Al2O3, SiO2 and TiO2 as Coatings for Safer LiNi0.8Co0.15Al0.05O2 Cathodes: Electrochemical Performance and Thermal Analysis by Accelerating Rate Calorimetry

Al2O3, SiO2 and TiO2 were coated on LiNi0.8Co0.15Al0.05O2 (NCA) in varying amounts and investigated regarding their influence on thermal stability and electrochemical performance. The presence of the coating on the surface was confirmed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Al2O3 showed agglomerated nano-scaled particles. SiO2 was coated homogeneously. TiO2 showed loose nano-scaled particles on the surface. The coated materials showed a decrease in capacity of ≈10 mAh g−1 with slightly higher cycling stability compared to the pristine NCA. Accelerating Rate Calorimetry (ARC) of the delithiated materials was conducted with 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) 3:7 (by weight) + 2 wt% vinylene carbonate (VC) + 0.5 wt% lithium bis(oxalate)borate (LiBOB). The TiO2 coating resulted in an increase of the onset temperature of thermal runaway (TdT/dt>0.2 K/min) of 5°C. Coatings with Al2O3 and SiO2 led to a significant increase of TdT/dt>0.2 K/min of up to 10 and 11°C, respectively. The coatings are able to lower the extent of reaction between the highly reactive oxygen evolving from the charged cathode at higher temperatures and the organic solvent-based electrolyte.