Modulated electrochemical force microscopy: Investigation of sodium-ion transport at hard carbon composite anodes.

For sodium (Na)-ion batteries (SIBs), the next generation of sustainable batteries, hard carbon (HC) composite electrodes are the most used anodes. Here, we demonstrate the potential of modulated electrochemical force microscopy (mec-AFM) to investigate electrochemical strain due to ion insertion at the electrolyte/electrode interface. HC composite anodes have a complex, multiphase structure, which include the HC particles, conductive carbon nanoparticles (carbon black) and the binder. To address the effect of the composite material on the sodium-ion transport, we employ mec-AFM. A HC composite anode was embedded in an epoxy-polymer matrix and was polished to expose a micro-sized area that enabled high-frequency modulation of the ion transport. We analyse the influence of the modulation on interfacial forces and its role in generating electrochemical strain in the composite anode. Multichannel mec-AFM imaging at varying electrode potentials revealed that the observed electrochemical strain predominantly occurred in the softer binder matrix rather than in the HC microparticles. Our findings underscore the significance of ionic transport pathways through the binder matrix and establish mec-AFM as a novel AFM-derived technique for visualising ion dynamics at battery interfaces.