Method for synthesizing cathode material by supercritical hydrothermal synthesis, cathode material and lithium ion battery

Abstract

The present invention discloses a method for synthesizing a cathode material by supercritical hydrothermal synthesis, a cathode material and a lithium-ion battery. The method includes: injecting water into a reactor and heating it, adjusting the pressure and temperature in the reactor, and after the supercritical reaction conditions are reached in the reactor, adding raw materials of the cathode material and introducing CO2 gas into the reactor to carry out the reaction to obtain the cathode material. The present invention uses the supercritical hydrothermal method to synthesize the cathode material, and can obtain a cathode material with a uniform primary particle size distribution and a small particle size. The primary particle size D 50 can be as small as within 100 nm, the particle size distribution is extremely narrow, and the prepared material exhibits excellent rate performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy materials, and relates to a method for synthesizing a cathode material by supercritical hydrothermal synthesis, a cathode material and a lithium ion battery, and particularly relates to a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium iron manganese phosphate, lithium iron manganese phosphate and a lithium ion battery. Background Art

[0002] At present, the commercially available cathode materials for lithium ion batteries mainly include lithium cobaltate, lithium manganate, lithium iron phosphate and ternary materials, etc. Among them, the polyanionic type LiMPO4 (M = Fe, Mn) materials with olivine structure are widely concerned because of their stable structure and excellent cycling performance during the process of lithium deintercalation and intercalation. In particular, the LiFePO4 material is currently widely used in power batteries. LiMnPO4 has a higher discharge platform (~3.8V) than LiFePO4, and its theoretical specific capacity is about 20% higher than that of LiFePO4, which is expected to improve the energy density of the material. However, its electrochemical performance fails to reach the ideal level due to its very poor electronic conductivity and lithium ion diffusion ability. The lithium iron manganese phosphate (LiMn x Fe 1-x PO4, 0 < x < 1) material formed by doping manganese into LiFePO4 can combine the advantages of both and meet the requirements of higher-performance power batteries.

[0003] The synthesis methods for lithium ion battery cathode materials mainly include high-temperature solid-phase method, sol-gel method, hydrothermal method, solvothermal method, co-precipitation method and microwave-assisted heating synthesis. These methods have the disadvantages of complex synthesis steps, long reaction time, too large particle size, poor size uniformity and poor stability. In addition, reducing the particle size can shorten the diffusion path of lithium ions and thus improve the electrochemical performance of the material, and using a mixture of small-sized particles and large-sized particle materials is an effective method to improve the tap density of the material and increase the energy density of the material. It is difficult to synthesize a cathode material with uniform morphology and ultrafine size by the above various synthesis methods. Therefore, it is very necessary to use an effective method to prepare a nano cathode particle material with uniform size and small particle size (particle size less than 100nm).

[0004] Supercritical fluid is a fluid with temperature and pressure both higher than the critical point. Its viscosity and diffusion coefficient are close to those of gases, while its density and solvation ability are close to those of liquids. The supercritical hydrothermal synthesis technology uses supercritical fluid as the reaction medium and undergoes hydrothermal reactions under supercritical conditions. It has been used in the synthesis of materials such as metal oxides and multi-component compounds, and has also been used in the synthesis of phosphate-based cathode materials. For example, some Chinese patents (CN102569800A, CN 112573499A, CN 102790216A) have used the supercritical hydrothermal method to prepare some nano LiFePO4 with relatively small sizes, but these particles are all larger than 100 nm. Currently, it is still difficult to prepare ultra-fine nano phosphate cathode materials with uniform morphology and size. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the purpose of the present invention is to provide a method for preparing cathode materials by supercritical solvothermal method, a cathode material and a lithium-ion battery. In particular, it is to provide a method for preparing uniform ultra-fine spherical nano lithium iron manganese phosphate, lithium iron manganese phosphate and a lithium-ion battery by supercritical hydrothermal synthesis.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a method for synthesizing a cathode material by supercritical hydrothermal method, and the method includes the following steps:

[0008] Inject water into the reactor and heat it, adjust the pressure in the reactor. When the supercritical point of water (such as 375 °C, 23 MPa) is reached in the reactor, or the water in the reactor reaches the supercritical state, add the raw materials of the cathode material into the reactor and introduce CO2 gas, and carry out the reaction to obtain the cathode material.

[0009] The present invention uses CO2-assisted supercritical hydrothermal method to synthesize cathode materials. Compared with pure supercritical water or supercritical carbon dioxide alone, this method not only combines the advantages of both supercritical water and CO2, but also CO2 and water produce a synergistic effect, making the reaction more efficient and the particle size control more uniform. During the reaction process, CO2 can not only prevent the particles from growing and aggregating too large, but also regulate the particle size by controlling the flow rate of CO2. In addition, CO2 and water produce a synergistic effect, thereby obtaining a cathode material with a uniform primary particle size distribution and a small particle size. The primary particle size D 50 can be as small as within 100 nm, and the diffusion path of Li + is smaller. At the same time, the particle size distribution is extremely narrow, and the difference between D 10 and D 90 does not exceed 200 nm. The combined effect of the above factors makes the lithium-ion battery prepared from the synthesized small-size materials exhibit better rate performance.

[0010] In one embodiment, the supercritical reaction is a continuous reaction, rather than a batch production by the one-pot method.

[0011] The preparation method of the present invention has a simple process, low cost, can effectively control the particle size of the product, has little impact on the environment, and is suitable for industrial production.

[0012] As a preferred technical solution of the method of the present invention, the heating temperature is 350 - 500 °C, such as 350 °C, 370 °C, 400 °C, 425 °C, 450 °C, 475 °C or 500 °C, etc., preferably 375 - 400 °C. The heating temperature affects the state of the supercritical fluid and the morphology, particle size and purity of the final product. If the heating temperature is too low, the solvent water cannot form a supercritical state, and the product particle size is too large; if the heating temperature is too high, the supercritical water seriously corrodes the reactor, requires higher equipment requirements, and in addition, the reaction is too fast, which is not conducive to the nucleation and growth of the material, the particle size distribution becomes wider, and other side reactions will occur and impurity phases will be generated.

[0013] Preferably, the pressure in the reactor is 23 - 30 MPa, such as 23 MPa, 25 MPa, 26 MPa, 28 MPa, 29 MPa or 30 MPa, etc., preferably 24 - 28 MPa. The pressure in the reactor affects the supercritical state and the morphology and size of the product. If the pressure is too small, the supercritical state cannot be formed, and the product particle size is too large; if the pressure is too large, the supercritical water seriously corrodes the reactor, requires higher equipment requirements, and the product particle size will also be too small.

[0014] Preferably, the flow rate of the CO2 gas introduced is 1 - 100 mL / min, such as 1 mL / min, 3 mL / min, 5 mL / min, 7 mL / min, 10 mL / min, 12 mL / min, 15 mL / min, 18 mL / min, 20 mL / min, 25 mL / min, 30 mL / min, 35 mL / min, 40 mL / min, 45 mL / min, 50 mL / min, 55 mL / min, 60 mL / min, 65 mL / min, 70 mL / min, 75 mL / min, 80 mL / min, 85 mL / min, 90 mL / min, 95 mL / min or 100 mL / min, etc., preferably 10 - 50 mL / min. The flow rate of the CO2 gas affects the state of the reaction system and the reaction time, and ultimately affects the material morphology and size. If the flow rate is too small, the material particles will increase; if the flow rate is too large, the temperature in the reaction system will drop too much, and the conditions of supercritical water cannot be achieved.

[0015] Preferably, the raw materials of the positive electrode material are added to the reactor in the form of a solution. In the solution system in the reactor, the concentration of the metal salt is 0.01 mol / L to 2 mol / L, such as 0.01 mol / L, 0.03 mol / L, 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.5 mol / L, 1.7 mol / L or 2 mol / L, etc. If the concentration of the metal salt is too low, the crystal nucleation will be slow, the particle size will be too small, and the yield will also be low; if the concentration of the metal salt is too high, it will lead to difficult dissolution, insufficient reaction, too fast crystal growth and nucleation, too large particle size, and may also produce impurity phases and may cause pipeline blockage. Preferably, it is 0.1 to 0.5 mol / L.

[0016] Preferably, the raw materials of the positive electrode material are divided into a lithium source and other raw materials.

[0017] The present invention does not limit the specific type of the lithium source, including but not limited to at least one of lithium hydroxide, lithium oxalate and lithium acetate.

[0018] Preferably, the other raw materials and the lithium source are respectively formulated into a first raw material solution and a second raw material solution, and are fed into the reactor in parallel.

[0019] Preferably, the feeding flow rates of the first raw material solution and the second raw material solution are 10 to 30 mL / min, such as 10 mL / min, 15 mL / min, 18 mL / min, 20 mL / min, 25 mL / min or 30 mL / min, etc.

[0020] Preferably, the feeding flow rates of the first raw material solution and the second raw material solution are equal.

[0021] In one embodiment, when the molar feed per unit time of the first raw material solution and the second raw material solution satisfies Li:(Fe + Mn)=3:1, the feeding flow rates of the first raw material solution and the second raw material solution are equal, and the feeding speed is 10 mL / min to 30 mL / min, such as 10 mL / min, 12 mL / min, 15 mL / min, 18 mL / min, 20 mL / min, 22 mL / min, 25 mL / min, 28 mL / min or 30 mL / min, etc.

[0022] Preferably, the other raw materials include an iron source, a manganese source and a phosphorus source, and preferably also include an antioxidant. The antioxidant can prevent some ions that are easily oxidized (such as Fe 2+ ) from being oxidized.

[0023] The present invention does not limit the specific types of iron source, manganese source and phosphorus source. Exemplarily, the iron source can be at least one of ferrous sulfate, ferrous acetate and ferrous nitrate; the manganese source can be at least one of manganese acetate, manganese sulfate, manganese nitrate and manganese chloride; the phosphorus source can be at least one of phosphoric acid, ammonium dihydrogen phosphate and ammonium phosphate.

[0024] Preferably, the antioxidant is a carbon-containing antioxidant. On the one hand, such an antioxidant plays an antioxidant role, and on the other hand, it can serve as a carbon source to ultimately form a carbon layer coating the cathode material, inhibiting excessive growth and agglomeration of particles, and forming a cathode material@carbon composite material.

[0025] Preferably, the antioxidant includes at least one of citric acid, sucrose and ascorbic acid, and is preferably ascorbic acid.

[0026] Preferably, the reaction time is 2 to 300 s, such as 2 s, 5 s, 7 s, 10 s, 12 s, 16 s, 20 s, 25 s, 30 s, 35 s, 40 s, 45 s, 50 s, 55 s, 60 s, 65 s, 70 s, 75 s, 80 s, 85 s, 90 s, 95 s, 100 s, 110 s, 115 s, 120 s, 130 s, 135 s, 140 s, 150 s, 155 s, 160 s, 170 s, 180 s, 190 s, 200 s, 215 s, 230 s, 245 s, 260 s, 270 s, 280 s or 300 s, etc., and is preferably 5 s to 200 s.

[0027] Preferably, the method further includes a step of sintering the reaction product.

[0028] In one embodiment, the reaction product is obtained by filtering and drying the reaction slurry.

[0029] The present invention does not specifically limit the drying method. For example, it can be vacuum drying. The temperature of vacuum drying can be 60 to 80 °C, such as 60 °C, 65 °C, 70 °C, 75 °C or 80 °C, etc. The time of vacuum drying can be 8 to 20 h, such as 8 h, 9 h, 10 h, 12 h, 13 h, 15 h, 16 h, 18 h or 20 h, etc.

[0030] Preferably, the sintering atmosphere is an inert atmosphere.

[0031] Preferably, the gas in the inert atmosphere includes at least one of nitrogen, argon or helium, and is preferably nitrogen.

[0032] Preferably, the heating rate of the sintering is 1 to 5 °C / min, such as 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min or 5 °C / min, etc.

[0033] Preferably, the sintering temperature is 300 to 700 °C, such as 300 °C, 325 °C, 350 °C, 370 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C or 700 °C, etc.

[0034] Preferably, the sintering time is 2 to 10 h, such as 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, etc.

[0035] The powder obtained before sintering is preliminary lithium iron manganese phosphate, and sintering helps to further improve its crystal crystallinity.

[0036] As a preferred technical solution of the method of the present invention, the method includes the following steps:

[0037] Step 1: Dissolve an iron source, a manganese source, a phosphorus source and an antioxidant in deoxygenated deionized water and mix evenly to obtain a first raw material liquid;

[0038] Prepare the Li salt into a separate aqueous solution to obtain a second raw material liquid;

[0039] Among them, the molar ratio of Li:P:Mn:Fe is 3:1:x:1-x, x is 0.2 to 0.8, and the solution temperature is 25 to 60 °C;

[0040] Step 2: First, introduce pure water into the tubular reactor, and adjust the temperature in the reactor to 350 to 500 °C and the pressure to 23 to 30 MPa through a high-pressure pump and a heater; then turn on the circulating water cooling system to cool the outlet pipeline so that the outlet material remains in a liquid phase state;

[0041] Step 3: After the temperature and pressure in the reactor reach the supercritical point of water, pump the first raw material liquid and the second raw material liquid into the tubular reactor at the same flow rate through a high-pressure pump for reaction, and at the same time pump CO2 gas into the tubular reactor. The reaction starts, and the raw materials stay in the reaction tube for 2 to 300 s. The fluid after the reaction is collected after passing through a cooling device and a pressure relief device for pressure relief;

[0042] Step 4: Filter the collected slurry and dry it in a vacuum drying oven at 60 to 80 °C for 8 to 20 h to obtain preliminary lithium iron manganese phosphate powder;

[0043] Step 5: Grind the above-mentioned preliminary lithium iron manganese phosphate powder and sinter it under an inert atmosphere. The heating rate is 1 to 5 °C / min, the sintering temperature is 300 to 700 °C, and the sintering time is 2 to 10 h, and finally a lithium iron manganese phosphate material with higher crystallinity is obtained.

[0044] In a second aspect, the present invention provides a positive electrode material prepared by the method as described in the first aspect. The particle size D of the positive electrode material 50Within 100 nm (such as 100 nm, 95 nm, 90 nm, 85 nm, 80 nm, 75 nm, 70 nm, 65 nm, 60 nm or 50 nm, etc.), D 10 and D 90 differ by no more than 200 nm (the difference is, for example, 200 nm, 180 nm, 150 nm, 140 nm, 120 nm, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm, 50 nm or 40 nm, etc.).

[0045] Preferably, the positive electrode material is lithium iron manganese phosphate.

[0046] Preferably, the surface of the positive electrode material is coated with a carbon layer.

[0047] By using the method of the present invention, ultrafine positive electrode materials can be prepared, and the particle size distribution is extremely narrow. When applied to batteries, it exhibits excellent rate performance.

[0048] In a third aspect, the present invention provides a lithium-ion battery. The positive electrode of the lithium-ion battery includes a current collector and a positive electrode active layer combined on the current collector. The positive electrode material in the positive electrode active layer includes the positive electrode material described in the second aspect or the positive electrode material prepared by the method described in the first aspect.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] (1) The present invention uses a supercritical hydrothermal method to synthesize the positive electrode material. CO2 can not only prevent the particles from growing and aggregating too large, but also regulate the particle size by controlling the flow rate of CO2. In addition, CO2 and water produce a synergistic effect, thereby obtaining a positive electrode material with a uniform primary particle size distribution and a small particle size. The primary particle size D 50 can be as small as within 100 nm, and the diffusion path of Li + is smaller. At the same time, the particle size distribution is extremely narrow, and D 10 and D 90 differ by no more than 200 nm. The combined effect of the above factors makes the lithium-ion battery prepared from the synthesized small-size material exhibit better rate performance.

[0051] (2) The preparation method of the present invention has a simple process, low cost, can effectively control the product particle size, has little impact on the environment, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a schematic flow chart of preparing ultrafine nano lithium iron manganese phosphate by the supercritical continuous hydrothermal method in the present invention.

[0053] Figure 2 is the ultrafine lithium iron manganese phosphate LiMn 0.7 Fe0.3 X-ray diffraction pattern of the PO4 material. The lower spectrum is the standard card (PDF-#13-0336).

[0054] Figure 3 is the ultrafine lithium manganese iron phosphate LiMn 0.7 Fe 0.3 transmission electron microscope image of the PO4 material.

[0055] Figure 4 is the charge-discharge curve of the lithium-ion secondary battery prepared with the ultrafine lithium manganese iron phosphate material in Example 1.

[0056] Figure 5 is the rate performance of the lithium-ion secondary batteries prepared with the ultrafine lithium manganese iron phosphate materials in Example 1 and Example 6 respectively. Detailed implementation manners

[0057] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and through specific implementation manners.

[0058] The following is a detailed description of the technical content, achieved objectives and effects of the present invention. The present invention will be further described below in conjunction with examples and with reference to the accompanying drawings / tables. Obviously, the described examples are only a part of the implementation manners of the present invention, rather than all of the implementation manners.

[0059] In one implementation manner of the present invention, a method for synthesizing a positive electrode material by supercritical hydrothermal method is provided. See the process flow chart in Figure 1 , including the following steps:

[0060] Step 1: Dissolve an iron source, a manganese source, a phosphorus source and an antioxidant in deoxygenated deionized water and mix evenly to obtain a first raw material solution;

[0061] Prepare the Li salt into a separate aqueous solution to obtain a second raw material solution;

[0062] wherein, the molar ratio of Li:P:Mn:Fe is 3:1:x:1-x, x is 0.2 to 0.8, and the solution temperature is 25 to 60 °C;

[0063] Step 2: First, introduce pure water into the tubular reactor, and adjust the temperature in the reactor to 350 to 500 °C and the pressure to 23 to 30 MPa through devices such as a high-pressure pump, a heater and a back pressure valve; then turn on the circulating water cooling system to cool the outlet pipeline so that the outlet material remains in a liquid phase state;

[0064] Step 3: After reaching the supercritical reaction conditions in the reactor, the raw material solution containing Fe, Mn, and P and the Li salt solution are pumped into the tubular reactor at the same flow rate through a high-pressure pump for reaction. At the same time, CO2 gas is also pumped into the tubular reactor to start continuous reaction. The raw materials stay in the reaction tube for a period of time, and the fluid after reaction is collected after being depressurized through a cooling device and a back-pressure device;

[0065] Step 4: Filter and dry the collected slurry to obtain preliminary lithium iron manganese phosphate powder;

[0066] Step 5: Grind the above-mentioned preliminary lithium iron manganese phosphate powder and sinter it under an inert atmosphere to finally obtain lithium iron manganese phosphate material, that is, obtain the positive electrode material.

[0067] For the convenience of comparison, in the following examples, the value of x in LiMn x Fe 1-x PO4 is 0.7, but it is not a limitation to the present invention. Other values of x within 0 to 1 are also applicable to the present invention, and other positive electrode materials are also applicable to the present invention.

[0068] Example 1

[0069] This example provides a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium iron manganese phosphate, including the following steps:

[0070] (1) Weigh 8.34 g (0.03 mol) of ferrous sulfate heptahydrate (FeSO4·7H2O), 10.57 g (0.07 mol) of manganese sulfate (MnSO4), and 9.8 g (0.1 mol) of phosphoric acid (H3PO4), dissolve them in 1000 mL of deoxygenated deionized water, and add 3 g of ascorbic acid and stir to dissolve to form the first raw material solution;

[0071] Weigh 7.18 g (0.3 mol) of lithium hydroxide (LiOH) and dissolve it in 1000 mL of deoxygenated deionized water to prepare a Li + raw material solution to obtain the second raw material solution.

[0072] (2) Pass pure water into the tubular reactor, adjust the temperature in the reactor to 375 °C and the pressure to 25 MPa through a high-pressure pump and a heater, then pump the first raw material solution, the second raw material solution, and CO2 gas into the tubular reactor at the same time to start continuous synthesis reaction. The feeding flow rates of the first raw material solution and the second raw material solution are both 20 mL / min. In the solution system in the reactor, the ratio of Li / Fe+Mn ion concentration is 3:1, the total concentration of metal salts is 0.4 mol / L, the flow rate of CO2 gas is 50 mL / min, and the reaction material stays in the reaction tube for 12 s. The reaction equation is:

[0073] 3LiOH + H3PO4 + 0.7FeSO4 + 0.3MnSO4 → LiMn 0.7 Fe 0.3 PO4 + Li2SO4 + 3H2O

[0074] (3) After the reaction is completed, the suspension containing product particles collected from the outlet is filtered, and the filtrate is dried in a vacuum drying oven at 60 °C, and then the dried powder is ground using a mortar. The ground powder is sintered at 500 °C for 2 h under a nitrogen atmosphere with a heating rate of 5 °C / min, and finally the ultrafine lithium iron manganese phosphate LiMn 0.7 Fe 0.3 PO4 material is obtained.

[0075] Figure 2 is the X-ray diffraction pattern of the ultrafine lithium iron manganese phosphate LiMn 0.7 Fe 0.3 PO4 material in Example 1. It can be seen from the figure that the product is pure lithium iron manganese phosphate with high crystallinity.

[0076] Figure 3 is the transmission electron microscope image of the ultrafine lithium iron manganese phosphate LiMn 0.7 Fe 0.3 PO4 material in Example 1. It can be seen from the figure that the lithium iron manganese phosphate particles are spherical-like, the primary particle size is ~40 nm, and the morphology and size distribution are uniform.

[0077] Figure 4 is the charge-discharge curve of the lithium-ion secondary battery prepared using the ultrafine lithium iron manganese phosphate material in Example 1 at 0.1C. It can be seen that there are 2 plateaus in the charge-discharge curve at ~3.5V and ~4.0V respectively, corresponding to the redox processes of Fe and Mn.

[0078] Example 2

[0079] This example provides a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium iron manganese phosphate, including the following steps:

[0080] (1) Weigh 5.22 g (0.03 mol) of ferrous acetate, 12.11 g (0.07 mol) of manganese acetate (MnSO4), and 11.50 g (0.1 mol) of ammonium dihydrogen phosphate (NH4H2PO4), dissolve them in 1000 mL of deoxygenated deionized water, and add 3 g of ascorbic acid and stir to dissolve to form the first raw material solution;

[0081] Weigh 7.18 g (0.3 mol) of lithium hydroxide (LiOH) and dissolve it in 1000 mL of deoxygenated deionized water to prepare a Li + raw material solution to obtain the second raw material solution.

[0082] (2) Pure water is introduced into the tubular reactor. After adjusting the temperature in the reactor to 375 °C and the pressure to 25 MPa through a high-pressure pump and a heater, the first feed liquid, the second feed liquid, and CO2 gas are simultaneously pumped into the tubular reactor to start continuous synthesis reaction. The feed flow rates of the first feed liquid and the second feed liquid are both 20 mL / min. In the solution system in the reactor, the total concentration of metal salts is 0.4 mol / L, the flow rate of CO2 gas is 40 mL / min, and the reaction material stays in the reaction tube for 12 s.

[0083] (3) After the reaction is completed, the suspension containing product particles collected from the outlet is filtered. The filtrate is dried in a vacuum drying oven at 60 °C, and then the dried powder is ground using a mortar. The ground powder is sintered at 500 °C for 2 h in an argon atmosphere with a heating rate of 5 °C / min, and finally ultrafine lithium manganese iron phosphate LiMn 0.7 Fe 0.3 PO4 material is obtained.

[0084] Example 3

[0085] This example provides a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium manganese iron phosphate, including the following steps:

[0086] (1) Weigh 5.39 g (0.03 mol) of ferrous nitrate (Fe(NO3)2), 12.53 g (0.07 mol) of manganese nitrate (Mn(NO3)2), and 11.50 g (0.1 mol) of ammonium dihydrogen phosphate (NH4H2PO4), dissolve them in 1000 mL of deoxygenated deionized water, and add 4 g of citric acid and stir to dissolve to form the first feed liquid;

[0087] Weigh 15.28 g (0.15 mol) of lithium oxalate (Li2C2O4), dissolve it in 1000 mL of deoxygenated deionized water to prepare Li + feed liquid, and obtain the second feed liquid.

[0088] (2) Pure water is introduced into the tubular reactor. After adjusting the temperature in the reactor to 400 °C and the pressure to 25 MPa through a high-pressure pump and a heater, the first feed liquid, the second feed liquid, and CO2 gas are simultaneously pumped into the tubular reactor to start continuous synthesis reaction. The feed flow rates of the first feed liquid and the second feed liquid are both 20 mL / min. In the solution system in the reactor, the total concentration of metal salts is 0.4 mol / L, the flow rate of CO2 gas is 35 mL / min, and the reaction material stays in the reaction tube for 15 s.

[0089] (3) After the reaction is completed, the suspension containing product particles collected from the outlet is filtered. The filtrate is dried in a vacuum drying oven at 60 °C, and then the dried powder is ground using a mortar. The ground powder is sintered at 420 °C for 2 h under a nitrogen atmosphere with a heating rate of 5 °C / min, and finally ultrafine lithium manganese iron phosphate LiMn 0.7 Fe 0.3 PO4 material is obtained.

[0090] Example 4

[0091] This example provides a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium manganese iron phosphate, including the following steps:

[0092] (1) Weigh 16.68 g (0.06 mol) of ferrous sulfate heptahydrate (FeSO4·7H2O), 21.14 g (0.14 mol) of manganese sulfate (MnSO4), and 19.60 g (0.2 mol) of phosphoric acid (H3PO4), dissolve them in 1000 mL of deoxygenated deionized water, and add 3 g of ascorbic acid and stir to dissolve to form the first raw material solution;

[0093] Weigh 14.36 g (0.6 mol) of lithium hydroxide (LiOH) and dissolve it in 1000 mL of deoxygenated deionized water to prepare a Li + raw material solution to obtain the second raw material solution.

[0094] (2) Pass pure water into the tubular reactor, adjust the temperature in the reactor to 375 °C and the pressure to 25 MPa through a high-pressure pump and a heater. Then, pump the first raw material solution, the second raw material solution, and CO2 gas into the tubular reactor simultaneously to start continuous synthesis reaction. The feeding flow rates of the first raw material solution and the second raw material solution are both 30 mL / min. In the solution system in the reactor, the total concentration of metal salts is 0.4 mol / L, the flow rate of CO2 gas is 30 mL / min, and the reaction material stays in the reaction tube for 12 s.

[0095] (3) After the reaction is completed, the suspension containing product particles collected from the outlet is filtered. The filtrate is dried in a vacuum drying oven at 60 °C, and then the dried powder is ground using a mortar. The ground powder is sintered at 500 °C for 2 h under a nitrogen atmosphere with a heating rate of 5 °C / min, and finally ultrafine lithium manganese iron phosphate LiMn 0.7 Fe 0.3 PO4 material is obtained.

[0096] Example 5

[0097] This example provides a preparation method for supercritical hydrothermal synthesis of uniform ultrafine spherical nano lithium manganese iron phosphate, including the following steps:

[0098] (1) Weigh 8.34 g (0.03 mol) of ferrous sulfate heptahydrate (FeSO4·7H2O), 10.57 g (0.07 mol) of manganese sulfate (MnSO4), and 9.8 g (0.1 mol) of phosphoric acid (H3PO4), dissolve them in 1000 mL of deoxygenated deionized water, and add 3 g of ascorbic acid, then stir to dissolve to form the first raw material solution;

[0099] Weigh 7.18 g (0.3 mol) of lithium hydroxide (LiOH) and dissolve it in 1000 mL of deoxygenated deionized water to prepare the Li + raw material solution, obtaining the second raw material solution.

[0100] (2) Pass pure water into the tubular reactor. After adjusting the temperature in the reactor to 375 °C and the pressure to 25 MPa through a high-pressure pump and a heater, pump the first raw material solution, the second raw material solution, and CO2 gas into the tubular reactor simultaneously to start the continuous synthesis reaction. The feeding flow rates of the first raw material solution and the second raw material solution are both 25 mL / min. In the solution system in the reactor, the total concentration of metal salts is 0.4 mol / L, the flow rate of CO2 gas is 20 mL / min, and the reaction material stays in the reaction tube for 12 s.

[0101] (3) After the reaction is completed, filter the suspension containing product particles collected from the outlet, place the filtrate in a vacuum drying oven at 80 °C for drying, then grind the dried powder using a mortar. Sinter the ground powder in a nitrogen atmosphere at 350 °C for 6 h with a heating rate of 4 °C / min. Finally, obtain the ultrafine lithium manganese iron phosphate LiMn 0.7 Fe 0.3 PO4 material.

[0102] Example 6

[0103] The difference from Example 1 is that the temperature in step (2) is 320 °C.

[0104] Example 7

[0105] The difference from Example 1 is that the temperature in step (2) is 550 °C.

[0106] Example 8

[0107] The difference from Example 1 is that the pressure in step (2) is 20 MPa.

[0108] Example 9

[0109] The difference from Example 1 is that the pressure in step (2) is 32 MPa.

[0110] Example 10

[0111] The difference from Example 1 is that in step (2), the flow rate of CO2 gas is 1 mL / min.

[0112] Example 11

[0113] The difference from Example 1 is that in step (2), the flow rate of CO2 gas is 100 mL / min.

[0114] Example 12

[0115] The difference from Example 1 is that in step (2), the residence time of the reaction material in the reaction tube is 300 s.

[0116] Comparative Example 1

[0117] The difference from Example 1 is that in step (2), no CO2 gas is introduced.

[0118] Comparative Example 2

[0119] The difference from Example 1 is that in step (2), the temperature is 320 °C and the pressure is 20 MPa.

[0120] Test:

[0121] Particle size distribution test: Tested by a laser particle size analyzer. The material is dispersed in water, a certain amount of dispersant is added, and then ultrasonically dispersed for 20 min, and the test is completed within 30 min. The results are shown in Table 1.

[0122] Lithium-ion secondary battery test:

[0123] The lithium-ion secondary battery is a button cell. The secondary battery includes necessary components such as a positive electrode, a negative electrode, a separator, and an electrolyte, and of course also includes other necessary or auxiliary components. Among them, the positive electrode includes a current collector and a positive electrode active layer combined on the current collector.

[0124] Among them, the positive electrode active material contained in the positive electrode active layer includes the ultrafine lithium iron phosphate manganese material provided in the above examples, or the ultrafine lithium iron phosphate manganese material prepared by the preparation method of the above application examples. In the examples, the mass percentage of the ultrafine lithium iron phosphate manganese material in the positive electrode active layer is 93%. The mass content of the binder in the positive electrode active layer is 4%, and the binder is polyvinylidene chloride. The content of the conductive agent in the positive electrode active layer is 3%, and the conductive agent is carbon black. The positive electrode current collector is aluminum foil. The preparation process of the positive electrode can be: mixing the positive electrode active material, the conductive agent and the binder to obtain an electrode paste, coating the electrode paste on the current collector, and preparing the positive electrode through steps such as drying, rolling, and die-cutting.

[0125] The negative electrode of the button battery is a lithium metal sheet, the separator is a porous polypropylene separator, and the electrolyte is 1 mol / L LiPF6 / EC + EMC + DEC (volume ratio 1:1:1).

[0126] In the glove box, assemble in the order of the negative electrode case, shrapnel, steel sheet, lithium sheet, separator, positive electrode sheet, and positive electrode case. During the process, 10 μL of electrolyte is injected, and then the button battery is sealed with a sealer. Electrochemical performance tests are carried out on each group of button batteries.

[0127] The battery rate performance test is carried out on a charge-discharge system produced by Shenzhen Neware Electronic Equipment Co., Ltd. The working steps include constant current charging, constant voltage charging, standing, constant current discharging, etc. The charge-discharge voltage window is 2.0 - 4.25 V, and the rate test is carried out at different charge-discharge currents. The charge and discharge are both carried out at room temperature. The results are shown in Table 2.

[0128] Table 1. Dynamic light scattering test results of ultrafine LiMn 0.7 Fe 0.3 PO4 materials.

[0129]

[0130] According to the data in Table 1, it can be seen that by comparing Examples 1, 2, and 4, as the CO2 flow rate increases, the particle size of the prepared lithium iron phosphate manganese decreases.

[0131] By comparing Example 1 and Example 6, it can be seen that when the reaction temperature is too low to reach the supercritical state of water, the particle size distribution of the synthesized particles becomes wider. By comparing Examples 1, 7 - 9, it can be seen that when the reaction temperature and pressure are too high or too low, the synthesized particle size distribution becomes wider.

[0132] By comparing Examples 1, 10, and 11, it can be known that too large or too small CO2 flow rate during the reaction will lead to uneven particle size distribution.

[0133] By comparing Example 1 and Comparative Example 1, it can be seen that when there is no auxiliary CO2 injection, the particle size of the synthesized lithium iron phosphate manganese is larger and the distribution is wider.

[0134] By comparing Examples 1 - 9 and Comparative Example 2, it can be seen that under the condition of reacting to prepare lithium iron phosphate manganese without reaching the supercritical state of water but only reaching the supercritical state of CO2, the particle size of the prepared particles is large and the distribution is wide.

[0135] Table 2. Particle size D 0.7 Fe 0.3 of ultrafine LiMn 50 PO4 materials and rate performance test results

[0136]

[0137] As can be seen from the data in Table 2, as the primary particle size of the LiMn 0.7 Fe 0.3 PO4 material increases, the rate performance of its battery shows a downward trend. This is because the diffusion path of small-sized particles of Li + is shorter, which is beneficial to the insertion and extraction during its charge and discharge process.

[0138] The applicant declares that the present invention illustrates the detailed method of the present invention through the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of each raw material of the product of the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for synthesizing a cathode material by supercritical hydrothermal synthesis, characterized in that, The method includes the following steps: Inject water into the reactor and heat it, adjust the pressure in the reactor. When the supercritical point of water is reached in the reactor, or the water in the reactor reaches the supercritical state, add the raw materials of the positive electrode material into the reactor and introduce CO2 gas, and carry out the reaction to obtain the positive electrode material; The raw materials of the positive electrode material are divided into a lithium source and other raw materials, and the other raw materials include an iron source, a manganese source and a phosphorus source; The heating temperature is 350°C to 500°C; the pressure in the reactor is 23 MPa to 30 MPa; the flow rate of the introduced CO2 gas is 10 mL / min to 50 mL / min; the reaction time is 5 s to 200 s.

2. The method according to claim 1, characterized in that, The heating temperature is 375°C to 400°C.

3. The method according to claim 1, characterized in that, The pressure in the reactor is 24 MPa to 28 MPa.

4. The method according to claim 1, characterized in that, The raw materials of the positive electrode material are added into the reactor in the form of a solution. In the solution system in the reactor, the total concentration of metal salts is 0.01 mol / L to 2 mol / L.

5. The method according to claim 4, characterized in that, The raw materials of the positive electrode material are added into the reactor in the form of a solution. In the solution system in the reactor, the total concentration of metal salts is 0.1 mol / L to 0.5 mol / L.

6. The method according to claim 1, characterized in that, The other raw materials and the lithium source are respectively formulated into a first raw material liquid and a second raw material liquid, and are fed into the reactor in parallel.

7. The method according to claim 6, characterized in that, The feeding flow rates of the first raw material liquid and the second raw material liquid are 10 mL / min to 30 mL / min.

8. The method according to claim 6, characterized in that, The feeding flow rates of the first raw material liquid and the second raw material liquid are equal.

9. The method according to claim 1, characterized in that, The other raw materials also include an antioxidant.

10. The method according to claim 9, characterized in that, The antioxidant is a carbon-containing antioxidant.

11. The method according to claim 9, characterized in that, The antioxidant includes at least one of citric acid, sucrose and ascorbic acid.

12. The method according to claim 11, characterized in that, The antioxidant is ascorbic acid.

13. The method according to claim 1, characterized in that, The method also includes the step of sintering the reaction product.

14. The method according to claim 13, characterized in that, The atmosphere for sintering is an inert atmosphere.

15. The method according to claim 14, characterized in that, The gas in the inert atmosphere includes at least one of nitrogen, argon or helium.

16. The method according to claim 15, characterized in that, The gas in the inert atmosphere is nitrogen.

17. The method according to claim 13, characterized in that, The heating rate for sintering is 1°C / min to 5°C / min.

18. The method according to claim 13, characterized in that, The sintering temperature is 300°C to 700°C.

19. The method according to claim 13, characterized in that, The sintering time is 2 h to 10 h.

20. A cathode material prepared by the method according to any one of claims 1-19, wherein the primary particle size D of the cathode material 50 is within 100 nm, and D 10 and D 90 differ by no more than 200 nm.

21. The cathode material according to claim 20, wherein The positive electrode material is lithium iron manganese phosphate.

22. The cathode material according to claim 20, wherein The surface of the positive electrode material is coated with a carbon layer.

23. A lithium-ion battery, wherein The positive electrode of the lithium-ion battery includes a current collector and a positive electrode active layer combined on the current collector, and the positive electrode material in the positive electrode active layer includes the positive electrode material according to any one of claims 20-22 or the positive electrode material prepared by the method according to any one of claims 1-19.

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