Lithium supplement method based on supercritical carbon dioxide, modified conductive agent and pole piece

Abstract

The invention relates to a lithium supplement method based on supercritical carbon dioxide, a modified conductive agent and a pole piece, the conductive agent and a catalyst are added into a lithium salt mixed solution, carbon dioxide is introduced, and a first mixture is obtained; the lithium salt mixed solution comprises a first lithium salt and a second lithium salt, and the decomposition temperature of the first lithium salt is lower than that of the second lithium salt; heating and pressurizing the first mixture to enable the carbon dioxide to reach a supercritical state, stirring, and reducing the pressure to normal pressure to obtain a second mixture; carrying out first heating treatment on the second mixture in a reducing atmosphere so as to decompose the first lithium salt into a lithium-containing intermediate product; carrying out second heating treatment to melt the second lithium salt and reduce the second lithium salt into metal lithium; and third heating treatment is carried out, so that the lithium-containing intermediate product is decomposed and reduced into metal lithium, and the lithiated conductive agent is obtained. The method is high in adaptability, lithium supplement agents do not need to be designed for the positive electrode and the negative electrode respectively, and the lithium supplement modified conductive agent is directly applied to the positive electrode or the negative electrode.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium supplementing method based on supercritical carbon dioxide, a modified conductive agent and a pole piece. BACKGROUND

[0002] During the first charge-discharge process of a lithium ion battery, the formation of a solid electrolyte interface (SEI) film or the occurrence of an irreversible side reaction will cause a large loss of active lithium, resulting in a decrease in capacity and a decrease in energy density of the battery cell. To solve this problem, there are two solutions in the industry: one is to select a battery cell main material with high first efficiency and design a suitable battery cell system under the premise of meeting the performance index requirements of the battery cell; the other is to supplement lithium to the battery cell to make up for the loss of active lithium during the first charge-discharge process.

[0003] In the industry, lithium supplementing for lithium ion batteries is divided into positive electrode side lithium supplementing and negative electrode side lithium supplementing.

[0004] Positive electrode lithium supplementing usually involves adding lithium supplementing additives (such as Li3NiO2, etc.) to the positive electrode slurry. During the first charge of the battery, these compounds release lithium ions to supplement the active lithium consumed by the SEI film in the negative electrode material.

[0005] Negative electrode lithium supplementing involves introducing an additional lithium source into the negative electrode active material to compensate for the active lithium consumed during the first charge-discharge process. Common negative electrode lithium supplementing methods include lithium foil supplementing, lithium powder supplementing, chemical supplementing and self-discharge mechanism lithiation, etc.

[0006] The above-mentioned positive electrode lithium supplementing method cannot be applied to negative electrode lithium supplementing, and similarly, the above-mentioned negative electrode lithium supplementing method cannot be applied to positive electrode lithium supplementing. Positive and negative electrode lithium supplementing need to be designed with lithium supplementing agents and supporting lithium supplementing methods, which have poor adaptability, SUMMARY The present application provides a lithium supplementing method based on supercritical carbon dioxide, a modified conductive agent and a pole piece. The present application has high adaptability and does not need to design lithium supplementing agents for positive and negative electrodes. The modified conductive agent for lithium supplementing can be directly applied to positive or negative electrodes.

[0007] In a first aspect, a lithium supplementing method based on supercritical carbon dioxide is provided, which comprises: adding a conductive agent and a catalyst to a lithium salt mixed solution, and introducing carbon dioxide to obtain a first mixture; the lithium salt mixed solution comprises a first lithium salt and a second lithium salt, and the decomposition temperature of the first lithium salt is lower than that of the second lithium salt; subjecting the first mixture to temperature and pressure increasing treatment to make the carbon dioxide reach a supercritical state, and then stirring and reducing the pressure to normal pressure to obtain a second mixture; performing a first temperature rising process on the second mixture under a reducing atmosphere to decompose the first lithium salt into a lithium-containing intermediate and a byproduct gas; performing a second temperature rising process to melt the second lithium salt and reduce it into metallic lithium and a byproduct gas; performing a third temperature rising process to decompose the lithium-containing intermediate and reduce it into metallic lithium and a byproduct gas, to obtain a lithiated conductive agent.

[0008] In some embodiments, the first lithium salt is lithium oxalate, the second lithium salt is lithium acetate, and the lithium-containing intermediate is lithium carbonate.

[0009] In some embodiments, the first temperature rising process comprises: rising the temperature to 250-280℃ at a temperature rising rate of 1-5℃ / min, and maintaining the temperature for 1-2h; the second temperature rising process comprises: rising the temperature to 340-360℃ at a temperature rising rate of 1-5℃ / min, and maintaining the temperature for 0.5-1h; the third temperature rising process comprises: rising the temperature to 400-450℃ at a temperature rising rate of 1-5℃ / min, and maintaining the temperature for 0.5-1h.

[0010] In some embodiments, before rising the temperature to 250-280℃, the first temperature rising process further comprises: first washing away the residual moisture in the second mixture, then drying at 80-100℃, then rising the temperature to 140-200℃ at a temperature rising rate of 1-5℃ / min, maintaining the temperature for 0.5-1h, and finally vacuumizing to remove the crystal water in the lithium acetate.

[0011] In some embodiments, after the first temperature rising process, the second temperature rising process, and / or the third temperature rising process, the lithium supplementing method further comprises: vacuumizing and back-filling a protective gas to remove the byproduct gas.

[0012] In some embodiments, after the third temperature rising process, the lithium supplementing method further comprises: first lowering the temperature to a preset temperature at a first temperature lowering rate, and then lowering the temperature to room temperature along with the furnace; and / or, the third temperature rising process comprises: vacuumizing to reduce the partial pressure of the byproduct gas.

[0013] In some embodiments, the catalyst comprises one or more of cobalt oxide Co3O4, iron trioxide Fe2O3, and molybdenum oxide / cobalt MoO2 / Co composite catalyst.

[0014] In some embodiments, the catalyst is added in an amount of 0.5-1.5wt% of the total mass of the lithium salt.

[0015] In some embodiments, the reducing atmosphere comprises a reducing gas and an inert carrier gas, the reducing gas comprises hydrogen, and the reducing gas accounts for 3-8% of the reducing atmosphere.

[0016] In some embodiments, the conductive agent comprises one or more of carbon nanotubes (CNTs), vapor-grown carbon fibers (VGCF), conductive carbon black, and acetylene black.

[0017] In some embodiments, the ratio of the total mass of the conductive agent and the lithium salt is (0.58-1.21): 1.

[0018] In some embodiments, the mass ratio of the first lithium salt and the second lithium salt is 1:(1-3).

[0019] In some embodiments, the lithium ion concentration in the lithium salt mixed solution is 0.4-0.8 mol / L.

[0020] In some embodiments, the solvent in the lithium salt mixed solution is an aqueous solution of methanol, ethanol, propanol, or acetone, and the volume ratio of methanol, ethanol, propanol, or acetone to water is 1:(1-1.3).

[0021] In some embodiments, the conductive agent and the catalyst are added to the lithium salt mixed solution, and carbon dioxide is introduced to obtain a first mixture, comprising: adding the conductive agent to a reaction kettle, and then adding the catalyst and the lithium salt mixed solution to the reaction kettle to obtain a reaction system, the volume of the reaction system accounts for 5-10% of the volume of the reaction kettle, the reaction kettle is pre-cooled to 5-10°C, and carbon dioxide is introduced to a pressure of 5-7 MPa to make the carbon dioxide in a liquid state.

[0022] In some embodiments, the temperature and pressure increasing process comprises: increasing the temperature to 30-50°C and increasing the pressure to 10-15 MPa.

[0023] In some embodiments, the depressurization rate when depressurizing to normal pressure is not greater than 1 MPa / min.

[0024] In a second aspect, a modified conductive agent is provided, which is prepared by the supercritical carbon dioxide-based lithium supplementing method as described in any of the above.

[0025] In a third aspect, a pole piece is provided, which comprises the modified conductive agent as described above.

[0026] In some embodiments, it further comprises a binder, a solid-state electrolyte, and an active material, wherein the mass ratio of the modified conductive agent, the binder, the solid-state electrolyte, and the active material is 1-5:3-5:0-40:50-96.

[0027] In some embodiments, the binder comprises one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyisobutylene (PIB), and styrene-ethylene-butylene-styrene block copolymer (SEBS). And / or, the solid-state electrolyte comprises one or more of a sulfide electrolyte, an oxide electrolyte, a halide electrolyte, and a polymer electrolyte. And / or, the active material is a positive electrode active material or a negative electrode active material, the positive electrode active material comprises one or more of a ternary positive electrode, lithium cobaltate, lithium iron phosphate, lithium manganate, lithium iron manganese phosphate, and a lithium-rich manganese-based material; and the negative electrode active material comprises one or more of a graphite-based negative electrode and a silicon-based negative electrode.

[0028] In some embodiments, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0029] In a fourth aspect, a battery is provided, comprising the electrode sheet of any of the above.

[0030] The technical solutions provided by the present application have the following beneficial effects: The present application uses supercritical carbon dioxide as a solvent. When stirring, the low viscosity, high diffusivity, and no solvent residue characteristics of supercritical CO2 are used to achieve uniform dispersion of lithium salt inside the conductive agent. Meanwhile, the gradient pressure reduction process induces directional crystallization of lithium salt, reducing surface agglomeration.

[0031] The present application uses first and second lithium salts with different decomposition temperatures as lithium supplement sources. In a step-by-step heating heat treatment process, the decomposition process is carried out in stages, and the melting filling synergistic effect is combined to achieve uniform deposition of lithium supplement components. Then, under the action of a reducing atmosphere and a catalyst, lithium salt generates metallic lithium and byproduct gas, avoiding the generation of inert substances, and chemically adsorbing to defect sites such as carboxyl and hydroxyl groups on the surface of the conductive agent to form lithium-carbon compounds, achieving the lithium supplement function of the modified conductive agent.

[0032] The present application uses a step-by-step heat treatment process to achieve stage-by-stage decomposition, avoid local overheating, and improve the structural integrity of the conductive agent.

[0033] Compared with the current positive and negative electrodes, the present application has high adaptability and does not need to design lithium supplement agents for positive and negative electrodes. The lithium supplement modified conductive agent can be directly applied to positive or negative electrodes. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced. Obviously, the drawings in the following description only constitute some of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative effort based on these drawings.

[0035] Figure 1 The 0.1C charge-discharge schematic diagram provided by the present application. DETAILED DESCRIPTION

[0036] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort belong to the scope of protection of the present application.

[0037] The present application provides a lithium supplement method based on supercritical carbon dioxide, which comprises the following steps: S1: adding a conductive agent and a catalyst into a lithium salt mixed solution, and introducing carbon dioxide to obtain a first mixture; the lithium salt mixed solution comprises a first lithium salt and a second lithium salt, and the decomposition temperature of the first lithium salt is lower than that of the second lithium salt.

[0038] In step S1, the lithium salt mixed solution is prepared first, and then the conductive agent is added after stirring to form a homogeneous system.

[0039] The first lithium salt can be lithium oxalate, and the second lithium salt can be lithium acetate.

[0040] S2: performing temperature and pressure increasing treatment on the first mixture to make the carbon dioxide reach a supercritical state, and then stirring and reducing the pressure to normal pressure to obtain a second mixture.

[0041] In step S2, the carbon dioxide is made to reach a supercritical state, and the lithium salt mixed solution is dispersed in the supercritical carbon dioxide. The lithium salt is dispersed more uniformly by virtue of the strong dispersing property of the supercritical carbon dioxide. In the present application, the supercritical carbon dioxide is used as a solvent. When stirring, the low viscosity, high diffusivity and no solvent residue characteristics of the supercritical CO2 are used to realize the uniform dispersion of the lithium salt in the conductive agent. Meanwhile, the gradient pressure reduction process induces the directional crystallization of the lithium salt, and reduces the surface agglomeration.

[0042] S3: performing first temperature increasing treatment on the second mixture under a reducing atmosphere to decompose the first lithium salt into a lithium-containing intermediate product and a byproduct gas.

[0043] In the present application, when lithium oxalate is used as the first lithium salt, the lithium-containing intermediate product obtained by decomposition is lithium carbonate. It can be understood that, in the first temperature rising treatment, the reducing atmosphere does not directly participate in the reaction, and the main role is to establish a reducing atmosphere, protect the functional groups such as carboxyl and hydroxyl on the surface of the conductive agent, and reserve the active sites for subsequent reactions. Under the action of the catalyst, lithium oxalate decomposes at the corresponding decomposition temperature to generate the lithium-containing intermediate product.

[0044] S4: After the first temperature rising treatment, a second temperature rising treatment is performed under a reducing atmosphere, so that the second lithium salt is decomposed into metallic lithium and byproduct gas.

[0045] In step S4, the molten second lithium salt can fill the pores generated by decomposition, promote the uniformity of internal lithium modification of the conductive agent, and avoid local aggregation.

[0046] In step S4, the reducing atmosphere participates in the reduction reaction, and Li + is reduced to metallic Li.

[0047] S5: After the second temperature rising treatment, a third temperature rising treatment is performed under a reducing atmosphere, so that the lithium-containing intermediate product is decomposed and reduced to metallic lithium and byproduct gas, to obtain the lithiated conductive agent.

[0048] In step S5, the reducing atmosphere does not directly participate in the reaction, and the main role is to establish a reducing atmosphere, protect the functional groups such as carboxyl and hydroxyl on the surface of the conductive agent, and reserve the active sites for subsequent reactions. Under the action of the catalyst, the lithium-containing intermediate product decomposes at the corresponding decomposition temperature to generate metallic lithium.

[0049] The present application generates metallic lithium and byproduct gas by steps S3-S5, avoids the generation of inert substances, and the byproduct gas can be subsequently discharged.

[0050] The present application uses first and second lithium salts with different decomposition temperatures as lithium supplement sources, and allows the decomposition process to be performed in stages in the step-by-step heating heat treatment process, while combining the synergistic effect of melting and filling, to realize uniform deposition of the lithium supplement component. Then, under the action of the reducing atmosphere and the catalyst, the lithium salt generates metallic lithium and byproduct gas, avoids the generation of inert substances, chemically adsorbs on the defect sites such as carboxyl and hydroxyl on the surface of the conductive agent, forms a lithium-carbon compound, and realizes the lithium supplement function of the modified conductive agent.

[0051] The present application uses three temperature rising treatments, and uses the step-by-step heat treatment process to realize stage-by-stage decomposition, avoid local overheating, and improve the structural integrity of the conductive agent.

[0052] Compared with the current positive and negative, the lithium supplement agent and the matching lithium supplement method need to be designed respectively, the adaptability of the application is high, the lithium supplement agent does not need to be designed respectively for the positive and negative, and the lithium supplement modified conductive agent can be directly applied to the positive or negative.

[0053] In order to realize the step-by-step process of lithium salt decomposition, and at the same time, combine the synergistic effect of melting filling, realize the uniform deposition of lithium supplement component, taking lithium oxalate and lithium acetate as an example, the first temperature rising treatment includes: according to the temperature rising rate of 1-5 ℃ / min, the temperature is raised to 250-280 ℃, and the temperature is kept for 1-2 h; The second temperature rising treatment includes: according to the temperature rising rate of 1-5 ℃ / min, the temperature is raised to 340-360 ℃, and the temperature is kept for 0.5-1 h; The third temperature rising treatment includes: according to the temperature rising rate of 1-5 ℃ / min, the temperature is raised to 400-450 ℃, and the temperature is kept for 0.5-1 h.

[0054] The temperature rising rate of 1-5 ℃ / min can ensure that the material is heated uniformly and the decomposition reaction in each stage is stable; correspondingly, the holding temperature and holding time of each stage are determined, so that the best decomposition process in each stage under the catalyst and reducing gas atmosphere, and ensure that each decomposition reaction is complete.

[0055] In order to avoid the influence of by-product gas, further, negative pressure exhaust can be used, specifically, after the first temperature rising treatment, the second temperature rising treatment and the third temperature rising treatment are finished, the lithium supplement method further includes: after the holding is finished, vacuumizing and backfilling protective gas to exhaust by-product gas. For example, as an example, after the holding is finished, vacuumizing to 10 -4 ~10 -2 Pa, and backfilling protective gas such as argon to exhaust by-product gas, after three cycles, reducing atmosphere is introduced again to normal pressure.

[0056] Further, before the temperature is raised to 250-280 ℃, the first temperature rising treatment further includes: first washing away the residual moisture in the second mixture, then drying treatment at 80-100 ℃, then according to the temperature rising rate of 1-5 ℃ / min, the temperature is raised to 140-200 ℃, and the temperature is kept for 0.5-1 h, finally vacuumizing to exclude the crystal water in lithium acetate.

[0057] The purpose of excluding residual moisture and crystal water is to prevent the reaction between produced metal lithium and water in the subsequent decomposition process.

[0058] Further, after the third temperature rising treatment, the lithium supplement method further comprises: first cooling to a preset temperature at a first cooling rate, and then cooling to room temperature with the furnace. For example, as an example, cooling to 150℃ at a cooling rate of 1-5℃ / min, and then cooling to room temperature with the furnace. In the two-stage cooling process, the uniform cooling in the early stage can avoid the damage to the material caused by rapid temperature change.

[0059] Further, the third temperature rising treatment further comprises: vacuumizing to reduce the partial pressure of the byproduct gas and promote the decomposition process.

[0060] The catalyst comprises one or more of cobalt oxide Co3O4, ferric oxide Fe2O3, and molybdenum oxide / cobalt MoO2 / Co composite catalyst. The catalyst is added in an amount of 0.5-1.5wt% of the total mass of the lithium salt.

[0061] In the above steps S3-S5, the reducing atmosphere comprises a reducing gas and an inert carrier gas, the reducing gas comprises hydrogen, and the proportion of the reducing gas in the reducing atmosphere is 3-8%. If the proportion is too low, it will not work, and if the proportion is too high, there will be too many side reactions.

[0062] The intermediate product of the decomposition of lithium oxalate and lithium acetate is lithium carbonate, which can be reduced by hydrogen to obtain metallic lithium and byproduct gas, including carbon dioxide and water vapor.

[0063] The conductive agent comprises one or more of carbon nanotubes CNTs, vapor-deposited carbon fibers VGCF, conductive carbon black, and acetylene black. The proportion of the conductive agent to the total mass of the lithium salt is (0.58-1.21):1.

[0064] In the above step S1, in the lithium salt mixed solution, the mass ratio of the first lithium salt to the second lithium salt is 1:(1-3).

[0065] In the lithium salt mixed solution, the solvent is an aqueous solution of methanol, ethanol, propanol, or acetone, and the volume ratio of methanol, ethanol, propanol, or acetone to water is 1:(1-1.3). Two kinds of solvents are used because the solubility of the two kinds of lithium salts in methanol, ethanol, propanol, or acetone is not good, and the solubility of pure water in supercritical carbon dioxide is not good.

[0066] Taking lithium oxalate and lithium acetate as an example, lithium acetate and lithium oxalate are dissolved in an ethanol / water mixed solvent in a mass ratio of 1:3, the volume ratio of ethanol to water is 1:(1-1.3), and the lithium ion concentration is controlled at 0.4-0.8mol / L. Ultrasonic assistance (20-40kHz) and mechanical stirring (200-500rpm) are turned on for 30-60 minutes to ensure that the lithium salt is completely dissolved and a homogeneous system is formed.

[0067] In the step S1, the conductive agent and the catalyst are added into the lithium salt mixed solution, and carbon dioxide is introduced to obtain the first mixture, including: adding the conductive agent into a reaction kettle, and then adding the catalyst and the lithium salt mixed solution into the reaction kettle to obtain a reaction system, the volume of the reaction system is 5-10% of the volume of the reaction kettle, the reaction kettle is pre-cooled to 5-110 DEG C, and carbon dioxide is introduced to 5-7 MPa to make the carbon dioxide in liquid state.

[0068] In the step S2, the temperature and pressure increasing treatment includes: increasing the temperature to 30-50 DEG C, and increasing the pressure to 10-15 MPa to make the carbon dioxide in supercritical state between liquid and gas.

[0069] In the step S2, the pressure decreasing rate is not more than 1 MPa / min when the pressure is decreased to normal pressure, which can make the lithium salt uniformly dispersed.

[0070] The application further provides a modified conductive agent prepared by the above-mentioned lithium supplementing method based on supercritical carbon dioxide.

[0071] The application further provides a pole piece comprising the modified conductive agent.

[0072] The pole piece further comprises a binder, a solid electrolyte and an active material, wherein the mass ratio of the modified conductive agent, the binder, the solid electrolyte and the active material is 1-5: 3-5: 0-40: 50-96.

[0073] The binder comprises one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyisobutylene (PIB) and styrene-ethylene-butylene-styrene block copolymer (SEBS). The solid electrolyte comprises one or more of sulfide electrolyte, oxide electrolyte, halide electrolyte and polymer electrolyte. The active material is a positive active material or a negative active material, the positive active material comprises one or more of ternary positive electrode, lithium cobaltate, lithium iron phosphate, lithium manganate, lithium iron manganese phosphate and lithium-rich manganese-based material, and the negative active material comprises one or more of graphite-based negative electrode and silicon-based negative electrode.

[0074] The pole piece is a positive pole piece or a negative pole piece.

[0075] The application further provides a battery comprising the pole piece.

[0076] At least one of the positive pole piece and the negative pole piece of the battery contains the modified conductive agent.

[0077] The present application is described in detail below through some embodiments.

[0078] Example 1 Step 1: Dissolve 0.7 g of lithium acetate and 0.3 g of lithium oxalate in an ethanol / water mixed solvent (volume ratio 1:1), control the lithium ion concentration at 0.5 mol / L, then add 0.01 g of Co3O4 catalyst.

[0079] Step 2: Turn on ultrasonic assistance (30 kHz) and mechanical stirring (400 rpm) for 40 minutes to ensure that the lithium salt is completely dissolved and a homogeneous system is formed.

[0080] Step 3: Place 0.85 g of carbon nanotubes CNTs to be modified by lithium supplementation in the reaction kettle, and place the homogeneous solution in Step 2 into the reaction kettle, while controlling the volume to be 8% of the reaction kettle, then pre-cool the reaction kettle to 10℃, and introduce CO2 to a pressure of 7 MPa, at which point the carbon dioxide is in a liquid state, obtaining a first mixture.

[0081] Step 4: Increase the temperature of the reaction kettle to 40℃ and pressurize to 12 MPa to make the system reach a supercritical state, and mechanically stir (3000 rpm) for 40 minutes.

[0082] Step 5: Slowly depressurize to atmospheric pressure at a rate of 0.5 MPa / min to allow the lithium salt to deposit directionally, obtaining a modified precursor material (i.e., a second mixture).

[0083] Step 6: Wash and filter the second mixture obtained in Step 5 with ethanol to remove residual moisture, and dry at 100℃, then place it in a high-temperature reaction kettle equipped with H2 / Ar gas circuit and vacuum system, first heat to about 160℃ at a rate of 2℃ / min under the protection of inert gas such as argon, and keep the temperature for 1 hour, then vacuum exhaust the waste gas.

[0084] Then heat to 265℃ at a rate of 2℃ / min and keep the temperature for 1 hour (H2 / Ar mixed gas, H2 ratio 5%, flow rate 50 mL / min), under the action of the catalyst, lithium oxalate decomposes at this temperature to generate loose porous Li2CO3 and CO gas, vacuum to 10 -3 Pa and backfill with a protective gas such as inert gas Ar to remove the byproduct gas; after three cycles of gas circulation, introduce H2 / Ar mixed gas again and maintain atmospheric pressure.

[0085] Step 7: Increase to 350℃ at a rate of 5℃ / min and keep the temperature for 0.5h to make lithium acetate melt and decompose into metallic lithium and byproduct gas, then vacuum to 10 -3 Pa and backfill with a protective gas to remove the byproduct gas, after the exhaust treatment is completed, introduce inert protective gas until the pressure is 0.8 MPa and keep the pressure for 1 hour.

[0086] In this process, lithium oxalate is completely decomposed, and lithium acetate is melted during heating (melting point ≥ 286°C) and fills the loose lithium carbonate pores generated in step 6 and the surface gaps of the conductive agent, avoiding local aggregation and further promoting the uniformity of lithium supplementation. During the holding process, the Li in the molten lithium acetate is reduced to liquid lithium under the action of the catalyst and the strong reducing hydrogen gas. The holding process is completed, and the tail gas is treated in the same way as in step 6. + The liquid lithium is generated at the reduction site. After the tail gas treatment is completed, inert protective gas is introduced until the pressure is ≥ 0.5 MPa, promoting the uniform distribution of the liquid lithium and chemical adsorption on the surface of the conductive agent to form a lithium-carbon composite structure and achieve pre-lithiation of the conductive agent.

[0087] After the tail gas treatment is completed, inert protective gas is introduced until the pressure is ≥ 0.5 MPa, promoting the uniform distribution of the liquid lithium and chemical adsorption on the surface of the conductive agent to form a lithium-carbon composite structure and achieve pre-lithiation of the conductive agent.

[0088] Step 8: increase to 400°C at 5°C / min, then vacuum to 10 -3 Pa, hold for 1 h, and vacuum to reduce the partial pressure of CO / CO2 gas generated after the reaction of lithium carbonate (Li2CO3), promoting the decomposition process of lithium carbonate (Li2CO3) to occur. The generated Li2O then undergoes a carbothermic reduction reaction on the surface of the conductive agent to generate metallic lithium, achieving pre-lithiation of the conductive agent. Then vacuum to 10 -3 Pa and backfill with protective gas, then vacuum-gas circulation for 3 times to ensure complete removal of byproducts, then decrease to 150°C at 5°C / min and decrease with the furnace, then decrease with the furnace again to obtain lithiumated carbon nanotubes.

[0089] Step 9: prepare the electrode sheet according to the mass ratio of lithiumated carbon nanotubes: PTFE: oxide solid electrolyte: silicon-carbon negative electrode = 3:3:14:80, and assemble it into a battery.

[0090] Example 2 The difference between this example and Example 1 is that in step 1, the lithium acetate is 0.5 g and the lithium oxalate is 0.5 g, and in step 3, the carbon nanotubes are replaced with conductive graphite with a weight of 0.70 g, and the rest is the same as Example 1.

[0091] Example 3 The difference between this example and Example 1 is that in step 3, the carbon nanotubes are replaced with conductive graphite, and in step 5, the pressure is reduced to atmospheric pressure at a rate of 1 MPa / min, and the rest is the same as Example 1.

[0092] Example 4 The difference between this example and Example 1 is that in step 3, the carbon nanotubes are replaced with VGCF, and in step 8, the holding time is shortened to 0.3 h, and the rest is the same as Example 1.

[0093] Example 5 The difference between this embodiment and embodiment 1 is only that 1.21 g of carbon nanotubes to be modified by lithium supplement is added in step 3, and the rest is the same as embodiment 1.

[0094] Embodiment 6 The difference between this embodiment and embodiment 1 is only that the decomposition temperature of lithium oxalate is adjusted from 265°C to 250°C in step 6.

[0095] Embodiment 7 The difference between this embodiment and embodiment 1 is only that the decomposition temperature of lithium oxalate is adjusted from 265°C to 280°C in step 6.

[0096] Embodiment 8 The difference between this embodiment and embodiment 1 is only that the catalyst is replaced by iron trioxide Fe2O3 in step 1, and the content is adjusted to 0.5wt%, and the rest is the same as embodiment 1.

[0097] Embodiment 9 The difference between this embodiment and embodiment 1 is only that the catalyst is replaced by molybdenum oxide / cobalt MoO2 / Co composite catalyst in step 1, and the content is adjusted to 1.5wt%, and the rest is the same as embodiment 1.

[0098] Embodiment 10 The difference between this embodiment and embodiment 1 is only that the lithium ion concentration is adjusted to 0.2mol / L by adjusting the solvent content in step 1, and the rest is the same as embodiment 1.

[0099] Embodiment 11 The difference between this embodiment and embodiment 1 is only that the pressure reduction process is reduced to atmospheric pressure at 5MPa / min in step 5, and the rest is the same as embodiment 1.

[0100] Embodiment 12 The difference between this embodiment and embodiment 1 is only that CO2 is not introduced in step 3, i.e. without supercritical CO2, and the rest is the same as embodiment 1.

[0101] Embodiment 13 The difference between this embodiment and embodiment 1 is that the amount of lithium acetate is replaced by the amount of lithium oxalate, ensuring consistent lithium ion concentration, and the rest is the same as embodiment 1.

[0102] Embodiment 14 The difference between this embodiment and embodiment 1 is that in step 9, the same kind of conductive agent without pre-lithiation is used during electrode preparation, and the rest is the same as embodiment 1.

[0103] Embodiment 15 The difference between this embodiment and embodiment 1 is that no catalyst is added in step 1, and the rest is the same as embodiment 1.

[0104] Example 16 The difference between this example and Example 1 is that the amount of lithium oxalate is replaced by the amount of lithium acetate, ensuring the consistency of lithium ion concentration, and the rest is the same as Example 1.

[0105] The mold cell batteries of each example were assembled, in which the lithium-indium alloy was used as the cathode. According to the gram capacity of the main material used and the gram capacity of the main material of the electrode sheet, the theoretical battery capacity was calculated, the charge and discharge were carried out at 0.1C current, and the initial efficiency of the battery was obtained. The battery was tested for the initial efficiency of the cell, and the test results are shown in Table 1 and Figure 1 .

[0106] Table 1

[0107] From the test data in Table 1, the initial coulombic efficiency of Examples 1-11 is higher than that of Example 12, which shows that using supercritical carbon dioxide as a solvent and using lithium sources with different decomposition temperatures for three times of temperature rising treatment can improve the initial coulombic efficiency.

[0108] Comparing Comparative Example 1 and Example 10, it can be seen that the lithium salt concentration in Step 1 is reduced, the pre-lithiation degree of the conductive agent is reduced, and the initial efficiency is relatively reduced.

[0109] Comparing Comparative Example 1 and Example 11, it can be seen that the depressurization rate is too fast, the lithium salt is not uniformly deposited, which affects the subsequent lithiation process, and the initial efficiency is relatively reduced.

[0110] Comparing Comparative Example 1 and Example 12, it can be seen that no CO2 is introduced, i.e. without the dispersion effect of supercritical carbon dioxide, the uniformity of lithium salt distribution is reduced, the pre-lithiation of the conductive agent is affected, and the final cell initial efficiency is reduced.

[0111] Comparing Comparative Example 1 and Examples 13 and 16, it can be seen that a single lithium salt composition cannot achieve the uniformity of the lithiumation modification of the conductive agent, thereby affecting the initial efficiency of the cell. For example, in Example 13, there is no lithium acetate component, i.e. there is no molten filling stage and the molten lithium ion directly reduces the distribution pre-lithiation process, which affects the uniformity and degree of pre-lithiation of the conductive agent, resulting in a reduction in the final cell initial efficiency.

[0112] Comparing Comparative Example 1 and Example 14, it can be seen that the pre-lithiated conductive agent can effectively improve the initial efficiency of the cell.

[0113] Comparing Comparative Example 1 and Example 15, it can be seen that without adding a catalyst, the decomposition reaction of lithium oxalate and lithium acetate is difficult to proceed, the conductive agent is not pre-lithiated, and the existing lithium salt and other impurities affect the final initial efficiency of the cell.

[0114] In the description of the present application, it should be noted that the terms "upper", "lower", and the like are used for indicating the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. Unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connecting" should be interpreted broadly, for example, can be fixed connection, can also be detachable connection, or integrally connected; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0115] It should be noted that in the present application, relational terms such as "first" and "second", and the like are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus including a series of elements includes not only those elements, but also other elements not explicitly listed, or other elements inherent in such a process, method, article or apparatus. Without more limitations, the element defined by the statement "comprising a" does not exclude the presence of other identical elements in the process, method, article or apparatus including the element.

[0116] The above is only a specific embodiment of the present application, which enables those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features applied herein.

Claims

1. A lithium supplementation method based on supercritical carbon dioxide, characterized in that: It includes: adding a conductive agent and a catalyst to a lithium salt mixed solution, and introducing carbon dioxide to obtain a first mixture; The lithium salt mixed solution includes a first lithium salt and a second lithium salt, and the decomposition temperature of the first lithium salt is lower than the decomposition temperature of the second lithium salt; The first mixture is subjected to a temperature and pressure treatment to make the carbon dioxide reach a supercritical state, and the pressure is reduced to normal pressure after stirring to obtain a second mixture; Under a reducing atmosphere, subjecting the second mixture to a first temperature increase treatment to decompose the first lithium salt into a lithium-containing intermediate product and a by-product gas; performing a second temperature increase treatment to melt the second lithium salt and reduce it to metallic lithium and byproduct gas; The third temperature-raising treatment is performed to decompose and reduce the lithium-containing intermediate product into metallic lithium and by-product gas, thereby obtaining a lithiated conductive agent.

2. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The first lithium salt is lithium oxalate, the second lithium salt is lithium acetate, and the lithium-containing intermediate product is lithium carbonate.

3. The lithium supplementation method based on supercritical carbon dioxide according to claim 2, characterized in that: The first heating treatment comprises: heating to 250-280°C at a heating rate of 1-5°C / min and keeping the temperature for 1-2 hours; The second heating treatment includes: heating to 340-360°C at a heating rate of 1-5°C / min and keeping the temperature for 0.5-1h; The third temperature rising treatment includes: heating to 400-450° C. at a heating rate of 1-5° C. / min, and keeping the temperature for 0.5-1 hour.

4. The lithium supplementation method based on supercritical carbon dioxide according to claim 3, characterized in that: Before heating to 250-280°C, the first heating treatment further includes: first washing away residual water in the second mixture, then drying at 80-100°C, then heating to 140-200°C at a heating rate of 1-5°C / min, keeping warm for 0.5-1h, and finally vacuuming to remove crystallization water in lithium acetate.

5. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: After the first temperature rising treatment, the second temperature rising treatment and / or the third temperature rising treatment are completed, the lithium replenishment method further includes: vacuuming and backfilling with protective gas to discharge by-product gas.

6. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: After the third temperature rising process, the lithium replenishing method further comprises: firstly cooling the temperature to a preset temperature at a first cooling rate, and then cooling the temperature to room temperature along with the furnace; And / or, the third temperature raising treatment includes: vacuuming to reduce the partial pressure of by-product gas.

7. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The catalyst includes one or more of cobalt oxide Co3O4, ferric oxide Fe2O3 and molybdenum oxide / cobalt MoO2 / Co composite catalyst.

8. The lithium supplementation method based on supercritical carbon dioxide according to claim 7, characterized in that: The amount of the catalyst added is 0.5-1.5 wt% of the total mass of the lithium salt.

9. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The reducing atmosphere includes a reducing gas and an inert carrier gas, the reducing gas includes hydrogen, and the reducing gas accounts for 3 to 8% of the reducing atmosphere.

10. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The conductive agent includes one or more of carbon nanotubes (CNTs), vapor-deposited carbon fibers (VGCF), conductive carbon black, and acetylene black.

11. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The ratio of the total mass of the conductive agent to the total mass of the lithium salt is (0.58-1.21):

1.

12. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The mass ratio of the first lithium salt to the second lithium salt is 1:(1-3).

13. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: In the lithium salt mixed solution, the lithium ion concentration is 0.4-0.8 mol / L.

14. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: In the lithium salt mixed solution, the solvent is an aqueous solution of methanol, ethanol, propanol or acetone, and the volume ratio of methanol, ethanol, propanol or acetone to water is 1: (1 to 1.3).

15. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: Adding a conductive agent and a catalyst to a lithium salt mixed solution and introducing carbon dioxide to obtain a first mixture includes: adding the conductive agent to a reactor, then adding the catalyst and the lithium salt mixed solution to the reactor to obtain a reaction system, wherein the volume of the reaction system is 5-10% of the volume of the reactor, precooling the reactor to 5-10°C, and introducing carbon dioxide to a pressure of 5-7 MPa to make the carbon dioxide in a liquid state.

16. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The temperature and pressure increasing treatment includes: increasing the temperature to 30-50° C. and increasing the pressure to 10-15 MPa.

17. The lithium supplementation method based on supercritical carbon dioxide according to claim 1, characterized in that: The pressure reduction rate when reducing the pressure to normal pressure shall not exceed 1MPa / min.

18. A modified conductive agent, characterized in that: It is prepared by the lithium supplementation method based on supercritical carbon dioxide as described in any one of claims 1 to 17.

19. A pole piece, characterized in that: It comprises the modified conductive agent as claimed in claim 18.

20. The pole piece according to claim 19, characterized in that: The invention also includes a binder, a solid electrolyte and an active material, wherein the mass ratio of the modified conductive agent: the binder: the solid electrolyte: the active material is 1-5: 3-5: 0-40: 50-96.

21. The pole piece according to claim 20, characterized in that: The binder includes one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyisobutylene (PIB) and styrene-ethylene-butylene-styrene block copolymer (SEBS); And / or, the solid electrolyte includes one or more of a sulfide electrolyte, an oxide electrolyte, a halide electrolyte and a polymer electrolyte; And / or, the active material is a positive electrode active material or a negative electrode active material, the positive electrode active material includes one or more of a ternary positive electrode, lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide, lithium manganese iron phosphate and a lithium-rich manganese-based material; the negative electrode active material includes one or more of a graphite negative electrode and a silicon-based negative electrode.

22. The pole piece according to claim 19, wherein: The electrode is a positive electrode or a negative electrode.

23. A battery, characterized in that: It comprises the pole piece according to any one of claims 19 to 22.

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