Full length articleRecycling of lithium, cobalt, nickel, and manganese from end-of-life lithium-ion battery of an electric vehicle using supercritical carbon dioxide
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Introduction
In 1991, Sony Corporation announced a new product called lithium-ion battery (LIB), which has several advantages including high energy density, low self-discharge rate and low maintenance. Nowadays, LIB dominates the market of portable electronics and electric vehicles.(Windisch-Kern et al., 2022) Electric vehicles (EV) are a key technology to mitigate the climate change impact from internal combustion engine vehicles.(Shafique et al., 2022) According to the International Energy Agency (2019), Bloomberg-NEF (2020) and Deloitte Insights (2020), the annual sales of EVs are projected to be 21–31 million cars in 2030 compared with one million cars in 2017.(Harper et al., 2019; Shafique et al., 2022) With conservative assumptions of an average battery pack weight of 250 kg and volume of 0.5 m3, the projected wastes would comprise around 5,250,000 tons and 10 million m3 of unprocessed waste in 2030.(Harper et al., 2019) Because of the large growing quantity of LIBs along with environmental and safety concerns, as well as valuable metals facing limited supply, effective recycling of end-of-life (EoL) LIBs has become imperative.
The high content of lithium (Li), nickel (Ni), manganese (Mn), and cobalt (Co) in EoL lithium-nickel-manganese-cobalt oxide (NMC) type LIB, widely used in EVs, can be regarded as a secondary resource for these metals.(Zhang et al., 2018). The typical life cycle of an EV battery is illustrated in Figure 1. The used batteries are removed from EVs and sent to either junkyard or battery refurbishing company for second-life application in stationary energy storage, or to raw material extraction and processing facilities. Although used batteries can have second-life applications, they eventually reach end-of-life and can be recycled which would in turn enable the circular economy. One ton of Li can be extracted from every 28 tons of EoL LIB, which compares favorably with the consumption of the primary resources, e.g., 250 tons of minerals or 750 tons of brine per ton of Li.(Larcher and Tarascon, 2015) The content of Ni, Mn and Co in NMC type LIB can be in the range of 10–30 wt%, 10–30 wt% 5–20 wt%, respectively. A LIB comprises the positive and negative electrodes, the electrolyte, separator, and casing. The positive electrode (cathode) is usually made of Li metal oxides, such as LiNixMnyCozO2 (NMC, x + y + z = 1), attaching to aluminum foil (current collector). The negative electrode (anode) is usually made of graphite and carbonaceous materials attached to copper foil (current collector). The electrolyte is composed of a soluble Li salt (e.g., LiClO4, LiBF4, LiPF6) and non-aqueous organic solvents (e.g., dimethyl sulfoxide, propylene carbonate), which allow Li+ to move through the separator.(Nayaka et al., 2016) Hence, the cathode material in LIB is usually targeted for recycling due to its embedded economic value from the metals.
Conventional recycling processes of LIBs mainly comprise three steps: pre-treatment, metal extraction, and product separation (Chan et al., 2022). The pre-treatment involves battery casing removal, separation of cathode material from the electrolyte, separator, current collectors and carbonaceous additives (Or et al., 2020). The disassembly of batteries is a complex task due to variations in battery design, size, and volume. In the DeMoBat project conducted at the Fraunhofer Institute, they investigated disassembly strategy and optimized them using heuristic optimization algorithms (Baazouzi et al., 2021). They also argued for the need of industrial disassembly systems to reach higher levels of circularity and move away from the rather simplistic treatment of EoL batteries with rough manual disassembly (Glöser-Chahoud et al., 2021). There are two basic pre-treatment approaches in industry before metal extraction. The first is the robust pyro-metallurgical processes that can treat different battery chemistries in a single process (Glöser-Chahoud et al., 2021). The second is the mechanical treatment which plays a minor role in industry as it is sensitive to contamination, requiring detailed disassembly and preselection of battery chemistries (Glöser-Chahoud et al., 2021). A fully automated disassembly process requires product specific investment that can be economically challenging because of constant changing production environments; therefore, many dismantling processes are still carried out manually or human robot collaboration approach (Herrmann and Kara, 2018). The next step is metal extraction, which can be achieved with different types of processes including pyrometallurgy, hydrometallurgy and bio-metallurgy. Among these techniques, hydrometallurgy is more promising for the recovery of valuable metals (Li, Ni, Mn and Co) from the cathode material using acids (e.g., HNO3, H3PO4, H2SO4, and HCl) along with reducing agents (e.g., H2O2, NaHSO3) (Chan et al., 2021). The last step is product separation that can be achieved using selective precipitation, solvent extraction, or electrodialysis (Chan et al., 2022; Lv et al., 2018). Direct recycling is another recycling approach for active materials in LIB without using hydrometallurgical processes for metal extraction and purification, it focuses on the extraction of the active material can be directly reintroduced into cell production. This approach can save energy and resources in production, but it requires high homogeneity of the cell chemistry of the processed batteries. In addition, the direct recycling approach cannot meet the technological development of cell chemistry since predominant active materials and their respective material ratio changes over time (Glöser-Chahoud et al., 2021).
There are many aspects that can be improved in the recycling process of LIBs. Hydrometallurgical processes are more benign in metal extraction than pyrometallurgical processes which require high energy input and emit hazardous gases (Joulié et al., 2014). However, hydrometallurgical processes utilize large amounts of alkaline or acidic media in combination with reducing agents to dissolve the EoL LIB in the leaching step, which generates hazardous waste stream (Bertuol et al., 2016).
Recently, the application of SCFE has gained interest for recycling strategic materials from end-of-life products because of its sustainability and efficiency (e.g. nickel metal hydride battery (Yao et al., 2017), neodymium iron boron magnet (Zhang et al., 2018) and fluorescent lamp (Shimizu et al., 2005; Zhang et al., 2022)). The SCFE is a mature technology. and it has been commercialized around 1980s to 2000s focusing on energy, chemical, food and pharmaceuticals (Zhang et al., 2022). The SCFE for metal was initially reported in 1990s (Laintz et al., 1992, 1991; Wai et al., 1993), the scholars introduced the concept of complexation of metal ions and organic liquid for improving the solubility of metal in supercritical carbon dioxide. Supercritical fluids have advantageous properties for extraction, such as low viscosity, high diffusivity, and solvation; hence, they are effective solvents for metal extraction. Supercritical carbon dioxide (sc-CO2) is the most used solvent, because of its moderate critical temperature (Tc = 31.1 °C) and pressure (7.37 MPa). In addition, sc-CO2 is inert, easily accessible, inexpensive, and easy to vent or conveniently recyclable by phase separation from products after extraction and depressurization (Yao et al., 2017; Zhang et al., 2018; Zhang et al., 2022).
Currently, the number of studies on using SCFE for recycling of LIBs is limited. There are only four studies in this field, and three focused on the recovery of the electrolyte from LIBs using SCFE (Grützke et al., 2014; Grützke et al., 2015; Liu et al., 2014). Feasibility tests were first conducted using sc-CO2 for extraction of electrolytes from commercial 18650 cells (a commercialized type of LIB) in a static autoclave (Grützke et al., 2014). The results confirmed the suitability of SCFE for the recovery of the organic carbonate solvents of LIB electrolytes. The same group developed a flow-through method for the extraction of LIB's electrolytes using sc-CO2 and liquid CO2 in the presence of different solvents and the results showed that extraction time and recovery rates significantly improved compared with their prior reported static extraction system (Grützke et al., 2015). Another group used the response surface methodology for optimizing the SCFE process for the recovery of the electrolyte from LIBs (Liu et al., 2014). They investigated pressure of 15–35 MPa, temperature of 40–50 °C and static extraction time of 45–75 min. The optimum extraction efficiency was 85% at 23 MPa, 40 °C, and 45 min.
Only one study focused on the metal recovery from the cathode material of LIBs using sc-CO2 (Bertuol et al., 2016). They used H2SO4 and H2O2 as extractant and they found higher Co extraction using SCFE than atmospheric pressure. The use of supercritical conditions reduced reaction time from 60 min (time required for 95% extraction of Co at atmospheric pressure) to 5 min, and it also reduced the consumption of H2O2 by half. This process is not standalone SCFE process rather a pressurized extraction process or supercritical fluid assisted extraction, because the acid solution does not dissolve in sc-CO2 (non-polar). The polarity difference exists between the solute and solvent, which can hinder the extraction of metal ions and organometallic compounds. Complexing (chelating) agents with high solubility in sc-CO2 are used to form complexes with the metal ions for accommodating the polarity difference and satisfying charge neutrality (Ding et al., 2016; Wai and Waller, 2000). Tributyl phosphate and nitric acid (TBP-HNO3) adduct was reported to be suitable for complexation with metals in sc-CO2 solvent. Stable hydrophobic complexes can form and dissolve in sc-CO2 up to 11 vol. % (Baek et al., 2016). Previous studies have successfully developed SCFE processes using TBP-HNO3 adduct for the extraction of rare earth metals from EoL products, such as neodymium-iron-boron magnet (Zhang et al., 2018), fluorescent lamps (Zhang et al., 2022) and nickel-metal hybrid batteries (Yao et al., 2017). They also found that other metals, such as iron (Fe) and aluminum (Al), can be extracted using the same system by tunning experimental conditions (Zhang et al., 2018; Zhang et al., 2022). It can be hypothesized that a similar SCFE process can extract Li, Ni, Mn and Co from EoL LIBs. Tributyl phosphate is a common extractant for Ni and Co ions in liquid-liquid extraction, as it has functional groups with strongly electron-donor oxygen atoms that can form complexes with diverse structure (Bogacki and Gajda, 2007).
In this work, a SCFE process that utilizes sc-CO2 in combination with TBP-HNO3 adduct is developed for the extraction of Li, Ni, Mn, and Co from an EoL LIB of an EV. A full factorial design of experiment is utilized for determining the effect of operating parameters including temperature (X1, °C), pressure (X2, MPa) and adduct to sample (A/S) ratio (X3, mL/g) and to optimize the process. Thorough characterizations of the cathode material before and after SCFE were conducted for elucidating the extraction mechanism. The results of this study confirm the feasibility of using SCFE for recycling of LIBs. The SCFE process consumes less hazardous chemicals compared with hydrometallurgical processes, and the CO2 solvent can be recycled by depressurization and phase change; thus, it is a greener and more efficient alternative for recycling of LIBs.
Section snippets
Materials
An EoL LIB (40 Ah high power superior lithium polymer cell) used in electric vehicle (confidential) was obtained from eCamion. The cell was disassembled from a 36 cells module, and the cell was in end-of-life state after over 500 discharge/recharge cycles. Tri-n-butyl phosphate (TBP, ≥ 98%) and nitric acid (HNO3) (15.7 M, 70 wt%) were acquired from VWR. Carbon dioxide (CO2, grade 4.0) was acquired from Linde Canada. Lithium nickel manganese cobalt oxide powder (<0.5 μm, >98%) was acquired from
Characterization
The cathode material comprises the following elements: nickel (Ni, 23.2 wt%), manganese (Mn, 18.9 wt%), cobalt (Co, 13.0 wt%), lithium (Li, 6.8 wt%), aluminum (Al, 0.12 wt%) and copper (Cu, 0.04 wt%) (Figure 2 a). To provided further proof of the reproducibility, the cathode material composition from another cell disassembled from the same module was characterized, and the chemical composition of the two cells were the same. According to ICP-OES results, the cathode material can written as Li(Ni
Conclusions
A SCFE process is developed for extraction of Li, Ni, Mn, and Co from an EoL LIB (NMC type, decommissioned from an electric vehicle after over 500 charge/discharge cycles) using sc-CO2 along with TBP-HNO3 adduct and H2O2 reducing agent. A fully crossed design was used to study the effect of operating parameters on extraction efficiency. This study used empirical model to optimize the process and 90% extraction efficiency for Li, Co, Mn, and Ni from the cathode material at 60°C, 31 MPa, 30 min,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the financial support provided by Natural Sciences and Engineering Research Council of Canada (NSERC) (Grant No. 498382) and Ministry of Economic Development, Job Creation and Trade (Grant No. 504535). We thank Dr. Raiden Acosta for help with XRD and Dr. Peter Brodersen for help with XPS analyses. We thank eCAMION for providing the end-of-life lithium-ion battery to support this study.
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