Life cycle analysis of three battery chemistries for PHEVs and BEVs; environmental impacts higher than expected
|Flow diagram of the battery system, defined by the functional unit of 50 MJ stored and delivered to the powertrain. Credit: ACS, Majeau-Bettez et al. Click to enlarge.|
Researchers from the Norwegian University of Science and Technology (NTNU) have performed life cycle assessments (LCA) of three batteries for plug-in hybrid (PHEV) and full performance battery electric (BEV) vehicles. They compiled a transparent life cycle inventory (LCI) in a component-wise manner for nickel metal hydride (NiMH), nickel cobalt manganese lithium-ion (NCM), and iron phosphate lithium-ion (LFP) batteries.
The battery systems were investigated with a functional unit based on energy storage, and environmental impacts were analyzed using midpoint indicators. On a per-storage basis, the team found that the NiMH technology was found to have the highest environmental impact, followed by NCM and then LFP, for all categories considered except ozone depletion potential. They also found higher life cycle global warming emissions than have been previously reported.
A paper on the study appears in the ACS journal Environmental Science & Technology.
Except for ozone depletion potential, the NiMH battery performs significantly worse than the two Li-ion batteries for all impact categories. This difference may be rationalized by the greater use phase efficiency of Li-ion relative to NiMH, and the fact that each kilogram of Li-ion battery is expected to store between 2 to 3 times more energy in the course of its lifetime. Moreover, the NCM and LFP batteries contain at least an order of magnitude less nickel and virtually no rare earth metals.
Among Li-ion batteries, our analysis points to overall environmental benefits of LFP relative to NCM, which can be explained by a greater lifetime expectancy and the use of less environmentally intensive materials. As an approximate indicator, if we assume a vehicle powertrain efficiency of 0.5 MJ·km-1, our results indicate an overall global warming impact of 35 gCO2-eq·km-1 for NiMH, 19 gCO2-eq·km-1 for NCM, and 14 gCO2-eq·km-1 for LFP.—Majeau-Bettez et al.
The team expressed its results for a given amount of energy (50 MJ) accumulated by the battery and then delivered to the powertrain. This approach is intuitive, representative of the purpose of the device, free of any assumption concerning the powertrain, and inclusive of the majority of battery characteristics, such as specific energy capacity, depth-of- discharge, cycle-life expectancy, and charge-discharge energy efficiency, they reasoned.
They did not define the functional unit in terms of driving distance or driving range, as such a functional unit would have been dependent on powertrain and driving cycle assumptions. As a result, they deemed any electricity consumed by the vehicle powertrain, including the energy requirements induced on the powertrain to transport the mass of the battery, beyond the system definition. This approach preserves the generality of the inventory, they argued, allowing it to be adapted to other specific systems of interest.
Among the findings from the study were:
For all three batteries, the manufacture energy requirements are a major cause of GWP. The production of polytetrafluoroethylene as dispersant/binder in the electrode paste is responsible for more than 97% of the ozone depletion potential of all three batteries, along with 14.15% of the GWP of the two Li-ion batteries, mostly due to the halogenated methane emissions of this value chain. The final shipping and the productions of the cell containers, module packaging, separator material, and electrolyte contribute relatively little to the environmental damage, with collectively less than 10% of any impact category.
While production of NiMH causes the least GWP impact per kilogram, its lower energy density makes it score worst both relative to its nominal energy capacity and our storage-based functional unit. Similarly, the GWP impacts of LFP and NCM production are roughly equal for a given mass or nominal energy capacity, but the greater life expectancy of LFP confers a net environmental advantage to this chemistry for a per-energy-delivered functional unit.
Differences in battery designs can lead to important variations in environmental impacts. For example, alternative materials have been used by the battery industry in lieu of polytetrafluoroethylene as binder and nickel foam as NiMH current collectors, both of which are identified in the study as especially environmentally consequential.
When altering the ratio of the components to optimize for either more energy or power density, a change of 25% in energy density changed the GWP impact of the life cycle of the battery by 3–10%.
The efficiency of the battery proves to be a crucial parameter, with an alteration of 5 percentage points (80 ± 5% for NiMH and 90 ± 5% for Li-ion) leading to 8.23% changes life cycle GWP due to changes in use phase electricity waste. A reduction of lifetime estimations by one-third increases all categories of impacts by 30–45%.
If average Chinese electricity mix is used for all inventoried production processes instead of average European electricity mix, the life cycle impacts of the batteries increase by 10.16% for GWP and by 10.29% for particulate matter and photochemical oxidant formation.
More than 70% of GWP emissions occurred in processes more than 6 tiers upstream of the use phase in the value chain.
Though associated with important uncertainties, our results point to a higher than expected level of environmental impacts for the production and use of traction batteries. This inventory and LCA provide a basis for further benchmarking and focused development policies for the industry.—Majeau-Bettez et al.
Guillaume Majeau-Bettez, Troy R. Hawkins, Anders Hammer Strømman (2011) Life Cycle Environmental Assessment of Lithium-Ion and Nickel Metal Hydride Batteries for Plug-In Hybrid and Battery Electric Vehicles. Environmental Science & Technology Article ASAP doi: 10.1021/es103607c