New Mechanistic Insights on Na-Ion Storage in Nongraphitizable Carbon

TLDR

Hard carbon is a promising anode for Na‑ion batteries, but its storage mechanism is traditionally described by the card‑house model, which posits intercalation between graphene sheets followed by pore filling. The study aimed to test this model by varying turbostratic nanodomain sizes and discovered that structural defects correlate with Na‑ion storage. The authors propose that Na‑ion storage in hard carbon occurs at defect sites, then proceeds via intercalation and finally pore filling. They found that increased structural defects enhance Na‑ion storage capacity.

Abstract

Nongraphitizable carbon, also known as hard carbon, is considered one of the most promising anodes for the emerging Na-ion batteries. The current mechanistic understanding of Na-ion storage in hard carbon is based on the "card-house" model first raised in the early 2000s. This model describes that Na-ion insertion occurs first through intercalation between graphene sheets in turbostratic nanodomains, followed by Na filling of the pores in the carbon structure. We tried to test this model by tuning the sizes of turbostratic nanodomains but revealed a correlation between the structural defects and Na-ion storage. Based on our experimental data, we propose an alternative perspective for sodiation of hard carbon that consists of Na-ion storage at defect sites, by intercalation and last via pore-filling.