cathode protection and repair strategies for commercially viable batteries.