Electrolyte solution, secondary battery, battery module, battery pack, and electric device

Abstract

The application provides an electrolyte and a secondary battery, a battery module, a battery pack and a power utilization device, wherein the electrolyte comprises: a film-forming additive, the film-forming additive contains a nitrate group in a chemical structure. By adding the film-forming additive containing the nitrate group in the chemical structure in the electrolyte, the nitrate group with high reduction reaction activity can participate in the formation of a sodium ion solvation sheath layer, a sodium metal surface is formed to be rich in NaN x O y and Na3N, and an SEI layer with high conductivity and high mechanical strength, reducing or even avoiding the growth of sodium dendrites. At the same time, the nitrate group has the function of adjusting the sodium metal deposition morphology, which can further reduce or even avoid the formation of sodium dendrites.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to an electrolyte, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Secondary batteries are widely used in various consumer electronics and electric vehicles due to their outstanding characteristics such as light weight, no pollution, and no memory effect. Lithium-ion batteries, with their advantages of high energy density, long lifespan, and no memory effect, have already achieved a dominant position in many areas of the energy storage market. However, lithium-ion batteries face the problems of scarce lithium resources and high costs. Sodium, compared to lithium, has the advantages of abundant reserves and low refining costs. Moreover, both belong to the same group, exhibit similar chemical properties, and have similar electrode potentials. Therefore, in recent years, sodium-ion batteries have received increasing attention from researchers as a potential energy storage technology.

[0003] Currently, sodium-ion batteries are being developed using various material systems. In existing electrolyte systems, due to uneven sodium deposition, sodium dendrite growth may occur on the electrodes after multiple charge-discharge cycles. This sodium dendrite growth will affect the coulombic efficiency and cycle performance of the sodium-ion battery. Summary of the Invention

[0004] Therefore, it is necessary to provide an electrolyte, a secondary battery, a battery module, a battery pack, and an electrical device to improve the situation of sodium dendrite growth on the electrodes and enhance the coulombic efficiency and cycle performance of sodium-ion batteries.

[0005] To achieve the above objectives, a first aspect of this application provides an electrolyte comprising a film-forming additive, wherein the film-forming additive contains a nitrate ester group in its chemical structure.

[0006] In inorganic nitrates, the N–O bond of the nitrate ion forms a resonance structure, with the negative charge of the nitrate ion concentrated on the oxygen atom, resulting in high reduction stability. However, in nitrate ester groups, the nitrate ion undergoes esterification with the hydroxyl group, disrupting the N–O resonance structure and thus increasing the reduction reactivity of the nitrate ester group. Compared to inorganic nitrates, nitrate ester groups exhibit higher reducing power. The electrolyte of this application, by adding a film-forming additive containing nitrate ester groups in its chemical structure, allows the highly reactive nitrate ester groups to participate in the formation of a sodium ion solvation sheath, forming a NaN-rich layer on the surface of metallic sodium. x O y The SEI layer, composed of Na3N and possessing high conductivity and high mechanical strength, reduces or even eliminates the growth of sodium dendrites. Simultaneously, the nitrate groups regulate the deposition morphology of sodium metal, further reducing or even preventing sodium dendrite formation.

[0007] In some embodiments, the number of carbon atoms in the chemical structure of the film-forming additive is denoted as N, and the number of carbon atoms in the chemical structure of the film-forming additive satisfies: 3≤N≤12; optionally, 3≤N≤8.

[0008] In some embodiments, the film-forming additive may further contain one or more of alkyl, cycloalkyl, heteroalkyl, heterocyclic, and alkylene groups in its chemical structure.

[0009] In some embodiments, the heteroatoms contained in the heteroalkyl group and the heterocyclic group each independently include oxygen atoms.

[0010] In some embodiments, the film-forming additive includes one or more of nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, pentylenetetrazolium nitrate, butyl nitrate, pentyl nitrate, and isoamyl nitrate.

[0011] In some embodiments, the film-forming additive accounts for 0.5% to 20% by mass in the electrolyte; optionally, it accounts for 2% to 10%.

[0012] In some embodiments, sodium salts are also included;

[0013] Optionally, the sodium salt includes one or more of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, and sodium bis(trifluoromethanesulfonyl)imide.

[0014] In some embodiments, the molar concentration of the sodium salt in the electrolyte is 0.5–8 mol / L; alternatively, it is 1–4 mol / L.

[0015] In some embodiments, ether solvents are also included;

[0016] Optionally, the ether solvent includes one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, diphenyl ether, and crown ether.

[0017] A second aspect of this application provides a secondary battery, including the electrolyte of the first aspect of this application.

[0018] In some embodiments, the secondary battery is a sodium-ion battery.

[0019] The third aspect of this application provides an electrical device, including one or more of the secondary battery of the second aspect of this application, the battery module of the third aspect of this application, and the battery pack of the fourth aspect of this application. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0021] Figure 2 yes Figure 1 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0022] Figure 3 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0023] Explanation of reference numerals in the attached figures:

[0024] 1. Secondary battery; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Electrical device. Detailed Implementation

[0025] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0027] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0028] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0029] In this application, if the unit for a data range is only followed by the right endpoint, it indicates that the units for the left and right endpoints are the same. For example, 1–4 mol / L means that the units for the left endpoint “1” and the right endpoint “4” are both mol / L (moles per liter).

[0030] In this application, the terms "multiple", "various", "multiple times", etc., unless otherwise specified, refer to a quantity greater than 2 or equal to 2. For example, "multiple" means two or more.

[0031] This document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0032] The "scope" disclosed in this application is defined by a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. The scope defined in this way may include end values ​​or not.

[0033] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows for temperature fluctuations within the precision range controlled by the instrument.

[0034] In recent years, sodium-ion batteries have attracted increasing attention from researchers as a potential energy storage technology. In existing sodium-ion electrolyte systems, uneven sodium deposition can lead to sodium dendrite growth on the electrodes after multiple charge-discharge cycles, affecting the coulombic efficiency and cycle performance of the battery. Related technologies often use inorganic nitrates as additives in the electrolyte; however, the effect of inorganic nitrates in improving sodium dendrite growth is not ideal.

[0035] To address the aforementioned problems, this application provides an electrolyte comprising a film-forming additive, wherein the chemical structure of the film-forming additive contains a nitrate ester group.

[0036] It should be noted that the nitrate ester group mentioned in this application can be either acyclic or cyclic; acyclic nitrate ester group refers to a nitrate ester group that is not in a cyclic structure, while cyclic nitrate ester group refers to a nitrate ester group that is in a cyclic structure. However, compared with film-forming additives containing acyclic nitrate ester groups, film-forming additives containing cyclic nitrate ester groups have greater steric hindrance when used in electrolytes, resulting in increased electrolyte viscosity and decreased electrolyte conductivity, severely affecting electrolyte performance.

[0037] It is understandable that in inorganic nitrates, the N–O bond of the nitrate ion is a resonance structure, and the negative charge of the nitrate ion is concentrated on the oxygen atom, resulting in high reduction stability of the nitrate ion. However, in nitrate ester groups, the nitrate ion undergoes esterification with the hydroxyl group, thus disrupting the N–O bond resonance structure and increasing the reduction reactivity of the nitrate ester group. Compared to inorganic nitrates, nitrate ester groups exhibit higher reducing power. The electrolyte of this application, by adding a film-forming additive containing nitrate ester groups in its chemical structure, allows the highly reactive nitrate ester groups to participate in the formation of a sodium ion solvation sheath, forming a NaN-rich layer on the surface of metallic sodium. x O y The SEI layer, composed of Na3N and possessing high conductivity and high mechanical strength, reduces or even eliminates the growth of sodium dendrites. Simultaneously, the nitrate groups regulate the deposition morphology of sodium metal, further reducing or even preventing sodium dendrite formation.

[0038] Furthermore, during the charge-discharge cycle of the secondary battery, the solvation effect of the nitrate ester group can continuously regulate the sodium deposition process; during the charge-discharge cycle of the secondary battery, the metallic sodium deposition layer can grow uniformly, significantly reducing or even avoiding the formation of sodium dendrites.

[0039] Through in-depth research, the applicant has discovered that, in addition to meeting the above-mentioned design conditions, the electrolyte of this application can further improve the coulombic efficiency and cycle performance of the secondary battery if it can optionally meet one or more of the following conditions.

[0040] In some embodiments, the number of carbon atoms in the chemical structure of the film-forming additive is denoted as N, and the number of carbon atoms in the chemical structure of the film-forming additive satisfies: 3 ≤ N ≤ 12; for example, it can be 4 ≤ N ≤ 12, 4 ≤ N ≤ 11, 5 ≤ N ≤ 10, or 6 ≤ N ≤ 9, etc. When the number of carbon atoms in the chemical structure of the film-forming additive is greater than the above range, due to the increased steric hindrance effect, its addition to the electrolyte will increase the electrolyte viscosity and reduce the electrolyte conductivity, seriously affecting the electrolyte performance. Further, the number of carbon atoms in the chemical structure of the film-forming additive satisfies: 3 ≤ N ≤ 8.

[0041] Understandably, N is a positive integer.

[0042] In some embodiments, the film-forming additive may further contain one or more of alkyl, cycloalkyl, heteroalkyl, heterocyclic, and alkylene groups in its chemical structure.

[0043] It should be noted that the term "alkyl" refers to a saturated hydrocarbon containing a primary (normal) carbon atom, or a secondary carbon atom, or a tertiary carbon atom, or a quaternary carbon atom, or a combination thereof. Phrases containing this term, such as "C1-C9 alkyl," refer to alkyl groups containing 1 to 9 carbon atoms, and each occurrence can independently be C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, C7 alkyl, C8 alkyl, or C9 alkyl. Suitable examples include, but are not limited to: 1-propyl (n-Pr, n-propyl, -CH2CH2CH3), 2-propyl (i-Pr, i-propyl, -CH(CH3)2), 1-butyl (n-Bu, n-butyl, -CH2CH2CH2CH3), 2-methyl-1-propyl (i-Bu, i-butyl, -CH2CH(CH3)2), 2-butyl (s-Bu, s-butyl, -CH(CH3)CH2CH3), 2-methyl-2-propyl ( t-Bu, t-butyl, -C(CH3)3), 1-pentyl (n-pentyl, -CH2CH2CH2CH2CH3), 2-pentyl (-CH(CH3)CH2CH2CH3), 3-pentyl (-CH(CH2CH3)2), 2-methyl-2-butyl (-C(CH3)2CH2CH3), 3-methyl-2-butyl (-CH(CH3)CH(CH3)2), 3-methyl-1-butyl (-CH2CH2CH(CH3)2) ), 2-methyl-1-butyl (-CH2CH(CH3)CH2CH3), 1-hexyl (-CH2CH2CH2CH2CH2CH3), 2-hexyl (-CH(CH3)CH2CH2CH2CH3), 3-hexyl (-CH(CH2CH3)(CH2CH2CH3)), 2-methyl-2-pentyl (-C(CH3)2CH2CH2CH3), 3-methyl-2-pentyl (-CH(CH3)CH(CH3)CH2 CH3), 4-methyl-2-pentyl (-CH(CH3)CH2CH(CH3)2), 3-methyl-3-pentyl (-C(CH3)(CH2CH3)2), 2-methyl-3-pentyl (-CH(CH2CH3)CH(CH3)2), 2,3-dimethyl-2-butyl (-C(CH3)2CH(CH3)2), 3,3-dimethyl-2-butyl (-CH(CH3)C(CH3)3 and octyl (-(CH2)7CH3).

[0044] The term "cycloalkyl" refers to a non-aromatic hydrocarbon containing a ring of carbon atoms, which can be monocycloalkyl, spirocycloalkyl, or bridged cycloalkyl. Phrases containing this term, such as "C3-C9 cycloalkyl," refer to cycloalkyl compounds containing 3 to 9 carbon atoms, and each occurrence can independently be C3, C4, C5, C6, C7, C8, or C9 cycloalkyl. Suitable examples include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl. Additionally, "cycloalkyl" may contain one or more double bonds; representative examples of cycloalkyl compounds containing double bonds include cyclopentenyl, cyclohexenyl, cyclohexadienyl, and cyclobutadienyl.

[0045] The term "heteroalkyl" refers to an alkyl group in which at least one carbon atom is replaced by a non-carbon atom, which can be an N atom, O atom, S atom, etc. For example, if the carbon atom in the alkyl group that is attached to the parent nucleus is replaced by a non-carbon atom, the resulting heteroalkyl group is an alkoxy group (e.g., -OCH3, etc.), an amine (e.g., -NHCH3, -N(CH3)2, etc.), or a thioalkyl group (e.g., -SCH3). If the carbon atom in the alkyl group that is not attached to the parent nucleus is replaced by a non-carbon atom, the resulting heteroalkyl group is an alkyl ether (e.g., -CH2CH2-O-CH3, etc.), an alkylamine (e.g., -CH2NHCH3, -CH2N(CH3)2, etc.), or a thioalkyl ether (e.g., -CH2-S-CH3). If the terminal carbon atom of the alkyl group is replaced by a non-carbon atom, the resulting heteroalkyl group is a hydroxyalkyl group (e.g., -CH2CH2-OH), an aminoalkyl group (e.g., -CH2NH2), or an alkyl mercapto group (e.g., -CH2CH2-SH). Phrases containing this term, such as “C2-C9 heteroalkyl”, refer to heteroalkyl groups containing 2 to 9 carbon atoms, and each time they appear, they can be independently C2 heteroalkyl, C3 heteroalkyl, C4 heteroalkyl, C5 heteroalkyl, C7 heteroalkyl, C8 heteroalkyl, or C9 heteroalkyl.

[0046] The term "heterocyclic group" refers to a cycloalkyl group in which at least one carbon atom is replaced by a non-carbon atom, which can be an N atom, O atom, S atom, etc., and can be a saturated ring or a partially unsaturated ring. Phrases containing this term, such as "C4-C9 heterocyclic group," refer to heterocyclic groups containing 4 to 9 carbon atoms, and each occurrence can be independently C4, C6, C7, C8, or C9 heteroalkyl. Suitable examples include, but are not limited to: dihydropyridyl, tetrahydropyridyl (piperidinyl), tetrahydrothiophenyl, sulfur-oxidized tetrahydrothiophenyl, tetrahydrofuranyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl, and dihydroindolyl.

[0047] The term "alkylene" refers to a hydrocarbon group derived from an alkyl group by removing one hydrogen atom, forming a group with two monovalent centers. This group can be a saturated branched alkyl group or a saturated straight-chain alkyl group. For example, "C1-C9 alkylene" means that the alkyl moiety contains 1 to 9 carbon atoms, and each occurrence can be independently C1, C2, C3, C4, C5, C6, C7, C8, or C9 alkylene. Suitable examples include, but are not limited to: methylene (-CH2-), 1,1-ethyl (-CH(CH3)-), 1,2-ethyl (-CH2CH2-), 1,1-propyl (-CH(CH2CH3)-), 1,2-propyl (-CH2CH(CH3)-), 1,3-propyl (-CH2CH2CH2-), and 1,4-butyl (-CH2CH2CH2CH2-).

[0048] In some of these embodiments, the heteroatoms contained in the heteroalkyl and heterocyclic groups each independently include an oxygen atom.

[0049] In some embodiments, the film-forming additive includes one or more of nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, pentylenetetrazolium nitrate, butyl nitrate, pentyl nitrate, and isoamyl nitrate.

[0050] In some embodiments, the mass percentage of the film-forming additive in the electrolyte is 0.5% to 20%. When the mass percentage of the film-forming additive in the electrolyte is lower than this range, it is difficult to ensure the formation of a stable SEI film. When the mass percentage of the film-forming additive in the electrolyte is higher than this range, it will cause an increase in electrolyte viscosity, reduce ionic conductivity, and affect the kinetic performance of the secondary battery. Simultaneously, an excessively high mass percentage of the film-forming additive in the electrolyte will increase the cost of the electrolyte. Optionally, the mass percentage of the film-forming additive in the electrolyte is 2% to 10%.

[0051] In some embodiments, the electrolyte further includes a sodium salt; optionally, the sodium salt includes one or more of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, and sodium bis(trifluoromethanesulfonyl)imide.

[0052] In some embodiments, the molar concentration of the sodium salt in the electrolyte is 0.5–8 mol / L; alternatively, it is 1–4 mol / L.

[0053] In some embodiments, the electrolyte also includes ether solvents; ether solvent molecules can build a stable electrode / electrolyte interface on the surface of sodium metal anode (including no anode), carbon material anode and other non-carbon material anodes, forming a stable SEI film and reducing electrochemical polarization.

[0054] Optionally, the ether solvent includes one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, diphenyl ether, and crown ether.

[0055] All of the above-mentioned raw materials, unless otherwise specified, can be obtained through commercial purchase.

[0056] Secondary batteries

[0057] This application also provides a secondary battery, including an electrolyte, which is the electrolyte provided above in this application.

[0058] A secondary battery also includes a positive electrode, a negative electrode, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The separator, positioned between the positive and negative electrodes, serves as a barrier. The electrolyte, acting as a conductor of ions, lies between the positive and negative electrodes.

[0059] Positive electrode sheet

[0060] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0061] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0062] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0063] In some embodiments, the positive electrode active material includes one or more of transition metal oxides, polyanionic compounds, and Prussian blue analogues.

[0064] As an example, the positive electrode active material can be selected from sodium-iron composite oxide (NaFeO2), sodium-cobalt composite oxide (NaCoO2), sodium-chromium composite oxide (NaCrO2), sodium-manganese composite oxide (NaMnO2), sodium-nickel composite oxide (NaNiO2), and sodium-nickel-titanium composite oxide (NaNi). 1 / 2Ti 1 / 2 O2), sodium nickel manganese composite oxide (NaNi) 1 / 2 Mn 1 / 2 O2), sodium iron manganese composite oxide (Na) 2 / 3 Fe 1 / 3 Mn 2 / 3 O2), sodium nickel cobalt manganese composite oxide (NaNi) 1 / 3 Co 1 / 3 Mn 1 / 3 O2), sodium iron phosphate (NaFePO4), sodium manganese phosphate (NaMn) P The present application may use materials such as O4, sodium cobalt phosphate (NaCoPO4), Prussian blue materials, and polyanionic materials (phosphates, fluorophosphates, pyrophosphates, sulfates), but this application is not limited to these materials. Other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used.

[0065] The positive electrode film may also optionally include a binder or conductive agent.

[0066] As an example, the adhesive may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0067] As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0068] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0069] Negative electrode sheet

[0070] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0071] As an example, the negative electrode current collector has two surfaces opposite each other in its own 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.

[0072] As an example, the negative electrode current collector can be a metal foil or a composite current collector. For example, copper foil can be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, copper 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.).

[0073] In some embodiments, the negative electrode active material includes one or more of sodium metal (including non-negative electrode), carbon materials, and other non-carbon materials.

[0074] As an example, the negative electrode active material may include at least one of the following materials: sodium metal, artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0075] In some embodiments, the negative electrode film layer may optionally include a binder. 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).

[0076] In some embodiments, the negative electrode film may optionally include a conductive agent. 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.

[0077] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0078] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0079] Separating membrane

[0080] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0081] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0082] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0083] In some embodiments, the secondary battery of this application is a sodium-ion secondary battery.

[0084] Secondary batteries can be prepared according to conventional methods in the field, such as winding (or stacking) the positive electrode, separator, and negative electrode in sequence, so that the separator is placed between the positive electrode and the negative electrode to play a role in isolation, thus obtaining a battery cell. The battery cell is placed in an outer package, electrolyte is injected and the package is sealed to obtain a secondary battery.

[0085] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 1.

[0086] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0087] 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. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0088] In some embodiments, refer to Figure 2 The outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The shell 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be placed on the opening to close the receiving cavity.

[0089] The positive electrode, negative electrode, and separator can be formed into electrode assembly 12 by a winding or stacking process. Electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in electrode assembly 12. The secondary battery 1 can contain one or more electrode assemblies 12, which can be adjusted according to requirements.

[0090] In some embodiments, secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in a battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0091] In a battery module, multiple secondary batteries can be arranged sequentially along the length of the module. Alternatively, they can be arranged in any other manner. Furthermore, these multiple secondary batteries can be secured using fasteners.

[0092] Optionally, the battery module may also include a housing with a receiving space in which multiple secondary batteries are housed.

[0093] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0094] The battery pack may include a battery box and multiple battery modules disposed within the battery box. The battery box includes an upper body and a lower body, with the upper body covering the lower body to form a closed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0095] Electrical appliances

[0096] This application also provides an electrical device, which includes at least one of the aforementioned secondary battery, battery module, or battery pack. The secondary battery, battery module, or battery pack can be used as a power source for the device or as an energy storage unit for the device. The device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0097] The device can be configured to use a secondary battery, battery module, or battery pack, depending on its usage requirements.

[0098] Figure 3 This is an example of an electrical device 2. This electrical device 2 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density of the secondary battery, a battery pack or battery module can be used.

[0099] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0100] The beneficial effects of this application are further illustrated below with reference to the embodiments.

[0101] Example

[0102] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0103] All materials used in the embodiments of this application are commercially available.

[0104] Preparation of primary and secondary batteries

[0105] Example 1

[0106] 1. Preparation of positive electrode sheet

[0107] 10 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone, and 10 wt% carbon black conductive agent and 80 wt% positive electrode active material Na4Fe3(PO4)2P2O7 were added to prepare a uniformly dispersed slurry. The slurry was uniformly coated on the surface of aluminum foil and then transferred to a vacuum drying oven for complete drying. The resulting electrode was rolled and then punched to obtain the positive electrode.

[0108] 2. Preparation of negative electrode sheet

[0109] 4 wt% carbon nanotube material and 1.6 wt% sodium carboxymethyl cellulose polymer binder were added to water and stirred into a uniform slurry. The slurry was coated on the surface of copper foil, transferred to a vacuum drying oven for complete drying, and then punched to obtain the negative electrode sheet.

[0110] 3. Preparation of electrolyte

[0111] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), sodium hexafluorophosphate (NaPF6, as sodium salt) and nitroglycerin (as film-forming additive) were dissolved in ethylene glycol dimethyl ether (as ether solvent) and stirred until homogeneous to obtain an electrolyte with a sodium salt concentration of 1 mol / L.

[0112] 4. Separating membrane

[0113] Polypropylene film is used as the separator.

[0114] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrolyte is then added to assemble the stacked secondary battery.

[0115] Example 2-23

[0116] The preparation methods of the secondary batteries in Examples 2-23 are basically the same as those in Example 1. The main difference is that at least one of the following is different when preparing the electrolyte: the type of film-forming additive and its mass ratio in the electrolyte, the type of sodium salt, and the type of ether solvent. See Table 1 below for details.

[0117] Comparative Examples 1-2

[0118] The preparation methods of the secondary batteries in Comparative Examples 1-2 are basically the same as those in Example 1. The main difference is that at least one of the following is different when preparing the electrolyte: the type of film-forming additive and its mass ratio in the electrolyte, the type of sodium salt, and the type of ether solvent. See Table 1 below for details.

[0119] II. Performance Testing

[0120] 1. Coulomb efficiency

[0121] Each of the secondary batteries prepared above was charged to 3.7V at 25°C with a constant current of 1 / 3C, and then charged at a constant voltage of 3.7V until the current dropped to 0.05C to obtain the first charge capacity (Cc1); then discharged to 2.5V with a constant current of 1 / 3C to obtain the first discharge capacity (Cd1), and the coulombic efficiency of the secondary battery was calculated according to the following formula.

[0122] Coulombic efficiency of a secondary battery = initial discharge capacity (Cd1) / initial charge capacity (Cc1).

[0123] 2. Capacity retention rate

[0124] Each of the prepared secondary batteries was charged at 25°C with a constant current of 1C to 3.7V, then charged at a constant voltage of 3.7V until the current dropped to 0.05C, and then discharged with a constant current of 1C to 2.5V to obtain the first discharge capacity (Cd1). This charging and discharging process was repeated until the nth cycle to obtain the discharge capacity of the sodium-ion battery after n cycles, denoted as Cdn. The capacity retention rate of the secondary battery was then calculated according to the following formula:

[0125] Capacity retention rate = discharge capacity after n cycles (Cdn) / discharge capacity in the first cycle (Cd1).

[0126] 3. DC internal resistance

[0127] DC internal resistance (DCR) refers to the resistance that current encounters inside the battery cell. After the battery discharges, the battery voltage will rebound due to polarization.

[0128] Each of the prepared secondary batteries was charged at 25°C with a constant current of 1C to 3.7V, then charged at a constant voltage of 3.7V until the current dropped to 0.05C, and then discharged at a constant current of 1C to 2.5V. After that, it was left to stand for 5 minutes (stabilization time) and then the next cycle was started. The battery voltage before the discharge stopped and the battery voltage after the battery voltage stabilized were recorded in each cycle. Then the DC impedance R = ΔU / I was calculated using the following formula (where: ΔU is the voltage difference, R is the DC resistance, and I is the discharge current).

[0129] 4. Sodium dendrites

[0130] After cycling each of the prepared secondary batteries 200 times according to the charge-discharge method described in section "2. Capacity Retention," the secondary batteries were disassembled in an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm). The surface morphology of the negative electrode was visually observed to determine whether sodium dendrites were formed. No white spots on the negative electrode indicated no sodium dendrites; scattered white spots indicated slight sodium dendrite formation; and dense white spots indicated severe sodium dendrite formation.

[0131] The performance test results of the above embodiments and comparative examples are shown in Table 1 below.

[0132] Table 1

[0133]

[0134]

[0135] Among them, NaPF6 is sodium hexafluorophosphate, NaDFOB is sodium difluorooxalate borate, NaBF4 is sodium tetrafluoroborate, NaBOB is sodium dioxalate borate, NaFSI is sodium bis(fluorosulfonyl)imide, and NaTFSI is sodium bis(trifluoromethylsulfonyl)imide.

[0136] As can be seen from the results of Examples 1-22 in Table 1, the electrolyte formed by using film-forming additives containing nitrate ester groups in the chemical structure, sodium salts and ether solvents can effectively improve the situation of sodium dendrite growth on the electrode and improve the coulombic efficiency and cycle performance of sodium-ion batteries.

[0137] A comparison of the results from Examples 1-12 and Comparative Example 2 shows that by adding a film-forming additive containing nitrate ester groups in its chemical structure to the electrolyte, the coulombic efficiency and capacity retention after 200 cycles of the secondary battery are significantly improved, the battery DCR after 200 cycles is significantly reduced, and sodium dendrites are significantly reduced or even eliminated. This indicates that by adding a film-forming additive containing nitrate ester groups in its chemical structure to the electrolyte, the highly reactive nitrate ester groups can participate in the formation of a sodium ion solvation sheath, forming a NaN-rich layer on the surface of metallic sodium. x O y The SEI layer, which contains Na3N and has high conductivity and high mechanical strength, reduces or even avoids the formation of sodium dendrites and improves the coulombic efficiency and cycle performance of secondary batteries.

[0138] The main difference between Examples 1 and Examples 8-13 lies in the different mass percentages of the film-forming additive in the electrolyte. In Example 12, the film-forming additive had the smallest mass percentage in the electrolyte. The coulombic efficiency and capacity retention after 200 cycles of the secondary battery in Example 12 were lower than those in Examples 1 and 8-11, while the battery DCR after 200 cycles was higher than that in Examples 1 and 8-11, and slight sodium dendrite formation was observed. This indicates that when the mass percentage of the film-forming additive in the electrolyte is less than 0.5%, it is difficult to ensure the formation of a stable SEI film, which is detrimental to improving the coulombic efficiency and cycle performance of the secondary battery, and cannot completely prevent the formation of sodium dendrites. In Example 13, the film-forming additive had the largest mass percentage in the electrolyte. The battery DCR after 200 cycles of the secondary battery in Example 13 was significantly higher than that in Examples 1 and 8-11. This indicates that when the mass percentage of the film-forming additive in the electrolyte is higher than 20%, it will cause an increase in electrolyte viscosity, reduce ionic conductivity, and affect the kinetic performance of the secondary battery.

[0139] The difference between Examples 1-3 and 6-7 and Comparative Example 1 lies in the type of film-forming additive in the electrolyte. The film-forming additives in Examples 1-3 and 6-7 contain nitrate ester groups in their chemical structure, while the film-forming additive in Comparative Example 1 is an inorganic nitrate. Compared with Comparative Example 1, the coulombic efficiency and capacity retention after 200 cycles of the secondary batteries in Examples 1-3 and 6-7 are significantly improved, the battery DCR after 200 cycles is significantly reduced, and no sodium dendrites are formed. The technicians analyzed that the reason is that the N-O bond of the nitrate ion in the inorganic nitrate is a resonance structure, and the negative charge of the nitrate ion is concentrated on the oxygen atom, resulting in high reduction stability of the nitrate ion. In contrast, the nitrate ion in the nitrate ester group undergoes an esterification reaction with the hydroxyl group, which destroys the resonance structure of the N-O bond in the nitrate ester group, thereby increasing the reduction reactivity of the nitrate ester group. Compared with inorganic nitrate, the nitrate ester group has higher reducing power.

[0140] The difference between Example 1 and Example 23 lies in the solvent used in the electrolyte; ether-based solvents were used in Example 1, while non-ether-based solvents were used in Example 23. Compared to Example 1, the coulombic efficiency and capacity retention after 200 cycles of the secondary battery in Example 23 were significantly reduced, while the battery DCR after 200 cycles was significantly increased, and sodium dendrites were formed. This indicates that ether-based solvent molecules can construct a stable electrode / electrolyte interface on the surface of sodium metal anodes (including those without anodes), carbon anodes, and other non-carbon anodes, forming a stable SEI film, reducing electrochemical polarization, and improving the coulombic efficiency and cycle performance of the secondary battery.

[0141] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0142] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A secondary battery, characterized in that, The secondary battery is a sodium metal battery, which includes an electrolyte. The electrolyte includes a film-forming additive and an ether solvent. The film-forming additive is nitroglycerin. The ether solvent includes one or more of the following: ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, diphenyl ether, and crown ether.

2. The secondary battery as described in claim 1, characterized in that, The film-forming additive accounts for 0.5-20% of the mass of the electrolyte.

3. The secondary battery as described in claim 2, characterized in that, The film-forming additive accounts for 2-10% of the mass of the electrolyte.

4. The secondary battery as described in claim 1, characterized in that, It also includes sodium salts.

5. The secondary battery as described in claim 4, characterized in that, The sodium salt includes one or more of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, and sodium bis(trifluoromethanesulfonyl)imide.

6. The secondary battery as described in claim 4, characterized in that, The molar concentration of the sodium salt in the electrolyte is 0.5~8 mol / L.

7. The secondary battery as described in claim 6, characterized in that, The molar concentration of the sodium salt in the electrolyte is 1~4 mol / L.

8. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 7.

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