Electrochemical Sodium-Ion Intercalation Reaction at Graphite Electrodes

Abstract

Introduction Sodium-ion batteries (SIBs) are widely focused on as post lithium-ion batteries (LIBs) from the view of element strategy[1]. Hard carbon electrode is often used as the negative electrode of sodium-ion battery. This is because the graphite electrodes show negligible reversible capacity different from LIBs. Graphite is cheaper and has higher density than hard carbon, and therefore, graphite negative electrode for SIBs should be quite attractive. However, it is well-known that sodium-ion insertion at graphite electrode is very difficult although the reason has not been clarified yet. In this study, the sodium-ion intercalation reaction at graphite electrodes in organic electrolyte solutions was investigated to understand the sodium-intercalation mechanism. Expetimental Electrochemical measurements were carried out using a three-electrode cell. Working electrode was graphite composite electrode (natural graphite and graphitized carbon nanosphere[2] (GCNS)). Reference electrode was Ag/Ag+ electrode and counter electrode was natural graphite composite electrode. The electrolyte solution was 0.9 mol kg- 1 sodium bis(fluorosulfonyl)amide (NaFSA)/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 by vol.). Cyclic voltammetry and charge-discharge measurements were carried out. To characterize the sodium intercalated graphite, Raman spectroscopy and X-ray diffraction (XRD) measurement were used. Results Figure 1 shows cyclic voltammograms of natural graphite composite electrode in 0.9 mol kg- 1 NaFSA/EC+DMC(1:1 by vol.). Reversible sodium-ion intercalation was not indicated. In charge-discharge curves, large irreversible capacity was observed at the 1st cycle. Reversible capacity was around 5 mAh g- 1. Thus, a small amount of reversible sodium-ion intercalation into graphite was indicated. The XRD patterns for graphite electrodes held at -3.03 V vs. Fc/Fc+ during from 1day to 7 days didn’t change compared with the XRD pattern for pristine graphite electrode. Figure 2 shows Raman spectra of the graphite electrode held at -3.03 V vs. Fc/Fc+. After 1day, G-band around 1580 cm- 1 was split and a new band around 1600 cm- 1 derived from intercalated species[3] appeared. The ratio of band intensity indicated the formation of stage-5 sodium graphite intercalation compound (Na-GIC). The intensity of the new band increased with increasing potential holding time, and the formation of lower stage Na-GIC was indicated. Therefore, the formation of Na-GIC was limited on the surface region of graphite. It suggests that the diffusion of sodium-ion in graphite is very slow.In the meeting, the results for GCNS will be reported. Acknowledgement This work was partially supported by ESICB, Kyoto University. Reference S. Komaba et al., Adv. Funct. Mater., 21 (2011) 3859. N. Yoshizawa et al., Mater. Chem. Phys., 121 (2010) 419. M. S. Dresselhaus and G. Dresselhaus., Adv. Phys., 51 (2002) 1. Figure 1