Sodium secondary battery and electric device

Abstract

The invention discloses a sodium secondary battery, electrolyte and electric equipment, the sodium secondary battery comprises a positive pole piece, electrolyte and a negative pole piece, the positive pole piece comprises a positive current collector and a positive active material layer arranged on at least one side of the positive current collector, and the negative pole piece comprises a negative active material layer arranged on at least one side of the negative current collector. The positive electrode active material layer comprises a first positive electrode active material layer and a second positive electrode active material layer which are sequentially stacked in the direction away from the positive electrode current collector, the first positive electrode active material layer comprises a first positive electrode active material, and the second positive electrode active material comprises a second positive electrode active material; the second positive electrode active material comprises Na4M3 (PO4) 2P2O7, and M comprises at least one of Fe, Co, Mn or Ni; the electrolyte includes a solvent including a linear ether and a cyclic ether. According to the sodium secondary battery, the generation of sodium dendrites can be reduced, and the cycle life is prolonged.

Description

Technical Field

[0001] The present application belongs to the field of secondary batteries, and specifically relates to a sodium secondary battery and electrical equipment. Background Art

[0002] Secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] As a promising energy storage technology, sodium secondary batteries (NSBs) offer broad development prospects due to their abundant raw materials, low manufacturing costs, and strong temperature adaptability. However, due to their system characteristics, dendrites are easily formed at the negative electrode of SSBs. The cumulative growth of these dendrites can cause a short circuit (internal short circuit) within the battery, leading to a rapid deterioration in battery capacity and significantly limiting the cycling stability of SSBs. Summary of the Invention

[0004] In view of the technical problems existing in the background technology, the present application provides a secondary battery, which aims to suppress the internal short circuit caused by the growth of sodium dendrites and improve the cycle performance of the sodium secondary battery.

[0005] In order to achieve the above-mentioned objectives, the first aspect of the present application provides a sodium secondary battery, which includes a positive electrode plate, an electrolyte and a negative electrode plate, wherein the positive electrode plate includes a positive electrode collector and a positive electrode active material layer provided on at least one side of the positive electrode collector, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer stacked in sequence along a direction away from the positive electrode collector, the first positive electrode active material layer includes a first positive electrode active material, the second positive electrode active material includes a second positive electrode active material, the second positive electrode active material includes Na4M3(PO4)2P2O7, and M includes at least one of Fe, Co, Mn or Ni; the electrolyte includes a solvent, and the solvent includes a linear ether and a cyclic ether.

[0006] The present application includes at least the following beneficial effects: in the sodium secondary battery of the present application, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer, and the positive electrode active material of the layer close to the electrolyte uses Na4M3(PO4)2P2O7, which is combined with the linear ether and cyclic ether of the electrolyte solvent to reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0007] In some embodiments, the residual alkali in the Na4M3(PO4)2P2O7 accounts for 0.05% to 2.5% by mass. This can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0008] In some embodiments, the residual alkali in the Na4M3(PO4)2P2O7 accounts for 0.05% to 0.5% by mass, thereby reducing the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0009] In some embodiments, the volume ratio of the linear ether to the cyclic ether is (60:40)-(90:10). This can reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0010] In some embodiments, a carbon coating material is formed on at least a portion of the surface of the Na4M3(PO4)2P2O7, and the mass of the carbon coating material accounts for 1.5% to 2.5% of the total mass of the second positive electrode active material. This can reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0011] In some embodiments, the first positive electrode active material layer further includes a third positive electrode active material, wherein the third positive electrode active material includes Na4M3(PO4)2P2O7, where M includes at least one of Fe, Co, Mn, or Ni. This can reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0012] In some embodiments, the mass of the third positive electrode active material accounts for 1% to 3% of the total mass of the first positive electrode active material layer, thereby reducing internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0013] In some embodiments, the positive electrode active material includes the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material, and the mass proportion of the Na4M3(PO4)2P2O7 is 5%-20% based on the total mass of the positive electrode active material. This can reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0014] In some embodiments, the electronic conductivity of the positive electrode plate is greater than or equal to 25 μS·cm -1 Thus, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0015] In some embodiments, the mass ratio of the first cathode active material to the second cathode active material is (80:20) to (95:5), thereby reducing internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0016] In some embodiments, the sodium secondary battery satisfies one or more of the following conditions: the ionic conductivity of the solvent is greater than or equal to 9 mS·cm -1 The linear ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether, or diethylene glycol diethyl ether; the cyclic ether includes at least one of tetrahydrofuran, methyltetrahydrofuran, or 1,3-dioxolane; the volume average particle size Dv50 of the Na4M3(PO4)2P2O7 is 2.5μm-3.5μm; the first positive electrode active material includes at least one of a sodium transition metal oxide, a polyanion compound, a Prussian blue-type sodium compound, and their respective modified compounds; and the second positive electrode active material includes Na4Fe3(PO4)2P2O7. Thus, internal short circuits caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, and the cycle performance of sodium secondary batteries can be improved.

[0017] In some embodiments, the first cathode active material comprises NaM1O2 or Na x R y P m O n At least one of the following: wherein M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, or Cu, 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb. Thus, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0018] In some embodiments, the electrolyte further includes a solvent, the first positive electrode active material includes NaM1O2, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, and the sodium secondary battery satisfies one or more of the following conditions: the surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2The thickness of the first positive electrode active material layer is 28.56 μm-33.9 μm; the surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 The thickness of the second positive electrode active material layer is 2.94 μm-11.7 μm; the compaction density of the positive electrode active material layer is 2.58 g / cm 3 -2.75g / cm 3 The compaction density of the first positive electrode active material layer is 2.8g / cm 3 -3.2g / cm 3 The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 Thus, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0019] In some embodiments, the first cathode active material comprises Na x R y P m O n , 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W or Pb, and the sodium secondary battery meets one or more of the following conditions: the surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2 The thickness of the first positive electrode active material layer is 4.70 μm-5.59 μm; the surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 The thickness of the second positive electrode active material layer is 2.94 μm-11.7 μm; the compaction density of the positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 ; The compaction density of the first positive electrode active material layer is

[0020] 1.7g / cm 3 -2g / cm 3 The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 Thus, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0021] In some embodiments, the sodium secondary battery satisfies one or more of the following conditions: based on the total mass of the first positive electrode active material layer, the mass proportion of the first positive electrode active material is 77%-80%; based on the total mass of the second positive electrode active material layer, the mass proportion of the second positive electrode active material is 90%-95%. This can reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0022] In some embodiments, the sodium secondary battery comprises a sodium metal battery, thereby reducing internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0023] In some embodiments, the negative electrode plate includes a negative electrode current collector and an interface modification layer disposed on at least one side of the negative electrode current collector, wherein the interface modification layer includes a binder and a conductive agent. This can reduce internal short circuits caused by sodium dendrite growth in sodium secondary batteries and improve the cycling performance of sodium secondary batteries.

[0024] In a third aspect of the present application, the present application proposes an electrical device comprising the secondary battery described in the first aspect of the present application.

[0025] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become obvious from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0027] Figure 1 Schematic diagram of a battery according to one embodiment of the present application.

[0028] Figure 2 yes Figure 1 An exploded view of a battery according to an embodiment of the present application is shown.

[0029] Figure 3 Schematic diagram of a battery module according to one embodiment of the present application.

[0030] Figure 4 Schematic diagram of a battery pack according to one embodiment of the present application.

[0031] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0032] Figure 6 Schematic diagram of an electrical device using a battery as a power source according to one embodiment of the present application.

[0033] Description of reference numerals:

[0034] 1 battery cell; 11 shell; 12 electrode assembly; 13 cover plate; 2 battery module; 3 battery pack; 31 upper box; 32 lower box. DETAILED DESCRIPTION

[0035] The following embodiments of the technical solution of the present application are described in detail. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only used as examples and are not intended to limit the scope of protection of the present application.

[0036] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0037] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0038] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0039] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0040] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0041] Currently, judging by market developments, the application of secondary batteries is becoming increasingly widespread. They are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power stations, but are also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric vehicles, as well as in military equipment and aerospace, among other fields.

[0042] As a storage technology with great potential, sodium secondary batteries show broad development prospects due to their abundant raw materials, low manufacturing cost, and strong temperature adaptability. However, due to its system characteristics, in sodium secondary batteries, the negative electrode is prone to dendrites. Taking the negative electrode-free sodium metal battery as an example, during charging, the sodium ions released from the positive electrode will be deposited on the negative electrode primer to form metallic sodium. The sodium metal deposition process is affected by factors such as the electrolyte, the isolation membrane, and the ion diffusion process of the positive electrode active material. Since the positive electrode active material has a strong oxidizing effect under high pressure, as the number of cycles increases, the side reactions on the surface of the positive electrode active material gradually increase, forming a gradually thickening CEI (Cathode Electrolyte Interface, positive electrode-electrolyte interface film), resulting in a gradual decrease in the rate of sodium ion insertion and extraction. At the same time, the side reactions will also cause pore blocking in the isolation membrane. In addition, the binding effect of the solvent in the electrolyte on sodium ions will also increase the resistance of sodium ions to escape from the solvent and deposit on the negative electrode. The above reasons will cause the sodium ions to form an uneven charge distribution during the transmission and deposition process, and gradually produce dendrites at the negative electrode. The cumulative growth of these dendrites will cause a short circuit (internal short) inside the battery, causing the battery capacity to deteriorate rapidly, greatly limiting the cycle stability of the sodium secondary battery.

[0043] In order to suppress the internal short circuit caused by the growth of sodium dendrites and improve the performance of the battery, a variety of improvement strategies have emerged. Optimizing the electrolyte composition, especially the use of additives, is the most commonly used method to improve the performance of sodium secondary batteries because it is simple and compatible with current industrial production lines. It is also one of the most widely used methods. Most additives help to form a stable SEI (Solid Electrolyte Interface) and further alleviate the problem of sodium dendrites, such as introducing fluorine-containing ions or solvents to preferentially participate in the formation of SEI at the negative electrode. However, preferential participation in interface construction leads to inevitable and irreversible consumption, and the role of fewer additives is limited; secondly, the battery will also experience capacity decay during long-term cycling. Although the addition of additives alleviates this problem, it increases the manufacturing cost of sodium secondary batteries, and fluorine-containing or other functional groups (such as CF, etc.) will produce adverse side reactions with sodium metal.

[0044] In the sodium secondary battery of the embodiment of the present application, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer, and the positive electrode active material of the layer close to the electrolyte uses Na4M3(PO4)2P2O7. The sodium ion diffusion coefficient of Na4M3(PO4)2P2O7 is high. When sodium ions enter the electrolyte from the surface of Na4M3(PO4)2P2O7, the ion transport generated is fast and the charge distribution is more uniform, so that the morphology of the negative electrode sodium metal deposition is more flat, and uniform sodium deposition is achieved, thereby reducing The internal short circuit of sodium secondary batteries is reduced; in addition, Na4M3(PO4)2P2O7 has excellent stability, which can reduce the oxidative decomposition reaction of the electrolyte at the interface between the positive electrode and the electrolyte, reduce the consumption of the electrolyte, the growth of the CEI film and the continuous occurrence of diaphragm plugging; More importantly, the experiment found that when the internal short circuit occurs during the cycle of the sodium secondary battery, the capacity of Na4M3(PO4)2P2O7 hardly decays, and the use of solvents containing linear ethers and cyclic ethers weakens the Na + The desolvation energy of the solvent reduces the effect of Na + When the generated sodium dendrites come into contact with Na4M3(PO4)2P2O7, the sodium dendrites will undergo oxidation reaction to become sodium ions. At the same time, the Na + It is quickly embedded in the Na4M3(PO4)2P2O7 structure, and the excellent reaction kinetics process reduces the consumption of active sodium at the interface, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0045] The sodium secondary battery disclosed in the embodiments of the present application can be used in various energy storage systems that use batteries as power sources or use batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the like, and spacecraft may include airplanes, rockets, space shuttles, and spacecraft, and the like.

[0046] In a first aspect, the present application provides a sodium secondary battery, comprising a positive electrode sheet, an electrolyte, and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer comprising a first positive electrode active material layer and a second positive electrode active material layer stacked sequentially in a direction away from the positive electrode current collector, the first positive electrode active material layer comprising a first positive electrode active material, the second positive electrode active material comprising a second positive electrode active material, the second positive electrode active material comprising Na4M3(PO4)2P2O7, where M comprises at least one of Fe, Co, Mn, or Ni; the electrolyte comprising a solvent, and the solvent comprising a linear ether and a cyclic ether.

[0047] In the sodium secondary battery of the embodiment of the present application, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer, and the positive electrode active material of the layer close to the electrolyte uses Na4M3(PO4)2P2O7. The sodium ion diffusion coefficient of Na4M3(PO4)2P2O7 is high. When the sodium ions enter the electrolyte from the surface of Na4M3(PO4)2P2O7, the ion transmission generated is fast and the charge distribution is more uniform, so that the morphology of the negative electrode sodium metal deposition is more flat, and uniform sodium deposition is achieved, thereby reducing The internal short circuit of sodium secondary batteries is reduced; in addition, Na4M3(PO4)2P2O7 has excellent stability, which can reduce the oxidative decomposition reaction of the electrolyte at the interface between the positive electrode and the electrolyte, reduce the consumption of the electrolyte, the growth of the CEI film and the continuous occurrence of diaphragm plugging; More importantly, the experiment found that when the internal short circuit occurs during the cycle of the sodium secondary battery, the capacity of Na4M3(PO4)2P2O7 hardly decays, and the use of solvents containing linear ethers and cyclic ethers weakens the Na + The desolvation energy of the solvent reduces the effect of Na + When the generated sodium dendrites come into contact with Na4M3(PO4)2P2O7, the internal short sodium dendrites will undergo oxidation reaction to become sodium ions. At the same time, the Na +It is quickly embedded in the Na4M3(PO4)2P2O7 structure, and the excellent reaction kinetics process reduces the consumption of active sodium at the interface, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0048] Taking all the above effects into consideration, when Na4M3(PO4)2P2O7 is used in combination with the first positive electrode active material and a solvent containing linear ether and cyclic ether, it helps the desolvation of sodium ions and the kinetic process of deposition at the negative electrode, and can form a protective layer on the surface of the first positive electrode active material layer, which can effectively improve the long-cycle stability of the sodium secondary battery. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas production performance of the sodium secondary battery are also improved.

[0049] It can be understood that the ether group of a linear ether is connected to two carbon atoms, which can be adjacent carbon atoms on the same carbon chain or carbon atoms on different carbon chains; the ether group of a cyclic ether is connected to the carbon atoms of a ring structure to form a cyclic ether.

[0050] It is understood that, in the embodiments of the present application, the linear ether and cyclic ether in the electrolyte can be determined by the following method:

[0051] Detection was performed by gas chromatography-mass spectrometry (GC-MS).

[0052] It can be understood that the first positive electrode active material layer and the second positive electrode active material layer of the embodiment of the present application can be seen by using a ZEISS Sigma300 scanning electron microscope to obtain an ion polished cross-sectional morphology (CP) image of the positive electrode sheet.

[0053] The materials of the first positive electrode active material and the second positive electrode active material can be determined by the following method:

[0054] According to the CP diagram obtained above, the specific positions of the first positive electrode active material layer and the second positive electrode active material layer are obtained, and then powder is scraped in sequence along the direction from the second positive electrode active material layer to the current collector, and the number of times the second positive electrode active material layer and the first positive electrode active material layer are scraped is recorded respectively. Then, according to the number of times the second positive electrode active material layer and the first positive electrode active material layer are scraped, the dried positive electrode sheet is scraped and sampled to obtain the upper layer powder (the powder in the second positive electrode active material), and then the powder is scraped and sampled again to obtain the bottom layer powder (the first positive electrode active material layer);

[0055] The upper and lower powders are dissolved in nitric acid and analyzed by inductively coupled plasma (ICP) spectroscopy, for example, according to standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, according to embodiments of the present application, measurements can be performed using an inductively coupled plasma emission spectrometer.

[0056] In some embodiments of the present application, the mass proportion of residual alkali in the Na4M3(PO4)2P2O7 is 0.05%-2.5%. For example, it can be 0.05%-2.4%, 0.1%-2%, 0.5%-1.5%, 1%-2%, etc. The residual alkali refers to alkaline substances such as sodium carbonate and sodium hydroxide that remain on the surface of the positive electrode active material. Controlling the content of residual alkali in Na4M3(PO4)2P2O7 within the above range can reduce the side reactions of Na4M3(PO4)2P2O7 during charging and discharging, improve the kinetics of Na4M3(PO4)2P2O7, improve the electronic conductivity of the second positive electrode active material, reduce gas production in the sodium secondary battery, reduce the probability of reaction between the second positive electrode active material and the electrolyte, and accelerate the oxidative dissolution of sodium dendrites when they are generated, thereby improving the cycle performance of the sodium secondary battery. In other embodiments of the present application, the mass proportion of residual alkali in the Na4M3(PO4)2P2O7 is 0.05%-0.5%.

[0057] It is understood that in the embodiments of the present application, the “mass proportion of residual alkali in the Na4M3(PO4)2P2O7” can be determined by the following method:

[0058] Test principle (acid-base titration):

[0059] A certain mass m of Na4M3(PO4)2P2O7 powder is titrated with a standard hydrochloric acid solution to the sodium bicarbonate and sodium carbonate in the cathode material. Using a pH electrode as the indicator electrode, the endpoint is determined by the potential jump produced. The gas volumes V1 and V2 corresponding to the pH mutation point are measured. The titration volume of the standard titration solution is determined based on the endpoint. The calculated mass of Na2CO3 and NaHCO3 is divided by the mass of the cathode active material, and the sodium ion content is used as the residual alkali content of the cathode active material.

[0060] Na2CO3%=(V2-V1)×C×106×100n / 1000m

[0061] NaHCO3%=V2×C×84×n×100 / 1000m

[0062] Na + % = V2×C×23×n×100 / 1000m

[0063] Where C is the concentration of the hydrochloric acid standard solution 0.05 mol / L; m is the mass of the sample, n is 100 mL / pipetting volume, 106 is the molecular weight of Na2CO3; 84 is the molecular weight of NaHCO3.

[0064] In some embodiments of the present application, a carbon coating material is formed on at least part of the surface of the Na4M3(PO4)2P2O7. Based on the total mass of the second positive electrode active material, the mass proportion of the carbon coating material is 1.5%-2.5%. For example, it can be 1.5%-2.4%, 1.7%-2.3%, 2%-2.2%, etc. The carbon coating material refers to a substance made of carbon formed on at least part of the surface of Na4M3(PO4)2P2O7. The carbon coating layer can improve the conductivity of Na4M3(PO4)2P2O7 and can isolate the electrolyte from Na4M3(PO4)2P2O7 to a certain extent, reduce side reactions, and improve the diffusion coefficient of sodium ions, thereby reducing the internal short of the sodium secondary battery containing it. When the internal short occurs, the generated sodium dendrites come into contact with Na4M3(PO4)2P2O7, and the sodium dendrites are rapidly oxidized to Na on the surface of Na4M3(PO4)2P2O7. + It is embedded in the Na4M3(PO4)2P2O7 structure, reducing the consumption of active sodium at the interface, reducing the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries, and improving the cycle performance of sodium secondary batteries.

[0065] It can be understood that in the embodiments of the present application, the preparation of the second positive electrode active material containing a carbon coating material is a method well known in the art. For example, carbon sources such as oleic acid, dopamine, and resin can be used with Na4M3(PO4)2P2O7 to form a uniform carbon coating material on the surface of Na4M3(PO4)2P2O7 during heat treatment.

[0066] In some embodiments of the present application, the volume ratio of the linear ether to the cyclic ether is (60:40)-(90:10), for example, it can be 60:40, 70:30, 80:20, 90:10, etc. The volume ratio of the linear ether to the cyclic ether is controlled within the above range, so that the solvent can dissolve the sodium salt and has a weak solvation effect on the sodium ions, which can reduce the excessive solvation effect on the sodium ions caused by too much linear ether, and can reduce the weak solubility of the solvent for the sodium salt caused by too little linear ether. When sodium dendrites are generated, it can help the sodium dendrites to be rapidly oxidized to Na4M3(PO4)2P2O7 on the surface. + It is embedded in the Na4M3(PO4)2P2O7 structure, which reduces the consumption of active sodium at the interface, reduces the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries, and can further improve the cycle performance of sodium secondary batteries.

[0067] It can be understood that the "volume ratio of linear ether and cyclic ether" is a definition known in the art, which can be determined by a method known in the art, for example, by using the following method:

[0068] Detection is carried out by gas chromatography-mass spectrometry (GC-MS). After disassembling the sodium secondary battery, part of the electrolyte is taken, separated by gas chromatography, and then detected and identified by mass spectrometry. The ether solvent is separated in the gas chromatograph, enters the mass spectrometer, and is qualitatively and quantitatively analyzed according to the mass-to-charge ratio of the fragment ions. Qualitative analysis is carried out according to the retention time and characteristic ions of each component in the mass spectrum, and quantitative analysis is carried out according to the peak area of each solvent.

[0069] In some embodiments of the present application, the electronic conductivity of the positive electrode sheet is greater than or equal to 25 μS·cm -1 , for example, can be 25 μS·cm -1 -80 μS·cm -1 , 30 μS·cm -1 -70 μS·cm -1 , 40 μS·cm -1 -60 μS·cm -1 , etc. The electronic conductivity of the positive electrode sheet is within the above range, the positive electrode sheet has good conductivity, and the weak solvation of linear ether and cyclic ether solvent helps to reduce the desolvation process of sodium ions, thereby improving the kinetic process of Na + ions. Excellent electronic and ionic kinetic processes can reduce the consumption of active sodium caused by side reactions when internal shorts occur, thereby improving the cycle performance of the secondary battery.

[0070] It can be understood that the "electronic conductivity of the positive electrode sheet" is a definition known in the art, which can be determined by a method known in the art, for example, by using the following method:

[0071] Cut the dried positive electrode film layer at the left, middle and right of the positive electrode sheet into small round pieces with a diameter of 3 mm. Turn on the power of the electrode resistance meter of Yuan Neng Technology, place it in the appropriate position of the electrode resistance meter "probe", input the thickness parameter of the electrode sheet, click the "start" button, and read the value when the value is stable. Each small round piece is tested at two positions, and the average value of six measurements is calculated, which is the electronic conductivity of the electrode sheet.

[0072] In some embodiments of the present application, the mass ratio of the first positive electrode active material to the second positive electrode active material is (80:20)-(95:5). For example, the mass ratio of the first positive electrode active material to the second positive electrode active material can be 80:20, 85:15, 90:10, 95:5, etc. Controlling the mass ratio of the first positive electrode active material to the second positive electrode active material within the above range can better match the use of Na4M3(PO4)2P2O7 with the first positive electrode active material and the solvent containing linear ether and cyclic ether, help the desolvation and deposition kinetics of sodium ions at the negative electrode, form a protective layer on the surface of the first positive electrode active material layer, effectively improve the long cycle stability of the sodium secondary battery, and also improve the storage performance and gas production performance of the sodium secondary battery due to the reduction of internal shorts and side reactions.

[0073] It can be understood that the "mass ratio of the first positive electrode active material to the second positive electrode active material" is a definition known in the art, which can be determined by methods known in the art, for example, the following method can be used to determine:

[0074] First, the thickness of the first positive electrode active material layer and the second positive electrode active material layer of the electrode sheet is photographed, and the ion polishing cross-section morphology (CP) of the positive electrode sheet can be obtained by using a ZEISS Sigma300 scanning electron microscope.

[0075] The mass of the first positive electrode active material and the second positive electrode active material can be determined by the following method:

[0076] According to the CP obtained above, the specific positions of the first positive electrode active material layer and the second positive electrode active material layer are obtained, then the powder is scraped in turn along the direction from the second positive electrode active material layer to the current collector, and the number of times of scraping the second positive electrode active material layer and the first positive electrode active material layer is recorded respectively, then the powder scraped from the positive electrode sheet after drying is sampled to obtain the mass of the upper layer powder (the second positive electrode active material) according to the number of times of scraping the second positive electrode active material layer and the first positive electrode active material layer, and then the mass of the bottom layer powder (the first positive electrode active material layer) is sampled by scraping, and the mass ratio of the first positive electrode active material to the second positive electrode active material can be obtained by calculation.

[0077] In some embodiments of the present application, the ionic conductivity of the solvent is greater than or equal to 9 mS·cm -1 . For example, the ionic conductivity of the solvent can be 9 mS·cm -1 -60 mS·cm -1 , 10 mS·cm -1 -50 mS·cm -1 , 20 mS·cm -1 -40 mS·cm -130 mS-cm -1 -35 mS-cm -1 By controlling the ionic conductivity of the solvent within the above range, the kinetics of the desolvation process of sodium ions is improved, a protective layer can be formed on the surface of the first positive electrode active material layer, and the long cycle stability of the sodium secondary battery can be effectively improved. In addition, due to the reduction of internal shorts and side reactions, the storage performance and gas generation performance of the sodium secondary battery are also improved.

[0078] In some embodiments of the present application, the linear ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether, or diethylene glycol diethyl ether. The above linear ether has good solubility for sodium salt and is compatible with various cyclic ethers. When used as a solvent in combination with cyclic ethers, the binding effect of the solvent on sodium ions is weak (weak solvation), making it easier for sodium ions to deposit from the solvent to the negative electrode, reducing the generation of sodium dendrites, and the probability of side reactions between the above linear ether and the positive electrode active material Na4M3(PO4)2P2O7 is low, which can further improve the cycle performance of the sodium secondary battery.

[0079] In some embodiments of the present application, the cyclic ether includes at least one of tetrahydrofuran (THF), methyl tetrahydrofuran, or 1,3-dioxolane. The above cyclic ether has weak solvation effect on sodium ions, i.e. weak binding effect on sodium ions when sodium ions deposit onto the negative electrode, making it easier for sodium ions to deposit from the solvent to the negative electrode, reducing the generation of sodium dendrites, and the probability of side reactions between the above linear ether and the positive electrode active material Na4M3(PO4)2P2O7 is low, which can further improve the cycle performance of the sodium secondary battery. + When sodium dendrites are generated, it can help the sodium dendrites to quickly oxidize into Na4M3(PO4)2P2O7 on the surface of Na4M3(PO4)2P2O7, and then embed into the structure of Na4M3(PO4)2P2O7, reducing the consumption of active sodium at the interface, and reducing the internal shorts caused by the growth of sodium dendrites in the sodium secondary battery, which can further improve the cycle performance of the sodium secondary battery.

[0080] In some embodiments of the present application, the volume average particle size Dv50 of the Na4M3(PO4)2P2O7 is 2.5-3.5 μm. For example, it can be 2.5-3.4 μm, 2.8-3.2 μm, 2.9-3 μm, etc. By controlling the volume average particle size Dv50 of Na4M3(PO4)2P2O7 within the above range, the ion transport generated when sodium ions enter the electrolyte from the surface of Na4M3(PO4)2P2O7 is faster and the charge distribution is more uniform, thereby making the morphology of the negative electrode sodium metal deposition more flat, achieving uniform sodium deposition, thereby reducing the internal shorts of the sodium secondary battery, and further improving the cycle performance of the sodium secondary battery.

[0081] It is understood that the volume average particle size Dv50 refers to the particle size corresponding to when the cumulative volume distribution percentage reaches 50%. The volume average particle size Dv50 of Na4M3(PO4)2P2O7 can be measured by methods known in the art, for example, by the following method:

[0082] Referring to the standard GB / T 19077-2016, a laser particle size analyzer (such as Malvern Master Sizer 3000) was used to measure the volume average particle size Dv50 of Na4M3(PO4)2P2O7.

[0083] In some embodiments of the present application, the first positive electrode active material includes at least one of a sodium transition metal oxide, a polyanion compound, a Prussian blue-based sodium compound, and modified compounds thereof. The first positive electrode active material, when combined with the second positive electrode active material, can further reduce the formation of sodium dendrites and improve the cycling performance of sodium secondary batteries.

[0084] As an example, the first positive electrode active material may include at least one of the following materials: a sodium transition metal oxide, a polyanion compound, a Prussian blue-based sodium compound, and their respective modified compounds. However, this application is not limited to these materials; other conventional materials that can be used as battery positive electrode active materials may also be used. The modified compounds of the above materials may be modified by doping and / or surface coating.

[0085] In some embodiments of the present application, the transition metal in the sodium transition metal oxide may be at least one of Ti, V, Mn, Co, Ni, Fe, Zn, V, Zr, Ce, Cr, and Cu. The chemical formula of the sodium transition metal oxide may satisfy Na y1 MO2, wherein M includes at least one of Ti, V, Mn, Co, Ni, Fe, Zn, V, Zr, Ce, Cr, and Cu, and 0<y1≤1.

[0086] In some embodiments of the present application, the polyanionic compound may be a compound having sodium ions, transition metal ions and tetrahedral (YO4) n1- A class of compounds with anionic units. Among them, the transition metal may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may include at least one of P, S, and Si; n1 represents (YO4) n1- valence.

[0087] In some embodiments of the present application, the polyanionic compound may also be a compound having sodium ions, transition metal ions, tetrahedral (YO4)n1- A class of compounds having anionic units and halogen anions. The transition metal can include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can include at least one of P, S, and Si, n1 represents the valence of (YO4) n1- , and the halogen can include at least one of F, Cl, and Br.

[0088] In some embodiments of the present application, the polyanionic compound can also be a class of compounds having sodium ions, tetrahedral (YO4) n1- anionic units, polyhedral (ZO y ) m1+ , and optional halogen anions. M can include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y can include at least one of P, S, and Si, n1 represents the valence of (YO4) n1- , Z represents a transition metal, and m1 represents the valence of (ZO y ) m1+ , and the halogen can include at least one of F, Cl, and Br.

[0089] As an example, the polyanionic compound can satisfy at least one of the chemical formulas NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM’PO4F (M’ includes at least one of V, Fe, Mn, and Ni), and Na3(VO y1 )2(PO4)2F 3-2y1 (0≤y1≤1).

[0090] In some embodiments of the present application, the Prussian blue compound can be a class of compounds having sodium ions, transition metal ions, and cyanide ions (CN - ). The transition metal can include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce.

[0091] As an example, the Prussian blue compound can satisfy the chemical formula Na a Me b Me’ c (CN)6, where Me and Me’ each independently include at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0

[0092] The battery's charge and discharge processes are accompanied by the deintercalation and consumption of Na, resulting in different molar contents of Na at different discharge states. The molar contents of Na in the positive electrode active materials listed in this application refer to the initial state of the material, i.e., the state before the materials are added. The molar contents of Na will change after the positive electrode active materials are applied to the battery system and undergo charge and discharge cycles.

[0093] In the list of positive electrode active materials for sodium ion batteries in this application, the molar content of O is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0094] In some other embodiments of the present application, the first positive electrode active material includes NaM1O2 or Na x R y P m O n At least one of, wherein M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W or Pb.

[0095] As an example, x may be 2.5-4.4, 3-4, 3.5-3.8, etc., y may be 1.5-3.4, 2-3, 2.5-2.8, etc., m may be 2.6-4.4, 3-4, 3.5-3.8, etc., and n may be 11.5-15.4, 12-15, 13-14, etc.

[0096] The above-mentioned first positive electrode active material has high capacity and strong stability, but the above-mentioned first positive electrode active material has a strong oxidation effect under high pressure. As the number of cycles increases, the side reactions on the surface of the positive electrode active material gradually increase, forming a gradually thickening CEI (Cathode Electrolyte Interface), which causes the deintercalation rate of sodium ions to gradually decrease. At the same time, the side reactions will also cause pore blocking problems in the isolation membrane. These problems are more prominent during the cycle process. Therefore, for the above-mentioned first positive electrode active material, when used in combination with Na4M3(PO4)2P2O7 and solvents containing linear ethers and cyclic ethers, it further helps the desolvation of sodium ions and the kinetic process of deposition at the negative electrode. A protective layer can be formed on the surface of the first positive electrode active material layer, which can further improve the long-cycle stability of the sodium secondary battery. At the same time, due to the reduction of internal shorts and side reactions, the storage performance and gas production performance of the sodium secondary battery are also improved.

[0097] In some other embodiments of the present application, the second positive electrode active material includes Na4Fe3(PO4)2P2O7 (NFPP), which is a fast ion conductor with excellent stability and electronic conductivity, and can improve the sodium ion conductivity and electronic conductivity of the sodium secondary battery. In addition, NFPP can oxidize sodium dendrites to Na in a solvent environment containing linear ethers and cyclic ethers. + It is embedded in the Na4M3(PO4)2P2O7 structure, reducing the consumption of active sodium at the interface, reducing the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries, and improving the cycle performance of sodium secondary batteries.

[0098] In some embodiments of the present application, the first positive electrode active material includes NaM1O2, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, and the sodium secondary battery meets one or more of the following conditions:

[0099] The compaction density of the positive electrode active material layer is 2.58 g / cm 3 -2.75g / cm 3 , for example, it can be 2.58g / cm 3 -2.74g / cm 3 , 2.6g / cm 3 -2.7g / cm 3 , 2.65g / cm 3 -2.69g / cm 3 Controlling the compaction density of the positive electrode active material layer within the above range can ensure good contact between the component powders of the positive electrode active material layer, and at the same time, the electrolyte can easily infiltrate the electrode, so that the electrode has excellent ionic conductivity and electronic conductivity in the electrolyte, which is conducive to the rapid and uniform deintercalation of sodium ions, can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery, and improve the cycle performance of the sodium secondary battery.

[0100] It is understood that the “compression density of the positive electrode active material layer” is a well-known definition in the art and can be measured using methods well-known in the art, for example, using the following method:

[0101] According to the embodiment of the present application, the compaction density PD of the positive electrode active material layer is obtained by measuring the mass of the positive electrode active material layer per unit area (g / cm 2 ) and the thickness of the positive electrode active material layer on one side (cm) (the number of sampling points is greater than 14). Specifically, the compacted density of the positive electrode active material layer PD = the mass of the positive electrode active material layer on one side per unit area (g / cm 2 ) / thickness of positive electrode active material layer (cm).

[0102] The compaction density of the first positive electrode active material layer is 2.8 g / cm 3 -3.2g / cm 3 , for example, 2.8 g / cm 3 -3.19g / cm 3 , 2.9g / cm 3 -3.1g / cm 3 , 3g / cm 3 -3.1g / cm 3 By controlling the compaction density of the first positive electrode active material layer within the above range, the energy density and cycle performance of the sodium secondary battery can be taken into account, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0103] The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 , for example, it can be 1.7 g / cm 3 -1.9g / cm 3 , 1.8g / cm 3 -1.9g / cm 3 The compaction density of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can block the sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer with the above surface density can also work together with the solvent to make the sodium dendrites quickly oxidized to Na on the surface of Na4M3(PO4)2P2O7 + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0104] It is understood that the “compacted density of the first positive electrode active material layer and the compacted density of the second positive electrode active material layer” are definitions well known in the art and can be measured using methods well known in the art, for example, using the following method:

[0105] According to the embodiment of the present application, the compaction density PD of the positive electrode film layer is obtained by measuring the mass of the positive electrode film layer per unit area (g / cm 2 ) and the thickness of the positive electrode film on one side (cm) (the number of collection points is greater than 14). Specifically, the compaction density of the positive electrode film PD1 = the mass of the positive electrode film on one side per unit area (g / cm 2 ) / thickness of positive electrode film (cm).

[0106] Then take a picture of the thickness of the first positive electrode active material layer and the second positive electrode active material layer of the electrode sheet, which can be seen by using a ZEISS Sigma300 scanning electron microscope to obtain an ion polished cross-sectional morphology (CP) image of the positive electrode sheet. According to the CP image obtained above, the specific positions of the first positive electrode active material layer and the second positive electrode active material layer are obtained, and then a positive electrode sheet of a fixed area is taken, and the powder is scraped in sequence along the direction from the second positive electrode active material layer to the current collector, and the number of times the powder is scraped to the second positive electrode active material layer is recorded respectively. Then, according to the number of times the powder is scraped from the second positive electrode active material layer, the dried positive electrode sheet is sampled to obtain the upper layer of powder and weigh the mass (powder in the second positive electrode active material), and the compacted density of the second positive electrode active material layer is obtained by calculation. PD2 = the mass of the second positive electrode active material per unit area on one side (g / cm 2 ) / thickness of the second positive electrode active material layer (cm). Then, the mass of the first positive electrode active material can be obtained based on the mass difference between the mass of the positive electrode sheet and the mass of the second active material, and the compaction density of the first positive electrode active material layer can be calculated. PD3 = mass of the first positive electrode active material per unit area (g / cm 2 ) / thickness of the first positive electrode active material layer (cm).

[0107] The surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2 , for example, it can be 8mg / cm 2 -9.4mg / cm 2 , 8.2mg / cm 2 -9.3mg / cm 2 , 8mg / cm 2 -9mg / cm 2 , 8.5mg / cm 2 -8.8mg / cm 2 Etc., by controlling the surface density of the first positive electrode active material layer within the above range, the sodium secondary battery can take into account both energy density and cycle performance, and sodium dendrites are less likely to occur in the first positive electrode active material layer, which can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0108] The thickness of the first positive electrode active material layer is 25μm-33.9μm, for example, it can be 25μm-33.5μm, 26μm-33μm, 27μm-32μm, 28μm-31μm, 29μm-30μm, etc. The thickness of the first positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and sodium dendrites are not easily generated in the first positive electrode active material layer, which can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0109] The surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 , for example, it can be 0.5 mg / cm 2 -1.9mg / cm 2 , 1mg / cm 2 -1.5mg / cm 2 , 1.2mg / cm 2 -1.3mg / cm 2 The surface density of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can block the sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer with the above surface density can also work together with the solvent to make the sodium dendrites quickly oxidized to Na on the surface of Na4M3(PO4)2P2O7 + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0110] It is understood that the “area density of the first positive electrode active material layer and the areal density of the second positive electrode active material layer” are definitions well known in the art and can be measured using methods well known in the art, for example, using the following method:

[0111] First, the thickness of the first positive electrode active material layer and the second positive electrode active material layer of the electrode piece is photographed, which can be seen by using a ZEISS Sigma300 scanning electron microscope to obtain an ion polished cross-sectional morphology (CP) image of the positive electrode piece.

[0112] Then weigh the mass of the first positive electrode active material and the second positive electrode active material, which can be measured by the following method:

[0113] According to the CP diagram obtained above, the specific positions of the first positive electrode active material layer and the second positive electrode active material layer are obtained, a positive electrode sheet of a fixed area is taken, and then the powder is scraped in sequence along the direction from the second positive electrode active material layer to the current collector, and the number of times the second positive electrode active material layer and the first positive electrode active material layer are scraped is recorded respectively, and then the dried positive electrode sheet is scraped and sampled according to the number of times the second positive electrode active material layer and the first positive electrode active material layer are scraped to obtain the upper layer of powder and weigh the mass (powder in the second positive electrode active material), and then the powder is scraped and sampled to obtain the mass of the bottom layer of powder (first positive electrode active material layer), and the formula: surface density of the material = mass of the active material / sheet area is used to obtain the surface density of the first positive electrode active material layer and the surface density of the second positive electrode active material layer.

[0114] The thickness of the second positive electrode active material layer is 2.5 μm-11.7 μm, for example, it can be 2.5 μm-11.5 μm, 3 μm-11 μm, 4 μm-10 μm, 5 μm-9 μm, 6 μm-8 μm, etc. The thickness of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can block the sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer of the above thickness can also work together with the solvent to rapidly oxidize the sodium dendrites on the surface of Na4M3(PO4)2P2O7 to become Na + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0115] It is understood that the “thickness of the first positive electrode active material layer and the thickness of the second positive electrode active material layer” are definitions well known in the art and can be measured using methods well known in the art, for example, using the following method:

[0116] According to the example of the present application, the thickness of the first positive electrode active material layer and the second positive electrode active material layer of the electrode piece is then photographed, which can be measured by obtaining an ion polishing cross-sectional morphology (CP) image of the positive electrode piece using a ZEISS Sigma300 scanning electron microscope.

[0117] In other embodiments of the present application, the first positive electrode active material includes Na x R y P m O n , 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W or Pb, and the sodium secondary battery meets one or more of the following conditions:

[0118] The compaction density of the positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 , for example, it can be 1.7 g / cm 3 -1.95g / cm 3 , 1.75g / cm 3 -1.9g / cm 3 , 1.8g / cm 3 -1.85g / cm 3Controlling the compaction density of the positive electrode active material layer within the above range can ensure good contact between the component powders of the positive electrode active material layer, and at the same time, the electrolyte can easily infiltrate the electrode, so that the electrode has excellent ionic conductivity and electronic conductivity in the electrolyte, which is conducive to the rapid and uniform deintercalation of sodium ions, can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery, and improve the cycle performance of the sodium secondary battery.

[0119] The compaction density of the first positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 , for example, 1.75 g / cm 3 -1.9g / cm 3 , 1.8g / cm 3 -1.85g / cm 3 By controlling the compaction density of the first positive electrode active material layer within the above range, the energy density and cycle performance of the sodium secondary battery can be taken into account, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0120] The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 , for example, it can be 1.7 g / cm 3 -1.9g / cm 3 , 1.8g / cm 3 -1.9g / cm 3 The compaction density of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can prevent sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer with the above compaction density can also work together with the solvent to make the sodium dendrites quickly oxidized to Na on the surface of Na4M3(PO4)2P2O7 + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0121] The surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2 , for example, it can be 8mg / cm 2 -9.4mg / cm 2 , 8.2mg / cm 2 -9.3mg / cm 2 , 8mg / cm 2 -9mg / cm 2 , 8.5mg / cm 2-8.8mg / cm 2 Etc., by controlling the surface density of the first positive electrode active material layer within the above range, the sodium secondary battery can take into account both energy density and cycle performance, and sodium dendrites are less likely to occur in the first positive electrode active material layer, which can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0122] The thickness of the first positive electrode active material layer is 40μm-55.9μm, for example, it can be 40μm-55.5μm, 45μm-55μm, 50μm-52μm, etc. The thickness of the first positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and sodium dendrites are not easily generated in the first positive electrode active material layer, which can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0123] The surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 , for example, it can be 0.5 mg / cm 2 -1.9mg / cm 2 , 1mg / cm 2 -1.5mg / cm 2 , 1.2mg / cm 2 -1.3mg / cm 2 The surface density of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can block the sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer with the above surface density can also work together with the solvent to make the sodium dendrites quickly oxidized to Na on the surface of Na4M3(PO4)2P2O7 + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0124] The thickness of the second positive electrode active material layer is 2.5 μm-11.7 μm, for example, it can be 2.5 μm-11.5 μm, 3 μm-11 μm, 4 μm-10 μm, 5 μm-9 μm, 6 μm-8 μm, etc. The thickness of the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can block the sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the second positive electrode active material layer of the above thickness can also work together with the solvent to rapidly oxidize the sodium dendrites on the surface of Na4M3(PO4)2P2O7 to become Na +It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0125] In some embodiments of the present application, based on the total mass of the first positive electrode active material layer, the mass proportion of the first positive electrode active material is 77%-80%, for example, it can be 77%-79%, 77%-78%, 78%-79%, etc., and the mass proportion of the first positive electrode active material is controlled within the above range. The energy density and cycle performance of the sodium secondary battery can be taken into account, the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery can be reduced, and the cycle performance of the sodium secondary battery can be improved.

[0126] It is understood that “the mass proportion of the first positive electrode active material based on the total mass of the first positive electrode active material layer” is a definition well known in the art and can be measured using methods well known in the art, for example, using the following method:

[0127] First, the thickness of the first positive electrode active material layer and the second positive electrode active material layer of the electrode piece is photographed, which can be seen by using a ZEISS Sigma300 scanning electron microscope to obtain an ion polished cross-sectional morphology (CP) image of the positive electrode piece.

[0128] The mass of the first positive electrode active material is then weighed, which can be determined by the following method:

[0129] Based on the CP diagram obtained above, the specific positions of the first and second positive electrode active material layers are determined. A fixed-area positive electrode sheet is then scraped sequentially from the second positive electrode active material layer to the current collector, with the number of scrapings across the second and first positive electrode active material layers recorded. The dried positive electrode sheet is then scraped to obtain an upper layer of powder, and its mass m2 (the total mass of the second positive electrode active material layer) is weighed. Subsequently, a further scraping sample is taken to obtain a lower layer of powder, m1 (the total mass of the first positive electrode active material layer). The lower layer of powder is dissolved in nitric acid, and the mass m3 of the first positive electrode active material is determined by inductively coupled plasma (ICP) spectrometry. The mass ratio of the first positive electrode active material, m3 / m1, can be obtained. For example, reference can be made to standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, according to the embodiments of the present application, an inductively coupled plasma optical emission spectrometer may be used for measurement.

[0130] In some embodiments of the present application, based on the total mass of the second positive electrode active material layer, the mass proportion of the second positive electrode active material is 90%-95%. For example, it can be 90%-94%, 91%-93%, 92%-93%, etc. The content of the second positive electrode active material in the second positive electrode active material layer is controlled within the above range, so that the sodium secondary battery can take into account both energy density and cycle performance, and can prevent sodium dendrites generated by the positive electrode active material layer. When sodium dendrites are generated, the above-mentioned second positive electrode active material layer can also work together with the solvent to cause the sodium dendrites to be rapidly oxidized to Na on the surface of Na4M3(PO4)2P2O7. + It is embedded in the Na4M3(PO4)2P2O7 structure, which can reduce the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0131] It is understood that “the mass proportion of the second positive electrode active material based on the total mass of the second positive electrode active material layer” is a definition well known in the art and can be measured using methods well known in the art, for example, using the following method:

[0132] The thicknesses of the first positive electrode active material layer and the second positive electrode active material layer in the embodiment of the present application can be seen by obtaining an ion polished cross-sectional morphology (CP) image of the positive electrode sheet using a ZEISS Sigma300 scanning electron microscope.

[0133] The materials of the first positive electrode active material and the second positive electrode active material can be determined by the following method:

[0134] According to the CP diagram obtained above, the specific positions of the first positive electrode active material layer and the second positive electrode active material layer are obtained, and then the powder is scraped in sequence along the direction from the second positive electrode active material layer to the current collector, and the number of times the powder is scraped to the second positive electrode active material layer is recorded respectively. Then, according to the number of times the powder is scraped from the second positive electrode active material layer, the dried positive electrode sheet is sampled to obtain the mass m2 of the upper layer powder (the total mass of the second positive electrode active material layer).

[0135] The upper layer of powder is dissolved in nitric acid, and the mass m1 of the second positive electrode active material is determined by inductively coupled plasma (ICP) spectrometry analysis to obtain the mass ratio m2 / m1 of the first positive electrode active material. For example, reference can be made to standards YS / T1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, according to embodiments of the present application, measurements can be performed using an inductively coupled plasma optical emission spectrometer.

[0136] In some embodiments of the present application, the first positive electrode active material layer further includes a third positive electrode active material, wherein the third positive electrode active material includes Na4M3(PO4)2P2O7, where M includes at least one of Fe, Co, Mn, or Ni. Thus, the addition of the third positive electrode active material to the first positive electrode active material layer can improve the sodium ion conductivity and electronic conductivity of the first positive electrode active material layer, further improve the sodium ion conductivity and electronic conductivity of the sodium secondary battery, reduce the generation of uneven sodium ion transport and deposition, reduce internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery, and improve the cycle performance of the sodium secondary battery.

[0137] It can be understood that the selection of the third positive electrode active material Na4M3(PO4)2P2O7 and the M element in the second positive electrode active material are independent of each other, and the M elements in the two can be the same or different.

[0138] In some embodiments of the present application, the mass proportion of the third positive electrode active material is 1%-3% based on the total mass of the first positive electrode active material layer. For example, it can be 1%-2.9%, 1.5%-2.5%, 2%-2.2%, etc. Controlling the mass proportion of the third positive electrode active material within the above range can reduce the slow sodium ion diffusion kinetics in the positive electrode sheet caused by too little third positive electrode active material, resulting in internal shorting and the consumption of active sodium after internal shorting. It can also reduce the reduction in positive electrode sheet capacity caused by excessive third positive electrode active material content, which affects the energy density of the sodium secondary battery, and can further reduce internal shorting caused by the growth of sodium dendrites in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0139] In some embodiments of the present application, the positive electrode active material includes the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material. Based on the total mass of the positive electrode active material, the mass proportion of the Na4M3(PO4)2P2O7 is 5%-20%. For example, it can be 5%-19%, 10%-15%, 12%-13%, etc. Controlling the mass proportion of Na4M3(PO4)2P2O7 in all positive electrode active materials within the above range is sufficient to improve the sodium ion conductivity and electronic conductivity of the entire positive electrode active material layer, and can also reduce the impact of excessive addition of Na4M3(PO4)2P2O7 on the capacity of the positive electrode sheet, thereby affecting the energy density of the sodium secondary battery, and can reduce internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0140] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active metal ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0141] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector.

[0142] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either or both of the two facing surfaces of the positive electrode current collector.

[0143] In some embodiments of the present application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0144] In some embodiments of the present application, the first positive electrode active material layer and the second positive electrode active material layer each independently and optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0145] In some embodiments of the present application, the first positive electrode active material layer and the second positive electrode active material layer each independently and optionally further include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0146] In some embodiments of the present application, a positive electrode sheet can be prepared by the following method: the components for preparing a positive electrode sheet, such as a first positive electrode active material, a conductive agent, a binder, a positive electrode irreversible additive, and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on a positive electrode current collector, and after drying, cold pressing, and other processes, a first positive electrode active material layer can be obtained. The preparation method of the second positive electrode active material layer is similar to that of the first positive electrode active material layer, and a positive electrode sheet can be obtained.

[0147] In some embodiments of the present application, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0148] In some embodiments of the present application, when the battery is a sodium ion battery, the electrolyte salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalatoborate, sodium tetrafluoroborate, sodium bisoxalatoborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethylsulfonate or sodium bis(trifluoromethylsulfonyl)imide.

[0149] In some embodiments of the present application, the solvent may also include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, ethylene glycol dimethyl ether, methyl ethyl sulfone or diethyl sulfone.

[0150] In some embodiments of the present application, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0151] In some embodiments of the present application, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer provided on at least one side of the negative electrode current collector.

[0152] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is provided on either or both of the two facing surfaces of the negative electrode current collector.

[0153] In some embodiments of the present application, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0154] In some embodiments of the present application, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and sodium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from at least one of elemental tin, a tin oxide compound, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0155] In some embodiments of the present application, the negative active material layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0156] In some embodiments of the present application, the negative active material layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0157] In some embodiments of the present application, the negative active material layer can further optionally include other additives, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0158] In some embodiments of the present application, the negative electrode sheet can be prepared by dispersing the above-described components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and then performing processes such as drying and cold pressing to obtain the negative electrode sheet.

[0159] In some embodiments of the present application, the secondary battery includes a sodium metal battery. In the above-mentioned secondary battery, the active metal ion sodium on the negative electrode plate is relatively active, and the deposition of the negative electrode sodium is easily affected, resulting in uneven deposition, which makes it easier to produce sodium dendrites. For the alkali metal battery of the embodiment of the present application, Na4M3(PO4)2P2O7, when used in conjunction with the first positive electrode active material and a solvent containing linear ether and cyclic ether, can further contribute to the desolvation of sodium ions and the kinetic process of deposition at the negative electrode, and can form a protective layer on the surface of the first positive electrode active material layer, which can effectively improve the long-cycle stability of the sodium metal battery. At the same time, due to the reduction of internal short and side reactions, the storage performance and gas production performance of the sodium secondary battery are also improved.

[0160] In some embodiments of the present application, when it is a negative electrode-free battery, the negative electrode plate includes a negative electrode current collector and an interface modification layer provided on at least one side of the negative electrode current collector, and the interface modification layer includes a binder and a conductive agent.

[0161] The binder and conductive agent of the negative electrode sheet have been described in detail above and will not be repeated here.

[0162] In some embodiments of the present application, the thickness of the interface modification layer is 0.6 μm-2 μm, for example, 0.6 μm-1.9 μm, 0.8 μm-1.7 μm, 1 μm-1.5 μm, etc.

[0163] In some embodiments of the present application, the sodium metal battery comprises a cathode-free sodium metal battery. That is, in the sodium metal battery, the cathode electrode comprises a cathode current collector, and the sodium metal battery deposits metallic sodium in situ on the cathode current collector during charging. Experiments have shown that, for such cathode-free sodium metal batteries, when Na4M3(PO4)2P2O7 is used in conjunction with the first cathode active material and a solvent containing a linear ether and a cyclic ether, it significantly reduces sodium dendrites, effectively improving the cycling stability of the cathode-free sodium metal battery, enabling the cathode-free sodium metal battery to cycle for over 1,000 cycles.

[0164] It is understandable that a battery without negative electrode means that no negative electrode active material is added during the battery preparation stage, but there is still a negative electrode current collector. A battery without negative electrode is just a special metal battery (such as lithium metal battery, sodium metal battery). It does not really have no negative electrode. The negative electrode in the actual working process still has active metal (such as lithium metal, sodium metal). The negative electrode of a battery without negative electrode includes a bare negative electrode current collector (such as copper). Taking lithium batteries as an example, during the battery charging process, active metal ions such as Li + The lithium metal is released from the positive electrode and deposited on the negative electrode current collector to form a lithium negative electrode. During the subsequent battery discharge process, the deposited lithium metal dissolves and re-inserts into the positive electrode.

[0165] In some embodiments of the present application, the battery includes a sodium metal negative electrode plate. In this case, the negative electrode plate includes a negative electrode current collector and an active material layer disposed on at least a portion of the surface of the negative electrode current collector. The active material layer includes at least one of elemental lithium metal or a lithium metal alloy. Alternatively, the active material includes at least one of sodium metal or a sodium metal alloy.

[0166] In other embodiments of the present application, the chemical formula of the sodium metal alloy is NaR1, and R1 includes at least one of tin, zinc, aluminum, magnesium, silver, gold, gallium, indium, platinum, boron, carbon or silicon.

[0167] In other embodiments of the present application, when a sodium metal negative electrode sheet is used, the preparation method is as follows: sodium foil or sodium metal alloy is coated on a current collector by single-sided rolling, and then cut into negative electrode sheets.

[0168] The present application has no particular limitation on the type of isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0169] In some embodiments of the present application, the material of the separator may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0170] The secondary battery of the present application includes a battery cell form, a battery module form and a battery pack form. The battery cell, battery module and battery pack of the present application are described below with reference to the accompanying drawings as appropriate.

[0171] In some embodiments of the present application, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a winding process or a lamination process.

[0172] In some embodiments of the present application, the battery cell may include an outer packaging, which may be used to encapsulate the electrode assembly and the electrolyte.

[0173] In some embodiments of the present application, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery may also be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0174] It can be understood that the sodium secondary battery mentioned above in this application is a battery monomer.

[0175] The shape of the battery cell is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure battery cell 1 as an example.

[0176] In some embodiments of the present application, referring to Figure 2 , the outer package can include a housing 11 and a cover plate 13. The housing 11 can include a bottom plate and a side plate connected to the bottom plate, which enclose a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can form an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 12. The number of electrode assemblies 12 contained in the battery cell 1 can be one or more, which can be selected by those skilled in the art according to the specific actual needs.

[0177] In some embodiments of the present application, the battery cell can be assembled into a battery module, and the number of battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0178] Figure 3 is a battery module 2 as an example. Referring to Figure 3 , in the battery module 2, a plurality of battery cells 1 can be arranged in sequence along the length direction of the battery module 2. Of course, it can also be arranged in any other way. Further, the plurality of battery cells 1 can be fixed by fasteners.

[0179] Optionally, the battery module 2 can also include a housing having a receiving space, and the plurality of battery cells 1 are received in the receiving space.

[0180] In some embodiments of the present application, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0181] Figure 4 and Figure 5 is a battery pack 3 as an example. Referring to Figure 4 and Figure 5 , the battery pack 3 can include a battery box and a plurality of battery modules 2 arranged in the battery box. The battery box includes an upper box body 31 and a lower box body 32, and the upper box body 31 can be provided on the lower box body 32 to form a closed space for receiving the battery modules 2. The plurality of battery modules 2 can be arranged in the battery box in any way.

[0182] In addition, the present application also provides an electric device, which includes the sodium secondary battery provided in the first aspect of the present application. The battery cell, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0183] As the electrical equipment, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0184] Figure 6 This is an example of an electric device. This electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0185] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0186] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0187] Example 1

[0188] 1. Preparation of positive electrode sheet

[0189] First, 10 wt% polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and 10 wt% carbon black conductive agent, 78 wt% first positive electrode active material Na4Mn3(PO4)2P2O7 and 2 wt% NFPP are added to make a uniformly dispersed slurry to obtain a slurry for the first positive electrode active material layer.

[0190] 2.5wt% polyvinylidene fluoride binder was fully dissolved in N-methyl pyrrolidone, 2.5wt% carbon black conductive agent and 95wt% NFPP second positive electrode active material were added to prepare a uniformly dispersed slurry (wherein the mass ratio of residual alkali in NFPP is 0.3%, the volume average particle size Dv50 of NFPP is 2.5μm; the mass ratio of carbon-coated material is 2%), to obtain the slurry of the second positive electrode active material layer. The mass ratio of the first positive electrode active material to the second positive electrode active material is 95:5.

[0191] The two slurries were uniformly coated on the surface of the aluminum foil by double-layer extrusion coating (wherein the mass ratio of NFPP to the total positive electrode active material is 6.65%), and then transferred to a vacuum drying oven for complete drying. The obtained electrode piece was rolled and then punched to obtain a positive electrode piece.

[0192] The electronic conductivity of the finally prepared positive electrode piece is 30μS·cm -1 , the surface density of the first positive electrode active material layer is 9.5mg / cm 2 ; the thickness of the first positive electrode active material layer is 52.8μm; the surface density of the second positive electrode active material layer is 0.5mg / cm 2 ; the thickness of the second positive electrode active material layer is 2.78μm; the compaction density of the positive electrode active material layer is 1.8g / cm 3 ; the compaction density of the first positive electrode active material layer is 1.8g / cm 3 ; the compaction density of the second positive electrode active material layer is 1.8g / cm 3 .

[0193] 2. Preparation of negative electrode piece

[0194] 5g of sodium hydroxymethyl cellulose (CMC) was weighed and stirred and dissolved in 1000mL of water, then 5g of single-walled carbon nanotubes was added, and a slurry was prepared after ultrasonic dispersion. The slurry was coated on the surface of a copper foil, then transferred to a vacuum drying oven for complete drying, and then cut and die cut to prepare a negative electrode piece without negative electrode structure. The coating weight of the negative electrode is 0.1mg / cm 2 , and the coating thickness is 1μm.

[0195] 3. Preparation of electrolyte

[0196] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), a certain amount of sodium hexafluorophosphate was dissolved in a mixed solvent of ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (wherein the volume ratio of DME and 1,3-dioxolane is 70:30), and stirred uniformly. The concentration of sodium hexafluorophosphate was controlled to be 1mol / L to form the final used electrolyte.

[0197] 4. Isolation film

[0198] Polypropylene film is used as the isolation film.

[0199] 5. Preparation of secondary batteries

[0200] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator acting as a barrier between the positive and negative electrodes. Tabs are then welded to the bare cell, which is then placed in an aluminum shell and baked at 80°C to remove moisture. The electrolyte is then injected and sealed to create an uncharged battery. The uncharged battery then undergoes a series of processes, including resting, hot and cold pressing, formation, shaping, and capacity testing, to produce a negative-electrode-free sodium secondary battery.

[0201] The preparation methods of the sodium secondary batteries of Examples 2-17 and Comparative Examples 1-4 are the same as those of Example 1, except that the process of preparing the electrolyte is different, as shown in Table 1.

[0202] Among them, in the solvent components in Table 1, the percentage is the percentage of a certain component in the total volume of the solvent. For example, the solvent of Example 1 includes 70% by volume of DME and 30% by volume of 1,3-dioxolane. Comparative Example 1 does not add the second positive electrode active material, wherein the second positive electrode active material layer includes 50 wt% of polyvinylidene fluoride binder and 50 wt% of carbon black conductive agent. In Example 17 and Comparative Examples 1-2, the surface density of the first positive electrode active material layer is 8 mg / cm 2 The thickness of the first positive electrode active material layer is 28.6 μm; the surface density of the second positive electrode active material layer is 2 mg / cm 2 The thickness of the second positive electrode active material layer is 10 μm; the compaction density of the positive electrode active material layer is 2.59 g / cm 3 The compaction density of the first positive electrode active material layer is 2.8 g / cm; the compaction density of the second positive electrode active material layer is 2 g / cm 3 .

[0203] Table 1

[0204]

[0205]

[0206] The storage capacity retention rate, cycle performance, and storage gas production volume of the batteries of Examples 1-17 and Comparative Examples 1-4 were characterized. The characterization results are shown in Table 2.

[0207] 1. Storage capacity retention test

[0208] The sodium ion battery was charged to 3.65V at a constant current of 0.2C at 25°C, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged to 1.5V at a constant current of 0.2C to obtain the discharge capacity before storage (Cd1); then the battery was charged to 3.65V at a constant current of 0.2C again, and then charged at a constant voltage of 3.65V until the current dropped to 0.05C. The battery was then placed in a 60°C constant temperature box for 30 days. After being taken out, the battery was placed at 25°C and charged to 3.65V at a constant current of 0.2C, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged to 1.5V at a constant current of 0.2C to obtain the discharge capacity after storage (Cd2), and the capacity retention rate of the sodium ion battery was calculated according to the following formula:

[0209] Storage capacity retention rate = Cd2 / Cd1×100%.

[0210] 2. Cyclic performance test

[0211] The cycling performance test process is as follows: at 25°C, the prepared battery was allowed to stand for 30 minutes, then discharged at a constant current of 0.33C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, allowed to stand for 1 hour, and then discharged at a constant current of 0.33C to 1.5V to obtain the initial capacity (C0); after standing for 1 hour, the battery was again charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, allowed to stand for 1 hour, and then discharged at a constant current of 0.33C to 1.5V to obtain the process capacity (C1). The above steps were repeated for the same battery, and the process capacity (Cn) of the battery after the 100th cycle was recorded at the same time. The capacity retention rate after 100 cycles = Cn / C1×100%. The test process of the comparative example and other embodiments is the same as above.

[0212] 3. Storage gas production volume test

[0213] Before testing the capacity, the volume of the cell (V1) was tested at 25°C using the drainage method. The sodium ion battery was charged to 3.65V at a constant current of 0.2C at 25°C, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged to 1.5V at a constant current of 0.2C to obtain the discharge capacity before storage (Cd1); the battery was then charged to 3.65V at a constant current of 0.2C again, and then charged at a constant voltage of 3.65V until the current dropped to 0.05C. The battery was then stored in a 60°C constant temperature box for 30 days. After being taken out, the battery was placed at 25°C to test the volume of the cell after storage (V2), and the gas production of the sodium ion battery was calculated according to the following formula:

[0214] Gas production = [volume of the cell after storage (V2) - volume of the cell before storage (V1)] / cell capacity Cd1. The results are shown in Table 2.

[0215] Table 2

[0216]

[0217] As can be seen from Table 2, in the sodium secondary batteries of Examples 1-17 of the present application, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer, and the positive electrode active material of a layer close to the electrolyte uses Na4M3(PO4)2P2O7, combined with linear ether and cyclic ether of the electrolyte solvent, the sodium secondary battery has excellent cycle performance, less gas production, and good high-temperature storage performance. Comparative Examples 1 and 3 do not use the first positive electrode active material and the second positive electrode active material, and the electrolytes of Comparative Examples 2 and 4 do not use linear ether and cyclic ether at the same time. The cycle performance, storage performance and gas production of the battery are significantly deteriorated. It can be seen that the sodium secondary battery of the embodiment of the present application can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery, improve the cycle performance and storage performance of the sodium secondary battery, and reduce gas production.

[0218] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A sodium secondary battery, characterized in that The sodium secondary battery comprises a positive electrode sheet, an electrolyte and a negative electrode sheet, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer stacked in sequence in a direction away from the positive electrode current collector, the first positive electrode active material layer includes a first positive electrode active material, the second positive electrode active material includes a second positive electrode active material, the second positive electrode active material includes Na4M3(PO4)2P2O7, and M includes at least one of Fe, Co, Mn or Ni; The electrolyte includes a solvent including a linear ether and a cyclic ether.

2. The sodium secondary battery according to claim 1, characterized in that In the Na4M3(PO4)2P2O7, the mass proportion of residual alkali is 0.05%-2.5%.

3. The sodium secondary battery according to claim 1 or 2, characterized in that In the Na4M3(PO4)2P2O7, the mass proportion of residual alkali is 0.05%-0.5%.

4. The sodium secondary battery according to any one of claims 1 to 3, characterized in that The volume ratio of the linear ether to the cyclic ether is (60:40)-(90:10).

5. The sodium secondary battery according to any one of claims 1 to 4, characterized in that A carbon coating material is formed on at least a portion of the surface of the Na4M3(PO4)2P2O7. Based on the total mass of the second positive electrode active material, the mass of the carbon coating material accounts for 1.5%-2.5%.

6. The sodium secondary battery according to any one of claims 1 to 5, characterized in that The mass ratio of the first positive electrode active material to the second positive electrode active material is (80:20)-(95:5).

7. The sodium secondary battery according to any one of claims 1 to 6, characterized in that The first positive electrode active material layer further includes a third positive electrode active material, wherein the third positive electrode active material includes Na4M3(PO4)2P2O7, where M includes at least one of Fe, Co, Mn or Ni.

8. The sodium secondary battery according to claim 7, characterized in that Based on the total mass of the first positive electrode active material layer, the mass proportion of the third positive electrode active material is 1%-3%.

9. The sodium secondary battery according to claim 7 or 8, characterized in that The positive electrode active material includes the first positive electrode active material, the second positive electrode active material and the third positive electrode active material. Based on the total mass of the positive electrode active material, the mass proportion of Na4M3(PO4)2P2O7 is 5%-20%.

10. The sodium secondary battery according to any one of claims 1 to 9, characterized in that The electronic conductivity of the positive electrode plate is greater than or equal to 25 μS·cm -1 .

11. The sodium secondary battery according to any one of claims 1 to 10, characterized in that The sodium secondary battery satisfies one or more of the following conditions: The ionic conductivity of the solvent is greater than or equal to 9 mS·cm -1 ; The linear ether comprises at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether or diethylene glycol diethyl ether; The cyclic ether includes at least one of tetrahydrofuran, methyltetrahydrofuran or 1,3-dioxolane; The volume average particle size Dv50 of the Na4M3(PO4)2P2O7 is 2.5 μm-3.5 μm; The first positive electrode active material comprises at least one of a sodium transition metal oxide, a polyanion compound, a Prussian blue sodium compound, and modified compounds thereof; The second positive electrode active material includes Na4Fe3(PO4)2P2O7.

12. The sodium secondary battery according to any one of claims 1 to 11, characterized in that The first positive electrode active material includes NaM1O2 or Na x R y P m O n At least one of, wherein M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W or Pb.

13. The sodium secondary battery according to any one of claims 1 to 12, characterized in that The first positive electrode active material includes NaM1O2, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, and the sodium secondary battery meets one or more of the following conditions: The surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2 ; The thickness of the first positive electrode active material layer is 25 μm-33.9 μm; The surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 ; The thickness of the second positive electrode active material layer is 2.5 μm-11.7 μm; The compaction density of the positive electrode active material layer is 2.58 g / cm 3 -2.75g / cm 3 ; The compaction density of the first positive electrode active material layer is 2.8 g / cm 3 -3.2g / cm 3 ; The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 .

14. The sodium secondary battery according to any one of claims 1 to 13, characterized in that The first positive electrode active material includes Na x R y P m O n , 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W or Pb, and the sodium secondary battery meets one or more of the following conditions: The surface density of the first positive electrode active material layer is 8 mg / cm 2 -9.5mg / cm 2 ; The thickness of the first positive electrode active material layer is 40 μm-55.9 μm; The surface density of the second positive electrode active material layer is 0.5 mg / cm 2 -2mg / cm 2 ; The thickness of the second positive electrode active material layer is 2.5 μm-11.7 μm; The compaction density of the positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 ; The compaction density of the first positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 ; The compaction density of the second positive electrode active material layer is 1.7 g / cm 3 -2g / cm 3 .

15. The sodium secondary battery according to any one of claims 1 to 14, characterized in that The sodium secondary battery satisfies one or more of the following conditions: Based on the total mass of the first positive electrode active material layer, the mass proportion of the first positive electrode active material is 77%-80%; Based on the total mass of the second positive electrode active material layer, the mass proportion of the second positive electrode active material is 90%-95%.

16. The sodium secondary battery according to any one of claims 1 to 15, characterized in that The sodium secondary battery includes a sodium metal battery.

17. The sodium secondary battery according to any one of claims 1 to 16, characterized in that The negative electrode plate includes a negative electrode current collector and an interface modification layer provided on at least one side of the negative electrode current collector, wherein the interface modification layer includes a binder and a conductive agent.

18. An electrical device, characterized in that: A sodium secondary battery comprising the sodium secondary battery according to any one of claims 1 to 17.

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