Secondary batteries and electrical devices

Abstract

The present invention relates to the field of electrochemical technology, and specifically discloses a secondary battery and an electrical device. The secondary battery of the present invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode active material in the positive electrode sheet comprises lithium iron manganese phosphate and lithium iron phosphate; the charging platform with the lowest potential in the charging curve of the secondary battery at 10°C and a 1C rate charge test is the first step platform; the secondary battery satisfies the following relationship: 2.9≤0.3×D span ×Q / U≤12.9; wherein, the D span is the particle size distribution coefficient of lithium iron phosphate; U is the potential value of the first step platform; Q is the ratio of the charging capacity corresponding to the first step platform to the total discharge capacity. The present invention comprehensively controls the particle size distribution coefficient of lithium iron phosphate and the voltage platform characteristics of the positive electrode active material, so that the secondary battery has excellent low-temperature fast charging capability and good cycle life.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technology, in particular to a secondary battery and an electrical device. Background Art

[0002] With the widespread application of lithium-ion batteries in electric vehicles, portable electronic devices and energy storage systems, the demand for high energy density and fast charging capability is increasing. Lithium manganese iron phosphate (LMFP), as a new type of cathode material, has attracted widespread attention due to its excellent energy density.

[0003] However, due to its inherent material properties, the traditional LMFP cathode material has limited lithium ion insertion and deintercalation rates during high-rate charging, resulting in low charging efficiency. Especially under low temperature conditions, the ability of lithium ions to intercalate and deintercalate in LMFP is limited, making the lithium intercalation ability of LMFP batteries poor. Under low-temperature fast charging conditions, the battery's charging capacity is low and the fast charging ability is very poor.

[0004] To solve this problem, existing technologies generally optimize fast charging capabilities by coating conductive additives, optimizing electrolyte formulations, or changing the microstructure of materials, but these methods are not sufficient to effectively improve the fast charging performance of batteries at low temperatures.

[0005] Therefore, it is necessary to develop a secondary battery with higher fast charging capability. Summary of the invention

[0006] The object of the present invention is to overcome the shortcomings of the prior art and provide a secondary battery and an electrical device. The secondary battery includes a positive electrode plate, and the positive electrode active material in the positive electrode plate includes lithium iron manganese phosphate and lithium iron phosphate. By mixing part of lithium iron phosphate into the lithium iron manganese phosphate and comprehensively controlling the particle size distribution coefficient of the lithium iron phosphate and the voltage platform characteristics of the positive electrode active material, the secondary battery has excellent low-temperature fast charging capability and the battery has a good cycle life.

[0007] To achieve the above object, in a first aspect of the present invention, the present invention provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium iron manganese phosphate and lithium iron phosphate;

[0008] The secondary battery is charged at 10° C. at a rate of 1C in a voltage range of 2.5 to 4.25 V to obtain a charging curve; the charging platform with the lowest potential in the charging curve is the first step platform;

[0009] The secondary battery satisfies the following relationship: 2.9≤0.3×Dspan ×Q / U≤12.9;

[0010] Among them, the D span is the particle size distribution coefficient of lithium iron phosphate;

[0011] U is the potential value of the first step platform, in V;

[0012] Q is the ratio of the charging capacity corresponding to the first step platform to the total discharge capacity, in %.

[0013] As a preferred embodiment of the present invention, the secondary battery satisfies the following relationship: 5.5≤0.3×D span ×Q / U≤8.8.

[0014] As a preferred embodiment of the present invention, the range of U is 3.1~3.4V.

[0015] As a further preferred embodiment of the present invention, the range of U is 3.2~3.25V.

[0016] As a preferred embodiment of the present invention, the range of Q is 10~40%.

[0017] As a further preferred embodiment of the present invention, the range of Q is 20~30%.

[0018] As a preferred embodiment of the present invention, the D span The range is 2.77~3.88.

[0019] As a further preferred embodiment of the present invention, the D span The range is 2.82~3.28.

[0020] As a preferred embodiment of the present invention, the mass ratio of the lithium manganese iron phosphate to the lithium iron phosphate is (2.3-9):1.

[0021] As a preferred embodiment of the present invention, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, the negative electrode active material layer contains negative electrode active material, and the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, SiOx, silicon carbon, and lithium titanate.

[0022] In a second aspect of the present invention, the present invention provides an electrical device comprising the secondary battery described above.

[0023] The beneficial effects of the present invention are:

[0024] The present invention provides a secondary battery and an electric device. The secondary battery of the present invention includes a positive electrode plate, in which the positive electrode active material includes lithium iron manganese phosphate and lithium iron phosphate, and by mixing part of lithium iron phosphate into the lithium iron manganese phosphate and comprehensively controlling the particle size distribution coefficient of the lithium iron phosphate and the voltage platform characteristics of the positive electrode active material, the secondary battery has excellent low-temperature fast charging capability and good high-temperature cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a QV curve diagram of the lithium-ion battery of Example 11.

[0026] Figure 2 dQ / dV-V curve of the lithium ion battery of Example 11. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] In the present invention, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0029] In the present invention, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0030] In the present invention, there is no particular limitation on the specific dispersion and stirring treatment methods.

[0031] The reagents or instruments used in the present invention without indicating the manufacturer are all conventional products that can be obtained through commercial purchase.

[0032] An embodiment of the present invention provides a secondary battery, including a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer contains a positive electrode active material, wherein the positive electrode active material includes lithium manganese iron phosphate (LMFP) and lithium iron phosphate (LFP); wherein the structural formula of LMFP is LiMn x Fe y M z n PO4; x is greater than 0 and less than 1; y is greater than 0 and less than 1; z is greater than or equal to 0 and less than 1, and x, y, z satisfy: 2(x+y)+n×z=2; wherein M is a doping element, and n is the valence state of the M element; the structural formula of LFP is LiFePO4.

[0033] The secondary battery is charged at 10° C. at a rate of 1C in a voltage range of 2.5 to 4.25 V to obtain a charging curve; the charging platform with the lowest potential in the charging curve is the first step platform;

[0034] The secondary battery satisfies the following relationship: 2.9≤0.3×D span ×Q / U≤12.9;

[0035] Among them, the D span is the particle size distribution coefficient of lithium iron phosphate;

[0036] U is the potential value of the first step platform, in V;

[0037] Q is the ratio of the charging capacity corresponding to the first step platform to the total discharge capacity, in %.

[0038] The present invention blends part of lithium iron phosphate into lithium manganese iron phosphate as a positive electrode active material and comprehensively controls the particle size distribution coefficient of lithium iron phosphate and the voltage platform characteristics of the positive electrode active material, so that the secondary battery has excellent low-temperature and high-temperature fast charging capabilities, the fast charging lithium insertion under low temperature conditions is greatly improved, and the battery has a good high-temperature cycle life.

[0039] In the present invention, the voltage platform characteristics of the positive electrode active material are controlled mainly by controlling the potential value of the first step platform (i.e., U value) when the battery is charged at 10°C and 1C rate, and Q is the ratio of the charging capacity corresponding to the first step platform to the total discharge capacity (i.e., Q value).

[0040] Those skilled in the art know that, due to the different charge and discharge voltages of manganese and iron, lithium manganese iron phosphate has two voltage platforms (manganese platform and iron platform) during charge and discharge. Based on the dual voltage platforms of lithium manganese iron phosphate, the present invention further adjusts the low-potential voltage platform by mixing lithium iron phosphate, so that U and Q of the voltage platform with the lowest potential (i.e., the first step platform) satisfy the above relationship.

[0041] The present invention has found that the U value and Q value are the main factors affecting the current density and internal resistance of the material during charging of the secondary battery. According to Joule's law W=I 2 Rt (W is heat, t is time, I is current, R is resistance), U value and Q value further affect the heat generated by the battery during charging. By properly controlling the U value and Q value, it helps to keep W in an appropriate range, allowing the secondary battery to continue to charge at a high rate in the initial stage of charging, thereby increasing the internal temperature of the battery, improving the subsequent lithium deintercalation ability of the positive and negative electrodes, and thus improving the fast charging capability.

[0042] However, the present invention further found that the introduction of the low-potential first-step platform by means of blending LMFP and LFP may also have a certain degree of negative impact on the high-temperature cycle life of the battery. During the high-temperature cycle of LMFP, active lithium is consumed, causing the positive electrode to quickly lose capacity on the low-potential platform, reducing the battery life. In addition, when the low-potential platform accounts for a large proportion of the total capacity (that is, the Q value is large), the charging pressure on the high-potential interval of LMPF is extremely large. During the high-temperature cycle of the battery, the positive electrode side is in a high-potential charging state for a long time, which will accelerate the degradation of the electrolyte and further affect the battery life.

[0043] D span is the particle size distribution coefficient of lithium iron phosphate, D span =(Dn90-Dn10) / Dn50, where Dn10, Dn50, and Dn90 are the particle sizes corresponding to the cumulative distribution percentage of lithium iron phosphate particles reaching 10%, 50%, and 90%, respectively. span The size reflects the uniformity of LFP particle size distribution. The uniformity of LFP particle size distribution affects the particle contact ability between LMFP and LFP in the positive electrode active material, which further affects the charging capacity of the secondary battery. In addition, the positive electrode active material may expand or shrink in volume during the cycle, causing mechanical stress; especially when the battery is charged under high temperature conditions, it will aggravate the possible local overheating phenomenon, further causing uneven stress in the positive electrode active material. The uniformity of lithium iron phosphate particle size distribution will affect the stress distribution of the positive electrode active material during the charge and discharge cycle, thereby affecting the high temperature cycle stability of the battery.

[0044] Since both the U value and the Q value are affected by the D spanIn order to take into account both low-temperature fast charging performance and high-temperature cycle performance, D needs to be strictly adjusted. span The relationship between U and Q is within an appropriate range. span , U and Q satisfy the relationship 2.9≤0.3×D span ×Q / U≤12.9, which enables the secondary battery to maintain a good high-temperature cycle life and the charging capacity is significantly improved under low-temperature conditions.

[0045] When 0.3×D span When the value of ×Q / U exceeds the upper limit of 12.9, the ability of positive and negative electrodes to deintercalate lithium cannot be significantly improved, and the low-temperature fast charging effect of the secondary battery is not significantly improved; when 0.3×D span When the value of ×Q / U exceeds the lower limit of 2.9, the secondary battery has insufficient high-rate charging capacity at low temperatures, which limits the improvement in fast charging effect and may also cause the cycle life of the secondary battery to decrease.

[0046] Exemplarily, in the present invention, 0.3×D span The value of ×Q / U can be 2.9, 3.0, 4.0, 5.0, 7.0, 9.0, 10.0, 11.0, 12.0, 12.5, or 12.9, or it can be an interval formed by any two of the above values.

[0047] In one embodiment, the secondary battery satisfies the following relationship: 5.5≤0.3×D span ×Q / U≤8.8.

[0048] When the above relationship is within the preferred range, the fast charging capability of the secondary battery is relatively better.

[0049] In one embodiment, the range of U is 3.1-3.4V. For example, U can be 3.10V, 3.15V, 3.20V, 3.25V, 3.30V, 3.35V, or 3.40V.

[0050] In one preferred embodiment, the range of U is 3.20~3.25V.

[0051] The potential (U) of the first step platform mainly affects the actual current I value and resistance R value of the positive electrode active material and the charging time t value when the secondary battery is charged. Specifically, when the U value is too low, the voltage at which the battery starts to extract lithium ions at a high flux is extremely low. At this time, the oxidation reaction is basically independent of the high potential platform reaction, resulting in a large current density of the actual reaction, significant battery polarization, and an increase in the battery internal resistance R, which is not conducive to the continuous reaction of the first step platform, resulting in an extremely small t value, and it is impossible to achieve heat generation of the battery during initial charging, and it is difficult to increase the battery operating temperature, and the effect of improving the fast charging ability of the battery at low temperatures is not good. If the U value is high, the battery heating can be improved to a certain extent, but when the U value is too high, it may cause the battery voltage to be higher during charging, the battery starts to generate heat later, and the effect of increasing the internal temperature rise of the battery is poor; and the first step platform reaction and the high potential platform reaction may overlap to a certain extent, resulting in uneven lithium desorption between platforms, large battery polarization, and increased material internal resistance, which makes the current too small, which is also not conducive to battery heat generation, and is even more unfavorable to the improvement of battery fast charging performance. In addition, excessive polarization may also lead to decreased battery stability and reduced cycle life.

[0052] In one embodiment, the range of Q is 10-40%. For example, Q can be 10%, 15%, 20%, 25%, 30%, 35%, or 40%.

[0053] In one preferred embodiment, the range of Q is 20-30%.

[0054] Q refers to the proportion of the charging capacity below the first-step platform to the total discharge capacity, also known as the first-step platform ratio. The size of the Q value mainly affects the current I value and the time t value when the battery is charging and generating heat. An appropriate platform ratio can ensure a larger current I value and increase the heat generated by the battery. When the Q value is too small, the first-step platform, as the voltage platform with the lowest potential, has a too low platform ratio. The low SOC platform needs to withstand a larger current density, resulting in insufficient initial high-rate charging capacity, which is not conducive to the battery's self-work and temperature rise. When the Q value is too large, the platform ratio is too high, and the internal resistance of the battery is low at low SOC. Although the internal resistance R is small, the temperature rise rate is insufficient due to low polarization, and the battery temperature is difficult to increase.

[0055] In the present invention, the values ​​of U and Q can be obtained from the charging curve of the secondary battery.

[0056] Specifically, U and Q can be measured by the following method:

[0057] (1) Constant capacity: The secondary battery was left at 25°C for 120 min, and then discharged at a constant current of 0.33C to 2.5V. After being left at 20 min, the battery was charged at a constant current and voltage of 0.33C to 4.25V, then charged at a constant voltage of 0.05C, and then discharged at a constant current of 0.33C to 2.5V. After the above steps were repeated once, the capacity Q0 of the second cycle was obtained.

[0058] (2) Drawing a charging curve: The secondary battery is charged at a constant current of 0.33C to 8% SOC, left to stand at 10°C for 120 min, and then charged at a constant current of 1C to an upper limit voltage of 4.25V to obtain a QV curve;

[0059] According to the QV curve, the voltage and power data of the nth data point are subtracted from the voltage and power data of the n+1th data point to obtain a dV and dQ data; all the data are processed in sequence to obtain a series of dV and dQ data; then dQ is divided by dV to obtain dQ / dV; dQ / dV is used as the ordinate and voltage is used as the abscissa to obtain a dQ / dV-V curve;

[0060] (3) The peak position of the characteristic peak with the minimum voltage in the dQ / dV-V curve (i.e., the potential corresponding to the first oxidation peak at a low potential from low to high) is the potential value of the first step platform (U), in V;

[0061] The capacity corresponding to the characteristic peak with the minimum voltage in the dQ / dV-V curve is Q1. The proportion of the charging capacity corresponding to the first step platform to the total charging capacity (Q) = Q1 / Q0 × 100%, and the unit of Q is %.

[0062] The values ​​of U and Q are mainly affected by the composition and structure of the positive electrode sheet, especially the positive electrode active material, such as the blending ratio of LMFP and LFP in the positive electrode active material, the particle coating of LFP, the mixing ratio of the raw materials for preparing LFP, the content of manganese in LMFP, the particle morphology and particle size distribution of the positive electrode active material, the crystal structure of the positive electrode active material, etc., all of which will affect the values ​​of U and Q. The values ​​of U and Q can be controlled by adjusting the above adjustments.

[0063] In one embodiment, the D span The range is 2.77~3.88. For example, the D span It can be 2.77, 2.80, 2.85, 3.0, 3.2, 3.5, 3.7, 3.75, 3.80, 3.85, 3.88.

[0064] In one preferred embodiment, the D span The range is 2.82~3.28.

[0065] The present invention has found that D span When the value of is within the above preferred range, the particle size distribution uniformity of lithium iron phosphate is suitable and can cooperate well with LMFP. span When the value of is large, it means that the particle size distribution of lithium iron phosphate is relatively wider and the particle size uniformity is poor, which may lead to poor low-temperature fast charging performance of the battery; when D span When the value is small, the particle size distribution of lithium iron phosphate is too concentrated, and the effective contact with LFMP particles is insufficient, which has limited improvement on the fast charging performance of the battery. Moreover, due to the overly concentrated particle size distribution, the stress generated during the high-temperature cycle charge and discharge of the battery is difficult to be effectively released, and the battery capacity decreases too quickly, resulting in poor high-temperature cycle performance of the battery.

[0066] In one embodiment, the lithium iron phosphate has a Dn90 particle size of 2.63-4.6 μm, a Dn50 particle size of 0.6-1.5 μm, and a Dn10 particle size of 0.3-0.45 μm.

[0067] When the particle size of lithium iron phosphate is within the appropriate range, it helps the battery's high-temperature cycle and power performance to be better overall. When the particle size is too small, it may cause an increase in side reactions and affect the battery's high-temperature cycle performance. When the particle size is too large, it may lead to poor battery dynamics and affect fast charging capabilities.

[0068] In the present invention, the present invention does not limit the testing method of the particle size of lithium iron phosphate, and those skilled in the art can detect the particle size of lithium iron phosphate according to conventional technical means.

[0069] For example, the particle size of lithium iron phosphate can be detected by the following method:

[0070] Take the empty positive electrode sheet and soak it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, take out the positive electrode sheet and dry it in a vacuum environment. Perform SEM (scanning electron microscope) + EDS (energy dispersive X-ray spectroscopy) test. In the SEM observation field, the LFP particle morphology is identified by EDS, and elements are collected to confirm the lithium iron phosphate particles.

[0071] The size of lithium iron phosphate particles was measured by MEARSURE NANO software on the SEM images. The diagonal line method was used to collect the size of lithium iron phosphate particles. After the sample volume reached more than 100, the particle size distribution was statistically analyzed, and the particle size related parameters of lithium iron phosphate were calculated: Dn10, Dn50, Dn90, and D span =(Dn90-Dn10) / Dn50.

[0072] In one embodiment, the mass ratio of lithium manganese iron phosphate to lithium iron phosphate is (2.3-9):1.

[0073] In the positive electrode active material, when LMFP and LFP are in an appropriate mass ratio, it helps the secondary battery to have a suitable voltage platform characteristic and good particle contact ability, and also helps to improve the structural stability of the secondary battery, so that it has a better cycle life during the cycle process.

[0074] In one embodiment, the molar content of manganese in the lithium manganese iron phosphate is 60-90% of the transition metal elements.

[0075] The present invention does not limit the method for detecting the molar content of manganese in lithium manganese iron phosphate as a percentage of transition metal elements, and those skilled in the art can detect the manganese content in lithium manganese iron phosphate according to conventional technical means.

[0076] For example, the molar content of manganese in lithium manganese iron phosphate as a percentage of transition metal elements can be detected by the following method:

[0077] The battery is disassembled to obtain the positive electrode plate, which is then processed to obtain the positive electrode active material powder. The lithium manganese iron phosphate particles are confirmed by SEM (scanning electron microscopy) and EDS (energy dispersive X-ray spectroscopy) tests to obtain the molar content of manganese in the lithium manganese iron phosphate as a percentage of the transition metal element.

[0078] In the present invention, the preparation method of LMFP is not limited, and those skilled in the art can prepare LMFP according to conventional technical means, such as a hydrothermal method or a solid phase method.

[0079] Exemplarily, the solid phase preparation method of LMFP may include the following steps:

[0080] Mixing a manganese source, an iron source, a phosphorus source, a lithium source and a carbon source (if any) in a certain molar ratio and ball milling;

[0081] The ball-milled product is sintered in an atmosphere with an oxygen concentration of less than 150 ppm to obtain LMFP.

[0082] The preparation method of LFP is similar to that of LMFP, except that no manganese source is added.

[0083] Exemplarily, the hydrothermal preparation method of LMFP may include the following steps:

[0084] The lithium source, manganese source, iron source, phosphorus source and carbon source (if any) are mixed, the mixed materials are placed in a reactor for reaction, and the product after the reaction is sintered under an inert atmosphere to obtain LMFP.

[0085] The preparation method of LFP is similar to that of LMFP, except that no manganese source is added.

[0086] For the hydrothermal preparation method, the reaction conditions in the reactor can be: temperature: 200-300°C; time: 2-3h; pressure 1.5-2.5Mpa; the sintering conditions can be: temperature 500-800°C, time 5-15h.

[0087] The lithium source may include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate or lithium acetate.

[0088] The phosphorus source may include at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, or lithium phosphate.

[0089] The iron source may include at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferrous carbonate heptahydrate, ferrous oxide, ferrous oxide or ferric oxalate.

[0090] The manganese source may include at least one of manganese carbonate, manganese sulfate, manganese sulfate monohydrate, manganese nitrate, manganese chloride, manganese oxalate, or manganese acetate.

[0091] The carbon source may include glucose and / or sucrose.

[0092] Ferromanganese phosphate can be selected as a manganese source, an iron source and a phosphorus source at the same time; ferric phosphate can be selected as a manganese source, an iron source and a phosphorus source at the same time; the lithium source can include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate or lithium acetate.

[0093] When preparing LMFP and / or LFP, a certain amount of doping element source such as vanadium source (vanadium pentoxide), tungsten source (ammonium metatungstate), titanium source (titanium oxide), magnesium source (magnesium carbonate), etc. can be mixed with manganese source (if any), iron source, phosphorus source, lithium source, etc. as needed to obtain LMFP and / or LFP containing a certain amount of doping elements.

[0094] Optionally, the solvent may be at least one of water and ethanol.

[0095] By adjusting the addition amount of manganese source, iron source and phosphoric acid, the proportion of manganese content in LMFP can be controlled.

[0096] In the preparation of LFP, the molar ratio of the iron source, the lithium source and the phosphorus source can be 1: (1.001~1.05): (1.001~1.06); the added amount of the carbon source can be 5~10 wt.% of the total weight of the iron source, the lithium source and the phosphorus source.

[0097] In addition to the above-mentioned positive electrode active material, the positive electrode active material layer may further include a conductive agent and a binder.

[0098] The conductive agent can be used as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. In the present invention, there is no particular limitation on the type of the conductive agent. Specifically, the conductive agent can be at least one of carbon nanotubes, carbon black or graphene.

[0099] The binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. In the present invention, the binder can be a conventional choice in the battery field. Specifically, the conductive agent can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC) or sodium alginate.

[0100] The present invention has no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause adverse chemical changes in the battery, and can use, for example: stainless steel, aluminum, nickel, titanium, fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc.

[0101] The positive electrode sheet of the present invention can be prepared according to conventional methods in the art, for example, the positive electrode active material, the conductive agent and the binder are dispersed in a solvent to form a uniform positive electrode slurry, the positive electrode slurry is coated on the positive electrode current collector, and after drying, rolling and other processes, the positive electrode sheet is obtained.

[0102] In one embodiment, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer contains a negative electrode active material, and the negative electrode active material includes at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, SiOx, silicon carbon, and lithium titanate.

[0103] The diaphragm is located between the positive electrode sheet and the negative electrode sheet, and is used to separate the positive electrode sheet and the negative electrode sheet to prevent the positive electrode sheet and the negative electrode sheet from contacting and short-circuiting. The diaphragm can be any material suitable for an electrochemical energy storage device isolation membrane in the art. Specifically, the diaphragm includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber.

[0104] The electrolyte of the present invention can be any electrolyte suitable for electrochemical energy storage devices in the art. The electrolyte includes an electrolyte and a solvent, and the electrolyte can generally include a lithium salt.

[0105] Specifically, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte in the electrolyte may be 0.5-5 mol / L.

[0106] Specifically, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0107] An embodiment of the present invention provides an electric device, comprising the secondary battery described above. The secondary battery serves as a power supply for the electric device.

[0108] The electrical device refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy or other energy forms, such as electric motors, electric heat engines, electric light sources, etc. Specifically, it may include but is not limited to mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices may be mobile phones, laptops, drones, sweeping robots, electronic cigarettes, etc.; electric vehicles may be pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0109] The present invention is further described below with reference to specific embodiments.

[0110] Example 1

[0111] Embodiment 1 provides a lithium ion battery, and the preparation method is as follows:

[0112] (1) Preparation of positive electrode

[0113] (1.1) Preparation of positive electrode active materials

[0114] Preparation of LMFP: According to LiMn 0.75 Fe 0.25 Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate, and phosphoric acid were weighed and mixed in molar proportions of the elements Li, Mn, Fe, and P in the PO4 chemical formula. The mixed materials were placed in a reactor and reacted at 250°C and 2MPa pressure for 2.5h. After being taken out, they were sintered at 600°C for 12h to obtain LMFP (referred to as LMFP-75);

[0115] Preparation of LFP: Disperse ferrous oxalate into deionized water, then add H2O2 and slowly stir, then add Li2CO3 and H3PO4, and add carbon source (sucrose), place the mixed materials in a reactor, and react at 220°C and 2.2MPa pressure for 3h to obtain LFP precursor;

[0116] The molar ratio of ferrous oxalate, lithium carbonate (Li2CO3) and phosphoric acid is 1:1.03:1.04; the amount of carbon source added accounts for 7.9wt.% of the total mass of ferrous oxalate, lithium carbonate and phosphoric acid

[0117] The LFP precursor was sintered in an Ar atmosphere at a temperature of 650°C for 10 h. After sintering, the LFP was crushed and sieved to obtain a particle size distribution coefficient satisfying Table 2.

[0118] (1.2) The LMFP and LFP are mixed uniformly in a mass ratio of 8:2 to obtain a positive electrode active material;

[0119] The positive electrode active material, binder (PVDF), conductive agent (SP), and conductive agent (CNT) are mixed evenly in NMP at a mass ratio of 97:1.5:1:0.5. Subsequently, the mixed positive electrode slurry is evenly coated on aluminum foil and dried in a vacuum oven at 100°C to obtain a positive electrode sheet, which is then cut into strips and then rolled to obtain a positive electrode sheet.

[0120] (2) Preparation of negative electrode

[0121] The negative electrode active material (artificial graphite), conductive agent (SP), and binder (carboxymethyl cellulose, CMC) are mixed in a mass ratio of 96.4:1:2.6, dispersed in deionized water, and made into negative electrode slurry by a wet process using a vacuum mixer. The negative electrode slurry is evenly coated on the negative electrode collector (copper foil), and the negative electrode collector coated with the negative electrode slurry is transferred to a vacuum environment in an oven at 100°C for drying for 12 hours, and then rolled and cut to obtain negative electrode sheets.

[0122] (3) Preparation of electrolyte

[0123] Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0124] (4) Preparation of diaphragm

[0125] Polyethylene (PE) diaphragm is used.

[0126] (5) Preparation of batteries

[0127] The positive electrode sheet, separator and negative electrode sheet prepared above are wound to obtain a bare cell without liquid injection; the bare cell and the adapter of the cover plate assembly are electrically connected, the bare cell is placed in a shell, the cover plate and the shell are welded together to assemble a battery; the prepared electrolyte is injected into the dried bare cell, and a lithium-ion battery is obtained after vacuum packaging, standing, formation, shaping, sorting and other processes.

[0128] Examples 2 to 14 and Comparative Examples 1 to 4

[0129] Examples 2 to 14 and Comparative Examples 1 to 4 respectively provide a lithium ion battery, and the preparation method is basically the same as that of Example 1, except that:

[0130] (1.1) In the preparation of LFP: the molar ratio of ferrous oxalate, lithium carbonate and phosphoric acid is shown in Table 1, the type and amount of carbon source added are shown in Table 1, and the sintering conditions are shown in Table 1;

[0131] (1.2) LMFP and LFP are mixed evenly according to the mass ratio shown in Table 1.

[0132] also:

[0133] The differences between the preparation methods of the lithium ion batteries of Examples 5, 8, and 12 and Example 1 include:

[0134] In Example 5 and Example 8, the preparation of LMFP is as follows: 0.80 Fe 0.20 Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate, and phosphoric acid are weighed and mixed in molar proportions of the elements Li, Mn, Fe, and P in the PO4 chemical formula. The mixed materials are placed in a reactor and reacted at 200°C and 3MPa pressure for 3h. After being taken out, they are sintered at 580°C for 10h to obtain LMFP (referred to as LMFP-80);

[0135] In Example 12, LMFP was prepared by 0.85 Fe 0.15Lithium hydroxide, manganese sulfate monohydrate, ferrous sulfate heptahydrate and phosphoric acid were weighed and mixed in the molar ratio of each element Li, Mn, Fe and P in the PO4 chemical formula. The mixed materials were placed in a reactor and reacted at 300°C and 2MPa pressure for 2h. After being taken out, they were sintered at 550°C for 12h to obtain LMFP (denoted as LMFP-85).

[0136] The differences between the preparation method of the lithium ion battery of Example 7 and Example 1 also include:

[0137] In the preparation of the negative electrode sheet, the negative electrode active material is a mixture of silicon-carbon material and artificial graphite, wherein the silicon-carbon material accounts for 15wt.% of the negative electrode active material, and the silicon element in the silicon-carbon material accounts for 45wt.%.

[0138] The differences between the preparation method of the lithium ion battery of Example 12 and Example 1 also include:

[0139] In the preparation of the negative electrode sheet, the negative electrode active material is lithium titanate (Li4Ti5O 12 ).

[0140] Comparative Example 5

[0141] Comparative Example 5 provides a lithium ion battery, and the preparation method is basically the same as that of Example 1, except that: in the preparation of the positive electrode active material, LFP is not prepared, that is, the positive electrode active material is only composed of LMFP-75.

[0142] Table 1

[0143]

[0144] The potential value (U) of the first step platform of the lithium-ion battery prepared in each embodiment and comparative example, the proportion of the charge capacity below the first step platform to the total discharge capacity (Q), and the D span The test results are shown in Table 2.

[0145] The QV curve and dQ / dV-V curve of the lithium ion battery of Example 11 are shown as follows: Figure 1 and Figure 2 shown.

[0146] Table 2

[0147]

[0148] *: Comparative Example 5 does not contain LFP, so there is no D of LFP. span , so 0.3×D is not calculated span×Q / U value; its positive electrode active material is composed of only a single LFMP-75, and the potential value of the first step platform (U value) is higher than that of other embodiments and comparative examples.

[0149] The low temperature fast charging performance and cycle life of lithium-ion batteries are tested. The specific methods are as follows:

[0150] (1) Low temperature fast charging performance:

[0151] At 25°C, the lithium-ion battery was discharged at a constant current of 0.33C to 2.5V, then charged at a constant current of 0.33C to 4.25V, charged at a constant voltage to 0.05C, and discharged at a constant current of 0.33C to 2.5V; after the above steps were repeated once, the capacity Q0 of the second cycle was obtained;

[0152] The lithium-ion battery was charged at 0.33C constant current to 8% SOC, and after standing at 10°C for 120 min, it was charged at 1C constant current with a cut-off upper limit voltage of 4.25V, and the capacity Q2 of 1C constant current charging was obtained;

[0153] 1C lithium insertion SOC = (Q2 / Q0×100%+8%) SOC;

[0154] The 1C lithium insertion SOC value reflects the lithium insertion capacity of lithium-ion batteries under low-temperature fast charging. The higher the 1C lithium insertion SOC value, the stronger the lithium insertion capacity and the better the low-temperature fast charging performance of the battery.

[0155] (2) High temperature cycle life

[0156] The lithium-ion battery was left to stand at 45°C for 120 minutes, fully charged at 1C constant current and constant voltage, with a cut-off voltage of 4.25V and a cut-off current of 0.33C. After standing for 10 minutes, it was discharged at 1C constant current to 2.5V. The above was one cycle. 200 cycles (200 circles) of charge and discharge were performed. The discharge capacity of the 200th cycle was divided by the full 1C discharge capacity of the second cycle to obtain the high-temperature cycle capacity retention rate.

[0157] The lithium ion batteries prepared in Example 1, Example 2 and Comparative Examples 1 and 2 were subjected to a cover plate temperature rise test in the following manner:

[0158] At 25°C, the lithium-ion battery was discharged at a constant current of 0.33C to 2.5V, then charged at a constant current of 0.33C to 4.25V, charged at a constant voltage to 0.05C, and discharged at a constant current of 0.33C to 2.5V; after the above steps were repeated once, the capacity Q0 of the second cycle was obtained;

[0159] The single cell cover of the above lithium-ion battery is connected to a temperature sensor; the lithium-ion battery is charged at a constant current of 0.33C to 8% SOC, and after standing at 10°C for 120 minutes, the temperature of the cell cover is recorded as T1°C; the lithium-ion battery is charged at a constant current of 1C, with a cut-off upper limit voltage of 4.25V, and the highest temperature of the cell cover during the charging process is recorded as T2°C; the cover temperature rise = T2-T1.

[0160] After testing, the temperature rise of the cover plate of the lithium ion battery of Example 1 and Example 2 was 6.6°C and 5.5°C respectively; the temperature rise of the cover plate of the lithium ion battery of Comparative Example 1 and Comparative Example 2 was 2.0°C and 1.5°C respectively. It can be seen that the temperature rise of the cover plate of the lithium ion battery prepared in the embodiment is improved to a certain extent during the charging process.

[0161] The test results of 1C lithium insertion SOC and cycle life are shown in Table 3.

[0162] Table 3

[0163]

[0164] According to the test results in Table 3, it can be seen that the lithium-ion batteries prepared in the embodiments of the present invention have good low-temperature fast charging performance and high-temperature cycle stability, 1C lithium insertion SOC ≥ 61%, and capacity retention rate ≥ 84% after 200 cycles at 45°C.

[0165] According to Examples 1 to 9, when 0.3×D span When the value of ×Q / U satisfies 5.5-8.8, the 1C lithium insertion SOC and high temperature cycle capacity retention rate of the lithium ion battery are relatively higher. span When 2.82~3.28 is further met, Q further meets 20~30%, and U further meets 3.2~3.25V, it will help lithium-ion batteries to have better low-temperature fast charging performance and high-temperature cycle stability.

[0166] According to Comparative Examples 1 to 4, when the lithium-ion battery 0.3×D span When the value of ×Q / U exceeds the scope of the technical solution of the present invention, its low-temperature fast-charging performance deteriorates seriously, and the 1C lithium-intercalation SOC does not exceed 50%; and the high-temperature cycle performance of the lithium-ion batteries of Comparative Examples 2 to 4 is poor, and the capacity retention rate does not exceed 76.2% after 200 cycles at 45°C. In Comparative Example 5, the positive electrode active material does not contain LFP, and the fast-charging capability of the battery is very poor, and the 1C lithium-intercalation SOC is only 41.2%.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium iron manganese phosphate and lithium iron phosphate; The secondary battery is charged at 10° C. at a rate of 1C in a voltage range of 2.5 to 4.25 V to obtain a charging curve; the charging platform with the lowest potential in the charging curve is the first step platform; The secondary battery satisfies the following relationship: 2.9≤0.3×D span ×Q / U≤12.9; Among them, the D span is the particle size distribution coefficient of lithium iron phosphate, D span =(Dn90-Dn10) / Dn50, where Dn10, Dn50, and Dn90 are the particle sizes corresponding to the cumulative distribution percentages of the number of lithium iron phosphate particles reaching 10%, 50%, and 90%, respectively; span The range is 2.77~3.88; U is the potential value of the first step platform, in V, and the range of U is 3.1-3.4V; Q is the ratio of the charging capacity corresponding to the first step platform to the total discharge capacity, expressed in %, and the range of Q is 10-40%.

2. The secondary battery according to claim 1, characterized in that: The secondary battery satisfies the following relationship: 5.5≤0.3×D span ×Q / U≤8.

8.

3. The secondary battery according to claim 1, characterized in that: The range of U is 3.2~3.25V.

4. The secondary battery according to claim 1, characterized in that: The range of Q is 20~30%.

5. The secondary battery according to claim 1, characterized in that: The D span The range is 2.82~3.

28.

6. The secondary battery according to claim 1, characterized in that: The mass ratio of the lithium manganese iron phosphate to the lithium iron phosphate is (2.3-9):

1.

7. The secondary battery according to claim 1, characterized in that: The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, the negative electrode active material layer contains negative electrode active materials, and the negative electrode active materials include at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, SiOx, silicon carbon, and lithium titanate.

8. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 7.

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