Composite positive electrode material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application provides a composite positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery. The composite positive electrode material comprises a first positive electrode active material, a second positive electrode active material and a third positive electrode active material. The first positive electrode active material is a P63mc crystal form positive electrode active material, the second positive electrode active material is a Fd3-m crystal form positive electrode active material, and the third positive electrode active material is a Pmnb crystal form positive electrode material. The average particle size of the first positive electrode active material is R1, the average particle size of the second positive electrode active material is R2, and the average particle size of the third positive electrode active material is R3. The following relationships are satisfied: R2 / R1<=0.414, R3 / R1<=0.225, and R3 / R2<=0.543. The composite positive electrode material provided by the application has good cycle performance, low-temperature performance and safety performance at a high voltage of 4.50 V and above, and has a high electrode sheet compaction density.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a composite positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Since commercialization in the 1990s, lithium batteries have been widely used due to their high energy density, high charging and discharging efficiency, small self-discharge, long service life and environmental friendliness. At present, they have been applied to consumer electronics, aerospace, military, electric tools and electric vehicles. With the development of technology, people's requirements for the endurance of lithium ion batteries are getting higher and higher in both consumer and power battery fields. Developing high energy density lithium batteries has become a key point. Developing high energy density lithium batteries can be approached from two aspects: one is to develop new high-capacity positive and negative electrode materials; the other is to improve the charging and discharging voltage of lithium batteries. Improving the charging and discharging voltage of lithium batteries can simultaneously improve the mass energy density and volume energy density of lithium batteries, and can also reduce the cost of lithium batteries, which has become a research hotspot.

[0003] For the positive electrode material, the high lithium extraction amount brought by high voltage greatly challenges its structural stability. As a high energy density positive electrode material, the lithium extraction amount of lithium cobaltate is as high as more than 73% above the voltage of 4.50V, which will cause the irreversible phase transition of O3 to H1-3 of lithium cobaltate, the rapid change of unit cell parameters and the collapse of structure, resulting in serious cycle decay and safety risk. The common modification method of lithium cobaltate at present is mainly to delay the phase transition voltage through bulk doping and surface coating, but for higher voltage of 4.55V and above, the degree of irreversible phase transition is intensified, and the effect of delaying phase transition through doping and coating is not good. In addition, due to the severe contraction of C-axis under high lithium extraction and the collapse of crystal structure, the safety risk is caused, and the improvement effect through doping and coating is very small. On the other hand, with the increase of the compaction density of the electrode sheet, the pressure on the positive electrode particles increases during rolling, greatly increasing the risk of particle breakage and cycle failure.

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

[0005] One of the purposes of the present application is to provide a composite positive electrode material to solve the problems of poor cycle performance, insufficient safety performance, poor low-temperature performance and low compaction density of the electrode sheet of the current lithium ion battery in view of the deficiencies of the prior art.

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

[0007] A composite positive electrode material, comprising a first positive electrode active material, a second positive electrode active material, and a third positive electrode active material, wherein the first positive electrode active material is a P63mc crystal form positive electrode active material, the second positive electrode active material is a crystal form positive electrode active material, and the third positive electrode active material is a Pmnb crystal form positive electrode material.

[0008] wherein the average particle size of the first positive electrode active material is R1, the average particle size of the second positive electrode active material is R2, and the average particle size of the third positive electrode active material is R3; and the following relationships are satisfied: R2 / R1≤0.414, R3 / R1≤0.225, and R3 / R2≤0.543.

[0009] Preferably, R1 satisfies 10 μm≤R1≤20 μm; R2 satisfies 2 μm≤R2≤8 μm; and R3 satisfies 0.2 μm≤R3≤4 μm.

[0010] Preferably, the composite positive electrode material has a chemical formula of LiMn a Fe 1-a-b N b PO4@[Li x Na y Co 1-z M z O2 / LiNi c Mn 2-c-d T d O4], wherein M, N, and T each include at least one of Al, Mg, Ti, La, Y, Zn, Ni, Mn, Co, V, and Si; wherein 0.7≤x<1, 0.01<y<0.1, and 0≤z<0.02; 0≤c<0.6, 0≤d<0.02; 0≤a<0.5, and 0≤b<0.02.

[0011] Preferably, the first positive electrode active material is Li x Na y Co 1-z M z O2, the second positive electrode active material is LiNi c Mn 2-c-d T d O4, and the third positive electrode active material is LiMn a Fe 1-a-b N b PO4.

[0012] Preferably, the mass of the first positive electrode active material is m1, the mass of the second positive electrode active material is m2, and the mass of the third positive electrode active material is m3, wherein m1, m2, and m3 satisfy the relationships: m1>m2≥m3, m1 / m2≥4, and m1 / m3≥15.

[0013] The second object of the present application is to provide a preparation method of the composite positive electrode material.

[0014] S1: uniformly mixing a cobalt salt solution and a salt solution of a doping element M according to a Co to M molar ratio of (1-z):z, adding an excess of a precipitant solution, stirring and filtering to obtain a precipitate; sintering the precipitate at 400-600°C for 6-12h to obtain a (Co 1-z M z )3O4 powder, wherein 0≤z<0.02;

[0015] S2: uniformly mixing the (Co 1-z M z )3O4 and a sodium salt according to a Na to Co molar ratio of (0.5-0.7):(1-1.2); sintering at 800-1100°C for 45-52h to obtain a Na n Co 1-z M z O2 having a P63mc crystal phase structure, wherein 0.5≤n≤0.7;

[0016] S3: dispersing the Na n Co 1-z M z O2 in a lithium salt solution, uniformly mixing according to a Li to Na molar ratio≥5, reacting in a 140-160°C reaction kettle for 22-26h, after pressure filtration, washing with deionized water, drying and mechanically crushing to obtain Li x Na y Co 1-z M z O2 positive electrode particles;

[0017] S4: uniformly mixing a +3 valence iron salt solution, a manganese salt solution and a salt solution of a doping element N according to a Mn, Fe, N molar ratio of a:(1-a-b):b to form a mixed solution, then mixing the mixed solution with a phosphate salt solution according to a (Fe+N+Mn) to P molar ratio of 1:1, filtering to obtain a Mn a Fe 1-a-b N b PO4 precipitate; then uniformly mixing the Mn a Fe 1-a-b N b PO4 precipitate with a lithium salt according to a P, Li molar ratio of (0.8-1):(1.2-1.4), high-temperature sintering at 700-1000°C in an oxygen atmosphere for 8-12h, and mechanically crushing to obtain a LiMn a Fe 1-a-b N b PO4 positive electrode material;

[0018] S5: the nickel salt solution, the manganese salt solution and the salt solution of the doping element T are mixed uniformly according to the molar ratio of Ni, Mn and T as c:(2-c-d):d to form a mixed solution, and then the mixed solution is mixed and precipitated with the carbonate solution according to the molar ratio of (Ni+T+Mn) to carbonate ions as 1:2, and then filtered to obtain Ni c Mn 2-c-d T d CO3 precipitate; then the Ni c Mn 2-c-d T d CO3 precipitate is mixed uniformly with the lithium salt according to the molar ratio of Li:(Ni+Mn+T) as (1.0-1.5):(1.8-2.4), and then high-temperature sintering is carried out at 750-1000 DEG C for 8-12h, and then mechanical crushing is carried out to obtain LiNi c Mn 2-c-d T d O4 positive electrode material;

[0019] S6: the materials prepared in the steps S3, S4 and S5 are mixed according to the ratio, and then sintering is carried out at 700-800 DEG C for 5-8h to obtain the composite positive electrode material.

[0020] The third object of the present application is to provide a positive electrode sheet comprising the composite positive electrode material according to any one of the above.

[0021] The fourth object of the present application is to provide a secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator interposed between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet according to the above.

[0022] Compared with the prior art, the composite positive electrode material provided by the present application has the advantages that: the composite positive electrode material has good cycle performance, low-temperature performance and safety performance at high voltage of 4.50V and above, and has high electrode sheet compaction density. The first positive electrode active material in the composite positive electrode material has a unique coplanar connection of LiO6 octahedron and CoO6 octahedron, only the sliding of CoO2 layer and the arrangement of Li vacancies occur at high voltage, has high reversibility, and avoids cycle decay caused by irreversible capacity loss; the second positive electrode active material has three-dimensional lithium ion diffusion channels, lithium ions can be quickly removed at high voltage, and the low-temperature discharge performance of the composite material is improved; the third positive electrode active material can maintain an intact crystal framework at high delithiation, relieve the large volume expansion of the first positive electrode active material during the delithiation process, avoid structure collapse, and improve the safety performance of the composite positive electrode material; the first positive electrode active material, the second positive electrode active material and the third positive electrode active material with different particle sizes are compounded, and the three-stage particle arrangement improves the electrode sheet compaction density, thereby improving the energy density. DETAILED DESCRIPTION

[0023] To make the technical solution and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0024] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0025] 1. Cathode material

[0026] The first aspect of this invention aims to provide a composite positive electrode material comprising a first positive electrode active material, a second positive electrode active material, and a third positive electrode active material, wherein the first positive electrode active material is a P63mc crystal form positive electrode active material, the second positive electrode active material is... The first positive electrode active material and the third positive electrode active material are Pmnb crystalline positive electrode materials; the average particle size of the first positive electrode active material is R1, the average particle size of the second positive electrode active material is R2, and the average particle size of the third positive electrode active material is R3; and they satisfy the following relationships: R2 / R1≤0.414, R3 / R1≤0.225, R3 / R2≤0.543.

[0027] The inventors discovered that maintaining the size of the composite cathode material particles within the aforementioned range has several advantages. First, the combination of three different particle sizes helps improve compaction density. Second, the first cathode active material exhibits high reversibility under high voltage, reducing irreversible capacity loss. The second cathode active material possesses three-dimensional lithium-ion diffusion channels, enabling rapid lithium-ion extraction under high voltage and improving the low-temperature performance of the cathode material. Third, during the delithiation process, the FePO4 structure of the third cathode active material is consistent with the bulk material, stabilizing the crystal framework, preventing structural collapse, and enhancing safety. The combined use of these three materials not only increases the compaction density of the cathode sheet but also contributes to high energy density in lithium-ion batteries and effectively improves the battery's cycle performance, low-temperature performance, and safety performance under high voltage.

[0028] A second aspect of the present invention provides a method for preparing a composite cathode material, comprising the following steps:

[0029] S1: Mix the cobalt salt solution and the salt solution of doped element M uniformly according to the molar ratio of Co to M of (1-z):z, add excess precipitant solution, stir and filter to obtain precipitate; sinter the precipitate at 400℃~600℃ for 6~12h to obtain (Co 1-z Mz )3O4 powder, wherein 0≤z<0.02;

[0030] S2: mixing (Co 1-z M z )3O4 with sodium salt in a molar ratio of Na to Co of (0.5-0.7):(1-1.2), and sintering at 800-1100°C for 45-52h to obtain Na n Co 1-z M z O2, wherein 0.5≤n≤0.7;

[0031] S3: dispersing Na n Co 1-z M z O2 in a lithium salt solution, and mixing uniformly in a molar ratio of Li to Na≥5, and reacting in a 140-160°C reactor for 22-26h, and after pressure filtration, washing with deionized water and drying, mechanically crushing to obtain Li x Na y Co 1-z M z O2 positive electrode particles;

[0032] S4: mixing uniformly a +3 valence iron salt solution, a manganese salt solution, and a salt solution of a doping element N in a molar ratio of Mn, Fe, N of a:(1-a-b):b to form a mixed solution, and mixing the mixed solution with a phosphate salt solution in a molar ratio of (Fe+N+Mn) to P of 1:1, and filtering to obtain Mn a Fe 1-a-b N b PO4 precipitate; and mixing uniformly the Mn a Fe 1-a-b N b PO4 precipitate with a lithium salt in a molar ratio of P, Li of (0.8-1):(1.2-1.4), and sintering at 700-1000°C in an oxygen atmosphere for 8-12h, and mechanically crushing to obtain LiMn a Fe 1-a-b N b PO4 positive electrode material;

[0033] S5: mixing uniformly a nickel salt solution, a manganese salt solution, and a salt solution of a doping element T in a molar ratio of Ni, Mn, T of c:(2-c-d):d to form a mixed solution, and mixing the mixed solution with a carbonate salt solution in a molar ratio of (Ni+T+Mn) to carbonate ions of 1:2 to precipitate, and filtering to obtain Ni c Mn 2-c-d T d CO3 precipitate; and mixing uniformly the Ni c Mn2-c-d T d CO3 precipitate and lithium salt are mixed uniformly according to the molar ratio of Li:(Ni+Mn+T) (1.0-1.5):(1.8-2.4), high-temperature sintering at 750-1000°C for 8-12h, and mechanical crushing to obtain LiNi c Mn 2-c-d T d O4 positive electrode material;

[0034] S6: the materials prepared in steps S3, S4 and S5 are mixed according to the proportion, sintered at 700-800°C for 5-8h, to obtain the composite positive electrode material.

[0035] In some embodiments, the sintering temperature in step S1 is 400-600°C, specifically 400°C, 450°C, 500°C, 550°C, 600°C, which can include but is not limited to the above-mentioned temperature values; the sintering time is 6-12h, specifically 6h, 7h, 8h, 9h, 10h, 11h, 12h, which can include but is not limited to the above-mentioned sintering time, preferably 10h.

[0036] In some embodiments, the sintering temperature in step S2 is 800-1100°C, specifically 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, which can include but is not limited to the above-mentioned sintering temperature; the sintering time is 45-52h, specifically 45h, 46h, 47h, 48h, 49h, 50h, 51h, 52h, which can include but is not limited to the above-mentioned sintering time, preferably 48h.

[0037] In some embodiments, the temperature of the reaction kettle in step S3 is 140-160°C, specifically 140°C, 145°C, 150°C, 155°C, 160°C, which can include but is not limited to the above-mentioned reaction kettle temperature, preferably 150°C; the reaction time in the reaction kettle is 22-26h, specifically 22h, 23h, 24h, 25h, 26h, which can include but is not limited to the above-mentioned reaction time, preferably 24h.

[0038] In some embodiments, step S4 is sintered at 700-1000°C in an oxygen atmosphere for 8-12h, the sintering temperature specifically being 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, which can include but is not limited to the above-mentioned sintering temperature; the sintering time is 8-12h, specifically 8h, 9h, 10h, 11h, 12h, which can include but is not limited to the above-mentioned sintering time, preferably 10h.

[0039] In some embodiments, the step S5 is sintered at 700-800℃ for 5-8h, the sintering temperature can be 700℃, 750℃, 800℃, can include but not limited to the above-mentioned sintering temperature, preferably 750℃; the sintering time is 5-8h, can be 5h, 6h, 7h, 8h, can include but not limited to the above-mentioned sintering time, preferably 6h.

[0040] In step S1, the cobalt salt solution can be one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt acetate, preferably cobalt sulfate; the M salt solution of the doping element can be one of aluminum sulfate, magnesium sulfate, preferably aluminum sulfate; the precipitant solution can be one of sodium carbonate, sodium hydroxide, preferably sodium carbonate. In addition, the (Co 1-z M z )3O4 powder prepared in step S1, the value of z is in the range of 0≤z<0.02, and too much doping is not conducive to particle growth.

[0041] In step S2, the molar ratio of Na to Co is (0.5-0.7):(1-1.2), which can be 0.5:1, 0.5:1.1, 0.5:1.2, 0.6:1, 0.6:1.1, 0.6:1.2, 0.7:1, 0.7:1.1, 0.7:1.2, can include but not limited to the above-mentioned values, preferably 0.7:1, wherein the P63mc structure of Na n Co 1-z M z O2, wherein 0.5≤n≤0.7.

[0042] In step S3, the lithium salt solution can be lithium nitrate, lithium chloride, lithium carbonate, lithium acetate, preferably lithium nitrate; in addition, the molar ratio of Li to Na in step 3 is ≥5, which can be 5, 6, 7, 8, 9, 10, 11; can include but not limited to the above-mentioned values, preferably, the molar ratio of Li / Na is 10, if the ratio of Li / Na is too low, it will cause low ion exchange efficiency, affecting the performance of the battery.

[0043] In step S4, the ferric salt solution includes one of ferric chloride, ferric sulfate, ferric nitrate, preferably ferric chloride; the manganese salt solution includes one of manganese chloride, manganese sulfate, manganese nitrate, preferably manganese chloride; the doping element N salt solution includes one of aluminum sulfate, magnesium sulfate, preferably aluminum sulfate; the lithium salt is in a P, Li molar ratio (0.8-1) : (1.2-1.4), specifically 0.8:1.2, 0.8:1.3, 0.8:1.4, 0.9:1.2, 0.9:1.3, 0.9:1.4, 1:1.2, 1:1.3, 1:1.4, can include but not limited to the above-mentioned values, preferably 1:1.2, and excess lithium can avoid the volatilization of Li in calcination to cause "poor lithium".

[0044] In step S5, the nickel salt solution includes one of nickel chloride, nickel sulfate, nickel nitrate, preferably nickel nitrate; the manganese salt solution includes one of manganese chloride, manganese sulfate, manganese nitrate, preferably manganese chloride; the doping element N salt solution includes one of aluminum sulfate, magnesium sulfate, preferably aluminum sulfate.

[0045] In step S5, the molar ratio of (Ni+T+Mn) to carbonate ions is (0.8-1.2) : (1.6-2.2), specifically 0.8:1.6, 0.8:1.7, 0.8:1.8, 0.8:1.9, 0.8:2.0, 0.8:2.1, 0.8:2.2, 0.9:1.6, 0.9:1.7, 0.9:1.8, 0.9:1.9, 0.9:2.0, 0.9:2.1, 0.9:2.2, 1.0:1.6, 1.0:1.7, 1.0:1.8, 1.0:1.9, 1.0:2.0, 1.0:2.1, 1.0:2.2, 1.1:1.6, 1.1:1.7, 1.1:1.8, 1.1:1.9, 1.1:2.0, 1.1:2.1, 1.1:2.2, 1.2:1.6, 1.2:1.7, 1.2:1.8, 1.2:1.9, 1.2:2.0, 1.2:2.1, 1.2:2.2; can include but not limited to the above-mentioned ratios, preferably 1:2, and excess carbonate ions are conducive to the full precipitation of metal ions.

[0046] In step S5, the molar ratio of Li:(Ni+Mn+T) is (1.0-1.5):(1.8-2.4), and can be 1.0:1.8, 1.0:1.9, 1.0:2.0, 1.0:2.1, 1.0:2.2, 1.0:2.3, 1.0:2.4, 1.1:1.8, 1.1:1.9, 1.1:2.0, 1.1:2.1, 1.1:2.2, 1.1:2.3, 1.1:2.4, 1.2:1.8, 1.2:1.9, 1.2:2.0, 1.2:2.1, 1.2:2.2, 1.2:2.3, 1.2:2.4, 1.3:1.8, 1.3:1.9, 1.3:2.0, 1.3:2.1, 1.3:2.2, 1.3:2.3, 1.3:2.4, 1.4:1.8, 1.4:1.9, 1.4:2.0, 1.4:2.1, 1.4:2.2, 1.4:2.3, 1.4:2.4, 1.5:1.8, 1.5:1.9, 1.5:2.0, 1.5:2.1, 1.5:2.2, 1.5:2.3, 1.5:2.4; can include but not limited to the above examples of the ratio, preferably 1.2:2; excess lithium salt can avoid high temperature calcination Li volatilization caused by "poor lithium".

[0047] Compared with the traditional single positive active material, the composite positive active material in the application is composed of three different crystal forms of active materials. The first positive active material has a high reversibility because only the CoO2 layer sliding and Li vacancy arrangement occur at high voltage due to the coplanar connection of the unique LiO6 octahedron and CoO6 octahedron, avoiding the cycle decay caused by irreversible capacity loss. The second positive active material has three-dimensional lithium ion diffusion channels, and lithium ions can be quickly removed at high voltage, improving the low-temperature discharge performance of the composite material. The third positive active material can maintain an intact crystal framework at high delithiation, relieving the large volume expansion of the first positive active material during the delithiation process, avoiding structure collapse, and improving the safety performance of the composite material. The first positive active material, the second positive active material, and the third positive active material with different particle sizes are compounded, and the three-level particle arrangement improves the compaction density of the electrode sheet, thereby improving the energy density.

[0048] 2. The positive electrode sheet

[0049] The third aspect of the application provides a positive electrode sheet, which comprises the positive active material described above, and specifically comprises a positive current collector and a positive active material layer coated on at least one surface of the positive current collector. The positive active material is the positive active material described in the application.

[0050] The positive current collector can be various materials suitable for use as a lithium ion battery positive current collector in the art, for example, the positive current collector can be, but is not limited to, a metal foil, and more specifically can be, but is not limited to, an aluminum foil.

[0051] 3. Secondary battery

[0052] The fourth aspect of the present application aims to provide a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet being the positive electrode sheet described above.

[0053] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based material, tin-based material, lithium titanate, or other metals capable of forming alloys with lithium. The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon alloy; and the tin-based material can be selected from one or more of elemental tin, tin oxide compound, tin alloy. The negative electrode current collector is generally a structure or part that collects current, and can be any material suitable for use as a negative electrode current collector in a lithium ion battery, for example, the negative electrode current collector can be, but not limited to, a metal foil, and more specifically can be, but not limited to, a copper foil.

[0054] The separator can be any material suitable for use as a separator in a lithium ion battery, for example, can be a combination of one or more of, but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber.

[0055] The secondary battery further comprises an electrolyte, which comprises an organic solvent, an electrolyte lithium salt, and an additive. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolyte, at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolyte, at least one of LiBF4, LiBOB, LiPF6, LiTFSI used in overcharge-preventing electrolyte, at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; a chain carbonate, including DFC, DMC, or EMC; a carboxylic acid ester, including MF, MA, EA, and MP. The additive includes, but not limited to, at least one of a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0056] In order to make the technical solutions and advantages of the present application clearer, the following will further describe the present application and its beneficial effects in detail in conjunction with specific embodiments, but the embodiments of the present application are not limited thereto.

[0057] Embodiment 1

[0058] The preparation method of the composite cathode material in this embodiment is as follows:

[0059] 1) uniformly mix a cobalt sulfate solution and an aluminum sulfate solution in a Co:Al molar ratio of 0.99:0.01, add an excess of sodium carbonate solution, stir, and filter to obtain a precipitate; sinter the precipitate at 550°C for 10h to obtain (Co 0.99 Al 0.01 )3O4 powder.

[0060] 2) uniformly mix (Co 0.99 Al 0.01 )3O4 and Na2CO3 in a Na:Co molar ratio of 0.7:1; sinter at 950°C for 48h to obtain Na 0.7 Co 0.99 Al 0.01 O2 with a P63mc structure.

[0061] 3) disperse Na 0.7 Co 0.99 Al 0.01 O2 in a lithium nitrate solution, uniformly mix in a Li:Na molar ratio of 10:1, react in a 150°C reaction kettle for 24h, after pressure filtration, wash with deionized water, dry, and mechanically crush to obtain Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 cathode material with a Dv50 of 13μm.

[0062] 4) uniformly mix a ferric sulfate solution, a manganese sulfate solution, and an aluminum sulfate solution in a Mn:Fe:Al molar ratio of 0.4:0.59:0.01, then mix and precipitate in a (Fe+Al+Mn):P molar ratio of 1:1, filter to obtain Mn 0.4 Fe 0.59 Al 0.01 PO4 precipitate; then uniformly mix the precipitate with lithium salt in a P:Li molar ratio of 1:1.2, sinter at 850°C in an oxygen atmosphere for 10h, and mechanically crush to obtain LiMn 0.4 Fe 0.59 Al 0.01 PO4 cathode material with a Dv50 of 1μm.

[0063] 5) Mix the nickel sulfate solution, manganese sulfate solution, aluminum sulfate solution uniformly according to the molar ratio of Ni, Mn, Al as 0.5:1.49:0.01, then mix with the sodium carbonate solution according to the molar ratio of (Ni+Al+Mn) to carbonate as 1:2, filter thoroughly to obtain Ni 0.5 Mn 1.49 Al 0.01 CO3 precipitate; then mix it with lithium hydroxide according to the molar ratio of Li:(Ni+Mn+Al) as 1.2:2, uniformly, sinter at high temperature of 750-1000℃ for 10h, mechanically crush to obtain LiNi 0.5 Mn 1.49 Al 0.01 O4 cathode material.

[0064] 6) Mix Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 prepared in step 3 and LiMn 0.4 Fe 0.59 Al 0.01 PO4 prepared in step 4 and LiNi 0.5 Mn 1.49 Al 0.01 O4 prepared in step 5 according to the mass ratio of 90:5:5, then sinter at 750℃ for 6h, to obtain the composite cathode material LMFP@[LCO / LNMO]-1.

[0065] Example 2

[0066] The difference between this example and example 1 is that in step (3), the Dv50 of Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 is 10μm.

[0067] The rest is the same as example 1, which will not be repeated here.

[0068] Example 3

[0069] The difference between this example and example 1 is that in step (3), the Dv50 of Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 is 20μm.

[0070] The rest is the same as example 1, which will not be repeated here.

[0071] Example 4

[0072] The difference between this example and Example 1 is that in step (5), LiNi 0.5 Mn 1.49 Al 0.01 The Dv50 of LiMn

[0073] The rest is the same as Example 1, which will not be repeated here.

[0074] Example 5

[0075] The difference between this example and Example 1 is that in step (5), LiNi 0.5 Mn 1.49 Al 0.01 The Dv50 of LiMn

[0076] The rest is the same as Example 1, which will not be repeated here.

[0077] Example 6

[0078] The difference between this example and Example 1 is that in step (4), LiMn 0.4 Fe 0.59 Al 0.01 The Dv50 of LiMn

[0079] The rest is the same as Example 1, which will not be repeated here.

[0080] Example 7

[0081] The difference between this example and Example 1 is that in step (4), LiMn 0.4 Fe 0.59 Al 0.01 The Dv50 of LiMn

[0082] The rest is the same as Example 1, which will not be repeated here.

[0083] Example 8

[0084] The difference between this example and Example 7 is that in step (5), LiNi 0.5 Mn 1.49 Al 0.01 The Dv50 of LiMn

[0085] The rest is the same as Example 1, which will not be repeated here.

[0086] Example 9

[0087] The difference between this example and Example 1 is that in step (6), Li 0.87 Na 0.13 Co 0.99 Al 0.01O2 cathode material, LiNi 0.5 Mn 1.49 Al 0.01 O4 cathode material and LiMn 0.4 Fe 0.59 Al 0.01 The mass ratio of PO4 cathode material is 75:20:5.

[0088] The rest is the same as in Example 1, and will not be repeated here.

[0089] Example 10

[0090] The difference between this embodiment and embodiment 1 is that, in step (6), Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 cathode material, LiNi 0.5 Mn 1.49 Al 0.01 O4 cathode material and LiMn 0.4 Fe 0.59 Al 0.01 The mass ratio of PO4 cathode material is 75:15:10.

[0091] The rest is the same as in Example 1, and will not be repeated here.

[0092] Example 11

[0093] The difference between this embodiment and embodiment 1 is that, in step (6), Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 cathode material, LiNi 0.s Mn 1.49 Al 0.01 O4 cathode material and LiMn 0.4 Fe 0.59 Al 0.01 The mass ratio of PO4 cathode material is 70:20:10.

[0094] The rest is the same as in Example 1, and will not be repeated here.

[0095] Comparative Example 1

[0096] 1) Mix cobalt sulfate solution and aluminum sulfate according to C o The mixture was thoroughly mixed with Al at a molar ratio of 0.99:0.01, and excess sodium carbonate solution was added. The mixture was stirred and filtered to obtain a precipitate. The precipitate was sintered at 550℃ for 10 h to obtain (Co). 0.99 Al 0.01 )3O4 powder;

[0097] 2) mix (C o0.99 Al 0.01 )3O4 and Na2CO3 uniformly according to the molar ratio of Na to Co of 0.7:1; high-temperature sintering at 950℃ for 48h to obtain Na 0.7 Co 0.99 Al 0.01 O2 with P63mc structure.

[0098] 3) disperse Na 0.7 Co 0.99 Al 0.01 O2 in lithium nitrate solution, mix uniformly according to the molar ratio of Li to Na of 10:1, react in a 150℃ reaction kettle for 24h, after pressure filtration, wash with deionized water and dry, mechanically crush to obtain Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 cathode material with Dv50 of 13μm.

[0099] 4) sinter the Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 cathode material prepared in step 3 at 750℃ for 6h to obtain the comparative example LCO-1.

[0100] Comparative example 2

[0101] 1) mix nickel sulfate solution, manganese sulfate solution, aluminum sulfate solution uniformly according to the molar ratio of Ni, Mn, Al of 0.5:1.49:0.01, then mix with sodium carbonate solution according to the molar ratio of (Ni+Al+Mn) to carbonate of 1:2, fully precipitate and filter to obtain Ni 0.5 Mn l.49 Al 0.01 CO3 precipitate; then mix with lithium hydroxide according to the molar ratio of Li:(Ni+Mn+Al) of 1.2:2, high-temperature sinter at 750-1000℃ for 10h, mechanically crush to obtain LiNi 0.5 Mn 1.49 Al 0.01 O4 cathode material with Dv50 of 4μm.

[0102] 2) sinter the LiNi 0.5 Mn 1.49 Al 0.01 O4 cathode material prepared in step 3 at 750℃ for 6h to obtain the comparative example LNMO-2.

[0103] Comparative example 3

[0104] 1) Iron sulfate solution, manganese sulfate solution, aluminum sulfate solution are mixed uniformly according to the molar ratio of Mn, Fe, Al as 0.4:0.59:0.01, and then mixed and precipitated according to the molar ratio of (Fe+Al+Mn) to P as 1:1, and then filtered to obtain Mn 0.4 Fe 0.59 Al 0.01 PO4precipitate; then the precipitate is mixed uniformly with lithium salt according to the molar ratio of P to Li as 1:1.2, and then sintered at 850°C under oxygen atmosphere for 10h, and then mechanically broken to obtain LiMn 0.4 Fe 0.59 Al 0.01 PO4cathode material with Dv50 of 1μm.

[0105] 2) The LiMn 0.4 Fe 0.59 Al 0.01 PO4cathode material prepared in step 1 is sintered at 750°C for 6h to obtain the comparative example LMFP-3.

[0106] The cathode materials obtained in examples 1-11 and comparative examples 1-3 are mixed uniformly according to the mass ratio of 97.9:1.3:0.8 of the cathode material, conductive agent and polyvinylidene fluoride as the binder to prepare cathode slurry. The cathode slurry is coated on the surface of the current collector aluminum foil according to the surface density of 220g / cm2, and then dried at 85°C and wound, and then the prepared aluminum foil with double positive active material layers is cold-pressed, and the compaction density of the electrode sheet is tested according to the pressure of 200T and the roll gap of 150μm. The cold-pressed electrode sheet is assembled into a lithium ion button cell, and the gram capacity test is carried out at 25°C, the charge and discharge conditions are 0.1C, and the charge and discharge range is 3.0V-4.60V; the 100cls cycle test is carried out at 45°C, the charge and discharge conditions are 1.0C, and the charge and discharge range is 3.0V-4.60V; the electrode sheet after the gram capacity test is dried at 100°C and then sent for DSC test, and the test conditions are: 25°C-450°C, the temperature rising rate is 10K / min, and the nitrogen protective atmosphere.

[0107] The test results are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] From the test results in Table 1, it can be seen that the composite cathode material has the advantages of high-temperature cycle, low-temperature discharge, safety performance and compaction density through the design of the three-stage grading of the P63mc crystal type cathode material, the Pmnb crystal type cathode material and the Pmnb crystal type cathode material.

[0112] From the test results of examples 1-11 and comparative examples 1-3, the positive electrode material Li 0.87 Na 0.13 Co 0.99 Al 0.01 O2 has good cycle stability, poor low-temperature and safety performance; The positive electrode material LiNi 0.5 Mn 1.49 Al 0.01 O4 has good low-temperature discharge performance, low specific capacity, and poor cycle and safety; the positive electrode material LiMn 0.4 Fe 0.59 Al 0.01 PO4 has high DSC peak temperature, good thermal stability, and good safety performance. The single particle size material in the comparative example has low electrode plate compaction density, and after three-stage particle matching, the electrode plate compaction is significantly improved. When the three-stage particle grading of examples 1, 2, 3, 4, 6 simultaneously satisfies the conditions of R2 / R1≤0.414, R3 / R1≤0.225, R3 / R2≤0.543, m1 / m2≥4, and m1 / m3≥15, the electrode plate compaction is as high as 4.25-4.30 g / cm 3 Overall, the composite material prepared in example 1 has high specific capacity, long cycle, low-temperature performance, high safety performance, and high compaction density.

[0113] In summary, the present application designs a functional composite material containing a P63mc crystal form positive electrode material, a Pmnb crystal form positive electrode material, and a three-stage grading of the three components. The composite positive electrode material improves the cycle, low-temperature performance, and safety performance at high voltage through the three components, and the three components are designed in three stages to improve the electrode plate compaction density. The P63mc crystal form positive electrode material has high reversibility at high voltage, reducing the irreversible capacity loss; the Pmnb crystal form positive electrode material has three-dimensional lithium ion diffusion channels, and the lithium ion can be quickly removed at high voltage, improving the low-temperature discharge performance of the composite material; the FePO4 in the Pmnb crystal form positive electrode material has the same structure as the bulk material during the delithiation process, which can stabilize the crystal framework and avoid structure collapse, improving the safety performance. The P63mc crystal form positive electrode material, the Pmnb crystal form positive electrode material, and the Pmnb crystal form positive electrode material are designed in three stages to improve the electrode plate compaction density, thereby improving the energy density.

[0114] Those skilled in the art can make various modifications and variations to the above embodiments based on the disclosure and teachings of this specification. Therefore, the present application should not be limited to the above specific embodiments, and any obvious modifications, replacements or variations made by those skilled in the art based on the present application shall fall within the scope of the present application. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation on the present application.

Claims

1. A composite cathode material, characterized in that, The composite positive electrode material comprises a first positive electrode active material, a second positive electrode active material and a third positive electrode active material, wherein the first positive electrode active material is a P63mc crystal form positive electrode active material, the second positive electrode active material is a Fdm crystal form positive electrode active material, and the third positive electrode active material is a Pmnb crystal form positive electrode material. The average particle size of the first positive electrode active material is R1, the average particle size of the second positive electrode active material is R2, and the average particle size of the third positive electrode active material is R3, and the following relationships are satisfied: R2 / R1≤0.414, R3 / R1≤0.225, and R3 / R2≤0.

543. The R1 satisfies 10 μm≤R1≤20 μm, the R2 satisfies 2 μm≤R2≤8 μm, and the R3 satisfies 0.2 μm≤R3≤4 μm. The chemical general formula of the composite positive electrode material is: LiMn a Fe 1-a-b N b PO4@[Li x Na y Co 1-z M z O2 / LiNi c Mn 2-c-d TdO4] M, N and T each comprise at least one of Al, Mg, Ti, La, Y, Zn, Ni, Mn, Co, V and Si, 0.7≤x<1, 0.01<y<0.1, 0≤z<0.02, 0≤c<0.6, 0≤d<0.02, 0≤a<0.5, and 0≤b<0.

02. Li x Na y Co 1-z M z O2, the second positive electrode active material is LiNi c Mn 2-c-d T d O4, the third positive electrode active material is LiMn a Fe 1-a-b N b PO4.

2. The composite cathode material of claim 1, wherein, The mass of the first positive electrode active material is m1, the mass of the second positive electrode active material is m2, and the mass of the third positive electrode active material is m3, wherein m1, m2 and m3 satisfy the following relationships: m1>m2≥m3, m1 / m2≥4, and m1 / m3≥15.

3. The method for preparing a composite cathode material according to any one of claims 1-2, characterized in that, The method comprises the following steps: S1: uniformly mix a cobalt salt solution and a salt solution of a doping element M according to a Co to M molar ratio of (1-z):z, add an excess of a precipitant solution, stir and filter to obtain a precipitate, sinter the precipitate at 400°C-600°C for 6-12h to obtain a (Co 1-z M z )3O4 powder, wherein 0≤z<0.02; S2: mixing (Co 1-z M z )3O4 with sodium salt in a molar ratio of Na to Co of (0.5-0.7):(1-1.2), and sintering at 800-1100°C for 45-52h to obtain Na n Co 1-z M z O2, wherein 0.5≤n≤0.7; S3: mix Na n Co 1-z M z O2 is dispersed in a lithium salt solution, mixed uniformly according to a molar ratio of Li to Na ≥ 5, and reacted in a 140-160°C reaction kettle for 22-26h. After pressure filtration, deionized water is used for washing and drying, and then mechanically broken to obtain Li x Na y Co 1-z M z O2 positive electrode particles; S4: the +3 valence iron salt solution, the manganese salt solution, the salt solution of the doping element N are mixed uniformly according to the molar ratio of Mn, Fe, N as a: (1-a-b): b, a mixed solution is formed, then the mixed solution is mixed with the phosphate salt solution according to the molar ratio of (Fe+N+Mn) to P as 1:1, filtration is carried out to obtain Mn a Fe 1-a-b N b PO4 precipitate, then the Mn a Fe 1-a-b N b PO4 precipitate is mixed uniformly with the lithium salt according to the molar ratio of P to Li (0.8~1): (1.2~1.4), high-temperature sintering is carried out under oxygen atmosphere at 700~1000℃ for 8~12h, and mechanical crushing is carried out to obtain Li Mn a Fe 1-a-b N b PO4 positive electrode material; S5: the nickel salt solution, the manganese salt solution, the salt solution of the doping element T are mixed uniformly according to the molar ratio of Ni, Mn, T as c: (2-c-d): d to form a mixed solution, and then the mixed solution is mixed with the carbonate solution according to the molar ratio of (Ni+T+Mn) to carbonate ions as 1:2, filtered to obtain Ni c Mn 2-c-d T d CO3precipitate, and then the Ni c Mn 2-c-d T d CO3precipitate is mixed with the lithium salt according to the molar ratio of Li: (Ni+Mn+T) (1.0~1.5):(1.8~2.4) uniformly, high-temperature sintered at 750~1000℃ for 8~12h, and mechanically broken to obtain LiNi c Mn 2-c-d T d O4cathode material; S6: The materials prepared in steps S3, S4 and S5 are mixed in a certain proportion, and then sintered at 700-800℃ for 5-8h to obtain the composite positive electrode material.

4. A positive electrode sheet characterized by comprising: The composite positive electrode material of any one of claims 1-2.

5. The positive electrode sheet according to claim 4, characterized by The positive electrode sheet compact density P satisfies: 3.6 g / cm 3 ≤ P < 4.45 g / cm 3 .

6. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, characterized by The positive electrode sheet of any one of claims 4-5. The positive electrode sheet of any one of claims 4-5.

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