Influence of Anion Structure of Ionic Liquid on the Electrochemical Performance of Si-Based Negative Electrode for Li-Ion Battery

Abstract

To use Li-ion battery as a power source in electric vehicles and stationary power supply systems, an additional increase in its energy density is required. While silicon (Si) is a promising active material for use as a negative electrode due to its high theoretical capacity, higher energy density leads to an increase in a risk of ignition of the battery. Electrolyte is one of the most important component determining the battery performance. Ionic liquids have received much attention as an alternative to a conventional organic electrolytes because of their excellent physicochemical properties, such as high thermal stability, negligible vapor pressure, and wide electrochemical window. We have previously reported that the electrochemical performance of Si electrode in ionic liquid electrolyte is superior to that in organic electrolyte,1 and that an initial charge-discharge capacity of Si electrode increases when ether groups is introduced to side chain of cation in ionic liquid; cation structure of ionic liquid influences the electrochemical performance of Si negative electrode.2 Anion of ionic liquid is also key factor because it should affect the solvation structure of Li+. However, the anion structure of ionic liquid has not yet been optimized. In this study, we investigated the effect of anion structure of ionic liquid on the electrochemical performance of Si negative electrode for Li-ion battery. Si electrode was prepared by a gas-deposition method, which is without any binder or conductive agent. We assembled 2032-type coin cell consisted of the fabricated Si electrode as working electrode, Li foil as counter electrode, and glass fiber filter as separator. The ionic liquid used in this study consists of 1-((2-methoxyethoxy)methyl)-1-methylpiperidinium (PP1MEM) cation and three types of anion, including bis(fluorosulfonyl)amide (FSA), bis(trifluoromethanesulfonyl)amide (TFSA), and tetrafluoroborate (BF4). The electrolyte solution used was 1 mol dm-3 LiX-dissolved in PP1MEM-X (X: FSA, TFSA, BF4); anion of Li salt was same as that of ionic liquid. 1 mol dm-3 TFSA in propylene carbonate (PC) was also used for comparison. A galvanostatic charge-discharge test was carried out in the voltage range between 0.005 and 2.000 V at 303 K under a current density of 0.42 A g-1 (0.12 C). The conductivity of ionic liquid electrolytes was investigated by an electrochemical impedance spectroscopic analysis in the frequency of 100 kHz to 500 Hz with amplitude of 10 mV. An initial discharge capacity of Si electrode was less than 800 mA h g-1 in BF4-based electrolyte solution. The cycle performance of Si electrode in BF4-based electrolyte was almost the same as that in PC-based electrolyte; the discharge capacity was about 170 mA h g-1 at 80th cycle in both electrolytes. In FSA- and TFSA-based electrolytes, on the other hand, Si negative electrode exhibited high initial discharge capacity of ca. 3000 mA h g-1. It is well-known that surface film is formed on the Si electrode during an initial charging process through the reductive decomposition of the electrolyte, and that the film works as protective film which prevent continuous decomposition of the electrolyte. Better cycle performance of Si electrode in FSA- and TFSA-based electrolytes should be arising from more protective film compared to the film formed in BF4- and PC-based electrolytes. In addition, the disintegration of Si electrode would be suppressed because of the uniform Li insertion into the Si electrode over the entire surface in these electrolytes.3 Si negative electrode showed an excellent high-rate performance in FSA-based electrolyte; a reversible capacity of 800 mA h g-1 was achieved even at the high current density of 6 C, while Si electrode showed the discharge capacity of 100 mA h g-1 in TFSA-based electrolyte. Conductivity of FSA-based electrolyte (2.06 mS cm-1) is more than tripled compared to that of TFSA-based electrolyte (0.66 mS cm-1). Consequently, it is considered that the conductivity significantly affects the rate performance. References 1) H Usui, T. Masuda, and H. Sakaguchi, Chem. Lett., 41(2012) 521. 2) M. Shimizu, H. Usui, K. Matsumoto, T. Nokami, T. Itoh, and H. Sakaguchi, J. Electrochem. Soc., 161(2014) A1765. 3) M. Shimizu, H. Usui, T. Suzumura, and H. Sakaguchi, J. Phys. Chem. C., 119 (2015) 2975. Acknowledgment This work was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant-in-Aid for Scientific Research B (Grant 24350094).