Biomass-Assisted Reductive Leaching in H2SO4 Medium for the Recovery of Valuable Metals from Spent Mixed-Type Lithium-Ion Batteries
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A hydrometallurgical method involving natural biomass waste as reductant was proposed for the treatment of spent mixed-type lithium-ion batteries. Results showed that almost complete dissolution of Li, Ni, Mn and nearly 90% dissolution of Co were achieved under the optimal conditions of H2SO4 concentration of 2 M, waste tea biomass dosage of 0.3 g/g, solid/ratio of 50 g L−1, temperature of 90°C and time of 120 min. The leaching kinetics was further investigated, and the activation energies were determined to be 1.7 kJ mol−1, 10.3 kJ mol−1, 10.1 kJ mol−1 and 10.9 kJ mol−1 for Li, Ni, Mn and Co, respectively. The cathode materials before leaching and the leaching residue were characterized with different analytical methods. The characterization results confirmed that the addition of the waste tea acted as reductant and resulted in better dissolution of the metals, supporting the principles of sustainable processes by decreasing the chemical consumption and integrating waste into a secondary use.
Lithium-ion batteries (LIBs) are currently essential components of modern technology and are used extensively as electrochemical power sources in portable electronics and hybrid and electric vehicles due to their characteristic light weight, high energy density and good performance.1, 2, 3 Such increased consumption and related reduction in the average battery lifespan have led to a significant amount of related end-of-life LIBs. This increasing amount of spent LIBs has resulted in them becoming the fastest growing electronic waste worldwide, which has created a global environmental issue.4, 5, 6, 7 Attempts in recent years to tackle the recycling of end-of-life LIBs have proved to be technologically challenging for a number of reasons: (1) they comprise a mixture of different elements that are highly integrated together; (2) the battery chemistry varies by manufacture; and (3) the chemical composition of these secondary materials is typically not available to recyclers. These factors require that recycling companies adapt to a continually evolving waste stream composition that primarily results from the non-standardized improvement of battery electrodes. Consequently, when the diversity of the end-of-life battery types and contemporary industrial sorting processes are considered, a focus on the effective treatment of spent mixed-type LIBs should enhance the possibilities for new and efficient industrial-scale battery recycling processes.
Moreover, reductants are used to achieve higher rates of valuable metals extraction from spent LIBs.19 Inorganic reductants, like H2O2,19 NaHSO3,26 and Na2S2O5,16 have been previously considered to offer high efficiencies. Nevertheless, H2O2 can be easily decomposed to oxygen gas, which reduces efficiency and complicates leaching operations, whereas the use of NaHSO3 or Na2S2O5, introduces impurities like Na+ that not only increase operational costs but also affect final product purity. In contrast, alternative organic reductants like ascorbic acids,18,27 glucose,28 and cellulose,29 have shown beneficial properties that include easy degradation and less secondary environmental pollution risk. Additionally, biomass has also been increasingly utilized as organic-based reductants due to their non-hazardous nature, good reactivity, availability and low cost.30
In this investigation, the possibility and the leaching kinetics of the cathode materials from spent mixed-type LIBs using a waste tea biomass as reductant in sulfuric acid medium were explored. Moreover, the tentative leaching mechanism was investigated by characterization of the materials before and after leaching by using x-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Overall, this study highlights that the use of waste tea as a biomass reducing agent in acid leaching offers a low cost and environmentally friendly approach for the recovery of valuable metals from the cathode materials of spent mixed-type LIBs.
Materials and Methods
Chemical composition of the calcined cathode materials
Waste tea biomass sourced from Changsha was dried and milled prior to use. Waste tea is a multicomponent mixture. It is known that crude protein, saccharides, and tea polyphenols are the main components within waste tea.30,32 Tea polyphenols have reducibility and the major polyphenols, e.g., catechins, are commonly composed of (–)-epicatechin (EC), (–)-epigallocatechin (EGC), (–)-epicatechin gallate (ECg) and (–)-epigallocatechin gallate (EGCg), etc.33
All chemical reagents used throughout the experiments were of analytical grade, and all the solutions were prepared or diluted using distilled water.
Concentrations of the metal ions were analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES; SPECTROBLUE SOP). Carbon contents were determined using a carbon and sulfur combustion–infrared absorption analyzer (CS-600; Leco, USA). The calcined cathode materials and the dried leaching residue were characterized by XRD (Rigaku D/max-2500) and SEM (TESCAN MIRA3 LMU) equipped with an energy dispersive spectrometer (EDS; Oxford X-Max20). FT-IR (Bruker) was used to identify the relevant vibrational bands in the range between 4000 cm−1 and 400 cm−1.
Results and Discussion
Optimized Leaching Conditions
As the initial H2SO4 concentration is increased from 0.5 M to 2 M, the leaching efficiencies of Li, Ni, Mn and Co are observed to increase from 92%, 73%, 86%, and 43% to 97%, 97%, 96% and 83%, respectively (Fig. 2b). When the H2SO4 concentration is further increased to 3 M, the leaching efficiency of Co reaches 92%, while for the other metals it remains nearly constant. Therefore, an initial H2SO4 concentration of 2 M is deemed to be optimal for the subsequent leaching experiments.
In contrast, when the S/L ratio is varied from 10 g L−1 to 125 g L−1, the leaching efficiencies of the metals display a decreasing trend (Fig. 2c). The leaching efficiencies for all the metals remain relatively unchanged when the S/L ratio is less than 50 g L−1. It is known that higher S/L ratios would increase the concentration of metal ions in the leaching solution.34 However, the leaching efficiencies of the metals are reduced, for instance, at a S/L ratio of 125 g L−1, extraction of Li, Ni, and Mn decrease to 93%, 91%, and 91%, respectively, whereas Co is only approximately 67%. The reason for this observation is two-fold, as increased slurry densities due to higher S/L ratio decrease the available surface area per unit volume of the leaching solution, which results in insufficient metal leaching, especially for Co. Consequently, a S/L ratio of 50 g L−1 is taken as the optimum value for the following experiments.
Figure 2d illustrates the influence of temperature on the leaching efficiencies of the metals between 40°C and 95°C in 2 M H2SO4, reductant dosage of 0.3 g/g, S/L ratio of 50 g L−1 and 60 min. As expected, the leaching efficiencies of Li, Ni, Mn and Co are improved with the increase of temperature. This is attributed to the fact that the elevated temperature allows more energetic and more frequent collisions that accelerate the leaching process. These results also indicate that a temperature of 90°C appears to be sufficient to maximize the leaching of the valuable metals.
Dissolution Kinetics of Li, Ni, Mn and Co
The leaching kinetics were further investigated by selecting a number of data examples and subjecting them to analysis via different separate models, which included the shrinking core model, the empirical logarithmic equation, and the Avrami equation. Further details of the equations related to these models are outlined in supplementary Table S-I.
Linear fitting experiments were carried out with the leaching data shown in Fig. 3. The correlations for Ni, Mn Co and Li with relatively low values of correlation coefficients (R2) suggest that none of the kinetic data of any of the leached metals conform to the shrinking core model (see supplementary Figs. S-1 and S-2). The values of R2 for Ni, Mn, Co and Li with the logarithmic rate law model (supplementary Fig.S-3) are correspondingly higher than 0.97, 0.94, 0.96 and 0.93, respectively, with the temperature ranging from 60°C to 90°C. Nevertheless, when the temperature is lower than 60°C, the logarithmic rate law model does not fit for Li and Co because of the low values of R2 (lower than 0.81 and 0.77). The plots of ln(− ln(1 − x)) versus lnt (in supplementary Fig. S-4) for Ni, Mn, Co and Li,show a good linear relationship for all the tested temperatures with R2 higher than 0.96, 0.93, 0.96 and 0.95, as listed in supplementary Table S-II. According to the kinetics studies reported by Li et al.35,36 when the slopes of the lines, the ‘n’ in the Avrami equation, are less than 0.5, the leaching is controlled by internal diffusion.
Characterization of Calcined Cathode Materials and Leaching Residue
It can be clearly observed that the major phases (LiCoO2, Li0.9Ni0.5Co0.5O2−x, and Li4Mn5O12) in the calcined cathode materials in Fig. 5a disappear when comparing the phases in the leaching residue in Fig. 5b. This confirms that the crystalline structure would break down to form water-soluble sulfates. It has been reported that the leaching efficiency of Co depended on the concentration of reductant used, and the species of Co2+ would be stable in the acidic solution.37 Furthermore, a newly formed CoSO4 H2O phase is also detected in the leaching residues, which implies that the dissolved Co2+ ions may be adsorbed to unreacted waste tea.38
A sustainable approach for the recovery of valuable metals from spent mixed-type LIBs has been investigated using waste tea biomass as a reductant in H2SO4 medium. The dissolution of Li, Ni, Mn and Co were found to be facilitated with the increase of acid concentration, temperature and time in the presence of waste tea biomass. The optimum leaching conditions were found to be H2SO4 concentration of 2 M, waste tea biomass dosage of 0.3 g/g, solid/ratio of 50 g L−1, temperature of 90°C and time of 120 min. Kinetic studies showed that the leaching of the metals followed the Avrami equation. The calculated activation energies of Li, Ni, Mn and Co were determined to be 1.7 kJ mol−1, 10.3 kJ mol−1, 10.1 kJ mol−1 and 10.9 kJ mol−1, respectively. According to the characterization results, the main substances contained in waste tea biomass could act as efficient reductants to accelerate the breakdown of active material lattice structures like LiCoO2, LiMn2O4, and LiNixCoyMn1−x−yO2. It can be concluded that the utilization of waste tea biomass would not only give a credible alternative to currently used chemical reductants but also provide an environmentally friendly route for the leaching of valuable metals from spent LIBs.
Open access funding provided by Aalto University. This work has been supported by Anhui Province Innovative Engineering Project for New Energy Vehicles and Intelligent Connected Vehicles in China. The authors would like to acknowledge the funding support from the Chinese Scholarship Council (No. 201806370026) and BATCircle (Grant Number 4853/31/2018) in Finland.
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