Pyrophosphate and ferric sodium phosphate composite material and preparation process, evaluation method, process optimization method and application thereof

Abstract

The invention belongs to the technical field of sodium-ion battery materials, and particularly relates to a pyrophosphoric acid and sodium ferric phosphate composite material as well as a preparation process, an evaluation method, a process optimization method and application thereof. The pyrophosphoric acid and ferric phosphate sodium composite material has a spinel type NaFePO4 phase; xRD diffraction is carried out on the pyrophosphate ferric phosphate sodium composite material under CuK alpha radiation, in an obtained XRD diffraction pattern, when 2 theta is 33.6 + / -0.2 degrees and 32.8 + / -0.2 degrees, a characteristic diffraction peak T of pyrophosphate ferric phosphate sodium and a characteristic diffraction peak O of spinel type NaFePO4 exist, the peak intensities are IT and IO respectively, the diffraction peak intensity ratio Ir is IT / IO, and 6.19 < = Ir < = 13. The invention creatively discovers that the pyrophosphate and sodium ferric phosphate composite material has a spinel type NaFePO4 phase with a specific content, so that the capacity performance of the material can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery materials, and in particular relates to a sodium iron pyrophosphate composite material and a preparation process, an evaluation method, a process optimization method and an application thereof. Background Art

[0002] Sodium-ion batteries are rocking-chair batteries, which are secondary batteries that rely on the insertion and extraction of ions between the positive and negative electrodes. Both the positive and negative electrode materials allow sodium ions to be reversibly inserted and extracted. Research on sodium-ion batteries has been going on since 1982, and has made great progress in recent years and has met the conditions for commercial application. At present, the positive electrode materials of sodium-ion batteries mainly include oxides, Prussian blue and polyanions.

[0003] As a polyanion sodium ion battery positive electrode material, sodium iron pyrophosphate composite material has a structurally stable sodium ion diffusion channel and has good cycle performance, as well as a moderate operating voltage and specific capacity. It is a cheap and promising sodium ion battery positive electrode material.

[0004] At present, the common synthesis method of this material is the liquid phase method, which has high requirements for reaction process conditions, complex production equipment, and difficulty in mass production, and is not suitable for large-scale industrial applications. In addition, there is also a solution for preparing sodium iron pyrophosphate material by wet homogeneous sand milling combined with spray drying, but in order to ensure the acquisition of pure phase Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), it is necessary to achieve uniform dispersion of the precursor salt at the nanoscale and control the particle size between 1 and 0.1 μm, which places high requirements on the operating process and reaction equipment; and the capacity performance of the sodium iron pyrophosphate composite material obtained by the existing synthesis process is limited, and there is still room for further improvement. Summary of the invention

[0005] In view of the above technical problems, one of the objectives of the present invention is to provide a sodium iron pyrophosphate composite material.

[0006] The second object of the present invention is to provide a preparation process of sodium iron pyrophosphate composite material.

[0007] A third object of the present invention is to provide an evaluation method for sodium iron pyrophosphate composite materials.

[0008] A fourth object of the present invention is to provide a method for optimizing the preparation process of sodium iron pyrophosphate composite material.

[0009] A fifth object of the present invention is to provide a sodium iron pyrophosphate composite material for use as a positive electrode material for a sodium ion battery in a sodium ion battery.

[0010] Compared with the existing technology that pursues the pure phase of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), the present invention creatively discovered that when the sodium iron pyrophosphate composite material has a specific content of spinel NaFePO 4 phase, the capacity performance of the sodium iron pyrophosphate composite material can be effectively improved, but the NaFePO 4 The phase may also play a certain role as a structural phase. When its content is too low, the structural stability of the material will be destroyed, thereby reducing the overall electrochemical performance; when its content is too high, the overall electrochemical performance will be reduced.

[0011] Therefore, the present invention provides a sodium iron pyrophosphate composite material, wherein the sodium iron pyrophosphate composite material has a spinel type NaFePO 4 phase; the sodium iron pyrophosphate composite material is subjected to XRD diffraction under CuKα radiation, and the obtained XRD diffraction pattern has characteristic diffraction peaks T of sodium iron pyrophosphate and spinel type NaFePO at 2θ of 33.6±0.2° and 32.8±0.2° 4 The characteristic diffraction peak O, whose peak intensity is I T and I O , let the diffraction peak intensity ratio Ir = I T / I O , where 6.19≤Ir≤13.

[0012] Furthermore, the sodium iron pyrophosphate composite material comprises sodium iron pyrophosphate and a carbon layer coated on the surface of the sodium iron pyrophosphate, and the chemical formula is Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C, where 0≤x≤0.02.

[0013] Further preferably, the carbon content of the sodium iron pyrophosphate composite material is 2-5wt%.

[0014] Further preferably, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in the range of 100-200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size of the secondary particles is mainly distributed in the range of 300-800 nm.

[0015] Based on NaFePO 4 Effect of phase content on material properties, NaFePO 4 Phase control is the most core difficulty in the synthesis of this material.

[0016] In this regard, the present invention also provides a preparation process of the sodium iron pyrophosphate composite material, comprising the following steps: (1) Weigh the synthetic Na according to the preset feeding ratio 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 The raw materials and an appropriate amount of carbon source are wet-grinded and mixed to obtain a mixture; (2) After the mixture is dried, it is sintered in a protective atmosphere according to preset sintering parameters to obtain a sodium iron pyrophosphate composite material Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C; Among them, 0≤x≤0.02.

[0017] Furthermore, in step (1): the synthesis of Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 The raw materials include an iron source and a sodium source.

[0018] Further preferably, the iron source is ammonium ferrous phosphate, and the sodium source includes sodium pyrophosphate and a P-free sodium source.

[0019] Further preferably, the P-free sodium source is selected from any one or more of sodium carbonate, ammonium bicarbonate, sodium acetate and sodium citrate.

[0020] More preferably, the molar ratio of ammonium ferrous phosphate, sodium pyrophosphate and the P-free sodium source is 3:0.96~1:0.5~0.54.

[0021] Furthermore, the carbon source is any one or more of glucose, sucrose, starch, and polyethylene glycol.

[0022] Further preferably, the amount of the carbon source is synthetic Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 75~20% of the mass of the raw materials.

[0023] Furthermore, the wet grinding and mixing time is 4 to 6 hours; the medium of the wet grinding is a solvent with a boiling point lower than 80° C., such as ethanol.

[0024] Furthermore, in step (1), the grinding method is vibration milling or rolling milling, and more preferably continuous rolling ball milling.

[0025] Furthermore, in step (2), the preset sintering parameters include the sintering heating rate, the sintering temperature and the sintering time.

[0026] More preferably, the heating rate of the sintering is 10-15° C. / min.

[0027] More preferably, the sintering temperature is 500-550°C.

[0028] More preferably, the sintering time is 8 to 12 hours.

[0029] Furthermore, in step (2), the protective atmosphere is an inert gas or nitrogen; more preferably, it is one of high-purity nitrogen, argon, and argon-hydrogen mixed gas.

[0030] The present invention discloses a method for evaluating a sodium iron pyrophosphate composite material, comprising the following steps: S1. Performing an XRD diffraction test on the sodium iron pyrophosphate composite material under CuKα radiation, and obtaining an XRD diffraction pattern, and calculating the corresponding performance test value according to the peak intensity of the required characteristic diffraction peak; S2. Obtain performance evaluation results by comparing performance test values ​​with performance reference values.

[0031] Further, The performance test value in step S1 is Ir', Ir'=I T ' / I O ', where I T ' is the peak intensity of the characteristic diffraction peak T at 2θ of 33.6±0.2° in the XRD diffraction pattern, I O ' is the peak intensity of the characteristic diffraction peak O of the XRD diffraction spectrum at 2θ of 32.8±0.2°; The performance reference value in step S2 is Ir, and its interval range is 6.19≤Ir≤13; when Ir' is within the interval range of Ir, the performance evaluation result of the material is excellent.

[0032] The present invention has found that there are two possible reaction pathways in the synthesis reaction of the sodium iron pyrophosphate composite material, namely: 4NH 4 FePO4 ·H 2 O+Na 2 CO 3 +Na 4 P 2 O 7 →2NaFePO 4 +2Na 2 FeP 2 O 7 +CO 2 +4NH 3 +6H 2 O(1) 6NH 4 FePO 4 ·H 2 O+2Na 2 CO 3 +Na 4 P 2 O 7 →2Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 )+6NH 3 +9H 2 O+2CO 2 (2) During the reaction, both reactions may occur and compete with each other. However, since reaction (1) has a greater entropy increase and a greater thermodynamic advantage, NaFePO 4 The phase formation temperature range is about 300~400℃, so the electrochemically inert NaFePO 4 Compared with reaction (2), the tendency of the target product to form is stronger.

[0033] Based on the above findings, the present invention can reduce the NaFePO by increasing the heating rate. 4 The phase formation time is controlled, and combined with the method of using lattice defects (controlling the iron-phosphorus ratio) to inhibit the formation of thermodynamically dominant impure phases, the content of inert phases can be reduced, thereby improving the material capacity performance.

[0034] In this regard, the present invention discloses a method for optimizing a preparation process of a sodium iron pyrophosphate composite material, wherein the sodium iron pyrophosphate composite material prepared by the preparation process of the composite sodium iron phosphate positive electrode material is evaluated using the above-mentioned sodium iron pyrophosphate composite material evaluation method, and further comprising the following steps: S3. By comparing the performance test value and the performance reference value, it is determined whether the performance test value is within the range of the performance reference value; If the performance test value is not within the range of the performance reference value, repeat steps (1), (2), S1, and S2, and optimize and adjust the process parameters in step (1) or (2) according to the comparison results until the performance test value is within the range of the performance reference value, and then output the optimized process parameters.

[0035] Furthermore, the process parameters in step (1) or (2) are optimized and adjusted according to the comparison results, wherein the process parameters include the amount of iron source added to the preset feed in step (1) and / or the preset sintering parameters in step (2).

[0036] Further preferably: when Ir' is less than the lower limit of the interval of Ir, the amount of iron source added is correspondingly reduced and / or the heating rate is increased; when Ir' is greater than the upper limit of the interval of Ir, the amount of iron source added is correspondingly increased and / or the heating rate is reduced.

[0037] The present invention also discloses the use of the sodium iron pyrophosphate composite material described above, or the sodium iron pyrophosphate composite material prepared by the preparation process described above, or the sodium iron pyrophosphate composite material evaluated as excellent by the evaluation method described above, or the sodium iron pyrophosphate composite material prepared by the sodium iron pyrophosphate composite material preparation process optimized by the optimization method described above as a positive electrode material for sodium ion batteries in sodium ion batteries.

[0038] The present invention has the following beneficial effects: (1) In the prior art, pure phase Na is generally obtained by process improvement. 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), and reduce the electrochemically inert spinel NaFePO 4 The present invention creatively finds that the sodium iron pyrophosphate composite material has a specific content of spinel NaFePO 4 phase, it can effectively improve the capacity performance of the material. Tests have shown that NaFePO 4 The sodium iron pyrophosphate composites have an average capacity that is about 10% higher than materials with lower or higher content.

[0039] (2) This application defines the spinel type NaFePO by the characteristic peaks and the ratio of the peak intensity in the XRD spectrum obtained by XRD diffraction of the sodium iron pyrophosphate composite material under CuKα radiation. 4 The content of the phase ensures that the capacity performance is improved while preventing its electrochemical inertness from affecting other electrochemical properties of the material.

[0040] (3) The present invention proposes a method based on NH 4 FePO 4 ·H 2 O is the iron source of the composite sodium iron phosphate solid phase synthesis method, the raw material cost is low, the formula is easy to control, the synthesized product NaFePO 4 The (Marticite) content is appropriate, the process is simple, the economic benefit is high, and the prepared material has excellent electrochemical properties.

[0041] (4) The evaluation method of the sodium iron pyrophosphate composite material of the present invention is used to evaluate the performance of the prepared composite sodium iron phosphate. If the evaluation fails, the process is adjusted to control the iron-phosphorus ratio and sintering temperature to reduce the NaFePO 4 The residence time in the phase temperature range controls the NaFePO 4 phase, thereby controlling the NaFePO 4 The content of the phase is controlled to obtain a process for preparing products with better performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] 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, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 This is the XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 1.

[0044] Figure 2 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Example 1.

[0045] Figure 3 It is the charge and discharge curve and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 1.

[0046] Figure 4 This is the XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 2.

[0047] Figure 5 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Example 2.

[0048] Figure 6 The charge and discharge curve and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 2.

[0049] Figure 7This is the XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 3.

[0050] Figure 8 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Example 3.

[0051] Fig. 9 The charge and discharge curves and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 3.

[0052] Fig.10 This is the XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 4.

[0053] Fig.11 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Example 4.

[0054] Fig.12 The charge and discharge curve and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 4.

[0055] Fig.13 This is the XRD diagram of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0056] Fig.14 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0057] Fig.15 The charge and discharge curves and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0058] Fig.16 This is the XRD diagram of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[0059] Fig.17 This is the SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[0060] Fig.18 The charge and discharge curves and cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 2. DETAILED DESCRIPTION

[0061] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the specification, but the protection scope of the present invention is not limited to the following specific embodiments.

[0062] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0063] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0064] Example 1 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And 5% glucose of the total mass of the above substances.

[0065] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0066] (3) After the sand-milled slurry is dried, the temperature is increased at a rate of 10°C / min in an inert gas atmosphere and sintered at 500°C for 8 h to obtain the sodium iron pyrophosphate composite material Na 4 Fe 2.94 (PO 4 ) 2 P 2 O 7 @C.

[0067] After testing, the XRD pattern after deducting the baseline is as follows Figure 1 As shown in the SEM images Figure 2 As shown, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in 100~200nm, and the secondary particles are formed by the agglomeration of the primary particles, and the size is mainly distributed in 300~800nm. The strongest characteristic diffraction peak peak intensity I of the diffraction peak at 2θ=33.6°±0.2° in the XRD diffraction pattern 1 obtained under CuKα radiation T , and NaFePO at 2θ=32.8°±0.2° 4 (Marticite) characteristic diffraction peak intensity I O , the peak intensity ratio after deducting the baseline is I T / I O For NaFePO in the material 4 (Marticite) relative content was evaluated. T / I O Defined as Ir, the Ir of the material in this embodiment is 6.19.

[0068] (4) Sodium iron pyrophosphate composite material Na prepared by the above method 4 Fe2.94 (PO 4 ) 2 P 2 O 7 @C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 2.94 (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Figure 3 As shown, its electrochemical 0.1C discharge capacity is 102.3mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.8%.

[0069] Example 2 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And glucose accounting for 10% of the total mass of the above substances.

[0070] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 5 hours.

[0071] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 13°C / min in an inert gas atmosphere and sintered at 530°C for 10 h to obtain the sodium iron pyrophosphate composite material Na 4 Fe 2.97 (PO 4 ) 2 P 2 O 7 @C. After testing, the XRD pattern after deducting the baseline is as follows Figure 4 As shown in the SEM images Figure 5 As shown, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in 100-200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800 nm. The material Ir of this embodiment is 12.6.

[0072] (4) Sodium iron pyrophosphate composite material Na prepared by the above method 4 Fe2.97 (PO 4 ) 2 P 2 O 7 @C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 2.97 (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Figure 6 As shown, the 0.1C discharge capacity is 109.1mAh / g, and the capacity retention rate after 100 cycles at 1C is 95.8%.

[0073] Example 3 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And glucose accounting for 10% of the total mass of the above substances.

[0074] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 5 hours.

[0075] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 13°C / min in an inert gas atmosphere and sintered at 530°C for 12 h to obtain the sodium iron pyrophosphate composite material Na 4 Fe 2.97 (PO 4 ) 2 P 2 O 7 @C. After testing, the XRD pattern after deducting the baseline is as follows Figure 7 As shown in the SEM images Figure 8 As shown, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in 100-200nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800nm. The material Ir of this embodiment is 13.

[0076] (4) Sodium iron pyrophosphate composite material Na prepared by the above method 4 Fe 2.97(PO 4 ) 2 P 2 O 7 @C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 2.97 (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Fig. 9 As shown, the 0.1C discharge capacity is 113.6mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.4%.

[0077] Example 4 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And glucose accounting for 20% of the total mass of the above substances.

[0078] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 6 hours.

[0079] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 15°C / min in an inert gas atmosphere and sintered at 550°C for 12 h to obtain the sodium iron pyrophosphate composite material Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 @C. After testing, the XRD pattern after deducting the baseline is as follows Fig.10 As shown in the SEM images Fig.11 As shown, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in 100-200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800 nm. The material Ir of this embodiment is 10.7.

[0080] (4) Sodium iron pyrophosphate composite material Na prepared by the above method 4 Fe 3 (PO 4) 2 P 2 O 7 @C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Fig.12 As shown, the 0.1C discharge capacity is 103.3mAh / g, and the capacity retention rate after 100 cycles at 1C is 98.03%.

[0081] Comparative Example 1 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And glucose accounting for 10% of the total mass of the above substances.

[0082] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0083] (3) After drying the sand-milled slurry, the temperature was raised in an inert gas atmosphere at a rate of 13°C / min in the temperature range of 25-300°C and 400-530°C, and at a rate of 2°C / min in the temperature range of 300-400°C, and sintered at 530°C for 12 h to obtain the composite material Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C, 0≤x≤0.02. After testing, the XRD pattern after deducting the baseline is as follows Fig.13 As shown in the SEM images Fig.14 As shown, the Ir of the material is 5.6.

[0084] In order to control the generation of the impurity phase, although the same raw material ratio as in Examples 2 and 3 was used in this comparative example, the final product did not have a clear chemical formula, so the above chemical formula containing x was used to represent it.

[0085] (4) Material Na prepared by the above method4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Fig.15 As shown, the 0.1C discharge capacity is 93.9 mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.9%.

[0086] Comparative Example 2 (1) Weigh NH 4 FePO 4 ·H 2 O、Na 2 CO 3 、Na 4 P 2 O 7 And glucose accounting for 10% of the total mass of the above substances.

[0087] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0088] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 15°C / min in an inert gas atmosphere and sintered at 530°C for 12 h to obtain the composite material Na 4 Fe 2.91 (PO 4 ) 2 P 2 O 7 @C. After testing, the XRD pattern after deducting the baseline is as follows Fig.16 As shown in the SEM images Fig.17 As shown, the Ir of the material is 14.2.

[0089] (4) Material Na prepared by the above method 4 Fe 2.91 (PO 4 ) 2 P 2 O 7@C and sodium metal sheet are assembled into a button battery for testing. Assembly of button battery: positive electrode material Na 4 Fe 2.91 (PO 4 ) 2 P 2 O 7 @C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator is Whatman GF / B, and the electrolyte formula is 1M NaClO 4 Dissolved in V% EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curve and cycle performance are shown in the figure Fig.18 As shown, the 0.1C discharge capacity is 98.4mAh / g, and the capacity retention rate after 100 cycles at 1C is 93.7%.

[0090] Table 1 The electrochemical properties and Ir values ​​of the embodiments and comparative examples are shown in Table 1. It can be seen from the table that when 6.19≤Ir≤13, the spinel type NaFePO 4 When the phase content is within a certain content range, the corresponding electrochemical properties such as battery capacity and cycle performance are good. Based on this discovery, the process parameters of the solid phase method for preparing composite sodium iron phosphate in the present invention can be evaluated and optimized, and the method for preparing composite sodium iron phosphate can be optimized by optimizing the ratio of raw materials in step (1), the heating rate and sintering temperature and the holding time in step (2).

[0091] Optimization of process parameters in Comparative Example 1: Example 2 is equivalent to the optimized condition result of Comparative Example 1. Compared with Example 2, when heating, NaFePO 4 The time in the phase formation temperature zone is prolonged, and the final Ir is too low and the performance is poor, which indicates that the control of the heating rate condition is not suitable for NaFePO 4 The feasibility and rationality of phase reduction.

[0092] Optimization of process parameters in Comparative Example 2: The optimized parameters of Examples 1 and 2 are equivalent to those of Comparative Example 2. Comparative Example 2 shows that NaFePO 4 Phase is also one of the structural phases of composite sodium iron phosphate. Too low a content will also destroy the structural stability of NFPP itself and reduce the material performance. In this comparative example 2, the Fe source ratio is further reduced (reduced by 3%). In the end, Ir is too high and the performance is poor. The decline in cycle performance indirectly indicates that the structural stability of the material itself is reduced in this case.

[0093] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A sodium iron pyrophosphate composite material, It is characterized in that The sodium iron pyrophosphate composite material has a spinel type NaFePO 4 phase; the sodium iron pyrophosphate composite material is subjected to XRD diffraction under CuKα radiation, and the obtained XRD diffraction pattern has characteristic diffraction peaks T of sodium iron pyrophosphate and spinel type NaFePO at 2θ of 33.6±0.2° and 32.8±0.2° 4 The characteristic diffraction peak O, whose peak intensity is I T and I O , let the diffraction peak intensity ratio Ir = I T / I O , where 6.19≤Ir≤13.

2. The sodium iron pyrophosphate composite material according to claim 1, It is characterized in that It includes sodium ferric pyrophosphate and a carbon layer coated on the surface of the sodium ferric pyrophosphate, and the chemical formula is Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C, where 0≤x≤0.02; Preferably, the carbon content of the sodium iron pyrophosphate composite material is 2-5wt%.

3. A preparation process of sodium iron pyrophosphate composite material, It is characterized in that The following steps are involved: (1) Weigh the synthetic Na according to the preset feeding ratio 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 The raw materials and an appropriate amount of carbon source are wet-grinded and mixed to obtain a mixture; (2) After the mixture is dried, it is sintered in a protective atmosphere according to preset sintering parameters to obtain a sodium iron pyrophosphate composite material Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 @C; Among them, 0≤x≤0.

02.

4. The preparation process according to claim 3, It is characterized in that In step (1): The synthetic Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 The raw materials include iron source and sodium source; Preferably, the iron source is ammonium ferrous phosphate, and the sodium source includes sodium pyrophosphate and a sodium source that does not contain P; Preferably, the P-free sodium source is selected from one or more of sodium carbonate, ammonium bicarbonate, sodium acetate and sodium citrate; Preferably, the molar ratio of ammonium ferrous phosphate, sodium pyrophosphate and the P-free sodium source is 3:0.96~1:0.5~0.

54.

5. The preparation process according to claim 3, It is characterized in that In step (1): The carbon source is one or more of glucose, sucrose, starch, and polyethylene glycol; Preferably, the amount of the carbon source is synthetic Na 4 Fe 3(1-x) (PO 4 ) 2 P 2 O 7 5~20% of the mass of the raw materials.

6. The preparation process according to claim 3, It is characterized in that In step (2), the preset sintering parameters include the sintering heating rate, the sintering temperature and the sintering time; Preferably, the heating rate of the sintering is 10-15°C / min; Preferably, the sintering temperature is 500-550°C; Preferably, the sintering time is 8 to 12 hours.

7. A method for evaluating sodium iron pyrophosphate composite materials, It is characterized in that The following steps are involved: S1. Performing an XRD diffraction test on the sodium iron pyrophosphate composite material under CuKα radiation, and obtaining an XRD diffraction pattern, and calculating the corresponding performance test value according to the peak intensity of the required characteristic diffraction peak; S2. Obtain performance evaluation results by comparing performance test values ​​with performance reference values.

8. The evaluation method according to claim 7, It is characterized in that In step S1, the performance test value is Ir', Ir'=I T ' / I O ', where I T ' is the peak intensity of the characteristic diffraction peak T at 2θ of 33.6±0.2° in the XRD diffraction pattern, I O ' is the peak intensity of the characteristic diffraction peak O of the XRD diffraction spectrum at 2θ of 32.8±0.2°; In step S2, the performance reference value is Ir, and its interval range is 6.19≤Ir≤13; when Ir' is within the interval range of Ir, the performance evaluation result of the material is excellent.

9. An optimization method for the preparation process of sodium iron pyrophosphate composite material, It is characterized in that The sodium iron pyrophosphate composite material prepared by the preparation process of any one of claims 3 to 6 or the sodium iron pyrophosphate composite material according to claim 1 or 2 is evaluated using the evaluation method according to claim 7 or 8, further comprising the following steps: S3. By comparing the performance test value and the performance reference value, it is determined whether the performance test value is within the range of the performance reference value; If the performance test value is not within the range of the performance reference value, repeat steps (1), (2), S1, and S2, and optimize and adjust the process parameters in step (1) or (2) according to the comparison results until the performance test value is within the range of the performance reference value, and then output the optimized process parameters.

10. The optimization method according to claim 9, It is characterized in that Optimizing and adjusting the process parameters in step (1) or (2) according to the comparison results, the process parameters including the amount of iron source added to the preset feed in step (1) and / or the preset sintering parameters in step (2); Preferably: when Ir' is less than the lower limit of the interval of Ir, the amount of iron source added is reduced and / or the heating rate is increased; when Ir' is greater than the upper limit of the interval of Ir, the amount of iron source added is increased and / or the heating rate is reduced.

11. The sodium iron pyrophosphate composite material according to claim 1 or 2, or the sodium iron pyrophosphate composite material prepared by the preparation process according to any one of claims 3 to 6, or the sodium iron pyrophosphate composite material evaluated as excellent by the evaluation method according to claim 7 or 8, or the sodium iron pyrophosphate composite material prepared by the sodium iron pyrophosphate composite material preparation process optimized by the optimization method according to claim 9 or 10, It is characterized in that Application as a positive electrode material in sodium ion batteries.

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