Doped High Voltage Spinel for High Energy Cathode

Abstract

High voltage spinel LiNi0.5Mn1.5O4 (LNMO) is a promising cathode material due to its Co-free nature, high operating voltage and high ionic conductivity. However, the rapid capacity fading, especially at high temperature, limits its commercial success. The root causes of this limitation are the dissolution of transition metals of Ni/Mn in the electrolyte and unstable crystal structure. Doping with other elements is a prevalent strategy to effectively improve the electrochemical performance of LNMO cathode. In this study, precursors doped with Ca and Sr were synthesized through a co-precipitation process and the doped LNMO was achieved by high temperature sintering precursors with lithium sources. The doping level in final LNMO products was controlled through adjusting the ratio of elements in the precursor solutions. Analysis on X-ray diffraction (XRD) showed that Ca and Sr were successfully incorporated into the LNMO crystal structure. The Rietveld refinement was used to further reveal the doping sites. The obtained products were evaluated as cathodes of lithium-ion batteries (LIBs). Compared to baseline undoped LNMO, doped cathodes delivered slightly lower capacity, but exhibited improved durability at room temperature and high temperature of 55 °C. Post-mortem analysis revealed that integrity of crystal structure was well preserved in doped LNMO after numerous cycles. It is believed that the improved electrochemical performance is ascribed to the strong affinity of Ca2+ and Sr2+ to oxygen, which could effectively alleviate the dissolution of Ni/Mn and stabilize the crystal structure. The effect of doping level on the electrochemical performance of LNMO was also investigated.