Electrolyte and alkali metal ion secondary battery

Abstract

The invention relates to the technical field of batteries, and particularly discloses an electrolyte and an alkali metal ion secondary battery. The electrolyte comprises an oxygen-containing compound containing 0.1%-5% of selenium. When the electrolyte is applied to the alkali metal ion secondary battery, the oxygen-containing compound of selenium can participate in formation of an SEI film, so that the SEI film contains the oxygen-containing compound of selenium, and the oxygen-containing compound of selenium has relatively high lithium / sodium / potassium ion transmission capability, so that the film-forming impedance can be greatly reduced, and the initial internal resistance of the battery is further reduced. And the SEI film formed in the invention has good stability at a high temperature and is not easy to decompose, so that the side reaction between the negative electrode and the electrolyte is reduced, the impedance and thickness increase of the battery at the high temperature are slowed down, and the thermal safety is improved. In addition, the SEI film formed by the invention has little resistance change at low temperature, and the growth of the new SEI film is slowed down, so that the battery has better low-temperature discharge performance and can be suitable for a cold environment.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to an electrolyte and an alkali metal ion secondary battery. Background Art

[0002] Alkali metal ion secondary batteries, such as lithium / sodium / potassium ion secondary batteries, are widely used in various fields due to their stable energy output and excellent electrochemical performance. Taking lithium ion secondary batteries as an example, a passivation film, namely the solid electrolyte interface film (SEI film), will be formed on the surface of the negative electrode material during the first charge and discharge process of the battery, and the SEI film will continue to grow during the battery cycle. Although the SEI film can block the direct contact between the electrolyte and the negative electrode sheet and prevent the solvent in the electrolyte from having side reactions on the negative electrode surface, which is beneficial to improve the cycle life of the battery, it also increases the electrochemical impedance of the battery, especially in high temperature environments, which will cause the internal resistance of the battery to increase significantly. Summary of the invention

[0003] In view of this, the present application provides an electrolyte for alkali metal ion secondary batteries, which aims to solve the problems of low-temperature discharge capacity attenuation, increased internal resistance and thickness at high temperatures, and poor thermal safety existing in existing batteries to a certain extent.

[0004] According to an embodiment of the present application, an electrolyte is provided for an alkali metal ion secondary battery, wherein the electrolyte comprises an oxygen-containing compound of selenium, and the content of the oxygen-containing compound of selenium is 0.1%-5% based on the mass of the electrolyte.

[0005] In some optional embodiments, the selenium oxygen-containing compound includes at least one of selenium dioxide, selenium trioxide, lithium selenate (Li2SeO4), lithium selenite (Li2SeO3), sodium selenate, sodium selenite, potassium selenate, and potassium selenite.

[0006] In some optional embodiments, based on the mass of the electrolyte, the content of the selenium oxygen-containing compound is 0.2%-2%.

[0007] In some optional embodiments, the electrolyte further includes at least one of fluoroethylene carbonate, a sulfur-containing organic compound, and a nitrile compound.

[0008] In some optional embodiments, the sulfur-containing organic compound includes at least one of a sulfonate compound and a sulfate compound.

[0009] In some optional embodiments, based on the mass of the electrolyte, the content of the fluoroethylene carbonate is 0.1%-30%.

[0010] In some optional embodiments, based on the mass of the electrolyte, the content of the fluoroethylene carbonate is 2%-15%.

[0011] In some optional embodiments, based on the mass of the electrolyte, the content of the sulfur-containing organic compound is 0.1%-5%.

[0012] In some optional embodiments, based on the mass of the electrolyte, the content of the sulfur-containing organic compound is 0.5%-1%.

[0013] In some optional embodiments, based on the mass of the electrolyte, the content of the nitrile compound is 0.1%-10%.

[0014] In some optional embodiments, based on the mass of the electrolyte, the content of the nitrile compound is 0.5%-2%.

[0015] Further, in some optional embodiments, the sulfonate compound includes at least one of 1,3-propane sultone, 1,3-propylene sultone, and methylene disulfonate.

[0016] Further, in some optional embodiments, the sulfate ester compound includes at least one of dimethyl sulfate, vinyl sulfate, propylene sulfate, ethylene sulfite, lithium methyl sulfate, and 1,3-propylene glycol sulfate.

[0017] Further, in some optional embodiments, the nitrile compound includes acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, ethylene glycol dipropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butene dinitrile, trans-hexene dinitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzene tetracarbonitrile, 2,3,5,6-pyrazine tetranitrile, sebacononitrile, azelaic acid dinitrile, dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butene dinitrile, trans-hexene dinitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexane tetranitrile, 1,2,4,5-benzene tetracarbonitrile, 2,3,5,6-pyrazine tetranitrile, At least one of benzene, pyridine-3,4-dicarbonitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyranidenemalononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarbonitrile, 1,3,5-cyclohexanetricarbonitrile, 1,2,3-propanetricarbonitrile, glyceroltricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propanetetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, di(cyanoethoxy)ethane, tris(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

[0018] According to an embodiment of the present application, in a second aspect, an alkali metal ion secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte described in the first aspect.

[0019] In some optional embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector, the negative electrode active layer includes selenium, and the content of the selenium element is not less than 10 ppm based on the mass of the negative electrode active layer.

[0020] In some optional embodiments, the alkali metal ion secondary battery satisfies at least one of the following conditions:

[0021] a. The thickness growth rate after storage at 85°C for 12 hours shall not exceed 6.8% and the internal resistance growth rate shall not exceed 23.7%;

[0022] b. The discharge capacity retention rate at -20°C is not less than 55.4%;

[0023] c. Can pass the hot box test at least 131°C.

[0024] In some optional embodiments, the alkali metal ion secondary battery includes any one of a lithium ion secondary battery, a sodium ion secondary battery, and a potassium ion secondary battery.

[0025] The technical solution of this application has the following advantages:

[0026] The electrolyte provided by the present application includes a certain amount of oxygen-containing compounds of selenium. When the electrolyte is applied to an alkali metal ion secondary battery, since the oxygen-containing compounds of selenium can participate in the formation of an SEI film, the oxygen-containing compounds of selenium are included in the SEI film, and the oxygen-containing compounds of selenium have a high lithium / sodium / potassium ion transmission capacity, so the film-forming impedance can be greatly reduced, thereby reducing the initial internal resistance of the battery. And the SEI film formed by the present application has good stability at high temperatures, is not easy to decompose, reduces the side reactions between the negative electrode and the electrolyte, slows down the impedance and thickness growth of the battery at high temperatures, and improves thermal safety. In addition, the resistance of the SEI film formed by the present application at low temperatures does not change much, and the growth of the new SEI film slows down, so that the battery has good low-temperature discharge performance and can be applied to cold environments.

[0027] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0028] The following examples are provided for a better understanding of the present application, but are not limited to the best implementation mode described, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the protection scope of the present application.

[0029] It should be noted that the stability of the SEI film in existing alkali metal ion secondary batteries is poor. It will decompose at high temperatures and undergo spontaneous exothermic reactions above 90°C, which may cause internal heating of the battery and even thermal runaway and safety accidents. After the SEI film decomposes, the interface side reactions between the electrolyte and the negative electrode will intensify, causing the internal resistance of the battery to increase. At the same time, during the battery charging and discharging process, the SEI film will re-form on the negative electrode surface and continue to grow and decompose in subsequent charge and discharge cycles, thereby increasing the thickness of the SEI film, which in turn causes the internal volume of the battery to expand, and ultimately increases the thickness of the battery. In addition, in a low temperature environment, the resistance of the SEI film will increase, causing the negative electrode potential to shift to a low potential, which makes it easier for lithium ions to be precipitated, resulting in the decomposition of the electrolyte to form a new SEI film. This process consumes active lithium ions in the battery, making the battery's discharge performance worse.

[0030] In order to solve the problems of low-temperature discharge capacity attenuation, increased internal resistance and thickness at high temperature, and poor thermal safety existing in lithium-ion batteries in the related art, according to the first aspect of the present application, an electrolyte is provided for alkali metal ion secondary batteries, the electrolyte comprising oxygen-containing compounds of selenium, and the content of the oxygen-containing compounds of selenium is 0.1%-5% based on the mass of the electrolyte.

[0031] Exemplarily, the content of oxygen-containing compounds of selenium in the electrolyte of the present application can be 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, etc. or within the range formed by any two of the above values. The oxygen-containing compounds of selenium can be, for example, at least one of selenium dioxide, selenium trioxide, lithium selenate (Li2SeO4), lithium selenite (Li2SeO3), sodium selenate, sodium selenite, potassium selenate, and potassium selenite.

[0032] The present application has found that by adding a certain amount of selenium oxygen-containing compounds to the electrolyte of the alkali metal ion secondary battery, it can participate in the formation of the SEI film, so that the SEI film contains selenium oxygen-containing compounds, and the selenium oxygen-containing compounds have a high lithium / sodium / potassium ion transmission capacity, so the film formation impedance can be greatly reduced, thereby reducing the initial internal resistance of the battery. And the SEI film formed by the present application has good stability at high temperatures, is not easy to decompose, reduces the side reactions between the negative electrode and the electrolyte, slows down the impedance and thickness growth of the battery at high temperatures, and improves thermal safety. In addition, the impedance of the SEI film formed by the present application at low temperatures does not change much, and the growth of the new SEI film slows down, so that the battery has good low-temperature discharge performance and can be used in cold environments. If the content of selenium oxygen-containing compounds in the electrolyte is too high, the content of selenium oxygen-containing compounds in the SEI film will be too high. On the contrary, if the content of selenium oxygen-containing compounds in the electrolyte is too low, the content of selenium oxygen-containing compounds in the SEI film will be too low. As a result, it is impossible to achieve the effects of improving the battery's low-temperature discharge performance, thermal safety, and alleviating the increase in battery internal resistance and thickness at high temperatures.

[0033] In some embodiments, the electrolyte further comprises at least one of fluoroethylene carbonate, sulfur-containing organic compounds, and nitrile compounds. Specifically:

[0034] As the main alkali metal ion secondary battery electrolyte additive, fluoroethylene carbonate (FEC) can participate in the formation of a SEI film rich in lithium / sodium / potassium fluoride, thereby ensuring that the SEI film forms a tight structure layer to prevent further decomposition of the electrolyte and improve the low-temperature performance of the electrolyte. Unfortunately, the above SEI film has poor conductivity for alkali metal (lithium / sodium / potassium) ions, resulting in a high initial impedance of the battery.

[0035] On this basis, the present application research found that by further adding FEC to the electrolyte containing selenium oxygen-containing compounds, FEC can jointly participate in the formation of SEI film with selenium oxygen-containing compounds, so that the SEI film contains both selenium oxygen-containing compounds and lithium / sodium / potassium fluoride components, further improving the stability of the SEI film and the ability to conduct alkali metal (lithium / sodium / potassium) ions, thereby improving the low-temperature discharge performance and thermal safety of the battery, and alleviating the increase in internal resistance and thickness of the battery at high temperatures.

[0036] In some embodiments, the content of the fluoroethylene carbonate is 0.1%-30% based on the mass of the electrolyte. Exemplarily, the content of FEC in the electrolyte of the present application can be 0.1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, etc. or within the range of any two of the above values.

[0037] In some embodiments, the electrolyte containing oxygen-containing compounds of selenium in the present application may further include sulfur-containing organic compounds, and the content of the sulfur-containing organic compounds is 0.1%-5% based on the mass of the electrolyte. The present application has found that the sulfur-containing organic compounds can participate in the formation of SEI film together with the oxygen-containing compounds of selenium, so that the SEI film contains both oxygen-containing compounds of selenium and sulfur-containing organic compounds, further improving the stability of the SEI film and reducing the film impedance, thereby improving the low-temperature discharge performance and thermal safety of the battery, and alleviating the increase of the internal resistance and thickness of the battery at high temperature.

[0038] Exemplarily, the content of sulfur-containing organic compounds in the electrolyte of the present application can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, etc. or within the range of any two of the above values. Further, the sulfur-containing organic compound includes at least one of sulfonate compounds and sulfate compounds, and the sulfonate compounds include at least one of 1,3-propane sultone (PS), 1,3-propylene sultone (PST), and methane disulfonic acid methylene ester. The sulfate compounds include at least one of dimethyl sulfate (DMS), vinyl sulfate (DTD), propylene sulfate, ethylene sulfite, lithium methyl sulfate, and 1,3-propylene glycol sulfate (PSa).

[0039] In some embodiments, the electrolyte containing oxygen-containing compounds of selenium in the present application may further include nitrile compounds, and the content of the nitrile compounds is 0.1%-10% based on the mass of the electrolyte. The present application has found that nitrile compounds can stabilize oxygen-containing compounds of selenium, so that oxygen-containing compounds of selenium can be reduced to form films more at the negative electrode instead of decomposing at the positive electrode, thereby enhancing the effect of oxygen-containing compounds of selenium, further improving the stability of SEI film and reducing film impedance, thereby improving the low-temperature discharge performance and thermal safety of the battery, and alleviating the increase in internal resistance and thickness of the battery at high temperature.

[0040] Illustratively, the content of nitrile compounds in the electrolyte of the present application may be 0.1%, 0.5%, 1%, 2%, 5%, 7%, 10%, etc., or within the range formed by any two of the above values. Further, the nitrile compound includes acetonitrile (AN), malononitrile, succinonitrile (SN), glutaronitrile (GN), adiponitrile (ADN), 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, ethylene glycol dipropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butene dinitrile, trans-hexene dinitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, sebacononitrile, azelaic acid dinitrile, dicyanobenzene, pyrazine tetranitrile, At least one of 3,4-pyridine dicarbonitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyrylidenemalononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazine dicarbonitrile, 1,3,5-cyclohexane tricarbonitrile, 1,2,3-propane tricarbonitrile, glycerol tricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propane tetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, di(cyanoethoxy)ethane (DENE), tris(cyanoethoxy)propane (TCP), and tetra(cyanoethoxy)butane.

[0041] It should be noted that in some embodiments of the present application, based on the mass of the electrolyte, an electrolyte that includes 0.2%-2% of oxygen-containing compounds of selenium, 2%-15% of fluoroethylene carbonate, 0.5%-1% of sulfur-containing organic compounds and 0.5%-2% of nitrile compounds can enable alkali metal ion secondary batteries to exhibit better low-temperature discharge performance and thermal safety, as well as smaller battery internal resistance and thickness changes at high temperatures.

[0042] It is understood that the electrolyte for alkali metal ion secondary batteries generally includes a solvent and an electrolyte and an additive dissolved in the solvent. In the present application, the additive includes at least one of the aforementioned oxygen-containing compounds of selenium, fluoroethylene carbonate, sulfur-containing organic compounds, and nitrile compounds. When the content of nitrile compounds in the electrolyte is higher than 5%, it can be considered as a solvent, and its main function is to dissolve the electrolyte, and when the content of nitrile compounds is less than or equal to 5%, it can be considered as an additive, and its main function is to improve battery performance.

[0043] Exemplarily, in a lithium ion secondary battery, the electrolyte includes one or a combination of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonate (LiSbF6), lithium difluorophosphate (LiPF2O2), 4,5-dicyano-2-trifluoromethylimidazolium lithium (LiDTI), lithium bis(oxalate borate) (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium nitrate (LiNO3), LiN(SO2RF)2, LiN(SO2F)(SO2RF), wherein RF=C n F 2n+1 , n is an integer from 1 to 10.

[0044] Exemplarily, the solvent includes ethylene carbonate (EC), propylene carbonate, butylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, fluorodimethyl carbonate, fluoroethyl methyl carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, etc. A mixture of one or more of the following in any proportion: ester, propyl butyrate, butyl butyrate, methyl difluoroacetate, ethyl difluoroacetate, γ-butyrolactone, γ-valerolactone, δ-valerolactone, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, fluoroether F-EPE, fluoroether D2, fluoroether HFPM, fluoroether MFE, fluoroether EME, acetonitrile, malononitrile, glutaronitrile, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, sulfolane, and dimethyl sulfoxide.

[0045] According to a second aspect of the present application, an alkali metal ion secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte described in the first aspect.

[0046] The alkali metal ion secondary battery containing the electrolyte described in the first aspect of the present application exhibits good low-temperature discharge performance, thermal safety, and small internal resistance and thickness changes of the battery at high temperatures.

[0047] It can be understood that the alkali metal ion secondary battery includes any one of a lithium ion secondary battery, a sodium ion secondary battery, and a potassium ion secondary battery. Hereinafter, a lithium ion secondary battery will be taken as an example for illustration.

[0048] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material can be a positive electrode active material known in the art for lithium ion secondary batteries.

[0049] As an example, the positive electrode active material may include at least one of the following materials: lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), lithium iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel-type lithium manganese oxide (LiMn2O4), spinel-type lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1), a lithium-rich material (such as lithium-rich nickel cobalt manganese oxide), manganese dioxide (MnO2), vanadium oxide, sulfur oxide, silicate oxide, and at least one of their respective modified compounds. The modified compound can be doping modification, surface coating modification, or doping and coating modification simultaneously of the positive electrode active material. The above materials can be used alone or in combination.

[0050] In some embodiments, the positive electrode active material layer may further include a binder. As an example, the binder includes but is not limited to at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0051] In some embodiments, the positive electrode active material layer may further include a conductive agent, which includes, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0052] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, which can be a negative electrode active material for lithium ion secondary batteries known in the art.

[0053] As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material includes at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material includes at least one of elemental tin, tin oxide compounds, and tin alloys.

[0054] In some embodiments, the negative electrode active layer may further include selenium, and the content of the selenium is not less than 10 ppm based on the mass of the negative electrode active layer. This indicates that the oxygen-containing compounds of selenium have participated in the formation process of the SEI film on the surface of the negative electrode sheet and constituted the SEI film.

[0055] In some embodiments, the negative electrode active material layer may further include a binder. As an example, the binder includes butadiene styrene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). At least one of the following.

[0056] In some embodiments, the negative electrode active material layer may further include a conductive agent, which includes, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0057] In some embodiments, the negative electrode active material layer may further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0058] In some optional embodiments, the alkali metal ion secondary battery satisfies at least one of the following conditions:

[0059] a. The thickness growth rate after storage at 85°C for 12 hours shall not exceed 6.8% and the internal resistance growth rate shall not exceed 23.7%;

[0060] b. The discharge capacity retention rate at -20°C is not less than 55.4%;

[0061] c. Can pass the hot box test at least 131°C.

[0062] The present application is described in further detail below in conjunction with specific examples, and these examples should not be construed as limiting the scope of protection claimed in the present application. Where specific experimental steps or conditions are not indicated in the examples and comparative examples, the operation or conditions of the conventional experimental steps described in the literature in this area can be carried out. Where the reagents or instruments used do not indicate the manufacturer, they are all conventional reagent products that can be obtained commercially. In all examples and comparative examples of the present application, the unit % represents the mass percentage content.

[0063] Example 1

[0064] This embodiment provides a method for preparing a lithium-ion secondary battery, comprising the following steps:

[0065] Step 1: Preparation of positive electrode

[0066] Lithium cobalt oxide, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97.6:1.4:1, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode slurry; the positive electrode slurry is coated on the front and back surfaces of an aluminum foil, and after baking and rolling, a positive electrode sheet is obtained.

[0067] Step 2: Preparation of negative electrode

[0068] The negative electrode active material is mixed with SP, carboxymethyl cellulose lithium (CMC-Li), and styrene butadiene rubber (SBR) in a mass ratio of 97:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry; wherein the negative electrode active material includes silicon carbon and graphite, the weight of silicon carbon accounts for 10% of the total weight of the negative electrode active material, and the remaining 90% is graphite. The negative electrode slurry is applied to both surfaces of the copper foil, and after baking and rolling, a negative electrode sheet is obtained.

[0069] Step 3: After the positive and negative electrode sheets are cut and sliced, they are wound with the diaphragm to obtain a roll core, wherein the diaphragm adopts a conventional ceramic-coated diaphragm, i.e., 1μm PVDF glue + 6μm PE base film + 1μm alumina ceramic coating + 1μm PVDF glue.

[0070] Step 4: The core is packaged, baked, injected, formed, sealed, sorted and OCV to obtain a lithium-ion battery; wherein, based on the mass of the electrolyte, the composition of the electrolyte is: LiPF614% and additives. The specific substance types and contents of the additives are shown in Table 1, and the rest is solvent (ethylene carbonate / diethyl carbonate=3:7, volume ratio).

[0071] The preparation methods of Examples 2-28 and Comparative Examples 1-3 are basically the same as those of Example 1, and the differences are shown in Table 1. Wherein " / " represents that the substance does not exist.

[0072] Table 1

[0073]

[0074]

[0075]

[0076] Test Case

[0077] 1. High temperature storage performance test

[0078] The 100% SOC lithium-ion secondary battery was placed at 85°C for 12 hours, and then the internal resistance of the battery was directly tested by an internal resistance tester to calculate the internal resistance growth rate. The AC signal frequency was set to 1KHz. At the same time, the thickness of the battery was tested in a hot state and the thickness expansion rate was calculated.

[0079] Note: The internal resistance and thickness growth rates are the growth rates between the 100% SOC battery and the initial 100% SOC battery.

[0080] 2. Hot box test

[0081] 131℃ hot box test

[0082] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature of the incubator is raised to 131°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0083] 132℃ hot box test

[0084] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature of the incubator is raised to 132°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0085] 133℃ hot box test

[0086] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature of the incubator is raised to 133°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0087] 134℃ hot box test

[0088] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature of the incubator is raised to 134°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0089] 135℃ hot box test

[0090] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature of the incubator is raised to 135°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0091] 136℃ hot box test

[0092] The lithium-ion secondary battery is charged at 25°C and charged to the upper limit voltage at 0.5C. The battery is placed in an incubator, and the temperature is raised to 136°C at a rate of 5°C / min, and maintained at this temperature for 60 minutes. The battery is considered to have passed the test if there is no smoke or fire.

[0093] 3. Low temperature discharge performance test

[0094] At 25°C, the lithium-ion secondary battery was charged to the upper limit voltage (4.5V) at 3.0C constant current, then charged to the current of 0.05C at 4.5V constant voltage, left to stand for 5 minutes, discharged to 3.0V at 0.5C, and the discharge capacity Q1 was recorded; then the battery was charged to the upper limit voltage (4.5V) at 3.0C constant current at -20°C, then charged to the current of 0.05C at 4.5V constant voltage, left to stand for 5 minutes, discharged to 3.0V at 0.5C, and the discharge capacity Q2 was recorded. Calculate the 3C discharge capacity retention rate Q2 / Q1.

[0095] 4. Selenium content test

[0096] After the lithium-ion secondary battery is formed and disassembled, the negative electrode sheet is cleaned with DMC, and a small amount of the negative electrode active layer is taken and the selenium content therein is tested by inductively coupled plasma optical emission spectrometry (ICP-OES). The test refers to GB / T 30902-2014.

[0097] With reference to GB / T 30902-2014, the selenium content in the electrolyte was tested by inductively coupled plasma optical emission spectrometry (ICP-OES), and the content of selenium oxygen-containing compounds was obtained by conversion.

[0098] The above test results are shown in Table 2.

[0099] Table 2

[0100]

[0101]

[0102]

[0103] From Tables 1 and 2, it can be seen that adding an appropriate amount of selenium oxygen-containing compound additives to the electrolyte can reduce the impedance growth and thickness growth of the battery during high-temperature storage, improve the battery hot box safety performance (which can be tested by a hot box at a higher temperature), and also improve the low-temperature discharge performance of the battery.

[0104] When the selenium oxygen-containing compound additive is used in combination with fluoroethylene carbonate (FEC), a sulfur-containing organic compound additive, and a nitrile compound additive, better battery performance can be obtained.

[0105] Excluding the influence of experimental errors and test errors, the higher the content of selenium oxygen-containing compound additives in the electrolyte, the higher the selenium content in the battery negative electrode sheet will be, proving that selenium oxygen-containing compound additives are involved in the formation process of the SEI film of the negative electrode sheet.

[0106] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection created by this application.

Claims

1. An electrolyte for an alkali metal ion secondary battery, characterized in that: The electrolyte includes oxygen-containing compounds of selenium. Based on the mass of the electrolyte, the content of the oxygen-containing compounds of selenium is 0.1%-5%.

2. The electrolyte according to claim 1, characterized in that The selenium oxygen-containing compound comprises at least one of selenium dioxide, selenium trioxide, lithium selenate, lithium selenite, sodium selenate, sodium selenite, potassium selenate and potassium selenite; And / or, based on the mass of the electrolyte, the content of the selenium oxygen-containing compound is 0.2%-2%.

3. The electrolyte according to claim 1 or 2, characterized in that The electrolyte further comprises at least one of fluoroethylene carbonate, sulfur-containing organic compounds, and nitrile compounds; Preferably, the sulfur-containing organic compound includes at least one of a sulfonate compound and a sulfate compound.

4. The electrolyte according to claim 3, characterized in that Based on the mass of the electrolyte, the content of the fluoroethylene carbonate is 0.1%-30%; And / or, based on the mass of the electrolyte, the content of the sulfur-containing organic compound is 0.1%-5%; And / or, based on the mass of the electrolyte, the content of the nitrile compound is 0.1%-10%.

5. The electrolyte according to claim 4, characterized in that Based on the mass of the electrolyte, the content of the fluoroethylene carbonate is 2%-15%; And / or, based on the mass of the electrolyte, the content of the sulfur-containing organic compound is 0.5%-1%; And / or, based on the mass of the electrolyte, the content of the nitrile compound is 0.5%-2%.

6. The electrolyte according to claim 3, characterized in that The sulfonate compound includes at least one of 1,3-propane sultone, 1,3-propylene sultone, and methylene disulfonate; And / or, the sulfate ester compound includes at least one of dimethyl sulfate, vinyl sulfate, propylene sulfate, ethylene sulfite, lithium methyl sulfate, and 1,3-propylene glycol cyclic sulfate; and / or, the nitrile compound includes acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetricarboxylic acid nitrile, 1,3,5-pentanetricarboxylic acid nitrile, ethylene glycol dipropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxycyclotriphosphazene, pentafluorophenoxycyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butene dinitrile, trans-hexene dinitrile, 3-(trimethylsilyloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetranitrile, 2,3,5,6-pyrazinetetranitrile, sebacononitrile, azelaic acid nitrile, dicyanobenzene, pyridine-3, At least one of 4-dinitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoroterephthalonitrile, 4-tetrahydrothiopyranidemalononitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarbonitrile, 1,3,5-cyclohexanetricarbonitrile, 1,2,3-propanetricarbonitrile, glyceroltricarbonitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propanetetracarbonitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonitrile, 7,7,8,8-tetracyanoquinodimethane, tetracyanoethylene, di(cyanoethoxy)ethane, tris(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

7. An alkali metal ion secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 6.

8. The alkali metal ion secondary battery according to claim 7, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector, the negative electrode active layer includes selenium, and the content of the selenium element is not less than 10 ppm based on the mass of the negative electrode active layer.

9. The alkali metal ion secondary battery according to claim 7, characterized in that: The alkali metal ion secondary battery satisfies at least one of the following conditions: a. The thickness growth rate after storage at 85°C for 12 hours shall not exceed 6.8% and the internal resistance growth rate shall not exceed 23.7%; b. The discharge capacity retention rate at -20°C is not less than 55.4%; c. Can pass the hot box test at least 131°C.

10. The alkali metal ion secondary battery according to any one of claims 7 to 9, characterized in that: The alkali metal ion secondary battery includes any one of a lithium ion secondary battery, a sodium ion secondary battery, and a potassium ion secondary battery.

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