A lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery

Abstract

This invention provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. The preparation method includes: placing a lithium iron phosphate cathode material precursor in a sealed container for sealed calcination to obtain the lithium iron phosphate cathode material; wherein, the self-generated gas generated during the vacuum sealed calcination increases the gas pressure inside the sealed container; by performing vacuum sealed calcination on the lithium iron phosphate cathode material precursor, the gas generated during the calcination process causes the internal pressure of the sealed container to continuously increase. The gradual increase in pressure reduces the lattice defects on the surface of lithium iron phosphate, improves the crystallinity of lithium iron phosphate, and ultimately increases the migration rate of lithium ions on the material surface, thereby improving the electrochemical performance of the lithium-ion battery.

Description

Technical Field

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

[0002] Currently, the most common method for preparing lithium iron phosphate cathode materials is solid-state synthesis. Solid-state synthesis requires high temperatures and must be carried out in an inert gas protected environment. It mainly includes high-temperature solid-state synthesis, mechanochemical method, and microwave sintering method.

[0003] Currently, high-temperature solid-state processing is widely used. The general process for preparing lithium iron phosphate by high-temperature solid-state method is to first ball-mill and mix the raw materials, then dry and / or pre-calcine them into a precursor, and finally sinter to obtain lithium iron phosphate.

[0004] The currently used high-temperature solid-state reaction method involves mixing ferrous salts (such as ferrous oxalate and ferrous acetate) with ammonium hydrogen phosphate and lithium salts (such as lithium carbonate and lithium hydroxide), and then calcining them in stages at 300-350℃ and 500-800℃ under an inert atmosphere (such as argon and nitrogen) to synthesize LiFePO4. The entire synthesis process releases ammonia gas, making it difficult to obtain pure LiFePO4. The reaction is incomplete and contains various impurities, such as ferric ions and pyrophosphates. This is mainly due to the incomplete reaction, resulting in low crystallinity. Furthermore, the use of inert gases during the reaction not only incurs additional costs but also requires careful attention to the influence of atmospheric oxygen on the reaction.

[0005] Therefore, there is a need to develop new methods for preparing lithium iron phosphate. Summary of the Invention

[0006] In view of the problems existing in the prior art, the present invention provides a lithium iron phosphate cathode material, its preparation method and lithium-ion battery. Based on the solid-state reaction method, the reaction pressure in the reaction process is changed by adjusting the loading amount of the lithium iron phosphate cathode material precursor. Through one pre-calcination and one vacuum-sealed calcination, the crystallinity of lithium iron phosphate is improved, and the influence of oxygen in the air on the reaction can be avoided. The lithium iron phosphate cathode material prepared by the present invention has high crystallinity and purity, thereby improving the ion mobility and material conductivity of lithium iron phosphate.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, the method comprising: placing a lithium iron phosphate cathode material precursor in a sealed container for vacuum-sealed calcination to obtain a lithium iron phosphate cathode material; wherein, the self-generated gas generated by the vacuum-sealed calcination increases the gas pressure inside the sealed container.

[0009] This invention involves placing the lithium iron phosphate cathode material precursor in a sealed container, then evacuating and sealing the container, and subjecting it to vacuum calcination at a high temperature. Because high-temperature calcination continues to generate gases (NH3 and / or CO2, etc.), this creates different internal pressures in sealed containers of different capacities. This internal pressure reduces the lattice defects of lithium iron phosphate, thereby producing highly crystalline lithium iron phosphate, ultimately increasing the migration rate of lithium ions on the material surface, and thus improving the electrochemical performance of lithium-ion batteries.

[0010] Preferably, the ratio between the volume of the sealed container and the mass of the lithium iron phosphate cathode material precursor during the vacuum-sealed calcination is 1–10:1 g / cm³. 3 For example, it could be 1:1 g / cm³ 3 2:1, 3:1 g / cm 3 4:1g / cm 3 5:1g / cm 3 6:1g / cm 3 7:1g / cm 3 8:1g / cm 3 9:1g / cm 3 Or 10:1g / cm 3 This includes, but is not limited to, the listed values; other unlisted values ​​within this range also apply.

[0011] Through extensive research, this invention has found that controlling the ratio between the volume of the sealed container and the mass of the lithium iron phosphate cathode material precursor to 1 to 10:1 will help improve the crystallinity of lithium iron phosphate and result in better electrochemical performance.

[0012] Preferably, at the initial stage of vacuum-sealed calcination, the absolute pressure range inside the sealed container is 0.01 Pa to 0.1 Pa, for example, it can be 0.01 Pa, 0.02 Pa, 0.03 Pa, 0.04 Pa, 0.05 Pa, 0.06 Pa, 0.07 Pa, 0.08 Pa, 0.09 Pa, or 0.1 Pa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0013] Preferably, the vacuum-sealed calcination temperature is 600–800°C, for example, it can be 600°C, 620°C, 640°C, 660°C, 680°C, 710°C, 730°C, 750°C, 770°C or 800°C, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0014] Preferably, the vacuum-sealed calcination time is 6 to 12 hours, for example, it can be 6 hours, 6.7 hours, 7.4 hours, 8 hours, 8.7 hours, 9.4 hours, 10 hours, 10.7 hours, 11.4 hours or 12 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] Preferably, the self-generated gas from the vacuum-sealed calcination includes any one or a combination of at least two of ammonia, carbon monoxide, or carbon dioxide, wherein typical but non-limiting combinations are a combination of ammonia and carbon monoxide, a combination of carbon dioxide and carbon monoxide, or a combination of ammonia and carbon dioxide.

[0016] Preferably, the preparation of the lithium iron phosphate cathode material precursor includes: mixing a lithium source, an iron source and a phosphorus source, and then sequentially ball-milling, drying and pre-calcining the mixture to obtain the lithium iron phosphate cathode material precursor.

[0017] The present invention first places the mixture in a non-sealed environment and removes most of the gas by pre-firing, so as to avoid the influence of excessive gas generated by the reaction on the product.

[0018] This invention does not impose any special limitations on ball milling; any ball milling process known to those skilled in the art for preparing lithium iron phosphate cathode material precursors can be used, and adjustments can be made according to actual circumstances.

[0019] Preferably, the lithium source includes any one or a combination of at least two of lithium acetate, lithium dihydrogen phosphate, lithium carbonate, or lithium hydroxide, wherein typical but non-limiting combinations are combinations of lithium acetate and lithium dihydrogen phosphate, combinations of lithium carbonate and lithium dihydrogen phosphate, combinations of lithium acetate and lithium carbonate, and combinations of lithium hydroxide and lithium dihydrogen phosphate.

[0020] Preferably, the iron source includes any one or a combination of at least two of ferrous oxalate, ferric nitrate, ferric phosphate, or ferric oxide, wherein typical but non-limiting combinations are the combination of ferrous oxalate and ferric nitrate, the combination of ferric phosphate and ferric nitrate, the combination of ferrous oxalate and ferric phosphate, the combination of ferric oxide and ferric nitrate, and the combination of ferrous oxalate and ferric oxide.

[0021] Preferably, the phosphorus source includes any one or a combination of at least two of lithium phosphate, iron phosphate, lithium dihydrogen phosphate, monoammonium phosphate, or phosphoric acid, wherein typical but non-limiting combinations are combinations of lithium phosphate and iron phosphate, combinations of lithium dihydrogen phosphate and iron phosphate, combinations of lithium phosphate and lithium dihydrogen phosphate, combinations of monoammonium phosphate and iron phosphate, and combinations of lithium phosphate and phosphoric acid.

[0022] Preferably, the pre-calcination temperature is 300-400℃, for example, it can be 300℃, 310℃, 320℃, 330℃, 345℃, 350℃, 360℃, 370℃, 380℃ or 400℃, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0023] Preferably, the pre-calcination time is 8 to 12 hours, for example, it can be 8 hours, 8.5 hours, 8.9 hours, 9.4 hours, 9.8 hours, 10.3 hours, 10.7 hours, 11.2 hours, 11.6 hours or 12 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] Preferably, the pre-calcination is carried out in a protective atmosphere.

[0025] Preferably, the protective atmosphere includes a nitrogen atmosphere.

[0026] Preferably, the pre-calcination and the vacuum-sealed calcination are carried out in the same sealed container, and the pre-calcination is not subject to vacuuming and sealing treatment; after the pre-calcination and before the vacuum-sealed calcination, the sealed container is first subjected to vacuuming and sealing treatment in sequence.

[0027] As a preferred embodiment of the first aspect of the present invention, a method for preparing a lithium iron phosphate cathode material is provided, the method comprising the following steps:

[0028] (1) Mix lithium source, iron source and ammonium phosphorus source, ball mill to mix evenly and then dry to obtain a well dried mixture;

[0029] The mixture was filled into a container and then pre-calcined under a nitrogen atmosphere for 6 hours to obtain a lithium iron phosphate cathode material precursor.

[0030] (2) The container containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated and sealed to form a sealed container. Then, vacuum sealing calcination is performed. The self-generated gas generated by the vacuum sealing calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to obtain the lithium iron phosphate cathode material.

[0031] Secondly, the present invention provides a lithium iron phosphate cathode material, wherein the lithium iron phosphate cathode material is prepared by the preparation method of the lithium iron phosphate cathode material described in the first aspect.

[0032] The lithium iron phosphate cathode material provided by the second aspect of the present invention has high crystallinity and excellent electrical properties, and has broad application prospects.

[0033] Thirdly, the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises the lithium iron phosphate cathode material described in the first aspect.

[0034] Preferably, the lithium-ion battery is a button cell battery.

[0035] The lithium-ion battery provided in the third aspect of the present invention has excellent cycle performance and high discharge efficiency.

[0036] Compared with the prior art, the present invention has at least the following beneficial effects:

[0037] (1) The preparation method of lithium iron phosphate cathode material provided by the present invention has high crystallinity and purity, which can improve the ion mobility and material conductivity of lithium iron phosphate.

[0038] (2) The lithium-ion battery provided by the present invention has excellent cycle performance and high discharge efficiency. Under preferred conditions, its capacity retention rate after 50 cycles is above 96.51%, its discharge efficiency is above 96.8%, its 0.1C charging capacity is above 156.5 mAh / g, and its 0.1C discharge capacity is above 151 mAh / g. Attached Figure Description

[0039] Figure 1 This is a flowchart of the preparation method of lithium iron phosphate cathode material provided in a specific embodiment of the present invention.

[0040] Figure 2 This is a comparison of the X-ray diffraction patterns of the lithium iron phosphate cathode materials prepared in Example 2 and Comparative Example 1 of the present invention. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0042] The present invention will now be described in further detail. However, the examples described below are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0043] As a specific embodiment of the present invention, a method for preparing lithium iron phosphate cathode material is provided, such as... Figure 1 As shown, the preparation method includes the following steps:

[0044] (1) Mix lithium source, iron source and phosphorus source, ball mill to mix evenly and then dry to obtain a well dried mixture;

[0045] The mixture was filled into a container and then pre-calcined under a nitrogen atmosphere for 6 hours to obtain a lithium iron phosphate cathode material precursor.

[0046] (2) The container containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated and sealed to form a sealed container. Then, vacuum sealing calcination is performed. The self-generated gas generated by the vacuum sealing calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to obtain the lithium iron phosphate cathode material.

[0047] It should be noted that the phrase "weighing the corresponding mass of lithium source, iron source and phosphate source according to a certain stoichiometric ratio" in this invention can be adjusted according to the ratio requirements of conventional lithium iron phosphate solid-phase synthesis method, and is not specifically limited in this invention; similarly, the ball milling, drying and other operation methods can also refer to conventional methods, and are not limited here.

[0048] Example 1

[0049] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0050] (1) Weigh lithium carbonate, ferric oxide and monoammonium phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 100℃ for 12 hours to obtain a well dried mixture.

[0051] 10g of the mixture was filled into a 30mL ampoule and then pre-calcined at 400°C for 6h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0052] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 600°C for 12 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (25°C) to obtain the lithium iron phosphate cathode material.

[0053] Example 2

[0054] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0055] (1) Weigh lithium carbonate, ferric oxide and monoammonium phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 100℃ for 12 hours to obtain a well dried mixture.

[0056] 10g of the mixture was filled into a 50mL ampoule and then pre-calcined at 400°C for 6h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0057] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 600°C for 12 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (25°C) to obtain the lithium iron phosphate cathode material.

[0058] Example 3

[0059] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0060] (1) Weigh lithium carbonate, ferric oxide and monoammonium phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 100℃ for 12 hours to obtain a well dried mixture.

[0061] 10g of the mixture was filled into an 80mL ampoule and then pre-calcined at 400°C for 6h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0062] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 600°C for 12 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (25°C) to obtain the lithium iron phosphate cathode material.

[0063] Example 4

[0064] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0065] (1) Weigh lithium carbonate, ferric oxide and monoammonium phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 100℃ for 12 hours to obtain a well dried mixture.

[0066] 10g of the mixture was filled into a 100mL ampoule and then pre-calcined at 400°C for 6h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0067] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 600°C for 12 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (25°C) to obtain the lithium iron phosphate cathode material.

[0068] Example 5

[0069] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0070] (1) Weigh lithium carbonate, ferric oxide and ammonium dihydrogen phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 120°C for 8 hours to obtain a well dried mixture.

[0071] 10g of the mixture was filled into an 80mL ampoule and then pre-calcined at 300°C for 12h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0072] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 800°C for 6 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (10°C) to obtain the lithium iron phosphate cathode material.

[0073] Example 6

[0074] This embodiment provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0075] (1) Weigh lithium acetate, phosphoric acid and ferrous phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 80°C for 15 hours to obtain a well dried mixture.

[0076] 10g of the mixture was filled into a 120mL ampoule and then pre-calcined at 400°C for 8h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0077] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is evacuated to a vacuum degree of 0.06 Pa and sealed. Then, it is vacuum sealed and calcined at 700°C for 10 hours. The self-generated gas generated by the vacuum sealed calcination increases the gas pressure in the sealed container. Then, it is naturally cooled to room temperature (30°C) to obtain the lithium iron phosphate cathode material.

[0078] Example 7

[0079] This embodiment provides a method for preparing lithium iron phosphate cathode material. Except for the ampoule volume being only 5 mL, the preparation method is the same as in Example 1, and will not be repeated here.

[0080] Example 8

[0081] This embodiment provides a method for preparing lithium iron phosphate cathode material. The preparation method is identical to that in Example 1, except that the ampoule volume is 300 mL.

[0082] Comparative Example 1

[0083] This embodiment provides a method for preparing lithium iron phosphate cathode material. Except for step (2), in which the ampoule is not vacuumed and sealed, the preparation method is the same as in embodiment 1, and will not be repeated here.

[0084] Specifically, the steps include the following:

[0085] (1) Weigh lithium carbonate, ferric oxide and monoammonium phosphate in a molar ratio of Li:Fe:P of 3:1:1, mix them evenly by ball milling and dry them in a vacuum oven at 100℃ for 12 hours to obtain a well dried mixture.

[0086] 10g of the mixture was filled into a 50mL ampoule and then pre-calcined at 400°C for 6h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material precursor.

[0087] (2) The ampoule containing the lithium iron phosphate cathode material precursor obtained in step (1) is calcined at 600°C for 12 hours in a nitrogen atmosphere (without vacuuming or sealing), and then naturally cooled to room temperature (25°C) to obtain the lithium iron phosphate cathode material.

[0088] Comparative Example 2

[0089] This embodiment provides a method for preparing lithium iron phosphate cathode material. Except for step (2), in which the ampoule is not vacuumed and is directly sealed, the preparation method is the same as in embodiment 1, and will not be repeated here.

[0090] Comparative Example 3

[0091] This embodiment provides a method for preparing lithium iron phosphate cathode material. Except for step (1), in which the ampoule is first evacuated and sealed before pre-calcination, the preparation method is the same as in Example 1, and will not be repeated here.

[0092] Test method: XRD was used to detect the crystallinity of lithium iron phosphate cathode material.

[0093] Figure 2This is a comparison of the X-ray diffraction (XRD) spectra of lithium iron phosphate in Example 2 and Comparative Example 1. Crystallinity refers to the number and arrangement of crystals in a sample. In XRD analysis, crystallinity can be determined by calculating the intensity and shape of diffraction peaks. The more crystals in the sample, the higher the intensity of the diffraction peaks, and the higher the crystallinity. Furthermore, a better arrangement of crystals results in sharper peaks and narrower peak widths, also indicating higher crystallinity. Figure 2 It can be found that by changing the amount of precursor in a closed container to generate different internal pressures, the crystallinity of lithium iron phosphate can be effectively improved.

[0094] The lithium iron phosphate positive electrode material was then assembled into a lithium-ion battery. The specific assembly process is as follows: using metallic lithium as the negative electrode, electrolyte purchased from Guangzhou Tinci Chemical Co., Ltd., acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, a positive electrode sheet was made. The battery was then assembled into a button cell for electrochemical performance testing. The assembly steps were as follows: first, the negative electrode shell was placed, followed by a spacer. After placing the spacer, 1-2 drops of electrolyte were added to the spacer. Then, the lithium electrode sheet was placed, and 5-6 drops of electrolyte were added to the lithium electrode sheet. Next, the separator was placed, and 3-4 drops of electrolyte were added to the separator. Then, the electrode sheet was placed, and 1-2 drops of electrolyte were added to the electrode sheet. Finally, the positive electrode shell was placed on top, and the assembled button cell was stamped into shape on a dedicated battery stamping press. The lithium-ion battery was then subjected to charge-discharge tests at a constant temperature of 25°C and cycled for 50 cycles. The capacity retention rate after 50 cycles was calculated.

[0095] The test results of the above embodiments and comparative examples are shown in Table 1.

[0096] Table 1

[0097]

[0098]

[0099] The following points can be observed from Table 1:

[0100] (1) As can be seen from the comprehensive examples 1 to 3, the preparation method of lithium iron phosphate cathode material provided by the present invention can improve the crystallinity of lithium iron phosphate and facilitate the migration of lithium ions from the bulk phase to the surface, thereby improving the utilization rate of lithium ions. On the other hand, it can also facilitate electron transport and improve the conductivity of the material. Its capacity retention rate after 50 cycles is above 96.51%, its discharge efficiency is above 96.8%, its 0.1C charging capacity is above 156.5 mAh / g, and its 0.1C discharge capacity is above 151 mAh / g.

[0101] (2) As can be seen from Examples 1 and 7-8, in Example 1, 10g of the mixture was filled into a 30mL ampoule. Compared to Examples 7-8, where the ampoule volumes were only 5mL and 300mL respectively, the capacity retention rate after 50 cycles in Example 1 was 96.51%, the discharge efficiency was 96.8%, the 0.1C charging capacity was 156.5mAh / g, and the 0.1C discharging capacity was 151mAh / g. In contrast, the capacity retention rates in Examples 7-8 were only 9... The figures of 4.13% and 94.02% indicate that, under the same reactant loading, an excessively small volume of the sealed container leads to excessive internal pressure, which may damage the reaction container and potentially cause a reverse reaction. Conversely, an excessively large volume of the sealed container under the same reactant loading results in insufficient internal pressure, failing to achieve the goal of improving the crystallinity of the product. This invention, by optimally controlling the ratio between the volume and loading of the vacuum-sealed container, can further improve the crystallinity of lithium iron phosphate and correspondingly improve the electrical properties of the material.

[0102] (3) As can be seen from the combined examples 1 and 1-3, in Example 1, the steps of first pre-calcining under non-vacuum sealing conditions to remove most of the gas and then vacuum sealing calcining under vacuum sealing conditions are used to gradually increase the pressure during the crystal growth process. Compared with Comparative Example 1, where step (2) does not involve vacuuming and sealing the ampoule, Comparative Example 2, where step (2) does not involve vacuuming the ampoule and only directly sealing it, and Comparative Example 3, where step (1) does not involve vacuuming the ampoule and sealing it before pre-calcining, the results are different. In Example 1, the capacity retention rate after 50 cycles was 96.51%, the discharge efficiency was 96.8%, the 0.1C charging capacity was 156.5 mAh / g, and the 0.1C discharging capacity was 151 mAh / g. In contrast, the capacity retention rate, capacity retention rate, and charge / discharge capacity of Comparative Examples 1 to 3 were all inferior to those of Example 1. This indicates that the preparation method of lithium iron phosphate cathode material provided by the present invention, through the combination of pre-calcination and subsequent vacuum-sealed calcination, and by controlling the vacuum and sealing conditions during the two calcination processes, is beneficial to improving the crystallinity of lithium iron phosphate cathode material.

[0103] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The preparation method includes: A mixture of lithium, iron and phosphorus sources is sequentially ball-milled, dried and pre-calcined to obtain a lithium iron phosphate cathode material precursor. The lithium iron phosphate cathode material precursor is placed in a sealed container and calcined under vacuum to obtain the lithium iron phosphate cathode material. The self-generated gas produced by the vacuum-sealed calcination increases the gas pressure inside the sealed container; the self-generated gas from the vacuum-sealed calcination includes any one or a combination of at least two of ammonia, carbon monoxide, or carbon dioxide. The pre-calcination and the vacuum-sealed calcination are carried out in the same sealed container, and the pre-calcination is not subject to vacuuming and sealing. After the pre-calcination and before the vacuum-sealed calcination, the sealed container is first vacuumed and sealed in sequence. The ratio between the volume of the sealed container and the mass of the lithium iron phosphate cathode material precursor during the vacuum-sealed calcination process is 1~10:1cm³. 3 / g.

2. The preparation method according to claim 1, characterized in that, At the initial stage of vacuum-sealed calcination, the absolute pressure range inside the sealed container is 0.01 Pa to 0.1 Pa.

3. The preparation method according to claim 1, characterized in that, The vacuum-sealed calcination temperature is 600~800℃.

4. The preparation method according to claim 1, characterized in that, The vacuum-sealed calcination time is 6~12 hours.

5. The preparation method according to claim 1, characterized in that, The pre-calcination temperature is 300~400℃.

6. The preparation method according to claim 1, characterized in that, The pre-calcination time is 8-12 hours.

7. The preparation method according to claim 1, characterized in that, The precalcination is carried out in a protective atmosphere.

8. The preparation method according to claim 7, characterized in that, The protective atmosphere includes a nitrogen atmosphere.

9. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by the method for preparing lithium iron phosphate cathode material according to any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium iron phosphate cathode material as described in claim 9.

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