Decomposition of phosphorus-containing additives at a charged NMC surface: atomistic modeling insights

Stabilizing the cathode/electrolyte interface at high voltage is necessary to achieve higher capacities while still maintaining capacity retention in Lithium-ion batteries. One strategy is through the use of additives in the electrolyte: components in low concentration (<10%) that have a lower anodic stability than the baseline electrolyte, so that during the initial cycles, the additive will decompose on the charged cathode surface preferentially over the baseline electrolyte. This reaction will then yield a layer which will inhibit further reaction between the electrolyte and the cathode surface. However, the mechanism of improvement remains unclear. In the present work, Density Functional Theory is used to gain insights and understanding on experimental results using a potentiostatic hold technique to evaluate cathode/electrolyte reactivity for two families of additives: phosphites and phosphates. Simulations indicate the susceptibility of the various additives to electrochemical and chemical oxidation, showing chemical oxidation to be much more likely with the phosphite moiety. The identity of the ligands on the phosphorus-containing additive can dramatically affect both the decomposition current and the cathode surface film.