Hydrometallurgical Synthesis of Nickel Nano-Sulﬁdes from Spent Lithium-Ion Batteries

: By developing technologies, spent lithium-ion batteries (LIBs) are considered a great source of nickel-cobalt ions to fabricate metal sulﬁde nanoparticles. However, the presence of unnecessary ions such as manganese and lithium may hamper the formation of pure metal sulﬁde nanostructures where the utilization of proper ligand(s) can be very critical. For this reason, the present study initially introduces a promising and cost-effective acidic leaching followed by precipitation and thermal treatment (~250 ◦ C, 1 h) processes to fabricate pure NiS nanopowders from the LIBs, while considering potassium amyl xanthate as the precipitating agent. The produced nanoparticle’s size and structure were characterized by commonly used and advanced analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photon correlation spectroscopy (PCS), and inductively coupled plasma optical emission spectrometry (ICP-OES). The elemental distributions and functional groups were analyzed through energy dispersive X-ray spectroscopy (EDS) and Fourier-transform infrared spectroscopy (FTIR) methods. The characteristic analyses of the spherical-like NiS particles showed a mean particle size of 61 ± 2 nm (by TEM) and 64 nm (through the PCS method) with high purities approved via XRD and EDS analyses. Moreover, comparing the obtained XRD data with the standard ones showed a perfect match with the hexagonal structure of the NiS phase. We ﬁnally concluded that such a presented approach is very practical and efﬁcient for converting the LIBs to an extremely valuable product. Future studies are suggested for the fundamental conceptualization of the processes.


Introduction
The annual production of lithium-ion batteries (LIBs) has considerably augmented due to the sharp increase in growth of electric vehicles and electronic devices over the last decades [1]. The high specific capacity and electrochemical specifications of nickel and cobalt transition metal chalcogenide compounds make them highly applicable for the LIBs. Among the cobalt and nickel cathode materials used in the LIBs, LiCoO 2 and LiCo x Ni 1−x O 2 (0 ≤ x ≤ 1) are the most common compositions. From an environmental point of view, LIBs are composed of active ingredients, organic compounds and inorganic compounds, which must be properly managed. The economical aspects represent the LIBs as a recyclable resource for rare metals (e.g., nickel, cobalt and lithium). To support the environment and economy, some governments have restricted landfill disposal and mandated the collecting and recycling of used rechargeable batteries. To manage these constant (K sp ) is an essential physicochemical parameter affecting the selective extraction of metals in hydrometallurgical methods. Different factors influence the K sp including solution pH, available anions, cations and organics. In this regard, various chemical reactions including metal ion hydrolysis, metal hydroxide precipitation and the formation of soluble metal xanthate complexes may occur [46,47]. A low decomposition temperature (~200 • C) and high solubility lead xanthates to be suitable ligands for synthesizing various metal sulfide nanomaterials [31]. While having no influence on manganese and lithium, xanthate ligands' chemical reaction with nickel and cobalt results in the precipitation and formation of nickel and cobalt xanthates in acid leached LIBs [48]. Further thermal treatments of the formed nickel and cobalt xanthate decompose the organic part of the metal's xanthate composition, and the nano-sulfur metal can remain.
The present study aims to fabricate nanostructure nickel sulfides from spent LIBs. In this regard, two environmentally harmful heavy metals such as nickel and cobalt are recovered and transformed into usable materials. The novelty of the current study is to synthesize NiS nanoparticles through a hydrometallurgical process combined with a thermal treatment using a xanthate ligand. To the best of our knowledge, this paper is the first novel work to address the high potential selectivity of xanthates as ligands to recover metals during hydrometallurgical procedures.

Materials and Reagents
In this work, cathode materials (CMs) were collected from spent LIBs (Samsung galaxy core prime battery, cellphone battery). The chemical analysis of the CM presents a chemical combination of the battery containing 19.9% Co, 27.7% Mn, 2.6% Ni, 4.6% Li and 45.2% C (as graphite). The other chemical reagents used in this study are sodium chloride (NaCl), sulfuric acid (H 2 SO 4 ) as the leaching agent, potassium amyl xanthate (99.5%, CH 3 (CH 2 ) 4 OCS 2 K, PAX) for the complexation of the PLS and ammonium hydroxide solution (NH 4 OH, 25-30%) for the washing process. All chemicals were purchased from the local suppliers (Kerman, Iran) with analytical grades.

Characterization Methods
The mineralogical identification of generated nanoparticles was studied with X-ray diffraction (XRD, X'pert-MPD Philips equipped with a detector using Co-Kα radiation, Westborough, MA, USA). The particle size distribution and morphology of the nanoparticles were analyzed through transmission electron microscopy (TEM; Philips CM20, Hamburg, Germany) and scanning electron microscopy (SEM, Tescan Vega-II, TESCAN, Brno, Czech Republic), respectively. Furthermore, the elemental distributions of the nanopowders were characterized by a multipoint energy-dispersive X-ray spectrometer (EDS, Inca 300, Oxford Instruments, Oxford, UK). A Fourier-transform infrared (FT-IR) spectrometer (Bruker Tensor 27 instrument with RT-DLATGS detector, Berlin, Germany) was used to identify the functional groups in the final product in the wave range of 400-4000 cm −1 . The pH of the PLS was continuously monitored by a pH-meter (827 pH Metrohm, Herisau, Switzerland). Additionally, inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer 7100 DV, PerkinElmer, Waltham, MA, USA) was used to exactly determine the elemental composition of the PLS and final product. TG-DTA (TG 209 F3 Tarsus ® , NETZSCH, Selb, Germany) was employed to determine the mechanism of decomposition of the nickel xanthate complex into the final NiS nanopowder.

Preparation of Spent Batteries
The LIBs were initially soaked into 1 M NaCl salt solution for eight hours followed by a drying process that lasted for 12 h at 60 • C. The cathode part of the LIBs was thereafter manually disassembled and sliced into 50 mm × 50 mm parts. Furthermore, muffle furnace (KLS-M, Tuttlingen, Germany) preheating (300 • C for 30 min) was applied to decompose the binder and other organic materials while simultaneously cathode materials were separated from the aluminum foil. Under this condition, cathode recovery from graphite was accomplished freely.

Leaching Process
Acidic leaching experiments (at 2 M H 2 SO 4 ) were performed within a 1 L stirred tank reactor (STR) at 85 • C, 700 rpm and 20% (w/v) pulp density for 120 min. Through a direct optimization process, 2% (v/v) oxygenated water (H 2 O 2 ) resulted in the highest recovery of nickel and cobalt (above 90%) due to its high reducing power in converting Co 3+ to Co 2+ and significantly increasing its dissolution from the recalcitrant crystalline structure of LiCoO 2 [49]. Table 1 presents the elemental compositions of the PLS and the filtered composition obtained using the (ICP-OES) analysis. Figure 1 shows a top-down and stepwise operational diagram of the synthesis of NiS nanoparticles. The prepared PLS underwent complexation using potassium amyl xanthate (1000 ppm) to obtain the coprecipitated nickel and cobalt xanthate residue. The filtered-precipitated residue was then washed up with ammonia solution (2M NH 4 OH) in order to separate the precipitated Ni from Co in the form of nickel ammonia sulfate. While nickel xanthate has a lower solubility (K sp ) over cobalt xanthate, manganese and lithium ions do not react with the xanthate ligands and remain as sulfate phases [50,51]. To again form a nickel xanthate complex, 1000 ppm PAX was added into the nickel-ammonia sulfate complex to precipitate a black-green residue. The cobalt xanthate (from the initial stage) and nickel xanthate (from the secondary stage) residues were filtered and subsequently washed with distilled water and ethanol and finally dried at 44 • C before thermal treatment in a muffle furnace (220 • C, 1 h) for the fabrication of overall nanostructure nickel and cobalt sulfides. It is worth noting that in this research work the main emphasis was on recycling nickel as NiS nanoparticles in particular. Therefore, the production of other existing metal ions such as cobalt, manganese and lithium is under investigation and will be the subject of presentation in future works.

X-Ray Powder Diffraction (XRD)
The XRD patterns for the as-synthesized NiS are shown in Figure 2. The characteristic diffraction peaks completely match the hexagonal structure of the NiS phase (Crystallography Open Database (COD) code: 1010435) with lattice parameters of a = 3.4 nm, b = 3.4 nm and c = 5.3 nm (space group P 63/m m c, #194). As shown in Table 2, the mean crystal-

X-Ray Powder Diffraction (XRD)
The XRD patterns for the as-synthesized NiS are shown in Figure 2. The characteristic diffraction peaks completely match the hexagonal structure of the NiS phase (Crystallography Open Database (COD) code: 1010435) with lattice parameters of a = 3.4 nm, b = 3.4 nm and c = 5.3 nm (space group P 63/m m c, #194). As shown in Table 2, the mean crystalline size was estimated to be approximately 23.45 nm according to the Scherrer equation: D = (Kλ)/(βcosθ) [52,53], where K is the grain shape factor (K= 0.98), λ is the incident X-ray wavelength (0.15418 Å), θ is the diffraction angle in degrees and β is the full width at half-maximum intensity (FWHM) in radians [19,52,54].    Figure 2. XRD patterns and crystallographic depiction of hexagonal nickel sulfide (NiS) nanoparticles.
The Rietveld method may calculate the crystallite size and/or microstrain of the as-synthesized NiS based on the change of the profile parameters when compared to a standard NiS crystal (Reference code: 96-101-0436). It is worth mentioning that the profile parameters depend on the instrumental settings used for the data collection and on the profile function used for the refinement (Size/strain analysis, help of software Malvern Panalytical's HighScorePlus, UK). Figure 3 demonstrates the TGA-DTA analysis of the nickel-xanthate complex. The first derivative of the curve manifested the occurrence of decomposition in a single step. The experimental mass lost at 220 • C was slightly lower (72.88%) than the lost amount that was calculated (76.36%) for the NiS formation from the complex. The XRD indicated that the NiS was the only phase formed ( Figure 2). However, the small discrepancy between the measured and calculated weight that was lost could be partially explained by the presence of water adsorption, sulfate and possibly other xanthate decomposition products on the surface of the NiS nanocrystals.  The straightforward diagram of the thermogravimetry (TG) analysis was in fact implicitly indicative of the purity of the Ni(S2CO(CH2)4CH3)2 complex, whose thermal decomposition at 220 °C resulted in the production of a pure and uniform NiS nanopowder. It is worth noting that the thermal analysis showed that a temperature of 220 °C was sufficient and suitable to completely convert the amylxanthate into the sulfide (Figure 3). Based on the TGA-DTA analysis, the reaction mechanism of metal sulfide formation from the nickel-xanthate complex pathway was proposed as presented in Figure 4. By adding the xanthate solution to the PLS in a dropwise manner, the following reaction actually occurred:   The straightforward diagram of the thermogravimetry (TG) analysis was in fact implicitly indicative of the purity of the Ni(S 2 CO(CH 2 ) 4 CH 3 ) 2 complex, whose thermal decomposition at 220 • C resulted in the production of a pure and uniform NiS nanopowder. It is worth noting that the thermal analysis showed that a temperature of 220 • C was sufficient and suitable to completely convert the amylxanthate into the sulfide (Figure 3). Based on the TGA-DTA analysis, the reaction mechanism of metal sulfide formation from the nickel-xanthate complex pathway was proposed as presented in Figure 4. The straightforward diagram of the thermogravimetry (TG) analysis was in fact implicitly indicative of the purity of the Ni(S2CO(CH2)4CH3)2 complex, whose thermal decomposition at 220 °C resulted in the production of a pure and uniform NiS nanopowder. It is worth noting that the thermal analysis showed that a temperature of 220 °C was sufficient and suitable to completely convert the amylxanthate into the sulfide (Figure 3). Based on the TGA-DTA analysis, the reaction mechanism of metal sulfide formation from the nickel-xanthate complex pathway was proposed as presented in Figure 4. By adding the xanthate solution to the PLS in a dropwise manner, the following reaction actually occurred: By adding the xanthate solution to the PLS in a dropwise manner, the following reaction actually occurred: Ni 2+ + 2KS 2 CO(CH 2 ) 4 CH 3 ) 2 (aq) → Ni (S 2 CO(CH 2 ) 4 CH 3 ) 2 (s) + 2K + (1) It was reported that the thermal decomposition of the solid product followed the Chugaev reaction pathway to produce alkene (C 2 H 4 ), carbonyl sulfide (COS) and alkyl thiol (R-SH) as volatiles, and NiS as a pure solid product [55,56].

Fourier-Transform Infrared (FTIR) Spectroscopy
The as-synthesized NiS was analyzed by the FTIR spectrum while considering the main absorption peaks at about 3434, 1631, 1138 and 641 cm −1 . The two absorption peaks of 3442 and 1631 cm −1 were attributed to the vibration of OH groups of H 2 O [19,52]. The peak at 2351 cm −1 can be assigned to the C-O bond originating from the adsorption of CO 2 (from the atmosphere and/or the reaction product) [19,54]. The bands at 638 cm −1 , 641 cm −1 and 964 cm −1 were related to the symmetrical and asymmetrical stretch of the Ni-S band in NiS particles [57]. The bending vibration of the sulfonated group appeared at 1138 cm −1 ( Figure 5). Furthermore, elemental analysis obtained by the ICP-OES method indicates a high purity (98.94%) of the fabricated nanopowders (Table 3). Additionally, the FTIR analysis ( Figure 5) implicitly indicates the high purity of the powder, since the presence of even very low amounts (below 1%) of impurities in the powder can show certain peaks in this analysis. Figure 6a illustrates the SEM photomicrograph of NiS nanoparticles. The agglomerated form of the nanoparticles was due to the thermal decomposition. The morphology of the NiS was characterized by spherical-shaped particles with a uniform distribution. peak at 2351 cm -1 can be assigned to the C-O bond originating from the adsorption of CO2 (from the atmosphere and/or the reaction product) [19,54]. The bands at 638 cm -1 , 641 cm -1 and 964 cm -1 were related to the symmetrical and asymmetrical stretch of the Ni-S band in NiS particles [57]. The bending vibration of the sulfonated group appeared at 1138 cm -1 ( Figure 5). Furthermore, elemental analysis obtained by the ICP-OES method indicates a high purity (98.94%) of the fabricated nanopowders (Table 3). Additionally, the FTIR analysis ( Figure 5) implicitly indicates the high purity of the powder, since the presence of even very low amounts (below 1%) of impurities in the powder can show certain peaks in this analysis.   Figure 6a illustrates the SEM photomicrograph of NiS nanoparticles. The agglomerated form of the nanoparticles was due to the thermal decomposition. The morphology of the NiS was characterized by spherical-shaped particles with a uniform distribution. The EDS reveals the purity of the powder, with only Ni and S elements at atomic percentages of 48.2 and 51.8, respectively (Figure 6b). The XRD, FTIR and EDS analyses also confirm the high purity of the nanoparticles (Figures 2, 5 and 6b) Figure 7 shows the TEM image of spherical NiS nanoparticles with an average diameter of 61 ± 2 nm. The particle size distribution (PSD) was evaluated based on 25 different images, counting about 250 particles (ImageJ 1.x software, the Laboratory for Optical and Computational Instrumentation (LOCI, University of Wisconsin, Madison, WI, USA)). The PSD showed a narrow size distribution range of 22 to 106 nm (average size of 61 nm).  The EDS reveals the purity of the powder, with only Ni and S elements at atomic percentages of 48.2 and 51.8, respectively (Figure 6b). The XRD, FTIR and EDS analyses also confirm the high purity of the nanoparticles (Figures 2, 5 and 6b) Figure 7 shows the TEM image of spherical NiS nanoparticles with an average diameter of 61 ± 2 nm. The particle size distribution (PSD) was evaluated based on 25 different images, counting about 250 particles (ImageJ 1.x software, the Laboratory for Optical and Computational Instrumentation (LOCI, University of Wisconsin, Madison, WI, USA)). The PSD showed a narrow size distribution range of 22 to 106 nm (average size of 61 nm). The EDS reveals the purity of the powder, with only Ni and S elements at atomic percentages of 48.2 and 51.8, respectively (Figure 6b). The XRD, FTIR and EDS analyses also confirm the high purity of the nanoparticles (Figures 2, 5 and 6b) Figure 7 shows the TEM image of spherical NiS nanoparticles with an average diameter of 61 ± 2 nm. The particle size distribution (PSD) was evaluated based on 25 different images, counting about 250 particles (ImageJ 1.x software, the Laboratory for Optical and Computational Instrumentation (LOCI, University of Wisconsin, Madison, WI, USA)). The PSD showed a narrow size distribution range of 22 to 106 nm (average size of 61 nm). As shown in Figure 7, the TEM analysis also constituted the other verification for the purity of the as-synthesized NiS, since the crystals were completely uniform without any other heterogeneous crystals. By comparing the results of the SEM (Figure 6a) and TEM (Figure 7a) images morphologically, one can observe that the SEM shows a nonspherical shape for the synthesized particles while the shape is relatively spherical when using the TEM. This is because the newly formed and somewhat incomplete particles, which are As shown in Figure 7, the TEM analysis also constituted the other verification for the purity of the as-synthesized NiS, since the crystals were completely uniform without any other heterogeneous crystals. By comparing the results of the SEM (Figure 6a) and TEM ( Figure 7a) images morphologically, one can observe that the SEM shows a nonspherical shape for the synthesized particles while the shape is relatively spherical when using the TEM. This is because the newly formed and somewhat incomplete particles, which are also characterized by XRD analysis, may change their morphology when heated for decomposition because of the phenomenon of sintering and agglomeration. Due to differences in the mechanisms of these two analyses, in the SEM analysis the as-synthesized particles are directly imaged after the furnace (220 • C). However, in the TEM analysis, which uses strong ultrasonication (40 kHz) for 30 min, particles are given a chance to completely differentiate and disperse. Thus, in terms of sintering, the morphology displayed in the SEM picture is somewhat different than that shown by the TEM, which is spherical and closer to the original particles.

Photon Correlation Spectroscopy (PCS)
By using the PCS particle size analysis, the size distribution of the NiS nanoparticles was also obtained ( Figure 8). By comparing the TEM and PCS analyses it was revealed that the average particle sizes of the NiS nanoparticles were about 61 and 64, respectively. The negligible difference in sizes can be related to the electrostatic forces between particles and, in the PCS analysis, to agglomeration. composition because of the phenomenon of sintering and agglomeration. Due to differences in the mechanisms of these two analyses, in the SEM analysis the as-synthesized particles are directly imaged after the furnace (220 °C). However, in the TEM analysis, which uses strong ultrasonication (40 kHz) for 30 min, particles are given a chance to completely differentiate and disperse. Thus, in terms of sintering, the morphology displayed in the SEM picture is somewhat different than that shown by the TEM, which is spherical and closer to the original particles.

Photon Correlation Spectroscopy (PCS)
By using the PCS particle size analysis, the size distribution of the NiS nanoparticles was also obtained ( Figure 8). By comparing the TEM and PCS analyses it was revealed that the average particle sizes of the NiS nanoparticles were about 61 and 64, respectively. The negligible difference in sizes can be related to the electrostatic forces between particles and, in the PCS analysis, to agglomeration.

Conclusions
The main motivation of this research is to recycle NiS nanopowders from spent LIBs. This was obtained through a facilitated method with mild conditions (complexation at an ambient temperature and then thermal decomposition at 220 °C for 1 h). The mineralogical and particle size properties of the produced nanoparticles were respectively characterized by XRD and TEM as well as PCS analyses. Additionally, their elemental distributions and functional groups were assessed by the ICP-OES, EDS and FTIR methods, respectively. The characteristic analysis of the as-prepared NiS identified a mean particle size of 61 ± 2 nm (by TEM) and 64 nm (through the PCS method) with high purities approved via XRD and EDS analyses. According to the obtained XRD data and the standard ones, they matched well with the hexagonal structure of the NiS phase. The results of the FTIR data disclosed that bands of 638, 641 and 964 cm -1 were related to the stretch of the NiS band. These results were highly correlated with the data obtained by ICP-OES. The morphological analyses using SEM and TEM disclosed a discrepancy with respect to the differences in the mechanisms of these two techniques. SEM displayed a non-spherical shape for the synthesized nanoparticles while the shape given by the TEM was relatively spherical. We finally concluded that the results of the present research could be considered a new path for producing extremely valuable and pure nanomaterials from industrial e-wastes without requiring high costs and complex processes like solvent extraction .

Conclusions
The main motivation of this research is to recycle NiS nanopowders from spent LIBs. This was obtained through a facilitated method with mild conditions (complexation at an ambient temperature and then thermal decomposition at 220 • C for 1 h). The mineralogical and particle size properties of the produced nanoparticles were respectively characterized by XRD and TEM as well as PCS analyses. Additionally, their elemental distributions and functional groups were assessed by the ICP-OES, EDS and FTIR methods, respectively. The characteristic analysis of the as-prepared NiS identified a mean particle size of 61 ± 2 nm (by TEM) and 64 nm (through the PCS method) with high purities approved via XRD and EDS analyses. According to the obtained XRD data and the standard ones, they matched well with the hexagonal structure of the NiS phase. The results of the FTIR data disclosed that bands of 638, 641 and 964 cm −1 were related to the stretch of the NiS band. These results were highly correlated with the data obtained by ICP-OES. The morphological analyses using SEM and TEM disclosed a discrepancy with respect to the differences in the mechanisms of these two techniques. SEM displayed a non-spherical shape for the synthesized nanoparticles while the shape given by the TEM was relatively spherical. We finally concluded that the results of the present research could be considered a new path for producing extremely valuable and pure nanomaterials from industrial e-wastes without requiring high costs and complex processes like solvent extraction.

Data Availability Statement:
The data presented in this study is available on request from the corresponding authors. The data is not publicly available due to the confidentiality of the research work.