Positive electrode material, positive electrode sheet, electrochemical device and electric device

Abstract

Disclosed in the present application are a positive electrode material, a positive electrode sheet, an electrochemical device and an electric device. The positive electrode material comprises an aggregate, wherein the aggregate comprises primary particles and carbon nanotubes among the primary particles. The aggregate satisfies the following relationship: 0.09≤a*c / b≤126, and b＝LCNT / d, wherein a is the molar ratio of manganese in the total amount of manganese and iron in the aggregate; c is the porosity of the aggregate, with the unit thereof being %; d is the diameter of the primary particles, with the unit thereof being nm; and LCNT is the average length of the carbon nanotubes in the aggregate, with the unit thereof being nm. In the present application, by inserting the carbon nanotubes inside the aggregate instead of on the surface thereof, and controlling a, b and c to satisfy 0.09≤a*c / b≤126, the electrical properties and structural stability of the positive electrode material can be effectively improved, such that the purpose of internal conduction of the positive electrode material can be achieved, an increase in ion transport impedance is avoided, sufficient internal dynamic transport capacity of particles is ensured, and the conductivity and the cycle performance thereof are also improved.

Description

A positive electrode material, a positive electrode sheet, an electrochemical device, and a power-using device

[0001] Cross-reference to related applications

[0002] This application claims priority to the Chinese patent application No. 202411305487.4, filed on September 19, 2024, and entitled "A positive electrode material, a positive electrode sheet, an electrochemical device, and a power-using device", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present application relates to the field of positive electrode materials, in particular to a positive electrode material, a positive electrode sheet, an electrochemical device, and a power-using device. BACKGROUND

[0004] Lithium manganese iron phosphate (LMFP) positive electrode material has attracted extensive attention in the field of electric vehicles and portable electronic devices due to its high energy density. Compared with lithium iron phosphate (LFP), LMFP improves the theoretical energy density of the battery by introducing manganese elements, making the battery have longer endurance.

[0005] However, LMFP material has poor electrical conductivity and extremely low lithium ion diffusion rate, which affects its discharge capacity, rate performance, and cycle performance. Currently, in order to improve its electrical conductivity, the particle size of LMFP material is often made small. However, when the particle size of LMFP is too small, the surface density cannot be improved. Some methods are to make manganese iron lithium into agglomerates to improve the surface density. However, the currently disclosed manganese iron lithium agglomerates have the problem of poor internal electrical conductivity, which further causes insufficient internal kinetic transmission. SUMMARY

[0006] Therefore, the technical problem to be solved by the present application is to overcome the defect that the agglomerates disclosed in the prior art have poor internal electrical conductivity, which leads to insufficient internal kinetic transmission, thereby providing a positive electrode material, a positive electrode sheet, an electrochemical device, and a power-using device to solve the above problems.

[0007] To achieve the above-mentioned purpose, the present application provides a positive electrode material, comprising an agglomerate of a chemical formula LiMn x Fe y M (1-x-y) PO4, wherein 0.5≤x<1, 0<y≤0.5; the agglomerate comprises primary particles and carbon nanotubes between the primary particles; the agglomerate satisfies the following relationship:

[0008] 0.09≤a*c / b≤126; b=L CNT / d;

[0009] wherein a is the molar ratio of manganese in the total amount of manganese and iron in the agglomerate; c is the porosity of the agglomerate, in %; d is the diameter of the primary particles, in nm; L is the average length of the carbon nanotubes in the agglomerate, in nm. CNT wherein a is the molar ratio of manganese in the total amount of manganese and iron in the agglomerate; c is the porosity of the agglomerate, in %; d is the diameter of the primary particles, in nm; L is the average length of the carbon nanotubes in the agglomerate, in nm.

[0010] In some embodiments, the chemical formula LiMn x Fe y M (1-x-y) PO4, M is a doping element.

[0011] The application also provides a positive electrode sheet comprising the positive electrode material described above.

[0012] The application provides an electrochemical device comprising the positive electrode sheet described above.

[0013] The application provides an electric device comprising the electrochemical device described above.

[0014] The application has the following beneficial effects:

[0015] The positive electrode material disclosed in the application can effectively improve the internal conductivity of the particles by optimizing the structure of the agglomerate of the chemical formula LiMn x Fe y M (1-x-y) PO4, and by inserting carbon nanotubes (CNT) inside the agglomerate instead of just wrapping them on the surface of the agglomerate. This provides a prerequisite for obtaining a battery with better performance. The above-mentioned insertion means that the CNT can be arranged between the primary particles and between several primary particles. In the application, in order to obtain a battery with higher energy density, the higher the molar ratio a of manganese, the better; at the same time, the increase of the value b can improve the conductivity; the increase of the porosity c can improve the wettability of the electrolyte to the material. However, if the molar ratio a of manganese is too high and the porosity c is too large, it will cause poor internal conductivity and increased impedance of the agglomerate, poor internal dynamics of the agglomerate, and affected fast charging performance of the battery; in addition, if the molar ratio of manganese is too high, the stability of the agglomerate structure will be poor, the risk of manganese dissolution will increase, and the cycle performance will be poor; therefore, by comprehensively adjusting the relationship between a, c and b, the internal conductivity of the agglomerate can be improved, the impedance can be reduced, the dynamics of lithium ions can be improved, the stability of the agglomerate structure can be improved, and the cycle performance can be improved, especially when 0.09≤a*c / b≤126, a positive electrode material with better internal conductivity and more stable agglomerate structure can be obtained, which has relatively high energy density while taking into account the fast charging performance and cycle performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the specific embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0017] Figure 1 is an SEM image of the agglomerates in Example 1 of the present application at a magnification of 5.00K, which is 5000.

[0018] Figure 2 is an SEM image of the agglomerates in Example 1 of the present application at a magnification of 20.00K, which is 20000.

[0019] Figure 3 is an SEM image of the agglomerates in Example 1 of the present application at a magnification of 50.00K, which is 50000. DETAILED DESCRIPTION

[0020] The following examples are provided to better further understand the present application and are not limited to the best mode, and do not limit the content and protection scope of the present application. Any person who obtains any product identical or similar to the present application under the inspiration of the present application or by combining the present application with other prior art features falls within the protection scope of the present application.

[0021] The specific experimental steps or conditions are not specified in the examples, and can be performed according to the conventional experimental steps or conditions described in the literature in the art. The reagents or instruments used are not specified by the manufacturer, and are conventional reagent products that can be obtained by purchase.

[0022] "ranges" disclosed herein are defined by both a lower and an upper limit, and the ranges are defined by selecting a lower limit and an upper limit, the selected lower limit and upper limit defining the boundaries of a particular range. Ranges defined by the limits can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to define a range. For example, if a range of 50% to 90% and a range of 60% to 80% are listed for a particular parameter, it is understood that a range of 50% to 80% and a range of 60% to 90% are also contemplated. In addition, if a minimum range value of 1 and 2 are listed, and if a maximum range value of 3, 4 and 5 are listed, then the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise stated, a numerical range "a to b" indicates a shorthand way of describing all the individual ranges between a and b, where a and b are both real numbers. For example, the numerical range "0.5 to 4" indicates that all the real numbers between "1 to 4" have been listed herein, e.g., 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, etc., and "0.5 to 4" is just a shorthand way of describing these numerical combinations.

[0023] The present application provides a positive electrode material, comprising agglomerates of chemical formula LiMn x Fe y M (1-x-y) PO4, wherein 0.5≤x<1, 0 CNT y≤0.5; M is a doping element, x+y≤1; the agglomerates comprise primary particles and carbon nanotubes between the primary particles; the agglomerates satisfy the following relationship:

[0024] 0.09≤a*c / b≤126;

[0025] Specifically, a is the molar ratio of manganese in the total amount of manganese and iron in the agglomerates;

[0026] c is the porosity of the agglomerates, in %;

[0027] b=L CNT / d; wherein d is the diameter of the primary particles, in nm; L CNT is the average length of the carbon nanotubes CNT in the agglomerates, in nm; by comprehensively adjusting the relationship between the average length of the carbon nanotubes CNT and the diameter of the primary particles, the internal conductivity of the agglomerates is improved, and the internal kinetic performance of the agglomerates is improved.

[0028] Agglomerates: refers to secondary particles in an agglomerated state formed by two or more primary particles, and the agglomerated state of the primary particles can be observed by using a scanning electron microscope (SEM) image.

[0029] Porosity: the porosity inside the agglomerate, which reflects the degree of accumulation of primary particles inside the agglomerate.

[0030] The present application optimizes the structure of the agglomerate of the chemical formula LiMn x Fe y M (1-x-y) By interpenetrating the carbon nanotubes inside the agglomerate instead of just wrapping them on the surface of the agglomerate, the conductive performance inside the particles can be effectively improved, thereby providing a prerequisite for obtaining a battery with better performance.

[0031] In order to obtain a battery with higher energy density, the higher the value of the manganese molar ratio a is, the better; at the same time, the increase of the value of b can improve the conductivity; the increase of the value of the porosity c can improve the wettability of the electrolyte to the material. However, if the manganese molar ratio a is too high and the porosity c is too large, it will cause poor conductivity, increased impedance, poor kinetic performance inside the agglomerate, and the fast charging performance of the battery will be affected. In addition, if the value of the manganese molar ratio a is too high, the stability of the agglomerate structure will be poor, the risk of manganese dissolution will increase, and the cycle performance will be poor. By controlling the value of b to be larger, the CNTs are interpenetrated inside the agglomerate, which not only can improve the conductivity, reduce the impedance, and improve the kinetic performance of lithium ions, but also can stabilize the agglomerate structure, improve the structural stability, and increase the cycle stability of the battery.

[0032] Therefore, by comprehensively adjusting the relationship between a, c and b, the present application can improve the conductivity inside the agglomerate, reduce the impedance, and improve the kinetic performance of lithium ions; at the same time, it can improve the structural stability of the agglomerate and improve the cycle performance; especially by controlling 0.09≤a*c / b≤126, a positive electrode material with better internal conductivity and more stable agglomerate structure can be obtained, which effectively balances the fast charging performance and cycle performance of the battery.

[0033] Specifically, the higher the manganese molar ratio a is, the worse the conductivity inside the agglomerate is, and at the same time, the structural stability of the agglomerate becomes poor, the risk of manganese dissolution increases, and the cycle performance becomes poor; the larger the porosity c is, the larger the gap between the primary particles and the particles is, and therefore the CNTs are needed to electrically connect and improve the structural stability of the agglomerate; that is, when the manganese molar ratio is large and / or the porosity is large, the value of b can be adjusted, that is, the relationship between the diameter of the primary particles and the diameter of the L CNT The present application researches and finds that when a*c / b is higher than 126, there may be problems of slow kinetic transmission rate due to poor internal conductivity of the material, and poor structural stability of the agglomerate, increased risk of manganese dissolution, and attenuated cycle performance. By controlling a*c / b to be no more than 126, the present application effectively balances the fast charging performance and cycle stability of the battery.

[0034] Meanwhile, the present application also finds that the value of a*c / b should not be too low. When the value of b is large, although the internal conductivity of the agglomerate can be improved, a too high value of b indicates that the length of CNT is too long, and / or the primary particle size is too small, and the CNT or primary particles in the agglomerate are too tightly packed, causing the lithium ion transmission to be blocked, the ion impedance to increase, and the kinetic performance to deteriorate. A too low value of manganese molar ratio a affects the average voltage, resulting in a low energy density of the battery. A too low value of porosity c results in a high degree of tightness between the primary particles, poor electrolyte impregnation efficiency, affected lithium ion transmission rate, and decreased fast charging performance of the battery. When a*c / b is lower than 0.09, there will be problems of low energy density and poor fast charging performance. By controlling a*c / b to be not less than 0.09, the present application effectively achieves the purpose of balancing the fast charging performance and energy density of the battery.

[0035] In summary, by controlling a*c / b within a certain range (0.09≤a*c / b≤126), a positive electrode material with better internal conductivity can be obtained, while the fast charging performance and energy density of the battery are balanced. In the present application, the value of a*c / b can be any value between 0.09 and 126, for example: 0.09, 0.1, 0.3, 0.5, 1, 3, 5, 7, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 126, etc.

[0036] Further, by controlling the value of a*c / b within a more preferred range of 0.6-20, the internal kinetic transmission performance of the present application is more optimal.

[0037] The present application further optimizes the molar ratio value a of manganese in the total amount of manganese and iron in the agglomerate; a too low value of a results in a low manganese content, affecting the average voltage and resulting in a low energy density; a too high value of a results in a too high manganese content, leading to poor kinetic performance, and poor structural stability of the agglomerate, increased risk of manganese dissolution, and poor cycle performance; therefore, in the present application, a ranges from 0.5 to 0.96, and is preferably from 0.6 to 0.85. For example, the a is controlled to be 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.96, etc.

[0038] The present application further optimizes the value of the porosity c of the agglomerate; a too low value of c results in a too low porosity, affecting the electrolyte impregnation and contact between the primary particles, and a too high value of c results in a too high porosity, leading to poor conductive contact between the particles; therefore, in the present application, the porosity c of the agglomerate is in %, and the value of c is 5-35, and is preferably 10-25; for example, the porosity c is controlled to be 5, 10, 12, 15, 20, 25, 30, 35, etc.

[0039] The value of the porosity c of the agglomerate is affected by the parameters of the spray drying process, such as the solid content, flow rate, pressure, etc. In the present application, the agglomerate porosity can be adjusted by adjusting the solid content.

[0040] The present application further optimizes the value range of b, i.e. controls the value of b to be 0.25-50, preferably 1-13, which can further improve the internal kinetic transmission capacity. Specifically, if the CNT length is too long and / or the primary particle size is too small, the CNTs or primary particles in the agglomerate are too tightly packed, the lithium ion transmission is blocked, the ion impedance increases, the kinetic performance deteriorates, and the cycle performance also deteriorates; while if the CNT length is too short and the primary particle size is too large, the function of improving the internal conductivity of the agglomerate cannot be achieved; therefore, when the primary particle diameter is relatively small, the primary particles in the agglomerate are packed relatively tightly, the distance between the two adjacent primary particles is small, L CNT can be controlled to be smaller, by controlling the value of L CNT / d in the range of 0.25-50, so that it can achieve the purpose of internal conductivity while avoiding the increase of ion transmission impedance, ensuring the sufficient internal kinetic transmission capacity of the particles and improving the conductivity.

[0041] Further, the L CNT is 100nm-1000nm, preferably 200nm-800nm; for example, the L CNT is 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, etc.

[0042] Further, the diameter d of the primary particle is 20nm-400nm, preferably 40nm-200nm; for example, the diameter d of the primary particle is 20nm, 30nm, 40nm, 50nm, 80nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, etc.

[0043] The above L CNT length and the diameter d of the primary particle are related to the process conditions of the preparation of the positive electrode material. In the present application, for example, the corresponding process conditions such as grinding time and rotation speed can be adjusted to achieve the purpose of adjusting the L CNT length and the diameter d of the primary particle.

[0044] The present application also discloses a positive electrode sheet comprising the above positive electrode material.

[0045] Further, the positive electrode sheet further comprises a positive electrode conductive agent, the positive electrode conductive agent is selected from conductive carbon black, carbon nanotube; the carbon nanotube comprises one or more of multi-walled carbon nanotube MWCNT, single-walled carbon nanotube SWCNT, carbon nanotube fiber VGCF, and the conductive carbon black can be selected as KB. Preferably, the positive electrode conductive agent is selected from conductive carbon black. In the present application, the conductive carbon black is preferably added, and the gaps between the agglomerates can be filled by the conductive carbon black, so that better effects of ensuring infiltration and conductive transmission can be achieved.

[0046] Hereinafter, specific embodiments of a positive electrode material, a positive electrode sheet, an electrochemical device and a power-using device of the present application are specifically described with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed description is omitted. For example, there are cases where detailed description of matters known well, repeated description of substantially the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0047] [Electrochemical device]

[0048] The electrochemical device in the present application is a secondary battery, also known as a rechargeable battery or a storage battery, which refers to a battery that can continue to be used by activating the active material through charging after the battery is discharged.

[0049] Generally, the secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. During the charging and discharging process of the battery, active ions (such as lithium ions) are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent short circuit between the positive electrode and the negative electrode, and at the same time to allow the active ions to pass through. The electrolyte is between the positive electrode sheet and the negative electrode sheet, mainly to conduct the active ions.

[0050] As an example, the preparation process of the secondary battery is as follows: the positive electrode sheet, the separator and the negative electrode sheet are stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, then the bare cell is obtained by winding; the bare cell is placed in an outer packaging shell, dried and then injected with electrolyte, and then subjected to processes such as vacuum packaging, standing, formation and shaping to obtain a secondary battery.

[0051] [Positive electrode sheet]

[0052] The positive electrode sheet generally comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, and the positive electrode film layer comprises a positive electrode active material, which is any one of the positive electrode materials disclosed in the present application.

[0053] As an example, the positive electrode material in the present application comprises a chemical formula LiMn xFe y M (1-x-y) PO4 agglomerates; the agglomerates include primary particles and carbon nanotubes between the primary particles.

[0054] The preparation method of the above positive electrode material is as follows: a chemical formula LiMn x Fe y M (1-x-y) PO4 precursor slurry is obtained, and then the precursor slurry is mixed with a carbon-coated source and a carbon nanotube conductive slurry for grinding and spray drying to obtain dry powder; the dry powder is sintered in a protective atmosphere, and the lithium manganese iron phosphate positive electrode material with CNTs formed in the agglomerates is obtained after cooling.

[0055] The process of preparing the precursor slurry by solid-phase grinding to obtain the positive electrode material is as follows: the required lithium source, manganese source, iron source and phosphorus source are weighed according to the molar ratio and added to deionized water, and a precursor slurry is obtained by first grinding; the carbon-coated source and the carbon nanotube conductive slurry are added to the precursor slurry and mixed uniformly with the precursor slurry, and second grinding is performed; the solid content is adjusted and spray drying is performed to obtain dry powder; the dry powder is sintered in a protective atmosphere, and the lithium manganese iron phosphate positive electrode material with CNTs formed in the agglomerates is obtained after cooling.

[0056] The process of preparing the precursor slurry by hydrothermal reaction to obtain the positive electrode material is as follows: the required manganese source and iron source for synthesizing the precursor slurry are weighed according to the molar ratio, added to a reaction kettle, deionized water is added, and reaction is performed; the lithium source and the phosphorus source are added to the reaction kettle, and after the addition is completed, the reaction is continued; the pH is adjusted and the temperature is raised to the reaction temperature, and after the reaction, a reaction slurry is obtained; the reaction slurry is filtered by a centrifuge and washed with water, and the solid composition after washing is the lithium manganese iron phosphate intermediate powder; the lithium manganese iron phosphate intermediate powder is first ground to obtain the precursor slurry. Finally, the precursor slurry, the carbon-coated source and the carbon nanotube conductive slurry are added to a mixing tank, deionized water is added after mixing, and then the mixture is fed into a sand mill for second grinding to obtain a slurry; the slurry is transferred to a spray pre-storage tank, the solid content of the slurry is adjusted, and spray drying is performed to obtain dry powder; the dry powder is sintered in a protective atmosphere, and the lithium manganese iron phosphate positive electrode material with CNTs formed in the agglomerates is obtained after cooling.

[0057] Meanwhile, during the preparation of the precursor slurry, a certain amount of doping elements (i.e., a chemical formula LiMn x Fe y M (1-x-y)M in PO4) is mixed with a manganese source, an iron source, a phosphorus source, and a lithium source, and the doping element is one or more of vanadium, tungsten, titanium, magnesium, etc., and the source of the doping element is, for example, a vanadium source (vanadium pentoxide), a tungsten source (ammonium metatungstate), a titanium source (titanium oxide), a magnesium source (magnesium carbonate), etc., to obtain a precursor slurry containing a certain amount of doping elements.

[0058] The preparation method of the positive electrode material can be used to prepare the positive electrode material, which comprises LiMn x Fe y M (1-x-y) The agglomerates of CNTs are formed inside the positive electrode material of the chemical formula LiMn

[0059] The lithium source, the manganese source, the iron source, the phosphorus source, and the doping element of the positive electrode material are selected from one or more of lithium carbonate, lithium hydroxide, manganese carbonate, manganese oxide, manganese oxalate, iron phosphate, magnesium oxide, lithium dihydrogen phosphate, ferrous oxalate, and diammonium hydrogen phosphate.

[0060] In the preparation method of the positive electrode material, the diameter d of the primary particles is related to the time and the rotation speed of the first grinding, and the length L of the carbon nanotubes is related to the rotation speed and the time of the second grinding. CNT The diameter d of the primary particles and the length L of the carbon nanotubes can be adjusted by changing the parameters of the first grinding and the second grinding. CNT .

[0061] The grinding time of the first grinding can be selected from 1-4h, for example, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, etc.

[0062] The grinding time of the second grinding can be selected from 0.5-4h, for example, 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, etc.

[0063] The porosity inside the agglomerates in the positive electrode material obtained by spray drying is affected by the solid content. Therefore, the solid content of the slurry for spray drying can be selected from 20%-60%, for example, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%.

[0064] In the hydrothermal reaction method, the pH value after adjusting the pH can be selected from 5-7, for example, 5, 6, or 7.

[0065] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector; and subjecting the positive electrode slurry to drying, rolling, cutting, and the like to obtain the positive electrode sheet.

[0066] In the present application, the binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector, and in the present application, the binder can be a conventional choice in the battery field. Specifically, the binder can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), or sodium alginate.

[0067] 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 for example, stainless steel, aluminum, nickel, titanium, baked carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like can be used.

[0068] [Negative electrode sheet]

[0069] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer including a negative electrode active material. As an example, the negative electrode active material includes one or more of natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, soft carbon, silicon, SiOx, silicon-carbon, Li4Ti5O 12

[0070] In some embodiments, the negative electrode film layer can also optionally include a conductive agent; as an example, the conductive agent can be selected from conductive carbon black.

[0071] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector; and subjecting the negative electrode slurry to drying, rolling, cutting, and the like to obtain the negative electrode sheet.

[0072] In some embodiments, as an example, the negative electrode current collector is a copper foil.

[0073] [Electrolyte]

[0074] ​The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application, and can be selected as desired. As an example, the electrolyte of the present application can be any of various electrolytes known to be suitable for use in electrochemical energy storage devices. The electrolyte includes an electrolyte and a solvent, and the electrolyte can generally include a lithium salt.

[0075] Specifically, the lithium salt includes 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 difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte can be 0.5 to 5 mol / L.

[0076] Specifically, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0077] [Separator]

[0078] In some embodiments, a secondary battery further includes a separator. The type of separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0079] In some embodiments, the separator is, for example, one of PP, PE, and PP / PF.

[0080] Example 1

[0081] An electrochemical device, for example, a secondary battery, includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0082] 1. Preparation of a positive electrode sheet

[0083] The specific preparation process of the positive electrode sheet is: obtaining a positive electrode active material, mixing the positive electrode active material, conductive carbon black, binder PVDF and carbon nanotube conductive agent according to a mass ratio of 96.5:1:2:0.5, adding a solvent NMP, stirring under the action of a vacuum stirrer until the system is uniform, obtaining a positive electrode slurry with a solid content of 55%; coating the positive electrode slurry on an aluminum foil to obtain a double-sided coated positive electrode sheet; and then drying, rolling, and cutting to obtain the positive electrode sheet. The SEM images of the agglomerates in the positive electrode sheet under different magnifications are shown in FIGS. 1-3.

[0084] The above process for obtaining the positive electrode active material is:

[0085] 1.1, solid phase grinding: first, based on the synthesis of lithium manganese iron phosphate material LiMn 0.9 Fe 0.1 PO4, the molar ratio of lithium source, manganese source, iron source and phosphorus source required by the theory is required, lithium carbonate, manganese oxide, iron phosphide and diammonium hydrogen phosphate are weighed respectively, and the raw materials are placed in the pre-mixed tank of the sand mill, mixed with deionized water to reach a solid content of 20%, and 0.5mm zirconium oxide balls are used as grinding media. The first grinding time is 0.5h, the grinding speed is 650r / min, and the precursor slurry is obtained.

[0086] 1.2, spray drying: add 20% of the solid mass of the precursor slurry as a carbon source, at the same time, add 0.6% of the solid mass of the precursor slurry as a carbon nanotube conductive slurry (the solid content of the carbon nanotube conductive slurry is 4%), mix the above-mentioned precursor slurry uniformly, and then grind under the condition of a grinding speed of 500r / min for 4h, adjust the solid content to 21.4%, and set the spray drying speed to 65L / min for spray drying.

[0087] 1.3, high-temperature sintering: the dried powder is transferred to a tube-type sintering furnace, heated to 650℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, sintered at 650℃ for 6h, and naturally cooled to room temperature to obtain lithium manganese iron phosphate positive electrode material with CNT formed inside the agglomerates.

[0088] 2, preparation of the negative electrode sheet

[0089] The specific preparation process is: mixing graphite, conductive carbon black, thickening agent CMC and binder SBR according to a mass ratio of 96.4:1:1.2:1.4, dispersing in deionized water to obtain a negative electrode slurry; coating the negative electrode slurry on a copper foil to obtain a double-sided coated electrode sheet; and then rolling, cutting to obtain the negative electrode sheet.

[0090] 3, obtaining the separator film

[0091] The separator film in this embodiment is a PP separator film.

[0092] 4. Preparation of electrolyte

[0093] The specific preparation process is as follows: ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, then the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0094] 5. Assembly of secondary battery

[0095] The specific process is as follows: the above positive electrode sheet, separator and negative electrode sheet are stacked in order, with the separator between the positive and negative electrode sheets to play a separating role, then the bare battery is obtained by winding; the bare battery is placed in an outer packaging shell, dried and then injected with electrolyte, and then subjected to vacuum packaging, standing, formation, shaping and other processes to obtain a lithium ion secondary battery.

[0096] Example 2

[0097] A secondary battery, which is different from example 1 only in that the preparation process of the positive electrode active material is different, and other aspects are the same as example 1, specifically:

[0098] The total moles of manganese and iron in the precursor slurry are the same, the manganese source and iron source are added in a molar ratio of 5:5, the first grinding time is 4h, the first grinding speed is 650r / min, the second grinding time is 1h, the second grinding speed is 300r / min, and the slurry solid content is adjusted to 45.5% before spraying.

[0099] Example 3

[0100] A secondary battery, which is different from example 1 only in that the preparation process of the positive electrode active material is different, and other aspects are the same as example 1, specifically:

[0101] 1.1, hydrothermal reaction method: first, based on the theoretical required molar ratio of synthesizing lithium manganese iron phosphate material LiMn 0.8 Fe 0.2 PO4, lithium carbonate, manganese oxide, iron phosphide and diammonium hydrogen phosphate are weighed respectively, manganese oxide and iron phosphide are added to the reaction kettle, deionized water is added and reacted for 1h, then lithium carbonate and diammonium hydrogen phosphate are added to the reaction kettle, and after the addition is completed, the reaction is continued for 1h. Add an appropriate amount of 5% sodium hydroxide solution to adjust the pH to 6, increase the temperature of the reaction kettle to 90℃, and react for 6h. The reaction slurry is filtered by centrifuge, washed with water, and the washed solid composition is LiMn 0.8 Fe 0.2PO4 intermediate powder. LiMn 0.8 Fe 0.2 PO4 intermediate powder was added into deionized water to reach 20% solid content, 0.5mm zirconia ball was used as grinding medium for the first time grinding, the first time grinding time was 2.0h, grinding speed was 400r / min, precursor slurry was obtained.

[0102] 1.2, spray drying: glucose was weighed according to 20% of the solid mass in the precursor slurry, CNT conductive slurry (solid content of CNT conductive slurry was 4%) was weighed according to 0.6% of the solid mass, and then added into the mixing tank, deionized water was added and mixed, and then transferred into the sand mill, the sand mill speed was adjusted to 400r / min, and the slurry was ground for 3h, then the slurry was transferred into the spray pre-storage tank, the solid content of the slurry was adjusted to 30%, and the spray drying speed was set to 65L / min for spray drying.

[0103] 1.3, high temperature sintering: the dried powder was transferred into a tube sintering furnace, and heated to 650℃ at a heating rate of 5℃ / min under nitrogen atmosphere, sintered at 650℃ for 6h, and naturally cooled to room temperature, to obtain LiMn 0.8 Fe 0.2 PO4 powder.

[0104] Example 4

[0105] A secondary battery, the difference from example 1 is only that the process of obtaining the positive active material is different in this example, and other aspects are completely the same as example 1, specifically:

[0106] The total mole amount of manganese and iron in the precursor slurry was the same, the manganese source and the iron source were added according to a mole ratio of 6:4, the first time grinding time was 3h, the first time grinding speed was 680r / min, the second time grinding time was 2h, the second time grinding speed was 400r / min, and the slurry solid content before spraying was adjusted to 40.5%.

[0107] Example 5

[0108] A secondary battery, the difference from example 1 is only that the process of obtaining the positive active material is different in this example, and other aspects are completely the same as example 1, specifically:

[0109] The total mole amount of manganese and iron in the precursor slurry was the same, the manganese source and the iron source were added according to a mole ratio of 3:1, the first time grinding time was 3h, the first time grinding speed was 650r / min, the second time grinding time was 3h, the second time grinding speed was 400r / min, and the slurry solid content before spraying was adjusted to 38.5%.

[0110] Example 6

[0111] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the other aspects are identical to those of Example 1, in particular:

[0112] The total molar amount of manganese and iron in the precursor slurry is the same, the manganese source and the iron source are added according to a molar ratio of 6:4, the first grinding time is 2.8h, the first grinding speed is 650r / min, the second grinding time is 1.5h, the second grinding speed is 350r / min, and the slurry solid content is adjusted to 42.8% before spraying.

[0113] Example 7

[0114] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the other aspects are identical to those of Example 1, in particular:

[0115] The total molar amount of manganese and iron in the precursor slurry is the same, the manganese source and the iron source are added according to a molar ratio of 24:1, the first grinding time is 3h, the first grinding speed is 680r / min, the second grinding time is 1h, the second grinding speed is 380r / min, and the slurry solid content is adjusted to 23.2% before spraying.

[0116] Example 8

[0117] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the other aspects are identical to those of Example 1, in particular:

[0118] The total molar amount of manganese and iron in the precursor slurry is the same, the manganese source and the iron source are added according to a molar ratio of 17:3, the first grinding time is 2.5h, the first grinding speed is 500r / min, the second grinding time is 3.8h, the second grinding speed is 500r / min, and the slurry solid content is adjusted to 28.5% before spraying.

[0119] Example 9

[0120] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the other aspects are identical to those of Example 1, in particular:

[0121] The total molar amount of manganese and iron in the precursor slurry is the same, the manganese source and the iron source are added according to a molar ratio of 13:7, the first grinding time is 0.5h, the first grinding speed is 500r / min, the second grinding time is 4h, the second grinding speed is 550r / min, and the slurry solid content is adjusted to 20% before spraying.

[0122] Example 10

[0123] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the details are as follows:

[0124] The total molar amount of manganese and iron in the precursor slurry is the same, and the manganese source and the iron source are added according to a molar ratio of 11:9. The first grinding is performed for 2 h at a speed of 400 r / min. The second grinding is performed for 4 h at a speed of 500 r / min. The solid content of the slurry before spraying is adjusted to 47.5%.

[0125] Example 11

[0126] A secondary battery, which is identical to that of Example 1 except that the preparation process of the positive active material is different in this example, and the details are as follows:

[0127] The total molar amount of manganese and iron in the precursor slurry is the same, and the manganese source and the iron source are added according to a molar ratio of 49:1. The first grinding is performed for 1 h at a speed of 300 r / min. The second grinding is performed for 2 h at a speed of 350 r / min. The solid content of the slurry before spraying is adjusted to 15.2%.

[0128] Example 12

[0129] A secondary battery, which is identical to that of Example 6 except that the preparation process of the positive active material is different in this example, and the details are as follows:

[0130] The total molar amount of manganese and iron in the precursor slurry is the same, and the manganese source and the iron source are added according to a molar ratio of 24:1. During the addition, 2000 ppm of a doping element accounting for the total molar amount of manganese and iron is mixed with the manganese source, the iron source, the phosphorus source, and the lithium source. The source of the doping element is vanadium pentoxide. The first grinding is performed for 3 h at a speed of 680 r / min. The second grinding is performed for 1 h at a speed of 380 r / min. The solid content of the slurry before spraying is adjusted to 20.4%.

[0131] Example 13

[0132] A secondary battery, which is identical to that of Example 1 except that the conductive agent used in the positive electrode sheet is replaced by conductive carbon black in this example, and the preparation process of the positive active material is different, and the details are as follows:

[0133] The total molar amount of manganese and iron in the precursor slurry is the same, and the manganese source and the iron source are added according to a molar ratio of 41:9. The first grinding is performed for 2 h at a speed of 400 r / min. The second grinding is performed for 3 h at a speed of 400 r / min. The solid content of the slurry before spraying is adjusted to 26.4%.

[0134] Comparative Example 1

[0135] A secondary battery, which is different from Example 1 only in that the obtaining process of the positive electrode active material is different in the present comparative example, is completely the same as Example 1, and the specific process is as follows:

[0136] Compared with Example 1, the molar ratio of manganese source and iron source of lithium manganese iron phosphate precursor is 24:1, the first grinding time is 0.5h, the first grinding speed is 650r / min, the second grinding time is 4h, the second grinding speed is 500r / min, and the solid content of the slurry before spraying is adjusted to 19.4%.

[0137] Comparative Example 2

[0138] A secondary battery, which is different from Example 1 only in that the obtaining process of the positive electrode active material is different in the present comparative example, is completely the same as Example 1, and the specific process is as follows:

[0139] Compared with Example 1, the molar ratio of manganese source and iron source of lithium manganese iron phosphate precursor is 6:4, the first grinding time is 4h, the first grinding speed is 650r / min, the second grinding time is 0.8h, the second grinding speed is 280r / min, and the solid content of the slurry before spraying is adjusted to 49.5%.

[0140] Comparative Example 3

[0141] A secondary battery, which is different from Example 1 only in that the obtaining process of the positive electrode active material is different in the present comparative example, is completely the same as Example 1, and the specific process is as follows:

[0142] 1. Preparation of positive electrode sheet

[0143] The specific preparation process of the positive electrode sheet is as follows: obtaining the positive electrode active material, mixing the positive electrode active material, conductive carbon black, binder PVDF, and carbon nanotube conductive agent according to the mass ratio of 96.5:1:2:0.5, adding solvent NMP, and stirring under the action of a vacuum stirrer until the system is uniform to obtain a positive electrode slurry with a solid content of 55%; coating the positive electrode slurry on an aluminum foil to obtain a double-sided coated positive electrode sheet; and then drying, rolling, and cutting to obtain the positive electrode sheet.

[0144] The above-mentioned obtaining process of the positive electrode active material is as follows:

[0145] 1.1. Solid phase grinding: first, based on the synthesis of lithium manganese iron phosphate material LiMn 0.8 Fe 0.2The molar ratio of lithium source, manganese source, iron source, and phosphorus source required by PO4 theory was required, and lithium carbonate, manganese oxide, iron phosphide, and diammonium hydrogen phosphate were weighed and placed into a sand mill pre-mixing tank. After mixing, deionized water was added to reach a solid content of 20%, 0.5mm zirconium oxide balls were used as grinding media, and the first grinding was performed for 0.5h at a grinding speed of 650r / min to obtain a precursor slurry.

[0146] 1.2, spray drying: 20% of the solid mass of the precursor slurry was added as a carbon source, and the precursor slurry was mixed uniformly. Then, the mixture was ground for 4h at a grinding speed of 500r / min, and the solid content was adjusted to 23.6%. The spray drying speed was set to 65L / min for spray drying.

[0147] 1.3, high-temperature sintering: the dried powder was transferred to a tube-type sintering furnace, heated to 650℃ under a nitrogen atmosphere, sintered at 650℃ for 6h, and naturally cooled to room temperature to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0148] 2, preparation of negative electrode sheet

[0149] The specific preparation process was as follows: graphite, conductive carbon black, thickening agent CMC, and binder SBR were mixed in a mass ratio of 96.4:1:1.2:1.4, and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry was coated on a copper foil to obtain a double-sided coated electrode sheet; then, the electrode sheet was rolled and cut to obtain a negative electrode sheet.

[0150] 3, obtaining of separator film

[0151] The separator film in this embodiment was a PP separator film.

[0152] 4, preparation of electrolyte

[0153] The specific preparation process was as follows: ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a lithium salt concentration of 1mol / L.

[0154] 5, assembly of secondary battery

[0155] The specific process was as follows: the above positive electrode sheet, separator film, and negative electrode sheet were stacked in order, with the separator film between the positive and negative electrode sheets to play a separating role, and then the bare cell was obtained by winding; the bare cell was placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum packaging, standing, formation, shaping, and other processes, a lithium ion secondary battery was obtained.

[0156] Comparative Example 4

[0157] A secondary battery, which is only different from Example 1 in that, in the present comparative example, CNT is not added in the lithium manganese iron phosphate material during the preparation process, but is directly prepared on the surface of the lithium manganese iron phosphate material, and the specific preparation process is as follows:

[0158] Preparation of the lithium manganese iron phosphate material: (1) based on the synthesis of lithium manganese iron phosphate material LiMn 0.9 Fe 0.1 The molar ratio of lithium source, manganese source, iron source and phosphorus source required by the theoretical lithium manganese iron phosphate material LiMn PO4was required, and the lithium source, manganese source, iron source and phosphorus source were weighed, and 20% of the total mass of the lithium source, manganese source, iron source and phosphorus source was weighed as a carbon source; the lithium source, manganese source, iron source and phosphorus source were added to water to achieve a solid content of 20%, and wet grinding was carried out; 0.5 mm zirconia balls were used as grinding media in the wet grinding process, the first grinding time was 0.5 h, and the grinding speed was 650 r / min, to obtain a precursor slurry; (2) the above ground precursor slurry was mixed with the carbon source, and then the second grinding was carried out under the condition of grinding speed 500 r / min, grinding time 4 h, and the solid content was adjusted to 22.1% for spray drying to obtain dry powder; (3) the dried powder was transferred to a tube furnace, and the temperature was raised to 650 ℃ under nitrogen atmosphere, sintered at 650 ℃ for 6 h, and naturally cooled to room temperature to obtain the lithium manganese iron phosphate material.

[0159] Preparation of the positive electrode composite material: CNT slurry was deposited on the surface of the lithium manganese iron phosphate material by atomic layer deposition (ALD) technology to obtain a positive electrode composite material with CNT coating, and the mass ratio of lithium manganese iron phosphate material to CNT in the positive electrode composite material was 100:0.6.

[0160] Preparation of the positive electrode sheet: the above positive electrode composite material, conductive carbon black, binder PVDF and carbon nanotube conductive agent were mixed in a mass ratio of 96.5:1:2:0.5, a solvent NMP was added, and the system was stirred to be uniform under the action of a vacuum stirrer to obtain a positive electrode slurry with a solid content of 55%; the positive electrode slurry was coated on an aluminum foil to obtain a double-sided coated positive electrode sheet; and then the positive electrode sheet was obtained after drying, rolling and cutting.

[0161] The process parameter conditions of the above Examples 1-13 and Comparative Examples 1-2 are shown in Table 1.

[0162] Table 1

[0163] Test:

[0164] 1. Test of the positive electrode material:

[0165] (1) Test of manganese molar ratio a:

[0166] Pre-treatment: disassemble the lithium ion battery in empty and electric state, get the positive electrode sheet, soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 min, take out, wash and dry to remove electrolyte and surface by-products on the surface of the positive electrode sheet; scrape the positive active material on the surface of the current collector, calcine at 400°C for 3 hours to remove the binder to obtain the positive active material powder.

[0167] Test: accurately weigh 0.5 g of the positive active material powder, disperse in 20 ml of water, then add 10 ml of nitric acid, mix uniformly and then heat treat, after the positive active material powder is dissolved, the material is diluted to 100 mL with water to obtain the test solution;

[0168] ICP test is performed on the test solution, and the test conditions are as follows: standard solution is prepared before testing, and the standard solution with a linear correlation coefficient of the concentration above 0.999 can be used as a normal standard solution. The concentration linear relationship is measured by diluting the 1000 mg / L standard solution with deionized water to different concentrations, wherein the different concentrations are generally 0 mg / 100 mL, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL, 5 mg / 100 mL, 10 mg / 100 mL, 20 mg / 100 mL, 50 mg / 100 mL, 100 mg / 100 mL, etc.

[0169] Select the element detection spectrum wavelength and set the experimental conditions: set appropriate ICP instrument working conditions according to the characteristics of the sample and the elements to be detected; for example: gas flow 0.5 L / min, power 1150 W, test Fe wavelength 259.94 nm, test Mn wavelength 257.61 nm.

[0170] (2) Average length L CNT , diameter d of primary particles and porosity c test:

[0171] Pre-treatment: disassemble the lithium ion battery in empty and electric state, get the positive electrode sheet, soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 min, take out, dry at room temperature with humidity ≤15%, and obtain the treated positive electrode sheet.

[0172] (2.1) Average length L CNT test: first paste a layer of conductive adhesive on the sample holder, stick the positive electrode sheet on the sample holder, then coat a layer of conductive film, and then observe under a scanning electron microscope. SEM is magnified to 50,000 times to find agglomerates, and then find CNT materials in the agglomerates. The length of the whole CNT is collected by the line method, and the average length of at least 3 CNTs collected at different positions in the page is taken as L CNT .

[0173] (2.2) Test of the diameter d of primary particles: first, a layer of conductive glue is pasted on the sample holder, then the positive electrode sheet is adhered to the sample holder, and then a conductive film is plated, and then observed under a scanning electron microscope. The SEM is magnified to 50,000 times to measure the particle size, and the size of the primary particles in the agglomerate is measured by the MEARSURE NANO software on the SEM picture. The diagonal line method is used to collect the size of the primary particles, and after collecting more than 100 samples, the particle size distribution is counted, and the average value of the diameter of the primary particles is calculated, that is, the diameter d of the primary particles.

[0174] (2.3) Test of the porosity c: 10 positive electrode sheets are taken, 10 regions of each positive electrode sheet are randomly selected, and cross-sectional SEM (scanning electron microscope) argon ion polishing CP (SEM-CP, cross-section polishing-scanning electron microscope) test is carried out, the magnification is 10,000 times, and each SEM-CP picture contains at least 5 spherical agglomerates. The software Nano Measurer is used for measurement, wherein the diameter of the cross-section of the agglomerate is R, the cross-section of the pore in the agglomerate is usually a long and narrow structure, the relatively longer distance of the pore in the cross-section is the length a, and the relatively shorter distance of the pore is the width b, then the cross-sectional area S1 of the agglomerate is π*(R / 2) 2 , and the area S2 of the pore in the cross-section of the agglomerate is a*b. The porosity of a single agglomerate is S2 / S1*100%, and the average value of the cumulative porosity of 50 agglomerates is the value of the porosity c of the active material, and the calculation formula is P=∑S2 / S1 / 50*100%.

[0175] 2. Performance test:

[0176] (1) 45℃ cycle capacity retention rate test

[0177] The lithium ion secondary batteries prepared from the examples and the comparative examples are subjected to cycle performance test according to the following procedure at 45℃:

[0178] 1) Constant current charging to 4.25V at 1C rate, constant voltage charging to current less than 0.05C, more than 3 times of working steps, recording the initial capacity C1 of the battery;

[0179] 2) Constant current charging to 4.25V at 1C rate, constant voltage charging to current less than 0.05C;

[0180] 3) Stand for 5 minutes;

[0181] 4) Discharge to 2.5V at 1C rate;

[0182] 5) Stand for 5 minutes;

[0183] The cycle test is performed according to the steps 2)-5) until the lithium ion secondary battery is cycled for 300 times, and the capacity C2 of the battery after 300 times of cycling is recorded.

[0184] 45 °C cycle capacity retention rate = C2 / C1*100%.

[0185] (2) DCR test

[0186] 1) Place the battery in a 25 °C oven until thermal equilibrium is reached;

[0187] 2) Charge at a 1 / 3C rate to 4.25 V, and then charge at a constant voltage until the current is less than 0.05C, and perform more than 3 times, and record the capacity C1 of the battery;

[0188] 3) Stand for 5 minutes;

[0189] 4) Adjust the load of the battery to 80% SOC at a 1 / 3C discharge rate;

[0190] 5) Discharge at a current of 1C rate for 18S, record the battery voltage U2, current I and battery voltage U1 after the battery voltage stabilizes before the discharge is stopped, calculate the DC internal resistance R1 according to the formula R1 = (U2-U1) / I, and the R1 is the 25 °C 1C discharge DCR.

[0191] The test results are shown in Tables 2 and 3 below.

[0192] Table 2

[0193] Table 3

[0194] From the data of Examples 1-13 and Comparative Examples 1-2 in Tables 2 and 3, it can be seen that by comprehensively controlling the average length L CNT of the nanotubes, the primary particle diameter d, the manganese molar ratio a, and the porosity c, the internal conductivity of the agglomerates can be improved, the kinetic performance of lithium ions can be improved, especially when 0.09≤a*c / b≤126, a positive electrode material with better internal conductivity and more stable agglomerate structure can be obtained, and the battery fast charging performance and cycle performance are also considered. Especially, by setting the length L CNT of the nanotubes, the primary particle diameter d, the manganese molar ratio a, and the porosity c within the range, the cycle performance can be further improved under the condition of relatively good fast charging performance; and by optimizing the value of a*c / b to be between 0.6-20, the cycle performance and fast charging performance of the battery can be more excellent.

[0195] And, by Examples 1-13 and Comparative Examples 3 and 4, it is known that Comparative Example 3 does not have CNTs inside the agglomerates, and Comparative Example 4 has CNTs coated on the surface of the agglomerates, and the performance of Comparative Examples 3 and 4 is worse than that of Examples 1-13; it is shown that when there are no CNTs inside the agglomerates, or only CNTs on the surface, the effect of improving the internal conductivity of the agglomerates, and improving the kinetic performance and cycle stability cannot be achieved. In the present application, by means of CNTs penetrating between primary particles inside the agglomerates, and by controlling the comprehensive relationship of a*c / b between the average length L of the nanotubes, the diameter d of the primary particles, the molar ratio a of manganese, and the porosity c in the agglomerates, the effect of improving the kinetic performance and cycle stability is achieved. CNT , the primary particle diameter d, the molar ratio a of manganese, and the porosity c in the agglomerates, the effect of improving the kinetic performance and cycle stability is achieved.

[0196] Obviously, the above examples are merely examples for the purpose of clear illustration, and are not limitations on the embodiments. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art. All embodiments do not need to be exhausted here. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A positive electrode material, characterized by, comprise a chemical formula LiMn x Fe y M (1-x-y) PO4, wherein 0.5≤x<1, 0 The agglomerate satisfies the following relationship: 0.09 < a*c / b < 126; b = L CNT / d; wherein a is the molar ratio of manganese to the total amount of manganese and iron in the aggregate; c is the porosity of the aggregate, in %; d is the diameter of the primary particles, in nm; L CNT L is the average length of the carbon nanotubes in the aggregate, in nm.

2. The positive electrode material of claim 1, wherein, a*c / b has a value of 0.6-20; and / or, a has a value of 0.5-0.96; and / or, c has a value of 5-35; and / or, b has a value of 0.25-50.

3. The positive electrode material according to claim 2, characterized in that, a has a value of 0.6-0.85; and / or, c has a value of 10-25; and / or, b has a value of 1-13.

4. The positive electrode material according to any one of claims 1-3, wherein The L CNT is 100 nm - 1000 nm; and / or, d is 20 nm-400 nm.

5. The positive electrode material according to claim 4, wherein The L CNT is 200 nm - 800 nm; and / or, d is 40 nm-200 nm.

6. A positive electrode sheet characterized by comprising: The positive electrode material according to any one of claims 1-5.

7. The positive electrode sheet according to claim 6, characterized by The positive electrode sheet further comprises a positive electrode conductive agent selected from one or more of conductive carbon black, carbon nanotubes.

8. The positive electrode sheet according to claim 7, characterized by The positive electrode conductive agent is selected from conductive carbon black.

9. An electrochemical device, characterized by, The positive electrode sheet according to any one of claims 6-8.

10. An electrical device, characterized by The electrochemical device according to claim 9.

11. The cathode material of claim 1, wherein, Chemical formula LiMn x Fe y M (1-x-y) PO4, M is a doping element.

Images