High-magnification lithium iron phosphate positive electrode material, method for producing the same, positive electrode and battery including the same

Abstract

The present invention provides a high-magnification lithium iron phosphate positive electrode material, comprising lithium iron phosphate and carbon coated on the surface of the lithium iron phosphate, the primary particle size of which is 30-70 nm. The material in the present invention has a small and uniform primary particle size, no large single crystal particles, and a large specific surface area, so that the battery manufactured therefrom has a large capacity, good cycling performance, and excellent rate and low temperature performance. The present invention further provides a method for manufacturing the high-magnification lithium iron phosphate positive electrode material, which has a simple manufacturing process, is environmentally friendly, and does not require the purchase of precursors and expensive and valuable equipment, resulting in low input costs.

Description

[Technical Field]

[0001] This application claims priority from a Chinese invention patent application filed on April 15, 2022, bearing application number 202210395265.0, entitled "High-magnification lithium iron phosphate positive electrode material, its manufacturing method, positive electrode, and battery containing the same," the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to the field of batteries, and in particular to high-power lithium iron phosphate cathode materials, methods for their manufacture, cathodes and batteries containing the same. [Background technology]

[0003] Currently, the automotive start-stop battery market is dominated by lead-acid batteries, with AGM lead-acid batteries being the predominant type used in the global start-stop battery field. From an application market perspective, start-stop batteries are widely used in the European and Japanese markets. This is likely due primarily to the impact of strict carbon emissions measures implemented in Europe and Japan. Research has revealed that in 2016, the annual installation rate of start-stop batteries in new vehicles in Europe and Japan was over 70% and over 50%, respectively, higher than the 15% installation rate in China.

[0004] In China, environmental issues are becoming increasingly serious, and measures such as carbon allowances and carbon credits are being actively implemented. These measures could potentially lead to significant investments by OEMs in terms of emissions reduction and energy conservation. By 2020, the average fuel economy of new passenger cars in China is expected to reach 5L / 100km, and the average fuel economy of commercial vehicles is expected to be 10% lower than 2015 levels. While these fuel economy targets differ significantly from those in Europe, they are likely to be on a par with those in Japan. Achieving such stringent targets will be a major challenge for OEMs. Furthermore, research has shown that the use of automotive start-stop systems can reduce fuel consumption on urban roads by more than 8%, reduce CO2 emissions, and enable quick and safe vehicle start-up. Therefore, the installation of start-stop power supplies is becoming an effective means for OEMs to reduce fuel consumption.

[0005] With the development of lithium iron phosphate and the continuous decline in the cost of lithium iron phosphate batteries, the application of lithium iron phosphate as a start-up / shutdown power source has become a trend of the times. However, at present, lithium iron phosphate still has certain deficiencies in terms of low temperature and power supply performance. [Summary of the Invention]

[0006] In view of this, the technical problem to be solved by the present invention is to provide a high-power lithium iron phosphate positive electrode material, a manufacturing method thereof, a positive electrode, and a battery including the same. The high-power lithium iron phosphate positive electrode material provided by the present invention has high capacity, good power supply performance, and excellent low-temperature performance and cycling performance.

[0007] The present invention provides a high-magnification lithium iron phosphate positive electrode material comprising lithium iron phosphate and carbon coated on the surface of the lithium iron phosphate, the primary particles of which have a particle size of 30 to 70 nm.

[0008] Specifically, the high-magnification lithium iron phosphate cathode material provided by the present invention is a secondary particle formed by aggregation of primary particles, and the primary particles include lithium iron phosphate and carbon coated on the surface of the lithium iron phosphate. The lithium iron phosphate is a lithium iron phosphate commonly known to those skilled in the art. The carbon is uniformly coated on the surface of the lithium iron phosphate, thereby enhancing its electronic conductivity. In one embodiment, the carbon content is 1 to 5 wt%, preferably 2.21 wt%.

[0009] The high-magnification lithium iron phosphate cathode material described in the present invention has a small primary particle size, which provides good electrochemical performance. In one embodiment, the primary particle size is 50 nm.

[0010] The high-power lithium iron phosphate positive electrode material provided by the present invention has a concentrated particle size distribution, no large single-crystal particles, and good electrochemical performance. In one embodiment, the high-power lithium iron phosphate positive electrode material has a D10 of 0.1-1 μm, a D50 of 1-5 μm, and a D90 of 6-9 μm.

[0011] The high-magnification lithium iron phosphate positive electrode material provided by the present invention has high porosity and, as a result of testing by the BET specific surface area test method, also has a large specific surface area, which is advantageous for obtaining good electrochemical performance. In one embodiment, the high-magnification lithium iron phosphate positive electrode material has a specific surface area of ​​15 to 25 m 2 / g, preferably 19.16m 2 / g.

[0012] The present invention provides a method for producing the above-mentioned high-magnification lithium iron phosphate positive electrode material, The method includes a step of mixing an organic iron compound, an organic phosphorus compound, and an organic lithium compound, granulating the granulated material, and then firing the granulated material to obtain a high-magnification lithium iron phosphate positive electrode material.

[0013] Specifically, in the present invention, first, an organoiron compound, an organophosphorus compound, and an organolithium compound are uniformly mixed by stirring at a temperature of 50 to 60°C at a speed of 300 to 500 r / min.

[0014] The present invention provides a method for producing and obtaining a high-magnification lithium iron phosphate positive electrode material having small particle size using an organoiron compound, an organophosphorus compound, and an organolithium compound as raw materials. In one embodiment, the organoiron compound includes one or more of iron gluconate, iron citrate, iron acetate, and iron glycine. In one embodiment, the organophosphorus compound includes one or more of tributyl phosphate and trioctyl phosphate. In one embodiment, the organolithium compound includes one or more of lithium acetate and lithium citrate. In one embodiment, the molar ratio of Fe, P, and Li in the organoiron compound, the organophosphorus compound, and the organolithium compound is 1:1:1 to 1.1, preferably 1:1:1.03 to 1.05.

[0015] In the present invention, the above-mentioned organoiron compound, organophosphorus compound, and organolithium compound are uniformly mixed by stirring, then granulated, and the granulated mixture is calcined. The granulation described in the present invention is carried out by a granulation method familiar to those skilled in the art, such as extrusion granulation, spray granulation, and centrifugal granulation. The calcination described in the present invention is carried out by a two-stage calcination method. Specifically, the temperature is raised to a first temperature at a first rate and maintained at that temperature for a first hour, and then the temperature is raised to a second temperature at a second rate and maintained at that temperature for a second hour.

[0016] In one embodiment, the firing step involves increasing the temperature to 250-400°C at a rate of 50-80°C / h and maintaining the temperature for 2-5 hours, followed by increasing the temperature to 600-800°C at a rate of 100-150°C / h and maintaining the temperature for 5-10 hours. In one embodiment, the firing step involves increasing the temperature to 300-350°C at a rate of 50-80°C / h and maintaining the temperature for 2-3 hours, followed by increasing the temperature to 680-720°C at a rate of 100-150°C / h and maintaining the temperature for 4-6 hours. The first rate described in the present invention increases the temperature at a slower rate than the second rate, thereby avoiding carbon consumption and preventing a high magnetic material content. The second temperature described in the present invention is higher than the first temperature, resulting in high particle crystallinity and allowing a low magnetic material content.

[0017] In the present invention, the organic iron compound, organic phosphorus compound, and organic lithium compound are uniformly mixed by stirring, granulated, and then fired in an inert gas atmosphere. Specifically, the firing described in the present invention is performed in a roller hearth furnace, which includes a first temperature-raising segment, a first temperature-retaining segment, a second temperature-raising segment, a second temperature-retaining segment, and a temperature-reducing segment, connected in series. An inert gas is flowed through the roller hearth furnace, and the volume of the inert gas flowed through the roller hearth furnace per hour is 10 to 15 times the furnace volume of the roller hearth furnace, thereby achieving an internal furnace pressure 50 to 100 Pa higher than the ambient atmospheric pressure. In one embodiment, the volume ratio of the inert gas in the first temperature-raising segment, the first temperature-retaining segment, the second temperature-raising segment, the second temperature-retaining segment, and the temperature-reducing segment is 2:1.5 to 2:1 to 2:0.5 to 1.2. In one embodiment, the firing is carried out in an atmosphere of helium gas, argon gas, or nitrogen gas.

[0018] In the present invention, the organic iron compound, organic phosphorus compound, and organic lithium compound are uniformly mixed, granulated, and then calcined. After calcination, the resulting material is subsequently crushed, classified, sieved, de-ironized, and packaged. The crushing, classification, sieving, de-ironized, and packaged processes described in the present invention are common material processing methods known to those skilled in the art.

[0019] Specifically, in the present invention, the material obtained by calcination is subjected to the following processes in sequence: airflow pulverization, classification with a classifying wheel, sieving with an ultrasonic vibrating sieve, iron removal with an electromagnetic iron remover, and packaging. In one embodiment, the pulverization gas used in the airflow pulverization is nitrogen gas at 130-150°C and 0.3-0.5 MPa, containing less than 100 ppm of oxygen gas and having a dew point below -20°C. In one embodiment, the classification with the classifying wheel uses a 15-25 cm classifying wheel and is performed at a rotation speed of 500-700 r / min. The classifying wheel has ceramic vanes uniformly distributed thereon, with the ceramic vanes spaced 1-1.5 cm apart. In one embodiment, the sieving with the ultrasonic vibrating sieve uses a sieve with a mesh size of 100-200 mesh and is performed at a power of 3-5 kW. In one embodiment, the iron removal is performed by a 15000GS electromagnetic iron remover, and the iron removal time is 15 seconds or more. In one embodiment, the packaging is performed at a temperature of 15 to 25°C.

[0020] The present invention provides a positive electrode comprising the lithium iron phosphate positive electrode material according to the present invention.

[0021] The present invention further provides a battery comprising the above positive electrode.

[0022] The high-power lithium iron phosphate positive electrode material provided by the present invention comprises lithium iron phosphate and carbon coated on the surface of the lithium iron phosphate, and its primary particle size is 30 to 70 nm. Because the primary particle size is small and uniform, there are no large single-crystal particles, and the specific surface area is large, batteries manufactured therefrom have large capacity, good cycling performance, and excellent power supply and low-temperature performance. Experiments have shown that the present invention achieves a discharge capacity of 140 mAh / g or more at 10 C and a capacity retention rate of 90% or more at -20°C, meeting the requirements for start-up and power outage power supplies. The present invention also provides a method for manufacturing the high-power lithium iron phosphate positive electrode material, which is simple, environmentally friendly, and has low input costs because it does not require the purchase of precursors or expensive equipment. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a low magnification scanning electron micrograph of a high magnification lithium iron phosphate cathode material according to the present invention. [Figure 2] 1 is a high magnification scanning electron micrograph of a high magnification lithium iron phosphate cathode material according to the present invention. [Figure 3] 1 is an XRD spectrum of a high magnification lithium iron phosphate cathode material according to the present invention. [Figure 4] 1C, 2C, 5C and 10C charge / discharge graphs of the high-magnification lithium iron phosphate cathode material according to the present invention. [Figure 5] 1C is a graph showing the ambient temperature cycling performance of the high-magnification lithium iron phosphate cathode material according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention discloses a high-power lithium iron phosphate cathode material, a method for manufacturing the same, a cathode, and a battery including the same. Those skilled in the art will be able to adapt the process parameters in light of the present disclosure. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present disclosure. The method and application of the present disclosure have been described in terms of preferred embodiments. It is clear that those skilled in the art can realize and apply the technology of the present disclosure by modifying, appropriately altering, or combining the methods and applications described herein without departing from the content, spirit, and scope of the present disclosure.

[0025] The following further illustrates the present invention in combination with examples. [Example]

[0026] Iron gluconate, tributyl phosphate, and lithium acetate are mixed and uniformly stirred at a high speed of 400 r / min at a temperature of 55°C to obtain a paste, where the molar ratio of Fe, P, and Li in the iron gluconate, phosphate, and lithium acetate is 1:1:1.04.

[0027] The resulting paste is extruded and granulated, then transferred to a sagger, which is then placed in a roller hearth furnace for firing under nitrogen gas protection. The roller hearth furnace includes a first heating segment, a first heat-holding segment, a second heating segment, a second heat-holding segment, and a heat-reducing segment, connected in series. The paste is heated to 320°C at a heating rate of 65°C / h in the first heating segment, and held at 320°C for 3 hours in the first heat-holding segment. The paste is then heated to 690°C at a heating rate of 120°C / h in the second heating segment, and held at 690°C for 5 hours in the second heat-holding segment. The paste is then cooled to 60°C or below in the heat-reducing segment before being removed from the furnace to obtain a fired material.

[0028] Each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment in the roller hearth furnace is provided with an outlet connected to an induced draft fan. Nitrogen gas flows into each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment, and the nitrogen gas flows in from the bottom of the kiln. The volume ratio of nitrogen gas flowing into the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment is 2:1.8:1.5:0.8:2. During the firing process, the volume of nitrogen gas flowing in per hour is 13 times the volume of the roller hearth furnace, and the pressure inside the furnace is 80 Pa higher than the ambient atmospheric pressure.

[0029] The calcined material is passed through a fluidized bed jet mill and pulverized and classified under conditions of 0.4 MPa and 140°C to obtain pulverized material. During the pulverization process, the airflow is nitrogen gas, with an oxygen content of less than 100 ppm and a dew point of less than -20°C. During the classification process, a classification wheel is used, rotating at a speed of 600 r / min. The classification wheel is made of ceramic and has a diameter of 20 cm. The classification wheel has uniformly distributed ceramic vanes spaced 1.2 cm apart. The pulverized material is then passed through an ultrasonic vibrating sieve to sieve the material. The sieved material is then retained in a 15,000 Gs electromagnetic de-ironizer for two-stage de-ironization, with a retention time of 15 seconds or more. The iron-free material is transported to a packaging room with constant temperature and humidity, where it is vacuum-packed in an environment with a humidity of 10% or less and a temperature of 25°C, to obtain a high-magnification lithium iron phosphate cathode material.

[0030] The shape and structure of the high-magnification lithium iron phosphate positive electrode material were characterized by scanning electron microscopy and XRD, and the results are shown in Figures 1, 2, and 3. Here, Figure 1 is a low-magnification scanning electron micrograph of the high-magnification lithium iron phosphate positive electrode material described in the present invention, Figure 2 is a high-magnification scanning electron micrograph of the high-magnification lithium iron phosphate positive electrode material described in the present invention, and Figure 3 is an XRD spectrum of the high-magnification lithium iron phosphate positive electrode material described in the present invention.

[0031] As can be seen from Figures 1 and 2, the primary particles of the high-magnification lithium iron phosphate positive electrode material have a very small particle size of about 50 nm and a highly concentrated distribution, with no large single-crystal particles and high particle sphericity. These factors are very advantageous for achieving excellent magnification performance. As can be seen from Figure 3, the lithium iron phosphate positive electrode material described in the present invention matches the standard diffraction peaks of lithium iron phosphate.

[0032] When expressing the high-magnification lithium iron phosphate cathode material in terms of chemical indices, the elemental composition is detected by ICP, the powder internal resistance is measured by the four-point probe method, the measurement pressure is 10 MPa, and the press density is measured at a pressure of 3 tons. The results are shown in Table 1.

[0033] [Table 1]

[0034] As can be seen from Table 1, the high-power lithium iron phosphate positive electrode material described in the present invention mainly contains Li, Fe, P, and C, and has a very low impurity content. The extremely low powder internal resistance of the high-power lithium iron phosphate positive electrode material described in the present invention also indicates that the carbon coating is very uniform and the electronic conductivity is good. Furthermore, because the high-power lithium iron phosphate positive electrode material described in the present invention is produced at a high sintering temperature, its particle crystallinity is high and the amount of iron leaching and the content of magnetic substances are low.

[0035] The high-magnification lithium iron phosphate positive electrode material is assembled into a button battery to demonstrate its performance. The lithium iron phosphate positive electrode material, SP conductive agent, and polyvinylidene fluoride (PVDF) are mixed in a 90:5:5 weight ratio using a supermixer. The mixture is then coated onto aluminum foil using an automatic coater. The coated aluminum foil is then dried in an oven. The dried aluminum foil is then roll-pressed at the required pressure density and cut into small circular pieces of the desired size. The mass is measured and then re-dried. The final aluminum foil serves as the positive electrode, the lithium sheet serves as the negative electrode, the lithium hexafluorophosphate electrolyte, and the polyethylene material serves as the separator. The positive electrode case, negative electrode case, positive electrode sheet, lithium sheet, separator, and electrolyte are assembled into a button battery as needed. The button battery is then hooked onto a battery test system and left to stand, after which testing is performed.

[0036] The assembled button battery was tested in a blue battery test system at a test temperature of 24-26°C. Charge capacities were obtained by charging at rates of 0.1C, 1C, 2C, 5C, and 10C to 3.75V, and then discharged at corresponding rates of 0.1C, 1C, 2C, 5C, and 10C to 2.0V to obtain discharge capacities. The test results are shown in Table 2 and Figure 4. Figure 4 shows the charge / discharge curves of the high-magnification lithium iron phosphate cathode material described in the present invention at 1C, 2C, 5C, and 10C. In Figure 4, a, b, c, and d are the charge / discharge curves at 1C, 2C, 5C, and 10C, respectively.

[0037] [Table 2]

[0038] As can be seen from Table 2 and FIG. 4, the capacity of the high-power lithium iron phosphate cathode material described in the present invention functions effectively, and the power-up performance and low-temperature performance are excellent.

[0039] Next, the high-power lithium iron phosphate cathode material described in the present invention was tested for room temperature cycling performance at 1C. Specifically, the high-power lithium iron phosphate cathode material described in the present invention was assembled into a 3 Ah pouch-type battery cell and tested at a test rate of 1C at a temperature of 25°C using a blue battery test system. The test results are shown in Figure 5. Figure 5 is a graph showing the room temperature cycling performance at 1C of the high-power lithium iron phosphate cathode material described in the present invention.

[0040] FIG. 5 clearly shows that the capacity retention rate of the high-magnification lithium iron phosphate positive electrode material according to the present invention after 2500 cycles is 96.6%, indicating that its cycling performance is excellent. [Example]

[0041] Iron citrate, trioctyl phosphate, and lithium acetate are mixed and uniformly stirred at a high speed of 400 r / min at a temperature of 55°C to obtain a paste, in which the molar ratio of Fe, P, and Li in the iron citrate, trioctyl phosphate, and lithium acetate is 1:1:1.07.

[0042] The resulting paste is extruded and granulated, then transferred to a sagger, which is then placed in a roller hearth furnace for firing under nitrogen gas protection. The roller hearth furnace includes a first heating segment, a first heat-holding segment, a second heating segment, a second heat-holding segment, and a heat-reducing segment, connected in series. The paste is heated to 370°C at a heating rate of 78°C / h in the first heating segment, and held at 370°C for 5 hours in the first heat-holding segment. The paste is then heated to 790°C at a heating rate of 145°C / h in the second heating segment, and held at 790°C for 10 hours in the second heat-holding segment. The paste is then cooled to 60°C or below in the heat-reducing segment before being removed from the furnace to obtain a fired material.

[0043] Each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment in the roller hearth furnace is provided with an outlet connected to an induced draft fan. Nitrogen gas flows into each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment, and the nitrogen gas flows in from the bottom of the kiln. The volume ratio of nitrogen gas flowing into the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment is 2:1.8:1.5:0.8:2. During the firing process, the volume of nitrogen gas flowing in per hour is 13 times the volume of the roller hearth furnace, and the pressure inside the furnace is 80 Pa higher than the ambient atmospheric pressure.

[0044] The calcined material is passed through a fluidized bed jet mill and pulverized and classified under conditions of 0.4 MPa and 140°C to obtain pulverized material. During the pulverization process, the airflow is nitrogen gas, with an oxygen content of less than 100 ppm and a dew point of less than -20°C. During the classification process, a classification wheel is used, rotating at a speed of 600 r / min. The classification wheel is made of ceramic and has a diameter of 20 cm. The classification wheel has uniformly distributed ceramic vanes spaced 1.2 cm apart. The pulverized material is then passed through an ultrasonic vibrating sieve to sieve the material. The sieved material is then retained in a 15,000 Gs electromagnetic de-ironizer for two-stage de-ironization, with a retention time of 15 seconds or more. The iron-free material is transported to a packaging room with constant temperature and humidity, where it is vacuum-packed in an environment with a humidity of 10% or less and a temperature of 25°C, to obtain a high-magnification lithium iron phosphate cathode material.

[0045] When the high-power lithium iron phosphate positive electrode material is expressed in terms of physical indices, the powder internal resistance is measured by a four-probe method at a measurement pressure of 10 MPa, and the press density is measured at a pressure of 3 tons. The test results show that the high-power lithium iron phosphate positive electrode material produced in this example has a primary particle size of 70 nm, a D50 of 3.1 μm, a powder internal resistance of 6.2 Ω·cm, a press density of 1.97 g / mL, and a specific surface area of ​​18.9 m2 / g. Similar to the high-power lithium iron phosphate positive electrode materials produced in Examples 1 and 2, the high-power lithium iron phosphate positive electrode material has low powder internal resistance and high specific surface area.

[0046] The high-magnification lithium iron phosphate positive electrode material is assembled into a button battery to demonstrate its performance. The lithium iron phosphate positive electrode material, SP conductive agent, and polyvinylidene fluoride (PVDF) are mixed in a 90:5:5 weight ratio using a supermixer. The mixture is then coated onto aluminum foil using an automatic coater. The coated aluminum foil is then dried in an oven. The dried aluminum foil is then roll-pressed at the required pressure density and cut into small circular pieces of the desired size. The mass is measured and then re-dried. The final aluminum foil serves as the positive electrode, the lithium sheet serves as the negative electrode, the lithium hexafluorophosphate electrolyte, and the polyethylene material serves as the separator. The positive electrode case, negative electrode case, positive electrode sheet, lithium sheet, separator, and electrolyte are assembled into a button battery as needed. The button battery is then hooked onto a battery test system and left to stand, after which testing is performed.

[0047] The performance is expressed as follows: The assembled button battery is placed in a blue battery test system and tested at a test temperature of 25°C. It is charged at a rate of 0.1C until it reaches 3.75V to obtain the charge capacity, and then discharged at a rate of 0.1C until it reaches 2.0V to obtain the discharge capacity. The test results are shown in Table 3.

[0048] [Table 3]

[0049] As can be seen from Table 3, the high-magnification lithium iron phosphate positive electrode material prepared in this example is almost the same as the high-magnification lithium iron phosphate positive electrode material prepared in Example 1, and has similar high specific capacity and high-magnification performance, as well as excellent low-temperature performance. [Example]

[0050] Iron acetate, tributyl phosphate, and lithium citrate are mixed and uniformly stirred at a high speed of 400 r / min at a temperature of 55°C to obtain a paste, where the molar ratio of Fe, P, and Li in the iron gluconate, phosphate, and lithium acetate is 1:1:1.05.

[0051] The resulting paste is extruded and granulated, then transferred to a sagger, which is then placed in a roller hearth furnace for firing under nitrogen gas protection. The roller hearth furnace includes a first heating segment, a first heat-holding segment, a second heating segment, a second heat-holding segment, and a heat-reducing segment, connected in series. The paste is heated to 320°C at a heating rate of 60°C / h in the first heating segment, and held at 320°C for 4 hours in the first heat-holding segment. Subsequently, the paste is heated to 690°C at a heating rate of 120°C / h in the second heating segment, and held at 690°C for 4 hours in the second heat-holding segment. The paste is then cooled to 60°C or below in the heat-reducing segment before being removed from the furnace to obtain a fired material.

[0052] Each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment in the roller hearth furnace is provided with an outlet connected to an induced draft fan. Nitrogen gas flows into each of the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment, and the nitrogen gas flows in from the bottom of the kiln. The volume ratio of nitrogen gas flowing into the first temperature-raising segment, first temperature-retaining segment, second temperature-raising segment, second temperature-retaining segment, and temperature-reducing segment is 2:1.8:1.5:0.9:2. During the firing process, the volume of nitrogen gas flowing in per hour is 12 times the volume of the roller hearth furnace, and the pressure inside the furnace is 70 Pa higher than the ambient atmospheric pressure.

[0053] The calcined material is passed through a fluidized bed jet mill and pulverized and classified under conditions of 0.4 MPa and 140°C to obtain pulverized material. During the pulverization process, the airflow is nitrogen gas, with an oxygen content of less than 100 ppm and a dew point of less than -20°C. During the classification process, a classification wheel is used, rotating at a speed of 600 r / min. The classification wheel is made of ceramic and has a diameter of 20 cm. The classification wheel has uniformly distributed ceramic vanes spaced 1.2 cm apart. The pulverized material is then passed through an ultrasonic vibrating sieve to sieve the material. The sieved material is then retained in a 15,000 Gs electromagnetic de-ironizer for two-stage de-ironization, with a retention time of 15 seconds or more. The iron-free material is transported to a packaging room with constant temperature and humidity, where it is vacuum-packed in an environment with a humidity of 10% or less and a temperature of 25°C, to obtain a high-magnification lithium iron phosphate cathode material.

[0054] When expressing the high-magnification lithium iron phosphate cathode material in terms of chemical indices, the elemental composition is detected by ICP, the powder internal resistance is measured by the four-point probe method, the measurement pressure is 10 MPa, and the press density is measured at a pressure of 3 tons. The results are shown in Table 4.

[0055] [Table 4]

[0056] As can be seen from Table 4, the high-magnification lithium iron phosphate positive electrode material produced in this example is similar to that produced in Example 1, containing mainly Li, Fe, P, and C, and has low powder internal resistance, low iron elution amount, and low magnetic substance content.

[0057] The high-magnification lithium iron phosphate positive electrode material is assembled into a button battery to demonstrate its performance. The lithium iron phosphate positive electrode material, SP conductive agent, and polyvinylidene fluoride (PVDF) are mixed uniformly in a 90:5:5 weight ratio using a supermixer and coated onto aluminum foil using an automatic coating machine. The coated aluminum foil is then dried in an oven. The dried aluminum foil is roll-pressed at the required pressure and cut into small circular pieces of the desired size. The mass is measured and then re-dried. The final aluminum foil serves as the positive electrode, the lithium sheet serves as the negative electrode, the lithium hexafluorophosphate electrolyte, and the polyethylene material serves as the separator. The positive electrode case, negative electrode case, positive electrode sheet, lithium sheet, separator, and electrolyte are assembled into a button battery as needed. The button battery is then hooked onto a battery test system and tested. The assembled button battery is then tested in a blue battery test system at a test temperature of 25°C. The charge capacity is obtained by charging at a rate of 0.1C until it reaches 3.75V, and then the discharge capacity is obtained by discharging at a corresponding rate of 0.1C until it reaches 2.0V. The test results are shown in Table 5.

[0058] [Table 5]

[0059] As can be seen from Table 5, the high-magnification lithium iron phosphate positive electrode material prepared in this example is similar to the high-magnification lithium iron phosphate positive electrode material prepared in Example 1, and has high specific capacity and high-magnification performance, and excellent low-temperature performance.

[0060] The above description is merely a preferred specific embodiment of the present invention, and the protection scope of the present invention is not limited thereby. Those skilled in the art can make equivalent substitutions or modifications based on the technical solutions and inventive ideas of the present invention within the technical scope disclosed in the present invention, and they will be considered to be covered within the protection scope of the present invention.

Claims

1. A method for producing a lithium iron phosphate cathode material, comprising: The method includes a step of mixing an organic iron compound, an organic phosphorus compound, and an organic lithium compound, granulating the granulated material, and then firing the granulated material to obtain a lithium iron phosphate positive electrode material; The average particle size of the primary particles of the obtained lithium iron phosphate positive electrode material is 30 nm to 70 nm, The firing step includes increasing the temperature to 250°C to 400°C at a first rate and maintaining the temperature for 2 to 5 hours, and then increasing the temperature to 600°C to 800°C at a second rate and maintaining the temperature for 5 to 10 hours; the first rate is 50°C / h to 80°C / h, and the second rate is 100°C / h to 150°C / h; the organic iron compound comprises one or more of iron gluconate, iron citrate, iron acetate, and iron glycylate; the organophosphorus compound includes one or more of tributyl phosphate, trioctyl phosphate; The method for producing a lithium iron phosphate positive electrode material, wherein the organolithium compound includes at least one of lithium acetate and lithium citrate.

2. 2. The method for producing a lithium iron phosphate positive electrode material according to claim 1, wherein the molar ratio of Fe, P, and Li in the organofiber compound, the organophosphorus compound, and the organolithium compound is 1:1:1 to 1.1.

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