A few years ago, if you had been asked “What has nanotechnology done for you lately?”, the answer could have been “not much.” Today, the situation has changed considerably. Because nanosize materials behave differently than larger-scale versions with the same chemical makeup, nanotechnology is having a pronounced effect in many areas.
One of these, power storage, is undergoing particularly rapid change as new materials are developed for rechargeable batteries, each promising to deliver better performance than the last. Figure 1 is a Ragone plot that relates energy density to power density for a number of battery chemistries.
Nickel-metal-hydride (Ni-MH) batteries have largely replaced nickel-cadmium (Ni-Cd) cells due partly to environmental concerns and because Ni-MH technology provides greater stored energy for a given battery weight. As shown in Figure 1, it has a greater energy density measured in W-h/kg.
Energy density also is called gravimetric-specific energy to distinguish it from volumetric-specific energy or the amount of energy stored per unit volume. Similarly, a comparison of the power available from different types of batteries is simplified by dealing with power density with units of W/kg.
Because both axes are densities, Figure 1 relates the amount of energy that can be delivered by a battery and the rate at which it is delivered. For example, at power levels approaching 1,000 on the horizontal scale for lead-acid (Pb/PbO2) batteries, much less energy is available than at lower power levels. You can get high amounts of power for short times such as when starting a car, but if you draw power from the battery at a much lower rate, more total energy is available.
The battery’s internal resistance is responsible for part of this phenomenon. At high discharge rates, the internal resistance drops the terminal voltage and dissipates some of the energy, reducing the cell capacity. The detailed internal construction and chemistry also limit discharge current depending on the type of battery.
The diagonal lines in Figure 1 correspond to discharge rates. For example, all the battery technologies perform well at a discharge rate numerically equal to the battery capacity (C). The total discharge time is one hour at this rate. Most of the technologies do not cope well with very high discharge rates above 10C, as shown by the falling energy density characteristic.
Development continues to improve the energy and power densities, temperature range, charge time, and most recently, the safety of batteries. These properties determine the suitability of a certain battery technology for a particular application.
For example, lead-acid batteries are low cost, but they rapidly discharge and are bulky and heavy. Their large capacity and good power density work well in cars, but they don’t match user expectations for a cell phone or portable computer power source. Ni-MH batteries are acceptable, but lithium-ion (Li-ion) cells are an even better fit.
Li-ion technology offers much higher specific energy than Ni-MH and has been popular in military and portable electronics applications that require lots of energy in a very small package. The flat Li-ion curve in Figure 1 means that a relatively high discharge rate can be supported without greatly reducing the battery capacity.
Like most battery technologies, there are many variants of Li-ion cells, each with its own performance characteristics. For example, some companies make two ranges of batteries. One features very high energy density, and the other has very high power density. Obviously, different types of applications require batteries with optimized performance.
Unfortunately, the basic Li-ion cell also has its drawbacks including possible thermal runaway leading to fire and explosion. In addition, because of the chemical processes involved in its operation, the basic Li-ion battery cannot be rapidly charged, its terminal voltage must not be allowed to drop below 2.0 V, and it can’t be charged at temperatures below 0°C or above 120°C. Nevertheless, billions of cells have been manufactured and remain in use.
In 2006 and 2007, several million Li-ion batteries were recalled by battery companies and the manufacturers of computers in which the batteries were used. Some computers actually caught fire because of internal shorts in the Li-ion battery packs that led to a thermal runaway condition. Typically, the problem was caused by small pieces of metal that contaminated the batteries during the manufacturing process. So, the immediate solution was to improve the process to eliminate possible contamination.
In addition, a great deal of work had already been done independently to eliminate thermal runaway regardless of cell contamination or possible damage. One of the more commercially successful cell variants to have resulted from this work replaced the typical cobalt oxide (LiCoO2) or manganese spinel cathode with an iron-phosphate one (LiFePO4). In this approach, the cell uses a graphite anode but one with special additives according to A123, the battery’s manufacturer.
Watching Ions at Work
When a Li-ion cell is charged, Li ions move from the positive cathode to the negative anode. The ion travel direction is reversed during discharge. Figure 2 shows a cell discharging and producing a voltage (V).
The cathode is represented by the red cube with ions stored in spaces within the crystal structure. Li ions are small but finite in size. Because of the large difference in the number of Li ions present in the cathode of a charged or discharged cell, cobalt- or manganese-based material slightly expands or contracts as the cell is cycled.
A graphite anode is represented by the layered purple structure shown in Figure 2. The process of storing Li ions in a layered material is called intercalation; that is, the ions occupy the spaces between the layers of carbon making up the graphite.
Compared to conventional Co- or Mn-based cathodes, LiFePO4 maintains its size regardless of a cell’s state of charge. This means that cells with LiFePO4 cathodes benefit from having one less built-in wear-out mechanism. Indeed, these cells typically remain useful after at least twice as many charge/discharge cycles as cobalt- or manganese-based cells: 1,000 vs. 500.
Because the traditional cathode materials have a relatively weak metal-oxygen bond, if the cell is overheated, oxygen can be released and react with the electrolyte, causing further heating leading to thermal runaway. The oxygen bonds in an iron-phosphate cathode are much stronger, requiring temperatures higher than 400°C before breaking, effectively eliminating thermal runaway.
The original work on LiFePO4 cathodes done at the University of Texas was improved upon by Yet-Ming Chiang and others at MIT by doping the material with aluminum, niobium, and zirconium. These changes improved characteristics significantly, especially the cathode’s electrical conductivity.1
Another approach to increasing conductivity is being commercialized by Phostech Lithium in cooperation with Hydro-Quebec and Université de Montréal, owners or co-owners of several patents on LiFePO4. In the process used by Phostech Lithium, nanoparticles of LiFePO4 are coated with carbon. A high energy density results, and the process retains the economic and safety benefits of the basic phosphate material.
Most of the Li-ion battery improvements reported to date involve changes to the cathode material. An exception is the lithium titanate development undertaken by Altairnano that replaces the traditional carbon anode.
As shown in Figure 2, during charge/discharge cycling, Li ions are added to or removed from the carbon anode. Intercalation causes a carbon anode to change size slightly because of the presence or absence of Li ions. In contrast, lithium titanate oxide (Li4Ti5O12) behaves structurally like LiFePO4 to the extent that both are low- or zero-strain materials in this application: Their structures don’t change size as Li ions are added or removed.
Eliminating the carbon anode wear-out mechanism greatly extends battery life. However, the improvement the new anode material makes to safety and fast charging is even more important.
In Li-ion cells that use a carbon anode, including those with LiFePO4 cathodes, a solid electrolyte interface (SEI) layer is formed on the surface of the anode when the cell is first charged. The SEI provides a safety layer between the highly reactive carbon anode and the electrolyte. At temperatures above 120°C, the SEI breaks down, and the anode can react violently with the electrolyte, further raising the cell temperature.
This process causes more of the SEI to break down and quickly leads to thermal runaway with cell destruction soon following. In normal operation, it is the somewhat porous SEI that limits the charging and discharging rates.
The SEI also figures prominently in low temperature operation. Rather than pass a small current below 0°C, the SEI becomes virtually nonconductive. This means that efforts to charge a cell at low temperatures cause Li ions to plate onto the SEI rather than to pass through it to the anode. This condition also can lead to thermal runaway.
Altairnano’s nLTO anode, where the n distinguishes nanosize Li4Ti5O12 from the bulk material, does not react with the electrolyte so no SEI is formed. This means that thermal runaway cannot occur.
In fact, eliminating the SEI is beneficial in several ways. Cells using nLTO anodes can be charged to 90% of room-temperature capacity at -30°C in 30 minutes. The new material increases charge/discharge cycles by at least a factor of 10 while improving both high- and low-temperature operation. In addition, the 2.0-V to 4.2-V window within which conventional Li-ion cells must operate has been extended from 0 V to about 5 V over a temperature range from -50°C to +260°C.
Where Can I Buy These Materials?
Before you rush out to your local chemical company, make sure your spray hydrolysis equipment is tuned up and ready to go. In a detailed technical paper, Altairnano described the process it uses to produce nLTO as well as several other similar materials. Basically, a very fine spray of a solution containing LTO is rapidly evaporated. This can be done by mixing hot air with the spray or by spraying onto a hot plate.
The paper’s authors commented that for TiO2 production, a higher concentration of metal in the oxide produced a less dense material. A higher temperature resulted in a coarser film. And, if evaporation is not complete because of too high a feed rate, solids form on the hot plate. It is assumed that similar trade-offs apply to production of nLTO.
The particles produced from the initial stage of the process range in size from a few micrometers to 100 µm. A second stage involving calcination with selected additives promotes crystallization into nanosize TiO2 particles. Further stages of milling, spray drying, and micronizing may be needed depending on the characteristics desired for the end product. The overall process addresses a range of materials and particle sizes.2
Both Phostech Lithium and Altairnano are running pilot production plants and sell their nanomaterials to interested battery companies. Of course, regardless of how promising a technology may be, there are many steps between invention and commercial success. Links made between these materials companies, battery makers, and battery users are the real keys to developing viable electric vehicles.
To date, Altairnano has demonstrated an all-electric vehicle using the company’s range of NanoSafe™ Batteries and is supplying the batteries to Phoenix Motor Cars (U.S.) and The Lightning Car Company (U.K.). One of the highly publicized advantages of Altairnano’s technology is a 10-minute charge time. It has been demonstrated but requires a 250-kW charger. Something more conservative such as 30 minutes is probably more practical although the rate is not limited by the batteries themselves, which is the point of advertising a 10-minute time.
A123 Systems has developed a line of Li-ion batteries based on LiFePO4 technology and applied them to the portable power tool industry. DeWalt tools use these batteries in a 36-V pack, which delivers much greater torque than earlier 18-V battery packs. Based on the power-tool success, A123 has been selected by General Motors to take part in battery development for the company’s future VOLT car.
A123’s patented Nanophosphate™ technology is derived from work initially done at MIT and claimed not to infringe in any way on the technology used by Phostech Lithium’s C-LiFePO4 compounds. The company’s website states that A123 has the largest Li-ion R&D team in North America. However, they are hardly alone.
Li-ion battery development is a widely studied topic with hundreds of materials under scrutiny. LiFePO4 is a good example of a very popular cathode material that has generated dozens of research papers. Some of them deal with the identical material, but often there are slight differences in surface treatment, doping, particle size, and preparation that affect behavior. Sometimes, the identical cathode material may be used, but the electrolyte, separator, or anode material is different. Simply understanding the relationships among current findings across a large number of research labs is daunting.
Further, the situation is exacerbated by the funding available for opportunity-driven, industry-based research vs. that found in academia. Various governments and agencies are bridging that gap by funding development work.
For example, the Batteries for Advanced Transportation Technologies (BATT) Program is supported by the U.S. Department of Energy Office of Vehicle Technologies to help develop high-performance rechargeable batteries for use in electric vehicles (EVs) and hybrid-electric vehicles. The work is carried out by the Lawrence Berkeley National Laboratory and managed by the Berkeley Electrochemical Research Council.
Summary Including Recent Findings
More than 100 papers about Li-ion battery technology were presented at the 212th Electrochemical Society (ECS) meeting in October 2007. Almost all were from academia with only a few battery and materials companies represented. Subjects covered included cathodes, anodes, electrolytes, basic battery technology, and the operation of lithium-based materials and systems.
Several papers and much of the work discussed in the BATT quarterly reports concerned LiFePO4. This material is being used commercially by concerns such as A123 yet has remained the subject of much research because of the apparent contradiction between its low conductivity and high charge/discharge rates.
A mathematical model concluded that transport of ions across the semi-insulating SEI layer is via a concentration gradient, not a potential gradient. This means that, in the extreme, a high charge/discharge rate battery could be made from an insulating material.3
Keeping with the LiFePO4 material, an ECS paper reported on solubility sensitivity to particle size; that is, the solubility of lithium ions in FePO4. The surface energies associated with several of the crystal facets are shown to differ significantly as do the potentials related to extraction/insertion of lithium from those surfaces. The authors demonstrate how these differences play a role in the thermodynamics and kinetics of lithiation and delithiation. They also consider the impact of surface and particle size on the thermodynamic properties.4
In another LiFePO4 paper, the cycling and discharge behaviors of material that included multiwalled carbon nanotubes (MWCN) were investigated. Carbon coating, metal dispersion, and doping have been used to improve the material’s conductivity, and in this case, MWCNs. The importance of this work is in its relationship to a sol-gel method of LiFePO4 production.5
At the anode, various materials other than the traditional carbon graphite are being explored. Sony has introduced a new Li-ion cell with a nanostructured anode made from a tin-cobalt-carbon (Sn-Co-C) composite. It provides a high specific capacity but costs much more than carbon.
Silicon also is attractive, having a very high specific capacity of 4,200 mAh/g. However, lithiation results in a 300% volume expansion so the silicon soon fractures and the huge capacity cannot be retained over more than a few charge/discharge cycles. A Stanford University researcher has reported the use of silicon nanowires as an anode. In this form, silicon appeared to withstand multiple lithiation/delithiation cycles without fracture.6
In yet another paper, the authors propose a carbon nanotube structure in place of the usual graphite anode. The self-supporting material is claimed to provide high specific capacity, low irreversible capacity loss, and low volume expansion.7
Finally, work at Argonne National Lab has characterized the performance of a cell using a nanostructured Li4Ti5O12 spinel. The researchers concluded that a much longer life can be expected for batteries using this anode material. It also contributes to good low-temperature operation and good safety characteristics. The only drawback of this type of cell is a relatively low output voltage of 2.5 V compared to 3.7 V for traditional cells.8
To summarize, there is no one Li-ion battery chemistry or construction best for all applications. Manufacturers have their sights set on the huge EV market but have only recently demonstrated safe batteries with the required charge/discharge characteristics. Cost, at least partially a function of production volume, is high at this time. Initially, the new batteries are powering small production runs of high-performance sports cars and environmentally friendly small SUVs.
Will your 2010 car be an EV? Can Li-ion batteries leapfrog fuel cell development and hybrids? Is there a new material being developed that will perform better than the current favorites LiFePO4 and Li4Ti5O12? Time will tell, but if none of these things happens, it won’t be for lack of effort.
References
1. Chiang, Y., et al., “Electronically conductive phospho-olivines as lithium storage electrodes,” Nature Materials, Sept. 22, 2002, pp. 123-128.
2. Verhulst., D., et al., “A New Process for the Production of Nano-sized TiO2 and Other Ceramic Oxides by Spray Hydrolysis,” Altair Nanomaterials.
3. Srinivasan, V., and Newman, J., “Mathematical Modeling of Advanced Li-ion Chemistries,” Lawrence Berkeley National Laboratory.
4. Ceder, G., et al., “Surface and Particle-size Effects on the Thermodynamics of LiFePO4 from First-Principles Simulations,” Department of Materials Science and Engineering, Massachusetts Institute of Technology.
5. Wang, W. and Kumta, P., “Further Enhancement of Rate Capability of Sol-Gel Derived Nanostructured LiFePO4 by Multi-Walled Carbon Nanotubes,” Department of Materials Science and Engineering, Carnegie Mellon University.
6. Cui, Y., et al., “High-Performance Lithium Battery Anodes Using Silicon Nanowires,”Nature Nanotechnology, December 2007.
7. Hossain, S., et al., “Carbon Fiber Composite—A High Capacity Anode for Lithium-ion Batteries,” LiTech.
8. Amine, K., et al., “Nano-Structured Li4Ti5O12 for High-Power Lithium-Ion Batteries,” Argonne National Laboratory.
June 2008