Life Cycle Assessment of Silicon Nanotube Based Lithium Ion Battery

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Life Cycle Assessment of Silicon Nanotube Based Lithium Ion Battery for Electric Vehicles Yelin Deng, Lulu Ma, Tonghui Li, Jianyang Li, and Chris Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04136 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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ACS Sustainable Chemistry & Engineering

Life Cycle Assessment of Silicon Nanotube Based Lithium Ion Battery for Electric Vehicles Yelin Deng,[a][b] Lulu Ma,[a] Tonghui Li, [a] Jianyang Li,[a] Chris Yuan*[a][c] a

Department of Mechanical Engineering, 3200 North Cramer Street, University of Wisconsin-

Milwaukee, 53211, WI, the United States; b Department

of Civil and Environmental Engineering, No.8 Jixue Road, Soochow University, 215131,

Suzhou, China; c Department

of Mechanical and Aerospace Engineering, 10900 Euclid Avenue, Case Western Reserve

University, Cleveland, OH, the United States *corresponding author: Tel: 216-368-5191; Fax: 216-368-6445; email: [email protected]

Abstract: The study presents a life cycle assessment (LCA) of a next generation lithium ion battery pack using silicon nanotube anode (SiNT), Nickel-Cobalt-Manganese oxide cathode, and lithium hexafluorophosphate electrolyte. The battery pack is characterized with 63 kWh capacity to power a midsized electric vehicle (EV) for a 320 km range. A novel LCA model is developed through the inventory analyses of the SiNT anode manufacturing conducted based on our lab-scale experimentation, and the inventory of the NMC-SiNT battery manufacturing is constructed from our industrial partners’ pilot-scale battery production facilities. The upstream and downstream inventory analyses are performed through professional LCA databases and public literature. The obtained impact results of the NMC-SiNT battery are benchmarked with those of a conventional NMC-Graphite battery pack under the same driving distance per charge baseline. The results show that the NMC-SiNT battery has comparable environmental impacts with the conventional NMC-Graphite battery, with 10%-17% higher impacts in global warming potential and fossil depletion potential while 39%-56% lower impacts in human toxicity, freshwater ecotoxicity, and marine toxicity. In this study, a sensitivity analysis is also performed to investigate the robustness and reliability of the LCA results. Finally, the paper conducted scenario analysis to identify potential ways to improve the environmental performance of the NMC-SiNT battery for future sustainable development in EVs’ application. Keywords: life cycle assessment, silicon nanotube anode, lithium ion battery, electric vehicle.

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Introduction Ground transportation generates 28% of total greenhouse gas emissions in the U.S 1. Electric vehicles (EVs) powered by a low carbon electricity source are considered as clean alternatives to conventional vehicles for reducing greenhouse gas emissions from the ground transportation sector 2. However, lithium ion batteries (LIBs) used in current EVs have limited driving range per charge and significant environmental impacts during their life cycles 3. For example, Chevrolet Volt and Nissan Leaf only have 53 and 151 miles driving range per charge

4, 5.

Past studies have demonstrated that the limited driving

range is a major barrier for large scale deployment of EVs in the commercial market 6. Furthermore, based on a recent review on environmental impacts of LIBs per Wh storage capacity incurs a cumulative energy demand of 138~550 (328 average) Wh, and causes greenhouse gas (GHG) emissions of 50~250 (110 average) g CO2eq indicating an energy-intensive process of battery production.7 In the past decades, next generation battery technologies with high energy density are under rapid research and development to extend the driving range of electric vehicles. Among various new battery technologies, silicon (Si) is recognized as the most promising material because Si has a high theoretical capacity of 4200 mAh g-1 8, which is almost 11 times of that of graphite as used in conventional LIBs 8. Moreover, Si has a 0.2-V discharging potential with respect to Li/Li+, which is lower than most other metal oxide alloy anodes 8. Silicon is an abundant material in nature as well. All these features make Si a promising anode material for lithium ion battery applications 8-14. However, commercial application of the silicon based batteries faces several technical challenges including over 400% volume expansion of Si during the lithiation processes, low conductivity of Si material, and slow lithium diffusion rate, etc.8. After intensive research in recent years, significant breakthroughs have been achieved in adopting Si in lithium ion batteries. The large volume expansion issue can be alleviated through using various nanostructured Si, e.g. porous Si 10, 15, Si nanopowder16, 17, Si nanosheet 18, Si nanotube (SiNT)8, 19, Si nanowires (SiNW) 20, 21, etc. Among these nano-structured Si, the silicon nanotube is considered a highly promising material for battery anode 8, 22-25, with the following reasons: First, the inner void structure of SiNTs provides extra space to accommodate the silicon volume expansion. Second, the nanoscale structure significantly increases specific surface area, and thus promotes diffusion rate of the lithium ions during battery cycling operations. In particular, the cycling performance of SiNT on lithium ion battery can be largely improved by coating a layer of carbon

26, 27,

sandwich structure with coatings

Ge 28, SiOx29, or TiO230 onto the surface of SiNT walls or creating a 29

on both the inner and outer layers, to further limits the volume

expansion. For example, the SiNT anode with both walls coated with SiOx when used as lithium ion battery anode presents 85% superior capacity retention after 6000 cycles with a reversible specific 2 ACS Paragon Plus Environment

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capacity of 1566 mAh g-1 and 99.9% coulombic efficiency 29. In addition, the fabrication techniques for SiNTs have been expanded from chemical vapor deposition (CVD) 23, 26, 31 to a solution based synthesis approach 24, 32, 33. The solution based synthesis first produces silica nanotubes through a template assisted method, and then converts the silica nanotubes to SiNTs through a chemical reduction process. This method has a high potential to produce SiNTs on a large scale33. The low conductivity problem of the silicon material can be resolved by adding such conductive additives as carbon black in the active material prior to electrode manufacturing. With such advancement, the silicon based batteries are entering commercial application for next generation EV applications 10, 13, 34. For example, the Tesla’s Model 3 claims using 10% weight fraction silicon as part of the anode in its battery pack 35. However, the life cycle environmental impacts of the SiNT based batteries can be significant but have never been studied before. The solution-based chemical synthesis of SiNT involves large inputs of toxic chemicals and generates significant amount of waste flows throughout the cradle-to-gate life cycle of the lithium ion battery, which may cause significant burdens on the environment and public health. For example, hydrazine, one of the raw materials used for the solution based SiNT synthesis, is a highly toxic chemical. Also, the conversion of silica nanotube to SiNT requires substantial energy input during the 660oC high temperature reduction process. Along the technological development, the environmental impacts of the SiNT battery should also be assessed and understood to ensure that the battery development follows a sustainable route. In this regard, life cycle assessment (LCA) is a powerful analysis tool for comprehensive environmental impact assessments. In literature, some LCA studies have been applied on conventional lithium ion batteries including NMC-graphite battery36, lithium manganese oxide (LMO)-graphite battery37, lithium iron phosphate (LFP) -graphite battery, etc.38. Past LCA studies are focusing on the impacts of greenhouse gases (GHG) emissions and primary energy demand 7. In recent years, some LCA studies have been conducted on next generation lithium ion batteries for electric vehicles, including lithium sulfur battery39, molybdenum disulfide battery40, silicon nanowire battery3, lithium air battery41, Sodium ion battery42. SiNT based lithium ion battery has been developed as a promising next generation battery technology, but no LCA study has been conducted to understand its environmental performance. In this paper, we report a LCA model developed through an attributional hybrid approach for comprehensive environmental impact assessment of the SiNT based LIB pack for EV applications, using SiNT anode coupled with traditional NMC (Nickel-Manganese-Cobalt) oxide cathode. In the analysis, the SiNT anode fabrication and the battery cell manufacturing are retrieved from our lab-scale data, and our industrial partner’s lithium ion battery production facilities, respectively. After, the ReCiPe method (version 1.08) is employed for the life cycle impact assessment 43. The environmental impacts of the NMC-SiNT battery 3 ACS Paragon Plus Environment


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are benchmarked with those of commercial NMC-graphite lithium ion battery for its environmental performance evaluation. Method Goal and scope definition This LCA aims at analyzing and understanding the environmental impacts of NMC-SiNT battery technology for EV applications. The LCA study will firstly explore the potential environmental profile of the NMC-SiNT battery at the pilot scale based on material synthesis, SiNT in particular, from our laboratory and battery cell assembly from a pilot scale LIB production plant. The environmental impacts of the pilot scale production are expected to reflect the situation for the NMC-SiNT in the early stage of the commercial use.

SiNT

Theoretical modeling Laboratory data

Template synthesis Silica deposition Template removal

Commercial database

Magnesium Reduction NCM LiPF6 ... Raw materials extraction and prepration

Component production

Battery pack manufacturing

Battery use

Battery disposal

Figure 1 Scope and boundary of the LCA for the NMC-SiNT battery. The LCA study will also examine several scenarios to estimate how the environmental profile of the NMC-SiNT battery evolves if the massive installation of the silicon battery is realized as well as the production of NMC-SiNT battery technology is optimized. An overview of the system boundary of this LCA is provided in Figure 1 below. The life cycle of the battery pack is divided into five stages: raw materials extraction, material processing, battery manufacturing, battery use, and battery disposal. In particular, the energy consumption of the EV during the use phase is analyzed to account for the mass reduction benefits of fuel economy by using the NMC-SiNT battery with a higher energy density. The hydrometallurgical process is modelled as the EoL scenario for the NMC-SiNT disposal. The credits of the recycled materials are not considered in the life cycle of the NMC-SiNT battery pack due to the fact that batteries are currently mostly not recycled, and thus, no qualified data are available. On the other 4 ACS Paragon Plus Environment

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hand, the potential benefits of the NMC-SiNT battery recycling are evaluated in the scenario analysis as a future improvement potential. The functional unit of the LCA is per kilometer driving for a mid-sized EV powered by the NMCSiNT battery pack under average U.S. operating conditions. The overall driving distance of the EV is set as 200,000 km, representing approximately 10-years of service life

44.

Meanwhile, the per km

environmental impacts of traditional LIB battery, i.e., the NMC-Graphite battery pack, is also assessed in this study to provide a benchmark for understanding the relative environmental performance of the NMCSiNT battery pack. NMC-SiNT battery pack configuration Using the BatPaC software of Argonne National Lab, the NMC-SiNT battery pack is configured with a 120 kW power output to sustain a 320 km driving distance per charge, to meet the DoE 2020 target (Figure 2, Detailed modeling information are provided in the excel spreadsheet in the Supporting Information). The first step is to derive the required battery capacity, which can sustain the 320km driving distance. Specifically, the traction energy (Etraction) is calculated for EV at two typical driving cycles. Namely, UDDS (Urban Dynamometer Driving Schedule) representing the local driving, and HWFET (The Highway Fuel Economy Test) for highway driving45. The involved parameters, A, B, and C, are rolling, rotating, and aerodynamic resistive coefficients, respectively. vi and ai are driving speed and acceleration under a specific driving cycle, respectively46. A partial kinetic energy will be re-accumulated by the battery pack through the regenerative braking system and store it in a battery to provide additional tractive energy. Kim et al. indicate that energy recapturing has an effect similar to mass reduction46. Therefore, the vehicle mass (M) is reduced by a factor of θμηcharging, where θ is the ratio of braking to kinetic energy, μ is the regenerative braking efficiency, ηcharging is the charging efficiency. The discharged energy (Edischarged) can then be estimated as a summation of a constant energy demand from auxiliary devices (Paux=750W including lighting, air conditioning, entertaining) and tractive energy corrected by efficiencies of the motor (ηmotor, 0.89 including a 0.97 controller efficiency), and transmission systems (ηtransmission, 0.93)46.The a and b factors represent the shares of the city (55%) and highway driving (45%) distances as in EPA fuel economy testing, respectively47. Finally, the nominal capacity (Enominal) of the NMC-SiNT battery pack is converted from the discharged energy by dividing the discharging efficiency (ηdischarging) and the user accessible ratio (UR), which is the accessible battery capacity divided by the total battery capacity. Detailed information for battery nominal capacity calculation is documented in Table S1~S2 of Supporting Information. 5 ACS Paragon Plus Environment


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Figure 2 Determination of the NMC-SiNT battery pack with 320 km range. Detailed modeling information is provided in the excel spreadsheet in the Supporting Information The second step involves the calculation of the area-specific impedance (ASI) of the electrode system, which is needed to calculate the discharging/charging efficiencies of the NMC-SiNT battery pack. Electrode ASIs at energy and power applications are calculated separately. The ASI of the electrode system is considered to be a sum of every charge transfer steps. The negative and positive electrodes have a porous structure where the Newman-Tobias method is applied to calculate the polarization48. The separator bulk, which contains separator and electrolyte, is considered to be an ionic conductive component. Thus, the ASI of the separator is calculated by the ratio of its thickness over the ionic conductivity (κ). The final step is to use the BatPac software49 to calculate the detailed configuration of the NMCSiNT battery pack. The cathode of the NMC-SiNT battery pack is configured with 95 wt% NMC oxide, 3 wt% carbon black, and 2% PVDF binder for a specific capacity of 180 mAh g-1 NMC, the same as the traditional NMC-Graphite battery. The anode is composed of SiNT, carbon black, and PVDF binder at the ratio of 7:2:1 by mass 24. A specific capacity of 2000 mAh g-1 SiNT from our experiment, as well as 1.1 NP ratio, is used to calculate the required mass of anode. Due to volumetric expansion of the SiNT after lithiation, the anode should be fabricated with voids to accommodate the expanded SiNT in addition to electrolyte fill. As the volume of silicon expands linearly relative to the progress of lithiation, and a full lithiation triggers 4.4 times of volume expansion50, we have determined 37% porosity by volume in the anode is needed to absorb the expanded SiNT. Additionally, 25% porosity in the anode is needed for electrolyte with the applied application rate of 0.5 mL g-1 SiNT (see Table S3 in Supporting information 6 ACS Paragon Plus Environment

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section for input parameters). The separator is a polypropylene-polyethylene-polypropylene trilayer membrane, with 20μm thickness and 39% porosity. Layers of electrodes and separators are sealed through a multilayer pouch welded with two terminals made of aluminium and copper for current collection. The electrolyte, which is lithium hexafluorophosphate (LiPF6) dissolved in dimethyl carbonate (DMC) and ethylene carbonate (EC) solvent at a ratio of 6:4 by volume, is used in the configured NMCSiNT battery. The configurations of the NMC-SiNT cells, module and pack packaging are directly taken from the outputs of the BatPac. We have determined that a 320 kg NMC-SiNT battery pack (63 kWh) with 384 cells in 12 modules is needed. With each cell operating at 3.65 V open circuit voltage of the NMC-SiNT cell, each cell is weighted at 573 grams containing 49 Ah capacity. The ASI of cell electrode system is calculated at 101.7 Ω cm2 and 32Ω cm2 for energy and power applications, respectively. These values are higher than those of conventional NMC-Graphite battery whose ASIs are recorded at 58 Ω cm2 and 33 Ω cm2, respectively 49. A higher ASI for the NMC-SiNT is expected due to the fact that SiNT is not conductive and the diffusion rate of lithium in SiNT anode is limited when compared with the graphite anode. By weight, the entire battery pack consists of 68.8% stacked cells, 5.8% module packaging, 9.6% cooling system, and 14.6% pack packing. The overall gravitational energy density of NMC-SiNT battery pack is obtained at 199 Wh kg-1. Detailed information on ASI calculation and BatPaC simulation of the NMC-SiNT can be found in Section 1.2 and Section 1.3, respectively, in the Supporting Information. Table 1 Comparison of technical parameters between NMC-SiNT and reference battery pack Parameters

NMC-SiNT

NMC-Graphite

Battery mass (kg)

320

417

Gravimetric energy density (Wh kg-1 battery)

199

160

Volumetric energy density (Wh L-1)

324

282

UR

85%

85%

Capacity (kWh)

63

66

Corresponding Vehicle mass (kg)

1824

1921

Driving distance per charge (km)

320

320

For benchmarking, a reference NMC-Graphite battery pack is configured using the BatPaC software in this study with a 66 kWh capacity to power the EV for the same 320 km driving distance per charge. Compared to the former 120 Wh kg-1 energy density in literature 36, the pack level energy density of the most recent NMC-Graphite battery is calculated at 160 Wh kg-1. Meanwhile, in literature, it is reported that 48-56 kWh battery capacity is required for a 320 km driving range16, 51. 7 ACS Paragon Plus Environment


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Key parameters for the configured NMC-SiNT and NMC-Graphite packs are presented in Table 1. It can be found that the derived configurations of the NMC-SiNT and NMC-Graphite battery packs account for 17% and 29% of total EV masses, respectively. The numbers agree quite well with current EV design where battery pack consists of 15%-25% of total vehicle mass for a mid-sized EV with four seats and sufficient luggage space39. These results also confirm that the battery design method applied in this study is valid and robust. Life cycle inventory analysis Inventories for the NMC-SiNT battery pack are derived from a hybrid method using our lab-scale experimentation, theoretical modeling, and literature survey. The inventory of the SiNT material is modelled through our lab processes to compute the material inputs and specific energy consumptions. The energy consumption of the cell manufacturing process of NMC-SiNT is derived from our actual measurements of a pilot-scale LIB cell manufacturing facility of Johnson Controls. The inventories for other components including the NMC cathode, electrolyte, separator, BMS, are calculated based on the outputs from the BatPac v3.0 software. The upstream and downstream processes are modeled with inventory data retrieved from the GaBi 6.0 professional database and Ecoinvent v3.3 database.

Detailed

information for inventories of the NMC-SiNT battery pack is provided in Section 2 of the Supporting Information. A brief introduction to SiNT synthesis, as well as the estimate of the NMC-SiNT cell manufacturing, is provided below. Inventory for the SiNT material. Since the SiNT material has not entered into commercial production, the inventory was estimated from the laboratory scale (Figure 3 presents the morphologies of the synthesized silicon nanotube from our experiments). The material inventory (Table 2) is based on the stoichiometric relationship and material production efficiencies obtained in our lab for the SiNT synthesis: template synthesis, silica coating, template removal and magnesiothermic reduction 24, 33. The cyclohexane solvent is expected to be on-site regenerated through thermal distillation. The cyclohexane solvent can be recycled with a rate of 90%~96% currently in industry52. Regeneration of cyclohexane can follow traditional distillation process, consuming 1.5 kg steam per kg cyclohexane recovered

53,52.

Meanwhile, 0.2 MJ electricity kg-1 recovered cyclohexane is needed during the whole distillation process53. The energy inventory of the SiNT synthesis is constructed based on a bottom-up approach. Each production step is decomposed into several unit operation processes including baseline consumption (e.g. lighting, ventilation, and space heating), stirring, reaction heating, centrifugation, drying and etc. To reflect the potential energy consumption for SiNT synthesis at the industrial scale, the specific energy consumptions (SECs) of these unit processes are calculated using mechanical and thermal analysis models, 8 ACS Paragon Plus Environment

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or directly using industrial data. The energy consumption for a production step is the summation of the SECs of the unit operations multiplying the volume/mass of the materials produced.

(a)

(b) Figure 3 Morphologies of the synthesized silicon nanotube (a) TEM of the synthesized SiNT (b) SEM of the SiNT structure

The emissions during the SiNT synthesis are monitored by residual gas analyzer (Extorr RGA) and ultra-fine particles counter (UCPC, TSI 3776). The on-site air emissions are obtained based on the experimental results. To extrapolate the emissions for potential industrial application, we applied the 90% removal efficiency for volatile organic carbon in industry54 and 30% removal rate for PM2.5 55. In this study, we have derived the emission factors of 0.063% and 0.055% for cyclohexane and diethyl amine respectively as well as 0.12 g PM2.5 emissions kg-1 SiNT synthesis. The resultant effluent collected from the SiNT synthesis contains magnesium ions, nickel ions, dissolved organic nitrogen (as in hydrazine and diethyl amine). For a potential industrial production, the nickel ions are modelled to be removed at a rate of 96% 56. Hydrazine can be degraded into N2. The dissolved organic nitrogen is removed at an efficiency of 60-80% depending on different technologies involved 57. In this study, we have applied a mean 70% removal efficiency for the organic nitrogen removal. The remaining diethyl amine is discharged into environment as COD and BOD. The conversion rate for diethyl amine is 1.31 gBOD and 2.95g COD per g diethyl amine emitted. The aggregated inventory results are presented in Table 2, with a detailed explanation of the SiNT synthesis (Figure S1 in the Supporting information) and inventory data provided in the Section 2.2 of Supporting Information. Inventory of the NMC-SiNT battery pack production The mass of the NMC-SiNT battery pack is estimated according to the outputs from the BatPac software to meet the range and power target. Detailed information is documented in the Supporting Information (Table S4 and Table S5). The energy 9 ACS Paragon Plus Environment


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consumption data for the NMC-SiNT battery production is directly applied according to the actual data measured from a pilot-scale LIB battery manufacturing facility of our industrial partner, Johnson Controls.

Table 2 Inventory data on synthesis of one kg SiNT material Inputs Materials NiCl2 Fresh cyclohexane

Unit g g

920 2730

Regenerated cyclohexane

g

51865

Surfactant

g

39714

hydrazine hydrate TEOS diethyl amine

g g g

682 26071 8263

Hydrochloric acid

g

19393

Magnesium Energy Heat

g

4762

MJ

19.6

kWh

20.4

kg

129

g g g g g g g g

34.4 4.5 0.12 238.1 46.0 1003 2275 1557.4

Electricity Waste treatment Hazardous waste treatment Emissions Cyclohexane to air Diethyl amine to air PM2.5 to air Magnesium to water Nickel emission to water BOD COD Silica solid waste

Input

Data source Table S7 in Supporting Information GLO: market for cyclohexane, ecoinvent 1.5 kg steam and 0.2 MJ electricity kg-1 recovered cyclohexane RER: ethoxylated alcohols (AE7), pertrochemical, at plant RoW: hydrazine production ecoinvent Table S6 in Supporting information GLO: market for diethyl amine, ecoinvent RER: market for hydrochloric acid production, without water, ecoinvent GLO: market for magnesium ecoinvent US: Process steam from natural gas 85% (eGrid) PE US: electricity grid mix PE CH: disposal, solvent mixture, 16.5% water, to hazardous waste incineration

Inventory for the battery use phase. The inventory for the NMC-SiNT battery use phase considers battery life and energy consumption during EV operations. The service life of the NMC-SiNT battery is determined by its degradation rate. In particular, the lifetime driving distance, T, is derived from Equation (1)36. 𝑇 = 𝐷0(𝑐 ―

𝑟𝑐2 2

)

(1)

where D0 is the initial driving distance per charge (320km), c is the cycling number; and r is the capacity decay rate. Considering a battery replacement at 30% of capacity loss, the capacity decay rate needs to be below 0.04% in order to sustain 200,000 km for a single NMC-SiNT battery pack. Currently, the lab10 ACS Paragon Plus Environment

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scale SiNT anode can be developed with a capacity decay rate as low as 0.004%29, indicating that a single NMC-SiNT battery pack could power the EV during its whole service life. Considering the stability of the SiNT anode may be different during real battery production, in this study the potential variations of battery decay rates and the resulted varying battery life are investigated in the later sensitivity analysis. The energy associated with the battery use phase considers the entire energy used to power the vehicle operation for the battery LCA. The reason for this selection is because a lower battery mass can also trigger mass saving for the entire vehicle leading to a higher fuel economy. To capture this mass saving benefit achieved by using a lighter silicon battery, the entire EV operation energy is then analysed in the LCA of the NMC-SiNT battery and compared with that of the NMC-Graphite battery. The energy consumption of the NMC-SiNT battery pack during the use phase is calculated using Equation (2) based on the specific energy consumption per km driven of the EV. The SEC (specific energy consumption) per km driven is a ratio between the discharged energy ( 𝐸𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒𝑑) from the battery pack and the driving distance per charge, the battery efficiency and the charger efficiency. Then the total energy consumption during a single battery life is calculated as the product of the specific energy consumption per km driven and the total distance of the EV driven during the use phase. In this study, the U.S. average electric grid is considered for electricity generation. The operation of the EV is 55% city and 45% highway driving, based on the U.S. national average data. 𝐸𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒𝑑

𝐸𝑢𝑠𝑒 = 𝐷𝜂𝑏𝑎𝑡𝑡𝑒𝑟𝑦𝜂𝑐ℎ𝑎𝑟𝑔𝑒𝑟𝑇

(2)

Calculation of the discharged energy is documented in Section 1.1 in the Supporting Information. The charger efficiency (𝜂𝑐ℎ𝑎𝑟𝑔𝑒𝑟) is set at 0.90. Battery efficiency (𝜂𝑏𝑎𝑡𝑡𝑒𝑟𝑦) equals to the product of charging efficiency and discharging efficiency. T is the total driving distance. With the ASIs, the cumulative discharging/charging efficiency of the NMC-SiNT battery pack is determined as 0.86 through the Joule’s law, which is comparable to the 0.90 efficiency as reported for the silicon nanowire battery pack 3, while lower than the 0.92-0.95 discharging/charging efficiency of the conventional NMC-Graphite battery. 11 ACS Paragon Plus Environment


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Inventory for the battery disposal The battery EoL treatment is modelled based on the following hydrometallurgical process based on Hawkins et al.58 The environmental impacts of the battery disposal process are 100% allocated to the life cycle of the NMC-SiNT battery pack. Firstly, the EV should be collected and battery pack should be removed. Energy consumption per kg battery removal is sourced from Hawkins et al., 0.023kWh kg-1 pack. Then the removed battery pack is emerged into salt solvent to deplete any residual capacity. Dunn et al.59 estimated that 0.001 kWh electricity kg-1 battery pack is needed to sustain the depletion procedure. Subsequently, the battery pack is cryogenically dissembled by man labor. The disassembly process separates the BMS and cooling systems, and cell modules. Cell modules are cut open by electric saw. Afterwards, electrodes are pulled out from cell container. Hawkins et al.58 estimated that 1.123kg liquid nitrogen kg-1 battery and 0.0227 kWh kg-1 battery are required accordingly. Finally, the battery active materials are dissolved in acid for further treatment. Life Cycle Impact assessment In this analysis, the ReCiPe method is adopted to compute the midpoint life cycle environmental impacts per km driven, for the EV using the NMC-SiNT battery pack. In a total, 13 impact categories are analyzed with the ReCiPe method, including global warming (GWP), fossil depletion (FDP), ozone depletion (ODP), photochemical oxidant formation (POFP), particulate matter formation (PMFP), terrestrial acidification (TAP), freshwater eutrophication (FEP), marine eutrophication (MEP), freshwater ecotoxicity (FETP), marine ecotoxicity (METP), terrestrial ecotoxicity (TETP), Human toxicity (HTP), and Metal Depletion (MDP). The MDP category should be interpreted with caution because the ReCiPe method version 1.08 allocates a higher weighting on manganese relative to other metals. Results and Discussion Material and primary energy flow analysis of the NMC-SiNT battery pack The material analysis provides a mass inventory of all materials needed to produce the specified NMCSiNT battery pack (Table S8 in the Supporting Information). As calculated, the 63 kWh NMC-SiNT battery pack needs 2049 kg total material inputs. Synthesis of the SiNT based anode is identified as the most material demanding process, which accounts for 84.0% of the total material inputs. The SiNT product requires intensive cyclohexane solvent and surfactant uses to produce the NiCl2-hydrazine nanorod template, which represents 31.0% and 22.7% of total material flows, respectively. Each gram of NiCl2-hydrazine nanorod template synthesis requires 34 g cyclohexane and 25 g surfactant to ensure a homogenous and well-dispersed nanorod templates solution. Furthermore, the limited 32% conversion rate of silicon transformation from silica nanotube to SiNT is another main reason for the significant 12 ACS Paragon Plus Environment

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material demand. The electrolyte used in the NMC-SiNT battery pack includes 4.1 kg LiPF6 salt and 33.3 kg EC/DMC solvent. In the NMC-SiNT battery pack, common plastics including 4.2 kg PE (polyethylene), 2.9 kg PP (polypropylene), and 4.4 kg ABS (Acrylonitrile-Butadiene-Styrene) are used mainly for the separator in the NMC-SiNT cell and lids in the module packaging, respectively. Furthermore, the housing of the pack packaging, positive current collector, an external conductor of the cooling system, conductive layers of the module packaging are the major outlets of the total 84.8 kg aluminium input, accounting for 64.1%, 16.9%, 10.4%, and 5.2%, respectively. Copper including copper wire in the NMC-SiNT battery weighs 36 kg in a total, indicating a rate of 0.6 kg copper use per kWh NMC-SiNT battery. When compared, the conventional LIB battery has a copper concentration of 1.2 kg per kWh capacity36. In addition, production of the battery tray requires 2.6 kg steel, which is used to produce straps of the pack packaging. The ethylene glycol (EG) coolant used for the NMC-SiNT battery is 0.37 kg kWh-1 to ensure no more than 1oC battery temperature rises during normal operation. This value is higher than the conventional NMCGraphite battery in which 0.31 kg kWh-1 coolant is used. The high coolant use complies with the fact that the NMC-SiNT battery has a lower energy efficiency and requires more coolant for heat dissipation.

Figure 4 primary energy consumption and distribution of NMC-SiNT battery pack. Numerical results are provided in Table S9 in Supporting Information. 13 ACS Paragon Plus Environment


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For the energy consumption, Figure 4 depicts the primary energy embodied in major materials, components, and processes of the NMC-SiNT battery during its whole life cycle. The primary energy refers to those nonrenewable energies extracted from nature which has not been refined into any form of secondary energy (e.g. electricity or steam). Adopting the concept of primary energy provides a comparable basis for different activities and materials. The overall primary energy use of the NMC-SiNT is calculated to be 2.82 MJ km-1, 60% denoting primary energy demand in the use phase. Next, to the battery use phase, battery manufacturing consumes 0.66 MJ km-1 of primary energy, in which silicon nanotube synthesis, electrode drying, and dry room conditioning represent 31%, 28%, and 21% of manufacturing primary energy consumptions, respectively. The significant primary energy demand of silicon nanotube synthesis is mainly caused by template material synthesis involving the NiCl2-hydrazine nanorod synthesis and surfactant treatment. Per kg SiNT requires 39.7 kg surfactant to treat the NiCl2hydrazine nanorod, which subsequently contributes to about 36% of total primary energy consumption use in SiNT synthesis. Meanwhile, the magnesium reduction consumes 13.7 kWh electricity and 12.4 MJ heat, representing 70% and 63% of total electricity and heat uses per kg SiNT synthesis, respectively. The primary energy demand of overall NMC-SiNT materials is 0.44 MJ km-1, about 65% of the primary energy consumed in battery manufacturing. From an energy perspective, the most significant material is magnesium, the reduction agent converting silica nanotube to silicon nanotube, which accounts for 23% of the total primary energy of the battery materials, followed by the TEOS (21%) and NMC (12%). Life Cycle Impact Assessment Results Environmental impact assessment and hotspot identification. The environmental impact hotspots during the life cycle of the NMC-SiNT battery pack are revealed by contribution analysis in Figure 5, which is configured to present two aggregated results: the pattern bars representing the contribution of environmental impacts of NMC-SiNT battery for three main life cycle stages: battery production, battery use, and disposal; The colored bars illustrating detailed environmental impact contributions to the battery production stage from each component of the NMC-SiNT battery pack and assembly. Production of the SiNT anode exhibits an impact fraction in the range of 35%-60% of total battery production impacts in most impact categories. The significant environmental impacts of the SiNT anode are driven by surfactant for the component materials such as the TEOS and magnesium for the silicon nanotube and significant use of surfactant for the silica nanotube synthesis. Next, to the SiNT synthesis, the NMC-SiNT battery cell manufacturing accounts for 46% in GWP and 37% in FDP. This is mainly caused by the intensive electricity use of battery manufacturing, in the amount of 544MJ kWh-1 pack (equivalent to 43.3 kWh kg-1 cell). A decomposition analysis reveals that manufacturing a single 49 Ah 14 ACS Paragon Plus Environment

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cell requires a total of 25.0 kWh electricity input, in which 10.4 kWh electricity is used in electrode to evaporate the NMP solvent, and 7.7 kWh electricity is consumed by heating and dehumidification machines in the dry room. In comparison, Ellingsen et al.36 estimated an energy consumption of 586~2138 MJ kWh-1 (or 162~644 kWh kWh-1 pack) for the conventional NMC-Graphite cell manufacturing from a pilot scale industrial plant. The lower end value of the Ellingsen et al.36 represents the situation of the full capacity operation of the pilot-scale plant. A recent report from Dai et al.60 presented a survey on energy consumption for the battery pack production based on different sources of the industrial data. Their results show that the total energy demand can be as high as 990~1941 MJ kWh-1 battery pack for a pilot scale battery manufacturing plant. Their surveyed values are in compatible to the energy consumption level reported in Ellingsen et al.36 as well as to the values in this study. As a result, the energy consumption data of battery pack from our pilot-scale measurement agree well with the results in the literature. For future large-scale industrial production, Dai et al.60 indicated that total energy demand can be reduced to 119~175 MJ kWh-1, which is 10~20% of the values in the pilot scale battery plant.

Figure 5 Life cycle environmental impact of NMC-SiNT battery per FU. The absolute values of the impacts are documented in Table S10 of Supporting Information The copper current collector, though only accounting for 8% of total battery mass, is the main toxicity source representing in around 28%-56% shares in FETP, METP, and HTP such impact categories due to the disposal of the sulfidic tailings during copper refinery. This finding aligns well with the previous result in Ellingsen et al.36. Meanwhile, copper is found to take a significant 57% share in FEP due to the disposal of the sulfidic tailing during copper production which results in large impacts in eutrophication. 15 ACS Paragon Plus Environment


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In this LCA study, the toxic impact of SiNT itself is not considered due to lack of characterization factors needed for the SiNT particles. However, current preliminary toxicological understanding on silicon nanomaterials indicates that toxicity effect of the SiNT nanomaterial is limited. According to the USEtox methodology, which is popularly used in quantifying the toxicity effect in the life cycle assessment61, the effect factor of SiNT could be very limited. Silicon nanoparticle is considerably reactive, and then, is highly likely to form silica after being discharged into the environment. The toxicological study has already shown that silicon nanoparticles exhibited no acute toxicity, and may be considered as biocompatible material 62, 63. For the total life cycle impacts of the 63 kWh NMC-SiNT battery pack, it is found that the battery production phase accounts for dominant shares in most impact categories in a range of 37%-99%. However, the battery use phase accounts for 63% and 58% in GWP and FDP, respectively. Since electricity is the only considered factor during the vehicle use phase, these environmental burdens are mainly associated with the electricity generation and distribution in the U.S. grid. Finally, impacts of battery hydrometallurgical disposal on average take only 1.5% of life cycle impacts. This indicates that direct environmental burdens from the battery recycling process are insignificant. A sensitivity analysis has been conducted on five key parameters including driving range, decay rate, total lifetime distance, battery efficiency, and battery decay rate to test the sensitivity of the LCA results on these parameters. The results are provided in Table S11 in the Supporting Information. In general, except for the battery decay rate, it is found that the life cycle impacts of the NMC-SiNT battery maintain considerable stability (-20%~20%) on most of these key parameters, which demonstrates a robust LCA study. On the other hand, the battery decay rate is found a highly sensitive indicator leading to 100%~150% changes in results. This result is reasonable because battery decay rate governs the battery life which dictates the number of battery packs needed for the EV service life. Comparison between NMC-SiNT and NMC-Graphite Battery Pack. Figure 6 shows a benchmarking of the LCA results between the NMC-SiNT battery pack and the conventional NMC-Graphite battery pack under the same driving range. As observed, the NMC-SiNT battery has comparable environmental impacts with the conventional NMC-Graphite battery. As shown in Figure 6, among 13 impact categories, the NMC-SiNT battery is identified to have 2%17% higher environmental impacts in 6 impact categories including GWP, FDP, ODP, PMFP, POFP, TAP and TETP. Notably, it is found that the NMC-SiNT battery generates 10~17% more impacts in GWP and FDP, respectively, than the conventional NMC-Graphite battery. For GHG emission, the production of NMC-SiNT battery emits 66.7 g CO2, eq km-1, which is 38% higher than that of the NMC-Graphite battery pack. The intensive GHG emissions in the production phase result in the higher life cycle GHG 16 ACS Paragon Plus Environment

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emission of the NMC-SiNT battery pack, even though a slight 2% GHG emission in the use phase reduction can be obtained due to the lightweight of the NMC-SiNT battery. The higher impacts in GWP and FDP of the NMC-SiNT battery are caused by intensive material and energy uses in the SiNT synthesis and relatively larger energy consumption in cell manufacturing. The SiNT synthesis triggers considerable higher impact in TETP, which contributes to 11% higher than that of the NMC-Graphite battery. The reason for such a high toxicity impact in TETP is caused by the hydrazine used for SiNT synthesis. Specifically, the SiNT synthesis accounts for 62% of total life cycle TETP impact for the NMC-SiNT battery, while the use of the hydrazine hydroxide represents 40% of TETP impact in the SiNT synthesis. In addition, the NMC-SiNT battery is found to have 25%-67% lower impacts in FETP, HTP, METP, FEP, MEP, and MDP, which are rewarded by the significant 30% lower mass of the NMC-SiNT battery compared to the NMC-Graphite, and thus requires less copper and transition metals.



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Figure 6 Life cycle impact benchmarking between NMC-SiNT and NMC-Graphite battery packs. Significance indicates the absolute number of the impact value. Units are shown on the left for each impact. Numerical results presented in Table S12 in Supporting Information From Figure 6, the life cycle environmental impacts of the NMC-SiNT battery in this study is found comparable with those of NMC-graphite battery. Considering our study is based on the pilot-scale data to represent the environmental profile of the NMC-SiNT in the near term, future industrial-scale production of SiNT battery will significantly lower its life cycle impacts, and hence the life cycle environmental impacts of the NMC-SiNT battery could be potentially lower than those of conventional NMC-graphite battery, which will make the NMC-SiNT battery an environmentally favorable power technology for future EVs. Future Improvement Potential for the SiNT anode Since the current design of the NMC-SiNT battery presents comparable environmental impacts in GWP and FDP, further improvement potentials need to be identified for future sustainable development of the NMC-SiNT battery technology based on the current LCA results. In these section, the potential improvements in environmental impact reductions for the industrial scale production of the NMC-SiNT cells as well as recycling credits from the NMC-SiNT EoL treatment are to be evaluated. Meanwhile, the relative position of the NMC-SiNT battery to other new generations of battery can be assessed. For the future large-scale production of the NMC-SiNT battery, both the materials use efficiency for electrode synthesis and energy consumption of the cell assembly can be upgraded due to both the technology improvements and economy of scale. The magnesiothermic reduction reaction may be optimized the parameters of reaction time, temperature, pressure, and molar ratio to improve the material efficiency for the SiNT synthesis. Meanwhile, the large dispersion equipment can help with improving the cyclohexane solvent use rate, and in turn the use of the auxiliary material. In the scenario analysis, we have evaluated the potential environmental impact reductions by improving the magnesiothermic conversion efficiency from the original 32% to 65% as well reducing cyclohexane solvent to 12.5 kg per kg template. Moreover, as energy consumption of the LIB manufacturing is related to the production scale, in the scenario analysis, the possible variations of the energy consumption in the actual industrial scale of productions of LIB battery are evaluated down to 8.7 kWh kg-1 cell implying 80% reduction from the original energy consumption level. Figure 7 depicts the cumulative benefits of the environmental impacts of the NMC-SiNT battery. Upgrading the production scale contributes to environmental impact reductions over a board range of categories. Significant impact reductions of 15%-50% are recorded in the categories of GWP, FDP, ODP, PMFP, POFP, TAP and TETP. These impact categories are sensitive to the production scale due to the fact that the energy consumption for battery assembly is the main 18 ACS Paragon Plus Environment

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contributor to these impacts. In particular, the GWP and FDP can be reduced by 50% (figure 7), making the NMC-SiNT a low environmental impact battery with a high driving range in the future compared to the conventional NMC-Graphite battery pack.

Figure 7 Potential environmental benefits for large-scale production of the NMC-SiNT battery Based on the survey from Boyden et al.64, for hydrometallurgical recycling, different recyclers recover different elements with copper, aluminum, steel, lithium and cobalt most commonly recycled. The estimated overall material recovery efficiencies are 55.6%~65.3% for current battery recyclers. Due to lack of specific material recovery efficiencies, we applied a single 57% material recovery efficiency for all materials. The recycling benefits of these recovered materials are considered as the negative environmental impacts as replacing the original materials use for the battery production. In particular, lithium is recycled to replace the production of the original lithium carbonate. The lithium carbonate is further reacted with lime to generate lithium hydroxide while cobalt is recycled as cobalt sulphate. Both of them are precursors for the NMC cathode paste. The recovered aluminium and copper are used to original metals production. The comparative environmental impacts of the battery production are presented below. It can be seen that battery recycling results in about 5% decrease of the total GWP emissions of the NMC-SiNT production. Such an insignificant value is caused by the fact that the pilot scale production of the NMC-SiNT battery pack is mainly dominated by SiNT synthesis and battery cell assembly. However, 25%-40% decreases of impacts in FETP, FEP, HTP, and METP can be observed due to the copper recycling credits (see Figure S2 in the supporting information). 19 ACS Paragon Plus Environment


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Besides, the specific capacity and decay rate of the SiNT anode is found having significant effects on the life cycle impacts of the NMC-SiNT battery pack. Future investigations are needed on optimal control of the size and morphology of the SiNT particles during the synthesis to ensure an improved specific capacity, to alleviate the decay rate and then reduce the total life cycle environmental impacts of the NMC-SiNT battery. Comparison between NMC-SiNT with other next-generation batteries. Multiple LCA studies have been reported in the literature on various future batteries such as a lithium-sulfur battery, silicon nanowire anode based battery, and sodium-ion battery (SIB). A benchmarking of technological performance and GWP emission between the SiNT battery and the other batteries (Lithium-sulfur battery, NMC-silicon nanowire battery and sodium-ion battery) is provided in Table S13 in the Supporting Information. Since these LCA literatures were constructed on different production scale, impact values of NMC-SiNT are accordingly adjusted to ensure a proper benchmarking. Specifically, the LCA study on the Li-Sulfur battery is modelled at the pilot scale. Thus, the comparison is made with the environmental impacts of the NMC-SiNT at the pilot scale. Meanwhile, literature studies on the NMC-SiNW and sodium ion batteries are constructed upon the projection of the industrial scale production. Accordingly, the corresponding impacts of the NMC-SiNT at the industrial scale are assessed in this study. From the benchmarking, the potential environmental impacts difference between the silicon nanotube battery and silicon nanowire battery are significant. Silicon nanowire battery pack is found to be produced at around 343 kg CO2e per kWh battery pack, which is around three times higher than the NMC-SiNT battery. The reason is caused by the fact that producing the silicon nanowire is much less efficient than the solvent based synthesis of the silicon nanotube. Producing one kg silicon nanowire requires 6.3 kg silica flour with 1296 MJ electricity consumption in grinding. When compared, the silicon nanotube anode is produced in a more efficient way which only requires 91.7 MJ total energy (72 MJ electricity and 19.7 MJ heat). The energy density of the NMC-SiNT battery pack is similar to that of the lithiumsulfur battery but the GWP per kWh of storage capacity from the NMC-SiNT battery is 25% higher than that of the lithium-sulfur battery. However, the lithium-sulfur battery, using the lithium metal as the anode, may raise a significant safety concern. Current sodium-ion battery technology has a low energy density, only half of the energy density of the SiNT battery at pack level, although the GWP of sodium ion battery is around 35% higher than that of the SiNT battery. Associated Content Supporting Information


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Details of NMC-SiNT battery pack configuration, life cycle inventories and numerical results from the life cycle assessment

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Life Cycle Assessment of Silicon Nanotube Based Lithium Ion Battery

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