Positive electrode active material, sodium-ion battery, preparation method therefor, and electrical device

Abstract

Current sodium ion batteries have relatively poor low-temperature resistance, which needs to be improved.SOLUTION: The present application provides a positive electrode active material, a sodium ion battery, a preparation method therefor, and an electrical device, and belongs to the technical field of secondary batteries. The positive active material includes a polyanionic material and Na4Fe3 (PO4) 2P2O7, and a mass of the Na4Fe3 (PO4) 2P2O7 accounts for 40% to 60% of a mass of the positive active material. In the present application, a combination of a polyanionic material and Na4Fe3 (PO4) 2P2O7 is used as a positive electrode active material, and the positive electrode active material has a high Na + diffusion coefficient, a high energy density and a good cycle property under a low-temperature environment due to the combination of the two materials, which is beneficial to improving the low-temperature performance of the sodium ion battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present application belongs to the technical field of secondary batteries, and particularly relates to a positive electrode active material, a sodium ion battery, a preparation method thereof, and an electric device. [Background technology]

[0002] The rapid growth in demand for electric motorcycles has led to an increase in the demand for batteries as a core component of these vehicles. Currently, rechargeable lithium-ion batteries (LIBs) are widely used in portable electronic devices and electric vehicles due to their long lifespan and high energy density. However, the limited availability of lithium has hindered the development of lithium-ion batteries. In contrast, sodium-ion batteries (SIBs) have attracted attention due to the abundant and widely distributed sodium resources. Sodium-ion batteries offer significant advantages for electric motorcycle applications, including light weight, moderate specific energy, low cost, a wide operating temperature range, and high safety.

[0003] However, the electrochemical performance of sodium-ion batteries (including capacity, efficiency, and energy density / power density) deteriorates significantly as temperature decreases. If the operating temperature drops below 0°C, the battery's performance is impaired.

[0004] The low temperature resistance of sodium ion batteries is important for meeting the actual usage requirements in extreme weather and different regions. However, current sodium ion batteries have relatively poor low temperature resistance, and improvement is required. Summary of the Invention [Problem to be solved by the invention]

[0005] The present application aims to provide a positive electrode active material, a sodium ion battery, a preparation method thereof, and an electrical device that can solve the above problems. [Means for solving the problem]

[0006] To achieve the above objectives, the present application adopts the following technical solutions:

[0007] The positive electrode active material includes a polyanionic material and Na4Fe3(PO4)2P2O7, and the mass of the Na4Fe3(PO4)2P2O7 accounts for 40% to 60% of the mass of the positive electrode active material.

[0008] In an embodiment of the present application, the polyanionic material comprises at least one of sodium iron phosphate, sodium iron pyrophosphate.

[0009] The present application further provides a sodium ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises the above-described positive electrode active material, the negative electrode comprises the negative electrode active material, the negative electrode active material comprises natural graphite, and the electrolyte comprises an ether-based electrolyte.

[0010] In an embodiment of the present application, the ether-based electrolyte solution includes an electrolyte and a solvent, the electrolyte includes NaPF6, and the solvent includes DME, DG, and TG; And / or, in the ether-based electrolytic solution, the concentration of the electrolyte is 1 to 1.8 mol / L.

[0011] The present application further provides a method for preparing the above-mentioned sodium-ion battery, which includes the steps of: preparing a positive electrode slurry using a positive electrode active material, and applying the positive electrode slurry to a first current collector to obtain a positive electrode; preparing a negative electrode slurry using a negative electrode active material, and applying the negative electrode slurry to a second current collector to obtain a negative electrode; and assembling the positive electrode, the negative electrode, and an electrolyte to obtain the sodium-ion battery.

[0012] In an embodiment of the present application, preparing a positive electrode slurry using the positive electrode active material includes mixing the positive electrode active material, a first binder, a first conductive agent, and a first solvent to obtain a positive electrode slurry; the first binder comprises polyvinylidene fluoride; the first conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black; The first solvent includes N-methylpyrrolidone.

[0013] In an embodiment of the present application, the first current collector comprises aluminum foil.

[0014] In an embodiment of the present application, preparing a negative electrode slurry using the negative electrode active material includes mixing the negative electrode active material, a second binder, a second conductive agent, and a second solvent to obtain the negative electrode slurry; the second binder contains at least one of styrene butadiene rubber and carboxymethyl cellulose, The second conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black.

[0015] In an embodiment of the present application, the second current collector comprises aluminum foil; and / or a positive electrode tab is welded to the positive electrode and a negative electrode tab is welded to the negative electrode, and the material of the positive electrode tab and the material of the negative electrode tab each independently include aluminum, nickel, or an aluminum-nickel clad material.

[0016] The present application further provides an electrical device, which comprises the sodium ion battery described above or a sodium ion battery prepared by the preparation method described above. [Effects of the Invention]

[0017] Compared with the prior art, the present application has the following beneficial effects:

[0018] In this application, a combination of a polyanion material and Na4Fe3(PO4)2P2O7 is used as a positive electrode active material. By combining these two materials, the positive electrode active material has a relatively high Na +It has a low diffusion coefficient, a relatively high energy density, and excellent cycle stability, and can contribute to improving the low-temperature usability of sodium-ion batteries.

[0019] The present application further provides a sodium ion battery. The sodium ion battery uses the above-mentioned positive electrode active material, and thereby, can maintain the sodium ion battery in a low-temperature environment. + The sodium ion battery has an excellent diffusion coefficient and a relatively high energy density. The sodium ion battery uses natural graphite as the negative electrode active material, and the natural graphite negative electrode has a charge storage mechanism based on solvent co-intercalation. The sodium ions can store charge without a desolvation process, contributing to improved low-temperature usability of the sodium ion battery. The sodium ion battery also uses an ether-based electrolyte, which has a relatively low freezing point, preventing the problem of electrolyte freezing in low-temperature environments and contributing to improved low-temperature usability of the sodium ion battery. In summary, the sodium ion battery according to the present application combines excellent low-temperature resistance, excellent cycle performance, relatively low production costs, a relatively wide usable temperature range, excellent electrochemical performance even in low-temperature environments down to -40°C, and is market-competitive.

[0020] The sodium ion battery prepared by the method for preparing a sodium ion battery according to the present application has excellent electrochemical performance even in a low-temperature environment, and the preparation method is simple to operate, has low production costs, and is suitable for mass production.

[0021] An electrical device using the sodium ion battery according to the present application can be used at relatively low temperatures, thereby extending the usable temperature range of current electrical devices. [Brief explanation of the drawings]

[0022] In order to more clearly describe the technical solutions of the embodiments of the present application, the drawings used in the embodiments will be briefly described below. The drawings described are only for illustrating some embodiments of the present application and are not intended to limit the scope of the present application.

[0023] [Figure 1]1 is a scanning electron microscope photograph of a positive electrode plate according to Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0024] The term "and / or" as used herein represents a relationship between related objects and represents a three-way relationship, such as A and / or B, where only A is present, both A and B are present, or only B is present.

[0025] In order to better explain the technical solution according to the present application, before describing the examples, the technical solution is described as follows.

[0026] The present application provides a positive electrode active material, which includes a polyanionic material and Na4Fe3(PO4)2P2O7, and the mass of the Na4Fe3(PO4)2P2O7 accounts for 40% to 60% of the mass of the positive electrode active material.

[0027] Na4Fe3(PO4)2P2O7 is Fe 2+ The compound is a blended phosphate compound containing phosphates of the Pn21a orthorhombic crystal system, with FeO6 octahedra connected by shared edges or shared angles, and PO4 tetrahedra connected to these FeO6 octahedra, forming layered units along the axial planes. These layered units are connected in the horizontal direction via P2O7 groups to form a three-dimensional framework structure. Thus, sodium ion diffusion channels exist in all three directions in the three-dimensional space, and this structure has four different sodium sites, which can contribute to the insertion and extraction of sodium ions, and the Na ions at the electrode-electrolyte interface are located at the electrode-electrolyte interface. + The exsolution process of NaFe(PO)2P2O7 is accelerated, suppressing electrode polarization, allowing the electrode to have sufficient discharge capacity and operate at a relatively low temperature. In other words, the structural characteristics of NaFe(PO)2P2O7 can contribute to the diffusion of sodium ions. Polyanionic materials have a 3D network structure, and Na + The diffusion path of Na is excellent even at low temperatures. +The diffusion rate of Na is relatively fast, which ensures excellent cycle stability and fast reaction rate of the battery. In low temperature environment, Na4Fe3(PO4)2P2O7 shows a better average discharge voltage (about 3.2V) than polyanionic materials. When charging and discharging, Na in Na4Fe3(PO4)2P2O7 + During the desorption or insertion process, a solid-solution reaction occurs in which some of the lattice is deformed, and the volume change during this process is relatively small, which contributes to maintaining the stability of the crystal structure. When the mass of Na4Fe3(PO4)2P2O7 accounts for 40% to 60% of the mass of the positive electrode active material, the positive electrode has excellent electrochemical performance. Specifically, sodium-ion batteries can operate stably at low temperatures of -40°C. If the content of Na4Fe3(PO4)2P2O7 added is too low, on the one hand, it is not possible to ensure that the manufactured sodium-ion battery will operate at temperatures below -40°C, and on the other hand, the cost of the battery will increase. If the content of Na4Fe3(PO4)2P2O7 added is too high, the energy density of the sodium-ion battery will be relatively low. This is because the theoretical specific capacity of Na4Fe3(PO4)2P2O7 is lower than that of the polyanionic material. For example, the theoretical specific capacity of Na4Fe3(PO4)2P2O7 is 129 mAh / g, while the theoretical specific capacity of the polyanionic material sodium iron phosphate is 154 mAh / g. In addition, the structural stability of the polyanionic material can prevent safety issues such as overcharge and overdischarge. If the content of Na4Fe3(PO4)2P2O7 is too high, the content of the polyanionic material will be relatively low, leading to battery safety issues and other related problems. In short, the positive electrode active material of the present application has a relatively high Na content in a low-temperature environment. + It has a low diffusion coefficient, a relatively high energy density, and excellent cycle stability, and can contribute to improving the low-temperature usability of sodium-ion batteries.

[0028] Exemplarily, the mass of the Na4Fe3(PO4)2P2O7 accounts for 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or 60% of the mass of the positive electrode active material.

[0029] In the examples of the present application, the polyanionic material includes at least one of sodium iron phosphate (NaFePO4) and sodium iron pyrophosphate (Na8Fe4(P2O7)5). When the above polyanionic material is mixed with Na4Fe3(PO4)2P2O7, the framework structure formed by blending phosphate ions and pyrophosphate ions can exist stably and form a sodium ion compound, contributing to improving the performance of sodium ion batteries in low-temperature environments. Furthermore, the above polyanionic material is relatively low in cost, contributing to reducing the production cost of sodium ion batteries and increasing their market competitiveness.

[0030] The present application further provides a sodium-ion battery. The sodium-ion battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes the above-described positive electrode active material. The negative electrode includes a negative electrode active material. The negative electrode active material includes natural graphite. The electrolyte includes an ether-based electrolyte.

[0031] The positive electrode of the sodium ion battery contains the above positive electrode active material, and thus the sodium ion battery can + The raw material for the negative electrode contains natural graphite, and Na + Both Na and solvent molecules are intercalated into natural graphite, which allows for fast reaction rates and indicates the possibility of operation at low temperatures. + During storage, the average discharge plateau of the natural graphite anode is +The voltage is approximately 0.6 V versus the negative electrode potential, which effectively prevents the risk of Na electroplating due to battery polarization under low-temperature conditions. Furthermore, natural graphite has excellent thermal stability, making it more suitable for low-temperature sodium-ion batteries than conventional negative electrode active materials (e.g., hard carbon). Furthermore, electrolytes with high freezing points tend to solidify in low-temperature environments, limiting the low-temperature usability of batteries. To address this technical issue, the present application uses an ether-based electrolyte, which has a lower freezing point and higher conductivity than other types of electrolytes (e.g., ester-based electrolytes), contributing to improved low-temperature operation performance of sodium-ion batteries.

[0032] In the embodiment of the present application, the ether-based electrolyte solution includes an electrolyte and a solvent, where the electrolyte includes NaPF6 and the solvent includes DME, DG, and TG, which can provide an efficient and stable ion conduction environment for the sodium ion battery, ensuring high performance and long life of the sodium ion battery.

[0033] Furthermore, the mass ratio of DME to DG to TG is 4:2:1.

[0034] And / or, in the ether-based electrolyte solution, the concentration of the electrolyte is 1 to 1.8 mol / L, for example, 1 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, or 1.8 mol / L. Within the above concentration range, the ether-based electrolyte solution can provide appropriate ion mobility and contribute to rapid transport of sodium ions in the electrolyte solution, thereby improving charge / discharge efficiency and power density.

[0035] In addition to a positive electrode, a negative electrode, and an electrolyte, a sodium ion battery further includes the necessary components of a conventional sodium ion battery. For example, a sodium ion battery further includes a separator and a steel case.

[0036] The present application further provides a method for preparing the above-mentioned sodium-ion battery, which includes the steps of: preparing a positive electrode slurry using a positive electrode active material, applying the positive electrode slurry to a first current collector to obtain a positive electrode; preparing a negative electrode slurry using a negative electrode active material, applying the negative electrode slurry to a second current collector to obtain a negative electrode; and assembling the positive electrode, the negative electrode, and an electrolyte to obtain a sodium-ion battery.

[0037] The sodium ion battery prepared by this preparation method has excellent electrochemical performance even in a low temperature environment, and the preparation method is simple to operate, has low production costs, and is suitable for mass production.

[0038] In an embodiment of the present application, preparing a positive electrode slurry using a positive electrode active material includes mixing the positive electrode active material, a first binder, a first conductive agent, and a first solvent to obtain the positive electrode slurry.

[0039] The first binder includes polyvinylidene fluoride (PVDF).

[0040] The first conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black.

[0041] The first solvent includes N-methylpyrrolidone.

[0042] For example, the mass ratio of the positive electrode active material, the first binder, and the first conductive agent is 96.5:2.5:3.5. The first conductive agent contains conductive carbon black and carbon nanotubes, and the mass ratio of the conductive carbon black to the carbon nanotubes is 2:1.5. The solid content of the positive electrode slurry is 50% to 60%.

[0043] In an embodiment of the present application, the first current collector comprises an aluminum foil, and the aluminum foil comprises at least one of a carbon-coated aluminum foil, a corona-treated aluminum foil, and a general aluminum foil.

[0044] Furthermore, after the positive electrode slurry is applied to the first current collector, the preparation method further includes drying, rolling, and cutting to obtain a positive electrode.

[0045] In the embodiment of the present application, preparing the negative electrode slurry using the negative electrode active material includes mixing the negative electrode active material, the second binder, the second conductive agent, and the second solvent to obtain the negative electrode slurry.

[0046] The second binder includes at least one of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC).

[0047] The second conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black.

[0048] Furthermore, the second solvent includes water.

[0049] For example, the mass ratio of the negative electrode active material, the second binder, the second conductive agent, and the second solvent is 94.5:5.755:4.55:110. The second binder includes styrene-butadiene rubber and carboxymethyl cellulose, and the mass ratio of the styrene-butadiene rubber to the carboxymethyl cellulose is 4.2:1.555. The second conductive agent includes conductive carbon black and carbon nanotubes, and the mass ratio of the conductive carbon black to the carbon nanotubes is 1.55:3.

[0050] Furthermore, the solid content of the negative electrode slurry is 40% to 45%.

[0051] In an embodiment of the present application, the second current collector comprises aluminum foil.

[0052] Furthermore, after the negative electrode slurry is applied to the second current collector, the preparation method further includes drying, rolling, and cutting to obtain a negative electrode.

[0053] And / or, a positive electrode tab is welded to the positive electrode, and a negative electrode tab is welded to the negative electrode. The material of the positive electrode tab and the material of the negative electrode tab each independently include aluminum, nickel, or an aluminum-nickel clad material. The positive electrode tab is connected to the positive electrode, and the negative electrode tab is connected to the negative electrode, thereby ensuring a smooth flow of electrical energy during charging and discharging of the battery.

[0054] The present application further provides an electrical device, which comprises the sodium ion battery described above or a sodium ion battery prepared by the preparation method described above.

[0055] Electrical equipment according to this application includes consumer electronics, electric transportation, portable power supplies and outdoor equipment, medical equipment, industrial equipment, etc. For example, consumer electronics includes smartphones, laptops, tablets, wearable devices, etc. Electric transportation includes electric bicycles, electric cars, electric boats, and drones. Portable power supplies and outdoor equipment includes portable power supplies, camping lanterns, outdoor gear, etc. Medical equipment includes portable medical equipment, emergency medical equipment, etc. Industrial equipment includes industrial robots, remote monitoring systems, automatic guided vehicles, etc.

[0056] The present application will be described in detail below with specific examples. Those skilled in the art will appreciate that the following examples are merely illustrative of the present application and are not intended to limit the scope of the present application. In the examples, specific conditions are not specified, and the experiments can be carried out under conventional conditions or under conditions recommended by the manufacturer. For reagents or equipment whose manufacturers are not specified, conventional commercially available products can be used.

[0057] Example 1 Below, we will use a cylindrical battery (diameter ≦ 18 mm, height ≦ 65 mm) with the model number 18650 as an example.

[0058] The positive electrode active material according to Example 1 contained sodium iron phosphate and Na4Fe3(PO4)2P2O7, with the mass of Na4Fe3(PO4)2P2O7 accounting for 50% of the mass of the positive electrode active material.

[0059] Preparation of positive electrode plate: The positive electrode active material, binder PVDF, conductive carbon black, and carbon nanotubes were dispersed in N-methylpyrrolidone in a mass ratio of 96.5:2.5:2:1.5 and uniformly mixed to obtain a positive electrode slurry. The positive electrode slurry had a solids content of 50% to 60% and a viscosity of 2000 to 3000 cp. The slurry to be applied was uniformly applied to the positive electrode current collector, i.e., aluminum foil, using an applicator. After drying, the electrode plate was pressed using a rolling machine and cut into four pieces using a slitter to obtain a positive electrode plate.

[0060] An electron microscope image of the positive electrode plate is shown in Figure 1. As shown in Figure 1, the electron microscope image shows that the granules are uniformly dispersed and there is no granule aggregation. This means that when the two materials, sodium iron phosphate and Na4Fe3(PO4)2P2O7, are mixed, they are uniformly dispersed, and the two materials have relatively compatible chemical and physical properties, forming a stable mixed system. The uniformly dispersed positive electrode material contributes to improving the electrochemical performance of the electrode plate.

[0061] Preparation of Negative Electrode Plate: To prepare the negative electrode plate, natural graphite (active material), SBR (binder), conductive carbon black, carbon nanotubes, CMC (binder), and water were used. The mass ratio of natural graphite, SBR, conductive carbon black, carbon nanotubes, CMC, and deionized water was 94.5:4.2:1.55:3:1.555:110. First, a colloidal solution was prepared by mixing the CMC, water, and carbon nanotubes (Step 1). Next, the negative electrode active material and SP conductive carbon black were uniformly mixed by dry mixing (Step 2). The colloidal solution was then added to the mixed dry powder in three batches (Step 2). Deionized water was then added to adjust the viscosity of the slurry. The mixture was homogenized overnight, and the next day, SBR was added to prevent demulsification of the SBR due to premature addition, turning the slurry into a jelly. This prepared negative electrode slurry. The solid content of the negative electrode slurry is 40% to 45%, and the viscosity is 2000 to 2500 cp. The slurry to be applied is uniformly applied to the negative electrode current collector, i.e., aluminum foil, using an application device, and after drying the electrode plate, it is pressed using a rolling machine and one plate is cut into four pieces using a slitter to obtain the negative electrode plate.

[0062] Preparation of a sodium-ion battery capable of withstanding temperatures of -40°C: After welding tabs to the prepared positive and negative plates, the positive and negative plates and a separator were wound together to obtain an electrode unit, which was then packaged in an 18650 steel case. After spot welding, charging, precharging, aging, and capacity grading, a sodium-ion battery was obtained. In Example 1, the electrolyte used in the process of preparing the sodium-ion battery was an ether-based electrolyte, which included an electrolyte and a solvent. The electrolyte was NaPF6, and the solvent was a mixture of DME, DG, and TG, with a mass ratio of DME, DG, and TG of 4:2:1. The electrolyte concentration in the ether-based electrolyte was 1.4 mol / L.

[0063] Example 2 A sodium ion battery was prepared by referring to the method of Example 1, but the only difference from Example 1 was that the mass of Na4Fe3(PO4)2P2O7 in the positive electrode active material of Example 2 accounted for 60% of the mass of the positive electrode active material, and the other steps were the same as those of Example 1.

[0064] Example 3 A sodium ion battery was prepared by referring to the method of Example 1, but the only difference from Example 1 was that the mass of Na4Fe3(PO4)2P2O7 in the positive electrode active material of Example 3 accounted for 40% of the mass of the positive electrode active material, and the other steps were the same as those of Example 1.

[0065] Example 4 A sodium ion battery was prepared with reference to the method of Example 1, except that in Example 4, the positive electrode active material was sodium iron pyrophosphate and Na4Fe3(PO4)2P2O7, and the mass of Na4Fe3(PO4)2P2O7 accounted for 50% of the mass of the positive electrode active material, and the other steps were the same as those of Example 1.

[0066] Example 5 A sodium ion battery was prepared by referring to the method of Example 4. However, Example 5 differed from Example 4 only in that the mass of NaFe(PO)P0 accounted for 60% of the mass of the positive electrode active material, and the other steps were the same as those of Example 4.

[0067] Example 6 A sodium ion battery was prepared by referring to the method of Example 4. However, Example 6 differed from Example 4 only in that the mass of NaFe(PO)P0 accounted for 40% of the mass of the positive electrode active material, and the other steps were the same as those of Example 4.

[0068] Comparative Example 1 A sodium ion battery was prepared by referring to the method of Example 1. Comparative Example 1 differed from Example 1 only in that the mass of NaFe(PO)P0 accounted for 20% of the mass of the positive electrode active material, and the other steps were the same as those of Example 1.

[0069] Comparative Example 2 A sodium ion battery was prepared by referring to the method of Example 1, but Comparative Example 2 differed from Example 1 only in that hard carbon was used as the negative electrode active material of the battery, and the other steps were the same as those of Example 1.

[0070] Comparative Example 3 A sodium ion battery was prepared with reference to the method of Example 1, but in Comparative Example 3, an ester-based electrolyte solution was used as the electrolyte solution of Comparative Example 3, the electrolyte was NaPF6, the solvent of the electrolyte solution was a mixture of DMC, EMC, PC and EC, the mass ratio of DMC, EMC, PC and EC was 1:1:1:2, and the concentration of the electrolyte solution was 1.4 mol / L, which was different from Example 1 only in that the other steps were the same as Example 1.

[0071] Comparative Example 4 A sodium ion battery was prepared with reference to the method according to Example 1, but in Comparative Example 4, the positive electrode active material was sodium copper nickel iron manganate (Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2), and the other steps were the same as in Example 1.

[0072] Performance Test The sodium ion batteries according to Examples 1 to 6 and Comparative Examples 1 to 4 were subjected to performance tests, and the test methods were as follows.

[0073] The sodium-ion batteries according to Examples 1 to 6 and Comparative Examples 1 to 4 were placed in environments at -10°C, -20°C, and -40°C, and a charge-discharge test was conducted at a low rate of current. First, the batteries were placed in freezers at -10°C, -20°C, and -40°C for 3 hours, respectively, and then the test was conducted in the following steps: (1) Charging to 3.7 V at a constant voltage of 0.2 C and stopping at 0.01 C; (2) Allowing to stand for 10 minutes; (3) Discharging to 1.5 V at 0.2 C; (4) Allowing to stand for 10 minutes. Steps (1) to (4) were repeated 250 times, and the capacity retention rate was recorded. The design capacity of the sodium-ion batteries was 1 Ah. The low-rate charge-discharge test method consisted of charging at 0.2 C and discharging at 0.2 C within a voltage range of 1.5 to 3.7 V at -40°C for 250 or more cycles. A capacity retention rate of 60% was considered to pass the test. In each experimental group, 16 batteries were used for parallel experiments, and the experimental batteries were relatively consistent. The pass rate is the ratio of the number of batteries that passed the test to the total number of batteries that participated in the test. The pass rates of Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 1.

[0074] [Table 1]

[0075] In the -40°C cycle test, the pass rates of Examples 1 to 6 were higher than those of Comparative Examples 1 to 4. Therefore, when the positive electrode active material contains a polyanionic material and Na4Fe3(PO4)2P2O7, and the mass of Na4Fe3(PO4)2P2O7 accounts for 40% to 60% of the mass of the positive electrode active material, the negative electrode active material is natural graphite, and the electrolyte is an ether-based electrolyte, sodium ions have a relatively long service life in a low-temperature environment of -40°C. The pass rates of Examples 1 and 4 were also higher than those of Examples 2, 3, 5, and 6. Therefore, it is preferable that the mass of Na4Fe3(PO4)2P2O7 accounts for 50% of the mass of the positive electrode active material, and in this case, the low-temperature cycle performance of the sodium ion battery is better.

[0076] Table 1 shows the test pass rate of 16 sets of parallel experiments, and Table 2 shows the test data of one set of the experiments.

[0077] [Table 2]

[0078] As can be seen from Table 2, Examples 1 to 6 maintained a capacity retention rate of 60% or more after 250 cycles under conditions of -40°C, while Comparative Examples 1 to 4 all had a capacity retention rate of 0 after 250 cycles in a -40°C environment. In other words, the low-temperature performance of the sodium-ion batteries of Examples 1 to 6 was superior to that of Comparative Examples 1 to 4. The positive electrode active material according to the present application uses a composite of a polyanionic material and Na4Fe3(PO4)2P2O7, with the mass of Na4Fe3(PO4)2P2O7 accounting for 40% to 60% of the mass of the positive electrode active material, natural graphite is used as the negative electrode active material, and an ether-based electrolyte is used, so that the prepared sodium-ion battery has excellent low-temperature resistance.

[0079] In addition, the batteries according to Example 1 and Comparative Example 1 were disassembled after five cycles at low temperature. After disassembly, no sodium deposition was observed on the surface of the negative electrode plate of Example 1, confirming that the insertion and desorption of sodium ions was normal during the initial cycles at low temperature for the battery according to Example 1. The negative electrode plate of Comparative Example 1 had dense white dots, indicating severe sodium deposition, which would affect the battery's subsequent cycles. The negative electrodes of Examples 2 to 6 after five cycles at low temperature were similar to Example 1, and the negative electrodes of Comparative Examples 2 and 3 after five cycles at low temperature were similar to Comparative Example 1. In other words, the batteries according to the Examples had better electrochemical and cycle performance than the comparative examples.

[0080] The above embodiments are merely for illustrating the technical solutions of the present application and are not intended to limit the same. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art may modify the technical solutions described in the above embodiments and make equivalent substitutions for some or all of the technical features therein. These modifications or substitutions do not deviate from the essence of the corresponding technical solutions and the scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode active material, The positive electrode active material is a polyanionic material and Na 4 Fe 3 (P.O. 4 ) 2 P 2 O 7 and 4 Fe 3 (P.O. 4 ) 2 P 2 O 7 The mass of the positive electrode active material is 40% to 60%. A positive electrode active material characterized by:

2. The polyanionic material includes at least one of sodium iron phosphate and sodium iron pyrophosphate. The positive electrode active material according to claim 1 .

3. a positive electrode, a negative electrode, and an electrolyte solution; The positive electrode comprises the positive electrode active material according to claim 1 or 2, the negative electrode includes a negative electrode active material, the negative electrode active material includes natural graphite, The electrolyte solution contains an ether-based electrolyte solution. A sodium-ion battery characterized by:

4. The ether-based electrolyte solution contains an electrolyte and a solvent, and the electrolyte is NaPF 6 wherein the solvent comprises DME, DG, and TG; and / or, in the ether-based electrolyte solution, the concentration of the electrolyte is 1 to 1.8 mol / L.

4. The sodium ion battery according to claim 3.

5. 4. A method for preparing the sodium ion battery of claim 3, comprising: preparing a positive electrode slurry using a positive electrode active material and applying the positive electrode slurry to a first current collector to obtain a positive electrode; preparing a negative electrode slurry using the negative electrode active material, and applying the negative electrode slurry to a second current collector to obtain a negative electrode; and assembling the positive electrode, the negative electrode, and an electrolyte to obtain the sodium ion battery. A method for preparing a sodium ion battery, comprising:

6. preparing the positive electrode slurry using the positive electrode active material includes mixing the positive electrode active material, a first binder, a first conductive agent, and a first solvent to obtain the positive electrode slurry; the first binder comprises polyvinylidene fluoride; the first conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black; The first solvent includes N-methylpyrrolidone.

6. The method of claim 5.

7. The first current collector includes aluminum foil.

7. The method of claim 6.

8. preparing the negative electrode slurry using the negative electrode active material includes mixing the negative electrode active material, a second binder, a second conductive agent, and a second solvent to obtain the negative electrode slurry; the second binder contains at least one of styrene butadiene rubber and carboxymethyl cellulose; The second conductive agent includes at least one of carbon nanotubes, conductive carbon black, conductive graphite, graphene, and acetylene black.

6. The method of claim 5.

9. the second current collector includes aluminum foil, and / or a positive electrode tab is welded to the positive electrode, a negative electrode tab is welded to the negative electrode, and the material of the positive electrode tab and the material of the negative electrode tab each independently include aluminum, nickel, or an aluminum-nickel clad material.

9. The method of claim 8.

10. An electrical device, The electrical device comprises the sodium ion battery according to claim 3. An electrical device characterized by:

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