A lithium manganese iron phosphate positive electrode material, a preparation method and application thereof

Abstract

The application provides a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, and comprises a fluorine-doped lithium manganese iron phosphate core and a coating layer coated on at least part of the surface of the fluorine-doped lithium manganese iron phosphate core; the coating layer comprises carbon elements and lithium-conducting metal oxides; the lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 130 mAh / g at 25 DEG C, and a capacity retention rate of not less than 95% after 200 cycles of charging and discharging under 1C condition; the lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 140 mAh / g at 45 DEG C, and a capacity retention rate of not less than 90% after 200 cycles of charging and discharging under 1C condition. The lithium manganese iron phosphate positive electrode material provided by the application has excellent cycle stability.

Description

Technical Field

[0001] The present invention relates to a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, belonging to the field of lithium ion batteries. Background Art

[0002] Lithium-ion batteries can be used in electric vehicles, energy storage systems and other fields to store electrical energy and release it when needed, thereby reducing the demand for traditional fuels, reducing greenhouse gas emissions and helping to achieve carbon neutrality.

[0003] Among them, lithium-ion battery positive electrode materials are an important component of lithium-ion batteries. The selection of positive electrode materials directly affects the battery's energy density, power density, and cycle stability. Currently, common lithium-ion battery positive electrode materials include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, and lithium manganese iron phosphate. Among them, lithium iron manganese phosphate electrode materials have excellent thermal and chemical stability, can resist the risk of thermal runaway at high temperatures or when overcharged, are relatively rich in elements, are environmentally friendly, and are relatively inexpensive. Therefore, lithium iron manganese phosphate has become a widely popular electrode material.

[0004] However, the current lithium manganese iron phosphate positive electrode material still has the problem of poor cycle stability. Summary of the Invention

[0005] The present invention provides a lithium iron manganese phosphate positive electrode material, which has relatively excellent cycle stability both at room temperature and at high temperature.

[0006] The present invention also provides a method for preparing a lithium iron manganese phosphate cathode material, which can be used to prepare the above-mentioned lithium iron manganese phosphate cathode material. The preparation method is simple to operate and can prepare a lithium iron manganese phosphate cathode material with excellent cycle stability.

[0007] The present invention also provides a positive electrode sheet having excellent cycle stability.

[0008] The present invention also provides a battery having excellent cycle stability.

[0009] In one aspect, the present invention provides a lithium iron manganese phosphate cathode material, comprising a fluorine-doped lithium iron manganese phosphate core and a coating layer coated on at least a portion of the surface of the fluorine-doped lithium iron manganese phosphate core;

[0010] The coating layer includes carbon elements and lithium conductive metal oxides;

[0011] The lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 130 mAh / g at 25°C, and a capacity retention rate of not less than 95% after 200 cycles of charge and discharge at 1C;

[0012] The lithium manganese iron phosphate positive electrode material is 45

[0013] The 1C discharge specific capacity at ℃ is not less than 140mAh / g, and the capacity retention rate after 200 cycles of charge and discharge at 1C is not less than 90%.

[0014] In the lithium manganese iron phosphate positive electrode material as described above, the thickness of the coating layer is 2-7 nm.

[0015] In the lithium manganese iron phosphate positive electrode material as described above, the mass proportion of the lithium conductive metal oxide is 0.5%-5%; and / or the mass proportion of the carbon element is 1%-10%.

[0016] The above-mentioned lithium manganese iron phosphate positive electrode material, the chemical formula of the fluorine-doped lithium manganese iron phosphate core is Li 1+ a Mn x Fe 1-x F a PO4, a is 0.001-0.03, x is 0.2-0.8.

[0017] In the lithium manganese iron phosphate positive electrode material described above, the particle size D50 of the fluorine-doped lithium manganese iron phosphate core is 0.3-0.6 μm.

[0018] In another aspect, the present invention provides a method for preparing the lithium manganese iron phosphate positive electrode material as described above, comprising the following steps:

[0019] 1) ball milling a mixture of a phosphorus source, a manganese source, an iron source, and a lithium source at a rotation speed of 100-3000 r / min for 0.5-24 h to obtain a lithium iron manganese phosphate precursor;

[0020] 2) mixing the lithium iron manganese phosphate precursor with a fluorine source, and pretreating the mixture at 200-500° C. for 0.5-5 h to obtain fluorine-doped nickel cobalt lithium iron manganese phosphate powder;

[0021] 3) The fluorine-doped nickel-cobalt-manganese-iron lithium powder is placed in a solvent and mixed evenly with an organic carbon source and a lithium conductive metal compound, followed by drying and calcining at 500-900° C. for 0.5-24 h to obtain the manganese-iron-phosphate lithium positive electrode material.

[0022] In the preparation method as described above, in step 1), the rotation speed is 300-500 r / min and the ball milling time is 1-10 h.

[0023] In the preparation method as described above, in step 3), the calcination temperature is 600-800° C. and the calcination time is 5-15 hours.

[0024] In another aspect, the present invention provides a positive electrode sheet, comprising the lithium iron manganese phosphate positive electrode material as described above or the lithium iron manganese phosphate positive electrode material prepared by the preparation method as described above.

[0025] In another aspect, the present invention provides a battery comprising the lithium manganese iron phosphate positive electrode material or the positive electrode sheet as described above.

[0026] The lithium manganese iron phosphate positive electrode material provided by the present invention is coated with a special coating layer, has relatively excellent capacity retention rate at both 25° C. and 45° C., and has relatively excellent cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 XRD diffraction pattern of the lithium manganese iron phosphate positive electrode material provided in Example 1 of the present invention;

[0028] Figure 2 A transmission electron microscope image of the lithium manganese iron phosphate positive electrode material provided in Example 2 of the present invention;

[0029] Figure 3 XRD diffraction pattern of the lithium manganese iron phosphate positive electrode material provided in Comparative Example 1 of the present invention;

[0030] Figure 4 This is an X-ray photoelectron spectrum of the lithium manganese iron phosphate positive electrode material provided in Comparative Example 2 of the present invention;

[0031] Figure 5 2 are the cycle number-discharge specific capacity curves of the button batteries prepared in Example 1 of the present invention and Comparative Example 1 respectively under the test condition of 25° C.;

[0032] Figure 6 2 are the cycle number-discharge specific capacity curves of the button batteries prepared in Example 1 of the present invention and Comparative Example 1 respectively under the test condition of 45°C. DETAILED DESCRIPTION

[0033] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0034] In one aspect, the present invention provides a lithium iron manganese phosphate cathode material, comprising a fluorine-doped lithium iron manganese phosphate core and a coating layer coated on at least a portion of the surface of the fluorine-doped lithium iron manganese phosphate core;

[0035] The coating layer includes carbon elements and lithium conductive metal oxides;

[0036] The lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 130 mAh / g at 25°C, and a capacity retention rate of not less than 95% after 200 cycles of charge and discharge at 1C;

[0037] The lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 140 mAh / g at 45° C., and a capacity retention rate of not less than 90% after 200 cycles of charge and discharge at 1C.

[0038] The lithium manganese iron phosphate positive electrode material provided by the present invention is a material with a coating structure, wherein the coating layer includes carbon elements and lithium conductive metal oxides, the core is a fluorine-doped lithium manganese iron phosphate positive electrode material, and the coating layer covers at least a portion of the surface of the core.

[0039] In the present invention, the fluorine in the fluorine-doped lithium manganese iron phosphate core exists in the form of lithium fluoride, which can be characterized by X-ray photoelectron spectroscopy, where a strong peak appears at the position of 685.5 eV in the X-ray photoelectron spectroscopy.

[0040] The present invention does not limit the specific type of metal ions in the lithium conductive metal oxide, and a suitable type can be selected according to actual conditions, such as niobium ions, titanium ions, tungsten ions, zirconium ions, etc.

[0041] In one embodiment, the lithium conductive metal oxide in the coating layer is Nb2O5.

[0042] The present invention does not limit the distribution form of carbon elements and lithium conductive metal oxides in the coating layer. In a specific embodiment, the carbon elements and lithium conductive metal oxides are alternately distributed using a transmission electron microscope. Figure 2 shown.

[0043] The specific structure and composition characteristics of lithium manganese iron phosphate positive electrode materials can be characterized by transmission electron microscopy.

[0044] The discharge specific capacity of the present invention is obtained based on the test of a button battery under the condition of a test voltage within the voltage range of 2-4.2V and a rate of 1C. The preparation method of the button battery is to prepare a slurry of the lithium manganese iron phosphate positive electrode material, carbon black, and PVDF provided by the present invention in a mass ratio of 90:5:5, apply it on aluminum foil, vacuum dry it at 110°C, slice it, and make a positive electrode sheet with a diameter of 9 mm and a thickness of 20 microns, with lithium metal as the negative electrode; the electrolyte is 1M LiPF6 electrolyte, and its solvent is EC:DEC:DMC (1:1:1), and the button battery is assembled in a glove box.

[0045] The capacity retention rate refers to the ratio of the initial capacity of a lithium battery to that of a lithium battery after a certain charge and discharge cycle. The capacity retention rate of the present invention refers to the ratio between the discharge specific capacity of the button battery after 200 cycles at a rate of 1C within a voltage range of 2-4.2 and the discharge specific capacity of the first cycle.

[0046] It will be appreciated that 25°C and 45°C refer to different test temperatures.

[0047] The lithium iron manganese phosphate cathode material provided by the present invention exhibits excellent discharge specific capacity and capacity retention at both 25°C and 45°C. Therefore, the lithium iron manganese phosphate cathode material provided by the present invention exhibits excellent cycling performance. The inventors believe that this may be due to the fact that fluorine doping effectively stabilizes the surface of the lithium iron manganese phosphate and facilitates the construction of a coating layer. The coating layer, comprising carbon and a lithium-conductive metal oxide, rapidly conducts electrons and lithium ions, effectively improving interfacial kinetics, reducing surface side reactions, and inhibiting manganese dissolution, thereby improving cycling performance.

[0048] Furthermore, in a specific embodiment of the present invention, the thickness of the coating layer is 2-7 nm.

[0049] Specifically, the thickness of the coating layer in the present invention refers to the average thickness of the lithium manganese iron phosphate positive electrode material, which can be determined by transmission electron microscopy.

[0050] For example, the thickness of the coating layer includes but is not limited to 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, or a range consisting of any two thereof.

[0051] When the coating layer is covered on at least part of the surface of the fluorine-doped lithium manganese iron phosphate core, the coating layer thickness within the above range can not only effectively increase electron transfer but also does not affect ion conduction. When used in lithium-ion batteries, it can improve the battery's rate performance, energy density and cycle performance.

[0052] Furthermore, in a specific embodiment of the present invention, the mass proportion of the lithium conductive metal oxide is 0.5%-5%; and / or the mass proportion of the carbon element is 1%-10%.

[0053] Specifically, the mass proportion of lithium conductive metal oxide refers to the mass proportion of lithium conductive metal oxide in the entire lithium manganese iron phosphate positive electrode material, which can be obtained by ICPMS testing.

[0054] Similarly, the mass proportion of carbon element refers to the mass proportion of carbon element in the overall lithium manganese iron phosphate positive electrode material, which can also be obtained by testing using a carbon-sulfur analyzer method.

[0055] In detail, the mass percentage of the lithium conductive metal oxide includes but is not limited to 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or a range between any two thereof.

[0056] The mass percentage of carbon element includes but is not limited to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range between any two thereof.

[0057] When the mass proportion of the lithium conductive metal oxide and the mass proportion of the carbon element are within the above range, the electronic conductivity and ion conductivity of the lithium manganese iron phosphate positive electrode material can be further improved on the basis of ensuring structural stability and cycle performance.

[0058] Furthermore, in one embodiment of the present invention, the chemical formula of the fluorine-doped lithium manganese iron phosphate core is Li 1+a Mn x Fe 1-x F a PO4, a is 0.001-0.03, x is 0.2-0.8.

[0059] Specifically, a includes but is not limited to 0.001, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03 or a range between any two of them.

[0060] x includes but is not limited to 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or a range between any two thereof.

[0061] When the chemical formula of the fluorine-doped manganese iron phosphate core conforms to the above rules, it not only improves the structural stability of the fluorine-doped lithium manganese iron phosphate cathode material, but also optimizes its electrochemical performance. This may be because fluorine forms stronger bonds with other components, reducing structural damage to the material during the electrochemical process, while promoting rapid diffusion and reaction rate of lithium ions in the material, thereby improving electrochemical performance.

[0062] Furthermore, in a specific embodiment of the present invention, the particle size D50 of the fluorine-doped lithium manganese iron phosphate core is 0.3-0.6 μm.

[0063] It can be understood that D50 is usually used to represent the average particle size of the fluorine-doped lithium manganese iron phosphate core, that is, the particle size corresponding to when the cumulative particle size distribution percentage of the fluorine-doped lithium manganese iron phosphate core reaches 50%.

[0064] Specifically, the particle size D50 of the fluorine-doped lithium manganese iron phosphate core includes but is not limited to 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm or a range consisting of any two of them.

[0065] When the D50 of the fluorine-doped lithium iron manganese phosphate core is within the above range, the core has a larger specific surface area, providing more active surface. Lithium ions can be quickly intercalated and deintercalated within the lithium iron manganese phosphate, increasing the lithium ion transfer rate and further reducing the impedance of the lithium iron manganese phosphate, thereby improving the cycling performance and rate performance of the lithium iron manganese phosphate.

[0066] In another aspect, the present invention provides a method for preparing the lithium manganese iron phosphate positive electrode material as described above, comprising the following steps:

[0067] 1) ball milling a mixture of a phosphorus source, a manganese source, an iron source, and a lithium source at a rotation speed of 100-3000 r / min for 0.5-24 h to obtain a lithium iron manganese phosphate precursor;

[0068] 2) mixing a lithium manganese iron phosphate precursor with a fluorine source, and pretreating the mixture at 200-500° C. for 0.5-5 h to obtain a fluorine-doped nickel cobalt manganese iron lithium powder;

[0069] 3) Fluorine-doped nickel-cobalt-manganese iron lithium powder is placed in a solvent and mixed evenly with an organic carbon source and a lithium conductive metal compound, followed by drying and calcining at 500-900° C. for 0.5-24 h to obtain a manganese iron phosphate lithium cathode material.

[0070] Specifically, in step 1), a phosphorus source, a manganese source, an iron source, and a lithium source are mixed, and then ball milled at a rotation speed of 100-3000 r / min for 0.5-24 h to obtain a lithium iron manganese phosphate precursor.

[0071] The lithium source is a substance containing lithium that can be used to prepare lithium iron manganese phosphate cathode materials. The present invention does not impose specific limitations on the choice of lithium source; commonly used materials in the art can be used. In one embodiment, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium nitrate, or lithium acetate.

[0072] A manganese source is a substance containing manganese that can be used to prepare lithium manganese iron phosphate cathode materials. The present invention does not impose specific restrictions on the choice of manganese source; commonly used materials in the art can be used. In one embodiment, the manganese source is selected from at least one of manganese acetate, manganese oxalate, manganese phosphate, and manganese tetraoxide.

[0073] A phosphorus source is a substance containing phosphorus that can be used to prepare lithium manganese iron phosphate cathode materials. The present invention does not impose specific limitations on the choice of phosphorus source; commonly used materials in the art can be used. In one embodiment, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, and lithium dihydrogen phosphate.

[0074] An iron source is a substance containing iron that can be used to prepare a lithium manganese iron phosphate cathode material. The present invention does not impose any specific restrictions on the choice of iron source; commonly used materials in the art can be used. In one embodiment, the iron source is at least one of iron phosphate, ferrous oxalate, and ferric oxide.

[0075] During the ball milling process, the ball milling effect can be promoted by adding a ball milling aid. The ball milling aid can be a common type in the art, such as ethanol, ethylene glycol, water, acetone, etc.

[0076] It is understood that the molar ratios among the phosphorus source, manganese source, iron source and lithium source can be adjusted as required.

[0077] In step 2), the lithium manganese iron phosphate is mixed with a fluorine source and pretreated at 200-500° C. for 0.5-5 h. The fluorine reacts with the residual lithium on the surface of the lithium manganese iron phosphate precursor to form lithium fluoride, thereby obtaining fluorine-doped lithium manganese iron phosphate powder.

[0078] A fluorine source is a substance containing fluorine that can be used to prepare lithium manganese iron phosphate cathode materials. The present invention does not impose any specific restrictions on the choice of fluorine source; commonly used substances in the art can be used. In one embodiment, the fluorine source is at least one of ammonium fluoride, trifluoroacetic acid, and trifluoromethanesulfonic acid.

[0079] It is understood that in order to avoid the influence of impurities on the reaction, the pretreatment can be performed under an inert gas atmosphere. In one embodiment, the inert gas is argon.

[0080] The lithium fluoride in the fluorine-doped lithium manganese iron phosphate powder can not only stabilize the surface, but also the fluorine element has high electronegativity, which is conducive to the negative charge of the surface, further attracting metal cations and facilitating the construction of the coating layer.

[0081] In step 3), the fluorine-doped nickel-cobalt-manganese iron lithium powder is placed in a solvent and mixed evenly with an organic carbon source and a lithium conductive metal compound, followed by drying and calcining at 500-900°C for 0.5-24h. During the calcination process, the carbon source that adsorbs metal ions is carbonized, and the metal ions react with the oxygen atoms in the carbon source to form metal oxides to obtain a lithium manganese iron phosphate positive electrode material.

[0082] An organic carbon source is a substance containing carbon that can be used to prepare lithium manganese iron phosphate cathode materials. The present invention does not impose any specific restrictions on the choice of organic carbon source; commonly used substances in the art can be used. In one embodiment, the organic carbon source is at least one of glucose, citric acid, starch, hydroxybenzoic acid, fructose, and glyceraldehyde.

[0083] The present invention does not limit the specific type of solvent, as long as it can dissolve the fluorine-doped lithium nickel cobalt manganese oxide powder, the carbon source and the lithium conductive metal compound. In a specific embodiment, the solvent includes at least one of water, ethanol, acetone and methanol.

[0084] It is understood that various substances in the solvent can be mixed evenly by stirring. In one embodiment, the stirring speed is 300-400 r / min and the stirring time is 30 min-10 h.

[0085] The present invention does not limit the specific parameters of drying, as long as the moisture can be removed. In one embodiment, the drying temperature is 80°C.

[0086] To avoid the influence of impurities on the reaction, pretreatment can be performed under an inert gas atmosphere. In one embodiment, the inert gas is argon.

[0087] The inventors discovered that the lithium iron manganese phosphate cathode material prepared using the preparation method of the lithium iron manganese phosphate cathode material provided by the present invention has excellent cycle performance. The inventors speculate that the reason may be that the polar groups in the organic carbon source bond with the metal ions, anchoring the metal ions in the organic carbon source. Through calcination, a carbon coating layer wrapped with metal oxide is formed on the surface of the lithium iron manganese phosphate electrode material, integrating the substance that can quickly conduct lithium ions and the electronic conductor into the same layer of coating layer, while achieving improved interface dynamic performance and stability without increasing the thickness of the coating layer. In addition, combined with surface fluorine doping, the coating layer is easier to construct on the surface and stabilizes the material surface, reducing the impact of changes in the electrode material on the coating layer during electrochemical cycling.

[0088] Furthermore, in a specific embodiment of the present invention, in step 1), the rotation speed is 300-500 r / min, and the ball milling time is 1-10 h.

[0089] In detail, the rotational speed includes but is not limited to 300r / min, 320r / min, 340r / min, 360r / min, 380r / min, 400r / min, 420r / min, 440r / min, 460r / min, 480r / min, 500r / min or a range consisting of any two of them.

[0090] The ball milling time includes but is not limited to 1 h, 2 h, 4 h, 6 h, 8 h, 10 h or a range consisting of any two thereof.

[0091] When the rotation speed and ball milling time are within the above range, the particle size distribution of the lithium manganese iron phosphate precursor can be better controlled, making the lithium manganese iron phosphate precursor more uniform, thereby affecting the cycle stability of the finally prepared lithium manganese iron phosphate positive electrode material.

[0092] Furthermore, in a specific embodiment of the present invention, in step 3), the calcination temperature is 600-800° C., and the calcination time is 5-15 hours.

[0093] In detail, the calcination temperature includes but is not limited to 600°C, 620°C, 640°C, 660°C, 680°C, 700°C, 720°C, 740°C, 760°C, 780°C, 800°C or a range consisting of any two thereof.

[0094] The calcination time includes, but is not limited to, 5 h, 8 h, 10 h, 12 h, 14 h, 15 h, or a range consisting of any two of these.

[0095] When the calcination temperature and time are within the above ranges, the thickness of the coating layer can be effectively adjusted, thereby improving the cycle performance of the lithium manganese iron phosphate positive electrode material.

[0096] In another aspect, the present invention provides a positive electrode sheet, comprising the lithium iron manganese phosphate positive electrode material as described above or the lithium iron manganese phosphate positive electrode material prepared by the preparation method as described above.

[0097] The positive electrode sheet of the present invention specifically comprises a positive electrode current collector and a positive electrode active layer formed of a lithium manganese iron phosphate positive electrode material and arranged on the surface of the positive electrode current collector.

[0098] When preparing a positive electrode sheet, for example, the lithium manganese iron phosphate positive electrode material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and the mixture is thoroughly stirred to form a uniform positive electrode slurry. The positive electrode slurry is evenly coated on the positive electrode current collector, and the positive electrode sheet is obtained after drying, rolling, and slitting. In one embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt.% of the lithium manganese iron phosphate positive electrode material, 0.5-15 wt.% of the conductive agent, and 0.5-15 wt.% of the binder. Furthermore, the positive electrode active layer comprises 80-98 wt.% of the lithium manganese iron phosphate positive electrode material, 1-10 wt.% of the conductive agent, and 1-10 wt.% of the binder.

[0099] Among them, the material of the positive electrode current collector can be at least one of aluminum foil and nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, and polyurethane.

[0100] Since the lithium manganese iron phosphate positive electrode material provided by the present invention has the characteristic of excellent cycle performance, the positive electrode sheet provided by the present invention also has the characteristic of excellent cycle performance.

[0101] In another aspect, the present invention provides a battery comprising the lithium manganese iron phosphate positive electrode material or the positive electrode sheet as described above.

[0102] It is conceivable that the battery provided by the present invention includes, in addition to the above-mentioned positive electrode sheet, a negative electrode sheet, an electrolyte and a separator.

[0103] The present invention is not strictly limited to the negative electrode active material in the negative electrode sheet, and can be at least one of the negative electrode active materials commonly used in lithium-ion batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microbeads, silicon-based negative electrode materials (mainly including silicon oxide, silicon-carbon negative electrode), tin-based negative electrode materials (mainly including tin, tin alloy), etc.

[0104] The present invention is not strictly limited to the choice of electrolyte, and can include one or more of the solvents commonly used in lithium-ion battery electrolytes, and the electrolyte lithium salts commonly used in lithium-ion electrolytes. For example, the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt can be, for example, one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0105] The present invention does not strictly limit the material selection of the diaphragm, and it can be a diaphragm material commonly used in lithium-ion batteries, such as polypropylene diaphragm (PP), polyethylene diaphragm (PE), polypropylene / polyethylene double-layer composite film (PP / PE), polyimide electrospun diaphragm (PI), polypropylene / polyethylene / polypropylene three-layer composite film (PP / PE / PP), cellulose non-woven fabric diaphragm, and one of the diaphragms with ceramic coating.

[0106] When preparing lithium-ion batteries, the positive electrode sheet, separator, and negative electrode sheet are wound or stacked to form a bare cell. The bare cell is then encapsulated in a pre-stamped aluminum-plastic film bag. After the encapsulated battery is dried at 85°C, the electrolyte is injected into the dried cell. The battery is then placed, formed, and sealed again to complete the lithium-ion battery production.

[0107] The preparation method of the lithium manganese iron phosphate positive electrode material provided by the present invention will be described in detail with reference to specific embodiments below.

[0108] Example 1

[0109] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0110] 1. Ferrous oxalate, ammonium dihydrogen phosphate, manganese sulfate and lithium carbonate were weighed and placed in a ball mill according to the molar ratio of Li, Mn, Fe and P of 1.004:0.6:0.4:1. The mixture was ball milled at a speed of 600 r / min for 2 h in the presence of ethanol and dried to obtain a lithium manganese iron phosphate precursor.

[0111] 2. The lithium manganese iron phosphate precursor and NH4F were mixed in a molar ratio of 100:0.4, and the mixture was placed in a ball mill for 0.5 h at a speed of 300 r / min. Then, it was placed in a tube furnace and pretreated in an argon atmosphere at 400 ° C for 2 h to obtain fluorine-doped lithium manganese iron phosphate powder.

[0112] 3. Take 1g of fluorine-doped lithium manganese iron phosphate powder and glucose and niobium oxalate in a ratio of 10:1.5:0.5 and place them in 50ml of ethanol. After stirring at a speed of 400r / min for 1h, evaporate to dryness at 80℃, then place it in a tube furnace and calcine it at 700℃ in an argon atmosphere for 10h to obtain lithium manganese iron phosphate positive electrode material.

[0113] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.004 Fe 0.6 Mn 0.4 F 0.004 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 5nm, the mass proportion of the lithium conductive metal oxide is 0.85wt.%, the mass proportion of the carbon element is 7.8wt.%, and the core particle size D50 of the fluorine-doped lithium manganese iron phosphate is 0.48μm.

[0114] The lithium manganese iron phosphate positive electrode material provided in this embodiment was tested by X-ray diffractometer. Figure 1 As shown, the XRD peaks of the lithium manganese iron phosphate positive electrode material are in good agreement with the standard card PDF#40-1499, indicating that pure phase lithium manganese iron phosphate was successfully prepared and the modification did not damage the material structure.

[0115] Example 2

[0116] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this embodiment is basically the same as that in Example 1, except that:

[0117] In step 2), the molar ratio of the lithium manganese iron phosphate precursor to NH4F is 100:0.3, to obtain fluorine-doped lithium manganese iron phosphate powder;

[0118] In step 3), the ratio of fluorine-doped lithium manganese iron phosphate powder to glucose and niobium oxalate is 10:1:0.3.

[0119] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.003 Fe 0.6 Mn 0.4 F 0.003 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 4.8nm, the mass proportion of the lithium conductive metal oxide is 0.5wt.%, the mass proportion of the carbon element is 5.7wt.%, and the core particle size D50 of the fluorine-doped lithium manganese iron phosphate is 0.46μm.

[0120] The lithium manganese iron phosphate positive electrode material provided in this embodiment was characterized by transmission electron microscopy. Figure 2 As shown, a coating layer appears on the surface of the lithium manganese iron phosphate positive electrode material, and the coating layer is an alternating distribution of niobium oxide and carbon.

[0121] Example 3

[0122] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0123] 1. Ferrous oxalate, ammonium dihydrogen phosphate, manganese sulfate and lithium carbonate were weighed and placed in a ball mill according to the molar ratio of Li, Mn, Fe and P of 1.006:0.6:0.4:1. The mixture was ball milled at a speed of 100 r / min for 24 h in the presence of ethanol and dried to obtain a lithium manganese iron phosphate precursor.

[0124] 2. The lithium manganese iron phosphate precursor and fluoroamide were mixed at a ratio of 100:0.2, and the mixture was placed in a ball mill for 0.5 h at a speed of 300 r / min. Then, it was placed in a tube furnace and pretreated in an argon atmosphere at 200°C for 5 h to obtain fluorine-doped lithium manganese iron phosphate powder.

[0125] 3. Take 1g of fluorine-doped lithium manganese iron phosphate powder and glucose and niobium chloride in a ratio of 10:0.8:0.5 and place them in 50ml of ethanol. After stirring at a speed of 400r / min for 1h, evaporate to dryness at 80℃, then place it in a tube furnace and calcine it at 600℃ in an argon atmosphere for 24h to obtain lithium manganese iron phosphate positive electrode material.

[0126] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.006 Fe 0.6 Mn 0.4 F 0.006 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 4nm, the mass proportion of the lithium conductive metal oxide is 0.88wt.%, the mass proportion of the carbon element is 4.1wt.%, and the core particle size D50 of the fluorine-doped lithium manganese iron phosphate is 0.4μm.

[0127] Example 4

[0128] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this embodiment includes the following steps:

[0129] 1. Ferrous sulfate, diammonium hydrogen phosphate, manganese sulfate and lithium carbonate were weighed according to the molar ratio of Li, Mn, Fe and P of 1.004:0.6:0.4:1 and placed in a ball mill. The mixture was ball milled at a speed of 3000 r / min for 0.5 h in the presence of ethanol and dried to obtain a lithium manganese iron phosphate precursor.

[0130] 2. The lithium manganese iron phosphate precursor and NH4F were mixed in a molar ratio of 100:0.4, and the mixture was placed in a ball mill for 0.5 h at a speed of 300 r / min. Then, it was placed in a tube furnace and pretreated in an argon atmosphere at 500°C for 0.5 h to obtain fluorine-doped lithium manganese iron phosphate powder.

[0131] 3. Take 1g of fluorine-doped lithium manganese iron phosphate powder, citric acid and tetrabutyl titanate niobium in a ratio of 10:1.2:1 and place it in 50ml of ethanol. After stirring at a speed of 350r / min for 1h, evaporate it to dryness at 80℃, and then place it in a tube furnace and calcine it at 800℃ in an argon atmosphere for 5h to obtain lithium manganese iron phosphate positive electrode material.

[0132] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.004 Fe 0.6 Mn 0.4 F 0.004 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 5nm, the mass proportion of the lithium conductive metal oxide is 1.2wt.%, the mass proportion of the carbon element is 6.4wt.%, and the core particle size D50 of the fluorine-doped lithium manganese iron phosphate is 0.46μm.

[0133] Example 5

[0134] The preparation method of lithium manganese iron phosphate provided in this embodiment is basically the same as that in Example 1, except that:

[0135] In step 1), the rotation speed is 600 r / min and the ball milling time is 5 h.

[0136] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.004 Fe 0.6 Mn 0.4 F 0.004PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 5nm, the mass proportion of the lithium conductive metal oxide is 0.84wt.%, the mass proportion of the carbon element is 7.9wt%, and the core particle size of the fluorine-doped lithium manganese iron phosphate is 0.35μm.

[0137] Example 6

[0138] The preparation method of lithium manganese iron phosphate provided in this embodiment is basically the same as that in Example 1, except that:

[0139] In step 3), the calcination temperature is 1000° C. and the time is 12 h.

[0140] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.004 Fe 0.6 Mn 0.4 F 0.004 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 4.8nm, the mass proportion of the lithium conductive metal oxide is 0.84wt.%, the mass proportion of the carbon element is 7.8wt%, and the core particle size of the fluorine-doped lithium manganese iron phosphate is 0.35μm.

[0141] Example 7

[0142] The preparation method of lithium manganese iron phosphate provided in this embodiment is basically the same as that in Example 1, except that:

[0143] In step 2), the molar ratio of Li, Mn, Fe, and P is 1.01:0.6:0.4:1. Ferrous sulfate, diammonium hydrogen phosphate, manganese sulfate, and lithium carbonate are weighed, and the molar ratio of lithium manganese iron phosphate precursor to NH4F is 100:1.

[0144] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.008 Fe 0.6 Mn 0.4 F 0.008 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 5nm, the mass proportion of the lithium conductive metal oxide is 0.85wt.%, the mass proportion of the carbon element is 7.7wt.%, and the core particle size of the fluorine-doped lithium manganese iron phosphate is 0.5μm.

[0145] Example 8

[0146] The preparation method of lithium manganese iron phosphate provided in this embodiment is basically the same as that in Example 1, except that:

[0147] In step 3), the ratio of fluorine-doped lithium manganese iron phosphate powder to glucose and niobium oxalate is 10:1.8:5.

[0148] The core chemical formula of the fluorine-doped lithium manganese iron phosphate positive electrode material is Li 1.004 Fe 0.6 Mn 0.4 F 0.004 PO4, the test shows that the coating layer thickness of the lithium manganese iron phosphate positive electrode material provided in this embodiment is 6.2nm, the mass proportion of the lithium conductive metal oxide is 8.4wt.%, the mass proportion of the carbon element is 7.2wt.%, and the core particle size of the fluorine-doped lithium manganese iron phosphate is 0.48μm.

[0149] Comparative Example 1

[0150] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this comparative example comprises the following steps:

[0151] 1. Ferrous oxalate, ammonium dihydrogen phosphate, manganese sulfate and lithium carbonate were weighed and placed in a ball mill according to the molar ratio of Li, Mn, Fe and P of 1.02:0.6:0.4:1. The mixture was ball milled at a speed of 600 r / min for 2 h in the presence of ethanol and dried to obtain a lithium manganese iron phosphate precursor.

[0152] 2. Place the lithium manganese iron phosphate precursor in a tube furnace and calcine it at 700°C in an argon atmosphere for 10 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0153] The lithium manganese iron phosphate positive electrode material provided in this comparative example was tested by X-ray diffractometer. Figure 3 As shown, the XRD peaks of the lithium manganese iron phosphate positive electrode material are in good agreement with those of the standard card PDF#40-1499, indicating that the lithium manganese iron phosphate positive electrode material is pure phase lithium manganese iron phosphate.

[0154] Comparative Example 2

[0155] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this comparative example comprises the following steps:

[0156] 1. Ferrous oxalate, ammonium dihydrogen phosphate, manganese sulfate and lithium carbonate were weighed and placed in a ball mill according to the molar ratio of Li, Mn, Fe and P of 1.004:0.6:0.4:1. The mixture was ball milled at a speed of 600 r / min for 2 h in the presence of ethanol and dried to obtain a lithium manganese iron phosphate precursor.

[0157] 2. The lithium manganese iron phosphate precursor and NH4F were mixed in a molar ratio of 100:0.4, and the mixture was placed in a ball mill for 0.5 h at a speed of 300 r / min. Then, it was placed in a tube furnace and pretreated in an argon atmosphere at 400 ° C for 2 h to obtain fluorine-doped lithium manganese iron phosphate powder.

[0158] 3. Place the fluorine-doped lithium manganese iron phosphate powder in a tube furnace and calcine it at 700°C in an argon atmosphere for 10 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0159] like Figure 4 As shown, the lithium manganese iron phosphate positive electrode material provided in this comparison was characterized by X-ray photoelectron spectroscopy, and a strong peak was found at around 685.5 eV, indicating the formation of lithium fluoride and the successful surface doping of the material.

[0160] Comparative Example 3

[0161] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this comparative example is basically the same as that in Example 1, except that:

[0162] In step 3), no niobium oxalate is added.

[0163] Comparative Example 4

[0164] The preparation method of the lithium manganese iron phosphate positive electrode material provided in this comparative example comprises the following steps:

[0165] 1. Ferrous oxalate, ammonium dihydrogen phosphate, manganese sulfate, lithium carbonate, and NH4F were weighed and placed in a ball mill according to the molar ratio of Li, Mn, Fe, P, and F being 1.004:0.6:0.4:1:0.0041. The mixture was ball milled at a speed of 600 r / min for 2 h in the presence of ethanol. The mixture was then placed in a tube furnace and pretreated at 400°C in an argon atmosphere for 2 h to obtain fluorine-doped lithium manganese iron phosphate powder.

[0166] 2. Take 1g of fluorine-doped lithium manganese iron phosphate powder and glucose and niobium oxalate in a ratio of 100:1.5:0.5 and place them in 50ml of ethanol. After stirring at a speed of 400r / min for 1h, evaporate to dryness at 80℃, then place it in a tube furnace and calcine it at 700℃ in an argon atmosphere for 10h to obtain lithium manganese iron phosphate positive electrode material.

[0167] Test Case

[0168] 1. Electrochemical performance test

[0169] The lithium manganese iron phosphate positive electrode material prepared in each embodiment and comparative example is made into a positive electrode sheet, and then assembled with a negative electrode sheet, an electrolyte, and a separator according to the following method to obtain a button battery. The method includes:

[0170] The positive electrode materials in the examples and comparative examples were mixed with conductive carbon black (SP) and PVDF in a weight ratio of 90%:5%:5% respectively, and the positive electrode slurry was obtained by dispersion. The slurry was coated on an aluminum foil current collector and rolled to obtain a surface density of 2.5 g / cm 2The positive electrode sheet was then punched out into a small disc with a diameter of 9 mm using a film mold. After drying and weighing, a button battery was assembled in a glove box under an Ar protective atmosphere using a 2025 button battery shell, a Li metal disc as the negative electrode, a 1M LiPF6 electrolyte, and an EC:DEC:DMC (1:1:1) solvent.

[0171] At 25°C and 45°C, the battery was charged to 4.50V at a constant current of 1C, then charged to 4.50V at a constant voltage of 0.05C, and then discharged to 2.0V at a discharge rate of 1C. This charge and discharge cycle was repeated 200 times. The discharge capacity Q1 at the first cycle to the discharge capacity Q at the 200th cycle at different temperatures was measured. 200 , calculate the capacity retention rate, see Table 1 for details.

[0172] Among them, the capacity retention rate Q = Q 200 / Q1*100%

[0173] Figure 5 The figures are the cycle number-discharge specific capacity curves of the button batteries prepared in Example 1 and Comparative Example 1 respectively under the test condition of 25°C.

[0174] Figure 6 The figures are the cycle number-discharge specific capacity curves of the button batteries prepared in Example 1 and Comparative Example 1 respectively under the test condition of 45°C.

[0175] Table 1

[0176]

[0177] 2. Mn 2+ , Fe 2+ Dissolution testing

[0178] The cycled battery was disassembled, the electrolyte collected, and the positive electrode was rinsed with ethylene carbonate solution, and the rinse liquid was collected. The two liquids were combined and subjected to inductively coupled plasma (ICP) testing to obtain data on transition metal ion dissolution.

[0179] The test results are shown in Table 2.

[0180] Table 2

[0181] Serial number <![CDATA[Mn 2+ Dissolution amount (ppm)]]> <![CDATA[Fe 2+ Dissolution amount (ppm)]]> Example 1 4.32 2.15 Example 2 4.36 2.38 Example 3 4.41 2.42 Example 4 4.25 2.17 Example 5 4.38 2.22 Example 6 4.46 2.25 Example 7 4.42 2.31 Example 8 4.39 2.19 Comparative Example 1 8.75 3.86 Comparative Example 2 4.82 2.54 Comparative Example 3 4.92 2.56 Comparative Example 4 5.02 2.67

[0182] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: It comprises a fluorine-doped lithium manganese iron phosphate core and a coating layer coated on at least a portion of the surface of the fluorine-doped lithium manganese iron phosphate core; The coating layer includes carbon elements and lithium conductive metal oxides; The lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 130 mAh / g at 25°C, and a capacity retention rate of not less than 95% after 200 cycles of charge and discharge at 1C; The lithium manganese iron phosphate positive electrode material has a 1C discharge specific capacity of not less than 140 mAh / g at 45°C, and a capacity retention rate of not less than 90% after 200 cycles of charge and discharge at 1C; The fluorine in the fluorine-doped lithium manganese iron phosphate core exists on the core surface in the form of lithium fluoride. The chemical formula of the fluorine-doped lithium manganese iron phosphate core is Li 1+a Mn x Fe 1-x F a PO4, a is 0.001-0.03, x is 0.2-0.

8.

2. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The thickness of the coating layer is 2-7 nm.

3. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that The mass proportion of the lithium conductive metal oxide is 0.5%-5%; and / or the mass proportion of the carbon element is 1%-10%.

4. The lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that The particle size D50 of the fluorine-doped lithium manganese iron phosphate core is 0.3-0.6 μm.

5. A method for preparing the lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: 1) ball milling a mixture of a phosphorus source, a manganese source, an iron source, and a lithium source at a rotation speed of 100-3000 r / min for 0.5-24 h to obtain a lithium iron manganese phosphate precursor; 2) mixing the lithium iron manganese phosphate precursor with a fluorine source, and pretreating the mixture at 200-500° C. for 0.5-5 h to obtain fluorine-doped nickel-cobalt-lithium iron manganese phosphate powder; 3) The fluorine-doped nickel-cobalt-manganese-iron lithium powder is mixed evenly with an organic carbon source and a lithium conductive metal compound in a solvent, followed by drying and calcining at 500-900° C. for 0.5-24 h to obtain the manganese-iron-phosphate lithium positive electrode material.

6. The preparation method according to claim 5, characterized in that In step 1), the rotation speed is 300-500 r / min, and the ball milling time is 1-10 h.

7. The preparation method according to claim 5 or 6, characterized in that: In step 3), the calcination temperature is 600-800°C and the time is 5-15 hours.

8. A positive electrode sheet, characterized in that: The invention relates to a lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4 or a lithium iron manganese phosphate positive electrode material prepared by the preparation method according to any one of claims 5 to 7.

9. A battery, characterized in that: The invention comprises the lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 4 or the positive electrode sheet according to claim 8.

Images