Crystal form of lithium ion battery additive ethylene trisulfate and preparation method thereof

Abstract

The invention belongs to the technical field of lithium ion battery materials, and particularly relates to a crystal form of a lithium ion battery additive ethylene trisulfate and a preparation method of the crystal form. An X-ray powder diffraction pattern of the crystal form contains 2theta values as follows: 11.5 + / -0.2 degrees, 17.7 + / -0.2 degrees, 19.8 + / -0.2 degrees, 20.2 + / -0.2 degrees, 20.7 + / -0.2 degrees, 21.5 + / -0.2 degrees and 24.7 + / -0.2 degrees; the crystal form provided by the invention is large in particle size, easy to remove water, capable of obtaining a product with low water content, high in stability, controllable in crystal form, low in requirements on storage conditions and capable of being stably stored, and the transportation cost and the production cost are saved; the use effect of the triethylene sulfate (TriDTD) as a battery additive can be enhanced, and the triethylene sulfate has relatively high cycle capacity retention rate and high-temperature storage capacity retention rate and relatively low internal resistance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a crystal form of a lithium ion battery additive ethylene trisulfate and a preparation method thereof. BACKGROUND

[0002] At present, most electric vehicles choose ternary materials or lithium iron phosphate materials as the positive electrode, and with the continuous improvement of cell assembly technology and battery management system, lithium ion batteries have been greatly improved in safety and electrochemical performance, so that the application prospect of lithium ion batteries in the field of electric vehicles has been further expanded. Considering the actual use conditions of vehicles, such as high-temperature exposure or use in high-latitude areas in winter, on the one hand, the vehicle battery has very high requirements for the endurance, and on the other hand, the performance of the battery under high-temperature and low-temperature conditions is also strict. The pursuit of the comprehensive performance of the power battery at high and low temperatures by the vehicle manufacturer, the ternary battery system also highlights the advantages more and more.

[0003] The electrolyte is the main medium for lithium ion conduction in the battery system, and has a great influence on the comprehensive performance of the battery. Compared with changing the battery structure design, adding functional additives to the electrolyte to improve the performance of the battery at high temperature, low temperature and long cycle is undoubtedly one of the most efficient ways to improve the comprehensive performance of the battery.

[0004] Organic cyclic sulfate additives are an important class of film-forming additives that can inhibit the decline in initial capacity, increase the initial discharge capacity, improve the high and low temperature cycle performance, reduce the battery expansion after high temperature storage, and improve the charge and discharge performance and cycle times of the battery.

[0005] Sulfate additives include vinyl sulfate (DTD), propylene sulfate (PS), bisulfate (BiDTD), and tri-sulfate (TriDTD), and their structural formulas are as follows: (DTD), (PS), (BiDTD), (TriDTD).

[0006] The sulfuric ester compound with multi-ring connection has a large molecular weight, and the additive can form an SEI layer at the anode surface or the cathode surface, reduce the battery swelling after high-temperature storage, and improve the discharge capacity at low temperature. However, the traditional sulfuric ester additive has its inherent defects, such as easy water absorption and hydrolysis in air, instability to heat, discoloration at room temperature, storage temperature of 2-8℃, and cold chain logistics, which brings many inconveniences to industrial production and transportation. It is particularly important to prepare a multi-ring connected sulfuric ester additive which can not only improve the battery performance well, but also has good thermal stability. In addition, there is still a need in the art to develop battery additives with better use effect to further improve the cycle capacity retention rate and high-temperature storage capacity retention rate of the battery, and reduce the internal resistance. SUMMARY

[0007] In view of the problems existing in the prior art, the purpose of the present application is to provide a crystal form of a lithium ion battery additive triethylene trisulfate (TriDTD) and a preparation method thereof. The crystal form is a stable crystal form, has large particle size and is easy to remove water, can obtain a product with low water content, has high stability, the crystal form is controllable, has low requirements on storage conditions, can be stored stably, saves transportation cost and production cost, can enhance the use effect of triethylene trisulfate (TriDTD) as a battery additive, has high cycle capacity retention rate and high-temperature storage capacity retention rate, and low internal resistance.

[0008] The present application provides a new crystal form A of a lithium ion battery additive triethylene trisulfate, which contains 2θ values of 11.5±0.2°, 17.7±0.2°, 19.8±0.2°, 20.2±0.2°, 20.7±0.2°, 21.5±0.2°, and 24.7±0.2° in the X-ray powder diffraction pattern; and the structural formula of the triethylene trisulfate is as follows: .

[0009] Triethylene trisulfate (TriDTD), as an important kind of sulfuric ester compound, has the following structural formula: , code: HF03, the addition of vinyl sulfate and bis-vinyl sulfate in ternary lithium battery can obviously inhibit the increase of impedance during high temperature storage process, the improvement effect of BiDTD is better than that of DTD. Tri-vinyl sulfate is connected with three sulfate rings, has larger molecular weight, helps to build more stable interface film, can effectively inhibit the continuous decomposition of electrolyte. At the same time, the introduction of more S heteroatoms can improve the ionic conductivity of SEI film and reduce the interface impedance, which will be beneficial to the improvement of the comprehensive electrochemical performance of the battery. Compared with DTD and BiDTD, the improvement effect of tri-vinyl sulfate (TriDTD) on impedance is better, and it has better effect in high and low temperature storage and cycling, which is mainly due to the three-ring structure of tri-vinyl sulfate, which increases the activity of the four tertiary carbon atoms shared, has lower reduction potential and is more likely to participate in the reaction, and the content of film-forming heteroatoms is higher, the ionic conductivity is better, so the electrochemical performance is better. Tri-vinyl sulfate can generate stable SEI film on the surface of graphite negative electrode, inhibit solvent decomposition and reduce the impedance of the battery. At the same time, compared with DTD and BiDTD, tri-vinyl sulfate can greatly improve the high and low temperature performance of the battery, and it has better electrochemical performance in the temperature range of-20~60℃. This shows that tri-vinyl sulfate has great potential to improve the application range of lithium ion battery.

[0010] The sulfate compounds mainly include tri-vinyl sulfate (TriDTD), bis-vinyl sulfate (BiDTD), vinyl sulfate (DTD), propylene sulfate (PS) and the like. The present application research finds that the crystal form has a significant influence on the stability of the tri-vinyl sulfate product. The prior art has few records on the preparation method of tri-vinyl sulfate (TriDTD), and the prepared tri-vinyl sulfate product has fine crystals and large specific surface area. When contacting with moisture, the stability of the product is greatly increased. During packaging, even if the vacuum extraction and inert gas replacement operations are increased, it is also difficult to completely replace the moisture between the solids, so low temperature (2~8℃) storage is required, which increases the difficulty of packaging and storage and shortens the effective period.

[0011] The present application research finds that the reasons affecting the stability of tri-vinyl sulfate (TriDTD) mainly include the following points: (1) Moisture residue and heat sensitivity: the water washing step in the product refining process leads to high water content, and the inherent thermal instability makes it easy to cause decomposition by using conventional or high temperature dehydration methods, so that the water is difficult to be completely removed.

[0012] (2) Crystal defects and mixing: the conventional preparation process is easy to obtain mixed crystal forms, and the crystal structure has defects, so that the chemical stability, especially the stability under high temperature and high humidity conditions, is far inferior to that of single crystal product.

[0013] (3) The whole process of thermal history: the compound is sensitive to heat, not only the drying temperature, including the whole process of heat history of reaction, crystallization, centrifugation will accumulate instability in the product, accelerate its decomposition during storage.

[0014] (4) Acid-base impurities catalysis: residual trace amounts of acidic or basic impurities can be used as a high-efficiency catalyst to induce and continuously accelerate the hydrolysis and ring-opening decomposition reaction of the compound.

[0015] (5) Metal ion synergistic catalysis: trace amounts of transition metal ions (such as Fe 3+ , Cu 2+ , residual metal catalysts) are strong Lewis acids, which can produce a synergistic effect with trace amounts of moisture, significantly reduce the decomposition reaction energy barrier, and sharply accelerate the decomposition.

[0016] The present application provides a new crystal form A of tri-sulfuric acid ethylene ester (TriDTD) through a large number of researches, the crystal structure is compact, has a smaller specific surface area, the surface area in contact with moisture is smaller, the product has higher stability, can avoid severe storage conditions, and reduces the cost of storage, transportation and use.

[0017] Preferably, the melting point of the crystal form A is 189.34-197.34℃, which is about 5℃ higher than the melting point of tri-sulfuric acid ethylene ester prepared by the existing process, has higher stability, and the decomposition temperature is 236.74-244.39℃; the particle size distribution peak is narrow, the crystal morphology is good, the crystal is uniform, the specific surface area is small, and the stability is better.

[0018] The present application also provides a preparation method of the crystal form A, comprising the following steps: (1) adding a solvent in D-mannitol, adding cesium carbonate, adding 1-(fluorosulfonyl)-3-methyl-1H-imidazole trifluoromethanesulfonate of formula A, reacting, removing the cesium carbonate after the reaction is completed, adding dilute hydrochloric acid dropwise, adjusting the pH to 5-6, stirring, filtering, and drying to obtain a crude tri-sulfuric acid ethylene ester, and the reaction equation is as follows: ; (2) warming the crude tri-sulfuric acid ethylene ester with a solvent, stirring until clear, then decolorizing and adsorbing metal impurities with activated carbon and mercaptosilica gel, then cooling to 12-25℃, adding a poor solvent under stirring, finally cooling to 10±5℃ for crystallization, filtering, and drying to obtain the crystal form A.

[0019] Preferably, in step (1), the solvent is acetonitrile, and the amount of acetonitrile is 8-15 times the mass of D-mannitol.

[0020] Preferably, in step (1), the molar ratio of D-mannitol: cesium carbonate: compound A is 1.0: 1.5~3.0: 3.0~5.0.

[0021] Preferably, in step (1), the reaction conditions are: under nitrogen protection, the reaction temperature is 20~30℃, and the reaction is stirred for 1~2 hours.

[0022] Preferably, in step (1), the concentration of the dilute hydrochloric acid is 5%.

[0023] Preferably, step (1) is as follows: add acetonitrile solvent to D-mannitol, add cesium carbonate, add compound A 1-(fluorosulfonyl)-3-methyl-1H-imidazolium trifluoromethanesulfonate, stir for 1-2 hours under nitrogen protection at a temperature of 20-30°C, after the reaction is complete, filter to remove cesium carbonate, add 5% hydrochloric acid dropwise to adjust the pH to 5-6, stir for 0.5 hours, filter, dry to obtain crude ethylene trisulfate, and the reaction equation is as follows; Preferably, in step (2), the solvent is one or two of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and the amount of the solvent used is 3 to 10 times the volume of crude ethylene trisulfate.

[0024] Preferably, in step (2), the undesirable solvent is one of n-hexane, cyclohexane, and n-heptane, and the amount of the undesirable solvent added is 10 to 30 times the volume of crude ethylene trisulfate.

[0025] Preferably, in step (2), the solvent is heated to 35~45°C and stirred until the solution is clear.

[0026] The above preparation method can produce crystal form A with very high product purity and crystal form purity, good stability, and low water content after drying.

[0027] The crystal form A prepared by this invention can be packaged in conventional packaging (inner layer is a high-density polyethylene drum; outer layer is an aluminum foil bag, vacuum packaged), and can be stored stably for a long time.

[0028] To further improve storage time and test the airtightness of the packaging, color-changing silica gel can be added between the high-density polyethylene drum and the vacuum aluminum foil bag. The addition of color-changing silica gel can absorb small amounts of moisture in the packaging, ensuring the stability and storage period of the product; it can also be used to judge the stability of the product based on the degree of color change of the silica gel when the packaging is damaged.

[0029] Compared with the prior art, the present invention has the following advantages: (1) The new crystal form A of the lithium-ion battery additive triethylene sulfate (TriDTD) provided by the present invention has a single-peak normal distribution in particle size distribution. It is easy to remove water when drying, and can obtain a product with low water content. It has high stability, controllable crystal form, low requirements for storage conditions, and can be stored stably, saving transportation and production costs.

[0030] (2) The crystal form provided by the present invention can enhance the effect of ethylene trisulfate (TriDTD) as a battery additive, and has a high cycle capacity retention rate and high temperature storage capacity retention rate, as well as a low internal resistance.

[0031] (3) The present invention greatly improves the stability of ethylene trisulfate (TriDTD), enabling stable storage and reducing the severity of storage conditions. Existing technologies require refrigeration at 2°C to 8°C and absolutely avoid storage at high temperatures (such as above 20°C), while the present invention, through crystallization control of the product, enables it to be stably stored at room temperature (10 to 30°C) for more than 2 years.

[0032] (4) The crystal form A prepared by the present invention has better performance in high and low temperature storage and charging and discharging processes of batteries. Attached Figure Description

[0033] Figure 1 This is the powder diffraction pattern of crystal form A; Figure 2 This is the powder diffraction pattern of crystal form B; Figure 3 This is a microscope image of crystal form A; Figure 4 This is a microscope image of crystal form B; Figure 5 This is the liquid phase spectrum of crystal form A; Figure 6 This is the DSC / TG spectrum of crystal form A; Figure 7 This is the 1H NMR spectrum of crystal form A; Figure 8 This is the liquid phase spectrum of crystal form B. Detailed Implementation

[0034] The present invention will be further illustrated below with specific embodiments. These embodiments are merely illustrative and do not limit the scope of the invention.

[0035] Example 1 A crystalline form A of ethylene trisulfate (TriDTD), a lithium-ion battery additive, has X-ray powder diffraction patterns containing 2θ values ​​of: 11.5±0.2°, 17.7±0.2°, 19.8±0.2°, 20.2±0.2°, 20.7±0.2°, 21.5±0.2°, and 24.7±0.2°.

[0036] The preparation method includes the following steps: (1) 182.0 g (1.0 mol, 1.0 eq) of D-mannitol was added to a reaction flask, along with 2184 g of acetonitrile solvent, 520 g (1.6 mol, 1.6 eq) of cesium carbonate, and 1004.8 g (3.2 mol, 3.2 eq) of compound A, 1-(fluorosulfonyl)-3-methyl-1H-imidazolium trifluoromethanesulfonate. The mixture was stirred for 1 h under nitrogen protection at 25 °C. After the reaction was complete, the cesium carbonate was removed by filtration, and 5% hydrochloric acid was added dropwise to adjust the pH to between 5 and 6. The mixture was stirred for 0.5 h, filtered, and dried to obtain crude ethylene trisulfate with a yield of 92% and 338.6 g. The reaction equation is as follows: .

[0037] (2) The crude ethylene trisulfate obtained by the above process was heated to 40°C with 1692.8g of dimethyl carbonate and stirred until dissolved. Then, it was decolorized and metal impurities were adsorbed with 0.5% activated carbon and 1% mercaptosilica. The temperature was then lowered to 18°C ​​and 6772g of n-heptane, a poor solvent, was added while stirring. Finally, the temperature was lowered to 10±5°C and kept at the temperature for 1h to crystallize. After filtration and drying, crystal form A was obtained with a yield of 93.5%, a purity of 99.9%, and 316.6g.

[0038] The prepared crystal form A was tested: The powder diffraction pattern of crystal form A is shown below. Figure 1 ,pass Figure 1 It can be seen that the X-ray powder diffraction pattern of crystal form A contains 2θ values ​​of 11.504°, 17.7552°, 19.899°, 20.296°, 20.712°, 21.565°, and 24.723°, indicating a very high crystal form purity. Microscopic photograph of crystal form A is shown below. Figure 3 The crystal form A prepared by the present invention has a good crystal morphology, and the crystal form is rod-shaped crystal with regular crystal structure and uniform texture.

[0039] The liquid phase spectrum of crystal form A is shown below. Figure 5 ,pass Figure 5 It can be seen that the crystal form A product prepared by this invention has high purity, with a liquid phase purity of over 99.99%; The DSC / TG spectrum of crystal form A is shown below. Figure 6 ,passFigure 6 The melting point of crystal form A is 189.34℃~197.34℃, and the decomposition temperature is 236.74℃~244.39℃, indicating that crystal form A has a high melting point and decomposition temperature and is not easily decomposed.

[0040] The 1H NMR spectrum of crystal form A is shown below. Figure 7 , 1H-NMR (600 MHz, CD3CN) δ(ppm): 5.52-5.49 (2H,m), 5.43-5.41 (2H, m), 5.09-5.06 (2H, m), 4.90-4.88 (2H, m).

[0041] Comparative Example 1 A method for preparing crystalline form B of ethylene trisulfate (TriDTD), a lithium-ion battery additive, includes the following steps: (1) 182.0 g (1.0 mol, 1.0 eq) of D-mannitol, 910 g of dimethyl carbonate and 496.0 g (4.0 mol, 4.0 eq) of thionyl chloride were mixed and subjected to esterification reaction at 60 °C. After 5 hours of reaction, the mixture was concentrated to dryness to obtain the intermediate product vinyl trisulfite. 0.5% mol of ruthenium trichloride catalyst was added, and then 3.1 mol of sodium periodate oxidant was added to oxidize the intermediate product to obtain crude vinyl trisulfite (TriDTD); 290.7 g, yield 79.0%; the reaction equation is as follows: .

[0042] (2) The crude ethylene trisulfate was dissolved by stirring with 10 times the volume of solvent acetonitrile. Then, it was decolorized and metal impurities were adsorbed by 0.5% activated carbon and 1% mercaptosilica. Purified water was added dropwise over 2 hours. After the addition was completed, the mixture was kept warm for 1 hour, filtered, and dried to obtain crystal form B with a weight of 247.1 g, a yield of 85%, and a purity of 95.39%.

[0043] During the preparation process, the hydroxyl groups of D-mannitol are replaced by Cl, generating chlorinated impurities. The presence of these impurities affects the purity of the product and makes it difficult to purify, resulting in a purity of less than 99%, which makes it unsuitable for use in lithium battery electrolytes.

[0044] The prepared crystal form B was tested: The powder diffraction pattern of crystal form B is shown below. Figure 2 ,pass Figure 2 It can be seen that the X-ray powder diffraction pattern of crystal form B contains 2θ values ​​of: 6.377°, 12.687°, 17.234°, 19.053°, 20.172°, and 24.301°. Microscopic images of crystal form B prepared in this embodiment are shown below.Figure 4 The photo shows that the tiny crystals aggregate to form small spherical particles with a large specific surface area, making them easy to decompose upon contact with moisture in the air. The liquid phase spectrum of crystal form B is shown below. Figure 8 ,pass Figure 8 It can be seen that the purity of crystal form B prepared in this comparative example is 95.39%; Tests showed that crystal form B has a melting point of 186.5℃~190.5℃, which is low, and a decomposition temperature of 230.51℃~237.26℃.

[0045] Performance testing: I. Storage Stability The crystal form A prepared in Example 1 and the crystal form B prepared in Comparative Example 1 were tested, and the test results are shown in Table 1.

[0046] Table 1 Note: The implementation standard is an internal corporate standard.

[0047] 2. Crystal form A and crystal form B are packaged using conventional packaging methods, as follows: the inner layer is a high-density polyethylene drum; the outer layer is an aluminum foil bag, vacuum packaged.

[0048] After packaging, accelerated stability tests were conducted: Crystal form A and crystal form B, packaged according to the above packaging, were subjected to accelerated tests (temperature 40℃±2℃, humidity 75%RH±5%RH, time 6 months, sampling points 1 month, 2 months, 3 months, 6 months), and the results are shown in the table below.

[0049] Table 2. Accelerated stability study of crystal form A Note: The implementation standard is an internal corporate standard.

[0050] Table 3. Accelerated stability study of crystal form B From the tables above, we can see that: (1) By controlling the crystallization conditions, dehydration process and optimizing the drying process, the present invention prepares a new crystal form A of ethylene trisulfate (TriDTD), which has better product purity, lower moisture content and superior product quality.

[0051] (2) The triethylene trisulfate (TriDTD) product prepared by the existing technology in Comparative Example 1 has poor quality. Even after purification, the product purity is only 95.39%, and the storage stability is poor. During the storage process, the acidity and moisture will increase, the decomposition rate will increase rapidly, and the trisulfate decomposition is serious. The present invention obtains a new crystal form A by controlling crystallization. The accelerated stability of the crystal form in Example 1 has a storage decomposition rate of only 0.12% after 6 months. By calculation, the storage time of the product at room temperature (10~30℃) can reach more than 2 years. This shows that the crystal form A obtained by the present invention by controlling crystallization has good stability.

[0052] (3) The crystal form A provided by this invention has a more significant advantage in storage. It can be stored stably for 6 months with a decomposition rate of only 0.12%. Different crystal forms have different stability because their molecular packing and interaction forces are different, resulting in different Gibbs free energies. Under specific conditions, only one crystal form is thermodynamically stable (with the lowest free energy), while other crystal forms are metastable. The crystal form A we prepared has the best thermodynamic stability and is the optimal crystal form with the highest storage stability.

[0053] II. Secondary Battery Performance Testing Since batteries spend most of their time in standby mode, changes in their state during storage are a crucial indicator of their marketability. To expedite the experiment, high-temperature storage was chosen to accelerate the reaction, simulating the self-discharge effects and aging processes that batteries undergo during routine storage.

[0054] Preparation of positive electrode: The preparation method of the positive electrode sheet is as follows: Lithium cobalt oxide (LiCoO2), polyvinylidene fluoride (PVDF), and conductive additive (Super P) are mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) is added as a solvent and stirred in a vacuum mixer until a uniform and fluid positive electrode active slurry is formed. The slurry is then uniformly coated on both surfaces of an aluminum foil. The coated aluminum foil is then dried, rolled, and slit to finally obtain the desired positive electrode sheet.

[0055] Preparation of negative electrode: The negative electrode sheet is prepared as follows: Artificial graphite, silicon suboxide, sodium carboxymethyl cellulose, styrene-butadiene rubber, and conductive carbon black are mixed in a mass ratio of 79.5:15:2.5:1.5:1.5. Deionized water is added, and the mixture is stirred under vacuum to form a homogeneous slurry. The slurry is then uniformly coated onto both sides of a copper foil, first dried at room temperature, then thoroughly dried at 80°C. Finally, the negative electrode sheet is obtained through cold pressing and slitting.

[0056] Preparation of electrolyte for blank control group: The electrolyte is prepared as follows: Ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate are mixed in a mass ratio of 10:20:40:30 as an organic solvent, and then 1 mol / L lithium hexafluorophosphate is added to form a lithium salt base system. Next, 5% fluoroethylene carbonate and 2% hexanetrionitrile (by mass of the total electrolyte) are introduced into the system as functional additives. After all components are thoroughly stirred and dissolved, and the moisture and free acid levels are tested and found to be within acceptable limits, the target electrolyte is obtained.

[0057] Preparation of electrolyte with added crystal form A: Add 1% by mass of crystal form A to the blank electrolyte; Preparation of electrolyte with added crystal form B: Add 1% by mass of crystal form B to the blank electrolyte.

[0058] Preparation of secondary batteries: The battery is manufactured as follows: First, the positive electrode, separator and negative electrode are stacked and wound into a cell in sequence; then the cell is placed in the outer packaging aluminum foil, each group of electrolytes is injected, and then vacuum sealing, standing, formation, shaping and sorting are carried out to finally obtain a battery with a working voltage range of 2.8V-4.25V.

[0059] After the battery is filled with electrolyte, it is activated using standard methods. After full activation, the battery is stored at 60°C for 7 days, and then its capacity retention is tested.

[0060] Test section: (1) Room temperature cycling performance test At 25°C, the secondary battery is charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 4.25V to a cutoff current of 0.05C, left to rest for 10 minutes, and then discharged at a constant current of 1C to 2.8V, left to rest for 5 minutes. This constitutes one charge-discharge cycle. The discharge capacity at this point is recorded as C0. This charge-discharge cycle process is repeated for the same secondary battery, and the discharge capacity C1 of the 300th cycle is recorded. The cycle capacity retention rate of the battery is P300 = C1 / C0 × 100%.

[0061] (2) High-temperature storage performance test At 25°C, the secondary battery is charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 4.25V to the cutoff current of 0.05C, left to stand for 10 minutes, and then discharged at a constant current of 1C to 2.8V, left to stand for 5 minutes. This constitutes one charge-discharge cycle. Record the discharge capacity D0 at this point.

[0062] Store the secondary battery in an environment of 60°C for 60 days. Afterward, remove the secondary battery and cool it down to a surface temperature of 25°C.

[0063] Subsequently, at 25°C, it was discharged at a constant current of 1C to 2.8V, then charged at a constant current of 1C to 4.25V, and then charged at a constant voltage of 4.25V to the cutoff current of 0.05C. After resting for 10 minutes, it was discharged at a constant current of 1C to the termination voltage of 2.8V to obtain the discharge capacity D1 after high-temperature storage.

[0064] High-temperature storage capacity retention rate (%) = (D1 / D0) × 100%.

[0065] (3) Power performance test The power performance of a secondary battery is characterized by its direct current resistance (DCR). Generally, the lower the DCR, the better the battery's power performance.

[0066] At 25°C, the secondary battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 4.25V to a cutoff current of 0.05C, allowed to stand for 30 minutes, and then discharged at a constant current of 1C for 0.5 hours, allowed to stand for 30 minutes, and the voltage V1 after standing was recorded. Subsequently, it was discharged at 4C for 30 seconds, and the voltage V2 at the end of the discharge was recorded.

[0067] The battery's DCR = (V1 - V2) / I, where I = the current corresponding to a 4C rate.

[0068] The test results are detailed in Table 4 below: Table 4 As can be seen from the table above, the secondary batteries with added crystal form A and crystal form B have higher cycle capacity retention and high-temperature storage capacity retention, as well as lower internal resistance compared to the blank group. The secondary batteries with added crystal form A are superior to those with crystal form B in terms of cycle capacity retention, high-temperature storage capacity retention, and reduced internal resistance.

[0069] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A crystalline form A of a lithium ion battery additive, ethylene trithionate, characterized by: The X-ray powder diffraction pattern thereof contains 2θ values of 11.5±0.2°, 17.7±0.2°, 19.8±0.2°, 20.2±0.2°, 20.7±0.2°, 21.5±0.2°, 24.7±0.2°; the structure of the trithioethylene sulfate is as follows: 。 2. The crystalline Form A of claim 1, characterized by: The melting point of the crystal form A is 189.34-197.34℃.

3. A process for preparing the crystalline Form A of claim 1 or 2, characterized in that: The method comprises the following steps: (1) adding a solvent in D-mannitol, adding cesium carbonate, adding a compound of formula A 1-(fluorosulfonyl)-3-methyl-1H-imidazole trifluoromethanesulfonate, carrying out a reaction, after the reaction is completed, removing the cesium carbonate by filtration, adding dilute hydrochloric acid dropwise, adjusting the pH to 5-6, stirring, filtering, drying, to obtain a trithioethylene sulfate crude product, and the reaction equation is as follows: ； (2) warming the trithioethylene sulfate crude product with a solvent, stirring until clear, then decolorizing and adsorbing metal impurities with activated carbon and mercaptosilica gel, then cooling to 12-25℃, adding a poor solvent under stirring, finally cooling to 10±5℃ and keeping the temperature, filtering, drying, to obtain the crystal form A.

4. The method of claim 3, wherein the crystalline Form A is prepared by: In step (1), the solvent is acetonitrile, and the amount of acetonitrile is 8-15 times the mass of D-mannitol.

5. The method of claim 3, wherein the crystalline Form A is prepared by: In step (1), the molar ratio of D-mannitol: cesium carbonate: compound of formula A is 1.0: 1.5-3.0: 3.0-5.

0.

6. A process for preparing the crystalline Form A of claim 3, characterized in that: In step (1), the reaction condition is: under the protection of nitrogen, the reaction temperature is 20-30℃, and the stirring reaction time is 1-2h.

7. A process for preparing the crystalline Form A of claim 3, characterized in that: In step (1), the concentration of the dilute hydrochloric acid is 5%.

8. A process for preparing the crystalline Form A of claim 3, characterized in that: In step (2), the solvent is one or two of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate, and the amount of the solvent is 3-10 times the volume of the trithioethylene sulfate crude product.

9. A process for preparing the crystalline Form A of claim 3, characterized in that: In step (2), the poor solvent is one of n-hexane, cyclohexane and n-heptane, and the amount of the poor solvent is 10-30 times the volume of the trithioethylene sulfate crude product.

10. A process for preparing the crystalline Form A of claim 3, characterized in that: In step (2), the temperature is warmed to 35-45℃ with a solvent until clear under stirring.

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