A Na4Fe3(po4)2(p2o7)@C lithium ion battery cathode material and cathode and battery

Abstract

The application belongs to the field of lithium ion batteries, and discloses a Na4Fe3(PO4)2(P2O7)@C lithium ion battery positive electrode material and a positive electrode and a battery. The Na4Fe3(PO4)2(P2O7)@C lithium ion battery positive electrode material is prepared by sequentially dissolving iron source compounds, phosphorus source compounds, sodium source compounds and organic small molecule carbon sources in water, and then performing spray drying and sintering under a protective atmosphere. The Na4Fe3(PO4)2(P2O7)@C is directly used as a lithium ion battery positive electrode material for the first time.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and more specifically, relates to a Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, cathode, and battery. Background Technology

[0002] The scarcity of fossil fuels and environmental pollution have driven the rapid development of renewable clean energy sources such as wind, solar, and tidal power. However, their volatility and intermittency make them unable to meet continuous and stable energy demands. Therefore, developing economical, environmentally friendly, and high-performance large-scale energy storage devices has become crucial for the development and application of renewable clean energy. Among various energy storage devices, lithium-ion batteries have demonstrated advantages in high energy density and good long-range cycle stability, and currently dominate the consumer electronics field. They are considered the most promising competitors for future new energy vehicles and large-scale grid energy storage systems, and are one of the most promising power sources for large-scale energy storage systems. However, limited lithium resources and rising lithium prices severely restrict their widespread application in large-scale energy storage systems.

[0003] Cathode materials play a crucial role in the cost and performance of lithium-ion batteries, and improving the cost-effectiveness of cathode materials is one of the most effective means to improve the overall cost-effectiveness of lithium-ion batteries. Using inexpensive raw materials and developing simple and economical production processes to produce cathode materials is one of the most effective ways to reduce the cost of lithium-ion batteries, thereby facilitating their widespread application in large-scale energy storage systems. In traditional lithium-ion batteries, both the cathode material and the electrolyte are lithium-containing substances, with Li acting as the charge carrier during charging and discharging. + Primarily supplied by the positive electrode, with the electrolyte serving only as an ion transport medium. The production process of lithium-containing positive electrode materials requires a large amount of lithium raw materials, significantly increasing the production cost. Lithium-containing positive electrode materials generally have a framework structure with large channels for Li-ion exchange. + Migration. Some sodium-based homologues of lithium-containing framework compounds have structures similar to those of lithium-containing framework compounds. Due to the abundant reserves, wide distribution, and relatively low price of sodium, the production cost of sodium-containing framework compounds is significantly reduced. Furthermore, sodium and oxygen form longer Na-O bonds, providing a more open framework structure; therefore, sodium-containing framework compounds generally have larger channels, allowing for faster ion migration. Using sodium-containing framework compounds with large channels as cathode materials for lithium-ion batteries will greatly improve their rate performance and reduce the production cost of lithium-ion battery cathode materials.

[0004] Transition metals commonly used as electrochemical redox centers include Ni, Co, Mn, Fe, and V. Among these, Fe is abundant and non-toxic on Earth. Due to the abundance and low price of both Na and Fe, secondary battery cathode materials using Fe as the redox center and Na as the initial framework structure and charge carrier have significant advantages in terms of cost, resources, and environmental friendliness. In recent years, the iron-based mixed polyanionic compound Na4Fe3(PO4)2(P2O7) has attracted widespread attention in the battery industry. Na4Fe3(PO4)2(P2O7) belongs to the orthorhombic crystal system, with its crystal framework composed of [FeO6] octahedra, [PO4] tetrahedra, and pyrophosphate [P2O7]. This pyrophosphate connection results in large channels in the crystal structure of Na4Fe3(PO4)2(P2O7), allowing Na… + This allows for storage and migration, resulting in Na4Fe3(PO4)2(P2O7) exhibiting excellent sodium storage properties.

[0005] Due to Li + The radius (0.76 Å) is greater than that of Na. + With a small radius (1.02 Å), the channels in the Na4Fe3(PO4)2(P2O7) crystal structure will be highly favorable for Li + The large-channel lithium storage of Na4Fe3(PO4)2(P2O7) allows for excellent rate performance. Indeed, the electrochemical behavior of desodium-treated Na4Fe3(PO4)2(P2O7) as a lithium storage material has been reported. However, in the reported literature, Na4Fe3(PO4)2(P2O7) requires chemical or electrochemical desodium removal before lithium storage, a very cumbersome step. This uneconomical production process increases the manufacturing cost of lithium-ion batteries and prevents their mass production.

[0006] If Na4Fe3(PO4)2(P2O7) could be directly used as the cathode material for lithium-ion batteries, the production cost of lithium-ion batteries would be greatly reduced. Barker once used Na3V2(PO4)2F3 as the cathode and Li electrolyte to form a battery, with graphite or Li4Ti5O as the anode. 12 He found that in both cases, the negative electrode reaction was based on a lithium insertion mechanism. Barker's research indicates that the Na extracted at the positive electrode... + Since it does not insert into typical lithium-ion battery anode materials, it does not disrupt the anode structure. Therefore, Na4Fe3(PO4)2(P2O7) is a potential high-rate lithium-ion battery cathode material. Therefore, this invention urgently aims to propose a resource-rich, low-cost, high-rate Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, as well as the cathode and battery itself. Summary of the Invention

[0007] The purpose of this invention is to address the scarcity and high cost of lithium resources by proposing a resource-rich, low-cost, high-rate Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, as well as the cathode and battery itself. This invention is the first to directly use Na4Fe3(PO4)2(P2O7)@C as a lithium-ion battery cathode material.

[0008] In this invention, the applicant first directly uses Na4Fe3(PO4)2(P2O7) as the positive electrode material of a lithium-ion battery. When Na4Fe3(PO4)2(P2O7) is matched with the lithium electrolyte and the negative electrode material, during the first charging process, Na4Fe3(PO4)2(P2O7) undergoes electrochemical desodium removal to form NaFe3(PO4)2(P2O7), and the Na released from Na4Fe3(PO4)2(P2O7)... + Li dissolves in the electrolyte, while Li dissociates from the lithium electrolyte. + Insert into the negative electrode. During the first discharge process, Li + It is extracted from the negative electrode, while the Li in the lithium electrolyte is also removed. + and a small amount of Na + It will insert into the NaFe3(PO4)2(P2O7) crystal framework. During subsequent charge and discharge processes, Li... + and a small amount of Na + Repeated insertion and extraction at the positive electrode, while only Li is inserted and extracted at the negative electrode. + Because the channels in Na4Fe3(PO4)2(P2O7) can themselves supply Na... + Migration, therefore Li + and a small amount of Na + It will not destroy the crystal structure of Na4Fe3(PO4)2(P2O7).

[0009] To achieve the above objectives, the first aspect of the present invention provides a Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, wherein the Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material is prepared by spray drying and sintering under a protective atmosphere after dissolving an iron source compound, a phosphorus source compound, a sodium source compound, and an organic small molecule carbon source in water in sequence.

[0010] In this invention, Na4Fe3(PO4)2(P2O7)@C is a carbon-coated spherical material with a gray-black powder appearance.

[0011] According to the present invention, preferably, the sodium source compound is one or more of sodium hydroxide, sodium acetate, sodium chloride, sodium nitrate, sodium carbonate, sodium formate, and sodium dihydrogen phosphate.

[0012] According to the present invention, preferably, the phosphorus source compound is one or more of sodium dihydrogen phosphate, phosphoric acid, metaphosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and triammonium phosphate.

[0013] According to the present invention, preferably, the iron source compound is one or more of ferric nitrate, ferric chloride, ferric citrate, and ferric ammonium citrate.

[0014] According to the present invention, preferably, the organic small molecule carbon source is one or more of glucose, sucrose, citric acid, fructose, benzoic acid, ascorbic acid, and malic acid.

[0015] According to the present invention, preferably, the amount of the organic small molecule carbon source is 8-30% of the mass of Na4Fe3(PO4)2(P2O7). Preferably, the organic small molecule carbon source is glucose, and the amount of glucose is 10%, 15%, or 20% of the mass of Na4Fe3(PO4)2(P2O7). Further, in the present invention, according to the amount of glucose added from low to high (10%, 15%, 20%), the Na4Fe3(PO4)2(P2O7)@C products are named NFPP@LC, NFPP@MC, and NFPP@HC, respectively.

[0016] According to the present invention, preferably, the mixed solution after the iron source compound, phosphorus source compound, sodium source compound, and organic small molecule carbon source are sequentially dissolved in water is subjected to the spray drying treatment to obtain a powder precursor; the total concentration of sodium ions in the mixed solution is 1-2 mol / L;

[0017] The spray drying temperature is 120-180℃, the fan frequency is 30-50Hz, and the flow rate of the mixed solution is 10-50mL / min.

[0018] According to the present invention, preferably, the sintering temperature is 450-550°C and the sintering time is 5-15 hours. In the present invention, since the sintering temperature is much lower than the calcination temperature (≥700°C) of traditional lithium-ion battery cathode materials, the present invention greatly reduces the energy consumption in the cathode material production process.

[0019] According to the present invention, preferably, the protective atmosphere is a mixture of argon and hydrogen, wherein the hydrogen accounts for 5%-10% of the total volume of the mixture.

[0020] The second aspect of the present invention provides a lithium-ion battery positive electrode, wherein the lithium-ion battery positive electrode is prepared by uniformly mixing positive electrode active material, conductive agent and binder to form a slurry, coating it on a current collector, and then drying it.

[0021] The positive electrode active material is the aforementioned Na4Fe3(PO4)2(P2O7)@C lithium-ion battery positive electrode material.

[0022] According to the present invention, preferably, the mass ratio of the positive electrode active material, the conductive agent and the binder is (70-80):(10-20):(5-15).

[0023] According to the present invention, preferably, the conductive agent is at least one selected from acetylene black, Super P, VGCF, carbon nanotubes, carbon nanofibers, graphene, and graphite.

[0024] According to the present invention, preferably, the adhesive is at least one selected from polyvinylidene fluoride, polytetrafluoroethylene, and polyacrylic acid.

[0025] According to the present invention, preferably, the drying process is carried out at a temperature of 95-105°C for a time of 10-15 hours.

[0026] A third aspect of the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery is the aforementioned positive electrode.

[0027] According to the present invention, preferably, the negative electrode of the lithium-ion battery is a lithium sheet.

[0028] According to the present invention, preferably, the electrolyte of the lithium-ion battery is prepared by mixing a lithium-containing compound with an organic solvent; the concentration of the lithium-containing compound in the electrolyte is 0.8-1.2M, and the lithium-containing compound is LiPF6; the organic solvent is a mixed solvent of EC and DMC, and the volume ratio of EC to DMC is (0.8-1.2):(0.8-1.2).

[0029] The beneficial effects of the technical solution of the present invention are as follows:

[0030] This invention uses inexpensive Na and Fe raw materials as starting materials and synthesizes Na4Fe3(PO4)2(P2O7)@C using a simple and scalable spray drying method. In this invention, carbon coating aims to prevent the agglomeration and growth of Na4Fe3(PO4)2(P2O7) grains while improving the electrical conductivity of Na4Fe3(PO4)2(P2O7).

[0031] This invention is the first to directly use Na4Fe3(PO4)2(P2O7)@C as a cathode material for lithium-ion batteries.

[0032] In batteries using lithium electrolyte and lithium metal anode, the optimized Na4Fe3(PO4)2P2O7@C exhibited high capacity (118.1 mAh g / L reversible capacity at 0.1C). -1 It exhibits excellent rate performance (74.9% of the reversible capacity at 20C compared to 0.1C) and outstanding cycling stability (74.8 mAhg of reversible capacity after 1000 cycles at 20C).-1 (Capacity retention rate of 84.6%). Na4Fe3(PO4)2P2O7@C addresses the problem that limited lithium resources and rising lithium prices severely restrict its widespread application in large-scale energy storage systems.

[0033] The availability of inexpensive raw materials, the simple and economical synthesis method, and the excellent lithium storage performance indicate that Na4Fe3(PO4)2(P2O7)@C can pave the way for the production of inexpensive, high-rate cathode materials for lithium-ion batteries, making it suitable for large-scale production applications.

[0034] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0035] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0036] Figure 1 The present invention provides a flow chart of the preparation of a Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material (Spray solution; Spray drying; Precursor; Calcinations) provided in Example 1 of the present invention.

[0037] Figure 2a A scanning electron microscope image of Na4Fe3(PO4)2(P2O7)@C (NFPP@HC) obtained in Example 3 of the present invention is shown.

[0038] Figure 2b A scanning electron microscope image of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention is shown.

[0039] Figure 2c shows a scanning electron microscope image of Na4Fe3(PO4)2(P2O7)@C (NFPP@LC) obtained in Example 1 of the present invention, as well as the corresponding elemental mappings of Na, Fe, P, O and C. Figure 2c-1 This is an SEM image. Figure 2c-2 This is the Na mapping (EDS diagram). Figure 2c-3 This is the Fe mapping (EDS diagram). Figure 2c-4 For P mapping (EDS graph). Figure 2c-5 For O mapping (EDS graph). Figure 2c-6 For C mapping (EDS graph)).

[0040] Figure 3 The XRD patterns (“Intensity”, “2θ (degree)”) of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown.

[0041] Figure 4a The Fourier transform infrared spectrum (“Absorbance”, “Wavenumber”) of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention is shown.

[0042] Figure 4b The Fourier transform infrared spectra of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown.

[0043] Figure 5a The first-cycle CV curves ("Current" and "Potential" voltage) of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown.

[0044] Figure 5b The first four cycles of the CV curves of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention are shown.

[0045] Figure 5c The third CV curve of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention is shown.

[0046] Figure 5d The first charge-discharge curves ("Voltage", "Specific Capacity") of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown.

[0047] Figure 5e The charge-discharge curves of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention are shown as the first four cycles ("1st", "2nd", "3rd", and "4th").

[0048] Figure 5f The third charge-discharge curve of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention is shown.

[0049] Figure 6aThe rate performance diagrams ("Discharge Capacity", "Cycle Number") of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown.

[0050] Figure 6b The charge-discharge curves of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention at different rates are shown.

[0051] Figure 6c The diagram shows the cycling performance of Na4Fe3(PO4)2(P2O7)@C (NFPP@MC) obtained in Example 2 of the present invention at 0.2C ("Coulombic efficiency", "Current rate", "ChargeCapacity").

[0052] Figure 6d The cycling performance of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention is shown in the figure at 5C.

[0053] Figure 6e The cycling performance of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention is shown in the graph at 20°C.

[0054] Figure 7a The EIS diagrams of Na4Fe3(PO4)2(P2O7)@C obtained in Examples 1-3 of the present invention are shown (NFPP@LCfresh (uncirculated NFPP@LC electrode), NFPP@MCfresh (uncirculated NFPP@MC electrode), NFPP@HCfresh (uncirculated NFPP@HC electrode), NFPP@LC 100th (circulated NFPP@LC electrode after 100 cycles at 0.2C), NFPP@MC100th (circulated NFPP@MC electrode after 100 cycles at 0.2C), and NFPP@HC 100th (circulated NFPP@HC electrode after 100 cycles at 0.2C).

[0055] Figure 7b The R obtained from Na4Fe3(PO4)2(P2O7)@C in Examples 1-3 of this invention is shown based on equivalent circuit fitting. ct Value ("Category" category). Detailed Implementation

[0056] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0057] Example 1

[0058] This embodiment provides a Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material. The preparation process of the Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material is as follows: Figure 1 As shown, it includes:

[0059] Add 12.12g of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), 6.24g of sodium dihydrogen phosphate (NaH2PO4), and glucose (C6H2PO4) to the solution. 12 O6) was mixed with 80 mL of water to obtain a mixed solution;

[0060] The mixed solution was spray-dried at 160°C to obtain a powder precursor;

[0061] The powder precursor was sintered at 500℃ in an Ar / H2 (5%) atmosphere for 10h to obtain Na4Fe3(PO4)2(P2O7)@C (a gray-black powder product).

[0062] The amount of glucose used is 10% of the mass of Na4Fe3(PO4)2(P2O7).

[0063] This embodiment also provides a lithium-ion battery positive electrode, which is made by uniformly mixing Na4Fe3(PO4)2(P2O7)@C, acetylene black and polyvinylidene fluoride to form a slurry, coating it on a current collector, drying it under vacuum at 100°C for 12 hours, and cutting it into a circular piece with a radius of 6 mm.

[0064] The mass ratio of Na4Fe3(PO4)2(P2O7)@C, acetylene black, and polyvinylidene fluoride is 75:15:10.

[0065] This embodiment also provides a lithium-ion battery, which is a 2032 type two-electrode coin cell battery, wherein the positive electrode of the lithium-ion battery is the aforementioned positive electrode;

[0066] Lithium foil is used as both the counter electrode and the reference electrode.

[0067] The electrolyte is 1 M LiPF6 dissolved in a 1:1 (V / V) mixture of EC / DMC.

[0068] The 2032 type two-electrode button cell was assembled in an argon-filled glove box, where the water and oxygen content was less than 10 ppm.

[0069] Example 2

[0070] The only difference between this embodiment and Embodiment 1 is that the amount of glucose used accounts for 15% of the mass of Na4Fe3(PO4)2(P2O7).

[0071] Example 3

[0072] The only difference between this embodiment and Embodiment 1 is that the amount of glucose used accounts for 20% of the mass of Na4Fe3(PO4)2(P2O7).

[0073] Test Example 1

[0074] This test example characterizes the Na4Fe3(PO4)2(P2O7)@C (NFPP@C) obtained in the above embodiments. Wherein:

[0075] The crystal structure of NFPP@C was identified using X-ray diffraction (XRD, ADWANCE D8, Bruker), with Cu Kα radiation (λ = 1.54056 Å), a tube voltage of 40 kV, a tube current of 20 mA, a 2θ angle range of 5° to 80°, and a scan rate of 2° / min. -1 .

[0076] The morphology and microstructure of the samples were observed using a field emission scanning electron microscope (SEM, JEOL, JSM7500F).

[0077] The Fourier transform infrared spectra of the samples were measured using a Nicolet iG50 Fourier transform infrared spectrometer (Thermo Fisher Scientific, USA), with a wavenumber range of 1400–500 cm⁻¹. -1 .

[0078] The results are as follows:

[0079] The morphology and elemental distribution of the three samples from Examples 1-3 were investigated using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). According to the SEM images in Figure 2, all three samples exhibited a regular spherical shape, with particle diameters ranging from 3 to 5 µm. The surfaces of the spheres were dense, and no obvious pores were observed. The EDS image of NFPP@LC is shown in Figure 2c. Na, Fe, O, P, and C elements were uniformly distributed within the NFPP@LC microspheres. This uniform elemental distribution confirms that spray drying is a simple and effective method for synthesizing pure-phase NFPP.

[0080] XRD patterns of NFPP@LC, NFPP@MC, and NFPP@HC microspheres are shown in [reference needed]. Figure 3The XRD diffraction peaks of the three samples in Examples 1-3 are consistent with the previously reported diffraction peaks of NFPP. All three samples belong to the orthorhombic crystal system, space group Pn21a. The absence of any impurity diffraction peaks confirms the high phase purity of the three samples. Figure 3 No typical diffraction peaks of crystalline carbon were observed in any of the samples, indicating that the carbon derived from the pyrolysis of glucose is amorphous carbon. The unit cell parameters of the three samples are shown in Table 1. It can be seen that the unit cell parameters of the three samples are similar, and are similar to the previously reported unit cell parameters of NFPP.

[0081] Table 1

[0082]

[0083] The structural characteristics of NFPP@C were further investigated using Fourier transform infrared spectroscopy (FT-IR). Figure 4a The Fourier transform infrared spectrum of NFPP@MC can be divided into three spectral regions. The region is 500-680 cm⁻¹. -1 The multiple absorption peaks appearing within this range are attributed to the OPO bending vibration of the PO4 group, located at 680-970 cm⁻¹. -1 The absorption peaks are attributed to the POP stretching vibration peaks of the P2O7 group, among which the absorption peak located in the high-frequency region (954 cm⁻¹) is particularly prominent. -1 903cm -1 and 887cm -1 This belongs to asymmetric stretching vibration, and the absorption peak is located at a low frequency (737 cm⁻¹). -1 This is a symmetrical stretching vibration. It is located at 970-1250 cm⁻¹. -1 The peak is the PO stretching vibration of the PO4 group. Figure 4b The Fourier transform infrared spectra of the three samples are also displayed. It can be seen that the vibrational spectra of the three samples are similar, indicating that the PO chemical environment of the three samples is the same.

[0084] Test Example 2

[0085] This test example performs electrochemical tests on the lithium-ion batteries obtained in the above embodiments. Wherein:

[0086] CV tests were performed on a PGSTAT 302N electrochemical workstation.

[0087] EIS testing was performed using a PGSTAT 302N electrochemical workstation, with the EIS detection frequency ranging from 100 kHz to 10 Hz.

[0088] Capacity, rate capability, and cycle performance were tested using a CT2001A battery tester, with a voltage range of 2.0 to 4.3V (vs. Li / Li). + Before testing, the battery should be left to stand for 5 hours.

[0089] The results are as follows:

[0090] The electrochemical behavior of NFPP@LC, NFPP@MC, and NFPP@HC samples in LiPF6 electrolyte was investigated using CV and galvanostatic charge-discharge tests. Lithium metal was used as the counter electrode, and the CV scan rate was 0.1 mV / s. -1 The voltage range is 2.0-4.3V (vs. Li / Li). + The result is:

[0091] The CV plots for the first lap of the three samples are shown below. Figure 5a Their anodic curves are similar to the CV behavior of NFPP in Na electrolyte, with two oxidation peaks at 3.30 V and 3.47 V indicating that sodium is extracted from different crystal sites. The two reduction peaks are located at 3.24 V and 3.30 V, respectively, with a voltage interval (0.06 V) smaller than that of NFPP in Na electrolyte, possibly due to the presence of Li. + This is caused by insertion into the NFPP lattice. Clearly, the redox peaks of NFPP@MC are the highest, indicating that this material has the fastest electrode process kinetics.

[0092] Figure 5b The image shows the CV curves for the first four cycles of NFPP@MC. Clearly, the CV shape of the second cycle is significantly different from the first. The voltage interval between the two oxide peaks in the second cycle (0.05V) is much smaller than that in the first cycle (0.17V). This is mainly due to the Na+ oxidation. + The desorption reaction transforms it into Li + This is caused by the extraction reaction. The reduction peak at 3.24V gradually disappears, while the position of the reduction peak at 3.30V remains unchanged. Furthermore, a pair of new redox peaks appear in the low-voltage region, indicating a minor structural rearrangement in the NFPP crystal. By the third cycle, the two oxidation peaks in the high-voltage region merge into one oxidation peak, the reduction peak at 3.24V disappears, and a weak reduction peak appears at 3.1V. The CV curve of the fourth cycle coincides with that of the third cycle. From... Figure 5b It can be seen that the anodic curve of NFPP@MC changes significantly, mainly due to Na + The desorption reaction transforms it into Li + This is caused by the extraction reaction. The slight changes in the NFPP@MC cathode curve indicate that Na... + The number of inserted NFPPs gradually decreases. Furthermore, the redox peak is lowest in the first round of CV because the first positive scan is mainly Na. + Extracted, negative scan showed some Na + Insertion. Due to Na + The radius is larger, and the migration rate is smaller than that of Li. + Therefore, the redox peak is lowest in the first CV cycle. With the increase of Li... +As the deintercalation / intercalation reaction gradually becomes dominant, the overall electrode kinetics accelerate, resulting in a gradual increase in peak current.

[0093] Figure 5c The CV plots for the third cycle of the three samples clearly show similar electrochemical behaviors at the three electrodes, which differ from the behavior of NFPP in Na electrolyte, but are similar to the behavior of electrochemically desodium-reduced NFPP in Li electrolyte. This indicates that the anodic and cathodic behaviors of the three samples in the third cycle are mainly similar to those of Li. + It is related to the de-intercalation / intercalation reaction.

[0094] Figure 5d The results show that the NFPP@LC, NFPP@MC, and NFPP@HC cathodes are at 0.2C (1C = 129 mA g). -1 Initial charge-discharge curves at 2.0-4.3V rates. The initial discharge capacity of both NFPP@LC and NFPP@MC is higher than the charge capacity, which may be due to some Na+. + This was caused by the material being extracted from the NFPP lattice and entering the electrolyte during the 5-hour settling period.

[0095] Figure 5e The charge / discharge curves of the first four cycles of NFPP@MC are shown, and the changing trend of the charge / discharge curves in the first four cycles is visible. Figure 5b The CV curves for the first four cycles show a consistent trend. The average voltage during the first charging process is higher than the average voltage during the subsequent charging processes. These CV results confirm that the first charge is primarily based on Na. + The deactivation mechanism, while the subsequent charging is mainly based on Li. + Extraction mechanism. Due to Li + The radius is much smaller than Na + radius, Li + The activation barrier for extraction is lower than that for Na. + The activation barrier that is removed leads to Li + The charging voltage decreases during the discharge process. Simultaneously, in the low-voltage region, the initial discharge voltage is lower than that of subsequent discharges. The above CV results confirm that during the first discharge, some Na... + It was inserted into the NFPP crystal framework. + The activation barrier required for insertion is high, resulting in a low discharge voltage. Subsequent discharges show a gradual increase in discharge voltage and capacity, indicating that Na... + The insertion amount gradually decreases, while Li + The insertion amount gradually increases. Li + It is easier to insert / extract from the NFPP crystal framework, therefore the charging voltage gradually decreases while the discharging voltage gradually increases. Due to Li + It is easier to insert / extract from the NFPP crystal framework, thus the subsequent charge and discharge capacity gradually increases.

[0096] Figure 5f The charge-discharge curves for the third cycle of the three samples are shown. Their behavior differs from that of NFPP in sodium electrolytes, but is very similar to that of desodium-treated NFPP in lithium electrolytes. The discharge capacities for the third cycle of NFPP@LC, NFPP@MC, and NFPP@HC are 113.0, 114.7, and 105.0 mAh g, respectively. -1 The coulombic efficiency of all three samples reached 100%.

[0097] Figure 6a The rate performance graphs of the three samples at room temperature are shown. The discharge specific capacities of NFPP@MC at 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 30, and 50C are 118.1, 116.6, 114.8, 112.9, 111.6, 106.7, 101.6, 87.4, 65.7, and 27.1 mAh g⁻¹, respectively. -1 The specific capacities of NFPP@LC and NFPP@HC at the same discharge rate are 117.6, 115.7, 113.2, 110.2, 107.4, 102.3, 93.9, 69.8, 39.4, and 12.2 mAh g, respectively. -1 and 114.2, 106.4, 101.0, 96.5, 92.3, 85.4, 77.9, 59.5, 36.0, 10.1 mAh g -1 Clearly, all three samples exhibited excellent rate performance, especially NFPP@MC. Notably, after cycling at an ultra-high current density (50C), when the current was reduced to 0.1C, the discharge capacities of NFPP@LC, NFPP@MC, and NFPP@HC were 114.8, 117.0, and 109.8 mAh g, respectively. -1 This indicates that NFPP can withstand Li + This involves charging and discharging at high current densities for charge carriers.

[0098] Figure 6bThe initial charge-discharge curves of NFPP@MC at rates of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 30, and 50C are shown. With increasing current density, polarization increases and capacity decreases. This indicates that the capacity reduction of NFPP at high current densities is due to increased polarization, not structural damage. Therefore, when the current density is reduced, the capacity of the electrode cycled at high current densities will recover. It is well known that phosphate materials are insulators with very low electronic conductivity, and the amorphous carbon formed by the pyrolysis of glucose at 500℃ also has low electronic conductivity. Therefore, the overall electronic conductivity of the NFPP@C electrode synthesized in this paper remains low. Adding highly conductive materials such as graphene to NFPP will significantly reduce NFPP polarization, thereby further improving the rate performance of NFPP.

[0099] Figure 6c The cycling performance of NFPP@MC at 0.2C is demonstrated. Clearly, the charge / discharge capacity gradually increases in the first few cycles. This is mainly due to Na... + The insertion / extraction amount gradually decreases, while Li + This is due to the gradual increase in insertion / extraction. The initial discharge capacity of NFPP@MC is 111.6 mAh g. -1 The discharge capacity gradually increases thereafter, reaching a maximum of 116.4 mAhg on the 15th cycle. -1 Subsequently, the capacity gradually decreases, but after 100 cycles, the discharge specific capacity can be maintained at 110.9 mAh g. -1 The capacity retention rate was 99.4% from the first lap to 100 laps, demonstrating the excellent cycling stability of NFPP@MC.

[0100] The cycling performance of the three electrodes at 5C is as follows: Figure 6d As shown, the initial discharge specific capacities of the NFPP@LC, NFPP@MC, and NFPP@HC electrodes are 102.4, 106.5, and 93.8 mAh g, respectively. -1 After 500 cycles, the discharge specific capacity reached 97.9, 99.0, and 73.8 mAh g, respectively. -1 The capacity decay rates per revolution were 0.009%, 0.014%, and 0.042%, respectively.

[0101] Figure 6e The cycling performance of the three electrodes at 20C was demonstrated, with initial discharge specific capacities of 80.7, 88.4, and 55.9 mAh g for the NFPP@LC, NFPP@MC, and NFPP@HC electrodes, respectively. -1 After 1000 cycles, the discharge specific capacities were 52.2, 74.8, and 27.8 mAh g, respectively. -1The capacity decay rates per cycle were 0.035%, 0.015%, and 0.050%, respectively. This shows that the three samples still exhibited excellent cycle stability at high current densities, indicating that NFPP@C can withstand long-range charge-discharge cycles at high current densities. Notably, the specific capacity of NFPP@MC reached its maximum at cycles 15, 21, and 35, respectively, when the current densities were 0.2, 5, and 20C. This is because at low current densities, more Na+ is released during each cycle. + Extracted from NFPP@MC. Therefore, at low current densities, Na + Li was quickly + replace.

[0102] Figure 7a The EIS curves for a three-electrode circuit without cycling (fresh) and a three-electrode circuit cyclicated 100 times at 0.2C are shown. It can be seen that all Nyquist plots consist of a semicircle in the high-frequency region and a sloping line in the low-frequency region; the semicircle and the sloping line are related to the charge transfer impedance and Warburg impedance, respectively. The corresponding equivalent circuit simulation diagram is shown below. Figure 7a , where R s and R ct Z represents electrolyte resistance and charge transfer resistance, respectively. w This indicates a Warburg resistor, and CPE represents an electric double-layer capacitor.

[0103] R obtained based on equivalent circuit fitting ct Values ​​such as Figure 7b As shown, the charge transfer impedance of a fresh NFPP@MC electrode is the lowest (236.5 Ω). Due to the formation of the SEI film, the R of a typically cycled electrode... ct It is much larger than the R of the fresh electrode. ct The charge transfer impedance of the NFPP three-electrode system decreased significantly after 100 cycles. This is because the main substance migrating from the fresh electrode during EIS testing is Na. + After 100 cycles, the electrode exhibited the following migration pattern during EIS testing: primarily Li. + Li + The radius is smaller than Na + radius, Li + Than Na + The migration rate is faster in the NFPP framework, so the R of the three electrodes after 100 cycles is higher. ct reduce. Figure 7b The main indirect explanation of the late insertion / extraction NFPP framework structure is Li + This further demonstrates that the structure of NFPP remains intact after long-range cycling. The charge transfer impedance of the NFPP@MC electrode after cycling remains the lowest (R0). ct =105.7Ω).

[0104] This invention successfully prepared carbon-coated NFPP@C microspheres (Na4Fe3(PO4)2(P2O7)@C) using inexpensive Na and Fe raw materials as starting materials via a simple and scalable spray drying method. This is the first time NFPP@C has been directly used as a cathode material in lithium-ion batteries. In lithium batteries with LiPF6 electrolyte and Li metal anode, the initial anodic scan behavior of NFPP@C was primarily based on Na… + The extraction reaction, and the behavior of the first cathode scan is mainly based on Li + Insertion reaction. As the CV cycle proceeds, Na... + The insertion / extraction response gradually weakens, while Li + The insertion / extraction reaction gradually becomes dominant, leading to a gradual decrease in the average charging voltage and a gradual increase in the average discharging voltage in subsequent cycles. Simultaneously, the R of the cycled NFPP@C electrode... ct The value is less than the R of a fresh NFPP@C electrode. ct Value, indicating Li + The migration rate in NFPP@C crystals is faster than that in Na. + Li + Repeated de-intercalation / extraction did not disrupt the crystal structure of NFPP@C. With the development of Li... + The insertion / extraction reaction gradually becomes dominant, and the electrode process kinetics become faster. Therefore, NFPP@C exhibits high capacity, high rate performance, and excellent cycle stability as a lithium storage material. Specifically, the optimized NFPP@MC achieves a discharge specific capacity of 118.1 mAh g⁻¹ at 0.1C. -1 Furthermore, after 1000 cycles at an ultra-high rate of 20C, the discharge specific capacity still reaches 74.8 mAh g. -1 The initial reversible capacity at 20C reached 74.9% of the initial reversible capacity at 0.1C. The availability of inexpensive raw materials, a simple production process, and excellent lithium storage performance indicate that NFPP is a very promising cathode material for lithium-ion batteries.

[0105] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0106] This invention belongs to the field of lithium-ion batteries and discloses a Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, a cathode, and a battery. The Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material is prepared by spray drying and sintering under a protective atmosphere after sequentially dissolving an iron source compound, a phosphorus source compound, a sodium source compound, and an organic small-molecule carbon source in water. This invention is the first to directly use Na4Fe3(PO4)2(P2O7)@C as a lithium-ion battery cathode material.

Claims

1. A Na4Fe3(PO4)2(P2O7)@C lithium-ion battery cathode material, characterized in that, The Na4Fe3(PO4)2(P2O7)@C lithium ion battery positive electrode material is a carbon-coated NFPP@C microsphere prepared by sequentially dissolving an iron source compound, a phosphorus source compound, a sodium source compound and an organic small molecule carbon source in water, and then performing spray drying and sintering in a protective atmosphere. The organic small molecule carbon source is one or more of glucose, sucrose, citric acid, fructose, benzoic acid, ascorbic acid and malic acid. The organic small molecule carbon source accounts for 8-30% of the mass of Na4Fe3(PO4)2(P2O7). 2.The Na4Fe3(PO4)2(P2O7)@C lithium ion battery positive electrode material according to claim 1, wherein The sodium source compound is one or more of sodium hydroxide, sodium acetate, sodium chloride, sodium nitrate, sodium carbonate, sodium formate and sodium dihydrogen phosphate. The phosphorus source compound is one or more of sodium dihydrogen phosphate, phosphoric acid, metaphosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and triammonium phosphate. The iron source compound is one or more of ferric nitrate, ferric chloride, ferric citrate and ferric ammonium citrate.

3. The Na4Fe3(P04)2(P207)@C lithium-ion battery cathode material of claim 1, wherein, After the mixed solution in which the iron source compound, the phosphorus source compound, the sodium source compound and the organic small molecule carbon source are sequentially dissolved in water is subjected to the spray drying treatment, a powder precursor is obtained; the total concentration of sodium ions in the mixed solution is 1-2 mol / L. The temperature of the spray drying is 120-180 ℃, the frequency of the fan is 30-50 Hz, and the flow rate of the mixed solution is 10-50 mL / min.

4. The Na4Fe3(P04)2(P207)@C lithium-ion battery cathode material of claim 1, wherein, The sintering temperature is 450-550 ℃, and the sintering time is 5-15 h. The protective atmosphere is a mixture of argon and hydrogen, and the hydrogen accounts for 5%-10% of the total volume of the mixture.

5. A lithium-ion battery cathode, characterized by, The lithium ion battery positive electrode is prepared by coating a slurry formed by uniformly mixing a positive electrode active material, a conductive agent and a binder on a current collector, and then performing a drying treatment. The positive electrode active material is the Na4Fe3(PO4)2(P2O7)@C lithium ion battery positive electrode material according to any one of claims 1-4.

6. The lithium-ion battery cathode of claim 5, wherein, The mass ratio of the positive electrode active material, the conductive agent and the binder is (70-80):(10-20):(5-15). 7.The lithium ion battery positive electrode according to claim 5, wherein The conductive agent is at least one of acetylene black, Super P, VGCF, carbon nanotubes, carbon nanofibers, graphene and graphite. The binder is at least one of polyvinylidene fluoride, polytetrafluoroethylene and polyacrylic acid. The temperature of the drying treatment is 95-105 ℃, and the time is 10-15 h.

8. A lithium-ion battery, characterized by The lithium ion battery positive electrode is the positive electrode according to any one of claims 5-7. 9.The lithium ion battery according to claim 8, wherein The negative electrode of the lithium ion battery is a lithium sheet. The electrolyte of the lithium ion battery is prepared by mixing a lithium-containing compound and an organic solvent; the concentration of the lithium-containing compound in the electrolyte is 0.8-1.2 M, the lithium-containing compound is LiPF6; and the organic solvent is a mixed solvent of EC and DMC, and the volume ratio of the EC and DMC is (0.8-1.2):(0.8-1.2).

Images