Composite material, positive electrode sheet, secondary battery, and electric device

Abstract

A composite material, a positive electrode sheet, a secondary battery, and an electric device. The composite material comprises a first positive electrode active material and a second positive electrode active material, and the chemical formula of the second positive electrode active material is xLi2MnO3(1-x)LiMO2, wherein M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of a lithium element in the second positive electrode active material is 7.5% to 8.6%. The composite material is capable of mitigating the problems of lower energy density and shorter cycle life of batteries caused by existing lithium-based positive electrode materials being prone to losing active lithium during cycling.

Description

A composite material, a positive electrode sheet, a secondary battery, and an electrical device.

[0001] This application claims priority to Chinese Patent Application No. 202411055489.2, filed on July 31, 2024, entitled "A Composite Material, Positive Electrode, Secondary Battery and Electrical Equipment", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery manufacturing technology, and in particular to a composite material, a positive electrode sheet, a secondary battery, and an electrical device. Background Technology

[0003] Currently, lithium-ion batteries are widely used in portable electronic products and new energy vehicles due to their advantages such as high operating voltage, long cycle life, high energy density, low self-discharge, and no memory effect.

[0004] However, existing lithium-based cathode materials are prone to losing active lithium during cycling, resulting in battery capacity decay. At the same time, because the anode material forms a solid electrolyte interface (SEI) film during the first lithium intercalation process, some active lithium is consumed, which further reduces the battery capacity of lithium-based cathode materials, seriously affecting the battery's energy density and lifespan.

[0005] Currently, traditional lithium replenishment agents can only replenish the active lithium consumed by the negative electrode material, and have a relatively low effect on improving the energy density and lifespan of lithium-based cathode battery systems. Summary of the Invention

[0006] The technical problem to be solved by this application is to provide a composite material, a positive electrode sheet, a secondary battery, and an electrical device to improve the problem that existing lithium-based positive electrode materials are prone to losing active lithium during cycling, resulting in low energy density and short cycle life of the battery.

[0007] To address the aforementioned problems, this application provides the following technical solution:

[0008] This application proposes a composite material comprising a first positive electrode active material and a second positive electrode active material. The chemical formula of the second positive electrode active material is xLi2MnO3·(1-x)LiMO2, wherein M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of lithium element in the second positive electrode active material is 7.5% to 8.6%.

[0009] Furthermore, in the composite material, the first-cycle activation coulombic efficiency of the second positive electrode active material at 4.4V is 68%–90%.

[0010] Furthermore, in the composite material, the manganese element accounts for 50% to 100% of the mass of all transition metal elements in the second positive electrode active material.

[0011] Furthermore, in the composite material, the mass ratio of the second positive electrode active material to the first positive electrode active material is (5-95):(95-5).

[0012] Furthermore, in the composite material, the mass ratio of the second positive electrode active material to the first positive electrode active material is (15-50):(50-85).

[0013] Furthermore, in the composite material, M is selected from one or more of Ni, Co, and Mn.

[0014] Furthermore, in the composite material, the first positive electrode active material is selected from one or more of lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganese oxide.

[0015] This application also proposes a positive electrode sheet, including a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the composite material as described above.

[0016] This application also proposes a secondary battery, including the positive electrode as described above.

[0017] Furthermore, the secondary battery also includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector;

[0018] The secondary battery satisfies the following condition: N / P = (1~1.08):1;

[0019] Wherein, N represents the molar mass of lithium ions extracted from the positive electrode, and P represents the molar mass of lithium ions that can be inserted into the negative electrode.

[0020] This application also proposes an electrical device, which includes the aforementioned secondary battery, serving as the power supply for the electrical device.

[0021] Compared with the prior art, the embodiments of this application have the following advantages:

[0022] In this embodiment, the provided composite material includes a first positive electrode active material and a second positive electrode active material. The chemical formula of the second positive electrode active material is xLi2MnO3·(1-x)LiMO2, where M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of lithium in the second positive electrode active material is 7.5% to 8.6%. Under the conditions that 0.05≤x≤0.2 and the mass percentage of lithium in the second positive electrode active material is 8.0% to 12%, the lithium-rich phase Li2MnO3 of the second positive electrode active material has sufficient irreversible lithium. When the above composite material is used as a positive electrode material, during the activation process, a portion of the irreversible lithium in Li2MnO3 can be activated with a higher activation voltage to replenish the lithium ion consumption caused by the formation of the SEI film. In subsequent cycling, another part of Li2MnO3 can provide reversible capacity and continuously replenish irreversible lithium, while LiMO2 can continuously provide reversible capacity, thus effectively improving the energy density and lifespan of the battery system. Therefore, the composite material provided in this application can improve the problem that existing lithium-based cathode materials are prone to losing active lithium during cycling, resulting in low battery energy density and short cycle life.

[0023] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0024] Figure 1 is a comparison chart of the cycle life of the batteries prepared in Example 1, Comparative Example 1 and Comparative Example 2 of this application. Detailed Implementation

[0025] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] This application provides a composite material comprising a first positive electrode active material and a second positive electrode active material. The chemical formula of the second positive electrode active material is xLi2MnO3·(1-x)LiMO2, wherein M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of lithium element in the second positive electrode active material is 7.5% to 8.6%.

[0027] The first positive electrode active material is different from the second positive electrode active material. Since the lithium-rich phase Li2MnO3 contains both irreversible and reversible lithium, while LiMO2 contains only reversible lithium, under the conditions that 0.05≤x≤0.2 and the mass percentage of lithium in the second positive electrode active material is 8.0% to 12%, the lithium-rich phase Li2MnO3 of the second positive electrode active material has sufficient irreversible lithium. When the above composite material is used as the positive electrode material, during the activation process, a portion of the irreversible lithium in Li2MnO3 can be activated at a higher activation voltage to replenish the lithium ion consumption caused by the formation of the SEI film. In the subsequent lower voltage cycling process, another part of Li2MnO3 can provide reversible capacity for lithium intercalation and deintercalation, and can also serve as irreversible lithium to continuously replenish the first positive electrode active material. LiMO2 can continuously provide reversible capacity for lithium intercalation and deintercalation, thus effectively improving the energy density and lifespan of the battery system.

[0028] Specifically, in Li₂MnO₃, some Li atoms occupy transition metal positions. During the first charge, lithium atoms are released from the lattice of the positive electrode, causing partial lattice collapse. During the first discharge, the released lithium atoms cannot completely return to the positive electrode, and the lithium atoms that cannot be re-inserted are pre-stored in the negative electrode. As the battery cycles, the continuous occurrence of side reactions at the electrode interfaces continuously consumes the active lithium in the battery system. Once the active lithium is consumed, the pre-stored lithium in the negative electrode replenishes the consumed lithium to improve cycle life.

[0029] In practical applications, aberration electron microscopy characterization can determine that the lithium within 0-2 nm of the surface of the second positive electrode active material is irreversible lithium, while the remaining lithium is reversible lithium.

[0030] Therefore, the composite material provided in this application can improve the problem that existing lithium-based cathode materials are prone to losing active lithium during cycling, resulting in low battery energy density and short cycle life.

[0031] In this embodiment, the second positive electrode active material is equivalent to a lithium replenisher with reversible lithium insertion / extraction function. The second positive electrode active material can be added to the positive electrode slurry coating process by positive electrode lithium replenishment to form the above-mentioned composite material. During the first charging process, the Li2MnO3 of the second positive electrode active material decomposes into MnO2 and active lithium. All or most of the active lithium replenishes the consumption of the negative electrode SEI. Only a small portion of the decomposed and released active lithium will be stored in the negative electrode as pre-stored lithium. In subsequent cycles, it will continue to provide active lithium, thereby playing a lithium replenishment role and improving the battery life.

[0032] By controlling the content and reversibility of the lithium-rich phase Li2MnO3 in the second positive electrode active material, the lithium replenishment efficiency of the second positive electrode active material can be effectively improved.

[0033] Optionally, x can be a range of one or any two of 0.05, 0.06, 0.08, 0.1, 0.05, and 0.2.

[0034] Optionally, the mass percentage of lithium in the second positive electrode active material can be one or any two of the following: 7.5%, 7.6%, 7.7%, 7.8%, 8.0%, 8.2%, 8.5%, and 8.6%.

[0035] Since only a small portion of the active lithium released from the decomposition of Li2MnO3 is stored as pre-stored lithium in the negative electrode, there is no need to leave excessive storage space in the negative electrode sheet, thus reducing the amount of negative electrode used and lowering costs.

[0036] In this embodiment, the ternary phase LiMO2 in the second positive electrode active material has the characteristic of reversible lithium insertion / extraction and its specific capacity is higher than 160mAh / g. Therefore, the composite material provided in this embodiment can provide reversible capacity and can continuously replenish lithium to improve the energy density and lifespan of the battery system.

[0037] Optionally, in the embodiments of this application, the specific capacity of the second positive electrode active material is 350-375 mAh / g, and the reversible capacity is 250-350 mAh / g.

[0038] In practical applications, during the activation process, a higher activation voltage can be used to activate some of the irreversible lithium Li2MnO3 in the second positive electrode active material for lithium replenishment. Then, cycling is performed within the electrochemical window of the positive electrode active material. This allows the second positive electrode active material to provide both reversible capacity and continuous lithium replenishment during cycling, thereby improving the energy density and lifespan of the battery system. In practical applications, by controlling the activation voltage, the amount of irreversible lithium extracted from the lithium-rich manganese-based cathode in the first cycle can be controlled to precisely replenish the lithium consumed in the first cycle. In subsequent cycles, the remaining irreversible lithium from the lithium-rich manganese-based cathode can be gradually released to replenish the active lithium consumed during the battery's lifespan.

[0039] Optionally, the activation voltage is 4.25 to 4.4V, specifically one or any two of 4.25V, 4.28V, 4.3V, 4.35V, and 4.4V. This ensures that the irreversible lithium released from the second positive electrode active material during the first charge process can just replenish the lithium ions consumed in the first charge, thus avoiding the problems of lithium plating on the negative electrode and the inability to fully utilize the capacity of the positive electrode in the first charge.

[0040] Optionally, in one embodiment, the first-cycle activation coulombic efficiency of the second positive electrode active material at 4.4V is 68% to 90%, enabling the second positive electrode active material to release sufficient active lithium through activation to replenish the active lithium consumed during the formation of the negative electrode SEI film. For example, the first-cycle activation coulombic efficiency of the second positive electrode active material at 4.4V is a value within the range of one or both of 68%, 69%, 70%, 72%, 74%, 78%, 80%, 85%, and 90%.

[0041] Optionally, in one embodiment, the manganese element accounts for 50% to 100% of the mass of all transition metals in the second positive electrode active material, so that the second positive electrode active material has enough manganese element to combine with lithium ions to form Li2MnO3, thereby ensuring that enough irreversible lithium can be released to replenish the SEI film and the lithium ion consumption caused by charge and discharge cycles, thereby improving the energy density and life of the battery system.

[0042] Optionally, in one embodiment, the mass ratio of the second positive electrode active material to the positive electrode active material in the composite material is (5-95):(95-5), which effectively balances the performance advantages of the positive electrode active material and the lithium replenishment performance of the second positive electrode active material. Optionally, the mass ratio of the second positive electrode active material to the positive electrode active material is (15-50):(50-85), which better balances the performance advantages of the positive electrode active material and the lithium replenishment performance of the second positive electrode active material. Optionally, the mass ratio of the second positive electrode active material to the first positive electrode active material can be one or any two of the following: 15:85, 20:80, 30:70, 40:60, 50:50.

[0043] Optionally, in one specific embodiment, the first positive electrode active material in the above-mentioned composite material is selected from one or more of lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganese oxide. Optionally, the first positive electrode active material can be lithium manganese iron phosphate, which not only matches well with the second positive electrode active material to give full play to the performance advantages of lithium manganese iron phosphate, but also effectively improves the problem of capacity decay caused by the loss of active lithium during cycling, thus affecting battery life.

[0044] Optionally, in one embodiment, M is selected from one or more of Ni, Co, and Mn.

[0045] This application also provides a positive electrode sheet, including a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the composite material as described above.

[0046] This application also provides a secondary battery, including the positive electrode as described above.

[0047] Optionally, in one embodiment, the secondary battery further includes a negative electrode sheet, which includes a negative current collector and a negative active material layer disposed on the negative current collector.

[0048] The above-mentioned secondary battery satisfies the following condition: N / P is (1~1.08):1;

[0049] Where N represents the molar mass of lithium ions extracted from the positive electrode, and P represents the molar mass of lithium ions that can be inserted into the negative electrode.

[0050] In this embodiment, because only a small portion of the active lithium released from the decomposition of Li2MnO3 in the second positive electrode active material (which serves as a lithium replenishment agent) is stored as pre-stored lithium in the negative electrode, there is no need to retain excessive storage space in the negative electrode. The ratio of the molar mass of lithium ions extracted from the positive electrode to the molar mass of lithium ions that can be inserted into the negative electrode only needs to be (1-1.08):1 to meet the lithium-ion cycling requirements. This not only prevents lithium plating at the negative electrode but also effectively improves the battery energy density. Therefore, this embodiment can reduce the amount of negative electrode used, effectively reducing costs.

[0051] Optionally, in one embodiment, the positive electrode sheet further includes a conductive agent and an adhesive, wherein the adhesive may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0052] Optionally, in one embodiment, the conductive agent can be acetylene black, carbon fiber, carbon nanotubes, Ketjen black, artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, etc.

[0053] In some embodiments, the positive electrode sheet is prepared as follows: the components used to prepare the positive electrode sheet, such as the second positive electrode active material including the above-mentioned positive electrode material, the first positive electrode active material, the binder and any other components, are dispersed in a solvent such as N-methylpyrrolidone to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector; after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0054] The other components mentioned above may include a dispersant, and the second positive electrode active material and the first positive electrode active material are pre-baked to a moisture content of less than or equal to 150 ppm.

[0055] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer can be a negative active material for batteries known in the art, such as artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, etc.

[0056] The secondary battery provided in this application embodiment also includes a separator and an electrolyte.

[0057] In this embodiment, the electrolyte acts as a conductor of ions between the positive and negative electrodes. In some implementations, the electrolyte comprises an electrolyte salt and a solvent, wherein the electrolyte salt is a lithium salt.

[0058] In practical applications, the positive electrode sheet is rolled and die-cut, then stacked with the separator and negative electrode sheet in sequence and packaged to obtain a bare cell. After baking, the bare cell is injected with electrolyte, formed, resealed and sorted to obtain the above-mentioned secondary battery.

[0059] The above-mentioned formation process, also known as the activation process, involves charging the bare cell in a constant current, constant voltage (CCCV) mode. Specifically, it is first charged at 0.2C to 4.4V, and then charged at a constant voltage of 4.4V to a cutoff current of 0.05C to complete the activation process of the bare cell. This activates some of the irreversible lithium Li2MnO3 in the second positive electrode active material to replenish lithium, so that the irreversible lithium extracted from the lithium-rich manganese base can replenish the lithium consumed in the first cycle due to the formation of the SEI film.

[0060] In actual use, the aforementioned secondary batteries are charged and discharged at 0.33C within the range of 4.35V-2.0V.

[0061] This application also proposes an electrical device including the aforementioned secondary battery, wherein the secondary battery is used as a power supply for the electrical device.

[0062] For the above-described positive electrode sheet, secondary battery embodiment, and electrical device embodiment, the positive electrode sheet includes a positive electrode active material layer, which includes the above-described composite material and can achieve the same technical effect. To avoid repetition, it will not be described again here. For relevant details, please refer to the description of the composite material embodiment.

[0063] To make the objectives, technical solutions, and beneficial effects of this application clearer, the application is further described below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application.

[0064] The present application will be described in detail below through embodiments.

[0065] Test methods

[0066] (1) Capacity testing:

[0067] Using a Blue Electric 5V 3A test cabinet, the formed battery was charged to 4.4V at a constant current and constant voltage of 0.33C, with a cutoff current of 0.02C; then discharged to 2.0V at a constant current of 0.33C. This charging and discharging process was repeated 3 times, and the discharge specific capacity of the 3rd cycle was taken as the rated capacity.

[0068] (2) Cyclic performance test:

[0069] Using a Blue Electric 5V 3A test cabinet, the formed battery was charged to 4.4V at a constant current and constant voltage of 0.33C, with a cutoff current of 0.02C; then discharged to 2.0V at a constant current of 0.33C. This charging and discharging process was repeated, and the discharge capacity corresponding to each cycle was recorded. The test was stopped when the discharge capacity gradually decreased to 80% of the initial capacity, and the number of cycles was recorded as a test indicator of cycle performance.

[0070] (3) First-cycle activation coulombic efficiency test of the second positive electrode active material:

[0071] Using a Blue Electric 5V 3A test cabinet, the unformed battery was charged to 4.4V at a constant current and constant voltage of 0.1C, with a cutoff current of 0.05C. The charging capacity was recorded as C1. After the battery was left to stand for 30 minutes, it was discharged to 2.0V at a constant current of 0.1C, with a cutoff current of C2. The coulombic efficiency was C2 / C1*100%.

[0072] Example 1

[0073] (1) Preparation of the positive electrode sheet:

[0074] a. The second positive electrode active material and the first positive electrode active material, lithium manganese iron phosphate (LMFP), are mixed at a mass ratio of 15:85 to obtain the above composite material, wherein the second positive electrode active material is 0.11Li2MnO3·0.89LiMn 0.65 Ni 0.35 O2.

[0075] b. The composite material, conductive agent acetylene black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed uniformly at a mass ratio of composite material:Super P:PVDF = 95:5:3, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a homogeneous slurry. The mixed slurry is then coated on both sides of an aluminum foil, controlling the electrode surface density to be 410 mg / cm³. 2 After high-temperature drying, rolling, cutting, and slitting, positive electrode sheets are prepared, wherein the compaction density after rolling is controlled to be 2.4 g / cm³. 3 .

[0076] (2) Preparation of negative electrode sheet

[0077] A negative electrode sheet is prepared by homogenizing graphite, styrene-butadiene rubber (SBR), conductive carbon black and sodium carboxymethyl cellulose (CMC) in a mass ratio of 100:3.4:1:1.6, uniformly coating it onto aluminum foil, drying it at high temperature, rolling it, cutting it into sheets and slitting it.

[0078] (3) Preparation of lithium-ion batteries

[0079] A bare cell is obtained by stacking positive electrode, negative electrode, and separator (PE separator). The bare cell is then placed in a casing and injected with electrolyte (the electrolyte is a mixture of carbonate solvent and 1M LiPF6). After encapsulation, drying, and capacity testing, a lithium-ion battery can be prepared.

[0080] Example 2

[0081] The difference between Example 2 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.05Li2MnO3·0.95LiMn 0.65 Ni 0.35 O2.

[0082] Example 3

[0083] The difference between Example 3 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.2Li2MnO3·0.8LiMnO2.

[0084] Example 4

[0085] The difference between Example 4 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.18Li2MnO3·0.82LiMn 0.65 Co 0.35 O2.

[0086] Example 5

[0087] The difference between Example 5 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.11Li2MnO3·0.89LiMnO2 and the mass ratio of the second positive electrode active material to the first positive electrode active material lithium manganese iron phosphate (LMFP) is adjusted to 50:50.

[0088] Example 6

[0089] The difference between Example 6 and Example 5 is that in step (a), the mass ratio of the second positive electrode active material to the first positive electrode active material lithium manganese iron phosphate (LMFP) is adjusted to 30:70.

[0090] Example 7

[0091] The difference between Example 7 and Example 1 is that in step (a), the mass ratio of the second positive electrode active material to the first positive electrode active material lithium manganese iron phosphate (LMFP) is adjusted to 5:95.

[0092] Example 8

[0093] The difference between Example 8 and Example 1 is that in step (a), the mass ratio of the second positive electrode active material to the first positive electrode active material lithium manganese iron phosphate (LMFP) is adjusted to 95:5.

[0094] Example 9

[0095] The difference between Example 9 and Example 1 is that in step (a), the first positive electrode active material is adjusted to lithium nickel cobalt manganese oxide.

[0096] Example 10

[0097] The difference between Example 10 and Example 1 is that in step (a), the first positive electrode active material is adjusted to lithium iron phosphate.

[0098] Example 11

[0099] The difference between Example 11 and Example 1 is that in step (a), the second positive electrode active material 0.18Li2MnO3·0.82LiMn 0.65 Ni 0.35 O2.

[0100] Example 12

[0101] The difference between Example 12 and Example 1 is that in step (a), the second positive electrode active material 0.06Li2MnO3·0.94LiMn 0.65 Ni 0.35 O2.

[0102] Example 13

[0103] The difference between Example 13 and Example 1 is that in step (a), the second positive electrode active material 0.19Li2MnO3·0.81LiMn 0.41 Ni 0.59 O2.

[0104] Comparative Example 1

[0105] The difference between Comparative Example 1 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.042Li2MnO3·0.958LiMnO2.

[0106] Comparative Example 2

[0107] The difference between Comparative Example 2 and Example 1 is that the positive electrode does not contain a second positive electrode active material.

[0108] Comparative Example 3

[0109] The difference between Comparative Example 3 and Example 1 is that in step (a), the second positive electrode active material is adjusted to Li2MnO3.

[0110] Comparative Example 4

[0111] The difference between Comparative Example 4 and Example 1 is that in step (a), the second positive electrode active material is adjusted to 0.02Li2MnO3·0.98LiMn 0.65 Ni 0.35 O2.

[0112] The process parameters for each embodiment and comparative example are shown in Table 1.

[0113] The first-cycle activation coulombic efficiency of the second positive electrode active material in each embodiment and comparative example was tested, and the test data are shown in Table 1.

[0114] The batteries prepared in each embodiment and comparative example were subjected to capacity testing and cycle performance testing. The test data are shown in Table 2. Figure 1 shows a comparison of the cycle life of Embodiment 1, Comparative Example 1, and Comparative Example 2.

[0115] Table 1

[0116] Table 2

[0117] As can be seen from Examples 1-8, 11-13 and Comparative Examples 1, 3, and 4, combined with Table 2, when the molar content of Li2MnO3 in the second positive electrode active material is between 5% and 20%, and the mass percentage of lithium element in the second positive electrode active material is between 7.5% and 8.6%, the second positive electrode active material has sufficient irreversible lithium, which can effectively replenish the SEI film and the active lithium consumed during cycling. The battery has higher energy density and lower cycle life.

[0118] As can be seen from Examples 1, 11-12 and Comparative Example 1 combined with Table 2, the first-cycle activation coulombic efficiency of the second positive electrode active material is directly related to its lithium replenishment performance. When the first-cycle activation coulombic efficiency is between 68% and 90%, it can release enough irreversible lithium to replenish the SEI film during the activation process, and can avoid excessive active lithium existing as pre-stored lithium in the negative electrode.

[0119] As can be seen from Examples 1-13 and Comparative Example 2, combined with Table 2, the addition of the second positive electrode active material can effectively improve the energy density and cycle life of the battery.

[0120] In this application, the provided composite material includes a first positive electrode active material and a second positive electrode active material. The chemical formula of the second positive electrode active material is xLi2MnO3·(1-x)LiMO2, where M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of lithium element in the second positive electrode active material is 7.5% to 8.6%. Under the conditions of 0.05≤x≤0.2 and the mass percentage of lithium element in the second positive electrode active material being 7.5% to 8.6%, the lithium-rich phase Li2MnO3 of the second positive electrode active material contains sufficient irreversible lithium. When the above composite material is used as a positive electrode material, during the activation process, a portion of the irreversible lithium in Li2MnO3 can be activated with a higher activation voltage to replenish the lithium ion consumption caused by the formation of the SEI film. In subsequent cycling, another portion of Li2MnO3 can provide reversible capacity and continuously replenish irreversible lithium, while LiMO2 can continuously provide reversible capacity, thus effectively improving the energy density and lifespan of the battery system. Therefore, the composite material provided in this application can improve the problem that existing lithium-based cathode materials are prone to losing active lithium during cycling, resulting in low battery energy density and short cycle life.

[0121] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0122] The above provides a detailed description of the composite material, positive electrode sheet, secondary battery, and electrical equipment provided in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A composite material, characterized in that, The composite material includes a first positive electrode active material and a second positive electrode active material. The chemical formula of the second positive electrode active material is xLi2MnO3·(1-x)LiMO2, where M is a transition metal element, 0.05≤x≤0.2, and the mass percentage of lithium element in the second positive electrode active material is 7.5% to 8.6%.

2. The composite material according to claim 1, characterized in that, The second positive electrode active material has a first-cycle activation coulombic efficiency of 68% to 90% at 4.4V.

3. The composite material according to claim 1, characterized in that, In the second positive electrode active material, manganese accounts for 50% to 100% of the mass of all transition metal elements in the second positive electrode active material.

4. The composite material according to claim 1, characterized in that, The mass ratio of the second positive electrode active material to the first positive electrode active material is (5-95):(5-95).

5. The composite material according to claim 1, characterized in that, The mass ratio of the second positive electrode active material to the first positive electrode active material is (15-50):(50-85).

6. The composite material according to claim 1, characterized in that, M is selected from one or more of Ni, Co, and Mn.

7. The composite material according to claim 1, characterized in that, The first positive electrode active material is selected from one or more of lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganese oxide.

8. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the composite material as described in any one of claims 1 to 7.

9. A secondary battery, characterized in that, Including the positive electrode sheet as described in claim 8.

10. The secondary battery according to claim 9, characterized in that, The secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; The secondary battery satisfies the following condition: N / P = (1~1.08):1; Wherein, N represents the molar mass of lithium ions extracted from the positive electrode, and P represents the molar mass of lithium ions that can be inserted into the negative electrode.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 9 or 10, wherein the secondary battery serves as the power supply for the electrical equipment.

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