Modified negative electrode current collector for sodium metal battery and preparation method of modified negative electrode current collector

Abstract

The invention provides a modified negative electrode current collector for a sodium metal battery and a preparation method of the modified negative electrode current collector, and belongs to the technical field of secondary batteries. The modified negative electrode current collector comprises a metal substrate and an electron-ion composite conductor coating, the electron-ion composite conductor coating is located on the surface of the metal substrate, and the electron-ion composite conductor coating is obtained by compounding any one of a zeolite molecular sieve and an expansive soil material with a graphitized carbon material. The method disclosed by the invention is simple and feasible, the obtained modified negative electrode current collector can be widely applied to a high-energy-density negative-electrode-free sodium metal battery system, and due to the fact that sodium metal deposition is more uniform and stable, compared with a negative electrode current collector modified only by a graphitized carbon material, the modified negative electrode current collector has better cycling stability.

Description

TECHNICAL FIELD

[0001] The application relates to a modified negative electrode current collector for a sodium metal battery and a preparation method thereof, and belongs to the technical field of secondary batteries. BACKGROUND

[0002] Traditional lithium ion and sodium ion batteries both rely on ion storage in carbon materials, which greatly limits the energy density of the batteries. A negative electrode-free sodium metal battery refers to a battery that only relies on a negative electrode current collector and does not use sodium metal before battery assembly, and the active sodium is entirely from a sodium-containing positive electrode material or a pre-sodiumized material. The negative electrode-free sodium metal battery has only a copper foil or an aluminum foil as a current collector on the negative electrode side (i.e., there is no active negative electrode material in the battery), and ions do not flow to the negative electrode, but sodium is deposited in a metallic form on the copper foil or the aluminum foil of the negative electrode current collector in situ during charging, and is converted into sodium ions again and migrates to the positive electrode during discharging. The negative electrode-free sodium metal battery has the advantages of high energy density, low battery production cost, safe production process, and the like, and is a future leading technology that surpasses current lithium ion batteries.

[0003] However, since the negative electrode does not provide active substances, the reversibility of sodium metal on the negative electrode current collector is required to be high. The applicant finds that, at present, through the compounding of an ether electrolyte, the coulombic efficiency of sodium metal on the aluminum foil current collector can be ensured to be more than 99%, but the high reversibility and dendrite-free sodium metal deposition morphology under high current and high capacity conditions cannot be met, which seriously affects the cycle life of the high-energy-density negative electrode-free sodium metal battery. SUMMARY

[0004] Therefore, the application provides a modified negative electrode current collector for a sodium metal battery. The electronic-ion composite conductor coating cooperatively improves the sodium metal deposition and dissolution process under large current and large capacity, improves the coulombic efficiency of the sodium metal negative electrode deposition and dissolution, ensures the long-life cycle stability of the negative electrode-free sodium metal battery under working conditions, and enables the modified negative electrode current collector to be widely applied to a high-energy-density negative electrode-free sodium metal battery system with an energy density of 170-350 Wh / kg.

[0005] Specifically, the application is implemented through the following scheme.

[0006] A modified negative electrode current collector for a sodium metal battery, the modified negative electrode current collector comprising a metal substrate and an electronic-ion composite conductor coating, the electronic-ion composite conductor coating being located on the surface of the metal substrate, the electronic-ion composite conductor coating being obtained by compounding any one of a zeolite molecular sieve and an expanded soil material with a graphitized carbon material, the zeolite molecular sieve or the expanded soil material:graphitized carbon material = 0.3-3:1.

[0007] The modified negative electrode current collector of the present application is composed of an aluminum metal substrate and an electronic-ion composite conductor coating on the surface, wherein the graphitized carbon material with high electronic conductivity is composed, which significantly improves the surface conductivity of the aluminum current collector and provides abundant nucleation sites for sodium metal deposition, promoting the uniform and dense deposition and stripping of sodium metal during charging and discharging; the zeolite molecular sieve or expanded soil material with high ionic conductivity can reduce the desolvation energy of the electrolyte at the interface, accelerate the ion transport capacity at the interface, and synergistically improve the deposition and stripping efficiency of sodium metal.

[0008] Further, as preferred:

[0009] The zeolite molecular sieve is at least one of natural clinoptilolite, mordenite, synthetic A-type zeolite, ZSM-5, X-type, and Y-type zeolite.

[0010] The expanded soil material is at least one of montmorillonite and kaolinite.

[0011] The graphitized carbon material is one or more of natural graphite, artificial graphite, partially high-temperature treated hard carbon (graphitized hard carbon), carbon black, acetylene black, mesocarbon microbeads (MCMB), carbon nanotubes (CNTs), graphene and its derivative materials.

[0012] The thickness of the electronic-ion composite conductor coating is 10 nm to 10 μm.

[0013] The sodium metal battery is a negative electrode-free sodium metal battery, which includes a modified negative electrode current collector, a positive electrode, a separator and an electrolyte. In the negative electrode-free battery system, the high efficiency of sodium deposition and stripping on the current collector is required, compared with the conventional aluminum foil current collector and the published functionalized current collector, the electronic-ion composite conductor coating of the present application synergistically improves the sodium metal deposition and stripping process under high current and large capacity, and the obtained modified negative electrode current collector can be widely used in high energy density negative electrode-free sodium metal battery system.

[0014] The positive electrode active material of the positive electrode is at least one of sodium iron pyrophosphate, sodium iron sulfate, sodium vanadium phosphate, sodium vanadium fluorophosphate, layered oxide, Prussian blue, Prussian white and the like. The positive electrode active material, conductive carbon black and polyvinylidene fluoride binder are uniformly mixed in a mass ratio of 9:0.5:0.5, fully stirred and mixed uniformly, coated on the surface of the aluminum foil, and the positive electrode is obtained.

[0015] The electrolyte is an organic solvent electrolyte of electrolyte salt, the organic solvent is at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and the electrolyte salt is one or more of sodium tetrafluoroborate, sodium hexafluorophosphate, sodium triflate, sodium bisoxalate borate, sodium difluoro oxalate borate, sodium difluoro bisoxalate phosphate, sodium tetrafluoro oxalate phosphate, sodium bis-trifluoromethylsulfonimide, sodium bisfluorosulfonylimide and sodium perchlorate.

[0016] The separator is at least one of a polypropylene (PP) separator, a polyethylene (PE) separator, or a composite separator modified by a ceramic coating such as aluminum oxide or zirconium oxide.

[0017] The metal substrate is an aluminum foil.

[0018] A preparation method of the modified negative electrode current collector for the sodium metal battery, wherein a zeolite molecular sieve or an expanded soil material is crushed into nodules, then subjected to vacuum heat treatment at 120-150 DEG C for 8-12 hours to remove impurity gases in the pores, and then mixed with graphitized carbon material and a binder to obtain a modified negative electrode current collector.

[0019] The binder is one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinyl alcohol (PVA) and polyacrylate emulsion (SBR).

[0020] In the coating, the mass fraction of the binder is 1-20%, and the remaining 80-99% is an active material composed of the zeolite molecular sieve or the expanded soil material and the graphitized carbon material.

[0021] The modified negative electrode current collector, the separator and the positive electrode are stacked in sequence, and the electrolyte is injected to obtain a negative electrode-free sodium metal battery.

[0022] The working principle and beneficial effects of the negative electrode-free sodium metal battery are summarized as follows:

[0023] The modified negative electrode current collector for the negative electrode-free sodium metal battery of the application comprises a metal substrate and a layer of electron-ion composite conductor coating modified thereon, which is composed of graphitized carbon material with high electron conductivity, significantly improving the surface conductivity of the aluminum current collector and providing abundant nucleation sites for sodium metal deposition, promoting the uniform and dense deposition and stripping of sodium metal during charging and discharging; zeolite molecular sieve or expanded soil material with high ion conductivity can reduce the desolvation energy of the electrolyte at the interface and accelerate the ion transport capacity at the interface, thereby synergistically improving the deposition and stripping efficiency of sodium metal. The method of the application is simple and feasible, and the obtained modified negative electrode current collector can be widely used in high-energy-density negative electrode-free sodium metal battery systems, and has better cycle stability than the negative electrode current collector modified only with graphitized carbon material due to the more uniform and stable sodium metal deposition. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Figure 1 is a structural schematic diagram of the negative electrode-free sodium metal battery of the application,

[0025] Reference numerals in the figure:

[0026] 1. positive electrode; 2. separator; 3. modified negative electrode current collector; 31. metal substrate; 32. electron-ion composite conductor coating. DETAILED DESCRIPTION

[0027] The following examples are further illustrations of the application, but do not limit the scope of the application.

[0028] Example 1

[0029] The modified negative electrode current collector for the sodium metal battery of the present embodiment comprises a metal substrate 31 and an electron-ion composite conductor coating 32, the metal substrate 31 is an aluminum foil, and the electron-ion composite conductor coating 32 is coated with a mixture of zeolite molecular sieve and graphitized carbon material as active material and a binder. Figure 1

[0030] Among them, the zeolite molecular sieve is 3A type zeolite, the feeding proportion is 45%; the graphitized carbon material is conductive graphite, the feeding proportion is 45%; and the binder is a CMC-SBR mixture, the feeding proportion is 10%.

[0031] The preparation process of the above modified negative electrode current collector is as follows:

[0032] Step one, take the zeolite molecular sieve, mill it for 120 minutes to obtain uniform micron-sized powder particles, and then transfer it to a high-temperature oven at 250°C for 12 hours to fully remove impurity gases in the pores;

[0033] ​Step two, the zeolite molecular sieve after heat treatment in step one is fully stirred with graphitized carbon material and binder, and after uniform mixing, the obtained slurry is uniformly coated on the surface of the metal substrate aluminum foil by using a scraper or an automatic coating device. After the solvent is volatilized, the modified negative electrode current collector is obtained.

[0034] In this embodiment, the thickness of the metal substrate 31 is 10 pm, and the thickness of the electron-ion composite conductor coating 32 is 3 pm.

[0035] The modified negative electrode current collector 3, the positive electrode 1, and the separator 2 obtained by modifying the above scheme are sequentially stacked as shown in Figure 1 The side of the electron-ion composite conductor coating 32 is close to the separator 2, and then the electrolyte is filled to obtain the negative electrode-free sodium metal battery.

[0036] Wherein:

[0037] The positive electrode active material sodium iron phosphate pyrophosphate of the sodium ion battery is uniformly mixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 9:0.5:0.5, fully stirred and mixed uniformly, and coated on one side surface of the aluminum foil to obtain the positive electrode sheet.

[0038] The separator is a polypropylene (PP) separator.

[0039] The composition of the electrolyte includes: a sodium tetrafluoroborate ethylene glycol dimethyl ether solution, and the mass concentration of sodium tetrafluoroborate is 10%.

[0040] Comparative Example 1

[0041] This comparative example has the same settings as Example 1, except that the negative electrode current collector of the negative electrode-free sodium metal battery is an aluminum foil.

[0042] Comparative Example 2

[0043] This comparative example has the same settings as Example 1, except that the modification process of the modified negative electrode current collector is as follows:

[0044] The graphitized carbon material and the binder are fully stirred at a mass ratio of 90:10, and after uniform mixing, the obtained slurry is uniformly coated on the surface of the metal substrate aluminum foil by using a scraper or an automatic coating device. After the solvent is volatilized, the modified negative electrode current collector is obtained.

[0045] Comparative Example 3

[0046] This comparative example has the same settings as Example 1, except that the modification process of the modified negative electrode current collector is as follows:

[0047] Step one, after ball milling zeolite molecular sieve for 120 min, micron-sized uniform powder particles are obtained, and the particles are transferred into a high-temperature oven at 250°C for 12 hours to fully remove impurity gases in the pores.

[0048] Step two, the treated zeolite molecular sieve, binder were fully stirred according to the mass ratio of 90:10, after mixing evenly, the obtained slurry was uniformly coated on the surface of the metal substrate aluminum foil by scraper or automatic coating equipment, and the modified negative electrode current collector was obtained after the solvent was volatilized.

[0049] The negative electrode current collectors of the above example 1 and comparative examples 1-3 and the negative electrode-free sodium metal batteries applied were tested for performance, and the results were as follows:

[0050] 1) Detection method or standard:

[0051] (1) Cycle performance test: charge to the upper limit cutoff voltage at 1.0C rate, stand for 30 min; discharge to the lower limit cutoff voltage at 1.0C rate, stand for 30 min; repeat the above steps 100 times, record and calculate the first cycle discharge specific capacity C1 and the 100th cycle discharge specific capacity C100, and the capacity retention rate is C100 / C1*100%.

[0052] (2) Rate discharge performance test: charge to the upper limit cutoff voltage at 1C current, stand for 30 min; discharge to the lower limit cutoff voltage at 1C current, record the specific capacity C1; stand for 30 min; then charge to the upper limit cutoff voltage at 1C current; stand for 30 min; then discharge to the lower limit cutoff voltage at 3C current, record the specific capacity C2; calculate the rate retention rate as C2 / C1*100%.

[0053] 2) Test results: see Table 1.

[0054] Table 1: Effect of different modification methods on electrochemical performance

[0055]

[0056] Compared with comparative example 1, example 1 improves the sodium metal negative electrode deposition and stripping coulombic efficiency by electronic-ion conductive synergistic modification of the current collector, which can improve the uniform and dense deposition and stripping of sodium metal during charging and discharging, reduce the loss of sodium active material, ensure the long cycle stability under the working conditions of the negative electrode-free sodium metal battery and the feasibility under large current density, and specifically show that the 1C first cycle coulombic efficiency is as high as 99.2%, the 3C rate retention rate is as high as 95.3%, and the 100 cycle capacity retention rate is as high as 97%. Although comparative example 2 only improves the conductivity and comparative example 3 only improves the ion conductivity, they can improve the performance to a certain extent, but the improvement is limited, especially the 100 cycle capacity retention rate is generally low.

[0057] Example 2

[0058] The present example is the same as the setting of Example 1, except that in Step 1, the coating thickness is changed to 15 μm, 12 μm, 10 μm, 8 μm, 6 μm, 4 μm and 1 μm, respectively.

[0059] The batteries corresponding to the different coatings and different coating thicknesses are subjected to performance testing, as shown in Table 2.

[0060] Table 2: Influence of different coating thicknesses

[0061]

[0062] The results in Table 2 show that, under the condition of using an electron-ion conductor composite coating, when the coating thickness is 8 μm or more, the initial coulombic efficiency and capacity retention are relatively poor, but the rate performance is improved to some extent; when the coating thickness is less than 8 μm, the initial coulombic efficiency and cycle capacity retention are good, and the rate performance is also improved, and the performance is best when the coating thickness is 3 μm (i.e. Example 1).

[0063] Example 3

[0064] The present example is the same as the setting of Example 1, except that in Step 1, the 3A zeolite molecular sieve is replaced by ZSM-5, X type, Y zeolite and montmorillonite, kaolinite material.

[0065] The batteries corresponding to the different coatings are subjected to performance testing, as shown in Table 3.

[0066] Table 3: Influence of different coating compositions on the electrochemical performance of the batteries

[0067]

[0068] The results in Table 3 show that, under the condition of using an electron-ion conductor composite coating, at least one of the zeolite molecular sieves, i.e. natural clinoptilolite, mordenite, synthetic A-type zeolite, ZSM-5, X type, Y zeolite, can achieve good initial coulombic efficiency, cycle capacity retention and rate performance, and ZSM-5, X type, Y zeolite, montmorillonite, kaolinite and synthetic A-type zeolite (e.g. 3A zeolite of Example 1) are preferred, and the corresponding batteries have good electrochemical performance, with the 1C initial coulombic efficiency being maintained at 95.3% or more, the 100-cycle capacity retention being 88.5% or more, and the 3C rate retention being 89.4% or more.

Claims

1. A modified negative electrode current collector for sodium metal batteries, characterized in that: The modified negative electrode current collector includes a metal substrate and an electron-ion composite conductor coating. The electron-ion composite conductor coating is located on the surface of the metal substrate. The electron-ion composite conductor coating is obtained by combining any one of zeolite molecular sieve or expanded clay material with graphitized carbon material, wherein the ratio of zeolite molecular sieve or expanded clay material to graphitized carbon material is 0.3 to 3:

1.

2. The modified negative electrode current collector for a sodium metal battery according to claim 1, characterized in that: The zeolite molecular sieve is at least one of natural clinoptilolite, mordenite, synthetic type A zeolite, ZSM-5, type X, and type Y zeolite.

3. The modified negative electrode current collector for a sodium metal battery according to claim 1, characterized in that: The expansive soil material is at least one of montmorillonite and kaolinite.

4. The modified negative electrode current collector for sodium metal batteries according to claim 1, characterized in that: The graphitized carbon material is one or more of the following: natural graphite, artificial graphite, graphitized hard carbon, carbon black, acetylene black, mesophase carbon microspheres, carbon nanotubes, graphene, and their derivatives.

5. A modified negative electrode current collector for a sodium metal battery according to claim 1, characterized in that: The thickness of the electron-ion composite conductor coating is 10 nm to 10 μm.

6. The modified negative electrode current collector for a sodium metal battery according to claim 1, characterized in that: The sodium metal battery is a negative electrode-free sodium metal battery, comprising a modified negative electrode current collector, a positive electrode, a separator, and an electrolyte. The positive electrode active material is at least one of sodium iron pyrophosphate, sodium iron sulfate, sodium vanadium phosphate, sodium vanadium fluorophosphate, layered oxides, Prussian blue, Prussian white, and their analogues. The electrolyte salt is one or more of sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoromethanesulfonate, sodium bis(oxalate)borate, sodium difluorooxalateborate, sodium difluorobis(oxalate)phosphate, sodium tetrafluorooxalate, sodium di(trifluoromethanesulfonate)imide, sodium difluorosulfonylimide, and sodium perchlorate. The separator is at least one of polypropylene (PP) separator, polyethylene (PE) separator, and ceramic-coated composite separator.

7. A modified negative electrode current collector for a sodium metal battery according to claim 1, characterized in that: The metal substrate is aluminum foil.

8. A method for preparing a modified negative electrode current collector for a sodium metal battery according to any one of claims 1 to 7, characterized in that: After zeolite molecular sieves or expanded clay materials are pulverized with ductile iron, they are vacuum heat-treated at 120–150 °C for 8–12 h, and then thoroughly mixed with graphitized carbon materials and binders. The mixture is then coated onto the surface of a metal substrate to obtain a modified negative electrode current collector.

9. The method for preparing a modified negative electrode current collector for a sodium metal battery according to claim 8, characterized in that: The adhesive is one or more of PVDF, CMC, PAA, PVA, and SBR.

10. The method for preparing a modified negative electrode current collector for a sodium metal battery according to claim 8, characterized in that: By stacking the modified negative electrode current collector, separator, and positive electrode in sequence and injecting electrolyte, a sodium metal battery without a negative electrode can be obtained.

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