A manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, a preparation method and application thereof

Abstract

The application provides a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material and a preparation method and application thereof, and belongs to the technical field of positive electrode materials.The application uses titanium white by-product ferrous sulfate as raw material, reduces trivalent iron by using a reducing agent, removes part of titanium and other impurities by flocculation after adding alkali to obtain refined ferrous sulfate with a certain content of manganese and titanium, then oxidizes to obtain iron phosphate by adding phosphoric acid and alkali, and finally obtains the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material by calcining after mixing with a lithium source and a carbon source.The application does not need to completely remove manganese and titanium in the titanium white by-product ferrous sulfate, but in-situ doping is carried out by taking the manganese and titanium as doping elements, the preparation process is simpler, uniform doping in the bulk phase is realized, the manganese and titanium double-doped elements have a synergistic effect, and the performance of the composite material is improved.In addition, the carbon material is formed after calcining by adding the carbon source, the conductivity of the composite material is improved, and the electrochemical performance of the composite material is further improved.

Description

Technical Field

[0001] This invention relates to the field of cathode material technology, and in particular to a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, its preparation method and application. Background Technology

[0002] Lithium iron phosphate (LFP) cathode materials are widely recognized as the most suitable cathode material for energy storage lithium-ion batteries due to their lack of scarce resources such as cobalt and nickel, as well as their advantages such as long cycle life and good safety. To prepare high-performance LFP cathode materials, in addition to strictly controlling the production process and equipment, strict requirements are also placed on the purity of raw materials, such as battery-grade lithium salts, phosphoric acid, and high-purity iron sources.

[0003] In my country's sulfuric acid process for titanium dioxide production, millions of tons of ferrous sulfate are produced annually as a byproduct. This ferrous sulfate contains numerous impurities, which are generally believed to affect the structure and properties of lithium iron phosphate (LiFePO4), thereby impacting battery performance. Synthesizing battery-grade anhydrous LiFePO4 requires complex, cumbersome, and costly processes to remove impurities from the raw materials, along with high-temperature calcination to remove the water of crystallization from the ferrous sulfate. On the other hand, to improve the performance of lithium iron phosphate (LiFePO4), bulk doping with elements into the LiFePO4 lattice during subsequent synthesis has become an important method for improving ion transport characteristics.

[0004] Although the production and doping processes of lithium iron phosphate are relatively mature, and many companies also use ferrous sulfate, a byproduct of titanium dioxide production, to prepare lithium iron phosphate electrode materials, the conventional method involves removing impurities from the ferrous sulfate byproduct, followed by high-temperature calcination for dehydration, and then adding doping elements. This method is relatively complex, and the material uniformity needs improvement. Therefore, simplifying the process and improving the performance of lithium iron phosphate remains a challenge for current technologies. Summary of the Invention

[0005] The purpose of this invention is to provide a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, its preparation method, and its applications. The preparation method provided by this invention is simple, and the prepared manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material exhibits excellent electrochemical performance.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, comprising the following steps:

[0008] (1) Ferrous sulfate, a by-product of titanium dioxide, is mixed with water and a reducing agent to carry out a reduction reaction, and a reduced solution is obtained.

[0009] (2) The reducing solution obtained in step (1) is mixed with alkali and subjected to hydrolysis to obtain hydrolysis products;

[0010] (3) The hydrolysis product obtained in step (2) is mixed with a flocculant and subjected to a flocculation reaction to obtain a refined ferrous sulfate solution; the mass content of manganese in the refined ferrous sulfate solution is 0.05-0.5%, and the mass content of titanium in the refined ferrous sulfate solution is 0.04-0.38%.

[0011] (4) The refined ferrous sulfate solution obtained in step (3) is mixed with phosphoric acid and alkali to carry out a precipitation reaction to obtain an intermediate product;

[0012] (5) The intermediate product obtained in step (4) is mixed with an oxidant and an acid to carry out an oxidation reaction to obtain a precursor;

[0013] (6) The precursor obtained in step (5) is mixed with a lithium source and a carbon source and then calcined to obtain a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material.

[0014] Preferably, in step (1), the mass ratio of the reducing agent to ferrous sulfate by-product of titanium dioxide is (0.5-2):100.

[0015] Preferably, the hydrolysis reaction temperature in step (2) is 40-70°C and the hydrolysis reaction time is 1-4 hours.

[0016] Preferably, in step (4), the molar ratio of phosphoric acid to ferrous ions in the purified ferrous sulfate solution is (1-1.2):1.

[0017] Preferably, the precipitation reaction time in step (4) is 0.5 to 2 hours.

[0018] Preferably, the molar ratio of the oxidant in step (5) to the iron in the intermediate product is (1.2 to 2.5): 1.

[0019] Preferably, in step (6), the mass ratio of carbon source to precursor is (7-9.5):100.

[0020] Preferably, in step (6), the molar ratio of lithium in the lithium source to iron in the precursor is (1.02-1.1):1.

[0021] This invention provides a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material prepared by the preparation method described in the above technical solution.

[0022] This invention also provides the application of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material described above as a cathode material for lithium-ion batteries.

[0023] This invention provides a method for preparing a manganese and titanium in-situ doped lithium iron phosphate composite material, comprising the following steps: (1) mixing ferrous sulfate, a byproduct of titanium dioxide production, with water and a reducing agent to carry out a reduction reaction, thereby obtaining a reduced solution; (2) mixing the reduced solution obtained in step (1) with an alkali to carry out a hydrolysis reaction, thereby obtaining a hydrolysis product; (3) mixing the hydrolysis product obtained in step (2) with a flocculant to carry out a flocculation reaction, thereby obtaining a refined ferrous sulfate solution; wherein the mass content of manganese in the refined ferrous sulfate solution is 0.05-0.5%, and the mass content of titanium in the refined ferrous sulfate solution is 0.04-0.38%; (4) mixing the refined ferrous sulfate solution obtained in step (3) with phosphoric acid and an alkali to carry out a precipitation reaction, thereby obtaining an intermediate product; (5) mixing the intermediate product obtained in step (4) with an oxidant and an acid to carry out an oxidation reaction, thereby obtaining a precursor; (6) calcining the precursor obtained in step (5) with a lithium source and a carbon source to obtain manganese and titanium in-situ doped lithium iron phosphate. This invention uses ferrous sulfate, a byproduct of titanium dioxide production, as raw material. A reducing agent is added to reduce ferric iron, an alkali is added to hydrolyze titanium, and a flocculant is added to remove a certain amount of Ti and other impurities, resulting in refined ferrous sulfate with a specific manganese and titanium content. Phosphoric acid and an alkali are then added to obtain manganese and titanium-doped ferrous phosphate in situ. An oxidizing agent is then added to oxidize the ferrous phosphate to iron phosphate. Finally, the mixture is mixed with a lithium source and a carbon source and calcined to obtain a carbon-containing lithium iron phosphate composite material with manganese and titanium in situ doped. This method eliminates the need to completely remove impurities from the ferrous sulfate byproduct of titanium dioxide production; instead, manganese and titanium are used as beneficial doping elements. After obtaining iron phosphate, no additional doping elements are needed, simplifying the preparation process. In-situ doping of manganese and titanium achieves uniform bulk doping, and the synergistic effect of manganese and titanium improves the performance of the composite material. The addition of a carbon source forms carbon materials after calcination, improving the conductivity of the composite material and further enhancing its electrochemical performance. The results of the embodiments show that the discharge capacity of the coin cell assembled from the composite material prepared in this invention is 145.2 mAh g⁻¹. -1 above. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of the preparation method of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material of the present invention;

[0025] Figure 2 The image shows the XRD pattern of Mn and Ti doped FePO4·2H2O prepared in Example 1 of this invention.

[0026] Figure 3 This is a SEM image of Mn and Ti doped FePO4·2H2O prepared in Example 1 of this invention;

[0027] Figure 4The image shows the XRD pattern of the Mn and Ti doped LiFePO4 / C composite material prepared in Example 2 of this invention.

[0028] Figure 5 This is a SEM image of the Mn and Ti doped LiFePO4 / C composite material prepared in Example 2 of this invention;

[0029] Figure 6 This is a SEM image of the Mn and Ti doped LiFePO4 / C composite material prepared in Example 5 of this invention;

[0030] Figure 7 This is a charge-discharge curve of a lithium-ion battery assembled from the Mn and Ti doped LiFePO4 / C composite material prepared in Example 2 of the present invention.

[0031] Figure 8 The image shows the charge-discharge curves of the lithium-ion battery assembled from the Mn and Ti doped LiFePO4 / C composite material prepared in Example 5 of this invention. Detailed Implementation

[0032] This invention provides a method for preparing a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, comprising the following steps:

[0033] (1) Ferrous sulfate, a by-product of titanium dioxide, is mixed with water and a reducing agent to carry out a reduction reaction, and a reduced solution is obtained.

[0034] (2) The reducing solution obtained in step (1) is mixed with alkali and subjected to hydrolysis to obtain hydrolysis products;

[0035] (3) The hydrolysis product obtained in step (2) is mixed with a flocculant and subjected to a flocculation reaction to obtain a refined ferrous sulfate solution; the mass content of manganese in the refined ferrous sulfate solution is 0.05-0.5%, and the mass content of titanium in the refined ferrous sulfate solution is 0.04-0.38%.

[0036] (4) The refined ferrous sulfate solution obtained in step (3) is mixed with phosphoric acid and alkali to carry out a precipitation reaction to obtain an intermediate product;

[0037] (5) The intermediate product obtained in step (4) is mixed with an oxidant and an acid to carry out an oxidation reaction to obtain a precursor;

[0038] (6) The precursor obtained in step (5) is mixed with a lithium source and a carbon source and then calcined to obtain a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material.

[0039] Unless otherwise specified, the present invention does not impose any special restrictions on the source of the raw materials, and commercially available products well known to those skilled in the art can be used.

[0040] This invention involves mixing ferrous sulfate, a byproduct of titanium dioxide production, with water and a reducing agent to undergo a reduction reaction, yielding a reduced solution. In this invention, the reduction reaction begins after the mixing of the ferrous sulfate, water, and reducing agent is completed. During the reduction reaction, the ferric ions in the ferrous sulfate react with the reducing agent to generate ferrous ions.

[0041] The present invention does not have any special limitation on the source of the by-product ferrous sulfate of titanium dioxide, and the by-product ferrous sulfate obtained in the sulfuric acid process of titanium dioxide production, which is well known to those skilled in the art, can be used.

[0042] In various embodiments of the present invention, the mass content of each element in the titanium dioxide by-product ferrous sulfate is: Fe 88.5%, Mg 1.8%, Mn 4.2%, Al 0.8%, Ca 0.6%, Zn 0.9%, Ti 1.8%, Na 0.5%, Cr 0.3%, Cu 0.3%, and Ni 0.3%.

[0043] In this invention, the preferred method for mixing the titanium dioxide by-product ferrous sulfate with water and reducing agent is to first mix the titanium dioxide by-product ferrous sulfate with water to obtain a mixed solution, and then add the reducing agent.

[0044] In this invention, the concentration of the mixed solution of ferrous sulfate (a byproduct of titanium dioxide production) and water is preferably 1–5 mol / L, more preferably 2–4 mol / L. By limiting the concentration of the mixed solution of ferrous sulfate (a byproduct of titanium dioxide production) and water to the above range, this invention allows for more complete dissolution of the ferrous sulfate.

[0045] In this invention, the reducing agent is preferably iron powder. The mass ratio of the reducing agent to ferrous sulfate (a byproduct of titanium dioxide production) is preferably (0.5–2):100, more preferably (1–1.5):100. In this invention, the reducing agent is used to reduce ferric iron in the ferrous sulfate produced as a byproduct of titanium dioxide production. By limiting the mass ratio of the reducing agent to ferrous sulfate within the above range, this invention enables a more complete reduction of ferric iron.

[0046] In this invention, the temperature of the reduction reaction is preferably 20–40°C, more preferably 20–30°C; the time of the reduction reaction is preferably 0.1–1 h, more preferably 0.5 h. By limiting the temperature and time of the reduction reaction within the above ranges, this invention enables a more complete reduction of ferric iron.

[0047] After obtaining the reducing solution, the present invention mixes the reducing solution with an alkali and carries out a hydrolysis reaction to obtain the hydrolysis product.

[0048] In this invention, the alkali is preferably ammonia. In this invention, the pH value of the mixed solution after mixing the reducing solution and the alkali is preferably 2-4, more preferably 3. This invention does not impose any special limitations on the concentration and amount of ammonia, as long as the pH value of the mixed solution after mixing the reducing solution and the alkali is within the above-mentioned range.

[0049] In this invention, the preferred temperature for the hydrolysis reaction is 40–70°C, more preferably 50–60°C; the preferred time for the hydrolysis reaction is 1–4 hours, more preferably 2–3 hours. In this invention, during the hydrolysis reaction, some titanium in the byproduct ferrous sulfate undergoes hydrolysis to produce a precipitate. By limiting the pH value, hydrolysis temperature, and time of the mixed solution within the above-mentioned ranges, this invention can adjust the degree of titanium hydrolysis and ensure the titanium content in the subsequent refined ferrous sulfate.

[0050] After obtaining the hydrolysis product, the present invention mixes the hydrolysis product with a flocculant to carry out a flocculation reaction to obtain a refined ferrous sulfate solution.

[0051] In this invention, the flocculant preferably includes polyacrylamide, sodium polyvinyl acetate, or polyethyleneimine.

[0052] In this invention, the preferred mass ratio of the flocculant to ferrous sulfate, a byproduct of titanium dioxide production, is (0.02–0.15):100, more preferably (0.05–0.1):100. In this invention, the flocculant is used to remove precipitates generated after Ti hydrolysis and impurities such as Na, Zn, Al, Cu, Ca, Cr, and Mg.

[0053] In this invention, the flocculation reaction time is preferably 3 to 10 minutes. By limiting the amount of flocculant and the flocculation reaction time within the above range, this invention enables the more complete removal of impurities from the titanium dioxide byproduct ferrous sulfate and ensures that the manganese and titanium content in the refined ferrous sulfate solution is within a certain range.

[0054] After the flocculation reaction is completed, the present invention preferably filters the product of the flocculation reaction to obtain a refined ferrous sulfate solution.

[0055] The present invention does not impose any special limitations on the operation of the filtration, and any filtration technology solution known to those skilled in the art can be used.

[0056] In this invention, the mass content of manganese in the refined ferrous sulfate solution is 0.05-0.5%, preferably 0.1-0.4%; the mass content of titanium in the refined ferrous sulfate solution is 0.04-0.38%, preferably 0.1-0.3%. By limiting the mass content of manganese and titanium in the refined ferrous sulfate solution to the above ranges, this invention can adjust the doping content of manganese and titanium in the prepared composite material. After manganese doping, a solid solution is formed, increasing the cell volume and ion diffusion channels, improving the lithium-ion diffusion rate, and enhancing rate performance. However, excessive manganese content leads to oxidation and formation of Mn. 3+ When dissolved in electrolyte solutions, it leads to a decrease in lithium intercalation capacity. Titanium doping is beneficial for improving conductivity and makes lithium iron phosphate tend to be nano-sized, thereby shortening the ion diffusion path and improving the rate performance of the material. However, excessive Ti will cause cell deformation, block the lithium ion transport channels, and reduce the diffusion coefficient.

[0057] When the manganese and titanium content in the refined ferrous sulfate solution is below the above-mentioned range, the present invention preferably adds manganese and titanium sources. In the present invention, the manganese source is preferably manganese oxalate; the titanium source is preferably titanium dioxide. The present invention does not have a special limitation on the amount of manganese and titanium sources added, as long as the manganese and titanium content in the refined ferrous sulfate solution is within the above-mentioned range.

[0058] After obtaining a refined ferrous sulfate solution, the present invention mixes the refined ferrous sulfate solution with phosphoric acid and alkali to carry out a precipitation reaction to obtain an intermediate product.

[0059] In this invention, the phosphoric acid is preferably phosphoric acid with a mass concentration of 85%. In this invention, the molar ratio of phosphoric acid to ferrous ions in the purified ferrous sulfate solution is preferably (1-1.2):1, more preferably (1.1-1.2):1. In this invention, the phosphoric acid and ferrous sulfate react to form ferrous phosphate. By limiting the molar ratio of phosphoric acid to ferrous ions in the purified ferrous sulfate solution to the above range, this invention enables a more complete reaction of the ferrous ions in the ferrous phosphate.

[0060] In this invention, the alkali preferably includes ammonia, sodium hydroxide, or potassium hydroxide.

[0061] In this invention, the pH value of the system after adding alkali is preferably 5.8–6.8, more preferably 6.0–6.6. This invention does not have a specific limitation on the amount of alkali added, as long as the pH value of the system after adding alkali is within the above range. Limiting the pH value of the system after adding alkali to the above range in this invention is beneficial for the complete precipitation of ferrous ions to form ferrous phosphate, while avoiding its hydrolysis to form Fe(OH)₂ precipitate, or even Fe(OH)₃.

[0062] In this invention, the precipitation reaction time is preferably 0.5 to 2 hours, more preferably 1 to 1.5 hours. By limiting the precipitation reaction time to the above range, this invention ensures complete precipitation of ferrous ions.

[0063] After the precipitation reaction is completed, the present invention preferably filters, washes and dries the product of the precipitation reaction in sequence to obtain an intermediate product.

[0064] The present invention does not impose any special limitations on the filtration, washing and drying operations, and any filtration, washing and drying technical solutions known to those skilled in the art can be used.

[0065] In this invention, the intermediate product is Fe3(PO4)2·8H2O, which is in situ doped with Mn and Ti.

[0066] After obtaining the intermediate product, the present invention mixes the intermediate product with an oxidant and an acid to carry out an oxidation reaction to obtain the precursor.

[0067] In this invention, the oxidant preferably includes hydrogen peroxide or sodium peroxide. In this invention, the molar ratio of the oxidant to iron in the intermediate product is preferably (1.2–2.5):1, more preferably (1.5–2):1. In this invention, the oxidant is used to oxidize the intermediate product ferrous phosphate to produce ferric phosphate. By limiting the amount of oxidant within the above range, this invention enables more complete oxidation of the intermediate product.

[0068] In this invention, the acid is preferably phosphoric acid.

[0069] In this invention, the pH value of the system after adding acid is preferably 1.2 to 1.8, more preferably 1.3 to 1.6. This invention does not have a specific limitation on the amount of acid added, as long as the pH value of the system after adding acid is within the above range. Limiting the pH value of the system after adding acid to the above range allows the oxidation reaction to proceed more fully.

[0070] In this invention, the temperature of the oxidation reaction is preferably 60–80°C, more preferably 70°C; the time of the oxidation reaction is preferably 0.25–2 h, more preferably 0.5–1.5 h. By limiting the temperature and time of the oxidation reaction within the above ranges, this invention ensures that the oxidation reaction proceeds fully.

[0071] After the oxidation reaction is completed, the product of the oxidation reaction is preferably filtered, washed and dried in sequence to obtain the precursor.

[0072] The present invention does not impose any special limitations on the filtration, washing and drying operations, and any filtration, washing and drying technical solutions known to those skilled in the art can be used.

[0073] In this invention, the precursor is nanoscale FePO4·2H2O doped with manganese and titanium in situ.

[0074] After obtaining the precursor, the present invention mixes the precursor with a lithium source and a carbon source and then calcines it to obtain a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material.

[0075] In this invention, the lithium source preferably includes lithium carbonate, lithium bicarbonate, or lithium oxalate.

[0076] In this invention, the preferred molar ratio of lithium in the lithium source to iron in the precursor is (1.02 to 1.1):1. By limiting the molar ratio of lithium in the lithium source to iron in the precursor within the above range, this invention ensures a more complete reaction of the precursor.

[0077] In this invention, the carbon source preferably includes polyethylene glycol 20000, glucose, or β-cyclodextrin.

[0078] In this invention, the preferred mass ratio of the carbon source to the precursor is (7-9.5):100, more preferably (7.5-9):100. By limiting the mass ratio of the carbon source to the precursor within the above range, this invention can regulate the carbon content in the composite material, improve its conductivity, and thus enhance its electrochemical performance.

[0079] In this invention, the calcination temperature is preferably 550–750°C, more preferably 600–700°C; the calcination time is preferably 8–12 h, more preferably 9–11 h. By limiting the calcination temperature and time to the above ranges, this invention enables the lithium source and precursor to react fully to form lithium iron phosphate, and the carbon source to react fully to form carbon materials.

[0080] After calcination, the present invention preferably cools the calcined product to obtain a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material.

[0081] The present invention does not impose any special limitations on the cooling operation; any cooling technology solution known to those skilled in the art can be used.

[0082] This invention incorporates a reducing agent to reduce ferric iron, an alkali to hydrolyze titanium, and a flocculant to remove a certain amount of Ti and other impurities, yielding refined ferrous sulfate with specific manganese and titanium content. Phosphoric acid and an alkali are then added, followed by an oxidant to obtain ferric phosphate. Finally, the mixture is mixed with a lithium source and a carbon source and calcined to obtain a carbon-doped lithium iron phosphate composite material with in-situ manganese and titanium doping. This process eliminates the need to completely remove impurities from the titanium dioxide byproduct ferrous sulfate, and no additional doping elements are required after obtaining ferric phosphate, simplifying the preparation process. In-situ doping of manganese and titanium achieves uniform bulk doping, and the synergistic effect of manganese and titanium enhances the composite material's performance. The addition of a carbon source forms carbon materials after calcination, improving the composite material's conductivity. Controlling the process parameters further enhances the electrochemical performance of the composite material.

[0083] The preferred flowchart of the preparation method of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material provided by the present invention is as follows: Figure 1 As shown, ferrous sulfate, a byproduct of titanium dioxide production, is subjected to Fe... 3+ Ion reduction, followed by selective impurity removal under controlled conditions, then in-situ precipitation of Mn / Ti-doped ferrous phosphate octahydrate, then oxidation precipitation of nano-sized iron phosphate dihydrate, and finally addition of lithium salt and carbon source to obtain in-situ doped lithium iron phosphate cathode material.

[0084] This invention provides a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material prepared by the preparation method described in the above technical solution.

[0085] The manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material prepared by this invention has excellent electrochemical performance.

[0086] This invention also provides the application of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material described above as a cathode material for lithium-ion batteries.

[0087] The present invention does not impose any special limitations on the application of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material as a cathode material for lithium-ion batteries. Any technical solution known to those skilled in the art for the application of manganese and titanium doped carbon-containing lithium iron phosphate composite material as a cathode material for lithium-ion batteries can be used.

[0088] The lithium-ion battery assembled using the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material described in this invention as the cathode material has a higher discharge capacity.

[0089] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0090] Example 1

[0091] (1) Ferrous sulfate by-product of titanium dioxide was mixed with water to obtain a ferrous sulfate solution with a concentration of 3 mol / L. Iron powder was added as a reducing agent (the mass ratio of iron powder to ferrous sulfate by-product was 0.5:100), and the reaction was carried out for 0.5 h to achieve complete reduction of ferric iron. Ammonia water was added until the pH reached 3.0, and the temperature was controlled at 65℃ for 2 h to allow some titanium ions to hydrolyze and form a precipitate. Polyacrylamide (the mass ratio of polyacrylamide to ferrous sulfate by-product was 0.05:100) was added to assist in adsorption and co-precipitation to remove impurity elements such as Na, Zn, Al, Cu, Ca, Cr, and Mg. After filtering the impurities, a refined ferrous sulfate solution with a certain amount of Mn and Ti ions was obtained, wherein the mass content of Mn was 0.15% and the mass content of Ti was 0.24%.

[0092] (2) Add phosphoric acid and ammonia to the refined ferrous sulfate solution obtained in step (1). The molar ratio of phosphoric acid to Fe in ferrous sulfate is 1:1. After adding ammonia, the pH value of the system is 6.5. The reaction time is 1h. After washing and drying the precipitate, Fe3(PO4)2·8H2O doped with Mn and Ti in situ is obtained.

[0093] (3) Hydrogen peroxide was added to Fe3(PO4)2·8H2O obtained in step (2), wherein the molar ratio of hydrogen peroxide to Fe in Fe3(PO4)2·8H2O was 1.2:1; the pH was controlled at 1.5 by adding phosphoric acid, the temperature was 80℃, and after reacting for 1 h, the white precipitate was washed and dried to obtain nano-sized FePO4·2H2O;

[0094] (4) Using the in-situ doped FePO4·2H2O obtained in step (3) as raw material, it is mixed with lithium carbonate and glucose in a certain proportion, wherein the molar ratio of Li:Fe is 1.07:1 and the mass ratio of glucose to FePO4·2H2O is 8.88:100. After calcination at 700℃ for 10h and cooling, Mn and Ti in-situ doped LiFePO4 / C composite material is obtained.

[0095] The XRD pattern of FePO4·2H2O prepared in Example 1 is shown below. Figure 2 As shown, from Figure 2 It can be seen that FePO4·2H2O is a typical monoclinic phase. The SEM image of FePO4·2H2O prepared in Example 1 is shown below. Figure 3 As shown, from Figure 3 It can be seen that the particle size of FePO4·2H2O is approximately 50 nm.

[0096] The Mn and Ti in-situ doped LiFePO4 / C composite material prepared in Example 1 has a carbon content of 1.36% and an electrical conductivity of 2.63E. -3 Scm -1 Its specific surface area is 7.31 m². 2 g -1 .

[0097] Example 2

[0098] (1) Ferrous sulfate by-product of titanium dioxide was mixed with water to obtain a ferrous sulfate solution with a concentration of 3 mol / L. Iron powder was added as a reducing agent (the mass ratio of iron powder to ferrous sulfate by-product was 1:100), and the reaction was carried out for 0.5 h to achieve complete reduction of ferric iron. Ammonia water was added until the pH was 3.5, the temperature was controlled at 50℃, and the holding time was 1 h to allow some titanium ions to hydrolyze and form a precipitate. Polyacrylamide (the mass ratio of polyacrylamide to ferrous sulfate by-product was 0.03:100) was added to assist in adsorption and co-precipitation to remove impurity elements such as Na, Zn, Al, Cu, Ca, Cr, and Mg. After filtering the impurities, a refined ferrous sulfate solution with a certain amount of Mn and Ti ions was obtained, wherein the mass content of Mn was 0.28% and the mass content of Ti was 0.35%.

[0099] (2) Add phosphoric acid and ammonia to the refined ferrous sulfate solution obtained in step (1). The molar ratio of phosphoric acid to Fe in ferrous sulfate is 1:1. After adding ammonia, the pH value of the system is 6.5. The reaction time is 1h. After washing and drying the precipitate, Fe3(PO4)2·8H2O doped with Mn and Ti in situ is obtained.

[0100] (3) Hydrogen peroxide was added to Fe3(PO4)2·8H2O obtained in step (2), wherein the molar ratio of hydrogen peroxide to Fe in Fe3(PO4)2·8H2O was 1.2:1; the pH was controlled at 1.5 by adding phosphoric acid, the temperature was 80℃, and after reacting for 1 h, the white precipitate was washed and dried to obtain nano-sized FePO4·2H2O;

[0101] (4) Using the in-situ doped FePO4·2H2O obtained in step (3) as raw material, it is mixed with lithium carbonate and glucose in a certain proportion, wherein the molar ratio of Li:Fe is 1.07:1 and the mass ratio of glucose to FePO4·2H2O is 8.88:100. After calcination at 700℃ for 10h and cooling, Mn and Ti in-situ doped LiFePO4 / C composite material is obtained.

[0102] The XRD pattern of the Mn and Ti in-situ doped LiFePO4 / C composite material prepared in Example 2 is shown below. Figure 4 As shown, from Figure 4It can be seen that Mn and Ti doping did not change the crystal structure of LiFePO4, which is consistent with the orthorhombic Pnma space group LiFePO4 (JCPDF83-2092). The SEM image of the Mn and Ti in-situ doped LiFePO4 / C composite material prepared in Example 2 is shown below. Figure 5 As shown, from Figure 5 It can be seen that the Mn and Ti in-situ doped LiFePO4 / C composite material is composed of micron-sized spherical particles, which are aggregated from small particles of about 200 nm.

[0103] The Mn and Ti in-situ doped LiFePO4 / C composite material prepared in Example 2 has a carbon content of 1.35% and an electrical conductivity of 8.77E. -3 Scm -1 Its specific surface area is 8.47 m². 2 g -1 .

[0104] Example 3

[0105] (1) Ferrous sulfate by-product of titanium dioxide was mixed with water to obtain a ferrous sulfate solution with a concentration of 3 mol / L. Iron powder was added as a reducing agent (the mass ratio of iron powder to ferrous sulfate by-product was 1:100), and the reaction was carried out for 0.5 h to achieve complete reduction of ferric iron. Ammonia water was added until the pH reached 3.0, and the temperature was controlled at 55℃ for 4 h to allow some titanium ions to hydrolyze and form a precipitate. Polyacrylamide (the mass ratio of polyacrylamide to ferrous sulfate by-product was 0.1:100) was added to assist in adsorption and co-precipitation to remove impurities such as Na, Zn, Al, Cu, Ca, Cr, and Mg. After filtering the impurities, a refined ferrous sulfate solution with a certain amount of Mn and Ti ions was obtained, wherein the mass content of Mn was 0.08% and the mass content of Ti was 0.27%.

[0106] (2) Phosphoric acid and ammonia were added to the refined ferrous sulfate solution obtained in step (1). The molar ratio of phosphoric acid to Fe in ferrous sulfate was 1.1:1. After adding ammonia, the pH of the system was 6.2. The reaction time was 1 h. The precipitate was washed and dried to obtain Fe3(PO4)2·8H2O doped with Mn and Ti in situ.

[0107] (3) Hydrogen peroxide was added to Fe3(PO4)2·8H2O obtained in step (2), wherein the molar ratio of hydrogen peroxide to Fe in Fe3(PO4)2·8H2O was 1.2:1; the pH was controlled at 1.5 by adding phosphoric acid, the temperature was 80℃, and after reacting for 1 h, the white precipitate was washed and dried to obtain nano-sized FePO4·2H2O;

[0108] (4) Using the in-situ doped FePO4·2H2O obtained in step (3) as raw material, it is mixed with lithium carbonate and glucose in a certain proportion, wherein the molar ratio of Li:Fe is 1.07:1 and the mass ratio of glucose to FePO4·2H2O is 8.88:100. After calcination at 700℃ for 10h and cooling, Mn and Ti in-situ doped LiFePO4 / C composite material is obtained.

[0109] The Mn and Ti in-situ doped LiFePO4 / C composite cathode material prepared in Example 3 has a carbon content of 1.4% and an electrical conductivity of 3.5E. -3 Scm -1 Its specific surface area is 7.44 m². 2 g -1 .

[0110] Example 4

[0111] (1) Ferrous sulfate by-product of titanium dioxide was mixed with water to obtain a ferrous sulfate solution with a concentration of 3 mol / L. Iron powder was added as a reducing agent (the mass ratio of iron powder to by-product ferrous sulfate was 1.5:100), and the reaction was carried out for 0.5 h to achieve complete reduction of ferric iron. Ammonia water was added until the pH was 3.0, the temperature was controlled at 55℃, and the holding time was 2 h to allow some titanium ions to hydrolyze and form a precipitate. Polyacrylamide (the mass ratio of polyacrylamide to by-product ferrous sulfate was 0.1:100) was added to assist in adsorption and co-precipitation to remove impurity elements such as Na, Zn, Al, Cu, Ca, Cr, and Mg. After filtering the impurities, a refined ferrous sulfate solution with a certain amount of Mn and Ti ions was obtained, wherein the mass content of Mn was 0.07% and the mass content of Ti was 0.29%.

[0112] (2) Add phosphoric acid and ammonia to the refined ferrous sulfate solution obtained in step (1). The molar ratio of phosphoric acid to Fe in ferrous sulfate is 1:1. After adding ammonia, the pH value of the system is 6.2. The reaction time is 1h. After washing and drying the precipitate, Fe3(PO4)2·8H2O doped with Mn and Ti in situ is obtained.

[0113] (3) Hydrogen peroxide was added to Fe3(PO4)2·8H2O obtained in step (2), wherein the molar ratio of hydrogen peroxide to Fe in Fe3(PO4)2·8H2O was 1.6:1; the pH was controlled at 1.5 by adding phosphoric acid, the temperature was 80℃, and after reacting for 1.5h, the white precipitate was washed and dried to obtain nano-sized FePO4·2H2O;

[0114] (4) Using the in-situ doped FePO4·2H2O obtained in step (3) as raw material, it is mixed with lithium carbonate and glucose in a certain proportion, wherein the molar ratio of Li:Fe is 1.07:1 and the mass ratio of glucose to FePO4·2H2O is 8.88:100. After calcination at 600℃ for 10h and cooling, Mn and Ti in-situ doped LiFePO4 / C composite material is obtained.

[0115] The Mn and Ti in-situ doped LiFePO4 / C composite cathode material prepared in Example 4 has a carbon content of 1.31% and an electrical conductivity of 6.56E. -4 Scm -1 Its specific surface area is 6.86 m². 2 g -1 .

[0116] Example 5

[0117] (1) Ferrous sulfate by-product of titanium dioxide was mixed with water to obtain a ferrous sulfate solution with a concentration of 3 mol / L. Iron powder was added as a reducing agent (the mass ratio of iron powder to ferrous sulfate by-product was 1:100), and the reaction was carried out for 0.5 h to achieve complete reduction of ferric iron. Ammonia water was added until the pH was 3.0, the temperature was controlled at 50℃, and the holding time was 1 h to allow some titanium ions to hydrolyze and form a precipitate. Polyacrylamide (the mass ratio of polyacrylamide to ferrous sulfate by-product was 0.03:100) was added to assist in adsorption and co-precipitation to remove impurity elements such as Na, Zn, Al, Cu, Ca, Cr, and Mg. After filtering the impurities, a refined ferrous sulfate solution with a certain amount of Mn and Ti ions was obtained, wherein the mass content of Mn was 0.3% and the mass content of Ti was 0.38%.

[0118] (2) Add phosphoric acid and ammonia to the refined ferrous sulfate solution obtained in step (1). The molar ratio of phosphoric acid to Fe in ferrous sulfate is 1:1. After adding ammonia, the pH value of the system is 6.5. The reaction time is 1h. After washing and drying the precipitate, Fe3(PO4)2·8H2O doped with Mn and Ti in situ is obtained.

[0119] (3) Hydrogen peroxide was added to Fe3(PO4)2·8H2O obtained in step (2), wherein the molar ratio of hydrogen peroxide to Fe in Fe3(PO4)2·8H2O was 1.2:1; the pH was controlled at 1.5 by adding phosphoric acid, the temperature was 80℃, and after reacting for 1 h, the white precipitate was washed and dried to obtain nano-sized FePO4·2H2O;

[0120] (4) Using the in-situ doped FePO4·2H2O obtained in step (3) as raw material, it is mixed with lithium carbonate and polyethylene glycol PEG20000 in a certain proportion, wherein the molar ratio of Li:Fe is 1.07:1, and the mass ratio of polyethylene glycol PEG20000 to FePO4·2H2O is 8.07:100. After calcination at 700℃ for 10h, it is cooled to obtain Mn and Ti in-situ doped LiFePO4 / C composite material.

[0121] The SEM image of the Mn and Ti in-situ doped LiFePO4 / C composite material prepared in Example 5 is shown below. Figure 6 As shown, the product has a carbon content of 1.11% and an electrical conductivity of 1.56E. -4 Scm -1 The specific surface area is 14.33 m². 2 g -1 .

[0122] Application examples

[0123] Using the Mn and Ti in-situ co-doped LiFePO4 / C composite materials prepared in Examples 2 and 5 as positive electrodes and lithium metal as the negative electrode, respectively, and 1M LiPF6 dissolved in EC (ethylene carbonate) / DMC (dimethyl carbonate) / EMC (ethyl methyl carbonate) (volume ratio 1 / 1 / 1) solvent as the electrolyte, the cells were encapsulated in coin cells in a glove box and subjected to constant current and constant voltage charge-discharge tests at a rate of 0.1C, with a voltage range of 2.0–3.75V. The charge-discharge curves of the Mn and Ti in-situ co-doped LiFePO4 / C composite materials prepared in Examples 2 and 5 are shown below. Figure 7 and Figure 8 As shown in the figure. Among them, the discharge capacity of the Mn and Ti in-situ co-doped LiFePO4 / C prepared in Example 2 is 149.6 mAh g. -1 The 3.3V platform has a capacity of 132.7mAh. -1 The platform ratio was 88.7%. The discharge capacity of the Mn and Ti in-situ co-doped LiFePO4 / C prepared in Example 5 was 145.2 mAh g. -1 The 3.3V platform has a capacity of 124.1mAh. -1 The platform's share is 85.5%.

[0124] In summary, the coin cells assembled from the composite materials prepared in this invention exhibit excellent electrochemical performance.

[0125] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material, comprising the following steps: (1) Ferrous sulfate, a byproduct of titanium dioxide production, is mixed with water and a reducing agent to carry out a reduction reaction, thereby obtaining a reduced solution; The mass ratio of the reducing agent to ferrous sulfate, a byproduct of titanium dioxide production, is (0.5~2):100; (2) The reducing solution obtained in step (1) is mixed with alkali and subjected to hydrolysis to obtain hydrolysis products; the temperature of the hydrolysis reaction is 40~70℃ and the time of the hydrolysis reaction is 1~4h. (3) The hydrolysis product obtained in step (2) is mixed with a flocculant to carry out a flocculation reaction, thereby obtaining a refined ferrous sulfate solution; the mass content of manganese in the refined ferrous sulfate solution is 0.05~0.5%, and the mass content of titanium in the refined ferrous sulfate solution is 0.04~0.38%; (4) The refined ferrous sulfate solution obtained in step (3) is mixed with phosphoric acid and alkali to carry out a precipitation reaction to obtain an intermediate product; (5) The intermediate product obtained in step (4) is mixed with an oxidant and an acid to carry out an oxidation reaction to obtain a precursor; (6) The precursor obtained in step (5) is mixed with a lithium source and a carbon source and then calcined to obtain a carbon-containing lithium iron phosphate composite material doped with manganese and titanium in situ.

2. The preparation method according to claim 1, characterized in that, In step (4), the molar ratio of ferrous ions in phosphoric acid and refined ferrous sulfate solution is (1~1.2):

1.

3. The preparation method according to claim 1, characterized in that, The precipitation reaction time in step (4) is 0.5~2h.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the oxidant to the iron in the intermediate product in step (5) is (1.2~2.5):

1.

5. The preparation method according to claim 1, characterized in that, In step (6), the mass ratio of carbon source to precursor is (7~9.5):

100.

6. The preparation method according to claim 1, characterized in that, In step (6), the molar ratio of lithium in the lithium source to iron in the precursor is (1.02~1.1):

1.

7. The manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the manganese and titanium in-situ doped carbon-containing lithium iron phosphate composite material of claim 7 as a cathode material for lithium-ion batteries.

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