A revised mechanistic model for sodium insertion in hard carbons

• Published in: Energy & environmental science Vol. 13; no. 1; pp. 3469 - 3479
• Main Authors: Au, Heather, Alptekin, Hande, Jensen, Anders C. S, Olsson, Emilia, O'Keefe, Christopher A, Smith, Thomas, Crespo-Ribadeneyra, Maria, Headen, Thomas F, Grey, Clare P, Cai, Qiong, Drew, Alan J, Titirici, Maria-Magdalena
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 14.10.2020
• Subjects: Binding sites; Defects; Interlayers; Low voltage; Metallicity; NMR; Nuclear magnetic resonance; Pore size; Porosity; Rechargeable batteries; Sodium; Sodium-ion batteries
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/d0ee01363c

Hard carbons have shown considerable promise as anodes for emerging sodium-ion battery technologies. Current understanding of sodium-storage behaviour in hard carbons attributes capacity to filling of graphitic interlayers and pores, and adsorption at defects, although there is still considerable debate regarding the voltages at which these mechanisms occur. Here, ex situ 23 Na solid-state NMR and total scattering studies on a systematically tuned series of hard carbons revealed the formation of increasingly metallic sodium clusters in direct correlation to the growing pore size, occurring only in samples which exhibited a low voltage plateau. Combining experimental results with DFT calculations, we propose a revised mechanistic model in which sodium ions store first simultaneously and continuously at defects, within interlayers and on pore surfaces. Once these higher energy binding sites are filled, pore filling occurs during the plateau region, where the densely confined sodium takes on a greater degree of metallicity. Sodium filling inside hard carbon pores demonstrates increasingly metallic character with increasing pore size.