Electrochemical and Structural Investigation on Ultrathin ALD ZnO and TiO2 Coated Lithium-Rich Layered Oxide Cathodes

Ultrathin coatings (1.5 +/- 0.3 nm) of titanium dioxide and zinc oxide were deposited on lithium-rich layered oxide cathodes (Li1.2Mn0.6Ni0.2O2, LLO) by atomic layer deposition (ALD). The structures, electrochemical performances, and thermal stabilities of these coatings were investigated. An ultrathin uniform coating was obtained for TiO2 but not for ZnO because of differences in the layer growth mechanism. Regarding the initial charge-discharge curves under a current density of 0.04 C rate, the TiO2 coated samples exhibited a higher discharge capacity, 242 mAhg(-1), compared with the ZnO coated samples, 220 mAhg(-1), or the pristine samples, 228 mAhg(-1). Both coated samples exhibited more stable cycling performance and thermal stability than the pristine samples. After 80 cycles under 0.5 C rate, the TiO2 and ZnO coated samples were found to have higher capacity retention (similar to 94% and 78%, respectively) than the pristine samples (68%). The reaction temperature of the exothermic peak of the TiO2 and ZnO coated samples at 4.8 V shifted to 280 degrees C with heat release of 88.7 J/g for TiO2 and 270 degrees C with heat release of 154.6 J/g for ZnO. This is compared with an exothermic peak at 258 degrees C with heat release of 253.5 J/g for the pristine sample. In particular, an enhanced rate capability was only observed for the TiO2 coated samples. When the current densities were higher than 2 C rate, the TiO2 coated samples exhibited superior capacities than the pristine and ZnO coated samples. At a current density of 5 and 10 C rate, the capacities were found to be 120 and 95 mAhg(-1). The improved electrochemical performances were mainly attributed to lower resistance of the charge transfer, which resulted from the layer morphology of the TiO2 film. This feature lead to more preactivation of LLO, smoother electron transport, and suppression of more side reactions, when compared with the island structure of the ZnO film.