Cation-doped modified sodium-rich NASICON type sodium ion battery positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of sodium-ion batteries, and discloses a preparation method and application of a cation-doped polyanion type sodium-ion battery positive electrode material, the chemical formula of the positive electrode material is Na < 4 + x > Fe < 3-y > TMy (PO4) 2P2O7, TM is one or more of transition metals with the same valence state as Fe < 2 + > in Mn, Co, Ni, Cu and Zn, x is more than 0 and less than or equal to 0.5, y is more than 0 and less than or equal to 0.4; precursor wet gel is obtained through a sol-gel process, and is sintered after being dried. The transition metal cations with the same valence state and different electronic structures are introduced into the Fe site of the material, and the electron transmission rate is effectively increased and the ion diffusion kinetics is enhanced by adjusting the Fe coordination environment; when the Na < 4 + x > Fe < 3-y > TMy (PO4) 2P2O7 positive electrode material prepared by the invention is applied to a sodium ion battery, high specific discharge capacity, excellent rate capability and cycle performance are shown.

Description

Technical Field

[0001] The present invention belongs to the field of energy storage materials, sodium ion battery positive electrode materials, and particularly relates to a cation-doped modified sodium-rich NASICON type sodium ion battery positive electrode material, sodium iron pyrophosphate, and a preparation method thereof. Background Art

[0002] With the increasing demand for new energy storage systems and limited lithium resources, sodium-ion batteries with abundant reserves, low costs and wide temperature range will become potential targets for future grid-scale storage applications.

[0003] Positive electrode materials play an important role in achieving high energy density and low-cost production of sodium-ion batteries. The current mainstream positive electrode materials are transition metal oxides, polyanionic compounds, and Prussian blue analogs. Taking into account the impact of cost, environment, and application scenarios, the polyanionic compound sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, denoted as NFPP) has a typical NASICON structure and has attracted much attention due to its high theoretical capacity (129mAh / g), small volume change during cycling (<4%), non-toxicity, low cost, and easy synthesis.

[0004] However, large size (PO4) 3- The inherent isolation properties of the groups lead to low electronic conductivity and slow ion diffusion of NFPP materials, which limits their further development in sodium ion batteries. Studies have shown that ion doping can effectively improve the electrochemical performance of positive electrode materials. How to select doping elements to specifically improve the electrochemical performance of positive electrode materials for sodium ion batteries remains an urgent problem to be solved. This invention is funded by the S&T Program of Energy Shaanxi Laboratory, Grant No. ESLB202402. Summary of the Invention

[0005] Purpose of the Invention

[0006] In order to solve the above problems, the present invention provides a transition metal cation-doped sodium-rich NASICON type sodium ion battery positive electrode material, aiming to improve the discharge capacity, rate performance and cycle performance of the NFPP material.

[0007] Solution

[0008] The present invention can be achieved through the following technical solutions:

[0009] A transition metal cation-doped sodium-rich polyanion positive electrode material, the chemical formula of the positive electrode material is Na 4+x Fe3-y TM y (PO4)2P2O7, wherein TM is a transition metal other than iron, wherein 0<x<0.5, 0<y<0.4.

[0010] The present invention also provides a method for preparing a transition metal cation-doped sodium-rich polyanion cathode material for a sodium ion battery, characterized in that it comprises the following steps:

[0011] (1) dissolving an iron salt, a transition metal salt, and a chelating agent in deionized water and stirring to obtain a mixed solution A; dissolving a sodium salt and a phosphate in deionized water and stirring to obtain a mixed solution B; the transition metal in the transition metal salt does not include iron;

[0012] (2) adding the mixed solution B to the mixed solution A at a constant rate of 20-50 μL / s to obtain a mixed solution C;

[0013] (3) stirring the mixed solution C in an oil bath at 70-90° C. for 0.1 h-1 h, then adding ethylene glycol dropwise to the mixed solution C and stirring in an oil bath at 100-130° C. for 0.5-2 h to form a gel D;

[0014] (4) drying the gel D at 70-90° C. to form an electrode material precursor E;

[0015] (5) Pre-sintering the precursor E in an inert atmosphere at 250-350° C. for 2-6 h, and then sintering at 500-650° C. for 8-12 h to obtain a transition metal cation-doped sodium-rich polyanion cathode material;

[0016] The composition of the positive electrode material is Na 4+x Fe 3-y TM y (PO4)2P2O7, wherein TM is a transition metal other than iron, wherein 0<x<0.5, 0<y<0.4.

[0017] The molar ratio of the sodium salt, iron salt, transition metal salt and phosphate in step (1) is 4+x:3-y:y:4, wherein 0<x<0.5 and 0<y<0.4; the molar ratio of the chelating agent to the iron salt is 1.2-1.5:1, and the molar volume of the sodium salt to the ethylene glycol is 2.8-2.9 mmol:100 μL.

[0018] Preferably, 0.2≤x≤0.4, 0.05≤y≤0.3, more preferably x is 0.3.

[0019] The sodium salt includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium pyrophosphate;

[0020] The iron salt includes at least one of ferrous acetate, ferrous acetate, ferrous oxalate, ferric nitrate, and ferrous sulfate;

[0021] The phosphate includes at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0022] The transition metal salt includes at least one of manganese salt, cobalt salt, nickel salt, copper salt and zinc salt;

[0023] The chelating agent includes at least one of citric acid, gluconic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

[0024] Most preferably, the positive electrode material is Na 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7、Na 4.3 Fe 2.95 Ni 0.05 (PO4)2P2O7 or Na 4.3 Fe 2.7 Ni 0.3 (PO4)2P2O7.

[0025] The drying method in step (4) can be at least one of forced air drying and vacuum drying.

[0026] The protective atmosphere in step (5) includes at least one of nitrogen, argon, and hydrogen, and the heating rates of the pre-sintering and high-temperature sintering processes are both 1-5°C / min.

[0027] The present invention also provides a transition metal cation-doped sodium-rich polyanion positive electrode material, which is characterized in that it is prepared according to the above-mentioned preparation method.

[0028] The present invention also provides an application of a transition metal cation-doped sodium-rich polyanion-type positive electrode material in a sodium ion battery, characterized in that the positive electrode material adopts the aforementioned transition metal cation-doped sodium-rich polyanion-type positive electrode material as an active material.

[0029] The present invention also provides a sodium ion battery comprising the above transition metal cation-doped sodium-rich polyanion positive electrode material. 4+x Fe 3-y TM y (PO4)2P2O7 material is used as the positive electrode, and the positive electrode shell, current collector, positive electrode, electrolyte, separator, electrolyte, negative electrode, and negative electrode shell are stacked and pressed in sequence to assemble into a sodium ion battery.

[0030] Beneficial effects

[0031] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0032] (1)Na 4+x Fe 3-y TM y In the (PO4)2P2O7 cathode material, TM doping of metals such as Mn, Co, Ni, Cu, and Zn to replace the active center Fe has a good effect on the high rate performance and long cycle stability of the material. After Ni doping, a moderate strength Fe-O covalent bond is formed in the above cathode materials, and some electrons are transferred from Ni to the active center. 2+ Transfer to Fe 3+ The unique electronic coupling of Na promotes efficient electron transport. Therefore, the appropriate amount of Ni doping plays a role in promoting the electron transfer rate and accelerating the Na + The effect of ion extraction / insertion dynamics can improve the discharge capacity, high rate performance and cycle performance of the material.

[0033] (2) Sodium-rich Na 4+x Fe 3-y TM y The additional Na in (PO4)2P2O7 cathode material + ions induce the occupancy and utilization of active Na1, Na3 and Na4 sites. In addition, some Fe 3+ Reduction to Fe 2+ , maintain the electrical balance of the crystal and enhance the stability of the crystal structure. 4+x Fe 3-y TM y (PO4)2P2O7 cathode material doping and Fe 2+ One or more TM elements in the transition metal group with the same valence are more conducive to improving the electrochemical performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0035] Figure 1 The Na prepared in Comparative Example 3 4.3 Fe3(PO4)2P2O7, Na prepared in Example 1 4.3 Fe 2.9 Mn 0.1 (PO4)2P2O7 and Na prepared in Example 5 4.3 Fe 2.9 Ni 0.1 Comparison of the XRD (X-ray diffraction) spectrum of (PO4)2P2O7 material with the standard card (PDF#89-0579);

[0036] Figure 2 Na prepared in Comparative Example 3 4.3 Fe3(PO4)2P2O7, Na prepared in Example 5 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7, Na prepared in Example 6 4.3 Fe 2.95 Ni 0.05 (PO4)2P2O7 and Na prepared in Example 7 4.3 Fe 2.7 Ni 0.3 Comparison of battery performance of (PO4)2P2O7, where (a)-(b) are the charge and discharge curves at 0.1C, (c) is the rate performance, and (d)-(e) are the cycle performance at 1C. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the embodiments can serve as a guide for those skilled in the art to make further improvements. It should be understood that the specific embodiments are only used to illustrate the present invention and do not constitute a limitation of the present invention in any way.

[0038] In the present invention, unless otherwise specified, the experimental methods used in the examples are conventional methods, and the materials and reagents used are all commercially available.

[0039] Example 1: (Preparation of Na by doping with transition metal element Mn 4.3 Fe 2.9 Mn 0.1 (PO4)2P2O7)

[0040] 2.89 mmol of sodium acetate, 1.95 mmol of ferric acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.067 mmol of manganese acetate and 2.52 mmol of citric acid were weighed according to the molar ratio of Na:Fe:P:Mn:citric acid = 4.3:2.9:4:0.1:3.75, and 100 μL of ethylene glycol was weighed; the citric acid, ferric acetate and manganese acetate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then added dropwise at a constant rate of 30 μL / s. The mixture B was added to form a mixture C; the mixture C was magnetically stirred in an 80°C oil bath for 0.5 h; ethylene glycol was then added to the mixture C, and magnetically stirred in an oil bath at 120°C for 1.5 h until a gel was formed; the gel was vacuum dried at 80°C for 8 h and then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, and an argon-hydrogen mixture (95:5) was passed through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6 h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10 h, and cooled in the furnace to obtain the desired Mn-doped Na 4.3 Fe 2.9 Mn 0.1 (PO4)2P2O7 positive electrode material.

[0041] Example 2: (Preparation of Na by doping with transition metal element Co) 4.3 Fe 2.9 Co 0.1 (PO4)2P2O7)

[0042] 2.89 mmol of sodium acetate, 1.95 mmol of ferric acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.067 mmol of cobalt acetate tetrahydrate and 2.52 mmol of citric acid were weighed according to the molar ratio of Na:Fe:P:Co:citric acid = 4.3:2.9:4:0.1:3.75, and 100 μL of ethylene glycol was weighed; the citric acid, ferric acetate and cobalt acetate tetrahydrate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then stirred at 30 μL / s The mixed solution B was dripped at a uniform speed to form a mixed solution C; the mixed solution C was magnetically stirred in an 80°C oil bath environment for 0.5h; ethylene glycol was then added to the mixed solution C, and magnetically stirred in an oil bath at 120°C for 1.5h until a gel was formed; the gel was vacuum dried at 80°C for 8h and then ground to obtain a precursor powder; the precursor powder was placed in a tubular furnace, and an argon-hydrogen mixture (95:5) was passed through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10h, and cooled in the furnace to obtain the desired Co-doped Na4.3 Fe 2.9 Co 0.1 (PO4)2P2O7 positive electrode material.

[0043] Example 3: (Preparation of Na by doping with transition metal element Cu 4.3 Fe 2.9 Cu 0.1 (PO4)2P2O7)

[0044] 2.89 mmol sodium acetate, 1.95 mmol ferric acetate, 2.69 mmol ammonium dihydrogen phosphate, 0.067 mmol copper acetate and 2.52 mmol citric acid were weighed according to the molar ratio of Na:Fe:P:Cu:citric acid = 4.3:2.9:4:0.1:3.75, and 100 μL of ethylene glycol was weighed; citric acid, ferric acetate and copper acetate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then added dropwise at a constant rate of 30 μL / s. The mixture B was added to form a mixture C; the mixture C was magnetically stirred in an 80°C oil bath environment for 0.5h; ethylene glycol was then added to the mixture C, and magnetically stirred in an oil bath at 120°C for 1.5h until a gel was formed; the gel was vacuum dried at 80°C for 8h and then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, and an argon-hydrogen mixture (95:5) was passed through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10h, and cooled in the furnace to obtain the desired Cu-doped Na 4.3 Fe 2.9 Cu 0.1 (PO4)2P2O7 positive electrode material.

[0045] Example 4: (Preparation of Na by doping with transition metal element Zn 4.3 Fe 2.9 Zn 0.1 (PO4)2P2O7)

[0046] According to the molar ratio of Na:Fe:P:Zn: citric acid = 4.3:2.9:4:0.1:3.75, 2.89 mmol of sodium acetate, 1.95 mmol of iron acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.067 mmol of zinc acetate dihydrate and 2.52 mmol of citric acid were weighed, and 100 μL of ethylene glycol was weighed; the citric acid, iron acetate and zinc acetate dihydrate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was uniformly dropped into the mixed solution B at a rate of 30 μL / s to form a mixed solution C; the mixed solution C was magnetically stirred at 80°C in an oil bath environment for 0.5 h; then the ethylene glycol was added to the mixed solution C, and magnetically stirred at 120°C in an oil bath for 1.5 h until a gel was formed; the gel was vacuum dried at 80°C for 8 h, then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, argon-hydrogen mixed gas (95:5) was passed through the furnace, heated to 300°C at a rate of 2°C / min, kept for 6 h, then cooled with the furnace and taken out, ground uniformly, then heated to 550°C and kept for 10 h, cooled with the furnace, and then ground uniformly to obtain the desired Zn-doped Na 4.3 Fe 2.9 Zn 0.1 (PO4)2P2O7 cathode material.

[0047] Example 5: Preparation of Na 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7)

[0048] According to the molar ratio of Na:Fe:P:Ni: citric acid = 4.3:2.9:4:0.1:3.75, 2.89 mmol of sodium acetate, 1.95 mmol of iron acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.067 mmol of nickel acetate tetrahydrate and 2.52 mmol of citric acid were weighed, and 100 μL of ethylene glycol was weighed; the citric acid, iron acetate and nickel acetate tetrahydrate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was uniformly dropped into the mixed solution B at a rate of 30 μL / s to form a mixed solution C; the mixed solution C was magnetically stirred at 80 °C in an oil bath environment for 0.5 h; then the ethylene glycol was added to the mixed solution C, which was magnetically stirred at 120 °C in an oil bath for 1.5 h until a gel was formed; the gel was vacuum dried at 80 °C for 8 h, then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, argon-hydrogen mixed gas (95:5) was passed through the furnace, heated to 300 °C at a rate of 2 °C / min, kept for 6 h, then cooled with the furnace and taken out, ground uniformly, then heated to 550 °C and kept for 10 h, cooled with the furnace, and then ground uniformly to obtain the desired Ni-doped Na 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7positive electrode material.

[0049] Example 6: Preparation of Na 4.3 Fe 2.95 Ni 0.05 (PO4)2P2O7)

[0050] According to the molar ratio of Na:Fe:P:Ni: citric acid = 4.3:2.95:4:0.05:3.75, 2.89 mmol of sodium acetate, 1.95 mmol of iron acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.033 mmol of nickel acetate tetrahydrate and 2.52 mmol of citric acid were weighed, and 100 μL of ethylene glycol was weighed; the citric acid, iron acetate and nickel acetate tetrahydrate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was uniformly dropped into the mixed solution B at a rate of 30 μL / s to form a mixed solution C; the mixed solution C was magnetically stirred at 80°C in an oil bath environment for 0.5 h; then the ethylene glycol was added to the mixed solution C, which was magnetically stirred at 120°C in an oil bath for 1.5 h until a gel was formed; the gel was vacuum dried at 80°C for 8 h, then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, argon-hydrogen mixed gas (95:5) was passed through the furnace, heated to 300°C at a rate of 2°C / min, kept for 6 h, then cooled with the furnace and taken out, ground uniformly, then heated to 550°C and kept for 10 h, cooled with the furnace, and then ground uniformly to obtain the desired Ni-doped Na 4.3 Fe 2.95 Ni 0.05 (PO4)2P2O7positive electrode material.

[0051] Example 7: Preparation of Na 4.3 Fe 2.7 Ni 0.3 (PO4)2P2O7)

[0052] According to the molar ratio of Na:Fe:P:Ni: citric acid = 4.3:2.7:4:0.3:3.75, 2.89 mmol of sodium acetate, 1.95 mmol of iron acetate, 2.69 mmol of ammonium dihydrogen phosphate, 0.22 mmol of nickel acetate tetrahydrate and 2.52 mmol of citric acid were weighed, and 100 μL of ethylene glycol was weighed; the citric acid, iron acetate and nickel acetate tetrahydrate were dissolved in deionized water, stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water, stirred until completely dissolved to obtain a mixed solution B; then the mixed solution A was uniformly dropped into the mixed solution B at a rate of 30 μL / s to form a mixed solution C; the mixed solution C was magnetically stirred at 80°C in an oil bath environment for 0.5 h; then the ethylene glycol was added to the mixed solution C, and magnetically stirred at 120°C in an oil bath for 1.5 h until a gel was formed; the gel was vacuum dried at 80°C for 8 h, then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, argon-hydrogen mixed gas (95:5) was passed through the furnace, heated to 300°C at a rate of 2°C / min, kept for 6 h, then taken out after the furnace was cooled, ground uniformly, then heated to 550°C and kept for 10 h, and then taken out after the furnace was cooled, to obtain the desired Na 4.3 Fe 2.7 Ni 0.3 (PO4)2P2O7 cathode material.

[0053] Comparative Example 1: (Na4Fe3(PO4)2P2O7)

[0054] According to the molar ratio of Na:Fe:P: citric acid = 4:3:4:3.75, 2.6 mmol of sodium acetate, 1.95 mmol of iron acetate, 2.6 mmol of ammonium dihydrogen phosphate and 2.52 mmol of citric acid were weighed, and 100 μL of ethylene glycol was weighed; the citric acid and iron acetate were dissolved in deionized water, stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water, stirred until completely dissolved to obtain a mixed solution B; then the mixed solution A was uniformly dropped into the mixed solution B at a rate of 30 μL / s to form a mixed solution C; the mixed solution C was magnetically stirred at 80°C in an oil bath environment for 0.5 h; then the ethylene glycol was added to the mixed solution C, and magnetically stirred at 120°C in an oil bath for 1.5 h until a gel was formed; the gel was vacuum dried at 80°C for 8 h, then ground to obtain a precursor powder; the precursor powder was placed in a tube furnace, argon-hydrogen mixed gas (95:5) was passed through the furnace, heated to 300°C at a rate of 2°C / min, kept for 6 h, then taken out after the furnace was cooled, ground uniformly, then heated to 550°C and kept for 10 h, and then taken out after the furnace was cooled, to obtain the desired Na4Fe3(PO4)2P2O7 cathode material.

[0055] Comparative Example 2: (Na 4.1 Fe3(PO4)2P2O7)

[0056] 2.67 mmol of sodium acetate, 1.95 mmol of ferric acetate, 2.6 mmol of ammonium dihydrogen phosphate, and 2.52 mmol of citric acid were weighed according to a molar ratio of Na:Fe:P:citric acid = 4.1:3:4:3.75, and 100 μL of ethylene glycol was weighed; the citric acid and ferric acetate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then dripped into the mixed solution B at a constant rate of 30 μL / s to form a mixed solution C. ; The mixed solution C was magnetically stirred in an 80°C oil bath for 0.5h; ethylene glycol was then added to the mixed solution C and magnetically stirred in a 120°C oil bath for 1.5h until a gel was formed; the gel was vacuum dried at 80°C for 8h and then ground to obtain a precursor powder; the precursor powder was placed in a tubular furnace, and an argon-hydrogen mixture (95:5) was passed through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10h, and cooled in the furnace to obtain the desired Na 4.1 Fe3(PO4)2P2O7 positive electrode material.

[0057] Comparative Example 3: (Na 4.3 Fe3(PO4)2P2O7)

[0058] 2.8 mmol of sodium acetate, 1.95 mmol of ferric acetate, 2.6 mmol of ammonium dihydrogen phosphate, and 2.52 mmol of citric acid were weighed according to a molar ratio of Na:Fe:P:citric acid = 4.3:3:4:3.75, and 100 μL of ethylene glycol was weighed; the citric acid and ferric acetate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then dripped into the mixed solution B at a constant rate of 30 μL / s to form a mixed solution C. ; The mixed solution C was magnetically stirred in an 80°C oil bath for 0.5h; ethylene glycol was then added to the mixed solution C and magnetically stirred in a 120°C oil bath for 1.5h until a gel was formed; the gel was vacuum dried at 80°C for 8h and then ground to obtain a precursor powder; the precursor powder was placed in a tubular furnace with an argon-hydrogen mixture (95:5) passing through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10h, and cooled in the furnace to obtain the desired Na 4.3 Fe3(PO4)2P2O7 positive electrode material.

[0059] Comparative Example 4: (Na 4.5 Fe3(PO4)2P2O7)

[0060] 2.93 mmol of sodium acetate, 1.95 mmol of ferric acetate, 2.6 mmol of ammonium dihydrogen phosphate, and 2.52 mmol of citric acid were weighed according to a molar ratio of Na:Fe:P:citric acid = 4.5:3:4:3.75, and 100 μL of ethylene glycol was weighed; the citric acid and ferric acetate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution A; the sodium acetate and ammonium dihydrogen phosphate were dissolved in deionized water and stirred until completely dissolved to obtain a mixed solution B; the mixed solution A was then dripped into the mixed solution B at a uniform rate of 30 μL / s to form a mixed solution C. ; The mixed solution C was magnetically stirred in an 80°C oil bath for 0.5h; ethylene glycol was then added to the mixed solution C and magnetically stirred in a 120°C oil bath for 1.5h until a gel was formed; the gel was vacuum dried at 80°C for 8h and then ground to obtain a precursor powder; the precursor powder was placed in a tubular furnace, and an argon-hydrogen mixture (95:5) was passed through the furnace, and heated to 300°C at a heating rate of 2°C / min, kept warm for 6h, and then cooled in the furnace, taken out and ground evenly, and then heated to 550°C and kept warm for 10h, and cooled in the furnace to obtain the desired Na 4.3 Fe3(PO4)2P2O7 positive electrode material.

[0061] Performance and effect test examples

[0062] Test Example 1

[0063] The positive electrode materials prepared in Comparative Example 3, Example 1 and Example 5 were subjected to X-ray diffraction test. Figure 1 As shown in the XRD patterns, the cathode materials prepared in Comparative Example 3, Example 1, and Example 5 match the standard PDF card (PDF#: 89-0579), corresponding to NFPP (orthorhombic, space group: Pn21a). No obvious impurity peaks or other structural phases were observed, indicating that the introduction of a small amount of doping elements has no effect on the crystal structure of the samples.

[0064] Test Example 2

[0065] The positive electrode materials prepared in Examples 5-7 and Comparative Examples 1, 3, and 4 were used as the active material, Ketjen black as the conductive agent, PVDF as the binder, and N-methyl-2-pyrrolidone (NMP) as the dispersant. A slurry was prepared at a mass ratio of 8:1:1 (solid content 14.8%) of positive electrode material: Ketjen black:PVDF. The slurry was coated onto aluminum foil and dried in a vacuum oven at 120°C for 12 hours. The resulting slurry was then cut into 12 mm discs to serve as the positive electrode sheets. A sodium metal sheet was then used as the negative electrode, a polypropylene microporous membrane as the separator, and the electrolyte was a 1 mol / L sodium salt of NaClO₄ dissolved in a 1:1 volume ratio of ethylene carbonate (EC) and diethyl carbonate (DEC), with an additional 5% fluoroethylene carbonate (FEC) additive. CR2032 button cells were fabricated in an argon-filled glove box.

[0066] The button cells prepared in Examples 5-7 and Comparative Example 3 were subjected to constant current charge and discharge tests in the voltage range of 1.8-4.0V and at a current density of 0.1C. The test results are as follows: Figure 2 As shown in 2a and 2b.

[0067] The button cells prepared in Examples 5-7 and Comparative Example 3 were tested for rate performance in the voltage range of 1.8-4.0 V and at current densities of 0.1C, 0.5C, 1C, 2C, 5C, 10C and 0.1C. The rate performance test results are shown in FIG. Figure 2 As shown in c.

[0068] The button cells prepared in Examples 5-7 and Comparative Example 3 were subjected to cycle stability tests in the voltage range of 1.8-4.0 V. The test results of 500 cycles at a current density of 1 C were as follows: Figure 2 d and 2e.

[0069] Table 1: Electrochemical performance test results of Examples 5-7 and Comparative Examples 1, 3, and 4

[0070]

[0071] From the above experimental data, it can be seen that in Comparative Example 3, compared with Examples 5, 6 and 7, no doping element nickel is introduced to partially replace the iron position in the material. Figure 2 Rate performance and cycle performance test, the positive electrode material Na prepared in Example 5 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7 has the highest discharge capacity at a high current density of 10C and the highest capacity retention after 500 cycles. This shows that the appropriate amount of nickel doping can reasonably adjust the Fe e gOrbital occupancy, and then by regulating the covalency of Fe-O, a comprehensive effect of promoting the electron transfer rate and enhancing the extraction / insertion kinetics of sodium ions is achieved.

[0072] As can be seen from Table 1, the sodium-rich comparative example 3 has better performance than the non-sodium-rich comparative example 1, and Examples 5-7 have better performance than comparative examples 3 and 1. This proves that the sodium-rich positive electrode material has a higher specific capacity, better rate performance and cycle performance than the non-sodium-rich positive electrode material. Doping transition metals in the sodium-rich positive electrode material is more conducive to improving the specific capacity, rate performance and cycle performance. This is because the additional sodium ion supplementation can increase the occupancy and utilization of Na1, Na3 and Na4 sites in the material, optimize the internal crystal structure of the material to the greatest extent, and enhance the ion transfer kinetics.

[0073] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. Within the scope of the claims, various modifications and variations that can be made by those skilled in the art without inventive effort remain within the scope of protection of the present invention.

Claims

1. A method for preparing a transition metal cation-doped sodium-rich polyanion cathode material for a sodium ion battery, characterized in that: The steps include: (1) dissolving an iron salt, a transition metal salt, and a chelating agent in deionized water and stirring to obtain a mixed solution A; dissolving a sodium salt and a phosphate in deionized water and stirring to obtain a mixed solution B; the transition metal in the transition metal salt does not include iron; (2) adding the mixed solution B to the mixed solution A at a constant rate of 20-50 μL / s to obtain a mixed solution C; (3) stirring the mixed solution C in an oil bath at 70-90° C. for 0.1 h-1 h, then adding ethylene glycol dropwise to the mixed solution C and stirring in an oil bath at 100-130° C. for 0.5-2 h to form a gel D; (4) drying the gel D at 70-90° C. to form an electrode material precursor E; (5) Pre-sintering the precursor E in an inert atmosphere at 250-350° C. for 2-6 h, and then sintering at 500-650° C. for 8-12 h to obtain a transition metal cation-doped sodium-rich polyanion cathode material; The composition of the positive electrode material is Na 4+x Fe 3-y TM y (PO4)2P2O7, wherein TM is a transition metal other than iron, wherein 0<x<0.5, 0<y<0.

4.

2. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 1, characterized in that: The molar ratio of the sodium salt, iron salt, transition metal salt and phosphate in step (1) is 4+x:3-y:y:4, wherein 0<x<0.5 and 0<y<0.4; the molar ratio of the chelating agent to the iron salt is 1.2-1.5:1, and the molar volume ratio of the sodium salt to ethylene glycol is 2.8-2.9 mmol:100 μL.

3. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 1 or 2, characterized in that: TM is one or more of Mn, Co, Ni, Cu, and Zn; 0.2≤x≤0.4, 0.05≤y≤0.

3.

4. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 3, characterized in that: x is 0.

3.

5. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 4, characterized in that: The sodium salt includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium pyrophosphate; The iron salt includes at least one of ferrous acetate, ferrous acetate, ferrous oxalate, ferric nitrate, and ferrous sulfate; The phosphate includes at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate; The transition metal salt includes at least one of a manganese salt, a cobalt salt, a nickel salt, a copper salt, and a zinc salt; The chelating agent includes at least one of citric acid, gluconic acid, ethylenediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

6. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 5, characterized in that: The positive electrode material is Na 4.3 Fe 2.9 Ni 0.1 (PO4)2P2O7、Na 4.3 Fe 2.95 Ni 0.05 (PO4)2P2O7 or Na 4.3 Fe 2.7 Ni 0.3 (PO4)2P2O7.

7. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 6, characterized in that: The drying method in step (4) can be at least one of forced air drying and vacuum drying.

8. The method for preparing a transition metal cation-doped sodium-rich polyanion cathode material according to claim 7, characterized in that: The protective atmosphere in step (5) includes at least one of nitrogen, argon, and hydrogen, and the heating rates of the pre-sintering and high-temperature sintering processes are both 1-5°C / min.

9. A transition metal cation-doped sodium-rich polyanion positive electrode material, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 8.

10. Use of the transition metal cation-doped sodium-rich polyanion cathode material in a sodium ion battery according to claim 9, characterized in that: The positive electrode material adopts the transition metal cation-doped sodium-rich polyanion positive electrode material as claimed in claim 9 as the active material.

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