Time resolved x-ray diffraction study of the thermal degradation of cathode materials used in Li-ion batteries

We have used both high-resolution and time-resolved XRD to monitor structural changes in charged Li _1-xNi _0.8Co _0.15Al _0.05O ₂ cathodes as a function of temperature. Studies were done on charged electrodes that were washed and dried and on electrodes in the presence of excess electrolyte. High-resolution XRD indicates that decomposition reactions can occur at temperatures as low as 130°C in charged cathodes, in the presence of electrolyte, if the lithium content is below Li _0.5. With or without electrolyte charged cathodes first convert from a layered structure to a disordered spinel structure, with the temperature of conversion decreasing with increased state of charge (SOC). In the presence of excess electrolyte the spinel converts, at higher temperatures, to a rock salt structure with the temperature of conversion remaining relatively constant, at about 300°C, from high SOC to low SOC. In the case of the washed dry samples the conversion to the disordered spinel structure is similar. However the conversion of the spinel to the rock salt structure occurs gradually above 300°C. In this case the conversion of the spinel to the rock salt structure probably occurs by loss of oxygen. In the presence of electrolyte the solvent acts as a reducing agent, hence the first order phase transformation. Overcharge reduces the conversion temperature for both the layered to spinel and the spinel to rocksalt phase reactions, in the presence of electrolyte. Decomposition reactions for the overcharged cathode material to the spinel can be seen at temperatures as low as 140°C. The spinel converts to the rock salt structure at 250°C.