Summary of Pretreatment of Waste Lithium-Ion Batteries and Recycling of Valuable Metal Materials: A Review

Abstract

The recycling of used lithium-ion batteries has become a growing concern. As a large number of rare metal elements are present in waste lithium-ion batteries, recycling them can significantly improve resource utilization and reduce the material cost of battery production. The process of recycling used lithium-ion batteries involves three main technology parts: pretreatment, material recovery, and cathode material recycling. Pretreatment includes discharge treatment, uniform crushing, and removing impurities. Material-recovery technology mainly involves traditional pyrometallurgical and hydrometallurgical technologies, as well as the developing biometallurgy technology. Analysis of existing data shows that pretreatment technology is crucial for the recycling of used lithium-ion batteries. Hydrometallurgical technology and pyro-hydrometallurgical technology are expected to be the most suitable industrialization technology paths in the future, with biometallurgical technology and direct recycling technology providing a low-pollution development direction. This article summarizes the different pretreatment techniques and valuable metal-recovery pathways. The advantages and disadvantages of each method were evaluated. The economic costs, environmental benefits, and degree of industrialization of each method were assessed. The possible development directions of various methods are summarized to provide reference for future research.

1. Introduction

The global new energy market has been proliferating in recent years. In 2022, over 7.8 million electric vehicles (EVs) were sold, marking a 68% year-on-year increase and constituting 10% of new vehicle sales [1]. Borgwardana’s 2023 performance report predicts EV sales will reach USD 2.3 to USD 2.8 billion in 2023. The company’s fourth-quarter 2023 reports highlight the significant market share of power batteries (as shown in

Table 1. Fourth quarter 2023 organic net sales change (unaudited in millions) [2].

(In Millions)	Q1 2022 Net Sales	FX	Q1 2023 Acquisition Impact	Organic Net Sales Change	Q1 2022 Net Sales	Organic Net Sales Change %
Air Management	USD 1768	USD (81)	USD 2	USD 290	USD 1979	16.4%
Drivetrain & Battery Systems	895	(32)	—	92	955	10.3%
Fuel Systems	591	(24)	—	1	568	0.2
ePropulsion	440	(19)	20	46	487	10.5%
Aftermarket	307	(6)	—	29	330	9.4%
Inter-segment eliminations	(127)	—	—	(12)	(139)	—
Net sales	USD 3874	USD (162)	USD 22	USD 446	USD 4180	11.5%

) [2]. Properly managing waste lithium-ion batteries is essential for social development and ecological protection [3].

The power battery contains high-value metal materials such as Li, Ni, Co, and Mn, enriching its economic value. Generally, graphite is used as the anode material, and there are various choices of cathode materials, such as (LiFePO₄) [4], (LiCoO₂), (Li(NiCo-Mn)O₂), (LiMn₂O₄), (LiV₂O₃), and (LiNiO₂) [5]. The electrolyte usually consists of a vinyl carbonate solvent [6], dimethyl sulfoxide, propylene, and a salt solution. Even though the types of batteries differ, their internal structures are very similar, indicating that most batteries are rich in high-value metal materials (as shown in Figure 1) [7]. The cost of these cathode materials can typically account for 20% or more of the entire battery’s price, as the proportion of these metal materials in nature is very small, and the mining cost is relatively high. Therefore, there is a high motivation for recycling waste lithium-ion batteries at the economic level.

Secondly, due to the complex structure of the battery, as shown in Figure 1, spent lithium-ion batteries will cause great harm to the environment and human health. The enrichment of metal materials in the battery makes it difficult to degrade under natural conditions. Some substances in the electrolyte will produce harmful gases during the degradation process. For example, LiPF₆ in water will react and generate HF, and P₂O₅ will be produced during incineration. Unhealthy substances can poison land resources, pollute groundwater resources and air, and ultimately lead to threats to human health [8]. Therefore, recycling used lithium-ion batteries is also necessary from an ecological and health perspective.

The first step in recycling batteries is to pre-treat used batteries, which includes discharge, crushing, and flotation. Currently, the more mature discharge schemes are the use of conductive salt solution [9], conductive metal powder, conductive graphite powder [10], and direct extrusion discharge. Among them, the salt solution discharge method is cost-effective. Crushing technology can be divided into dry crushing and wet crushing, as well as hammer crushing and shear crushing. Mechanical crushing is a prevalent and effective method in industrial production. Finally, flotation technology is used to remove some impurities from the material [11]. The second step is the material-recovery phase. In this article, we will introduce pyrometallurgical technology [12], hydrometallurgical technology [13], biometallurgical technology [14], and direct repair technology.

2. Pre-Processing Technology

According to a study by Yu Dawei’s group, it is important to note that the pre-processing technology of lithium-ion batteries is crucial for their recycling. After pre-processing lithium-ion batteries, we can effectively prevent environmental pollution caused by the liquid in the batteries and make better use of the precious metals found in spent lithium-ion batteries [15].

2.1. Discharge

When recycling spent lithium-ion batteries, it is important to note that the battery will not be fully discharged. Before mechanical treatment, the battery has to undergo a discharge treatment to reduce the risk of cell explosion in the subsequent process [16]. Typically, NaCl and FeSO₄ solutions are used to soak the discharge of the recycled materials. This kind of technology effectively discharges the battery at a very low cost, but it also has a disadvantage—it can easily produce explosive gases like H₂ [17].

Jiang’s team conducted a study on the impact of different salt solutions on battery discharge. They compared the metal ion residue and discharge time of 5% Na₂S solution, NaCl, Na₂SO₄, and Na₂CO₃ discharge solutions. The results indicated that the Na₂S solution had only 0.2 M Li, 0.3 M Ni, and 0.011 M cobalt plasma at the end of discharge. Additionally, the metal ion content in the precipitate was lower than that of other discharge solutions (see

Table 2. Mass fraction of the major element ions in supernatant [18] (10⁻⁶).

Solution System	Li	Ni	Co	Mn	Cu	Fe	Al	P
NaCl	15.200	0.500	15.200	0.030	0.010	4.900	93.000	116.000
Na2SO4	3.400	0.500	0.490	0.020	0.010	1.200	161.000	12.300
Na2S	0.200	0.300	0.011	0.020	0.001	0.200	4.500	0.030
Na2CO3	0.400	0.200	0.008	0.010	0.010	0.400	2.000	0.020

and

Table 3. Mass fraction of major elements in precipitates [18] (%).

Solution System	Li	Ni	Co	Mn	Cu	Fe	Al	P	S
NaCl	0.033	0.420	0.044	0.072	0.065	36.600	19.400	0.032	0.021
Na2SO4	0.014	0.470	0.016	0.038	0.036	44.700	10.800	0.210	1.670
Na2S	0.007	0.003	0.005	0.008	0.004	26.500	0.180	0.008	61.400

) [18]. However, the discharge process took longer. This method has minimal corrosive effects on the battery, resulting in low environmental pollution and good emission control, but the efficiency is relatively low.

Yao’s team explored the effect of FeSO₄ solution on battery discharge. FeSO₄, NaCl, and MnSO₄ solutions were used to conduct parallel experiments with the discharge rate and metal ion residue detection. A fast discharge model using FeSO₄ solution reduces the residual voltage to 1.0 V at 125 min. A total discharge model using NaCl solution and FeSO₄ solution reduces the residual voltage to 0.5 V or less in 183 min. At the same time, nitrogen, carbon dioxide, and water vapor were mainly excluded in experiments with FeSO₄ solutions. Cu and Fe are mainly residuals in the solution. The results show that the FeSO₄ discharge solution has less pollution to the environment and has high discharge efficiency (the process is shown in Figure 2) [19].

2.2. Crushing, Screening, and Sorting

Currently, crushing technologies mainly include hammer crushing, wet crushing, and shear crushing [20,21]. After crushing, the materials need to be sorted layer by layer to separate the plastic, separator, metal shell, and electrode powder. The electrode powder consists of graphite as well as valuable metal oxides, which require further processing.

Direct screening allows for the enrichment of particles of a specific size. Nan’s group used a mechanical disassembly method to effectively separate the metal casing while exposing the valuable materials inside the battery using a sieve [22]. Granata’s team obtained electrode powder, which accounts for 49% of the original mass of used lithium-ion batteries, after two crushing and mesh screening, while the rest of the metal casing remained on the sieve [20]. Maschler’s team grinding and sieving analysis of used lithium-ion batteries showed that the coarse particles (+200 μm) were mainly composed of ferrous metal shells, copper foil, and aluminum foil, while most of the substances containing Co and Li were concentrated in −100 μm. Later, more than 90% of the Co and Li were concentrated in −200 μM by fine crushing, magnetic separation, and fractionation [23].

Magnetic screening is an effective method for separating the metal shell and internal components of used lithium-ion batteries. Shin’s team utilized waste lithium-ion batteries and conducted two rounds of crushing and screening in addition to one round of magnetic separation to separate them from the plastic packaging. During the initial crushing vibration, the magnetic separator successfully separated the shell material in the battery, while during the second crushing vibration, the cathode material was completely separated from the aluminum foil [24]. Li’s team proposed a new method involving anaerobic roasting and wet magnetic separation to treat powders enriched with LiCoO₂ and graphite after mechanical crushing. Through high-temperature burning, metal cobalt and lithium carbonate are directly obtained. Subsequently, the powder is treated with water using a magnetic stirrer, allowing the lithium carbonate to dissolve directly, the metal cobalt to be adsorbed on the magnetic stirrer, and the graphite to precipitate at the bottom of the water, thereby completing the separation of substances without the need for additional chemicals [25]. This method enables the eco-friendly and efficient recycling of used lithium-ion batteries.

The flotation process primarily utilizes hydrophilic cathode material, which sinks to the bottom of the water when wet. On the other hand, the anode material, graphite, is not hydrophilic and will adhere to air bubbles and float on the liquid’s surface. N-dodecane, kerosene, and similar substances are commonly used as collectors in flotation, while methyl isobutyl methanol (MIBC), octanol, pine oil, and others are used as foaming agents. Recent studies have shown that using the Fenton reagent in flotation can help reduce the impact of the organic film in batteries on the flotation process compared to direct flotation. Yu’s study found that the Fenton reagent increased the LiCoO₂ grade from 55.63% to 90% for the same raw material [26]. Fenton’s reagent can enhance the mineral grade of the obtained substances by breaking down the organic film through the generation of hydroxyl radicals.

In general, there are three main methods of battery sorting technology. The results show that the direct sieving method is low-cost and effective for the separation of metal shell and electrode material. However, due to the mechanical structure, it is not possible to subdivide too small particles. The magnetic separation method is an efficient method of separating metal components from internal materials and can achieve very little pollution and waste through process optimization design, which is a green sorting method, but the equipment cost is high, and the economic cost of large-scale use needs to be considered. Finally, there is the flotation method, which makes good use of the hydrophilic and hydrophobic properties of different substances. However, it will be affected by the battery separator, which will lead to a decrease in efficiency. While Fenton’s reagent is a good solution to this problem, it does require the pH of the reaction. In addition, the handling of organic reagents is an issue that must be considered.

3. The Technology of the Material-Recycling Steps

Now the spent battery technologies include a pyrometallurgical method, hydromel-metallurgical method, direct recovery method, and metallurgy method [27]. Figure 3 shows the path [28]. The first three techniques have been in development for a long time and they are more mature. The metallurgy one is currently more in the laboratory technology stage.

3.1. Combination of Pyrometallurgical Technology and Hydrometallurgical Technology

Pyrometallurgy is the process of using high-temperature heat to separate target metal materials from impurities through physical and chemical reactions. The use of pyrometallurgical technology is a classic treatment method for waste resources. High-temperature treatment can destroy the complex structure in the battery and melt the metals. The separation of the essential components in the cathode of a lithium-ion battery is achieved via a high-temperature method [29]. In general, manganese and lithium produce oxides in oxygen-rich environments. Other metals are alloyed at high temperatures, which makes it relatively easy to separate the relevant metal materials from waste lithium-ion battery materials [30]. However, from the perspective of environmental protection, pyrometallurgical technology has to produce a large number of harmful gases and sewage. And in the smelting process, there will be more loss of valuable metal materials. Therefore, from the perspective of the recycling and environmental benefits of the whole process, pyrometallurgical technology is facing great challenges [31,32]. Therefore, an increasing number of scholars have started studying the combination of pyrometallurgy and hydrometallurgy. They aim to utilize the characteristics of pyrometallurgy to eliminate complicated pretreatment steps and, at the same time, employ hydrometallurgy to achieve higher metal-recovery efficiency. (as shown in Figure 4).

Dang’s team investigated the effect of Gibbs’s free energy and reaction equilibrium constants using calcium chloride to affect metallurgical reactions on the pyrometallurgical treatment of used batteries. They added CaCl₂ at a ratio of 1.8:1 Cl:Li, and roasted at a temperature of not less than 500 °C. The results showed that LiCl produced in the chlorination reaction was a good chlorine donor, and the final optimal roasting conditions at 800 °C for 60 min, Cl/Li molar ratio of 1.8:1, and subsequent leaching conditions at 60 °C for 30 min, with a water/calcination mass ratio of 30:1, could produce a maximum lithium-recovery rate of 90.58% [33].

Gou Haipeng researched the use of pyrometallurgy to extract metals from waste batteries in a nitrogen atmosphere. Nitrogen was used as the gas atmosphere in the experiment, and activated carbon and alkali absorption devices were used to adsorb harmful gases. The results indicated that at 650 degrees Celsius, a black powder rich in graphite and metal can be obtained. The percentage of this powder is shown in

Table 4. Proportion of different metals after purification [34].

Elements	Cu	Al	Li	Ni	Co	Mn
Content	10.72	6.15	2.92	16.08	5.81	4.95

[34]. This method resolves the exhaust gases generated during the process and eliminates the risk of explosion. Afterward, the powder needs to undergo treatment using hydrometallurgical methods.

Meng studied the effect of synergistic pyrometallurgy with organic waste on the pyro-wet treatment of spent lithium-ion batteries. In his experiments, he used waste plastics as carbon and hydrogen sources to produce reducing agents to achieve the reduction of valuable metals. The experimental results show that LiCoO₂ is completely decomposed into Li₂CO₃, CoO, Co, and chloride for 90 min at 450 °C with a PVC/LCO mass ratio of 1:1. High recoveries of Li (92.50%) and Co (94.85%) can be achieved by water immersion. It is important to note that the Cl compounds released by the decomposition of PVC may be in metal chlorides. During the pyrolysis process, PVC can initially decompose to form carbon, gaseous hydrocarbons, and hydrochloric acid [35].

Liu studied the reactions of different spent lithium-ion batteries (LCO, LMO, NMC) and benzene-containing plastics (polycarbonate, polyethylene terephthalate, polybutylene terephthalate, polystyrene) in sealed reactors. According to the experimental results, the pyrolysis of the two together produces Li₂CO₃, metals, and metal oxides. The use of closed reactors can make the polycyclic aromatic hydrocarbons and lithium transition metal oxides undergo sufficient reduction reactions, and the lithium recoveries of LCO, LMO, and NMC can reach 98.3%, 99.9%, and 97.5% [36].

Recycling used lithium-ion batteries using a single pyrometallurgical technology is challenging due to environmental and economic reasons [37]. Currently, efforts are focused on combining pyrometallurgy with hydrometallurgy to create a more efficient recovery process that eliminates complex pretreatment steps. However, this approach requires significant equipment investment and still poses a gas pollution issue. Collaborative pyrolysis technology offers a promising solution by efficiently utilizing plastic waste and used batteries, but optimizing the process parameters, conditions, and equipment selection is crucial. When using household waste as a source of carbon and hydrogen, the presence of inorganic substances like minerals can significantly impact the recovery of valuable metals in cathode materials through pyrolysis. Additionally, addressing the emission of polluting gases during pyrometallurgy remains an important challenge to address [38].

3.2. The Inorganic Acid Hydrometallurgy Technology

The hydrometallurgical method is a method in which the useful metals contained in the raw materials are transferred to the liquid phase through chemical reactions after contact with aqueous solutions or other liquids, and then the various useful metals contained in the liquid phase are separated and enriched and finally recovered in the form of metals or other compounds. It mainly includes unit operation processes such as leaching, liquid-solid separation, solution purification, metal extraction in solution, and wastewater treatment. Compared to the limitation of the pyrometallurgy tech, the hydrometallurgy tech is used more widely than the former, and the recycling effect is better too [39].

Now, hydrometallurgy technology can process popular lithium iron phosphate batteries, ternary lithium batteries, nickel-metal hydride batteries, and so on [40,41]. The hydrometallurgy technology can be widely used, and there is in-depth study in this area. Based on the chemical properties of the leaching solution, the hydrometallurgical method can be roughly divided into two groups: the inorganic acid leaching method and the organic acid leaching method [42]. The concentration, temperature, solid–liquid ratio, and pH value of the reagents used in hydrometallurgy are all decisive factors.

Inorganic acid leaching is a well-established technique for extracting high-purity valuable metals from spent lithium-ion battery materials. This technique primarily utilizes solutions such as HCl, HNO₃, and H₂SO₄. The final effect of these three leaching solutions varies due to their chemical properties. When using sulfuric acid and nitric acid as extractors, hydrogen peroxide needs to be added as an auxiliary infusion agent to achieve a better leaching effect [43]. Wang’s group uses sulfuric acid as an extractor to process spent lithium-ion batteries. Let Li, Co, Ni, Mn, and other valuable metal elements be leached. At the same time, add the KMnO₄ to the leachate to selectively react with Mn to generate MnO₂ and manganese hydroxide. Then Ni was extracted and separated using butanediketoxime, followed by selective precipitation of Co by sodium hydroxide (pH = 11). Finally, the Na₂Co₃ was used to form the Li₂CO₃ precipitation of Li in solution. The results of the test showed that it can achieve extraction purities of Li, Mn, Co, and Ni of 96.97%, 98.23%, 96.94%, and 97.43%, respectively [44].

Barik’s team investigated the effect of HCl as the leaching agent and HClO as the impurity remover on the extraction of used lithium-ion battery materials. They used a multi-dimensional, single-variable experiment to explore the optimal extraction conditions, and the results showed that the temperature was 50 °C, the concentration was 1.75 mol/L HCl, and the condition was within 90 min. The leaching efficiency of O²⁻ and Mn²⁺ was higher than 99%. Also, 90% total Co recovery and 95% total Mn recovery can be achieved when the treatment scale is scaled [45].

Dorella’s group also used H₂SO₄ and H₂O₂ as the leaching agent to leach the cathode material of lithium cobalt oxide battery. Then they used NH₄OH to remove Al from the leachate. Finally, Cyanex272 was used for extraction to separate the cobalt from the leachate. A study shows when the H₂SO₄ concentration is 6%, the H₂O₂ concentration is 1%, the solid–liquid ratio is 1:30, and the leaching rates of Co and Li can reach 80% and 95%, respectively [46].

The hydrometallurgical treatment of waste lithium-ion batteries using inorganic acids is an effective, stable, and widely applicable method. The extractant is cost-effective and has high extraction efficiency. However, a common disadvantage is the strong corrosive effect on the equipment. Additionally, the reuse performance of the extractant is poor. From an economic point of view, inorganic acid extraction is a technology with a small upfront investment, but the equipment maintenance cost is high in the later stage. Industrialization is relatively easy and suitable for large-scale production. From an environmental point of view, the inorganic acid extraction method produces a large amount of wastewater and complex waste gases, which needs to be purified before it can be used.

Table 5. Leaching effects of inorganic acids.

Reagent	Type of Material	Conditions	Efficiency	References
H2SO4 with H2O2	Spent LiBs	2.00 M H2SO4, 4.0% H2O2, 50 °C, 120 min, S/L 50 g·L−1	>98% Co and Li were recovered	[43]
HNO3 with H2O2	Spent LiBs	1.00 M HNO3, 1.7% H2O2, 75 °C, 60 min, S/L 20 g·L−1	>95% Co and Li were recovered	[43]
HCl	Spent LiBs	1.75 M HCl, 50 °C, 120 min, S/L 20 g·L−1	90% Co and 99% Mn were recovered	[45]
H2SO4 and H2O2; Cyanex272	The cathode material of lithium cobalt oxide battery	6% H2SO4, 1% H2O2, S/L 30 g·L	80% Co and 95% Li were recovered	[46]

provides a summary of the above technical paths.

3.3. The Organic Acid Leaching Method

The inorganic acid leaching method offers several technical advantages such as good scale efficiency, a simple technical process, and moderate cost. However, it generates a large amount of harmful gases and leads to strong corrosion of the recycling equipment, significantly reducing its service life [47]. On the other hand, the organic acid leaching method has emerged as a major research focus in hydrometallurgy due to its low pollution and low corrosive properties [48,49].

Organic acids can often be chelated with metal lithium ions due to their chemical structure and physical and chemical properties. The chelation reaction between organic acids and metal ions is usually multi-point, involving a variety of molecular forces. A strong chelating structure can be formed. So organic acid extraction can achieve the leaching effect of strong inorganic acids in a low acidity state. At the same time, many organic acids have their own reducing ability such as oxalic acid, citric acid, and ascorbic acid. As a leaching agent can promote the metal ions from a high-valence state to a low-valence state, researchers are trying to replace inorganic acids with organic acids to become the leaching agent of spent lithium-ion batteries [50]. The hydrometallurgical process of organic acids is mainly divided into several important steps: pretreatment, extraction, separation, and reverse extraction (as shown in Figure 5).

Meng’s team studied the extraction of valuable metals from used lithium-ion batteries using malic acid as the extractant, and they used 1.5 M malic acid, 8 V voltage, 300 r/min, 60 °C, and extraction for 30 min. The results showed that the extraction rates of Li, Co, Ni, and Mn were 100%, 99.87%, 99.58%, and 99.82%, respectively [52]. Pant and Dolker proposed to use citrus juice (CJ) components to help dispose of spent lithium-ion batteries in their study. CJ is rich in citric acid malic acid and other organic acids. They can be used well as a complexing agent for ascorbic acid and citrus flavonoids for the reduction of a variety of metals. In the experiment, the positive electrode leachate was obtained by this method and then the pH value of Na₂CO₃ was adjusted to achieve selective precipitation. It is calculated that the method can achieve Li, Mn, Ni, and Co leaching rates of 100%, 99%, 98%, and 94% [53]. Sun’s group conducts leaching treatment with LiCoO₂ by combining maleic acid and SnCl₂. It was found that the leaching rates of Li and Co reached 98.67% and 97.5%, respectively, under the conditions of 1 mol/L maleic acid and 0.3 mol/L SnCl₂ [54].

In summary, organic acids can be used as extractants to obtain better extraction results under milder conditions. This can greatly reduce the degree of corrosion in the operating equipment. However, compared with the low price of inorganic acids, the price of organic acids is higher, and the cost of use is relatively large. Yang studied the extraction effect of a mixture of acetic acid and ascorbic acid. They experimented with different concentration ratios and experimental conditions. Finally, it was found that 1 M acetic acid was mixed with 0.1 M ascorbic acid 1:1, working voltage of 4 V, reaction temperature of 25 °C, and reaction time of 70 min. The leaching rates of Ni, Co, Mn, and Li reached 99.8%, 99.8%, 99.8%, and 99.9%, respectively. At the same time, Yang proved that the surface chemical reaction of the metal determines the speed of the reaction [55]. This process also shows that chelation reactions are very important in organic acid leaching. This is also the difference between organic acid leaching and inorganic acid leaching.

Su proposed the use of EDTA as a leaching-reducing agent to treat waste lithium-ion battery materials. Under the condition of 0.5 M EDTA, a solid–liquid ratio of 30:1 mL/g, 353 K, 2 h can achieve a metal ion leaching rate of 99.9%. In the experiment, the reaction thermodynamic mechanism proved that there is a synergistic leaching effect of Fe and Al. EDTA can also be recycled after separating the various ions by precipitation (as shown in Figure 6) [56].

In conclusion, the above two methods show that the mechanism of organic acid extraction is not only the solubility of ions in different solutions, but also the complexation effect of organic solvents can enhance the extraction effect, and the recycling performance of the organic extraction system is better, which can reduce the increase in the cost of extractants.

On top of the non-selective leaching methods described above, selective extraction methods have also been proposed in some studies. Selective extraction of partial ions is achieved by different metal salts having different solubilities in the same extractant. Rouquette used the different solubility of different transition metal salts in oxalic acid as the main driving force for selective extraction. Under the conditions of 60 °C, 60 min, and 0.6 M oxalic acid, the leaching rate of lithium reached 98.8%. The leaching rate of cobalt and nickel is only 1.5%. Efficient recovery of lithium is achieved [57].

Wang Kun used a completely different extraction strategy in his study. The simulated complex metal ion solution was extracted using an extraction system consisting of 0.4 mol/L 2-thenoyltrifluoroacetone (HTTA), 0.4 mol/L trioctylphosphine oxide (TOPO), and ionic liquid (IL) 1-butyl-3-methylimidazolium bis (trifluo-romethyl sulfonyl) imide. Under the conditions of O/A = 1/1, 25 °C, 20 min, and pH = 3.65, the extraction rates of Co, Mn, and Ni reached 99.54%, 99.85%, and 99.68%, while the extraction rate of Li was only 2.84%. This achieves the effect of selective extraction [58]. The above two solutions can achieve high-purity extraction of Li to prepare for the subsequent efficient recovery of metal materials and the production of high-quality recycled cathode materials.

Table 6. Leaching effects of organic acids .

Reagent	Type of Material	Conditions	Efficiency	References
Malic acid	Cathode waste materials	1.5 M malic acid, 8 V, 300 r/min, 60 °C, 30 min	100% Li, 99.87% Co, 99.58% Ni and 99.82% Mn were recovered	[52]
Acetic acid, Ascorbic acid	Spent LiBs	1.00 M acetic acid, 0.10 M ascorbic acid, 4 V, 25 °C, 70 min	99.8% Ni, 99.8% Co, 99.8% Mn, 99.9% Li were recovered	[55]
EDTA	Spent LiBs	0.50 M EDTA, 353 K, 120 min, S/L 30 mL/g	99% metal were recovered	[56]
Gluconic acid	Spent LiBs	1.2 M gluconic acid, H2O2 1.6 vol%, S/L 25 g/L, 75 °C, 192 min	Over 98% Li, Co, Mn, over 80% Ni were recovered	[59]
Ascorbic acid or Employing hexuronic acid	Spent LiBs	0.8 M acids, 70 °C, 60 min, S/L 50 g/L	100% Li, 99.5% Cowere recovered	[60]
DL-lactic acid	Spent LiBs	1.00 M DL-lactic acid, 6% H2O2, 60 °C, S/L 10 g/L, 60 min	99.8% Li, 99% Co, were recovered	[61]

will show some representative techniques.

In summary, the organic acid leaching method can achieve the extraction effect of inorganic acids under milder extraction conditions under more extreme pH conditions, which can reduce the corrosion rate of the equipment. The upfront investment cost of organic acid extraction is relatively high, and the price of reagents is also high. However, it can effectively solve the problem of poor circulation of inorganic acids and serious corrosion of equipment. Similarly, the exhaust gases obtained by the organic acid extraction method are complex and can be used in other industrial processes after purification. With the reasonable circulation design, the wastewater generated by the organic acid extraction method can be greatly reduced and the environmental pollution can be reduced [62]. There is still great room for the development of the organic acid leaching method in the future. We need to focus on finding an organic acid leaching agent with high unit leaching efficiency and relatively low cost.

3.4. The Direct Recycling Technology

The charging and discharging of lithium-ion batteries is essentially the cyclic process of directional movement and removal of Li⁺ inside the battery. As the charging and discharging process progresses, the crystal structure of the cathode material of lithium-ion batteries continues to expand and contract. As a result, the battery loosens the cathode material structure, which is reflected in the macroscopic decline of the capacity of the battery [63].

The direct recycling method repairs the cathode material of lithium-ion batteries that have been retired because of battery capacity decline through some specific physical and chemical reactions. Now the main technical paths include atmospheric pressure lithiation of low eutectic solvents, hydrothermal lithiation, electrochemical method, solid phase method, and so on [64].

In Ji’s solid-state method research, dimethyl carbonate was used to clean disassembled mobile phone batteries. The batteries were then calcined into spent LiCoO₂ (S-LCO) powder in the air. Annealing with a 1.03 ratio of lithium carbonate to Li/Co mixture at 850 °C for 10 h was carried out. The experimental results suggest that this process smoothens the surface of LCO and forms H-LCO, which helps repair the damaged structure. The resulting product is mixed with Co₃O₄ at 300 °C and further annealed to create a spinel protective layer, forming the regenerated LCO (HS-LCO) (this is shown in Figure 7B). HS-LCO demonstrates a high specific capacity retention rate of 85.9% after 100 cycles (as shown in Figure 8) [65].

Chen et al. used LiOH as a lithium source and tartaric acid as a reducing agent in their study. After hydrothermal treatment, the regenerated LFP was annealed in Ar gas at 700 °C for 2 h (as shown in Figure 7C). The characterization image shows that the lattice distortion and fringes disappeared after processing. Due to the combination of hydrothermal treatment and annealing, the discharge capacity of the regenerative battery material at 1 °C is 145.92 mAh·g. Experiments show that the discharge performance of the repaired material is greatly increased (as shown in Figure 9) [66].

Wang’s team conducted experiments using eutectic solvent culture. Direct remediation of degraded LiCoO₂ was performed using LiCl–CH₄N₂O deep eutectic solvent (DES) (as shown in Figure 7A). The results suggest that DES is not used to dissolve LiCoO₂, but is used directly as a carrier for selective supplementation of lithium and cobalt. Lithium supplementation restored LiCoO₂ to a capacity of 130 mAh·g (0.1 C rate) at different states of charge, while cobalt supplementation increased capacity retention by 90% after 100 cycles [67]. Zhou’s team researched methods to recover unused lithium and manganese from LiMn₂O₄ using a pH gradient created in neutral water. The study found that lithium and manganese ions can be extracted from LiMn₂O₄ at low pH in the anode chamber, while manganese oxide can be precipitated at high pH in the cathode chamber. Additionally, hydrogen and oxygen are produced as useful byproducts. Lithium can be recovered in the form of lithium carbonate, and the LiMn₂O₄ can be re-synthesized, completing the process loop (As shown in Figure 7D). The study also carefully examined the specific roles of various factors in the leaching of LiMn₂O₄ and the resulting phase changes [68].

Table 7. Summary of different direct recycling techniques.

Method Type	Type of Material	Conditions	Efficiency	References
Solid-state methode	LCO	Solid-state process with direct mixing	152.4 mA h g−1 at 0.2 C and the capacity decay rate per cycle was only 0.0313 mA h g−1	[69]
	LCO	Solid-state process with a mixture of S-LCO, Li2CO3 and Co3O4	135.7 mA h g−1 at 0.5 C after 250 cycles, superior capacity retention	[65]
Hydrothermal method	LCO	Hydrothermal treatments with short annealing and adopted a Ni and Mn co-doped strategy	160.23 mA h g−1 at 1 C and capacity retention of 91.2% after 100 cycles	[70]
	LCO	Hydrothermal treatment 220 °C for 4 h with short annealing (800 °C for 4 h)	48.2 mAh g−1 (150 mA g−1, 3.0–4.3 V), 91.2% after 100 cycles	[71]
Eutectic medium method	LOC	Eutectic salts LiOH-KOH-Li2CO3	144 mA h g−1, an excellent cycling stability of 92.5% capacity retention after 200 cycles at 0.2 C	[72]
	LOC	Eutectic salts Li2CO3, LiOH·H2O and LiAc·2H2O	160 mAh·g−1 between 3.0–4.3 V. The discharge capacity after cycling for 50 times is still 145.2 mAh·g−1	[73]
Electrochemical method	LOC	Electrodeposition	Delivering a specific capacity of an initial discharge capacity of 127.2 mA h g−1 at 0.1 C	[74]
	LOC	Electrodeposition	The discharge capacity is 160.1 mA h g−1 in the initial cycle and 146.2 mA h g−1 in the 100th cycle, and the reversible specific capacity is up to 144.2 mA h g−1 at 5 C.	[75]

summarizes the results of technical research in different directions.

In short, direct recycling is an emerging technology with the potential to significantly reduce the use of chemical reagents and be environmentally friendly. Additionally, it exhibits strong selectivity for specific metal ions and has demonstrated promising results in laboratory settings. However, as of now, this technology is only being investigated on a small scale in laboratories and has not been applied on an industrial level. Processing the cathode material is currently not feasible.

3.5. The Biometallurgy Method

Biometallurgical technology refers to the method of using microorganisms as the main body to dissolve metal ions into the leaching solution or enrich them in microorganisms by using microbial catalytic oxidation. It mainly includes four main steps: bioleaching, bioabsorption, bioselection and enrichment, and waste biological treatment [76].

Biometallurgy technology has been developed alongside bioengineering. However, biometallurgy technology has been significantly hindered by its low recovery efficiency and selectivity. To overcome this technical barrier, researchers have shifted their focus to developing new strains [77]. Gumulya illustrates that microorganisms used in metallurgical technology have good acidophilic, autotrophic, and heavy metal resistance. This allows them to obtain energy from metal oxide at lower pH and in a relatively high concentration of heavy metal ions. Through genetic modification technology, these sexual transformations can be made more significant, and finally suitable for industrial production [78].

Roy’s team studied the extraction effect of A. ferroxidase under specified conditions. They find the optimal extraction conditions by controlling the inoculum amount, pH, liquid ratio, temperature, and rotational speed. The experimental results showed that when the pH value was equal to 2, 10% of the bacterial solution was added, and the inoculation amount of 100 g/L of the strain could obtain a good effect of 94% Co extraction rate [79].

Ghassa’s team used a consortium of moderately thermophilic bacteria as the extraction microorganism. At pH 1.8, 10% inoculation at 45 °C, and 130 rpm, the extraction of 99.9% Co, 99.7% Ni, and 84% Li was achieved [80]. Bahaloo-Horeh’s team used Aspergillus niger at 26.478 (g L⁻¹) sucrose concentration, 3.45% (V V⁻¹) inoculum amount, and pH 5.44. They achieved an extraction rate of 100% of Cu, 100% of Li, and 77% of Mn. They also tried other extraction conditions and finally achieved satisfactory results (as shown in

Table 8. Leaching effects of bioleaching.

Microorganism	Type of Material	Conditions	Efficiency	References
A. ferrooxidans	A mixture of LiCoO2-based spent LiBs	pH 2; 10% (v/v) Modified 9 K medium at pulp density 100 g/L 30 °C and 160 rpm in 72 h	Co (94%) and Li (60%) were recovered	[76]
Consortium of moderately thermophilic bacteria	Waste LiB cells	pH 1.8; 10% inoculation at 45 °C and 130 rpm	Co recoveries reached 99.9%, Ni recoveries reached 99.7%, and Li recoveries reached 84%	[80]
Aspergillus niger	Spent LiBs	At 26.478 (g L−1) sucrose concentration, 3.45% (V V−1) inoculum amount, pH 5.44	100% Cu, 100% Li, 77% Mn, and 75% Al occurred at 2% (w v−1) pulp density; 64% Co and 54% Ni recovery occurred at 1% (w v−1)	[81]
Aspergillus niger	Spent LiBs	At a pulp density of 1% (w/v), the adapted Aspergillus niger leached	100% Li, 94% Cu, 72% Mn, 62% Al, 45% Ni, and 38% Co were recovered	[82]

) [81].

Biometallurgy technology has shown promise in achieving improved selectivity and higher extraction rates, while also causing less pollution in laboratory settings. However, the impact of heavy metal poisoning on microorganisms, the high cost of cultivating microorganisms, and the challenges associated with recycling make the process economically costly. Moreover, the extraction efficiency tends to decrease when dealing with high concentrations of metal ions.

Chen’s team pointed out in a study that Ni²⁺ produces nickel ion stress when processing spent lithium-ion battery materials. This threatens the health of microorganisms and results in a decrease in the treatment effect. By studying the mechanism of sulfadoxine on nickel ion stress, they looked for the improvement of biological resistance to nickel ion stress by increasing the activity of ATP-reducing coenzymes. At the same time, RNA-sep genes identify what we know differentially expressed by nickel ion stress. That provided ideas for understanding metallurgy technology (as shown in Figure 10) [83].

All in all, the factors influencing biometallurgy technology’s effectiveness are primarily two-fold. First, there is the microbial species, and second, there is the influence of the environment on the microorganisms themselves [84]. Biometallurgy is undoubtedly a green technology. The waste gas produced mainly comes from microbial catalytic oxidation reactions, and the pH value and other conditions of wastewater are also relatively mild. However, at present, its cost is high, and the efficiency does not have much advantage over traditional technologies.

3.6. Summary of Technologies for the Material-Recycling Phase

In general, the primary methods for recycling mature materials are pyrometallurgy and hydrometallurgy. Direct repair technology and biometallurgy require significant development time to become industrialized. Pyrometallurgy technology is relatively easy to achieve. However, it consumes a lot of energy and the quality and recovery efficiency of material recovery are not high. Therefore, it is necessary to combine it with certain hydrometallurgical technologies to achieve a high recovery rate [85].

Currently, hydrometallurgy technology is expected to become a more widely used recycling method in the future. It offers better recovery efficiency, lower energy consumption, and causes slightly less direct pollution to the environment compared to pyrometallurgy. However, the use of organic acids for hydrometallurgy is still in the developmental stage [86]. Additionally, the hydrometallurgical technology using inorganic acids results in high losses on equipment [87]. Proper treatment of a large volume of toxic and harmful waste gas and waste liquid generated in the recycling process is a significant issue that needs to be addressed. At present, some scholars are studying the use of waste gas mineralization technology, which can also be applied to the recovery process to reduce CO₂ and other gas emissions [88].

At present, biometallurgy and direct repair technology can effectively treat waste lithium-ion batteries under specific laboratory conditions. However, biometallurgy technology faces two major challenges: cost and the difficulty of finding suitable tolerance microorganisms [89]. Direct repair technology is not efficient, and the cost is high. Economic and environmental indicators are challenging to judge with data. A summary in

Table 9. Summary of comprehensive evaluation and development of various recycling technologies.

Technology	The Level of Technical Difficulty	Economies of Scale Costs	Degree of Industrialization	Environmentally Friendly	Major Contaminants	Technological Development Points
Pyrometallurgy	Low	Low	High	Low	Sewage, exhaust gas, metal waste residue	Use low-energy devices in combination with other methods.
Pyrometallurgy–hydrometallurgy	Medium	Medium	Medium	Medium	Sewage, exhaust gas, metal waste residue, organic solvents	The use of organic waste synergistic pyrometallurgy to achieve waste utilization, cleaner solvents
Hydrometallurgy	Medium	Low	High	Medium	Sewage, organic solvents, exhaust gases	A cleaner extraction system with a better recycling process design
Direct recycling	High	High	Low	High	Sewage, exhaust gases	Embark on an industrial
Biometallurgy	High	High	Low	High	Sewage, waste microorganisms	Microorganisms that are more suitable for working in extreme environments, industrial exploration

will provide an overview of the current status of relevant technologies based on “high”, “medium”, and “low” judgments from multiple dimensions.

In the short term, the recycling of waste lithium-ion batteries should focus on developing wet-firing mixing and hydrometallurgical technologies that use organic acids. In the long term, biometallurgy technology could offer a more environmentally friendly approach to recycling lithium-ion batteries.

4. Conclusions

It is essential to note that, in addition to recycling electrode materials and precursors from spent lithium-ion batteries, significant profits can also be gained from the pretreatment process. The metals, such as aluminum, copper, and iron, recovered during the pretreatment phase hold great value [90]. Pyrometallurgical technology can process large quantities of materials while also being very simple but with high energy consumption and low recovery of valuable metals. The advantages of hydrometallurgical technology are that the recovery efficiency of valuable metals is high, the application scenarios are very wide, and the application is also very flexible. Among them, organic acid extraction is becoming a green and effective process method [91]. The main disadvantage of hydrometallurgical technology is that it has a long process flow, which leads to a slightly more complex process flow. Metals from spent lithium-ion batteries are recycled, mainly focusing on pyrometallurgical or hydrometallurgical methods or a combination of the two [92]. Biometallurgical technology belongs among the current cutting-edge technology, it has the advantages of being green and effective, and the bioleaching process is inevitably accompanied by an increase in Ni²⁺ accumulation in the later stage because the battery is rich in Ni. Therefore, how to reduce the impact of this problem on microorganisms will be a major concern in the future. At the same time, most of the results are currently obtained in the laboratory; achieving industrialization requires more exploration.