Electrochemical Characterization of Li2s-P2S5 Glass-Ceramic Solid Electrolyte/Li Metal Electrode Interface
All solid-state Li rechargeable batteries are considered as promising candidates for post Li-ion batteries because non-flammability of inorganic solid electrolyte enhances their safety and reliability and Li metal can be used for a negative electrode. Sulfide-based solid electrolytes like Li2S-P2S5 showed high lithium ion conductivity of about 10-4 S cm-1 at 25 °C. In general, glass crystallization results in lower conductivities. However, 70Li2S-30P2S5 glass-ceramic obtained by crystallization of the glass showed conductivity of about 10-3 S cm-1 at room temperature because enhancement of the conductivity of the glass-ceramic came from the precipitation of superionic Li7P3S11 crystal in the glass-ceramic.1
Previously, we investigated the charge transfer reaction at the sulfide-based solid electrolyte/Li metal electrode interface and organic liquid electrolyte/Li interface by electrochemical methods with microelectrodes to evaluate activation energy (Ea) of each process in charging. For the liquid electrolyte, the Ea for charge transfer reaction was much higher than that for ionic conduction. This suggests that the desolvation step of solvated Li+ has a large barrier. On the other hand, for the sulfide-based glass electrolyte, the Ea for charge transfer was comparable to that for ionic conduction due to the lacking of the desolvation step.2
In this study, the Ea for charge transfer reaction at the 70Li2S-30P2S5 glass-ceramic/Li metal electrode interface was evaluated, and it was compared with the Ea for the charge transfer at the Li2S-P2S5glass/Li metal electrode interface.
70Li2S-30P2S5 glass was prepared by mechanical milling of Li2S and P2S5 mixed powders for 12h. 70Li2S-30P2S5 glass-ceramics were prepared using one-step heat treatment 280 °C for 2h. 1 For electrochemical measurements, a two-electrode cell was fabricated. The glass-ceramic electrolyte powder was pelletized and sandwiched with Ni microelectrode (Φ50 μm) as working electrode and Li disk (Φ8 mm) attached on the stainless steel current collector as counter and reference electrode. All procedure was executed in Ar-filled glove box. Li was deposited on the working electrode at -150 mV (vs. Li/Li+) and the potential was stepped to -10, +10, -20, +20 mV (vs. OCV) until it reached ±150 mV. Each potential was held for 2 s. All measurements were performed at various temperatures.
Results and Discussion
To evaluate i0, the steady-state current (i) measured at each overpotential (η) by potential step method was inserted into Allen-Hickling equation.
Fig. 1 shows Allen-Hickling plots by the potential step method at various temperatures. All plots exhibited a linear relationship in the η range from -80 to +80 mV. From the intercept of each plot, i0 was evaluated and then i0 was plotted against the reciprocal of absolute temperature to calculate Eausing Arrhenius equation.
Fig. 2 shows the Arrhenius plot of i0 obtained from potential step method. We successfully measured the Ea of charge transfer reaction at the interface between glass ceramic electrolyte and Li metal electrode. The Eafor the charge transfer was larger than that for ionic conduction, suggesting that the mechanism of charge transfer reaction at glass-ceramic/Li metal electrode interface is different from that for glass/Li metal electrode interface.
1) K. Minami, A. Hayashi, M. Tatsumisago, J. Am. Ceram. Soc., 94, 1779 (2011).