Chemical & Engineering News
October 13, 1997
Copyright © 1997 by the American Chemical Society
ELECTRIC VEHICLES GEAR UP
Battery range, cost, and life limitations are gradually being overcomeSophie L. Wilkinson
W ithin just a few years, state mandates will put thousands of electric vehicles on American roads in an effort to curb urban pollution. Government officials, car companies, and suppliers of the batteries that power electric vehicles are hashing out the dates and quotas that will govern their marketing. And they are involved in extensive and expensive research to meet these looming deadlines.
The fundamental concept is engaging: Power a car with a battery and shift the tailpipe pollution out of the city back upstream to an electric utility, where environmental impact can be curtailed. That said, the hurdles to successful commercialization are still significant. The key to making electric vehicles a mass-market product is to improve the battery. Whether based on nickel-cadmium, lead-acid, nickel-metal hydride, lithium-ion or lithium-polymer, or any of the other technologies under development, these batteries must overcome high cost, low driving range, and limited life. And these difficulties are evidenced by the mixed feelings voiced by those involved in their evolution.
Although some firms in the field, particularly small start-ups, seem to relish their participation in the development of battery-powered vehicles, the major manufacturers, with higher business costs, are acting as a result of government pressure. The development of electric vehicles is "largely being driven by requirements in California, which has the most severe air pollution problem in the country," says Ken Heitner, program manager for electric vehicle advanced battery development at the Department of Energy (DOE). "Recent activity in New York and Massachusetts is also important."
Marlyn J. Stroven, manager for electric-vehicle driveline, battery, climate-control, and electromechanical systems at Ford Motor Co., Dearborn, Mich., agrees. "I doubt any manufacturer would voluntarily have gone into production with an electric vehicle they have to sell at two and one-half to three times the cost of their other comparable products," he says. Although there is a "high percentage of people that would give serious consideration to buying an electric vehicle, that interest disappears real quickly when the cost gets more than 10% above an internal-combustion vehicle."
The toughest challenge is being able to offer a vehicle that "isn't going to cost the customer more for less," Stroven adds. Because electric vehicles serve a niche market, their price doesn't benefit from economies of scale that customers expect from a high-volume manufacturer like Ford, Stroven notes. "When customers walk into a Ford or Toyota dealership, they're not used to paying more than twice what they would pay for a comparable car that has an internal combustion engine in it," Stroven says. "In fact, it will be many years before Ford or any other manufacturer is able to make any profit on their electric vehicles."
Customers may need to go through a mental shift to really see the benefits of an electric vehicle. "An electric vehicle is very nice to drive," Stroven says. "They are extremely quiet, you don't feel any shifting, and they have very good performance, particularly at low speeds." And because an electric vehicle can be plugged in and refueled overnight at the customer's house, "every morning when you get in your vehicle you are leaving with the equivalent of a full tank of fuel," Stroven says. "That's a little different from thinking, 'I have to fill up twice a week.'"
Charging stations are "springing up all over the country, at shopping centers, city halls, McDonald's," says Larry R. Stadtner, chairman and president of Zebra Motors, Alameda, Calif. They're also appearing at parking lots, hotels, and mass-transit stations. Eventually, he says, "it will be like a parking meter, where you put a quarter in and recharge your car while you're at the office." He points out that the "cost to recharge an electric car is far cheaper than filling a car with gas." Stadtner estimates that recharging will cost about $1.
Public charging stations as well as home outlets are being developed to easily recharge electric vehicles.
And electric vehicles have lower operating costs, he says. "There's nothing to tune up, there's no oil, and there are fewer moving parts."
Nevertheless, Stroven believes electric cars are likely to be bought as second cars, or to be occasionally supplemented with a rental car for long trips. Commuters and those who like to be the first to adopt any new technology are likely customers for electric vehicles, Stadtner says. He notes that "80% of vehicle trips are within the range of an electric car." Typically, electric vehicles are limited to between 50 and 100 miles per charge.
About 100 commuters are participating in the National Station Car Association's test of the "station car" concept. Small battery-powered cars are rented out on an as-needed basis for local trips between a user's home and a mass-transit station or between a station and work. The cars, which can be reserved in advance, can also be used for errands during the day, or short evening or weekend trips. The association notes that most transit station parking lots fill up by 7:30 AM, but the station cars are guaranteed a parking space. So commuters who arrive after 7:30 won't be forced to drive all the way to work.
Rather than concentrating solely on retail customers, the car companies can also go after markets that have an incentive to go electric. Ford, for example, has targeted the utilities, some of which along with government agencies are obligated by the federal Energy Policy Act to purchase a percentage of vehicles that can operate on alternative fuels for their vehicle fleets.
The next step for Ford is to "go after customers who have high disposable income, who are accustomed to spending much more on a second car than a lot of people do for their first car," Stroven says. These customers might currently buy a vehicle such as a Ford Explorer, a BMW 328, or a small Mercedes as their second car, Stroven says-in other words, "someone willing to spend $30,000 or more."
The smaller start-ups have an advantage over the large established firms in this respect. Zebra, for example, expects to market its Model Z for $19,700, "comparable with a midsized internal-combustion vehicle," Stadtner says. While contending that "our car is better looking," he admits it is a "no-frills car, that doesn't have the same fit and finish as a Ford car." But lack of high overhead also keeps the price down. "We're not going to have to recoup $500 million in our first 35 vehicles," he says with a laugh.
Just how big can the electric vehicle niche get? "If I put on my rose-colored glasses, and if everything went extremely well and we were able to drive our battery costs down to our long-term targets, this could become as much as 10% of our volume," Ford's Stroven says.
California has mandated that 10% of the new cars and light trucks manufactured for sale in the state must be nonpolluting as of 2003, says Edwin Riddell, business area manager for electric transportation, customer systems group, at the Electric Power Research Institute (EPRI), Palo Alto, Calif. California's Air Resources Board, a department of California's Environmental Protection Agency, expects this will put 800,000 electric vehicles on the state's roads by 2010. EPRI estimates that a fleet of 2 million electric vehicles would cut urban emissions by 160,000 tons per year and save 60,000 barrels of oil per day.
Chrysler, Ford, General Motors, Honda, Mazda, Nissan, and Toyota-the seven highest volume sellers in California- are the companies that must meet California's mandates. However tough, those requirements seem easier to meet than the original California mandate-now dropped-that 2% of vehicles sold be zero-emission by 1998, with the percentage increasing in following years.
Other states are working on similar regulations. Massachusetts, for example, has a 2003 mandate like California's. And New York is seeking to impose a 2% minimum by 1998 and 10% by 2003-but its mandate may be held up by a lawsuit. DOE's Heitner also notes that EPA's new ozone and particulate matter standards may impact state and local requirements for cleaning up vehicle emissions (C&EN, April 14, page 10).
When the seven U.S. and Japanese car makers were aiming to meet the original 1998 California mandates, they focused on lead-acid battery technology, "because that was the only battery technology that was commercially available to support that timing," Stroven says. In the near future, the emphasis will shift to nickel-metal hydride (NiMH) batteries. He believes most car makers will be using NiMH when the mandates go back into place in 2003. After that, the U.S. and Japanese industries will probably shift to lithium-based batteries as they become available, he says.
In Europe, electric vehicles are available that run on nickel-cadmium batteries, Stroven says. He expects the European market will make the transition to NiMH, though German firms are also working on sodium-nickel chloride batteries.
"The first NiMH batteries are prototypes and are going to be built in limited quantities," Stroven says. "The idea is to gain experience with them in the field under various operating conditions and the different ways customers use the product."
Batteries are "sensitive to being abused," Heitner admits. "People are capable of beating up conventional cars, and electric cars will be sensitive to that also." That abuse could be administered through a lot of hard accelerations and decelerations, or repeatedly driving until the battery is almost fully discharged.
Quick-charging can also be rough on a battery-a shame because this might be one way to appeal to customers concerned about the vehicles' lack of range. "We're trying to understand that phenomenon and in the future design batteries so they can accept quick charge," Heitner says. AeroVironment of Monrovia, Calif., has begun marketing to industrial customers a charger for lead-acid batteries that recharges to 80% of normal capacity in less than 20 minutes.
For now, NiMH batteries can regain as much as 85% of their normal capacity in a half-hour. Normal recharge typically takes about six to eight hours. "That fits in nicely with the fact that utilities have a big dip in their load at night," Heitner says. "They would offer low-cost, off-peak electricity at that time."
Theoretically, the recharge time could be squeezed down to just five or 10 minutes, "the equivalent of a stop at a gasoline station," Heitner says. "But you have to develop an awful lot of power to recharge batteries quickly."
Drivers could get around this issue by exchanging a used battery for a charged one when they pull into a service station. But so far this has only been developed for large vehicles such as buses, Heitner says.
Israeli engineers have developed an exchangeable zinc-air battery with a range of 300 to 400 miles that is being tested in Europe for use in vans and other large vehicles. Europeans also have had exchangeable lead-acid batteries for years, Heitner says. In an automobile, however, "a battery is too much wrapped up by the vehicle to do that easily."
Much of the U.S. development work on advanced-battery technology is being funded through cooperative arrangements among DOE, EPRI, a number of utilities, and the U.S. Advanced Battery Consortium (USABC), Heitner says. Ford, General Motors, and Chrysler set up the USABC partnership in 1991 to develop advanced batteries, and it has since linked up with DOE and EPRI. The partners have committed hundreds of millions of dollars to the research and development effort.
Individually, these entities are working on developing the technology of several different battery types. But as a group, these partners "don't do any work on lead-acid or nickel-cadmium batteries, which we consider a current technology," Heitner says. "The main batteries we are working on now are nickel-metal hydride and lithium batteries."
Michael C. Saft, director of marketing, SAFT America, Cockeysville, Md., believes that NiMH is already a "fairly well developed product," and the challenge is to "move it from the pilot stage into mass production."
Lithium-ion batteries have further to go, with engineering improvements and exhaustive testing needed to "ensure the cell will be safe in an abusive condition," Saft says. The organic solvent in these batteries could burn if the battery were punctured or exposed to flame, he explains. Saft draws an analogy with gasoline, which carries a similar risk, but which has been successfully managed through engineering.
Europe's consortium-Battery Research & Development in Europe, or BRADE-is focusing on NiMH and lithium-ion batteries, Stroven says. And Japan's Ministry of International Trade & Industry (MITI) is sponsoring a large program with the Lithium Battery Energy Storage Technology Research Association (LIBES), Tokyo, to develop lithium batteries for electric vehicles as well as for utility load leveling, Heitner says.
The different battery technologies offer a broad range of performance, though developers hope they can all be improved. For example, lead-acid batteries can store about 35 watt-hours per kg, Stroven says, "and that is probably its major Achilles' heel. To get any kind of reasonable energy levels, the battery weight gets to be very high." That limits a vehicle running on this type of battery to a range of about 50 to 70 miles, he says. (Of course, range depends on the size of the vehicle and battery used, so the ranges given in this article can only provide a rough comparison between battery types.)
Lead-acid batteries also have a poor cycle life, down around 300 to 350 recharge cycles, says Boone B. Owens, adjunct professor in the department of chemical engineering and materials science at the University of Minnesota, Minneapolis. Owens is also a consultant in the battery field and a technical specialist with Research International, Seattle. And if the battery is abused, it can give off hydrogen gas, Saft says. In addition, once the battery is exhausted, the lead must be recovered.
Specific energy for nickel-cadmium batteries is around 45 watt-hours per kg, Saft says, while range is about 65 to 85 miles. Cost per kilowatt-hour is about $300 to $500, and lifetime is about 1,000 cycles, he says.
Like valve-regulated lead-acid batteries, NiMH batteries are also sealed and maintenance-free, but their life is quite a bit longer, around eight years, Stroven says. And their current specific energy is up around the high 60s to low 70s watt-hours per kg, roughly double that of lead-acid batteries, Stroven says. That boosts vehicle range to about 100 to 120 miles, which would satisfy the demands of both fleet and retail customers, he adds.
"Most people, retail customers particularly, want to have a 100-mile range, even though they may not necessarily need it on a day-to-day basis," Stroven notes. In this case, though, the customers must be considered right. "You have to make them feel comfortable or they won't consider buying your vehicle," he says.
Unfortunately, NiMH batteries are relatively expensive, he notes. "Even in extremely high volumes, we would never be able to meet our target cost," which USABC has set for the long term at $100 per kWh. Stroven pegs NiMH costs at about $300 to $700 per kWh, while lead-acid is down around $150 to $200. However, the lead-acid battery also has a short life, wearing out after just a couple of years of continuous use, according to Stroven.
NiMH batteries are "one of the safer systems out there," Saft says, "and they are environmentally benign. There are no materials that can't be recycled or that would create any problems."
Zinc-air batteries are "getting a fresh look," Owens says, though they have some drawbacks. One company working in this area is B.A.T. International, Burbank, Calif. The firm, which is collaborating with battery manufacturer Kummerow Corp. of North America, Burbank, notes that "although the zinc-air has enormous energy capacity, it does not give up this energy very quickly," making it difficult to accelerate at highway speeds.
B.A.T. International's minivan powered by zinc-air batteries traveled a record 478 miles on a single charge.
Zinc-air batteries include a zinc plate, pellets, or powder as the anode and a catalyst-containing cathode that draws oxygen from the air, says Bill Wason, chief executive officer of B.A.T. subsidiary Ultra-Force Battery, Burbank. The electrolyte is a potassium hydroxide solution. The reaction yields a slurry containing zinc, which has to be recovered to recharge the battery. The air electrode will disintegrate if that is done while the zinc is in the battery, Wason says. So the zinc is removed from the battery for recharging. Drivers of zinc-air vehicles would refuel by swapping their used battery pack for a new one, leaving the used pack behind to be recharged.
B.A.T. says the batteries are expected to cost just $100 per kWh when produced in volume, to have a range of 250 to 300 miles, and to last four years. Energy storage will be around 160 watt-hours per kg. The company, which is also working on nickel-cadmium and NiMH batteries, expects to have its zinc-air battery available for fleet vehicles within a year.
Lithium-ion batteries, familiar to notebook computer owners, can store considerably more energy than NiMH on a weight basis, as much as 140 to 150 watt-hours per kg. But, Heitner says, "on a volume basis they are neck and neck." So, Stroven notes, "even though you can get appreciably more energy for the weight, you have to find a place to put it." Range may be about 150 to 200 miles for lithium-ion batteries and perhaps as much as 250 to 300 miles-"the same as a gasoline vehicle"-for lithium-polymer batteries, adds Riddell.
Lithium battery life is expected to be about five or six years, and costs will probably be around $180 to $220 per kWh in high volume, Stroven says.
Saft is more optimistic and believes lithium-ion batteries could get closer to USABC's midterm goal of $150 per kWh and will last eight to 10 years, in part, because they are sealed.
Saft notes that the batteries contain some materials such as lithium salts that will "have to be recovered" when the battery life is exhausted, "though there is nothing that would be considered really hazardous."
Another crucial issue concerns performance as the battery discharges. "The battery has to deliver enough power on a consistent basis to make sure the vehicle accelerates properly even as it's being discharged," Heitner says. "The vehicle shouldn't become sluggish and unsafe."
The emphasis in advanced battery R&D is on energy density, cycle life, safety/reliability, and cost, Owens says. Unfortunately, there's a trade-off between energy content and cycle life, with a low-energy-density design generally lasting longer than a high-energy-density design, he says. Thermal management, charge time, and discharge rate (acceleration requires rapid discharge) also are important issues.
And battery management itself is critical. "This is not a simple battery like in an electric shaver or flashlight, where you just turn it on and use it, and plug it in to charge it," Owens says. Electric vehicle battery packs are complex, containing several hundred individual electrochemical cells. With these batteries, "you need to make sure that all of the cells that make up the battery are operating correctly, that they're not being overcharged or overdischarged excessively. If a few cells short out here and there you won't necessarily see that, and it can lead to system failure," Owens says.
Much of the advanced battery R&D is focused on lithium batteries. Lithium metal is very reactive and energetic as well as lightweight, making it attractive for electric vehicle batteries, Owens adds. In the 1970s and 1980s, early rechargeable lithium batteries for such products as watches and electric wheelchairs used lithium as the anode and compounds such as titanium disulfide or molybdenum disulfide as the cathode, he points out. As the battery discharged, the lithium metal oxidized and lithium ions traveled through the electrolyte and intercalated into the cathode.
Recharging reversed the process, Owens says, but it didn't do it well. Rather than re-forming a flat electrode, the lithium would plate back out in the shape of dendrites, some of which could flake off and short out the battery, he says. This could lead to overheating of the organic liquid electrolyte and possible fires.
Lithium-ion batteries for products such as camcorders and laptop computers can avoid these problems by replacing the lithium anode with one in which lithium ions are intercalated into various forms of carbon such as graphite. But carbon increases the size of the battery, so its energy density drops, and some of the carbons are "rather special" and drive up the cost, Owens says.
Lithium-ion batteries with cobalt oxides as the cathode are already being evaluated in prototype batteries for vehicles, though they suffer from an environmental point of view from cobalt's heavy-metal status. Interest is now shifting to nickel oxide cathodes, Owens notes. High cost is an issue with both, though manganese oxides may in the future solve that problem. "Any of these cathodes could, in principle, be modified to work with a lithium metal anode if we ever get back to that," he says.
Lithium cells, which at up to 4 V run at a much higher voltage than zinc-air (1.2 V), NiMH (1.3 to 1.4 V), and lead-acid (2.1 V), are more demanding on their electrolytes, Owens says. The electrolyte consequently must be more stable thermodynamically. For lithium-ion batteries, the electrolytes are lithium salts such as lithium hexafluoroarsenate (arsenic is a drawback) or lithium hexafluorophosphate dissolved in organic acids, he says.
The liquid electrolyte can be replaced with a solid conducting polymer electrolyte similar to polyethylene oxide containing a lithium salt, forming a lithium-polymer battery. This has the benefit that the electrolyte won't vaporize as it warms up. However, "diffusion of ions is somewhat restricted in the polymer matrix and so ionic conductivity is low," Owens explains. That limits the discharge rate. But heating the polymer above about 60 °C changes the polymer structure, making it less viscous, enhancing ion movement and battery performance, he says.
One appealing feature of polymer electrolytes is that they could be formed as a thin membrane, which could be coated inexpensively with an anode on one side and a cathode on the other, Owens says. Internal resistance could be addressed by making the membranes very thin. But that leads to issues with manufacturing precision, how to stack these into a large battery, how to manage individual cells, and how to regulate heat flow, he says.
3M and electric utility company Hydro-Québec, Montreal, are heading a project with DOE, Argonne National Laboratory, and Lawrence Berkeley National Laboratory to develop these batteries, Heitner says. 3M is aiming for commercial launch in electric vehicles in 2000 at the earliest.
There is also a lithium-ion battery category between solid and liquid electrolytes, in which a gel electrolyte is formed by adding a low molecular weight organic component to the polymer, Owens says. This can plasticize or swell the polymer, enhancing ionic conductivity at ambient temperatures.
Longer range research in lithium and lithium-ion battery technology remains focused on new kinds of electrodes, Owens says. Cathode materials with very high energy density such as sulfur compounds or vanadium oxides are under study. And there is interest in returning to lithium itself to reduce the complexity and hence the cost of the battery, Owens notes.
NiMH is also the focus of considerable research efforts. The active material in the cathode is nickel hydroxide. The anode is a hydrogen-storing alloy, primarily made of rare-earth materials and nickel. Other components, amounting to about 20% of the anode material, are manipulated to control cycle life, rate capability, and power, says Uwe Köhler, project manager for NiMH battery development at VARTA Batterie, Kelkheim, Germany. VARTA is working on lead-acid and NiMH batteries for electric vehicles, and is collaborating with Duracell on the lithium-ion battery for USABC.
Researchers are boosting NiMH performance by raising conductivity, tinkering with the physical structure of the cell (including the separation between the electrodes), and changing the kinetics (and hence the rate capability) of the battery's active materials. Different alloys, in which the relative amounts of additives such as cobalt, manganese, and aluminum are shifted, can be discharged at different rates. In fact, says Köhler, "almost the whole periodic system can be introduced as an alloy component." Researchers are also working with nickel-titanium alloys.
The electrolyte, which is primarily aqueous potassium hydroxide with lithium hydroxide and sodium hydroxide, can also be varied, Köhler says. "The higher the potassium hydroxide, the better the rate capability. But for other reasons, for instance for improving the temperature range, additives like sodium or lithium can be advantageous."
Which of all these battery types will win out? Although Saft sees lithium-ion batteries as a "product the automotive industry can standardize around," he also believes a whole range of battery technologies will coexist for "a good period of time," just as other vehicle options do. For instance, "not everybody wants to drive a V-8; some people will drive a four-cylinder vehicle," Saft says. With all the roadblocks in the path of electric vehicles, they may seem to be more trouble than they are worth. "But in the context of meeting a definition of a zero-emissions vehicle at the point where the vehicle is used, electric is the only thing we know how to do today," Stroven says. Other options don't appear to be practical in the near term. Cars that run on fuel cells, for example, still require a battery, are expensive, and may be several years from market, he says. Nevertheless, "just about everybody in the industry is working on them," he says.
Hybrid vehicles also require an auxiliary power source, whether it be a turbine, diesel, or gasoline engine, "so they aren't emission free," Stroven says. And if a hybrid were designed to run off clean-burning natural gas, "the infrastructure wouldn't be there for refueling," he says. They are also pricey, since they "have most of the cost issues associated with an electric, and the cost associated with having an auxiliary power unit," Stroven says.
Ironically, it's possible that car makers may not even need to rely on these exotic technologies. "It's not clear that you can't push conventional technology to extremely low levels of emissions, using modifications such as alternate fuel or improved emission controls," Heitner says. One example he mentions is Honda's "very clean" natural gas vehicle. But this won't stop development of other options. "What we're trying to do is create alternatives and let people try them out," Heitner says.
So what's actually available on the electric-vehicle market, or coming in the near term?
Electric vehicles that are already or soon will be on the market include GM's Saturn EV1 (left) and Ford's Ranger EV.
GM's Saturn EV1 two-seat car and Chevrolet S-10 electric pickup are already available with lead-acid batteries. GM Ovonic, a joint venture between GM and Ovonic Battery Co. that is working in cooperation with DOE, will introduce NiMH batteries in the EV1 and S-10 next year.
Toyota offers the RAV4-EV sport-utility vehicle in Japan (and for 1998 fleet models in the U.S.) and Honda is marketing the EV Plus, both of which run on NiMH batteries developed by Matsushita. Toyota also markets an electric bus in Japan that runs on lead-acid batteries.
Ford's compact pickup truck, the Ranger, will debut with a valve-regulated, maintenance-free, 2,000-lb lead-acid battery in the 1998 model year, Stroven says. And Ford will use the NiMH battery in its 1999 Ranger EV vehicles. Initial target customers include utilities and fleets.
Chrysler's Dodge Caravan and Plymouth Voyager EPIC (Electric Powered Interurban Commuter) minivans will come out with advanced lead-acid batteries in 1998 for California fleets. The company is also working with SAFT on NiMH for the minivan. SAFT's nickel-cadmium batteries are being used in cars such as the electric Peugeot 106 and Renault Clio in Europe.
Nissan will introduce an electric vehicle in Japan's retail market early next year, and later in 1998 it will bring the Altra EV compact van to the U.S. fleet market. The vehicles will run on lithium-ion batteries developed by Sony, which is the "world leader" in this sector, Heitner says.
Many other firms are working to bring electric vehicles to the market. Stadtner expects the Zebra Model Z two-seat convertible will be available in first-quarter 1998. It will run on advanced lead-acid batteries that take four hours to charge. In the next generation of vehicles, the company will switch to Unison Batteries' NiMH batteries, which will be leased to customers.
Corbin-Pacific, Castroville, Calif., is marketing the three-wheel, one-passenger Sparrow, powered by a lead-acid battery. The car was designed for commuters and inner-city driving. The company notes that the vehicle can use car-pool lanes because it can be registered as a motorcycle and carries "100% of passenger capacity when driving alone."
In 1899, the electric "la Jamais Contente" (top left) set a world land speed record of more than 100 km per hour. Zebra Motors Model Z (top right) has a sleek look but is a "no-frills car," designed to keep the price down. Corbin-Pacific's three-wheeled one-passenger Sparrow (bottom) is designed for commuting.
While electric vehicles seemingly have caught the public's attention only recently, development in the field has been going on for decades. The first electric vehicle may have been built as early as the 1830s. In 1899, Belgian Camille Jenatzy reportedly drove his electric car, "la Jamais Contente" ("Never Satisfied" or "Never Happy") to a world land speed record in France, topping 100 km per hour. By 1900, more electric vehicles were registered in the U.S. than steam or gasoline vehicles, but the introduction of the convenient self-starter for gasoline-powered vehicles and Ford's Model T, and the spread of gas stations, diminished the relative popularity of electric vehicles. Perhaps that trend is nearing reversal.