Negative Electrode Properties of Carbon-Coated Si Leaf Powder for Lithium-Ion Batteries

In recent years, Li-Si alloy anodes have attracted much attention for large-format lithium ion batteries (FLLIBs) because of their high theoretical capacity (ca. 4200 mAh g) [1]. However, the poor cycleability and high irreversible capacity (Cirr) in the 1 charge/discharge cycle are big problems for practical use. The poor capacity retention is ascribed to a large volume change during the charge/discharge cycling, which leads to particle fracture and electrochemical pulverization [2]. In previous studies [3-7], we reported that the use of Si thin flakes, i.e. Si Leaf Powder (Si-LP), Oike & Co., Ltd.), is effective to improve the cycleability by relaxization of the stress due to volume change. However, the high Cirr still remained in the 1 cycle. In this study, we prepared crabon-coated Si-LPs (Si-LP@C) to reduce the Cirr, and examined the negative electrode properties. Si-LP powder (thickness: 100 nm) was mixed with citric acid in ethanol, filtered and heated at 600C or 700C in Ar atmosphere. The Si-LP@C samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, thermogravimetry-differential thermal analysis (TGDTA), etc. The test electrode was fabricated by coating slurry on Cu foil as a current collector. The slurry was prepared by mixing 85.3 wt% Si-LPs, 4.9 wt% Ketjen Black (KB) as a conductive agent and 9.8 wt% carboxymethyl cellulose sodium (NaCMC) salt as a binder. The electrochemical properties were evaluated by constant current-constant voltage (CC-CV) charge/ discharge tests with a coin-type cell. The counter electrode was Li foil. The electrolyte solution was 1 M LiPF6 dissolved in EC/DEC (1:1 by volume). Fig. 1 shows TEM images of the Si-LP@C(600C) and Si-LP@C(700C). For each sample, carbon layer was homogeneously coated on the surface of Si-LP@C. Their thicknesses were 6-8 nm and 8-10 nm for the SiLP@C(600C) and Si-LP@C(700C), respectively. The carbon layer slightly became thinner by an increase in the heat-treatment temperature, indicating promotion of the graphitization of carbon layer. The presence of the carbon layer on the Si-LP@Cs was also confirmed by Raman spectra. From the TG-DTA data, the amount of carbon layer was estimated to be ca. 14 wt% for the both SiLP@Cs. Fig. 2 shows XRD patterns of the Si-LP@Cs. The Si-LP@C(600C) almost kept the amorphous phase of Si after the heat-treatment, while the Si-LP@C(700C) partially possessed crystalline Si generated by the heating. In the previous studies [3-7], we reported that the amorphous phase of Si contributes to the good cycleability in addition to the shape of thin flake. It is therefore interesting to investigate the difference of the electrode properties between the Si-LP@C(600C) and Si-LP@C(700C). Table 1 summarized the initial discharge and irreversible capacities of the Si-LP and SiLP@C electrodes at C/6 rate. For both Si-LP@Cs, the Cirr in the 1 cycle was successfully reduced down to a half (around 1100 mAh g) of the non-coated one (2336 mAh g) though the charge/discharge capacity was almost kept (over 2100 mAh g at the 5 cycle). This suggests that the carbon-coating is an effective technique to suppress the excessive electrolyte decomposition on the surface of Si-LPs. Further characterization and investigation on cycleability for the Si-LP@Cs are now in progress and will be present.