Mechanistic understanding of sodium storage from operando stress measurements in hard-carbon thin-films

Abstract

Hard carbon is a promising anode material for sodium-ion batteries, yet the sodium-storage mechanism in hard carbon remains debated. Here, operando substrate-curvature measurements on hard-carbon thin films were used to probe the volume-expansion-induced stress during sodiation and desodiation. The volume-expansion-induced stress rises steeply in the higher-voltage sloping region during initial sodiation, then it increases at a slower rate in the low-voltage plateau region, followed by a second steep slope below approximately 0.025 V at the end of sodiation. These regimes in stress evolution are consistent with intercalation-dominated storage in the sloping region, pore filling or adsorption in the plateau region, and a plating process at low potentials. The stress relaxation profile evolves asymmetrically during desodiation, indicating that sodium removal from hard carbon is not a simple reversal of sodium insertion into it. The irreversibility in sodiation-desodiation profiles of hard carbon is associated with sodium retention and/or (micro)structural rearrangement.