Pre-sodium hard carbon intermediate buffer film for improving first-week coulomb efficiency of sodium ion battery, preparation method and pre-sodium method

Abstract

The present application relates to a kind of pre-sodium hard carbon intermediate buffer film for improving the first week coulomb efficiency of sodium ion battery and preparation method and pre-sodium method.The hard carbon is considered as a kind of extremely commercial value sodium ion battery negative electrode material due to having good structural stability and low temperature performance and high sodium storage specific capacity.But the lower first coulomb efficiency and specific capacity of hard carbon material limit its industrial application in sodium ion battery.Therefore, we first propose by regulating the different proportion of hard carbon and soft carbon, thereby preparing the pre-sodium hard carbon intermediate film with reasonable structure, porous structure, adjustable electronic and ion channel, the first week coulomb efficiency of sodium ion battery applied is as high as 121.1%, and the battery has good cycle and rate performance.The sodium ion battery prepared by the method has the advantages of high first week coulomb efficiency, good cycle life, cost advantage, simple operation, suitable for industrial production and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of sodium ion batteries, and relates to a pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method thereof and a method for pre-sodium of a sodium ion battery negative electrode sheet. BACKGROUND

[0002] Due to limited lithium resource reserves and uneven distribution, the cost of metal lithium is continuously increasing, thereby limiting the further development of lithium ion batteries, especially the large-scale application in the energy storage field. The energy storage mechanism of sodium ion batteries is similar to that of lithium ion batteries, and sodium resources are abundant and low in cost, so sodium ion batteries are expected to replace lithium ion batteries and be widely applied in large-scale energy storage fields.

[0003] Hard carbon is considered to be a highly commercial sodium ion battery negative electrode material due to its good structural stability, low temperature performance and high sodium storage specific capacity. The structure of hard carbon material is usually complex, and most of the internal structure is disordered amorphous structure, defects and various pore structures. When the hard carbon material is used as a sodium ion battery negative electrode material, the binding energy between the internal porous structure or defect site and the sodium ion is too large, the sodium ion cannot undergo reversible reaction after electrochemical reaction, and irreversible loss of sodium ion occurs, thereby causing the first cycle coulombic efficiency of the hard carbon material in the sodium ion battery to be low. On the other hand, since most hard carbon materials have rich pore structures, they have a large specific surface area, and during the first charge and discharge of the sodium ion battery, the electrolyte will decompose on the surface of the hard carbon material and a series of side reactions will occur, forming an unstable solid electrolyte (SEI) film, thereby consuming a large amount of sodium ions and causing the first cycle coulombic efficiency of the sodium ion battery to be low. In a sodium ion full battery system, all irreversible sodium ions in the system are completely provided by the positive electrode material, and the amount of positive electrode material in the full battery is limited, so in this case, the first cycle coulombic efficiency of the negative electrode material plays a crucial role in improving the energy density and cycle life of the battery. Therefore, the first cycle coulombic efficiency of the sodium ion battery directly determines its commercialization process.

[0004] In order to improve the first cycle coulombic efficiency of the sodium ion battery, a pre-sodium method can be used. The current common pre-sodium methods for sodium ion batteries mainly include the following: in-situ doping pre-sodium, electrochemical pre-sodium, chemical pre-sodium and contact pre-sodium. Among them, the contact pre-sodium method is to directly contact the metal sodium sheet with the battery negative electrode sheet for pre-sodium, the electrolyte is used to wet the interface between the sodium sheet and the negative electrode sheet, and the contact pressure between the sodium sheet and the negative electrode sheet is adjusted by applying an external force, so as to control the depth and uniformity of the pre-sodium. This method is conducive to controlling the pre-sodium degree of the electrode, but the production process conditions are high and it is not easy to scale up. SUMMARY

[0005] Technical problems to be solved

[0006] In order to avoid the shortcomings of the prior art, the present application provides a pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method and a pre-sodium method, and a preparation method and application of a pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery which can be prepared on a commercial scale. By adjusting the different proportions of hard carbon and soft carbon, a pre-sodium hard carbon intermediate film with reasonable structure, porous structure, adjustable electron and ion channels is prepared, which has a first cycle coulombic efficiency of up to 121.1% when applied to a sodium ion battery, and the battery has good cycle and rate performance. The sodium ion battery prepared by the method has the advantages of high first cycle coulombic efficiency, good cycle life, cost advantage, simple operation and suitability for industrial production.

[0007] Technical scheme

[0008] A pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, characterized in that the mass ratio of hard carbon to soft carbon is 7:3; the hard carbon is obtained by carbonizing phenolic resin in an inert atmosphere at 600-900°C; and the soft carbon is obtained by carbonizing pitch in an inert atmosphere at 600-900°C.

[0009] The pitch is one or more of different softening point coal pitch or petroleum pitch.

[0010] A method for preparing the pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, characterized by the following steps:

[0011] Step 1: Dissolve pitch with different softening points, phenolic resin and toluene, stir at room temperature for 1-10 hours, then filter to obtain a solution, and then distill the solution under reduced pressure to obtain a precursor of porous hard carbon material;

[0012] The ratio of the pitch, phenolic resin and toluene is 1-10g:1-30g:40-300ml;

[0013] Step 2: Mix the precursor of the porous hard carbon material with an activating agent and a carbonate salt in a mass ratio of 1-10:1-10:1-20, then move the mixed powder into an alumina crucible, and calcine at a temperature of 600-1000°C for 1-5h in a nitrogen or Ar atmosphere; wash with 0.1-5mol / L hydrochloric acid to remove excess activating agent and carbonate salt, wash with deionized water until the pH is 7, and then dry in a 100°C oven to obtain a porous hard carbon material

[0014] Step 3: The porous hard carbon, sodium salt, film-forming additive, and organic solvent are mixed in a mass ratio of 1-10:1-10:1-30:1-50, and stirred for 2-6 hours to form a viscous slurry. The viscous slurry is poured into a polytetrafluoroethylene (PTFE) container, and dried in a vacuum at 80-120°C for 6-12 hours to obtain a pre-sodium hard carbon intermediate buffer film with a porous structure and adjusted electron and ion channels.

[0015] The activator is selected from KOH, NaOH, H3PO4, ZnCl2; and the carbonate is selected from Na2CO3, NaHCO3.

[0016] The sodium salt is selected from at least one of NaTFSI, NaClO4, NaBF4, and NaDFOB.

[0017] The film-forming additive is selected from at least one of PVDF, PEO, PVP, PAN, PC, and PMMA.

[0018] In step 1, the stirring speed of the toluene solution is 500-1000 r / min, and the temperature for reduced pressure distillation treatment is 60-100°C, with a stirring speed of 300-1000 r / min.

[0019] In step 2, the mixing is performed by ball milling the powder at a speed of 400-800 r / min for 2-24 hours; and step 2 is heated in a tube furnace at a heating rate of 10°C / min. -1

[0020] The organic solvent in step 3 is selected from at least one of N,N-dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, dimethyl sulfoxide, chloroform, and toluene.

[0021] A method for pre-sodium of a sodium-ion battery negative electrode sheet using the hard carbon intermediate buffer film prepared by the preparation method, characterized in that: under an inert atmosphere, the hard carbon negative electrode sheet, the hard carbon intermediate buffer film, and the sodium foil are placed in order from bottom to top, 10-100 μL of electrolyte is added on the surface of the hard carbon negative electrode sheet, and pre-sodium is performed under a pressure of 1-10 kg, and the pre-sodium time is 0.1-10 hours.

[0022] Advantages

[0023] ​The application provides a pre-sodiumized hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method and a pre-sodiumization method. The application provides a pre-sodiumized hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method and a pre-sodiumization method. The application provides a pre-sodiumized hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method and a pre-sodiumization method.

[0024] The application provides a pre-sodiumized hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, a preparation method and a pre-sodiumization method. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 SEM microstructure of the porous hard carbon material prepared from the precursors with different proportions in the comparative example 1;

[0026] Figure 2 Defect degree of the porous hard carbon material prepared from the precursors with different proportions in the comparative example 1;

[0027] Figure 3 Pore structure of the hard carbon material prepared from the precursors with different proportions in the comparative example 1;

[0028] Figure 4 SEM cross-sectional view of the pre-sodiumized hard carbon intermediate buffer film prepared in the application example 2;

[0029] Figure 5 Schematic diagram of pre-sodiumization treatment of the hard carbon negative electrode sheet of the sodium ion battery in the application example 3;

[0030] Figure 6 Ion conductivity and electronic conductivity of the hard carbon intermediate buffer films prepared from sodium salts with different contents in the comparative examples 1-4;

[0031] Figure 7 The first circle coulombic efficiency graph of the hard carbon negative electrode sheet in the coin half-cell after pre-sodium of 0, 3, 6, 9 minutes in Comparative Example 5; DETAILED DESCRIPTION

[0032] The application will be further described in conjunction with examples and drawings:

[0033] The structure of the hard carbon material is generally complex, and most of the internal structure is disordered amorphous structure, defects and various pore structures. When it is used as a negative electrode material of a sodium ion battery, the binding energy between the internal pore structure or defect site and the sodium ion is too large, the sodium ion cannot undergo a reversible reaction after electrochemical reaction, and the irreversible loss of sodium ion is formed, thus causing the first coulombic efficiency of the hard carbon material in the sodium ion battery to be low. On the other hand, since most of the hard carbon material has a rich pore structure, it has a large specific surface area, and the electrolyte will decompose on the surface of the hard carbon material during the first charge and discharge of the sodium ion battery, a series of side reactions will occur, and an unstable solid electrolyte (SEI) film will be formed, thereby consuming a large amount of sodium ions, resulting in a low first cycle coulombic efficiency of the sodium ion battery. In the sodium ion full battery system, all the irreversible sodium ions in the system are completely provided by the positive electrode material, and the amount of the positive electrode material in the full battery is limited, so in this case, the first cycle coulombic efficiency of the negative electrode material plays a crucial role in improving the energy density and cycle life of the battery. Therefore, the first cycle coulombic efficiency of the sodium ion battery directly determines its commercialization process. Based on this, the present application first proposes to adjust the different proportions of hard carbon and soft carbon to prepare a pre-sodium hard carbon intermediate film with a reasonable structure, a porous structure, adjustable electronic and ionic channels, and a high first cycle coulombic efficiency of up to 121.1% when applied to a sodium ion battery. The sodium ion battery prepared by the method has the advantages of high first cycle coulombic efficiency, good cycle life, cost advantage, simple operation, and is suitable for industrial production.

[0034] The pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, characterized in that the mass ratio of hard carbon to soft carbon is 7:3, the hard carbon is obtained by carbonizing phenolic resin in an inert atmosphere (nitrogen, argon, helium, etc.) at 600-900℃; and the soft carbon is obtained by carbonizing pitch (petroleum pitch, coal pitch) with different softening points (118℃, 150℃, 250℃, 280℃) in an inert atmosphere (nitrogen, argon, helium, etc.) at 600-900℃.

[0035] The method for preparing the pre-sodium hard carbon intermediate buffer film for improving the first cycle coulombic efficiency of a sodium ion battery, characterized in that the steps are as follows:

[0036] Step 1: Dissolve the asphalt (118℃, 150℃, 250℃, 280℃), phenolic resin (70℃, 85℃, 110℃) with different softening points and toluene, stir at room temperature for 1-10 hours, then filter to obtain the solution, and then distill the solution under reduced pressure to obtain the precursor of the porous hard carbon material;

[0037] The ratio of asphalt, phenolic resin and toluene is 1-10g:1-30g:40-300ml;

[0038] Step 2: Mix the precursor of the porous hard carbon material with the activator and carbonate in a mass ratio of 1-10:1-10:1-20, then move the mixed powder into an alumina crucible, and calcine at a temperature of 600-1000℃ under nitrogen or Ar atmosphere for 1-5h; clean with 0.1-5mol / L hydrochloric acid to remove excess activator and carbonate, wash with deionized water until PH=7, and then dry in a 100℃ oven to obtain the porous hard carbon material

[0039] Step 3: Mix the porous hard carbon, sodium salt, film-forming additive and organic solvent uniformly in a mass ratio of 1-10:1-10:1-30:1-50, stir for 2-6 hours to form a viscous slurry, pour the viscous slurry into a polytetrafluoroethylene PTFE container, and dry in a vacuum at 80-120℃ for 6-12 hours to obtain a pre-sodium hard carbon intermediate buffer film with a porous structure and adjusted electronic and ionic channels.

[0040] Example 1

[0041] 1) Preparation of the precursor of the porous hard carbon material: mix asphalt and phenolic resin in a mass ratio of 1:0 / 9:1 / 8:2 / 7:3 / 5:5 / 0:1 with 50ml toluene, stir at room temperature for 2 hours, then filter to obtain the solution, and then distill the solution under reduced pressure to obtain the precursor of the porous hard carbon material;

[0042] 2) Preparation of the porous hard carbon material: mix the precursor of the porous hard carbon material with NaOH and Na2CO3 in a ratio of 3:3:1 by ball milling for 1 hour, then move the mixed powder into an alumina crucible, and heat to 900℃ at a heating rate of 10℃ / min in a tube furnace under nitrogen (Ar) atmosphere for 2 hours, cool down with the furnace, then rinse with 2mol / L HCl solution, wash with deionized water until PH=7, and dry in a 100℃ oven to obtain the porous hard carbon material. -1 -1 2) Preparation of the porous hard carbon material: mix the precursor of the porous hard carbon material with NaOH and Na2CO3 in a ratio of 3:3:1 by ball milling for 1 hour, then move the mixed powder into an alumina crucible, and heat to 900℃ at a heating rate of 10℃ / min in a tube furnace under nitrogen (Ar) atmosphere for 2 hours, cool down with the furnace, then rinse with 2mol / L HCl solution, wash with deionized water until PH=7, and dry in a 100℃ oven to obtain the porous hard carbon material.

[0043] ​3) Preparation of pre-sodium hard carbon intermediate buffer film: porous hard carbon, NaClO4 and PVDF are dispersed in 4ml N-methyl pyrrolidone in a ratio of 5:1:2 to form a viscous slurry after stirring for 6 hours, the viscous slurry is dispersed and cast in a polytetrafluoroethylene (PTFE) mold, dried in a vacuum at 90℃ for 12 hours, and a pre-sodium hard carbon intermediate buffer film with a thickness of 271μm is obtained.

[0044] 4) The negative electrode material current collector of the sodium ion battery is a pure copper foil with a thickness of 12μm, the negative electrode active material is a commercial hard carbon material, and it also contains a conductive agent carbon black, CMC (sodium carboxymethyl cellulose), SBR (styrene butadiene rubber); wherein the mass ratio of the commercial hard carbon material, the conductive agent carbon black, CMC, SBR and CNT (carbon nanotube) is 91:4.9:1.5:2.5:0.1.

[0045] The microstructure of the porous hard carbon material prepared by using different proportions of precursors in Comparative Example 1 is shown in Figure 1 , the material defect degree is shown in Figure 2 , and the pore structure of the hard carbon material prepared by different proportions of precursors is shown in Figure 3 .

[0046] Example 2

[0047] 1) Preparation of precursor of porous hard carbon material: 7g of pitch, 3g of phenolic resin and 50ml of toluene are mixed, stirred at room temperature for 2 hours, then filtered to obtain a solution, and then the solution is distilled under reduced pressure to obtain the precursor of the porous hard carbon material;

[0048] 2) Preparation of porous hard carbon material: the precursor of the porous hard carbon material is ball milled with NaOH and Na2CO3 in a ratio of 3:3:1 for 1 hour, and then the mixed powder is moved into an alumina crucible, heated to 900℃ at a heating rate of 10℃ / min in a tube furnace under nitrogen (Ar) atmosphere, and kept for 2 hours, then cooled with the furnace, washed with 2mol / L HCl solution, washed with deionized water until PH=7, and dried in a 100℃ oven to obtain the porous hard carbon material. -1 -1

[0049] 3) Preparation of pre-sodium hard carbon intermediate buffer film: porous hard carbon, NaClO4 and PVDF are dispersed in 4ml N-methyl pyrrolidone in a ratio of 5:2:2 to form a viscous slurry after stirring for 6 hours, the viscous slurry is dispersed and cast in a polytetrafluoroethylene (PTFE) mold, dried in a vacuum at 90℃ for 12 hours, and a pre-sodium hard carbon intermediate buffer film with a thickness of 271μm is obtained, the SEM cross-section of which is shown in Figure 4 .

[0050] ​​4) The current collector of the sodium-ion battery negative electrode material is a pure copper foil with a thickness of 12μm. The negative electrode active material is a commercial hard carbon material, which also includes conductive carbon black, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber). The mass ratio of commercial hard carbon material, conductive carbon black, CMC, SBR and CNT (carbon nanotubes) is 91:4.9:1.5:2.5:0.1.

[0051] Example 3

[0052] 1) Preparation of precursor of porous hard carbon material: 7g asphalt, 3g phenolic resin and 50ml toluene were mixed and stirred at room temperature for 2 hours. After filtration, a solution was obtained. Then the solution was distilled under reduced pressure to obtain the precursor of porous hard carbon material.

[0053] 2) Preparation of porous hard carbon materials: The precursor of porous hard carbon materials was ball-milled with NaOH and Na2CO3 in a ratio of 3:3:1 for 1 hour at a ball mill speed of 400 r / min. The mixed powder was then transferred to an alumina crucible and heated in a tube furnace under a nitrogen (Ar) atmosphere at 10 °C for 1 minute. -1 Heating rate: Heat to 900℃ and hold for 2 hours; after furnace cooling, use 2 mol L... -1 The material was rinsed and filtered with HCl solution, washed with deionized water until pH=7, and dried in an oven at 100℃ to obtain porous hard carbon material.

[0054] 3) Preparation of pre-sodium hard carbon intermediate buffer membrane: Porous hard carbon, NaClO4 and PVDF were dispersed in 4 ml of N-methylpyrrolidone in a ratio of 5:3:2 and stirred for 6 hours to form a viscous slurry. The viscous slurry was dispersed and poured into a polytetrafluoroethylene (PTFE) mold and dried in vacuum at 90°C for 12 hours to obtain a pre-sodium hard carbon intermediate buffer membrane with a thickness of 271 μm.

[0055] 4) The current collector of the sodium-ion battery negative electrode material is a pure copper foil with a thickness of 12μm. The negative electrode active material is a commercial hard carbon material, which also includes conductive carbon black, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber). The mass ratio of commercial hard carbon material, conductive carbon black, CMC, SBR and CNT (carbon nanotubes) is 91:4.9:1.5:2.5:0.1.

[0056] Under inert atmosphere conditions, see Figure 5 As shown, a sodium-ion negative electrode, a pre-sodium-treated hard carbon intermediate buffer film, and a sodium sheet are placed sequentially from bottom to top. 50 μL of electrolyte is added to the interface of the sodium-ion negative electrode and pre-sodium-treated for 6 minutes under a pressure of 5 kg to finally obtain the pre-sodium-treated hard carbon negative electrode material.

[0057] Example 4

[0058] 1) Preparation of precursor of porous hard carbon material: 7g asphalt, 3g phenolic resin and 50ml toluene were mixed and stirred at room temperature for 2 hours. After filtration, a solution was obtained. Then the solution was distilled under reduced pressure to obtain the precursor of porous hard carbon material.

[0059] 2) Preparation of porous hard carbon materials: The precursor of porous hard carbon materials was ball-milled with NaOH and Na2CO3 in a ratio of 3:3:1 for 1 hour at a ball mill speed of 400 r / min. The mixed powder was then transferred to an alumina crucible and heated in a tube furnace under a nitrogen (Ar) atmosphere at 10 °C for 1 minute. -1 Heating rate: Heat to 900℃ and hold for 2 hours; after furnace cooling, use 2 mol L... -1 The material was rinsed and filtered with HCl solution, washed with deionized water until pH=7, and dried in an oven at 100℃ to obtain porous hard carbon material.

[0060] 3) Preparation of pre-sodium-treated hard carbon intermediate buffer membrane: Porous hard carbon, NaClO4, and PVDF were dispersed in 4 ml of N-methylpyrrolidone at a ratio of 5:1:2 / 5:2:2 / 5:3:2 / 5:4:2 and stirred for 6 hours to form a viscous slurry. The viscous slurry was then poured into a polytetrafluoroethylene (PTFE) mold and dried in a vacuum at 90°C for 12 hours. The ionic conductivity and electronic conductivity of the hard carbon intermediate buffer membranes prepared using different sodium salt contents in this example are shown in the columnar diagram. Figure 6 As shown, from Figure 6 It can be seen that as the amount of sodium salt added increases, the electronic conductivity of the hard carbon intermediate buffer membrane gradually increases, while the ionic conductivity gradually decreases.

[0061] Example 5

[0062] 1) Preparation of precursor of porous hard carbon material: 7g asphalt, 3g phenolic resin and 50ml toluene were mixed and stirred at room temperature for 2 hours. After filtration, a solution was obtained. Then the solution was distilled under reduced pressure to obtain the precursor of porous hard carbon material.

[0063] 2) Preparation of porous hard carbon materials: The precursor of porous hard carbon materials was ball-milled with NaOH and Na2CO3 in a ratio of 3:3:1 for 1 hour at a ball mill speed of 400 r / min. The mixed powder was then transferred to an alumina crucible and heated in a tube furnace under a nitrogen (Ar) atmosphere at 10 °C for 1 minute. -1 Heating rate: Heat to 900℃ and hold for 2 hours; after furnace cooling, use 2 mol L... -1 The material was rinsed and filtered with HCl solution, washed with deionized water until pH=7, and dried in an oven at 100℃ to obtain porous hard carbon material.

[0064] 3) Preparation of pre-sodiation hard carbon intermediate buffer film: porous hard carbon, NaClO4 and PVDF were dispersed in 4 ml of N-methyl pyrrolidone in a ratio of 5:3:2 and stirred for 6 hours to form a viscous slurry, the viscous slurry was dispersed and cast in a polytetrafluoroethylene (PTFE) mold, and dried in a vacuum at 90°C for 12 hours.

[0065] The present embodiment provides the application of the pre-sodiation hard carbon intermediate buffer film prepared by the above method in the hard carbon negative electrode of sodium ion battery, specifically:

[0066] Under the condition of inert atmosphere, the sodium battery negative electrode sheet, the pre-sodiation hard carbon intermediate buffer film and the sodium sheet were placed in order from bottom to top, 50 μL of electrolyte was added at the interface of the sodium battery negative electrode sheet, and pre-sodiation was carried out for 0, 3, 6 and 9 minutes under the condition of 5 kg pressure, and finally the pre-sodiation hard carbon negative electrode material was obtained. Figure 7 The first circle charge-discharge curve diagram of the hard carbon negative electrode sheet provided by the present embodiment after pre-sodiation for 0, 3, 6 and 9 minutes was obtained, and it can be seen from the diagram that the first circle coulombic efficiency of the hard carbon material pre-sodiated for 9 minutes was the best, which was 121.1%.

Claims

1. A pre-sodiated hard carbon interlayer buffer film for improving the first cycle coulombic efficiency of a sodium-ion battery, characterized by The mass ratio of the hard carbon and the soft carbon is 7:3; the hard carbon is obtained by carbonizing phenolic resin in an inert atmosphere at 600-900 DEG C; and the soft carbon is obtained by carbonizing pitch in an inert atmosphere at 600-900 DEG C; The pre-sodium hard carbon intermediate buffer film has a porous structure and adjusts electron and ion channels. The preparation method of the pre-sodium hard carbon intermediate buffer film comprises: Step 1: dissolving pitch, phenolic resin and toluene with different softening points, stirring at room temperature for 1-10 hours, and then obtaining a solution after filtration, and then obtaining a precursor of the porous hard carbon material by distilling the solution under reduced pressure; The ratio of the pitch, the phenolic resin and the toluene is 1-10 g: 1-30 g: 40-300 ml; Step 2: mixing the precursor of the porous hard carbon material with an activating agent and a carbonate salt at a mass ratio of 1-10: 1-10: 1-20, then moving the mixed powder into an alumina crucible, and then baking the powder in a nitrogen or Ar atmosphere at a temperature of 600-1000 DEG C for 1-5 hours; cleaning with 0.1-5 mol / L hydrochloric acid to remove excess activating agent and carbonate salt, washing with deionized water until PH=7, and then drying in a 100 DEG C oven to obtain the porous hard carbon material; Step 3: uniformly mixing the porous hard carbon, a sodium salt, a film-forming additive and an organic solvent at a mass ratio of 1-10: 1-10: 1-30: 1-50, stirring for 2-6 hours to form a viscous slurry, pouring the viscous slurry into a polytetrafluoroethylene PTFE container, and drying the viscous slurry in a vacuum at 80-120 DEG C for 6-12 hours to obtain the pre-sodium hard carbon intermediate buffer film with a porous structure and adjusting electron and ion channels.

2. The pre-sodiated hard carbon interlayer buffer film for improving the first cycle coulombic efficiency of a sodium-ion battery according to claim 1, characterized in that: The pitch is one or more of different softening point coal pitch or petroleum pitch.

3. The pre-sodded hard carbon interlayer buffer film for improving the first cycle coulombic efficiency of sodium ion batteries according to claim 1, characterized in that: The activating agent is selected from KOH, NaOH, H3PO4 and ZnCl2; and the carbonate salt is selected from Na2CO3 and NaHCO3.

4. The pre-sodded hard carbon interlayer of claim 1, wherein: The sodium salt is at least one of NaTFSI, NaClO4, NaBF4 and NaDFOB.

5. The pre-sodded hard carbon interlayer of claim 1, wherein: The film-forming additive is at least one of PVDF, PEO, PVP, PAN, PC and PMMA.

6. The pre-sodded hard carbon interlayer of claim 1, wherein: In step 1, the stirring speed is 500-1000 r / min when dissolving with toluene; and the temperature of the reduced pressure distillation treatment is 60-100 DEG C.

7. The pre-sodded hard carbon interlayer of claim 1, wherein: The step 2 mixing: ball-milling the powder at a speed of 400-800 r / min for 2-24 h; the step 2 heating in a tube resistance furnace at a heating speed of 10 ℃min -1 .

8. The pre-sodded hard carbon interlayer of claim 1, wherein: In step 3, the organic solvent is at least one of N,N-dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, dimethyl sulfoxide, chloroform and toluene.

9. A method for pre-sodiation of a sodium-ion battery negative electrode sheet using a hard carbon intermediate buffer film prepared by the preparation method of any one of claims 1-8, characterized in that: In an inert atmosphere, sequentially placing a hard carbon negative electrode sheet, a hard carbon intermediate buffer film and a sodium foil from bottom to top, and dropping 10-100 μL of electrolyte on the surface of the hard carbon negative electrode sheet, and then pre-sodium under a pressure of 1-10 kg, and the pre-sodium time is 0.1-10 hours.

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