Interfacial Chemical Stability of Doped Li7-XLa3Zr2 -X(Nb/TaX)O12 with LiCoO2 Electrode Material

Abstract

Garnet-type oxide solid state electrolytes (SEs) are promising candidates due to their potential high ionic conductivity and chemical stability with Li for the replacement of flammable liquid electrolytes used in the current Li-ion batteries. Intense research has been focused on optimising the interface of garnets with Li, with the positive side to be somehow neglected. A significant issue of oxide SEs, hurdling their industrial application, is their poor contact with positive materials due to their rigidity. Poor interfacial contact leads to the need of co-sintering at high temperatures (> 400 °C), which enables side reactions and unwanted processes like cation intermixing, diffusion of elements and formation of non-conductive secondary mixed phases.In this work the chemical stability of Ta and Nb doped LLZO garnet SE was tested in contact with LiCoO2, a common positive material used in the battery industry. Initially, mixed powders of SE and active material were heat treated and studied using Raman spectroscopy and scanning electron microscopy to observe side reactions. In order to improve contact between the two materials, instead of already synthesized crystalline powder, nitrate precursors were also used for in-situ synthesis of LiCoO2 material in contact with the garnet. Mixtures of the SE powder and nitrate precursors for LiCoO2 were also heat-treated at various temperatures for different time periods in different gas atmospheres. The heat-treated materials were then analysed extensively using scanning electron microscopy coupled with energy dispersive X-Ray element mapping and with Raman microscopy to prove the existence or absence of secondary mixed phases at given conditions. Additionally, the optimized heat-treatment conditions were adapted for layered garnet SE/LiCoO2 samples to study the actual electrolyte/electrode interface at the same conditions used for powder mixtures. Based on the results the symmetrical LiCoO2/garnet SE/LiCoO2 cells will be prepared and tested electrochemically.