A modified positive electrode material and its preparation method and application

Abstract

The present invention belongs to the field of battery technology, and specifically relates to a modified positive electrode material, a preparation method thereof, and an application thereof. The preparation method comprises the following steps: adjusting the pH of a mixed solution of a nickel salt, a silicon source, and a lithium salt to 6-9, adding ascorbic acid and a fast ion conductor, and mixing to obtain a precursor solution; adding lithium iron phosphate to the precursor solution, mixing and then heating, drying, and grinding to obtain a composite precursor powder; ball-milling the composite precursor powder, heating it under an inert atmosphere, and cooling and grinding to obtain the modified positive electrode material. The present invention uses LFP with good stability to coat a lithium nickel silicate material to obtain a modified positive electrode material, thereby improving the maximum discharge capacity, cycle performance, and storage performance of the lithium nickel silicate material.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to a modified positive electrode material and a preparation method and application thereof. Background Art

[0002] With the oil crisis and environmental concerns, countries have been competing to develop hybrid and electric vehicles in recent years, creating a favorable opportunity for the development of lithium-ion batteries for automotive applications. Cathode materials are key to the performance of lithium-ion batteries. Currently, commercially available cathode materials are primarily cobalt compounds, but cobalt resources are limited, toxic, poses a significant environmental risk, and presents poor safety concerns.

[0003] In recent years, lithium nickel silicate (LiNiSiO4) cathode materials have received widespread attention. Compared with traditional cathodes, they are rich in raw materials, inexpensive, environmentally friendly, and have excellent thermal stability and safety. In particular, silicate cathodes have a high voltage platform, which allows two lithium ions to be inserted and removed, resulting in a high theoretical specific capacity (330mAh / g), which is twice that of phosphate cathode materials. However, studies on the stability and surface properties of lithium nickel silicate cathode materials have found that when the material is exposed to air for a long time, Li2CO3 will form on the surface, resulting in a decrease in cycling and storage capacity.

[0004] Based on the above research, it is necessary to provide a modified lithium nickel silicate material to overcome the intrinsic defects of lithium nickel silicate from the material itself, and fully consider the material properties of LiNiSiO4, and adopt a specific modification method to greatly improve the storage performance and cycle performance of the obtained material. Summary of the Invention

[0005] The present invention aims to solve the above problems and provides a modified positive electrode material, a preparation method and application thereof. Taking full account of the intrinsic defects of lithium nickel silicate material, the material is coated with LFP (LiFePO4, lithium iron phosphate) material with good stability to form a core-shell structure. To a certain extent, the intrinsic defects of lithium nickel silicate material are overcome, the stability of lithium nickel silicate material is improved, and the storage performance and cycle performance of the material are greatly improved.

[0006] According to the technical solution of the present invention, the method for preparing the modified positive electrode material comprises the following steps:

[0007] S1: adjusting the pH of the mixed solution of nickel salt, silicon source and lithium salt to 6-9, adding ascorbic acid and fast ion conductor, and mixing to obtain a precursor solution;

[0008] S2: adding lithium iron phosphate to the precursor solution, mixing, heating, drying, and grinding to obtain a composite precursor powder;

[0009] S3: ball-milling the composite precursor powder, heating it under an inert atmosphere, and grinding it after cooling to obtain the modified positive electrode material.

[0010] Specifically, in step S1, ascorbic acid is used as a reducing agent and chelating agent to prevent oxidation of the reactants; the fast ion conductor is used to improve the conductivity of the material and ensure the capacity of the material; in step S2, the composite precursor powder is a composite precursor powder of LiFePO4 mixed with nickel silicate; the modified positive electrode material obtained in step S2 is a lithium iron phosphate-coated lithium nickel silicate positive electrode material.

[0011] In the present invention, LFP with good stability is used to coat the lithium nickel silicate material. The coating layer (LFP) material can improve the surface of the substrate (lithium nickel silicate), such as intrinsic defects in the cross section or surface, reduce the surface area of ​​the substrate, and reduce the consumption of Li ions. Since Li2CO3 is an irreversible compound, LFP coating can effectively isolate the air, avoid the formation of Li2CO3, and extend the service life of the lithium nickel silicate. LFP itself also has good stability. Introducing LFP to synthesize the composite material can fully improve the storage performance and cycle performance of the material.

[0012] Furthermore, the nickel salt is selected from at least one of nickel sulfate and nickel oxalate; the silicon source is selected from at least one of ethyl orthosilicate and methyl silicate; and the lithium salt is selected from at least one of lithium hydroxide and lithium carbonate.

[0013] Furthermore, in the mixed solution of nickel salt, silicon source and lithium salt, the molar ratio of nickel salt, silicon source and lithium salt is 1:(0.8-1.2):(1-3).

[0014] Furthermore, in step S1, the pH of the mixed solution of nickel salt, silicon source, and lithium salt is adjusted to 6-9 using ammonia. Ammonia is a weak base and has a better adjustment effect than a stronger base. However, using other bases containing metal cations, such as sodium hydroxide, for adjustment may introduce sodium ions, forming impurity ions that affect material properties.

[0015] Furthermore, in step S1, the molar ratio of the added ascorbic acid to the nickel salt is (0.03-0.04):1.

[0016] Furthermore, in step S1, the molar ratio of the added fast ion conductor to the nickel salt is (0.03-0.08):1.

[0017] Furthermore, the fast ion conductor is LiTi2(PO4)3 (LTP for short), which has three-dimensional [Ti2(PO4)3] - Skeleton, Li + It can move in the narrow gaps between different lattice nodes in LiTi2(PO4)3, and therefore has higher ionic conductivity.

[0018] Furthermore, in the composite precursor powder, the mass fraction of lithium iron phosphate is 5-12%.

[0019] Furthermore, in step S2, the temperature of the heating treatment is 50-90° C., and the time of the heating treatment is 4-8 hours.

[0020] Furthermore, in step S2, the drying temperature is 80-120°C, preferably 100°C.

[0021] Furthermore, in step S3, the ball milling speed is 800-1000 / 900-1100 r / min (wherein, 800-1000 is the rotation speed, and 900-1100 is the revolution speed), and the ball milling time is 0.5-2 h.

[0022] Furthermore, in step S3, the inert atmosphere is selected from one or more of nitrogen, helium, and argon.

[0023] Furthermore, in step S3, the temperature of the heating treatment is 500-1000°C, the time of the heating treatment is 8-15h, and the heating rate is 1-10°C·min -1 .

[0024] A second aspect of the present invention provides a modified positive electrode material, comprising lithium nickel silicate and lithium iron phosphate coated on the surface of the lithium nickel silicate, wherein the lithium iron phosphate accounts for 5-12% of the total mass of the modified positive electrode material.

[0025] Furthermore, the modified positive electrode material is prepared by the above preparation method.

[0026] A third aspect of the present invention provides a lithium-ion secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode material layer, wherein the positive electrode material layer comprises the above-mentioned modified positive electrode material.

[0027] The technical solution of the present invention has the following advantages over the existing technology: the present invention uses LFP with good stability to coat the lithium nickel silicate material to obtain a modified positive electrode material, thereby improving the maximum discharge capacity (from 144 mAh / g to 151 mAh / g), cycle performance (250-cycle capacity retention rate increased from 89% to 94.2%) and storage performance (45°C 100% SOC, increased from 2.8 months to 4 months) of the lithium nickel silicate material. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific examples so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention.

[0029] The present invention provides a modified positive electrode material, namely, a lithium iron phosphate-coated lithium nickel silicate positive electrode material, wherein the lithium iron phosphate accounts for 5-12% of the total mass of the modified positive electrode material.

[0030] The preparation method of the modified positive electrode material can be as follows:

[0031] S1: Add nickel salt, silicon source (dissolved in ethanol or other alcohol) and lithium salt in a molar ratio of 1: (0.8-1.2): (1-3) to water, stir evenly on a constant temperature magnetic stirrer, measure the pH value and adjust it to 6-9 (for example, 7.5) with ammonia water; add ascorbic acid and fast ion conductor, and mix well to obtain a precursor solution

[0032] Wherein, the nickel salt is selected from at least one of nickel sulfate and nickel oxalate; the silicon source is selected from at least one of ethyl orthosilicate and methyl silicate; the lithium salt is selected from at least one of lithium hydroxide and lithium carbonate;

[0033] The molar ratio of ascorbic acid to nickel salt is (0.03-0.04):1; the fast ion conductor is LTP, and the molar ratio of the fast ion conductor to nickel salt is (0.03-0.08):1;

[0034] S2: Under water bath heating conditions, add LiFePO4 powder to the above precursor solution, the amount of LiFePO4 powder added is calculated based on the mass fraction of LiFePO4 in the final composite material (LiFePO4 mixed with nickel silicate composite precursor powder) being 5-12%. After ultrasonic vibration for 2-5 minutes, transfer the mixture into a stainless steel reaction kettle with polytetrafluoroethylene, and under water bath heating conditions, react at 50-90°C for 4-8 hours. Then, place the product in an oven to dry at 80-120°C, and grind to obtain LiFePO4 mixed with nickel silicate composite precursor powder;

[0035] S3: Use ball milling method, after the ball milling time is 0.5-2h at the stirring shaft speed of 800-1000 / 900-1100r / min, the powder is placed in a porcelain boat, placed in a tube furnace, and an inert gas (such as nitrogen, helium, argon, etc.) is introduced to remove the air in the furnace; then, the powder is heated at 1-10℃·min -1 The temperature is raised to 500-1000°C at a rate of 0.5°C, heat treated in a flowing inert atmosphere for 8-15h, naturally cooled to room temperature, taken out, and ground into fine powder in an agate mortar to obtain lithium iron phosphate-coated lithium nickel silicate positive electrode material.

[0036] The modified positive electrode material can be used to prepare positive electrode sheets and used in lithium-ion secondary batteries.

[0037] Example 1:

[0038] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material, and the preparation method is as follows:

[0039] S1. 0.7 mol NiSO4·6H2O, 0.7 mol ethyl orthosilicate (C8H 20 O4Si, dissolved in ethanol) and 2.1 mol LiOH·H2O were added to 50 mL of water in sequence and stirred evenly to obtain a first mixed solution. Ammonia solution was added to the first mixed solution under continuous stirring to adjust the pH of the reaction system to 7.5. 0.035 mol ascorbic acid and 0.05 mol fast ion conductor LTP were added thereto and stirred evenly. Stirring was continued for 10 minutes to obtain a second mixed solution.

[0040] S2. Under water bath heating conditions, add LiFePO4 powder with a particle size of 300nm to the above-mentioned second mixed liquid. The amount of LiFePO4 powder is calculated based on the mass fraction of LiFePO4 in the final composite material (LiFePO4 mixed with nickel silicate composite precursor powder) being 8%. After ultrasonic vibration for 3 minutes, transfer the mixture into a stainless steel reaction kettle with polytetrafluoroethylene, and react at 80°C for 6 hours under water bath heating conditions. Place the product in an oven and dry it at 100°C. After grinding, obtain LiFePO4 mixed with nickel silicate composite precursor powder.

[0041] S3. After ball milling for 1 h at a stirring shaft speed of 900 / 1000 r / min, the powder was placed in a porcelain boat and placed in a tube furnace. High-purity nitrogen (purity ≥ 99.999%) was introduced into the air in the furnace for 10 min; then the mixture was heated at 5 °C·min -1 The temperature was raised to 700°C at a rate of 0.5°C, heat treated (sintered) in a flowing nitrogen atmosphere for 10 hours, naturally cooled to room temperature (25±5°C), taken out, and ground into fine powder in an agate mortar to obtain lithium iron phosphate-coated lithium nickel silicate positive electrode material.

[0042] Example 2:

[0043] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The difference between its preparation method and that of Example 1 is that the amount of fast ion conductor LTP added to S1 is 0.03 mol, and the rest is consistent with Example 1.

[0044] Example 3:

[0045] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The difference between its preparation method and that of Example 1 is that the amount of fast ion conductor LTP added to S1 is 0.08 mol, and the rest is consistent with Example 1.

[0046] Example 4:

[0047] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the reaction temperature in S2 is 50° C., and the rest is the same as that of Example 1.

[0048] Example 5:

[0049] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The difference between its preparation method and that of Example 1 is that the reaction temperature in S2 is 90° C., and the rest is consistent with Example 1.

[0050] Example 6:

[0051] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the mass fraction of lithium iron phosphate added to S2 is 5%; the rest is consistent with Example 1.

[0052] Example 7:

[0053] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the mass fraction of lithium iron phosphate added to S2 is 12%, and the rest is consistent with Example 1.

[0054] Example 8:

[0055] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the rotation speed of the ball mill stirring shaft in S3 is 800 / 900 r / min, and the rest is consistent with Example 1.

[0056] Example 9:

[0057] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the rotation speed of the ball mill stirring shaft in S3 is 1000 / 1100 r / min, and the rest is consistent with Example 1.

[0058] Example 10:

[0059] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the sintering temperature in S3 is 500° C., and the rest is consistent with Example 1.

[0060] Example 11:

[0061] This embodiment provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The preparation method thereof is different from that of Example 1 in that the sintering temperature in S3 is increased to 1000° C., and the rest is the same as that of Example 1.

[0062] Comparative Example 1:

[0063] This comparative example provides a positive electrode material, and its preparation method is different from that of Example 1 in that LiFePO4 is not added to S2, and the second mixed solution is directly reacted at 80°C for 6 hours under water bath heating conditions, and the product is placed in an oven at 100°C for drying and ground to obtain nickel silicate composite precursor powder. The rest is consistent with Example 1.

[0064] Comparative Example 2:

[0065] This comparative example provides a positive electrode material. The difference between its preparation method and that of Example 1 is that the ball milling method is not used in S3, and direct sintering is used. The rest is the same as that of Example 1.

[0066] Comparative Example 3:

[0067] This comparative example provides a positive electrode material. The difference between its preparation method and that of Example 1 is that no fast ion conductor LTP is added to S1, and the rest is consistent with Example 1.

[0068] Comparative Example 4:

[0069] This comparative example provides a lithium iron phosphate-coated lithium nickel silicate positive electrode material. The difference between its preparation method and that of Example 1 is that the sintering temperature in S3 is increased to 250° C., and the rest is consistent with Example 1.

[0070] Result analysis:

[0071] Lithium-ion batteries were prepared using the positive electrode materials in Examples 1-11 and Comparative Examples 1-4, and tested. The results are shown in Table 1.

[0072] The preparation method of the lithium-ion battery is as follows:

[0073] Preparation of positive electrode sheets: The positive electrode active material NCM811, the conductive agent acetylene black, and the binder PVDF were mixed in a mass ratio of 96:2:2, and the solvent NMP was added. The mixture was stirred in a vacuum mixer until the system was uniform to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. The positive electrode sheets were then cold pressed and cut into pieces.

[0074] Preparation of negative electrode sheets: Graphite (a negative electrode active material) or a mixture of graphite and other active materials in different mass ratios, conductive agent SP, CMC, and binder SBR are mixed in a mass ratio of 96.4:1:1.2:1.4, deionized water is added as a solvent, and the mixture is stirred in a vacuum mixer until the system becomes uniform to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for further drying, and then cold pressed and cut to obtain negative electrode sheets;

[0075] Electrolyte material: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (EDC) were mixed in a volume ratio of 1:1:1, and then 1 mol / L Li PF6 was added and mixed evenly to prepare an electrolyte;

[0076] Battery assembly: The positive electrode sheet, separator and negative electrode sheet are stacked in sequence, then wound into a battery cell and placed in a soft-pack shell. After top and side sealing, liquid injection, formation and sorting, a lithium-ion battery is obtained.

[0077] The test items and methods are as follows:

[0078] (1) Gram capacity test: The lithium-ion battery is left at 25°C for 30 minutes, discharged at an IC constant current, left at 10 minutes, charged at a 1C constant current, discharged at a 0.05C constant voltage, left at 5 minutes, and discharged at a 1C constant current. The discharge capacity at this time is the capacity of the battery at 1C. The gram capacity of the positive electrode material is calculated based on the capacity data.

[0079] (2) Cycle performance test: The lithium-ion battery was placed at 25°C for 30 minutes, discharged at a constant current of 1C, and allowed to stand for 10 minutes. It was then charged at a constant current and constant voltage of 1C and allowed to stand for 10 minutes. The full charge and discharge cycle was repeated 250 times, and the capacity retention rate was calculated.

[0080] (3) Storage performance test: The lithium-ion battery was placed at 25°C for 30 minutes, discharged at a constant current of 1C, placed for 10 minutes, charged at a constant current and constant voltage of 1C, placed for 10 minutes, and stored at an ambient temperature of 60°C at 100% SOC (state of charge). The storage time to reach 80% SOC was recorded.

[0081] Table 1

[0082]

[0083] The results show that Example 1 is the preferred solution. Examples 1-3 compare the effect of the fast ion conductor LTP content on material properties, indicating that reasonable control of the LTP content is crucial to the specific capacity of the positive electrode material and the cycle performance of the lithium ion battery. LiTi2(PO4)3 (abbreviated as LTP) is a fast ion conductor with three-dimensional [Ti2(PO4)3] - Skeleton. + It can move within the narrow gaps between different lattice nodes in LiTi2(PO4)3, thus having high ionic conductivity. Due to the low conductivity of the LFP coating itself, too low an amount of LTP (no addition in Comparative Example 3) is insufficient to improve the conductivity of the positive electrode material. If the amount is too high, the product particles will agglomerate, hindering the improvement of the electrochemical performance of the lithium-ion battery.

[0084] Examples 1, 4, and 5 compare the effect of reaction temperature on material properties. The hydrolysis of ethyl orthosilicate (hydrolysis of ethyl orthosilicate to form hydroxylated products and corresponding alcohols, wherein the hydroxylated products are also called silicic acid; during the hydrolysis, dehydration condensation reactions occur between the silicic acids or between the silicic acid and ethyl orthosilicate to form a colloidal mixture) can be carried out at room temperature, but the temperature still has an effect on its reaction rate. For example, when the temperature is increased from room temperature to 80°C, the hydrolysis reaction rate increases significantly, and the degree of hydrolysis is also very complete. However, further increases in temperature, such as above 80°C, have little effect on the hydrolysis rate, and the dehydration reaction is incomplete. If the temperature is below 80°C, the hydrolysis reaction rate decreases again. Incomplete hydrolysis can cause the material in the preparation process to easily break, thereby affecting the cycle life of the lithium-ion battery and the gram capacity of the positive electrode material.

[0085] Examples 1, 6, and 7 compare the effects of lithium iron phosphate content on material performance. They show that increasing the amount of lithium iron phosphate coating reduces the cathode material's conductivity, impacting its capacity, while decreasing the amount of lithium iron phosphate coating slightly decreases its stability. In contrast, without lithium iron phosphate coating (Comparative Example 1), the cathode material's gram capacity and lithium-ion battery cycling stability remain relatively low.

[0086] Examples 1, 8, and 9 compare the effects of ball milling parameters on material properties. Combined with Comparative Example 2, it is shown that the evenly dispersed LiFePO4 particles reduce the transmission distance of lithium ions and improve ion and electron transmission, so that the prepared lithium-ion battery has better cycle performance.

[0087] Examples 1, 10, and 11 compare the effects of sintering parameters on material properties. One reason for the optimal sintering temperature of 700°C may be that temperatures below 700°C, particularly below 500°C (Comparative Example 4), result in low crystallinity in the lithium nickel silicate crystals, which prevent the formation of a complete lattice and large grains, leading to a low initial charge-discharge capacity. Excessively high temperatures (e.g., exceeding 1000°C) cause the particles to grow, hindering the passage of lithium ions and thus reducing the initial discharge capacity and cycle performance of the lithium-ion battery.

[0088] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a modified positive electrode material, characterized in that: The following steps are included: S1: adjusting the pH value of the mixed solution of nickel salt, silicon source and lithium salt to 6-9, adding ascorbic acid and fast ion conductor, and mixing to obtain a precursor solution; S2: adding lithium iron phosphate to the precursor solution, mixing, heating, drying, and grinding to obtain a composite precursor powder; S3: ball-milling the composite precursor powder, heating it under an inert atmosphere, and grinding it after cooling to obtain the modified positive electrode material.

2. The preparation method according to claim 1, wherein The nickel salt is selected from at least one of nickel sulfate and nickel oxalate; the silicon source is selected from at least one of ethyl orthosilicate and methyl silicate; and the lithium salt is selected from at least one of lithium hydroxide and lithium carbonate.

3. The preparation method according to claim 1 or 2, wherein In the mixed solution of the nickel salt, the silicon source and the lithium salt, the molar ratio of the nickel salt, the silicon source and the lithium salt is 1:(0.8-1.2):(1-3).

4. The preparation method according to claim 1, wherein In step S1, the molar ratio of the added ascorbic acid to the nickel salt is (0.03-0.04):

1.

5. The preparation method according to claim 1, wherein In step S1, the molar ratio of the added fast ion conductor to the nickel salt is (0.03-0.08):

1.

6. The preparation method according to claim 1, wherein In the composite precursor powder, the mass fraction of lithium iron phosphate is 5-12%.

7. The preparation method according to claim 1, wherein In step S2, the heating treatment temperature is 50-90° C., and the heating treatment time is 4-8 h.

8. The preparation method according to claim 1, wherein In step S3, the temperature of the heating treatment is 500-1000° C., and the time of the heating treatment is 8-15 hours.

9. A modified positive electrode material, characterized in that Prepared by the preparation method according to any one of claims 1-8, comprising lithium nickel silicate and lithium iron phosphate coated on the surface of the lithium nickel silicate, wherein the lithium iron phosphate accounts for 5-12% of the total mass of the modified positive electrode material.

10. A lithium-ion secondary battery comprising a positive electrode plate, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode material layer, wherein: The positive electrode material layer comprises the modified positive electrode material according to claim 9.

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