Expanded graphite positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the field of batteries, and particularly discloses an expanded graphite positive electrode material as well as a preparation method and application thereof. The preparation raw materials of the expanded graphite positive electrode material comprise a graphite source, an acidic oxidant, a metal salt and / or a hydrate thereof; the mass volume ratio of the graphite source to the acidic oxidant is 1g: (7-25) mL; the mass ratio of the graphite source to the metal salt and / or the hydrate thereof is 1: (0.7-3). The expanded graphite positive electrode material disclosed by the invention has the advantages that the graphite interlayer spacing is increased, more anion sites are provided, the anion intercalation resistance is reduced, and the volume change of graphite during intercalation is small, so that the specific capacity, the rate and the cycle performance of a dual-ion battery are improved; and due to larger interlayer spacing, the stripping of the graphite sheet layer in the reaction process is greatly reduced, and the cycling stability of the expanded graphite positive electrode material is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of batteries, and particularly relates to an expanded graphite positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Due to high cost of transition metals (such as cobalt), low reserves of lithium resources, and serious environmental pollution, the large-scale energy storage development of lithium ion batteries is restricted, and high-safety low-cost large-scale energy storage technology needs to be developed. Among many new batteries, the dual-ion battery is different from the traditional rocking chair battery, and is not a single ion deintercalation mechanism, but in the charging and discharging process, the anions and cations in the electrolyte are used as active materials, and the anions and cations are oxidized and reduced with the positive electrode and the negative electrode respectively; the diffusion kinetics, rate performance and power density are improved, and the working voltage of the dual-ion battery is high, and the energy density is higher. Organic materials, metal organic frameworks (MOFs) and graphite materials are the most common three kinds of positive electrode materials of the dual-ion battery. The organic materials generally have problems of poor electronic conductivity, low capacity, single energy storage mechanism, limited active sites and the like; the conductivity of the MOFs is low and the material is easy to dissolve. The graphite material has excellent comprehensive performance, has advantages of high working potential, high safety and low cost, and is the most common positive electrode material of the dual-ion battery. However, due to insufficient interlayer spacing and anion storage sites, the intercalation reaction capacity of the graphite is limited, and in the deep charging stage, the strong mutual repulsion of the anions makes the anion transmission resistance between the graphite layers large, the anion intercalation voltage is increased, and the transmission kinetics is limited. In addition, the anion intercalation causes the expansion of the graphite interlayer, which will cause obvious volume change (>130%), and long-time charging and discharging cycle will cause the graphite structure to fail. Therefore, it is necessary to improve the capacity of the graphite-based positive electrode and relieve the expansion caused by the intercalation reaction to improve the cycle stability. SUMMARY

[0003] In order to overcome at least one technical problem existing in the prior art, one of the purposes of the present application is to provide an expanded graphite positive electrode material, the interlayer spacing of the positive electrode material is increased, the interlayer resistance is reduced, more storage sites can be provided for the anion intercalation reaction of the positive electrode end of the dual-ion battery, the transmission kinetics is accelerated, the volume change caused by the anion intercalation reaction is relieved, and the electrolyte infiltration and anion diffusion are facilitated, thereby improving the capacity, rate and cycle life of the dual-ion battery assembled by the expanded graphite positive electrode material.

[0004] The second purpose of the present application is to provide a preparation method of the expanded graphite positive electrode material, the preparation method prepares the expanded graphite positive electrode material by a metal salt intercalation thermal expansion method, the interlayer spacing of the graphite is increased, the transmission kinetics of the anion is accelerated, a simple and low-cost preparation process is adopted, and the preparation process is suitable for large-scale production.

[0005] The third object of the present application is to provide a dual-ion battery with high energy density and power density, high first charge-discharge efficiency, and good cycle stability.

[0006] The fourth object of the present application is to provide application of the above-mentioned expanded graphite positive electrode material and / or the preparation method of the above-mentioned expanded graphite positive electrode material in the field of batteries.

[0007] To achieve the above-mentioned objects, the technical solution adopted by the present application is: The first aspect of the present application provides an expanded graphite positive electrode material, and the preparation raw material of the expanded graphite positive electrode material comprises a graphite source, an acidic oxidizing agent, a metal salt and / or a hydrate thereof. The mass-volume ratio of the graphite source and the acidic oxidizing agent is 1g:(7-25)mL. The mass ratio of the graphite source and the metal salt and / or the hydrate thereof is 1:(0.7-3).

[0008] In the present application, the acidic oxidizing agent is used to open the graphite layers in the graphite source, so that the metal salt can enter the interlayer and insert into the interlayer, thereby preliminarily increasing the graphite layer spacing to obtain expandable graphite; and after high-temperature pyrolysis and acid washing, the expanded graphite positive electrode material is obtained.

[0009] In some embodiments of the present application, the preparation raw material of the expanded graphite positive electrode material further comprises an acid solution; the acid solution is used to remove the metal oxide in the expanded graphite.

[0010] In some embodiments of the present application, the mass-volume ratio of the graphite source and the acidic oxidizing agent is any one of 1g:7mL, 1g:8mL, 1g:10mL, 1g:12mL, 1g:14mL, 1g:16mL, 1g:18mL, 1g:20mL, 1g:22mL, 1g:24mL, 1g:25mL or a range value formed by any two thereof.

[0011] In some embodiments of the present application, the mass ratio of the graphite source and the metal salt and / or the hydrate thereof is any one of 1:0.7, 1:0.9, 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8, 1:2, 1:2.2, 1:2.4, 1:2.6, 1:2.8, 1:3 or a range value formed by any two thereof.

[0012] In some embodiments of the present application, the expanded graphite positive electrode material has a lamellar structure, and the lamellar spacing is 0.335-0.34 nm. The expanded graphite positive electrode material in the present application has been expanded in the direction perpendicular to its lamellar structure, has a larger interlayer spacing, and has a certain degree of disorder and defects. The three-dimensional structure after expansion can provide more storage sites for the anion intercalation reaction of the positive electrode end of the dual-ion battery, can also alleviate the volume change caused by the anion intercalation reaction, and is helpful for the infiltration of the electrolyte and the diffusion of the anion, thereby improving the capacity, rate and cycle life of the positive electrode material of the dual-ion battery.

[0013] In some embodiments of the present application, the graphite source includes at least one of natural graphite, artificial graphite, and graphite recovered from waste batteries.

[0014] In some embodiments of the present application, the natural graphite includes at least one of flaky graphite, earthy graphite, and massive graphite. The natural graphite is derived from graphite deposits.

[0015] In some embodiments of the present application, the artificial graphite includes at least one of single crystal graphite, polycrystalline graphite, pyrolytic graphite, and graphite fiber. The artificial graphite is prepared from petroleum coke and pitch coke which are easy to graphitize.

[0016] In some embodiments of the present application, the graphite recovered from waste batteries includes at least one of graphite recovered from waste lithium-ion batteries, graphite recovered from waste lithium metal batteries, and graphite recovered from waste sodium-ion batteries. The graphite recovered from waste batteries is a recycled material, which is environmentally friendly and green. Moreover, the material has been repeatedly cycled and ions have been repeatedly intercalated / deintercalated during the operation of the battery, so that the interlayer spacing is larger than that of natural graphite and artificial graphite, which is more conducive to exciting anion storage. Under the action of an acidic oxidizing agent opening the interlayer spacing of graphite, metal salts can be inserted more quickly and in larger amounts to expand the graphite layers, and the thermal decomposition products of metal salt hydrates further expand the graphite layers.

[0017] In some embodiments of the present application, the graphite source is selected from at least one of flaky graphite, earthy graphite, massive graphite, single crystal graphite, polycrystalline graphite, pyrolytic graphite, graphite fiber, graphite recovered from waste lithium-ion batteries, graphite recovered from waste lithium metal batteries, and graphite recovered from waste sodium-ion batteries.

[0018] In the present application, the graphite source forms a three-dimensional structure after expansion, which can provide more sites for the anion intercalation reaction of the positive electrode end of the dual-ion battery, can also alleviate the volume change caused by the anion intercalation reaction, and is helpful for the infiltration of the electrolyte and the diffusion of the anion. In addition, the graphite source in the present application is low in cost, widely available, green and environmentally friendly, and conducive to commercial development.

[0019] In some embodiments of the present application, the acid oxidant comprises at least one of nitric acid, nitrous acid, sulfuric acid, permanganic acid, hypochlorous acid, chloric acid, chlorous acid, perchloric acid.

[0020] In some embodiments of the present application, the nitric acid comprises at least one of concentrated nitric acid, dilute nitric acid.

[0021] In some embodiments of the present application, the metal salt comprises at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium chloride, lithium sulfate, lithium carbonate, lithium nitrate, sodium chloride, sodium carbonate, sodium bicarbonate, sodium phosphate, sodium nitrate, sodium phosphite, sodium sulfite, sodium acetate, sodium formate, sodium propionate, sodium acrylate, sodium benzoate, sodium hypochlorite, sodium chlorate, sodium perchlorate, sodium pyrophosphate, sodium thiosulfate, sodium persulfate, potassium chloride, potassium nitrate, potassium sulfate, potassium dihydrogen phosphate, potassium hydrogen phosphate, potassium carbonate, potassium bicarbonate, magnesium citrate, magnesium glycinate, magnesium chloride, magnesium sulfate, magnesium lactate, magnesium carbonate, magnesium sulfide, magnesium chlorate, calcium gluconate, calcium hydrogen phosphate, calcium lactate, calcium chloride, calcium oxalate, calcium carbonate, calcium phosphate, calcium fluoride, calcium nitrate, calcium chlorate, calcium perchlorate, calcium bicarbonate, calcium dihydrogen phosphate.

[0022] A second aspect of the present application provides a preparation method of the expanded graphite positive electrode material of the first aspect of the present application, comprising the following steps: mixing preparation raw materials comprising a graphite source, an acid oxidant, a metal salt and / or a hydrate thereof to obtain expandable graphite; subjecting the expandable graphite to thermal expansion treatment to obtain expanded graphite; acid washing the expanded graphite to obtain the expanded graphite positive electrode material.

[0023] The preparation method in the present application utilizes an acid oxidant to open the graphite interlayer, so that the metal salt and / or the hydrate thereof enters the graphite interlayer and intercalates in the interlayer to obtain expandable graphite; during the thermal expansion treatment of the expandable graphite, the metal salt and / or the hydrate in the interlayer decomposes and generates metal oxide and gas, and the generated gas can further expand the graphite interlayer spacing. The metal oxide remaining in the graphite interlayer after decomposition needs to be removed by acid washing, so as to wash away the metal oxide in the graphite interlayer, and thus obtain the expanded graphite positive electrode material in the present application.

[0024] The acid oxide and the metal salt and / or the hydrate thereof are used to prepare the expanded graphite, the acid oxide first opens the graphite interlayer, and then the metal salt and / or the hydrate thereof is inserted and the strong oxidizing property further opens the graphite layer, in the thermal expansion process, the gas generated by the decomposition of the metal salt and / or the hydrate thereof further expands the graphite interlayer, the multiple expansion mechanism can prepare the expanded graphite positive electrode material with high expansion volume and excellent structure performance, and the expanded graphite preparation process has the characteristics of high reaction efficiency, good controllability and good comprehensive economy; the intercalation agent used in the preparation of the existing expanded graphite, i.e. glacial acetic acid, can only provide a protonic acid environment, the intercalation depth is shallow, the oxidation efficiency is low, the glacial acetic acid has strong corrosivity, a large amount of glacial acetic acid is required in the preparation of the expanded graphite, the reaction cost is increased, and the glacial acetic acid has a low boiling point of 118 DEG C, which will volatilize in advance in the subsequent high-temperature expansion (about 900 DEG C), resulting in insufficient residual intercalation.

[0025] In some embodiments of the present application, the temperature of the mixing is 15-40 DEG C; in some embodiments of the present application, the temperature of the mixing is any one of 15 DEG C, 16 DEG C, 18 DEG C, 20 DEG C, 22 DEG C, 24 DEG C, 25 DEG C, 26 DEG C, 28 DEG C, 30 DEG C, 32 DEG C, 34 DEG C, 35 DEG C, 36 DEG C, 38 DEG C, 40 DEG C or a range value formed by any two of them.

[0026] In some embodiments of the present application, the mixing time is 6-48h; in some embodiments of the present application, the mixing time is any one of 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, 26h, 28h, 30h, 32h, 34h, 36h, 38h, 40h, 42h, 44h, 46h, 48h or a range value formed by any two of them.

[0027] In some embodiments of the present application, the temperature of the thermal expansion treatment is 400-1200 DEG C; in some embodiments of the present application, the temperature of the thermal expansion treatment is any one of 400 DEG C, 500 DEG C, 600 DEG C, 700 DEG C, 800 DEG C, 900 DEG C, 1000 DEG C, 1100 DEG C, 1200 DEG C or a range value formed by any two of them.

[0028] In some embodiments of the present application, the time of the thermal expansion treatment is 60-150s; in some embodiments of the present application, the time of the thermal expansion treatment is any one of 60s, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s, 150s or a range value formed by any two of them.

[0029] In some embodiments of the present application, the atmosphere of the thermal expansion treatment is selected from at least one of air atmosphere, nitrogen atmosphere and argon atmosphere.

[0030] In some embodiments of the present application, the thermal expansion treatment is performed in a tube furnace.

[0031] In some embodiments of the present application, the expanded graphite further comprises a cooling step after the thermal expansion treatment step.

[0032] In some embodiments of the present application, the cooling step is specifically natural cooling of the product after the thermal expansion treatment.

[0033] In some embodiments of the present application, the acid washing step is acid washing with an acid solution. The acid washing removes metal oxides between layers of the expanded graphite.

[0034] In some embodiments of the present application, the mass-to-volume ratio of the expanded graphite to the acid solution is 1 g: (15-60) mL; in some embodiments of the present application, the mass-to-volume ratio of the expanded graphite to the acid solution is any one of 1 g: 15 mL, 1 g: 18 mL, 1 g: 20 mL, 1 g: 22 mL, 1 g: 25 mL, 1 g: 28 mL, 1 g: 30 mL, 1 g: 32 mL, 1 g: 35 mL, 1 g: 38 mL, 1 g: 40 mL, 1 g: 42 mL, 1 g: 45 mL, 1 g: 48 mL, 1 g: 50 mL, 1 g: 52 mL, 1 g: 55 mL, 1 g: 58 mL, 1 g: 60 mL or a range value formed by any two of them.

[0035] In some embodiments of the present application, the acid washing time is 6-15 h; in some embodiments of the present application, the acid washing time is any one of 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h or a range value formed by any two of them.

[0036] In some embodiments of the present application, the acid solution used in the acid washing comprises at least one of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid, hydrofluoric acid, oxalic acid, nitrous acid.

[0037] In some embodiments of the present application, the expandable graphite further comprises a washing step, a suction filtration step and a drying step in sequence; the washing step, the suction filtration step and the drying step in sequence are after the mixing step.

[0038] In some embodiments of the present application, the washing is washing with water. In some embodiments of the present application, the washing is washing with distilled water until the pH of the wastewater after washing is 7.

[0039] In some embodiments of the present application, the drying temperature is 75-85°C.

[0040] In some embodiments of the present application, the drying time is 10-20h.

[0041] A third aspect of the present application provides a dual-ion battery comprising the expanded graphite positive electrode material of the first aspect of the present application, or the expanded graphite positive electrode material prepared by the method of the second aspect of the present application.

[0042] In some embodiments of the present application, the dual-ion battery comprises a negative electrode, a positive electrode, an electrolyte and a separator, and the positive electrode material is the expanded graphite positive electrode material of the first aspect of the present application.

[0043] A fourth aspect of the present application provides the use of the expanded graphite positive electrode material of the first aspect of the present application and / or the expanded graphite positive electrode material prepared by the method of the second aspect of the present application in the field of batteries.

[0044] The expanded graphite positive electrode material of the present application has an increased interlayer spacing of graphite, provides more anion sites, reduces the intercalation resistance of anions and the volume change of graphite during intercalation, thereby improving the specific capacity, rate and cycle performance of the dual-ion battery; due to the larger interlayer spacing, the peeling of graphite layers during the reaction process is greatly reduced, and the cycle stability of the expanded graphite positive electrode material is improved.

[0045] The dual-ion battery assembled from the expanded graphite positive electrode material of the present application has high working voltage, fast reaction kinetics, high energy density and power density, high first charge-discharge efficiency, good cycle stability and other performances, and has great application prospects in the field of new energy storage. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 SEM images of the flake graphite in Comparative Example 2 and the expanded graphite prepared in Example 1.

[0047] Figure 2 XRD images of the expanded graphite in Example 1 and the flake graphite in Comparative Example 2.

[0048] Figure 3 Raman curve graphs of the expanded graphite in Example 1 and the flake graphite in Comparative Example 2.

[0049] Figure 4 Constant current charge-discharge curve graph of the dual-ion battery prepared in Example 1 at 1C.

[0050] Figure 5 Long cycle curve graph of the dual-ion battery prepared in Example 1 at 2C.

[0051] Figure 6Figure for rate capability test of the dual-ion battery prepared in Example 1. DETAILED DESCRIPTION

[0052] The present application will be further described in details with reference to the accompanying drawings and examples, but the implementation and protection of the present application are not limited thereto. It should be noted that the following processes not specifically described in details are realized or understood by those skilled in the art according to the prior art. The reagents or instruments not specified by the manufacturer are conventional products that can be purchased on the market.

[0053] Example 1 This example provides a preparation method of expanded graphite positive electrode material, comprising the following steps: 3 g of graphite recovered from waste lithium batteries (used as a graphite source) is added into a mixed solution of 40 mL of concentrated nitric acid and 4 g of magnesium chloride hexahydrate, and magnetically stirred at room temperature for 24 h. The magnesium chloride is intercalated into the graphite interlayer to obtain a graphite acid solution; The graphite acid solution is slowly introduced into distilled water, and suction filtered and washed with water until the pH is 7. The MgCl2 in the graphite interlayer will absorb H2O to crystallize MgCl2 6H2O during the water washing process. The sample is placed in a vacuum drying oven at 80°C for 12 h to obtain expandable graphite; The expandable graphite is subjected to thermal expansion treatment, specifically: placed in a 900°C tube furnace for calcination for 90 s. The MgCl2 6H2O in the graphite interlayer is decomposed by heat to produce HCl and water vapor, which expands the graphite interlayer. After taking out, it is cooled at room temperature to obtain expanded graphite; The expanded graphite is acid washed: 30 mL of dilute hydrochloric acid is used to wash off the MgO in the interlayer of the expanded graphite (1 g) to obtain an expanded graphite positive electrode material with increased interlayer spacing.

[0054] The graphite is opened by the action of the acid oxidant (i.e. concentrated nitric acid), and the metal salt hydrate (i.e. MgCl2 6H2O) exists in the graphite interlayer to form expandable graphite. The expandable graphite is subjected to thermal expansion treatment to obtain expanded graphite.

[0055] The example also provides a dual-ion battery prepared by the preparation method comprising the following steps: mixing the positive electrode material (i.e. the expanded graphite positive electrode material prepared in the example) with a PVDF binder and an SP conductive agent in a mass ratio of 8:1:1, adding NMP after grinding for 30 minutes, stirring to a paste, and coating on an aluminum foil to prepare a dual-ion battery electrode. Li sheet is used as a counter electrode, 1 mol / L LiPF6 solution is used as an electrolyte, the solvent in the electrolyte is a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1, and a Li-C dual-ion battery is assembled.

[0056] During the charging process of the dual-ion battery, the interlayer spacing of the graphite increases, the van der Waals force between the layers decreases, the resistance of the anion intercalation decreases, the anion has more intercalation sites, the intercalation capacity is improved, and the transmission kinetics of the anion is accelerated, thereby improving the rate performance and capacity of the dual-ion battery. In addition, after the interlayer spacing increases, the peeling of the graphite layer caused by the insertion and extraction of the anion is alleviated, and the cycle stability of the battery is improved.

[0057] The graphite recovered from the waste lithium battery is introduced into a mixed solution of concentrated nitric acid and magnesium chloride hexahydrate and stirred uniformly. The concentrated nitric acid opens the graphite interlayer, and the metal salt is inserted to form MgCl2·6H2O / graphite. The graphite layer increases the interlayer spacing to accommodate MgCl2·6H2O. Under the action of high temperature, magnesium chloride hexahydrate decomposes to produce hydrogen chloride and water vapor, and the gas molecules overflow from the graphite layer to further expand the graphite layer (0.33795 nm). MgO remains between the graphite layers and is washed away with dilute hydrochloric acid. The use of this expanded graphite positive electrode material with large interlayer spacing greatly improves the storage kinetics of the anion. The intercalation mechanism of conventional graphite intercalation agents (such as glacial acetic acid) is not as effective as the multiple expansion of the graphite layer by the metal salt (hydrate), so the expansion effect is weaker than that of the metal salt (including its hydrate).

[0058] Examples 2-8 The preparation method of the expanded graphite positive electrode material in examples 2-8 is different from that of example 1 only in that an equal amount of other graphite sources is used instead of the graphite recovered from the waste lithium battery in example 1. In example 2, flaky graphite is used as the graphite source; in example 3, soil block graphite is used as the graphite source; in example 4, blocky graphite is used as the graphite source; in example 5, single crystal graphite is used as the graphite source; in example 6, polycrystalline graphite is used as the graphite source; in example 7, pyrolytic graphite is used as the graphite source; and in example 8, graphite fiber is used as the graphite source.

[0059] The dual-ion battery in examples 2-8 is prepared by referring to the preparation method of the dual-ion battery in example 1.

[0060] Comparative examples 1-8 The difference between the dual-ion battery in Comparative Examples 1-8 and the dual-ion battery in Example 1 is only that an equal amount of a graphite source is directly used as the positive electrode material.

[0061] The SEM images of the unexpanded flake graphite and the expanded graphite positive electrode material prepared in Example 1 are tested by a scanning electron microscope, and are specifically as shown in Figure 1 , wherein, Figure 1 (a) in the figure is an SEM image of the unexpanded flake graphite; Figure 1 (b) and Figure 1 (c) in the figure are SEM images of the expanded graphite positive electrode material prepared in Example 1 with a scale of 5 μm and a scale of 1 μm, respectively. As can be seen from Figure 1 , the expanded graphite positive electrode material prepared in Example 1 has a uniform micro-morphology and a uniform graphite sheet structure. Compared with the unexpanded flake graphite (interlayer spacing of 0.33471 nm), the expanded graphite positive electrode material prepared in Example 1 has expanded in the direction perpendicular to the graphite sheet layer structure, and the interlayer spacing is increased, which is 0.33795 nm, while the interlayer spacing of the graphite recovered from the waste lithium-ion battery is 0.33504 nm, and there is a certain degree of disorder and defects. The three-dimensional structure of the expanded graphite positive electrode material obtained after such expansion can provide more reaction sites for the intercalation reaction of anions, at the same time, it can also relieve the volume change caused by the intercalation reaction, and is helpful for the infiltration of the electrolyte and the diffusion of the anions.

[0062] The XRD images of the unexpanded flake graphite in Comparative Example 2 and the expanded graphite positive electrode material prepared in Example 1 are tested by an XRD tester, and are specifically as shown in Figure 2 . Compared with the graphite recovered from the waste lithium battery in Comparative Example 1 ((002) peak crystal face diffraction peak position angle at 26.52°), Figure 2 , the (002) peak crystal face diffraction peak position angle of the expanded graphite positive electrode material prepared in Example 1 is 26.35°, and the (002) peak crystal face diffraction peak position angle of the unexpanded flake graphite in Comparative Example 2 is 26.61°, and it is found by the Bragg formula 2dsinθ = nλ that the graphite interlayer of the expanded graphite prepared in Example 1 is increased.

[0063] The Raman curves of the unexpanded flake graphite in Comparative Example 2 and the expanded graphite positive electrode material prepared in Example 1 are tested by a Raman tester, and are specifically as shown in Figure 3 . As can be seen from Figure 3 , the use of Raman spectrum to characterize graphite generally has three relatively obvious characteristic peaks, a D peak at 1350 cm -1 , a G peak at 1580 cm -1 , and a 2D peak at 2700 cm -1of the 2D peak. The Raman spectrum of an ordered, defect-free graphite sample is observed to have no D peak or a weak D peak; the higher the degree of disorder and the more defects in the graphite sample, the stronger the D peak. The unexpanded flake graphite in Comparative Example 2 has two obvious peaks at 1580 cm -1 and 2700 cm -1 , which are the G peak and the 2D peak often observed in natural graphite samples. The expanded graphite anode material sample prepared in Example 1 has a relatively obvious D peak at 1350 cm -1 in addition to the G peak at 1580 cm -1 and the 2D peak at 2700 cm -1 , which indicates that the expanded graphite anode material in Example 1 has a certain degree of disorder and defects.

[0064] The dual-ion batteries prepared from Examples 1-8 and Comparative Examples 1-8 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the test results are shown in Table 1. The constant-current charge-discharge curve of the dual-ion battery prepared in Example 1 at 1 C is shown in Figure 4 , the long cycle curve at 2 C is shown in Figure 5 , and the rate performance data are shown in Figure 6 .

[0065] Table 1: Electrochemical performance of the dual-ion batteries of Examples 1-8 and Comparative Examples 1-8

[0066] In the present application, the "cycle number" in all tables is defined as the cycle number at which the capacity retention rate decays to 80% under 2 C charge-discharge conditions.

[0067] As can be seen from Table 1, the electrochemical performance of the batteries using expanded graphite anode materials prepared from different graphite sources as the positive electrode material varies greatly, and the performance of the expanded graphite anode material prepared from the graphite recovered from waste lithium batteries is superior to that of the other graphite sources. Compared with the untreated graphite sources in Comparative Examples 1-8, the expanded graphite anode materials prepared by the method of the present application in Examples 1-8 as the positive electrode material significantly improve the specific capacity of the batteries at 1 C and 5 C and the long cycle performance at 2 C.

[0068] The expanded graphite positive electrode material prepared in Example 1 is assembled into a dual-ion battery, and the discharge specific capacity thereof is increased to 142 mAh / g at a current density of 1 C; after the charge-discharge rate is increased from 1 C to 5 C, the discharge specific capacity is 97 mAh / g, and after the rate is reduced to 1 C again, the capacity is restored to 140 mAh / g, and the rate performance is significantly improved. In addition, the expanded graphite positive electrode material can alleviate the volume change caused by anion intercalation, so that the assembled expanded graphite novel dual-ion battery has excellent cycle life; after 500 cycles at a current density of 1 C, the specific capacity is still 139 mAh / g, and the capacity retention rate is as high as 98%; and the capacity retention rate is not less than 80% after 3848 cycles at 2 C.

[0069] Examples 9-16 The difference between the preparation method of the expanded graphite positive electrode material in Examples 9-16 and Example 1 is only that an equal amount of other acidic oxidants is used instead of concentrated nitric acid in Example 1, and the specific types of the acidic oxidants used in each example are shown in Table 2 below.

[0070] The dual-ion battery in Examples 9-16 is prepared by referring to the preparation method of the dual-ion battery in Example 1.

[0071] The dual-ion battery prepared in Examples 9-16 is subjected to charge-discharge tests at different rates, and long cycle tests at 2 C, and the specific test results are shown in Table 2 below.

[0072] Table 2 Comparison of electrochemical performance of expanded graphite positive electrode materials prepared based on different acidic oxidants

[0073] As can be seen from Table 2, the electrochemical performance of the expanded graphite positive electrode materials prepared by different acidic oxidants is different, and the effect of concentrated nitric acid as an acidic oxidant is better than that of other acidic oxidants.

[0074] While the acidic oxidant opens the graphite layer, the graphite is intercalated with a metal salt, and after filtration, washing and drying, the metal exists in the form of a metal salt and / or a hydrate between the graphite layers, forming an expandable graphite. A very critical point is the type of metal salt hydrate. The following uses different metal salts (including hydrates) to prepare expanded graphite positive electrode materials.

[0075] Examples 17-69 The difference between the preparation method of the expanded graphite positive electrode material in Examples 17-69 and Example 1 is only that an equal amount of other metal salts (including hydrates) is used instead of magnesium chloride hexahydrate in Example 1, and the specific types of the metal salts (including hydrates) used in each example are shown in Table 3 below.

[0076] The dual-ion batteries in Examples 17-69 were prepared according to the preparation method of the dual-ion battery in Example 1.

[0077] The dual-ion batteries prepared in Examples 17-69 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the specific test results are shown in Table 3.

[0078] Table 3 Performance comparison of expanded graphite positive electrodes prepared based on different metal salts (including hydrates)

[0079] As can be seen from Table 3, the performance of the expanded graphite positive electrode materials prepared using different metal salts (including hydrates) has certain differences, and the electrochemical performance of the expanded graphite positive electrode material prepared using magnesium chloride hexahydrate is better than that of other metal salts (including hydrates).

[0080] Examples 70-95 The preparation method of the expanded graphite positive electrode material in Examples 70-95 is different from that of Example 1 only in that the amount of the graphite source is different and / or the volume-mass ratio of the acidic oxidant to the metal salt (including hydrates) is different, as shown in Table 4.

[0081] The dual-ion batteries in Examples 70-95 were prepared according to the preparation method of the dual-ion battery in Example 1.

[0082] The dual-ion batteries prepared in Examples 70-95 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the specific test results are shown in Table 4.

[0083] Table 4 Performance comparison of expanded graphite positive electrodes prepared based on different amounts of raw materials

[0084] As can be seen from Table 5, the amount of the graphite source and the amount and ratio of the acidic oxidant and the metal salt (including hydrates) affect the performance of the prepared expanded graphite positive electrode material.

[0085] Examples 96-102 The preparation method of the expanded graphite positive electrode material in Examples 96-102 is different from that of Example 1 only in that the stirring time is different, as shown in Table 5.

[0086] The dual-ion batteries in Examples 96-102 were prepared according to the preparation method of the dual-ion battery in Example 1.

[0087] The dual-ion batteries prepared in Examples 96-102 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the specific test results are shown in Table 5.

[0088] Table 5 Performance comparison of expanded graphite positive electrode materials prepared based on different stirring times

[0089] As can be seen from Table 5, the stirring time of graphite, acidic oxide and metal salt hydrate has a great influence on the performance of the prepared expanded graphite positive electrode material, and the stirring time is preferably 18-30 h.

[0090] Examples 103-137 The preparation method of the expanded graphite positive electrode material in Examples 103-137 is different from that of Example 1 only in that the thermal expansion temperature and / or thermal expansion time are different, and the specific values are shown in Table 6.

[0091] The dual-ion batteries in Examples 103-137 were prepared by referring to the preparation method of the dual-ion batteries in Example 1.

[0092] The dual-ion batteries prepared in Examples 103-137 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the specific test results are shown in Table 6.

[0093] Table 6 Performance comparison of expanded graphite positive electrode materials prepared at different thermal expansion temperatures and times

[0094] As can be seen from Table 6, the thermal expansion temperature and time parameters have a great influence on the performance of the prepared expanded graphite positive electrode material, and the thermal expansion temperature and time need to be reasonably controlled. If the thermal expansion is excessive, it will affect the graphite layer. It is preferred that the thermal expansion is carried out at 700-900℃.

[0095] Examples 138-152 The preparation method of the expanded graphite positive electrode material in Examples 138-152 is different from that of Example 1 only in that the amount of acid used during pickling and / or the pickling time are different, and the amount of acid used corresponds to the amount used for 1 g of expanded graphite, and the specific values are shown in Table 7.

[0096] The dual-ion batteries in Examples 138-152 were prepared by referring to the preparation method of the dual-ion batteries in Example 1.

[0097] The dual-ion batteries prepared in Examples 138-152 were subjected to charge-discharge tests at different rates and long cycle tests at 2 C, and the specific test results are shown in Table 7.

[0098] Table 7 Performance comparison of expanded graphite anodes prepared by different amounts of dilute hydrochloric acid and pickling time

[0099] As can be seen from Table 7, the amount of dilute hydrochloric acid and the pickling time have a great influence on the performance of the prepared expanded graphite anode, and sufficient amount of dilute hydrochloric acid and pickling time can completely remove MgO in the graphite layer.

[0100] Comparative Examples 9-20 The difference between the preparation method of the expanded graphite anode material in Comparative Examples 9-20 and Example 1 is only that an equal amount of oxidizing agent is used instead of the acidic oxidizing agent in Example 1, and an equal amount of intercalation agent is used instead of magnesium chloride hexahydrate in Example 1, as shown in Table 8 below.

[0101] The dual-ion battery in Comparative Examples 9-20 was prepared by referring to the preparation method of the dual-ion battery in Example 1.

[0102] The dual-ion battery prepared in Comparative Examples 9-20 was subjected to charge-discharge test at different rates, and long cycle test at 2 C, and the specific test results are shown in Table 8 below.

[0103] Table 8 Performance comparison of expanded graphite anodes prepared by conventional method

[0104] As can be seen from Table 8, compared with Example 1, the electrochemical performance of the expanded graphite anode material prepared by the conventional method in Comparative Examples 9-20 is significantly reduced.

[0105] The Raman data of the expanded graphite anode material prepared in Example 1 and Comparative Example 15 were tested by Raman testing instrument, and the test results showed that in the expanded graphite anode material prepared in Example 1, in addition to the G peak at 1580 cm -1 nearby and the 2D peak at 2700 cm -1 nearby commonly seen in graphite, a relatively obvious D peak also appeared at 1350 cm -1 nearby, I D / I G was 0.428; while the I D / I G of the expanded graphite anode material prepared by using hydrogen peroxide oxidizing agent and glacial acetic acid intercalation agent in Comparative Example 15 was 0.302, further indicating that the expanded graphite anode material prepared by the preparation method in the present application has a higher degree of disorder, providing more defect sites for anion storage.

[0106] In summary, the application forms the expandable graphite containing metal salt hydrate by fully mixing the metal salt or its hydrate, the graphite source and the acidic oxidant, the metal salt can be inserted between the graphite layers in the case of the acidic oxidant opening the interlayer of graphite, and after the suction filtration, washing and drying, the expandable graphite containing metal salt hydrate between layers is formed. After the thermal expansion treatment of the expandable graphite, the storage kinetics of the anion of the large interlayer spacing expandable graphite positive electrode material is greatly improved, the cycle life is improved while the excellent rate performance is maintained. The preparation process of the expandable graphite positive electrode material in the application is simple, the raw materials are abundant and cheap, and it is suitable for large-scale production and can be widely applied in the energy storage field.

[0107] The above has described the embodiments of the application in detail, but the application is not limited to the above embodiments, and various changes can be made within the knowledge range of the ordinary skill in the art without departing from the purpose of the application. In addition, the embodiments of the application and the features in the embodiments can be combined with each other without conflict.

Claims

1. An expanded graphite cathode material, characterized in that: The raw materials for preparing the expanded graphite cathode material include a graphite source, an acidic oxidant, a metal salt and / or its hydrate; The mass-to-volume ratio of the graphite source to the acidic oxidant is 1 g : (7~25) mL; The mass ratio of the graphite source to the metal salt and / or its hydrate is 1:(0.7~3).

2. The expanded graphite cathode material according to claim 1, characterized in that: The expanded graphite cathode material has a layered structure with a layer spacing of 0.335~0.34nm.

3. The expanded graphite cathode material according to claim 1, characterized in that: The graphite source includes at least one of natural graphite, artificial graphite, and graphite recovered from waste batteries. And / or, the acidic oxidizing agent includes at least one of nitric acid, nitrous acid, sulfuric acid, permanganic acid, hypochlorous acid, chloric acid, chlorous acid, and perchloric acid; And / or, the metal salt comprises at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium chloride, lithium sulfate, lithium carbonate, lithium nitrate, sodium chloride, sodium carbonate, sodium bicarbonate, sodium phosphate, sodium nitrate, sodium phosphite, sodium sulfite, sodium acetate, sodium formate, sodium propionate, sodium acrylate, sodium benzoate, sodium hypochlorite, sodium chlorate, sodium perchlorate, sodium pyrophosphate, sodium thiosulfate, sodium persulfate, potassium chloride, potassium nitrate, potassium sulfate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium carbonate, potassium bicarbonate, magnesium citrate, magnesium glycinate, magnesium chloride, magnesium sulfate, magnesium lactate, magnesium carbonate, magnesium sulfide, magnesium chlorate, calcium gluconate, calcium hydrogen phosphate, calcium lactate, calcium chloride, calcium oxalate, calcium carbonate, calcium phosphate, calcium fluoride, calcium nitrate, calcium chlorate, calcium perchlorate, calcium bicarbonate, and calcium dihydrogen phosphate.

4. The method for preparing the expanded graphite cathode material according to any one of claims 1 to 3, characterized in that: Includes the following steps: The raw materials, including graphite source, acidic oxidant, metal salt and / or its hydrate, are mixed to obtain expandable graphite; The expandable graphite is subjected to thermal expansion treatment to obtain expanded graphite; The expanded graphite is acid-washed to obtain the expanded graphite cathode material.

5. The method for preparing the expanded graphite cathode material according to claim 4, characterized in that: The mixing time is 6 to 48 hours.

6. The method for preparing the expanded graphite cathode material according to claim 4, characterized in that: The temperature of the thermal expansion treatment is 400~1200℃; and / or the time of the thermal expansion treatment is 60~150s.

7. The method for preparing the expanded graphite cathode material according to claim 4, characterized in that: The pickling step has at least one of the following characteristics: (a1) The pickling step is to pickle with acid solution, and the mass-volume ratio of expanded graphite to acid solution is 1g:(15~60)mL; (a2) The pickling time is 6~15h; (a3) The acid solution used in the pickling includes at least one of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid, hydrofluoric acid, oxalic acid, and nitrous acid.

8. The method for preparing the expanded graphite cathode material according to claim 4, characterized in that: The preparation of the expandable graphite also includes the steps of washing, filtration, and drying in sequence; the steps of washing, filtration, and drying are performed after the mixing step.

9. A dual-ion battery, characterized in that: It includes the expanded graphite cathode material according to any one of claims 1 to 3, or the expanded graphite cathode material prepared by the preparation method according to any one of claims 4 to 8.

10. The application of the expanded graphite cathode material according to any one of claims 1 to 3 and / or the preparation method of the expanded graphite cathode material according to any one of claims 4 to 8 in the field of batteries.

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