Simple surface treatment improves performance of carbon materials for sodium ion battery anodes

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Hard carbons are the most extended anode materials for sodium-ion batteries (SIBs); however, they suffer from several limitations such as low stability, poor rate performance and low initial Coulombic efficiency (iCE). Herein, a simple, fast, and low-cost surface treatment at room temperature using short-chain organic molecules: 3-mercaptopropionic acid (MPA), 1,2-ethanedithiol (EDT) and oxalic acid (OxA) has been applied to a hard carbon (C1400). The carbons treated with sulfur containing molecules (MPA or EDT) exhibit higher capacity (12 % capacity enhancement after 100th cycles at C/10 and 18 % enhancement at 1C vs. C1400). The introduction of these ligands leads to improved micropore blockage, helping in the reversible insertion of Na ions. Moreover, ex-situ X-ray photoelectron spectroscopy (XPS) analyses demonstrate that thiol functional groups promote the formation of favorable NaF and Na2O-rich solid electrolyte interfaces (SEI) leading to and faster sodium diffusion in the plateau region. Additionally, MPA and EDT treatments have been applied to a soft carbon (Vulcan XC-72R) resulting in a substantial 30 % capacity improvement after 100 cycles at 1C. These results demonstrate the wide applicability of the method as a straightforward and efficient strategy for improving the electrochemical properties of carbon anodes used in SIBs.