A positive electrode active material for sodium batteries and a preparation method and application thereof

Abstract

The application provides a positive electrode active material for a sodium battery and a preparation method and application thereof. A molecular general formula of the positive electrode active material comprises Na x+b M y [M'(CN)6] z ·(RCOO) b ·(c-H2O) 6‑6z‑b ·(f-H2O) a wherein c-H2O represents coordinated water, f-H2O represents free water, RCOO represents a-1 valence anion group, 0 The positive electrode active material is a low-water-content, high-sodium-content and high-gram-capacity Prussian blue framework compound, and can be used to provide a sodium battery with high energy density, good cycle stability and high safety performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode active material for sodium batteries and a preparation method and application thereof. Background Art

[0002] Prussian blue-like framework compounds have three-dimensional channels for rapid migration of sodium ions and are ideal positive electrode active materials for sodium batteries. However, this type of framework compound easily absorbs water, causing the material's specific capacity to rapidly decay, and even causing battery bloating, which seriously affects the normal performance of the battery. There are literature reports on reducing the water content of such compounds by introducing neutral coordinating groups, but such materials are still prone to absorbing large amounts of water during subsequent storage and application. In addition, with the development of sodium battery technology, the market's requirements for the energy density and reversible capacity of sodium batteries are getting higher and higher, and electrode sodium supplementation has gradually become a necessary issue for sodium batteries. At present, the sodium supplements commonly used in the industry include sodium metal strips, sodium metal powders, sodium oxides, etc. Most of these sodium supplements have very strict environmental requirements, and the uniform dispersion of sodium supplements is also a considerable problem. Summary of the Invention

[0003] In view of this, the present application provides a positive electrode active material for a sodium battery, which is a Prussian blue-like framework compound with low water content, high sodium content, and high gram capacity, and can be used to provide a sodium battery with high energy density, good cycle stability and high safety performance.

[0004] The first aspect of the present application provides a positive electrode active material for a sodium battery, wherein the molecular formula of the positive electrode active material includes Na x+b M y [M'(CN)6] z ·(RCOO) b (c-H2O) 6-6z-b (f-H2O) a , wherein c-H2O represents coordinated water, f-H2O represents free water, RCOO represents a -1 valence anionic group, 0<x≤2, 0<y≤1, 0<z<1, 0<b<6-6z, a>0; R includes one or more of substituted or unsubstituted aliphatic groups, substituted or unsubstituted aromatic groups; M elements and M' elements each independently include transition elements.

[0005] The above-mentioned positive electrode active material is a Prussian blue-like framework compound, wherein -COO - The group has a strong coordination effect and can replace part of the coordinated water (c-H2O) in the lattice of the original Prussian blue framework compound, thereby reducing the water content of the above-mentioned positive electrode active materials. More importantly, -COO - The group is an anionic group, which can introduce more Na into the original framework compound through charge interaction.+ , to obtain the above compound Na x+b M y [M’(CN)6] z ·(RCOO) b ·(c-H2O) 6-6z-b ·(f-H2O) a , compared with the material without introducing Na + , the content of active Na + in the positive active material is significantly improved, thereby imparting the material with higher specific capacity, and further reducing the water content of the positive active material.

[0006] The second aspect of the present application provides a preparation method of a positive active material for sodium batteries, comprising the following steps:

[0007] (1) dehydrating a first raw material to obtain a second raw material; wherein the molecular formula of the first raw material comprises Na x M y [M’(CN)6] z ·(c-H2O) 6-6z ·(f-H2O) a , c-H2O represents coordinated water, f-H2O represents free water, 0

[0008] (2) soaking the second raw material in a sodium carboxylate solution in a dew point control environment to obtain a suspension; wherein the molecular formula of the sodium carboxylate comprises RCOO - Na + , R comprises one or more of a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aromatic group;

[0009] (3) performing solid-liquid separation on the suspension, and vacuum drying the obtained solid to obtain a positive active material for sodium batteries;

[0010] The molecular formula of the positive active material comprises Na x+b M y [M’(CN)6] z ·(RCOO) b ·(c-H2O) 6-6z-b ·(f-H2O) a , 0

[0011] The above preparation method is simple to operate, has strong controllability, has a wide source of raw materials, and has high production efficiency, and can be used for large-scale industrial production.

[0012] The third aspect of the present application provides a positive electrode sheet, which comprises the positive electrode active material for sodium batteries provided by the first aspect of the present application.

[0013] The fourth aspect of the present application provides a sodium battery, which comprises the positive electrode sheet provided by the third aspect of the present application.

[0014] The sodium battery has a high energy density, good cycle stability and safety performance. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1A A schematic diagram of the crystal structure of the Prussian blue analog framework compound with coordinated water;

[0016] Figure 1B A schematic diagram of the cross-sectional structure of the crystal structure of the Prussian blue analog framework compound from a certain perspective;

[0017] Figure 2 A schematic diagram of the cross-sectional structure of the crystal structure of the positive electrode active material provided by an embodiment of the present application from a certain perspective. DETAILED DESCRIPTION

[0018] The Prussian blue analog framework compound has a three-dimensional channel for rapid migration of sodium ions, and is an ideal positive electrode active material for sodium batteries. However, the framework structure of the compound can easily cause the compound to absorb water, resulting in rapid decay of the material's specific capacity, and even cause the battery to swell, which seriously affects the normal performance of the battery and restricts the application of this type of material in the battery field.

[0019] Generally, the water molecules in the Prussian blue analog framework compound exist in two forms: coordinated water (c-H2O) and free water (f-H2O). The coordinated water mainly comes from the absence of ligands at the defects of the compound lattice, and the water molecules replace the original coordinated cyan groups to form coordination with the central metal ions, thereby maintaining the basic structure of the central metal ions (see the structural schematic diagram in Figure 1A and Figure 1B ). The molecular formula thereof can be represented as Na x M y [M’(CN)6] z ·(c-H2O) 6-6z ·(f-H2O) a, 0 < x < 2, 0 < y < 1, 0 < z < 1, the M element and the M' element each independently comprise a transition element. In addition, the water absorption of the above-mentioned material also reflects the easy absorption of free water, and part of the free water also exists in the three-dimensional channels of the above-mentioned compound. The existence of the above-mentioned two types of water not only increases the water content of the compound, but also may affect the migration of Na ions in the three-dimensional channels. At present, there are documents that report that the introduction of neutral coordination groups can reduce the water content of such compounds, but such materials are difficult to effectively prevent the material from continuing to absorb water (mainly free water) in the subsequent storage and application process, and the improvement effect of the material is limited. In addition, the content of active sodium ions in the Prussian blue analog framework compound needs to be further improved to reduce the dependence on sodium supplementing agents in the subsequent specific application process.

[0020] To solve the above-mentioned problems, the embodiment of the present application provides a positive electrode active material for a sodium battery, the molecular general formula of which comprises Na x+b M y [M'(CN)6] z ·(RCOO) b ·(c-H2O) 6-6z-b ·(f-H2O) a , 0 < x < 2, 0 < y < 1, 0 < z < 1, 0 < b < 6-6z, a > 0; R comprises one or more of a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aromatic group; the M element and the M' element each independently comprise a transition element.

[0021] In the present application, the above-mentioned M element and the M' element can be the same or different. Exemplarily, the above-mentioned transition element comprises but is not limited to one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Sn, In and Cd, etc.

[0022] In the above-mentioned positive electrode active material, one [M'(CN)6] m- group has 6 coordination sites, and when multiple coordination sites generated by the vacancy of one [M'(CN)6] m- group are simultaneously occupied by RCOO - groups, the R groups of multiple RCOO - groups can be the same or different. In other words, assuming that the group on the No. 1 coordination site is R1COO - , and the group on the No. 2 coordination site is R2COO - , the above-mentioned R1 and R2 can be the same or different.

[0023] —COO - group has a strong coordination effect, not only can successfully replace the coordination water in the lattice of the Prussian blue analog framework compound, and the —COO- The binding force between the group and the central transition metal ion is strong, and it can continuously and stably play a role in occupying the space, so it can stably maintain the water content of the positive electrode active material at a very low level (see Figure 2 Schematic diagram of the structure in ). And RCOO - The volume of the group is larger than that of the water molecule (here refers to the coordinated water c-H2O occupied in the original compound), and the space it occupies is large, which compresses the space originally belonging to free water. Therefore, it can further reduce the overall water content of the positive electrode active material, significantly reduce the risk of the positive electrode active material absorbing water again during storage and application, fully improve the cycle stability and safety performance of the final sodium battery, and extend the cycle service life of the sodium battery.

[0024] In addition, RCOO - The groups are electronegative, and each RCOO - The groups can introduce a Na + , and form a stable Na x+b M y [M'(CN)6] z ·(RCOO) b (c-H2O) 6-6z-b (f-H2O) a Compound, and Na + Always exists in [M'(CN)6] m- The coordination site is near the compound, and the part in the compound is relatively uniform, which can significantly and uniformly improve the active Na + Similarly, the introduction of Na + It can also compress the space for free water, thereby further reducing the water content of the positive electrode active material. The above-mentioned positive electrode active material has a higher specific capacity and lower water content, which can further improve the reversible capacity, energy density and cycle performance of the final sodium battery.

[0025] In some embodiments of the present application, the water content of the positive electrode active material is in the range of 500 ppm to 2000 ppm. The water content here refers to the water content of the positive electrode active material stored in a dry or low humidity environment after being prepared, i.e., the intrinsic water content of the material, and in addition, the water content here refers to the water content of the positive electrode active material as a whole, without specifically distinguishing between coordinated water and free water. In the present application, the water content of the positive electrode active material can be tested by a Karl Fischer moisture meter. For example, the water content of the positive electrode active material can be 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm, etc.

[0026] In some embodiments of the present application, R includes one or more of substituted or unsubstituted aliphatic groups, substituted or unsubstituted aromatic groups, the substituted or unsubstituted aliphatic groups including substituted or unsubstituted C 1-8 alkyl groups, C 2-8 alkyl groups, C 2-8 alkynyl groups, C 3-8 cycloalkyl groups; and the substituted or unsubstituted aromatic groups including substituted or unsubstituted phenyl groups. In some specific embodiments of the present application, the R is preferably a C1alkyl group. At this time, the R group has a more suitable spatial volume, which can make it easier for the RCOO - group to replace the coordinated water of the original Prussian blue framework compound, and can fully occupy the position of the free water in the original framework structure while leaving sufficient space for Na + + and further reducing the risk of the RCOO - group affecting the destruction of the original framework structure, thereby fully ensuring that the water content of the positive electrode active material is low and the reversible capacity is high.

[0027] For example, the above-mentioned R group can be a methyl group, an ethyl group, a propyl group, a butyl group, an ethenyl group, a propenyl group, an allyl group, an ethynyl group, a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a phenyl group, etc.

[0028] In some embodiments of the present application, the substituted groups of the above-mentioned R group each independently include, but are not limited to, halogen atoms. For example, the substituted groups can be F atoms, Cl atoms, Br atoms, I atoms, etc. At this time, for example, the RCOO -The group may be a halogenated acetic acid group, a halogenated propionic acid group, a halogenated benzoic acid group, etc. In some specific embodiments, the substituent of the R group is a F atom. The F atom is a strongly hydrophobic group, and its hydrophobic effect can further weaken the ability of the positive electrode active material to absorb water from the environment, thereby further improving the cycle stability and safety performance of the final sodium battery, and further reducing the risk of the positive electrode active material absorbing water again during storage and application.

[0029] In some embodiments of the present application, The value of is greater than or equal to 1. That is, the theoretical molar ratio of sodium ions to transition metal ions (M and M') in the positive electrode active material is greater than or equal to 1. At this time, the active Na + The higher the content of , the more fully the gram capacity of the positive electrode active material can be improved. The value of can be 1, 1.010, 1.015, 1.016, 1.017, 1.018, 1.019, 1.020, 1.021, 1.022, 1.023, 1.024, 1.025, 1.026, 1.027, 1.028, 1.029, 1.030, 1.031, 1.032, 1.033, 1.034, 1.035, 1.040, 1.045, 1.050, 1.053, etc.

[0030] In some embodiments of the present application, the gram capacity of the positive electrode active material is greater than or equal to 142 mAh / g. For example, the gram capacity of the material can be 142 mAh / g, 143 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g, 151 mAh / g, and the like, and higher. In the present application, the above-mentioned positive electrode active material has a low coordinated water content and a high content of active Na ions, thereby resulting in a high gram capacity of the material.

[0031] Accordingly, the present invention also provides a method for preparing a positive electrode active material for a sodium battery, comprising the following steps:

[0032] (1) Dehydrating the first raw material to obtain a second raw material; wherein the molecular formula of the first raw material includes Na x M y [M'(CN)6] z (c-H2O) 6-6z (f-H2O) a , c-H2O represents coordinated water, f-H2O represents free water, 0<x≤2, 0<y≤1, 0<z<1, a>0; M element and M' element each independently include a transition element;

[0033] (2) in a dew point control environment, the second raw material is placed in a sodium carboxylate solution for soaking treatment, to obtain a suspension; wherein the molecular general formula of the sodium carboxylate includes RCOO - Na + , R includes one or more of substituted or unsubstituted aliphatic groups, substituted or unsubstituted aromatic groups;

[0034] (3) the suspension is subjected to solid-liquid separation treatment, and the obtained solid is subjected to vacuum drying treatment, to obtain a positive electrode active material for sodium batteries;

[0035] The molecular general formula of the positive electrode active material includes Na x+b M y [M'(CN)6] z ·(RCOO) b ·(c-H2O) 6-6z-b ·(f-H2O) a , wherein 0

[0036] Step (1) is a dehydration treatment for removing free water in the first raw material, which can clear the obstacle for the RCOO - group to smoothly enter the framework structure of the first raw material compound and smoothly replace the coordinated water, so that as long as the dehydrated first raw material (i.e. the second raw material) is soaked in the corresponding sodium carboxylate under certain conditions, and then the solvent is removed by drying, the positive electrode active material provided in the embodiments of the present application can be obtained.

[0037] The dew point in chemical industry is defined as the temperature when unsaturated air is cooled to saturation. The dew point control environment in step (2) specifically refers to that the humidity of the operating environment in step (2) is low or reaches the saturation humidity (i.e. high degree of dryness). Dew point control is a process condition control often involved in the preparation of batteries and related materials. Soaking treatment under a dew point control environment can effectively prevent the sodium carboxylate solution / second raw material from absorbing a large amount of water in the environment during the process, which is conducive to ensuring the quality of the positive electrode active material prepared subsequently. If the environmental humidity is too large (i.e. not a dew point control environment at the operating temperature), the above-mentioned solution and the second raw material will absorb water, and especially the second raw material will absorb water in the environment, which will cause the free water removed in step (1) to return to the raw material, hindering the smooth replacement of the RCOO - group.

[0038] Exemplarily, when the operating temperature is 18-25°C, the dew point control environment refers to that the environmental humidity is less than or equal to 1% RH, and the dew point temperature is equal to or less than -35°C.

[0039] The preparation method has simple operation, strong controllability, wide raw material source and high production efficiency, and can be used for large-scale industrial production.

[0040] In some embodiments of the present application, the water content of the positive electrode active material obtained after the vacuum drying treatment in step (3) is in the range of 500 ppm-2000 ppm.

[0041] In some embodiments of the present application, the dehydration treatment in step (1) is performed at a temperature of 85°C-200°C and a vacuum degree of less than or equal to 20 mTorr for 6h-48h. At this time, the free water in the first raw material can be removed more fully.

[0042] In some embodiments of the present application, the soaking treatment time in step (2) is 6h-24h. The soaking time can be controlled according to the type of sodium carboxylate solution, and in general, the time is controlled in the above range, which can ensure that the content of RCOO - group and Na + is high, and the water content of the finally prepared positive electrode active material is low and the specific capacity is high.

[0043] In some embodiments of the present application, the molar concentration of the sodium carboxylate solution is 0.1 mol / L-2 mol / L. The molar concentration can affect the preparation speed and the content of RCOO - group and Na + in the final positive electrode active material, and the molar concentration of the sodium carboxylate can be determined according to the actual production needs. Exemplarily, the molar concentration of the sodium carboxylate in the above sodium carboxylate solution can be 0.1 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2.0 mol / L, etc.

[0044] In some embodiments of the present application, the solvent of the sodium carboxylate includes but is not limited to one or more of methanol, ethanol, isopropanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide.

[0045] Exemplarily, the above sodium carboxylate solution can be sodium acetate, sodium propionate, sodium benzoate, sodium fluoroacetate, sodium fluoropropionate, sodium fluorobenzoate, etc.

[0046] In some embodiments of the present application, the drying treatment in step (3) is performed at a temperature of 50-150°C and a vacuum degree of less than or equal to 20 mTorr for 6-48 h. At this time, the excess solvent in the solid material can be removed more completely.

[0047] In some embodiments of the present application, the solid-liquid separation treatment in step (3) includes, but is not limited to, centrifugation, filtration, and the like.

[0048] The present application also provides a positive electrode sheet, which comprises the positive electrode active material for sodium batteries provided by the present application.

[0049] In the present application, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode sheet, wherein the positive electrode active material layer comprises the positive electrode active material for sodium batteries, a binder, and optionally a conductive agent.

[0050] In some embodiments of the present application, the positive electrode sheet can be prepared by the following steps:

[0051] In some embodiments of the present application, the positive electrode active material for sodium batteries provided by the present application can be formed on a positive electrode current collector, and then rolled and cut to obtain a positive electrode sheet. Specifically, a slurry containing the positive electrode active material for sodium batteries can be coated on a positive electrode current collector, and then dried, rolled, and cut to obtain a positive electrode sheet.

[0052] In some embodiments of the present application, the positive electrode current collector includes, but is not limited to, an aluminum foil, a copper foil, and the like. The binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefins (such as polyethylene, polypropylene, polystyrene, and the like), sodium carboxymethyl cellulose (CMC), and sodium alginate.

[0053] In some embodiments of the present application, the conductive agent is a material known to those skilled in the art. For example, the conductive agent can be carbon nanotubes (CNT), carbon fiber conductive agent (HV), carbon black (such as acetylene black, ketjen black), furnace black, graphene, and the like.

[0054] The present application also provides a sodium battery comprising the positive electrode sheet provided by the present application. The sodium battery has a high energy density, good cycle stability, and safety performance.

[0055] In some embodiments of the present application, the positive electrode sheet provided above and the conventional separator and negative electrode sheet in the art can be sequentially stacked or wound to obtain a dry battery, and then subsequent processes are performed to obtain a soft package and / or hard shell battery.

[0056] The negative electrode sheet can be a negative current collector (including but not limited to aluminum foil, copper foil, etc.) coated with a negative electrode active material layer. The negative electrode active material is a material known to those skilled in the art, and can be, for example, graphene, hard carbon, soft carbon, carbon nanotubes, silicon-carbon composite materials, etc.

[0057] The technical solutions of the present application are further described in detail in the following embodiments.

[0058] Embodiment 1

[0059] (1) The first raw material-Na 1.86 Mn[Fe(CN)6] 0.965 ·(c-H2O) 0.21 ·(f-H2O) 1.79 is dried in an environment with a vacuum degree less than 20 mTorr and a temperature of 150°C for 24 hours to remove free water in the first raw material, thereby obtaining a second raw material.

[0060] (2) In a glove box, the second raw material is soaked in a ligand solution for 12 hours in a 25°C dew point control environment (humidity is 1% RH, dew point temperature is ≤ 35°C), thereby obtaining a suspension; wherein the ligand solution is a 1.0 mol / L sodium acetate isopropyl alcohol solution.

[0061] (3) The suspension is subjected to centrifugal treatment to separate out a solid, and the solid is placed in a vacuum degree less than 20 mTorr and a temperature of 70°C to dry and remove excess solvent. The finally obtained positive electrode active material has a general molecular formula of: Na 2.03 Mn[Fe(CN)6] 0.965 ·(CH3COO) 0.17 ·(c-H2O) 0.04 ·(f-H2O) 0.01 .

[0062] Embodiment 2

[0063] The difference from Embodiment 1 is that the ligand solution used in step (2) is a 1.0 mol / L sodium trifluoroacetate isopropyl alcohol solution. The finally obtained positive electrode active material has a general molecular formula of: Na 2.03 Mn[Fe(CN)6] 0.965 ·(CF3COO) 0.17 ·(c-H2O) 0.04 ·(f-H2O) 0.003.

[0064] Example 3

[0065] The difference from Example 1 is only that the ligand solution used in step (2) is a 1.0 mol / L sodium benzoate isopropyl alcohol solution. The final positive electrode active material has a molecular formula of: Na 2.00 Mn[Fe(CN)6] 0.965 ·(C6H5COO) 0.14 ·(c-H2O) 0.07 ·(f-H2O) 0.004 .

[0066] Example 4

[0067] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium 2-propylhexanoate isopropyl alcohol solution for 12 h. The final positive electrode active material has a molecular formula of: Na 1.98 Mn[Fe(CN)6] 0.965 ·(C8H 17 COO) 0.12 ·(c-H2O) 0.09 ·(f-H2O) 0.003 .

[0068] Example 5

[0069] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium acrylate isopropyl alcohol solution for 12 h. The final positive electrode active material has a molecular formula of: Na 2.01 Mn[Fe(CN)6] 0.965 ·(C2H3COO) 0.15 ·(c-H2O) 0.06 ·(f-H2O) 0.004 .

[0070] Example 6

[0071] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium propargylate isopropyl alcohol solution for 12 h. The final positive electrode active material has a molecular formula of: Na 2.02 Mn[Fe(CN)6] 0.965 ·(C2HCOO) 0.16 ·(c-H2O) 0.05 ·(f-H2O) 0.017 .

[0072] Example 7

[0073] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium cyclohexanecarboxylate isopropyl alcohol solution for 12 h. The molecular formula of the final positive electrode active material is: Na 1.99 Mn[Fe(CN)6] 0.965 ·(C6H 11 COO) 0.13 ·(c-H2O) 0.08 ·(f-H2O) 0.006 .

[0074] Example 8

[0075] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium cyclohexanecarboxylate isopropyl alcohol solution for 12 h. The molecular formula of the final positive electrode active material is: Na 2.03 Mn[Fe(CN)6] 0.965 ·(CCl3COO) 0.17 ·(c-H2O) 0.04. ·(f-H2O) 0.011 .

[0076] Example 9

[0077] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium cyclohexanecarboxylate isopropyl alcohol solution for 12 h. The molecular formula of the final positive electrode active material is: Na 2.02 Mn[Fe(CN)6] 0.965 ·(CH3COO) 0.10 ·(C6H5COO) 0.06 ·(c-H2O) 0.05 ·(f-H2O) 0.01 .

[0078] Example 10

[0079] The difference from Example 1 is only that in step (2), the above-mentioned second raw material is soaked in a 1.0 mol / L sodium cyclohexanecarboxylate isopropyl alcohol solution for 12 h. The molecular formula of the final positive electrode active material is: Na 1.99 Mn[Fe(CN)6] 0.965 ·(CH3COO) 0.13 ·(c-H2O) 0.08 ·(f-H2O) 0.011 .

[0080] Example 11

[0081] The difference from Example 1 is only that: (2) in a glove box, the above-mentioned second raw material is soaked in a 2.0 mol / L sodium acetate isopropyl alcohol solution for 12 h in a 25℃ controlled dew point environment to obtain a suspension; and the molecular formula of the final obtained positive electrode active material is: Na 2.03 Mn[Fe(CN)6] 0.965 ·(CH3COO) 0.17 ·(c-H2O) 0.04 ·(f-H2O) 0.004

[0082] Example 12

[0083] The difference from Example 1 is only that: in step (1), the first raw material-Na 1.86 Mn[Fe(CN)6] 0.965 ·(c-H2O) 0.21 ·(f-H2O) 1.79 is placed in a vacuum degree less than 20 mTorr and a temperature of 150℃ environment for drying for 24 h to remove free water in the above-mentioned first raw material to obtain a second raw material. The molecular formula of the final obtained positive electrode active material is: Na 2.02 Ni[Fe(CN)6] 0.965 ·(CH3COO) 0.16 ·(c-H2O) 0.05 ·(f-H2O) 0.01

[0084] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0085] Comparative Example 1

[0086] (1) The first raw material-Na 1.86 Mn[Fe(CN)6] 0.965 ·(c-H2O) 0.21 ·(f-H2O) 1.79 is placed in a vacuum degree less than 20 mTorr and a temperature of 150℃ environment for drying for 24 h to remove free water in the above-mentioned first raw material to obtain a second raw material.

[0087] (2) In a glove box, it is strictly stored in a room temperature dew point control environment for 12 h.

[0088] (3) After being taken out from the glove box, it is dried in a vacuum degree less than 20 mTorr and a temperature of 70℃ for 12 h to obtain a positive electrode active material Na 1.86 Mn[Fe(CN)6] 0.965 ·(c-H2O) 0.21 ·(f-H2O)​​0.045 .

[0089] Comparative Example 2

[0090] (1) The first raw material-Na 1.86 Mn[Fe(CN)6] 0.965 ·(c-H2O) 0.21 ·(f-H2O) 1.79 was dried in a vacuum degree of less than 20 mTorr and at a temperature of 150°C for 24 h to remove free water in the above first raw material, to obtain a second raw material.

[0091] (2) In a glove box, the above second raw material was soaked in isopropanol for 12 h in a 25°C dew point control environment to obtain a suspension;

[0092] (3) The suspension was subjected to centrifugal treatment to separate out a solid, and the solid was placed in a condition of a vacuum degree of less than 20 mTorr and at a temperature of 70°C to dry and remove excess solvent, to obtain a positive electrode active material Na 1.86 Mn[Fe(CN)6] 0.965 ·(CH3CHOHCH3) 0.02 ·(c-H2O) 0.19 ·(f-H2O) 0.024 .

[0093] Comparative Example 3

[0094] The difference from Comparative Example 2 is only that in step (2), the second raw material was soaked in a ligand solution for 12 h to obtain a suspension; wherein the ligand solution is a 1.0 mol / L (same as Example 1) acetonitrile isopropanol solution to obtain a positive electrode active material Na 1.86 Mn[Fe(CN)6] 0.965 ·(CH3CN) 0.12 ·(c-H2O) 0.09 ·(f-H2O) 0.011 .

[0095] Comparative Example 4

[0096] The difference from Comparative Example 2 is only that in step (2), the second raw material was soaked in a ligand solution for 12 h to obtain a suspension; wherein the ligand solution is a 1.0 mol / L methanesulfonic acid isopropanol solution. To obtain a positive electrode active material Na 1.86 Mn[Fe(CN)6] 0.965 ·(CH3SO3) 0.11 ·(c-H2O) 0.10 ·(f-H2O) 0.011 .

[0097] Comparative Example 5

[0098] The difference between the comparative example 2 is only that: in step (2), the second raw material is soaked in the ligand solution for 12 h to obtain a suspension; wherein the ligand solution is a sodium ethoxide isopropyl alcohol solution with a concentration of 1.0 mol / L. The positive electrode active material Na 1.95 Mn[Fe(CN)6] 0.965 ·(C2H5O) 0.09 ·(c-H2O) 0.12 ·(f-H2O) 0.017 .

[0099] Table 1: Parameter condition summary table of each example and comparative example

[0100]

[0101] The performance of the positive electrode active materials of each example and comparative example is tested.

[0102] (1) Water content test

[0103] The Karl Fischer moisture meter is used to test the water content in the material, and the test cutoff temperature of the water content is 170°C. Specifically, a) the positive electrode active material obtained after the above drying treatment is tested for its water content; b) each of the above positive electrode active materials is placed in a constant humidity oven with a humidity of 60% RH (RH is relative humidity) for 1 h, and then the sample moisture is tested, and the test results are summarized in Table 2.

[0104] (2) Gravimetric capacity test

[0105] a) The positive electrode active material, conductive agent-carbon black, and binder-PVDF provided by each example and comparative example are added to the solvent N-methyl pyrrolidone (NMP) in a mass ratio of 8:1:1, then stirred in a vacuum stirrer to form a stable and uniform positive electrode slurry. The above positive electrode slurry is coated on the current collector-aluminum foil to obtain a positive electrode sheet;

[0106] b) A metal sodium sheet with appropriate size is cut as a negative electrode sheet;

[0107] c) Dissolve 1 mol of sodium salt-NaPF6 in 1 L of organic solvent (the volume ratio of ethylene carbonate and diethyl carbonate is 1:1) to obtain an electrolyte;

[0108] d) In the glove box, the positive electrode sheet, the separator, and the negative electrode sheet prepared in step (2) are alternately stacked under Ar atmosphere, and the electrolyte is injected to assemble a CR2032 button cell.

[0109] e) The above each coin cell was charged at 0.2C constant current to 4V cut-off, and then rested for 5min, and discharged at 0.2C current density to 2.5V. The gram capacity of the material was determined, and the results were summarized in Table 3.

[0110] (3) The actual ratio of the molar ratio of sodium ions to transition metal ions in the positive electrode active material was determined:

[0111] The contents of Na element and M, M' elements in the above each positive electrode active material were determined by inductively coupled atomic emission spectrometer (ICP-AES), and the actual molar ratio of sodium ions to transition metal ions was calculated. The results are summarized in Table 3.

[0112] (4) Charge-discharge cycle curve test

[0113] On the LAND CT 2001C secondary battery performance testing device, under the condition of 25±1℃, the battery was tested by charge-discharge cycle at 0.2C. The steps were as follows: resting for 10min; constant voltage charging to 4.0V / 0.05C cut-off; resting for 10min; constant current discharging to 2.0V, which was one cycle. The steps were repeated for 100 cycles. Each example and comparative example selected 5 batteries for testing, and the average value of 5 batteries was taken for each test result. The test results of each battery were summarized in Table 3.

[0114] Table 2: Summary of water content test results of positive electrode active materials of each example and comparative example

[0115]

[0116]

[0117] Table 3: Summary of electrochemical performance of positive electrode active materials of each example and comparative example

[0118]

[0119]

[0120] The data in Table 2 showed that RCOO - The ligand can successfully replace the coordinated water in the raw material Prussian blue compound, and significantly reduce the water content of the material (comparative example and comparative example 1). Comparative examples 2-5 can find that isopropanol cannot significantly reduce the water content of the material, indicating that isopropanol plays a role of dissolution in the ligand solution, and the water removal effect of neutral ligands such as acetonitrile and methanesulfonic acid after substitution is not as good as RCOO - ligand in the examples of the present application, fully illustrating the superiority of carboxyl group, and illustrating that Na +The introduction of can further reduce the water content of the material. Even though the ligand used in Comparative Example 5 is sodium ethoxide, a certain amount of Na ions can be introduced into the material, but CH3CH2O - has a coordination ability weaker than RCOO - , so that its ability to replace the coordinated water is relatively weak, and the water content of the modified material is higher and the water resistance is poorer. In addition, the water content of the positive electrode active material of Example 2 after being exposed to 60% RH for 1 h is still lower than that of the positive electrode active material of Comparative Examples 1 and 2 just after drying, indicating that the R group substituted by F atoms can greatly improve the water resistance of the positive electrode active material during the short-term environmental exposure during the process connection in the application process. The positive electrode active material provided in the examples has high practicability.

[0121] The data in Table 3 show that the positive electrode active material provided in the examples has a higher gram capacity, indicating that the presence of Na + acts as a "sodium supplement". In addition, the water content of the material is highly related to the cycle life of the battery. The battery of Example 2 shows the best cycle retention rate, and the capacity retention rate can reach 96% after 100 cycles at a rate of 0.2C. The sodium ethoxide in Comparative Example 5 has a certain sodium supplement effect, but the gram capacity and cycle performance of the material are significantly poorer than those of the examples.

[0122] The above describes exemplary embodiments of the present application. It should be noted that those skilled in the art can make some improvements and refinements without departing from the principles of the present application, and these improvements and refinements are also considered within the scope of protection of the present application.

Claims

1. A positive electrode active material for a sodium battery, characterized in that The molecular formula of the positive electrode active material includes Na x+b M y [M'(CN)6] z ·(RCOO) b (c-H2O) 6-6z-b (f-H2O) a , wherein c-H2O represents coordinated water, f-H2O represents free water, RCOO represents a -1 valence anionic group, 0<x≤2, 0<y≤1, 0<z<1, 0<b<6-6z, a>0; R includes one or more of substituted or unsubstituted aliphatic groups, substituted or unsubstituted aromatic groups; M elements and M' elements each independently include transition elements.

2. The positive electrode active material for sodium battery according to claim 1, characterized in that The value of is greater than or equal to 1.

3. The positive electrode active material for sodium battery according to claim 1, characterized in that The substituted or unsubstituted aliphatic group includes substituted or unsubstituted C 1-8 Alkyl, C 2-8 Hydrocarbon, C 2-8 Alkynyl, C 3-8 cycloalkyl; the substituted or unsubstituted aryl group includes a substituted or unsubstituted phenyl group.

4. The positive electrode active material for sodium battery according to claim 1, characterized in that The substituent groups in the substituted aliphatic group and the substituent groups in the substituted aryl group each independently include a halogen atom.

5. The positive electrode active material for sodium battery according to claim 1, characterized in that The transition elements include one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Sn, In and Cd.

6. The positive electrode active material for sodium battery according to claim 1, characterized in that The gram capacity of the positive electrode active material is greater than or equal to 142 mAh / g.

7. A method for preparing a positive electrode active material for a sodium battery, characterized in that: The following steps are involved: (1) Dehydrating the first raw material to obtain a second raw material; wherein the molecular formula of the first raw material includes Na x M y [M'(CN)6] z (c-H2O) 6-6z (f-H2O) a , c-H2O represents coordinated water, f-H2O represents free water, 0<x≤2, 0<y≤1, 0<z<1, a>0; M element and M' element each independently include a transition element; (2) In a dew point controlled environment, the second raw material is immersed in a sodium carboxylate solution to obtain a suspension; wherein the molecular formula of the sodium carboxylate includes RCOO - Na + , R includes one or more of substituted or unsubstituted aliphatic groups, substituted or unsubstituted aryl groups; (3) subjecting the suspension to solid-liquid separation, and subjecting the obtained solid to vacuum drying to obtain a positive electrode active material for a sodium battery; The molecular formula of the positive electrode active material includes Na x+b M y [M'(CN)6] z ·(RCOO) b (c-H2O) 6-6z-b (f-H2O) a , wherein 0<x≤2, 0<y≤1, 0<z<1, 0<b<6-6z, a>0, and RCOO represents a -1 valence anionic group.

8. The preparation method according to claim 7, characterized in that The water content of the positive electrode active material obtained by the vacuum drying treatment in step (3) is in the range of 500ppm-2000ppm.

9. The preparation method according to claim 7, characterized in that The conditions for the dehydration treatment in step (1) are: dehydration for 6h-48h in an environment with a temperature of 85°C-200°C and a vacuum degree of less than or equal to 20mTorr.

10. The preparation method according to claim 7, characterized in that The soaking time in step (2) is 6 hours to 24 hours.

11. The preparation method according to claim 7, characterized in that The molar concentration of the sodium carboxylate solution is 0.1 mol / L-2 mol / L; the solvent of the sodium carboxylate includes one or more of water, methanol, ethanol, isopropanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, and N,N-diethylformamide.

12. The preparation method according to claim 7, characterized in that The drying conditions in step (3) are: drying at a temperature of 50° C. to 150° C. and a vacuum degree of less than or equal to 20 mTorr for 6 h to 48 h.

13. A positive electrode plate, characterized in that: The positive electrode sheet comprises the positive electrode active material for a sodium battery according to any one of claims 1 to 6.

14. A sodium battery, characterized in that: The sodium battery comprises the positive electrode sheet according to claim 13.

Images