Development of a Thin-Film NMC System for the Investigation and Suppression of Harmful Reactions Occurring on the Surface of Cathode Materials in Li-Ion Batteries

Abstract

Materials with the stoichiometry Li(NixMnyCoz)O2 (NMC) have emerged as promising candidates for use as cathodes in Li-ion batteries for electric vehicles. This is primarily due to their high capacity, which increases with the nickel content (Ni-rich NMC). However, the increase in nickel also leads to a decrease in the stability of the material, hindering market introduction [1]. Much of these stability issues emanate from the surface of the material and move subsurface towards the bulk during battery operation [2]. The internal electrode architecture of traditional composite battery cathodes, containing active materials, carbon additives, and binders, complicates the study of the surface reactions. For this reason, we developed a thin-film model system of Ni-rich NMC with a high surface area-to-volume ratio to exacerbate the effect of the surface reactions, enabling easier characterization of the surface without influence of the bulk.The interfacial reactions occurring when NMC is exposed to air was investigated using the thin-film system. These experiments were supported by thermodynamic calculations to propose a mechanism of what reactions occur on the surface. Our study showed that the surface of NMC is stable in inert environments, however, when exposed to air (in particular H2O and CO2) the active material at the surface reacts to form Li2CO3, coinciding with loss of Li2O and protonation of the material, forming transition metal oxyhydroxides. We found that this phenomenon can be reversed by performing a heat treatment, moving Li2O back into the film. However, during the annealing process, an additional reaction between the film and substrate takes place. This, once again, leads to the active material being deficient in Li2O. To compensate for this deficiency, Li2CO3 was deposited onto NMC prior to annealing. The thermodynamical properties of each film were approximated from their electrochemical behavior. It was found that the film treated with Li2CO3 before annealing was thermodynamically similar to what is expected for an NMC cathode. These results contribute to the understanding of the interfacial reactions that take place on the surface of NMC cathodes. Additionally, the study proposes strategies to mitigate and compensate for these reactions.