Life Cycle Environmental Impact of High-Capacity Lithium Ion

LCA is a systematic tool widely adopted for comprehensive environmental ... while other data are retrieved from GaBi 6 professional database or modele...
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Life Cycle Environmental Impact of High-Capacity Lithium Ion Battery with Silicon Nanowires Anode for Electric Vehicles Bingbing Li, Xianfeng Gao, Jianyang Li, and Chris Yuan* Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States S Supporting Information *

ABSTRACT: Although silicon nanowires (SiNW) have been widely studied as an ideal material for developing high-capacity lithium ion batteries (LIBs) for electric vehicles (EVs), little is known about the environmental impacts of such a new EV battery pack during its whole life cycle. This paper reports a life cycle assessment (LCA) of a high-capacity LIB pack using SiNW prepared via metal-assisted chemical etching as anode material. The LCA study is conducted based on the average U.S. driving and electricity supply conditions. Nanowastes and nanoparticle emissions from the SiNW synthesis are also characterized and reported. The LCA results show that over 50% of most characterized impacts are generated from the battery operations, while the battery anode with SiNW material contributes to around 15% of global warming potential and 10% of human toxicity potential. Overall the life cycle impacts of this new battery pack are moderately higher than those of conventional LIBs but could be actually comparable when considering the uncertainties and scale-up potential of the technology. These results are encouraging because they not only provide a solid base for sustainable development of next generation LIBs but also confirm that appropriate nanomanufacturing technologies could be used in sustainable product development.

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INTRODUCTION

significantly increase the battery energy density is through use of a novel anode material.4,5 When compared with graphite anode material, silicon has a high theoretical specific capacity (4200 mAh/g) and has been widely recognized as the ideal anode material for high-capacity LIBs because of its high specific capacity and abundant availability.2,6,7 Currently, silicon has limited applications in commercial LIBs because bulk silicon materials used on the battery suffer from a drastic volume change (up to 400%) during charging/discharging, which leads to cracking and even pulverization of the active materials in the battery anode and results in large irreversible capacity and severe capacity fading.6,7 Recently it has been found that reducing the silicon size below 150 nm can effectively address the volume expansion issue and obtain high-capacity LIBs.8 As a result, high-capacity LIB technologies are currently under development at lab scale with silicon nanostructured anode materials.9−11 In particular, silicon nanowires (SiNW) are widely studied as a promising anode material for high-capacity LIBs due to its low cost of fabrication and volume production potential.7,10,11 Although using silicon nanomaterials on LIB is technically beneficial, the associated environmental impacts might be of concern because

Background and Context. Battery-powered electric vehicles (EVs) are considered a future mode of ground transportation due to their environmental and technological advantages such as no tailpipe emissions in operations, reduced consumption of fossil oil, quiet and smooth operation, stronger acceleration, and requiring less maintenance.1 Large fleet deployment of EVs is expected in the following decades. The United States plans to have one million EVs on road by 2015.2 As predicted, the EV deployment in 2030 will account for 24− 46% of U.S. light-vehicle fleet and could reduce 20−69% of greenhouse gas (GHG) emissions from U.S. light-vehicles from 2005 levels.3 Although EVs have a promising future, the major challenge facing their large fleet deployment is the battery technology. Current EVs are all powered by lithium ion battery (LIB) due to its high energy density and lightweight advantages. However, current LIBs are made of graphite anode which has a relatively small specific capacity (372 mAh/g), and can only store a limited amount of energy to power the vehicle for a short driving distance. Extending the driving range of EVs has to increase the energy storage of LIBs which need to use a highcapacity anode. Currently, intensive efforts are put on development of new materials that would enable batteries to have a high energy density for the full-range EV operations.4 As the significant improvement of energy density in alternative cathode material is limited, the most promising way to © 2014 American Chemical Society

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Figure 1. System boundary of LCA for lithium ion battery.

nanoscale manufacturing is usually energy intensive,12,13 relying heavily on toxic chemicals for manufacturing operations,14,15 and generating a large proportion of wastes and emissions including nanowastes and nanoparticle emissions from the nanomanufacturing process.13,16−18 Literature Review. Review of the literature indicates SiNW for LIB anode are mainly prepared in two ways: chemical vapor deposition (CVD)7,11,19 or wet chemistry method.10,20,21 When compared, wet chemistry method is better than CVD in terms of both economic and environmental performance.22 In particular, a recently developed wet chemistry technique using metal-assisted chemical etching of silicon powder to produce nanowire structure, is easily scalable and costeffective.20 Using SiNWs prepared by this technology in LIB anode has obtained a high capacity at 2400 mAh/g.21 This technology has a promising future for large-scale manufacturing of high-capacity EV batteries in the future.20,21 Although silicon nanomaterials are considered the ideal anode material for high-capacity LIBs, until now there has been no study conducted on the potential environmental impacts of LIBs with silicon nanomaterial for future EV applications. As the application of LIBs on EVs began in 2008,23 and on massproduced EVs just began in 2010,24,25 there is much space to improve the sustainability performance of the LIB technology for future large-scale applications. In this point, a pilot study on the potential environmental impacts of a future LIB technology based on silicon nanomaterial would provide solid data and information support for sustainable development of the technology in the coming years. Currently, the environmental impact studies on LIBs for EVs are quite limited since the technology was adopted only a few years ago. We did a rigorous literature review and found only several studies on the environmental impacts of conventional LIBs using graphite anode. For example, Zackrisson et al. performed a LCA on conventional LIBs and found that nearly 70% of global warming, 50% of photochemical smog, and 75% of acidification are generated during the usage phase of the EV if used in China.26 Notter et al. studied the potential environmental impacts of conventional LIBs and found a conventional LIB pack contributes to about 15% of the total environmental impact of a VW Golf-sized EV.27 Majeau-Bettez et al. conducted a LCA on nickel metal hydride battery and two types of conventional LIBs and found that the environmental impacts of nickel metal hydride battery are higher than those of lithium ion batteries.28 Samaras and Meisterling investigated life cycle GHG emissions from plug-in EV, and found an EV could reduce 32% of GHG emissions from conventional vehicles.29 Gaines et al. presented a LCA of conventional LIB recycling and potential impacts on production life cycle burdens for plug-

in Hybrid EV, and found the recycling of LIB materials potentially reduced the material production energy by as much as 50%.30 Wender applied LCA methods to quantify the energy trade-offs associated with single-wall carbon nanotube (SWCNT) based LIBs, and revealed both challenges and values of the batteries from large energy demands of nanomanufacturing processes.31 Yu conducted environmental impact assessment through a combination of the LCA, Ecoindicator 99 system, and Monte Carlo simulation (MCS) to compare LIBs and Ni-MH batteries under the uncertainty of cycle performance, and found that the environmental impact of Li-ion battery is lower than that of the Ni-MH battery.32 Recently, U.S. EPA published an LCA study of LIBs for EVs, with a focus on conventional LIB using carbon graphite anode, but incorporating a SWCNT anode.33 Using data from both lab-scale experiment and database, the EPA study reported that the cradle-to-gate environmental impacts of SWCNT anodes are thousands of times higher than conventional graphite anodes but may be significantly reduced in future large-scale productions.33 In this study, the incorporation of SWCNT anode with a capacity of 1100 mAh/g34 is mainly for improving the energy density and associated impacts in manufacturing and supply chains over conventional LIBs. Significance and Purpose. This paper reports a life cycle environmental impact study on a high-capacity LIB pack for EV, using SiNWs anode material produced from metal-assisted chemical etching method. SiNW is selected for the pilot study due to its promising application potential on future large fleet EVs. The metal-assisted chemical etching method is used for the SiNW synthesis due to its simplicity, low cost, and scalable advantages. This study, as a pilot investigation on a specific SiNW production route for next-generation LIB technology development, could provide solid data and information support on the selection and development of the SiNW and similar anode materials for high-capacity LIBs. Although future industrial-scale productions will be different from current labscales, the fundamental mechanism and processes should be similar. Such first-hand information and data from this research can provide valuable feedback for sustainable development of next-generation lithium ion battery technologies.

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METHODS AND DEFINITIONS Life Cycle Assessment (LCA). LCA is a systematic tool widely adopted for comprehensive environmental impact assessment of a product from cradle to grave.35 LCA considers trade-offs in materials selection and performance and between different categories of environmental impacts, and identifies areas of concerns for environmental improvement. In this study, LCA is applied on a LIB pack using SiNW anode for 3048

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Figure 2. Prepared and characterized silicon nanowires.

thermodynamically efficient.20 In our experiments, the SiNW structures are successfully developed on the surface of the microscale silicon matrix, as shown in Figure 2. The SiNW structures are characterized and confirmed with X-ray diffraction (XRD). Battery System. The prepared SiNWs are then mixed with carbon black and CMC binder (8:1:1, weight ratio) and fabricated into LIB anode. The cathode is the lithium−nickel− manganese−cobalt-oxide (LiNi1/3Mn1/3Co1/3O2, NMC) because of its high specific energy in good match with silicon anode. The assumed capacity of the SiNW anode is 2400 mAh/ g. The separator is commercial monolayer polyethylene (PE) with 40% porosity. The electrolyte is the LiPF6. The cell casing is made of aluminum foil and polypropylene resin. Considering the high capacity of each LIB cell with SiNW anode, the battery pack configured is a 43.2 KWh battery system composed of 12 modules with each module containing 12 LIB prismatic cells. The battery pack has a total weight of 120 kg. Each LIB cell has a 3.65-V voltage and 27-Ah capacity. The battery management system (BMS) installed in the battery pack is modeled on current commercial BMS systems which are made of copper foil, stainless steel, and printed circuit boards.33 The mass of the BMS is 2% of the total battery weight in which copper wires share 50%, stainless steel shares 40%, and printed circuit board shares 10%.33 The battery pack housing is made of polyethylene terephthalate, which takes a share of 17% of the battery mass.36 The passive cooling system is made of stainless steel and aluminum, with a share of 0.5% and 16.2% of the battery total mass, respectively.37 Battery Use and EV Operation. The use of the 43.2 kwh battery pack is supposed on board an average midsized EV with a weight of 4270 lb (1936.8 kg),38 and an average driving distance of 200 000 km during a 10-year service life.39 A single battery pack is assumed to power the vehicle during its whole

powering EVs. The goal is to understand the potential environmental impacts of such a high-capacity LIB pack for future EV application, so as to provide quantitative data and information to support sustainable development of nextgeneration LIB technology for EVs. The life cycle of the battery pack is divided into six stages, including material extraction, material processing, component manufacturing, battery manufacturing, battery use, and end-of-life. As the LIB is to provide power for EVs, the functional unit is selected as one average kilometer driven by an EV powered by the LIB pack under average U.S. operating conditions. The boundary is defined as such to include those materials and energies as well as wastes and emissions directly associated with the battery pack’s life cycle activities, as illustrated in Figure 1. A comprehensive life cycle inventory is compiled, with the manufacturing data mainly collected from our lab experimentation, while other data are retrieved from GaBi 6 professional database or modeled from actual industrial operating conditions. Life cycle impact assessment is conducted with GaBi 6 using its standard impact categories and characterization factors. Silicon Nanowire Preparation. Commercial silicon powders in 44-μm diameter (325 mesh, 99.99%, Sigma Aldrich) are used in preparing multidimensional SiNW structure via metal-assisted chemical etching. The process works in such a way that the silicon surface is oxidized in H2O2 with a metal such as Ag, Au, Pt, Pd, and Cu, used as catalyst, then the silicon oxide is dissolved in an HF solution to form nanowire structures.20 In our experiment, AgNO3 is used as the Ag source of catalyst and HF is used as the etching agent. Further detailed experimental process is provided in the Supporting Information (SI). The determined standard reduction potential for this etching process is only 2.69 V which means this etching process is 3049

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kg. Assembly of a single punch cell with above-prepared components requires 0.299 MJ/kg only, and battery pack production including stacking LIB cells, winding control wires, and assembling the BMS, cooling system, and battery pack housing consumes a total of 0.412 MJ/kg. These energy data are consistent with the results published in the literature.27 The energy consumption of the battery pack during its whole life cycle are mainly in the categories of grid electricity supply (25373.5 MJ) for production and battery operations, diesel combustion (107.6 MJ) for transportation and some onsite activities, and thermal energy generated from natural gas combustion (2519.7 MJ) for process heating and treatment. The grid electricity used in this analysis is average U.S. electricity mix with 89.56% of nonrenewable energies.41 For U.S. grid electricity supply, the average transformation and distribution efficiency is 91.4%.48 For battery energy analysis in the use phase, 90% of charge/discharge efficiency is used for battery with SiNW anode (a little bit lower than that of conventional battery with graphite anode within 96−100%49),50 and the average electricity required for EV operation is 164.8 Wh/km.33 The energy consumptions for material recovery at end-of-life stage of the LIB pack are modeled based on real industrial LIB recycling processes by Toxco and Umicore.51,52 The consumed energy in the life cycle of the battery pack, depending on the fuel types, contributes to emissions such as CO2, CH4, SO2, and NO2. In this analysis, eGrid model is used to trace the energy consumed during the whole life cycle of the battery pack to the original fuel types and quantities under U.S. average electricity mix condition,41 and the retrieved fuel data are then used in the GaBi 6 software for environmental impact assessment. Besides energy consumption, the other major part of LCI data is the inputs and outputs of various types of materials and emissions during the whole life cycle of the LIB pack. The material compositions of the battery pack using SiNW and carbon black anodes are shown in Table S1 and S2, respectively, in the SI. A detailed material flow structure for the LIB pack production is illustrated in SI Figure S1. Detailed material inputs and outputs related to the production of each battery component and major intermediate materials used in the battery production are shown from SI Tables S4−S22. The industrial production of the SiNW is modeled based on a scaleup analysis of our lab experimentation in which 80 mg of SiNWs is obtained from metal-assisted chemical etching of 400 mg of silicon powder. For a conservative analysis, the lab-scale data on SiNW synthesis are linearly scaled up to the scale of producing 1 kg of SiNWs, as shown in SI Table S10. Another major source of the environmental impacts from the LIB pack using SiNW anode is the nanowastes and nanoparticle emissions generated from the SiNW synthesis process. Although current environmental impact assessment methods such as GaBi do not include such nanowastes and emissions and accordingly are not able to assess their potential impacts, inclusion of the nanowastes data and information in the life cycle inventory could support future research on development of environmental impact assessment methods for nanowastes, or provide decision support for reducing the quantity of nanowastes during the scale-up of the technology. Nanowastes are different from their bulk materials since the toxicity of nanomaterials is linked to their multivariant attributes such as mass, shape, particle size, particle number, and surface charge, etc.16,53 In this study, the nanowastes from the SiNW synthesis process have no surface charge and hence are characterized on

life. The driving mix of the EV is 55% urban and 45% highway.40 The average U.S. electricity mix is supposed for the battery charging and operations.41 Reference Battery System. A conventional battery pack using graphite anode with the same amount of power output (43.2 KWh) is chosen as a basis for comparison of the life cycle impact results. The conventional battery pack has 36 modules and each module has 12 cells. The conventional battery pack has a total weight of 360 kg. Other battery components and the battery use conditions are the same as those listed above for the battery with SiNW anode. End-of-Life of Battery Pack. The battery pack is expected to be recycled at the end-of-life stage. The recycling is mainly for material recovery, and is assumed using current recycling technologies including direct physical recycling,42 and hydrometallurgical43 and pyrometallurgical recovery.44 In this analysis, the recovered materials are not treated as credits in the raw material extraction and processing phase because of the high quality and purity requirements of battery materials. Only the energy consumption during recycling process of material was considered in this manuscript. SiNW materials are not targeted for recovery after 10 years of charging/discharging.45

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RESULTS AND DISCUSSION Life Cycle Inventory Analysis. LCI analysis of the battery pack is conducted based on the goal and scope defined above. The LCI analysis of the anode and cell manufacturing are conducted using process-based LCA method, with the SiNW synthesis data collected from our lab experimentation and the LIB cell manufacturing data collected from the industrial LIB cell prototyping facilities (Johnson Controls’ advanced battery lab). The LCI data for other stages are retrieved from GaBi 6 professional database. The energy consumed throughout the life cycle of the battery pack is a major part of the LCI data. Energy is required by all the LCA activities ranging from material extraction to end-oflife (Figure S2 in the SI). As calculated, the embedded total energy in the 43.2 KWh battery pack is 10.94 GJ. Following the battery pack manufacturing processes, a unit energy flow of the LIB pack is shown in SI Figure S2, in which the energy use of each process is normalized by the total weight of the battery pack (120 kg). The energy consumption from battery pack quality testing and validation (0.023 MJ/kg) is also included in the analysis. Major energy intensive processes identified in the production of the LIB pack include silicon powder production (74.63 MJ/kg), SiNW preparation (4.44 MJ/kg), aluminum foil production (3.273 MJ/kg), NMC production (2.525 MJ/kg), and copper foil production (0.77 MJ/kg). As calculated, the anode manufacturing processes including mixing, coating, drying, calendaring, and slitting using the prepared SiNWs, carbon black, CMC binder, and copper foil consume 0.394 MJ per kg of battery produced, which is in good agreement with the literature result reported in ref 27. Among the unit processes, the SiNW fabrication from silica sand extraction, carbothermic reduction, chemical purification, chemical deposition, and ball-milling, consume an amount of energy as high as 79.07 MJ/kg. This is the largest amount of energy flow path in this LIB battery pack manufacturing system, mainly due to the use of high-temperature processes such as the carbothermic reduction (∼2000 °C), the fluid bed reaction (∼760 °C), gasification (∼1100 °C), and deposition (∼1100 °C).46,47 When compared, other LIB components including cathode, separator, electrolyte, and casing together consume 11.367 MJ/ 3050

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Figure 3. Nanowastes from SiNW synthesis. (a) Silicon wastes with silver on the surface. (b) Silver nanowastes. (c) EDS spectrum of Si and Ag wastes. (d) Nanoparticle size distributions in the etching solution.

Figure 4. Life cycle environmental impacts of the NMC-SiNW lithium ion battery pack: abiotic depletion potential (ADP), global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), ozone depletion potential (ODP), photochemical oxidation potential (POP), ecological toxicity potential (ETP), and human toxicity potential (HTP). The absolute values of the impacts are provided in Tables S24 and S25 in the SI.

the mass, shape, particle size, and particle number (mass information shown in SI Table S10; shape, material compositions, and size distribution shown in Figure 3; particle size data shown in SI Table S3). The nanowastes are analyzed for both the solid nanowastes and the nanoparticles in the etching solution. Figure 3a is the silicon nanowastes with the reduced Ag catalyst deposited on the silicon surface, after the centrifuging separation from the SiNW product. Figure 3b is the Ag dendrite nanowastes on the

surface of the silicon matrix. Figure 3c is the energy dispersive X-ray spectroscopy (EDS) characterization of the solid nanowaste compositions which confirms the wastes are composed of silicon and silver only. Figure 3d is the Zetapotential analysis of the nanoparticle size and distribution within the etching solution, with average diameter obtained at 705.8 nm and geometric standard deviation at 1.703. Additional nanoparticle emission data from the SiNW synthesis process are listed in SI Table S3. 3051

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Figure 5. Life cycle impact benchmarking between LIB packs with SiNW and graphite anode. Units of the X-axis values are different and shown under each impact category name on Y-axis.

Life Cycle Impact Assessment. Figure 4 shows the generations of conventional environmental impacts from various life cycle stages of the LIB pack using SiNW anode, as characterized by GaBi impact assessment method. The environmental impacts of SiNW nanowastes and emissions are not included in the impacts as no characterization factors and metrics are available in the conventional impact assessment methods. In current LCA studies of nanoproducts such as nanosilver T-shirt reported by Walser et al.,53 the impacts of nanospecific effects are not included. However, research on incorporating the nanomaterials’ impacts into LCA is ongoing and may be accomplished in the near future. For example, Eckelman et al. has already developed characterization factors for evaluating the aquatic ecotoxicity of carbon nanotubes after their releases into freshwater.54 Using the GaBi impact methods, the life cycle environmental impacts of the LIB pack are illustrated in terms of the defined functional unit (i.e., one kilometer of EV driving) in Figure 4. The life cycle impact distributions among the six life cycle phases are expressed by the patterned bars, while the life cycle impact contributions of LIB components are shown by the colored bars which are the total impacts from such upstream stages including material extraction, material processing, component manufacture, and battery assembly. The results show that most of the life cycle impacts are generated in the battery use and material production stages. Battery use stage alone contributes to more than half of the life cycle impacts in categories such as ADP (51%), GWP (56%), AP (52%), EP (51%), POP (54%), and HTP (51%), while most impacts in ODP and EDP categories are generated from material extraction stage (58% and 85%, respectively). These results are in good agreement with the results published in refs 27, 29, and 55. The largest impact from the battery use stage is mainly from the primary energy consumption (2.94 × 105 MJ) during the 10-year service life of the EV. The total primary energy consumption in the use stage is about 18 times that of the embedded energy in the battery pack. The results demonstrate that the major opportunity for reducing the life cycle impacts of the battery pack is to use clean energy supply for battery operation, such as solar and wind electricity, which could reduce these environmental impacts significantly.56 The SiNW anode as produced with large amount of embedded

energy and toxic chemicals, contributes to 15% of GWP, 18% of ADP, 17% of POP, and 10% of HTP, respectively, to each corresponding life cycle impact category of the battery pack. In summary, the battery components (including anode, cathode, electrolyte, separator, cell casing, BMS, cooling system, and pack housing) together take a share of each corresponding impact ranging between 21% (HTP) and 77% (ETP). Absolute values of the impacts from each battery component and individual life cycle stages are presented in SI Tables S24 and S25. Comparison of the SiNW and Conventional LIB Packs. As the SiNW based LIBs are supposed to replace conventional graphite based LIBs to extend the driving range of the EVs, here a comparison is conducted between the life cycle impacts of such two battery packs, with the same power output (43.2 KWH). To validate the LCA model we developed for this study, we have benchmarked our LCA results of the conventional battery pack with the published results in literature.27 As shown in SI Table S26, our LCA results on conventional LIBs are in good agreement with the LCA data in published literature. For instance, our study obtained 2.2 MJ/ km of life cycle energy and 0.155 kg CO2‑eq/km of GWP, comparable to 1.97 MJ/km of energy and 0.181 kg CO2‑eq/km of GWP in Notter’s work,27 and 0.93 MJ/km of energy and 0.935 kg CO2‑eq/km of GWP in Zackrisson’s work.26 The minor differences might be from the data sources in that we used lab-scale manufacturing data and GaBi 6 professional database, while Notter and Zackrisson used Ecoinvent database. Also, the battery cathode material in our study is NMC, but LiMn2O4 is used in Notter’s analysis and LiFePO4 is used in Zackrisson’s study. With the validated LCA model, the life cycle impacts of the two battery packs are characterized using GaBi methods and benchmarked among each impact category in Figure 5. The results demonstrate that the compositions of the life cycle impacts from individual life cycle stages of the two battery packs are quite different. For example, the life cycle impacts of the conventional battery pack using carbon graphite are dominated by the battery use phase which contributes to 78% of ADP, 78% of GWP, 64% of AP, 75% of EP, 80% of POP, and 83% of HTP. Whereas for the battery pack using SiNW anode, the contributions of battery use stage are much 3052

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The LCA results may also be influenced by several factors in the analysis. In this study, we performed a sensitivity analysis on the effects of multiple factors including the cathode material, the service life of battery pack, the electricity mix for battery charging, and the operating geographic region of the EVs. The cathode materials, besides NMC, may also use LiMn2O4 and LiFePO4 in the battery pack. Here we compared the life cycle impacts of these three cathode materials used with SiNW anode in the battery pack. The results demonstrate that most impact categories are varied by less than 10%, as shown in SI Figure S6 and Table S28. Currently the LIB pack is supposed to be used for 10 years, but its service time may vary in real applications. We evaluated the sensitivity of the life cycle impact results on the battery service life varying at 5, 10, and 15 years. The results showed a slight increasing of total impacts in the range of 0.45−39% when the service life is extended by every 5 years. The results are shown in SI Figure S7. In addition, the geographic differences of the EVs in different U.S. regions may also affect the LCA results. Here we investigated the sensitivity of the LCA results in four selected different regions including Washington, DC, Alaska, Maine, and Washington. Using eGrid data, the characterized GWP and AP will increase by 104% and 462%, respectively, in Washington, DC, and will decrease by 76% and 93%, respectively, in the state of Washington. The results are shown in SI Figure S8. All the above analyses including the life cycle inventory analysis, impact analysis, uncertainty, and sensitivity analysis together confirm that the LIB pack using SiNW anode from metal-assisted chemical etching could have environmental impacts comparable with those of conventional battery pack, while significantly increasing the battery energy storage and extending the driving range of EVs in the future.

lower because of the increased impacts from the battery production. This results from the increased energy storage capacity and the extended driving range of EVs, as well as the reduced mass of the battery pack. As calculated, the mass reduction from 360-kg conventional LIB pack to 120-kg SiNW battery pack could result in a 12.5% of primary energy saving, and 12.3−13.5% of impact savings on various impact categories during the usage of the battery packs (SI Table S27). During the SiNW battery production, a total of 0.077 kg CO2‑eq is generated, in comparison with 0.029 kg CO2‑eq generated from conventional battery production, per km of EV driving. As shown in Figure 5, the life cycle impacts of the battery pack with SiNW anode are roughly 6−43% higher than the corresponding impacts of the conventional battery pack. The largest difference (43%) is in HTP category which is attributed to the use of toxic chemicals (HF and HNO3) in synthesis of the SiNW materials. Overall the differences of the impacts between the two battery packs, in average, are moderate. Considering the uncertainties in the lab-scale inventory data and the potential of impact reduction in future industrial scale productions, we can say the life cycle environmental impacts of the two battery packs are comparable with each other.57 This is encouraging since some nanoscale manufacturing technologies could generate impacts several orders of magnitude higher than those of conventional manufacturing technologies.12,33 This study reveals that appropriate nanomanufacturing technologies could be employed to enhance the technical performance of manufactured products while maintaining the same level of environmental impacts. Uncertainty and Sensitivity Analysis. Because this metal-assisted chemical etching has not yet applied in industrial scale, and the manufacturing process parameters are not yet set, the LCA manufacturing data and analysis based on the lab experimentation will induce some uncertainties in the LCA results. In the analysis, this work is for pilot-scale laboratory designs but scaling up to commercial production will likely lead to change of efficiencies in material and energy use and chemical recycling. This may shift the overall results by orders of magnitude. For example, typical industrial scale production has an energy consumption several orders of magnitude lower than lab-scale experiments.12 Besides, the SiNW are synthesized with 80% of the silicon powders ended as wastes in the experiments. The use of toxic chemicals, including HF and HNO3, may also be reduced per unit mass of the SiNW synthesized. Also, the technology selected in this study is metalassisted chemical etching, while some other techniques such as CVD or laser-assisted processes may also be used in preparing SiNW materials, but these technologies may largely increase the energy consumption of SiNW synthesis by more than 2 orders of magnitude as estimated from data published in the literature.58 Another major source of uncertainty comes from the battery capacity and cycling performance of the battery pack using SiNW anode material. Theoretically, the SiNW can produce more than 11 times of specific capacity (∼4200 mAh/g) than conventional carbon graphite, while current experiments can only obtain ∼2400 mAh/g of stable capacity from this technique.21 As the battery capacity dictates the amount of energy storage in a single charge and determines the amounts of electricity consumption during the use phase of the battery pack, the LCA results could be affected with some uncertainties.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed LCA information and inventory data. This information is available free of charge via the Internet at http://pubs.acs. org/

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: 414-229-5639; fax: 414-2296958. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from National Science Foundation (CBET1351602), the Research Growth Initiative (RGI) of University of Wisconsin-Milwaukee, and Johnson Controls Inc. are gratefully acknowledged.

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REFERENCES

(1) U.S. EPA and U.S. DOE. Electric Vehicles. http://www. fueleconomy.gov/feg/evtech.shtml (August 23, 2013). (2) U.S. DOE. One Million Electric Vehicles by 2015; U.S. Department of Energy: Washington, DC, February 8, 2011; p 7. (3) Becker, T. A.; Sidhu, I.; Tenderich, B. Electric Vehicles in the United States: A New Model with Forecasts to 2030; 2009.1.v.2.0; Center for Entrepreneurship and Technology, University of California, Berkeley: Berkeley, CA, August 24, 2009; p 36.

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