Regulating Dynamic Electrochemical Interface of LiNi_0.5Mn_1.5O₄ Spinel Cathode for Realizing Simultaneous Mn and Ni Redox in Rechargeable Lithium Batteries

The exploding electric-vehicle market requires cost-effective high-energy materials for rechargeable lithium batteries. The manganese-rich spinel oxide LiNi_0.5Mn_1.5O₄ (LNMO) can store a capacity greater than 200 mAh g⁻¹ based on the multi-cation (Ni²⁺/Ni⁴⁺ and Mn³⁺/Mn⁴⁺) redox centers. However, its practical capacity is limited to Ni²⁺/Ni⁴⁺ redox (135 mAh g⁻¹) due to the poor reversibility of Mn³⁺/Mn⁴⁺ redox. This instability is generally attributed to the Jahn–Teller distortion of Mn³⁺ and its disproportionation, which leads to severe Mn dissolution. Herein, for the first time, the excellent reversibility of Mn³⁺/Mn⁴⁺ redox within 2.3–4.3 V is demonstrated, requiring revisiting the previous theory. LNMO loses capacity only within a wide voltage range of 2.3–4.9 V. It is revealed that a dynamic evolution of the electrochemical interface, for example, potential-driven rocksalt phase formation and decomposition, repeatedly occurs during cycling. The interfacial evolution induces electrolyte degradation and surface passivation, impeding the charge-transfer reactions. It is further demonstrated that stabilizing the interface by electrolyte modification extends the cycle life of LNMO while using the multi-cation redox, enabling 71.5% capacity retention of LNMO after 500 cycles. The unveiled dynamic oxide interface will propose a new guideline for developing Mn-rich cathodes by realizing the reversible Mn redox.