Lithium-ion batteries are composed of positive and negative electrode sheets, binders, electrolytes and separators. In industry, manufacturers mainly use lithium cobalt oxide, lithium manganate, nickel cobalt lithium manganate ternary materials and lithium iron phosphate as the positive electrode materials of lithium-ion batteries, and natural graphite and artificial graphite as negative electrode active materials. Polyvinylidene fluoride (PVDF) is a widely used cathode binder with high viscosity and good chemical stability and physical properties. Industrially produced lithium-ion batteries mainly use a solution of electrolyte lithium hexafluorophosphate (LiPF6) and an organic solvent as the electrolyte, and use organic membranes, such as porous polyethylene (PE) and polypropylene (PP) and other polymers as battery separators. Lithium-ion batteries are generally considered to be environmentally friendly and pollution-free green batteries, but improper recycling of lithium-ion batteries can also cause pollution. Although lithium-ion batteries do not contain toxic heavy metals such as mercury, cadmium, and lead, the positive and negative electrode materials and electrolytes of the battery still have a great impact on the environment and human body. If ordinary garbage disposal methods are used to dispose of lithium-ion batteries (landfill, incineration, composting, etc.), cobalt, nickel, lithium, manganese and other metals in the battery, as well as various organic and inorganic compounds, will cause metal pollution, organic pollution, and dust pollution. , acid-base pollution. Lithium ion electrolyte machine conversion products, such as LiPF6, lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiCF3SO3), hydrofluoric acid (HF), etc., solvents and hydrolysis products such as ethylene glycol dimethyl ether ( DME), methanol, formic acid, etc. are all toxic substances. Therefore, waste lithium-ion batteries need to be recycled to reduce the harm to the natural environment and human health.
1. Production and use of lithium-ion batteries
Lithium-ion batteries have the advantages of high energy density, high voltage, small self-discharge, good cycle performance, safe operation, etc., and are relatively friendly to the natural environment, so they are widely used in electronic products such as mobile phones, tablet computers, notebook computers and digital cameras Wait. In addition, lithium-ion batteries are widely used in energy storage power systems such as water power, thermal power, wind power and solar energy, and have gradually become the best choice for power batteries. The emergence of lithium iron phosphate batteries has promoted the development and application of lithium ion batteries in the electric vehicle industry. With the gradual increase of people's demand for electronic products and the gradual acceleration of the replacement of electronic products, and affected by the rapid development of new energy vehicles, the global market demand for lithium-ion batteries is increasing, and the growth rate of battery production is increasing year by year. .
The huge market demand for lithium-ion batteries, 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 response to the huge market Demand, manufacturers need to produce a large number of lithium-ion batteries to supply the market. At present, the cathode materials for the production of lithium ion batteries mainly include lithium cobalt oxide, lithium manganate, nickel cobalt lithium manganate ternary materials and lithium iron phosphate, etc. Therefore, 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, cobalt is a scarce strategic metal in my country, mainly imported meet growing demand. The content of some metals in waste lithium-ion batteries is higher than that in natural ores. Therefore, in the case of increasing shortage of production resources, recycling and processing waste batteries has certain economic value.
2. Lithium-ion battery recycling technology
The recycling process of waste lithium-ion batteries mainly includes pretreatment, secondary treatment and advanced treatment. Since part of the electricity still remains in the waste battery, the pretreatment process includes deep discharge process, crushing, and physical sorting; the purpose of secondary treatment is to achieve complete separation of positive and negative electrode active materials and substrates. Heat treatment methods and organic solvent dissolution methods are commonly used. , lye dissolution method and electrolysis method to achieve complete separation of the two; advanced treatment mainly includes two processes of leaching and separation and purification to extract valuable metal materials. According to the classification of extraction process, battery recycling methods can be mainly divided into three categories: dry recycling, wet recycling and biological recycling.
1. Dry recycling
Dry recovery refers to the direct recovery of materials or valuable metals without media such as solutions. Among them, the main methods used are physical sorting method and high temperature pyrolysis method.
(1) Physical sorting method
The physical sorting method refers to the disassembly and separation of the battery, and the battery components such as electrode actives, current collectors and battery shells are crushed, sieved, magnetically separated, finely pulverized and classified to obtain valuable high-content substances. . A method proposed by engineers to recover Li and Co from lithium-ion battery waste liquid using sulfuric acid and hydrogen peroxide includes two processes: physical separation of metal-containing particles and chemical leaching. Among them, the physical separation process includes crushing, screening, magnetic separation, fine crushing and classification. In the experiment, a set of crushers with rotating and fixed blades were used for crushing, and sieves with different apertures were used to classify the crushed materials, and magnetic separation was used for further processing to prepare for the subsequent chemical leaching process.
The engineers developed a new method for recovering cobalt and lithium from lithium-sulfur battery waste using a mechanochemical method based on a grinding technique and a water leaching process. The method utilizes a planetary ball mill to co-grind lithium cobaltate (LiCoO2) and polyvinyl chloride (PVC) in air to mechanochemically form Co and lithium chloride (LiCl). Subsequently, the ground product was dispersed in water to extract chlorides. Grinding promotes mechanochemical reactions. The extraction yields of both Co and Li were improved as the grinding progressed. Grinding for 30 min resulted in the recovery of over 90% Co and nearly 100% Li. Meanwhile, about 90% of the chlorine in the PVC samples has been converted to inorganic chlorides.
The operation of the physical separation method is relatively simple, but it is not easy to completely separate the lithium-ion battery, and during the screening and magnetic separation, mechanical entrainment losses are prone to occur, and it is difficult to achieve complete separation and recovery of metals.
(2) High temperature pyrolysis method
The high temperature pyrolysis method refers to decomposing the lithium battery materials after preliminary separation such as physical crushing, and decomposing them at high temperature, and removing the organic binder, so as to separate the constituent materials of the lithium battery. At the same time, the metal and its compounds in the lithium battery can be oxidized, reduced and decomposed, volatilized in the form of steam, and then collected by methods such as condensation.
The researchers used a high-temperature pyrolysis method to prepare LiCoO2 from waste lithium-ion batteries. The researchers first heat-treated the LIB samples in a muffle furnace at 100-150 °C for 1 h. Second, the heat-treated battery was shredded to release the electrode material. The samples were disassembled with a high-speed pulverizer specially designed for this study and classified by size ranging from 1 to 50 mm. Then, 2-step heat treatment was performed in a furnace, the first heat treatment at 100-500 °C for 30 min, and the second heat treatment at 300-500 °C for 1 h, and the electrode material was released from the current collector by vibration screening. Next, by burning at a temperature of 500-900° C. for 0.5-2 h, the carbon and the binder are burned off, and the cathode active material LiCoO 2 is obtained. Experimental data show that the carbon and binder are burned off at 800°C.
The high temperature pyrolysis treatment technology has simple process, convenient operation, fast reaction speed and high efficiency in high temperature environment, and can effectively remove the binder; and this method does not require high components of raw materials, and is more suitable for processing large or complex materials. Battery. However, this method requires high equipment; during the treatment process, the decomposition of organic matter in the battery will produce harmful gases, which is not friendly to the environment. It is necessary to increase purification and recovery equipment to absorb and purify harmful gases to prevent secondary pollution. Therefore, the processing cost of this method is high.
2. Wet recycling
The wet recycling process is to crush and dissolve the waste batteries, and then use suitable chemical reagents to selectively separate the metal elements in the leaching solution to produce high-grade cobalt metal or lithium carbonate, which can be directly recycled. Wet recycling is more suitable for recycling waste lithium batteries with relatively single chemical composition, and its equipment investment cost is low, which is suitable for the recovery of small and medium-scale waste lithium batteries. Therefore, this method is currently widely used.
(1) Alkali-acid leaching method
This method is often used to separate aluminum foils because the positive electrode material of lithium-ion batteries does not dissolve in the lye solution, while the base aluminum foil will dissolve in the lye solution. When recovering Co and Li in batteries, Zhang Yang et al. used alkali to remove aluminum in advance, and then used dilute acid solution to destroy the adhesion of organic matter and copper foil. However, the alkaline leaching method cannot completely remove PVDF, which has an adverse effect on the subsequent leaching.
Most of the positive active materials in lithium-ion batteries can be dissolved in acid, so the pre-treated electrode materials can be leached with acid solution to achieve the separation of active materials and current collectors, and then combined with the principle of neutralization reaction to target metal Precipitation and purification are carried out to achieve the purpose of recovering high-purity components.
The acid solutions used in the acid leaching method include traditional inorganic acids, including hydrochloric acid, sulfuric acid, and nitric acid. However, in the process of leaching with inorganic strong acids, harmful gases such as chlorine (Cl2) and sulfur trioxide (SO3) that have an impact on the environment are often generated, so researchers try to use organic acids to treat waste lithium batteries, such as citric acid , oxalic acid, malic acid, ascorbic acid, glycine, etc. Li et al. used hydrochloric acid to dissolve the recovered electrodes. Since the efficiency of the acid leaching process may be affected by the hydrogen ion (H+) concentration, temperature, reaction time and solid-liquid ratio (S/L), in order to optimize the operating conditions of the acid leaching process, experiments were designed to explore the reaction time, H+ concentration and temperature effects. The experimental data show that when the temperature is 80 °C, the H+ concentration is 4 mol/L, the reaction time is 2 h, and the leaching efficiency is the highest. Among them, 97% of Li and 99% of Co in the electrode material are dissolved. Zhou Tao et al. used malic acid as the leaching agent and hydrogen peroxide as the reducing agent to reduce the leaching of the positive electrode active material obtained by pretreatment, and studied the influence of different reaction conditions on the leaching rate of Li, Co, Ni, and Mn in the malic acid leaching solution. optimal reaction conditions. The research data show that when the temperature is 80℃, the malic acid concentration is 1.2mol/L, the liquid-liquid volume ratio is 1.5%, the solid-liquid ratio is 40g/L, and the reaction time is 30min, the efficiency of malic acid leaching is the highest. The leaching rates of Co, Ni and Mn reached 98.9%, 94.3%, 95.1% and 96.4%, respectively. However, compared with inorganic acids, the cost of leaching with organic acids is higher.
(2) Organic solvent extraction method
The organic solvent extraction method utilizes the principle of "similar compatibility" and uses a suitable organic solvent to physically dissolve the organic binder, thereby weakening the adhesion between the material and the foil and separating the two.
Contestabile et al. used N-methylpyrrolidone (NMP) to selectively separate the components in order to better recover the active material of the electrode when recycling lithium cobalt oxide batteries. NMP is a good solvent for PVDF (solubility around 200 g/kg) and has a relatively high boiling point, around 200°C. The study utilized NMP to treat the active material at approximately 100 °C for 1 h, effectively achieving the separation of the film from its support, and thus recovering the metallic form of Cu by simply filtering it out of the NMP (N-methylpyrrolidone) solution. and Al. Another benefit of this method is that the recovered Cu and Al metals can be directly reused after sufficient cleaning. 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 the cathode material from the aluminum foil when recycling cathode waste for lithium-ion batteries. The waste lithium-ion battery used in the experiment used polytetrafluoroethylene (PTFE) as the organic binder, and the effects of TFA concentration, liquid-solid ratio (L/S), reaction temperature and time on the separation efficiency of cathode material and aluminum foil were systematically studied . The experimental results show that in the TFA solution with a mass fraction of 15, the liquid-solid ratio is 8.0 mL/g, and the reaction temperature is 40 ℃, and the cathode material can be completely separated under appropriate stirring for 180 min.
The experimental conditions for separating materials and foils by organic solvent extraction method are relatively mild, but organic solvents have certain toxicity, which may cause harm to the health of operators. At the same time, due to the different production processes of lithium-ion batteries by different manufacturers, the binders selected are different. Therefore, for different production processes, manufacturers need to choose different organic solvents when recycling and disposing of waste lithium batteries. In addition, cost is also an important consideration for large-scale recycling processing operations at an industrial level. Therefore, it is very important to choose a solvent with a wide range of sources, reasonable price, low toxicity and no harm, and wide applicability.
(3) Ion exchange method
The ion exchange method refers to the use of ion exchange resins to achieve metal separation and extraction with different adsorption coefficients of the metal ion complexes to be collected. After acid leaching treatment of the electrode material, Wang Xiaofeng et al. added an appropriate amount of ammonia water to the solution to adjust the pH value of the solution, and reacted with the metal ions in the solution to generate [Co(NH3)6]2+, [Ni(NH3) 6] 2+ and other complex ions, and continuously introduce pure oxygen into the solution for oxidation. Then, different concentrations of ammonia sulfate solution are repeatedly passed through the weakly acidic cation exchange resin to selectively elute the nickel complex and trivalent cobalt ammine complex on the ion exchange resin respectively. Finally, the cobalt complex was completely eluted with 5% H2SO4 solution, and the cation exchange resin was regenerated at the same time, and the cobalt and nickel metals in the eluate were recovered by oxalate respectively. The ion exchange method has a simple process and is relatively easy to operate.
Mishra et al. used inorganic acids and Thiobacillus ferrooxidans to leach metals from waste lithium-ion batteries, and used S and ferrous ions (Fe2+) to generate metabolites such as H2SO4 and Fe3+ in the leaching medium. These metabolites help dissolve metals in spent batteries. The study found that cobalt biodissolves faster than lithium. As the dissolution process progresses, iron ions react with the metals in the residue to precipitate, resulting in a decrease in the concentration of ferrous ions in the solution, and as the metal concentration in the waste sample increases, cell growth is blocked and the dissolution rate slows down . In addition, higher solid/liquid ratios also affect the rate of metal dissolution. Zeng et al. used Thiobacillus acidophilus ferrooxidans to bioleaching metal cobalt from waste lithium-ion batteries. Unlike Mishra et al., this study used copper as a catalyst to analyze the effect of copper ions on the bioleaching of LiCoO2 by Thiobacillus acidophilus ferrooxidans. . The results showed that almost all cobalt (99.9%) entered the solution after bioleaching for 6 days when the Cu ion concentration was 0.75 g/L, while in the absence of copper ions, only 43.1% of the cobalt was bioleached after 10 days of reaction time. Cobalt dissolves. In the presence of copper ions, the cobalt dissolution efficiency of spent lithium-ion batteries is improved. In addition, Zeng et al. also studied the catalytic mechanism and explained the dissolution effect of copper ions on cobalt, in which LiCoO2 and copper ions undergo a cation exchange reaction to form copper cobaltate (CuCo2O4) on the surface of the sample, which is easily dissolved by iron ions.
Bioleaching has low cost, high recovery efficiency, less pollution and consumption, less impact on the environment, and the microorganisms can be reused. However, it is difficult to cultivate high-efficiency microorganisms, the treatment period is long, and the control of leaching conditions are several major problems required by this method.
4. Combined recycling method
The waste lithium battery recycling process has its own advantages and disadvantages. At present, there have been researches on recycling methods that combine and optimize various processes to give full play to the advantages of various recycling methods and maximize economic benefits.