Nanomaterials for energy storage and conversion
Nanomaterials are increasingly playing an active role by either increasing the efficiency of the energy storage and conversion processes or by improving device design and performance.
Ford Motor Company
Progress in the area of nanoscience and nanotechnology has pervaded almost all areas of science and technology. Over last couple of decades the ability to manipulate and control materials at an atomic and molecular level (nanometer range) and subsequent understanding of the fundamental processes at nanoscale have led to new avenues. The knowledge thus acquired can be translated into innovative processes, leading to design or fabrication of better products. More importantly, new scientific phenomenon and processes have emerged that could provide either revolutionary or novel solutions to the energy, environmental, and sustainable mobility challenges that will face humanity in the 21st century.
With demand for clean and sustainable energy sources increasing at an exponential rate, new material technologies are being explored that could provide cost-effective and environmentally clean solutions to the world's energy problems. Developments in the areas of alternative fuels or energy storage technologies like advanced batteries, fuel cells, ultra capacitors and biofuels are emerging as strong contenders to petroleum-based sources. Energy derivable from clean and renewable sources like solar and wind power have tremendous potential, but the practical use of these sources of energy requires efficient electrical energy storage (EES) technologies that can provide uninterrupted power on demand. In all of these new technologies, nanomaterials are increasingly playing an active role by either increasing the efficiency of the energy storage and conversion processes or by improving device design and performance. Figure 1 shows some of the applications which are using nanostructured materials as the "building block" for the next generation of technologies. In most cases, however, the use of nanoscale materials is a logical extension of the current technology.
The primary components of EES systems constitute chemical (batteries) and capacitive storage. Although electrochemistry is the main guiding principle behind both the technologies, there exists one basic fundamental difference. In the case of chemical storage, the reactants are stored in the cell to produce electricity whereas in capacitive storage, it is charge that is stored across the double layer. With regard to EES materials, perhaps the most visible application of nanomaterials is in the area of super capacitors and fuel cells. Developments in nanostructured carbon materials have optimised super capacitor performance by increasing the surface area (thus increasing capacitance) while also allowing for a pore size distribution that permits better electrolyte accessibility and hence increased power delivery capability. Reducing the use of precious metal catalysts without significantly compromising the activity or performance is one of the key challenges in the area of fuel cell research.
Developments in nanocatalyst materials have shown promising results in minimising the precious metals thereby reducing the cost. For example, nano-carbon supports like nanofibers, aerogels and mesoporus carbon have been used in polymer-electrolyte membrane fuel cells (PEMFC) and have allowed for a significant reduction in the amount of platinum catalyst (less than 0.5 mg/cm-2) without compromising with the cell performance.
Nanomaterials for batteries
In order to meet the need for vehicles with improved fuel economy, many automotive companies now offer gas-electric hybrids (HEVs) that utilise large batteries (> 1kWh) to store energy recovered from braking events. There is also much interest in the development of plug-in-hybrids (PHEVs), which have large batteries (> 5kWh) that can be recharged from the power grid. Although current HEVs use batteries based on nickel-metal hydride (NiMH) chemistry, there is much interest in replacing them with lithium-ion batteries because of their larger gravimetric and volumetric energy density. Realising the potential of this technology in making PHEVs and HEVs a reality, there has been tremendous efforts in industry, governmental agencies, and academia to accelerate the development of lithium-ion battery. The offices of the Freedom Car of the DOE in association with United States Automotive Battery Consortium (USABC) have mandated specifications and requirements, both in terms of performance and cost for lithium-ion cell technology.
From materials point of view, significant improvements in the areas of lithium-ion battery cathodes, anodes, electrolytes, and separators are needed in order to meet the required energy density, rate capability and the operating temperature range. It is expected that the use of nanomaterial-based anodes and cathodes will be required to meet the requirements of the batteries used in the next generation HEVs and PHEVs. The high-capacity and high-rate cathode materials in use today have secondary particle sizes in the range of 5-15 microns, which comprise primary particles having diameters in the range of tens to hundreds of nanometers. From the perspective of lithium-ion transport, nanostructured materials offer a shorter path length for lithium-ion diffusion compared to micron or sub millimeter-sized particles and hence offer better capacity utilisation and discharge / charge rates. Under a simplistic assumption, the characteristic time constant for diffusion is given by t = L2/D, where L is the diffusion length and D is the diffusion coefficient. Therefore, the time for intercalation varies as square of the length scale and should be faster for smaller particle domains.
The increased surface area allows the electrolyte to surround individual particles for better accessibility of the electro-active material. It is worth mentioning that the increase in surface area however, could be a potential impediment for the cell performance and life as it could accelerate unwanted reduction-oxidation chemistry that occurs on the electrode material surface. Interestingly, to alleviate this problem researchers have found another nanotechnology-based solution that involves chemically coating the particle surface with a few-nanometer-thick layer of amorphous carbon or other suitable inorganic material. The presence of such a film avoids the surface chemistry as well as increases the intrinsic conductivity of the particle without affecting the transport of lithium-ions into and out of the core of the particle. Therefore, it is becoming increasingly evident that at a materials level going over to the nanometer-sized particle is particularly advantageous.
Another fundamental challenge is developing electrode materials that have both a large ionic and electronic conductivity, which are requirements for maximising the ability to quickly discharge and recharge the battery. Towards this end, there have been significant advances made in the areas of nanocoating processes, design of porous nanostructured electrodes, nanowire-based electrode synthesis methods, and the incorporation of carbon nanotube and nanostructures in electrode materials which are beginning to make the transition from the laboratory to manufacturing scales that will be required for commercial products. There are some more fundamental changes that occur at small particles sizes as reported recently in literature, the most noticeable among them are, (a) the chemical potential for Li+ and electrons could be altered compared to their bulk value affecting the reaction thermodynamics, (b) extended solid-solution composition can exist for nanoparticle compared to bulk materials and (c) the ability of nanostructured electrodes to withstand more mechanical stress / strain upon lithium insertion.
Promising nanomaterials for Li-ion battery technology
As mentioned earlier, the major limitation in rate capabilities of Li-ion batteries arises on account of the slow solid-sate diffusion of Li+ within the electrode materials. In this case, going over to nanostructured design of electrodes is particularly appealing because in this case the distance the Li+ diffusion is limited to the diameter of the nanoparticle. Recent advances in nanostructured tin (Sn), silicon (Si), nickel (Ni), cobalt (Co), and intermetallic alloys (Cu6Sn5, InSb, Cu2Sb) as replacements for carbon-based anodes have resulted in batteries with higher specific capacity and enhanced cycle life. Anodes made from nanostructured lithium titanate (Li4Ti5O12) are another promising replacement for the carbon anode. Specifically, they enable a very high rate capability and do not suffer strain upon lithium intercalation. Also, unlike carbon anodes, no surface electrolyte interface (SEI) layer is formed on this material. Disruption of this layer in conventional lithium-ion batteries is a primary source of capacity and power fade.
One drawback of lithium titanate anodes is their higher potential relative to carbon anodes, which has the effect of reducing battery specific energy. With respect to the cathode (the positive electrode), one of the most successful applications of nanomaterials has been the recent commercialisation of batteries based on lithium iron phosphate (LiFePO4). This cathode consists of nanometer-sized, amorphous-carbon-coated LiFePO4 particles that yield exceptional rate capability and stability. The presence of the carbon nanolayer is critical for improving the electrical conductivity as well as facilitating the transport of lithium-ions into and out of the LiFePO4 domains. This approach has also been extended recently to other high capacity layered cathode materials like LiMnPO4.
Other potential electroactive nanoscale materials (for lithium storage) that are in the early stages of development but which show some promise include V2O5, MnO2, Co3O4 and CuV2O6. Other exploratory research directions attempt to exploit the shape, morphology, and assembly of nanoparticles to enhance both specific capacity as well as rate. Recently, Chan and co-workers demonstrated a silicon-nanowire-based anode with a capacity close to the theoretical limit of 4200mAh/g. Another example of a development that could increase the volume fraction of electroactive material (and hence increase battery-specific energy) is replacing the relatively large amount of carbonaceous materials normally added to the cathode (~ 6wt per cent) with a small amount of carbon nanotubes (~ 0.1wt per cent).
Commercialisation of nanomaterials
Over the last few years developments in low cost and scalable nanomaterials synthesis and manufacturing methods have resulted in mass-scale production of a limited number of electrode materials that can potentially meet the production volumes required for automotive applications. Table 1 summarises some of the recent successes in the area of nanostructured electrode materials for HEV and PHEV battery applications. This is however, by no means a comprehensive list of all the nanoscale battery materials, but gives a flavour of the range of nanomaterials that are already beginning to enter the product landscape for automotive applications.
Several automotive OEMs have announced plans to introduce vehicles that use lithium- battery technology. Notable among them is the recent announcement by Mercedes-Benz to introduce Lithium-ion batteries in their upcoming S-class hybrids, which are scheduled to be in the marketplace by mid-2009. Their Li-ion battery packs are being developed in partnership with Johnson Controls-SAFT. GM has already announced last year of its intention to use Lithium-ion batteries for their Chevrolet Volt PHEV. GM is partnering with Hitachi for its next generation hybrids, which are scheduled for production in 2012. LG-Chem will begin supplying lithium-polymer rechargeable batteries for Hyundai hybrid vehicles, which will be introduced in the marketplace in the 2009 timeframe. Other companies are following the trend to switch over from Ni-MH to Li-ion batteries for their HEVs. These developments will lead to further efforts to scale-up the manufacturing processes for Li-ion batteries (for automotive use) and will have a significant impact on reducing their cost and spearhead the research activity in the area of basic energy storage.
It is clear that nanotechnology is a key enabler for the success of next-generation battery chemistries for developing high energy and power cells for automotive HEV and PHEV applications. It may be useful to view this specific discipline more as a process-based technology that is redefining the entire product landscape enabling smaller and more efficient batteries. In addition to superior performance, in order to be considered successful for automotive applications, the battery technology must meet cost and all vehicle operating and life requirements. If nanotechnology for automotive energy storage applications is to be viewed as something more than just hype, it must be able to satisfy all these demanding performance, manufacturing, and cost goals.
Jagjit Nanda is currently working as a member technical staff at Research and Advanced Engineering unit (formerly known as Ford Research Laboratory) Ford Motor Co. Dr. Nanda obtained his Masters in Physics form Jadavapur University Kolkata in 1993, followed by PhD in Solid State Chemistry from Indian Institute of Science, Bangalore in 2000. After a 2 year post-doctoral work at Stanford University, Dr. Nanda was a research member of the soft-matter nanotechnology and advanced spectroscopy team at Los Alamos National Laboratory from 2002-2005. His current research interest is in the area of electrochemical energy storage and application.