Lithium ion secondary battery, positive electrode active material and power-consuming device

Abstract

The present application relates to a lithium-ion secondary battery, a positive electrode active material, and an electrical device. The lithium-ion secondary battery includes a positive electrode plate, which includes a positive electrode film layer; the positive electrode film layer includes a positive electrode active material, which includes a mixture of a first lithium-containing phosphate material in a granular form and a second lithium-containing phosphate material in a rod-shaped form. The first and second lithium-containing phosphate materials have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction pattern, and the intensity ratio of diffraction peak A to diffraction peak B is 1:1. A / I B ; The second lithium phosphate material satisfies: 1.1≥I A / I B ≥0.95; I of the second lithium-containing phosphate material A / I B Greater than the I of the first lithium-containing phosphate material A / I B ; Both the first lithium-containing phosphate material and the second lithium-containing phosphate material contain a carbon coating layer, and the amount of carbon coating in the first lithium-containing phosphate material is less than the amount of carbon coating in the second lithium-containing phosphate material.

Description

[0001] Related applications

[0002] This application is a divisional application of the Chinese patent application submitted by the applicant to the China Patent Office on November 4, 2024, with application number 2024115563633 and titled "Lithium-ion secondary batteries, positive electrode active materials and electrical devices". Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a lithium-ion secondary battery, a positive electrode active material, and an electrical device. Background Art

[0004] In recent years, the application of lithium-ion secondary batteries has become increasingly widespread. They are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. Among them, lithium-ion phosphate batteries such as lithium iron phosphate have achieved significant development.

[0005] As demand for batteries increases, higher requirements are placed on their energy density and low-temperature performance. However, lithium-phosphate materials, such as lithium iron phosphate, have inherently low electronic conductivity, which significantly reduces their ion diffusion rate at low temperatures. This in turn affects their performance under low-temperature conditions, particularly energy density, capacity retention, and power performance. Summary of the Invention

[0006] In order to achieve the above objectives, the first aspect of the present application provides a lithium-ion secondary battery, a positive electrode active material and an electrical device with good energy density, capacity retention and power performance under low temperature conditions.

[0007] In a first aspect of the present application, a lithium-ion secondary battery is provided, comprising a positive electrode plate, wherein the positive electrode plate comprises a positive electrode film layer; the positive electrode film layer comprises a positive electrode active material, wherein the positive electrode active material comprises a mixture of a first lithium-containing phosphate material containing particles and a second lithium-containing phosphate material containing rods, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction pattern, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1:1. A / I B ; The second lithium phosphate material satisfies: 1.1≥I A / I B ≥0.95; I of the second lithium-containing phosphate material A / I B greater than the first lithium-containing phosphate material I A / I B; Both the first lithium-containing phosphate material and the second lithium-containing phosphate material contain a carbon coating layer, and the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, and the carbon coating amount refers to the mass content of carbon.

[0008] Thus, the lithium-ion secondary battery of the present application is mainly improved from the positive electrode sheet, specifically, by grading lithium-containing phosphate materials with different morphologies of granular and rod-shaped to obtain a better dense packing effect, thereby improving the compaction density of the positive electrode sheet and thus improving the energy density of the lithium-ion secondary battery; in addition, the second lithium-containing phosphate material used has a diffraction peak at a specific position, and the intensity ratio of diffraction peak A to diffraction peak B is limited to 1. A / I B , and controlling the difference in carbon coating amount between the first lithium-containing phosphate material and the second lithium-containing phosphate material and I A / I B The difference can improve the low-temperature performance of lithium-ion secondary batteries, while taking into account good energy density, capacity retention and power performance under low-temperature conditions.

[0009] In some embodiments, in the positive electrode active material, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material.

[0010] In some embodiments, the mass content of the second lithium-containing phosphate material is 5% to 50% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

[0011] In some embodiments, the mass content of the second lithium-containing phosphate material is 10% to 30% of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material.

[0012] As the amount of the second lithium-containing phosphate material increases, the power performance of the battery is improved; however, a larger amount of the second lithium-containing phosphate material will affect the energy density; by controlling the mass content of the second lithium-containing phosphate within the above range, it is beneficial to balance the low-temperature performance and volume energy density of the lithium-ion secondary battery, so that the lithium-ion secondary battery has both good low-temperature performance and volume energy density.

[0013] In some embodiments, at least one of the following conditions is met:

[0014] (1) The carbon coating amount in the first lithium-containing phosphate material is 1% to 1.3%;

[0015] (2) The carbon coating amount in the second lithium-containing phosphate material is 1.3% to 1.8%;

[0016] (3) The difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% to 0.8%.

[0017] Controlling the difference in the amount of carbon coating is beneficial to further improving the low-temperature performance of the secondary battery, while taking into account better energy density, capacity retention and power performance under low-temperature conditions.

[0018] In some embodiments, at least one of the following conditions is met:

[0019] (1) The first lithium-containing phosphate material satisfies: 0.92≥I A / I B ≥0.8;

[0020] (2) The second lithium-containing phosphate material satisfies: 1.05≥I A / I B ≥0.95.

[0021] Second lithium-containing phosphate material I A / I B Within this range, it can further improve the low-temperature performance of the lithium-ion secondary battery.

[0022] In some embodiments, the average primary particle size of the first lithium-containing phosphate material is greater than the average primary particle size of the second lithium-containing phosphate material.

[0023] A larger carbon coating inhibits the particle size growth of the lithium-containing phosphate material, making it difficult to obtain larger particles, thereby reducing the compaction density of the first lithium-containing phosphate material. Furthermore, increasing the carbon coating amount of the first lithium-containing phosphate material reduces its specific capacity utilization. Therefore, the coating amount of the first lithium-containing phosphate material with a larger particle size is controlled to be smaller. The first lithium-containing phosphate material primarily provides a high compaction density, thereby increasing the energy density of the battery. The second lithium-containing phosphate material has a smaller average primary particle size and a greater probability of side reactions. Therefore, it is necessary to increase its carbon coating amount to improve the integrity of the carbon coating, reduce the probability of side reactions, and improve the effective capacity utilization and electronic conductivity of the second lithium-containing phosphate material. Therefore, the coating amount of the second lithium-containing phosphate material with a smaller particle size is controlled to be larger. The second lithium-containing phosphate material primarily provides high kinetic performance, thereby improving the power performance of the battery. In this way, the combination of the first lithium-containing phosphate material with a larger particle size and a smaller coating amount and the second lithium-containing phosphate material with a smaller particle size and a larger coating amount can achieve lithium-ion secondary batteries with improved energy density, capacity retention, and power performance under low-temperature conditions.

[0024] In some embodiments, the ratio of the average primary particle size of the first lithium-containing phosphate material to the average primary particle size of the second lithium-containing phosphate material is 3 to 40. By controlling this ratio, the second lithium-containing phosphate material can better fill the gaps in the first lithium-containing phosphate material, achieving a good dense packing effect. At the same time, the second lithium-containing phosphate material with a smaller particle size can better play its role in improving the power performance of the battery.

[0025] In some embodiments, the average primary particle size of the first lithium-containing phosphate material is 700 nm to 1500 nm.

[0026] In some embodiments, the average primary particle size of the first lithium-containing phosphate material is 700 nm to 1200 nm.

[0027] By controlling the average primary particle size of the first lithium-containing phosphate material within the above range, the first lithium-containing phosphate material and the second lithium-containing phosphate material can better fill each other, achieve better tight stacking, and improve the overall compaction density of the positive electrode film layer, thereby taking into account good power performance, high capacity retention rate and energy density.

[0028] In some embodiments, the average primary particle size of the second lithium-containing phosphate material is 20 nm to 260 nm.

[0029] In some embodiments, the average primary particle size of the second lithium-containing phosphate material is 80 nm to 160 nm.

[0030] The increased average particle size of the primary particles of the second lithium-containing phosphate material increases the transport path between the particles. Therefore, controlling the average particle size of the primary particles of the second lithium-containing phosphate material within the aforementioned range can improve the mobility of lithium ions in the positive electrode active material, thereby facilitating the lithium-ion secondary battery's ability to maintain good low-temperature capacity retention, power performance, and energy density. Furthermore, combining this with the larger particle size of the first lithium-containing phosphate material can increase the compaction density, thereby improving the battery's energy density.

[0031] In some embodiments, the aspect ratio of the second lithium-containing phosphate material is (1.1-3.9):1.

[0032] The aspect ratio of the second lithium-containing phosphate material is controlled within the above range, so that the rod-shaped lithium-containing phosphate material in the second lithium-containing phosphate material is mainly short rod-shaped, which can provide a better exposure effect of the (010) crystal plane, which is conducive to better low-temperature performance. In addition, the aspect ratio within the above range makes its specific surface area within a suitable range, which is conducive to controlling side reactions and improving battery life. In addition, the upper limit of the aspect ratio is further controlled to control the compaction density of the positive electrode sheet within a suitable range, which is conducive to improving the energy density of the battery.

[0033] In some embodiments, at least one of the following conditions is met:

[0034] (1) The powder compaction density of the first lithium-containing phosphate material under a pressure of 3t is 2.38 g / cm 3 ~2.68 g / cm 3 ;

[0035] (2) The powder compaction density of the second lithium-containing phosphate material under a pressure of 3t is 1.85 g / cm 3 ~2.35 g / cm 3 ;

[0036] (3) The compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g / cm 3 ~2.8g / cm 3 .

[0037] In some embodiments, the chemical formula of the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material is Li β Fe α M (1-α) PO4, wherein 0.2≤α≤1, 1≤β≤1.1, and M includes at least one of Ti, V, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, and Sr.

[0038] In some embodiments, in the positive electrode film layer, the total mass content of the positive electrode active material is 92% to 98%.

[0039] In some embodiments, the positive electrode film layer further includes a binder, and the positive electrode film layer satisfies at least one of the following characteristics:

[0040] (1) In the positive electrode film layer, the mass content of the binder is 1% to 4%;

[0041] (2) The binder includes at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0042] In some embodiments, the positive electrode film layer further includes a conductive agent, and the positive electrode film layer satisfies at least one of the following characteristics:

[0043] (1) In the positive electrode film layer, the mass content of the conductive agent is 1% to 4%;

[0044] (2) The conductive agent includes at least one of conductive carbon, metal fiber and organic conductive polymer.

[0045] In some embodiments, the conductive carbon includes at least one of carbon nanotubes, graphite, graphene, carbon black, Ketjen black, carbon dots, and carbon nanofibers, and the carbon black includes acetylene black.

[0046] In some embodiments, the lithium-ion secondary battery further includes an electrolyte containing additives, including at least one of 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazolium bis(trifluoromethanesulfonamide), aluminum oxide-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide, lithium difluorobisoxalatoborate, fluoroethylene carbonate, and 1,3-dioxolane. The inclusion of these additives in the electrolyte can improve the low-temperature performance of the battery.

[0047] In a second aspect of the present application, a positive electrode active material is provided, wherein the positive electrode active material comprises a mixture of a first lithium-containing phosphate material containing particles and a second lithium-containing phosphate material containing rods, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction pattern, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1:1. A / I B ; The second lithium phosphate material satisfies: 1.1≥I A / I B ≥0.95; I of the second lithium-containing phosphate material A / I B greater than the first lithium-containing phosphate material I A / I B ; Both the first lithium-containing phosphate material and the second lithium-containing phosphate material contain a carbon coating layer, and the amount of carbon coating in the first lithium-containing phosphate material is less than the amount of carbon coating in the second lithium-containing phosphate material.

[0048] In some embodiments, the positive electrode active material is the positive electrode active material in the lithium-ion secondary battery provided in the first aspect of the present application.

[0049] In a third aspect of the present application, an electrical device is provided, comprising the lithium-ion secondary battery provided in the first aspect of the present application and at least one of the positive electrode active materials provided in the second aspect of the present application.

[0050] The electric device of the present application includes the lithium-ion secondary battery provided by the present application, and thus has at least the same advantages as the lithium-ion secondary battery.

[0051] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the accompanying drawings should not be considered as limiting the scope of the disclosed application, the embodiments or examples currently described, and any of the best modes currently understood for these applications. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the accompanying drawings:

[0053] Figure 1 Schematic diagram of a lithium-ion secondary battery according to one embodiment of the present application.

[0054] Figure 2 yes Figure 1 FIG. 1 is an exploded view of a lithium-ion secondary battery according to an embodiment of the present application.

[0055] Figure 3 Schematic diagram of a battery module according to one embodiment of the present application.

[0056] Figure 4 Schematic diagram of a battery pack according to one embodiment of the present application.

[0057] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0058] Figure 6 Schematic diagram of an electrical device using a lithium-ion secondary battery according to one embodiment of the present application as a power source.

[0059] Figure 7 3 is an SEM image of the first lithium-containing phosphate material and the second lithium-containing phosphate material in Example 1.

[0060] Figure 8This is the XRD pattern of the second lithium-containing phosphate material prepared in Example 1 of the present application.

[0061] Figure 9 yes Figure 8 Magnified view at diffraction angles 2Theta between 24° and 31°.

[0062] Description of reference numerals:

[0063] 1. Battery pack; 2. Upper box; 3. Lower box; 4. Battery module; 5. Battery cell; 51. Housing; 52. Electrode assembly; 53. Cover; 6. Electrical device. DETAILED DESCRIPTION

[0064] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0065] " scope " disclosed in the present application can be limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and any end value can be included or not included independently, and can be arbitrarily combined, that is, any lower limit can form a scope with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 are also listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is merely an abbreviation for a combination of these values. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when a parameter is expressed as an integer selected from "2-10", this is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0066] In this application, "a plurality of" or "a plurality of" refers to a number greater than or equal to 2 unless otherwise specified. For example, "one or more" means one or more than or equal to two.

[0067] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0068] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor does it necessarily refer to independent or alternative embodiments that are mutually exclusive with other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments. References to "implementations" herein have a similar understanding.

[0069] It will be appreciated by those skilled in the art that, in the methods of various embodiments or examples, the order in which the steps are written does not imply a strict order of execution and does not constitute any limitation on the implementation process, and the detailed order of execution of each step should be determined by its function and possible inherent logic. Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0070] In this application, open technical features or technical solutions described with words such as "contain," "include," and "includes" do not exclude additional members beyond the listed members unless otherwise specified, and can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions that include additional members beyond the listed members. For example, if A includes a1, a2, and a3, it may or may not include other members unless otherwise specified. This can be regarded as providing both the feature or solution of "A consists of a1, a2, and a3" and the feature or solution of "A includes not only a1, a2, and a3, but also other members."

[0071] In this application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0072] In this application, the terms "optionally," "optional," and "optional" are optional and refer to either option being present or absent. If a technical solution contains multiple "options," each option is considered independent unless otherwise specified and there are no conflicts or constraints.

[0073] As described in the background, increasing demand for batteries has led to higher requirements for energy density and low-temperature performance. However, lithium-phosphate materials, such as lithium iron phosphate, inherently have low electronic conductivity, significantly reducing their ion diffusion rate at low temperatures. This, in turn, affects their performance under low-temperature conditions, particularly energy density, capacity retention, and power performance.

[0074] Therefore, how to improve the energy density, capacity retention and power performance of lithium-ion secondary batteries under low temperature conditions is an urgent problem to be solved.

[0075] Based on this, one embodiment of the present application provides a lithium-ion secondary battery, a positive electrode sheet, a positive electrode active material, and an electrical device. The following will describe the positive electrode sheet and the positive electrode active material in detail in conjunction with the lithium-ion secondary battery.

[0076] One embodiment of the present application provides a lithium-ion secondary battery comprising a positive electrode plate, the positive electrode plate comprising a positive electrode film layer; the positive electrode film layer comprising a positive electrode active material, the positive electrode active material comprising a mixture of a first lithium-containing phosphate material comprising particles and a second lithium-containing phosphate material comprising rods. In other words, the first lithium-containing phosphate material comprises a lithium-containing phosphate material comprising particles, and the second lithium-containing phosphate material comprises a lithium-containing phosphate material comprising rods.

[0077] The first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in the X-ray diffraction spectrum, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1: A / I B The second lithium-containing phosphate material satisfies: 1.1≥I A / I B ≥0.95. I of the second lithium-containing phosphate material A / I B greater than the first lithium-containing phosphate material I A / I B .

[0078] The first lithium-containing phosphate material and the second lithium-containing phosphate material both contain a carbon coating layer. The carbon coating amount in the first lithium-containing phosphate material is less than that in the second lithium-containing phosphate material. The carbon coating amount refers to the mass content of carbon.

[0079] Taking lithium iron phosphate as an example, the lithium phosphate material contains a diffraction peak A between 29° and 30° in the X-ray diffraction spectrum. The diffraction peak A corresponds to the (010) crystal plane of the lithium iron phosphate material. Peak A and peak B are the two main peaks of the lithium iron phosphate material. The intensity ratio of peak A (010) can indirectly represent the exposure ratio of the (010) crystal plane. Further, in the present application, the first lithium iron phosphate material containing particles I A / I B In the range of 0.8 to 0.92, as an example, the I of the first lithium iron phosphate material A / I B It can be 0.8, 0.82, 0.85, 0.86, 0.88, 0.89, 0.9, 0.91, 0.92, or within the range formed by any two of the above point values ​​as end values.

[0080] By controlling the intensity ratio of the diffraction peak of the second lithium-containing phosphate material in the X-ray diffraction pattern A / I B In the above range, the ratio of the (010) crystal plane exposure of the second lithium-containing phosphate material can be regulated, thereby regulating the electrochemical reaction kinetics of the second lithium-containing phosphate material. A / I B The higher the value, the higher the exposure ratio of the (010) crystal plane of the second lithium-containing phosphate material. This is presumably because during the electrochemical reaction, lithium ions undergo insertion and extraction reactions along the b-axis in the second lithium-containing phosphate material. Therefore, a higher exposure ratio of the (010) crystal plane is more conducive to improving the electrochemical reaction kinetics, thereby enabling the second lithium-containing phosphate material to exhibit excellent low-temperature performance when used in lithium-ion secondary batteries.

[0081] At the same time, the carbon coating amount of the second lithium-containing phosphate material is controlled to be large, and the electronic conductivity of the second lithium-containing phosphate material is further improved, which is beneficial to improving its power performance. At the same time, its coating amount can be increased, the coating integrity can be improved, the side reaction of the second lithium-containing phosphate material can be reduced, and the consumption of the active component of the second lithium-containing phosphate material can be reduced, which is beneficial to improving the capacity retention rate of the battery; the I of the first lithium-containing phosphate material A / I B Less than the I of the second lithium-containing phosphate material A / I B , which is beneficial to improving the compaction density of the first lithium-containing phosphate material, while more carbon has an adverse effect on the compaction density of the first lithium-containing phosphate material, and more carbon will affect the gram capacity of the first lithium-containing phosphate material; therefore, the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, which is beneficial to improving the energy density, capacity retention rate and power performance of the lithium-ion secondary battery under low temperature conditions.

[0082] Thus, the lithium-ion secondary battery of the present application is mainly improved from the positive electrode sheet, specifically, by grading lithium-containing phosphate materials with different morphologies of granular and rod-shaped to obtain a good dense packing effect, thereby improving the compaction density of the positive electrode sheet and thus improving the energy density of the lithium-ion secondary battery; in addition, the second lithium-containing phosphate material used has a specific intensity ratio of diffraction peak A to diffraction peak B of 1:1. A / I B , and controlling the difference in carbon coating amount between the first lithium-containing phosphate material and the second lithium-containing phosphate material and I A / I B The difference can improve the low-temperature performance of lithium-ion secondary batteries, while taking into account good energy density, capacity retention and power performance under low-temperature conditions.

[0083] In some embodiments, the difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% to 0.8%. As an example, the difference is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or a range formed by any two of the above values. It can further be 0.3% to 0.8%, and even more preferably 0.5% to 0.8%. Controlling this difference in carbon coating amount is beneficial to further improving the low-temperature performance of the secondary battery, while achieving better energy density, capacity retention, and power performance under low-temperature conditions.

[0084] In some embodiments, the carbon coating amount of the first lithium-containing phosphate material is 1% to 1.3%; as an example, the carbon coating amount of the first lithium-containing phosphate material can be 1%, 1.1%, 1.2%, 1.3%, or within a range consisting of any two of the above point values ​​as end values.

[0085] In some embodiments, the carbon coating amount of the second lithium-containing phosphate material is 1.3%~1.8%; as an example, the carbon coating amount of the second lithium-containing phosphate material can be 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or within a range consisting of any two of the above point values ​​as end values.

[0086] The following test methods can be used to qualitatively and quantitatively test the positive electrode active materials in the positive electrode sheets of lithium-ion secondary batteries:

[0087] 1) Battery disassembly

[0088] a. Fully discharge the battery until the voltage reaches 2.0 V.

[0089] b. Then disassemble the battery in the glove box and control the humidity in the glove box to less than 2%;

[0090] c. Separate the positive electrode from the battery, then soak and fully wash off the electrolyte on the positive electrode with DMC (dimethyl carbonate);

[0091] d. Take out the positive electrode and place it in a vacuum drying oven at 80°C for 5 h.

[0092] 2) Separation of positive electrode active materials

[0093] a. Separating the dried positive electrode film and the positive electrode current collector of the positive electrode sheet;

[0094] b. The separated positive electrode film layer was ground into a powder and dissolved in NMP (N-methylpyrrolidone) under heating conditions, and then filtered and the residue was collected, and the dissolution and filtration steps were repeated until the binder was fully removed;

[0095] c. Filter and dry the mixture after fully washing away the binder.

[0096] 3) Conductive agent separation

[0097] The dried mixture powder is dispersed in a liquid medium (the density of the liquid medium is greater than 1.2 g / cm 3 , such as nitrobenzene, bromobenzene or carbon tetrachloride, etc.), and stirred to make it uniform and basically uniformly mixed. After standing for a long enough time, the solution is separated into layers, the upper layer is a conductive agent such as conductive carbon, and the lower layer is a positive electrode active material.

[0098] 4) Separation of the first lithium-containing phosphate material and the second lithium-containing phosphate material

[0099] Principle: Since the density difference between the first lithium-containing phosphate material and the second lithium-containing phosphate material is small, the two cannot be separated by density difference; when separating the first lithium-containing phosphate material and the second lithium-containing phosphate material, it is necessary to further utilize the difference in their surface wettability: the carbon coating amount of the first lithium-containing phosphate material is less than the carbon coating amount of the second lithium-containing phosphate material, so the hydrophilicity of the first lithium-containing phosphate material is greater than that of the second lithium-containing phosphate material; therefore, bubble flotation is used for separation in a hydrophilic liquid medium. The first lithium-containing phosphate material has a greater hydrophilicity and is more easily precipitated, while the second lithium-containing phosphate material has a less hydrophilicity and is floated.

[0100] Specific steps:

[0101] a. The positive electrode active material in step 3) is dried to obtain a mixture powder;

[0102] b. Similar to the coal screening process, select a suitable hydrophilic liquid medium (such as water, ethylene glycol or glycerol, etc.) and a suitable flotation agent (such as fat oil 190), and use bubble flotation method for separation.

[0103] The specific steps are as follows: taking 1 kg of the mixed positive electrode material in a liquid medium, adjusting it to a relatively suitable solid content, adding a flotation agent, stirring to generate bubbles, collecting the floating foam, washing and drying it, and then obtaining the second lithium-containing phosphate material; collecting the slurry at the bottom, washing and drying it, and then obtaining the first lithium-containing phosphate material.

[0104] c. Drying the separated first lithium-containing phosphate material and the second lithium-containing phosphate material separately, and performing subsequent characterization analysis on each.

[0105] 5) I A / I B Detection method:

[0106] A certain amount of the separated second lithium-containing phosphate material was weighed, and the diffraction peak intensity ratio of the second lithium-containing phosphate material was tested by X-ray. The second lithium-containing phosphate material was placed on the test platform of the X-ray diffractometer (model Shimadzu XRD-7000) using a copper target X-ray diffractometer. The starting angle of the scan was 10°, the ending angle was 90°, and the step size was 0.013. Then, the test was started to obtain a diffraction pattern of the second lithium-containing phosphate material in the diffraction angle range of 10° to 90°. The intensity ratio I of the diffraction peak was determined according to the diffraction pattern. A / I B .

[0107] Then, the separated first lithium-containing phosphate material and the second lithium-containing phosphate material were tested by ICP according to GB / T 33822-2017 to obtain their respective carbon coating amounts.

[0108] As an example, the intensity ratio of the diffraction peak A to the diffraction peak B of the second lithium-containing phosphate material is A / I B It can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.02, 1.04, 1.05, 1.06, 1.08, 1.1, or within the range formed by any two of the above point values ​​as end values. A / I B Satisfaction: I A / I B ≥0.96; further, I A / I B ≥0.98.

[0109] In some embodiments, the intensity ratio of the diffraction peak A to the diffraction peak B of the second lithium-containing phosphate material is A / I B Satisfy: 1.1≥I A / I B ≥0.95; further, 1.05≥IA / I B ≥0.95; further, 1.05≥I A / I B ≥0.96. Within this range, it can further improve the low-temperature performance of lithium-ion secondary batteries.

[0110] The first lithium-containing phosphate material and the second lithium-containing phosphate material of the present application can be purchased commercially. The present application has no particular limitation on the preparation method of the second lithium-containing phosphate material, as long as the purpose of the present application can be achieved.

[0111] In some embodiments, the second lithium-containing phosphate material can be prepared by a liquid phase synthesis method. Taking lithium iron phosphate as an example, the method includes the following steps:

[0112] Step A: Mix an iron source, a lithium source and a phosphorus source in a certain proportion, add a solvent and mix to obtain a mixed solution A.

[0113] Furthermore, the solvent is selected from alcohol, water or a mixed solvent of alcohol and water. Furthermore, the alcohol is selected from one of ethanol, methanol, ethylene glycol and propylene glycol. Furthermore, the iron source includes but is not limited to at least one of divalent iron salts such as ferrous sulfate, ferrous chloride, ferrous acetate and ferrous oxalate. Furthermore, the phosphorus source includes but is not limited to at least one of phosphoric acid, ammonium dihydrogen phosphate, (NH4)2HPO4, LiH2PO4, Li3PO4 and (NH4)3PO4. Furthermore, the lithium source includes but is not limited to at least one of lithium hydroxide, lithium oxide, lithium chloride, lithium nitrite, lithium nitrate, lithium oxalate, lithium carbonate, lithium acetate, lithium phosphate, lithium dihydrogen phosphate and lithium hydrogen phosphate.

[0114] Furthermore, the amounts of the iron source, lithium source, and phosphorus source added are based on a molar concentration ratio of 0.5 mol:0.65 mol:1.5 mol of iron atoms, phosphorus atoms, and lithium atoms.

[0115] Step B: After adding a surfactant to the mixed solution A, a pH adjuster is added to adjust the pH of the mixed solution A to 7-8 to obtain a mixed solution B.

[0116] The surfactant includes but is not limited to at least one of sodium citrate, sodium lactate, sodium malate, and sodium tartrate. Furthermore, as an example, the mass ratio of the surfactant to the mixed solution A is 1:100.

[0117] Step C: Transfer the mixed solution B to a reactor, seal it and react it at a temperature of 170°C to 250°C for 1 hour to 6 hours. After cooling, filter and wash the precipitate obtained by the reaction to obtain a lithium iron phosphate material.

[0118] During the hydrothermal precipitation process, the (010) crystal plane of lithium iron phosphate exposes more iron ions. Due to the coordination effect of the surfactant, they are adsorbed on the (010) surface, limiting the further growth rate of the (010) plane, causing the thickness in the (010) crystal direction to decrease. The energy required for the lithium ion deintercalation reaction along the (010) crystal direction is the lowest, which can significantly improve the lithium ion transmission speed and reduce the solid-phase reaction diffusion impedance.

[0119] Furthermore, the method may further include step D: drying the lithium iron phosphate material obtained in step C, uniformly mixing the material with a carbon source, and then sintering and carbonizing the material in a nitrogen inert atmosphere to obtain a carbon-coated lithium iron phosphate material.

[0120] Furthermore, the carbon source includes but is not limited to at least one of glucose, sucrose, starch, and polyethylene glycol. Furthermore, the sintering and carbonization temperature is 750° C. to 800° C. Furthermore, the mass ratio of the lithium iron phosphate material obtained in step C to the carbon source is 20:1.

[0121] In the present application, the intensity ratio of the diffraction peak I can be controlled by controlling the hydrothermal reaction time t in step C, the concentration C of the hydrothermal reactant in step C, the concentration Cx of the surfactant added to the hydrothermal reactant in step C, or the selection of the surfactant. A / I B The applicant found that as the reaction time increased, A / I B The value increases continuously, but when the reaction time continues to increase, I A / I B The value will show a downward trend. Based on this, the liquid phase reaction time is regulated to be 1 h to 6 h. Within this reaction time range, I A / I B The value increases with the extension of reaction time. The synthesis process of the first granular lithium-containing phosphate material and the second lithium-containing phosphate material is different. The second lithium-containing phosphate material adopts the preparation method of liquid phase reaction to facilitate the better adjustment of the crystal surface exposure ratio, that is, I A / I B .

[0122] In some embodiments, in the positive electrode active material, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material.

[0123] In some embodiments, the mass content of the second lithium-containing phosphate material is 5% to 50%, and further can be 10% to 30%, of the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material. As an example, the mass content can be 5%, 8%, 10%, 12%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range formed by any two of the above values.

[0124] As the amount of the second lithium-containing phosphate material increases, the power performance of the battery is improved; however, a larger amount of the second lithium-containing phosphate material will affect the energy density; by controlling the mass content of the second lithium-containing phosphate material within the above range, it is beneficial to balance the low-temperature performance and volume energy density of the lithium-ion secondary battery, so that the lithium-ion secondary battery has both good low-temperature performance and volume energy density.

[0125] The mass content of the second lithium-containing phosphate material in the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material in the positive electrode plate of the battery can be detected by the following method:

[0126] The first lithium-containing phosphate material and the second lithium-containing phosphate material are separated by steps 1) to 4) in the above-mentioned qualitative and quantitative testing of the positive electrode active material in the positive electrode sheet of the lithium-ion secondary battery, and are weighed to obtain their respective masses. In this way, the mass ratio of the first lithium-containing phosphate material and the second lithium-containing phosphate material in the positive electrode sheet can be obtained.

[0127] The primary average particle size refers to the average particle size of the primary particles. The primary particles in this application do not have obvious agglomeration interfaces in the particle cross-section image, but may contain tiny pores and point and line defects. The "primary average particle size" refers to the average of the primary particle sizes of all particles, which is numerically equal to the total particle size divided by the total number of particles.

[0128] In some embodiments, the average primary particle size of the first lithium-containing phosphate material is greater than the average primary particle size of the second lithium-containing phosphate material.

[0129] A larger carbon coating inhibits the particle size growth of the lithium-containing phosphate material, making it difficult to obtain larger particles, thereby reducing the compaction density of the first lithium-containing phosphate material. Furthermore, increasing the carbon coating amount of the first lithium-containing phosphate material reduces its specific capacity utilization. Therefore, the coating amount of the first lithium-containing phosphate material with a larger particle size is controlled to be smaller. The first lithium-containing phosphate material primarily provides a high compaction density, thereby increasing the energy density of the battery. The second lithium-containing phosphate material has a smaller average primary particle size and a greater probability of side reactions. Therefore, it is necessary to increase its carbon coating amount to improve the integrity of the carbon coating, reduce the probability of side reactions, and improve the effective capacity utilization and electronic conductivity of the second lithium-containing phosphate material. Therefore, the coating amount of the second lithium-containing phosphate material with a smaller particle size is controlled to be larger. The second lithium-containing phosphate material primarily provides high kinetic performance, thereby improving the power performance of the battery. In this way, the combination of the first lithium-containing phosphate material with a larger particle size and a smaller coating amount and the second lithium-containing phosphate material with a smaller particle size and a larger coating amount can achieve lithium-ion secondary batteries with improved energy density, capacity retention, and power performance under low-temperature conditions.

[0130] Furthermore, the ratio of the average primary particle size of the first lithium-containing phosphate material to the average primary particle size of the second lithium-containing phosphate material is 3-40. For example, it can be 3, 3.1, 3.5, 4, 5, 6, 7, 8, 8.5, 8.75, 9, 10, 12, 15, 16, 17, 18, 18.75, 19, 20, 25, 30, 35, 40, or any two of the above values ​​as end points. It can also be 5-15, and further can be 5-10. By controlling this ratio, the second lithium-containing phosphate material can better fill the gaps in the first lithium-containing phosphate material, achieving a better dense packing effect. At the same time, the second lithium-containing phosphate material with a smaller particle size can better play its role in improving the power performance of the battery.

[0131] In some embodiments, the average primary particle size of the first lithium-containing phosphate material is 700 nm to 1500 nm. For example, the average primary particle size of the first lithium-containing phosphate material is 700 nm, 750 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, or 1500 nm, or is within a range consisting of any two of the foregoing values. Furthermore, the average primary particle size of the first lithium-containing phosphate material can be 700 nm to 1200 nm.

[0132] By controlling the average primary particle size of the first lithium-containing phosphate material within the above range, the first lithium-containing phosphate material and the second lithium-containing phosphate material can better fill each other, achieve better tight stacking, and improve the overall compaction density of the positive electrode film layer, thereby taking into account good power performance, high capacity retention rate and energy density.

[0133] In some embodiments, the primary particle size distribution of the first lithium-containing phosphate material is in the range of 600 nm to 3000 nm. The particle size distribution range refers to the interval from the minimum to the maximum particle size in the material, which reflects the overall span of the particle size in the material.

[0134] In some embodiments, the average primary particle size of the second lithium-containing phosphate material is 20 nm to 260 nm. As an example, the average primary particle size of the second lithium-containing phosphate material is 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 180 nm, 200 nm, 210 nm, 220 nm, 240 nm, 250 nm, 260 nm, or within a range consisting of any two of the above values. Furthermore, the average primary particle size of the second lithium-containing phosphate material can be 80 nm to 160 nm.

[0135] The increased average particle size of the primary particles of the second lithium-containing phosphate material increases the transport path between the particles. Therefore, controlling the average particle size of the primary particles of the second lithium-containing phosphate material within the aforementioned range can improve the mobility of lithium ions in the positive electrode active material, thereby facilitating the lithium-ion secondary battery's ability to maintain good low-temperature capacity retention, power performance, and energy density. Furthermore, combining this with the larger particle size of the first lithium-containing phosphate material can increase the compaction density, thereby improving the battery's energy density.

[0136] In some embodiments, the second lithium-containing phosphate material has an aspect ratio of (1.1-3.9):1. For example, the aspect ratio can be 1.1:1, 1.2:1, 1.4:1, 1.5:1, 1.8:1, 2:1, 2.5:1, 2.6:1, 3:1, 3.5:1, 3.6:1, 3.9:1, or a range between any two of the aforementioned values. Furthermore, the second lithium-containing phosphate material has an aspect ratio of (1.5-2.6):1. The aspect ratio of the second lithium-containing phosphate material is controlled within the above range, so that the rod-shaped lithium-containing phosphate material in the second lithium-containing phosphate material is mainly short rod-shaped, which can provide a better exposure effect of the (010) crystal plane, which is conducive to better low-temperature performance. In addition, the aspect ratio within the above range makes its specific surface area within a suitable range, which is conducive to controlling side reactions and improving battery life. In addition, the upper limit of the aspect ratio is further controlled to control the compaction density of the positive electrode sheet within a suitable range, which is conducive to improving the energy density of the battery.

[0137] It is understood that the granular shape includes one or more of a spherical shape and a spherical-like shape.

[0138] The average primary particle size of the first lithium-containing phosphate material and the average primary particle size of the second lithium-containing phosphate material in the positive electrode sheet, as well as the aspect ratio of the second lithium-containing phosphate material, can be detected by the following method.

[0139] The positive electrode sheet is cut open perpendicularly to the large surface of the positive electrode sheet using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The particle size of the lithium-containing phosphate material is statistically analyzed using a length diameter statistical method (its length diameter is used as the primary particle size). Specifically, the total number of lithium-containing phosphate primary particles and the sum of the primary particle sizes of the lithium-containing phosphate primary particles can be counted in the scanning electron microscope photograph. The average primary particle size of the lithium-containing phosphate material particles = the primary particle size of the total lithium-containing phosphate material / the total number of lithium-containing phosphate materials.

[0140] In the above process, the total number of lithium-containing phosphate particles with a primary particle size greater than 600 nm in the scanning electron microscope photograph and the sum of the primary particle sizes of the lithium-containing phosphate particles with a primary particle size greater than 600 nm are counted separately, that is, the total number of the first lithium-containing phosphate particles and the sum of the primary particle sizes; the primary average particle size of the first lithium-containing phosphate material particles = the sum of the primary particle sizes of the first lithium-containing phosphate material / the total number of the first lithium-containing phosphate material.

[0141] In the above process, the total number of lithium-containing phosphate particles with a primary particle size less than or equal to 600 nm and the sum of the primary particle sizes of the lithium-containing phosphate particles with a primary particle size less than or equal to 600 nm are counted separately, which is the total number of the second lithium-containing phosphate particles and the sum of the primary particle sizes; the primary average particle size of the second lithium-containing phosphate material particles = the sum of the primary particle sizes of the second lithium-containing phosphate material / the total number of the second lithium-containing phosphate material.

[0142] In the above process, when the particle size of the lithium-containing phosphate material is statistically analyzed using the length-diameter statistical method, the long diameter a and the short diameter b of each particle are measured, and their ratio, which is the aspect ratio of the particle, is obtained; the first lithium-containing phosphate material and the second lithium-containing phosphate material can be intuitively detected and distinguished.

[0143] In some embodiments, the first lithium-containing phosphate material has a powder compaction density of 2.38 g / cm3 at a pressure of 3 t. 3 ~2.68 g / cm 3 In some embodiments, the second lithium-containing phosphate material has a powder compaction density of 1.85 g / cm3 at a pressure of 3 t. 3 ~2.35 g / cm 3 .

[0144] In some embodiments, the compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g / cm 3 ~2.8g / cm 3 .

[0145] In this application, the pole piece compaction density test method is:

[0146] When the electrode is coated on one side, the compaction density of the film layer on one side of the electrode = m / (V1-V2). When the electrode is coated on both sides, the compaction density of the film layer on one side of the electrode = m / [2×(V1-V2)], where m represents the weight of the film layer, V1 represents the volume of the electrode, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the electrode. The product of the surface area of ​​the electrode and the thickness of the electrode is the volume V1 of the electrode, and the product of the surface area of ​​the electrode and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the electrode are obtained by measuring the thickness of the empty foil in the tab area and the thickness of the film area with a micrometer.

[0147] In some embodiments, the chemical formula of the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material is Li β Fe α M (1-α)PO4, wherein 0.2≤α≤1, 1≤β≤1.1, and M is a doping metal element, including but not limited to at least one of Ti, V, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, and Sr. It is understood that the specific types of lithium-containing phosphates in the first lithium-containing phosphate material and the second lithium-containing phosphate material may be the same or different.

[0148] The lithium-containing phosphate includes doped or undoped lithium iron phosphate. As an example, the first lithium-containing phosphate material and the second lithium-containing phosphate material may include at least one of undoped lithium iron phosphate (LiFePO4), lithium iron manganese phosphate, lithium iron cobalt phosphate, and lithium iron nickel phosphate.

[0149] It is understandable that the battery will be accompanied by lithium (Li) deintercalation and consumption during the charge and discharge process, and the content of Li in the positive electrode active material is different when the battery is discharged to different states. In the list of positive electrode active materials in this application, unless otherwise specified, the Li content is the initial state of the material. The positive electrode active material is applied to the positive electrode plate in the battery system, and after the charge and discharge cycle, the Li content in the positive electrode active material contained in the plate will usually change. Among them, the Li content can be measured by molar content, but is not limited to this. Regarding "the Li content is the initial state of the material", the initial state of the material refers to the state before the material is added to the positive electrode slurry. It is understandable that new materials obtained by appropriate modification on the basis of the listed positive electrode active materials are also within the scope of positive electrode active materials. The aforementioned appropriate modification refers to acceptable modification methods for positive electrode active materials, and non-limiting examples include coating modification.

[0150] In the examples of positive electrode active materials in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual O content will fluctuate. The O content can be measured by molar content, but is not limited to this.

[0151] In some embodiments, in the positive electrode film layer, the total mass content of the positive electrode active material is 92%~98%. As an example, it can be 92%, 93%, 94%, 95%, 96%, 97%, 98%, or within the range consisting of any two of the above point values ​​as end values.

[0152] Furthermore, the positive electrode film layer also includes a binder. Furthermore, in the positive electrode film layer, the binder has a mass content of 1% to 4%, and as an example, can be 1%, 2%, 3%, 4%, or within a range consisting of any two of the above values ​​as end values.

[0153] Furthermore, the binder includes at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0154] Furthermore, the positive electrode film layer also includes a conductive agent.

[0155] Furthermore, the mass content of the conductive agent is 1% to 4%. As an example, it can be 1%, 2%, 3%, 4%, or within a range consisting of any two of the above point values ​​as end values.

[0156] Furthermore, the conductive agent includes conductive carbon, metal fibers, and organic conductive polymers. The carbon nanofibers include, but are not limited to, vapor-phase carbon nanofibers. Furthermore, the conductive carbon includes at least one of carbon nanotubes, graphite, graphene, carbon black, Ketjen black, carbon dots, and carbon nanofibers. Furthermore, the carbon black includes acetylene black.

[0157] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a positive electrode slurry; the positive electrode slurry is coated on at least one side of the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained. The type of solvent can be selected from but not limited to any one of the aforementioned embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode current collector coated with the positive electrode slurry can be on a single surface of the positive electrode current collector or on both surfaces of the positive electrode current collector. The solid content of the positive electrode slurry can be 40wt%~80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000mPa·s~25000mPa·s. When coating the positive electrode slurry, the coating unit area density based on dry weight (excluding solvent) can be 15mg / cm 2 ~35mg / cm 2 .

[0158] It is understood that another embodiment of the present application further provides a method for preparing the above-mentioned lithium-ion secondary battery, which includes the step of forming the above-mentioned positive electrode sheet.

[0159] The positive electrode sheet includes not only the positive electrode film layer mentioned above, but also a positive electrode current collector. The positive electrode film layer is provided on at least one surface of the positive electrode current collector.

[0160] As a non-limiting example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is disposed on either or both of the two facing surfaces of the positive electrode current collector.

[0161] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on a polymer material substrate. Non-limiting examples of the metal material in the positive electrode current collector may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. Non-limiting examples of the polymer material substrate in the positive electrode current collector may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0162] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0163] Negative electrode

[0164] The negative electrode plate includes a negative electrode current collector. Further, the negative electrode plate may also include a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0165] As a non-limiting example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0166] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be obtained by forming a metal material on a polymer substrate. Non-limiting examples of the metal material in the negative electrode current collector may include one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymer substrate in the negative electrode current collector may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0167] In some of these embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As a non-limiting example, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0168] In some embodiments, the negative electrode active material layer may further include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0169] In some embodiments, the negative electrode active material layer may further include a conductive agent, which may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0170] In some embodiments, the negative electrode active material layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0171] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on at least one side of the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained. The surface of the negative electrode current collector coated with the negative electrode slurry can be a single surface of the negative electrode current collector or two surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 40wt%~60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000~10000mPa·s. When coating the negative electrode slurry, the coating unit surface density based on dry weight (excluding solvent) can be 75~220 g / m 2 The compaction density of the negative electrode sheet can be 1.0 g / cm 3 ~ 1.8 g / cm 3 .

[0172] electrolytes

[0173] The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0174] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0175] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP).

[0176] In some embodiments, the solvent includes at least one of an ether solvent, an ester solvent, and a sulfone solvent.

[0177] As an example, the ether solvent may include at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), and 1,3-dioxolane (DOL);

[0178] For example, the ester solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), γ-butyrolactone (BL), 1,3-propane sultone (1,3-PS), methyl propionate (MP), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), propyl propionate (PP), and ethyl butyrate (EB). Furthermore, the ester solvent may include ethyl propionate (EP). Ethyl propionate (EP) may improve low-temperature performance when added to the solvent.

[0179] As an example, the sulfone-based solvent includes dimethyl sulfoxide (DMSO).

[0180] Furthermore, the solvent includes an ester solvent and an ether solvent.

[0181] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0182] Further, the additive for improving low temperature performance may include at least one of an ionic liquid type additive, lithium difluorobis(oxalato)borate (LiODFB), fluoroethylene carbonate (FEC), and 1,3-dioxolane (DOL).

[0183] Furthermore, ionic liquid additives include, but are not limited to, at least one of 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazolium bis(trifluoromethanesulfonamide) and aluminum oxide-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide (Al2O3-PY-TFSI). These additives can reduce the activation energy of ion transport, thereby reducing charge transfer resistance at low temperatures. This can effectively improve the low-temperature performance of lithium-ion secondary batteries, maintaining stable capacity retention at -20°C. Among them, lithium difluorobisoxalatoborate (LiODFB) can reduce charge transfer resistance and enhance low-temperature performance. Fluorinated ethylene carbonate (FEC) can reduce electrolyte viscosity, increase low-temperature conductivity, and improve low-temperature capacity and power. 1,3-Dioxolane (DOL) has a low freezing point and low viscosity, reducing the charge transfer resistance of the electrolyte at low temperatures, increasing the Li ion migration rate, and improving low-temperature performance.

[0184] Isolation film

[0185] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0186] In some embodiments, the material of the separator may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0187] In some embodiments, the isolation film has a thickness of 6 μm to 40 μm, and may be 12 μm to 20 μm.

[0188] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0189] In some embodiments, the lithium-ion secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0190] In some embodiments, the outer packaging of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium-ion secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic. Further, non-limiting examples of the plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0191] Another embodiment of the present application provides an electrical device comprising the lithium-ion secondary battery.

[0192] The lithium-ion secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0193] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0194] In this application, unless otherwise specified, a "battery cell" refers to a basic unit that can achieve the mutual conversion of chemical energy and electrical energy. Further, generally speaking, it includes at least a positive electrode plate, a negative electrode plate, and an electrolyte. During the battery's charge and discharge process, active ions are embedded in and out of the positive and negative electrode plates. The electrolyte plays the role of conducting active ions between the positive and negative electrode plates.

[0195] The present application has no particular restrictions on the shape of the battery cell, which can be cylindrical, square or any other shape. For example, Figure 1 The lithium-ion secondary battery shown is a single battery cell, which is an example of a battery cell having an exemplary square structure.

[0196] In some embodiments, the lithium-ion secondary battery may include an outer packaging. The outer packaging may be used to encapsulate the electrode assembly and electrolyte. In some embodiments, the outer packaging of the lithium-ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium-ion secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic. Further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0197] In some of these embodiments, reference Figure 2The outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to actual needs.

[0198] In some embodiments, the lithium-ion secondary battery may be a battery module or a battery pack. A battery module includes at least one battery cell. A battery module may contain one or more battery cells, and those skilled in the art may select an appropriate number based on the application and capacity of the battery module.

[0199] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of battery cells 5 may further be fixed by fasteners.

[0200] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0201] In some embodiments, the battery modules may be assembled into a battery pack. The battery pack may contain one or more battery modules. Those skilled in the art may select an appropriate number based on the application and capacity of the battery pack.

[0202] Figure 4 and Figure 5 The battery pack 1 is used as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0203] In addition, one embodiment of the present application further provides an electrical device, which includes the lithium-ion secondary battery provided herein. The lithium-ion secondary battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Among them, mobile devices may be, for example, mobile phones, laptop computers, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited to these.

[0204] As an electrical device, a lithium-ion secondary battery can be selected according to its usage requirements.

[0205] Figure 6 The power consumption device 6 is used as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the power consumption device's requirements for high power and high energy density of lithium-ion secondary batteries, a battery pack or battery module can be used.

[0206] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a lithium-ion secondary battery as a power source.

[0207] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0208] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0209] Example 1

[0210] (1) Preparation of positive electrode sheet

[0211] 1. The first lithium iron phosphate material (granular)

[0212] The first lithium iron phosphate material uses carbon-coated LiFePO4 with an average primary particle size of 800nm, and its particle size distribution is within the range of 600nm to 3000nm. The mass content of carbon is 1.1%, and the powder compaction density of the first lithium iron phosphate material under 3t pressure is 2.55 g / cm 3 .

[0213] 2. Preparation of the second lithium iron phosphate material (rod-shaped):

[0214] Step A: Ferrous sulfate, lithium hydroxide and phosphoric acid are mixed with water in an amount such that the molar concentration ratio of iron atom, phosphorus atom and lithium atom is 0.5 mol:0.65 mol:1.5 mol to obtain a mixed solution A.

[0215] Step B: After adding the surfactant sodium citrate to the mixed solution A, the mass ratio of the surfactant to the mixed solution A is 1:100, and then adding a pH regulator to adjust the pH of the mixed solution A to 7-8 to obtain a mixed solution B.

[0216] Step C: Transfer the mixed solution B to a reactor, seal it and react at a temperature of 200° C. for 1 to 6 hours. After cooling, filter and wash the precipitate obtained by the reaction to obtain a lithium iron phosphate material.

[0217] Step D: The lithium iron phosphate material obtained in step C is dried and uniformly mixed with carbon source glucose (the mass ratio of lithium iron phosphate material to carbon source is 20:1), and then sintered and carbonized at 800°C in a nitrogen inert atmosphere to obtain carbon-coated LiFePO4, i.e., the second lithium iron phosphate material, whose average primary particle size is 80nm, the mass content of the carbon coating is 1.7%, and the powder compaction density of the second lithium iron phosphate material under a pressure of 3t is 2.1g / cm 3 .

[0218] 3. Preparation of positive electrode sheet

[0219] The first lithium iron phosphate material and the second lithium iron phosphate material are mixed at a mass ratio of 9:1 to form a positive electrode active material.

[0220] The positive electrode active material, conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed uniformly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 95:3:2 to obtain a positive electrode slurry with a viscosity of 20,000 mPa·s.

[0221] The positive electrode slurry was prepared at 280 g / m 2The coating weight is evenly coated on the positive electrode current collector aluminum foil with a thickness of 15μm, and dried at 100℃ to form a positive electrode film layer; the positive electrode sheet is obtained through drying, cold pressing, slitting, cutting and other processes; the compaction density of the positive electrode sheet is 2.6g / cm 3 .

[0222] (2) Preparation of negative electrode sheet:

[0223] The negative electrode active material graphite, conductive agent conductive carbon black, binder styrene butadiene rubber and sodium carboxymethyl cellulose were mixed in a mass ratio of 93:3:2:2, deionized water was added as a solvent, and the mixture was stirred under a vacuum stirrer until the system became uniform to obtain a negative electrode slurry with a viscosity of 20000 mPa·s; the obtained negative electrode slurry was heated at 200 g / m 2 The coating weight is evenly coated on the surface of a copper foil with a thickness of 8μm, dried in an oven at 100℃, and then the negative electrode sheet is obtained through processes such as cold pressing, tab forming, and slitting.

[0224] (3) Isolation film

[0225] A 9μm thick polyethylene isolation film was selected.

[0226] (4) Preparation of electrolyte

[0227] In an environment with a water content of less than 10 ppm, the non-aqueous organic solvents ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain an electrolyte solvent. The lithium salt LiPF6 is then dissolved in the mixed solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0228] (5) Preparation of batteries:

[0229] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound using a winding machine to form a bare cell. The electrode assembly is placed in an outer square shell, dried, and then injected with electrolyte. After vacuum packaging, static standing, formation, and exhaust, the lithium-ion secondary battery is finally obtained. The electrolyte injection coefficient is 3.6g / Ah.

[0230] Comparative Examples 1-2 and Examples 2-6

[0231] It is basically the same as Example 1, except that the strength of the second lithium iron phosphate material is greater than that of the first lithium iron phosphate material. A / I B , at least one of the average primary particle size and the aspect ratio is different. The strength ratio of the second lithium iron phosphate material in each embodiment is 1 A / I B The average particle size of the primary particles and the aspect ratio can be adjusted by adjusting the hydrothermal reaction time in step C. The specific parameters are shown in Table 1.

[0232] Comparative Example 3

[0233] Comparative Example 3 is basically the same as Example 1, except that the strength of the second lithium iron phosphate material is greater than that of the first lithium iron phosphate material. A / I B As shown in Table 1, it is prepared by solid phase synthesis method and is in the form of spherical particles.

[0234] Examples 7-10

[0235] It is basically the same as Example 1, except that in the preparation step of the positive electrode plate, the mixing mass ratio of the first lithium iron phosphate and the second lithium iron phosphate is different, so the mass content of the second lithium iron phosphate in the total amount of the first lithium iron phosphate and the second lithium iron phosphate is different, as shown in Table 1.

[0236] Examples 11-12

[0237] It is basically the same as Example 1, except that the average particle size of the primary particles of the first lithium iron phosphate material used is different, and its particle size distribution is also in the range of 600nm~3000nm, and the mass content of carbon is the same as that of Example 1; the first lithium iron phosphate material in Examples 1, 11~12 has an average particle size of 1000nm. A / I B They are 0.88, 0.89 and 0.85 respectively.

[0238] The parameters are shown in Table 1.

[0239] The following is a performance test.

[0240] (1) Scanning electron microscope (SEM) test.

[0241] The first lithium iron phosphate material and the second lithium iron phosphate material prepared in Example 1 were subjected to scanning electron microscopy testing. The results were as follows: Figure 7 As shown in (A) and (B).

[0242] (2) XRD test (X-ray test).

[0243] The carbon-coated lithium iron phosphate material obtained in step D of Example 1 was subjected to XRD detection; specifically, a copper target X-ray diffractometer was used, and the substance to be detected was placed on the test platform of the X-ray diffractometer (model Shimadzu XRD-7000). The starting angle of the scan was 10°, the ending angle was 90°, and the step length was 0.013. Then, the test was started to obtain a diffraction pattern of the second active material in the diffraction angle range of 10° to 90°. The intensity ratio of the diffraction peak was determined according to the diffraction pattern. A / I B .

[0244] The obtained XRD pattern is as follows Figure 8 and Figure 9 As shown, in the X-ray diffraction pattern, there is a diffraction peak A between 29° and 30°, and a diffraction peak B between 25° and 26°, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1 A / I B =0.96.

[0245] (3) -20℃ capacity retention test:

[0246] The lithium ion secondary batteries prepared in the examples and comparative examples were kept at 25°C for 2 hours, then charged at a constant current rate of 0.33C to 3.65V, and then charged at a constant voltage rate of 0.05C at 3.65V. After charging, the tested batteries were left at 25°C for 2 hours, and then discharged at a DC rate of 0.5C to 2.5V. The discharge capacity at room temperature was recorded as C0;

[0247] The lithium-ion secondary batteries prepared in each example and comparative example were kept at 25°C for 2 hours, then charged at a constant current rate of 0.33C to 3.65V. They were then charged at a constant voltage rate of 0.05C at 3.65V. After charging, the tested batteries were allowed to stand at -20°C for 2 hours, then discharged at a DC rate of 0.5C to 2.5V. The -20°C discharge capacity was recorded as C1. The capacity retention rate of the lithium-ion secondary battery at -20°C was: (C1 / C0) × 100%.

[0248] (4) -20℃ power performance test:

[0249] Capacity calibration: The lithium-ion secondary batteries prepared in the examples and comparative examples were kept at 25°C for 2 hours, then charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage of 0.05C at 3.65V. After charging, the tested batteries were left at 25°C for 2 hours, and then discharged at a DC current of 0.33C to 2.5V. The discharge capacity at room temperature was recorded as C0.

[0250] Adjust the SOC (state of charge): After keeping the calibrated lithium-ion secondary battery at 25°C for 2 hours, discharge it at a discharge rate of 1 / 3C0 for 144 minutes to adjust the capacity of the lithium-ion secondary battery to 20% SOC;

[0251] Power test: After a 20% SOC lithium-ion secondary battery is left at -20°C for 2 hours, it is discharged at a rate of 3C0 for 30 seconds under a pulse current I. The voltage before 3C0 discharge is recorded as V1, and the voltage at the end of the 30-second discharge is recorded as V2. The value DCR (DCR) of (V1-V2) / I is calculated. This data can be used to characterize the battery's power performance. A smaller DCR value indicates a higher power performance.

[0252] (5) Volume energy density E at -20℃ x calculate:

[0253] The lithium-ion secondary batteries prepared in the examples and comparative examples were kept at 25°C for 2 hours, and then charged to 3.65V at a constant current of 0.33C. They were then charged to 0.05C at a constant voltage of 3.65V. After charging, the tested batteries were left at -20°C for 2 hours, and then discharged to 2.5V at a direct current of 0.33C. The discharge energy E0 (Wh) and the coating area S (dm2) of the lithium battery were recorded. 2 ) and the thickness of the positive electrode sheet h (dm), then the volume energy density E x =E0 / (S×h), unit is Wh / dm 3 .

[0254] Some parameters and performance test results of each embodiment and comparative example are shown in Table 1.

[0255] Table 1

[0256]

[0257] In Table 1, the first LFP and the second LFP are the first lithium iron phosphate material and the second lithium iron phosphate material, respectively. The mass content of the second LFP refers to the mass content of the second lithium iron phosphate material in the total amount of the first lithium iron phosphate material and the second lithium iron phosphate material.

[0258] In Comparative Example 1, the positive electrode active material used in the positive electrode plate is exclusively the second lithium iron phosphate material, which exhibits good low-temperature capacity retention and low-temperature power performance, but has a low energy density due to its low compaction density. In Comparative Example 2, the positive electrode active material used in the positive electrode plate is exclusively the first lithium iron phosphate material, which exhibits poor low-temperature capacity retention and low-temperature power performance. Therefore, despite its high compaction density, its energy density is still reduced.

[0259] Comparative Example 3 uses 1 A / I B The first lithium iron phosphate material with a lower capacity has an improved capacity retention rate and energy density compared to Comparative Example 2, but its DCR value at low temperature is still in a relatively high range, indicating that its improvement in power performance is relatively small.

[0260] Compared with Comparative Examples 1 to 3, the positive electrode active material of the positive electrode plate of each embodiment adopts a mixture of the first lithium iron phosphate material and the second lithium iron phosphate material, which can take into account good low-temperature capacity retention, low-temperature power performance and energy density.

[0261] It can be seen from Examples 1 to 3 that the second lithium iron phosphate material A / I BThe value increases in the range of 0.95~1.05, and its DCR value at low temperature decreases, indicating that its low-temperature power performance is improved, and the low-temperature capacity retention rate and energy density are also improved. Through Examples 1 and 4~6, it can be seen that the average particle size of the primary particles of the second lithium iron phosphate material increases. Since the transmission path between the particles becomes longer, its low-temperature DCR value increases. Controlling the aspect ratio of the second lithium iron phosphate material within a suitable range so that the rod shape of the second lithium iron phosphate is mainly short rod shape can provide a better (010) crystal plane exposure effect, which is conducive to improving low-temperature performance and obtaining better capacity retention rate and energy density. Through Examples 1 and 7~10, it can be seen that as the amount of the second lithium iron phosphate material increases, its low-temperature DCR value decreases, the low-temperature power performance and capacity retention rate are greatly improved, and the energy density first increases and then decreases. This is because the amount of the second lithium iron phosphate material is large, which makes the amount of the first lithium iron phosphate material small, thus resulting in a decrease in energy density. It can be seen from Examples 1 and 11-12 that further controlling the average primary particle size of the first lithium iron phosphate material can achieve both a lower low-temperature DCR value (i.e., better power performance) and a higher capacity retention rate and energy density.

[0262] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0263] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A lithium-ion secondary battery, characterized in that: The invention comprises a positive electrode plate, wherein the positive electrode plate comprises a positive electrode film layer; the positive electrode film layer comprises a positive electrode active material, wherein the positive electrode active material comprises a mixture of a first lithium-containing phosphate material in a granular form and a second lithium-containing phosphate material in a rod-shaped form, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction pattern, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1:

1. A / I B ; The second lithium phosphate material satisfies: 1.1≥I A / I B ≥0.95; I of the second lithium-containing phosphate material A / I B greater than the first lithium-containing phosphate material I A / I B ; Both the first lithium-containing phosphate material and the second lithium-containing phosphate material contain a carbon coating layer, the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, the carbon coating amount refers to the mass content of carbon, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material, and the average particle size of the primary particles of the first lithium-containing phosphate material is greater than the average particle size of the primary particles of the second lithium-containing phosphate material.

2. The lithium-ion secondary battery according to claim 1, wherein The mass content of the second lithium-containing phosphate material in the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material is 5% to 50%.

3. The lithium-ion secondary battery according to claim 2, wherein The mass content of the second lithium-containing phosphate material in the total mass of the first lithium-containing phosphate material and the second lithium-containing phosphate material is 10% to 30%.

4. The lithium-ion secondary battery according to claim 1, wherein At least one of the following conditions is met: (1) The carbon coating amount in the first lithium-containing phosphate material is 1% to 1.3%; (2) The carbon coating amount in the second lithium-containing phosphate material is 1.3% to 1.8%; (3) The difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.1% to 0.8%.

5. The lithium-ion secondary battery according to claim 4, wherein The difference between the carbon coating amount in the second lithium-containing phosphate material and the carbon coating amount in the first lithium-containing phosphate material is 0.5% to 0.8%.

6. The lithium-ion secondary battery according to claim 1, wherein At least one of the following conditions is met: (1) The first lithium-containing phosphate material satisfies: 0.92≥I A / I B ≥0.8; (2) The second lithium-containing phosphate material satisfies: 1.05≥I A / I B ≥0.

95.

7. The lithium-ion secondary battery according to claim 1, wherein The ratio of the average primary particle size of the first lithium-containing phosphate material to the average primary particle size of the second lithium-containing phosphate material is 3-40.

8. The lithium-ion secondary battery according to claim 1, wherein The average primary particle size of the first lithium-containing phosphate material is 700 nm to 1500 nm.

9. The lithium-ion secondary battery according to claim 8, wherein The average primary particle size of the first lithium-containing phosphate material is 700 nm to 1200 nm.

10. The lithium ion secondary battery according to any one of claims 1 to 9, wherein The average primary particle size of the second lithium-containing phosphate material is 20 nm to 260 nm.

11. The lithium-ion secondary battery according to claim 10, wherein The average primary particle size of the second lithium-containing phosphate material is 80 nm to 160 nm.

12. The lithium ion secondary battery according to any one of claims 1 to 9, wherein The aspect ratio of the second lithium-containing phosphate material is (1.1-3.9):

1.

13. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: At least one of the following conditions is met: (1) The powder compaction density of the first lithium-containing phosphate material under a pressure of 3t is 2.38 g / cm 3 ~2.68 g / cm 3 ; (2) The powder compaction density of the second lithium-containing phosphate material under a pressure of 3t is 1.85 g / cm 3 ~2.35 g / cm 3 ; (3) The compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g / cm 3 ~2.8g / cm 3 .

14. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: The chemical formula of the lithium-containing phosphate in the first lithium-containing phosphate material and the second lithium-containing phosphate material is Li β Fe α M (1-α) PO4, wherein 0.2≤α≤1, 1≤β≤1.1, and M includes at least one of Ti, V, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, and Sr.

15. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: In the positive electrode film layer, the total mass content of the positive electrode active material is 92% to 98%.

16. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: The positive electrode film layer further includes a binder, and the positive electrode film layer satisfies at least one of the following characteristics: (1) In the positive electrode film layer, the mass content of the binder is 1% to 4%; (2) The binder includes at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyurethane, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

17. The lithium ion secondary battery according to any one of claims 1 to 9, wherein: The positive electrode film layer further includes a conductive agent, and the positive electrode film layer satisfies at least one of the following characteristics: (1) In the positive electrode film layer, the mass content of the conductive agent is 1% to 4%; (2) The conductive agent includes at least one of conductive carbon, metal fiber and organic conductive polymer.

18. The lithium ion secondary battery according to claim 17, wherein The conductive carbon includes at least one of carbon nanotubes, graphite, graphene, carbon black, Ketjen black, carbon dots and carbon nanofibers, and the carbon black includes acetylene black.

19. The lithium ion secondary battery according to any one of claims 1 to 9, wherein The lithium-ion secondary battery also includes an electrolyte, which includes additives. The additives include at least one of 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazole bis(trifluoromethanesulfonamide), aluminum oxide-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide, lithium difluorobisoxalatoborate, fluoroethylene carbonate, and 1,3-dioxolane.

20. A positive electrode active material, characterized in that The positive electrode active material comprises a mixture of a first lithium-containing phosphate material in a granular form and a second lithium-containing phosphate material in a rod-shaped form, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material have a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26° in an X-ray diffraction pattern, and the intensity ratio of the diffraction peak A to the diffraction peak B is 1:

1. A / I B ; The second lithium phosphate material satisfies: 1.1≥I A / I B ≥0.95; I of the second lithium-containing phosphate material A / I B greater than the first lithium-containing phosphate material I A / I B ; Both the first lithium-containing phosphate material and the second lithium-containing phosphate material contain a carbon coating layer, the carbon coating amount in the first lithium-containing phosphate material is less than the carbon coating amount in the second lithium-containing phosphate material, the carbon coating amount refers to the mass content of carbon, the mass content of the first lithium-containing phosphate material is greater than or equal to the mass content of the second lithium-containing phosphate material, and the average particle size of the primary particles of the first lithium-containing phosphate material is greater than the average particle size of the primary particles of the second lithium-containing phosphate material.

21. The positive electrode active material according to claim 20, wherein The positive electrode active material is the positive electrode active material according to any one of claims 2 to 19.

22. An electrical device, characterized in that: A lithium ion secondary battery comprising the lithium ion secondary battery according to any one of claims 1 to 19 and at least one of the positive electrode active materials according to claim 20 or 21.

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