Rare earth element doped polyanion compound and preparation method thereof

Abstract

The invention discloses a rare earth element doped polyanion compound and a preparation method thereof, and the preparation method comprises the following steps: dissolving 0.9-1.1 mmol of manganese acetate tetrahydrate, 2.9-3.1 mmol of ammonium dihydrogen phosphate and 2.9-3.1 mmol of sodium acetate in 20ml of deionized water to obtain a solution A; 0.9 mmol to 1.1 mmol of butyl titanate is dissolved in 10 ml of an ethanol solution, and a solution B is obtained; 0.005 mmol to 0.015 mmol of rare earth element oxide is dissolved in 20 ml of an ethanol solution, and a solution C is obtained; dropwise adding the solution B and the solution C into the solution A, uniformly mixing to obtain a solution D, and immediately sealing; heating the solution D to 75-85 DEG C, keeping the temperature for 0.8-1.5 hours, stirring in an open manner until water is evaporated to dryness to obtain faint yellow gel, and then putting the faint yellow gel into a drying box of 75-85 DEG C to be subjected to vacuum drying for 0.8-1.2 hours to obtain a precursor; and grinding the precursor, sintering in a microwave tube furnace, cooling, and uniformly grinding to obtain the rare earth element doped polyanion compound. The electrochemical performance of the polyanion compound can be effectively improved, and the prepared rare earth element doped polyanion compound is stable in structure.

Description

Technical Field

[0001] The invention belongs to the technical field of new energy materials and relates to a rare earth element doped polyanion compound and a preparation method thereof. Background Art

[0002] With the rapid development of new energy technologies, sodium-ion batteries, as a potential energy storage device, have received extensive attention and research. Polyanionic compounds are widely used in energy storage devices such as sodium-ion batteries due to their advantages such as open structural framework, large gap channels and high ion mobility. Among them, NASICON-type sodium titanium phosphate materials (NTP) derived from polyanion systems are considered to be a promising sodium-ion battery active material due to their high theoretical capacity, good thermal stability, low cost, environmental friendliness and good tolerance to overcharge and overdischarge.

[0003] However, sodium titanium phosphate materials have low electronic conductivity, which limits their further application in the field of sodium ion batteries to a certain extent. Although the sodium manganese titanium phosphate material (NMTP) synthesized based on sodium titanium phosphate has overcome the disadvantage of low electronic conductivity of polyanion system materials to a certain extent, since the radius of manganese ions is smaller than that of sodium ions, manganese ions are easy to occupy the active sites of sodium, resulting in sodium-manganese mixing, which is not conducive to the continuous transmission of electrons inside the solid phase and affects the electrochemical performance. At the same time, polyanion compounds are rigid inorganic compounds. When sodium ions are embedded and extracted, they are affected by manganese ions and the material structure is prone to irreversible phase changes, resulting in poor long-cycle performance of the battery and reduced capacity.

[0004] Therefore, how to optimize the structure of polyanion compounds and improve their electrochemical properties to meet the demand of sodium ion batteries for high-performance positive electrode materials is the research focus of technicians in this field. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a rare earth element doped polyanion compound and a preparation method thereof. The present invention can effectively improve the electrochemical properties of the polyanion compound, and the prepared rare earth element doped polyanion compound has a stable structure, so as to meet the demand of sodium ion batteries for high-performance positive electrode materials.

[0006] The technical solution of the present invention is achieved in this way: A method for preparing a rare earth element-doped polyanion compound comprises the following steps: (1) Dissolve 0.9-1.1 mmol manganese acetate tetrahydrate, 2.9-3.1 mmol ammonium dihydrogen phosphate and 2.9-3.1 mmol sodium acetate in 20 ml of deionized water to obtain solution A; dissolve 0.9-1.1 mmol butyl titanate in 10 ml of ethanol solution to obtain solution B; dissolve 0.005-0.015 mmol rare earth element oxide in 20 ml of ethanol solution to obtain solution C; (2) Add solution B and solution C dropwise into solution A, mix well to obtain solution D, and seal immediately; (3) Heat solution D to 75-85°C, keep warm for 0.5-1.5 h, then stir in the open air until the water evaporates to obtain a light yellow gel, and then place in a drying oven at 75-85°C and vacuum dry for 10-15 h to obtain a precursor; (4) Grinding the precursor and placing it in a microwave tube furnace, then heating it to 600-700° C. at a heating rate of 3-5° C. / min under an inert atmosphere, keeping the temperature for 0.8-1.2 h, and grinding it evenly after cooling to obtain the rare earth element-doped polyanion compound.

[0007] The present invention dopes a trace amount of rare earth elements into the sodium manganese titanium phosphate material. The rare earth elements can replace part of the manganese element in the oxygen manganese octahedron, thereby avoiding the phenomenon of mixed arrangement of sodium ions and manganese ions due to similar radii, and further avoiding the problem of unsmooth sodium ion escape / embedding.

[0008] Studies have shown that the heating rate also affects the material structure. When the heating rate is too fast (≥10℃ / min), defects will occur in the crystal, thus affecting the structural properties of the material, so the heating rate needs to be controlled. The rare earth element doped polyanion compound prepared by microwave heating at 600~700℃ has a better effect and uniform particles. In particular, when microwave heating is performed at 650℃, the rare earth element doped polyanion compound has the best morphology effect.

[0009] Furthermore, the rare earth element oxide is yttrium oxide or scandium oxide.

[0010] Here, the two oxides of scandium and yttrium have a good effect on improving the battery performance, and the ionic radius of yttrium and scandium is slightly larger than that of manganese ions, so it is not easy for sodium ions to be mixed after substitution.

[0011] Furthermore, during vacuum drying, the vacuum degree is 0.005~0.01MPa.

[0012] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention uses yttrium oxide (Y 2 O 3 ) and scandium oxide (Sc 2 O3 ) as rare earth element oxides, Y in yttrium oxide and scandium oxide 3+ and Sc 3+ The ions have a stable electronic configuration and a stable electronic structure; at the same time, both yttrium oxide and scandium oxide have good physical and chemical stability. For example, yttrium oxide is insoluble in water and alkali, soluble in acid, and has high thermal stability, corrosion resistance and high thermal conductivity. These properties allow yttrium oxide to remain stable under extreme conditions such as high temperature and high pressure, thus helping to improve the electrochemical properties of doped polyanion compounds, especially the rate performance and cycle stability of sodium ion batteries.

[0013] 2. The rare earth element doped polyanion compound Na prepared by the present invention n RMnTi(PO 4 ) z , where R is a rare earth element Sc or Y. Octahedral MO 6 (M is transition metal Mn and Ti) and RO 6 Together with the tetrahedral anionic group (PO 4 ) 3- By connecting in a corner / edge sharing manner, a stable and open framework structure is formed for sodium ions to embed and escape. During the charging process, the oxidation migration of divalent manganese ions with a smaller radius to a thermodynamically stable state is avoided, thereby stabilizing the voltage of the charge and discharge curve and maintaining the capacity release of the material in the effective voltage window. At the same time, the presence of trivalent manganese ions is suppressed, the Jahn-Teller effect is slowed down, and the structural stability and kinetic properties of rare earth element-doped polyanion compounds are improved.

[0014] 3. The present invention adopts the sol-gel method to synthesize the polyanionic compound precursor. The raw materials and reagents used are cheap, the synthesis method is non-toxic and harmless, simple and efficient, and the trace rare earth element doping is within the cost controllable range. At the same time, the rapidity, uniformity, selectivity and high efficiency and energy saving characteristics of microwave heating are utilized to achieve good environmental and energy benefits. In addition, the entire reaction process is carried out under the protection of an inert atmosphere, which can effectively reduce the oxidation loss of the material and further improve the yield of the target product. The yield of the present invention can reach 92%. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 -SEM image of Y-NMTP prepared in Example 1 at a magnification of 500 times.

[0016] Figure 2 -SEM image of Y-NMTP prepared in Example 2 magnified 500 times.

[0017] Figure 3 -SEM image of Y-NMTP prepared in Example 3 at 500 times magnification.

[0018] Figure 4 -SEM image of Sc-NMTP prepared in Example 4 at a magnification of 500 times.

[0019] Figure 5 -SEM image of NMTP prepared in Comparative Example 1 at a magnification of 500 times.

[0020] Figure 6 -SEM image of the NTP prepared in Comparative Example 2 at a magnification of 500 times.

[0021] Figure 7 -XRD patterns of Y-NMTP, Sc-NMTP, NMTP and NTP prepared in Example 1, Example 4, Comparative Example 1 and Comparative Example 2.

[0022] Figure 8 -Rate cycle spectra of Y-NMTP, Sc-NMTP and NMTP prepared in Example 1, Example 4 and Comparative Example 1.

[0023] Fig. 9 -Charge and discharge curves of Y-NMTP, Sc-NMTP and NMTP prepared in Example 1, Example 4 and Comparative Example 1.

[0024] Fig.10 -CV curves of Y-NMTP, Sc-NMTP and NMTP prepared in Example 1, Example 4 and Comparative Example 1.

[0025] Fig.11 -XRD patterns of NMTP, Y-NMTP-1 and Sc-NMTP-1 prepared in Comparative Example 1, Comparative Example 3 and Comparative Example 4.

[0026] Fig.12 -Long cycle diagrams of Y-NMTP-2 and Sc-NMTP-2 prepared in Comparative Examples 5 and 6. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0028] Example 1 A method for preparing a rare earth element-doped polyanion compound comprises the following steps: (1) Dissolve 1 mmol of manganese acetate tetrahydrate, 3 mmol of ammonium dihydrogen phosphate, and 3 mmol of sodium acetate in 20 ml of deionized water, referred to as solution A; (2) Dissolve 1 mmol of butyl titanate in 10 ml of ethanol solution, referred to as solution B; (3) Dissolve 0.01 mmol of yttrium oxide in 20 ml of ethanol solution, referred to as solution C; (4) Add solution B and solution C gradually into solution A at a constant rate to ensure that they are fully mixed; (5) The mixed solution is sealed, heated to 80°C, kept warm for 1 hour, and then stirred in the open air until the water is evaporated to obtain a light yellow gel, which is then placed in a drying oven at 80°C for 12 hours to obtain a precursor; (6) Grind the precursor material in step 5, weigh an appropriate amount of powder and place it in a microwave tube furnace. Under the protection of an inert atmosphere, heat it to 650°C at a heating rate of 5°C / min, keep it warm for 1 hour, and grind it evenly after cooling to obtain yttrium-doped sodium manganese titanium phosphate Y-NMTP.

[0029] Example 2 This embodiment is the same as embodiment 1, except that the microwave heating temperature in step (6) of this embodiment is 600°C.

[0030] Example 3 This embodiment is the same as embodiment 1, except that the microwave heating temperature in step (6) of this embodiment is 700°C.

[0031] Example 4 This example is the same as Example 1, except that in step (3) of this example, 0.01 mmol of scandium oxide is used instead of yttrium oxide to obtain scandium-doped sodium manganese titanium phosphate Sc-NMTP.

[0032] Comparative Example 1 A method for preparing a polyanion compound comprises the following steps: (1) Dissolve 1 mmol of manganese acetate tetrahydrate, 3 mmol of ammonium dihydrogen phosphate, and 3 mmol of sodium acetate in 20 ml of deionized water, referred to as solution A; (2) Dissolve 1 mmol of butyl titanate in 10 ml of ethanol solution, referred to as solution B; (3) Add solution B to solution A gradually at a constant rate to ensure that the mixture is fully mixed. (4) The mixed solution is sealed, heated to 80°C, kept warm for 1 hour, and then stirred in the open air until the water is evaporated to obtain a light yellow gel, which is then placed in a drying oven at 80°C for 12 hours to obtain a precursor; (5) Grind the precursor material in step 4, weigh an appropriate amount of powder and place it in a microwave tube furnace. Under the protection of an inert atmosphere, heat it to 650°C at a heating rate of 5°C / min, keep it warm for 1 hour, and grind it evenly after cooling to obtain sodium manganese titanium phosphate NMTP.

[0033] Comparative Example 2 A method for preparing a polyanion compound comprises the following steps: (1) Dissolve 3 mmol of ammonium dihydrogen phosphate and 3 mmol of sodium acetate in 20 ml of deionized water, referred to as solution A; (2) Dissolve 1 mmol of butyl titanate in 10 ml of ethanol solution, referred to as solution B; (3) Add solution B to solution A gradually at a constant rate to ensure that the mixture is fully mixed. (4) The mixed solution is sealed, heated to 80°C, kept warm for 1 hour, and then stirred in the open air until the water is evaporated to obtain a light yellow gel, which is then placed in a drying oven at 80°C for 12 hours to obtain a precursor; (5) Grind the precursor material in step 4, weigh an appropriate amount of powder and place it in a microwave tube furnace. Under the protection of an inert atmosphere, heat it to 650°C at a heating rate of 5°C / min, keep it warm for 1 hour, and grind it evenly after cooling to obtain sodium titanium phosphate NTP.

[0034] Comparative Example 3 This example is the same as Example 1, except that in step (3) of this example, the amount of yttrium oxide is 0.02 mmol, and yttrium-doped sodium manganese titanium phosphate Y-NMTP-1 is obtained.

[0035] Comparative Example 4 This example is the same as Example 1, except that in step (3) of this example, the amount of scandium oxide is 0.02 mmol, and scandium-doped sodium manganese titanium phosphate Sc-NMTP-1 is obtained.

[0036] Comparative Example 5 This embodiment is the same as embodiment 1, except that the heating rate in step (6) of this embodiment is 10° C. / min, and yttrium-doped sodium manganese titanium phosphate Y-NMTP-2 is obtained.

[0037] Comparative Example 6 This embodiment is the same as embodiment 4, except that the heating rate in step (6) of this embodiment is 10° C. / min, and scandium-doped sodium manganese titanium phosphate Sc-NMTP-2 is obtained.

[0038] 1. The yields of yttrium-doped sodium manganese titanium phosphate Y-NMTP obtained in Examples 1 to 3 are 89%, 92%, and 90%, respectively (yield = actual mass of Y-NMTP / theoretical mass of Y-NMTP × 100%). The SEM images of Y-NMTP obtained in Examples 1 to 3, magnified 500 times, are as follows: Figure 1 , Figure 2 and Figure 3As shown in the figure, the Y-NMTP prepared by microwave heating at 650°C has the best effect, maintaining excellent morphology and no agglomeration. However, after high-temperature calcination, the low-melting-point substances remaining in the reaction, such as acetate, will be carbonized and then coated on the surface of the material to form a dense carbon layer, effectively improving the electronic conductivity of the polyanionic compound. The Y-NMTP prepared by microwave heating at 600°C is sintered more completely, and the grain distribution is also not significantly affected by the carbon coating shell; the Y-NMTP prepared by microwave heating at 700°C is sintered a little too much, which makes the low-melting-point impurities unevenly carbonized and agglomerates.

[0039] 2. The SEM image of scandium-doped sodium manganese titanium phosphate Sc-NMTP prepared in Example 4 is as follows: Figure 4 As shown, the morphology is excellent and the carbon coating is obvious.

[0040] 3. The SEM images of pure sodium manganese titanium phosphate NMTP and pure sodium titanium phosphate NTP obtained in Comparative Example 1 and Comparative Example 2 are as follows: Figure 5 and Figure 6 As shown in the figure, it can be seen that the prepared NMTP is also affected by the carbon coating shell, but some areas show a clear grain arrangement with a relatively uniform size. NTP also shows a relatively uniform grain distribution in some areas, and the crystal structure is more obvious.

[0041] 5. The XRD spectra of Y-NMTP, Sc-NMTP, NMTP and NTP obtained in Examples 1 and 4 and Comparative Examples 1 and 2 are as follows: Figure 7 As shown. Based on the diffraction peak position of NTP, the corresponding peak positions of Sc-NMTP, Y-NMTP, and NMTP shifted to varying degrees at diffraction angles of 24˚, 29˚, and 33˚. This is because the manganese oxide octahedron replaced part of the titanium oxide octahedron. The difference in the radius of titanium and manganese caused the crystal plane spacing involved in diffraction to deviate. Based on the NMTP spectrum, it was found that after doping with trace rare earth elements Sc and Y, peak shifts occurred at diffraction angles equal to 20˚ and 23˚, indicating that trace rare earth elements were successfully doped, and manganese in part of the manganese oxide octahedron was replaced by rare earth elements, which better stabilized the structure of the active material. Among them, the diffraction angle shift at 20˚ after yttrium doping was more obvious.

[0042] 6. The Y-NMTP, Sc-NMTP, and NMTP prepared in Examples 1, 4, and Comparative Example 1 were mixed and coated with conductive carbon black and PVDF to prepare electrodes, and sodium ion batteries were assembled for rate cycle testing, wherein the mass ratio of Y-NMTP, Sc-NMTP, NMTP to conductive carbon black and PVDF was 8:1:1. Specifically, the raw materials were mixed according to the above ratios, and an appropriate amount of N-methylpyrrolidone was added for grinding until a black and shiny slurry without obvious particles was formed. Aluminum foil was selected as the current collector for coating, and the pieces were cut after drying to a diameter of 12 mm. When assembling the battery, a metal sodium sheet was used as the negative electrode, and a CR2032 sodium ion battery was assembled in the order of positive electrode shell, gasket, positive electrode material, diaphragm, negative electrode material, gasket, and negative electrode shell for testing.

[0043] The results of the rate cycle test are as follows: Figure 8 As shown in the figure, after being assembled into sodium ion batteries, Y-NMTP, Sc-NMTP and NMTP all showed good capacity performance. The discharge specific capacities at 0.1C rate were 127mAh / g, 122mAh / g and 114mAh / g, respectively. The initial discharge specific capacities at 0.2C rate were 121mAh / g, 117mAh / g and 110mAh / g, respectively. The initial discharge specific capacities at 0.5C rate were 113mAh / g, 111mAh / g and 108mAh / g, respectively. The initial discharge specific capacities at 1C rate were 110mAh / g, 108mAh / g and 98mAh / g, respectively. The initial discharge specific capacities at 2C rate were 103mAh / g, 102mAh / g and 93mAh / g, respectively. The initial discharge capacity at C rate is 93mAh / g, 88mAh / g, and 81mAh / g, and the initial discharge capacity at 10C rate is 75mAh / g, 69mAh / g, and 55mAh / g. When the current density is increased in the opposite direction, the discharge capacity at the corresponding rate is slightly reduced, but Y-NMTP shows the best rate performance. After 200 cycles at 1C, the specific capacity of Y-NMTP, Sc-NMTP and NMTP decreased from the initial 110mAh / g, 108mAh / g, and 98mAh / g to 107mAh / g, 104mAh / g, and 93mAh / g, and the capacity retention rates were 98.19%, 97.69%, and 96.03%, respectively. It can be seen that Y-NMTP maintains the highest capacity and cycle stability, which may be because the radius of yttrium ions is larger than that of scandium ions, thereby better slowing down the phase change of material structure and performance caused by manganese and sodium ion doping.

[0044] The charge and discharge curves are as follows Fig. 9 As shown in the figure, the corresponding curves of Y-NMTP, Sc-NMTP and NMTP all show three charge-discharge platforms, corresponding to Mn 3+ / 4+, 3.6V Mn 2+ / 3+ and 2.1V Ti 3+ / 4+ At the same time, corresponding to the three redox peaks shown in the CV curve, at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C, the Figure 8 The specific capacity of the cycling analysis is basically the same. Fig.10 As shown in the figure, compared with the first three cycles of NMTP, the redox peak of Y-NMTP has the highest repetition degree after fitting, followed by Sc-NMTP, indicating that rare earth element doping is conducive to stabilizing the structure of the active material and showing the best charge and discharge performance, showing the excellent electrochemical performance of the material. 2+ / 3+ There is a large deviation from the redox state, indicating that there is an irreversible slight phase change in the structure of the material at this time.

[0045] 7. In Comparative Examples 3 to 4, the amount of yttrium oxide and scandium oxide added is doubled. Fig.11 As shown, Figure 7 The shifts of the corresponding diffraction peaks shown are not much different. Therefore, based on cost considerations, yttrium oxide and scandium oxide are added in an amount of 0.01 mmol to allow rare earth elements to replace part of the manganese element in the oxygen-manganese octahedron, thereby avoiding the phenomenon of mixed arrangement of sodium ions and manganese ions due to their similar radii, and thus avoiding the problem of smooth sodium ion escape / embedding.

[0046] 8. In Comparative Examples 5 to 6, the heating rate is increased, such as Fig.12 As shown in the figure, the retention rates of Y-NMTP-2 and Sc-NMTP-2 decreased significantly in the long cycle of 1000 cycles, and their initial capacities were only 100mAh / g and 88mAh / g respectively, indicating that too fast a heating rate may cause the crystal to be in multiple thermodynamic metastable states during the formation process, thereby generating lattice defects and affecting the structural properties of the material.

[0047] Finally, it should be noted that the above embodiments of the present invention are only examples for illustrating the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all implementation methods here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for preparing a rare earth element doped polyanion compound, characterized in that: The specific steps include: (1) Dissolve 0.9-1.1 mmol manganese acetate tetrahydrate, 2.9-3.1 mmol ammonium dihydrogen phosphate and 2.9-3.1 mmol sodium acetate in 20 ml of deionized water to obtain solution A; dissolve 0.9-1.1 mmol butyl titanate in 10 ml of ethanol solution to obtain solution B; dissolve 0.005-0.015 mmol rare earth element oxide in 20 ml of ethanol solution to obtain solution C; (2) Add solution B and solution C dropwise into solution A, mix well to obtain solution D, and seal immediately; (3) Heat solution D to 75-85°C, keep warm for 0.5-1.5 h, then stir in the open air until the water evaporates to obtain a light yellow gel, and then place in a drying oven at 75-85°C and vacuum dry for 10-15 h to obtain a precursor; (4) Grinding the precursor and placing it in a microwave tube furnace, then heating it to 600-700° C. at a heating rate of 1-5° C. / min under an inert atmosphere, keeping the temperature for 0.8-1.2 h, and grinding it evenly after cooling to obtain the rare earth element-doped polyanion compound.

2. The method for preparing a rare earth element doped polyanion compound according to claim 1, characterized in that: The rare earth element oxide is yttrium oxide or scandium oxide.

3. The method for preparing a rare earth element-doped polyanion compound according to claim 1, characterized in that: During vacuum drying, the vacuum degree is 0.005~0.01MPa.

4. The method for preparing a rare earth element doped polyanion compound according to claim 1, characterized in that: In step (4), the microwave heating temperature is 650°C.

5. A rare earth element doped polyanion compound, characterized in that: The polyanion compound doped with a rare earth element is prepared by the method for preparing the polyanion compound doped with a rare earth element as described in any one of claims 1 to 4.

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