Compatibility study of oxide and olivine cathode materials with lithium aluminum titanium phosphate

Abstract

The compatibility of the solid electrolyte Li1.5Al0.5Ti1.5(PO4)3 (LATP) with the cathode materials LiCoO2, LiMn2O4, LiCoPO4, LiFePO4, and LiMn0.5Fe0.5PO4 was investigated in a co-sintering study. Mixtures of LATP and the different cathode materials were sintered at various temperatures and subsequently analyzed by thermal analysis, X-ray diffraction, and electron microscopy. Oxide cathode materials display a rapid decomposition reaction with the electrolyte material even at temperatures as low as 500 °C, while olivine cathode materials are much more stable. The oxide cathode materials tend to decompose to lithium-free compounds, leaving lithium to form Li3PO4 and other metal phosphates. In contrast, the olivine cathode materials decompose to mixed phosphates, which can, in part, still be electrochemically active. Among the olivine cathode materials, LiFePO4 demonstrated the most promising results. No secondary phases were detected by X-ray diffraction after sintering a LATP/LiFePO4 mixture at temperatures as high as 700 °C. Electron microscopy revealed a small secondary phase probably consisting of Li2FeTi(PO4)3, which is ionically conductive and should be electrochemically active as well.