Preparation method and application of lithium iron titanate / carbon quantum dot composite material

Abstract

The present invention provides a preparation method and application of a lithium iron titanate / carbon quantum dot composite material, belonging to the field of electrochemical energy storage technology. The preparation method comprises the following steps: uniformly mixing anatase and sodium hydroxide, ultrasonically treating, and hydrothermally activating the mixture; repeatedly centrifuging and adding deionized water to the activated product, sodium titanate, until the supernatant is neutral, and drying to obtain an acid-washed product; grinding, sieving, and calcining the acid-washed product to obtain bronze-phase TiO2(B); using citric acid as a chelating agent, dissolving CH3COOLi·2H2O, FeCl2·4H2O, TiO2(B), and carbon quantum dots in anhydrous ethanol, stirring in a water bath to obtain a gel, vacuum drying and sieving, and calcining under an inert atmosphere to obtain a pre-calcined material; and sequentially ball milling and sieving the pre-calcined material, followed by final calcination under an inert atmosphere to obtain a Li2FeTiO4 / carbon quantum dot composite material. The present invention combines carbon quantum dots with lithium iron titanate to form a lithium-ion battery positive electrode composite material with enhanced electrochemical performance. The preparation process is simple, low-cost, and suitable for large-scale commercial production.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy storage, and in particular to a preparation method and application of a lithium iron titanate / carbon quantum dot composite material. Background Art

[0002] With the rapid development of society and the improvement of people's living standards, energy demand continues to grow. Traditional fossil fuels are not only limited in resources, but also produce a large amount of greenhouse gases and other pollutants during use, which has a serious impact on the environment. Therefore, the development of clean and renewable new energy materials has become a key research direction. As a high-efficiency energy storage device, lithium-ion batteries have been widely used in electric vehicles, portable electronic devices and other fields. However, with the continuous growth of energy demand and the rapid development of science and technology, higher requirements are placed on the energy density, cycle stability and safety of lithium-ion batteries. As an important component of lithium-ion batteries, the physical and chemical properties of the positive electrode material determine the battery's capacity, charge and discharge speed and service life. Therefore, the research and development of high-performance positive electrode materials has become the key to the development of lithium-ion battery technology.

[0003] The Li2FeTiO4 cathode material system has attracted significant attention due to its ability to achieve high capacity through coordinated redox reactions of transition metal cations and anions, and has been extensively explored by researchers for application in lithium-ion battery energy storage. However, Li2FeTiO4 lacks ion and electron transport pathways, resulting in slow kinetics and poor electrochemical performance, significantly limiting its application in electrochemical energy storage. Currently, the Ti source for preparing Li2FeTiO4 cathode materials is typically anatase and rutile TiO2. However, bronze-phase TiO2, as a metastable state, is currently understudied and underappreciated. Using it as a Ti source for lithium iron titanate cathode materials is a worthy topic, and further research is needed to determine how to prepare bronze-phase TiO2 as a Ti source for lithium-ion battery cathode materials.

[0004] On the other hand, it has been reported that combining nanomaterials can enhance reaction kinetics, reduce interfacial resistance, and promote rapid electron and ion transport, potentially enabling efficient lithium storage. However, systematic reports on the combination of nanomaterials with Li2FeTiO4 within cathode material systems are currently lacking. In particular, it remains unknown how to optimize the interfacial interaction between nanomaterials and Li2FeTiO4 to reduce interfacial resistance and thereby regulate their electrochemical effects on the Li2FeTiO4 material.

[0005] At the same time, the type of nanocomposite material is of great significance for obtaining materials with stable structure and excellent performance. Related reports suggest that although graphene has excellent conductivity and a special two-dimensional structure, it can form a close bond with Li2FeTiO4 particles to improve the overall conductivity of the composite material. However, graphene materials are highly hydrophilic and easily adsorb water molecules to produce chemical reactions, thereby affecting the stability and electrochemical performance of the composite material. Carbon quantum dots (CDs), as a new type of zero-dimensional carbon material, have the characteristics of excellent conductivity, good chemical stability and high specific surface area, and are considered to be an ideal modifier.

[0006] Although carbon quantum dots have certain advantages in the field of electrochemistry as a conductive material, how to effectively combine carbon quantum dots with Li2FeTiO4 and fully utilize their functions in specific applications remains a technical challenge. Although the application of carbon quantum dots in battery materials has been studied, how to ensure their uniform dispersion and form a stable composite with lithium iron titanate materials remains a technical difficulty. In particular, how to control the composite process through methods such as sol-gel method to ensure the interaction between lithium iron titanate and carbon quantum dots and avoid agglomeration and instability caused by excessive carbon quantum dots is a technical problem that needs to be solved urgently. Summary of the Invention

[0007] To address the aforementioned issues with existing technologies, the present invention provides a method for preparing and applying a lithium iron titanate / carbon quantum dot composite material. This method combines carbon quantum dots with lithium iron titanate to form a lithium-ion battery cathode composite material with enhanced electrochemical performance. The preparation process is simple, low-cost, and suitable for large-scale commercial production.

[0008] The technical solutions of the present invention are as follows:

[0009] A method for preparing a lithium iron titanate / carbon quantum dot composite material, the preparation method comprising the following steps:

[0010] (1) Anatase and sodium hydroxide are used as raw materials, uniformly mixed in deionized water, and then ultrasonically treated and hydrothermally activated to obtain an activated product, sodium titanate;

[0011] (2) Repeatedly centrifuging and adding deionized water to the activated product sodium titanate until the pH of the supernatant is ≤10, then adding dilute hydrochloric acid and stirring for 2-4 hours, repeating the operation until the supernatant is neutral, and drying to obtain an acid-washed product;

[0012] (3) grinding, sieving, and calcining the pickled product to obtain bronze-phase TiO2 (B);

[0013] (4) Citric acid was used as a chelating agent, CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) were used as Li, Fe, and Ti sources, respectively, and a certain mass fraction of carbon quantum dots were dissolved in anhydrous ethanol;

[0014] (5) stirring the mixture obtained in step (4) in a water bath to obtain a gel, vacuum drying and sieving, and calcining under an inert atmosphere to obtain a pre-calcined material;

[0015] (6) The pre-fired material is ball-milled and sieved in sequence, and finally sintered under an inert atmosphere to obtain a Li2FeTiO4 / carbon quantum dot composite material.

[0016] Preferably, the mass ratio of anatase to sodium hydroxide in step (1) is 1:34-37, and the ultrasonic treatment time is 20-40 min.

[0017] Preferably, the hydrothermal activation in step (1) is carried out in a sealed reactor, the activation temperature is 150-170°C, the heating rate is 7-8°C / s, and the hydrothermal activation time is 20-24h.

[0018] Furthermore, the anatase in step (1) is commercial anatase; 70-85 ml of deionized water is used for every 1 g of anatase.

[0019] Preferably, the concentration of the dilute hydrochloric acid in step (2) is 0.1-0.2 M; the drying temperature is 70-90° C., and the drying time is 10-12 h.

[0020] Furthermore, the neutrality of the supernatant in step (2) means that its pH is about 7.

[0021] Preferably, the screening in step (3) is through a 150-200 mesh standard sieve; the calcination atmosphere is air, the calcination temperature is 340-360°C, the time is 2-4h, and the heating rate is 4-6°C / min.

[0022] Preferably, the mass fraction of the carbon quantum dots in step (4) is 1-5% of the sum of the Li, Fe and Ti sources; and the molar ratio of the citric acid to the sum of the Fe ions and the Ti ions is 1-2:1.

[0023] Preferably, the mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) in step (4) is 2.56:2.49:1.

[0024] Furthermore, in step (4), 8-16 ml of anhydrous ethanol is used per 1 g of citric acid.

[0025] Preferably, the water bath temperature in step (5) is 60-70° C., and the stirring time is 4-6 h; the drying temperature is 100-120° C., and the time is 10-12 h; and the sieving is through a 60-100 mesh standard sieve.

[0026] Preferably, the calcination atmosphere in step (5) is argon, the calcination temperature is 400-600°C, the time is 7-10h, and the heating rate is 4-6°C / min.

[0027] Preferably, the screening in step (6) is through a 100-300 mesh standard sieve; the calcination atmosphere is argon, the calcination temperature is 600-700°C, the time is 7-9h, and the heating rate is 4-6°C / min.

[0028] Furthermore, the ball milling process described in step (6) uses anhydrous ethanol as the medium and the ball milling time is 2-4 hours.

[0029] The reaction principles of the above steps and the factors affecting the process parameter settings are as follows:

[0030] The activation step (1) effectively breaks and restructures the ionic and covalent bonds between the titanium oxide in anatase and the ionic bonds between the sodium hydroxide, thereby obtaining sodium titanate. Excessively high activation temperatures may lead to overreaction or decomposition of the raw materials, while too low a temperature may not achieve the desired activation effect. The same applies to the holding time during activation. If the holding time is too short, the raw materials will not react sufficiently and the desired activation effect will not be achieved. If the holding time is too long, the raw materials may overreact or consume unnecessary energy.

[0031] The purpose of pickling in step (2) is to remove residual Na + , effectively remove impurities and improve the purity of TiO2(B).

[0032] The calcination in step (3) can make the chemical bonds in the raw materials more easily broken and recombined, which is conducive to the formation of bronze phase titanium dioxide, namely TiO2 (B) crystal phase. The preparation conditions of bronze phase titanium dioxide are different from those of other phases of titanium dioxide. For example, for the preparation of anatase titanium dioxide, polyvinyl alcohol is usually used as a dispersant, and a hydrothermal reaction is carried out at a high temperature (200°C). After acid washing and high-temperature calcination, anatase titanium dioxide is finally obtained; this is because anatase titanium dioxide is generally more stable at higher temperatures, so the reaction conditions are relatively strong and the reaction temperature is relatively high, which is conducive to the formation of anatase titanium dioxide.

[0033] The present invention uses commercial anatase TiO2 and sodium hydroxide for hydrothermal activation at a relatively low temperature (around 160°C), followed by acid washing and low-temperature calcination (around 350°C) to produce bronze-phase titanium dioxide. Because anatase TiO2 already has a relatively stable crystal structure, the hydrothermal reaction and acid exchange transform the sodium titanate into titanate nanotubes. The calcination process more effectively maintains the crystal form of the bronze-phase titanium dioxide (TiO2(B)).

[0034] Anatase TiO2 readily transforms to rutile or other crystalline forms at higher temperatures, but under certain conditions (such as appropriate hydrothermal activation and acid exchange treatment), it tends to form bronze TiO2 (B). Excessively high calcination temperatures can lead to phase transformation or grain growth, increasing the formation of by-products. Appropriate calcination times help ensure the full removal of impurities while avoiding degradation of product performance due to over-calcination.

[0035] If the water bath temperature in step (5) is too high or the stirring time is too short, the reactants may not be fully dissolved and mixed, and the gel may be unevenly formed, affecting the structure and properties of the composite material during the subsequent calcination process. On the contrary, if the water bath temperature is too low or the stirring time is too long, energy consumption and time costs will be increased, which is not conducive to industrial production.

[0036] If the calcination temperature in step (6) is too high, the Li2FeTiO4 grains will grow excessively, reducing the specific surface area and ion diffusion rate of the composite material. At the same time, too high a temperature may also cause the carbon quantum dots to agglomerate or sinter, reducing their dispersibility and stability in the composite material. On the contrary, if the calcination temperature is too low or the time is insufficient, the reactants cannot fully react, and unreacted residues or impurities will exist, affecting their purity and performance.

[0037] The present invention also provides an application of the lithium iron titanate / carbon quantum dot composite material obtained by the preparation method. The lithium iron titanate / carbon quantum dot composite material is used as a positive electrode material for lithium-ion batteries to achieve efficient and stable lithium storage.

[0038] The beneficial technical effects of the present invention are:

[0039] 1. The present invention prepares Li2FeTiO4 / CDs composite material by preparing bronze-phase titanium dioxide as a Ti source. TiO2(B) has a unique crystal structure and electronic properties, which can effectively promote the insertion and deinsertion process of lithium ions, further optimize the transmission path of lithium ions, reduce diffusion resistance, and thus significantly improve the charge and discharge rate and rate performance of lithium-ion batteries.

[0040] 2. This invention utilizes a sol-gel method to develop and manufacture Li2FeTiO4 / carbon quantum dot composites for lithium-ion battery research and production. By regulating the mass fraction of carbon quantum dots in the precursor, the dosage and distribution of the carbon quantum dots can be precisely controlled. The stability and high performance of the resulting composite material have been verified and optimized through multiple rounds of experiments. The CDs are evenly dispersed on the surface of the Li2FeTiO4 particles, forming a denser coating, thereby effectively improving the composite's electrical conductivity and lithium ion diffusion rate.

[0041] 3. Carbon quantum dot nanomaterials, as conductive bridges, not only improve the conductivity of materials but also, through their unique structural and electronic properties, reduce the resistance to electron transport within the material and optimize charge distribution within the composite material, reducing charge accumulation. This, on the one hand, improves the conductivity and structural stability of the positive electrode material, and on the other hand, effectively increases the charge transfer rate, reduces interfacial impedance, improves the battery's rate performance, and extends the cycle life of lithium-ion batteries. Experimental results show that lithium iron titanate composites with an appropriate amount of carbon quantum dots exhibit significant performance improvements at different discharge rates. In particular, at high discharge rates, the composites exhibit more stable capacity retention and excellent cycle performance.

[0042] 4. The present invention cleverly combines bronze-phase titanium dioxide and carbon quantum dots with iron-based disordered rock salt lithium iron titanate positive electrode materials to improve the safety performance of lithium-ion batteries. The preparation cost of carbon quantum dots is relatively low, which helps to reduce the overall cost of the composite material. The present invention has a high reproducibility and a simple preparation process, making it suitable for large-scale commercial production. Therefore, the preparation method of the lithium iron titanate / carbon quantum dot composite material provided by the present invention provides new ideas and methods for the development of high-performance lithium-ion battery positive electrode materials, and has important scientific significance and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Flow chart for preparing Li2FeTiO4 / CDs composite materials for Examples 1-6;

[0044] Figure 2 SEM image of TiO2 (B) prepared in Example 1;

[0045] Figure 3 This is a SEM image of the Li2FeTiO4 / CDs composite material prepared in Example 4;

[0046] Figure 4 The charge-discharge curves of the first three cycles of the Li2FeTiO4 / CDs composite material prepared in Example 4;

[0047] Figure 5 Rate performance test of the Li2FeTiO4 / CDs composite materials prepared in Examples 1-5;

[0048] Figure 6 Cyclic performance test of the Li2FeTiO4 / CDs composite materials prepared in Examples 1-5;

[0049] Figure 7 The cycling performance tests of the Li2FeTiO4 materials and Li2FeTiO4 / CDs composite materials prepared in Example 4 and Comparative Example 1 were performed. DETAILED DESCRIPTION

[0050] The present invention is described in detail below with reference to the accompanying drawings and embodiments. It is apparent that the embodiments described are only a portion of the embodiments of the present invention, rather than all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0051] If there is no special description of raw materials or processing techniques, it means that they are all conventional commercially available raw materials or conventional processing techniques in this field.

[0052] The carbon quantum dots (CDs) used in the following examples were prepared using a citric acid hydrothermal method. Specifically, 0.2 g of citric acid and 20 mL of a 0.1 mol / L NaOH solution were transferred to a 50 mL Teflon-lined stainless steel autoclave and heated to 200°C for 3 hours at a heating rate of 5°C / min. After the reaction, the autoclave was allowed to cool naturally, and the aqueous solution was centrifuged to obtain a carbon dot solution. Residual NaOH was separated using a dialysis membrane, and the treated solution was freeze-dried to obtain carbon dot powder (CDs) with an average particle size of 3 nm.

[0053] Example 1-5:

[0054] (1) The preparation process of Examples 1-5 is as follows Figure 1 As shown, TiO2 (B) was prepared using 0.315g of commercial anatase and 11.2g of sodium hydroxide as raw materials. The mixture was uniformly mixed in 24.5ml of deionized water. The mixture was then ultrasonically treated for 30 minutes, transferred to a sealed reactor, and hydrothermally activated by heating to 160°C at a rate of 7.7s / °C for 24 hours.

[0055] (2) The activated product sodium titanate was subjected to a "centrifugation-addition of deionized water-pH measurement" process until the pH of the supernatant was ≤ 10, 0.1 M dilute hydrochloric acid was added and stirred, and the "centrifugation-addition of deionized water-pH measurement" process was repeated until the pH of the supernatant was 7, and dried at 80°C for 12 h to obtain an acid-washed product;

[0056] (3) grinding the acid-washed product and passing it through a 200-mesh standard sieve, and calcining the sieved sample in an air atmosphere at a temperature of 350°C for 3 hours at a heating rate of 5°C / min to obtain TiO2 (B);

[0057] (4) Using citric acid as a chelating agent, CH3COOLi·2H2O as a Li source, FeCl2·4H2O as an Fe source, and TiO2(B) as a Ti source, the mixtures were weighed according to a molar ratio of n(citric acid):n(transition metal ions Fe and Ti)=1.25:1. Using ethanol as a medium, 12 ml of anhydrous ethanol was added to every 1 gram of citric acid. The mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) was 2.56:2.49:1. In Examples 1-5, carbon quantum dots were added at a mass of 1%, 2%, 3%, 4%, and 5% of the sum of the masses of the Li, Fe, and Ti sources, respectively, and the mixtures were thoroughly mixed.

[0058] (5) Stirring in a water bath at 65°C for 5 h to obtain a wet gel, vacuum drying at 120°C for 12 h and passing through an 80-mesh standard sieve to obtain a dry gel; calcining the dry gel at a high temperature in an argon atmosphere at a temperature of 500°C, a time of 8 h, and a heating rate of 5°C / min to obtain a pre-burned material.

[0059] (6) The pre-calcined sample was ball-milled for 3 h and passed through a 200-mesh standard sieve, and then calcined at a high temperature in an argon atmosphere at a temperature of 650°C for 8 h at a heating rate of 5°C / min to obtain a sample.

[0060] (7) The above-obtained Li2FeTiO4 / CDs composite cathode material (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) were combined to form a cathode slurry. The slurry was evenly spread on aluminum foil and dried at 80°C overnight. The electrode pieces were cut into 10 mm pieces, the loading amount was controlled at 1.0-1.3 mg, argon was used as the filling gas, Celgard2400 was used as the separator, Li was used as the counter electrode, KLD-LFP01 was used as the electrolyte, and a CR2032 battery shell was used to assemble a half-cell in a glove box and measure its electrochemical performance.

[0061] The SEM of TiO2 (B) prepared in Example 1 is as follows: Figure 2 As shown in the figure, the prepared material has a relatively regular flake or rod-like structure, a relatively smooth surface and high crystallinity. The SEM morphology shows obvious crystal planes and a hierarchical structure, showing the typical characteristics of bronze-phase TiO2.

[0062] The SEM of the Li2FeTiO4 / CDs composite material prepared in Example 4 is as follows: Figure 3As shown in the figure, the sample prepared in Example 4 exhibits large, regular particle size and relatively uniform distribution of carbon quantum dots. This demonstrates that the addition of CDs not only increases ion diffusion rate and storage capacity, but also effectively enhances charge and discharge performance. Furthermore, CDs help maintain the material's stable structure and mitigate capacity decay.

[0063] Example 6:

[0064] (1) In this example, TiO2 (B) was prepared using 0.315 g of commercial anatase and 10.71 g of sodium hydroxide as raw materials. The mixture was uniformly mixed in 22.05 ml of deionized water. The mixture was then ultrasonically treated for 20 min, transferred to a sealed reactor, and hydrothermally activated by heating to 150°C at a rate of 7 s / °C and maintaining the temperature for 20 h.

[0065] (2) The activated product sodium titanate was subjected to a "centrifugation-addition of deionized water-pH measurement" treatment until the pH of the supernatant was ≤ 10, 0.1 M dilute hydrochloric acid was added and stirred, and the "centrifugation-addition of deionized water-pH measurement" treatment was repeated until the pH of the supernatant was 7, and the product was dried at 70°C for 10 h to obtain an acid-washed product.

[0066] (3) grinding the acid-washed product and passing it through a 150-mesh standard sieve, and calcining the sieved sample in an air atmosphere at a temperature of 340°C for 2 h at a heating rate of 4°C / min to obtain TiO2 (B);

[0067] (4) Using citric acid as a chelating agent, CH3COOLi·2H2O as a Li source, FeCl2·4H2O as an Fe source, and TiO2(B) as a Ti source, the mixture was weighed according to a molar ratio of n(citric acid):n(transition metal ions Fe and Ti)=1.25:1. Using ethanol as a medium, 8 ml of anhydrous ethanol was required for every 1 gram of citric acid. The mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) was weighed to be 2.56:2.49:1. Carbon quantum dots (2% of the total mass of the Li, Fe, and Ti sources) were added and mixed thoroughly.

[0068] (5) Stirring in a water bath at 60°C for 4 hours to obtain a wet gel, vacuum drying at 100°C for 10 hours and passing through a 60-mesh standard sieve to obtain a dry gel; calcining the dry gel at a high temperature in an argon atmosphere at a temperature of 400°C, a time of 7 hours, and a heating rate of 4°C / min to obtain a pre-burned material.

[0069] (6) The pre-calcined sample was ball-milled for 2 h and passed through a 100-mesh standard sieve, and then calcined at a high temperature in an argon atmosphere at a temperature of 600 °C, a time of 7 h, and a heating rate of 4 °C / min to obtain a sample.

[0070] (7) The Li2FeTiO4 / CDs composite cathode material obtained above was assembled into a half-cell, and the specific method was the same as that in Example 1.

[0071] Example 7:

[0072] (1) In this example, TiO2 (B) was prepared using 0.315 g of commercial anatase and 11.67 g of sodium hydroxide as raw materials. The mixture was uniformly mixed in 26.78 ml of deionized water. The mixture was then ultrasonically treated for 40 min, transferred to a sealed reactor, and hydrothermally activated by heating to 170°C at a rate of 8°C / s and maintaining the temperature for 24 h.

[0073] (2) The activated product sodium titanate was subjected to a "centrifugation-addition of deionized water-pH measurement" process until the pH of the supernatant was ≤ 10, 0.2 M dilute hydrochloric acid was added and stirred, and the "centrifugation-addition of deionized water-pH measurement" process was repeated until the pH of the supernatant was 7, and dried at 90°C for 12 h to obtain an acid-washed product;

[0074] (3) grinding the acid-washed product and passing it through a 200-mesh standard sieve, and calcining the sieved sample in an air atmosphere at a temperature of 360°C for 4 hours at a heating rate of 6°C / min to obtain TiO2 (B);

[0075] (4) Using citric acid as a chelating agent, CH3COOLi·2H2O as a Li source, FeCl2·4H2O as an Fe source, and TiO2(B) as a Ti source, the mixture was weighed according to a molar ratio of n(citric acid):n(transition metal ions Fe and Ti)=1.25:1. Using ethanol as a medium, 16 ml of anhydrous ethanol was required for every 1 gram of citric acid. The mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) was weighed at a mass ratio of 2.56:2.49:1. 3% of the total mass of the Li, Fe, and Ti sources of carbon quantum dots was added and mixed thoroughly.

[0076] (5) Stirring in a water bath at 70°C for 6 hours to obtain a wet gel, vacuum drying at 120°C for 12 hours and passing through a 100-mesh standard sieve to obtain a dry gel; calcining the dry gel at a high temperature in an argon atmosphere at a temperature of 600°C, a time of 10 hours, and a heating rate of 6°C / min to obtain a pre-burned material.

[0077] (6) The pre-calcined sample was ball-milled for 4 h and passed through a 300-mesh standard sieve, and then calcined at a high temperature in an argon atmosphere at a temperature of 700 °C, a time of 9 h, and a heating rate of 6 °C / min to obtain a sample.

[0078] (7) The Li2FeTiO4 / CDs composite cathode material obtained above was assembled into a half-cell, and the specific method was the same as that in Example 1.

[0079] Comparative Example 1:

[0080] (1) In this comparative example, TiO2 (B) was prepared using 0.315 g of commercial anatase and 11.2 g of sodium hydroxide as raw materials. The mixture was uniformly mixed in 24.5 ml of deionized water. The mixture was then ultrasonically treated for 30 min, transferred to a sealed reactor, and hydrothermally activated by heating to 160°C at a rate of 7.7 s / °C and maintaining the temperature for 24 h.

[0081] (2) The activated product sodium titanate was subjected to a "centrifugation-addition of deionized water-pH measurement" process until the pH of the supernatant was ≤ 10, 0.1 M dilute hydrochloric acid was added and stirred, and the "centrifugation-addition of deionized water-pH measurement" process was repeated until the pH of the supernatant was 7, and dried at 80°C for 12 h to obtain an acid-washed product;

[0082] (3) grinding the acid-washed product and passing it through a 200-mesh standard sieve. The sieved sample was calcined in an air atmosphere at a temperature of 350°C for 3 hours at a heating rate of 5°C / min to obtain TiO2 (B);

[0083] (4) Using citric acid as a chelating agent, CH3COOLi·2H2O as a Li source, FeCl2·4H2O as a Fe source, and TiO2(B) as a Ti source, the mixture was weighed according to a molar ratio of n(citric acid):n(transition metal ions Fe and Ti)=1.25:1. Using ethanol as a medium, 12 ml of anhydrous ethanol was required for every 1 gram of citric acid. The mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) was weighed to be 2.56:2.49:1. 0% carbon quantum dots were added and mixed thoroughly.

[0084] (5) Stirring in a water bath at 65°C for 5 h to obtain a wet gel, vacuum drying at 120°C for 12 h and passing through an 80-mesh standard sieve to obtain a dry gel; calcining the dry gel at a high temperature in an argon atmosphere at a temperature of 500°C, a time of 8 h, and a heating rate of 5°C / min to obtain a pre-burned material.

[0085] (6) The pre-calcined sample was ball-milled for 3 h and passed through a 200-mesh standard sieve, and then calcined at a high temperature in an argon atmosphere at a temperature of 650°C for 5 h at a heating rate of 5°C / min to obtain a sample.

[0086] (7) The Li2FeTiO4 / CDs composite cathode material obtained above was assembled into a half-cell, and the specific method was the same as that in Example 1.

[0087] Test example:

[0088] 1. Electrochemical performance test of Li2FeTiO4 / CDs composite materials:

[0089] The electrochemical properties of the samples were determined by assembling a half-cell using a CHI660E electrochemical workstation and a lithium electrode as the counter electrode.

[0090] The first three charge-discharge curves of the Li2FeTiO4 / CDs composite cathode material prepared in Example 4 are as follows: Figure 4 As shown, the battery's cyclic charge and discharge voltage range is 1.5-4.8V. During the first charge process, the voltage rose rapidly from about 2.6V to about 4.0V, then rose gently to 4.5V, and a clear platform appeared, which is related to the redox process in the Li2FeTiO4 / CDs composite material. During the first discharge, the voltage dropped rapidly from 4.8V to about 3.4V, and then the voltage gradually dropped to about 1.7V and a platform appeared, indicating that the battery underwent a typical intercalation / deintercalation reaction during the charge and discharge process. The first charge and discharge specific capacities were 202.6mAh / g (charge) and 147.48mAh / g (discharge), respectively, showing good energy storage capacity. The charge and discharge curves of the second and third cycles overlapped highly, indicating that the battery had good electrochemical stability and small capacity decay in subsequent cycles, demonstrating the excellent cycle stability of the material in long-term use.

[0091] The rate performance test of the Li2FeTiO4 / CDs composite materials prepared in Examples 1-5 is as follows: Figure 5 As shown. The results show that the rate performance of Example 4 is significantly better than that of samples with other composite ratios. After 30 cycles, the capacity of Example 4 recovered to 119.4 mAh / g, and the capacity retention rate reached 84.6%, showing strong stability and high conductivity. Therefore, the compounding of an appropriate amount of carbon quantum dots significantly improves the conductivity and rate performance of the material, can effectively improve the conductivity of electrons and ions, enhance structural stability, and reduce capacity attenuation during high-rate discharge. However, too high or too low a composite ratio will cause the material to form an unstable structure and agglomeration, significantly reducing its electrochemical properties.

[0092] The cycle performance test of the Li2FeTiO4 / CDs composite material prepared in Examples 1-5 is as follows Figure 6 As shown, the initial discharge capacity of the sample in Example 1 is 125.14 mAh / g, the initial discharge capacity of the sample in Example 2 is 118.61 mAh / g, the initial discharge capacity of the sample in Example 3 is 110.83 mAh / g, the initial discharge capacity of the sample in Example 4 is 140.16 mAh / g, and the initial discharge capacity of the sample in Example 5 is 132.28 mAh / g. The electrochemical performance of Example 4 is significantly better than that of the other examples, showing better cycle stability and lower capacity decay.

[0093] The initial discharge specific capacities of Example 6 and Example 7 are 107.51 mAh / g and 101.78 mAh / g, respectively, which are slightly lower than those of Example 2 (118.61 mAh / g) and Example 3 (110.83 mAh / g). According to the results, due to the differences in factors such as raw material ratio, reaction conditions and calcination temperature during the preparation processes of Example 6 and Example 7, there are differences in the crystal structure, surface properties, particle size distribution and other aspects of the materials. These structural changes directly affect the electrical conductivity, ion diffusion rate and electrochemical stability of the material, thereby leading to a decrease in battery performance. Therefore, the optimization of preparation conditions is crucial in improving the performance of lithium-ion batteries, especially in controlling the appropriate material microstructure and chemical composition while ensuring high initial discharge specific capacity and low capacity decay, to promote efficient transfer and reaction of charge. In addition, the electrochemical performance results of Example 6 and Example 7 also provide an important experimental basis for further adjusting the synthesis process parameters and improving the material structure.

[0094] The cycle performance test of the Li2FeTiO4 / CDs composite material prepared in Example 4 and Comparative Example 1 is as follows: Figure 7 As shown, the first-cycle discharge specific capacities of Example 4 and Comparative Example 1 are 140.16 mAh / g and 92.2 mAh / g, respectively. The discharge specific capacities after 30 cycles are 118.59 mAh / g and 47.2 mAh / g, respectively, and the capacity retention rates are 84.61% and 51.19%, respectively. Compared with the sample of Comparative Example 1, the cycle performance of the sample of Example 4 is improved by about 33%. This shows that the addition of CDs effectively enhances the conductivity between Li2FeTiO4 molecules and between them and the electrolyte, improves the charge and discharge performance of the battery, and at the same time improves the structural stability of the material. In addition, carbon quantum dots have excellent electrical conductivity and a high specific surface area, which can reduce the resistance of the battery during the charge and discharge process and promote the rapid diffusion of lithium ions. At the same time, carbon quantum dots can improve the structural stability of the material, reduce volume expansion during repeated charge and discharge, and further improve the long-term stability of the material.

[0095] This invention utilizes bronze-phase titanium dioxide as a Ti source. Its unique crystal structure and excellent electronic properties effectively facilitate the insertion and deinsertion of lithium ions, thereby optimizing the lithium ion transport pathway. Furthermore, the composite carbon quantum dot nanomaterial acts as a conductive bridge, reducing the internal electron transport resistance and improving charge distribution within the composite material, thereby reducing charge accumulation. These improvements enhance the conductivity and structural stability of the Li2FeTiO4 material, significantly extending the cycle life of lithium-ion batteries.

[0096] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A method for preparing a lithium iron titanate / carbon quantum dot composite material, characterized in that: The preparation method comprises the following steps: (1) Anatase and sodium hydroxide are used as raw materials, uniformly mixed in deionized water, and then ultrasonically treated and hydrothermally activated to obtain an activated product, sodium titanate; (2) Repeatedly centrifuging and adding deionized water to the activated product sodium titanate until the pH of the supernatant is ≤10, then adding dilute hydrochloric acid and stirring for 2-4 hours, repeating the operation until the supernatant is neutral, and drying to obtain an acid-washed product; (3) grinding, sieving, and calcining the pickled product to obtain bronze-phase TiO2 (B); (4) Citric acid was used as a chelating agent, CH3COOLi·2H2O, FeCl2·4H2O, and TiO2(B) were used as Li, Fe, and Ti sources, respectively, and a certain mass fraction of carbon quantum dots were dissolved in anhydrous ethanol; (5) stirring the mixture obtained in step (4) in a water bath to obtain a gel, vacuum drying and sieving, and calcining under an inert atmosphere to obtain a pre-calcined material; (6) The pre-fired material is ball-milled and sieved in sequence, and finally sintered under an inert atmosphere to obtain a Li2FeTiO4 / carbon quantum dot composite material.

2. The preparation method according to claim 1, characterized in that The mass ratio of anatase to sodium hydroxide in step (1) is 1:34-37, and the ultrasonic treatment time is 20-40 min; The hydrothermal activation in step (1) is carried out in a sealed reactor, the activation temperature is 150-170°C, the heating rate is 7-8°C / s, and the hydrothermal activation time is 20-24h.

3. The preparation method according to claim 1, characterized in that The concentration of the dilute hydrochloric acid in step (2) is 0.1-0.2M; the drying temperature is 70-90°C, and the drying time is 10-12h.

4. The preparation method according to claim 1, characterized in that The sieving in step (3) is through a 150-200 mesh standard sieve; the calcination atmosphere is air, the calcination temperature is 340-360°C, the calcination time is 2-4h, and the heating rate is 4-6°C / min.

5. The preparation method according to claim 1, characterized in that The mass fraction of the carbon quantum dots in step (4) is 1-5% of the sum of the Li, Fe and Ti sources; the molar ratio of the citric acid to the sum of the Fe ions and the Ti ions is 1-2:

1.

6. The preparation method according to claim 1, characterized in that The mass ratio of CH3COOLi·2H2O, FeCl2·4H2O, and TiO2 (B) in step (4) is 2.56:2.49:

1.

7. The preparation method according to claim 1, characterized in that The water bath temperature in step (5) is 60-70° C., and the stirring time is 4-6 h; the drying temperature is 100-120° C., and the time is 10-12 h; and the sieving is through a 60-100 mesh standard sieve.

8. The preparation method according to claim 1, characterized in that The calcination atmosphere in step (5) is argon, the calcination temperature is 400-600°C, the time is 7-10h, and the heating rate is 4-6°C / min.

9. The preparation method according to claim 1, characterized in that The sieving in step (6) is through a 100-300 mesh standard sieve; the calcination atmosphere is argon, the calcination temperature is 600-700°C, the time is 7-9h, and the heating rate is 4-6°C / min.

10. Application of the lithium iron titanate / carbon quantum dot composite material obtained by the preparation method according to any one of claims 1 to 9, characterized in that: The lithium iron titanate / carbon quantum dot composite material is used as a positive electrode material for lithium-ion batteries to achieve efficient and stable lithium storage.

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