Battery

Abstract

The invention discloses a battery, and belongs to the technical field of secondary batteries. The battery comprises a positive plate, the positive plate comprises a positive active material layer, the positive active material layer comprises a positive active material, and the positive active material comprises first lithium iron phosphate and second lithium iron phosphate. The positive active material of the battery comprises lithium iron phosphate, the capacity retention ratio of the battery at-10 DEG C is set as a, the discharge slope at 2.5 V in a discharge curve when the battery discharges at the rate of 0.3 C is set as b, and a / b is controlled in a suitable range, so that the battery has good cycle performance, and meanwhile, the capacity retention ratio of the battery at-10 DEG C is set as a, and the discharge slope at 2.5 V in a discharge curve at the rate of 0.3 C is set as b. The battery has good rate capability; by compounding two kinds of lithium iron phosphate with different particle sizes, a continuous conductive network can be formed on the positive electrode active material layer, so that the structural stability of the material is further improved, the contact resistance of a positive electrode interface is reduced, and the rate capability and the cycle performance of the battery are comprehensively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a battery. BACKGROUND

[0002] Lithium iron phosphate battery is a lithium ion battery using lithium iron phosphate (LiFePO4, LFP for short) as a positive active material. Lithium iron phosphate battery has the characteristics of high safety, long service life, large capacity, good high-temperature performance, no memory effect and no pollution, and has been widely used in electric vehicles, energy storage systems and other fields.

[0003] However, the existing lithium iron phosphate battery has poor cycle performance, which is specifically manifested in that the battery capacity decreases at low temperature, the cycle life decreases, and even the battery cannot work normally at low temperature (such as below-20℃).

[0004] Therefore, it is necessary to provide a technical solution to solve the above problems. SUMMARY

[0005] Based on the defects of the prior art, the purpose of the present application is to provide a battery with good cycle performance.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] A battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material layer, the positive electrode active material layer comprises a positive electrode active material, the positive electrode active material comprises first lithium iron phosphate and second lithium iron phosphate, the average particle size of the first lithium iron phosphate is D1, the average particle size of the second lithium iron phosphate is D2, 0.8μm≤D1≤2μm, and 0.1μm≤D2≤0.75μm.

[0008] The capacity retention rate of the battery is a, wherein the capacity retention rate of the battery refers to the percentage of the discharge capacity of the battery measured after the battery is placed at-10℃ for 6h to the discharge capacity of the battery at 25℃.

[0009] The absolute value of the discharge slope of the battery at 2.5V in the discharge curve of the battery is |b|, wherein the discharge curve of the battery is the discharge curve measured by discharging the battery at-10℃ for 6h at a rate of 0.3C.

[0010] a and |b| satisfy: 18.5≤a / |b|≤82.

[0011] Compared with the prior art, the present application has the following beneficial effects:

[0012] The positive active material of the battery described in the present application comprises lithium iron phosphate, the capacity retention rate of the battery at-10℃ is set as a, the absolute value of the discharge slope at 2.5V in the discharge curve of the battery when the battery is discharged at a rate of 0.3C is set as |b|, and a / |b| is controlled within a suitable range, so that the battery has good rate performance while ensuring good cycle performance.

[0013] The present application can form a continuous conductive network in the positive active material layer by compounding two different particle sizes of lithium iron phosphate, further improve the structural stability of the material, reduce the positive electrode interface contact resistance, and comprehensively improve the rate performance and cycle performance of the battery. DETAILED DESCRIPTION

[0014] In order to better illustrate the purpose, technical scheme and advantages of the present application, the present application will be further described below in combination with specific examples and comparative examples, the purpose of which is to understand the content of the present application in detail, rather than to limit the present application. All other examples obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0015] In the present application, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating the number of the technical features indicated. Therefore, the features limited by "first" and "second" can include at least one of the features, explicitly or implicitly.

[0016] A battery comprises a positive electrode sheet, the positive electrode sheet comprises a positive active material layer, the positive active material layer comprises a positive active material, the positive active material comprises first lithium iron phosphate and second lithium iron phosphate, the average particle size of the first lithium iron phosphate is D1, the average particle size of the second lithium iron phosphate is D2, 0.8μm≤D1≤2μm, 0.1μm≤D2≤0.75μm;

[0017] The capacity retention rate of the battery is a, the capacity retention rate of the battery refers to the percentage of the discharge capacity measured after the battery is placed at-10℃ for 6h to the discharge capacity of the battery at 25℃;

[0018] The absolute value of the discharge slope at 2.5V in the discharge curve of the battery is |b|, the discharge curve of the battery is the discharge curve measured when the battery is discharged at a rate of 0.3C after being placed at-10℃ for 6h;

[0019] a and |b| satisfy: 18.5≤a / |b|≤82.

[0020] The application sets the capacity retention rate of the battery at -10℃ as a, sets the absolute value of the discharge slope at 2.5V in the discharge curve when the battery is discharged at a rate of 0.3C as |b|, controls a / |b| in a suitable range, so that the battery has good cycle performance and good rate performance at the same time.

[0021] The application can improve the compaction density of the positive active material layer, reduce the side reaction of the electrolyte and the positive electrode interface and the electrode expansion stress by controlling the average particle size of the first lithium iron phosphate in the positive active material layer to be in the range of 0.8μm-2μm; the application can shorten the lithium ion diffusion path and reduce the ion migration resistance by controlling the average particle size of the second lithium iron phosphate in the positive active material layer to be in the range of 0.1μm-0.75μm; the first lithium iron phosphate and the second lithium iron phosphate are compounded to form a continuous conductive network in the positive active material layer, further improve the structural stability of the material, reduce the positive electrode interface contact resistance, and comprehensively improve the rate performance and cycle performance of the battery.

[0022] For example, the average particle size of the first lithium iron phosphate can be 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2μm or a range formed by any two of the above values.

[0023] For example, the average particle size of the second lithium iron phosphate can be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.75μm or a range formed by any two of the above values.

[0024] For example, a / |b| can be 18.5, 20, 25, 29.5, 30, 35, 40, 44, 45, 50, 55, 60, 65, 70, 75, 80, 82 or a range formed by any two of the above values.

[0025] In some embodiments, a and |b| satisfy: 29.5≤a / |b|≤44. When a and |b| meet this condition, the cycle performance and rate performance of the battery are better.

[0026] In some embodiments, 45%≤a≤90%, and further preferably, 65%≤a≤75%.

[0027] By controlling 'a' within a suitable range, this application can reduce the lithium-ion diffusion resistance at low temperatures, ensure the reactivity of the active material, and improve the lithium-ion insertion / extraction efficiency, thereby ensuring the specific capacity of the battery. In addition, it can also avoid the irreversible thickening of the SEI film caused by lithium deposition on the negative electrode surface due to lithium-ion diffusion obstruction, reduce the consumption of active lithium, and thus improve the cycle performance of the battery.

[0028] For example, 'a' can be 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or a range of any two sets of values.

[0029] In some embodiments, 0.01 ≤ |b| ≤ 0.03, and more preferably, 0.015 ≤ |b| ≤ 0.025.

[0030] This application controls |b| within a suitable range, resulting in a smooth and appropriate discharge curve for the battery, minimal internal polarization, and a high lithium-ion diffusion rate. This ensures that the positive electrode active material is fully utilized, thereby guaranteeing the battery's specific capacity and improving its rate performance. It also avoids or reduces the oxidative decomposition of the electrolyte under high voltage due to slow battery discharge, thereby reducing the consumption of active lithium and improving the battery's cycle performance.

[0031] For example, |b| can be 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019, 0.02, 0.021, 0.022, 0.023, 0.024, 0.025, 0.026, 0.027, 0.028, 0.029, 0.03, or a range consisting of any two sets of values.

[0032] In some embodiments, the ratio of the first lithium iron phosphate to the second lithium iron phosphate in the positive electrode active material layer is 0.2 to 0.5.

[0033] For example, the ratio of the first lithium iron phosphate to the second lithium iron phosphate in the positive electrode active material layer can be 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, or a range of any two sets of values.

[0034] The first lithium iron phosphate can improve the compaction density of the positive electrode active material layer, thereby improving the mechanical strength of the positive electrode sheet, reducing the volume change of the positive electrode sheet during charging and discharging, and prolonging the cycle life of the battery, but it will prolong the diffusion path of lithium ions, increase the migration resistance of lithium ions, and reduce the rate performance of the battery.

[0035] The second lithium iron phosphate can provide a short-range lithium ion diffusion path, improve the lithium ion migration rate, and improve the rate performance of the battery, but its average particle size is larger, so its specific surface area is larger, which can easily cause side reactions between the positive electrode active material layer and the electrolyte, resulting in a decrease in the cycle performance of the battery.

[0036] Controlling the ratio of the first lithium iron phosphate to the second lithium iron phosphate in the positive electrode active material layer to be within the range of 0.2 to 0.5 can balance the diffusion efficiency of lithium ions in the positive electrode active material layer and the structural stability of the positive electrode active material layer, and avoid the performance short board of the battery caused by a single particle.

[0037] In some embodiments, the first lithium iron phosphate is coated with a first carbon layer, and the thickness of the first carbon layer is 1 nm to 10 nm.

[0038] For example, the thickness of the first carbon layer can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or a range formed by any two of the above values.

[0039] In some embodiments, the second lithium iron phosphate is coated with a second carbon layer, and the thickness of the second carbon layer is 1 nm to 10 nm.

[0040] For example, the thickness of the second carbon layer can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or a range formed by any two of the above values.

[0041] The first carbon layer coated with the first lithium iron phosphate and the second carbon layer coated with the second lithium iron phosphate can form a conductive network on the surface of the lithium iron phosphate particles, improve the conductivity of the positive electrode active material, reduce the charge transfer resistance, and also inhibit the agglomeration of the particles and shorten the diffusion path of lithium ions. In addition, the carbon layers can also optimize the interface between the positive electrode and the electrolyte, and inhibit the active lithium loss caused by the continuous thickening of the SEI film during the cycle. Therefore, by controlling the first carbon layer and the second carbon layer within a certain range, the rate performance and cycle performance of the battery can be improved.

[0042] In some embodiments, the porosity of the positive electrode active material layer is 22% to 30%.

[0043] The application can appropriately increase the porosity of the positive active material layer, increase the electrolyte infiltration channels in the positive active material layer, reduce the migration resistance of lithium ions, and ensure the loading amount of the active material in the positive plate, thereby comprehensively improving the rate performance and cycle performance of the battery.

[0044] For example, the porosity of the positive active material layer can be 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range formed by any two of the above values.

[0045] In some embodiments, the positive active material further comprises a doping element, the doping element comprising at least one of V, Ni, Ce, Mg, and Zr, and the content of the doping element in the positive active material is 700 ppm to 3000 ppm. The inventors have found that the doping element widens the lithium ion diffusion channel by introducing vacancies or changing the lattice parameters, thereby shortening the ion migration path, and can also inhibit Fe 2+ oxidation and phase change stress, thereby significantly improving the structural integrity of the positive active material particles during the charge and discharge cycles of the battery.

[0046] In some embodiments, the molar ratio of Fe to P in the positive active material is (0.96-0.98):1. The inventors have found that if the molar ratio of Fe to P in the positive active material is too large, the dissolution of Fe 3+ will be intensified during the cycle, leading to the collapse of the positive structure and the decline of the cycle performance of the battery; if the molar ratio of Fe to P in the positive active material is too small, Li3PO4 and other insulating phases may be generated, hindering the electron transmission, increasing the internal resistance of the battery, and causing the capacity to decay rapidly during high-rate charge and discharge.

[0047] In some embodiments, the preparation method of the positive active material comprises but is not limited to the following steps:

[0048] The lithium source, the iron phosphate precursor, and the doping source are mixed, and the carbon source and the dispersant are added and mixed to obtain a mixture; the mixture is ground and dried to obtain a precursor powder A; the obtained precursor powder A is calcined under the protection of an inert atmosphere, and then magnetically separated and crushed to obtain a first lithium iron phosphate particle material, the first lithium iron phosphate particle material comprising a first lithium iron phosphate and a first carbon layer coated on the surface of the first lithium iron phosphate;

[0049] Mixing a lithium source, an iron phosphate precursor and a doping source, adding a carbon source and a dispersant to obtain a mixture; grinding and drying the mixture to obtain a precursor powder B; calcining the precursor powder B under the protection of an inert atmosphere, then removing magnetism and crushing to obtain a second lithium iron phosphate particle material, wherein the second lithium iron phosphate particle material comprises a second lithium iron phosphate and a second carbon layer coated on the surface of the second lithium iron phosphate.

[0050] Mixing the first lithium iron phosphate particle material and the second lithium iron phosphate particle material to obtain a positive electrode active material.

[0051] The grinding method includes but is not limited to at least one of ball milling and sand milling.

[0052] In the preparation of the first lithium iron phosphate particle material, the molar ratio of the doping source to the iron phosphate precursor is (0.001-0.003):1; the grinding conditions are that the particle size of the grinding medium is 0.3-0.35 mm, the mass ratio of the grinding medium to the mixture is (6-10):1, the grinding speed is 1000-2000 r / min, and the grinding time is 1-4 h; the calcining conditions are that the calcining time is 10-20 h and the calcining temperature is 750-850℃; the particle size of the first lithium iron phosphate can be controlled by adjusting at least one of the particle size of the grinding medium, the mass ratio of the grinding medium to the mixture, the grinding speed, the grinding time, the calcining time and the calcining temperature; the thickness of the first carbon layer can be controlled by adjusting at least one of the addition amount of the carbon source, the calcining time and the calcining temperature; and the doping content of the doping element in the first lithium iron phosphate can be controlled by adjusting at least one of the addition amount of the doping source, the calcining time and the calcining temperature.

[0053] In the preparation of the second lithium iron phosphate particle material, the molar ratio of the doping source to the iron phosphate precursor is (0.001-0.003):1; the grinding conditions are that the particle size of the grinding medium is 0.2-0.25 mm, the mass ratio of the grinding medium to the mixture is (6-10):1, the grinding speed is 1000-2000 r / min, and the grinding time is 1-4 h; the calcining conditions are that the calcining time is 6-12 h and the calcining temperature is 650-750℃; the particle size of the second lithium iron phosphate can be controlled by adjusting at least one of the particle size of the grinding medium, the mass ratio of the grinding medium to the mixture, the grinding speed, the grinding time, the calcining time and the calcining temperature; the thickness of the second carbon layer can be controlled by adjusting at least one of the addition amount of the carbon source, the calcining time and the calcining temperature; and the doping content of the doping element in the second lithium iron phosphate can be controlled by adjusting at least one of the addition amount of the doping source, the calcining time and the calcining temperature.

[0054] In this application, the particle size of the first lithium iron phosphate and the particle size of the second lithium iron phosphate in the positive electrode active material layer, as well as the mass ratio of the first lithium iron phosphate particle material and the second lithium iron phosphate particle material, can be adjusted to control the quantity ratio of the first lithium iron phosphate and the second lithium iron phosphate in the positive electrode active material layer.

[0055] In the above-mentioned method for preparing positive electrode active material, the lithium source may include at least one of an inorganic salt containing Li, a hydroxide containing Li, and an oxide containing Li; the doping source may include at least one of an inorganic salt containing a dopant element, a hydroxide containing a dopant element, and an oxide containing a dopant element.

[0056] This application does not specifically limit the method of controlling 'a'. For example, 'a' can be controlled by adjusting the calcination temperature, grinding time, etc., in the preparation method of the positive electrode active material. Specifically, the higher the calcination temperature, the larger the particle size of the positive electrode active material, the longer the diffusion path of lithium ions inside the particles, and the smaller 'a'; the longer the grinding time, the smaller 'a'.

[0057] This application does not specifically limit the method of controlling |b|. For example, it can be controlled by adjusting the content of doping elements and the Fe / P ratio of the precursor. The higher the content of doping elements, the more stable the positive electrode active material and the larger |b|. The larger the Fe / P ratio of the precursor and the smaller the particle size of the positive electrode active material, the steeper the discharge curve and the smaller |b|.

[0058] In some embodiments, the positive electrode active material layer further includes a conductive agent, and the conductive agent in the positive electrode active material layer includes at least one of acetylene black, Ketjen black, Super P, carbon nanotubes, conductive graphite, graphene, and conductive carbon fiber.

[0059] In some embodiments, the mass percentage of the conductive agent in the positive electrode active material layer is 0.05–10%.

[0060] For example, the mass percentage of the conductive agent in the positive electrode active material layer can be 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two sets of values ​​therein.

[0061] In some embodiments, the positive electrode active material layer further includes a binder, wherein the binder in the positive electrode active material layer includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyimide, and polyamide-imide.

[0062] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 0.1% to 10%.

[0063] For example, the mass percentage of the binder in the positive electrode active material layer can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values.

[0064] In some embodiments, the positive electrode sheet includes a positive current collector, and at least one surface of the positive current collector is provided with the positive active material layer.

[0065] The positive current collector can be a metal foil or a composite current collector. For example, the metal foil may include aluminum foil. The composite current collector may include a substrate layer and a metal layer formed on at least one surface of the substrate layer. The composite current collector can be obtained by forming a metal material on a polymer substrate. The substrate layer is a polymer material, such as at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE). The metal layer may be made of at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy.

[0066] In this application, the positive electrode sheet can be prepared according to conventional methods in the art, for example, by dispersing the positive electrode active material, conductive agent and binder in a solvent to obtain a positive electrode slurry, coating the positive electrode slurry onto at least one surface of the positive electrode current collector, and then drying, rolling and cutting to obtain the positive electrode sheet.

[0067] In some embodiments, the battery further includes an electrolyte comprising a solvent and a lithium salt, wherein the concentration of the lithium salt in the electrolyte is 0.5 mol / L to 2 mol / L. The inventors have discovered that controlling the concentration of the lithium salt in the electrolyte within a suitable range can, on the one hand, reduce the viscosity of the electrolyte, promote lithium-ion diffusion, and improve the rate performance of the battery; on the other hand, it can reduce free solvent molecules, significantly reduce solvent co-intercalation and electrolyte decomposition, thereby suppressing the loss of active lithium caused by repeated SEI film rupture and regeneration, and improving the cycle performance of the battery. In some embodiments, the solvent in the electrolyte includes at least one selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, ethyl propionate, ethylene carbonate, propylene carbonate, fluoroethylene carbonate, ethyl difluorocarbonate, dimethyl sulfate, adiponitrile, and ethylene glycol dimethyl ether.

[0068] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiN(SO2F)2), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(SO2CF3)2), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium dioxolaneborate (LiB(C2O4)2), and lithium difluorooxolaneborate (LiBF2C2O4).

[0069] In some embodiments, the battery further includes a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, SiO2. x Silicon-carbon, Li4Ti5O 12 At least one of the following, wherein the compaction density of the coating on the negative electrode sheet is 1.5 g / cm³. 3 ~1.7g / cm 3 .

[0070] For example, the compaction density of the coating on the negative electrode sheet can be 1.5 g / cm³. 3 1.55g / cm 3 1.6g / cm 3 1.65g / cm 3 1.7g / cm 3 Or a range consisting of any two sets of values.

[0071] The inventors discovered through research that controlling the compaction density of the coating on the negative electrode sheet within a suitable range can, on the one hand, ensure that the electrolyte fully wets the negative electrode sheet, keep the lithium-ion diffusion channels unobstructed, and improve the rate performance of the battery; on the other hand, it can prevent excessive pressure from causing the active material particles to break, the SEI film to repeatedly rupture and regenerate, accelerate the loss of active lithium, and improve the cycle performance of the battery.

[0072] In some embodiments, the negative electrode active material layer further includes a conductive agent, which includes at least one of acetylene black, Ketjen black, Super P, carbon nanotubes, conductive graphite, graphene, and conductive carbon fiber.

[0073] In some embodiments, the negative electrode active material layer further includes a binder, wherein the binder in the negative electrode active material layer includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyethylene oxide, styrene-butadiene rubber, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyimide, and polyamide-imide.

[0074] In some embodiments, the negative electrode active material layer comprises the following components by mass percentage: 90.0%–98.0% negative electrode active material, 0.1%–1% conductive agent, and 0.5%–10% binder.

[0075] For example, in the negative electrode active material layer, the mass percentage of the negative electrode active material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or any two of these values; the mass percentage of the conductive agent can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values; the mass percentage of the binder can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values.

[0076] In some embodiments, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector.

[0077] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, the metal foil may include copper foil. The composite current collector may include a substrate layer and a metal layer formed on at least one surface of the substrate layer; the composite current collector may be obtained by forming a metal material on a polymer substrate; the substrate layer is a polymer material, such as at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE); the metal layer may be made of at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy.

[0078] In this application, the negative electrode sheet can be prepared according to conventional methods in the art, for example, by dispersing the negative electrode active material, conductive agent and binder in a solvent to obtain a negative electrode slurry, coating the negative electrode slurry onto at least one surface of the negative electrode current collector, and then drying, rolling and cutting to obtain the negative electrode sheet.

[0079] In one embodiment, the separator membrane includes a base membrane, which includes at least one of polyethylene, polypropylene, polyamide, and aramid.

[0080] Optionally, the diaphragm may further include an adhesive layer and / or a ceramic layer. The adhesive layer may be made of at least one of polyvinylidene fluoride, polymethyl methacrylate, aramid, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, or polyaniline; the ceramic layer may be made of at least one of boehmite, alumina, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, aluminum nitride, and magnesium nitride.

[0081] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments.

[0082] Unless otherwise specified, all components and raw materials used in the embodiments and comparative examples of this application are commercially available, and the same type of components and raw materials are used in each parallel experiment.

[0083] Example 1

[0084] An embodiment of the lithium-ion battery described in this application includes a method for preparing the lithium-ion battery comprising the following steps:

[0085] S1. Preparation of positive electrode active material:

[0086] S11. Lithium carbonate, iron phosphate precursor, and V-source vanadium pentoxide are mixed. The molar ratio of lithium carbonate to iron phosphate precursor is 1:2. The molar ratio of V-source vanadium pentoxide to iron phosphate precursor is shown in Table 1. Then, a carbon source (glucose) and PEG are added to disperse and prepare a mixture (the mass content of carbon source in the mixture is shown in Table 1). Then, the mixture and ethanol are added to a ball mill at a mass ratio of 1:2. Zirconium balls with a diameter of 0.30 mm are added to the ball mill as abrasive at a ball-to-material ratio (i.e., the mass ratio of the mixture to the zirconium balls) of 8:1. The ball milling is carried out at a speed of 1400 rpm for the milling time shown in Table 1. Then, the mixture is spray-dried to obtain precursor powder A. The precursor powder A is calcined in a nitrogen atmosphere for 12 h at the calcination temperature shown in Table 1. Then, the mixture is demagnetized and subjected to air jet milling at a pressure of 350 kPa to obtain the first lithium iron phosphate particle material.

[0087] S12. Lithium carbonate, iron phosphate precursor, and V-source vanadium pentoxide are mixed according to the stoichiometric ratio. The molar ratio of lithium carbonate to iron phosphate precursor is 1:2. The molar ratio of V-source vanadium pentoxide to iron phosphate precursor is shown in Table 1. Then, carbon source (glucose) and PEG are added to disperse and prepare a mixture (the mass content of carbon source in the mixture is 8%). Then, the mixture and ethanol are added to a ball mill at a mass ratio of 1:2. Zirconium balls with a diameter of 0.20 mm are added to the ball mill as abrasive according to a ball-to-material ratio (i.e., the mass ratio of mixture to zirconium balls) of 8:1. The ball milling is performed at a speed of 1400 rpm. The ball milling time is shown in Table 1. Spray drying is performed to obtain precursor powder B. The obtained precursor powder B is calcined under a nitrogen atmosphere for 8 hours. The calcination temperature is shown in Table 1. Then, it is demagnetized and subjected to air jet milling under a pressure of 350 kPa to obtain the second lithium iron phosphate particle material.

[0088] S13. The first lithium iron phosphate particle material and the second lithium iron phosphate particle material are mixed according to the mass ratio shown in Table 1. After being evenly dispersed, they are calcined at 765°C for 10 hours under a nitrogen atmosphere and then subjected to air jet pulverization at a pressure of 350 kPa to obtain the positive electrode active material.

[0089] S2, Preparation of the positive electrode:

[0090] The positive electrode active material, conductive agent Super P, carbon nanotubes, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96.5:0.5:1:2.0. The solvent NMP was added, and the mixture was stirred under vacuum until the system was homogeneous to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated on two opposite surfaces of the positive electrode current collector aluminum foil along its thickness direction. After being dried at room temperature, it was transferred to an oven for further drying. Then, it was rolled and cut to obtain the positive electrode sheet.

[0091] S2, Preparation of the negative electrode:

[0092] Artificial graphite, conductive agent Super P, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 96.4:0.6:1.6:1.4. Deionized water was added, and the mixture was stirred under vacuum until the system was homogeneous to obtain a negative electrode slurry. The negative electrode slurry was coated on one side of a copper foil, dried at room temperature, and then transferred to an oven for further drying. After rolling and cutting, the negative electrode sheet was obtained.

[0093] S3. Preparation of electrolyte:

[0094] EC, EMC and DMC were mixed in a mass ratio of 3:4:3 to obtain a solvent. The fully dried lithium salt LiPF6 was dissolved in the obtained organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0095] S4. Preparation of the diaphragm:

[0096] PE is selected as the base film, Al2O3 is selected as the coating, and polyvinylidene fluoride (PVDF) is selected as the adhesive layer. An adhesive layer is set on one side of the base film, and a coating layer and an adhesive layer are set on the other side in sequence. The thickness of the adhesive layer is 2μm, the thickness of the base film is 9μm, and the thickness of the coating layer is 3μm.

[0097] S5. Stack the obtained positive electrode sheet, separator, and negative electrode sheet in sequence, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play a role in isolation, and then wind them to obtain a bare cell; place the bare cell in an outer packaging shell, dry it, inject electrolyte, and then go through vacuum sealing, standing, formation, shaping and other processes to obtain a lithium-ion battery.

[0098] Examples 2-17 and Comparative Examples 1-6

[0099] The differences between Examples 2-17 and Comparative Examples 1-6 and Example 1 are as follows:

[0100] Examples 2-16 and Comparative Examples 1-6 changed at least one of the following parameters in step S11: ball milling time, calcination temperature of precursor powder A, molar ratio of V source to iron phosphate precursor in step S11, and mass content of carbon source in the mixture in step S11. See Table 1 for details.

[0101] Examples 2-16 and Comparative Examples 1 and 3-6 also changed at least one of the following parameters in step S12: ball milling time, calcination temperature of precursor powder B, molar ratio of V source to iron phosphate precursor in step S12, and mass content of carbon source in the mixture in step S12. See Table 1 for details.

[0102] In Example 17, the V source was not used in either step S11 or step S12;

[0103] In Comparative Example 2, steps S12 and S13 were not implemented. In step S2, the first lithium iron phosphate obtained in step S1 was directly used as the positive electrode active material.

[0104] The mass ratios of the first lithium iron phosphate and the second lithium iron phosphate in step S13 of Examples 2-17 and Comparative Examples 1, 3-6 are shown in Table 1.

[0105] Performance testing

[0106] Using the batteries provided in the above embodiments and comparative examples as test objects, the following tests were conducted:

[0107] 1. The test methods for the particle size of lithium iron phosphate, the particle size of lithium iron phosphate, and the ratio of the number of lithium iron phosphate to lithium iron phosphate are as follows:

[0108] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for discharge treatment. Then the positive electrode was disassembled and soaked in DMC solution at 25°C for 4 hours, vacuum dried, and a 1cm×1cm area in the middle region of the electrode was cut as a test piece.

[0109] Focused ion beam scanning electron microscopy (SEM-FIB) was used, with the following test conditions: cutting voltage 30 kV, cutting beam current 3 nA, cutting range 70 × 50 × 9 μm, interlayer spacing 20 mm, imaging voltage 2 kV, imaging beam current 0.8 nA, resolution 3072 × 2048, and working distance 3.5 mm. Avizo was used to calculate the number and size of particles within a 1 / 3 region, specifically a 23.98 × 23.98 × 7.96 μm area. Particles were categorized according to the size of the first lithium iron phosphate (LFP) particles and the size and morphology of the second LFP particles. The ratio of the number of first LFP particles to the number of second LFP particles was calculated. The particle sizes of all first LFP particles and second LFP particles within this region were statistically analyzed, and the average values ​​were taken to obtain the average particle size of the first LFP (denoted as D1) and the average particle size of the second LFP (denoted as D2).

[0110] 2. The testing methods for the thickness of the first carbon layer and the thickness of the second carbon layer are as follows:

[0111] 1) Battery disassembly and electrode cleaning: Disassemble the battery in the empty state, remove one positive electrode, and then soak it in an inert solvent (such as DMC, i.e., dimethyl carbonate) in a glove box for 2 hours or clean the electrode with ultrasound (avoid contact with water and oxygen, and the cleaning time is usually 10-30 seconds) to remove soluble electrolyte residue, and then vacuum dry it.

[0112] 2) TEM sample preparation: The particles are scraped in a glove box, and then the powder is dispersed in ethanol or methanol and sonicated for more than 15 minutes to ensure uniform dispersion; the dispersion is dropped onto a copper grid of a carbon support film, dried, and then tested.

[0113] 3) TEM test carbon layer thickness: Find particles with a diameter of 0.8μm to 2μm in TEM, take pictures of the carbon coating on the surface, select an electron beam acceleration voltage of 200kV, take a magnification of 300-500KX, and take the width at the edge and the junction of light and dark colors, which is the thickness of the first carbon layer.

[0114] Following the above method, particles with a diameter in the range of 0.1 μm to 0.75 μm were found in TEM, and the thickness of the second carbon layer was tested.

[0115] 3. The method for testing the porosity of the positive electrode active material layer is as follows:

[0116] 1) Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V, take the empty battery, disassemble the positive electrode, soak the positive electrode in dimethyl carbonate (DMC) solution for 4 hours; and air dry.

[0117] 2) Subsequently, the positive electrode sheet was cut into circular sheets with a diameter of 12mm using an electrode sheet punching machine. Simultaneously, the thickness of the positive electrode sheet and the positive current collector was measured using a thickness gauge, denoted as h1 and h2 respectively. The mass was weighed using a balance with an accuracy of 0.00001g and recorded as m1. According to the formula V=π×r 2 Calculate the volume V of the cut positive electrode sheet using the formula ×(h1-h2); immerse the positive electrode sheet in a sealed container with a certain volume of hexadecane for 1 hour (the volume of hexadecane in the sealed solution is not required, but the amount must be sufficient to completely submerge the electrode sheet); after 1 hour, remove the electrode sheet with tweezers and place it on filter paper to absorb dry until constant weight (generally, 1 hour is sufficient to absorb dry to constant weight). Weigh it using a balance and record the mass as m2. Calculate the porosity using the formula X / v, where X=(m2-m1) / ρ, and ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0118] 4. The mass content of doped elements and the Fe / P ratio in the positive electrode active material layer are tested using the following methods:

[0119] Discharge the battery to the lower limit voltage (2.5V) at 0.33C, disassemble the empty lithium-ion battery, disassemble the lithium-ion battery to obtain the electrode, soak the positive electrode in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, and air dry it at room temperature with humidity ≤15%.

[0120] Based on the positive electrode, the positive electrode active material layer on the surface of the current collector is scraped off, calcined at 400℃ for 3 hours, and then washed and dried to obtain powder;

[0121] Accurately weigh a certain amount of powder, disperse it in 20ml of water, add 10ml of nitric acid, mix well, and then heat it until the powder dissolves. Dilute the material with water to 100mL to obtain the test solution. Perform ICP testing on the test solution. Before testing, a standard solution must be prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard. The 1000 mg / L standard solution was diluted with deionized water to different concentrations (generally 0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL). The element detection wavelength was selected, and the experimental conditions were set: according to the characteristics of the sample and the element to be detected, the appropriate ICP instrument operating conditions were set: gas flow rate 0.5 L / min, power 1150 W, and the element detection wavelength was selected, depending on the element to be tested (e.g., Fe wavelength 259.94 nm). The ICP testing software's self-analysis function can read the mass content of dopant element V, iron, and phosphorus in the sample, and calculate the molar ratio of iron and phosphorus in the sample (denoted as Fe / P).

[0122] 5. Absolute values ​​of battery capacity retention rate and discharge slope:

[0123] 5.1 Capacity Retention Rate: Using the batteries from the above embodiments and comparative examples as test objects, the specific test steps are as follows:

[0124] (1) At 25℃, let stand for 2 hours;

[0125] (2) Vent at 25℃ and 0.3C for 30 minutes;

[0126] (3) Charge at 25℃ and 0.3C to 3.65V / 0.05C, then let stand for 30 minutes;

[0127] (4) Discharge to 2.5V at 0.3C at 25℃ and let stand for 30 minutes;

[0128] (5) Repeat steps 3)-4) for 3 cycles and record the discharge capacity of the third cycle as Q1;

[0129] (6) Charge at 0.3C to 3.65V / 0.05C full charge, then let stand for 30 minutes;

[0130] (7) Pause, cool to -10℃, and let stand for 6 hours;

[0131] (8) The capacity of the low-temperature discharge from 0.3C to 2.0V is recorded as Q2, and the low-temperature capacity retention rate is Q2 / Q1*100%.

[0132] 5.2 Discharge Slope: Using the batteries from the above embodiments and comparative examples as test objects, the specific test steps are as follows:

[0133] (1) Plot the voltage and capacity from the original data of low-temperature discharge at -10℃, with the voltage range being 2.8V-2.0V and the corresponding capacity data;

[0134] (2) Plot a scatter plot with smooth lines, with capacity as the horizontal axis and voltage as the vertical axis;

[0135] (3) Select the trend line in the chart element, add a trend line and add a formula. The absolute value of the slope in the formula is denoted as |b|.

[0136] 6. The test method for the cycle performance of the battery is as follows:

[0137] Using the batteries from the above embodiments and comparative examples as test objects, the specific test steps are as follows:

[0138] The test environment temperature was 25℃. After the battery was left to stand for 2 hours, it was discharged to 2.5V at 0.3C.

[0139] 1) Charge at a constant current rate of 1C to 3.65V, and then charge at a constant voltage until the current drops to 0.05C;

[0140] 2) Let stand for 30 minutes;

[0141] 3) Discharge to 2.5V at a 1C rate;

[0142] 4) Let stand for 30 minutes.

[0143] Perform cycle tests according to steps 1)-4) until the capacity of the lithium-ion battery is less than 80% of the initial capacity, and record the number of cycles.

[0144] 7. The test method for the rate performance of a battery is as follows:

[0145] The assembled single-cell full cell underwent a capacity test, as detailed below:

[0146] (1) Vent at 25℃ and 0.3C for 30 minutes;

[0147] (2) Charge at 25℃ and 0.3C to 3.65V / 0.05C, then let stand for 30 minutes;

[0148] (3) Discharge to 2.5V at 0.3C at 25℃ and let stand for 30 minutes;

[0149] (4) Repeat steps 3)-4) for 2 cycles;

[0150] (5) Charge at 25℃ with 1C until fully charged to 3.65V / 0.05C, then let stand for 30 minutes;

[0151] (6) At 25℃, discharge at 1C to 2.5V, let stand for 30 minutes and record the 1C discharge capacity as Q3;

[0152] (7) The discharge capacity is calculated using the following formula: 1C discharge capacity = Q3 / [(weight of positive electrode sheet - weight of foil) × mass percentage of positive electrode active material].

[0153] The test results are shown in Tables 1 and 2 below.

[0154] Table 1

[0155]

[0156]

[0157] Table 2

[0158]

[0159]

[0160] As can be seen from Tables 1 and 2, each embodiment of this application uses two different particle sizes of lithium iron phosphate compounded together, and by comprehensively controlling the capacity retention rate a of the battery and the absolute value of the discharge slope at 2.5V in the discharge curve when the battery is discharged at a rate of 0.3C, |b| is controlled within a suitable range. This ensures that the battery has good cycle performance while also having good rate performance, so that the 1C discharge capacity of the battery is not less than 125mAh / g and the number of cycles at 25℃ is not less than 800.

[0161] When a / |b| is preferably in the range of 29.5 to 44, the battery has better cycle performance and rate performance.

[0162] Compared with the embodiments, in Comparative Example 1, a / |b| is too large, and a≤45%, |b|≤0.01. Although the battery has good rate performance, the cycle performance of the battery is significantly deteriorated, with only 564 cycles at 25°C.

[0163] Compared with the embodiments, the positive electrode active material in Comparative Example 2 only uses lithium iron phosphate particles, which lengthens the lithium-ion transport path and a≤45%, |b|≥0.03, a / |b| is too small. Although the battery has good cycle performance, the rate performance of the battery is significantly reduced, and the 1C discharge capacity of the battery is only 95.3mAh / g.

[0164] Compared with the embodiments, in Comparative Example 3, a / |b| is too large and |b|≤0.01. Although the battery has good rate performance, the cycle performance of the battery is significantly deteriorated, with only 619 cycles at 25°C.

[0165] Compared with the embodiments, in Comparative Example 4, a / |b| is too small, and a≤45%, |b|≥0.03, resulting in a significant decrease in the rate performance of the battery, with the 1C discharge capacity of the battery being only 95.8mAh / g.

[0166] Compared with the embodiments, although in Comparative Example 5, a satisfies the range of 45% to 90% and |b| also satisfies the range of 0.01 to 0.03, a / |b| is too small, and the rate performance of the battery is significantly reduced.

[0167] Compared with the embodiments, although in Comparative Example 6, a satisfies the range of 45% to 90% and |b| also satisfies the range of 0.01 to 0.03, a / |b| is too large, and the cycle performance of the battery is significantly reduced.

[0168] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A battery, characterized in that, The device includes a positive electrode sheet, which includes a positive electrode active material layer, which includes a positive electrode active material, which includes a first lithium iron phosphate and a second lithium iron phosphate. The average particle size of the first lithium iron phosphate is D1, and the average particle size of the second lithium iron phosphate is D2, where 0.8 μm ≤ D1 ≤ 2 μm and 0.1 μm ≤ D2 ≤ 0.75 μm. The capacity retention rate of the battery is a, which refers to the percentage of the discharge capacity measured after the battery is placed at -10℃ for 6 hours to the discharge capacity of the battery at 25℃. The absolute value of the discharge slope at 2.5V in the discharge curve of the battery is |b|. The discharge curve of the battery is the discharge curve measured by discharging the battery at a rate of 0.3C after placing it at -10℃ for 6 hours. a and |b| satisfy: 18.5≤a / |b|≤82.

2. The battery as described in claim 1, characterized in that, a and |b| satisfy: 29.5≤a / |b|≤44.

3. The battery as described in claim 1, characterized in that, 45% ≤ a ≤ 90%, and for further preference, 65% ≤ a ≤ 75%; And / or, 0.01≤|b|≤0.03, more preferably, 0.015≤|b|≤0.

025.

4. The battery as described in claim 1, characterized in that, The ratio of the first lithium iron phosphate to the second lithium iron phosphate in the positive electrode active material layer is 0.2 to 0.

5.

5. The battery as described in claim 1, characterized in that, The first lithium iron phosphate is coated with a first carbon layer, the thickness of which is 1 nm to 10 nm. And / or, the second lithium iron phosphate is coated with a second carbon layer, the thickness of which is 1 nm to 10 nm.

6. The battery as claimed in claim 1, characterized in that, The porosity of the positive electrode active material layer is 22% to 30%.

7. The battery as claimed in claim 1, characterized in that, The positive electrode active material layer further includes doping elements, including at least one of V, Ni, Ce, Mg, and Zr, and the mass content of the doping elements in the positive electrode active material layer is 700ppm to 3000ppm.

8. The battery as claimed in claim 1, characterized in that, The molar ratio of Fe to P in the positive electrode active material layer is (0.96–0.98):

1.

9. The battery as claimed in claim 1, characterized in that, The battery also includes an electrolyte, which comprises a solvent and a lithium salt, wherein the concentration of the lithium salt in the electrolyte is 0.5 mol / L to 2 mol / L.

10. The battery as claimed in claim 1, characterized in that, The battery further includes a negative electrode sheet, which comprises a negative electrode active material layer. The negative electrode active material layer comprises a negative electrode active material, including natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, and SiO₂. x Silicon-carbon, Li4Ti5O 12 At least one of the following, wherein the compaction density of the coating on the negative electrode sheet is 1.5 g / cm³. 3 ~1.7g / cm 3 .

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