Polyanionic material for sodium ion battery

Abstract

The invention discloses a polyanionic material for a sodium ion battery. The chemical general formula of the polyanionic material is Na4VMn (1-x) Yx (PO4) 3, wherein Y is Ni or Zn, and x is more than 0 and less than or equal to 0.4. According to the invention, the rate capability, long-term circulation capability and low-temperature kinetics of the low-vanadium polyanion cathode in the sodium-ion battery are greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly relates to a polyanionic material for sodium-ion batteries. Background Art

[0002] Compared with lithium, sodium resources are more abundant and the cost is lower. Therefore, sodium-ion batteries have become one of the research hotspots and industrial development directions in recent years. However, at present, a mature industrial chain for sodium-ion batteries has not been formed, and there are still some deficiencies in the positive electrode material, negative electrode material, electrolyte, etc., which greatly limit their wide application. Developing high-performance positive electrode materials is regarded as one of the important breakthroughs to rapidly promote their commercialization process.

[0003] At present, the positive electrode materials for sodium-ion batteries mainly include layered oxides, polyanions, Prussian blue / white, and organic positive electrodes, etc. Among them, polyanion positive electrode materials have the advantages of excellent structural stability, electrochemical stability, and abundant ion diffusion channels, etc., which endow sodium-ion batteries with higher safety characteristics and will show great application prospects in the fields of large-scale energy storage and low-speed vehicles. Taking phosphate as an example, it contains special tetrahedral PO 4 units with strong covalent bonds, resulting in the relative isolation of valence electrons from polyanions. This special three-dimensional (3D) stereoscopic structure is very conducive to the insertion and extraction behavior of sodium ions because the smaller energy orbitals transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), usually accompanied by a multi-electron mechanism. Therefore, understanding the unique electronic structure is a good way to develop practical polyanionic electrode materials for sodium-ion batteries. At present, the most representative polyanionic electrode materials are Na 3 V 2 (PO 4 ) 3 and NaTi 2 (PO 4 ) 2 for sodium-based cathode and anode materials, respectively. Both show excellent electrochemical performance and broad prospects in the development and application of sodium-ion batteries. The recently emerged carbon phosphate Na 3 MnCO 3 PO 4 and amorphous FePO4 also contribute to further expanding the research scope of polyanionic sodium-ion batteries. However, the typical low conductivity and relatively low capacity performance of the above materials still limit their development. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a polyanionic material for sodium-ion batteries.

[0005] The present invention discloses a polyanionic material for sodium-ion batteries. The chemical general formula of the polyanionic material is: Na 4 VMn 1-x Y x (PO4) 3 ; wherein, Y is Ni or Zn, and 0 < x ≤ 0.4.

[0006] As a further improvement of the present invention, a carbon-coated layer is formed on the surface of the polyanionic material. The carbon-coated layer is obtained by carbonizing sucrose and citric acid, and its chemical general formula is: Na 4 VMn 1-x Y x (PO4) 3 @C.

[0007] As a further improvement of the present invention, the chemical general formula of the polyanionic material is: Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C.

[0008] As a further improvement of the present invention, the chemical general formula of the polyanionic material is: Na 4 VMn 0.8 Zn 0.2 (PO4) 3 @C.

[0009] As a further improvement of the present invention, the polyanionic material is prepared by a chelating agent-assisted sol-gel method.

[0010] As a further improvement of the present invention, the specific preparation method includes:

[0011] Adding a sodium source, a vanadium source, a manganese source, a nickel source / zinc source, a phosphorus source, and a gelling agent / chelating agent into an aqueous solvent, and stirring evenly to obtain a sol;

[0012] Placing the sol in an open container and performing microwave radiation to obtain a gel or semi-gel;

[0013] Performing vacuum freeze-drying on the gel or semi-gel to obtain a xerogel;

[0014] First grinding the xerogel, and then performing calcination treatment under a protective atmosphere to obtain a sodium vanadium phosphate cathode material for sodium batteries.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] The present invention greatly improves the rate capability, long-term cycling ability, and low-temperature kinetics of the low-vanadium polyanionic cathode in sodium-ion batteries. Brief Description of the Drawings

[0017] Figure 1 The crystal structure and phase composition of the polyanionic material for sodium-ion batteries disclosed in the present invention.

[0018] Figure 2 The SEM morphology of the polyanionic material for sodium-ion batteries disclosed in the present invention;

[0019] Figure 3 The electrochemical performance of the polyanionic material for sodium-ion batteries disclosed in the present invention;

[0020] Figure 4 The XRD and XPS diagrams of the polyanionic material for sodium-ion batteries disclosed in the present invention;

[0021] Figure 5 The low-temperature performance of the polyanionic material for sodium-ion batteries disclosed in the present invention. Detailed implementation manners

[0022] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] The following further describes the present invention in detail with reference to the accompanying drawings:

[0024] Existing Na 4 VMn(PO4) 3 materials have broad application prospects in high-energy-density SIBs (theoretically about 390 Wh kg-1). When assembled with a hard carbon anode in a pouch cell, the estimated energy density can reach ≈200 Wh kg-1. However, the Jahn-Teller effect caused by the presence of Mn 3+ may lead to serious structural collapse, resulting in a decrease in the capacity of SIBs. Therefore, introducing individual manganese has its advantages and disadvantages, and further regulation is needed to solve this problem. To improve the electrochemical performance of V-Mn polyanionic materials, the present invention introduces additional heteroatoms through bimetallic doping, utilizing the synergistic effect of bimetallic substitution, that is, promoting metal properties and conductivity, and further stabilizing the structure.

[0025] Specifically, the present invention provides a polyanionic material for sodium-ion batteries, and its chemical general formula is Na 4 VMn 0.7 Ni 0.3 (PO4) 3@C. The polyanionic material is prepared by a chelating agent-assisted sol-gel method. The specific preparation method includes:

[0026] 1) Adding a sodium source, a vanadium source, a manganese source, a nickel source / zinc source, a phosphorus source, and a gelling agent / chelating agent to an aqueous solvent, stirring evenly to obtain a sol;

[0027] 2) Placing the sol in an open container and performing microwave radiation to obtain a gel or semi-gel;

[0028] 3) Performing vacuum freeze-drying on the gel or semi-gel to obtain a dry gel;

[0029] 4) First grinding the dry gel, and then performing calcination treatment under a protective atmosphere to obtain a sodium vanadium phosphate cathode material for sodium batteries.

[0030] Among them, the sodium source includes at least one of sodium dihydrogen phosphate, sodium acetate dihydrate, sodium hydroxide, sodium carbonate, and sodium phosphate; the vanadium source includes at least one of vanadium pentoxide, ammonium metavanadate, and vanadium trioxide; the phosphorus source includes at least one of ammonium phosphate, diammonium phosphate, ammonium dihydrogen phosphate, phosphoric acid, and sodium dihydrogen phosphate; the gelling agent / chelating agent includes at least one of citric acid, ascorbic acid, oxalic acid, and gluconic acid.

[0031] On the one hand, the synthesis route of the present invention can reduce the gelation time and improve the uniformity of the gel, so as to easily obtain a particle size distribution with uniform and controllable size; on the other hand, it can prepare a porous structure material, which is beneficial to increasing the contact area between the electrode material and the electrolyte, increasing the active sites of sodium ions, and shortening the diffusion path of sodium ions when in use, thereby improving its rate performance and cycling performance.

[0032] In addition to the increased rate capability at 25 °C (67 mA h g -1 at 3Ag -l ) and the extended cycle life (74.6% of 8000 cycles at 2Ag -1 ), this polyanionic material also exhibits a high discharge capacity at -40 °C (82 mAh g -1 at 20 mAg -2 ), excellent rate performance (60 mAh g -1 at 400 mAg -3 ) and excellent cycling performance (capacity retention rate of 90.4% after 500 cycles at 100 mAg -3 ), demonstrating enhanced low-temperature kinetics. Density functional theory (DFT) calculations reveal that introducing bimetals into sodium vanadium phosphate can effectively reduce the band gap, bring stronger metallic properties, and accelerate Na + / e -Transfer and form a robust structure.

[0033] Perform X-ray diffraction (XRD) to identify the crystal structure and phase composition of the synthesized vanadium-based polyanion material, as Figure 1 shown. Among them, as Figure 1 shown in a, the prepared Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C can be incorporated into the trigonal phase with the R-3c space group, and the exact parameters are and Figure 1 Figure b shows the crystal structure model of Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C. In this 3D open framework, the interconnected Na+ channels are constructed by sharing all corners of the island-like Ni / Mn / VO6 octahedra with the PO4 tetrahedra. Measure X-ray photoelectron spectroscopy (XPS) to study the valence states of the elements contained in the prepared materials. Mn 2p and Ni 2p peaks are detected in the full XPS spectrum ( Figure 1 c), indicating that Mn and Ni ions are favorably introduced into the Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C material. As Figure 1 shown in d, the peaks at 516.79 and 523.6 eV are attributed to V 3+ 's V 2p 3 / 2 and V 2p 1 / 2 . In the Mn 2p spectrum ( Figure 1 e), the peaks at 641.6 and 653.9 eV are pointed to Mn 2+ 's Mn 2p 3 / 2 and Mn 2p 1 / 2 , and there is also a satellite peak (646.7 eV). In addition, the peaks at 873.8 and 856.1 eV correspond to Ni 2+ and a pair of satellite peaks (860.9 and 879.9 eV), indicating the successful incorporation of nickel ( Figure 1 f). The inductively coupled plasma (ICP) results show that the content ratio of V:Mn:Ni in NVMNP is 1:0.7:0.3. The valence states of NMVP@C and NVP@C have been detected. All Na / Mn / V / P / O / C elements are present in the XPS full spectrum of NMVP@C (Figure S2a, Supporting Information), and Mn 2+ / Mn 3+ coexist in the Mn2p narrow spectrum due to surface oxidation.

[0034] The morphology of vanadium-based polyanions was observed using a scanning electron microscope (SEM), as Figure 2 shown; among them, in Figure 2 a, b, the overall NVMNP@C particles are oval-shaped, with a size range of 100 - 300 nm, and the surface carbon coating obtained by carbonization of sucrose and citric acid can be clearly observed. SEM observation of NVMP@C shows similarities with NVMNP@C, indicating that the doping of nickel has little effect on the morphology of this material. However, as prepared, NVP@C is slightly different, showing an aggregated overall appearance and individual particle sizes less than 100 nm. Transmission electron microscopy (TEM) was used to further investigate NVMNP@C. As Figure 2 shown in c, d, particles (100 - 300 nm) were clearly observed in NVMNP@C, consistent with the SEM results. Through Figure 2 high-resolution transmission electron microscopy (HRTEM) in e, f, clear lattice spacings of 0.39 and 0.28 nm could be indexed to NVMNP@C, respectively, confirming the high crystallinity of the synthesized material. In addition, a thin carbon coating of ≈4 nm was detected ( Figure 2 f). Energy-dispersive X-ray spectroscopy results showed uniform distributions of Na, V, Mn, Ni, P, O, and C elements, confirming the presence of dual ions in the NVMNP@C cathode.

[0035] The assembled half-cells based on the prepared cathode and sodium metal anode were initially cycled at ambient temperature using a galvanostatic charge-discharge (GCD) technique. In the voltage range of 2.5 - 3.8 V, in addition to the reaction of the V 3+ / V 4+ pair, there is also a redox reaction of Mn 4 VMn 0.7 Ni 0.3 (PO4) 3 @C, which provides partial capacity. 2+ / Mn 3+ It can be concluded from a that the prepared Na Figure 3 VMn 4 Ni 0.7 (PO4) 0.3 @C electrode material exhibits the best electrochemical performance (excellent discharge capacity and rate performance). According to the comparison of the charge-discharge curves of all Na4VMnxNi1-xPO4@C (0.5 ≤ x ≤ 0.9) cathode materials, Na 3 VMn 4 Ni 0.7 (PO4) 0.3 @C shows the smallest polarization. In addition, three cathodes (Na 3 VMn 4 VMn 0.7Ni 0.3 (PO4) 3 @C, Na 4 VMn(PO 4 ) 3 @C and Na 3 V 2 (PO4) 3 @C) rate performance ( Figure 3 b). At a moderate rate of 0.05 Ag -1 the discharge capacity of the NVMNP@C cathode is 107 mA h g -1 and when the current density increases, the discharge capacities of the NVMNP@C cathode at 0.1, 0.2, 0.5, 1, 1.5, and 2 Ag -1 are 97, 92, 86, 81, 78, and 71 mA h g -1 respectively. When cycled at 3 Ag -1 the cathode can still maintain a high capacity of 67 mA h g -l indicating its excellent high-rate performance of the NVMNP@C cathode. The GCD curve of NVMNP@C as Figure 3 shown in c clearly shows two discharge plateaus at 3.4 / 3.6 V. Impedance and conductivity were also tested to further verify the excellent electrochemical performance of Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C. Na 4 VMn 0.7 Ni 0.3 (PO4) 3 @C shows the lowest impedance and resistivity, as well as the highest conductivity among all samples. Next, cyclic comparisons of NVMNP@C, NVMP@C, and NVP@C cathodes were performed. This NVMNP@C has an initial capacity of 88 mA h g -1 and still maintains 94.2% of its capacity after 200 cycles at 0.2 A g -l which is better than NVMP@C (72.9%) and NVPC@ (85%). In addition, the NVMNP@C cathode shows excellent cycling performance at 2 Ag-1 ( Figure 3 e), with an initial capacity of 72 mAh g -l and still maintains 74.6% of its capacity after 8000 cycles, with an average Coulombic efficiency (CE) of approximately 100%, which is better than NMVP@C (<60% after 1800 cycles) and NVP@C (70% after 1000 cycles). It should be noted that the GCD curve of NVMNP@C at 2 Ag -1Under different cycles (the 100th, 500th, 1000th, 2000th, 3000th, 6000th, and 8000th), the cathode shows similar profiles, confirming excellent reversibility. Even at a higher mass loading (1 mg cm -2 ), NVMNP@C maintains 70 mAh g -l at a high current density of 1 A g-1 during 500 cycles, also showing great rate capacity and excellent cycling performance.

[0036] Ex-situ XRD and XPS were measured to detect the NVMNP@C electrode within the working window of 2.5 - 3.8 V, as Figure 4 shown. Figure 4 a, b show the ex-situ XRD patterns from 10 - 60° and the enlarged patterns from 31 - 32.5°. During charging (from the open circuit voltage (OCV) to full charge at 3.8 V), the diffraction peaks of (104), (113), (221), (300), (314), and (226) shift to the right, corresponding to sodium extraction, and the phase at 3.8 V is consistent with sodium-deficient Na4-xVMn0.7Ni0.3(PO4)3@C. When discharging to 2.5 V, the migrated diffraction peaks fully recover to the sodium-rich origin state of the NVMNP@C phase. Ex-situ XPS was performed to verify the valence state evolution of V and Mn elements in various charge / discharge states. In Figure 4 c, d, the peak values of V 2p3 / 2 and Mn 2p1 / 2 at the origin of the NVMNP@C electrode match well with V3+ (517.0 eV) and Mn2+ (653.9 eV). When fully charged to 3.8 V, the value of V2p3 / 2 increases to 517.4 eV (V4+), and Mn 2p1 / 2 increases to 654.5 eV (Mn3+), indicating that the working platforms at 3.4 and 3.6 V are generated by V3+ / V4+ and Mn2+ / Mn3+. When discharging to 2.5 V, Mn 2p1 / 2 and V 2p3 / 2 transform back to the original state, i.e., the repetition of Mn2+ and V3+. In addition, the ex-situ XPS spectra of Ni 2p in different states are shown in Figure S13 for supporting information. When the battery is charged to 3.8 V and discharged to 2.5 V, the binding energies of Ni 2p3 / 2 and the satellite remain almost unchanged, further confirming that nickel does not participate in the electrochemical process. Therefore, the ex-situ XRD and XPS results further confirm NVMNP@C.

[0037] Based on the above advantages, the synthesized NVMNP@C cathode material also shows excellent low-temperature adaptability at -40 °C. The rate performance of NVMNP@C and NVMP@C Figure 5 a compares the samples (charging current density: 20 mA gauge pressure). This NVMNP@C cathode at 20 mA g-1 It can provide a high capacity of 82 mAh g⁻² and has a continuous and stable discharge capacity from 20 mA g –1 to 100 mA g –2 . In addition, at a high discharge current density of 400 mA g -1 , a cathode of 60 mAh g⁻¹ NVMNP@C (40 mA hg -1 ) can be achieved, further verifying the cathode of NVMNP@C at low temperatures. In addition, the NVMNP@C cathode also exhibits a high degree of reversibility. When it returns to 20 mAg -1 , it can provide 81 mAh g -2 . CV measurements were also carried out at -40 °C, which further indicates that NVMNP@C has fast low-temperature reaction kinetics. Figure 5 b shows the NVMNP@C from 20 to 400 mAg -1 . As low as -40 °C, NVMNP@C still maintains a low polarization (<0.1 V) at 20 mAg -1 ; even when discharging rapidly at 400 mA g⁻¹, the electrode can still maintain a working platform >3 V, which is superior to the NVMP@C cathode. Service life and cycle stability The cathode of NVMNP@C at low temperatures has been greatly improved, which can be verified from the cycling performance at different rates. When cycling at a current of 20 mA g -1 , the NVMNP@C cathode shows 82 mAh g -1 , and remains 81 mAh g -1 after 160 cycles, equivalent to a high retention rate of up to 98.2%. In comparison, NVMP@C shows a lower capacity of 59 mAh g -1 , and is still 91.5% after the same number of cycles ( Figure 5 c). When the discharge rate is further increased to 100 mAg⁻¹, after 200 cycles, NVMNP@C remains 72 mAh g -1 with a capacity of 94.9%, and its cycling performance is also superior to the cathode (72.1%) in Figure S16 of NVMP@C, supporting information. In addition, even when undergoing a rapid charge / discharge process (100 mA g⁻¹), NVMNP@C can still provide a high initial capacity of 73 mA h g -1 (CE: 90.5%) and remains 90.4% after 230 cycles, while the NVMP@C sample only provides 51 mA h g⁻¹ and rapidly decays to 60% of its capacity after the same number of cycles ( Figure 5 d). Figure 5The GCD curves in e show almost the same shape during the 1st, 10th, 50th, 100th, 200th, and 230th cycles, revealing NVMNP@C at a temperature of -40°C. In addition, the electrochemical impedance spectroscopy (EIS) at -40°C was also tested to illustrate the excellent low-temperature adaptability of the NVMNP@C cathode.

[0038] The present invention greatly improves the rate capability, long-term cycling ability, and low-temperature kinetics of low-vanadium polyanion cathodes in sodium-ion batteries; it exhibits excellent electrochemical performance at 25°C, with a delivery rate capacity of 67 mA h g -1 at 3 A g -l and a long-term cycling ability of 74.6% after 8000 cycles at 2 A g -1 At -40°C, it is 60 mAh g -1 at 400 mA g -2 and a capacity retention rate of 90.4% after 230 cycles at 100 mA g -1 . This unique work opens up a viable approach for high-performance SIB cathodes at low temperatures.

[0039] The present invention synthesized Na 4 VMn 0.8 Zn 0.2 (PO4) 3 @C by a citric acid-assisted sol-gel method. Characterizations using XRD, XPS, HRTEM, and Raman spectroscopy confirmed the high purity of these materials. Among them, Na 4 VMn 0.8 Zn 0.2 (PO4) 3 @C showed the best performance, with lower internal resistance, stronger diffusion kinetics, achieved a stable capacity of 72 mAh g-1 at a temperature of 10°C, a retention rate of 83% after 3000 cycles, and had a high rate tolerance at a temperature of 50°C. Operando XRD studies revealed a highly reversible two-phase charge storage mechanism, in-situ XPS analysis confirmed the charge storage mechanism of the cathode, while electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) analysis confirmed the reduced polarization of the NVMP cathode after doping with zinc. These research results indicate that Na 4 VMn 0.8 Zn 0.2 (PO4) 3 @C is a promising SIB cathode material.

[0040] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sodium ion battery polyanion material, characterized in that: The chemical formula of the polyanionic material is: Na4VMn 1-x Y x (PO4)3; wherein Y is Ni or Zn, 0<x≤0.

4.

2. The sodium ion battery polyanionic material according to claim 1, characterized in that: The surface of the polyanionic material is formed with a coated carbon layer, which is obtained by carbonizing sucrose and citric acid, and its chemical formula is: Na4VMn 1-x Y x (PO4)3@C.

3. The sodium ion battery polyanionic material according to claim 2, characterized in that: The chemical formula of the polyanionic material is: Na4VMn 0.7 Ni 0.3 (PO4)3@C.

4. The sodium ion battery polyanionic material according to claim 2, characterized in that: The chemical formula of the polyanionic material is: Na4VMn 0.8 Zn 0.2 (PO4)3@C.

5. The sodium ion battery polyanionic material according to claim 2, characterized in that: The polyanionic material is prepared by a chelating agent-assisted sol-gel method.

6. The sodium ion battery polyanionic material according to claim 5, characterized in that: The specific preparation method comprises: Adding a sodium source, a vanadium source, a manganese source, a nickel source / zinc source, a phosphorus source and a gelling agent / chelating agent into an aqueous solvent and stirring the mixture uniformly to obtain a sol; The sol is placed in an open container and subjected to microwave irradiation to obtain a gel or semi-gel; The gel or semi-gel is subjected to vacuum freeze drying to obtain a xerogel; The dry gel is firstly ground and then calcined under a protective atmosphere to obtain sodium vanadium phosphate positive electrode material for sodium battery.

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