Secondary Li-ion batteries outperform all other battery chemistries currently on the market with respect to energy density per weight and volume. As logical consequence Li-ion batteries dominate areas such as portable electronics, power tools and electric mobility. For the lattermost in particular, there is a difficult compromise between energy density and safety when using Li-ion chemistries, and safer high-performance chemistries are keenly sought. For electric vehicle (EV) applications, power and range requirements mean that the most common positive electrode is based on the LiNi_xMn_yCo_zO₂(NMC) system. Within this system increasing the Ni content results in higher capacity values, but to the detriment of the capacity retention and the safety rating. One way to mitigate cycle life and safety issues is to apply a thin protective coating onto the active material surface. Coatings of alumina applied by atomic layer deposition (ALD) have been shown to improve the cycle life of high Ni NMC cathodes [1-3], but ALD is not a technique which will scale easily and cheaply to the production volumes needed for battery manufacturing.

Here we present a comparative study exploring the possibility of ultra-thin alumina and silica coatings on NMC 811 using wet chemical methods. NMC 811 was synthesised by a citric acid-aided gelling method with subsequent annealing. Alumina coatings were processed by a solution drying method employing NMC 811 particles dispersed in an aqueous Al(NO₃)₃ solution. The solution was sonicated and evaporated under vigorous stirring. Subsequently, the powder was heat-treated to form the desired alumina coating. The temperature and dwelling time were controlled so as to mitigate extensive Al diffusion into the layered structure and the formation of a LiAlO₂ surface layer. [4] The silica coatings were processed by a solution drying process employing either a tetraethyl orthosilicate solution, or an aqueous solution with the addition of functionalised silanes. The powder/solution mixture was treated and dried in the same manner as for the alumina coating. After the solution was dried, the powders were heat treated to form an ultra-thin silica coating. The electrochemical performance and cycling stability of the coated NMC 811 materials are compared to the as-synthesised material, and morphology and chemical composition of the coatings and interface were analysed by electron microscopy and electron energy loss spectroscopy. Finally, the thermal stability of the charged (delithiated) material was determined by differential scanning calometry.

1. Mohanty, D., et al., Modification of Ni-Rich FCG NMC and NCA Cathodes by Atomic Layer Deposition: Preventing Surface Phase Transitions for High-Voltage Lithium-Ion Batteries. Sci Rep, 2016. 6: p. 26532.
2. Laskar, M.R., et al., Atomic Layer Deposition of Al2O3-Ga2O3 Alloy Coatings for Li[Ni0.5Mn0.3Co0.2]O2 Cathode to Improve Rate Performance in Li-Ion Battery. ACS Appl Mater Interfaces, 2016. 8(16): p. 10572-80.
3. Wise, A.M., et al., Effect of Al2O3 Coating on Stabilizing LiNi0.4Mn0.4Co0.2O2 Cathodes. Chemistry of Materials, 2015. 27(17): p. 6146-6154.
4. Han, B., et al., From Coating to Dopant: How the Transition Metal Composition Affects Alumina Coatings on Ni-Rich Cathodes. ACS Appl Mater Interfaces, 2017. 9(47): p. 41291-41302.