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 battery positive electrode materials, in particular to a modified ferric sodium pyrophosphate positive electrode material and a preparation method and application thereof. The modified ferric sodium pyrophosphate positive electrode material comprises a Na4Fe3 (PO4) 2P2O7 inner core, and a Na < 4 + x > Fe < 3 + x > (PO4) 2 + xP2O7 middle coating layer and a Na < 4 + y > Fe < 3 + y > (PO4) 2 + yP2O7 outer coating layer which sequentially coat the surface of the Na4Fe3 (PO4) 2P2O7 inner core, x is greater than or equal to 0.1 and less than 0.5, y is greater than 0.1 and less than or equal to 0.5, and y-x is greater than or equal to 0.1. The modified sodium ferric phosphate pyrophosphate positive electrode material provided by the invention has a multi-level core-shell structure, and a gradient coating layer is formed by gradient doping of NaFePO4, so that the conductivity and the structural stability of the modified sodium ferric phosphate pyrophosphate positive electrode material are effectively improved, and the modified sodium ferric phosphate pyrophosphate positive electrode material has relatively high discharge capacity and excellent rate capability and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery positive electrode materials, in particular to a modified sodium ferric pyrophosphate positive electrode material and a preparation method and application thereof, and specifically to a modified sodium ferric pyrophosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a sodium ion battery. Background Art

[0002] With the intensification 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 resources have the inherent advantages of low cost, wide distribution and abundant reserves. Therefore, the large-scale application of sodium-ion batteries is expected to change the market structure of intermittent renewable energy. The development of electrode materials is one of the most important methods to improve sodium-ion batteries.

[0003] Among the positive electrode materials for sodium ion batteries, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) has the advantages of low raw material price, stable structure, high theoretical capacity (129mAh / g), high working voltage (3.1V, Na + / Na) and low volume expansion (4%), and is considered to be the most promising positive electrode material for sodium ion batteries. However, existing NFPP materials still have problems such as low discharge specific capacity and poor cycle stability.

[0004] Therefore, it is desirable to provide a new modified sodium iron pyrophosphate cathode material. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a modified sodium pyrophosphate iron phosphate positive electrode material and a preparation method and application thereof. The modified sodium pyrophosphate iron phosphate positive electrode material provided by the present invention has a multi-level core-shell structure, and a gradient coating layer is formed by gradient doping NaFePO4, which effectively improves the conductivity and structural stability of the modified sodium pyrophosphate iron phosphate positive electrode material, thereby having a higher discharge capacity and excellent rate performance and cycle performance.

[0006] In a first aspect, the present invention provides a modified sodium iron pyrophosphate positive electrode material, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4+x Fe 3+x (PO4) 2+x P2O7 intermediate coating and Na 4+y Fe 3+y (PO4) 2+y P2O7 outer coating layer, wherein 0.1≤x<0.5, 0.1<y≤0.5, and yx≥0.1.

[0007] The modified sodium iron pyrophosphate positive electrode material provided by the present invention has a multi-level core-shell structure, and a gradient coating layer is formed by gradient doping NaFePO4, which effectively improves the conductivity and structural stability of the modified sodium iron pyrophosphate positive electrode material, thereby having a higher discharge capacity and excellent rate performance and cycle performance. Specifically:

[0008] The modified sodium iron pyrophosphate positive electrode material provided by the present invention has a multi-layer core-shell structure, that is, two layers of NFPP materials with different NaFePO4 doping ratios are coated on the surface of the Na4Fe3(PO4)2P2O7 (NFPP) core. Compared with the positive electrode material with a single shell structure, the multi-layer core-shell structure has a larger specific surface area per unit volume and more interfaces. The multiple shells arranged in sequence from the outside to the inside also construct a unique time-space ordered structure, that is, the electrochemical reaction must first pass through the outer shell before reaching the inner shell, and it must follow this order in time and space. The multiple shells physically isolate the material into multiple relatively independent spaces, and each space can be given its own independent characteristics. In the modified sodium iron pyrophosphate positive electrode material of the present invention, the NaFePO4 content in the molecular formula of the three-layer NFPP structure increases from the inside to the outside, and as the Na + The increase of the first discharge capacity gradually increases. At the same time, the phosphotyrite-type NaFePO4 phase can cause Na + The migration barrier of Na is reduced, providing capacity for the low potential area. Due to the existence of the multilayer structure, the interface between the shells allows Na + Ions diffuse and further activate the NaFePO4 phase, so the NaFePO4 nanodomain will produce a new permeation interface between the newly generated amorphous phase and the remaining grains, ultimately leading to the activation of the entire nanodomain from the outside to the inside. Moreover, compared with diffusion doping and uniform doping, the modified sodium iron pyrophosphate positive electrode material provided by the present invention is gradient doped, which can not only play the dual role of surface element-rich doping and bulk element-poor doping, but also, on the one hand, a stable doping layer is constructed near the surface, the surface structure is stabilized, and the formation of microcracks is inhibited. On the other hand, the amount of non-chemically active dopants can be reduced, the surface and bulk stability of the oxide positive electrode material can be improved, and the stress during the charge and discharge process can be alleviated. Therefore, the shell structure provided by the present invention can effectively improve the conductivity and structural stability of the sodium iron pyrophosphate positive electrode material, and further improve the discharge capacity and rate performance of the material. In addition, the gradient coating layer of the present invention can isolate the core NFPP from the electrolyte, prevent it from direct contact with the electrolyte, alleviate the cycle attenuation failure caused by electrolyte corrosion, and maintain a relatively stable Na even at high voltage (4.2V voltage). + Insertion / extraction transition and stable phase change, significantly improved cycle performance at high temperature and better cycle life.

[0009] The molecular formula of the intermediate coating layer of the present invention is Na 4+x Fe 3+x (PO4) 2+x P2O7, which is composed of Na4Fe3(PO4)2P2O7 and NaFePO4 in a stoichiometric ratio of 1:x, hereinafter abbreviated as NFPP-4+x.

[0010] The molecular formula of the outer coating layer of the present invention is Na 4+y Fe 3+y (PO4) 2+y P2O7, which is composed of Na4Fe3(PO4)2P2O7 and NaFePO4 in a stoichiometric ratio of 1:y, hereinafter abbreviated as NFPP-4+y.

[0011] As a preferred technical solution of the present invention, x=0.25, y=0.5, that is, the middle coating layer is NFPP-4.25, and the outer coating layer is NFPP-4.5. In this case, the modified sodium iron pyrophosphate positive electrode material provided by the present invention comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 P2O7 outer coating.

[0012] Na in the process of de-sodiumization of NFPP and NFPP-4.5 + The ion diffusion coefficient is at the same level of 10 -10 ~10 - 9 cm 2 s -1 In addition, the interfacial charge transfer resistance of the two materials is also very close (NFPP is 220.5Ω, NFPP-4.5 is 204.8Ω). Therefore, the introduction of phosphotyrite-type NaFePO4 phase as a coating material in the NFPP material will not have a negative impact on the structural stability and kinetic performance, but will increase the capacity delivery. However, the applicant found that there is an upper limit to the proportion of NaFePO4 coexisting with NFPP. Excessive NaFePO4 will cause the material to produce Na-deficient Fe3(PO4)2 and Fe4(P2O7)3, which will not be fully activated and affect the capacity. Therefore, when the outer coating layer is NFPP-4.5 and the middle coating layer is NFPP-4.25, a gradient transition can provide more ideal electrochemical performance.

[0013] As a preferred technical solution of the present invention, the mass ratio of the inner core, the intermediate coating layer and the outer coating layer is 1:(1.7-4.1):(7.5-17.3), for example, 1:1.7:7.5, 1:2.5:12.5, 1:3.2:13.8, 1:4.1:17.3, etc., preferably 1:3.2:13.8.

[0014] In a second aspect, the present invention provides a method for preparing the modified sodium iron pyrophosphate positive electrode material according to the first aspect, the preparation method comprising:

[0015] (1) mixing a first sodium source, a first iron source, a first phosphorus source, a reducing agent and a solvent, heating, drying and sintering to obtain a Na4Fe3(PO4)2P2O7 core;

[0016] (2) mixing the Na4Fe3(PO4)2P2O7 core with a second sodium source, a second iron source, a second phosphorus source, a reducing agent and a solvent, heating, drying and sintering to obtain a sodium iron pyrophosphate positive electrode material coated with an intermediate coating layer;

[0017] (3) The sodium iron pyrophosphate positive electrode material coated with the intermediate coating layer is mixed with a third sodium source, a third iron source, a third phosphorus source, a reducing agent and a solvent, and heated, dried and sintered to obtain the modified sodium iron pyrophosphate positive electrode material.

[0018] The modified sodium iron pyrophosphate positive electrode material prepared by the preparation method of the present invention has good spherical morphology and cycle stability.

[0019] As a preferred technical solution of the present invention, the first sodium source, the second sodium source and the third sodium source each independently include any one or more of disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3) and sodium acetate (CH3COONa).

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

[0021] As a preferred technical solution of the present invention, the second iron source and the third iron source each independently include any one or more of ferric nitrate (Fe(NO3)3), ferric sulfate (Fe2(SO4)3) and ferric citrate (FeC6H5O7).

[0022] As a preferred technical solution of the present invention, the first phosphorus source includes any one or more of disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), iron phosphate (FePO4), diammonium hydrogen phosphate ((NH4)2HPO4) and ammonium dihydrogen phosphate (NH4H2PO4).

[0023] As a preferred technical solution of the present invention, the second phosphorus source and the third phosphorus source each independently include any one or more of disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), diammonium hydrogen phosphate ((NH4)2HPO4) and diammonium dihydrogen phosphate (NH4H2PO4).

[0024] As a preferred technical solution of the present invention, the molar ratio of sodium, iron and phosphorus in the first sodium source, the first iron source and the first phosphorus source is (4-4.18):(2.82-3):4, for example, 4:2.82:4, 4.1:2.92:4, 4.18:3:4, etc., preferably (4-4.12):(2.91-3):4.

[0025] As a preferred technical solution of the present invention, the molar ratio of sodium element, iron element and phosphorus element in the second sodium source, the second iron source and the second phosphorus source is (4+x):(3+x):(4+x).

[0026] As a preferred technical solution of the present invention, the molar ratio of sodium element, iron element and phosphorus element in the third sodium source, the third iron source and the third phosphorus source is (4+y):(3+y):(4+y).

[0027] As a preferred technical solution of the present invention, the Fe 3+ The concentration of the ions is 2.82-3 mol / L, for example, 2.82 mol / L, 2.85 mol / L, 2.9 mol / L, 2.95 mol / L, 3 mol / L, etc., preferably 2.91-3 mol / L.

[0028] As a preferred technical solution of the present invention, the Fe 3+ The concentration of the ions is 3.1-3.5 mol / L, for example, 3.1 mol / L, 3.2 mol / L, 3.3 mol / L, 3.4 mol / L, 3.5 mol / L, etc., preferably 3.25 mol / L.

[0029] As a preferred technical solution of the present invention, the Fe 3+The concentration of the ions is 3.3-3.8 mol / L, for example, 3.3 mol / L, 3.4 mol / L, 3.5 mol / L, 3.6 mol / L, 3.7 mol / L, 3.8 mol / L, etc., preferably 3.5 mol / L.

[0030] As a preferred technical solution of the present invention, the reducing agent includes citric acid (C6H8O7), oxalic acid (C2H2O4) sucrose (C 12 H 22 O 11 ) and ascorbic acid, any one or more thereof.

[0031] As a preferred technical solution of the present invention, the molar ratio of the reducing agent in each step to the iron element in the corresponding iron source is 0.36-0.84:1, such as 0.36:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, etc. It can be understood that the reducing agent and the corresponding iron source in each step refer to the reducing agent and the iron source in the same step, for example, the reducing agent and the corresponding iron source in step (1) refer to the reducing agent and the first iron source in step (1).

[0032] As a preferred technical solution of the present invention, the reducing agent is ascorbic acid, and the molar ratio of ascorbic acid to the iron element in the corresponding iron source is 0.36:1; or, the reducing agent is citric acid, and the molar ratio of citric acid to the iron element in the corresponding iron source is 0.84:1; or, the reducing agent is sucrose or oxalic acid, and the molar ratio of sucrose or oxalic acid to the iron element in the corresponding iron source is 0.5:1.

[0033] As a preferred technical solution of the present invention, the solvent includes deionized water.

[0034] As a preferred technical solution of the present invention, the mass ratio of the Na4Fe3(PO4)2P2O7 core to the mass of the sodium element, iron element and phosphorus element in the second sodium source, the second iron source and the second phosphorus source is 0.4-0.9:1, for example, 0.4:1, 0.6:1, 0.8:1, 0.9:1, etc., preferably 0.5:1.

[0035] As a preferred technical solution of the present invention, the mass ratio of the sodium iron pyrophosphate positive electrode material coated by the intermediate coating layer to the sodium element, iron element and phosphorus element in the third sodium source, the third iron source and the third phosphorus source is 0.4-0.9:1, preferably 0.5:1.

[0036] As a preferred technical solution of the present invention, the mixing method is sand grinding, and the sand grinding time is 0.1-5h, for example, 0.1h, 0.5h, 1h, 2h, 3h, 4h, 5h, etc.

[0037] As a preferred technical solution of the present invention, the heating temperature is 50-80°C, such as 50°C, 60°C, 70°C, 80°C, etc., and the heating is performed until the solution is in a gel state.

[0038] As a preferred technical solution of the present invention, the drying temperature is 110-125°C, for example, 110°C, 115°C, 120°C, 125°C, etc., preferably 120°C; the drying time is 18-36h, for example, 18h, 24h, 30h, 36h, etc., preferably 24h.

[0039] As a preferred technical solution of the present invention, the sintering method is to calcine at 350°C for 3-8h in an inert gas atmosphere, and then calcine at 550-650°C for 8-18h, with a heating rate of 2°C / min, wherein the 3-8h can be 3h, 4h, 5h, 6h, 7h, 8h, etc., the 550-650°C can be 550°C, 580°C, 600°C, 620°C, 650°C, etc., the 8-18h can be 8h, 10h, 12h, 14h, 16h, 18h, etc., and the inert gas can be any one or more of nitrogen, argon or helium.

[0040] As a preferred technical solution of the present invention, grinding is performed before and after sintering, wherein before sintering, grinding is performed until there is no granular feeling when touched, and after sintering, grinding is performed until D50 is 9-15 μm.

[0041] In a third aspect, the present invention provides a positive electrode plate, which includes the modified sodium iron pyrophosphate positive electrode material described in the first aspect or the modified sodium iron pyrophosphate positive electrode material prepared by the preparation method described in the second aspect.

[0042] In a fourth aspect, the present invention provides a sodium ion battery, comprising the positive electrode sheet described in the third aspect.

[0043] The technical solution provided by the embodiment of the present invention has the following advantages compared with the prior art:

[0044] The modified sodium iron pyrophosphate positive electrode material provided by the present invention has a multi-layer core-shell structure, and a gradient coating layer is formed by gradient doping NaFePO4, which effectively improves the conductivity and structural stability of the modified sodium iron pyrophosphate positive electrode material, thereby having a higher discharge capacity and excellent rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0047] Figure 1 This is the XRD diffraction pattern of the modified sodium iron pyrophosphate positive electrode material prepared in Example 1 of the present invention;

[0048] Figure 2 The first cycle charge and discharge curve of the button cell assembled with the positive electrode materials prepared in Example 1 and Comparative Examples 1 and 2 of the present invention at 0.1C;

[0049] Figure 3 The figure is a rate performance diagram of button-type batteries assembled with the positive electrode materials prepared in Example 1 and Comparative Examples 1 and 2 of the present invention;

[0050] Figure 4 The cycle performance diagram of button-type batteries assembled with the positive electrode materials prepared in Example 1 and Comparative Examples 1 and 2 of the present invention at 1C;

[0051] Figure 5 The first cycle charge and discharge curve of a sodium ion full battery assembled with the modified sodium iron pyrophosphate positive electrode material prepared in Example 1 of the present invention at 0.2C;

[0052] Figure 6 This is a cycle performance diagram of a sodium ion full battery assembled with the modified sodium iron pyrophosphate positive electrode material prepared in Example 1 of the present invention at 1C. DETAILED DESCRIPTION

[0053] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0054] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0055] Example 1

[0056] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25(PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 outer coating layer, the preparation method comprises the following steps:

[0057] (1) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4:3:4; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.84 times the molar amount of citric acid of the Fe element was added at the same time, and sand milled for 1 hour; heated and stirred at 55°C until the solution became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 5 hours, and a second stage of 650°C for 12 hours; after grinding, the core material was sieved through a 300-mesh sieve.

[0058] (2) Sodium bicarbonate, ferrous sulfate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.25:3.25:4.25; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.25 mol / L, and 0.52 times the molar amount of oxalic acid of the Fe element was added at the same time; the core material in step 1 was weighed according to 0.5 times the mass of the (Na+Fe+P) elements in the solution, and added to the above solution, and stirred evenly; heated and stirred at 65°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 4h, and a second stage of 600°C for 14h; after grinding, the single-layer coated positive electrode material was obtained.

[0059] (3) Sodium acetate, ferric nitrate and ammonium dihydrogen phosphate were weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added to deionized water and mixed to prepare a solution with an Fe concentration of 3.5 mol / L, and glucose was added at 0.40 times the molar amount of Fe element; the single-layer coated positive electrode material obtained in step 2 was weighed according to 0.5 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 55°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 550°C for 12h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained through a 300-mesh sieve, and its XRD diffraction pattern was as follows: Figure 1 shown.

[0060] Example 2

[0061] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 outer coating layer, the preparation method comprises the following steps:

[0062] (1) Sodium oxalate, ferric phosphate and sodium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4:3:4; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.36 times the molar amount of citric acid of the Fe element was added at the same time, and sand milled for 3 h; heated and stirred at 55°C until the solution became a gel, transferred to an oven and dried at 120°C for 24 h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 3°C / min, a first stage of 350°C for 5 h, and a second stage of 550°C for 18 h; after grinding, the core material was sieved through a 300-mesh sieve.

[0063] (2) Sodium acetate, ferric citrate and sodium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.25:3.25:4.25; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.25 mol / L, and sucrose was added in an amount 0.50 times the molar amount of the Fe element; the core material obtained in step 1 was weighed according to 0.5 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 65°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 600°C for 14h; after grinding, the single-layer coated positive electrode material was obtained.

[0064] (3) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added to deionized water and mixed to prepare a solution with an Fe concentration of 3.5 mol / L, and sucrose was added in an amount equal to 0.45 times the molar amount of the Fe element; the single-layer coated positive electrode material obtained in step 2 was weighed according to 0.5 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 75°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 570°C for 14h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained.

[0065] Example 3

[0066] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 outer coating layer, the preparation method comprises the following steps:

[0067] (1) According to the molar ratio of elements Na:Fe:P=4:3:4, disodium hydrogen phosphate, iron oxide, and ammonium dihydrogen phosphate were calculated and weighed; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.56 times the molar amount of oxalic acid of the Fe element was added at the same time, and sand milled for 5 hours; heated and stirred at 75°C until the solution became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under helium protection, with a heating rate of 2°C / min, a first stage of 350°C for 7 hours, and a second stage of 550°C for 16 hours; after grinding, the core material was sieved through a 300-mesh sieve.

[0068] (2) Sodium carbonate, ferric nitrate and diammonium hydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.25:3.25:4.25; the above substances were added to deionized water and mixed to prepare a solution with an Fe concentration of 3.25 mol / L, and ascorbic acid was added in an amount 0.36 times the molar amount of the Fe element; the core material in step 1 was weighed according to 0.7 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 60°C until the solution became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 5 hours, and a second stage of 600°C for 14 hours; after grinding, the single-layer coated positive electrode material was obtained.

[0069] (3) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added to deionized water and mixed to prepare a solution with an Fe concentration of 3.5 mol / L, and sucrose was added in an amount equal to 0.45 times the molar amount of the Fe element; the single-layer coated positive electrode material obtained in step 2 was weighed according to 0.6 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 75°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under helium protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 590°C for 16h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained.

[0070] Example 4

[0071] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 outer coating layer, the preparation method comprises the following steps:

[0072] (1) Sodium acetate, ferrous sulfate and ammonium dihydrogen phosphate were weighed according to the molar ratio of elements Na:Fe:P=4:3:4; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.5 times the molar amount of sucrose of the Fe element was added; the solution was heated and stirred at 60°C until it became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 4 hours, and a second stage of 580°C for 15 hours; after grinding, the core material was sieved through a 300-mesh sieve.

[0073] (2) Sodium oxalate, ferrous sulfate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.25:3.25:4.25; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.25 mol / L, and 0.8 times the molar amount of citric acid of the Fe element was added at the same time; the core material obtained in step 1 was weighed according to 0.4 times the mass of the (Na+Fe+P) elements in the solution, and added to the above solution, and stirred evenly; heated and stirred at 75°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 4h, and a second stage of 610°C for 13h; after grinding, the single-layer coated positive electrode material was obtained.

[0074] (3) Sodium bicarbonate, ferric citrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.5 mol / L, and sucrose was added in an amount equivalent to 0.36 times the molar amount of the Fe element; the single-layer coated positive electrode material obtained in step 2 was weighed according to 0.9 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 70°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under helium protection, with a heating rate of 2°C / min, a first stage of 350°C for 4h, and a second stage of 630°C for 10h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained.

[0075] Example 5

[0076] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 outer coating layer, the preparation method comprises the following steps:

[0077] (1) Sodium dihydrogen phosphate, ferric citrate and sodium bicarbonate were calculated and weighed according to the molar ratio of the elements Na:Fe:P=4:3:4; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.52 times the molar amount of oxalic acid of the Fe element was added; the solution was heated and stirred at 55°C until it became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 3°C / min, a first stage of 350°C for 5 hours, and a second stage of 550°C for 18 hours; after grinding, the core material was sieved through a 300-mesh sieve.

[0078] (2) Calculate and weigh ferric citrate and sodium dihydrogen phosphate according to the molar ratio of elements Na:Fe:P=4.25:3.25:4.25; add the above substances together into deionized water and mix to prepare a solution with an Fe concentration of 3.25 mol / L, and add ascorbic acid in an amount 0.36 times the molar amount of the Fe element; weigh the core material obtained in step 1 according to 0.6 times the mass of the (Na+Fe+P) element in the solution, add it to the above solution, and stir evenly; heat and stir at 65°C until the solution becomes a gel, transfer it to an oven and dry it at 120°C for 24 hours, and then grind it into an amorphous powder; put the above powder into a sagger, and sinter it in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 3 hours, and a second stage of 600°C for 14 hours; after grinding, pass through a 300-mesh sieve to obtain a single-layer coated positive electrode material.

[0079] (3) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.5 mol / L, and ascorbic acid was added in an amount 0.36 times the molar amount of the Fe element; the single-layer coated positive electrode material obtained in step 2 was weighed according to 0.5 times the mass of the (Na+Fe+P) element in the solution, and added to the above solution, and stirred evenly; heated and stirred at 75°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under nitrogen protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 650°C for 10h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained.

[0080] Example 6

[0081] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.1 Fe 3.1 (PO4) 2.1x P2O7 intermediate coating and Na 4.5 Fe 3.5 (PO4) 2.5 P2O7 outer coating;

[0082] The preparation method is the same as that of Example 1, except that:

[0083] In step (2) of this embodiment, sodium bicarbonate, ferrous sulfate and ammonium dihydrogen phosphate are calculated and weighed according to the molar ratio of the elements Na:Fe:P=4.1:3.1:4.1.

[0084] Example 7

[0085] This embodiment provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.35 Fe 3.35 (PO4) 2.35 P2O7 intermediate coating and Na 4.45 Fe 3.45 (PO4) 2.45 P2O7 outer coating;

[0086] The preparation method is the same as that of Example 1, except that:

[0087] In this embodiment, step (2) calculates and weighs sodium bicarbonate, ferric sulfate, and ammonium dihydrogen phosphate according to the molar ratio of elements Na:Fe:P=4.35:3.35:4.35; step (3) calculates and weighs sodium acetate, ferric nitrate, and ammonium dihydrogen phosphate according to the molar ratio of elements Na:Fe:P=4.45:3.45:4.45.

[0088] Comparative Example 1

[0089] This comparative example provides a sodium iron pyrophosphate positive electrode material Na4Fe3(PO4)2P2O7 and a preparation method thereof, the preparation method comprising the following steps:

[0090] Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of the elements Na:Fe:P=4:3:4; the above substances were added into deionized water and mixed to prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and citric acid in an equal molar amount to the Fe element was added at the same time; the solution was heated and stirred at 55°C until it became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 3 hours, and a second stage of 550°C for 12 hours; after grinding, the uncoated positive electrode material was sieved through a 300-mesh sieve.

[0091] Comparative Example 2

[0092] This comparative example provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core coated with Na 4.5 Fe 3.5 (PO4) 2.5 The P2O7 coating layer, the preparation method comprises the following steps:

[0093] (1) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of the elements Na:Fe:P=4:3:4; the above substances were added into deionized water to mix and prepare a solution with an Fe concentration of 3 mol / L, stirred evenly, and 0.84 times the molar amount of citric acid of the Fe element was added; the solution was heated and stirred at 55°C until it became a gel, transferred to an oven and dried at 120°C for 24 hours, and then ground into an amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 3 hours, and a second stage of 550°C for 12 hours; after grinding, the core material was sieved through a 300-mesh sieve.

[0094] (2) Sodium carbonate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of elements Na:Fe:P=4.5:3.5:4.5; the above substances were added together into deionized water to mix and prepare a solution with an Fe concentration of 3.5 mol / L, and glucose was added at 0.40 times the molar amount of Fe element; the core material obtained in step 1 was weighed according to 0.5 times the mass of (Na+Fe+P) elements in the solution, and added to the above solution, and stirred evenly; heated and stirred at 55°C until the solution became a gel, transferred to an oven and dried at 120°C for 24h, and then ground into amorphous powder; the above powder was loaded into a sagger, and sintered in a box furnace under argon protection, with a heating rate of 2°C / min, a first stage of 350°C for 3h, and a second stage of 550°C for 12h; after grinding, the modified sodium iron pyrophosphate positive electrode material was obtained by sieving.

[0095] Comparative Example 3

[0096] This comparative example provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 intermediate coating and Na 4.6 Fe 3.6 (PO4) 2.6 P2O7 outer coating;

[0097] The preparation method is the same as that of Example 1, except that:

[0098] In step (3) of this comparative example, sodium acetate, ferric nitrate and ammonium dihydrogen phosphate were calculated and weighed according to the molar ratio of the elements Na:Fe:P=4.6:3.6:4.6.

[0099] Comparative Example 4

[0100] This comparative example provides a modified sodium iron pyrophosphate positive electrode material and a preparation method thereof, wherein the modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4.5 Fe 3.5 (PO4) 2.5 P2O7 intermediate coating and Na 4.25 Fe 3.25 (PO4) 2.25 P2O7 outer coating;

[0101] The preparation method is the same as that of Example 1, except that step (3) is swapped with step (2).

[0102] Performance Test 1

[0103] (1) Assembling button cells: The positive electrode materials prepared in the examples and comparative examples were used as positive electrode active materials, respectively, and mixed with conductive acetylene black (AB) and a binder (4 wt.% polyvinylidene fluoride (PVDF) dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1) to prepare positive electrode slurry. The slurry was homogenized with a vibrating homogenizer for at least 15 minutes, and then evenly coated on aluminum foil with a 150 μm scraper, and dried in a vacuum oven at 100 ° C for 10 hours until NMP and residual moisture were completely volatilized. The loading amount of the positive electrode active material was about 2.5 mg / cm 2In a glove box with argon atmosphere (H2O / O2<0.01ppm), button cells were made using the above-mentioned electrode, sodium sheet, glass fiber membrane and electrolyte (5Vol% fluoroethylene carbonate was added to 1mol / L NaClO4 propylene carbonate solution) as reference electrode, counter electrode, diaphragm and electrolyte, respectively.

[0104] (2) Conduct constant current charge and discharge experiments at different rates on the battery test system. The nominal specific capacity is 129 mAh g -1 The voltage window is 1.80-4.20V, and the charge and discharge rates are 0.1C, 1C, 5C, 10C, 20C, 30C, 40C, and 50C in sequence, with each rate cycled 5 times.

[0105] The results are shown in Table 1 and Figure 2-4 As shown, Figure 2 The first cycle charge and discharge curve of the button cell assembled with the positive electrode materials prepared in Example 1, Comparative Examples 1 and 2 at 0.1C, Figure 3 The figure is a rate performance diagram of button-type batteries assembled with the positive electrode materials prepared in Example 1, Comparative Examples 1 and 2. Figure 4 This is a cycle performance diagram of button cells assembled with the positive electrode materials prepared in Example 1 and Comparative Examples 1 and 2 at 1C.

[0106] The results in Table 1 are as follows:

[0107] Table 1

[0108]

[0109] It can be found from Table 1 that the button cell assembled using the modified sodium iron pyrophosphate positive electrode material prepared by the present invention has a high discharge capacity and first coulombic efficiency as well as excellent rate performance.

[0110] Performance Test 2

[0111] (1) Assemble a 3500mAh sodium-ion full battery: The positive electrode materials prepared in the examples and comparative examples are used as positive electrode active materials, respectively, and mixed with AB and PVDF binders in a mass ratio of 9.5:0.25:0.25 to prepare positive electrode slurry. The negative electrode slurry is prepared by mixing hard carbon, AB and binder carboxymethyl cellulose and binder styrene butadiene rubber in a mass ratio of 9.6:0.2:0.1:0.1. The positive and negative electrode capacity ratio is 1.12. The electrolyte is: 1 mol / L NaPF6, the solvent is propylene carbonate / ethylene carbonate / diethyl carbonate (PC / EC / DEC, volume ratio 1:1:1), and fluoroethylene carbonate (2 vol% FEC). The amount of electrolyte used is 6.5 gAh -1 . PP / PE / PP film is used as the diaphragm.

[0112] (2) Nominal specific capacity is 129 mAh g -1 The voltage window is 1.0-3.65V. After 5 cycles at a rate of 0.2C, it is cycled 700 times at a rate of 1C.

[0113] The test results are shown in Table 2 and Figure 5-6 As shown, Figure 5 The first cycle charge and discharge curve of the sodium ion full battery assembled with the modified sodium iron pyrophosphate positive electrode material prepared in Example 1 at 0.2C, Figure 6 This is a cycle performance diagram of a sodium ion full battery assembled with the modified sodium iron pyrophosphate positive electrode material prepared in Example 1 at 1C.

[0114] The results in Table 2 are as follows:

[0115] Table 2

[0116]

[0117]

[0118] It can be found from Table 2 that the sodium ion battery assembled using the modified sodium iron pyrophosphate positive electrode material prepared by the present invention has excellent electrochemical properties such as high discharge capacity and good cycle stability.

[0119] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0120] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments described herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modified sodium iron pyrophosphate positive electrode material, characterized in that: The modified sodium iron pyrophosphate positive electrode material comprises a Na4Fe3(PO4)2P2O7 core and Na 4+x Fe 3+x (PO4) 2+x P2O7 intermediate coating and Na 4+y Fe 3+y (PO4) 2+y P2O7 outer coating layer, wherein 0.1≤x<0.5, 0.1<y≤0.5, and yx≥0.

1.

2. The modified sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that x=0.25, y=0.

5.

3. The modified sodium iron pyrophosphate positive electrode material according to claim 1 or 2, characterized in that: The mass ratio of the inner core, the middle coating layer and the outer coating layer is 1:(1.7-4.1):(7.5-17.3).

4. The method for preparing the modified sodium iron pyrophosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises: (1) mixing a first sodium source, a first iron source, a first phosphorus source, a reducing agent and a solvent, heating, drying and sintering to obtain a Na4Fe3(PO4)2P2O7 core; (2) mixing the Na4Fe3(PO4)2P2O7 core with a second sodium source, a second iron source, a second phosphorus source, a reducing agent and a solvent, heating, drying and sintering to obtain a sodium iron pyrophosphate positive electrode material coated with an intermediate coating layer; (3) The sodium iron pyrophosphate positive electrode material coated with the intermediate coating layer is mixed with a third sodium source, a third iron source, a third phosphorus source, a reducing agent and a solvent, and heated, dried and sintered to obtain the modified sodium iron pyrophosphate positive electrode material.

5. The preparation method according to claim 4, characterized in that: The first sodium source, the second sodium source and the third sodium source each independently include any one or more of disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate and sodium acetate; and / or, the first iron source comprises any one or more of ferric oxalate, ferric oxide, ferric phosphate, ferric nitrate, ferric sulfate and ferric citrate; And / or, the second iron source and the third iron source each independently include any one or more of ferric nitrate, ferric sulfate and ferric citrate; and / or, the first phosphorus source includes any one or more of disodium hydrogen phosphate, sodium dihydrogen phosphate, iron phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate; And / or, the second phosphorus source and the third phosphorus source each independently include any one or more of disodium hydrogen phosphate, sodium dihydrogen phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

6. The preparation method according to claim 4 or 5, characterized in that: The molar ratio of sodium, iron and phosphorus in the first sodium source, the first iron source and the first phosphorus source is (4-4.18):(2.82-3):4, preferably (4-4.12):(2.91-3):4; And / or, the molar ratio of sodium element, iron element and phosphorus element in the second sodium source, the second iron source and the second phosphorus source is (4+x):(3+x):(4+x); And / or, the molar ratio of sodium element, iron element and phosphorus element in the third sodium source, the third iron source and the third phosphorus source is (4+y):(3+y):(4+y).

7. The preparation method according to any one of claims 4 to 6, characterized in that: The reducing agent includes any one or more of citric acid, oxalic acid, sucrose and ascorbic acid; and / or, the molar ratio of the reducing agent in each step to the iron element in the corresponding iron source is 0.36-0.84:1; And / or, the solvent comprises deionized water.

8. The preparation method according to any one of claims 4 to 7, characterized in that: The mixing method is sand grinding, and the sand grinding time is 0.1-5h; And / or, the heating temperature is 50-80° C., and the heating is performed until the solution is in a gel state; And / or, the drying temperature is 110-125°C, and the drying time is 18-36h; And / or, the sintering method is calcining at 350°C for 3-8h, then calcining at 550-650°C for 8-18h under an inert gas atmosphere, with a heating rate of 2°C / min; And / or, grinding is performed before and after the sintering.

9. A positive electrode sheet, characterized in that: The positive electrode plate comprises the modified sodium iron pyrophosphate positive electrode material according to any one of claims 1 to 3 or the modified sodium iron pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 4 to 8.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises the positive electrode sheet according to claim 9.

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