Real-time measurement of sodiation induced stress in hard carbon composite electrodes

Durability of hard carbon (HC) electrodes is becoming important owing to their emergence as potential sodium-ion battery anodes. The volume expansion/contraction during electrochemical cycling of HC against sodium may result in significant mechanical degradation and capacity fading. From this perspective, the real-time stress evolution in composite HC electrodes was measured as a function of capacity for several sodiation/de-sodiation cycles using substrate-curvature method. The effect of polymer binder on the stress evolution was investigated with two different binder systems: 1) Carboxymethyl cellulose/Styrene-butadiene rubber (CMC/SBR) and 2) polyvinylidene fluoride (PVdF). The HC electrodes exhibited similar stress evolution pattern upon electrochemical cycling irrespective of the binder. Compressive stress was generated during sodiation which increased to a peak value of approximately – 6.8 MPa in case of HC-CMC/SBR and – 5.9 MPa in case of HC-PVdF at the end of sodiation. Upon desodiation there was elastic unloading and the stress became tensile with cycling reaching a peak value of ∼3.3 MPa and ∼1.5 MPa in case of HC-CMC/SBR and HC-PVdF, respectively, at the end of desodiation. Insights were provided about the effect of stress hysteresis on the cell performance. Some preliminary observations on sodiation mechanism were also made based on the stress measurements.