A positive electrode material, a secondary battery, and an electrical device.

Abstract

This application discloses a cathode material, a secondary battery, and an electrical device, relating to the field of battery technology. The cathode material provided in this application introduces vanadium, a modified element, into the core, and forms an interface layer comprising vanadium on at least a portion of the core's surface. Simultaneously, a conductive layer is introduced on at least a portion of the interface layer's surface. The resulting cathode material exhibits good bulk intrinsic conductivity and excellent structural stability. The corresponding secondary battery possesses high initial discharge specific capacity, as well as excellent initial coulombic efficiency and cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode material, a secondary battery, and an electrical device. Background Technology

[0002] The cathode material of a secondary battery is crucial in determining its energy density, safety, and cycle life. Lithium manganese iron phosphate (LiMn) with an olivine structure is a key example. x Fe 1-x PO4 material is used due to its high operating voltage (approximately 4.1V vs. Li / Li). + Mn has attracted widespread attention due to its high theoretical specific capacity and superior thermal stability, and is considered a strong candidate for the next generation of high-performance power batteries. However, its large-scale application is limited by two inherent defects: first, its intrinsic electronic conductivity and ionic conductivity are low, resulting in poor coulombic efficiency in the first cycle of the corresponding secondary battery; second, during charge-discharge cycles, especially at high voltages, Mn... 3+ The Jahn-Teller effect can cause lattice distortion and lead to the dissolution of manganese into the electrolyte, resulting in loss of active material and destruction of the interface structure, thereby accelerating capacity decay. Summary of the Invention

[0003] The purpose of this application is to solve the technical problems of poor bulk intrinsic conductivity and insufficient structural stability of cathode materials in the prior art, which leads to low capacity, poor first-cycle coulombic efficiency and poor cycle performance of the corresponding secondary batteries. A cathode material with good bulk intrinsic conductivity and structural stability is proposed, and the corresponding secondary battery has high first-cycle discharge specific capacity and first-cycle coulombic efficiency, as well as excellent cycle performance.

[0004] To achieve the above objectives, a first aspect of this application provides a positive electrode material, the positive electrode material comprising a core, an interface layer disposed on at least a portion of the surface of the core, and a conductive layer disposed on at least a portion of the surface of the interface layer; The core comprises lithium manganese iron phosphate and modified vanadium. The interface layer includes vanadium; In X-ray photoelectron spectroscopy (XPS) testing, vanadium 2P in the interface layer 2 / 3 After fitting, the spectrum showed two characteristic peaks at binding energies of 516.0 ± 0.1 eV and 517.2 ± 0.1 eV; the core contains vanadium 2P. 2 / 3 After fitting, the spectrum showed a characteristic peak at a binding energy of 517.2 ± 0.1 eV.

[0005] As an embodiment of this application, the percentage of vanadium atoms in the interface layer is greater than the percentage of vanadium atoms in the core.

[0006] As an embodiment of this application, the percentage of vanadium atoms in the interface layer is 10% to 15%.

[0007] As an embodiment of this application, the atomic percentage of vanadium in the core is 0.01% to 0.07%.

[0008] As an embodiment of this application, the core also includes modified aluminum.

[0009] As an embodiment of this application, the chemical formula of the core is Li. a (Mn) 1-x-y Fe x Al y P 1-z V z O4, where 0.4≤x≤0.6, 0.003≤y≤0.01, 0.001≤z≤0.005, and 0.95≤a≤1.05.

[0010] As an embodiment of this application, the conductivity of the conductive layer at 25°C is 70 S / cm to 100 S / cm.

[0011] As an embodiment of this application, the conductive layer comprises polypyrrole.

[0012] As an embodiment of this application, in X-ray photoelectron spectroscopy testing, the nitrogen 1s spectrum of the polypyrrole, after fitting, has three characteristic peaks at binding energies of 399.8 ± 0.1 eV, 401.2 ± 0.1 eV, and 400.5~401.0 eV.

[0013] As an embodiment of this application, the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms is 20% to 40%.

[0014] As an embodiment of this application, the number-average molecular weight of the polypyrrole is 5000 g / mol to 50000 g / mol.

[0015] As an embodiment of this application, the thickness of the interface layer is 0.5nm~1.5nm.

[0016] In one embodiment of this application, the thickness of the conductive layer is 0.6 nm to 2.8 nm.

[0017] As an embodiment of this application, the Dv50 of the cathode material is 0.5μm~2μm.

[0018] A second aspect of this application provides a secondary battery comprising the positive electrode material described in this application.

[0019] A third aspect of this application provides an electrical device comprising the secondary battery described in this application.

[0020] Compared with the prior art, the beneficial effects of this application are: The cathode material provided in this application introduces vanadium, a modified element, into the core, and forms an interface layer including vanadium on at least a portion of the surface of the core. Simultaneously, a conductive layer is introduced on at least a portion of the surface of the interface layer. The resulting cathode material exhibits good bulk intrinsic conductivity and excellent structural stability. Consequently, the resulting secondary battery has a high first-cycle discharge specific capacity, as well as excellent first-cycle coulombic efficiency and cycle performance. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0023] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0024] In one embodiment of this application, a positive electrode material is provided, the positive electrode material comprising a core, an interface layer disposed on at least a portion of the surface of the core, and a conductive layer disposed on at least a portion of the surface of the interface layer. The core comprises lithium manganese iron phosphate and modified vanadium. The interface layer includes vanadium; In X-ray photoelectron spectroscopy (XPS) testing, vanadium 2P in the interface layer 2 / 3 After fitting, the spectrum showed two characteristic peaks at binding energies of 516.0 ± 0.1 eV and 517.2 ± 0.1 eV; the core contains vanadium 2P. 2 / 3 After fitting, the spectrum showed a characteristic peak at a binding energy of 517.2 ± 0.1 eV.

[0025] This application research found that the cathode material provided by this application, by introducing the modified element vanadium into the core and setting an interface layer including vanadium on at least a part of the surface of the core, and introducing a conductive layer on at least a part of the surface of the interface layer, the cathode material has good bulk intrinsic conductivity and excellent structural stability. The corresponding secondary battery has high first-cycle discharge specific capacity, as well as excellent first-cycle coulombic efficiency and cycle performance.

[0026] Specifically, firstly, the core includes the modified element vanadium, V 5+ With P 5+ The price states are the same, but V 5+ The radius (0.54 Å) is slightly larger than P. 5+ The radius (0.31 Å). Under the corresponding synthetic conditions, V can partially replace P in the lattice, which in 2P 2 / 3 After fitting, the spectrum shows a characteristic peak at a binding energy of 517.2 ± 0.1 eV (i.e., mainly V). 5+ (existing in the form of [VO4] tetrahedron), which can form a more robust [VO4] tetrahedron, thereby further improving the stability of the kernel structure.

[0027] Secondly, an interface layer is introduced into at least part of the surface of the core, and the interface layer includes vanadium, while the vanadium in the interface layer is in 2P 2 / 3 After fitting, the spectrum showed two characteristic peaks at binding energies of 516.0 ± 0.1 eV and 517.2 ± 0.1 eV (i.e., in terms of V). 4+ and V 5+ Vanadium exists in the form of an amorphous or low-crystallinity interface layer in the interface layer, in an amorphous or defective state environment. This effectively acts as a bridge between the core and the conductive layer, anchors the conductive layer, improves the adhesion between the conductive layer and the interface layer, and thus enhances the stability of the overall structure.

[0028] Thirdly, introducing a conductive layer on at least part of the surface of the interface layer can not only effectively improve the electronic / ionic conductivity and increase the capacity, but also effectively isolate the electrolyte and improve the structural stability.

[0029] In some embodiments, the percentage of vanadium atoms in the interface layer is greater than the percentage of vanadium atoms in the core.

[0030] This study found that vanadium in the interface layer is formed by segregation from the bulk phase to the core surface during the preparation of the cathode material due to high temperature and subsequent cooling. During the segregation process, it will further combine with elements such as oxygen. At the same time, lithium in lithium manganese iron phosphate, which is rich in lithium, may also exist in the interface layer. The atomic percentage of vanadium in the interface layer is greater than that in the core, which can achieve a smooth transition of vanadium content, thereby achieving a bridging effect on the core and the conductive layer, and thus improving the structural stability of the cathode material.

[0031] In some embodiments, the percentage of vanadium atoms in the interface layer is 10% to 15%.

[0032] For example, the percentage of vanadium atoms in the interface layer can be any point value or any two points between 10% and 15%, such as 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, etc.

[0033] It should be noted that the method for testing the percentage of vanadium atoms in the interface layer is as follows: the sample is prepared using a focused ion beam, and the cross-section of the particles in the interface layer region is quantitatively analyzed at multiple points using a scanning transmission electron microscope-energy dispersive X-ray spectrometer, and the arithmetic mean is taken.

[0034] This study found that the percentage of vanadium atoms in the interface layer affects its bridging effect on the core and conductive layer, as well as its chemical anchoring effect on the conductive layer; it also affects the diffusion efficiency of lithium ions. When the percentage of vanadium atoms in the interface layer is further selected within the above range, the resulting cathode material has better structural stability and conductivity, resulting in higher first-cycle coulombic efficiency, better cycle performance, and higher first-cycle discharge specific capacity of the secondary battery.

[0035] In some embodiments, the percentage of vanadium atoms in the core is 0.01% to 0.07%.

[0036] For example, the percentage of vanadium atoms in the core can be any point value or any two points between 0.01% and 0.07%, such as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, etc.

[0037] It should be noted that the method for testing the percentage of vanadium atoms in the core is as follows: the sample is prepared using a focused ion beam, and the region inside the particle that avoids the interface layer is quantitatively analyzed at multiple points using a scanning transmission electron microscope-energy dispersive X-ray spectrometer, and the arithmetic mean is taken.

[0038] This study found that the percentage of vanadium atoms in the core affects its substitution effect on P, thereby affecting the structural stability of the formed [VO4] tetrahedron and consequently the overall structural stability of the cathode material. When the percentage of vanadium atoms in the core is further selected within the above range, the secondary battery prepared subsequently exhibits better overall performance.

[0039] In some embodiments, the core further includes the modified element aluminum.

[0040] This application research found that Al 3+ radius and Fe 2+ and Mn 2+ The radii are close, and the Al-O bond energy (~512 kJ / mol) is much stronger than that of Fe-O (~409 kJ / mol) and Mn-O (~402 kJ / mol). When Al occupies Fe and / or Mn sites, it enhances the stability of the lithium manganese iron phosphate olivine structure and effectively suppresses lattice collapse and volume change during the cycling process.

[0041] In some embodiments, the core has the chemical formula Li. a (Mn) 1-x-y Fe x Al y P 1-z V z O4, where 0.4≤x≤0.6, 0.003≤y≤0.01, 0.001≤z≤0.005, and 0.95≤a≤1.05.

[0042] In some embodiments, the conductivity of the conductive layer at 25°C is 70 S / cm to 100 S / cm.

[0043] For example, the conductivity of the conductive layer at 25°C can be any point value or any two-point range value between 70S / cm and 100S / cm, such as 70S / cm, 75S / cm, 80S / cm, 85S / cm, 90S / cm, 95S / cm, 100S / cm, etc.

[0044] It should be noted that the test method for the conductivity of the conductive layer at 25°C is as follows: take the positive electrode material, use solvent dissolution or ultrasonic peeling to separate and collect the conductive layer from the surface of the positive electrode material, dry the collected conductive layer material and press it into a sheet, and use the four-probe method to measure it at 25°C.

[0045] This study found that the conductivity of the conductive layer affects the ion / electron transport efficiency, as well as the degree of material activity in the core; it also affects Mn. 3+The dissolution of the conductive layer and its barrier effect on the electrolyte; when the conductivity of the conductive layer at 25°C is further selected to be within the above range, the secondary battery prepared subsequently has a higher first-cycle discharge specific capacity, better cycle performance, and higher first-cycle coulombic efficiency.

[0046] In some embodiments, the conductive layer comprises polypyrrole.

[0047] This study found that further selection of polypyrrole into the conductive layer can better establish chemical bonds with vanadium between the interface layers, thereby effectively improving the bonding force between the conductive layer and the interface layer, enhancing the structural stability of the positive electrode active material, and thus improving the overall performance of the corresponding secondary battery.

[0048] In some embodiments, during X-ray photoelectron spectroscopy testing, the nitrogen 1s spectrum in the polypyrrole, after fitting, exhibits three characteristic peaks at binding energies of 399.8 ± 0.1 eV, 401.2 ± 0.1 eV, and 400.5~401.0 eV.

[0049] This study found that among the above characteristic peaks, 399.8 ± 0.1 eV is the inherent -NH- peak in polypyrrole, and 401.2 ± 0.1 eV is the doped -N peak. + - Peak, 400.5~401.0 eV is V 5+ The peak of nitrogen coordination; that is, N can better enhance the connection between layers by coordinating with V, thereby improving the structural stability of the cathode material and the overall performance of the secondary battery.

[0050] In some embodiments, the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms is 20% to 40%.

[0051] For example, the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms can be any point value or any two points between 20% and 40%, such as 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, etc.

[0052] It should be noted that the method for testing the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms is as follows: After treating the cathode material with N-methylpyrrolidone, X-ray photoelectron spectroscopy (XPS) is used to perform peak fitting on the nitrogen 1s spectrum. The peak area of ​​charged nitrogen is calculated as the proportion of the total nitrogen peak area. Among them, the peak at 399.8 ± 0.1 eV is assigned to neutral nitrogen (-NH-), and the peak at 401.2 ± 0.1 eV is assigned to charged nitrogen (-N-). + -), the proportion of charged nitrogen atoms is calculated by dividing the area of ​​charged nitrogen peaks by the area of ​​total nitrogen peaks.

[0053] This study found that the ratio of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms affects the ion / electron conduction capability of the cathode material, as well as the regularity of the polypyrrole chain segments, thus affecting the structural stability. When the ratio of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms is further selected within the above range, the resulting secondary battery has higher first-cycle coulombic efficiency and capacity, and better cycle performance.

[0054] In some embodiments, the number-average molecular weight of the polypyrrole is 5000 g / mol to 50000 g / mol.

[0055] For example, the number-average molecular weight of the polypyrrole can be any point value or any two points between 5000 g / mol and 50000 g / mol, such as 5000 g / mol, 10000 g / mol, 15000 g / mol, 20000 g / mol, 25000 g / mol, 30000 g / mol, 35000 g / mol, 40000 g / mol, 45000 g / mol, 50000 g / mol, etc.

[0056] It should be noted that the number-average molecular weight of the polypyrrole is determined by the following method: taking the positive electrode material, separating and collecting the conductive layer from the surface of the positive electrode material by solvent dissolution or ultrasonic exfoliation, and measuring the collected conductive layer material using gel permeation chromatography with polystyrene as a standard.

[0057] This study found that the number-average molecular weight of polypyrrole affects the structural continuity, compactness, and mechanical strength of the conductive layer, thereby affecting the structural stability of the conductive layer and its protective effect on the core. It also affects the bonding ability with the interface layer, thus affecting the structural stability of the cathode material. When the number-average molecular weight of polypyrrole is further selected within the above range, the resulting secondary battery has higher capacity, higher first-cycle coulombic efficiency, and better cycle performance.

[0058] In some embodiments, the thickness of the interface layer is 0.5 nm to 1.5 nm.

[0059] For example, the thickness of the interface layer can be any point value or any two-point range value between 0.5nm and 1.5nm, such as 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1.0nm, 1.1nm, 1.2nm, 1.3nm, 1.4nm, 1.5nm, etc.

[0060] It should be noted that the test method for the thickness of the interface layer is as follows: the contrast difference at the edge of the cathode material particles is observed using a high-resolution transmission electron microscope, the distance from the outermost surface of the particle to the location where the contrast of the core crystal appears is measured, and the arithmetic mean is taken.

[0061] This study found that the thickness of the interface layer affects the ion / electron conduction efficiency of the cathode material, as well as the chemical anchoring effect of the conductive layer, thereby affecting the structural stability of the cathode material. When the thickness of the interface layer is further selected within the above range, the obtained cathode material has a high intrinsic bulk conductivity and excellent structural stability. The corresponding secondary battery has a high first-cycle coulombic efficiency, excellent cycle performance, and high capacity.

[0062] In some embodiments, the thickness of the conductive layer is 0.6 nm to 2.8 nm.

[0063] For example, the thickness of the conductive layer can be any point value or any two-point range value between 0.6nm and 2.8nm, such as 0.6nm, 0.8nm, 1.0nm, 1.2nm, 1.4nm, 1.6nm, 1.8nm, 2.0nm, 2.2nm, 2.4nm, 2.6nm, 2.8nm, etc.

[0064] It should be noted that the test method for the thickness of the conductive layer is as follows: the amorphous contrast region of the outermost layer of the positive electrode material particles is observed using a high-resolution transmission electron microscope, the distance from the outermost surface of the particles to the location where the interface layer appears is measured, and the arithmetic mean is taken.

[0065] This study found that the thickness of the conductive layer affects the ion / electron conduction efficiency of the cathode material, as well as its mechanical toughness, thus impacting its buffering effect against core volume expansion. Furthermore, the thickness of the conductive layer also affects the strength of physical isolation, thereby influencing the degree of side reactions. Simultaneously, the thickness of the conductive layer also affects the utilization rate of the active material, thus affecting the capacity utilization rate. When the thickness of the conductive layer is further selected within the aforementioned range, the resulting secondary battery exhibits higher capacity, higher first-cycle coulombic efficiency, and better cycle performance.

[0066] In some embodiments, the Dv50 of the cathode material is 0.5 μm to 2 μm.

[0067] For example, the Dv50 of the cathode material can be any point value or any two-point range value between 0.5μm and 2μm, such as 0.5μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, etc.

[0068] It should be noted that the Dv50 test method for the cathode material is as follows: the cathode material is tested using a laser particle size analyzer with water as the dispersion medium, and the particle size value corresponding to the cumulative volume distribution reaching 50% is taken.

[0069] This study found that the Dv50 of the cathode material affects the length of the lithium-ion solid-phase diffusion path and the degree of side reactions, thereby affecting Mn. 3+ The degree of dissolution also affects the interface stability and processing performance; when the Dv50 of the cathode material is further selected within the above range, the capacity, first-cycle coulombic efficiency and cycle performance of the corresponding secondary battery can be better improved.

[0070] In some embodiments, the method for preparing the positive electrode material includes the following steps: (1) Dissolve aluminum, iron and manganese sources in deionized water in stoichiometric proportions to obtain solution A; (2) Dissolve the vanadium source in a stoichiometric ratio in an aqueous solution of citric acid (C6H8O7) to obtain a yellow transparent solution, which is solution B; (3) Dissolve the phosphorus source and lithium source in deionized water in a stoichiometric ratio to obtain solution C; (4) Mix solution A and solution B and add solution C dropwise to obtain a mixed solution; then stir and evaporate the mixed solution at 80±5℃ until a viscous gel is formed; (5) After drying the viscous gel, it is pre-sintered in an inert gas environment. After the pre-sintering is completed, it is sintered, then cooled and ground to obtain an intermediate. (6) The intermediate was added to ethanol and ultrasonically dispersed to form a suspension. Then, the polymer monomer was added dropwise under ice bath conditions, followed by the addition of an aqueous solution of oxidant and stirring. After the reaction was completed, the mixture was filtered, washed, dried and pulverized to obtain the positive electrode material.

[0071] In some embodiments, in step (1), the aluminum source includes at least one of hydrated aluminum nitrate (Al(NO3)3·9H2O), aluminum sulfate (Al2(SO4)3), aluminum acetate (Al(CH3COO)3), and aluminum isopropoxide (Al(Oi-Pr)3); the iron source includes at least one of hydrated ferric nitrate (Fe(NO3)3·9H2O), ferrous sulfate (FeSO4·7H2O), ferric acetate (Fe(CH3COO)3), and ferric citrate; and the manganese source includes at least one of hydrated manganese acetate (Mn(CH3COO)2·4H2O), manganese sulfate (MnSO4·H2O), manganese nitrate (Mn(NO3)2), and manganese carbonate (MnCO3).

[0072] In some embodiments, in step (2), the vanadium source includes at least one of ammonium metavanadate (NH4VO3), vanadium oxysulfate (VOSO4), vanadium oxyacetylacetonate (VO(acac)2), and vanadium pentoxide (V2O5).

[0073] In some embodiments, in step (3), the phosphorus source includes at least one of ammonium dihydrogen phosphate (NH4H2PO4), diammonium hydrogen phosphate ((NH4)2HPO4), phosphoric acid (H3PO4), and ammonium phosphate ((NH4)3PO4); the lithium source includes at least one of lithium carbonate (Li2CO3), lithium hydroxide (LiOH·H2O), lithium acetate (CH3COOLi), and lithium nitrate (LiNO3).

[0074] In some embodiments, in step (5), the drying temperature is 110℃~130℃ and the drying time is 10h~14h.

[0075] In some embodiments, in step (5), the inert gas includes at least one of helium, argon, and neon.

[0076] In some embodiments, in step (5), the pre-sintering temperature is 300℃~400℃ and the pre-sintering time is 3h~5h.

[0077] In some embodiments, in step (5), the sintering process involves heating to 600°C to 750°C at a heating rate of 2°C / min to 8°C / min and then holding for 5 to 10 hours.

[0078] In some embodiments, in step (5), the cooling process is to cool down to 20°C to 30°C at a cooling rate of 1°C / min to 15°C / min.

[0079] In some embodiments, in step (5), the grinding speed is 200 rpm to 400 rpm and the grinding time is 1 h to 5 h.

[0080] In some embodiments, the ultrasonic dispersion time in step (6) is 25-35 min.

[0081] In some embodiments, in step (6), the polymer monomer includes any one of pyrrole, N-methylpyrrole, 3,4-ethylenedioxythiophene (EDOT), and aniline; the oxidant includes any one of sodium persulfate, ammonium persulfate ((NH4)2S2O8), potassium persulfate (K2S2O8), and ferric chloride (FeCl3).

[0082] In some embodiments, in step (6), the mass percentage of the oxidant in the aqueous oxidant solution is 20% to 30%.

[0083] In some embodiments, in step (6), the molar ratio of oxidant to polymer monomer is (0.7~1.5):1.

[0084] In some embodiments, the stirring reaction time in step (6) is 10h to 14h.

[0085] In some embodiments, in step (6), the drying temperature is 55°C to 65°C and the drying time is 10h to 14h.

[0086] It should be noted that the chemical formula of the core can be changed by controlling the amount of phosphorus source, manganese source, iron source, vanadium source and aluminum source added in steps (1) to (3).

[0087] It should be noted that by controlling the amount of vanadium source added in step (2), or by controlling the sintering temperature and time and cooling rate in step (5), the core chemical formula can be changed, and the percentage of vanadium atoms in the core and the percentage of vanadium atoms in the interface layer can also be changed, as well as the thickness of the interface layer can be changed.

[0088] It should be noted that by controlling the amount of polymer monomer added in step (6) or the stirring reaction time, the thickness of the conductive layer can be changed, and the conductivity of the conductive layer at 25°C can be changed; the number-average molecular weight of the polymer in the conductive layer can also be changed.

[0089] It should be noted that by controlling the molar ratio of oxidant to polymer monomer, reaction temperature, reaction time, and type of oxidant in step (6), the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms (i.e., doping degree) can be changed. Specifically, increasing the molar ratio of oxidant to monomer, extending the reaction time, or selecting an oxidant with stronger oxidizing power can all help to increase the doping degree of polypyrrole.

[0090] In one embodiment of this application, a secondary battery is provided, including the positive electrode material described in this application.

[0091] In some embodiments, the secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator.

[0092] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer includes the positive electrode material, positive conductive agent, and positive binder described in this application.

[0093] This application does not limit the positive electrode conductive agent; any known positive electrode conductive agent can be used. For example, the positive electrode conductive agent may be at least one of acetylene black, graphene, and carbon nanotubes (CNTs).

[0094] This application does not limit the positive electrode binder; any known positive electrode binder can be used. For example, the positive electrode binder may be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium polyacrylate (PAANa), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR).

[0095] In some embodiments, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector; the negative active material layer includes a negative active material, a negative conductive agent, and a negative binder.

[0096] This application does not limit the negative electrode active material; any known negative electrode active material can be used. For example, the negative electrode active material may be at least one of artificial graphite, natural graphite, silicon carbide, and silicon oxide.

[0097] This application does not limit the choice of the negative electrode conductive agent; any known negative electrode conductive agent can be used. For example, the negative electrode conductive agent may be at least one of acetylene black, graphene, and carbon nanotubes (CNTs).

[0098] This application does not limit the negative electrode binder; any known negative electrode binder can be used. Exemplarily, the negative electrode binder may be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium polyacrylate (PAANa), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR).

[0099] In some embodiments, the electrolyte comprises an organic solvent and a lithium salt.

[0100] This application does not limit the choice of organic solvent; any known organic solvent may be used. For example, the organic solvent may be at least one of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate.

[0101] This application does not limit the choice of lithium salt; any known lithium salt can be used. For example, the lithium salt may be at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0102] This application does not limit the choice of diaphragm; any known diaphragm can be used. For example, the diaphragm may be a polyethylene film.

[0103] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0104] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, 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., but are not limited thereto.

[0105] Example 1 This application provides a positive electrode material and a secondary battery. The preparation method of the positive electrode material and the secondary battery includes the following steps: (1) Preparation of cathode materials S1. Dissolve 0.15g of aluminum source (Al(NO3)3·9H2O), 6.06g of iron source (Fe(NO3)3·9H2O) and 5.88g of manganese source (Mn(CH3COO)2·4H2O) in 100mL of deionized water to obtain solution A; (2) Dissolve 0.11g of vanadium source (NH4VO3) in 50mL of citric acid aqueous solution (citric acid concentration is 10%) to obtain a yellow transparent solution, which is solution B; (3) Dissolve 2.30g of phosphorus source (NH4H2PO4) and 1.48g of lithium source (Li2CO3) in deionized water to obtain solution C; (4) Slowly add solution A to solution B, then add solution C dropwise to obtain a mixed solution; then stir and evaporate the mixed solution at 80±5℃ until a viscous gel is formed; (5) After the viscous gel was vacuum dried at 120°C for 12 hours, it was pre-sintered in an inert gas environment (argon environment) (temperature 350°C, time 4 hours). After the pre-sintering was completed, it was sintered again (heated to 700°C at a heating rate of 5°C / min and held for 10 hours). Then it was cooled (cooled to 25°C at a cooling rate of 8°C / min). The cooled product was placed in a planetary ball mill and ball-milled at a speed of 300 rpm for 2 hours. The intermediate was obtained after grinding. (6) Add 10g of the above intermediate to 100mL of ethanol and ultrasonically disperse for 30min to form a suspension. Then keep stirring (stirring speed is 400rpm) and add 0.23g of polymer monomer (pyrrole) dropwise under ice bath conditions. Then add an oxidant aqueous solution (sodium persulfate aqueous solution, temperature is 0℃, the mass percentage of sodium persulfate in the sodium persulfate aqueous solution is 25%) and stir for 12h (molar ratio of sodium persulfate to polymer monomer is 1:1). The solution turns black. After the reaction is completed, filter and collect the filter residue. Wash it with deionized water and ethanol in sequence until the filtrate is colorless. Then dry the solid under vacuum at 60℃ for 12h and then pulverize it to obtain the positive electrode material. (2) Preparation of positive electrode sheet The above-mentioned positive electrode material, positive electrode conductive agent (Super P), and positive electrode binder (polyvinylidene fluoride) are mixed evenly in a mass ratio of 90:5:5, and then uniformly dispersed in N-methylpyrrolidone (NMP) to form a slurry. The mixed slurry is coated on both sides of aluminum foil, and after baking, rolling, and cutting, the positive electrode sheet is obtained. (3) Preparation of negative electrode sheet The negative electrode active material (graphite), negative electrode binder (sodium carboxymethyl cellulose, CMC-Na), negative electrode binder (styrene-butadiene rubber, SBR), and negative electrode conductive agent (Super P) are added to deionized water in a mass ratio of 90:3:3:4 and stirred to form a uniform slurry. The mixed slurry is coated on both sides of copper foil, and then baked, rolled, and cut into sheets to obtain the negative electrode sheet. (4) Preparation of electrolyte Using ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 as solvents, lithium hexafluorophosphate was dissolved in the solvent to prepare an electrolyte with a lithium hexafluorophosphate molar concentration of 1 mol / L. (5) Preparation of secondary batteries The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding, hot pressing and shaping, and welding of the tabs, a bare battery is obtained. The bare battery is placed in an outer aluminum-plastic film and baked in an oven at 85±10 ℃ for 24 h. The electrolyte is injected into the dried battery, and the battery is allowed to stand, form, and be capacity tested to obtain a secondary battery.

[0106] Examples 2-3 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the amount of vanadium source added in step (2) is adjusted (the amount of vanadium source added is reduced and increased respectively. The amount of vanadium source added in Example 2 is 0.028g and the amount of vanadium source added in Example 3 is 0.19g), so as to change the percentage of vanadium atoms in the core and thus achieve the parameters in Tables 1 to 2.

[0107] Example 4 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that no aluminum source is added in step (1), so that there is no aluminum in the core, thereby achieving the parameters in Tables 1 to 2.

[0108] Examples 5-6 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the heating rate and cooling rate of sintering in step (5) are adjusted to change the thickness of the interface layer, thereby achieving the parameters in Tables 1-2. Specifically, in Example 5, the heating rate was 8°C / min and the cooling rate was 12°C / min; in Example 6, the heating rate was 2°C / min and the cooling rate was 4°C / min.

[0109] Examples 7-8 This application provides a cathode material and a secondary battery. The difference between the preparation method of the cathode material and the secondary battery and that of Example 1 is that the cooling rate after sintering in step (5) is adjusted (Example 7 uses a faster cooling rate of 12℃ / min to suppress vanadium segregation; Example 8 uses a slower cooling rate of 3℃ / min to promote vanadium segregation), while keeping the amount of vanadium source added in step (2) consistent with that in Example 1, so as to change the percentage of vanadium atoms in the interface layer, thereby achieving the parameters in Tables 1 to 2.

[0110] Examples 9-11 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the amount of polymer monomer added in step (6) is adjusted to change the thickness of the conductive layer, thereby achieving the parameters in Tables 1-2. Specifically, in Example 9, the amount of polymer monomer added was 0.38g; in Example 10, the amount of polymer monomer added was 0.12g; and in Example 11, the amount of polymer monomer added was 0.54g.

[0111] Examples 12-13 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the molar ratio of sodium persulfate to pyrrole monomer in step (6) is adjusted to change the conductivity of the conductive layer and the proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms, thereby achieving the parameters in Tables 1-2. Specifically, in Example 12, the molar ratio of sodium persulfate to polymer monomer was 0.7:1; in Example 13, the molar ratio of sodium persulfate to polymer monomer was 1.5:1.

[0112] Examples 14-16 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the reaction temperature and time in step (6) are adjusted to change the conductivity of the conductive layer and the number-average molecular weight of polypyrrole, thereby achieving the parameters in Tables 1-2. Specifically, in Example 14, the reaction temperature was 0°C and the reaction time was 24 hours; in Example 15, the reaction temperature was 0°C and the reaction time was 6 hours; and in Example 16, the reaction temperature was 5°C and the reaction time was 24 hours.

[0113] Examples 17-19 This application provides a positive electrode material and a secondary battery. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the ball milling time and rotation speed in step (5) are adjusted to change the Dv50 of the positive electrode material, thereby achieving the parameters in Tables 1-2. Specifically, in Example 17, the ball milling speed was 200 rpm and the time was 2 hours; in Example 18, the ball milling speed was 400 rpm and the time was 2 hours; and in Example 19, the ball milling speed was 200 rpm and the time was 1 hour.

[0114] Comparative Example 1 This application provides a positive electrode material and a secondary battery in comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the same amount of vanadium source as in Example 1 is added in step (2), but the sintering temperature in step (5) is reduced to 600°C, the holding time is shortened to 5h, and slow cooling (cooling rate 2°C / min) is adopted to promote the full segregation of vanadium to the particle surface to form an interface layer, thereby reducing the entry of vanadium into the crystal lattice and realizing a structure with no vanadium core and vanadium-containing interface layer, thus achieving the parameters in Tables 1 to 2.

[0115] Comparative Example 2 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that aluminum source is not added in step (1), and the same amount of vanadium source as in Example 1 is added in step (2). However, the sintering temperature in step (5) is reduced to 600°C, the holding time is shortened to 5h, and slow cooling (cooling rate 2°C / min) is used to promote the full segregation of vanadium to the particle surface to form an interface layer, thereby reducing the entry of vanadium into the crystal lattice and realizing a structure with no vanadium core and vanadium-containing interface layer, thus achieving the parameters in Tables 1 to 2.

[0116] Comparative Example 3 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the cooling rate in step (5) is adjusted. Specifically, rapid cooling (cooling rate of 15°C / min) is adopted to suppress the segregation of vanadium to the surface and achieve no vanadium in the interface layer, thereby achieving the parameters in Tables 1-2.

[0117] Comparative Example 4 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that after sintering in step (5), liquid nitrogen quenching is used with a cooling rate >100℃ / min to completely suppress vanadium segregation, so that no interface layer is formed (i.e., the interface layer thickness is 0). At the same time, the conductive layer is still coated in step (6), that is, no interface layer is constructed, so that no interface layer is achieved, thereby achieving the parameters in Tables 1 to 2.

[0118] Comparative Example 5 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that the sintering atmosphere in step (5) is adjusted to be air, which causes a change in the valence state of vanadium, thereby realizing vanadium 2P in the interface layer. 2 / 3 After fitting, the spectrum shows a characteristic peak at a binding energy of 516.0 ± 0.1 eV, thus achieving the parameters in Table 1.

[0119] Comparative Example 6 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that step (6) is omitted, that is, no conductive layer is coated, so that there is no conductive layer, thereby achieving the parameters in Table 1.

[0120] Comparative Example 7 This application provides a positive electrode material and a secondary battery in a comparative example. The difference between the preparation method of the positive electrode material and the secondary battery and that of Example 1 is that liquid nitrogen quenching (cooling rate >100℃ / min) is used in step (5) to suppress the formation of the interface layer, and the conductive layer coating in step (6) is omitted, thereby realizing a positive electrode material without an interface layer and a conductive layer, and achieving the parameters in Tables 1 to 2.

[0121] The percentage of vanadium atoms in the core (Q1), the chemical formula of the core, and the vanadium 2P content in the core. 2 / 3 After the spectrum is fitted, check whether there is a characteristic peak at the binding energy of 517.2 ± 0.1 eV (marked as √ if present, × if absent), the thickness of the interface layer h1, the percentage of vanadium atoms in the interface layer Q2, and the vanadium 2P content in the interface layer. 2 / 3After fitting the spectrum, we determine whether there are two characteristic peaks at binding energies of 516.0 ± 0.1 eV and 517.2 ± 0.1 eV (√ if present, × if absent). We also determine the thickness h2 of the conductive layer, the conductivity k of the conductive layer at 25℃, the proportion s of charged nitrogen atoms in the polypyrrole molecular chain to the total nitrogen atoms, the number-average molecular weight Mn of polypyrrole, and whether there are three characteristic peaks at binding energies of 399.8 ± 0.1 eV, 401.2 ± 0.1 eV, and 400.5~401.0 eV after fitting the nitrogen 1s spectrum in polypyrrole (√ if present, × if absent). The Dv50 of the cathode material is shown in Tables 1 and 2. Table 1 Table 2 The prepared cathode material and secondary battery were subjected to performance tests, specifically: 1. The powder conductivity of the positive electrode material was measured using a four-probe tester on isostatically compacted powder discs; 2. The secondary battery was subjected to electrochemical performance testing on the Blue Electric test system under a constant temperature environment of 25°C. The charge and discharge regime was as follows: within the voltage range of 2.5~4.3 V, the first charge and discharge was performed at a rate of 0.5 C to evaluate the first-cycle coulombic efficiency and the first-cycle discharge specific capacity. Subsequently, constant current charge and discharge cycle tests were performed at a rate of 1 C up to 150 cycles to evaluate cycle stability. The results are shown in Table 3. Table 3 As can be seen from Table 3, when the technical solution provided in this application is adopted, the obtained cathode material has excellent conductivity, with a conductivity of 420 × 10⁻⁶. -7 With an S / m or higher, the resulting secondary battery has a high capacity and first-cycle coulombic efficiency, and excellent cycle performance; specifically, the first-cycle discharge specific capacity of the obtained secondary battery is above 152.8 mAh / g, the first-cycle coulombic efficiency is above 91.8%, and the capacity retention rate after 150 cycles is above 87.5%. As can be seen from the examples and comparative examples, significant effects can be achieved with the structure of the cathode material given in this application.

[0122] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A positive electrode material, characterized in that, The positive electrode material includes a core, an interface layer disposed on at least a portion of the surface of the core, and a conductive layer disposed on at least a portion of the surface of the interface layer. The core comprises lithium manganese iron phosphate and modified vanadium. The interface layer includes vanadium; In X-ray photoelectron spectroscopy (XPS) testing, vanadium 2P in the interface layer 2 / 3 After fitting, the spectrum showed two characteristic peaks at binding energies of 516.0 ± 0.1 eV and 517.2 ± 0.1 eV; the core contains vanadium 2P. 2 / 3 After fitting, the spectrum showed a characteristic peak at a binding energy of 517.2 ± 0.1 eV.

2. The cathode material according to claim 1, characterized in that, The percentage of vanadium atoms in the interface layer is greater than the percentage of vanadium atoms in the core.

3. The cathode material according to claim 2, characterized in that, The percentage of vanadium atoms in the interface layer is 10% to 15%. And / or, the percentage of vanadium atoms in the core is 0.01% to 0.07%.

4. The cathode material according to claim 1, characterized in that, The core also includes the modified element aluminum.

5. The positive electrode material according to claim 4, characterized in that, The chemical formula of the core is Li a (Mn) 1-x- y Fe x Al y P 1-z V z O4, Wherein, 0.4≤x≤0.6, 0.003≤y≤0.01, 0.001≤z≤0.005, and 0.95≤a≤1.

05.

6. The cathode material according to claim 1, characterized in that, The conductivity of the conductive layer at 25°C is 70 S / cm to 100 S / cm.

7. The positive electrode material according to claim 1, characterized in that, The conductive layer comprises polypyrrole.

8. The cathode material according to claim 7, characterized in that, In X-ray photoelectron spectroscopy, the nitrogen 1s spectrum of the polypyrrole, after fitting, showed three characteristic peaks at binding energies of 399.8 ± 0.1 eV, 401.2 ± 0.1 eV, and 400.5~401.0 eV.

9. The positive electrode material according to claim 7, characterized in that, Satisfy at least one of the following: (1) The proportion of charged nitrogen atoms in the polypyrrole molecular chain to the total number of nitrogen atoms is 20% to 40%; (2) The number average molecular weight of the polypyrrole is 5000 g / mol to 50000 g / mol.

10. The cathode material according to claim 1, characterized in that, Satisfy at least one of the following: (1) The thickness of the interface layer is 0.5 nm to 1.5 nm; (2) The thickness of the conductive layer is 0.6 nm to 2.8 nm; (3) The Dv50 of the positive electrode material is 0.5μm~2μm.

11. A secondary battery, characterized in that, The secondary battery includes the positive electrode material as described in any one of claims 1 to 9.

12. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 10.

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