Lithium iron phosphate positive electrode sheet, preparation method thereof and lithium ion battery

Abstract

The application provides a lithium iron phosphate positive electrode sheet, a preparation method thereof and a lithium ion battery. The lithium iron phosphate positive electrode sheet comprises a current collector and a positive active layer on the surface of the current collector. The surface of the current collector contains carbon. The material of the positive active layer comprises stacked lithium iron phosphate nanosheets and a carbon layer coated on the surface of the lithium iron phosphate nanosheets. The lithium iron phosphate nanosheets have a (010) plane as a stacking plane. In the lithium iron phosphate positive electrode sheet, the lithium iron phosphate nanosheets are stacked on the current collector along the (010) plane, which shortens the diffusion path of lithium ions, reduces the charge transfer impedance and ion diffusion impedance of the lithium iron phosphate reaction, and further helps to improve the rate performance of the lithium iron phosphate positive electrode sheet. The presence of the carbon layer in the lithium iron phosphate positive electrode sheet helps to improve the electrical conductivity and stability of the positive active layer. On the other hand, the carbon layer and the current collector are connected through a carbon bond, which helps to improve the interaction force between the positive active layer and the current collector.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate cathode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, as one of the most promising new energy sources, are the preferred energy source for electric and hybrid vehicles. Among them, lithium iron phosphate (LiFePO4) materials have been widely used in power batteries due to their advantages such as low raw material cost, stable structure, and high safety. Lithium iron phosphate has a typical olivine structure, possesses a high theoretical capacity (170mAh / g), and a sufficient operating voltage (3.45V vs. LiFePO4). + Lithium iron phosphate (LiPO4) boasts a long cycle life (greater than 2000 cycles). However, it has a significant drawback: slow lithium-ion diffusion rate, resulting in poor electrochemical performance. Within the LiFePO4 lattice, lithium ions diffuse along the b-axis, i.e., the

[010] direction, and intercalate / deintercalate at the interface where LiFePO4 and FePO4 crystal structures coexist. Therefore, due to the anisotropy of the LiFePO4 crystal structure, adjusting the preference of the (010) plane can effectively promote lithium-ion diffusion, thereby improving the rate performance of LiFePO4.

[0003] However, controlling the preference of the (010) plane in the LiFePO4 crystal structure implies that the material has strong anisotropy, often exhibiting a sheet-like or plate-like structure. It is difficult to ensure the same orientation of sheet-like or plate-like lithium iron phosphate materials during slurry preparation and coating. This not only leads to poor processing performance of the electrode sheet during coating (low compaction and rough surface), but also results in the random distribution of lithium iron phosphate nanomaterials in the finished product. The high anisotropy increases the ion diffusion path, leading to poor actual rate performance. Therefore, there is an urgent need to develop a lithium iron phosphate electrode sheet that can guarantee both high compaction and high rate performance. Summary of the Invention

[0004] The main objective of this invention is to provide a lithium iron phosphate cathode sheet, its preparation method, and a lithium-ion battery, in order to solve the problem of low rate performance of lithium iron phosphate cathode sheets in the prior art.

[0005] To achieve the above objectives, according to one aspect of the present invention, a lithium iron phosphate cathode is provided, comprising a current collector and a cathode active layer located on its surface, the surface of the current collector containing carbon, and the material of the cathode active layer comprising stacked lithium iron phosphate nanosheets and a carbon layer coated on the surface of the lithium iron phosphate nanosheets, wherein the lithium iron phosphate nanosheets are stacked with the (010) face as the stacking face.

[0006] The diffusion of lithium ions in lithium iron phosphate mainly occurs along the

[010] direction. In the lithium iron phosphate cathode of this application, lithium iron phosphate nanosheets are stacked on the current collector along the (010) plane, which helps to increase the charge transfer area of ​​the lithium iron phosphate cathode during charging and discharging, shorten the diffusion path of lithium ions, reduce the charge transfer impedance and ion diffusion impedance of the lithium iron phosphate reaction, and thus help to improve the rate performance of the lithium iron phosphate cathode. The presence of a carbon layer in the lithium iron phosphate cathode helps to improve the conductivity and stability of the cathode active layer on the one hand; on the other hand, the carbon layer is connected to the current collector through carbon bonds, which helps to improve the interaction force between the cathode active layer and the current collector.

[0007] Furthermore, the average planar diameter of the above-mentioned lithium iron phosphate nanosheets is 100nm~300nm, and the average thickness is 10nm~30nm.

[0008] The average planar diameter of lithium iron phosphate nanosheets within the aforementioned range helps to provide more lithium-ion reaction sites, thereby contributing to an increase in the lithium-ion insertion / extraction rate. The thickness of lithium iron phosphate nanosheets within the aforementioned range helps to shorten the diffusion path of lithium ions within the material, thus benefiting the rate performance of lithium iron phosphate cathodes.

[0009] Furthermore, the mass ratio of the lithium iron phosphate nanosheets to the carbon layer is 1:0.012~0.070; and / or, the current collector is carbon paper and / or carbon fiber cloth.

[0010] Controlling the mass ratio of lithium iron phosphate nanosheets to carbon layers within the above range helps to improve the cycle stability and rate performance of the positive electrode active layer.

[0011] According to another aspect of the present invention, a method for preparing the aforementioned lithium iron phosphate cathode is provided, the method comprising: step S1, mixing raw materials including a lithium source, an iron source, a phosphorus source, a reducing agent, water and a surfactant to obtain a mixture; step S2, heat-treating a current collector in an oxygen-containing gas to obtain a heat-treated current collector; step S3, placing the heat-treated current collector and the mixture in a reactor for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets, wherein the current collector is placed horizontally at the bottom of the reactor; and step S4, calcining the current collector loaded with lithium iron phosphate nanosheets in a protective gas to obtain a lithium iron phosphate cathode.

[0012] The reducing agent reduces ferric iron to ferrous iron, while the surfactant regulates the morphology and growth direction of lithium iron phosphate to form (010)-faceted stacked lithium iron phosphate nanosheets. The surfactant also serves as a carbon source, and subsequent calcination forms a carbon layer on the surface of the lithium iron phosphate nanosheets. In step S2, heat treatment of the current collector in an oxygen-containing gas increases the number of oxygen-containing functional groups on the current collector surface, thereby improving the adsorption capacity of the surfactant on the lithium iron phosphate nanosheet surface. In step S3, the heat-treated current collector and the mixture are placed in a reactor for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets. Calcination of the current collector loaded with lithium iron phosphate nanosheets converts the surfactant into a carbon layer. The carbon layer is connected to the current collector via carbon bonds, which helps improve the interaction force between the current collector and the positive electrode active layer, as well as the crystallinity of the lithium iron phosphate nanosheets.

[0013] Furthermore, in step S3 above, the temperature of the hydrothermal reaction is 150~300℃; and / or, the time of the hydrothermal reaction is 6~10h.

[0014] Controlling the temperature and time of the hydrothermal reaction within the above range helps to improve the purity and morphological consistency of lithium iron phosphate.

[0015] Furthermore, in step S4 above, the heating rate of calcination is 0.5~10℃ / min; and / or, the calcination temperature is 600~850℃; and / or, the holding time of calcination is 2~6h.

[0016] Controlling the heating rate, temperature, and holding time during calcination within the above-mentioned range helps to improve the crystallinity of lithium iron phosphate and the efficiency of surfactant conversion into carbon layers.

[0017] Furthermore, in step S2 above, the heat treatment temperature is 400~500℃; and / or, the heat treatment time is 2~6h; and / or, the oxygen volume content in the oxygen-containing gas is not less than 21%.

[0018] Controlling the temperature and time of heat treatment within the above range helps to improve the interaction force between the surfactant and the current collector.

[0019] Further, in step S1 above, the volume ratio of water to reducing agent is 1~2:3; and / or, the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.9~1.1):(0.9~1.1):(0.9~1.1); and / or, the mass percentage of water in the raw materials is 20%~40%; and / or, the mass percentage of the surfactant is 2%~10% of the total mass of the lithium source, iron source, and phosphorus source; and / or, the reducing agent is ethylene glycol and / or ethanol; and / or, the surfactant is selected from polyvinyl alcohol and polyvinylpyrrolidine. The surfactant is selected from one or more of ketones and sodium dodecylbenzenesulfonate; preferably, the surfactant is polyvinyl alcohol, and the number average molecular weight of polyvinyl alcohol is 170,000 to 220,000 g / mol; and / or, the lithium source is selected from one or more of lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide and lithium sulfate; and / or, the iron source is selected from one or more of ferrous chloride, ferrous sulfate, ferrous oxalate, ferrous acetate, ferric nitrate and ferric dihydrogen phosphate; and / or, the phosphorus source is selected from one or more of phosphoric acid, ferric dihydrogen phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0020] Controlling the volume ratio of water to reducing agent within the aforementioned range helps improve the formation efficiency of lithium iron phosphate nanosheets. Preferably, controlling the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source within the aforementioned range helps reduce impurity generation. Preferably, controlling the mass percentage of water in the raw materials within the aforementioned range helps control the viscosity of the reaction system, thereby contributing to improved lithium iron phosphate nanosheet formation efficiency. Preferably, controlling the mass percentage of the surfactant relative to the total mass of the lithium, iron, and phosphorus sources within the aforementioned range helps control the mass ratio of lithium iron phosphate nanosheets to the carbon layer within a suitable range, thereby contributing to improved rate performance and cycle stability of the lithium iron phosphate cathode.

[0021] Furthermore, step S3 above also includes washing and drying the product after the hydrothermal reaction in sequence to obtain a current collector loaded with lithium iron phosphate nanosheets; preferably, the drying is carried out in a vacuum drying oven; and / or, the drying temperature is 60~100℃; and / or, the drying time is 3~6h.

[0022] Washing and drying the products after the hydrothermal reaction helps to further reduce the impurity content in lithium iron phosphate nanosheets.

[0023] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode is the aforementioned lithium iron phosphate positive electrode.

[0024] The aforementioned lithium-ion batteries have high rate performance.

[0025] By applying the technical solution of this invention, the presence of lithium iron phosphate in the form of nanosheets helps to shorten the diffusion path of lithium ions within the material, thereby helping to reduce charge transfer resistance and ion diffusion resistance, and thus helping to improve the rate performance of the lithium iron phosphate cathode. The diffusion of lithium ions in lithium iron phosphate mainly occurs along the

[010] direction. In the lithium iron phosphate cathode of this application, lithium iron phosphate nanosheets are stacked along the (010) plane on the current collector, which helps to increase the charge transfer area of ​​the lithium iron phosphate cathode during charging and discharging, shorten the diffusion path of lithium ions, reduce the charge transfer resistance and ion diffusion resistance of the lithium iron phosphate reaction, and thus help to improve the rate performance of the lithium iron phosphate cathode. The presence of a carbon layer in the lithium iron phosphate cathode helps to improve the conductivity and stability of the cathode active layer; on the other hand, the carbon layer is connected to the current collector through carbon bonds, which helps to improve the interaction force between the cathode active layer and the current collector. Traditional lithium iron phosphate cathode sheets contain binders and conductive agents, while the lithium iron phosphate cathode sheet of this application does not contain binders and conductive agents, and there is no need to prepare the cathode material into a slurry for coating, thus avoiding the problem of difficult coating process. Therefore, the production cost of the lithium iron phosphate cathode sheet of this application is lower. Attached Figure Description

[0026] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0027] Figure 1 The charge-discharge curves of the lithium iron phosphate cathode in Example 1 of this application at a rate of 0.2C are shown.

[0028] Figure 2 The diagram shows the cycling performance of the lithium iron phosphate cathode in Example 1 of this application at rates of 0.2C, 0.5C, 1C, and 2C.

[0029] Figure 3 An SEM image of a cross-section of a lithium iron phosphate cathode sheet in Embodiment 1 of this application is shown. Detailed Implementation

[0030] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0031] As analyzed in the background section of this application, the existing lithium iron phosphate cathode has a low rate performance. In order to solve the above problems, this application provides a lithium iron phosphate cathode, its preparation method, and a lithium-ion battery.

[0032] In a typical embodiment of this application, a lithium iron phosphate cathode is provided, comprising a current collector and a cathode active layer located on its surface. The surface of the current collector contains carbon, and the material of the cathode active layer comprises stacked lithium iron phosphate nanosheets and a carbon layer coated on the surface of the lithium iron phosphate nanosheets, wherein the lithium iron phosphate nanosheets are stacked with the (010) face as the stacking face.

[0033] The presence of lithium iron phosphate in the form of nanosheets helps to shorten the diffusion path of lithium ions within the material, thereby reducing charge transfer resistance and ion diffusion resistance, and thus improving the rate performance of the lithium iron phosphate cathode. Lithium ions diffuse mainly along the

[010] direction in lithium iron phosphate. In the lithium iron phosphate cathode of this application, lithium iron phosphate nanosheets are stacked along the (010) plane on the current collector, which helps to increase the charge transfer area of ​​the lithium iron phosphate cathode during charging and discharging, shorten the diffusion path of lithium ions, reduce the charge transfer resistance and ion diffusion resistance of the lithium iron phosphate reaction, and thus help to improve the rate performance of the lithium iron phosphate cathode. The presence of a carbon layer in the lithium iron phosphate cathode helps to improve the conductivity and stability of the cathode active layer; on the other hand, the carbon layer is connected to the current collector through carbon bonds, which helps to improve the interaction force between the cathode active layer and the current collector. Traditional lithium iron phosphate cathode sheets contain binders and conductive agents, while the lithium iron phosphate cathode sheet of this application does not contain binders and conductive agents, and there is no need to prepare the cathode material into a slurry for coating, thus avoiding the problem of difficult coating process. Therefore, the production cost of the lithium iron phosphate cathode sheet of this application is lower.

[0034] In one embodiment of this application, the average planar diameter of the lithium iron phosphate nanosheets is 100nm~300nm, and the average thickness is 10nm~30nm.

[0035] Preferably controlling the average planar diameter of the lithium iron phosphate nanosheets within the aforementioned range helps provide more lithium-ion reaction sites, thereby improving the lithium-ion insertion / extraction rate and consequently increasing the specific capacity of the lithium iron phosphate cathode. Simultaneously, nanosheets within the aforementioned size range help maintain good interlayer contact during stacking, thus contributing to uniform charge distribution and rapid charge transport. Preferably controlling the thickness of the lithium iron phosphate nanosheets within the aforementioned range helps shorten the lithium-ion diffusion path within the material, thereby improving the rate performance of the lithium iron phosphate cathode.

[0036] In one embodiment of this application, the mass ratio of the lithium iron phosphate nanosheets to the carbon layer is 1:0.012~0.070; and / or, the current collector is carbon paper and / or carbon fiber cloth.

[0037] Insufficient carbon layer content is detrimental to improving the conductivity and structural stability of the positive electrode active layer. Excessive carbon layer content is detrimental to improving the specific capacity and energy density of the lithium iron phosphate positive electrode. Preferably, controlling the mass ratio of lithium iron phosphate nanosheets to carbon layer within the aforementioned range helps improve the cycle stability and rate performance of the positive electrode active layer.

[0038] In another typical embodiment of this application, a method for preparing the aforementioned lithium iron phosphate cathode is provided. The method includes: step S1, mixing raw materials including a lithium source, an iron source, a phosphorus source, a reducing agent, water, and a surfactant to obtain a mixture; step S2, heat-treating the current collector in an oxygen-containing gas to obtain a heat-treated current collector; step S3, placing the heat-treated current collector and the mixture in a reactor for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets, wherein the current collector is placed horizontally at the bottom of the reactor; and step S4, calcining the current collector loaded with lithium iron phosphate nanosheets in a protective gas to obtain a lithium iron phosphate cathode.

[0039] In step S1, raw materials including lithium source, iron source, phosphorus source, reducing agent, water, and surfactant are mixed to obtain a mixture. The reducing agent reduces ferric iron to ferrous iron, and the surfactant regulates the morphology and growth direction of lithium iron phosphate to form (010)-faceted stacked lithium iron phosphate nanosheets. The surfactant also serves as a carbon source, which, after calcination, forms a carbon layer on the surface of the lithium iron phosphate nanosheets. The presence of the carbon layer helps improve the conductivity and stability of the positive electrode active layer; furthermore, the carbon layer is connected to the current collector through carbon bonds, which helps improve the interaction force between the positive electrode active layer and the current collector. In step S2, the current collector is heat-treated in an oxygen-containing gas, which helps increase the number of oxygen-containing functional groups on the surface of the current collector, thereby improving the adsorption capacity of surfactants on the surface of lithium iron phosphate nanosheets. In step S3, the heat-treated current collector and the mixture are placed in a reactor for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets. Calcining the current collector loaded with lithium iron phosphate nanosheets converts the surfactant into a carbon layer. The carbon layer is connected to the current collector through carbon bonds, which helps to improve the interaction force between the current collector and the positive electrode active layer and improve the crystallinity of the lithium iron phosphate nanosheets.

[0040] Including but not limited to, the protective gas mentioned above is selected from any one or more of argon, nitrogen and helium.

[0041] In one embodiment of this application, step S1 includes: step S11, adding lithium source, iron source and phosphorus source to a certain amount of water and reducing agent mixed solution and stirring to dissolve, to obtain mixed solution A; step S12, heating and dissolving surfactant in water at 70~90℃ to prepare surfactant aqueous solution, adding it to mixed solution A and stirring evenly to obtain mixed solution B; using mixed solution B as the mixed liquid.

[0042] In one embodiment of this application, in step S3 above, the temperature of the hydrothermal reaction is 150~300℃; and / or, the time of the hydrothermal reaction is 6~10h.

[0043] Insufficient temperature or time in the hydrothermal reaction is detrimental to reducing the impurity content in lithium iron phosphate. During the hydrothermal reaction, the precursor gradually forms lithium iron phosphate nanosheets under the induction of surfactants. Under the mutual attraction between the current collector and the surfactant, the surfactant-coated lithium iron phosphate nanosheets gradually stack on the carbon paper. Preferably, controlling the temperature and time of the hydrothermal reaction within the above-mentioned range helps to improve the purity and morphological consistency of lithium iron phosphate.

[0044] In one embodiment of this application, in step S4 above, the heating rate of calcination is 0.5~10℃ / min; and / or, the calcination temperature is 600~850℃; and / or, the calcination holding time is 2~6h.

[0045] If the calcination temperature is too low or the holding time is too short, it will hinder the improvement of the crystallinity of lithium iron phosphate nanosheets. If the calcination temperature is too high or the holding time is too long, it will hinder the reduction of impurities such as iron phosphide and lithium phosphate. It is preferable to control the heating rate, temperature and holding time of calcination within the above range, which will help improve the crystallinity of lithium iron phosphate and the efficiency of surfactant conversion into carbon layer.

[0046] In order to improve the interaction force between the surfactant and the current collector, in one embodiment of this application, it is preferred that in step S2 above, the heat treatment temperature is 400~500℃; and / or the heat treatment time is 2~6h; and / or the oxygen volume content in the oxygen-containing gas is not less than 21%.

[0047] In one embodiment of this application, in step S1 above, the volume ratio of water to reducing agent is 1~2:3; and / or, the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.9~1.1):(0.9~1.1):(0.9~1.1); and / or, the mass percentage of water in the raw materials is 20%~40%; and / or, the mass percentage of the surfactant is 2%~10% of the total mass of the lithium source, iron source, and phosphorus source; and / or, the reducing agent is ethylene glycol and / or ethanol; and / or, the surfactant is selected from polyvinyl alcohol and polyethylene glycol. The surfactant is selected from one or more of pyrrolidone and sodium dodecylbenzenesulfonate; preferably, the surfactant is polyvinyl alcohol, and the number average molecular weight of polyvinyl alcohol is 170,000 to 220,000 g / mol; and / or, the lithium source is selected from one or more of lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide and lithium sulfate; and / or, the iron source is selected from one or more of ferrous chloride, ferrous sulfate, ferrous oxalate, ferrous acetate, ferric nitrate and ferric dihydrogen phosphate; and / or, the phosphorus source is selected from one or more of phosphoric acid, ferric dihydrogen phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0048] Excessive or insufficient reducing agent content is detrimental to the formation of lithium iron phosphate nanosheets. It is preferable to control the volume ratio of water to reducing agent within the aforementioned range, which helps improve the formation efficiency of lithium iron phosphate nanosheets. Ethylene glycol is preferred as the reducing agent, as its high viscosity helps slow down diffusion and thus inhibit nucleation growth. Controlling the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source within the aforementioned range helps reduce impurity generation. Controlling the mass percentage of water in the raw materials within the aforementioned range helps control the viscosity of the reaction system, thereby improving the formation efficiency of lithium iron phosphate nanosheets. During the reaction, the surfactant adsorbs onto the (010) facet to inhibit the vertical growth of lithium iron phosphate, thereby inducing the formation of lithium iron phosphate nanosheets with (010) faces. During calcination, the surfactant is converted into an inorganic carbon layer and uniformly coats the lithium iron phosphate nanosheets. Too low a surfactant content is detrimental to improving the uniformity of the carbon layer coating, thus hindering the improvement of the conductivity of the positive electrode active layer; too high a surfactant content is detrimental to improving the specific capacity of the material. Preferably, controlling the mass of the surfactant relative to the total mass of the lithium, iron, and phosphorus sources within the aforementioned range helps to control the mass ratio of lithium iron phosphate nanosheets to the carbon layer within a suitable range, thereby contributing to improving the rate performance and cycle stability of the lithium iron phosphate positive electrode. Polyvinyl alcohol is preferably used as the surfactant, and controlling the number-average molecular weight of polyvinyl alcohol within the aforementioned range helps to generate more lithium iron phosphate nanosheets with (010) faces, thus contributing to improving the rate performance of the lithium iron phosphate positive electrode. Controlling the types of lithium, iron, and phosphorus sources within the aforementioned range helps to improve the synergistic effect between the components, thereby further contributing to improving the rate performance of the lithium iron phosphate positive electrode.

[0049] To further reduce the impurity content in lithium iron phosphate nanosheets and thereby improve the rate performance of lithium iron phosphate cathode sheets, in one embodiment of this application, step S3 preferably further includes washing and drying the product after hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets, preferably the drying is carried out in a vacuum drying oven; and / or the drying temperature is 60~100℃; and / or the drying time is 3~6h.

[0050] In another typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode is the aforementioned lithium iron phosphate positive electrode.

[0051] Because the lithium-ion battery contains the lithium iron phosphate cathode material of this application, the lithium-ion battery has a high rate performance.

[0052] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0053] Example 1

[0054] (1) Lithium hydroxide, ferrous chloride and ammonium dihydrogen phosphate were added to a mixed solution of water and ethylene glycol in a Li:Fe:P molar ratio of 1:1:1 and stirred to dissolve, thus obtaining solution A;

[0055] (2) Polyvinyl alcohol is heated and dissolved in water to make a polyvinyl alcohol aqueous solution. It is added to solution A and stirred evenly to obtain solution B. Polyvinyl alcohol accounts for 5% of the total mass of lithium hydroxide, ferrous chloride and ammonium dihydrogen phosphate. The number average molecular weight of polyvinyl alcohol is 170000 g / mol. The mass ratio of water in solution B is 40%, and the volume ratio of water to ethylene glycol is 2:3.

[0056] (3) Heat-treat the carbon paper at 500°C for 2 hours in an air atmosphere to obtain heat-treated carbon paper;

[0057] (4) The heat-treated carbon paper and solution B are transferred together into a sealed reactor for hydrothermal reaction, wherein the carbon paper is placed in the inner lining of the reactor and reacted at 200°C for 8 hours.

[0058] (5) After cooling, collect the carbon paper loaded with lithium iron phosphate nanosheets at the bottom, wash it with deionized water and put it into a vacuum drying oven for drying. The temperature of the vacuum drying oven is set to 70°C and the drying time is 4 hours.

[0059] (6) The dried carbon paper loaded with lithium iron phosphate nanosheets was calcined at 650°C for 4 hours under a nitrogen atmosphere with a heating rate of 5°C / min to obtain a lithium iron phosphate cathode. The mass ratio of lithium iron phosphate nanosheets to carbon layer in the lithium iron phosphate cathode was 1:0.03. The average planar diameter of the lithium iron phosphate nanosheets was 154 nm and the average thickness was 19 nm.

[0060] Example 2

[0061] The difference from Example 1 is that the hydrothermal reaction temperature is 150°C and the hydrothermal reaction time is 10 hours, ultimately yielding a lithium iron phosphate cathode sheet.

[0062] Example 3

[0063] The difference from Example 1 is that the hydrothermal reaction temperature is 300°C and the hydrothermal reaction time is 6 hours, ultimately yielding a lithium iron phosphate cathode sheet.

[0064] Example 4

[0065] The difference from Example 1 is that the hydrothermal reaction temperature is 350°C and the hydrothermal reaction time is 5 hours, ultimately yielding a lithium iron phosphate cathode sheet.

[0066] Example 5

[0067] The difference from Example 1 is that the calcination heating rate is 0.5℃ / min, the calcination temperature is 850℃, and the calcination holding time is 2h, finally obtaining a lithium iron phosphate cathode sheet, wherein the average planar diameter of the lithium iron phosphate nanosheet is 253nm and the average thickness is 27nm.

[0068] Example 6

[0069] The difference from Example 1 is that the calcination heating rate is 10℃ / min, the calcination temperature is 600℃, and the calcination holding time is 6h, finally obtaining a lithium iron phosphate cathode sheet, wherein the average planar diameter of the lithium iron phosphate nanosheet is 121nm and the average thickness is 13nm.

[0070] Example 7

[0071] The difference from Example 1 is that the calcination heating rate is 15℃ / min, the calcination temperature is 900℃, and the calcination holding time is 1h, finally obtaining a lithium iron phosphate cathode sheet, wherein the average planar diameter of the lithium iron phosphate nanosheet is 306nm and the average thickness is 32nm.

[0072] Example 8

[0073] The difference from Example 1 is that the number average molecular weight of polyvinyl alcohol is 220,000 g / mol, and lithium iron phosphate cathode is finally obtained.

[0074] Example 9

[0075] The difference from Example 1 is that the number average molecular weight of polyvinyl alcohol is 350,000 g / mol, and lithium iron phosphate cathode is finally obtained.

[0076] Example 10

[0077] The difference from Example 1 is that polyvinyl alcohol accounts for 10% of the total mass of lithium hydroxide, ferrous chloride and ammonium dihydrogen phosphate, and finally a lithium iron phosphate cathode is obtained. The mass ratio of lithium iron phosphate nanosheets to carbon layer in the lithium iron phosphate cathode is 1:0.07.

[0078] Example 11

[0079] The difference from Example 1 is that polyvinyl alcohol accounts for 15% of the total mass of lithium hydroxide, ferrous chloride and ammonium dihydrogen phosphate, and finally a lithium iron phosphate cathode is obtained. The mass ratio of lithium iron phosphate nanosheets to carbon layer in the lithium iron phosphate cathode is 1:0.10.

[0080] Example 12

[0081] The difference from Example 1 is that (1) lithium nitrate, ferrous oxalate and diammonium hydrogen phosphate were added to a mixed solution of water and ethylene glycol in a Li:Fe:P molar ratio of 1.1:0.9:1.1, stirred and dissolved to obtain solution A;

[0082] (2) Polyvinyl alcohol is heated and dissolved in water to make a polyvinyl alcohol aqueous solution. It is added to solution A and stirred evenly to obtain solution B. Polyvinyl alcohol accounts for 5% of the total mass of lithium nitrate, ferrous oxalate and diammonium hydrogen phosphate. The number average molecular weight of polyvinyl alcohol is 170000 g / mol. The mass ratio of water in solution B is 20%, and the volume ratio of water to ethylene glycol is 1:3.

[0083] (3) Heat-treat the carbon paper at 500°C for 2 hours in an air atmosphere to obtain heat-treated carbon paper;

[0084] (4) The heat-treated carbon paper and solution B are transferred together into a sealed reactor for hydrothermal reaction, wherein the carbon paper is placed in the inner lining of the reactor and reacted at 200°C for 8 hours.

[0085] (5) After cooling, collect the carbon paper loaded with lithium iron phosphate nanosheets at the bottom, wash it with deionized water and put it into a vacuum drying oven for drying. The temperature of the vacuum drying oven is set to 60°C and the drying time is 6 hours.

[0086] (6) The dried carbon paper loaded with lithium iron phosphate nanosheets was calcined at 650°C for 4 hours under a nitrogen atmosphere with a heating rate of 5°C / min to obtain lithium iron phosphate cathode sheet.

[0087] Example 13

[0088] The difference from Example 1 is that (1) lithium carbonate, ferrous oxalate and phosphoric acid are added to a mixed solution of water and ethylene glycol in a Li:Fe:P molar ratio of 0.9:1.1:0.9, and stirred to dissolve, to obtain solution A;

[0089] (2) Polyvinyl alcohol is heated and dissolved in water to make a polyvinyl alcohol aqueous solution. It is added to solution A and stirred evenly to obtain solution B. Polyvinyl alcohol accounts for 2% of the total mass of lithium carbonate, ferrous oxalate and phosphoric acid. The number average molecular weight of polyvinyl alcohol is 170000 g / mol. The mass ratio of water in solution B is 40%, and the volume ratio of water to ethylene glycol is 2:3.

[0090] (3) Heat-treat the carbon paper at 500°C for 2 hours in an air atmosphere to obtain heat-treated carbon paper;

[0091] (4) The heat-treated carbon paper and solution B are transferred together into a sealed reactor for hydrothermal reaction, wherein the carbon paper is placed in the inner lining of the reactor and reacted at 200°C for 8 hours.

[0092] (5) After cooling, collect the carbon paper loaded with lithium iron phosphate nanosheets at the bottom, wash it with deionized water and put it into a vacuum drying oven for drying. The temperature of the vacuum drying oven is set to 100°C and the drying time is 3 hours.

[0093] (6) The dried carbon paper loaded with lithium iron phosphate nanosheets was calcined at 650°C for 4 hours under a nitrogen atmosphere with a heating rate of 5°C / min to obtain lithium iron phosphate cathode sheet.

[0094] Comparative Example 1

[0095] The difference from Example 1 is that carbon paper is not added in step (4). To obtain the final cathode material, polyvinylidene fluoride (PVDF) is first dissolved in N-methylpyrrolidone solvent (NMP), wherein the mass ratio of PVDF is 5%. The cathode material, PVDF, and carbon black SuperP are mixed in a mass ratio of 8:1:1 and coated onto the surface of carbon paper. The mixture is then dried in an oven to obtain lithium iron phosphate cathode sheets.

[0096] Comparative Example 2

[0097] The difference from Example 1 is that polyvinyl alcohol is not added in step (2) and carbon paper is not added in step (4). To obtain the final cathode material, polyvinylidene fluoride (PVDF) is first dissolved in N-methylpyrrolidone solvent (NMP), wherein the mass ratio of PVDF is 5%. The cathode material, PVDF, and carbon black SuperP are mixed in a mass ratio of 8:1:1 and the resulting slurry is coated on the surface of carbon paper and dried in an oven to obtain lithium iron phosphate cathode sheet.

[0098] Performance testing

[0099] The lithium iron phosphate cathode and lithium sheet prepared in the examples and comparative examples were used as counter electrodes and reference electrodes, and 1 mol / L lithium hexafluorophosphate electrolyte were used to assemble coin cells. The assembled coin cells were subjected to constant current charge-discharge tests at rates of 0.2C, 0.5C, 1C, and 2C. The initial discharge specific capacity at different rates was recorded. The charge-discharge window was set to 2.5V~4.0V. The test results are shown in Table 1.

[0100] Table 1

[0101]

[0102] Figure 1 This is a charge-discharge curve of the lithium iron phosphate cathode in Example 1 of this application at a rate of 0.2C. Figure 1 It can be seen that there is a flat charge-discharge plateau, no side reactions are observed, and the polarization of the reaction is small.

[0103] Figure 2 This is a cycle performance diagram of the lithium iron phosphate cathode in Example 1 of this application at rates of 0.2C, 0.5C, 1C, and 2C. The lithium iron phosphate cathode was subjected to three charge-discharge cycles sequentially at rates of 0.2C, 0.5C, 1C, and 2C. Figure 2 The average discharge capacity at 0.2C is 158.2 mAh / g, at 0.5C it is 152.4 mAh / g, at 1C it is 145.5 mAh / g, and at 2C it is 135.1 mAh / g, demonstrating good rate performance.

[0104] Figure 3 This is a SEM image of the cross-section of the lithium iron phosphate cathode sheet in Example 1 of this application. Figure 3 It can be seen that lithium iron phosphate nanosheets grow with the (010) facet as the preferred facet.

[0105] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0106] The presence of lithium iron phosphate in the form of nanosheets helps to shorten the diffusion path of lithium ions within the material, thereby reducing charge transfer resistance and ion diffusion resistance, and thus improving the rate performance of the lithium iron phosphate cathode. Lithium ions diffuse mainly along the

[010] direction in lithium iron phosphate. In the lithium iron phosphate cathode of this application, lithium iron phosphate nanosheets are stacked along the (010) plane on the current collector, which helps to increase the charge transfer area of ​​the lithium iron phosphate cathode during charging and discharging, shorten the diffusion path of lithium ions, reduce the charge transfer resistance and ion diffusion resistance of the lithium iron phosphate reaction, and thus help to improve the rate performance of the lithium iron phosphate cathode. The presence of a carbon layer in the lithium iron phosphate cathode helps to improve the conductivity and stability of the cathode active layer; on the other hand, the carbon layer is connected to the current collector through carbon bonds, which helps to improve the interaction force between the cathode active layer and the current collector. Traditional lithium iron phosphate cathode sheets contain binders and conductive agents, while the lithium iron phosphate cathode sheet of this application does not contain binders and conductive agents, and there is no need to prepare the cathode material into a slurry for coating, thus avoiding the problem of difficult coating process. Therefore, the production cost of the lithium iron phosphate cathode sheet of this application is lower.

[0107] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode sheet, comprising a current collector and a positive electrode active layer located on its surface, characterized in that, The surface of the current collector contains carbon, and the material of the positive electrode active layer includes stacked lithium iron phosphate nanosheets and a carbon layer coating the surface of the lithium iron phosphate nanosheets, wherein the lithium iron phosphate nanosheets have the (010) face as the stacking face; the preparation method of the lithium iron phosphate positive electrode includes: Step S1: Mix the raw materials including lithium source, iron source, phosphorus source, reducing agent, water and surfactant to obtain a mixture; Step S2: The current collector is heat-treated in an oxygen-containing gas to obtain a heat-treated current collector. Step S3: The heat-treated current collector and the mixture are placed in a reaction vessel for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets, wherein the current collector is placed flat at the bottom of the reaction vessel; Step S4: The current collector loaded with lithium iron phosphate nanosheets is calcined in a protective gas to obtain the lithium iron phosphate cathode.

2. The lithium iron phosphate cathode sheet according to claim 1, characterized in that, The lithium iron phosphate nanosheets have an average planar diameter of 100nm~300nm and an average thickness of 10nm~30nm.

3. The lithium iron phosphate cathode sheet according to claim 1 or 2, characterized in that, The mass ratio of the lithium iron phosphate nanosheets to the carbon layer is 1:0.012~0.070; and / or, the current collector is carbon paper and / or carbon fiber cloth.

4. A method for preparing a lithium iron phosphate cathode sheet according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1: Mix the raw materials including lithium source, iron source, phosphorus source, reducing agent, water and surfactant to obtain a mixture; Step S2: The current collector is heat-treated in an oxygen-containing gas to obtain a heat-treated current collector. Step S3: The heat-treated current collector and the mixture are placed in a reaction vessel for hydrothermal reaction to obtain a current collector loaded with lithium iron phosphate nanosheets, wherein the current collector is placed flat at the bottom of the reaction vessel; Step S4: The current collector loaded with lithium iron phosphate nanosheets is calcined in a protective gas to obtain the lithium iron phosphate cathode.

5. The preparation method according to claim 4, characterized in that, In step S3, the temperature of the hydrothermal reaction is 150~300℃; and / or, the time of the hydrothermal reaction is 6~10h.

6. The preparation method according to claim 4 or 5, characterized in that, In step S4, the heating rate of calcination is 0.5~10℃ / min; and / or, the calcination temperature is 600~850℃; and / or, the calcination holding time is 2~6h.

7. The preparation method according to claim 4 or 5, characterized in that, In step S2, the temperature of the heat treatment is 400~500℃; and / or, the time of the heat treatment is 2~6h; and / or, the volume content of oxygen in the oxygen-containing gas is not less than 21%.

8. The preparation method according to claim 4 or 5, characterized in that, In step S1, the volume ratio of water to reducing agent is 1~2:3; And / or, the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.9~1.1):(0.9~1.1):(0.9~1.1). And / or, the water content in the raw material is 20% to 40% by mass; And / or, the surfactant accounts for 2% to 10% of the total mass of the lithium source, the iron source, and the phosphorus source; And / or, the reducing agent is ethylene glycol and / or ethanol; And / or, the surfactant is selected from any one or more of polyvinyl alcohol, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate; And / or, the lithium source is selected from any one or more of lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide, and lithium sulfate; And / or, the iron source is selected from any one or more of ferrous chloride, ferrous sulfate, ferrous oxalate, ferrous acetate, ferric nitrate, and ferric dihydrogen phosphate; And / or, the phosphorus source is selected from any one or more of phosphoric acid, ferric dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

9. The preparation method according to claim 8, characterized in that, The surfactant is polyvinyl alcohol, and the number average molecular weight of the polyvinyl alcohol is 170,000 to 220,000 g / mol.

10. The preparation method according to claim 4 or 5, characterized in that, Step S3 further includes washing and drying the product after the hydrothermal reaction to obtain the current collector loaded with lithium iron phosphate nanosheets.

11. The preparation method according to claim 10, characterized in that, The drying is carried out in a vacuum drying oven; and / or the drying temperature is 60~100℃; and / or the drying time is 3~6h.

12. A lithium-ion battery, comprising a positive electrode, an electrolyte, and a negative electrode, characterized in that, The positive electrode is a lithium iron phosphate positive electrode as described in any one of claims 1 to 3.

Images