Effect of Particle Size on the Stability of Dense Si Electrodes
Authors: Lee Pui-kit , Yu Denis Yau Wai 
Published: February 2020

Silicon has been widely studied as a next-generation anode material for Li-ion batteries, as it has a high gravimetric capacity of up to 4000 mAh g-1 and a low average discharge voltage of about 0.4V versus Li/Li+. The biggest challenge is to maintain reversibility of the electrode despite the large volume expansion of more than 300% from the alloy reaction. Nano-materials (nanowire, nanoparticles, porous framework etc.) [1,2] and composites with carbon and graphene [3] were shown to give high capacity and good cycle stability. However, the additional space in the material leads to low packing density (typically less than 0.2 g cm-3) and thus low volumetric density.
For practical applications, stable dense Si electrodes are desirable. Though, electrodes with bulk silicon show poor cycle performance, typically attributed to factors such as particle cracking, electrode delamination, binder breakdown and loss of connectivity between particles. We have previously shown that it is possible to increase the reversibility of thickness change of electrodes with 10-20 mm silicon particles with the use of a polyimide binder [4]. Though, capacity fading is still observed with extended cycles. Characterization of the cycled electrode using scanning electron microscopy shows micro-cracks on the surface of the large silicon particles, suggesting that the initial particles are probably too big. In this study, we investigate the effect of particle size on the stability of dense silicon electrodes.
Bulk commercial silicon with a particle size of 10 to 20 µm from Sigma Aldrich is used as the starting material. The material is broken down into smaller sizes with a planetary ball mill with different rotation speed and time. The resulting samples are then mixed with acetylene black (Alfa Aesar) and polyimide binder in a weight ratio of 6:2:2 to form the electrodes. Typical electrode thickness is between 15-30 µm with a packing density of about 1.5-1.6 g cm-
2. Electrode loading is about 1.2-1.3 mg silicon cm-2. The electrodes are assembled in 2032 coin-type half-cells with Li metal as counter electrodes for electrochemical measurements. The electrodes are tested with a constant current rate of 250mA g-1 between 0 and 1 V vs. Li/Li+.

The cycle performance of three silicon electrodes with particle size of 10-20 µm (bulk), 1-10 µm (200 rpm) and less than 1 µm (500 rpm) is shown in figure 1. The electrode with bulk Si particle exhibits rapid capacity decay with cycle. The capacity retention is 41% after 100 cycles. In comparison, the capacity retention after 100 cycles is improved to 61.1% and 71.9% with samples milled at 200rpm and 500rpm, respectively. This is attributed to less particle cracking with the smaller particle size. Further works to optimize the particle size and binder, as well as to characterize and measure volume expansion of the electrode are underway, and the results will be presented at the meeting.
Reference:
1) H. Kim, B. Han, J. Choo, and J. Cho. Angew. Chem., Int. Ed. 47, 10151–10154 (2008)
2) Y. Ru, D.G. Evans, H. Zhu, and W. Yang. RSC adv. 4, 71-75 (2014)
3) M. Zhou, X. Li, B. Wang, Y. Zhang, J. Ning, Z. Xiao, X. Zhang, Y. Chang, and L. Zhi. Nano Lett. 15, 6222–6228 (2015)
4) D. Y. W. Yu, M. Zhao, and H. E. Hoster. ChemElectroChem. 2, 1090–1095 (2015)

Figure 1