On resistance and capacity of LiNi_(1/3)Mn_(1/3)Co_(1/3)O_2 under high voltage operation

摘要

Operating commercial LiNixCoyMn1-x -yO2(NMCs)/ graphite cells at a higher voltage cut-off would deliver a higher energy density. This protocol has been broadly investigated in the literature, and connected with the occurrence of a rapid and severe degradation. In particular, these studies point to a de-coupling between capacity fade (mostly located on graphite) and impedance rise (mostly located on NMC). However, in the present work we unveil a non-negligible contribution of NMC111 to the total capacity fade, not reported in other studies. This unexpected feature is addressed by means of an experimental and modelling approach apt to unveil the causes behind it, and to quantify the relative impact of different, concurrent ageing mechanisms. For this purpose, a physics-based model including different ageing modes is proposed, and cross-validated on Direct and Alternate Current measurements. The fitting reveals that the capacity loss on NMC111 is in fact coupled to its characteristic impedance rise, and the parameters thus extracted are further validated by means of surface and bulk analytical techniques. In this way, the physical validity of these parameters is confirmed, and they can thus be used for lifetime prediction of NMC/graphite cells operated at high voltage. In addition, we investigate how the occurrence of a non-negligible capacity loss on NMC111 impacts the uneven stoichiometric drift occurring in the jelly roll of commercial cells, while demonstrating how lab-scale cells can still be used for representing the behaviour of commercial devices. It is revealed how high temperatures and localized Li plating can potentially push NMC111 above the chosen upper voltage cut-off, with a consequent increase in the degradation rate at cell level.