Anodes made out of silicon combined with sulfur doped graphene for high energy density for lithium ion batteries

The most common anodes in commercial lithium ion batteries are graphite intercalation anodes. Graphite is a form of solid carbon which consists of stacked layers with each layer taking the form of a 2D honeycomb lattice. In the process of intercalation, positively charged lithium ions slip between the graphite layers while the carbon layers gains electrons to balance the overall charge in the intercalation compound. Graphite anodes are comparatively cheap to manufacture and they have very long cycle life. However, there energy density is relative modest.

Silicon has been frequently explored as an alternative to graphite for lithium ion battery anodes. When graphite has has absorbed a maximum capacity of lithium ions it contains one lithium atom for each 6 carbon atoms. When silicon as absorbed a maxim capacity of lithium it contains 15 lithium atoms for each 4 atoms of silicon. As a result the theoretical maximum energy density of a silicon anode is much higher (nearly ten times) than that of a graphite anode.

However, the high ratio of lithium to silicon in the fully charged state leads to a large increase in the anode volume (nearly 300%) which leads to silicon particle cracking and degradation of the anode performance after a relative small number of cycles.

An article published by chemical engineering researchers at the University of Waterloo in Ontario, Canada entitled Evidence of covalent synergy in silicon–sulfur–graphene yielding highly efficient and long-life lithium-ion batteries points the way to a possible silicon anode design which overcomes the cycle life problem.

This anode design embeds silicon nano-particles in a matrix of sulfur doped graphene and polyacrylonitrile (PAN). Graphene is a single atom thick two dimensional hexagonal lattice of carbon atoms. The hexagonal layers of carbon mention above in the discussion of graphite anodes are in fact graphene if they can be separated from the other layers of carbon which from graphite particles are composed. This separation can be achieved by various methods. One method is to disperse graphite particles in an organic solvent which separates the hexagonal layers.

Doping the graphene with sulfur helps to create strong chemical bonds with the silicon nano-particles. The precise role played by PAN, which is a synthetic, semi-crystalline organic polymer resin, with the linear formula (C3H3N)n is not clear to me, but apparently it is needed to stabilize the overall electrode structure.

The Waterloo researchers claim to have build an anode that maintain a specific current capacity of >1000mah/gm (the theoretical maximum for graphite is 372mah/gm) for 2275 cycles. The incredibly precise number of 2275 cycles probably means that they had a single sample which failed after exactly this many cycles. This sample provides an existence theorem for a long cycle life silicon anode, but does not prove that such results can be achieved on a consistent basis. Furthermore even longer cycle life is desirable that was demonstrated in the experiment are desirable for certain applications.

Interestingly this anode configuration does not prevent the fracturing of the silicon particles as they repeatedly expand while absorbing lithium ions. Electron microscopy of the the anode at the end of life shows that the particles have fractured into small pieces, but the pieces remain bound to the sulfur doped graphene and apparently continue to function as effective absorbers of lithium ions over many charge cycles of the battery.

Fabrication of the electrodes sounds comparatively easy. The silicon particles, sulfur doped graphene, and PAN are mixed together with graphite oxide and the organic solvent dimethylformamide under ultrasonic radiation. The resulting slurry is coated onto copper sheets and dried by a heat treatment.

If this research ultimately results in a practical battery design one of the key cost issues will be the production of the graphene itself. Graphene is being intensively researched for a variety of applications of because of its unique chemical, electronic and mechanical properties. However, economic methods for the large scale production of graphene have not yet been developed. When and if such processes are developed then this anode design may be a candidate for improved energy density lithium ion batteries.