Surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and preparation method thereof

Abstract

The invention discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof. The method comprises the following steps: carrying out Fe coating and doping double-reinforcement modification on a sodium vanadium phosphate (NVP) material by utilizing a decomposition induced precipitation (DIP) mechanism of hexamethylenetetramine (HMTA), firstly dissolving the prepared NVP material in ethanol, then adding HMTA and Fe (NO3) 3.9 H2O, and carrying out centrifugation and high-temperature calcination to prepare a final sample NVP-coated Fe. The rate capability of a battery can be effectively improved when the material is applied to a sodium ion positive electrode material. The method is simple to operate and high in repeatability, and a new way is provided for designing and preparing the sodium ion battery with good stability and excellent rate capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of the design and preparation of cathode materials for sodium-ion batteries, and specifically relates to a sodium vanadium phosphate high-rate sodium-ion battery material with dual enhancement of surface coating and doping, a preparation method thereof, and an application in a cathode of a sodium-ion battery. Background Art

[0002] Since the breakthrough research on lithium-ion batteries won the Nobel Prize in Chemistry in 2019, lithium-ion batteries have achieved rapid application and innovation in multiple fields with the advantages of high energy and power density, bringing great convenience to human life. With the increasing demand for lithium-ion batteries in human society, some defects of lithium-ion batteries have gradually emerged: one is the relative scarcity of lithium resources; the second is the safety problems of lithium-ion batteries; the third is that the working temperature range of lithium-ion batteries is relatively narrow. The above defects have greatly restricted the large-scale development and application of lithium-ion batteries. Based on this, people turn their attention to sodium, which is in the same main group as lithium, and the development of sodium-ion batteries has gradually become a research hotspot. Compared with lithium resources, sodium resources are more abundant, more widely distributed, and have low costs. At the same time, through experimental verification, sodium-ion batteries have a wider working temperature range compared with lithium ions and can work normally at -20°C. Sodium-ion batteries not only adapt to a wider working environment but also have higher safety. Therefore, during the rapid development of lithium-ion batteries, promoting sodium-ion battery technology simultaneously helps to relieve the economic pressure brought by the scarcity of lithium resources on the one hand, and helps to promote the upgrade of battery technology and the diversified development of new energy technology on the other hand, realizing sustainable energy development.

[0003] The pursuit of electrode materials with high energy density and long cycle stability has become a research hotspot for scientific researchers. Currently, the mainstream cathode materials include layered oxides, polyanion compounds, and Prussian blue analogs. Among them, polyanion compounds have a relatively high redox potential due to the inductive effect, and have a stable structural framework and good safety, becoming one of the most promising candidate materials for commercialization. However, due to the separation between M-O octahedrons, the electronic conductivity is low and the electrochemical kinetics is poor, resulting in a large overpotential. At the same time, due to the relatively large molecular weight of non-active polyanions, the tap density is low (compared with layered oxides), resulting in problems such as small specific capacity, poor rate performance, and low conductivity. These restrictive factors hinder its large-scale application and need to be solved urgently. Sodium vanadium phosphate is the most commercially promising polyanion compound, has a sodium superionic conductor (NASICON) structure, and can provide a stable three-dimensional ion transport channel, which is beneficial to the rapid diffusion of sodium ions. Its theoretical specific capacity is 117.6 mAh / g, and the working voltage is about 3.4 V (relative to Na / Na +) It exhibits a relatively high energy density and good cycle stability. However, the intrinsic electronic conductivity of sodium vanadium phosphate is relatively low, resulting in poor rate performance, which has become a major obstacle to its commercialization. To solve the problem of poor rate performance of sodium vanadium phosphate, element doping or coating methods are used to improve its conductivity. Coating materials with good conductivity such as carbon can accelerate the sodium ion transport kinetics and optimize the electrochemical performance of the material; doping heteroatoms can change the electronic structure of the material itself and increase the conductivity. Despite these significant progress made by doping and coating treatments respectively, there are still a few studies attempting to combine the two to construct a double-enhanced synergistic modification of coating and doping to prepare electrode materials with excellent rate performance and long cycle stability.

[0004] The present invention proposes to use the decomposition-induced precipitation (DIP) mechanism of hexamethylenetetramine (HMTA) to perform Fe(OH) coating on the surface of sodium vanadium phosphate 3 treatment, followed by high-temperature calcination to form a double-enhanced modified structure of coating and doping. The introduction of Fe can improve the conductivity of sodium vanadium phosphate and at the same time activate the crystal lattice to improve its rate performance. After the coating and doping treatment of Fe, a cathode material for sodium-ion batteries with excellent rate performance and long cycle stability is prepared. Summary of the Invention

[0005] To solve the above-mentioned existing problems, the present invention provides a method for using the decomposition-induced precipitation (DIP) mechanism of hexamethylenetetramine (HMTA) to perform Fe(OH) coating on the surface of sodium vanadium phosphate 3 treatment, followed by high-temperature calcination to form a double-enhanced modified structure of coating and doping. The introduction of Fe can improve the conductivity of sodium vanadium phosphate and at the same time activate the crystal lattice to improve its rate performance. The coating treatment and doping double-enhanced modification of Fe achieve the goal of having excellent rate performance and long cycle stability at the same time.

[0006] The technical solution proposed by the present invention is as follows:

[0007] The present invention discloses a high-rate sodium-ion battery material with double-enhanced surface coating and doping of sodium vanadium phosphate and its preparation method, including the following steps:

[0008] (1) Dissolve the prepared sodium vanadium phosphate in a certain amount of absolute ethanol, ultrasonicate for 30 min and continuously stir to disperse it evenly in ethanol.

[0009] (2) Add a certain amount of hexamethylenetetramine and iron(III) nitrate nonahydrate to the above solution and stir to completely dissolve them.

[0010] (3) Transfer the solution in step (2) to an oil bath at a certain temperature and heat it for a certain period of time.

[0011] (4) Subsequently, centrifuge the solution in step (3), wash it three times with absolute ethanol, obtain the precipitate and place it in an oven at a certain temperature for drying.

[0012] (5) Grind the obtained precursor evenly, conduct calcination treatment in a tubular furnace filled with an argon atmosphere, and obtain the final sample NVP@Fe after calcination at a certain temperature for a certain period of time.

[0013] As a preferred scheme, in step (1), the mass of the prepared NVP added is 0.5 - 3.0 g, and it is dissolved in 30 - 50 mL of absolute ethanol.

[0014] As a preferred scheme, in step (2), the mass of hexamethylenetetramine added is 20 - 200 mg, and the mass of ferric nitrate nonahydrate added is 20 - 200 mg.

[0015] As a preferred scheme, in step (3), the temperature of the oil bath is 50 - 70 °C, and the reaction time is 1 - 5 h.

[0016] As a preferred scheme, in step (5), the optimal temperature for high-temperature calcination is 300 - 500 °C, the heating rate is 2 - 5 °C / min, and the high-temperature calcination time is 1 - 6 h.

[0017] Compared with the prior art, the advantages of the technology of the present invention are as follows:

[0018] (1) It can perform simple post-treatment on the produced sodium vanadium phosphate to improve its rate performance.

[0019] (2) The double enhancement modification of Fe coating and doping together improves the conductivity of sodium vanadium phosphate, retains the original long-cycle stability of sodium vanadium phosphate while improving its rate performance. Description of the Drawings

[0020] Figure 1 XRD pattern and SEM image of the NVP@Fe material prepared in Example 1.

[0021] Figure 2 TEM image and corresponding EDS image of the NVP@Fe material in Example 1.

[0022] Figure 3 XPS spectrum of the NVP@Fe prepared in Example 1.

[0023] Figure 4 GCD and CV data graphs of the sodium-ion batteries assembled in Example 1 and Example 4.

[0024] Figure 5 Charge-discharge curves at different rates and cycle stability graphs of the sodium-ion batteries assembled in Example 1 and Example 4.

[0025] Figure 6 Electrochemical impedance diagrams of the sodium-ion batteries assembled for Example 1 and Example 4 at different cycle numbers. Detailed implementation manners

[0026] The present invention will be described more comprehensively below in conjunction with specific embodiments and the accompanying drawings, but the present invention is not limited thereto.

[0027] The experimental methods in the following embodiments are all conventional methods unless otherwise specified.

[0028] The materials, reagents, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0029] Example 1

[0030] Preparation of NVP@Fe material

[0031] The obtained pretreated powder was ground evenly and calcined in a tubular furnace filled with an argon atmosphere. The program was set to hold at 500 °C for 3 h, with a heating rate of 3 °C / min, and finally the product sample was obtained.

[0032] (1) Dissolve 1 g of the prepared sodium vanadium phosphate in a certain amount of absolute ethanol, sonicate for 30 min and continuously stir to disperse it evenly in ethanol.

[0033] (2) Add 50 mg of hexamethylenetetramine and 50 mg of iron(III) nitrate nonahydrate to the above solution and stir until completely dissolved.

[0034] (3) Transfer the mixed solution in step (2) to an oil bath at 60 °C and heat for 3 h.

[0035] (4) Subsequently, centrifuge the solution in step (3), wash it three times with absolute ethanol, obtain the precipitate and place it in an oven at 80 °C to dry.

[0036] (5) Grind the obtained precursor evenly and perform calcination treatment in a tubular furnace filled with an argon atmosphere. Set the heating rate to 3 °C / min and calcine at 500 °C for 3 h to obtain the final sample NVP@Fe. For comparison, the untreated sodium vanadium phosphate was directly assembled into a battery for relevant tests.

[0037] For the original NVP and the prepared NVP@Fe materials, the XRD is as Figure 1 shown in a-b. The XRD results of the original NVP and NVP@Fe can both correspond to the standard card of Na 3 V 2 (PO 4 ) 3 , indicating that the coating and doping treatments did not change the original structure of NVP.

[0038] The morphological structure of the prepared NVP@Fe material is as follows Figure 1 As shown in c-d, the SEM image shows an irregular blocky morphology with particle agglomeration, and the basic particle size is about 300 nm.

[0039] Example 2

[0040] Proof of coating and element doping of NVP@Fe material

[0041] The NVP@Fe material obtained in Example 1 was observed by transmission electron microscopy. It can be seen that the (110) crystal plane is exposed on the surface, and the lattice spacing is 0.431 nm. The (110) lattice spacing of standard NVP is 0.437 nm. This is due to the formation of doping by Fe coating on the surface. The corresponding EDS results show that the elements Na, V, Fe, P, and O are evenly distributed without segregation. The above results indicate that the Fe element has been successfully surface-coated and bulk-doped.

[0042] Example 3

[0043] Analysis of the element valence states of NVP@Fe material

[0044] The NVP@Fe material obtained in Example 1 was analyzed by X-ray photoelectron spectroscopy. The XPS results show that the V element is +3 valence, and the Fe element also shows +3 valence. The Fe element replaces the V element equivalently without charge compensation. In addition, the Na and P elements both show single valence states, and the Fe doping does not affect them. The XPS results can further prove the successful doping of Fe and show a single valence state.

[0045] Example 4

[0046] Preparation of NVP@Fe material into electrode wafers

[0047] The obtained NVP@Fe material, carbon black, and polyvinylidene fluoride (PVDF) were mixed and dispersed in N-methylpyrrolidone (NMP) solution in a mass ratio of 8:1:1 to form a uniform slurry. Then, the above slurry was coated on aluminum foil and dried in a vacuum oven at 100 °C, and then cut into 12 mm wafers. The mass loading of the positive electrode is about 2.0 - 2.5 mg / cm 2 .

[0048] Example 5

[0049] Application of NVP@Fe material to sodium-ion batteries.

[0050] Using the positive electrode wafer obtained in Example 4 as the positive electrode, metallic sodium as the negative electrode, and 1.0 M sodium perchlorate NaClO 4With an electrolyte of EC:DEC = 1:1 Vol% and 5.0% FEC, glass fiber (Whatman, GF / D) was used as the separator. It was assembled into a CR2032 coin-type battery in a glove box with water and oxygen concentrations below 0.1 ppm. After the assembled battery was left standing for 12 h, electrochemical tests were carried out.

[0051] Figure 4 a-b are the GCD curves of the NVP@Fe battery at low rates and different rates (1C = 117.6 mA / g) respectively. According to Figure 4 the GCD curve of a, NVP@Fe shows a single charging platform during charging and two discharging platforms during discharging. After multiple cycles, the charging and discharging platforms of the GCD curve remain unchanged and have a high degree of overlap, indicating its good stability; according to Figure 4 the GCD curve of b, NVP@Fe maintains the same charging and discharging platforms as at low rates at different rates and has less capacity attenuation, indicating its excellent rate performance. Figure 4 c-d are the CV curves of NVP@Fe. In the voltage range of 2.5 - 3.8 V, NVP@Fe has an oxidation peak and two reduction peaks, which is consistent with the number of charging and discharging platforms of the GCD curve, and the CV curve also shows a high degree of overlap, further proving its excellent stability. At the same time, as the scan rate increases, the oxidation and reduction peaks show regular shifts and no new peaks appear or peaks disappear, which further proves its excellent rate performance.

[0052] Figure 5 shows the rate performance and cycle stability of NVP@Fe and NVP.

[0053] Rate tests were carried out in the range of 2.5 - 3.8 V. It was found that after the Fe coating and doping treatment, the rate performance of the material was significantly improved. Especially at high rates of 10C and 20C, the capacity retention rate of the material was as high as 93.8%, and the rate performance was greatly improved. At the same time, cycle tests were carried out at 1C and 10C rates in the range of 2.5 - 3.8 V. NVP@Fe all showed good stability, and the capacity retention rates were 98.56% and 97.50% respectively, which is consistent with Figure 3 the high degree of overlap result of the CV curve. The Fe coating and doping treatment did not change the original cycle stability of NVP, but greatly improved its rate performance.

[0054] Figure 6 shows the electrochemical impedance (EIS) test carried out on the batteries assembled with NVP@Fe and NVP.

[0055] Figure 6 a shows the EIS diagram of NVP@Fe after one cycle. The charge transfer resistance after fitting is 258.3 Ω. Figure 6Figure b shows the EIS diagram after one cycle of unprocessed NVP. After fitting, the charge transfer resistance is 766.9 Ω. Figure 6 Figures c-d respectively compare the EIS diagrams of NVP@Fe and NVP after five cycles and ten cycles. It can be seen that the charge transfer resistance of both shows a decreasing trend, indicating that there is an activation process in both. Moreover, the charge transfer resistance of NVP@Fe is always smaller than that of NVP, indicating that the former has a smaller mass transfer resistance and better rate performance.

Claims

1. The present invention discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof, characterized in that: The following steps are involved: (1) Dissolve the prepared sodium vanadium phosphate in a certain amount of anhydrous ethanol. (2) Add a certain amount of hexamethylenetetramine and ferric nitrate nonahydrate to the above solution and stir until they are completely dissolved. (3) The solution of step (2) is transferred to an oil bath at a certain temperature for heating reaction for a certain period of time. (4) Subsequently, the solution in step (3) is centrifuged, washed three times with anhydrous ethanol, and the precipitate is dried in an oven at a certain temperature. (5) The obtained precursor is ground evenly, and calcined in a tubular furnace filled with argon atmosphere. After calcination at a certain temperature for a certain time, the final sample NVP@Fe is obtained.

2. The present invention according to claim 1 discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof, characterized in that: In the step (1), 0.5-3.0 g of the prepared NVP is added and dissolved in 30-50 mL of anhydrous ethanol.

3. The invention according to claim 2 discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof, characterized in that: In the step (2), the mass of the added hexamethylenetetramine is 20-200 mg, and the mass of the added ferric nitrate nonahydrate is 20-200 mg.

4. The invention according to claim 3 discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof, characterized in that: In the step (3), the temperature of the oil bath is 50-70° C., and the reaction time is 1 to 5 hours.

5. The invention according to claim 5 discloses a surface-coated and doped double-enhanced sodium vanadium phosphate high-rate sodium ion battery material and a preparation method thereof, characterized in that: In the step (5), the optimum temperature for high-temperature calcination is 300-500° C., the heating rate is 2-5° C. / min, and the high-temperature calcination time is 1-6 hours.

6. NVP@Fe obtained by the preparation method according to any one of claims 1 to 5, characterized in that: Using NVP@Fe as the sodium ion positive electrode material, the dual enhanced modification effects of Fe surface coating and doping are used to improve the cycle stability and rate performance of sodium ion batteries.

Images