Revealing the effects of microstructural changes of graphite anodes during cycling on their lithium intercalation kinetics utilizing operando XRD

Abstract

Concerning cell ageing and lithium plating, electrode kinetics – in particular the one of the anode - plays an important role. In our study, we applied a holistic physicochemical post-mortem analysis including operando XRD and SEM-studies as well as electrochemical analysis to get deeper insights into the changes of the microstructure and phase composition of graphite anodes from LiNi0.6Co0.2Mn0.2O2//Graphite pouch cells during cyclic ageing and their influence on the lithium intercalation kinetics. The key findings are as follows: Li plating and loss of cyclable lithium is the main source of capacity fade of the investigated cells. Cycling causes pore clogging of the anode leading to an increased transport resistance for Li-ions and slower charge transfer reactions. Visual inspections and operando XRD measurements in turn indicate an inhomogeneous Li intercalation over the thickness of the electrode resulting in a Li gradient. In the cycled electrode LiC6 forms in surface-near regions, whereas in deeper regions LiC12 formation has not finished yet. As a result, Li plating occurs as the surface-near graphite is already fully intercalated and Li-ion transport to deeper regions is hindered due to pore clogging. By combining complementary methods we could consistently prove the hypothesis of pore clogging, deterioration of anode kinetics and subsequent inhomogeneous lithiation which is supposed to trigger the observed Li-plating.