Lithium iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof.The lithium iron phosphate positive electrode material comprises matrix particles and a carbon coating layer coating the surfaces of the matrix particles; the carbon source forming the carbon coating layer comprises a first carbon source, and the first carbon source comprises a divalent iron chelating agent; the carbon coating layer of the lithium iron phosphate positive electrode material comprises SP2 hybrid carbon and SP3 hybrid carbon. The doping of the Ti element and the M element in the matrix particles and the arrangement of the two carbon coating layers are combined, so that the capacity of the material and the exertion of energy efficiency are remarkably improved.

Description

TECHNICAL FIELD

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

[0002] Lithium iron phosphate (LiFePO4) is a typical lithium intercalation polyanion compound with an olivine orthorhombic structure. The compound is intercalated with lithium ions through Fe 3+ / Fe 2+ Redox pairs participate in electrochemical reactions, and the Fermi level of the Fe 3+ / Fe 2+ Redox ion pair is lowered due to the influence of XO4 n- , and the voltage of the battery is also increased. In addition, there is a strong P-O covalent bond in LiFePO4, which is very beneficial to the stability of the olivine structure, and also contributes to the safety of the electrode material. The open-circuit voltage of the olivine structure LiFePO4 is moderate, 3.4V (Vs. Li), the theoretical specific capacity (170mAh / g) is high, the material skeleton is quite stable, and the performance can still be maintained after multiple cycles, the cost is low, and there is no pollution. It is a high-power lithium ion secondary battery cathode material with great development potential and competitiveness.

[0003] Due to its own structure, the lithium iron phosphate material has low electronic / ionic conductivity and low lithium ion diffusion rate, which will reduce its energy efficiency and limit its further practical application. Although the existing technology often uses carbon doping or material particle nanocrystallization to improve the energy efficiency of lithium iron phosphate material, such solutions may reduce the specific capacity of the overall material due to material doping, or cause the product to agglomerate, and ultimately fail to achieve the desired effect. SUMMARY

[0004] The present application provides a lithium iron phosphate cathode material and a preparation method and application thereof to solve the problems of low specific capacity and low energy efficiency of lithium iron phosphate in the prior art.

[0005] To this end, in a first aspect, the present application provides a lithium iron phosphate cathode material, which comprises a base particle and a carbon coating layer coated on the surface of the base particle; the base particle is selected from a Ti and M co-doped lithium iron phosphate particle; wherein M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; the carbon source forming the carbon coating layer comprises a first carbon source, and the first carbon source comprises a divalent iron chelating agent; the carbon coating layer of the lithium iron phosphate cathode material comprises SP 2 hybrid carbon and SP 3 hybrid carbon.

[0006] In some embodiments, the chemical formula of the matrix particles is as follows: Li 1+a Fe x Ti y M z (PO4) 1+b , where 0.01≤a≤0.03; 0.9≤x≤1.0; 0.01≤y≤0.02; 0.001≤z≤0.01; 0≤b≤0.05.

[0007] In some implementations, SP 3 Hybridized carbon and SP 2 The mass ratio of hybrid carbon is 0.2 to 1.2, and can be selected as 0.4 to 0.8.

[0008] In some embodiments, the lithium iron phosphate cathode material I D / I G The ratio is 0.5~1.5:1, and can be selected as 1.0~1.2:1.

[0009] In some embodiments, the carbon source forming the coating layer further includes a second carbon source and a third carbon source, wherein the second carbon source includes an organic small molecule carbon source and the third carbon source includes an organic polymer carbon source.

[0010] In some embodiments, the second carbon source includes one or both of glucose and sucrose.

[0011] In some embodiments, the third carbon source includes one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone.

[0012] In some embodiments, the ferrous chelating agent includes one or more of oxalic acid, citric acid, EDTA, and malonic acid.

[0013] In some embodiments, the carbon coating layer comprises the SP 2 Carbon layer A, mainly composed of hybrid carbon, and SP 3 Carbon layer B, mainly composed of hybrid carbon.

[0014] In some embodiments, the mass ratio of the sum of the second and third carbon sources to the mass of the first carbon source is 0.5 to 5:1, and can be selected as 1.3 to 3.0:1.

[0015] In some embodiments, the carbon content is 0.8wt%-1.6wt% based on the total mass of the lithium iron phosphate cathode material; optionally, it is 1.0wt%-1.4wt%.

[0016] In some embodiments, the particle size Dv10, Dv50, Dv90 of the lithium iron phosphate positive electrode material satisfies 0.1 μm≤Dv10≤1 μm, 0.5 μm≤Dv50≤2.0 μm, 2 μm≤Dv90≤10 μm, 1.0≤(Dv90-Dv10) / Dv50≤5.0; optionally, 0.3 μm≤Dv10≤0.5 μm, 0.8 μm≤Dv50≤1.6 μm, 3 μm≤Dv90≤5 μm, 1.5≤(Dv90-Dv10) / Dv50≤3.0.

[0017] In some embodiments, the specific surface area BET of the lithium iron phosphate positive electrode material is 5-20 m 2 / g, optionally, the BET is 9-12 m 2 / g.

[0018] In a second aspect, the application provides a preparation method of the lithium iron phosphate positive electrode material, comprising the following steps: mixing iron phosphate, a lithium source, a first carbon source, a second carbon source, a third carbon source, a titanium source, an oxide or salt containing element M, and water, and grinding, spray drying, and sintering to obtain the lithium iron phosphate positive electrode material; wherein the first carbon source comprises a divalent iron chelating agent; the second carbon source comprises an organic small molecule carbon source, and the third carbon source comprises an organic polymer carbon source; M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb.

[0019] In some embodiments, the divalent iron chelating agent comprises one or more of oxalic acid, citric acid, EDTA, and malonic acid.

[0020] In some embodiments, the second carbon source comprises one or both of glucose and sucrose. In some embodiments, the third carbon source comprises one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone.

[0021] In some embodiments, the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source is 0.5-5:1, optionally 1.3-3.0:1.

[0022] In some embodiments, in the sintering process, the sintering temperature is 700-850 ℃, and the time is 5-15 h, preferably, the sintering temperature is 750-820 ℃, and the time is 8-12 h.

[0023] In some embodiments, the inlet temperature of the spray drying is 280-300 ℃, and the outlet temperature is 80-100 ℃.

[0024] In some embodiments, the lithium source comprises one or more of lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium dihydrogen phosphate.

[0025] In some embodiments, the particle size Dv50 of the slurry after grinding is 100-1000 nm, and optionally 300-600 nm.

[0026] In some embodiments, the titanium source is selected from titanium dioxide.

[0027] In some embodiments, the oxide or salt containing M element is selected from one or more of oxide containing M element, nitrate containing M element, sulfate containing M element, hydrochloride containing M element, phosphate containing M element.

[0028] In some embodiments, the oxide or salt containing M element is selected from oxide containing M element.

[0029] In some embodiments, the oxide or salt containing M element is selected from one or more of V2O5, MgO, Al2O3.

[0030] In some embodiments, the mass of the lithium source accounts for 25%-30% of the mass of the iron phosphate.

[0031] In some embodiments, the mass of the first carbon source accounts for 2%-10% of the mass of the iron phosphate.

[0032] In some embodiments, the mass of the second carbon source accounts for 2%-10% of the mass of the iron phosphate.

[0033] In some embodiments, the mass of the third carbon source accounts for 2%-10% of the mass of the iron phosphate.

[0034] In some embodiments, the mass of the titanium source accounts for 0.5%-1.2% of the mass of the iron phosphate.

[0035] In some embodiments, the mass of the oxide or salt containing M element accounts for 0.1%-0.3% of the mass of the iron phosphate.

[0036] In a third aspect, the present application provides a positive electrode tab, comprising: a positive electrode current collector, and a positive electrode active material layer arranged on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the lithium iron phosphate positive electrode material of any one of the first aspect or the lithium iron phosphate positive electrode material prepared by the preparation method of the second aspect.

[0037] In a fourth aspect, a secondary battery comprises the positive electrode tab of the third aspect. In a fifth aspect, an electric device comprises the secondary battery of the fourth aspect.

[0038] The technical solution of this invention has the following advantages: 1. The lithium iron phosphate cathode material provided by the present invention comprises matrix particles and a carbon coating layer covering the surface of the matrix particles; the matrix particles are selected from lithium iron phosphate particles co-doped with Ti and M; wherein M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; the carbon source forming the carbon coating layer comprises a first carbon source, the first carbon source comprising a divalent iron chelating agent; the carbon coating layer of the lithium iron phosphate cathode material comprises SP 2 Hybrid carbon and SP 3 Hybridized carbon. In the lithium iron phosphate cathode material of the present invention, on the one hand, Ti element replaces Fe sites, generating Li vacancies to achieve charge balance and forming more Li. + Migration pathways; the d orbitals of Ti participate in the formation of new band structures, narrowing the band gap and increasing electron mobility; furthermore, the small ionic radius of Ti can inhibit grain growth and improve the reactivity of lithium iron phosphate particles. Moreover, M elements, including one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb, can partially substitute Li sites, Fe sites, or both in the lithium iron phosphate lattice. When M elements substitute Li sites in the lattice, Li vacancies are generated, and these vacancies are adjacent Li... + Migration provides more space and pathways, thus significantly improving the lithium-ion diffusion coefficient; when the M element replaces the Fe site in the lattice, Fe appears in the lattice. 2+ and Fe 3+ The coexisting mixed valence states can introduce electrons and holes, forming an electronic conductive network. The synergistic effect of Ti and M elements significantly improves the material's capacity.

[0039] On the other hand, unlike conventional double-layer carbon coating, the lithium iron phosphate cathode material of this application includes SP. 3 Hybrid carbon and SP 2 Hybrid carbon. SP 3 Hybrid carbon is mainly produced by the thermal decomposition of a second carbon source at high temperatures. The carbon layer formed by the second carbon source is unevenly distributed and has a porous structure on the surface of lithium iron phosphate, which is beneficial to improving ionic conductivity; SP 2 Hybrid carbon is mainly produced by the thermal decomposition of a third carbon source at high temperatures. The carbon layer formed by the third carbon source forms a relatively uniform three-dimensional conductive network on the surface of lithium iron phosphate, which has high electronic conductivity and steric hindrance effect, thereby inhibiting the growth of lithium iron phosphate particles and giving it high energy efficiency.

[0040] The first carbon source (i.e., the ferrous chelating agent) readily reacts with Fe. 2+The in-situ carbon layer formed on the surface of the substrate particles by chelation, and the divalent iron chelator decomposes to produce a reducing atmosphere on the surface of lithium iron phosphate during sintering, creating a strong reducing environment in the reaction system, effectively preventing Fe 2+ oxidation to Fe 3+ , so that the final synthesized lithium iron phosphate hybrid phase is reduced and the crystal form is more complete; in addition, the in-situ carbon layer promotes the formation of small particles, the lithium ion diffusion path is short, thereby improving the kinetic performance of the lithium iron phosphate material, and thus facilitating the capacity and energy efficiency of the material.

[0041] 2. The lithium iron phosphate positive electrode material provided by the present application controls the mass ratio of SP 3 hybrid carbon to SP 2 hybrid carbon to be 0.2-1.2, especially 0.4-0.8; which can avoid the mass ratio being too high to cause the SP 3 hybrid carbon to be too large to reduce the electronic conductivity of the carbon layer; and the uniformity of the carbon layer coating is moderate, avoiding the carbon layer coating being too uniform, which affects the migration of lithium ions, thereby further improving the energy efficiency of the battery.

[0042] By controlling I D / I G is 0.5-1.5, especially 1.0-1.2; which can further improve the energy efficiency of the battery.

[0043] 3. The lithium iron phosphate positive electrode material provided by the present application controls the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source to be 0.5-5:1, especially 1.3-3.0:1; which can avoid the mass ratio being too large to provide an insufficient reducing environment, and there may be Fe 3+ locally, affecting the integrity of the crystal structure and thus the energy efficiency of the battery; in addition, it can also improve the formation of small crystalline particles that are less affected by the in-situ carbon layer, thereby affecting the energy efficiency of the lithium iron phosphate; if the mass ratio is too low, it will cause a large part of the particles to be not coated by the carbon layer, affecting the overall capacity.

[0044] 4. The lithium iron phosphate positive electrode material provided by the present application, the carbon source forming the carbon coating layer further comprises a second carbon source and a third carbon source, the second carbon source comprises one or both of glucose and sucrose; the third carbon source comprises one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone. The first carbon source has a relatively low decomposition temperature and decomposes preferentially during sintering to produce reducing gas, inhibiting particle growth and improving the integrity of the crystal structure; the second and third carbon sources have relatively high decomposition temperatures and form SP 3 hybrid carbon-based carbon layer and SP 2The carbon layer mainly composed of hybrid carbon has higher energy efficiency.

[0045] 5.The preparation method of the lithium iron phosphate positive electrode material provided by the application, comprising the following steps: mixing iron phosphate, a lithium source, a first carbon source, a second carbon source, a third carbon source, a titanium source, an oxide or a salt containing an element M, and water to perform grinding, and then performing spray drying and sintering to obtain the lithium iron phosphate positive electrode material; wherein the second carbon source comprises an organic small-molecule carbon source, and the third carbon source comprises an organic polymer carbon source; M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; the element Ti and the element M synergistically act to significantly improve the capacity of the material, and the addition of the three carbon sources is beneficial to the capacity and energy efficiency of the material, thereby greatly improving the problems of low specific capacity and low energy efficiency of the lithium iron phosphate. BRIEF DESCRIPTION OF DRAWINGS

[0046] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings described below are some embodiments of the application, and those skilled in the art can also obtain other drawings according to these drawings without any creative effort.

[0047] Figure 1 is the Raman spectrum of the lithium iron phosphate positive electrode material prepared in Example 1. DETAILED DESCRIPTION In order to better further understand the application, the following examples are provided, but the following examples do not constitute limitations on the content and protection scope of the application, and any product which is the same or similar to the application obtained by anyone under the inspiration of the application or by combining the application with other prior art features falls within the protection scope of the application.

[0048] If the specific experimental steps or conditions are not specified in the examples, they are operated according to the conventional experimental steps or conditions in the art. If the reagents or instruments used are not specified by the manufacturer, they are conventional reagent products or instruments which can be obtained by market purchase.

[0049] In a first aspect, the application provides a lithium iron phosphate positive electrode material, which comprises base particles and a carbon coating layer coated on the surface of the base particles; the base particles are selected from Ti and M co-doped lithium iron phosphate particles; wherein M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; the carbon source forming the carbon coating layer comprises a first carbon source, and the first carbon source comprises a divalent iron chelating agent; the carbon coating layer of the lithium iron phosphate positive electrode material comprises SP 2 Hybrid carbon and SP 3Hybridized carbon. In the lithium iron phosphate cathode material of the present invention, on the one hand, Ti element replaces Fe sites, generating Li vacancies to achieve charge balance and forming more Li. + Migration pathways; the d orbitals of Ti participate in the formation of new band structures, narrowing the band gap and increasing electron mobility; in addition, the small ionic radius of Ti can also inhibit grain growth and improve the reactivity of lithium iron phosphate particles. M elements, including one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb, can partially substitute Li sites, Fe sites, or both in the lithium iron phosphate lattice. When M elements substitute Li sites in the lattice, Li vacancies are generated, and these vacancies are adjacent Li... + Migration provides more space and pathways, thus significantly improving the lithium-ion diffusion coefficient; when the M element replaces the Fe site in the lattice, Fe appears in the lattice. 2+ and Fe 3+ The coexisting mixed valence states can introduce electrons and holes, forming an electronic conductive network. The synergistic effect of Ti and M elements significantly improves the material's capacity.

[0050] On the other hand, unlike conventional double-layer carbon coating, the first coating layer of this application includes SP. 3 Hybrid carbon and SP 2 Hybrid carbon. SP 3 Hybrid carbon is mainly produced by the thermal decomposition of a second carbon source at high temperatures. The carbon layer formed by the second carbon source is unevenly distributed and has a porous structure on the surface of lithium iron phosphate, which is beneficial to improving ionic conductivity; SP 2 Hybrid carbon is mainly produced by the thermal decomposition of a third carbon source at high temperatures. The carbon layer formed by the third carbon source creates a relatively uniform three-dimensional conductive network on the surface of lithium iron phosphate, exhibiting high electronic conductivity and steric hindrance, thereby inhibiting the growth of lithium iron phosphate particles and resulting in high energy efficiency. The first carbon source (i.e., the divalent iron chelating agent) readily reacts with Fe. 2+ The in-situ carbon layer formed on the surface of the matrix particles through chelation is generated during the sintering process by the decomposition of these carbon sources on the lithium iron phosphate surface. This creates a strong reducing environment in the reaction system, effectively preventing Fe from being absorbed. 2+ Oxidized to Fe 3+ This reduces the impurity phase in the final synthesized lithium iron phosphate and makes the crystal structure more complete. In addition, the in-situ carbon layer promotes the formation of small particles and shortens the lithium ion diffusion path, thereby improving the kinetic performance of lithium iron phosphate materials and thus contributing to the material's capacity and energy efficiency.

[0051] In some alternative embodiments, the chemical formula of the matrix particles is as follows: Li 1+a Fe x Ti y M z (PO4) 1+bwherein 0.01≤a≤0.03; 0.9≤x≤1.0; 0.01≤y≤0.02; 0.001≤z≤0.01; 0≤b≤0.05. For example, a can be 0.03, 0.028, 0.025, 0.023, 0.02, 0.017, 0.015, 0.012, 0.01, or within a range between any two of the above values; x can be 1.0, 0.99, 0.98, 0.97, 0.96, 0.95, 0.94, 0.93, 0.92, 0.91, 0.9, or within a range between any two of the above values; y can be 0.02, 0.019, 0.018, 0.017, 0.016, 0.015, 0.014, 0.013, 0.012, 0.011, 0.01, or within a range between any two of the above values; z can be 0.01, 0.009, 0.008, 0.007, 0.006, 0.005, 0.004, 0.003, 0.002, 0.001; b can be 0.05, 0.045, 0.04, 0.035, 0.03, 0.025, 0.02, 0.015, 0.01, 0.005, 0, or within a range between any two of the above values.

[0052] In some alternative embodiments, the SP 3 The mass ratio of the hybrid carbon to the SP 2 The mass ratio of the hybrid carbon to the SP 3 The mass ratio of the hybrid carbon to the SP 2 The mass ratio of the hybrid carbon to the SP

[0053] In some alternative embodiments, the I D / I G is 0.5-1.5. For example, the I D / I G may be 0.5, 0.6, 0.7, 0.8, 0.9, 1.2, 1.5, or within a range between any two of the above values.

[0054] In some alternative embodiments, the carbon source for forming the carbon coating layer further comprises a second carbon source and a third carbon source, the second carbon source comprises an organic small molecule carbon source, and the third carbon source comprises an organic polymer carbon source. For example, the second carbon source comprises one or both of glucose and sucrose; and the third carbon source comprises one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone.

[0055] In some alternative embodiments, the divalent iron chelator comprises one or more of oxalic acid, citric acid, EDTA, malonic acid.

[0056] In some alternative embodiments, the carbon coating layer comprises SP 2 A carbon layer A mainly composed of hybrid carbon and a carbon layer B mainly composed of SP 3 A carbon layer B mainly composed of hybrid carbon.

[0057] In some alternative embodiments, the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source is 0.5-5:1. For example, the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source can be 0.5:1, 1:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, or within a range defined by any two of the above values.

[0058] In some alternative embodiments, the carbon content is 0.8wt%-1.6wt% based on the total mass of the lithium iron phosphate positive electrode material. For example, the carbon content can be 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, or within a range defined by any two of the above values, based on the total mass of the lithium iron phosphate positive electrode material.

[0059] In some alternative embodiments, the particle size Dv10, Dv50, Dv90 of the lithium iron phosphate positive electrode material satisfies 0.1μm≤Dv10≤1μm, 0.5μm≤Dv50≤2.0μm, 2μm≤Dv90≤10μm, 1.0≤(Dv90-Dv10) / Dv50≤5.0. For example, the particle size Dv10 of the lithium iron phosphate positive electrode material can be 1μm, 0.9μm, 0.8μm, 0.7μm, 0.6μm, 0.5μm, 0.3μm, 0.2μm, 0.1μm, or within a range defined by any two of the above values; the Dv50 can be 2μm, 1.8μm, 1.6μm, 1.4μm, 1.2μm, 1μm, 0.8μm, 0.5μm, or within a range defined by any two of the above values; the Dv90 can be 10μm, 9μm, 8μm, 7μm, 6μm, 5μm, 3μm, 2μm, or within a range defined by any two of the above values; and the (Dv90-Dv10) / Dv50 can be 5μm, 4.5μm, 4μm, 3.5μm, 3μm, 2.5μm, 2μm, 1.5μm, 1μm, or within a range defined by any two of the above values.

[0060] In some alternative embodiments, the specific surface area BET of the lithium iron phosphate positive electrode material is 5-20m 2 / g. For example, the specific surface area BET of the lithium iron phosphate positive electrode material can be 5 m 2 / g, 7 m 2 / g, 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 14 m 2 / g, 16 m 2 / g, 18 m 2 / g, 20 m 2 / g or within a range between any two of the above values.

[0061] In a second aspect, the present application provides a preparation method of a lithium iron phosphate positive electrode material, comprising: mixing iron phosphate, a lithium source, a first carbon source, a second carbon source, a third carbon source, a titanium source, an oxide or salt containing element M, and water, and grinding, spray drying, and sintering to obtain the lithium iron phosphate positive electrode material; the second carbon source comprises an organic small molecule carbon source, and the third carbon source comprises an organic polymer carbon source; M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb.

[0062] In some optional embodiments, the divalent iron chelator comprises one or more of oxalic acid, citric acid, EDTA, and malonic acid.

[0063] In some optional embodiments, the second carbon source comprises one or both of glucose and sucrose.

[0064] In some optional embodiments, the third carbon source comprises one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone.

[0065] In some optional embodiments, the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source is 0.5-5:1. For example, the mass ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source can be 0.5:1, 1:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, or within a range between any two of the above values.

[0066] In some optional embodiments, in the sintering process, the sintering temperature is 700-850°C, and the time is 5-15 h. For example, the sintering temperature can be 700°C, 750°C, 800°C, 850°C, or within a range between any two of the above values; and the sintering time can be 5 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 15 h, or within a range between any two of the above values.

[0067] In some alternative embodiments, the inlet temperature of the spray drying is 280-300°C, the outlet temperature of the spray drying is 80-100°C, and the moisture of the final spray material is <2%. For example, the inlet temperature of the spray drying can be 280°C, 285°C, 290°C, 295°C, 300°C, or within a range between any two of the above values; the outlet temperature of the spray drying can be 80°C, 85°C, 90°C, 95°C, 100°C, or within a range between any two of the above values.

[0068] In some alternative embodiments, the lithium source comprises one or more of lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium dihydrogen phosphate.

[0069] In some alternative embodiments, the particle size Dv50 of the slurry after grinding is 100-1000 nm. For example, the particle size Dv50 of the slurry after grinding can be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, or within a range between any two of the above values.

[0070] In some alternative embodiments, the titanium source is selected from titanium dioxide.

[0071] In some alternative embodiments, the oxide or salt containing M element is selected from one or more of an oxide containing M element, a nitrate containing M element, a sulfate containing M element, a hydrochloride containing M element, a phosphate containing M element.

[0072] In some embodiments, the mass of the lithium source accounts for 25-30% of the mass of the iron phosphate. For example, the mass of the lithium source accounts for 25%, 26%, 27%, 28%, 29%, 30%, or within a range between any two of the above values of the mass of the iron phosphate.

[0073] In some embodiments, the mass of the first carbon source accounts for 2-10% of the mass of the iron phosphate. For example, the mass of the first carbon source accounts for 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within a range between any two of the above values of the mass of the iron phosphate.

[0074] In some embodiments, the mass of the second carbon source accounts for 2-10% of the mass of the iron phosphate. For example, the mass of the second carbon source accounts for 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or within a range between any two of the above values of the mass of the iron phosphate.

[0075] In some embodiments, the third carbon source has a mass of 2% to 10% of the mass of the iron phosphate. For example, the third carbon source has a mass of 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or within a range between any two of the aforementioned values of the mass of the iron phosphate.

[0076] In some embodiments, the titanium source has a mass of 0.5% to 1.2% of the mass of the iron phosphate. For example, the titanium source has a mass of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2% or within a range between any two of the aforementioned values of the mass of the iron phosphate.

[0077] In some embodiments, the oxide or salt containing the element M has a mass of 0.1% to 0.3% of the mass of the iron phosphate. For example, the oxide or salt containing the element M has a mass of 0.1%, 0.15%, 0.2%, 0.25%, 0.3% or within a range between any two of the aforementioned values of the mass of the iron phosphate.

[0078] In a third aspect, the present application provides a positive electrode tab, comprising: a positive electrode current collector, and a positive electrode active material layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the lithium iron phosphate positive electrode material of the first aspect or the lithium iron phosphate positive electrode material prepared by the preparation method of the second aspect.

[0079] For example, the positive electrode current collector has two opposite surfaces in the thickness direction of the positive electrode current collector, and the positive electrode active material layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0080] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, the positive electrode current collector can be an aluminum foil. The composite current collector can comprise a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0081] In some embodiments, the positive electrode active material layer can further comprise a binder. For example, the binder can comprise at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0082] In some embodiments, the binder accounts for 0.1-3.5%, optionally 0.5-2.5%, of the total weight of the positive electrode active material layer.

[0083] In some embodiments, the positive electrode active material layer optionally further comprises a conductive agent. As an example, the conductive agent can comprise at least one of super-P, acetylene black, carbon black (e.g., conductive carbon black), ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0084] In some embodiments, the conductive agent accounts for 0.05-5%, optionally 0.5-3%, of the total weight of the positive electrode active material layer.

[0085] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and subjecting to drying, cold pressing, and the like to obtain the positive electrode sheet.

[0086] In a fourth aspect, the present application provides a secondary battery comprising the positive electrode sheet of the third aspect.

[0087] In some embodiments, the secondary battery further comprises a negative electrode sheet, an electrolyte, and a separator.

[0088] In some embodiments, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a negative electrode active material.

[0089] As an example, the negative electrode current collector has two opposite surfaces in the thickness direction of itself, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0090] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector can comprise a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base layer (such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0091] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0092] In some embodiments, the negative film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0093] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0094] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na), etc.

[0095] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative current collector, and then drying, cold-pressing, etc., to obtain the negative electrode sheet.

[0096] The electrolyte serves to conduct ions between the positive electrode sheet and the negative electrode sheet. The present application does not have a specific limitation on the type of electrolyte, which can be selected as needed. For example, the electrolyte can be in a liquid state, a gel state, or a full solid state.

[0097] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0098] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0099] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, butane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0100] In some embodiments, the electrolyte solution can further optionally include an additive. The additive can include, for example, a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc. For example, the additive can be at least one of fluoroethylene carbonate, trifluoroethyl methyl carbonate, 1,3-propane sultone, propylene carbonate, methyl ethyl carbonate, ethylene sulfate, trialkynylphosphine sulfate, lithium difluorophosphate.

[0101] In some embodiments, the secondary battery can further include a separator. The type of the separator is not particularly limited in the present application, and any known porous separator having good chemical stability and mechanical stability can be used.

[0102] In some embodiments, the separator can be made of at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0103] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to manufacture an electrode assembly through a winding process or a stacking process.

[0104] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte solution described above.

[0105] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be used.

[0106] The shape of the secondary battery is not particularly limited in the present application, and the secondary battery can be in a cylindrical shape, a square shape, or any other shape.

[0107] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0108] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0109] In the fifth aspect, the application provides a power consuming device comprising the secondary battery of the fourth aspect.

[0110] In some embodiments, the above-mentioned power consuming device can also comprise a battery module or a battery pack assembled from the above-mentioned secondary battery. The secondary battery, the battery module, or the battery pack can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include but is not limited to mobile devices (such as mobile phones, notebook computers, 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, etc.

[0111] As the power consuming device, the secondary battery, the battery module or the battery pack can be selected according to the use requirements thereof. As an example of the power consuming device, a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc., in order to meet the requirements of high power and high energy density of the secondary battery for the power consuming device, a battery pack or a battery module can be used.

[0112] As another example of the device, it can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a secondary battery can be used as a power source.

[0113] The application will be further described in detail below in combination with specific embodiments, which cannot be understood as limiting the scope of the application claimed.

[0114] I. Raw materials and sources:

[0115] II. Test methods: (1) SEM test: First, a layer of conductive adhesive is adhered to the copper base, then the sample is dipped with a sampling rod and adhered to the conductive adhesive, then the powder adhered to the surface is blown away with a cotton ball, and then it is placed under a scanning electron microscope (Zeiss Sigma500) for observation.

[0116] (2) Crystal structure test: X-ray powder diffractometer (PANalytical Aeris) was used, Cu Kα ray was used as radiation source, the scanning diffraction angle range was 10°-80°, and the scanning rate was 0.09° / S.

[0117] (3) Raman test: in the Raman spectrum determination process, the microscope was 50x, the scanning type was surface scanning, the scanning power was 0.5%, the exposure time was 1 s, and the scanning range was 200-2000 cm -1 -1,000 cm -1 -955 cm -1 -1,000 cm - -1,000 cm - -1,000 cm - -1,000 cm - -1,000 cm 3 -1,000 cm D -1,000 cm G -1,000 cm

[0118] Meanwhile, Raman spectrum can provide sp 2 and sp 3 hybridization information. Carbon peak decomposition was performed on the Raman spectrum, and the characteristic peak with the maximum peak intensity in the range of 1,300 cm 1 -1,400 cm 1 -1,400 cm 3 was attributed to sp 3 disordered carbon structure, and the peak area of the characteristic peak was obtained by integration, which was denoted as |sp 1 |. The characteristic peak with the maximum peak intensity in the range of 1,550 cm 1 -1,650 cm 2 -1,650 cm 2 -1,650 cm 3 -1,650 cm 2 -1,650 cm 3 -1,650 cm 2The ratio can reflect the graphitization characteristics and electronic conductivity of the surface carbon layer.

[0119] Referring to Figure 1 as shown, Figure 1 is a Raman spectrum of the lithium iron phosphate positive electrode material prepared in Example 1.

[0120] (4) Carbon content: referring to the national standard GB / T 20123-2006, the sample is heated and combusted in the oxygen gas flow of a high-frequency induction furnace, and the generated carbon dioxide is carried by oxygen to the measuring chamber of an infrared analyzer. Since carbon dioxide absorbs infrared energy of a certain wavelength, its absorption energy is proportional to the concentration. The carbon content of the sample is tested according to the change of the energy received by the detector.

[0121] (5) Particle size Dv10, Dv50, Dv90: pure water is used as the dispersion medium, 0.025 g of the material is dispersed in 30 mL of pure water to meet the light shielding requirement, and then a Malvern 3000 particle size tester is used for testing.

[0122] (6) Specific surface area BET: referring to the national standard GB / T 19587-2017, the sample is degassed after being heated and purged, and then low-temperature physical adsorption is carried out in liquid nitrogen. When the adsorption reaches equilibrium, the adsorption amount of the monolayer on the surface of the sample is measured by using an automatic specific surface area tester, and then the specific surface area of the sample is calculated by using the BET adsorption theory.

[0123] Example 1 The present embodiment provides a preparation method of a lithium iron phosphate positive electrode material, which comprises the following steps: Phosphorus acid iron, lithium carbonate, TiO2, V2O5, and oxalic acid, glucose, PEG2000 are added to pure water to obtain a slurry (solid content is 37%~42%), which is sand ground and then spray dried. The mass of the phosphorus acid iron is 250 kg, the mass of the lithium carbonate accounts for 25% of the mass of the phosphorus acid iron, the masses of TiO2 and V2O5 are 0.80% and 0.18% of the mass of the phosphorus acid iron respectively, and the masses of the oxalic acid, glucose and PEG2000 are 6%, 7% and 4% of the mass of the phosphorus acid iron respectively. The slurry is sand ground until the particle size Dv50 is 350 nm. The inlet temperature of the spray drying is 260°C, and the outlet temperature is 90°C. The spray drying obtains a spray material with a water content of less than 2%. Then the spray material is loaded into a graphite crucible with a weight of 7 kg and enters a roller kiln for sintering. The sintering temperature is 780°C, and the holding time is 8 h. The entire sintering process is carried out in a nitrogen atmosphere, and the oxygen content is less than 10 ppm. The sintered material is crushed, sieved and de-ironed to obtain the final lithium iron phosphate positive electrode active material (the lithium iron phosphate positive electrode material includes a molecular formula of Li 1.015 FeTi 0.015 V 0.003 (PO4) 1.03matrix particles of LiFePO4).

[0124] Experimental Example 2 The present embodiment provides a preparation method of lithium iron phosphate positive electrode material, comprising the following steps: adding iron phosphate, lithium hydroxide monohydrate, TiO2, MgO and EDTA, sucrose, polyacrylic acid (weight average molecular weight 2000) into pure water to obtain a slurry (solid content 37%), and then performing sand milling and spray drying, wherein the mass of iron phosphate is 250 kg, the mass of lithium hydroxide monohydrate accounts for 28.5% of the mass of iron phosphate, the masses of TiO2 and MgO are 0.8% and 0.25% of the mass of iron phosphate respectively, and the masses of EDTA, sucrose and polyacrylic acid are 6%, 4% and 4% of the mass of iron phosphate respectively. The slurry is sand milled until the particle size Dv50 is 420 nm. The inlet temperature of spray drying is 300°C, and the outlet temperature is 100°C. The spray-dried material has a water content of less than 2%. Then, the spray-dried material is loaded into graphite crucibles in a weight of 7 kg and enters a roller kiln for sintering, the sintering temperature is 790°C, and the holding time is 8 h. The entire sintering process is performed in a nitrogen atmosphere, and the oxygen content is less than 10 ppm. The sintered material is crushed, sieved and de-ironed to obtain the final lithium iron phosphate positive electrode active material (the lithium iron phosphate positive electrode material comprises matrix particles of LiFePO4). 1.012 FeTi 0.015 Mg 0.009 (PO4) 1.03 matrix particles).

[0125] Experimental Example 3 The present embodiment provides a preparation method of lithium iron phosphate positive electrode material, comprising the following steps: adding iron phosphate, lithium carbonate, TiO2, alumina and citric acid, glucose, starch into pure water to obtain a slurry (solid content 37%), and then performing sand milling and spray drying, wherein the mass of iron phosphate is 250 kg, the mass of lithium carbonate accounts for 25% of the mass of iron phosphate, the masses of TiO2 and alumina are 0.8% and 0.2% of the mass of iron phosphate respectively, and the masses of citric acid, glucose and starch are 5%, 7% and 6% of the mass of iron phosphate respectively. The slurry is sand milled until the particle size Dv50 is 400 nm. The inlet temperature of spray drying is 280°C, and the outlet temperature is 80°C. The spray-dried material has a water content of less than 2%. Then, the spray-dried material is loaded into graphite crucibles in a weight of 7 kg and enters a roller kiln for sintering, the sintering temperature is 750°C, and the holding time is 12 h. The entire sintering process is performed in a nitrogen atmosphere, and the oxygen content is less than 10 ppm. The sintered material is crushed, sieved and de-ironed to obtain the final lithium iron phosphate positive electrode active material (the lithium iron phosphate positive electrode material comprises matrix particles of LiFePO4). 1.012 FeTi 0.015 Al 0.006 (PO4) 1.03 matrix particles).

[0126] Example 4 The difference from Example 1 is that the mass of glucose and PEG2000 is adjusted to 8% and 3% of the mass of iron phosphate respectively, and the total amount of glucose and PEG2000 remains unchanged.

[0127] Example 5 The difference from Example 1 is that the mass of glucose and PEG2000 is adjusted to 3% and 8% of the mass of iron phosphate respectively, and the total amount of glucose and PEG2000 remains unchanged.

[0128] Example 6 The difference from Example 1 is that the mass of glucose and PEG2000 is adjusted to 2% and 9% of the mass of iron phosphate respectively, and the sintering temperature is increased to 850°C, so that I D / I G <0.5.

[0129] Example 7 The difference from Example 1 is that the mass of glucose and PEG2000 is adjusted to 9% and 2% of the mass of iron phosphate respectively, and the sintering temperature is decreased to 700°C, so that I D / I G >1.5.

[0130] Example 8 The difference from Example 1 is that the mass of oxalic acid, glucose and PEG2000 is adjusted to 9%, 5% and 3% of the mass of iron phosphate respectively, and the total amount of oxalic acid, glucose and PEG2000 remains unchanged.

[0131] Example 9 The difference from Example 1 is that the mass of oxalic acid, glucose and PEG2000 is adjusted to 3%, 9% and 5% of the mass of iron phosphate respectively, and the total amount of oxalic acid, glucose and PEG2000 remains unchanged.

[0132] Comparative Example 1 The difference from Example 1 is that PEG2000 is omitted, and the amount of glucose is increased to 11% of the mass of iron phosphate.

[0133] Comparative Example 2 The difference from Example 1 is that glucose is omitted, and the amount of PEG2000 is increased to 11% of the mass of iron phosphate.

[0134] Comparative Example 3 The difference from Example 7 is that oxalic acid is omitted, and the amount of glucose is increased to 10% of the mass of iron phosphate, and the amount of PEG2000 is increased to 7% of the mass of iron phosphate.

[0135] Table 1 Physical parameters of lithium iron phosphate positive electrode materials

[0136] Table 2 Particle size parameters of lithium iron phosphate positive electrode materials

[0137] Experimental Example 1 The lithium iron phosphate positive electrode materials prepared in each example and comparative example were assembled into button cells in the following manner: lithium iron phosphate positive electrode material, conductive agent (acetylene black), and binder (PVDF) were dispersed in a dispersing agent (NMP) at a mass ratio of 94:4:2, and homogenized for 15 min to make them uniformly mixed. Then the uniformly mixed slurry was coated on an aluminum foil current collector on a heated flat coating machine, and placed in a vacuum oven for drying at 120°C for 6 h. Then the roll gap of the calender was adjusted to zero, and the dried electrode sheet was calendered 4-5 times until the compacted density was 2.5-2.7 g / cm 3 A pure lithium sheet with a diameter of 18 mm was used as the negative electrode, and an electrolyte was selected from XZB LBC338A1. The assembly into C2032 button cells was performed in an Ar-filled glove box.

[0138] Capacity test: The button cell half-battery prepared was placed in a thermostat for testing, and the test temperature was 25±1°C. A new battery tester was used, and the working steps were set as follows: first, charge at 0.1C rate to 3.75V, then constant voltage charging current drops to 0.02C, test the initial charge capacity C1 of the button cell, then discharge at 0.1C rate to 2.0V, test the initial discharge capacity C2 of the button cell; then discharge to 3.2V and 2.0V at a current of 1.0C in turn, test the discharge capacity of the button cell in turn C3, C4, calculate the first efficiency and energy efficiency according to the following formula, first efficiency = C2 / C1*100%, energy efficiency = C3 / C4*100%.

[0139] Table 3 Test results

[0140] As can be seen from the above table, compared with Comparative Examples 1-3, the discharge specific capacity, first efficiency, and energy efficiency of the battery of each example of the present application are significantly improved.

[0141] Compared with Example 1, Examples 4-7 use SP 3 / SP 2 or I D / I G limited to the preferred range, which can further improve the discharge specific capacity, first efficiency, and energy efficiency of the battery.

[0142] Compared with Example 1, Examples 8-9 can further improve the discharge specific capacity, the first efficiency and the energy efficiency of the battery by limiting the ratio of the sum of the mass of the second carbon source and the third carbon source to the mass of the first carbon source within a preferred range.

[0143] Obviously, the above examples are only illustrative examples for the sake of clarity, and are not limitations on the embodiments. Based on the above description, other different forms of changes or variations can also be made by those of ordinary skill in the art. All embodiments do not need to be exhausted here, and obvious changes or variations derived therefrom are still within 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 matrix particles and a carbon coating layer covering the surface of the matrix particles; the matrix particles are selected from lithium iron phosphate particles co-doped with Ti and M; wherein M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; the carbon source forming the carbon coating layer includes a first carbon source, which includes a divalent iron chelating agent; the carbon coating layer of the lithium iron phosphate cathode material includes SP. 2 Hybrid carbon and SP 3 Hybridized carbon.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following: A. The chemical formula of the matrix particles is as follows: Li 1+a Fe x Ti y M z (PO4) 1+b Where, 0.01≤a≤0.03; 0.9≤x≤1.0; 0.01≤y≤0.02; 0.001≤z≤0.01; 0≤b≤0.05; B. SP in the lithium iron phosphate cathode material 3 Hybridized carbon and SP 2 The mass ratio of hybrid carbon is 0.2~1.2, and can be selected as 0.4~0.8; C. The lithium iron phosphate cathode material I D / I G The value is 0.5~1.5, and can be selected as 1.0~1.2; D. The carbon source forming the carbon coating layer further includes a second carbon source and a third carbon source, wherein the second carbon source includes an organic small molecule carbon source and the third carbon source includes an organic polymer carbon source; Optionally, the second carbon source includes one or both of glucose and sucrose; Optionally, the third carbon source includes one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone; E. The ferrous chelating agent includes one or more of oxalic acid, citric acid, EDTA, and malonic acid; F. The carbon coating layer comprises the SP 2 Carbon layer A, mainly composed of hybrid carbon, and SP 3 Carbon layer B, mainly composed of hybrid carbon.

3. The lithium iron phosphate cathode material according to claim 2, characterized in that, The mass ratio of the sum of the second and third carbon sources to the mass of the first carbon source is 0.5 to 5:1, and can be selected as 1.3 to 3.0:

1.

4. The lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, Based on the total mass of the lithium iron phosphate cathode material, the carbon content is 0.8wt%-1.6wt%; it can be selected as 1.0wt%-1.4wt%.

5. The lithium iron phosphate cathode material according to any one of claims 1-4, characterized in that, The particle sizes Dv10, Dv50, and Dv90 of the lithium iron phosphate cathode material satisfy the following: 0.1μm≤Dv10≤1μm, 0.5μm≤Dv50≤2.0μm, 2μm≤Dv90≤10μm, and 1.0≤(Dv90-Dv10) / Dv50≤5.0; optionally, 0.3μm≤Dv10≤0.5μm, 0.8μm≤Dv50≤1.6μm, 3μm≤Dv90≤5μm, and 1.5≤(Dv90-Dv10) / Dv50≤3.

0.

6. The lithium iron phosphate cathode material according to claim 1, characterized in that, The specific surface area (BET) of the lithium iron phosphate cathode material is 5~20m². 2 / g, optional, BET is 9~12m 2 / g.

7. A method for preparing a lithium iron phosphate cathode material, characterized in that, The process includes the following steps: mixing and grinding iron phosphate, a lithium source, a first carbon source, a second carbon source, a third carbon source, a titanium source, an oxide or salt containing element M, and water; followed by spray drying and sintering to obtain lithium iron phosphate cathode material; wherein, the first carbon source includes a divalent iron chelating agent; the second carbon source includes an organic small molecule carbon source; and the third carbon source includes an organic polymer carbon source; M is selected from one or more of Mg, Co, Al, Cr, Zr, Ce, Mo, Mn, V, and Nb; Optionally, the ferrous chelating agent includes one or more of oxalic acid, citric acid, EDTA, and malonic acid; Optionally, the second carbon source includes one or both of glucose and sucrose; Optionally, the third carbon source includes one or more of polyethylene glycol, starch, polyacrylic acid, and polyvinylpyrrolidone; Optionally, the mass ratio of the sum of the masses of the second and third carbon sources to the mass of the first carbon source is 0.5 to 5:1, and optionally 1.3 to 3.0:1; Optionally, during the sintering process, the sintering temperature is 700~850℃ and the time is 5~15h; preferably, the sintering temperature is 750~820℃ and the time is 8~12h. Optionally, the inlet temperature of the spray dryer is 280-300℃, and the outlet temperature is 80-100℃. Optionally, the lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, and lithium dihydrogen phosphate; Optionally, the particle size Dv50 of the ground slurry is 100~1000nm, and can be 300~600nm; Optionally, the titanium source is selected from titanium dioxide; Optionally, the oxide or salt containing element M is selected from one or more of oxides containing element M, nitrates containing element M, sulfates containing element M, hydrochlorides containing element M, and phosphates containing element M; further, it can be selected as an oxide containing element M; even more preferably, it can be selected from one or more of V2O5, MgO, and Al2O3. Optionally, the lithium source accounts for 25%-30% of the mass of iron phosphate; Optionally, the mass of the first carbon source accounts for 2%-10% of the mass of iron phosphate; Optionally, the second carbon source accounts for 2%-10% of the mass of iron phosphate; Optionally, the third carbon source accounts for 2%-10% of the mass of iron phosphate; Optionally, the titanium source accounts for 0.5%-1.2% of the mass of iron phosphate; Optionally, the mass of the oxide or salt containing element M accounts for 0.1%-0.3% of the mass of iron phosphate.

8. A positive electrode sheet, characterized in that, include: Positive current collector, and A positive electrode active material layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the lithium iron phosphate positive electrode material according to any one of claims 1-6 or the lithium iron phosphate positive electrode material prepared by the preparation method according to claim 7.

9. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 8.

10. An electrical device, characterized in that, Includes the secondary battery as described in claim 9.

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