A sodium-ion battery and a preparation method thereof

Abstract

The application discloses a kind of sodium ion batteries, including positive pole piece, negative pole piece, by controlling its positive pole piece and negative pole piece capacity ratio N / P ratio, the capacity design value of negative pole piece is less than the capacity of positive pole piece, so that a part of sodium ion is first embedded into the active substance of negative pole piece during charging process, since the design capacity of negative pole piece is less than the capacity of positive pole piece, so that there is no enough space to accommodate sodium ion from positive pole piece in the active substance of negative pole piece, during the process of continuing charging, sodium ion is then precipitated in the pore of negative pole piece in the form of sodium metal particles, and exists in the form of granular solid sodium metal in the pore of negative pole piece.The application improves the weight specific energy of negative electrode, reduces the weight and thickness of negative pole piece, which improves the energy density of sodium ion battery and reduces the cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, in particular to a sodium ion battery and a preparation method thereof. BACKGROUND

[0002] Sodium ion batteries have the potential to replace lithium ion batteries in some application fields, such as data centers, communication base stations, large-scale energy storage fields, unsubsidized electric vehicles, and low-temperature applications, due to their advantages such as being not limited by resources and having good low-temperature performance.

[0003] Sodium ion batteries are a relatively new discipline, and their scientific principles and the like are still in the exploratory stage. For example, the storage mechanism of sodium ions in hard carbon is currently controversial in the academic community at home and abroad. However, as summarized in the three academic papers (1. New Mechanistic Insights on Na-Ion Storage in Nongraphitizable Carbon. C. Bommier et al. / Nano Lett. 2015, 15, 5888-5892; 2. Revealing sodium ion storage mechanism in hard carbon. S. Alvin et al. / Carbon 145 (2019) 67-81; 3. Hard carbons for sodium-ion batteries: Structure, analysis, sustainability, and electrochemistry. X. Dou et al. / Materials Today Volume 23 March 2019;), the physical and chemical environment of sodium ion storage in hard carbon can exist in three ways inside the hard carbon, (1) the edge and defect point of the carbon surface, (2) the closed Angstrom (or sub-nanometer) voids formed by the disordered stacking of turbostratic nanodomains (TN) formed by graphene sheets, and (3) the interlayer spacing voids between the graphene sheets of the turbostratic graphite domains. Figure 10 2 ​The hybrid connected carbon atoms are closely packed into a single layer two-dimensional honeycomb lattice structure. The carbon atoms have four valence electrons, of which three electrons form sp2 bonds, i.e., each carbon atom contributes an unpaired electron in a pz orbital, and the pz orbitals of adjacent atoms in a vertical direction to the plane can form a π bond, and the newly formed π bond is in a half-filled state. Studies have shown that the coordination number of carbon atoms in graphene is 3, the bond length between every two adjacent carbon atoms is 1.42 x 10-10 meters, and the angle between the bonds is 120°. In addition to the σ bond linked to other carbon atoms to form a honeycomb layered structure of hexagonal rings, the pz orbital of each carbon atom perpendicular to the layer plane can form a multi-atom π bond (similar to a benzene ring) throughout the layer, thus having excellent electrical conductivity.

[0004] The definition of a cluster in physics is that a cluster is a relatively stable micro or submicroscopic aggregate composed of several to thousands of atoms, molecules or ions through physical or chemical binding forces. A cluster is a new level of material structure between atoms, molecules and macroscopic solid matter, and represents the initial state of condensed matter. Clusters exist widely in nature and human practical activities. Through the study of clusters, this new growth point in materials science can be discovered, understood and utilized. The spatial scale of a cluster is in the range of several angstroms to several hundred angstroms. Describing it with inorganic molecules is too large, and describing it with small pieces of solid is too small. Many properties are neither the same as a single atom or molecule nor the same as a solid or liquid, and cannot be obtained by simple linear extension or interpolation of the properties of the two. Therefore, a cluster is considered to be a new level of material structure between atoms, molecules and macroscopic solid matter, a transitional state of various substances from atoms and molecules to bulk matter, or in other words, it represents the initial state of condensed matter. The basic problem of cluster science research is to understand how clusters evolve from atoms and molecules step by step, and how the structure and properties of clusters change with this evolution, and specifically when the size is large enough to transition into a macroscopic solid. Cluster science is in the field of multi-disciplinary intersection. Concepts and methods from atomic and molecular physics, condensed matter physics, quantum chemistry, structural chemistry, atomic cluster chemistry, surface science and materials science are intertwined to form the central issues of current cluster research, and are developing into a new discipline between atomic and molecular physics and solid state physics.

[0005] Sodium metal cluster is a relatively stable micro or submicroscopic aggregate composed of several or even hundreds of atoms, molecules or ions by physical or chemical binding force, whose physical and chemical properties vary with the number of atoms contained. Cluster is a concept of material scale angstrom, sub-nanometer material. The spatial scale of the cluster is in the range of several angstroms to several hundred angstroms, which is too small to be described by inorganic molecules and too large to be described by small solid, many properties are different from single atom molecule, solid and liquid, and cannot be obtained by simple linear extension or interpolation of the properties of the two. Therefore, people regard the cluster as a new level of material structure between atoms, molecules and macroscopic solid substances, a transition state of the transformation of various substances from atoms and molecules to bulk materials, or a representative of the initial state of condensed matter.

[0006] And the sodium metal particles (existing in the pores of the coating of the pole piece, formed by the pores between the solid particles of the active material hard carbon and the conductive agent and the binder, etc.) in the present application are micron-sized solid metals, which are the concept of solid metals in general, and have essential differences from sodium metal clusters.

[0007] Regarding the porosity of the electrode pole piece, the sodium ion battery electrode pole piece is composed of powder particles (such as hard carbon active material powder, conductive agent powder particles, etc.) plus binder, and due to the rough surface and irregular shape of the powder particles, there must be pores between the particles when they are stacked. Therefore, there are pores in the electrode pole piece coating, and the porosity is usually about 30%. When the densities of the active material, conductive agent and binder are used to calculate the porosity, the calculated porosity is the pore between the particles, and does not include the angstrom (sub-nanometer) void inside the active material particles.

[0008] In summary, the pore size of the battery pole piece is multi-scale, and generally the pores formed between the solid particles of the active material hard carbon and the conductive agent and the binder, etc. are in the micron to sub-micron size, while the internal void of the active material hard carbon particles is in the angstrom to sub-nanometer level.

[0009] The porosity is the volume percentage of the pores formed between the solid particles of the active material hard carbon and the conductive agent and the binder, etc. in the electrode pole piece to the total volume of the electrode pole piece (excluding the angstrom to sub-nanometer void inside the active material hard carbon particles), and the porosity of the electrode pole piece directly reflects the compactness of the pole piece and is closely related to the compaction density value. The porosity of the electrode pole piece is shown in formula (1):

[0010] Porosity = 1-Vi / V

[0011] In the formula: Vi represents the total volume of each solid phase component of the electrode plate coating (excluding the metal current collector), including the volume of solid components such as active materials, binders, and conductive agents; V represents the overall volume of the electrode plate coating. The volume of each component is the weight of each component divided by the true density of each component, and the volume of the coating can be obtained by dividing the coating weight of the electrode plate by the electrode plate compaction density, which is obtained by dividing the electrode plate area density by the electrode plate thickness.

[0012] The following figure is a schematic diagram of the storage mechanism of sodium ions in hard carbon inside the invention Figure 10 .

[0013] The sodium ion battery of the present application generally uses a hard carbon negative electrode. Unlike the graphite negative electrode used in lithium ion batteries, due to the radius of sodium ion being 0.102 nm, which is larger than the radius of lithium ion (0.076 nm), and due to the small interlayer spacing of graphite molecules (the interlayer spacing of graphite molecules is generally D002 crystal plane spacing of 0.34 nm), the sodium ion with a larger radius can only be inserted into the interlayer of graphite with the help of ether solvent and by increasing the thermal Gibbs energy, and it causes the graphite to expand by more than 100%. Therefore, the traditional graphite negative electrode used in lithium ion batteries cannot be used as the negative electrode of sodium ion batteries. Therefore, it is necessary to find a carbon material that can insert sodium ions. Hard carbon material has a low true density value (the true density of hard carbon is 1.45 g / cm 3 The micro-pores formed by the disordered stacking of the turbine-like graphite domains formed by the graphene sheets inside the hard carbon are much larger than the ordered stacking micro-pores of the graphene sheet layers inside the graphite, and the true density value of the graphite is 2.2 g / cm 3 The highly ordered stacking of the graphene sheet layers inside the graphite active material has much smaller internal pores than the internal pores of the hard carbon, about 34% smaller, and the true density ratio of the hard carbon to the graphite is 1.45 / 2.2 = 65.9%. Within the effective potential window, sodium ions can be inserted into the angstrom (or sub-nanometer) pores between the disordered turbine-like graphite domains formed by the graphene sheets in the hard carbon material to form sodium metal clusters, or into the graphene sheet spacing of the turbine-like graphite domains in the hard carbon material. Due to the large sodium insertion pore space of the hard carbon, the hard carbon material is selected as the preferred negative electrode active material for sodium insertion. The current research direction of sodium ion battery negative electrode materials includes hard carbon, titanium-based oxides and alloys, phosphorus-carbon or metal phosphide and pre-sodium metal phosphide materials, etc. The research on hard carbon is the most and the currently commercialized sodium ion battery also mainly uses hard carbon material as the negative electrode. However, the specific capacity of the hard carbon negative electrode used in sodium ion batteries is relatively low compared to the specific capacity of graphite, and the cost is high. Therefore, it is necessary to further improve the energy density of sodium ion batteries and further reduce the cost.

[0014] Therefore, a sodium ion battery and a preparation method thereof are provided to solve the above problems. SUMMARY

[0015] The purpose of the present application is to provide a sodium ion battery, including a positive electrode sheet and a negative electrode sheet, by controlling the capacity ratio N / P of the positive and negative electrode sheets, the capacity design value of the negative electrode sheet is less than that of the positive electrode sheet, so that during the charging process, a part of sodium ions is first embedded into the active material hard carbon particles of the negative electrode sheet. Due to the design capacity of the negative electrode sheet being less than that of the positive electrode sheet, there is not enough space in the active material hard carbon of the negative electrode sheet to accommodate sodium ions from the positive electrode sheet. During the continued charging process, sodium ions are then precipitated in the pores of the negative electrode sheet in the form of sodium metal particles, and exist in the pores of the negative electrode sheet in the form of granular solid sodium metal. After charging is completed, the negative terminal becomes an alloy electrode sheet formed by sodium ions and sodium metal clusters, granular solid sodium metal and hard carbon negative electrode, that is, an alloy formed by sodium ions embedded in the hard carbon active material and a composite negative electrode sheet formed by the existence of granular solid sodium metal in the pores of the negative electrode sheet coating. The obtained sodium ion battery is a composite sodium battery of sodium ion / sodium metal cluster-hard carbon alloy battery and granular solid sodium metal battery. The battery can be used as a secondary rechargeable sodium ion battery, and can also be used as a primary sodium metal battery. The rated voltage is 1.5-3.5V, which can replace high-cost lithium metal primary batteries, and also becomes an electrochemical device of a composite sodium battery of hard carbon type sodium ion battery and granular sodium metal battery.

[0016] Further, the negative electrode material is a hard carbon material or other type of sodium-embedded material, the sodium-embedded material is one or more of a titanium-based layered oxide, a tin alloy, a phosphide, or a phosphorus-carbon material, the phosphorus-carbon material is a nanoparticle composite of phosphorus and carbon coating, and the phosphide is Sn3P4, SbP, or a pre-sodiumized phosphorus-carbon material.

[0017] Further, the positive electrode material is one of a multi-element layered oxide sodium salt, Prussian blue, sodium manganate, and a polyanion sodium salt, the polyanion sodium salt is one of a phosphate salt and a sulfate salt sodium compound (such as sodium iron pyrophosphate, sodium vanadium phosphate, sodium iron sulfate, etc.).

[0018] Further, the preparation steps of the Prussian sodium positive electrode include:

[0019] Step 1: Preparation of oxygen-free water: nitrogen is continuously injected into deionized water for more than one hour to remove oxygen in the water, or deionized water is heated to 100°C and stirred under vacuum to remove oxygen in the water, or iron chips are used with water for more than 24 hours to remove oxygen in the water, and oxygen-free water is obtained.

[0020] Step two: slurry and positive electrode tab preparation of Prussian white type positive electrode: using the prepared deoxygenated water, vacuum stirring or inert gas protection stirring to prepare the positive electrode slurry, the prepared tab is vacuum dried at a temperature higher than 150℃ for at least 4 hours to remove the crystal water of the active material in the positive electrode tab, or the prepared battery is vacuum dried at a temperature higher than 150℃ for at least 4 hours before electrolyte injection to remove the crystal water of the active material in the positive electrode tab, to obtain the positive electrode active material layered oxide, the positive electrode active material layered oxide, binder and conductive agent are dissolved and uniformly mixed in NMP according to a certain mass ratio to prepare the positive electrode slurry, which is coated on the current collector and formed by rolling after drying.

[0021] Step three: add conductive agent, binder and solvent to the negative electrode tab to prepare the negative electrode slurry, which is coated on the current collector and formed by rolling after drying;

[0022] Step four: preparation of electrolyte: dissolve the electrolyte in the electrolyte solvent to obtain the electrolyte.

[0023] Step five: full battery assembly: cut the positive electrode tab, negative electrode tab and separator to the corresponding size, and then wind them into a dry battery by a winding machine, and then perform welding, aluminum plastic film packaging, high temperature baking, injection, formation, vacuuming, secondary packaging, formation and capacity test processes to prepare a cylindrical soft package sodium ion composite battery.

[0024] Further, the current collector of the positive and negative electrode tabs is at least one of metal foil, foam current collector, metal mesh current collector, carbon felt current collector, carbon cloth current collector and carbon paper current collector.

[0025] Further, the conductive agent includes at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, graphene and fullerene.

[0026] Further, the compaction density of the negative electrode tab is 0.5-1.8, the internal porosity of the tab is 1-99%, and the active material coating thickness of the negative electrode tab is greater than 10 microns.

[0027] Further, the separator substrate is an olefin, which is one of polyethylene, polypropylene, polyimide PI high temperature separator, paper fiber or glass fiber woven high temperature separator, and at least one surface of the separator substrate layer is coated with a surface treatment layer, which is one of a polymer layer, an inorganic layer or a composite polymer and inorganic layer.

[0028] Further, the material of the electrolyte can be at least one of sodium perchlorate, sodium hexafluorophosphate, sodium bisfluorosulfonylimide, sodium bis-trifluoromethanesulfonylimide, sodium triflate, sodium tetrafluoroborate, sodium difluorophosphate and sodium tetraphenylborate.

[0029] Further, the electrolyte solvent is at least one of fluorinated carbonate (such as 3,3,3-trifluoropropyl carbonate, fluoroethylene carbonate, methyl 2,2,2-trifluoroethyl carbonate), 1,3-dioxolan-2-one, 4-[2,3,3,3-tetrafluoro-2-(trifluoromethyl)propyl], vinyl carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, methyl acetate, ethyl propionate, fluoroethylene carbonate, or ether, ethylene glycol dimethyl ether, 1,3-dioxolane, diethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl tert-butyl ether, hydrofluoroether, fluoroether D2, preferably ether or fluoroether, fluorinated carbonate solvent, and the electrolyte concentration is 0.1-5 mol.

[0030] Compared with the prior art, the application has the following advantages:

[0031] The sodium ion battery of the application is a composite sodium battery of sodium ion / sodium metal cluster-hard carbon alloy battery and particle type solid sodium metal battery, which has great advantages compared with the battery with conventional pure sodium metal electrode. The sodium metal particles in the composite electrode are naturally formed in the battery formation and charging process, without the need for a special anhydrous and oxygen-free environment. The pure sodium metal electrode is difficult to make and has high cost. The sodium metal is easily oxidized in the air, so the sodium metal electrode must be processed in an anhydrous and oxygen-free environment under inert gas protection. In addition, sodium dendrites are easily generated in the cycle, which may cause short circuit of the battery.

[0032] The N / P ratio of the application is less than 1, and the capacity of the negative electrode hard carbon material is less than that of the positive electrode. Therefore, part of the capacity of the negative electrode is replaced by sodium metal particles with higher theoretical specific capacity, which improves the weight specific capacity of the negative electrode and reduces the weight of the negative electrode and the thickness of the negative electrode. The use amount of the active material of the negative electrode can be reduced to improve the energy density of the sodium ion battery. In addition, the reduction of the thickness of the negative electrode also reduces the internal resistance of the battery, thereby improving the discharge rate performance of the battery. The sodium metal particles filled in the pores of the electrode improve the conductivity of the negative electrode due to the excellent conductivity of sodium metal, and also reduce the direct current resistance of the battery, thereby improving the discharge rate performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a data schematic diagram of the negative electrode electrode in Example 1 of the application;

[0034] Figure 2 is a structure schematic diagram of the negative electrode electrode in Example 1 of the application;

[0035] Figure 3is a data schematic diagram of the positive electrode tab in Example 1 of the present application;

[0036] Figure 4 is a structure schematic diagram of the positive electrode tab in Example 1 of the present application;

[0037] Figure 5 is a data schematic diagram of the negative electrode tab in Comparative Example 1 of the present application;

[0038] Figure 6 is a structure schematic diagram of the negative electrode tab in Comparative Example 1 of the present application;

[0039] Figure 7 is a data schematic diagram of the positive electrode tab in Comparative Example 1 of the present application;

[0040] Figure 8 is a structure schematic diagram of the positive electrode tab in Comparative Example 1 of the present application;

[0041] Figure 9 is a cycle life test curve diagram of Example 1 of the present application.

[0042] Figure 10 is a schematic diagram of the storage mechanism of sodium ions inside the hard carbon. DETAILED DESCRIPTION

[0043] The present application provides a sodium ion battery and a preparation method thereof.

[0044] 1. Selection of materials for manufacturing the negative electrode tab of the sodium ion battery

[0045] The negative electrode tab can be manufactured by adding a conductive agent, a binder, and a suitable solvent to the hard carbon material to form a negative electrode slurry, coating the slurry on a current collector composed of a metal base, drying, and then forming by rolling.

[0046] As the conductive agent, acetylene black, chlorophyll black, carbon nanofiber, carbon nanotube, graphene, or carbon fiber can be used, and the addition amount varies depending on the type of conductive agent used. The preferred proportion of the added conductive agent is 0.5 to 15% by weight, wherein the amount of hard carbon material + the amount of binder + the amount of conductive agent = 100% by weight, more preferably 0.5 to 7% by weight, and particularly preferably 0.5 to 5% by weight.

[0047] As the binder, if it does not react with the electrolyte such as a complex of PVDF, polytetrafluoroethylene, and SBR and CMC, there is no particular limitation, wherein PVDF, PVDF attached to the surface of the active material rarely hinders the movement of sodium ions, is preferred for obtaining good input-output characteristics, in order to dissolve PVDF and form a slurry, a polar solvent such as N-methylpyrrolidone is preferably used, but water-based latex such as butadiene-styrene rubber, sodium polyacrylate, and CMC dissolved in water can also be used, if the amount of the binder added is too much, the resistance of the electrode obtained becomes large, thus the internal resistance of the battery becomes large, and the battery characteristics decrease, which is undesirable. In addition, if the amount of the binder added is too small, the combination of the negative electrode material particles with each other and with the current collector material is insufficient, the preferred amount of the binder to be added varies depending on the type of the binder used, but the binder of the PVDF type is preferably 0.5 to 10% by weight;

[0048] On the other hand, in the binder using water as the solvent, a plurality of binders such as a complex of SBR and CMC are often used in combination, as the total amount of the total binder used, it is preferably 0.5 to 5% by weight, more preferably 1 to 4% by weight, the electrode active material layer is basically formed on both surfaces of the current collector, but can be formed on one surface as needed, the thicker the electrode active material layer, the less the current collector and the separator, thus it is preferred to be high-capacity, but the thinner the electrode active material layer, the more advantageous it is to improve the input-output characteristics, so if the active material layer is too thick, the input-output characteristics decrease, the preferred thickness of the one-surface active material layer is not limited, in the range of 10 μm to 1000 μm, preferably 10 to 130 μm, more preferably 20 to 75 μm, particularly 20 to 50 μm.

[0049] The negative electrode tab generally has a current collector, and the current collector can use a foil material of stainless steel, copper, aluminum, nickel, or carbon, wherein copper foil or aluminum foil is preferred.

[0050] 2. Selection of materials for manufacturing the positive electrode tab of the sodium ion battery

[0051] In the case of forming the negative electrode tab of the sodium ion secondary battery using the negative electrode material of the present application, there is no particular limitation on the corresponding positive electrode material, separator, and electrolyte and other materials of the battery, and various sodium-containing positive electrode materials, separators, and electrolytes, etc. commonly used for sodium ion secondary batteries can be used.

[0052] The positive electrode tab can include a positive electrode active material, and can further include a conductive aid, a binder, or both, and the complex ratio of the positive electrode active material and other materials in the positive electrode active material layer is not limited as long as the effect of the present application is achieved, and can be appropriately determined. As the positive electrode active material, it can be a sodium salt of Prussian blue, a sodium salt of a multi-element layered oxide, a sodium salt of a manganese acid, a polyanion-type sodium salt including sodium iron sulfate, sodium iron pyrophosphate, and sodium vanadium phosphate.

[0053] The positive electrode generally has a current collector, and as the positive electrode current collector, a foil of stainless steel, copper, aluminum, nickel, or carbon can be used, with an aluminum foil being preferred.

[0054] The corresponding separator material for the sodium-ion battery is not particularly limited, and various separators used for lithium-ion batteries, sodium-ion secondary batteries, and capacitors can be used, and the separator can be an olefin-based separator such as polyethylene or polypropylene; a high-temperature separator such as polyimide PI; a high-temperature separator woven with paper fibers or glass fibers, with a paper separator being preferred.

[0055] The corresponding electrolyte material for the sodium-ion battery is not particularly limited, and various electrolyte solvents used for lithium-ion batteries and capacitors can be used, and the electrolyte sodium salt material can be at least one of sodium perchlorate, sodium hexafluorophosphate, sodium bisfluorosulfonylimide, sodium bis-trifluoromethylsulfonylimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium acetate, and sodium tetraphenylborate. The electrolyte solvent can be at least one of fluorinated carbonate such as 3,3,3-trifluoropropyl carbonate, fluorinated ethylene carbonate, methyl 2,2,2-trifluoroethyl carbonate; 1,3-dioxolane-2-ketone, 4-[2,3,3,3-tetrafluoro-2-(trifluoromethyl)propyl], or at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl acetate, ethyl propionate, ethyl acetate, or diethyl ether, dimethyl ether, 1,3-dioxolane, diethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl tert-butyl ether, or hydrofluoroether, fluorinated ether D2, with ether-based solvents or fluorinated ethers, fluorinated carbonate solvents being preferred, and the electrolyte concentration being 0.1-5 mol.

[0056] Example 1:

[0057] All of the components, percentages, and ratios are based on weight, the N / P ratio of the battery is set to 0.4 / 1, and the battery capacity is 400 mAh, and the sodium-ion production process of the present example is as follows:

[0058] A. Negative electrode sheet production and negative electrode slurry preparation: a negative electrode hard carbon material (full battery specific capacity 210-280 mAh) of model CNY5, conductive carbon black, binder SBR, and thickening agent CMC are compounded in deionized water at a mass ratio of 91.5%:4.5%; 2.5%:1.5%, respectively, to prepare a negative electrode slurry, and a transfer coater is used to coat the negative electrode slurry on the surface of a 9-μm-thick carbon-coated aluminum foil according to the weight requirement per unit area of the negative electrode active material, and the coated foil is dried, and then a roll press is used to roll the coated foil at a design compaction density of 0.9 g / cm 3 The final negative electrode sheet is obtained, as shown in FIG. 1.Figure 1 , 2 The prepared electrode sheet surface density, compaction density, thickness, true density of each solid substance, porosity are shown in FIGS. 1-4.

[0059] B. Positive electrode sheet preparation: The positive electrode active material layered oxide (full cell specific capacity 65mAh-100mAh), 1:2 mixture of binder fluorine-containing PVDF and fluorine-free PAN, carbon nanotube CNT, conductive carbon black were dissolved and uniformly mixed in NMP according to the mass ratio of 94.5%:2.5%:1.0%:2.0%, and then dispersed to prepare a positive electrode slurry. The positive electrode slurry was coated on the surface of a 9um thick aluminum foil by a coating machine according to the positive electrode active material unit area mass requirement and dried, and then the coated electrode sheet was rolled by a roller press at a design compaction density of 2.7-3.2g / cm 3 The final positive electrode sheet is shown in FIGS. 5-8. Figure 3 , 4

[0060] C. Preparation of electrolyte: Sodium hexafluorophosphate with a concentration of 1mol / L was dissolved in DME solvent to obtain an electrolyte.

[0061] D. Full cell assembly: The positive electrode sheet, negative electrode sheet, and separator film were cut to the corresponding size and wound into a dry cell by a winding machine. Then, the dry cell was subjected to welding, aluminum plastic film packaging, high temperature baking, liquid injection, formation, degassing, secondary packaging, formation, and capacity distribution processes to prepare a 400mAh cylindrical soft-pack sodium ion composite battery.

[0062] The electrolyte injection amount was set to 2.7g.

[0063] Comparative Example 1:

[0064] All components, percentages, and ratios are based on weight, the N / P ratio of the battery is 1.1 / 1, and the battery capacity is 300mAh. The sodium ion preparation process of the present comparative example is as follows:

[0065] A. Negative electrode sheet preparation: The negative electrode slurry was prepared in the same way as the negative electrode sheet preparation of Example 1. The negative electrode sheet coating thickness, surface density parameters, compaction density, true density of each solid substance, porosity, and electrode sheet length and width parameters were adjusted according to the N / P ratio of 1.1 / 1, as shown in FIGS. 9-12. Figure 5 , 6

[0066] ​​B. Positive electrode tab making: the positive electrode active material layered oxide, the binder fluorine-containing PVDF and the mixture without fluorine PAN, carbon nanotube CNT, conductive carbon black are dissolved, dispersed and uniformly mixed in NMP to prepare a positive electrode slurry according to the mass ratio of 94.5%:2.5%:1.0%:2.0%, and then coated on the surface of a 9um-thick aluminum foil according to the positive electrode active material unit area mass requirement by using a coating machine and dried, and then the coated tab is rolled by a roller press at a design compaction density of 2.7-3.5 mg / mm 3 The final positive electrode tab is shown in FIGS. 1-3. Figure 7 , 8

[0067] C. Electrolyte preparation: sodium hexafluorophosphate with a concentration of 1 mol / L is dissolved in DME solvent to obtain an electrolyte.

[0068] D. Full battery assembly: the positive electrode tab, the negative electrode tab and the separator film are cut into corresponding sizes and wound into a dry cell by a winding machine, and then the dry cell is subjected to welding, aluminum-plastic film packaging, high-temperature baking, liquid injection, formation, gas extraction, secondary packaging, formation and capacity distribution process to prepare a 300mAh cylindrical soft-pack sodium ion composite battery.

[0069] The electrolyte injection amount is set to 2.7g.

[0070] Example 2:

[0071] ​Example 2, different from example 1, the positive active material is adjusted to be Prussian white, and 3% by weight of Prussian white is replaced by sodium acetate (sodium oxalate), i.e. the weight composition of the active material is 97% Prussian white + 3% by weight of sodium acetate (sodium oxalate). The rest of the negative electrode plate, electrolyte and the like are made the same as example 1, specifically, the positive active material Prussian white, sodium acetate, binder SBR, thickening agent CMC, conductive carbon black (Super-P) are compounded uniformly in deoxygenated deionized water according to the mass ratio of 93.12%:2.88%:2%:1%:4% to prepare a positive electrode slurry, wherein the sodium acetate is dissolved in deionized water (the sodium acetate is dispersed in the positive electrode slurry at a molecular level, and the best dispersion effect is obtained), and a transfer coating machine is used to coat the surface of the 9um thick aluminum foil according to the positive active material unit area mass requirement and dry, during the drying process, the size of the sodium acetate is uniformly precipitated in the gap between the conductive agent and the active material Prussian white of the positive electrode plate in the form of particles smaller than nanoscale, and then the coated electrode plate is processed by a rolling machine at a design compaction density of 1-1.5mg / mm3 to prepare the final positive electrode plate. The battery prepared in example 2 has a first formation charging voltage of 4.1V, during the formation charging process, the sodium ions of sodium acetate are transferred to the negative electrode end and embedded into the negative electrode plate, and carbon dioxide gas is generated at the same time, the carbon dioxide gas is extracted outside the battery during the vacuum pumping sealing process of the battery or the negative pressure formation process, only leaving sodium atoms inside the battery, which improves the first efficiency of the battery by 3%, the weight of the battery is also reduced, the capacity of the battery remains the same as example 1, and the cost of the battery is reduced by more than 1% (because the cost of sodium acetate is only half of that of the positive active material).

[0072] As can be seen from example 2, by dissolving sodium acetate in deionized water (sodium acetate is dispersed in the positive electrode slurry at a molecular level, and the best dispersion effect is obtained), the first efficiency of the battery is improved, the weight of the battery is also reduced, the capacity of the battery remains the same, and the cost of the battery is reduced (because the cost of sodium acetate is only half of that of the positive active material)

[0073] Compare the performance data of example 1 and comparative example 1:

[0074] According to the surface density value and active material content of the positive and negative electrode plates prepared according to example 1 and the active material gram capacity value, the N / P ratio of the battery of example 1 is 0.4 / 1, the actual test battery capacity is 402.0mAh, the design capacity of the positive active material is 465.2mAh, the first efficiency is 86.41%, the porosity of the negative electrode plate is tested by the n-butanol method, and the porosity of the negative electrode plate is 35.53%, the total volume of the pores in the negative electrode plate is obtained by multiplying the porosity of the negative electrode plate by the volume of the negative electrode plate V-Vi=0.81-0.522=0.288cm 3 , according to the theoretical density of sodium metal 0.97g / cm3 The weight of the sodium metal particles that can be stored in the pores inside the negative electrode sheet is 0.288 cm3*0.97 g / cm3= 0.279 g, and the specific capacity of the sodium metal is 1166 mAh / g. The maximum capacity of the sodium metal particles that can be stored in the pores inside the negative electrode sheet is 0.279 g*1166 mAh / g = 325.7 mAh. 3 The weight of the sodium metal particles that can be stored in the pores inside the negative electrode sheet is 0.288 cm3*0.97 g / cm3= 0.279 g, and the specific capacity of the sodium metal is 1166 mAh / g. The maximum capacity of the sodium metal particles that can be stored in the pores inside the negative electrode sheet is 0.279 g*1166 mAh / g = 325.7 mAh.

[0075] According to the face density value, the active material content, and the active material specific capacity value of the positive and negative electrode sheets prepared according to Comparative Example 1, the N / P ratio of the battery of Comparative Example 1 is 1.1 / 1, and the actual test battery capacity is 323.2 mAh, wherein the design capacity of the positive electrode active material is 371.49 mAh, and the first efficiency is 87%.

[0076] It can be seen from the comparison between Example 1 and Comparative Example 1 that the battery capacity of Comparative Example 1 is 80 mAh less than that of Example 1, that is, the battery capacity of Comparative Example 1 is only 80.39% of that of Example 1 (323.2 / 402 = 80.39%), and the volume specific energy density of Example 1 is more than 19.61% higher than that of Comparative Example 1. Comparative Example 1 is the production and manufacturing method of a conventional sodium ion battery, which shows that the sodium metal particle and sodium metal cluster / hard carbon alloy composite sodium battery of the present application has improved volume specific energy density compared with the conventional sodium ion battery.

[0077] The weight specific energy density of the battery of Example 1 is 0.402 Ah*2.9 V / 13.6 g = 85.72 Wh / kg, and the weight specific energy density of the battery of Comparative Example 1 is 0.323 Ah*2.94 V / 12.44 g = 76.33 Wh / kg. Therefore, the weight specific energy density of Example 1 is 1.123 times (85.72 Wh / kg / 76.33 Wh / kg = 1.123) higher than that of Comparative Example 1, which is more than 12%. This also shows that the sodium metal particle and sodium metal cluster / hard carbon alloy composite sodium battery of the present application has improved weight specific energy density compared with the conventional sodium ion battery.

[0078] It can be seen from the comparison between Example 1 and Comparative Example 1 that the battery of Example 1 has an N / P ratio less than 1, which is actually 0.4. After the full-charged battery cell is disassembled, the negative electrode sheet is observed using an electron microscope EDS to observe the dissection interface. It is observed that the pores in the electrode sheet are filled with sodium metal particles. For Comparative Example 1, the N / P ratio is greater than 1, which is actually 1.1. After the full-charged battery cell is disassembled, the negative electrode thickness is observed using an electron microscope EDS to observe the dissection interface, and no sodium metal particles are observed.

[0079] The internal resistance of the battery of Example 1 is 55 mΩ, and the cycle life test curve is as follows: Figure 9As shown, the charge-discharge voltage is set to 3.45V-1.5V, the charge-discharge current is 1.5C, that is, 600mA, the charge-discharge cycle is performed for 300 times, the battery capacity and average voltage value corresponding to the discharge from 3.45V to 1.5V after 300 cycles are recorded, the capacity retention rate is more than 90%, the initial average voltage value is 2.91V, and the capacity retention rate calculation method is: the percentage of the battery discharge capacity value at the 300th week divided by the initial battery discharge capacity value.

[0080] In summary, the application controls the N / P ratio of the positive and negative electrodes to be less than 1, that is, the capacity ratio of the positive electrode plate to the negative electrode plate is less than 1, and the specific N / P ratio is 0.99-0.1, that is, the capacity of the designed negative electrode plate is less than the capacity of the positive electrode plate. During the charging process of the battery, a part of sodium ions is inserted into the interlayer spacing and defects of the hard carbon negative electrode. Since the capacity of the negative electrode plate is less than the capacity of the positive electrode plate, there is not enough space in the molecular structure of the negative active material to accommodate all the sodium ions from the positive electrode. Therefore, a part of the sodium ions of the positive electrode will be precipitated in the pores of the negative electrode plate in the form of sodium metal during the charging process, and will exist in the pores of the negative electrode plate in the form of granular solid sodium metal. After charging, the negative electrode becomes a negative electrode plate of the alloy of sodium ion / hard carbon and granular sodium metal, forming a composite sodium battery of hard carbon type sodium ion battery and granular sodium metal battery, which has low cost and high performance, and forms an electrochemical device of the composite sodium battery of hard carbon type sodium ion battery and granular sodium metal battery.

[0081] The technical solutions in the embodiments of the application will be clearly and completely described in combination with the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0082] In the description of the application, it should be noted that the terms "upper", "lower", "inner", "outer", "front end", "rear end", "two ends", "one end", "the other end" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore cannot be understood as a limitation on the application. In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance.

[0083] In the description of the present application, it should be noted that unless specifically stated and limited otherwise, the terms "mounting", "provided with", "connected" and the like, should be interpreted broadly, for example, "connected" can be fixed connection, can also be detachable connection, or integrally connected; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

Claims

1. A sodium-ion battery, comprising a positive electrode and a negative electrode, characterized in that: By controlling the N / P capacity ratio of the positive and negative electrodes, the design capacity of the negative electrode is made smaller than that of the positive electrode. The compaction density of the negative electrode is 0.5-1.8, and the internal porosity is 1-35.56%. During charging, some sodium ions are first embedded in the active material of the negative electrode. Because the design capacity of the negative electrode is smaller than that of the positive electrode, there is not enough space in the active material of the negative electrode to accommodate the sodium ions from the positive electrode. During continued charging, the sodium ions are precipitated in the pores of the negative electrode as sodium metal particles, existing in the pores of the negative electrode as particulate solid sodium metal. The shape of the sodium metal particles is similar to the shape of the pores in the electrode, being spherical or elliptical. The tadpole-shaped electrode, after charging, results in a negative electrode that is an alloy of sodium ions and hard carbon. Specifically, it is a negative electrode sheet composed of an alloy formed by sodium ions embedded within sodium metal clusters in hard carbon and a mixture of granular sodium metal. The resulting sodium-ion battery is a composite sodium battery combining a sodium ion / sodium metal cluster-hard carbon alloy battery and a granular solid sodium metal battery. This battery can be used as a rechargeable sodium-ion battery or a primary sodium metal battery, with a rated voltage of 1.5-3.5V. It can replace high-cost lithium metal primary batteries and serves as an electrochemical device for a composite sodium battery combining hard carbon sodium-ion batteries and granular sodium metal batteries. The negative electrode sheet has a coating thickness of more than 3 micrometers containing active material, and the negative electrode material is hard carbon.

2. A sodium-ion battery according to claim 1, characterized in that, The positive electrode material is one of Prussian white, sodium salt of multi-layered oxide, sodium manganate, or polyanionic sodium salt, wherein the polyanionic sodium salt is one of sodium ferric sulfate, sodium ferric pyrophosphate, or sodium vanadium phosphate.

3. The method for preparing a sodium-ion battery according to claim 1, characterized in that, The preparation steps include: Step 1: Preparation of deoxygenated water: Nitrogen gas is continuously injected into deionized water for more than one hour to remove oxygen from the water, or deionized water is heated to 100°C and stirred under vacuum to remove oxygen from the water, or iron filings are coexisted with water for more than 24 hours to remove oxygen from the water, thus obtaining deoxygenated water. Step 2: Preparation of Prussian white cathode slurry and cathode sheet: Using the prepared deoxygenated water, the cathode slurry is prepared by vacuum stirring or stirring under inert gas protection. The obtained cathode sheet is vacuum dried at a temperature above 150°C for at least 4 hours to remove the water of crystallization of the active material in the cathode sheet. Alternatively, the prepared battery cell is vacuum dried at a temperature above 150°C for at least 4 hours before electrolyte injection to remove the water of crystallization of the active material in the cathode sheet, resulting in a layered oxide cathode active material. The layered oxide cathode active material, binder, and conductive agent are dissolved and uniformly mixed in NMP at a certain mass ratio, dispersed, and the cathode slurry is prepared. The cathode slurry is coated on the current collector, dried, and then rolled into shape. Step 3: Add conductive agent, binder and solvent to the negative electrode sheet to make negative electrode slurry, coat it on the current collector, dry it and then roll it into shape; Step 4: Prepare the electrolyte: Dissolve the electrolyte in an electrolyte solvent to obtain the electrolyte solution; Step 5: Full battery assembly: The positive electrode, negative electrode, and separator are cut to the appropriate size and wound into dry cells by a winding machine. Then, the cells are processed through welding, aluminum-plastic film encapsulation, high-temperature baking, liquid injection, formation, degassing, secondary encapsulation, and capacity testing to produce cylindrical soft-pack sodium-ion composite batteries.

4. The method for preparing a sodium-ion battery according to claim 3, characterized in that, The current collectors of the positive and negative electrodes are at least one of metal foil, foam current collectors, metal mesh current collectors, carbon felt current collectors, carbon cloth current collectors, and carbon paper current collectors.

5. The method for preparing a sodium-ion battery according to claim 3, characterized in that, The conductive agent includes at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene.

6. The method for preparing a sodium-ion battery according to claim 3, characterized in that, The membrane substrate is an olefin-based or polyimide (PI) high-temperature membrane, a high-temperature membrane woven from paper fiber or glass fiber, and at least one surface of the membrane substrate layer is coated with a surface treatment layer, which is one of a polymer layer, an inorganic layer, or a layer formed by a composite polymer and an inorganic material. The olefin is one of polyethylene or polypropylene.

7. The method for preparing a sodium-ion battery according to claim 3, characterized in that, The electrolyte is made of at least one of the following: sodium perchlorate, sodium hexafluorophosphate, sodium difluorosulfonamide, sodium bis(trifluoromethylsulfonyl)imide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium tetraphenylborate, and sodium chloride.

8. The method for preparing a sodium-ion battery according to claim 3, characterized in that, The electrolyte solvent is a fluorinated carbonate, wherein the fluorinated carbonate is at least one of 3,3,3-trifluoropropyl carbonate, fluoroethylene carbonate, methyl 2,2,2-trifluoroethyl carbonate, 1,3-dioxacyclopentan-2-one, 4-[2,3,3,3-tetrafluoro-2-(trifluoromethyl)propyl], ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, methyl acetate, ethyl propionate, fluorinated vinyl carbonate, or diethyl ether, ethylene glycol dimethyl ether, 1,3-dioxacyclopentanone, diethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl tert-butyl ether, hydrofluoroether, and fluorinated ether D2, and the concentration of the electrolyte is 0.1-5 mol.

9. A method for preparing a sodium-ion battery according to claim 8, characterized in that, The electrolyte solvent is selected from ether solvents, fluorinated ethers, or fluorinated carbonate solvents.

Images