Iron phosphate dihydrate and preparation method thereof, lithium iron phosphate material, electrochemical device and electronic equipment

Abstract

The invention discloses iron phosphate dihydrate and a preparation method thereof, a lithium iron phosphate material, an electrochemical device and electronic equipment. The iron phosphate dehydrate comprises a monoclinic phase and an orthorhombic phase, and the crystal phase ratio d of the monoclinic phase to the orthorhombic phase is more than or equal to 1.5 and less than or equal to 6. When the iron phosphate dehydrate is used as a raw material to prepare the lithium iron phosphate material, the compaction density and the comprehensive electrochemical performance, especially the dynamic performance, of the lithium iron phosphate material can be remarkably improved. An electrochemical device (especially a lithium ion battery) obtained by using the lithium iron phosphate material as a positive electrode material has excellent capacity performance under different multiplying power.

Description

Technical Field

[0001] This invention relates to an iron phosphate dihydrate and its preparation method, lithium iron phosphate materials, electrochemical devices, and electronic devices. Background Technology

[0002] The current mainstream production process for lithium iron phosphate involves mixing, grinding, spray drying, sintering, and crushing iron phosphate dihydrate (FePO4·2H2O) with a lithium source to obtain lithium iron phosphate material. In this process, the raw materials in each step have a fundamental impact on the performance of the lithium iron phosphate material; specifically, the phase composition and properties of iron phosphate dihydrate have a substantial influence on the performance of both iron phosphate and lithium iron phosphate materials.

[0003] The current main process routes for preparing ferric phosphate dihydrate are characterized by long processing times, low efficiency, and high energy consumption. Furthermore, the process is difficult to control with high reaction precision, which leads to poor product consistency and affects the performance of ferric phosphate dihydrate.

[0004] Therefore, how to strengthen the process control of ferric phosphate dihydrate and improve product performance is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] To address the aforementioned technical problems in the prior art, this invention provides iron phosphate dihydrate, its preparation method, lithium iron phosphate materials, electrochemical devices, and electronic devices. Using iron phosphate dihydrate as a raw material to prepare lithium iron phosphate materials significantly improves the compaction density and overall electrochemical performance, especially the kinetic performance, of the lithium iron phosphate materials. Electrochemical devices (especially lithium-ion batteries) using this lithium iron phosphate material as a cathode material exhibit excellent capacity performance at different rate capabilities.

[0006] The present invention solves the above-mentioned technical problems through the following technical solution:

[0007] A first aspect of the present invention provides ferric phosphate dihydrate, comprising a monoclinic phase and an orthorhombic phase, wherein the crystal phase ratio d of the monoclinic phase and the orthorhombic phase satisfies: 1.5 ≤ d ≤ 6.

[0008] A second aspect of the present invention provides a method for preparing ferric phosphate dihydrate, comprising the following steps:

[0009] S1. An oxidation precipitation reaction is carried out on a mixed solution containing an iron source, a phosphorus source and an oxidant to obtain the first slurry;

[0010] S2. The first slurry is aged to obtain the second slurry;

[0011] S3. The second slurry is subjected to solid-liquid separation, and the resulting solid is dried by Joule heat flash drying.

[0012] A third aspect of the present invention provides ferric phosphate dihydrate, which is prepared by the method for preparing ferric phosphate dihydrate as described above.

[0013] A fourth aspect of the present invention provides a method for preparing lithium iron phosphate material, which uses iron phosphate dihydrate as described above.

[0014] The fifth aspect of the present invention provides a lithium iron phosphate material, which is prepared by the method for preparing lithium iron phosphate material as described above.

[0015] A sixth aspect of the present invention provides an electrochemical device comprising the lithium iron phosphate material as described above.

[0016] A seventh aspect of the present invention provides an electronic device comprising the electrochemical device described above.

[0017] The positive and progressive effects of this invention are as follows:

[0018] 1. This invention uses Joule heating flash drying technology to prepare ferric phosphate dihydrate, which shortens the drying time from the traditional hours to minutes or even seconds, increasing production efficiency by tens of times. Joule heating directly heats the material in bulk, with extremely high thermal efficiency. Compared with traditional ovens, it significantly reduces the energy consumption of the drying process, achieving green and efficient production.

[0019] 2. This invention utilizes the instantaneous heating characteristics of Joule heating to rapidly remove moisture while perfectly maintaining the ideal monoclinic-orthorhombic phase ratio formed by aging, avoiding secondary phase transformation and particle agglomeration. The resulting ferric phosphate dihydrate has an ideal monoclinic-orthorhombic phase ratio and good dispersibility.

[0020] 3. Using the iron phosphate dihydrate obtained by this method as a raw material to prepare lithium iron phosphate materials can significantly improve the compaction density and overall electrochemical performance, especially the kinetic performance, of lithium iron phosphate materials. Electrochemical devices (especially lithium-ion batteries) using this lithium iron phosphate material as a cathode material exhibit excellent capacity performance at different rate capabilities. Attached Figure Description

[0021] Figure 1 The image shows an XRD comparison of ferric phosphate dihydrate and orthorhombic phase in Example 1 and Comparative Example 1.

[0022] Figure 2 The image shows a comparison of XRD patterns of ferric phosphate dihydrate and monoclinic phase in Example 1 and Comparative Example 1.

[0023] Figure 3 for Figure 1 The enlarged view of the corresponding horizontal axis within the range of 10° to 20°.

[0024] Figure 4 for Figure 1 The enlarged view of the corresponding horizontal axis within the range of 25° to 35°. Detailed Implementation

[0025] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0026] Ferric phosphate dihydrate

[0027] A first aspect of the present invention provides ferric phosphate dihydrate, comprising a monoclinic phase and an orthorhombic phase, wherein the crystal phase ratio d of the monoclinic phase and the orthorhombic phase satisfies: 1.5 ≤ d ≤ 6.

[0028] In this invention, the crystal phase ratio refers to the ratio of the sum of the integral intensities of the XRD characteristic peaks of the corresponding phases. The crystal phase ratio of the monoclinic phase to the orthorhombic phase is the ratio of the sum of the integral intensities of the XRD characteristic peaks corresponding to the monoclinic phase to the sum of the integral intensities of the XRD characteristic peaks corresponding to the orthorhombic phase.

[0029] In this invention, the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is calculated based on the characteristic peak of 2θ not greater than 45° in the XRD pattern of ferric phosphate dihydrate.

[0030] In some preferred embodiments, the crystal phase ratio d of the monoclinic phase and the orthorhombic phase satisfies: 2≤d≤5, for example, 3 or 4.

[0031] In some embodiments, the specific surface area of ​​the ferric phosphate dihydrate is 26-70 m². 2 / g, preferably 30~55 m 2 / g, for example, 40 m 2 / g or 50 m 2 / g.

[0032] In some embodiments, the molar ratio of iron to phosphorus in the iron phosphate dihydrate is 0.95 to 0.99, for example, 0.960, 0.965, 0.970 or 0.980.

[0033] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 2, and the specific surface area of ​​the ferric phosphate dihydrate is 50 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.960.

[0034] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 1.5, and the specific surface area of ​​the ferric phosphate dihydrate is 40 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.965.

[0035] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 3, and the specific surface area of ​​the ferric phosphate dihydrate is 26 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.990.

[0036] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 4, and the specific surface area of ​​the ferric phosphate dihydrate is 70 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.950.

[0037] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 5, and the specific surface area of ​​the ferric phosphate dihydrate is 55 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.980.

[0038] In one specific embodiment, the ferric phosphate dihydrate satisfies the following conditions: the crystal phase ratio d of the monoclinic phase and the orthorhombic phase is 6, and the specific surface area of ​​the ferric phosphate dihydrate is 30 m². 2 / g, wherein the molar ratio of iron to phosphorus in the iron dihydrate is 0.970.

[0039] In this invention, ferric phosphate dihydrate having the above structure, properties and performance can be prepared by the method for preparing ferric phosphate dihydrate according to the second aspect of this invention.

[0040] A second aspect of the present invention provides a method for preparing ferric phosphate dihydrate, comprising the following steps:

[0041] S1. An oxidation precipitation reaction is carried out on a mixed solution containing an iron source, a phosphorus source and an oxidant to obtain the first slurry;

[0042] S2. The first slurry is aged to obtain the second slurry;

[0043] S3. The second slurry is subjected to solid-liquid separation, and the resulting solid is dried by Joule heat flash drying.

[0044] In the existing process route of iron phosphate dihydrate, by precisely controlling the temperature, time and phosphorus source addition during the aging process, iron phosphate dihydrate containing both monoclinic and orthorhombic phases can be prepared, which improves the compaction density and initial discharge capacity of the final lithium iron phosphate material. However, the kinetic performance of the final lithium iron phosphate material is still unsatisfactory. The inventors found through research that the reason why the kinetic performance of the lithium iron phosphate material obtained above is still unsatisfactory is that the lithium iron phosphate material obtained by oven drying after aging has potential quality risks. Specifically, during the long drying process, the lithium iron phosphate material particles will agglomerate and the internal structure will change. For example, the conversion from orthorhombic phase to monoclinic phase is difficult to control, which affects the consistency and final performance of the product. In addition, oven drying also has the following problems: (1) low efficiency: oven drying usually takes several hours or even longer, which is one of the main time-consuming links in the entire production process; (2) high energy consumption: the long heating process consumes a lot of energy, which increases the production cost.

[0045] In some implementations, in step S3, the Joule heat flash drying satisfies the following conditions: the voltage is 10~50 V, for example, 25 V, 30 V or 40 V.

[0046] In some implementations, in step S3, the Joule heat flash drying meets the following requirements: the current is 10~100 A, for example, 20 A, 30 A, 35 A or 50 A.

[0047] In some implementations, in step S3, the drying time of the Joule heat flash drying is 30~300 s, preferably 60~240 s, for example 120 s or 200 s.

[0048] In some embodiments, in step S3, the Joule heat flash drying is carried out under an inert atmosphere. The inert atmosphere can be an inert atmosphere conventionally used in the art, such as a nitrogen atmosphere.

[0049] In some implementations, in step S1, the iron source includes one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, ferric nitrate, and ferric acetate, for example, ferrous sulfate.

[0050] In some embodiments, in step S1, the phosphorus source includes one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, for example, ammonium dihydrogen phosphate.

[0051] In some implementations, the oxidant in step S1 includes hydrogen peroxide.

[0052] In some implementations, in step S1, the molar ratio of the iron source, phosphorus source and oxidant is 1:1:(0.55~0.65), for example, 1:1:0.6.

[0053] In some embodiments, in step S1, the temperature of the oxidation precipitation reaction is 40~65°C, for example 55°C.

[0054] In some implementations, the oxidation precipitation reaction in step S1 takes 0.5 to 3 hours, for example, 1.5 hours.

[0055] In some implementations, in step S1, the first slurry comprises amorphous ferric phosphate dihydrate.

[0056] In this invention, the aging treatment refers to standing at a certain temperature. This aging treatment allows amorphous ferric phosphate dihydrate to redissolve and rearrange, promoting its transformation from amorphous to a crystal with a specific ratio of orthorhombic and monoclinic phases.

[0057] In some implementations, in step S2, the aging treatment temperature is 70~85°C, for example 72°C, 75°C, 78°C or 83°C.

[0058] In some implementations, the aging process in step S2 is carried out for 3 to 7 hours, for example, 4 hours, 4.5 hours, 5 hours, 6 hours or 6.5 hours.

[0059] In some implementations, the phosphorus source added in step S2 during the aging process includes phosphoric acid.

[0060] In some implementations, in step S2, the molar ratio of phosphorus in the phosphorus source added during the aging process to iron in the iron source mentioned in step S1 is 0.22 to 0.28, for example, 0.23, 0.25 or 0.26.

[0061] In some implementations, in step S2, the second slurry comprises ferric phosphate dihydrate, which includes a monoclinic phase and an orthorhombic phase.

[0062] In some implementations, step S3, following the Joule heat flash drying, further includes pulverization and sieving.

[0063] The sieving can be performed in accordance with conventional practices in the field, such as passing through a 400-mesh sieve.

[0064] A third aspect of the present invention provides ferric phosphate dihydrate, which is prepared by the method for preparing ferric phosphate dihydrate as described above.

[0065] In this invention, the structure, properties, and performance of the ferric phosphate dihydrate are the same as those of the ferric phosphate dihydrate provided in the first aspect of this invention.

[0066] Lithium iron phosphate materials

[0067] A fourth aspect of the present invention provides a method for preparing lithium iron phosphate material, which uses iron phosphate dihydrate as described above.

[0068] In this invention, the preparation method of the lithium iron phosphate material can be carried out in accordance with conventional methods in the art, preferably including the following steps: spray drying and sintering a mixture containing the iron phosphate dihydrate, lithium source and carbon source in sequence to obtain lithium iron phosphate material.

[0069] The lithium source can be any lithium source conventionally used in the preparation of lithium iron phosphate materials, such as lithium carbonate.

[0070] The carbon source can be any carbon source conventionally used in the preparation of lithium iron phosphate materials, such as glucose.

[0071] The molar ratio of iron phosphate dihydrate, lithium source, and carbon source is, for example, 1:1:0.1.

[0072] The mixture preferably further includes a dopant. The dopant can be any dopant conventionally used in the preparation of lithium iron phosphate materials, and is preferably a titanium source, such as titanium dioxide.

[0073] The molar ratio of the iron phosphate dihydrate to the dopant is, for example, 1:0.012.

[0074] In some specific embodiments, the lithium source is lithium carbonate, the carbon source is glucose, and the mixture further includes a dopant, which is titanium dioxide; wherein the molar ratio of iron phosphate dihydrate, lithium carbonate, glucose and titanium dioxide is 1:1:0.1:0.012.

[0075] Preferably, the spray drying process further includes a grinding process; the grinding process is, for example, sand milling; the sand film is, for example, sand milled to a particle size D50 of 400 nm.

[0076] The sintering can be carried out in an inert atmosphere, such as in a nitrogen atmosphere.

[0077] The sintering temperature is, for example, 780°C.

[0078] The sintering time is, for example, 10 hours. The sintering time refers to the time spent holding the sample at the sintering temperature.

[0079] The fifth aspect of the present invention provides a lithium iron phosphate material, which is prepared by the method for preparing lithium iron phosphate material as described above.

[0080] In this invention, the lithium iron phosphate material containing doped elements or with a coating layer is also within the scope of the lithium iron phosphate material of this invention.

[0081] In some embodiments, the compaction density of the lithium iron phosphate material is 2.65 g / cm³. 3 More preferably, it is 2.65~2.75 g / cm³. 3 For example, 2.67 g / cm³ 3 2.68 g / cm 3 2.69 g / cm 3 2.70 g / cm 3 Or 2.71 g / cm 3 .

[0082] Electrochemical device

[0083] A sixth aspect of the present invention provides an electrochemical device comprising the lithium iron phosphate material as described above.

[0084] In this invention, the electrochemical device is preferably a battery.

[0085] In this invention, the electrochemical device is preferably a lithium-ion battery. The lithium-ion battery can be a liquid battery, a solid-state battery, or a semi-solid-state battery. For example, a liquid lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; a solid lithium-ion battery includes a positive electrode, a negative electrode, and a solid electrolyte. The type of battery does not limit the scope of protection of this invention.

[0086] The following uses a liquid battery as a specific embodiment to illustrate the technical content of the present invention.

[0087] In one alternative embodiment, the electrochemical device is a lithium-ion battery; the lithium-ion battery includes a negative electrode, a positive electrode, an electrolyte, and a separator, wherein the positive electrode comprises lithium iron phosphate material as described above.

[0088] Positive electrode film

[0089] In this invention, the positive electrode sheet may include a positive current collector and a positive electrode material layer, wherein the positive electrode material layer is disposed on at least one surface of the positive current collector; the positive electrode material layer includes lithium iron phosphate material as described above.

[0090] In some embodiments, the positive electrode material layer further includes a conductive agent. The conductive agent is a reagent used to ensure that the electrode has good charge-discharge performance. It can be selected from graphite materials such as natural graphite and artificial graphite; carbon black materials such as conductive carbon black (Super P), acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; carbon nanotubes; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; and conductive metal oxides or polyphenylene derivatives such as titanium dioxide, for example, conductive carbon black.

[0091] In some embodiments, the positive electrode material layer further includes a binder. The binder may be a component that facilitates bonding between the positive electrode material and the conductive agent, and also facilitates bonding between the positive electrode material and the positive electrode current collector. It can typically be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers, such as PVDF.

[0092] In some implementations, the positive electrode material layer includes lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride.

[0093] In some specific implementations, the mass ratio of the lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride is 90:5:5.

[0094] In this invention, the positive electrode current collector can be a conventional positive electrode current collector in the art. For the positive electrode current collector, materials that do not cause chemical changes and have high conductivity can be used without limitation. For example, commonly used materials include stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel materials surface-treated with carbon, nickel, titanium, silver, etc. To enhance adhesion, micro-embossing can be formed on the surface of the positive electrode current collector. The positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, or porous bodies.

[0095] In some alternative embodiments, the positive current collector is a carbon-coated aluminum foil.

[0096] In this invention, the positive electrode sheet can be prepared using methods conventional in the art.

[0097] In some alternative embodiments, the method for preparing the positive electrode includes the following steps:

[0098] After mixing the components of the positive electrode material layer in a certain mass ratio, a solvent is added and mixed evenly to obtain a positive electrode slurry. Then, the positive electrode slurry is uniformly coated on at least one surface of the positive electrode current collector. After drying, rolling, slitting and other processes, a positive electrode sheet is prepared.

[0099] negative electrode sheet

[0100] In some implementations, the negative electrode is a lithium electrode.

[0101] In other embodiments, the negative electrode sheet may include a negative electrode current collector and a negative electrode material layer, the negative electrode material layer being disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a negative electrode material.

[0102] In this invention, the negative electrode material in the negative electrode material layer can be a negative electrode material conventionally used in the art, such as graphite-based negative electrode material, silicon-oxygen-based negative electrode material, or silicon-carbon-based negative electrode material.

[0103] In some embodiments, the negative electrode material includes one or more of lithium titanate, artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon suboxide, and silicon carbide.

[0104] In some implementations, the negative electrode material layer further includes a conductive agent.

[0105] The conductive agent is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, specific materials that can be used include: graphite, such as natural or artificial graphite; carbon-based materials, such as conductive carbon black (Super P), carbon nanotubes (CNTs), acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives.

[0106] In some implementations, the negative electrode material layer further includes a binder.

[0107] The type of adhesive is not particularly limited and can be selected from polyvinylidene fluoride, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid (PAA), ethylene-propylene-diene terpolymer and its sulfonates, styrene-butadiene rubber (SBR), fluororubber and various copolymers.

[0108] In some implementations, the negative electrode material layer also includes a thickener.

[0109] The addition of the thickener can increase the viscosity of the system of each component in the negative electrode slurry. It can be a thickener commonly used in the art to prepare negative electrode sheets, such as sodium carboxymethyl cellulose (CMC).

[0110] In this invention, the negative electrode current collector can be a conventional negative electrode current collector in the art. The negative electrode current collector, serving as the substrate supporting the negative electrode material layer, is typically a metal foil with a thickness of 3-500 μm. There are no particular limitations on the material, as long as it has high conductivity and does not produce a chemical reaction in the secondary battery system. For example, it can be a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel, carbon, etc. The negative electrode current collector usually has a smooth surface, but fine textures can also be formed on its surface to improve the adhesion between the negative electrode material layer and the current collector. Besides foil, the negative electrode current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or non-woven fabric. Generally, the negative electrode current collector is copper foil.

[0111] In some embodiments, the method for preparing the negative electrode sheet includes the following steps: coating the negative electrode slurry obtained by thoroughly mixing the components of the negative electrode material layer in a solvent onto at least one surface of the negative electrode current collector, drying, cold pressing, and slitting to obtain the final product.

[0112] electrolyte

[0113] In some embodiments, the electrolyte may be a conventional electrolyte used in batteries, typically including non-aqueous solvents and lithium salts.

[0114] In this invention, the non-aqueous solvent can be a conventional non-aqueous solvent in the art.

[0115] In some embodiments, the non-aqueous solvent preferably comprises ester solvents and / or dimethyl sulfoxide (DMSO), more preferably carbonate solvents. The carbonate solvent may optionally be one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate, propylene carbonate, and butyl carbonate (BC). The non-aqueous solvent may also include ethyl acetate.

[0116] In this invention, the lithium salt can be a conventional lithium salt in the art, preferably one or more of LiPF6, LiBF4, LiClO4, LiCF3SO3 and LiN(CF3SO2)2, for example, LiPF6.

[0117] In this invention, the electrolyte may include additives, which may be conventional additives in the art, such as fluoroethylene carbonate (FEC).

[0118] In some embodiments, the electrolyte includes LiPF6, EC, DEC, and PC.

[0119] The volume ratio of EC, DEC and PC is, for example, 1:1:1.

[0120] The concentration of the lithium salt is, for example, 1M.

[0121] In some embodiments, the electrolyte can be prepared by conventional methods in the art. Optionally, it can be prepared by the following method: mixing the various non-aqueous solvents in proportion in an argon atmosphere glove box with a water content of <10 ppm, and then adding a fully dried lithium salt and mixing evenly to obtain the electrolyte.

[0122] diaphragm

[0123] In some alternative embodiments, the diaphragm may be a polypropylene membrane or a polyethylene membrane, for example, a polypropylene membrane.

[0124] In this invention, the method for preparing the lithium-ion battery can be a conventional method in the art. It can be that the positive electrode, separator, and negative electrode are wound in sequence to obtain the battery cell, which is then packaged in a packaging shell and injected with the electrolyte. Alternatively, the positive electrode, separator, and negative electrode are stacked in sequence to obtain the battery cell, which is then packaged in a packaging shell and injected with the electrolyte. After that, the lithium-ion battery is obtained through processes such as settling, hot and cold pressing, formation, clamping, and capacity testing.

[0125] A seventh aspect of the present invention provides an electronic device comprising the electrochemical device described above.

[0126] For example, the electronic devices described in this invention may be, but are not limited to, mobile devices (such as mobile phones, tablets, laptops, video recorders, portable printers / copiers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems and backup power supplies, etc.

[0127] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0128] The reagents and raw materials used in this invention are all commercially available.

[0129] Example 1

[0130] 1. Preparation of ferric phosphate dihydrate

[0131] S1. Oxidation precipitation reaction: The mixed solution of ferrous sulfate, ammonium dihydrogen phosphate and 35% hydrogen peroxide solution is pumped into the reactor according to the molar ratio of ferrous sulfate, ammonium dihydrogen phosphate and 35% hydrogen peroxide solution 1:1:0.6 (in terms of the number of moles of H2O2), and reacted at 55℃ for 1.5h to obtain the first slurry containing amorphous ferric phosphate dihydrate.

[0132] S2. Aging treatment: Add phosphoric acid (the molar ratio of phosphorus in phosphoric acid to iron in ferrous sulfate in step S1 is 0.25) to the first slurry and let it stand at 78°C for 4.5 h to obtain a second slurry containing both monoclinic and orthorhombic phases.

[0133] S3. Solid-liquid separation and Joule flash drying: The second slurry is filtered by pressure to obtain a wet filter cake; the wet filter cake is then subjected to Joule flash drying, specifically: the filter cake is evenly spread on a carbon felt, placed in a reaction chamber under a nitrogen atmosphere, and a voltage of 30V and a current of 50A are applied for 60 seconds.

[0134] The dried product was pulverized and passed through a 400-mesh sieve to obtain ferric phosphate dihydrate powder.

[0135] 2. Preparation of Lithium Iron Phosphate Materials

[0136] The lithium iron phosphate material was obtained by mixing iron phosphate dihydrate with lithium carbonate, glucose, and titanium dioxide (the molar ratio of iron phosphate dihydrate:lithium carbonate:glucose:titanium dioxide was 1:1:0.1:0.012), milling (controlling the particle size D50=400nm), spray drying, and sintering at 780℃ for 10h in a nitrogen atmosphere.

[0137] Example 2

[0138] The difference between this embodiment and embodiment 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.28 and it is left to stand at 85°C for 3 hours; (2) in step S3, a voltage of 10V and a current of 10A are applied and the current is applied for 300 seconds.

[0139] The other steps and conditions are the same as in Example 1.

[0140] Example 3

[0141] The difference between this embodiment and embodiment 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.22 and it is left to stand at 72°C for 6 hours; (2) in step S3, a voltage of 50V and a current of 100A are applied and the current is applied for 30 seconds.

[0142] The other steps and conditions are the same as in Example 1.

[0143] Example 4

[0144] The difference between this embodiment and embodiment 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.25 and it is left to stand at 83°C for 6.5h; (2) in step S3, a voltage of 25V and a current of 35A are applied and the current is applied for 120 seconds.

[0145] The other steps and conditions are the same as in Example 1.

[0146] Example 5

[0147] The difference between this embodiment and embodiment 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.23 and it is left to stand at 75°C for 5 hours; (2) in step S3, a voltage of 30V and a current of 30A are applied and the current is applied for 200 seconds.

[0148] The other steps and conditions are the same as in Example 1.

[0149] Example 6

[0150] The difference between this embodiment and embodiment 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.26 and it is left to stand at 78°C for 4 hours; (2) in step S3, a voltage of 40V and a current of 20A are applied and the current is applied for 240 seconds.

[0151] The other steps and conditions are the same as in Example 1.

[0152] Comparative Example 1

[0153] The difference between this comparative example and Example 1 is that in the preparation of ferric phosphate dihydrate, in step S3, the moist filter cake is not subjected to Joule heat flash drying, but is placed in an 80°C forced-air oven for drying for 6 hours.

[0154] The other steps and conditions are the same as in Example 1.

[0155] Comparative Example 2

[0156] The difference between this comparative example and Example 1 is that in the preparation of iron phosphate dihydrate, (1) in step S2, P:Fe is 0.24 and it is left to stand at 70°C for 7 hours; (2) in step S3, a voltage of 20V and a current of 30A are applied and the current is applied for 100 seconds.

[0157] The other steps and conditions are the same as in Example 1.

[0158] Example 1

[0159] The crystal phase ratio d and specific surface area of ​​monoclinic and orthorhombic phases were tested for the iron phosphate dihydrate prepared in Examples 1-6 and Comparative Examples 1-2, respectively. The compaction density of the prepared lithium iron phosphate materials was also tested. The specific testing methods are as follows:

[0160] 1. The phase ratio d of monoclinic and orthorhombic phases in ferric phosphate dihydrate

[0161] The ferric phosphate dihydrate prepared in each example and comparative example was tested using an XRD diffractometer, and the XRD patterns were obtained. The comparison diagrams of ferric phosphate dihydrate from Example 1 and Comparative Example 1 with the orthorhombic phase XRD (PDF#33-0667) are shown below. Figure 1 As shown; the comparison diagrams of ferric phosphate dihydrate and monoclinic phase XRD (PDF#33-0666) of Example 1 and Comparative Example 1 are as follows. Figure 2 As shown; Figure 1 The enlarged views of the corresponding horizontal coordinates within the range of 10° to 20° and the enlarged views of the corresponding horizontal coordinates within the range of 25° to 35° are shown below. Figure 3 and Figure 4 As shown.

[0162] from Figure 2 It can be seen that both Example 1 and Comparative Example 1 contain a monoclinic phase of ferric phosphate dihydrate; from Figure 3 and Figure 4 It can be seen that the XRD spectrum of ferric phosphate dihydrate obtained in Example 1 has characteristic peaks of (1 1 1) (2θ = 16.08°), (1 2 2) (2θ = 28.64°), (3 1 1) (2θ = 29.74°), and (1 31) (2θ = 30.28°) belonging to the orthorhombic phase of ferric phosphate dihydrate, while these are not present in Comparative Example 1. This proves that both the monoclinic and orthorhombic phases exist in the ferric phosphate dihydrate obtained in Example 1, while the ferric phosphate dihydrate obtained in Comparative Example 1 only contains the monoclinic phase of ferric phosphate dihydrate.

[0163] The crystal phase ratios d of the oblique and orthorhombic phases were calculated based on the XRD spectra obtained from the above tests, as follows:

[0164] The crystal phase ratio refers to the ratio of the sum of the integrated intensities of the XRD characteristic peaks of corresponding phases. The crystal phase ratio of the monoclinic phase to the orthorhombic phase is the ratio of the sum of the integrated intensities of the XRD characteristic peaks corresponding to the monoclinic phase to the sum of the integrated intensities of the XRD characteristic peaks corresponding to the orthorhombic phase. In the XRD spectrum of ferric phosphate dihydrate, the characteristic peaks with 2θ greater than 45° have relatively low intensities. In this invention, the crystal phase ratios of the monoclinic and orthorhombic phases are calculated based on the characteristic peaks with 2θ not greater than 45° in the XRD spectrum of ferric phosphate dihydrate. The specific results are shown in Table 1.

[0165] 2. Specific surface area of ​​ferric phosphate dihydrate

[0166] The determination was performed using the volumetric method specified in GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0167] Testing instrument: Micromeritics TriStar II Plus surface area and pore size analyzer.

[0168] Test steps:

[0169] Accurately weigh the ferric phosphate dihydrate powder sample to be tested and place it in a sample tube. Heat the sample under vacuum conditions, with a degassing temperature of 120℃~150℃. Simultaneously, evacuate the system vacuum to < 10. -2 After degassing (Pa), turn off the heating system and allow the sample tube to cool naturally to room temperature under vacuum. Then, remove the sample tube and accurately weigh the total mass of the sample tube containing the sample using a balance; this will be used to calculate the net mass of the sample later. Ensure the BET analyzer is in normal working order and set the correct test parameters, setting the adsorbate to high-purity nitrogen (purity ≥99.999%) and the test temperature to liquid nitrogen temperature (77K). After the test, the software automatically calculates the final specific surface area value, in m². 2 / g. The test results are shown in Table 1.

[0170] 3. Compacted density of lithium iron phosphate materials

[0171] Testing instrument: Yuaneng Technology Powder Resistivity Compacted Density Tester PRCD3100.

[0172] Test Procedure: Accurately weigh 1.0 g of the lithium iron phosphate material to be tested using a balance. Evenly pack the sample into the cavity of the dedicated tableting mold and place the upper pressure rod. Place the mold in the center of the lower pressure plate of the compactor and set the test parameters. Apply a pressure of 3 tons (T) for compaction. After the test, the instrument automatically calculates and displays the compacted density of the powder (unit: g / cm³). 3 The test results are shown in Table 1.

[0173] Table 1

[0174]

[0175] Example 2

[0176] The lithium iron phosphate materials prepared in Examples 1-6 and Comparative Examples 1-2 were used as cathode materials for the following cathode sheet preparation and battery assembly, specifically:

[0177] Preparation of positive electrode:

[0178] (1) Slurry mixing: Lithium iron phosphate material, conductive carbon black (Super P) and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 90:5:5, and an appropriate amount of N-methylpyrrolidone (NMP) solvent is added and stirred evenly to form a positive electrode slurry.

[0179] (2) Coating and drying: The prepared positive electrode slurry was then uniformly coated onto carbon-coated aluminum foil and dried in a vacuum drying oven at 110°C for 2 hours.

[0180] (3) Rolling and cutting: The dried electrode sheet is rolled under 3T pressure and the positive electrode sheet is cut out by a tablet press.

[0181] Battery assembly:

[0182] The negative electrode uses commercial lithium foil, and the separator is Celgard 2400 polypropylene (PP) membrane. The positive electrode and lithium foil are assembled in an argon-filled glove box with water and oxygen content of less than 5 ppm. At the same time, 10 drops of electrolyte are added to obtain the CR2032 button half cell.

[0183] After assembly, let it stand in a constant temperature cabinet at 25℃ for 12 hours.

[0184] The electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) in a volume ratio of 1:1:1, with a lithium hexafluorophosphate concentration of 1.0 M.

[0185] For the lithium iron phosphate materials prepared in Examples 1-6 and Comparative Examples 1-2, six coin half-cells were assembled according to the above method, and the following electrochemical performance tests were performed. The average value of the test results of the corresponding six coin half-cells was taken as the final test result.

[0186] Electrochemical performance testing:

[0187] The discharge capacity was tested using the Blue Electric M340A testing system. The capacity of the button half-cell prepared above was 2.7 mAh; the discharge capacity was tested at the following rates: 0.1C current was 0.27 mA, 1C current was 2.7 mA, and 5C current was 13.5 mA.

[0188] The specific testing method is as follows:

[0189] (1) 0.1C discharge capacity test method: In the voltage range of 2.0~3.75V, charge at a constant current density of 0.1C to 3.75V, then charge at a constant voltage until the current is less than 0.01C; then discharge at a constant current density of 0.1C to 2.0V. Record the discharge capacity of this process, which is the 0.1C discharge capacity.

[0190] (2) 1C discharge capacity test method: Based on the battery that has undergone the above 0.1C discharge capacity test, the 1C discharge capacity test is carried out. Specifically: within the voltage range of 2.0~3.75V, the battery is charged at a constant current density of 0.1C to 3.75V, and then charged at a constant voltage until the current is less than 0.01C; subsequently, it is discharged at a constant current density of 1C to 2.0V. The discharge capacity of this process is recorded, which is the 1C discharge capacity.

[0191] (3) 5C discharge capacity test method: Based on the battery that has undergone the above 1C discharge capacity test, the 5C discharge capacity test is carried out. Specifically: within the voltage range of 2.0~3.75V, the battery is charged at a constant current density of 0.1C to 3.75V, and then charged at a constant voltage until the current is less than 0.01C; subsequently, it is discharged at a constant current density of 5C to 2.0V. The discharge capacity of this process is recorded, which is the 5C discharge capacity.

[0192] The final test results are shown in Table 2.

[0193] Table 2

[0194]

[0195] As shown in Tables 1 and 2, the lithium iron phosphate materials prepared in Examples 1-6 of this invention using iron phosphate dihydrate with a specific ratio of monoclinic and orthorhombic phases as raw materials exhibit excellent compaction density, specifically reaching 2.65 g / cm³. 3 In summary, electrochemical devices (especially lithium-ion batteries) using this lithium iron phosphate material as the cathode material exhibit excellent capacity performance at different discharge rates. Specifically, the discharge capacity at 0.1C can reach over 160 mAh / g, the discharge capacity at 1C can reach over 141 mAh / g, and even over 145 mAh / g, and the discharge capacity at 5C can reach over 110 mAh / g, and even over 120 mAh / g.

[0196] As can be seen from Example 1 and Comparative Example 1, the iron phosphate dihydrate obtained in Comparative Example 1 does not contain an orthorhombic phase, but is only a pure monoclinic phase. The compaction density of the lithium iron phosphate material prepared using this iron phosphate dihydrate as a raw material is significantly reduced. The capacity performance of lithium-ion batteries obtained using this lithium iron phosphate material as a cathode material is worse, especially the 5C discharge capacity is significantly reduced.

[0197] As shown in Example 1 and Comparative Example 2, even when iron phosphate dihydrate contains both monoclinic and orthorhombic phases, the compaction density of lithium iron phosphate material prepared using this iron phosphate dihydrate as a raw material is significantly reduced when the proportion of monoclinic and orthorhombic phases is too low (Comparative Example 2). The capacity performance of lithium-ion batteries obtained using this lithium iron phosphate material as a cathode material is worse, especially the 5C discharge capacity is significantly reduced.

[0198] As can be seen from Examples 1 to 6, when the ratio d of the monoclinic phase and orthorhombic phase of the obtained iron phosphate dihydrate is in the range of 1.5 to 2 or 4 to 6, the lithium iron phosphate material prepared using this iron phosphate dihydrate as raw material has better capacity performance as the cathode material, especially the 5C discharge capacity.

[0199] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and the scope of protection of the present invention is defined by the appended claims. Those skilled in the art can make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but all such changes and modifications fall within the scope of protection of the present invention.

Claims

1. A type of ferric phosphate dihydrate, characterized in that, It includes a monoclinic phase and an orthorhombic phase, and the crystal phase ratio d of the monoclinic phase and the orthorhombic phase satisfies: 1.5≤d≤6.

2. The ferric phosphate dihydrate according to claim 1, characterized in that, It meets one or more of the following conditions: (a) The specific surface area of ​​the ferric phosphate dihydrate is 26~70 m². 2 / g; (b) The molar ratio of iron to phosphorus in the iron phosphate dihydrate is 0.95 to 0.99; (c) The crystal phase ratio d of the monoclinic phase and the orthorhombic phase satisfies: 2≤d≤5.

3. A method for preparing ferric phosphate dihydrate, characterized in that, It includes the following steps: S1. An oxidation precipitation reaction is carried out on a mixed solution containing an iron source, a phosphorus source and an oxidant to obtain the first slurry; S2. The first slurry is aged to obtain the second slurry; S3. The second slurry is subjected to solid-liquid separation, and the resulting solid is dried by Joule heat flash drying.

4. The method for preparing ferric phosphate dihydrate according to claim 3, characterized in that, The Joule heat flash drying meets one or more of the following conditions: (a) The voltage is 10~50V; (b) Current is 10~100A; (c) The drying time is 30~300s; (d) The process is carried out under an inert atmosphere.

5. The method for preparing ferric phosphate dihydrate according to claim 3, characterized in that, It meets one or more of the following conditions: (a) In step S1, the iron source includes one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, ferric nitrate, and ferric acetate; (b) In step S1, the phosphorus source includes one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; (c) In step S1, the oxidant includes hydrogen peroxide; (d) In step S1, the molar ratio of the iron source, phosphorus source and oxidant is 1:1:(0.55~0.65). (e) In step S1, the temperature of the oxidation precipitation reaction is 40~65℃; (f) In step S1, the oxidation precipitation reaction takes 0.5 to 3 hours; (g) In step S1, the first slurry comprises amorphous ferric phosphate dihydrate; (h) In step S2, the aging treatment temperature is 70~85℃; (i) In step S2, the aging process takes 3 to 7 hours; (j) In step S2, the phosphorus source added during the aging process includes phosphoric acid; (k) In step S2, the molar ratio of phosphorus in the phosphorus source added during the aging process to iron in the iron source mentioned in step S1 is 0.22 to 0.

28. (l) In step S2, the second slurry includes ferric phosphate dihydrate, which contains a monoclinic phase and an orthorhombic phase; (m) In step S3, the Joule heat flash drying process further includes pulverization and sieving.

6. A type of ferric phosphate dihydrate, characterized in that, It is prepared by the method of any one of claims 3 to 5 for the preparation of ferric phosphate dihydrate.

7. A method for preparing lithium iron phosphate material, characterized in that, It uses ferric phosphate dihydrate as described in any one of claims 1, 2 and 6.

8. A lithium iron phosphate material, characterized in that, It is prepared using the method for preparing lithium iron phosphate material as described in claim 7.

9. An electrochemical device, characterized in that, It includes the lithium iron phosphate material as described in claim 8.

10. An electronic device, characterized in that, It includes the electrochemical device as described in claim 9.

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