Nowadays, lithium-ion batteries (LIBs) are the most consumed energy storage and supply devices for portable electronics. Their characteristics (high energy density, long cycle life, high roundtrip efficiency, wide range of operating temperature, high reliability, safety, fast recharge and low self-discharge rate) make them the best option for numerous applications. LIBs play a vital role in the development of electric vehicles and in the storage of energy from renewable sources [1
], which will contribute to decreasing the global carbon emissions. The increasing demand for LIBs from the sectors of portable electronics and transportation comes together with an increasing interest for recycling of spent batteries [2
The main limitations for the implementation of proper recycling procedures are the lack of standards for the physical formats and the frequent appearance of new batteries with different chemistries [1
]. There are normally three different shapes: prismatic, pouch (or polymer) and cylindrical cells. With respect to the chemical composition, LIBs consist of a cathode, an anode, an organic electrolyte and a separator. In most of the LIBs currently commercialized, the anode is made of graphite cast over copper foil. Nowadays, alternative anodes are under study, such as Li4
, graphite nanotubes or nanoparticles, or compounds of Si and Sn. The cathode, supported on an aluminum foil, is usually built from transition metal oxides, such as LiMn2
(NMC) and (NiCoAl)O2
]. Most of the toxic and valuable metals are found in the cathode, which represent approximately 25–30% of the total battery mass.
According to the European Union, some components of LIBs, such as cobalt, phosphorous and natural graphite, are classified as “Critical Raw Materials” (CRMs) due to their high supply risk and economic importance [6
]. Hence, recycling technologies for any CRM-containing waste are in the spotlight of the EU development plan. Nowadays, most battery recycling technologies focus on the recovery of Co, as it is considered the bottleneck in the battery industry, leaving the recuperation of Li in the background [1
]. However, the increase in the LIBs market may place Li on the list of critical materials by 2030 [1
]. Indeed, the increasing demand of Li for batteries may soon produce a supply shortage, which promotes the investigation of Li-free alternatives such as sodium or aluminum ion batteries [9
]. In addition to the supply chain risk, some of the materials used in LIBs represent a threat to the environment, for example, the emissions of fluoride gas from battery fires or the aqueous contamination from the metals in the cathode. This environmental threat also motivates the development of recycling initiatives [10
Regarding the currently existing LIBs recycling technologies, they can be classified into pyrometallurgical, hydrometallurgical, biometallurgical and combined techniques. Pyrometallurgical processes use high temperature smelting to recover cobalt, copper and nickel alloys [2
]. Hydrometallurgical processes use chemical acidic leaching to dissolve the metal containing components, followed by chemical separation and recovery [12
]. In biometallurgical processes, microbial activity promotes the production of inorganic and organic acids to leach metals from spent LIBs [13
]. The main limitation of these techniques is that lithium is lost in slag, and they have high energy and chemical reagents consumption [3
]. Up to 50 companies around the world recycle lithium-ion battery at some scale, most of them located in China, South Korea, Europe and North America [4
]. Some important companies, such as Umicore, Duesenfeld, Toxco and Recupyl have developed industrial-scale recycling processes applying the aforementioned techniques [15
As an innovative recycling process, Villen-Guzman et al. [17
] proposed the application of electrodialytic remediation (EDR) to LIB residues. This technique is based on the use of ion-exchange membranes for the selective separation of ions from liquid matrices by means of an applied electric potential. The EDR has been successfully used for the remediation of wet solid matrices and aqueous suspensions (e.g., polluted soil, treated timber waste, fly ash, wastewater sludge and harbor sediments [18
To the extent of our knowledge, there are no previous experimental studies evaluating the application of EDR to LIBs, apart from initial unpublished experiments carried out in our group. These previous experiments tested the recovery of Li and Co from LiCoO2 particles, placed in the central section of a standard three-compartment ED cell, using an anion-exchange membrane at the anode compartment and a cation-exchange membrane (CEM) at the cathode compartment. In those experiments, some limiting drawbacks were found, such as the difficulty to attain a proper stirring of the suspension within the cell, a significant deposition of particles at the surface of the ion-exchange membranes and the formation of chloride gas at the anode.
With the aim of tackling the aforementioned limitations, a novel experimental setup based on a hydrometallurgical–electrodialytic treatment is proposed in this paper. This experimental system basically consists of an electrodialytic cell using CEMs for the separation of the central compartment from both electrode compartments, while the dissolution of the LiCoO2 particles is carried out outside the electrodialytic cell, hindering the undissolved particles to reach the membranes and avoiding fouling. In order to achieve the proof of concept, the experiments presented here were carried out on new LiCoO2 particles.
2. Materials and Methods
2.1. Extraction Analysis
In order to study the kinetics of the dissolution of LiCoO2 particles, a number of extractions were carried out on LiCoO2 powder (Alfa Aesar, 97%) in HCl 0.1 M solution. Batch-extraction experiments were carried out in well-stirred 50 mL polypropylene vessels containing LiCoO2 powder suspended in the extracting solution using a liquid/solid ratio of L:S = 200, namely 125 mg of solid powder suspended in 25 mL of 0.1 M HCl solution. The vessels with the suspensions were stirred in a rotatory shaking table at room temperature, and they were withdrawn and analyzed at different times (up to 6 days) to obtain the transient concentration profile of and in the aqueous phase. With the aim of determining the initial amount of metals, microwave-assisted acid digestion was carried out. All the samples were filtered using 0.60 µm glass-fiber (Macherey-Nagel (MN) GF-3) and analyzed for Co and Li using atomic absorption spectrophotometry (Varian SpectrAA 1101).
In hydrometallurgical processes, different extracting agents (organic and inorganic acids) have been tested [3
]. In this study, 0.1 M HCl was used to extract Co and Li from LiCoO2
. The choice of the extracting agent was done to evaluate the capability of chloride ions as extraction agent, as it may act as a reducing agent of
2.2. Solid Surface Characterization
X-ray photoelectron spectroscopy (XPS) measurements were performed with a Physical Electronics PHI 5701 spectrometer with a multi-channel hemispherical electron analyzer. Samples of solid before and after extraction were mounted on a sample holder without adhesive tape and kept overnight at high vacuum in the preparation chamber before being transferred to the analysis chamber for testing. The photoelectron lines Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 at 932.7, 368.3 and 84.0 eV were selected to calibrate the spectrometer energy scale. High-resolution spectra were collected in the constant pass energy mode at 29.35 eV operating at a given take-off angle of 45°. The adventitious carbon (C1 s at 284.8 eV) was used as charge reference. The PHI ACCESS ESCA-V8.0 software package was used for acquisition and data analysis.
2.3. Hydrometallurgical–Electrodialytic Experiments
The combined hydrometallurgical–electrodialytic experiments were carried out in duplicate, using cells electrically connected in series, i.e., the cathode of one of the ED cells was connected to the anode of the other one, assuring that both cells were submitted to the same electrical current. The experiments were performed at constant current of 50 mA by means of a DC power supply unit (Genesys RDK Lambda GEN 600-2.6), which corresponded to a current density of 1 mA cm−2
referred to the internal diameter of the ED cell. The experimental system is schematically presented in Figure 1
Each ED cylindrical cell was built using three methacrylate compartments (4 cm length and 8 cm internal diameter). The central compartment of the electrodialytic cell was connected with an external vessel and a separatory funnel of 100 mL capacity. A volume of 350 mL of the suspension with the LiCoO2 particles in 0.1 M HCl (L:S = 200) was initially placed in the magnetically and well stirred vessel. The suspension was continuously pumped from the vessel to the central compartment, passing through the separatory funnel using a four-channel Watson Marlow 302S peristaltic pump. The conical shape of the separatory funnel helps the solid particles to not follow the liquid phase containing the extracted metals and therefore prevents the particles from reaching the central compartment of the ED cell. A fiberglass filter was added at the upper end of the separatory funnel to assure that no solid particles were dragged by the aqueous flow. The liquid phase continuously flowed back to the vessel from the central compartment. The flow rate was 0.2 mL s−1, optimized to assure the retention of the undissolved particles in the separatory funnel.
The central compartment was separated from both electrode compartments by means of CEMs (Neosepta CMX-fg standard grade; electrical resistance 1.8 , thickness 0.16 mm, stability pH 0–14). The commercial anode consisted of the base titanium material covered with mixed oxides, mainly Ir, to prevent their oxidation (Metakem GmbH), and the cathode was stainless steel. The catholyte was 0.1 M HCl, similar to the extracting solution. In turn, the anolyte was 0.1 M solution, as it could not be further oxidized at the anode. The use of CEMs to separate the anode and the central compartments hindered the passing of towards the anode and the corresponding oxidation to chlorine gas. This configuration allows the regeneration and reuse of the extracting solution, which is an advantage of this technique due to the continual addition of extracting agent solution not being necessary.
Liquid samples of 10 mL from the catholyte and the central compartments were withdrawn twice a day during the experimental time (6 days). With the aim of keeping the liquid volume constant, 10 mL of 0.1 M HCl solution was added to the central compartment after sampling. The metal concentration of all liquid samples was determined by an AAS Varian SpectrAA-110.
At the end of the experiment, the cells were disassembled and the cathodes were soaked in 1:1 solution for 24 h to determine the metal electrodeposited on them. Similarly, the CEMs were soaked in 1 M solution for 24 h to determine the amount of metal accumulated. The membranes used in these experiments are stable in strong acids, which allows low pH regeneration. With the aim of complete global mass balance, the fiberglass was also soaked in concentrated acid to extract the metals accumulated. Particles attached to the fiberglass used in the separator funnel were considered as part of the central compartment solution.