Ambient storage derived surface contamination of NCM811 and NCM111: Performance implications and mitigation strategies

The quality of metal oxide-based battery active materials is compromised by surface contamination from storage and handling at ambient conditions. We present a detailed analysis of the true nature and the quantity of the surface contaminants on two different cathode active materials, the widely used LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) and the Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811). We process these materials in three distinct conditions "wet" (excessive exposure to moisture), "dry" (standard drying of as-received materials), and "calcined" (heat-treatment of cathode powders). Surface contaminants are then quantified by thermogravimetric analysis coupled with mass spectrometry (TGA-MS), and their reactivity with an ethylene carbonate-based electrolyte is evaluated using on-line mass spectrometry (OMS). We demonstrate that not only the commonly assumed LiOH and Li₂CO₃ residues account for NCM performance deterioration upon storage in moisture and CO₂ containing atmosphere, but also basic transition metal hydroxides/carbonates formed on the material surface. Eventually, we showcase a thermal treatment that removes these transition metal based surface contaminants and leads to superior cycling stability.