In situ probing of interfacial kinetics for studying the electrochemical properties of active nano/micro-particles and the state of Li-ion batteries

It is critical to monitor the state of health (SOH) of the Li-ion batteries to ensure a safe operation and to extend the service life of the batteries in electric vehicles. In this work, we demonstrated that the equivalent capacitance (C-p) and resistance (R-p) of the electrode interface derived using a first-order RC equivalent circuit under a large galvanostatic pulse (LGPM) condition can be correlated with SOH. For both the cathode and the anode, the interfacial kinetics of Li-ions were analyzed to study the electrochemical properties of active particles. The RC parameters of the equivalent circuit were correlated with the diffusion kinetics of Li-ions near the interface between the electrolyte and the active nano/micro-particles during fast charging/discharging. For fresh LiFePO4 (LFP)/Li half-cells, the values and the change of C-p and R-p were explained using the hypothesis of interparticle ion transport under a non-equilibrium condition. For graphite/Li half-cells, the buffering of Li-ions by the solid-electrolyte interphase (SEI) layer was speculated to affect C-p and R-p under a non-equilibrium condition. In commercial LFP/graphite batteries, the C-p values of unhealthy batteries were found to be higher than those of healthy batteries. In further tests, the C-p values of the half cells with the graphite anode recovered from the unhealthy batteries were found to be higher than those of the half cells with graphite from the healthy batteries. The half cells with LFP from the unhealthy batteries behaved similarly to those with LFP from the healthy batteries. With additional analysis on the microstructure, we proposed that the deterioration of the LFP/graphite batteries was mostly due to the formation of a thicker SEI on the graphite anode. The method developed in this work can be integrated in EVs at a low calculation cost. More importantly, we gained a better understanding of the interfacial kinetics of Li-ions during a non-equilibrium process.