Method for extracting and preparing metal lithium from waste lithium ion batteries

Abstract

The invention belongs to the field of resource recycling of waste lithium iron phosphate batteries, and particularly discloses a method for extracting and preparing metal lithium from waste lithium ion batteries, which comprises the following steps: taking a waste positive plate disassembled from the waste lithium ion batteries as an anode, and taking an electrode of which the surface is compounded with a modified layer as a cathode; respectively conductively connecting the anode and the cathode with a positive electrode and a negative electrode of a power supply and carrying out electrochemical treatment in the electrolyte, deintercalating lithium ions in the waste positive plate and depositing on the cathode to form metal lithium; the modified layer on the cathode comprises a binder and a lithium-loving modified material; wherein the lithium-loving modified material is at least one of Li3N, LiF, Li3PO4 and Al2O3, and the lithium-loving modified material is at least one of Li3N, LiF, Li3PO4 and Al2O3; the electrolyte is a hydrophobic solution containing a conductive lithium salt and a deep eutectic solvent; the electrochemical treatment process comprises a pulse charging process which is performed in advance and a constant-voltage charging process which is performed subsequently. According to the method, high-value recycling of waste lithium iron phosphate battery resources can be realized.

Description

Technical Field

[0001] The invention belongs to the field of recycling waste lithium-ion batteries, and in particular relates to a method for extracting lithium from waste positive electrodes. Background Art

[0002] With the high-quality development of human society and the rapid upgrading of science and technology, new energy electric vehicles have been widely used in daily life. According to SNE research, the global power battery installation will reach 705 GWh in 2023, of which lithium iron phosphate accounts for more than 50%. The service life of lithium iron phosphate batteries is between 5 and 8 years. With the use of a large number of power batteries, the peak period of power battery scrapping will come in the next few years. Faced with such a large amount of waste in the future, it is urgent to take effective measures to dispose of lithium iron phosphate batteries.

[0003] Lithium iron phosphate batteries contain fluorine electrolyte and binder PVDF, which will pollute the environment if not properly disposed of. At the same time, lithium iron phosphate batteries contain about 3% lithium and about 25% iron. If metal resources can be efficiently recycled, it will not only ease my country's dependence on imported lithium resources, but also solve the harm of lithium iron phosphate batteries to the environment.

[0004] There are many methods for recycling spent lithium iron phosphate batteries. The main process flow includes: battery pretreatment, battery crushing, metal current collector sorting, metal element leaching, element separation, etc. Methods for element separation include: wet leaching and electrochemical lithium removal. Wet leaching of metal elements is described in the paper: Sustainable and Facile Methodfor the Selective Recovery of Lithium from Cathode Scrap of Spent LiFePO 4 Batteries, at a solid-liquid ratio of 300 g L -1 , at 25°C, with Na 2 S 2 O 4 As an oxidant, LiFePO 4 Oxidized to FePO 4 , release Li + The lithium leaching rate reached 99%, and then the pH value of the liquid phase was adjusted to gradually separate the valuable components. The electrochemical extraction of metal elements is described in the paper: A Green and Cost-Effective Method for Production of LiOHfrom Spent LiFePO 4 Based on the suspended electrolysis process of electrodialysis, a cation exchange membrane is constructed to separate the anode and cathode chambers. The anode electrolyte is Li 2SO 4 solution, the cathode electrolyte is LiOH solution, LiFePO 4 The powder was added to the anode electrolyzer and the current density was 60 mA cm -2 , temperature is 80℃, solid-liquid ratio is 100 gL -1 Under the conditions of LiFePO 4 The leaching rate was 96.74%, and the converted LiOH•H 2 The purity of O reached 99.13%, and the current efficiency of the whole process was 85.81%. This scheme essentially converts LiFePO 4 The lithium is separated and the water electrolysis reaction occurs to generate OH - , thus generating LiOH•H in the cathode region 2 O.

[0005] Although hydrometallurgy has many advantages, the whole process of wet leaching is long, and a large number of acid and alkali reagents are used. A large amount of waste liquid needs to be treated later, and there is a risk of secondary pollution. The electrochemical recovery process is carried out in an aqueous solution system. It is inevitable that water electrolysis side reactions will occur during the electrolysis process, resulting in low current efficiency in the whole process and energy waste. In addition, the lithium element of retired lithium iron phosphate batteries is enriched in the form of ions, which need to be further purified and separated and converted into various lithium products. The process of generating lithium products is long and has many process pollutants, which is not conducive to the rapid disposal of a large number of retired lithium iron phosphate batteries. Therefore, the development of a short-process direct extraction technology for metallic lithium from retired lithium iron phosphate batteries can shorten the lithium element recovery chain and achieve rapid reuse of resources. Summary of the invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for extracting and preparing metallic lithium from waste lithium-ion batteries, aiming to directly prepare metallic lithium products with high yield and high purity from waste lithium-ion batteries.

[0007] At present, the production processes of high-purity lithium metal include molten salt electrolysis and vacuum thermal reduction. Molten salt electrolysis is the main method for industrial preparation of lithium metal, and 90% of lithium metal comes from this process. The molten salt electrolysis method is to electrolyze LiCl-KCl molten salt at about 420℃ and 6~10V voltage. The electrodes undergo redox reactions, the anode releases chlorine gas, and the cathode generates liquid lithium metal. The purity of lithium can reach 99%. The molten salt electrolysis method has the advantages of strong operability and mature technology, but the process has high requirements for the purity of raw materials LiCl and KCl, and the cost of raw materials is high; LiCl is extremely volatile at high temperature, causing serious corrosion to the electrolytic cell and electrodes, resulting in high maintenance costs in the production process; the chlorine gas released during the production process has the risk of polluting the environment. At present, the cost of producing 1Kg of lithium metal in the lithium metal production process is about 800 yuan. The production cost of lithium metal is high, so the development of efficient, energy-saving and convenient lithium metal extraction technology is of great significance to the development of my country's lithium industry.

[0008] Therefore, developing a process for directly extracting lithium metal from retired lithium battery positive electrode materials can shorten the lithium recovery chain, achieve rapid reuse of resources, and greatly reduce the cost of lithium metal production.

[0009] In view of the above problems, the present invention provides the following solutions after in-depth research:

[0010] A method for extracting and preparing metallic lithium from waste lithium-ion batteries, using a waste positive electrode sheet disassembled from waste lithium-ion batteries as an anode, and an electrode with a modified layer on the surface as a cathode; the anode and the cathode are respectively conductively connected to the positive electrode and the negative electrode of a power source and electrochemically treated in an electrolyte, lithium ions in the waste positive electrode sheet are deintercalated and deposited on the cathode to form metallic lithium;

[0011] The modified layer on the cathode includes a binder and a lithium-philic modified material; wherein the lithium-philic modified material is Li 3 N, LiF, Li 3 PO 4 、Al 2 O 3 At least one of;

[0012] The electrolyte is a hydrophobic solution containing a conductive lithium salt and a low eutectic solvent;

[0013] The electrochemical treatment process includes a preliminary pulse charging process and a subsequent constant voltage charging process.

[0014] The present invention innovatively uses the waste positive electrode sheet to be processed directly as the anode, places it and the cathode together in the electrolyte to construct an electrochemical system, and performs electrochemical treatment under the special pulse-constant voltage mechanism. In this way, there is no need to crush, peel, or perform other operations on the positive electrode sheet, and the constraints of the SEI on the surface of the positive electrode sheet and the lattice of the positive electrode material can be broken, so that metallic lithium products can be directly obtained in a lossless, efficient, high-yield, and high-purity manner.

[0015] The waste positive electrode sheet is a positive electrode sheet obtained by disassembling waste lithium-ion batteries. The disassembly method can be well known in the industry. In the present invention, there is no need to perform peeling and other treatments on the waste positive electrode sheet, and it can be directly used as an anode for the electrochemical treatment. In the present invention, considering the preparation scale, the size of the waste positive electrode sheet can be slightly adjusted as needed.

[0016] The process of the present invention has good universality and can be applied to extracting lithium from any waste lithium-ion battery and preparing metallic lithium. For example, as an optional solution, the waste positive electrode sheet includes a current collector and a positive electrode material composited on its surface; wherein the positive electrode material includes at least one positive electrode active material selected from lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel-cobalt binary material, and nickel-cobalt-manganese ternary material.

[0017] In the present invention, a conductive agent and a binder are allowed to exist in the positive electrode material.

[0018] In the present invention, in the positive electrode material, the content of the positive electrode active material is above 50 wt.%, and considering the value of the recycling process, it can be further 80-95 wt.%.

[0019] The present invention shows that the combination of the cathode, electrolyte type and charging mechanism of electrochemical treatment is the key to synergistically breaking through the SEI and lattice of waste positive electrode sheets and improving the direct production rate, yield and purity of metallic lithium.

[0020] In the present invention, the cathode includes a current collector and a modified layer composited on its surface. The modified layer can be a homogeneous or heterogeneous material layer of a binder-lithiophilic modified material. For example, in the present invention, the modified layer includes a binder layer on the surface of the current collector and a composite layer of a binder-lithiophilic modified material composited on the surface of the binder layer.

[0021] Preferably, the current collector comprises copper foil, lithium foil, aluminum foil or nickel foil.

[0022] Preferably, the thickness of the current collector is between 1 μm and 50 μm, and can further be between 5 and 15 μm.

[0023] Preferably, the binder includes at least one of PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), PAN (polyacrylonitrile), and PEO.

[0024] Preferably, the thickness of the adhesive layer is 100-2000 nm, and further can be 300-1000 nm.

[0025] In the composite layer, the content of the lithium-philic modified material is 10-70%, preferably 30-60%.

[0026] Preferably, the thickness of the composite layer is 100 nm to 1000 nm, and further can be 300 to 1000 nm.

[0027] In a preferred embodiment of the present invention, in the composite layer, the content of the lithium-philic modified material is arranged to decrease along the thickness direction. The current collector side refers to the bottom surface, and the thickness direction refers to the direction from the bottom surface to the surface.

[0028] The research of the present invention unexpectedly shows that the cathode with the lithium-philic modified material arranged in a decreasing manner, combined with the electrolyte and the charging mechanism, can unexpectedly further achieve synergy and further enhance the yield and purity of metallic lithium directly produced from waste positive electrode sheets.

[0029] In the present invention, in the electrolyte, the low eutectic solvent includes a quaternary ammonium salt-amide; further, it may include at least one of tetraethylammonium chloride / caprolactam, tetrabutylammonium chloride / caprolactam, tetrabutylammonium acetate / caprolactam, ethyleneimine quaternary ammonium / caprolactam, octanamide gemini quaternary ammonium salt / caprolactam, double-chain quaternary ammonium salt / caprolactam, benzalkonium chloride / caprolactam, benzyltriethylammonium chloride / caprolactam, and tetrabutylammonium hydrogen sulfate / caprolactam.

[0030] The electrolyte described in the present invention may also contain a diluent solvent, which includes at least one of toluene, xylene, carbon tetrachloride, tetrahydrofuran, tetrahydrofuran, tetrafluoroethane, tetrachloromethane, ethyl tetrafluoroacetate, chloromethane, dichloromethane, difluoromethane, dichloroethane, trifluoroethane, etc.

[0031] Preferably, the conductive lithium salt comprises LiPF 6 , LiBOB, LiTFSI, LiFSI, LiNO3, LiCLO 4 , LiBF 4 At least one of .

[0032] Preferably, in the electrolyte, the concentration of the conductive lithium salt is 0.1M~3M, and can further be 1~2M.

[0033] Preferably, before the electrochemical operation, the anode and cathode can be soaked in the electrolyte for 1 to 10 hours first.

[0034] In the present invention, the plate distance between the anode and the cathode is between 1 cm and 10 cm; and the cathode area is 1 to 1.2 times that of the anode area.

[0035] In the present invention, the pulsed voltage during the pulsed charging process is set by Asin(ωx + φ)+k, where 0 < x < π / 2ω, the amplitude of the sine curve 0 < A ≤ 0.2, 2.6 < k < 4.2, 0 < ω ≤ 0.8; preferably 0 < A ≤ 0.1, 2.8 < k < 4.2, 0 < ω ≤ 0.5. Research in the present invention shows that by adopting this preferred pulsed mode and combining with the process described in the present invention, excellent combined synergistic effects can be obtained, which helps to further improve the lithium extraction effect.

[0036] Preferably, the maximum voltage of the pulsed charging is 4 to 4.5 V, and further can be 4.2 to 4.3 V.

[0037] Preferably, the number of sine waves is not less than 10 times, preferably 10 to 20 times.

[0038] In the present invention, after the pulsed charging reaches the maximum voltage, constant voltage charging is carried out.

[0039] Preferably, the treatment time of the constant voltage charging is more than 1 hour, preferably 1 to 12 hours, and further 3 to 6 hours.

[0040] In the present invention, the electrochemical treatment process can be carried out in any electrolytic cell. As an optional scheme, the present invention can be carried out in a unidirectional circulation electrolytic cell, which includes a plurality of cathode groups and anode groups arranged side by side. The cathodes and anodes in the cathode groups and anode groups are arranged alternately. A delivery pump for pushing the electrolyte to move unidirectionally between the gaps of each anode and each cathode, and an external circulation pump for circulating the electrolyte from the end of the unidirectional movement to the starting end of the unidirectional movement are also provided in the electrolytic cell.

[0041] In the present invention, the temperature in the electrochemical treatment stage is 30°C to 60°C.

[0042] Beneficial effects

[0043] Innovatively, the present invention assembles the cathode with the modified layer and the electrolyte of the deep eutectic solvent to form an electrochemical system, and further combines with the pulsed-constant voltage electrochemical mechanism, so as to achieve synergy, break through the physical and chemical structural characteristics of the waste cathode sheet, and can directly prepare metallic lithium with high efficiency, high yield and high purity.

[0044] In addition, the present invention also shows that the use of a cathode with a gradient content of modified materials, especially a decreasing content of modified materials, combined with the electrolyte and charging mechanism, can further achieve synergy and further enhance the yield and purity of direct extraction and preparation of metallic lithium. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a top view schematic diagram of a device for directly extracting metallic lithium from waste lithium iron phosphate;

[0046] Figure 2 This is an optical image of the electrochemical preparation of metallic lithium in Example 1. DETAILED DESCRIPTION

[0047] The present invention is suitable for non-destructive lithium extraction and preparation of metallic lithium from any waste lithium-ion battery. As an illustrative solution, the following case uses waste lithium iron phosphate battery as a typical case, and the technical solution is: using waste lithium iron phosphate positive electrode sheet as anode, lithium-philic current collector as cathode, low eutectic solvent as electrolyte, customizing charging system, and constructing electrochemical system.

[0048] The waste lithium iron phosphate electrodes used come from the positive electrodes of commercial lithium iron phosphate batteries and cannot be recycled. The mass fraction of lithium iron phosphate on the positive electrode is not less than 90%.

[0049] The lithium iron phosphate positive electrode is treated by immersion in a lithium-containing solution (electrolyte).

[0050] The solvent of the lithium-containing solution is a low eutectic solvent: the low eutectic solvent includes a quaternary ammonium salt-amide; further, it may include at least one of tetraethylammonium chloride / caprolactam, tetrabutylammonium chloride / caprolactam, tetrabutylammonium acetate / caprolactam, ethyleneimine quaternary ammonium / caprolactam, octylamide gemini quaternary ammonium salt / caprolactam, double-chain quaternary ammonium salt / caprolactam, benzalkonium chloride / caprolactam, benzyltriethylammonium chloride / caprolactam, and tetrabutylammonium hydrogen sulfate / caprolactam; the lithium salt of the lithium-containing solution is a conductive lithium salt including LiPF 6 , LiBOB, LiTFSI, LiFSI, LiNO3, LiCLO 4 , LiBF 4 , at least one of the following.

[0051] The lithium salt concentration of the lithium solution is between 0.1 M and 3 M.

[0052] The immersion time of the lithium iron phosphate positive electrode is between 1 h and 5 h, so that the lithium ions can penetrate into the lithium iron phosphate.

[0053] The immersion temperature of lithium iron phosphate positive electrode is 30℃~60℃.

[0054] After infiltration, the electrolyte contact angle of the lithium iron phosphate electrode is between 1° and 5°.

[0055] The area of ​​the lithium iron phosphate cathode is 5 cm 2 ~1 m 2 between.

[0056] The lithium-philic metal lithium electrode (also referred to as cathode in the present invention) used in the present invention comprises a current collector and a lithium-philic artificial interface (modified layer) composited on its surface. The lithium-philic artificial interface layer comprises a binder (organic matter) and a modified material (inorganic matter), wherein the organic matter includes: PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), PAN (polyacrylonitrile), PEO, etc.; the inorganic matter includes: Li 3 N, LiF, Li 2 O 3 Si, Li 3 PO 4 Lithium-conducting inorganic substances.

[0057] The gradient of the lithium-philic artificial interface layer is mainly inorganic matter inside and mainly organic matter outside. The lithium-philic artificial interface layer is constructed by coating on the current collector after stirring.

[0058] The lithium-philic artificial interface layer is coated on the current collector, and the current collector used is a lithium-philic current collector, and the material is copper foil, lithium foil, aluminum foil, nickel foil, etc. The thickness of the current collector used is between 1 μm and 50 μm.

[0059] The configuration of the lithium-philic artificial interface layer slurry is composed of a solvent and a solute, wherein the solvent is one or more of NMP (N-methylpyrrolidone) and DMF (N,N-dimethylformamide).

[0060] The solute mass fraction of the slurry configuration is: 5%~20%, the stirring method is a paddle mixer or a propeller stirring paddle, the speed is at least 1000 r / min, the shear modulus is 1000-2000 Pa, the pressure is normal pressure, the temperature is 30℃~50℃, and the time is 0.5 h~5 h.

[0061] The lithium-philic artificial interface layer is constructed by first coating an inorganic slurry on a lithium-philic current collector with a coating thickness of 100 nm to 100 μm; after drying, coating an organic slurry on the inorganic layer with a coating thickness of 100 nm to 100 μm.

[0062] The lithium-philic artificial interface layer is a composite artificial interface of external organic matter and internal inorganic matter, and its total thickness is controlled between 200nm~200μm, and the thickness ratio of the organic layer and the inorganic layer of the artificial interface layer is 1:1.

[0063] On the lithium-philic current collector, the coating thickness is between 100 nm and 100 μm.

[0064] The thickness of the lithium-philic metal lithium electrode is between 10 μm and 10 cm, and the area is 5 cm 2 ~1 m 2 between.

[0065] The applicable charge density range of the lithium-philic metal lithium electrode is: 0.1 mA cm -2 ~500 cm -2 between;

[0066] The density of deposited lithium metal on the lithiophilic lithium metal electrode is between 85% and 99%.

[0067] The amount of lithium metal deposited by the lithium-philic metal lithium electrode is: 1 mg cm -2 ~500 g cm -2 .

[0068] The lithium-philic metal lithium electrode can deposit metal lithium with a thickness ranging from 1 μm to 100 cm.

[0069] The present invention constructs an electrochemical system, and the electrolyte used is a low eutectic solvent electrolyte.

[0070] In the present invention, the solvent of the lithium-containing solution is a low eutectic solvent: quaternary ammonium salt-amide, polyol, etc., wherein the quaternary ammonium salt-amide includes: tetraethylammonium chloride / caprolactam, tetrabutylammonium chloride / caprolactam, tetrabutylammonium acetate / caprolactam, etc., and the polyol includes: ethylene glycol, glycerol, butanediol, xylitol; the lithium salt of the lithium-containing solution is lithium hexafluorophosphate, LiTFSI, etc.

[0071] In the present invention, the infiltration liquid solvent is one or more of the above-mentioned low eutectic solvents, and the lithium salt is one of the above-mentioned lithium salts.

[0072] In the present invention, the lithium salt concentration of the lithium solution is between 0.1M and 3M.

[0073] In the present invention, the electrochemical delithiation temperature of the lithium iron phosphate positive electrode is 30° C. to 60° C., and appropriate heating is conducive to the rapid delithiation of the electrode.

[0074] In the present invention, the electrolyte used is to completely immerse the anode lithium iron phosphate and immerse 90% of the cathode area.

[0075] In the present invention, the electrochemical system constructed, the distance between the positive and negative electrodes, the electrode area ratio, and the number of positive and negative electrode plates all need to be regulated.

[0076] In the present invention, the distance between the positive and negative electrodes is maintained between 1 cm and 10 cm, and the cathode electrode area: anode electrode area is 1:1-1.5:1, that is, the cathode area is 0-20% larger than the anode area.

[0077] In the present invention, for the constructed electrochemical system, a special charging regime needs to be formulated to force lithium deintercalation from lithium iron phosphate.

[0078] In the present invention, the charging regime of the constructed electrochemical system consists of pulsed charging and constant voltage charging. Among them, the pulsed voltage is set according to the sine curve Asin(ωx + φ) + k, and the maximum voltage of the pulsed charging is 4.3 V.

[0079] In the present invention, for the above-mentioned pulsed charging regime, the voltage increase varies according to the sine curve Asin(ωx + φ) + k, where 0 < x < π / 2ω, the amplitude of the sine curve 0 < A ≤ 0.2, 2.6 < k < 4.2, 0 < ω ≤ 0.8; preferably 0 < A ≤ 0.1, 2.8 < k < 4.2, 0 < ω ≤ 0.5.

[0080] In the present invention, the above-mentioned pulsed charging regime is composed of a combination of multiple repeated sine curves, and the number of sine waves is not less than 10 times until the system voltage reaches 4.3 V.

[0081] In the present invention, during the above-mentioned pulsed process, the current changes with the change of voltage, and the practical current range is 0.01 mA cm -2 ~ 100 mA cm -2 , and the specific value is determined by the sine curve of the pulse.

[0082] In the present invention, the constant voltage charging after pulsed charging is carried out at a constant voltage of 4.3 V, and the constant voltage time is between 1 h and 12 h.

[0083] In the present invention, for the constructed electrochemical system, lithium deintercalation occurs during the charging of lithium iron phosphate at the anode, and at the same time, lithium ions at the cathode are directly converted into metallic lithium. The top view schematic diagram of the device is as Figure 1 shown. To achieve efficient lithium deintercalation, multiple positive and negative electrodes are connected inside the device, and the number of positive and negative electrodes is not less than 10 respectively;

[0084] The device is in a protective gas to maintain the purity of metallic lithium, and the device is airtight; the gas that can be introduced is one of protective gases such as argon, and the gas flow rate is controlled at 5 mL min -1 ~5 L min -1 .

[0085] In the present invention, the liquid inside the device can flow unidirectionally to keep the lithium ion concentration in the electrolyte uniform and reduce the concentration polarization on the electrode surface. A pump is installed on one side of the electrolytic cell to push the electrolyte in the electrolytic cell to flow back, and the flow rate of the pumped liquid is controlled at 10 ml min -1 ~2000 ml min -1 . The amount of the pumped liquid is determined by the amount of waste lithium iron phosphate to be treated. For example, the amount of the pumped liquid per minute is 5% - 30% of the mass of lithium iron phosphate.

[0086] In the present invention, the external reflux device may include a purification device for capturing materials such as lithium iron phosphate particles, conductive carbon and binder scattered in the electrolyte. For example, the particulate matter is captured by using a multi-layer gradient filter layer, wherein the number of filter layers is not less than 5 layers, and the filter gradients are: 1000 μm, 500 μm, 200 μm, 50 μm, 20 μm, 5 μm, 1 μm, etc., to ensure that 99% of the particles are effectively filtered. Furthermore, the material of the gradient filter layer is polyester filter cloth. In order to ensure efficient filtration of the external reflux device, a pressure pump needs to be added to ensure the delivery rate of the liquid. The pumping pressure is maintained at 0.1 MPa~10 MPa.

[0087] The present invention also provides a method for extracting the above-mentioned prepared metallic lithium. After the metallic lithium deposition reaches 5 mm to 10 cm, it can be collected. The specific method is: after the cathode is taken out of the electrolytic cell, it is peeled off in a drying room.

[0088] In the present invention, the metal lithium collection process is to first peel off the surface composite artificial interface layer, then heat-melt the metal lithium, and then soften and collect it. For example, NMP or DMF solvent can be used to elute the artificial interface layer, and then the metal lithium on the electrode plate is heated and softened before collection. Preferably, the softening temperature is 100°C to 200°C.

[0089] The advantages of the present invention are:

[0090] (1) In the present invention, waste lithium iron phosphate is used as a lithium resource, and a special electrochemical system is applied to achieve forced delithiation of lithium iron phosphate. At the same time, the delithiation is electrodeposited as metallic lithium at the cathode, so that the extraction rate of lithium in the waste lithium iron phosphate is not less than 98%, and the purity of the metallic lithium is not less than 99%.

[0091] (2) In the prior art, since the crystal structure in waste lithium iron phosphate is destroyed, it is difficult to achieve lithium extraction by conventional charging system. Through the combination of the cathode, electrolyte and pulse-constant voltage charging mechanism, the microscopic expansion and contraction of the crystal structure can be achieved, thereby widening the lithium conduction channel, achieving efficient lithium extraction, and being able to directly prepare dense and uniform metallic lithium. The process technology of the present invention has a short resource recovery process, low cost, and easy operation, and achieves high-value recovery of lithium resources in waste lithium iron phosphate. DETAILED DESCRIPTION

[0093] The following is a specific description of the preferred embodiments of the present invention, which does not constitute any limitation to the present invention. That is, the present invention is not meant to be limited to the above embodiments, and common modifications or substitutes in the technical field are all included in the scope defined by the claims of this application.

[0094] Embodiment 1:

[0095] (1) Waste positive electrode (anode)

[0096] After a company disassembled its waste lithium iron phosphate soft-pack batteries, lithium resources were efficiently recycled. The size of the electrode sheet after disassembly was 10 cm × 20 cm, of which the surface density of lithium iron phosphate in a single positive electrode sheet was 15 mg cm -2 , the mass of single-sided lithium iron phosphate is 3g.

[0097] Before operation, soak in electrolyte for 2 h;

[0098] (2) Cathode

[0099] The cathode electrode used is a copper foil composited with a modified layer, wherein the thickness of the copper foil is 10 μm, the area is 1.1 times the area of ​​the positive electrode, and the length and width are 11 cm×22 cm.

[0100] The modified layer includes an organic layer (binder layer, PVDF layer) on the surface of the current collector, and a composite layer composited on the surface of the organic layer, wherein the composite layer is an inorganic material (Li 3 N), a composite layer of organic matter (PVDF), the preparation steps are:

[0101] Organic matter (PVDF) is dispersed in dispersant (NMP) to obtain an organic solution. Inorganic matter and organic matter are dispersed in dispersant (NMP) to obtain a composite solution.

[0102] An organic solution is pre-coated on the copper foil to form an organic layer (PVDF layer, with a thickness of 500nm), and then composite solutions with high inorganic concentration and low inorganic concentration are sequentially coated to form a composite layer; wherein the total thickness of the composite layer is 500nm; the inorganic concentrations of the composite solutions with high and low inorganic concentrations are 30 g / L and 10 g / L, respectively, and the weight of the coated inorganics in each coating solution is the same, that is, in this case, the weight of the coated inorganics in the composite solution with high inorganic concentration is the same as the weight of the coated inorganics in the composite solution with low inorganic concentration, except that the dispersed concentration of the inorganics decreases from the bottom layer to the surface, and the coating is dried after coating;

[0103] Before operation, soak in electrolyte for 2 h;

[0104] (3): Electrolyte composition:

[0105] Tetraethylammonium chloride / caprolactam is used as solvent, lithium hexafluorophosphate is used as lithium salt, and the electrolyte concentration is 1.5 M.

[0106] (4): Assembling the electrochemical system

[0107] Select 15 pieces of the above-mentioned waste lithium iron phosphate positive electrode (anode), coat the above-mentioned composite artificial interface layer on 15 pieces of copper foil (cathode), set the cathode and cathode alternately, and control the distance between the cathode and cathode to be 5 cm, configure the above-mentioned electrolyte, and construct an electrochemical lithium metal recovery system (each anode is connected to the positive electrode of an external power supply, and each cathode is connected to the negative electrode of the external power supply). The charging system consists of pulse charging and constant voltage charging, wherein the pulse voltage is set by the sinusoidal curve Asin(ωx+φ)+k, wherein A=0.1, initial k=2.8, ω=0.1, therefore, during the charging process, the number of sine waves is 16 times, until the voltage reaches 4.3V, and then it is converted to constant voltage charging, and the constant voltage time is 5 h. The circulating gas of the device is nitrogen, and the gas flow rate is controlled at 1 L min -1 The external electrolyte return device has a pumping liquid flow rate of 500 ml min -1 The external purification and collection device is an 8-layer gradient filter with a particle size of 1000 μm, 500 μm, 200 μm, 50 μm, 20 μm, 5 μm, 1 μm, and 500 nm, and the filter material is polyester filter cloth. The external circulation system applies a pressure of 5 MPa. After electrochemical delithiation, the residual lithium content in the waste lithium iron phosphate pole piece is less than 3% after detection by ICP and XRF and other equipment. The waste lithium iron phosphate pole piece is then taken out and replaced with a batch of waste lithium iron phosphate pole pieces. The above electrochemical system is repeated to convert the lithium in the waste lithium iron phosphate pole piece into metallic lithium. After 500 cycles, the mass of metallic lithium stably deposited on a single cathode was 27 g and the thickness was 28.5 μm. After 1000 cycles, the mass of metallic lithium stably deposited on a single cathode was 52 g and the thickness was 59 μm. The artificial interface on the cathode surface was eluted with NMP solvent, and the electrode was placed in a vacuum heater and heated to 180°C to collect the softened metallic lithium.

[0108] Example 2

[0109] Compared with Example 1, the only difference is that the structure of the cathode in step 2 is changed. Specifically, after coating the organic layer, the same amount of inorganic and organic matter is divided into 5 equal parts, and dispersed into composite liquids with concentrations of 50 g / L, 40 g / L, 30 g / L, 10 g / L, and 5 g / L with a solvent, and then coated in 5 stages in sequence, and cured to obtain a composite layer with a decreasing inorganic concentration. Other operations and parameters are the same as the modified layer in Example 1.

[0110] Example 3

[0111] Compared with Example 2, the only difference is that the structure of the cathode in step 2 is changed. Specifically, after coating the organic layer, it is coated with a composite solution with an inorganic concentration of 5 g / L, 10 g / L, 30 g / L, 40 g / L, and 50 g / L in sequence, and solidified to obtain a composite layer with a successively set inorganic concentration. Other operations and parameters are the same as in Example 2.

[0112] Example 4

[0113] Compared with Example 1, the only difference is that in step 2, the type of inorganic substance or the structure of the modified layer is changed, specifically:

[0114] Group A: Inorganic substances are Li with a weight ratio of 1:1 3 N and LiF;

[0115] Group B: Inorganic substance is LiF;

[0116] Group C: Mixing the modified organic matter and inorganic matter to form a homogeneous solution, and coating the solution to obtain a modified layer with a homogeneous distribution of inorganic matter;

[0117] D: The thickness of the organic layer was changed to 400 nm and the total thickness of the composite layer was 700 nm.

[0118] Other operations and parameters are the same as in Example 1.

[0119] Comparative Example 1

[0120] Compared with Example 1, the only difference is that the cathode is a copper foil without artificial interface layer protection, and other operations and parameters are the same as those in Example 1.

[0121] Comparative Example 2

[0122] Compared with Example 1, the only difference is that Li 2 SO 4 Replace the inorganic component Li 3 N, other operations and parameters are the same as in Example 1.

[0123] Comparative Example 3

[0124] Compared with Example 1, the only difference is that the electrolyte system is changed, for example, fluoroethylene carbonate (FEC) is used to replace the low eutectic solvent, and other operations and parameters are the same as Example 1.

[0125] The results of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1:

[0126]

[0127] In summary, the electrochemical direct lithium extraction technology of the process described in the present invention can reuse all materials, is environmentally friendly, has low pollution, has high resource recovery efficiency, and has high purity of prepared metallic lithium, low cost, and strong operability. In particular, the preferred combination of inorganic materials and the concentration reduction scheme can achieve better performance.

[0128] Example 5: Electrochemical process pulse voltage condition regulation:

[0129] Compared with Example 1, the only difference is that the sine curve Asin(ωx+φ)+k is set to A=0.15, the initial k=2.9, ω=0.15, the direct voltage is 4.4V, the number of sine waves is 10 times, the constant voltage time is 4 h, and the other operations and parameters are the same as Example 1.

[0130] Example 6: Electrochemical process pulse voltage condition regulation:

[0131] Compared with Example 1, the only difference is that the voltage rising method is changed, specifically: the charging system is composed of pulse charging and constant voltage charging, wherein the pulse voltage is set by a sine curve Asin(ωx+φ)+k, wherein A=0.5, initial k=2.8, ω=0.8, therefore, during the charging process, the number of sine waves is 3 times, until the voltage reaches 4.3V, and then it is converted to constant voltage charging, and the constant voltage time is 5 h.

[0132] Comparative Example 4: Pulse voltage condition regulation in electrochemical process:

[0133] Compared with Example 1, the only difference is that the voltage rising method is changed, specifically: the charging system consists of step charging and constant voltage charging, wherein the charging voltage rises in a step form, the step rising voltage number is 0.1 V, the step number is 15, and during the charging process, until the voltage reaches 4.3 V, it is converted to constant voltage charging, and the constant voltage time is 5 h.

[0134] Comparative Example 5: Voltage Condition Regulation of Electrochemical Process:

[0135] Compared with Example 1, the only difference is that the voltage rising mode is changed, specifically: the charging system is constant voltage charging, wherein the electrochemical process is only constant voltage charging, and the constant voltage time is 5 h.

[0136] Comparative Example 6: Voltage Condition Regulation of Electrochemical Process:

[0137] Compared with Example 1, the only difference is that the voltage rising mode is changed, specifically: the charging system consists of constant current charging and constant voltage charging, wherein the constant current charging is charged at a rate of 0.5 C until the voltage reaches 4.3V, and then changes to constant voltage charging, and the constant voltage time is 5 h.

[0138] Comparative Example 7: Control of pulse voltage conditions in electrochemical process:

[0139] Compared with Example 1, the only difference is that the voltage rising method is changed, specifically: the charging system is pulse charging, wherein the pulse voltage is set by a sine curve Asin(ωx+φ)+k, wherein A=0.1, initial k=2.8, ω=0.1, therefore, during the charging process, the number of sine waves is 16 times, until the voltage reaches 4.3V, and then the charging is terminated.

[0140] The results of Examples 5-6 and Comparative Examples 4-7 are shown in Table 2:

[0141]

[0142] It can be seen from Examples 1 to 7 that the use of the gradient composition artificial interface layer modified electrode and the sinusoidal pulse-constant voltage treatment described in the present invention, combined with the joint control of the conditions, can unexpectedly synergistically optimize the treatment effect.

Claims

1. A method for extracting and preparing metallic lithium from waste lithium-ion batteries, characterized in that: The waste positive electrode sheet disassembled from waste lithium-ion batteries is used as the anode, and the electrode with a modified layer on the surface is used as the cathode; the anode and the cathode are respectively connected to the positive electrode and the negative electrode of the power source and electrochemically treated in the electrolyte, so that the lithium ions in the waste positive electrode sheet are deintercalated and deposited on the cathode to form metallic lithium; The modified layer on the cathode includes a binder and a lithium-philic modified material; wherein the lithium-philic modified material is at least one of Li3N, LiF, Li3PO4, and Al2O3; The electrolyte is a hydrophobic solution containing a conductive lithium salt and a low eutectic solvent; The electrochemical treatment process includes a preliminary pulse charging process and a subsequent constant voltage charging process.

2. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The waste positive electrode sheet is a positive electrode sheet disassembled from waste lithium-ion batteries; Preferably, the waste positive electrode sheet comprises a current collector and a positive electrode material composited on its surface; wherein the positive electrode material comprises at least one positive electrode active material selected from lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel-cobalt binary material, and nickel-cobalt-manganese ternary material; Preferably, the positive electrode material also allows the presence of a conductive agent and a binder; Preferably, in the positive electrode material, the content of the positive electrode active material is above 50 wt.%, preferably 80-95 wt.%.

3. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The cathode comprises a current collector and a modified layer composited on the surface thereof, wherein the modified layer comprises a binder layer on the surface of the current collector and a composite layer of a binder-lithiophilic modified material composited on the surface of the binder layer; Preferably, the current collector comprises copper foil, lithium foil, aluminum foil or nickel foil; Preferably, the thickness of the current collector is between 1 μm and 50 μm, and may further be between 5 and 15 μm; Preferably, the binder includes at least one of PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), PAN (polyacrylonitrile), and PEO; Preferably, the thickness of the adhesive layer is 100-2000 nm.

4. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 3, characterized in that: In the composite layer, the content of the lithium-philic modified material is 10-70%, preferably 30-60%; Preferably, the thickness of the composite layer is 100 nm to 2000 nm; Preferably, in the composite layer, the content of the lithium-philic modified material is arranged to decrease gradually along the thickness direction.

5. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: In the electrolyte, the low eutectic solvent includes quaternary ammonium salt-amide; further, it may include at least one of tetraethylammonium chloride / caprolactam, tetrabutylammonium chloride / caprolactam, tetrabutylammonium acetate / caprolactam, ethyleneimine quaternary ammonium / caprolactam, octylamide gemini quaternary ammonium salt / caprolactam, double-chain quaternary ammonium salt / caprolactam, benzalkonium chloride / caprolactam, benzyltriethylammonium chloride / caprolactam, and tetrabutylammonium hydrogen sulfate / caprolactam; Preferably, the electrolyte is further allowed to contain a dilution solvent, and the dilution solvent includes at least one of toluene, xylene, carbon tetrachloride, tetrahydrofuran, tetrahydrofuran, tetrafluoroethane, tetrachloromethane, ethyl tetrafluoroacetate, chloromethane, dichloromethane, difluoromethane, dichloroethane, trifluoroethane, etc.; Preferably, the conductive lithium salt includes at least one of LiPF6, LiBOB, LiTFSI, LiFSI, LiNO3, LiCLO4, and LiBF4; Preferably, in the electrolyte, the concentration of the conductive lithium salt is 0.1 M to 3 M; Preferably, before the electrochemical operation, the anode and cathode can be soaked in the electrolyte for 1 to 10 h.

6. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The plate distance between the anode and the cathode is between 1 cm and 10 cm; and the cathode area is 1 to 1.2 times that of the anode area.

7. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The pulse voltage in the pulse charging process is set by Asin(ωx + φ) + k, where 0 < x < π / 2ω, the amplitude of the sine curve 0 < A ≤ 0.2, 2.6 < k < 4.2, 0 < ω ≤ 0.8; preferably 0 < A ≤ 0.1, 2.8 < k < 4.2, 0 < ω ≤ 0.5; Preferably, the maximum voltage of the pulse charging is 4 to 4.5 V; Preferably, the number of sine waves is not less than 10 times, preferably 10 to 20 times.

8. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: After the pulse charging reaches the maximum voltage, constant voltage charging is carried out; Preferably, the treatment time of the constant voltage charging is more than 1 h, preferably 1 to 12 h, and further preferably 3 to 6 h.

9. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The electrochemical treatment process is carried out in a unidirectional circulation electrolytic cell, which includes a plurality of cathode groups and anode groups arranged side by side, the cathodes and anodes in the cathode groups and anode groups are arranged alternately, and a delivery pump for pushing the electrolyte to move unidirectionally between the gaps of each anode and each cathode, and an external circulation pump for circulating the electrolyte from the end of the unidirectional movement to the start of the unidirectional movement are also provided in the electrolytic cell.

10. The method for extracting and preparing metallic lithium from waste lithium-ion batteries according to claim 1, characterized in that: The temperature in the electrochemical treatment stage is 30°C to 60°C.

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