Probing Sodium Structures and Dynamics in Hard Carbon via Operando NMR
Sodium-ion batteries present an attractive alternative to lithium-ion batteries due to their lower environmental impact, enhanced safety, and competitive energy density. A critical factor in improving these batteries is the choice of anode material, which significantly influences performance, stability, and the formation of the solid electrolyte interphase (SEI). Hard carbons, which are non-graphitizable carbons, have emerged as leading candidates for sodium-ion battery anodes.
This study investigates the mechanisms of sodium storage in hard carbon negative electrodes using advanced characterization techniques. Operando, in situ and ex situ solid-state nuclear magnetic resonance (NMR) spectroscopy was employed alongside scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. These methods provided insights into sodium structures, dynamics and the SEI composition.
Key findings include the influence of carbonization temperature on sodium storage. At higher temperatures, larger pores facilitate the formation of quasi-metallic sodium clusters, improving sodium storage capacity. Additionally, the SEI was found to consist of sodium fluoride (NaF) forming on the hard carbon surface, with sodium carbonate (Na₂CO₃) embedded within the layer. A comprehensive mechanism for Na+ ion behavior in hard carbon was proposed: during initial sodiation, Na+ ions adsorb on the hard carbon surface and insert between graphene layers and then fill the pore walls and at the end quasi-metallic Na clusters are formed if the pores are large enough. During desodiation, this process reverses. Underpotential sodiation leads to Na metal plating, with distinct NMR chemical shifts reflecting the different properties of plated Na and dendrites. These insights could pave the way for optimizing sodium-ion battery anodes for enhanced efficiency and longevity.
The authors of this research are Matej Gabrijelčič, Blaž Tratnik, Gregor Kapun, Elena Tchernychova, Nataša Zabukovec Logar, Andraž Krajnc, Robert Dominko, and Alen Vižintin from National Institute of Chemistry.
The work is published in Journal of Materials Chemistry A.
Link: https://pubs.rsc.org/en/content/articlelanding/2025/ta/d4ta07135b
Cover graphics: In this study, we aim to (i) provide an in-depth analysis of the formation and composition of SEI, (ii) understand and differentiate the sodium storage processes, and (iii) distinguish between Na metal plating and dendrite growth.
Contact for further information: matej.gabrijelcic(at)ki.si and alen.vizintin(at)ki.si
This article is dedicated to the memory of Prof. Dr. Gregor Mali.