Co-sintering Reaction Analysis of LiCoO2 Cathodes and NASICON-Type LATP Solid Electrolytes Studied by Experimental and Computational Methods

LiCoO2 (LCO), a practical cathode material for Li-ion batteries, and Li1.3Al0.3Ti1.7(PO4)3 (LATP), an oxide-based solid electrolyte, were mixed at different ratios and cosintered at various temperatures. The reaction products in the crystalline and amorphous phases formed during sintering were quantitatively analyzed by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES). LCO and LATP reacted with each other to form Co3O4, Li3PO4, and amorphous-TiO2 in either mixing ratios when sintering at relatively low temperatures (<500 °C). At the sintering temperatures between 500 and 900 °C, LiCoPO4, CoTiO3, and rutile-TiO2 were formed for LCO and LATP at a volume ratio of 3:7; Co2TiO4, CoTiO3, and Co3O4 at a volume ratio of 5:5; and Li2TiO3, Co2TiO4, and Co3O4 at a volume ratio of 7:3. We conducted thermodynamic calculations of the same system at various temperatures under an oxygen partial pressure of 0.21 atm to compare with actual reaction products determined by XRD and XANES. At 900 °C, the experimentally observed actual reaction products were reasonably reproduced by the thermodynamic calculations except for a few discrepancies. The discrepancies between the identified reaction products and the predicted thermodynamically stable phases were attributed to (1) the formation of alternative phases that have Gibbs free energies comparable to the predicted most stable phases and (2) the occurrence of phase transformation of high temperature phases during the cooling process to perform product analysis at room temperature. Meanwhile, as the temperature became lower from 900 °C, discrepancies between experimental results and calculations became more prominent, so that the starting materials and reaction products increased and decreased, respectively, probably due to the slow kinetic process such as mutual diffusion as evidenced by elongating the sintering time and using smaller grain sizes.