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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 potentially improve battery performance. == Silicon == 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 have 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 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. Additional research has involved depositing a carbon coating onto the silicon nanowires, which helps stabilize the material such that a stable solid electrolyte interphase (SEI) forms. An SEI is an inevitable byproduct of the electrochemistry that occurs in the battery; its formation contributes to decreased capacity in the battery since it is an electrically insulating phase (despite being ionically conductive). It can also dissolve and reform over multiple battery cycles. Hence, a stable SEI is preferable in order to prevent continued capacity loss as the battery is used. When carbon is coated onto silicon nanowires, capacity retention has been observed at 89% of the initial capacity after 200 cycles. This capacity retention is on par with that of graphitic anodes today. One design uses a stainless steel anode covered in silicon nanowires. Silicon stores ten times more lithium than graphite, offering increased energy density. The large surface area increases the anode's power density, allowing for fast charging and high current delivery. The anode was invented at Stanford University in 2007. In September 2010, researchers demonstrated 250 charge cycles maintaining above 80 percent of initial storage capacity. However, some studies pointed out that Si nanowire anodes shows significant fade in energy capacity with more charge cycles caused by the volumetric expansion of Si nanowires during lithiation process. Researchers has proposed many solutions to remedy this problem: published results in 2012 showed doping impurities to the nanowire anode improves the battery performance, and it is showed that phosphorus doped Si nanowires achieved better performance compared with boron and undoped nanowire electrode; researchers also demonstrated the possibility of sustaining an 85% of initial capacity after cycling over 6,000 times by replacing nominally undoped silicon anode into a doubled-walled silicon nanotube with silicon oxide ion-permeating layer as coating. The silicon nanowire-based battery cell also provides opportunity for dimensional flexible energy source, which would also leads to the development of wearable technological device. Scientist from Rice University showed this possibility by depositing porous copper nanoshells around the silicon nanowire within a polymer matrix. This lithium-polymer silicon nanowire battery (LIOPSIL) has a sufficient operational full cell voltage of 3.4V and is mechanically flexible and scalable. Commercialization was originally expected to occur in 2012, but was later deferred to 2014.〔 A related company, Amprius, shipped a related device with silicon and other materials in 2013. Canonical announced on July 22, 2013, that its Ubuntu Edge smartphone would contain a silicon-anode lithium-ion battery. In January 2015, EaglePicher announced that it has signed an engineering agreement and a license agreement with OneD Material, a spin-out from Nanosys, to vertically integrate the production of a "silicon nanowire on graphite" anode called SiNANOde(tm) into new high energy density cells and batteries manufactured in Joplin, MO.〔http://www.eaglepicher.com/news/eaglepicher-news〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nanowire battery」の詳細全文を読む スポンサード リンク
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