Sodium metal battery and electrochemical device

Abstract

To provide a sodium metal battery in which sodium metal can form one uniform sodium deposition layer on a surface of a negative electrode current collector in a charge / discharge process and reversibility of the charge / discharge process is secured, and an electrochemical device.SOLUTION: The present application relates to a sodium metal battery having a positive electrode sheet and a negative electrode sheet that is a negative electrode current collector, and a thickness of a sodium layer being equal to or greater than 30nm and deposited in situ on the negative electrode current collector after first charge and discharge, and to an electrochemical device. The present application utilizes an initial irreversible capacity of a positive electrode material and cell design optimization. After initial charge and discharge of a cell, an amount of sodium metal remains enough such that a sodium deposition layer having a predetermined thickness can be uniformly formed on a surface of the negative electrode current collector, thereby avoiding higher nucleation energy required to deposit sodium on a surface of the current collector in a subsequent charge and discharge cycle process, reducing the overall deposition overpotential, and ensuring deposition uniformity of a sodium metal and reversibility of the charge and discharge process.SELECTED DRAWING: None

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to Chinese Patent Application No. 202110742607.7, entitled "Sodium Metal Battery and Electrochemical Device," filed on June 26, 2021, the entire contents of which are incorporated herein by reference.

[0002] This application relates to the technical field of sodium batteries, and in particular to sodium metal batteries and electrochemical devices. [Background technology]

[0003] As lithium-ion battery technology is increasingly adopted in markets such as consumer electronics, electric vehicles, and power storage, the scarcity of lithium resources has become a growing concern. Sodium-based batteries have gradually gained attention due to the Earth's high abundance of sodium, and they hold a strategic position in cost-sensitive applications such as power storage. Due to metallic sodium's higher reduction potential and larger relative molecular mass than metallic lithium, sodium-ion batteries, which operate on a similar principle, have significantly lower energy densities than lithium-ion batteries. Furthermore, the larger ionic radius of sodium ions can lead to greater volume expansion during insertion and extraction into and from the cathode and anode materials, reducing the battery's cycling reversibility. These factors have significantly limited the application and widespread use of sodium-ion batteries. Advances in electrolyte and additive technologies and surface modification technologies have significantly improved the long-standing academic problem of sodium dendrite growth due to uneven metal surface deposition, potentially leading to significant improvements in product safety. This has brought high-energy-density sodium metal anodes back into the realm of possibility.

[0004] To further increase cell energy density, "anode-less" sodium metal batteries have also been developed, in which sodium from the cathode material is desorbed and deposited in situ on the anode current collector. This significantly improves cell manufacturability and safety by eliminating the need for pre-coating / deposition of highly active sodium metal on the anode side. However, anode-less sodium metal batteries require higher overpotentials for deposition on the anode current collector surface, which is also prone to uneven sodium deposition, resulting in severe side reactions with the electrolyte and high consumption of active sodium, ultimately affecting the cell's cycle performance. Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above, in order to overcome the above-mentioned drawbacks, the present application provides a sodium metal battery and an electrochemical device in which sodium metal can form a uniform sodium deposition layer on the surface of the negative electrode current collector during the charge and discharge process, thereby ensuring reversibility of the charge and discharge process. [Means for solving the problem]

[0006] In a first aspect, the present application provides a sodium metal battery including a positive electrode sheet and a negative electrode sheet that is a negative electrode current collector, wherein a thickness of a sodium layer in situ deposited on the negative electrode current collector after an initial charge / discharge is ≥ 30 nm.

[0007] In the sodium metal battery of the present application, the negative electrode active material is formed in situ by depositing sodium released from the positive electrode. After the initial charge / discharge of the cell, due to the incomplete reversibility of the initial sodium release / insertion from the positive electrode active material, some sodium metal remains on the negative electrode side and cannot return to the positive electrode. Due to the non-uniformity of the negative electrode current collector surface and the limited high activity of the reaction between sodium metal and the electrolyte, when the total amount of residual sodium metal is low, there is a significant non-uniformity in its distribution on the current collector surface. The active sodium-retaining regions have a lower nucleation energy (corresponding to a lower deposition overpotential) than the non-sodium-retaining regions, making it easier for sodium metal to deposit during subsequent charging. This exacerbates the problem of non-uniform sodium deposition, resulting in severe side reactions between the highly active regions (tip, dendrite regions) and the electrolyte, ultimately resulting in the consumption of active sodium and a decline in battery performance. This application utilizes the initial irreversible capacity of the positive electrode material and cell design optimization to ensure that a sufficiently large amount of sodium metal remains on the current collector after the first charge / discharge of the cell so that a uniform sodium deposition layer of a predetermined thickness can be formed on the current collector surface, thereby avoiding the higher nucleation energy required to deposit sodium on the current collector surface during subsequent charge / discharge cycles, reducing the overall deposition overpotential, and ensuring uniform sodium metal deposition and reversibility of the charge / discharge cycle. Specifically, the sodium deposition thickness on the negative electrode after the first charge / discharge of the cell is required to be ≥ 30 nm.

[0008] In some alternative embodiments, the initial charge capacity Q C mAh / g, initial discharge capacity Q D mAh / g, applied mass of positive electrode active material C W g / cm 2 and the theoretical volumetric capacity of sodium metal, X mAh / cm 3 but,

number

[0009] In some alternative embodiments, the negative electrode current collector comprises an aluminum-based current collector, the aluminum-based current collector comprising: (1) The aluminum-based current collector includes at least one of aluminum foil and aluminum alloy foil; (2) The aluminum-based current collector is an aluminum-based composite current collector including a polymer base film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer base film; (3) The aluminum-based current collector is an aluminum-based composite current collector including a polymer base film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer base film, and the polymer base film is any one of polyamide, polyester terephthalate, polyimide, polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, polyparaphenylene terephthalamide, ethylene propylene rubber, polyoxymethylene, epoxy resin, phenolic resin, polytetrafluoroethylene, polyvinylidene fluoride, silicone rubber, and polycarbonate; (4) The aluminum-based current collector has at least one of the following technical features: the surface roughness is 0.3 μm to 1.5 μm.

[0010] In some alternative embodiments, a conductive coating including an adhesive and a conductive agent containing at least one of metal, conductive carbon, conductive polymer, and conductive ceramic material is provided on at least a portion of the surface of the negative electrode current collector. In some alternative embodiments, the conductive coating comprises: (5) The metal has a body-centered cubic structure and contains any one of α-Fe, V, Nb, Cr, Mo, Ta, and W; (6) The conductive carbon includes at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene; (7) The conductive polymer includes any one of polyaniline, polythiophene, polypyrrole, and polyphenylacetylene; (8) The conductive ceramic material contains at least one of TiB2, TiC, and B4C3; (9) The adhesive contains any one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene butadiene rubber, sodium alginate, lithium / sodium polyacrylate, polytetrafluoroethylene, polyimide, and polyurethane; (10) The mass ratio of the adhesive to the conductive agent is 1:(1 to 30).

[0011] In some alternative embodiments, the conductive coating has a thickness of 1 μm to 10 μm.

[0012] In some alternative embodiments, the conductive coating can be formed by any of the following methods: transfer coating, extrusion coating, and spray coating.

[0013] In some alternative embodiments, the positive electrode active material includes at least one of a sodium transition metal oxide, a polyanion-type compound, and a Prussian blue-based compound.

[0014] In some alternative embodiments, the battery has an initial coulombic efficiency of 80% to 99%.

[0015] In a second aspect, the present application provides an electrochemical device comprising a sodium metal battery according to the first aspect. [Effects of the Invention]

[0016] The beneficial effects of the present application are as follows: (1) This application utilizes the initial irreversible capacity of the positive electrode material and optimized cell design to ensure that a sufficiently large amount of sodium metal remains on the current collector surface after the first charge / discharge of the cell so that a uniform sodium deposition layer with a predetermined thickness can be formed on the current collector surface. This avoids the higher nucleation energy required to deposit sodium on the current collector surface during subsequent charge / discharge cycles, reduces the overall deposition overpotential, and ensures uniform sodium metal deposition and reversibility of the charge / discharge process. (2) In the present application, by providing a conductive coating on the surface of the negative electrode current collector, the overpotential required for sodium deposition can be further reduced, thereby ensuring uniform deposition of sodium metal. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following are optional embodiments of the examples of the present application, and it should be noted that those skilled in the art may make some further improvements and modifications without departing from the principles of the examples of the present application, and these improvements and modifications should also be considered to fall within the protection scope of the examples of the present application.

[0018] The terms used in the examples of this application are merely for the purpose of describing particular examples and are not intended to limit the scope of this application. As used in the examples of this application and the appended claims, the singular forms "a," "the," and "the" are intended to include the plural forms unless the context clearly dictates otherwise.

[0019] An embodiment of the present application provides a sodium metal battery, which may include at least one of a pouch, a rectangular aluminum housing, a rectangular steel housing, a cylindrical aluminum housing, and a cylindrical steel housing battery, and the battery includes a positive electrode sheet and a negative electrode sheet that is an aluminum-based current collector, and after the first charge and discharge, a thickness of a sodium layer deposited in situ on the aluminum-based current collector is ≧30 nm.

[0020] In contrast to the above technical solution, the sodium metal battery of the present application does not require the use of a negative electrode active material; the negative electrode active material is formed in situ by depositing sodium released from the positive electrode. After the initial charge / discharge of the cell, due to the incomplete reversibility of the initial release / insertion of sodium from the positive electrode active material, some sodium metal remains on the negative electrode side and cannot return to the positive electrode. Due to the non-uniformity of the negative electrode current collector surface and the limited high activity of the reaction between sodium metal and the electrolyte, when the total amount of residual metal sodium is low, there is a significant non-uniformity in its distribution on the negative electrode current collector surface. The active sodium-remaining regions have a lower nucleation energy (corresponding to a lower deposition overpotential) than the non-sodium-remaining regions, making it easier for sodium metal to deposit during subsequent charging. This exacerbates the problem of non-uniform sodium deposition, resulting in severe side reactions between the highly active regions (tip, dendrite regions) and the electrolyte, ultimately resulting in the consumption of active sodium and a decline in battery performance. This application utilizes the initial irreversible capacity of the positive electrode material and cell design optimization to ensure that a sufficient amount of sodium metal remains on the current collector surface after the first charge / discharge of the cell so that a uniform sodium deposition layer of a predetermined thickness can be formed on the current collector surface. This avoids the higher nucleation energy required to deposit sodium on the current collector surface during subsequent charge / discharge cycles, reduces the overall deposition overpotential, and ensures uniform sodium metal deposition and reversibility of the charge / discharge cycle. The sodium deposition layer has a thickness of ≥ 30 nm, specifically, the thickness of the sodium deposition layer can be 30 nm, 31 nm, 32 nm, 33 nm, 34 nm, 35 nm, 36 nm, 37 nm, 38 nm, 39 nm, 40 nm, etc., but is not limited thereto. A sodium deposition layer thickness of 30 nm or greater can meet the sodium deposition requirement on the negative electrode while also satisfying some of the sodium consumption associated with the formation of by-products during the reaction of the negative electrode sodium metal with the electrolyte.

[0021] In some embodiments, the initial charge capacity Q of the positive electrode active material in the positive electrode sheet C mAh / g, initial discharge capacity Q D mAh / g, applied mass of positive electrode active material C W g / cm 2and the theoretical volumetric capacity of sodium metal, X mAh / cm 3 but,

number

[0022] In the above formula (I), the theoretical volumetric capacity of sodium metal is X mAh / cm 3 =1166mAh / g*0.97g / cm 3 , 1166 mAh / g is the theoretical reversible specific capacity of sodium metal, and 0.97 g / cm 3 is the theoretical density of sodium metal, and 10 7 The units are converted from cm to nm. By limiting the initial charge / discharge capacity and coating mass of the positive electrode material within the above ranges, the cell can ensure that sufficient sodium remains on the negative electrode side after the initial charge / discharge. The consumption of some sodium due to the reaction with the electrolyte to form by-products is also taken into consideration. If the above cell design value is less than 300 nm, the amount of active sodium remaining on the negative electrode current collector surface after the initial charge / discharge is insufficient, resulting in incomplete coverage of the current collector surface. If the above cell design value is greater than 5000 nm, the initial coulombic efficiency of the positive electrode material will be low or the coating mass of the material will be too high. The former is detrimental to the cell's energy density, while the latter is detrimental to the cell's final cycle performance due to issues such as powder shedding and poor wetting caused by a thick sheet. Neither of these is practical.

[0023] In some embodiments, the initial coulombic efficiency of the battery may be 80% to 99%. When the initial coulombic efficiency of the battery is greater than 99%, the initial irreversible capacity of the positive electrode material is low. To ensure a sufficient sodium deposition thickness on the negative electrode side after the first charge / discharge, an excessively large coating weight of the positive electrode material is required, which is likely to cause problems such as powder shedding during cell production and brittle sheets after cold pressing, which is unfavorable for batch production of cells. When the initial coulombic efficiency of the battery is less than 80%, the initial irreversible capacity of the positive electrode material is too large, the reversible capacity of the material is low, and the cell energy density is low, which significantly reduces its practicality.

[0024] In some embodiments, the negative electrode current collector used in the negative electrode sheet may include at least one of a metal foil current collector, a metal foam current collector, a metal mesh current collector, a carbon felt current collector, a carbon cloth current collector, and a carbon paper current collector. Because sodium ions do not form alloys with aluminum, an aluminum-based current collector may be used to reduce cost and weight. The aluminum-based current collector may be any of aluminum foil, aluminum alloy foil, and aluminum-based composite current collectors. The aluminum-based composite current collector may include a polymer base film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer base film. Optionally, the aluminum-based composite current collector may have a "sandwich" structure, with a polymer base film in the middle and aluminum foil or aluminum alloy foil on both sides, and further with aluminum foil on one side and aluminum alloy foil on the other side of the polymer base film. The polymer base film may be any of polyamide, polyester terephthalate, polyimide, polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, polyparaphenylene terephthalamide, ethylene propylene rubber, polyoxymethylene, epoxy resin, phenolic resin, polytetrafluoroethylene, polyvinylidene fluoride, silicone rubber, and polycarbonate. Alternatively, the present application selects an aluminum-based composite current collector, which has better ductility and is advantageous in maintaining the integrity of the electrode during sodium deposition / desorption.

[0025] In some embodiments, the surface roughness of the aluminum-based current collector may be 0.3 μm to 1.5 μm, specifically, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.0 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, etc., but is not limited thereto. Keeping the surface roughness of the aluminum-based current collector within the above range ensures that the deposited sodium has a suitable bonding strength with the aluminum-based current collector. When the roughness is less than 0.3 μm, the surface of the aluminum-based current collector is too smooth, and the bonding strength between the deposited sodium and the aluminum-based current collector is insufficient, making it prone to peeling and powder shedding during use, resulting in a loss of contact with the conductive network and electrical insulation, which will affect the capacity and cycle life of the cell. When the roughness is greater than 1.5 μm, sodium is prone to be unevenly deposited at localized high-activity tips, making it easier for dendrites to form, posing a safety risk to the cell.

[0026] In some embodiments, a conductive coating is applied to at least a portion of the surface of the negative electrode current collector, and the conductive coating may include a conductive agent and an adhesive, and the conductive agent may include at least one of metal, conductive carbon, conductive polymer, and conductive ceramic material. In the present application, by applying a conductive coating to the surface of the negative electrode current collector, if the separator between the positive and negative electrodes is damaged, the negative electrode current collector is short-circuited to the positive electrode current collector through the conductive coating, preventing thermal runaway due to an internal short circuit in the cell. Furthermore, the short-circuiting between the negative electrode current collector and the positive electrode current collector through the conductive coating allows energy within the cell to be quickly consumed, thereby avoiding thermal runaway in the battery cell. Furthermore, the conductive coating reduces the contact resistance between sodium metal and the negative electrode current collector, increasing the force between the sodium metal and the negative electrode current collector and preventing the sodium metal layer from peeling off. The thickness of the conductive coating may be 1 μm to 10 μm, specifically 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc., but is not limited thereto. If the conductive coating thickness is greater than 10 μm, a certain energy density loss will occur, and if the conductive coating thickness is less than 1 μm, the coating distribution will be uneven and the corresponding function will not be achieved.

[0027] The conductive coating may be a metal layer, and the metal may have a body-centered cubic structure and may include any one of α-Fe, V, Nb, Cr, Mo, Ta, and W. The conductive carbon may include at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene. The conductive polymer may include any one of polyaniline, polythiophene, polypyrrole, and polyphenylacetylene. The conductive ceramic material may include at least one of TiB2, TiC, and B4C3.

[0028] The adhesive may include any of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene butadiene rubber, sodium alginate, lithium polyacrylate, sodium polyacrylate, polytetrafluoroethylene, polyimide, and polyurethane. The mass ratio of the adhesive to the conductive agent may be 1:1 (1 to 30), specifically, 1:1, 1:5, 1:10, 1:15, 1:20, 1:25, 1:30, etc., but is not limited thereto. If the amount of adhesive is too small, the conductive coating is likely to fall off. If the amount of adhesive is too large, the bonding strength between the aluminum-based current collector and sodium metal is weakened. Therefore, by forming a conductive coating using an adhesive and a conductive agent, not only can the resistance be reduced, but the bonding strength between the aluminum-based current collector and sodium metal can be strengthened, which can further reduce the overpotential of sodium deposition and improve the cycle performance of the cell.

[0029] The conductive coating can be made of a conductive material such as metal or conductive ceramic. The conductive material can partially or completely cover the surface of the aluminum-based current collector. The conductive coating not only reduces resistance but also strengthens the bonding strength between the aluminum-based current collector and the sodium metal. The conductive coating can be formed by transfer coating, extrusion coating, or spray coating. Specifically, the conductive coating can be prepared by adding an adhesive and a conductive agent to a water solvent in a predetermined ratio and stirring for 6 to 8 hours to homogenize the mixture to obtain a conductive slurry. The conductive slurry can then be applied to a perforated current collector using a gravure coater and baked to obtain the conductive coating.

[0030] In some embodiments, the positive electrode active material may include at least one of a sodium transition metal oxide, a polyanion-type compound, and a Prussian blue-based compound. In the sodium transition metal oxide, the transition metal may be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide may be, for example, Na xIt may be MO2, where M may be one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1. The polyanion-type compound includes one or more of sodium vanadium trifluorophosphate Na3V2(PO4)2F3, sodium vanadium fluorophosphate NaVPO4F, sodium vanadium phosphate Na3V2(PO4)3, Na4Fe3(PO4)2P2O7, NaFePO4, and Na3V2(PO4)3. The Prussian blue-based compound is Na x M 1 M 2 (CN)6, where M 1 , M 2 is one or more of Fe, Mn, Co, Ni, Cu, Zn, Cr, Ti, V, Zr, and Ce, and 0 < x ≤ 2.

[0031] An adhesive and / or a conductive agent may be further added to the positive electrode active material. The types of the adhesive and the conductive agent are not limited, and those skilled in the art can select according to actual needs. For example, the above adhesive may be one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR), and the above conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, carbon nanotubes, graphene, and carbon nanofibers.

[0032] The material of the positive electrode current collector is not limited, and those skilled in the art can select according to actual needs. Preferably, a metal may be used. The metal may include, for example, but is not limited to, aluminum foil.

[0033] When fabricating the positive electrode sheet by a general method in this field, usually, the positive electrode active material, the selectable conductive agent, and the adhesive may be dispersed in a solvent. Generally, N-methylpyrrolidone (NMP) may be selected as the solvent to form a uniform positive electrode slurry. The positive electrode slurry is applied to at least one surface of the positive electrode current collector, and processes such as baking and cold pressing are performed to obtain the positive electrode sheet.

[0034] Furthermore, the electrochemical device may further include a separator, which is placed between the positive electrode and the negative electrode to prevent short circuits. The material and shape of the separator do not need to be specially selected and can be selected by those skilled in the art according to actual needs.

[0035] In some embodiments, the separator may include a substrate layer, and the substrate layer may be a nonwoven fabric, a membrane, or a composite membrane having a porous structure. In some embodiments, the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. In some embodiments, the material of the substrate layer may include a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0036] In some embodiments, at least one surface of the substrate layer is provided with a surface treatment layer. In some embodiments, the surface treatment layer can be a polymer layer, an inorganic layer, or a mixed polymer-inorganic layer. In some embodiments, the polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylic ester polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0037] In some embodiments, the inorganic layer may include inorganic particles and an adhesive. In some embodiments, the inorganic particles may include a combination of one or more of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate.

[0038] In some embodiments, the adhesive may include one or more combinations of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0039] The electrochemical device may further include an electrolyte, which may include a sodium salt and an organic solvent. Specifically, the organic solvent in the electrolyte is not particularly limited, and may be any organic solvent commonly used in the art for electrolytes. For example, the organic solvent may be at least one selected from ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propylidene carbonate, methyl acetate, ethyl propionate, fluoroethylene carbonate, ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and methyl tert-butyl ether. Preferably, an ether-based solvent is selected to control the deposition morphology of sodium ions and thereby suppress the bulk growth of sodium dendrites. In the electrochemical device of the present application, the sodium salt in the electrolyte is not particularly limited, and may be any sodium salt commonly used in the art for electrolytes. For example, the sodium salt may be at least one selected from sodium hexafluorophosphate, sodium bisfluorosulfonylimide, sodium bistrifluoromethanesulfonimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium perchlorate, and sodium chloride.

[0040] In the electrochemical device of the present application, suitable additives may be added to the electrolyte to improve the performance of the electrolyte.

[0041] The electrochemical device of the present application is not particularly limited in its application and can be used in any electronic device known in the prior art. In some embodiments, the electrochemical device of the present application can be used in, but is not limited to, notebook computers, pen-input personal computers, mobile personal computers, electronic book players, mobile phones, portable fax machines, portable copiers, portable printers, headphone stereos, video recorders, LCD televisions, handheld vacuum cleaners, portable CD players, minidiscs, transceivers, electronic organizers, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, mopeds, bicycles, lighting fixtures, toys, game machines, clocks, power tools, flash devices, cameras, large-scale household storage batteries, power storage, and sodium ion capacitors.

[0042] The following examples are provided to more specifically describe the disclosure of the present application, and are for illustrative purposes only, since various modifications and variations within the scope of the disclosure will be apparent to those skilled in the art. Unless otherwise specified, all parts, percentages, and proportions in the following examples are by weight, and all reagents used in the examples are commercially available or synthesized by conventional methods and can be used directly without further treatment. All equipment used in the examples is commercially available.

[0043] Example 1

[0044] (1) Preparation of negative electrode: A negative electrode was prepared using an aluminum foil with a thickness of 12 μm as a negative electrode current collector, and the roughness of the negative electrode current collector was 0.5 μm.

[0045] (2) Preparation of the positive electrode: The positive electrode active material NaFePO, the adhesive polyvinylidene fluoride (PVDF), and the conductive carbon black (Super-P) were uniformly mixed in a mass ratio of 96%:2%:2% in N-methylpyrrolidone (NMP) solvent to prepare a positive electrode slurry. The positive electrode active material was applied to the surface of an aluminum foil using a squeeze coater according to the required unit area mass of the positive electrode active material, and the aluminum foil was baked. The applied sheet was then coated with a 2.5 g / cm2 coating using a cold press machine. 3 The final positive electrode sheet was obtained by cold pressing at a design compaction ratio of 1000 to 10000, where the positive electrode material and coating mass of each example are shown in Table 1.

[0046] (3) Preparation of electrolyte: NaPF6 with a concentration of 1 mol / L was dissolved in a solvent in which diethylene glycol dimethyl ether / tetraethylene glycol dimethyl ether were mixed in a volume ratio of 1:1 to obtain an electrolyte.

[0047] (4) Battery Assembly: Coin-type batteries were used to evaluate the electrical properties of the positive electrode material, such as the specific capacity and initial coulombic efficiency. The positive electrode sheet was punched into small circular pieces with a diameter of 14 mm using a puncher, and each positive electrode sheet was weighed using a balance. In a dry room, coin-type half-cells were assembled from small circular negative electrodes, separators (Celgard 2300 model), sodium sheets (16 mm diameter), and coin-type battery cases. An electrolyte solution consisting of 1 mol / L NaPF6 dissolved in a 1:1 volumetric mixture of diethylene glycol dimethyl ether and tetraglyme was added dropwise, and the coin-type half-cells were finally packed using a coin-type battery packing device to obtain coin-type half-cells.

[0048] (5) Assembling the entire battery

[0049] The energy density and cycle performance of the cell were tested using the entire battery. The positive and negative electrode sheets and separator were cut to the appropriate size and wound on a winder to form a dry battery cell. A 10 Ah pouch sodium metal battery was then fabricated using the standard process flow, including welding, aluminum plastic film packing, electrolyte injection, chemical formation, gas extraction, secondary packing, and filling. The electrolyte injection rate was set at 3 g / Ah.

[0050] Examples 2-3 and Comparative Examples 1-2

[0051] The difference from Example 1 is that the initial coulombic efficiency of the battery cell was adjusted to change the design value of the cell; see Table 1 below for details.

[0052] Example 4 and Comparative Examples 3 to 4

[0053] The difference from Example 1 is that the design values ​​of the cell were changed by adjusting the applied mass of the active material; see Table 1 below for details.

[0054] Examples 5 to 8 and Comparative Examples 5 to 6

[0055] The difference from Example 1 is that the roughness of the negative electrode current collector was adjusted.

[0056] Examples 9 to 11 and Comparative Examples 7 to 8

[0057] The difference from Example 1 is that the conductive coating was increased and the thickness of the conductive coating was adjusted. [Table 1-1] [Table 1-2] [Table 1-3]

[0058] (Performance test)

[0059] (1) Positive electrode material specific capacity test:

[0060] A charge / discharge test was performed on a coin-type cell containing the positive electrode material using a battery tester to evaluate the electrochemical characteristics of the cell. The charge / discharge voltage was set to 2.5 V to 3.65 V, and the charge / discharge current was set to 50 mA / g. The initial charge / discharge capacity of the battery was read. The specific charge / discharge capacity of the positive electrode material was calculated using the following formula:

number

[0061] (2) Full battery test:

[0062] The electrochemical characteristics of the cell were evaluated by performing a charge / discharge test on the cell using a battery tester. The charge / discharge voltage was set to 2.5 V to 3.65 V, and the charge / discharge current was set to 1 A (0.1 C). The cell capacity and average voltage plateau corresponding to the initial charge / discharge and the discharge from 3.65 V to 2.5 V after 200 charge / discharge cycles were recorded. The cell was weighed using an electronic balance with a precision of 1 / 1000, and the gravimetric energy density, volumetric energy density, and capacity retention after 200 cycles were calculated using the following equations:

number

number

[0063] (3) Sodium Deposition Thickness Test:

[0064] After the first charge / discharge, the cell was disassembled, and the negative electrode interface was observed using SEM. The sodium deposition layer (sodium element content ≥ 80%) was confirmed using EDS, and the thickness of this layer was measured.

[0065] (4) Deposition overpotential test:

[0066] After the first charge and discharge, the cell was disassembled, the negative electrode sheet was removed and punched, and a coin-type half cell was assembled with a separator, a sodium sheet, and an electrolyte. The discharge voltage of the coin-type battery was -100mV vs Na / Na + and set the current density to 1mA / cm 2 The lowest voltage point in the capacity-voltage discharge curve was read and taken as the overpotential for sodium deposition on the negative electrode sheet. The test results are shown in Table 2. [Table 2]

[0067] As can be seen from a comparison of Examples 1-4 and Comparative Examples 1-4, by selecting positive electrode materials with different Coulombic efficiencies and controlling the coating mass, the cells formed a sodium deposition layer of a predetermined thickness on the negative electrode surface after the initial charge / discharge. During subsequent charging and discharging, the negative electrode deposition overpotential was significantly reduced, resulting in more uniform sodium deposition, which was beneficial for improving cell cycling performance. When a positive electrode material with higher initial Coulombic efficiency was selected (Comparative Example 1), even with a high coating mass, there was a limit to the sodium deposition thickness after the initial charge / discharge. A uniform sodium deposition layer could not be formed, resulting in a relatively high negative electrode deposition overpotential and a significant decrease in cell cycling performance. When a positive electrode material with low initial Coulombic efficiency was used (Comparative Example 2), the cell cycling performance improved, but the cell energy density was low, making it unsuitable for practical use. When a thicker sodium deposition layer was obtained by increasing the coating mass (Comparative Example 3), the negative electrode deposition overpotential was reduced, but the sheet was too thick, which not only caused problems with powder shedding during the processing / winding process but also hindered electrolyte penetration, preventing a significant improvement in cell cycling performance. When the coating mass was relatively small (Comparative Example 4), the sodium deposition thickness was insufficient to achieve the effect of reducing the deposition overpotential, and the improvement in the cell cycle performance was not obvious, and at the same time, it was disadvantageous to the improvement of the cell energy density.

[0068] As can be seen from a comparison of Examples 5 to 8 and Comparative Examples 5 to 6, by limiting the surface roughness of the aluminum-based current collector within the limited range in Examples 5 to 8, it was possible to ensure an appropriate bonding strength between the deposited sodium and the aluminum-based current collector. If the surface roughness of the aluminum-based current collector was too small (Comparative Example 5), the bonding strength between the sodium deposition layer and the current collector was weak, causing peeling and resulting in electrical insulation and loss of activity. If the surface roughness of the aluminum-based current collector was too large (Comparative Example 6), sodium deposition became uneven at localized tip positions of the current collector, making it easier for sodium dendrites to form and causing severe side reactions with the electrolyte, which in turn reduced the electrical characteristics of the cell and increased the risk of short circuits.

[0069] As can be seen from a comparison between Examples 9 to 11 and Comparative Examples 7 and 8, by applying a conductive coating to the surface of an aluminum-based current collector and keeping the thickness of the conductive coating within the selectable range of this application (Examples 9 to 11), it is possible to further reduce the sodium deposition overpotential and further improve the cycle performance of the cell. However, when the conductive coating was applied too thinly (Comparative Example 7), it was difficult for the conductive coating to cover the entire current collector, and the local nucleation energy in the uncoated areas was somewhat high, which did not significantly reduce the overall deposition overpotential. However, when the conductive coating was applied too thickly (Comparative Example 8), it was disadvantageous to improving the cell energy density.

[0070] In summary, the present application utilizes the initial irreversible capacity of the positive electrode material and optimized cell design to ensure that a sufficiently large amount of sodium metal remains on the current collector after the first charge / discharge of the cell so that a uniform sodium deposition layer having a predetermined thickness can be formed on the current collector surface, thereby avoiding the higher nucleation energy required to deposit sodium on the current collector surface during subsequent charge / discharge cycles, reducing the overall deposition overpotential, and ensuring uniform sodium metal deposition and reversibility of the charge / discharge process.

[0071] The above is merely a preferred embodiment of the present application, and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and principles of the present application shall fall within the scope of protection of the present application.

Claims

1. A sodium metal battery including a positive electrode sheet and a negative electrode sheet including a negative electrode current collector, the negative electrode active material of the negative electrode sheet is sodium metal formed by depositing sodium released from the positive electrode; The initial charge capacity Q of the positive electrode active material in the positive electrode sheet C mAh / g, initial discharge capacity Q D mAh / g, applied mass C of positive electrode active material W g / cm 2 and the theoretical volumetric capacity of sodium metal X mAh / cm 3 but, [Equation 1] Sodium metal batteries.

2. The negative electrode current collector includes an aluminum-based current collector, and the aluminum-based current collector is (1) The aluminum-based current collector includes at least one of aluminum foil and aluminum alloy foil; (2) The aluminum-based current collector is an aluminum-based composite current collector including a polymer base film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer base film; (3) The aluminum-based current collector is an aluminum-based composite current collector including a polymer base film and aluminum foil and / or aluminum alloy foil formed on both sides of the polymer base film, and the polymer base film is any one of polyamide, polyester terephthalate, polyimide, polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, polyparaphenylene terephthalamide, ethylene propylene rubber, polyoxymethylene, epoxy resin, phenolic resin, polytetrafluoroethylene, polyvinylidene fluoride, silicone rubber, and polycarbonate. The sodium metal battery according to claim 1, comprising at least one of the following technical features:

3. a conductive coating is provided on at least a portion of the surface of the negative electrode current collector; 2. The sodium metal battery of claim 1, wherein the conductive coating contains a conductive agent including at least one of a metal, a conductive carbon, a conductive polymer, and a conductive ceramic material, and an adhesive.

4. The conductive coating comprises: (5) The metal has a body-centered cubic structure and includes any one of α-Fe, V, Nb, Cr, Mo, Ta, and W; (6) The conductive carbon includes at least one of conductive carbon black, graphite, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and fullerene; (7) The conductive polymer includes any one of polyaniline, polythiophene, polypyrrole, and polyphenylacetylene; (8) The conductive ceramic material is TiB 2 , TiC, B 4 C 3 containing at least one of (9) The adhesive includes any one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene butadiene rubber, sodium alginate, lithium / sodium polyacrylate, polytetrafluoroethylene, polyimide, and polyurethane; (10) The sodium metal battery according to claim 3, which includes at least one of the technical features: a mass ratio of the adhesive to the conductive agent is 1:(1 to 30).

5. 4. The sodium metal battery of claim 3, wherein the conductive coating has a thickness of 1 μm to 10 μm.

6. 2. The sodium metal battery according to claim 1, wherein the positive electrode active material comprises at least one of a sodium transition metal oxide, a polyanion-type compound, and a Prussian blue-based compound.

7. 10. The sodium metal battery of claim 1, wherein the initial coulombic efficiency is between 80% and 99%.

8. An electrochemical device comprising the sodium metal battery of any one of claims 1 to 7.

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