Novel Surface Modification of Graphite with Excellent Performance for Sodium Ion Battery

Recently, sodium–ion batteries (SIBs) have attracted increasing attention because of the wide availability and relative low cost of sodium, as compared with lithium. In addition, SIBs could possess similar chemical nature to lithium-ion batteries (LIBs). Nevertheless, Na–ion is about 55% larger than the Li–ion, resulting in serious obstruction to obtain high reversibility of Na–ion insertion and extraction during the charging/ discharging process. As a common anodic material for LIBs, graphite is not suitable for SIBs because Na–ion is known to hardly intercalate into graphite.

    Up to this point, several investigations have demonstrated that the structure and surface modification of carbon materials are significantly related to the capacity and cycling stability of SIBs. For example, hard carbon materials show high capacities of 200–300 mAh/g as anodes for NIBs. Furthermore, nitrogen–doped carbon materials have been developed to provide significant enhancement in capacity, presumably because of increasing reactivity and electronic conductivity. Graphene–based carbon material has also attracted attention because of its high specific surface area and superior conductivity. However, it must be noted that graphene–based carbon materials possess low volumetric capacity owing to low tap density (< 0.02 g/cm³), which comes from large amounts of hydrogen or significant nanoporosity. Overall, to date, there has not been report on high capacity and good cycling stability of carbon material for SIBs at high temperatures.

    It has been known that several alkali atoms, including Li, K, Rb and Cs, can readily form intercalation compounds with graphite. Contrary to the heavy alkali metals, the intercalation of Na into graphite is known to be difficult. Therefore, ionic size is not the major effect that precludes the intercalation of Na ion. In this study, we report modified graphite by partial oxidation of graphite flakes to introduce oxygen-containing functional groups, such as hydroxyl, carboxyl, and ether groups, randomly into the layered graphene matrix.Compared with graphite, the partially oxidized graphite (POG) has an expanded interlayer distance and thus an enhanced micropores and nanometer channels, facilitating Na-ion diffusion. Meanwhile, the presence of oxygen atoms within the graphite structure is shown to remarkably enhance Na-ion intercalation capacity. The un-oxidized portion of the graphite maintains high crystallinity and provides bulk electronic conductivity.

    The other purpose of this report is to improve the surface conductivity of POG and to simultaneously maintain the expanded interlayer distance by introducing conducting polymer, poly(3,4-ethylene-dioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS). The PEDOT-PSS conducting polymer is commercially available in aqueous dispersion, which can easily be casted into film electrode at low temperatures (<100 ^oC). PEDOT:PSS provides high electric conductivity (ca. 550 S cm^-1) and the films can be heated in air at 100 ^oC for over several hours with only few change in its conductivity. It has been widely used as an antistatic coating material, and as a hole-transport electrode for rechargeable polymer batteries or optoelectronic devices.

    For application of an anode of SIBs, this novel POG electrode provides a high reversible capacity of 344 mAh/g (Fig. 1) and retains 77.1% after 2000 cycles at room temperature in NaPF₆/EC/DEC (Fig. 2a) and 70.3% after 1000 cycles in NaPF₆/EC/PC (Fig. 2b). It is found that the PEDOT:PSS film can prevent the co-intercalation of solvent into the graphite layer even in EC/PC solvent. The crystalline structures of the POG materials have been characterized in detail by high-resolution TEM as well as in situ synchrotron XRD.