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

Abstract

The present application relates to a lithium-ion secondary battery, a positive electrode active material and an electric device. The lithium-ion secondary battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode film layer; the positive electrode film layer comprises a positive electrode active material; the positive electrode active material comprises a mixture of a granular first lithium-containing phosphate material and a rod-shaped second lithium-containing phosphate material; the X-ray diffraction patterns of the first and second lithium-containing phosphate materials have a diffraction peak A of between 29° and 30° and a diffraction peak B of between 25° and 26°, and the intensity ratio of the diffraction peak A to the diffraction peak B is IA / IB; the second lithium-containing phosphate material satisfies: 1.1≥IA / IB≥0.95; the ratio IA / IB of the second lithium-containing phosphate material is greater than the ratio IA / IB of the first lithium-containing phosphate material; and the first lithium-containing phosphate material and the second lithium-containing phosphate material both contain carbon coating layers, and the coating amount of carbon in the first lithium-containing phosphate material is less than the coating amount of carbon in the second lithium-containing phosphate material.

Description

Lithium-ion secondary batteries, positive electrode active materials and electrical devices

[0001] Related applications

[0002] This application claims priority to Chinese Patent Application No. 2024115563633, filed on November 4, 2024, entitled "Lithium-ion Secondary Battery, Positive Electrode Active Material and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This 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 Technology

[0004] In recent years, with the increasingly wide application of lithium-ion rechargeable batteries, they have been widely used in energy storage power 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 many other fields. Among them, lithium iron phosphate batteries and other lithium-containing phosphate batteries have achieved tremendous development.

[0005] With the increasing demand for batteries, higher requirements are being placed on their energy density and low-temperature performance. However, lithium iron phosphate materials and other lithium phosphate materials have inherently low electronic conductivity, which leads to a significant decrease in ion diffusion rate at low temperatures. This, in turn, affects their performance under low-temperature conditions, particularly their energy density, capacity retention, and power performance. Summary of the Invention

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

[0007] In a first aspect, this application provides a lithium-ion secondary battery, comprising a positive electrode sheet, the positive electrode sheet including a positive electrode film layer; the positive electrode film layer including a positive electrode active material, the positive electrode active material comprising a mixture containing a particulate first lithium-containing phosphate material and a rod-shaped second lithium-containing phosphate material, wherein the first lithium-containing phosphate material and the second lithium-containing phosphate material exhibit diffraction peak A between 29° and 30° and 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 I. A / I B The second lithium phosphate material satisfies: 1.1 ≥ I A / I B ≥0.95; I of the second lithium phosphate material A / IB I greater than the first lithium phosphate material A / I B Both the first lithium phosphate material and the second lithium phosphate material contain a carbon coating layer. The amount of carbon coating in the first lithium phosphate material is less than the amount of carbon coating in the second lithium phosphate material. The amount of carbon coating refers to the mass content of carbon.

[0008] Thus, the lithium-ion secondary battery of this application mainly improves the positive electrode sheet. Specifically, it achieves a better compact packing effect by grading lithium phosphate materials with different morphologies, such as granular and rod-shaped materials, thereby increasing 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 phosphate material used has diffraction peaks at specific positions, and the intensity ratio of diffraction peak A to diffraction peak B is limited. A / I B And control the difference in carbon coating amount between the first lithium phosphate material and the second lithium phosphate material and I A / I B The difference is that it improves 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, the mass content of the first lithium phosphate material in the positive electrode active material is greater than or equal to the mass content of the second lithium phosphate material.

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

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

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

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

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

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

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

[0017] Controlling the difference in carbon coating amount is beneficial to further improve the low-temperature performance of secondary batteries, 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 satisfied:

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

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

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

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

[0023] Larger carbon coating inhibits the growth of lithium phosphate particles, making it difficult to obtain larger particles and thus reducing the compaction density of the first lithium phosphate material. Furthermore, for the first lithium phosphate material, increased carbon coating reduces its specific capacity. Therefore, the coating amount of the first lithium phosphate material with larger particle size is kept relatively small. The first lithium phosphate material mainly serves to provide high compaction density to improve the battery's energy density. The second lithium phosphate material has a smaller average primary particle size, resulting in a higher probability of side reactions. Therefore, its carbon coating amount needs to be increased to improve carbon coating integrity, reduce the probability of side reactions, and improve the effective capacity and electronic conductivity of the second lithium phosphate material. Therefore, the coating amount of the second lithium phosphate material with smaller particle size is kept relatively large. The second lithium phosphate material mainly serves to improve kinetic performance, thereby improving the battery's power performance. Thus, the combined use of a first lithium phosphate material with larger particle size and smaller coating amount and a second lithium phosphate material with smaller particle size and larger coating amount allows the lithium-ion secondary battery to achieve better 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 phosphate material to the average primary particle size of the second lithium phosphate material is 3 to 40. By controlling this ratio, the second lithium phosphate material can better fill the gaps in the first lithium phosphate material, achieving a better compact packing effect. At the same time, it allows the smaller-sized second lithium phosphate material to better play its role in improving the power performance of the battery.

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

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

[0027] By controlling the average particle size of the first lithium phosphate material within the aforementioned range, the first lithium phosphate material and the second lithium phosphate material can better fill each other, achieve better compaction, and improve the overall compaction density of the cathode film layer, thereby achieving good power performance, high capacity retention, and energy density.

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

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

[0030] The increased average particle size of the primary particles in the second lithium phosphate material lengthens the transport path between particles. Therefore, controlling the average particle size of the primary particles in the second lithium phosphate material within the aforementioned range allows lithium ions to exhibit a good migration rate in the positive electrode active material, which is beneficial for lithium-ion secondary batteries to maintain good capacity retention, power performance, and energy density under low-temperature conditions. Simultaneously, combining this with the aforementioned larger-particle-size first lithium 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 phosphate material is (1.1 to 3.9):1.

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

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

[0034] (1) The compacted density of the first lithium phosphate material under a pressure of 3t is 2.38 g / cm³. 3 ~2.68g / cm 3 ;

[0035] (2) The compacted density of the second lithium phosphate material under a pressure of 3t is 1.85 g / cm³. 3 ~2.35g / 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 / cm³. 3 ~2.8g / cm 3 .

[0037] In some embodiments, the lithium phosphate in the first and second lithium phosphate materials has the chemical formula 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, the total mass content of the positive electrode active material in the positive electrode film layer 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 adhesive 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 fluorinated acrylate resin.

[0042] In some embodiments, the positive electrode film further includes a conductive agent, and the positive electrode film 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, wherein the carbon black includes acetylene black.

[0046] In some embodiments, the lithium-ion secondary battery further includes an electrolyte comprising additives, the additives being at least one selected from 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazolium bis(trifluoromethanesulfonamide), alumina-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide, lithium difluorodioxolane borate, fluoroethylene carbonate, and 1,3-dioxane. The presence of these additives in the electrolyte can improve the low-temperature performance of the battery.

[0047] In a second aspect, this application provides a positive electrode active material comprising a mixture of a first lithium phosphate material in particulate form and a second lithium phosphate material in rod form. The first and second lithium phosphate materials exhibit diffraction peak A between 29° and 30° and diffraction peak B between 25° and 26° in an X-ray diffraction pattern. The intensity ratio of diffraction peak A to diffraction peak B is I. A / I B The second lithium phosphate material satisfies: 1.1 ≥ I A / I B ≥0.95; I of the second lithium phosphate material A / I B I greater than the first lithium phosphate material A / I B Both the first lithium phosphate material and the second lithium phosphate material contain a carbon coating layer. The amount of carbon coating in the first lithium phosphate material is less than the amount of carbon coating in the second lithium phosphate material. The amount of carbon coating refers to the mass content of carbon.

[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 this application.

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

[0050] The electrical device of this application includes the lithium-ion secondary battery provided in this application, and therefore has at least the same advantages as the lithium-ion secondary battery.

[0051] Details of one or more embodiments of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0052] To better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments or examples, or the best mode of conduct of these applications as currently understood. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0053] Figure 1 is a schematic diagram of a lithium-ion secondary battery according to an embodiment of this application.

[0054] Figure 2 is an exploded view of a lithium-ion secondary battery according to an embodiment of this application shown in Figure 1.

[0055] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0056] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0057] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0058] Figure 6 is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to an embodiment of this application.

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

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

[0061] Figure 9 is an enlarged view of Figure 8 at diffraction angles 2Theta ranging from 24° to 31°.

[0062] Explanation of reference numerals in the attached drawings: 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module; 5. Battery cell; 51. Housing; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. Detailed Implementation

[0063] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0064] The "range" disclosed in this application can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be combined arbitrarily, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this document; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, stating that a parameter is an integer ≥2 is equivalent to disclosing that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, stating that a parameter is an integer selected from "2-10" is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0065] In this application, the terms "multiple" or "various" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0066] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0067] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0068] Those skilled in the art will understand that the order in which the steps are written in the methods of various embodiments or examples does not imply a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0069] In this application, open-ended technical features or solutions described using terms such as "containing," "including," or "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, if A includes a1, a2, and a3, it may also include other members or exclude additional members unless otherwise specified. This can be considered as providing both the feature or solution that "A consists of a1, a2, and a3" and the feature or solution that "A includes not only a1, a2, and a3, but also other members."

[0070] In this application, unless otherwise specified, A (e.g., B) means that B is a non-limiting example of A, and it is understood that A is not limited to B.

[0071] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it means that it is selected from either "with" or "without." If there are multiple "optional" entries in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" entry shall be independent.

[0072] As described in the background section, with the increasing demand for batteries, higher requirements are being placed on battery energy density, low-temperature performance, and other aspects. However, lithium iron phosphate materials and other lithium-containing phosphate materials have inherently low electronic conductivity, which significantly reduces their ion diffusion rate at low temperatures, thus affecting their performance under low-temperature conditions, especially their energy density, capacity retention, and power performance.

[0073] Therefore, improving the energy density, capacity retention, and power performance of lithium-ion secondary batteries under low-temperature conditions is an urgent problem to be solved.

[0074] Based on this, one embodiment of this application provides a lithium-ion secondary battery, a positive electrode sheet, a positive electrode active material, and an electrical device. The positive electrode sheet and positive electrode active material will be described in detail below in conjunction with the lithium-ion secondary battery.

[0075] One embodiment of this application provides a lithium-ion secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive electrode film layer; the positive electrode film layer including a positive electrode active material, the positive electrode active material including a mixture of a first lithium-containing phosphate material containing granules and a second lithium-containing phosphate material containing rods. In other words, the first lithium-containing phosphate material includes granular lithium-containing phosphate material; the second lithium-containing phosphate material includes rod-shaped lithium-containing phosphate material.

[0076] The first lithium phosphate material and the second lithium phosphate material exhibit diffraction peak A between 29° and 30° and diffraction peak B between 25° and 26° in their X-ray diffraction patterns. The intensity ratio of diffraction peak A to diffraction peak B is I. A / I B The second lithium phosphate material satisfies: 1.1 ≥ I A / I B ≥0.95. The second lithium phosphate material I A / I B I greater than the first lithium phosphate material A / I B .

[0077] 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 that in the second lithium-containing phosphate material. The carbon coating amount refers to the mass content of carbon.

[0078] Taking lithium iron phosphate as an example, in the X-ray diffraction pattern, there is a diffraction peak A between 29° and 30°. Diffraction peak A corresponds to the (010) crystal plane of lithium iron phosphate. Peaks A and B are the two main peaks of lithium iron phosphate. The intensity ratio of peak A (010) can indirectly represent the exposure ratio of the (010) crystal plane. Furthermore, in this application, the I containing particulate first lithium iron phosphate material... 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 any two of the above point values ​​as end values ​​within the range.

[0079] By controlling the intensity ratio of the diffraction peaks of the second lithium phosphate material in the X-ray diffraction pattern to I... A / I B Within the aforementioned range, the proportion of (010) crystal planes exposed in the second lithium phosphate material can be controlled, thereby regulating the electrochemical reaction kinetics performance of the second lithium phosphate material. Specifically, the I... A / I B The higher the value, the higher the proportion of the (010) crystal plane exposed in the second lithium phosphate material. It is speculated that this may be because during the electrochemical reaction, lithium ions undergo insertion and extraction reactions along the b-axis in the second lithium phosphate material. Therefore, the higher the proportion of the (010) crystal plane exposed, the more beneficial it is to improve the electrochemical reaction kinetics performance, thus enabling the second lithium phosphate material to exhibit excellent low-temperature performance when applied to lithium-ion secondary batteries.

[0080] Meanwhile, controlling the carbon coating amount of the second lithium phosphate material to be larger further improves the electronic conductivity of the second lithium phosphate material, thereby improving its power performance. Increasing the coating amount also improves coating integrity, reduces side reactions in the second lithium phosphate material, and reduces the consumption of active components, thus improving battery capacity retention. The first lithium phosphate material's I... A / I B I is smaller than that of the second lithium phosphate material A / I B The carbon content is beneficial to improving the compaction density of the first lithium phosphate material, while a large amount of carbon has an adverse effect on the compaction density of the first lithium phosphate material, and a large amount of carbon will affect the specific capacity of the first lithium phosphate material. Therefore, the carbon coating amount in the first lithium phosphate material is less than that in the second lithium phosphate material, which is beneficial to improving the energy density, capacity retention rate and power performance of lithium-ion secondary batteries under low temperature conditions.

[0081] Thus, the lithium-ion secondary battery of this application mainly improves the positive electrode sheet. Specifically, it achieves a good close packing effect by grading lithium phosphate materials with different morphologies, such as granular and rod-shaped materials, thereby increasing 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 phosphate material used has a specific intensity ratio I between diffraction peak A and diffraction peak B. A / I B And control the difference in carbon coating amount between the first lithium phosphate material and the second lithium phosphate material and I A / I B The difference is that it improves 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.

[0082] In some embodiments, the difference between the carbon coating amount in the second lithium phosphate material and the carbon coating amount in the first lithium phosphate material is 0.1% to 0.8%; as an example, this difference is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or within a range formed by any two of the above values ​​as endpoints; further, it can be 0.3% to 0.8%; even further, it can be 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 taking into account better energy density, capacity retention, and power performance under low-temperature conditions.

[0083] In some embodiments, the carbon coating of the first lithium phosphate material is 1% to 1.3%; as an example, the carbon coating of the first lithium phosphate material can be 1%, 1.1%, 1.2%, 1.3%, or within the range formed by any two of the above point values ​​as endpoints.

[0084] In some embodiments, the carbon coating content of the second lithium phosphate material is 1.3% to 1.8%; as an example, the carbon coating content of the second lithium phosphate material can be 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or within the range formed by any two of the above point values ​​as endpoints.

[0085] The following testing methods can be used for qualitative and quantitative testing of the positive electrode active material in the positive electrode sheet of a lithium-ion secondary battery:

[0086] 1) Battery disassembly

[0087] a. Discharge the battery to a full discharge state with a voltage of 2.0V;

[0088] b. Then disassemble the battery in the glove box and control the humidity in the glove box to be below 2%;

[0089] c. Separate the positive electrode from the battery, then soak it in DMC (dimethyl carbonate) and wash it thoroughly to remove the electrolyte from the positive electrode;

[0090] d. Remove the positive electrode and place it in a vacuum drying oven, then dry it at 80°C for 5 hours.

[0091] 2) Separation of positive electrode active material

[0092] a. Separate the positive electrode film and the positive electrode current collector from the dried positive electrode sheet;

[0093] b. Grind the separated positive electrode film into powder, dissolve it with NMP (N-methylpyrrolidone) under heating conditions, then filter and collect the filter residue. Repeat the dissolution and filtration steps until the binder is completely removed.

[0094] c. Filter and dry the mixture after thoroughly washing away the adhesive.

[0095] 3) Separation of conductive agent

[0096] The dried mixture powder is dispersed in a liquid medium (the density of the liquid medium is greater than 1.2 g / cm³). 3 (e.g., nitrobenzene, bromobenzene, or carbon tetrachloride), stir to make it uniformly mixed, and let it stand for a sufficient time. The solution will separate into layers, with the upper layer being a conductive agent such as conductive carbon and the lower layer being the positive electrode active material.

[0097] 4) Separation of the first lithium-containing phosphate material and the second lithium-containing phosphate material

[0098] Principle: Since the density difference between the first lithium phosphate material and the second lithium phosphate material is small, they cannot be separated by density difference alone. When separating the first lithium phosphate material and the second lithium phosphate material, it is necessary to further utilize the difference in their surface wettability: the carbon coating of the first lithium phosphate material is less than that of the second lithium phosphate material, therefore the first lithium phosphate material is more hydrophilic than the second lithium phosphate material. Therefore, the bubble flotation method is used for separation in a hydrophilic liquid medium. The first lithium phosphate material, being more hydrophilic, is easier to precipitate, while the second lithium phosphate material, being less hydrophilic, is floated.

[0099] Specific steps:

[0100] a. Dry the lower layer of positive electrode active material in step 3) to obtain a mixture powder;

[0101] b. Similar to the coal screening process, a suitable hydrophilic liquid medium (e.g., water, ethylene glycol, or glycerol) and a suitable flotation agent (e.g., grease 190) are selected, and separation is carried out by bubble flotation.

[0102] The specific steps are as follows: Take 1 kg of the mixed positive electrode material in the liquid medium, adjust it to a suitable solid content, add flotation agent, stir to generate bubbles, collect the floating foam, wash and dry it to obtain the second lithium phosphate material; collect the slurry at the bottom, wash and dry it to obtain the first lithium phosphate material.

[0103] c. The separated first lithium phosphate material and the second lithium phosphate material were dried separately and then subjected to subsequent characterization and analysis.

[0104] 5) The second lithium phosphate material was subjected to I... A / I B Detection method:

[0105] A certain amount of the separated second lithium-containing phosphate material was weighed, and the intensity ratio of the diffraction peaks of the second lithium-containing phosphate material was tested by X-ray diffraction. Using a copper target X-ray diffractometer, the second lithium-containing phosphate material was placed on the testing platform of the X-ray diffractometer (model Shimadzu XRD-7000). The scanning start angle was 10°, the end angle was 90°, and the step size was 0.013. Then the test was started, and the diffraction pattern of the second lithium-containing phosphate material in the diffraction angle range of 10° to 90° was obtained. The intensity ratio I of the diffraction peaks was determined according to the diffraction pattern. A / I B .

[0106] Then, the carbon coating amount of the first and second lithium phosphate materials was obtained by ICP testing according to GB / T 33822-2017.

[0107] As an example, the intensity ratio of diffraction peak A to diffraction peak B of the second lithium phosphate material is I. 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 any two of the above point values ​​as endpoints within a range. Further, I A / I B Satisfy: I A / I B ≥0.96; Furthermore, I A / I B ≥0.98.

[0108] In some embodiments, the intensity ratio of diffraction peak A to diffraction peak B of the second lithium phosphate material is I. A / I B Satisfy: 1.1≥I A / I B ≥0.95; further, 1.05≥I A / IB ≥0.95; furthermore, 1.05≥I A / I B ≥0.96. Within this range, it can further improve the low-temperature performance of lithium-ion secondary batteries.

[0109] The first and second lithium-containing phosphate materials of this application can be commercially available. This application does not impose any particular restrictions on the preparation method of the second lithium-containing phosphate material, as long as it achieves the purpose of this application.

[0110] In some embodiments, the second lithium-containing phosphate material can be synthesized by a liquid-phase method. Taking lithium iron phosphate as an example, the method includes the following steps:

[0111] Step A: Mix the iron source, lithium source and phosphorus source in a certain proportion, add solvent and mix to obtain mixture A.

[0112] Further, the solvent is selected from alcohol, water, or a mixture of alcohol and water. Further, the alcohol is selected from ethanol, methanol, ethylene glycol, and glycerol. Further, 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. Further, 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. Further, 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.

[0113] Furthermore, the amounts of iron source, lithium source and phosphorus source added are based on the molar concentration ratio of iron atom, phosphorus atom and lithium atom of 0.5mol:0.65mol:1.5mol.

[0114] Step B: After adding a surfactant to mixture A, add a pH adjuster to adjust the pH of mixture A to 7-8, thus obtaining mixture B.

[0115] The surfactant includes, but is not limited to, at least one of sodium citrate, sodium lactate, sodium malate, and sodium tartrate. Further, as an example, the mass ratio of the surfactant to mixture A is 1:100.

[0116] Step C: Transfer the mixture B to the reaction vessel, seal it and react at a temperature of 170℃~250℃ for 1h~6h. After cooling, filter and wash the precipitate obtained from the reaction to obtain lithium iron phosphate material.

[0117] During hydrothermal precipitation, the (010) crystal facet of lithium iron phosphate exposes more iron ions. Due to the coordination effect of surfactants, these ions are adsorbed onto the (010) surface, limiting the further growth rate of the (010) facet and reducing the thickness of the (010) crystal direction. Since the energy required for the lithium ion insertion / extraction reaction along the (010) crystal direction is the lowest, the lithium ion transport speed can be significantly improved, and the diffusion resistance of the solid-phase reaction can be reduced.

[0118] Further, it may include step D: drying the lithium iron phosphate material obtained in step C and uniformly mixing it with a carbon source, and then sintering and carbonizing it in a nitrogen inert atmosphere to obtain carbon-coated lithium iron phosphate material.

[0119] 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℃~800℃. Furthermore, the mass ratio of lithium iron phosphate material obtained in step C to the carbon source is 20:1.

[0120] In this application, the intensity ratio of the diffraction peak can be controlled by adjusting parameters such as the hydrothermal reaction time t in step C, the concentration C of the hydrothermal reactants in step C, the concentration Cx of the surfactant added to the hydrothermal reactants in step C, or the type of surfactant selected. A / I B The applicant has discovered that as response time increases, I A / I B The value keeps increasing, but as the reaction time continues to extend, I... A / I B The value will show a downward trend. Based on this, this application adjusts the liquid phase reaction time to 1h to 6h. Within this reaction time range, I A / I B The value increases with increasing reaction time. The synthesis processes for the granular first lithium-containing phosphate material and the second lithium-containing phosphate material differ. The second lithium-containing phosphate material is prepared using a liquid-phase reaction method, which allows for better adjustment of the crystal facet exposure ratio, i.e., I0. A / I B .

[0121] In some embodiments, the mass content of the first lithium phosphate material in the positive electrode active material is greater than or equal to the mass content of the second lithium phosphate material.

[0122] In some embodiments, the mass content of the second lithium phosphate material is 5% to 50% of the total mass of the first lithium phosphate material and the second lithium phosphate material, and may further be 10% to 30%. As an example, this mass content may be 5%, 8%, 10%, 12%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or within a range formed by any two of the above point values ​​as endpoints.

[0123] As the amount of the second lithium phosphate material increases, the power performance of the battery is improved; however, a large amount of the second lithium phosphate material will affect the energy density. By controlling the mass content of the second lithium phosphate material within the above range, it is beneficial to balance the low-temperature performance and volumetric energy density of the lithium-ion secondary battery, so that the lithium-ion secondary battery has both good low-temperature performance and volumetric energy density.

[0124] The following method can be used to detect the mass content of the second lithium-containing phosphate material in the total mass of the first and second lithium-containing phosphate materials in the positive electrode of a battery:

[0125] In the qualitative and quantitative testing of the positive electrode active material in the positive electrode sheet of the lithium-ion secondary battery, steps 1) to 4) are used to obtain the separated first lithium phosphate material and second lithium phosphate material. The mass of each material is weighed to obtain the mass ratio of the first lithium phosphate material and the second lithium phosphate material in the positive electrode sheet.

[0126] The primary average particle size refers to the average particle size of the primary particles. In this application, the primary particles do not have obvious agglomeration interfaces in the particle cross-section diagram, but may contain tiny pores and point or line defects. The "primary average particle size" refers to the average primary particle size of all particles, numerically equal to the total particle size value divided by the total number of particles.

[0127] In some embodiments, the average primary particle size of the first lithium phosphate material is greater than that of the second lithium phosphate material.

[0128] Larger carbon coating inhibits the growth of lithium phosphate particles, making it difficult to obtain larger particles and thus reducing the compaction density of the first lithium phosphate material. Furthermore, for the first lithium phosphate material, increased carbon coating reduces its specific capacity. Therefore, the coating amount of the first lithium phosphate material with larger particle size is kept relatively small. The first lithium phosphate material mainly serves to provide high compaction density to improve the battery's energy density. The second lithium phosphate material has a smaller average primary particle size, resulting in a higher probability of side reactions. Therefore, its carbon coating amount needs to be increased to improve carbon coating integrity, reduce the probability of side reactions, and improve the effective capacity and electronic conductivity of the second lithium phosphate material. Therefore, the coating amount of the second lithium phosphate material with smaller particle size is kept relatively large. The second lithium phosphate material mainly serves to improve kinetic performance, thereby improving the battery's power performance. Thus, the combined use of a first lithium phosphate material with larger particle size and smaller coating amount and a second lithium phosphate material with smaller particle size and larger coating amount allows the lithium-ion secondary battery to achieve better energy density, capacity retention, and power performance under low-temperature conditions.

[0129] Furthermore, the ratio of the average primary particle size of the first lithium phosphate material to the average primary particle size of the second lithium phosphate material is 3 to 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, or 40, or any two of the above values ​​as endpoints; it can be selected as 5 to 15, and more specifically, 5 to 10. By controlling this ratio, the second lithium phosphate material can better fill the gaps in the first lithium phosphate material, achieving a better compact packing effect. Simultaneously, it allows the smaller-sized second lithium phosphate material to better exert its role in improving the battery's power performance.

[0130] In some embodiments, the average primary particle size of the first lithium phosphate material is 700 nm to 1500 nm. For example, the average primary particle size of the first lithium 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 a range formed by any two of the above values ​​as endpoints. Further, the average primary particle size of the first lithium phosphate material can be 700 nm to 1200 nm.

[0131] By controlling the average particle size of the first lithium phosphate material within the aforementioned range, the first lithium phosphate material and the second lithium phosphate material can better fill each other, achieve better compaction, and improve the overall compaction density of the cathode film layer, thereby achieving good power performance, high capacity retention, and energy density.

[0132] In some embodiments, the primary particle size distribution range of the first lithium phosphate material is 600 nm to 3000 nm. The particle size distribution range refers to the interval between the minimum and maximum particle sizes in the material, reflecting the overall span of particle size in the material.

[0133] In some embodiments, the average primary particle size of the second lithium phosphate material is 20 nm to 260 nm. As an example, the average primary particle size of the second lithium 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, or 260 nm, or within a range defined by any two of the above values ​​as endpoints. Further, the average primary particle size of the second lithium phosphate material can be 80 nm to 160 nm.

[0134] The increased average particle size of the primary particles in the second lithium phosphate material lengthens the transport path between particles. Therefore, controlling the average particle size of the primary particles in the second lithium phosphate material within the aforementioned range allows lithium ions to exhibit a good migration rate in the positive electrode active material, which is beneficial for lithium-ion secondary batteries to maintain good capacity retention, power performance, and energy density under low-temperature conditions. Simultaneously, combining this with the aforementioned larger-particle-size first lithium phosphate material can increase the compaction density, thereby improving the battery's energy density.

[0135] In some embodiments, the aspect ratio of the second lithium phosphate material is (1.1 to 3.9):1. As an example, this 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 any of the above values ​​as endpoints. Further, the aspect ratio of the second lithium phosphate material is (1.5 to 2.6):1. By controlling the aspect ratio of the second lithium phosphate material within the above-mentioned range, the rod-shaped lithium phosphate material in the second lithium phosphate material is mainly short rod-shaped, which can provide a better exposure effect of the (010) crystal plane, which is beneficial to the better performance at low temperature. In addition, the aspect ratio within the above-mentioned range keeps the specific surface area within a suitable range, which is beneficial to controlling side reactions and improving battery life. Furthermore, by further controlling the upper limit of the aspect ratio, the compaction density of the positive electrode sheet is controlled within a suitable range, which is beneficial to improving the energy density of the battery.

[0136] Understandably, granular includes one or more of spherical and near-spherical shapes.

[0137] The average primary particle size of the first lithium phosphate material and the average primary particle size of the second lithium phosphate material in the positive electrode sheet, as well as the aspect ratio of the second lithium phosphate material, can all be detected by the following methods.

[0138] The positive electrode sheet was cut open perpendicular to the large surface of the positive electrode sheet using an argon ion beam to expose the cross-section. The cross-section was photographed using a scanning electron microscope (SEM). The particle size of the lithium phosphate material was statistically analyzed using the major diameter statistical method (using its major diameter as the primary particle size). Specifically, the total number of primary lithium phosphate particles and the primary particle size of the primary lithium phosphate particles were counted in the SEM images. The average primary particle size of the lithium phosphate material particles = the primary particle size of the total lithium phosphate material / the total number of lithium phosphate materials.

[0139] In the above process, the total number of lithium phosphate particles with a primary diameter greater than 600 nm in the scanning electron microscope images is counted separately, and the sum of the primary diameters of the lithium phosphate particles with a primary diameter greater than 600 nm is also counted. These are the total number of lithium phosphate particles and the sum of their primary diameters. The average primary diameter of the first lithium phosphate material particles = the sum of the primary diameters of the first lithium phosphate material / the total number of the first lithium phosphate material.

[0140] In the above process, the total number of lithium phosphate particles with a primary particle size less than or equal to 600 nm and the sum of the primary particle sizes of lithium phosphate particles with a primary particle size less than or equal to 600 nm are separately counted, which are the total number of lithium phosphate particles and the sum of their primary particle sizes; the primary average particle size of the second lithium phosphate material particles = the sum of the primary particle sizes of the second lithium phosphate material / the total number of the second lithium phosphate material.

[0141] In the above process, when using the length-diameter statistical method to statistically analyze the particle size of lithium phosphate materials, the length-diameter a and the short-diameter b of each particle are measured, and their ratio is obtained, which is the length-diameter ratio of the particle; the first lithium phosphate material and the second lithium phosphate material can be directly detected and distinguished.

[0142] In some embodiments, the compacted density of the first lithium phosphate material powder under a pressure of 3t is 2.38 g / cm³. 3 ~2.68g / cm 3 In some embodiments, the compacted density of the second lithium phosphate material at a pressure of 3t is 1.85 g / cm³. 3 ~2.35g / cm 3 .

[0143] In some embodiments, the compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g / cm³. 3 ~2.8g / cm 3 .

[0144] In this application, the electrode compaction density test method is as follows:

[0145] When the electrode is coated on one side only, the compaction density of the film layer on one side of the electrode is m / (V1-V2). When the electrode is coated on both sides only, the compaction density of the film layer on one side of the electrode is 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 of the electrode, V1. 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 film area in the film area using a micrometer.

[0146] In some embodiments, the lithium phosphate in the first and second lithium phosphate materials has the chemical formula 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. Understandably, the specific types of lithium phosphate in the first lithium phosphate material and the second lithium phosphate material may be the same or different.

[0147] Lithium-containing phosphates include doped or undoped lithium iron phosphate. As an example, the first and second lithium-containing phosphate materials may include at least one of undoped lithium iron phosphate (LiFePO4), lithium manganese iron phosphate, lithium cobalt iron phosphate, and lithium nickel iron phosphate.

[0148] Understandably, lithium (Li) is intercalated and deintercalated during the charging and discharging process of a battery, and the Li content in the positive electrode active material varies depending on the state of discharge. In the examples of positive electrode active materials listed in this application, unless otherwise specified, the Li content refers to the initial state of the material. When positive electrode active materials are applied to the positive electrode sheet in a battery system, the Li content in the positive electrode active material typically changes after charge-discharge cycles. The Li content can be measured using molar content, but is not limited to this. Regarding "Li content refers to the initial state of the material," the initial state of the material refers to its state before being added to the positive electrode slurry. It is understood that new materials obtained by appropriately modifying 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.

[0149] 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 changes in the molar content of oxygen, and the actual O content will fluctuate. The O content can be measured in molar content, but is not limited to this.

[0150] In some embodiments, the total mass content of the positive electrode active material in the positive electrode film layer is 92% to 98%. For example, it can be 92%, 93%, 94%, 95%, 96%, 97%, 98%, or within the range formed by any two of the above point values ​​as endpoints.

[0151] Furthermore, the positive electrode film layer also includes a binder. Further, in the positive electrode film layer, the mass content of the binder is 1% to 4%, for example, it can be 1%, 2%, 3%, 4%, or within the range formed by any two of the above points as endpoints.

[0152] Furthermore, the binder includes at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose, polyurethane, PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0153] Furthermore, the positive electrode film also includes a conductive agent.

[0154] Furthermore, the mass content of the conductive agent is 1% to 4%, for example, it can be 1%, 2%, 3%, 4%, or within the range formed by any two of the above point values ​​as end values.

[0155] Further, the conductive agent includes at least one of conductive carbon, metal fibers, and organic conductive polymers. Further, the conductive carbon includes at least one of carbon nanotubes, graphite, graphene, carbon black, Ketjen black, carbon dots, and carbon nanofibers. Further, carbon black includes acetylene black. Carbon nanofibers include, but are not limited to, fumed carbon nanofibers.

[0156] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive active material, conductive agent, binder, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry onto at least one surface of the positive electrode current collector, and obtaining the positive electrode sheet after drying, cold pressing, and other processes. The solvent can be selected from, but is not limited to, any of the solvents described in the foregoing embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode current collector coated with the positive electrode slurry can be a single surface of the positive electrode current collector or both surfaces of the positive electrode current collector. The solid content of the positive electrode slurry can be 40wt% to 80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000 mPa·s to 25000 mPa·s. When coating the positive electrode slurry, the coating unit areal density (dry weight, minus solvent) can be 15 mg / cm³. 2 ~35mg / cm 2 .

[0157] It is understood that another embodiment of this application also provides a method for preparing the above-mentioned lithium-ion secondary battery, which includes the step of forming the above-mentioned positive electrode sheet.

[0158] In addition to the aforementioned positive electrode film layer, the positive electrode also includes a positive electrode current collector. The positive electrode film layer is disposed on at least one surface of the positive electrode current collector.

[0159] As a non-limiting example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0160] 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 substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the positive electrode current collector may include one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limiting examples of the polymer material substrate in the positive electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0161] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The positive and negative electrodes are positioned opposite each other, and the separator is placed between them, primarily to prevent short circuits while allowing ions to pass through.

[0162] Negative electrode sheet

[0163] The negative electrode sheet includes a negative current collector. Further, the negative electrode sheet may also include a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer comprising a negative active material.

[0164] As a non-limiting example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0165] 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 polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be obtained by forming a metal material on the polymeric material 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 polymeric material substrate in the negative electrode current collector may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0166] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. 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, and lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional 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.

[0167] In some embodiments, the negative electrode active material layer may optionally 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).

[0168] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0169] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0170] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (a non-limiting example of a solvent is deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto at least one surface of a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing, and other processes. 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 both surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 40wt% to 60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000 to 10000 mPa·s. When coating the negative electrode slurry, the coating unit areal density (dry weight, minus solvent) can be 75 to 220 g / m². 2 The compaction density of the negative electrode sheet can be 1.0 g / cm³. 3 ~1.8g / cm 3 .

[0171] electrolytes

[0172] Electrolytes function to conduct ions between the positive and negative electrode plates. This application does not impose any particular restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or completely solid.

[0173] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0174] 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 difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0175] In some embodiments, the solvent includes at least one of ether solvents, ester solvents, and sulfone solvents.

[0176] 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);

[0177] As an example, the ester solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate (BC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), γ-butyrolactone (BL), 1,3-propanesulfonate lactone (1,3-PS), methyl propionate (MP), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), propyl propionate (PP), and ethyl butyrate (EB). Further, the ester solvent may include ethyl propionate (EP), the addition of which can improve low-temperature performance.

[0178] As an example, the sulfone solvent includes dimethyl sulfoxide (DMSO).

[0179] Furthermore, the solvent includes ester solvents and ether solvents.

[0180] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0181] Furthermore, additives that improve low-temperature performance may include at least one of ionic liquid additives, lithium difluorodioxolane borate (LiODFB), fluoroethylene carbonate (FEC), and 1,3-dioxane (DOL).

[0182] Furthermore, ionic liquid additives include, but are not limited to, at least one of 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazolium bis(trifluoromethanesulfonamide) and alumina-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide (Al2O3-PY-TFSI), which can reduce the ion transport activation energy, thereby reducing the charge transfer resistance at low temperatures and effectively improving the low-temperature performance of lithium-ion secondary batteries, maintaining stable capacity retention at -20°C. Among these, lithium difluorodioxolane borate (LiODFB) can reduce charge transfer resistance and improve low-temperature performance. Fluorinated ethylene carbonate (FEC) can reduce electrolyte viscosity, increase low-temperature conductivity, and improve low-temperature capacity and power. 1,3-Dioxapentane (DOL) has low freezing point and low viscosity, which can reduce the charge transfer resistance of the electrolyte at low temperatures, increase the migration rate of Li ions, and improve low-temperature performance.

[0183] Separating membrane

[0184] In some embodiments, the lithium-ion secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0185] In some embodiments, the material of the separator may include one or more of glass fiber, nonwoven 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.

[0186] In some embodiments, the thickness of the isolation membrane is 6 μm to 40 μm, and optionally 12 μm to 20 μm.

[0187] In some embodiments, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding process or a stacking process.

[0188] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0189] In some embodiments, the outer packaging of the lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the lithium-ion secondary battery can also be a soft pack, such as a pouch. The soft pack can be made of plastic; further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0190] Another embodiment of this application provides an electrical device. This electrical device includes the aforementioned lithium-ion secondary battery.

[0191] The lithium-ion secondary battery and power supply device of this application will be described below with appropriate reference to the accompanying drawings.

[0192] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0193] In this application, unless otherwise specified, "cell battery" refers to the basic unit capable of converting chemical energy into electrical energy, and generally includes at least a positive electrode, a negative electrode, and an electrolyte. During the charging and discharging process of the battery, active ions move back and forth between the positive and negative electrode plates, inserting and extracting. The electrolyte acts as a conductor for the active ions between the positive and negative electrode plates.

[0194] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, the lithium-ion secondary battery shown in Figure 1 is a battery cell, which serves as an example of a square-structured battery cell.

[0195] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode components and electrolyte described above. In some embodiments, the outer packaging of the lithium-ion secondary battery may be a rigid 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 pack, such as a pouch. The material of the soft pack may be plastic; further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0196] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to actual needs.

[0197] In some embodiments, the lithium-ion secondary battery can be a battery module or a battery pack. A battery module includes at least one battery cell. The number of battery cells in a battery module can be one or more, and those skilled in the art can select an appropriate number based on the application and capacity of the battery module.

[0198] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0199] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0200] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more. Those skilled in the art can select an appropriate number according to the application and capacity of the battery pack.

[0201] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0202] In addition, one embodiment of this application also provides an electrical device, which includes the lithium-ion secondary battery provided in this application. The lithium-ion secondary battery can be used as a power source for the electrical device or as an energy storage unit of 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, laptops, 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.

[0203] As an electrical device, lithium-ion rechargeable batteries can be selected according to its usage requirements.

[0204] Figure 6 shows an example of an electrical device 6. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium-ion secondary battery for this electrical device, a battery pack or battery module can be used.

[0205] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion rechargeable battery as their power source.

[0206] 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 embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0207] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0208] Example 1

[0209] (1) Preparation of positive electrode sheet

[0210] 1. First-stage lithium iron phosphate material (granular)

[0211] The first lithium iron phosphate material uses carbon-coated LiFePO4 with an average primary particle size of 800 nm, and its particle size distribution ranges from 600 nm to 3000 nm. The carbon mass content is 1.1%, and the powder compaction density of the first lithium iron phosphate material under a pressure of 3 tons is 2.55 g / cm³. 3 .

[0212] 2. Preparation of lithium iron phosphate material (rod form):

[0213] Step A: Add ferrous sulfate, lithium hydroxide, and phosphoric acid to water in a molar ratio of 0.5 mol: 0.65 mol: 1.5 mol for iron, phosphorus, and lithium atoms to obtain mixture A.

[0214] Step B: After adding the surfactant sodium citrate to mixture A, the mass ratio of surfactant to mixture A is 1:100. Then, add a pH adjuster to adjust the pH of mixture A to 7-8 to obtain mixture B.

[0215] Step C: Transfer the mixture B to the reaction vessel, seal it and react at 200°C for 1 to 6 hours. After cooling, filter and wash the precipitate obtained from the reaction to obtain lithium iron phosphate material.

[0216] Step D: After drying the lithium iron phosphate material obtained in Step C, it is uniformly mixed with the carbon source glucose (the mass ratio of lithium iron phosphate material to carbon source is 20:1). Then, it is sintered and carbonized at 800℃ in a nitrogen inert atmosphere to obtain carbon-coated LiFePO4, i.e., the second lithium iron phosphate material. Its primary particle average size is 80nm, its carbon coating mass content is 1.7%, and the powder compaction density of the second lithium iron phosphate material under 3t pressure is 2.1g / cm³. 3 .

[0217] 3. Preparation of the positive electrode sheet

[0218] The first lithium iron phosphate material and the second lithium iron phosphate material were mixed at a mass ratio of 9:1 to form the positive electrode active material.

[0219] The above-mentioned positive electrode active material, conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed evenly 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 20000 mPa·s.

[0220] The positive electrode slurry was prepared at 280g / m 2The coating weight is uniformly coated onto a 15μm thick aluminum foil for the positive electrode current collector, and dried at 100℃ to form a positive electrode film. Through drying, cold pressing, slitting, and cutting processes, the positive electrode sheet is obtained; the compacted density of the positive electrode sheet is 2.6 g / cm³. 3 .

[0221] (2) Preparation of negative electrode sheet:

[0222] Graphite (negative electrode active material), conductive carbon black (conductive agent), styrene-butadiene rubber (binder), and sodium carboxymethyl cellulose (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 vacuum until the system was homogeneous, yielding a negative electrode slurry with a viscosity of 20000 mPa·s. The prepared negative electrode slurry was then subjected to a 200 g / m³... 2 The coating weight is evenly coated on the surface of copper foil with a thickness of 8μm, dried in an oven at 100℃, and then the negative electrode sheet is obtained through cold pressing, tab forming, slitting and other processes.

[0223] (3) Separating membrane

[0224] A 9μm thick polyethylene separator film was selected.

[0225] (4) Preparation of electrolyte

[0226] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate are mixed in a volume ratio of 1:1:1 to obtain an electrolyte solvent. Then, lithium salt LiPF6 is dissolved in the mixed solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0227] (5) Battery fabrication:

[0228] The positive electrode, separator, and negative electrode are stacked in sequence and wound using a winding machine to obtain a bare battery cell. The electrode assembly is placed in an outer shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and degassing, a lithium-ion secondary battery is finally obtained. The electrolyte injection coefficient is 3.6 g / Ah.

[0229] Comparative Examples 1-2 and Examples 2-6

[0230] It is basically the same as Example 1, except that the strength of the second lithium iron phosphate material is higher than that of I. A / I B The average particle size and aspect ratio of the primary particles are different. The strength ratio of the second lithium iron phosphate material in each embodiment is I. A / I B The average particle size and aspect ratio of the primary particles can be adjusted by adjusting the hydrothermal reaction time in step C. Specific parameters are shown in Table 1.

[0231] Comparative Example 3

[0232] Comparative Example 3 is basically the same as Example 1, except that the strength of the second lithium iron phosphate material is higher than that of I. A / I B As shown in Table 1, it is prepared by solid-phase synthesis and is in the form of spherical particles.

[0233] Examples 7-10

[0234] It is basically the same as Example 1, except that the mixing mass ratio of the first lithium iron phosphate and the second lithium iron phosphate is different in the preparation steps of the positive electrode. Therefore, the mass content of the second lithium iron phosphate is different in the total amount of the first lithium iron phosphate and the second lithium iron phosphate, as shown in Table 1.

[0235] Examples 11-12

[0236] It is basically the same as Example 1, except that the average particle size of the primary lithium iron phosphate material used is different, and its particle size distribution is also in the range of 600nm to 3000nm. The carbon mass content is the same as in Example 1; the first lithium iron phosphate material in Examples 1 and 11-12 has a different I... A / I B The values ​​are 0.88, 0.89, and 0.85, respectively.

[0237] The parameters are shown in Table 1.

[0238] The following are performance tests.

[0239] (1) Scanning electron microscopy (SEM) test.

[0240] The first lithium iron phosphate material and the second lithium iron phosphate material prepared in Example 1 were tested by scanning electron microscopy, and the results are shown in Figure 7(A) and (B), respectively.

[0241] (2) XRD test (X-ray test).

[0242] The carbon-coated lithium iron phosphate material obtained in step D of Example 1 was subjected to XRD analysis. Specifically, using a copper target X-ray diffractometer, the material to be tested was placed on the test platform of the X-ray diffractometer (model Shimadzu XRD-7000). The scanning start angle was 10°, the end angle was 90°, and the step size was 0.013. Then, the test was started, and the diffraction pattern of the second active material in the diffraction angle range of 10° to 90° was obtained. The intensity ratio of the diffraction peaks was determined based on the diffraction pattern. A / I B .

[0243] The obtained XRD patterns, as shown in Figures 8 and 9, show a diffraction peak A between 29° and 30° and a diffraction peak B between 25° and 26°. The intensity ratio of diffraction peak A to diffraction peak B is I. A / I B =0.96.

[0244] (3) -20℃ capacity retention test:

[0245] The lithium-ion secondary batteries prepared in each embodiment and comparative example were kept at 25°C for 2 hours, then charged at a constant current rate of 0.33C to 3.65V, and charged at a constant voltage of 3.65V to 0.05C. After charging, the batteries were left to stand at 25°C for 2 hours, and then discharged at a DC rate of 0.5C to 2.5V. Their room temperature discharge capacity was recorded as C0.

[0246] The lithium-ion secondary batteries prepared in each embodiment and comparative example 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 batteries were placed at -20°C for 2 hours, and then discharged at a DC rate of 0.5C to 2.5V. The discharge capacity at -20°C was recorded as C1. The capacity retention rate of the lithium-ion secondary batteries at -20°C is (C1 / C0)×100%.

[0247] (4) Power performance test at -20℃:

[0248] Capacity calibration: 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 of 0.33C to 3.65V, and charged at a constant voltage of 3.65V to 0.05C. After charging, the batteries were left to stand 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.

[0249] Adjusting SOC (State of Charge): After keeping the calibrated lithium-ion secondary battery at 25°C for 2 hours, discharge it at 1 / 3C0 discharge rate for 144 minutes to adjust the capacity of the lithium-ion secondary battery to 20% SOC.

[0250] Power Test: After a 20% SOC lithium-ion secondary battery is left to stand at -20℃ for 2 hours, it is discharged for 30 seconds at a 3C0 discharge rate under a pulse current I. The voltage before 3C0 discharge is recorded as V1, and the voltage at the end of 30 seconds discharge is recorded as V2. The value of (V1-V2) / I, DCR, is calculated. This data can characterize the power performance of the battery. The smaller the DCR value, the greater the power performance of the battery.

[0251] (5) Volumetric energy density E at -20℃ x calculate:

[0252] The lithium-ion secondary batteries prepared in each embodiment and comparative example 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 3.65V to 0.05C. After charging, the batteries were left to stand at -20°C for 2 hours, and then discharged at a DC current of 0.33C to 2.5V. The discharge energy E0 (Wh) and the coating area S (dm²) of the positive electrode were recorded. 2 Given the thickness h (dm) of the positive electrode, the volumetric energy density E x =E0 / (S×h), unit is Wh / dm 3 .

[0253] Table 1 shows some parameters and performance test results of each embodiment and comparative example.

[0254] Table 1

[0255] In Table 1, the first LFP and the second LFP represent the first and second lithium iron phosphate materials, 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 both the first and second lithium iron phosphate materials.

[0256] In Comparative Example 1, the positive electrode active material is only lithium iron phosphate, which has good low-temperature capacity retention and low-temperature power performance, but its energy density is low due to its low compaction density. In Comparative Example 2, the positive electrode active material is only lithium iron phosphate, which has poor low-temperature capacity retention and low-temperature power performance. Therefore, although its electrode compaction density is high, its energy density is still reduced.

[0257] Comparative Example 3 uses I A / I B The lower-grade lithium iron phosphate material showed improved capacity retention and energy density compared to Comparative Example 2, but its DCR value at low temperatures remained high, indicating minimal improvement in power performance.

[0258] Compared to Comparative Examples 1-3, the positive electrode active material of each embodiment uses a mixture of first lithium iron phosphate material and second lithium iron phosphate material, which can achieve good low-temperature capacity retention, low-temperature power performance and energy density.

[0259] As can be seen from Examples 1-3, the second lithium iron phosphate material I A / I BAs the value increases within the range of 0.95 to 1.05, the low-temperature DCR value decreases, indicating improved low-temperature power performance, low-temperature capacity retention, and energy density. Examples 1 and 4-6 show that an increase in the average primary particle size of the second lithium iron phosphate material leads to a longer transport path between particles, thus increasing the low-temperature DCR value. Controlling the aspect ratio of the second lithium iron phosphate material within a suitable range, ensuring that the rod-shaped form is primarily short rods, provides better exposure of the (010) crystal plane, which is beneficial for improving low-temperature performance and achieving better capacity retention and energy density. Examples 1 and 7-10 show that as the amount of the second lithium iron phosphate material increases, its low-temperature DCR value decreases, while low-temperature power performance and capacity retention are significantly improved. The energy density initially increases and then decreases because the larger amount of the second lithium iron phosphate material results in a smaller amount of the first lithium iron phosphate material, leading to a decrease in energy density. As can be seen from Examples 1 and 11-12, further controlling the average particle size of the primary particles of the first lithium iron phosphate material can achieve a balance between a lower low-temperature DCR value (i.e., better power performance), higher capacity retention, and higher energy density.

[0260] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above 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.

[0261] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A lithium-ion secondary battery, wherein, The device includes a positive electrode sheet, which comprises a positive electrode film layer. The positive electrode film layer comprises a positive electrode active material, which comprises a mixture of a first lithium phosphate material containing granules and a second lithium phosphate material containing rods. The first and second lithium phosphate materials exhibit diffraction peak A between 29° and 30° and diffraction peak B between 25° and 26° in their X-ray diffraction patterns. The intensity ratio of diffraction peak A to diffraction peak B is I. A / I B The second lithium phosphate material satisfies: 1.1 ≥ I A / I B ≥0.95; I of the second lithium phosphate material A / I B I greater than the first lithium phosphate material A / I B Both the first lithium phosphate material and the second lithium phosphate material contain a carbon coating layer. The amount of carbon coating in the first lithium phosphate material is less than the amount of carbon coating in the second lithium phosphate material. The amount of carbon coating refers to the mass content of carbon.

2. The lithium-ion secondary battery as described in claim 1, wherein, In the positive electrode active material, the mass content of the first lithium phosphate material is greater than or equal to the mass content of the second lithium phosphate material.

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

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

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

6. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein, At least one of the following conditions must be met: (1) The first lithium phosphate material satisfies: 0.92 ≥ I A / I B ≥0.8; (2) The second lithium phosphate material satisfies: 1.05 ≥ I A / I B ≥0.

95.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, wherein, The average primary particle size of the first lithium phosphate material is greater than that of the second lithium phosphate material.

8. The lithium-ion secondary battery as described in claim 7, wherein, The ratio of the average primary particle size of the first lithium phosphate material to the average primary particle size of the second lithium phosphate material is 3 to 40.

9. The lithium-ion secondary battery according to any one of claims 1 to 8, wherein, The average particle size of the primary particles in the first lithium phosphate material is 700 nm to 1500 nm.

10. The lithium-ion secondary battery as described in claim 8, wherein, The average particle size of the primary particles in the first lithium phosphate material is 700 nm to 1200 nm.

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

12. The lithium-ion secondary battery as described in claim 11, wherein, The average particle size of the primary particles in the second lithium phosphate material is 80 nm to 160 nm.

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

1.

14. The lithium-ion secondary battery according to any one of claims 1 to 13, wherein, At least one of the following conditions must be met: (1) The compacted density of the first lithium phosphate material under a pressure of 3t is 2.38 g / cm³. 3 ~2.68g / cm 3 ; (2) The compacted density of the second lithium phosphate material under a pressure of 3t is 1.85 g / cm³. 3 ~2.35g / cm 3 ; (3) The compaction density of the positive electrode film layer on one side of the positive electrode sheet is 2.45 g / cm³. 3 ~2.8g / cm 3 .

15. The lithium-ion secondary battery according to any one of claims 1 to 14, wherein, The chemical formula of the lithium phosphate in the first lithium phosphate material and the second lithium 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.

16. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein, In the positive electrode film layer, the total mass content of the positive electrode active material is 92% to 98%.

17. The lithium-ion secondary battery according to any one of claims 1 to 16, 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 adhesive 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 fluorinated acrylate resin.

18. The lithium-ion secondary battery according to any one of claims 1 to 17, 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.

19. The lithium-ion secondary battery as described in claim 18, characterized in that, 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.

20. The lithium-ion secondary battery according to any one of claims 1 to 19, wherein, The lithium-ion secondary battery further includes an electrolyte, which includes additives, the additives being at least one selected from 1-methyl-3-[(3-trimethoxysilyl)-propyl]imidazolium bis(trifluoromethanesulfonamide), alumina-grafted 1-methyl-3-propylpyrrolidine bis(trifluoromethanesulfonic acid)imide, lithium difluorodioxolane borate, fluoroethylene carbonate, and 1,3-dioxane.

21. A positive electrode active material, wherein, The positive electrode active material comprises a mixture of a first lithium phosphate material in particulate form and a second lithium phosphate material in rod form. The first and second lithium phosphate materials exhibit diffraction peak A between 29° and 30° and diffraction peak B between 25° and 26° in their X-ray diffraction patterns. The intensity ratio of diffraction peak A to diffraction peak B is I. A / I B The second lithium phosphate material satisfies: 1.1 ≥ I A / I B ≥0.95; I of the second lithium phosphate material A / I B I greater than the first lithium phosphate material A / I B Both the first lithium phosphate material and the second lithium phosphate material contain a carbon coating layer. The amount of carbon coating in the first lithium phosphate material is less than the amount of carbon coating in the second lithium phosphate material. The amount of carbon coating refers to the mass content of carbon.

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

23. An electrical appliance, wherein, It includes at least one of the lithium-ion secondary batteries according to any one of claims 1 to 20 and the positive electrode active material according to claim 21 or 22.

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