Coupling differential voltage analysis and distribution of relaxation times I: Evaluating the impact of NMC811 coating in the degradation of NMC811/Gr cells

Abstract

This paper focuses on the coupling of the Differential Voltage Analysis (dV/dQ vs Q) and the Distribution of Relaxation Times (DRT), whose benefits to electrodes development are illustrated by the tracking of degradation effects in the cycling aging of NMC811/Gr coin cells, with and without an NMC electrode coating. The former technique allows monitoring of individual electrode loss of active material (LAM), chemistry change and state of charge (SOC) alignment of the electrodes, whose variation is related mainly to loss of Li inventory (LLI). DRT is a time domain representation of the impedance that offers a high resolution of the resistance processes, complementing the DVA analysis by bringing kinetics degradation of the cells and by identifying in which electrode the main resistance processes are originated. The protocol inserting those techniques in an aging regime is a non-invasive approach, and it indicated the consumption of Li + ions as the main reason for capacity decay in both cells, with reduced or minimal NMC LAM and chemistry change when coating is applied. From the perspective of power decay, the coating decelerated the impedance rise (IR), mostly related to resistance processes in the positive electrode. • DVA and DRT coupling approach tracked electrodes contribution to degradation. • NMC particle coating prevented loss of mass and slowed down cell impedance increase. • Non-invasive operando approach tracks the evolution of the degradation effects.