Influence of Electrolyte Anions on Lithium-Ion Transfer Kinetics at Electrode/Electrolyte Interfaces
Authors: Hiraoka Kou , Kondo Yasuyuki , Katayama Yu , Yamada Yuki 
Published: December 2024

Introduction

Large charge-transfer resistances in lithium-ion batteries (LIBs) induced by the significant activation energy (E
a) of Li+ transfer at the electrode/electrolyte interface are responsible for the slow charge-discharge rates of LIBs. Therefore, it is necessary to improve the rate performance of LIBs by reducing the E
a of interfacial Li+ transfer for the widespread use of electric vehicles with LIBs. Previous studies have shown that the E
a of interfacial Li+ transfer depends on the de-solvation energies of electrolyte solvents and can be reduced by adopting weakly coordinating solvents[1]. However, in such electrolytes, Li+ is easily interacted with anions to form ion pairs. Therefore, the anion species may affect the E
a in weakly coordinating solvent-based electrolytes. For example, Ugata et al. investigated the E
a of Li+ transfer at the interface between positive electrodes and highly concentrated electrolytes where anion coordination to Li+ becomes dominant[2]. They suggested that the E
a in highly concentrated electrolytes is determined by electrolyte fluidity (viscosity), not the Li+-anion interaction. However, the effect of anion properties on the E
a in weakly coordinating solvent-based electrolytes has yet to be clarified. In this study, the charge-transfer resistances and the E
a of interfacial Li+ transfer in weakly coordinating solvent-based electrolytes with various anions were investigated to gain insight into the determining factors for E
a.

Experimental

The electrolytes were prepared by mixing a weakly coordinating solvent such as dimethyl carbonate (DMC) and lithium perchlorate (LiClO4) or lithium bis(fluorosulfonyl)imide (LiFSI) as Li salts with a concentration of 1 mol L-1 (M). Electrochemical measurements (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) were performed using a three-electrode cell with a Li4Ti5O12 (LTO) thin-film electrode as the working electrode and Li metal as the reference and counter electrodes. The LTO thin-film electrode was used as a model electrode to eliminate other inessential effects of binders and conductive carbon[3]. EIS measurements were conducted over a frequency range from 100 kHz to 10 mHz with an applied voltage amplitude of 5 mV at various temperatures. The E
a was evaluated from the temperature dependence of the inverse of Li+ transfer resistances at the electrode/electrolyte interface (1/R
ct) using the Arrhenius equation.

Results and Discussion

The Nyquist plots of LTO thin-film electrodes showed a potential-dependent semi-circle at around 1.56 V vs. Li/Li+ in the middle-frequency region, which was assigned to R
ct
. The E
a of interfacial Li+ transfer was evaluated from the temperature dependence of 1/R
ct. The 1 M LiClO4/DMC electrolyte gave low E
a of <50 kJ mol-1 as compared to those in conventional ethylene carbonate (EC)-based electrolyte (>50 kJ mol-1), which is consistent with the previous studies[1], [3]. In contrast, the E
a in 1 M LiFSI/DMC electrolyte was almost twice as large as that of 1 M LiClO4/DMC. It should be noted that the viscosities of these two electrolytes are almost the same. These results suggest that the anion species affect the kinetics of interfacial Li+ transfer. In the presentation, the E
a of interfacial Li+ transfer in various weakly coordinating solvent-based electrolytes will be presented to discuss the determining factors of E
a.

Reference

[1] Y. Kondo et al., ACS Applied Materials & Interfaces 2022 14 (20), 22706-22718

[2] Y. Ugata. et al., The Journal of Physical Chemistry, 2023, 127: 3977-3987

[3] Y. Ishihara. et al., ECS Electrochemistry Letters, 2014, 3 (8): A83-A86