A composite positive electrode sheet for a sodium-ion secondary battery and a sodium-ion battery

Abstract

The present application relates to a kind of sodium ion secondary battery composite positive electrode sheet and sodium ion battery, and the composite positive electrode sheet includes existing sodium ion battery positive electrode material and / or iron-based polyanion compound.The existing sodium ion battery positive electrode material includes transition metal oxide and / or prussian blue compound.The composite positive electrode sheet of the present application includes three: 1) after existing sodium ion battery positive electrode material and iron-based polyanion compound are uniformly mixed, it is used to prepare single-layer coated positive electrode sheet;2) iron-based polyanion compound, existing sodium ion battery positive electrode material and iron-based polyanion compound are coated layer by layer to form sandwiched positive electrode sheet;3) existing sodium ion battery positive electrode material and iron-based polyanion compound are coated layer by layer to form double-coated composite positive electrode sheet.The present application includes the secondary battery using the above composite positive electrode sheet.Compared with prior art, the sodium ion battery including the composite positive electrode sheet of the present application has excellent cycle stability, and preparation method is simple, suitable for large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion secondary batteries and materials, and particularly relates to a composite positive electrode sheet for a sodium ion secondary battery and a sodium ion battery. BACKGROUND

[0002] Compared with lithium ion secondary batteries, sodium ion secondary batteries have significant advantages such as low price and high safety, and have similar preparation process and working principle to lithium ion batteries, and have great application prospect in large-scale energy storage field. However, there are still many technical difficulties to be overcome in sodium ion secondary batteries.

[0003] At present, there are three main categories of sodium ion battery cathode materials with application prospects, including layered transition metal oxides, prussian blue type materials and polyanion compound materials. Due to the unstable crystal structure characteristics in the charging and discharging process, the cycle stability of the layered transition metal oxide cathode material is usually poor. In order to improve the long-term cycle life of the oxide type cathode material, the invention patents (CN110233252A, CN110838576A, CN111082058A) respectively disclose the measures for improving the cycle stability of the material by coating other substances on the surface of the active material, specifically, the above-mentioned invention patents all take layered transition metal oxides as the core, and realize the tight coating of electrochemically inert metal carbonate (Na2CO3), electrochemically inert oxide (aluminum oxide Al2O3) and phosphate (NaTi2(PO4)3) on the surface of the layered oxide particles by secondary reaction and calcination, which can effectively improve the crystal structure stability of the cathode material in the long-term cycle and the cycle life of the electrode sheet by inhibiting the volume strain of the active particle crystal and reducing the side reactions caused by the direct contact of the active material and the electrolyte. However, it should be pointed out that in the above-mentioned method, since the coating material has no electrochemical activity, part of the capacity needs to be sacrificed as a price, and increasing the coating amount will reduce the charge and discharge capacity per unit mass of the positive electrode, thereby inevitably reducing the overall energy density of the sodium ion battery, which is not conducive to large-scale production and use.

[0004] Prussian blue type cathode material has large ion diffusion channel and open crystal framework structure, which is very beneficial to the embedding and extraction of sodium ions. However, the current mainstream material synthesis method mainly based on coprecipitation process cannot avoid the existence of complex water residue and crystal defects in the product. The residual complex water will be released to the electrolyte and form corrosive acidic substances with the electrolyte salt in the charging and discharging process, which will further aggravate the structure damage at the original defect of the prussian blue material and the transition metal dissolution, leading to rapid deterioration of the cycle performance. By compounding a layer of other type of positive electrode material with good cycle stability and nanoscale particle size on the surface of the prussian blue type material particles or the electrode interface, the performance degradation of the prussian blue type electrode can be inhibited. SUMMARY

[0005] The present application aims to overcome the defects of the prior art mentioned above and provide a composite positive electrode tab which does not reduce the overall reversible specific capacity of the positive electrode side and significantly improves the cycle stability, and a simple preparation method thereof, which can be used for large-scale production.

[0006] The object of the present application can be achieved by the following technical solutions:

[0007] The inventors found that iron-based pyrophosphate / phosphate polyanion compounds, which have been proven to be effective for sodium battery positive electrode materials due to their open three-dimensional crystal framework and ultra-fast ion diffusion rate, have very high crystal structure stability and high sodium storage activity (theoretical discharge specific capacity of 97-129 mAh g -1 ), and are an ideal positive electrode composite synergistic material. There is no literature or patent reporting the preparation process of composite tabs for enhancing the cycle stability of layered oxide materials and Prussian blue type materials by using iron-based polyanion compounds.

[0008] On the one hand, the "blending" strategy of mixing nanoscale iron-based pyrophosphate / phosphate polyanion compound particles into layered transition metal oxide or Prussian blue type compound materials with larger particle size can make the iron-based polyanion compound particles uniformly fill between the layered transition metal oxide or Prussian blue type compound material particles, realize the close combination with the main positive electrode material matrix, and obtain a composite positive electrode material. Combined with the excellent sodium storage performance and structural stability of iron-based polyanion compounds, not only can the direct contact between the active material and the electrolyte be effectively separated, reducing the occurrence of side reactions, but also the volume strain of the main positive electrode material during charging and discharging can be effectively relieved, thereby effectively inhibiting the volume expansion problem of the positive electrode side, improving the cycle stability of the composite positive electrode material and the cycle life of the sodium ion battery.

[0009] On the other hand, the surface of the layered transition metal oxide or Prussian blue type compound material coating layer, or both the surface and the bottom, can also be coated with an iron-based polyanion compound coating layer by double-layer coating or sandwich coating, which can effectively inhibit the harmful side reactions between the transition metal oxide and the Prussian blue type compound and the electrolyte, and effectively improve the long-term cycle stability of the composite positive electrode tab.

[0010] The specific application contents are as follows:

[0011] A composite positive electrode for a sodium ion secondary battery, which comprises an existing sodium ion battery positive electrode material and an iron-based polyanion compound.

[0012] Further, the existing sodium-ion battery cathode material includes a transition metal oxide and / or a Prussian blue compound, and the mass ratio of the iron-based polyanion compound to the existing sodium-ion battery cathode material is (0.05-1):1.

[0013] Further, the transition metal oxide material has a molecular formula of Na m M n O p wherein M in the molecular formula contains at least one of Mn and Ni, and can optionally contain one or more of Li, Mg, Al, Fe, Co, Cu, Zn, Ca, Sr, Ce, Cr, Ti, Zr, Sn, V, Nb, Sb, or Mo, and m>0.44, n>1, p>2, the values of m, n, and p satisfy the charge balance of the chemical formula.

[0014] Further, the Prussian blue compound material has a molecular formula of A x P[R(CN)6] y P and R are each one or more of Mn, Fe, Ni, Co, Cu, Ce, Cr, Ti, Zn, and V; and the values of x and y satisfy the charge balance of the chemical formula.

[0015] Further, the iron-based polyanion compound is one or more of Na4Fe x M y (PO4)2P2O7 / C, Na4Fe x M y PO4P2O7F3 / C, Na3Fe x M y PO4F / C, and Na2Fe x M y P2O7 / C, wherein M contains one or more of Ni, Mn, Co, Cu, Zn, Mg, Al, Ca, Sr, Ce, Ti, Zr, Sn, V, Nb, Sb, or Mo, and the values of x and y satisfy the charge balance of the chemical formula.

[0016] A positive electrode sheet for a sodium-ion secondary battery, which includes the composite cathode material as described above, a binder, and a conductive additive.

[0017] Further, the positive electrode sheet is a sandwiched positive electrode sheet formed by layer-by-layer coating of the iron-based polyanion compound, the existing sodium-ion battery cathode material, and the iron-based polyanion compound;

[0018] or a double-coating composite positive electrode sheet formed by layer-by-layer coating of the existing sodium-ion battery cathode material and the iron-based polyanion compound;

[0019] Or mix the existing sodium ion battery positive electrode material and iron-based polyanion compound, and then single-layer coat the positive electrode sheet.

[0020] Further, the conductive additive is a high-conductivity carbon material, specifically including one or more of graphene, carbon nanotubes, carbon fibers, acetylene black, conductive carbon black, or conductive graphite; the mass ratio of the conductive agent in the sodium ion secondary battery positive electrode sheet is 0.5-10wt%.

[0021] The binder is a high-molecular polymer, specifically including one or more of polyvinyl alcohol (PVA), sodium hydroxymethyl cellulose (CMC), fluorine-containing polyolefins (PTFE, PVDF, or VGDF), polyurethane, polyolefins (PP or PE), or SBR rubber; the mass ratio of the binder in the sodium ion secondary battery positive electrode sheet is 2-10wt%.

[0022] Further, the specific preparation method of the positive electrode sheet is as follows: uniformly dispersing the positive electrode material, the binder, and the conductive additive in a solvent to form a slurry, coating the slurry on the surface of the current collector, and then performing air drying and vacuum drying, and roller pressing to obtain the sodium ion secondary battery positive electrode sheet.

[0023] The coating is performed by spraying, extrusion scraping, or silk screen printing.

[0024] A sodium ion secondary battery, which comprises the positive electrode sheet as described above.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] (1) On the one hand, the present application utilizes the super-stable crystal structure advantage of the iron-based polyanion compound in the charging and discharging process to effectively buffer the volume stress change of the active material components loaded on the sodium ion battery positive electrode sheet in the charging and discharging process, thereby improving the damage of the volume expansion of the positive electrode material in the charging and discharging process to the cycle performance of the battery; on the other hand, the iron-based polyanion compound nanoparticles are covered on the surface of the layered transition metal oxide and Prussian blue compound material, which can inhibit the occurrence of harmful side reactions at the electrode / electrolyte interface; therefore, the charging and discharging cycle stability of the composite positive electrode sheet provided by the present application is obviously improved compared with the existing sodium ion battery positive electrode prepared based on the layered transition metal oxide or the Prussian blue compound alone.

[0027] (2) The present application does not reduce the overall reversible specific capacity of the positive electrode, and is simple and convenient to operate, low in cost, and has great potential for large-scale production. The sodium ion battery containing the composite positive electrode has excellent cycle life. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1A scanning electron microscope (SEM) image of the composite cathode material S1 prepared in Example 1 of the present application;

[0029] Figure 2 The Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 The composite cathode obtained by mixing O2 and Na4Fe3(PO4)2P2O7 / C at a mass ratio of 1:1 has a specific capacity of 150mAh g-1 at 100mA g-1 -1 Constant current charge-discharge cycle performance.

[0030] Figure 3 The Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 The O2 cathode has a specific capacity of 130mAh g-1 at 100mA g-1 -1 Constant current charge-discharge cycle performance. DETAILED DESCRIPTION

[0031] The present application will be described in detail below with reference to the accompanying drawings and specific examples. Obviously, the scope of protection of the present application is not limited to the following examples.

[0032] Example 1

[0033] A layered transition metal oxide Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 O2 material and a sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7 / C were placed in a 25mL volume mixing box at a mass ratio of 1:1, and mixed on a THINKY MIXER ARE-310 self-rotation and revolution stirrer at a speed of 650, 2000, 650r / min for 3, 5, 2 minutes, respectively, to prepare a composite cathode material S1 for a sodium-ion secondary battery as shown in Figure 1 .

[0034] Preparation of the cathode sheet: the composite cathode material, conductive agent and adhesive were mixed at a mass ratio of 90:5:5 to obtain a uniform slurry, which was uniformly coated on an aluminum foil, dried, and then cut to obtain a circular sheet with a diameter of about 14mm.

[0035] Sodium-ion half battery assembly: In an argon-protected glove box, using metal Na sheet and glass fiber (GFD) with a thickness of 1 mm as the counter electrode and the separator respectively, an electrolyte with sodium hexafluorophosphate as the electrolyte and carbonate compound as the solvent was used. In the working voltage range of 2.0-4.3 V, the charge-discharge cycle performance test was carried out at a current density of 1C rate, Figure 2 As shown in the figure, the capacity retention rate of the prepared battery in the voltage range of 2.0-4.3 V is 88.9% after 200 cycles.

[0036] Comparative Example 1

[0037] In order to compare the excellent cycle performance of the prepared sodium-ion secondary battery composite positive electrode of the present application, in this embodiment, only the layered transition metal oxide material Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 O2 was used as the positive active material to obtain the electrode D1.

[0038] The electrode preparation, sodium-ion battery assembly and test part were carried out under the same process and conditions as the previous embodiment, and the electrode charge-discharge cycle performance test was carried out for 200 cycles with a capacity retention rate of 72.1%.

[0039] Example 2

[0040] The layered transition metal oxide material Na 0.71 Ni 0.25 Mg 0.06 Mn 0.64 Zr 0.03 O2 and the sodium iron pyrophosphate material Na4Fe3(PO4)2P2O7 / C were placed in a small box with a volume of 25 mL in a mass ratio of 10:1, and were mixed on a THINKY MIXER ARE-310 self-rotation, revolution mixer at speeds of 650, 2000, 650 r / min for 3, 5, 2 minutes respectively to prepare a composite positive electrode material S2 for sodium-ion secondary battery.

[0041] The assembly and test of the sodium-ion secondary battery were carried out in the same way as in Example 1, and the capacity retention rate of the battery after 200 cycles was 90.0%.

[0042] Comparative Example 2

[0043] In order to compare the excellent cycle performance of the prepared sodium-ion secondary battery composite positive electrode material of the present application, in this embodiment, only Na 0.7 Ni 0.25 Mg 0.06 Mn 0.64 Zr 0.03O2 is used as the positive electrode active material to obtain electrode D2.

[0044] The electrode preparation, sodium-ion half-cell assembly, and testing were all carried out using the same processes and conditions as in the aforementioned embodiments. The battery's capacity retention rate was 86.2% after 200 charge-discharge cycle tests within a voltage range of 2.0-4.3V.

[0045] Example 3

[0046] The manganese-iron-based Prussian blue compound material Na 1.6 Mn[Fe(CN)6] 0.9 Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7 / C) was placed in a 25mL container at a mass ratio of 4:1 and mixed for 3, 5 and 2 minutes respectively on a THINKY MIXER ARE-310 rotary mixer at speeds of 650, 2000 and 650 r / min to obtain the composite cathode material S3 for sodium-ion secondary batteries.

[0047] The sodium-ion secondary battery was assembled and tested in the same manner as in Example 1. After 200 cycles in the voltage range of 2.0-4.0V, the capacity retention rate was 85.7%.

[0048] Example 4

[0049] Layered transition metal oxide material NaNi 0.33 Fe 0.33 Mn 0.33 O2 and sodium iron pyrophosphate material Na2FeP2O7 were placed in a 25mL container at a mass ratio of 4:1. Under argon gas protection, they were mixed for 3, 5 and 2 minutes respectively on a THINKY MIXER ARE-310 rotary mixer at speeds of 650, 2000 and 650 r / min to obtain composite cathode material S4 for sodium-ion secondary batteries.

[0050] The sodium-ion secondary battery was assembled and tested in the same manner as in Example 1. The battery retained 89.3% of its capacity after 200 cycles in a voltage range of 2.0-4.0V.

[0051] Comparative Example 3

[0052] To illustrate the superior cycle performance of the sodium-ion secondary battery composite cathode material prepared in this invention, only NaNi was used in this embodiment. 0.33 Fe 0.33 Mn 0.33 O2 is used as the positive electrode active material to obtain electrode D2.

[0053] The electrode sheet preparation, sodium-ion secondary battery assembly and testing part were all carried out using the same process and conditions as in the foregoing Example 4. The capacity retention rate after 200 cycles was 78.2%.

[0054] Example 5

[0055] The layered transition metal oxide material Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 O2 and the sodium iron pyrophosphate material Na2FeP2O7 / C were placed in a small box with a volume of 25 mL in a mass ratio of 3:1, and were mixed on a THINKY MIXER ARE-310 self-rotation, revolution stirrer at speeds of 650, 2000, 650 r / min for 3, 5, 2 minutes, respectively, to prepare a composite positive electrode material S5 for sodium-ion secondary batteries.

[0056] The assembly and testing of the sodium-ion secondary battery were carried out in the same way as in Example 1. The capacity retention rate of the battery after 200 cycles was 89.5%.

[0057] Example 6

[0058] The sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 / C material was stirred uniformly with the conductive carbon SP and the binder PVDF in a ratio of 8:1:1 to obtain a uniform slurry, and a coating with a thickness of 5 um was coated on the surface of an aluminum foil. After the electrode sheet obtained in the first step was dried, the Prussian blue compound material Na 1.6 Mn[Fe(CN)6] 0.9 was stirred uniformly with the SP and PVDF in a ratio of 8:1:1, and the uniform slurry obtained was further coated on the surface of the first layer of material to form a coating with a thickness of 100 um. After the electrode sheet obtained in the second step was dried, a coating with a thickness of 20 um was further coated on the surface of the second layer of material, and after drying and rolling, a "sandwiched" composite electrode S6 was finally prepared. The assembly and testing of the sodium-ion secondary battery were carried out in the same way as in Example 1. The capacity retention rate of the battery obtained after 200 cycles was 87.1%.

[0059] Comparative Example 4

[0060] In order to compare and illustrate the excellent cycle performance of the composite positive electrode material of the sodium-ion secondary battery prepared in the present application, only the Na 1.6 Mn[Fe(CN)6] 0.9 was used as the positive electrode active material in this example to obtain an electrode sheet D2.

[0061] The electrode sheet preparation, sodium-ion secondary battery assembly and testing part are all carried out by using the same process and conditions as those in the above-mentioned Example 3. The capacity retention rate of the battery after 200 cycles is 73.2%.

[0062] Example 7

[0063] Na 0.67 Ni 0.25 Mg 0.08 Mn 0.66 Sn 0.01 O2 and conductive carbon SP, binder PVDF are stirred uniformly in a ratio of 8:1:1 to obtain a uniform slurry, and a coating layer with a thickness of 200 um is sprayed on the surface of the aluminum foil. After drying the electrode sheet obtained in the first step, Na4Fe3PO4P2O7F3 / C material is uniformly stirred with SP and PVDF in a ratio of 8:1:1, and the obtained uniform slurry is continuously sprayed on the surface of the first layer of material to cover a coating layer with a thickness of 20 um. After drying and rolling, a composite positive electrode sheet S7 for sodium-ion secondary battery is prepared. The sodium-ion secondary battery is assembled and tested in the same way as in Example 1. In the voltage range of 2.0-4.3V, the capacity retention rate of the battery after 200 cycles is 88.9%.

[0064] The above S1-S7 and D1-D4 are assembled into sodium-ion batteries as positive electrodes, respectively. The capacity retention rate of the electrode after 200 cycles of 1C rate cycle test is shown in Table 1. The detection results here are based on the assembly into CR2032 button cell batteries.

[0065] Table 1

[0066] Number Capacity retention rate (%) S1 88.9 S2 90.0 S3 85.7 S4 89.3 S5 89.5 S6 87.1 S7 88.9 D1 72.1 D2 86.2 D3 78.2 D4 73.2

[0067] As can be seen from the above test examples, the capacity retention rate of the sodium-ion secondary battery assembled by using the composite positive electrode prepared by the "blending" and "secondary coating" process of the present application has been significantly improved.

[0068] The above description is only the preferred embodiments of the present application, and is not intended to limit the present application in other forms. Any person skilled in the art can modify or change the above-mentioned technical content to obtain equivalent embodiments. However, any simple modification, equivalent change and modification of the above-mentioned embodiments based on the technical essence of the present application still falls within the protection scope of the present application.

Claims

1. A composite positive electrode for a sodium-ion secondary battery, characterized by comprising a sodium transition metal oxide and a carbon material. The composite cathode comprises an existing sodium-ion battery cathode material and a Fe-based polyanion compound; The existing sodium-ion battery cathode material comprises a layered transition metal oxide and / or a Prussian blue compound, and the mass ratio of the Fe-based polyanion compound to the existing sodium-ion battery cathode material is (0.05-1): 1; The layered transition metal oxide material has the molecular formula Na. m M n O p In the molecular formula, M contains at least one element, Mn and Ni, and may also selectively contain one or more of Li, Mg, Al, Fe, Co, Cu, Zn, Ca, Sr, Ce, Cr, Ti, Zr, Sn, V, Nb, Sb or Mo, and m≥0.44, n≥1, p≥2, and the values ​​of m, n, and p satisfy the charge balance of the chemical formula. The Prussian blue compound material has a molecular formula of A x P[R(CN)6] y P and R are respectively one or more of Mn, Fe, Ni, Co, Cu, Ce, Cr, Ti, Zn, and V; and x and y satisfy the charge balance of the chemical formula. The iron-based polyanionic compound is Na4Fe x M y (PO4)2P2O7 / C and Na2Fe x M y one or more of the group consisting of P2O7 / C molecular formula, wherein M comprises one or more of Ni, Mn, Co, Cu, Zn, Mg, Al, Ca, Sr, Ce, Ti, Zr, Sn, Nb, Sb or Mo, x, y values meet the charge balance of the chemical formula.

2. A composite cathode electrode sheet for a sodium-ion secondary battery, characterized by comprising a sodium-ion cathode active material and a sodium-ion cathode binder. The composite cathode sheet comprises the composite cathode of claim 1, a binder and a conductive additive.

3. The composite cathode sheet for a sodium-ion secondary battery according to claim 2, characterized by, The composite cathode sheet is a sandwiched composite cathode sheet formed by layer-by-layer coating of the Fe-based polyanion compound, the existing sodium-ion battery cathode material and the Fe-based polyanion compound. Or a double-coating composite cathode sheet formed by layer-by-layer coating of the existing sodium-ion battery cathode material and the Fe-based polyanion compound. Or a single-layer composite cathode sheet formed by mixing the existing sodium-ion battery cathode material and the Fe-based polyanion compound.

4. The composite cathode sheet for a sodium-ion secondary battery according to claim 2, characterized by, The conductive additive is a high-conductivity carbon material, specifically including one or more of graphene, carbon nanotubes, carbon fibers, acetylene black, conductive carbon black or conductive graphite; the mass ratio of the conductive additive in the sodium-ion secondary battery composite cathode sheet is 0.5-10wt%. The binder is a high-molecular polymer, specifically including one or more of polyvinyl alcohol (PVA), sodium carboxymethylcellulose (CMC), PTFE, PVDF, polyurethane, PP, PE or SBR rubber; the mass ratio of the binder in the sodium-ion secondary battery composite cathode sheet is 2-10wt%.

5. The composite cathode sheet for a sodium-ion secondary battery according to claim 2, characterized by, The specific preparation method of the composite cathode sheet is: uniformly dispersing the cathode material, the binder and the conductive additive in a solvent to form a slurry, coating the slurry on the surface of a current collector, and then performing air drying and vacuum drying and rolling to obtain a sodium-ion secondary battery composite cathode sheet. The coating is performed by spraying, extrusion and blade coating or screen printing.

6. A sodium-ion secondary battery, characterized by, The secondary battery comprises the composite cathode sheet of any one of claims 2-5.

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