Lithium ion batteries are composed of positive and negative plates, binders, electrolytes and diaphragms. In industry, lithium cobalt, lithium manganate, lithium nickel cobalt manganate and lithium ferrous phosphate are used as cathode materials for lithium ion batteries, and natural graphite and artificial graphite are used as negative active materials. Polyvinylidene fluoride (PVDF) is a widely used cathode binder with high viscosity, good chemical stability and physical properties. Industrial lithium-ion batteries mainly use electrolyte lithium hexafluorophosphate (LiPF6) and organic solvent solution as electrolyte, and organic membranes such as porous polyethylene (PE) and polypropylene (PP) as separators. Lithium-ion batteries are generally regarded as environmentally friendly and pollution-free green batteries, but improper recovery of lithium-ion batteries will also cause pollution. Although lithium-ion batteries do not contain toxic heavy metals such as mercury, cadmium and lead, the positive and negative materials and electrolytes of lithium-ion batteries still have a great impact on the environment and human body. If lithium-ion batteries (landfill, incineration, composting, etc.) are treated by ordinary waste treatment methods, cobalt, nickel, lithium, manganese and various organic and inorganic compounds in the batteries will cause metal pollution, organic matter pollution, dust pollution, acid-base pollution. Machine transformants of lithium ion electrolyte, such as LiPF6, LiAsF6, LiCF3SO3 and HF, are toxic substances in solvents and hydrolysates such as diethylene glycol dimethyl ether (DME), methanol and formic acid. Therefore, waste lithium-ion batteries need to be recycled to reduce the harm to the natural environment and human health.
I. Production and Use of Lithium Ion Batteries
Lithium-ion batteries are widely used in electronic products, such as mobile phones, tablet computers, laptops and digital cameras, because of their advantages of high energy density, high voltage, low self-discharge, good cycle performance, safe operation, and relatively friendly to the natural environment. In addition, lithium-ion batteries have been widely used in energy storage power systems, such as water power, firepower, wind power and solar energy, and gradually become the best choice for power batteries. The appearance of lithium iron phosphate batteries has promoted the development and application of lithium ion batteries in electric vehicle industry. With the increasing demand for electronic products and the acceleration of renewal of electronic products, and the impact of the rapid development of new energy vehicles, the demand for lithium-ion batteries in the global market is growing, and the growth rate of battery production is increasing year by year.
The huge demand for lithium-ion batteries in the market, on the one hand, will lead to a large number of waste batteries in the future. How to deal with these waste lithium-ion batteries to reduce their impact on the environment is an urgent problem to be solved. On the other hand, in order to meet the huge demand of the market, manufacturers need to produce a large number of lithium-ion batteries to supply the market. Field. At present, the cathode materials for lithium ion batteries mainly include lithium cobalt oxide, lithium manganate, lithium nickel cobalt manganate and lithium ferrous phosphate. Therefore, the waste lithium ion batteries contain more cobalt (Co), lithium (Li), nickel (Ni), manganese (Mn), copper (Cu), iron (Fe) and other metal resources, including a variety of rare metal resources. China is a scarce strategic metal, mainly to meet the growing demand by importing. The metal content of waste lithium-ion batteries is higher than that of natural ores. Therefore, under the situation of increasing shortage of production resources, recycling waste lithium-ion batteries has certain economic value.
II. Recycling Technology of Lithium Ion Batteries
The recovery process of waste lithium ion batteries mainly includes pretreatment, secondary treatment and advanced treatment. The pretreatment process includes deep discharging, crushing and physical separation because of the residual electricity in waste batteries. The purpose of secondary treatment is to achieve the complete separation of positive and negative active materials from the base. The two processes are usually realized by heat treatment, organic solvent dissolution, alkali solution dissolution and electrolysis. Separation; advanced treatment mainly includes two processes: leaching and separation and purification to extract valuable metal materials. According to the extraction process, the recovery methods of batteries can be divided into three categories: dry recovery, wet recovery and biological recovery.
1. Dry recovery
Dry recovery refers to the direct recovery of materials or valuable metals without solution or other media. Among them, the main methods used are physical separation and pyrolysis at high temperature.
(1) Physical sorting
Physical separation means that the battery is disassembled and separated, and valuable high-content substances are obtained by crushing, sieving, magnetic separation, fine grinding and classification of battery components, such as electrode active substances, collector and battery shell. Shin et al. proposed a method to recover Li and Co from waste liquor of lithium ion batteries by sulfuric acid and hydrogen peroxide, which includes physical separation of metal particles and chemical leaching. The physical separation process includes crushing, screening, magnetic separation, fine crushing and classification. In the experiment, a group of rotating and fixed blade crushers were used for crushing, and sieves with different apertures were used to classify and crush materials, and magnetic separation was used for further treatment to prepare for the subsequent chemical leaching process.
Shu et al. developed a new mechanochemical method to recover cobalt and lithium from lithium-sulfur battery waste based on the grinding technology and water leaching process developed by Zhang et al., Lee et al. and Saeki et al. In this method, lithium cobalt oxide (LiCoO 2) and polyvinyl chloride (PVC) are ground together in air by planetary ball mill to form Co and lithium chloride (LiCl) in a mechanochemical manner. The grinding products were then dispersed in water to extract chloride. Grinding promotes mechanochemical reactions. With the development of grinding, the extraction yields of Co and Li are improved. More than 90% Co and nearly 100% lithium were recovered after 30 minutes of grinding. At the same time, about 90% of chlorine in PVC samples has been converted into inorganic chloride.
Physical separation method is easy to operate, but it is not easy to completely separate lithium-ion batteries. In screening and magnetic separation, it is easy to have mechanical entrainment loss, and it is difficult to achieve complete separation and recovery of metals.
(2) pyrolysis at high temperature
High temperature pyrolysis refers to the decomposition of lithium battery materials after physical crushing and other preliminary separation treatment at high temperature, and the removal of organic adhesives, thereby separating the components of lithium battery materials. At the same time, the metals and their compounds in lithium batteries can be oxidized, reduced and decomposed, volatilized in the form of steam, and then collected by condensation and other methods.
Lee et al. used waste lithium-ion batteries to prepare LiCoO 2 by pyrolysis at high temperature. Lee et al. first heat-treated LIB samples in a muffle furnace for 1 hour at a temperature of 100-150 C. Secondly, the heat treated batteries are shredded to release the electrode material. Samples are disassembled by a high-speed crusher specially designed for this study. The size ranges from 1 mm to 50 mm. Then, two steps of heat treatment were carried out in the furnace. The first heat treatment lasted for 30 minutes at 100-500 C and the second heat treatment lasted for 1 hour at 300-500 C. The electrode material was released from the collector through vibration screening. Next, the cathode active material LiCoO 2 was obtained by burning carbon and adhesives for 0.5-2 h at 500-900 C. The experimental data show that carbon and adhesives are burned off at 800 C.
High temperature pyrolysis process is simple, easy to operate, fast reaction speed and high efficiency in high temperature environment, which can effectively remove adhesives; moreover, the method does not require high composition of raw materials and is more suitable for handling large or complex batteries. However, this method requires higher equipment; in the process of treatment, the organic matter decomposition of batteries will produce harmful gases, which is not friendly to the environment. It is necessary to increase purification and recovery equipment, absorb and purify harmful gases, and prevent secondary pollution. Therefore, the processing cost of this method is high.
2. Wet recovery
Wet recovery process is to dissolve waste batteries after crushing, then selectively separate metal elements from leaching solution by using appropriate chemical reagents, and produce high-grade cobalt or lithium carbonate, etc. for direct recovery. Wet recovery process is more suitable for the recovery of waste lithium batteries with relatively single chemical composition, and its equipment investment cost is low. It is suitable for the recovery of small and medium-sized waste lithium batteries. Therefore, this method is widely used at present.
(1) Alkali-acid leaching
Because the cathode material of lithium ion batteries can not dissolve in alkali solution, and the base aluminum foil can dissolve in alkali solution, this method is often used to separate aluminum foil. When recovering Co and Li from batteries, Zhang Yang et al. used alkali to remove Al in advance, and then used dilute acid to immerse to destroy the adhesion of organic matter to copper foil. However, alkali leaching can not completely remove PVDF, which has adverse effects on subsequent leaching.
Most of the positive active materials in lithium ion batteries can be dissolved in acid, so the pretreated electrode materials can be leached with acid solution to separate the active materials from the collector, and then precipitate and purify the target metals according to the principle of neutralization reaction, so as to achieve the purpose of recovering high purity components.
The traditional inorganic acids, including hydrochloric acid, sulfuric acid and nitric acid, are used in acid leaching. However, in the process of leaching with inorganic strong acid, harmful gases such as chlorine (Cl2) and sulfur trioxide (SO3) are often produced, so researchers try to use organic acid to treat waste lithium batteries, such as citric acid, oxalic acid, malic acid, ascorbic acid and glycine. Li et al dissolve the recovered electrodes with hydrochloric acid. Because the efficiency of acid leaching process may be affected by hydrogen ion concentration, temperature, reaction time and solid-liquid ratio (S/L), in order to optimize the operation conditions of acid leaching process, experiments were designed to explore the effects of reaction time, H+ concentration and temperature. The experimental data show that the leaching efficiency is the highest when the H+ concentration is 4 mol/L and the reaction time is 2 h at 80 C. Among them, 97% of L I and 99% of CO in the electrode material are dissolved. Zhou Tao et al. used malic acid as leaching agent and hydrogen peroxide as reducing agent to reductive leaching of positive active substances obtained from pretreatment, and studied the effect of different reaction conditions on the leaching rate of Li, Co, Ni and Mn in malic acid leaching solution, so as to find out the best reaction conditions. The results show that the leaching efficiency of malic acid is the highest when the temperature is 80 C, the concentration of malic acid is 1.2 mol/L, the volume ratio of liquid to liquid is 1.5%, the solid-liquid ratio is 40 g/L and the reaction time is 30 minutes. The leaching rates of Li, Co, Ni and Mn are 98.9%, 94.3%, 95.1% and 96.4%, respectively. However, compared with inorganic acid, the cost of leaching with organic acid is higher.
(2) Organic solvent extraction
The organic solvent extraction method uses the principle of "similar compatibility" to dissolve the organic binder physically with suitable organic solvents, thus weakening the adhesion between the material and the foil and separating them.
Contestabile et al. used N-methyl pyrrolidone (NMP) to selectively separate components in order to better recover the active materials of the electrodes when recovering lithium cobalt oxide batteries. NMP is a good solvent for PVDF (solubility is about 200 g/kg), and its boiling point is high, about 200 C. The active material was treated by NMP at about 100 C for 1 h, which effectively separated the film from its carrier. Therefore, the metal forms of copper and Al were recovered by simply filtering it out of NMP (N-methyl pyrrolidone) solution. Another advantage of this method is that the recovered Cu and Al metals can be reused directly after being fully cleaned. In addition, the recovered NMP can be recycled. Because of its high solubility in PVDF, it can be reused many times. Zhang et al. used trifluoroacetic acid (TFA) to separate cathode material from aluminium foil when recycling cathode waste for lithium ion batteries. Polytetrafluoroethylene (PTFE) was used as an organic binder for waste lithium ion batteries. The effects of TFA concentration, liquid-solid ratio (L/S), reaction temperature and time on the separation efficiency of cathode materials and aluminum foil were systematically studied. The experimental results show that the cathode material can be completely separated when the liquid-solid ratio is 8.0 mL/g in TFA solution with mass fraction of 15 and the reaction temperature is 40 C and the reaction time is 180 min under proper stirring.
The experimental conditions for separating materials and foils by organic solvent extraction are mild, but organic solvents have certain toxicity, which may cause harm to the health of operators. At the same time, due to the different technology of lithium-ion batteries made by different manufacturers, different binders are selected. Therefore, different organic solvents are needed for different manufacturing processes when recycling waste lithium-ion batteries. In addition, cost is also an important consideration for large-scale recycling operations at the industrial level. Therefore, it is very important to select a solvent with wide source, suitable price, low toxicity, harmless and wide applicability.
(3) Ion exchange method
Ion exchange method refers to the separation and extraction of metals by using different adsorption coefficients of ion exchange resins for metal ion complexes to be collected. After acid leaching, Wang Xiaofeng et al. added appropriate amount of ammonia water to the solution, adjusted the pH value of the solution, reacted with metal ions in the solution, and formed [Co(NH3)6]2+, [Ni(NH3)6]2+ plasma complexes, which were continuously oxidized into the solution with pure oxygen. Then, the nickel complexes and cobalt-ammonia complexes on ion exchange resins were selectively eluted by repeatedly passing weak acid cation exchange resins with ammonia sulfate solutions of different concentrations. Finally, the cobalt complex was completely eluted by 5% H2SO4 solution, and the cation exchange resin was regenerated. The cobalt and nickel metals in the eluent were recovered by oxalate, respectively. Ion exchange process is simple and easy to operate.
3. Biological recovery
Mishra et al. leached metals from spent lithium-ion batteries by inorganic acid and acidophilic Thiobacillus ferrooxidans, and used S and ferrous ions (Fe2+) to produce H2SO4, Fe3+ and other metabolites in the leaching medium. These metabolites help dissolve metals in spent batteries. It was found that the dissolution rate of cobalt was faster than that of lithium. As the dissolution process proceeds, iron ions react with metal residues and precipitate, resulting in a reduction of ferrous ion concentration in the solution. With the increase of metal concentration in waste samples, cell growth is blocked and the dissolution rate slows down. In addition, higher solid/liquid ratio also affects the dissolution rate of metals. Zeng et al. used Thiobacillus ferrooxidans to bioleach cobalt from spent lithium-ion batteries, which was different from Mishra et al. In this study, copper was used as catalyst to analyze the effect of copper ions on Bioleaching of LiCoO 2 by Thiobacillus ferrooxidans. The results showed that almost all cobalt (99.9%) entered the solution after 6 days of bioleaching at the concentration of 0.75g/L of copper ion, while only 43.1% of cobalt dissolved after 10 days of reaction without copper ion. In the presence of copper ions, the cobalt dissolution efficiency of waste lithium ion batteries is improved. In addition, Zeng et al. studied the catalytic mechanism and explained the dissolution of cobalt by copper ions. LiCoO 2 reacted with copper ions to form copper cobalt oxide (CuCoO 4) on the surface of the sample, which was easily dissolved by iron ions.
Bioleaching method has low cost, high recovery efficiency, less pollution and consumption, less impact on the environment, and microorganisms can be reused. However, it is difficult to cultivate efficient microorganisms, long treatment period and control of leaching conditions.
4. Joint recovery method
Recycling technology of waste lithium batteries has its own advantages and disadvantages. At present, there are many methods to combine and optimize the recycling technology, so as to give full play to the advantages of various recycling methods and maximize economic benefits.
