Characterization of the solid/electrolyte interphase at hard carbon anodes via scanning (electrochemical) probe microscopy

Nanostructural properties impacting ion and electron transfer processes at the electrolyte-electrode interface are of pivotal importance for the overall performance of rechargeable batteries such as sodium-ion batteries (SIBs). Compared to lithium-ion batteries (LIBs), less is known about the solid electrolyte interphase (SEI) formation at the negative electrode of SIBs. In this contribution, we present conductive atomic force microscopy (AFM), AFM-based nanomechanical mappings and AFM-scanning electrochemical microscopy (AFM-SECM) measurements addressing the interphase formation on hard carbon (HC) providing insight on the effect of additives like 4-fluoro-1,3-dioxolan-2-one (FEC) and the potential cumulative impact of the electrochemical and nanomechanical properties. The additive strongly influences the adhesion forces, whereas in contrast the Young's modulus appears significantly increased after cycling independent of an added additive to the electrolyte solution. AFM-SECM images before and after cycling clearly indicate the formation of an electrically insulating interphase layer. In addition, the effective heterogeneous rate constant of electron transfer was determined at single HC particles.