A team from Samsung R&D and Shinshu University has developed a silicon/soft-carbon nanohybrid anode material for high performance lithium-ion batteries (LIBs). The material, which is composed of micronized silicon coated with “soft-carbon” dispersed in soft-carbon matrix at nanometer level, is characterized with abundant nanosized voids (nanovoids) (diameter of ~70 nm) and hard bulk skeletal structure.
As described in a paper in the Journal of Power Sources, the material’s volume expansion ratio is 6.9% at a capacity level of 1100 mAh/g. This electrode capacity is approximately three times larger than that of graphite-based electrode currently used in LIB. Furthermore, the electrode retained 80.9% of its capacity at 250 cycles in a full cell with a LiCoO2 counter electrode. Addition of 5 wt % fluoroethylene carbonate (FEC) to the electrolyte improved the retention up to 81.3% after 300 cycles.
|A structural model of the Si/soft-carbon nanohybrid material. Kobayashi et al. Click to enlarge.|
The greatest challenges that obstruct the practical use of silicon-based materials for LIB is their short cycle life that arises from the alloying of silicon and lithium during their charging and discharging processes and the change in their big volume through large expansion and contraction during the repeated desorption of lithium from the alloy.
… Numerous studies to overcome this lifetime problem have already been conducted, including some reports that a longer battery lifetime can be achieved by lowering the absolute value of the volume change that occurs when silicon and lithium are alloyed; in these cases, longer lifetimes were achieved through the fabrication of thinned silicon electrodes and micronization of the silicon. However, the thin-film method has several shortcomings in terms of production costs that hinder its industrial development; specifically, the required manufacturing equipment is expensive, and its production speed is low compared to that of the method of applying an electrode paste, which is commonly practiced to produce LIB. Furthermore, micronized silicon forms SiO2 via oxidation by oxygen when handled in ordinary atmosphere, which results in increased irreversible capacity during the initial charging of LIB prepared using this method. A large number of studies have been conducted on the conjugation of silicon and graphite; however, uniform silicone graphite composites are difficult to prepare, while the amount of added silicon is difficult to increase.—Kobayashi et al.
To synthesize their silicon/soft-carbon hybrid, the team first prepared a slurry of silicon/ethanol. This mixture was milled with Φ0.2-mm zirconia balls in an SC10 wet bead mill for 5 h and then with Φ0.05-mm zirconia balls for 18 h. The milled mixture was combined with citric acid and silicon in the ratio of 16.7:83.3–66.7:33.3 wt%. It was then transferred to an electric furnace for calcination under flowing argon to obtain carbon-coated silicon granules.
The carbonization/calcination procedure consisted of preserving and drying the sample at 60 ˚C for 1 h to remove ethanol, heating it at 500-900 C for 4 h, and then left it to cool to room temperature.
The resulting carbon-coated silicon was mixed with poly-vinyl chloride, the soft-carbon precursor, in an agate mortar. It was set in an electric furnace for calcination; this process consisted of heating the sample to a predetermined temperature at a heating rate of 5 C/min, holding the sample at 600 C for 1 h, and then allowing it to cool naturally. The sample was milled using an agate mortar or a planetary ball mill after calcination.
The researchers assessed the electrochemical performance of the both the the carbon-coated silicon and the carbon-coated silicon/soft-carbon nanohybrid materials.
While the carbon-coating improves the cycle performance of Si, the results were still insufficient for practical applications, they found.
Because soft-carbon contains numerous gaps between its turbostratic [a crystal structure in which basal planes have slipped out of alignment] carbon units, unlike graphite, and has a harder structure than graphite, the soft-carbon should be an optimal matrix material for suppressing and assimilating the volume changes of silicon via its conjugation with silicon. Furthermore, the electrode characteristics of soft-carbon differ from those of graphite. The electrode potential gradually approaches toward the potential of lithium during the lithium-ion intercalation process. On the contrary, graphite shows a plateau potential. In the case of graphite and silicon composites that have been well studied in the past, silicon is always fully charged against graphite because the alloying potential of silicon with lithium ions is nobler than the lithium-ion intercalation potential of graphite. Such situation facilitates the degradation of Si because silicon rapidly degrades when it is fully charged and discharged. Moreover, the ratio of Si to graphite is fairly small for the nanocomposites of silicon and graphite. Preparation of nanocomposites of silicon and graphite requires high temperature; high temperature condition tends to generate SiC, which adversely affects battery performance. In contrast, the silicon/soft-carbon nanohybrid materials prepared in the present study have no limitations on the Si content because soft-carbon is synthesized at several hundred degrees, which is less than the formation temperature of SiC. It can suppress the deterioration of silicon due to the full charging and discharging through control of its charge amount in the soft-carbon matrix.
… It should be a promising next-generation and high-capacity electrode material for replacing graphite. It will positively affect battery systems for portable electronics as well as for electric vehicles. In the future, this material could be developed into a battery system with greater capacity and longer cycle life by combining it with optimized positive electrode materials (e.g., Co- or Ni-, Mn- based materials) and electrolytes.—Kobayashi et al.
Naoya Kobayashi, Yuki Inden, Morinobu Endo (2016) “Silicon/soft-carbon nanohybrid material with low expansion for high capacity and long cycle life lithium-ion battery,” Journal of Power Sources, Volume 326, Pages 235-241 doi: 10.1016/j.jpowsour.2016.06.117