Nanowire batteries

A nanowire battery uses nanowires to increase the surface area of one or both of its electrodes. Some designs (silicon, germanium and transition metal oxides), variations of the lithium-ion battery have been announced, although none are commercially available. All of the concepts replace the traditional graphite anode and could improve battery performance.

Silicon is a desirable material for lithium battery anodes because it offers extremely desirable material properties. Silicon has a low discharge potential and a high theoretical charge capacity ten times higher than that of typical graphite anodes currently used in industry. Nanowires could improve these properties by increasing the amount of available surface area in contact with the electrolyte, thereby increasing the anode’s power density and allowing for faster charging and higher current delivery. However, the use of silicon anodes in batteries has been limited by the volume expansion during lithiation. Silicon swells by 400% as it intercalates lithium during charging, resulting in degradation of the material. This volume expansion occurs anisotropically, caused by crack propagation immediately following a moving lithiation front. These cracks result in pulverization and substantial capacity loss noticeable within the first few cycles.

Research done at Stanford University indicates that silicon nanowires (SiNWs) grown directly on the current collector (via VLS growth methods) are able to circumvent the negative effects associated with volume expansion. This geometry lends itself to several advantages. First, the nanowire diameter allows for improved accommodation of volume changes during lithiation without fracture. Second, each nanowire is attached to the current collector such that each can contribute to the overall capacity. Third, the nanowires are direct pathways for charge transport; in particle-based electrodes, charges are forced to navigate interparticle contact areas (a less efficient process). Silicon Nanowires have a theoretical capacity of roughly 4,200 mAh g^-1, which is larger than the capacity of other forms of silicon. This value indicates a significant improvement over graphite, which has a theoretical capacity of 372 mAh g^-1.

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