Effect of HF in LiPF 6 Based Electrolytes on the Properties of Surface Passivation Films Formed on Graphite Electrodes i
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T. SATO *, M. DESCHAMPS, H. SUZUKI, H. OTA, H. ASAHINA AND S. MORI * Tsukuba Research Center, Mitsubishi Chemical Corporation, 8-3-1, Chuo, Ami, Ibaraki, Japan, [email protected] ABSTRACT
The effect of hydrofluoric acid (HF) concentration in conventional LiPF 6 /EC+DEC electrolyte on the properties of solid electrolyte interphase (SEI) films formed on synthetic graphite electrode and the electrochemical performance were studied using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), elemental analysis and electrochemical measurement. The morphology and composition of SEI are affected by a small amount of HF in the electrolyte. Higher F content in the SEI was observed when high HF electrolyte were used, although the irreversible capacities used for formation of SEI were nearly the same value. The cycleability of the synthetic graphite electrode was greatly improved when surface film contains larger amount of LiF. INTRODUCTION Interest in the formation of the solid electrolyte interphase (SEI) on graphite electrode
has recently increased to improve the performance of lithium ion batteries. It is well known that surface passivating film play a key role in the cycling of lithium ion cell. The composition of SEI is extensively analyzed by some researchers concluding lithium carbonate and lithium alkylcarbonates are the main components when ethylene and diethyl carbonates (EC+DEC) solution is used [1,2]. In case of LiPF 6 containing electrolytes, lithium fluoride (LiF) is also analysed. Aurbach et al. proposed several possible reactions scheme to produce LiF [3]. However, it is not clearly understood the effect of HF, one of the major impurities found in LiPF 6 , on the formation of SEI and the graphite electrode performance [1-4]. This work aims at a comparative study of graphite electrodes in electrolyte solutions based on LiPF 6 and EC+DEC containing different HF concentrations. HF effect on electrode performances and properties of SEI were explored. EXPERIMENT EC and DEC (Mitsubishi Chemical Co., battery grade) were used without further purification. LiPF 6 was obtained from Hashimoto (battery grade). Electrolyte solutions, 1M 457 Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
LiPF 6 / EC+DEC (1:1), were used in this study and the different concentrations of HF were obtained by adding water to this electrolyte. The water content was measured by Karl-Fischer titration (Hiranuma, AQ7) and the HF concentration was determined by NaOH titration after 2 weeks storage. The electrolyte solutions used in this study are listed in Table I. In this paper, note that the titlated acid value was calculated as HF concentration. Electrochemical measurements were performed using CR2016 coin-type cell. KS44 synthetic graphite (Timcal) was used as working electrode and lithium metal (Honjo Metal) was used as reference and counter electrode. All cells were assembled in glove box (VAC) filled with dry Ar. Impedance spectroscopy was performed using the same coin cell at an open-ci
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