Influence of Al₂O₃ Coatings on HF Induced Transition Metal Dissolution from Lithium-Ion Cathodes

Transition metal (TM) dissolution from oxide cathode materials is a major challenge limiting the performance of modern Li-ion batteries. Coating the cathode materials with thin protective layers has proved to be a successful strategy to prolong their lifetime. Yet, there is a lack of fundamental understanding of the working mechanisms of the coating. Herein, the effect of the most commonly employed coating material, Al₂O₃, on suppressing hydrofluoric acid(HF)-induced TM dissolution from two state-of-the-art cathode materials, LiMn₂O₄ and LiNi_0.8Mn_0.1Co_0.1O₂, is investigated. Karl Fischer titration, fluoride selective probe and inductively coupled plasma optical emission spectrometry are coupled to determine the evolution of H₂O, HF and TM concentrations, respectively, when the active materials come in contact with the aged electrolyte. The coating reduces the extent of TM dissolution, in part due to the ability of Al₂O₃ to scavenge HF and reduce the acidity of the electrolyte. Delithiation of the cathode materials, however, increases the extent of TM dissolution, likely because of the higher vulnerability of surface TMs in +IV oxidation state towards HF attack. In conclusion, the current study evidences the important role of acid-base reactions in governing TM dissolution in Li-ion batteries and shows that coatings enhance the chemical integrity of the cathode towards an acidic electrolyte.