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| Fig. 1: A schematic of a prismatic form factor battery. (Source: Wikimedia Commons) |
Battery-powered electric vehicles have garnered attention recently due to their emergence into the otherwise stagnant American automobile industry. The Silicon Valley startup Tesla Motors, Inc. has led the way with their production of the Roadster and Model S causing companies like Nissan and Chevy to release their entries into the "green" automobile race, the Nissan Leaf and Chevy Volt respectively. The production and distribution of these automobiles have brought about campaigns by these companies promising that these cars produce zero emissions and are more environmentally-friendly than their internal combustion engine (ICE) counterparts. It is important to note, however, that the production of the batteries used to power these automobiles require processing that can be energy intensive and materials that can be toxic both to humans and to the environment. A global warming potential (GWP) analysis is performed here, comparing the production of a battery pack and the electricity it uses over its life cycle with the gasoline consumed over the life cycle of an ICE automobile.
For the purposes of this analysis, a battery pack consisting of lithium-ion cells is examined. Lithium ion cells have seen wide implementation in the production of battery-powered electric vehicles due to their high energy density and high electrochemical potential. High energy density allows for a pack of lithium ion cells to hold more charge than a lead acid pack of the same mass, for example. A high electrochemical potential translates into a higher nominal voltage per cell; a typical cell voltage for a lithium ion cell is around 3.7 Volts, while a typical cell voltage for a lead acid battery is about 2.0 Volts. A higher nominal voltage allows for a cell to put out higher power at the same current, or discharge rate; A lithium ion cell with a voltage of 3.7 Volts discharged at 1 Ampere will produce 3.7 Watts, while a lead acid cell with a voltage of 2.0 Volts discharged at 1 Ampere will produce only 2.0 Watts. For implementation in an automobile, a higher energy density allows for more range on an electric vehicle while a high cell voltage can allow for better power output and performance.
In particular, the type of lithium ion chemistry that will be used for this analysis is a nickel cobalt manganese (NCM) cathode material, graphite anode, and organic electrolyte containing the lithium salt. Batteries work on the basis of charge separation and coupled reduction-oxidation (redox) reactions. During discharge, an oxidation reaction occurs at the anode, or negative electrode, pushing electrons into an external circuit powering a load, an electric motor in this case. These electrons are ultimately supplied back to the cathode, or positive electrode, where a reduction reaction occurs completing the circuit. During this process, excess negative charge is built up on the cathode and excess positive charge is built up on the anode. Thus, an electrolyte with counter ions is required to supply cations, or positively charged ions, to the cathode and anions, or negatively charged ions, to the anode. In secondary, or rechargeable, batteries, these reduction-oxidation reactions can be run in reverse to charge the battery. In lithium ion batteries, lithium ions participate in the reduction-oxidation reactions by the process of intercalation, or insertion into the materials that comprise the electrodes in the battery. [1]
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| Fig. 2: A diagram showing components in Li-ion cell manufacturing. [2] (Courtesy of the U.S. Environmental Protection Agency) |
To assemble a full cell, a few more elements are required to successfully extract charge from the electrochemical system. A porous separator is required to keep separate the two electrodes, but also allow for free movement of the the lithium ions and counter-ions to move freely between the electrodes. In the absence of a separator, the two electrodes would electrochemically short and a runaway reaction would occur. In fact, this is a common failure mechanism for cells that have undergone mechanical trauma or penetration; sufficient resistive heating from the high voltage runaway chemical reaction heats the organic electrolyte and causes combustion. Current collectors are needed to extract charge from the electrodes and act as highways for electron transport from relatively resistive composite electrode materials to the exterior terminals. Finally, housings are needed to enclose the electrochemical cell and provide neat exterior connections for their implementation in a battery pack. Two of the most common form factors used in lithium ion cells are 18650 cells, popularized by Tesla, and prismatic cells. A prismatic cell can be seen in Fig. 1. [2]
To compare a lithium ion NCM battery pack and its life cycle electricity use to an ICE vehicles life cycle gasoline use, the metric of global warming potential (GWP) will be used. Global warming potential is the contribution of the production of a material or combustion of a fuel to global warming by means of related greenhouse gas (GHG) emissions. The main contributors to GWP are carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) and are measured in terms of mass units of CO2 equivalent. One kilogram of CO2 represents one kilogram of CO2 equivalent, one kilogram of CH4 25 kilograms of CO2 equivalent, and one kilogram of N2O represents 298 kilograms of CO2 equivalent. [3]
For the purposes of this analysis, an automobile with a 60 kWh Li-NCM pack was chosen. A gas mileage for the comparison ICE automobile was chosen to be 30 miles per gallon. This analysis disregards manufacturing phase for the electric vehicle save the battery pack and the manufacturing phase for the ICE vehicle; this is known as a strictly tank-to- wheels GWP analysis. [3]
A life cycle analysis (LCA) done by the EPA calculated the GWP for the production of a Li-NCM battery pack and found the battery component phase to be 121 kg CO2 equivalent per kWh of capacity. For a 60 kWh pack, the GWP associated with producing the pack amount to 7260 kg CO2 equivalent. Also, the use phase was found to have a relationship of 120 g of CO2 equivalent per kilometer driven, or 193 g of CO2 equivalent per mile. [2] Tables detailing the battery manufacturing and use phase of Li-NCM batteries can be found in Table 1.
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| Table 1: Global warming potentials of Li-ion battery phases. [2] The units are kg of CO2 equivalent per kWh capacity. (Courtesy of the U.S. Environmental Protection Agency) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
For the ICE vehicle, weighted averages for vehicle emissions in 2005 were used to calculate the GWP per mile driven. Nitrous oxide emissions were found to be 0.0079 grams per mile, or 2.35 g of CO2 equivalent per mile, and methane emissions were found to be 0.0147 grams per mile, or 0.368 g of CO2 equivalent per mile. Carbon dioxide emissions were found to be 8.81 kg CO2 equivalent per gallon and with a fuel efficiency of 30 miles per gallon, emissions on a per mile basis were found to be 294 g of CO2 equivalent per mile driven. The total amount of GWP per mile driven for an ICE vehicle is 297 g CO2 equivalent per mile. [4]
Setting these two relationships together and finding a linear intersection, a distance of 41,017 miles is needed before a crossover is seen in the global warming potential of a battery-powered electric vehicle and an ICE vehicle. Lifetimes of battery packs have been estimated at 125,000 miles. [5] This tank-to-wheels GWP analysis shows that a crossover occurs at about one-third of the vehicle's lifetime at which point an ICE vehicle has a higher global warming potential than a battery-powered electric vehicle.
The results of this analysis are not as drastic as one would expect and certainly not as eye-opening as the companies touting the "greenness" of the cars they sell would have you believe, but battery-powered electric vehicles show promise for decreasing greenhouse gas emissions caused by the transportation sector. Transportation currently accounts for around one quarter of energy-related greenhouse gas emissions and with business-as-usual projections, meaning no change in regulation or energy mix change, energy-related greenhouse gas emissions are projected to increase by nearly 50% by 2030 and by more than 85% by 2050. [3] The use of batteries to power electric vehicles has only recently been brought to the mass market and battery engineering for this purpose has not been extensively studied or optimized; there is room for improvement in the production of battery materials for use in electric vehicles at large scale. Battery powered electric vehicles certainly have a place in the discussion as transportation for a future with a growing energy demand and a need for a reduction in greenhouse gas emissions.
© Camilo Cabrera. 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 commercial rights, are reserved to the author.
[1] R. Nie, "Electric Vehicle Battery Pollution," Physics 240, Stanford University, Fall 2010.
[2] "Application of Life-Cycle Assessment to Nanoscale Technology: Lithium-ion Batteries for Electric Vehicles," U.S. Environmental Protection Agency, EPA 744-R-12-001, April 2013.
[3] H. Ma et al., "A New Comparison Between the Life Cycle Greenhouse Gas Emissions of Battery Electric Vehicles and Internal Combustion Vehicles," Energy Policy 44, 140 (2012).
[4] "Direct Emissions from Mobile Combustion Sources," U. S. Environmental Protection Agency, EPA 430-K-08-004, May 2008.
[5] T. R. Hawkins, O. M. Gausen and A. H. Stromman, "Environmental Impacts of Hybrid and Electric Vehicles - a Review," Int. J. Life Cycle Assess. 17, 997 (2012).
