Lithium iron phosphate positive electrode material, preparation method and application

Abstract

The invention discloses a lithium iron phosphate positive electrode material, a preparation method and application, the lithium iron phosphate positive electrode material comprises a matrix and a carbon coating layer coating the matrix, the corrosion resistance omega of the lithium iron phosphate positive electrode material is 0%-40%, omega = (betaI1) / (alphaI0), and alpha is the mass fraction of iron elements except ferrous iron in the lithium iron phosphate positive electrode material; beta is the mass fraction of elemental iron in the lithium iron phosphate positive electrode material; i1 / I0 is related to the diffraction intensity of a specified crystal face in an XRD (X-Ray Diffraction) diagram of the lithium iron phosphate positive electrode material and the diffraction intensity of a corresponding crystal face in a standard XRD card; the specified crystal face is one of crystal faces (101), (112), (011), (010) and (001). The corrosion resistance omega is limited in a reasonable range, so that the cycle performance and the rate capability can be considered.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode materials, in particular to a lithium iron phosphate cathode material, a preparation method and application. BACKGROUND

[0002] With the rapid development of lithium ion battery technology, lithium iron phosphate (LiFePO4) has become one of the widely used cathode materials in power batteries and energy storage systems due to its high safety, long cycle life, low cost and environmental friendliness. In practical applications, in order to improve the electronic conductivity and ion transport performance of lithium iron phosphate materials, a conductive carbon layer is usually formed on the surface of the lithium iron phosphate materials by carbon coating technology to improve their electrochemical performance.

[0003] The carbon coating process mainly includes solid phase method, sol-gel method, hydrothermal method and spray drying method, etc. By introducing organic carbon sources (such as glucose, sucrose, citric acid, pitch, etc.) on the surface of the precursor or finished material, a uniform or partially covered carbon layer structure is formed by high temperature pyrolysis. The carbon layer can effectively enhance the electronic conduction between particles, inhibit the side reaction between active materials and electrolyte, thereby improving the cycle stability and coulombic efficiency of the material.

[0004] However, in the actual preparation process, the distribution of the carbon coating layer is often difficult to accurately control. On the one hand, if the carbon source is not mixed uniformly or the pyrolysis conditions are not appropriate, the carbon coating layer will have different thicknesses, resulting in local thinning or missing areas, which will cause structural degradation of the material during long-term charging and discharging, and thus significantly reduce its cycle performance. On the other hand, in order to pursue uniformity of coating, the process parameters may be excessively optimized, which may result in a too dense or continuous carbon layer, hindering the rapid diffusion of lithium ions on the surface of the electrode material, thereby limiting the rate performance of the material. Therefore, how to ensure good cycle performance while taking into account excellent rate performance has become a key challenge for current carbon-coated lithium iron phosphate cathode materials. SUMMARY

[0005] The purpose of the present application is to provide a lithium iron phosphate cathode material, a preparation method and application, which is beneficial to lithium ion batteries with both cycle performance and rate performance.

[0006] The present application is implemented as follows:

[0007] In a first aspect, the present application provides a lithium iron phosphate cathode material, which comprises a substrate and a carbon coating layer coating the substrate, and the corrosion resistance strength ω of the lithium iron phosphate cathode material is 0%-40%, and ω=(βI1) / (αI0), wherein,

[0008] α is the mass fraction of iron elements other than divalent iron in the lithium iron phosphate cathode material;

[0009] β is the mass fraction of elemental iron in the lithium iron phosphate positive electrode material;

[0010] I0 is the standard diffraction intensity of the specified crystal plane recorded in the standard XRD card in the lithium iron phosphate crystal;

[0011] I1 = I' 最强 I1' / I 最强 ; wherein I' 最强 is the strongest peak in the standard XRD card; I 最强 is the diffraction intensity of the crystal plane corresponding to I' 最强 in the XRD pattern of the lithium iron phosphate positive electrode material; I1' is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of the lithium iron phosphate positive electrode material;

[0012] The specified crystal plane is one of (101), (112), (011), (010), and (001).

[0013] In an optional embodiment, the substrate is lithium iron phosphate doped with titanium;

[0014] And / or, α is 0%-1.5%;

[0015] And / or, β is 0%-0.2%;

[0016] And / or, I1 / I0 is 1.2-3.

[0017] In an optional embodiment, the detection method of α includes: taking a lithium iron phosphate positive electrode material sample m0 = 5.0000 g ± 0.0010 g, soaking it in 10 ml of dilute sulfuric acid with a concentration of 0.01 mol / L at 18-30°C for 10 min, and detecting the concentration c0 g / ml of dissolved iron ions by potential titration method, α = (c0*10 / m0)*100%;

[0018] And / or, the detection method of β includes: taking a lithium iron phosphate positive electrode material sample m1 = 5.0000 g ± 0.0010 g, reacting it with 10 ml of copper sulfate solution with a concentration of 0.1 mol / L at 18-30°C for 10 min, and then detecting the concentration c1 g / ml of iron ions in the solution by ICP, β = (c1*10 / m1)*100%.

[0019] In a second aspect, the present application provides a preparation method of the lithium iron phosphate positive electrode material of any one of the preceding embodiments, comprising:

[0020] Placing a pre-reaction liquid containing a lithium source, an iron source, a phosphorus source, a first titanium source, an additive, and an inducer under hydrothermal reaction conditions to perform hydrothermal reaction, to obtain a post-hydrothermal reaction liquid; the inducer includes at least one of citric acid, piperidine, and isopropyl alcohol;

[0021] Separating the solid phase in the hydrothermal reaction post-liquid, and sequentially performing acid washing, water washing, tetravalent titanium source soaking and drying on the solid phase to obtain a to-be-calcined material;

[0022] Performing reduction calcination and magnetic removal on the to-be-calcined material in sequence to obtain the lithium iron phosphate positive electrode material.

[0023] In an optional embodiment, the molar ratio of lithium source, phosphorus source, iron source, inducer, and first titanium source in the pre-reaction liquid is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1);

[0024] And / or, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate;

[0025] And / or, the additive is selected from ascorbic acid;

[0026] And / or, the molar ratio of the additive to iron in the iron source is 1.5%-2.5%;

[0027] And / or, the mass fraction of water in the pre-reaction liquid is 40%-60%.

[0028] In an optional embodiment, the temperature of the hydrothermal reaction is 160°C-200°C; and the hydrothermal reaction time is 5h-7h;

[0029] And / or, the pH of the pre-reaction liquid is 6.0-8.0;

[0030] And / or, the washing liquid used for acid washing is dilute sulfuric acid with a concentration of 0.005mol / L-0.1mol / L.

[0031] In an optional embodiment, the tetravalent titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate;

[0032] And / or, the soaking solution used in the tetravalent titanium source soaking step is a 0.1mol / L-0.2mol / L tetravalent titanium source solution, and the mass ratio of the solid phase to the soaking solution in the tetravalent titanium source soaking step is 1:(1-3), and the soaking time is 8min-12min.

[0033] In an optional embodiment, the temperature of the reduction calcination step is 550°C-750°C, and the time is 2h-6h;

[0034] And / or, the atmosphere of the reduction calcination step includes inert gas and carbon monoxide, and the volume fraction of carbon monoxide in the atmosphere is 1%-3%.

[0035] In a third aspect, the present application provides a pole piece comprising the lithium iron phosphate positive electrode material according to any one of the preceding embodiments.

[0036] In a fourth aspect, the present application provides a lithium ion battery comprising the pole piece according to the preceding embodiments.

[0037] The present application has the following beneficial effects:

[0038] The increase of the corrosion resistance strength ω of the lithium iron phosphate positive electrode material is conducive to the improvement of the cycle performance, but if the corrosion resistance strength ω is too high, the crystal growth direction will change, resulting in too small migration rate of lithium ions along the b-axis, which will lead to the reduction of the rate performance, and the corrosion resistance strength ω of the lithium iron phosphate positive electrode material in the present application is limited within a reasonable range, which is conducive to the consideration of both cycle performance and rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0040] Figure 1 XRD pattern of the lithium iron phosphate positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely. The specific conditions are not specified in the embodiments, and the conventional conditions or the conditions suggested by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be purchased on the market.

[0042] The embodiments of the present application provide a lithium iron phosphate positive electrode material, wherein the corrosion resistance strength ω of the lithium iron phosphate positive electrode material is 0%-40%, and ω=(βI1) / (αI0), wherein,

[0043] α is the mass fraction of iron elements other than divalent iron in the lithium iron phosphate positive electrode material;

[0044] β is the mass fraction of elemental iron in the lithium iron phosphate positive electrode material;

[0045] I0 is the standard diffraction intensity recorded in the standard XRD card for a specified crystal face in the lithium iron phosphate crystal;

[0046] I1 = I' 最强 I1' / I 最强 ; wherein I' 最强I0is the diffraction intensity of the strongest peak in the standard XRD card; I is the diffraction intensity of the strongest peak in the XRD pattern of the lithium iron phosphate positive electrode material; 最强 I' is the diffraction intensity of the strongest peak in the XRD pattern of the lithium iron phosphate positive electrode material; I0is the diffraction intensity of the strongest peak in the standard XRD card; 最强 I' is the diffraction intensity of the strongest peak in the XRD pattern of the lithium iron phosphate positive electrode material; I0is the diffraction intensity of the strongest peak in the standard XRD card;

[0047] The specified crystal face is one of (101), (112), (011), (010), and (001).

[0048] The applicant found that increasing the corrosion resistance strength ω of the lithium iron phosphate positive electrode material is conducive to improving the cycle performance, but if the corrosion resistance strength ω is too high, the carbon coating layer is too uniform and dense, which will lead to a decrease in the rate performance, therefore, the corrosion resistance strength ω of the lithium iron phosphate positive electrode material in the present application is limited in the range of 0%-40%, for example, 0%, 4.4%, 8.9%, 13.3%, 17.8%, 22.2%, 26.7%, 31.1%, 35.6%, and 40%, which is conducive to taking into account the cycle performance and the rate performance.

[0049] The corrosion resistance strength ω in the present application is related to the exposed crystal face of the lithium iron phosphate positive electrode material, and the density and uniformity of the carbon coating layer on the surface of the lithium iron phosphate positive electrode material are different when the exposed crystal face of the lithium iron phosphate positive electrode material is different, and increasing the proportion of the specified crystal face such as (101), (112), (011), (010), and (001) is conducive to improving the density and uniformity of the carbon coating layer, and further conducive to improving the cycle performance.

[0050] In an optional embodiment, the substrate is lithium iron phosphate doped with titanium, and the titanium doping is conducive to taking into account the cycle performance and the rate performance.

[0051] In an optional embodiment, α is 0%-1.5%, for example, 0%, 0.17%, 0.33%, 0.5%, 0.67%, 0.83%, 1.0%, 1.17%, 1.33%, and 1.5%;

[0052] And / or, β is 0%-0.2%, for example, 0%, 0.02%, 0.04%, 0.06%, 0.08%, 0.10%, 0.12%, 0.14%, 0.16%, 0.18%, and 0.2%.

[0053] In an optional embodiment, I1 / I0is 1.2-3, for example, 1.2, 1.3, 1.4, 1.5, 1.8, 2.0, 2.2, 2.4, 2.8, 3.0, and can also be other values in the range of 1.2-3.

[0054] The trivalent iron ions or metallic iron existing in the lithium iron phosphate positive electrode material are easily dissolved by hydrofluoric acid, migrated to the negative electrode and deposited on the surface of the negative electrode, and the electrochemical deposition is easy to induce internal short circuit of the battery, so the trivalent iron ions or metallic iron should be reduced as much as possible.

[0055] In an optional embodiment, the detection method of alpha includes: placing a lithium iron phosphate positive electrode material sample m0=5.0000g-0.0010g in 18-30℃ dilute sulfuric acid 10ml with a concentration of 0.01mol / L for 10min, detecting the concentration c0 g / ml of the dissolved iron ions by the potential titration method, and alpha=(c0*10 / m0)*100%.

[0056] The lithium iron phosphate crystal structure is stable, and the low hydrogen ion concentration of dilute sulfuric acid is not enough to destroy the crystal lattice; while the trivalent iron compound is mostly weak acid salt or oxide, which can be gradually dissolved in dilute sulfuric acid to generate soluble Fe 3 + sulfate; the metallic iron is a active metal, which can displace the dilute sulfuric acid to realize the dissolution of the metallic iron.

[0057] In an optional embodiment, the detection method of beta includes: placing a lithium iron phosphate positive electrode material sample m1=5.0000g±0.0010g in 18-30℃ copper sulfate solution 10ml with a concentration of 0.1mol / L for 10min, and then detecting the concentration c1 g / ml of the iron ions in the solution by ICP, and beta=(c1*10 / m1)*100%.

[0058] The metallic iron is a active metal, which can displace the copper in the copper sulfate solution, and the content of the displaced iron ions can be used to calculate the mass of the metallic iron in the lithium iron phosphate positive electrode material, and then calculate beta.

[0059] The embodiment of the application further provides a preparation method of the lithium iron phosphate positive electrode material.

[0060] The reaction front liquid containing a lithium source, an iron source, a phosphorus source, a first titanium source, an additive and an inducing agent is placed under hydrothermal reaction conditions to perform hydrothermal reaction, so as to obtain a post-hydrothermal reaction liquid; the inducing agent includes at least one of citric acid, piperidine and isopropyl alcohol;

[0061] The solid phase in the post-hydrothermal reaction liquid is separated out, and the solid phase is sequentially subjected to acid washing, water washing, tetravalent titanium source soaking and drying, so as to obtain a to-be-calcined material;

[0062] The to-be-calcined material is sequentially subjected to reduction calcination and magnetic removal, so as to obtain the lithium iron phosphate positive electrode material.

[0063] In the preparation method of the lithium iron phosphate positive electrode material, because the activation energies of the crystal faces of the lithium iron phosphate are inconsistent, in the hydrothermal process, the crystal faces with high activation energy grow first and finally close, leaving the crystal faces with low activation energy exposed. In the present application, the addition of the inducer can change the growth order of the crystal faces with different activation energies, so that the specified crystal face of the lithium iron phosphate obtained finally is exposed, thereby affecting the uniformity of the carbon coating layer, and the lithium iron phosphate with a corrosion resistance strength ω in a reasonable range is obtained. Specifically, the -OH in isopropyl alcohol can prevent the rapid growth of the surface with high activation energy, and finally the surface with high activation energy is exposed; the N lone pair of electrons in piperidine has strong binding ability with the non-polar crystal face, preventing the growth of the high non-polar crystal face, and through different inducers, the purpose of inducing the growth or exposure of different crystal faces can be achieved.

[0064] Further, in the present application, the product is soaked in a titanium source after the hydrothermal reaction, so that in the subsequent reduction and calcination process, the titanium source existing on the surface of the lithium iron phosphate can act as a protective agent to avoid the excessive reduction of high-valence iron to generate elemental iron and avoid the excessive content of elemental iron.

[0065] In an optional embodiment, the molar ratio of the lithium source, the phosphorus source, the iron source, the inducer, and the first titanium source in the reaction solution is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1), for example, (1.00:0.98:1.00:0.02:0.01), (1.01:0.982:1.01:0.03:0.02), (1.02:0.984:1.02:0.04:0.03), (1.03:0.986:1.03:0.05:0.04), (1.04:0.988:1.04:0.06:0.05), (1.05:0.99:1.05:0.07:0.06), (1.045:0.992:1.045:0.08:0.07), (1.035:0.994:1.035:0.09:0.08), (1.025:0.996:1.025:0.095:0.09), (1.015:0.998:1.015:0.10:0.10); the inducer can also act as a carbon source to form a carbon coating layer after calcination, thereby improving the cycle performance of the lithium iron phosphate positive electrode material. It should be noted that the lithium source in the reaction solution can be lithium carbonate or lithium hydroxide, etc.

[0066] In an optional embodiment, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate.

[0067] In an optional embodiment, the additive is selected from ascorbic acid, which can prevent the oxidation of divalent iron;

[0068] and / or, the ratio of the additive to the substance amount of iron in the iron source is 1.5%-2.5%, for example, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%.

[0069] The sufficient amount of the additive can reduce the oxidation of the divalent iron.

[0070] and / or, the temperature of the hydrothermal reaction is 160°C-200°C, for example, 160°C, 164°C, 168°C, 172°C, 176°C, 180°C, 184°C, 188°C, 192°C, 196°C, 200°C; the hydrothermal reaction time is 5h-7h, for example, 5h, 5.2h, 5.4h, 5.6h, 5.8h, 6.0h, 6.2h, 6.4h, 6.6h, 6.8h, 7h;

[0071] and / or, the pH of the pre-reaction solution is 6.0-8.0, for example, 6.0, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, 8.0.

[0072] and / or, the mass fraction of water in the pre-reaction solution is 40%-60%, for example, 40%, 45%, 50%, 55%, 60%.

[0073] During the hydrothermal reaction, since the precipitation solubility product of titanium phosphate is less than that of iron phosphate, the doping of titanium ions in the hydrothermal process can be achieved.

[0074] In the optional embodiment, the pickling solution is dilute sulfuric acid with a concentration of 0.005mol / L-0.1mol / L, which reduces the impurities in the lithium iron phosphate, and the concentration of the dilute sulfuric acid is, for example, 0.005mol / L, 0.010mol / L, 0.050mol / L, 0.055mol / L, 0.060mol / L, 0.065mol / L, 0.070mol / L, 0.075mol / L, 0.080mol / L, 0.085mol / L, 0.090mol / L, 0.095mol / L, 0.100mol / L.

[0075] In the optional embodiment, the source of positive tetravalent titanium is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate;

[0076] And / or, the concentration of the tetravalent titanium source solution used in the tetravalent titanium source soaking step is 0.1 mol / L-0.2 mol / L, for example 0.10 mol / L, 0.11 mol / L, 0.12 mol / L, 0.13 mol / L, 0.14 mol / L, 0.15 mol / L, 0.16 mol / L, 0.17 mol / L, 0.18 mol / L, 0.19 mol / L, 0.20 mol / L, the mass ratio of the solid phase to the soaking solution in the tetravalent titanium source soaking step is 1:(1-3), for example 1:1, 1:1.22, 1:1.44, 1:1.67, 1:1.89, 1:2.11, 1:2.33, 1:2.56, 1:2.78, 1:3; the soaking time is 8 min-12 min, for example 8 min, 8.4 min, 8.8 min, 9.2 min, 9.6 min, 10 min, 10.4 min, 10.8 min, 11.2 min, 11.6 min, 12 min.

[0077] The introduction of a sufficient amount of tetravalent titanium source is beneficial to reduce the elemental iron generated in the reducing calcination step, but an excessive amount of tetravalent titanium source will lead to an excessively high titanium doping amount, which is not conducive to the balance of cycle performance and rate performance. Therefore, it is necessary to reasonably set the concentration of the tetravalent titanium source in the soaking solution and the parameters such as the amount of the soaking solution and the soaking time.

[0078] In an optional embodiment, the temperature of the reducing calcination step is 550℃-750℃, for example 550℃, 570℃, 590℃, 610℃, 630℃, 650℃, 670℃, 690℃, 710℃, 730℃, 750℃; the time is 2h-6h, for example 2h, 2.4h, 2.8h, 3.2h, 3.6h, 4.0h, 4.4h, 4.8h, 5.2h, 5.6h, 6h;

[0079] And / or, the atmosphere of the reducing calcination step includes inert gas and carbon monoxide, and the volume fraction of carbon monoxide in the atmosphere is 1%-3%, for example 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%.

[0080] The reducing calcination can reduce the trivalent iron possibly existing in the precursor to divalent iron, while inhibiting the oxidation of the divalent iron to trivalent iron at high temperature, which is beneficial to generate pure-phase lithium iron phosphate and reduce the structural defects and electrochemical performance decline caused by the existence of trivalent iron.

[0081] The embodiments of the present application also provide a pole piece comprising the lithium iron phosphate positive electrode material according to any one of the preceding embodiments.

[0082] The embodiment of the present application also provides a lithium ion battery comprising the pole piece.

[0083] The features and performances of the present application are further described in detail below in combination with embodiments.

[0084] Embodiment 1

[0085] The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, and specifically comprises the following steps:

[0086] Nitrogen is introduced into a hydrothermal reactor containing deionized water to remove oxygen in the deionized water, lithium hydroxide, phosphoric acid, ferrous sulfate and titanium oxalate are weighed according to a mass ratio of 1.05:1:1.02:0.02 and then added into the hydrothermal reactor, ascorbic acid with a mass ratio of 2% of the mass ratio of iron in the ferrous sulfate is weighed and added into the reactor, a PH regulator is added into the reactor to adjust the PH to 7, and a reaction liquid with a water mass fraction of 50% is obtained.

[0087] The temperature in the hydrothermal reactor is adjusted to 180℃, and hydrothermal reaction is performed for 6h, after the hydrothermal reaction is completed, the temperature is lowered, and solid-liquid separation is performed on the reaction liquid after the hydrothermal reaction, the solid phase is cleaned with dilute sulfuric acid with a concentration of 0.01mol / L and then deionized water, then the solid phase after the cleaning is immersed in a tetrabutyl titanate aqueous solution with a mass concentration of 0.2mol / L, the mass ratio of the solid phase after the cleaning to the tetrabutyl titanate aqueous solution is 1:2, the immersion time is 10min, after the immersion is completed, the solid phase is separated out and then vacuum dried, and a calcination material is obtained.

[0088] The calcination material is placed in a nitrogen / carbon monoxide mixed gas atmosphere for reduction calcination, the volume fraction of carbon monoxide in the mixed gas atmosphere is 2%, the calcination temperature is 700℃, the calcination time is 4h, after the calcination is completed, the lithium iron phosphate positive electrode material is obtained by cooling and removing magnetism, and an XRD pattern is as shown in Figure 1 .

[0089] Embodiment 2

[0090] The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, and specifically comprises the following steps:

[0091] Nitrogen is introduced into a hydrothermal reactor containing deionized water to remove oxygen in the deionized water, lithium hydroxide, phosphoric acid, ferrous sulfate and titanium oxalate are weighed according to a mass ratio of 1.05:1:1.02:0.02 and then added into the hydrothermal reactor, ascorbic acid with a mass ratio of 2% of the mass ratio of iron in the ferrous sulfate is weighed and added into the reactor, a PH regulator is added into the reactor to adjust the PH to 7, and a reaction liquid with a water mass fraction of 50% is obtained.

[0092] Adjust the temperature of the hydrothermal reactor to 180℃ and hydrothermal reaction for 6h. After the hydrothermal reaction, cool down and separate the solid and liquid. Clean the solid with dilute sulfuric acid with a concentration of 0.01mol / L, then clean with deionized water. Then soak the solid in a titanium ethyl ester aqueous solution with a concentration of 0.1mol / L, wherein the mass ratio of the solid to the titanium ethyl ester aqueous solution is 1:1, and the soaking time is 10min. After soaking, separate the solid and dry it in vacuum to obtain the calcination material.

[0093] Put the calcination material in a nitrogen / carbon monoxide mixed atmosphere and perform reduction calcination. The volume fraction of carbon monoxide in the mixed atmosphere is 1%, the calcination temperature is 650℃, and the calcination time is 6h. After calcination, cool down and remove the magnet to obtain the lithium iron phosphate positive electrode material.

[0094] Example 3

[0095] The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which specifically comprises the following steps:

[0096] Introduce nitrogen into the hydrothermal reactor to remove oxygen in the deionized water. Weigh lithium hydroxide, phosphoric acid, ferrous oxalate, and titanium sulfate according to the molar ratio of 1.05:0.98:1.0:0.01, and then add them into the hydrothermal reactor. Weigh ascorbic acid with a molar ratio of 2% of the iron in the ferrous oxalate, and add it into the reactor. Weigh the inducer citric acid with a molar ratio of 2% of the ferrous oxalate, and add it into the reactor. Adjust the pH to 7 to obtain a reaction solution with a water mass fraction of 50%.

[0097] Adjust the temperature of the hydrothermal reactor to 180℃ and hydrothermal reaction for 6h. After the hydrothermal reaction, cool down and separate the solid and liquid. Clean the solid with dilute sulfuric acid with a concentration of 0.01mol / L, then clean with deionized water. Then soak the solid in a titanium ethyl ester aqueous solution with a concentration of 0.1mol / L, wherein the mass ratio of the solid to the titanium ethyl ester aqueous solution is 1:1, and the soaking time is 10min. After soaking, separate the solid and dry it in vacuum to obtain the calcination material.

[0098] Put the calcination material in a nitrogen / carbon monoxide mixed atmosphere and perform reduction calcination. The volume fraction of carbon monoxide in the mixed atmosphere is 1%, the calcination temperature is 650℃, and the calcination time is 6h. After calcination, cool down and remove the magnet to obtain the lithium iron phosphate positive electrode material.

[0099] Example 4

[0100] The embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which specifically comprises the following steps:

[0101] The lithium iron phosphate positive electrode material is prepared by the following steps.

[0102] The temperature of the hydrothermal reaction kettle is adjusted to 180℃, and hydrothermal reaction is performed for 6h. After the hydrothermal reaction is completed, the temperature is lowered, and the solid-liquid separation is performed on the hydrothermal reaction solution. The solid phase is cleaned with dilute sulfuric acid with a concentration of 0.01mol / L, and then cleaned with deionized water. Then, the solid phase after cleaning is immersed in a tetrabutyl titanate aqueous solution with a concentration of 0.2mol / L, wherein the mass ratio of the solid phase to the tetrabutyl titanate aqueous solution is 1:1, and the immersion time is 10min. After the immersion is completed, the solid phase is separated and vacuum dried to obtain the calcination material.

[0103] The calcination material is placed in a nitrogen / carbon monoxide mixed atmosphere for reduction calcination. The volume fraction of carbon monoxide in the mixed atmosphere is 1%, the calcination temperature is 700℃, and the calcination time is 2h. After the calcination is completed, the lithium iron phosphate positive electrode material is obtained by cooling and removing the magnetism.

[0104] Example 5

[0105] The example provides a preparation method of a lithium iron phosphate positive electrode material. The difference from the example 1 is that the inducer citric acid with a mass fraction of 5% of the ferrous sulfate is added into the reaction kettle, so that ω is within a reasonable range, but I1 / I0 is too low.

[0106] Example 6

[0107] The example provides a preparation method of a lithium iron phosphate positive electrode material. The difference from the example 1 is that the mass ratio of the solid phase to the tetrabutyl titanate aqueous solution is 1:1, and the immersion time is 10min, so that α increases.

[0108] Example 7

[0109] The example provides a preparation method of a lithium iron phosphate positive electrode material. The difference from the example 1 is that the volume fraction of carbon monoxide in the mixed atmosphere is 5%, so that β increases.

[0110] Comparative Example 1

[0111] The comparative example provides a preparation method of a lithium iron phosphate positive electrode material. The difference from the example 4 is that the first titanium source and the tetravalent titanium source are not introduced, and the inducer is replaced by a carbon source. The preparation method specifically comprises the following steps.

[0112] The water hydrothermal reactor is purged with nitrogen to remove oxygen in the deionized water. Lithium hydroxide, phosphoric acid, and ferrous sulfate are weighed according to the molar ratio of 1.05:0.98:1.0, and then added to the water hydrothermal reactor. Ascorbic acid is weighed at 2% of the amount of iron in the ferrous sulfate. Glucose is added as a carbon source at 5% of the amount of ferrous sulfate. The pH is adjusted to 7, and a reaction solution with a water mass fraction of 50% is obtained.

[0113] The temperature of the water hydrothermal reactor is adjusted to 180℃, and the water hydrothermal reaction is carried out for 6h. After the water hydrothermal reaction is completed, the temperature is lowered, and the solid-liquid separation is performed on the water hydrothermal reaction solution. The solid is washed with dilute sulfuric acid with a concentration of 0.01mol / L, and then washed with deionized water. After vacuum drying, the calcined material is obtained.

[0114] The calcined material is heated and calcined in a nitrogen atmosphere. The volume fraction of carbon monoxide in the mixed atmosphere is 2%, the calcination temperature is 700℃, and the calcination time is 2h. After calcination, the lithium iron phosphate positive electrode material is obtained by cooling and removing the magnet.

[0115] Comparative Example 2

[0116] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the first titanium source is replaced with an equal amount of zirconium sulfate, and the corrosion resistance strength ω is too large.

[0117] Comparative Example 3

[0118] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material. The main difference from Example 1 is that the titanium tetrabutoxide is replaced with an equal amount of zirconium tetraethoxide, and the corrosion resistance strength ω is too large.

[0119] Test Example 1:

[0120] The test results of the lithium iron phosphate positive electrode material prepared in the above examples and comparative examples are shown in Table 1, and the test methods are as follows:

[0121] The detection method of α includes: taking a lithium iron phosphate positive electrode material sample m0=5.0000g±0.0010g, immersing it in 10ml of dilute sulfuric acid with a concentration of 0.01mol / L at 18-30℃ for 10min, and detecting the concentration c0 g / ml of dissolved iron ions by potential titration method, α=(c0*10 / m0)*100%;

[0122] The detection method of β includes: taking a lithium iron phosphate positive electrode material sample m1=5.0000g±0.0010g, immersing it in 10ml of copper sulfate solution with a concentration of 0.1mol / L at 18-30℃ for 10min, and then detecting the concentration c1 g / ml of iron ions in the solution by ICP, β=(c1*10 / m1)*100%.

[0123] The corrosion resistance strength ω is calculated from the maximum value of I1 / I0 corresponding to the (101), (112), (011), (010), (001) crystal face in Table 1.

[0124] I0 is the standard diffraction intensity of the specified crystal face in the standard XRD card of lithium iron phosphate crystal;

[0125] I1 = I` 最强 I1` / I 最强 ; wherein I` 最强 is the strongest peak in the standard XRD card; I 最强 is the diffraction intensity of the crystal face corresponding to I` 最强 in the XRD pattern of the lithium iron phosphate cathode material; I1` is the diffraction intensity of the crystal face corresponding to I0 in the XRD pattern of the lithium iron phosphate cathode material.

[0126] Test Example 2:

[0127] The lithium iron phosphate cathode material prepared in each of the above examples and comparative examples is prepared into an electrode sheet, and then assembled into a battery. The specific method comprises: mixing lithium iron phosphate, conductive carbon black and PVDF in a mass ratio of 8:1:1 into an NMP solvent for magnetic stirring to form a uniform slurry, coating the slurry on an aluminum foil for vacuum drying at 100°C for 12h, cutting the prepared electrode sheet into 12mm circular sheets under a slicing machine, and controlling the load of each electrode to be 1.5mg / cm 2 -3mg / cm 2 The electrode sheet is assembled into a button cell in a glove box, with lithium metal as the negative electrode, Celgrad2400 as the separator, and 1mol / L LiPF6 organic solution (solvent mixed by EC, DEC, DMC in a volume ratio of EC:DEC:DMC=1:1:1) as the electrolyte to assemble into a CR2032 button cell.

[0128] Performance test method: constant current charge and discharge test is carried out by using a blue battery test cabinet.

[0129] Table 1

[0130]

[0131] Note: The specified crystal face in the table is the crystal face corresponding to I1 / I0 used to calculate the corrosion resistance strength ω.

[0132] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material comprises a matrix and a carbon coating layer covering the matrix. The corrosion resistance ω of the lithium iron phosphate cathode material is 0%-40%, and ω=(βI1) / (αI0), where, α represents the mass fraction of iron (excluding ferrous iron) in the lithium iron phosphate cathode material. β represents the mass fraction of elemental iron in the lithium iron phosphate cathode material; I0 is the standard diffraction intensity of a specified crystal plane in a lithium iron phosphate crystal as recorded on a standard XRD card. I1=Iˋ 最强 I1ˋ / I 最强 Among them, Iˋ 最强 The strongest peak in the standard XRD pattern; I 最强 The XRD pattern of lithium iron phosphate cathode material with Iˋ 最强 The diffraction intensity of the corresponding crystal plane; I1ˋ is the diffraction intensity of the crystal plane corresponding to I0 in the XRD pattern of lithium iron phosphate cathode material; The specified crystal plane is one of the (101), (112), (011), (010), and (001) crystal planes.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The matrix is ​​lithium iron phosphate doped with titanium; And / or, α is 0%-1.5%; And / or, β is 0%-0.2%; And / or, I1 / I0 is 1.2-3.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The detection method for α includes: taking a lithium iron phosphate cathode material sample m0 = 5.0000g ± 0.0010g and immersing it in 10ml of dilute sulfuric acid with a concentration of 0.01mol / L at 18℃-30℃ for 10min, and detecting the concentration of dissolved iron ions c0g / ml by potentiometric titration, α = (c0*10 / m0)*100%; And / or, the detection method for β includes: taking a lithium iron phosphate cathode material sample m1 = 5.0000 g ± 0.0010 g and placing it in 10 ml of a 0.1 mol / L copper sulfate solution at 18-30℃ and reacting for 10 min, then using ICP to detect the iron ion concentration c1 g / ml in the solution, and β = (c1*10 / m1)*100%.

4. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, include: The pre-reaction liquid containing lithium source, iron source, phosphorus source, first titanium source, additives and inducer is subjected to hydrothermal reaction under hydrothermal reaction conditions to obtain the post-reaction liquid; the inducer includes at least one of citric acid, piperidine and isopropanol; The solid phase in the liquid after the hydrothermal reaction is separated, and the solid phase is then subjected to acid washing, water washing, soaking in tetravalent titanium source and drying in sequence to obtain the material to be calcined; The material to be calcined is subjected to reducing calcination and demagnetization in sequence to obtain the lithium iron phosphate cathode material.

5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The molar ratio of lithium source, phosphorus source, iron source, inducer, and first titanium source in the pre-reaction liquid is (1.0-1.05):(0.98-1):(1.0-1.05):(0.02-0.1):(0.01-0.1). And / or, the first titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate; And / or, the additive is selected from ascorbic acid; And / or, the additive is in the molar ratio of iron to iron in the iron source to 1.5%-2.5%; And / or, the mass fraction of water in the pre-reaction liquid is 40%-60%.

6. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The hydrothermal reaction temperature is 160℃-200℃, and the hydrothermal reaction time is 5h-7h; And / or, the pH of the solution before the reaction is 6.0-8.0; And / or, the pickling solution used is dilute sulfuric acid with a concentration of 0.005 mol / L to 0.1 mol / L.

7. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The tetravalent titanium source is selected from at least one of tetrabutyl titanate, ethyl titanate, titanium sulfate, and titanium oxalate. And / or, the immersion solution used in the tetravalent titanium source immersion step is a tetravalent titanium source solution of 0.1 mol / L-0.2 mol / L, the mass ratio of solid phase to immersion solution in the tetravalent titanium source immersion step is 1:(1-3), and the immersion time is 8 min-12 min.

8. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The temperature for the reducing calcination step is 550℃-750℃, and the time is 2h-6h. And / or, the atmosphere of the reducing calcination step includes an inert gas and carbon monoxide, wherein the volume fraction of carbon monoxide in the atmosphere is 1%-3%.

9. An electrode sheet, characterized in that, Including the lithium iron phosphate cathode material as described in any one of claims 1-3.

10. A lithium-ion battery, characterized in that, Includes the electrode sheet as described in claim 9.

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