Surface Structure Evolution and its Impact on the Electrochemical Performances of Aqueous-Processed High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathodes in Lithium-Ion Batteries

LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode in lithium-ion batteries (LIBs) due to its high operating voltage and open Li⁺ diffusion framework. However, the instability of the electrode–electrolyte interface and the negative environmental impact of electrode fabrication processes limit its practical application. Therefore, switching electrode processing conditions to aqueous and understanding the accompanying surface structural evolution are imperative. Here, water-treated, poly(acrylic acid) (PAA)-treated, and H₃PO₄-treated LNMO, labeled as W-LNMO, A-LNMO, and H-LNMO, are studied systematically. W-LNMO shows a high concentration of Mn³⁺ induced by Li loss while a conformal PAA layer formed on A-LNMO reduces this phenomenon. H-LNMO displays a second MnPO₄∙H₂O phase. Upon cycling, a fast capacity decay is observed in W-LNMO while an extra plateau at ≈2.7 V appears in the initial charging, corresponding to a two-phase transition. A surface reconstruction layer from a spinel to a rock-salt phase with a reductive Mn²⁺ segregation is observed in W-LNMO after 105 cycles. The PAA layer persists on A-LNMO and alleviates the capacity decay. H-LNMO delivers a relatively low capacity due to the formation of a MnPO₄∙H₂O phase. This study provides new insights into manipulating the surface chemistry of LNMO cathodes to enable aqueous, large-scale processingin LIBs.