Influence of Heat Treatment on Charge/Discharge Characteristics of Sn-Ag Alloy Plating Films for Li-Ion Battery Anode

Introduction

Lithium ion batteries have energy densities higher than those of conventional rechargeable batteries. Although graphite is currently used as the anode active material in lithium ion batteries, it is necessary to consider alternative materials for obtaining higher capacity. In this research, tin (991 mAh g^-1) with a capacity higher than graphite (372 mAh g^-1) was used as an active material. However, during charging and discharging of tin, it pulverizes due to a large volume change, and the cycling characteristics are deteriorated. Therefore, alloys such as Sn-Cu and Sn-Ni have been examined for suppressing volume change. The Sn-Ag alloy¹⁾ especially has been reported as an anode material with excellent cycling characteristics. The charge/discharge characteristic of an Sn-Ag alloy plating film produced by electrodeposition were reported.²⁾ SANYO has reported³⁾ that the cycling characteristics of tin were improved by heat treatment.

In this study, we explored the influence of heat treatment of the Sn-Ag alloy plating film on copper on the charge/discharge characteristics.

 

Experimental

An Sn-Ag alloy plating bath⁴⁾ containing 1 M K₄P₂O₇�+0.25 M Sn₂P₂O₇�+2 M KI�+0.02 M AgI was prepared. Electroplating was carried out by the current regulating method at 25°C. A pure copper plate and a platinum plate were used as the cathode and anode, respectively. The current density was changed and Sn-Ag alloy plating films with various compositions were produced and subjected to stress relief heat treatment.

The microstructure of the Sn-Ag alloy plating films was examined by field-emission scanning electron microscopy (SEM). The phase structure of the deposits was analyzed by X-ray diffraction (XRD).

Electrochemical studies of the Sn-Ag alloy plating films were carried out with coin cells that were assembled in an Ar-filled glove box. Each coin cell consisted of a lithium foil as a counter electrode and the Sn-Ag alloy plating film as a working electrode. The electrolyte was 1 M LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol%). Cycling tests were performed in the range 0.02-1.5 V (vs. Li/Li⁺) at a constant temperature of 25°C.

 

Results and Discussion

By changing the current density, a silver content in the range 5-60 at% in the Sn-Ag alloy plating films was obtained. The films were found to consist of a beta-Sn phase and an Ag₃Sn phase by XRD. From the Sn-Ag alloy phase diagram, it was confirmed that the obtained Sn-Ag alloy plating films were an equilibrium phase. Moreover, when the silver content in the plating film was increased, there was a tendency for the roughness  of the film surface to increase.

Figure 1 shows the cycling characteristics of the Sn-10 at% Ag alloy plating film before and after heat treatment. In the first cycle, the discharge capacities of both films are almost the same (about 600 mAh g^-1). After 3 cycles, the discharge capacity of the film before heat treatment decreased abruptly at the initial stage and remained at about 100 mAh g^-1. In contrast, the heat-treated films showed a small deterioration in capacity at the initial stage and subsequently maintained a relatively higher capacity of about 400 mAh g^-1.

References

1)  Jingtian Yin, Masashi Wada, Seiji Yoshida, Kouji Ishihara, Shigeo Tanase and Tetsuo Sakai, Journal of The Electrochemical Society, 150(8), A1129-A1135 (2003).

2)  Koichi Nishimura and Susumu Arai, Abstract of 80^th Electrochemical Society, p481 (2013)

3)  Noriyuki Tamura, Ryuji Ohshita, Masahisa Fujimoto, Shin Fujitani, Maruo Kamino and Ikuo Yonezu, Journal of Power Sources, 107, 48-55 (2002).

4)  Susumu Arai and Tohru Watanabe, Electrochemistry, 65(12), 1097-1101 (1997).