Positive plate, preparation method thereof and lithium ion battery

Abstract

The invention relates to the field of lithium ion batteries, and discloses a positive plate, a preparation method thereof and a lithium ion battery. The positive plate comprises a current collector, a middle layer arranged on the surface of the current collector, and a surface layer arranged on the surface of one side, far away from the current collector, in the middle layer, wherein the intermediate layer contains a positive electrode active material, a first conductive agent and a binder; the surface layer is provided with a three-dimensional conductive network formed by a second conductive agent, and the surface layer does not contain a binder; the ratio of the porosity of the surface layer to the porosity of the middle layer is greater than or equal to 1.4: 1. The positive plate has a specific double-layer structure, and compared with a conventional positive plate, the positive plate can enable the lithium ion battery to have higher rate retention rate, first-cycle reversible gram capacity and capacity retention rate, can effectively avoid overall expansion of the battery in the cycle process, and inhibits the surface lithium precipitation phenomenon of the negative electrode due to overlarge pressure.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, specifically to a positive electrode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] During the initial charge and discharge of a lithium-ion battery, a solid electrolyte interphase (SEI) film forms on the surface of the negative electrode material, permanently consuming lithium from the positive electrode and resulting in low initial coulombic efficiency and energy density. This is particularly true for silicon-based negative electrode materials, where the significant volume change of silicon during lithium insertion / extraction can easily lead to structural collapse and capacity decay, causing instability in the SEI film. Furthermore, the continuous formation and destruction of the SEI film continuously consumes lithium ions, while a stable SEI film is a key factor in extending battery cycle life. Therefore, the use of silicon-based materials in lithium-ion batteries still faces significant challenges.

[0003] The most effective solution to the above problems is to use pre-lithiation technology to add a small amount of lithium source before the electrode is officially charged and discharged, to make up for the excess lithium consumed in the reaction, to replenish the side reactions in the SEI film formation process and the consumption of cathode lithium, to a certain extent reducing the volume expansion of silicon anode, thereby improving the overall performance of lithium-ion battery.

[0004] Lithium replenishment technologies are divided into negative electrode lithium replenishment and positive electrode lithium replenishment. Among them, negative electrode lithium replenishment technologies, such as lithium metal powder and lithium foil technologies, are relatively mature, but safety issues and high costs remain major obstacles to their commercialization. In contrast, positive electrode lithium replenishment often involves pre-lithiation of the positive electrode, which directly incorporates lithium replenishing agents into the material system or directly uses lithium-rich positive electrodes. This approach offers advantages such as good process safety, simple operation, and low cost, and is more compatible with existing lithium battery manufacturing processes, making it a promising lithium replenishment method.

[0005] A typical method for applying lithium replenishing agents to the positive electrode is to add a small amount of high-capacity lithium replenishing material during the positive electrode slurry preparation process. Another method is to coat the surface of the positive electrode sheet with a slurry formed by combining high-capacity lithium replenishing materials. For the former method, since most lithium replenishing materials have poor conductivity, when added during the positive electrode slurry preparation, they share a small proportion of conductive agent with a large amount of positive electrode material, making it difficult for the lithium replenishing material to decompose completely during the first week of charging, posing a risk of continued decomposition and gas generation. After some lithium replenishing material decomposes, it leaves pores inside the positive electrode material layer, which will damage the original positive electrode structure, interrupt the electronic conduction path, and deteriorate the electronic conductivity of the electrode. In addition, most lithium replenishing materials are strongly alkaline. When added during the conventional positive electrode slurry preparation, the alkaline groups will attack the CF bonds of the PVDF binder, causing the binder to deactivate, increasing the slurry viscosity, or even making the slurry jelly-like and unable to complete the positive electrode coating. For the latter method, there are many voids in the coated but not rolled positive electrode sheet. When the lithium replenishing agent slurry is coated on the surface, it will enter the lower positive electrode sheet through the voids, decompose and destroy the original positive electrode structure. On the other hand, the surface of the rolled positive electrode sheet is smooth and requires the assistance of a binder to coat the surface of the lithium replenishing agent slurry. When the lithium replenishing agent slurry is coated on the surface, a coating composed of binder and conductive agent will be left on the positive electrode surface. At this time, the binder (binder insulation) is often useless and reduces the conductivity of the electrode sheet, affecting the energy density of the cell. Summary of the Invention

[0006] This invention addresses the problems of poor electronic and ionic conductivity of existing lithium-ion battery positive electrodes, excessive expansion of the negative electrode during cycling leading to excessive pressure and lithium deposition between electrodes, and the need for further improvement in the capacity and energy density of lithium-ion batteries. It provides a positive electrode, its preparation method, and a lithium-ion battery.

[0007] To achieve the above objectives, a first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising: a current collector, an intermediate layer disposed on the surface of the current collector, and a surface layer disposed on a side surface of the intermediate layer away from the current collector;

[0008] The intermediate layer contains a positive electrode active material, a first conductive agent, and a binder; the surface layer has a three-dimensional conductive network composed of a second conductive agent, and the surface layer does not contain a binder.

[0009] The porosity of the surface layer: the porosity of the intermediate layer ≥ 1.4:1.

[0010] A second aspect of the present invention provides a method for preparing a positive electrode sheet, the method comprising:

[0011] (1) A positive electrode slurry containing a positive electrode active material, a first conductive agent and a binder is coated onto a current collector and then dried to obtain a first electrode sheet with an active material layer.

[0012] (2) A lithium replenishing agent slurry containing a lithium replenishing agent and a second conductive agent is coated on the surface of the active material layer of the first electrode, and then rolled to obtain a second electrode with a surface lithium replenishing agent layer.

[0013] (3) Use the second electrode to assemble a battery, and perform formation and capacity testing on the battery to decompose the lithium replenishing agent. Then disassemble the capacity-tested battery to obtain the positive electrode.

[0014] Wherein, the median pore size of the pores in the active material layer is less than the D50 of the lithium replenishing agent, and the median pore size of the pores in the active material layer is not less than 100 nm;

[0015] The lithium supplement slurry does not contain a binder.

[0016] The third aspect of the present invention provides a positive electrode sheet prepared by the preparation method described in the second aspect above.

[0017] A fourth aspect of the present invention provides a lithium-ion battery comprising the positive electrode sheet described in the first or third aspect above.

[0018] Through the above technical solution, the present invention has the following beneficial effects:

[0019] (1) The positive electrode provided by the present invention has a double-layer structure, including a porous surface layer and a relatively less porous intermediate layer. The surface layer has a loose and porous three-dimensional conductive network composed of a conductive agent and does not contain a binder, which makes the positive electrode have high electronic conductivity and ionic conductivity. The three-dimensional conductive network on the surface is easily compressed under pressure, which can effectively reduce the internal pressure after the battery expands during cycling and avoid the risk of lithium plating.

[0020] (2) The positive electrode preparation method provided by the present invention forms a positive electrode active material layer on the surface of the current collector. By controlling the median pore size of the pores in the positive electrode active material layer to be less than the D50 of the lithium replenishing agent, and the median pore size of the pores in the active material layer to be not less than 100 nm, the binder-free lithium replenishing agent slurry can be effectively coated on the surface of the positive electrode active material layer, and the lithium replenishing agent will not enter the underlying positive electrode active material layer and damage the electrode structure. After that, the whole assembly is rolled to the target compaction density, and then assembled into a battery, formed, and tested for capacity, the above-mentioned positive electrode with a double-layer structure can be obtained. In the above preparation process, a lithium-ion battery is also produced (without disassembling the battery to extract the positive electrode separately). Compared with the battery using a conventional positive electrode, the lithium-ion battery has a higher rate retention rate, first-cycle reversible specific capacity, and capacity retention rate, and can effectively avoid the overall expansion of the battery during cycling, thereby alleviating the surface lithium deposition phenomenon of the negative electrode due to excessive pressure. Detailed Implementation

[0021] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0022] The first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising: a current collector, an intermediate layer disposed on the surface of the current collector, and a surface layer disposed on a side surface of the intermediate layer away from the current collector;

[0023] The intermediate layer contains a positive electrode active material, a first conductive agent, and a binder; the surface layer has a three-dimensional conductive network composed of a second conductive agent, and the surface layer does not contain a binder.

[0024] The porosity of the surface layer: the porosity of the intermediate layer ≥ 1.4:1.

[0025] The positive electrode sheet provided by this invention has a specific double-layer structure disposed on a current collector. The surface layer has a loose, porous three-dimensional conductive network composed of a conductive agent. This surface layer has high porosity and numerous large pores, thus possessing strong electrolyte storage capacity and enhancing ionic conductivity. Simultaneously, the surface layer contains no binder (binders have insulating properties), only a loose, porous three-dimensional conductive network, thus exhibiting strong electronic conductivity. Furthermore, compared to conventional positive electrode surface coatings containing binders (such as PVDF), the surface layer of the positive electrode sheet of this invention, due to its loose, porous three-dimensional conductive network, has lower hardness and is easily compressible under pressure. This ensures that the electrodes can contact each other and charge / discharge normally before battery cycle expansion, while preventing excessive internal pressure and lithium plating after expansion.

[0026] According to the present invention, in the positive electrode, the porosity of the surface layer to the porosity of the intermediate layer is ≥1.4:1. The surface layer has a sponge-like porous structure with a wide pore size distribution, which can improve the absorption capacity of the positive electrode to the electrolyte, provide an active ion-conducting network and an electron-conducting network for the electrode, thereby effectively improving the rate performance of the electrode and avoiding lithium plating at high rates.

[0027] According to the present invention, in the positive electrode sheet, preferably, the porosity of the surface layer is 50-99%, and the porosity of the intermediate layer is 25-45%.

[0028] According to a preferred embodiment of the present invention, in the positive electrode, the porosity of the surface layer is 70-90%, and the porosity of the surface layer to the porosity of the intermediate layer is (1.8-3):1, which is more conducive to providing a rich ion-conducting network for the positive electrode, reducing the concentration polarization of the electrode surface, bringing better battery rate performance, and making the surface layer more likely to collapse and compress as the electrode thickens, providing a larger expansion space for the battery.

[0029] In this invention, the porosity of the surface layer and the porosity of the intermediate layer of the positive electrode are determined by mercury intrusion porosimetry.

[0030] According to the present invention, in the positive electrode sheet, the surface layer has a large number of micron-sized macropores. Preferably, the pore size of the surface layer is 0.005-20 μm, and more preferably 0.5-10 μm. These micron-sized macropores can improve the porosity of the surface layer, accommodate more electrolyte, improve the rate performance of the battery, improve the overall expansion of the battery, and suppress premature lithium plating on the negative electrode surface.

[0031] According to the present invention, in the positive electrode sheet, the intermediate layer has relatively small pores, preferably, the pore diameter of the intermediate layer is 5-1000 nm, more preferably 10-700 nm.

[0032] According to the present invention, in the positive electrode sheet, the lower limit of the pore diameter range refers to the minimum value among all pore diameters in the layer (intermediate layer or surface layer) where the pore is located, and the upper limit of the pore diameter range refers to the maximum value among all pore diameters in the layer (intermediate layer or surface layer) where the pore is located.

[0033] According to the present invention, in the positive electrode sheet, the thickness ratio of the intermediate layer to the surface layer is (4-90):1. Using this ratio range allows for the provision of suitable expansion space for the lithium-ion battery, avoiding insufficient expansion space leading to lithium plating on the negative electrode or excessive expansion space resulting in energy density loss. Preferably, the thickness ratio of the intermediate layer to the surface layer is (10-65):1.

[0034] According to the present invention, in the positive electrode sheet, preferably, the thickness of the intermediate layer is 20-200 μm.

[0035] According to the present invention, there is no particular limitation on the selection of the positive electrode active material in the positive electrode sheet, and conventional materials that can be used as positive electrode active materials for lithium-ion batteries can be used. Preferably, the positive electrode active material can be selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, and lithium-rich manganese-based positive electrode materials.

[0036] According to the present invention, there is no particular limitation on the selection of the first conductive agent and the second conductive agent in the positive electrode sheet, and conventional materials that can be used as conductive agents for lithium-ion battery electrodes can be used. Preferably, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene, and carbon fiber;

[0037] According to the present invention, the range of materials that can be selected for the binder in the positive electrode sheet is relatively wide, and materials conventionally used as binders for battery positive and negative electrode slurries can be used. Preferably, the binder can be selected from acrylic resin (PAA) and / or polyvinylidene fluoride (PVDF).

[0038] According to the present invention, the positive electrode sheet can produce a large thickness change under the action of external force and has the characteristic of being easily compressible under pressure.

[0039] According to a preferred embodiment of the present invention, in the positive electrode sheet, the porosity of the surface layer is 70-80%, and the porosity of the surface layer to the porosity of the intermediate layer is (2-3):1. The pore size of the surface layer is 0.7-10 μm, the pore size of the intermediate layer is 10-700 nm, the thickness of the intermediate layer is 60-100 μm, and the thickness ratio of the intermediate layer to the surface layer is (10-30):1. This can bring higher rate retention, first-cycle reversible specific capacity, and capacity retention to the lithium-ion battery, effectively avoiding overall battery expansion and surface lithium deposition during cycling.

[0040] A second aspect of the present invention provides a method for preparing a positive electrode sheet, the method comprising:

[0041] (1) A positive electrode slurry containing a positive electrode active material, a first conductive agent and a binder is coated onto a current collector and then dried to obtain a first electrode sheet with an active material layer.

[0042] (2) A lithium replenishing agent slurry containing a lithium replenishing agent and a second conductive agent is coated on the surface of the active material layer of the first electrode, and then rolled to obtain a second electrode with a surface lithium replenishing agent layer.

[0043] (3) Use the second electrode to assemble a battery, and perform formation and capacity testing on the battery to decompose the lithium replenishing agent. Then disassemble the capacity-tested battery to obtain the positive electrode.

[0044] Wherein, the median pore size of the pores in the active material layer is less than the D50 of the lithium replenishing agent, and the median pore size of the pores in the active material layer is not less than 100 nm;

[0045] The lithium supplement slurry does not contain a binder.

[0046] According to the present invention, in the method for preparing the positive electrode sheet, step (1) has a relatively broad definition regarding the composition of the positive electrode slurry, which can be prepared by combining conventional lithium-ion battery positive electrode slurry components in the art. Preferably, based on the total weight of the positive electrode slurry, the content of the positive electrode active material in the positive electrode slurry is 52-84% by weight, the content of the first conductive agent is 0.5-2% by weight, the content of the binder is 0.5-1% by weight, and the content of the solvent is 15-45% by weight.

[0047] According to the present invention, in the method for preparing the positive electrode sheet, in step (1), the positive electrode slurry can be coated onto the current collector using conventional methods in the art, and the present invention does not have any particular limitation on this.

[0048] According to the present invention, in the method for preparing the positive electrode sheet, in step (1), preferably, the coating surface density of the positive electrode slurry can be 50-500 g / m². 2 .

[0049] According to the present invention, in the method for preparing the positive electrode, step (1) further includes drying the wet film obtained after coating to form an active material layer.

[0050] According to the present invention, in the method for preparing the positive electrode sheet, in step (1), if the median pore size of the pores in the active material layer formed by coating the positive electrode slurry onto the current collector and drying it is already smaller than the D50 of the lithium replenishing agent (since it has not been rolled after coating, it can be assumed that the median pore size of the pores in the active material layer will not be less than 100 nm), step (2) can be performed directly, or the active material layer formed after coating can be pre-rolled (but the median pore size of the pores in the active material layer after pre-rolling should not be less than 100 nm, otherwise it will affect the coating of the lithium replenishing agent slurry), and then step (2) can be performed. From the perspective of simplifying the operation process, it is preferable not to perform pre-rolling in the above situation. If the median pore size of the active material layer formed by coating the positive electrode slurry onto the current collector and drying it is not less than the D50 of the lithium replenishing agent, the active material layer formed after coating needs to be pre-rolled to reduce the pore size inside the active material layer until the median pore size of the active material layer is less than the D50 of the lithium replenishing agent (but the median pore size of the active material layer after pre-rolling should not be less than 100nm, otherwise it will affect the coating of the lithium replenishing agent slurry). This allows the binder-free lithium replenishing agent slurry to be effectively coated on the surface of the positive electrode active material layer and prevents the lithium replenishing agent slurry from entering the positive electrode active material layer through the large pores of the lower layer and damaging the electrode structure.

[0051] According to the present invention, in the method for preparing the positive electrode, the active material layer is finally formed into the intermediate layer of the positive electrode product after subsequent steps (2) and (3).

[0052] According to the present invention, in the method for preparing the positive electrode sheet, in step (2), the lithium replenishing agent slurry does not contain a binder. On the one hand, this can avoid the situation where the binder is deactivated by alkali, which would increase the viscosity of the slurry and affect the coating. The relatively low viscosity of the lithium replenishing agent slurry is conducive to reducing the coating surface density, increasing the battery energy density, and highlighting the lithium replenishing advantage. On the other hand, it can ensure that the surface layer of the prepared positive electrode sheet does not contain a binder (the binder has insulating properties), thereby improving the electronic conductivity of the positive electrode sheet.

[0053] According to the present invention, preferably, based on the total weight of the lithium replenishing agent slurry, the lithium replenishing agent content in the lithium replenishing agent slurry is 60-85% by weight, the amount of the second conductive agent is 0.1-30% by weight, and the solvent content is 10-20% by weight.

[0054] According to the present invention, preferably, the content of the second conductive agent in the lithium replenishing agent slurry is 0.1-20% by weight, which is more conducive to constructing a porous conductive agent surface layer with a suitable thickness, better improving the rate performance of the battery, and providing suitable expansion reserve space.

[0055] According to the present invention, in the preparation method of the positive electrode, in step (2), the lithium replenishing agent slurry can be coated on the surface of the active material layer of the first electrode using conventional methods in the art, and the present invention does not have any particular limitation on this.

[0056] According to the present invention, in the method for preparing the positive electrode, in step (2), preferably, the coating surface density of the lithium replenishing agent slurry can be 1-20 g / m². 2 .

[0057] According to the present invention, in the method for preparing the positive electrode, step (2) further includes drying the wet film obtained after coating to form a surface lithium replenishing agent layer.

[0058] According to the present invention, in the method for preparing the positive electrode sheet, in step (2), the active material layer is rolled to a target compaction density by the rolling process. In this invention, the target compaction density is related to the positive electrode active material used in the positive electrode sheet, and the target compaction density can be a value known in the art. For example, for lithium iron phosphate positive electrodes, the corresponding target compaction density can be 2.35-2.55 g / cm³. 3 For nickel-cobalt-manganese ternary cathodes, the corresponding target compaction density can be 3.3-3.6 g / cm³. 3 .

[0059] According to the present invention, in the method for preparing the positive electrode, the surface lithium replenishing agent layer is finally formed into the surface layer of the positive electrode product after subsequent step (3).

[0060] According to the present invention, the method for preparing the positive electrode sheet does not particularly limit the selection of the positive electrode active material, and conventional materials that can be used as positive electrode active materials for lithium-ion batteries can be used. Preferably, the positive electrode active material can be selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, and lithium-rich manganese-based positive electrode materials.

[0061] According to the present invention, in the method for preparing the positive electrode, there is no particular limitation on the selection of the first conductive agent and the second conductive agent, and conventional materials that can be used as conductive agents for lithium-ion battery electrodes can be used. Preferably, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene, and carbon fiber.

[0062] According to the present invention, the method for preparing the positive electrode sheet allows for a wide range of choices for the binder, and conventional materials in the art that can be used as binders for battery positive and negative electrode slurries can be employed. Preferably, the binder can be selected from acrylic resin (PAA) and / or polyvinylidene fluoride (PVDF).

[0063] According to the present invention, in the method for preparing the positive electrode sheet, the lithium replenishing agent is a lithium replenishing agent material that can generate gas upon decomposition. The gas generated by the decomposition of the lithium replenishing agent can participate in the film formation reaction of the negative electrode SEI film in the battery and promote an increase in the proportion of inorganic SEI film components, which is beneficial to improving the cycle stability of the battery. The lithium replenishing agent is preferably at least one of lithium oxalate, lithium squaric acid, lithium oxide, lithium peroxide, and lithium nitride.

[0064] According to the present invention, in the method for preparing the positive electrode, preferably, the D50 of the lithium supplement is 0.3-1 μm.

[0065] According to the present invention, in the preparation method of the positive electrode sheet, in step (3), the second electrode sheet is die-cut and assembled into a battery with the negative electrode sheet, separator and electrolyte. After aging, the battery is formed and capacity-divided to decompose the lithium replenishing agent and fully utilize its capacity. Then the capacity-divided battery is disassembled to obtain the positive electrode sheet.

[0066] According to the present invention, in the preparation method of the positive electrode, step (3) does not have any particular limitations on the negative electrode, the separator, and the electrolyte, and conventional choices in the art can be adopted. The present invention will not elaborate further here.

[0067] According to the present invention, in the preparation method of the positive electrode sheet, in step (3), conventional methods and parameters in the art can be used for the formation and capacity separation, as long as the lithium supplement can be completely decomposed and the capacity can be fully utilized after the formation and capacity separation. The present invention does not have any particular limitation on this.

[0068] The positive electrode preparation method provided by this invention involves coating a positive electrode active material layer onto a current collector, and then controlling the median pore size of the pores in the positive electrode active material layer to be less than the D50 of the lithium replenishing agent, while ensuring that the median pore size is not less than 100 nm. This allows the binder-free lithium replenishing agent slurry to be effectively coated on the surface of the positive electrode active material layer, preventing the lithium replenishing agent from entering the positive electrode active material layer through the larger pores in the lower layer (thus preventing the formation of large pores in the positive electrode active material layer after formation, which would affect the electronic conductivity pathway). Afterward, the entire assembly is rolled to the target compaction density, and then assembled into a battery, formed, and tested for capacity, resulting in a finished product. A positive electrode sheet with a specific double-layer structure (intermediate layer and surface layer) is obtained. The surface layer of the positive electrode sheet has a loose, porous three-dimensional conductive network composed of a conductive agent. The surface layer has high porosity and numerous large pores, with a porosity ratio of the surface layer to the intermediate layer ≥ 1.4:1. Furthermore, the surface layer contains no binder, only a loose, porous three-dimensional conductive network, thus exhibiting strong ionic and electronic conductivity. Additionally, the surface layer of the positive electrode sheet is easily compressible under pressure, ensuring that the electrodes can contact each other and charge / discharge normally before battery cycle expansion, while preventing excessive internal pressure and lithium plating after expansion. The positive electrode sheet prepared using this method, compared to conventional positive electrode sheets, enables lithium-ion batteries to exhibit higher rate retention, first-cycle reversible specific capacity, and capacity retention, and effectively avoids overall battery expansion and surface lithium plating during cycling.

[0069] The third aspect of the present invention provides a positive electrode sheet prepared by the preparation method described in the second aspect above.

[0070] According to the present invention, the positive electrode sheet prepared by the preparation method described in the second aspect above has the same structure, parameters and performance as the positive electrode sheet described in the first aspect above, and will not be described again here.

[0071] A fourth aspect of the present invention provides a lithium-ion battery comprising the positive electrode sheet described in the first or third aspect above.

[0072] The lithium-ion battery provided by the present invention uses the positive electrode sheet described in the first or third aspect above. Compared with lithium-ion batteries using conventional positive electrode sheets, it has a higher rate retention rate, first-cycle reversible specific capacity, and capacity retention rate, and can effectively avoid lithium deposition on the negative electrode surface after cycling.

[0073] The present invention will be described in detail below through examples. Unless otherwise specified, the following examples and comparative examples are all conventional methods; the reagents and materials mentioned are commercially available unless otherwise specified.

[0074] Example 1

[0075] (1) The positive electrode material LFP, binder PVDF, conductive agent SP and NMP are mixed into a slurry to obtain a positive electrode slurry (in this positive electrode slurry, based on the total weight of the slurry, the content of LFP is 55% by weight, the content of PVDF is 1% by weight, the content of SP is 2% by weight, and the content of solvent NMP is 42% by weight); then the positive electrode slurry is coated on an aluminum foil with a thickness of 12 μm (the coating surface density is 225 g / m²). 2 And dry to obtain the first electrode;

[0076] (2) Lithium oxalate (D50 is 500 nm), conductive agent SP, conductive agent CNT and solvent (NMP) are mixed into a slurry to obtain a lithium replenishing agent slurry (in this lithium replenishing agent slurry, based on the total weight of the slurry, the content of lithium oxalate is 70% by weight, the total content of SP and CNT is 10% by weight, the weight ratio of SP:CNT is 1:1, and the content of NMP is 20% by weight); then the lithium replenishing agent slurry is coated on the surface of the active material layer of the first electrode (the coating surface density is 15 g / m²). 2 After drying, the electrode is rolled to achieve a target compaction density of 2.5 g / cm³. 3 The second electrode is obtained;

[0077] (3) The second electrode sheet is die-cut and assembled with the graphite negative electrode sheet, commercially available separator (Shanghai Enjie ND9T22-R01), and lithium hexafluorophosphate electrolyte to form a lithium-ion battery L1. Then, the battery L1 is aged, formed (formation temperature is 45℃, constant current charging at a step current of 0.05C / 0.1C / 0.2C to 4.3V, and then charging at 0.33C to 4.5V), and capacity tested. The capacity tested battery L1 is disassembled to obtain the formed positive electrode sheet (denoted as P1).

[0078] Example 2

[0079] The method is the same as in Example 1, except that in step (1), the first electrode obtained after drying is pre-rolled to a compaction density of 2.2 g / cm³. 3 (i.e., pre-rolling to 88% of the target compaction density), with other steps and conditions the same as in Example 1, to obtain the formed positive electrode sheet (denoted as P2) and the capacity-separated lithium-ion battery L2.

[0080] Example 3

[0081] The method is the same as in Example 1, except that in step (1), the first electrode obtained after drying is pre-rolled to a compaction density of 2.3 g / cm³. 3 (i.e., pre-rolling to 92% of the target compaction density), with other steps and conditions the same as in Example 1, to obtain the formed positive electrode sheet (denoted as P3) and the capacity-balanced lithium-ion battery L3.

[0082] Example 4

[0083] The method is the same as in Example 1, except that in step (1), the first electrode obtained after drying is pre-rolled to a compaction density of 2.1 g / cm³. 3 (i.e., pre-rolling to 84% of the target compaction density), with other steps and conditions the same as in Example 1, to obtain the formed positive electrode sheet (denoted as P4) and the capacity-balanced lithium-ion battery L4.

[0084] Comparative Example 1

[0085] The method is the same as in Example 1, except that in step (1), the compaction density of the first electrode obtained after drying, pre-rolled to the active material layer, is 2.5 g / cm³. 3 (i.e., directly roll-pressed to the target compaction density). Other steps and conditions are the same as in Example 1. It was found that in step (2), the lithium supplement slurry could not be coated on the surface of the active material layer of the first electrode, and ultimately a double-layer positive electrode could not be prepared.

[0086] Comparative Example 2

[0087] The method of Example 1 is followed, except that in step (2), the composition of the lithium replenishing agent slurry is adjusted as follows: based on the total weight of the slurry, the content of lithium oxalate is 65% by weight, the total content of SP and CNT is 10% by weight, the weight ratio of SP:CNT is 1:1, the content of binder PVDF is 5% by weight, the content of solvent NMP is 20% by weight, and the coating surface density of the lithium replenishing agent slurry is 17 g / m². 2 The other steps and conditions are the same as in Example 1, resulting in a positive electrode sheet (denoted as DP2) after formation and a lithium-ion battery DL2 after capacity testing.

[0088] Comparative Example 3

[0089] (1) The cathode material LFP, lithium oxalate (D50 of 500 nm), binder PVDF, conductive agent SP and solvent NMP are mixed into a slurry to obtain a mixed slurry (in this mixed slurry, based on the total weight of the slurry, the content of LFP is 51% by weight, the content of lithium oxalate is 3% by weight, the content of PVDF is 1% by weight, the content of SP is 2% by weight, and the content of solvent NMP is 43% by weight); then the mixed slurry is coated on an aluminum foil with a thickness of 12 μm (coating surface density is 238 g / m²). 2 After drying, it is rolled until the compacted density is 2.5 g / cm³. 3 A composite electrode is obtained;

[0090] (2) The above composite electrode sheet is die-cut and assembled with graphite negative electrode sheet, commercially available separator (Shanghai Enjie ND9T22-R01) and lithium hexafluorophosphate electrolyte to form a lithium-ion battery. Then the battery is aged, formed (formation temperature is 45℃, constant current charging at 0.05C / 0.1C / 0.2C step current to 4.3V, then charging at 0.33C to 4.5V), and capacity tested. The capacity tested battery is disassembled to obtain the formed positive electrode sheet (denoted as DP3) and the capacity tested lithium-ion battery DL3.

[0091] Comparative Example 4

[0092] (1) The positive electrode material LFP, binder PVDF, conductive agent SP and solvent NMP are mixed into a slurry to obtain a mixed slurry (in this positive electrode slurry, based on the total weight of the slurry, the content of LFP is 54% by weight, the content of PVDF is 1% by weight, the content of SP is 2% by weight, and the content of solvent NMP is 43% by weight); then the mixed slurry is coated on an aluminum foil with a thickness of 12 μm (the coating surface density is 225 g / cm³). 2 After drying, it is rolled until the compacted density is 2.5 g / cm³. 3 To obtain the electrode sheet;

[0093] (2) The above-mentioned electrode sheet is die-cut and assembled with graphite negative electrode sheet, commercially available separator (Shanghai Enjie ND9T22-R01) and lithium hexafluorophosphate electrolyte to form a lithium-ion battery. Then the battery is aged, formed (formation temperature is 45℃, constant current charging at 0.05C / 0.1C / 0.2C step current to 4.3V, then charging at 0.33C to 4.5V), and capacity tested. The capacity tested battery is disassembled to obtain the formed positive electrode sheet (denoted as DP4) and lithium-ion battery DL4.

[0094] Comparative Example 5

[0095] The method of Example 1 is followed, except that in step (2), the lithium oxalate D50 is 200nm. All other steps and conditions are the same as in Example 1, and the positive electrode sheet (denoted as DP5) after formation and the lithium-ion battery DL5 after capacity testing are obtained.

[0096] The first electrode and the positive electrode (i.e., the positive electrode after formation) involved in Examples 1-4 and Comparative Examples 1-5 were subjected to index testing, and the testing methods are as follows:

[0097] Median pore size of the pores in the active material layer of the first electrode: determined by mercury intrusion porosimetry;

[0098] Thickness of the middle and surface layers of the positive electrode sheet: SEM cross-section observation and thickness measurement;

[0099] Total porosity and pore size distribution of the positive electrode sheet: determined by mercury porosimetry;

[0100] Porosity and pore size distribution of the intermediate layer of the positive electrode sheet: The surface layer of the positive electrode sheet was removed by polishing, and the porosity and pore size distribution of the intermediate layer were determined by mercury intrusion porosimetry.

[0101] The porosity and pore size distribution of the surface layer of the positive electrode sheet are calculated using the following formula.

[0102] Where X is the porosity of the surface layer (%), Y is the porosity of the intermediate layer (%), Z is the total porosity of the positive electrode sheet (%); a is the thickness of the surface layer (unit: μm), and b is the thickness of the intermediate layer (unit: μm).

[0103] The surface resistance and volume resistance of the positive electrode of the product were measured using a two-probe test.

[0104] Thickness change rate of the positive electrode sheet under 1MPa pressure: The thickness of the positive electrode sheet (before applying 1MPa pressure) and the thickness of the positive electrode sheet after applying 1MPa pressure were measured using a micrometer. The thickness change rate was calculated using the formula (thickness of the positive electrode sheet after applying 1MPa pressure - thickness of the positive electrode sheet before applying 1MPa pressure) / thickness of the positive electrode sheet before applying 1MPa pressure × 100%.

[0105] The test results are shown in Tables 1, 2, 3 and 4.

[0106] Table 1

[0107]

[0108] Table 2

[0109]

[0110]

[0111] Table 3

[0112]

[0113] Table 4

[0114]

[0115] As can be seen from the data in Tables 1-4, the positive electrode sheets P1-P4 prepared by the method of the present invention have a porous surface layer and a relatively less porous intermediate layer. The surface layer has a loose and porous three-dimensional conductive network composed of conductive agent and does not contain binder. The overall porosity of the positive electrode sheet is high, the surface resistance and volume resistance are low, and it has high electronic conductivity and ionic conductivity. Moreover, the thickness change rate of the positive electrode sheet under 1 MPa pressure is significantly greater than that of the positive electrode sheets DP2-DP5 prepared in the comparative example, and it has the characteristic of being easily compressible under pressure.

[0116] Test case

[0117] The lithium-ion batteries prepared in the above embodiments and comparative examples were subjected to electrical performance tests, wherein...

[0118] ACR: Measured using a voltage internal resistance meter;

[0119] DCR: Measured using the Blue Electricity testing system;

[0120] Rate retention rate from 1C to 0.1C: Using the Blue Electric test system, the system first charges the battery to 3.65V using a constant current of 0.33C, then discharges it using a constant current of 1C; then it charges the battery again using a constant current of 0.33C, then discharges it using a constant current of 1C; the rate retention rate is calculated using the discharge capacity from the two tests.

[0121] First-week reversible capacity: Using the Blue Electricity test system, after formation (with the same formation conditions as in the previous embodiment), a 0.1C discharge was performed to measure the discharge capacity and the first-week reversible capacity.

[0122] 0.1C capacity retention rate after 500 cycles: Using the Blue Electric test system, 500 cycles were first performed with a constant current charge / discharge rate of 1C / 1C, followed by one cycle of constant current charge / discharge at a charge / discharge rate of 0.33C / 0.1C. The discharge capacity at 0.1C was compared with the reversible capacity at the beginning of the cycle to obtain the 0.1C capacity retention rate after 500 cycles.

[0123] Battery thickness expansion rate after 500 cycles: The battery thickness was measured using a micrometer after the first reversible specific capacity test. After the battery was cycled for 500 cycles, the battery thickness was measured again, and the battery thickness expansion rate was calculated.

[0124] Lithium plating on the negative electrode of the battery after 500 cycles: visual inspection after disassembling the battery.

[0125] The results are shown in Table 5.

[0126] Table 5

[0127]

[0128] As can be seen from the data in Table 5, under the above test conditions, the lithium-ion batteries L1-L4 using the positive electrode sheet provided by the present invention have lower internal resistance than batteries DL2-DL5 during operation, and have higher rate retention rate, first-cycle reversible specific capacity and capacity retention rate. After 500 cycles, the overall expansion of the battery is lower, which can avoid lithium deposition on the negative electrode surface.

[0129] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes: a current collector, an intermediate layer disposed on the surface of the current collector, and a surface layer disposed on the side of the intermediate layer away from the current collector. The intermediate layer contains a positive electrode active material, a first conductive agent, and a binder; the surface layer has a three-dimensional conductive network composed of a second conductive agent, and the surface layer does not contain a binder. The porosity of the surface layer: the porosity of the intermediate layer ≥ 1.4:

1.

2. The positive electrode according to claim 1, wherein, The porosity of the surface layer is 50-99%, and the porosity of the intermediate layer is 25-45%. Preferably, the porosity of the surface layer is 70-90%, and the porosity of the surface layer to the porosity of the intermediate layer is (1.8-3):1; Preferably, the pore size of the surface layer is 0.005-20 μm, more preferably 0.5-10 μm; the pore size of the intermediate layer is 5-1000 nm, more preferably 10-700 nm.

3. The positive electrode according to claim 1 or 2, wherein, The thickness ratio of the intermediate layer to the surface layer is (4-90):1, preferably (10-65):1; And / or, the thickness of the intermediate layer is 20-200 μm.

4. The positive electrode sheet according to any one of claims 1-3, wherein, The positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, and lithium-rich manganese-based positive electrode materials. And / or, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene and carbon fiber; And / or, the adhesive is selected from acrylic resins and / or polyvinylidene fluoride.

5. A method for preparing a positive electrode sheet, characterized in that, The method includes: (1) A positive electrode slurry containing a positive electrode active material, a first conductive agent and a binder is coated onto a current collector and then dried to obtain a first electrode sheet with an active material layer. (2) A lithium replenishing agent slurry containing a lithium replenishing agent and a second conductive agent is coated on the surface of the active material layer of the first electrode, and then rolled to obtain a second electrode with a surface lithium replenishing agent layer. (3) Use the second electrode to assemble a battery, and perform formation and capacity testing on the battery to decompose the lithium replenishing agent. Then disassemble the capacity-tested battery to obtain the positive electrode. Wherein, the median pore size of the pores in the active material layer is less than the D50 of the lithium replenishing agent, and the median pore size of the pores in the active material layer is not less than 100 nm; The lithium supplement slurry does not contain a binder.

6. The preparation method according to claim 5, wherein, The content of the second conductive agent in the lithium replenishing agent slurry is 0.1-20% by weight.

7. The preparation method according to claim 5 or 6, wherein, The coating surface density of the positive electrode slurry is 50-500 g / m³. 2 ; And / or, the coating surface density of the lithium replenishing agent slurry is 1-20 g / m². 2 .

8. The preparation method according to any one of claims 5-7, wherein, The positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium cobalt oxide, and lithium-rich manganese-based positive electrode materials. And / or, the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene and carbon fiber; And / or, the adhesive is selected from acrylic resins and / or polyvinylidene fluoride; And / or, the lithium replenishing agent is a lithium replenishing agent material that can generate gas after decomposition, preferably at least one of lithium oxalate, lithium squarate, lithium oxide, lithium peroxide and lithium nitride.

9. The positive electrode sheet prepared by the preparation method according to any one of claims 5-8.

10. A lithium-ion battery comprising the positive electrode sheet according to any one of claims 1-4 and 9.

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