Preparation method of flaky iron phosphate and preparation method of flaky lithium iron phosphate

Abstract

The present disclosure discloses a method for preparing flaky iron phosphate and a method for preparing flaky lithium iron phosphate. The present disclosure introduces electric pulses during the preparation of iron phosphate, which can control the reaction rate during the synthesis of iron phosphate and reduce the impurity ions included during the precipitation of iron phosphate. At the same time, flaky iron phosphate can be obtained, and then flaky lithium iron phosphate with good electrochemical performance can be obtained. In addition, the presence of electric pulses is conducive to the generation of smaller-scale flaky iron phosphate and the improvement of the density of iron phosphate, thereby improving the electrochemical performance of lithium iron phosphate as a positive electrode material.

Description

Technical Field

[0001] The present disclosure relates to the technical field of positive electrode materials, and in particular to a method for preparing flaky iron phosphate and a method for preparing flaky lithium iron phosphate. Background Art

[0002] Lithium iron phosphate batteries are safe, pollution-free, come from a wide range of raw materials, and offer excellent cycle stability. With their widespread application, demand for lithium iron phosphate is growing. Currently, the primary iron source for lithium iron phosphate production is ferrous sulfate, derived primarily from titanium dioxide byproducts and steel pickling products. With the rapid development of new energy sources, the supply and demand of iron sources is becoming increasingly strained.

[0003] Russia, Canada, Australia, Brazil, and China are rich in laterite nickel ore resources. The nickel-iron alloy obtained by reduction roasting laterite nickel ore contains over 60% iron, making it an ideal iron source. By separating nickel and iron from the nickel-iron alloy, high-purity iron oxide can be obtained. For example, Chinese patent CN114132969A discloses a method for preparing high-purity iron oxide using the nickel-iron alloy as a raw material. The method involves dissolving the nickel-iron alloy, adjusting the pH, settling iron, washing, and calcining the alloy to obtain high-grade iron oxide. High-purity iron oxide or iron hydroxide can be directly used to synthesize iron phosphate. However, the molar ratio of phosphoric acid to ferric iron compounds is high during the synthesis of iron phosphate, resulting in high phosphoric acid consumption. Furthermore, during the high-temperature reaction between phosphoric acid and ferric iron to form iron phosphate, the iron red reacts incompletely, resulting in entrainment or inclusion in the finished product, reducing the purity of the product. This also affects the electrochemical performance of lithium iron phosphate synthesized using this iron phosphate as a raw material. Chinese patent CN116409830A discloses a method for comprehensive resource utilization of nickel-iron alloys, comprising leaching the nickel-iron alloy with sulfuric acid, filtering to obtain a first filtrate and a filter residue; adding ammonium phosphate, ammonium monohydrogen phosphate, or ammonium dihydrogen phosphate to the first filtrate, then adding hydrogen peroxide and ammonia water to precipitate iron, and filtering to obtain a second filtrate and a precipitate. The precipitated iron phosphate also contains a large amount of nickel ions, which requires multiple washings to remove, which consumes a lot of water and increases costs.

[0004] In view of this, the present disclosure is proposed. Summary of the Invention

[0005] The purpose of the present disclosure is to provide a method for preparing flaky iron phosphate and a method for preparing flaky lithium iron phosphate, which can avoid the use of a complexing agent while ensuring the slow precipitation of ferrophosphide to obtain flaky iron phosphate and further obtain flaky lithium iron phosphate.

[0006] The present disclosure is achieved as follows:

[0007] In a first aspect, the present disclosure provides a method for preparing flake iron phosphate, comprising: subjecting a ferrophosphorus solution to a first electric pulse condition to perform a first precipitation reaction to obtain a reaction solution containing flake iron phosphate;

[0008] The ferrophosphorus solution includes iron ions and phosphate radicals but does not include a complexing agent. The pH value of the ferrophosphorus solution is 1.5-3.5.

[0009] In some embodiments, the concentration of phosphate in the ferrophosphorus solution is 0.5 mol / L to 3 mol / L.

[0010] In some embodiments, the molar ratio of phosphate to iron in the ferrophosphorus solution is (1.01-2):1.

[0011] In some embodiments, the temperature of the first precipitation reaction is 80° C.-95° C., and the time is 2 h to 6 h.

[0012] In some embodiments, the first electrical pulse signal has a pulse number of 5 to 10 times / s and a frequency of 10 kHz to 20 kHz.

[0013] In some embodiments, the device for generating the first electric pulse signal is an electric pulse descaling device.

[0014] In some embodiments, the first electric pulse signal generating device is disposed on the outer wall of the reactor used for the first precipitation reaction near the feed port.

[0015] In some embodiments, the preparation of ferrophosphorus solution is also included:

[0016] Oxidation precipitation of iron: under the condition of the second electric pulse, adding an oxidant and a precipitant to the acidic nickel-iron solution to oxidize and precipitate iron, and adjusting the pH to maintain at 1.5-3.5, and obtaining an iron-containing precipitate and an iron precipitation mother liquor through filter pressing;

[0017] Dissolve, use phosphoric acid to dissolve the iron-containing precipitate, and then add water to dilute it to obtain the ferrophosphorus solution.

[0018] In some embodiments, the second electric pulse signal has a pulse number of 10 times / s to 30 times / s and a frequency of 20 kHz to 50 kHz.

[0019] In some embodiments, the device for generating the second electric pulse signal is an electric pulse descaling device.

[0020] In some embodiments, the second electric pulse signal generating device is disposed on the outer wall of the reactor used for iron oxidation precipitation, near the feed port.

[0021] In some embodiments, the acidic nickel-iron solution is obtained by dissolving a nickel-iron alloy in an acid solution and filtering the solution.

[0022] In some embodiments, the nickel-iron alloy is obtained by reduction roasting of laterite nickel ore, and the iron content in the nickel-iron alloy is 60 wt% to 85 wt%, and the impurity content is less than 2 wt%.

[0023] In some embodiments, the acid solution is at least one of sulfuric acid and hydrochloric acid with a concentration of 2 mol / L to 8 mol / L.

[0024] In some embodiments, the solid-to-liquid ratio of the nickel-iron alloy when dissolved in the acid solution is 1 g / mL to 50 g / mL.

[0025] In some embodiments, the oxidant is at least one of hydrogen peroxide, oxygen, air, ozone and sodium persulfate.

[0026] In some embodiments, the amount of the oxidant added is 1.2 to 3 times the molar amount of iron in the acidic nickel-iron solution.

[0027] In some embodiments, the precipitant is at least one of ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonia water.

[0028] In some embodiments, the amount of the precipitant added is 1.5 to 5 times the molar amount of iron in the acidic nickel-iron solution.

[0029] In some embodiments, the concentration of phosphoric acid used in the dissolving step is 20 wt % to 40 wt %.

[0030] In some embodiments, the iron-containing precipitate is washed with water before the dissolving step.

[0031] In some embodiments, the reaction liquid containing flaky ferric phosphate is subjected to solid-liquid separation, and the separated solid is dried to obtain flaky ferric phosphate dihydrate, and the drying temperature is 100° C. to 300° C. and the drying time is 6 h to 12 h.

[0032] In some embodiments, the dried flaky ferric phosphate dihydrate is calcined at 650° C. to 900° C. for 1 to 2 hours to obtain flaky anhydrous ferric phosphate.

[0033] In a second aspect, the present disclosure provides a method for preparing flaky lithium iron phosphate, comprising sintering a mixture containing the flaky iron phosphate according to any one of the aforementioned embodiments, a lithium source, and a carbon source to obtain lithium iron phosphate.

[0034] In some embodiments, the molar ratio of the flaky iron phosphate to the lithium salt is 1:1.01 to 1.03.

[0035] In some embodiments, the lithium source is at least one of lithium phosphate, lithium hydroxide, and lithium carbonate.

[0036] In some embodiments, the amount of the carbon source added is 3% to 10% of the mass of the ferric phosphate.

[0037] In some embodiments, the carbon source is at least one of glucose, sucrose, citric acid, polyethylene glycol, cyclodextrin, polyvinyl alcohol, phenolic resin, polyacrylonitrile, starch and cellulose.

[0038] In some embodiments, the sintering is performed at 600° C. to 750° C. in an inert atmosphere for 3 h to 10 h.

[0039] The present disclosure has the following beneficial effects:

[0040] The present invention introduces electric pulses when preparing iron phosphate, so that water breaks into single water molecules when passing through the electric pulse generating device. The polar water molecules wrap around the iron ions and phosphate radicals, reducing the chance of collision between the iron ions and phosphate radicals. In this way, the reaction rate during the synthesis of iron phosphate is controlled, the impurity ions included in the precipitation of iron phosphate are reduced, and the purity of iron phosphate is improved. At the same time, flaky iron phosphate can be obtained. The lithium iron phosphate prepared using the flaky iron phosphate as a raw material can inherit the flaky morphology of iron phosphate to a certain extent, thereby obtaining flaky lithium iron phosphate with good electrochemical properties.

[0041] In the prior art, the particle size of lithium iron phosphate prepared by using a complexing agent can reach more than 1 μm. The embodiment of the present disclosure continuously turns on the electric pulse device during the precipitation process, which can generate flaky iron phosphate with a scale of about 500 nm. At the same time, it is beneficial to increase the density of iron phosphate, thereby improving the electrochemical performance of the positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0043] Figure 1 This is a flow chart for preparing lithium iron phosphate in Example 1.

[0044] Figure 2 This is the SEM image of the iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0045] To make the purpose, technical solutions, and advantages of the embodiments of the present disclosure more clear, the technical solutions in the embodiments of the present disclosure are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer were followed. Reagents or instruments used where the manufacturer is not specified are conventional products that can be purchased commercially.

[0046] This embodiment provides a method for preparing flake iron phosphate, comprising: subjecting a ferrophosphorus solution to a first electric pulse condition to perform a first precipitation reaction to obtain a reaction solution containing flake iron phosphate;

[0047] The ferrophosphorus solution includes iron ions and phosphate radicals but does not include a complexing agent. The pH value of the ferrophosphorus solution is 1.5-3.5.

[0048] The migration path of ions in the flaky morphology of lithium iron phosphate along the thickness direction is short, which can shorten the migration distance between ions and improve conductivity, so the electrochemical performance as a positive electrode material is better. In the prior art, a complexing agent is usually used in the iron phosphate precipitation process to reduce the precipitation rate of iron phosphate. However, the introduction of the complexing agent will generate a rod-shaped morphology of dihydrate iron phosphate, and the obtained lithium iron phosphate is also rod-shaped, with poor electrochemical performance. In this embodiment, an electric pulse is introduced when preparing iron phosphate, so that water breaks into single water molecules when passing through the electric pulse generating device. The polar water molecules are wrapped around the iron ions and phosphate, so that the collision opportunities between the iron ions and phosphate are reduced, thereby controlling the reaction rate during the synthesis of iron phosphate, reducing the impurity ions included in the precipitation of iron phosphate, and improving the purity of iron phosphate. At the same time, flaky iron phosphate can be obtained. The lithium iron phosphate prepared using the flaky iron phosphate as the raw material can inherit the flaky morphology of iron phosphate to a certain extent, thereby obtaining flaky lithium iron phosphate with better electrochemical performance.

[0049] In this embodiment, the electric pulse device is continuously turned on during the precipitation process, which can generate smaller-scale flake iron phosphate and is conducive to increasing the density of iron phosphate, thereby improving the electrochemical performance of the positive electrode material.

[0050] In some embodiments, the concentration of phosphate in the ferrophosphorus solution is 0.5 mol / L to 3 mol / L, specifically 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, or any value between 0.5 mol / L and 3 mol / L.

[0051] In some embodiments, the molar ratio of phosphate to iron in the ferrophosphorus solution is (1.01-2):1, specifically 1.0.1:1, 1.2.1:1, 1.4.1:1, 1.6.1:1, 1.8.1:1, 2:1 or any value between (1.01-2):1.

[0052] The relatively low concentrations of phosphate and iron ions in the ferrophosphorus solution facilitate the production of round, uniformly sized ferrophosphate precipitates. Furthermore, a slight excess of phosphate is recommended to improve the utilization of the iron ions, and the excess phosphate can be recycled if necessary.

[0053] In some embodiments, the temperature of the first precipitation reaction is 80°C-95°C, and the time is 2h~6h. Specifically, the temperature can be 80°C, 83°C, 86°C, 89°C, 92°C, 95°C or any value between 80°C and 95°C, and the time can be 2h, 3h, 4h, 5h, 6h or any value between 2h and 6h.

[0054] Since the concentrations of phosphate and iron ions in the ferrophosphorus solution are relatively low, in order to allow the reaction to proceed smoothly, the temperature of the first precipitation reaction is relatively high. Within the temperature range specified in this embodiment, the precipitation is carried out for 2h to 6h, and the median particle size of the ferrophosphate precipitate can reach 0.3μm-0.7μm.

[0055] In some embodiments, the pulse number of the first electric pulse signal is 5 times / s to 10 times / s, specifically 5 times / s, 6 times / s, 7 times / s, 8 times / s, 9 times / s, 10 times / s or any value between 5 times / s and 10 times / s, and the frequency is 10kHz to 20kHz, specifically 10kHz, 12kHz, 14kHz, 16kHz, 18kHz, 20kHz or any value between 10kHz and 20kHz.

[0056] In some embodiments, the device for generating the first electric pulse signal is an electric pulse descaling device.

[0057] In some embodiments, the first electric pulse signal generating device is disposed on the outer wall of the reactor used for the first precipitation reaction near the feed port.

[0058] The electric pulse signal generator can be a commercially available electric pulse descaling device, such as the Vulcan S100 electric pulse descaling device. The electric pulse device is positioned on the outer wall of the reactor. Specifically, positioning the electric pulse device near the feed inlet facilitates quick entry of the material into the electric pulse environment upon entering the reactor.

[0059] In some embodiments, the preparation of ferrophosphorus solution is also included:

[0060] Oxidation precipitation of iron: under the condition of a second electric pulse, adding an oxidant and a precipitant to the acidic nickel-iron solution to oxidize and precipitate iron, and adjusting the pH to maintain at 1.5 to 3.5, specifically 1.5, 2.0, 2.5, 3.0, 3.5 or any value between 1.5 and 3.5, and obtaining an iron-containing precipitate and an iron precipitation mother liquor by filter pressing;

[0061] Dissolving, using phosphoric acid or a combination of phosphoric acid and hydrochloric acid to dissolve the iron-containing precipitate, and then diluting with water to obtain the ferrophosphorus solution.

[0062] Ferric phosphate is prepared using nickel-iron alloy as raw material. The resulting acidic nickel-iron solution contains not only iron but also a large amount of nickel. In order to minimize the content of impurity ions in the ferric phosphate, an oxidant and a precipitant can be added first to initially precipitate the iron therein, and then the iron and phosphate are dissolved to obtain a ferrophosphorus solution containing iron and phosphate with a relatively low impurity content.

[0063] In order to ensure the dissolution efficiency of the iron-containing precipitate, the concentration of phosphoric acid used in the dissolution step should not be too low, but the concentration of phosphate and iron ions in the ferrophosphorus solution does not need to be too high. Therefore, water is added to the dissolved liquid for dilution. Under normal circumstances, dilution of 2 to 3 times is sufficient to make the ferrophosphorus solution reach the specified concentration.

[0064] When the iron-containing precipitate is iron hydroxide, phosphoric acid can be used to dissolve it. When the iron-containing precipitate is iron phosphate, a certain amount of phosphoric acid can be added first to make the phosphate ions and iron ions reach a specified ratio, and then hydrochloric acid is added until the iron phosphate is completely dissolved, and then diluted with water. If necessary, hydrochloric acid or ammonia water can be used to adjust the pH to the specified range.

[0065] In some embodiments, the second electric pulse signal has a pulse number of 10 times / s to 30 times / s, specifically 10 times / s, 15 times / s, 20 times / s, 25 times / s, 30 times / s, or any value between 10 times / s and 30 times / s; and a frequency of 20kHz to 50kHz, specifically 20kHz, 25kHz, 30kHz, 35kHz, 40kHz, 45kHz, 50kHz, or any value between 20kHz and 50kHz. In this embodiment, since the nickel content in the acidic nickel-iron solution is relatively high, in order to reduce the inclusion of impurities, the second electric pulse signal has a relatively high pulse number and frequency to reduce the precipitation rate.

[0066] It should be noted that, except for the first precipitation reaction and the ferrous iron oxidation reaction, other steps such as the dissolution step can be carried out under electric pulse conditions or not under electric pulse conditions, which has little effect on the ferric phosphate product.

[0067] In some embodiments, the device for generating the second electric pulse signal is an electric pulse descaling device.

[0068] In some embodiments, the second electric pulse signal generating device is disposed on the outer wall of the reactor used for iron oxidation precipitation, near the feed port.

[0069] The electric pulse signal generator can be a commercially available electric pulse descaling device, such as the Vulcan S100 electric pulse descaling device. The electric pulse device is positioned on the outer wall of the reactor. Specifically, positioning the electric pulse device near the feed inlet facilitates quick entry of the material into the electric pulse environment upon entering the reactor.

[0070] In some embodiments, the acidic nickel-iron solution is obtained by dissolving a nickel-iron alloy in an acid solution and filtering the solution.

[0071] In some embodiments, the nickel-iron alloy is obtained by reduction roasting of laterite nickel ore, and the iron content in the nickel-iron alloy is 60 wt% to 85 wt%, and the impurity content is less than 2 wt%.

[0072] The mass fraction of nickel in nickel-iron alloy can be as high as nearly 40wt%. Even if a chelating agent is added during the precipitation process, it is difficult to reduce the nickel content in ferric phosphate: if the amount of chelating agent added is small, the ferric phosphate will contain more impurities such as nickel. If too much chelating agent is added, some of it may be entrained in the ferric phosphate precipitate, resulting in a decrease in the purity of the ferric phosphate.

[0073] In some embodiments, the acid solution is at least one of sulfuric acid and hydrochloric acid with a concentration of 2 mol / L to 8 mol / L. Specifically, the concentration of the acid solution can be 2 mol / L, 4 mol / L, 6 mol / L, 8 mol / L or any value between 2 mol / L and 8 mol / L, and can be sulfuric acid, hydrochloric acid or a mixed acid consisting of sulfuric acid and hydrochloric acid.

[0074] In some embodiments, the solid-to-liquid ratio of the nickel-iron alloy when dissolved in the acid solution is 1 g / mL to 50 g / mL, specifically, 1 g / mL, 5 g / mL, 10 g / mL, 20 g / mL, 30 g / mL, 40 g / mL, 50 g / mL or any value between 1 g / mL and 50 g / mL.

[0075] In some embodiments, the oxidant is at least one of hydrogen peroxide, oxygen, air, ozone and sodium persulfate, for example, any one of them, or a mixture of oxygen and ozone, or a mixture of air, ozone and sodium persulfate.

[0076] In some embodiments, the amount of the oxidant added is 1.2 to 3 times the molar amount of iron in the acidic nickel-iron solution.

[0077] In some embodiments, the precipitant is at least one of ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate and ammonia water, for example, it can be any one of them, or a mixture of ammonium phosphate and ammonium hydrogen phosphate, or a mixture of ammonium hydrogen phosphate, ammonium dihydrogen phosphate and ammonia water.

[0078] In some embodiments, the amount of the precipitant added is 1.5 to 5 times the molar amount of iron in the acidic nickel-iron solution.

[0079] In some embodiments, the concentration of phosphoric acid used in the dissolving step is 20 wt % to 40 wt %.

[0080] When the precipitant is phosphate, the iron-containing precipitate is ferric phosphate. At this time, when dissolving the ferric phosphate, the amount of phosphoric acid added only needs to be enough to dissolve the ferric phosphate. However, when the precipitant includes ammonia water, the iron-containing precipitate contains ferric hydroxide. At this time, in order to achieve a suitable ratio of phosphorus and iron in the ferrophosphorus solution, in addition to dissolving the ferric hydroxide, additional phosphoric acid needs to be added.

[0081] It should be noted that, in some embodiments, if the pH value of a part of the solution, such as the ferrophosphorus solution, does not reach the specified range, a conventional acid or base may be added to adjust the pH, such as adding sulfuric acid or strong sodium hydroxide.

[0082] In some embodiments, the iron-containing precipitate is washed with water before the dissolution step. In this embodiment, if two precipitations are combined with electric pulse treatment, the nickel ions in the obtained iron phosphate can be reduced to below the required range by washing the iron-containing precipitate with water at most once.

[0083] In some embodiments, the reaction liquid containing flaky ferric phosphate is subjected to solid-liquid separation, and the separated solid is dried to obtain flaky ferric phosphate dihydrate, and the drying temperature is 100°C to 300°C, specifically, it can be 100°C, 150°C, 200°C, 250°C, 300°C or any value between 100°C and 300°C; the drying time is 6h to 12h, specifically, it can be 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value between 6h and 12h.

[0084] In some embodiments, the dried flaky ferric phosphate dihydrate is calcined at 650° C. to 900° C. for 1 to 2 hours to obtain flaky anhydrous ferric phosphate. Specifically, the calcination temperature can be 650° C., 700° C., 750° C., 800° C., 850° C., 900° C., or any value between 650° C. and 900° C.; and the calcination time can be 1.0 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2.0 hours, or any value between 1 hour and 2 hours.

[0085] In a second aspect, the present disclosure provides a method for preparing flaky lithium iron phosphate, comprising sintering a mixture of the flaky iron phosphate described in any one of the aforementioned embodiments, a lithium source, and a carbon source to obtain lithium iron phosphate. Figure 1 shown.

[0086] In some embodiments, the molar ratio of the flaky iron phosphate to the lithium salt is 1:1.01 to 1.03, specifically 1:1.01, 1:1.02, 1.03, or any value between 1:1.01 and 1.03.

[0087] In some embodiments, the lithium source is at least one of lithium phosphate, lithium hydroxide, and lithium carbonate.

[0088] In some embodiments, the added amount of the carbon source is 3% to 10% of the mass of the ferric phosphate, specifically 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value between 3% and 10%.

[0089] In some embodiments, the carbon source is at least one of glucose, sucrose, citric acid, polyethylene glycol, cyclodextrin, polyvinyl alcohol, phenolic resin, polyacrylonitrile, starch and cellulose.

[0090] In some embodiments, the sintering is performed at 600° C. to 750° C. in an inert atmosphere for 3 to 10 hours. Specifically, the sintering temperature can be 600° C., 650° C., 700° C., 750° C., or any value between 600° C. and 750° C., and the sintering time can be 3 hours, 6 hours, 8 hours, 10 hours, or any value between 3 hours and 10 hours.

[0091] The features and properties of the present disclosure are further described in detail below with reference to the examples, wherein if the reaction temperature is not specified, it is room temperature (18°C-25°C).

[0092] Example 1

[0093] This embodiment provides a method for preparing flaky lithium iron phosphate, which specifically includes the following steps:

[0094] S1: Dissolve nickel-iron alloy (by mass: nickel content 25%, iron content 74%, other impurity metals such as Mn, Mg, Co, etc., totaling 1%) in 5 mol / L sulfuric acid solution with a solid-liquid ratio of 40 g / ml. After dissolution, filter and collect the filtrate to obtain an acidic nickel-iron solution.

[0095] S2: Turn on the electric pulse device and add hydrogen peroxide (1.5 times the molar amount of iron) to the acidic nickel-iron solution for oxidation precipitation. The pH is controlled to 2.8 with 3 mol / L ammonia. After filter pressing, a crude ferric hydroxide precipitate and an iron precipitation mother liquor are obtained. The electric pulse device is then turned off. The electric pulse device emits an electric pulse signal at a rate of 20 pulses / s and a frequency of 30 kHz.

[0096] S3: Wash the crude ferric hydroxide precipitate once with pure water. The washing method is to immerse the crude ferric hydroxide in pure water and stir at a speed of 100 rpm, with a solid-liquid ratio of 20g / 100ml. After washing, dry the crude ferric hydroxide at 60°C for 12 hours to obtain a ferric hydroxide precipitate.

[0097] S4: Phosphoric acid is added to the ferric hydroxide precipitate to dissolve it, and filtered to obtain a concentrated ferrophosphorus solution. The molar ratio of phosphoric acid to iron in ferric hydroxide is 1.5:1, and the concentration of phosphoric acid is 30 wt%.

[0098] S5: Turn on the electric pulse device and dilute the concentrated ferrophosphorus solution with pure water to a phosphate concentration of 1.5 mol / L and an iron ion concentration of 1 mol / L. Control the pH to 2.5 with ammonia and phosphoric acid. Heat the solution to 85°C and allow it to react for 4 hours to produce ferrophosphate precipitation. Turn off the electric pulse device. The electric pulse device generates an electric pulse signal at a rate of 7 pulses / s and a frequency of 15 kHz.

[0099] S6: Dry the ferric phosphate precipitate at 150°C for 8 hours to obtain ferric phosphate dihydrate. Calcine the dried ferric phosphate dihydrate at 700°C for 1.5 hours to obtain anhydrous ferric phosphate, the morphology of which is as follows: Figure 2 shown.

[0100] S7: Mix the obtained anhydrous ferric phosphate with lithium hydroxide and glucose in a molar ratio of 1:1.01 (5% of the mass of the ferric phosphate), and sinter at 750° C. in a nitrogen atmosphere for 6 hours to obtain lithium iron phosphate.

[0101] Example 2:

[0102] This embodiment provides a method for preparing flaky lithium iron phosphate, which specifically includes the following steps:

[0103] S1: Dissolve nickel-iron alloy (by mass: nickel content 25%, iron content 74%, other impurity metals such as Mn, Mg, Co, etc., totaling 1%) in 5 mol / L sulfuric acid solution with a solid-liquid ratio of 40 g / ml. After dissolution, filter and collect the filtrate to obtain an acidic nickel-iron solution.

[0104] S2: Turn on the electric pulse device and add hydrogen peroxide (1.5 times the molar amount of iron) and ammonium phosphate (1.3 times the molar amount of iron) to the acidic nickel-iron solution for oxidation precipitation. The pH is controlled to 1.8 with 3 mol / L ammonia water. Press filteration is performed to obtain a crude iron phosphate precipitate and an iron precipitation mother liquor. The electric pulse device emits an electric pulse signal at a rate of 20 pulses / s and a frequency of 30 kHz.

[0105] S3: Wash the crude iron phosphate precipitate once with pure water.

[0106] S4: Phosphoric acid and hydrochloric acid are added to the crude ferric phosphate precipitate to dissolve it, and filtered to obtain a concentrated ferric phosphate solution. The phosphoric acid is first added at a molar ratio of 0.1:1 to the iron in the crude ferric phosphate, and then hydrochloric acid is added until the solid is completely dissolved. The concentrations of hydrochloric acid and phosphoric acid used are both 30%. The electric pulse signal emitted by the electric pulse device is 20 pulses / s and the frequency is 30 kHz.

[0107] S5: Dilute the concentrated ferrophosphorus solution with pure water to obtain a solution with a phosphate concentration of 1.1 mol / L and an iron ion concentration of 1 mol / L. Control the pH to 2.1 with ammonia and phosphoric acid. Heat the solution to 85°C and react for 4 hours to obtain ferrophosphate precipitation. Turn off the electric pulse device. The electric pulse device generates an electric pulse signal at a rate of 7 pulses / s and a frequency of 15 kHz.

[0108] S6: drying the ferric phosphate precipitate at 150° C. for 8 hours to obtain ferric phosphate dihydrate, and calcining the dried ferric phosphate dihydrate at 700° C. for 1.5 hours to obtain anhydrous ferric phosphate.

[0109] S7: Mix the obtained anhydrous ferric phosphate with lithium hydroxide and glucose in a molar ratio of 1:1.01 (5% of the mass of the ferric phosphate), and sinter at 750° C. in a nitrogen atmosphere for 6 hours to obtain lithium iron phosphate.

[0110] Example 3:

[0111] This embodiment provides a method for preparing flaky lithium iron phosphate, which specifically includes the following steps:

[0112] S1: Dissolve nickel-iron alloy (by mass: nickel content 25%, iron content 74%, other impurity metals such as Mn, Mg, Co, etc., totaling 1%) in 2 mol / L sulfuric acid solution with a solid-liquid ratio of 50 g / ml. After dissolution, filter and collect the filtrate to obtain an acidic nickel-iron solution.

[0113] S2: Turn on the electric pulse device and add hydrogen peroxide (1.5 times the molar amount of iron) and ammonium phosphate (1.3 times the molar amount of iron) to the acidic nickel-iron solution for oxidation precipitation. The pH is controlled to 2.5 with 3 mol / L ammonia water. Press filteration is performed to obtain a crude iron phosphate precipitate and an iron precipitation mother liquor. The electric pulse device emits an electric pulse signal at a rate of 10 pulses / s and a frequency of 20 kHz.

[0114] S3: adding phosphoric acid and hydrochloric acid to the crude ferric phosphate precipitate to dissolve it, and filtering to obtain a concentrated ferrophosphorus solution, wherein the ratio of the molar amount of phosphoric acid added to the iron in the crude ferric phosphate is 0.5:1, and then adding hydrochloric acid until the solid is completely dissolved, and the concentrations of the hydrochloric acid and phosphoric acid used are both 20%.

[0115] S4: Turn on the electric pulse device and dilute the concentrated ferrophosphorus solution with pure water to obtain a solution with a phosphate concentration of 1.5 mol / L and an iron ion concentration of 1 mol / L. The pH is controlled to 2.7 with ammonia and phosphoric acid, and the temperature is raised to 80°C for 6 hours to obtain a ferrophosphate precipitate. The electric pulse signal emitted by the electric pulse device is 5 pulses / s and the frequency is 10 kHz.

[0116] S5: drying the ferric phosphate precipitate at 100° C. for 12 hours to obtain ferric phosphate dihydrate, and calcining the dried ferric phosphate dihydrate at 800° C. for 10 hours to obtain anhydrous ferric phosphate.

[0117] S6: Mix the obtained anhydrous ferric phosphate with lithium hydroxide and glucose in a molar ratio of 1:1.02 (3% of the mass of the ferric phosphate), and sinter at 600° C. in a nitrogen atmosphere for 10 hours to obtain lithium iron phosphate.

[0118] Example 4:

[0119] This embodiment provides a method for preparing flaky lithium iron phosphate, which specifically includes the following steps:

[0120] S1: Dissolve nickel-iron alloy (by mass: nickel content 25%, iron content 74%, other impurity metals such as Mn, Mg, Co, etc., accounting for a total of 1%) in 8 mol / L sulfuric acid solution with a solid-liquid ratio of 2 g / ml. After dissolution, filter and collect the filtrate to obtain an acidic nickel-iron solution.

[0121] S2: Turn on the electric pulse device and add hydrogen peroxide (1.5 times the molar amount of iron) and ammonium phosphate (1.3 times the molar amount of iron) to the acidic nickel-iron solution for oxidation precipitation. The pH is controlled to 3.5 with 3 mol / L ammonia water. Press filteration is performed to obtain a crude iron phosphate precipitate and an iron precipitation mother liquor. The electric pulse device emits an electric pulse signal at a rate of 30 pulses / s and a frequency of 50 kHz.

[0122] S3: Wash the crude iron phosphate precipitate once with pure water.

[0123] S4: adding phosphoric acid and hydrochloric acid to the crude ferric phosphate precipitate to dissolve it, and filtering to obtain a concentrated ferrophosphorus solution, wherein the ratio of the molar amount of phosphoric acid added to the iron in the crude ferric phosphate is 1:1, and then hydrochloric acid is added until the solid is completely dissolved, and the concentrations of the hydrochloric acid and phosphoric acid used are both 40%.

[0124] S5: Turn on the electric pulse device and dilute the concentrated ferrophosphorus solution with pure water to obtain a solution with a phosphate concentration of 1.8 mol / L and an iron ion concentration of 0.9 mol / L. The pH is controlled to 2.3 with ammonia and phosphoric acid. The solution is heated to 95°C and reacted for 2 hours to obtain a ferrophosphate precipitate. The electric pulse device is then turned off. The electric pulse device emits an electric pulse signal at a rate of 10 pulses / s and a frequency of 20 kHz.

[0125] S6: drying the ferric phosphate precipitate at 300° C. for 6 hours to obtain ferric phosphate dihydrate, and calcining the dried ferric phosphate dihydrate at 900° C. for 3 hours to obtain anhydrous ferric phosphate.

[0126] S7: Mix the obtained anhydrous ferric phosphate with lithium hydroxide and glucose in a molar ratio of 1:1.03 (10% of the mass of the ferric phosphate) and sinter at 650° C. in a nitrogen atmosphere for 3 h to obtain lithium iron phosphate.

[0127] Example 5

[0128] The only difference from Example 1 is that in step S2, the number of pulses of the electric pulse signal emitted by the electric pulse device is 7 times / s and the frequency is 15 kHz.

[0129] Example 6

[0130] The only difference from Example 1 is that in step S4, the phosphoric acid concentration is 10%.

[0131] Example 7

[0132] The only difference from Example 1 is that in step S5, the concentrated ferrophosphorus solution is diluted with pure water to obtain a solution with a phosphate concentration of 2.7 mol / L and an iron ion concentration of 1.8 mol / L, the pH is controlled to 2.5 using ammonia and phosphoric acid, and the temperature is raised to 85°C and reacted for 4 hours to obtain ferric phosphate precipitate.

[0133] Example 8

[0134] The only difference from Example 1 is that in step S5, the concentrated ferrophosphorus solution is diluted with pure water to obtain a solution with a phosphate concentration of 0.5 mol / L and an iron ion concentration of 0.33 mol / L, the pH is controlled to 2.5 using ammonia and phosphoric acid, and the temperature is raised to 85°C and reacted for 4 hours to obtain ferric phosphate precipitate.

[0135] Example 9

[0136] The only difference from Example 1 is that in step S4, the molar ratio of phosphoric acid to iron in ferric hydroxide is 3:1.

[0137] Example 10

[0138] The only difference from Example 1 is that in step S4, the molar ratio of phosphoric acid to iron in ferric hydroxide is 0.9:1.

[0139] Example 11

[0140] The only difference from Example 1 is that in step S5, the temperature is raised to 70° C. and the reaction is carried out for 4 hours to obtain iron phosphate precipitate.

[0141] Example 12

[0142] The only difference from Example 1 is that in step S5, the temperature is raised to 110° C. and the reaction is carried out for 4 hours to obtain iron phosphate precipitate.

[0143] Example 13

[0144] The only difference from Example 1 is that in step S5, the number of pulses of the electric pulse signal emitted by the electric pulse device is 20 times / s and the frequency is 30 kHz.

[0145] Example 14

[0146] The only difference from Example 1 is that step S3 is omitted.

[0147] Comparative Example 1:

[0148] The only difference from Example 1 is that the electric pulse device is not turned on in steps S2 and S5.

[0149] Comparative Example 2:

[0150] The only difference from Example 1 is that the electric pulse device is not turned on in step S5.

[0151] Comparative Example 3:

[0152] The only difference from Comparative Example 1 is that the electric pulse device is not turned on in step S5 and citric acid is added as a complexing agent. The mass fraction of the citric acid is 10%, and the ratio of the added volume to the solution volume is 0.01:1.

[0153] The resistivity of the lithium iron phosphate material was tested using a solid conductivity tester. Acetylene black was used as a conductive agent, and PVDF was used as a binder. Lithium iron phosphate, acetylene black, PVDF, and NMP were mixed in a mass ratio of 8:1:1:40. After stirring, the mixture was coated on aluminum foil to prepare a positive electrode sheet. A button battery was prepared using a metal lithium sheet as the negative electrode. The assembled button battery was tested for 0.1C discharge capacity using a CT2001A LAND battery tester in the charge and discharge voltage range of 2.75V to 4.3V and a test temperature of 25°C. The results are shown in the following table.

[0154]

[0155]

[0156] Industrial Applicability

[0157] The present invention introduces electric pulses when preparing iron phosphate, so that water breaks into single water molecules when passing through the electric pulse generating device. The polar water molecules wrap around the iron ions and phosphate radicals, reducing the chance of collision between the iron ions and phosphate radicals. This controls the reaction rate during the synthesis of iron phosphate, reduces the impurity ions included in the precipitation of iron phosphate, improves the purity of iron phosphate, and can obtain flaky iron phosphate. The lithium iron phosphate prepared using the flaky iron phosphate as a raw material can inherit the flaky morphology of iron phosphate to a certain extent, thereby obtaining flaky lithium iron phosphate with better electrochemical performance. In addition, the presence of electric pulses can generate smaller-scale flaky iron phosphate, which is beneficial to increase the density of iron phosphate, thereby improving the electrochemical performance of the positive electrode material lithium iron phosphate.

Claims

1. A method for preparing flaky iron phosphate, characterized in that: include: subjecting the ferrophosphorus solution to a first precipitation reaction under a first electric pulse condition to obtain a reaction solution containing flake ferric phosphate, wherein the first electric pulse signal has a pulse rate of 5 to 10 times per second and a frequency of 10 to 20 kHz; The ferrophosphorus solution includes iron ions and phosphate radicals but does not include a complexing agent. The pH value of the ferrophosphorus solution is 1.5-3.

5.

2. The method for preparing flaky ferric phosphate according to claim 1, wherein: The concentration of phosphate in the ferrophosphorus solution is 0.5-3 mol / L.

3. The method for preparing flaky iron phosphate according to claim 2, wherein: The molar ratio of phosphate to iron in the ferrophosphorus solution is (1.01-2):

1.

4. The method for preparing flaky ferric phosphate according to claim 3, wherein: The temperature of the first precipitation reaction is 80° C.-95° C., and the time is 2 h to 6 h.

5. The method for preparing flaky ferric phosphate according to any one of claims 1 to 4, characterized in that: The device for generating the first electric pulse signal is an electric pulse descaling device.

6. The method for preparing flaky ferric phosphate according to any one of claims 1 to 5, characterized in that: The first electric pulse signal generating device is arranged on the outer wall of the reactor used for the first precipitation reaction near the feed port.

7. The method for preparing flaky ferric phosphate according to any one of claims 1 to 6, characterized in that: Also included is the preparation of ferrophosphorus solution: Oxidation precipitation of iron: under the condition of the second electric pulse, adding an oxidant and a precipitant to the acidic nickel-iron solution to oxidize and precipitate iron, and adjusting the pH to maintain at 1.5-3.5, and obtaining an iron-containing precipitate and an iron precipitation mother liquor through filter pressing; Dissolving, using phosphoric acid or a combination of phosphoric acid and hydrochloric acid to dissolve the iron-containing precipitate, and then diluting with water to obtain the ferrophosphorus solution.

8. The method for preparing flaky ferric phosphate according to claim 7, characterized in that: The second electric pulse signal has a pulse number of 10 times / s to 30 times / s and a frequency of 20 kHz to 50 kHz.

9. The method for preparing flaky iron phosphate according to claim 7 or 8, characterized in that: The device for generating the second electric pulse signal is an electric pulse descaling device.

10. The method for preparing flaky iron phosphate according to any one of claims 7 to 9, characterized in that: The second electric pulse signal generating device is arranged on the outer wall of the reactor used for iron oxidation precipitation, near the feed port.

11. The method for preparing flaky ferric phosphate according to any one of claims 7 to 10, characterized in that: The acidic nickel-iron solution is obtained by dissolving nickel-iron alloy in an acid solution and filtering the solution.

12. The method for preparing flaky ferric phosphate according to claim 11, characterized in that: The nickel-iron alloy is obtained by reduction roasting of laterite nickel ore. The iron content in the nickel-iron alloy is 60wt% to 85wt%, and the impurity content is less than 2wt%.

13. The method for preparing flaky ferric phosphate according to claim 11 or 12, characterized in that: The acid solution is at least one of sulfuric acid and hydrochloric acid with a concentration of 2 mol / L to 8 mol / L.

14. The method for preparing flaky iron phosphate according to any one of claims 11 to 13, characterized in that: When the nickel-iron alloy is dissolved in an acid solution, the solid-liquid ratio is 1 g / mL to 50 g / mL.

15. The method for preparing flaky iron phosphate according to any one of claims 7 to 14, characterized in that: The oxidant is at least one of hydrogen peroxide, oxygen, air, ozone and sodium persulfate.

16. The method for preparing flaky iron phosphate according to any one of claims 7 to 15, characterized in that: The added amount of the oxidant is 1.2 to 3 times the molar amount of the iron element in the acidic nickel-iron solution.

17. The method for preparing flaky ferric phosphate according to any one of claims 7 to 16, characterized in that: The precipitant is at least one of ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate and ammonia water.

18. The method for preparing flaky ferric phosphate according to any one of claims 7 to 17, characterized in that: The amount of the precipitant added is 1.5 to 5 times the molar amount of the iron element in the acidic nickel-iron solution.

19. The method for preparing flaky iron phosphate according to any one of claims 7 to 18, characterized in that: The concentration of phosphoric acid used in the dissolving step is 20 wt % to 40 wt %.

20. The method for preparing flaky iron phosphate according to any one of claims 7 to 19, characterized in that: Before the dissolving step, the iron-containing precipitate is washed with water.

21. The method for preparing flaky ferric phosphate according to any one of claims 1 to 20, characterized in that: The reaction liquid containing flaky ferric phosphate is subjected to solid-liquid separation, and the separated solid is dried to obtain flaky ferric phosphate dihydrate, wherein the drying temperature is 100° C. to 300° C. and the drying time is 6 hours to 12 hours.

22. The method for preparing flaky ferric phosphate according to claim 21, wherein: The dried flaky ferric phosphate dihydrate is calcined at 650° C. to 900° C. for 1 to 2 hours to obtain flaky anhydrous ferric phosphate.

23. The method for preparing flaky ferric phosphate according to claim 22, wherein: The median particle size of the flaky anhydrous ferric phosphate is 0.3 μm-0.7 μm.

24. A method for preparing flaky lithium iron phosphate, characterized in that: A mixture containing the flaky iron phosphate according to any one of claims 1 to 23, a lithium source and a carbon source is sintered to obtain lithium iron phosphate.

25. The method for preparing flaky ferric phosphate according to claim 24, characterized in that: The molar ratio of the flaky iron phosphate to the lithium salt is 1:1.01-1.

03.

26. The method for preparing flaky ferric phosphate according to claim 24 or 25, characterized in that: The lithium source is at least one of lithium phosphate, lithium hydroxide and lithium carbonate.

27. The method for preparing flaky iron phosphate according to any one of claims 24 to 26, characterized in that: The added amount of the carbon source is 3% to 10% of the mass of the ferric phosphate.

28. The method for preparing flaky iron phosphate according to any one of claims 24 to 27, characterized in that: The carbon source is at least one of glucose, sucrose, citric acid, polyethylene glycol, cyclodextrin, polyvinyl alcohol, phenolic resin, polyacrylonitrile, starch and cellulose.

29. The method for preparing flaky ferric phosphate according to any one of claims 24 to 28, characterized in that: The sintering is carried out at 600° C. to 750° C. in an inert atmosphere for 3 hours to 10 hours.

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