The potential use of silicon in lithium-ion batteries as a means of improving energy density has long been sitting in the open, but the relative lack of operational stability with silicon-based anodes has remained something of a stumbling block (though, some batteries do already incorporate limited amounts).
New research from the University of Waterloo is claiming to have overcome this (to a degree), though — through the use of observed covalent interactions between sulfur nanoparticles, sulfur-doped graphene, and cyclized polyacrylonitrile (with regard to the alteration of electrode structures).
To put it in perhaps plainer (if oversimplified) language, typically, silicon use tends to result in a relatively fragile structure, and the new research claims to have found a means of improving structural stability while still using silicon. If true, that could mean an improvement in commercial battery energy density, and perhaps costs.
As I am certainly not a battery technology specialist, I’m simply going to include some excerpts here from the research paper itself to provide some details:
This hierarchical structure stabilized the solid electrolyte interphase leading to superior reversible capacity of over 1,000 mAh g−1 for 2,275 cycles at 2 A g−1. Furthermore, the nanoarchitectured design lowered the contact of the electrolyte to the electrode leading to not only high coulombic efficiency of 99.9% but also maintaining high stability even with high electrode loading associated with 3.4 mAh cm−2. The excellent performance combined with the simplistic, scalable and non-hazardous approach render the process as a very promising candidate for Li-ion battery technology.
…Based on our DFT model, the Si atom has covalent interactions with a sulfur atom in SG and two adjacent carbon atoms. The equivalent strength of this covalent interaction is similar to that of a single covalent bond. This interaction may not involve the Si atom reacting directly with sulfur to form either SiS or SiS2, as this would require debonding of sulfur from within the graphene matrix, and may result in electrode degradation. In the case of Si clusters (to simulate nanoparticles), only a small portion of the silicon atoms form this covalent interaction with the SG. We believe that this type of Si does not participate in alloy formation with lithium; however, provides an anchoring site for the majority of Si atoms within the nanoparticle that are readily available for alloying/dealloying, thereby contributing to the observed capacity.
It can be seen that Si binds more strongly to SG than on G. One reason is the covalent interaction of Si atoms with the sulfur atom. The second reason is because the increased charge density on the defective (with nanoholes) carbon adjacent to sulfur. This indicates a covalent synergy for the interaction between Si and SG leading to a superior material electrochemical performance, which has not been seen with Si–G. It is clearly shown that, even after 2,275 cycles of charge/discharge, the amorphous SiNP re-organized into channels of the cyclized PAN and the sulfur pathway on graphene, as seen in Fig. 6.
In summary, the novel design of a Si-based electrode through the covalent binding of commercial SiNP and SG along with cyclized PAN offers exceptional potential in the practical utilization of Si anodes for LIB technologies. This covalent synergy enables superior cycling stability along with a high aerial capacity of the electrode, which is close to that of commercial technologies. Such a rational design and scalable fabrication paves the way for the real application of Si anodes in high-performance LIBs. The interaction between S and Si plays a critical role of improving the long-term cycle stability, in addition, the synergistic effect of the covalent bonds between Si–S, the facilitated charge transfer by 3-D graphene network and cyclized PAN and the improved electrode integrity all contributed to the superior cycle performance.
Those wanting more details, or to get a better idea of the research methodology, can find the paper here.
As always with battery research, it’s hard to say if practical applications will arrive or be seen anytime in the near future… or anytime in the future, for that matter. The researchers involved in the work, of course, think that it will.
“Battery researchers have been working with silicon for a while but now we’ve found a way to overcome a critical challenge,” stated Zhongwei Chen, a professor of chemical engineering at Waterloo. “There’s been considerable investor interest in the new technology and we expect to have it commercialized and on the market within the next year.”
“The economical flash heat treatment creates uniquely structured silicon anode materials that deliver extended cycle life to more than 2000 cycles with increased energy capacity of the battery.”
(h/t to “Ktowntslafan” on the Tesla Motors Club forums)