Environmental and economic evaluation of remanufacturing lithium-ion batteries from electric vehicles

Waste Management 102 (2020) 579–586

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Environmental and economic evaluation of remanufacturing lithium-ion batteries from electric vehicles Siqin Xiong a,b, Junping Ji a,c,⇑, Xiaoming Ma a,b a

School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China College of Environmental Sciences and Engineering, Peking University, Beijing 100080, China c Energy Analysis and Environmental Impacts Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA b

a r t i c l e

i n f o

Article history: Received 2 December 2018 Revised 17 October 2019 Accepted 11 November 2019

Keywords: Battery remanufacturing Lithium-ion batteries Environmental impact Economic evaluation

a b s t r a c t The environmental threats posed by spent lithium-ion batteries (LIBs) and the future supply risks of battery components for electric vehicles can be simultaneously addressed by remanufacturing spent electric vehicle LIBs. To figure out the feasibility of battery remanufacturing, this paper quantifies the environmental impacts and costs of the remanufacturing of lithium-nickel-manganese-cobalt oxide battery cells and compares the results with the production of batteries from virgin materials. Based on the EverBatt model, a China-specific database of hydrometallurgical remanufacturing process is established. The results indicate that the reductions in energy consumption and greenhouse gas emissions by battery remanufacturing are 8.55% and 6.62%, respectively. From the economic standpoint, the potential cost-saving from battery remanufacturing is approximately $1.87 kg
1 cell produced. Through a sensitivity analysis, LIB remanufacturing is found to be economically viable until the purchase price of spent batteries rises to $2.87 kg
1. Furthermore, the impact of battery type variability is prominent, whereas the influence of recovery efficiency is limited. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The increasing adoption of electric vehicles (EVs) can reap significant environmental benefits while also reducing dependence on fossil fuels (Li et al., 2018). Having recognized these benefits, most countries are speeding ahead with EVs. In China, EVs accounted for approximately 4.40% of all 28.08 million automobiles sold in 2018, which is much higher than the U.S. (just over 2%) and Europe (around 3%) (MIIT, 2019). The desirable characteristics of lithium-ion batteries (LIBs), such as high energy density, low self-discharge rate, and long lifespan, have made them superior to other batteries as power sources for EVs (Nitta et al., 2015). As millions of LIBs for vehicles sold over the past decade are reaching their end-of-life (EOL), numerous scrap batteries need to be properly disposed of to prevent negative environmental impacts. The manufacturing of LIBs with spent batteries, i.e., remanufacturing, is now seen as a promising EOL option for EV batteries, for its potential to reduce the environmental impacts of both the ⇑ Corresponding author at: School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China. E-mail addresses: [email protected] (S. Xiong), [email protected] (J. Ji), [email protected] (X. Ma). https://doi.org/10.1016/j.wasman.2019.11.013 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.

waste disposal and battery production. Meanwhile, producing with spent batteries would prevent possible price surges and supply disruptions of battery materials, particularly when the supplies of materials are highly dependent on imports, like cobalt (Co) and nickel (Ni) in China (Canals Casals and Amante García, 2016; Sommer et al., 2015). While the importance of recycling is subjectively recognized, more efforts are required on the quantitative analysis to confirm the feasibility of using recycled materials to replace virgin materials for battery production. Previous studies have attempted to estimate the environmental impacts caused by battery recovering, as listed in Table 1. From the economic standpoint, Foster et al. (2014) found that remanufacturing can save approximately 40% over new battery produce. Kampker et al. (2016) calculated that remanufacturing of LIBs potentially offers cost-savings up to €60 kWh
1. Qiao et al. (2019) indicated that the gross income of battery recycling is approximately $0.74 kg
1. The profitability of battery remanufacturing has also been claimed by Sanfélix et al. (2016), Rohr et al. (2017), and Li et al. (2018). Nonetheless, Song et al. (2017) was more pessimistic and concluded that waste battery recycling enterprises might sustain economic losses when only spent batteries are used as the material inputs during production because of the low utilization rates of waste batteries. It is seen that a large deviation is observed in both environmental analysis and economic evaluation. The

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Table 1 Previous studies on the environmental impacts of battery recycling or remanufacturing. Reference

Recycled battery chemistry

Recycled materials

Recycling method

Results or conclusions

Dewulf et al. (2010)

LIB

Co, Ni

Linda Gaines et al. (2011) Dunn et al. (2012)

Various cathode chemistries LMO

Including Co, Ni, Al, Cu, electrolyte solvent, anode LMO, Al, Cu

Pyrometallurgical recycling, Hydrometallurgical recycling Pyrometallurgical recycling (Umicore; Toxco); direct recycling (eco-bat) Direct physical recycling

Oliveira et al. (2015)

LMO, LFP

Li, Co, steel, nonferrous materials

Hydrometallurgical recycling

Dunn et al. (2015)

Various cathode chemistries

Pyrometallurgical recycling; intermediate recycling; direct recycling

Lettieri et al. (2016)

LIB

Co-containing materials (pyrometallurgical and intermediate recycling); cathode materials (direct recycling) Li-salt, Co, Ni, iron, steel

The recycling scenario results in 51.3% natural resource savings. Through the Umicore process, recovery of Co and Ni saves about 70% of the energy needed for their production from sulfide ores Approximately 48% of energy during material production can be reduced. The benefit of metal recycling is negligible because the materials and energy use in the recycling processes is intensive. The GHG emission reduction potential is 60% to 75% for the pyrometallurgical process, 11% to 91% for the intermediate recycling process and 81% to 98% for the direct recycling process

Han Hao et al. (2017)

NMC

NMC, steel, Cu

Hydrometallurgical recycling

Rebecca and Ciez (2018)

NMC, NCA, LFP

Co, Ni, Cu, iron, cement slag (pyrometallurgical); Li2CO3, Co, Ni, Mn, iron (hydrometallurgical); cell hardware, cathode precursor (direct cathode recycling)

Pyrometallurgical recycling; Hydrometallurgical recycling; direct cathode recycling

Hydrometallurgical recycling

reasons could mainly be attributed to three parts: different recycling techniques, unclear sources of data or poor data quality, and regional disparity. Specifically, different recycling approaches, including physical mechanisms, pyrometallurgical treatments, and hydrometallurgical treatments are used in different studies. Meanwhile, the used reagents, recovery efficiency rates, and the materials that are desired to be recycled vary a lot among papers. Secondly, most studies based their evaluation on modelling and sourced the data from the available database or previous studies, without providing detailed inventory. Another contributor to the uncertainty is the regional disparity in the emission-intensity of the electricity and material prices. For example, since the electricity structure in China is coal-dominated, keeping all other parameters the same, the greenhouse gas (GHG) emissions of products production would be higher than that in most European countries. Although crucial insights have been provided by previous studies, some information is outdated, particularly as the EV batteries have rapidly evolved in recent years. Besides, as above mentioned, large deviations in previous results have been observed, which highlight the necessity for studies to provide localized analysis, as well as detailed data. Moreover, few studies have attempted to simultaneously perform environmental and economic evaluation concerning battery remanufacturing. As the two aspects should complement each other in the decision process, it is of great advantage to combine the environmental and economic analysis within the same framework. This study aims to calculate the environmental impacts and costs of battery manufacturing with recycled materials, and then compare the results with the manufacturing of LIBs using virgin materials. The environmental impacts are represented by energy consumption and GHG emissions, as the primary purpose to promote EVs is to decarbonize the transportation sector. The prevailing hydrometallurgical recycling technique is employed and a China-specific database is established based on our best estimation. Furthermore, the robustness of the results is identified by a sensitivity analysis regarding the key parameters. By doing so, this

The disposal phase has a minor impact on the total environmental burdens. The reduction of energy consumption and GHG emissions are 32.1 GJ and 5.1 tons per EOL EV. About 10% of the life cycle GHG emissions can be reduced by recycling. The pyrometallurgical and hydrometallurgical processes do not significantly reduce life-cycle GHG emissions while the direct cathode recycling has the potential to reduce emissions for NMC and NCA

study contributes to helping battery producers or recyclers better understand the environmental and economic viability of battery remanufacturing, eliminating the uncertainties associated with the evaluation, and promoting the transportation sector in reducing environmental burdens and dependency on imported materials. 2. Methodology and data 2.1. Battery cell model Although a wide variety of LIBs have been developed, the most prevalent cathode components are the lithium (Li)-Ni-manganese (Mn)–Co oxide (NMC) and Lithium iron phosphate (LFP) cathodes, whereas the anode is typically graphite. However, it is noticeable that LFP is limited by its lower energy density, while the NMC battery is more likely to dominate the EV battery market in the near future due to its superior electrochemical performance (Zubi et al., 2018). Additionally, the NMC type, which contains relatively high Co, provides a strong incentive for recyclers, while current Li prices are too low, which makes the recovery of LFP economically attractive. In this study, the recycling of NMC batteries is discussed to represent the environmental and economic merits of recovering spent LIBs. Although NMC cells of different stoichiometric ratios are available in the battery market, this paper uses the prismatic NMC 111 battery as the reference type and discusses other types of NMC in Section 4.2. Based on the Battery Performance and Cost (BatPaC) model (ANL, 2016; Nelson et al., 2012), the energy density of NMC 111 cell is assumed to be 164.37 Wh/kg, and the proportion of material components are shown in Table 2. 2.2. System boundary This paper splits the entire remanufacturing cycle into three stages: recycling, cathode remanufacturing, and cell remanufacturing

S. Xiong et al. / Waste Management 102 (2020) 579–586 Table 2 Material components of the NMC 111 cell. NMC 111 Active cathode material Graphite Carbon black Binder: PVDF Copper Al Electrolyte: LiPF6 Electrolyte: EC Electrolyte: DMC Plastic: PP Plastic: PE Plastic: PET

34.10% 19.00% 2.30% 2.90% 16.40% 8.20% 2.20% 6.20% 6.20% 1.90% 0.30% 0.30%

Note: EC: ethylene carbonate; DMC: dimethyl carbonate; PP: polypropylene; PE: polyethylene; PET: polyethylene terephthalate.

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evaluates the impacts of battery remanufacturing at the cell level, because information regarding the recycling of battery pack components is quite limited. The functional unit in this analysis is based on per kg of cell produced. The potential environmental and economic benefits are attributed to the reuse of Co, Ni, Mn, the sale of Aluminum (Al), copper (Cu), and the reduced need for purchased fuels, which can be provided by burning the polyvinylidene difluoride (PVDF) and electrolyte. Furthermore, Li is not recycled in our research, mainly because of its relatively adequate reserves. Based on previous literature, Li has also been excluded in most recycling plants (Boxall et al., 2018; Song et al., 2017; Zeng et al., 2015). Other battery constituents, like graphite, separator, and carbon black, are collected as scrap or are landfilled for their cheap prices, small mass proportions in batteries, or fewer energy requirements during production (Olivetti et al., 2017).

2.3. Remanufacturing approach

processes. The manufacturing process with raw materials is presented for comparison, as the system boundary shown in Fig. 1. The recyclable and valuable metals in spent LIBs are recycled in the recycling stage while other chemistries are either landfilled or burned out for energy. As for remanufacturing the cathode, the recycled product would be coprecipitated and then sintered with the blended lithium carbonate (Li2CO3) to produce NMC cathode powder. In the process of cell remanufacturing, the inputs for energy and other materials except the cathode are assumed to be identical to those for the manufacturing with virgin materials. The specific process of each stage is described in Section 2.3. As the calculation of battery collection and transportation is highly dependent on the distance between the recycling stations to the recycling factory, the collection and transportation stages are excluded from the scope of this study. China is chosen as the target region as it currently has the biggest EV battery market in the world, and tons of spent batteries are awaiting appropriate treatment. It should be noted that this study

Based on previous studies, three recycling pathways have been used for LIBs: pyrometallurgy, hydrometallurgy, and direct recycling (Dunn et al., 2015; Rebecca and Ciez, 2018). Pyrometallurgy recovers a mixed metal alloy including Co, Ni, Cu, and iron by oxidation and reduction reactions with high temperature (Yun et al., 2018). However, the intensive energy consumption and hazardous gas emissions of this approach make it hardly advisable in the long term (Chen et al., 2015). Direct recycling seems appealing as the cathode materials are expected to be directly recovered without great chemical changes (Linda, 2014). However, the immature recycling techniques limit its application in China. The hydrometallurgy method uses in-solution chemistries to extract recyclable materials, mainly involving processes of solvent extraction and acid leaching. In fact, the series of tailorable processes of the hydrometallurgy approach resembles the extraction of virgin materials. Thus, most constituents of a cell could be recovered in a form that facilitates simplified procedures to reproduce batteries (Kushnir, 2015). Additionally, this method is already industrialized

Fig. 1. System boundary of this study. Note: hydrometallurgical recycling process (in green), cathode remanufacturing process (in yellow), cell remanufacturing process with recycled or virgin materials (in blue), cathode manufacturing with virgin materials (in orange). Objects in oval represent the purchased materials and the objects in green diamond boxes are sold out to generate revenue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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by many recycling businesses, like Retriev Technologies, Accurec, and Bangpu (Gies, 2015; Han Hao et al., 2017). Hence, the hydrometallurgy process is applied in this study to evaluate the recycling benefits. The hydrometallurgy recycling process comprises four main steps: discharge and disassembly; shredding and calcination; physical separation; and leaching. More specifically, spent batteries are initially discharged to prevent short-circuiting by soaking in brine, then are manually disassembled to remove the external cell case. The dissembled batteries are then calcined at a relatively high temperature to eliminate the binder and electrolyte. In this study, N-Methyl-2-pyrrolidone (NMP) is used to dissolve the binder, while water is used to dissolve the binder from the anode, leaving the Al and Cu foil to be recycled. Next, the batteries are undergone several physical separation processes to separate the metal and plastic materials, including density separation, froth flotation, and filter pressing. After drying the cathode residue, the metals within it are leached out by sulfuric acid, in the presence of hydrogen peroxide as the reducing agent. In the cathode remanufacturing process, the recycled Co/Ni/ Mn-containing solution is coprecipitated with ammonium hydroxide (NH4OH) and sodium hydroxide (NaOH). After filtrating, drying, and preparing via a ball-mill, the metal powders are finally sintered in a sintering furnace with the blended lithium carbonate (Li2CO3), aiming to produce NMC cathode powder. In the cell manufacturing stage, the positive and negative electrode materials are mixed, using an NMP/water solvent to make the electrode material slurry for subsequent electrode coating. Then, these materials are coated, calendered, and then slit and dried in vacuum. Following this, cells are stacked and electrolyte is filled, and the final sealing and testing processes are performed. 2.4. Quantitative method To facilitate the calculation, the Excel-based EverBatt model is used, which is developed by the Argonne National Lab, to benchmark the environmental impacts and costs of battery manufacturing from recycled materials against that from virgin materials (ANL, 2018a; Qiang Dai et al., 2019). This model leverages previous BatPaC model, which is designed to count the cost of battery manufacturing, and The Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) model (ANL, 2018b; Dunn et al., 2015), which accommodates thousands of background data, and can be used to quantify the life cycle environmental impacts of transportation-related facilities. Although the EverBatt model has only been developed recently, the BatPaC and GREET models, which EverBatt model is based on, have been widely used in previous life cycle assessments of LIBs or EVs (Ciez and Whitacre, 2017; Dunn et al., 2012). Considering that Co is the most desirable and valuable metal to be recycled, we assume that Co comes completely from the recycled spent batteries in the remanufacturing scenario. Then, the demands for spent batteries that are required to extract adequate Co content are calculated based on Equation (1):

ALIBs ¼ 1=bcathode  bCo  acathode  acell

ð1Þ

where ALIBs denotes the required amount of spent LIBs; bcathode and bCo refer to the recovery efficiency of the cathode and Co, respectively; acathode represents the yield rate of active cathode material per cell. The metal recovery efficiency values and material yield rates are shown in Table 3. acell denotes the cell acceptance rate after testing in the manufacturing process and is assumed to be 95%. The environmental impacts of each process are calculated based on the materials and energy flows through the process by the following equation:

Table 3 Recovery efficiencies in the recycling process and material yield rates in the remanufacturing process. Recovery efficiency (%) Cu

70.00%

Al

70.00%

NMC 111 Co2+ in Co salt/ oxide Ni2+ in Ni salt/ oxide Mn2+ in Mn salt/ oxide

EI ¼

X

mi  eii þ

i

Yield rate (%) 92.20%

90.00% 90.00%

Active cathode material Active anode material Al foil Cu foil

90.00%

Separator

98.00%

90.00%

Electrolyte

94.00%

X

qj  eij þ P

92.20% 90.20% 90.20%

ð2Þ

j

where EI refers to energy consumption or GHG emissions for each process; mi denotes the mass of material i consumed in the process; qj denotes the quantity of fuel type j consumed in the process; eii and eij denote the energy or GHG emission intensity for material i or fuel j; P denotes the energy or GHG emissions from the process as a result of combustion or thermal decomposition of materials, and this could be negative when the energy released by combustion is returned to the system boundary. The economic evaluation involves the material expenditure, operating labor costs, energy and other utility fees, fixed charges, variable overhead, and general expenses, as well as the revenues generated by selling out Al and Cu as downcycling materials. The cost analysis for the recycling and cathode production process is based on a production model, as summarized in Table A1, whereas the analysis for cell remanufacturing is based on Table A2. The entire cost of the battery remanufacturing can be calculated from the following equation:

Cremanufacturing ¼ C recycle þ C cathode þ C cell


X

mk  pk

ð3Þ

k

where Cremanufacturing denotes the total cost of battery remanufacturing, Crecycle and Ccathode , and Ccell are the cost of the recycling, cathode remanufacturing, and cell remanufacturing stages, respectively; mk is the mass of material k that could be sold, including Al and Cu in this study, and pk denotes its unit price. 2.5. Data To evaluate the environmental impacts of battery recovery, the default values of energy consumption in the EverBatt model are used, which are based on the process-based analysis or literature reviews (Majeau-Bettez et al., 2011; Dunn et al., 2015; J.B. Dunn et al., 2012; Dunn et al., 2016). Supplementary Tables A3–A6. provide the material and energy inputs of each stage and Table A7. presents the GHG emission factor for the electricity in China. For the cost analysis, the material and fuel costs are calculated by summing the costs of each material and fuel input while the labor costs are determined by the average wage of workers in China and the number of required workers for operating the processes. It should be noted that the labor costs in our study only include the payments for the operating manpower of the plant, whereas the costs for administers, sellers, and others are categorized as sales and administration fees. Obviously, the prices of material purchases are crucial for a cost-benefit analysis, particularly when the prices of battery-related materials are highly volatile and regionally disparate. Therefore, some adjustments are made to the default values in the EverBatt model, after verification

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with some material suppliers and online rechecking the current material prices in China, as illustrated in Table A8. Excluding the purchased equipment costs, material expenditure, utility fees and operating labor costs, the amounts of other types of costs and investments are estimated by assuming fixed relative cost ratios of these modeled costs. The annual production capacity of cell manufacturing plants is set as 10,000 metric tons of cells, for both the production with virgin materials and recycled content. This assumed annual production capacity is within the limited range of 1000–100,000 metric tons of cells in the Everbatt model and approaches the capacity of a medium-level battery producer in China. Considering that the battery recycling industry is still in an early phase, the rated capacity of a plant is hardly satisfied, and this paper assumes a relatively conservative equipment efficiency of 65%. Table A9. provides detailed information regarding the plant facilities.

3. Results

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the required fuel inputs, underlining the necessity of figuring out the most energy-intensive recycling processes for further improvement. Therefore, the energy requirements of each equipment at the given capacity rate are further investigated. This analysis does not aim to compare the results with the production with virgin materials, as the data of the equipment-based manufacturing with virgin materials is inaccessible. Table A10. presents the equipment-based electricity requirements in each process, given the 65% equipment efficiency. The results indicate that 55.77 MJ/kg energy is required in the recycling stage, which is nearly 2 times greater than the figure calculated by the previous process-based analysis. The reason for this is that most energy in the process-based recycling process is supplied by natural gas, but is totally provided by electricity in this equipment -based modeling. The shredding and calcination processes are the most energy-intensive steps, in which the binder and other impurities from the cathode materials are removed under high temperature. In addition, the gas and water treatment equipment contributes to 17.62% of the total power requirement.

3.1. Environmental assessments 3.2. Economic analysis The results indicate that the entire remanufacturing process consumes 149.80 MJ/kg energy, coupled with 10.53 kg/kg GHG emissions, while the total energy consumption for battery manufacturing from virgin materials is approximately 163.81 MJ/kg and the GHG emissions are estimated at 11.28 kg/kg. Therefore, the potential reduction of energy consumption and GHG emissions from battery production with recycled compounds are 8.55% and 6.62%, respectively. Fig. 2 shows the energy contribution of each process. More specifically, the recycling process consumes 29.61 MJ/kg energy and releases 2.37 kg/kg GHG emissions. More than half of the energy consumption in the recycling stage comes from the fuel inputs to initiate and catalyze the chemical reactions. For the cathode manufacturing process, 18.80 MJ/kg of energy is required and the associated GHG emissions are 5.83 kg/kg, accounting for 17.62% and 30.48% of the overall energy consumption and GHG emissions, respectively. Fuel inputs and materials are in roughly equal parts responsible for this total energy consumption in the cathode manufacturing process. Cathode remanufacturing dominates the total energy consumption and GHG emissions of the entire remanufacturing cycle, accounting for 62.61% and 50.15%, respectively. Among the cell remanufacturing process, 46.33% of the energy consumed is for assembling the cell, and 26.92% is contributable to the graphite anode. Noticeably, 60.29% of the energy consumed in the recycling process and 49.31% in the entire remanufacturing cycle results from

Through economic evaluation, it is found that 8.17% of costs can be saved by replacing valuable virgin materials with recycled components, where the cost for virgin production is $22.68 kg
1 and that for spent LIBs recovering is about $20.81 kg
1. The recycling process accounts for 25.91% of the total costs whereas the cathode remanufacturing comprises 17.41%. As the majority of components are purchased in the cell remanufacturing process, it is the costliest stage. The costs in the recycling and cathode manufacturing stage add up to $9.02 kg
1. Comparing this figure with the calculated costs of the cathode production with virgin materials, $1.29 kg
1 of the costs can be saved. In this sense, the reduced costs represent the benefits by recovering the Co, Ni and Mn materials, accounting for 68.86% of the total cost savings. In addition, the revenues generated by selling the recycled Al and Cu account for 4.03% and 27.11% of the total cost reductions, respectively. As visualized in Fig. 3, materials play a substantial role in the total costs, accounting for 64.28%. Besides, 6.43% of the costs are attributable to the utilities while only 1.14% are paid for the operating manpower, due to the relatively cheaper labor costs in China. Additionally, the recycling process is characterized by high initial investment costs, leading to a relatively high share of fixed charges at currently low processing quantities. As other expenses are estimated based on the fixed cost ratios, our discussion focuses on the costs of raw materials, operating labor and utilities. For the

Fig. 2. Contribution of energy consumption in the entire remanufacturing cycle.

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Fig. 3. Cost breakdown of the entire remanufacturing process.

is challenging because it heavily relies on the metal market, changing battery technology, and recycling policies. To identify the impact of the purchasing cost of spent LIBs on the economic evaluation, we assumed the price increases to $2.5 kg
1 of spent battery or decreases to $1.5 kg
1, respectively. The results show that when the price increases to $2.5 kg
1, the costs per kg of battery produced will increase to $21.91 kg
1 and when the price decreases, the overall cost will reduce to $19.70 kg
1. It is found that the elasticity of the total costs to the spent battery price is approximately 21.30%; thus, the breakeven price for spent LIBs is estimated at $2.87 kg
1. Our economic evaluation results show that battery recovering is beneficial in the current battery market, but the breakeven price, which is not likely but still possible, marks the upper limit for remanufacturing costs. 4.2. Discussion on the battery type variability

recycling phase, the dominance of the material costs is evident, which accounts for 60.03% of the total cost in this stage. Among the material expenses, the expenses paid for buying the spent batteries represent 87.05% of the overall expenditure, due to the high price of the valuable metals in these batteries. The utility fees used to purchase electricity, natural gas and process water account for 11.24%. In terms of the cathode remanufacturing process, 38.24% of the costs are attributable to the purchase of raw materials to produce the cathode, among which 88.67% are used for purchasing Li2CO3. As little energy is required and can be provided by natural gas, just 2.07% of costs are spent on utilities in this process. In the cell remanufacturing process, $12.38 kg
1 of costs are required, of which 73.76% are spent on the material purchases and 5.61% are spent on utilities, mainly for providing electricity to assemble cells. For all of the three processes, the contribution of operating labor costs is below 2%, as shown in Fig. 4. 4. Sensitivity analysis As the recycling industry is still in infancy, its environmental and economic evaluation faces significant uncertainties. A sensitivity analysis is performed to assess the impact of the main uncertainties on the evaluation results. 4.1. Discussion on the purchase price of spent LIBs As mentioned above, the spent battery is a major cost contributor in the recycling process, as well as in the overall recovering cycle. However, the prediction of the spent LIB purchasing price

Fig. 4. Cost of raw materials, utilities and operating labor in each process.

The baseline scenario assumes that only spent NMC 111 batteries are collected to reproduce NMC 111 cells. However, LIB technology is ever-evolving to achieve better performance, leading to the diversity of batteries in the market, as well as in the recycled collections (Wang et al., 2014). In particular, NMC 532 and NMC 622 materials are entering the battery industry as the next-generation cathodes as battery manufacturers are moving towards less-Co and high-Ni compounds to lower manufacturing costs. Therefore, different scenarios concerning the variety of battery types of collected batteries and desirable products are discussed. NMC 111 and NMC 622 batteries are chosen as the representative types, as they represent the state-of-art and high-Ni trend of battery technology, while NMC 532 batteries can be regarded as the intermediate state of them. The mixing degree refers to the proportion of undesirable batteries collected, e.g., the proportion of NMC 622 batteries in the NMC 111 battery remanufacturing scenario. The changes in energy consumption and cost with the mixing degree are represented in Fig. 5, while changes in GHG emissions are not plotted as they closely resemble that of energy consumption. To remanufacture NMC 111 batteries, the energy consumption monotonically increases with the increasing mixing degree. Furthermore, when all collected batteries are NMC 622 batteries, the 171.42 MJ/kg of energy consumption overpasses that of processing with virgin materials, which is 163.81 MJ/kg. However, when NMC 622 batteries are targeted to be produced, the energy consumption first increases, then decreases once the mixing degree nears to 50%. The minimum energy consumption is located where mixing does not occur. Notably, no matter how it is mixed, energy consumption for recovering NMC 622 batteries is always below that of the production with virgin materials, consolidating the environmental benefits of battery recovery, even if battery technology is moving towards less Co-containing materials. From the economic perspective, the cost-change trend of NMC 111battery remanufacturing is similar to that of NMC 622 remanufacturing and the cost rises up and then decreases with the increasing of the mixing degree. The superior case is when the mix is prevented, while the worst scenario is when the mixing ratio is approximately 70% for NMC 111 and 30% for NMC 622 battery remanufacturing, that is, 70% of the collected batteries are NMC 622 batteries. For remanufacturing of NMC 111 batteries, the costs in mixing scenarios are all higher than the non-mixing scenario, but less than the production costs with virgin materials. When the mixing degree is higher than 80% for remanufacturing of NMC 622 batteries, the cost is even less than the non-mixing scenario. The key point of this sensitivity analysis is that producing NMC 622 batteries with a certain amount of mixed spent NMC 111 batteries is both environmentally beneficial and economically viable. This conclusion exactly conforms to the practical needs. Additionally,

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Fig. 5. Impact of the battery type variability on energy consumption and costs.

the results inspire that more efforts can be focused on developing comprehensive recycling process lines, so as to simultaneously recover various battery chemistries (Yang et al., 2017). 4.3. Discussion on the recovery efficiency This paper claims a uniform 90% recovery rate for the recyclable metals, but according to relevant policy, the comprehensive recovery rates should be up to 98% for Co and Ni under the hydrometallurgy process (MIIT, 2016). Besides, some previous experimental researches also indicate recovery efficiencies could approach 100% (Song et al., 2017). Thus, higher efficiencies (98%) for recyclable metals are assumed to conduct a sensitivity analysis. The results indicate that the energy use is approximately 147.63 MJ/ kg of cell produced, which is only 1.45% less than the baseline scenario, along with 1.04% less GHG emissions. Additionally, the overall cost reduces to $20.31 kg
1, which is 2.40% less than the baseline scenario. Therefore, the environmental and economic benefits gained by improving the recovery efficiency are limited. 5. Discussion and conclusions This paper performs a quantitative analysis to explore the environmental and economic feasibility of battery remanufacturing. The results show that, through hydrometallurgy routes, 149.80 MJ/kg energy is consumed, coupled with 10.53 kg/kg GHG emissions for the remanufacturing of NMC 111 batteries, representing an 8.55% energy reduction and a 6.62% GHG emission mitigation in comparison to the battery production with virgin materials. In addition, a reduced cost of $1.87 kg
1 is realized by using recycled materials in production, displaying high economic effectiveness. Besides, our results confirm the economic feasibility of battery remanufacturing under a 65% production scale, which large battery recyclers in China have already surpassed. As the number of batteries available for recycling increases, along with the development of automatic technology and increasingly advanced mechanics, higher operating efficiencies and more cost reductions can both be achieved. From the environmental aspect, the recycling stage accounts for approximately 20% of the total energy consumption, among which over 60% is consumed by fuel inputs. Besides, the calciner, gas and

water treatment are the major contributors to energy consumption, all of which are used to remove impurities or dispose of waste. Cathode remanufacturing and cell remanufacturing respectively account for 17.62% and 62.62% of the total energy consumption. From the economic standpoint, 25.91% of the total costs are spent in the recycling stage, while the cathode remanufacturing and cell remanufacturing stages account for 17.41% and 59.48%, respectively. The recovery of Co, Ni and Mn materials contribute to 68.86% of the total cost savings and the revenues generated by selling the recycled Al and Cu account for 4.03% and 27.11%, respectively. The robustness of the results is confirmed by the sensitivity analysis regarding the purchase price for spent batteries, the variability of the battery type, and the recycling efficiency for different recyclable materials. The purchase price for spent batteries plays a determinative role in the profitability of battery recovery. It is estimated that the recovering would be economically sound until the purchase price for spent batteries reaches $2.87 kg
1. Through the discussion of the battery type variability, it is found that producing NMC 622 batteries with a certain amount of mixed spent NMC 111 batteries is not only environmentally and economically beneficial, but also aligns with the practical needs. The consequential reduction from higher recovery efficiencies is below 3%, for both environmental and economic aspects, but higher efficiencies would still be favored to maximize the potential benefits. Our findings quantitatively validate the environmental and economic viability of battery remanufacturing, which facilitates to boost the EV battery recycling adoption. However, several gaps remain to be covered by further research, and some challenges need to be solved in industrial practice. For example, experimental research should be conducted to test whether the quality of remanufactured LIBs is able to reach the same quality level as that produced with virgin materials. Moreover, this study only focuses on the hydrometallurgical recycling process because it is the prevailing approach in the current industry. Further analysis could be expanded to cover other recycling approaches or other reagents used in the recycling process if more relevant information is available. Declaration of Competing Interest The authors declared that there is no conflict of interest.

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