A New Prospect for Stabilization of Graphite Electrode/Electrolyte Interface in Bis(fluorosulfonyl)imide Anion-based Ionic Liquid Electrolyte

Abstract

We investigated effects of high lithium bis(fluorosulfonyl)imide (LiFSI) salt concentration or heat treatment before initial charge on charge-discharge performance of a graphite electrode in a 1-ethyl-3-methylimidazolium (EMImFSI)-based ionic liquid (IL) electrolyte. LiF was observed at the surface of graphite electrodes taken out from 2.00 mol kg −1 LiFSI/EMImFSI system and from 0.32 mol kg −1 LiFSI/EMImFSI system with pre-heat treatment before the initial charge. The surface LiF effectively suppresses the reductive decomposition of EMIm + . As a result, the irreversible capacity of initial cycle was significantly suppressed and the coulombic efficiency of subsequent cycles was greatly improved.

Insight into the Solid Electrolyte Interphase Formation in Bis(fluorosulfonyl)Imide Based Ionic Liquid Electrolytes

“…It has been calculated that the reduction potential of LiFSI could be as high as 2.3 V for the concentrated LiFSI/DME electrolyte in lithium sulfur batteries, though a high thermal energy is needed to initiate the reduction [44] . A similar observation was reported by Uchida et al [45] . with the use of the LiFSI/EMIm.FSI ionic liquid electrolyte, where LiF on the surface of graphite was observed, indicating the reduction of FSI − anion.…”

“…with the use of the LiFSI/EMIm.FSI ionic liquid electrolyte, where LiF on the surface of graphite was observed, indicating the reduction of FSI − anion. It is reported that a high temperature pretreatment is needed (or cycling at elevated temperatures) for the relatively low concentration LiFSI/EMIm.FSI (0.32 m ) electrolyte to form a LiF rich SEI layer on the graphite through a non‐electrochemical mechanism [45] . Recently, Alvarado et al.…”

“…It is evident from the F 1s spectra in Figure 6 and Table 3 that the major contribution from the FSI salt is LiF (685.7 eV), which starts to form already at OCV, builds on the surface during discharge, through the voltage at 1.8 V, and until the end of discharge at 0.005 V. [ 40,50 ] It is more surprising to observe LiF formed at OCV, which suggests that its formation must come from chemical reactions rather than electrochemical reactions. The possible reaction of surface functional groups of OMC with FSI anions is ruled out from the fact that LiF was also observed at OCV of a graphite base Li metal cell in a 2.00 mol kg −1 LiFSI/EMIm.FSI electrolyte [45] . Therefore, the only way to produce LiF at OCV must originate from the instability of the imidazolium ionic liquid at the lithium negative electrode and the presence of trace moisture in the electrolyte via the following routes; that is, imidazolium carbenes generated at the negative electrode (route ii) diffuse to the positive electrode where they react with trace water to regenerate EMIm cation and simultaneously generate hydroxide (route iii) that initiates chemical reaction with FSI anion to form LiF and other salt (sulfonate) (route iv) [ 54,57 ] :…”

Preface for the 66th Special Feature “Novel Aspects and Approaches to Experimental Methods for Electrochemistry”

“…In 2018, he moved to AIST as a researcher. Uchida has been extensively engaged in research on lithium secondary batteries focusing on positive electrode materials, [185][186][187] composite electrode design, 188,189 and electrode/electrolyte interfaces, 190,191 one of which has received the Excellent Paper Award of ECSJ in 2018. 186 In recent years, he has also focused on elucidating ion transport phenomena in non-aqueous electrolytes and has published several papers on this subject.…”

Lithium-ion battery performance enhanced by the combination of Si thin flake anodes and binary ionic liquid systems

“…[26][27][28] In the IL electrolytes used in the present study, the SEI layer is believed to be generated by the decomposition of FSA and TFSA anions. 14,16,[29][30][31][32][33] Fig. 5 1 eV), which is known as one of the SEI components having high Li-ion conductivity.…”