Preparation method of nanometer cobalt phosphate and application thereof

Abstract

This invention discloses a method for preparing nano-cobalt phosphate and its application, belonging to the field of new energy technology. The preparation method provided by this invention includes the following steps: S1. A cobalt salt solution, a phosphorus source solution, and a pH adjuster are simultaneously introduced into a stirred base liquid; the base liquid is water; the concentration of the cobalt salt solution is 1–3 mol / L; during the simultaneous introduction process, the pH of the resulting mixture is controlled to be 8–10, and the temperature is controlled to be 20–40℃; S2. The solid product obtained in step S1 is calcined. The preparation method provided by this invention can prepare nano-cobalt phosphate with relatively uniform particle size and pure composition at a mild temperature without the use of thickeners or other additives. This invention also provides applications of the above preparation method.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, and in particular to a method for preparing nano-cobalt phosphate and its application. Background Technology

[0002] To improve the energy density of lithium-ion batteries, cathode materials are being developed towards higher voltage levels. While high-voltage cathode materials with high delithiation states exhibit good energy density, they also possess strong oxidizing properties and are prone to side reactions with organic electrolytes, leading to a decrease in battery cycle life and safety. Related technologies indicate that surface coating modification of cathode materials can mitigate the performance degradation caused by high voltage conditions to some extent. Among the many materials suitable for surface coating modification, cobalt phosphate can not only improve the cycle performance of cathode materials but also enhance their high-temperature storage performance.

[0003] The particle size of the coating material can affect the uniformity of its coating on the cathode material to a certain extent, so the preparation of nano-cobalt phosphate is a research hotspot.

[0004] Currently reported methods for preparing nano-cobalt phosphate mainly include microwave heating and hydrothermal methods. Both methods require stringent reaction conditions, sophisticated equipment, and low production capacity, making mass production impossible. For example, one technique uses a surfactant-assisted hydrothermal method to prepare purple, sheet-like Co3(OH)2(HPO4)2 nanosheets. This method requires high reaction temperatures, sophisticated equipment, and low yield. Furthermore, the addition of surfactants introduces impurities, increasing costs and making it unsuitable for large-scale production. Another technique uses a co-precipitation method, where soluble phosphates are added to soluble cobalt salts to obtain a cobalt phosphate precipitate. The reaction system is then adjusted to neutral with ammonia, and finally filtered, washed, and dried to obtain cobalt phosphate powder. The cobalt phosphate material prepared by this method has an uncontrollable particle size, with a median diameter of 1–25 micrometers. The relatively large size and poor uniformity of the particle size make it unsuitable for forming a uniform and dense coating layer when used for cathode material coating. For example, some technologies use a co-precipitation method to prepare nano-cobalt phosphate by adding a thickener to a solution of cobalt salt and phosphate. However, the materials prepared by this method tend to agglomerate into spheres and introduce impurities, which increases costs.

[0005] In summary, with existing technologies, to obtain nanoscale cobalt phosphate materials with uniform particle size, one can either use hydrothermal or microwave heating methods, which require extremely high-end equipment and are not easy to industrialize, or use co-precipitation methods, which introduce thickeners or surfactants, increasing the risk of introducing impurities into the product. Summary of the Invention

[0006] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes a method for preparing nano-cobalt phosphate, which can prepare nano-cobalt phosphate with relatively uniform particle size and pure composition at a mild temperature and without the use of thickeners or other additives.

[0007] The present invention also provides applications of the above preparation method.

[0008] According to an embodiment of the first aspect of the present invention, a method for preparing nano-cobalt phosphate is provided, the method comprising the following steps:

[0009] S1. Cobalt salt solution, phosphorus source solution and pH adjuster are introduced concurrently into the bottom liquid under stirring; the concentration of the cobalt salt solution is 1-3 mol / L; during the concurrent introduction process, the pH of the resulting mixture is controlled to be 8-10 and the temperature is 20-40℃.

[0010] S2. The solid product obtained from calcination step S1.

[0011] The preparation method according to embodiments of the present invention has at least the following beneficial effects:

[0012] In traditional techniques, the preparation of nano-cobalt phosphate by co-precipitation usually requires the introduction of surfactants or thickeners; otherwise, severe agglomeration will occur, resulting in inconsistent and uncontrolled particle sizes. This introduces impurities into the cobalt phosphate, affecting its performance as a cathode material coating. Furthermore, the usable concentration range of thickeners or surfactants is very narrow, which is not conducive to industrial production and promotion.

[0013] The preparation method provided by this invention produces nano-cobalt phosphate with uniform morphology and better dispersibility without the use of thickeners, surfactants or other auxiliary additives. Furthermore, since no additives are introduced, the preparation method has a wider operating window, and the resulting nano-cobalt phosphate has higher purity and better electrochemical performance.

[0014] This invention adjusts the nucleation amount within the reaction system per unit time by adjusting the reaction pH, temperature, cobalt salt solution concentration, and flow rate, ensuring that the solid product obtained in step S1 is in the nucleation stage. Simultaneously, by controlling the temperature, the primary particles of the obtained cobalt phosphate are refined, ultimately controlling the particle size of the nano-cobalt phosphate (≤200 nm), improving its morphological uniformity, and preventing agglomeration. Specifically, if the concentration of the cobalt salt solution is too low, the nucleation amount decreases, and the obtained nano-cobalt phosphate tends to grow larger rather than being in the nucleation stage; if the concentration of the cobalt salt solution is too high, it may exceed its saturation concentration, causing cobalt salt precipitation, which in turn leads to a decrease in the uniformity of the obtained nano-cobalt phosphate particle size. Because the nano-cobalt phosphate prepared by this method has the above advantages, it is expected to serve as a cathode material coating, forming a uniform and stable coating layer and exhibiting good electrochemical performance.

[0015] Regarding the Ksp of cobalt phosphate, precipitation occurs at a pH of around 4. Furthermore, as the pH increases, the disorder of the cobalt phosphate product increases, with primary particles being predominantly plate-like and large in size, not belonging to nanomaterials. Therefore, traditional co-precipitation techniques typically employ acidic or neutral environments. This invention creatively utilizes an alkaline environment, where the cobalt phosphate growth pattern differs from that under acidic to neutral conditions. Combined with temperature and other conditions, this overcomes the problem of large and disordered cobalt phosphate particles. Further, in the preparation method, if the pH value is below the range required by this invention, the resulting cobalt phosphate is plate-like and large in size, within the micrometer range, which is unfavorable for forming a uniform coating layer. If the pH value is above the range required by this invention, the synthesized cobalt phosphate may exhibit other impurities, which is detrimental to the electrochemical performance of the coating layer.

[0016] In the preparation method provided by this invention, the stirring state provides a good mass transfer environment, avoiding problems such as uneven particle size caused by local high concentrations in the reaction vessel. Overall, the preparation method provided by this invention has the advantages of low temperature, mild conditions, cost reduction and energy saving; in addition, the preparation method is simple to operate, has good reproducibility, and is conducive to industrialization.

[0017] According to some embodiments of the present invention, in step S1, the molar ratio of cobalt in the cobalt salt solution to phosphorus in the phosphorus source solution is 1:0.5 to 1. The chemical formula of the nano-cobalt phosphate is Co3(PO4)2, wherein the theoretical molar ratio of cobalt to phosphorus is approximately 1:0.67. The cobalt / phosphorus ratio range provided by the present invention is near the above-mentioned theoretical molar ratio, thereby providing higher controllability of the morphology of the intermediate product obtained in step S1, ensuring the formation of the nano-cobalt phosphate; and the content of unreacted cobalt in the cobalt salt solution is very low, avoiding cost waste.

[0018] According to some embodiments of the present invention, in step S1, the molar ratio of cobalt in the cobalt salt solution to phosphorus in the phosphorus source solution is 1:0.6 to 0.9. For example, it may be approximately 1:0.7 or 1:0.8.

[0019] According to some embodiments of the present invention, in step S1, the cobalt salt in the cobalt salt solution includes at least one of cobalt sulfate, cobalt chloride, cobalt nitrate, and cobalt acetate.

[0020] According to some embodiments of the present invention, in step S1, the concentration of the cobalt salt solution is 1.1 to 2.5 mol / L. For example, it can be about 1.2 mol / L, 1.5 mol / L, 2 mol / L, or 2.3 mol / L.

[0021] According to some embodiments of the present invention, in step S1, the flow rate of the cobalt salt solution is 150-600 mL / min.

[0022] According to some embodiments of the present invention, in step S1, the flow rate of the cobalt salt solution is 200–580 mL / min. Specifically, it can be approximately 250 mL / min, 450 mL / min, or 550 mL / min.

[0023] According to some embodiments of the present invention, in step S1, the phosphorus source in the phosphorus source solution includes at least one of phosphoric acid, monoammonium phosphate, diammonium phosphate and sodium phosphate.

[0024] According to some embodiments of the present invention, in step S1, the concentration of the phosphorus source solution is 1–18 mol / L. The concentration of the phosphorus source solution has a certain matching relationship with the concentration and flow rate of the cobalt salt solution; it also has a certain matching relationship with the type (solubility) of the phosphorus source. For example, when the phosphorus source is phosphoric acid, the concentration of the phosphorus source solution can be ≥14 mol / L, while when the phosphorus source is monoammonium phosphate, a concentration of approximately 2 mol / L is suitable.

[0025] According to some embodiments of the present invention, in step S1, the solute in the phosphorus source solution is phosphoric acid, and the concentration is 2 to 15 mol / L. For example, it can be about 3 mol / L or 14 mol / L.

[0026] According to some embodiments of the present invention, in step S1, the solute in the phosphorus source solution is diammonium phosphate with a concentration of 1.5–3 mol / L. For example, it can be approximately 2 mol / L.

[0027] According to some embodiments of the present invention, in step S1, the pH adjuster includes at least one of ammonia and liquid alkali.

[0028] According to some embodiments of the present invention, the liquid alkali comprises an aqueous solution of sodium hydroxide.

[0029] According to some embodiments of the present invention, the mass concentration of the sodium hydroxide aqueous solution is 30-35%. Specifically, it can be about 32%.

[0030] The type of pH adjuster and the pH value have a certain compatibility. For example, when the pH adjuster is selected from ammonia, the pH value should be between 8.9 and 10. When the pH adjuster includes liquid alkali, the pH value can be satisfied within the range of 8 to 10, for example, it can be selected between 8 and 9.

[0031] The type of pH adjuster and the pH of the mixture obtained by parallel flow have a certain synergistic effect. Combined with the temperature control in step S1, the size of the obtained primary nano-cobalt phosphate particles is strictly controlled, avoiding problems such as excessively large primary particle size and agglomeration.

[0032] According to some embodiments of the present invention, the pH adjuster comprises ammonia and liquid alkali. The volume ratio of ammonia to liquid alkali is 10–15:1. Specifically, it can be approximately 12.5:1.

[0033] In step S1, the pH is the process pH.

[0034] According to some embodiments of the present invention, in step S1, during the parallel flow process, the pH is 8.5 to 9.5. For example, it can specifically be 8.9 to 9.0.

[0035] According to some embodiments of the present invention, in step S1, during the parallel flow process, the pH is 9.2 to 9.8. For example, it can be 9.3 to 9.4; or 9.4 to 9.5.

[0036] According to some embodiments of the present invention, in step S1, the base liquid accounts for 40-60% of the volume of the reaction vessel used in the preparation method. For example, it can specifically be about 50%.

[0037] According to some embodiments of the present invention, in step S1, the base liquid is water. Using water as the base liquid allows for better control of the morphology of the obtained cobalt phosphate nanoparticles compared to using a salt solution or a base liquid containing crystal nuclei.

[0038] According to some embodiments of the present invention, in step S1, the stirring speed is 300 to 600 r / min. For example, it can be approximately 200 r / min, 450 r / min, or 500 r / min.

[0039] According to some embodiments of the present invention, in step S1, the process temperature is 25–35°C. For example, it can be approximately 30°C.

[0040] According to some embodiments of the present invention, in step S1, the duration of the parallel flow is 0.5 to 2 hours. Within this time range, the production capacity of the nano-cobalt phosphate can be guaranteed, and the nano-cobalt phosphate can avoid undergoing a growth stage, ensuring that it is in the nucleation stage and has a nanoparticle size.

[0041] According to some embodiments of the present invention, in step S1, the duration of the parallel flow is 1 to 1.5 hours.

[0042] According to some embodiments of the present invention, in step S1, the container holding the bottom liquid includes a reaction vessel.

[0043] According to some embodiments of the present invention, step S1 further includes, after the parallel flow, sequentially performing solid-liquid separation, solid material washing, drying, and crushing. That is, after the cobalt salt solution and phosphorus source solution are completely introduced in parallel, solid-liquid separation is performed immediately, without any reaction or aging operations in between; thus, the solid products obtained in step S1 are all in the nucleation stage and have not yet had time to grow and aggregate, resulting in small and uniform particle size of the obtained nano-cobalt phosphate.

[0044] According to some embodiments of the present invention, the solid substance is washed by water. The endpoint of the water washing is when the washing solution is nearly neutral (unless otherwise specified, near neutral is pH 6.8–7.2).

[0045] According to some embodiments of the present invention, the drying method includes at least one of forced-air drying, vacuum drying, and freeze drying. The drying temperature is 80–150°C. For example, it can be approximately 120°C. In actual production, the drying time may be related to the production volume; for example, a drying time of ≥10 hours can be selected; more specifically, a drying time of approximately 12 hours can be selected. In actual production, a suitable drying method can be selected according to the requirements of energy consumption and equipment, as long as the drying function can be achieved; considering the cost, the present invention preferably adopts a drying method.

[0046] According to some embodiments of the present invention, the solid product obtained in step S1 is amorphous cobalt phosphate.

[0047] According to some embodiments of the present invention, in step S2, the calcination includes a first stage of isothermal treatment and a second stage of isothermal treatment performed sequentially. This can convert the amorphous cobalt phosphate obtained in step S1 into crystalline cobalt phosphate. If the solid product obtained in step S1 contains other impurities and is not simply amorphous cobalt phosphate, the product after calcination in step S2 may also contain impurities, which may negatively impact electrochemical performance.

[0048] According to some embodiments of the present invention, the temperature of the first stage of constant temperature is 200-300°C. The first stage of constant temperature is equivalent to a preheating process, which can avoid damage to the equipment caused by a sudden and rapid increase in temperature from room temperature to high temperature, and can also avoid agglomeration and over-sintering of materials caused by a one-step increase in temperature to the high temperature zone.

[0049] According to some embodiments of the present invention, the temperature for the first stage of constant temperature is 220–280°C. For example, it can be approximately 250°C.

[0050] According to some embodiments of the present invention, the holding time for the first stage of constant temperature is 0.5 to 1 hour.

[0051] According to some embodiments of the present invention, the heating rate from the start of calcination to the first stage of isothermal temperature is 4–8 °C / min. For example, it can be approximately 5 °C / min or 6 °C / min.

[0052] According to some embodiments of the present invention, the temperature for the second stage of isothermal control is 400–600°C. This second stage of isothermal control is used to promote the phase transformation of the solid product obtained in step S1, changing it from an amorphous state to a crystalline state. Excessive temperature will lead to primary particle agglomeration, while insufficient temperature will not cause phase change.

[0053] According to some embodiments of the present invention, the temperature for the second stage of constant temperature is 450–580°C. For example, it can be approximately 480°C, 500°C, 520°C, or 550°C.

[0054] According to some embodiments of the present invention, the heat preservation time for the second stage of constant temperature is 1 to 4 hours.

[0055] According to some embodiments of the present invention, the duration of the second stage of constant temperature maintenance is 2 to 3.5 hours. For example, it can be approximately 2.5 hours or 3 hours.

[0056] According to some embodiments of the present invention, the heating rate from the first stage of isothermal control to the second stage of isothermal control is 4 to 8 °C / min. For example, it can be about 5 °C / min or 6 °C / min.

[0057] During the calcination process, the steps, temperature, and duration of each step worked synergistically to avoid problems such as over-sintering and agglomeration of particles, and also ensured the consistency of the phase, particle size, dispersibility, and morphology of the obtained nano-cobalt phosphate.

[0058] According to some embodiments of the present invention, in step S2, the calcination is carried out in a protective atmosphere. The protective atmosphere includes at least one of nitrogen and argon, specifically nitrogen.

[0059] According to some embodiments of the present invention, step S2 further includes crushing and sieving sequentially after the calcination. The crushing method includes mechanical crushing and air jet milling. The sieving includes passing the material through two 325-mesh sieves. This dissociates the slight agglomerates introduced during the calcination process without damaging the primary nanoparticles of cobalt phosphate.

[0060] According to some embodiments of the present invention, the particle size of the nano-cobalt phosphate is ≤200 nm. For example, it can be 100–150 nm.

[0061] According to an embodiment of the second aspect of the present invention, the application of the preparation method described herein in the preparation of lithium-ion battery cathode materials is provided.

[0062] Since the application employs all the technical solutions of the preparation methods described in the above embodiments, it possesses at least all the beneficial effects brought about by the technical solutions of the above embodiments. Specifically, in the preparation of lithium-ion battery cathode materials, the nano-cobalt phosphate can form a uniform and stable coating layer, thereby improving the high-voltage performance, cycle performance, and safety performance of the resulting lithium-ion battery cathode material.

[0063] According to some embodiments of the present invention, the lithium-ion battery cathode material has a core-shell structure, wherein the core is the cathode active material and the shell is the nano-cobalt phosphate.

[0064] According to some embodiments of the present invention, the positive electrode active material includes at least one of lithium cobalt oxide, ternary cathode material, and lithium iron phosphate. Since lithium cobalt oxide and cobalt phosphate have the same metal ion, when the positive electrode active material includes lithium cobalt oxide, the resulting lithium-ion battery cathode material exhibits higher core-shell bonding strength and superior electrochemical performance.

[0065] According to some embodiments of the present invention, a method for preparing the lithium-ion battery cathode material includes mixing the cathode active material and the nano-cobalt phosphate. The mixing includes at least one of wet mixing and dry mixing. The mixing also includes at least one of ball milling and stirring.

[0066] Unless otherwise specified, "constant temperature" in this invention means that the temperature fluctuates within ±10℃.

[0067] Unless otherwise specified, the water used in this invention is pure water with a specific conductivity of 1-10 μs / cm.

[0068] Unless otherwise specified, the term "about" in this invention actually means that the error is allowed to be within ±2%, for example, about 100 is actually 100 ± 2% × 100.

[0069] Unless otherwise specified, "between" in this invention includes the number itself, for example, "between 2 and 3" includes the endpoint values ​​2 and 3.

[0070] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0071] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0072] Figure 1 This is an SEM image of the semi-finished product obtained in step S1 of Embodiment 1 of the present invention.

[0073] Figure 2This is a SEM image of the nano-cobalt phosphate obtained in Example 1 of the present invention.

[0074] Figure 3 This is a SEM image of the nano-cobalt phosphate obtained in Example 2 of the present invention.

[0075] Figure 4 This is a SEM image of the nano-cobalt phosphate obtained in Example 3 of the present invention.

[0076] Figure 5 This is a SEM image of cobalt phosphate obtained in Comparative Example 1 of this invention.

[0077] Figure 6 This is a SEM image of cobalt phosphate obtained in Comparative Example 2 of this invention.

[0078] Figure 7 This is a SEM image of cobalt phosphate obtained in Comparative Example 3 of this invention.

[0079] Figure 8 These are the XRD patterns of cobalt phosphate obtained in Example 2 and Comparative Example 1 of this invention. Detailed Implementation

[0080] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0081] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0082] Example 1

[0083] This example demonstrates the preparation of nano-cobalt phosphate, with the following specific steps:

[0084] S1. Add 50% pure water (based on the total volume of the reactor) to the reactor as the reaction base liquid. Set the stirring speed to 450 r / min and the temperature to 30℃. Introduce cobalt sulfate solution, phosphoric acid solution, and liquid alkali (32 wt% sodium hydroxide aqueous solution) into the reactor in parallel. The feed flow rate of cobalt sulfate solution is 480 mL / min and the concentration is 1.5 mol / L. The concentration of phosphoric acid solution is 14.7 mol / L. Control the molar ratio of cobalt to phosphorus during the feeding process to 1:0.67. Adjust the pH of the reaction process to 8.9-9.0 with liquid alkali. The total feeding time is 1 h.

[0085] After the co-current feeding is completed, the resulting mixture is subjected to solid-liquid separation. The solid material is washed with water until the washing liquid is nearly neutral (the dehydration time after the last wash is about 12 hours to reduce the water content in the material to be dried). Then, it is dried at 120℃ for 24 hours to obtain a semi-finished product.

[0086] S2: The semi-finished product obtained from calcination step S1, wherein the calcination process is as follows: the temperature is increased to 250℃ at a rate of 6℃ / min and held for 0.5h, and then increased to 550℃ at a rate of 6℃ / min and held for 2.5h. The calcined product is then subjected to mechanical crushing, air jet milling, and two 325-mesh sieves.

[0087] Example 2

[0088] This example demonstrates the preparation of nano-cobalt phosphate, with the following specific steps:

[0089] S1. Add 40% pure water (based on the total volume of the reactor) to the reactor as the reaction base liquid. Set the stirring speed to 200 r / min and the temperature to 35℃. Introduce cobalt sulfate solution, diammonium phosphate solution, and ammonia water into the reactor in parallel. The feed flow rate of cobalt sulfate solution is 580 mL / min and the concentration is 1.1 mol / L. The concentration of diammonium phosphate solution is 2 mol / L. Control the molar ratio of cobalt to phosphorus during the feeding process to 1:0.9. Adjust the pH of the reaction process to 9.4-9.5 with ammonia water. The total feeding time is 1 h.

[0090] After the co-current feeding is completed, the resulting mixture is subjected to solid-liquid separation. The solid material is washed with water until the washing liquid is nearly neutral (the dehydration time after the last wash is about 12 hours to reduce the water content in the material to be dried). After centrifugation for 12 hours, it is dried at 120℃ for 24 hours to obtain a semi-finished product.

[0091] S2: The semi-finished product obtained from calcination step S1, wherein the calcination process is as follows: the temperature is increased to 280℃ at a rate of 5℃ / min and held for 0.5h, and then increased to 480℃ at a rate of 5℃ / min and held for 3.5h. The calcined product is then subjected to mechanical crushing, air jet milling, and two 325-mesh sieves.

[0092] Example 3

[0093] This example demonstrates the preparation of nano-cobalt phosphate, with the following specific steps:

[0094] S1. Add 50% pure water (based on the total volume of the reactor) to the reactor as the reaction base liquid. Set the stirring speed to 500 r / min and the temperature to 30℃. Add cobalt sulfate solution, monoammonium phosphate solution, and a mixture of ammonia and liquid alkali in parallel to the reactor. The feed flow rate of cobalt sulfate solution is 240 mL / min, and the concentration is 2.37 mol / L. The concentration of monoammonium phosphate solution is 2 mol / L. In the mixture of ammonia and liquid alkali, the volume ratio of ammonia (6 mol / L) to liquid alkali (32% sodium hydroxide solution) is 12.5:1. Adjust the reaction pH to 9.3-9.4 with the mixture of ammonia and liquid alkali. The total feeding time is 1 h.

[0095] After the co-current feeding is completed, the resulting mixture is subjected to solid-liquid separation. The solid material is washed with water until the washing liquid is nearly neutral (the dehydration time after the last wash is about 12 hours to reduce the water content in the material to be dried). After centrifugation for 12 hours, it is dried at 120℃ for 24 hours to obtain a semi-finished product.

[0096] S2: The semi-finished product obtained from calcination step S1; the calcination process is as follows: the temperature is increased to 200℃ at a rate of 4℃ / min and held for 0.5h, then increased to 520℃ at a rate of 4℃ / min and held for 3h. The calcined product is then subjected to mechanical crushing, air jet milling, and two 325-mesh sieves.

[0097] Comparative Example 1

[0098] This example prepares cobalt phosphate, and the specific steps differ from those in Example 1 as follows:

[0099] The difference between this example and Example 1 is that in step S1, the pH of the process is controlled to be 7.0 to 7.2.

[0100] Comparative Example 2

[0101] This example prepares a cobalt phosphate, which differs from Example 1 in that:

[0102] In step S1, the cobalt sulfate solution from Example 1 is mixed with pure water as the base solution, and the resulting mixture is used as the base solution. Then, the same phosphoric acid solution (flow rate as in Example 1) and liquid alkali are simultaneously introduced into it. The flow rate of the liquid alkali is adjusted according to the target pH of 8.9–9.0.

[0103] Comparative Example 3

[0104] This example prepares a cobalt phosphate, which differs from Example 1 in that:

[0105] In step S1, the process pH is not controlled; only after the cobalt salt solution and phosphorus source solution are introduced in parallel, the pH is adjusted to 8.9-9.0 using the same liquid alkali as in Example 1.

[0106] Comparative Example 4

[0107] This example demonstrates the preparation of cobalt phosphate, specifically:

[0108] Cobalt phosphate material was prepared according to Example 2 in CN 112938918 A.

[0109] Test case

[0110] This example tested the morphology of the semi-finished products obtained in step S1 of Examples 1-3 and Comparative Examples 1-4, and the cobalt phosphate obtained in step S2. The specific testing method was scanning electron microscopy (SEM), and the particle size of the obtained cobalt phosphate was measured from the SEM images. The results show:

[0111] In Examples 1-3, the primary particles of the semi-finished product were nanoparticles. The finished product obtained after calcination largely inherited the morphology of the semi-finished product, remaining nanoparticles with a particle size between 150 and 200 nm. The overall dispersibility was good, meeting the requirements for cathode material coating. Specific test results are as follows... Figures 1-4 As shown, the morphology of the semi-finished products in Examples 2 and 3 is similar to that in Example 1.

[0112] In Comparative Examples 1-3, if a near-neutral pH is used, along with the flow rate and other parameters of the cobalt salt solution of this invention, flake-like cobalt phosphate is synthesized, with a flake diameter even exceeding 5 μm. If the feeding order is changed, the resulting cobalt phosphate still exhibits a flake-like morphology, but the uniformity decreases significantly. If the process pH is not controlled, and the pH is only adjusted at the reaction endpoint, the cobalt phosphate tends to form a layered structure composed of stacked lamellar structures, with a larger particle size. Based on the morphological results, it can be seen that the cobalt phosphate obtained in Comparative Examples 1-3, when used as a coating material, cannot form a uniform coating layer, resulting in a relatively small improvement in the electrochemical performance of the coated positive electrode active material. The morphological results of the cobalt phosphate obtained in Comparative Examples 1-3 are as follows: Figures 5-7 As shown.

[0113] In Comparative Example 4, cobalt phosphate was prepared using a traditional co-precipitation method with added thickener. The morphology of the co-precipitated cobalt phosphate under SEM was similar to that of the semi-finished product in the examples. After calcining the cobalt phosphate obtained in Comparative Example 4 using step S2 of Example 1, the product exhibited a certain degree of agglomeration. Furthermore, in Comparative Example 4, a relatively large amount of thickener was added. If the co-precipitated product was first coated onto the surface of lithium cobalt oxide before calcination, a large amount of carbon dioxide and water would be generated during the calcination process. These gaseous components significantly degraded the electrochemical performance of the coated cathode material.

[0114] This example also tested the XRD patterns of cobalt phosphate obtained in Examples 1-3 and Comparative Examples 1-3. The results showed that the patterns of cobalt phosphate obtained in the examples and comparative examples were similar, indicating that the nano-cobalt phosphate prepared by this invention, like cobalt phosphate prepared by conventional methods, has good crystallinity. In addition, it has a small and uniform particle size, therefore it is expected to be used as a coating agent to significantly improve the electrochemical performance of cathode materials. The specific XRD test results are as follows: Figure 8 As shown, the XRD patterns are similar between the embodiments and between the comparative examples.

[0115] This example also tested the electrochemical performance of the cathode materials obtained by coating lithium cobalt oxide with cobalt phosphate in Examples 1-3 and Comparative Examples 1-4. The coating method was as follows: cobalt phosphate and lithium cobalt oxide were ball-milled at a mass ratio of 3:97 and then heat-treated. The heat treatment temperature was 700°C and the duration was 2 hours. A cathode containing the obtained cathode material was prepared, and lithium metal was used as the anode to form a coin cell. The obtained coin cell was cycled at a voltage range of 3-4.6V, a temperature of 45°C, and a 1C / 1C rate (set 1C = 170 mAh / g). The results showed that the cathode material prepared with cobalt phosphate in Examples 1-3 had better cycle performance than that in Comparative Examples 1-4 under the above conditions.

[0116] In summary, the preparation method provided by this invention has a synergistic effect between the preparation steps and the parameters of the preparation steps, which can prepare spherical cobalt phosphate with good dispersibility and uniform morphology without the use of surfactants / thickeners. The obtained nano-cobalt phosphate is expected to be used in the coating of lithium-ion battery cathode materials, and is expected to obtain uniform and stable coating, and improve the cycle performance, high pressure and high temperature performance of the obtained cathode material.

[0117] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A method for preparing nano-cobalt phosphate for coating lithium-ion battery cathode materials, characterized in that, The preparation method includes the following steps: S1. Cobalt salt solution, phosphorus source solution, and pH adjuster are introduced concurrently into a stirred base liquid; the concentration of the cobalt salt solution is 1~3 mol / L; during the concurrent introduction process, the pH of the resulting mixture is controlled at 8.5~9.5, and the temperature is controlled at 20~40℃; after the concurrent introduction is completed, solid-liquid separation is performed; the duration of the concurrent introduction is 0.5~2 hours. S2. The solid product obtained from calcination step S1; The calcination includes a first stage of constant temperature and a second stage of constant temperature, performed sequentially. The temperature for the first stage of constant temperature is 200~300℃; the temperature for the second stage of constant temperature is 400~600℃. The particle size of the nano-cobalt phosphate is between 150 and 200 nm.

2. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of cobalt in the cobalt salt solution to phosphorus in the phosphorus source solution is 1:0.5~1; and / or, the flow rate of the cobalt salt solution is 150~600 mL / min.

3. The preparation method according to claim 1, characterized in that, In step S1, the cobalt salt in the cobalt salt solution includes at least one of cobalt sulfate, cobalt chloride, cobalt nitrate, and cobalt acetate.

4. The preparation method according to claim 1, characterized in that, In step S1, the phosphorus source in the phosphorus source solution includes at least one of phosphoric acid, monoammonium phosphate, diammonium phosphate, and sodium phosphate; and / or, the concentration of the phosphorus source solution is 1~18 mol / L.

5. The preparation method according to claim 1, characterized in that, In step S1, the pH adjuster includes at least one of ammonia and liquid alkali.

6. The preparation method according to claim 1, characterized in that, In step S1, the base liquid accounts for 40-60% of the volume of the reaction vessel used in the preparation method; and / or, the base liquid is water.

7. The application of the preparation method according to any one of claims 1 to 6 in the preparation of lithium-ion battery cathode materials.

8. The application according to claim 7, characterized in that, The lithium-ion battery cathode material has a core-shell structure, wherein the core is the cathode active material and the shell is the nano-cobalt phosphate; and / or, the cathode active material includes at least one of lithium cobalt oxide, ternary cathode material and lithium iron phosphate.

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