The capacity of lithium-ion batteries could be increased up to six times by using anodes made of silicon instead of graphite, new research suggests.

Lithium-ion batteries provide laptops, smart phones and tablet computers with energy. However, their capacity when it comes to applications such as electric vehicles may be limited because electrode materials such as graphite are able to stably adsorb only a limited number of lithium ions.

Semiconductor materials such as silicon are receiving attention as alternative electrodes for lithium batteries. However, while bulk silicon is able to absorb large quantities of lithium, the migration of the lithium ions destroys the silicon’s crystal structure. This can swell the volume by a factor of three, which leads to major mechanical stresses.

A team from the Helmholtz-Zentrum Berlin (HZB) Institute of Soft Matter and Functional Materials has observed in detail how lithium ions migrate into thin films of silicon. Through neutron measurements, the team demonstrated that lithium ions do not penetrate deeply into the silicon. During the charge cycle, a 20-nm anode layer develops containing a high proportion of lithium—meaning that extremely thin layers of silicon would be sufficient to achieve the maximal load of lithium.

Lithium ions migrate through the electrolyte into the layer of crystalline silicon. Image credit: HZB.Lithium ions migrate through the electrolyte into the layer of crystalline silicon. Image credit: HZB.“We were able to precisely track where the lithium ions adsorb in the silicon electrode using neutron reflectometry methods, and also how fast they were moving," says Dr. Beatrix-Kamelia Seidlhofer, who carried out the experiments.

She discovered two different zones during her investigations. Near the boundary to the electrolytes, a roughly 20-nm layer formed that had high lithium content: 25 lithium atoms were lodged among 10 silicon atoms. A second adjacent layer contained one lithium atom for 10 silicon atoms. Both layers together were less than 100 nm thick after the second charging cycle.

After discharge, roughly one lithium ion per silicon node in the electrode remained in the silicon boundary layer exposed to the electrolytes. The researchers calculated that the theoretical maximum capacity of these types of silicon-lithium batteries lies at about 2300 mAh/g, which they say is more than six times the theoretical maximum attainable capacity for a lithium-ion battery constructed with graphite (372 mAh/g).

The researchers say their findings could improve the design of silicon electrodes. Very thin silicon films should be sufficient for adsorbing the maximum possible amount of lithium, which in turn would save on material and energy consumed during manufacturing.

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