Positive electrode sheet, method for manufacturing the same, and battery

Abstract

The application provides a positive electrode sheet, a preparation method thereof and a battery. The positive electrode sheet comprises a carbon-coated current collector and a positive electrode active layer. The material of the positive electrode active layer comprises a positive electrode active material and a carbon layer coated on the surface of the positive electrode active material. The positive electrode active layer has a porous structure, and the average pore diameter of the positive electrode active layer is 0.2-1 mu m. The porosity of the positive electrode active layer is 10-40%. The positive electrode active layer of the application has an ordered porous structure. The existence of the structure helps to shorten the migration path of ions, thereby helping to promote the infiltration of electrolyte in the pores and the rapid migration of ions. Controlling the average pore diameter of the positive electrode active layer in the range of 0.2-1 mu m and the porosity of the positive electrode active layer in the range of 10-40% helps to improve the migration rate of ions and the infiltration rate of electrolyte while ensuring the stability of the structure of the positive electrode active layer, thereby helping to improve the rate performance and production efficiency of the battery.

Description

TECHNICAL FIELD

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

[0002] The positive electrode is one of the key factors determining the electrochemical performance, safety and cost of the battery. In order to promote ion transmission and improve the kinetics of electrochemical reaction, designing suitable positive electrode materials and electrode structures is an effective way to reduce the average free path of ions and electrons and improve the reaction rate. Currently, the positive electrode sheet is prepared by coating a mixed slurry of positive electrode material, conductive agent and binder on a metal foil (current collector), and then drying and rolling to obtain the electrode sheet. The electrode sheet prepared by this process has disordered and twisted ion diffusion channels, which hinders the diffusion of ions and limits the rate performance of the battery. SUMMARY

[0003] The main purpose of the present application is to provide a positive electrode sheet, a preparation method thereof and a battery, so as to solve the problem of low ion transmission efficiency of the positive electrode sheet in the prior art.

[0004] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a positive electrode sheet is provided, which comprises a carbon-coated current collector and a positive electrode active layer, the material of the positive electrode active layer comprises a positive electrode active material and a carbon layer coated on the surface of the positive electrode active material, the positive electrode active layer has a porous structure, and the average pore size thereof is 0.2-1 μm, and the porosity of the positive electrode active layer is 10-40%.

[0005] Controlling the average pore size of the positive electrode active layer in the range of 0.2-1 μm and the porosity of the positive electrode active layer in the range of 10-40% helps to improve the migration rate of ions and the infiltration rate of electrolyte while ensuring the stability of the structure of the positive electrode active layer, thereby helping to improve the rate performance and production efficiency of the battery.

[0006] Further, the thickness of the carbon layer is 1-10 nm; and / or, the mass ratio of the positive electrode active material to the carbon layer is 1:0.01-0.05.

[0007] The carbon layer with the above-mentioned thickness helps to improve the bonding force between the positive electrode active layer and the carbon-coated current collector. The mass ratio of the positive electrode active material to the carbon layer in the above-mentioned range helps to improve the electrical conductivity and structural stability of the positive electrode active layer.

[0008] Further, the thickness of the positive electrode active layer is 100-400 μm; and / or, the carbon-coated current collector is a carbon-coated aluminum foil and / or a carbon-coated copper foil; and / or, the positive electrode active material is a polyanion-based positive electrode active material; preferably, the polyanion-based positive electrode active material is selected from any one or more of a phosphate-based positive electrode active material, a sulfate-based positive electrode active material, and a silicate-based positive electrode active material.

[0009] The positive electrode active layer having the above thickness helps to further improve the ion migration rate and the electrolyte infiltration rate. The carbon-coated current collector of the above kind helps to improve the conductivity of the carbon-coated current collector. The positive electrode active material of the above kind helps to further improve the energy density of the positive electrode active layer.

[0010] According to another aspect of the present application, there is provided a method for preparing the aforementioned positive electrode sheet, which comprises: step S1, mixing each precursor raw material including a positive electrode active material, a carbon source, a dispersant, a carbonaceous conductive agent, and a solvent, and then performing a grinding treatment to obtain a slurry; step S2, pre-cooling the carbon-coated current collector to the freezing point of the slurry or below, and then coating the slurry on the carbon-coated current collector, and after the slurry is solidified, a positive electrode sheet preform is formed; and step S3, sequentially performing a freeze-drying and a calcination treatment on the positive electrode sheet preform to obtain the positive electrode sheet; wherein the calcination treatment is performed in a protective gas.

[0011] The grinding treatment in step S1 helps to improve the uniformity of the dispersion between each component; when the slurry is moved onto the carbon-coated current collector at low temperature in step S2, a temperature gradient field is formed between the bottom and the top of the slurry, and the solvent at the bottom begins to nucleate and crystallize when it is cooled to the freezing point, while the solute in the slurry is extruded by the crystal to form dense pore walls, and finally an array of ice crystals with parallel to each other is formed. After the solidified slurry is treated by a freeze-drying machine, the crystal sublimates and is removed, leaving the pore channels with a directional structure and the densified precursor pore walls. During the sintering process, the carbon in the carbon source, the dispersant, and the carbonaceous conductive agent fuses together to form a carbon layer, and the carbon layer fuses with the carbon material on the surface of the carbon-coated foil to form a firm interface rivet active material, so that the positive electrode active layer with a specific pore structure and the carbon-coated current collector are tightly combined to form the positive electrode sheet, avoiding the use of a binder. The preparation method of the present application integrates the material synthesis process and the electrode sheet manufacturing process into a one-step process, which improves the electrical performance and greatly improves the production efficiency.

[0012] Further, in step S2, the surface temperature of the carbon-coated current collector is 0-50°C, preferably -20- -40°C.

[0013] Controlling the surface temperature of the carbon-coated current collector in the above range helps to improve the orderliness of the pore structure of the positive electrode active layer.

[0014] Further, in the step S2, the vacuum degree of the freeze-drying is ≤ 10 Pa; and / or, the cold trap temperature of the freeze-drying is ≤ -50℃; and / or, the drying time of the freeze-drying is 1-24 h; and / or, the protective gas is selected from any one or more of nitrogen, argon, nitrogen-hydrogen mixed gas, and argon-hydrogen mixed gas; and / or, the temperature increasing rate of the calcination treatment is 1-5℃ / min; and / or, the temperature of the calcination treatment is 300-550℃; and / or, the holding time of the calcination treatment is 5-20 h.

[0015] Controlling the vacuum degree, the cold trap temperature, and the drying time of the freeze-drying within the above ranges helps to improve the efficiency of solvent removal. Controlling the type of the protective gas within the above ranges helps to enrich the selectivity of the type of the gas. Controlling the temperature increasing rate, the temperature, and the holding time of the calcination treatment within the above ranges helps to improve the efficiency of forming the positive electrode active material.

[0016] Further, in the step S1, the grinding treatment includes sequentially performing a rough grinding treatment and a sand grinding treatment, the rotation speed of the rough grinding treatment is 100-500 rpm; and / or, the time of the rough grinding treatment is 0.5-4 h; and / or, the rotation speed of the sand grinding treatment is 1000-3000 rpm; and / or, the time of the sand grinding treatment is 0.5-6 h.

[0017] Controlling the conditions of the rough grinding treatment within the above ranges helps to improve the uniformity of the dispersion of the components.

[0018] Further, the ratio of the total mass of each of the precursor raw materials of the positive electrode active material, the mass of the carbon source, the mass of the dispersant, and the mass of the carbonaceous conductive agent is 28:1.1-2.5:0.1-0.2:0.2; and / or, the ratio of the total mass of each of the precursor raw materials of the positive electrode active material, the carbon source, the dispersant, and the carbonaceous conductive agent to the volume of the solvent is 2.95-6.04:1 kg / L.

[0019] Preferably, the ratio of the total mass of each of the precursor raw materials of the positive electrode active material, the mass of the carbon source, the mass of the dispersant, and the mass of the carbonaceous conductive agent is controlled within the above ranges, which helps to fully exert the synergistic effect among the components, thereby helping to form the carbon layer on the surface of the positive electrode active material. Preferably, the ratio of the total mass of each of the precursor raw materials of the positive electrode active material, the carbon source, the dispersant, and the carbonaceous conductive agent to the volume of the solvent is controlled within the above ranges, which helps to improve the dispersibility of the components and to control the average pore size and the porosity of the positive electrode active layer finally formed.

[0020] Further, the carbon source is selected from any one or more of oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, citric acid, soluble starch, ascorbic acid, sucrose, and glucose; and / or, the dispersant is selected from any one or more of sodium carboxymethyl cellulose, polyacrylic acid, and polycarboxylic acid; and / or, the carbonaceous conductive agent is selected from any one or more of carbon black Super P, conductive graphite, carbon nanotube, and graphene; and / or, the solvent is selected from any one or more of water, t-butyl alcohol, and acetonitrile.

[0021] Preferably, the types of the carbon source, the dispersant, and the carbonaceous conductive agent are controlled within the above ranges, which helps to improve the synergistic effect among the three, thereby helping to form a carbon layer with stable structure. Preferably, the type of the solvent is controlled within the above range, which helps to improve the dispersibility of each component and control the average pore size and porosity of the positive electrode active layer finally formed.

[0022] According to yet another aspect of the present application, there is provided a battery comprising a positive electrode sheet, an electrolyte, and a negative electrode sheet, the positive electrode sheet being the aforementioned positive electrode sheet.

[0023] The above battery has high cycle stability and rate capability.

[0024] By applying the technical solution of the present application, the positive electrode active layer of the present application has a porous structure arranged in order, which helps to shorten the migration path of ions, thereby helping to promote the infiltration of electrolyte in the pores and the rapid migration of ions. Controlling the average pore size of the positive electrode active layer within the range of 0.2-1 μm and the porosity of the positive electrode active layer within the range of 10-40% helps to improve the migration rate of ions and the infiltration rate of electrolyte while ensuring the stability of the structure of the positive electrode active layer, thereby helping to improve the rate capability and production efficiency of the battery. The presence of the carbon layer in the positive electrode active layer helps to improve the conductivity of the positive electrode active layer, and the carbon layer can serve as a physical protective layer to reduce the risk of side reactions between the positive electrode active material and the electrolyte, thereby helping to improve the cycle stability and safety of the battery. More importantly, the carbon layer is fused with the carbon material on the surface of the carbon-coated current collector, forming a firm interface, which helps to improve the bonding force between the positive electrode active layer and the carbon-coated current collector, thereby helping to reduce the risk of shedding or pulverization of the positive electrode active layer during charging and discharging, and further helping to improve the long-term stability and service life of the battery. Moreover, the positive electrode active layer of the present application does not contain a binder, which on the one hand helps to further reduce the resistance of ion transmission, and on the other hand, helps to reduce the pollution to the environment. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an exemplary embodiment of the application and together with the description, serve to explain the application without imposing undue limitation thereon. In the drawings:

[0026] Figure 1 An SEM image of the positive electrode active layer in Example 1 of the present application is shown at a low magnification;

[0027] Figure 2 An SEM image of the positive electrode active layer in Example 1 of the present application is shown at a high magnification. DETAILED DESCRIPTION

[0028] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0029] As analyzed in the background section of the present application, the positive electrode sheet in the prior art has the problem of low ion transmission efficiency. In order to solve the above problems, the present application provides a positive electrode sheet and a preparation method thereof, and a battery.

[0030] In a typical embodiment of the present application, a positive electrode sheet is provided, comprising a carbon-coated current collector and a positive electrode active layer, the material of the positive electrode active layer comprising a positive electrode active material and a carbon layer coated on the surface of the positive electrode active material, the positive electrode active layer having a porous structure, and the average pore size thereof being 0.2-1 μm, and the porosity of the positive electrode active layer being 10-40%.

[0031] The positive electrode active layer of the present application is a porous structure arranged in order, and the existence of this structure helps to shorten the migration path of ions, thereby helping to promote the infiltration of electrolyte in the pores and the rapid migration of ions. Controlling the average pore size of the positive electrode active layer in the range of 0.2-1 μm and the porosity of the positive electrode active layer in the range of 10-40% helps to improve the migration rate of ions and the infiltration rate of electrolyte while ensuring the stability of the structure of the positive electrode active layer, thereby helping to improve the rate performance and production efficiency of the battery. The existence of the carbon layer in the positive electrode active layer helps to improve the conductivity of the positive electrode active layer, and the carbon layer can act as a physical protective layer to reduce the risk of side reactions between the positive electrode active material and the electrolyte, thereby helping to improve the cycle stability and safety of the battery. More importantly, the carbon layer is fused with the carbon material on the surface of the carbon-coated current collector to form a firm interface, which helps to improve the bonding force between the positive electrode active layer and the carbon-coated current collector, thereby helping to reduce the risk of shedding or pulverization of the positive electrode active layer during charging and discharging, and further helping to improve the long-term stability and life of the battery. Moreover, the positive electrode active layer of the present application does not contain a binder, which on the one hand helps to further reduce the resistance of ion transmission, and on the other hand helps to reduce pollution to the environment.

[0032] In an embodiment of the present application, the thickness of the carbon layer is 1-10 nm; and / or, the mass ratio of the positive electrode active material to the carbon layer is 1:0.01-0.05.

[0033] Preferably, the thickness of the carbon layer is controlled within the above range, which helps to improve the binding force between the positive electrode active layer and the carbon-coated current collector. Preferably, the mass ratio of the positive electrode active material to the carbon layer is controlled within the above range, which helps to improve the conductivity and structural stability of the positive electrode active layer while ensuring that the positive electrode active layer has a relatively high energy density.

[0034] In an embodiment of the present application, the thickness of the positive electrode active layer is 100-400 μm; and / or, the carbon-coated current collector is a carbon-coated aluminum foil and / or a carbon-coated copper foil; and / or, the positive electrode active material is a polyanion-based positive electrode active material; preferably, the polyanion-based positive electrode active material is selected from any one or more of a phosphate-based positive electrode active material, a sulfate-based positive electrode active material, and a silicate-based positive electrode active material.

[0035] A positive electrode active layer that is too thin is not conducive to improving the porosity of the positive electrode active layer, and a positive electrode active layer that is too thick is not conducive to controlling the pore size of the positive electrode active layer and improving the uniformity of the pore structure. Preferably, the thickness of the positive electrode active layer is controlled within the above range, which helps to further improve the migration rate of ions and the infiltration rate of electrolyte by controlling the uniformity of the pore structure and the porosity and average pore size of the positive electrode active layer within a suitable range. Preferably, the material of the carbon-coated current collector is controlled within the above range, which helps to improve the conductivity of the carbon-coated current collector. Preferably, the type of the positive electrode active material is controlled within the above range, which helps to further improve the energy density of the positive electrode active layer.

[0036] The chemical formula of the positive electrode active material includes but is not limited to: Na x M y (X a O b ) z Z w , wherein M is selected from any one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, and Nb; X is selected from any one or more of Si, S, P, As, B, Mo, W, and Ge; Z is P2O7; 1≤x≤6, 0.3≤y≤4, 1≤a≤4, 4≤b≤15, 0≤z≤4, 0≤w≤2. Optionally, the positive electrode active material is Na3Fe2(SO4)3 and / or Na4Fe3(PO4)2(P2O7), and other types of positive electrode active materials that are more suitable for batteries can also be selected according to actual needs.

[0037] In another typical embodiment of the present application, a preparation method of the aforementioned positive electrode sheet is provided, which comprises: step S1, mixing each precursor raw material of the positive electrode active material, a carbon source, a dispersant, a carbon conductive agent and a solvent, and then performing grinding treatment to obtain a slurry; step S2, pre-cooling the carbon-coated current collector to the freezing point of the slurry or below, and then coating the slurry on the carbon-coated current collector, and after the slurry is solidified, a positive electrode sheet preform is formed; step S3, sequentially performing freeze-drying and calcination treatment on the positive electrode sheet preform to obtain the positive electrode sheet; wherein the calcination treatment is performed in a protective gas.

[0038] In step S1, after mixing each precursor raw material of the positive electrode active material, the carbon source, the dispersant, the carbon conductive agent and the solvent, grinding treatment is performed, which helps to improve the uniformity of the dispersion between each component, thereby obtaining a viscous and uniform slurry; in step S2, when the slurry is moved to the low-temperature carbon-coated current collector, a temperature gradient field is formed between the bottom and the top of the slurry, and the solvent at the bottom begins to nucleate and crystallize when the temperature is lowered to the freezing point, while the solute in the slurry is extruded by the crystal to form a dense pore wall. As the temperature continues to decrease, the solidified crystals gradually grow to the surface along the direction of the temperature field, and because of the difference in nucleation sites, an array of ice crystals parallel to each other is finally formed. After the solidified slurry is treated by the freeze-drying machine, the crystals sublimate and are removed, leaving behind the pores with a directional structure and the densified precursor pore wall. During the sintering process, the carbon in the carbon source, the dispersant and the carbon conductive agent fuses together to form a carbon layer, and the carbon layer fuses with the carbon material on the surface of the carbon-coated foil to form a firm interface rivet active material, so that the positive electrode active layer with a specific pore structure and the carbon-coated current collector are tightly combined to form the positive electrode sheet, avoiding the use of a binder. In the preparation method of the present application, the material synthesis process and the electrode sheet manufacturing process are integrated into a one-step process, which improves the electrical performance and greatly improves the production efficiency.

[0039] The types of each precursor raw material of the positive electrode active material can be selected according to the composition of the positive electrode active material to be finally formed. The precursor raw material of the positive electrode active material includes, but is not limited to, a sodium source, a metal source and an anion source.

[0040] Optionally, the sodium source is selected from any one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate and sodium dihydrogen pyrophosphate.

[0041] Optionally, the metal source is selected from any one or more of iron phosphate, iron sulfate, iron chloride, iron nitrate and ferrous oxalate.

[0042] Optionally, the anion source is selected from any one or more of phosphoric acid, sodium sulfate, sodium silicate and sodium fluoride.

[0043] In an embodiment of the present application, in step S2, the carbon-coated current collector is placed on the surface of the continuously flowing base band, so that the carbon-coated current collector is in a continuously flowing state, the continuously flowing carbon-coated current collector is pre-cooled to the freezing point of the slurry or below, the slurry is continuously coated on the continuously flowing carbon-coated current collector by using a doctor blade, and after the slurry is solidified, the positive electrode sheet preform is formed. Preferably, the slit of the doctor blade is 100-400 μm, and the speed of the continuously moving carbon-coated current collector is 0.1-10 m / min.

[0044] In order to further improve the order and porosity of the pore structure of the positive electrode active layer, in an embodiment of the present application, preferably in the above step S2, the surface of the carbon-coated current collector is at 0-50°C, preferably at -20- -40°C.

[0045] In an embodiment of the present application, in the above step S2, the vacuum degree of the freeze-drying is ≤10 Pa; and / or, the cold trap temperature of the freeze-drying is ≤-50°C; and / or, the drying time of the freeze-drying is 1-24 h; and / or, the protective gas is selected from any one or more of nitrogen, argon, nitrogen-hydrogen mixed gas and argon-hydrogen mixed gas; and / or, the temperature rising rate of the calcination treatment is 1-5°C / min; and / or, the temperature of the calcination treatment is 300-550°C; and / or, the holding time of the calcination treatment is 5-20 h.

[0046] Preferably, the vacuum degree, the cold trap temperature and the drying time of the freeze-drying are controlled within the above ranges, which helps to improve the efficiency of solvent removal. Preferably, the type of the protective gas is controlled within the above ranges, which helps to improve the selectivity of the type of the gas. Preferably, the temperature rising rate, the temperature and the holding time of the calcination treatment are controlled within the above ranges, which helps to improve the formation efficiency of the positive electrode active material while avoiding the reaction between the protective gas and the carbon-coated current collector.

[0047] In order to improve the uniformity of the dispersion of the components and thus improve the rate performance of the positive electrode sheet, in an embodiment of the present application, preferably in the above step S1, the grinding treatment comprises sequentially performing coarse grinding treatment and sand grinding treatment, the rotation speed of the coarse grinding treatment is 100-500 rpm; and / or, the time of the coarse grinding treatment is 0.5-4 h; and / or, the rotation speed of the sand grinding treatment is 1000-3000 rpm; and / or, the time of the sand grinding treatment is 0.5-6 h.

[0048] In an embodiment of the present application, the ratio of the total mass of each precursor raw material of the positive electrode active material, the mass of the carbon source, the mass of the dispersant and the mass of the carbonaceous conductive agent is 28:1.1-2.5:0.1-0.2:0.2; and / or, the ratio of the total mass of each precursor raw material of the positive electrode active material, the carbon source, the dispersant and the carbonaceous conductive agent to the volume of the solvent is 2.95-6.04:1 kg / L.

[0049] Preferably, the ratio of the total mass of each precursor raw material of the positive electrode active material, the mass of the carbon source, the mass of the dispersant, and the mass of the carbonaceous conductive agent is controlled within the above range, which helps to fully exert the synergistic effect between the components, thereby helping to form a carbon layer on the surface of the positive electrode active material. The presence of the carbon layer helps to improve the conductivity of the positive electrode active layer, and the carbon layer can act as a physical protective layer to reduce the risk of side reactions between the positive electrode active material and the electrolyte, thereby helping to improve the cycle stability and safety of the battery. More importantly, the carbon layer is integrated with the carbon material on the surface of the carbon-coated current collector, forming a firm interface, which helps to improve the adhesion between the positive electrode active layer and the carbon-coated current collector, thereby helping to reduce the risk of shedding or pulverization of the positive electrode active layer during charging and discharging, and further helping to improve the long-term stability and life of the battery. Preferably, the ratio of the total mass of each precursor raw material of the positive electrode active material, the carbon source, the dispersant, and the carbonaceous conductive agent to the volume of the solvent is controlled within the above range, which helps to improve the dispersibility of the components and control the average pore size and porosity of the finally formed positive electrode active layer.

[0050] In an embodiment of the present application, the carbon source is selected from any one or more of oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, citric acid, soluble starch, ascorbic acid, sucrose, and glucose; and / or, the dispersant is selected from any one or more of sodium carboxymethyl cellulose, polyacrylic acid, and polycarboxylic acid; and / or, the carbonaceous conductive agent is selected from any one or more of carbon black Super P, conductive graphite, carbon nanotubes, and graphene; and / or, the solvent is selected from any one or more of water, tert-butyl alcohol, and xylene.

[0051] Preferably, the types of the carbon source, the dispersant, and the carbonaceous conductive agent are controlled within the above range, which helps to improve the synergistic effect between the three, thereby helping to form a carbon layer with stable structure. Preferably, the type of the solvent is controlled within the above range, which helps to improve the dispersibility of the components and control the average pore size and porosity of the finally formed positive electrode active layer. Different solvents have different freezing points, and the freezing point can be adjusted by mixing solvents, and the water-based and organic-based dispersion systems can be adapted.

[0052] In another typical embodiment of the present application, a battery is provided, comprising a positive electrode sheet, an electrolyte, and a negative electrode sheet, wherein the positive electrode sheet is the aforementioned positive electrode sheet.

[0053] Since the above battery has the positive electrode sheet of the present application, the battery has high cycle stability and rate capability.

[0054] The beneficial effects of the present application will be further illustrated below with examples.

[0055] Example 1

[0056] Take 28 kg of iron phosphate, sodium carbonate, and phosphoric acid with a molar ratio of Na:Fe:P of 4:3:4, add 1.6 kg of glucose, 0.2 kg of sodium carboxymethyl cellulose, and 0.2 kg of carbon conductive agent carbon black Super P to 10 L of water and stir until uniform, then transfer to a coarse grinder and a sand mill in sequence, coarse grind at 100 rpm for 4 h, and sand grind at 1000 rpm for 6 h to obtain a viscous and uniform precursor slurry. Place a carbon-coated aluminum foil as a carrier film on the surface of the base strip, embed a temperature control device at the bottom of the base strip to lower the temperature of the carbon-coated aluminum foil surface to -20℃, and move at a speed of 1 m / min. Transfer the precursor slurry to the slurry tank of the casting machine, uniformly coat it on the surface of the carbon-coated aluminum foil through a 200 μm doctor blade gap, and then transfer it to a freeze dryer after the slurry is completely frozen. Set the vacuum degree to 10 Pa and the cold trap temperature to -50℃, and dry for 6 h. Place the dried sample in a nitrogen atmosphere, heat it to 500℃ at a rate of 2℃ / min, and calcine it for 10 h to obtain a positive electrode sheet. The thickness of the positive electrode active layer in the positive electrode sheet is 200 μm, the positive electrode active material in the positive electrode active layer is Na4Fe3(PO4)2(P2O7), the thickness of the carbon layer coated on the surface of the positive electrode active material is 3 nm, and the mass ratio of the positive electrode active material to the carbon layer is 1:0.02.

[0057] Example 2

[0058] The difference from Example 1 is that the mass of glucose is 1.9 kg, the mass of sodium carboxymethyl cellulose is 0.1 kg, the mass of carbon conductive agent carbon black Super P is 0.2 kg, and the volume of water is 5 L. The final positive electrode sheet has a carbon layer coated on the surface of the positive electrode active material with a thickness of 5 nm, and the mass ratio of the positive electrode active material to the carbon layer is 1:0.025.

[0059] Example 3

[0060] The difference from Example 1 is that the doctor blade gap is 400 μm, and the final positive electrode sheet has a positive electrode active layer with a thickness of 400 μm.

[0061] Example 4

[0062] The difference from Example 1 is that the moving speed of the carbon-coated aluminum foil is 5 m / min, and the final positive electrode sheet is obtained.

[0063] Example 5

[0064] The difference from Example 1 is that the surface temperature of the carbon-coated aluminum foil is -40℃, and the final positive electrode sheet is obtained.

[0065] Example 6

[0066] The iron phosphate, sodium sulfate were taken 28 kg material according to the molar ratio of Na:Fe:S 2:1:2, which was mixed with 1.6 kg of glucose, 0.2 kg of polyacrylic acid, 0.2 kg of carbon conductive agent carbon black Super P into 10 L water and stirred uniformly, and then transferred into a coarse grinder and a sand mill in sequence, and ground at 100 rpm for 2 h and at 2000 rpm for 4 h respectively to obtain a viscous and uniform precursor slurry. A carbon-coated aluminum foil was placed on the surface of the base tape as a carrier film, and a temperature control device was embedded at the bottom of the base tape to reduce the temperature of the surface of the carbon-coated aluminum foil to-20℃ and move at a speed of 1 m / min. The precursor slurry was transferred to the slurry tank of the casting machine, and then uniformly coated on the surface of the carbon-coated aluminum foil through a 200 μm doctor blade gap. After the slurry was completely frozen, it was transferred into a freeze dryer, the vacuum degree was set to 10 Pa, the cold trap temperature was set to-50℃, and the drying time was 6 h. The dried sample was placed in a nitrogen atmosphere, heated to 350℃ at a rate of 2℃ / min, and calcined for 10 h to obtain a positive electrode sheet. The thickness of the positive electrode active layer in the positive electrode sheet was 200 μm, the positive electrode active material in the positive electrode active layer was Na3Fe2(SO4)3, the thickness of the carbon layer coated on the surface of the positive electrode active material was 3.5 nm, and the mass ratio of the positive electrode active material to the carbon layer was 1:0.022.

[0067] Example 7

[0068] The difference from Example 1 is that the mass of glucose is 1.1 kg, the mass of sodium carboxymethyl cellulose is 0.2 kg, and the mass of carbon conductive agent carbon black Super P is 0.2 kg. Finally, a positive electrode sheet is obtained, the thickness of the carbon layer coated on the surface of the positive electrode active material is 1 nm, and the mass ratio of the positive electrode active material to the carbon layer is 1:0.01.

[0069] Example 8

[0070] The difference from Example 1 is that the mass of glucose is 2.5 kg, the mass of sodium carboxymethyl cellulose is 0.2 kg, and the mass of carbon conductive agent carbon black Super P is 0.2 kg. Finally, a positive electrode sheet is obtained, the thickness of the carbon layer coated on the surface of the positive electrode active material is 10 nm, and the mass ratio of the positive electrode active material to the carbon layer is 1:0.05.

[0071] Example 9

[0072] The difference from Example 1 is that the mass of glucose is 0.8 kg, the mass of sodium carboxymethyl cellulose is 0.2 kg, and the mass of carbon conductive agent carbon black Super P is 0.2 kg. Finally, a positive electrode sheet is obtained, the thickness of the carbon layer coated on the surface of the positive electrode active material is 0.5 nm, and the mass ratio of the positive electrode active material to the carbon layer is 1:0.008.

[0073] Example 10

[0074] The difference from Example 1 is that the temperature increasing rate of the calcination treatment is 2℃ / min, the temperature of the calcination treatment is 600℃, and the holding time of the calcination treatment is 5h, and finally the positive electrode sheet is obtained.

[0075] Example 11

[0076] The difference from Example 1 is that the rotation speed of the rough grinding treatment is 500rpm, the time of the rough grinding treatment is 0.5h, the rotation speed of the sand grinding treatment is 3000rpm, and the time of the sand grinding treatment is 0.5h.

[0077] Example 12

[0078] The difference from Example 1 is that the rotation speed of the rough grinding treatment is 90rpm, the time of the rough grinding treatment is 1h, the rotation speed of the sand grinding treatment is 1500rpm, and the time of the sand grinding treatment is 3h.

[0079] Example 13

[0080] The difference from Example 1 is that the surface temperature of the carbon-coated aluminum foil is -5℃, and finally the positive electrode sheet is obtained.

[0081] Comparative Example 1

[0082] The difference from Example 1 is that the precursor slurry is placed in a 60℃ vacuum drying oven for drying for 24h. The dried sample is placed in a nitrogen atmosphere, and heated to a temperature of 500℃ at a rate of 2℃ / min, and held for 10h to obtain a positive electrode material. The positive electrode material is mixed with N-methyl pyrrolidone (NMP), polyvinylidene fluoride (PVDF), and carbon black Super P in a mass ratio of 47.5:50:1.25:1.25, and the obtained slurry is coated on the surface of a carbon-coated aluminum foil, and placed in an oven for drying to obtain a positive electrode sheet.

[0083] Comparative Example 2

[0084] The difference from Example 6 is that the precursor slurry is placed in a 60℃ vacuum drying oven for drying for 24h. The dried sample is placed in a nitrogen atmosphere, and heated to a temperature of 350℃ at a rate of 2℃ / min, and held for 10h to obtain a positive electrode material. The positive electrode material is mixed with N-methyl pyrrolidone (NMP), polyvinylidene fluoride (PVDF), and carbon black Super P in a mass ratio of 47.5:50:1.25:1.25, and the obtained slurry is coated on the surface of a carbon-coated aluminum foil, and placed in an oven for drying to obtain a positive electrode sheet.

[0085] The positive electrode active layer on the positive electrode sheet obtained in the examples is measured for porosity and average pore size, and the measurement results are shown in Table 1.

[0086] The positive electrode sheet prepared from the examples and the comparative examples was assembled into button cells in a glove box with water oxygen less than 0.01 ppm, and the discharge capacity at different rates was tested at room temperature, and the specific results are shown in Table 1.

[0087] Table 1

[0088]

[0089] As shown in Table 1, the batteries of Examples 1-6 have better rate performance, and the discharge capacity of the examples is significantly higher than that of the comparative examples when the current density reaches 10C. This is because the precursor particles are pushed away and extruded to form pore walls under the action of the ice crystals growing from bottom to top. After freeze-drying and high-temperature sintering, the precursor particles react to form positive electrode materials, and these materials are integrated into dense pore walls after melting. This directional pore structure reduces the penetration path of the electrolyte, improves the wettability of the electrolyte, reduces the ion transmission distance, and improves the rate performance of the material, especially at high rates. The advantages of this directional structure are more obvious.

[0090] Figure 1 The SEM image of the positive electrode active layer in Example 1 of the present application at a low magnification shows that the positive electrode active layer is an ordered pore structure. Figure 1

[0091] The SEM image of the positive electrode active layer in Example 1 of the present application at a high magnification shows that the positive electrode active layer is a pore wall formed by the fusion of positive electrode material particles. Figure 2 Figure 2 From the above description, it can be seen that the above-mentioned examples of the present application achieve the following technical effects:

[0092] From the above description, it can be seen that the above-mentioned examples of the present application achieve the following technical effects:

[0093] ​The positive electrode active layer of the present application is a porous structure arranged in order, the existence of the structure helps to shorten the migration path of ions, thereby helping to promote the infiltration of electrolyte in the pores and the rapid migration of ions. Controlling the average pore size of the positive electrode active layer in the range of 0.2-1 μm and the porosity of the positive electrode active layer in the range of 10-40% helps to improve the migration rate of ions and the infiltration rate of electrolyte while ensuring the stability of the structure of the positive electrode active layer, thereby helping to improve the rate performance and production efficiency of the battery. The existence of the carbon layer in the positive electrode active layer helps to improve the conductivity of the positive electrode active layer, and the carbon layer can act as a physical protective layer to reduce the risk of side reactions between the positive electrode active material and the electrolyte, thereby helping to improve the cycle stability and safety of the battery. More importantly, the carbon layer is fused with the carbon material on the surface of the carbon-coated current collector, forming a firm interface, which helps to improve the bonding force between the positive electrode active layer and the carbon-coated current collector, thereby helping to reduce the risk of shedding or pulverization of the positive electrode active layer during charging and discharging, and further helping to improve the long-term stability and life of the battery. Moreover, the positive electrode active layer of the present application does not contain a binder, which on the one hand helps to further reduce the resistance of ion transmission, and on the other hand helps to reduce pollution to the environment.

[0094] The above is only an embodiment of the present application and is not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode sheet comprising a carbon-coated current collector and a positive electrode active layer, characterized by, The material of the positive electrode active layer comprises a positive electrode active material and a carbon layer coated on the surface of the positive electrode active material, the positive electrode active layer has a porous structure, and the average pore diameter thereof is 0.2-1 μm, and the porosity of the positive electrode active layer is 10-40%; the preparation method of the positive electrode sheet comprises: Step S1, mixing each precursor raw material comprising a positive electrode active material, a carbon source, a dispersing agent, a carbonaceous conductive agent and a solvent, and then performing grinding treatment to obtain a slurry; Step S2, pre-cooling a carbon-coated current collector to the freezing point of the slurry or below, coating the slurry on the carbon-coated current collector, and forming a positive electrode sheet preform after the slurry is solidified; Step S3, sequentially performing freeze-drying and calcination treatment on the positive electrode sheet preform to obtain the positive electrode sheet; wherein the calcination treatment is performed in a protective gas.

2. The positive electrode sheet according to claim 1, characterized by The thickness of the carbon layer is 1-10 nm; and / or, the mass ratio of the positive electrode active material to the carbon layer is 1:0.01-0.

05.

3. The positive electrode sheet according to claim 1 or 2, characterized by, The thickness of the positive electrode active layer is 100-400 μm; and / or, the carbon-coated current collector is a carbon-coated aluminum foil and / or a carbon-coated copper foil; and / or, the positive electrode active material is a polyanion type positive electrode active material.

4. The positive electrode sheet according to claim 3, characterized by The polyanion type positive electrode active material is selected from any one or more of a phosphate type positive electrode active material, a sulfate type positive electrode active material and a silicate type positive electrode active material.

5. A method for producing the positive electrode sheet according to any one of claims 1 to 4, characterized by, The preparation method comprises: Step S1, mixing each precursor raw material comprising a positive electrode active material, a carbon source, a dispersing agent, a carbonaceous conductive agent and a solvent, and then performing grinding treatment to obtain a slurry; Step S2, pre-cooling a carbon-coated current collector to the freezing point of the slurry or below, coating the slurry on the carbon-coated current collector, and forming a positive electrode sheet preform after the slurry is solidified; Step S3, sequentially performing freeze-drying and calcination treatment on the positive electrode sheet preform to obtain the positive electrode sheet; wherein the calcination treatment is performed in a protective gas.

6. The production method according to claim 5, wherein In the step S2, the surface temperature of the carbon-coated current collector is 0℃-50℃.

7. The preparation method according to claim 6, characterized in that, The surface temperature of the carbon-coated current collector is -20℃-40℃.

8. The production method according to any one of claims 5 to 7, characterized by, In the step S2, the vacuum degree of the freeze-drying is ≤10 Pa; and / or, the cold trap temperature of the freeze-drying is ≤-50℃; and / or, the drying time of the freeze-drying is 1-24 h; and / or, the protective gas is selected from any one or more of nitrogen, argon, nitrogen-hydrogen mixed gas and argon-hydrogen mixed gas; and / or, the temperature increasing rate of the calcination treatment is 1-5℃ / min; and / or, the temperature of the calcination treatment is 300-550℃; and / or, the holding time of the calcination treatment is 5-20 h.

9. The production method according to any one of claims 5 to 7, characterized by, In the step S1, the grinding treatment comprises sequentially performing coarse grinding treatment and sand grinding treatment, the rotation speed of the coarse grinding treatment is 100-500 rpm; and / or, the time of the coarse grinding treatment is 0.5-4 h; and / or, the rotation speed of the sand grinding treatment is 1000-3000 rpm; and / or, the time of the sand grinding treatment is 0.5-6 h.

10. The production method according to any one of claims 5 to 7, characterized by, The ratio of the total mass of each precursor raw material of the positive electrode active material, the mass of the carbon source, the mass of the dispersing agent, and the mass of the carbonaceous conductive agent is 28:1.1-2.5:0.1-0.2:0.2; and / or, the ratio of the total mass of each precursor raw material of the positive electrode active material, the carbon source, the dispersing agent, and the carbonaceous conductive agent to the volume of the solvent is 2.95-6.04:1 kg / L.

11. The production method according to any one of claims 5 to 7, characterized by, The carbon source is selected from any one or more of formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, soluble starch, ascorbic acid, sucrose, and glucose; and / or, the dispersing agent is selected from any one or more of sodium carboxymethyl cellulose, polyacrylic acid, and polycarboxylic acid; and / or, the carbonaceous conductive agent is selected from any one or more of carbon black Super P, conductive graphite, carbon nanotubes, and graphene; and / or, the solvent is selected from any one or more of water, tert-butyl alcohol, and cyclohexene.

12. A battery comprising a positive electrode sheet, an electrolyte, and a negative electrode sheet, characterized by The positive electrode sheet is the positive electrode sheet of any one of claims 1-4.

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