Probing sodium structures and dynamics in hard carbon for Na-ion batteries using 23Na operando solid-state NMR spectroscopy

Abstract

Hard carbons are promising negative electrode materials for Na-ion batteries (SIBs), and the process of (de)insertion of Na⁺ ions into/from hard carbons has attracted much attention in recent research. Being a relatively new technology compared to lithium-ion batteries, the precise operational mechanism and degradation pathways of SIBs remain elusive. In this investigation, we focus on elucidating the physical and electrochemical attributes of corncob-derived hard carbon synthesized at 1400 °C using operando, in situ and ex situ solid-state NMR spectroscopy techniques, complemented by other advanced characterization methods, such as SEM-EDX, XPS and electrochemistry. Through the analysis of ²³Na NMR spectra of hard carbon, we gained insights into sodium insertion within the active material, which is characterized by the shifting NMR peak during the (de)sodiation process. Furthermore, our findings revealed the formation of a solid electrolyte interphase (SEI) at the hard carbon electrode and pore surface, predominantly in the form of Na₂CO₃ in the bulk of the SEI, and NaF at the SEI surface, alongside the occurrence and differentiation of plating and metal sodium dendrites.