Lithium iron phosphate positive electrode material, preparation method and application thereof
By directly coating the surface of the lithium iron phosphate substrate with a Ge2Sb2Te5 coating layer, the low-temperature and rate performance issues of lithium iron phosphate cathode materials were solved, resulting in better battery cycle performance and capacity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SVOLT ENERGY TECHNOLOGY CO LTD
- Filing Date
- 2022-10-08
- Publication Date
- 2026-06-23
AI Technical Summary
Existing lithium iron phosphate cathode materials have poor low-temperature performance and rate performance, especially in terms of cycle performance, which fails to meet stringent requirements.
A Ge2Sb2Te5 coating layer is directly coated onto the surface of a lithium iron phosphate substrate. The hydrophilic groups are used to achieve uniform and dense coating, which reduces the lithium ion diffusion path and improves the conductivity and cycle performance of the material.
It significantly improves the rate, cycle and capacity performance of lithium iron phosphate cathode materials, especially their performance under low temperature conditions. The battery performs excellently in both high-rate and low-temperature environments.
Smart Images
Figure CN115528240B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a lithium iron phosphate cathode material, its preparation method and application. Background Technology
[0002] Currently, many systems are being researched for use as cathode materials in lithium-ion batteries, but only lithium cobalt oxide (LiCoO2), LiMn2O4, LiFePO4, and ternary composite oxides have achieved large-scale industrialization. LiCoO2 is expensive and has poor safety performance. LiMn2O4 is relatively cheaper than LiCoO2 and has slightly higher thermal stability, but its capacity is lower and its high-temperature performance is poor. Novel ternary composite oxides such as Li... 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and LiCoO2 share the same structure and possess high energy density, but the poor safety performance of ternary materials has been a persistent problem in the battery industry, remaining unresolved to this day. In 1997, Goodenough's research group first reported the lithium-ion cathode material LiFePO4, with a theoretical specific capacity of 170 mAh / g, exceeding the actual discharge specific capacity of commercially available LiCoO2. Furthermore, LiFePO4 exhibits excellent cycle performance, with a stable discharge plateau around 3.45V. LiFePO4 is currently the primary electrode material used in power batteries, its main advantages including a stable voltage plateau, inexpensive and abundant raw materials, environmental friendliness, low toxicity, and high safety due to its excellent stability.
[0003] LiFePO4 has an orthorhombic olivine-type crystal structure, belonging to the Pnmb space group, and its lattice constant is [not specified]. The crystal structure of LiFePO4 remains stable at 400℃, greatly improving its cycle performance and safety. Lithium ions migrate along one-dimensional channels in the LiFePO4 lattice, significantly limiting their diffusion rate. Furthermore, these one-dimensional channels are easily blocked by impurity defects, further reducing its ionic conductivity. Due to the very strong bonds between O atoms and Fe and P atoms, the LiFePO4 structure exhibits excellent high-temperature stability compared to layered structures such as LiCoO2. However, the strong PO bonds also lead to a lower ion diffusion rate (10 [not specified]). -13 -10 -16 cm -2 ·S -1 ) and electronic conductivity (-10 -19 cm -2 ·S -1 () is relatively low.
[0004] The low ionic and electronic conductivity reduces its actual discharge capacity, causes severe polarization, and results in unsatisfactory rate and low-temperature performance.
[0005] Improving the low-temperature and rate performance of lithium iron phosphate (LFP) materials is a pressing desire for materials researchers and manufacturers. Reducing particle size can improve its electrochemical performance; coating its surface with a conductive amorphous carbon network not only increases electronic conductivity but also inhibits grain growth, thus effectively improving ionic conductivity; and high-valence cation doping of Li or Fe sites to form P-type semiconductors further enhances conductivity. These modification methods significantly impact both ionic and electronic conductivity, thereby improving discharge capacity, cycle life, and rate performance. Currently, methods for synthesizing LFP include high-temperature solid-state synthesis, sol-gel synthesis, hydrothermal synthesis, carbothermal reduction, and spray pyrolysis, but only the high-temperature solid-state synthesis is widely used in industrial production.
[0006] CN102394312A discloses a method for improving the low-temperature performance of lithium iron phosphate. This method uses lithium manganese oxide and lithium iron phosphate as the positive electrode active materials, then prepares a slurry with the active materials, a conductive agent, and a binder, and coats it onto a current collector to form a positive electrode sheet. While the use of lithium manganese oxide in this method improves the low-temperature performance of the positive electrode active material, its rate performance at low temperatures remains poor.
[0007] CN107768667A discloses a low-temperature cycling lithium iron phosphate power battery and its preparation method. Using lithium iron phosphate cathode material, it can achieve a discharge capacity of -30℃ and 0.5C, and the capacity retention rate after 250 cycles is 80% of that at room temperature. Although the performance is relatively excellent, it still cannot meet the stringent requirements of a high rate of 1C and a capacity retention rate of more than 80% after 500 cycles.
[0008] Therefore, improving the low-temperature performance of lithium iron phosphate cathode materials, especially their cycle performance and rate performance, is an urgent technical problem to be solved. Summary of the Invention
[0009] The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method, and its applications. In this invention, a Ge2Sb2Te5 coating layer is directly coated on the surface of a lithium iron phosphate matrix containing hydrophilic groups, achieving uniform and dense coating, and effectively improving the rate capability, cycle life, and capacity of the lithium iron phosphate cathode material, especially its performance at low temperatures.
[0010] To achieve this objective, the present invention employs the following technical solution:
[0011] In a first aspect, the present invention provides a lithium iron phosphate cathode material, the lithium iron phosphate cathode material comprising a lithium iron phosphate matrix having hydrophilic groups on its surface and a coating layer covering the surface of the lithium iron phosphate matrix having hydrophilic groups, the coating layer comprising Ge2Sb2Te5.
[0012] In this invention, the Ge2Sb2Te5 coating layer is directly coated on the surface of the lithium iron phosphate substrate containing hydrophilic groups, meaning there are no other coating layers in between. Ge2Sb2Te5 is in direct contact with the surface of the lithium iron phosphate material, reducing the diffusion path of lithium ions and achieving uniform and dense coating. This also effectively improves the rate, cycle, and capacity of the lithium iron phosphate cathode material, especially its performance at low temperatures. If Ge2Sb2Te5 is coated on the surface of a carbon-coated lithium iron phosphate substrate, meaning Ge2Sb2Te5 is in direct contact with the carbon layer, the carbon coating layer itself does not have hydrophilic groups, making it impossible for the carbon-coated lithium iron phosphate material to bond tightly with the Ge2Sb2Te5 coating layer. This results in poor coating effect and deterioration in rate, low-temperature, and cycle performance.
[0013] In this invention, the unmodified lithium iron phosphate material (the modified material has hydrophilic groups on its surface) is a hydrophilic inorganic material. The hydrophilic groups are used to modify its surface to obtain a lithium iron phosphate matrix containing hydrophilic groups, thereby achieving a coating layer containing Ge2Sb2Te5, and achieving a tight coating.
[0014] Furthermore, if the lithium iron phosphate material is coated with a carbon layer, the surface cannot be modified with hydrophilic groups. Without the presence of hydrophilic groups, it is impossible to achieve a tight bond with the coating layer Ge2Sb2Te5, resulting in a poor coating effect and failing to achieve the expected electrical performance.
[0015] It should be noted that the preparation method of lithium iron phosphate material with hydrophilic inorganic material properties is common knowledge in the art, that is, lithium iron phosphate material prepared by ferrous oxalate process can be used. For example, the present invention provides a preparation method: glucose, lithium dihydrogen phosphate and ferrous oxalate are mixed and slurried, and then spray dried and sintered in sequence (under high-purity nitrogen atmosphere, at 750°C) to obtain lithium iron phosphate cathode material.
[0016] Preferably, the mass ratio of the coating layer to the lithium iron phosphate matrix containing hydrophilic groups is (0.06 to 0.12):1, for example, 0.06:1, 0.07:1, 0.08:1, 0.09:1, 0.10:1, 0.11:1 or 0.12:1, etc.
[0017] In this invention, if the mass ratio of the coating layer to the lithium iron phosphate matrix containing hydrophilic groups is too small, it will not be conducive to the uniformity of the coating and the coating effect will be poor. If the mass ratio is too large, it will lead to the coating layer being too thick, which will affect the lithium ion insertion / extraction rate.
[0018] Preferably, the thickness of the coating layer is 10-30 nm, such as 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm or 30 nm, and more preferably 20-30 nm.
[0019] In this invention, a coating thickness of 20–30 nm can better improve the lithium-ion transport rate.
[0020] Preferably, the hydrophilic groups in the lithium iron phosphate matrix containing hydrophilic groups include any one or a combination of at least two of hydroxyl, carboxyl, amino, or aldehyde groups.
[0021] In a second aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material as described in the first aspect, the method comprising the following steps:
[0022] A solution containing hydrophilic groups of lithium iron phosphate matrix is mixed with a coating material and coated, then sintered to obtain the lithium iron phosphate cathode material;
[0023] The coating material includes Ge2Sb2Te5.
[0024] In this invention, the lithium iron phosphate substrate contains hydrophilic groups on its surface. Therefore, coating is not required by complex processing methods such as magnetron sputtering or deposition. Instead, it can be obtained by simple liquid-phase mixing, coating, and sintering. Furthermore, the hydrophilic groups on the surface of the lithium iron phosphate substrate easily form strong chemical bonds with Ge2Sb2Te5, showing a tendency to form a water molecule structure, which can result in a uniform and dense coating layer.
[0025] Preferably, the method for preparing the solution of the lithium iron phosphate matrix containing hydrophilic groups includes:
[0026] Unmodified lithium iron phosphate material was mixed with a solvent, dispersed by ball milling, and then a modifier containing hydrophilic groups was added and ball milling continued to obtain a solution of lithium iron phosphate matrix containing hydrophilic groups.
[0027] In this invention, ball milling dispersion causes the unmodified lithium iron phosphate material to take the form of spherical or near-spherical shapes, which is more conducive to obtaining a uniform coating layer in subsequent modification and coating processes. Furthermore, the unmodified lithium iron phosphate material is a hydrophilic inorganic material, and hydrophilic groups are easier to modify on the material surface.
[0028] In this invention, the hydrophilic groups in the modifier include carboxylic acid groups, sulfonic acid groups, sulfuric acid groups, phosphate groups, amino groups, or quaternary ammonium groups, etc. The modifier contains hydrophilic groups and does not affect the structure and performance of the material. This invention is applicable to all such modifiers.
[0029] Preferably, the D50 of the unmodified lithium iron phosphate material is 200-1200nm, such as 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm or 1200nm, and more preferably 200-500nm.
[0030] Preferably, the solid content of the unmodified lithium iron phosphate material in the mixed solution is ≤30wt%, for example, 5wt%, 8wt%, 10wt%, 13wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, or 30wt%, preferably 5-30wt%, and more preferably 15-30wt%.
[0031] Preferably, the mass of the modifier containing hydrophilic groups is 1.5 to 2.5 wt% of the mass of the unmodified lithium iron phosphate material, for example, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, or 2.5 wt%.
[0032] In this invention, if the mass of the modifier is too small, it will affect the continuity of the coating process and result in uneven coating. On the other hand, if the mass of the modifier is too large, it will lead to the introduction of non-system substances, which, if they cannot be removed later, will affect the electrical properties of the material.
[0033] Preferably, the solvent includes an alcohol, and more preferably methanol.
[0034] In this invention, alcohol solvents are used to achieve better dispersion, while methanol enables dispersion between particles after ball milling, avoiding the agglomeration of excessively small particles.
[0035] Preferably, the method of mixed coating includes ball milling coating.
[0036] Preferably, the mixture is centrifuged, filtered, and vacuum dried sequentially after coating.
[0037] Preferably, the vacuum drying temperature is 100-120°C, such as 100°C, 103°C, 105°C, 108°C, 110°C, 113°C, 115°C, 118°C, or 120°C.
[0038] Preferably, the sintering temperature is 600-700℃, such as 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃ or 700℃.
[0039] In this invention, if the sintering temperature is too low, it is not conducive to the growth of the crystal structure, and the uniformity and density of the coating layer will deteriorate. If the sintering temperature is too high, it will lead to the formation of non-lithium iron phosphate impurity phases, which will affect the electrical performance.
[0040] As a preferred technical solution, the preparation method includes the following steps:
[0041] (1) Mix the unmodified lithium iron phosphate material with a solvent. The solid content of the unmodified lithium iron phosphate material in the mixed solution is 15-30 wt%. Disperse by ball milling, add a modifier containing hydrophilic groups and continue ball milling to obtain a solution of lithium iron phosphate matrix containing hydrophilic groups.
[0042] (2) A solution of lithium iron phosphate matrix containing hydrophilic groups is ball-milled with a coating material, centrifuged, filtered, vacuum dried at 100-120°C, and sintered at 600-700°C to obtain the lithium iron phosphate cathode material.
[0043] In step (1), the mass of the modifier containing hydrophilic groups is 1.5 to 2.5 wt% of the mass of the unmodified lithium iron phosphate material, and the coating material includes Ge2Sb2Te5.
[0044] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the lithium iron phosphate cathode material as described in the first aspect.
[0045] Compared with the prior art, the present invention has the following beneficial effects:
[0046] (1) In this invention, the Ge2Sb2Te5 coating layer is directly coated on the surface of the lithium iron phosphate matrix containing hydrophilic groups, i.e., there are no other coating layers in between. The Ge2Sb2Te5 directly contacts the surface of the lithium iron phosphate material, reducing the diffusion path of lithium ions and achieving uniform and dense coating. It also effectively improves the rate, cycle and capacity of the lithium iron phosphate cathode material, especially its performance at low temperatures. The battery uses the lithium iron phosphate material provided by this invention as the cathode. At the same time, the thickness of the Ge2Sb2Te5 coating layer in the cathode material, the D50 of the unmodified lithium iron phosphate material and the amount of modifier added are controlled. The battery can cycle at least 4803 times or even more in a charge-discharge environment of 25°C, 2.0-3.65V, 6C before decaying to 80%. The battery can achieve a discharge capacity of more than 15.1Ah in a charge-discharge environment of -40°C, 1.8-3.65V, 1C.
[0047] (2) The preparation method provided by the present invention obtains a uniform and dense coating layer through simple liquid phase coating and sintering, without the need for complicated preparation process, and is suitable for large-scale production. Attached Figure Description
[0048] Figure 1 The image shows a SEM image of the lithium iron phosphate cathode material provided in Example 1.
[0049] Figure 2 SEM image of the lithium iron phosphate cathode material provided for Comparative Example 1.
[0050] Figure 3 A comparison chart of the cycle performance of the batteries provided in Example 1 and Comparative Example 1. Detailed Implementation
[0051] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.
[0052] By way of example, the present invention provides a detailed method for preparing an unmodified lithium iron phosphate material:
[0053] Glucose, lithium dihydrogen phosphate, ferrous oxalate, and deionized water were mixed to form a slurry, which was then coarsely ground, finely ground, and spray-dried to form a light yellow powder. Finally, the powder was sintered at 750°C under a high-purity nitrogen atmosphere to obtain a modified lithium iron phosphate material. The following examples and comparative examples all used the unmodified lithium iron phosphate material obtained from the preparation for modification treatment.
[0054] Example 1
[0055] This embodiment provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix with hydrophilic groups on its surface and a coating layer covering the surface of the lithium iron phosphate matrix with hydrophilic groups. The coating layer includes Ge2Sb2Te5, the mass ratio of the coating layer to the lithium iron phosphate matrix with hydrophilic groups is 0.06:1, and the thickness of the coating layer is 20 nm.
[0056] The preparation method of the lithium iron phosphate cathode material is as follows:
[0057] 1) Lithium iron phosphate with a secondary particle size of 200 nm (D50) was dispersed in methanol to obtain dispersion A with a solid content of 15 wt%.
[0058] 2) The lithium iron phosphate in dispersion A was dispersed by ball milling.
[0059] 3) Add a hydrophilic modifier containing aldehyde groups (uronic acid) to dispersion A. The amount of modifier added is 2 wt% of the mass of unmodified lithium iron phosphate. Disperse by ball milling to modify the surface of lithium iron phosphate and obtain dispersion B (a solution of lithium iron phosphate with hydrophilic groups on the surface).
[0060] 4) Add the coating material Ge2Sb2Te5 to the dispersion B and disperse by ball milling to obtain the dispersion C in which the coating material Ge2Sb2Te5 is deposited on the surface of lithium iron phosphate;
[0061] 5) The dispersion C was centrifuged, filtered, vacuum dried at 120℃, and calcined at 700℃ to finally obtain a modified coated lithium iron phosphate material with a coating thickness of 20nm.
[0062] Example 2
[0063] This embodiment provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix with hydrophilic groups on its surface and a coating layer covering the surface of the lithium iron phosphate matrix with hydrophilic groups. The coating layer includes Ge2Sb2Te5, the mass ratio of the coating layer to the lithium iron phosphate matrix with hydrophilic groups is 0.12:1, and the thickness of the coating layer is 30 nm.
[0064] The preparation method of the lithium iron phosphate cathode material is as follows:
[0065] 1) Lithium iron phosphate with a secondary particle size of 400 nm (D50) was dispersed in methanol to obtain a dispersion A with a solid content of 20 wt%.
[0066] 2) The lithium iron phosphate in dispersion A was dispersed by ball milling.
[0067] 3) Add a hydrophilic modifier (polyacrylic acid) containing hydroxyl groups to dispersion A. The amount of modifier added is 2.5 wt% of the mass of unmodified lithium iron phosphate. Disperse by ball milling to modify the surface of lithium iron phosphate and obtain dispersion B (a solution of lithium iron phosphate with hydrophilic groups on the surface).
[0068] 4) Add the coating material Ge2Sb2Te5 to the dispersion B and disperse by ball milling to obtain the dispersion C in which the coating material Ge2Sb2Te5 is deposited on the surface of lithium iron phosphate;
[0069] 5) The dispersion C was centrifuged, filtered, vacuum dried at 100℃, and calcined at 650℃ to finally obtain a modified coated lithium iron phosphate material with a coating thickness of 30nm.
[0070] Example 3
[0071] This embodiment provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix with hydrophilic groups on its surface and a coating layer covering the surface of the lithium iron phosphate matrix with hydrophilic groups. The coating layer includes Ge2Sb2Te5, the mass ratio of the coating layer to the lithium iron phosphate matrix with hydrophilic groups is 0.1:1, and the thickness of the coating layer is 25 nm.
[0072] The preparation method of the lithium iron phosphate cathode material is as follows:
[0073] 1) Dispersion A with a solid content of 10wt% was obtained by dispersing lithium iron phosphate with secondary particle size of 500nm (D50) in methanol.
[0074] 2) The lithium iron phosphate in dispersion A was dispersed by ball milling.
[0075] 3) Add a hydrophilic modifier containing carboxyl groups (sodium polyacrylate) to dispersion A. The amount of modifier added is 1.5 wt% of the mass of unmodified lithium iron phosphate. Disperse by ball milling to modify the surface of lithium iron phosphate and obtain dispersion B (a solution of lithium iron phosphate with hydrophilic groups on the surface).
[0076] 4) Add the coating material Ge2Sb2Te5 to the dispersion B and disperse by ball milling to obtain the dispersion C in which the coating material Ge2Sb2Te5 is deposited on the surface of lithium iron phosphate;
[0077] 5) The dispersion C was centrifuged, filtered, vacuum dried at 110℃, and calcined at 600℃ to finally obtain a modified coated lithium iron phosphate material with a coating thickness of 25nm.
[0078] Example 4
[0079] The difference between this embodiment and Embodiment 1 is that the thickness of the coating layer in this embodiment is 15 nm, and the mass ratio of the coating layer to the lithium iron phosphate matrix containing hydrophilic groups is 0.05:1.
[0080] The remaining preparation methods and parameters are consistent with those in Example 1.
[0081] Example 5
[0082] The difference between this embodiment and Embodiment 1 is that the thickness of the coating layer in this embodiment is 35 nm, and the mass ratio of the coating layer to the lithium iron phosphate matrix containing hydrophilic groups is 0.15:1.
[0083] The remaining preparation methods and parameters are consistent with those in Example 1.
[0084] Example 6
[0085] The difference between this embodiment and Embodiment 1 is that in step 3) of this embodiment, the amount of modifier added is 1 wt% of the mass of the unmodified lithium iron phosphate.
[0086] The remaining preparation methods and parameters are consistent with those in Example 1.
[0087] Example 7
[0088] The difference between this embodiment and Embodiment 1 is that in step 3) of this embodiment, the amount of modifier added is 3 wt% of the mass of the unmodified lithium iron phosphate.
[0089] The remaining preparation methods and parameters are consistent with those in Example 1.
[0090] Example 8
[0091] The difference between this embodiment and Embodiment 1 is that the D50 of lithium iron phosphate in step 1) of this embodiment is 800nm.
[0092] The remaining preparation methods and parameters are consistent with those in Example 1.
[0093] Comparative Example 1
[0094] The difference between this comparative example and Example 1 is that this comparative example does not modify or coat the lithium iron phosphate material.
[0095] Figure 1 The SEM image of the lithium iron phosphate cathode material provided in Example 1 is shown. Figure 2 The SEM image of the lithium iron phosphate cathode material provided in Comparative Example 1 is shown. Figure 1 and Figure 2 The comparison shows that the lithium iron phosphate material prepared by the embodiment of the present invention has a smooth surface and a better coating effect, while the unmodified lithium iron phosphate material in Comparative Example 1 has a rough surface and obvious particle aggregation.
[0096] Figure 3 A comparison chart of the cycle performance of the batteries provided in Example 1 and Comparative Example 1, from... Figure 3 It can be seen that the lithium iron phosphate cathode material prepared using Example 1 of the present invention has significantly better cycle performance than the uncoated lithium iron phosphate in Comparative Example 1, and also shows that the lithium iron phosphate cathode material provided by the present invention has good cycle performance at high rates.
[0097] Comparative Example 2
[0098] The difference between this comparative example and Example 1 is that this comparative example does not modify the lithium iron phosphate material, but directly coats it.
[0099] The remaining preparation methods and parameters are consistent with those in Example 1.
[0100] Comparative Example 3
[0101] The difference between this comparative example and Example 1 is that the unmodified material is carbon-coated lithium iron phosphate material, that is, step 1) is carbon-coated lithium iron phosphate material.
[0102] The remaining preparation methods and parameters are consistent with those in Example 1.
[0103] The lithium iron phosphate cathode materials and conductive carbon black provided in Examples 1-8 and Comparative Examples 1-3 were added to PVDF adhesive (the mass ratio of lithium iron phosphate powder: conductive agent: PVDF adhesive was 96.8:1.2:2) to obtain a cathode slurry. The cathode slurry was coated on the surface of the cathode current collector and dried by roller pressing to prepare a cathode sheet. The cathode sheet and graphite anode sheet were used for full-cell assembly verification and the corresponding electrical performance was tested.
[0104] The electrochemical performance of the batteries provided in Examples 1-8 and Comparative Examples 1-3 was tested under the following conditions:
[0105] (1) Charge and discharge tests were conducted at 25℃, 2.0-3.65V, and 6C.
[0106] (2) Charge and discharge tests were conducted at -40℃, 1.8-3.65V, and 1C.
[0107] The results of the above tests are shown in Table 1.
[0108] Table 1
[0109]
[0110]
[0111] The data results from Examples 1, 4, and 5 show that if the coating layer is too thin and has too little mass, it is not conducive to reducing the contact area between the electrolyte and the material, and the improvement effect on low temperature and cycle performance is not good. On the other hand, if the coating layer is too thick and has too much mass, it will lead to a decrease in lithium ion migration rate and a deterioration in performance.
[0112] The data from Examples 1, 6, and 7 show that adding too much modifier increases the content of non-system substances, and the introduction of impurities deteriorates the performance. Adding too little modifier makes it difficult to achieve the modification effect, resulting in poor performance.
[0113] The data results from Examples 1 and 8 show that if the D50 of the lithium iron phosphate material is too large, it will affect the uniformity and density of the coating and thus the electrical performance.
[0114] The data from Example 1 and Comparative Example 1 show that without modification and coating of lithium iron phosphate, it is impossible to suppress the contact between the electrolyte and the material, improve the migration rate of lithium ions, and improve cycle, low temperature and rate performance.
[0115] The data from Example 1 and Comparative Example 2 show that directly coating lithium iron phosphate without modifying its hydrophilic groups results in poor coating effect, which fails to effectively inhibit the contact between the electrolyte and the material, leading to poor performance.
[0116] The data from Example 1 and Comparative Example 3 show that by further modifying lithium iron phosphate coated with a carbon layer by coating it with hydrophilic groups and then using the preparation method provided by this invention, it is impossible to obtain a uniformly coated and dense modified material, and the effect of improved electrical performance that can be achieved in this paper cannot be obtained.
[0117] In summary, in this invention, the Ge2Sb2Te5 coating layer is directly coated on the surface of the lithium iron phosphate matrix containing hydrophilic groups, meaning there are no other coating layers in between. The Ge2Sb2Te5 directly contacts the surface of the lithium iron phosphate material, reducing the diffusion path of lithium ions and achieving a uniform and dense coating. This also effectively improves the rate capability, cycle life, and capacity of the lithium iron phosphate cathode material, especially its performance at low temperatures. Using the lithium iron phosphate material provided by this invention as the cathode, and by controlling the thickness of the Ge2Sb2Te5 coating layer, the D50 of the unmodified lithium iron phosphate material, and the amount of modifier added, the battery can withstand at least 4803 cycles or more in a 25°C, 2.0-3.65V, 6C charge / discharge environment before its capacity decays to 80%. In a -40°C, 1.8-3.65V, 1C charge / discharge environment, the discharge capacity can reach over 15.1Ah.
[0118] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.
Claims
1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material includes a lithium iron phosphate matrix with hydrophilic groups on its surface and a coating layer covering the surface of the lithium iron phosphate matrix with hydrophilic groups, wherein the coating layer includes Ge2Sb2Te5. The mass ratio of the coating layer to the lithium iron phosphate matrix containing hydrophilic groups is (0.06~0.12):1; The hydrophilic groups in the lithium iron phosphate matrix containing hydrophilic groups include any one or a combination of at least two of hydroxyl, carboxyl, amino, or aldehyde groups. The lithium iron phosphate cathode material is prepared by the following method, which includes the following steps: A solution containing hydrophilic groups of lithium iron phosphate matrix is mixed with a coating material and coated, then sintered to obtain the lithium iron phosphate cathode material; The coating material includes Ge2Sb2Te5.
2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The thickness of the coating layer is 10~30nm.
3. The lithium iron phosphate cathode material according to claim 2, characterized in that, The thickness of the coating layer is 20~30nm.
4. A method for preparing the lithium iron phosphate cathode material as described in any one of claims 1-3, characterized in that, The preparation method includes the following steps: A solution containing hydrophilic groups of lithium iron phosphate matrix is mixed with a coating material and coated, then sintered to obtain the lithium iron phosphate cathode material; The coating material includes Ge2Sb2Te5.
5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The method for preparing the solution of the lithium iron phosphate matrix containing hydrophilic groups includes: Unmodified lithium iron phosphate material was mixed with a solvent, dispersed by ball milling, and then a modifier containing hydrophilic groups was added and ball milling continued to obtain a solution of lithium iron phosphate matrix containing hydrophilic groups.
6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The D50 of the unmodified lithium iron phosphate material is 200~1200nm.
7. The method for preparing the lithium iron phosphate cathode material according to claim 6, characterized in that, The D50 of the unmodified lithium iron phosphate material is 200~500nm.
8. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, In the mixed solution, the solid content of the unmodified lithium iron phosphate material is ≤30wt%.
9. The method for preparing the lithium iron phosphate cathode material according to claim 8, characterized in that, The solid content of the unmodified lithium iron phosphate material in the mixed solution is 5-30 wt%.
10. The method for preparing the lithium iron phosphate cathode material according to claim 9, characterized in that, The solid content of unmodified lithium iron phosphate materials is 15~30wt%.
11. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The mass of the modifier containing hydrophilic groups is 1.5 to 2.5 wt% of the mass of the unmodified lithium iron phosphate material.
12. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The solvent includes alcohols.
13. The method for preparing lithium iron phosphate cathode material according to claim 12, characterized in that, The solvent includes methanol.
14. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The method of hybrid coating includes ball milling coating.
15. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The mixture is then centrifuged, filtered, and vacuum dried sequentially.
16. The method for preparing lithium iron phosphate cathode material according to claim 15, characterized in that, The vacuum drying temperature is 100~120℃.
17. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The sintering temperature is 600~700℃.
18. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The preparation method includes the following steps: (1) Mix the unmodified lithium iron phosphate material with a solvent. The solid content of the unmodified lithium iron phosphate material in the mixed solution is 15~30wt%. Disperse by ball milling, add a modifier containing hydrophilic groups and continue ball milling to obtain a solution of lithium iron phosphate matrix containing hydrophilic groups. (2) The solution of lithium iron phosphate matrix containing hydrophilic groups is ball-milled with the coating material, centrifuged, filtered, vacuum dried at 100~120℃, and sintered at 600~700℃ to obtain the lithium iron phosphate cathode material; In step (1), the mass of the modifier containing hydrophilic groups is 1.5 to 2.5 wt% of the mass of the unmodified lithium iron phosphate material, and the coating material includes Ge2Sb2Te5.
19. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium iron phosphate cathode material as described in any one of claims 1-3.