Surface coating

Size-tailored NCM622 cathode material was received from BASF SE, dried in a vacuum for 12 h and stored in an argon glovebox (MBraun, [O₂] and [H₂O] < 0.1 ppm)¹⁹. 1 M lithium ethoxide solution was prepared by reacting absolute ethanol (6 ppm H₂O, 99%, Merck) with lithium metal (Albemarle Germany GmbH). Ethanol was dried over sodium metal and diethyl phthalate (99%, Merck), refluxed for 2 h, distilled and stored over 0.3 nm molecular sieve (Merck). For 0.5 M niobium ethoxide solution, Nb(OCH₂CH₃)₅ (99.95%, Sigma-Aldrich) was dissolved in absolute ethanol. All of the synthesis steps were performed under an argon atmosphere. To prepare Li₂CO₃/LiNbO₃-coated NCM622 cathode material, an amount of 5.94 g of NCM622 powder was added to 812 µL of niobium ethoxide solution. Next, the relevant amount of 1 M lithium ethoxide solution was added and the mixture was sonicated for 30 min at room temperature, followed by drying in a vacuum. The resultant powder was ground using mortar and pestle and then heated in a quartz tube furnace for 2 h at 300 °C in air (5 °C/min heating rate). Sample S6 was prepared following the same procedure, except that the final heating step was performed under oxygen flow (99.998%, Air Liquide). The uncoated (pristine) and coated NCM622 cathode materials were stored under an argon atmosphere.