Si-Based Negative Elctrodes for Lithium-Ion Batteries Utilizing Carbon Paper Matrix

Introduction

   Silicon-based materials have potential high capacities exceeding that of graphite. However, their poor cycleability due to the intrinsic and inevitable volume change during cycling is a significant problem from  a practical point of view. Our preliminary study on silicon material taught us following points:

   - Tight contact between silicon and conductive carbon is effective.

   - Long-range conductive pass by carbon fiber or nickel foam is also effective other than the short-range one by carbon powder.

   - PVDF is insufficient to fix silicon powders against their volume change.

   - Three-dimensional conductive network with much pore volume is effective to absorb volume change.

Based on the above, we developed Si/C/CP electrodes, consisting of a silicon/carbon composite deposited onto a carbon paper (CP). The Si/C/CP electrode prepared from silicon and PVC (polyvinyl chloride) of 10:90 in weight showed an initial capacity of 1720 mAhg^-1 and Coulombic efficiency of 90%, and sable cycleability [1]. However, this electrode showed 25% increase in thickness after 100 cycles in a liquid electrolyte, which is serious in practical use. We recently found that PEO(polyethylene)-based polymer electrolyte could suppress the increase in thickness of the Si/C/CP electrode.

Experimental

   Si and PVC (10:90) were mixed in THF to prepare a slurry, and a carbon paper was immersed the in the slurry, after dying, it was heated at 900 ºC for 2 h in 2% H₂-Ar. The weight ratio composing the resultant Si/C/CP was 10/10/100. The amount of Si in the electrode was 0.6 mg cm^-2.

   The PEO-based polymer electrolyte consisted of PEO (Aldrich, average molecular weight: 6x10⁵) and LiTFSI with molecular ratio of Li:O = 1:18. The polymer electrolyte sheet contained a non-woven texture of polymer materials for reinforcement. The sheet thickness was approximately 220 μm.

   We used Pouch cells of Li/liTFSI-PEO₁₈/Si/C/CP to measure the performance of the Si/C/CP electrodes. The two sheets of the Si/C/CP electrode and PEO were assembled and preheated at 80^oC for 3 h to ensure tight contact and penetration of the PEO electrolyte into the electrode. The cells were charged and discharged at constant current of 0.1 A g^-1 to Si/C composite at 60^oC between 0.02 and 1.5 V.

Results and discussion

   Fig. 1 illustrates the charge-discharge cycle test result of Li/LiTFSI-PEO₁₈/Si/C/CP pouch cell, which shows an initial capacity of 980 mAh g^-1 and 710 mAh g^-1 after 250 cycles. The Coulombic efficiency at the first cycle was 77%, which was lower than that in organic liquid electrolyte, showing 90%, but as the same level as that shown by MCMB electrode in PEO electrolyte system [2]. After 100 cycles, Coulombic efficiency kept 100% .

   The cross-sectional SEM images for Si/C/CP electrode after 250 cycles revealed no change from the initial thickness of 145μm, which is a significant improvement from the thickness change of 25% for the same electrode after 40 cycles in liquid organic electrolyte. Two reasons are supposed to account for this improvement, mechanical and chemical one. The former might be caused by the tight and soft contact of PEO sheet to Si/C/CP electrode to suppress and confined the mechanical stress by the volume change of Si into the open pores of the carbon paper. The latter means the qualitatively and quantitatively different SEI layers formed by different electrolytes. The analysis to elucidate the mechanism will be shown at the presentation.

References

 [1] Q. Si, M. Matsui, T. Horiba, O. Yamamoto,

Y. Takeda, N. Seki, N. Imanishi, J. Power Sources 241 (2013) 744-750.

[2] M. Kawakubo, Y. Takeda, O. Yamamoto, N. Imanishi, J. Power Sources, 225(2013) 187-191.