Tailoring the room-temperature miscibility gap in ordered spinel LiNi0.5Mn1.5O4 cathodes by multi-element doping†

In the pursuit of improved lithium-ion battery performance, the ordered spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathode material stands out as a promising candidate because of its 5 V level operating voltage. However, problems of capacity degradation due to both leaching of Mn²⁺via a side reaction with electrolyte and a large strain formed inside the particle by the two-phase reaction model hinder its practical application. This study investigates the effect of multiple element doping (Si, Ti, and Ge) on charge/discharge reaction mechanisms of LNMO cathodes to mitigate the capacity degradation, caused by the large strain. Structural and electrochemical changes induced by doping were analyzed coupled with operando XRD, ex situ XAFS, and theoretical calculations with a universal neural network potential. The results demonstrate that multi-element doping enhances structural stability, tailored charge/discharge reaction mechanisms, and improves cyclability. The doped LNMO cathode exhibited an altered charge–discharge voltage profile showing the deviation of the capacity–voltage curve from the constant potential, suggesting a shift in the reaction mechanism toward a solid solution model. The corresponding solid solution ranges outside the miscibility gap (Li_1.0NMO/Li_0.5NMO and Li_0.5NMO/Li_0.0NMO) due to multiple-element doping were found to significantly reduce strain formation, leading to enhanced performance. Overall, this study provides a comprehensive understanding of the effects of multi-element doping on LNMO cathodes and highlights the potential of this strategy to address the limitations of current lithium-ion battery technology.