Method for accurately controlling titanium doping in iron phosphate synthesis process and application thereof

Abstract

The method for controlling titanium doping in the iron phosphate synthesis process comprises the following steps that S1, a phosphorus source, an iron source, a titanium source and water are mixed and dissolved, and molten metal is prepared; s2, respectively preparing titanium phosphate seed crystal slurry and iron phosphate seed crystal slurry; s3, the iron phosphate seed crystal slurry and the titanium phosphate seed crystal slurry are added into the molten metal for a heating reaction, and titanium-doped iron phosphate slurry is obtained; in the molten metal, the concentration of the iron element is 60-70 g / L, the molar ratio of the iron element to the phosphorus element is (0.98-1): 1, and the mass ratio of the iron element to the titanium element is (95-100): 1. By controlling the concentration of Fe and Ti in molten metal and adding seed crystals, the precipitation rate of Fe and Ti in the synthesis process is regulated and controlled, so that titanium doping is accurately realized, and iron phosphate products with multiple series of titanium contents can be synthesized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium battery precursor material preparation, in particular to a method for precisely controlling titanium doping in the synthesis of iron phosphate and application thereof. BACKGROUND

[0002] Doping is considered as a method that can effectively improve the intrinsic conductivity and ion conductivity of lithium iron phosphate. Metal element doping can inhibit the growth of particles, be beneficial to the nanocrystallization of particles, and improve the electrochemical performance of the material. Metal element doping replaces lithium or iron in lithium iron phosphate, induces lattice distortion and produces lattice defects, and weakens the Li-O bond. At present, titanium-doped lithium iron phosphate has become one of the mainstreams. Titanium ions can replace iron ions into the iron phosphate lattice, increase the interplanar spacing and specific surface area, and thus be beneficial to the deintercalation of lithium ions. Titanium doping can improve the discharge capacity and cycle performance of lithium ion batteries. Currently, there are three ways for titanium-doped lithium iron phosphate: first, in the preparation process of iron phosphate, titanium salt solution is mixed with the precursor of iron phosphate, and titanium element is uniformly doped by controlling the reaction conditions (such as pH value); second, high-temperature solid-phase method: nano-titanium dioxide is doped during the high-temperature sintering of iron phosphate and lithium carbonate; third, wet high-energy ball milling: different titanium content iron phosphate is mixed with other components by using wet high-energy ball milling technology, and titanium-doped lithium iron phosphate is obtained after high-temperature sintering. However, it is difficult to achieve uniform distribution of titanium doping by high-temperature solid-phase doping and wet high-energy ball milling. The position with high titanium content is easy to form a heterogeneous phase, resulting in a decrease in active lithium content. At the same time, due to the limitation of the preparation process of high-temperature solid-phase sintering, doping needs to be carried out at high temperature, which has high energy consumption. SUMMARY

[0003] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the present application proposes a method for precisely controlling titanium doping in the synthesis of iron phosphate and application thereof. By controlling Fe / Ti and adding titanium phosphate seed crystals, the synthesis process of iron phosphate is optimized, and a high-performance lithium iron phosphate precursor is prepared.

[0004] According to a first aspect of the present application, a method for controlling titanium doping in the synthesis of iron phosphate is proposed, comprising the following steps: S1: mixing and dissolving a phosphorus source, an iron source, a titanium source and water to prepare a metal liquid; S2: preparing a titanium phosphate seed slurry and an iron phosphate seed slurry, respectively; S3: adding the iron phosphate seed slurry and the titanium phosphate seed slurry to the metal liquid for heating reaction to obtain a titanium-doped iron phosphate slurry; The concentration of iron element in the molten metal is 60-70 g / L, the molar ratio of iron element to phosphorus element is (0.98-1):1, and the mass ratio of iron element to titanium element is (95-100):1.

[0005] By controlling the concentrations of Fe and Ti in the molten metal and adding seed crystals, the precipitation rates of Fe and Ti in the synthesis process are regulated, the titanium doping can be accurately realized, and by regulating the relative concentrations of Fe and Ti in the molten metal, a plurality of series of phosphorus iron products with different titanium contents can be synthesized.

[0006] In some embodiments, in step S2, the preparation of the titanium phosphate seed slurry comprises: mixing titanium phosphate and water to prepare a slurry, and grinding the obtained slurry to obtain a titanium phosphate seed slurry. And / or, the preparation of the iron phosphate seed slurry comprises: mixing iron phosphate and water to prepare a slurry, and grinding the obtained slurry to obtain an iron phosphate seed slurry.

[0007] In some embodiments, the grinding comprises: controlling the grinding speed to 500-1500 r / min for cyclic grinding, and the number of cyclic grinding is ≥2 times.

[0008] In some embodiments, in step S2, the solid content of the titanium phosphate seed slurry is 20%-30%, and the solid content of the iron phosphate seed slurry is 25%-40%.

[0009] In some embodiments, the titanium phosphate is single-crystal titanium phosphate, and / or the iron phosphate is monoclinic iron phosphate.

[0010] Single-crystal titanium phosphate refers to a whole titanium phosphate crystal composed of a continuous, defect-free crystal structure, with highly ordered and consistent atomic arrangement, and the crystal plane direction remains unique and continuously distributed in the whole crystal. In contrast, polycrystalline titanium phosphate is composed of multiple small single-crystal titanium phosphates arranged randomly, with grain boundaries between the grains, and the atomic arrangement at the grain boundaries is discontinuous, resulting in anisotropy and non-uniformity of the performance of polycrystalline titanium phosphate. Single-crystal titanium phosphate has no grain boundaries, thus providing a low-impedance ion transport channel, while polycrystalline titanium phosphate has grain boundaries, which sometimes become a "short circuit" path for electronic conduction, which reduces the ion migration number and efficiency.

[0011] In some preferred embodiments, the specific surface area of the titanium phosphate seed slurry is 15-25 m 2 / g.

[0012] By controlling the crystal type, particle size and specific surface area of the titanium phosphate seed, the crystal type, particle size and specific surface area of the generated titanium phosphate can be induced and controlled.

[0013] In some preferred embodiments, the iron phosphate source for preparing the iron phosphate seed slurry is derived from a previous batch of titanium-doped iron phosphate slurry. The titanium-doped iron phosphate slurry described in step S3 is filtered and washed to obtain the iron phosphate.

[0014] In some embodiments, the seed particle size of the titanium phosphate seed slurry and the iron phosphate seed slurry is independently 1-2 μm. By controlling the seed particle size of the iron phosphate seed slurry, the particle size of the finished iron phosphate product can be prevented from being too large, which can result in a decrease in electrochemical performance.

[0015] In some embodiments, the phosphorus source is at least one selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, and the like; and / or, the iron source is at least one selected from the group consisting of ferrous sulfate, ferric sulfate, ferrous oxalate, iron powder, red iron oxide, ferrous nitrate, or ferric nitrate; and / or, the titanium source is at least one selected from the group consisting of titanyl sulfate and titanium dioxide.

[0016] In some embodiments, when the iron source is selected from iron powder or red iron oxide, step S1 further comprises adding phosphoric acid to dissolve the iron source; when the titanium source is selected from titanium dioxide, step S1 further comprises using concentrated sulfuric acid to dissolve the titanium dioxide to generate a titanyl sulfate solution, which is then diluted for later use.

[0017] In some embodiments, an iron lithium phosphate lithium extraction residue acid solution is used as both the phosphorus source and the iron source, the iron lithium phosphate lithium extraction residue acid solution has an iron content of 65-70 g / L, a molar ratio of iron to phosphorus of (0.95-1.0): 1, and a molar ratio of sulfur to iron of (0.95-1): 1.

[0018] In some embodiments, the metal solution comprises Fe 2+ In some embodiments, step S3 further comprises adding hydrogen peroxide to the metal solution, the amount of hydrogen peroxide added is 1.5-2 times the theoretical amount required to oxidize all Fe 2+ in the metal solution to Fe 3+ , and the concentration of the hydrogen peroxide is 15%-30%.

[0019] In some embodiments, in step S3, the heating reaction is a heat preservation reaction at 90-95 °C for 5-10 h, and the temperature at which the metal solution is mixed with the iron phosphate seed slurry and the titanium phosphate seed slurry is 50-55 °C.

[0020] In some embodiments, in step S3, the amount of the added iron phosphate seed slurry is determined as 5% to 15% of the total mass of the dihydrate iron phosphate when the iron phosphate seed has converted all the iron element in the metal liquid into dihydrate iron phosphate; and / or, the amount of the added titanium phosphate seed slurry is determined as 0.5% to 3% of the total mass of the titanium phosphate when the titanium phosphate seed has converted all the titanium element in the metal liquid into titanium phosphate.

[0021] According to a second aspect of the present application, a method for preparing titanium-doped anhydrous iron phosphate is provided, which comprises the steps of the method of the first aspect, and further comprises the following steps: After filtering, washing and drying the titanium-doped iron phosphate slurry, the titanium-doped anhydrous iron phosphate is obtained by calcination; The calcination temperature is 550 to 700℃, and the calcination time is 4 to 7h.

[0022] In some embodiments, the washing comprises: adding pure water to perform multiple times of leaching and slurry washing on the titanium-doped iron phosphate slurry until the conductivity of the washing water is ≤500μS / m.

[0023] According to a third aspect of the present application, the method of the first aspect and the second aspect is applied in the preparation of lithium battery cathode materials.

[0024] According to an embodiment of the present application, at least the following beneficial effects are achieved: 1. In the metal liquid preparation process of the present application, the content of titanium in the product is precisely controlled by strictly controlling the Fe / Ti indicators, and then the titanium-doped amount in the product is precisely realized by adding iron phosphate seeds and titanium phosphate seeds. The precipitation rates of iron and titanium in the iron phosphate synthesis process are stably achieved to be more than 97% and 99%, respectively. In the traditional synthesis process, a very high activation energy (i.e. "nucleation energy barrier") is required for the formation of iron phosphate and titanium phosphate nuclei from the metal liquid phase, which leads to slow reaction and difficulty in controlling the precipitation rates of iron and titanium. By adding iron phosphate seeds and titanium phosphate seeds, the present application provides a ready-made and stable solid-liquid interface, so that solute molecules can be directly and orderly arranged on the existing crystal lattice of the seeds, without the need to form a new interface. The addition of seeds greatly reduces the energy barrier required for the entire precipitation process, making the precipitation more likely to occur at a lower supersaturation. At the same time, by changing Fe / Ti in the metal liquid preparation process, different titanium series of iron phosphate products can be flexibly prepared.

[0025] 2. The present application controls the particle size of the single-crystal titanium phosphate by grinding, increases the specific surface area of the seeds, and makes them more uniformly dispersed in the titanium-doped iron phosphate.

[0026] 3. By adding titanium phosphate seed crystals, the crystal structure of titanium phosphate in the synthesis process of iron phosphate can be accurately controlled, which is conducive to the formation of single crystal compounds and improves the electrochemical performance compared to polycrystalline titanium phosphate. BRIEF DESCRIPTION OF DRAWINGS

[0027] The present application will be further described below in conjunction with the accompanying drawings and examples, in which: Figure 1 The process flow chart of Example 1 of the present application is shown in Figure 1. Figure 2 The XRD comparison chart of the anhydrous iron phosphate prepared in Example 1 of the present application and the standard card is shown in Figure 2. DETAILED DESCRIPTION

[0028] The concept and technical effects of the present application will be described below in conjunction with examples to fully understand the purpose, features and effects of the present application.

[0029] The raw materials, reagents or devices used in the following examples are commercially available or can be obtained by known methods unless otherwise specified.

[0030] Example 1 The present embodiment provides a method for accurately controlling titanium doping in the synthesis process of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid, and titanyl sulfate solution into pure water, stir for 0.5 h, then filter to obtain a clear mixed liquid as the metal liquid, and sample detection of Fe, P, and Ti contents is Fe: 65.66 g / L, P: 36.7 g / L, and Ti: 0.675 g / L, respectively, and the molar ratio Fe / P = 0.992 and the mass ratio Fe / Ti = 97.27 are calculated; Step 2, preparation of titanium phosphate and iron phosphate seed slurry: accurately weigh 20 g of single crystal titanium phosphate into 60 mL of pure water, stir, then add to a grinding tank, and perform wet ball milling, controlling the rotation speed at 1000 r / min during the grinding process, and grinding for 2 h with 2 cycles of grinding, sample detection of particle size, and stopping grinding when D50 reaches 1 μm, and taking out as seed crystals for use; accurately weigh 115.5 g of iron phosphate into 214.5 mL of pure water, stir, then add to a grinding tank, and perform wet ball milling, controlling the rotation speed at 1000 r / min during the grinding process, and grinding for 2 h with 2 cycles of grinding, sample detection of particle size, and stopping grinding when D50 reaches 1 μm, and taking out as seed crystals for use; Step 3, iron phosphate synthesis: take the 5L metal liquid in step 1 into the synthesis reactor, start stirring for about 10min, start heating to 50℃, weigh 580g, 27.5% concentration of hydrogen peroxide, add to the reactor with peristaltic pump, hydrogen peroxide adding time is 2h. Add 313g, 35% solid content of iron phosphate seed slurry and 1g, 25% solid content of titanium phosphate seed slurry (before weighing the seed, the iron phosphate and titanium phosphate seed slurry have been fully stirred evenly) stirring for 0.5h, continue to heat to 93℃, keep the temperature for 7h to get titanium doped iron phosphate slurry. Among them, the amount of iron phosphate seed added is 10% of the total mass of Fe in the metal liquid converted to dihydrate iron phosphate, and the amount of titanium phosphate seed added is 2% of the total mass of Ti in the metal liquid converted to titanium phosphate; Step 4, washing and drying: the titanium doped iron phosphate slurry in step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, and pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500μS / m. Combine all the washing water and filtrate to obtain the mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace at 600℃ for calcination for 6h, then cooled and taken out, crushed to obtain titanium doped iron phosphate.

[0031] Example 2 The present embodiment provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: take 5L lithium extraction residue sulfuric acid solution, sample and detect Fe, P, S, Ti, according to the detection results, add phosphoric acid, titanyl sulfate solution and pure water to prepare a metal liquid with Fe: 65.2g / L, P: 36.51g / L, Ti: 0.672g / L, molar ratio Fe / P=0.990, mass ratio Fe / Ti=97.02; Step 2, preparation of titanium phosphate and iron phosphate seed slurry: accurately weigh 20g of single crystal titanium phosphate and add it to 60mL of pure water, stir, then add it to the grinding tank and perform wet ball milling, control the rotation speed at 1000r / min during the grinding process, cycle grinding for 2 times, grinding time 2h, sample and detect the particle size, stop grinding when D50 reaches 1μm, take out as seed for use; accurately weigh 115.5g of iron phosphate and add it to 214.5mL of pure water, stir, then add it to the grinding tank and perform wet ball milling, control the rotation speed at 1000r / min during the grinding process, cycle grinding for 2 times, grinding time 2h, sample and detect the particle size, stop grinding when D50 reaches 1μm, take out as seed for use; Step 3, iron phosphate synthesis: Take the lithium extraction residue sulfuric acid solution of lithium iron phosphate of step 1, 5L, into the synthesis reactor, start stirring for about 10min, start heating to 50℃, add 311g, solid content 35% of iron phosphate seed slurry and 1g, solid content 25% of titanium phosphate seed slurry (before weighing the seed, the iron phosphate and titanium phosphate seed slurries have been fully stirred and uniformly), stir for 0.5h, continue to heat to 93℃, keep the temperature for 7h, then take out the slurry. Among them, the amount of iron phosphate seed added is 10% of the total amount of Fe in the metal liquid converted to iron phosphate dihydrate, and the amount of titanium phosphate seed added is 2% of the total amount of Ti in the metal liquid converted to titanium phosphate; Step 4, washing and drying: the titanium-doped iron phosphate slurry of step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500μS / m, and all the washing water and filtrate are combined to obtain a mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace for calcination at 600℃ for 6h, then cooled and taken out, crushed to obtain titanium-doped iron phosphate.

[0032] Example 3 The present embodiment provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid, and titanyl sulfate solution into pure water, stir for 0.5h, then filter to obtain a clear mixed liquid with Fe, P, and Ti contents of Fe: 63.54g / L, P: 35.51g / L, and Ti: 0.662g / L, respectively, as the metal liquid, and calculate the molar ratio Fe / P=0.992 and the mass ratio Fe / Ti=95.98, respectively; Step 2, preparation of titanium phosphate and iron phosphate seed slurry: accurately weigh 20g of polycrystalline titanium phosphate into 60mL of pure water, stir, then add to a grinding tank for wet ball milling, control the rotation speed at 600r / min during the grinding process, cycle for 2 times, and grind for 2h, then stop the grinding after the D50 reaches 2μm, and take out as seed for use; accurately weigh 115.5g of iron phosphate into 214.5mL of pure water, stir, then add to a grinding tank for wet ball milling, control the rotation speed at 1000r / min during the grinding process, cycle for 2 times, and grind for 2h, then stop the grinding after the D50 reaches 1μm, and take out as seed for use; Step 3, iron phosphate synthesis: take the metal liquid of step 1, 5L, into the synthesis reactor, start stirring for about 10min, heat the metal liquid to 50℃, weigh hydrogen peroxide, and add it to the reactor with a peristaltic pump, the amount of hydrogen peroxide added is Fe 2+ all oxidized to Fe3+ Theoretical amount of 1.6 times, hydrogen peroxide addition time is 2h. Respectively add 303g, solid content of 35% of iron phosphate seed slurry and 0.97g, solid content of 25% of titanium phosphate seed slurry (before weighing the seed, the iron phosphate and titanium phosphate seed slurry have been fully stirred uniformly), constant temperature reaction 0.5h, continue to heat to 92℃, keep the temperature for 8h, prepare titanium doped iron phosphate slurry. Among them, the addition amount of iron phosphate seed is 10% of the total amount of Fe in the metal liquid converted into iron phosphate dihydrate, and the addition amount of titanium phosphate seed is 2% of the total amount of Ti in the metal liquid converted into titanium phosphate.

[0033] Step 4, washing and drying: the titanium doped iron phosphate slurry of step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, and the crude iron phosphate is washed with pure water for multiple times until the conductivity of the washing water is ≤500μS / m, and all the washing water and filtrate are combined to obtain a mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace at 700℃ for calcination for 6h, then cooled and taken out, and crushed to obtain titanium doped iron phosphate.

[0034] Comparative Example 1 The present embodiment provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid, and titanyl sulfate solution into pure water, stir for 0.5h, and then filter to obtain a clear mixed liquid with Fe, P, and Ti contents of Fe: 62.34g / L, P: 34.84g / L, and Ti: 0.643g / L, respectively, as the metal liquid, and the molar ratio Fe / P=0.992 and the mass ratio Fe / Ti=96.95 are calculated respectively; Step 2, preparation of iron phosphate seed slurry: accurately weigh 115.5g of iron phosphate into 214.5mL of pure water, stir, and then add to a grinding tank for wet ball milling, control the rotation speed at 1000r / min during the grinding process, and cycle the grinding for 2 times, with a grinding time of 2h, take a sample to detect the particle size, and stop the grinding when the D50 reaches 1μm, and then take out as seed for use; Step 3, synthesis of iron phosphate: take the 5 L metal liquid in step 1 into a synthesis reactor, start stirring for about 10 min, start heating to 50℃, weigh 580 g of hydrogen peroxide solution with a concentration of 27.5%, add it to the reactor with a peristaltic pump, the hydrogen peroxide solution is added for 2 h, add 297 g of iron phosphate seed slurry with a solid content of 35% and 0.23 g of single crystal titanium phosphate solid (the iron phosphate seed slurry is fully stirred before the seed is weighed), stir for 0.5 h, continue to heat to 93℃, and react for 7 h after keeping the temperature, to obtain titanium-doped iron phosphate slurry. Among them, the amount of iron phosphate seed added is 12% of the total amount of iron in the metal liquid converted into dihydrate iron phosphate, and the amount of single crystal titanium phosphate solid added is 1% of the total amount of titanium in the metal liquid converted into titanium phosphate.

[0035] Step 4, washing and drying: the titanium-doped iron phosphate slurry in step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, and pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500 μS / m, and all the washing water and filtrate are combined to obtain a mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace at 600℃ for calcination for 6 h, then cooled and taken out, crushed to obtain titanium-doped iron phosphate.

[0036] Comparative Example 2 The present comparative example provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, preparation of metal liquid: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid, and titanyl sulfate solution into pure water, stir for 0.5 h, and then filter to obtain a clear mixed liquid with Fe, P, and Ti contents of Fe: 65.23 g / L, P: 36.56 g / L, and Ti: 0.675 g / L, respectively, as a metal liquid, and the molar ratio Fe / P = 0.989 and the mass ratio Fe / Ti = 96.64 are calculated respectively; Step 2, preparation of iron phosphate seed slurry: accurately weigh 108.9 g of iron phosphate into 254.1 mL of pure water, stir, and then add to a grinding tank for wet ball milling, control the rotation speed at 1000 r / min during the grinding process, circulate for 2 times, and grind for 2 h, take a sample to detect the particle size, and stop grinding when the D50 reaches 1 μm, and then take out as seed for use; Step 3, iron phosphate synthesis: Take the metal liquid 5L of step 1 into the synthesis reactor, start stirring for about 10min, start heating to 50℃, take 990g, 15% concentration of hydrogen peroxide, add to the reactor with peristaltic pump, hydrogen peroxide adding time is 2h. Add 363g, solid content is 30% iron phosphate seed slurry (before weighing the seed, the iron phosphate seed slurry has been fully stirred evenly) stirring for 0.5h, continue to heat to 93℃, keep the reaction for 7h, get the titanium doped iron phosphate slurry. Among them, the amount of iron phosphate seed added is 10% of the total amount of Fe in the metal liquid converted into iron phosphate dihydrate; Step 4, washing and drying: the titanium doped iron phosphate slurry of step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500μS / m, all the washing water and filtrate are combined to obtain the mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace for calcination at 600℃ for 6h, then cooled and taken out, crushed to obtain titanium doped iron phosphate.

[0037] Comparative example 3 The present comparative example provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid, and titanyl sulfate solution into pure water, stir for 0.5h, then filter to obtain a clear mixed liquid with Fe, P, and Ti contents of Fe: 67.56g / L, P: 37.63g / L, and Ti: 0.689g / L, respectively, as the metal liquid, and the molar ratio Fe / P=0.996 and the mass ratio Fe / Ti=98.06 are calculated respectively; Step 2, preparation of titanium phosphate and iron phosphate seed slurry: accurately weigh 20g of single crystal titanium phosphate into 60mL of pure water, stir, then add to a grinding tank for wet ball milling, control the rotation speed at 600r / min during the grinding process, cycle for 2 times, and the grinding time is 2h, take a sample to detect the particle size, stop grinding when the D50 reaches 2μm, and take out as seed for use; accurately weigh 115.5g of iron phosphate into 214.5mL of pure water, stir, then add to a grinding tank for wet ball milling, control the rotation speed at 1000r / min during the grinding process, cycle for 2 times, and the grinding time is 2h, take a sample to detect the particle size, stop grinding when the D50 reaches 1μm, and take out as seed for use; Step 3, iron phosphate synthesis: take the metal liquid 5L of step 1 into the synthesis reactor, start stirring for about 10min, start heating to 50℃, take 990g, 15% concentration of hydrogen peroxide, add to the reactor with peristaltic pump, hydrogen peroxide adding time is 2h. Add 363g, solid content is 30% iron phosphate seed slurry (before weighing the seed, the iron phosphate seed slurry has been fully stirred evenly) stirring for 0.5h, continue to heat to 93℃, keep the reaction for 7h, get the titanium doped iron phosphate slurry. Among them, the amount of iron phosphate seed added is 10% of the total amount of Fe in the metal liquid converted into iron phosphate dihydrate; 2+ all oxidized to Fe 3+Theoretical amount of 1.6 times, hydrogen peroxide addition time is 2h. Respectively add 322g, solid content is 35% iron phosphate seed slurry and 1g, solid content is 25% titanium phosphate seed slurry (before weighing the seed, the iron phosphate and titanium phosphate seed slurry have been fully stirred uniformly), constant temperature reaction 0.5h, continue to heat to 95℃, keep the reaction for 10h, and titanium doped iron phosphate slurry is prepared. Among them, the addition amount of iron phosphate seed is 7% of the total amount of Fe in the metal liquid converted into iron phosphate dihydrate, and the addition amount of titanium phosphate seed is 2% of the total amount of Ti in the metal liquid converted into titanium phosphate; Step 4, washing and drying: the titanium doped iron phosphate slurry of step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, and pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500μS / m. All washing water and filtrate are combined to obtain a mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace at 650℃ for calcination for 5h, then cooled and taken out, and crushed to obtain titanium doped iron phosphate.

[0038] Comparative Example 4 This comparative example provides a method for accurately controlling titanium doping in the synthesis of iron phosphate, comprising the following steps: Step 1, metal liquid preparation: accurately weigh ferrous sulfate heptahydrate (Fe: 20.25%, Ti: 0.005%), phosphoric acid (H3PO4: 87%) and titanyl sulfate solution into pure water, stir for 0.5h, then filter to obtain a clear mixed liquid with Fe, P and Ti contents of Fe: 63.54g / L, P: 35.51g / L and Ti: 0.7g / L as the metal liquid, respectively calculating the molar ratio Fe / P=0.992 and the mass ratio Fe / Ti=90.77; Step 2, preparation of iron phosphate seed slurry: accurately weigh 115.5g of iron phosphate into 214.5mL of pure water, stir, then add to a grinding tank for wet ball milling, control the rotation speed at 1000r / min during the grinding process, cycle the grinding for 2 times, and the grinding time is 2h. After the D50 reaches 1μm, stop the grinding, and take out as seed for use; Step 3, iron phosphate synthesis: take 5L of the metal liquid of step 1 and transfer it to a synthesis reaction kettle, start stirring for about 10min, heat the metal liquid to 50℃, weigh hydrogen peroxide, and add it to the reaction kettle by peristaltic pump. The addition amount of hydrogen peroxide is Fe 2+ All oxidized to Fe 3+Theoretical amount of 1.6 times, hydrogen peroxide addition time is 2h. Add 303g, solid content is 35% of iron phosphate seed slurry (before weighing the seed, the iron phosphate seed slurry has been fully stirred evenly), constant temperature reaction 0.5h, continue to heat to 92℃, keep the temperature for 8h, titanium doped iron phosphate slurry is prepared. Among them, the amount of iron phosphate seed slurry is 10% of the total amount of Fe in the metal liquid converted into iron phosphate dihydrate.

[0039] Step 4, washing and drying: the titanium doped iron phosphate slurry of step 3 is subjected to solid-liquid separation to obtain crude iron phosphate and filtrate, and pure water is added to wash the crude iron phosphate for multiple times until the conductivity of the washing water is ≤500μS / m, and all the washing water and filtrate are combined to obtain the mother liquor. Part of the washed iron phosphate filter cake is used to prepare the iron phosphate seed slurry in step 2, and the remaining iron phosphate filter cake is transferred to a crucible and placed in a muffle furnace at 700℃ for calcination for 6h, then cooled and taken out, crushed to obtain titanium doped iron phosphate.

[0040] Test example The metal liquid indicators prepared in examples 1-3 and comparative examples 1-4 are shown in table 1, wherein Fe / P is the molar ratio, and Fe / Ti is the mass ratio: Table 1

[0041] According to , , the precipitation rates of Fe and Ti in the preparation of anhydrous iron phosphate in examples 1-3 and comparative examples 1-4 are calculated respectively, as shown in table 2: Table 2

[0042] The chemical and physical indicators of the anhydrous iron phosphate prepared in examples 1-3 and comparative examples 1-4 are shown in table 3: Table 3

[0043] The lithium iron phosphate obtained in examples 1-3 and comparative examples 1-4 is ground uniformly with lithium carbonate and sucrose, sintered into lithium iron phosphate, and used as a positive electrode material to prepare a button cell for testing: The battery grade lithium carbonate: iron phosphate: sucrose with a mass ratio of 17.5:66:16.5 is accurately weighed and mixed, added to a proper amount of pure water for wet grinding for 3h, and then dried by spray drying to obtain dry powder. The obtained powder is weighed and placed in a sintering kiln, which is moved into a calcining furnace, and high-purity nitrogen is introduced for primary pre-sintering. The sintering temperature is controlled at 400℃, and the sintering time is 5h. After cooling, it is taken out, crushed and mixed, and then subjected to secondary sintering. The sintering temperature is controlled at 800℃, and the sintering time is 8h. After cooling, it is taken out and crushed to obtain lithium iron phosphate positive electrode material.

[0044] The positive electrode material, acetylene black and PVDF with a mass ratio of 90:5:5 were weighed, mixed and ground uniformly, and then pressed into a positive electrode film with a uniform thickness on a roll mill. The obtained positive electrode film was placed in a vacuum drying oven and dried at 120°C for 12h. The dried positive electrode film was tightly pressed with an aluminum foil to obtain a positive electrode sheet.

[0045] Battery assembly: The prepared positive electrode sheet was placed in an argon-filled glove box, a lithium sheet was used as the negative electrode of the battery, the electrolyte was 1mol / L LiPF6(EC+DEC, volume ratio 1:1), the separator was Celgard2400 type, and a CR2032 button cell was assembled. After assembly, the battery was taken out and sealed, and rested for 12 hours before testing.

[0046] The first charge specific capacity and the first discharge specific capacity at 0.1C were tested at a charge cut-off voltage of 4V and a discharge cut-off voltage of 2V; the discharge specific capacity at 1C was tested at a charge cut-off voltage of 3.75V and a discharge cut-off voltage of 2V, and the results are recorded in Table 4.

[0047] Table 4

[0048] As can be seen from Tables 2-4, even though the metal liquid sources of Example 1 and Example 2 are different, the titanium precipitation rate and doping amount in the synthesis of iron phosphate are basically unaffected; as can be seen from Comparative Example 1, Example 3, and Comparative Example 1, Comparative Example 3, the addition amount of titanium phosphate seed crystals, the particle size of titanium phosphate seed crystals, and the crystal form (single crystal, polycrystal) of titanium phosphate have basically no effect on the doping amount of titanium phosphate; as can be seen from Comparative Example 1, Comparative Example 2 and Comparative Example 4, when no titanium phosphate seed crystals are doped, the titanium doping amount is significantly reduced, and it is necessary to increase the titanium concentration in the metal liquid and reduce the Fe / Ti mass ratio to achieve the same doping amount; according to Comparative Example 2 and Comparative Example 4, without adding titanium phosphate seed crystals, the titanium precipitation rate is higher than 90%, but there is a certain deviation, which shows that the Fe / Ti in the metal liquid will affect the titanium precipitation rate. The present application preliminarily realizes the precise doping of titanium by controlling the Fe / Ti in the metal liquid, and then adds titanium phosphate seed crystals to make the titanium precipitation rate in the synthesis of iron phosphate ≥99%, thereby deeply realizing the precise doping of titanium.

[0049] Comparative Example 1 and Comparative Example 2, the amount of titanium doping is low, the electrochemical performance of iron phosphate is slightly lower; Comparative Example 1 and Comparative Example 1, the titanium phosphate seed is not ground, the electrochemical performance of iron phosphate is slightly reduced, because grinding can reduce the particle size of titanium phosphate particles, increase its specific surface area, make it more evenly dispersed in the LFP precursor, avoid phase separation caused by local enrichment; Comparative Example 1 and Example 3, single crystal titanium phosphate as seed is higher than that of polycrystalline titanium phosphate as seed doped and synthesized iron phosphate, because TiO4 2+ and PO4 3- The titanium phosphate generated in the synthesis of iron phosphate is consistent with the crystal form of the seed, the single crystal material has a long-range ordered lattice structure, which can provide more uniform ion diffusion channels and reduce the hindrance of grain boundaries to Li⁺ migration, thereby improving the rate performance.

[0050] The above describes the embodiments of the present application in detail, but the present application is not limited to the above embodiments, and various changes can be made within the knowledge range possessed by those skilled in the art without departing from the purpose of the present application. In addition, the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for controlling titanium doping during the synthesis of iron phosphate, characterized in that, Includes the following steps: S1: Mix and dissolve phosphorus source, iron source, titanium source and water to prepare a metal liquid; S2: Prepare titanium phosphate seed slurry and iron phosphate seed slurry respectively; S3: Add the iron phosphate seed slurry and the titanium phosphate seed slurry to the molten metal and heat them to react, thereby obtaining titanium-doped iron phosphate slurry; In the molten metal, the concentration of iron is 60-70 g / L, the molar ratio of iron to phosphorus is (0.98-1):1, and the mass ratio of iron to titanium is (95-100):

1.

2. The method according to claim 1, characterized in that, In step S2, the preparation of titanium phosphate seed slurry includes: mixing titanium phosphate and water to form a slurry, grinding the resulting slurry, and obtaining titanium phosphate seed slurry; And / or, the preparation of the iron phosphate seed slurry includes: mixing iron phosphate and water to form a slurry, grinding the resulting slurry, and obtaining the iron phosphate seed slurry.

3. The method according to claim 1 or 2, characterized in that, In step S2, the solid content of the titanium phosphate seed slurry is 20%~30%, and the solid content of the iron phosphate seed slurry is 25%~40%.

4. The method according to claim 2, characterized in that, The titanium phosphate is monocrystalline titanium phosphate; and / or, the iron phosphate is monoclinic iron phosphate.

5. The method according to claim 2, characterized in that, The seed particle size of the titanium phosphate seed slurry and the iron phosphate seed slurry is independently 1~2μm.

6. The method according to claim 1, characterized in that, The phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, or ammonium monohydrogen phosphate. And / or, the iron source is selected from at least one of ferrous sulfate, ferric sulfate, ferrous oxalate, iron powder, iron oxide red, ferrous nitrate, or ferric nitrate; And / or, the titanium source is selected from at least one of titanium oxysulfate or titanium dioxide.

7. The method according to claim 1, characterized in that, In step S3, the heating reaction is carried out by raising the temperature to 90~95℃ and then holding the reaction at that temperature for 5~10 hours; the temperature when the molten metal is mixed with the iron phosphate seed slurry and the titanium phosphate seed slurry is 50~55℃.

8. The method according to claim 1, characterized in that, In step S3, the amount of iron phosphate seed slurry added is determined according to 5% to 15% of the total mass of iron phosphate dihydrate when all iron elements in the molten metal are converted into iron phosphate dihydrate; and / or, the amount of titanium phosphate seed slurry added is determined according to 0.5% to 3% of the total mass of titanium phosphate when all titanium elements in the molten metal are converted into titanium phosphate.

9. A method for preparing titanium-doped anhydrous iron phosphate, comprising the steps of the method according to any one of claims 1-8, characterized in that, It also includes the following steps: The titanium-doped iron phosphate slurry was filtered, washed, dried, and then calcined to obtain titanium-doped anhydrous iron phosphate. The roasting temperature is 550~700℃, and the roasting time is 4~7h.

10. The application of the method according to any one of claims 1-9 in the preparation of lithium battery cathode materials.

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