Secondary batteries and electrical equipment

Abstract

The embodiments of the present application disclose a secondary battery and an electrical device. The secondary battery is formed by stacking a first active material layer and a second active material layer on a positive electrode current collector of a positive electrode sheet. The first active material layer and the second active material layer constitute the positive electrode active material layer of the positive electrode sheet, and the manganese content of the first manganese iron lithium oxide in the first active material layer is higher than the manganese content of the first manganese iron lithium oxide in the second active material layer. The second active material layer with a low manganese content is coated on the first active material layer, which reduces the grain boundary impedance between different materials and enhances the interlayer bonding force between the first active material layer and the second active material layer. The manganese-poor interface of the second active material layer can reduce the manganese content in contact with the electrolyte, thereby reducing manganese dissolution. When assembled into a secondary battery, a secondary battery with high energy density, good safety performance, long cycle life, and high cost performance can be obtained.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and electrical equipment. Background Art

[0002] Lithium manganese iron phosphate (LiMn x Fe 1-x PO4) is doped with a certain proportion of manganese on the basis of lithium iron phosphate. On the basis of not changing the original olivine structure, it inherits the advantages of lithium iron phosphate such as low cost, high thermal stability, and high safety, and also gives play to the advantages of manganese, thereby improving the safety and stability of the positive electrode material, expanding the voltage window, and increasing the theoretical energy density.

[0003] However, lithium manganese iron phosphate also has defects that limit its development and application. Manganese materials generally have Jahn-Teller effect, and manganese ions (Mn 3+ ) undergoes disproportionation and dissolution, and the precipitated manganese ions reach the negative electrode through the electrolyte, causing damage and reorganization of the SEI film, consuming electrolyte and active lithium, and resulting in a decrease in the theoretical specific capacity of the secondary battery. In addition, the dissolution of manganese ions causes a manganese-deficient phase in the positive electrode, lattice distortion, structural collapse, and reduced material stability, affecting the electrochemical properties of the secondary battery, such as cycle stability and service life. Summary of the Invention

[0004] The embodiments of the present application provide a secondary battery and an electrical device, which can solve the problem of reduced cycle stability and service life caused by manganese ion dissolution in existing lithium manganese iron phosphate secondary batteries.

[0005] A first aspect of the present application provides a secondary battery, comprising a positive electrode plate, the positive electrode plate comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode active material layer comprising a first active material layer and a second active material layer, the first active material layer being disposed between the positive electrode current collector and the second active material layer; the first active material layer comprising a first manganese iron lithium oxide, and the second active material layer comprising a second manganese iron lithium oxide; wherein, based on the total molar amount of metal elements other than lithium, the molar percentage of manganese element in the first manganese iron lithium oxide is n1%, and the molar percentage of manganese element in the second manganese iron lithium oxide is n2%, satisfying n1>n2.

[0006] Optional, 30≤n1-n2≤80.

[0007] Optional, 50≤n1≤90.

[0008] Optional, n2≤20.

[0009] Optionally, the average particle size of the first manganese iron lithium oxide is D1, and the average particle size of the second manganese iron lithium oxide is D2, satisfying D1>D2.

[0010] Optionally, 500nm≤D1≤900nm.

[0011] Optionally, 200nm≤D2≤500nm.

[0012] Optionally, based on the total mass of the first manganese iron lithium oxide and the second manganese iron lithium oxide in the positive electrode active material layer being 100%, the mass percentage of the first manganese iron lithium oxide in the first active material layer is greater than or equal to 80%.

[0013] Optionally, the general formula of the first manganese iron lithium oxide includes Li a Mn x Fe 1-x PO4, wherein 0.6≤x≤0.8, 0.95≤a≤1.2; and / or, the general formula of the second manganese iron lithium oxide includes Li b Mn y Fe 1-y PO4, where 0<y≤0.2, 0.95≤b≤1.2.

[0014] The second aspect of the present application provides an electrical device comprising the aforementioned secondary battery, wherein the electrical device includes but is not limited to electric toys, electric tools, battery vehicles, electric vehicles, energy storage devices, ships, spacecraft, etc.

[0015] The beneficial effect of the present application is to provide a secondary battery and an electrical device having the secondary battery, wherein the secondary battery is formed by stacking a first active material layer and a second active material layer on a positive electrode current collector of a positive electrode plate, the first active material layer and the second active material layer constitute the positive electrode active material layer of the positive electrode plate, and the manganese content in the first manganese iron lithium oxide in the first active material layer is higher than the manganese content in the first manganese iron lithium oxide in the second active material layer, the second active material layer with a low manganese content is coated on the first active material layer, the first active material layer and the second active material layer both contain manganese iron lithium oxide, thereby reducing the grain boundary resistance between different materials and enhancing the interlayer bonding force between the first active material layer and the second active material layer, the manganese-poor interface of the second active material layer can reduce the manganese content in contact with the electrolyte, thereby reducing manganese dissolution, and assembling them into a secondary battery can obtain a secondary battery with high energy density, good safety performance, long cycle life and high cost performance. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application and are not used to limit the present application. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the said features.

[0017] In the detailed description and claims, a list of items linked by the term "at least one of" can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can contain a single element or multiple elements. Item B can contain a single element or multiple elements. Item C can contain a single element or multiple elements. The term "at least one of" has the same meaning as the term "at least one of."

[0018] In this specification, a numerical range expressed using “to” means a range including the numerical values ​​described before and after “to” as the minimum value and the maximum value, respectively.

[0019] Embodiments of the present application provide a secondary battery, a method for preparing a secondary battery, and an electrical device having the secondary battery. The secondary battery is formed by stacking a first active material layer and a second active material layer on a positive current collector of a positive electrode sheet. The first active material layer and the second active material layer constitute the positive electrode active material layer of the positive electrode sheet, and the manganese content of the first manganese iron lithium oxide in the first active material layer is higher than the manganese content of the first manganese iron lithium oxide in the second active material layer. The second active material layer with a low manganese content is coated on the first active material layer. The first active material layer and the second active material layer both contain manganese iron lithium oxide, which reduces the grain boundary resistance between different materials and enhances the interlayer bonding force between the first active material layer and the second active material layer. The manganese-poor interface of the second active material layer can reduce the manganese content in contact with the electrolyte, thereby reducing manganese dissolution. When assembled into a secondary battery, a secondary battery with high energy density, good safety performance, long cycle life, and high cost performance can be obtained.

[0020] In one embodiment of the present application, a secondary battery is provided. The secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a casing.

[0021] I. Positive electrode

[0022] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0023] Positive electrode active material layer

[0024] The positive electrode active material layer includes a first active material layer and a second active material layer, wherein the first active material layer is disposed between the positive electrode current collector and the second active material layer.

[0025] In some embodiments, the first active material layer includes a first manganese iron lithium oxide, and the molar percentage of manganese element in the first manganese iron lithium oxide is n1% based on the total molar amount of metal elements other than lithium; the second active material layer includes a second manganese iron lithium oxide, and the molar percentage of manganese element in the second manganese iron lithium oxide is n2% based on the total molar amount of metal elements other than lithium, and n1>n2 is satisfied.

[0026] Because the molar percentage of manganese in the first manganese iron lithium oxide is n1%, it is higher than the molar percentage of manganese in the second manganese iron lithium oxide, which is n2%. That is, the manganese ratio (manganese content) in the first active material layer (equivalent to the inner layer) close to the positive electrode current collector is higher, and the manganese ratio (manganese content) in the second active material layer (equivalent to the outer layer) away from the positive electrode current collector is lower, forming a double-layer composite structure positive electrode sheet with high manganese in the inner layer and low manganese in the outer layer. The low manganese manganese iron lithium oxide used in the outer layer can reduce the manganese content in contact with the electrolyte, thereby reducing manganese dissolution. In addition, the manganese iron lithium oxide in the first active material layer and the second active material layer both have an olivine structure, which reduces the grain boundary resistance between different materials and improves the material bonding.

[0027] In some embodiments, 30≤n1-n2≤80, specifically, the value of n1-n2 includes 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80 or a range consisting of any of the above values.

[0028] In some embodiments, n1 ≥ 50. Specifically, n1 includes 50, 55, 60, 65, 70, 75, 80, 90, 95, 99, or a range consisting of any of the foregoing values. Preferably, 60 ≤ n1 ≤ 70. This can increase the voltage platform of the positive electrode active material, thereby increasing the energy density of the battery.

[0029] In some embodiments, 0 < n2 ≤ 20, specifically, n1 includes 1, 5, 10, 15, 20, or a range consisting of any of the foregoing values. When 0 < n2 ≤ 20, the lithium iron manganese oxide has a higher bulk electronic conductivity and lithium ion diffusion rate, further reducing manganese dissolution and accelerating lithium ion conduction efficiency, thereby improving the rate capability and cycle stability of the battery system.

[0030] In some embodiments, the first manganese iron lithium oxide has a general formula comprising Li a Mn x Fe 1-x PO4, wherein 0.6≤x≤0.8, 0.95≤a≤1.2, specifically, x includes any one of 0.6, 0.65, 0.7, 0.75, 0.8, then the general formula of the first manganese iron lithium oxide includes LiMn 0.6 Fe 0.4 PO4、LiMn 0.65 Fe 0.35 PO4、LiMn 0.7 Fe 0.3 PO4、LiMn 0.75 Fe 0.25 PO4、LiMn 0.8 Fe 0.2 Any one of PO4.

[0031] In some embodiments, the second manganese iron lithium oxide has a general formula of LiMn y Fe 1-y PO4, wherein 0<y≤0.2, 0.95≤b≤1.2, specifically, y includes any one of 0.1, 0.13, 0.15, 0.18, 0.2, then the general formula of the first manganese iron lithium oxide includes LiMn 0.1 Fe 0.9 PO4、LiMn 0.13 Fe 0.87 PO4、LiMn 0.15 Fe 0.85 PO4、LiMn 0.18 Fe 0.82 PO4、LiMn 0.2 Fe 0.8 Any one of PO4.

[0032] In some embodiments, based on the total mass of the first manganese iron lithium oxide and the second manganese iron lithium oxide in the positive electrode active material layer being 100%, the mass percentage of the first manganese iron lithium oxide in the first active material layer is greater than or equal to 80%, and the mass percentage of the second manganese iron lithium oxide in the second active material layer is less than or equal to 20%. For example, if the mass percentage of the first manganese iron lithium oxide is 85%, the mass percentage of the second manganese iron lithium oxide is 15%; if the mass percentage of the first manganese iron lithium oxide is 90%, the mass percentage of the second manganese iron lithium oxide is 10%; if the mass percentage of the first manganese iron lithium oxide is 83%, the mass percentage of the second manganese iron lithium oxide is 17%; and if the mass percentage of the first manganese iron lithium oxide is 88%, the mass percentage of the second manganese iron lithium oxide is 12%.

[0033] Adjusting the ratio of the first manganese iron lithium oxide content in the first active material layer to the second manganese iron lithium oxide content in the second active material layer can achieve an optimal ratio within a certain range, ensuring that manganese dissolution is reduced without compromising the voltage range, thereby achieving stable cycling of the secondary battery. Furthermore, by controlling the mass percentage of the first manganese iron lithium oxide in the first active material layer to be greater than the mass percentage of the second manganese iron lithium oxide in the second active material layer, such that the mass percentage of the first manganese iron lithium oxide is greater than or equal to 80%, and the mass percentage of the second manganese iron lithium oxide in the second active material layer is less than or equal to 20%, sufficient manganese is ensured to stabilize the electrode, improving the cycling stability of the positive electrode active material, expanding the voltage range of the secondary battery, and achieving high energy density. The ratio of the first manganese iron lithium oxide to the second manganese iron lithium oxide can be adjusted by parameters such as surface density and thickness.

[0034] In some embodiments, the average particle size of the first manganese iron lithium oxide is D1, and the average particle size of the second manganese iron lithium oxide is D2, satisfying D1>D2.

[0035] In some embodiments, the second manganese iron lithium oxide exists in a granular form with a particle size of 200nm to 500nm, specifically, it can be 200, 250, 300, 350, 400, 450, 500 or a range consisting of any two numbers therein. The first manganese iron lithium oxide exists in a granular form with a particle size of 500 to 900nm, specifically, it can be 500, 550, 600, 650, 700, 750, 800, 850, 900 or a range consisting of any two numbers therein. The small particle size of the second manganese iron lithium oxide in the second active material layer can help to increase the compaction density and form a tight manganese iron lithium oxide outer layer. Manganese iron lithium oxide particles with different particle sizes at the interface between the first active material layer and the second active material layer can help to combine more tightly.

[0036] In some embodiments, the positive electrode active material layer further includes a positive electrode conductor and a positive electrode binder.

[0037] Positive electrode conductive agent

[0038] There is no limitation on the type of positive electrode conductive agent; any known conductive agent may be used. Examples of positive electrode conductive agents include, but are not limited to, graphites such as natural graphite and artificial graphite; carbon blacks such as acetylene black; carbon materials such as amorphous carbon such as needle coke; carbon nanotubes; and graphene. These positive electrode conductive agents may be used alone or in any combination.

[0039] positive electrode binder

[0040] There is no particular restriction on the type of positive electrode binder used in the manufacture of the positive electrode active material layer. In the case of a coating method, any material that can be dissolved or dispersed in the liquid medium used in the manufacture of the electrode can be used. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; styrene-butadiene-styrene block copolymers or their hydrogenates, ethylene-propylene-diene terpolymers (EPDM), Thermoplastic elastomer polymers such as styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers, or their hydrogenated forms; soft resin polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorinated polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; and polymer compositions with ion conductivity for alkali metal ions (particularly lithium ions). These positive electrode binders may be used alone or in any combination.

[0041] positive electrode current collector

[0042] The type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; and composite materials formed by polymers and metal layers. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0043] There is no particular limitation on the form of the positive electrode current collector. When the positive electrode current collector is a metal material, the form of the positive electrode current collector may include, but is not limited to, metal foil, metal cylinder, metal strip roll, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector may include, but is not limited to, carbon plate, carbon film, carbon cylinder, etc. In some embodiments, the positive electrode current collector is a metal foil. In some embodiments, the metal foil is mesh-shaped. There is no particular limitation on the thickness of the metal foil. In some embodiments, the thickness of the metal foil is greater than 1 μm, greater than 3 μm, or greater than 5 μm. In some embodiments, the thickness of the metal foil is less than 1 mm, less than 100 μm, or less than 50 μm. In some embodiments, the thickness of the metal foil is within the range formed by any two of the above values.

[0044] II. Negative electrode

[0045] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains graphite.

[0046] The negative electrode sheet can be single-sided or double-sided. In single-sided applications, the negative electrode active material layer is located on one surface of the negative electrode current collector. In double-sided applications, the negative electrode active material layer is located on both surfaces of the negative electrode current collector. A negative electrode sheet can also have both single-sided and double-sided negative electrode sheet regions.

[0047] negative electrode current collector

[0048] In some embodiments, the negative electrode current collector includes, but is not limited to, metal foil, metal cylinder, metal coil, metal plate, metal film, expanded metal, stamped metal, foamed metal, etc. In some embodiments, the negative electrode current collector is metal foil. In some embodiments, the negative electrode current collector is aluminum foil or copper foil. As used herein, the term "copper foil" includes copper alloy foil.

[0049] In some embodiments, the negative electrode current collector is a conductive resin. In some embodiments, the conductive resin comprises a film obtained by evaporating copper on a polypropylene film.

[0050] Negative electrode active material layer

[0051] The negative electrode active material layer can be one or more layers, and each layer in the multiple layers can contain the same or different negative electrode active materials. A negative electrode active material is any substance that can reversibly intercalate and deintercalate metal ions such as lithium ions. In some embodiments, the charge capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent lithium metal from precipitating on the negative electrode during charging.

[0052] In some embodiments, the thickness of the negative electrode active material layer refers to the thickness of the negative electrode active material layer coated on a single side of the negative electrode current collector. In some embodiments, the thickness of the single-sided negative electrode active material layer is 15 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 20 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 30 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 150 μm or less. In some embodiments, the thickness of the single-sided negative electrode active material layer is 120 μm or less. In some embodiments, the thickness of the single-sided negative electrode active material layer is 100 μm or less. In some embodiments, the thickness of the negative electrode active material layer is within the range consisting of any two of the above values. When the thickness of the negative electrode active material layer is within the above range, the electrolyte can penetrate into the vicinity of the negative electrode current collector interface, thereby improving the charge and discharge characteristics of the secondary battery at high current density; at the same time, the volume ratio of the negative electrode current collector to the negative electrode active material is within an appropriate range, which can ensure the capacity of the secondary battery.

[0053] In some embodiments, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductor, and a negative electrode binder.

[0054] negative electrode active material

[0055] In some embodiments, the negative electrode active material can be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, silicon-based materials, and tin-based materials, with graphite being more preferred.

[0056] Negative electrode conductive agent

[0057] In some embodiments, the conductive agent includes one or more of carbon black, graphite, carbon fiber, carbon nanotube, or graphene, preferably carbon black.

[0058] negative electrode binder

[0059] The binder can improve the bonding between the negative electrode active materials. The type of binder is not particularly limited, as long as it is stable to the electrolyte or solvent used in electrode manufacturing. In some embodiments, the binder includes sodium carboxymethyl cellulose and styrene-butadiene rubber.

[0060] Preparation of negative electrode sheet

[0061] The negative electrode active material graphite, conductive agent CNT, thickener CMC, and binder SBR were mixed in a mass ratio of 96.5:0.8:0.9:1.8, and deionized water was added as a solvent and vacuum stirred until the system became uniform to obtain the negative electrode slurry. The negative electrode slurry was then evenly coated on the upper and lower surfaces of the negative electrode current collector copper foil. After drying at room temperature, it was transferred to an oven for further drying to a compaction density of 1.7 g / cm3 The negative electrode sheet is obtained by cold pressing and cutting under the conditions.

[0062] III. Electrolyte

[0063] In some embodiments, the electrolyte comprises a lithium salt and a solvent. In some embodiments, the electrolyte further comprises an additive.

[0064] lithium salts

[0065] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, organic lithium borate, lithium perchlorate, and sulfonimide lithium salts. The content of the lithium salt is not particularly limited as long as it does not impair the effects of the present application.

[0066] solvent

[0067] In some embodiments, the electrolyte further comprises any non-aqueous solvent known in the art that can be used as a solvent for an electrolyte.

[0068] In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of the following: cyclic carbonates and linear carbonates.

[0069] In some embodiments, examples of the cyclic carbonate may include, but are not limited to, one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate. In some embodiments, the cyclic carbonate has 3-6 carbon atoms.

[0070] In some embodiments, examples of the chain carbonates may include, but are not limited to, one or more of the following: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, di-n-propyl carbonate, etc. Examples of fluorine-substituted chain carbonates may include, but are not limited to, one or more of the following: bis(fluoromethyl) carbonate, bis(difluoromethyl) carbonate, bis(trifluoromethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2-difluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2,2-difluoroethyl methyl carbonate, and 2,2,2-trifluoroethyl methyl carbonate.

[0071] additive

[0072] In some embodiments, examples of the additive may include, but are not limited to, one or more of the following: fluorocarbonate, ethylene carbonate containing a carbon-carbon double bond, a compound containing a sulfur-oxygen double bond, and an acid anhydride.

[0073] IV, diaphragm

[0074] To prevent short circuits, a separator is typically placed between the positive and negative electrodes. In this case, the electrolyte of the present application is typically infiltrated into the separator for use. The separator can be made of any common material, such as polyethylene, polypropylene, polyvinylidene fluoride, and various composite membranes. Polypropylene membranes are particularly preferred.

[0075] In some embodiments, a method for preparing a secondary battery includes the following steps:

[0076] 1) Preparation of lithium manganese iron phosphate slurry

[0077] The first lithium manganese iron phosphate, a conductive agent, and a binder are added to a stirring tank filled with N-methylpyrrolidone (NMP) to prepare a slurry A with uniform mixing and stable viscosity; the second lithium manganese iron phosphate, a conductive agent, and a binder are added to a stirring tank filled with NMP to prepare a slurry C with uniform mixing and stable viscosity;

[0078] 2) Preparation of positive electrode sheet

[0079] Slurry A was applied to one side of an aluminum foil serving as a positive electrode current collector and dried. Slurry A of corresponding thickness and mass was then applied to the other surface of the aluminum foil and dried to obtain aluminum foil layers coated with the slurry on both sides. The aluminum foil layers were then rolled to a thickness of 0.12 mm on each side, forming first active material layers on both surfaces of the positive electrode current collector.

[0080] Then, coating C is applied on both sides of the first active material layer, dried, and then rolled until the coating thickness on one side of the positive electrode current collector is about 0.14 mm. The positive electrode sheet is obtained after slitting and cutting.

[0081] 3) Making batteries

[0082] The prepared positive electrode sheets, negative electrode sheets and separators are wound or stacked to make battery cells, which are then processed through processes such as liquid injection, formation, and capacity division to make corresponding secondary batteries.

[0083] The following describes the method for preparing the secondary battery provided by this application in conjunction with specific embodiments:

[0084] Example 1

[0085] (1) Preparation of positive electrode sheet

[0086] The average particle size of LiMn is 600 nm. 0.6 Fe 0.4PO4, conductive agent acetylene black and binder PVDF were mixed in a mass ratio of 88:4:8 and added into a stirring tank filled with N-methylpyrrolidone solution (NMP) to prepare a slurry A with uniform mixing and stable viscosity; LiMn with an average particle size of 400nm was added. 0.1 Fe 0.9 PO4 was mixed with the conductive agent acetylene black and the binder PVDF in a mass ratio of 88:4:8, and then added into a stirring tank filled with NMP to prepare a slurry C with uniform mixing and stable viscosity;

[0087] Slurry A is coated on one side of the aluminum foil and dried. After drying, slurry A is coated on the other surface of the aluminum foil and dried to obtain a first active material layer coated with slurry A on both sides, which is rolled to 0.12 mm on each side. Slurry C is then coated on the first active material layer, both sides are required to be coated, dried, and then rolled to a coating thickness of about 0.14 mm on a single surface of the positive electrode current collector to form a second active material layer. After slitting and cutting, the positive electrode sheet is obtained. Among them, LiMn in the positive electrode sheet 0.6 Fe 0.4 PO4 and LiMn 0.1 Fe 0.9 The mass ratio of PO4 is 9:1;

[0088] (2) Preparation of negative electrode sheet

[0089] The negative electrode active material graphite, conductive agent CNT, thickener CMC, and binder SBR were mixed in a mass ratio of 96.5:0.8:0.9:1.8, and deionized water was added as a solvent. The mixture was vacuum stirred until the system was uniform to obtain the negative electrode slurry, which was then evenly coated on the upper and lower surfaces of the negative electrode current collector copper foil. After drying at room temperature, the mixture was transferred to an oven for further drying to a compaction density of 1.7 g / cm 3 The negative electrode sheet is obtained by cold pressing, slitting and cutting under the conditions.

[0090] (3) Preparation of electrolyte

[0091] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0092] (4) Preparation of isolation membrane

[0093] A polyethylene film is selected as the separator.

[0094] (5) Preparation of lithium-ion batteries

[0095] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain a bare cell; the bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0096] Example 2

[0097] The difference between this embodiment and embodiment 1 is that the LiMn 0.6 Fe 0.4 PO4 and LiMn 0.1 Fe 0.9 The mass ratio of PO4 is 8:2.

[0098] Example 3

[0099] The difference between this embodiment and embodiment 1 is that LiMn 0.2 Fe 0.8 PO4 instead of LiMn 0.1 Fe 0.9 PO4.

[0100] Example 4

[0101] The difference between this embodiment and embodiment 2 is that LiMn 0.2 Fe 0.8 PO4 instead of LiMn 0.1 Fe 0.9 PO4.

[0102] Example 5

[0103] The difference between this embodiment and embodiment 1 is that LiMn 0.7 Fe 0.3 PO4 instead of LiMn 0.6 Fe 0.4 PO4.

[0104] Example 6

[0105] The difference between this embodiment and embodiment 5 is that the LiMn 0.7 Fe 0.3 PO4 and LiMn 0.1 Fe 0.9 The mass ratio of PO4 is 8:2.

[0106] Example 7

[0107] The difference between this embodiment and embodiment 5 is that LiMn 0.2 Fe 0.8 PO4 instead of LiMn 0.1 Fe 0.9 PO4.

[0108] Example 8

[0109] The difference between this embodiment and embodiment 6 is that LiMn 0.2 Fe 0.8 PO4 instead of LiMn 0.1 Fe 0.9 PO4.

[0110] Example 9

[0111] The difference between this embodiment and embodiment 3 is that the LiMn 0.6 Fe 0.4 PO4 and LiMn 0.2 Fe 0.8 The mass ratio of PO4 is 7:3.

[0112] Example 10

[0113] The difference between this embodiment and embodiment 3 is that the LiMn 0.6 Fe 0.4 PO4 and LiMn 0.2 Fe 0.8 The mass ratio of PO4 is 9.5:0.5.

[0114] Example 11

[0115] The difference between this embodiment and embodiment 3 is that LiMn 0.3 Fe 0.7 PO4 instead of LiMn 0.2 Fe 0.8 PO4.

[0116] Example 12

[0117] The difference between this embodiment and embodiment 3 is that LiMn 0.5 Fe 0.5 PO4 instead of LiMn 0.6 Fe 0.4 PO4.

[0118] Example 13

[0119] The difference between this embodiment and embodiment 1 is that LiMn 0.8 Fe 0.2 PO4 instead of LiMn 0.6 Fe 0.4 PO4.

[0120] Example 14

[0121] The difference between this embodiment and embodiment 1 is that LiMn 0.9 Fe0.1 PO4 instead of LiMn 0.6 Fe 0.4 PO4.

[0122] Example 15

[0123] The difference between this embodiment and embodiment 1 is that LiMn 0.6 Fe 0.4 The average particle size of PO4 is 500nm, LiMn 0.1 Fe 0.9 The average particle size of PO4 is 300nm.

[0124] Example 16

[0125] The difference between this embodiment and embodiment 1 is that LiMn 0.6 Fe 0.4 The average particle size of PO4 is 900nm, LiMn 0.1 Fe 0.9 The average particle size of PO4 is 500nm.

[0126] Example 17

[0127] The difference between this embodiment and embodiment 1 is that LiMn 0.6 Fe 0.4 The average particle size of PO4 is 400nm, LiMn 0.1 Fe 0.9 The average particle size of PO4 is 600nm.

[0128] Example 18

[0129] The difference between this embodiment and embodiment 1 is that LiMn 0.6 Fe 0.4 The average particle size of PO4 is 600nm, LiMn 0.1 Fe 0.9 The average particle size of PO4 is 200nm.

[0130] Example 19

[0131] The difference between this embodiment and embodiment 1 is that LiMn 0.6 Fe 0.4 The average particle size of PO4 is 400nm, LiMn 0.1 Fe 0.9 The average particle size of PO4 is 200nm.

[0132] Comparative Example 1

[0133] 1) LiMn with an average particle size of 600 nm 0.6 Fe 0.4PO4 is mixed with the conductive agent acetylene black and the binder PVDF in a mass ratio of 88:4:8, and then added into a stirring tank filled with N-methylpyrrolidone solution (NMP) to prepare a slurry with uniform mixing and stable viscosity;

[0134] 2) coating the slurry on one side of an aluminum foil serving as a positive electrode current collector, drying the slurry, and then coating the other side of the aluminum foil with the slurry of corresponding thickness and mass, drying the slurry, to obtain aluminum foil layers coated with the slurry on both sides, rolling the aluminum foil layers until each side has a thickness of 0.14 mm, and then slitting and cutting the aluminum foil layers to obtain positive electrode sheets.

[0135] 3) The positive electrode sheet, the negative electrode sheet described in Example 1, and the isolation film are stacked in order, so that the isolation film is located between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with the electrolyte described in Example 1. After vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0136] Comparative Example 2

[0137] 1) LiMn with an average particle size of 600 nm 0.7 Fe 0.3 PO4 is mixed with conductive agent acetylene black and binder PVDF in a mass ratio of 88:4:8, and then added into a stirring tank filled with N-methylpyrrolidone solution (NMP) to form a slurry with uniform mixing and stable viscosity.

[0138] 2) coating the slurry on one side of the aluminum foil and drying it. After drying, coating the slurry of corresponding thickness and mass on the other surface of the aluminum foil and drying it to obtain aluminum foil layers coated with the slurry on both sides. Rolling the aluminum foil layers until the thickness of each side is 0.14 mm, slitting and cutting the aluminum foil layers to obtain positive electrode sheets.

[0139] 3) The positive electrode sheet, the negative electrode sheet described in Example 1, and the isolation film are stacked in order, so that the isolation film is located between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain a bare battery cell; the bare battery cell is placed in an outer packaging shell, dried, and then injected with the electrolyte described in Example 1. After vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0140] Next, the test involved in the present invention will be described.

[0141] Manganese dissolution test of positive electrode sheet before preparing secondary battery:

[0142] The instrument used is an optical spectrum analyzer (ICP), and the specific test steps are as follows:

[0143] 5 g of each of the positive electrode sheets of Examples 1-19 and Comparative Examples 1-2 were weighed and placed in a beaker containing 100 mL of hydrochloric acid (with a concentration range of 0.01 mol / L), stirred, allowed to stand, and filtered to obtain a sample solution.

[0144] Take 1 mL of sample solution, add a small amount of nitric acid to acidify it, make up to volume, and test it by ICP to obtain the concentration of manganese in the sample solution (unit: mg / L).

[0145] Capacity test:

[0146] The lithium-ion batteries prepared in Examples 1-19 and Comparative Examples 1-2 were subjected to charge and discharge tests using a charge and discharge test system at 25±0.5°C. The charge and discharge conditions were: charge termination voltage 4.3V; discharge termination voltage 2.8V; and charge and discharge current density: 0.1C.

[0147] Cyclic performance test:

[0148] The lithium-ion batteries prepared in Examples 1-19 and Comparative Examples 1-2 were subjected to charge and discharge tests using a charge and discharge test system at 25±0.5°C. The charge and discharge conditions were as follows: charge termination voltage 4.3V; discharge termination voltage 2.8V; charge and discharge current density: 1C. The cycle performance was the cycle performance of 200 cycles under 1C conditions. The results are shown in Table 1.

[0149] Manganese dissolution test of positive electrode material after 200 cycles:

[0150] After the lithium-ion battery was subjected to 200 cycles of the cycle test, the negative electrode of the lithium-ion battery was disassembled and dissolved with a 0.1 mol / L HCl aqueous solution. The amount of manganese ions in the HCl solution was measured by atomic absorption spectrometry (AAS) to compare the cyclic manganese dissolution of the lithium-ion batteries in Examples 1-19 and Comparative Examples 1-2.

[0151] Table 1

[0152]

[0153]

[0154]

[0155] As shown in Table 1, the present embodiment uses a second active material layer with a lower manganese content as a protective outer layer and a first active material layer with a higher manganese content as an inner layer, forming a double-layer composite structure for the positive electrode active material layer. The first manganese iron lithium oxide in the first active material layer and the second manganese iron lithium oxide in the second active material layer both have excellent electrochemical properties, which can improve the cycle performance of the secondary battery. Compared with the comparative example, the double-layer composite structure of the embodiment effectively reduces manganese dissolution and improves the electrochemical performance of the secondary battery.

[0156] Comparing Examples 1 and 2 with Examples 3 and 4, and comparing Examples 5 and 6 with Examples 7 and 8, LiMn 0.2 Fe 0.8 The PO4 protective layer is more effective than LiMn 0.1 Fe 0.9 PO4 protective layer has a good effect, which is different from LiMn 0.2 Fe 0.8 This is related to the smaller grains of PO4. The smaller the grain size, the shorter the lithium ion extraction / embedding path in the active material, which can increase the diffusion rate of lithium ions.

[0157] Comparing Examples 1, 2, 3, and 4 with Examples 5, 6, 7, and 8, respectively, and comparing Examples 3, 7, and 12, it can be seen that the electrochemical performance is better when the Mn:Fe ratio is 6:4, such as the electrochemical performance in terms of gram capacity and capacity retention rate shown in Table 1.

[0158] Comparing Examples 1 and 3 with Examples 2 and 4, respectively, and comparing Examples 3, 4, 9, and 10, it can be seen that the first active material layer with a higher manganese content accounts for 90% of the mass of the positive electrode active material layer, which is an optimal ratio and exhibits better electrochemical performance in terms of gram capacity and capacity retention. A second active material layer with a lower manganese content can improve conductivity but suffers some loss in stability and energy density. A higher mass proportion of the second active material layer in the positive electrode active material layer results in a slight decrease in the gram capacity of the secondary battery. Therefore, the mass proportion of the second active material layer in the positive electrode active material layer should not be too high, as long as it can effectively inhibit manganese dissolution.

[0159] The above is a detailed introduction to a secondary battery and electrical equipment provided in the embodiments of the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for those skilled in the art, based on the ideas of the present application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A secondary battery comprising a positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; The positive electrode active material layer includes a first active material layer and a second active material layer, wherein the first active material layer is disposed between the positive electrode current collector and the second active material layer; The active material of the first active material layer is a first manganese iron lithium oxide, and the active material of the second active material layer is a second manganese iron lithium oxide; Wherein, based on the total molar amount of metal elements other than lithium, the molar percentage of manganese element in the first manganese iron lithium oxide is n1%, and the molar percentage of manganese element in the second manganese iron lithium oxide is n2%, satisfying n1>n2; 30≤n1-n2≤80; Based on the total mass of the first manganese iron lithium oxide and the second manganese iron lithium oxide in the positive electrode active material layer being 100%, the mass percentage of the first manganese iron lithium oxide in the first active material layer is greater than or equal to 80%; the general formula of the first manganese iron lithium oxide is Li a Mn x Fe 1-x PO4, wherein 0.6≤x≤0.8, 0.95≤a≤1.2; the general formula of the second manganese iron lithium oxide is Li b Mn y Fe 1-y PO4, where 0<y≤0.2, 0.95≤b≤1.

2.

2. The secondary battery according to claim 1, wherein The average particle size of the first manganese iron lithium oxide is D1, and the average particle size of the second manganese iron lithium oxide is D2, satisfying D1>D2.

3. The secondary battery according to claim 2, wherein 500nm≤D1≤900nm.

4. The secondary battery according to claim 2, wherein: 200nm≤D2≤500nm.

5. An electrical device, characterized in that: A secondary battery according to any one of claims 1 to 4.

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