Abstract
This work was dedicated to the understanding of the surface degradation mechanisms and reactivity of Ni-rich NMC materials, with a focus on the reactions that generate gas. In a first approach, the exact nature and accurate quantification of the water-soluble species left from synthesis or produced from specific atmosphere-exposure were investigated through complementary techniques as titration, AAS, ICP-OES and FTIR. This comparative study unveils the reactivity of five commercial NMC materials, as function of the increasing nickel content. TG/MS analyses of these materials allowed the proposition of a novel mechanism that explains (i) the degradation of Ni-rich NMC materials towards air, producing soluble and insoluble surface species; and (ii) the partial recovery of the electrochemical performances of exposed materials, after annealing at relatively low temperature. Some of those NMC surface species were proved to chemically react with the electrolyte components into LiPO2F2. With the aim of quantifying gaseous products, first, different separation conditions were studied by making use of a GC-BID apparatus. The analysis of gases produced from a defined storage protocol, evidenced the consumption of C2H4 and the multiple sources of CO2; both having in common, the reactivity of delithiated NMC. Lastly, being produced from in-situ chemical reactions and described in the literature as an efficient electrolyte additive, LiPO2F2 was evaluated as a solution for enhancing electrochemical performances of Ni-rich NMC materials-based cells. The analysis of solid, liquid and gaseous degradation products after cycling helps clarify the action mechanism through which it can improve the capacity retention and seemingly reduces gassing. Surprisingly, a premature capacity loss was observed when cycling NMC 811-containing cells, at 55 ºC. Through electron microscopy analyses, it was hypothesized that it was due to the particle disintegration, caused by the reactions between the electrolyte and grains boundaries interphase