Effect of Electrolyte on Sodium-Ion Storage Behavior into Non-Graphitizable Carbon Negative Electrode

Abstract

Introduction Sodium-ion batteries (SIBs) are expected to be an alternative power source to lithium-ion batteries (LIBs) because their abundant resources reduce the cost of SIBs. As the negative electrode of SIBs, non-graphitizable carbon recieves a lot of attention because it can store ions not only in the graphene interlayer but also in its internal pores.[1] By controlling the pores, it is possible to achieve larger reversible capacity of more than 400 mAh g−1.[2] While many researchers have studied non-graphitizable carbon from the viewpoint of thermodynamics, there are a few reports that pay attention to kinetic viewpoint. The kinetic viewpoint is a very important factor because it affects the rate performance of the battery.We have focused on the interfacial reactions of non-graphitizable carbon electrodes and found that the charge-transfer resistance and the activation energy of the charge-transfer reaction in SIBs were larger than those in LIBs.[3] This result and the Lewis acidity indicated that the desolvation process was not identified as the rate determining step in the charge-transfer reaction. One of the significant factors responsible for the increased resistance is the effect of the solid electrolyte interphase (SEI). Objective In this work, we used six types of electrolytes and formed SEI on the surface of non-graphitizable carbon. The interfacial reaction of non-graphitizable carbon with SEI derived from these electrolytes was investigated.In this paper, we report about the effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon electrode. Experimental Non-graphitizable carbon heat-treated at 2273 K (HC-2000) was used as the negative electrode material. A three-electrode cell was used for electrochemical measurements. HC-2000 composite electrode, natural graphite composite electrode, and sodium metal were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolytes were prepared using sodium bis(trifluoromethanesulfonyl) amide (NaTFSA), sodium bis(fluorosulfonyl) amide (NaFSA), and NaPF6 as sodium salts and ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 by volume) and fluoroethylene carbonate (FEC) as solvents.Cyclic voltammetry (CV) was performed to form SEI on HC-2000. The scan rate and range were 0.1 mV s−1 and 0–2.5 V, respectively. After CV, electrochemical impedance spectroscopy (EIS) was performed with an electrode potential of 0.2 V and an AC amplitude of 10 mV in the frequency range of 100 kHz–10 mHz. In addition, X-ray photoelectron spectroscopy (XPS) was used to analyze the electrode after CV measurements with respect to SEI. Results and discussion Figure 1 shows the Nyquist plots obtained from the EIS analyses. The semicircles in the low frequency region were attributed to the charge-transfer resistances. Among EC+DEC solvents, the system of NaFSA had the largest charge-transfer resistance. For NaTFSA and NaPF6, the charge-transfer resistances increased in the FEC solvent, while no significant differences in the charge-transfer resistance were observed for the NaFSA system in the EC+DEC and FEC solvents.The SEI components of the HC-2000 composite electrode in each electrolyte were determined by XPS analysis after CV measurements. In NaFSA/EC+DEC system, the amount of NaF was remarkably large, while the amount of NaF was relatively small in NaTFSA/EC+DEC and NaPF6/EC+DEC systems. On the other hand, in FEC solvents, the amount of NaF increased in NaTFSA and NaPF6 systems. The systems with higher NaF contents had the larger charge-transfer resistances. This result indicates that NaF was involved in the charge-transfer resistance.Finally, EIS analyses were performed at different temperatures. The activation energies were calculated using Arrhenius plots and the Arrhenius equation. The values of the activation energies were in the range of 60–70 kJ mol−1. There was a slight variation in the values, but they were within the error range and were not significantly different for all systems. These results showed that NaF affects the frequency factor rather than the activation energy. Conclusion The effects of SEI on the interfacial reaction between electrolyte and non-graphitizable carbon were investigated. The systems with the large charge-transfer resistance had a large amount of NaF. NaF did not affect the activation energy of charge-transfer resistance but did affect the frequency factor.