An electrolyte and an alkali metal ion secondary battery

Abstract

This application relates to the field of battery technology, specifically disclosing an electrolyte and an alkali metal ion secondary battery. The electrolyte includes 0.1%-5% selenium-containing oxygen compounds. When this electrolyte is applied to an alkali metal ion secondary battery, the selenium-containing oxygen compounds can participate in the formation of an SEI film, resulting in the SEI film containing selenium-containing oxygen compounds. Since selenium-containing oxygen compounds have high lithium / sodium / potassium ion transport capabilities, this significantly reduces film-forming resistance, thereby reducing the initial internal resistance of the battery. Furthermore, the SEI film formed in this application exhibits good stability at high temperatures and is not prone to decomposition, reducing side reactions between the negative electrode and the electrolyte. This slows down the increase in impedance and thickness of the battery at high temperatures, improving thermal safety. Additionally, the resistance change of the SEI film formed in this application is minimal at low temperatures, and the growth of the new SEI film is slower, thus giving the battery good low-temperature discharge performance, making it suitable for cold environments.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to an electrolyte and an alkali metal ion secondary battery. Background Technology

[0002] Alkali metal ion secondary batteries, such as lithium / sodium / potassium ion batteries, are widely used in various fields due to their stable energy output and excellent electrochemical performance. Taking lithium-ion batteries as an example, a passivation film, namely the solid electrolyte interface (SEI) film, forms on the surface of the negative electrode material during the first charge and discharge process, and the SEI film continues to grow during battery cycling. Although the SEI film can prevent direct contact between the electrolyte and the negative electrode, preventing side reactions of the solvent in the electrolyte on the negative electrode surface and thus improving the cycle life of the battery, it also increases the electrochemical impedance of the battery, especially at high temperatures, which can cause a significant increase in the battery's internal resistance. Summary of the Invention

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

[0004] According to an embodiment of this application, an electrolyte is provided for use in an alkali metal ion secondary battery. The electrolyte includes 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 alternative embodiments, the selenium-containing 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 alternative embodiments, the content of selenium oxygenated compounds is 0.2%-2% based on the mass of the electrolyte.

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

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

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

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

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

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

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

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

[0015] Furthermore, in some optional embodiments, the sulfonate compound includes at least one of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, and methanedisulfonate.

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

[0017] Further, in some optional embodiments, the nitrile compounds include acetonitrile, malononitrile, butadionitrile, 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-difluorobutadionitrile, tricyanobenzene, acrylonitrile, crotonitrile, trans-butenedionitrile, trans-hexenedionitrile, 3-(trimethylsiloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, decanonitrile, nonadionitrile, dicyano At least one of the following: benzene, pyridine-3,4-dianitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorobutadiene, tetrafluoroterephthalonitrile, 4-tetrahydrothiamethylenemalonium, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarboxynitrile, 1,3,5-cyclohexanetrionitrile, 1,2,3-propanetricarboxynitrile, glycerol trionitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propanetetracarbonylnitrile, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonylnitrile, 7,7,8,8-tetracyano-terebenzoquinone dimethyl ether, tetracyanoethylene, di(cyanoethoxy)ethane, tri(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

[0018] According to an embodiment of this application, in a second aspect, an alkali metal ion secondary battery is provided, comprising a positive electrode, a negative electrode, 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 including selenium element, and the content of selenium element is not less than 10 ppm based on the mass of the negative electrode active layer.

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

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

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

[0023] c. Can be tested in a hot chamber at at least 131°C.

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

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

[0026] The electrolyte provided in this application includes a certain amount of selenium-containing oxygen compounds. When this electrolyte is applied to an alkali metal ion secondary battery, the selenium-containing oxygen compounds can participate in the formation of the SEI film, resulting in the SEI film containing selenium-containing oxygen compounds. Since selenium-containing oxygen compounds have high lithium / sodium / potassium ion transport capabilities, this significantly reduces film-forming impedance, thereby lowering the initial internal resistance of the battery. Furthermore, the SEI film formed in this application exhibits good stability at high temperatures and is not prone to decomposition, reducing side reactions between the negative electrode and the electrolyte. This slows down the impedance and thickness growth of the battery at high temperatures, improving thermal safety. In addition, the resistance change of the SEI film formed in this application is minimal at low temperatures, and the growth of the new SEI film is slower, thus giving the battery better low-temperature discharge performance, making it suitable for cold environments.

[0027] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Detailed Implementation

[0028] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0029] It should be noted that the SEI film in existing alkali metal ion secondary batteries has poor stability and decomposes at high temperatures, undergoing a spontaneous exothermic reaction above 90°C. This can lead to internal heating within the battery, and even thermal runaway and safety accidents. After the SEI film decomposes, the interfacial side reactions between the electrolyte and the negative electrode intensify, increasing the battery's internal resistance. Simultaneously, during battery charging and discharging, the SEI film reforms on the negative electrode surface and continues to grow and decompose in subsequent charge-discharge cycles, resulting in increased SEI film thickness and subsequent internal volume expansion, ultimately increasing the battery's overall thickness. Furthermore, at low temperatures, the resistance of the SEI film increases, causing the negative electrode potential to shift to lower levels. This makes it easier for lithium ions to be deposited, leading to electrolyte decomposition and the formation of a new SEI film. This process consumes active lithium ions in the battery, resulting in poorer discharge performance.

[0030] To address the problems of low-temperature discharge capacity decay, increased internal resistance and thickness at high temperatures, and poor thermal safety in lithium-ion batteries in related technologies, according to a first aspect of this application, an electrolyte is provided for use in alkali metal ion secondary batteries. The electrolyte includes selenium-containing oxygen compounds, and the content of the selenium-containing oxygen compounds is 0.1%-5% based on the mass of the electrolyte.

[0031] For example, the content of selenium oxygen-containing compounds in the electrolyte of this application may be 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, etc., or within any range of two of the above values. The selenium oxygen-containing compounds may 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] This application research found that by adding a certain amount of selenium-containing oxygenated compounds to the electrolyte of an alkali metal ion secondary battery, an SEI film can be formed, resulting in the SEI film containing selenium-containing oxygenated compounds. These selenium-containing oxygenated compounds have high lithium / sodium / potassium ion transport capabilities, thus significantly reducing film-forming impedance and consequently lowering the battery's initial internal resistance. Furthermore, the SEI film formed in this application exhibits good stability at high temperatures and is not prone to decomposition, reducing side reactions between the negative electrode and the electrolyte. This slows down the impedance and thickness growth of the battery at high temperatures, improving thermal safety. In addition, the SEI film formed in this application shows little impedance change at low temperatures, and the growth of the new SEI film is slower, thus giving the battery better low-temperature discharge performance, making it suitable for cold environments. If the content of selenium oxygenates in the electrolyte is too high, it will lead to an excessively high content of selenium oxygenates in the SEI film. Conversely, if the content of selenium oxygenates in the electrolyte is too low, it will result in an excessively low content of selenium oxygenates in the SEI film. Neither of these methods can achieve the desired effect of improving the battery's low-temperature discharge performance, thermal safety, or mitigating the increase in battery internal resistance and thickness at high temperatures.

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

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

[0035] Based on this, this application research found that by further adding FEC to an electrolyte containing selenium-containing oxygenated compounds, FEC can participate in the formation of an SEI film together with the selenium-containing oxygenated compounds. This allows the SEI film to simultaneously contain selenium-containing oxygenated compounds and lithium / sodium / potassium fluoride components, further improving the stability of the SEI film and its ability to conduct alkali metal (lithium / sodium / potassium) ions. This improves the low-temperature discharge performance and thermal safety of the battery, and alleviates the increase in battery internal resistance and thickness at high temperatures.

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

[0037] In some embodiments, the electrolyte containing selenium-containing oxygenated compounds in this application may further include sulfur-containing organic compounds, wherein the content of the sulfur-containing organic compounds is 0.1%-5% based on the mass of the electrolyte. This application has found that sulfur-containing organic compounds can co-form the SEI film with selenium-containing oxygenated compounds, resulting in the SEI film simultaneously containing both selenium-containing oxygenated compounds and sulfur-containing organic compounds. This further improves the stability of the SEI film and reduces its impedance, thereby improving the low-temperature discharge performance and thermal safety of the battery, and alleviating the increase in battery internal resistance and thickness at high temperatures.

[0038] For example, the content of sulfur-containing organic compounds in the electrolyte of this application may be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or within any two of the above values. Further, the sulfur-containing organic compounds include at least one of sulfonate compounds and sulfate compounds. The sulfonate compounds include at least one of 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), and methanedisulfonate. The sulfate compounds include at least one of dimethyl sulfate (DMS), vinyl sulfate (DTD), propylene sulfate, ethylene sulfite, lithium methyl sulfate, and 1,3-propanediol sulfate (PSa).

[0039] In some embodiments, the electrolyte containing selenium-containing oxygenated compounds in this application may further include nitrile compounds, wherein the content of the nitrile compounds is 0.1%-10% based on the mass of the electrolyte. This application has found that nitrile compounds can stabilize selenium-containing oxygenated compounds, allowing them to be reduced to a film at the negative electrode rather than decomposed at the positive electrode. This enhances the effectiveness of the selenium-containing oxygenated compounds, further improving the stability of the SEI film and reducing film impedance, thereby improving the low-temperature discharge performance and thermal safety of the battery, and mitigating the increase in battery internal resistance and thickness at high temperatures.

[0040] For example, the content of nitrile compounds in the electrolyte of this application may be 0.1%, 0.5%, 1%, 2%, 5%, 7%, 10%, etc., or within any two of the above values. Further, the nitrile compounds include acetonitrile (AN), malononitrile, butadionitrile (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-difluorobutadionitrile, tricyanobenzene, acrylonitrile, crotonitrile, trans-butenedionitrile, trans-hexenedionitrile, 3-(trimethylsiloxy)propionitrile, 1,3,4,6-hexanetetranitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetranitrile, decanonitrile, nonadionitrile, dicyanobenzene, pyrazine, etc. At least one of the following: pyridine-3,4-dionitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorobutadione, tetrafluoroterephthalonitrile, 4-tetrahydrothiamethylenemalonium, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarboxylon, 1,3,5-cyclohexanetrionitrile, 1,2,3-propanetricarboxylon, glycerol trionitrile, tris(3-cyanopropyl)phosphate, 1,1,3,3-propanetetracarbonyl, 1,2,2,3-tetracyanopropane, 3-methyl-3-propyl-cyclopropane-1,1,2,2-tetracarbonyl, 7,7,8,8-tetracyano-terebenzoquinone dimethyl ether, tetracyanoethylene, di(cyanoethoxy)ethane (DENE), tri(cyanoethoxy)propane (TCP), and tetra(cyanoethoxy)butane.

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

[0042] It is understood that electrolytes used in alkali metal ion secondary batteries typically include a solvent, an electrolyte dissolved in the solvent, and additives. In this application, the additives include at least one of the aforementioned selenium-containing oxygen compounds, 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, whose main function is to dissolve the electrolyte; while when the content of nitrile compounds is lower than or equal to 5%, it can be considered as an additive, whose main function is to improve battery performance.

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

[0044] For example, the solvent includes ethylene carbonate (EC), propylene carbonate, butene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, dimethyl fluorocarbonate, methyl ethyl fluorocarbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl ethyl 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, and ethyl butyrate. A mixture of one or more of the following: esters, 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-dioxopentane, 1,4-dioxane, sulfolane, and dimethyl sulfoxide, in any proportion.

[0045] According to a second aspect of this application, an alkali metal ion secondary battery is provided, comprising a positive electrode, a negative electrode, 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 and thermal safety, as well as a small internal resistance and thickness change of the battery at high temperature.

[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 adopt the positive electrode active materials 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 lithium manganese oxide (LiMn2O4), spinel 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 simultaneous doping and coating modification 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), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0051] In some embodiments, the positive electrode active material layer may further include a conductive agent. As an example, the conductive agent 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 surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, which may be a negative electrode active material known in the art for use in lithium-ion secondary batteries.

[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, and 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 selenium content is not less than 10 ppm based on the mass of the negative electrode active layer. This indicates that selenium-containing oxygen compounds have participated in the SEI film formation process on the surface of the negative electrode and constitute the SEI film.

[0055] In some embodiments, the negative electrode active material layer may further include a binder. As an example, the binder includes, but is not limited to, 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).

[0056] In some embodiments, the negative electrode active material layer may further include a conductive agent. As an example, the conductive agent 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 also include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

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

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

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

[0061] c. Can be tested in a hot chamber at at least 131°C.

[0062] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Where the manufacturers of reagents or instruments are not specified, they are all commercially available conventional reagent products. In all embodiments and comparative examples of this application, the unit % represents mass percentage content.

[0063] Example 1

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

[0065] Step 1: Preparation of positive electrode sheet

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

[0067] Step 2: Negative electrode preparation

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

[0069] Step 3: After the positive and negative electrode sheets are slit and formed, they are wound with the separator to obtain the core. The separator is a conventional ceramic-coated separator, namely 1μm PVDF adhesive + 6μm PE base film + 1μm alumina ceramic coating + 1μm PVDF adhesive.

[0070] Step 4: The core is packaged, baked, injected with electrolyte, formed, resealed, sorted and OCV is applied to obtain a lithium-ion battery; wherein, based on the mass of the electrolyte, the composition of the electrolyte is: 14% LiPF6 and additives, the specific types and contents of the additives are shown in Table 1, and the remainder 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, with the differences shown in Table 1. " / " indicates that the substance is not present.

[0072] Table 1

[0073]

[0074]

[0075]

[0076] Test case

[0077] 1. High-temperature storage performance test

[0078] A 100% SOC lithium-ion secondary battery was placed at 85°C for 12 hours, and then its internal resistance was directly measured using an internal resistance tester to calculate the internal resistance growth rate. The AC signal frequency was set to 1 kHz. Simultaneously, the battery thickness was measured under thermal conditions, and the thickness expansion rate was calculated.

[0079] Note: The growth rates of internal resistance and thickness are the growth rates between a 100% SOC battery and an 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 at a rate of 0.5C to the upper limit voltage. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0083] 132℃ Hot Box Test

[0084] The lithium-ion secondary battery is charged at 25°C to the upper limit voltage at a rate of 0.5C. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0085] 133℃ Hot Box Test

[0086] The lithium-ion secondary battery is charged at 25°C at a rate of 0.5C to the upper limit voltage. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0087] 134℃ Hot Box Test

[0088] The lithium-ion secondary battery is charged at 25°C to the upper limit voltage at a rate of 0.5C. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0089] 135℃ Hot Box Test

[0090] The lithium-ion secondary battery is charged at 25°C at a rate of 0.5C to the upper limit voltage. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0091] 136℃ Hot Box Test

[0092] The lithium-ion secondary battery is charged at 25°C at a rate of 0.5C to the upper limit voltage. The battery is then placed in a temperature chamber, which is heated 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 it does not smoke or catch fire.

[0093] 3. Low-temperature discharge performance test

[0094] At 25℃, a lithium-ion secondary battery is charged at a constant current of 3.0C to the upper limit voltage (4.5V), then charged at a constant voltage of 4.5V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at 0.5C to the 3.0V cutoff point. The discharge capacity Q1 is recorded. Then, at -20℃, the battery is charged at a constant current of 3.0C to the upper limit voltage (4.5V), then charged at a constant voltage of 4.5V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at 0.5C to the 3.0V cutoff point. The discharge capacity Q2 is 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, a small amount of the negative electrode active layer is taken, and the selenium content is tested by inductively coupled plasma optical emission spectrometry (ICP-OES) in accordance with GB / T 30902-2014.

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

[0098] Please see Table 2 for the test results.

[0099] Table 2

[0100]

[0101]

[0102]

[0103] As can be seen from Tables 1 and 2, adding an appropriate amount of selenium-containing oxygenated compound additives to the electrolyte can reduce the impedance growth and thickness growth during the high-temperature storage process of the battery, improve the battery's thermal box safety performance (which can be tested in a thermal box at higher temperatures), and also improve the battery's low-temperature discharge performance.

[0104] When selenium-containing oxygenated compound additives are used in combination with fluoroethylene carbonate (FEC), sulfur-containing organic compound additives, and nitrile compound additives, better battery performance can be obtained.

[0105] Excluding the influence of experimental and testing errors, the higher the content of selenium oxygenated compound additives in the electrolyte, the higher the selenium content in the negative electrode of the battery, proving that selenium oxygenated compound additives participate in the formation process of the SEI film of the negative electrode.

[0106] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. An electrolyte for an alkali metal ion secondary battery, characterized by comprising: 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; The oxygen-containing compound of selenium comprises at least one of selenium dioxide, selenium trioxide, lithium selenate, lithium selenite, sodium selenate, sodium selenite, potassium selenate, and potassium selenite; The electrolyte further comprises fluoroethylene carbonate, a sulfur-containing organic compound, and a nitrile compound; The content of the fluoroethylene carbonate is 0.1%-30% based on the mass of the electrolyte; The content of the sulfur-containing organic compound is 0.1%-4% based on the mass of the electrolyte; The content of the nitrile compound is 0.1%-7% based on the mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that, The content of the oxygen-containing compound of selenium is 0.2%-2% based on the mass of the electrolyte.

3. The electrolyte according to claim 1 or 2, characterized in that, The sulfur-containing organic compound comprises at least one of a sulfonate compound and a sulfate compound.

4. The electrolyte of claim 1, wherein The content of the fluoroethylene carbonate is 2%-15% based on the mass of the electrolyte; The content of the sulfur-containing organic compound is 0.5%-1% based on the mass of the electrolyte; The content of the nitrile compound is 0.5%-2% based on the mass of the electrolyte.

5. The electrolyte of claim 3, wherein The sulfonate compound comprises at least one of 1,3-propane sultone, 1,3-propene sultone, and methylene methane disulfonate. The sulfate compound comprises at least one of dimethyl sulfate, ethylene sulfate, propylene sulfate, ethylene sulfite, lithium methyl sulfate, and 1,3-propanediol cyclic sulfate.

6. The electrolyte of claim 1, wherein The nitrile compound comprises at least one of acetonitrile, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, ethylene glycol bispropionitrile ether, hexafluorocyclotriphosphazene, pentafluoroethoxy cyclotriphosphazene, pentafluorophenoxy cyclotriphosphazene, 1,4-dicyano-2-butene, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoromalonic nitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, crotononitrile, trans-butenedinitrile, trans-hexenedinitrile, 3-(trimethylsiloxy)propionitrile, 1,3,4,6-hexanetetracarbonitrile, 1,2,4,5-benzenetetracarbonitrile, 2,3,5,6-pyrazinetetracarbonitrile, decanedinitrile, nonanedinitrile, dicyanobenzene, pyridine-3,4-dicarbonitrile, 2,5-dicyanopyridine, 2,2,3,3-tetrafluorosuccinonitrile, tetrafluoro-p-xylylene dinitrile, 4-tetrahydrothiopyran methylene succinonitrile, 1,4,5,6-tetrahydro-5,6-dioxo-2,3-pyrazinedicarbonitrile, 1,3,5-cyclohexanetricarbonitrile, 1,2,3-propanetricarbonitrile, glycerol tricarbonitrile, 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-tetracyano-p-quinodimethane, tetracyanoethylene, bis(cyanoethoxy)ethane, tris(cyanoethoxy)propane, and tetra(cyanoethoxy)butane.

7. An alkali metal-ion secondary battery characterized by comprising: The positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte solution according to any one of claims 1 to 6.

8. The alkali-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, and the negative electrode active layer includes selenium, and the content of the selenium is not less than 10 ppm based on the mass of the negative electrode active layer.

9. The alkali-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 is not more than 6.8% and the internal resistance growth rate is not more than 23.7% after storage at 85°C for 12h; b. The discharge capacity retention rate at -20°C is not less than 55.4%; c. The battery can pass a hot box test of at least 131°C.

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

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