Electrochemical Performance of Gas-Atomized Si-Alloy Anode for Lithium-Ion Battery

Introduction

 Si is focused as high capacity active anode material of lithium-ion battery. However, the large volume changes that occur during charge-discharge cycles cause cracking and falling conductivity of electrode. In order to prevent from these problems, various materilas that have structure covering Si surface by some substances such as SiOx have been studied^1,2.

 We have improved conductivity of Si anode material by fabricating the unique microstructure that crystalized alloy-matrix phases around Si phases³.Here, the Si-alloy powder was prepared by gas atomization. One of the important features of gas atomization process is to be able to produce quenched powder in large volume.

 In this study, we investigated influence of Si composition and paticle size of gas-atomized Si-alloys on electrochemical properties for anode material such as cycle stability, initial clulombic efficiency, swelling of electrode during charge-discharge cycles.

Experiment

 Si-alloy powder (d50 = 18 µm) was prepared by gas atomization. Fine powder (d50 = 2~5 µm) was obtained by planetary ball mill. An electrode was prepared by coating slurry of 80 mass% Si-alloy powder, 5 mass% ketjen black and 15 mass% polyamic-acid on SUS foil. The electrode was tested in a 2032-type coin cell assembled using a lithium foil anode, a separator and electrolyte (1M LiPF₆ in EC/DEC (1:1 volume ratio)). The charge-discharge performance was evaluated at 25 degrees C and the current density of 0.2 mA (1st cycle), 0.2C-rate (from 2nd cycle to 50th cycle) in the potential range of 0.002-1.0 V vs. Li/Li⁺.

Results and Discussions

 Fig.1 shows SEM image of Si-Sn-Fe-Cu alloy powder. The microstructure of the powder is composed of three phases (Si, Si-Fe and Sn-Cu phases). It can be seen that the Si phase is surrounded by Si-Fe and Sn-Cu phases.

 Fig.2 shows cycle life performance of Si-alloy powder anodes having different particle size. When patricle size is large (d50 = 18 µm), the discharge capacity dropped with increasing cycle number. On the other hand, cycle stability of the fine powder (d50 = 2 µm) was drastically improved.

 Fig.3 shows the relationship between the initial columbic efficiency and the Si composition of the Si-alloy. The initial coulombic efficiency decreaced by the particle size reduction and improved by increasing of the Si contens in the Si-alloy.

 It was clarified that the particle size and the Si contents in the Si-alloy powder affected the electrochemical performance.

References

1.T.Morita, N.Takami, J.Electrochem. Soc., 153(2), A425(2006)

2.M.Miyachi et al., J.Electrochem. Soc., 152(10), A2089(2005)

3.Y.Tago et al., The Electrochemical Society in Japan 80^th Spring meeting proceedings,1H22(2013)