Investigation of sodium insertion in hard carbon with operando small angle neutron scattering

Sodium-ion battery technology is a promising and more sustainable alternative to its more conventional lithium-ion based counterpart. The most common anode material for these systems is a disordered form of graphite known as hard carbon. The inherent disorder in these carbons results in multiple possible pathways for sodium storage making the characterisation of sodiation mechanisms during cycling highly challenging. Here, we report an operando small angle neutron scattering (SANS) investigation of sodiation in a commercial hard carbon using a custom electrochemical cell. We demonstrate that it is possible to discern different sodiation mechanisms throughout cycling and provide supporting evidence for a three-stage model in which sodium ions are first adsorbed onto the surface of particles, then intercalated into the graphene layers, and finally inserted into the nanopores during the electrochemical stage known as the plateau region. This study showcases the unique capabilities of operando SANS for the characterisation of sodiation mechanisms of carbon-based, disordered, porous materials.