Behind the "retirement wave" of ternary 3.7V 18650 lifepo4 battery is the carnival of wet recycling technology

With the rapid growth of the market share of ternary lithium-ion batteries, retired ternary lithium-ion batteries have experienced explosive growth. Therefore, the recycling of ternary lithium-ion battery electrode materials has become a new focus in the battery industry.

Under the dual stimulation of national industrial policies and market demand, the output of ternary materials has continued to grow. With the development of my country's new energy vehicle industry and the industry's requirements for the cruising range of electric vehicles, ternary materials with high energy density have been obtained. With widespread application, the market share of ternary lithium-ion batteries will further increase in the future.

The service life of lithium-ion batteries, especially the power batteries of new energy vehicles, is usually three to five years, and Co, Li and Ni in ternary lithium-ion batteries are all high-value metals.

Therefore, recycling and reusing retired power batteries will produce considerable economic and social benefits.

At present, the recycling of decommissioned lithium-ion battery materials is mainly divided into two categories: pyrometallurgy and hydrometallurgy.

Pyrometallurgy directly uses high-temperature treatment to extract metals or metal oxides from electrodes. The process is simple, but the purity of the recovered materials is low. Organic matter such as electrolytes and binders in decommissioned batteries will produce harmful gases due to high-temperature reactions and require installation. Supporting facilities are provided for secondary waste gas treatment.

Hydrometallurgy involves first dismantling the battery casing, crushing and screening to obtain electrode materials. The valuable metals in the electrode materials are leached in acid or biological solutions and then separated to obtain the corresponding salts or oxides of each metal.

The operating conditions of wet recycling are mild, the metal recovery rate is high, and the product has fewer impurities. The recycled products can directly enter the production process of new electrode materials, realizing a closed cycle of valuable metal materials, so it has become a research hotspot at home and abroad. .

So, what are the steps in the wet recycling process of retired ternary lithium battery materials?

one

The main steps of the wet recycling process for decommissioned ternary lithium battery electrodes include pretreatment, pretreatment, leaching of valuable metals, separation and extraction of valuable metals, etc.

Pre-processing involves physical discharge and disassembly of retired ternary batteries. The state of charge of retired lithium-ion batteries is uneven, and there will be varying degrees of residual power.

Lithium-ion batteries are in a charged state and are prone to dangerous phenomena such as spontaneous combustion, short circuits, and explosions when dismantling them. Therefore, from a safety perspective, the battery must be fully discharged before dismantling it.

At present, there are two most common methods for discharging retired batteries. One is to perform physical discharge on a charge and discharge instrument; the other is to fully discharge by immersing it in a salt solution of a certain concentration.

Generally, lithium-ion battery packs are wrapped in plastic shells, and single lithium-ion batteries are wrapped in metal shells or aluminum-plastic films. After the fully discharged battery pack is disassembled, the single cells are sorted, and the single cells are then sorted into batteries. Shell, electrode, diaphragm, etc.

In the dismantling part, most of them are dismantled by hand at present. The materials recovered by manual dismantling have less impurities and the purity of the recovered products is high, but the efficiency is low, the processing volume is small, and the danger is high.

Pretreatment is the separation of electrode active materials and current collectors. The electrodes not only contain active materials, but also conductive carbon, binders, current collectors and other substances. The electrodes also need to undergo pretreatment to separate the active materials from current collectors to obtain valuable metals. Solid powder, which is beneficial to subsequent leaching.

Commonly used pretreatment methods include heat treatment, mechanical separation, solvent and alkali dissolution.

The heat treatment method takes advantage of the different decomposition temperatures of each substance in the electrode material. The high temperature causes the binder to decompose and fail, and the active material falls off the current collector.

The heat treatment method is simple to operate and can effectively separate the active material from the aluminum foil. The recycled active material introduces less impurities and has high purity, making it suitable for large-scale industrial applications.

Since the heat treatment method needs to be carried out under high temperature conditions, the energy consumption is high, and the organic solvent remaining in the electrode will produce harmful gases after high temperature, and special equipment is required for purification treatment.

The mechanical separation method uses the physical characteristics of the components, such as differences in particle size, magnetic properties, density, etc. to achieve the separation of each component.

Due to the complex structure of lithium-ion batteries, the chemical composition includes a variety of metals, organic substances, and inorganic substances. It is difficult to achieve a high degree of separation of each component by mechanical separation method, so the recycled products have high impurity content and low purity.

The solvent method uses the principle of similar miscibility and uses an organic solvent with the same polarity as the binder to dissolve the binder, thereby achieving the separation of the active material and the current collector.

Figure 1. Process of solvent treatment of retired batteries

Among them, the more commonly used solvent is N-methylpyrrolidone, which can promote the dissolution of binders in retired battery recycling.

In addition, the active materials obtained by the solvent method have higher purity, so the assembled batteries also have excellent performance. The solvent method is less destructive to the materials and simplifies the separation process. However, the commonly used organic solvents are expensive, highly toxic, and harmful to the human body. Harmful to health.

The alkali dissolution method mainly uses strong alkali to selectively dissolve aluminum foil, while the valuable metals Li, Ni, Co and Mn in the electrode material are basically insoluble, thus achieving the separation of aluminum foil and active materials.

The high concentration of alkali in the alkali solution method is harmful to the environment and the human body, and there are more binders and conductive carbon black residues, which affects the subsequent leaching of valuable metals.

two

Leaching valuable metals is to dissolve retired ternary lithium-ion battery electrode active materials so that valuable metals such as Li, Co, Ni, and Mn can be efficiently leached into the solution, which is the core of wet recovery technology.

At present, the main methods for leaching valuable metals from ternary materials are acid leaching and biological leaching. Adding an appropriate amount of reducing agent to the leach solution can effectively improve the leaching efficiency of valuable metals.

The main function of the reducing agent (H2O2, NaHSO3, glucose, etc.) is to reduce the high valence Co and Mn in the solid phase into more easily soluble Co2+ and Mn2+, thereby increasing the leaching rate of Co and Mn.

The acid leaching method can transfer most of the metal ions of solid powder to the acid solution, and can effectively separate from some conductive carbon, binder and other residual components, providing raw material liquid for subsequent separation and purification.

The acid used in the acid leaching method is divided into inorganic acid and organic acid.

Inorganic acids can dissociate hydrogen ions, show strong acidity, and have strong leaching effects on Li, Co, Mn, and N.

Inorganic acid leaching mainly uses hydrochloric acid, sulfuric acid, nitric acid, etc. as leaching agents, and then processes the acid leaching liquid and residue. In the inorganic acid leaching system, there are many solvent systems that use H2SO4 as the leaching agent and H2O2 as the reducing agent.

Inorganic acids are widely available and relatively cheap, but they are prone to produce Cl2, SO3, NOx acidic gases and waste liquids, posing potential threats to the environment and human health. In addition, the strong acidity and corrosiveness of inorganic acids place high demands on production equipment, which increases recycling costs to a certain extent.

Compared with inorganic acid, organic acid leaching of valuable metals is less corrosive to the equipment and instruments used. The waste liquid produced in the process is easy to biodegrade, has low environmental pollution hazard, is suitable for recycling, and also shows good performance on valuable metals. Excellent leaching ability.

In recent years, the technology of using organic acids such as formic acid, tartaric acid, citric acid, malic acid, etc. as leaching agents has been favored by researchers.

Among many organic acids, citric acid can dissociate into H+, HCit2-, H2Cit- and Cit3- in aqueous solution, which is beneficial to the leaching of metals.

Malic acid is an environmentally friendly binary organic weak acid. It contains 1 hydroxyl group and 2 carboxyl groups. It combines with transition metals to form a complex. Since it has its own hydroxyl group, it has certain reducing properties and can promote forward direction in the leaching reaction. reaction effect.

In addition to different types of acid leaching agents and reducing agents that have an important impact on metal leaching, the concentration of the leaching agent, the amount of reducing agent, the liquid-to-solid ratio, and the leaching time and temperature are also important factors affecting metal leaching.

Bioleaching utilizes the special selectivity of microbial bacteria to achieve the leaching and dissolution of metals. Bioleaching has the advantages of low acid consumption, high metal dissolution rate, environmental friendliness, and mild operating conditions, and is gradually used in the recovery of valuable metals.

Bioleaching is a potential technology for recovering valuable metals from retired batteries in the future. However, in solutions with higher metal solubility, the bacterial flora is easily inactivated and the leaching efficiency is affected. Therefore, the bioleaching method is only suitable for low-solubility metals. in solution.

In addition, biological flora requires a long cultivation period and has strict environmental requirements, which limits its industrial application. Therefore, it is necessary to further increase the culture speed of bacterial strains, the adsorption speed of metal ions, etc. to increase the leaching rate of metal ions.

three

The separation and extraction of valuable metals is to separate and extract valuable metals from the leach solution. Currently, there are mainly extraction methods and precipitation methods.

The extraction method is to select one or a mixture of several organic solvents as the extraction agent to form a coordination complex with the target metal ions, and then transfer the metal ions of the coordination complex to another organic solvent to achieve extraction. Separation and purification of different metal ions.

The extraction method has low energy consumption, simple equipment operation, good separation effect, and high purity of recovered metals. However, the extraction agents are mostly toxic organic solvents, which are relatively expensive and the separation process is complicated.

For systems containing Li, Ni, Co, and Mn, the commonly used extraction agents include trimethylphosphonic acid, diphosphoric acid, diethylphosphonic acid mono-2-ethyl ester, etc.

The precipitation method is to add a suitable precipitant to the leach solution, so that the metal ions and the precipitant form a compound that is difficult to dissolve, thereby achieving the separation of metals.

For acid-leached nickel-cobalt-manganese electrode materials, the precipitation method can achieve purification, impurity removal and precipitation separation of metal ions.

The precipitation method has simple steps, lower cost, shorter process, and higher metal ion recovery efficiency. However, Ni, Co, and Mn are transition metals with similar chemical properties, and they are prone to co-precipitation, resulting in low product purity.

Therefore, during the precipitation process, it is important to select the appropriate precipitant and control the precipitation conditions.

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