Research in lithium-ion batteries

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Research in Lithium-ion Batteries has produced many proposed refinements of lithium-ion batteries. Areas on research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.

Anode[edit]

Lithium ion battery Anodes have traditionally been made of graphite. At this time, significant other classes of lithium-ion battery anode materials have been proposed and evaluated as alternatives to graphite, especially in cases where niche applications require novel approaches.

Titanium dioxide[edit]

In 1984, researchers at Bell Labs reported the synthesis and evaluation of a series of lithiated titanates. Of specific interest were the anatase form of titanium dioxide and the lithium spinel LiTi2O4 [1] Anatase has been observed to have a maximum capacity of 150 mAh/g (0.5Li/Ti)with the capacity limited by the availability of crystallographic vacancies in the framework. The TiO2 polytype brookite has also been evaluated and found to be electrochemically active when produced as nanoparticles with a capacity approximately half that of anatase (0.25Li/Ti). In 2014, researchers at Nanyang Technological University used used a materials derived from a titanium dioxide gel derived from naturally spherical titanium dioxide particles into nanotubes[2] In addition a non-naturally occurring electrochemically active titanate referred to as TiO2(B)can be made by ion-exchange followed by dehydration of the potassium titanate K2Ti4O9 [3]. This layered oxide can be produced in multiple forms including nanowires, nanotubes, or oblong particles with an observed capacity of 210 mAh/g in the voltage window 1.5-2.0V (vs Li).

Niobates[edit]

In 2011, Lu et al., reported reversible electrochemical activity in the porous niobate KNb5O13 [4]. This material inserted approximately 3.5Li per formula unit (about 125 mAh/g) at a voltage near 1.3V (vs Li). This lower voltage (compared to titantes) is useful in systems where higher energy density is desirable without significant SEI formation as it operates above the typical electrolyte breakdown voltage.

Lithium[edit]

Lithium anodes have been used for the first lithium-ion batteries in the previous century, based on the TiS
2
/Li
cell chemistry, but were eventually replaced due to dendrite formation which caused internal short-circuits and was a fire hazard.[5][6] Replaced in commercial cell designs in the late-1970s by graphite carbon, effort continued in areas that required lithium, including charged cathodes such as manganese dioxide, vanadium pentoxide, or molybdenum oxide and some polymer electrolyte based cell designs. The interest in lithium metal anodes was re-established with the increased interest in high capacity lithium-air battery and lithium-sulfur battery systems.

Research to inhibit dendrite formation has been an active area due in part to the need for a stable anode for these new beyond-lithium energy storage chemistries. Doron Aurbach and co-workers at Bar-Ilan University have extensively studied the role of solvent and salt in the formation of films on the lithium surface. Notable observations were the addition of LiNO3, dioxolane, and hexafluoroarsenate salts all appeared to create films that inhibit dendrite formation while incorporating reduced Li3As as a lithium-ion conductive component. [7] [8]

Carbon[edit]

Various forms of carbon are used in lithium-ion battery cell configurations. Besides graphite poorly or non-electrochemically active types of carbon are used in cells such as CNTs, carbon black, grapheme, grapheme oxides, or MWCNTs.

Recent work includes efforts in 2014 by researchers at Northwestern University who found that metallic single-walled carbon nanotubes (SWCNTs) accommodate lithium much more efficiently than their semiconducting counterparts. If made denser, semiconducting SWCNT films take up lithium at levels comparable to metallic SWCNTs.[9]

In 2015 hydrogen-treated graphene nanofoam electrodes in LIBs showed higher capacity and faster transport. Chemical synthesis methods used in standard anode manufacture leave significant amounts of atomic hydrogen. Experiments and multiscale calculations revealed that low-temperature hydrogen treatment of defect-rich graphene can improve rate capacity. The hydrogen interacts with the graphene defects to open gaps to facilitate lithium penetration, improving transport. Additional reversible capacity is provided by enhanced lithium binding near edges, where hydrogen is most likely to bind.[10] Rate capacities increased by 17–43% at 200 mA/g.[11] In 2015, researchers in China used porous graphene as the material for a lithium ion battery anode in order to increase the specific capacity and binding energy between lithium atoms at the anode. The properties of the battery can be tuned by applying strain. The binding energy increases as biaxial strain is applied.[12]

Silicon[edit]

Silicon is an earth abundant element, and is fairly inexpensive to refine to high purity. When alloyed with lithium it has a theoretical capacity of ~3,600 milliampere hours per gram (mAh/g), which is nearly 10 times the energy density of graphite electrodes (372 mAh/g).[13] One of silicon's inherent traits, unlike carbon, is the expansion of the lattice structure by as much as 400% upon full lithiation (charging). For bulk electrodes, this causes great structural stress gradients within the expanding material, inevitably leading to fractures and mechanical failure, which significantly limits the lifetime of the silicon anodes.[14][15] In 2011, a group of researchers assembled data tables that summarized the morphology, composition, and method of preparation of those nanoscale and nanostructured silicon anodes, along with their electrochemical performance.[16]

Porous silicon nanoparticles are more reactive than bulk silicon materials and tend to have a higher weight percentage of silica as a result of the smaller size. Porous materials allow for internal volume expansion to help control overall materials expansion. Methods include a silicon anode with an energy density above 1,100 mAh/g and a durability of 600 cycles that used porous silicon particles using ball-milling and stain-etching.[13] In 2013, researchers developed a battery made from porous silicon nanoparticles.[17][18] Below are various structural morphologies attempted to overcome issue with silicon's intrinsic properties.

Silicon Encapsulation[edit]

In 2016 researchers announced a method for caging 3 nm-diameter silicon particles in a shell of graphene. The particles were first coated with nickel. Graphene layers then coated the metal. Acid dissolved the nickel, leaving enough of a void within the cage for the silicon to expand. The particles broke into smaller pieces, but remained functional within the cages. [19][20]

In 2014 researchers encapsulated silicon nanoparticles inside carbon shells, and then encapsulated clusters of the shells with more carbon. The shells provide enough room inside to allow the nanoparticles to swell and shrink without damaging the shells, improving durability.[21]

Silicon Nanowire[edit]

Silicon Nanofiber[edit]

In 2015 a prototype electrode was demonstrated that consists of sponge-like silicon nanofibers increases Columbic efficiency and avoids the physical damage from silicon's expansion/contractions. The nanofibers were created by applying a high voltage between a rotating drum and a nozzle emitting a solution of tetraethyl orthosilicate (TEOS). The material was then exposed to magnesium vapors. The nanofibers contain 10 nm diameter nanopores on their surface. Along with additional gaps in the fiber network, these allow for silicon to expand without damaging the cell. Three other factors reduce expansion: a 1 nm shell of silicon dioxide; a second carbon coating that creates a buffer layer; and the 8-25 nm fiber size, which is below the size at which silicon tends to fracture.[22]

Conventional lithium-ion cells use binders to hold together the active material and keep it in contact with the current collectors. These inactive materials make the battery bigger and heavier. Experimental binderless batteries do not scale because their active materials can be produced only in small quantities. The prototype has no need for current collectors, polymer binders or conductive powder additives. Silicon comprises over 80 percent of the electrode by weight. The electrode delivered 802 mAh/g after more than 600 cycles, with a Coulombic efficiency of 99.9 percent.[22]

Tin[edit]

In 2013, researchers at Washington State University developed a tin electrode technology that they predicted would triple the energy density of lithium ion batteries. The technology involves using standard electroplating processes to create tin nanoneedles that do not short circuit when the tin expands by one third during charging.[23][24]

Cu6Sn5[edit]

A material traditionally found in lead-free solders, Cu6Sn5 has become an attractive potential anode material due to its high theoretical specific capacity, resistance against Li metal plating compared to carbon-based anodes, and ambient stability[25][26]. Lithium intercalation occurs through an alloying process where two lithium ions are inserted per Cu6Sn5 formula unit to produce a distinct crystal structure in the form of Li2CuSn. This phase transformation is accompanied by drastic volumetric expansion, up to 257%[25], presenting a major challenge toward commercial use. The stresses would loosen electrical contact to the current collector without some control mechanism. Current research focuses on investigating alloying and low dimensional geometries to mitigate mechanical stress during lithiation. Alloying tin with elements that do not react with lithium, such as copper, has been shown to reduce stress. As for low dimensional applications, thin films have been produced with discharge capacities of 1127 mAhg-1 due to lithium ion storage at grain boundaries and defect sites[25]. Other approaches include making nanocomposites with Cu6Sn5 at its core with a nonreactive outer shell, SnO2-c hybrids have been shown to be effective[27], to accommodate volume changes and overall stability over cycles. No commercially viable anode of pure Cu6Sn5 has been produced due to reliability concerns over cycling- primarily due to the large volume expansion during intercalation.

Copper antimonide[edit]

In 2015 researchers announced a solid-state 3-D battery annode made of copper antimonide. It is electroplated onto a copper foam. The anode is then layered with a solid polymer electrolyte that provides a physical barrier across which ions (but not electrons) can travel. The cathode is an inky slurry. The volumetric energy density was up to twice as much energy conventional batteries. The solid electrolyte prevents dendrite formation.[28]

Semi-solid[edit]

In 2016 researchers announced an anode composed of a slurry of Lithium-iron phosphate and graphite with a liquid electrolyte. They claimed that the technique increased safety (the anode could be deformed without damage) and energy density.[29] A flow battery without carbon, called Solid Dispersion Redox Flow Battery, was reported, proposing increased energy density and high operating efficiencies.[30][31] A review of different semi-solid battery systems can be found here.[32]

Cathode[edit]

As of 2016, cathodes are typically made of a material containing lithium.

Vanadium[edit]

In 2007, Subaru introduced a battery with double the energy density while only taking 15 minutes for an 80% charge. They used vanadium, which is able to load two to three times more lithium ions onto the cathode.[33] In 2013 researchers announced hierarchical vanadium oxide nanoflowers (V10O24·nH2O) synthesized by a reaction of vanadium foil in a NaCl aqueous solution. Electrochemical tests demonstrate deliver high reversible specific capacities with 100% coulombic efficiency, especially at high C rates (e.g., 140 mAh g−1 at 10 C).[34] In 2014, researchers announced the use of vanadate-borate glasses (V2O5 – LiBO2 glass with reduced graphite oxide) as a cathode material. The cathode achieved around 1000 Wh/kg with high specific capacities in the range of ~ 300 mAh/g for the first 100 cycles.[35] In 2017, researchers announced the use of V
2
O
5
as a cathode material in concert with a lithium metal anode (to supply lithium ions).[36]

Cobalt[edit]

In 2014, researchers at the School of Engineering at the University of Tokyo and Nippon Shokubai discovered that adding cobalt to the lithium oxide crystal structure gave it seven times the energy density.[37][38] In 2017, researchers at University of Virginia reported a scalable method to produce sub-micrometer scale lithium cobalt oxide.[39]

Graphene/lithium metal[edit]

In 2014, researchers at Rensselaer Polytechnic Institute developed an all carbon battery that improves energy density and cycle durability. After over 1,000 charges, the battery showed highly stable performance. The new battery uses an anode and cathode made from graphene with metallic lithium and without cobalt.[40]

Disordered materials[edit]

In 2014, researchers at Massachusetts Institute of Technology found that creating lithium-ion batteries with disorder in the materials they are composed of achieved 660 watt-hours per kilogram at 2.5 volts.[41]

In 2015 researchers blended powdered vanadium pentoxide with borate compounds at 900 C and quickly cooled the melt to form glass. The resulting paper-thin sheets were then crushed into a powder to increase their surface area. The powder was coated with reduced graphite oxide (RGO) to increase conductivity while protecting the electrode. The coated powder was used for the battery cathodes. Trials indicated that capacity was quite stable at high discharge rates and remained consistently over 100 charge/discharge cycles. Energy density reached around 1,000 watt-hours per kilogram and a discharge capacity that exceeded 300 mAh/g.[42]

Graphene oxide coated sulfur[edit]

In 2014, researchers at USC Viterbi School of Engineering used a graphite oxide coated sulfur cathode to create a battery with 800 mAh/g for 1,000 cycles of charge/discharge, over 5 times the energy density of commercial cathodes. Sulfur is abundant, low cost and has low toxicity. Sulfur has been a promising cathode candidate owing to its high theoretical energy density, over 10 times that of metal oxide or phosphate cathodes. However, sulfur's low cycle durability has prevented its commercialization. Graphene oxide coating over sulfur is claimed to solve the cycle durability problem. Graphene oxide high surface area, chemical stability, mechanical strength and flexibility.[13]

Nanophosphate[edit]

In 2012, researchers at A123 Systems developed a battery that operates in extreme temperatures without the need for thermal management material. It went through 2,000 full charge-discharge cycles at 45 C while maintaining over 90% energy density. It does this using a nanophosphate positive electrode.[43][44]

Transition-metal Oxides[edit]

In 2000, researchers from the Université de Picardie Jules Verne examined the use of nano-sized transition-metal oxides as cathode materials. The metals used were cobalt, nickel, copper and iron and they proved to have capacities of 700 mA h/g and maintain full capacity for 100 cycles. These promising results show that transition-metal oxides may be useful in ensuring the integrity of the lithium-ion battery over many discharge-recharge cycles.[45]

Seawater[edit]

In 2012, researchers at Polyplus Corporation created a battery with an energy density more than triple that of traditional lithium-ion batteries using seawater. Its energy density is 1,300 W·h/kg, which is a lot more than the traditional 400 W·h/kg. It has a solid lithium positive electrode and a solid electrolyte. It could be used in underwater applications.[46]

Purpurin[edit]

In 2012, researchers at Rice University, The City College of New York and U.S. Army Research Laboratory found that using purpurin (1,2,4-Trihydroxyanthraquinone) in the cathode is more environmentally friendly than using the traditional lithium cobalt oxide.[47]

Three-dimensional nanostructure[edit]

In 2011, researchers at University of Illinois at Urbana-Champaign discovered that wrapping a thin film into a three-dimensional nanostructure can decrease charge time by a factor of 10 to 100. The technology is also capable of delivering a higher voltage output.[48] In 2013, the team improved the microbattery design, delivering 30 times the energy density 1,000x faster charging.[49] The technology also delivers better power density than supercapacitors. The device achieved a power density of 7.4 W/cm2/mm.[50]

Lithium[edit]

Lithium nickel manganese cobalt oxide[edit]

In 2009, researchers at Nissan announced a lithium nickel manganese cobalt oxide cathode (NMC). The new battery offered twice the energy density.[51][needs update]

Lithium iron phosphate[edit]

In 2009, scientists at Massachusetts Institute of Technology created nanoball batteries that increased charge rates 100 times. They are capable of a 10-second re-charge of a cell phone battery and a 5-minute re-charge of an electric car battery. The cathode is composed of nanosized balls of lithium iron phosphate. The rapid charging is because the nanoballs transmit electrons to the surface of the cathode at a much higher rate. The batteries have also shown higher energy density, power density and cycle durability.[52][53]

Lithium manganese silicon oxide[edit]

A “lithium orthosilicate-related” cathode compound,  Li
2
MnSiO
4
, was able to support a charging capacity of 335 mAh/g.[54] Li2MnSiO4@C porous nanoboxes were synthesized via a wet-chemistry solid-state reaction method. The material displayed a hollow nanostructure with a crystalline porous shell composed of phase-pure Li2MnSiO4 nanocrystals. Powder X-ray diffraction patterns and transmission electron microscopy images revealed that the high phase purity and porous nanobox architecture were achieved via monodispersed MnCO3@SiO2 core–shell nanocubes with controlled shell thickness.[55]

Air[edit]

In 2013, researchers at MIT used a genetically modified virus called M13 to create crosslinked manganese oxide nanowire electrodes covered in spikes that more than double the surface area of the electrode along with its energy density. 3-5 weight-percent palladium increases conductivity. This room temperature process is water-based. Specific capacity of 7,340 mAh/ gc+catalyst) of specific energy at 0.4  A/ gc.[56]

In 2009, researchers at the University of Dayton Research Institute announced a solid-state battery with higher energy density that uses air as its cathode. When fully developed, the energy density could exceed 1,000 Wh/kg.[57][58]

Iron fluoride[edit]

Iron fluoride, a potential intercalation-conversion cathode, presents a high theoretical energy density of 1922 Wh kg−1. This material displays poor electrochemical reversibility. When doped with cobalt and oxygen, reversibility improves to over 1000 cycles and capacity reaches 420 mAh g−1. Doping changes the reaction from less-reversible intercalation-conversion to a highly reversible intercalation-extrusion.[59]

Analysis technique[edit]

In 2014, researchers at Brookhaven National Laboratory conducted three studies that concluded the usage of nanoscale coatings and other methods could be used to improve the cycle durability of batteries.[60]

In 2014, researchers at Nissan announced a new analytic technique to allow them to observe how cathodes operate.[61]

In 2014, researchers at Helmholtz-Zentrum Berlin found that a lithium-rich cathode material ((x)Li
2
MnO
3
(1-x)LiMO
2
) could be charged and discharged rapidly or at higher currents. In the formula, "M" stands for a transition metal. The material had twice the regular amount of lithium and smaller amounts of rare, toxic elements like nickel and cobalt. The technique allowed them to determine that the battery's rapid energy density drop was due to the rearrangement of oxygen atoms.[62]

In 2014, researchers at Technical University of Munich used a neutron beam to observe when metallic lithium forms during charging without cutting the battery open. Metallic lithium formations lead to a reduced cycle durability and short circuits.[63]

In 2014, researchers at Michigan Technological University discovered atomic shuffling when using transmission electron microscopy. They took a closer look at how the ions move into and out of the anode causing stress.[64]

In 2017, researchers at University of Virginia reported a flow based technique (Dispersed Particle Resistance, or DPR) to measure the performance of active materials. DPR was mainly correlated with rate capabilities of active materials.[65][66][67]

Electrolyte[edit]

Currently, electrolytes are typically made of lithium salts in a liquid organic solvent. Research centers on increased safety via reduced flammability and reducing shorts via preventing dendrites.

Additive[edit]

In 2015 researchers announced that a mixture of lithium nitrate and lithium polysulfide formed a solid and stable interface between the electrode and the electrolyte and that it prevented dendrite formation. The prototype device ran with 99 percent efficiency over 300 charging cycles.[68]

Copper[edit]

In 2014, researchers at Stanford University discovered that adding a copper nanolayer to the electrolyte can detect fires by responding to a drop in the voltage caused by a dendrite, most likely formed during charging.[69]

Kevlar[edit]

In 2015 a battery using a separator membrane made of nanofibers extracted from Kevlar was demonstrated. It prevents dendrite growth because its pores are only 15-20 nm across, smaller than dendrites' 20- to 50-nm nanoscale tips, but large enough to allow individual lithium ions to pass. The membrane can be much thinner than existing separators.[70] Kevlar is an insulator and offers good heat resistance. The university has founded a spin-off company, Elegus Technologies, to further develop and commercialize the technology. Production is expected to begin toward the end of 2016.[71]

Perfluoropolyether[edit]

In 2014, researchers at University of North Carolina found a way to replace the electrolyte’s flammable organic solvent with nonflammable perfluoropolyether (PFPE). PFPE is usually used as an industrial lubricant, e.g., to prevent marine life from sticking to the ship bottoms. The material exhibited unprecedented high transference numbers and low electrochemical polarization, indicative of a higher cycle durability.[72]

Sticky[edit]

In 2014, researchers at Washington State University developed a chewing gum like substance that may replace liquid electrolytes. This new material contains liquid, but is sticky, which eliminates the fire hazard. This material is flexible, suggesting use in bendable electronics in the future.[73]

Solid-state[edit]

While no solid-state batteries have reached the market, multiple groups are researching this alternative. The notion is that solid-state designs are safer because they prevent dendrites from causing short circuits. They also have the potential to substantially increase energy density because their solid nature prevents dendrite formation and allows the use of pure metallic lithium anodes. They may have other benefits such as lower temperature operation.

In 2015 researchers announced an electrolyte using superionic lithium-ion conductors, which are compounds of lithium, germanium, phosphorus and sulfur.[74]

Glassy Electrolytes[edit]

In March 2017, researchers announced a solid-state battery with a glassy ferroelectric electrolyte of lithium, oxygen, and chlorine ions doped with barium, a lithium metal anode, and a composite cathode in contact with a copper substrate. A spring behind the copper cathode substrate holds the layers together as the electrodes change thickness. The cathode comprises particles of sulfur "redox center", carbon, and electrolyte. During discharge, the lithium ions plate the cathode with lithium metal and the sulfur is not reduced unless irreversible deep discharge occurs. The thickened cathode is a compact way to store the used lithium. During recharge, this lithium moves back into the glassy electrolyte and eventually plates the anode, which thickens. No dendrites form.[75] The cell has 3 times the energy density of conventional lithium-ion batteries. An extended life of more than 1,200 cycles was demonstrated. The design also allows the substitution of sodium for lithium minimizing lithium environmental issues.[76]

Lithium[edit]

Salt[edit]

In 2015 researchers announced a new electrolyte completely eliminates dendrites and promises to increase battery efficiency and vastly improve current carrying capacity.[77]

The material was 99% efficient and was compatible with a lithium metal anode. The electrolyte used lithium bis(fluorosulfonyl)imide salt, an organosilicon compound added to the solvent dimethoxyethanein. Instead of dendrites, the electrode developed a thin sheet of lithium nodules that did not extend into the electrolyte and risk short-circuiting the battery. The device survived more than 1,000 charge/discharge cycles producing 98.4 percent of its initial charge, with a current of around 4 milliamps per square centimeter.[77]

Thiophosphate[edit]

In 2015, researchers worked with a lithium carbon fluoride battery. They incorporated a solid lithium thiophosphate electrolyte wherein the electrolyte and the cathode worked in cooperation, resulting in capacity 26 percent. Under discharge, the electrolyte generates a lithium fluoride salt that further catalyzes the electrochemical activity, converting an inactive component to an active one. More significantly, the technique was expected to substantially increase battery life.[78]

Superhalogen[edit]

Conventional electrolytes generally contain halogens, which are toxic. In 2015 researchers claimed that these materials could be replaced with non-toxic superhalogens with no compromise in performance. In superhalogens the vertical electron detachment energies of the moieties that make up the negative ions are larger than those of any halogen atom.[79] The researchers also found that the procedure outlined for Li-ion batteries is equally valid for other metal-ion batteries, such as sodium-ion or magnesium-ion batteries.[80]

Water-in-salt[edit]

In 2015, researchers at the University of Maryland and the Army Research Laboratory showed significantly increased stable potential windows for aqueous electrolytes with very high salt concentration.[81][82][83] By increasing the molality of Bis(trifluoromethane)sulfonimide lithium salt to 21 m, the potential window could be increased from 1.23 to 3 V due to the formation of SEI on the anode electrode, which has previously only been accomplished with non-aqueous electrolytes.[84] Using aqueous rather than organic electrolyte could significantly improve the safety of Li-ion batteries.[81]

Design and management[edit]

Charging[edit]

In 2014, researchers at MIT, Sandia National Laboratories, Samsung Advanced Institute of Technology America and Lawrence Berkeley National Laboratory discovered that uniform charging could be used with increased charge speed to speed up battery charging. This discovery could also increase cycle durability to ten years. Traditionally slower charging prevented overheating, which shortens cycle durability. The researchers used a particle accelerator to learn that in conventional devices each increment of charge is absorbed by a single or a small number of particles until they are charged, then moves on. By distributing charge/discharge circuitry throughout the electrode, heating and degradation could be reduced while allowing much greater power density.[85][86]

In 2014, researchers at Qnovo developed software for a smartphone and a computer chip capable of speeding up re-charge time by a factor of 3-6, while also increasing cycle durability. The technology is able to understand how the battery needs to be charged most effectively, while avoiding the formation of dendrites.[87]

In 2014, StoreDot announced it had started working on a technology called multifunction electrode (MFE), that will enable future electric vehicles to fully charge in only 5 minutes. The MFE is a combination of a conductive polymer and metal oxide.[88]

Management[edit]

Durability[edit]

In 2014, independent researchers from Canada announced a battery management system that increased cycles four-fold, that with specific energy of 110 – 175 Wh/kg using a battery pack architecture and controlling algorithm that allows it to fully utilize the active materials in battery cells. The process maintains lithium-ion diffusion at optimal levels and eliminates concentration polarization, thus allowing the ions to be more uniformly attached/detached to the cathode. The SEI layer remains stable, preventing energy density losses.[89][90]

Thermal[edit]

In 2016 researchers announced a reversible shutdown system for preventing thermal runaway. The system employed a thermoresponsive polymer switching material. This material consists of electrochemically stable, graphene-coated, spiky nickel nanoparticles in a polymer matrix with a high thermal expansion coefficient. Film electrical conductivity at ambient temperature was up to 50 S cm−1. Conductivity decreases within one second by 107-108 at the transition temperature and spontaneously recovers at room temperature. The system offers 103–104x greater sensitivity than previous devices.[91][92]

Flexibility[edit]

In 2014, multiple research teams and vendors demonstrated flexible battery technologies for potential use in textiles and other applications.

One technique made li-ion batteries flexible, bendable, twistable and crunchable using the Miura fold. This discovery uses conventional materials and could be commercialized for foldable smartphones and other applications.[93]

Another approached used carbon nanotube fiber yarns. The 1 mm diameter fibers were claimed to be lightweight enough to create weavable and wearable textile batteries. The yarn was capable of storing nearly 71 mAh/g. Lithium manganate (LMO) particles were deposited on a carbon nanotube (CNT) sheet to create a CNT-LMO composite yarn for the cathode. The anode composite yarns sandwiched a CNT sheet between two silicon-coated CNT sheets. When separately rolled up and then wound together separated by a gel electrolyte the two fibers form a battery. They can also be wound onto a polymer fiber, for adding to an existing textile. When silicon fibers charge and discharge, the silicon expands in volume up to 300 percent, damaging the fiber. The CNT layer between the silicon-coated sheet buffered the silicon's volume change and held it in place.[94]

A third approach produced rechargeable batteries that can be printed cheaply on commonly used industrial screen printers. The batteries used a zinc charge carrier with a solid polymer electrolyte that prevents dendrite formation and provides greater stability. The device survived 1,000 bending cycles without damage.[95]

A fourth group created a device that is one hundredth of an inch thick and doubles as a supercapacitor. The technique involved etching a 900 nanometer-thick layer of Nickel(II) fluoride with regularly spaced five nanometer holes to increase capacity. The device used an electrolyte made of potassium hydroxide in polyvinyl alcohol. The device can also be used as a supercapacitor. Rapid charging allows supercapacitor-like rapid discharge, while charging with a lower current rate provides slower discharge. It retained 76 percent of its original capacity after 10,000 charge-discharge cycles and 1,000 bending cycles. Energy density was measured at 384 Wh/kg, and power density at 112 kW/kg.[96]

Volume Expansion[edit]

Current research has been primarily focused on finding new materials and characterizing by means of specific capacity (mAh/g), which provides a good metric to compare and contrast all electrode materials. Recently, some of the more promising materials are showing some large volume expansions which need to be considered upon engineering devices. Lesser known to this realm of data is the volumetric capacity (mAh/cm3) of various materials to their design.

Nanotechnology[edit]

Researchers have taken various approaches to improving performance and other characteristics by using nanostructured materials. One strategy is to increase electrode surface area. Another is to reduce the distance between electrodes to reduce transport distances. A third is to allow the use of materials that exhibit unacceptable flaws when use in bulk forms, such as silicon.

Finally, adjusting the geometries of the electrodes, e.g., by interdigitating anode and cathode units variously as rows of anodes and cathodes, alternating anodes and cathodes, hexagonally packed 1:2 anodes:cathodes and alternating anodic and cathodic triangular poles. One electrode can be nested within another.

Carbon nanotubes and nanowires have been examined for various purposes, as have aerogels and other novel bulk materials.

Finally, various nanocoatings have been examined, to increase electrode stability and performance.

Nanosensors is now being integrated in to each cell of the battery. This will help to monitor the state of charge in real time which will be helpful not only for security reason but also be useful to maximize the use of the battery.[97]

Economy[edit]

In 2016, researchers from CMU found that prismatic cells are more likely to benefit from production scaling than cylindrical cells.[98][99]

See also[edit]

References[edit]

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