Sodium ferric sulfate composite material and preparation method thereof, and sodium ion battery

Abstract

This invention relates to the field of battery technology, specifically to a sodium iron sulfate composite material and its preparation method, and a sodium-ion battery. The sodium-ion battery was tested under the following charge-discharge conditions: first, the sodium-ion battery was charged at a constant current to 4.5V, and then discharged at a constant current to 1.0V; this allowed the sodium iron sulfate composite material in the sodium-ion battery to successively complete the Fe... 3+ / Fe 2+ Redox reaction with Fe 2+ / Fe 0 Redox reactions enable sodium-ion batteries to provide an actual discharge specific capacity exceeding the theoretical discharge specific capacity. Within the discharge cutoff voltage range of 1.5V–1.0V, Fe... 2+ / Fe 0 This redox reaction provides additional discharge capacity to the sodium-ion battery, thus avoiding irreversible damage to the sodium iron sulfate composite material. This allows the sodium iron sulfate composite material to maintain its stability while significantly improving the discharge capacity of the sodium-ion battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a sodium iron sulfate composite material and its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are considered an important supplement to lithium-ion batteries in large-scale energy storage due to the abundant and inexpensive availability of sodium resources. Among the reported cathode materials for sodium-ion batteries, iron-based sulfate materials have attracted widespread attention due to their high operating voltage, excellent structural stability, and high thermal safety. Of these, Alluaudite-type sodium iron sulfate (Na₂) is particularly noteworthy. 2+2x Fe 2-x (SO4)3) exhibits the highest Fe content among iron-based cathode materials to date. 3+ / Fe 2+ Redox potential (approximately 3.8V vs. Na) + / Na), while having lower Na), + Its migration barrier and good rate performance make it a cathode material with great potential for commercial applications.

[0003] However, existing alluaudite-type sodium iron sulfate cathode materials still face the problem of limited reversible capacity in practical applications. Within the conventional operating voltage window (typically 2.0-4.5V vs. Na+ / Na), these materials primarily utilize Fe... 3+ / Fe 2+ Discharge specific capacity is generated by redox reaction, and is based on Fe 3+ / Fe 2+ The maximum amount of electricity released when all active materials participate in the electrochemical reaction during a redox reaction is the theoretical discharge specific capacity. When sodium ferric sulfate is used as a cathode material for sodium ions, its theoretical discharge specific capacity is approximately 110 mAh g. -1 In practical sodium-ion batteries, due to factors such as the low intrinsic electronic conductivity of sodium iron sulfate material, limited electrochemical reaction kinetics, and side reactions of carbonate-based electrolytes in the high-voltage range, the actual reversible discharge specific capacity often falls short of the theoretical value. For example, non-stoichiometric Na... 2+2x Fe 2-x (SO4)3 series materials are obtained through Na + It partially occupies Fe sites, which to some extent masks structural stability and ion transport performance, but its reversible capacity is still smaller than that of Fe. 3+ / Fe 2+ The theoretical discharge specific capacity of redox reactions.

[0004] To improve the electrochemical performance of sodium ferric sulfate cathode materials, researchers in this field have proposed various modification strategies, including morphological design, carbon material composites, and heterostructure construction. While carbon coating can effectively improve electronic conductivity and structural stability, the introduction of carbon materials increases the proportion of inactive components, reducing the overall energy density of the electrode. Heterostructure strategies can improve overall performance through the synergistic effect of coupling different phases; however, they still fail to fundamentally overcome the discharge specific capacity limitation caused by redox reactions. In summary, how to overcome the limitations of Fe... 3+ / Fe 2+ The theoretical discharge specific capacity limitation of redox reactions and the need to significantly improve their discharge specific capacity have become key technical problems that urgently need to be solved in the field of this material. Summary of the Invention

[0005] To address the technical problem that the discharge specific capacity of sodium-ion batteries is limited by the theoretical discharge specific capacity in existing technologies, this invention provides a sodium iron sulfate composite material, its preparation method, and a sodium-ion battery.

[0006] This invention is achieved using the following technical solution: a sodium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, all made of a sodium iron sulfate composite material. The sodium-ion battery has a discharge cutoff voltage of 1.0V and a charging cutoff voltage of 4.5V. The actual discharge specific capacity of the sodium-ion battery can be increased to exceed the theoretical discharge specific capacity through the following charge-discharge test. The charge-discharge test process is as follows: first, the sodium-ion battery is charged at a constant current to 4.5V, and then discharged at a constant current to 1.0V; wherein, during the discharge stage: the sodium iron sulfate composite material in the positive electrode completes Fe... 3+ / Fe 2+ Redox reactions thus provide Fe-based 3+ / Fe 2+ This redox reaction generates a specific discharge capacity, which is achieved through Fe in the discharge voltage range of 1.5V to 1.0V. 2+ Reduced to Fe 0 This provides additional discharge capacity for sodium-ion batteries; through the aforementioned electrochemical rate test, the sodium iron sulfate composite material in sodium-ion batteries was successively completed to achieve Fe... 3+ / Fe 2+ Redox reaction with Fe 2+ / Fe 0 The redox reaction enables sodium-ion batteries to provide an actual discharge capacity that exceeds the theoretical discharge capacity.

[0007] As a further improvement of the present invention, the positive electrode also includes polyvinylidene fluoride, acetylene black, and aluminum foil; the positive electrode is prepared as follows: polyvinylidene fluoride, acetylene black, and sodium ferric sulfate composite material are mixed evenly in a mass ratio of 1:1.33:7.67, and then a solvent with a mass-volume ratio of 0.23g:1mL to sodium ferric sulfate composite material is added and stirred to form a uniform slurry; the slurry is coated on aluminum foil and vacuum dried to form the positive electrode.

[0008] As a further improvement of the present invention, the negative electrode is a sodium metal sheet.

[0009] As a further improvement of the present invention, the diaphragm is a glass fiber filter membrane.

[0010] As a further improvement of the present invention, the electrolyte is a mixture of ethylene carbonate and diethyl carbonate containing 1 mol / L sodium perchlorate; wherein the volume ratio of ethylene carbonate to diethyl carbonate is 1:1.

[0011] As a further improvement of the present invention, the vacuum drying temperature is 80℃~100℃ and the time is 12h.

[0012] As a further improvement of the present invention, the sodium-ion battery is a sodium-ion button battery; the sodium-ion button battery also includes a negative electrode shell, a spring, a gasket, and a positive electrode shell; when assembling the sodium-ion button battery, the assembly sequence starting from the positive electrode shell is as follows: positive electrode shell, positive electrode, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell.

[0013] This invention also includes a sodium ferric sulfate composite material, which is used as a raw material for the positive electrode in a sodium-ion battery as described above; the chemical formula of the sodium ferric sulfate composite material is Na. 2+2x Fe 2-x (SO4)3@C, where 0≤x≤0.5.

[0014] The present invention also includes a method for preparing the sodium ferric sulfate composite material as described above, comprising: weighing raw materials in an inert atmosphere with an iron source and a sodium source at a molar ratio of 1.8:1.2, and adding 10 wt% of a carbon source; ball milling the raw materials in an inert atmosphere to obtain a precursor mixture; and solid-state sintering the precursor mixture under a protective atmosphere to obtain the sodium ferric sulfate composite material.

[0015] As a further improvement of the present invention, the solid-state sintering temperature is 300℃-400℃ and the time is 12-24h.

[0016] As a further improvement of the present invention, the iron source is one or more of ferrous sulfate, ferrous oxalate, ferrous sulfate heptahydrate, and ferrous sulfate monohydrate.

[0017] As a further improvement of the present invention, the nano source is one or more of sodium carbonate and sodium sulfate.

[0018] As a further improvement of the present invention, the ball milling process adopts a planetary ball mill for cyclic vacuum ball milling. The cyclic process of vacuum ball milling is as follows: first rotate forward for 30 minutes, then rotate backward for 30 minutes, and stop ball milling for 10 minutes after rotating backward; repeat the above operation until the total ball milling time reaches 12 hours.

[0019] The technical solution provided by this invention has the following beneficial effects: (1) The sodium-ion battery provided by the present invention discharges within a discharge voltage range of 4.5-1.0V, enabling the positive electrode of the sodium-ion battery to discharge within a discharge voltage range of 4.5-1.5V based on Fe. 3+ / Fe 2+ The discharge specific capacity formed by this redox reaction can also be maintained within the discharge voltage range of 1.5V to 1.0V by Fe. 2+ Reduced to Fe 0 This provides additional discharge specific capacity for sodium ions. Furthermore, the sum of the discharge specific capacities across the two voltage ranges is significantly greater than that of existing technologies based on Fe. 3+ / Fe 2+ The theoretical discharge specific capacity formed by this redox reaction enables the sodium-ion battery of this scheme to provide an actual discharge specific capacity that far exceeds the theoretical discharge specific capacity, thereby achieving the goal of significantly improving the discharge specific capacity of the sodium-ion battery.

[0020] (2) The sodium-ion battery provided by the present invention can not only pass through Fe within the range of 1.5V-1.0V discharge cutoff voltage. 2+ / Fe 0 This redox reaction provides additional discharge specific capacity to the sodium-ion battery, resulting in a significantly higher actual discharge specific capacity for the sodium-ion battery based on Fe. 3+ / Fe 2+ The theoretical discharge specific capacity formed by this redox reaction, and the capacity recovery rate exceeding 98% within the discharge cutoff voltage range of 1.5V-1.0V, demonstrate that the redox reaction of the sodium-ion battery provided in this scheme, based on the low-voltage platform excited by deep discharge (i.e., discharge cutoff voltage of 1.5-1.0V), has good reversibility, and within this low-voltage range, it is based on Fe... 2+ / Fe 0 This redox reaction provides additional discharge specific capacity, thus preventing irreversible damage to the sodium iron sulfate composite material within this low-pressure range. This allows for a significant increase in the discharge specific capacity of the sodium-ion battery while maintaining the stability of the sodium iron sulfate composite material. Attached Figure Description

[0021] Figure 1 The first charge-discharge curves of the three sodium-ion batteries (the first group, the second group, and the third group) in the performance testing section of this invention are shown.

[0022] Figure 2 The first charge-discharge curves of the fourth and fifth groups of sodium-ion batteries in the performance testing section of this invention are shown.

[0023] Figure 3 This is a cyclic voltammetry curve of the first group of sodium-ion batteries in the performance test section of this invention when the discharge cutoff voltage is adjusted to 1.0V.

[0024] Figure 4 This is a cyclic voltammetry curve of the second group of sodium-ion batteries in the performance test section of this invention when the discharge cutoff voltage is adjusted to 1.5V.

[0025] Figure 5 This is a rate performance diagram of the first group of sodium-ion batteries, the second group of sodium-ion batteries, and the third group of sodium-ion batteries in the performance test section of this invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] In existing technologies, sodium-ion batteries containing a sodium iron sulfate composite material as the positive electrode are typically tested for their discharge specific capacity within a voltage range of 4.5V to 2.0V when performing electrochemical rate testing. Within this voltage range, the discharge specific capacity is generally obtained by utilizing Fe... 3+ / Fe 2+Discharge specific capacity is generated by redox reactions. However, due to factors such as the low intrinsic electronic conductivity of sodium iron sulfate composite materials, limited electrochemical reaction kinetics, and side reactions of carbonate-based electrolytes in the high-voltage range, the discharge specific capacity of sodium-ion batteries in the discharge voltage range of 4.5V-2.0V is difficult to reach its theoretical value. Discharge specific capacity is the amount of electricity that can be released per unit weight or volume of battery material. It is a core indicator for measuring battery performance; a higher value indicates a more durable battery of the same size or weight. Therefore, exploring how to improve the discharge specific capacity of sodium-ion batteries is a key technical problem that urgently needs to be solved by those skilled in the art. Based on this, this embodiment provides a sodium-ion battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte containing sodium iron sulfate composite materials. The discharge cutoff voltage of the sodium-ion battery is 1.0V, and the charging cutoff voltage is 4.5V. The actual discharge specific capacity of the sodium-ion battery is increased to far exceed the theoretical discharge specific capacity through the following charge-discharge test. The charge-discharge test process is as follows: the sodium-ion battery is first charged at a constant current to 4.5V, and then discharged at a constant current to 1.0V. During the discharge phase: the sodium ferric sulfate composite material in the positive electrode completes the Fe process within the voltage range of 4.5V to 1.5V. 3+ / Fe 2 + Redox reactions thus provide Fe-based 3+ / Fe 2+ The discharge specific capacity formed by this redox reaction is within the discharge voltage range of 1.5V to 1.0V through Fe 2+ Reduced to Fe 0 This provides additional discharge specific capacity for sodium ions. Through the aforementioned electrochemical rate tests, the Fe-sodium sulfate composite material in sodium-ion batteries was subsequently completed. 3+ / Fe 2+ Redox reaction with Fe 2+ / Fe 0 The redox reaction enables sodium-ion batteries to provide an actual discharge capacity that exceeds the theoretical discharge capacity.

[0028] Among them, based on Fe 3+ / Fe 2+ The discharge specific capacity formed by this redox reaction is less than or equal to that based on Fe. 3+ / Fe 2+ The theoretical discharge specific capacity is formed by this redox reaction. This means that when using existing conventional discharge voltages to perform electrochemical rate tests on the sodium-ion battery of this scheme, its discharge specific capacity is difficult to exceed its theoretical discharge specific capacity. However, in this scheme, by discharging within the discharge voltage range of 4.5-1.0V, the positive electrode of the sodium-ion battery can not only achieve a discharge specific capacity within the 4.5-1.5V range based on Fe... 3+ / Fe 2+The discharge specific capacity formed by this redox reaction can also be maintained within the discharge voltage range of 1.5V to 1.0V by Fe. 2+ Reduced to Fe 0 This provides additional discharge specific capacity for sodium ions. Furthermore, the sum of the discharge specific capacities across the two voltage ranges is significantly greater than that of existing technologies based on Fe. 3+ / Fe 2+ The theoretical discharge specific capacity formed by this redox reaction enables the sodium-ion battery of this scheme to provide an actual discharge specific capacity that exceeds the theoretical discharge specific capacity, thereby achieving the goal of improving the discharge specific capacity of the sodium-ion battery.

[0029] The positive electrode also includes polyvinylidene fluoride (PVDF), acetylene black, and aluminum foil. The positive electrode can be prepared as follows: PVDF, acetylene black, and sodium ferric sulfate composite material are mixed uniformly in a mass ratio of 1:1.33:7.67. Then, a solvent with a mass-to-volume ratio of 0.23 g:1 mL to the sodium ferric sulfate composite material is added and stirred to form a uniform slurry. The slurry is coated onto aluminum foil and vacuum dried at 80-100℃ for 12 hours to prepare the positive electrode.

[0030] N-methylpyrrolidone can be used as a solvent.

[0031] For sodium ferric sulfate composite materials, the chemical formula is Na. 2+2x Fe 2-x (SO4)3@C, where 0 ≤ x ≤ 0.5. In a preferred embodiment of this solution, x = 0.2, that is, the chemical formula of the sodium ferric sulfate composite material at this time is as follows: Na 2.4 Fe 1.8 (SO4)3@C.

[0032] The preparation method of sodium ferric sulfate composite material is as follows: Iron source and sodium source are weighed in a molar ratio of 1.8:1.2, and 10 wt% carbon source is added; the above raw materials are ball-milled and mixed in an inert atmosphere to obtain a precursor mixture. Then, under a protective atmosphere, the precursor mixture is solid-state sintered to obtain the sodium ferric sulfate composite material.

[0033] When preparing sodium ferric sulfate composite materials, the iron source can be one or more of ferrous sulfate, ferric oxalate, ferrous sulfate heptahydrate, and ferrous sulfate monohydrate; the sodium source can be one or a mixture of sodium carbonate and sodium sulfate. If the selected iron and sodium sources contain sulfate, no additional sulfate source is needed. If neither the selected iron nor sodium source contains sulfate, an additional sulfate source needs to be added, and ammonium sulfate can be selected as the added sulfate source. The carbon source can be Ketjen black.

[0034] For ball milling the iron, sodium, and carbon sources, a planetary ball mill can be used for vacuum ball milling. The rotation speed can be set to 600 r / min, and the milling time can be 12 hours. The ball milling program for vacuum ball milling is as follows: first rotate clockwise for 30 minutes, then counterclockwise for 30 minutes, and stop for 10 minutes after each hour of operation. Repeat the above ball milling program until the total ball milling time reaches 12 hours to obtain the precursor mixture. After obtaining the precursor mixture, it can be placed in a tube furnace and sintered under an inert atmosphere at a sintering temperature of 350℃ for 12 hours. After sintering, the product is allowed to cool naturally to room temperature, then removed, ground, and filtered through a 325-mesh sieve to obtain the sodium ferric sulfate composite material. The sodium ferric sulfate composite material described above can achieve high purity, uniform elemental distribution, fine grains, and high electrochemical activity by using vacuum ball milling combined with solid-state sintering. The use of vacuum ball milling allows for more thorough contact between raw materials, resulting in better mixing uniformity. Sieving disperses the material, which is beneficial for subsequent pulping processes. A one-step sintering process directly reacts to form the phase, further improving the uniformity of elemental distribution in the resulting sodium ferric sulfate composite material. Furthermore, vacuum ball milling, through mechanochemical action, refines particles, introduces defects, and increases specific surface area, thereby reducing the activation energy required for ion diffusion, solid-state reaction, and lattice crystallization during subsequent sintering—essentially lowering the "overall sintering energy barrier." The vacuum environment also prevents oxidation or contamination, which is beneficial for activation.

[0035] The heating rate during solid-state sintering can be 5℃ / min.

[0036] The negative electrode can be a sodium metal sheet, the separator can be glass fiber, and the electrolyte can be a mixture of ethylene carbonate and diethyl carbonate containing 1 mol / L sodium perchlorate. The volume ratio of ethylene carbonate to diethyl carbonate is 1:1. The main raw material of the positive electrode is sodium iron sulfate composite material, which has the following advantages: (1) Iron, sulfur and sodium in sodium iron sulfate composite material are elements with high abundance in the earth's crust, and there are no precious metals such as cobalt and nickel, which makes the raw material cost of making the positive electrode extremely low and suitable for large-scale energy storage scenarios; (2) It is one of the positive electrode materials of polyanionic sodium-ion batteries. Its crystal structure is similar to a three-dimensional framework structure, which makes the skeleton stable when sodium ions are inserted / extracted. It has the high safety, long cycle life and high working voltage common to polyanionic materials. (3) It has good chemical stability with sodium perchlorate-carbonate electrolyte and is not prone to dissolution and side reactions. The advantages of using sodium metal sheets as the negative electrode are as follows: (1) Using sodium metal sheets can ensure that the sodium-ion battery has sufficient internal sodium source and no problem of insufficient sodium insertion, thereby making the sodium-ion battery have high coulombic efficiency in the first cycle; (2) The sodium metal potential is close to the sodium standard electrode potential, which can increase the overall output voltage of the sodium-ion battery. The advantages of using glass fiber filter membrane as the separator are as follows: (1) Glass fiber filter membrane has high porosity, large liquid absorption, and good liquid retention. It can continuously provide electrolyte for the solid-liquid interface between the active material of the positive electrode and the electrolyte and the solid-liquid interface between the active material of the negative electrode and the electrolyte, reduce polarization, and improve the charge-discharge rate and cycle performance of the sodium-ion battery. (2) The fiber interwoven structure of glass fiber filter membrane has good toughness, which can suppress the dendrite penetration of sodium metal to a certain extent and alleviate the micro short circuit caused by sodium dendrite, thereby making it suitable for the sodium metal negative electrode system. (3) Glass fiber does not react chemically with metallic sodium, sodium perchlorate electrolyte, or the positive electrode, thus enabling the sodium-ion battery to cycle stably for a long time. (3) The electrolyte is a mixture of ethylene carbonate and diethyl carbonate containing 1 mol / L sodium perchlorate, which has the following advantages: it can form a dense and stable solid interface film (SEI) on the surface of metallic sodium, thereby inhibiting the continuous corrosion of the sodium metal and the decomposition of the electrolyte, and improving the cycle stability of the sodium-ion battery. In summary, this scheme uses low-cost sodium iron sulfate composite material as the main raw material to make the positive electrode, high-capacity metallic sodium as the negative electrode, and high-safety glass fiber filter membrane as the separator and high-conductivity sodium perchlorate as the electrolyte. This can build a high-potential low-cost sodium-ion battery system, which makes the sodium-ion battery have the characteristics of low raw material cost, high energy density, simple preparation process, good safety, and excellent cycle performance.

[0037] The sodium-ion battery provided in this solution can be a sodium-ion coin cell battery, which may also include a negative electrode casing, a spring contact, a gasket, and a positive electrode casing. The assembly of the sodium-ion coin cell battery begins with the positive electrode casing, and the assembly sequence is as follows: positive electrode casing, positive electrode, electrolyte, separator, electrolyte, negative electrode, gasket, spring contact, and negative electrode casing. Since this solution does not involve any improvement to the assembly process of the sodium-ion battery, the specific assembly process of the sodium-ion battery in this solution can refer to the assembly process of existing technologies.

[0038] Performance testing To verify the performance of the sodium-ion battery provided in this embodiment, the technicians verified the scheme of Example 1 in subsequent test cases and experiments, synthesized related intermediate products (such as sodium iron sulfate composite material) and the final finished sodium-ion battery, and conducted the following comparative analysis on the performance of the prepared sodium-ion battery in subsequent experiments.

[0039] Test Example 1 This test example provides a specific fabrication process for a sodium-ion battery, which includes the following specific steps: (I) Preparation of sodium ferric sulfate composite material The chemical formula of the sodium ferric sulfate composite material is as follows: Na 2.4 Fe 1.8 (SO4)3@C. The preparation process is as follows: Ferrous sulfate and sodium sulfate are weighed at a molar ratio of 1.8:1.2, and 10 wt% Ketjen black is added. In an inert gas atmosphere, ferrous sulfate, sodium sulfate, and Ketjen black are vacuum ball-milled using a planetary ball mill at a speed of 600 r / min for 12 hours. The ball milling program is as follows: first, rotate clockwise for 30 minutes, then counterclockwise for 30 minutes, with a 1-hour rest period after each milling. This process is repeated until the total milling time reaches 12 hours, yielding the precursor mixture. The precursor mixture is placed in a tube furnace and subjected to one-step solid-state sintering under inert gas protection at a temperature of 350℃ for 12 hours. After sintering, the product is naturally cooled to room temperature, removed, ground, and then sieved through a 325-mesh sieve to obtain the sodium ferric sulfate composite material.

[0040] (II) Preparation of the positive electrode The sodium ferric sulfate composite material obtained in step (I), acetylene black, and polyvinylidene fluoride were uniformly mixed at a mass ratio of 7.67:1.33:1. Then, N-methylpyrrolidone (N-methylpyrrolidone) with a mass-to-volume ratio of 0.23 g:1 mL to the sodium ferric sulfate composite material was added as a solvent and stirred to form a uniform slurry. The slurry was then coated onto aluminum foil and dried in a vacuum drying oven at 80°C for 12 hours to produce the positive electrode.

[0041] (III) Assembling sodium-ion batteries A sodium metal sheet was used as the negative electrode (i.e., the counter electrode), a glass fiber filter membrane as the separator, and a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1 mol / L sodium perchlorate (NaClO4) as the electrolyte, with a volume ratio of EC to DEC of 1:1. In a glove box filled with high-purity argon, the prepared positive electrode, separator, and sodium metal sheet were assembled into a CR2032 coin-type sodium-ion battery in the following order: positive electrode shell, positive electrode, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell.

[0042] Comparative Example 1 Comparative Example 1 provides a specific sodium-ion battery preparation process that is exactly the same as that of Test Example 1 in terms of preparation process and raw materials. The difference is that 10% wt of Ketjen Black was not added when preparing the sodium iron sulfate composite material.

[0043] Four sodium-ion batteries were prepared according to the method in Test Example 1, and one sodium-ion battery was prepared according to the method in Comparative Example 1. A total of five sodium-ion batteries were obtained. The four sodium-ion batteries prepared in Test Example 1 were named, sequentially, the first sodium-ion battery, the second sodium-ion battery, the third sodium-ion battery, and the fourth sodium-ion battery; the sodium-ion battery prepared in Comparative Example 1 was named the fifth sodium-ion battery. Electrochemical rate testing was performed on each of the five sodium-ion batteries, and the specific process is as follows: The first group of sodium-ion batteries was left to stand at room temperature for 4 hours to ensure that the electrolyte fully wetted the electrodes and separator. Constant current charge-discharge rate tests were then conducted using a battery testing system, with the voltage range set to 1.0-4.5V (vs Na / Na). + The charge / discharge current density is 1C = 100 mAh g. -1 First, the sodium-ion battery was activated and the electrode interface was stabilized by performing one charge-discharge cycle at a current of 0.1C. Then, rate tests were conducted sequentially at gradients of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with the charging current consistently maintained at 0.2C throughout the tests; only the discharging current was varied. Each rate increment was cycled for five consecutive times, with a 5-minute rest period between charge and discharge cycles to minimize concentration polarization. After all high-rate tests were completed, the battery was switched back to 0.1C for charge-discharge testing to examine its capacity recovery capability. The discharge specific capacity and charge-discharge voltage curves at different rates were recorded, and the capacity recovery rate at each rate was calculated.

[0044] The second group of sodium-ion batteries was placed at room temperature for 4 hours to ensure that the electrolyte fully wetted the electrodes and separator. Constant current charge-discharge rate tests were conducted using a battery testing system, with the voltage range set to 1.5-4.5V (vs Na / Na). +The charge / discharge current density is 1C = 100 mAh g. -1 First, the sodium-ion battery was activated and the electrode interface was stabilized by performing one charge-discharge cycle at a current of 0.1C. Then, rate tests were conducted sequentially at gradients of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with the charging current consistently maintained at 0.2C throughout the tests; only the discharging current was varied. Each rate increment was cycled for five consecutive times, with a 5-minute rest period between charge and discharge cycles to minimize concentration polarization. After all high-rate tests were completed, the battery was switched back to 0.1C for charge-discharge testing to examine its capacity recovery capability. The discharge specific capacity and charge-discharge voltage curves at different rates were recorded, and the capacity recovery rate at each rate was calculated.

[0045] The third group of sodium-ion batteries was placed at room temperature for 4 hours to ensure that the electrolyte fully wetted the electrodes and separator. Constant current charge-discharge rate tests were conducted using a battery testing system, with the voltage range set to 2.0-4.5V (vs Na / Na). + The charge / discharge current density is 1C = 100 mAh g. -1 First, the sodium-ion battery was activated and the electrode interface was stabilized by performing one charge-discharge cycle at a current of 0.1C. Then, rate tests were conducted sequentially at gradients of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with the charging current consistently maintained at 0.2C throughout the tests; only the discharging current was varied. Each rate increment was cycled for five consecutive times, with a 5-minute rest period between charge and discharge cycles to minimize concentration polarization. After all high-rate tests were completed, the battery was switched back to 0.1C for charge-discharge testing to examine its capacity recovery capability. The discharge specific capacity and charge-discharge voltage curves at different rates were recorded, and the capacity recovery rate at each rate was calculated.

[0046] The fourth group of sodium-ion batteries was placed at room temperature for 4 hours to ensure that the electrolyte fully wetted the electrodes and separator. Constant current charge-discharge rate tests were conducted using a battery testing system, with the voltage range set to 2.5-4.5V (vs Na / Na). + The charge / discharge current density is 1C = 100 mAh g. -1First, the sodium-ion battery was activated and the electrode interface was stabilized by performing one charge-discharge cycle at a current of 0.1C. Then, rate tests were conducted sequentially at gradients of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with the charging current consistently maintained at 0.2C throughout the tests; only the discharging current was varied. Each rate increment was cycled for five consecutive times, with a 5-minute rest period between charge and discharge cycles to minimize concentration polarization. After all high-rate tests were completed, the battery was switched back to 0.1C for charge-discharge testing to examine its capacity recovery capability. The discharge specific capacity and charge-discharge voltage curves at different rates were recorded, and the capacity recovery rate at each rate was calculated.

[0047] The fifth group of sodium-ion batteries was placed at room temperature for 4 hours to ensure that the electrolyte fully wetted the electrodes and separator. Constant current charge-discharge rate tests were conducted using a battery testing system, with the voltage range set to 2-4.5V (vs Na / Na+) and the charge / discharge point current density set to 1C = 100 mAh g⁻¹. -1 First, the sodium-ion battery was activated and the electrode interface was stabilized by performing one charge-discharge cycle at a current of 0.1C. Then, rate tests were conducted sequentially at gradients of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with the charging current consistently maintained at 0.2C throughout the tests; only the discharging current was varied. Each rate increment was cycled for five consecutive times, with a 5-minute rest period between charge and discharge cycles to minimize concentration polarization. After all high-rate tests were completed, the battery was switched back to 0.1C for charge-discharge testing to examine its capacity recovery capability. The discharge specific capacity and charge-discharge voltage curves at different rates were recorded, and the capacity recovery rate at each rate was calculated.

[0048] The above operations can be used to obtain the following: Figures 1 to 5 .in, Figure 1 The graph shows the first charge-discharge curves of the first, second, and third groups of sodium-ion batteries. Figure 2 The first charge-discharge curves for the fourth and fifth sodium-ion battery groups are shown. Figure 3 The diagram shows the cyclic voltammetry curves of the first group of sodium-ion batteries when the discharge cutoff voltage is adjusted to 1.0V. Figure 4 The diagram shows the cyclic voltammetry curves of the second group of sodium-ion batteries when the discharge cutoff voltage is adjusted to 1.5V. Figure 5 This shows the rate performance diagrams for the first, second, and third groups of sodium-ion batteries. (The text then abruptly shifts to a seemingly unrelated topic: "Through analysis...") Figure 1 and Figure 2 The data in the table can be processed to obtain Table 1.

[0049] Table 1 compares the electrochemical characteristics of different groups of sodium-ion batteries. Based on the data in Table 1, the following analysis can be performed: (I) Electrochemical behavior under conventional discharge cutoff voltage The test results of the fourth group of sodium-ion batteries show that when the discharge cutoff voltage is set to 2.5V, the specific discharge capacity of the sodium-ion battery is 98.22 mAh g. -1 Approaching sodium ferric sulfate composite material based on Fe 3+ / Fe 2+ The theoretical discharge capacity of the redox reaction (approximately 110 mAh g) -1 Furthermore, its capacity recovery rate after rate cycling is 95.24%. This indicates that the sodium-ion battery of this scheme has acceptable cycle reversibility of its internal sodium iron sulfate composite material within the conventional voltage window. However, its reversible discharge specific capacity is limited by the theoretical threshold upper limit of single-electron reaction, making further breakthroughs difficult.

[0050] Looking at the test results of the fifth group of sodium-ion batteries, we can see that when the discharge cutoff voltage is set to 2.0V, the discharge specific capacity of the sodium-ion battery is only 67.7 mAh g. -1 The capacity recovery rate was 90.76%. Therefore, it can be seen that the discharge specific capacity and capacity recovery rate of the fifth group of sodium-ion batteries are lower than those of the remaining four groups. Furthermore, the fifth group of sodium-ion batteries did not contain Ketjen Black during preparation. The background technology of this solution also describes that: under carbon-free conditions, sodium ferric sulfate undergoes the process of producing divalent iron (Fe2+). 2+ It is easily oxidized to ferric iron (Fe3+). 3+ This leads to the formation of impurity phases in the product, making it difficult to obtain pure Alluaudite-type sodium iron sulfate. Furthermore, the presence of these impurity phases not only reduces the effective content of the active material but may also hinder Na+ diffusion channels and increase electrode polarization, thus severely degrading the electrochemical performance of sodium iron sulfate. Therefore, the test results of the fifth group of sodium-ion batteries show that the very low discharge specific capacity is not simply due to the discharge cutoff voltage setting, but rather a result of the poor intrinsic quality of the sodium iron sulfate composite material under uncoated conditions. This also indirectly confirms the necessity of carbon coating in the preparation of sodium iron sulfate composite materials.

[0051] (II) Electrochemical behavior under deep discharge conditions Combined with Table 1, Figures 3 to 5 Data analysis shows that for the third group of sodium-ion batteries, the discharge specific capacity at a discharge cutoff voltage of 2.0V is approximately 102.0 mAh g. -1 For the fourth group of sodium-ion batteries, the discharge specific capacity at a discharge cutoff voltage of 2.5V is approximately 98.22 mAh g⁻¹. -1The discharge specific capacity values ​​of the two groups of sodium-ion batteries mentioned above are not significantly different, and are basically within the theoretical discharge specific capacity range of the Alluaudite type sodium iron sulfate reaction. This indicates that the discharge depth of 2.0V is insufficient to effectively stimulate the additional redox activity of sodium-ion batteries.

[0052] Looking at the test data for the second group of sodium-ion batteries, with the discharge cutoff voltage set to 1.5V, their discharge specific capacity reached 105.9 mAh g⁻¹. -1 The discharge specific capacity is slightly improved compared to discharge cutoff voltages of 2.0V and 2.5V. Combined with... Figure 2 Analysis of the discharge curve suggests that the 1.5V discharge cutoff voltage has begun to reach Fe. 2+ / Fe 0 The onset potential of a redox reaction, such as Figure 4 As shown, however, due to the limited depth of discharge, Fe 2+ / Fe 0 The additional discharge capacity provided by the redox reaction has not been fully released. It is also noteworthy that the capacity recovery rate after rate cycling is as high as 98.64% at a discharge cutoff voltage of 1.5V, indicating that the sodium-ion battery of this design exhibits good chemical reversibility even at partially excited low voltages (i.e., 1.5V).

[0053] Looking at the test data for the first group of sodium-ion batteries, with the discharge cutoff voltage set to 1.0V, their discharge specific capacity reached 187.4 mAh g⁻¹. -1 Its discharge specific capacity is the highest among the five groups of sodium-ion batteries. This is compared to the theoretical discharge specific capacity of the sodium iron sulfate composite material (approximately 100 mAh g⁻¹). -1 In terms of efficiency, this represents an improvement of approximately 87.4%. This result strongly demonstrates that under deep discharge conditions of 1.0V, the redox activity of Fe in the sodium iron sulfate composite material is more fully utilized: firstly, sodium-ion batteries can complete the Fe redox activity within the discharge voltage range of 4.5V-1.5V. 3+ →Fe 2+ The redox reaction occurs, and secondly, the sodium-ion battery continues to undergo Fe reactions within the discharge voltage range of 1.5V-1.0V. 2+ →Fe 0 The redox reaction; the two redox reactions in the above two stages proceed sequentially, providing sodium-ion batteries with far superior performance compared to Fe. 3+ / Fe 2+ This single-reaction theory demonstrates the actual discharge specific capacity of sodium-ion batteries, thereby significantly improving their actual discharge specific capacity. Furthermore, the first set of sodium-ion batteries not only exhibits discharge specific capacity far exceeding that of Fe...3+ / Fe 2 + The actual discharge specific capacity under this single-reaction theory, and its capacity recovery rate after rate cycling as high as 99.38%, far superior to the capacity recovery rates of the second group of sodium-ion batteries (98.64%) and the third group of sodium-ion batteries (94.32%). This indicates that the sodium-ion battery provided by this scheme can provide additional discharge specific capacity by initiating a new redox reaction at a low voltage plateau (i.e., 1.5V-1.0V) through deep discharge to 1.0V, and the discharge specific capacity obtained at a discharge cutoff voltage set to 1.0V still exhibits excellent electrochemical reversibility and structural stability.

[0054] (III) General Laws of the Influence of Discharge Cut-off Voltage Gradient on the Performance of Sodium-ion Batteries Analysis of test data from the first, second, third, and fourth groups of sodium-ion batteries reveals that as the discharge cutoff voltage gradually decreases from 2.5V to 1.0V, the discharge specific capacity of the sodium-ion batteries initially increases slightly, then experiences a significant jump at 1.0V. Specifically, when the discharge cutoff voltage decreases from 2.5V to 2.0V, the discharge specific capacity of the sodium-ion batteries is 98 mAh g⁻¹. -1 Up to 102 mAh g -1 Within the specified range, the variation is relatively small; when the discharge cutoff voltage drops from 2.0V to 1.5V, the discharge specific capacity of the sodium-ion battery increases from 102 mAhg. -1 Increased to 105.9 mAh g -1 The increase was approximately 3.8%; when the discharge cutoff voltage was further reduced from 1.5V to 1.0V, its discharge specific capacity increased from 105.9 mAh g⁻¹. -1 Increased to 187.4 mAh g -1 The increase was as high as approximately 77.0%, and this stage saw a breakthrough in the discharge specific capacity of sodium-ion batteries, which clearly corresponds to the Fe... 2+ / Fe 0 This redox reaction is fully activated and carried out within the discharge voltage range of 1.5V-1.0V. Furthermore, considering the capacity recovery rate after rate cycling, the first, second, third, and fourth groups of sodium-ion batteries all exhibit a capacity recovery rate exceeding 94% at their respective discharge cutoff voltages. Moreover, the capacity recovery rates of these two groups of sodium-ion batteries exceed 98% at discharge cutoff voltages of 1.5V and 1.0V. This demonstrates that the redox reaction generated by the sodium-ion battery provided in this solution, based on the low-voltage platform excited by deep discharge (i.e., discharge cutoff voltage of 1.5-1.0V), has good reversibility, and within this low-voltage range, it is based on Fe... 2+ / Fe 0 This redox reaction provides additional discharge specific capacity, thus preventing irreversible damage to the sodium ferric sulfate composite material within this low-pressure range.

[0055] (Ⅳ)Fe 2+ / Fe 0 Capacity contribution analysis of reaction activation The first group of sodium-ion batteries had a discharge cutoff voltage of 1.0V and a discharge specific capacity of 187.4 mAh g. -1 The discharge cutoff voltage of the fourth group of sodium-ion batteries is 2.5V, and the discharge specific capacity is 98.22 mAh g. -1 A comparison shows that the difference in discharge specific capacity between the two is approximately 89.18 mAh g. -1 During the process of adjusting the discharge cutoff voltage from 2.5V to 1.0V, Fe will be excited. 2+ / Fe 0 This redox reaction results in an actual discharge specific capacity far exceeding the theoretical capacity, enabling the sodium-ion battery of this design to provide 187.4 mAh g⁻¹ at a discharge cutoff voltage of 1.0V. -1 The specific discharge capacity.

[0056] In summary, this solution, by adjusting the discharge cutoff voltage to 1.0V, can effectively activate Fe in the sodium-ion battery. 2+ / Fe 0 This additional redox reaction provides additional discharge specific capacity. The sodium-ion battery in this design achieves Fe by discharging within the discharge voltage range of 4.5V-1.0V. 3+ / Fe 2+ with Fe 2+ / Fe 0 The sequential two-stage redox reaction breaks through the theoretical discharge specific capacity limit of traditional sodium-ion batteries, significantly improving their discharge specific capacity. Simultaneously, the sodium-ion battery exhibits excellent electrochemical reversibility and rate performance at a discharge cutoff voltage of 1.5V-1.0V, confirming its reversible electrochemical sodium storage behavior. Furthermore, this scheme is simple to operate; a significant improvement in electrochemical performance can be achieved simply by adjusting the discharge cutoff voltage parameter, providing a practical and feasible technical approach for performance optimization of sodium iron sulfate composite materials.

[0057] The basic principles, main features, and advantages of this invention have been described above. Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made without departing from the spirit and scope of the invention, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection claimed by this invention is defined by the appended claims and their equivalents.

Claims

1. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte containing sodium iron sulfate composite material; the discharge cutoff voltage of the sodium-ion battery is 1.0V, and the charging cutoff voltage is 4.5V; the sodium-ion battery can increase its actual discharge specific capacity to exceed the theoretical discharge specific capacity through the following charge and discharge tests. The charge / discharge test process is as follows: First, the sodium-ion battery is charged to 4.5V under constant current, and then discharged to 1.0V under constant current. During the discharge phase, the sodium iron sulfate composite material in the positive electrode completes the Fe... 3+ / Fe 2+ Redox reactions thus provide Fe-based 3+ / Fe 2+ This redox reaction generates a specific discharge capacity, which is achieved through Fe in the discharge voltage range of 1.5V to 1.0V. 2+ Reduced to Fe 0 This provides additional discharge capacity for sodium-ion batteries; through the aforementioned electrochemical rate test, the sodium iron sulfate composite material in sodium-ion batteries was successively completed to achieve Fe... 3+ / Fe 2+ Redox reaction with Fe 2+ / Fe 0 The redox reaction enables sodium-ion batteries to provide an actual discharge capacity that exceeds the theoretical discharge capacity.

2. The sodium-ion battery as described in claim 1, characterized in that, The positive electrode also includes polyvinylidene fluoride, acetylene black, and aluminum foil; the positive electrode is prepared as follows: the polyvinylidene fluoride, the acetylene black, and the sodium ferric sulfate composite material are mixed evenly in a mass ratio of 1:1.33:7.67, and then a solvent with a mass-volume ratio of 0.23g:1mL to the sodium ferric sulfate composite material is added and stirred to form a uniform slurry; the slurry is coated on aluminum foil and vacuum dried to form the positive electrode.

3. The sodium-ion battery as described in claim 1, characterized in that, The negative electrode is a sodium metal sheet; And / or, the diaphragm is a glass fiber filter membrane.

4. The sodium-ion battery as described in claim 1, characterized in that, The electrolyte is a mixture of ethylene carbonate and diethyl carbonate containing 1 mol / L sodium perchlorate; wherein the volume ratio of ethylene carbonate to diethyl carbonate is 1:

1.

5. The sodium-ion battery as described in claim 2, characterized in that, The vacuum drying temperature is 80℃~100℃, and the time is 12h.

6. The sodium-ion battery as described in claim 2, characterized in that, The sodium-ion battery is a sodium-ion button cell; the sodium-ion button cell also includes a negative electrode shell, a spring, a gasket, and a positive electrode shell; when assembling the sodium-ion button cell, the assembly sequence starting from the positive electrode shell is as follows: positive electrode shell, positive electrode, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell.

7. A sodium ferric sulfate composite material, characterized in that, It is used as a raw material for the positive electrode in a sodium-ion battery as described in any one of claims 1-6; the chemical formula of the sodium iron sulfate composite material is Na. 2+2x Fe 2-x (SO4)3@C, where 0≤x≤0.

5.

8. A method for preparing the sodium ferric sulfate composite material as described in claim 7, characterized in that, It includes: In an inert atmosphere, iron source and nano source are weighed in a molar ratio of 1.8:1.2, and 10 wt% carbon source is added; the above raw materials are ball-milled and mixed in an inert atmosphere to obtain a precursor mixture. The sodium ferric sulfate composite material was prepared by solid-state sintering of the precursor mixture under a protective atmosphere.

9. The method for preparing the sodium ferric sulfate composite material as described in claim 8, characterized in that, The solid-state sintering temperature is 300℃-400℃, and the time is 12-24h. And / or, the iron source is one or more of ferrous sulfate, ferrous oxalate, ferrous sulfate heptahydrate, and ferrous sulfate monohydrate; Alternatively, the sodium source may be one or more of sodium carbonate and sodium sulfate.

10. The method for preparing the sodium ferric sulfate composite material as described in claim 8, characterized in that, The ball milling process uses a planetary ball mill for cyclic vacuum ball milling. The cyclic process of vacuum ball milling is as follows: first rotate clockwise for 30 minutes, then counterclockwise for 30 minutes, and stop ball milling for 10 minutes after counterclockwise rotation; repeat the above operation until the total ball milling time reaches 12 hours.

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