According to Stanford assistant professor of materials science and engineering, Yi Cui that nano-sized silicon structures can absorb lithium without breaking down is “a revolutionary development.” At institutions around the globe, material scientists are working on how to extend this capacity over more charge / discharge cycles.
With a theoretical capacity some 10 times that of graphite, silicon anodes could contribute to a doubling of the capacity of graphite-anode Li-ion batteries. However, a silicon anode experiences a large volume expansion during lithium-ion insertion and a consequent shrinkage during extraction; this leads to severe particle pulverization, resulting in quick failure of the electrode structure and resulting capacity fade with cycling.
Green Car Congress reports that researchers at the Pacific Northwest National Laboratory have been able to show 1600 mAh/g after 40 charging / discharging cycles. They accomplished this with a high-capacity silicon anode material.
Material scientists were able to take the nanopore structure, coat the Si surface, and include elastic carbon among the silicon particles provides. They hope this approach will lead to a cost-effective way to use large, micrometer-sized Si particles in Li-ion batteries.
Dr. Jason Zhang and the PNNL research team are addressing that challenge by designing a silicon particle architecture that would maintain structural integrity. The porous structure of the Si helps accommodate the large volume variations that occur during the Li insertion / extraction processes.
Chemical vapor deposition (CVD) of carbon coatings and highly elastic Ketjen Black (KB) carbon were used to improve the electrical conductivity throughout all cycling stages. The team placed these anodes between graphene—planar sheets of bonded carbon atoms—to maintain strong electrical contact between silicon particles.
The PNNL research team continues to improve the performance and long-term stability of the silicon anodes from 40 to 50 charging/discharging cycles today to a goal of about 500 cycles in the future. One solution may be the development of a better binder that can maintain improved mechanical and electrical contact. This method has potential for much greater cyclability while maintaining high energy density.
Editor’s note: As explanation of the title, readers may note that Battelle has operated PNNL for DOE and its predecessors since 1965.