All-Solid-State Three-Electrode Cells with Reduced Li4Ti5O12 Reference Electrode

Abstract

Introduction All-solid-state lithium-ion batteries (ASS-LIBs) are expected as next-generation LIBs because of high safety standard and power characteristics. However, ASS-LIBs still have several problems, such as short cycle life and high mass ratio of solid electrolytes. In order to solve these problems, it is necessary to clarify operation mechanisms of ASS-LIBs. In this study, we focused on electrochemical measurements, which have high versatility and high time resolution. Although three-electrode cells have been used in the electrochemical measurements of conventional LIBs to clarify the behavior of single electrodes, there have been few reports on three-electrode cells in ASS systems. [1] Moreover, electrochemical impedance spectroscopy (EIS) with three-electrode cells has never been reported in ASS-LIB systems. Thus, we synthesized chemically reduced Li4Ti5O12 (R-LTO) as an active material of the reference electrode and fabricated ASS three-electrode cells, and investigated electrochemical properties of LiCoO2 (LCO) composite electrode in Li-In|Li10GeP2S12(LGPS)|LCO cells by charge/discharge measurements and EIS measurements. Experimental methods R-LTO powder was synthesized by mixing of white Li4Ti5O12 powder with a tetrahydrofuran solution of lithium-naphthalenide. R-LTO reference electrode was fabricated by coating of the slurry composed of bluish black R-LTO powder, N-methyl-2-pyrrolidone and polyvinylidene fluoride on a nickel mesh current-collector. ASS three-electrode cell consisted of LCO composite electrode (LiNbO3-coated LCO:LGPS=7:3 wt%) as the working electrode, R-LTO reference electrode, LGPS as the solid electrolyte, and Li-In alloy as the counter electrode. Electrochemical measurements were carried out with a multichannel electrochemical system (VSP-300, Biologic). Results and Discussion Charge/discharge curves of the LCO electrodes and Li-In electrodes showed plateaus at around 2.35 V and –0.95 V vs. R-LTO, respectively. Assuming that the redox potential of Li7Ti5O12/ Li4Ti5O12 is 1.55 V vs. Li/Li+, we can calculate the plateau potentials of the LCO and Li-In electrodes to be 3.90 V and 0.60 V vs. Li/Li+, respectively. This clearly shows that the R-LTO reference electrodes successfully worked as an LTO redox couple. In the EIS measurements, the impedances of the LCO and the Li-In were successfully separated with the three-electrode cells since the impedance measured without the reference electrode agreed well with the combined impedance from those of the LCO and Li-In electrodes, which were separately measured with the reference electrode. We also investigated the temperature dependence and the state-of-charge (SOC) dependence of the LCO impedance. Figure 1 (a) shows the Nyquist plots obtained at 273, 288 and 298 K, and Figure 1 (b) gives the Arrhenius plots of two resistance values, R 2 and R 3, calculated with the equivalent circuit shown in the inset of Fig. 1 (a). The activation energy of R 3 was calculated to be 37 kJ mol–1, which is similar to that of the charge transfer resistance of LCO reported in ASS systems [2,3]. In addition, the SOC dependence of R 3 was similar to that of the charge transfer resistance observed for LCO electrode in a liquid electrolyte system [4]. From these results, R 3 can be assign to the charge transfer resistance. The activation energy of R 2 was 57 kJ mol–1, which is much higher than that of ionic conductivity of LGPS and also inconsistent with that of LCO electronic conductivity. Considering the SOC independence of R 2, we can assign R 2 to some interface or interphase resistance in the composite electrodes. The data with other electrode materials will be also reported in the presentation.