Positive plate, preparation method thereof and battery

Abstract

The invention relates to the field of batteries, in particular to a positive plate, a preparation method thereof and a battery, the positive plate comprises a current collector, a first active material layer and a second active material layer which are arranged in sequence; the first active material layer has a mesoporous structure; the first active material layer comprises a first active material, and the first active material comprises NaNi < y > Fe < z > Mn < x > O < 2 >, y is less than or equal to 0.4, 0lt; z < = 0.6, 0lt; x is smaller than or equal to 0.6, and y + z + x = 1; the second active material layer comprises a second active material and a polymer, the second active material comprises LiNi < m > ConMn < 1-m-n > O < 2 >, and 0 lt; m < = 1, 0lt; n < = 1, and m + n = 1. The positive plate provided by the invention can reduce the consumption of active sodium or active lithium, effectively improve the wettability and ion migration rate of an electrolyte, reduce polarization and improve the cycle performance.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular, to a positive electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] In recent years, sodium-ion batteries have attracted wide attention due to their advantages such as rich resources, good safety, low cost, and good low-temperature performance. Currently, as potential substitutes for lithium iron phosphate batteries, sodium-ion batteries are promising to be applied in the fields of energy storage and power; therefore, it is necessary to further improve their cycling performance and energy density to meet actual requirements.

[0003] Generally, the means to improve energy density mainly include increasing voltage and capacity. Currently, the capacity is mainly increased by designing a composite electrode sheet, while avoiding direct contact between the electrolyte and the cracked positive electrode material, thereby improving the cycle performance. However, due to the multi-layer structure, composite electrode sheets usually have problems such as poor wettability and large polarization, which affect the battery performance; existing technologies such as CN116864630A solve the problems of poor wettability and large polarization by introducing magnetic substances to create pores, but this method is not applicable to iron-based positive electrode materials and has certain drawbacks. Summary of the Invention

[0004] In view of this, the present invention is committed to providing a positive electrode sheet, a preparation method thereof, and a battery to solve the problems of poor wettability and large polarization existing in composite electrode sheets in the prior art due to their multi-layer structure.

[0005] To solve the above technical problems, the present application is implemented as follows:

[0006] The present invention provides a positive electrode sheet, which includes a current collector, at least one first active material layer disposed on at least one surface in the thickness direction of the current collector, and a second active material layer disposed on the surface of the first active material layer away from the current collector;

[0007] The first active material layer has a mesoporous structure; the first active material layer includes a first active material, and the first active material includes NaNi y Fe z Mn x O2, where 0 < y ≤ 0.4, 0 < z ≤ 0.6, 0 < x ≤ 0.6, and y + z + x = 1;

[0008] The second active material layer includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m-n O2, where 0 < m ≤ 1, 0 < n ≤ 1, and m + n = 1.

[0009] Optionally, the thickness ratio of the first active material layer to the second active material layer is 1:(0.4 - 1.2), preferably 1:(0.9 - 1); optionally, the molar ratio of the first active material to the second active material is (1.8 - 8):1, preferably (6 - 7):1; optionally, in the second active material layer, the molar ratio of the second active material to the polymer is (40 - 190):1, preferably (60 - 70):1; optionally, the polymer includes at least one of polyvinyl alcohol, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, and sodium carboxymethyl cellulose.

[0010] Optionally, the porosity of the first active material layer is 20 - 40%, preferably 25 - 30%; optionally, the mesopore diameter of the first active material layer is 40 - 100 nm, preferably 60 - 65 nm.

[0011] Optionally, the second active material layer has a mesoporous structure; optionally, the porosity of the second active material layer is 15 - 30%, preferably 20 - 25%; optionally, the mesopore diameter of the second active material layer is 40 - 100 nm, preferably 60 - 65 nm.

[0012] Optionally, the first active material layer further includes a first conductive agent and a first binder; the mass ratio of the first active material, the first conductive agent, and the first binder in the first active material layer is (94 - 99):(1 - 5):(1 - 3), preferably (96 - 97):(1.8 - 2.1):(0.8 - 1.1); optionally, the second active material layer further includes a second conductive agent and a second binder; the mass ratio of the second active material, the second conductive agent, and the second binder in the second active material layer is (94 - 99):(1 - 5):(1 - 5), preferably (96 - 97.5):(1.5 - 2):(0.9 - 1.2); optionally, each of the first conductive agent and the second conductive agent independently includes at least one of conductive carbon black, acetylene black, graphene, conductive graphite, carbon nanotubes, and carbon fibers; optionally, each of the first binder and the second binder independently includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, or sodium carboxymethyl cellulose.

[0013] The second aspect of the present invention provides a method for preparing a positive electrode sheet, including the following steps:

[0014] S1. Coating a first slurry on at least one surface in the thickness direction of the current collector and performing a first drying treatment to obtain a current collector provided with a first active material layer;

[0015] S2. Coating the second slurry on the surface of the current collector provided with the first active material layer and performing a second drying treatment to obtain a first positive electrode sheet; performing a rolling treatment and a third drying treatment on the first positive electrode sheet;

[0016] Among them, the first slurry includes a first active material and a first pore-forming agent, and the first active material includes NaNi y Fe z Mn x O2, where 0 < y ≤ 0.4, 0 < z ≤ 0.6, 0 < x ≤ 0.6, and y + z + x = 1;

[0017] The second slurry includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m-n O2, where 0 < m ≤ 1, 0 < n ≤ 1, and m + n = 1.

[0018] Optionally, the first pore-forming agent includes ammonium carbonate and / or ammonium bicarbonate, and the molar ratio of the first pore-forming agent to the first active material is 1:(5900 - 30000), preferably 1:(29000 - 30000); optionally, the second slurry further includes a second pore-forming agent; the second pore-forming agent includes ammonium carbonate and / or ammonium bicarbonate; the molar ratio of the second pore-forming agent to the second active material is 1:(1000 - 7000), preferably 1:(3500 - 4000).

[0019] Optionally, the preparation steps of the first slurry include: mixing the first active material, the first pore-forming agent, a first conductive agent, a first binder, and a first solvent and then performing a first stirring treatment; optionally, the preparation steps of the second slurry include: mixing a second active material, a second conductive agent, a second binder, and a second solvent and then performing a second stirring treatment to obtain an intermediate slurry; mixing the polymer and a third solvent and then performing a dispersion treatment to obtain a polymer solution; mixing the intermediate slurry with the polymer solution; optionally, the first solvent includes N-methylpyrrolidone; the second solvent includes N-methylpyrrolidone; the third solvent includes water.

[0020] Optionally, in step S1, the conditions of the first drying treatment include: temperature is 100 - 120 °C, time is 2 - 4 h; optionally, in step S2, the conditions of the second drying treatment include: temperature is 100 - 120 °C, time is 2 - 4 h; the conditions of the third drying treatment include: temperature is 60 - 80 °C, time is 8 - 16 h.

[0021] The third aspect of the present invention provides a battery, which includes a positive electrode sheet, and the positive electrode sheet is the above-mentioned positive electrode sheet and / or the positive electrode sheet prepared according to the above preparation method.

[0022] Through the above technical solutions, the beneficial technical effects of the present invention are as follows:

[0023] (1) The positive electrode sheet of the present invention includes a current collector, a first active material layer, and a second active material layer. The first active material layer includes a first active material NaNi y Fe z Mn x O2 and has a mesoporous structure. The second active material layer includes a second active material LiNi m Co n Mn 1-m-n O2 and a polymer. Since the lithium-ion SEI film is more stable than the sodium-ion SEI film, by sequentially arranging coatings containing NaNi y Fe z Mn x O2 and LiNi m Co n Mn 1-m-n O2, introducing LiNi y Fe z Mn x O2 into NaNi m Co n Mn 1-m-n O2 can optimize the SEI film to reduce the consumption of active sodium and thus improve the capacity; at the same time, the second active material layer containing LiNi m Co n Mn 1-m-n O2 can further improve the capacity and thus improve the energy density of the positive electrode sheet; the polymer is an interfacial film, which can effectively prevent the cracked positive electrode sheet from directly contacting the electrolyte during the charge and discharge cycle, reduce the consumption of active sodium or active lithium, and further improve the cycle; the directionally designed porous structure can improve the pore structure of the positive electrode sheet, effectively improve the wettability of the electrolyte and the ion migration rate, reduce polarization, and thus the cycle performance of the positive electrode sheet is improved.

[0024] (2) The preparation method of the positive electrode sheet of the present invention is to mix a slurry containing an active material NaNi y Fe z Mn x O2 and a pore-forming agent with a slurry containing an active material LiNi m Co n Mn 1-m-nA method of sequentially coating a slurry of O2 and a polymer to obtain a positive electrode sheet; the preparation method is simple and efficient; by introducing a pore-forming agent to directionally design a porous structure, the pore structure of the positive electrode sheet is improved, the wettability of the electrolyte and the ion migration rate can be effectively improved, and the polarization is reduced. Therefore, the cycle performance of the positive electrode sheet is improved; compared with the method of introducing magnetic substances to form pores, the method of introducing a pore-forming agent to directionally design a porous structure in the present invention is more suitable for the main iron-based positive electrode material.

[0025] Other features and advantages of the present invention will be described in detail in the following specific implementation section. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the following specific implementation, they are used to explain the present invention, but do not constitute a limitation to the present invention.

[0027] Figure 1 The structure diagram of the positive electrode sheet in the present invention is shown. SPECIFIC IMPLEMENTATION MODE

[0028] The present invention discloses a positive electrode sheet, a preparation method thereof, and a battery. Those skilled in the art can draw on the content of this article and appropriately modify the process parameters to achieve it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all regarded as included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. Relevant personnel can obviously make changes or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0029] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0030] The endpoints and any values in the ranges disclosed herein are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0031] If there is no special indication, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions.

[0032] If there is no special indication, all technical features and optional technical features of this application can be combined with each other to form new technical solutions.

[0033] If there is no special indication, the "including" and "comprising" mentioned in this application mean open-ended, and can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or can only include or comprise the listed components.

[0034] In order to solve the problems of poor wettability and large polarization that usually exist in composite electrode sheets due to the multi-layer structure in the prior art, the present invention adopts the following technical solutions:

[0035] The present invention provides a positive electrode sheet, as Figure 1 shown, the positive electrode sheet includes a current collector, a first active material layer provided on at least one surface in the thickness direction of the current collector, and a second active material layer provided on the surface of the first active material layer away from the current collector;

[0036] The first active material layer has a mesoporous structure; the first active material layer includes a first active material, and the first active material includes NaNi y Fe z Mn x O2, wherein, 0 < y ≤ 0.4, 0 < z ≤ 0.6, 0 < x ≤ 0.6, y + z + x = 1;

[0037] The second active material layer includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m-n O2, wherein, 0 < m ≤ 1, 0 < n ≤ 1, m + n = 1.

[0038] The positive electrode sheet of the present invention includes a current collector, a first active material layer and a second active material layer, wherein the first active material layer includes a first active material NaNi y Fe z Mn xO2 and has a mesoporous structure. The second active material layer includes a second active material LiNi m Co n Mn 1-m-n O2 and a polymer. Since the lithium-ion SEI film is more stable than the sodium-ion SEI film, by sequentially setting coatings containing NaNi y Fe z Mn x O2, LiNi m Co n Mn 1-m-n O2, introducing LiNi y Fe z Mn x O2 into NaNi m Co n Mn 1-m-n O2 can optimize the SEI film to reduce the consumption of active sodium and thus improve the capacity; at the same time, the second active material layer containing LiNi m Co n Mn 1-m-n O2 can further increase the capacity and thus improve the energy density of the positive electrode sheet; the polymer is an interfacial film, which can effectively prevent the cracked positive electrode sheet from directly contacting the electrolyte during the charge and discharge cycle, reduce the consumption of active sodium or active lithium, and further improve the cycle performance; the directionally designed porous structure improves the pore structure of the positive electrode sheet, can effectively improve the wettability of the electrolyte and the ion migration rate, reduce polarization, and thus the cycle performance of the positive electrode sheet is improved.

[0039] According to the present invention, the thickness ratio of the first active material layer to the second active material layer is 1:(0.4 - 1.2). As an example, the thickness ratio of the first active material layer to the second active material layer can be any value among 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.1 or 1:1.2 or any value within the range value composed of any two of the above values. Preferably, the thickness ratio of the first active material layer to the second active material layer is 1:(0.9 - 1).

[0040] According to the present invention, the molar ratio of the first active material to the second active material is (1.8 - 8):1. As an example, the molar ratio of the first active material to the second active material can be any value among 1.8:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1 or 8:1 or any value within the range value composed of any two of the above values. Preferably, the molar ratio of the first active material to the second active material is (6 - 7):1.

[0041] According to the present invention, in the second active material layer, the molar ratio of the second active material to the polymer is (40 to 190):1. In the present invention, in the second active material layer, an appropriate molar ratio of the second active material to the polymer helps to ensure the basic performance of the battery while protecting the interface to further improve the performance of the battery cell. As an example, in the second active material layer, the molar ratio of the second active material to the polymer can be any value among 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1, 120:1, 140:1, 150:1, 160:1, 180:1 or 190:1, or any value within the range value composed of any two of the above values. In the present invention, if the molar ratio of the second active material to the polymer in the second active material layer is too large, the effect of protecting the interface will not be obvious and thus the modification will not be real. If the molar ratio of the second active material to the polymer is too small, the battery performance will be limited. Preferably, in the second active material layer, the molar ratio of the second active material to the polymer is (60 to 70):1.

[0042] In a preferred embodiment of the present invention, the polymer includes at least one of polyvinyl alcohol, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, and sodium carboxymethyl cellulose.

[0043] According to the present invention, the porosity of the first active material layer is 20 to 40%. In the present invention, an appropriate porosity of the first active material layer helps to improve the wetting performance of the electrode sheet and increase the charge and discharge efficiency. As an example, the porosity of the first active material layer can be any value among 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38% or 40%, or any value within the range value composed of any two of the above values. In the present invention, if the porosity of the first active material layer is too high, the capacity utilization, safety and charge and discharge efficiency will be reduced. If the porosity of the first active material layer is too low, the electrolyte infiltration will be insufficient, affecting the battery kinetic performance. Preferably, the porosity of the first active material layer is 25 to 30%.

[0044] According to the present invention, the mesopore diameter of the first active material layer is 40 to 100 nm. In the present invention, the appropriate mesopore diameter of the first active material layer helps to improve the electrolyte infiltration and charge-discharge efficiency and ensure safety performance. As an example, the mesopore diameter of the first active material layer can be any value among 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm, or any value within the range value composed of any two of the above values. In the present invention, if the mesopore diameter of the first active material layer is too large, it will cause the formation of metal dendrites, leading to safety problems and reducing battery performance. If the mesopore diameter of the first active material layer is too small, it will affect the electrolyte infiltration effect. Preferably, the mesopore diameter of the first active material layer is 60 to 65 nm.

[0045] According to the present invention, the second active material layer has a mesoporous structure. In the present invention, the second active material layer is on the outer layer of the positive electrode plate relative to the first active material layer and is more easily infiltrated by the electrolyte. Therefore, the second active material layer can be provided with a mesoporous structure or not. When the second active material layer is provided with a mesoporous structure, it helps to improve the electrolyte infiltration and charge-discharge efficiency.

[0046] According to the present invention, the porosity of the second active material layer is 15 to 30%. In the present invention, the appropriate porosity in the second active material layer helps to improve the infiltration performance of the electrode plate and charge-discharge efficiency. As an example, the porosity of the second active material layer can be any value among 15%, 18%, 20%, 21%, 22%, 25%, 26%, 28% or 30%, or any value within the range value composed of any two of the above values. In the present invention, if the porosity of the second active material layer is too high, it will cause a decrease in capacity utilization, safety and charge-discharge efficiency; if the porosity of the second active material is too low, it will cause insufficient electrolyte infiltration and affect the battery kinetics performance. Preferably, the porosity of the second active material layer is 20 to 25%.

[0047] According to the present invention, the mesopore diameter of the second active material layer is 40 to 100 nm. In the present invention, an appropriate mesopore diameter in the second active material layer helps to improve the electrolyte infiltration and charge-discharge efficiency and ensure safety performance. As an example, the mesopore diameter of the second active material layer can be any value among 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm, or any value within the range formed by any two of the above values. In the present invention, if the mesopore diameter of the second active material layer is too large, it will cause the formation of metal dendrites, leading to safety problems and reducing battery performance. If the mesopore diameter of the second active material layer is too small, it will affect the electrolyte infiltration effect. Preferably, the mesopore diameter of the second active material layer is 60 to 65 nm.

[0048] According to the present invention, the first active material layer further includes a first conductive agent and a first binder.

[0049] According to the present invention, the mass ratio of the first active material, the first conductive agent, and the first binder in the first active material layer is (94 to 99):(1 to 5):(1 to 3). As an example, the mass ratio of the first active material, the first conductive agent, and the first binder in the first active material layer can be any value among 94:1:1, 94:1:3, 94:2:1, 94:2:2, 94:2:3, 95:3:2, 96:4:1, 97:1:1, 97:3:2, 98:2:3 or 98:3:3, or any value within the range formed by any two of the above values. Preferably, the mass ratio of the first active material, the first conductive agent, and the first binder in the first active material layer is (96 to 97):(1.8 to 2.1):(0.8 to 1.1).

[0050] According to the present invention, the second active material layer further includes a second conductive agent and a second binder.

[0051] According to the present invention, the mass ratio of the second active material, the second conductive agent, and the second binder in the second active material layer is (94 to 99):(1 to 5):(1 to 5). As an example, the mass ratio of the second active material, the second conductive agent, and the second binder can be any value among 94:1:1, 94:1:3, 94:2:1, 94:2:2, 94:2:3, 95:3:2, 96:4:1, 97:1:1, 97:3:2, 98:2:3 or 98:3:3, or any value within the range formed by any two of the above values. Preferably, the mass ratio of the second active material, the second conductive agent, and the second binder in the second active material layer is (96 to 97.5):(1.5 to 2):(0.9 to 1.2);

[0052] In a preferred embodiment of the present invention, the first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, acetylene black, graphene, conductive graphite, conductive carbon nanotubes, and conductive carbon fibers.

[0053] In a preferred embodiment of the present invention, the first binder and the second binder each independently include at least one of polytetrafluoroethylene, polyvinylidene fluoride, or sodium carboxymethyl cellulose.

[0054] The second aspect of the present invention provides a method for preparing a positive electrode sheet, comprising the following steps:

[0055] S1. Coating a first slurry on at least one surface in the thickness direction of a current collector and performing a first drying treatment to obtain a current collector provided with a first active material layer;

[0056] S2. Coating a second slurry on the surface of the current collector provided with the first active material layer and performing a second drying treatment to obtain a first positive electrode sheet; performing a rolling treatment and a third drying treatment on the first positive electrode sheet;

[0057] Wherein, the first slurry includes a first active material and a first pore-forming agent, and the first active material includes NaNi y Fe z Mn x O2, wherein, 0 < y ≤ 0.4, 0 < z ≤ 0.6, 0 < x ≤ 0.6, and y + z + x = 1;

[0058] The second slurry includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m-n O2, wherein, 0 < m ≤ 1, 0 < n ≤ 1, and m + n = 1.

[0059] The method for preparing the positive electrode sheet of the present invention obtains a positive electrode sheet by successively coating a slurry containing the active material NaNi y Fe z Mn x O2 and a pore-forming agent and a slurry containing the active material LiNi m Co n Mn 1-m-n O2 and a polymer; by successively arranging coatings containing NaNi y Fe z Mn x O2, LiNi m Co n Mn 1-m-n O2, in NaNi y Fe z Mn xIntroducing LiNi into O2 m Co n Mn 1-m-n Introducing O2 can optimize the SEI film, reduce the consumption of active sodium, and thus improve the capacity. Compared with other methods of doping lithium elements at the material level and performing multi-step blending during the homogenization or coating process, the solution described in the present invention is simpler and more efficient. At the same time, the second active material layer containing LiNi m Co n Mn 1-m-n O2 can further improve the capacity and thus increase the energy density of the positive electrode sheet. The polymer is an interfacial film, which can effectively prevent the cracked positive electrode sheet from directly contacting the electrolyte during the charge and discharge cycle, reduce the consumption of active sodium or active lithium, and further improve the cycle performance. By introducing a pore-forming agent to directionally design a porous structure and improve the pore structure of the positive electrode sheet, the wettability of the electrolyte and the ion migration rate can be effectively improved, and the polarization can be reduced. Therefore, the cycle performance of the positive electrode sheet is improved. Compared with the method of introducing magnetic substances to form pores, the method of introducing a pore-forming agent to directionally design a porous structure in the present invention is more suitable for the main iron-based positive electrode material.

[0060] Optionally, the first pore-forming agent includes ammonium carbonate and / or ammonium bicarbonate.

[0061] According to the present invention, the molar ratio of the first pore-forming agent to the first active material is 1:(5900 - 30000). In the present invention, an appropriate molar ratio of the first pore-forming agent to the first active material helps to increase the wetting effect of the coating layer, improve the battery performance while ensuring the safety performance. As an example, the molar ratio of the first pore-forming agent to the first active material can be any value among 1:5900, 1:6000, 1:10000, 1:15000, 1:20000, 1:25000, or 1:30000, or any value within the range composed of any two of the above values. In the present invention, if the molar ratio of the first pore-forming agent to the first active material is too high, it will cause the pore size to be too large or the number of pores to be too many, deteriorating the capacity performance and charge and discharge efficiency while increasing the wetting. If the molar ratio of the first pore-forming agent to the first active material is too low, it will cause insufficient pore formation and a decrease in the wetting improvement effect. Preferably, the molar ratio of the first pore-forming agent to the first active material is 1:(29000 - 30000).

[0062] According to the present invention, the second slurry further includes a second pore-forming agent.

[0063] Optionally, the second pore-forming agent includes ammonium carbonate and / or ammonium bicarbonate.

[0064] According to the present invention, the molar ratio of the second pore-forming agent to the second active substance is 1:(1000 - 7000). In the present invention, an appropriate molar ratio of the second pore-forming agent to the second active substance helps to increase the wetting effect of the coating layer, improve the battery performance and ensure the safety performance at the same time. As an example, the molar ratio of the second pore-forming agent to the second active substance can be any value among 1:1000, 1:2000, 1:3000, 1:4000, 1:5000, 1:6000 or 1:7000, or any value within the range value composed of any two of the above values. In the present invention, if the molar ratio of the second pore-forming agent to the second active substance is too large, it will cause the pore diameter to be too large or the number of pores to be too many, deteriorating the capacity performance and charge-discharge efficiency while increasing the wetting; if the molar ratio of the second pore-forming agent to the second active substance is too low, it will cause insufficient pore formation and a decline in the wetting improvement effect. Preferably, the molar ratio of the second pore-forming agent to the second active substance is 1:(3500 - 4000).

[0065] Optionally, the preparation steps of the first slurry include: mixing the first active substance, the first pore-forming agent, the first conductive agent, the first binder and the first solvent, and then performing a first stirring treatment.

[0066] Optionally, the preparation steps of the second slurry include: mixing the second active substance, the second conductive agent, the second binder and the second solvent, and then performing a second stirring treatment to obtain an intermediate slurry; mixing the polymer and the third solvent and then performing a dispersion treatment to obtain a polymer solution; mixing the intermediate slurry with the polymer solution.

[0067] Optionally, the first solvent includes N-methylpyrrolidone; the second solvent includes N-methylpyrrolidone; the third solvent includes water.

[0068] According to the present invention, in step S1, the conditions of the first drying treatment include: the temperature is 100 - 120°C and the time is 2 - 4 h. As an example, in step S1, the conditions of the first drying treatment include: the temperature can be any value among 100°C, 102°C, 105°C, 108°C, 110°C, 112°C, 115°C, 118°C or 120°C, or any value within the range value composed of any two of the above values, and the time can be any value among 2 h, 2.5 h, 3 h, 3.5 h or 4 h, or any value within the range value composed of any two of the above values.

[0069] According to the present invention, in step S2, the conditions of the second drying treatment include: the temperature is 100-120 °C, and the time is 2-4 h. As an example, in step S2, the conditions of the second drying treatment include: the temperature can be any value among 100 °C, 102 °C, 105 °C, 108 °C, 110 °C, 112 °C, 115 °C, 118 °C or 120 °C, or any value within the range formed by any two of the above values, and the time can be any value among 2 h, 2.5 h, 3 h, 3.5 h or 4 h, or any value within the range formed by any two of the above values.

[0070] According to the present invention, in step S2, the conditions of the third drying treatment include: the temperature is 60-80 °C, and the time is 8-16 h. As an example, in step S2, the conditions of the third drying treatment include: the temperature can be any value among 60 °C, 62 °C, 65 °C, 68 °C, 70 °C, 72 °C, 75 °C, 78 °C or 80 °C, or any value within the range formed by any two of the above values, and the time can be any value among 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h or 20 h, or any value within the range formed by any two of the above values.

[0071] The third aspect of the present invention provides a battery, which includes a positive electrode sheet, and the positive electrode sheet is the above-mentioned positive electrode sheet and / or the positive electrode sheet prepared according to the above preparation method.

[0072] The present invention will be further described in detail below through examples. All the raw materials used in the examples can be obtained through commercial channels.

[0073] Example 1

[0074] The preparation of the positive electrode sheet includes the following steps;

[0075] (1) 6.8 g of active material NaNi 0.33 Fe 0.33 Mn 0.33 O2, 1.2 g of conductive agent carbon black (Super-P), 0.07 g of binder polyvinylidene fluoride (PVDF), and 0.18 mg of pore-forming agent ammonium carbonate are uniformly dispersed in the solvent N-methylpyrrolidone (NMP) and mixed to obtain a first slurry; the first slurry is coated on an aluminum foil current collector and dried at 110 °C for 2 h to obtain a current collector provided with a layer of active material layer;

[0076] (2) 0.72 g of active material LiNi 0.33 Co 0.33 Mn 0.330.15 g of conductive agent carbon black (Super-P), 7 mg of binder polyvinylidene fluoride (PVDF), and 0.18 mg of pore-forming agent ammonium carbonate were uniformly dispersed in the solvent N-methylpyrrolidone (NMP) and mixed to obtain an intermediate slurry; 3 g of polyvinyl alcohol (PVA) was dissolved in 100 mL of deionized water, stirred at 98 °C for 2 h until clarified, cooled to 30 °C, and then the intermediate slurry was added dropwise under continuous stirring, and stirred continuously for 15 min to obtain a second slurry; the second slurry was coated on the surface of the current collector provided with the first active material layer, and then dried at 110 °C for 2 h to obtain a first positive electrode sheet; after the first positive electrode sheet was roll-pressed, it was vacuum-dried at 70 °C for 12 h to obtain a positive electrode sheet.

[0077] Example 2

[0078] The preparation method of the positive electrode sheet in this example is the same as that in Example 1 as a whole, except that: the mass of the pore-forming agent ammonium carbonate in the first slurry and the intermediate slurry is 0.36 mg.

[0079] Example 3

[0080] The preparation method of the positive electrode sheet in this example is the same as that in Example 1 as a whole, except that: the mass of the pore-forming agent ammonium carbonate in the first slurry and the intermediate slurry is 0.54 mg.

[0081] Example 4

[0082] The preparation method of the positive electrode sheet in this example is the same as that in Example 1 as a whole, except that: the mass of the pore-forming agent ammonium carbonate in the first slurry and the intermediate slurry is 0.72 mg.

[0083] Example 5

[0084] The preparation method of the positive electrode sheet in this example is the same as that in Example 1 as a whole, except that: ammonium bicarbonate is used as the pore-forming agent in the first slurry and the intermediate slurry.

[0085] Example 6

[0086] The preparation method of the positive electrode sheet in this example is the same as that in Example 2 as a whole, except that: ammonium bicarbonate is used as the pore-forming agent in the first slurry and the intermediate slurry.

[0087] Example 7

[0088] The preparation method of the positive electrode sheet in this example is the same as that in Example 3 as a whole, except that: ammonium bicarbonate is used as the pore-forming agent in the first slurry and the intermediate slurry.

[0089] Example 8

[0090] The preparation method of the positive electrode sheet in this example is the same as that in Example 4 as a whole, except that: ammonium bicarbonate is used as the pore-forming agent in the first slurry and the intermediate slurry.

[0091] Comparative Example 1

[0092] Preparation of the positive electrode sheet includes the following steps;

[0093] (1) 6.8 g of active material NaNi 0.33 Fe 0.33 Mn 0.33 O2, 1.5 g of conductive agent carbon black (Super-P), and 0.07 g of binder polyvinylidene fluoride (PVDF) are uniformly dispersed in the solvent N-methylpyrrolidone (NMP) and mixed to obtain the first slurry; the first slurry is coated on one side of the aluminum foil current collector and vacuum dried at 80 °C for 12 h to obtain the positive electrode sheet.

[0094] Comparative Example 2

[0095] In this comparative example, the preparation method of the positive electrode sheet is generally the same as that in Example 1, except that: no pore-forming agent is added to the first slurry and the intermediate slurry.

[0096] Test Example 1

[0097] Preparation of the hybrid ion battery includes the following steps:

[0098] The positive electrode sheets prepared in Examples 1 to 8 and Comparative Examples 1 to 2 are respectively cut into 12 mm circular positive electrode sheets, a sodium metal sheet with a diameter of 14 mm and a thickness of 0.2 mm is used as the negative electrode, a 0.1 mol / L sodium perchlorate / ethylene carbonate / dimethyl carbonate (sodium perchlorate: ethylene carbonate: dimethyl carbonate = 1:1:1) solution is used as the electrolyte, and a Whatman GF / F glass fiber with a diameter of 16 mm is used as the separator, and a CR2032 button battery is assembled in an argon glove box. The cycle performance of the above battery is tested, and the test results are shown in Table 1.

[0099] The cycle performance test of the battery includes: using a constant current charge and discharge mode, and performing charge and discharge tests at a current density of 0.1C. The test items include: 0.1C charge and discharge, 3C charge and discharge of the material in the sodium ion battery, and the capacity retention rate at 1C for 100 cls.

[0100] (1) Cycle capacity retention rate test: Charge and discharge 100 cycles at a 1C rate, and calculate the capacity retention rate. The test is carried out under the conditions that the discharge cut-off voltage is 2.0V and the charge cut-off voltage is 4V.

[0101] Cycle performance test: Let the hybrid ion battery stand at 25 °C for 30 min, charge it at a constant current of 1C to 4V, then charge it at a constant voltage to 0.05C, and then discharge it at a constant current of 1C to 2V. Record the discharge capacity at this time as the discharge capacity of the first cycle, stand for 10 min, and record the capacity of the lithium ion battery when cycling to the 100th cycle.

[0102] Retention rate of the 100th cycle capacity = Discharge capacity of the 100th cycle / Discharge capacity of the 1st cycle.

[0103] (2) First cycle discharge capacity test: Using the constant current charge-discharge mode, under the conditions that the discharge cut-off voltage is 2.0 V and the charge cut-off voltage is 4 V, charge and discharge are carried out at a current density of 0.1 C to test the discharge capacity.

[0104] (3) 3C capacity retention rate test:

[0105] 3C discharge capacity test: Using the constant current charge-discharge mode, let the hybrid ion battery stand at 25 °C for 30 min, charge it at a constant current of 3C to 4 V, then charge it at a constant voltage to 0.05C, and then discharge it at a constant current of 2C to 2 V, and record the discharge capacity at this time as the 3C discharge capacity.

[0106] 3C capacity retention rate = 3C discharge capacity / 0.1C discharge capacity.

[0107] Table 1

[0108]

[0109] As can be seen from Table 1, compared with the comparative example, the composite electrode sheet prepared by the present invention has more excellent discharge capacity and capacity retention rate during the charge-discharge process. Moreover, within the ranges of NaNi 0.33 Fe 0.33 Mn 0.33 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2 and the ratio, etc., excellent electrical properties are all shown. Among them, relatively speaking, the electrical properties of the positive electrode sheet prepared in Example 4 are better.

[0110] In summary, the positive electrode sheet prepared by the present invention can significantly improve the capacity performance and cycle performance of traditional sodium ion batteries.

[0111] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can still be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet comprises a current collector, a first active material layer disposed on at least one surface of the current collector in a thickness direction, and a second active material layer disposed on a surface of the first active material layer away from the current collector; The first active material layer has a mesoporous structure; the first active material layer includes a first active material, and the first active material includes NaNi y Fe z Mn x O2, where 0 <y≤0.4,0<z≤0.6,0<x≤0.6,y+z+x=1; The second active material layer includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m-n O2, where 0 <m≤1,0<n≤1,m+n=1。 2. The positive electrode sheet according to claim 1, characterized in that: The thickness ratio of the first active material layer to the second active material layer is 1:(0.4-1.2), preferably 1:(0.9-1); The molar ratio of the first active substance to the second active substance is (1.8-8):1, preferably (6-7):1; In the second active material layer, the molar ratio of the second active material to the polymer is (40-190):1, preferably (60-70):1; Optionally, the polymer includes at least one of polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride and sodium carboxymethyl cellulose.

3. The positive electrode sheet according to claim 1, characterized in that: The porosity of the first active material layer is 20-40%, preferably 25-30%; The mesopore diameter of the first active material layer is 40 to 100 nm, preferably 60 to 65 nm.

4. The positive electrode sheet according to claim 1, characterized in that: The second active material layer has a mesoporous structure; The porosity of the second active material layer is 15 to 30%, preferably 20 to 25%; The mesopore diameter of the second active material layer is 40 to 100 nm, preferably 60 to 65 nm.

5. The positive electrode sheet according to claim 1, characterized in that: The first active material layer further includes a first conductive agent and a first binder; the mass ratio of the first active material, the first conductive agent, and the first binder in the first active material layer is (94-99): (1-5): (1-3), preferably (96-97): (1.8-2.1): (0.8-1.1); and / or, The second active material layer further includes a second conductive agent and a second binder; the mass ratio of the second active material, the second conductive agent, and the second binder in the second active material layer is (94-99): (1-5): (1-5), preferably (96-97.5): (1.5-2): (0.9-1.2); Optionally, the first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, acetylene black, graphene, conductive graphite, conductive carbon tube and conductive carbon fiber; Optionally, the first binder and the second binder each independently include at least one of polytetrafluoroethylene, polyvinylidene fluoride or sodium carboxymethyl cellulose.

6. A method for preparing a positive electrode sheet, characterized in that: The following steps are involved: S1, applying the first slurry to at least one surface of the current collector in the thickness direction and performing a first drying process to obtain a current collector provided with a first active material layer; S2, applying the second slurry to the surface of the current collector provided with the first active material layer and performing a second drying process to obtain a first positive electrode sheet; performing a roll pressing process and a third drying process on the first positive electrode sheet; The first slurry includes a first active material and a first pore-forming agent, and the first active material includes NaNi y Fe z Mn x O2, where 0 <y≤0.4,0<z≤0.6,0<x≤0.6,y+z+x=1; The second slurry includes a second active material and a polymer, and the second active material includes LiNi m Co n Mn 1-m- n O2, where 0 <m≤1,0<n≤1,m+n=1。 7. The preparation method according to claim 6, characterized in that: The first pore-forming agent includes ammonium carbonate and / or ammonium bicarbonate, and the molar ratio of the first pore-forming agent to the first active substance is 1:(5900-30000), preferably 1:(29000-30000); Optionally, the second slurry further includes a second pore former; the second pore former includes ammonium carbonate and / or ammonium bicarbonate; the molar ratio of the second pore former to the second active substance is 1:(1000-7000), preferably 1:(3500-4000).

8. The preparation method according to claim 6 or 7, characterized in that: The preparation step of the first slurry includes: mixing the first active material, the first pore-forming agent, the first conductive agent, the first binder and the first solvent and performing a first stirring treatment; and / or, The preparation steps of the second slurry include: mixing the second active material, the second conductive agent, the second binder and the second solvent and performing a second stirring process to obtain an intermediate slurry; mixing the polymer and the third solvent and performing a dispersion process to obtain a polymer solution; mixing the intermediate slurry with the polymer solution; Optionally, the first solvent includes N-methylpyrrolidone; the second solvent includes N-methylpyrrolidone; and the third solvent includes water.

9. The preparation method according to claim 7, characterized in that: In step S1, the conditions of the first drying treatment include: a temperature of 100 to 120°C and a time of 2 to 4 hours; In step S2, the conditions of the second drying treatment include: temperature of 100-120°C and time of 2-4 hours; the conditions of the third drying treatment include: temperature of 60-80°C and time of 8-16 hours.

10. A battery, characterized in that: The battery comprises a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 6, and / or the positive electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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