Sodium secondary battery and electric device

Abstract

Provided are a sodium secondary battery and an electric device. The sodium secondary battery comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode active material, the ratio of the actual discharge specific capacity of the negative electrode active material measured by a three-stage gradual discharge method of discharging at a 0.05C rate first and then discharging at a 40 muA and 10 muA current in a 1.0V-0.5V voltage interval to the theoretical discharge specific capacity of the negative electrode active material is a; the electrolyte comprises a first component, the first component is a fluorinated carbonate compound, the mass fraction of the fluorinated carbonate compound in the total mass of the electrolyte is b, and a and b satisfy: 1<=a / b<=270. The sodium secondary battery can reduce the volume expansion rate of the battery after high-temperature storage, and improve the low-temperature charging performance and cycle stability.

Description

Sodium secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311485871.2, filed on November 9, 2023, entitled “Sodium Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a sodium secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the widespread use of secondary batteries, higher requirements have been placed on their cycle performance and service life.

[0005] In terms of resources and cost, sodium secondary batteries have greater advantages than lithium secondary batteries, but sodium secondary batteries produce serious gas, which limits their further application.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a sodium secondary battery for reducing gas production of the sodium secondary battery and improving the cycle stability of the battery.

[0008] A first aspect of the present application provides a sodium secondary battery, the sodium secondary battery comprising a negative electrode plate and an electrolyte, the negative electrode plate comprising a negative electrode active material, the negative electrode active material being discharged in a voltage range of 1.0V-0.5V at a rate of 0.05C and then discharged at currents of 40μA and 10μA in a three-stage stepwise discharge method, wherein the actual discharge specific capacity measured is a to the theoretical discharge specific capacity of the negative electrode active material; the electrolyte comprising a first component, the first component being a fluorinated carbonate compound, the mass proportion of the fluorinated carbonate compound being b based on the total mass of the electrolyte, and a and b satisfy: 1≤a / b≤270, and can optionally be 1≤a / b≤100.

[0009] The solid electrolyte interface film (SEI film) on the surface of the negative electrode is prone to dissolution or decomposition in the voltage range of 1.0V-0.5V, exposing the negative electrode to the electrolyte for reaction. After the solvent molecules decompose on the surface of the electrode, they easily produce gas, which makes the battery have a high expansion rate and produces soluble by-products. These by-products will trigger irreversible side reactions, reducing the cycle stability and kinetic performance of the entire system. The higher the actual discharge capacity of the negative electrode active material in the voltage range of 1.0V-0.5V, the more SEI films will be formed by hard carbon defects and surface functional groups during the sodium absorption process, and the more SEI films will dissolve or decompose when the negative electrode voltage is in the voltage range of 1.0V-0.5V during discharge. Fluorinated carbonate compounds have a lower LUMO energy due to the electron-withdrawing effect of fluorine atoms, and can be reduced to form SEI films on the surface of the negative electrode in the relatively high voltage range of 1.0V-0.5V to compensate for the increased battery gas production and decreased cycle stability caused by the decomposition of the SEI film.

[0010] A negative electrode and electrolyte with an a / b ratio within the appropriate range can synergistically improve the stability of the SEI film, thereby reducing gas production in the secondary battery and enhancing the battery's kinetic performance and cycling stability. When a and b satisfy 1≤a / b≤100 in the secondary battery, the battery's volume expansion rate, low-temperature charging performance, and room-temperature cycling capacity retention are further improved.

[0011] In any embodiment, the actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V and then discharged at currents of 40μA and 10μA is 9mAh / g-140mAh / g, optionally 18mAh / g-70mAh / g.

[0012] When the actual discharge capacity of the negative electrode active material in the voltage range of 1.0V-0.5V is within the appropriate range, the battery has low gas production, excellent kinetic performance and cycle stability. A negative electrode sheet with an actual discharge capacity of 18mAh / g-70mAh / g in the voltage range of 1.0V-0.5V can achieve low gas production, high cycle stability and high energy density.

[0013] In any embodiment, based on the total mass of the electrolyte, the mass proportion b of the fluorinated carbonate compound is 0.05% to 10%, and can be optionally 0.2% to 10%.

[0014] When the mass proportion b of fluoroethylene carbonate in the electrolyte is 0.05%-10% based on the total mass of the electrolyte, the battery has a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance, and room-temperature cycling capacity retention. When the mass proportion b of fluoroethylene carbonate in the electrolyte is 0.2%-10%, the battery volume expansion rate after high-temperature storage is further reduced, and the low-temperature charging performance and room-temperature cycling capacity retention are improved.

[0015] In any embodiment, the fluorinated carbonate compound includes a compound represented by formula I,

[0016] Wherein, R1, R2, R3, and R4 are independently hydrogen atoms, halogen atoms, C 1-6 Hydrocarbon, C 1-3 Halogenated alkyl, C 1-3 Alkoxy, C 1-3 At least one of a haloalkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

[0017] The cyclic fluorinated carbonate in which at least one of R1, R2, R3, and R4 is a fluorine atom is easy to open the ring and form an SEI film on the surface of the negative electrode, thereby reducing battery gas production and improving battery kinetic performance and cycle stability.

[0018] In any embodiment, the fluorinated carbonate compound comprises at least one of the following compounds,

[0019] In any embodiment, the negative electrode plate includes a negative electrode film layer, and the negative electrode film layer contains calcium.

[0020] The introduction of calcium into the negative electrode film layer can induce the deposition of sodium ions, help inhibit the formation of sodium dendrites, and reduce the oxidation and gas production of unstable components generated by sodium dendrites at the negative electrode, thereby reducing the gas production of the negative electrode during discharge and reducing the volume expansion rate of the battery after high-temperature storage.

[0021] In any embodiment, based on the total mass of the negative electrode film layer, the mass proportion of the calcium element in the negative electrode film layer is d, and the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at a current of 40μA and 10μA in a three-stage step-by-step discharge method to the theoretical discharge specific capacity of the negative electrode active material is c, and d and c satisfy: 2.5×10 -6 ≤d / c≤4.5×10 -3 , optional 5.5×10 - 5 ≤d / c≤3.1×10 -3.

[0022] Unlike lithium secondary batteries, sodium secondary batteries often use hard carbon as their negative electrode active material. The capacity of hard carbon mainly includes two stages. The first stage is between 1.5V and 0.1V (vs Na / Na + ) capacity comes from Na + The adsorption process at the hard carbon surface defects, the second stage at 0.1V (vsNa / Na + )The following capacity contributions come from Na + Filling process in hard carbon micropores. In order to improve the negative electrode capacity, the porosity of hard carbon is often increased in the prior art. However, Na + The potential during the hard carbon micropore filling process is close to the potential of metallic sodium deposition (0V), which can easily cause sodium precipitation problems during the charging process. The precipitated sodium dendrites are extremely reactive and react rapidly with the electrolyte to produce a large amount of gas and unstable by-products. These unstable substances are prone to oxidative decomposition as the negative electrode potential increases during discharge, resulting in insufficient film-forming driving force. In addition, unstable organic by-products are easily soluble in the electrolyte, causing the SEI film to be in a cyclical process of dissolution and repair, exacerbating the deterioration of battery gas production and cycle stability.

[0023] Calcium in the negative electrode film can induce sodium ion deposition at the negative electrode, helping to inhibit sodium dendrite formation and thereby reducing the unstable components generated by these dendrites. When the d / c ratio is within the appropriate range, the interaction between the calcium in the negative electrode film and the active material in the negative electrode sheet allows the secondary battery to maintain high capacity and energy density while exhibiting low gas production, high kinetic performance, and cycling stability.

[0024] In any embodiment, based on the total mass of the negative electrode film layer, the mass proportion d of the calcium element in the negative electrode film layer is 2 ppm to 3000 ppm, and can be optionally 40 ppm to 2300 ppm.

[0025] When the mass fraction (d) of calcium in the negative electrode film is within an appropriate range, it can not only reduce the negative impact of excessive calcium content on the capacity and impedance of the secondary battery, but also fully utilize the calcium's role in inhibiting dendrites and reducing gas production. This reduces battery gas production while improving the battery's low-temperature charging performance and room-temperature cycle capacity retention. When the mass fraction (d) of calcium in the negative electrode film is between 40ppm and 2300ppm, the battery's gas production rate is further reduced and the cycle stability is further improved.

[0026] In any embodiment, the actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V and then discharged at currents of 40μA and 10μA is 100mAh / g-300mAh / g, optionally 200mAh / g-250mAh / g.

[0027] The negative electrode active material's actual discharge capacity within the voltage range of 0.1V-0.005V is within an appropriate range, which can reduce the battery's gas production after high-temperature storage and improve the battery's kinetic performance and cycle stability. When the actual discharge capacity of the negative electrode active material is 200mAh / g-250mAh / g within the voltage range of 0.1V-0.005V, it can further achieve a balance between high energy density and low gas production.

[0028] In any embodiment, the negative electrode active material includes hard carbon.

[0029] In any embodiment, the sodium secondary battery further includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode active material.

[0030] In any embodiment, the positive electrode active material further contains copper element. Based on the total mass of the positive electrode active material, the mass proportion of the copper element is 0.01% to 23%, and optionally 6.5% to 18%.

[0031] The positive electrode active material containing copper elements has a more stable structure and can further improve the cycle stability of the battery.

[0032] The mass ratio of copper elements is within the appropriate range. While the battery cycle stability is improved, the copper element will not be converted into Cu under high voltage. 3+ , causing the electrolyte to decompose faster due to its high oxidizing properties, worsening battery gassing. A copper content of 6.5% to 18% by weight can further balance low gassing and high cycle stability for secondary batteries.

[0033] In any embodiment, the positive electrode active material includes a sodium transition metal oxide, and the sodium transition metal oxide includes Na m Cu n X o Fe p Mn q O 2-s， Wherein X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, and Ba, <m≤1,0≤n≤0.5,0≤o<0.5,0≤p≤0.5,0<q≤0.68,n+o+p+q=1,0≤s<0.2;

[0034] In some embodiments, the sodium transition metal oxide comprises Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2、Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 ]O2、Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 ]At least one of O2.

[0035] Sodium transition metal oxide cathode active materials have high voltage. While contributing to battery capacity, the anionic oxygen in these materials also generates a large amount of proton hydrogen, accelerating the oxidation and gassing of unstable components in the negative electrode, leading to severe gassing at the negative electrode side. The combined effects of the negative electrode plate and the fluorinated carbonate compound in the electrolyte provided in the embodiments of this application can effectively reduce battery gassing while increasing battery capacity and energy density.

[0036] In any embodiment, the electrolyte further includes a second component, which is one or more of vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, maleic anhydride, succinic anhydride, sodium difluorooxalatoborate, triallyl phosphate, sodium bis(oxalatoborate), sodium tetrafluorooxalatophosphate, sodium difluorobis(oxalatophosphate), sodium difluorophosphate, and sodium fluorosulfonate.

[0037] The SEI film formed at the negative electrode interface of sodium secondary batteries primarily consists of sodium alkyl carbonate and sodium carbonate. However, sodium alkyl carbonate has a greater solubility in electrolyte solvents than lithium alkyl carbonate, making the SEI film of sodium secondary batteries highly unstable. This leads to continuous side reactions between the electrolyte and the negative electrode, resulting in poor secondary battery cycle performance. A second component containing unsaturated functional groups can be reduced to film at the negative electrode before the solvent, interacting with the fluorocarbonate compound to inhibit the formation of easily soluble substances such as sodium alkyl carbonate. This, combined with the negative electrode plate, reduces battery gas production and improves battery cycle stability.

[0038] In any embodiment, based on the total mass of the electrolyte, the mass proportion of the second component is 0.01% to 10%, and optionally 0.1% to 5%.

[0039] The mass proportion of the second component is within the above range, which can improve the gas production of the battery while controlling the thickness of the SEI film, thereby achieving low impedance and low gas production of the battery at the same time.

[0040] A second aspect of the present application further provides an electrical device comprising the sodium secondary battery of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1 is a schematic diagram of a charge-discharge curve test of a negative electrode sheet according to an embodiment of the present application;

[0042] FIG2 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0043] FIG3 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG2 ;

[0044] FIG4 is a schematic diagram of a battery module according to an embodiment of the present application;

[0045] FIG5 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0046] FIG6 is an exploded view of the battery pack shown in FIG5 according to an embodiment of the present application;

[0047] FIG. 7 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0048] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 sodium secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0049] Below, the embodiments of the sodium secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0050] " 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.

[0051] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0052] 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.

[0053] 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.

[0054] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0055] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0056] It is generally believed in the prior art that the gas production of sodium secondary batteries mainly comes from the oxidation of electrolyte by the positive electrode active material at high voltage. Therefore, the prior art often adopts the scheme of coating the positive electrode active material and forming a film on the positive electrode surface to reduce the gas production of secondary batteries. During the research process, the applicant found that another important factor in the gas production of sodium secondary batteries lies in the negative electrode. The solid electrolyte interface (SEI film) on the surface of the negative electrode plays a key role in reducing the gas production of the negative electrode. However, during the charge and discharge cycle of the battery, the SEI film is in a cyclic process of dissolution and repair, which can easily lead to the deterioration of battery gas production and cycle stability.

[0057] [Sodium secondary battery]

[0058] Based on this, the present application proposes a sodium secondary battery, which includes a negative electrode plate and an electrolyte. The negative electrode plate includes a negative electrode active material. The negative electrode active material is discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA in a three-stage step-by-step discharge method. The ratio of the actual discharge specific capacity to the theoretical discharge specific capacity of the negative electrode active material is a; the electrolyte includes a first component, the first component includes a fluorinated carbonate compound, and the mass proportion of the fluorinated carbonate compound based on the total mass of the electrolyte is b, and a and b satisfy: 1≤a / b≤270. In some embodiments, a and b satisfy: 1≤a / b≤100.

[0059] Sodium secondary battery is a secondary battery that mainly relies on the movement of sodium ions between the positive and negative electrodes to work.

[0060] The actual discharge capacity of the negative electrode active material can be measured using the charge-discharge curve of a button cell. The test method is a three-stage stepwise discharge method, first discharging at a 0.05C rate, then discharging at currents of 40μA and 10μA to reduce the phenomenon of incomplete capacity utilization caused by polarization under high-rate discharge. As an example of the three-stage stepwise discharge method, the negative electrode sheet of the sodium secondary battery was punched into a small disc with a diameter of 14mm. This disc was used as the positive electrode in the button cell. A metal sodium sheet was used as the negative electrode, and a 1.3 mol / L sodium hexafluorophosphate solution was used as the electrolyte. The solvents in the electrolyte included ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, with a mass ratio of 1:2:2. The button cell was assembled and constant current charge and discharge tests were performed in the voltage range of 0.005-2V. The charge-discharge curve diagram is shown in Figure 1. During the discharge process, the battery was discharged at a constant current rate of 0.05C to 0.005V. After standing until the voltage returned to a stable value E, it was discharged at a constant current rate of 40μA to 0.005V. After standing until the voltage returned to a stable value F, it was discharged at a constant current rate of 10μA to 0.005V. During the charging process, it was charged at a constant current rate of 0.05C to 2V. During the second cycle of the above charge and discharge cycle, the total capacity (mAh) of the discharge process in the voltage range of 1.0V-0.5V was divided by the mass (g) of the negative electrode active material in the negative electrode sheet and recorded as the actual discharge specific capacity (mAh / g) of the negative electrode active material in the voltage range of 1.0V-0.5V. As shown in Figure 1, the difference between the specific capacity B corresponding to 0.5V and the specific capacity A corresponding to 1V in the discharge curve is the actual discharge specific capacity (mAh / g) of the negative electrode active material in the voltage range of 1.0V-0.5V. The charge and discharge curves can be measured by any electrochemical testing system in the art. As an example, the charge and discharge curves are obtained by testing using a CT3002A 1U model instrument of a blue electric test system.

[0061] In some embodiments, the negative electrode active material includes hard carbon, which has a theoretical discharge capacity of 300 mAh / g.

[0062] The fluorinated carbonate compound refers to a compound containing a carbonate group (-OC(O)O-) in which at least one hydrogen atom is replaced by a fluorine atom. It can be a linear compound or a cyclic compound.

[0063] The organic components of the solid electrolyte interface film (SEI film) on the surface of the negative electrode are prone to dissolution or decomposition in the voltage range of 1.0V-0.5V, exposing the negative electrode to the electrolyte for reaction. The solvent molecules will continue to reduce and decompose on the surface of the electrode to produce gas, which makes the battery have a high expansion rate and produces soluble by-products. These by-products will trigger irreversible side reactions, reducing the cycle stability and kinetic performance of the entire system. The higher the actual discharge capacity of the negative electrode active material in the voltage range of 1.0V-0.5V, the more SEI films will be formed by hard carbon defects and surface functional groups during the sodium absorption process, and the more SEI films will dissolve or decompose when the negative electrode voltage is in the voltage range of 1.0V-0.5V during discharge. Fluorinated carbonate compounds can increase the electron-accepting ability of the central atom by virtue of the electron-withdrawing effect of fluorine atoms. In the relatively high voltage range of 1.0V-0.5V, they are reduced to form SEI films on the surface of the negative electrode to compensate for the increased battery gas production and decreased cycle stability caused by the decomposition of the SEI film.

[0064] In some embodiments, the value of a / b can be selected from 1, 1.3, 2.7, 3, 5, 6.1, 10, 13.3, 15, 20, 23.3, 30, 40, 46.7, 50, 60, 66.7, 70, 80, 90, 100, 150, 200, 250, 266.7, 270, or any range therebetween.

[0065] The negative electrode and electrolyte within this range can synergistically improve the stability of the SEI film, thereby reducing gas production in the secondary battery and improving the battery's kinetic performance and cycling stability. When a and b satisfy 1≤a / b≤100 in the secondary battery, the battery's volume expansion rate, low-temperature charging performance, and room-temperature cycling capacity retention are further improved.

[0066] In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage step-discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA, is 9mAh / g-140mAh / g. In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage step-discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA, is 18mAh / g-70mAh / g.

[0067] In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage step-by-step discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V and then discharged at currents of 40μA and 10μA can be selected to be 9mAh / g, 15mAh / g, 18mAh / g, 20mAh / g, 30mAh / g, 40mAh / g, 50mAh / g, 60mAh / g, 70mAh / g, 80mAh / g, 90mAh / g, 100mAh / g, 110mAh / g, 120mAh / g, 130mAh / g, 140mAh / g or any value therebetween.

[0068] The actual discharge capacity of the negative electrode active material, measured by a three-stage step-by-step discharge method in the voltage range of 1.0V-0.5V, is first discharged at a rate of 0.05C, followed by discharges at currents of 40μA and 10μA. This can be manipulated by changing the preparation process of the negative electrode active material. Using hard carbon as an example, by varying the pyrolysis temperature and adjusting the surface defects and porosity of the negative electrode active material, the capacity of the negative electrode active material can be controlled in different voltage ranges. Increasing the pyrolysis temperature of hard carbon helps reduce surface defects, decreases sodium adsorption, and reduces its actual discharge capacity in the voltage range of 1.0V-0.5V.

[0069] Batteries within this range have low gas production, excellent kinetics, and cycle stability. Anode sheets with an actual discharge capacity of 18 mAh / g to 70 mAh / g in the 1.0 V to 0.5 V voltage range offer a balance of low gas production, high cycle stability, and high energy density.

[0070] In some embodiments, the mass proportion b of the fluorinated carbonate compound is 0.05% to 10% based on the total mass of the electrolyte. In some embodiments, the mass proportion b of the fluorinated carbonate compound is 0.2% to 10% based on the total mass of the electrolyte.

[0071] In some embodiments, based on the total mass of the electrolyte, the mass proportion b of the fluorinated carbonate compound can be selected as 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value therebetween.

[0072] When the mass proportion b of fluoroethylene carbonate in the electrolyte is 0.05%-10% based on the total mass of the electrolyte, the battery has a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance, and room-temperature cycling capacity retention. When the mass proportion b of fluoroethylene carbonate in the electrolyte is 0.2%-10% based on the total mass of the electrolyte, the battery volume expansion rate after high-temperature storage can be further reduced, and the low-temperature charging performance and room-temperature cycling capacity retention can be improved.

[0073] In some embodiments, the fluorinated carbonate compound includes a compound represented by Formula I,

[0074] Wherein, R1, R2, R3, and R4 are independently hydrogen atoms, halogen atoms, C 1-6 Hydrocarbon, C 1-3 Halogenated alkyl, C 1-3 Alkoxy, C 1-3 At least one of a haloalkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

[0075] As used herein, the term "halogen atom" refers to elements of Group VIIA of the periodic system, including but not limited to: F, Cl, Br, and I.

[0076] In this article, the term “C 1-6 "Hydrocarbon" refers to a group containing carbon and hydrogen atoms including 1-6 carbon atoms. In some embodiments, C 1-6 The hydrocarbon group does not include an unsaturated bond and is an alkyl group, and as examples, includes but is not limited to methyl, ethyl, propyl, isopropyl, 2-methyl-1-propyl, 2-methyl-2-propyl, 2-methyl-1-butyl, 3-methyl-1-butyl, 2-methyl-3-butyl, 2,2-dimethyl-1-propyl, 2-methyl-1-pentyl, 3-methyl-1-pentyl, 4-methyl-1-pentyl, 2-methyl-2-pentyl, 3-methyl-2-pentyl, 4-methyl-2-pentyl, 2,2-dimethyl-1-butyl, 3,3-dimethyl-1-butyl, 2-ethyl-1-butyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, neopentyl, and hexyl. In some embodiments, C 1-6 The hydrocarbon group includes an unsaturated bond and is a cycloalkyl group, an alkenyl group, an alkynyl group, or an aryl group, and includes, by way of example, but not limited to, a cyclopropyl group, a cyclobutyl group, a vinyl group, a 1-propenyl group or a 2-propenyl group, a phenyl group, and a naphthyl group.

[0077] In this article, the term “C 1-3 "Haloalkyl" refers to a C 1-3 Alkyl groups include, but are not limited to, -CF3, -CF2CH2, and -CF2CH2CH3.

[0078] In this article, the term “C 1-3 "Alkoxy" refers to a C-type group connected to the main carbon chain through an oxygen atom. 1-3 Alkyl groups include, by way of example, but are not limited to, methoxy (CH3O-), ethoxy (C2H5O-), and propoxy (C3H7O-).

[0079] In this article, the term “C 1-3 "Haloalkoxy" refers to a C 1-3 Alkoxy.

[0080] As used herein, the term "ester group" refers to a -COO- group.

[0081] As used herein, the term "cyano" refers to a -CN group.

[0082] As used herein, the term "sulfonic acid" refers to a -SO3H group.

[0083] As used herein, the term "isocyanate" refers to an -NCO group.

[0084] The cyclic fluorinated carbonate in which at least one of R1, R2, R3, and R4 is a fluorine atom is easy to open the ring and form an SEI film on the surface of the negative electrode, thereby reducing battery gas production and improving battery kinetic performance and cycle stability.

[0085] In some embodiments, the fluorinated carbonate compound comprises at least one of the following compounds,

[0086] In some embodiments, the negative electrode plate includes a negative electrode film layer, and the negative electrode film layer contains calcium.

[0087] It is understood that calcium can be introduced into the negative electrode membrane in any form. In some embodiments, calcium is introduced into the negative electrode membrane in the form of calcium oxide or calcium salt. In some embodiments, calcium is introduced into the negative electrode membrane in the form of CaO.

[0088] The introduction of calcium into the negative electrode film layer can induce the deposition of sodium ions, help inhibit the formation of sodium dendrites, and reduce the oxidation and gas production of unstable components generated by sodium dendrites at the negative electrode, thereby reducing the gas production of the negative electrode during discharge and reducing the volume expansion rate of the battery after high-temperature storage.

[0089] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion of the calcium element in the negative electrode film layer is d, and the negative electrode active material is discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at a current of 40μA and 10μA in a three-stage step-by-step discharge method to the theoretical discharge capacity of the negative electrode active material is c, and d and c satisfy: 2.5×10 -6 ≤d / c≤4.5×10 -3 In some embodiments, d and c satisfy: 5.5×10 -5 ≤d / c≤3.1×10 -3 .

[0090] In some embodiments, the value of d / c can be selected as 2.5×10 -6 , 3×10 -6 , 4.5×10 - 6 , 1.0×10 -5 , 1.1×10 -5 , 1.3×10 -5 , 1.4×10 -5 , 5.5×10 -5 , 1.0×10 -4 , 1.2×10 - 4 , 1.4×10 -4 , 1.5×10 -4 , 3.0×10 -4 , 4.0×10 -4 , 5.0×10 -4 , 6.0×10 -4 , 7.0×10 - 4 , 8.0×10 -4 , 9.0×10 -4 , 1.0×10 -3 , 2.0×10 -3 , 3.0×10 -3 , 4.0×10 -3 , 4.5×10 - 3 or any value in between.

[0091] The actual discharge capacity of the negative electrode active material can be measured through the charge-discharge curve of a button cell. The test method is a three-stage step-by-step discharge method, first discharging at a rate of 0.05C, then discharging at currents of 40μA and 10μA to reduce the phenomenon of incomplete capacity due to polarization under high-rate discharge. As an example, the negative electrode sheet of the sodium secondary battery is punched into a small disc with a diameter of 14mm and used as the positive electrode in the button cell. A metal sodium sheet is used as the negative electrode, and a 1.3 mol / L sodium hexafluorophosphate solution is used as the electrolyte. The solvents in the electrolyte include ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The mass ratio of the three solvents is 1:2:2. The button cell is assembled and constant current charge and discharge tests are performed in the voltage range of 0.005V-2V. The charge and discharge curve diagram is shown in Figure 1. During the discharge process, the battery was discharged at a constant current rate of 0.05C to 0.005V. After standing until the voltage returned to a stable value E, the battery was discharged at a constant current rate of 40μA to 0.005V. After standing until the voltage returned to a stable value F, the battery was discharged at a constant current rate of 10μA to 0.005V. During the charging process, the battery was charged at a constant current rate of 0.05C to 2V. The total capacity of the discharge process in the voltage range of 0.1V-0.005V during the second cycle of the above charge-discharge cycle was divided by the mass of the negative electrode active material in the negative electrode sheet to obtain the actual discharge specific capacity (mAh / g) of the negative electrode active material in the voltage range of 0.1V-0.005V. As shown in Figure 1, the difference between the specific capacity D corresponding to the discharge at a constant current rate of 10μA to 0.005V and the specific capacity C at 0.1V in the discharge curve is the actual discharge specific capacity (mAh / g) of the negative electrode sheet in the voltage range of 0.1V-0.005V.

[0092] In some embodiments, the negative electrode active material includes hard carbon, which has a theoretical discharge capacity of 300 mAh / g.

[0093] Unlike lithium secondary batteries, sodium secondary batteries often use hard carbon as their negative electrode active material. The capacity of hard carbon mainly includes two stages. The first stage is between 1.5V and 0.1V (vs Na / Na + ) capacity comes from Na + The adsorption process at the hard carbon surface defects, the second stage at 0.1V (vsNa / Na + )The following capacity contributions come from Na + Filling process in hard carbon micropores. In order to improve the negative electrode capacity, the porosity of hard carbon is often increased in the prior art. However, Na +The potential during the hard carbon micropore filling process is close to the potential of metallic sodium deposition (0V), which can easily cause sodium precipitation problems during the charging process. The precipitated sodium dendrites are extremely reactive and react rapidly with the electrolyte to produce a large amount of gas and unstable by-products. These unstable substances are prone to oxidative decomposition as the negative electrode potential increases during discharge, resulting in insufficient film-forming driving force. In addition, unstable organic by-products are easily soluble in the electrolyte, causing the SEI film to be in a cyclical process of dissolution and repair, exacerbating the deterioration of battery gas production and cycle stability.

[0094] Calcium in the negative electrode film can induce sodium ion deposition at the negative electrode, helping to inhibit sodium dendrite formation and thereby reduce the unstable components generated by these dendrites. When the d / c ratio is within the appropriate range, the interaction between the calcium in the negative electrode film and the active material in the negative electrode sheet allows the secondary battery to maintain high capacity and energy density while exhibiting low gas production, high kinetic performance, and cycling stability.

[0095] In some embodiments, based on the total mass of the negative electrode film layer, the mass fraction d of the calcium element in the negative electrode film layer is 2 ppm to 3000 ppm. In some embodiments, based on the total mass of the negative electrode film layer, the mass fraction d of the calcium element in the negative electrode film layer is 40 ppm to 2300 ppm.

[0096] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion d of the calcium element in the negative electrode film layer is 2ppm, 10ppm, 40ppm, 80ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1500ppm, 2000ppm, 2300ppm, 2500ppm, 3000ppm or any value therebetween.

[0097] In this article, ppm means parts per million.

[0098] When the mass fraction (d) of calcium in the negative electrode film is within an appropriate range, it can not only reduce the negative impact of excessive calcium content on the capacity and impedance of the secondary battery, but also fully utilize the calcium's role in inhibiting dendrites and reducing gas production. This reduces battery gas production while improving the battery's low-temperature charging performance and room-temperature cycle capacity retention. When the mass fraction (d) of calcium in the negative electrode film is between 40ppm and 2300ppm, the battery's gas production rate is further reduced and the cycle stability is further improved.

[0099] In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage step-discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at currents of 40μA and 10μA, is 100mAh / g-300mAh / g. In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage step-discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at currents of 40μA and 10μA, is 200mAh / g-250mAh / g.

[0100] In some embodiments, the actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V and then discharged at currents of 40μA and 10μA can be 100mAh / g, 150mAh / g, 200mAh / g, 250mAh / g, 300mAh / g or any value therebetween.

[0101] The actual discharge capacity of negative electrode active materials in the 0.1V-0.005V voltage range can be tuned by modifying the preparation process. Using hard carbon as an example, by varying the pyrolysis temperature, the pore size and content of the negative electrode active material can be adjusted to achieve control of the capacity of the negative electrode active material across different voltage ranges. Increasing the pyrolysis temperature helps induce the formation of ordered micropores in the hard carbon, thereby increasing the actual discharge capacity of the negative electrode active material in the 0.1V-0.005V voltage range.

[0102] The negative electrode active material's actual discharge capacity within the voltage range of 0.1V-0.005V is within an appropriate range, which can reduce the battery's gas production after high-temperature storage and improve the battery's kinetic performance and cycle stability. When the actual discharge capacity of the negative electrode active material is 200mAh / g-250mAh / g within the voltage range of 0.1V-0.005V, it can further achieve a balance between high energy density and low gas production.

[0103] In some embodiments, the negative electrode active material includes one or more of hard carbon, metallic sodium, sodium-tin alloy, and metal oxide.

[0104] In some embodiments, the negative electrode active material includes hard carbon.

[0105] In some embodiments, hard carbon is a negative electrode active material with a particle size of 1um to 50um prepared by calcining biomass material at 600℃-1000℃ in an inert environment for 1h-5h, grinding it for 1h-4h, and then calcining it again at 1100℃-1800℃ in an inert environment for 2h-8h.

[0106] In some embodiments, the biomass material includes one or more of peanut shells, straw, sawdust, walnut shells, bagasse, rice bran, wheat husks, coconut shells, apricot shells, wood, lignin, and papermaking waste residue.

[0107] In some embodiments, the primary calcination temperature may be 600° C., 700° C., 800° C., 900° C., 1000° C., or any range therebetween.

[0108] In some embodiments, the grinding time can be selected as 1 h, 2 h, 3 h, 4 h, or any range therebetween.

[0109] In some embodiments, the primary calcination time may be 1 h, 2 h, 3 h, 4 h, 5 h, or any range therebetween.

[0110] In some embodiments, the temperature of the secondary calcination may be 1100° C., 1200° C., 1300° C., 1400° C., 1500° C., 1600° C., 1700° C., 1800° C., or any range therebetween.

[0111] In some embodiments, the secondary calcination time may be 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, or any range therebetween.

[0112] In some embodiments, the particle size of the hard carbon is 1 um, 5 um, 10 um, 20 um, 30 um, 40 um, 50 um, or any range therebetween.

[0113] In some embodiments, the sodium secondary battery further includes a positive electrode sheet, the positive electrode sheet includes a positive electrode film layer, and the positive electrode film layer includes a positive electrode active material.

[0114] In some embodiments, the positive electrode active material further contains copper element. Based on the total mass of the positive electrode active material, the mass proportion of the copper element is 0.01% to 23%, and optionally 6.5% to 18%.

[0115] In some embodiments, based on the total mass of the positive electrode active material, the mass proportion of the copper element is 0.01%, 4%, 6.5%, 10%, 13%, 15%, 18%, 20%, 23% or any numerical range therebetween.

[0116] The positive electrode active material containing copper elements has a more stable structure and can further improve the cycle stability of the battery.

[0117] The mass ratio of copper elements is within the appropriate range. While the battery cycle stability is improved, the copper element will not be converted into Cu under high voltage. 3+, which causes the electrolyte to decompose rapidly under its high oxidizing property, severely deteriorating the gas generation phenomenon of the battery. When the mass percentage of copper element is within the range of 6.5% to 18%, the low gas generation amount and high cycle stability of the secondary battery can be further balanced.

[0118] In some embodiments, the positive electrode active material can be the positive electrode active material for batteries well-known in the art. As an example, the positive electrode active material can include at least one of the following materials: Prussian blue analogs, sodium-containing phosphates, sodium-containing transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as the positive electrode active material of the battery can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, the Prussian blue analog is Na x P[R(CN)6] δ ·zH2O, where P and R are each independently selected from at least one of transition metal elements, 0 < x ≤ 2, 0 < δ ≤ 1, and 0 ≤ z ≤ 10; the sodium-containing phosphate is Na b Me c (PO4) d O2X, where A is one or more of H, Li, Na, K, and NH4, Me is one or more of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, and Zn, X is one or more of F, Cl, and Br, 0 < b ≤ 4, 0 < c ≤ 2, 1 ≤ d ≤ 3; the sodium-containing transition metal oxide is Na a M b N c Fe d Mn e O2, M and N include at least one of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb, 0.05 ≤ b ≤ 0.2, 0.2 ≤ c ≤ 0.3, 0.2 ≤ d ≤ 0.3, 0.3 ≤ e ≤ 0.4, 0.75 ≤ a / (b + c + d + e) ≤ 1;

[0119] In some embodiments, the positive electrode active material includes a sodium transition metal oxide, and the sodium transition metal oxide includes Na m Cu n X o Fe p Mn q O 2-s，Wherein X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, and Ba, <m≤1,0≤n≤0.5,0≤o<0.5,0≤p≤0.5,0<q≤0.68,n+o+p+q=1,0≤s<0.2。

[0120] Sodium transition metal oxide cathode active materials have high voltage. While contributing to battery capacity, the anionic oxygen in these materials also generates a large amount of proton hydrogen, accelerating the oxidation and gassing of unstable components in the negative electrode, leading to severe gassing at the negative electrode side. The combined effects of the negative electrode plate and the fluorinated carbonate compound in the electrolyte provided in the embodiments of this application can effectively reduce battery gassing while increasing battery capacity and energy density.

[0121] In some embodiments, the sodium transition metal oxide comprises Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2、Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 ]O2、Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 ]At least one of O2.

[0122] In some embodiments, the electrolyte further includes a second component, which is one or more of vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, maleic anhydride, succinic anhydride, sodium difluorooxalatoborate, triallyl phosphate, sodium bis(oxalatoborate), sodium tetrafluorooxalatophosphate, sodium difluorobis(oxalatophosphate), sodium difluorophosphate, and sodium fluorosulfonate.

[0123] The SEI film formed at the negative electrode interface of sodium secondary batteries primarily consists of sodium alkyl carbonate and sodium carbonate. However, sodium alkyl carbonate has a greater solubility in electrolyte solvents than lithium alkyl carbonate, making the SEI film of sodium secondary batteries highly unstable. This leads to continuous side reactions between the electrolyte and the negative electrode, resulting in poor secondary battery cycle performance. A second component containing unsaturated functional groups can be reduced to film at the negative electrode before the solvent, interacting with the fluorocarbonate compound to inhibit the formation of easily soluble substances such as sodium alkyl carbonate. This, combined with the negative electrode plate, reduces battery gas production and improves battery cycle stability.

[0124] In some embodiments, the second component accounts for 0.01% to 10% by weight based on the total weight of the electrolyte. In some embodiments, the second component accounts for 0.1% to 5% by weight based on the total weight of the electrolyte.

[0125] In some embodiments, based on the total mass of the electrolyte, the mass proportion of the second component can be selected to be 0.01%, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value therebetween.

[0126] The mass proportion of the second component is within the above range, which can improve the gas production of the battery while controlling the thickness of the SEI film, thereby achieving low impedance and low gas production of the battery at the same time.

[0127] In some embodiments, the electrolyte includes an electrolyte salt selected from at least one of NaPF6, NaBF4, NaN(SO2F)2(NaFSI), NaClO4, NaAsF6, NaB(C2O4)2(NaBOB), NaBF2(C2O4)(NaDFOB), NaN(SO2RF)2, and NaN(SO2F)(SO2RF), wherein RF is represented by C b F 2b+1 , b is an integer between 1 and 10, and can be optionally an integer between 1 and 3.

[0128] In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2F)2, NaN(CF3SO2)2, NaB(C2O4)2, and NaBF2(C2O4). In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2RF)2, and NaBF2(C2O4). In some embodiments, RF is -CF3, -C2F5, or -CF2CF2CF3.

[0129] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of a chain carbonate, a chain carboxylate, a cyclic carbonic acid, an ether solvent, a sulfone solvent, and a nitrile solvent. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). In some embodiments, the chain carboxylate includes at least one of methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), ethyl butyrate (EB), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the linear carboxylic acid ester includes at least one of methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the ether solvent includes at least one of dioxolane (DOL), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2Me-THF), tetrahydropyran (THP), 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (DG), 1,2-diethoxyethane, and 1,2-dibutoxyethane.

[0130] [Positive electrode]

[0131] The positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0132] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0133] In some embodiments, 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.).

[0134] In some embodiments, the positive electrode film layer may further 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.

[0135] In some embodiments, the positive electrode film layer may further include a conductive agent. For 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.

[0136] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder 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 the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0137] [Negative electrode]

[0138] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0139] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0140] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0141] In some embodiments, the negative electrode film layer may further include a binder. For example, the binder may 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).

[0142] In some embodiments, the negative electrode film layer may further include a conductive agent. For example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0143] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0144] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0145] [Isolation film]

[0146] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0147] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can 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 can be the same or different, without particular limitation.

[0148] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0149] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0150] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0151] In the present application, the shape of the sodium secondary battery includes but is not limited to cylindrical, square or any other shape. For example, FIG2 shows a sodium secondary battery 5 with a square structure as an example.

[0152] In some embodiments, referring to FIG3 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the sodium secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0153] In some embodiments, sodium secondary batteries can be assembled into a battery module. The number of sodium secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0154] Figure 4 illustrates an exemplary battery module 4. Referring to Figure 4 , within battery module 4, multiple sodium secondary batteries 5 may be arranged sequentially along the length of battery module 4. Of course, any other arrangement is also possible. Furthermore, these multiple sodium secondary batteries 5 may be secured using fasteners.

[0155] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of sodium secondary batteries 5 are accommodated in the accommodation space.

[0156] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0157] Figures 5 and 6 illustrate an example battery pack 1. Referring to Figures 5 and 6 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0158] In addition, the present application also provides an electrical device, which includes at least one of the sodium secondary battery, battery module, or battery pack provided in the present application. The sodium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical 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.

[0159] As an electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0160] Figure 7 shows an example of an electric device. This device can be a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of sodium secondary batteries, a battery pack or battery module can be used.

[0161] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a sodium secondary battery as a power source.

[0162] Example

[0163] 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.

[0164] 1. Preparation method

[0165] Example 1:

[0166] 1) Electrolyte

[0167] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a 30 / 70 mass ratio, and 1M NaPF6 sodium salt was dissolved. Then, fluoroethylene carbonate was added and stirred evenly to prepare an electrolyte. The mass proportion of fluoroethylene carbonate was 1% based on the total mass of the electrolyte.

[0168] 2) Preparation of positive electrode active materials

[0169] Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 Preparation of O2: 0.39 mol Na2CO3, 0.22 mol CuO, 0.06 mol Fe2O3, and 0.67 mol MnO2 precursors are ball-milled in a ball mill with ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0170] 3) Preparation of positive electrode sheet C

[0171] The positive electrode active material Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain a positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The density is evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil is dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0172] 4) Preparation of negative electrode active materials

[0173] The biomass material coconut shell was calcined at 800°C for 2 hours in a tube furnace containing an argon atmosphere, then washed with hydrochloric acid and deionized water and dried respectively. After grinding it for 2 hours, it was calcined at 1550°C for 4 hours in a tube furnace with an argon atmosphere to obtain the target material H2 with a particle size of 10 μm. The test showed that its actual discharge specific capacity in the voltage range of 0.5V-1V was 40mAh / g, and the actual discharge specific capacity in the voltage range of 0.1V-0.005V was 220mAh / g. For specific test methods, please refer to the "Actual discharge specific capacity test of negative electrode active materials" section below.

[0174] 5) Preparation of negative electrode sheet C

[0175] The negative electrode active material H2, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the slurry dry material (i.e., the total mass of the negative electrode active material H2, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na), and CaO) was 100 ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 13 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0176] 6) Isolation film

[0177] A 9 μm polyethylene (PE) porous polymer film was used as the separator.

[0178] 7) Preparation of batteries

[0179] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to isolate the positive and negative electrode sheets. The bare battery cell is wound, the tabs are welded, and the bare battery cell is placed in an outer package. The above-prepared electrolyte is injected into the dried battery cell, and then the sodium secondary battery product of Example 1 is obtained after packaging, standing, formation, shaping, and capacity testing.

[0180] The preparation methods of the sodium secondary batteries of Examples 2-4 are substantially the same as the preparation method of Example 1, except that the type of the fluorinated carbonate compound is adjusted. Specific parameters are shown in Table 1.

[0181] The preparation methods of the sodium secondary batteries of Examples 5-8 are basically the same as the preparation method of Example 1, except that the mass ratio of the fluorinated carbonate compound is adjusted. Specific parameters are shown in Table 1.

[0182] The preparation methods of the sodium secondary batteries of Examples 9-12 are basically the same as the preparation method of Example 1, except that the preparation process of the negative electrode active material and the negative electrode plate is adjusted to adjust the actual discharge specific capacity of the negative electrode active material in the negative electrode plate in the voltage range of 1.0V-0.5V and the actual discharge specific capacity in the voltage range of 0.1V-0.005V. The specific parameters are shown in Table 1. The preparation process is as follows:

[0183] In Example 9, the actual discharge specific capacity of the negative electrode active material in the negative electrode plate A was measured by a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA. The actual discharge specific capacity was 9mAh / g; the actual discharge specific capacity was measured by a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at currents of 40μA and 10μA. The detailed test method is shown in the test method section below. The negative electrode plate preparation method is as follows:

[0184] The biomass material coconut shell was calcined at 800°C for 2 hours in a tube furnace containing an argon atmosphere, then washed with hydrochloric acid and deionized water and dried respectively. After grinding it for 2 hours, it was calcined at 1650°C for 6 hours in a tube furnace containing an argon atmosphere to obtain the target material H3 with a particle size of 20 μm.

[0185] The negative electrode active material H3, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the dry material was 100ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14g (dry weight) / 1540.25mm 2The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 13 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0186] The actual discharge specific capacity of the negative electrode active material in the negative electrode plate B in Example 10 is 18.3 mAh / g in the voltage range of 1.0 V to 0.5 V, and the actual discharge specific capacity in the voltage range of 0.1 V to 0.005 V is 250 mAh / g. The detailed test method is shown in the test method section below. The preparation method is as follows:

[0187] The negative electrode active material (30wt% H2 and 70wt% H3), conductive agent acetylene black, binder styrene butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC-Na) were fully stirred and mixed in a deionized water solvent system according to a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the dry material was 100ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 13 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0188] In Example 11, the actual discharge capacity of the negative electrode active material in the negative electrode plate E was measured by a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA. The actual discharge capacity was 140mAh / g; the actual discharge capacity was 100mAh / g in a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at currents of 40μA and 10μA. The detailed test method is shown in the test method section below. The negative electrode plate preparation method is as follows:

[0189] The biomass material coconut shell was calcined at 800°C for 2 h in a tube furnace containing an argon atmosphere, then washed with hydrochloric acid and deionized water and dried respectively. After grinding it for 2 h, it was calcined at 1150°C for 2 h in a tube furnace containing an argon atmosphere to obtain the target material H1 with a particle size of 2 μm.

[0190] The negative electrode active material H1, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the dry material was 100ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14g (dry weight) / 1540.25mm 2The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 13 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0191] The actual discharge specific capacity of the negative electrode active material in the negative electrode plate D in Example 12 was measured by a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA. The actual discharge specific capacity was 70mAh / g; the actual discharge specific capacity was measured by a three-stage step-by-step discharge method in which the negative electrode active material was first discharged at a rate of 0.05C in the voltage range of 0.1V-0V, and then discharged at currents of 40μA and 10μA. The detailed test method is shown in the test method section below. The preparation method is:

[0192] The negative electrode active material (30wt% H1 and 70wt% H2), conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were fully stirred and mixed in a deionized water solvent system according to a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the dry material was 100ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 8 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0193] The preparation methods of the sodium secondary batteries of Examples 13-18 are basically the same as the preparation method of Example 1, except that the mass ratio of calcium element in the negative electrode film layer is adjusted by adding different amounts of CaO to the negative electrode slurry. Specific parameters are shown in Table 1.

[0194] The preparation methods of the sodium secondary batteries of Examples 19-22 are substantially the same as those of Example 1, except that the preparation processes of the positive electrode active material and the positive electrode sheet are adjusted to adjust the mass ratio of the copper element in the positive electrode active material. Specific parameters are shown in Table 1. The preparation process is as follows:

[0195] In Example 19, the copper element mass percentage of the positive electrode active material in the positive electrode plate A is 0%, and the preparation method is:

[0196] Na 1 / 2 Fe 1 / 2 Mn 1 / 2Preparation of O2: 0.25 mol Na2CO3, 0.25 mol Fe2O3, and 0.5 mol MnO2 precursors are ball-milled in a ball mill with ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0197] The positive electrode active material Na 1 / 2 Fe 1 / 2 Mn 1 / 2 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain a positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0198] In Example 20, the copper element mass ratio of the positive electrode active material in the positive electrode plate B is 6.5%, and the preparation method is:

[0199] The positive electrode active material (50 wt% Na 1 / 2 Fe 1 / 2 Mn 1 / 2 O2 and 50wt% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) in a weight ratio of 90:5:5 in N-methylpyrrolidone solvent system and fully stirred and mixed to obtain positive electrode slurry; the positive electrode slurry was 0.28g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0200] In Example 21, the copper element in the positive electrode active material of the positive electrode plate E accounts for 23% by mass, and the preparation method is as follows:

[0201] Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2Preparation of O2: 0.45 mol Na2CO3, 0.4 mol CuO, 0.05 mol Fe2O3, and 0.5 mol MnO2 precursors are ball-milled in a ball mill with ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0202] The positive electrode active material Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain a positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0203] In Example 22, the copper element mass ratio of the positive electrode active material in the positive electrode plate D is 18%, and the preparation method is as follows:

[0204] The positive electrode active material (50 wt% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2 and 50wt% Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) in a weight ratio of 90:5:5 in N-methylpyrrolidone solvent system and fully stirred and mixed to obtain positive electrode slurry; the positive electrode slurry was 0.28g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0205] The preparation method of the sodium secondary battery of Examples 23-25 ​​is basically the same as the preparation method of Example 1, except that a second component is added. Specific parameters are shown in Table 1.

[0206] The preparation method of the sodium secondary battery of Comparative Example 1 is substantially the same as that of Example 9, except that in Comparative Example 1, the mass proportion of fluoroethylene carbonate in the electrolyte is 0.01%;

[0207] The preparation method of the sodium secondary battery of Comparative Example 2 is substantially the same as that of Example 1, except that the electrolyte does not include a fluorinated carbonate compound.

[0208] 2. Battery performance test

[0209] 1. High temperature storage volume change rate

[0210] At 25°C, the new sodium secondary batteries prepared in the examples and comparative examples were left for 5 minutes, charged to 4.0V at a constant current rate of 1C, and then charged at a constant voltage to a current less than or equal to 0.05C. After that, they were left for 5 minutes, and then discharged to 1.5V at a constant current rate of 1C. The volume V1 of the battery was tested by the drainage method. The battery was then placed in a 60°C oven and stored for 2 months. The battery was taken out and the test volume was V2. The volume change rate of the battery was =(V2-V1) / V1×100%.

[0211] 2. Charging performance at -10℃

[0212] A three-electrode battery containing a reference was prepared, where the reference electrode was sodium vanadium phosphate. The battery was charged at 25°C at a constant current of 1C to a voltage of 4.0V, then charged at a constant voltage to a current less than or equal to 0.05C, then left for 5 minutes, and then discharged at a constant current of 1C to 1.5V, and the discharge capacity was recorded as C1. The battery was then placed in a -10°C environment and allowed to rest for 2 hours, and charged at a constant current of 0.1C to a voltage of 4.0V. The charging capacity before the negative electrode potential was compared to the reference potential of -3.377V was C2, and the -10°C charging capacity of the battery was = C2 / C1×100%.

[0213] 3. Mass energy density

[0214] Battery cell capacity test: Allow the battery cell to rest at 25°C for 2 hours, ensuring the cell temperature is 25°C. At 25°C, charge the battery cell at 0.1C to the charge cutoff voltage. Continue constant voltage charging at the same charge cutoff voltage until the current reaches 0.05C, at which point the charge is cutoff (where C represents the rated capacity of the battery cell). Allow the battery cell to rest at 25°C for 1 hour. At 25°C, discharge the battery cell at 0.1C to the discharge cutoff voltage. Record the total discharge capacity (C0) and the total discharge energy (E0).

[0215] Battery cell weight measurement: Place the battery cell on an electronic balance until the weight stabilizes, and read the battery cell weight value M0.

[0216] Energy density calculation: Battery cell discharge energy E0 / battery cell weight M0 is the energy density of the battery cell.

[0217] 4. Battery cycle capacity retention rate

[0218] At 25°C, the prepared battery was charged to 4.0V at a constant current of 1C, then charged at a constant voltage of 4.0V until the current dropped to 0.05C. After standing for 5 minutes, it was discharged to 1.5V at a constant current of 1C. This was the first charge / discharge cycle of the battery, and the discharge capacity of this time was recorded as the discharge capacity of the battery in the first cycle (C0). The above steps were repeated for the same battery, and the discharge capacity of the battery after the 400th cycle was (C1). The capacity retention rate after 400 cycles = C1 / C0×100%.

[0219] 5. Actual discharge capacity test of negative electrode active materials

[0220] The negative electrode sheets in the comparative example and the embodiment were punched into small discs with a diameter of 14 mm and used as the positive electrode in the button battery. A metal sodium sheet was used as the negative electrode, a polypropylene film was used as the separator, and a 1.3 mol / L sodium hexafluorophosphate solution was used as the electrolyte. The solvent in the electrolyte included ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the mass ratio of the three solvents was 1:2:2. A button battery was assembled as the test electrolyte, and a constant current charge and discharge test was performed in the voltage range of 0.005-2V. During the discharge process, the battery was discharged at a constant current rate of 0.05C to 0.005V, and then the battery was allowed to stand until the voltage returned to a stable value. The battery was discharged at a constant current rate of 40μA to 0.005V, and then the battery was allowed to stand until the voltage returned to a stable value. The battery was discharged at a constant current of 10μA to 0.005V, and the charging process was charged to 2V at a constant current of 0.05C. The total discharge capacity (mAh) of the second discharge process in the voltage range of 1.0V-0.5V in the above charge and discharge cycle was divided by the mass (g) of the negative electrode active material in the negative electrode sheet, and recorded as the actual discharge specific capacity of the negative electrode active material at 1.0V-0.5V (unit: mAh / g). The total discharge capacity (mAh) of the second discharge process in the voltage range of 0.1V-0.005V in the above charge and discharge cycle was divided by the mass (g) of the negative electrode active material in the negative electrode sheet, and recorded as the actual discharge specific capacity (unit: mAh / g) of the negative electrode active material at 0.1V-0.005V.

[0221] The theoretical discharge capacity of the negative electrode active material hard carbon is 300 mAh / g.

[0222] 6. Determination of the mass ratio of calcium in the negative electrode film

[0223] The mass percentage of calcium in the negative electrode film can be determined by referring to the general principle EPA 6010D-2014 and using inductively coupled plasma atomic emission spectrometry. The mass percentage of silicon in the negative electrode film is calculated by dividing the mass of calcium in the negative electrode film sample by the mass of the negative electrode film sample.

[0224] 7. Determination of the mass ratio of copper element in positive electrode active material

[0225] The mass percentage of Cu in the positive electrode active material can be determined by referring to the general principle EPA 6010D-2014 and using inductively coupled plasma atomic emission spectrometry. The mass percentage of Cu in the positive electrode active material is calculated by dividing the mass of the Cu in the positive electrode active material sample by the mass of the positive electrode active material sample.

[0226] 3. Analysis of test results of various embodiments and comparative examples

[0227] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in the table below.

[0228] Table 1

[0229] Table 2

[0230] Table 3

[0231] Table 4

[0232] According to the above results, the sodium secondary batteries in Examples 1-25 all include a negative electrode plate and an electrolyte, the negative electrode plate includes a negative electrode active material, and the ratio of the actual discharge specific capacity of the negative electrode active material in the voltage range of 1.0V-0.5V to the theoretical discharge specific capacity of the negative electrode active material is a; the electrolyte includes a first component, the first component is a fluorinated carbonate compound, and the mass proportion of the fluorinated carbonate compound based on the total mass of the electrolyte is b; and a and b satisfy: 1≤a / b≤270.

[0233] From the comparison of the embodiment and the comparative example, it can be seen that when a and b satisfy: 1≤a / b≤270, the battery has a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance and room-temperature cycle capacity retention rate.

[0234] As can be seen from the examples, when a and b satisfy 1≤a / b≤100, the volume expansion rate, low-temperature charging performance and room-temperature cycle capacity retention rate of the battery are further improved.

[0235] As shown in Examples 1 and 5-8, when the mass percentage (b) of fluoroethylene carbonate in the electrolyte is 0.05%-10% based on the total mass of the electrolyte, the battery has a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance, and room-temperature cycling capacity retention. When the mass percentage (b) of fluoroethylene carbonate in the electrolyte is 0.2%-10%, the volume expansion rate after high-temperature storage is further reduced, and the low-temperature charging performance and room-temperature cycling capacity retention are improved.

[0236] As can be seen from Examples 1 and 9-12, when the actual discharge capacity of the negative electrode active material in the voltage range of 1.0V-0.5V is 9mAh / g-140mAh / g, and when the actual discharge capacity in the voltage range of 0.1V-0.005V is 100mAh / g-300mAh / g, the battery has a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance, and room temperature cycle capacity retention rate. When the discharge capacity of the negative electrode active material in the voltage range of 1.0V-0.5V is 18mAh / g-70mAh / g, and when the discharge capacity in the voltage range of 1.0V-0.005V is 200mAh / g-250mAh / g, the battery can take into account a low volume expansion rate after high-temperature storage, excellent low-temperature charging performance, room temperature cycle capacity retention rate, and high energy density.

[0237] As can be seen from Examples 1 and 9-18, based on the total mass of the negative electrode film layer, the mass proportion of the calcium element in the negative electrode film layer is d, the ratio of the actual discharge specific capacity of the negative electrode active material in the voltage range of 0.1V-0.005V to the theoretical discharge specific capacity of the negative electrode active material is c, and when d and c satisfy: d / c=2.5×10-6≤d / c≤4.5×10-3, the battery has a low battery volume expansion rate after high-temperature storage, excellent low-temperature charging performance, and room temperature cycle capacity retention rate. When d and c satisfy 5.5×10-5≤d / c≤3.1×10-3, the battery has both a low battery volume expansion rate after high-temperature storage and excellent low-temperature charging performance and room temperature cycle capacity retention rate.

[0238] It can be seen from Example 1 and Examples 13-18 that, based on the total mass of the negative electrode film layer, when the mass proportion d of the calcium element in the negative electrode film layer is 40ppm to 3000ppm, the volume expansion rate of the battery after high-temperature storage can be further reduced, and the low-temperature charging performance and the normal temperature cycle capacity retention rate can be further improved.

[0239] As shown in Examples 1 and 19-22, when the mass percentage of copper, based on the total mass of the positive electrode active material, is 0% to 23%, the battery exhibits low volume expansion after high-temperature storage, excellent low-temperature charging performance, and room-temperature cycling capacity retention. When the mass percentage of copper is 6.5% to 18%, the battery achieves both low volume expansion after high-temperature storage and excellent low-temperature charging performance and room-temperature cycling capacity retention.

[0240] It can be seen from Example 1 and Examples 23-25 ​​that by adding the second component to the electrolyte, the battery has a lower battery volume expansion rate after high-temperature storage, more excellent low-temperature charging performance and room-temperature cycle capacity retention rate.

[0241] 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, wherein: include: A negative electrode plate, wherein the negative electrode plate comprises a negative electrode active material, wherein the negative electrode active material is first discharged at a rate of 0.05C in a voltage range of 1.0V-0.5V, and then discharged at currents of 40μA and 10μA in a three-stage stepwise discharge method, and the ratio of the actual discharge specific capacity measured to the theoretical discharge specific capacity of the negative electrode active material is a; and An electrolyte, wherein the electrolyte comprises a first component, wherein the first component is a fluorinated carbonate compound, and the mass proportion of the fluorinated carbonate compound based on the total mass of the electrolyte is b; And a and b satisfy: 1≤a / b≤270.

2. The sodium secondary battery according to claim 1, wherein a and b satisfy: 1≤a / b≤100.

3. The sodium secondary battery according to claim 1, wherein The actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in a voltage range of 1.0V-0.5V and then discharged at currents of 40μA and 10μA is 9mAh / g-140mAh / g.

4. The sodium secondary battery according to claim 1, wherein The actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in a voltage range of 1.0V-0.5V and then discharged at currents of 40μA and 10μA is 18mAh / g-70mAh / g.

5. The sodium secondary battery according to claim 1, wherein Based on the total mass of the electrolyte, the mass proportion b of the fluorinated carbonate compound is 0.05% to 10%.

6. The sodium secondary battery according to claim 1, wherein Based on the total mass of the electrolyte, the mass proportion b of the fluorinated carbonate compound is 0.2% to 10%.

7. The sodium secondary battery according to claim 1, wherein The fluorocarbonate compound includes a compound represented by formula I, Wherein, R1, R2, R3, and R4 each independently include a hydrogen atom, a halogen atom, a C 1-6 Hydrocarbon, C 1-3 Haloalkyl, C 1-3 Alkoxy, C 1-3 At least one of a halogenated alkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

8. The sodium secondary battery according to any one of claims 1 to 7, wherein The fluorinated carbonate compound comprises at least one of the following compounds, 9. The sodium secondary battery according to any one of claims 1 to 7, wherein The negative electrode plate includes a negative electrode film layer, and the negative electrode film layer contains calcium.

10. The sodium secondary battery according to claim 9, wherein Based on the total mass of the negative electrode film layer, the mass proportion of calcium element in the negative electrode film layer is d, and the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V, and then discharged at a current of 40μA and 10μA in a three-stage stepwise discharge method to measure the actual discharge specific capacity of the negative electrode active material is c, and d and c satisfy: 2.5×10 -6 ≤d / c≤4.5×10 -3 .

11. The sodium secondary battery according to claim 10, wherein d and c meet: 5.5×10 -5 ≤d / c≤3.1×10 -3 .

12. The sodium secondary battery according to claim 9, wherein Based on the total mass of the negative electrode film layer, the mass proportion d of the calcium element in the negative electrode film layer is 2 ppm to 3000 ppm.

13. The sodium secondary battery according to claim 9, wherein Based on the total mass of the negative electrode film layer, the mass proportion d of the calcium element in the negative electrode film layer is 40 ppm to 2300 ppm.

14. The sodium secondary battery according to any one of claims 1 to 7, wherein The actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method of first discharging at a rate of 0.05C in a voltage range of 0.1V-0.005V and then discharging at currents of 40μA and 10μA is 100mAh / g-300mAh / g.

15. The sodium secondary battery according to any one of claims 1 to 7, wherein The actual discharge specific capacity of the negative electrode active material measured by a three-stage stepwise discharge method in which the negative electrode active material is first discharged at a rate of 0.05C in the voltage range of 0.1V-0.005V and then discharged at currents of 40μA and 10μA is 200mAh / g-250mAh / g.

16. The sodium secondary battery according to any one of claims 1 to 7, wherein The negative electrode active material includes hard carbon.

17. The sodium secondary battery according to any one of claims 1 to 7, wherein The sodium secondary battery also includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode active material.

18. The sodium secondary battery according to claim 17, wherein The positive electrode active material further contains copper element, and based on the total mass of the positive electrode active material, the mass proportion of the copper element is 0.01% to 23%.

19. The sodium secondary battery according to claim 18, wherein Based on the total mass of the positive electrode active material, the mass proportion of the copper element is 6.5% to 18%.

20. The sodium secondary battery according to claim 17, wherein The positive electrode active material includes a sodium transition metal oxide, and the sodium transition metal oxide includes Na m Cu n X o Fe p Mn q O 2-s , wherein X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, and Ba, 0 <m≤1,0≤n≤0.5,0≤o<0.5,0≤p≤0.5,0<q≤0.68,n+o+p+q=1,0≤s<0.2。 21. The sodium secondary battery according to claim 20, wherein The sodium transition metal oxide includes Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2、Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 ]O2、Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 ]At least one of O2.

22. The sodium secondary battery according to any one of claims 1 to 7, wherein The electrolyte also includes a second component, which is one or more of vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, maleic anhydride, succinic anhydride, sodium difluorooxalatoborate, triallyl phosphate, sodium bisoxalatoborate, sodium tetrafluorooxalate phosphate, sodium difluorobisoxalate phosphate, sodium difluorophosphate, and sodium fluorosulfonate.

23. The sodium secondary battery according to claim 22, wherein: Based on the total mass of the electrolyte, the mass proportion of the second component is 0.01% to 10%.

24. The sodium secondary battery according to claim 22 or 23, wherein Based on the total mass of the electrolyte, the mass proportion of the second component is 0.1% to 5%.

25. An electrical device, wherein: A sodium secondary battery comprising the sodium secondary battery according to any one of claims 1 to 24.

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