All solid-state Li-ion batteries (LIBs) are considered as promising candidates for post LIBs. Recently, some inorganic sulfides have been reported as highly conductive Li-ion conductors. Hayashi et al. found that Li₂S-P₂S₅ glassy solid electrolytes showed high ionic conductivity comparable to liquid electrolytes [1]. Furthermore, the Li₂S-P₂S₅ solid electrolytes are so soft and flexible that a closer contact interface between an electrode and solid electrolyte without any voids or cracks is formed by hot pressing technique, which is quite different from the oxide solid electrolyte/electrode interphase. Previously, we investigated the charge transfer reaction at the interface between Li₂S-P₂S₅ solid electrolyte and Li metal electrode by electrochemical methods with microelectrodes [2, 3]. In this study, deposition/dissolution of Li at various metal negative electrodes (In, Al, Sn) with alloying was investigated. Activation energy (E_a) for charge transfer reaction at the interface between these electrodes and solid electrolyte was evaluated to discuss suitability for the negative electrode of all solid-state LIBs.

Experiments

 Li₂S-P₂S₅ solid electrolyte powders were prepared with Li₂S and P₂S₅ powders by mechanical milling technique in an Ar-filled glove box [1]. For electrochemical measurements, a two-electrode cell was prepared. Four metal disk electrodes (φ 500 μm) were used as the working electrode and Li disk with 8 mm in diameter was used as the counter and reference electrodes. Li was deposited on the working electrode at -150 mV (vs. Li/Li⁺) and the potential was stepped as -10, +10, -20, +20 mV (vs. OCV) until the potential reached at ±150 mV.

Results and discussion

 To evaluate exchange current density i₀, steady-state current i and overpotential η were inserted into Allen-Hickling equation [Eq (1)];

 ln[i/{1-exp(Fη/RT)}] = ln i₀-αF/RT    (1)

 where F, R, T and α are Faraday constant, gas constant, absolute temperature and transfer coefficient, respectively. The value of i₀ was evaluated from the y-intercept of each plot and replotted against the reciprocal of absolute temperature to calculate the E_a for the Li⁺/Li couple reactions using the following Arrhenius equation;

 ln i₀ = lnA-(E_a/RT)   (2)

where A is frequency factor. Figure 1 shows Arrhenius plots of i₀ for four metal negative electrodes and Figure 2 shows E_a values calculated with Arrhenius plots. The In and Al electrodes had E_a value close to Li electrode. On the other hand, the Sn electrode had the E_a value larger than other metals. To investigate the reason why the Sn electrode showed high E_a value, we tried to find the rate determining steps of the charge transfer reaction between Li₂S-P₂S₅ solid electrolyte and alloy negative electrodes by using electrochemical methods. We found the rate determining steps for the In and Al electrodes were formation of Li alloy but that for the Sn electrode was the diffusion of Li in Sn metal. This difference suggests the diffusion rate of Li in Sn will be lower than that in In and Al. These results suggest that the In and Al electrodes are suitable for the negative electrode of all solid-state LIBs compared to the Sn electrode.

References

[1] A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc., 84, 477 (2001).

[2] M. Chiku, W. Tsujiwaki, E. Higuchi, H. Inoue, Electrochemistry, 80, 740 (2012).

[3] M. Chiku, W. Tsujiwaki, E. Higuchi, H. Inoue, J. Power Sources, 244, 675 (2013)