Compatibility Study of LiCoO₂ and LiCoPO₄ Cathode Materials with NASICON-Type Solid Electrolyte LATP

All-solid-state lithium-ion batteries (ASSLIBs), particularly those employing oxide-based solid electrolytes, are expected to be the next generation of rechargeable batteries because they guarantee safety and dependability by replacing highly combustible organic electrolyte solutions with solid electrolytes. In contrast to conventional liquid LIBs, in which electrolyte/electrode interfaces can be formed spontaneously by injecting liquid electrolyte into cells, the formation of a well-defined solid electrolyte/electrode interfaces with a high ionic conductivity is a key challenge for the development of ASSLIBs.<br/>The conventional method for obtaining the oxide-based solid electrolyte/electrode material interfaces is high-temperature sintering, as these materials have a high melting point and exhibits poor plasticity. However, co-sintering of solid electrolytes and electrode materials often leads to undesired side reactions. Thus, understanding the reaction between solid electrolyte and electrode materials is especially important not only for fundamental science but also for a possible synthetic route to form a well-defined interface with a high ionic conductivity.<br/>In this study, we performed the compatibility assessment of a highly ion-conducting solid electrolyte, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) with a high-capacity cathode material, LiCoO₂ (LCO) and a high-voltage cathode material, LiCoPO₄ (LCP) based on thermodynamic calculations. The calculation results showed that co-sintering LATP and LCO react with each other to form completely different chemical species instead of forming a well-defined interface. On the other hand, co-sintering LATP and LCP does not cause sever reaction. Thus, we experimentally co-sintered the composite of LATP/LCO and LATP/LCP as combination that reacts intensely and a combination that maintains their crystalline structure and characterized their property.<br/>In the intensely reactive combination LATP/LCO, various mixing ratios of LATP and LCO were sintered at various temperatures, and the crystalline phases formed by the reaction were identified and quantified by Rietveld analysis of X-ray diffraction (XRD) and phases including amorphous phase by linear combination fitting of X-ray absorption fine structure (XAFS). The results showed that regardless of the mixing ratio, LATP and LCO react at 300°C, which is relatively low for sintering temperatures, to form Co₃O₄, Li₃PO₄ and amorphous-TiO₂. Further increasing the sintering temperature, it was found that the chemical species formed by the reaction depended on the mixing ratio of LATP and LCO. In addition, we used thermodynamic calculations to calculate the chemical species produced at each temperature and mixing ratio and compared these with the experimentally observed chemical species. For sintering temperatures between 900°C and 700°C, the chemical species observed experimentally and those predicted to be formed by thermodynamic calculations were in reasonable agreement. On the other hand, for sintering temperatures below 600°C, there was a significant disagreement between the calculation and experimental results.<br/>In the maintaining their crystalline structure combination LATP/LCP, we co-sintered the mixture of LCP and LATP at 800°C and characterized the structure of their bulk and interfaces by XRD, XAFS and microscopic techniques. XRD showed that LATP and LCP maintained their crystalline structures after sintering. XAFS results for Ti and Co K-edges showed no change in chemical state around Ti before and after sintering, whereas a slight change was observed for Co, suggesting the small amount of LCP was decomposed to form Co-containing reaction products due to sintering. The microscopic measurements clearly imaged the formation of Co-containing amorphous layers at the grain boundaries within LATP/LCP composites. XAFS and electron energy loss spectroscopy analyses identified the major species of Co-containing amorphous layers to be CoO and Li₃PO_4.