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Ananowire battery usesnanowires to increase the surface area of one or both of itselectrodes, which improves the capacity of the battery. Some designs (silicon, germanium andtransition metal oxides), variations of thelithium-ion battery have been announced, although none are commercially available. All of the concepts replace the traditionalgraphite anode and could improve battery performance. Each type of nanowire battery has specific advantages and disadvantages, but a challenge common to all of them is their fragility.[1]
Silicon is an attractive material for applications aslithium battery anodes because of its discharge potential and 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, increasing the anode’spower density and allowing for faster charging and discharging. However, silicon swells by up to 400% as italloys with lithium during charging, causing it to break down. This volume expansion occursanisotropically, 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.[2]
Nanowires may help mitigate the volume expansion. The small nanowire diameter allows for improved accommodation of volume changes during lithiation. Another advantage is that, because all nanowire are attached to the current collector, they can serve as direct pathways for charge transport. By contrast, in particle-based electrodes, charges are forced to move from particle to particle, a less efficient process. Silicon nanowires have a theoretical capacity of roughly 4,200 mAh g−1, larger than that of other forms of silicon and much larger than that of graphite (372 mAh g−1).[3]
Like graphite anodes, silicon anodes formpassivation layers (solid-electrolyte interphases) on their surfaces during the first charge cycle. Coating silicon nanowires with carbon can improve the stability of these layers.[4]
Doping impurities, such as phorphorus or boron, into the nanowire anode can also improve performance by increasing the conductivity.[5]
An anode usinggermanium nanowire was claimed to have the ability to increase the energy density and cycle durability of lithium-ion batteries. Like silicon, germanium has a high theoretical capacity (1600 mAh g-1), expands during charging, and disintegrates after a small number of cycles.[6][7] However, germanium is 400 times more effective atintercalating lithium than silicon, making it an attractive anode material. The anodes claimed to retain capacities of 900 mAh/g after 1100 cycles, even at discharge rates of 20–100C. This performance was attributed to a restructuring of the nanowires that occurs within the first 100 cycles to form a mechanically robust, continuously porous network. Once formed, the restructured anode loses only 0.01% of capacity per cycle thereafter.[8] The material forms a stable structure after these initial cycles that is capable of withstanding pulverization. In 2014, researchers developed a simple way to produce nanowires of germanium from anaqueous solution.[9]
Transition metal oxides (TMO), such as Cr2O3, Fe2O3, MnO2, Co3O4 and PbO2, have many advantages as anode materials over conventional cell materials for lithium-ion battery (LIB) and other battery systems.[10][11][12] Some of them possess high theoretical energy capacity, and are naturally abundant, non-toxic and also environmentally friendly. As the concept of the nanostructured battery electrode has been introduced, experimentalists start to look into the possibility of TMO-based nanowires as electrode materials. Some recent investigations into this concept are discussed in the following subsection.
Lead-acid battery is the oldest type of rechargeable battery cell. Even though the raw material (PbO2) for the cell production is fairly accessible and cheap, lead-acid battery cells have relatively smallspecific energy.[13] The paste thickening effect (volumetric expansion effect) during the operation cycle also blocks the effective flow of the electrolyte. These problems limited the potential of the cell to accomplish some energy-intensive tasks.
In 2014, experimentalist successfully obtained PbO2 nanowire through simple templateelectrodeposition. The performance of this nanowire as anodefor lead-acid battery has also been evaluated. Due to largely increased surface area, this cell was able to deliver an almost constant capacity of about 190 mAh g−1 even after 1,000 cycles.[14][15] This result showed this nanostructured PbO2 as a fairly promising substitute for the normal lead-acid anode.
MnO2 has always been a good candidate forelectrode materials due to its high energy capacity, non-toxicity and cost effectiveness. However, lithium-ion insertion into the crystal matrix during charging/discharging cycle would cause significant volumetric expansion. To counteract this effect during operation cycle, scientists recently proposed the idea of producing a Li-enriched MnO2 nanowire with a nominal stoichiometry of Li2MnO3 as anode materials forLIB. This new proposed anode materials enable the battery cell to reach an energy capacity of 1279 mAh g−1 at current density of 500 mA even after 500 cycles.[16] This performance is much higher than that of pure MnO2 anode or MnO2 nanowire anode cells.
Heterojunction of different transition metal oxides would sometimes provide the potential of a more well-rounded performance of LIBs.
In 2013, researchers successfully synthesized a branched Co3O4/Fe2O3 nanowireheterostructure usinghydrothermal method. Thisheterojunction can be used as an alternative anode for the LIB cell. At operation, Co3O4 promotes a more efficient ionic transport, while Fe2O3 enhances the theoretical capacity of the cell by increasing the surface area. A high reversible capacity of 980 mAh g−1 was reported.[17]
The possibility of fabrication heterogeneous ZnCo2O4/NiO nanowire arrays anode has also been explored in some studies.[18] However, the efficiency of this material as anode is still to be evaluated.
In 2016, researchers at theUniversity of California, Irvine announced the invention of a nanowire material capable of over 200,000 charge cycles without any breakage of the nanowires. The technology could lead to batteries that never need to be replaced in most applications. Thegold nanowires are strengthened by amanganese dioxide shell encased in an Plexiglas-likegel electrolyte. The combination is reliable and resistant to failure. After cycling a test electrode about 200,000 times, no loss of capacity or power, nor fracturing of any nanowires occurred.[19]
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