Preparation and purification method of sodium bisoxalate borate and application thereof in secondary battery

Abstract

The application provides a preparation and purification method of sodium bisoxalate borate, which comprises the following steps: firstly, mixing oxalic acid dihydrate, boric acid and sodium compounds, ball milling treatment with acetone as a grinding solvent to obtain sodium bisoxalate borate crude product; secondly, dissolving the crude product in water, adjusting the pH of the solution, evaporating and crystallizing, washing the product with hot water and performing suction filtration to obtain a filter cake; thirdly, adding the filter cake into a good solvent to filter insoluble substances to obtain a filtrate; performing reduced pressure distillation on the filtrate to obtain a concentrated solution; finally, adding the concentrated solution into a poor solvent to precipitate crystals, washing and drying to obtain high-purity sodium bisoxalate borate. The application also provides an electrolyte taking the above sodium bisoxalate borate as a main salt, and a sodium ion secondary battery containing the electrolyte. The sodium bisoxalate borate prepared by the application has high purity, good crystal structure and excellent solubility in an organic solvent, can improve the ionic conductivity and electrochemical performance of the electrolyte, and further improves the cycle performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical materials, specifically relating to a method for preparing and purifying sodium bis(oxalate-borate), an electrolyte prepared using sodium bis(oxalate-borate) as the main salt, and the application of the electrolyte in sodium-ion secondary batteries. Background Technology

[0002] Sodium-ion batteries (NIBs) are among the most promising reusable batteries and other stationary energy storage systems currently available. Although still in the basic research stage, sodium-ion batteries offer advantages over lithium-ion batteries in terms of lower cost and greater durability, and are expected to have a brighter future in the energy storage field. To achieve better performance, sodium-ion batteries need to meet the prerequisites of providing high energy density and high power density. Currently, the most commonly used electrolytes for NIBs are NaClO4 or NaPF6. Other salts used for sodium electrolytes are also rare, such as NaN(CF3CO2)2, Na(CH3)C6H4SO3, NaTDI, and NaPDI. However, these sodium salts have drawbacks to varying degrees. For example, NaN(CF3CO2)2 has problems with aluminum corrosion; NaClO4 is potentially explosive, unsafe, and difficult to dry; at room temperature, NaTDI and NaPDI have lower electrical conductivity in PC than NaClO4 and NaPF6; NaPF6 and NaBF4 have poor compatibility with cathode materials, low thermal stability, are sensitive to moisture, and generate highly corrosive HF.

[0003] Dioxalate borate has only recently been studied in sodium-ion batteries. One method for preparing sodium bisoxalate borate involves reacting oxalic acid, boric acid, and sodium salt in a liquid phase. For example, CN115557980B discloses a synthesis and purification process for sodium bisoxalate borate: oxalic acid, boric acid, and sodium source are dissolved in a certain amount of deionized water to obtain corresponding solutions. The reaction is carried out under heating conditions in a water bath. The product is filtered and dried to obtain crude sodium bisoxalate borate. Finally, the crude product is recrystallized in water and vacuum dried to obtain the pure product. This method yields high-purity sodium bisoxalate borate through cooling crystallization in water; however, this purification method alters the crystal structure of sodium bisoxalate borate, resulting in decreased solubility.

[0004] On the other hand, sodium bis(oxalateborate) is obtained from oxalic acid, boric acid, and sodium salts via a solid-solid reaction. For example, Lanzhou University of Technology (CN110305153A) disclosed a method for synthesizing sodium bis(oxalateborate) and its application: A compound of boric acid, oxalic acid, and sodium is dried at high temperature, mixed, and compressed into tablets. A solid-solid reaction is then carried out at high temperature to obtain a white solid. The white solid is then dried, dissolved in acetonitrile or dimethyl carbonate, filtered, and the solvent is evaporated to obtain the final product, sodium bis(oxalateborate). This method uses a tableting process, resulting in insufficient contact between the raw materials and a low conversion rate. Furthermore, acetonitrile and dimethyl carbonate have low solubility for sodium bis(oxalateborate), making them unsuitable for purification.

[0005] Based on this, a method for preparing high-purity sodium bis(oxalatoborate) with a suitable crystal structure and excellent solubility in organic solvents is provided. This method can not only improve the ionic conductivity and electrochemical performance of the electrolyte, but also help improve the cycle performance of the battery. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing and purifying sodium bis(oxalatoborate) with high purity, good crystal structure, and high solubility in organic solvents.

[0007] The second objective of this invention is to provide an electrolyte prepared with sodium bis(oxalateborate) as the main salt.

[0008] The third objective of this invention is to provide an application of an electrolyte prepared with sodium bis(oxalate-borate) as the main salt in sodium-ion secondary batteries.

[0009] One of the technical solutions adopted to achieve the objective of this invention is: to provide a method for preparing and purifying sodium bis(oxalateborate), comprising the following steps:

[0010] S1. Mix oxalic acid dihydrate, boric acid, and sodium in a certain proportion, and ball mill them with acetone as a grinding aid to obtain crude sodium bis(oxalate-borate).

[0011] S2. Dissolve crude sodium bis(oxalate)borate in water, adjust the pH of the solution to neutral, and heat to evaporate and crystallize; wash the evaporated crystallized product in hot water, filter it while hot, and obtain filter cake;

[0012] S3. Add the filter cake to a good solvent and heat to dissolve it. Filter out the insoluble substances to obtain the filtrate. Distill the filtrate under reduced pressure to obtain the concentrated solution.

[0013] S4. Add the concentrated solution to a poor solvent to obtain precipitated crystals; wash and vacuum dry the precipitated crystals to obtain high-purity sodium bis(oxalate-borate).

[0014] The general idea of ​​the preparation and purification method of sodium bis(oxalate-borate) provided by this invention is as follows:

[0015] First, ball milling is used to ensure the complete reaction of oxalic acid dihydrate, boric acid, and sodium. Acetone is used as a grinding aid during ball milling, which not only prevents the material from separating from the steel balls during the milling process but also ensures more uniform mixing of the material in the solvent, resulting in a more complete reaction. Furthermore, acetone has low oxygen reactivity, which will not adversely affect the reaction process, and its volatility makes it easy to remove after the reaction. Second, the pH of the crude sodium bis(oxalate-borate) solution is adjusted to neutralize any unreacted free acid after the reaction. Impurities formed during neutralization are more easily washed away with water in the next step. Finally, the product is first added to a good solvent to remove inorganic impurities that were not completely removed by water. After vacuum distillation, crystals are precipitated using a poor solvent to further purify the product, thereby obtaining a high-purity sodium bis(oxalate-borate) product. The sodium bis(oxalateborate) prepared by this invention has high purity and excellent solubility in organic solvents such as trimethyl phosphate. Electrolytes prepared with these two as the main salt and solvent, respectively, have higher ionic conductivity and electrochemical performance, which helps to improve the cycle performance of the battery.

[0016] Further, in step S1, the sodium compound includes one or more combinations of sodium acetate, sodium hydroxide, sodium oxide, sodium carbonate, sodium nitrate, sodium oxalate, sodium chloride, and sodium sulfate.

[0017] Furthermore, in step S1, the molar ratio of the compounds of oxalic acid dihydrate, boric acid, and sodium is 2:1:(0.5-1).

[0018] Furthermore, the ball milling process employs a high-energy ball mill. Compared to other types of ball mills, the high-energy ball mill generates more intense collisions during milling, resulting in finer and more uniform material grinding. It also generates more heat, which is more conducive to the reaction. Preferably, the grinding media used in the ball milling process are stainless steel balls.

[0019] Furthermore, the ball milling process is performed at a rotational speed of 30–50 Hz for 6–12 hours. During ball milling, the volume ratio of the total material volume (reactant + steel balls) to acetone is 1:(1–1.1), and the total material volume is less than 2 / 3 of the volume of the ball mill jar. After ball milling, the acetone solvent can be removed by forced-air drying, followed by sieving of the steel balls and product, and grinding to obtain crude sodium bis(oxalate-borate).

[0020] Preferably, in step S1, the oxalic acid dihydrate and boric acid in the raw materials can be pre-treated by ball milling with a portion of acetone solvent for 1-3 hours. Then, a sodium compound and the remaining acetone solvent are added to the pre-treated product, and ball milling is continued for 6-12 hours to obtain the product. This operation avoids the strong alkaline sodium hydroxide from destroying the cyclic structure of bis(oxalic acid-boric acid) and generating other byproducts.

[0021] Further, in step S2, the pH of the solution is adjusted using a sodium hydroxide solution with a concentration of 0.5 to 1 mol / L; preferably, the mass ratio of the evaporated crystallization product to hot water is 1:(5 to 10).

[0022] In step S2 of this invention, the product of evaporation and crystallization is washed with hot water, which can improve the solubility of impurities and accelerate the dissolution rate of impurities. Hot filtration can prevent impurities from recrystallizing and precipitating after the water cools down, thus affecting the purity of the product. Preferably, the temperature of the hot water is 60-80°C, and the washing time of the evaporation and crystallization product in the hot water is 2-5 minutes. Preferably, the temperature of evaporation and crystallization is 100-110°C.

[0023] Further, in step S3, the benign solvent is selected from one or more combinations of trimethyl phosphate, triethyl phosphate, and N-methylpyrrolidone.

[0024] Furthermore, in step S3, the temperature for heating and dissolving is 60–80°C, and the temperature for vacuum distillation is 80–90°C.

[0025] Further, in step S4, the undesirable solvent is selected from one or more combinations of dichloromethane, dichloroethane, acetonitrile, ethylene carbonate, propylene carbonate, and dimethyl carbonate.

[0026] In steps S3 and S4 of this invention, a concentrated solution is first obtained by vacuum distillation, and then treated with a poor solvent, which can save the amount of poor solvent used. The poor solvent used is a volatile solvent, which has the advantages of being separable, easy to recover and reuse.

[0027] Preferably, in step S4, the precipitated crystals are washed multiple times with a poor solvent, centrifuged and dried to remove the poor solvent, and then vacuum dried.

[0028] Furthermore, the vacuum degree of the vacuum drying is 0 to -0.1 MPa, the vacuum drying temperature is 80 to 100°C, and the vacuum drying time is 8 to 12 hours.

[0029] Preferably, the vacuum drying equipment contains a desiccant, which includes one or more of calcium sulfate, calcium chloride, silica gel, and activated alumina.

[0030] The technical solution adopted to achieve the second objective of this invention is: to provide an electrolyte comprising a sodium salt, an organic solvent, and additives; wherein the sodium salt is sodium bis(oxalate-borate) prepared and purified by the method described in the first objective of this invention.

[0031] Further, the organic solvent includes one or more combinations of trimethyl phosphate, triethyl phosphate, N-methylpyrrolidone, and dimethyl sulfoxide. The above organic solvents can meet the dissolution requirements of sodium bis(oxalato)borate. Preferably, trimethyl phosphate is used as the organic solvent, as its solubility for sodium bis(oxalato)borate is higher than that of triethyl phosphate, and its electrochemical performance is superior to that of N-methylpyrrolidone and dimethyl sulfoxide. Studies have found that using trimethyl phosphate as an organic solvent to dissolve sodium bis(oxalato)borate can achieve the same electrochemical performance as conventional solvents at a concentration of 1 mol / L under low sodium salt treatment conditions (0.38–0.5 mol / L).

[0032] Furthermore, the additive includes one or more combinations of fluoroethylene carbonate, sodium nitrate, vinylene carbonate, 1,4-butane sulfonyl lactone, 2,4-butane sulfonyl lactone, propylene sulfate, and 1,3-propene sulfonate lactone.

[0033] Furthermore, the concentration of the sodium bis(oxalateborate) in the organic solvent is 0.38–1 mol / L.

[0034] The technical solution adopted to achieve the third objective of this invention is to provide a sodium-ion secondary battery, which includes a positive electrode material, a negative electrode material, a separator, and an electrolyte as described in the second objective of this invention.

[0035] Preferably, the separator is a glass fiber separator. Compared with separators made of other materials, glass fiber has better wettability to the electrolyte, and its moderate thickness helps to maintain the electrochemical performance of the battery.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) The present invention provides a method for preparing and purifying sodium bis(oxalateborate), which uses ball milling to achieve full reaction of the material. In the ball milling process, acetone is used as a grinding aid solvent. The addition of acetone can not only prevent the material from separating from the steel balls during the ball milling process, but also make the material mix more evenly in the solvent and the reaction more complete. In addition, the oxygen activity on acetone is poor and will not have an adverse effect on the reaction process. It has the characteristic of being volatile and is easy to remove after the reaction is completed.

[0038] (2) The present invention provides a method for preparing and purifying sodium bis(oxalato)borate, which utilizes unreacted free acid in an alkaline solution, removes soluble impurities in the solution by washing with hot water, and then combines purification with good solvent and crystallization with poor solvent, which significantly improves the purity of the product sodium bis(oxalato)borate and obtains a product with a crystal structure that meets the requirements.

[0039] (3) The sodium bis(oxalate-borate) prepared in this invention has a yield of approximately 83% and a purity >99%. It exhibits excellent solubility in organic solvents, such as trimethyl phosphate. Electrolytes prepared using both as the main salt and solvent, respectively, have higher ionic conductivity and electrochemical performance, which helps improve the cycle performance of the battery. The sodium bis(oxalate-borate) provided by this invention, when used as the main electrolyte salt in the manufacturing process of sodium-ion batteries, can meet the quality and safety requirements of sodium-ion batteries. The sodium-ion batteries produced have the advantages of low cost, good electrochemical performance, and good cycle performance. Attached Figure Description

[0040] Figure 1 The nuclear magnetic resonance boron spectrum of sodium bis(oxalateborate) prepared in Example 1;

[0041] Figure 2 Comparison of XRD patterns of sodium bis(oxalateborate) prepared in Example 1 before and after drying;

[0042] Figure 3 The image shows a comparison of the XRD patterns of sodium bis(oxalateborate) prepared in Comparative Example 1 before and after drying.

[0043] Figure 4 This is a comparison diagram of the solubility of sodium bis(oxalateborate) prepared in trimethyl phosphate between Example 1 and Comparative Example 1.

[0044] Figure 5 The first-cycle charge-discharge curve of sodium vanadium phosphate / sodium coin cell fabricated for Application Example 1;

[0045] Figure 6 Cycle performance graph of sodium vanadium phosphate / sodium coin cell fabricated for Application Example 1;

[0046] Figure 7 The first-cycle charge-discharge curve of sodium vanadium phosphate / hard carbon coin cell fabricated for Application Example 2;

[0047] Figure 8 Cycle performance graph of sodium vanadium phosphate / hard carbon coin cell fabricated for application example 2. Detailed Implementation

[0048] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0049] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0050] The present invention will be further described below with reference to specific embodiments, but these are not intended to limit the scope of the invention.

[0051] Example 1

[0052] This embodiment provides a method for preparing and purifying sodium bis(oxalateborate), including the following steps:

[0053] Step 1: Prepare raw materials according to the stoichiometric ratio of oxalic acid dihydrate: boric acid: sodium hydroxide 2:1:1. Add 12.6g of oxalic acid dihydrate and 3.09g of boric acid to four stainless steel ball mill jars, and then add 30mL of acetone. Use a high-energy ball mill for 2 hours at a ball milling frequency of 40.69Hz. Then add 2g of sodium hydroxide and 10mL of acetone to each jar and continue high-energy ball milling for 12 hours. After ball milling, place the ball mill jars in an 80℃ forced-air drying oven to dry the acetone. Sieve the stainless steel beads and the product. Grind the product into powder using a mortar and pestle. Place the powder in an 80℃ vacuum drying oven for 12 hours to obtain crude sodium dioxalate borate.

[0054] Step 2: Measure 400 mL of deionized water, heat it to 80 °C, dissolve the crude sodium bis(oxalate-borate) in the deionized water, monitor the pH value using pH paper, slowly add sodium hydroxide to neutralize the aqueous solution, and then heat to 100-110 °C to evaporate and crystallize; place the crystals in 80 °C deionized water and stir for 5 min (the mass ratio of the evaporated crystallization product to the hot water is 1:8). After completion, filter the mixed solution while it is still hot using a sintered glass funnel and retain the filter cake;

[0055] Step 3: Add 310 mL of N-methylpyrrolidone to the filter cake, heat to 60 °C to dissolve, and filter out the insoluble substances; distill the filtrate under reduced pressure at 85 °C until crystals begin to precipitate, then stop distillation and cool to room temperature;

[0056] Step 4: Add 500 mL of dichloromethane to the concentrate until all crystals precipitate; finally, wash the precipitated crystals three times with dichloromethane, centrifuge to remove the solvent, and place them in a 100°C vacuum drying oven containing one cup of calcium chloride to dry for 12 hours to obtain about 35 g of sodium bis(oxalato)borate, with a yield of about 83% and a purity of >99%.

[0057] The NMR boron spectrum of sodium bis(oxalateborate) prepared in this embodiment is shown in the figure. Figure 1 As shown in the figure, the NMR boron spectrum of lithium bis(oxalateborate) is given for reference.

[0058] The XRD pattern of sodium bis(oxalateborate) prepared in this embodiment is shown below. Figure 2 As shown, the diffraction peaks are sharp and there are no obvious impurity phase peaks, indicating that a relatively pure sodium bis(oxalateborate) body was obtained, which can be used as a raw material for secondary battery electrolyte.

[0059] Example 2

[0060] This embodiment provides a method for preparing and purifying sodium bis(oxalateborate), including the following steps:

[0061] Step 1: Prepare raw materials according to the stoichiometric ratio of oxalic acid dihydrate: boric acid: sodium hydroxide 2:1:1. Add 12.6g of oxalic acid dihydrate and 3.09g of boric acid to four stainless steel ball mill jars, and then add 15mL of acetone. Use a high-energy ball mill for 2 hours at a frequency of 30Hz. Then add 2g of sodium hydroxide and 5mL of acetone to each jar and continue high-energy ball milling for 6 hours. After ball milling, place the ball mill jars in an 80℃ forced-air drying oven to dry the acetone. Sieve the stainless steel beads and the product. Grind the product into powder using a mortar and pestle. Place the powder in an 80℃ vacuum drying oven for 12 hours to obtain crude sodium dioxalate borate.

[0062] Step 2: Measure 300 mL of deionized water, heat it to 80 °C, dissolve the crude sodium bis(oxalate-borate) in the deionized water, monitor the pH value using pH paper, slowly add sodium hydroxide to neutralize the aqueous solution, and then heat to 100-110 °C to evaporate and crystallize; place the crystals in 60 °C deionized water and stir for 5 min (the mass ratio of the evaporated crystallization product to the hot water is 1:10). After completion, filter the mixed solution while it is still hot using a sintered glass funnel and retain the filter cake;

[0063] Step 3: Add 500 mL of trimethyl phosphate to the filter cake, heat to 60 °C to dissolve, and filter out the insoluble substances; distill the filtrate under reduced pressure at 80 °C until crystals begin to precipitate, then stop distillation and cool to room temperature;

[0064] Step 4: Add 500 mL of dichloroethane to the concentrate until all crystals precipitate; finally, wash the precipitated crystals three times with dichloromethane, centrifuge to remove the solvent, and place them in an 80°C vacuum drying oven containing one cup of calcium chloride to dry for 8 hours to obtain sodium bis(oxalateborate) product with a purity >99%.

[0065] Example 3

[0066] This embodiment provides a method for preparing and purifying sodium bis(oxalateborate), including the following steps:

[0067] Step 1: Prepare raw materials according to the stoichiometric ratio of oxalic acid dihydrate: boric acid: sodium hydroxide 2:1:1. Add 12.6g of oxalic acid dihydrate and 3.09g of boric acid to four stainless steel ball mill jars, and then add 30mL of acetone. Use a high-energy ball mill for 2 hours at a frequency of 50Hz. Then add 2g of sodium hydroxide and 10mL of acetone to each jar and continue high-energy ball milling for 8 hours. After ball milling, place the ball mill jars in an 80℃ forced-air drying oven to dry the acetone. Sieve the stainless steel beads and the product. Grind the product into powder using a mortar and pestle. Place the powder in an 80℃ vacuum drying oven for 12 hours to obtain crude sodium dioxalate borate.

[0068] Step 2: Measure 400 mL of deionized water, heat it to 80 °C, dissolve the crude sodium bis(oxalate-borate) in the deionized water, monitor the pH value using pH paper, slowly add sodium hydroxide to neutralize the aqueous solution, and then heat to 100-110 °C to evaporate and crystallize; place the crystals in 70 °C deionized water and stir for 5 min (the mass ratio of the evaporated crystallization product to the hot water is 1:10). After completion, filter the mixed solution while it is still hot using a sintered glass funnel and retain the filter cake;

[0069] Step 3: Add the filter cake to 530 mL of triethyl phosphate, heat to 60 °C to dissolve, and filter out the insoluble substances; distill the filtrate under reduced pressure at 85 °C until crystals begin to precipitate, then stop distillation and cool to room temperature;

[0070] Step 4: Add 500 mL of dimethyl carbonate to the concentrate until all crystals precipitate; finally, wash the precipitated crystals three times with dichloromethane, centrifuge to remove the solvent, and place them in a 100°C vacuum drying oven containing one cup of calcium chloride to dry for 12 hours to obtain sodium bis(oxalate)borate product with a purity >99%.

[0071] Example 4

[0072] This embodiment provides a method for preparing and purifying sodium bis(oxalateborate), including the following steps:

[0073] Step 1: Prepare raw materials according to the stoichiometric ratio of oxalic acid dihydrate: boric acid: sodium carbonate 2:1:0.5. Add 12.6g of oxalic acid dihydrate and 3.09g of boric acid to four stainless steel ball mill jars, and then add 30mL of acetone. Use a high-energy ball mill for 2 hours at a ball milling frequency of 40.69Hz. Then add 1g of sodium carbonate and 10mL of acetone to each jar and continue high-energy ball milling for 12 hours. After ball milling, place the ball mill jars in an 80℃ forced-air drying oven to dry the acetone. Sieve the stainless steel beads and the product. Grind the product into powder using a mortar and pestle. Place the powder in an 80℃ vacuum drying oven for 12 hours to obtain crude sodium dioxalate borate.

[0074] Step 2: Measure 400 mL of deionized water, heat it to 80 °C, dissolve the crude sodium bis(oxalate-borate) in the deionized water, monitor the pH value using pH paper, slowly add sodium hydroxide to neutralize the aqueous solution, and then heat to 100-110 °C to evaporate and crystallize; place the crystals in 80 °C deionized water and stir for 5 min (the mass ratio of the evaporated crystallization product to the hot water is 1:5). After completion, filter the mixed solution while it is still hot using a sintered glass funnel and retain the filter cake;

[0075] Step 3: Add 310 mL of N-methylpyrrolidone to the filter cake, heat to 60 °C to dissolve, and filter out the insoluble substances; distill the filtrate under reduced pressure at 85 °C until crystals begin to precipitate, then stop distillation and cool to room temperature;

[0076] Step 4: Add 500 mL of dichloromethane to the concentrate until all crystals precipitate; finally, wash the precipitated crystals three times with dichloromethane, centrifuge to remove the solvent, and place them in a 90°C vacuum drying oven containing one cup of calcium chloride to dry for 12 hours to obtain sodium bis(oxalateborate) product with a purity >99%.

[0077] Example 5

[0078] This embodiment provides a method for preparing and purifying sodium bis(oxalateborate), including the following steps:

[0079] Step 1: Prepare raw materials according to the stoichiometric ratio of oxalic acid dihydrate: boric acid: sodium chloride 2:1:1. Add 12.6g of oxalic acid dihydrate and 3.09g of boric acid to four stainless steel ball mill jars, and then add 30mL of acetone. Use a high-energy ball mill for 2 hours at a ball milling frequency of 40.69Hz. Then add 2g of sodium chloride and 10mL of acetone to each jar and continue high-energy ball milling for 12 hours. After ball milling, place the ball mill jars in an 80℃ forced-air drying oven to dry the acetone. Sieve the stainless steel beads and the product. Grind the product into powder using a mortar and pestle. Place the powder in an 80℃ vacuum drying oven for 12 hours to obtain crude sodium dioxalate borate.

[0080] Step 2: Measure 400 mL of deionized water, heat it to 80 °C, dissolve the crude sodium bis(oxalate-borate) in the deionized water, monitor the pH value using pH paper, slowly add sodium hydroxide to neutralize the aqueous solution, and then heat to 100-110 °C to evaporate and crystallize; place the crystals in 80 °C deionized water and stir for 5 min (the mass ratio of the evaporated crystallization product to the hot water is 1:8). After completion, filter the mixed solution while it is still hot using a sintered glass funnel and retain the filter cake;

[0081] Step 3: Add 310 mL of N-methylpyrrolidone to the filter cake, heat to 60 °C to dissolve, and filter out the insoluble substances; distill the filtrate under reduced pressure at 90 °C until crystals begin to precipitate, then stop distillation and cool to room temperature;

[0082] Step 4: Add 500 mL of dichloromethane to the concentrate until all crystals precipitate; finally, wash the precipitated crystals three times with dichloromethane, centrifuge to remove the solvent, and place them in a 100°C vacuum drying oven containing one cup of calcium chloride to dry for 12 hours to obtain sodium bis(oxalateborate) product with a purity >99%.

[0083] Comparative Example 1

[0084] This comparative example uses a solution method and combines cooling crystallization in water to obtain sodium bis(oxalato)borate, including the following steps:

[0085] Step 1: Weigh 37.8g of oxalic acid dihydrate and dissolve it in 100mL of deionized water to obtain an oxalic acid aqueous solution; dissolve 9.27g of boric acid in 50mL of deionized water to obtain a boric acid aqueous solution; and dissolve 6g of sodium hydroxide in 20mL of deionized water to obtain a sodium hydroxide aqueous solution.

[0086] Step 2: After completely dissolving the weighed boric acid and oxalic acid dihydrate in deionized water, slowly add sodium hydroxide solution dropwise. After the addition is complete, heat the mixed solution in an oil bath at 80°C for 8 hours.

[0087] Step 3: Stop heating the liquid after the reaction is complete, cool it to room temperature in the air, and then place it in a 5°C refrigerator to allow it to cool and crystallize for 24 hours. After discarding the supernatant, white crystals are obtained.

[0088] Step 4: The obtained white crystals were heated in a vacuum drying oven at 160℃ for 12 hours until completely dried, yielding approximately 16.8g of sodium bis(oxalato)borate, with a first-time conversion rate of approximately 54%.

[0089] The XRD pattern of sodium bis(oxalateborate) prepared in this comparative example is shown below. Figure 3 As shown, by Figure 3 It can be seen that the sodium bis(oxalateborate) purified by water cooling recrystallization in the comparative example does not meet the requirements in terms of crystal structure, whether it is freshly crystallized or after drying. Figure 2 The crystal structure of the product obtained in the examples remained consistent before and after drying, meeting the requirements for high purity preparation.

[0090] Sodium bis(oxalate-borate) prepared in Example 1 and the comparative example were added to trimethyl phosphate (the dissolution test temperature was heated from room temperature to 60°C, and the concentration of sodium bis(oxalate-borate) in trimethyl phosphate was 0.4 mol / L). Their solubility in organic solvents was tested, and the test results are shown below. Figure 4 .Depend on Figure 4 It can be seen that, under the same concentration conditions, the sodium bis(oxalate-borate) prepared in Example 1 was completely dissolved, while the sodium bis(oxalate-borate) prepared in Comparative Example 1 could not achieve a good dissolution effect even under heating conditions.

[0091] Application Example 1

[0092] The electrolyte was prepared using sodium bis(oxalate-borate) as the main salt, as prepared in Example 1. The preparation process was carried out in an argon-atmosphere glove box (moisture content < 1 ppm). The electrolyte included sodium bis(oxalate-borate), a salt solvent, and a film-forming additive. The concentration of sodium bis(oxalate-borate) in the salt solvent was 0.4 mol / L, the salt solvent was trimethyl phosphate, and the film-forming additive was fluoroethylene carbonate, added at 2% of the total electrolyte volume.

[0093] The above electrolyte was used in the fabrication of sodium vanadium phosphate / sodium coin cells. The positive electrode was sodium vanadium phosphate, the negative electrode was a sodium sheet, and the separator was made of glass fiber (Whatman GF / F). The cycle performance of the sodium vanadium phosphate / sodium coin cells fabricated in this application example is as follows: Figure 5 The charge / discharge curve for the first week is as follows: Figure 6 The battery underwent 0.5C charge-discharge cycling in the first week, followed by 1C current density cycling. The discharge specific capacity in the first week was 110.9 mAh / g, with a coulombic efficiency of 99.7%. The average coulombic efficiency over 1000 cycles was 99.4%, and the discharge specific capacity at 1000 cycles was still 86.4 mAh / g, demonstrating a capacity retention of 77.9%. This exhibits excellent battery cycle performance.

[0094] Application Example 2

[0095] The electrolyte was prepared using sodium bis(oxalate-borate) as the main salt, as prepared in Example 1. The preparation process was carried out in an argon-atmosphere glove box (moisture content < 1 ppm). The electrolyte included sodium bis(oxalate-borate), a salt solvent, and a film-forming additive. The concentration of sodium bis(oxalate-borate) in the salt solvent was 0.4 mol / L, and the salt solvent used was trimethyl phosphate. The film-forming additive consisted of fluoroethylene carbonate, sodium nitrate, and vinylene carbonate, with each of the three components added accounting for 2% of the total electrolyte volume.

[0096] The above electrolyte was used in the fabrication of sodium vanadium phosphate / hard carbon coin cells. The positive electrode was sodium vanadium phosphate, the negative electrode was hard carbon, and the separator was made of glass fiber (Whatman GF / F). The cycle performance of the sodium vanadium phosphate / hard carbon coin cell fabricated in this application example is as follows: Figure 7 The charge / discharge curve for the first week is as follows: Figure 8 The first week involved 0.5C charge-discharge cycling, followed by cycling at a 1C current density. The discharge specific capacity in the first week was 78.55 mAh / g, with a coulombic efficiency of 68.3%. The average coulombic efficiency over 1000 cycles was 99.8%, and the discharge specific capacity at 1000 cycles was 44.75 mAh / g, with a capacity retention of 57%. This demonstrates excellent electrochemical performance.

[0097] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the content of this specification should be included within the protection scope of the present invention.

Claims

1. An electrolyte comprising a sodium salt, an organic solvent, and an additive; characterized in that, The sodium salt is sodium bis(oxalato)borate; the organic solvent is trimethyl phosphate; the additives include one or more combinations of fluoroethylene carbonate, sodium nitrate, vinylene carbonate, 1,4-butane sulpholactone, 2,4-butane sulpholactone, propylene sulfate, and 1,3-propene sulpholactone; the concentration of sodium bis(oxalato)borate in the organic solvent is 0.38~0.5 mol / L; The method for preparing and purifying sodium bis(oxalate-borate) includes the following steps: S1. Mix oxalic acid dihydrate, boric acid, and sodium in a certain proportion, and ball mill them with acetone as a grinding aid to obtain crude sodium bis(oxalate-borate). S2. Dissolve crude sodium bis(oxalate)borate in water, adjust the pH of the solution to neutral, and heat to evaporate and crystallize; wash the evaporated crystallized product in hot water, filter it while hot, and obtain filter cake; S3. Add the filter cake to a good solvent and heat to dissolve it. Filter out the insoluble substances to obtain a filtrate. Distill the filtrate under reduced pressure to obtain a concentrated solution. The good solvent is selected from one or more combinations of trimethyl phosphate, triethyl phosphate, and N-methylpyrrolidone. S4. Add the concentrated solution to a poor solvent to obtain precipitated crystals; wash and vacuum dry the precipitated crystals to obtain high-purity sodium bis(oxalate-borate).

2. The electrolyte according to claim 1, characterized in that, In step S1, the sodium compound includes one or more combinations of sodium acetate, sodium hydroxide, sodium oxide, sodium carbonate, sodium nitrate, sodium oxalate, sodium chloride, and sodium sulfate.

3. The electrolyte according to claim 1, characterized in that, In step S1, the molar ratio of the compounds of oxalic acid dihydrate, boric acid, and sodium is 2:1:(0.5~1).

4. The electrolyte according to claim 1, characterized in that, In step S1, the rotational speed of the ball milling process is 30~50Hz, and the ball milling time is 6~12h.

5. The electrolyte according to claim 1, characterized in that, In step S2, the pH of the solution is adjusted using a sodium hydroxide solution with a concentration of 0.5~1 mol / L.

6. The electrolyte according to claim 1, characterized in that, In step S4, the undesirable solvent is selected from one or more combinations of dichloromethane, dichloroethane, ethylene carbonate, propylene carbonate, and dimethyl carbonate.

7. A sodium-ion secondary battery, characterized in that, It includes a positive electrode material, a negative electrode material, a separator, and an electrolyte according to any one of claims 1-6.

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