Manipulating Adsorption–Insertion Mechanisms in Nanostructured Carbon Materials for High‐Efficiency Sodium Ion Storage

TLDR

Hard carbon is a promising anode for sodium‑ion batteries, yet its low Coulombic efficiency remains a major obstacle. The study synthesizes nanostructured hard carbon materials with controlled architectures to improve sodium‑ion battery performance. Using in‑situ X‑ray diffraction, NMR, EPR, electrochemical tests, and simulations, the authors establish an adsorption–intercalation mechanism in which Na ions first adsorb on defect sites of hard carbon, then intercalate into graphitic layers, producing a sloping voltage followed by a flat low‑voltage plateau. The work shows that enhancing the flat‑plateau intercalation while suppressing the sloping region yields a nonporous hard carbon material with high reversible capacity and Coulombic efficiency suitable for practical use.

Abstract

Hard carbon is one of the most promising anode materials for sodium‐ion batteries, but the low Coulombic efficiency is still a key barrier. In this paper, a series of nanostructured hard carbon materials with controlled architectures is synthesized. Using a combination of in situ X‐ray diffraction mapping, ex situ nuclear magnetic resonance (NMR), electron paramagnetic resonance, electrochemical techniques, and simulations, an “adsorption–intercalation” mechanism is established for Na ion storage. During the initial stages of Na insertion, Na ions adsorb on the defect sites of hard carbon with a wide adsorption energy distribution, producing a sloping voltage profile. In the second stage, Na ions intercalate into graphitic layers with suitable spacing to form NaC x compounds similar to the Li ion intercalation process in graphite, producing a flat low voltage plateau. The cation intercalation with a flat voltage plateau should be enhanced and the sloping region should be avoided. Guided by this knowledge, nonporous hard carbon material has been developed which has achieved high reversible capacity and Coulombic efficiency to fulfill practical application.