Insight into the interplay of the additive and different calcination process for the cobalt-free high voltage spinel LiNi_0.5Mn_1.5O₄ cathode

Solid-state reaction heterogeneity during the calcination process of cathode materials can significantly influence the physicochemical and electrochemical properties of the cathode material, highlighting the importance of carefully controlling the calcination parameters. In this study, by using the high-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathode as an example, the effect of NH₄H₂PO₄ additive with two different synthesis processes was systematically examined. It is demonstrated that the LNMO with NH₄H₂PO₄ additive in the 2nd calcination process (LNMO-1S2P) exhibits the most stable full cell cycling performances compared to the LNMO-1P, which has 84 % capacity retention after 300 cycles at 25 ℃ and 80 % after 50 cycles at 45 ℃. With various ex-situ characterizations such as XAFS, solid-state NMR, neutron powder diffraction, and in-situ XRD during charging, it is revealed that the LNMO-1S2P cathode formed a robust cathode electrolyte interface (CEI) by surface P element enrichment, moreover with a smoothed high voltage phase transformation, therefore significantly inhibited the lattice change and reduced the Mn ion dissolution during the cycling. In conclusion, this work reveals the interplay between the additive and the calcination process, offering new insights for the rational synthesis of transition metal oxide cathode materials.