Si Nanoparticles inside a Carbon Matrix As an Anode for Next Generation LIB
Authors: Iwashima Toshiki , Arakawa Taichi , Misaki Hidehiko , Ishiduka Yuto , Maniwa Atsushi , Watanabe Toshinari , Kajita Tetsuya , Abe Masanori , Takato Shuji 
Published: December 2020

1. Introduction

Lithium-ion batteries (LIB) are used in many applications such as cellular phones, mobile computers and electrical vehicles since they have high capacity and long cycle life. There is a continuing demand for advanced LIBs with higher energy density. However, graphite, conventional anode materials for commercial LIBs, are approaching their theoretical capacity limits. In this context, Si anodes have attracted much attention for the alternatives of graphite anodes because of their high capacity, low cost and environmental benignity. However, Si anodes suffer from drastic volume change and huge stress generation during the lithiation and delithiation process, which causes fracture of Si and deformation of the electrode. Such unfavorable phenomena arising from the severe volume change results in the poor cycle ability of Si anodes. These crucial problems hamper the direct utilization of Si anodes for next-generation LIBs.
Amorphous SiO
x
(x<2) anodes have shown the cycle performance improvement since the amorphous structure and SiO2 phase serve as the buffer layer which suppresses the volume expansion during the lithiation delithiation process. However, the irreversible capacity of the first cycle is large due to the formation of inactive Li-Si-O phase from SiO2 phase. [1-2]
Much research on composite anodes containing Si and graphite has been investigated, and improvement of the cycle performance of composite electrodes have been demonstrated. However, the amount of Si in the composite electrode is limited since there is a strong trade-off relationship between the capacity and the cycle performance in the composite electrode. [3-4]
Recently, we have successfully synthesized Si nanoparticles inside a Carbon Matrix (SCM). Here, we show the electrochemical Li storage performance of SCM.

2. Experimental

Synthesis of
Si nanoparticles with Voids inside a Carbon Matrix (SCM)

We synthesized SCM through chemical techniques. The cross-sectional SEM image of SCM is shown in Fig. 1. Si nanoparticles with voids inside the carbon matrix are observed.

Electrode preparation

SCM powder as active material, carbon powder as conductive additive and polyacrylate as binder at a mass ratio of 92:1:7 were mixed in water to form slurry. The slurry was coated onto Cu foil (10μm thickness) and dried at 90 ºC for 5 min, and then heated in vacuum at 100 ºC overnight. The electrode was punched into discs typically with a diameter of 13.8 mm. The weight of active material in the electrode is ranged from 3.0 to 4.0 mg.

CR2032 cell preparation

Coin type cell (CR2032) was assembled in the Ar-filled glove box. Li metal (Honjyou metal) was used as the counter electrode. Polypropylene was used as the separator. The liquid electrolyte used was 1.2 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 vol %) with fluoroethylene carbonate (FEC) (2 vol %).

3. Results

First charge –discharge curves of the SCM electrode with current rate of 0.1 C is shown in Fig. 2. During the first cycle, the electrode exhibited a reversible capacity of 680 mAhg-1 with an efficiency of 75.6 %. Cycle performance of the SCM electrode and shape of the SCM particle after cycling will be reported and discussed.

4. Reference

[1] J. Yang et al. SiOx-based anodes for secondary lithium batteries. Solid State Ionics
152-153, 125-129 (2002).
[2] M. N. Obrovac et al. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev.
114, 11444−11502 (2014).
[3] Y. liu et al. Silicon/carbon composites as anode materials for Li-ion batteries. Electrochem. Solid State Lett.
7 (10), A369-372 (2004).
[4] U. Kasavajjula et al. Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells. J. Power Sources
163, 1003-1039 (2007)

Figure 1