The energy storage market is expected to grow to over $125 billion on the next ten years. Nanomaterials will play a major part in this growth, especially in Lithium-ion batteries.
Energy conversion and storage is a growing problem in society. Rechargeable Lithium-ion batteries (LIB) have been the main energy storage medium for mobile applications over the past two decades. They are in demand for consumer electronics, electric vehicles and grid-scale stationary energy storage. However, there are significant drawbacks in LIB, such as the low availability and high cost of lithium, coupled with the environmental impact of extracting and disposing of this highly reactive ion. Market demand for lighter, thinner and higher capacity LIB, as well as increased competition from fuel cells for powering electric vehicles, necessitate ongoing research for new materials with improved properties over that of state-of-the-art.
Lithium-ion batteries
Lithium-ion batteries (LIB) are have widely used in portable electronics. They have relatively high voltages, energy densities and long cycle lives. LIB usually consist of a negative electrode (anode, e.g. graphite), a positive electrode (cathode, e.g. LiCoO2), a lithium ion-conducting electrolyte and certain membranes.
LIB is more expensive than other rechargeable batteries, and will become more so. Lithium costs from around $100 per kg to $2000 per kg for the extra pure lithium that is needed for the most efficient batteries. Technology for extracting lithium is also quite costly, it is difficult to store, and is becoming increasingly rare. At the rate humans use lithium now, and with the projected increase in demand for it, experts predict that we will run out long before the year 2100. Additionally, LIB still exhibit modest energy capacities that seem to have reached their asymptotic values with the present combination of graphite at the anode and insertion oxide or phosphate materials at the cathode. They have a theoretical energy density approaching 1 kWh/kg, but in practice do not deliver anywhere near this.
Future development will require devices that possess super high energy and power density, and excellent cycling stability. New applications, particularly for all-electric vehicles are pushing the development of electrode materials with higher Li storage capabilities, for both electrodes.
Nanomaterials
Nanostructured materials are allowing companies to develop the next generation of LIB devices with high power density, high energy density and high safety for application in sectors such as hybrid electric vehicles (HEV), plug in hybrid electric vehicles (PHEV) and pure electric vehicles (PEV). Other markets include consumer electronics, medical devices, power tools and military satellites. Advantages of nanomaterials are:
• Nanoscale shortens lithium-ion diffusion length
• New reactions at nanoscale are not possible with bulk materials
• Nanoscale combining with electronic conductive coating improves electronic transport
• Decreased mechanical stresses due to volume change lead to increased cyclability and lifetime
• Nanoscale enhances the electrode capability of Li storage
• Ordered mesoporous structure favours both Li storage and fast electrode kinetics
• Nano-structure enhances cycle stability.
For anodes in LIB, nanostructured materials such as nanowires, nanorods, nanotubes and 3D porous particles are used. Si- or Sn-based nanomaterials dispersed in a carbon-based matrix are also incorporated into LIB. For cathodes nanoparticle metal oxides or polyanion-based compounds are most investigated.
Samsung SDI, the world’s largest LIB maker has recently made a major investment in graphene producer XG Sciences. Automotive companies such as Chrysler utilize nanomaterials in LIB in their electric vehicles to improve battery capacity, cycle life, and charge-discharge rates while a high degree of safety. With more battery power, automobiles can travel farther on a single charge and accelerate more quickly and safely. For automobile and battery manufacturers, more battery power can reduce the number of batteries typically required in today’s hybrid and electric vehicles. Other large multinational companies developing nanomaterial based LIB products include GE, Panasonic Sanyo, Matsushita Industrial Co., Ltd., NEC, Toshiba, LG Chem, Samsung and Sony for application in areas such as cell phones and PCs, medical devices, military applications and cordless power tools.
Nanoparticles
Metal-oxides nanoparticles are utilized in high-energy-density LIB. Cobalt oxide and nickel oxide nanoparticles are utilized as anodes in LIB and exhibit good electrochemical performance. Molybdenum oxide nanoparticles have been employed as the negative electrode in LIB. The dramatically improved capacity and rate capability could possibly make these materials suitable for EV applications.
Silicon nanoparticles are promising for high-capacity anode materials for LIB. However, challenges including short cycle life and scalability hinder its widespread implementation.
A123 Energy Solutions (http://a123energy.com) develops and manufactures advanced lithium ion batteries and battery systems for the transportation, electric grid services and consumer markets. A123 has developed several battery configurations built around its Nanophosphate cell chemistry. SONY’s Nexelion battery also incorporates nanostructured SnCo anodes.
Graphene
Batteries are one of the limiting factors in current mobile electronic devices. Graphene has the potential to significantly increase the lifetime of LIB. Properties of graphene that are attractive for LIB include:
• High intrinsic conductivity (7580 vs. 500–3000 S m-1)
• High aspect ratio ( ~10 vs. 1)
• Large specific surface area
• Inert basal surface.
• Transparent electrode easily doped for electron/hole conducting media.
Graphene can be utilized as in LIB as:
• Anode active material
• Hybrid active material
• Electrode conductive additive.
However, the facile synthesis of graphene anode materials for high performance LIB, especially suitable for mass-scalable preparation remains challenging. Properly formulated graphene could greatly improve the energy & power density of LIB and is being widely researched and developed. In one potential process, graphene inks can be coupled with traditional printing process (R2R) to scale up printable batteries and supercapacitors. In batteries, graphene can be used as conductive additive as a substitute or addition to carbon black, as a current collector material and as a composite material with the electroactive compound.
The main LIB players such as Asahi Kasei, Hitachi, LG Chem, Mitsubishi Chemical Panasonic, Samsung SDI and Toshiba all have activities in graphene development and production. Target markets for graphene enhanced LIB include:
• Electric vehicles that require advanced battery technology to become more affordable
• Consumer electronic devices allowing for lighter weight for portability and improvement in LIB runtime including power tools and cellphones and laptops.
• Medical/Space applications.
• Defence (UAVs/robotics and portable electronics).
Vorbeck Materials (www.vorbeck.com), XG Sciences (http://xgsciences.com), Angstron Materials (www.angstronmaterials.com), and Grafoid (http://grafoid.com) are graphene producers developing LIB technologies. Vorbeck Materials is focusing on next generation lithium sulfur technology, spurred by a 2012 Department of Energy ARPA-E award. This technology could produce batteries with 10 times the energy density of current lithium-ion technology, paving the way to a widespread adoption of electric vehicles and machinery.
Graphene Batteries AS (www.graphenebatteries.no) develops “Battery Grade Graphene”, primarily for the LIB market. The technology involves synthesis, modification and formulation of graphene to be used in battery electrodes as an additive. The company has been awarded a competitive grant from Research Council of Norway as part of the ENERGIX program. The $2M project in collaboration with Institute for Energy Technology will support development of the company’s Silicon/graphene composite materials for LIB.
XG Sciences, Inc. (http://xgsciences.com) is producing anode materials for LIB with four times the capacity of conventional anodes. In November 2013 XG Sciences was awarded $1 million in funding from the US Department of Energy for continued development of these lithium-ion battery materials.
Vorbeck Materials is focusing on next generation lithium sulfur technology, spurred by a 2012 Department of Energy ARPA-E award. This technology could produce batteries with 10 times the energy density of current lithium-ion technology, paving the way to a widespread adoption in electric vehicles and machinery. This is in partnership with Pacific Northwest Nationals Laboratory (PNNL) and Princeton University. Vor-charge™ graphene composite anode is a fully formulated anode composite with Li-ion storage material and graphene mixed. The highly conductive anode leads to less internal resistance and low temperatures during rapid charge and discharge. Vor-charge™ offers long cycle life as there is no graphite particle fragmentation and loss of capacity due to large volume expansion and contraction.
Carbon nanotubes
Carbon nanotubes (CNTs) are a candidate material for use in LIB due to their unique set of electrochemical and mechanical properties. The use of carbon nanotubes for battery electrodes could produce a tenfold increase in the amount of power delivered from a given weight of material when compared to a conventional lithium-ion battery.
CNTs have the intrinsic characteristics desired in material used as electrodes in batteries and capacitors: high surface area and good electrical conductivity.
CNTs are currently used as the filler for anode materials used in LIB. The incorporation of CNTs as a conductive additive at a lower weight loading than conventional carbons, like carbon black and graphite, presents a more effective strategy to establish an electrical percolation network. In addition, CNTs have the capability to be assembled into free-standing electrodes (absent of any binder or current collector) as an active lithium ion storage material or as a physical support for ultra-high capacity anode materials like silicon or germanium. The use of carbon nanotubes for battery electrodes is expected to produce a tenfold increase in the amount of power delivered from a given weight of material when compared to a conventional LIB.
Companies developing CNTs for conductive LIB additves include ArkNano (www.arknano.com), Contour Energy Systems, Inc. (www.contourenergy.com) and GS Nanotech Co., Ltd. (www.gsnanotech.co.kr), EcoloCap Solutions, Inc. (www.ecolocap.com) claims to have successfully tested a carbon-nanotube LIB that is 99 percent efficient.
Image 1: Nano Lithium X Battery (Image courtesy EcoloCap Solutions, Inc.).
Nanofibers
Nanofiber-based polymeric battery separators boost the performance and safety of LIB. While the initial uses for the separator are in hybrid and electric vehicle batteries, the technology can also be utilized for batteries in renewable energy, grid applications, and specialty consumer applications, including laptops, cell phones and power tools, among others. The layered structure encountered in “platelet” nanofibers has been found to be an ideal material for lithium intercalation. Moreover, batteries fabricated with nanofiber electrodes can operate at temperatures as low as 233K. MD Nanotech Corporation (www.mdnanotech.jp), a spin-out from Mitsubishi Materials Corporation, is producing carbon nanofibers for batteries and conductive materials. Dreamweaver International (www.dreamweaverintl.com) is developing nanofibers as battery separators. DuPont (www.dupont.com) has also brought to market a nanofiber-based polymeric battery separator that boosts the performance and safety of lithium ion batteries. DuPont™ Energain™ battery separators can increase power 15 to 30 percent, increase battery life by up to 20 percent and improve battery safety by providing stability at high temperatures. DuPont estimates that, by 2015, the market for high-performance LIB alone will total more than $7 billion annually, primarily for electric vehicle applications and some photovoltaics and grid storage. The Energain™ battery separators are produced into a web using a proprietary spinning process that creates continuous filaments with diameters between 200 and 1,000 nanometers. The separators exhibit stability and low shrinkage in high temperatures and are highly saturable in electyrolyte liquids. The result is more efficient operation, longer battery life and improved safety. Batteries containing Energain™ separators can be quickly recharged, deliver improved performance and reduce the number of batteries needed by up to one-third for hybrid vehicles.
Silicon nanowires
Silicon, which exhibits the highest known Li-alloying capacity is one of the most promising anode materials. However, Li alloying with Si is accompanied by a large volume change which induces cracking and rapid pulverization of Si-based anodes. Significant improvements in the anode’s lifetime as well as charge–discharge rates have been obtained over the past few years by employing Si nanostructures, particularly nanowires.
Advantages of nanowires in LIB include good strain relaxation: large surface area and shorter distance for Li diffusion; interface control (better cycle life); and continuous electron transport pathway. The high theoretical charge capacity (~4200 mAh/g) of Si, comparatively low discharge potential and availability have made silicon a desirable battery anode material. However, induced stress upon lithiation has led to the aforementioned pulverization of bulk and layered silicon making it inappropriate for use in batteries.
Nanostructured silicon has the property to relax the strain built during charge/discharge cycles. However, even Si nanowire-based structures still exhibit limited cycling stability for extended numbers of cycles, with the specific capacity retention with cycling not showing significant improvements over commercial carbon-based anode materials.
Amprius (www.amprius.com) is developing LIBs incorporating silicon nanowires as anode. The carbon cathode is replaced with nanostructured silicon films. Amprius is already supplying smartphone and tablet OEMs with its first two product families. Amprius’ first smartphone battery, with 1850 mAh capacity, offers 580 Watt-hours per liter, compared to about 530 Wh/L for existing high-end batteries. Amprius is also offering a 4060 mAh, 600 Wh/L battery for tablets, and has also signed contracts with its OEM customers to design batteries that meet custom specifications. In 2014, Amprius is seeking to mass produce a second-generation battery with 670 Wh/L performance and has recently secured $30 million in funding. A third-generation model could go into volume production the following year, and would provide upwards of 700 Wh/L.
Image 2: Amprius Li-ion Cell (Image courtesy Amprius)
Recent research news
Researchers at Pacific Northwest National Laboratory (PNNL) have developed a battery incorporating fluorinated graphene. Read more: http://pubs.rsc.org/en/content/articlelanding/2013/ta/c3ta11710c#!divAbstract, http://www.nature.com/srep/2014/140121/srep03790/full/srep03790.html
Researchers at Kansas State University have developed composite paper utilizing graphene nano-sheets that can be used as a negative electrode in sodium-ion batteries. The researchers are working to commercialize the technology, with assistance from the university’s Institute of Commercialization. They also are exploring lithium and sodium storage in other nanomaterials. Read more: http://pubs.acs.org/doi/abs/10.1021/nn406156b
Researchers at A*STAR Institute of Materials Research and Engineering in Singapore, in collaboration with Fudan University in China, has developed a carbon nanotube electrode that can alleviate recharging problems in lithium–oxygen batteries, utilizing three-dimensional nickel foam. Read more: http://www.sciencedirect.com/science/article/pii/S0378775313008902
Researchers at the Materials and Surface Science Institute (MSSI), University of Limerick have developed a technology that more than doubles the capacity of lithium-ion battery anodes and retains this high capacity even after being charged and discharged over 1000 times. The research utilizes Germanium Nanowire-Based LIB anodes. Read more: http://dx.doi.org/doi:10.1021/nl403979s
Researchers from Stanford University and the SLAC National Accelerator Laboratory have investigated a pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes. As a result of this arrangement, the solid-electrolyte interphase in the LIB remains stable and spatially confined, resulting in superior cyclability (97% capacity retention after 1,000 cycles) Read more: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.6.html
