Electrolyte, secondary battery and electronic device

Abstract

The present disclosure provides an electrolyte a secondary battery and an electronic device. The electrolyte includes a sodium salt, a fluorinated sulfone compound, and an ether compound.

Description

[0001] ELECTROLYTE, SECONDARY BATTERY AND ELECTRONIC DEVICE

[0002] FIELD

[0003] The present disclosure relates to the technical field of batteries, and more particularly to an electrolyte, a secondary battery and an electronic device.

[0004] BACKGROUND

[0005] The existing secondary batteries include lithium ion batteries and sodium ion batteries, which have different requirements for electrolytes. In the sodium ion secondary batteries, the anode active material is usually made of hard carbon, which has low conductivity, large specific surface area, and low sodium intercalation potential, such that side reactions may happen, leading to sodium precipitation and electrolyte decomposition. In addition, due to the sodium storage mechanism, the negative electrode has metallic sodium characteristics, such that the residual sodium of the anode active material may be reacted with the electrolyte solvents, resulting in the electrolyte degradation. A surface of the cathode active material of the sodium ion battery has a high content of sodium hydroxide or sodium carbonate, which has a high Na / Na+potential, and the electrolyte may be oxidized and decomposed, which affects the cycle life of the sodium ion batteries.

[0006] SUMMARY

[0007] The present disclosure provides an electrolyte, a secondary battery and an electronic device.

[0008] In a first aspect, embodiments of the present disclosure provide an electrolyte for a sodium ion battery. The electrolyte includes a sodium salt; a sulfone compound, including a non-fluorinated sulfone compound and a fluorinated sulfone compound; and an ether compound.

[0009] In some embodiments, the fluorinated sulfone compound includes at least one selected from compounds having a general formula (I): where R1 and R2 are independently selected from a substituted or unsubstituted C1-C5 alkyl group, optionally are linked to form a ring, optionally, at least one of R1 or R2 includes a fluorine atom.

[0010] In some embodiments, the fluorinated sulfone compound includes at least one selected from trifluoromethyl ethyl sulfone, trifluoroethyl methyl sulfone, trifluoromethyl propyl sulfone, trifluoromethyl isopropyl sulfone, trifluoropropyl methyl sulfone, l,l,2,2-tetrafluoro-3- (m ethyl sulfonyl) propane, 2-fluorotetramethylsulfone, and 3-fluorotetramethylsulfone.

[0011] In some embodiments, the non-fluorinated sulfone compound comprises at least one selected from tetramethylene sulfone, methyl sulfonyl methane, and ethyl methyl sulfone.

[0012] In some embodiments, a mass ratio of the fluorinated sulfone compound to the nonfluorinated sulfone compound is ranged from 0.2: 1 to 2: 1, optionally 0.5: 1 to 2: 1.

[0013] In some embodiments, the fluorinated sulfone compound has a dielectric constant of greater than 25, optionally greater than 40.

[0014] In some embodiments, the fluorinated sulfone compound has a decomposition voltage of greater than 5.2V vs. Na / Na+, optionally greater than 5.5 V vs. Na / Na+.

[0015] In some embodiments, the ether compound includes at least one selected from compounds having a general formula (II):

[0016] R3 R4

[0017] (ii) where R3 and R4 are independently selected from a substituted or unsubstituted Ci-Cs alkyl group, optionally are linked to form a ring, optionally at least one of R3 or R4 includes a fluorine atom.

[0018] In some embodiments, the ether compound includes at least one selected from 1,2- dimethoxyethane, diglyme, tetrahydrofuran, and 1,3-dioxolane.

[0019] In some embodiments, the ether compound has a viscosity no more than 2 mPa- S'1at 25 °C, optionally no more than 1.5 mPa- S'1.

[0020] In some embodiments, based on a total weight of the electrolyte, the electrolyte includes 5wt% to 20wt% of the sodium salt, 5wt% to 30wt% of the fluorinated sulfone compound and 5wt% to 50wt% of the ether compound.

[0021] In some embodiments, the electrolyte further includes a carbonate solvent. Optionally, the electrolyte includes 5 wt% to 60 wt% carbonate solvent based on a total weight of the electrolyte. In some embodiments, the electrolyte further includes an additive. The additive is at least one selected from vinylene carbonate, fluoroethylene carbonate, 1,3,2-dioxathiolane 2,2-dioxide,

[0022] 1.3-propanesulfolactone, tri(trimethylsilyl) phosphate, tri(trimethylsilyl) phosphite, tri(trimethylsilyl) borate, 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide methylene disulfonate, 4-vinyl-

[0023] 1.3-dioxolan-2-one, l-propenyl-l,3-propanesulfolactone, succinonitrile, adiponitrile, hexane trinitrile, sodium difluorophosphate, sodium-difluoro(oxalato)borate, disodium monofluorophosphate, and sodium tetrafluoroborate.

[0024] In a second aspect, embodiments of the present disclosure provide a secondary battery. The secondary battery includes the electrolyte according to any one of embodiments in a first aspect of the present disclosure.

[0025] In a third aspect, embodiments of the present disclosure provide an electronic device. The electronic device includes the secondary battery according to any one of embodiments in a second aspect of the present disclosure.

[0026] According to the embodiments of the present disclosure, the electrolyte includes the sodium salt, the non-fluorinated sulfone compound, the fluorinated sulfone compound, and the ether compound. The combination of the non-fluorinated sulfone compound, the fluorinated sulfone compound and the ether compound allows the electrolyte to have moderate viscosity, and improved dielectric constant, conductivity and decomposition voltage, which reduces gas produced from a surface of the cathode active material in the sodium ion batteries and improves the performance of the secondary battery at a high temperature and a high voltage.

[0027] Additional aspects and advantages of the present disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure.

[0028] DETAILED DESCRIPTION

[0029] Reference will now be made in detail to embodiments. The implementations set forth in the following description of the embodiments do not represent all implementations consistent with the present disclosure.

[0030] The embodiments described here are only part of the embodiments of the present disclosure and are not all embodiments of the present disclosure. Based on the embodiments of the present disclosure, other embodiments obtained by those skilled in the art without creative work are within the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present disclosure belongs. The terms used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. The terms “comprises” and “includes” and any variation thereof in the description and claims of the present disclosure are intended to indicate a non-exclusive inclusion.

[0031] Terms used herein in embodiments of the present disclosure are only for the purpose of describing specific embodiments, but should not be construed to limit the present disclosure. As used in the embodiments of the present disclosure and the appended claims, “a / an”, and “the” in singular forms are intended to include plural forms, unless clearly indicated in the context otherwise. It should also be understood that, the term “and / or” used herein represents and contains any or all possible combinations of one or more associated listed items.

[0032] Term “optionally” means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0033] Term “approximately” or “about” are used, this term may mean that there can be a variance in value of up to ±10%, of up to 5%, of up to 2%, of up to 1%, of up to 0.5%, of up to 0.1%, or up to 0.01%.

[0034] Term “range” disclosed in the present disclosure is defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range defined in this way can be inclusive or exclusive, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60- 110 and 80-120 are also obtained. In addition, if the listed minimum values are 1 and 2, and if the listed maximum values are 3, 4 and 5, the ranges of 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5 may be obtained. In the present disclosure, unless otherwise specified, the numerical range “a-b” means the abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range “0-5” means that all the real numbers between “0-5” have been listed, and “0-5” is only the abbreviated representation of these numerical combinations. In addition, when a parameter is an integer >2, it 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.

[0035] The term “alkyl” refers to a saturated hydrocarbon group, having a linear or branched structure. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl (e.g. n- propyl, isopropyl), butyl (e.g. n-butyl, isobutyl, sec-butyl, tert-butyl). For example, Ci-s alkyl refers to an alkyl group including 1 to 8 carbon atoms, which at each occurrence, independently of one another, may be a Ci alkyl group, a C2 alkyl group, a C3 alkyl group, a C4 alkyl group, a C5 alkyl group, a C>, alkyl group, a C7 alkyl group, or a Cs alkyl group. Examples of suitable alkyl groups include, but are not limited to, methyl (Me, -CEE), ethyl (Et, -CH2CH3), 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-l -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-l -butyl (-CH2CH(CH3)CH2CH3), 1 -hexyl (- CH2CH2CH2CH2CH2CH3), 2-hexyl (-CH(CH3)CH2CH2CH2CH3), 3 -hexyl (- CH(CH2CH3)(CH2CH2CH3)), 2-methyl-2-pentyl (-QCJE^CJfcCJfcCJE), 3-methyl-2-pentyl (- CH(CH3)CH(CH3)CH2CH3), 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).

[0036] In the related art, in the sodium ion batteries, the anode active material, such as a hard carbon may cause sodium precipitation, and the cathode active material has a surface with a high Na+content, which leads to electrolyte decomposition and gas production, affecting the performance of the sodium ion batteries.

[0037] An object of the present disclosure is to provide an electrolyte, a secondary battery and an electronic device. The electrolyte has moderate viscosity, and improved dielectric constant, conductivity and decomposition voltage, which reduces gas production on the surface of the cathode active material in the sodium ion batteries and improves the performance of the secondary battery at a high temperature and a high voltage.

[0038] In a first aspect, embodiments of the present disclosure provide an electrolyte for a sodium ion battery. The electrolyte includes a sodium salt; a sulfone compound including a nonfluorinated sulfone compound and a fluorinated sulfone compound; and an ether compound.

[0039] In the electrolyte for the sodium ion battery, since the fluorine atom of the fluorinated sulfone compound has a strong electron absorption effect, the fluorinated sulfone compound is easy to be reduced to form a film on a negative electrode of the battery, and the film may prevent the negative electrode from sodium precipitation. The fluorinated sulfone compound has a high decomposition voltage, which prevents the electrolyte from decomposition, thereby improving the oxidation resistance of the electrolyte at the positive electrode side, and avoiding gas produced from the electrolyte. In addition, the electrolyte includes the non-fluorinated sulfone compound and the fluorinated sulfone compound, which may ensure that the electrolyte has a sufficient reducing ability so as to be reduced to form a film on the negative electrode of the battery, and avoid sodium precipitation due to the reduction reaction caused by the excessive electron absorption effect of fluorine atoms in the fluorinated sulfone compounds. The fluorinated sulfone compounds have lower viscosity, higher oxidation resistance, and lower melting point than the non-fluorinated sulfone compounds, which may reduce a precipitation temperature of sulfone solvents. The ether compound and Na+may form a complex, which improves the solubility and the ionic conductivity of the sodium salt of the electrolyte. By combining the fluorinated sulfone compound and the ether compound, the electrolyte has moderate viscosity, dielectric constant, and decomposition voltage, which further improves the solubility and the ion conductivity of the sodium salt in the electrolyte, thereby improving performance of the battery at a high temperature and a high voltage.

[0040] In some embodiments, the sulfone compound includes at least one selected from compounds having a general formula (I): where R1 and R2 are independently selected from a substituted or unsubstituted C1-C5 alkyl group, optionally are linked to form a ring, optionally, at least one of R1 or R2 includes a fluorine atom.

[0041] In some embodiments, the fluorinated sulfone compound includes at least one selected from trifluoromethyl ethyl sulfone, trifluoroethyl methyl sulfone, trifluoromethyl propyl sulfone, trifluoromethyl isopropyl sulfone, trifluoropropyl methyl sulfone, 1, 1,2,2- tetrafluoro-3 -(methyl sulfonyl) propane, 2-fluorotetram ethyl sulfone, and 3- fluorotetram ethyl sulfone; optionally selected from trifluoromethyl ethyl sulfone, trifluoroethyl methyl sulfone, trifluoromethyl propyl sulfone, trifluoromethyl isopropyl sulfone, 2-fluorotetramethyl sulfone, and 3 -fluorotetramethyl sulfone; optionally selected from 2-fluorotetramethylsulfone, and 3-fluorotetramethylsulfone.

[0042] In some embodiments, the non-fluorinated sulfone compound includes at least one selected from tetramethylene sulfone, methyl sulfonyl methane, and ethyl methyl sulfone.

[0043] In some embodiments, a mass ratio of the fluorinated sulfone compound to the nonfluorinated sulfone compound is ranged from 0.2: 1 to 2: 1, optionally 0.5: 1 to 2: 1. In an embodiment, when the electrolyte includes the non-fluorinated sulfone compound and the fluorinated sulfone compound, and the sulfone compounds are ranged from 5 wt% to 10 wt% based on a total weight of the electrolyte, the non-fluorinated sulfone compound and the fluorinated sulfone compound may be uniformly distributed. In an embodiment, when the sulfone compounds are ranged from 10 wt% to 20 wt% based on a total weight of the electrolyte, a mass ratio of the fluorinated sulfone compound to the non-fluorinated sulfone compound is greater than 0.2: 1. In an embodiment, when the sulfone compounds are ranged from 20 wt% to 30 wt% based on a total weight of the electrolyte, a mass ratio of the fluorinated sulfone compound to the non-fluorinated sulfone compound is greater than 0.5: 1.

[0044] In some embodiments, the fluorinated sulfone compound has a dielectric constant greater than 25, optionally greater than 40. The fluorinated sulfone compound has a high dielectric constant, such that the electrolyte has high solubility for sodium salts, thereby prevents the sodium salts from being precipitated.

[0045] In some embodiments, the fluorinated sulfone compound has a decomposition voltage of greater than 5.2V vs. Na / Na+, optionally greater than 5.5V vs. Na / Na+. In this way, the electrolyte has a decomposition voltage higher than a working voltage of the sodium ion battery (<4.0 V), such that the electrolyte is not easily to be decomposed, thereby improving the oxidation resistance of the electrolyte at the positive electrode side. In addition, the fluorinated sulfone compound is mixed with the ether compound, which avoids the problem of ether solvents being prone to oxidation due to their lower decomposition voltage (about 3.8V vs. Na / Na+).

[0046] In some embodiments, the ether compound includes at least one selected from compounds having a general formula (II):

[0047] R3 R4

[0048] (ii) where R3 and R4 are independently selected from a substituted or unsubstituted Ci-Cs alkyl group, optionally are linked to form a ring, optionally at least one of R3 or R4 includes a fluorine atom.

[0049] In some embodiments, the ether compound includes at least one selected from 1,2- dimethoxyethane, diglyme, tetrahydrofuran, and 1,3-dioxolane; optionally selected from 1,2- dimethoxyethane, and diglyme.

[0050] In some embodiments, the ether compound has a viscosity no more than 2 mPa- S'1at 25 °C, optionally no more than 1.5 mPa- S'1. The ether compound has a low viscosity, which is mixed with the sulfone compound (having a viscosity of greater than 10 mPa- S'1) to allow the electrolyte to have a moderate viscosity, thereby improving the solubility and the ionic conductivity of the electrolyte salts.

[0051] In some embodiments, the electrolyte includes 5wt% to 20wt% of the sodium salt, 5wt% to 30wt% of the fluorinated sulfone compound and 5wt% to 50wt% of the ether compound based on a total weight of the electrolyte.

[0052] In some embodiments, the electrolyte further includes a carbonate solvent. In some embodiments, the electrolyte includes 5 wt% to 60 wt% of carbonate solvent based on a total weight of the electrolyte.

[0053] In some embodiments, the electrolyte includes at least one selected from ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0054] In some embodiments, the electrolyte further includes an additive. In some embodiments, the additive is at least one selected from vinylene carbonate, fluoroethylene carbonate, 1,3,2- dioxathiolane 2,2-dioxide, 1,3 -propanesulfolactone, tri(trimethylsilyl) phosphate, tri(trimethylsilyl) phosphite, tri(trimethylsilyl) borate, 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide methylene disulfonate, 4-vinyl-l,3-dioxolan-2-one, l-propenyl-l,3-propanesulfolactone, succinonitrile, adiponitrile, hexane trinitrile, sodium difluorophosphate, sodium- difluoro(oxalato)borate, disodium monofluorophosphate, and sodium tetrafluorob orate.

[0055] In some embodiments, the sodium salt is at least one selected from sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, 4,5-dicyano-2- (trifluoromethyl) imidazole sodium, or 4,5-dicyano-2-(pentafluoroethyl) imidazole sodium; optionally at least one selected from sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide. For example, the sodium salt is hexafluorophosphate.

[0056] In some embodiments, the electrolyte includes 8% to 20% of hexafluorophosphate based on a total weight of the electrolyte; optionally, 10% to 18%; optionally, 13% to 16%. Alternatively, the electrolyte includes 5% to 15% sodium bis(fluorosulfonyl)imide based on a total weight of the electrolyte; optionally, 8% to 13%; optionally, 10% to 12%. Alternatively, the electrolyte includes 5% to 15% sodium bis(trifluoromethylsulfonyl)imide based on a total weight of the electrolyte; optionally, 8% to 13%; optionally, 10% to 12%.

[0057] Embodiments of the present disclosure also provide a method for preparing an electrolyte. The method includes mixing a sodium salt, a sulfone compound including a non-fluorinated sulfone compound and a fluorinated sulfone compound, and an ether compound to obtain the electrolyte.

[0058] In some embodiments, the method further includes adding a carbonate solvent into the electrolyte; optionally, the electrolyte comprises 5 wt% to 60 wt% carbonate solvent based on a total weight of the electrolyte.

[0059] In some embodiments, the method further includes adding an additive into the electrolyte.

[0060] In a second aspect, embodiments of the present disclosure provide a secondary battery. The secondary battery includes the electrolyte according to any one of the embodiments in a first aspect of the present disclosure.

[0061] The secondary battery is a sodium ion battery. The sodium ion battery includes a positive electrode plate, a negative electrode plate and the electrolyte of the present disclosure.

[0062] In the sodium ion battery, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. The positive electrode film layer includes the cathode active material.

[0063] As an example, the positive electrode current collector has two surfaces along a thickness direction thereof and facing in opposite directions, and the positive electrode film layer is provided on either or both of the two surfaces.

[0064] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil may be used. The composite current collector may include a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector may be formed by forming a metallic material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a high molecular material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0065] In the present disclosure, the cathode active material is a compound capable of reversibly intercalating and deintercalating Na+. As an example, the cathode active material includes transition metal oxides, polyanionic compounds, Prussian blue (PB) and its analogues (PBAs), and the like.

[0066] In some embodiments, the cathode active material is a transition metal oxide, for example, a sodium-containing composite oxide represented by NaxMCh or NayNfcCU (where M is a transition metal, 0 < x < l, 0 < y < 2), a spinel-like oxide, and a metal chalcogenide of a layered structure or an olivine structure. For example, the cathode active material is selected from sodium cobalt oxide such as NaCoCh, sodium manganese oxide such as NaM CU, sodium nickel oxide such as NaNiCh, sodium titanium oxide such as Na^Tis CU, sodium manganese nickel composite oxide and sodium manganese nickel cobalt composite oxide; a material having an olivine-type crystalline structure such as NaMPCh (M = Fe, Mn, Ni).

[0067] In some embodiments, the cathode active material is optionally a sodium-containing composite oxide of a layered structure or a spinel structure, for example, NaCoCh, NaM CU, NaNiCh, NaNii / 2Mni / 2O2, NaNii / sMni / sCoi / sCh, NaNio.eMno.2Coo.2O2, NaNii-x-y-zCoxAlyMgzO2 (where 0 < x < 1, 0 < y < 0.1, 0 < z < 0.1, 0 < 1-x-y-z < 1). In some embodiments, a part of the constituent elements of the sodium-containing composite oxides may be substituted with an additional element such as Ge, Ti, Zr, Mg, Al, Mo, and Sn.

[0068] In some embodiments, the cathode active material is a polyanionic compound. As an example, the polyanionic compound may be a class of compounds having sodium ions, transition metal ions, and tetrahedral (YCU)11' anionic units. The transition metal may include at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Y may be at least one selected from P, S and Si, and n in a group of (YCU)11' represents a valence of the group. The polyanionic compound may be a class of compounds having sodium ions, transition metal ions, tetrahedral (YCU)11' anion units, and halide anions. The transition metal may include at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Y may be at least one selected from P, S and Si, n in a group of (YCU)11' represents a valence of the group, and halogen may include at least one selected from F, Cl, and Br. The polyanionic compounds may be a class of compounds having sodium ions, tetrahedral (YC )11' anionic units, polyhedral units (ZOy)m+and optionally halide anions. Y may be at least one selected from P, S and Si, n in a group of (YC )11' represents a valence of the group, Z represents a transition metal, which may be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, m in a group of (ZOy)m+represents a valence of the group, and halogen may include at least one selected from F, Cl, and Br. The polyanionic compound is, for example, at least one selected from NaFePC , Na3V2(PO4)3, NaM’PO4F (M’ is at least one selected from V, Fe, Mn and Ni) and Na3(VOy)2(PO4)2F3.2y(0 < y < 1).

[0069] In some embodiments, the cathode active material is a Prussian blue (PB) and its analogues (PBAs). As an example, Prussian blue (PB) and its analogues (PB As) may be a class of compounds having a sodium ion, a transition metal ion, and a cyanide ion (CN‘). The transition metal may include at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Prussian blue (PB) and its analogues (PBAs) are for example NaaMebMe’c(CN)6, where Me and Me’ each independently include at least one selected from Ni, Cu, Fe, Mn, Co and Zn, where 0 <a < 2, 0 <b <1, and 0 <c <1.

[0070] In some embodiments, the positive electrode film layer optionally includes a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene- hexafluoropropylene copolymer, or fluoroacrylate resin.

[0071] In some embodiments, the positive electrode film layer includes the binder of 0.1 to 3.5%, optionally 0.5 to 2.5% by weight.

[0072] In some embodiments, the positive electrode film layer optionally includes a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0073] In some embodiments, the positive electrode film layer includes the conductive agent of 0.05% to 5%, optionally 0.5% to 3% by weight.

[0074] In some embodiments, the positive electrode plate may be prepared by: dispersing the above- mentioned components for preparing the positive electrode plate, such as the cathode active material, the conductive agent, the binder and any other components in a solvent (such as N-methyl pyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and obtaining the positive electrode plate after drying, cold pressing and other processes.

[0075] In the sodium ion battery, the negative electrode plate typically includes a negative current collector and an anode active material layer disposed on the negative current collector. The anode active material layer includes the anode active material.

[0076] The negative electrode plate may include only the negative electrode current collector, i.e. not include the anode active material. The negative electrode plate may include a pre-deposited metal phase on the negative current collector. The negative electrode current collector may be made of a conventional metal foil, a carbon-coated metal foil, or a porous metal plate. As an example, a copper foil or an aluminum foil may be used as the negative electrode current collector.

[0077] Specific type of the anode active material is not limited, and an active material known in the art which can be used for a negative electrode of a sodium ion battery can be used, and those skilled in the art can make a selection according to actual requirements. As an example, the anode active material may include, but is not limited to, one or more of a sodium metal, a carbon material, an alloy material, a transition metal oxide and / or sulfide, a phosphorus-based material, a titanate material. Specifically, the carbon material may include one or more of hard carbon, soft carbon, amorphous carbon, nano- structured carbon materials; the alloy material may include an alloy material formed by one or more of Si, Ge, Sn, Pb and Sb; the transition metal oxides and sulfides have a general formula MxNy, where M includes one or more of Fe, Co, Ni, Mn, Sn, Mo, Sb and V, and N includes O or S; the phosphorus-based material may include one or more of red phosphorus, white phosphorus and black phosphorus; the titanate material may include one or more of ISfeTisO?, ISfeTieOn, Na^isOu, Li^isOu and NaTi2(PC>4)3. These materials are commercially available.

[0078] The anode active material layer further generally optionally includes a conductive agent for improving an electrical conductivity of the anode active material layer, and a binder for binding the anode active material and the conductive agent to the negative electrode current collector. The types of the conductive agent and the binder are not particularly limited, and may be selected according to actual requirements.

[0079] As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. As an example, the binder may include at least one selected from styrene butadiene rubber (SBR), styreneic block copolymers (SBCs), water-based acrylic resin, and carboxymethyl cellulose (CMC).

[0080] The anode active material layer may optionally include a thickener such as carboxymethyl cellulose (CMC). The present disclosure is not limited to the above, and other materials which can be used as the thickener for the negative electrode plate of the sodium-ion battery may be used in the present disclosure.

[0081] In the sodium ion battery, the electrolyte may be the electrolyte according to any one of the embodiments in a first aspect of the present disclosure.

[0082] In some embodiments, the sodium ion battery may further include a separator. The present disclosure is not particularly limited to the above-mentioned separator, and any known separator having a porous structure and electrochemical stability and mechanical stability may be selected according to actual requirements, and may be, for example, a single-layer or multi-layer film including a material selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride.

[0083] In some embodiments, the secondary battery may be a battery module or a battery pack, which may be applied in electronic devices, such as mobile terminals and vehicles.

[0084] In some embodiments, the secondary battery includes an outer package. The outer package is used to encapsulate the electrodes and the electrolyte.

[0085] In some embodiments, the outer package of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell and the like. Alternatively, the outer package of the secondary battery may be a soft package, such as a soft bag. The soft bag may be made of a polymer material such as plastics, polypropylene, polybutylene terephthalate and polybutylene succinate.

[0086] The shape of the secondary battery may be cylindrical, square or any other shape, which is not limited in the present disclosure.

[0087] The secondary battery provided in the second aspect of the preset disclosure has the same advantage as the electrolyte in the first aspect of the preset disclosure, which will not be repeated.

[0088] In a third aspect, embodiments of the present disclosure provide an electronic device. The electronic device includes the secondary battery according to any one of the embodiments in a second aspect of the present disclosure.

[0089] In some embodiments, the electronic device may be electric cars, electric motorcycles, computers, mobile phones, smart watches, and so on.

[0090] Hereinafter, the present disclosure will be further described with Examples, but the present disclosure is not limited to these Examples.

[0091] It is noted that materials and reagents used in the embodiments may be obtained commercially or prepared according to common methods in the related art.

[0092] Examples

[0093] Inventive Example 1 (IE1)

[0094] Preparation of Electrolyte

[0095] 20 parts by mass of sodium hexafluorophosphate, NaPF6(NPF), 15 parts by mass of trifluoromethyl ethyl sulfone (FMES), 15 parts by mass of ethyl methyl sulfone (EMS), 50 parts by mas of 1,2-dimethoxy ethane (DME) were mixed to obtain an electrolyte.

[0096] Preparation of Battery

[0097] NaNio.33Feo.33Mno.33O2 (NFM111) is used to prepare a positive electrode, hard carbon is used to prepare a negative electrode, a PE separator is used as a separator, and the above-obtained electrolyte is used as an electrolyte, these components were assembled in a glove box under an argon atmosphere to form a battery.

[0098] Inventive Example 2 (IE2)

[0099] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 8.7 parts by mass of FMES, 8.7 parts by mass of EMS, 6.35 parts by mas of DME, 13.05 parts by mass of PC, 46.2 parts by mass of EMC, 1 part by mass of fluoroethylene carbonate (FEC), 2 parts by mass of 1,3-propanesulfolactone (PS), and 1 part of 1,3,2-dioxathiolane 2,2-dioxide (DTD) were mixed to obtain an electrolyte.

[0100] Inventive Example 2 (IE3)

[0101] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 10.7 parts by mass of 3 -fluorotetramethyl sulfone (3FTMS), 10.7 parts by mass of tetramethylene sulfone (TMS), 6.35 parts by mas of DME, 13.05 parts by mass of PC, 46.2 parts by mass of EMC were mixed to obtain an electrolyte.

[0102] Inventive Example 4 (IE4)

[0103] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 16 parts by mass of NPF, 15 parts by mass of FMES, 15 parts by mass of EMS, 30 parts by mas of DME, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte. Comparative Example 1 (CE1)

[0104] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 41.5 parts by mass of PC, 41.5 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0105] Comparative Example 2 (CE2)

[0106] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 20.75 parts by mass of DME, 20.75 parts by mass of PC, 41.5 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0107] Comparative Example 3 (CE3)

[0108] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 10.75 parts by mass of 2FTMS, 30.75 parts by mass of PC, 41.5 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0109] Comparative Example 4 (CE4)

[0110] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 10 parts by mass of tetramethylene sulfone (TMS), 10 parts by mass of DME, 25 parts by mass of PC, 38 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0111] Comparative Example 5 (CE5)

[0112] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 10 parts by mass of methyl sulfonyl methane (MSM), 10 parts by mass of DME, 25 parts by mass of PC, 38 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0113] Comparative Example 6 (CE6)

[0114] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 13 parts by mass of NPF, 10 parts by mass of ethyl methyl sulfone (EMS), 10 parts by mass of DME, 25 parts by mass of PC, 38 parts by mass of EMC, 1 part by mass of FEC, 2 parts by mass of PS, and 1 part of DTD were mixed to obtain an electrolyte.

[0115] Comparative Example 7 (CE7)

[0116] A battery was prepared in the same manner as in IE 1 except that, in the preparation of the electrolyte, 20 parts by mass of NPF, and 30 parts by mass of 2-fluorotetram ethyl sulfone (2FTMS) and 50 parts by mas of 1,2-dimethoxy ethane (DME) were mixed to obtain an electrolyte.

[0117] Tests

[0118] Evaluation of Cycling Performance at Room temperature

[0119] Under an environment temperature at 25°C, the batteries of Inventive Examples and Comparative Examples are charged in a 1 C constant current and constant voltage charging manner where a cutoff current is 0.05 C, then the batteries are discharged at a constant current of 1 C. A range of a testing voltage is from 1.5 V to 4.0 V. One charge and one discharge represent one cycle, and 400 cycles are performed at 25°C.

[0120] Capacity retention rate after 400 cycles (%) = (discharge capacity at 400thcycle / discharge capacity at 1stcycle) X 100%

[0121] Evaluation of gas volume

[0122] Gas volumes of the batteries of Inventive Examples and Comparative Examples are measured by a drainage method. A dry weight of the battery is measured as ml. The battery is placed into deionized water at 25 °C, and the weight of the battery is measured as m2. An initial volume of the battery is calculated by VI = (ml - m2) / p water. The battery after 400 cycles is placed into deionized water at 25 °C, and the weight of the battery is measured as m3. A volume of the battery after 400 cycles is calculated by V2 = (ml - m3) / p water. Gas volume is calculated by V3 = V2 - VI = (m2 - m3) / p water, where p water 1 g / cm3.

[0123] Evaluation of storage at 60°C

[0124] The batteries of Inventive Examples and Comparative Examples are fully charged and stored at 60°C for 42 days, and the reversible capacity retention rates of the batteries are measured.

[0125] Reversible capacity after 42 days (%) = (discharge capacity at 42thday / discharge capacity at 1stday) X 100%

[0126] Evaluation of ionic conductivity

[0127] Ionic conductivities of the electrolytes obtained by Inventive Examples and Comparative Examples are measure by Mettler Toledo S700-K at 25 °C.

[0128] Table 1 shows the capacity retention rate and gas volume after 400 cycles, capacity retention rate at 60 °C for 42 days of the batteries, and ionic conductivities of the electrolytes in Inventive Examples and Comparative Examples. Table 1

[0129] According to Table 1, the electrolytes of Inventive Examples of the present disclosure including the sulfone compound and the ether compound exhibit a high ionic conductivity, and the corresponding batteries exhibit a high capacity retention rate and a low gas volume after 400 cycles, and a high capacity retention rate at 60 °C after 42 days.

[0130] It can be seen from above table that IE1 to IE4 have good capacity retention rates after 400 cycles. Without wishing to be bound by any theory, both the non-fluorinated sulfone compound and the fluorinated sulfone compound are used, which may ensure that the electrolyte has a sufficient reducing ability to form a film on the negative electrode of the battery, and avoid sodium precipitation due to the reduction reaction caused by the excessive electron absorption effect of fluorine atoms in the fluorinated sulfone compounds, thus improving the capacity retention rate.

[0131] The electrolyte of CE1 only includes the carbonate solvent. When the electrolyte is used for the sodium ion battery, the gas volume is significantly increased, and the ionic conductivity of the electrolyte is poor. For CE2, the ether compound is added into the electrolyte compared to CE1. The ionic conductivity of the electrolyte is improved, but the gas volume is still high.

[0132] For CE3, the fluorinated sulfone compound is added into the electrolyte compared to CE1. The gas volume of the battery is decreased, but the capacity retention rate of the battery and the ionic conductivity of the electrolyte are still low.

[0133] For CE4 to CE6, the non-fluorinated sulfone compound and the ether compound are added into the electrolyte, and for CE7, the fluorinated sulfone compound and the ether compound are added into the electrolyte. However, such examples exhibit poor performances compared to IE1 to IE4 that uses the non-fluorinated sulfone compound, the fluorinated sulfone compound and the ether compound to obtain the electrolyte.

[0134] Reference throughout this specification to “an embodiment,” “some embodiments,” “one embodiment”, “another example,” “an example,” “a specific example,” or “some examples,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. Thus, the appearances of the phrases such as “in some embodiments,” “in one embodiment”, “in an embodiment”, “in another example,” “in an example,” “in a specific example,” or “in some examples,” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0135] Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed here. This application is intended to cover any variations, uses, or adaptations of the disclosure following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art. It is intended that the specification and examples be considered as illustrative only, with a true scope and spirit of the disclosure being indicated by the following claims.

Claims

CLAIMS1. An electrolyte for a sodium ion battery, comprising: a sodium salt; a sulfone compound, comprising a non-fluorinated sulfone compound and a fluorinated sulfone compound; and an ether compound.

2. The electrolyte according to claim 1, wherein the sulfone compound comprises at least one selected from compounds having a general formula (I):where R1 and R2 are independently selected from a substituted or unsubstituted C1-C5 alkyl group, optionally are linked to form a ring, optionally, at least one of R1 or R2 comprises a fluorine atom.

3. The electrolyte according to claim 1 or 2, wherein the fluorinated sulfone compound comprises at least one selected from trifluoromethyl ethyl sulfone, trifluoroethyl methyl sulfone, trifluoromethyl propyl sulfone, trifluoromethyl isopropyl sulfone, trifluoropropyl methyl sulfone, l,l,2,2-tetrafluoro-3-(methylsulfonyl) propane, 2-fluorotetram ethyl sulfone, and 3- fluorotetramethylsulfone.

4. The electrolyte according to any one of claims 1 to 3, wherein the non-fluorinated sulfone compound comprises at least one selected from tetramethylene sulfone, methyl sulfonyl methane, and ethyl methyl sulfone.

5. The electrolyte according to any one claims 1 to 4, wherein a mass ratio of the fluorinated sulfone compound to the non-fluorinated sulfone compound is ranged from 0.2: 1 to 2: 1, optionally 0.5: 1 to 2: 1.

6. The electrolyte according to any one of claims 1 to 5, wherein the fluorinated sulfone compound has a dielectric constant of greater than 25, optionally greater than 40.

7. The electrolyte according to any one of claims 1 to 6, wherein the fluorinated sulfone compound has a decomposition voltage of greater than 5.2V vs. Na / Na+, optionally greater than 5.5V vs. Na / Na+.

8. The electrolyte according to any one of claims 1 to 7, wherein the ether compound comprises at least one selected from compounds having a general formula (II):3 4(ii) where R3 and R4 are independently selected from a substituted or unsubstituted Ci-Cs alkyl group, optionally are linked to form a ring, optionally at least one of R3 or R4 comprises a fluorine atom.

9. The electrolyte according to any one of claims 1 to 8, wherein the ether compound comprises at least one selected from 1,2-dimethoxy ethane, diglyme, tetrahydrofuran, and 1,3- di oxolane.

10. The electrolyte according to any one of claims 1 to 9, wherein the ether compound has a viscosity no more than 2 mPa- S'1at 25 °C, optionally no more than 1.5 mPa- S'1.

11. The electrolyte according to any one of claims 1 to 10, wherein based on a total weight of the electrolyte, the electrolyte comprises 5wt% to 20wt% of the sodium salt, 5wt% to 30wt% of the sulfone compound and 5wt% to 50wt% of the ether compound.

12. The electrolyte according to any one of claims 1 to 11, further comprising a carbonate solvent; optionally, the electrolyte comprises 5 wt% to 60 wt% of carbonate solvent based on a total weight of the electrolyte.

13. The electrolyte according to any one of claims 1 to 12, further comprising an additive;wherein the additive is at least one selected from vinylene carbonate, fluoroethylene carbonate, 1,3,2-dioxathiolane 2,2-dioxide, 1,3-propanesulfolactone, tri(trimethylsilyl) phosphate, tri(trimethylsilyl) phosphite, tri(trimethylsilyl) borate, 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide methylene disulfonate, 4-vinyl-l,3-dioxolan-2-one, l-propenyl-l,3-propanesulfolactone, succinonitrile, adiponitrile, hexane trinitrile, sodium difluorophosphate, sodium- difluoro(oxalato)borate, disodium monofluorophosphate, and sodium tetrafluoroborate.

14. A secondary battery comprising the electrolyte according to any one of claims 1 to 13.

15. An electronic device comprising the secondary battery according to claim 14.

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