The synthesis and characterization of a hard carbon anode material for room-temperature sodium-ion battery is being reported. The carbon material was prepared via two-step process including hydrothermal carbonization (HTC) and pyrolysis. Similar carbon materials were recently synthesized and investigated by our group as supercapacitor electrode materials [1,2]. Sodium-ion batteries have emerged as a promising candidate for large-scale energy storage due to sodium’s abundance and low cost of raw materials. However, significant work is needed on the development of Na-ion intercalation anodes [3,4].

The materials were investigated using SEM, HR-TEM, gas sorption techniques and Raman spectroscopy. Electrochemical characterization has been carried out using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge method (GCD).

The glucose derived carbon particles have a round shape and consist of both ordered and disordered graphitic domains, as can be seen from HR-TEM images (Figure 1a).

The electrode slurry was prepared by mixing the carbon active material, conductive additive (Super P) and polyvinylidene difluoride (PVdF) binder in a 75:15:10 mass ratio. The mixed slurry was cast onto copper foil using doctor-blade technique. The cast electrodes were dried under vacuum for 24h. The half-cells (EL-Cell GmbH) were assembled in an Argon-filled glovebox (O₂ < 0.1 ppm, H₂O < 0.1 ppm). Sodium metal was used as counter and reference electrode and 1M NaClO₄ in propylene carbonate (PC) as the electrolyte.

Cyclic voltammograms measured at 0.5 mV s^-1 potential scan rate are shown in Figure 1b. The identical shape of the CVs indicates good reversibility. Very well developed peaks at E = 10 mV and E = 100 mV suggest that reversible Na-ion intercalation is taking place in the material. However, the electrode seems to exhibit some capacitive properties in the potential region from 225 mV to 1 V, after which the current density drops suggesting that the electrode is charged positively and therefore Na-ions are replaced by bigger and more solvated ClO₄^- anions. Galvanostatic charge-discharge curves in Figure 1c show discharge capacity values of 250 mAh g^-1 at 50 mA g^-1 current density. The half-cells were cycled between 0.005 and 1.5 V vs Na/Na⁺.

The effect of co-intercalating solvents [5–8] will be analysed using electrochemical techniques together with ex situ/in situ Raman spectroscopy data.

Acknowledgements

The present study was supported by the Estonian Centre of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13, NAMUR project 3.2.0304.12-0397 and projects 3.2.0302.10-0169 and 3.2.0302.10-0165.

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