Sodium-Ion Transfer Reaction at the Interface between Carbon Nanosphere Electrodes and Electrolytes

Abstract

Introduction Sodium-ion batteries (SIBs) are widely focused on as post lithium-ion batteries from the view of element strategy[1]. Hence, SIBs are attractive for large scale energy storage like electric vehicles. For the large scale energy storage, batteries with high rate capability are desired. Carbon nanosphere (CNS) covered with basal planes is expected as the candidate for carbon negative electrode material with high rate capability because CNS electrodes showed the high rate capability in LIBs owing to small particle sizes and short diffusion distances[2]. To enhance the rate capability of electrode materials, the internal resistances of electrodes should be investigated. In LIBs, the interfacial lithium-ion transfer at the interface between electrodes and electrolytes is one of the rate determining step[3]. However, the kinetic properties of interfacial sodium-ion transfer at the interface between carbon negative electrodes and electrolytes is unclear. In this study, the activation energies of interfacial sodium-ion transfer reaction at the interface between CNS electrodes and organic electrolyte solutions were evaluated. Experimental Electrochemical measurements were carried out using a three-electrode cell. Working electrode was CNS composite electrode. CNS treated at 1100°C and 2900°C were used and denoted as CNS-1100 and CNS-2900, respectively. Reference electrode was Ag/Ag+ electrode and counter electrode was natural graphite composite electrode. The electrolyte solutions were 0.9, 4 mol kg- 1 sodium bis(trifluoromethanesulfonyl)amide (NaTFSA)/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 by vol.) and 0.7 mol kg- 1 NaTFSA/fluoroethylene carbonate (FEC). Hereafter, all potentials are referred to as Fc/Fc+. Cyclic voltammetry (CV) was conducted between open circuit potential (OCV) and various potentials, and the scan rate was set at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was carried out with ac amplitude of 10 mV in a frequency range of 100 kHz − 10 mHz at temperatures ranging from 30°C to 10°C. Results Figure 1 shows the cyclic voltammograms of CNS-1100 composite electrodes in 0.9 mol kg- 1 NaTFSA/EC+DMC(1:1 by vol.). The irreversible currents were observed below −2 V and more than half of the irreversible currents disappeared from the 2nd cycle, indicating the formation of solid electrolyte interphase on CNS electrodes at the same manner of lithium ion-system. The redox peaks also appeared around −3 V, and were assigned to the peaks of reversible sodium-ion insertion into/from CNS-1100. Figure 2 shows the Nyquist plots of CNS-1100 composite electrode in 0.9 mol kg−1 NaTFSA/EC+DMC(1:1 by vol.). In the Nyquist plots, based on the potential dependence of the semi-circle, the semi-circle at the middle frequency region was assigned to the charge-transfer resistance. The same kinds of electrochemical behaviors were observed in the cases of other electrolytes and CNS-2900 electrodes.In the meeting, the activation energies of interfacial sodium-ion transfer will be reported. Acknowledgement This work was partially supported by ESICB, Kyoto University.