Lithium-ion battery recycling technology
The recycling process of spent lithium-ion batteries mainly includes pretreatment, secondary treatment, and advanced treatment. Since spent batteries still retain some charge, the pretreatment process includes deep discharge, crushing, and physical sorting. Secondary treatment aims to completely separate the positive and negative electrode active materials from the substrate, commonly achieved through methods such as thermal treatment, organic solvent dissolution, alkaline dissolution, and electrolysis. Advanced treatment mainly includes leaching and separation purification to extract valuable metal materials. Based on extraction processes, battery recycling methods can be broadly classified into three categories: dry recycling, wet recycling, and biological recycling.
1. Dry recycling
Dry recycling refers to the direct recovery of materials or valuable metals without the use of solutions or other media. Important methods used include physical sorting and high-temperature pyrolysis.
(1) Physical sorting method
Physical sorting refers to the disassembly and separation of batteries, where battery components such as electrode actives, current collectors, and battery casings are crushed, sieved, magnetically separated, finely pulverized, and classified to obtain valuable high-content substances. Shin et al. proposed a method for recovering Li and Co from lithium-ion battery waste liquid using sulfuric acid and hydrogen peroxide, which includes two processes: physical separation of metal-containing particles and chemical leaching. The physical separation process includes crushing, sieving, magnetic separation, fine crushing, and classification. The experiment used a set of rotating and fixed blade crushers for crushing, sieves of different aperture sizes to classify the crushed materials, and magnetic separation for further processing, preparing for the subsequent chemical leaching process.
Building upon the grinding technology and water leaching process developed by Zhang et al., Lee et al., and Saeki et al., Shu et al. developed a novel mechanochemical method for recovering cobalt and lithium from lithium-sulfur battery waste. This method utilizes a planetary ball mill to co-mill lithium cobalt oxide (LiCoO2) and polyvinyl chloride (PVC) in air, mechanochemically forming Co and lithium chloride (LiCl). Subsequently, the milled products are dispersed in water to extract the chlorides. Grinding promotes the mechanochemical reaction. With continued grinding, the extraction yields of both Co and Li are increased. A 30-minute grinding process resulted in the recovery of over 90% of Co and nearly 100% of the lithium. Simultaneously, approximately 90% of the chlorine in the PVC sample was converted into inorganic chlorides.
Physical sorting is relatively simple to operate, but it is not easy to completely separate lithium-ion batteries. Furthermore, mechanical entrainment losses are likely to occur during screening and magnetic separation, making it difficult to achieve complete separation and recycling of metals.
(2) High-temperature pyrolysis method
High-temperature pyrolysis refers to the process of calcining and decomposing lithium-ion battery materials that have undergone preliminary physical separation treatments such as crushing at high temperatures. This removes the organic binders and separates the constituent materials of the lithium-ion battery. Simultaneously, it can also cause the metals and their compounds in the lithium-ion battery to undergo oxidation-reduction and decomposition, volatilizing them as vapor, which is then collected using methods such as condensation.
Lee et al. employed a high-temperature pyrolysis method to prepare LiCoO2 from spent lithium-ion batteries. First, they heat-treated the LIB samples in a muffle furnace at 100–150°C for 1 hour. Next, they shredded the heat-treated batteries to release the electrode materials. The samples were disassembled using a high-speed shredder specifically designed for this study, sorted by size (range 1–50 mm). Then, a two-step heat treatment was performed in the furnace: a first treatment at 100–500°C for 30 minutes, followed by a second treatment at 300–500°C for 1 hour. The electrode materials were then released from the current collector by vibration sieving. Finally, the carbon and binder were burned off at 500–900°C for 0.5–2 hours to obtain the cathode active material, LiCoO2. Experimental data showed that the carbon and binder were burned off at 800°C.
High-temperature pyrolysis is a simple and convenient process with rapid and efficient reaction at high temperatures, effectively removing binders. It also has low requirements for the composition of raw materials, making it suitable for processing large quantities or complex batteries. However, this method requires sophisticated equipment; the decomposition of organic matter from the batteries during processing releases harmful gases, which is environmentally unfriendly. Additional purification and recovery equipment is needed to absorb and purify these harmful gases and prevent secondary pollution. Therefore, this method has a high processing cost.
2. Wet recycling
The wet recycling process involves crushing and dissolving waste batteries, then using appropriate chemical reagents to selectively separate metal elements from the leaching solution, producing high-grade cobalt metal or lithium carbonate, which are then directly recycled. Wet recycling is well-suited for recovering waste lithium-ion batteries with relatively simple chemical compositions. Its equipment investment costs are relatively low, making it suitable for small to medium-scale waste lithium-ion battery recycling. Therefore, this method is currently widely used.
(1) Alkali-acid leaching method
Since the positive electrode material of lithium-ion batteries is insoluble in alkaline solutions, while the substrate aluminum foil is, this method is often used to separate the aluminum foil. Zhang Yang et al., when recovering Co and Li from batteries, first used alkali leaching to remove aluminum, and then used dilute acid to soak the copper foil to break the adhesion between the organic matter and the copper foil. However, the alkali leaching method cannot completely remove PVDF, which has an adverse effect on subsequent leaching.
Most of the positive electrode active materials in lithium-ion batteries are soluble in acid. Therefore, the pre-treated electrode materials can be leached with acid solution to separate the active materials from the current collector. Then, the target metal can be precipitated and purified by the principle of neutralization reaction, thereby achieving the purpose of recovering high-purity components.
Acid leaching utilizes traditional inorganic acids, including hydrochloric acid, sulfuric acid, and nitric acid. However, the leaching process using strong inorganic acids often produces harmful gases such as chlorine (Cl2) and sulfur trioxide (SO3), which have environmental impacts. Therefore, researchers have attempted to use organic acids to treat spent lithium-ion batteries, such as citric acid, oxalic acid, malic acid, ascorbic acid, and glycine. Li et al. used hydrochloric acid to dissolve and recover electrodes. Since the efficiency of the acid leaching process can be affected by hydrogen ion (H+) concentration, temperature, reaction time, and solid-liquid ratio (S/L), experiments were designed to explore the effects of reaction time, H+ concentration, and temperature to optimize the operating conditions. Experimental data showed that the highest leaching efficiency was achieved at a temperature of 80℃, an H+ concentration of 4 mol/L, and a reaction time of 2 h, during which 97% of the Li and 99% of the Co in the electrode material were dissolved. Zhou Tao et al. used malic acid as the leaching agent and hydrogen peroxide as the reducing agent to perform reductive leaching of pretreated positive electrode active materials. They investigated the effects of different reaction conditions on the leaching rates of Li, Co, Ni, and Mn in the malic acid leachate to identify the optimal reaction conditions. The data showed that the highest leaching efficiency was achieved when the temperature was 80℃, the malic acid concentration was 1.2 mol/L, the liquid-to-liquid volume ratio was 1.5%, the solid-to-liquid ratio was 40 g/L, and the reaction time was 30 min. The leaching rates of Li, Co, Ni, and Mn reached 98.9%, 94.3%, 95.1%, and 96.4%, respectively. However, compared to inorganic acids, leaching with organic acids is more expensive.
(2) Organic solvent extraction method
Organic solvent extraction utilizes the principle of "like dissolves like" to physically dissolve organic binders using suitable organic solvents, thereby weakening the adhesion between the material and the foil and separating the two.
Contestabile et al., in recycling lithium cobalt oxide batteries, utilized N-methylpyrrolidone (NMP) for selective separation of components to better recover the active materials of the electrodes. NMP is a good solvent for PVDF (solubility approximately 200 g/kg) and has a high boiling point of about 200 °C. Their study used NMP to treat the active material at approximately 100 °C for 1 hour, effectively separating the film from its support. This allowed for the recovery of Cu and Al in their metallic forms by simply filtering the film from the NMP solution. Another advantage of this method is that the recovered Cu and Al can be directly reused after thorough cleaning. Furthermore, the recovered NMP can be recycled due to its high solubility in PVDF, allowing for multiple reuses. Zhang et al., in recycling cathode waste from lithium-ion batteries, used trifluoroacetic acid (TFA) to separate the cathode material from the aluminum foil. The waste lithium-ion batteries used in the experiment employed polytetrafluoroethylene (PTFE) as an organic binder. The effects of TFA concentration, liquid-to-solid ratio (L/S), reaction temperature, and time on the separation efficiency of the cathode material and aluminum foil were systematically studied. The experimental results showed that in a TFA solution with a mass fraction of 15%, a liquid-to-solid ratio of 8.0 mL/g, a reaction temperature of 40 °C, and a reaction time of 180 min under appropriate stirring, the cathode material could be completely separated.
While the experimental conditions for separating materials and foils using organic solvent extraction are relatively mild, organic solvents possess a degree of toxicity, potentially posing a health hazard to operators. Furthermore, due to variations in lithium-ion battery manufacturing processes among different manufacturers, the binders they choose differ. Therefore, manufacturers must select different organic solvents for recycling spent lithium-ion batteries, depending on the specific manufacturing process. In addition, cost is a significant consideration for large-scale industrial-scale recycling operations. Thus, selecting a widely available, affordable, low-toxicity, and widely applicable solvent is crucial.
