Secondary battery, electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries. The secondary battery is configured by using an olivine phosphate material and a lithium-containing oxide material as a positive electrode active material, and simultaneously regulating the charging behavior of the positive electrode material and the negative electrode material and the impedance of the positive electrode plate to construct a synergistic relationship, so that the high-temperature cycle performance of the secondary battery can be effectively guaranteed at a high level, and the fast-charging performance can be realized.

Description

[0001] This application is a divisional application of CN120184179A (application date May 20, 2025, application number 202510647658X, invention title: a secondary battery and an electrical device). Technical Field

[0002] This application relates to the field of battery technology, specifically to a secondary battery and an electrical device. Background Technology

[0003] Olivine-type phosphate cathode material system secondary batteries have ideal long cycle life and the production cost is much lower than that of layered ternary cathode material system secondary batteries. However, olivine-type phosphate cathode material has a slow ion transport rate and a strong oxidation tendency at high temperature. Therefore, the cycle performance of secondary batteries prepared with it deteriorates severely under high temperature operating environment.

[0004] In addition to high-temperature cycling performance and room-temperature cycling performance, secondary batteries are also expected to have ideal fast-charging capabilities when applied to devices. However, this performance involves the energy density and power characteristics of the cathode material and the corresponding lithium intercalation polarization of the anode, which are difficult to significantly improve by adjusting a single factor. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. The secondary battery described in this application uses olivine phosphate material and lithium oxide material as positive electrode active material, and at the same time controls the charging behavior of positive electrode material and negative electrode material and the synergistic relationship constructed by the impedance of positive electrode sheet, which can effectively ensure its high-temperature cycle performance while achieving fast charging performance.

[0006] To achieve the above objectives, in a first aspect of this application, a secondary battery is provided, the secondary battery comprising a positive electrode and a negative electrode, the positive electrode comprising a positive active material, the positive active material comprising an olivine phosphate and a lithium-containing oxide; the olivine phosphate comprising at least one of lithium manganese iron phosphate and doped lithium manganese iron phosphate; the lithium-containing oxide comprising at least one of lithium nickel cobalt manganese oxide and doped lithium nickel cobalt manganese oxide. The secondary battery satisfies (a×b) / c = 0.6~5 V. 2 ·Ω; Wherein, aV is the slope of the curve segment between the first oxidation plateau and the second oxidation plateau in the charging curve of the secondary battery obtained at 25℃ and in the working voltage range of 2.5~4.25V, the voltage range of the first oxidation plateau is 3.3~3.5V, and the voltage range of the second oxidation plateau is 4.0~4.1V; and a = 8~30V; bΩ is the bulk lithium-ion transport impedance of the positive electrode sheet under EIS test at 25℃ and a working voltage range of 2.5~4.25V, corresponding to the valley between the peaks of the first and second curves in the dQdV-V curves obtained by the secondary battery at 25℃. c V -1 The rate of decrease of the negative electrode potential of a secondary battery during charging at 25°C within the operating voltage range of 3.7~4.25V.

[0007] In a second aspect, this application also provides an electrical device including the secondary battery, which is used as a power source in the electrical device.

[0008] The beneficial effects of this application are as follows: This application provides a secondary battery and an electrical device. The secondary battery described in this application uses a composite of olivine phosphate material and lithium oxide material as the positive electrode active material, and simultaneously regulates the charging behavior of the positive electrode material and the impedance of the positive electrode sheet to build a synergistic relationship. This can effectively ensure that its high-temperature cycle performance is at a high level while also achieving fast charging performance. Attached Figure Description

[0009] Figure 1 The dQ / dV-V curve obtained when the secondary battery described in Embodiment 23 of the present invention is subjected to test a. Figure 2 The dQ / dV-V curve obtained during the c test of the secondary battery described in Embodiment 23 of the present invention; Figure 3 The negative parameter curve is obtained when the secondary battery described in Embodiment 23 of the present invention is subjected to the c test. Detailed Implementation

[0010] 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.

[0011] 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.

[0012] 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.

[0013] The present application is further illustrated below with specific embodiments: A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a positive active material, which includes an olivine phosphate and a lithium-containing oxide. The olivine phosphate includes at least one of lithium manganese iron phosphate and doped lithium manganese iron phosphate. The lithium-containing oxide includes at least one of lithium nickel cobalt manganese oxide and doped lithium nickel cobalt manganese oxide. The secondary battery satisfies (a×b) / c = 0.6~5 V. 2 ·Ω; Wherein, aV is the slope of the curve segment between the first oxidation plateau and the second oxidation plateau in the charging curve of the secondary battery obtained at 25℃ and in the working voltage range of 2.5~4.25V, the voltage range of the first oxidation plateau is 3.3~3.5V, and the voltage range of the second oxidation plateau is 4.0~4.1V; and a = 8~30V; bΩ is the bulk lithium-ion transport impedance of the positive electrode sheet under EIS test at 25℃ and a working voltage range of 2.5~4.25V, corresponding to the valley between the peaks of the first and second curves in the dQdV-V curves obtained by the secondary battery at 25℃. c V -1 The rate of decrease of the negative electrode potential of a secondary battery during charging at 25°C within the operating voltage range of 3.7~4.25V.

[0014] In this application's technical solution, to balance the fast-charging performance and high-temperature cycling performance of the secondary battery, olivine phosphate and lithium-containing oxide are used as the positive electrode active material. Simultaneously, the charging behavior of the positive and negative electrode materials and the ion impedance of the positive electrode sheet are controlled. Here, 'a' represents the slope of the voltage transition region curve between the two oxidation plateaus in the charging curve during the charging process of the secondary battery. This slope is related to the balance of electron / ion transport rates in the secondary battery and the activity of interfacial side reactions during lithium intercalation / deintercalation of the positive electrode active material. This parameter is also related to 'c', which is the rate of decrease in negative electrode potential caused by lithium intercalation and polarization during the charging process of the negative electrode active material in the secondary battery. The curve contains a potential rise region, where 'a' is related to the negative electrode potential rise region. When 'a' is too small, the polarization degree of the conversion region increases, and the negative electrode potential rise effect caused by lithium intercalation and polarization deteriorates. The rate of decrease of the negative electrode potential during the fast charging stage increases, which leads to a decrease in the charge and discharge capacity of the secondary battery at high rates and a decrease in fast charging capability. When 'a' is too large, the oxidation activity of the olivine phosphate material in the positive electrode active material increases due to low ion transport efficiency, and the high-temperature cycle performance of the secondary battery decreases. Therefore, by using 'b' and 'c' together to form a relationship with 'a' and achieve synchronous regulation, it is possible to effectively ensure that the secondary battery can achieve both excellent fast charging performance and high-temperature cycle stability when using two composite positive electrode active materials.

[0015] Specifically, the test method for a is as follows: disassemble the secondary battery in the discharged state, take out the positive electrode and the negative electrode, soak them in dimethyl carbonate (DMC) solution for 1 hour, and then take them out and dry them. After reassembling the positive and negative electrodes into a single battery cell (the electrolyte formulation used was a 1:1:1 volume ratio mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate as a solvent, containing 1 mol / L LiPF6 as the lithium salt), the battery was charged at 25°C using a LAND system. First, it was charged to 4.1V at a 0.02C rate, then charged at a constant current rate of 0.33C to 4.25V, followed by constant voltage charging until the current was less than or equal to 0.05C. After standing for 10 minutes, it was discharged at a constant current rate of 0.33C to 2.5V. This cycle was repeated for two weeks with a constant capacity setting. The device signal acquisition frequency was 100ms. The two distinct charging (oxidation) plateaus in the second week's charging curve were analyzed. The first oxidation plateau, with a low potential (3.3~3.5V), corresponds to Fe... 2+ The oxidation reaction, with a high potential (4.0~4.1V) second oxidation plateau corresponding to Mn 2+ The oxidation reaction. The slope region between the two oxidation plateaus is the voltage transition region. The data is differentiated once, and a dQ / dV-V curve is plotted to determine the Fe... 2+ / Mn 2+The voltage corresponding to the valley between oxidation peaks (that is, the position of the lowest point on the vertical axis, the lowest point of the y-value in the curve), and the ratio of the SOC and voltage difference 100ms before and after this voltage, is the slope of the voltage transition zone a=(U2-U1) / (SOC2-SOC1), where the SOC and voltage corresponding to the first 100ms are SOC1 and U1, respectively; and the SOC and voltage corresponding to the last 100ms are SOC2 and U2, respectively.

[0016] Specifically, the test method for b is as follows: After charging the secondary battery at 0.33C to the SOC corresponding to the valley value (i.e., the lowest potential point between the two oxidation plateaus in the charging curve), the positive electrode is disassembled and soaked in dimethyl carbonate (DMC) solution for 1 hour, then removed and dried; subsequently, a half-cell is assembled using a lithium metal sheet as the counter electrode (the electrolyte formula used is: ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate are mixed in a volume ratio of 1:1:1 as a solvent, and the electrolyte contains 1 mol / L LiPF6 as a lithium salt). Then, at 25°C, when the battery reaches equilibrium (i.e., after the battery is charged, it is left to stand at 25°C for more than 2 hours), an AC impedance test (5mV) is performed using an electrochemical workstation. The obtained data is analyzed by DRT to obtain 10 5 The bulk ionic impedance at a frequency of ~0.01Hz is called b.

[0017] Specifically, the test method for c is as follows: disassemble the secondary battery in an empty state, remove the positive electrode and the negative electrode, soak the positive electrode and the negative electrode in DMC solution for 1 hour, remove and dry them; after reassembling the positive electrode and the negative electrode into a single cell (the electrolyte formula used is: ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate are mixed in a volume ratio of 1:1:1 as a solvent), charge it to 4.1V at a rate of 0.02C, then charge it to 4.25V at a constant current of 0.33C, and finally charge it to less than or equal to 0.05C at a constant voltage. After standing for 10 minutes, discharge at a constant current rate of 0.33C to 2.5V. Then, at 25℃, use a LAND system to adjust the charge to 10% SOC at a rate of 0.33C, and start charging at 4C, stopping at the upper limit voltage of 4.25V or the negative parameter potential of 0mV. Then, charge in descending order at a rate of 0.2C until it reaches the charging capacity of the second cycle of constant capacity or 4.25V. Plot the dQ / dV-V curves and select Fe. 2+ / Mn 2+ The negative parameter curve corresponding to the charging segment where the valley (the lowest point of the y-value of the curve) between the two oxidation plateaus (3.7~4.25V) is located is selected. The coordinate point (SOC3, U3) corresponding to the highest voltage value and the end point of the charging curve (SOC4, 0) are selected. The ratio of the difference between the two points (SOC4-SOC3) / U3 is c.

[0018] In some embodiments, the secondary battery satisfies: (a×b) / c=0.6 V 2 ·Ω, 0.8 V 2 ·Ω、1V 2 ·Ω, 1.1 V 2 ·Ω, 1.4 V 2 ·Ω, 1.5 V 2 ·Ω, 1.8 V 2 ·Ω、2V 2 ·Ω, 2.2 V 2 ·Ω, 2.5 V 2 ·Ω, 2.8 V 2 ·Ω、3V 2 ·Ω, 3.5 V 2 ·Ω, 3.6 V 2 ·Ω, 4V 2 ·Ω, 4.5 V 2 ·Ω, 5V 2 The range of values ​​for one or both of Ω.

[0019] In some embodiments, the secondary battery satisfies: (a×b) / c = 1.5~3.6 V 2 ·Ω.

[0020] As mentioned above, the interaction between the charging behavior of the positive and negative electrodes and the ion impedance of a secondary battery can effectively ensure that both fast charging performance and high-temperature cycling performance are maintained at a high level. When the slope of the voltage conversion curve segment during charging is too large, the electron / ion transport balance is altered, and the oxidation activity of the positive electrode active material may increase. Further control of b and c is needed to avoid a decrease in the high-temperature cycling performance of the secondary battery. If a is too small, the overall lithium insertion efficiency of the secondary battery will decrease, and the fast charging capability cannot be guaranteed. Therefore, it is necessary to adjust the ion impedance of the corresponding positive electrode and the charging behavior of the negative electrode active material. On the other hand, changes in b and c will also affect the structural stability, interface stability, and lithium-ion insertion / extraction efficiency of the positive and negative electrode active materials. It is necessary to adjust the range of a simultaneously to ensure that the secondary battery can not only form a stable interface film at the interface of the negative electrode during charging and discharging, thus improving the lithium insertion activity of the negative electrode, but also ensure that the positive electrode active material has strong structural stability, low metal ion dissolution rate, and high ion transport rate. When the (a×b) / c relationship constructed by the three key parameters is controlled within the above range, the fast charging performance and high-temperature cycling performance of the secondary battery can be further improved.

[0021] In some implementations, a = 8~30 V.

[0022] In some implementations, a is a range of one or both of the following: 8 V, 10 V, 12 V, 13 V, 14 V, 15 V, 18 V, 20 V, 22 V, 24 V, 26 V, 27 V, 28 V, 29 V, and 30 V.

[0023] In some implementations, a = 13~27 V.

[0024] As the slope of the charging curve segment in the voltage conversion region between oxidation platforms, 'a' is directly related to the cycle stability of the positive electrode active material interface and the ion transport efficiency of the secondary battery under high temperature conditions. By regulating 'a' within the above range, the balance between the overall charging behavior and cycle stability of the secondary battery can be further optimized, thereby improving the overall electrochemical performance.

[0025] In some implementations, b = 0.35~0.66Ω.

[0026] In some implementations, b is a range of one or any two of the following: 0.35Ω, 0.4Ω, 0.42Ω, 0.45Ω, 0.48Ω, 0.5Ω, 0.52Ω, 0.54Ω, 0.55Ω, 0.58Ω, 0.6Ω, 0.62Ω, 0.64Ω, and 0.66Ω.

[0027] In some implementations, b = 0.4~0.56Ω.

[0028] The ion resistance of the positive electrode is related to its ion transport rate during charging and the stability of the positive active material on the positive electrode. When a positive electrode with the above-mentioned ion resistance is selected to construct a secondary battery, the product has a longer cycle life at high temperature.

[0029] In some implementations, c = 2.8~7.3 V -1 .

[0030] In some implementations, c = 2.8 V -1 3V -1 3.4 V -1 3.5 V -1 3.8 V -1 4V -1 4.1 V -1 4.3 V -1 4.6 V -1 5V -1 5.5 V -1 6 V -1 6.5 V -1 6.8 V -1 7 V -17.3 V -1 The range of one or any two of them.

[0031] In some implementations, c = 3.5~6 V -1 .

[0032] During charging, the potential drop rate caused by lithium intercalation and polarization at the negative electrode will change the (a×b) / c ratio, which in turn affects the fast charging performance and cycle performance of the secondary battery. When the drop rate is controlled within the above range, the electrochemical performance of the product is better.

[0033] In some embodiments, the olivine-type phosphate includes at least one of lithium manganese iron phosphate and doped lithium manganese iron phosphate.

[0034] For example, in some embodiments, the olivine-type phosphate has the structural formula LiFe. x Mn y PO4, where x = 0.15~0.4, y = 0.6~0.85.

[0035] In some embodiments, the olivine-type phosphate is modified by coating.

[0036] For example, in some embodiments, the olivine-type phosphate is carbon-coated manganese iron phosphate.

[0037] In some embodiments, the olivine-type phosphate may also be doped with a metal element, including at least one selected from vanadium, tungsten, titanium, and magnesium.

[0038] Those skilled in the art will understand that carbon coating can not only improve the dispersibility of olivine phosphate and prevent particle agglomeration, but also improve the conductivity of the material, thereby increasing the efficiency of ion / electron transport.

[0039] In some embodiments, the manganese content in the olivine phosphate is 60-85% of the total transition metal content.

[0040] In some embodiments, the lithium oxide includes at least one of lithium nickel cobalt manganese oxide, doped lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel manganese oxide, and lithium manganese-rich oxide.

[0041] For example, in some embodiments, the lithium oxide has the structural formula Li. x Ni d Mn e Co f N gO2, where x = 1~1.1, d = 0.6~0.92, e = 0.03~0.3, f = 0.02~0.35, 0 ≤ g < 0.1, and N is at least one of Al, Na, Ti, Nb, Zr, W, Fe, and Cr.

[0042] In some embodiments, in the lithium oxide, nickel accounts for 60-92% of the total transition metal content, and cobalt accounts for 2-35% of the total transition metal content.

[0043] Using common lithium manganese iron phosphate and lithium nickel cobalt manganese oxide as the positive electrode active materials, when applied to the secondary battery described in this application, the problems of lithium manganese iron phosphate transport rate and manganese dissolution can be overcome by adjusting (a×b) / c, while the overall energy density of the secondary battery can be improved, and the lithium insertion / extraction rate of the positive electrode active material is greatly improved.

[0044] Meanwhile, the ranges of a, b, and c in the secondary battery can be comprehensively adjusted through process parameters during the preparation of the positive electrode material, including but not limited to the types of positive and negative electrode materials, electrode formulation, preparation process, and electrolyte formulation. Specifically, this can be achieved by adjusting the Mn content and primary particle size in olivine phosphate; the Ni content and primary particle size in lithium oxide; the mass ratio of the two positive electrode materials; the porosity in the positive electrode active material layer; the type of conductive agent; the type of negative electrode material; and the composition and proportion of the electrolyte. Furthermore, the physicochemical parameters such as the primary particle size can be adjusted by those skilled in the art during the synthesis process through preparation parameters. For example, when preparing olivine phosphate using a liquid-phase method, the primary particle size of the olivine positive electrode material can be effectively controlled by adjusting the ion concentration in the reaction solution and the final sintering temperature. Similarly, the primary particle size of lithium oxide can also be adjusted by controlling the precursor particle size, adjusting the transition metal molar ratio, and the final sintering temperature.

[0045] Furthermore, those skilled in the art can also adjust the three parameters a, b, and c by means of, but not limited to, the selection of negative electrode materials, the special formulation of electrolytes, and even the formation process of secondary batteries, and are not limited to the specific embodiments described and listed in this application.

[0046] In some embodiments, the olivine-type phosphate can be lithium manganese iron phosphate, and the preparation method can be implemented as follows: Manganese, iron, and phosphorus sources are mixed and dispersed in a solvent, followed by the addition of a lithium source. The mixture is reacted at 140-160°C for 8-12 hours. The resulting product is then mixed with a carbon source and dispersed in a solvent. The resulting mixture is spray-dried and calcined under a protective atmosphere to obtain lithium manganese iron phosphate.

[0047] Specifically, the molar ratio of the manganese source, iron source, and phosphorus source is (0.6~0.85):(0.15~0.4):(1.02~1.05). Specifically, the solvent is water; Specifically, the calcination treatment is carried out at a temperature of 550~680℃ for 5~8 hours.

[0048] In some embodiments, the lithium-containing oxide is lithium nickel cobalt manganese oxide, and the preparation method can be implemented as follows: The nickel source, cobalt source, manganese source and alkali are mixed and then reacted at 80~100℃ for 16~24h. The resulting precursor is mixed with a lithium source and then calcined to obtain the lithium nickel cobalt manganese oxide.

[0049] Specifically, the molar ratio of the nickel source, cobalt source, manganese source, and alkali is (0.6~0.92):(0.02~0.35):(0.03~0.3):(2.1~2.5). Specifically, the calcination treatment is carried out at a temperature of 730~850℃ for 6~9 hours.

[0050] In this invention, the preparation methods of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide are not limited. Those skilled in the art can prepare the lithium manganese iron phosphate or lithium nickel cobalt manganese oxide by the above methods or other methods according to conventional technical means.

[0051] The manganese sources used include, but are not limited to, at least one of manganese tetroxide, manganese nitrate, manganese carbonate, manganese oxalate, manganese sulfate, manganese chloride, and manganese acetate; The iron source used includes, but is not limited to, at least one of the following: ferric phosphate, ferrous phosphate, ferric hydroxide, ferrous hydroxide, ferric carbonate, ferrous carbonate, ferric acetate, ferrous acetate, ferric oxide, ferric tetroxide, ferrous oxalate, and ferric oxalate. The cobalt source includes, but is not limited to, at least one of cobalt sulfate, cobalt nitrate, cobalt oxalate, cobalt acetate, and cobalt chloride; The phosphorus source used includes, but is not limited to, at least one of lithium dihydrogen phosphate, lithium phosphate, diammonium hydrogen phosphate, and ammonium phosphate; The lithium source used includes, but is not limited to, at least one of lithium dihydrogen phosphate, lithium phosphate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium citrate, and lithium acetate; The carbon source used includes, but is not limited to, at least one of glucose (GLC), sucrose, polyethylene glycol (PEG), and polyvinyl alcohol.

[0052] In addition, when preparing lithium manganese iron phosphate, a certain amount of dopant element source (if any) can be mixed with manganese source, iron source, phosphorus source and lithium source as needed. Dopant element sources include vanadium source (such as vanadium pentoxide), tungsten source (such as ammonium metatungstate), titanium source (such as titanium oxide), magnesium source (such as magnesium carbonate), etc., to obtain doped lithium manganese iron phosphate containing a certain amount of dopant element.

[0053] In some embodiments, the mass ratio of the olivine-type phosphate to the lithium oxide is (97:3) to (70:30).

[0054] For example, in some embodiments, the mass ratio of the olivine-type phosphate to the lithium oxide is a range of one or both of the following: 97:3, 95:5, 93:7, 90:10, 87:13, 85:15, 80:20, 75:15, and 70:30.

[0055] It should be noted that the mass ratio of the olivine-type phosphate to the lithium oxide can be confirmed by the following method: The positive electrode sheet is disassembled from the empty-charge secondary battery, then soaked in dimethyl carbonate (DMC), dried, and the positive electrode active material layer on the positive electrode sheet is scraped off for EDS (Energy Dispersive Spectrometer) testing. At a magnification of 15k, the morphology and size of the material in the electrode sheet are observed. The elemental composition and content (normalized atomic percentage) of particles of different shapes and sizes in the field of view are tested using EDS point-scan signal acquisition. At least three similar particles are selected for testing to obtain accurate parallel sample test results. Based on this data, the material type is identified, and the average molar ratio of each transition metal element to the total transition metal elements is obtained, which is the chemical composition of each component of the positive electrode main material in the electrode sheet. Subsequently, the sample was subjected to ICP testing (0.5g of the scraped positive electrode active material powder was accurately weighed, dispersed in 20mL of water, and then 10mL of nitric acid (HNO3 mass percentage of 66%) was added. The mixture was dispersed and heated until the positive electrode active material powder was completely dissolved, and then diluted to 100mL with water to obtain the test solution; the test solution was subjected to ICP testing, and the ICP instrument operating conditions were set as follows: gas flow rate 0.5L / min, power 1150W, to perform ICP testing), to confirm the corresponding chemical composition element concentration of the positive electrode active material (i.e., the specific molar percentage of chemical elements); to confirm the molar ratio of various transition metals in the electrode sheet to the total metal, and combined with the actual metal molar ratio of various materials confirmed by EDS above, the mass ratio of the positive electrode material with a specific metal element to the total material was calculated as θ = ICP molar ratio / EDS molar ratio, and the mass ratio of the remaining other types of materials was 1-θ. The mass ratio of the two positive electrode active materials was determined according to the chemical composition, and the mass content of each element in the olivine phosphate and lithium oxide alone was also confirmed by the above methods.

[0056] In some embodiments, the average primary particle size of the olivine-type phosphate is 80-200 nm.

[0057] In some embodiments, the average primary particle size of the lithium oxide is 1.5 to 2.2 μm.

[0058] It should be noted that the average primary particle size of olivine phosphate and lithium oxide in the positive electrode active material described in this application can be confirmed by the following method: The positive electrode sheet is disassembled from the secondary battery, then soaked in dimethyl carbonate (DMC), dried, and the positive electrode active material layer on the positive electrode sheet is scraped off for EDS energy dispersive spectroscopy. Under magnification, spot scanning is used to identify olivine phosphate and layered lithium oxide particles. Then, under a 10k magnification scanning electron microscope, three regions of the layered lithium oxide are selected to obtain particle morphology. The primary particles of the layered lithium oxide are measured using the "cross-hatching method" in nanomeasurer software. 200 samples are tested, and the statistical results are the average primary particle size of the lithium oxide. Similarly, under a 30k magnification scanning electron microscope, three regions of olivine phosphate are selected to obtain particle morphology. The diagonal length of the primary olivine phosphate particles is measured using nanomeasurer software. 80 samples are tested, and the statistical results of the three regions are the average primary particle size of the olivine phosphate.

[0059] In some embodiments, the secondary battery further includes an electrolyte comprising a solvent and a lithium salt.

[0060] For example, in some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0061] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0062] In some embodiments, the concentration of lithium salt in the electrolyte is 0.9~2 mol / L.

[0063] In some embodiments, the concentration of lithium salt in the electrolyte is one or any two of the following: 0.9 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, and 2 mol / L.

[0064] When the lithium salt concentration is preferably within the above range, the lithium ion transport capability can be further optimized, the overall impedance of the secondary battery system can be reduced, the negative electrode potential rise phenomenon can be improved, and the probability of excessive negative electrode potential drop rate can be further reduced, resulting in better fast charging performance of the secondary battery.

[0065] In addition, the electrolyte may also contain additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery high-temperature performance, additives that improve battery overcharge performance, additives that improve battery low-temperature performance, etc.

[0066] The battery may further include a separator located between the positive and negative electrode plates to separate them and prevent short circuits caused by contact. The separator can be any suitable battery separator material in the art. Exemplarily, the separator includes, but is not limited to, at least one of polypropylene and polyethylene.

[0067] In some embodiments, the positive electrode includes a current collector and a positive active material layer, wherein the positive active material layer includes a positive active material.

[0068] More preferably, the positive electrode active material layer further includes a binder and a conductive agent.

[0069] More preferably, in the positive electrode active material layer, the mass ratio of the positive electrode active material, the conductive agent, and the binder is (94~97.5):(0.8~2):(1.5~4).

[0070] In some embodiments, the negative electrode sheet includes a current collector and a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material; More preferably, the negative electrode active material layer includes a negative electrode active material, a binder, and a conductive agent.

[0071] More preferably, in the negative electrode active material layer, the mass ratio of the negative electrode active material, the conductive agent, and the binder is (96~98):(1~2):(1~3).

[0072] It should be noted that the conductive agents in the positive and negative electrode active material layers are used to provide conductivity, and any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. For example, the conductive agents in the positive electrode active material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0073] The binder in the positive electrode active material layer and the negative electrode active material layer is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable binding properties and does not significantly cause adverse chemical changes in the battery. Illustratively, the binder in the positive electrode active material layer includes, but is not limited to, fluorine-containing polyolefin binders, and fluorine-containing polyolefin binders include, but are not limited to, polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers or modified (e.g., modified by carboxylic acid, acrylic acid, acrylonitrile, etc.) derivatives thereof.

[0074] In some embodiments, the negative electrode active material is at least one of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, SiO f (0<f<2, such as f=1), silicon-carbon, and lithium titanate.

[0075] In some embodiments, the porosity of the positive electrode active material layer is 10~20%; In some embodiments, the method for testing the porosity of the positive electrode active material layer is: Disassemble the secondary battery to obtain a positive electrode plate, soak the positive electrode plate in DMC at room temperature for 60 min, take it out, and dry it in a room temperature environment with humidity ≤15%, then cut the electrode plate into a circular sheet with a diameter of 12 mm using an electrode plate puncher, and measure the thicknesses of the electrode plate and the current collector with a thickness gauge, which are recorded as h1 and h2 respectively; Weigh its mass with a balance with an accuracy of 0.00001 g, and record it as m1; According to the formula v=πr 2 Calculate the volume of the cut electrode sheet with (h1-h2); Then soak the cut electrode sheet in a closed container containing a certain volume of hexadecane for 1 hour (the volume of hexadecane in the closed solution is not required, but the required amount must be sufficient to ensure that the electrode sheet is completely immersed therein; take out the cut electrode sheet with tweezers, place it on filter paper and blot until the mass is constant (generally, 1 hour of blotting to constant mass is sufficient), weigh its mass with a balance and record it as m2.

[0076] Finally, according to the formula calculate the porosity, where ρ is 0.7734g / cm 3 , which is the density of hexadecane.

[0077] In some embodiments, the conductive agent in the positive electrode active material layer comprises carbon nanotubes, the carbon nanotubes have a diameter of 8~15 nm, and a specific surface area of 220~300 m 2 / g.

[0078] When carbon nanotubes are used as conductive agents, they can form point-to-line stacking when combined with active material particles, optimizing the contact between particles. This helps to improve the conductivity of the olivine phosphate surface in the active material, thereby improving the battery's high-temperature life, reducing battery impedance growth, and further enhancing the battery's high-temperature cycle performance and fast-charging performance. By adjusting the porosity of the positive electrode to the preferred range of 10-20%, the inactive and ineffective areas between particles can be further reduced, the conductivity between particles can be improved, and the high-temperature performance of the secondary battery can be further optimized and improved.

[0079] In some embodiments, the degree of graphitization of the negative electrode active material is 91-95%; In some embodiments, the negative electrode active material layer comprises graphite, wherein the average particle diameter of the graphite is 8-13 μm.

[0080] In some embodiments, the graphitization degree of the graphite is tested by disassembling the secondary battery in an empty state, scraping off the powder from the obtained negative electrode sheet, and then testing it according to the interlayer spacing d002 and graphitization degree test methods in the national standard GB / T24533-2019.

[0081] The degree of graphitization of the negative electrode active material and the particle size of the graphite particles contained in the electrode sheet have an impact on the bulk transport performance of the negative electrode active material of the secondary battery. When the degree of graphitization of the preferred material and / or the average particle diameter of the graphite particles in the electrode sheet material are within the above range, the material of the electrode active material layer has more isotropic structures and a suitable amount of vacancy structure defects, which is more conducive to improving the bulk transport performance of the negative electrode active material, thereby improving the fast charging performance of the secondary battery.

[0082] In some embodiments, the solvent in the electrolyte includes linear carbonate, which accounts for 50-90% of the total mass of the electrolyte; Specifically, the linear carbonate includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; In some embodiments, the solvent in the electrolyte further includes carboxylic acid ester solvents; Specifically, the carboxylic acid ester solvent includes at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; In some embodiments, the solvent in the electrolyte also includes ether solvents; In some embodiments, the solvent in the electrolyte further includes cyclic carbonates; Specifically, the cyclic carbonate includes at least one of ethylene carbonate and propylene carbonate; the ether solvent includes at least one of ethylene glycol dimethyl ether and tetrahydrofuran.

[0083] More preferably, when the electrolyte contains solvents other than linear carbonate, the total volume ratio of linear carbonate to other types of solvents is (50~90):(10~50).

[0084] In some embodiments, the additives in the electrolyte include at least one of vinylene carbonate (VC) and vinyl sulfate (DTD), wherein the vinylene carbonate and vinyl sulfate account for 0.2 to 1% of the total mass of the electrolyte.

[0085] Specifically, when the additives in the electrolyte contain both VC and DTD, the mass ratio of the two is not subject to any additional limitation and can be adjusted by those skilled in the art according to actual needs.

[0086] When linear carbonate is preferably used as the solvent component, and vinylene carbonate and vinyl sulfate are used as additive components, and the component content is optimized within the above range, a thin and dense SEI friction layer will be formed on the negative electrode during the charging and discharging process of the secondary battery. This will suppress the tendency of internal impedance to increase caused by solvent decomposition in the electrolyte, optimize the rate of negative electrode potential drop, and thus optimize the overall performance of the secondary battery.

[0087] In some embodiments, the mass percentage of solvent and additive in the electrolyte is determined by the following methods to determine their types and mass percentages: (1) Use a battery charging and discharging device to discharge the secondary battery. The specific discharge conditions are: current 0.33C, cutoff voltage 2.5V. Then, disassemble the battery in a glove box (H2O≤0.1ppm, O2≤0.1ppm) to collect the electrolyte. There are three methods for collecting the electrolyte: After opening the battery cover, ① if there is free electrolyte, use a pipette to collect the electrolyte into a 5mL sample tube and seal it with sealing glue to prevent electrolyte leakage. ② if there is no free electrolyte, use a hydraulic press (Beijing Heng'ao Technology Co., Ltd. FY-30 hydraulic press) to continuously pressurize until free electrolyte appears, collect the electrolyte into a sample tube and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, put the battery into an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic oscillator and oscillate for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing glue. (2) The collected electrolyte sample was injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe to obtain GC-MS chromatograms. Additives and solvents were prepared into EMC solutions of different concentrations and injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain GC-MS chromatograms of the standard substances. The GC-MS chromatograms of the electrolyte to be tested were compared with the standard GC-MS chromatograms for qualitative analysis to determine whether the electrolyte to be tested contained the above-mentioned additives and solvents (for example, if a characteristic peak appeared at the position of VC in the standard chromatogram of the electrolyte to be tested, it was determined that the electrolyte to be tested contained VC, and other components were deduced in the same way). Then, the content of each component was determined according to the peak area of ​​each component in the electrolyte to be tested.

[0088] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention: Example 1 A secondary battery, the preparation method comprising the following steps: (1.1) Preparation of olivine-type lithium manganese iron phosphate: MnSO4, FeSO4 and H3PO4 were mixed evenly in a molar ratio of 7:3:10. After mixing with water, a mixed solution with a transition metal element concentration of AM was prepared. Then, ascorbic acid and LiOH were added. The amount of ascorbic acid added was 0.2% of the total molar amount of MnSO4, FeSO4 and H3PO4, and the amount of LiOH added was 3 times the molar amount of H3PO4. The resulting mixture was stirred at 70℃ for 6 h, then pressurized and heated to 150℃ for 10 h. The solid was separated and dried. The resulting powder was then dispersed in water, and 21 wt% of glucose was added. The powder was spray-dried and finally calcined at B℃ for 6 h under a nitrogen atmosphere to obtain carbon-coated lithium manganese iron phosphate. (1.2) Preparation of lithium nickel cobalt manganese oxide containing lithium oxide: Nickel acetate, cobalt acetate, manganese acetate and sodium hydroxide were mixed and dissolved in water at a molar ratio of 6.5:2.5:1:10. The mixture was reacted at 80°C for 24 h. The solid was separated. The precursor with an average particle size of C μm was mixed with lithium hydroxide at a stoichiometric ratio and calcined at D°C for 8 h in air atmosphere to obtain the lithium nickel cobalt manganese oxide. The preparation and product parameters of the lithium manganese iron phosphate and lithium nickel cobalt manganese oxide are shown in Tables 1 and 2. (1.3) Preparation of positive electrode sheet: The carbon-coated lithium manganese iron phosphate and lithium nickel cobalt manganese oxide obtained above are mixed at a mass ratio of 9:1 to obtain positive electrode active material. The positive electrode active material, conductive carbon nanotubes and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 96:2:2. The slurry is prepared by vacuum stirring and then coated on both sides of the current collector aluminum foil. After drying, cold pressing and cutting, the positive electrode sheet is obtained. (2) Preparation of negative electrode sheet: The negative electrode active material artificial graphite, conductive agent acetylene black and binder sodium carboxymethyl cellulose are dispersed in water at a mass ratio of 96.4:1:2.6, and a slurry is prepared by vacuum stirring. The slurry is then coated on both sides of the current collector copper foil, and after drying, cold pressing and cutting, the negative electrode sheet is obtained. (3) Electrolyte preparation: dimethyl carbonate (dimethyl carbonate) and ethylene carbonate (ethylene carbonate) are prepared into a solvent, and then lithium hexafluorophosphate is added to prepare a mixture with a concentration of 1 mol / L and additives VC and DTD to obtain the electrolyte; wherein the total mass ratio of VC and DTD in the electrolyte is 0.23%, and the mass ratio between VC and DTD is 3:1; the proportion of linear carbonate in the total mass of the electrolyte is shown in Table 3. (4) The positive electrode, separator (commercially available PE separator) and negative electrode are stacked or wound in sequence to form a battery cell. The battery cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the secondary battery is obtained.

[0089] The parameters of each secondary battery are shown in Tables 3 and 4.

[0090] Examples 2-23, Examples 25-28 A secondary battery differs from Example 1 only in that the parameters and conditions during its preparation are different, as shown in Tables 1-4.

[0091] In Example 2, when preparing the electrolyte, dimethyl carbonate was replaced with ethyl methyl carbonate, ethylene carbonate was replaced with propylene carbonate, and diethanol dimethyl ether, an ether solvent, was also introduced into the solvent. The proportion of linear carbonate in the total mass of the electrolyte is shown in Table 3.

[0092] In Example 3, tetrahydrofuran, an ether solvent, was also introduced into the solvent. The proportion of linear carbonate in the total mass of the electrolyte is shown in Table 3. In Example 4, the mass ratio between VC and DTD is 2:3.

[0093] The molar content of manganese in lithium iron phosphate is achieved by adjusting the molar ratio of MnSO4 and FeSO4 in the added materials (while keeping the total molar amounts of Mn and Fe constant); the molar content of nickel and cobalt in lithium nickel cobalt manganese oxide is achieved by adjusting the molar ratio of nickel acetate and cobalt acetate in the added materials (while keeping the total molar amounts of Ni, Co and Mn constant); and a, b, and c are adjusted by the parameters in Tables 1 to 3.

[0094] Example 24 A secondary battery differs from Example 1 only in that the parameters and conditions during its preparation are different. In addition to artificial graphite, the negative electrode active material also contains silicon-carbon material, wherein the mass content of silicon-carbon material in the negative electrode sheet (excluding the current collector) is 2%.

[0095] Comparative Examples 1-4 A secondary battery differs from Example 1 only in that the parameters and conditions during its preparation are different, as shown in Tables 1-4.

[0096] Comparative Examples 5-6 A secondary battery differs from Example 1 only in that the parameters and conditions during its preparation are different. In addition to artificial graphite, the negative electrode active material also contains silicon-carbon material, wherein the mass content of silicon-carbon material in the negative electrode sheet (excluding the current collector) is 2%, as shown in Tables 1-4.

[0097] The confirmation methods for a, b, and c in Table 4 are as described above. Taking Example 23 as an example, during test a, the assembled single battery cell was subjected to cyclic capacitance testing, and the resulting charging curve is as follows. Figure 1 The derivative shown yields the dQ / dV-V curve, which determines Fe. 2+ / Mn 2+ The voltage corresponding to the valley between oxidation peaks, and the ratio of the SOC and voltage difference 100ms before and after this voltage, is the slope of the voltage transition zone a=(U2-U1) / (SOC2-SOC1). During test b, the impedance of the assembled half-cell is tested, and the obtained data is analyzed by DRT to obtain b. The data obtained after charging the secondary battery during test c is as follows: Figure 2 The processing shown yields the dQ / dV-V curve, followed by the plotting of the negative parameter curve, as shown. Figure 3 As shown, select the coordinate point (SOC3, U3) corresponding to the highest voltage value and the end point of the charging curve (SOC4, 0). The difference ratio between the two points (SOC4-SOC3) / U3 is c.

[0098] Table 1 Table 2 Table 3 Table 4 Example of effect The secondary batteries obtained in each embodiment and comparative example were tested as follows: (1) Fast charging performance test: During the assembly of the secondary battery, a copper wire was added to the negative electrode side. After assembling the secondary battery, it was left to stand for 24 hours, then charged at 0.02C to 4.1V, then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage to 0.05C. After standing for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V to complete the formation. Then it was discharged at a constant current of 0.33C to 4.25V, then charged at a constant voltage to 0.05C. After standing for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. This was the first cycle of capacity determination. After two cycles of capacity determination, the copper wire was first lithium-plated at a rate of 0.02C for 4 hours, and then charged at a rate of 0.33C to 10% of the capacity of the second cycle of capacity determination. Next, constant current charging was performed at a 4C rate until the negative parameter potential reached 0 or the terminal voltage reached 4.25V. Then, decremental charging was performed at a 0.2C gradient, with the cutoff condition remaining unchanged, until the charging rate dropped to 0.4C / 0.2C / 0.1C / 0.05C, at which point the battery reached 100% of its second week's charging capacity. The test temperature was 25℃, and the device signal acquisition frequency was 100ms. After the test, the charging time t required for the secondary battery to go from 10% to 80% SOC was calculated. (2) High temperature cycle performance test: The secondary battery was left to stand for 24 hours, then charged to 4.1V at 0.02C, then charged to 4.25V at a constant current of 0.33C, and then charged to 0.05C at a constant voltage. After standing for 10 minutes, the battery was discharged at a constant current of 0.33C to 2.5V to complete formation. Then, it was charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage of 0.05C. After standing for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. This was one cycle of capacity determination. After two cycles of capacity determination, the secondary battery was subjected to the following cycle at 45°C according to the determined capacity: charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 0.05C to cut off, and after standing for 10 minutes, discharged at a constant current of 1C to 2.5V. The discharge capacity under this cycle was recorded as Q1. After 200 cycles in this manner, the discharge capacity of the 200th cycle was recorded as Q200. The high-temperature cycle capacity retention rate of the secondary battery was calculated as H% = 100% × Q200 / Q1.

[0099] The test results are shown in Table 5.

[0100] Table 5 As can be seen from Table 5: (1) The secondary battery described in this application uses olivine phosphate and lithium oxide as positive electrode active materials. At the same time, the charging behavior of the positive and negative electrode materials of the secondary battery and the ion impedance of the positive electrode sheet are controlled. The relationship (a×b) / c of ​​the key parameters is constructed and the formula range is adjusted to 0.6~5, so that the two positive electrode active materials can exert a good synergistic effect. The secondary battery has both excellent fast charging performance and high temperature cycling performance. When the fast charging performance test is carried out, the charging time required from 10% to 80% SOC can be within 28 minutes. After 200 cycles at 45℃, the capacity retention rate of the secondary battery can be more than 92%.

[0101] (2) As can be seen from Examples 1 to 23 and Examples 25 to 28, when constructing (a×b) / c in the secondary battery, if the slope of the voltage conversion curve segment during charging is too large, the electron / ion transport balance will change, and the oxidation activity of the positive electrode active material may increase. It is necessary to further regulate b and c to avoid the decline in the high-temperature cycle performance of the secondary battery. If a is too small, the overall lithium insertion efficiency of the secondary battery will decrease, and the fast charging capability will not be guaranteed. Therefore, it is necessary to adjust the ion impedance of the corresponding positive electrode and the charging behavior of the negative electrode active material. On the other hand, the changes in b and c will also affect the structural stability, interface stability and lithium ion insertion / extraction efficiency of the positive and negative electrode active materials. That is, the range of (a×b) / c will directly affect the electrochemical behavior of the secondary battery. When (a×b) / c is preferably in the range of 1.5 to 3.6, the overall lithium ion insertion / extraction efficiency of the secondary battery is higher, and the material stability is also better. The fast charging time can be further shortened to within 25.3 min, and the high-temperature cycle capacity can be further increased to more than 94.5%.

[0102] Furthermore, as the slope of the charging curve segment in the voltage conversion region between oxidation platforms, 'a' is directly related to the cycle interface stability of the positive electrode active material and the ion transport efficiency of the secondary battery under high temperature conditions. By regulating 'a', the balance between charging behavior and cycle stability of the secondary battery can be improved to a certain extent. Similarly, the ion impedance 'b' of the positive electrode is related to its ion transport rate during charging and the stability of the positive electrode active material on the positive electrode. The potential drop rate caused by lithium intercalation and polarization during charging of the negative electrode will cause changes in (a×b) / c, which will affect the fast charging performance and cycle performance of the secondary battery. Optimizing the range of 'b' and 'c' can also improve the overall electrochemical behavior of the secondary battery. When (a×b) / c is in the range of 1.5~3.6, and 'a' is preferably in the range of 13~27, and / or 'b' is preferably in the range of 0.4~0.56Ω, and / or 'c' is preferably in the range of 3.5~6Ω, the performance of the secondary battery can be further improved. The shortest charging time can be only 18 minutes, while the cycle capacity retention rate can reach up to 97%.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a positive active material, which includes olivine phosphate and lithium oxide. The olivine phosphate includes at least one of lithium manganese iron phosphate and doped lithium manganese iron phosphate. The lithium oxide includes at least one of lithium nickel cobalt manganese oxide and doped lithium nickel cobalt manganese oxide. The secondary battery satisfies (a×b) / c = 0.6~5 V. 2 ·Ω; Where aV is the slope of the curve segment between the first and second oxidation plateaus in the charging curve of the secondary battery at 25℃ and a working voltage range of 2.5~4.25V. The voltage range of the first oxidation plateau is 3.3~3.5V, and the voltage range of the second oxidation plateau is 4.0~4.1V; a = 8~30; during the test, the slope region between the two oxidation plateaus is the voltage conversion region. The obtained charging curve data is differentiated once, and a dQ / dV-V curve is plotted to determine Fe. 2+ / Mn 2+ The voltage corresponding to the valley between oxidation peaks, and the ratio of the SOC and voltage difference 100ms before and after this voltage, is the slope of the voltage transition region a=(U2-U1) / (SOC2-SOC1), where the SOC and voltage corresponding to the first 100ms are SOC1 and U1, respectively; and the SOC and voltage corresponding to the last 100ms are SOC2 and U2, respectively. bΩ represents the Fe content in the dQ / dV-V curve of the secondary battery at 25℃ within the operating voltage range of 2.5~4.25V. 2+ / Mn 2+ The EIS test of the positive electrode sheet at the SOC corresponds to the valley between the peaks of the first and second curves corresponding to the two oxidation plateaus. 5 Bulk lithium-ion transport impedance at a frequency of ~0.01Hz; b = 0.35~0.66; b was measured using an electrochemical workstation for AC impedance testing, and the data was obtained through DRT analysis. c V -1 The rate of decrease of the negative electrode potential of a secondary battery during charging at 25°C within a working voltage range of 3.7~4.25V is defined as c = 2.8~7.

3. During testing, c is charged starting at 4C and stopped at the upper limit voltage of 4.25V or a negative parameter potential of 0mV. Then, charging is performed in descending order at 0.2C increments until the charge reaches the second week of constant capacity or 4.25V. The resulting charging curve data are plotted as a dQ / dV-V curve, and Fe is selected as the optimal value. 2+ / Mn 2+ The negative parameter curve corresponding to the charging segment where the valley between the two oxidation platforms is located is obtained by selecting the coordinate point (SOC3, U3) corresponding to the highest voltage value and the end point (SOC4, 0) of the charging curve, and then calculating it using c=(SOC4-SOC3) / U3.

2. The secondary battery as described in claim 1, characterized in that, The secondary battery satisfies: (a×b) / c = 1.5~3.6V 2 ·Ω.

3. The secondary battery as described in claim 1, characterized in that, a = 13~27, and / or b = 0.4~0.56, and / or c = 3.5~6.

4. The secondary battery as described in claim 1, characterized in that, The olivine-type phosphate contains 60-85% manganese as a molar proportion of the total transition metal elements, and / or the average primary particle size of the olivine-type phosphate is 80-200 nm.

5. The secondary battery as described in claim 1, characterized in that, The lithium-containing oxide also includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel manganese oxide, and lithium manganese-rich oxide.

6. The secondary battery as described in claim 1, characterized in that, The lithium oxide contains nickel, which accounts for 60-92% of the total transition metals, and cobalt, which accounts for 2-35% of the total transition metals, and / or the average primary particle size of the lithium oxide is 1.5-2.2 μm.

7. The secondary battery as described in claim 1, characterized in that, The mass ratio of the olivine-type phosphate to the lithium oxide is (97:3) to (70:30).

8. The secondary battery as described in claim 1, characterized in that, The olivine-type phosphate has the structural formula LiFe. x Mn y PO4, where x = 0.15~0.4, y = 0.6~0.

85.

9. The secondary battery as described in claim 1, characterized in that, The olivine-type phosphate was modified by coating.

10. The secondary battery as described in claim 1, characterized in that, The doped lithium manganese iron phosphate contains a doped metal element, which includes at least one of vanadium, tungsten, titanium, and magnesium.

11. The secondary battery as described in claim 1, characterized in that, The structural formula of lithium oxide is Li x Ni d Mn e Co f N g O2, where x = 1~1.1, d = 0.6~0.92, e = 0.03~0.3, f = 0.02~0.35, 0 ≤ g.

12. The secondary battery as described in claim 1, characterized in that, The secondary battery further includes an electrolyte, which includes a solvent, and the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

13. The secondary battery as described in claim 1, characterized in that, The secondary battery further includes an electrolyte comprising a lithium salt, wherein the lithium salt comprises at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

14. The secondary battery as described in claim 13, characterized in that, The concentration of lithium salt in the electrolyte is 0.9~2 mol / L.

15. The secondary battery as described in claim 1, characterized in that, Satisfy at least one of the following (a) to (f): (a) The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the porosity of the positive electrode active material layer is 10~20%; (b) The positive electrode sheet includes a positive active material layer, the positive active material layer includes a conductive agent, the conductive agent includes carbon nanotubes, the carbon nanotubes have an average diameter of 8~15nm and a specific surface area of ​​220~300m². 2 / g; (c) The secondary battery further includes a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, the graphitization degree of the negative electrode active material being 91~95%; (d) The secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes graphite, and the average particle diameter of the graphite is 8~13μm; (e) The secondary battery further includes an electrolyte, the electrolyte including a solvent, the solvent including a linear carbonate, the linear carbonate accounting for 50-90% of the total mass of the electrolyte; (f) The secondary battery further includes an electrolyte, which includes additives, including at least one of vinylene carbonate and vinyl sulfate, wherein the vinylene carbonate and vinyl sulfate account for 0.2 to 1% of the total mass of the electrolyte.

16. An electrical appliance, characterized in that, The secondary battery includes the secondary battery described in any one of claims 1 to 15, wherein the secondary battery is used as a power supply in an electrical device.

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