Secondary battery

Abstract

The present application provides a secondary battery. The secondary battery comprises a positive pole piece, a negative pole piece, a diaphragm and an electrolyte, the positive pole piece comprises a positive active material, the positive active material comprises LiMnaFe1-aPO4, a is greater than 0 and less than 1, the electrolyte comprises an additive A and an additive B, the additive A is selected from a compound shown in a formula I, and the additive B is selected from a compound shown in a formula II; and the additive B is selected from at least one of vinylene carbonate, fluoroethylene carbonate and vinylethylene carbonate. The additive A and the additive B are combined and applied to a lithium manganese iron phosphate system, so that a secondary battery has relatively low initial impedance and excellent normal-temperature cycle performance, high-temperature storage performance and high-temperature cycle performance, and transition metal ion deposition in a positive electrode material can be obviously reduced. # imgabs0 #

Description

Technical Field

[0001] The present application relates to the field of electrochemistry technology, and particularly to a secondary battery. Background Art

[0002] With the application and popularization of power batteries in electric vehicles, the energy density of batteries has received increasing attention and challenges. Compared with lithium iron phosphate, lithium manganese iron phosphate (LMFP) has a higher plateau voltage, so it is a more ideal cathode material for high-energy-density power batteries. However, during the charge and discharge process, manganese in LMFP has a relatively serious Jahn-Teller effect and there is a problem of manganese dissolution, resulting in poor cycle performance, and this situation will become more serious under high-temperature and high-pressure conditions. In the prior art, in order to meet the requirement of energy density, LMFP often requires a relatively high proportion of manganese. However, a high proportion of manganese will lead to a decrease in the conductivity of LMFP. Therefore, methods such as reducing the particle size, surface carbon coating, and doping with other metal elements are often used to improve the conductivity. However, these methods for improving conductivity will reduce the energy density of LMFP. At the same time, the materials prepared by the above methods are prone to problems such as slurry gelation, film cracking, and powder falling during the homogenization and coating processes, which limit the further development of LMFP.

[0003] The electrolyte is a component that plays a key role in the performance of secondary batteries. At the same time, electrolyte additives are the most economical and effective method to improve battery performance. They have the advantages of low dosage and strong pertinence, and have little impact on the manufacturing cost and production process of batteries. Therefore, developing an electrolyte that matches the LMFP cathode material has very important research significance and commercial value in the field of secondary batteries. Summary of the Invention

[0004] The purpose of the present application is to provide a secondary battery, which has a lower initial impedance, excellent normal-temperature cycle performance, high-temperature storage performance, and high-temperature cycle performance, and can significantly reduce the deposition of transition metal ions in the cathode material.

[0005] The present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes LiMn a Fe 1-a PO 4 , 0 < a < 1, the electrolyte includes additive A and additive B, and additive A is selected from at least one of the compounds shown in formula I;

[0006] Wherein, R 1 , R 2 , R 3 each independently selected from a hydrogen atom, a fluorine atom, an unsubstituted or fluorine-substituted C1 -C 4 alkyl, C 2 -C 4 alkenyl, C 1 -C 4 alkoxy, C 6 -C 12 at least one of aryl, amino, cyano and acyl; x, y, and z are each independently 0, 1, or 2;

[0007] The additive B is selected from at least one of vinylene carbonate, fluoroethylene carbonate, and ethylene vinylene carbonate;

[0008] Based on the mass of the electrolyte, the mass percentage of the additive A is w 1 , w 1 is 0.1% to 3%; the mass percentage of the additive B is w 2 , w 2 is 0.1% to 5%.

[0009] In one embodiment of the present application, w 1 is 0.3% to 1%, and w 2 is 0.5% to 2%.

[0010] In one embodiment of the present application, the mass ratio of the additive A to the additive B, w 1 / w 2 is 0.03 to 6.67.

[0011] In one embodiment of the present application, w 1 / w 2 is 0.15 to 2.

[0012] In one embodiment of the present application, the compound represented by Formula I is selected from at least one of the following compounds: .

[0013] In one embodiment of the present application, the electrolyte further includes an electrolyte, and the electrolyte is selected from LiPF 6 , LiAsF 6 , LiClO 4 , LiSO 3 CF3 , LiBF 4 , LiB(C 2 O 4 ), 2 , LiBF 2 C 2 O 4 , LiN(SO 2 F) 2 , LiN(SO 2 CF 3 ), 2 , LiPO 2 F 2 , LiPF 2 (C 2 O 4 ), 2 and LiPF 4 C 2 O 4 ; Based on the mass of the electrolyte, the mass percentage of the electrolyte is 6% - 20%.

[0014] In an embodiment of the present application, the electrolyte further includes a solvent, and the solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ - butyrolactone, sulfolane, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, propyl acetate, methyl propionate, 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, and 2,2 - difluoroethyl acetate; based on the mass of the electrolyte, the mass percentage of the solvent is 72% - 93%.

[0015] In an embodiment of the present application, the positive electrode active material further includes at least one of LiNi b Co c Mn 1-b-c O 2 and LiFePO 4 , where 0 < b < 1 and 0 < c < 1.

[0016] In an embodiment of the present application, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material is selected from at least one of carbon - based materials, silicon - based materials, lithium metal, and lithium titanate.

[0017] Advantages of the present application:

[0018] The present application provides a secondary battery. The secondary battery of the present application includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes LiMn a Fe 1-a PO 4, where 0 < a < 1, the electrolyte includes additive A and additive B. The compound shown by additive A contains a benzene ring, and the large π bond on the benzene ring will affect the overall activity, reduce its energy barrier, make the polymerization reaction of the cyano group easier, and thus can form a film on the negative electrode; also, since additive A has more than three cyano groups, they can form a cross-linked structure, which can well inhibit the reduction decomposition of the solvent and electrolyte, making the secondary battery have a lower impedance at the initial stage of cycling. At the same time, the cross-linked structure also significantly reduces the uneven charge distribution, reduces the deposition of Mn on the negative electrode. Since a high-density Mn deposition cannot be formed, the toxicity to the secondary battery is greatly reduced, and the situation of the negative electrode peeling and sticking to the separator is significantly improved. When additive A and additive B are used in combination, additive B will also participate in the composition of the cross-linked structure to form a solid electrolyte interface film (Solid Electrolyte Interface, SEI film) with high mechanical strength, strong ionic conductivity, and high uniformity. And additive B will continuously repair the damaged SEI film during long-term cycling. The combination of the two additives is applied to the LMFP system, and the mass percentage contents of additive A and additive B are limited within the scope of this application, which can make the secondary battery have a lower initial impedance, excellent normal-temperature cycling performance, high-temperature storage performance, and high-temperature cycling performance, and can significantly reduce the deposition of transition metal ions in the positive electrode material.

[0019] Of course, it is not necessary for any product or method implementing this application to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of this application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following-described drawings are only some embodiments of this application. For those of ordinary skill in the art, other embodiments can also be obtained based on these drawings.

[0021] Figure 1 Differential capacity curves of the lithium-ion batteries prepared for Example 1-1, Comparative Example 1-3, and Comparative Example 1-14. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following will clearly and completely describe the technical solutions in this application. Obviously, the described embodiments are only some embodiments of this application, rather than all embodiments. All other embodiments obtained by those skilled in the art based on this application belong to the scope of protection of this application.

[0023] It should be noted that in the specific embodiments of this application, a lithium-ion battery is taken as an example of the secondary battery to explain this application, but the secondary battery of this application is not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0024] The present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes lithium manganese iron phosphate LiMn a Fe 1-a PO 4 , where 0 < a < 1, the electrolyte includes additive A and additive B, and additive A is selected from at least one of the compounds shown in Formula I;

[0025] wherein, R 1 , R 2 , R 3 are each independently selected from a hydrogen atom, a fluorine atom, an unsubstituted or fluorine-substituted C 1 -C 4 alkyl group, a C 2 -C 4 alkenyl group, a C 1 -C 4 alkoxy group, a C 6 -C 12 aryl group, an amino group, a cyano group, and an acyl group; x, y, and z are each independently 0, 1, or 2;

[0026] Additive B is selected from at least one of vinylene carbonate, fluoroethylene carbonate, and ethylene vinylene carbonate;

[0027] Based on the mass of the electrolyte, the mass percentage content of additive A is w 1 , w 1 is 0.1% to 3%, preferably 0.3% to 1%; the mass percentage content of additive B is w 2 , w 2 is 0.1% to 5%, preferably 0.5% to 2%. For example, based on the mass of the electrolyte, the mass percentage content w 1 of additive A can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a range composed of any two of these values; the mass percentage content w 2 of additive B can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or a range composed of any two of these values.

[0028] In the LMFP system, Mn ions are continuously dissolved from the positive electrode due to disproportionation reactions. The dissolved Mn ions, due to their higher valence states, are more likely to be concentrated and deposited in areas with high charge density on the negative electrode. The deposited Mn will damage the SEI film and promote the decomposition of the electrolyte, and also lead to further uneven charge distribution. This, in turn, will cause the continuously dissolved Mn ions to be continuously deposited in the same place, forming a vicious cycle. The aggregation of a large number of Mn ions will also cause some Mn ions to be co-inserted into the negative electrode after solvation, resulting in local stress expansion and structural collapse of the negative electrode, peeling of the negative electrode and adhesion to the separator, causing capacity attenuation.

[0029] It has been found in the research that conventional nitrile additives (such as 1,3,6 - hexanetricarbonitrile) are generally considered not easily oxidized and have strong coordination ability, which can complex the dissolved transition metal ions on the positive electrode, thereby improving the performance of secondary batteries. However, the cyanide group has a strong complexation with Co and Ni and a weak complexation with Mn. Therefore, its effect on the dissolution of Mn in the LMFP system is small, and the improvement of battery performance is limited. Moreover, conventional nitrile substances are also difficult to form a film by reduction. The compound of additive A in this application contains a benzene ring, and the large π bond on the benzene ring will affect the overall activity, reducing its energy barrier and making the polymerization reaction of the cyanide group easier, so that it can form a film on the negative electrode. Also, since additive A has more than three cyanide groups, they can form a cross-linked structure, which can well inhibit the reduction decomposition of the solvent and electrolyte, making the secondary battery have a lower impedance in the initial stage of cycling. At the same time, the cross-linked structure also significantly reduces the uneven charge distribution. The dissolved Mn ions are difficult to deposit on the negative electrode due to the uniform charge density, and the total amount of Mn deposited on the negative electrode is reduced. Since a high-density Mn deposit cannot be formed, the toxicity to the secondary battery is greatly reduced, and the situation of the negative electrode peeling and adhering to the separator is significantly improved. When additive A and additive B are used in combination, additive B will also participate in the formation of the cross-linked structure, forming an SEI film with high mechanical strength, high ionic conductivity and high uniformity. And additive B will continuously repair the damaged SEI film during long-term cycling. The two additives are used in combination in the LMFP system, and the mass percentage contents of additive A and additive B are limited within the scope of this application, which can make the secondary battery have a lower initial impedance, excellent normal-temperature cycling performance, high-temperature storage performance and high-temperature cycling performance, and can significantly reduce the deposition of transition metal ions in the positive electrode material.

[0030] In one embodiment of the present application, the mass ratio w 1 / w 2 of the additive A and the additive B is 0.03 to 6.67, preferably 0.15 to 2. For example, the mass ratio w 1 / w 2It can be 0.03, 0.15, 0.5, 1, 2, 4, 6, 6.67 or a range composed of any two of these values. The mass ratio w of additive A and additive B 1 / w 2 Limited within the above range, it can further enable the secondary battery to have a lower initial impedance, excellent normal temperature cycle performance, high temperature storage performance and high temperature cycle performance, and can significantly reduce the deposition of transition metal ions in the cathode material.

[0031] In one embodiment of the present application, the compound shown in Formula I is selected from at least one of the following compounds: 。

[0032] In one embodiment of the present application, the electrolyte further includes an electrolyte, and the electrolyte is selected from LiPF 6 、LiAsF 6 、LiClO 4 、LiSO 3 CF 3 、LiBF 4 、LiB(C 2 O 4 ) 2 、LiBF 2 C 2 O 4 、LiN(SO 2 F) 2 、LiN(SO 2 CF 3 ) 2 、LiPO 2 F 2 、LiPF 2 (C 2 O 4 ) 2 and LiPF 4 C 2 O 4at least one of them; based on the mass of the electrolyte, the mass percentage of the electrolyte is 6% to 20%. For example, based on the mass of the electrolyte, the mass percentage of the electrolyte can be 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20% or a range composed of any two of these values. The electrolyte includes the above electrolyte and controls the mass percentage of the electrolyte within the scope of this application, which can make the electrolyte have high ionic conductivity and good electrochemical stability, and further improve the cycling performance of the secondary battery.

[0033] In an embodiment of the present application, the electrolyte further includes a solvent, and the solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, propyl acetate, methyl propionate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 2,2-difluoroethyl acetate; based on the mass of the electrolyte, the mass percentage of the solvent is 72% to 93%. For example, based on the mass of the electrolyte, the mass percentage of the solvent can be 72%, 75%, 78%, 80%, 85%, 88%, 91%, 93% or a range composed of any two of these values. The electrolyte includes the above solvent and controls the mass percentage of the solvent within the scope of this application, which can make the electrolyte have appropriate viscosity, high ionic conductivity and good electrochemical stability, and can further improve the cycling performance of the secondary battery.

[0034] In an embodiment of the present application, the positive electrode active material further includes lithium nickel cobalt manganese oxide LiNi b Co c Mn 1-b-c O 2 and lithium iron phosphate LiFePO 4 at least one of them, 0 < b < 1, 0 < c < 1.

[0035] In an embodiment of the present application, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material is selected from at least one of carbon-based materials, silicon-based materials, lithium metal, and lithium titanate.

[0036] The present application places no particular restrictions on carbon-based materials and silicon-based materials, as long as the purpose of this application can be achieved. For example, carbon-based materials can include, but are not limited to, at least one of artificial graphite, natural graphite, soft carbon, hard carbon, and mesophase microcarbon spheres; silicon-based materials can include, but are not limited to, at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0037] In the present application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector, and the negative electrode material layer includes a negative electrode active material.

[0038] The above-mentioned "negative electrode material layer provided on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be provided on one surface of the negative electrode current collector along its own thickness direction, or can be provided on both surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be a partial area of the surface of the negative electrode current collector. There is no special limitation in the present application, as long as the purpose of the present application can be achieved.

[0039] The present application does not particularly limit the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0040] The present application does not particularly limit the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 40 μm to 60 μm, and the thickness of the negative electrode current collector is 8 μm to 14 μm.

[0041] In an embodiment of the present application, the negative electrode material layer may further include a conductive agent and a binder. The present application does not particularly limit the types of the conductive agent and the binder, as long as the purpose of the present application can be achieved. For example, the conductive agent can include but is not limited to at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, graphene, and carbon fiber. The above carbon nanotubes can include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fiber can include but is not limited to vapor-grown carbon fiber (VGCF) and / or nanofiber. The binder can include 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). The present application does not particularly limit the mass ratio of the negative electrode active material, the conductive agent, and the binder in the negative electrode material layer, and those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0042] The negative electrode material layer may further include a thickening agent, which may include but is not limited to at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose. The present application does not particularly limit the mass ratio of the negative electrode active material, conductive agent, binder, and thickening agent in the negative electrode material layer. Those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0043] Optionally, the negative electrode tab may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. The present application does not particularly limit the composition of the conductive layer, and it may be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a binder. The present application does not particularly limit the conductive agent and the binder in the conductive layer. For example, it may be at least one of the above-mentioned conductive agents and the above-mentioned binders.

[0044] The present application does not particularly limit the preparation method of the negative electrode tab as long as the purpose of the present application can be achieved. For example, the negative electrode active material, conductive agent, thickening agent, and binder can be mixed in a certain proportion, and deionized water is added and stirred evenly to obtain a negative electrode slurry with a solid content of 40wt% to 60wt%. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector, and after drying, a negative electrode tab with a single-sided coated negative electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode tab with a double-sided coated negative electrode material layer is obtained. After cold pressing, edge trimming, slicing, and striping, it is dried under vacuum conditions after striping, and the tab is welded to obtain the negative electrode tab.

[0045] In the present application, the positive electrode tab includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, and the positive electrode material layer includes a positive electrode active material.

[0046] The above-mentioned "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be provided on one surface of the positive electrode current collector along its thickness direction, or can be provided on two surfaces of the positive electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of the positive electrode current collector surface or a partial area of the positive electrode current collector surface. The present application does not particularly limit as long as the purpose of the present application can be achieved.

[0047] The present application has no particular limitation on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil, aluminum alloy foil, nickel foil or nickel alloy can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0048] The present application has no particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 60 μm to 90 μm, and the thickness of the positive electrode current collector is 10 μm to 20 μm.

[0049] In some embodiments of the present application, the positive electrode material layer may further include a conductive agent and a binder. The present application has no particular limitation on the types of the conductive agent and the binder, as long as the purpose of the present application can be achieved. For example, the conductive agent can include but is not limited to at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene and carbon fiber. The above carbon nanotubes can include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fiber can include but is not limited to vapor-grown carbon fiber (VGCF) and / or nanofiber. The binder can include 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 fluorinated acrylate resin. The present application has no particular limitation on the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode material layer, and those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0050] Optionally, the positive electrode plate may further include a conductive layer, and the conductive layer is located between the positive electrode current collector and the positive electrode material layer. The composition of the conductive layer has no particular limitation and can be a commonly used conductive layer in the art. The conductive layer includes a conductive agent and a binder. The present application has no particular limitation on the conductive agent and the binder in the conductive layer. For example, it can be at least one of the above conductive agents and the above binders.

[0051] The present application does not particularly limit the preparation method of the positive electrode sheet, as long as the object of the present application can be achieved. For example, the positive electrode active material, conductive agent, and binder are mixed in a certain proportion, and N-methylpyrrolidone (NMP) is added and stirred evenly to obtain a positive electrode slurry with a solid content of 40 wt% to 65 wt%. The positive electrode slurry is evenly coated on one surface of the positive electrode current collector, and after drying, a positive electrode sheet with a single-sided coated positive electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the positive electrode current collector, and after drying, a positive electrode sheet with a double-sided coated positive electrode material layer is obtained. After cold pressing, trimming, slicing, and slitting, it is dried under vacuum conditions after slitting, and the electrode tab is welded to obtain the positive electrode sheet.

[0052] The present application does not particularly limit the separator, as long as the object of the present application can be achieved. For example, the material of the separator can be selected from, but not limited to, at least one of polyethylene (PE), polypropylene (PP), glass fiber, polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), and polyamide (PA). The type of the separator can include at least one of a woven film, a ceramic separator, a non-woven fabric, a microporous film, a composite film, a rolled film, and a spun film.

[0053] In the present application, the thickness of the separator is not particularly limited, as long as the object of the present application can be achieved. For example, the thickness of the separator can be 4 μm to 20 μm.

[0054] In the present application, the lithium-ion battery further includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, as well as other components known in the field of lithium-ion batteries. The present application does not limit the above other components. The present application does not particularly limit the housing, and it can be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal, and the present application does not limit the type of metal, and a metal hard shell housing known in the art can be used, as long as the object of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0055] The preparation process of the lithium-ion battery of the present application is well-known to those skilled in the art, and the present application has no particular limitation. For example, the preparation process of the lithium-ion battery can include, but not be limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly, placing the electrode assembly into the housing, injecting the electrolyte into the housing, and obtaining the lithium-ion battery through processes such as vacuum packaging, standing, formation, and grading.

[0056] Examples

[0057] Hereinafter, examples and comparative examples are given to illustrate the embodiments of the present application more specifically. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0058] Test methods and equipment:

[0059] Initial DCIR test

[0060] Place the lithium-ion battery in an incubator at 25°C and let it stand for 0.5 hour to make the lithium-ion battery reach a constant temperature. Then charge the lithium-ion battery at a constant current of 1C until the voltage reaches 4.3V, let it stand for 5 minutes, then discharge it at a constant current of 1C for 30 minutes and then let it stand for 1 hour, and record the voltage at this time as V 0 , and then at a current I corresponding to a 2C rate 1 Discharge for 10s, and record the corresponding voltage as V 1 . The DCIR of the lithium-ion battery at 50% state of charge (SOC), that is, DCIR = (V 0 - V 1 ) / I 1 , and the unit is mΩ.

[0061] Room temperature cycle performance test

[0062] Place the lithium-ion battery in an incubator at 25°C and let it stand for 0.5 hour to make the lithium-ion battery reach a constant temperature. Charge it at a constant current of 1C until the voltage reaches 4.3V, then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C, and then discharge it at a constant current of 1C until the voltage reaches 2.5V, and record the initial discharge capacity as A 0 , and take this as one charge-discharge cycle, repeat the above charge-discharge cycle 1500 times, and obtain the discharge capacity A of the 1500th cycle 1500 .

[0063] Capacity retention rate at 25°C = A 1500 / A 0 × 100%.

[0064] High temperature storage performance test

[0065] Place the lithium-ion battery in an incubator at 25°C and let it stand for 0.5 hour to make the lithium-ion battery reach a constant temperature. Charge it at a constant current of 1C until the voltage reaches 4.3V, then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C, and then discharge it at a constant current of 1C until the voltage reaches 2.5V, and record the initial discharge capacity as B 0; Then charge the lithium-ion battery at a constant current of 1C until the voltage reaches 4.3V, then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C. After that, place the lithium-ion battery in an explosion-proof oven at 60°C and store it for 60 days. Then take out the lithium-ion battery and cool it to room temperature. At 25°C, discharge the lithium-ion battery at a constant current of 1C until the voltage reaches 2.5V, and record the discharge capacity at this time as B 1 ; At 25°C, charge the lithium-ion battery at a constant current of 1C until the voltage reaches 4.3V, then charge it at a constant voltage of 4.3V until the cut-off current is 0.05C, and then discharge it at a constant current of 1C until the voltage reaches 2.5V. Take this as one charge-discharge cycle and repeat the above charge-discharge cycle 3 times, and record the discharge capacity of the third cycle as B 2 .

[0066] Retention rate at 60°C = B 1 / B 0 ×100%; Recovery rate at 60°C = B 2 / B 0 ×100%.

[0067] High-temperature cycle performance test

[0068] Place the lithium-ion battery in an incubator at 45°C and let it stand for 3 hours to reach a constant temperature. Then charge the lithium-ion battery at a constant current of 1C until the voltage reaches 4.3V, then charge it at a constant voltage of 4.3V until the current is 0.05C, and then discharge it at a constant current of 1C until the voltage reaches 2.5V, and record the initial discharge capacity as C 0 , take this as one charge-discharge cycle and repeat the above charge-discharge cycle 800 times, and record the discharge capacity of the 800th cycle as C 800 .

[0069] Retention rate at 45°C = C 800 / C 0 ×100%.

[0070] Measurement of manganese deposition amount

[0071] Disassemble a lithium-ion battery that has undergone 800 high-temperature cycles, take out the negative electrode sheet, rinse the negative electrode sheet three times with dimethyl carbonate, then soak the negative electrode sheet in deionized water. Wait until the negative electrode material falls off the copper foil, discard the copper foil, filter and dry the negative electrode material. Weigh 0.5 g of the negative electrode material and place it in a 300 mL glass beaker. Slowly add 10 mL of 65 wt% nitric acid and 10 mL of 98 wt% sulfuric acid. Place it on an electric furnace, cover it with a watch glass, and heat until the sample dissolves (becomes milky white). Take it off and cool it. Add 10 mL of 36 wt% hydrochloric acid and heat until the salts dissolve (the solution becomes clear). After taking it off and cooling, filter it with a medium-speed quantitative filter paper. Rinse the filter paper and the beaker clean and transfer them to a 250 mL volumetric flask for volume fixation. Then perform ICP injection testing. Specifically, use an inductively coupled plasma optical emission spectrometer (Agilent ICP-OES 5110) to directly read the manganese content in the negative electrode material.

[0072] Differential capacity curve test

[0073] The dQ / dV curve represents the capacity fluctuation of the material within a unit voltage range and can be recorded and calculated through a conventional charge-discharge testing device. By differentiating the first charging curve, the dQ / dV curve can be obtained, which can reveal the activation energy and reaction rate of different electrochemical reactions inside the battery. As shown in the following formula, where Q is the charge quantity, V is the voltage, and I(t) is the current at time t.

[0074]

[0075] Example 1-1

[0076] <Preparation of electrolyte>

[0077] In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), mix ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) evenly according to a mass ratio of 20:4:21:41 to obtain a basic solvent. Subsequently, add lithium hexafluorophosphate (LiPF 6 ), the compound shown in Formula I-1, and vinylene carbonate (VC), dissolve and mix them evenly to obtain an electrolyte; among them, based on the mass of the electrolyte, the mass percentage content of LiPF 6 is 13%, the mass percentage content of the compound shown in Formula I-1 is 0.5%, the mass percentage content of VC is 1.5%, and the balance is the basic solvent.

[0078] <Preparation of positive electrode sheet>

[0079] The positive electrode active material lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4, the conductive agent Super P, the binder polyvinylidene fluoride (PVDF), and carbon nanotubes (CNT) are mixed in a mass ratio of 96.5:1.5:1:1, N-methylpyrrolidone (NMP) is added, and they are stirred evenly under the action of a vacuum mixer to obtain a positive electrode slurry with a solid content of 53 wt%; the positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 16 μm, and the coating amount is 250 g / m 2 , and the compaction density is 2.3 g / cm 3 . After drying at 85 °C, a positive electrode plate with a single-sided coated positive electrode material layer with a coating thickness of 76 μm is obtained; the above steps are repeated on the other surface of the positive electrode current collector aluminum foil, and a positive electrode plate with a double-sided coated positive electrode material layer is obtained; after cold pressing, trimming, cutting, and slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the tab is welded to obtain a positive electrode plate with a specification of 70 mm × 54 mm.

[0080] <Preparation of negative electrode plate>

[0081] The negative electrode active material artificial graphite, the conductive agent Super P, the thickener sodium carboxymethyl cellulose (CMC), and the binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 95:1.5:1:2.5, deionized water is added, and they are stirred evenly under the action of a vacuum mixer to obtain a negative electrode slurry with a solid content of 49 wt%; the negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 9 μm, and the coating amount is 154 g / m 2 , and the compaction density is 1.6 g / cm 3 . After drying at 85 °C, a negative electrode plate with a single-sided coated negative electrode material layer with a coating thickness of 49 μm is obtained; the above steps are repeated on the other surface of the negative electrode current collector copper foil, and a negative electrode plate with a double-sided coated negative electrode material layer is obtained; after cold pressing, trimming, cutting, and slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the tab is welded to obtain a negative electrode plate with a specification of 74 mm × 58 mm.

[0082] <Preparation of separator>

[0083] A polypropylene porous membrane with a thickness of 16 µm is used as the separator.

[0084] <Preparation of lithium-ion battery>

[0085] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 85 °C, and the prepared electrolyte is injected with a liquid injection coefficient of 4.5 g / Ah. After vacuum packaging, standing, formation, and grading processes, a lithium-ion battery is obtained with a specification of 4.0 mm × 80 mm × 100 mm. Among them, the standing time is 24 hours, the formation current is 0.5 C, the upper limit voltage of formation is 4.2 V, the formation temperature is 45 °C, the grading is to charge at 0.5 C to 4.3 V, then charge at a constant voltage of 4.3 V until the current is 0.05 C, hold for 10 minutes, and then discharge at 0.5 C to 2.5 V, and then repeat the above steps at 1 C.

[0086] Examples 1-2 to Examples 1-16

[0087] Except for adjusting the type of additive A according to Table 1 in <Preparation of Electrolyte>, the rest is the same as Example 1-1.

[0088] Examples 1-17 to Examples 1-19

[0089] Except for adjusting the type of additive B according to Table 1 in <Preparation of Electrolyte>, the rest is the same as Example 1-1.

[0090] Examples 1-20 to Examples 1-27

[0091] Except for adjusting the mass percentage content of additive A and additive B according to Table 1 in <Preparation of Electrolyte>, and the mass percentage content of the base solvent changes accordingly, while the percentage content of other components in the electrolyte remains unchanged, the rest is the same as Example 1-1.

[0092] Examples 1-28 to Examples 1-31

[0093] Except for adjusting the type and mass percentage content of additive B according to Table 1 in <Preparation of Electrolyte>, and the mass percentage content of the base solvent changes accordingly, while the percentage content of other components in the electrolyte remains unchanged, the rest is the same as Example 1-1.

[0094] Example 2-1

[0095] Except in <Preparation of Positive Electrode Sheet>, the positive active materials lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 , lithium nickel cobalt manganese oxide NCM811, conductive agent Super P, binder polyvinylidene fluoride (PVDF), and carbon nanotubes (CNT) are mixed in a mass ratio of 29:67.5:1.5:1:1, the rest is the same as Example 1-1.

[0096] Example 2-2

[0097] Except that in the preparation of the positive electrode sheet, the positive electrode active material lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO 4 , lithium iron phosphate LFP, conductive agent Super P, binder polyvinylidene fluoride (PVDF) and carbon nanotubes (CNT) are mixed according to a mass ratio of 77:19.5:1.5:1:1, the rest is the same as Example 1-1.

[0098] Comparative Example 1-1

[0099] Except that in the preparation of the electrolyte, additives A and B are not added, the mass percentage content of the basic solvent changes accordingly, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0100] Comparative Example 1-2

[0101] Except that in the preparation of the electrolyte, additive B is not added, the mass percentage content of the basic solvent changes accordingly, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0102] Comparative Example 1-3

[0103] Except that in the preparation of the electrolyte, additive A is not added, the mass percentage content of the basic solvent changes accordingly, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0104] Comparative Example 1-4

[0105] Except that in the preparation of the electrolyte, additive B is not added, the mass percentage content of additive A is adjusted according to Table 1, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0106] Comparative Example 1-5

[0107] Except that in the preparation of the electrolyte, additive A is not added, the mass percentage content of additive B is adjusted according to Table 1, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0108] Comparative Examples 1-6 to 1-9

[0109] Except that in the preparation of the electrolyte, the mass percentage contents of additives A and B are adjusted according to Table 1, the mass percentage content of the basic solvent changes accordingly, and the percentage contents of other components in the electrolyte remain unchanged, the rest is the same as Example 1-1.

[0110] Comparative Examples 1-10 to Comparative Examples 1-13

[0111] Except that in the <Preparation of Electrolyte>, the type of Additive B was adjusted according to Table 1 and the mass percentage content thereof changed, and the mass percentage content of the base solvent changed accordingly, while the percentage contents of other components in the electrolyte remained unchanged, the rest was the same as in Example 1-1.

[0112] Comparative Example 1-14

[0113] Except that in the <Preparation of Electrolyte>, the type of Additive A was adjusted according to Table 1, the rest was the same as in Example 1-1.

[0114] Comparative Example 2-1

[0115] Except that in the <Preparation of Electrolyte>, Additive A was not added and the mass percentage content of the base solvent changed accordingly, while the percentage contents of other components in the electrolyte remained unchanged, the rest was the same as in Example 2-1.

[0116] Comparative Example 2-2

[0117] Except that in the <Preparation of Electrolyte>, Additive A was not added and the mass percentage content of the base solvent changed accordingly, while the percentage contents of other components in the electrolyte remained unchanged, the rest was the same as in Example 2-2.

[0118] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 2.

[0119] Table 1

[0120] In Table 1, " / " indicates no relevant parameters, "FEC" is fluoroethylene carbonate, and "VEC" is vinylene carbonate.

[0121] Table 2

[0122] As can be seen from the above Examples 1-1 to 1-31, Examples 2-1 to 2-2, Comparative Examples 1-1 to 1-13, and Comparative Examples 2-1 to 2-2, when the electrolytes that do not add Additive A and Additive B simultaneously are applied to lithium-ion batteries, or the contents of Additive A and Additive B are not within the scope of this application, the lithium-ion batteries have low capacity retention rates at 25 °C, 60 °C, 60 °C capacity recovery rate and 45 °C capacity retention rate, as well as a high manganese content. When the electrolyte that simultaneously adds Additive A and Additive B is applied to the lithium-ion battery and the contents of Additive A and Additive B are adjusted within the scope of this application, the lithium-ion battery has high capacity retention rates at 25 °C, 60 °C, 60 °C capacity recovery rate and 45 °C capacity retention rate, as well as a low initial impedance and manganese content. The above results show that when Additive A and Additive B are used in combination and the mass percentage contents of Additive A and Additive B are limited within the scope of this application, the lithium-ion battery can have a low initial impedance, excellent normal temperature cycling performance, high temperature storage performance and high temperature cycling performance, and can significantly reduce the deposition of transition metal ions in the cathode material.

[0123] As can be seen from Example 1-1 and Comparative Example 1-14, compared with the conventional nitrile additive 1,3,6-hexanetricarbonitrile, when Additive A of this application is applied to a lithium-ion battery, the lithium-ion battery has high capacity retention rates at 25 °C, 60 °C, 60 °C capacity recovery rate and 45 °C capacity retention rate, as well as a low initial impedance and manganese content. In the research, it was found that conventional nitrile additives (such as 1,3,6-hexanetricarbonitrile in Comparative Example 1-14) are generally considered not easy to be oxidized and have strong coordination ability, which can complex the dissolved transition metal ions on the positive electrode, thereby improving the battery performance; however, the complexation of the cyano group with Co and Ni is strong, and the complexation with Mn is weak, so the effect on the dissolution of Mn in the LMFP system is small, and the improvement of the battery performance is limited. Moreover, conventional nitrile substances are also difficult to form a reduction film. For example Figure 1 as shown in Comparative Examples 1-3 and 1-14 of Figure 1As shown in Example 1-1, it will be preferentially reduced at around 1.8V. It will be reduced and decomposed by itself to form dimers / trimers, and then further form conjugated oligomers and polymers. Since additive A has more than three cyano groups, they can form a cross-linked structure, which can well inhibit the reduction and decomposition of the solvent and lithium salt, making the secondary battery have a lower impedance at the initial stage of cycling. At the same time, the cross-linked structure also significantly reduces the uneven charge distribution, reduces the deposition of Mn on the negative electrode. Since a high-density Mn deposition cannot be formed, the toxicity to the secondary battery is greatly reduced, and the situation of the negative electrode peeling and sticking to the separator is significantly improved. When additive A is used in combination with additive B, additive B will also participate in the composition of the cross-linked structure to form a SEI film with high mechanical strength, high ionic conductivity and high uniformity. And additive B will continuously repair the damaged SEI film during long cycling. Finally, it shows a lower initial impedance, excellent room temperature cycling performance, high temperature storage performance and high temperature cycling performance in terms of battery performance.

[0124] It can also be seen from Example 2-1 to Example 2-2 and Comparative Example 2-1 to Comparative Example 2-2 that LiMn 0.6 Fe 0.4 PO 4 When used in combination with NCM811 or LFP as the positive electrode active material, the lithium-ion battery can have a lower initial impedance, better room temperature cycling performance, high temperature storage performance and high temperature cycling performance, and can more significantly reduce the deposition of transition metal ions in the positive electrode material.

[0125] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; characterized in that: The positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes LiMn a Fe 1-a PO4, 0<a<1, the electrolyte comprises an additive A and an additive B, and the additive A is selected from at least one of the compounds shown in formula I; ； wherein R1, R2, and R3 are each independently selected from a hydrogen atom, a fluorine atom, an unsubstituted or fluorine-substituted C1-C4 alkyl group, a C2-C4 alkenyl group, a C1-C4 alkoxy group, a C6-C 12 At least one of aryl, amino, cyano and acyl; x, y, z are each independently 0, 1, 2; The additive B is selected from at least one of vinylene carbonate, fluoroethylene carbonate and vinyl ethylene carbonate; Based on the mass of the electrolyte, the mass percentage of the additive A is w1, and w1 is 0.1%~3%; the mass percentage of the additive B is w2, and w2 is 0.1%~5%.

2. The secondary battery according to claim 1, characterized in that: w1 is 0.3%~1%, w2 is 0.5%~2%.

3. The secondary battery according to claim 1, characterized in that: The mass ratio w1 / w2 of the additive A to the additive B is 0.03-6.

67.

4. The secondary battery according to claim 3, characterized in that: w1 / w2 is 0.15~2.

5. The secondary battery according to claim 1, characterized in that: The compound represented by formula I is selected from at least one of the following compounds: 。 6. The secondary battery according to claim 1, characterized in that: The electrolyte also includes an electrolyte, which is selected from at least one of LiPF6, LiAsF6, LiClO4, LiSO3CF3, LiBF4, LiB(C2O4)2, LiBF2C2O4, LiN(SO2F)2, LiN(SO2CF3)2, LiPO2F2, LiPF2(C2O4)2 and LiPF4C2O4; based on the mass of the electrolyte, the mass percentage of the electrolyte is 6%~20%.

7. The secondary battery according to claim 1, characterized in that: The electrolyte also includes a solvent, and the solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, cyclopentane, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, propyl acetate, methyl propionate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 2,2-difluoroethyl acetate; based on the mass of the electrolyte, the mass percentage of the solvent is 72%~93%.

8. The secondary battery according to claim 1, characterized in that: The positive electrode active material also includes LiNi b Co c Mn 1-b-c At least one of O2 and LiFePO4, 0<b<1, 0<c<1.

9. The secondary battery according to claim 1, characterized in that: The negative electrode plate includes a negative electrode active material, and the negative electrode active material is selected from at least one of a carbon-based material, a silicon-based material, lithium metal and lithium titanate.

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