Positive electrode composite, method for producing the same, and use thereof

Abstract

The application provides a positive electrode composite material and a preparation method and application thereof. The positive electrode composite material comprises first positive electrode particles and second positive electrode particles, the first positive electrode particles are LiM x Fe 1‑x PO4, 0 < x ≤ 0.2, and M elements are selected from transition elements; from the inside to the surface of the first positive electrode particles, the content of the M elements presents a decreasing trend; the second positive electrode particles are LiFe(P 1‑y A y )O4, 0

Description

TECHNICAL FIELD

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

[0002] Lithium iron phosphate material is widely used as a positive electrode material of a secondary battery due to its good stability, high safety performance, long service life and no memory effect, and the lithium iron phosphate battery is widely applied in automobiles, special aerospace and consumer electronics. However, due to the limitation of its own olivine structure, the intrinsic conductivity of the lithium iron phosphate material is low, and the lithium ion conductivity is not high, which leads to poor rate performance and low temperature performance, thereby limiting the further application of the lithium iron phosphate material. SUMMARY

[0003] In view of this, the present application provides a positive electrode composite material and a preparation method and application thereof, which can have good rate performance and low temperature performance.

[0004] The first aspect of the present application provides a positive electrode composite material, comprising first positive electrode particles and second positive electrode particles, wherein the first positive electrode particles are LiM x Fe 1-x PO4, 0 < x ≤ 0.2, and the M element is selected from transition elements; the content of the M element decreases from the inside to the surface of the first positive electrode particles; and the second positive electrode particles are LiFe (P 1-y A y )O4, 0 < y ≤ 0.05, and the A element includes S and / or N.

[0005] The M element doped in the iron position of the lithium iron phosphate can improve the ion conductivity of the first positive electrode particles, and the content of the M element in the first positive electrode particles decreases from the inside to the surface, which can reduce the interface resistance of the first positive electrode particles and further optimize the Li + ion diffusion channel in the first positive electrode particles. The S and / or N element doped in the phosphorus position of the lithium iron phosphate can effectively widen the Li + channel, thereby improving the lithium ion diffusion coefficient of the material, and further improving the rate performance and low temperature performance of the material. The first positive electrode particles and the second positive electrode particles can play a synergistic effect, further improving the electronic conductivity and ion conductivity of the positive electrode composite material.

[0006] The second aspect of the present application provides a preparation method of a positive electrode composite material, comprising: preparing the first positive electrode particles, mixing the first positive electrode particles with the second positive electrode particles to obtain the positive electrode composite material.

[0007] The above preparation method is simple in steps, high in process reliability and production efficiency, and suitable for large-scale industrial production.

[0008] The third aspect of the present application provides a positive electrode including the positive electrode composite provided by the first aspect of the present application. Due to the positive electrode composite provided by the present application, the positive electrode can be used to provide a battery with better rate performance and low temperature performance.

[0009] The fourth aspect of the present application provides a secondary battery including a negative electrode, the positive electrode provided by the third aspect of the present application, and an electrolyte between the positive electrode and the negative electrode. Due to the positive electrode provided by the present application, the secondary battery has better rate performance and performs better in a low temperature environment.

[0010] The fifth aspect of the present application provides a power consuming device including the secondary battery provided by the fourth aspect of the present application. Due to the secondary battery provided by the fifth aspect of the present application, the power consuming device has higher market competitiveness. DETAILED DESCRIPTION

[0011] The embodiment of the present application provides a positive electrode composite including first positive electrode particles and second positive electrode particles, wherein the first positive electrode particles are LiM x Fe 1-x PO4, 0 < x ≤ 0.2, and M elements are selected from transition elements; from the inside to the surface of the first positive electrode particles, the content of the M elements shows a decreasing trend, that is, the value of x in LiM x Fe 1-x PO4 shows a decreasing trend; in the embodiment of the present application, the content of the M elements in the first positive electrode particles shows a decreasing trend from the inside to the surface, which can be linearly decreasing or non-linearly decreasing, and only needs to show a decreasing trend, and does not need to strictly control the gradual decrease in the above direction. For example, from the inside to the surface of the first positive electrode particles, the content of the M elements can be sequentially increasing-decreasing-increasing, or sequentially decreasing-increasing, and the like, as long as the content of the M elements at the surface is greater than that at the inside. Of course, in some specific embodiments, the content of the M elements gradually decreases from the inside to the surface of the first positive electrode particles. It should be further pointed out that the above-mentioned “inside” is a concept relative to the surface of the first positive electrode particles, and can be the center of the first positive electrode particles, but is not limited to the center of the first positive electrode particles.

[0012] The second positive electrode particles are LiFe(P 1-y A y )O4, 0 < y ≤ 0.05, and A elements include S and / or N.

[0013] Transition metal ions M m+ (m > 0) are doped in the iron position of lithium iron phosphate, defects are generated in the lithium iron phosphate lattice, and lattice distortion is caused, so that the Li +diffusion rate and electron conductivity inside the first positive electrode particles, so as to improve the rate performance and low temperature performance of the first positive electrode particles; more importantly, the content of the doping element M in the first positive electrode particles decreases from the inside to the surface, which can improve the interface resistance of the first positive electrode particles and further widen the Li + ion diffusion channels of the first positive electrode particles, so as to further improve the Li + diffusion coefficient of the first positive electrode particles. The S (sulfur) element and / or N (nitrogen) element doped in the phosphorus position of lithium iron phosphate can improve the lithium ion diffusion coefficient of the material, so as to improve the rate performance and low temperature performance of the material. The first positive electrode particles and the second positive electrode particles can play a synergistic effect, further improving the electron conductivity and ion conductivity performance of the positive electrode composite material. Therefore, the above-mentioned positive electrode composite material can also provide a lithium iron phosphate battery with better rate performance and low temperature performance.

[0014] In the embodiments of the present application, the M elements in the first positive electrode particles are all doped in the lithium iron phosphate in the bulk phase, and the A elements in the second positive electrode particles are all doped in the lithium iron phosphate in the bulk phase.

[0015] In the embodiments of the present application, the M elements selected from transition elements means that the M elements are selected from transition elements other than Fe. In the embodiments of the present application, the M elements include but are not limited to one or more of Mo, Co, V, Mn, Ni, Zn, Cu, Cr, Ti, etc. Among them, different doping elements have some differences in the influence on the electrochemical performance of the material, for example, Mo element doped in the iron position can also slightly improve the capacity of the material; V element doped in the iron position is more conducive to enhancing the conductivity inside the material lattice.

[0016] In the embodiments of the present application, the x represents the molar percentage of the M element in LiM x Fe 1-x PO4, controlling the doping amount of M element 0 + x≤0.2 can avoid Li / Fe anti-site defects caused by excessive M element doping, even destroy the material structure, hinder Li 1-y transportation. Exemplarily, the value of x can be 0.001, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.048, 0.05, 0.08, 0.100, 0.120, 0.150, 0.180, 0.200, etc.

[0017] In the embodiments of the present application, the y represents the molar percentage of the A element in LiFe(P 1-y A y)O4, the doping amount of A element is controlled as 0 < y < 0.05, and excessive A element doping at the phosphorus site is not conducive to the stability of the material. Illustratively, the value of y can be 0.001, 0.005, 0.01, 0.012, 0.015, 0.018, 0.019, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.050, etc.

[0018] In some embodiments of the present application, the Dv50 of the first positive electrode particles is in the range of 0.1 μm-0.5 μm. In this way, the Li + transport path in the first positive electrode particles is more suitable for reducing the Li + transport time, thereby improving the Li + transport efficiency; at the same time, the smaller particle size can also increase the contact area of the first positive electrode particles and the conductive particles, thereby improving the electronic conduction efficiency in the battery, and further improving the rate performance of the battery. Illustratively, the Dv50 of the first positive electrode particles can be 0.10 μm, 0.12 μm, 0.15 μm, 0.18 μm, 0.20 μm, 0.22 μm, 0.25 μm, 0.28 μm, 0.30 μm, 0.32 μm, 0.35 μm, 0.38 μm, 0.40 μm, 0.42 μm, 0.45 μm, 0.48 μm, 0.50 μm, etc. In the embodiments of the present application, the Dv50 of the first positive electrode particles refers to the particle size corresponding to the cumulative particle size distribution percentage of 50% of the first positive electrode particles, which can be tested by a laser particle size instrument.

[0019] In some embodiments of the present application, the Dv50 of the second positive electrode particles is in the range of 0.75 μm-2.0 μm. Illustratively, the Dv50 of the second positive electrode particles can be 0.75 μm, 0.80 μm, 1.0 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, etc. In this way, the specific surface area of the second positive electrode particles is more suitable, the Li + diffusion path is also shorter, and at the same time, it is beneficial to achieve a higher compaction density. In addition, controlling the particle size of the second positive electrode particles in the above range can provide higher specific capacity, thereby facilitating the improvement of the energy density of the final battery.

[0020] In some embodiments of the present application, the Dv50 of the first positive electrode particles is in the range of 0.1 μm-0.5 μm, and the Dv50 of the second positive electrode particles is in the range of 0.75 μm-2.0 μm. The particle size matching of the first positive electrode particles and the second positive electrode particles can improve the compaction density and specific capacity of the positive electrode composite, thereby improving the energy density of the final battery, and thus enabling the final battery to have higher rate performance, better low-temperature performance, and higher energy density.

[0021] In some embodiments of the present application, the mass ratio of the first positive electrode particles to the second positive electrode particles is in the range of 1:(1-6). The mass ratio of the first positive electrode particles to the second positive electrode particles is appropriate, which can take into account the rate and low-temperature performance of the battery, and is also conducive to achieving higher compaction density, thereby improving the energy density of the battery. For example, the mass ratio of the first positive electrode particles to the second positive electrode particles can be 1:1.0, 1:1.5, 1:2.0, 1:2.5, 1:3.0, 1:3.5, 1:4.0, 1:4.5, 1:5.0, 1:5.5, 1:6.0, etc.

[0022] In some embodiments of the present application, the surface of the first positive electrode particles does not contain the M element.

[0023] In some embodiments of the present application, the surface of the first positive electrode particles further has a coating layer; wherein the material of the coating layer comprises a conductive carbon material and / or a fast ion conductor material. The conductive carbon material coated on the surface of the first positive electrode particles can accelerate the transmission of electrons at the interface, and can further improve the rate performance and low-temperature performance of the first positive electrode particles. The fast ion conductor material can improve the lithium ion transmission characteristics of the first positive electrode particles, thereby facilitating the rate and low-temperature performance of the first positive electrode particles. In addition, the coating layer can stabilize the interface of the first positive electrode particles, and can weaken the side reactions thereof during the charge and discharge cycle. The surface of the first positive electrode particles can have a carbon coating layer or a fast ion conductor coating layer, or the surface of the first positive electrode particles can have a carbon coating layer and a fast ion conductor coating layer stacked thereon, or the surface of the first positive electrode particles can have a mixed coating layer of carbon material and fast ion conductor material.

[0024] In some embodiments of the present application, the thickness of the coating layer on the surface of the first positive electrode particles is 3 nm-8 nm. In this way, the diffusion path of lithium ions in the first positive electrode particles is appropriately controlled, which facilitates achieving higher compaction density of the final positive electrode composite while taking into account better rate and low-temperature performance. For example, the thickness of the coating layer on the surface of the first positive electrode particles can be, but is not limited to, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, etc.

[0025] In some embodiments of the present application, the surface of the second positive electrode particle also has a coating layer; the material of the coating layer of the second positive electrode particle includes a conductive carbon material and / or a fast ion conductor material. Specifically, see the coating layer in the first positive electrode particle for analogy, which will not be described here.

[0026] The present application also provides a preparation method of the positive electrode composite material, comprising:

[0027] Preparation of the first positive electrode particle, mixing the first positive electrode particle with the second positive electrode particle to obtain the positive electrode composite material.

[0028] The above preparation method only needs to simply mix the first positive electrode particle and the second positive electrode particle to obtain the positive electrode composite material, which has simple steps and strong process reliability, and can realize large-scale industrial production. In the present application, the first positive electrode particle and the second positive electrode particle can be mixed in a mixer or other instruments known to those skilled in the art.

[0029] In some embodiments of the present application, the first positive electrode particle is prepared by a liquid phase method, and the second positive electrode particle is prepared by a solid phase method; wherein the liquid phase method includes but is not limited to hydrothermal method. The particle size of the first positive electrode particle prepared by the liquid phase method is small (Dv50 can be controlled within the range of 0.1 μm-0.5 μm), and the particle size distribution is narrow, so that the Li + diffusion coefficient of the first positive electrode particle is high; the particle size of the second positive electrode particle prepared by the solid phase method is relatively larger (Dv50 can be controlled within the range of 0.75 μm-2.0 μm), so that the positive electrode composite material obtained after mixing the two can achieve high compaction density and specific capacity, and the final battery can have high rate performance and energy density, and excellent cycle performance.

[0030] In some embodiments of the present application, the preparation of the first positive electrode particle comprises:

[0031] S01, under a first temperature, adding a lithium-containing solution, a phosphorus-containing solution and a M-containing solution to the stirring iron-containing solution in parallel flow, and gradually slowing down the parallel flow dropping speed of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution;

[0032] S02, keeping the first temperature for a first time, and cooling to obtain the first positive electrode particle. In the above preparation, the parallel flow dropping speed of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution is gradually slowed down, so that the M element in the first positive electrode particle gradually decreases from the inside to the surface layer.

[0033] In some embodiments of the present application, the flow rates of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution in step S01 are different, which specifically refers to the flow rates of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution before being combined are different. In some specific embodiments, the flow rate of the lithium-containing solution decreases from 2.6 L / h to 1.5 L / h, the flow rate of the phosphorus-containing solution decreases from 2.7 L / h to 1.5 L / h, and the flow rate of the M-containing solution decreases from 0.8 L / h to 0.2 L / h. In some embodiments of the present application, the flow rates of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution are different.

[0034] In the embodiments of the present application, the combined flow rate of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution can be linearly changed or non-linearly changed. In some embodiments of the present application, the solute of the above-mentioned iron-containing solution includes but is not limited to one or more of ferric sulfate heptahydrate, ferric chloride, ferric nitrate and other trivalent soluble iron compounds.

[0035] In some embodiments of the present application, the molar concentration of Fe in the above-mentioned iron-containing solution is 1.5 mol / L-2.1 mol / L. The molar concentration of Li in the above-mentioned lithium-containing solution is 1.0 mol / L-1.45 mol / L. The molar concentration of P in the above-mentioned phosphorus-containing solution is 1.5 mol / L-2.1 mol / L. The molar concentration of M in the above-mentioned M-containing solution is 0.05 mol / L-0.09 mol / L. Controlling the molar content of the elements in the above-mentioned solutions within the above-mentioned ranges is more conducive to the control of the flow rates of the solutions, thereby facilitating the sufficient reaction; in particular, controlling the molar concentration of M within the above-mentioned range is conducive to the gradient construction of M in the first positive electrode particles. Specifically, the molar concentration of Fe in the above-mentioned iron-containing solution can be, for example, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2.0 mol / L, 2.1 mol / L, etc. Specifically, the molar concentration of Li in the above-mentioned lithium-containing solution can be, for example, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.45 mol / L, etc. Specifically, the molar concentration of P in the above-mentioned phosphorus-containing solution can be, for example, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2.0 mol / L, 2.1 mol / L, etc. Specifically, the molar concentration of M in the above-mentioned M-containing solution can be, for example, 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, 0.09 mol / L.

[0036] In some embodiments of the present application, the solute of the lithium-containing solution includes, but is not limited to, one or more of lithium hydroxide, lithium chloride, lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium phosphate, and lithium nitrate.

[0037] In some embodiments of the present application, the solute of the phosphorus-containing solution includes, but is not limited to, any one or more of phosphoric acid, lithium phosphate, lithium dihydrogen phosphate, lithium monohydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0038] In some embodiments of the present application, the solute of the M-containing solution can be determined according to the specific selection of the M element. For example, when the M element includes a Mn element, the solute of the M-containing solution includes, but is not limited to, one or more of manganese carbonate, manganese oxide, manganese sulfate, and manganese nitrate; when the M element includes a Ti element, the solute of the M-containing solution includes, but is not limited to, at least one of titanium oxide, tetrabutyl titanate, isopropyl titanate, and tetraethyl titanate.

[0039] In some embodiments of the present application, the first temperature in steps S01 and S02 is in the range of 80-200°C. Controlling the first temperature in the above range can successfully prepare the lithium iron phosphate material doped with the M element at the iron site, improve the uniformity of the crystal structure, and control the particle size and surface roughness of the first positive electrode particles to be small. For example, the first temperature can be, but is not limited to, 80°C, 90°C, 100°C, 120°C, 150°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, and the like.

[0040] In some embodiments of the present application, the first time in step S02 is 4-6h. For example, the time can be, but is not limited to, 4.0h, 4.5h, 5.0h, 5.5h, 6.0h, and the like. A suitable holding time can ensure that the stability and electrochemical performance of the obtained first positive electrode particles are good.

[0041] In some embodiments of the present application, at 160-200°C, the lithium-containing solution, the phosphorus-containing solution, and the M-containing solution are added dropwise to the stirring iron-containing solution, the dropwise addition speed of the lithium-containing solution, the phosphorus-containing solution, and the M-containing solution is gradually slowed down, and the mixture is held for 4-6h to obtain the first positive electrode particles.

[0042] In some embodiments of the present application, the preparation of the first positive electrode particles further comprises a step S03: mixing the material obtained after cooling in S02 with a carbon source and calcining under a protective atmosphere. In this way, a coating layer (specifically a carbon coating layer) can be formed on the surface of the first positive electrode particles to improve the electrical conductivity of the first positive electrode particles, and the crystal structure of the first positive electrode particles can be further optimized, thereby optimizing the electrochemical performance of the first positive electrode particles. In embodiments of the present application, the protective atmosphere described above includes but is not limited to a nitrogen atmosphere, and the calcination temperature described above can be a calcination temperature well known to those skilled in the art, for example, 700-820°C. In embodiments of the present application, the carbon source described above can be selected from materials commonly used by those skilled in the art, and exemplarily, the carbon source described above includes but is not limited to one or more of sucrose, water-soluble phenol-formaldehyde resin, glucose, polyethylene glycol, hydroxymethyl cellulose, polyacrylamide, starch, polyvinyl alcohol, conductive carbon tube, graphene, etc.

[0043] In some embodiments of the present application, step S03 can also be: placing the material obtained after cooling in S02 in a solid-phase mixing device (for example, a ball mill) filled with a fast-ion conductor material, mixing, so as to form a fast-ion conductor coating layer on the surface of LiM x Fe 1-x PO4.

[0044] In some embodiments of the present application, the preparation of the second positive electrode particles comprises: weighing lithium source, iron source, phosphorus source and A element source according to the molecular formula LiFe(P 1-y A y )O4 of the second positive electrode particles to be prepared, mixing to obtain a first mixture, and sintering the first mixture to obtain the second positive electrode particles. The sintering temperature described above can be any temperature well known to those skilled in the art, for example, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C, 820°C.

[0045] In some specific embodiments of the present application, a carbon source is further added to the first mixture described above. In this way, a carbon coating layer can also be formed on the surface of the second positive electrode particles.

[0046] Embodiments of the present application also provide a positive electrode comprising the positive electrode composite material provided by embodiments of the present application. Due to the positive electrode composite material provided by the present application, the positive electrode can be used to provide a battery with good rate performance and low-temperature performance.

[0047] In some embodiments of the present application, the positive electrode comprises a positive electrode current collector and a positive electrode material layer arranged on at least one side surface of the positive electrode current collector, and the positive electrode material layer comprises the positive electrode composite material described above, a binder and an optional conductive agent.

[0048] In the embodiments of the present application, the binder can be any known binder suitable for the positive electrode of a lithium ion battery. Illustratively, the binder includes, but is not limited to, at least one of sodium hydroxyl cellulose (CMC), polyvinylidene fluoride (PVDF), and styrene butadiene rubber (SBR).

[0049] In the embodiments of the present application, the conductive agent can be any known conductive agent suitable for the positive electrode of a lithium ion battery. Illustratively, the conductive agent includes, but is not limited to, at least one of super p, acetylene black, graphene, and carbon nanotube.

[0050] The embodiments of the present application also provide a secondary battery including the positive electrode provided by the embodiments of the present application, a negative electrode, and an electrolyte between the positive electrode and the negative electrode. Since the positive electrode provided by the present application is used, the secondary battery has better rate performance and performs better in a low-temperature environment. In some embodiments, the secondary battery can also achieve a higher energy density.

[0051] In some embodiments, the secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the separator is arranged between the positive electrode and the negative electrode. Alternatively, in other embodiments, the battery cell can include the positive electrode, the negative electrode, and a solid-state electrolyte provided by the embodiments of the present application, and the solid-state electrolyte is arranged between the positive electrode and the negative electrode. In yet other embodiments, the secondary battery includes a positive electrode, a negative electrode, a solid-state electrolyte, and an electrolyte, and the solid-state electrolyte is arranged between the positive electrode and the negative electrode; or the secondary battery includes a positive electrode, a negative electrode, and a gel-state electrolyte, and the gel-state electrolyte is arranged between the positive electrode and the negative electrode.

[0052] The embodiments of the present application also provide an electrical equipment including the secondary battery provided by the embodiments of the present application. Since the secondary battery provided by the embodiments of the present application is used to supply power to the electrical equipment, the electrical equipment performs better in a low-temperature environment and has strong market competitiveness.

[0053] In some embodiments of the present application, the electrical equipment includes, but is not limited to, a vehicle, or a mobile phone, a notebook computer, a tablet computer, a smart watch, and other 3C electronic consumer goods.

[0054] The technical solutions of the present application are described in detail in the following embodiments.

[0055] Embodiment 1

[0056] (1) at a first temperature (specifically 80℃), a lithium-containing solution (specifically lithium hydroxide aqueous solution, the molar concentration of Li element is 1.45mol / L), a phosphorus-containing solution (specifically phosphoric acid, the molar concentration of P element is 1.8mol / L) and an M-containing solution (specifically vanadium hydroxide aqueous solution, the molar concentration of V element is 0.05mol / L) are added dropwise into an iron-containing solution (specifically iron sulfate aqueous solution, the molar concentration of Fe element is 1.8mol / L) under stirring, and the dropping speed of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution gradually decreases, the flow rate of the lithium-containing solution decreases from 2.6L / h to 1.5L / h, the flow rate of the phosphorus-containing solution decreases from 2.7L / h to 1.5L / h, and the flow rate of the M-containing solution decreases from 0.8L / h to 0.2L / h.

[0057] After the dropwise addition is completed, the hydrothermal reaction is carried out for a first time (specifically 4h) at a constant temperature, and then the reaction system is naturally cooled to room temperature, filtered to obtain a solidified product, and then the solidified product is sequentially washed and dried to obtain the first positive electrode particles, and the Dv50 of the first positive electrode particles is 0.55μm.

[0058] (2) the first positive electrode particles and second positive electrode particles (specifically LiFe(P 1-0.02 N 0.02 )O4) prepared by a solid phase method are mixed according to a mass ratio of 1:4 to obtain the positive electrode composite material, and the Dv50 of the second positive electrode particles is 0.95μm.

[0059] Example 2

[0060] The difference from Example 1 is that the material obtained after drying in step (1) in Example 1 is mixed with a carbon source (specifically glucose), and then calcined under a nitrogen atmosphere to obtain the first positive electrode particles with a carbon coating layer on the surface. The average thickness of the carbon coating layer is 3nm-8nm.

[0061] Example 3

[0062] The difference from Example 2 is that the Dv50 of the first positive electrode particles is 0.66μm, the Dv50 of the second positive electrode particles is 1.25μm, and the mass ratio of the first positive electrode particles to the second positive electrode particles is 1:5.2.

[0063] Example 4

[0064] The difference from Example 2 is that the Dv50 of the first positive electrode particles is 0.43μm, the Dv50 of the second positive electrode particles is 1.05μm, and the mass ratio of the first positive electrode particles to the second positive electrode particles is 1:2.33.

[0065] Example 5

[0066] The difference from Example 2 is that the Dv50 of the first positive electrode particle is 0.35 μm, the Dv50 of the second positive electrode particle is 1.32 μm, and the mass ratio of the first positive electrode particle to the second positive electrode particle is 1:1.5.

[0067] Example 6

[0068] The difference from Example 2 is that the Dv50 of the first positive electrode particle is 0.47 μm, the Dv50 of the second positive electrode particle is 1.48 μm, and the mass ratio of the first positive electrode particle to the second positive electrode particle is 1:1.

[0069] Example 7

[0070] The difference from Example 2 is that the mass ratio of the first positive electrode particle to the second positive electrode particle is 1:2.

[0071] To highlight the beneficial effects of the embodiments of this application, the following comparative examples are provided.

[0072] Comparative Example 1

[0073] (1) At a first temperature (specifically 80°C), a lithium-containing solution (specifically an aqueous solution of ferric sulfate), a phosphorus-containing solution (specifically phosphoric acid) and an M-containing solution (specifically an aqueous solution of vanadium hydroxide) are added dropwise to an iron-containing solution (specifically an aqueous solution of ferric sulfate) under stirring, and the dropwise addition rates of the lithium-containing solution, the phosphorus-containing solution and the M-containing solution gradually decrease.

[0074] After the addition is complete, the material is kept at a constant temperature for 4 hours to carry out a hydrothermal reaction. It is then naturally cooled to room temperature, filtered, and a solidified material is obtained. The solidified material is then washed and dried to obtain the positive electrode active material, which has a Dv50 of 0.55 μm.

[0075] Comparative Example 2

[0076] The difference from Example 1 is that the first positive electrode particle in Example 1 is replaced with LiV. x Fe 1-x PO4(LiV x Fe 1-x The total content of V element in PO4 is the same as in Example 1, but the dropping rate of the solution containing M element is fixed.

[0077] Performance testing

[0078] (1) Preparation of test batteries: The positive electrode composite materials, PVDF, carbon nanotubes and graphene were mixed in a mass ratio of 95:2:2:1 and dispersed in N-methylpyrrolidone to obtain a positive electrode slurry. The positive electrode slurry was coated on a positive electrode current collector (specifically aluminum foil), and after drying, rolling and slitting, the positive electrodes of each embodiment and comparative example were obtained, and the areal density and compaction density of the positive electrode were controlled to be consistent.

[0079] The positive electrode of each example and comparative example was assembled into a button cell together with a negative electrode (the negative electrode included a copper foil and a negative active material layer provided on the surface of the copper foil, the mass ratio of graphite: Super C: SBR: CMC in the negative active material layer was 94:2:3:1), and a separator.

[0080] (2) Measurement of lithium ion diffusion coefficient: After the test battery of each example and comparative example prepared above was kept at -10°C for 10 h, charge-discharge cycle test was carried out at a current of 3C rate at a voltage of 2.0-3.8 V, and after 10 cycles, the state of charge of the battery was adjusted to 50% SOC, electrochemical impedance spectroscopy (EIS) was tested, the Warburg impedance factor σ was obtained, and the active ion diffusion coefficient of each active material was calculated according to the following formula:

[0081]

[0082] wherein D represents the active ion diffusion coefficient of the active material, R is the gas constant, R = 8.314 J / (mol·K); T is 263.15 K; n is the number of electrons per mole participating in the electrochemical reaction (for Li + , the value is 1), A is the active area of the positive electrode; F is the Faraday constant, specifically 96485 C / mol, and C is the concentration of the active ion Li + in the positive electrode material, and the results are summarized in Table 1.

[0083] (3) The test battery of each example and comparative example prepared above was placed at 25°C, and was charged at a constant current and constant voltage between a voltage range of 2.5-4.25 V to 3.8 V at a current of 0.1C, and the cutoff current was 0.02C during the constant voltage charging, the first charge capacity was recorded, the first charge gram capacity of the battery was calculated, and the first charge gram capacity of each example and comparative example 2 = first charge capacity / mass of the positive electrode composite material (or, the first charge gram capacity of comparative example 1 = first charge capacity / mass of the positive electrode active material of comparative example 1), and the results are summarized in Table 1.

[0084] (4) Test of 3C discharge ratio:

[0085] First, the test battery of each example and comparative example was charged at a constant current of 1 / 3C to 3.8 V / cell at room temperature 25±5°C, and was left for 30 min; then, it was discharged at a constant current of 1 / 3C to 2.0 V / cell, and the initial discharge capacity was recorded; at 25±5°C, it was charged at a constant current of 1 / 3C to 3.8 V / cell, and was left for 30 min; then, it was discharged at a constant current of 3C to 2.0 V / cell, and was left for 1 h; the 3C discharge capacity was recorded; and the 3C discharge ratio = 3C discharge capacity / initial discharge capacity, and the results are summarized in Table 1.

[0086] Discharge ratio test at -20℃:

[0087] First, the test batteries of each example and comparative example were placed at room temperature 25±5℃, and charged at 1 / 3C constant current to 3.8V / cell, and then discharged at 1 / 3C constant current to 2.0V / cell, and the initial discharge capacity was recorded; at 25±5℃, charged at 1 / 3C constant current to 3.8V / cell; then stored at -20℃ for 6h, and discharged at 1 / 3C constant current to 2.0V / cell at -20℃, and the discharge capacity at -20℃ was recorded. The discharge ratio at -20℃ = discharge capacity at -20℃ / initial discharge capacity, and the results are summarized in Table 1.

[0088] Table 1

[0089]

[0090] From the results in Table 1, it can be seen that the positive electrode composite material provided by the examples of the present application has a higher lithium ion diffusion coefficient, so as to exhibit better rate performance and low temperature performance, and the positive electrode composite material of the examples of the present application also has a higher first charge gram capacity.

[0091] The above is an exemplary embodiment of the present application, it should be noted that for those skilled in the art, without departing from the principles of the present application, can make several improvements and refinements, these improvements and refinements are also considered to be within the scope of protection of the present application.

Claims

1. A positive electrode composite material, characterized in that, Comprising a first positive electrode particle and a second positive electrode particle, the first positive electrode particle being LiM x Fe 1-x PO4, where 0 < x ≤ 0.2, M is selected from transition elements, and M includes one or more of Mo, Co, V, Mn, Ni, Zn, Cu, Cr, Ti, and M is doped at the iron site of lithium iron phosphate; from the inside to the surface of the first positive electrode particle, the content of M shows a decreasing trend; the second positive electrode particle is LiFe(P 1-y A y )O4, where 0 < y ≤ 0.05, A includes S and / or N, and A is doped at the phosphorus site of lithium iron phosphate.

2. The positive electrode composite material according to claim 1, characterized in that, The Dv50 of the first positive electrode particle is in the range of 0.1μm-0.5μm.

3. The positive electrode composite material according to claim 1, characterized in that, The Dv50 of the second positive electrode particle is in the range of 0.75μm-2.0μm.

4. The positive electrode composite material according to claim 1, characterized in that, The mass ratio of the first positive electrode particle to the second positive electrode particle is in the range of 1:(1-6).

5. The positive electrode composite material according to any one of claims 1-4, characterized in that, The surface of the first positive electrode particle also has a coating layer; the material of the coating layer includes conductive carbon and / or fast ion conductor materials.

6. The method for preparing the positive electrode composite material according to any one of claims 1-5, characterized in that, include: The first positive electrode particle is prepared, and the first positive electrode particle is mixed with the second positive electrode particle to obtain the positive electrode composite material.

7. The preparation method according to claim 6, characterized in that, The preparation of the first positive electrode particle includes: At a first temperature, a lithium-containing solution, a phosphorus-containing solution, and a solution containing M are added dropwise in a co-current manner to an iron-containing solution under stirring, and the dropwise addition rates of the lithium-containing solution, the phosphorus-containing solution, and the solution containing M are gradually reduced. The material is kept at the first temperature for a first time, then cooled and subjected to solid-liquid separation, washing, and drying in sequence to obtain the first positive electrode particles.

8. The preparation method according to claim 7, characterized in that, The first temperature is 80℃-200℃, and the first time is 4h-6h.

9. The preparation method according to claim 7, characterized in that, It also includes mixing the dried material with a carbon source and calcining it under a protective atmosphere.

10. A positive electrode, characterized in that, Including the positive electrode composite material as described in any one of claims 1-5.

11. A secondary battery, characterized in that, The secondary battery includes a negative electrode, a positive electrode as described in claim 10, and an electrolyte located between the positive electrode and the negative electrode.

12. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 11.

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