Lithium iron phosphate positive electrode material and preparation method thereof, positive plate, battery, battery pack and electric equipment

Abstract

The invention provides a lithium iron phosphate positive electrode material and a preparation method thereof, a positive plate, a battery, a battery pack and electric equipment. The lithium iron phosphate positive electrode material comprises a lithium iron phosphate matrix and a carbon coating layer coating the lithium iron phosphate matrix, the lithium iron phosphate positive electrode material satisfies 5 < = (AD / AG) * S + sigma (AA / Av) < = 45; wherein AD is the peak area of a D peak in a Raman spectrum of the lithium iron phosphate positive electrode material; aG is the peak area of a G peak in the Raman spectrum of the lithium iron phosphate positive electrode material; av is the peak area of a symmetrical telescopic vibration peak of a phosphate group in a Raman spectrum of the lithium iron phosphate positive electrode material; sigma (GA / Av) is the standard deviation of AG / Av; s is the circularity of the lithium iron phosphate positive electrode material. The conductivity of the lithium iron phosphate positive electrode material can be improved, so that the battery resistance is reduced, and the rate capability of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a lithium iron phosphate cathode material and its preparation method, cathode sheet, battery, battery pack, and electrical equipment. Background Technology

[0002] In lithium-ion batteries, the performance of the cathode material directly affects the battery's energy density, cycle life, rate charge / discharge capability, and safety. Due to its advantages such as low cost, environmental friendliness, and high structural stability, lithium iron phosphate cathode materials have seen continuous market demand growth. However, their inherent low electronic / ionic conductivity and slow lithium-ion diffusion kinetics result in batteries generally exhibiting high impedance and poor rate performance, issues that urgently need to be addressed. Summary of the Invention

[0003] This invention provides a lithium iron phosphate cathode material and its preparation method, a cathode sheet, a battery, a battery pack, and electrical equipment, which can improve the conductivity and other properties of the lithium iron phosphate cathode material, thereby reducing the battery resistance and improving the battery's rate performance.

[0004] One aspect of the present invention provides a lithium iron phosphate cathode material, comprising a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix; the lithium iron phosphate cathode material satisfies 5 ≤ ​​(A D / A G )×S+σ(A G / A v )≤45; where A D The peak area of ​​peak D in the Raman spectrum of the lithium iron phosphate cathode material; A G A is the peak area of ​​peak G in the Raman spectrum of the lithium iron phosphate cathode material; v σ(A) represents the peak area of ​​the symmetric stretching vibration peak of the phosphate group in the Raman spectrum of the lithium iron phosphate cathode material. G / A v ) is A G / A v The standard deviation of S; S is the sphericity of the lithium iron phosphate cathode material.

[0005] According to one embodiment of the present invention, the A D / A G The value is 1 to 4; and / or, the value of S is 0.6 to 1; and / or, the value of σ(A) is 1 to 4. G / A v The range is 5 to 40.

[0006] According to one embodiment of the present invention, the chemical formula of the lithium iron phosphate matrix is ​​LiFe 1-x M xPO4, 0≤x<1; M includes at least one of Mn, Zn, Ni, Mg, and Al.

[0007] According to one embodiment of the present invention, the average particle size of the lithium iron phosphate cathode material is 50 nm to 1500 nm; and / or, the thickness of the carbon coating layer is greater than 0 nm and less than or equal to 30 nm.

[0008] According to one embodiment of the present invention, the average particle size of the lithium iron phosphate cathode material is 500 nm to 900 nm; and / or, the thickness of the carbon coating layer is 4 nm to 8 nm.

[0009] In another aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps: subjecting a mixture comprising a lithium source, an iron source, a phosphorus source and a first carbon source to a first sintering to obtain an intermediate; and subjecting the intermediate to a second carbon source and then subjecting it to a second sintering to obtain the lithium iron phosphate cathode material.

[0010] According to one embodiment of the present invention, the molar ratio of the lithium source to the iron source is (0.95-1.10):1; and / or, the molar ratio of the phosphorus source to the iron source is (0.95-1.10):1.

[0011] According to one embodiment of the present invention, the preparation process of the mixture includes: mixing a first material comprising a lithium source, an iron source, and a phosphorus source with a first carbon source, and then performing ball milling to obtain a second material; spray drying the second material to obtain the mixture; preferably, the mass ratio of the first material to the first carbon source is 100:(5-30); preferably, the first material further includes a manganese source; the molar ratio of the lithium source to the manganese source is 1:(0-0.3); preferably, the particle size D of the second material is... 50 The particle size range is 200nm to 950nm; preferably, the particle size range of the mixture is 10μm to 80μm.

[0012] According to one embodiment of the present invention, the first sintering is carried out under an inert atmosphere; and / or, the mass ratio of the intermediate to the second carbon source is 100:(5-20); and / or, the temperature of the first sintering is 700℃~800℃; and / or, the time of the first sintering is 10h~24h; and / or, the temperature of the second sintering is 600℃~800℃; and / or, the time of the second sintering is 10h~24h.

[0013] In another aspect, the present invention provides a positive electrode sheet comprising the above-described lithium iron phosphate positive electrode material or a lithium iron phosphate positive electrode material prepared according to the above-described method for preparing lithium iron phosphate positive electrode material.

[0014] In another aspect, the present invention provides a battery comprising the above-described positive electrode.

[0015] In another aspect, the present invention provides a battery pack comprising at least two batteries connected to each other.

[0016] In another aspect, the present invention provides an electrical device including the battery or the battery pack described above.

[0017] The lithium iron phosphate cathode material and its preparation method, cathode sheet, battery, battery pack, and electrical equipment provided in this invention embodiment include a lithium iron phosphate substrate and a carbon coating layer, which are processed by A... G / A v The degree of graphitization of the carbon coating layer is characterized; the higher the degree of graphitization, the better the conductivity of the lithium iron phosphate cathode material. This is achieved through σ(A) G / A v ) characterizes the uniformity of carbon coating distribution, σ(A G / A v The smaller the value of A, the better the uniformity of the carbon coating layer. The morphology of the lithium iron phosphate cathode material is also characterized by the roundness S. The closer S is to 1, the closer the cross-section of the lithium iron phosphate cathode material particles is to a circle, the lower the risk of cracking at high temperatures, and the more beneficial it is to the continuity of the carbon coating layer. This invention achieves this through synergistic regulation of A. G / A v σ(A) G / A v Key parameters such as roundness S should be ensured to satisfy 5 ≤ (A) D / A G )×S+σ(A G / A v With a carbon coating thickness of ≤45, the uniformity of carbon coating distribution and conductivity are improved, which helps to reduce battery polarization, improve battery rate performance, and reduce battery impedance, giving the battery excellent fast charge and discharge capabilities. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0019] Figure 1 This is the Raman spectrum of Example 1 of the present invention;

[0020] Figure 2 This is a scanning electron microscope (SEM) image of Embodiment 1 of the present invention.

[0021] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] In existing technologies, carbon coating modification of lithium iron phosphate cathode materials mainly employs liquid-phase coating or chemical vapor deposition (CVD). Liquid-phase coating typically uses glucose, sucrose, polyethylene glycol, etc., as carbon sources, forming a carbon layer on the material surface through steps such as mixing, ball milling, and spray drying. Chemical vapor deposition (CVD) typically uses gaseous carbon sources such as methane (CH4), acetylene (C2H2), and ethylene (C2H4), or volatile liquid carbon sources such as toluene and benzene (after vaporization) as carbon precursors. The carbon source is introduced into a high-temperature reactor via a carrier gas, where it undergoes pyrolysis, cracking, and deposition reactions on the material particle surface under an inert atmosphere, ultimately forming a carbon layer on the material surface.

[0024] However, the following problems exist in the current carbon coating modification of lithium iron phosphate cathode materials: (1) It is difficult to accurately control the uniformity of the carbon coating layer on the surface of lithium iron phosphate cathode materials, resulting in some lithium iron phosphate cathode material particles not being effectively coated, thus reducing conductivity; (2) An excessively thick carbon coating layer will reduce the tap density of lithium iron phosphate cathode materials, thereby affecting the energy density of the battery; (3) An excessively high degree of graphitization of the carbon coating layer may lead to cracking of the carbon coating layer or poor electrolyte wettability.

[0025] According to the inventors' research, the D peak (characteristic peak wavenumber of 1350 cm⁻¹) corresponding to amorphous carbon in the Raman spectrum of lithium iron phosphate cathode material can be used to identify the characteristic peak wavenumber of 1350 cm⁻¹. -1 The peak area A D And the corresponding G peak of graphitized carbon (characteristic peak wavenumber 1590 cm⁻¹). -1 The peak area A G The ratio of [value] to [value] describes the degree of graphitization of the carbon coating; additionally, it can be described by A [value]. G / A v Standard deviation σ(A) G / A v ) describes the coating effectiveness of the carbon coating layer, where A vThe Raman spectrum of lithium iron phosphate cathode material shows the symmetric stretching vibration peak corresponding to the phosphate group (characteristic peak wavenumber 945 cm⁻¹). -1 ~955cm -1 The peak area of ​​lithium iron phosphate cathode material can be measured; at the same time, the morphological characteristics of lithium iron phosphate cathode material can be described by the roundness S.

[0026] In view of this, embodiments of the present invention provide a lithium iron phosphate cathode material, comprising a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix; the lithium iron phosphate cathode material satisfies 5 ≤ ​​(A D / A G )×S+σ(A G / A v )≤45; where A D The peak area of ​​peak D in the Raman spectrum of lithium iron phosphate cathode material; A G A is the peak area of ​​the G peak in the Raman spectrum of lithium iron phosphate cathode material; v The peak area of ​​the symmetric stretching vibration peak of the phosphate group in the Raman spectrum of lithium iron phosphate cathode material; σ(A G / A v ) is A G / A v The standard deviation of S; S is the sphericity of the lithium iron phosphate cathode material.

[0027] According to the inventor's research, A D / A G This represents the ratio of the relative content of amorphous carbon to graphitized carbon in the carbon coating layer. A smaller ratio indicates a higher degree of graphitization in the carbon coating layer. Compared to amorphous carbon, graphitized carbon has better conductivity; therefore, a higher degree of graphitization in the carbon coating layer results in better conductivity for the lithium iron phosphate cathode material. Generally, the preparation process of a high-graphitization carbon coating layer requires high-temperature annealing. G / A v The ratio of the carbon coating layer to the lithium iron phosphate matrix is ​​given. Uneven distribution of the carbon coating layer will lead to variations in A content at different test locations. G / A v Different, therefore A G / A v Standard deviation (σ(A)) G / A v )) can be used to characterize the uniformity of carbon coating distribution, σ(A G / A vThe smaller the value, the better the uniformity of the carbon coating layer. The roundness S of lithium iron phosphate cathode materials refers to the ratio of the equivalent particle size to the longest side of the lithium iron phosphate cathode material particles. The equivalent particle size is the diameter of a circle with the same cross-sectional area as the lithium iron phosphate cathode material particle, and the longest side is the longest line connecting the edges of the cross-section of the lithium iron phosphate cathode material particle. The closer the roundness S is to 1, the closer the cross-section of the lithium iron phosphate cathode material particle is to a circle, and the lower the risk of cracking at high temperatures.

[0028] When the lithium iron phosphate cathode material satisfies the above formula, the graphitization degree of the carbon coating layer in the lithium iron phosphate cathode material is high, resulting in good conductivity. Simultaneously, the high sphericity S of the lithium iron phosphate cathode material reduces the risk of cracking of the carbon coating layer during high-temperature annealing. Furthermore, the high sphericity S indicates uniform surface curvature of the lithium iron phosphate cathode material particles, making it easier for the carbon coating layer to continuously cover the material; and σ(A) G / A v The high carbon coating level indicates that the carbon coating layer is evenly distributed on the surface of the lithium iron phosphate substrate and has good coating effectiveness, which can synergistically improve the conductivity of the lithium iron phosphate cathode material.

[0029] In summary, the lithium iron phosphate cathode material provided by this invention achieves synergistic regulation of A G / A v σ(A) G / A v By adjusting key parameters such as sphericity (S), the uniformity of carbon coating distribution and conductivity are improved, which helps reduce battery polarization, improves battery rate performance, and reduces battery impedance, enabling the battery to have excellent fast charge and discharge capabilities.

[0030] For example, (A) D / A G )×S+σ(A G / A v () can be a range consisting of 4, 5, 10, 15, 20, 25, 30, 35, 40, 45 or any two of them.

[0031] In this invention, the A of the lithium iron phosphate cathode material G / A v σ(A) G / A v The test can be performed as follows: Raman spectroscopy is used to perform Raman tests on lithium iron phosphate cathode materials or cathode sheets containing lithium iron phosphate cathode materials to obtain the Raman spectrum of the lithium iron phosphate cathode material or cathode sheet. The peak area A corresponding to the D peak of amorphous carbon in the Raman spectrum is then calculated by fitting the spectrum. D The peak area A corresponding to the G peak of graphitized carbon. G And the peak area A of the symmetric stretching vibration peak of the phosphate group.v Further calculations yielded A G / A v σ(A) G / A v ).

[0032] In this invention, the roundness S of the lithium iron phosphate cathode material can be tested in the following way: A scanning electron microscope (SEM) is used to perform SEM testing on the lithium iron phosphate cathode material or the cathode sheet containing the lithium iron phosphate cathode material to obtain an SEM image of the lithium iron phosphate cathode material or cathode sheet. Image recognition software (e.g., Mipar) or manual measurement is used to measure the cross-section of the lithium iron phosphate cathode material particles in the SEM image to obtain the longest connecting line (longest side) on the particle edge line, the shortest connecting line (shortest side) on the particle edge line, and the cross-sectional area of ​​the particle. The diameter (equivalent particle size) of the circle with the same cross-sectional area as the particle is calculated. Based on the formula: roundness S = equivalent particle size / longest side, the roundness S of the lithium iron phosphate cathode material is obtained.

[0033] In some embodiments, A D / A G It can be 1 to 4, for example, it can be a range consisting of 1, 1.5, 2, 2.5, 3, 3.5, 4, or any two of them. A D / A G A value not less than 1 can prevent excessive graphitization of the carbon coating layer, which is more conducive to improving the wettability of the electrolyte to the lithium iron phosphate cathode material; at the same time, A D / A G A value not exceeding 4 is more conducive to improving the graphitization degree of the carbon coating layer, thereby improving the conductivity of the lithium iron phosphate cathode material, reducing the battery impedance, and enhancing the battery's rate performance.

[0034] In some embodiments, S can be 0.6 to 1, for example, it can be a range of 0.6, 0.7, 0.8, 0.9, 1 or any two of these. S not less than 0.6 is more conducive to improving the surface curvature uniformity of lithium iron phosphate cathode material particles, promoting the continuous coverage of the carbon coating layer, and at the same time, it is more conducive to reducing the risk of cracking of the carbon coating layer during high-temperature annealing.

[0035] In some embodiments, σ(A) G / A v ) can be 5~40, for example, it can be a range of 5, 10, 15, 20, 25, 30, 45, 40 or any two of them. σ(A) G / A vWithin the above range, it is more conducive to improving the uniformity of carbon coating distribution, thereby improving the conductivity of lithium iron phosphate cathode material and optimizing the electrochemical performance of battery.

[0036] In some embodiments, the chemical formula of the lithium iron phosphate matrix is ​​LiFe 1-x M x PO4, 0≤x<1; M includes at least one of Mn, Zn, Ni, Mg, and Al.

[0037] In some specific embodiments, the average particle size of the lithium iron phosphate cathode material can be from 50 nm to 1500 nm, for example, it can be a range of 50 nm, 250 nm, 500 nm, 750 nm, 1000 nm, 1250 nm, 1500 nm, or any combination thereof. An average particle size within this range is more advantageous for balancing the specific surface area and compaction density of the lithium iron phosphate cathode material, thereby improving the battery's capacity performance.

[0038] In some preferred embodiments, the average particle size of the lithium iron phosphate cathode material can be 500 nm to 900 nm.

[0039] In some specific embodiments, the thickness of the carbon coating layer can be greater than 0 nm and less than or equal to 30 nm, for example, it can be a range of 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, or any combination thereof. A carbon coating layer thickness greater than 0 nm is more conducive to improving the continuity of the carbon coating layer and the conductivity of the lithium iron phosphate cathode material; at the same time, a carbon coating layer thickness not exceeding 30 nm is more conducive to improving the tap density of the lithium iron phosphate cathode material and improving the impedance and rate performance of the battery.

[0040] In some preferred embodiments, the thickness of the carbon coating layer can be 4 nm to 8 nm.

[0041] In this embodiment of the invention, the average particle size and the thickness of the carbon coating layer of the lithium iron phosphate cathode material can be tested using conventional techniques in the art, without any particular limitation. For example, the thickness of the carbon coating layer can be obtained by testing the lithium iron phosphate cathode material or cathode sheet using a transmission electron microscope (TEM), and the average particle size of the lithium iron phosphate cathode material can be obtained by testing the lithium iron phosphate cathode material or cathode sheet using image recognition software (e.g., Mipar).

[0042] It should be noted that the positive electrode sheet used for the test can be obtained by disassembling the battery.

[0043] The present invention also provides a method for preparing lithium iron phosphate cathode material, comprising the following steps: subjecting a mixture comprising a lithium source, an iron source, a phosphorus source and a first carbon source to a first sintering to obtain an intermediate; and subjecting the intermediate to a second carbon source and then subjecting it to a second sintering to obtain the lithium iron phosphate cathode material.

[0044] According to the inventor's research, the first carbon source and the second carbon source together form a carbon coating layer, and the two sintering processes (including the first sintering and the second sintering) can improve the continuity and uniformity of the carbon coating layer.

[0045] In some embodiments, the molar ratio of lithium source to iron source can be (0.95-1.10):1. A molar ratio of lithium source to iron source of not less than 0.95 is more conducive to refining grains, alleviating sintering fusion between grains, generating sufficient lithium iron phosphate material, reducing the generation of impurity phases, thereby promoting the uniformity of distribution of lithium iron phosphate cathode material and optimizing the rate performance of the battery. At the same time, a molar ratio of not more than 1.10 is more conducive to reducing the generation of low-activity impurity phases such as lithium phosphate (Li3PO4), thereby maintaining the regular morphology of lithium iron phosphate cathode material, improving the continuity of electronic conduction pathways, reducing battery internal resistance, and also more conducive to reducing adhesion between lithium iron phosphate cathode material particles, thereby improving the dispersibility of lithium iron phosphate cathode material.

[0046] In some embodiments, the molar ratio of phosphorus source to iron source can be (0.95-1.10):1. A molar ratio of phosphorus source to iron source of not less than 0.95 is more conducive to avoiding the formation of iron oxide impurity phase, thereby maintaining the regular morphology of lithium iron phosphate cathode material, improving the continuity of electron conduction pathway, and reducing battery internal resistance. At the same time, a molar ratio of not more than 1.10 is more conducive to increasing the iron content in the structure of lithium iron phosphate cathode material, providing iron ion support for crystal growth, thereby improving the particle crystallinity of lithium iron phosphate cathode material, improving the tap density of lithium iron phosphate cathode material, and further improving the compaction density of cathode sheet, reducing battery internal resistance, and enhancing battery rate performance.

[0047] In some embodiments, the preparation process of the mixture includes: mixing a first material comprising a lithium source, an iron source, and a phosphorus source with a first carbon source, followed by ball milling to obtain a second material; and spray drying the second material to obtain the mixture. Ball milling and spray drying are more conducive to improving the density of the mixture, optimizing the particle size of the lithium iron phosphate matrix, and promoting the uniform distribution of the lithium iron phosphate cathode material, thereby reducing the battery's internal resistance and enhancing its rate performance.

[0048] In this embodiment of the invention, ball milling can be performed using conventional techniques in the art, and there are no particular limitations on this; for example, ball milling can be performed using a ball milling machine. In this embodiment of the invention, spray drying can be performed using conventional techniques in the art, and there are no particular limitations on this; for example, ball milling can be performed using a spray dryer.

[0049] In some specific embodiments, the mass ratio of the first material to the first carbon source can be 100:(5-30). A mass ratio of the first material to the first carbon source within the above range is more conducive to optimizing the thickness of the carbon coating layer, while also improving the conductivity and tap density of the lithium iron phosphate cathode material, thereby reducing the internal resistance of the battery and improving the rate performance of the battery.

[0050] In some specific embodiments, the first material also includes a manganese source, which is more conducive to improving the energy density and low-temperature performance of the battery, and is beneficial to industrial production.

[0051] In some specific embodiments, the molar ratio of lithium source to manganese source can be 1:(0-0.3).

[0052] In some specific embodiments, the particle size D of the second material 50 It can be from 200nm to 950nm, for example, it can be a range of 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, 850nm, 900nm, 950nm or any combination thereof.

[0053] In this application, the particle size D of the second material 50 This refers to the particle size Dv 50 That is, the particle size of 50% of the material particles by volume in this Dv 50 Values ​​below.

[0054] In some embodiments, the particle size range of the mixture can be 10 μm to 80 μm, and the particle size of the mixture can be, for example, a range of 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, or any combination thereof. A particle size range within the above-mentioned range is more conducive to optimizing the particle size of the lithium iron phosphate matrix, promoting the uniformity of the distribution of the lithium iron phosphate cathode material, thereby reducing the battery's internal resistance and enhancing the battery's rate performance.

[0055] In this invention, "the particle size range of the mixture can be 10μm~80μm" means that the overall particle size range of the mixture can be 10μm~80μm.

[0056] In this embodiment of the invention, the particle size of the second material or mixture can be tested using conventional techniques in the art, without any particular limitation. For example, it can be tested using a laser particle size analyzer.

[0057] In some embodiments, the first sintering is carried out under an inert atmosphere, which is more conducive to avoiding the generation of oxidative impurities, thereby protecting the valence stability of the active elements in the lithium iron phosphate cathode material, and is also more conducive to improving the crystallinity composition and particle size uniformity of the lithium iron phosphate cathode material, thereby optimizing the electrochemical performance of the battery.

[0058] In some specific embodiments, the first sintering temperature can be 700℃~800℃, for example, a range of 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, or any combination thereof; the first sintering time can be 10h~24h, for example, a range of 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, or any combination thereof. A first sintering temperature and time within the above ranges are more conducive to forming a dense and regularly morphologically regular intermediate, while reducing the occurrence of side reactions.

[0059] In some embodiments, the mass ratio of the intermediate to the second carbon source can be 100:(5-20). A mass ratio of the intermediate to the second carbon source within the above range is more conducive to optimizing the thickness of the carbon coating layer, while also improving the conductivity and tap density of the lithium iron phosphate cathode material, thereby reducing the battery's internal resistance and improving the battery's rate performance.

[0060] In some specific embodiments, the second sintering temperature can be 600℃~800℃, for example, it can be a range of 600℃, 650℃, 700℃, 750℃, 800℃ or any two of them; in some embodiments, the second sintering time can be 10h~24h, for example, it can be a range of 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h or any two of them.

[0061] In practice, a second carbon source can be introduced through chemical vapor deposition (CVD), which is more conducive to improving the uniformity of carbon coating distribution, thereby improving the conductivity of lithium iron phosphate cathode material and synergistically improving the battery's internal resistance and rate performance.

[0062] The present invention also provides a positive electrode sheet, comprising the above-described lithium iron phosphate positive electrode material or a lithium iron phosphate positive electrode material prepared according to the above-described method for preparing lithium iron phosphate positive electrode material. This positive electrode sheet has advantages corresponding to the above-described lithium iron phosphate positive electrode material, which will not be elaborated further.

[0063] Specifically, the positive electrode sheet includes a positive current collector and a positive active layer formed of the aforementioned lithium iron phosphate positive electrode material disposed on the surface of the positive current collector.

[0064] In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% lithium iron phosphate positive electrode material, 0.5-15 wt% conductive agent, and 0.5-15 wt% binder. Further, the positive electrode active layer comprises 80-98 wt% lithium iron phosphate positive electrode material, 1-10 wt% conductive agent, and 1-10 wt% binder.

[0065] The positive electrode current collector material may include at least one of aluminum foil and nickel foil; the conductive agent may be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, carbon fiber, and carbon nanotubes (CNTs); the binder may be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0066] In this embodiment of the invention, the positive electrode sheet can be prepared by conventional coating method. For example, the lithium iron phosphate positive electrode material of the present invention can be dispersed with conductive agent, binder and other raw materials used to form the positive electrode active layer in N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and cutting, the positive electrode sheet is obtained.

[0067] This invention also provides a battery comprising the aforementioned positive electrode. This battery has advantages corresponding to the aforementioned positive electrode, which will not be elaborated further.

[0068] It is conceivable that the battery provided by the present invention, in addition to the aforementioned positive electrode, also includes a negative electrode, an electrolyte, and a separator.

[0069] Specifically, a battery may include a cell and a casing for encapsulating the cell. The cell includes a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. Electrolyte is injected into the casing to wet the cell.

[0070] This invention does not strictly limit the negative electrode active material in the negative electrode sheet. It can be a negative electrode active material commonly used in batteries, such as at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrode), and tin-based negative electrode materials (mainly including tin and tin alloy).

[0071] This invention does not strictly limit the choice of electrolyte. Conventional electrolytes in the art can be used, such as non-aqueous electrolytes. The electrolyte includes an organic solvent and an electrolyte salt. The organic solvent can include one or more solvents commonly used in current battery electrolytes. The electrolyte salt can include lithium salts, specifically lithium salts commonly used in current lithium-ion electrolytes. For example, the solvent can include one or more of ethylene carbonate (EC), propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc. The lithium salt can include one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0072] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0073] The battery of the present invention can be manufactured according to conventional methods in the art. For example, when manufacturing the battery, the positive electrode sheet, separator and negative electrode sheet are wound or stacked to obtain a bare cell, and the bare cell is packaged into a pre-stamped shell (such as an aluminum-plastic film bag). After the packaged battery precursor is dried at 85°C, the electrolyte is injected into the dried battery precursor. After the process of resting, formation and secondary sealing, the battery manufacturing is completed.

[0074] The present invention also provides a battery pack comprising at least two interconnected batteries as described above. This battery pack has advantages corresponding to the batteries described above, which will not be elaborated further.

[0075] Generally, a battery pack includes multiple batteries as individual cells, which are connected to form the battery pack. These batteries can be electrically connected using methods conventional in the art, such as series connection, parallel connection, or a combination of these connection methods, without any particular limitation.

[0076] The present invention also provides an electrical device, including the battery or battery pack described above, which has advantages corresponding to the battery or battery pack described above, and will not be described in detail here.

[0077] The electrical equipment used in the embodiments of the present invention can be conventional electrical equipment in the art, such as power equipment (e.g., electric vehicles, electric cars), electronic equipment (e.g., mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (e.g., watches, bracelets, VR glasses, etc.), energy storage power stations, etc., and there are no particular limitations on this.

[0078] The present invention will be further described below through specific embodiments. Unless otherwise specified, all raw materials and reagents used in the following embodiments and comparative examples are commercially available conventional types.

[0079] Example 1

[0080] (1) Preparation of lithium iron phosphate cathode material

[0081] The lithium source, iron source, and phosphorus source were mixed according to the molar ratio Li:Fe:P:M=1.02:1:1:0 to obtain the first material, wherein the lithium source is lithium carbonate, and the iron source and phosphorus source are iron and phosphorus. The first carbon source was added to the first material, and the mass ratio of the first material to the first carbon source was 100:10, wherein the first carbon source was polyethylene glycol and white sugar with a mass ratio of 30:70.

[0082] The above material system was placed in a ball mill, and water was added as a solvent to obtain a second material. The particle size D of the second material was... 50 It is 400nm.

[0083] The second material is spray-dried to obtain a mixture.

[0084] Under a nitrogen atmosphere, the mixture was subjected to a first sintering process, with the temperature increased to 750°C at a heating rate of 3°C / min, and held for sintering for 18 hours. Then, the temperature was reduced to <80°C to obtain an intermediate.

[0085] The intermediate material was mixed with the second carbon source (a 5% polyethylene glycol solution by mass volume) at a mass ratio of 100:5 and then stirred and dried until the water / ethanol was completely evaporated. The second sintering was carried out under an inert atmosphere, with the temperature increased to 700°C at a heating rate of 3°C / min and held for sintering for 18 hours. Then the temperature was cooled to <80°C and subjected to gas crushing treatment to obtain lithium iron phosphate cathode material with the chemical formula LiFePO4.

[0086] The average particle size of the lithium iron phosphate cathode material is 320 nm, and the thickness of the carbon coating layer in the lithium iron phosphate cathode material is 4 nm.

[0087] (2) Preparation of positive electrode sheet

[0088] The above-mentioned lithium iron phosphate cathode material was mixed with a binder (polyvinylidene fluoride, PVDF) and a conductive agent (carbon nanotubes, CNT) in a mass ratio of 90:5:5. An appropriate amount of solvent (N-methyl ketone pyrrole, NMP) was added, and the mixture was stirred until homogeneous to obtain a cathode slurry. This cathode slurry was then coated onto both sides of a cathode current collector (carbon-coated aluminum foil), dried, rolled, and cut to obtain a cathode sheet. The areal density of the cathode sheet was 450 g / m³. 2 The compaction density of the positive electrode is 2.6 g / cm³. 3 .

[0089] (3) Battery preparation

[0090] The above-mentioned positive electrode, separator (PE separator) and negative electrode (graphite electrode) are stacked in an alternating manner to obtain a stacked battery cell. The battery cell is then packaged in a pre-stamped aluminum-plastic film bag. After the packaged battery precursor is dried at 85°C, the electrolyte is injected into the dried battery precursor. After packaging, settling, formation and capacity testing, the battery is obtained.

[0091] The electrolyte contains LiPF6 as the lithium salt with a concentration of 1.0 mol / L, and EC and DMC as solvents with a volume ratio of EC:DMC=1:1.

[0092] Compared with Example 1, the particle size D of the second material in Examples 2-13 and Comparative Examples 1-7 is different. 50 By adjusting the ball milling speed to different sizes (specific values ​​are shown in Table 1), other distinguishing parameters are as follows:

[0093] Example 2

[0094] Compared with Example 1, the difference lies in the molar ratio of each element in the first material being Li:Fe:P:M = 1.03:1:1:0. All other conditions are the same as in Example 1.

[0095] Example 3

[0096] Compared with Example 1, the difference is that the molar ratio of each element in the first material is Li:Fe:P:M=1.01:1:1:0, and the other conditions are the same as in Example 1.

[0097] Example 4

[0098] The difference from Example 1 is that glucose was used as the first carbon source. All other conditions were the same as in Example 1.

[0099] Example 5

[0100] The difference from Example 1 is that the mass ratio of the first material to the first carbon source is 100:15. All other conditions are the same as in Example 1.

[0101] Example 6

[0102] The difference from Example 1 is that the first carbon source used is sucrose. All other conditions are the same as in Example 1.

[0103] Example 7

[0104] The difference from Example 1 is that the mass ratio of the first material to the first carbon source is 100:20. All other conditions are the same as in Example 1.

[0105] Example 8

[0106] The difference from Example 1 is that the sintering temperature for the second sintering is 650°C. All other conditions are the same as in Example 1.

[0107] Example 9

[0108] The difference from Example 1 is that the sintering temperature for the second sintering is 780°C. All other conditions are the same as in Example 1.

[0109] Example 10

[0110] Compared to Example 1, the difference lies in the molar ratio of each element in the first material being Li:Fe:P:M = 1.02:0.99:1:0.01, and the mass ratio of the first material to the first carbon source being 100:15. All other conditions are the same as in Example 1, and the chemical formula of the lithium iron phosphate matrix in the obtained lithium iron phosphate cathode material is LiFe 0.99 Mn 0.01 PO4.

[0111] Example 11

[0112] Compared to Example 1, the difference lies in the molar ratio of each element in the first material: Li:Fe:P:M = 1.02:0.7:1:0.3. All other conditions are the same as in Example 1, and the chemical formula of the lithium iron phosphate matrix in the obtained lithium iron phosphate cathode material is LiFe 0.7 Mn 0.3 PO4.

[0113] Example 12

[0114] Compared to Example 1, the difference lies in the molar ratio of each element in the first material: Li:Fe:P:M = 1.02:0.6:1:0.4. All other conditions are the same as in Example 1, and the chemical formula of the lithium iron phosphate matrix in the obtained lithium iron phosphate cathode material is LiFe 0.6 Mn 0.4 PO4.

[0115] Example 13

[0116] Compared with Example 1, the difference is that the heating rate of the second sintering is 2.85℃ / min, and the other conditions are the same as those in Example 1.

[0117] Comparative Example 1

[0118] Compared to Example 1, the difference lies in that after obtaining the intermediate, it is subjected to gas-shearing treatment to obtain the lithium iron phosphate cathode material, i.e., the second sintering step is not included. The remaining conditions are the same as in Example 1.

[0119] Comparative Example 2

[0120] Compared with Comparative Example 1, the difference is that the molar ratio of each element in the first material is Li:Fe:P:M=1.03:1:1:0, and the other conditions are the same as those in Comparative Example 1.

[0121] Comparative Example 3

[0122] Compared with Comparative Example 1, the difference is that the molar ratio of each element in the first material is Li:Fe:P:M=1.01:1:1:0, and the other conditions are the same as those in Comparative Example 1.

[0123] Comparative Example 4

[0124] The difference from Comparative Example 1 is that the first carbon source used is glucose, while the other conditions are the same as those in Comparative Example 1.

[0125] Comparative Example 5

[0126] Compared with Comparative Example 1, the difference is that the mass ratio of the first material to the first carbon source is 100:15, and the other conditions are the same as those in Comparative Example 1.

[0127] Comparative Example 6

[0128] The difference from Comparative Example 1 is that the first carbon source used is sucrose, while the other conditions are the same as those in Comparative Example 1.

[0129] Comparative Example 7

[0130] Compared with Comparative Example 1, the difference is that the mass ratio of the first material to the first carbon source is 100:20, and the other conditions are the same as those in Comparative Example 1.

[0131] In addition, the lithium iron phosphate cathode materials prepared in the examples and comparative examples were subjected to the following tests.

[0132] (1) Particle size test

[0133] Approximately 0.5 g of the second material powder was dispersed in water, and three drops of 1 wt% Triton were added as a dispersant. The mixture was sonicated for 3 minutes to obtain a mixed solution. The mixed solution was tested using a Mastersizer 3000 laser particle size analyzer, and the particle size at which the cumulative particle size distribution reached 50% was taken as Dv. 50 The particle size D of the second material is obtained. 50 .

[0134] (2) Raman spectroscopy test

[0135] The equipment used for testing was a Renishaw Qontor Raman spectrometer, with a laser intensity of 1-5% (care should be taken not to burn the sample to prevent the introduction of impurity peaks that could affect the results), an exposure time of 30 seconds, and a test range of 500-2000 cm⁻¹. -1 To obtain Raman spectra (e.g.) Figure 1 ).

[0136] Then, the Raman spectrum was analyzed using the Wire software tool. First, the Cosmic ray removal button was clicked to remove background noise peaks. Then, Analysis-curve fit was used to perform peak fitting, obtaining peak D (characteristic peak wavenumber of 1350 cm⁻¹). -1 ) and G peak (characteristic peak wavenumber 1590 cm⁻¹) -1 The area ratio of (A) D / A G The peak G and the symmetric stretching vibration peak of the phosphate group (characteristic peak wavenumber 945 cm⁻¹) -1 ~955cm -1 The area ratio of (A) D / A v ), for the measured A D / A G Take the average value to obtain the A value of the lithium iron phosphate cathode material or cathode sheet. D / A G For the measured A D / A v Take the standard deviation to obtain σ(A) G / A v ).

[0137] The following tests were performed on the positive electrode sheets prepared in the examples and comparative examples.

[0138] (3) SEM test

[0139] The positive electrode sheet was cut into 15*15mm samples and photographed using a SU8600 scanning electron microscope (SEM). The single-magnification image capture range was ≥5 groups at 2K, 5K, 10K, and 20K to obtain SEM images of the lithium iron phosphate positive electrode material in the positive electrode sheet (e.g., Figure 2 ).

[0140] The cross-section of lithium iron phosphate cathode material particles in the SEM image was measured using the image recognition software Mipar. The longest connecting line (longest side) and the shortest connecting line (shortest side) on the particle edge line were obtained, as well as the cross-sectional area of ​​the particle. The diameter (equivalent particle size) of the circle with the same cross-sectional area as the particle was calculated. According to the roundness S = equivalent particle size / longest side, the roundness S of the lithium iron phosphate cathode material was obtained.

[0141] In addition, the average particle size of the lithium iron phosphate cathode material was obtained by measuring the cross-sectional area using the image recognition software Mipar.

[0142] (4) Transmission electron microscopy (TEM) test

[0143] The cross-section of the positive electrode was obtained using FIB (Film Injection Biometry). This cross-section included several cross-sections of the positive electrode active material. TEM (Transmission Electron Microscopy) was performed on these cross-sections, and the thickness of the carbon coating layer was statistically analyzed from at least 50 TEM images. When the carbon coating layer thickness was non-uniform, the average thickness from multiple TEM fields was used to determine the final carbon coating layer thickness.

[0144] (5) Test of the areal density of the positive electrode

[0145] Using a sampler with a diameter of R=20mm, take five small circles (intervals <10mm) consecutively along the length of the positive electrode sheet from the center (avoiding the thinned area, distance >30mm) to obtain a sample. Turn on the electronic scale, adjust the small water droplet on the left side of the scale to the center position, press the zero button on the left side to zero it, and then weigh the weight m0 of a single sample. Calculate the theoretical mass m1 of the foil material of the sample based on the foil thickness; use the formula ρ s =(m0-m1) / (π*(R / 2))Calculate the surface density ρ of the sample s Calculate the average of the areal densities of all samples as the areal density of the positive electrode.

[0146] (6) Test of compaction density of positive electrode sheet

[0147] The thickness h1 of the positive electrode and the thickness h0 of the optical foil were measured using a Marl thickness gauge, based on the compaction density ρ. v =ρ s The compaction density ρ of the positive electrode is calculated using / (h1-h0). v .

[0148] The following tests were performed on the batteries prepared in the examples and comparative examples.

[0149] (7) Ratio performance test

[0150] At 25℃, the battery was fully charged at 1 / 3C and fully discharged at 1 / 3C for 3 cycles, and the actual discharge capacity of the third cycle was recorded as the initial capacity E0. Subsequently, the battery was fully charged at 1 / 3C and fully discharged at 5C for 3 cycles, and the discharge capacity of the third cycle was recorded as the rate capacity E1. The rate performance was evaluated using E1 / E0. More than 2 batteries were selected from each batch for testing, and the average value of the two batteries with a range of less than 2% was taken as the final result.

[0151] The rate performance level is defined as follows: the target value of rate performance is 95%. When the actual test value is less than 95% of the target value, the performance is considered to be at level C; when the actual test value is greater than or equal to 95% of the target value and less than 108% of the target value, the battery performance is considered to be at level B; and when the actual test value is greater than or equal to 108% of the target value, the battery performance is considered to be at level A.

[0152] (8) Impedance test

[0153] The DC internal resistance (DCIR) was used to evaluate the battery's impedance level. At 25°C, the battery was fully charged at 1 / 3C and fully discharged at 1C for three cycles. The actual discharge capacity of the third cycle was recorded as the initial capacity C0. After fully charging the battery at 1 / 3C, it was discharged at 1C0 for 24 minutes to adjust to 60% SOC (State of Charge). The battery was then left to rest at room temperature for 2 hours, and the end voltage V0 was recorded. Subsequently, it was charged at 3C for 30 seconds, and the cutoff voltage V1 was recorded. The DCIR was calculated using DCIR = (V1 - V0) / 3C. More than two batteries from each batch were selected for testing, and the average value of the two batteries with a range less than 2% was taken as the final result.

[0154] The impedance performance level is defined as follows: the target value of impedance performance is 2.5 mΩ. When the actual test value is less than 95% of the target value, the performance is considered to be at level A; when the actual test value is greater than or equal to 95% of the target value and less than 107% of the target value, the battery performance is considered to be at level B; and when the actual test value is greater than or equal to 107% of the target value, the battery performance is considered to be at level C.

[0155] The chemical formula of the lithium iron phosphate matrix, the average particle size of the lithium iron phosphate cathode material, the thickness of the carbon coating layer, and the particle size D of the second material were determined. 50 The results are summarized in Table 1.

[0156] Table 1

[0157]

[0158] A of lithium iron phosphate cathode material D / A G σ(A) G / A v ), S, (A D / A G )×S+σ(A G / A v Table 2 summarizes the rate performance level and impedance performance level of the battery.

[0159] Table 2

[0160]

[0161] Compared to Comparative Examples 1-7, the lithium iron phosphate cathode materials prepared in Examples 1-13 of this invention satisfy 5 ≤ (A D / A G )×S+σ(A G / A v With a value of ≤45, the battery exhibits superior rate performance and impedance performance.

[0162] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate positive electrode material comprises a lithium iron phosphate base and a carbon coating layer coated on the lithium iron phosphate base. The lithium iron phosphate positive electrode material satisfies 5≤(A D / A G )×S+σ(A G / A v )≤45; wherein A D is a peak area of a D peak in a Raman spectrum of the lithium iron phosphate positive electrode material; A G is a peak area of a G peak in a Raman spectrum of the lithium iron phosphate positive electrode material; A v is a peak area of a symmetric stretching vibration peak of a phosphate group in a Raman spectrum of the lithium iron phosphate positive electrode material; σ(A G / A v ) is a standard deviation of A G / A v ; and S is a circularity of the lithium iron phosphate positive electrode material.

2. The lithium iron phosphate positive electrode material according to claim 1, wherein, The A D / A G is 1-4; and / or, the S is 0.6-1; and / or, the σ(A G / A v ) is 5 to 40.

3. The lithium iron phosphate cathode material of claim 1, wherein, The chemical formula of the lithium iron phosphate matrix is LiFe 1-x M x PO4, 0≤x<1; M comprises at least one of Mn, Zn, Ni, Mg, and Al.

4. The lithium iron phosphate positive electrode material according to claim 1, wherein, the average particle size of the lithium iron phosphate positive electrode material is 50 nm-1500 nm; and / or, the thickness of the carbon coating layer is greater than 0 nm and less than or equal to 30 nm.

5. The lithium iron phosphate positive electrode material according to claim 1, wherein, the average particle size of the lithium iron phosphate positive electrode material is 500 nm-900 nm; and / or, the thickness of the carbon coating layer is 4 nm-8 nm.

6. A method of producing the lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that The method comprises the following steps: sintering a mixture comprising a lithium source, an iron source, a phosphorus source, and a first carbon source to obtain an intermediate; mixing the intermediate with a second carbon source and then sintering to obtain the lithium iron phosphate positive electrode material.

7. The method of claim 6, wherein the lithium iron phosphate cathode material is prepared by the steps of: The molar ratio of the lithium source to the iron source is (0.95-1.10):1; ​ and / or, the molar ratio of the phosphorus source to the iron source is (0.95-1.10):

1.

8. The method for preparing the lithium iron phosphate positive electrode material according to claim 6, wherein, the preparation process of the mixture comprises: mixing a first material comprising a lithium source, an iron source, and a phosphorus source with a first carbon source, ball milling to obtain a second material, and then spray drying the second material to obtain the mixture; Preferably, the mass ratio of the first material to the first carbon source is 100:(5-30); Preferably, the first material further comprises a manganese source, and the molar ratio of the lithium source to the manganese source is 1:(0-0.3); Preferably, the particle size D 50 is from 200 nm to 950 nm; Preferably, the particle size of the mixture ranges from 10 μm to 80 μm.

9. The method for preparing the lithium iron phosphate cathode material according to claim 6, characterized in that, The first sintering is performed in an inert atmosphere; and / or, the mass ratio of the intermediate to the second carbon source is 100:(5-20); and / or, the temperature of the first sintering is 700°C-800°C; and / or, the time of the first sintering is 10 h-24 h; and / or, the temperature of the second sintering is 600°C-800°C; and / or, the time of the second sintering is 10 h-24 h.

10. A positive electrode sheet characterized by comprising: The lithium iron phosphate positive electrode material comprises the lithium iron phosphate positive electrode material according to any one of claims 1-5 or the lithium iron phosphate positive electrode material prepared by the method according to any one of claims 6-9.

11. A battery, characterized by The positive electrode sheet comprises the positive electrode material according to claim 10.

12. A battery pack, characterized by The battery comprises at least two batteries connected to each other, and each battery is the battery according to claim 11.

13. An electrical device, characterized by The battery pack comprises the battery according to claim 11 or the battery according to claim 12.

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