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
Silicon is an attractive material for applications as lithium battery anodes because it offers advantageous material properties. In particular, 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.
The extensive 2007 Review Article by Kasavajjula et al. summarizes early research on silicon-based anodes for lithium-ion secondary cells. In particular, Hong Li et al showed in 2000 that the electrochemical insertion of lithium ions in silicon nanoparticles and silicon nanowires leads to the formation of an amorphous Li-Si alloy. The same year, Bo Gao and his doctoral advisor, Professor Otto Zhou described the cycling of electrochemical cells with anodes comprising silicon nanowires, with a reversible capacity ranging from at least approximately 900 to 1500 mAh/g.
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 in its fully lithiated state of LiC6.
Additional research has involved depositing carbon coatings onto 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, thereby 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 exhibit significant fade in energy capacity with more charge cycles; this is caused by the volumetric expansion of silicon nanowires during lithiation process. Researchers has proposed many solutions to remedy this problem: published results in 2012 showed that doping impurities to the nanowire anode improves the battery performance, and it was found that phosphorus-doped Si nanowires achieved better performance when compared with boron and undoped nanowire electrodes; researchers also demonstrated the possibility of sustaining an 85% of initial capacity after cycling over 6,000 times by placing a 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.
Germanium
An anode using germanium 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. However, germanium is 400 times more effective at intercalating 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. 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 an aqueous solution.
Transition metal oxides
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. Some of them possess high theoretical energy capacity, and are naturally abundant, non-toxic and also environmental friendly. As the concept of the nanostructred 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 oxide anode
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 small specific energy. 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 template electrodeposition. The performance of this nanowire as anode for 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. This result showed this nanostructured PbO2 as a fairly promising substitute for the normal lead-acid anode.
Manganese oxide
MnO2 has always been a good candidate for electrode 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 for LIB. 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. This performance is much higher than that of pure MnO2 anode or MnO2 nanowire anode cells.
Heterostructure TMOs
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 nanowire heterostructure using hydrothermal method. This heterojunction 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.
The possibility of fabrication heterogeneous ZnCo2O4/NiO nanowire arrays anode has also been explored in some studies. However, the efficiency of this material as anode is still to be evaluated.
Gold
In 2016, researchers at the University 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. The gold nanowires are strengthened by a manganese dioxide shell encased in an Plexiglas-like gel 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.
