The current state of energy and environment issues has received increasing attention from the general public over the past decades. With the industrial advancement in the past 200 years comes along the energy crisis and environment threats. Vehicles are becoming more commonly seen in every household as social and economic development continues to strive. The automobile industry consumes a significant amount of overall energy produced, and contributes vastly to the environmental problems, such as urban air pollution. Approximately 80% of national CO emission is attributed to transportation sector. Air pollutants result in chronic diseases in humans. [1] As a result, both academia and manufacturing industries are actively seeking solutions to achieve greener vehicles with cleaner tailpipe emission.
Electric vehicle (EV) is a potent solution to relieve energy crisis and reduce environmental pollution. However, its technology is not recent. Electric cars were first developed in Europe in the late 19th century. The production of electric cars peaked in 1920s when gasoline-powered and steam-power cars suffered from immature technology. The development of electric cars was much hindered in the early 20th century by the mass production of internal combustion engine vehicles and reduction of manufacturing costs and oil prices. Electric vehicle development revived in the 1990s with the renewal of several legislations on emission standards and the concern of energy security. Though increasing interest and efforts have been devoted in electric vehicle development in the recent decades, the popularization of electric cars has been much confronted by its battery technology.
Batteries serve as an intermediate to store and transform chemical energy and electrical energy through reversible chemical reactions. Many types of batteries have been used in electric cars, notably lead acid battery (Pb-Acid), nickel metal hydride battery (NiMH), nickel iron battery. Other batteries, such as metal-air battery and semi-solid battery have been actively explored. Lithium-ion batteries remain a potent candidate in electric vehicle operation, for their high energy density compared with Pb-Acid, NiMH, and Nickel-Cadmium batteries. [2] The performance of batteries, such as energy density and capacity, are determined by its electrochemical components and properties, namely electrodes (anode and cathode) and electrolyte. At present, graphite serves as the major material for anode. Cathode materials dominantly determine the energy density of present lithium-ion batteries, and among all, the most potent for electric vehicle are: lithium-nickel-cobalt-aluminum (NCA), lithium-nickel-manganese-cobalt (NMC), lithium-manganese-spinel (LMS), lithium titanate (LTO), and lithium-iron phosphate (LFP). [3,4]
The current technology only achieved specific power, power density and cycle life among a spectrum of goals set by the U.S. Advanced Battery Consortium (USABC), while energy density and cost remains far from target. [5] Despite the well-recognized energy storage capability, the future prevalence of electric vehicles over traditional gasoline-powered engines remains uncertain because of the low energy density of lithium-ion batteries compared with gasoline, which is 180 Wh/kg and 3000 Wh/L, respectively. [3] Limited energy density restricts the extension of travel range per charge. Increasing the energy storage density by double in the next decade with a target travel range of 500 km per charge has been generally targeted by automobile industries. [3] Safety is not addressed in the USABC goals, which overlooks the inherent threats posed by flammable organic solvents and active lithium. LFP is considered as the most promising cathode material, for its improving safety and inexpensive element iron. [2] Sustainability and green concepts have been emphasized in future battery design. Recycle-ability, renewability, and refilling cells are actively explored.
In the technical perspectives, the balance among a spectrum of dimensions, such as specific power and energy, safety, performance, and life span will continue to be strived in the coming decade, while cost remains the major obstacle in gaining the market share on the business side. [6]
Safety is by far the most important concern for electric cars. With active and flammable chemicals on board, an overcharged battery, a high discharge rate or a short circuit easily causes a thermal runaway reaction. Automobile manufacturers either choose installment of enhanced safety monitoring system or inherently safe batteries, such as LFP. [2]
Life span of battery is generally defined as the time when a battery is degraded to 70% of its original capacity in most publications. [6] More batteries than needed are usually installed in electric cars to guarantee the stated vehicle life span, taking cell aging and degradation into consideration. However, this increases electric vehicle size and weight, and inherently decreases efficiency whenever an electric car is in operation. Warranty program is likely to be introduced in the future to promptly change degraded batteries.
Charging time is another concern for EV current and potential customers. Long charging time is identified as the major inconvenience by the market. A charging period of overnight or less is much desired by the market. [6] Fast charging requires enhanced cooling system to avoid thermal runaway and ensure safety. Standardization of electricity system and battery pack is also required to realize fast and convenient charging.
Performance in different climate conditions poses another technical challenge. Operating temperature of lithium-ion batteries is usually within the range of -30°C to 60°C. [2] However, present technology hardly avoid degradation when operating over a wide range of temperature. To avoid temperature restrictions in EV mobility, more capable battery system for wider climate variance is desired in the future.
The cost is the major hurdle for the popularization of electric cars. General target of batteries is set at $200-300 per kWh, while the current cost of lithium-ion battery pack is around $800 - $1000 per kWh. [6] Incentive programs have been introduced in several countries in support of EV. The breakeven time of electric car owners depends on individual driving behaviors and difference in oil and electricity prices. A shorter breakeven time is expected with government incentives.
Electric cars and related battery industries are booming business in the next decade, in seeking better technology and gaining more market share. Players with better solution to the current inherent technical hurdles will gain more advantages in the near-term future. Long-term popularization of EV will ensure energy security and reduce pollution from transportation. However, the long-term mass acceptance of EV depends on that whether battery technology is further improved and manufacturing cost is lowered.
© Yuan Zhong. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commerical rights, are reserved to the author.
[1] "Our Nation's Air - Status and Trends Through 2008," U.S. Environmental Protection Agency, EPA-454/R-09-002, Feb 2010.
[2] Amirault et al., "The Electric Vehicle Battery Landscape: Opportunities and Challenges," University of California, Berkeley, December 2009.
[3] J.-M. Tarascon, "Key Challenges in Future Li-battery Research," Phil. Trans. Roy. Soc. A. 368, 3227 (2010).
[4] Axsen et al., "Batteries for Plug-in Hybrid Electric Vehicles (PHEVs): Goals and the State of Technology circa 2008," Institute of Transportation Studies, University of California, Davis, UCD-ITS-RR-08-14, May 2008.
[5] "Multi-Year Program Plan 2011 - 2015," Vehicle Technologies Program, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, December 2010.
[6] Pesaran et al., " Battery Requirements for Plug-In Hybrid Electric Vehicles-Analysis and Rational," U.S. National Renewable Energy Laboratory, NREL/CP-540-42240 July 2009.