Research in lithium-ion batteries
Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Ideas used have focused on improving energy density, safety, charge time, cycle durability, flexibility, and cost. As of 2014 few of these innovations had appeared in commercial products.
- 1 Anode
- 1.1 Titanium dioxide with aluminum
- 1.2 Titanium dioxide
- 1.3 Lithium
- 1.4 Carbon
- 1.5 Silicon
- 1.6 Tin
- 1.7 Nanowire
- 1.8 Nickel-fluoride
- 1.9 Copper
- 1.10 Iron-phosphate
- 1.11 Lithium metal foil
- 1.12 Aluminum/titanium
- 2 Cathode
- 2.1 Vanadium
- 2.2 Cobalt
- 2.3 Graphene/lithium metal
- 2.4 Disordered materials
- 2.5 Graphene oxide coated sulfur
- 2.6 Nanophosphate
- 2.7 Seawater
- 2.8 Purpurin
- 2.9 Three-dimensional nanostructure
- 2.10 Lithium
- 2.11 Air
- 2.12 Analysis technique
- 3 Electrolyte
- 4 Design and management
- 5 Nanotechnology
- 6 See also
- 7 References
Titanium dioxide with aluminum
In 2015, researchers at Massachusetts Institute of Technology developed a quick charge battery that has four times the energy density of typical lithium-ion batteries. The battery uses tiny capsules of titanium dioxide filled with aluminium. The aluminium yolk has space to expand and contract inside the shell. This overcomes previous problems of using aluminium as a battery anode.
In 2014, researchers at Nanyang Technological University used titanium dioxide in an anode and achieved 10,000 charging cycles. The battery can be charged to 70% in two minutes. They used a gel material made from titanium dioxide, an abundant, cheap and safe material found in soil. They developed a simple method to turn naturally spherical titanium dioxide particles into nanotubes. This nanostructure sped up the charging reaction.
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 abandoned due to dendrite formation, causing internal short-circuits and fire hazard. In 2014, researchers at Stanford University discovered that a pure-lithium anode increased energy density 400%. Researchers claimed that the anode did not expand during charging. This is done by building nanospheres, which are protective layers of interconnected carbon domes on top of the anode.
Also in 2014, a second technique was announced by Cornell University researchers that added halogenated lithium salts to the liquid electrolyte. This prevented the formation of battery-destroying metal dendrites as the battery went through charge/discharge cycles.
In 2014, researchers at Oak Ridge National Laboratory developed an anode from modified carbon black from discarded tires. The new technology is environmentally friendly. It also has a higher energy density.
In 2014, researchers at University of California, Riverside developed a battery that charges up to 16 times faster with 60% additional energy density. They use a three-dimensional, cone-shaped cluster of carbon nanotubes.
That same year, researchers at Northwestern University 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.
Heating polystyrene packing peanuts to between 500 and 900 °C (932 to 1,652 °F) in an inert atmosphere, in either the presence or absence of a transition metal salt catalyst produced either nanoparticles or microsheets that made excellent anodes. The sheets were about one tenth the thickness of graphite anodes, reducing charging times and exhibiting less electrical resistance. Specific capacity reached 420 mAh/g, vs the theoretical 372 mAh/g maximum for graphite. The anodes survived 300 charging cycles without a significant capaicty loss. The microsheets' porous structure exposed more contact area between the anode and the liquid electrolyte.
In 2015 researchers announced a one-step process for using natural silk to create 4.7% nitrogen-doped carbon-based nanosheets that reversibly 1865 mA h/g over 10,000 cycles with only a 9 percent capacity loss. The surface area was SBET: 2494 m2/g, hierarchical pore volume was 2.28 cm3/g. Capacitance reached 242 F/g and energy density was 102 W h/kg (48 W h/L).
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. Rate capacities increased by 17-43% at 200 mA/g.
In 2016 researchers pyrolized bee and cattail pollen to produce materials for an anode. Pyrolysis in an argon container produced pure carbon. The resulting carbon particles were heated at a lower temperature of around 300 °C (572 °F) in the presence of oxygen, which created pores in the carbon, increasing its energy capacity. In prototype devices at temperatures of 25 and 50 °C (77 and 122 °F) achieved gravimetric capacity of 590 mAh/g at 50 °C and 382 mAh/g at 25 °C.
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). 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. 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.  Below are various structural morphologies attempted to overcome issue with silicon's intrinsic properties.
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. The cages also prevented the silicon from reacting with the electrolyte.
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.
Flash heat treatment
In 2015 researchers announced a flash heat treatment for fabricating silicon anodes that minimizes volume expansion while boosting the performance and cycle capability of lithium-ion batteries. The material delivered 1,000 mAh g−1 for 2,275 cycles at 2 A g−1with increased energy capacity, by minimizing volume expansion.
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.
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.
In 2014, researchers developed a silicon anode with an energy density above 1,100 mAh/g and a durability of 600 cycles, making their anode nearly three times more powerful and longer lasting than a typical commercial anode. They used porous silicon particles using ball-milling and stain-etching.
In 2013, researchers developed a battery with three times the energy density of a conventional li-ion, and can be recharged in less time. It utilizes anodes made from porous silicon nanoparticles.
In 2014, researchers at University of California, Riverside announced an anode made from high-quartz sand collected from Cedar Creek Reservoir in Texas. They milled the sand to the nanometer scale and purified it, producing a similar color and texture to powdered sugar. Grinding salt and magnesium into the purified quartz and heating removed oxygen from the quartz, resulting in pure silicon with a porous, sponge-like consistency. After an extensive low current density activation process, at a discharge rate at C/2 tested over 1000 cycles, the half cell demonstrated a reversible capacity of 1024 mAhg−1and a Coulombic efficiency of 99.1% using a lithium metal counter electrode.
In 2014, researchers at Pacific Northwest National Laboratory discovered that spongy silicon doubles the energy density of lithium-ion batteries. A mesoporous silicon sponge that is capable of being filled with silicon rather than the silicon expanding. Silicon typically expands to 400% during charging, with the new technology only expanding 30%
In 2013, researchers at Stanford University developed a battery that maintains high energy density through 5,000 cycles. They used silicon and conducting polymer hydrogel, a spongy substance similar to the material used in soft contact lenses and other household products. This process doesn't cause fires. It is also inexpensive.
Silicon oxide-coated silicon nanotube
In 2012, researchers at Stanford and SLAC developed a battery with superior durability. It maintains 85% of the energy density after 6,000 cycles. They are using a double-walled silicon nanotube coated with a thin layer of silicon oxide. This strong outer layer keeps the outside wall of the nanotube from expanding.
In 2011, researchers from Northwestern University developed a battery that increased cycle durability and energy density by up to a factor of ten. They sandwiched clusters of silicon between graphene sheets. They used a redox process to create in-plane defects (10 to 20 nanometers) in the graphene sheets so the lithium ions would have a "shortcut" into the anode and be stored there by reaction with silicon.
In 2011, researchers at United States Department of Energy national laboratories developed a battery anode that can absorb eight times the amount of lithium. The polymer binds closely to silicon particles while they expand and shrink.
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.
In 2007, researchers at Stanford University invented the nanowire battery, which improved battery performance. It uses nanowires to increase the surface area of one or both electrodes. Both replace the traditional graphite anode. One uses silicon, while the other uses germanium.
In 2014, researchers at Rice University announced a method to create a flexible, long-lasting battery. They used nanoporous nickel(II) fluoride electrodes layered around a solid electrolyte without using lithium. The device retained 76% of its energy density after 10,000 charge-discharge cycles and 1,000 bending cycles.
In 2006, researchers developed a battery using nanotechnology that improves energy density by several times. Active materials are applied in a very thin film to copper nanorods anchored to sheets of copper foil The nanorods supply 50 cm2 of active material per cm2 of substrate.
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.
In 2009, researchers at MIT developed a battery using genetically engineered viruses to make a more environmentally friendly battery. In 2015, another MIT group announced a flexible, puncture-resilient battery with fewer, thicker electrodes that used a semisolid aqueous suspension lithium-iorn-phosphate (LFP)/lithium-titanium-phosphate (LTP) to achieve higher energy density than a conventional aqueous vanadium-redox flow battery. Using suspended particles instead of solid slabs greatly reduces the tortuosity (path length of charged particles as they move through the material).
Lithium metal foil
Historically, lithium metal was used only for non-rechargeable cells, because it tends to react with the electrolyte, trapping the lithium ions and preventing more and more of them from participating in future charge/discharge cycles. The reaction also creates dendrites, metal spikes that can cause short circuits and heating that can ignite the flammable electrolyte. Lithium metal remains a subject of interest because of its potential to increased energy density by 2x or more.
In 2015, an MIT spinoff company, SolidEnergy, demonstrated a battery that uses a thin sheet of lithium-metal foil. The company claimed to have solved multiple issues, including safety and lifetime.
SolidEnergy uses a combination of solid and liquid electrolytes. The solid electrolyte is applied to the lithium-metal foil—the ions don’t have far to travel through this thin material, so it does not matter that they move relatively slowly. Once through the solid electrolyte, they reach a non-flammable liquid electrolyte, which ferries them to the opposite electrode. The electrolyte has additives that prevent the lithium metal from reacting with it and that prevent dendrites.
SolidEnergy’s technology does not require new manufacturing equipment and can be recharged 300 times while retaining 80 percent of its storage capacity. It works at room temperature, whereas some other lithium-metal batteries operate at much higher temperatures.
In 2014 Seeo demonstrated a prototype of a solid-state battery, replacing the traditional liquid electrolyte with two polymer layers. One is soft and conducts ions; the other is hard and prevents dendrite formation. Battery charge cycling had yet to be assessed.
In 2015 an anode made from aluminum/titanium "yolk-and-shell" nanoparticles was introduced. The nanoparticles have a solid titanium outer shell and an inner aluminum "yolk" that can expand and contract within the shell, storing and releasing ions without damaging the structure of the electrode. Like lithium or silicon, aluminum can store much more energy per unit weight than graphite.
Production involved placing 50 nm-diameter aluminum particles in a solution of sulfuric acid and titanium oxysulfate. This coated the nanoparticles in a hard titanium shell three to four nanometers thick. A few hours in the acid shrank the aluminum particles to about 30 nanometers without affecting the outer shell. This gave the aluminum nanoparticles room to expand considerably as they absorbed lithium, without damaging the cell's electric contacts. The anode can reportedly store 1.2 Ah/g at a normal charging rate, declining to 0.66 Ah/g at a higher rate.
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.
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.
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.
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.
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.
Graphene oxide coated sulfur
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.
In 2012, researchers at A123 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.
In 2012, researchers at Polyplus Corporation created a battery with an energy density more than triple that of traditional lithium-ion batteries using seawater. It's 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.
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.
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. In 2013, the team improved the microbattery design, delivering 30 times the energy density 1,000x faster charging. The technology also delivers better power density than supercapacitors. The device achieved a power density of 7.4 W/cm2/mm.
Lithium nickel manganese cobalt oxide
Lithium iron phosphate
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.
Lithium manganese silicon oxide
A “lithium orthosilicate-related” compound, Li
4, cathode was able to support a charging capppacity of 335 mAh/g (milliAmpere-hours per gram). 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.
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 g−1c.
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.
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.
In 2014, researchers at Helmholtz-Zentrum Berlin found that a lithium-rich cathode material ((x)Li
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.
In 2014, researchers at Technische Universität München 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.
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.
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.
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.
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. 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.
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.
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.
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 may have other benefits ranging from lower temperature operation to increased energy density.
In 2015 researchers announced an electrolyte using superionic lithium-ion conductors, which are compounds of lithium, germanium, phosphorus and sulfur.
In 2015 researchers announced a new electrolyte completely eliminates dendrites and promises to increase battery efficiency and vastly improve current carrying capacity.
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.
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.
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. 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.
Design and management
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.
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.
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.
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.
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.
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.
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 silion-coated sheet buffered the silicon's volume change and held it in place.
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.
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.
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 know to this realm of data is the volumetric capacity (mAh/cm3) of various materials to their design.
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.
Finally, various nanocoatings have been examined, to increase electrode stability and performance.
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