Towards Sustainable Lithium-Ion Battery Recycling: Advancements in Circular Hydrometallurgy

Abstract

The growing demand for lithium-ion batteries (LIBs) has led to significant environmental and resource challenges, such as the toxicity of LIBs’ waste, which pose severe environmental and health risks, and the criticality of some of their components. Efficient recycling processes are essential to mitigate these issues, promoting the recovery of valuable materials and reducing environmental pollution. This review explores the application of electrodialysis in the process of recycling LIBs to contribute to the principles of circular hydrometallurgy. The article is structured to provide a comprehensive understanding of the topic, starting with an overview of the environmental and resource challenges associated with manufacturing LIBs. Then the current recycling processes are presented, focusing on hydrometallurgical methods. The concept of circular hydrometallurgy is introduced, emphasizing sustainable resource recovery. The electrodialysis technique is described in this context, highlighting its integration into the process of recycling LIBs to separate and recover valuable metals. Finally, the article addresses the challenges and limitations of the electrodialysis technique, such as energy consumption and system optimization, and identifies areas for future research and development. Through this analysis, the review aimed to contribute to advancing the development of more sustainable and efficient LIB recycling technologies, ensuring a safer and more environmentally friendly approach to the management of batteries’ lifecycle.

1. Introduction

Lithium-ion batteries (LIBs) are energy storage devices that have become essential in our modern society. These batteries were discovered in the 1970s by the 2019 Nobel laureates (John B. Goodenough, Akira Yoshino, and M. Stanley Whittingham) and were commercialized in the 1990s by Sony [1]. Since then, these batteries have been widely used in the portable electronics sector, thanks to their features, including their high charge voltage and energy density, long lifespan, good safety performance, and low self-discharge rate [2]. Furthermore, their use in the electric vehicle (EV) and renewable energy storage sectors has made them a key component in transitioning towards a decarbonized and clean energy system.

In 2022, the global capacity of the LIB market exceeded 500 GWh. Considering the sustainable development scenario (SDS), this capacity is estimated to reach 6 TWh by 2040 (Figure 1) [3]. This increase in demand is directly related to the rise of electric vehicles. In 2020, the number of electric vehicles on the world’s roads reached 10 million. According to the International Energy Agency [4], annual sales are estimated to reach 40 million and 80 million vehicles by 2030 and 2040, respectively. This increase in demand for LIBs, coupled with their lifespan, has led to an increase in the generation of LIB waste. The total amount of spent batteries for EV and storage applications is under 2 GWh. However, the volume of batteries reaching the end of their first life will increase to around 100 GWh in 2024, which is expected to reach 1.3 TWh by 2040 [3].

Lithium-ion batteries (LIBs) mainly consist of four components: the cathode material, the anode material, a current collector, and an electrolyte (Figure 2). The cathode is composed of materials such as metal chalcogens, transition metal oxides, and polyanionic compounds, with common commercial materials including lithium cobalt oxide (LiCoO₂, LCO), lithium nickel cobalt manganese oxide (LiNi_1−x−yCo_xMn_yO₂, NCM), and lithium nickel cobalt aluminum oxide (LiNi_0.80Co_0.15Al_0.05O₂, NCA). These materials are mixed with conductive black carbon to enhance their electrical conductivity. The anode, typically made of graphite, can also include alternatives such as lithium titanium oxide (LTO). The electrolyte, a mixture of lithium salts dissolved in organic solvents, enables the mobility of lithium ions, with lithium hexafluorophosphate (LiPF₆) being the most common. Lastly, the separator, usually made of polypropylene or polyethylene, is a crucial safety element that prevents contact between the cathode and anode, thus avoiding short circuits.

Understanding LIBs’s composition is essential to evaluate the potential risks associated with the increasing demand for and waste generation of batteries, and the importance of managing these residues to make them a true enabler of green transition. Moreover, the manufacturing of LIBs relies on raw materials classified as critical, creating concerns about the supply chains’ vulnerabilities and ethical issues related to mining practices. To address these challenges, a shift toward a circular economy is proposed, emphasizing the extension of battery life, designing for reuse, and recycling of battery wastes, aiming to decrease the environmental impact of these residues and recover the valuable components contained in these electronic wastes. In other words, LIB wastes would become a secondary source known as urban mining, and recycling can contribute to a sustainable supply chain.

Current LIB recycling methods primarily involve metallurgical processes. These techniques typically submit the batteries to mechanical and thermal pretreatment to facilitate the dissolution step via acid leaching (hydrometallurgy). However, these processes are usually associated with a high energy demand, consumption of water and reactants, and the generation of large volumes of wastewater. Therefore, the hydrometallurgical process must be redesigned to become more sustainable.

By exploring the intersection of the environmental and resource challenges, the current recycling methods, and the principles of circular hydrometallurgy, this review paper aimed to provide a comprehensive overview of how electrodialysis can enhance the efficiency and sustainability of LIB recycling. While previous literature has addressed aspects of LIB recycling or electrodialysis individually, the novelty of this review consists of integrating these topics to examine the potential of electrodialysis within the context of LIB recycling. This integration can contribute to the field by consolidating the current knowledge and identifying gaps in the existing research, offering valuable insights for advancing sustainable recycling practices.

2. Methodology and Structure of the Article

This review was conducted according to the Kitchenham framework for literature reviews [5], encompassing the three main stages: planning, conducting, and reporting the results. In the planning stage, different research questions were formulated, based on the novelty of this review (

Table 1. Research questions.

	Research Questions
Q1	What are the challenges of the manufacture and recycling process of LIBs?
Q2	What are the current processes and technologies for recycling lithium-ion batteries?
Q3	Can electrodialysis contribute to the concept of circular hydrometallurgy recycling?

).

To answer the research questions, different keywords, such as lithium-ion batteries, battery recycling, circular hydrometallurgy, and electrodialysis, were selected to filter the information in different databases (Scopus, Web of Science, MDPI, ScienceDirect). Once the data had been extracted and the articles selected, the results were presented and discussed according to the following structure. First, a general introduction is presented. The current section outlines the structure of this review and the methodology used to select the information. Then Section 3 discusses the environmental and resources challenges in the manufacture of LIBs, highlighting the need for effective recycling methods. Section 4 reviews the different LIB recycling processes and introduces the concept of circular hydrometallurgy, emphasizing the importance of resource recovery and waste minimization. Then the electrodialysis technique is presented in Section 5. Section 6 contextualizes electrodialysis within circular hydrometallurgy and its application in the process of LIB recycling. Finally, Section 7 presents the challenges and limitations associated with the implementation of electrodialysis, followed by the conclusion of the current review.

3. Environmental and Resource Challenges in the Manufacture of LIBs

LIBs and their waste contain highly toxic metals and organic solvents that threaten the environment and human health [6]. In the case of metals such as cobalt and nickel, prolonged exposure to these metals can lead to respiratory problems and dermatitis, and, in some cases, even lung cancer and damage to the central nervous system. Additionally, graphite, used in the anode, can react with oxidizing agents such as fluorine and produce CO and other toxic gases. The same problems apply to the LiPF₆ used in the electrolyte, which is highly corrosive and volatile and can form HF, P₂O₅, and Li₂O upon contact with air. All these substances can enter the environment directly or indirectly, leading to environmental pollution. Therefore, improper management of these waste materials results in various environmental issues that are further exacerbated by their severe impacts on ecosystems and human health. Despite these significant environmental challenges, the replacement of LIBs with alternative technologies is not yet feasible on a large scale due to the current lack of viable and scalable alternatives that offer the performance and efficiency of LIBs. Consequently, recycling LIBs has become essential to mitigate their environmental impact.

On the other hand, the rise of LIB technology entails an increased demand for resources for manufacturing. Some of the most commonly used materials in commercial LIBs, such as lithium and cobalt, are classified by the European Union as critical raw materials [7]. In the latest update of this list, published in March 2023, new metals have been included, with nickel and manganese standing out due to their importance in the LIB industry. This classification is based on the current and future assessment of these materials’ economic value and supply risk.

Most mines for these materials are located outside of Europe, in particular points of the world, making the European Union highly reliant on these strategic raw materials for industrial and technological development. Currently, approximately 74% of battery materials are supplied by China and a few countries in Africa (e.g., the Democratic Republic of Congo and South Africa) and Latin America (e.g., Chile and Peru) [7]. As a result, natural disasters or political conflicts in these countries can affect the availability of these materials. Moreover, numerous social issues are associated with mining these raw materials [8]. In addition to the environmental impact caused by mining activities (water and soil pollution, ecosystem loss, emissions), the lack of regulation in these countries leads to conflicts between communities and mining companies over land use. Furthermore, inadequate worker safety, human rights abuses (such as child labor), and corruption highlight the necessity of developing new strategies to make these metals more sustainable.

The demand for primary materials for batteries, as well as their criticality, can be decreased by the adoption of circular economy strategies. On one hand, they extend the lifespan of batteries and design batteries to be reused. On the other hand, recycling spent batteries focuses not only on decreasing the environmental impact of these residues but also on recovering the valuable components contained in these electronic wastes. In other words, LIB wastes would become a source of secondary material, which is known as urban mining. By the year 2040 (Figure 3), it is estimated that 51% of the cobalt, 42% of the nickel, and 23% of the manganese consumed for battery production in the European Union could come from recycling. However, it is estimated that only 9% of lithium will come from secondary sources [9].

Key aspects for increasing the volume/quantity of secondary raw materials from battery wastes include the traceability and identification of batteries throughout their life, maximizing the collection of spent batteries, and developing high-quality recycling technologies. In this regard, legislation plays an important role, as it sets the target for each stage of the lifecycle.

The European Union announced a new battery regulation (EU2023/1542) in 2023 to ensure the sustainability and safety of batteries in the European market. Before 2023, the recycling of batteries in Europe was regulated by the Batteries Directive (2006/66/EG) [10]. Although this legislation had been updated, a new version was required to include measures that Europe must adopt to meet the objectives of the EU Green Deal, the Circular Economy Action Plan, and the New Industrial Strategy. The New Batteries Regulation introduced, for the first time, the minimum requirements for collection and recycling efficiency for lithium-based batteries. The collection of waste batteries is a fundamental step in the recovery of valuable materials. The collection targets for portable batteries and electric vehicles have been set at 73% and 61%, respectively, by the end of 2031. Regarding recycling efficiency, the targets for lithium-based batteries have been set at 65% by the end of 2025 and 70% by the end of 2030. Moreover, all recycling technology shall achieve at least 90% Co, 90% Cu, 50% Li, and 90% Ni for recovery yield [11].

Despite the significant environmental challenges of LIBs related to the criticality of materials and their social-environmental issues, replacing LIBs with alternative technologies is yet to be feasible on a large scale. Currently, no viable options match the performance and efficiency of LIBs. Consequently, LIB recycling has become essential to mitigate their environmental impact. In this context, research on the recycling and utilization of LIB wastes has become a global research hotspot. Although various battery recycling technologies based on metallurgical methods are available today, they are still looking for a perfect solution [12]. The key (and most difficult) point of these recycling processes is recovering metals selectively with high purity from the battery wastes due to the variety of cathode chemistries used in commercial LIBs [13]. Many efforts must be made to develop efficient technologies to increase the recycling yield and metal purity and make the process environmentally sustainable and economically viable.

4. Overview of Recycling Processes for Lithium-Ion Batteries

Recycling is the third step in the waste management hierarchy, with reducing (designing LIBs with long lifespans) and reusing being preferable instead of recycling. However, reducing and reusing only postpone recycling, which is the ideal eventual fate for all LIBs. The current technologies for LIB recycling are based on metallurgical processes. Due to the complex structures and the high number of materials in LIBs, a pretreatment step is required to facilitate recycling [14].

The preliminary treatment comprises three major stages: discharge, dismantling, and separation (if required) [15]. The steps of separation consist of physical (flotation), mechanical (shredding and crushing), thermal (organic solvent evaporation), chemical (dissolution of binder or foils), or mechanic-chemical processes [16]. The primary purposes of the pretreatment step are enriching the metallic fraction, lowering the volume of waste, enhancing the recovery rate, and managing the safety issues. Once the battery has been pretreated, it is subjected to pyrometallurgy, hydrometallurgy, or direct recycling [14], as shown in Figure 4.

Pyrometallurgical processes use high temperatures to recover and purify valuable metals in the battery’s materials. This process usually involves two stages [13]. The battery is initially subjected to combustion within a smelting furnace operating under a vacuum or an inert atmosphere. The constituents present in LIBs undergo a heating process exceeding their respective melting points, facilitating the separation of metallic components in the molten state by reduction reactions and the subsequent generation of distinct immiscible molten strata [17]. The main outputs of the smelting procedure include metal alloys and slag. Valuable elements such as Co, Ni, and Cu are typically found in the metal alloy, whereas the slag fraction commonly contains Li, Mn, and Al [18]. In the second stage, metal alloys are further separated to recover pure materials via hydrometallurgical methods. Therefore, pyrometallurgical processes alone are not able to purify the metals and it aims to transform the metals into favorable phases to facilitate the subsequent leaching process [16].

Pyrometallurgy is a simple and mature recycling technique due to its flexibility. In this method, sorting and reducing the size are unnecessary, as a mixture of batteries can be burned together. Thus, the pretreatment step is optional. However, several disadvantages limit this technique. The first is the generation of toxic gases and the high energy consumption during the smelting process. Secondly, the fact that further processing is required to separate the metals from the alloys increases the cost of the recycling process. Moreover, only a few materials from LIBs can be recovered. The process recovers Co and Ni from the cathode and Cu from the anode’s current collector. However, other materials are lost in the slag (such as lithium and manganese) or evaporated during the process (such as electrolytes and plastics). The technique has been profitable due to the high cobalt content in LIBs. However, the industry is moving toward reducing the cobalt content. Thus, the process of pyrometallurgy may not work well for the new low-cobalt LIBs [13].

Hydrometallurgical processes use an aqueous solution to extract and separate the metals from LIBs. This process involves two main stages: leaching and separation. In the first stage, the pretreated battery materials are treated using acid and reducing agents to convert the metals contained in the solid into soluble metal ions. A wide range of lixiviants have been used to enhance the process, including inorganic (H₂SO₄, HCl, HNO₃) and organic acids (citric acid, tartaric, and oxalic), combined with the use of reductant agents. Once the metals have dissolved in the solution, they are selectively separated by chemical precipitation, solvent extraction, ion exchange, or electrodeposition [19].

Compared with the pyrometallurgical process, hydrometallurgy presents relevant advantages, such as lower energy consumption, lower emissions, higher recovery of materials (most constituents of LIBs can be recovered), and higher purity of the product, which make this method attractive for recycling LIBs. However, this technique also has some drawbacks. One of them is the need for sorting, which requires more storage space and increases the cost and complexity of the overall process. On the other hand, selectively separating the metals in the solution (such as Co, Ni, Mn, Fe, Cu) can become a challenge due to their similar chemical properties. Moreover, the amount of chemicals and water required in the process and the treatment of wastewater increase the cost [13]. In order to improve the profitability and sustainability of battery recycling, it is important to simplify the process, enhance the rate of metal recovery, and reduce the consumption of reactants.

Direct recycling is an emerging method of recovery that directly recovers the active materials of LIBs to be reconditioned without destroying the original compounds’ structure. In this process, the battery is disassembled, and its constituents are separated by physical methods, magnetic separation, and moderate thermal processing to avoid chemical breakdown of the active materials [13]. The purification of active materials involves repairing bulk defects by hydrothermal, solid-phase, electrochemical, and chemical re-lithiation.

The direct recycling method includes a series of advantages compared with the pyrometallurgical and hydrometallurgical methods, such as a relatively more straightforward process with lower emissions and less secondary pollution. Moreover, the active materials can be directly reused after regeneration. However, this technique is still limited and only exists at the laboratory scale [20]. The variety of the cathodes’ chemistry can result in poor efficiency of recovery, and it is a challenge to guarantee the high purity and crystal structure required by industrial battery standards. Thus, rigorous sorting and separation are required for the exact chemistry of the materials [21]. Considering the characteristics of different battery wastes (type, chemistry, and condition), it is difficult to think of direct recycling as an industrial method to manage battery wastes. Recent progress has mainly focused on recycling electrode scraps from battery manufacturers because their chemistry is well known [13].

Currently, pyrometallurgical and hydrometallurgical processes are widely utilized in the industry. Pyrometallurgical methods are used due to their flexibility in terms of the battery feedstock, their simplicity, and reliability, while hydrometallurgical methods create fewer emissions and offer superior effectiveness. It must be noted that none of the existing recycling methods offers a perfect recovery solution for all components of LIBs with low energy consumption and a low cost and without generating harmful gases and wastewater effluent. Table 2 summarizes the advantages, disadvantages, and challenges of each recycling method.

The recycling of spent LIBs requires more effort and investment to achieve more effective and greener solutions to recover not only valuable metals but also to dispose of substances that are adverse to the ecosystem correctly. In this context, the advantages of the hydrometallurgical method make this method one of the most promising in battery recycling. Proof of that is the number of studies over the past decade that have focused on mitigating their disadvantages and increasing their recovery rate and effectiveness [22]. Hence, the hydrometallurgical process needs to develop new strategies to ensure the sustainability of the technique.

Table 2. Advantages, disadvantages and challenges of different recycling process (adapted from [16,23]).

Process	Advantages	Disadvantages	Challenges
Pyrometallurgy	Simple operation No requirement for pretreatment High efficiency	High energy consumption More waste gasses Low efficiency of recovery Li and Mn are not recovered	Decreasing energy consumption, pollution, emissions and environmental hazards Combining it with hydrometallurgy
Hydrometa-llurgy	High recovery rate High purity product Low energy consumption Less waste gas High efficiency	More consumption of water and chemical reactants More wastewater Long process	Wastewater treatment Optimization of the process Circular hydrometallurgy
Direct recycling	Short recovery route Low energy consumption Environmentally friendly High recovery rate	High operational and equipment requirements Incomplete recovery	Decreasing recovery costs Decreasing the requirements for categories Further optimization of the products’ performance

Table 2. Advantages, disadvantages and challenges of different recycling process (adapted from [16,23]).

Process	Advantages	Disadvantages	Challenges
Pyrometallurgy	Simple operation No requirement for pretreatment High efficiency	High energy consumption More waste gasses Low efficiency of recovery Li and Mn are not recovered	Decreasing energy consumption, pollution, emissions and environmental hazards Combining it with hydrometallurgy
Hydrometa-llurgy	High recovery rate High purity product Low energy consumption Less waste gas High efficiency	More consumption of water and chemical reactants More wastewater Long process	Wastewater treatment Optimization of the process Circular hydrometallurgy
Direct recycling	Short recovery route Low energy consumption Environmentally friendly High recovery rate	High operational and equipment requirements Incomplete recovery	Decreasing recovery costs Decreasing the requirements for categories Further optimization of the products’ performance

4.1. The Hydrometallurgical Process of LIB Recycling

Hydrometallurgy is a chemical metallurgical process that involves the use of an aqueous solution to recover metals from ores. It has been widely used in mining and metallurgical industries, and in the past few decades, as aforementioned, its uses have been extended to the LIB recycling sector [24]. The hydrometallurgical process is divided into two main stages: leaching and separation of the metals. Several studies have focused on the optimization of parameters controlling these stages to increase the yield of extraction and the efficiency of recovery [19,25].

Leaching is usually carried out using acids, H₂SO₄ being the most used at the industrial scale [26]. However, other acids, such as HCl and HNO₃, have gained attention because they are much easier to regenerate than H₂SO₄ [27]. The use of organic acids has also been investigated to develop a more sustainable process [28]. In addition to acids, the leaching stage also requires the use of a strong reductant agent to promote the dissolution of metals. Although several reductant agents have been investigated, such as ascorbic acid [29] or current collector scraps [30], H₂O₂ is the most used in research and on the industrial scale [25]. Despite H₂O₂ being widely known as an oxidizing agent, its use as a reducing agent is justified due to the high reduction potential of Co³⁺/Co²⁺. In addition to the type and concentration of the extractant and reductant agents, other parameters controlling the leaching step are the temperature, the solid–liquid ratio, and the stirring mechanism.

Regarding the separation stage, different techniques have been studied to separate metals selectively once they have been solubilized. The most used at the industrial scale is chemical precipitation, which requires the use of an alkaline solution to neutralize and precipitate metals [31]. The mechanism of separation depends on the solubility of the metal compounds under specific conditions of temperature and pH. The most commonly used precipitant is CO₃²⁻ (as Na₂CO₃ or CaCO₃), as it can form insoluble compounds with almost all high-value metals [16]. Other precipitants, such as Na₃PO₄ [32], phosphoric acid [33], and oxalic acid [34], have also been reported. The advantages of this technique are the low cost, the low equipment requirements, and its simple operation. However, the main limitation is the difficulty of selectively separating the battery metals, due to their properties being so similar, so they precipitate at similar pH values [16]. An effective approach is coprecipitating them and sintering the precursor directly into NMC cathodes [35].

Solvent extraction is another technique used to separate metals. In solvent extraction, the driving mechanism is the different solubilities of various metal ions in an organic solvent versus an aqueous liquid. Several efficient solvents are used, including 5-nonylsacysalicylaldoxime, di-(2-ethylhexyl)phosphoric acid (DEHPA) and Cyanex 272 [13]. This technique has the advantages of a short duration and the high purity of the product, but the high cost of solvents and the complexity of the process limit its application. Electrochemical deposition is another method used to separate and recover the target metals [36]. The principle of this technique is the differences in the metals’ redox potential. This technique is mainly focused on the recovery of cobalt [37]. Moreover, the application of ion-exchange membranes and electrodialysis also have been reported as a technique to separate and concentrate metals from LIBs via leaching solutions [38].

As observed, both stages require a large amount of reagents and water, with the subsequent generation of residues and wastewater. It is one of the drawbacks of conventional hydrometallurgy, which can be described as predominantly linear in the sense that the reagents consumed and are not regenerated for subsequent reuse. It must be noted that many hydrometallurgical processes leave a large carbon footprint because producing hydrometallurgical reagents requires high energy consumption. Nowadays, there is more consciousness-raising regarding environmental issues, and it has also reached the extractive metallurgical sector. In this context, the hydrometallurgical process must be redesigned to become more sustainable. The transition from a linear to a circular economy could be a solution to face this challenge.

4.2. Circular Hydrometallurgy

Circular hydrometallurgy (CH) involves designing energy-efficient and resource-efficient unit processes that consume the minimum quantities of reagents and produce as little waste as possible. The basis of the circular approach is the regeneration and reuse of every reagent in the process. This refers not only to the acids and bases used for leaching and controlling the pH, but also the reducing agents, oxidizing agents, and other auxiliary agents used in the process. Moreover, the circular model should emphasize decreasing water and energy consumption.

To consolidate the concept of circular hydrometallurgical processes, Binnemans and Tom [27] defined the “12 principles of circular hydrometallurgy”. In this guide, they proposed different approaches to set a basis for identifying future research in the field of hydrometallurgy, providing a benchmark of sustainability for technological development. The 12 principles are shown in Figure 5. Some of these principles are more general, such as regenerating reagents, preventing waste, using benign chemicals, or maximizing efficiency in terms of mass, energy, time, and space. Other principles are more specific, such as decreasing the activation energy or electrifying the process wherever possible.

It must be noted that these principles are not independent and can be combined to strengthen the development of a circular hydrometallurgical flowsheet. By far, Principle 1 (regenerating reagents) is the most important because the circular process cannot be designed without regeneration. However, there are synergies between the principles. For example, integrating materials and energy flows (Principle 5) results in higher efficiencies (Principle 4). At the same time, a more efficient process with a lower consumption of reagents (Principle 4) leads to less need for regeneration of the reagents (Principle 1). Decreasing the activation energy (Principle 7) of hydrometallurgical reactions maximizes the process’s efficiency in terms of mass, energy, space, and time (Principle 4). But there is also a critical interaction among the principles, and optimizing a process focused on one principle could result in worse performance of the other principles. For example, the regeneration of reagents is enhanced if the consumption of acid is decreased. However, a low acid concentration slows down the leaching kinetics, affecting the efficiency in terms of space and time [27]. Hence, the development of CH involves a comprehensive analysis of the different principles and their interactions.

In the field of LIB recycling, as aforementioned, the hydrometallurgical process is one of the most commonly used techniques. Hence, implementing the principles of circular hydrometallurgy can lead to significant environmental benefits. By following principles such as regenerating reagents, closing water loops, preventing waste, and maximizing efficiency, the recycling process will become more resource-efficient and minimize the generation of waste [39]. This approach can result in lower energy consumption, reduced CO₂ emissions, and higher recovery rates of valuable metals such as lithium, cobalt, and nickel.

5. Electrodialysis

Electrodialysis (ED) is a membrane separation process in which ions are transferred from one solution to another through selective ion-exchange membranes, using an electric field as the driving force [40]. ED is a mature technology in the field of brackish water desalination. In the last decades, the development of new membranes has allowed for an extension of their application into the food, drug, and chemical processing industry, including wastewater treatment [41,42]. This extension also includes the remediation of soil and other solid matrices such as sewage sludge, ash, or marine sediments [43].

5.1. Basic Principles of Electrodialysis

The operational mechanism of electrodialysis (ED) involves the movement of cations and anions through cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs), respectively. This migration is driven by an electric field applied between two electrodes. When an electric field is applied, the ions are primarily transported by electromigration. However, at the boundary layers near the membranes and electrodes, high concentration gradients make diffusion transport significant. The combination of these transport mechanisms, known as electro-diffusion, is described by the Nernst–Planck transport equation [42]. Moreover, the application of an electrical current between the electrodes promotes the dissociation of water at the electrode’s surface:

$$\mathrm{Anode} {2\mathrm{H}}_2\mathrm{O}\leftrightarrows {4\mathrm{H}}^{+}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}+4{\mathrm{e}}^{-}$$

(1)

$$\mathrm{Cathode} {2\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\leftrightarrows {2\mathrm{O}\mathrm{H}}^{-}+{\mathrm{H}}_{2\left(\mathrm{g}\right)}$$

(2)

An ED cell unit is formed by an AEM, a CEM, and a channel between the membranes, where the concentrated solution is recirculated. Anions are transported to the anolyte compartment, while cations are moved to the catholyte compartment, leading to a decrease in the concentration of the feed stream. The combination of several ED cell units results in a multichannel cell (Figure 6), which produces diluted and concentrated outflows [44]. In terms of structure, these cell units are stacked between two electrodes, an anode, and a cathode, which are usually made of materials such as titanium coated with mixed metal oxides to enhance conductivity and resist corrosion. The membranes used in ED systems are specifically designed to allow selective ion transport. CEMs contain negatively charged functional groups that attract and allow the passage of cations while repelling anions. In contrast, AEMs contain positively charged functional groups that attract and allow the passage of anions while repelling cations. These membranes are typically made from polymers [42]. The configuration of the ED system can vary depending on the application. For example, in desalination, a series–parallel arrangement is often used to optimize the removal of salts from the feed water. In these systems, the feed water passes through multiple stages of cell pairs, each stage progressively reducing the ion concentration in the dilution stream while increasing it in the concentration stream [45].

The efficiency of the ED process, as well as the quality of its output and its cost-effectiveness, is greatly influenced by the operations’ conditions. These influencing factors can be classified in four categories: the intensity of the electric current, the initial solution’s concentration, the feeding rate, and the presence of co-ions. Figure 7 summarizes the influence of different operational parameters on the technique’s performance and the phenomena impacted. Hopsort et al. [46] discussed the impacts of these parameters on the ED process and provided a comprehensive review to understand how the technique can be optimized.

The current applied across the electrodialysis cell is a crucial parameter, since it determines the driving force for the migration of ions. Higher current intensities increase the migration rate of ions, enhancing the efficiency of separating the ions and reducing the membrane area required, as well as the operation time. However, excessive intensity can lead to an increased voltage drop, which can lead to higher electrical resistance with a subsequent increase in the energy demands [47].

The performance of electrodialysis significantly impacted by the composition of the feed solution, including the ion concentration and the presence of multivalent ions. Higher ion concentrations typically improve the current’s efficiency, as there are more ions available for being transported [48]. Moreover, increasing the initial concentration of the feed results in the final products being more concentrated. Nevertheless, ion selectivity is often decreased when the membranes are exposed to a solution with elevated concentration, due to phenomena such as back-diffusion and the reduced capability of the membranes to exclude charges [49,50].

Another important operational parameter of ED is the flow rate of the feed and the concentration and dilution streams. The flow rate influences the rate of mass transfer and the overall efficiency of the ED process. Higher flow rates can reduce the concentration’s polarization and enhance the transport of ions. However, if the flow rates are too high, they might increase pressure drops and energy consumption [51,52].

Achieving optimal performance in ED requires balancing various parameters to enhance the migration of ion while minimizing energy consumption and maintaining selectivity. The value of each parameter will also depend on the final ED application.

5.2. Comparison with Other Separation Techniques

In addition to electrodialysis, there are other techniques for separating metals used at the industrial scale, such as precipitation, solvent extraction, and ion exchange. Each method has a series of advantages but also disadvantages, which limit the implementation of the technique. Table 3 summarizes the pros and contras of each separation technique.

Precipitation offers a cost-effective and simple approach to separate metals, often using inexpensive reagents. This method is simple to implement and can achieve selective separation when the appropriate precipitating agents are selected [31]. However, precipitation requires the use of chemical reagents, increasing the cost and environmental impact of the technique. Moreover, it generates solid waste, which must be managed and disposed of properly. Precise control over the process can be also challenging due to the risk of co-precipitation of undesired metals. Scaling up of the precipitation process can be complex due to the need for a large volume of reagents and the management of solid wastes [53].

Solvent extraction is another widely used method for its high selectivity and effective separation of specific metals from complex mixtures [54,55]. It is also scalable for industrial applications and can be integrated into existing hydrometallurgical processes. However, solvent extraction requires hazardous and flammable organic solvents, posing environmental and safety risks. The process generates organic waste that requires proper disposal, and the cost of solvents and their regeneration can be significant. Additionally, solvent extraction still demands substantial energy inputs, particularly for regeneration of the solvent [56].

Ion exchange is characterized by its high selectivity, with resins designed to remove specific ions from solutions [57,58]. The resins can be regenerated and reused multiple times, making the process cost-effective over the long term. However, ion exchange resins can be fouled or exhausted, requiring regeneration or replacement. The regeneration process often involves chemicals, introducing additional costs and environmental concerns. High-quality ion exchange resins are expensive, and the initial setup cost can be significant. Moreover, the method requires careful control and monitoring to prevent breakthrough and ensure consistent performance.

Electrodialysis is characterized by its selective removal of ions and energy efficiency. This method facilitates the selective extraction of valuable metals, while consuming less energy [59]. Additionally, electrodialysis requires minimal consumption of chemicals, reducing the risk of secondary pollution. It is also highly scalable, making it suitable for various operational scales. However, electrodialysis faces challenges such as fouling of the membrane, which reduces the efficiency over time and requires regular maintenance. Moreover, the initial setup investment can be high due to the cost of the membranes and equipment [41].

While each separation method has its own set of strengths and weaknesses, electrodialysis offers significant benefits in terms of environmental impact and energy consumption when renewable energy is used, making it a promising tool for enhancing circular hydrometallurgy. However, its practical implementation requires us to address the challenges related to maintenance of the membrane and the initial investment. Compared with precipitation, solvent extraction, and ion exchange, electrodialysis stands out for its low consumption of chemicals and energy efficiency, although it shares some common challenges such as maintenance and initial setup costs.

Table 3. Advantages and disadvantages of different separation methods [31,41,54,58].

Method	Advantages	Disadvantages
Precipitation	Simple process Cost-effective Selective with proper chemical agents	High consumption of chemical reagents Generation of solid waste Limited control and co-precipitation Complex scalability
Solvent extraction	High selectivity Scalable Established method in the industry	Consumption of organic solvents Generation of organic wastes Significant cost of solvent High energy consumption
Ion exchange	High selectivity Low energy consumption Reusable resins	Fouling of the resin Operational complexity to prevent breakthrough Chemicals agent required for resins regeneration High costs of resin
Electrodialysis	Selective ion removal Energy efficient Low consumption of chemicals Scalable	Fouling of the membrane High initial cost Limited to ionic species Dependency on electrical energy

Table 3. Advantages and disadvantages of different separation methods [31,41,54,58].

Method	Advantages	Disadvantages
Precipitation	Simple process Cost-effective Selective with proper chemical agents	High consumption of chemical reagents Generation of solid waste Limited control and co-precipitation Complex scalability
Solvent extraction	High selectivity Scalable Established method in the industry	Consumption of organic solvents Generation of organic wastes Significant cost of solvent High energy consumption
Ion exchange	High selectivity Low energy consumption Reusable resins	Fouling of the resin Operational complexity to prevent breakthrough Chemicals agent required for resins regeneration High costs of resin
Electrodialysis	Selective ion removal Energy efficient Low consumption of chemicals Scalable	Fouling of the membrane High initial cost Limited to ionic species Dependency on electrical energy

6. Role of Electrodialysis in Circular Hydrometallurgy

ED aligns with several principles of circular hydrometallurgical recycling of LIBs. ED can contribute to a more sustainable and environmentally friendly recycling process, enhancing the recovery and purification of valuable metals, reducing the consumption of chemicals and energy, treating wastewater, and regenerating the chemicals of the process. In the following subsection, different studies regarding the application of ED to the process of LIB recycling are discussed.

6.1. Separation of Metals

ED is gaining attention to be applied in the field of LIB recycling, specifically to separate and concentrate metals once the solid has dissolved. For this purpose, ED must be considered an emerging green process that can recover valuable metals from waste LIBs [60]. The main challenge of ED is the difficulty of separating metal ions with similar charges due to the low selectivity of the ion exchange membranes. In order to enhance the selectivity of the process, various chelating agents, such as ethylenediaminetetraacetic acid (EDTA), citric acid, malic acid, and lactic acid, are used to create negatively charged complexes that can be separated from the cations. Furthermore, advancements in membrane technologies have facilitated the introduction of novel cell configurations, expanding the application of ED [48]. The use of monovalent exchange membranes (i.e., membranes that only allow the passage of ions with one negative or positive charge) is used to selectively separate lithium from the rest of the metals in the leachate solution.

Song and Zhao [32] studied the application of ED to concentrate lithium ions from low-lithium and high–salt solutions, usually produced in the process of LIB recycling. First, the solution was purified by adjusting the solution’s pH to 12 to remove the metals’ impurities. Then lithium was precipitated by sodium phosphate. The solid obtained was again dissolved in acid as the anolyte, and the Li⁺ and PO₄⁻ were separated by ED with cation-exchange membranes (DuPont NAFION117). The concentration of lithium in the catholyte solution was 22.5 g/L, and Na₂CO₃ was added to precipitate the Li⁺ as a Li₂CO₃ product. This process allowed the enrichment of the lithium solution to facilitate the obtention of Li₂CO_3.

Gmar et al. [61] investigated the efficiency of the ED process in selectively recovering lithium (I) from cobalt (II), nickel (II), and manganese (II) using monovalent ion exchange membranes. The study demonstrated that the Neosepta^® monovalent-selective cation exchange membrane exhibited high selectivity and faradic efficiency for lithium over divalent cations. This process of electrodialysis effectively concentrated lithium, enabling the production of high-grade lithium salts after subsequent precipitation steps. The primary challenge identified was managing the limiting current’s density to prevent the precipitation of divalent cations within the membrane’s pores. Despite this, the technology shows promising for integration into comprehensive hydrometallurgical recycling flowsheets, providing an efficient method for recovering lithium from spent batteries.

Xing et al. [62] and Iizuka et al. [63] studied the effect of adding a chelating agent on the performance of the ED approach to separate metals from leachate solutions of LIBs. The chelating agent’s role depended on the aqueous effluent’s transitional metal content. Basically, the chelating agent worked by forming negative complexes with the metal ions present in the solution, except for lithium ions. Thus, under the effect of the applied current, the lithium ions were separated from the metal complex ions, which were recovered in another cell compartment. The selectivity for each metal in the recovery cell was about 99%.

Chan et al. [64] examined the use of electrodialysis to separate and recover lithium, nickel, manganese, and cobalt from discarded lithium-ion batteries (LIBs). The method involved three stages of electrodialysis (ED) paired with EDTA, using an anion exchange membrane (AEM, PCA PC 400D) and a cation exchange membrane (CEM, Neosepta CMX) (Figure 8). Different EDTA–metal complexes formed at each stage, depending on the solution’s pH. In the first stage, with the solution’s pH being approximately 2, EDTA formed complexes with nickel, leading to the recovery of about 99.3% of the nickel in the Ni-EDTA compartment, while cobalt, manganese, and lithium were collected in the metal compartment. In the second stage, increasing the pH of the metal outflow from the first stage to 3 facilitated the formation of Co–EDTA complexes, allowing 87.3% of the cobalt to be separated. In the final stage, a monovalent cation exchange membrane (Neosepta CMS) was used to achieve separation of 99% of the lithium from manganese. The study also described processes for EDTA-based decomplexation and metal purification, with all the recovered metals achieving a purity of over 99%.

Chen et al. [65] investigated a novel approach to separate and recover lithium (Li), cobalt (Co), and nickel (Ni) from spent lithium-ion batteries using a combination of electrodialysis (ED) and coordination of metals. Figure 9 presents the experimental setup designed. The cell contained five compartments and four ion-selective membranes: a commercial cationic exchange membrane (CEM), a monovalent cationic exchange membrane (CIMS), an anion exchange membrane (AEM), and a tailor-made polymer inclusion membrane (PIM). Under the influence of an electric field, the negatively charged cobalt-based complex migrated into the chamber (S1), Li⁺ passed through the CIMS into another chamber (S2), and Ni²⁺ remained in the feed solution (Chamber F). This study evaluated the coordination capacity of different ligands (EDTA, NH₄SCN, and HCl) with Co²⁺ and Ni²⁺, finding that NH₄SCN showed the highest selectivity for cobalt over nickel. The research further explored the impact of the concentration of ligands and the current density on the performance in terms of separation. The optimal conditions achieved a high flux of cobalt and lithium ions while minimizing the transport of nickel, demonstrating the potential of NH₄SCN for enhancing the one-step process of separation. Additionally, the feasibility of using solar energy to drive the electrodialysis system was tested, showing promising results for sustainable and efficient recovery of metals.

6.2. Combination of the Leaching and Separation Stages

Although ED is commonly operated with a solution, it can also be applied to treat solid matrices or suspensions. The application of this technique to solids began to be studied in 1992 at the Technical University of Denmark (DTU), and it was patented in 1995 (PCT/DK95/00209) to improve the technique of electrokinetic remediation (EKR). Figure 10a schematically illustrates the patented experimental system. It consists of a cell with three compartments: anodic, central, and cathodic. The solid is placed in the central compartment and separated from the electrolytic compartments by ion exchange membranes. Between the anodic compartment and the central compartment, an anionic exchange membrane (AEM) is used to prevent the passage of cations from the anolyte to the solid matrix or suspension while allowing the passage of anions from the central compartment to the anolyte. Similar behavior occurs in the cation exchange membrane (CEM) placed between the central and cathodic compartments, which allows the passage of positively charged species to the catholyte and prevents the passage of anions from the catholyte to the solid matrix or suspension.

Figure 10b illustrates another configuration of ED cells patented by the DTU (PCT/EP2014/068956). This cell is formed of two compartments, separated by a CEM. The solid suspension is in the anode compartment. This configuration allows the acidification of the compartment due to water oxidation, promoting the mobilization of metals, and decreasing the amount of acid added to the system.

This technique has been applied in numerous studies for the remediation of soils and solid matrices (e.g., wood, marine sediments, fly ash, and mine tailings) contaminated by inorganic and organic pollutants at a laboratory scale [66,67,68,69,70,71,72]. The inorganic pollutants can be divided into cationic metals (e.g., lead and cadmium), anionic metals (e.g., arsenic and chromium), and radionuclides (e.g., strontium and uranium). Applying an electric current promoted the electromigration of ions through the soil or solid matrices [73], cleaning the solid matrices. Considering the characteristics of LIB waste, a solid matrix containing metals, ED could be applied to extract and recover these metals.

Villen et al. [38] reviewed the state of the art of electrodialytic remediation techniques and analyzed their potential to be applied in battery recycling. They proposed to connect the extraction step with the separation step to occur in parallel in an ED cell such as that represented in Figure 10b. The residue of LIBs, once pretreated and crushed, would be placed in the anolyte compartment, where the particles would be dissolved. At the same time, the dissolved metal ions would be continuously removed from the anolyte and sent to the catholyte compartment, where they would be collected. One of the advantages of this technique is the continuous acidification of the extraction medium due to water oxidation, resulting in a decrease in the acid required. In the same study, the authors highlighted the need to optimize the most relevant parameters of this technique, such as the extracting agent, the working pH, the S/L ratio, the energy density, the cell’s configuration, and the type of stirring, depending on the waste to be treated and the components to be recovered.

Cerrillo et al. [74] introduced an innovative method that combined hydrometallurgical extraction with electrodialysis to selectively recover lithium (Li) and cobalt (Co) from LiCoO₂ cathodes. This combined approach reduced the need for a leaching solution by regenerating the acid through electrolysis. The extracting agent selected was a 0.1 M HCl solution with a solid-to-liquid ratio of 5 g/L. The experimental setup designed in that study (Figure 11) consisted of an electrodialytic cell with three compartments using cation exchange membranes (Neopsepta CMX-fd CEMs) to separate the central compartment from both electrode compartments (anode and cathode). Moreover, the central compartment was connected to an external vessel where dissolution of the solids was performed. The LiCoO₂–HCl suspension was filtered before recirculation into the cell compartment to avoid fouling of the membrane. A constant electrical current of 50 mA, equating to a current density of 1 mA cm⁻², was maintained. This method achieved a recovery rate of 62% of the Li and 33% of the Co in the catholyte, being 80% of the cobalt electrodeposited on the cathode’s surface.

Diaz et al. [75] reported an electrochemical-based method for leaching metals from active materials of LIBs (Figure 12). The process was performed in a two-compartment electrochemical cell separated by a bipolar membrane. In the catholyte compartment, the solid was placed with the Fe²⁺, acting as a reducing agent (Fe³⁺/Fe²⁺). This cell configuration decreased the acid requirements, since the protons were produced electrochemically. On the other hand, the reducing agent Fe²⁺ could be regenerated at the cathode’s surface to be used again in the leaching medium. A membrane was used to avoid the oxidation of Fe²⁺ to Fe³⁺ at the anode’s surface. With this design, leaching efficiencies of over 96% for Li, Co, Mn, and Ni were achieved, with a pulp density of 240 g/L and a low concentration of mineral acid. Moreover, the process could reduce the cost of chemicals and energy by 80% compared with peroxide-based hydrometallurgical leaching.

Electrochemical leaching and ED have the potential to be applied in processes of LIB recycling, not only to enhance the yield of extraction but also to decrease the chemical and energy consumption of the process. Nevertheless, optimizing the technique must be addressed to make it competitive at an industrial scale.

6.3. Regeneration of Reactants

ED has been used in various processes to generate reactants such as H₂SO₄ or HCl and NaOH from sodium sulfate solutions or brines [76,77]; neutralize acidic and basic effluent waste streams by converting chemical energy into electric energy [78]; recover and purify inorganic acids, bases, and salts; and to produce organic acids [52,79]. ED has been also studied for its potential to regenerate the reactants used for recycling LIBs.

Kang et al. [80] proposed the use of ED to generate LiOH and H₂SO₄ from a Li₂SO₄ solution. The feed solution was a by-product of the hydrometallurgical process of recycling LIBs, where the metals were leached with H₂SO₄ and precipitated using a LiOH solution. An ED cell with three compartments was used, with a dilution channel where the Li₂SO₄ solution was introduced, and two concentration channels, one for H₂SO₄ and the other for LiOH solutions (Figure 13). The results indicated a high ion-recovery ratio for Li⁺ and SO₄²⁻ of 94.3% and 87.5% at a current density of 833 A m⁻². This proposal has huge potential because the electrogenerated acids and bases could be reused in the recycling process, creating a closed-loop process.

Along the same lines, Asadi et al. [81] evaluated the utilization of electrodialysis for a closed-loop process of LIB recycling (Figure 14). They studied the different stages in the hydrometallurgical process, incorporating electrodialysis as the last stage to prevent the formation of acidic or sodium-enriched waste streams and to regenerate the acids and precipitants. The use of H₂SO₄ and H₂O₂ as leaching agents and LiOH as a precipitant resulted in high recovery rates for Ni (99.8%), Co (99.6%), and Mn (99.5%). Due to the high solubility of Li, it remained solubilized after the precipitation stage, resulting in a solution rich in soluble Li₂SO_4. Electrodialysis was used to regenerate LiOH and H₂SO₄ from the Li₂SO₄ solution, in both the batch and continuous modes. Under the same experimental conditions (48 ED unit cells, 96 min, 1100 A m⁻²), the production rate of LiOH was 14.5 kmol/h in the continuous mode while the production rate was 18.2 kmol/h in the batch mode. The results indicated that batch mode provided a more efficient production rate than the continuous mode. Moreover, the batch mode enabled operation with better control. These findings validated the effectiveness and precision of the ED process in the recovery of lithium, highlighting its potential for industrial applications.

7. Challenges and Limitations of Electrodialysis

ED is a promising technology for being implemented in the process of circular hydrometallurgical recycling. Additionally, the application of electrodialysis technology in other sustainable fields, such as regeneration of metals and alloys, demonstrates its broader potential and the importance of developing this technology. However, this technology faces several challenges that must be addressed to make the process more efficient and cost-effective [41].

A significant challenge for electrodialysis is the cost. The initial investment required for the membrane stack and the membranes is quite high, which poses a barrier to scaling up the process economically. It could limit the widespread implementation of ED technology. Moreover, the energy requirements also present a limitation for the implementation of ED. The process demands high overpotential to operate, due to the resistance within the cells’ components, such as the membranes and solutions. This high energy consumption results in an increased operational cost, affecting the economic viability of the process.

The membranes’ performance and maintenance are additional critical issues. The membranes used in ED are prone to fouling and clogging, which can reduce their life span and efficiency. Issues of reusability and selectivity further complicate the operation, contributing to the high overall operating cost. The selectivity of the membranes is particularly problematic when a mixture of metals needs to be separated. Currently, there is a lack of effective selective membranes for the separation of certain metals, limiting the versatility of the technology.

To overcome these challenges, the development of new materials and optimization of the operational parameters are crucial [46]. Advances in membrane technology can enhance the selectivity and reduce the energy requirements, while improved operational parameters can mitigate issues related to fouling and clogging of the membranes. Research in these fields is crucial to mitigate the current limitations of ED. Furthermore, strategies need to be developed to address practical issues such as the production of metal concentrates, the formation of metallic scale on the membranes, and challenges associated with scaling up the electrodialysis system. Addressing these issues is essential for improving the efficiency and effectiveness of the process.

8. Conclusions

Electrodialysis has significant potential to advance the principles of circular hydrometallurgy, particularly in the sustainable processing and recycling of waste lithium-ion batteries (LIBs). As a membrane-based separation technology, electrodialysis can efficiently recover valuable metals and purify the processing streams by selectively transporting ions through ion-exchange membranes under an electric field. This selective separation of ions is crucial for isolating high-purity metal salts, such as lithium, cobalt, and nickel, from complex LIB leachates. By integrating electrodialysis into the hydrometallurgical process, it is possible to minimize waste, reduce the need for chemical reagents, and lower energy consumption, aligning with the circular economy’s goal of resource efficiency. Furthermore, the use of electrodialysis can facilitate the recirculation of the processing water, decreasing the overall environmental footprint of battery recycling operations. Thus, electrodialysis not only enhances the recovery of critical materials but also promotes the sustainable and economical management of resources within the hydrometallurgical processes.