Modified ferric sodium pyrophosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of sodium ion batteries, in particular to a modified ferric sodium pyrophosphate positive electrode material as well as a preparation method and application thereof. The modified ferric sodium pyrophosphate positive electrode material is Na < 4-x > A < x > Fe < 3-y > M < y > (PO4) < 2-z > E < z > (P2O7) w, wherein A is selected from any one or more of alkali metal ions or alkaline earth metal ions of which the ion radius is greater than Na < + >; m is selected from any one or more of transition metal ions; e is selected from any one or more of SiO4 < 4->, SO4 < 2->, BO3 < 3-> or F <->; 0.1 < = x < = 0.3, 0.1 < = y < = 0.3, 0.04 < = z < = 0.1, and the value of w enables positive and negative charges to be balanced. The stability of a crystal structure is optimized through multi-site multi-element doping, the ion / electron conduction efficiency is improved, and the oxidation reduction behavior is regulated and controlled, so that the cycle life, the rate capability and the energy density of the sodium ferric pyrophosphate positive electrode material are comprehensively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, in particular to a modified sodium iron pyrophosphate phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the aggravation of environmental pollution and the continuous depletion of fossil energy, the development of renewable clean energy is particularly urgent. Compared with traditional lithium ion batteries, sodium element resources have inherent advantages such as low cost, wide distribution and abundant reserves, so sodium ion batteries are expected to realize large-scale application and even monopolize the intermittent renewable energy market. Among them, the development of electrode materials is one of the most important goals to improve sodium ion batteries.

[0003] Na4Fe3(PO4)2P2O7 as a sodium battery positive electrode material has the advantages of low cost, stable structure, unique three-dimensional sodium ion diffusion channel, moderate working voltage, etc. However, its disadvantages are also obvious, such as low electronic and ionic conductivity, which limits the battery capacity and power density; the impurity phases such as NaFeP2O7 and NaFePO4 generated during preparation will hinder the diffusion of sodium ions, reduce the reversible capacity and rate performance, and the poor cycle stability also becomes an obstacle to its wide application.

[0004] Therefore, it is desirable to provide a new modified sodium iron pyrophosphate phosphate positive electrode material and a preparation method thereof. SUMMARY

[0005] In order to solve the above technical problems, the present application provides a modified sodium iron pyrophosphate phosphate positive electrode material and a preparation method and application thereof. The modified sodium iron pyrophosphate phosphate positive electrode material provided by the present application optimizes the crystal structure stability, improves the ion / electron conduction efficiency and regulates the redox behavior through the synergistic effect of multi-site and multi-element doping, comprehensively improves the cycle life, rate performance and energy density of the sodium iron pyrophosphate phosphate positive electrode material, thereby effectively improving the electrochemical performance of the sodium ion battery using the same.

[0006] In a first aspect, the present application provides a modified sodium iron pyrophosphate phosphate positive electrode material, wherein the modified sodium iron pyrophosphate phosphate positive electrode material is Na 4-x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w , wherein A is selected from any one or more of alkali metal ions or alkaline earth metal ions with an ionic radius greater than that of Na + ; M is selected from any one or more of transition metal ions; E is selected from any one or more of SiO4 4- , SO4 2- , BO3 3- , or F - ; 0.1≤x≤0.3, 0.1≤y≤0.3, 0.04≤z≤0.1, the value of w makes Na 4-x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w .

[0007] The modified sodium iron pyrophosphate positive electrode material provided by the application optimizes the crystal structure stability, improves the ion / electron conduction efficiency and regulates the redox behavior through the synergistic effect of multi-site and multi-element doping, comprehensively improves the cycle life, rate performance and energy density of the sodium iron pyrophosphate positive electrode material, thereby effectively improving the electrochemical performance of the sodium ion battery using the same, and specifically: The modified sodium iron pyrophosphate positive electrode material provided by the application improves the crystal structure stability of the material through the synergistic effect of doping alkali metal, alkaline earth metal, transition metal and anion, inhibits lattice collapse and volume change, reduces ion mixing and defects; at the same time, the sodium ion conduction performance is optimized, the diffusion channel is widened, the migration energy barrier is reduced; the redox behavior can also be regulated, the redox potential is improved, the reversible capacity is increased, the electronic conductivity is improved, and the cycle stability and comprehensive service performance are further enhanced. The modified sodium iron pyrophosphate positive electrode material provided by the application determines the appropriate doping ratio according to the crystal structure stability, electrochemical performance and solid solution limit of the doping element, and the chemical formula is Na 4- x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w , the alkali metal ion or alkaline earth metal ion with an ionic radius greater than Na + is used to replace Na + , the value of x is in the range of 0.1-0.3, which can optimize Na +The diffusion channel can also avoid the precipitation of impurity phases or excessive lattice distortion caused by excessive doping; the transition metal ions are partially replaced by Fe 2+ The value of y is in the range of 0.1-0.3, which can introduce new redox active sites to improve the voltage or capacity, and can maintain the structural charge balance by means of valence matching, to prevent the electronic transmission from being blocked or the transition metal ions from gathering due to excessive doping; the anions such as SiO4 4- , SO4 2- , BO3 3- , and F - are partially replaced by PO4 3- , and the value of z is in the range of 0.04-0.1, which can improve the stability by enhancing the rigidity of the polyhedron or optimizing the ion interaction, while avoiding the blockage of the ion conduction path caused by excessive damage to the connection mode of the original framework structure. The modified sodium iron pyrophosphate positive electrode material with multi-element and multi-site doping provided by the application can achieve a good balance between structural stability, ion / electron conduction and electrochemical activity, and provides a reliable foundation for improving the comprehensive performance of the material.

[0008] Excessive doping can cause a series of negative effects: the doping ions exceeding the solid solubility limit will precipitate in the form of impurity phases (such as K3PO4, SrFeO3, etc.), which can damage the structural integrity of the crystal, cause the diffusion channel of Na + to be blocked, and the ion conductivity to be greatly reduced; excessive large-radius ions (such as Ba 2+ ) can excessively expand the lattice, weaken the transition metal-oxygen bond energy, and exacerbate the volume expansion during charging and discharging, which can easily cause the particles to crack during the cycle process; excessive high-valence ions (such as Zr 4+ ) can cause lattice charge imbalance, generate a large number of oxygen vacancies or Na + vacancies, enhance the interface reaction between the electrolyte and the material, and cause the capacity to rapidly decay; in addition, excessive anion doping can destroy the symmetry of the polyhedron connection, reduce the electronic conductivity of the material, and cause the rate performance to deteriorate.

[0009] As a preferred technical solution of the application, the A is selected from any one or more of K + , Sr 2+ , and Ba 2+ .

[0010] As a preferred technical solution of the application, the M is selected from Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , V 3+ , Cr 3+ , Ti 4+ , and Zr 4+any one or more of the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, and Pb.

[0011] The modified sodium iron phosphate positive electrode material provided by the application has a series of excellent performances. 1. Improving the stability of the crystal structure. The alkali metal ions or alkaline earth metal ions with large radius partially replace Na + , which can reduce the structure shrinkage and expansion caused by Na + deintercalation by expanding the lattice spacing, for example, K + can increase the lattice parameter and reduce the volume change rate; the high-valence transition metal ions such as Zr 4+ , Ti 4+ partially replace Fe 2+ , which can stabilize the NASICON framework by means of stronger bonding effect and inhibit the crack of the crystal boundary; the rigid anions such as SO4 2- partially replace PO4 3- , which can enhance the polyhedron connection strength due to the higher S─O bond energy, reduce the framework collapse at high temperature, and the divalent charge of the alkaline earth metal can also balance the valence fluctuation of the transition metal and reduce the lattice defect density, and the high electronegativity of F - can fix the position of oxygen atoms and reduce the oxygen vacancy to avoid structural disorder.

[0012] 2. Optimizing the sodium ion conduction performance. The optimization of the sodium ion conduction performance by element doping is the key to improve the rate performance of the material, which is mainly to widen the diffusion channel and reduce the migration energy barrier. After the large ions such as K + , Sr 2+ replace Na + , the sodium layer spacing can be expanded and the diffusion channel cross-sectional area can be increased, for example, the K + doping can make the Na + diffusion coefficient increase by two orders of magnitude; when the small-size anions such as BO3 3- replace PO4 3- , the lattice parameter can be shortened due to the shorter B─O bond length, so that the diffusion path of Na + along the c axis is smoother; when the transition metals such as Ti 4+ , V 3+ replace Fe 2+ , the covalence of the transition metal-oxygen bond can be adjusted to reduce the electrostatic binding of Na + and reduce the migration energy barrier; the high electronegativity of F - can neutralize the positive charge of the channel wall and weaken the interaction between Na + and the lattice, which can significantly improve the ion conduction capacity even in a low-temperature environment.

[0013] 3. Controlling redox behavior and electrochemical performance. Controlling redox behavior and electrochemical performance is an important goal of element doping, aiming to improve the voltage platform, specific capacity and energy density of modified sodium iron phosphate cathode materials. Transition metals such as Mn 3+ / Mn 2+ , V 3+ / V 4+ replace Fe 3+ / Fe 2+ , which can increase the average voltage of the material by 0.2-0.7 V due to its higher redox potential; anions such as SO4 2- S 6+ has a higher electronegativity than P 5+ , which can also increase the redox potential of transition metals through the "induction effect". At the same time, F - , SO4 2- and other anion doping can activate the oxygen anion redox reaction, realize multi-electron transfer and increase the reversible capacity; V 3+ , Ti 4+ and other multi-valence transition metals can introduce additional redox sites to further improve the specific capacity, and Cu 2+ partially replacing Fe 2+ can adjust the energy band structure, reduce the band gap, enhance the electronic conductivity and promote electron transfer.

[0014] 4. Enhancing cycle stability and comprehensive service performance. Element doping enhances the cycle stability and comprehensive service performance, making the material more suitable for actual application requirements. SO4 2- can enhance the stability of transition metal-oxygen framework and reduce the dissolution rate of Fe 2+ ; Sr 2+ , Ba 2+ and other alkaline earth metals can reduce the leaching of H + from the electrolyte into transition metals and improve the residual rate of transition metals during the cycle process. In terms of high rate performance, the fast electron transfer ability of Cu + / Cu 2+ can improve the charge transfer and improve the capacity retention rate at high rate; the small size of BO3 3- can refine the grain size and shorten the Na + diffusion path, significantly improving the capacity retention rate at low temperature, so that the material can maintain good performance in a wide temperature range.

[0015] 5. Synergistic effect realizes comprehensive performance leap. The synergistic effect of multi-element and multi-site doping realizes performance leap through function complementation, and the core is synergistic superposition of different doping mechanisms: the structural support of alkali metal / alkaline earth metal and the electronic state regulation of transition metal cooperate, which can break through the balance between structural stability and electrochemical activity; the transition metal and anion cooperate, which can simultaneously optimize electronic conduction and ion diffusion, and avoid the "conduction bottleneck" of single doping; the alkali metal / alkaline earth metal and anion cooperate, which can consider lattice rigidity and interface stability. This multi-dimensional synergy covers key indicators such as structure, conduction, and redox, breaks through the performance limitations of single doping, and makes the comprehensive performance of the modified sodium iron pyrophosphate positive electrode material provided by the application close to the optimal value.

[0016] In a second aspect, the application provides a preparation method of the modified sodium iron pyrophosphate positive electrode material in the first aspect, and the preparation method comprises: (1) placing a sodium source, an iron source, a phosphorus source, A a source, M a source, E a source, a reducing agent, and a carbon source in a solvent water and mixing uniformly to obtain a mixed solution; (2) heating the mixed solution to a gel state, drying, and sintering to obtain the modified sodium iron pyrophosphate positive electrode material.

[0017] The preparation method of the modified sodium iron pyrophosphate positive electrode material provided by the application has simple process and strong operability, and is easy to realize mass production.

[0018] As a preferred technical solution of the application, the product Na 4-x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w The mass ratio of the sodium source, the iron source, the phosphorus source, the reducing agent, and the carbon source is 1:(0.8-2.4):(0.01-0.05), for example, 1:0.8:0.01, 1:1.2:0.02, 1:1.6:0.03, 1:2:0.04, 1:2.4:0.05, etc.

[0019] As a preferred technical solution of the application, the sodium source includes any one or more of sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sodium silicate (Na2SiO3), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4), and sodium acetate (CH3COONa).

[0020] As a preferred technical solution of the present application, the iron source includes any one or more of ferric oxalate (Fe2(C2O4)3), iron oxide (Fe2O3), iron phosphate (FePO4), ferric nitrate (Fe(NO3)3), ferric sulfate (Fe2(SO4)3) and ferric citrate (FeC6H5O7).

[0021] As a preferred technical solution of the present application, the phosphorus source includes any one or more of disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), iron phosphate (FePO4), diammonium hydrogen phosphate ((NH4)2HPO4), ammonium dihydrogen phosphate (NH4H2PO4) and phosphoric acid (H3PO4).

[0022] As a preferred technical solution of the present application, the A source includes any one or more of a salt or base of element A.

[0023] As a preferred technical solution of the present application, the A source includes any one or more of potassium sulfate (K2SO4), potassium carbonate (K2CO3), potassium hydroxide (KOH), strontium carbonate (SrCO3), barium sulfate (BaSO4) and barium carbonate (BaCO3).

[0024] As a preferred technical solution of the present application, the M source includes any one or more of an ester, oxide or salt of element M.

[0025] As a preferred technical solution of the present application, the M source includes any one or more of trimanganese tetroxide (Mn3O4), manganese acetate (Mn(CH3COO)2), cobalt oxide (CoO), nickel oxide (NiO), copper oxide (CuO), basic copper carbonate (Cu2(OH)2CO3), zinc oxide (ZnO), vanadium pentoxide (V2O5), chromium sesquioxide (Cr2O3), titanium dioxide (TiO2), titanyl sulfate (TiOSO4), tetrabutyl titanate (C 16 H 36 O4Ti), titanium tetrachloride (TiCl4) and zirconium nitrate (Zr(NO3)4).

[0026] As a preferred technical solution of the present application, the E source includes any one or more of silicon dioxide (SiO2), sodium silicate (Na2SiO3), sulfuric acid (H2SO4), sodium sulfate (Na2SO4), boric acid (H3BO3), hydrofluoric acid (HF) and ammonium bifluoride (NH4HF2).

[0027] As a preferred technical solution of the present application, the reducing agent includes any one or more of citric acid (C6H8O7), oxalic acid (C2H2O4) and ascorbic acid (C6H8O6).

[0028] As a preferred technical solution of the present application, the carbon source comprises any one or more of acetylene black, graphene oxide, Super P, sucrose (C 12 H 22 O 11 ) and glucose (C6H 12 O6).

[0029] As a preferred technical solution of the present application, the mixing method is sand milling, and the sand milling time is 3.5-10 h, such as 3.5 h, 5 h, 8 h, 10 h, etc.

[0030] As a preferred technical solution of the present application, the solid content of the mixed solution is 50-60%, such as 50%, 52%, 54%, 56%, 58%, 60%, etc.

[0031] As a preferred technical solution of the present application, the heating temperature is 45-60°C, such as 45°C, 50°C, 55°C, 60°C, etc.

[0032] As a preferred technical solution of the present application, the drying temperature is 100-120°C, such as 100°C, 105°C, 110°C, 115°C, 120°C, etc.

[0033] As a preferred technical solution of the present application, the drying time is 12-24 h, such as 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, etc.

[0034] As a preferred technical solution of the present application, the sintering is performed in an inert gas atmosphere, and the inert gas can be any one or more of nitrogen, argon or helium.

[0035] As a preferred technical solution of the present application, the sintering method is sintering at 350°C for 3-8 h (such as 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, etc.), followed by sintering at 550-650°C (such as 550°C, 580°C, 600°C, 620°C, 650°C, etc.) for 8-18 h (such as 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, etc.).

[0036] As a preferred technical solution of the present application, the sintering temperature rising rate is 2-5°C / min, such as 2°C / min, 3°C / min, 4°C / min, 5°C / min, etc.

[0037] In a third aspect, the present application provides a positive electrode sheet, wherein the positive electrode sheet comprises the modified sodium pyrophosphate ferric phosphate positive electrode material according to the first aspect or the modified sodium pyrophosphate ferric phosphate positive electrode material prepared by the preparation method according to the second aspect.

[0038] In a fourth aspect, the present application provides a sodium ion battery, wherein the sodium ion battery comprises the positive electrode sheet according to the third aspect.

[0039] Compared with the prior art, the technical scheme provided by the embodiments of the present application has the following advantages: The modified sodium pyrophosphate ferric phosphate positive electrode material provided by the present application optimizes the structural stability of the crystal, improves the ion / electron conduction efficiency and regulates the redox behavior through the synergistic effect of multi-site and multi-element doping, comprehensively improves the cycle life, rate performance and energy density of the sodium pyrophosphate ferric phosphate positive electrode material, and thus effectively improves the electrochemical performance of the sodium ion battery using the same. BRIEF DESCRIPTION OF DRAWINGS

[0040] The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments consistent with the present application and, together with the specification, serve to explain the principles of the present application.

[0041] In order to more clearly illustrate the technical scheme in the embodiments of the present application or the prior art, the accompanying drawings required to be used in the embodiments or the prior art description will be briefly introduced as follows, and obviously, other accompanying drawings can also be obtained by those skilled in the art without creative labor.

[0042] Figure 1 XRD pattern of the modified sodium pyrophosphate ferric phosphate positive electrode material prepared for Example 1 and standard XRD pattern of sodium pyrophosphate ferric phosphate; Figure 2 First circle charge-discharge curve of the button cell assembled by the positive electrode material prepared for Examples 1-3 and Comparative Examples 1-2 at 0.1C; Figure 3 Rate performance graph of the button cell assembled by the positive electrode material prepared for Examples 1-3 and Comparative Examples 1-2 at different rates of 0.1-30C; Figure 4 Long cycle curve graph of the button cell assembled by the positive electrode material prepared for Examples 1-3 and Comparative Examples 1-2 at 1C. DETAILED DESCRIPTION

[0043] In order to more clearly illustrate the above-mentioned purposes, features and advantages of the present application, the scheme of the present application will be further described below. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0044] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. The present application, however, can be practiced in a variety of different manners and embodiments than those described herein; clearly, the scope of the embodiments described in the specification is not to be understood as being limited to the embodiments described herein but rather only by the claims.

[0045] Example 1 The present example provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, the modified sodium iron pyrophosphate positive electrode material is Na 3.8 K 0.2 Fe 2.9 Ti 0.1 (PO4) 1.92 (BO3) 0.08 (P2O7) 1.05 ; The preparation method comprises the following steps: (1) taking sodium carbonate as the Na source, potassium carbonate as the A source, iron oxalate as the iron source, titanium dioxide as the M source, ammonium dihydrogen phosphate as the phosphorus source, boric acid as the E source, citric acid as the reducing agent, and glucose as the carbon source, weighing the above-mentioned raw materials according to Na:K:Fe:Ti:P:B=3.8:0.2:2.9:0.1:4.02:0.08, and the mass ratio of the final product to the reducing agent and the carbon source is 1:1.6:0.03, adding deionized water according to the solid content of 60% and stirring uniformly, sand grinding for 4 h to obtain a mixed solution; (2) heating and stirring the mixed solution at 55°C until the water is completely evaporated to a gel state, transferring to an oven and drying at 120°C for 12 h to obtain an amorphous powder. The above-mentioned powder is loaded into a crucible, and sintering is carried out in an argon atmosphere in a box furnace, the heating rate is 2°C / min, the first stage is 350°C for 5 h, and the second stage is 550°C for 12 h; after grinding, the powder is sieved through a 300 mesh sieve to obtain the modified sodium iron pyrophosphate positive electrode material.

[0046] Example 2 The present example provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, the modified sodium iron pyrophosphate positive electrode material is Na 3.9 Sr 0.1 Fe 2.75 Zr 0.25 (PO4) 1.96 (SiO4) 0.04 (P2O7) 1.14 ; The preparation method comprises the following steps: (1) taking sodium acetate as the Na source, strontium carbonate as the A source, iron nitrate as the iron source, zirconium nitrate as the M source, ammonium dihydrogen phosphate as the phosphorus source, and sodium silicate as the EUsing ascorbic acid as a reducing agent and graphene oxide as a carbon source, the above raw materials were weighed according to the following formula: Na:Sr:Fe:Zr:P:Si=3.9:0.1:2.75:0.25:4.24:0.04, and the mass ratio of the final product to the reducing agent and carbon source was 1:1.2:0.03. Deionized water was added according to a solid content of 55%, and the mixture was stirred evenly and milled for 6 hours to obtain a mixed solution. (2) The mixed solution was heated and stirred at 60°C until the water was completely evaporated and a gel-like state was formed. It was then transferred to an oven and dried at 110°C for 18 h to obtain an amorphous powder. The powder was placed in a crucible and sintered in a box furnace under argon protection. The heating rate was 2°C / min, the first stage was held at 350°C for 4 h, and the second stage was held at 650°C for 10 h. After grinding, it was passed through a 300-mesh sieve to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0047] Example 3 This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material is Na... 3.9 Ba 0.1 Fe 2.84 V 0.16 (PO4) 1.95 F 0.05 (P2O7) 1.09 ; The preparation method includes the following steps: (1) Using sodium sulfate as the Na source and barium carbonate as the Na source A Iron source: ferric sulfate, vanadium pentoxide. M Sodium dihydrogen phosphate is the phosphorus source, and hydrofluoric acid is the phosphorus source. E Using oxalic acid as a reducing agent and acetylene black as a carbon source, the above raw materials were weighed according to the following formula: Na:Ca:Fe:V:P:F=3.9:0.1:2.84:0.16:4.13:0.05, and the mass ratio of the final product to the reducing agent and carbon source was 1:2.0:0.01. Deionized water was added according to a solid content of 55%, and the mixture was stirred evenly. The mixture was then milled for 8 hours to obtain a mixed solution. (2) The mixed solution was heated and stirred at 45°C until the water was completely evaporated and a gel-like state was formed. It was then transferred to an oven and dried at 115°C for 24 hours to obtain an amorphous powder. The powder was placed in a crucible and sintered in a box furnace under argon protection. The heating rate was 2°C / min, the first stage was held at 350°C for 5 hours, and the second stage was held at 550°C for 12 hours. After grinding, it was passed through a 300-mesh sieve to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0048] Example 4 This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material is Na...3.7 K 0.3 Fe 2.7 Cu 0.3 (PO4) 1.9 (SO4) 0.1 (P2O7) 1.025 ; The preparation method comprises the following steps: (1) taking sodium carbonate as the Na source, potassium hydroxide as the A source, iron oxalate as the iron source, copper oxide as the M source, ammonium dihydrogen phosphate as the phosphorus source, potassium sulfate as the E source, citric acid as the reducing agent, and glucose as the carbon source, weighing the above-mentioned raw materials according to Na:K:Fe:Cu:P:S = 3.7:0.3:2.7:0.3:3.95:0.1, and the mass ratio of the final product to the reducing agent and the carbon source is 1:0.8:0.05, adding deionized water to uniformly stir according to the solid content of 60%, and sand grinding for 10 h to obtain a mixed solution; (2) heating and stirring the mixed solution at 55°C until the water is completely evaporated to a gel state, transferring to an oven to dry at 100°C for 24 h to obtain an amorphous powder. The above-mentioned powder is loaded into a crucible, and sintering is carried out in a box furnace under argon protection, the heating rate is 2°C / min, the first stage is 350°C for 5 h, and the second stage is 650°C for 8 h; after grinding, the powder is sieved through a 300-mesh sieve to obtain the modified sodium iron pyrophosphate phosphate positive electrode material.

[0049] Comparative Example 1 The present comparative example provides a sodium iron pyrophosphate phosphate positive electrode material Na4Fe3(PO4)2P2O7 and a preparation method thereof, the preparation method comprising the following steps: (1) taking sodium dihydrogen phosphate as the Na source and the phosphorus source, iron nitrate as the Fe source, citric acid as the reducing agent, and graphene oxide as the carbon source, weighing the above-mentioned raw materials according to Na:Fe:P = 4:3:4, and the mass ratio of the final product to the reducing agent and the carbon source is 1:1.6:0.03, adding deionized water to uniformly stir according to the solid content of 55%, and sand grinding for 4 h to obtain a mixed solution; (2) heating and stirring the mixed solution at 55°C until the water is completely evaporated to a gel state, transferring to an oven to dry at 115°C for 24 h to obtain an amorphous powder. The above-mentioned powder is loaded into a crucible, and sintering is carried out in a box furnace under nitrogen protection, the heating rate is 2°C / min, the first stage is 350°C for 8 h, and the second stage is 560°C for 10 h; after grinding, the powder is sieved through a 300-mesh sieve to obtain a non-doped sodium iron pyrophosphate phosphate positive electrode material.

[0050] Comparative Example 2 The present comparative example provides a modified sodium iron pyrophosphate phosphate positive electrode material and a preparation method thereof, the modified sodium iron pyrophosphate phosphate positive electrode material being Na 3.5 Sr0.5 Fe 2.5 Cu 0.5 (PO4) 1.7 (SO4) 0.3 (P2O7) 1.2 ; The preparation method includes the following steps: (1) Using sodium hydroxide as the Na source and strontium carbonate as the Na source. A Iron source: ferric nitrate, basic copper carbonate. M Phosphorus source: ammonium dihydrogen phosphate, ferric sulfate. E Using ascorbic acid as a reducing agent and acetylene black as a carbon source, the above raw materials were weighed according to the following ratios: Na:Sr:Fe:Cu:P:S = 3.5:0.5:2.5:0.5:4.1:0.3, and the mass ratio of the final product to the reducing agent and carbon source was 1:2.0:0.01. Deionized water was added according to a solid content of 60%, and the mixture was stirred evenly and then milled for 4 hours to obtain a mixed solution. (2) The mixed solution was heated and stirred at 55°C until the water was completely evaporated and a gel-like state was formed. It was then transferred to an oven and dried at 120°C for 12 h to obtain an amorphous powder. The powder was placed in a crucible and sintered in a box furnace under argon protection. The heating rate was 2°C / min, the first stage was held at 350°C for 5 h, and the second stage was held at 550°C for 12 h. After grinding, it was passed through a 300-mesh sieve to obtain an over-doped modified sodium iron pyrophosphate cathode material.

[0051] Comparative Example 3 This comparative example provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material is Na... 3.8 K 0.2 Fe 2.5 Ti 0.5 (PO4) 1.92 (BO3) 0.08 (P2O7) 1.25 The preparation method is as described in Example 1.

[0052] Comparative Example 4 This comparative example provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material is Na... 3.5 K 0.5 Fe 2.9 Ti 0.1 (PO4) 1.92 (BO3) 0.08 (P2O7) 1.05 The preparation method is as described in Example 1.

[0053] Comparative Example 5 This comparative example provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material is Na... 3.8 K 0.2 Fe 2.9 Ti 0.1 (PO4) 1.92 (BO3) 0.3 (P2O7) 0.885 The preparation method is as described in Example 1.

[0054] Performance Test 1 The modified sodium iron phosphate pyrophosphate cathode material prepared in the examples was subjected to XRD testing.

[0055] The XRD pattern of the modified sodium iron phosphate pyrophosphate cathode material prepared in Example 1 and the standard XRD pattern of sodium iron phosphate pyrophosphate are shown below. Figure 1 As shown.

[0056] Performance Test 2 1. The positive electrode materials prepared in the examples and comparative examples were used as positive electrode active materials to prepare positive electrode sheets and assemble coin cells.

[0057] The positive electrode materials prepared in the examples and comparative examples were used as positive electrode active materials and conductive acetylene black (AB) and binder (4), respectively. wt .% polyvinylidene fluoride (PVDF) was dispersed in a mass ratio of 8:1:1 in N The positive electrode slurry was prepared by mixing with NMP (nitromethylpyrrolidone). The slurry was homogenized using a vibratory homogenizer for at least 15 min, then uniformly coated onto aluminum foil using a 150 μm doctor blade. It was then dried in a vacuum oven at 100°C for 10 h until all NMP and residual moisture had evaporated. The loading of the positive electrode active material was approximately 2.5 mg / cm³. 2 In a glove box under an argon atmosphere (H2O / O2 < 0.01 ppm), a coin cell was fabricated using the above-mentioned electrode, sodium sheet, glass fiber membrane, and electrolyte (5 vol% fluoroethylene carbonate added to a 1 mol / L NaClO4 propylene carbonate solution) as the reference electrode, counter electrode, separator, and electrolyte, respectively.

[0058] 2. (1) Test the coin cell prepared in step 1 on a battery testing system at a rate of 129 mAh g. -1 The nominal specific capacity is first activated by charging and discharging at a rate of 0.1 C for 3 cycles, and then charged at a rate of 1 C for 0 min, 12 min, 24 min, 36 min, and 48 min respectively before the program ends. The battery is then disassembled and the electrode plates are removed to obtain electrode plates with a charge SOC of 0%, 20%, 40%, 60%, and 80%.

[0059] (2) The above pole pieces are respectively soaked in 10 g electrolyte (1 mol / L NaClO4 propylene carbonate solution with 5 vol% fluoroethylene carbonate) and placed at 40°C for 168 h. After taking out the pole pieces, 10 mL electrolyte is filtered and tested for ICP-Fe to obtain the dissolution amount of Fe in the electrolyte at 0%, 20%, 40%, 60%, 80% charged SOC. The formula is as follows:

[0060] The dissolution amount of Fe in the active material is obtained, and the results are shown in Table 1: Table 1

[0061] It can be found from Table 1 that the dissolution amount of Fe in the modified sodium pyrophosphate iron phosphate positive material prepared by the present application is much lower than that of the comparative examples, indicating that the modified sodium pyrophosphate iron phosphate positive material has excellent structure stability when the element doping amount is within the range defined by the present application.

[0062] 3. The coin cell prepared in step 1 is tested for ion diffusion coefficient of the positive material by constant current intermittent titration method (GITT), and the results are shown in Table 2: Table 2

[0063] It can be found from Table 2 that the average ion diffusion coefficient of the modified sodium pyrophosphate iron phosphate positive material prepared by the present application is much higher than that of the comparative examples, indicating that the modified sodium pyrophosphate iron phosphate positive material provided by the present application has excellent ion conductivity.

[0064] 4. The coin cell prepared in step 1 is subjected to different rate constant current charge and discharge experiments on a battery test system. The nominal specific capacity is set to 129 mAh g -1 , the voltage window is 1.80-4.20 V, and the charge and discharge rate is sequentially performed according to 0.1 C, 0.2C, 0.5 C, 1 C, 5 C, 10 C, 20 C, 30 C, 40C, 50 C, and each rate is cycled for 5 times.

[0065] The results are shown in Table 3 and Figures 2-3 , wherein Figure 2 is the first cycle charge and discharge curve of the coin cell assembled by the positive material prepared in Examples 1-3 and Comparative Examples 1-2 at 0.1C; Figure 3 is the rate performance graph of the coin cell assembled by the positive material prepared in Examples 1-3 and Comparative Examples 1-2 at 0.1-30C different rates.

[0066] 5. The button cell prepared in step 1 was subjected to 1 C constant current charge-discharge test on a battery test system. The nominal specific capacity was set to 129 mAh g-1, the voltage window was set to 1.80-4.20 V, and the cell was first subjected to 0.1 C charge-discharge cycle for 3 cycles, and then subjected to 1 C charge-discharge cycle. -1

[0067] Figure 4 The long cycle curves of the button cells assembled with the positive electrode materials prepared in examples 1-3 and comparative examples 1-2 were shown in the following figures.

[0068] The results of table 3 were shown as follows: Table 3

[0069] From table 3 and Figures 2-4 It can be found that the 0.1 C discharge capacity, rate performance and cycle performance of the button cell prepared by the examples of the present application are all greatly improved compared with comparative example 1, and from example 1 and comparative examples 2-5, it can be found that too much doping will lead to poor performance, which indicates that the battery prepared by using the modified sodium iron phosphate pyrophosphate positive electrode material provided by the present application has excellent rate performance and cycle performance.

[0070] It should be noted that, in this document, relational terms such as“first” and“second”, and the like, are used solely to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms“comprises”,“comprising”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by“comprises a...” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0071] The foregoing is merely illustrative of the principles of the application and various modifications can be made by persons skilled in the art without departing from the scope and nature of the teaching disclosed in the embodiments. Accordingly, while the present application is presented in terms of embodiments, it should be appreciated that the application is not limited to these embodiments. Rather, the application is most generally defined by the following claims as construed in light of the prior art.​

Claims

1. A modified sodium iron phosphate pyrophosphate cathode material, characterized in that, The modified sodium ferric pyrophosphate cathode material is Na. 4-x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w , Wherein, A is selected from those with an ionic radius greater than Na. + Any one or more of the alkali metal ions or alkaline earth metal ions. M is selected from any one or more transition metal ions; E is selected from SiO4 4- SO4 2- BO3 3- or F - Any one or more of the following; Given 0.1≤x≤0.3, 0.1≤y≤0.3, 0.04≤z≤0.1, the value of w makes Na... 4-x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w The positive and negative charges are balanced.

2. The modified sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The A is selected from K. + 、Sr 2 + Ba 2+ Any one or more of the following; And / or, the M is selected from Mn 2+ Co 2+ Ni 2+ Cu 2+ Zn 2+ V 3+ Cr 3+ Ti 4+ and Zr 4+ Any one or more of the following.

3. The method for preparing the modified sodium iron phosphate pyrophosphate cathode material as described in claim 1 or 2, characterized in that, The preparation method includes: (1) Sodium source, iron source, phosphorus source, A source, M source, E The source, reducing agent, and carbon source are mixed in a solvent water to obtain a mixed solution; (2) The mixed solution is heated until it becomes gel-like, dried and sintered to obtain the modified sodium iron phosphate pyrophosphate cathode material.

4. The preparation method according to claim 3, characterized in that, The product Na obtained by the preparation method is described above. 4- x A x Fe 3-y M y (PO4) 2-z E z (P2O7) w The mass ratio of the reducing agent to the carbon source is 1:(0.8-2.4):(0.01-0.05).

5. The preparation method according to claim 3 or 4, characterized in that, The sodium source includes any one or more of sodium hydroxide, sodium carbonate, sodium sulfate, sodium silicate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium acetate; And / or, the iron source includes any one or more of ferric oxalate, ferric oxide, ferric phosphate, ferric nitrate, ferric sulfate, and ferric citrate; And / or, the phosphorus source includes any one or more of disodium hydrogen phosphate, sodium dihydrogen phosphate, ferric phosphate, diammonium hydrogen phosphate, diammonium dihydrogen phosphate, and phosphoric acid.

6. The preparation method according to any one of claims 3-5, characterized in that, The source A includes any one or more salts or bases of element A; And / or, the source of M includes any one or more of the esters, oxides or salts of element M; And / or, the E source includes any one or more of silicon dioxide, sodium silicate, sulfuric acid, sodium sulfate, boric acid, hydrofluoric acid, and ammonium bifluoride; And / or, the reducing agent includes any one or more of citric acid, oxalic acid, and ascorbic acid; And / or, the carbon source includes any one or more of acetylene black, graphene oxide, Super P, sucrose, and glucose.

7. The preparation method according to any one of claims 3-6, characterized in that, The mixing method is sand milling, and the sand milling time is 3.5-10 h; And / or, the heating temperature is 45-60°C; And / or, the drying temperature is 100-120°C; And / or, the drying time is 12-24 h.

8. The preparation method according to any one of claims 3-6, characterized in that, The sintering is carried out in an inert gas atmosphere; And / or, the sintering method is to sinter at 350°C for 3-8 h, followed by sintering at 550-650°C for 8-18 h; And / or, the sintering heating rate is 2-5°C / min.

9. A positive electrode sheet, characterized in that, The positive electrode sheet comprises the modified sodium iron phosphate pyrophosphate positive electrode material according to claim 1 or 2, or the modified sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 3-7.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet as described in claim 9.

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