6LR61 alkaline battery.Recycling technology of valuable metals in waste lithium-ion batteries?

　　In modern life, electronic communication equipment such as cameras, camcorders, laptops, and mobile phones using lithium-ion batteries have been widely used by people. The main components of a lithium-ion battery are the positive electrode, negative electrode, separator and electrolyte. The battery positive electrode is composed of positive active materials, conductive agents, binders, current collectors, etc. The battery negative electrode is mainly composed of negative active materials, current collectors, etc. A separator made of polymer separates the positive and negative electrodes. The electrolyte plays a role in charging and discharging the battery. However, lithium-ion batteries have a limited service life, usually less than 3 years. Waste batteries contain toxic substances that can cause damage to soil and water quality in the environment. These toxic substances spread into the bodies of humans and animals and can harm human health. Recycling and reusing valuable metals can not only improve the environment, but also improve the economic benefits of enterprises. Therefore, green recovery and reuse technology of valuable metals in waste lithium-ion batteries has become a research hotspot in recent years [1-2]. This article mainly reviews domestic and foreign processes for recycling valuable metals in used lithium-ion batteries, and looks forward to the development trend of recycling technology.

　　1 Current status of research at home and abroad

　　In practical applications, the core recycling technologies are mainly divided into two categories: fire method and wet method. The fire method is a process of heating under high temperature conditions and extracting or separating non-ferrous metals from battery materials based on the physical properties (melting point, vapor pressure) of different metals. Wet method is a recycling process that uses acid, alkali or organic solvents to leach valuable metal components in batteries. The recycling process can be roughly divided into three steps: battery pre-treatment, separation of active materials and current collectors, and recovery and reuse of valuable metals.

　　1.1 Pre-processing of used lithium-ion batteries

　　1.1.1 Discharge

　　Used lithium-ion batteries have residual power inside them. In order to prevent accidents during battery disassembly, the battery must be discharged before disassembly. Treatment methods include physical discharge method and chemical discharge method. The physical discharge method mainly uses low temperature forced discharge. This method is suitable for small batch production. Umicore and Toxco companies in the United States use liquid nitrogen to pretreat the battery at low temperature and safely crush the battery at a temperature of -198°C. However, this method The equipment requirements are high. Chemical discharge method mainly uses electrolysis to discharge. The electrolyte is mostly sodium chloride solution. When the battery is placed in the solution, the positive and negative electrodes of the battery are short-circuited in the conductive liquid, quickly achieving complete discharge of the battery. The disadvantage of this method is that the electrolyte concentration and temperature will affect the battery discharge speed, and the valuable metals in the battery will dissolve into the conductive solution, reducing the metal recovery rate. At the same time, solutions containing valuable metals are highly polluting, making recycling difficult and increasing recycling costs [3-4].

　　1.1.2 Dismantling and breaking

　　In the laboratory, most batteries are disassembled and separated manually because of their small size. In actual production, mechanical crushing is often used to dismantle batteries. One method of mechanical crushing is the wet method. The wet method uses various acidic and alkaline solutions as transfer media to transfer metal ions from the electrode material to the leachate, and then removes the metal ions from the solution in the form of salts, oxides, etc. through ion exchange, precipitation, adsorption, etc. Extract it. Wet recycling technology is relatively complex, but has a high recovery rate of valuable metals. It is currently the main technology used to deal with waste nickel-metal hydride batteries and lithium-ion batteries. Wang Yuansun and others[5-6] tried to soak the battery in dilute alkaline water and then crush it. This method can reduce the production of HF, but it cannot effectively recover the fluorine-containing electrolyte, thus easily causing secondary pollution. Another method is the dry method. Dry methods mainly include mechanical separation methods and high-temperature pyrolysis methods (or high-temperature metallurgical methods). The advantages of the mechanical sorting recycling process are short and highly targeted. It is the preliminary stage to achieve metal separation and recycling. He [7] et al. compared the different effects of wet crushing and mechanical sorting methods on the recycling and processing of used lithium-ion batteries. The results showed that mechanical sorting crushing will not break the battery components into fine particles that are easily mixed together. , the recovery rate is higher. However, mechanical sorting recycling cannot completely separate the components in used lithium-ion batteries. People have tried to use high-temperature pyrolysis, that is, heating the battery in a muffle furnace to remove the organic solvent in the battery. Joo [8] et al. used a combination of mechanical sorting and high-temperature pyrolysis to efficiently recover cobalt and lithium from used lithium cobalt oxide batteries. However, high-temperature pyrolysis can also cause negative effects. For example, harmful gases are produced during high-temperature treatment, which can easily cause explosions. Therefore, purification devices need to be installed.

　　1.2 Separation of active materials and current collectors

　　The separation of the positive active material and the aluminum foil current collector mainly uses two methods including organic solvent dissolution and pyrolysis. Organic solvent discharge mainly uses organic solvents to dissolve PVDF to separate the positive active material from the current collector. Zeng[9] used NMP to soak the electrode sheets to achieve effective separation of active materials and current collectors in the battery. Yang [10] used the organic solvent DMAC (N, N-dimethylacetamide) to dissolve and removed the binder on the current collector under the process conditions of 100°C and 60 minutes. However, the active material particles obtained by this recycling method are small, solid-liquid separation is difficult, and the recycling investment is large. The pyrolysis method separates the cathode material and active body at high temperatures. Daniel[11] and others used a high-temperature treatment method in a vacuum environment to decompose the organic matter in the current collector at high temperature (600 ℃), and part of the cathode material was separated from the aluminum foil. When the temperature was greater than 650 ℃, The aluminum foil and cathode materials are both granular and mixed together. This method will produce harmful gases and pollute the air.

　　1.3 Separation, recovery and utilization of valuable metals

　　The recycling of valuable metals in used lithium-ion batteries mainly involves the recycling of positive active materials. The cathode recycling and treatment methods mainly include biological methods, high-temperature combustion methods, acid dissolution methods and electrochemical dissolution methods.

　　1.3.1 Biological law

　　The biological method uses the metabolic function of microorganisms to convert the metal elements in the positive electrode into soluble compounds and selectively dissolve them. After obtaining the metal solution, inorganic acids are used to separate the components of the positive electrode material, and finally achieve the separation and recovery of valuable metals. . Jia Zhihui [12] et al. used Ferrooxidans and Thiobacillus thiooxidans to treat waste lithium-ion batteries. This method has low recycling cost and is easy to implement under normal temperature and pressure process conditions. However, the disadvantage of this method is that the bacteria are difficult to cultivate and the leachate is difficult to separate. Zeng [13] and others used acidophilic bacteria to use sulfur and ferrous ions as energy sources, metabolize to produce products such as sulfuric acid and iron ions, and dissolve the metal elements in used lithium-ion batteries. However, higher contents of Fe(III) co-precipitate with other metal elements, which will reduce the solubility of metals, affect the growth rate of biological cells, and reduce the metal dissolution rate. Biological methods have the characteristics of low cost, low pollution and reusability, and have become an important development direction of recycling technology for waste lithium ion valuable metals. However, there are also problems that need to be solved, such as the selection and cultivation of microbial strains, optimal leaching conditions, and the bioleaching mechanism of metals.

　　1.3.2 High temperature combustion method

　　The high-temperature combustion method refers to soaking the dismantled cathode material in an organic solvent and then burning it at high temperature to obtain valuable metals. Japan's Sony and Sumitomo companies soaked used lithium-ion batteries in oxalic acid and then incinerated them at 1,000°C to remove the electrolyte and separator, and cracked the battery. The residual material after incineration was screened and magnetically separated. To separate metals such as Fe, Cu, and Al. The results show that when the oxalic acid concentration is 1.00 mol·L-1, the material-liquid ratio is 40~45 g·L-1, and the solubility is optimal when stirring at 80°C for 15~20 minutes. Japan's Matsuda Mitsuyo and others soaked the cathode material and then crushed it using a mechanical breakage method. After mechanical crushing, they used high-temperature heat treatment in a muffle furnace, flotation and other means to separate the metal. However, this method consumes a lot of energy, has high temperature, and will produce waste gas that pollutes the environment. The metal obtained has high impurity content and requires further purification to obtain high-purity metal materials.

　　1.3.3 Acid dissolution method

　　This method refers to using acid to dissolve the cathode material, and then using an organic extractant to extract the metals in the solution to separate the metal ions and obtain valuable metals after treatment. He Lipo [14] and others used 1.5 mol/L and 0.9 mol/L H2SO4 and H2O2 to dissolve lithium cobalt oxide, the cathode material of lithium-ion batteries, at 80°C. Zhou Tao [15] and others used the cobalt ion solution obtained above, used the extraction agent AcorgaM5640 to extract copper, and used Cyanex 272 to extract cobalt. The recovery rate of copper reached 98%, and the recovery rate of cobalt was 97%. The remaining lithium can be used with sodium carbonate. Let it settle out. Wang [16] et al. used hydrochloric acid to dissolve the cathode material, and PC-88A was used as the extraction agent to extract cobalt ions. After subsequent processing, cobalt sulfate was obtained. The advantage of this method is that the metal obtained is of high purity. The disadvantages are that the extraction agent is expensive, toxic, harmful to the human body, and the processing process is complicated.

　　1.3.4 Electrochemical dissolution method

　　This method uses the positive electrode material as the cathode, lead as the anode, uses a mixture of inorganic acid (sulfuric acid or hydrochloric acid) and citric acid or hydrogen peroxide as the electrolyte, conducts electrolysis experiments, precipitates cobalt plasma, and then uses an extraction agent to extract the metal. Chang Wei [17] et al. used 0.4mol/L sulfuric acid and 36g/L citric acid as the electrolyte. After electrolysis for 120 minutes at 25°C, the cobalt leaching rate reached 90.85% and the aluminum dissolution rate was 5.8%. Lu Xiuyuan [18] et al. adopted an orthogonal experimental method, using 3 mol/L sulfuric acid and 2.4 mol/L hydrogen peroxide. The reaction time was 20 minutes, and the cobalt leaching rate was as high as 99.6%. The electrochemical dissolution method is relatively simple and easy to implement, and has a high leaching rate of valuable metals, but it consumes a lot of energy during the electrolysis process. Therefore, the electrochemical method still needs to be improved to make it suitable for large-scale production. During the electrolysis process, the electrolysis reaction equation that occurs is:

　　cathode:

　　LiCoO2+4H++e-=Li++Co2++2H2O2H++2e=H2(g)

　　anode:

　　2H2O-4e-=O2(g)+4H+

　　2. Recycling of used lithium-ion batteries

　　(1) During the dismantling and crushing process of used lithium-ion batteries, the separation effect is still not ideal. Therefore, safely and effectively splitting and crushing used lithium-ion batteries is a prerequisite for recycling used batteries.

　　(2) In the current research process of valuable metals in waste lithium-ion batteries, wet methods are mainly used in the recycling process of valuable metals. This method uses chemical substances such as acids and alkalis, which will produce harmful waste gas and waste liquid, causing certain harm to people and the environment. Therefore, secondary pollution during the process is also an important issue that needs to be solved.

　　(3) In the process of recycling valuable metals from used lithium-ion batteries, most of the research is focused on the recycling of valuable metals in cathode materials. The negative electrode and electrolyte were ignored. In particular, electrolytes are mostly composed of high-concentration organic solvents, electrolyte lithium salts, additives and other raw materials. These substances are toxic and pollute the environment. Therefore, alternatives to these materials should be found to reduce the harm of electrolytes to the environment.

　　(4) Most current research focuses on lithium iron phosphate batteries among used lithium-ion batteries, and there is less research on lithium nickel cobalt manganate and lithium iron phosphate batteries. Therefore, the scope of research should be expanded and recycling processes for different types of lithium-ion batteries should be developed so that valuable metals from all types of waste lithium-ion batteries can be efficiently recycled.

　　3 Conclusion

　　To sum up, the recycling of used lithium-ion batteries is still in the laboratory stage, and the industrialization process is relatively slow. In the recycling and processing of used lithium-ion batteries, there are still questions about how to carry out safe disassembly, how to improve the recovery rate of valuable metals in positive electrode materials while avoiding secondary pollution, how to greenly process the electrolyte in used batteries, and how to effectively improve the recycling process. issues such as economic benefits and improved environmental effects. Therefore, there is an urgent need to strengthen research on the recycling, processing and utilization of lithium-ion batteries in the future to truly realize the green recovery and recycling of used batteries.

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