Graphene composite conductive agent, sodium ion battery and preparation method

Abstract

The application discloses a kind of graphene composite conductive agent, sodium ion battery and preparation method, it is related to sodium ion battery positive electrode material technical field, to solve the problem that conductive agent in sodium ion battery needs to be improved;The application includes that graphene raw material is homogenized and emulsified ultrasonically to obtain uniform graphene dispersion liquid;Centrifugal is obtained different dispersivity graphene solution, take centrifugal upper layer solution as A, centrifugal lower layer slurry as B;Alkaline substance is added to B, and after being dispersed sufficiently, it is dried with A respectively, after drying, heat annealing treatment is carried out respectively;The powder obtained after heat annealing of A, B is respectively wet pre-mixing homogenization treatment in organic solvent to obtain pre-dispersion liquid C and pre-dispersion liquid D, the mass concentration of pre-dispersion liquid C is lower than pre-dispersion liquid D;Pre-dispersion liquid C and pre-dispersion liquid D are mixed according to certain proportion to obtain graphene composite conductive agent;The process of the application is simple, environmental protection is easy to operate, and the sodium ion battery prepared using the conductive agent has excellent rate and cycle performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium-ion battery positive electrode materials, in particular to a graphene composite conductive agent, a sodium-ion battery and a preparation method. BACKGROUND

[0002] Sodium-ion batteries are a type of secondary battery technology similar to lithium-ion batteries, but use sodium ions as the intercalation / deintercalation ions in the battery. Compared with lithium-ion batteries, sodium-ion batteries are relatively new in research, but have received extensive attention and research in recent years.

[0003] Compared with lithium-ion batteries, sodium-ion batteries have the following advantages: compared with lithium, sodium resources are more abundant and have a wider geographical distribution. Therefore, by using sodium-ion batteries, a more sustainable battery material and energy supply chain can be achieved. Compared with lithium-ion batteries, the material cost of sodium-ion batteries is much lower, which makes sodium-ion batteries have potential in large-scale applications. Sodium ions have a larger ionic radius and lower electrochemical activity, which makes sodium-ion batteries have better safety and stability to a certain extent. At the same time, with the continuous growth of global energy demand and the emphasis on renewable energy, sodium-ion batteries are considered to be a more cost-effective and scalable energy storage technology, which can be used in the fields of grid energy storage and renewable energy storage.

[0004] The positive electrode materials of sodium-ion batteries mainly include three categories: layered metal oxides, polyanion compounds, and Prussian blue analogues. Despite the potential advantages of sodium-ion batteries, there are still some challenges in practical applications. For example, the relatively large size of sodium ions can cause volume changes in battery materials and permeability problems in electrolytes, affecting the cycle life and energy density of the battery. Patent CN115036510B discloses a graphene / carbon black composite conductive agent and its preparation method and application. Graphite material is used as raw material, and intercalated graphite in the form of flocculation is obtained by electrolytic reaction. The intercalated graphite and carbon black are pre-mixed and then subjected to homogenization treatment in a liquid solvent to obtain an additive-free graphene / carbon black composite conductive agent. The obtained conductive material maintains good electrochemical performance of the battery with reduced additive amount. However, the intercalated graphene in this method has certain defectiveness, and the quality of the graphene powder after physical mixing is uneven, with the phenomenon of insufficient peeling. The conductivity of the overall composite conductive agent needs to be improved. Patent CN111710863B discloses a preparation method of a graphene conductive agent and its application in lithium-ion battery negative electrodes. The invention provides a graphene conductive agent, which includes graphene material, dispersant, other additives, and water. The graphene is peeled off and dispersed during the mixing and stirring process to obtain 1-10 layers of graphene and 10-100 layers of graphene microsheets. The rigid and flexible graphene bridges each other to effectively improve the specific capacity, rate performance, and low-temperature performance of the battery. However, the use of dispersants and additives in the preparation process reduces the content of active substances. At the same time, the proportion of few-layer and multi-layer graphene peeled off by this method cannot be controlled, which has limited improvement on the performance of the battery. SUMMARY

[0005] The purpose of the present application is to provide a graphene composite conductive agent, a sodium-ion battery, and a preparation method to solve the problem that the conductive agent in the sodium-ion battery needs to be improved.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solution: a preparation method of a graphene composite conductive agent, comprising the following specific steps:

[0007] S1. Homogenously emulsifying and ultrasonically treating the graphene raw material to obtain a uniform graphene dispersion liquid;

[0008] S2. Centrifuging to obtain graphene solutions with different dispersities, taking the upper layer solution as A and the lower layer slurry as B, the number of graphene layers in A is less than that in B;

[0009] S3. Adding an alkaline substance to B and dispersing it thoroughly, then drying A and B respectively, and then performing heat annealing treatment; wetly pre-mixing and homogenizing the powders obtained after heat annealing of A and B in an organic solvent to obtain pre-dispersion liquid C and pre-dispersion liquid D, the mass concentration of pre-dispersion liquid C is lower than that of pre-dispersion liquid D;

[0010] S4. Mixing pre-dispersion liquid C and pre-dispersion liquid D in a certain proportion to obtain graphene composite conductive agent.

[0011] Preferably, in the step S1, the stirring speed of the homogenization emulsification is 500-8000 rpm, the ultrasonic power is 200-3000 W, and the specific surface area of the graphene raw material in the graphene dispersion liquid is 500-3000 m 2 / g.

[0012] Preferably, the graphene raw material is prepared by the following method: taking a flexible graphite material as a raw material, taking an oxygen-containing acid salt as an electrolyte, adding a complexing agent, electrolyzing to obtain graphene powder, filtering, washing, and ultrasonic dispersion to obtain the graphene raw material; wherein the flexible graphite material is one or more of graphite paper, graphite rod, graphite plate, and graphite foil; the oxygen-containing acid salt is one or more of sulfate, persulfate, perchlorate, nitrate, and phosphate; the complexing agent is one or more of citrate, pyrophosphate, thiosulfate, and sulfite; the voltage applied in the electrolysis reaction is 5-30 V, and the electrolysis time is 20-360 min; and the content of the complexing agent is 0-10%.

[0013] Preferably, in the step S2, the centrifugal speed is 500-10000 rpm, the centrifugal time is 30-180 min, the number of graphene layers in A is 1-5 layers, and the number of graphene layers in B is 5-20 layers.

[0014] Preferably, in the step S3, the alkaline substance added to B is selected from NaOH, KOH, NaHCO3, Na2CO3, K2CO3, and KHCO3, and the concentration is 0.1-10 M; the organic solvent is one or more of N-methyl pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide, and N-vinyl pyrrolidone.

[0015] Preferably, in the step S3, the graphene thermal annealing treatment is carried out in a protective atmosphere or a reducing atmosphere, the thermal annealing treatment temperature is 300-1000℃, and the treatment time is 30-240 min, wherein the particle size D50 of the powder obtained after the thermal annealing of B is 1-40 μm.

[0016] Preferably, in the step S3, the wet pre-mixing homogenization treatment is one or more of ball milling, sand milling, homogenization, and ultrasonic, wherein the stirring speed of the homogenization is 500-8000 rpm / min, and the mixing time is 30-120 min; the mass concentration of the pre-dispersion liquid C is 1-50 mg / ml, and the mass concentration of the pre-dispersion liquid D is 1-100 mg / ml.

[0017] Preferably, the volume ratio of the pre-dispersion liquid C and the pre-dispersion liquid D in the graphene composite conductive agent is 3:1-1:20.

[0018] The application provides another technical scheme: a graphene composite conductive agent obtained by the preparation method.

[0019] The application also provides another technical scheme: a sodium ion battery using the conductive agent, and the mass ratio of active material, binder and conductive agent in the positive electrode formula is (8-9.5):(0.1-1):(0.1-1).

[0020] Compared with the prior art, the application has the beneficial effects that:

[0021] 1. The graphene composite conductive agent, the sodium ion battery and the preparation method, the conductive agent comprises few-layer graphene and multi-layer hole-shaped graphene, on the one hand, the point-surface contact of the graphene conductive agent and the electrode material is realized, the defect that the conductive agent is prone to agglomeration is overcome, and the internal resistance of the battery is reduced; on the other hand, the hole structure enriches the electronic conduction network, increases the wettability of the electrolyte, and makes the battery still have high specific capacity and cycle stability under high active material load.

[0022] 2. The graphene composite conductive agent, the sodium ion battery and the preparation method, the process is simple, environmentally friendly and easy to operate, and no strong oxidizing agent such as concentrated sulfuric acid and potassium permanganate is added in the preparation process; at the same time, no surfactant or dispersant is used, and the influence of non-active substances on the performance of the battery is reduced.

[0023] 3. The graphene composite conductive agent, the sodium ion battery and the preparation method, the graphene is prepared by using an electrochemical method, and the size of the graphene raw material is uniform and the number of layers is controllable by adjusting parameters. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The AFM image of graphene in the dispersion liquid A of the embodiment 1 of the application

[0025] Figure 2 The AFM image of graphene in the dispersion liquid B of the embodiment 1 of the application

[0026] Figure 3 The powder particle size distribution diagram of graphene after heat annealing treatment in the dispersion liquid B of the embodiment 1 of the application. DETAILED DESCRIPTION

[0027] A preparation method of a graphene composite conductive agent, comprising the following specific steps:

[0028] S1. Homogenously emulsify the graphene raw material to obtain a uniform graphene dispersion liquid; wherein the stirring speed of the homogenously emulsifying can be further preferably 500-8000 rpm, the ultrasonic power can be further preferably 200-3000 W, and the specific surface area of the graphene raw material in the graphene dispersion liquid is preferably 500-3000 m 2 / g. Increasing the specific surface area can increase the wettability of the electrolyte and the loading capacity of the active material.

[0029] S2. Obtain graphene solutions with different dispersities by centrifugation, take the upper layer solution as A and the lower layer slurry as B, the number of graphene layers in A is less than that in B; the centrifugal speed in this step is preferably controlled at 500-10000 rpm, the centrifugation time is preferably 30-180 min, the number of graphene layers in A is preferably 1-5 layers, and the number of graphene layers in B is preferably 5-20 layers.

[0030] S3. Add an alkaline substance such as NaOH, KOH, NaHCO3, Na2CO3, K2CO3, KHCO3, etc. to B, the concentration is generally 0.1-10 M, preferably 0.1-5 M, after sufficient dispersion, dry A and B separately, the drying method is preferably spray drying, freeze drying, vacuum drying, vacuum freeze drying, etc., after drying, heat annealing treatment is carried out, which is generally carried out in a protective atmosphere or a reducing atmosphere, such as nitrogen, argon, hydrogen, etc., or a mixture gas, the heat annealing treatment temperature is preferably 300-1000℃, and the treatment time is preferably 30-240 min, wherein the powder particle size D50 of B after heat annealing is preferably 1-40 μm, and the powder particle size of A after heat annealing is not limited, but since A powder is few-layer graphene, its particle size is generally smaller than that of B, or at least comparable; the powders obtained after heat annealing of A and B are respectively subjected to wet pre-mixing homogenization treatment in an organic solvent to obtain pre-dispersion liquid C and pre-dispersion liquid D, the mass concentration of pre-dispersion liquid C is lower than that of pre-dispersion liquid D, further preferably, the mass concentration of pre-dispersion liquid C can be 1-50 mg / ml, and the mass concentration of pre-dispersion liquid D can be 1-100 mg / ml; the organic solvent can be selected from one or more of N-methyl pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide, N-vinyl pyrrolidone; the wet pre-mixing homogenization treatment can be one or more of ball milling, sand milling, homogenization, ultrasonic, and the two are preferably homogenized first, the stirring speed is preferably 500-8000 rpm / min, and the mixing time is preferably 30-120 min.

[0031] S4. Mix pre-dispersion liquid C and pre-dispersion liquid D in a certain proportion to obtain a graphene composite conductive agent.

[0032] In a preferred embodiment, the graphene raw material in step S1 is prepared by the following method: using a flexible graphite material as a raw material, using an oxygen-containing acid salt as an electrolyte, adding a complexing agent, electrolyzing to obtain graphene powder, filtering, washing, and ultrasonic dispersion to obtain the graphene raw material; wherein the flexible graphite material is one or more of graphite paper, graphite rod, graphite plate, and graphite foil; the oxygen-containing acid salt is one or more of sulfate, persulfate, perchlorate, nitrate, and phosphate; the complexing agent is one or more of citrate, pyrophosphate, thiosulfate, and sulfite; the voltage applied in the electrolysis reaction is 5-30 V, and the electrolysis time is 20-360 min; the content of the complexing agent is 0-10%; of course, the graphene raw material can also be prepared by other methods or purchased directly from the market, but the electrochemical method can be used to prepare graphene with different oxidation degrees, and by adjusting the parameters, graphene raw material with uniform size and controllable layer number can be obtained, which is also beneficial to the method of the present application.

[0033] In a preferred embodiment, the volume ratio of the pre-dispersion C and the pre-dispersion D in the graphene composite conductive agent is 3:1-1:20.

[0034] In addition, after the conductive agent is prepared by the above method, the following method can be used to prepare a sodium ion battery:

[0035] The mass ratio of the active material, the binder, and the conductive agent in the positive electrode formula is preferably (8-9.5):(0.1-1):(0.1-1), and more preferably 8:1:1.

[0036] The active material can be selected from the following conventional options: layered metal oxides NaNi 0.5 Mn 0.2 Ti 0.3 O2, NaLi 1 / 3 Ti 1 / 6Mn 1 / 2 O2, polyanion compounds NaFePO4, NaMnPO4, Na3V2(PO4)3, Prussian blue analogs Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Ni[Fe(CN)6];

[0037] The binder can be a mixture of the following conventional options: polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polyacrylonitrile (PAN), styrene-butadiene rubber (SBR) emulsion, and carboxymethyl cellulose (CMC).

[0038] The types and amounts of other components of the sodium ion battery are conventional amounts, for example, the negative electrode is hard carbon, and the present application will not be described here.

[0039] The sodium ion battery has a surface loading of (4-20) ± 0.5 mg / cm 2 , the capacity retention rate of 100 cycles under 0.2C current condition is not less than 85%, and the capacity retention rate of 300 cycles under 1C current condition is not less than 80%.

[0040] The present invention will be described in further detail below in conjunction with specific embodiments. In this embodiment, the active material is a Prussian blue analogue Na2Fe[Fe(CN)6], but this should not be understood as limiting the scope of the present invention to the following embodiments.

[0041] Example 1

[0042] (1) Electrochemical preparation of graphene: 5 mm thick flexible graphite paper was used as the cathode and anode, and 1 M ammonium sulfate was used as the electrolyte. Electrolysis was performed at 10 V for 90 min. Graphene powder was obtained after electrolysis. The collected graphene powder was filtered, washed three times with deionized water, and ultrasonically dispersed at 1000 W for 2 h.

[0043] (2) Further, the ultrasonic treatment was continued for 2 h under the homogenization condition of 2000 rpm to obtain a graphene dispersion with good dispersion.

[0044] Centrifuge at 5000 rpm for 1 hour, take the upper layer of the centrifuged solution as A, and the lower layer of the centrifuged slurry as B. After AFM detection, the thickness of graphene in dispersion A is 0.4-1.7 nm, and the number of layers is calculated to be 1-5. Figure 1 As shown in the AFM image of graphene randomly selected from dispersion A, it can be seen that the graphene thickness is around 1.5nm. The graphene thickness in dispersion B is 3 to 6.5nm, and the number of graphene layers is calculated to be 10-19 layers. Figure 2 As shown in the AFM image of graphene randomly selected from dispersion B, the graphene thickness is less than 7 nm. Since electrochemically prepared graphene has certain oxygen-containing functional groups and wrinkles, the actual number of layers is lower than the calculated value.

[0045] (3) Dispersion A was spray-dried to obtain powder A. 1M NaOH solution was added to dispersion B and the mixture was stirred and dispersed at 1500 rpm for 48 hours, followed by cold drying to obtain powder B. Powder A was thermally annealed at 600°C in an Ar atmosphere. Powder B was thermally annealed at 700°C in an Ar atmosphere. Powder A was dispersed in N-methylpyrrolidone solvent and stirred at 1000 rpm for 30 minutes to obtain dispersion C with a graphene concentration of 30 mg / ml. Powder B was dispersed in N-methylpyrrolidone solvent and stirred at 5000 rpm for 60 minutes and sand-milled at a frequency of 30 Hz for 2 hours to obtain dispersion D with a graphene concentration of 80 mg / ml.

[0046] (4) The above dispersion C and dispersion D were used as conductive agents in a ratio of 2:1; and the active material: graphene conductive agent: PVDF were mixed in a mass ratio of 8:1:1; stirred at 3000 rpm in a ball mill for 2 hours to obtain a positive electrode slurry of graphene composite conductive slurry. Then, the positive electrode slurry was coated at a thickness of 300 μm, rolled, and vacuum dried in a vacuum oven at 80°C for 12 hours to obtain an electrode sheet, and assembled into a button cell for testing. Among them, the active material surface loading was 20.3 mg / cm 2 .

[0047] Comparative Example 1

[0048] Commercially available Super P was used as a conductive agent and mixed in a mass ratio of 8:1:1 of active material: graphene conductive agent: PVDF. The mixture was stirred in a ball mill at 3000 rpm for 2 hours to obtain a positive electrode slurry of graphene composite conductive slurry. The positive electrode slurry was then coated at a thickness of 300 μm, rolled, and vacuum-dried in a vacuum oven at 80°C for 12 hours to obtain an electrode sheet. The electrode sheet was then assembled into a button cell for testing. The active material surface loading was 20.1 mg / cm 2 .

[0049] Example 2

[0050] (1) Electrochemical preparation of graphene: 8 mm thick flexible graphite paper was used as the cathode and anode, and 1 M sodium persulfate and 0.001 M sodium citrate were used as the electrolyte. A voltage of 15 V was applied and electrolysis was performed for 80 min. Graphene powder was obtained after electrolysis. The collected graphene powder was filtered, washed three times with deionized water, and ultrasonically dispersed at 600 W for 2 h.

[0051] (2) Further, the ultrasonic treatment was continued for 1 h under the homogenization condition of 3000 rpm to obtain a graphene dispersion with good dispersion.

[0052] Centrifuge at 2000 rpm for 1 hour. The upper layer is designated as solution A, and the lower layer is designated as slurry B. AFM analysis revealed that the graphene thickness in dispersion A ranged from 0.4 to 1.4 nm, with a calculated number of layers ranging from 1 to 4. The graphene thickness in dispersion B ranged from 2.5 to 6 nm, resulting in a calculated number of 8 to 18 layers.

[0053] (3) The dispersion liquid A was spray dried to obtain powder A. The dispersion liquid B was added with 3M K2CO3 solution and stirred at 500 rpm for 48 h, and then vacuum dried to obtain powder B. The powder A was heat annealed at 500°C under N2 atmosphere. The powder B was heat annealed at 750°C under Ar atmosphere. The powder A was dispersed in N-methyl pyrrolidone solvent, and stirred at 1000 rpm for 30 min to obtain dispersion liquid C with a graphene concentration of 20 mg / ml. The powder B was dispersed in N-methyl pyrrolidone solvent, and stirred at 3000 rpm for 120 min, and then sand-milled in a sand mill with a frequency of 60 Hz for 2 h to obtain dispersion liquid D with a graphene concentration of 30 mg / ml.

[0054] (4) The dispersion liquid C and the dispersion liquid D were mixed as conductive agents in a ratio of 1:1, and mixed with an active material, graphene conductive agent and PVDF in a mass ratio of 8:1:1. The mixture was stirred in a ball mill at 3000 rpm for 1 h to obtain a positive electrode slurry of a graphene composite conductive slurry. Then, the slurry was coated to a thickness of 200 μm, and vacuum dried at 100°C for 10 h in a vacuum oven to obtain an electrode sheet, which was assembled into a button cell for testing.

[0055] Example 3

[0056] (1) The graphene was prepared by an electrochemical method. A flexible graphite paper with a thickness of 5 mm was used as an anode and a cathode, 1M sodium phosphate and 0.01M sodium pyrophosphate were used as electrolyte, and a voltage of 20V was applied for 60 min. After the electrolysis, graphene powder was obtained, which was collected and filtered, washed with deionized water for 3 times, and dispersed by ultrasonic at a power of 600 W for 2 h.

[0057] (2) Further, the graphene dispersion liquid was obtained by further ultrasonic dispersion at 2000 rpm for 2 h.

[0058] The upper layer solution was taken as A and the lower layer slurry was taken as B after centrifugation at 4000 rpm for 50 min. AFM detection showed that the thickness of graphene in the dispersion liquid A was 0.3-1.1 nm, and the number of layers was 1-3. The thickness of graphene in the dispersion liquid B was 1.6-5 nm, and the number of layers was 5-15.

[0059] (3) The dispersion liquid A was spray dried to obtain powder A. The dispersion liquid B was added with 1M NaOH solution and stirred at 2000 rpm for 24h, and then vacuum dried to obtain powder B. The powder A was heat annealed at 600°C under N2 atmosphere. The powder B was heat annealed at 650°C under N2 atmosphere. The powder A was dispersed in N,N dimethylformamide solvent, and stirred at 1000 rpm for 30min to obtain dispersion liquid C with a graphene concentration of 20mg / ml. The powder B was dispersed in N,N dimethylformamide solvent, and stirred at 3000 rpm for 120min. The mixture was sand milled in a sand mill with a frequency of 60Hz for 2h to obtain dispersion liquid D with a graphene concentration of 30mg / ml.

[0060] (4) The dispersion liquid C and dispersion liquid D were mixed as conductive agent in a ratio of 1:1; and mixed in a mass ratio of active material: graphene conductive agent: PVDF = 8:1:1. The mixture was stirred in a ball mill at 2500 rpm for 2h to obtain positive electrode slurry of graphene composite conductive slurry. Then, the slurry was coated at a thickness of 250μm, and the electrode sheet was obtained after vacuum drying at 60°C in a vacuum oven for 20h, and a button cell was assembled for testing.

[0061] Example 4

[0062] (1) The graphene was prepared by electrochemical method. A flexible graphite paper with a thickness of 3mm was used as anode and cathode, and 0.8M sodium nitrate and 0.05M sodium sulfite were used as electrolyte. A voltage of 16V was applied for 50min. After the electrolysis, graphene powder was obtained, which was collected and filtered, washed with deionized water for 3 times, and dispersed by ultrasonic at a power of 600W for 2h.

[0063] (2) Further, the graphene dispersion liquid was obtained by ultrasonic at 3000 rpm for 2h.

[0064] The mixture was centrifuged at 6000 rpm for 90min. The upper layer solution was taken as A, and the lower layer slurry was taken as B. AFM detection showed that the thickness of graphene in dispersion liquid A was 0.6-1.6nm, and the number of layers was calculated to be 2-5 layers. The thickness of graphene in dispersion liquid B was 3-6.7nm, and the number of layers was calculated to be 10-20 layers.

[0065] (3) The dispersion liquid A was spray-dried to obtain powder A. The dispersion liquid B was added with 1M NaHC03solution and stirred at a speed of 1000 rpm for 12 h, and then freeze-dried to obtain powder B. The powder A was heat-annealed at 600°C under Ar atmosphere. The powder B was heat-annealed at 800°C under Ar atmosphere. The powder A was dispersed in N, N-dimethylacetamide solvent under a homogenization condition of 2000 rpm, and stirred for 30 min to obtain dispersion liquid C with a graphene concentration of 40 mg / ml. The powder B was dispersed in N, N-dimethylacetamide solvent under a homogenization condition of 6000 rpm, and stirred for 120 min. The mixture was sand-milled in a sand mill with a frequency of 50 Hz for 2 h to obtain dispersion liquid D with a graphene concentration of 60 mg / ml.

[0066] (4) The dispersion liquid C and the dispersion liquid D were mixed as conductive agents in a ratio of 2:3, and mixed in a mass ratio of active material: graphene conductive agent: PVDF = 8:1:1. The mixture was stirred in a ball mill at a speed of 2000 rpm for 2 h to obtain a positive electrode slurry of graphene composite conductive slurry. Then, the slurry was coated at a thickness of 150 μm, and vacuum-dried at 80°C in a vacuum oven for 20 h to obtain an electrode sheet, which was assembled into a button cell for testing.

[0067] Example 5

[0068] (1) The graphene was prepared by an electrochemical method. A flexible graphite paper with a thickness of 6 mm was used as an anode and a cathode, 2M sodium persulfate and 0.01M sodium thiosulfate were used as electrolyte, and a voltage of 10V was applied for 40 min. After the electrolysis, graphene powder was obtained, which was collected and filtered, washed with deionized water for 3 times, and dispersed by ultrasonic at a power of 300 W for 2 h.

[0069] (2) Further, the graphene dispersion liquid was obtained by further ultrasonic treatment at a homogenization condition of 5000 rpm for 2 h.

[0070] The upper layer solution was taken as A and the lower layer slurry was taken as B after centrifugation at a speed of 9000 rpm for 30 min. AFM detection showed that the thickness of graphene in the dispersion liquid A was 0.6-1.3 nm, and the number of layers was calculated to be 2-4 layers. The thickness of graphene in the dispersion liquid B was 2.7-5.4 nm, and the number of layers was calculated to be 8-16 layers.

[0071] (3) The dispersion liquid A was spray dried to obtain powder A. The dispersion liquid B was added with 2M KHCO3 solution and stirred at 800 rpm for 24 h, and then freeze-dried to obtain powder B. The powder A was heat annealed at 550°C under Ar atmosphere. The powder B was heat annealed at 650°C under Ar atmosphere. The powder A was dispersed in N-vinyl pyrrolidone solvent, and stirred at 1000 rpm for 60 min to obtain dispersion liquid C with a graphene concentration of 35 mg / ml. The powder B was dispersed in N-vinyl pyrrolidone solvent, and stirred at 5000 rpm for 90 min. The mixture was sand-milled in a sand mill with a frequency of 60 Hz for 2 h to obtain dispersion liquid D with a graphene concentration of 70 mg / ml.

[0072] (4) The dispersion liquid C and dispersion liquid D were mixed as conductive agents in a ratio of 1:3, and mixed with active material, graphene conductive agent and PVDF in a mass ratio of 8:1:1. The mixture was stirred in a ball mill at 3500 rpm for 1 h to obtain positive electrode slurry of graphene composite conductive slurry. Then, the slurry was coated at a thickness of 250 μm, and the electrode sheet was obtained after vacuum drying at 80°C in a vacuum oven for 20 h, and a button cell was assembled for testing.

[0073] Example 6

[0074] (1) The graphene was prepared by electrochemical method. The flexible graphite paper with a thickness of 6 mm was used as anode and cathode, 2M sodium persulfate and 0.01M sodium thiosulfate were used as electrolyte, and a voltage of 10V was applied for 40 min. The graphene powder was obtained after electrolysis, and then collected and filtered, washed with deionized water for 3 times, and dispersed by ultrasonic at a power of 300 W for 2 h.

[0075] (2) Further, the graphene dispersion liquid with good dispersibility was obtained by continuing ultrasonic at 3000 rpm for 1 h.

[0076] The upper layer solution was taken as A and the lower layer slurry was taken as B after centrifugation at 3000 rpm for 120 min. AFM detection showed that the thickness of graphene in dispersion liquid A was 0.3-0.7 nm, and the number of layers was calculated to be 1-2. The thickness of graphene in dispersion liquid B was 2-6.7 nm, and the number of layers was calculated to be 6-20.

[0077] (3) The dispersion liquid A was spray dried to obtain powder A. The dispersion liquid B was added with 0.1 M NaHC03solution and stirred at 1000 rpm for 24 h, and then freeze-dried to obtain powder B. The powder A was heat annealed at 800 °C under Ar atmosphere. The powder B was heat annealed at 700 °C under Ar atmosphere. The powder A was dispersed in N-methyl pyrrolidone solvent, and stirred at 2000 rpm for 60 min to obtain dispersion liquid C with a graphene concentration of 50 mg / ml. The powder B was dispersed in N-methyl pyrrolidone solvent, and stirred at 6000 rpm for 100 min. The mixture was sand-milled in a frequency of 60 Hz sand mill for 2 h to obtain dispersion liquid D with a graphene concentration of 75 mg / ml.

[0078] (4) The dispersion liquid C and dispersion liquid D were mixed as conductive agents in a ratio of 1:1, and mixed with active material: graphene conductive agent: PVDF in a mass ratio of 8:1:1. The mixture was stirred in a ball mill at 3000 rpm for 2 h to obtain a positive electrode slurry of graphene composite conductive slurry. Then, the slurry was coated at a thickness of 250 μm, and the electrode sheet was obtained after vacuum drying at 80 °C in a vacuum oven for 10 h. The button cell was assembled and tested.

[0079] In the above examples and comparative examples, the components, assemblies, and testing methods of the sodium ion battery were the related materials and technologies commonly used by those skilled in the art. For details, refer to the following: the positive electrode material was coated on one side of the aluminum foil surface, and the positive electrode sheet was obtained after drying. The button cell was assembled in the order of negative electrode shell, sodium sheet, electrolyte, separator, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell, wherein the electrolyte was 1 M NaCl04, the solvent was an EC / DMC mixture (mass ratio 1:1), the separator was a GF / C type glass fiber, the test voltage was 2-4.5 V, and the charging was performed by a constant current step, followed by discharging by a constant current step. The theoretical capacity of the positive electrode material was 140 mAh / g. The rate performance and cycle performance of the button cell were tested, and the results are shown in Tables 1 and 2:

[0080] Table 1: Rate performance test results

[0081]

[0082] Table 2: Cycle performance test results

[0083]

[0084] The above merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto, any person skilled in the art within the scope of the present application disclosed in the technical range, can easily think of the changes or replacements, should be covered within the scope of protection of the present application. Therefore, the scope of protection of the present application should be limited to the scope of protection defined by the claims.

[0085] The present application is not detailed, are well known to those skilled in the art of the present technology.

Claims

1. A method for preparing a graphene composite conductive agent, characterized in that: The method comprises the following specific steps: S1. Homogenously emulsifying the graphene raw material by ultrasonic to obtain a uniform graphene dispersion liquid; S2. Centrifuging to obtain graphene solutions with different dispersities, taking the upper layer solution as A and the lower layer slurry as B, the number of graphene layers in A is less than that in B, the number of graphene layers in A ranges from 1 to 5 layers, and the number of graphene layers in B ranges from 5 to 20 layers; S3. Adding an alkaline substance to B, fully dispersing, drying, and then performing heat annealing treatment; wetly pre-mixing and homogenizing the powders obtained by heat annealing of A and B in an organic solvent to obtain pre-dispersion liquid C and pre-dispersion liquid D, the mass concentration of pre-dispersion liquid C is lower than that of pre-dispersion liquid D; S4. Mixing pre-dispersion liquid C and pre-dispersion liquid D at a certain ratio to obtain a graphene composite conductive agent. The graphene raw material is prepared by using a flexible graphite material as a raw material, an oxygen-containing acid salt as an electrolyte, and adding a complexing agent, performing electrolytic reaction to obtain graphene powder, filtering, washing, and ultrasonic dispersion to obtain the graphene raw material; wherein the flexible graphite material is one or more of graphite paper, graphite rod, graphite plate, and graphite foil; the oxygen-containing acid salt is one or more of a sulfate, a persulfate, a perchlorate, a nitrate, and a phosphate; the complexing agent is one or more of a citrate, a pyrophosphate, a thiosulfate, and a sulfite; the voltage applied in the electrolytic reaction ranges from 5 to 30 V, and the electrolysis time ranges from 20 to 360 min; and the content of the complexing agent ranges from 0 to 10%.

2. The method for preparing a graphene composite conductive agent according to claim 1, wherein: The stirring speed of the homogenization emulsification is 500-8000 rpm, the ultrasonic power is 200-3000 W, and the specific surface area of the graphene raw material in the graphene dispersion liquid is 500-3000 m 2 / g.

3. The method for preparing the graphene composite conductive agent according to claim 1, wherein: In the step S2, the centrifugal speed ranges from 500 to 10,000 rpm, and the centrifugal time ranges from 30 to 180 min.

4. The method for preparing the graphene composite conductive agent according to claim 1, wherein: In the step S3, the alkaline substance added to B is selected from NaOH, KOH, NaHCO3, Na2CO3, K2CO3, and KHCO3, and the concentration ranges from 0.1 to 10 M; the organic solvent is one or more of N-methyl pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide, and N-vinyl pyrrolidone.

5. The method for preparing the graphene composite conductive agent according to claim 1, wherein: In the step S3, the graphene heat annealing treatment is performed in a protective atmosphere or a reducing atmosphere, the heat annealing treatment temperature ranges from 300 to 1,000 ℃, and the treatment time ranges from 30 to 240 min, wherein the particle size D50 of the powder obtained by heat annealing of B ranges from 1 to 40 μm.

6. The method for preparing the graphene composite conductive agent according to claim 1, wherein: In the step S3, the wet pre-mixing and homogenizing treatment is one or more of ball milling, sand milling, homogenizing, and ultrasonic, wherein the stirring speed of homogenizing ranges from 500 to 8,000 rpm / min, and the mixing time ranges from 30 to 120 min; the mass concentration of pre-dispersion liquid C ranges from 1 to 50 mg / ml, and the mass concentration of pre-dispersion liquid D ranges from 1 to 100 mg / ml.

7. The method for preparing the graphene composite conductive agent according to claim 1, wherein: In the graphene composite conductive agent, the volume ratio of pre-dispersion liquid C to pre-dispersion liquid D ranges from 3:1 to 1:

20.

8. A graphene composite conductive agent obtained by the preparation method according to any one of claims 1 to 7.

9. A sodium-ion battery, characterized in that: The graphene composite conductive agent is used in claim 8, and the mass ratio of active material, binder, and conductive agent in the positive electrode formula is (8-9.5):(0.1-1):(0.1-1).

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