A low-temperature-resistant lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The present invention provides a low-temperature resistant lithium iron phosphate cathode material and a preparation method thereof, comprising the following steps: (1) adding a polyurethane resin and a surfactant to water, stirring evenly, continuing to add anhydrous ferric phosphate, lithium carbonate, rubidium carbonate and sodium carbonate, stirring evenly, and obtaining a mixture A; (2) grinding the mixture A with a grinder, spray drying, and obtaining powder A; and calcining under a nitrogen atmosphere to obtain metal-doped carbon-coated lithium iron phosphate; (3) mixing metal-doped carbon-coated lithium iron phosphate, lanthanum phosphate, glucose, modified graphene oxide and water, stirring evenly, and obtaining a mixture B; (4) grinding the mixture B with a grinder, spray drying, and obtaining powder B, and calcining under a nitrogen atmosphere to obtain a low-temperature resistant lithium iron phosphate cathode material. The lithium iron phosphate cathode material of the present invention improves the low-temperature performance of the lithium iron phosphate cathode material and increases the median voltage of discharge.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery positive electrode materials, and particularly relates to a low-temperature resistant lithium iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] New energy technologies are widely recognized as the high-tech of the 21st century. As a crucial component of this new energy sector, the battery industry has become a new hotspot in global economic development. Lithium-ion batteries are now widely used as a vital energy source, playing a crucial role in fields ranging from electronic communications to transportation, and boasting broad application prospects. Lithium-ion batteries represent the epitome of modern high-performance batteries, offering high operating voltage, high specific energy, high capacity, low self-discharge, excellent cyclability, long service life, light weight, and compact size. They are an ideal power source for portable electronic devices such as mobile phones and laptops. Lithium-ion batteries primarily consist of a positive electrode, a negative electrode, an electrolyte that conducts lithium ions, and a separator that separates the positive and negative electrodes. The electrochemical performance of lithium-ion batteries depends primarily on the structure and properties of the electrode and dielectric materials used. The selection and quality of the electrode materials, in particular, directly determine the battery's characteristics and price. Cathode materials account for over 40% of a battery's total cost, and their performance directly impacts various performance indicators. Therefore, cathode materials play a central role in lithium-ion batteries.

[0003] Lithium iron phosphate (LIFP) batteries, a novel lithium-ion battery electrode material, represent a new generation of green, high-energy batteries with superior performance and have become a key focus of high-tech development. They are characterized by high voltage, high capacity, low power consumption, no memory effect, compact size, low internal resistance, low self-discharge, high cycle life, low cost, non-toxicity, and environmental friendliness. These characteristics have led to their application in numerous consumer and specialty applications, including mobile phones, laptops, camcorders, and digital cameras. As a lithium-ion battery cathode material, LFP batteries are currently one of the safest. This safe and stable cathode material ensures their safety under abusive conditions such as crushing, overcharging, and short-circuiting. Due to their safety and stability, LFP batteries have become a key development direction for lithium-ion power batteries. In recent years, LFP power batteries, favored by global lithium battery experts for their absolute safety, reliability, extremely long cycle life, and stable discharge profile, have led to rapid growth. LiFP power batteries can be said to completely eliminate the safety risks associated with lithium cobalt oxide and lithium manganese oxide batteries. However, the performance of lithium iron phosphate batteries decreases significantly at low temperatures, limiting their use in winter and in high-altitude areas. The low-temperature operating environment of lithium-ion batteries primarily occurs in winter and at high latitudes and altitudes, where low temperatures can affect the performance and lifespan of lithium-ion batteries and even cause serious safety issues. Furthermore, the low temperature slows the rate of lithium insertion into graphite, making it easy for metallic lithium to precipitate on the negative electrode surface, forming lithium dendrites that can penetrate the diaphragm and cause internal short circuits in the battery. Therefore, improving the low-temperature performance of lithium-ion batteries is crucial for their use in high-altitude areas. Summary of the Invention

[0004] The present invention provides a low-temperature resistant lithium iron phosphate positive electrode material and a preparation method thereof. The lithium iron phosphate is coated with a lanthanide metal phosphate, thereby improving the performance of the lithium iron phosphate positive electrode material and increasing the median discharge voltage, making the lithium iron phosphate positive electrode material suitable for low-temperature environments.

[0005] A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material comprises the following steps:

[0006] (1) adding 0.12-0.15 parts by mass of polyurethane resin and 0.04-0.06 parts by mass of surfactant to 10 parts by mass of water, stirring evenly, and then adding 1 part by mass of anhydrous ferric phosphate, 0.20-0.24 parts by mass of lithium carbonate, 0.03-0.04 parts by mass of rubidium carbonate and 0.01-0.02 parts by mass of sodium carbonate, stirring evenly to obtain a mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2-3:1:0.4-0.7;

[0007] (2) Grinding the mixture A with a grinder and spray drying to obtain powder A; calcining the powder A at 750-800° C. for 6-7 h under a nitrogen atmosphere to obtain metal-doped-carbon-coated lithium iron phosphate;

[0008] (3) mixing metal-doped carbon-coated lithium iron phosphate, lanthanum phosphate, glucose, modified graphene oxide, and water in a mass ratio of 1:0.03-0.04:0.03-0.04:0.01-0.02:10-15, stirring uniformly, and obtaining a mixture B;

[0009] (4) Grinding the mixed material B with a grinder and spray drying to obtain powder B, and calcining the powder B at 750-800° C. for 9-10 h in a nitrogen atmosphere to obtain a low-temperature resistant lithium iron phosphate positive electrode material.

[0010] Furthermore, the preparation method of the modified allyl alcohol polyoxyethylene ether is as follows: 30-35 parts by mass of a polysiloxane containing an SiH bond at the end group and 7-10 parts of allyl alcohol polyoxyethylene ether are mixed evenly, the mixture is heated to 80-100° C. and stirred for reaction for 50-60 minutes, 3-5 parts by mass of chloroplatinic acid and 5-6 parts by mass of 2,3-dimethylmaleic anhydride are added and mixed evenly, the mixture is heated to 85-90° C., the mixture is reacted for 3-5 hours, and the mixture is cooled to room temperature to obtain the modified allyl alcohol polyoxyethylene ether.

[0011] The Antarctic region experiences perennially low temperatures and strong winds. For example, at Kunlun Station, located at the highest altitude in Antarctica, the average annual temperature falls below -50°C. Currently, lithium iron phosphate batteries, known for their high specific power and energy density and environmental friendliness, are widely used in Antarctic research. However, commercially available lithium iron phosphate batteries perform better at high temperatures, but their capacity significantly decreases at low temperatures. After storage at low temperatures for a period of time, the initial discharge capacity decreases. The present invention attempts to improve the discharge capacity after storage at low temperatures through a combination of elemental doping and surfactants. Extensive experiments have shown that adding rubidium and sodium in a specific ratio is more effective, but commercially available surfactants do not provide an ideal dispersion effect on the entire system. The inventors have improved the discharge performance of lithium iron phosphate batteries after storage at low temperatures by adding a specific ratio of rubidium and sodium, as well as a surfactant composed of a commercially available surfactant and modified allyl alcohol polyoxyethylene ether. It is speculated that doping with rubidium and sodium can reduce the polarization effect of the crystal structure, while the addition of surfactants can regulate the formation of the crystal structure and the uniformity and density of the carbon coating, synergistically improving the battery's discharge performance at low temperatures.

[0012] Furthermore, the polyurethane resin is a water-soluble polyurethane resin with a solid content of 30±1 wt % and a viscosity of 60-85 cps.

[0013] Furthermore, in the step (2), the mixture A is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0014] Furthermore, in the step (4), the mixture B is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0015] Furthermore, the molar number of SiH bonds in the polysiloxane containing SiH bonds at the terminal groups is 1.2-1.4 mmol / g.

[0016] Furthermore, the preparation method of the modified graphene oxide is:

[0017] (1) mixing 1 part by mass of graphene oxide and 17-19 parts by mass of 18-20 wt% ammonia water, stirring and reacting at 30° C. for 10-15 h, heating to 65° C. and reacting for 10-14 h, filtering, washing, and vacuum drying to obtain amino-modified graphene oxide;

[0018] (2) 1 part by mass of amino-modified graphene oxide, 1.3-1.4 parts by mass of 3,5-diethyl-2,4-toluenediamine, 7-8 parts by mass of a 50 wt% isopropyl alcohol aqueous solution, and 2-3 parts by mass of a 2.5 mol / L hydrochloric acid aqueous solution were mixed, and 4-5 parts by mass of a 0.7 mol / L ammonium persulfate aqueous solution were added dropwise at 0°C. The mixture was stirred for 9-10 hours, filtered, washed, and vacuum-dried to obtain modified graphene oxide.

[0019] The sheet diameter of the graphene oxide is 0.4-0.8 μm.

[0020] Furthermore, the polysorbate is Tween-60; and the alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether.

[0021] In order to modify the lattice matching degree at the interface between iron phosphate and lithium iron phosphate during the lithium deintercalation process of ultra-low temperature lithium iron phosphate composite materials and improve the cycle stability of lithium iron phosphate batteries at low temperatures, the present invention uses lanthanum phosphate and glucose for secondary coating, but it is found that the lanthanum phosphate coating is not uniform, and the effect of improving the cycle stability at low temperatures is not ideal. The inventors tried to improve the dispersibility of the system by adding graphene oxide, but the compatibility of graphene oxide with the other components is not good. Therefore, the inventors further modified the graphene oxide to improve the cycle stability of the positive electrode material at low temperatures. It is speculated that the introduced surface groups make the system more compatible, and lanthanum phosphate and carbon simultaneously have better uniformity in the coating of metal-doped-carbon-coated lithium iron phosphate.

[0022] The present invention also provides a low-temperature resistant lithium iron phosphate positive electrode material prepared by the above preparation method.

[0023] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0024] 1. The present invention provides a low-temperature resistant lithium iron phosphate positive electrode material and a preparation method thereof. The lithium iron phosphate is coated with a lanthanide metal phosphate, which improves the performance of the lithium iron phosphate positive electrode material and increases the median discharge voltage, making the lithium iron phosphate positive electrode material suitable for low-temperature environments.

[0025] 2. The present invention attempts to improve the discharge capacity after low-temperature storage through the combined effects of elemental doping and surfactants. Extensive experiments have shown that adding rubidium and sodium in specific ratios is more effective, but commercially available surfactants do not provide an ideal dispersion effect on the overall system. The inventors have improved the discharge performance of lithium iron phosphate batteries after low-temperature storage by adding specific ratios of rubidium and sodium, as well as a surfactant composed of a commercially available surfactant and modified allyl alcohol polyoxyethylene ether.

[0026] 3. To improve the lattice matching of the ultra-low-temperature lithium iron phosphate composite material at the interface between the iron phosphate and lithium iron phosphate phases during the lithium insertion and extraction process, and thus improve the low-temperature cycling stability of the lithium iron phosphate battery, the present invention uses lanthanum phosphate and glucose for secondary coating. However, it was found that the lanthanum phosphate coating was uneven and the effect of improving the low-temperature cycling stability was not ideal. The inventors attempted to improve the dispersion of the system by adding graphene oxide, but the compatibility of graphene oxide with the other components was also poor. Therefore, the inventors further modified the graphene oxide to improve the low-temperature cycling stability of the positive electrode material. DETAILED DESCRIPTION

[0027] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0028] Example 1

[0029] This embodiment provides a low-temperature resistant lithium iron phosphate positive electrode material, and the preparation method thereof includes the following steps:

[0030] (1) Add 0.14 parts by mass of polyurethane resin and 0.05 parts by mass of surfactant to 10 parts by mass of water, stir evenly, continue to add 1 part by mass of anhydrous ferric phosphate, 0.22 parts by mass of lithium carbonate, 0.03 parts by mass of rubidium carbonate and 0.02 parts by mass of sodium carbonate, stir evenly to obtain a mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2.5:1:0.6;

[0031] (2) Grinding the mixture A with a grinder and spray drying to obtain powder A; calcining the powder A at 770° C. for 6.5 h under a nitrogen atmosphere to obtain metal-doped-carbon-coated lithium iron phosphate;

[0032] (3) mixing metal-doped carbon-coated lithium iron phosphate, lanthanum phosphate, glucose, modified graphene oxide, and water in a mass ratio of 1:0.03:0.04:0.01:13, stirring uniformly, and obtaining a mixture B;

[0033] (4) Grinding the mixed material B with a grinder and spray drying to obtain powder B, and calcining the powder B at 780° C. for 9.5 h in a nitrogen atmosphere to obtain a low-temperature resistant lithium iron phosphate positive electrode material.

[0034] The preparation method of the modified allyl alcohol polyoxyethylene ether is as follows: 33 parts by mass of polysiloxane containing SiH bonds at the end groups and 8 parts of allyl alcohol polyoxyethylene ether (Hubei Jiufenglong Chemical Co., Ltd., HPEG2400) are mixed evenly, the mixture is heated to 90° C. and stirred for reaction for 55 minutes, 4 parts by mass of chloroplatinic acid and 5.5 parts by mass of 2,3-dimethylmaleic anhydride are added and mixed evenly, the mixture is heated to 88° C., reacted for 4 hours, and cooled to room temperature to obtain the modified allyl alcohol polyoxyethylene ether.

[0035] The polyurethane resin is a water-soluble polyurethane resin with a solid content of 30 wt % and a viscosity of 70 cps, and was purchased from Shanghai Qizhan New Material Technology Co., Ltd.

[0036] In the step (2), the mixed material A is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0037] In the step (4), the mixed material B is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0038] The molar number of SiH bonds in the polysiloxane containing SiH bonds at the terminal groups was 1.3 mmol / g and was purchased from Shanghai Guishan Polymer Materials Co., Ltd.

[0039] The preparation method of the modified graphene oxide is:

[0040] (1) 1 part by mass of graphene oxide and 18 parts by mass of 19 wt% ammonia water were mixed, stirred at 30° C. for 13 h, heated to 65° C. for 12 h, filtered, washed, and vacuum dried to obtain amino-modified graphene oxide;

[0041] (2) 1 part by mass of amino-modified graphene oxide, 1.3 parts by mass of 3,5-diethyl-2,4-toluenediamine, 7.5 parts by mass of a 50 wt% aqueous solution of isopropyl alcohol, and 2.5 parts by mass of a 2.5 mol / L aqueous solution of hydrochloric acid were mixed, and 4.5 parts by mass of a 0.7 mol / L aqueous solution of ammonium persulfate were added dropwise at 0°C. The mixture was stirred for 9.5 hours, filtered, washed, and vacuum-dried to obtain modified graphene oxide.

[0042] The graphene oxide has a sheet diameter of 0.4-0.8 μm and was purchased from Xianfeng Nano.

[0043] The polysorbate is Tween-60, and the alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether, both of which were purchased from Jinan Xinshuangyue Chemical Co., Ltd.

[0044] Example 2

[0045] This embodiment provides a low-temperature resistant lithium iron phosphate positive electrode material, and the preparation method thereof includes the following steps:

[0046] (1) Add 0.12 parts by mass of polyurethane resin and 0.06 parts by mass of surfactant to 10 parts by mass of water, stir evenly, continue to add 1 part by mass of anhydrous ferric phosphate, 0.20 parts by mass of lithium carbonate, 0.04 parts by mass of rubidium carbonate and 0.01 parts by mass of sodium carbonate, stir evenly to obtain a mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2:1:0.7;

[0047] (2) Grinding the mixture A with a grinder and spray drying to obtain powder A; calcining the powder A at 780° C. for 6.5 h under a nitrogen atmosphere to obtain metal-doped-carbon-coated lithium iron phosphate;

[0048] (3) mixing metal-doped carbon-coated lithium iron phosphate, lanthanum phosphate, glucose, modified graphene oxide, and water in a mass ratio of 1:0.03:0.04:0.01:15, stirring uniformly, and obtaining a mixture B;

[0049] (4) Grinding the mixed material B with a grinder and spray drying to obtain powder B, and calcining the powder B at 750° C. for 10 h in a nitrogen atmosphere to obtain a low-temperature resistant lithium iron phosphate positive electrode material.

[0050] The preparation method of the modified allyl alcohol polyoxyethylene ether is as follows: 30 parts by mass of polysiloxane containing SiH bonds at the end groups and 10 parts of allyl alcohol polyoxyethylene ether (Hubei Jiufenglong Chemical Co., Ltd., HPEG2400) are mixed evenly, the mixture is heated to 80° C. and stirred for reaction for 60 minutes, 3 parts by mass of chloroplatinic acid and 6 parts by mass of 2,3-dimethylmaleic anhydride are added and mixed evenly, the mixture is heated to 85° C., reacted for 5 hours, and cooled to room temperature to obtain the modified allyl alcohol polyoxyethylene ether.

[0051] The polyurethane resin is a water-soluble polyurethane resin with a solid content of 30 wt % and a viscosity of 80 cps, and was purchased from Shanghai Qizhan New Material Technology Co., Ltd.

[0052] In the step (2), the mixed material A is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0053] In the step (4), the mixed material B is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

[0054] The molar number of SiH bonds in the polysiloxane containing SiH bonds at the terminal groups was 1.2 mmol / g and was purchased from Shanghai Guishan Polymer Materials Co., Ltd.

[0055] The preparation method of the modified graphene oxide is:

[0056] (1) 1 part by mass of graphene oxide and 19 parts by mass of 18 wt% ammonia water were mixed, stirred and reacted at 30° C. for 10 h, heated to 65° C. and reacted for 14 h, filtered, washed, and vacuum dried to obtain amino-modified graphene oxide;

[0057] (2) 1 part by mass of amino-modified graphene oxide, 1.3 parts by mass of 3,5-diethyl-2,4-toluenediamine, 8 parts by mass of a 50 wt% aqueous solution of isopropyl alcohol, and 2 parts by mass of a 2.5 mol / L aqueous solution of hydrochloric acid were mixed, and 5 parts by mass of a 0.7 mol / L aqueous solution of ammonium persulfate were added dropwise at 0°C. The mixture was stirred for 9 hours, filtered, washed, and vacuum-dried to obtain modified graphene oxide.

[0058] The graphene oxide has a sheet diameter of 0.4-0.8 μm and was purchased from Xianfeng Nano.

[0059] The polysorbate is Tween-60, and the alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether, both of which were purchased from Jinan Xinshuangyue Chemical Co., Ltd.

[0060] Comparative Example 1

[0061] The difference between this comparative example and Example 1 is that the amount of doping elements added is different.

[0062] Specifically: (1) adding 0.14 parts by mass of polyurethane resin and 0.05 parts by mass of surfactant to 10 parts by mass of water, stirring evenly, and then adding 1 part by mass of anhydrous ferric phosphate, 0.22 parts by mass of lithium carbonate, 0.01 parts by mass of rubidium carbonate and 0.05 parts by mass of sodium carbonate, stirring evenly to obtain a mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2.5:1:0.6.

[0063] Comparative Example 2

[0064] The difference between this comparative example and Example 1 is that the doping elements are different.

[0065] Specifically: (1) add 0.14 parts by mass of polyurethane resin and 0.05 parts by mass of surfactant to 10 parts by mass of water, stir evenly, continue to add 1 part by mass of anhydrous ferric phosphate, 0.22 parts by mass of lithium carbonate, 0.01 parts by mass of aluminum oxide and 0.04 parts by mass of zirconium nitrate, stir evenly, and obtain mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2.5:1:0.6.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 1:1:1.

[0068] Comparative Example 4

[0069] The difference between this comparative example and Example 1 is that the allyl alcohol polyoxyethylene ether is not modified.

[0070] Specifically, the surfactant is a mixture of polysorbate, alkylphenol polyoxyethylene ether, and allyl alcohol polyoxyethylene ether in a mass ratio of 3:1:0.5. Allyl alcohol polyoxyethylene ether was purchased from Hubei Jiufenglong Chemical Co., Ltd., HPEG2400.

[0071] Comparative Example 5

[0072] The difference between this comparative example and Example 1 is that the surfactant is a compound of Span-80 (purchased from Shandong Weilun Chemical Co., Ltd.), fatty alcohol polyoxyethylene ether AEO-9 (purchased from Shandong Weilun Chemical Co., Ltd.) and modified allyl alcohol polyoxyethylene ether in a mass ratio of 3:1:0.5.

[0073] Comparative Example 6

[0074] The difference between this comparative example and Example 1 is that the modified graphene oxide is replaced by graphene oxide. The graphene oxide has a sheet diameter of 0.4-0.8 μm and is purchased from Xianfeng Nano.

[0075] Comparative Example 7

[0076] The difference between this comparative example and Example 1 is that the modified graphene oxide is prepared by mixing 1 part by mass of graphene oxide and 18 parts by mass of 20 wt% ammonia water, stirring and reacting at 30°C for 15 hours, heating to 65°C and reacting for 14 hours, filtering, washing, and vacuum drying to obtain modified graphene oxide.

[0077] This comparative example differs from Example 1 in that the molar number of SiH bonds in the polysiloxane containing SiH bonds at the terminal groups is 0.8 mmol / g. The polysiloxane was purchased from Shanghai Guishan Polymer Materials Co., Ltd. The polyurethane resin was a water-soluble polyurethane resin with a solid content of 30 wt% and a viscosity of 259 cps. The polyurethane resin was purchased from Shanghai Qizhan New Materials Technology Co., Ltd. In step (2), the mixed material A was ground using a grinder to a particle size D50 of 0.5-0.6 μm.

[0078] Comparative Example 9

[0079] The difference between this comparative example and Example 1 is that in step (4), the mixed material B is ground with a grinder to a particle size D50 of 1-2 μm. The graphene oxide flakes have a diameter of 5-10 μm and are purchased from Xianfeng Nano.

[0080] Performance Testing

[0081] The lithium iron phosphate cathode materials prepared in Examples 1-2 and Comparative Examples 1-9 were composed, by weight, of 90% lithium iron phosphate cathode material, 5% binder, and 5% conductive agent. The conductive agent was SuperP (conductive carbon black), and the binder was PVDF (polyvinylidene fluoride). A battery was assembled using this cathode as the positive electrode, lithium metal as the negative electrode, and lithium hexafluorophosphate as the electrolyte. The charge and discharge voltage range was 2.5V-4.3V, and the current was 0.1C. Performance testing was conducted, and the results are shown in Table 1.

[0082] (1) Determine the first discharge specific capacity at 25°C;

[0083] (2) After the battery is placed at -50°C for 24 hours, the initial discharge capacity is measured; the capacity retention rate after 500 cycles at -50°C is measured; Capacity retention rate = (discharge capacity after 500 cycles at -50°C / initial discharge capacity after 24 hours at -50°C) × 100%

[0084] (3) The voltage at which the battery is discharged at -50°C until half of its capacity remains is the median voltage of low-temperature discharge.

[0085] Table 1 Performance test results

[0086]

[0087]

[0088] As shown in Examples 1-2, the lithium iron phosphate cathode material prepared by the present invention still has a good initial discharge specific capacity after being placed for 24 hours, solving the problem of a significant decrease in initial discharge specific capacity of existing lithium iron phosphate cathode materials after being placed at low temperatures. In addition, the lithium iron phosphate cathode material of the present invention has high cycle stability when used at low temperatures. Comparative Examples 1-9 show that the performance of the lithium iron phosphate cathode material decreases to varying degrees when the raw materials and preparation methods are changed.

[0089] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material, characterized in that: The preparation method is: (1) Add 0.12-0.15 parts of polyurethane resin and 0.04-0.06 parts of surfactant to 10 parts of water, stir evenly, add 1 part of anhydrous ferric phosphate, 0.20-0.24 parts of lithium carbonate, 0.03-0.04 parts of rubidium carbonate and 0.01-0.02 parts of sodium carbonate, stir evenly to obtain a mixture A; wherein the surfactant is a compound of polysorbate, alkylphenol polyoxyethylene ether and modified allyl alcohol polyoxyethylene ether in a mass ratio of 2-3:1:0.4-0.7; (2) Grinding the mixture A with a grinder, spray drying, and obtaining powder A, followed by calcining at 750-800° C. for 6-7 h under a nitrogen atmosphere to obtain metal-doped carbon-coated lithium iron phosphate; (3) mixing metal-doped carbon-coated lithium iron phosphate, lanthanum phosphate, glucose, modified graphene oxide, and water in a mass ratio of 1:0.03-0.04:0.03-0.04:0.01-0.02:10-15, stirring uniformly, and obtaining a mixture B; (4) Grinding the mixture B with a grinder, spray drying, and obtaining powder B, and then calcining at 750-800° C. for 9-10 h under a nitrogen atmosphere to obtain a low-temperature resistant lithium iron phosphate cathode material; Preparation method of modified allyl alcohol polyoxyethylene ether: 30-35 parts of polysiloxane containing Si-H bonds at the end groups and 7-10 parts of allyl alcohol polyoxyethylene ether are mixed evenly, heated to 80-100° C. and stirred for reaction for 50-60 minutes, 3-5 parts of chloroplatinic acid and 5-6 parts of 2,3-dimethylmaleic anhydride are added and mixed evenly, heated to 85-90° C., reacted for 3-5 hours, and cooled to room temperature to obtain modified allyl alcohol polyoxyethylene ether; Preparation method of modified graphene oxide: 1 part of graphene oxide and 17-19 parts of 18-20wt% ammonia water are mixed, stirred and reacted at 30°C for 10-15h, heated to 65°C and reacted for 10-14h, filtered, washed, and vacuum dried to obtain amino-modified graphene oxide; 1 part of amino-modified graphene oxide, 1.3-1.4 parts of 3,5-diethyl-2,4-toluenediamine, 7-8 parts of a 50wt% isopropanol aqueous solution, and 2-3 parts of a 2.5mol / L hydrochloric acid aqueous solution are mixed, 4-5 parts of a 0.7mol / L ammonium persulfate aqueous solution are added dropwise at 0°C, stirred and reacted for 9-10h, filtered, washed, and vacuum dried to obtain modified graphene oxide; The molar number of Si-H bonds in the polysiloxane containing Si-H bonds at the terminal groups is 1.2-1.4 mmol / g; The above parts are parts by mass.

2. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material according to claim 1, characterized in that: The polyurethane resin is a water-soluble polyurethane resin with a solid content of 30±1 wt % and a viscosity of 60-85 cps.

3. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material according to claim 1, characterized in that: (2) The mixed material A is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

4. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material according to claim 1, characterized in that: (4) The mixed material B is ground with a grinder to a particle size D50 of 0.2-0.3 μm.

5. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material according to claim 1, characterized in that: The sheet diameter of graphene oxide is 0.4-0.8 μm.

6. A method for preparing a low-temperature resistant lithium iron phosphate positive electrode material according to claim 1, characterized in that: The polysorbate is Tween-60; the alkylphenol polyoxyethylene ether is OP-12.

7. A low-temperature-resistant lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 6.

Images