Secondary battery

Abstract

The invention discloses a secondary battery, and belongs to the technical field of batteries, according to the secondary battery, the mass ratio of an iron element to a phosphorus element in a positive active material layer in a positive pole piece and the self-discharge rate of the secondary battery are synchronously regulated and controlled, so that the dynamic capacity of the secondary battery can be effectively improved; and meanwhile, the self-discharge behavior of the secondary battery is coordinated, and the polarization effect in the secondary battery is reduced, so that the secondary battery can still realize excellent cycling stability at a relatively high temperature.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery. Background Technology

[0002] Phosphorus-iron-based secondary batteries (such as lithium iron phosphate batteries) have advantages in terms of thermal runaway risk due to their predominantly olivine structure and low redox potential, exhibiting good safety performance, good cycle stability, and low production cost. However, when used as energy storage systems for electric vehicles, their insufficient theoretical specific capacity and volumetric energy density make it difficult to meet the requirements for long driving range. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. By simultaneously regulating the mass ratio of iron and phosphorus elements in the positive electrode active material layer of the positive electrode sheet and the self-discharge rate of the secondary battery, the dynamic capacity of the secondary battery can be effectively improved. At the same time, the self-discharge behavior of the secondary battery is coordinated, the polarization effect inside the secondary battery is reduced, and the secondary battery can still achieve excellent cycle stability at high temperatures.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, the secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive active material layer, the positive active material layer comprising a positive active material, the positive active material containing iron and phosphorus elements;

[0005] The secondary battery satisfies: a×b=0.01~0.12;

[0006] Where a is the mass ratio of iron to phosphorus in the positive electrode active material layer, and b is the self-discharge rate of the secondary battery.

[0007] The beneficial effects of this application are as follows:

[0008] This application provides a secondary battery that can effectively improve the dynamic capacity of the secondary battery by simultaneously regulating the mass ratio of iron and phosphorus elements in the positive electrode active material layer of the positive electrode sheet and the self-discharge rate of the secondary battery. At the same time, it can coordinate the self-discharge behavior of the secondary battery, reduce the polarization effect inside the secondary battery, and enable the secondary battery to achieve excellent cycle stability at high temperatures. Detailed Implementation

[0009] 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.

[0010] 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.

[0011] 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.

[0012] The present application is further illustrated below with specific embodiments:

[0013] A secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive active material layer, the positive active material layer includes a positive active material, and the positive active material contains iron and phosphorus.

[0014] The secondary battery satisfies: a×b=0.01~0.12;

[0015] Where a is the mass ratio of iron to phosphorus in the positive electrode active material layer, and b is the self-discharge rate of the secondary battery.

[0016] To overcome the shortcomings of existing phosphorus-iron-based secondary batteries in simultaneously achieving high capacity and good cycle stability under thermal conditions, this application provides a secondary battery solution. By simultaneously controlling the product of the mass ratio of iron and phosphorus in the positive electrode active material layer and the self-discharge rate of the secondary battery within a specific range, the secondary battery not only possesses high cycle capacity but also maintains good cycle stability under high-temperature conditions. In this secondary battery, when the mass ratio 'a' of iron and phosphorus increases, the relative proportion of iron in the material increases, providing more lithium-ion storage sites and thus improving the lithium-ion insertion / extraction capacity. However, this control leads to an excess of iron in the material, resulting in a simultaneous increase in the relative content of phosphides (such as iron phosphide and ferrous phosphide), which in turn forms magnetic impurities. These substances gradually dissolve in the battery during cycling. The polarization effect migrates to the negative electrode in the electrolyte, increasing the probability of micro-short circuits. This ultimately leads to an increased self-discharge rate, decreased voltage, and poorer cycle stability in the secondary battery. Therefore, it is necessary to simultaneously regulate the self-discharge rate of the secondary battery to keep the internal polarization effect within a low range, ensuring that both overcharging and over-discharging are at low levels, maintaining capacity improvement, and achieving good cycle stability. On the other hand, when 'a' is small, that is, when the relative proportion of iron is small, the utilization rate of active materials decreases. In this case, the self-discharge rate of the secondary battery can be simultaneously regulated to maintain a balance between capacity and cycle stability. However, if the product of the two exceeds the range defined in this application, it may lead to the secondary battery failing to achieve both expected effects, or even resulting in a serious situation of low capacity and poor cycle stability.

[0017] In some implementations, a×b = one or any two of the following values: 0.01, 0.02, 0.03, 0.03, 0.04, 0.05, 0.06, 0.07, 0.075, 0.08, 0.09, 0.1, 0.11, 0.12.

[0018] More preferably, a×b = 0.03 to 0.075.

[0019] As mentioned above, in the technical solution of this application, the synchronous regulation of the iron-phosphorus mass ratio in the positive electrode active material layer and the self-discharge rate of the secondary battery is crucial to ensure the expected comprehensive performance of the secondary battery. When the product of the two is further optimized within the above range, it can not only ensure better lithium insertion / extraction activity of the positive electrode active material particles and a larger overall kinetic capacity, but also reduce the impact of magnetic impurities formed by phosphides on the secondary battery, resulting in a lower degree of internal polarization effect and better cycle stability of the secondary battery under thermal conditions.

[0020] In some implementations, a = 0.9 to 1.1.

[0021] In some implementations, a is a range of one or any two of the following: 0.9, 0.92, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, 1.02, 1.05, 1.08, 1.1.

[0022] More preferably, a = 0.96 to 0.99.

[0023] When the secondary battery satisfies that a×b is within a defined range, the iron-phosphorus mass ratio of the secondary battery can be further optimized to be within the above range. At this time, the lithium deintercalation activity and deintercalation sufficiency of the primary particles in the positive electrode active material are better, the secondary battery can achieve a larger charge and discharge capacity, and the probability of phosphide generation can also be effectively controlled, reducing the polarization effect of the secondary battery and further improving the cycle stability of the secondary battery.

[0024] In some embodiments, the mass ratio 'a' of iron to phosphorus in the positive electrode active material layer of the secondary battery can be confirmed by, but is not limited to, the following method: The secondary battery is discharged, then disassembled. The resulting positive electrode sheet is pre-soaked in dimethyl carbonate (DMC) for 30 minutes, dried, and then powdered. 0.2 ± 0.0020 g of the resulting powder is weighed and placed in a beaker, and 10 mL of aqua regia is added. The beaker is then heated at 160°C for 40 minutes, cooled, and subjected to solid-liquid separation. The resulting solution is placed in a 50 mL centrifuge tube, and the beaker wall is washed with a 2% nitric acid solution. The resulting washing solution is also placed in the centrifuge tube, and the volume is adjusted to 100 mL to obtain the test solution. The test solution is then subjected to ICP testing using a Thermo Fisher Scientific iCAP instrument. PRO, with the following operating conditions set: gas flow rate 0.5L / min, power 1150W, confirm the iron and phosphorus concentrations (i.e., the specific molar percentage of chemical elements) in the positive electrode active material layer, from which the mass content of iron and phosphorus can be calculated.

[0025] In some implementations, b = 0.01 to 0.12.

[0026] In some implementations, b is a range of one or both of the following: 0.01, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.11, and 0.12.

[0027] More preferably, b = 0.03 to 0.08.

[0028] The change in the self-discharge rate of a secondary battery will affect its capacity and capacity stability. When the self-discharge rate b of the secondary battery is further optimized within the above range, the probability of overcharging and over-discharging during charge-discharge cycles is lower, the lithium intercalation / deintercalation stability of the secondary battery is better, and better overall electrochemical performance can be achieved.

[0029] In some embodiments, the self-discharge rate b of the secondary battery can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged, disassembled to obtain the positive electrode sheet, then the electrolyte byproducts on the surface are washed with an inert solution DMC, dried, and then 6 positive electrode sheets, 7 graphite negative electrode sheets, and a composite coating consisting of a 9μm thick PE base film coated with a 3μm thick alumina coating on one side and a 2μm thick PVDF coating on both sides are used as a separator. The battery is assembled on a stacking machine, tape is applied, and then it is hot-pressed on a hot press (temperature 75℃, pressure 1000N, time 3s), wrapped with aluminum-plastic film, sealed, and dried for 24 hours. The moisture content of the mixed sample is tested using a Carfelsh moisture meter and meets the following requirement: Within 0.00ppm, it can flow normally. After adding electrolyte according to the electrolyte injection coefficient of 4.0, it is placed at high temperature for 24 hours. The specific preparation method of the electrolyte is as follows: EC, EMC and DEC are compounded in a mass ratio of 1:1:1 as solvent. Then, based on the total mass of the electrolyte, lithium hexafluorophosphate is added to prepare a lithium hexafluorophosphate concentration of 1mol / L as electrolyte. After standing, the obtained battery is formed according to the following formation process: after being placed at 45℃ for 24 hours, it is placed in a glass clamp with a clamp pressure of 0.5MPa and left to stand for 10 minutes. First, it is charged at a rate of 0.02C for 300 minutes and then stopped. After standing for 10 minutes, it is charged at a rate of 0.1C for 300 minutes and then stopped. After the battery is placed at 45℃ for 24 hours and then resealed, it is subjected to the following steps for capacity determination: the battery is placed at 25℃ for 10 minutes, then charged at a constant current (0.33C) to the upper voltage limit of 3.65V, charged at a constant voltage until the cutoff current is less than or equal to 0.05C, placed for 10 minutes, and then discharged at a constant current (0.33C) to the lower voltage limit of 2.5V. This is one cycle, and the cycle is repeated three times. The discharge capacity of the third cycle is used as the capacity determination. After the battery is removed from the machine, a 2Ah soft-pack battery is obtained. The assembled battery is pre-assembled using the LAND system at 25℃. The battery is then left to stand until the single-step time is ≥2h, charged at a rate of 0.33C until the voltage is ≥3.65V, charged at a constant voltage of 3.65V until the current rate is ≤0.05C, left to stand until the single-step time is ≥5min, and discharged at a rate of 0.33C until the voltage is ≤2.5V. This process is repeated twice, and the discharge capacity of the second discharge is recorded as Q0. Then, the battery is charged at a constant current of 0.33C for 40min and left to stand for another 4h. The battery is then transferred to -20℃ and left to stand for 8h, and then to 25℃ and left to stand for 8h. At this time, the open circuit voltage OCV1 of the battery is measured with a voltmeter. The battery is then left to stand at 25℃ for 48h, and the open circuit voltage OCV2 of the battery is measured again with a voltmeter. Therefore, b = (OCV1 - OCV2) / 48.

[0030] In some embodiments, the positive electrode active material layer also contains at least one of iron phosphide and ferrous phosphide.

[0031] In some embodiments, the total content of iron phosphide and ferrous phosphide in the positive electrode active material layer is ≤8ppm.

[0032] More preferably, the total content of iron phosphide and ferrous phosphide in the positive electrode active material layer is 1 to 8 ppm.

[0033] In some embodiments, the total content of ferric phosphide and ferrous phosphide in the positive electrode active material layer can be confirmed by, but is not limited to, the following methods: After the secondary battery is discharged, it is then disassembled. The obtained positive electrode sheet is soaked in DMC to remove electrolyte components and pre-remove by-products. Then, 200g of powder is obtained by scraping the positive electrode active material layer from multiple positive electrode sheets and placing it in a polypropylene sample container. A magnetic rod and 300mL of ultrapure water are added. The sample container is sealed and placed on a can mill ball mill and ball milled at 80rpm for 60min. Then, the magnetic rod is retained, and the remaining sample is cleaned and added... Wash the magnetic rod with ultrapure water in a tank, then transfer it to a 250mL beaker. Immerse the rod in ultrapure water and ultrasonically clean it. Remove the water and soak it in 60mL of 10% hydrochloric acid solution for 30min. Then add 10mL of aqua regia and 50mL of ultrapure water to the beaker and soak it. Heat to boiling at 230℃ for 30min, then allow it to cool naturally to room temperature. Rinse the magnetic rod three times with a small amount of ultrapure water. Transfer the rinsed solution to a 50mL volumetric flask and record the solution as -50. Turn on the ThermoFisher Scientific iCAP PRO series ICP analyzer (RF power 1150W, nebulizer flow and auxiliary gas flow set to 0.5L / min, rinsing time 30s). Click "ICP-Expert" to enter the instrument interface, then click "Method" to select the magnetic material, edit the sample information and save it, then click "Analyze". The accuracy of the standard curve test needs to be above 0.999. After the analysis is completed, the data is read, and the injection tube is placed in 2% dilute nitric acid to rinse the injection system for 5 minutes, and then placed in distilled water to rinse the injection system for 5 minutes. The content of magnetic material in the test solution-50 is the total content of iron phosphide and ferrous phosphide in the positive electrode active material layer.

[0034] In some embodiments, the positive electrode active material includes at least one of lithium iron phosphate and doped lithium iron phosphate.

[0035] In some embodiments, the positive electrode active material also contains lithium supplementation additives.

[0036] More preferably, the lithium supplementation additive includes lithium iron ferrite rich in lithium.

[0037] More preferably, the lithium supplementation additive includes Li5FeO4.

[0038] More preferably, the mass percentage of lithium supplementation additive in the positive electrode active material layer is 0.5% to 5%.

[0039] When the iron-phosphorus mass ratio and self-discharge rate of the positive electrode active material layer are simultaneously regulated in the secondary battery, the introduction of lithium-replenishing additives into the positive electrode active material can further effectively improve the energy density of the secondary battery during the charging and discharging process, resulting in a higher discharge capacity. At the same time, optimizing the content of lithium-replenishing additives can ensure that the probability of lithium plating caused by lithium-replenishing additives is controlled at a low level, avoiding the risk of battery micro-short circuit due to lithium dendrite growth, thereby improving the cycle life of the battery.

[0040] In some embodiments, the mass percentage of lithium-supplementing additives in the positive electrode active material layer can be confirmed by, but is not limited to, the following methods: The secondary battery is subjected to discharge treatment, followed by disassembly and removal of the positive electrode sheet. The electrode is then immersed in DMC solvent in a glove box for 2 hours to remove soluble electrolyte residue, followed by vacuum drying. The electrode sample is then cut into small pieces (5×5mm) in a glove box for XPS testing. The testing uses a 120W monochromatic Al Kα X-ray source; the energy resolution is set to less than or equal to 0.48 eV; the test beam spot size is 400 micrometers; the test items selected are full-spectrum testing, narrow-spectrum testing, and depth profiling (10 times); the tested and etched elements are Li, Fe, Ni, C, O, and P; the etching depth is 200 nm; the obtained test results are then processed using Thermal Avantage software for peak segmentation; the Fe content of the positive electrode active material layer after etching is obtained. 3+ Peak area (at 710–711 eV), denoted as SFe 3+ And the O1S peak area, denoted as S O1s, then the content of lithium supplementation additive in the positive electrode active material layer = S Fe 3+ / S O1s.

[0041] In some embodiments, the positive electrode active material layer also contains dopant elements;

[0042] More preferably, the doping element includes at least one of magnesium and titanium.

[0043] More preferably, the magnesium element exists in ionic form, and the ion has a valence state of +2.

[0044] More preferably, the titanium element exists in ionic form, and the ion has a valence state of +4.

[0045] When magnesium or titanium is further introduced into the positive electrode active material layer in this application, the two, as heterovalent dopants, can generate electron-hole or local charge compensation effects, enhance electronic conductivity, suppress lattice distortion and oxygen release during high-voltage charging and discharging, maintain unobstructed ion diffusion channels, and at the same time, magnesium can anchor lattice oxygen and reduce Fe reduction driven by oxygen loss, while titanium can passivate the surface and inhibit Fe dissolution, further suppressing the generation of phosphides during the use of the secondary battery and improving the cycle stability of the secondary battery under thermal conditions.

[0046] More preferably, the concentration of doped elements in the positive electrode active material layer is 2000–6000 ppm.

[0047] In some embodiments, the concentration of dopant elements in the positive electrode active material layer can be determined by, but is not limited to, the following methods:

[0048] The secondary battery after discharge treatment was disassembled, the positive electrode was removed and soaked in DMC solvent for 2 hours, dried, and 0.2±0.002g of the powder obtained by scraping the positive electrode was weighed, soaked in 10mL aqua regia and heated at 160℃ for 40min, cooled, and the resulting mixture was transferred to a 50mL centrifuge tube and diluted to volume with 2wt% nitric acid solution. The diluted solution was filtered into another centrifuge tube (30mL) using quantitative filter paper, and tested using an ICP device (ICP-thermoscientific, iCAPPRO). The concentration of titanium and magnesium doping elements was confirmed by calculating Ti(323.452) / Mg(279.553) using the standard curve method.

[0049] In some embodiments, the positive electrode active material includes doped lithium iron phosphate, wherein the doping element in the doped lithium iron phosphate may be, but is not limited to, at least one of magnesium, aluminum, titanium, and manganese.

[0050] In some embodiments, the positive electrode active material is further provided with a coating layer containing carbon.

[0051] More preferably, the average thickness of the coating layer is 2 to 10 nm.

[0052] When the mass ratio of iron to phosphorus in the positive electrode active material of a secondary battery is optimized and controlled, a carbon-containing coating layer is further formed on its surface. Preferably, the thickness of the coating layer is within the above-mentioned range. This not only forms a continuous conductive film on the surface of the active particles, further improving the conductivity of the material, but also physically blocks the active material from direct contact with the electrolyte, reducing electrolyte corrosion, transition metal dissolution, and the generation of by-reaction products (such as iron phosphide and ferrous phosphide). This improves the overall stability of the material and gives the secondary battery a longer lifespan during cycling.

[0053] In some embodiments, the average thickness of the coating layer can be determined by, but is not limited to, the following methods:

[0054] The secondary battery was discharged, and then the positive electrode was disassembled. The positive electrode was scraped to remove powder, and the resulting powder was dispersed in ethanol and ultrasonically treated for 10 minutes. Then, samples were taken and transferred to a copper grid for transmission electron microscopy, dried, and the thickness of the particle coating was observed and measured at a discharge magnification of 500 kx under a transmission electron microscope. The number of samples measured was 20, and the average thickness of the coating layer of the samples was calculated as the average thickness of the coating layer.

[0055] In some embodiments, the positive electrode active material can be prepared by, but is not limited to, the following method: mixing carbon source, iron source, phosphorus source and lithium source doping element precursors in a solvent, grinding the resulting mixture, spray drying the resulting powder particles, calcining, removing impurities, pulverizing and mixing with lithium supplementation additives and dispersing evenly to obtain the positive electrode active material.

[0056] In some embodiments, the solvent includes at least one of water and ethanol.

[0057] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate.

[0058] The phosphorus source includes at least one of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, and iron phosphate.

[0059] More preferably, the phosphorus source and the iron source can be the same, such as iron phosphate.

[0060] The iron source includes at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferric oxide, ferric oxide, and ferric oxalate.

[0061] The carbon source includes at least one of glucose, fructose, and organic polymers.

[0062] The precursor of the doped element includes an oxide or salt thereof containing the doped element.

[0063] In some embodiments, the grinding process takes 2.5 to 3.5 hours and rotates at a speed of 1000 to 2000 r / min.

[0064] Specifically, the grinding process is carried out by ball milling, which uses zirconia balls with an average particle size of 0.3 to 0.35 mm and a ball-to-material ratio of (5 to 10):1.

[0065] In some embodiments, the spray drying speed is 100,000 to 130,000 r / min, the inlet temperature is 200 to 290°C, and the outlet temperature is 50 to 120°C.

[0066] In some embodiments, the calcination temperature is 710–840°C, the heating rate is 3–5°C / min, and the calcination time is 6–14 h.

[0067] In some embodiments, the pulverization is performed using air jet milling, and the pressure during air jet milling is 300–500 kPa.

[0068] It should be noted that, in this application, the mass ratio of iron to phosphorus in the positive electrode active material and the self-discharge rate of the secondary battery can be controlled by adjusting the parameters during the preparation of the positive electrode active material through material addition, grinding, calcination, and pulverization, but are not limited to this.

[0069] In some embodiments, 'a' can be controlled by the addition ratio of phosphorus source and iron source, while 'b' can be controlled by the addition amount of carbon source, the temperature and time during calcination, and the addition amount of lithium supplementation additive. In addition, 'b' can also be controlled by the addition ratio of conductive agent and the slurry solid content during the preparation of the positive electrode active material layer.

[0070] In some embodiments, the average diameter of the positive electrode active material is 300–700 μm;

[0071] In some embodiments, the method for testing the average diameter of the positive electrode active material can be as follows: disassemble the lithium-ion battery to obtain the positive electrode sheet, then scrape off the positive electrode active material layer on the positive electrode sheet to form powder, evenly sprinkle it on the conductive adhesive, and directly place it in a scanning electron microscope. Select a SEM image with a magnification of 30K and clear image, and use elemental analysis to confirm and identify the positive electrode active material particles. Then, use particle size measurement software (Nano Measurer) to measure the particle size distribution of these particles (measure 400 particles) to obtain the average diameter of the positive electrode active material.

[0072] In some embodiments, the positive electrode active material layer further includes carbon materials.

[0073] Carbon materials, especially graphite and graphene with high conductivity and high adsorption efficiency, can improve the overall conductivity of the positive electrode active material layer. Those skilled in the art can add a certain amount and configuration of carbon materials to the positive electrode active material layer to compound the lithium intercalation / deintercalation active material, as long as it does not affect the control of key parameters in the secondary battery described in this application.

[0074] In some embodiments, the positive electrode active material layer in the positive electrode sheet includes a positive electrode active material, a binder, and a conductive agent, wherein the mass percentage of the positive electrode active material in the positive electrode active material layer is 91-99%.

[0075] In some embodiments, the binder 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 adhesion and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorinated polyolefin binders, including but not limited to polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acids, acrylic acid, acrylonitrile, etc.).

[0076] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0077] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 1% to 4%, such as 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4%, or any range formed by any two of the above values.

[0078] In some embodiments, the conductive agent is used to provide conductivity. 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. Exemplary examples of 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.

[0079] In some embodiments, the mass percentage of the conductive agent in the positive electrode active material layer is 0.2% to 3%, such as 0.2%, 0.5%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 3%, or any range formed by any two of the above values.

[0080] In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following method: dispersing the positive active material, binder, and conductive agent evenly in a solvent to prepare a slurry, then coating it onto a current collector and drying it to obtain the positive electrode sheet.

[0081] In some embodiments, the secondary electrode further includes a negative electrode, a separator, and an electrolyte.

[0082] In some embodiments, the electrolyte includes additives, solvents, and lithium salts.

[0083] 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.

[0084] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0085] 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.

[0086] In some embodiments, the additives include, but are not limited to, vinylene carbonate.

[0087] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, which includes a negative electrode active material, including at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0088] The negative electrode active material layer may also contain conductive agents and / or binders and / or dispersants.

[0089] The conductive agent in the negative electrode active material layer is used to provide conductivity. 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 agent includes, but is 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, acetylene black, Ketjen black, etc.

[0090] In some embodiments, the mass percentage of the conductive agent in the negative electrode active material layer is 0.4% to 2%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any range formed by two of the above values.

[0091] The binder in the negative electrode active material layer is used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. For example, the binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and aqueous acrylic resin.

[0092] In some embodiments, the mass percentage of the binder in the negative electrode active material layer is 1% to 4.5%, such as 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, or any range formed by any two of the above values.

[0093] In some embodiments, the negative electrode active material layer in the negative electrode sheet includes a negative electrode active material, a binder, a dispersant, and a conductive agent, and the mass percentage of the negative electrode active material in the negative electrode active material layer is 70-99%.

[0094] More preferably, the negative electrode active material comprises primary particles and secondary particles.

[0095] More preferably, the particle size D of the negative electrode active material v50 It is 5–15 μm.

[0096] More preferably, the particle size D of the single particle v50 The size is 5–10 μm, more preferably 6.5–9 μm.

[0097] More preferably, the particle size D of the secondary particles v50 The value is 10–15 μm, more preferably 11–13.5 μm.

[0098] When the positive electrode active material in the positive electrode sheet is controlled by limiting the proportion of elements, the size of the primary and secondary particles in the negative electrode material is further controlled. Preferably within the above range, the ion diffusion channel of lithium ions during insertion and extraction can be effectively optimized, so that the secondary battery can still achieve a relatively stable insertion and extraction effect at a larger charging rate or higher temperature. The probability of puncture risk caused by lithium dendrite formation in the electrode sheet is lower, and the cycle stability of the secondary battery is better.

[0099] More preferably, the mass ratio of the single-stage particles to the secondary-stage particles is 1:(1-3).

[0100] In some embodiments, the porosity of the diaphragm is 35-50%.

[0101] More preferably, the porosity of the diaphragm is a range of one or both of 35%, 36%, 38%, 40%, 42%, 45%, 48%, and 50%.

[0102] In the secondary battery described in this application, when the porosity of the separator is preferably within the above-mentioned range, it can not only effectively expand the transport channels of lithium ions during transport and improve the dynamic performance of the secondary battery, but also avoid excessively increasing the contact area of ​​by-products generated by the electrode and electrolyte during cycling, so that the cycle performance of the secondary battery is maintained at a better level.

[0103] In some embodiments, the porosity of the diaphragm can be controlled by introducing a pore-forming agent during the preparation process and adjusting the content of the pore-forming agent. However, this is not the only method that can be used by those skilled in the art to control the porosity of the diaphragm.

[0104] In some embodiments, the diaphragm includes a base layer and further includes a ceramic coating and / or a polymer coating disposed on the base layer.

[0105] In some embodiments, the substrate layer includes at least one of polypropylene, polyethylene, and polyethylene terephthalate, and more preferably, the thickness of the substrate layer is 8 to 10 μm.

[0106] In some embodiments, the ceramic coating includes an alumina coating, the thickness of the ceramic coating is 2 to 4 μm, and the ceramic coating can be applied to one side or both sides.

[0107] In some embodiments, the polymer coating includes a PVDF coating with a thickness of 1 to 3 μm, and the PVDF coating may be applied on one side or both sides.

[0108] As mentioned above, in addition to introducing pore-forming agents, those skilled in the art can use other means to adjust the porosity of the membrane. For example, when a composite substrate layer with different components is used, the porosity of the membrane will be different.

[0109] 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:

[0110] Example 1

[0111] A secondary battery, the preparation method comprising the following steps:

[0112] (1) Preparation of lithium iron phosphate: Lithium phosphate, iron phosphate, carbon source glucose and PEG (the mass content of carbon source in the mixture is 5% and the mass ratio of glucose to PEG is 2:1) and dopant precursor magnesium oxide (3264ppm based on the total added materials in the mixture) were dispersed according to the stoichiometric ratio to prepare a mixture. The mixture was then added to ethanol and ball-milled at 1400rpm for 3h with 0.30mm zirconium balls as grinding material at a ball-to-material ratio of 8:1. The mixture was then spray-dried at 110000r / min (inlet air temperature 220℃, outlet air temperature 75℃). The resulting powder was calcined under a nitrogen atmosphere, demagnetized, and then air-jet pulverized under a pressure of 350KPa to obtain lithium iron phosphate. The surface of the material particles has a coating layer.

[0113] (2) Preparation of the positive electrode sheet: A slurry with a solid content of 65.4% was prepared by mixing doped lithium iron phosphate, conductive agent SP, binder polyvinylidene fluoride, and lithium supplementation additive in a mass ratio of 95:1:2:2 using N-methylpyrrolidone as a solvent. This slurry was then coated onto carbon-coated aluminum foil for the current collector, with a coating density set at 450 g / cm³. 2 After drying, cold pressing, and slitting, the material is rolled to obtain the positive electrode sheet, which has a compacted density of 2.6 g / cm³. 3 ;

[0114] (3) Preparation of the negative electrode sheet: The negative electrode material artificial graphite, conductive agent SP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.4:0.6:2.4:0.6. The mixture is vacuum stirred to prepare a slurry, which is then coated onto the current collector copper foil. After coating, drying, cold pressing, slitting, and rolling, the negative electrode sheet is obtained. The areal density of the negative electrode sheet is 207 g / m³. 2 The compacted density is 1.66 g / cm³. 3 .

[0115] (4) Preparation of electrolyte: EC, EMC and DMC are mixed in a mass ratio of 3:4:3 as solvent, and then lithium hexafluorophosphate is added based on the total mass of the electrolyte to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L.

[0116] (5) Separator: The separator is a commercially available product. The base membrane is a 9μm thick PE membrane with an alumina coating on one side with a thickness of 3μm. The outer layers of both sides of the separator are also respectively coated with 2μm thick PVDF.

[0117] (6) The positive electrode, separator and negative electrode are stacked in sequence to form a cell. The 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.

[0118] Examples 2-19, Comparative Examples 1-4

[0119] A secondary battery differs from Example 1 only in the parameters and conditions used in its preparation, as shown in Tables 1 and 2. When the doping element in the positive electrode active material layer is Ti, the precursor used in the preparation is titanium dioxide. Meanwhile, the content of lithium-supplementing additives and conductive agents in the positive electrode active material layer is adjusted such that the content of binder remains unchanged, while the content of lithium iron phosphate particles is increased or decreased to ensure that the total content of all components is 100%.

[0120] Table 1

[0121]

[0122]

[0123]

[0124] Table 2

[0125]

[0126]

[0127] Example of effect

[0128] The secondary batteries obtained in each embodiment and comparative example were tested as follows:

[0129] (1) Secondary battery discharge capacity test: Each secondary battery was tested at 25℃ at a 0.33C rate: the test voltage was 2.5~3.65V, and the battery was left to stand until the single step time was ≥2h, charged at a 0.33C rate until the voltage was ≥3.65V, charged at a constant voltage of 3.65V until the current rate was ≤0.05C, left to stand until the single step time was ≥5min, and discharged at a 0.33C rate until the voltage was ≤2.5V; left to stand until the single step time was ≥5min, charged at a 0.33C rate until the voltage was ≥3.65V, charged at a constant voltage of 3.65V until the current rate was ≤0.05C, left to stand until the single step time was ≥5min, and discharged at a 0.33C rate until the voltage was ≤2.5V. The discharge capacity of the second cycle was recorded as Q1.

[0130] (2) Thermal environment cycle stability test of secondary batteries: The secondary batteries obtained in each embodiment and comparative example were first fixed at 0.3C for two cycles, and then transferred to a 45℃ heat preservation box. They were charged at 1C rate in the empty state until the voltage was ≥3.65V, and charged at 3.65V constant voltage until the current rate was ≤0.05C cutoff. They were left to stand until the single step time was ≥5min, and discharged at 1C rate until the voltage was ≤2.5V. They were charged and discharged at 1C / 1C rate in the working voltage range of 2.5~3.65V until the SOH dropped to 80% of the initial value. The number of capacity retention cycles at 80% SOH was recorded.

[0131] The test results are shown in Table 3.

[0132] Table 3

[0133]

[0134]

[0135] As can be seen from Table 3:

[0136] The secondary battery described in this application has good dynamic capacity. The discharge capacity of the secondary batteries in each embodiment at 0.33C is above 1.5mAh, and the polarization effect inside the battery is small. The number of cycles to maintain 80% capacity at higher temperatures can reach more than 700 cycles. This is mainly due to the synchronous control of the mass ratio 'a' of iron and phosphorus elements in the positive electrode active material layer and the battery self-discharge rate 'b' during the setting of the secondary battery. That is, when a×b = 0.01 to 0.12, the secondary battery can achieve a balance between capacity and cycle stability. In contrast, the secondary batteries obtained in Comparative Examples 1 to 4 are difficult to achieve the same effect.

[0137] Furthermore, when a×b is preferably in the range of 0.03 to 0.075, not only can the lithium intercalation / deintercalation activity of the positive electrode active material particles be better and the overall kinetic capacity be larger, but also the magnetic impurities formed by phosphides have less impact on the secondary battery, the internal polarization effect of the secondary battery is low, and the cycle stability of the secondary battery under thermal environment is also better.

[0138] Furthermore, the iron-phosphorus mass ratio 'a' of the secondary battery can be further optimized to be within the range of 0.96 to 0.99. At this range, the lithium intercalation / deintercalation activity and sufficiency of the primary particles in the positive electrode active material are better, the secondary battery can achieve a larger charge-discharge capacity, and the probability of phosphide formation can be effectively controlled, reducing the polarization effect of the secondary battery and further improving the cycle stability of the secondary battery. The change in the self-discharge rate 'b' of the secondary battery will affect its capacity and capacity stability. When the self-discharge rate 'b' of the secondary battery is further optimized to be within the range of 0.03 to 0.08, the probability of overcharging and over-discharging during charge-discharge cycles is lower, the lithium intercalation / deintercalation stability of the secondary battery is better, and better overall electrochemical performance can be achieved.

Claims

1. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode active material layer, the positive electrode active material layer comprises a positive electrode active material, the positive electrode active material contains iron elements and phosphorus elements; The secondary battery satisfies a×b=0.01-0.12; Wherein a is the mass ratio of iron elements and phosphorus elements in the positive electrode active material layer, and b is the self-discharge rate of the secondary battery.

2. The secondary battery according to claim 1, wherein The a×b=0.03-0.

075.

3. The secondary battery according to claim 1, wherein The a=0.9-1.1, and / or the b=0.01-0.

12.

4. The secondary battery according to claim 3, wherein the negative electrode is a lithium metal electrode. The a=0.96-0.99, and / or the b=0.03-0.

08.

5. The secondary battery according to claim 1, wherein The positive electrode active material layer also contains at least one of iron phosphide and ferrous phosphide.

6. The secondary battery according to claim 5, wherein the negative electrode is a lithium metal electrode. The total content of iron phosphide and ferrous phosphide in the positive electrode active material layer is ≤8ppm.

7. The secondary battery according to claim 1, wherein The positive electrode active material comprises at least one of lithium iron phosphate and doped lithium iron phosphate; the positive electrode active material also contains a lithium supplement additive; and the lithium supplement additive comprises lithium-rich lithium ferrite.

8. The secondary battery according to claim 7, wherein the negative electrode is a lithium metal electrode. The mass percentage content of the lithium supplement additive in the positive electrode active material layer is 0.5-5%.

9. The secondary battery of claim 1, wherein The positive electrode active material layer also contains a doping element; the doping element comprises at least one of magnesium elements and titanium elements.

10. The secondary battery according to claim 9, wherein the negative electrode is a lithium metal electrode. The concentration of the doping element in the positive electrode active material layer is 2000-6000ppm.

11. The secondary battery as described in claim 1, characterized in that, The positive electrode active material is also provided with a coating layer; the coating layer contains carbon, and / or the average thickness of the coating layer is 2-10nm.

12. The secondary battery of claim 1, wherein The secondary battery further contains a negative electrode sheet including a negative electrode active material including single particles and secondary particles; a particle diameter D v50 of the negative electrode active material is 5 to 15 μm.

13. The secondary battery as described in claim 1, characterized in that, The secondary battery also contains a diaphragm, and the porosity of the diaphragm is 35-50%.

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