Low negative pressure sodium battery and preparation method thereof

Abstract

The application relates to the technical field of electrochemical energy storage, and discloses a low-negative-pressure sodium battery which comprises a negative electrode material, a positive electrode material, an electrolyte and a diaphragm, wherein the negative electrode material is hard carbon particles which are pre-lithiated and have a particle size of 5-10 mu m and are coated with a polyvinylidene fluoride-hexafluoropropylene functional layer; and a preparation method of the low-negative-pressure sodium battery is also disclosed, and the method comprises the following steps: S1, negative electrode material preparation: hard carbon particles and lithium metal sheets are mixed at a mass ratio of 1:0.4-0.6, and are heat-treated at 400-450 DEG C for 2-3 h. By adopting the pre-lithiated hard carbon negative electrode material, the stability of the battery under high-rate and long-cycle conditions is remarkably improved; by adopting the core-shell structure positive electrode material design, the structural stability and capacity retention rate of the battery in a high-temperature environment are enhanced; and by optimizing the electrolyte formula, the ion conductivity and decomposition resistance of the battery are improved.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, specifically to a low negative pressure sodium battery and its preparation method. Background Technology

[0002] Sodium-ion batteries have become a research hotspot in recent years due to their advantages such as low cost and abundant resources, especially in large-scale energy storage and low-cost applications, where they show greater potential than lithium-ion batteries. However, despite the theoretically promising application prospects of sodium-ion batteries, some technical bottlenecks in existing technologies still restrict their performance and application expansion, particularly in areas such as high temperature, high-rate discharge, and long-term cycle stability.

[0003] First, the selection of anode materials for sodium-ion batteries has always been one of the bottlenecks restricting their performance improvement. Currently, hard carbon is considered the most promising anode material due to its high specific capacity, good conductivity, and good cycle performance. However, a major problem with hard carbon materials in practical applications is that they are prone to significant capacity decay at high-rate discharge. This is because the structure of hard carbon is relatively loose, and the insertion and extraction of sodium ions within it can easily lead to structural collapse, thus affecting the battery's cycle life and capacity retention. In addition, the pore structure and surface properties of hard carbon are also key factors affecting its electrochemical performance. Many existing technologies use simple surface coatings or additives to improve the performance of hard carbon, but these methods have failed to fundamentally solve its poor rate performance and stability problems under high loads.

[0004] Secondly, the cathode material for sodium-ion batteries is also a key research focus. Layered oxide materials, as a major choice for cathode materials, have attracted widespread attention due to their high theoretical capacity and good conductivity. However, layered oxide materials are prone to phase transitions during long-term use, leading to a gradual decline in their capacity and affecting the overall performance of the battery. This phase transition phenomenon is particularly pronounced at high temperatures, resulting in material structural instability and a significant decrease in the battery's energy density and safety. To address this issue, some researchers have attempted to improve the high-temperature stability of layered oxide materials by doping elements or constructing composite materials. However, these methods often fail to effectively solve the phase transition problem, and some materials are costly and have complex preparation processes, making large-scale application difficult.

[0005] Electrolyte stability is also a key factor affecting the performance of sodium-ion batteries. Most sodium-ion batteries use sodium hexafluorophosphate (NaPF6) as the sodium salt and a carbonate solvent system. Although sodium hexafluorophosphate has good electrochemical stability, its electrolyte performance often deteriorates significantly under high voltage and high temperature conditions. Especially under high temperature conditions, electrolyte decomposition and sodium salt deposition can lead to increased internal resistance, electrolyte failure, or even short circuits, severely affecting battery safety and lifespan. Moreover, in existing sodium-ion battery electrolyte systems, the types and amounts of additives used have not yet achieved optimal stability and power characteristics, limiting the battery's high-rate performance and long cycle life.

[0006] In addition, existing sodium-ion batteries exhibit poor stability at high temperatures, which is particularly prominent in many applications. At high temperatures, irreversible reactions may occur in battery materials and electrolytes, leading to increased internal pressure, capacity degradation, and even safety accidents. Because temperature changes accelerate electrolyte degradation and structural damage to electrode materials, traditional sodium-ion batteries often cannot operate for extended periods at high temperatures, limiting their use in high-temperature applications.

[0007] In summary, while existing sodium-ion battery technology has shown significant potential in some areas, it still suffers from considerable technical shortcomings due to issues such as the rate performance of anode materials, the high-temperature stability of cathode materials, and the stability of electrolytes. Optimizing the battery's material system and improving its stability under high-temperature and high-rate conditions remains a major challenge in current sodium-ion battery research. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a low negative pressure sodium battery and its preparation method, which solves the problems of battery capacity degradation, poor rate performance, and instability at high temperatures in existing technologies.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a low negative pressure sodium battery, comprising a negative electrode material, a positive electrode material, an electrolyte, and a separator, wherein:

[0010] The negative electrode material is a pre-lithiated hard carbon particle with a particle size of 5-10 μm and a polyvinylidene fluoride-hexafluoropropylene functional layer coated on the surface.

[0011] The cathode material has a core-shell structure, including:

[0012] The core layer is composed of layered oxide material NaNi x Mn 1-x Composed of O2, where x is 0.7-0.8;

[0013] The shell is composed of sodium phosphate, and the shell mass accounts for 5-15% of the total mass of the positive electrode;

[0014] The electrolyte consists of sodium hexafluorophosphate, ethylene carbonate, and dimethyl carbonate in a volume ratio of 1:1, with a sodium salt concentration of 0.8-1.2M. The additives include fluoroethylene carbonate at a ratio of 2-5%, and N-methyl-N-propylpyrrolidone difluorosulfonylimide at a ratio of 15-25%.

[0015] The diaphragm is a porous diaphragm with a porosity of 40-50% and a thickness of 16-25 μm.

[0016] Preferably, the porosity of the hard carbon particles in the negative electrode material is 25-40%.

[0017] Preferably, the core layer grain size of the cathode material is 1-5 μm.

[0018] A method for preparing a low negative pressure sodium battery includes the following steps:

[0019] S1. Preparation of negative electrode material: Hard carbon particles and lithium metal sheets are mixed at a mass ratio of 1:0.4-0.6, heat-treated at 400-450℃ for 2-3 hours, cooled, coated with 3-6% polyvinylidene fluoride-hexafluoropropylene solution, dried, and then heat-treated at 70-100℃; S2. Preparation of positive electrode material:

[0020] Sodium carbonate, nickel oxide and manganese oxide are weighed in a molar ratio of 1:0.7-0.8:0.2-0.3, ball-milled for 10-12 hours, pre-fired at 500-550℃ for 5-7 hours, and then sintered at 850-900℃ for 10-12 hours to form the core layer.

[0021] The core material is coated with sodium phosphate solution, dried, and then sintered at 500-550℃ for 3-5 hours to form a shell.

[0022] S3. Electrolyte preparation: Dissolve 0.8-1.2M sodium hexafluorophosphate in EC:DMC mixed solvent with a volume ratio of 1:1, add 2-5% fluoroethylene carbonate and 15-25% N-methyl-N-propylpyrrolidone difluorosulfonylimide, and stir until homogeneous;

[0023] S4. Battery assembly: The positive electrode, separator and negative electrode are stacked in sequence, electrolyte is injected and then sealed, and 3-5 pre-cycles are performed at a rate of 0.05C for optimal selection. The heat treatment atmosphere in the preparation of negative electrode material in step S1 is high-purity argon gas, and the argon gas flow rate is 50-100 ml / min.

[0024] Preferably, the cathode material preparation in step S2 is carried out by an impregnation coating method.

[0025] Preferably, in step S3, the stirring speed during electrolyte preparation is 300-600 rpm, the stirring time is 2-4 h, and the water content in the solution is controlled below 10 ppm.

[0026] This invention provides a low-negative-pressure sodium battery and its preparation method. It has the following beneficial effects:

[0027] 1. This invention adopts a technical solution of pre-lithiation treatment of hard carbon particles, which enables the negative electrode material to have higher conductivity and better sodium ion intercalation capability, significantly improving the cycle stability of the battery. Compared with the traditional solution of directly using hard carbon materials in the prior art, the pre-lithiation treatment can reduce the adsorption of lithium ions on the surface of hard carbon particles, avoid the problems of capacity decay and battery performance degradation during the cycle, and significantly improve the service life and stability of low negative pressure sodium batteries.

[0028] 2. By introducing a core-shell structure into the cathode material, this invention optimizes the conduction and stability of sodium ions in the cathode material. The core layer adopts a layered oxide material, and the shell layer is a sodium phosphate coating. Compared with the simple synthesis method of traditional cathode materials, this invention can effectively avoid electrolyte decomposition and thermal runaway problems that occur when the battery is overcharged. This design significantly improves the safety of the battery under extreme conditions and improves the performance degradation problem of the battery after long-term use.

[0029] 3. This invention employs a specific electrolyte formulation that combines sodium hexafluorophosphate with a mixed solvent of ethylene carbonate and dimethyl carbonate, and adds fluoroethylene carbonate and N-methyl-N-propylpyrrolidone difluorosulfonylimide. This innovative formulation significantly improves the battery's performance in low-temperature environments, especially in terms of charge-discharge rate and cycle stability. Compared with traditional electrolyte formulations, the new electrolyte can maintain higher ion conductivity at low temperatures, greatly improving the battery's operating efficiency in cold environments. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Please see the appendix Figure 1 This invention provides a low negative pressure sodium battery, comprising a negative electrode material, a positive electrode material, an electrolyte, and a separator, wherein:

[0033] The negative electrode material is a pre-lithiated hard carbon particle with a particle size of 5-10 μm and a polyvinylidene fluoride-hexafluoropropylene functional layer coated on the surface.

[0034] The cathode material has a core-shell structure, including:

[0035] The core layer is composed of layered oxide material NaNi x Mn 1-x Composed of O2, where x is 0.7-0.8;

[0036] The shell is composed of sodium phosphate, and the shell mass accounts for 5-15% of the total mass of the positive electrode;

[0037] The electrolyte consists of sodium hexafluorophosphate, ethylene carbonate, and dimethyl carbonate in a volume ratio of 1:1, with a sodium salt concentration of 0.8-1.2M. The additives include fluoroethylene carbonate at a ratio of 2-5%, and N-methyl-N-propylpyrrolidone difluorosulfonylimide at a ratio of 15-25%.

[0038] The diaphragm is a porous diaphragm with a porosity of 40-50% and a thickness of 16-25 μm.

[0039] The porosity of hard carbon particles in the negative electrode material is 25-40%.

[0040] The core layer grain size of the cathode material is 1-5 μm.

[0041] A method for preparing a low negative pressure sodium battery, characterized by comprising the following steps:

[0042] S1. Preparation of negative electrode material: Hard carbon particles and lithium metal sheets are mixed at a mass ratio of 1:0.4-0.6, heat-treated at 400-450℃ for 2-3 hours, cooled, coated with 3-6% polyvinylidene fluoride-hexafluoropropylene solution, dried, and then heat-treated at 70-100℃; S2. Preparation of positive electrode material:

[0043] Sodium carbonate, nickel oxide and manganese oxide are weighed in a molar ratio of 1:0.7-0.8:0.2-0.3, ball-milled for 10-12 hours, pre-fired at 500-550℃ for 5-7 hours, and then sintered at 850-900℃ for 10-12 hours to form the core layer.

[0044] The core material is coated with sodium phosphate solution, dried, and then sintered at 500-550℃ for 3-5 hours to form a shell.

[0045] S3. Electrolyte preparation: Dissolve 0.8-1.2M sodium hexafluorophosphate in EC:DMC mixed solvent with a volume ratio of 1:1, add 2-5% fluoroethylene carbonate and 15-25% N-methyl-N-propylpyrrolidone difluorosulfonylimide, and stir until homogeneous;

[0046] S4. Battery assembly: Stack the positive electrode, separator and negative electrode in sequence, inject electrolyte and seal, and perform 3-5 pre-cycles at 0.05C rate.

[0047] The heat treatment atmosphere in the preparation of the negative electrode material in step S1 is high-purity argon, and the argon flow rate is 50-100 ml / min.

[0048] The S2 step involves an impregnation coating method in the preparation of the cathode material.

[0049] In step S3, the stirring speed during electrolyte preparation is 300-600 rpm, the stirring time is 2-4 hours, and the water content of the solution is controlled below 10 ppm.

[0050] Example 1: Preparation of a high-efficiency, low-negative-pressure sodium battery

[0051] Step 1: Preparation of negative electrode material

[0052] Hard carbon particles and lithium metal sheets were uniformly mixed at a mass ratio of 1:0.5. The mixture was placed in a high-purity argon atmosphere with a flow rate controlled at 80 mL / min and heat-treated at 400 °C for 2 h. After heat treatment, it was cooled to room temperature. Then, a 3.5% polyvinylidene fluoride-hexafluoropropylene solution was coated evenly and dried at 70 °C, and finally heat-treated at 100 °C for 1 h.

[0053] Step 2: Preparation of cathode material

[0054] Sodium carbonate, nickel oxide, and manganese oxide were weighed and mixed in a molar ratio of 1:0.75:0.25, and ball-milled for 12 hours. The ball-milled powder was pre-calcined at 500℃ for 6 hours, and then sintered at 850℃ for 11 hours to obtain the core layer. The surface of the core material was coated with sodium phosphate solution, and then sintered at 500℃ for 4 hours to form the shell layer. The cathode material finally obtained a layered structure, with the core layer grain size being approximately 3 μm and the shell layer accounting for 8% of the total mass.

[0055] Step 3: Preparation of electrolyte

[0056] Dissolve 1M sodium hexafluorophosphate in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1, and stir until homogeneous. Then, add 3% fluoroethylene carbonate and 20% N-methyl-N-propylpyrrolidone difluorosulfonylimide, and stir for 4 hours to ensure complete homogeneity of the solution. During stirring, maintain the water content in the solution below 10 ppm.

[0057] Step 4: Battery Assembly

[0058] The negative electrode, positive electrode, and porous separator were stacked sequentially, with the separator porosity maintained at 45% and a thickness of approximately 20 μm. The prepared electrolyte was added, and the battery was sealed and subjected to three pre-cycles at a 0.05C rate. After pre-cycles, the battery exhibited good charge-discharge performance and performed well at low temperatures.

[0059] Example 2: Low negative pressure sodium battery with optimized negative electrode material and electrolyte formulation

[0060] Step 1: Preparation of negative electrode material

[0061] Hard carbon particles and lithium metal sheets were mixed at a mass ratio of 1:0.45. After uniform mixing, the mixture was heated to 420°C under an argon atmosphere and held for 2.5 hours. After cooling to room temperature, a 4% polyvinylidene fluoride-hexafluoropropylene solution was applied and coated evenly. The mixture was then placed in an oven and dried at 80°C for 3 hours, followed by heat treatment at 90°C for 1 hour.

[0062] Step 2: Preparation of cathode material

[0063] Sodium carbonate, nickel oxide, and manganese oxide were weighed and mixed in a molar ratio of 1:0.8:0.2. After ball milling for 10 hours, the mixture was pre-calcined at 510℃ for 6 hours, and then sintered at 870℃ for 11 hours to form the core layer. The core material was then immersed in a sodium phosphate solution, coated, and sintered at 530℃ for 4 hours to form the shell layer. The grain size of this cathode material's layered structure was controlled within 2 μm, and the shell layer accounted for 10% of the total cathode mass.

[0064] Step 3: Preparation of electrolyte

[0065] Dissolve 0.9 M sodium hexafluorophosphate in an equal volume of a mixed solvent of ethylene carbonate and dimethyl carbonate. Add 4% fluoroethylene carbonate and 18% N-methyl-N-propylpyrrolidone difluorosulfonylimide, and stir until homogeneous for 3 hours. Maintain the water content in the solution below 15 ppm.

[0066] Step 4: Battery Assembly

[0067] The electrodes are stacked in the order of negative electrode, separator, and positive electrode, with the separator porosity maintained at 42% and a thickness of 18 μm. After injecting the above electrolyte, the battery is sealed and subjected to four pre-cycles, each charged and discharged at a rate of 0.05C to ensure the battery's capacity stability.

[0068] Example 3: Low-negative-pressure sodium batteries on a fully automated production line

[0069] Step 1: Preparation of negative electrode material

[0070] Hard carbon particles were mixed with lithium metal sheets at a mass ratio of 1:0.55 and heated at 430°C for 2.5 h in a high-purity argon atmosphere with an argon flow rate of 70 mL / min. After cooling to room temperature, the mixture was coated with a 5% polyvinylidene fluoride-hexafluoropropylene solution and dried at 80°C for 3 h. Finally, it was heat-treated at 95°C for 1 h to ensure the stability of the negative electrode material.

[0071] Step 2: Preparation of cathode material

[0072] Sodium carbonate, nickel oxide, and manganese oxide were weighed and mixed in a molar ratio of 1:0.7:0.3. After ball milling for 12 hours, the mixture was pre-calcined at 520℃ for 7 hours and then sintered at 880℃ for 12 hours to form the core layer. A sodium phosphate solution was coated onto the surface of the core material, dried, and then sintered at 530℃ for 5 hours to form the shell layer. The final layered structure of the cathode material had a grain size of 4 μm, and the shell layer accounted for 6% of the total mass.

[0073] Step 3: Preparation of electrolyte

[0074] 1.0 M sodium hexafluorophosphate was dissolved in an equal volume of a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC). Then, 3% fluoroethylene carbonate and 22% N-methyl-N-propylpyrrolidone difluorosulfonylimide were added, and the mixture was stirred for 4 hours until homogeneous. Finally, the water content in the solution was controlled to be below 12 ppm.

[0075] Step 4: Battery Assembly

[0076] The negative electrode, separator, and positive electrode are stacked sequentially using an automated production line. The separator porosity is controlled at 50%, and the thickness is 20 μm. After electrolyte injection, the battery is immediately sealed and subjected to five pre-cycles at a rate of 0.05C.

[0077] Example 4: Preparation of high-performance sodium batteries under low-temperature conditions

[0078] Step 1: Preparation of negative electrode material

[0079] Hard carbon particles and lithium metal sheets were mixed at a mass ratio of 1:0.5 and heat-treated in an argon atmosphere at 440℃ for 2 hours. After cooling, the surface was coated with a 4% polyvinylidene fluoride-hexafluoropropylene solution, dried, and then heat-treated at 90℃ for 1 hour.

[0080] Step 2: Preparation of cathode material

[0081] Sodium carbonate, nickel oxide, and manganese oxide were mixed in a molar ratio of 1:0.7:0.3, ball-milled for 10 hours, pre-calcined at 500℃ for 7 hours, and then sintered at 850℃ for 12 hours to form the core layer. A sodium phosphate solution was coated on the surface of the core material, dried, and then sintered at 520℃ for 5 hours to form the shell layer.

[0082] Step 3: Preparation of electrolyte

[0083] Dissolve 0.8M sodium hexafluorophosphate in an equal volume of a mixed solvent of ethylene carbonate and dimethyl carbonate, add 3% fluoroethylene carbonate and 15% N-methyl-N-propylpyrrolidone difluorosulfonylimide, stir until homogeneous, and ensure that the water content in the solution is below 10 ppm.

[0084] Step 4: Battery Assembly

[0085] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator porosity controlled at 45% and a thickness of 18 μm. After injecting electrolyte, the mixture is sealed and pre-circulated 5 times at a rate of 0.05C.

[0086] Comparative Example 1: The anode material was not pre-lithiated.

[0087] Preparation steps of Comparative Example 1:

[0088] Anode material preparation: Hard carbon particles and lithium metal sheets are mixed at a mass ratio of 1:1 and heat-treated at 400℃ for 2 hours. After cooling, the surface is not coated with any functional layer and proceeds directly to the next step.

[0089] Cathode material preparation: Sodium carbonate, nickel oxide and manganese oxide were weighed in a molar ratio of 1:0.75:0.25, ball-milled for 12 hours, pre-calcined at 500℃ for 6 hours, and then sintered at 850℃ for 11 hours to form the core layer; the core material was coated with sodium phosphate solution, dried and then sintered at 550℃ for 4 hours to form the shell layer.

[0090] Electrolyte preparation: Dissolve 1M sodium hexafluorophosphate in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1:1), add 5% fluoroethylene carbonate and 20% N-methyl-N-propylpyrrolidone difluorosulfonylimide, stir for 2 hours, and control the water content in the solution to below 20 ppm.

[0091] Battery assembly: The positive electrode, separator (porosity 40%, thickness 20μm) and negative electrode are stacked in sequence, electrolyte is injected and then sealed, pre-cycled 5 times at a rate of 0.1C.

[0092] Comparative Example 2: Adjustment of the proportion of the core layer of the cathode material

[0093] Preparation steps of Comparative Example 2:

[0094] Anode material preparation: Following step S1 in Example 1, pre-lithiated hard carbon particles were coated with a polyvinylidene fluoride-hexafluoropropylene functional layer.

[0095] Cathode material preparation: Sodium carbonate, nickel oxide and manganese oxide were weighed in a molar ratio of 1:0.5:0.5, ball-milled for 12 hours, pre-calcined at 500℃ for 5 hours, and then sintered at 850℃ for 10 hours to form the core layer; the core material was coated with sodium phosphate solution, dried and then sintered at 550℃ for 5 hours to form the shell layer.

[0096] Electrolyte preparation: Dissolve 1M sodium hexafluorophosphate in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1:1), add 3% fluoroethylene carbonate and 18% N-methyl-N-propylpyrrolidone difluorosulfonylimide, stir for 3 hours, and control the water content in the solution to be below 15 ppm.

[0097] Battery assembly: The positive electrode, separator (porosity 45%, thickness 22μm) and negative electrode are stacked in sequence, electrolyte is injected and then sealed, pre-cycled 5 times at a rate of 0.05C.

[0098] Comparative Example 3: Changes in the proportion of additives in the electrolyte

[0099] Preparation steps of Comparative Example 3:

[0100] Anode material preparation: Unlithiated hard carbon particles were coated with a polyvinylidene fluoride-hexafluoropropylene functional layer. The hard carbon particles had a particle size of 8 μm and a porosity of 35%.

[0101] Cathode material preparation: Sodium carbonate, nickel oxide and manganese oxide were weighed in a molar ratio of 1:0.7:0.3, ball-milled for 10 hours, pre-calcined at 500℃ for 6 hours, and then sintered at 850℃ for 11 hours to form the core layer; the core material was coated with sodium phosphate solution, dried and then sintered at 530℃ for 3 hours to form the shell layer.

[0102] Electrolyte preparation: Dissolve 1.0M sodium hexafluorophosphate in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1:1), add 1% fluoroethylene carbonate and 10% N-methyl-N-propylpyrrolidone difluorosulfonylimide, stir for 2 hours, and control the water content in the solution to be below 25 ppm.

[0103] Battery assembly: The positive electrode, separator (porosity 42%, thickness 18μm) and negative electrode are stacked in sequence, electrolyte is injected and then sealed, pre-cycled 3 times at a rate of 0.1C.

[0104] Comparative Example 4: Adjustment of Pre-Cycle Count During Battery Assembly

[0105] Preparation steps of Comparative Example 4:

[0106] Anode material preparation: Hard carbon particles that have undergone pre-lithiation treatment are used, and the surface is coated with a polyvinylidene fluoride-hexafluoropropylene functional layer. The hard carbon particles have a particle size of 7 μm and a porosity of 30%.

[0107] Cathode material preparation: Sodium carbonate, nickel oxide and manganese oxide were weighed in a molar ratio of 1:0.7:0.2, ball-milled for 11 hours, pre-calcined at 510℃ for 6 hours, and then sintered at 860℃ for 11 hours to form the core layer; the core material was coated with sodium phosphate solution, dried and then sintered at 540℃ for 4 hours to form the shell layer.

[0108] Electrolyte preparation: Dissolve 0.9M sodium hexafluorophosphate in a mixed solvent of EC and DMC (volume ratio 1:1), add 4% fluoroethylene carbonate and 17% N-methyl-N-propylpyrrolidone difluorosulfonylimide, stir for 2 hours, and control the water content in the solution to be below 10 ppm.

[0109] Battery assembly: The positive electrode, separator (porosity 43%, thickness 19μm) and negative electrode are stacked in sequence, electrolyte is injected and then sealed, pre-cycled 7 times at a rate of 0.1C.

[0110] Experiment 1: Charge-discharge performance test under low temperature environment

[0111] Experimental steps:

[0112] Battery fabrication:

[0113] Two sets of sodium batteries were prepared according to Example 1 and Comparative Example 1. The experimental group (Example 1) used a pre-lithiated hard carbon particle negative electrode, a core-shell structure positive electrode, and a specific electrolyte and porous membrane; Comparative Example 1 did not perform pre-lithiation treatment on the negative electrode material and used conventional sodium cobalt oxide as the positive electrode material.

[0114] During battery assembly, the positive electrode, separator, and negative electrode are stacked in sequence to ensure the battery assembly is sealed. Electrolyte is then injected and the battery is encapsulated.

[0115] Environmental condition settings:

[0116] The battery was placed in ambient temperatures of -20℃, 0℃, and 25℃ for charge and discharge tests.

[0117] All batteries were pre-cycled 5 times at -20°C to ensure battery stability.

[0118] Charge-discharge cycles:

[0119] Charge / discharge tests were conducted using a constant current charge / discharge device at different temperatures. The current for each charge / discharge cycle was 0.2C of the battery capacity (i.e., the battery charge / discharge rate was 20% of its rated capacity). The battery was charged to 3.2V and then discharged to 2.0V. The charge / discharge curves and changes in battery capacity were recorded.

[0120] Record the battery voltage, discharge capacity, and internal resistance during charging at different ambient temperatures.

[0121] Data collection and analysis:

[0122] The charge / discharge curves for each battery group record include charging time, maximum voltage, current intensity, and charging efficiency.

[0123] After testing, the internal resistance and discharge capacity of each battery were measured. The final data will be used to compare the performance differences between the example and comparative batteries under different temperature conditions.

[0124] Experimental data:

[0125] Table 1: Charge-discharge performance data under low temperature conditions

[0126]

[0127] In summary, the experimental data clearly demonstrate the excellent performance of Example 1 battery in low-temperature environments, especially at -20°C, where Example 1 exhibits superior discharge capacity and internal resistance compared to Comparative Example 1 battery. It can be inferred that the pre-lithiation treatment of hard carbon particles improves battery performance in low-temperature environments by introducing an additional lithium source into the anode material. This pre-lithiation treatment not only increases the porosity of the anode but also effectively enhances its conductivity, thus providing more stable charge-discharge performance at low temperatures.

[0128] Furthermore, the combination of the layered oxide cathode material and the sodium phosphate shell in Example 1 helps improve the structural stability of the cathode, especially at low temperatures when the electrolyte viscosity increases. This reduces the stability of the cathode material and the overall internal resistance of the battery, further optimizing the battery's cycle performance. This aligns with the mechanism in this invention, fully utilizing the advantages of the pre-lithiated anode material and addressing the problem of rapid capacity decay at low temperatures by optimizing the cathode material.

[0129] In contrast, Comparative Example 1, lacking pre-lithiation treatment, exhibited lower conductivity in the negative electrode and poorer structural stability in the positive electrode material, leading to increased internal resistance and more pronounced capacity decay at low temperatures. In summary, the design of Example 1 not only effectively improves charge-discharge performance at low temperatures but also extends battery life, providing a more efficient and reliable battery solution for applications such as new energy storage and electric vehicles.

[0130] Experiment 2: Cyclic stability and capacity retention test

[0131] Experimental steps:

[0132] Battery fabrication:

[0133] Two sodium batteries were prepared according to Example 2 and Comparative Example 2. Example 2 used a pre-lithiated hard carbon particle anode, a core-shell structure for the cathode, and a mixed solution of sodium hexafluorophosphate and dimethyl carbonate (DMC) with added fluoroethylene carbonate and N-methyl-N-propylpyrrolidone difluorosulfonylimide. Comparative Example 2 used conventional sodium anode material (without pre-lithiation) and a sodium cobalt oxide cathode, with a conventional sodium hexafluorophosphate and dimethyl carbonate (DMC) solution as the electrolyte.

[0134] Ambient temperature setting:

[0135] All batteries were tested at room temperature (25°C) for 300 cycles. During the test, the battery's discharge capacity and internal resistance were measured after every 50 charge-discharge cycles. Each charge cycle was performed to 3.2V and then discharged to 2.0V.

[0136] Charge-discharge cycle process:

[0137] The battery was charged and discharged at a rate of 0.5C, and the discharge capacity and internal resistance were recorded for each cycle. After every 50 cycles, the battery's voltage, capacity, internal resistance, and other data were tested and recorded.

[0138] At the end of each charge and discharge cycle, the battery internal resistance is tested, and the electrochemical impedance spectroscopy of the battery is measured using the AC impedance method.

[0139] Data collection and analysis:

[0140] The initial capacity and capacity decay during cycling of each battery group were statistically analyzed.

[0141] Data from each cycle is recorded and then compared based on the battery's overall cycle stability.

[0142] Experimental data:

[0143] Table 2: Cyclic stability and capacity retention test data

[0144]

[0145]

[0146] In summary, the experimental results show that Example 2 exhibits significantly better capacity retention than Comparative Example 2 after 300 charge-discharge cycles. The battery in Example 2 maintains a high capacity after 300 cycles with a relatively small increase in internal resistance. This is closely related to the technical solution of this invention, especially the pre-lithiation treatment of the negative electrode material. The pre-lithiation process improves the stability and conductivity of the hard carbon particles, resulting in better battery stability after long-term cycling.

[0147] Meanwhile, the core-shell structure design of the cathode material also contributes to the battery's capacity retention during long-cycle charge-discharge processes. The combination of the layered oxide core layer and the sodium phosphate shell reduces structural damage and capacity decay during high-rate charge-discharge. Therefore, the battery's cycle performance is effectively improved, and it can maintain a low internal resistance during long-term use.

[0148] In contrast, the battery in Comparative Example 2, where the negative electrode material was not pre-lithiated, resulted in poor conductivity during cycling, while the positive electrode material lacked structural stability. Consequently, the battery's internal resistance increased and its capacity significantly decreased with increasing usage cycles. This further validates the crucial role of pre-lithiated hard carbon particles and core-shell structured positive electrodes in improving battery cycle performance. This technical solution not only improves battery cycle life but also reduces energy loss, providing a more reliable solution for practical applications.

[0149] Experiment 3: Rate Performance Test

[0150] Experimental steps:

[0151] Battery fabrication:

[0152] Two sodium batteries were prepared according to Example 3 and Comparative Example 3. Example 3 used a pre-lithiated hard carbon particle anode, a core-shell structure as the cathode, and a mixed solution of sodium hexafluorophosphate and ethylene carbonate, with the addition of fluoroethylene carbonate and N-methyl-N-propylpyrrolidone difluorosulfonylimide. Comparative Example 3 used a conventional hard carbon anode, sodium cobalt oxide as the cathode, and a solution of sodium hexafluorophosphate and dimethyl carbonate (DMC) as the electrolyte.

[0153] Ratio testing conditions:

[0154] The battery was tested in a constant temperature environment of 25°C. Different charge / discharge rates were set for testing: 0.1C, 0.5C, 1C, 2C, and 5C. The charging voltage at each rate was 3.2V, and the discharge cut-off voltage was 2.0V. Each charge / discharge cycle was performed at least twice, and the discharge capacity at each rate was recorded.

[0155] Test steps:

[0156] Each battery was first pre-charged and discharged twice at a 0.1C rate to stabilize its performance. Then, charge and discharge tests were performed sequentially at 0.5C, 1C, 2C, and 5C rates. After each charge and discharge cycle, the battery's discharge capacity, internal resistance, and voltage data were recorded.

[0157] After each test, the internal resistance of the battery was measured using AC impedance spectroscopy to analyze the battery's electrochemical performance.

[0158] Data collection:

[0159] Record and analyze the battery's discharge capacity, internal resistance changes, and charging / discharging voltage characteristics at each rate. Test the relationship between capacity decay and internal resistance increase at each rate.

[0160] Experimental data:

[0161] Table 3: Ratio Performance Test Data

[0162]

[0163] In summary, the battery of Example 3 demonstrated significant superiority in rate performance testing, particularly maintaining a relatively high discharge capacity under high-rate charge-discharge conditions, with a minimal increase in internal resistance. This is because the pre-lithiated hard carbon particle anode effectively enhances the battery's conductivity at high rates, enabling efficient charge-discharge cycles in a short time without significant capacity decay. Simultaneously, the core-shell structure of the cathode material also improves stability at high rates; the sodium phosphate shell effectively prevents structural collapse, thereby reducing the increase in internal resistance.

[0164] In contrast, the battery in Comparative Example 3 performed poorly at high rates, especially above 1C, where its discharge capacity decreased significantly and its internal resistance increased substantially. This phenomenon can be attributed to the untreated hard carbon particle anode, which exhibits high charge transfer resistance during high-rate charge-discharge cycles, leading to a decline in battery performance. Simultaneously, the sodium cobalt oxide cathode material suffers from insufficient stability, particularly at high rates, where irreversible changes in the electrode material easily occur, resulting in decreased internal conductivity and consequently poor rate performance.

[0165] In summary, the battery of Example 3 outperforms Comparative Example 3 in rate performance, demonstrating that the combination of the pre-lithiated hard carbon anode and the core-shell structure cathode of this invention significantly enhances high-rate performance. These structural optimizations effectively improve the battery's conductivity and stability, making it particularly suitable for applications requiring high-rate charge and discharge.

[0166] Experiment 4: Cyclic Stability Test

[0167] Experimental steps:

[0168] Battery fabrication:

[0169] Two sodium batteries were prepared according to Example 4 and Comparative Example 4. Example 4 used pre-lithiated hard carbon particles as the negative electrode, a core-shell structure as the positive electrode, and a mixed solution of sodium hexafluorophosphate, ethylene carbonate, fluoroethylene carbonate, and N-methyl-N-propylpyrrolidone difluorosulfonylimide as the electrolyte. Comparative Example 4 used conventional hard carbon negative electrode materials, sodium cobalt oxide as the positive electrode, and a mixed solution of sodium hexafluorophosphate and dimethyl carbonate as the electrolyte.

[0170] Loop test conditions:

[0171] The battery was first subjected to two pre-cycles (0.1C rate, charging cut-off voltage of 3.2V, discharging cut-off voltage of 2.0V). Subsequently, it was set to a 1C rate for charging and discharging, and the discharge capacity was recorded after each cycle. The test cycle consisted of 300 cycles.

[0172] Test steps:

[0173] After initial charge-discharge at 0.1C, each battery was cycled at 1C. The capacity and voltage changes during each discharge and charge / discharge cycle were recorded. After each test, the battery's internal resistance was recorded, and electrochemical impedance spectroscopy (EIS) was used to detect the internal resistance and analyze the performance degradation of the battery during multiple cycles.

[0174] Data collection:

[0175] Record and compare the battery's capacity retention, internal resistance increase, and voltage change after 300 cycles to evaluate the battery's cycle stability and long-term performance.

[0176] Experimental data:

[0177] Table 4: Cyclic Stability Test Data

[0178]

[0179] In summary, the sodium battery of Example 4 maintained a relatively high discharge capacity and a small increase in internal resistance after 300 cycles. This indicates that the pre-lithiated hard carbon particle anode and the core-shell structure cathode material used in this invention can effectively suppress capacity decay and internal resistance increase during long-term cycling. This phenomenon may be related to the pre-lithiation treatment of the hard carbon particles, which increases the stability and conductivity of the anode and reduces structural changes in the electrode material during cycling. Furthermore, the core-shell structure of the cathode effectively protects the layered oxide material, preventing its disintegration during repeated charge-discharge cycles, thereby extending the battery's lifespan.

[0180] In Comparative Example 4, the battery exhibited significant capacity decay and rapid internal resistance increase under the same cycle test conditions. As the number of cycles increased, the battery capacity gradually decreased while the internal resistance also increased significantly, indicating that conventional hard carbon anodes and sodium cobalt oxide cathodes suffer severe performance degradation under high-rate charge-discharge and long-term cycling. Sodium cobalt oxide cathodes, in particular, are prone to structural collapse, leading to increased internal resistance and capacity decay.

[0181] By comparison, the battery in Example 4 exhibits excellent cycle stability, demonstrating that reasonable electrode material design and electrolyte formulation optimization can significantly improve the long-cycle performance of the battery. The battery's capacity retention and internal resistance variation trends showcase the advantages of this invention in cycle performance, making it particularly suitable for energy storage devices requiring long lifespan and stable performance.

[0182] Experiment 5: High Temperature Stability Test

[0183] Experimental steps:

[0184] Battery fabrication:

[0185] Two sodium batteries were prepared according to Example 5 and Comparative Example 5, respectively. Example 5 used pre-lithiated hard carbon particles as the negative electrode and a core-shell structure as the positive electrode. The electrolyte was a mixed solution of sodium hexafluorophosphate, ethylene carbonate, fluoroethylene carbonate, and N-methyl-N-propylpyrrolidone difluorosulfonylimide. Comparative Example 5 used a conventional hard carbon negative electrode and a sodium cobalt oxide positive electrode, with a mixed solution of sodium hexafluorophosphate and dimethyl carbonate as the electrolyte.

[0186] High-temperature environment testing conditions:

[0187] The two sets of batteries were placed in temperature environments of 45℃, 55℃, and 65℃ for charge-discharge tests. Under each temperature environment, the batteries were charged and discharged 100 times at a rate of 0.1C to evaluate the cycle performance and high-temperature stability of the batteries at different temperatures.

[0188] Test steps:

[0189] The battery was subjected to 100 charge-discharge cycles each at 45℃, 55℃, and 65℃. Changes in capacity, internal resistance, and voltage range before and after the tests were observed. Following the high-temperature cycling test, electrochemical impedance spectroscopy (EIS) was used to detect the battery's internal resistance and analyze the impact of high temperatures on battery performance.

[0190] Data collection:

[0191] After each cycle, the discharge capacity was recorded, and the battery capacity retention, internal resistance increase, and voltage range changes during charge and discharge were tested at different temperatures. Finally, the performance differences between the two groups of batteries at different temperatures were compared.

[0192] Experimental data:

[0193] Table 5: High Temperature Stability Test Data

[0194]

[0195] In summary, the battery of Example 5 significantly outperformed Comparative Example 5 in high-temperature environments. Even after 100 cycles at 65°C, the battery of Example 5 maintained a high discharge capacity and a slower increase in internal resistance. This phenomenon can be attributed to the pre-lithiation treatment of hard carbon particles and the core-shell structure of the positive electrode design. The pre-lithiation hard carbon anode provides more stable cycle performance at high temperatures, reduces excessive expansion and structural failure of the electrode material, and particularly improves thermal stability at high temperatures.

[0196] In contrast, the battery in Comparative Example 5 exhibited significant degradation at high temperatures. As temperature increased, the battery's discharge capacity gradually decreased, while its internal resistance increased rapidly. This indicates that conventional hard carbon anodes and sodium cobalt oxide cathodes are more prone to structural changes under high-temperature conditions, leading to capacity degradation and increased internal resistance. This difference reflects the significant advantage of the battery material of this invention in terms of high-temperature stability, demonstrating stronger resistance to thermal failure.

[0197] This invention optimizes the design of electrode materials, particularly demonstrating excellent cycle stability under high-temperature conditions. This indicates that the battery of Example 5 not only exhibits excellent performance at room temperature but also effectively maintains good capacity retention and low internal resistance at high temperatures, greatly expanding its applicability in high-temperature applications. For example, in fields with high temperature adaptability requirements, such as electric vehicles and energy storage systems, the battery of this invention has greater market potential.

[0198] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A sodium battery, characterized in that, It includes negative electrode materials, positive electrode materials, electrolyte, and separator, among which: The negative electrode material is a pre-lithiated hard carbon particle with a particle size of 5-10µm, and the surface is coated with a polyvinylidene fluoride-hexafluoropropylene functional layer. The cathode material has a core-shell structure, including: The core layer is made of layered oxide material The composition, where x is 0.7-0.8; The shell is composed of sodium phosphate, and its mass accounts for 5-15% of the total mass of the positive electrode. The electrolyte comprises sodium hexafluorophosphate, ethylene carbonate, and dimethyl carbonate in a volume ratio of 1:1, with a sodium salt concentration of 0.8-1.2 M. Additives include fluoroethylene carbonate at a ratio of 2-5% and N-methyl-N-propylpyrrolidone difluorosulfonylimide at a ratio of 15-25%. The diaphragm is a porous diaphragm with a porosity of 40-50% and a thickness of 16-25µm.

2. A sodium battery according to claim 1, characterized in that, The porosity of the hard carbon particles in the negative electrode material is 25-40%.

3. A sodium battery according to claim 1, characterized in that, The core layer grain size of the cathode material is 1-5µm.

4. A method for preparing a sodium battery, characterized in that, Includes the following steps: S1. Preparation of negative electrode material: Hard carbon particles and lithium metal sheets are mixed at a mass ratio of 1:0.4-0.6, and heat-treated at 400-450℃ for 2-3 hours. After cooling, the surface is coated with 3-6% polyvinylidene fluoride-hexafluoropropylene solution, dried, and then heat-treated at 70-100℃. S2, Preparation of cathode materials: Sodium carbonate, nickel oxide and manganese oxide are weighed in a molar ratio of 1:0.7-0.8:0.2-0.3, ball-milled for 10-12 hours, pre-fired at 500-550℃ for 5-7 hours, and then sintered at 850-900℃ for 10-12 hours to form the core layer. The core material is coated with sodium phosphate solution, dried, and then sintered at 500-550℃ for 3-5 hours to form a shell. S3. Electrolyte preparation: Dissolve 0.8-1.2M sodium hexafluorophosphate in EC:DMC mixed solvent at a volume ratio of 1:1, add 2-5% fluoroethylene carbonate and 15-25% N-methyl-N-propylpyrrolidone difluorosulfonylimide, and stir until homogeneous; S4. Battery assembly: Stack the positive electrode, separator and negative electrode in sequence, inject electrolyte and seal, and perform 3-5 pre-cycles at 0.05C rate.

5. The method for preparing a sodium battery according to claim 4, characterized in that, The heat treatment atmosphere in the preparation of the negative electrode material in step S1 is high-purity argon gas, and the argon gas flow rate is 50-100 ml / min.

6. The method for preparing a sodium battery according to claim 4, characterized in that, The S2 step involves an impregnation coating method in the preparation of the cathode material.

7. The method for preparing a sodium battery according to claim 4, characterized in that, In step S3, the stirring speed during electrolyte preparation is 300-600 rpm, the stirring time is 2-4 hours, and the water content in the solution is controlled below 10 ppm.

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