Positive electrode sheet, method for manufacturing the same, lithium ion battery, and electric device

Abstract

The application relates to a positive electrode sheet and a preparation method thereof, a battery and a power utilization device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer located on at least one side surface of the positive electrode current collector. The positive electrode film layer comprises a first film layer, a second film layer and a third film layer which are sequentially arranged on the positive electrode current collector. The first film layer comprises a phosphate positive electrode material containing an iron element. The second film layer comprises a conductive polymer. The third film layer comprises a ternary positive electrode material. When the positive electrode sheet is used in a battery, the third film layer comprising the ternary positive electrode material can effectively block the migration of iron ions generated in the phosphate positive electrode material containing the iron element in the first film layer to the negative electrode in the charging and discharging process, and prevent the iron ions from being deposited on the negative electrode in the charging and discharging process of the battery. The second film layer is arranged, more iron adsorption sites can be provided, the contact interface between the first film layer and the second film layer can be improved, and the overall electrochemical performance of the battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive plate, a preparation method thereof, a lithium ion battery and an electric device. BACKGROUND

[0002] In recent years, during the cycle process of lithium iron phosphate (LFP) batteries, iron dissolution occurs in the positive electrode material, resulting in capacity attenuation of lithium ion batteries. This phenomenon is more serious at high temperatures, ultimately leading to the problem of cycle diving of the battery. Therefore, the related technology of lithium iron phosphate batteries still needs to be improved. SUMMARY

[0003] The present application aims to at least solve one of the technical problems in the related art. To this end, the present application provides a positive plate that significantly improves the cycle stability and service life of the battery under high temperature conditions.

[0004] The first aspect of the present application provides a positive plate. According to embodiments of the present application, the positive plate includes a positive current collector and a positive film layer located on at least one side surface of the positive current collector; the positive film layer includes a first film layer, a second film layer and a third film layer which are sequentially stacked on the positive current collector; the first film layer includes an iron-containing phosphate positive material; the second film layer includes a conductive polymer; and the third film layer includes a ternary positive material. The positive plate of the present application can significantly improve the cycle stability and service life of the battery under high temperature conditions, making it more suitable for application requirements in high temperature environments.

[0005] According to embodiments of the present application, the positive film layer further includes a fourth film layer, the fourth film layer is located on the surface of the third film layer away from the second film layer, and the fourth film layer includes an iron-philic metal oxide.

[0006] According to embodiments of the present application, the iron-philic metal oxide includes at least one of iron oxide and titanium oxide.

[0007] According to embodiments of the present application, the phosphate positive material includes at least one of lithium iron phosphate and lithium manganese iron phosphate, preferably lithium iron phosphate;

[0008] The ternary positive material includes at least one of nickel-cobalt-manganese ternary positive material and nickel-cobalt-aluminum ternary positive material, preferably nickel-cobalt-manganese ternary positive material.

[0009] According to embodiments of the present application, the nickel-cobalt-manganese ternary positive material has the following chemical composition:

[0010] Li(Ni x Co y Mn z )Q r O2

[0011] wherein x = 0.3-1, y = 0.01-0.5, z = 0.01-0.5, and r = 0-0.1.

[0012] The Q element includes at least one of yttrium, zirconium, titanium, niobium, aluminum, magnesium, molybdenum, and niobium.

[0013] According to an embodiment of the present application, the ternary positive electrode material includes at least one of spherical nanoparticles, nanowires, and nanosheets.

[0014] According to an embodiment of the present application, the D50 particle size of the spherical nanoparticles is 2-5 μm.

[0015] The aspect ratio of the nanowires is 2-5.

[0016] The aspect ratio of the nanosheets is 2-5.

[0017] According to an embodiment of the present application, the conductive polymer includes at least one of polyaniline and polypyrrole.

[0018] According to an embodiment of the present application, the second film layer further includes an iron ion adsorbent.

[0019] According to an embodiment of the present application, the iron ion adsorbent includes at least one of manganese-based ferrite (MnFe2O4), Fe3O4 magnetic nanomaterial, ethylenediaminetetraacetic acid (EDTA), and tannic acid (TA); and / or

[0020] The mass percentage content of the iron ion adsorbent is 5%-20% based on the total mass of the second film layer.

[0021] According to an embodiment of the present application, the thickness ratio of the first film layer, the second film layer, and the third film layer is (0.5-1):(0.02-0.2):(0.02-0.3).

[0022] In a second aspect, the present application provides a method for preparing the positive electrode sheet described above. According to an embodiment of the present application, the method includes: coating a first slurry on at least one side surface of a positive electrode current collector and drying to form a first film layer; coating a second slurry on the surface of the first film layer away from the current collector and drying to form a second film layer; and coating a third slurry on the surface of the second film layer away from the current collector to form a third film layer.

[0023] According to an embodiment of the present application, the method further includes coating a fourth slurry on the surface of the third film layer away from the current collector and drying to form a fourth film layer.

[0024] In a third aspect, the present application provides a lithium ion battery. According to an embodiment of the present application, the lithium ion battery comprises the positive electrode sheet as described above. During the operation of the lithium ion battery, the second film layer and the third film layer serve as a barrier to intercept the iron ions generated in the first film layer, preventing the iron ions from depositing on the negative electrode during the charging and discharging of the battery, thereby improving the cycle stability of the battery under high temperature conditions, prolonging the service life of the battery, and making the battery more suitable for application in high temperature environments.

[0025] In a fourth aspect, the present application provides an electric device. According to an embodiment of the present application, the electric device comprises the lithium ion battery as described above. The electric device has all the features and advantages of the lithium ion battery as described above, which will not be repeated here. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a structural schematic diagram of a positive electrode sheet according to an embodiment of the present application.

[0027] Figure 2 is a structural schematic diagram of a positive electrode sheet according to another embodiment of the present application.

[0028] Figure 3 is a structural schematic diagram of a positive electrode sheet according to yet another embodiment of the present application.

[0029] Figure 4 is a SEM-EDS characterization result of the longitudinal section of the third film layer (i.e., the NCM layer) of the positive electrode sheet in the lithium ion battery according to Embodiment 1 of the present application.

[0030] Figure 5 is a TOF-SIMS characterization result of the iron content of the surface of the positive electrode sheet close to the separator in the lithium ion battery according to Embodiment 1 of the present application. DETAILED DESCRIPTION

[0031] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as limiting the present application.

[0032] The present application is based on the discovery and understanding of the inventors on the following facts and problems:

[0033] Iron leaching is an important factor causing capacity attenuation of LFP (lithium iron phosphate) batteries. The iron leaching process and the series of reactions caused by the leaching can be summarized as the following five reactions:

[0034] 1) LiPF6 hydrolysis produces HF, LiF and POF3: As LFP contains a small amount of water in the battery, the hydrolysis of LiPF6-containing electrolyte is widespread. In addition to the influence of water, electrolyte oxidation can also cause LiPF6 hydrolysis to produce protons.

[0035] 2) LFP surface etched by HF: HF in the electrolyte reacts with the surface of LFP, specifically, 4 equivalents of HF can react with LiFePO4 to generate H2, LiF, H3PO4 and insoluble FeF3. In the presence of HF, LFP is etched, leading to surface degradation, followed by exchange of protons and Fe 2+ / Fe 3+ in the electrolyte.

[0036] 3) Solvated Fe 2+ / Fe 3+ migration to the negative electrode. Solvated Fe 2+ / Fe 3+ is formed by the dissolution of Fe compounds.

[0037] 4) Reduction of solvated Fe 2+ / Fe 3+ . Once the Fe complex reaches the negative electrode, Fe 2+ / Fe 3+ may be reduced to Fe 0 and deposited on the negative electrode, eventually embedded in the SEI film.

[0038] 5) The impact of metal Fe deposition into the SEI film. Once Fe reaches the negative electrode, it diffuses into the inner layer of the SEI film and can cause the occurrence of lithium precipitation. In addition, studies have shown that the dissolution of transition metals can lead to the degradation of the SEI film, and the organic components of the SEI film can decompose into inorganic lithium carbonate, thus leading to the continuous growth of the SEI.

[0039] The entire reaction process can be summarized as follows:

[0040] ① LiPF6 → LiF + PF5

[0041] ② PF5 + H2O → POF3 + 2HF

[0042] ③ POF3 + H2O → HPO2F2 + HF

[0043] ④ LiPF6 + 2H2O → LiPO2F2 + 4HF

[0044] ⑤ 4HF + LiFePO4 → FeF3 + 0.5H2 + LiF + H3PO4

[0045] The means for inhibiting the iron dissolution of lithium iron phosphate (LiFePO4) usually involve the modification of electrolyte (electrolyte additives, etc.), the surface treatment of electrode materials (carbon coating, etc.), the optimization of battery structure (charging and discharging system of the battery, the control of state of charge (SOC) and the temperature management of the battery, etc.), the use of organic lithium salt (such as 3,4-dihydroxybenzonitrile dilithium, Li2DHBN), mechanical chemical activation, closed-loop regeneration method, etc. However, the various methods reported so far still have some shortcomings or potential problems: electrolyte additives may increase the impedance of the battery, reduce the thermal stability of the electrolyte, carbon coating may increase the production cost, the use of organic lithium salt has harsh conditions, the complexity and cost of the production process, the need for specific technology and equipment, etc.

[0046] In order to overcome the above-mentioned shortcomings to some extent, the present application relates to an improved positive electrode sheet which can intercept the migration of Fe ions dissolved in the positive electrode material to the negative electrode.

[0047] Therefore, in the first aspect of the present application, a positive electrode sheet is provided. According to the embodiments of the application, with reference to Figure 1 The positive electrode sheet comprises a positive electrode current collector 10 and a positive electrode film layer 20 located on at least one side surface of the positive electrode current collector; the positive electrode film layer 20 comprises a first film layer 21, a second film layer 22 and a third film layer 23 which are sequentially stacked on the positive electrode current collector 10; the first film layer 21 comprises an iron-containing phosphate positive electrode material; the second film layer 22 comprises a conductive polymer; and the third film layer 23 comprises a ternary positive electrode material. When the positive electrode sheet is used in a battery, during the charging and discharging cycle, the third film layer comprising a ternary positive electrode material not only has good electrochemical performance, but also can effectively block the migration of iron ions (Fe2+ / Fe3+) possibly produced in the iron-containing phosphate positive electrode material in the first film layer to the negative electrode during the charging and discharging process; thereby, the third film layer acts as a barrier, intercepting the iron ions in the first film layer and preventing them from depositing on the negative electrode during the charging and discharging process of the battery. The second film layer is provided, on the one hand, to provide more iron adsorption sites and improve the interception effect of iron ions, and on the other hand, the conductive polymer has a certain flexibility, which can improve the contact interface between the first film layer and the second film layer; and on the other hand, the conductive polymer can also improve the conductivity of the electrode and optimize the overall electrochemical performance of the battery, thereby improving the overall electrochemical performance of the battery. Therefore, the positive electrode sheet of the embodiments of the present application can significantly improve the cycle stability and service life of the battery under high temperature conditions, making it more suitable for application requirements in high temperature environments.

[0048] Specifically, the third film layer can achieve good interception effect of iron ions, mainly due to the dual action mechanism of the ternary positive electrode material. Specifically, the ternary positive electrode material has a certain water absorption, which enables it to play a role of water remover in the internal environment of the battery. During the manufacturing and use of the lithium ion battery, the presence of water is inevitable, and the water in the electrolyte will react with the lithium salt in the electrolyte to generate corrosive substances such as hydrofluoric acid, which will cause damage to the electrode material and accelerate the dissolution of iron ions in the phosphate positive electrode material. The water absorption of the ternary positive electrode material can effectively absorb the trace amount of water in the battery, reduce the generation of corrosive substances, thereby slowing down the dissolution behavior of iron ions in the phosphate positive electrode material, and improving the chemical stability and cycle life of the battery.

[0049] In addition, there may be residual alkaline substances such as lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) on the surface of the ternary positive electrode material. These alkaline substances form a stable chemical environment on the surface of the ternary positive electrode material, providing active sites for the deposition of iron ions. When the phosphate positive electrode material releases iron ions during charging and discharging, these sites can capture iron ions, preventing them from further diffusing to the negative electrode. This capture not only reduces the deposition of iron ions on the negative electrode, but also reduces the increase in internal resistance of the battery caused by the deposition of iron ions, thereby maintaining the electrochemical performance of the battery.

[0050] In summary, the water absorption of the ternary positive electrode material and the presence of surface alkaline substances can effectively intercept iron ions, not only improving the stability of the battery in high temperature environment, but also prolonging the service life of the battery, providing strong technical support for the application of lithium ion batteries in harsh conditions.

[0051] According to the embodiments of the present application, the positive electrode current collector can be a metal current collector or a composite current collector. For example, the metal current collector includes but is not limited to an aluminum foil current collector; the composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base film. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base film (such as a base film of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0052] According to the embodiments of the present application, the iron-containing phosphate positive electrode material can include at least one of lithium iron phosphate and lithium manganese iron phosphate, and as an example, the iron-containing phosphate positive electrode material can be lithium iron phosphate.

[0053] According to embodiments of the present application, in addition to the aforementioned iron element-containing phosphate positive electrode material, the first film layer can further include a binder, a conductive agent, and optionally a functional additive. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene-fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene-fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin; and the conductive agent can include at least one of super conductive carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0054] According to embodiments of the present application, the conductive polymer in the second film layer can include at least one of polyaniline and polypyrrole. In this way, the interface contact between the first film layer and the second film layer can be effectively improved, the overall electrochemical performance of the battery using the positive electrode sheet can be improved as a whole, and more iron ion adsorption sites can be provided to improve the effect of intercepting iron ions.

[0055] According to embodiments of the present application, the second film layer further includes an iron ion adsorbent. In this way, more iron ion adsorption sites can be provided to effectively improve the effect of intercepting iron ions.

[0056] According to embodiments of the present application, the iron ion adsorbent can include at least one of a manganese-based ferrite (MnFe2O4), Fe3O4 magnetic nanomaterial, ethylenediaminetetraacetic acid (EDTA), and tannic acid (TA). In this way, the effect of adsorbing iron ions is better.

[0057] According to embodiments of the present application, the mass percentage content of the iron ion adsorbent in the second film layer is 5% to 20%, specifically, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc., based on the total mass of the second film layer. In this way, while improving the effect of intercepting iron ions, other negative effects can be substantially avoided.

[0058] According to embodiments of the present application, the ternary positive electrode material used in the third film layer includes at least one of a nickel-cobalt-manganese ternary positive electrode material and a nickel-cobalt-aluminum ternary positive electrode material. As an example, the ternary positive electrode material used in the third film layer is a nickel-cobalt-manganese ternary positive electrode material.

[0059] It can be understood that the nickel-cobalt-manganese ternary positive electrode material refers to lithium nickel cobalt manganese oxide, and the nickel-cobalt-aluminum ternary positive electrode material refers to lithium nickel cobalt aluminum oxide. In this article, the nickel-cobalt-manganese ternary positive electrode material and the nickel-cobalt-aluminum ternary positive electrode material are understood in a broad sense, which can include intrinsic materials without modification, or materials with doping and / or coating modification.

[0060] In addition, the nickel-cobalt-manganese ternary positive electrode material and the nickel-cobalt-aluminum ternary positive electrode material can be divided into low-nickel material, medium-nickel material, high-nickel material and super-high-nickel material according to the nickel content. The ternary positive electrode materials with different nickel contents mentioned above can all be used in the third film layer. It can be understood that by accurately adjusting the molar ratio of nickel (Ni), cobalt (Co) and manganese (Mn) or the molar ratio of nickel (Ni), cobalt (Co) and aluminum (Al) in the ternary positive electrode material, the optimization of the electronic structure and chemical stability of the material can be realized. For example, increasing the content of manganese can improve the stability and safety of the material, and moderately increasing the content of nickel can help improve the capacity. Through such proportion adjustment, the third film layer can not only provide stronger iron ion adsorption capacity, but also maintain the structural integrity during the charging and discharging process, thereby effectively inhibiting the dissolution of iron ions in the phosphate positive electrode material. As an example, in order to obtain better use effect, a medium-nickel ternary positive electrode material can be used in the third film layer.

[0061] In some embodiments, the nickel-cobalt-manganese ternary positive electrode material has the following chemical composition:

[0062] Li(Ni x Co y Mn z )Q r O2

[0063] wherein x = 0.3-1, y = 0.01-0.5, z = 0.01-0.5, and r = 0-0.1.

[0064] The Q element includes at least one of yttrium, zirconium, titanium, niobium, aluminum, magnesium, molybdenum, and niobium.

[0065] As a specific example, the nickel-cobalt-manganese ternary positive electrode material is LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622).

[0066] Thus, it has better effect of intercepting iron ions, and is conducive to improving the capacity and cycle stability of the battery using the positive electrode sheet.

[0067] According to the embodiments of the application, in order to obtain better effect of intercepting iron ions, the morphology of the ternary positive electrode material particles can be adjusted. In some embodiments, the ternary positive electrode material includes at least one of spherical nanoparticles, nanowires and nanosheets. Thus, the specific surface area of the ternary positive electrode material can be effectively increased. Such high specific surface area nanostructure not only can provide more active sites for the adsorption of iron ions, but also can shorten the diffusion path of lithium ions, thereby improving the charging and discharging rate and cycle stability of the battery.

[0068] According to the embodiments of the application, the D50 particle size of the spherical nanoparticles is 2-5 mu m. Specifically, 2 mu m, 2.5 mu m, 3 mu m, 3.5 mu m, 4 mu m, 4.5 mu m, 5 mu m, etc.

[0069] In the present application, D50 refers to the particle size corresponding to the volume cumulative particle size distribution percentage of 50% of a sample. Its physical meaning is that the particles with a particle size greater than it account for 50% of the volume, and the particles with a particle size less than it also account for 50% of the volume. D50 is also called median diameter or median particle size. It can be detected by a laser particle size analyzer.

[0070] According to the embodiments of the application, the aspect ratio of the nanowire is 2-5, specifically, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.

[0071] In the present application, the aspect ratio of the nanowire refers to the ratio of the length of the nanowire to the diameter.

[0072] According to the embodiments of the application, the aspect ratio of the nanosheet is 2-5, specifically, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.

[0073] In the present application, the aspect ratio of the nanosheet refers to the ratio of the maximum distance between any two points on the nanosheet profile line to the minimum distance between any two points on the nanosheet profile line.

[0074] The ternary positive electrode material satisfying the above size has a larger specific surface area and is more likely to capture iron ions.

[0075] According to the embodiments of the application, with reference to Figure 3 , the positive electrode film layer 20 further comprises a fourth film layer 24 located on the surface of the third film layer 23 away from the second film layer 22, and the fourth film layer 24 comprises iron-philic metal oxide. In this way, the adsorption sites of iron ions can be effectively increased, not only the capture efficiency of the third film layer for iron ions can be improved, but also a more stable SEI film can be formed in the battery charging and discharging process, the decomposition of electrolyte is slowed down, and the service life of the battery is prolonged.

[0076] According to the embodiments of the application, the iron-philic metal oxide comprises at least one of iron oxide and titanium oxide. In this way, a better iron ion adsorption effect can be achieved.

[0077] According to the embodiments of the application, the thickness ratio of the first film layer, the second film layer and the third film layer can be (0.5-1):(0.02-0.2):(0.02-0.3). In this way, the area close to the first film layer has higher iron ion adsorption capacity and better chemical stability.

[0078] In a second aspect, the application provides a method for preparing the positive electrode sheet described above. According to an embodiment of the application, the method comprises:

[0079] S1: coating a first slurry on at least one side surface of the positive electrode current collector and drying to form a first film layer.

[0080] Specifically, in this step, the raw materials of the first film layer can be formulated into a first slurry, and then the first slurry is coated on at least one side surface of the positive electrode current collector, and then dried to form the first film layer.

[0081] Specifically, the specific operations of coating the slurry and drying are not particularly limited in the application, and can be flexibly selected according to actual needs. For example, it can be performed according to conventional techniques.

[0082] In some embodiments, the raw materials of the first film layer include a phosphate positive electrode material containing an iron element, a binder, a conductive agent, and a first solvent. The phosphate positive electrode material containing an iron element, the binder, and the conductive agent can be consistent with the description above, and will not be repeated here. The first solvent includes but is not limited to N-methyl pyrrolidone (NMP) and the like.

[0083] S2: coating a second slurry on the surface of the first film layer away from the current collector and drying to form a second film layer.

[0084] Specifically, in this step, the raw materials of the second film layer can be formulated into a second slurry, and then the second slurry is coated on the surface of the first film layer away from the positive electrode current collector, and then dried to form the second film layer.

[0085] Specifically, the specific operations of coating the slurry and drying are not particularly limited in the application, and can be flexibly selected according to actual needs. For example, it can be performed according to conventional techniques.

[0086] In some embodiments, the raw materials of the second film layer include a conductive polymer and a second solvent. The conductive polymer can be consistent with the description above, and will not be repeated here. The second solvent includes but is not limited to NMP.

[0087] In some embodiments, the raw materials of the second film layer include a conductive polymer, an iron ion adsorbent, and a second solvent. The iron ion adsorbent can be consistent with the description above, and will not be repeated here.

[0088] S3: coating a third slurry on the surface of the second film layer away from the current collector to form a third film layer.

[0089] Specifically, in this step, the raw materials of the third film layer can be formulated into a third slurry, and then the third slurry is coated on the surface of the second film layer away from the positive electrode current collector, and then dried to form the third film layer.

[0090] Specifically, the specific operations of coating the slurry and drying are not particularly limited in the present application, and can be flexibly selected according to actual needs. For example, they can be performed according to conventional techniques.

[0091] In some embodiments, the raw material of the third film layer includes a ternary positive electrode material, a binder, a conductive agent, and a third solvent. The ternary positive electrode material, the binder, and the conductive agent can be consistent with the foregoing description and will not be repeated here. The third solvent includes, but is not limited to, N-methyl pyrrolidone (NMP) and the like.

[0092] According to embodiments of the application, the method further includes coating a fourth slurry on the surface of the third film layer away from the current collector and drying to form a fourth film layer.

[0093] Specifically, in this step, the raw material of the fourth film layer can be prepared into a fourth slurry, and then the fourth slurry is coated on the surface of the third film layer away from the positive electrode current collector, and then dried to form the fourth film layer.

[0094] Specifically, the specific operations of coating the slurry and drying are not particularly limited in the present application, and can be flexibly selected according to actual needs. For example, they can be performed according to conventional techniques.

[0095] In some embodiments, the raw material of the fourth film layer includes an iron-philic metal oxide, a film-forming aid, and a third solvent. The iron-philic metal oxide can be consistent with the foregoing description and will not be repeated here. The film-forming aid includes, but is not limited to, PVDF and the like, and the third solvent includes, but is not limited to, NMP and the like.

[0096] In a third aspect, the present application provides a lithium ion battery. According to embodiments of the application, the lithium ion battery includes the positive electrode sheet described above. In the working process of the lithium ion battery, the second film layer and the third film layer act as a barrier to intercept the iron ions generated in the first film layer, preventing them from depositing on the negative electrode during the charging and discharging process of the battery. Not only does this improve the cycle stability of the battery under high temperature conditions, but it also prolongs the service life of the battery, making it more suitable for application requirements in high temperature environments.

[0097] It can be understood that the specific type of the battery is not particularly limited, for example, including but not limited to lithium ion batteries, sodium ion batteries, and the like, and the battery can be a square cell, a soft package battery, a cylindrical battery, and the like, and can be in the form of a battery monomer, a battery module, a battery pack, and the like.

[0098] It can also be understood that in addition to the positive electrode sheet described above, the battery can also include other conventional battery necessary structures and components. For example, it can include an outer package, and an electrode assembly and an electrolyte contained in the outer package. The electrode assembly can include the positive electrode sheet, the negative electrode sheet, and the separator between the positive electrode sheet and the negative electrode sheet.

[0099] As an example, the negative electrode sheet can include a negative electrode current collector and a negative electrode film layer located on at least one side surface of the negative electrode current collector, and the negative electrode film layer generally contains a negative electrode active material, a conductive agent, and a binder. The electrolyte solution can generally include an electrolyte salt and a solvent. The above-mentioned negative electrode active material, binder, conductive agent, current collector, electrolyte salt, and solvent can be selected according to conventional techniques, and the present application is not particularly limited.

[0100] In a fourth aspect, the present application provides a power consuming device. According to embodiments of the present application, the power consuming device includes the above-mentioned lithium ion battery. The power consuming device has all the features and advantages of the above-mentioned lithium ion battery, which will not be repeated here.

[0101] According to embodiments of the present application, the power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto. The battery can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device.

[0102] It can be understood that, in addition to the above-mentioned lithium ion battery, the power consuming device can also include the necessary structures and components of conventional power consuming devices. For example, in the case of an electric vehicle, it can include a vehicle body, a vehicle window, a chassis, an engine, a seat, a tire, and other necessary structures and components, which will not be repeated here.

[0103] Embodiments of the present application will be described in detail below.

[0104] The lithium ion batteries of Examples 1-9 and Comparative Example 1 below were prepared in the following manner, with the specific parameters differing as shown in Table 1:

[0105] (1) Preparation of the positive electrode sheet of the comparative example

[0106] The positive electrode active material LFP, the conductive agent acetylene black and carbon nanotube CNT, and the binder PVDF were mixed in a mass ratio of 97.5:0.5:2, and the solvent NMP was added to form a uniform slurry. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and after oven drying, the positive electrode sheet was formed by rolling, slitting, and die cutting.

[0107] (2) Preparation of the positive electrode sheet of the example

[0108] The positive electrode active material LFP, the conductive agent acetylene black and carbon nanotube CNT, and the binder PVDF were mixed in a mass ratio of 97.5:0.5:2, and the solvent NMP was added to form a uniform slurry. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and a first film layer was formed;

[0109] The conductive polymer polyaniline and the solvent NMP are mixed, stirred into a uniform slurry, and uniformly coated on the surface of the first film layer away from the current collector to form a second film layer.

[0110] The positive electrode active material NCM622 (D50 particle size of 4 μm), the conductive agent acetylene black and the carbon nanotube CNT, and the binder PVDF are mixed in a mass ratio of 97.5:0.5:2, the solvent NMP is added, and a uniform slurry is stirred. The positive electrode slurry is uniformly coated on the surface of the second film layer away from the current collector, and after oven drying, it is rolled, cut, and die cut into a positive electrode tab.

[0111] (3) Preparation of the negative electrode tab

[0112] The negative electrode active material graphite, the conductive agent, the binder SBR, and the thickening agent CMC are mixed in a mass ratio of 95.5:1:1.5:2, deionized water is added, and a uniform slurry is stirred. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and after oven drying, it is rolled to obtain a negative electrode tab.

[0113] (4) Preparation of the electrolyte

[0114] Vinyl carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1:1 and then added with vinylene carbonate VC to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L.

[0115] (5) Preparation of the separator film

[0116] A polyethylene film coated with ceramic glue on both sides is selected as the separator film.

[0117] (6) Preparation of the lithium ion battery

[0118] The above positive electrode tab, the separator film, and the negative electrode tab are stacked in order, with the separator film between the positive and negative electrode tabs to separate the positive and negative electrodes. Then, the tab is obtained by stacking, the tab is placed in an outer packaging shell, the electrolyte is injected, and the lithium ion battery is obtained after packaging, standing, formation, and capacity distribution processes.

[0119] Example 10

[0120] The preparation method is the same as above, except that the conductive polymer polyaniline, the iron ion adsorbent Fe3O4, and the solvent NMP are mixed, stirred into a uniform slurry, and uniformly coated on the surface of the first film layer away from the current collector to form a second film layer. The mass percentage of the iron ion adsorbent Fe3O4 in the total mass of the second film layer is 10%.

[0121] Example 11

[0122] The preparation method is the same as above, except that Fe2O3 and film-forming aid PVDF are mixed in a mass ratio of 95:5, solvent NMP is added, and stirring is performed to form a uniform slurry, which is uniformly coated on the surface of the third film layer away from the current collector, and after oven drying, the positive electrode sheet is formed by rolling, cutting, and die cutting.

[0123] Example 12

[0124] The preparation method is the same as above, except that the NCM622 particles are nanowires (aspect ratio of 4).

[0125] Example 13

[0126] The preparation method is the same as above, except that the NCM622 particles are nanosheets (aspect ratio of 4).

[0127] Example 14

[0128] The preparation method is the same as above, except that doped NCM: LiNi 0.6 Co 0.15 Mn 0.2 Zr 0.05 O2.

[0129] Example 15

[0130] The preparation method is the same as above, except that low-nickel NCM: LiNi 0.5 Co 0.2 Mn 0.3 O2.

[0131] Example 16

[0132] The preparation method is the same as above, except that high-nickel NCM: LiNi 0.8 Co 0.1 Mn 0.1 O2.

[0133] Example 17

[0134] The preparation method is the same as above, except that NCA: LiNi 0.8 Co 0.15 Al 0.05 O2.

[0135] Table 1

[0136]

[0137]

[0138] Note: The content of ternary cathode material refers to the mass percentage of ternary cathode material in the third film layer based on the total mass of the cathode film layer; the initial VC residual refers to the VC content in the electrolyte when the electrolyte is prepared.

[0139] Battery performance test

[0140] SEM-EDS, TOF-SIMS, 45°C cycle capacity retention, VC consumption, and iron dissolution amount tests were performed on the examples.

[0141] The test method is as follows:

[0142] 1) SEM-EDS test: The disassembled cathode sheet is cleaned with anhydrous DMC (dimethyl carbonate), and after the solvent is volatilized, a 1 cm x 1 cm electrode sheet is taken, and argon ion sputtering is used to obtain a smooth cross section of the cathode sheet. The sample is attached to the sample stage using conductive glue, ensuring good electrical conductivity. During sample preparation, the sample should be protected from contamination and damage, and the original structure and morphology of the observation surface should be maintained. The conditions of each step are controlled to ensure the quality of the sample. The SEM-EDS information of the selected area at different magnifications is obtained, and the Fe element content distribution is obtained. The test results of Example 1 are shown in Figure 4 . It can be seen from Figure 4 that after cycling, the iron ion content of the NCM layer increases, and the closer to the LFP layer, the higher the content, indicating that the iron is intercepted by the NCM layer after being released from the LFP layer.

[0143] 2) TOF-SIMS test: The disassembled cathode sheet is cleaned with anhydrous DMC, and after the solvent is volatilized, a 1 cm x 1 cm electrode sheet is taken for surface element distribution analysis of the cathode sheet. The test results of Fe element distribution in Example 1 are shown in Figure 5 , Figure 5 The left side of the figure is the test result before cycling, and the right side is the test result after cycling. From Figure 5 it can be seen that the iron ion content on the surface of the NCM layer (close to the separator side) after 300 cycles does not increase significantly, indicating that the iron is intercepted by the NCM layer after being released from the LFP layer.

[0144] 3) The battery is tested for 45°C cycle capacity retention, and the test process is as follows:

[0145] Test procedure: Environmental temperature: 45°C, rest for 5h before testing; 45°C cycle test is performed according to the following procedure:

[0146] ① Charge: 1C constant current charging to 3.8V, rest for 10min;

[0147] ② Discharge: 1C constant current discharge to 2.0V, rest for 10min.

[0148] ③ Cycle 300 times.

[0149] 4) VC residual test for the battery, the test process is as follows:

[0150] ① The battery is discharged at 1 / 3C current to 2.0V at room temperature;

[0151] ② In the glove box, EP is quantitatively added to the battery according to the ratio of ethyl propionate (EP): battery injection volume = 1:1, and the sealing machine is sealed;

[0152] ③ The shaker is shaken at a frequency of 75Hz for three days, and then the ultrasonic cleaner is ultrasonically cleaned for 30min, so that the EP is fully mixed with the electrolyte in the battery;

[0153] ④ The mixed solution is taken for GC-MS to obtain the peak area of each component, and the concentration of each component in the mixed solution is calculated based on the standard curve;

[0154] ⑤ The known EP addition amount is used to quantitatively deduce the content of each component of the electrolyte.

[0155] 5) Negative electrode sheet Fe content test: the disassembled negative electrode sheet is cleaned with anhydrous DMC, and after the solvent is volatilized, the negative electrode material is scraped off from the electrode sheet and transferred into a sample tube for ICP test.

[0156] Table 2

[0157]

[0158]

[0159] Note: The formation VC residual refers to the VC content in the electrolyte after the battery is formed, the formation Fe content refers to the Fe content in the negative electrode sheet after the battery is formed, the 300-cycle VC residual refers to the VC content in the electrolyte after the battery is cycled for 300 cycles, and the 300-cycle Fe content refers to the Fe content in the negative electrode sheet after the battery is cycled for 300 cycles.

[0160] From the above tests, it can be seen that the battery coated with the NCM layer shows an extended cycle life, and the inflection point of performance decline is obviously delayed compared with the comparative example. This result shows that the introduction of the NCM layer effectively improves the cycle stability of the battery.

[0161] Further, through disassembly and chemical composition analysis of the cycled battery, it is found that in the single LFP system battery of the comparative example 1, after the VC additive is consumed, the iron ion dissolution amount increases significantly, resulting in an inflection point of battery performance. In the battery of the embodiment, the positive electrode sheet is coated with the second film layer and the third film layer, and even after the VC additive is consumed, the iron ion dissolution amount on the negative electrode side is still within the error range and can be almost ignored.

[0162] Based on the experimental data and analysis results, it can be concluded that the coating of the second film layer and the third film layer provides additional deposition sites for iron ions, thereby inhibiting the dissolution of iron ions during the charging and discharging process of the battery. This inhibition slows down the decomposition of the electrolyte and the degradation of the SEI film, maintains a stable chemical environment inside the battery, and thus significantly improves the cycle performance of the battery. In addition, the experimental results also show that the introduction of the second film layer and the third film layer does not negatively affect the initial capacity or discharge platform of the battery.

[0163] In summary, by coating the second film layer and the third film layer on the surface of the LFP electrode, the present application successfully achieves effective control of the iron ion dissolution behavior, significantly improves the cycle stability and service life of the battery. This finding provides a new technical approach for the design and optimization of lithium-ion batteries, and has important practical application value and market potential.

[0164] In the description of the present application, it should be understood that the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0165] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the present application and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0166] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A positive electrode plate, characterized in that, It includes a positive current collector and a positive electrode film layer located on at least one side surface of the positive current collector; The positive electrode film layer includes a first film layer, a second film layer and a third film layer sequentially stacked on the positive electrode current collector; The first film layer includes an iron-containing phosphate cathode material; The second film layer includes a conductive polymer; The third film layer includes a ternary cathode material.

2. The positive electrode sheet according to claim 1, characterized in that, The positive electrode film layer further includes a fourth film layer, which is located on the surface of the third film layer away from the second film layer, and the fourth film layer includes a siderophile metal oxide.

3. The positive electrode sheet according to claim 2, characterized in that, The siderophile metal oxide includes at least one of iron oxide and titanium oxide.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The phosphate cathode material includes at least one of lithium iron phosphate and lithium manganese iron phosphate; and / or The ternary cathode material includes at least one of nickel-cobalt-manganese ternary cathode material and nickel-cobalt-aluminum ternary cathode material.

5. The positive electrode sheet according to claim 4, characterized in that, The phosphate cathode material includes lithium iron phosphate; and / or The ternary cathode material includes nickel-cobalt-manganese ternary cathode material.

6. The positive electrode sheet according to claim 4, characterized in that, The nickel-cobalt-manganese ternary cathode material has the following chemical composition: Li (Ni x Co y Mr z )Q r O2 Where x = 0.3~1, y = 0.01~0.5, z = 0.01~0.5, r = 0~0.1; The element Q includes at least one of yttrium, zirconium, titanium, niobium, aluminum, magnesium, molybdenum, and niobium.

7. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The ternary cathode material includes at least one of spherical nanoparticles, nanowires, and nanosheets.

8. The positive electrode sheet according to claim 7, characterized in that, The spherical nanoparticles have a D50 particle size of 2 μm to 5 μm; and / or The aspect ratio of the nanowires is 2 to 5; and / or The aspect ratio of the nanosheets is 2 to 5.

9. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The conductive polymer includes at least one of polyaniline and polypyrrole.

10. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The second membrane layer also includes an iron ion adsorbent.

11. The positive electrode sheet according to claim 10, characterized in that, The iron ion adsorbent comprises at least one of manganese-based ferrite, Fe3O4 magnetic nanomaterials, ethylenediaminetetraacetic acid, and tannic acid; and / or Based on the total mass of the second membrane layer, the mass percentage of the iron ion adsorbent is 5% to 20%.

12. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The thickness ratio of the first film layer, the second film layer, and the third film layer is (0.5-1):(0.02~0.2):(0.02~0.3).

13. A method for preparing a positive electrode sheet according to any one of claims 1 to 12, characterized in that, include: A first slurry is coated on at least one side of the positive electrode current collector and dried to form a first film layer; A second slurry is coated onto the surface of the first film layer away from the current collector and dried to form a second film layer; A third slurry is coated on the surface of the second film layer away from the current collector to form a third film layer.

14. The method according to claim 13, characterized in that, It also includes coating the surface of the third film layer away from the current collector with a fourth slurry and drying it to form a fourth film layer.

15. A lithium-ion battery, characterized in that, The positive electrode includes any one of claims 1 to 12.

16. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 15.

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