Current Battery Technology and Fully Electric Vehicles: A Review
by Radu Gogoana
Rising oil prices, government regulation and the threat of global warming are driving the development of clean, renewable automotive propulsion technologies. This has led to vehicles powered by fuel cells, hydrogen, ethanol, methanol, and even compressed air; each approach has its limitations, and so far the only one being mass produced is a hybrid between electric and gasoline power. Vehicles powered purely by electricity offer a renewable, zero-emissions alternative that has been explored for over a century; battery-powered cars were introduced in the late 1800s, even before internal combustion automobiles succeeded them.The factor limiting the success of electric vehicles for the past hundred years has been the storage of electrical energy; batteries have hindered the performance, range and cost of electric cars to a point where they are not viable for everyday consumer use. However, recent developments in lithium-ion cell technology have shown promising results: the improvements in energy density, capacity and cycle life are allowing fully electric passenger vehicles to reach a new level of performance, where they can compete on the same playing field as their combustion-powered counterparts.
The current automobile market is dominated by internal combustion engines, and recently we have seen the introduction of hybrid gasoline-electric vehicles to boost fuel economy. These cars use battery power during slow speed (stop-and-go) driving and when short bursts of power are needed for acceleration, but the car's overall energy supply is still derived from gasoline. Electric motors provide 100% of available torque from zero RPM, and effectively complement the power band of a gasoline engine, which peaks at high RPM's. However, cell technology has still not been able to provide the energy storage capacity to make a battery the vehicle's primary power source. A fully electric vehicle places high demands on its battery pack: it must have a high energy storage density, be able to provide high discharge rates, allow for rapid recharge times, undergo thousands of charge/discharge cycles without significant capacity loss, handle vibrations, be environmentally safe, while meeting the practical demands of manufacturing viability, cost effectiveness, crash worthiness, and safety standards associated with storing a large amount of energy in a concentrated package (Conte, 2006).
The hardest challenges facing battery technologies have historically been cycle-ability and energy density. The lead-acid (Pb) batteries that have been around for over a century are extremely heavy (low energy density), environmentally unsafe (lead is toxic), and can only handle up to 200 full charge/discharge cycles before their capacity dips below 80% of original. Nickel cadmium (NiCd) and nickel metal-hydride (Ni-MH) batteries have been in development for over two decades, and Ni-MH batteries are currently being used in gasoline-hybrid electric vehicles (Affanni et al., 2005). They are chosen because of their high discharge rate capability, cycle life (800+), cost, tolerance for abuse, and overall reliability. Ni-MH cells have been around for a long time, and their charge characteristics are predictable enough to green-light their mass production for hybrid vehicles. However, the major drawback of Ni-MH batteries is energy density; a Ni-MH battery pack with enough energy to provide a 300 mile range for an electric vehicle would be very large and heavy; this added weight would severely detract from the vehicle's overall performance. The latest developments in rechargeable battery technology involve compounds derived from lithium; it is the lightest and one of the most chemically reactive metals. This combination holds promising potential: lithium's reactivity allows for high chemical energy storage capacities, and its light weight is conducive to the energy-to-weight density required for fully-electric automotive applications. Currently developments have been centered on improving the ideal chemistry of lithium-ion cells, and maximizing the surface area of the anode and cathode in order to raise the energy density and allowable drain rates. The following is a review of the recent research being done with lithium-ion batteries, and the manufacturing developments that have ensued.
Cell Chemistry Developments
Rechargeable lithium batteries have been developed since the mid 1980's; in 1991, Sony introduced the first lithium-ion cell to the consumer market as an answer to the existing problems with battery technology. It offered twice the energy density of the Ni-Cd batteries of the day and could sustain a higher average voltage over its discharge cycle. These cells were not without their own drawbacks; they had a limited cycle life, offered low discharge rates, and the cell monitoring electronics of the time would not allow for safe use in large battery pack assemblies (Armand, 2001). Intensive developments in the decade following its introduction have raised cell capacity more than twice, and have allowed for much higher current drain rates.
The latest developments in rechargeable battery technology involve compounds derived from lithium; it is the lightest, and one of the most chemically reactive, metals.
Significant improvements have also been made to the cathode side of the lithium-ion battery; this has traditionally been constructed from LiCoO2 (Kang et al., 2006). Compounds of cobalt offer a high cycle life and good specific capacities; however, cobalt is expensive to produce, and is also slightly toxic to the environment. Alternative anode compositions such as LiNi.5Mn.5O2 have been laboratory-tested to show higher specific capacities and discharge rates; however, nickel and manganese are still relatively expensive, and these cells have not been fully field tested to conclude real-time production feasibility (2006). Such materials are viable for small battery packs such as the ones found in cell phones and laptops: in such quantities, the cost of materials is not a very pressing issue. However, electric vehicles will require the assembly of thousands of cells into a battery pack; on a large scale, materials costs play a much higher role.
Developments in Cell Surface Area
The most promising recent developments are in the field of nanotechnology. A battery works by having two different materials immersed in an electrolyte solution, with ions flowing through the solution and plating to one side of the battery. Lithium plating occurs on the anode side of the battery, creating a volumetric expansion of up to 200% during the process. Graphite has been traditionally used for anode construction because its semi-porous structure allows for the volumetric expansion of lithium plating without cracking or structural damage to the anode. However, graphite anodes are limited by their specific capacity of 372 mAh/g (Panero et al., 2004). Other metals such as aluminum and tin have higher specific capacities, but their smooth non-porous surfaces do not allow for the volumetric expansion of lithium plating, and thus suffer from cracking and destruction during the charge-cycling process. However, nanotechnology has enabled the erection of 3D structures on the surface of the anode; this surface tolerates the expansion of lithium plating, thus allowing the high-capacity metal alloy anode to undergo multiple charge/discharge cycles. This has an added advantage of increasing the overall surface area of the anode, allowing for higher possible drain rates (2004).
The studies conducted by Nam et al., Panero et al. and Son et al., have explored three-dimensional surface structures created with nanotube growth, nanowire assemblies, viruses, and specialized conventional assembly methods. Lithography, block copolymerization and layer-by-layer material depositing are the conventional methods which all allow for such surfaces; however, they are extremely costly. Another approach involves the genetic modification of viruses; they are grown in controlled conditions that allow their frames to plate with the desired anode alloys, and then these structures are left behind as mini-wires on the anode surface. This method can also be used for the production of extremely small batteries on a nano-scale. However, the most promising development efforts focus on synthesis of nanowires; these allow for a flexible, lightweight anode with a very high surface area. They are constructed via dipping methods that are feasible on a mass-production scale, thus promising to lower the eventual production costs of these batteries (Nam et al., 2006).
To this end, multiple studies have pointed to the development of LiFePO4 and LiFeS2 as promising alternatives (Son et al., 2004). They utilize compounds of iron, which are cheap and nontoxic. LiFePO4 anodes have a lower energy density than those of cobalt, but are less expensive, easier to manufacture and offer very high cycle life (2004). LiFeS2 anodes offer almost twice the energy density of cobalt, but have yet to be fully tested. Both of these developing technologies have the potential of lowering the manufacturing costs of lithium-ion batteries to a point where they can be mass-produced for electrical vehicle use (Ritchie, 2004). LiFePO4 cathodes have also proven to be more thermally stable than those of LiCoO2, LiNiO2 and LiMn2O4, potentially adding to the safety factor that can help to prevent the thermal runaway of these cells. Laboratory cycling tests have also shown that increasing the surface area of the LiFePO4 particle has led to increases in cell capacity (Takahashi et al., 2002).
The testing of nano-particle LiFePO4 surfaces have shown promising results; by creating 3-dimensional surface structures, researchers were able to increase the capacity of the cathode to three times what its ideal limit was thought to be (Son et al., 2004). Work still needs to be done on improving the cyclability of this nano-particle LiFePO4 combination, but the nano-particle surface has allowed for a significant step forward in the search for a cheaper, non-toxic cathode alloy.
The testing of nano-particle LiFePO4 surfaces has shown promising results; by creating 3-dimensional surface structures, researchers were able to increase the capacity of the cathode to three times what its ideal limit was thought to be.
Another important factor inhibiting the mass production of batteries suitable for electric vehicle use is safety. Lithium-ion batteries contain a flammable organic electrolyte; if a cell is physically damaged, manufactured of impure materials, improperly charged, drained too quickly, or short-circuited, it could trigger its own fiery meltdown. If the cells are assembled in a battery pack, the heat from one cell melting down can cause the adjacent cells to do the same, thus starting a chain reaction throughout the battery pack. This was a problem with a certain batch of laptop batteries manufactured by Sony; millions had to be recalled because there were impurities in the cell divider alloys. This problem caused some cells to internally short-circuit, leading to cell destruction and, consequently, violent destruction of the entire battery pack (Sony Support, 2007). These safety problems are mitigated by computerized monitoring to control charging and discharging rates, along with temperature controls monitoring certain sectors of the battery pack. Proper cooling and airflow must be provided to prevent overheating, along with cell "firewalls" built into sectors of the battery pack to prevent catastrophic meltdowns. These risks are inherent in electric vehicles, and these safety problems would have still existed with older battery technologies, although to a somewhat lesser extent. Problems like this arise in any situation where there is a concentrated amount of energy stored in a small space; gasoline tank fires are still a problem with internal-combustion automobiles. Proper pack management and safety monitoring are necessary to prevent the over-charging, over-discharging, and temperature fluctuations associated with cell overheating, decreased cycle life, and catastrophic failure (Affanni et al., 2005).
A123 Systems has already begun manufacturing batteries which fit the above criteria, using phosphates of iron and 3D nanostructures on the cell anode. These cells are being mass produced for DeWalt's line of portable power tools, and will be used by General Motors in their Chevy Volt plug-in hybrid passenger car (scheduled for launch in 2010). These cells have been extensively tested, and have proven themselves over 2000 charge/discharge cycles (a123 systems, 2007). As a safety demonstration, a nail was driven through all of the layers of a fully charged cell to internally short-circuit the battery; the cell did not burst or catch fire like most other cells; it simply vented smoke (Linton, 2007). This is due to the lack of oxygen in the electrolyte used in the a123 battery (Vieau, 2007). This is due to the lack of oxygen in the electrolyte used in the a123 battery. (Vieau, 2007) These results follow the trends that the latest cell research has pointed to regarding the thermal stability, cyclability and nano-technology surface area improvements on LiFePO4 cells.
A small private company named Tesla Motors has already come out with a fully electric sports car, priced at $98,000. This vehicle uses a battery pack constructed from 6831 consumer-grade conventional lithium ion cells, with a projected vehicle cycle life of 100,000 miles. With this battery pack, ranges of up to 245 miles are achievable (Tesla Motors, 2007). If battery development continues in the direction of nano-phosphate lithium ion batteries, this range value should keep increasing and cell manufacturing costs should keep decreasing.
Current research efforts regarding lithium-ion batteries are being concentrated in the field of nano-surface development. Scientists are taking different approaches to achieve the same goal, but there is a consensus that increasing the surface area of the anode and cathode with nanostructures is an effective way of upping the capacity, cycle life and drain capacity of lithium-ion batteries. These are relatively recent developments, and such batteries are only now entering mass production. However, breakthroughs are being made using cheaper, less toxic battery materials; scientists predict that the cost of high capacity, high power density lithium-ion batteries will fall dramatically in upcoming years. This will enable the cost effective, durable, energy dense battery packs needed for the consumer-level production of fully electric automobiles.
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