Positive electrode sheet, battery comprising the same, and electric device

Abstract

The present application relates to a kind of positive pole piece, battery and electric device comprising the positive pole piece, belong to battery technical field.The positive pole piece of the present application includes positive current collector and the positive active material layer of being arranged on the at least one surface of the positive current collector, positive active material layer includes positive active material, positive active material includes lithium manganese iron phosphate and ternary positive electrode material, positive pole piece satisfies: 0.09≤a / b≤5.1, wherein, a is the mass percentage content of Ni element in positive active material layer, unit is %;B is the longitudinal wettability of positive pole piece, unit is %.The present application is by LMFP and ternary positive electrode material compound, and the content of Ni element in positive active material layer and the longitudinal wettability of positive pole piece are reasonably controlled, after positive pole piece is applied in battery, can significantly shorten the charging time of battery in high pressure stage, simultaneously realizes the balance of lithium ion transport rate and electron transport rate in the longitudinal direction of positive pole piece, effectively improves the fast-charging performance and high temperature cycle performance of battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode sheet, a battery containing the positive electrode sheet and an electric device. BACKGROUND

[0002] Lithium manganese iron phosphate (LMFP) material has high voltage platform, large theoretical specific capacity, wide source and environmental friendliness, and is considered as a very promising positive active material of lithium ion battery.

[0003] In the charging process of the LMFP battery, the battery is first charged to the cut-off voltage in a constant current charging mode, and then charged to the cut-off current in a constant voltage charging mode. However, when the battery is charged to a high SOC, the electronic and ionic conductivities of the LMFP itself and the surface interface are poor, which leads to the deterioration of the fast charging performance of the battery. At the same time, under high SOC, the LMFP and the electrolyte are both under high voltage and small current continuous charging, the positive electrode and the electrolyte continuously react, the effective solvent in the electrolyte is continuously consumed, the DCR of the battery continuously increases, the charge-discharge polarization of the LMFP continuously increases, and before the cut-off of the charge-discharge voltage, the positive active material cannot participate in the electrochemical reaction in time, and the released active lithium continuously decreases, resulting in poor cycle performance.

[0004] Therefore, it is necessary to improve the fast charging performance and high temperature cycle performance of the LMFP battery. SUMMARY

[0005] The present application aims to overcome the deficiencies of the prior art and provide a positive electrode sheet, a battery containing the positive electrode sheet and an electric device, so as to improve the fast charging performance and high temperature cycle performance of the LMFP battery.

[0006] To achieve the above-mentioned purpose, in a first aspect, the present application provides a positive electrode sheet, comprising a positive current collector and a positive active material layer arranged on at least one surface of the positive current collector, the positive active material layer containing a positive active material, the positive active material comprising lithium manganese iron phosphate and a ternary positive material, the positive electrode sheet satisfying:

[0007] 0.09≤a / b≤5.1,

[0008] wherein a% is the mass percentage content of Ni element in the positive active material layer;

[0009] b% is the longitudinal wettability of the positive electrode sheet.

[0010] In a second aspect, the present application provides a battery comprising the positive electrode sheet.

[0011] In a third aspect, the present application provides an electric device comprising the battery.

[0012] Compared with the prior art, the beneficial effects of the present application are: by compounding LMFP with ternary positive electrode material, and adjusting the content of Ni element in the positive electrode active material layer and the longitudinal wettability of the positive electrode plate to meet a specific relationship, the positive electrode plate can significantly shorten the charging time of the battery in the high pressure stage after being applied in the battery, and at the same time, the balance of lithium ion transmission rate and electron transmission rate of the positive electrode plate in the longitudinal direction is realized, and the fast charging performance and high temperature cycle performance of the battery are effectively improved. DETAILED DESCRIPTION

[0013] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0014] In the present application, the technical features described in an open manner include both the closed technical solutions consisting of the listed features and the open technical solutions containing the listed features.

[0015] In the present application, if no special description is made, the numerical range is regarded as continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0016] In the present application, the specific dispersion and stirring treatment method is not particularly limited.

[0017] The reagents or instruments used in the present application are not specified by the manufacturer, and are all conventional products that can be obtained by purchase.

[0018] In the present application, the expressions such as "first time", "second time" and the like are not used to limit the number of times.

[0019] Positive electrode plate

[0020] The present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the positive electrode active material comprising lithium iron manganese phosphate and a ternary positive electrode material, the positive electrode plate satisfying:

[0021] 0.09≤a / b≤5.1,

[0022] wherein a% is the mass percentage content of Ni element in the positive electrode active material layer;

[0023] b% is the longitudinal wettability of the positive electrode tab.

[0024] The electronic and ionic conductivities of LMFP itself are poor, and the electronic and ionic conductivities of the ternary positive electrode material are better. This is because the ternary positive electrode material is a layered oxide, and the layered structure makes Li + The transmission path is shorter, and the elements Co and Ni contained therein are more conducive to the electronic conductivity of the material. Doping the ternary positive electrode material in the LMFP can promote the further deintercalation of lithium of the LMFP at high SOC, reduce the time of deintercalation of lithium at the constant voltage stage, thereby reducing the proportion of the constant voltage stage in the charging process and reducing the high-voltage charging time. At the same time, the doping of the ternary positive electrode material can also make up for the electronic conductivity required for the electrochemical reaction of the LMFP at high SOC before the potential cut-off, promote the LMFP to release more active lithium, and improve the cycle performance. In addition, the ternary positive electrode material is mainly composed of large particles, and the physical structure is stronger; the LMFP is mainly composed of small particles, and the material physical structure is looser. After rolling, the stress between the two materials is released, and the compaction density is improved, but this also makes it more difficult for the electrolyte to wet the positive electrode tab, and the lithium ion transmission resistance increases.

[0025] The longitudinal wettability (b%) of the positive electrode sheet refers to the wettability of the electrolyte to the positive electrode sheet in the direction perpendicular to the positive electrode current collector (i.e., in the thickness direction of the positive electrode current collector), which is calculated according to the mass change of the positive electrode sheet before and after being soaked in the electrolyte. During the fast charging process, the high current density requires the efficient transmission of electrolyte ions in a short time. Insufficient longitudinal wettability will increase the electrolyte concentration gradient in the deep layer of the electrode sheet, limiting the ion transmission. When the thickness of the electrode sheet is large, and in the design of high-rate fast charging, the requirement for longitudinal wettability is more stringent, directly affecting the utilization rate of deep active material, and the electrolyte distribution in the entire thickness of the electrode sheet must be uniform to avoid the formation of "dead zones" for ion transmission. The impact of insufficient overall wettability is mainly the uneven reaction in the surface area, while insufficient longitudinal wettability directly "invalidates" or reduces the reaction rate of the active material in the deep layer. The degradation problem in the deep layer is more hidden and difficult to be detected in time by ordinary detection means. Long-term accumulation of degradation will lead to irreversible performance degradation of the battery. The improvement of the longitudinal wettability of the positive electrode sheet, i.e., the improvement of the wettability in the longitudinal direction of the positive electrode sheet, helps to improve the lithium ion transmission rate in the longitudinal direction of the positive electrode sheet and improve the fast charging performance of the battery. However, too high longitudinal wettability of the positive electrode sheet means that the structure of the positive active material layer is too loose in the longitudinal direction, which will lead to poor electronic contact and poor conductivity of the positive active material in the longitudinal direction, and the electrolyte is more prone to side reactions with the positive active material during the cycle process, resulting in increased impedance. The longitudinal wettability of the positive electrode sheet should not be too low, otherwise the wettability in the longitudinal direction of the positive electrode sheet is poor, the lithium ion transmission rate is poor, and the wettability of the electrolyte between the particles in the positive active material layer is insufficient, leading to rapid degradation of the battery cycle performance.

[0026] The value of the longitudinal wettability (b%) of the positive electrode sheet can be adjusted by adjusting the type of ternary positive electrode material and / or LMFP (such as selecting single crystal or polycrystal for the ternary positive electrode material and selecting single crystal or polycrystal for the LMFP), the content of the conductive agent, or adjusting the pressure during the rolling process. Among them, the single crystal of the ternary positive electrode material is a primary particle of the ternary positive electrode material; the polycrystal of the ternary positive electrode material refers to a secondary particle of the ternary positive electrode material composed of multiple primary particles; the single crystal of the LMFP refers to a primary particle without agglomeration or a secondary particle with low agglomeration degree (two or three primary particles aggregated); and the polycrystal of the LMFP refers to an LMFP agglomerate, which is a secondary particle aggregated by more than three (i.e., four or more) primary particles.

[0027] As for the detection method of the longitudinal wettability (b%) of the positive electrode sheet, the present application does not make any limitation, and those skilled in the art can detect the longitudinal wettability (b%) of the positive electrode sheet according to conventional technical means. For example, the longitudinal wettability (b%) of the positive electrode sheet can be detected by the following method:

[0028] After the emptying of the positive electrode tab, dimethyl carbonate is soaked for 2h, and drying is carried out at 80℃. After drying, the size is cut to 1.5*1.5cm, and the mass M0 of the tab is measured (unit g). Then, the tab is hot-pressed together with a polymer film containing a circular hole with a radius r=0.5cm. After hot-pressing, the positive electrode tab is covered with a polymer film, and the above-mentioned circular hole is located at the center of the polymer film. The prepared electrolyte (solvent: EC and EMC, mass ratio of EC: EMC=3:7, containing 1mol / L lithium salt, and lithium salt is lithium hexafluorophosphate) is added to the circular hole of the polymer film on the surface of the tab at a drop rate of 0.5mL / min. The amount of electrolyte added is 2mL. After the electrolyte diffuses for 2min, the mass M1 of the tab is measured (unit g). The longitudinal wettability (b%) of the positive electrode tab is calculated according to the following formula,

[0029] b%= (M1-M0) / M0x100%.

[0030] The material of the polymer film can be selected from any polymer film material that cannot permeate electrolyte and does not react with electrolyte at room temperature and high temperature, such as PET-based-EVA hot melt adhesive composite film.

[0031] The mass percentage of Ni element (a%) in the positive electrode active material layer affects the fast charging performance, cycle performance and safety performance of the battery. When the mass percentage of Ni element in the positive electrode active material layer is too high, the proportion of ternary positive electrode material is too high, the probability of high reactivity between the ternary positive electrode material and the electrolyte increases, the DCR of the battery increases too fast, and the cycle performance, fast charging performance and safety performance all decrease; when it is too low, the proportion of ternary positive electrode material in the positive electrode active material is too low, which cannot effectively improve the ionic conductivity and electronic conductivity of the positive electrode active material, resulting in poor cycle performance and fast charging performance of the battery.

[0032] The value of the mass percentage of Ni element (a%) in the positive electrode active material layer can be adjusted by adjusting the ratio of LMFP to ternary positive electrode material, the molar proportion of nickel element in the structural formula of ternary positive electrode material, etc.

[0033] The detection method of the mass percentage of Ni element (a%) in the positive electrode active material layer is not limited in the present application, and the content of Ni element in the positive electrode active material layer can be detected by conventional technical means by those skilled in the art. For example, the mass percentage of Ni element (a%) in the positive electrode active material layer can be detected by the following method:

[0034] Disassemble the air-electric lithium ion battery to obtain the positive electrode sheet. The positive electrode sheet is soaked in DMC (dimethyl carbonate) at room temperature (25°C, the same below) for 60 min to remove the residual electrolyte and by-products on the surface of the positive electrode sheet. The positive electrode active material layer on the surface of the current collector is scraped off and calcined at 400°C for 3 hours to remove the binder, etc. Then the positive electrode sheet is tapped to obtain the positive electrode active material powder.

[0035] The 0.5 g of the positive electrode active material powder is accurately weighed, dispersed in 20 mL of water, and then 10 mL of nitric acid (HNO3 with a mass percentage of 65%-68%) is added. The dispersion is heated until the positive electrode active material powder is completely dissolved. The solution is diluted with water to 100 mL to obtain a test solution. The test solution is subjected to ICP test. Before the test, a standard solution is prepared. The standard solution of Ni element with a concentration of 1000 mg / L is diluted with deionized water to different concentrations (generally 0, 1 mg / 100 mL, 2 mg / 100 mL, and 3 mg / 100 mL). The relationship between the concentration of the standard Ni element and the peak area is calculated by test and calculation. The linear correlation coefficient of the relationship is above 0.999, which can be used as a normal standard. Then, according to the relationship and the corresponding peak area of the test sample, the content a% of Ni element in the positive electrode active material layer is calculated. The working conditions of the ICP instrument are set as follows: gas flow 0.5 L / min, power 1150 W, selected element Ni, and wavelength 231.604 nm.

[0036] The mass percentage of Ni element in the positive electrode active material layer and the longitudinal wettability of the positive electrode sheet affect the fast charging performance and high-temperature cycle performance of the battery to different degrees, and have certain mutual influence. It is difficult to control a single variable to achieve good fast charging performance and high-temperature cycle performance of the battery. The present application adjusts the mass percentage of Ni element in the positive electrode active material layer and the longitudinal wettability of the positive electrode sheet to meet the above specific relationship by compounding the LMFP with the ternary positive electrode material, so that the charging time of the battery containing the positive electrode sheet is significantly shortened in the high-voltage stage, and the balance of the lithium ion transmission rate and the electron transmission rate of the positive electrode sheet in the longitudinal direction is realized, effectively improving the fast charging performance and high-temperature cycle performance of the battery.

[0037] For example, the value of a / b can be selected as 0.09, 0.1, 0.3, 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.1 or the interval range formed by any two of the above values.

[0038] In one preferred embodiment, the positive electrode plate satisfies 0.17≤a / b≤2.4. The value of a / b is controlled in the range of 0.17-2.4 to make the fast charging performance and high-temperature cycle performance of the battery better.

[0039] In some embodiments, the mass percentage content (a%) of Ni element in the positive electrode active material layer ranges from 0.9% to 47%, such as 0.9%, 1.5%, 2%, 5%, 10%, 20%, 30%, 40%, 43%, 47%, or a range formed by any two of the above values.

[0040] In one preferred embodiment, the mass percentage content (a%) of Ni element in the positive electrode active material layer ranges from 2% to 42%.

[0041] When the mass percentage content (a%) of Ni element in the positive electrode active material layer ranges from 0.9% to 47%, especially from 2% to 42%, the fast charging performance and high-temperature cycle performance are better while having good safety performance.

[0042] In some embodiments, the longitudinal wettability (b%) of the positive electrode plate ranges from 5% to 40%. For example, the b% is 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a range formed by any two of the above values.

[0043] In one preferred embodiment, the longitudinal wettability (b%) of the positive electrode plate ranges from 10% to 35%.

[0044] When the longitudinal wettability (b%) of the positive electrode plate ranges from 5% to 40%, especially from 10% to 35%, the structure density of the positive electrode active material layer is appropriate, not only the positive electrode active material has good electronic contact and good conductivity in the longitudinal direction, but also the side reaction between the electrolyte and the positive electrode active material is less and the impedance is low during the cycle process. At the same time, the longitudinal wettability of the positive electrode plate is better, the lithium ion transmission rate in the longitudinal direction of the positive electrode plate is higher, and the cycle performance of the battery is better, thereby making the fast charging performance and high-temperature cycle performance of the battery better.

[0045] In some embodiments, the mass percentage of the ternary cathode material in the cathode active material is 3% to 93%, and the mass percentage of the lithium manganese iron phosphate in the cathode active material is 7% to 97%. For example, the mass percentage of the ternary cathode material in the cathode active material is 3%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 93%, or any range formed by any two of the above values; and the mass percentage of the lithium manganese iron phosphate in the cathode active material is 7%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 97%, or any range formed by any two of the above values.

[0046] When the mass percentage of the ternary cathode material and the mass percentage of the lithium manganese iron phosphate in the cathode active material are within the above ranges, the electronic and ionic conductivities of the cathode active material are both good, the time for deintercalation of lithium in the constant-voltage stage at high SOC is short, the proportion of the constant-voltage stage in the charging process is small, and the release of active lithium by LMFP at high SOC is well promoted. In addition, the compaction density of the cathode sheet is appropriate, the electrolyte is well infiltrated into the cathode sheet, the lithium ion transmission resistance is small, which helps to improve the fast-charging performance and high-temperature cycle performance of the battery.

[0047] In some embodiments, the molar content of manganese in the lithium manganese iron phosphate, calculated based on the total moles of metal elements other than lithium, is 0.5 to 0.99, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99, or any range formed by any two of the above values. When the molar content of manganese in the lithium manganese iron phosphate, calculated based on the total moles of metal elements other than lithium, is 0.6 to 0.8, the battery can have high energy density, fast-charging performance, and cycle performance.

[0048] In some embodiments, the chemical formula of the lithium manganese iron phosphate is LiMn d Fe e Q f nPO4, wherein, 0 < d < 1, 0 < e < 1, 0 ≤ f < 1, Q refers to a doping element, n refers to the valence of the doping element, and satisfies: 2(d+e)+n·f=2. Wherein, d can be selected as 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99 or an interval range formed by any two of the above values; e can be selected as 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 or an interval range formed by any two of the above values. The lithium manganese iron phosphate can contain no doping element or can contain a doping element. The present application does not limit the type of Q element, for example, Q includes but is not limited to at least one of V, W, Ti, Mg. In one of the embodiments, the surface of the lithium manganese iron phosphate is partially or entirely coated with a carbon layer.

[0049] In the present application, the preparation method of the lithium manganese iron phosphate is not limited, and those skilled in the art can prepare the lithium manganese iron phosphate according to conventional technical means. For example, the preparation method of the lithium manganese iron phosphate comprises the following steps:

[0050] The manganese source, the iron source, the phosphorus source, the lithium source and the first carbon source are mixed, and then subjected to first grinding, first spray drying and first sintering to obtain a lithium manganese iron phosphate precursor;

[0051] The obtained lithium manganese iron phosphate precursor and the second carbon source are mixed, and then subjected to second grinding, second spray drying, second sintering and iron removal by sieving to obtain the lithium manganese iron phosphate.

[0052] For example, in the process of preparing the lithium manganese iron phosphate precursor, the manganese source includes but is not limited to at least one of trimanganese tetraoxide, manganese nitrate, manganese carbonate, manganese oxalate, manganese sulfate, manganese chloride and manganese acetate;

[0053] and / or, the iron source includes but is not limited to at least one of iron phosphate, ferrous phosphate, iron hydroxide, ferrous hydroxide, iron carbonate, ferrous carbonate, iron acetate, ferrous acetate, diiron trioxide, trimanganese tetraoxide, ferrous oxalate and iron oxalate;

[0054] and / or, the phosphorus source includes but is not limited to at least one of lithium dihydrogen phosphate, lithium phosphate, diammonium hydrogen phosphate and ammonium phosphate;

[0055] and / or, the lithium source includes but is not limited to at least one of lithium dihydrogen phosphate, lithium phosphate, lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium citrate and lithium acetate;

[0056] And / or, the first carbon source used includes but is not limited to at least one of glucose (GLC), sucrose, polyethylene glycol (PEG), polyvinyl alcohol. In addition, in the preparation of the LMFP, a certain amount of doping element source (if any) can be mixed with the manganese source, iron source, phosphorus source and lithium source as needed, such as vanadium source (such as di vanadium pentoxide), tungsten source (such as ammonium metatungstate), titanium source (such as titanium oxide), magnesium source (such as magnesium carbonate) and the like, in order to obtain LMFP containing a certain amount of doping elements.

[0057] In some embodiments, the amount of manganese source, iron source, phosphorus source and lithium source used in the preparation of the lithium manganese iron phosphate precursor can be selected to satisfy: the molar amount of Li element: the sum of the molar amount of Mn element and the molar amount of Fe element: the molar amount of P element = (1-1.04):1:(1-1.02) (such as 1:1:1, 1:1:1.02, 1.04:1:1 or the interval range formed by any two of the above values), the molar amount of Mn element is more than 50% of the sum of the molar amount of Mn element and the molar amount of Fe element, such as 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or the interval range formed by any two of the above values. In one preferred embodiment, the molar amount of Mn element is 65% to 80% of the sum of the molar amount of Mn element and the molar amount of Fe element.

[0058] In some embodiments, the amount of carbon residue in the lithium manganese iron phosphate precursor is 0.1wt.% to 0.5wt.%, for example 0.1wt.%, 0.2wt.%, 0.3wt.%, 0.4wt.%, 0.5wt.% or the interval range formed by any two of the above values. In some embodiments, a dispersing agent can also be added during the first grinding process, wherein the dispersing agent can be selected from at least one of water and ethanol.

[0059] In some embodiments, the first grinding can be selected to use ball milling, wherein the ball milling beads can use zirconium oxide and the like. In one embodiment, the first grinding satisfies: ball milling time 0-48h, ball milling times 1-6 times.

[0060] In one embodiment, the pressure of the first spray drying can be selected to be 0.1-1.2MPa, for example 0.1MPa, 0.3MPa, 0.5MPa, 0.8MPa, 1MPa, 1.2MPa or the interval range formed by any two of the above values.

[0061] In one embodiment, the sintering atmosphere of the first sintering is an inert atmosphere, such as a nitrogen atmosphere.

[0062] In one embodiment, the first sintering temperature can be selected from 300-680 °C, for example 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 680 °C, or a range between any two of the foregoing values.

[0063] In one embodiment, the first sintering time can be selected from 2-18 h, for example 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, or a range between any two of the foregoing values.

[0064] For example, the second carbon source can include, but is not limited to, at least one of glucose (GLC), sucrose, polyethylene glycol (PEG), and polyvinyl alcohol.

[0065] In some embodiments, the residual carbon content in the lithium iron manganese phosphate can be selected from 1.4 wt.%-2.6 wt.%, for example 1.4 wt.%, 1.8 wt.%, 2.0 wt.%, 2.2 wt.%, 2.4 wt.%, 2.6 wt.%, or a range between any two of the foregoing values.

[0066] In some embodiments, the second grinding can be selected from ball milling, wherein the ball milling beads can be selected from zirconia, etc. In one embodiment, the second grinding satisfies: ball milling time 0-48 h, and ball milling times 1-6.

[0067] In one embodiment, the second spray drying pressure can be selected from 0.1-1.5 MPa, for example 0.1 MPa, 0.3 MPa, 0.5 MPa, 0.8 MPa, 1 MPa, 1.2 MPa, 1.5 MPa, or a range between any two of the foregoing values.

[0068] In one embodiment, the second sintering atmosphere is an inert atmosphere, for example a nitrogen atmosphere.

[0069] In one embodiment, the second sintering temperature can be selected from 400-800 °C, for example 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 620 °C, 640 °C, 660 °C, 680 °C, 700 °C, 720 °C, 750 °C, 800 °C, or a range between any two of the foregoing values.

[0070] In one embodiment, the second sintering time can be selected from 3-18 h, for example 3 h, 5 h, 7 h, 10 h, 12 h, 15 h, 18 h, or a range between any two of the foregoing values.

[0071] In the preparation of lithium manganese iron phosphate, the carbon source can be added in two steps as described above, or only added in the preparation of lithium manganese iron phosphate using lithium manganese iron phosphate precursor, and no carbon source is added in the preparation of lithium manganese iron phosphate precursor.

[0072] In order to obtain lithium manganese iron phosphate single crystal, centrifugal spraying equipment can be selected for the second spray drying, and a gas crushing process needs to be added after the second calcination is completed. The type of spraying equipment during the first spray drying is not limited, such as centrifugal spraying equipment or two-fluid spraying equipment. In order to obtain lithium manganese iron phosphate polycrystal, two-fluid spraying equipment can be selected for the second spray drying. The type of spraying equipment during the first spray drying is not limited, such as centrifugal spraying equipment or two-fluid spraying equipment.

[0073] In some embodiments, the mole content of nickel element in the ternary positive electrode material, calculated based on the total moles of metal elements other than lithium, ranges from 0.1 to 0.92. For example, the mole content of nickel element in the ternary positive electrode material, calculated based on the total moles of metal elements other than lithium, is 0.1 to 0.92, such as 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.63, 0.65, 0.67, 0.70, 0.73, 0.75, 0.77, 0.80, 0.83, 0.85, 0.87, 0.90, 0.92, or an interval range formed by any two of the above values. When the mole content of nickel element in the ternary positive electrode material, calculated based on the total moles of metal elements other than lithium, is greater than or equal to 0.6, the ternary positive electrode material is a high-nickel material, which is beneficial to obtain higher energy density of the battery.

[0074] In some embodiments, the chemical formula of the ternary positive electrode material is LiNi x Co y Mn (1-x-y) O2, wherein,

[0075] x is 0.1 to 0.92, such as 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.63, 0.65, 0.67, 0.70, 0.73, 0.75, 0.77, 0.80, 0.83, 0.85, 0.87, 0.90, 0.92, or an interval range formed by any two of the above values;

[0076] y is 0.05-0.35, such as 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.220, 0.240, 0.260, 0.280, 0.3, 0.32, 0.35, or an interval range formed by any two of the above values.

[0077] When the molar content of manganese element in the lithium manganese iron phosphate is ≥0.75 (such as 0.75, 0.77, 0.80, 0.83, 0.85, 0.87, 0.90, 0.92, or an interval range formed by any two of the above values) in terms of the total moles of metal elements other than lithium, the b% is ≥10% (such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, or an interval range formed by any two of the above values), so as to better balance the kinetic performance of the ternary positive electrode material itself and the wetting effect of the electrolyte on the positive electrode plate, and make the fast charging performance and the cycle performance of the battery better.

[0078] When the molar content of manganese element in the lithium manganese iron phosphate is ≥0.75 (such as 0.75, 0.77, 0.80, 0.83, 0.85, 0.87, 0.90, 0.92, or an interval range formed by any two of the above values) in terms of the total moles of metal elements other than lithium, preferably, the ternary positive electrode material is polycrystalline, so as to improve the wetting effect of the electrolyte on the positive electrode plate, and improve the fast charging performance and the cycle performance.

[0079] The ternary positive electrode material can be free of the doping element M or can contain the doping element M. The present application does not limit the type of the doping element M, which can be at least one of niobium, titanium, tantalum, tin, and lanthanide metal elements.

[0080] In the present application, the preparation method of the ternary positive electrode material is not limited, and the ternary positive electrode material can be prepared by the skilled person in the art according to conventional technical means. Illustratively, the preparation method of the ternary positive electrode material comprises the following steps:

[0081] The ternary positive electrode material precursor and the lithium source are mixed and then sintered to obtain the ternary positive electrode material.

[0082] The ternary positive electrode material precursor contains Ni, Co, and Mn in a target stoichiometric ratio, and the ternary positive electrode material precursor is one or more of oxides, hydroxides, and carbonates of Ni, Co, and Mn, for example, the ternary positive electrode material precursor is a hydroxide of Ni, Co, and Mn.

[0083] The ternary positive electrode material precursor can be obtained by a method known in the art, for example, by a coprecipitation method, a gel method or a solid phase method. As an example, the preparation method of the ternary positive electrode material precursor comprises the following steps:

[0084] The Ni source, the Co source and the Mn source are dispersed in a solvent to obtain a mixed solution;

[0085] The mixed solution, a strong alkali solution and a complexing agent solution are simultaneously pumped into a stirred reaction kettle, the pH value of the reaction solution is controlled to be 10-13, the temperature in the reaction kettle is controlled to be 25-90°C, and inert gas protection is provided during the reaction; after the reaction is completed, aging, filtration, washing and vacuum drying are performed to obtain a hydroxide containing Ni, Co and Mn, i.e. the ternary positive electrode material precursor.

[0086] In the preparation of the ternary positive electrode material precursor, the Ni source used includes but is not limited to at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate and nickel acetate;

[0087] and / or, the Co source used includes but is not limited to at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate and cobalt acetate;

[0088] and / or, the Mn source used includes but is not limited to at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate and manganese acetate;

[0089] and / or, the alkali of the strong alkali solution used includes at least one of sodium hydroxide and potassium hydroxide;

[0090] and / or, the complexing agent in the complexing agent solution used includes ammonia.

[0091] In the preparation of the ternary positive electrode material precursor, the amount of the Ni source, the Co source and the Mn source used can be selected to satisfy: Ni element molar amount: Co element molar amount: Mn element molar amount = (92-10): (5-35): (3-80). By adjusting the molar amount of Ni element in the Ni source in the total molar amount of Ni element, Co element and Mn element in the Ni source, the Co source and the Mn source, the molar proportion of nickel element in the structural formula of the ternary positive electrode material can be controlled.

[0092] In the preparation of the ternary positive electrode material, the Li source used includes but is not limited to at least one of lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium acetate (CH3COOLi), lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium nitrate (LiNO3).

[0093] In the process of preparing the ternary cathode material, the amount of the lithium source and the ternary cathode material precursor satisfies: the molar amount of Li element: the sum of the molar amounts of Ni, Co and Mn elements = (0.99-1.09):(0.99-1.02).

[0094] The sintering atmosphere for the mixing and sintering can be an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere; the ternary cathode material precursor and the lithium source can be mixed by using a ball mill mixer or a high-speed mixer in the process of preparing the ternary cathode material from the ternary cathode material precursor; and the sintering atmosphere for the sintering is an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere. In order to obtain a single crystal of the ternary cathode material, the sintering temperature is 850-1100°C, and a gas crushing process needs to be added after the sintering is completed. In order to obtain a polycrystal of the ternary cathode material, the sintering temperature is 700-850°C.

[0095] In addition, in the preparation of the ternary cathode material, a certain amount of a source of a doping element (if any) can be dispersed in a solvent together with the Ni source, the Co source and the Mn source to prepare the ternary cathode material precursor, so as to obtain a ternary cathode material containing a certain amount of a doping element, such as a niobium source, a titanium source, a tantalum source, a tin source, a lanthanide metal element source, etc.

[0096] In addition, the ternary cathode material precursor can be subjected to a coating process. Specifically, a coating material is coated on the surface of the cathode active material by using a dry coating method (high-temperature solid-phase method), and the surface of the cathode active material is partially or entirely coated with a coating layer formed by the coating material. The coating layer contains at least one element (hereinafter referred to as “coating element”) selected from the group consisting of aluminum (Al), titanium (Ti), tungsten (W), boron (B), phosphorus (P), cobalt (Co), yttrium (Y) and silicon (Si).

[0097] In some embodiments, the mass percentage of the cathode active material in the cathode active material layer is 94%-97.5%, such as 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5% or a range formed by any two of the above values.

[0098] In addition to the above-mentioned cathode active material, the cathode active material layer further contains a conductive agent and a binder.

[0099] The conductive agent in the positive electrode active material layer is used to provide electrical conductivity, and any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Illustratively, the conductive agent in the positive electrode active material layer includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, fullerenes, and the like, wherein the carbon fibers are, for example, carbon nanofibers and the like; the carbon black is, for example, SP (Super P, hereinafter the same), acetylene black, Ketjen black, and the like.

[0100] In some embodiments, the mass percentage of the conductive agent in the positive electrode active material layer is 1.0% to 2.0%, such as 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, or a range formed by any two of the above values.

[0101] The binder in the positive electrode active material layer is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector, and any binder can be used without particular limitation, as long as it has suitable binder properties and does not significantly cause adverse chemical changes in the battery. Illustratively, the binder in the positive electrode active material layer includes, but is not limited to, fluorine-containing polyolefin-based binders, which include, but are not limited to, polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers, or modified (e.g., carboxylic acid, acrylic acid, acrylonitrile, and the like) derivatives thereof, and the like.

[0102] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 1% to 4.0%, 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%, 4.0%, or a range formed by any two of the above values.

[0103] The positive electrode current collector is not particularly limited in the present application, as long as it has electrical conductivity without causing adverse chemical changes in the battery, and can be, for example, aluminum, nickel, titanium, stainless steel, baked carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like.

[0104] The positive electrode tab of the present application 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 obtain a positive electrode slurry, and the positive electrode slurry is coated on at least one side of the positive electrode current collector, followed by drying, rolling, cutting, and the like to obtain the positive electrode tab. The solvent includes, but is not limited to, at least one of N-methylpyrrolidone (NMP) and deionized water.

[0105] Battery

[0106] The application also provides a battery comprising the positive electrode sheet, the negative electrode sheet and the electrolyte.

[0107] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material.

[0108] The negative electrode active material is not particularly limited in the application. Exemplarily, the negative electrode active material comprises, but is not limited to, natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, SiO f at least one of the following: (0 < f < 2, such as f = 1), silicon-carbon, Li4Ti5O 12 .

[0109] In some embodiments, the mass percentage of the negative electrode active material in the negative electrode active material layer is 94% to 97.5%. For example, the content of the negative electrode active material in the negative electrode active material layer is 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5% or a range formed by any two of the above values.

[0110] The negative electrode active material layer can further comprise a conductive agent and / or a binder.

[0111] The conductive agent in the negative electrode active material layer is used to provide conductivity, and any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplarily, the conductive agent in the negative electrode active material layer comprises, but is not limited to, at least one of the following: carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, fullerenes, wherein the carbon fibers are, for example, carbon nanofibers and the like; the carbon black is, for example, SP, acetylene black, Ketjen black and the like.

[0112] 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 a range formed by any two of the above values.

[0113] The binder in the negative active material layer is used to improve the adhesion between the negative active material particles and the adhesion between the negative active material and the negative current collector, and any binder can be used without particular limitation as long as it has suitable binder properties and does not significantly cause adverse chemical changes in the battery. Exemplary binders in the negative active material layer include, but are not limited to, at least one of carboxymethyl cellulose (CMC), styrene butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and an aqueous acrylic resin.

[0114] In some embodiments, the negative active material layer has a mass percentage of the binder in the range of 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or a range formed by any two of the above values.

[0115] The negative current collector is not particularly limited in the present application as long as it has electrical conductivity without causing adverse chemical changes in the battery, and examples include copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surface-treated with at least one of carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0116] The electrolyte solution can be selected from various electrolyte solutions suitable for batteries in the art. The electrolyte solution includes an electrolyte and a solvent, and the electrolyte can generally include a lithium salt.

[0117] Exemplary lithium salts include, but are not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalato borate (LiDFOB), lithium bisoxalato borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalato phosphate (LiDFOP), and lithium tetrafluorodioxalato phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte solution can be selected in the range of 0.5 to 5 mol / L.

[0118] Exemplarily, the solvent includes, but is not limited to, at least one of ethylene carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), 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), methyl ethyl sulfone (EMS), diethyl sulfone (ESE). The mass percentage of the solvent in the electrolyte can be selected as 70% to 98%.

[0119] In addition, the electrolyte can further comprise an additive. Exemplarily, the additive can include a negative electrode film-forming additive, can also include a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive capable of improving high-temperature performance of the battery, an additive capable of improving overcharge performance of the battery, an additive capable of improving low-temperature performance of the battery, etc.

[0120] The battery can further comprise a separator located between the positive electrode sheet and the negative electrode sheet, for spacing the positive electrode sheet and the negative electrode sheet and preventing the positive electrode sheet and the negative electrode sheet from being in contact and short-circuiting. The separator can be any separator film material suitable for a battery in the art. Exemplarily, the separator includes, but is not limited to, polypropylene,

[0121] at least one of polyethylene.

[0122] Electric device

[0123] The present application further provides an electric device comprising the battery. The battery serves as a power supply for the electric device.

[0124] The electric device refers to any device capable of utilizing electric energy and converting it into mechanical energy, thermal energy, light energy, or other one or more forms of energy, such as an electric motor, an electric heating machine, an electric light source, etc. Specifically, the electric device can include, but is not limited to, a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc. The mobile device can be a mobile phone, a notebook computer, a drone, a sweeping robot, an electronic cigarette, etc. The electric vehicle can be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.

[0125] The present application is further described below with specific examples. It should be noted that the sintering below is performed in an air atmosphere unless otherwise specified:

[0126] Example 1

[0127] The present embodiment provides a lithium ion battery, and the specific preparation method is as follows:

[0128] (1) Preparation of positive electrode sheet

[0129] (1.1) Preparation of LMFP material

[0130] According to the molar ratio of Li, Mn, Fe and P in the chemical formula of LiMn f Fe 1-f PO4(f value in Table 1), Mn3O4, FePO4 and LiH2PO4 are added in a ball mill, then glucose and polyethylene glycol are added, water is used as a dispersant, the raw materials are ground at a speed of 2000 rpm for 12 hours, repeated grinding three times, then first spray drying is carried out under a pressure of 1 MPa, and then sintering is carried out at 400°C for 8 hours to obtain a lithium manganese iron phosphate precursor, wherein the mass ratio of glucose to polyvinyl alcohol is 1:3, and the total amount of glucose and polyvinyl alcohol is 15wt.% of the total amount of Mn3O4, FePO4 and LiH2PO4.

[0131] The obtained LMFP precursor is mixed with glucose and polyethylene glycol, water is used as a dispersant, the raw materials are ground at a speed of 4000 rpm for 18 hours, repeated grinding three times, then second spray drying is carried out under a pressure of 1 MPa, and the spray drying equipment used is shown in Table 1, then sintering is carried out for 10 hours, the sintering temperature is shown in Table 1, and iron is removed by sieving to obtain LMFP, wherein the mass ratio of glucose to polyvinyl alcohol is 1:2, and the total amount of glucose and polyvinyl alcohol is 20wt.% of the total amount of Mn3O4, FePO4 and LiH2PO4.

[0132] (1.2) Preparation of ternary positive electrode material

[0133] According to the molar ratio of Ni, Co and Mn in the chemical formula of LiNi x Co y Mn (1-x-y) O2, nickel sulfate, cobalt sulfate and manganese sulfate (x and y values are shown in Table 1) are weighed and dispersed in water to obtain a mixed solution;

[0134] The obtained mixed solution is delivered into a reaction kettle, oxygen is introduced as a protective gas, NaOH aqueous solution is added as a precipitating agent, and ammonia water is added as a complexing agent, the ammonia water concentration and the amount of addition are adjusted to control the pH of the solution (i.e. the pH of the precursor reaction, see Table 1 for details), the temperature in the reaction kettle is 50°C, the reaction is carried out for 5 hours, and the product is filtered and dried to obtain a ternary positive electrode material precursor;

[0135] The obtained ternary positive electrode material precursor and LiOH are mixed and sintered for 8 h, and the sintering temperature is shown in Table 1, to obtain a primary sintering material, wherein the dosages of the ternary positive electrode material precursor and LiOH satisfy: the molar amount of Li element: the sum of the molar amounts of Ni, Co and Mn elements = 1.05:1.

[0136] (1.3) Preparation of the positive electrode sheet

[0137] The above lithium manganese iron phosphate and the ternary positive electrode material are mixed according to the ratio in Table 1 as the positive electrode active material, and the positive electrode active material, the binder PVDF, the conductive agent SP and the conductive carbon nanotube are mixed according to the mass ratio of 97:1:1.7:0.3, and are dispersed in NMP to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of the aluminum foil, and then is subjected to rolling and cutting to obtain a positive electrode sheet.

[0138] (2) Preparation of the negative electrode sheet

[0139] The artificial graphite negative electrode active material, the conductive agent SP and the binder CMC are mixed according to the mass ratio of 96.4:1:2.6, and are dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of the copper foil, and then is subjected to drying, rolling and cutting to obtain a negative electrode sheet.

[0140] (3) Preparation of the electrolyte

[0141] Vinyl carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed according to the volume ratio of 1:1:1 to obtain a mixed organic solvent, and then the dry lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0142] (4) Preparation of the separator

[0143] A polyethylene (PE) separator is used.

[0144] (5) Assembly and formation

[0145] The above positive electrode sheet, the separator and the negative electrode sheet are stacked in order, and the separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and then are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, and after drying, an electrolyte is injected. After vacuum packaging, standing, formation and shaping, a lithium ion battery is obtained.

[0146] Examples 2-17 and Comparative Examples 1-2

[0147] The examples and comparative examples all provide a lithium ion battery, and the preparation method is close to that of Example 1, and the difference lies in that:

[0148] (S1) In step (1.1), the values of f, the equipment used for the second spray drying, and whether gas crushing is performed after sintering are shown in Table 1 (if gas crushing is not mentioned, it means that gas crushing is not performed);

[0149] (S2) In step (1.2), the values of x and y and the sintering temperature are shown in Table 1;

[0150] (S3) In step (1.3), the ratio of lithium manganese iron phosphate to ternary positive electrode material is adjusted, and the proportion of SP in the total weight of positive electrode active material, PVDF, SP, and carbon nanotubes is adjusted, and the proportion of positive electrode active material in the total weight of positive electrode active material, PVDF, SP, and carbon nanotubes is adjusted accordingly. The specific proportion of ternary positive electrode material in the positive electrode active material and the proportion of SP in the positive electrode active material layer are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] The content a% of Ni element in the positive electrode active material layer and the longitudinal wettability b% of the positive electrode sheet in each example and the comparative example were detected by the following method, and the test results are shown in Table 2:

[0155] The content a% of Ni element in the positive electrode active material layer: disassemble the lithium ion battery to obtain the positive electrode sheet, immerse the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 min, take out and dry at room temperature with humidity ≤ 15%; scrape off the positive electrode active material layer on the surface of the current collector, calcine at 400°C for 3 hours to remove the binder, then tap the electrode sheet to obtain positive electrode active material powder; accurately weigh 0.5 g of positive electrode active material powder, disperse in 20 mL of water, then add 10 mL of nitric acid (HNO3 content is about 37 wt.%), disperse and heat treat until the positive electrode active material powder is completely dissolved, dilute to 100 mL with water to obtain a test solution; perform ICP test on the test solution. Before testing, a standard solution with a Ni element concentration of 1000 mg / L is diluted to different concentrations (0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL) with deionized water, respectively. The relationship between the concentration of the standard and the peak area is calculated by testing and calculating, and the linear correlation coefficient of the relationship is above 0.999, which can be used as a normal standard. Then, according to the relationship and the corresponding peak area of the test sample, the content a% of Ni element in the positive electrode active material layer is calculated, wherein the ICP instrument working condition is set as follows: gas flow 0.5 L / min, power 1150 W, select element test Ni, wavelength 231.604 nm;

[0156] The longitudinal wettability b% of the positive electrode tab: after the positive electrode tab was discharged, it was soaked in dimethyl carbonate for 2 h, and dried at 80°C. The dried tab was cut into a size of 1.5*1.5 cm, and the mass M0 (unit: g) of the tab was measured. The tab was then hot-pressed with a polymer film (PET-based-EVA hot melt adhesive composite film) containing a circular hole with a radius r = 0.5 cm. The surface of the positive electrode tab was covered with the polymer film, and the circular hole was located at the center of the polymer film. The electrolyte (solvent: EC and EMC, mass ratio of EC: EMC = 3:7, containing 1 mol / L lithium salt, and the lithium salt was lithium hexafluorophosphate) was prepared in advance and was dropped onto the circular hole of the polymer film on the surface of the tab at a rate of 0.5 mL / min. The amount of electrolyte added was 2 mL. After the electrolyte diffused for 2 min, the mass M1 (unit: g) of the tab was measured. The longitudinal wettability (b%) of the positive electrode tab was calculated according to the following formula,

[0157] b% = (M1- M0) / M0 x 100%.

[0158] The positive electrode tabs or lithium ion batteries obtained in the examples and comparative examples were tested for performance, and the test results are shown in Table 2. The specific test methods are as follows:

[0159] Fast charging test method: the above positive electrode tab was combined with metal lithium to form a half-cell, and the electrolyte used was the same as in Example 1. The half-cell was charged at 0.33C to an upper limit voltage of 4.25V at 25°C, and then charged at a constant voltage until the current was less than or equal to 0.05C. Then, the half-cell was discharged at 0.33C to 2.5V, which was taken as one cycle. After two cycles of charging and discharging, the discharge capacity of the second cycle was taken as the capacity of the battery. Then, the battery was tested for fast charging at a charge rate of 1C / 2C / 6C, and the discharge rate, the charging cutoff condition, and the discharge cutoff condition were the same as before. The 2C fast charging performance was evaluated by the 2C constant current ratio, which was calculated as follows: 2C constant current ratio = 2C constant current charging capacity / 2C total charging capacity x 100%. The higher the constant current ratio, the better the fast charging performance.

[0160] High-temperature cycle performance test: after the battery was formed and the capacity was fixed, the battery was placed at 45°C for 2 h, then discharged at 0.33C to 2.5V, and then placed for 10 min. Then, the battery was charged at 1C to 4.25V, and the cutoff current was 0.05C at 4.25V. The battery was placed for 10 min, and then discharged at 1C to 2.5V. The 1C discharge capacity of the first cycle was taken as the initial first cycle discharge capacity. The ratio of the discharge capacity after 500 cycles of 1C charging and discharging to the initial first cycle discharge capacity was taken as the discharge capacity retention rate of the battery at high temperature. The cutoff conditions for 1C charging and discharging were the same as above, i.e., the battery was charged to an upper limit voltage of 4.25V, and the cutoff current was less than or equal to 0.05C when the battery was discharged to 2.5V.

[0161] Table 2

[0162]

[0163] The 2C constant current ratio of the battery prepared by each embodiment of the present application is all greater than or equal to 66%, and the capacity retention rate after 500 cycles of 1C at 45℃ is greater than or equal to 66.5%. It can be seen that the battery containing the positive electrode sheet of the present application has excellent fast charging performance and high-temperature cycle performance.

[0164] From the comparison of examples 1-5 and comparative examples 6-10, and the comparison of examples 11-12 and comparative example 13, it can be seen that when the mass percentage content of Ni element in the positive active material layer and the longitudinal wettability of the positive electrode sheet meet the preferred range described in the present application, the fast charging performance and high-temperature cycle performance of the battery are relatively better.

[0165] From the comparison of examples 1-5 and comparative examples 11-12, it can be seen that when the positive electrode sheet meets 0.31≤a / b≤3.9, the fast charging performance and high-temperature cycle performance of the battery are relatively better.

[0166] From the comparison of examples 1-5 and comparative examples 11-12, it can be seen that when the positive electrode sheet meets 0.31≤a / b≤3.9, the fast charging performance and high-temperature cycle performance of the battery are relatively better.

[0167] From the comparison of example 4 and comparative examples 16-17, it can be seen that when the molar content of manganese element is greater than or equal to 0.75, calculated by the total number of moles of metal elements other than lithium in the lithium manganese iron phosphate, and b% is greater than or equal to 10%, the fast charging performance and high-temperature cycle performance of the battery are relatively better.

[0168] Finally, it should be noted that the above examples are only used to illustrate the technical solutions herein and do not limit the scope of protection. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions herein can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions herein.

Claims

1. A positive electrode plate, characterized in that, The cathode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a positive active material, including lithium manganese iron phosphate and ternary positive electrode materials. The positive electrode sheet satisfies the following conditions: 0.09≤a / b≤5.1 Where a% is the mass percentage of Ni element in the positive electrode active material layer; b% represents the longitudinal wettability of the positive electrode sheet, which is detected using the following method: Take the de-energized positive electrode sheet, soak it in dimethyl carbonate for 2 hours, dry it at 80℃, cut it into 1.5*1.5cm size, and weigh the electrode sheet M0g. The electrode sheet and a polymer film containing a circular hole with a radius of 0.5 cm are hot-pressed together, and the surface of the positive electrode sheet is covered with a polymer film after hot pressing. Add the electrolyte dropwise into the round hole at a rate of 0.5 mL / min. The amount of electrolyte added is 2 mL. After the electrolyte has diffused for 2 minutes, weigh the electrode mass M1 g. b% = (M1 - M0) / M0 × 100%. The electrolyte contains 1 mol / L lithium hexafluorophosphate, and the solvents are ethylene carbonate and methyl ethyl carbonate, with a mass ratio of ethylene carbonate: methyl ethyl carbonate = 3:

7.

2. The positive electrode sheet as described in claim 1, characterized in that, The positive electrode sheet satisfies: 0.17≤a / b≤2.

4.

3. The positive electrode sheet as described in claim 1, characterized in that, The range of a% is 0.9% to 47%.

4. The positive electrode sheet as described in claim 3, characterized in that, The range of a% is 2% to 42%.

5. The positive electrode sheet as described in claim 1, characterized in that, The range of b% is 5% to 40%.

6. The positive electrode sheet as described in claim 5, characterized in that, The range of b% is 10% to 35%.

7. The positive electrode sheet as described in claim 1, characterized in that, In the lithium manganese iron phosphate, the molar content of manganese, excluding lithium, ranges from 0.6 to 0.8 based on the total molar number of metal elements; in the ternary cathode material, the molar content of nickel, excluding lithium, ranges from 0.1 to 0.92 based on the total molar number of metal elements.

8. The positive electrode sheet as described in claim 7, characterized in that, In the lithium manganese iron phosphate, when the molar content of manganese is ≥0.75 based on the total molar number of metal elements other than lithium, the b% is ≥10%.

9. The positive electrode sheet as described in claim 7 or 8, characterized in that, In the lithium manganese iron phosphate, when the molar content of manganese is ≥0.75 based on the total molar number of metal elements other than lithium, the ternary cathode material is polycrystalline.

10. A battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1 to 9.

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

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