Composite positive plate, preparation method thereof and sodium ion battery

Abstract

The invention provides a composite positive plate, a preparation method thereof and a sodium ion battery, and relates to the technical field of batteries. The composite positive plate provided by the invention comprises a current collector and a positive active material layer coated on the current collector, the positive electrode active material layer comprises a positive electrode main material, a conductive agent and a binder; the positive electrode main material comprises composite ferric phosphate sodium and an O3-phase layered oxide; the mass ratio of the composite sodium ferric phosphate to the O3-phase layered oxide is (1: 1)-(9: 1). The composite positive plate is large in compaction density and high in gram volume; the surface of the composite sodium ferric phosphate is coated with glycerophosphate, the specific surface area of the composite sodium ferric phosphate can be reduced, the material is not prone to agglomeration due to the negative electricity characteristic of the glycerophosphate, meanwhile, compared with the composite sodium ferric phosphate, glycerin groups have more affinity with a solvent, the solvent can fully wet the surface of the material, and the surface of the material is more stable. The final solid content of system homogenate is further improved, and the solvent loss is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a composite positive electrode sheet and a preparation method thereof, and a sodium ion battery. Background Art

[0002] Sodium-ion batteries (SIBs) are considered a potential alternative to lithium-ion batteries due to the widespread distribution and abundant reserves of sodium. Among the three main types of cathode materials currently used in SIBs, polyanionic cathode materials possess stable crystal structures, low volume changes during intercalation and deintercalation, and exhibit excellent cycling performance. Composite sodium iron phosphate (SFP) has a structure similar to iron phosphate and, therefore, similar properties to lithium iron phosphate. It exhibits excellent cycling stability and safety, making it an ideal raw material for energy storage cells.

[0003] However, the true density of composite sodium iron phosphate itself is not high, which results in a low upper limit for the electrode's compaction density and a low volumetric energy density. Existing methods for increasing primary particle size, particle shaping, and mixing large and small particles are unable to effectively improve the electrode's compaction density. Furthermore, the theoretical gram capacity of composite sodium iron phosphate is only 129 mAh / g, resulting in a low gravimetric energy density for the battery cell. Currently, the gram capacity of composite sodium iron phosphate is approaching 85% of the theoretical value, making further improvement difficult.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The first object of the present invention is to provide a composite positive electrode sheet to solve the above problems.

[0006] The second object of the present invention is to provide a method for preparing the composite positive electrode sheet.

[0007] A third object of the present invention is to provide a sodium ion battery.

[0008] In order to achieve the above objectives, the following technical solutions are adopted:

[0009] In a first aspect, the present invention provides a composite positive electrode sheet, comprising a current collector and a positive electrode active material layer coated on the current collector;

[0010] The positive electrode active material layer includes a positive electrode main material, a conductive agent and a binder;

[0011] The positive electrode main material includes composite sodium iron phosphate and O3 phase layered oxide;

[0012] The mass ratio of the composite sodium iron phosphate and the O3 phase layered oxide is 1:1-9:1;

[0013] The molecular formula of the composite sodium iron phosphate is Na x+2y Fe x+y(PO4) x (P2O7) y ; Among them, 0≤x≤4, 0≤y≤4, 0.5≤x / y≤4;

[0014] The molecular formula of the O3 phase layered oxide is NaM a Ni b Fe c Mn d O2; wherein M is one of Ni, Co, V, Cr, Ti, Fe, Cu or Zn, 0≤a≤0.15, 0.05≤b≤0.3, 0.3≤c≤0.5, 0.3≤d≤0.5, a+b+c+d=1;

[0015] The surface of the composite sodium iron phosphate is coated with a conductive material and glycerol phosphate in sequence.

[0016] As a further technical solution, the particle size of the composite sodium iron phosphate D50 is d1, and the particle size of the O3 phase layered oxide D50 is d2; wherein, d1 is 2-5 μm, d2 is 8-20 μm, and √2+1≤d2 / d1≤8.

[0017] As a further technical solution, the conductive material includes carbon, and the mass of the conductive material is 1%-3% of the mass of the composite sodium iron phosphate;

[0018] And / or, the mass of the glycerophosphate is 0.05%-0.5% of the mass of the composite sodium iron phosphate.

[0019] As a further technical solution, the positive electrode active material layer includes, by mass percentage, 1%-4% of a conductive agent, 1.5%-5% of a binder, and the remainder is the positive electrode main material.

[0020] As a further technical solution, the conductive agent includes at least one of conductive carbon black or carbon nanotubes; the carbon nanotubes include multi-walled carbon nanotubes; the multi-walled carbon nanotubes have an aspect ratio of 1000-10000;

[0021] The binder includes PVDF.

[0022] As a further technical solution, the current collector includes aluminum foil.

[0023] In a second aspect, the present invention provides a method for preparing the composite positive electrode sheet, comprising the following steps:

[0024] s1. The surface of the composite sodium iron phosphate coated with a conductive material, glycerol and a catalyst are mixed, and the reaction obtains a composite sodium iron phosphate surface coated with glycerol phosphate;

[0025] s2. The composite sodium iron phosphate obtained in step s1 is mixed with the O3 layered oxide phase and then mixed with a conductive agent, a binder and a solvent to obtain a positive electrode slurry;

[0026] s3. Coat the positive electrode slurry on the current collector to prepare a composite positive electrode sheet.

[0027] As a further technical solution, in step s1, the mass ratio of the composite sodium iron phosphate, glycerol and catalyst is (400-500):(0.8-1.2):(0.4-0.6);

[0028] The catalyst includes cyclohexane;

[0029] The reaction temperature is 75-85° C., and the reaction time is 4-6 hours.

[0030] As a further technical solution, the solvent includes N-methylpyrrolidone;

[0031] The solid content of the positive electrode slurry is 50%-60%.

[0032] In a third aspect, the present invention provides a sodium ion battery, wherein the sodium ion battery uses the composite positive electrode sheet as the positive electrode sheet.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The composite positive electrode sheet provided by the present invention includes a current collector and a positive electrode active material layer coated on the current collector; the positive electrode active material layer includes a composite sodium iron phosphate and an O3 phase layered oxide in a specific ratio. On the one hand, the charge and discharge range of the composite sodium iron phosphate does not involve the potential of oxygen evolution and drastic volume change of the O3 phase layered oxide, so the O3 phase layered oxide can exhibit ultra-high stability and extremely low gas production during the cycle; on the other hand, the higher gram capacity of the O3 phase layered oxide can effectively improve the gram capacity of the system, and the high compaction and low specific surface characteristics of the O3 phase layered oxide can also effectively improve the final solid content of the system homogenate and the compaction density of the electrode sheet. In addition, in the present invention, the surface of the composite sodium iron phosphate is coated with glycerol phosphate, which can reduce the specific surface area of ​​the composite sodium iron phosphate, and the negatively charged characteristic of glycerol phosphate makes the material less likely to agglomerate. At the same time, the glycerol group has a higher affinity with the solvent than the composite sodium iron phosphate, which can make the solvent fully wet the surface of the material, which helps to further increase the final solid content of the system homogenate and reduce solvent loss. DETAILED DESCRIPTION

[0035] Below in conjunction with embodiment and example, embodiment of the present invention is described in detail, but those skilled in the art will appreciate that the following embodiment and example are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiment in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work premise all fall within the scope of protection of the present invention. Unspecified conditions are carried out according to the conditions of normal conditions or manufacturer's recommendations. Reagents used or instruments not specified by the manufacturer are conventional products that can be purchased commercially.

[0036] In a first aspect, the present invention provides a composite positive electrode sheet, comprising a current collector and a positive electrode active material layer coated on the current collector;

[0037] The positive electrode active material layer includes a positive electrode main material, a conductive agent and a binder;

[0038] The positive electrode main material includes composite sodium iron phosphate and O3 phase layered oxide;

[0039] The mass ratio of the composite sodium iron phosphate and the O3 phase layered oxide can be, for example, but not limited to, 1:1, 3:1, 5:1, 7:1 or 9:1;

[0040] The molecular formula of the composite sodium iron phosphate is Na x+2y Fe x+y (PO4) x (P2O7) y ; Among them, 0≤x≤4, 0≤y≤4, 0.5≤x / y≤4;

[0041] The molecular formula of the O3 phase layered oxide is NaM a Ni b Fe c Mn d O2; wherein M is one of Ni, Co, V, Cr, Ti, Fe, Cu or Zn, 0≤a≤0.15, 0.05≤b≤0.3, 0.3≤c≤0.5, 0.3≤d≤0.5, a+b+c+d=1;

[0042] The surface of the composite sodium iron phosphate is coated with a conductive material and glycerol phosphate in sequence.

[0043] The composite positive electrode sheet provided by the present invention includes a current collector and a positive electrode active material layer coated on the current collector; the positive electrode active material layer includes a composite sodium iron phosphate and an O3 phase layered oxide in a specific ratio. On the one hand, the charge and discharge range of the composite sodium iron phosphate does not involve the potential of oxygen evolution and drastic volume change of the O3 phase layered oxide, so the O3 phase layered oxide can exhibit ultra-high stability and extremely low gas production during the cycle; on the other hand, the higher gram capacity of the O3 phase layered oxide can effectively improve the gram capacity of the system, and the high compaction and low specific surface characteristics of the O3 phase layered oxide can also effectively improve the final solid content of the system homogenate and the compaction density of the electrode sheet. In addition, in the present invention, the surface of the composite sodium iron phosphate is coated with glycerol phosphate, which can reduce the specific surface area of ​​the composite sodium iron phosphate, and the negatively charged characteristic of glycerol phosphate makes the material less likely to agglomerate. At the same time, the glycerol group has a higher affinity with the solvent than the composite sodium iron phosphate, which can make the solvent fully wet the surface of the material, which helps to further increase the final solid content of the system homogenate and reduce solvent loss.

[0044] In some optional embodiments, the D50 particle size of the composite sodium iron phosphate is d1, and the D50 particle size of the O3 phase layered oxide is d2; wherein, the range of d1 is 2-5 μm, the range of d2 is 8-20 μm, and √2+1≤d2 / d1≤8. The composite sodium iron phosphate can exert the highest gram capacity and exhibit a higher compaction density within this particle size range, and can more effectively improve the compaction density of the electrode when combined with the O3 phase layered oxide within a specific range.

[0045] In some optional embodiments, the conductive material includes carbon, and the mass of the conductive material is 1%-3% of the mass of the composite sodium iron phosphate, for example, but not limited to 1%, 2% or 3%.

[0046] Coating the surface of composite sodium iron phosphate with conductive materials helps to improve the conductivity of the material and promote its function.

[0047] And / or, the mass of the glycerophosphate is 0.05%-0.5% of the mass of the composite sodium iron phosphate, for example, but not limited to 0.05%, 0.1%, 0.2%, 0.3%, 0.4% or 0.5%.

[0048] In some optional embodiments, the positive electrode active material layer includes, by mass percentage, 1%-4% of a conductive agent, 1.5%-5% of a binder, and the remainder is a positive electrode main material.

[0049] In some optional embodiments, the conductive agent includes but is not limited to at least one of conductive carbon black or carbon nanotubes, or other conductive agents well known to those skilled in the art;

[0050] The carbon nanotubes are preferably multi-walled carbon nanotubes; the aspect ratio of the multi-walled carbon nanotubes can be, for example, but not limited to, 1000, 2000, 4000, 6000, 8000 or 10000. Carbon nanotubes within this range are not easy to agglomerate and can effectively build a conductive network between particles, further improving the specific capacity of the active material.

[0051] In some optional embodiments, the binder includes but is not limited to PVDF, or other binders well known to those skilled in the art.

[0052] In some optional embodiments, the current collector comprises aluminum foil.

[0053] In a second aspect, the present invention provides a method for preparing the composite positive electrode sheet, comprising the following steps:

[0054] s1. The surface of the composite sodium iron phosphate coated with a conductive material, glycerol and a catalyst are mixed, and the reaction obtains a composite sodium iron phosphate surface coated with glycerol phosphate;

[0055] s2. The composite sodium iron phosphate obtained in step s1 is mixed with the O3 layered oxide phase and then mixed with a conductive agent, a binder and a solvent to obtain a positive electrode slurry;

[0056] s3. Coat the positive electrode slurry on the current collector to prepare a composite positive electrode sheet.

[0057] The preparation method provided by the present invention is simple and convenient, and the prepared composite positive electrode sheet has a large compaction density and a high gram capacity, which can effectively improve the capacity of the battery.

[0058] In some optional embodiments, in step s1, the mass ratio of the composite sodium iron phosphate, glycerol and catalyst can be, for example, but not limited to, 400:1.2:0.4, 450:1:0.5, or 500:0.8:0.6;

[0059] The catalyst includes cyclohexane to catalyze the formation of glycerophosphate on the surface of the nano-ferric phosphate.

[0060] The reaction temperature may be, but is not limited to, 75° C., 77° C., 79° C., 81° C., 83° C. or 85° C., and the reaction time may be, but is not limited to, 4 h, 5 h or 6 h.

[0061] In some optional embodiments, the solvent includes but is not limited to N-methylpyrrolidone, or other solvents well known to those skilled in the art;

[0062] The solid content of the positive electrode slurry may be, for example, but not limited to, 50%, 52%, 54%, 56%, 58% or 60%.

[0063] In a third aspect, the present invention provides a sodium ion battery, wherein the sodium ion battery uses the composite positive electrode sheet as the positive electrode sheet.

[0064] The battery has high capacity.

[0065] The present invention is further described below by means of specific examples and comparative examples. However, it should be understood that these examples are merely provided for more detailed description and are not to be construed as limiting the present invention in any form.

[0066] Unless otherwise specified, in the following examples and comparative examples, the composite sodium ferric phosphate is a composite sodium ferric phosphate having carbon coated on the surface, wherein the coating amount in Example 1 is 2%, the coating amount in Example 2 is 1%, and the coating amount in Example 3 is 3%. The other examples and comparative examples are the same as Example 1.

[0067] Example 1

[0068] A composite positive electrode sheet comprising an aluminum foil and a positive electrode active material layer on the aluminum foil;

[0069] In terms of mass percentage, the positive electrode active material layer includes 2% of a conductive agent (conductive carbon black and multi-walled carbon nanotubes, with a mass ratio of 2:1 and an aspect ratio of the multi-walled carbon nanotubes of 5000), 2% of PVDF, and the remainder being the positive electrode main material;

[0070] The main material of the positive electrode includes composite sodium iron phosphate and O3 phase layered oxide;

[0071] The mass ratio of composite sodium iron phosphate and O3-phase layered oxide is 85:15;

[0072] The molecular formula of compound sodium iron phosphate is Na3Fe2(PO4)(P2O7);

[0073] The molecular formula of the O3 phase layered oxide is NaTi 0.1 Ni 0.2 Fe 0.3 Mn 0.4 O2;

[0074] The D50 particle size of the composite sodium iron phosphate is d1, and the D50 particle size of the O3 phase layered oxide is d2; wherein, d1 is 3 μm, and d2 is 14 μm;

[0075] The surface of the composite sodium ferric phosphate is coated with glycerophosphate. According to testing, the mass of the glycerophosphate is 0.2% of the mass of the composite sodium ferric phosphate.

[0076] The preparation method of the composite positive electrode sheet includes:

[0077] 1. Place composite sodium iron phosphate, glycerol and cyclohexane in a blender at a mass ratio of 450:1:0.5, maintain the vacuum degree of the blender at <-0.08 MPa, and keep the mixture at 80°C for 5 hours.

[0078] 2. Add O3-phase layered oxide to the stirrer and stir for 0.5h. The mass ratio of O3-phase layered oxide to the composite sodium iron phosphate added in step 1 is 15:85.

[0079] 3. Add PVDF, conductive carbon black and multi-walled carbon nanotubes to the blender;

[0080] 4. Add NMP to the mixer to make the solid content of the slurry range from 50% to 60%;

[0081] 5. Apply the prepared slurry on aluminum foil to obtain the desired composite positive electrode sheet.

[0082] Example 2

[0083] A composite positive electrode sheet comprising an aluminum foil and a positive electrode active material layer on the aluminum foil;

[0084] By mass percentage, the positive electrode active material layer includes 1% of a conductive agent (conductive carbon black and multi-walled carbon nanotubes, with a mass ratio of 2:1 and an aspect ratio of the multi-walled carbon nanotubes of 5000), 1.5% of PVDF, and the remainder being the positive electrode main material;

[0085] The main material of the positive electrode includes composite sodium iron phosphate and O3 phase layered oxide;

[0086] The mass ratio of composite sodium iron phosphate and O3-phase layered oxide is 1:1;

[0087] The molecular formula of compound sodium iron phosphate is Na 2.4 Fe 1.4 (PO4) 0.4 (P2O7);

[0088] The molecular formula of the O3 phase layered oxide is NaNi 0.3 Fe 0.3 Mn 0.4 O2;

[0089] The D50 particle size of the composite sodium iron phosphate is d1, and the D50 particle size of the O3 phase layered oxide is d2; wherein, d1 is 2 μm, and d2 is 16 μm;

[0090] The surface of the composite sodium ferric phosphate is coated with glycerophosphate. According to testing, the mass of the glycerophosphate is 0.05% of the mass of the composite sodium ferric phosphate.

[0091] The preparation method of the composite positive electrode sheet includes:

[0092] 1. Place composite sodium iron phosphate, glycerol and cyclohexane in a blender at a mass ratio of 700:1:0.5, maintain the vacuum degree of the blender at <-0.08 MPa, and keep the mixture at 75°C for 6 hours.

[0093] 2. Add O3-phase layered oxide to the stirrer and stir for 0.5h. The mass ratio of O3-phase layered oxide to the composite sodium iron phosphate added in step 1 is 1:1.

[0094] 3. Add PVDF, conductive carbon black and multi-walled carbon nanotubes to the blender;

[0095] 4. Add NMP to the mixer to make the solid content of the slurry range from 50% to 60%;

[0096] 5. Apply the prepared slurry on aluminum foil to obtain the desired composite positive electrode sheet.

[0097] Example 3

[0098] A composite positive electrode sheet comprising an aluminum foil and a positive electrode active material layer on the aluminum foil;

[0099] By mass percentage, the positive electrode active material layer includes 4% of a conductive agent (conductive carbon black and multi-walled carbon nanotubes, with a mass ratio of 2:1 and an aspect ratio of the multi-walled carbon nanotubes of 5000), 5% of PVDF, and the remainder being the positive electrode main material;

[0100] The main material of the positive electrode includes composite sodium iron phosphate and O3 phase layered oxide;

[0101] The mass ratio of composite sodium iron phosphate and O3-phase layered oxide is 1:9;

[0102] The molecular formula of compound sodium iron phosphate is Na2Fe 1.5 (PO4)(P2O7) 0.5 ;

[0103] The molecular formula of the O3 phase layered oxide is NaTi 0.15 Ni 0.05 Fe 0.3 Mn 0.5 O2;

[0104] The D50 particle size of the composite sodium iron phosphate is d1, and the D50 particle size of the O3 phase layered oxide is d2; wherein, d1 is 5 μm, and d2 is 13 μm;

[0105] The surface of the composite sodium ferric phosphate is coated with glycerophosphate. According to testing, the mass of the glycerophosphate is 0.5% of the mass of the composite sodium ferric phosphate.

[0106] The preparation method of the composite positive electrode sheet includes:

[0107] 1. Place composite sodium iron phosphate, glycerol and cyclohexane in a blender at a mass ratio of 300:1:0.5, maintain the vacuum degree of the blender at <-0.08 MPa, and keep the mixture at 85°C for 4 hours.

[0108] 2. Add O3-phase layered oxide to the stirrer and stir for 0.5h. The mass ratio of O3-phase layered oxide to the composite sodium iron phosphate added in step 1 is 1:9.

[0109] 3. Add PVDF, conductive carbon black and multi-walled carbon nanotubes to the blender;

[0110] 4. Add NMP to the mixer to make the solid content of the slurry range from 50% to 60%;

[0111] 5. Apply the prepared slurry on aluminum foil to obtain the desired composite positive electrode sheet.

[0112] Example 4

[0113] A positive electrode sheet, which differs from Example 1 in that the particle size of the composite sodium iron phosphate D50 is d1, and the particle size of the O3 phase layered oxide D50 is d2; wherein d1 is 1 μm and d2 is 25 μm.

[0114] Example 5

[0115] A positive electrode sheet, which differs from Example 1 in that the particle size of the composite sodium iron phosphate D50 is d1, and the particle size of the O3 phase layered oxide D50 is d2; wherein d1 is 10 μm and d2 is 5 μm.

[0116] Example 6

[0117] A positive electrode sheet, which differs from Example 1 in that the aspect ratio of the multi-walled carbon nanotubes is 100.

[0118] Example 7

[0119] A positive electrode sheet, which differs from Example 1 in that the molecular formula of the composite sodium iron phosphate is Na6Fe5(PO4)4(P2O7); the molecular formula of the O3 phase layered oxide is NaTi 0.05 Ni 0.15 Fe 0.5 Mn 0.3 O2.

[0120] Example 8

[0121] A positive electrode sheet, which differs from Example 1 in that the molecular formula of the composite sodium iron phosphate is Na5Fe3(PO4)1(P2O7)2; the molecular formula of the O3 phase layered oxide is NaTi 0.1 Ni 0.3 Fe 0.3 Mn 0.3O2.

[0122] Comparative Example 1

[0123] A positive electrode sheet, which differs from Example 1 in that the surface of the composite sodium iron phosphate is not coated with glycerol phosphate.

[0124] Comparative Example 2

[0125] A positive electrode sheet, which differs from Example 1 in that the main positive electrode material is composite sodium iron phosphate.

[0126] Comparative Example 3

[0127] A positive electrode sheet, which differs from Example 1 in that the main material of the positive electrode is entirely O3 phase layered oxide.

[0128] Comparative Example 4

[0129] A positive electrode sheet, which differs from Example 1 in that the molecular formula of the composite sodium iron phosphate is Na 2.2 Fe 1.2 (PO4) 0.2 (P2O7).

[0130] Comparative Example 5

[0131] A positive electrode sheet, which differs from Example 1 in that the molecular formula of the O3 phase layered oxide is NaNi 0.5 Fe 0.4 Mn 0.1 O2.

[0132] Comparative Example 6

[0133] A positive electrode sheet, which differs from Example 1 in that composite sodium iron phosphate without carbon coating on the surface is used as raw material.

[0134] Test Example 1

[0135] The positive electrode sheets provided in Examples 1-6 and Comparative Examples 1-5 were respectively combined with negative electrode sheets and electrolytes to prepare sodium ion batteries, wherein the negative electrode sheets were hard carbon and the electrolytes were propylene carbonate (PC) solutions containing 1.0M NaPF6 and 3.0% FEC.

[0136] The process characteristics and electrochemical performance of the sodium ion battery prepared above were tested, and the results are as follows:

[0137] Table 1 Process characteristics and electrochemical properties of the cells prepared in each embodiment and comparative example

[0138]

[0139] It can be seen from the above table that each embodiment provided by the present invention has good electrochemical performance. Comparing Examples 1-3, it can be found that when there are more O3 layered oxides in the system; although the compaction density and gram capacity are improved, the cycle performance of the battery cell deteriorates and the gas production increases.

[0140] Comparing Examples 4 and 5 with Example 1, if the gap between the two material particles is too small or too large, the compaction density will be reduced, the cycle performance will be reduced, and a good mixing effect cannot be achieved;

[0141] Comparing Example 6 with Example 1, the aspect ratio of the CNT is too small, which does not provide a good conductive effect, resulting in reduced specific capacity and cycle performance of the battery cell.

[0142] Comparing Comparative Example 1 with Example 1, without glycerophosphate coating, the solid content of the slurry is reduced, the dispersion effect is deteriorated, the gram capacity is reduced, and the cycle performance is slightly reduced;

[0143] Comparing Comparative Examples 2 and 3 with Example 1, the single composite sodium iron phosphate in Comparative Example 2 is poor in solid content, compacted density and gram capacity, and the single O3 phase layered oxide in Comparative Example 3 is poor in gram capacity and cycle performance;

[0144] Comparing Comparative Examples 4 and 5 with Example 1, it was found that the change in the ratio of phosphate and pyrophosphate in the composite sodium iron phosphate in Example 4 would affect the gram capacity of the battery cell. The increase in the nickel content in Comparative Example 5 caused an increase in the gram capacity, but also made the cycle performance worse.

[0145] In summary, the positive electrode active material provided by the embodiments of the present invention has the following characteristics:

[0146] 1) The high compaction characteristics of the O3 phase layered oxide effectively improve the overall compaction density of the electrode.

[0147] 2) The higher specific capacity of O3 phase layered oxides effectively improves the specific capacity of the entire system, thereby increasing the capacity of the battery cell.

[0148] 3) The low surface area of ​​the O3 phase layered oxide and the dispersing effect of glycerol phosphate increase the solid content of the system slurry and reduce the loss of NMP in the process.

[0149] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite positive electrode sheet, characterized in that: It includes a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer includes a positive electrode main material, a conductive agent and a binder; The positive electrode main material includes composite sodium iron phosphate and O3 phase layered oxide; The mass ratio of the composite sodium iron phosphate and the O3 phase layered oxide is 1:1-9:1; The molecular formula of the composite sodium iron phosphate is Na x+2y Fe x+y (PO4) x (P2O7) y ; Among them, 0≤x≤4, 0≤y≤4, 0.5≤x / y≤4; The molecular formula of the O3 phase layered oxide is NaM a Ni b Fe c Mn d O2; wherein M is one of Ni, Co, V, Cr, Ti, Fe, Cu or Zn, 0≤a≤0.15, 0.05≤b≤0.3, 0.3≤c≤0.5, 0.3≤d≤0.5, a+b+c+d=1; The surface of the composite sodium iron phosphate is coated with a conductive material and glycerol phosphate in sequence.

2. The composite positive electrode sheet according to claim 1, characterized in that: The particle size of the composite sodium iron phosphate D50 is d1, and the particle size of the O3 phase layered oxide D50 is d2; wherein, d1 is 2-5 μm, d2 is 8-20 μm, and √2+1≤d2 / d1≤8.

3. The composite positive electrode sheet according to claim 1, characterized in that: The conductive material includes carbon, and the mass of the conductive material is 1%-3% of the mass of the composite sodium iron phosphate; And / or, the mass of the glycerophosphate is 0.05%-0.5% of the mass of the composite sodium iron phosphate.

4. The composite positive electrode sheet according to claim 1, characterized in that: Calculated by mass percentage, the positive electrode active material layer includes 1%-4% of a conductive agent, 1.5%-5% of a binder, and the remainder is a positive electrode main material.

5. The composite positive electrode sheet according to claim 1, characterized in that: The conductive agent comprises at least one of conductive carbon black or carbon nanotubes; the carbon nanotubes comprise multi-walled carbon nanotubes; the multi-walled carbon nanotubes have an aspect ratio of 1000-10000; The binder includes PVDF.

6. The composite positive electrode sheet according to claim 1, characterized in that: The current collector includes aluminum foil.

7. The method for preparing the composite positive electrode sheet according to any one of claims 1 to 6, characterized in that: The following steps are involved: s1. The surface of the composite sodium iron phosphate coated with a conductive material, glycerol and a catalyst are mixed, and the reaction obtains a composite sodium iron phosphate surface coated with glycerol phosphate; s2. The composite sodium iron phosphate obtained in step s1 is mixed with the O3 phase layered oxide, and then mixed with a conductive agent, a binder and a solvent to obtain a positive electrode slurry; s3. Coat the positive electrode slurry on the current collector to prepare a composite positive electrode sheet.

8. The preparation method according to claim 7, characterized in that In step s1, the mass ratio of the composite sodium iron phosphate, glycerol and catalyst is (400-500):(0.8-1.2):(0.4-0.6); The catalyst includes cyclohexane; The reaction temperature is 75-85° C., and the reaction time is 4-6 hours.

9. The preparation method according to claim 7, characterized in that The solvent includes N-methylpyrrolidone; The solid content of the positive electrode slurry is 50%-60%.

10. A sodium ion battery, characterized in that: The sodium ion battery uses the composite positive electrode sheet according to any one of claims 1 to 6 as a positive electrode sheet.

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