Preparation method of sulfonic acid lithiated graphene, sulfonic acid lithiated graphene and diaphragm and battery applying sulfonic acid lithiated graphene

Abstract

The invention provides a preparation method of sulfonic acid lithiated graphene, the sulfonic acid lithiated graphene, and a diaphragm and a battery applying the sulfonic acid lithiated graphene. The preparation method comprises the following steps: S1, dispersing graphene oxide in water, and adjusting the pH value of a dispersion liquid to 8.5-10 to obtain a graphene oxide dispersion liquid; s2, adding a reducing agent into the graphene oxide dispersion liquid, and reacting at 65-75 DEG C for 0.75-1.5 h to obtain reduced graphene after the reaction is finished; s3, the reduced graphene is dispersed in water and placed in a microwave reactor for microwave treatment, concentrated sulfuric acid is dropwise added for a reaction for 5-8 h, and the technological conditions of microwave treatment are as follows: the temperature is 58-62 DEG C, the power is 340-360 W, and the frequency is 2300-2600 MHz; and S4, after the reaction is finished, adding lithium hydroxide into the solution until the pH value of the solution is not less than 10, filtering and washing to obtain the sulfonic acid lithiated graphene. The prepared sulfonic acid lithiated graphene is applied to modification of a diaphragm, and the electrochemical performance of a battery prepared from the sulfonic acid lithiated graphene is effectively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of lithium sulfonate graphene, lithium sulfonate graphene, and a separator and a battery using the same. BACKGROUND

[0002] The energy density of a conventional lithium ion battery using graphite as a negative electrode has reached the limit. However, the energy density of a lithium metal battery using lithium metal as a negative electrode can be further improved. However, the separator used in the lithium metal battery is mainly a commercial lithium ion battery separator. Such a commercial separator has problems such as a large pore, a low mechanical strength, and a low lithium ion transference number (<0.3), and when it is applied to the lithium metal battery, a large lithium ion concentration polarization, non-uniform lithium deposition, and other problems are caused, resulting in the formation of lithium dendrites and uncontrollable nucleation growth during the cycle process, the puncture of the separator, the short circuit of the battery, and safety hazards. At the same time, the repeated dissolution / deposition of the lithium dendrites further causes the formation of dead lithium in some parts due to the loss of electronic contact, and the further reduction of the battery capacity and cycle life.

[0003] At present, the modification strategies for the separator mainly include the following two aspects: (1) coating lithium ion-conducting ceramic particles and metal organic frameworks on the surface of the separator, and using the adsorption of metal ions to anions to effectively homogenize the lithium ion flux and reduce the concentration polarization of lithium ions. However, although such a coating can well inhibit the initial formation of dendrites, its mechanical strength is usually limited. Once a particularly stubborn lithium dendrite begins to grow due to the volume expansion of the battery during the cycle process or other reasons, the soft ceramic / MOF coating may not be able to effectively physically block the puncture of the dendrites. (2) coating an electronic-conducting carbon carrier on the surface of the separator to enhance the mechanical properties of the separator, improve the ability of the separator to resist lithium dendrites, and reduce the short circuit risk of the battery. However, a simple carbon coating may not be able to actively guide the uniform deposition of lithium ions. If the ion flow itself is not uniform, lithium still tends to nucleate and grow at some points. Although the hard carbon layer can block the dendrites, it may force lithium to accumulate between the coating and the negative electrode, resulting in a huge pressure and even damage to the battery structure.

[0004] In summary, a single modification strategy cannot simultaneously meet the application of the separator in the lithium metal battery. Therefore, there is an urgent need to develop a separator that simultaneously considers ion, electron conduction, and mechanical strength to solve the above problems. SUMMARY

[0005] To solve the problems in the prior art, the application provides a preparation method of lithium sulfonate graphene, lithium sulfonate graphene and a separator and a battery using the same.

[0006] According to a first aspect of the application, a preparation method of lithium sulfonate graphene is provided, comprising the following steps: S1. dispersing graphene oxide in water, adjusting the pH of the dispersion to 8.5-10 to obtain a graphene oxide dispersion; S2. adding a reducing agent to the graphene oxide dispersion, reacting at 65-75 DEG C for 0.75-1.5 h, and obtaining reduced graphene after the reaction; S3. dispersing the reduced graphene in water, placing it in a microwave reactor, and adding concentrated sulfuric acid dropwise while microwave processing for 5-8 h; the process conditions for microwave processing are as follows: temperature is 58-62 DEG C, power is 340-360 W, and frequency is 2300-2600 MHz; S4. after the reaction, adding lithium hydroxide to the solution until the pH of the solution is 9.5-11, filtering and washing to obtain lithium sulfonate graphene.

[0007] Graphene oxide (GO) has poor conductivity and contains a large number of oxygen-containing functional groups (such as epoxy, hydroxyl, carboxyl), so it is very unstable and has many defects. Direct sulfonation of graphene oxide cannot restore its electronic conductivity, and the sulfonation reaction conditions are severe. These strong acids and high temperatures will further attack and destroy the fragile skeleton of GO, possibly leading to carbon-carbon bond rupture, breaking large pieces of GO into small pieces, destroying the integrity of its two-dimensional structure, and adversely affecting the ion / electron conductivity of the subsequent product. It also reduces its mechanical strength. Therefore, graphene oxide needs to be reduced first to form reduced graphene (rGO).

[0008] The quality of the currently commercially provided reduced graphene is uneven, and the reduction degree thereof cannot be determined. The dispersion of the reduced graphene in an aqueous solution is closely related to the reduction degree of the graphene, the higher the reduction degree of the rGO is, the fewer the hydrophilic groups on the surface of the rGO are, the stronger the hydrophobicity is, and the worse the dispersion in water is, and the rGO is easy to re-agglomerate and precipitate. In the present application, the pH is first adjusted to 8.5-10, and then a reducing agent is added to react at a relatively low temperature, and the reduction condition is relatively mild, so that a part of the oxygen-containing functional groups can be reserved, these groups can provide more reaction sites for the subsequent sulfonation reaction, and at the same time, the dispersion is improved, the subsequent sulfonation reaction is more sufficient, the graphene skeleton after reduction has higher stability, can withstand the severe conditions of concentrated sulfuric acid and microwaves in the subsequent steps, avoids the structure from being excessively damaged, and the electrochemical performance of the final sulfonated lithium graphene is improved.

[0009] In the sulfonation reaction, the present application adopts a microwave and concentrated sulfuric acid cooperative reaction, microwave heating is bulk heating, the energy directly acts on the molecules, the heating speed is fast, the thermal efficiency is high, the thermal gradient problem of traditional oil bath heating is avoided, the sulfonation reaction is more uniform, and the product quality is more uniform. Moreover, in the microwave reaction process of the present application, the concentrated sulfuric acid is added in a dropwise manner, which avoids the damage to the graphene structure (such as generating too many defects or breaking the layers) caused by local overheating due to the instantaneous addition of a large amount of concentrated sulfuric acid, realizes the completion of the sulfonation reaction under relatively mild conditions, and guarantees the structural integrity of the final product, so that the final product can better balance the ion / electron conduction effect and the mechanical strength, and the electrochemical performance is optimized. In addition, the dropwise addition of concentrated sulfuric acid also avoids the problem of a large safety risk caused by an instantaneous reaction that is too intense.

[0010] Finally, lithium hydroxide is added to convert the sulfonic acid group from the acidic sulfonic acid type (-SO3H) to the neutral lithium sulfonate type (-SO3Li), and remove the excess sulfuric acid in the reaction, so as to obtain the final product sulfonated lithium graphene. Lithium hydroxide is added to oxidize lithium to a solution pH of 9.5-11, and the process will be accompanied by precipitation. Too high pH will introduce lithium hydroxide impurities, and too low pH will cause incomplete lithiumization of -SO3H. It should be noted that lithium hydroxide has better overall effect than other basic lithium compounds or lithium salt + basic substances, for example, although lithium carbonate is theoretically feasible, the by-product is CO2 gas, which will produce bubbles and easily cause the slurry to overflow, causing safety hazards. The use of lithium salt + basic substance theoretically requires the use of alkali (such as NaOH) to neutralize the remaining sulfuric acid, and then a soluble lithium salt is added to exchange Na + to Li +, generating target product lithium sulfonate and sodium chloride (NaCl). Although such operation is chemically feasible, it introduces various ionic impurities, making subsequent purification very difficult, and the presence of these ions affects the electrochemical performance of the final product, lithium sulfonate graphene.

[0011] Preferably, in S1, the mass fraction of graphene oxide in the graphene oxide dispersion solution is 0.05-0.3%.

[0012] Preferably, in S1, the pH is adjusted to 8.5-10 using an alkaline solution. Preferably, the alkaline solution is a weak alkaline solution. Preferably, the weak alkaline solution includes at least one of sodium carbonate solution, sodium bicarbonate solution, and ammonia water.

[0013] Preferably, in S1, before adjusting the pH of the dispersion solution to 8.5-10, the solution is subjected to ultrasonic crushing and ultrasonic dispersion. Preferably, the ultrasonic crushing time is not less than 30 min. Preferably, the ultrasonic dispersion time is not less than 30 min.

[0014] Preferably, in S2, before adding the reducing agent to the graphene oxide dispersion solution for reaction, the graphene oxide dispersion solution is subjected to centrifugal treatment. Preferably, in the centrifugal treatment, the centrifuge speed is 2000-3000 rpm / min, and the centrifugal time is 25-35 min. The original graphene powder is a polydisperse system, which contains not only single-layer graphene, but also oligolayer thin nanosheets, and inevitably contains thick sheet aggregates that are not completely exfoliated and some over-oxidized carbon fragments. The main function of centrifugation is to perform screening and grading, and to ensure the uniformity of the reaction precursor as much as possible, thereby ensuring the uniformity and effectiveness of the subsequent reaction.

[0015] Preferably, in S2, the amount of reducing agent is calculated based on a molar ratio of reducing agent to graphene oxide of 1-10:1. Preferably, the reducing agent includes at least one of sodium borohydride, sodium ascorbate, and hydrazine hydrate. Preferably, the reducing agent is sodium borohydride.

[0016] Preferably, in S2, after the reaction is completed, the reduced graphene is obtained by suction filtration and washing treatment.

[0017] Preferably, in S3, the reduced graphene is dispersed in water, the pH of the dispersion solution is adjusted to neutral, and ultrasonic dispersion treatment is performed. Preferably, the ultrasonic dispersion time is not less than 30 min. Preferably, after the ultrasonic dispersion treatment is completed, the dispersion solution is cooled in an ice bath. Preferably, the dispersion solution is cooled to room temperature.

[0018] Preferably, the conditions of the microwave treatment are achieved by the following operations: in S3, the microwave reactor is heated to 58-62°C at a heating rate of 0.7-1.5°C / min, and then kept at 340-360W, 2300-2600MHz for 8-12min, with the temperature, power and frequency kept unchanged.

[0019] Preferably, in S3, the molar ratio of H2SO4 to reduced graphene in the concentrated sulfuric acid solution is (1-10):1 during the microwave reaction.

[0020] Preferably, in S3, the concentration of the concentrated sulfuric acid solution is 95-98%.

[0021] Preferably, in S3, the dropping time of the concentrated sulfuric acid solution is not less than 45min; preferably, in S3, the dropping time of the concentrated sulfuric acid solution is 45-60min. Slow dropping ensures that the concentrated sulfuric acid can be rapidly dispersed and diluted in the reaction system, avoiding any local area with too high acid concentration. This prevents the graphene carbon skeleton from being subjected to too severe chemical attack, resulting in excessive oxidation, carbon ring rupture or uncontrollable defects. At the same time, slow dropping makes it more likely for the sulfonic acid groups (-SO3H) to be uniformly grafted on the active sites on the surface and edges of the graphene sheets, resulting in more uniform distribution of sulfonic acid groups in the final product, smoother lithium ion transmission path and smaller resistance, thereby obtaining higher and more stable ionic conductivity. Moreover, the operation of slow dropping allows the reaction heat to be released slowly, which is more matched with the above-mentioned specific microwave reaction conditions, ensuring that the entire sulfonation reaction process is always carried out under the preset mild temperature conditions, which is conducive to controlled functional group reaction and inhibits uncontrollable carbon skeleton destruction side reactions, ensuring the quality of the final product and further ensuring its more excellent electrochemical performance.

[0022] Preferably, in S3, stirring is also accompanied during the process of dropping the concentrated sulfuric acid solution and after the dropping of the concentrated sulfuric acid. Preferably, the stirring speed is not less than 500 rpm / min.

[0023] According to a second aspect of the present application, a lithium sulfonated graphene is provided, which is prepared by any of the above-mentioned methods for preparing lithium sulfonated graphene. The lithium sulfonated graphene prepared by the method provided by the present application has lower impurity ions, more excellent electrochemical performance and mechanical strength, and is more conducive to the modification of battery products such as separators.

[0024] According to a third aspect of the present application, a separator is provided, comprising a base film and a functional coating; the functional coating comprises the above-mentioned sulfonated graphene. Specifically, firstly, the two-dimensional graphene sheet layer can improve the mechanical strength of the separator, and at the same time, the two-dimensional sheet structure of graphene can further adjust the pore structure of the separator, homogenize the lithium ion transmission flux, be beneficial to the dense deposition of lithium ions, avoid causing dead lithium, reduce the growth of lithium dendrites, and make the battery more durable. Moreover, graphene as an electronic conductor will be lithiated in the cycle process, thereby enhancing the ability to conduct lithium ions, and thereby reducing the concentration polarization. Secondly, the lithium sulfonate groups on the surface of graphene can adsorb anions in the electrolyte, thereby anchoring the anions and improving the migration number of lithium ions. At the same time, lithium sulfonate itself can act as a lithium ion transport site, which is beneficial to the rapid migration of lithium ions. The principle of anchoring anions is that after the lithium sulfonate groups on the surface of graphene are dissociated, the lithium ions (Li + ) generated thereby act as a medium to effectively coordinate and anchor the free anions (such as PF6 - ) in the electrolyte by forming ion associates such as (-SO3 - )—Li + —(PF6 - ), thereby limiting the long-range migration of anions. Furthermore, the sulfonated graphene of the present application can further reduce the contact angle of the surface of the separator, and thus can improve the wettability of the electrolyte, thereby further improving the transmission efficiency of lithium ions and the cycle life of the battery.

[0025] Preferably, the functional coating further comprises a binder. The binder is beneficial to the uniform dispersion of the sulfonated graphene, and at the same time enhances the wettability of the separator to the electrolyte. Preferably, the binder comprises at least one of PVDF (polyvinylidene fluoride), PEO (polyethylene oxide), CMC (carboxymethyl cellulose), and PAN (polyacrylonitrile). Preferably, the binder is PVDF. Preferably, the molecular weight of the PVDF is not less than 1 million.

[0026] Preferably, the base film comprises at least one of Celgard 2500 separator, cellulose separator, / polyimide separator, and PE separator.

[0027] Preferably, the thickness of the functional coating is 5-15 μm. The thickness of the functional coating within the above range can ensure the formation of a continuous and effective ion homogenization layer, and meanwhile control the ion transmission resistance within an acceptable range. Meanwhile, the thickness is sufficient to establish a perfect conductive network to achieve electric field homogenization, and due to its limited thickness and compounding with the binder, the electronic conductivity is controlled at a "just enough but not high" level, avoiding the risk of significant self-discharge or short circuit. Furthermore, the thickness can provide significant mechanical reinforcement. Too thin will not provide sufficient protection; too thick will make the separator brittle and affect flexibility. Therefore, the thickness of the functional coating within the above range maximizes the dual effects of "ion flow homogenization" and "electric field homogenization" without significantly increasing the internal resistance of the battery and the loss of energy density, thereby actively and efficiently inhibiting the growth of lithium dendrites. Meanwhile, the thickness ensures the mechanical integrity, flexibility and safety of the coating, and ultimately synergistically improves the cycle life, rate performance and safety of the battery.

[0028] Preferably, the thickness of the base film is 15-40 μm.

[0029] Preferably, the preparation method of the separator comprises the following steps: dispersing the lithium sulfonated graphene in an organic solvent, then adding a binder and mixing uniformly, coating the obtained mixed solution on the base film, drying to obtain the separator.

[0030] Preferably, after the lithium sulfonated graphene is dispersed in the organic solvent, ultrasonic crushing is performed. Preferably, the time for ultrasonic crushing is not less than 30 min.

[0031] Preferably, after the binder is added, the mixed solution is stirred uniformly at 65-75 °C.

[0032] Preferably, the mass ratio of the lithium sulfonated graphene to the binder is 0.005-0.05:10.

[0033] Preferably, the mass fraction of the lithium sulfonated graphene in the dispersion thereof with the organic solvent is 0.1-0.5%.

[0034] Preferably, the mass fraction of the binder in the mixed solution is 5-10%.

[0035] Preferably, after the mixed solution is coated on the base film, vacuum drying is performed at 60-80 °C for 20-28 h.

[0036] According to a fourth aspect of the present application, a battery is provided, comprising a positive electrode, a negative electrode, an electrolyte and any of the above separators, the separator being disposed between the positive electrode and the negative electrode. The battery prepared by using the above lithium sulfonated graphene modified separator has improved kinetic performance, including significantly improved ion diffusion rate, and the cycle life and capacity retention rate of the battery are significantly improved.

[0037] Preferably, the battery is a lithium metal battery. For the lithium metal battery, the problem of lithium dendrite is more serious, therefore, the lithium sulfonate graphene provided by the present application is more suitable for the lithium metal battery, and can effectively improve the problem of lithium dendrite of the lithium metal battery, thus the capacity and cycle life of the lithium metal battery can be obviously improved.

[0038] Preferably, in the lithium metal battery, the negative electrode comprises lithium metal. Preferably, in the lithium metal battery, the positive electrode comprises lithium iron phosphate. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 The SEM picture and the energy spectrum distribution test results of the lithium sulfonate graphene prepared in Example 2 are shown, wherein (a) is the SEM picture of the lithium sulfonate graphene, (b), (c) and (d) are the C, O and S element distribution diagrams corresponding to the SEM picture of (a), respectively.

[0040] Figure 2 The test results of the infrared spectrum of the lithium sulfonate graphene prepared in Example 2 are shown.

[0041] Figure 3 The test results of the Raman spectrum of the lithium sulfonate graphene prepared in Example 2 are shown.

[0042] Figure 4 The SEM pictures of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b) are shown.

[0043] Figure 5 The test results of the surface electrolyte (1M LiPF6 EC / DEC) contact angle of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b) are shown.

[0044] Figure 6 The test results of the conductivity of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b) are shown.

[0045] Figure 7 The test results of the transference number of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b) are shown.

[0046] Figure 8 The polarization test results of the Li||Li symmetric battery prepared in Example 2 (a) and Comparative Example 1 (b) are shown.

[0047] Figure 9 Results of the cycle performance test of the Li||LiFeP04 battery prepared in Example 2(a) and Comparative Example 1(b). DETAILED DESCRIPTION

[0048] In order to better understand the technical scheme of the present application, the technical scheme in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, not all.

[0049] Example 1 1. Preparation of lithium sulfonate graphene The lithium sulfonate graphene of the present embodiment was prepared according to the following steps: S1. 0.1 g of graphene oxide was ultrasonically broken and dispersed in 100 mL of distilled water, and then the pH of the dispersion was adjusted to 9 using a 5 wt % aqueous sodium carbonate solution to obtain a graphene oxide dispersion; S2. After ultrasonic treatment of the graphene oxide dispersion for 30 min, the solution after ultrasonic treatment was separated by a centrifuge for 30 min to remove unreacted graphite, and the graphene oxide was redispersed in distilled water after centrifugation at 2000 rpm. Then, a sodium borohydride solution (0.78 g of sodium borohydride was weighed and dissolved in 2 mL of distilled water by stirring) was added, and the reaction was carried out at 70°C for 1 h. After the reaction was completed, distilled water was used for washing until the pH value reached 7; S3. The reduced graphene collected by suction filtration was redispersed in 100 mL of distilled water, and ultrasonic breaking and oscillation were performed for 30 min. After cooling to room temperature using an ice bath, it was placed in a microwave reactor, and the microwave reactor was heated to 60°C at a heating rate of 1°C / min. Then, concentrated sulfuric acid solution was added dropwise under the conditions of 350 W and a frequency of 2450 MHz for 10 min. The temperature, power, and frequency were kept constant. Then, concentrated sulfuric acid solution was added dropwise, and the reaction was carried out for 6 h. Stirring was performed during the reaction at a stirring speed of not less than 500 rpm / min. The addition of concentrated sulfuric acid and the reaction after the addition of concentrated sulfuric acid were both maintained with stirring. The molar ratio of H2SO4 to reduced graphene in the concentrated sulfuric acid solution was controlled to be 1:1, and the concentration of the concentrated sulfuric acid solution was 95-98%. S4. After the reaction was completed, lithium hydroxide was added to the solution to make the pH of the solution 10, and a precipitate was generated. Filtration and repeated washing with water and ethanol were performed to obtain lithium sulfonate graphene.

[0050] 2. Preparation of the separator Take 0.01 g of the above prepared sulfonated lithium graphene and disperse it in 10 g of DMF solvent, ultrasonic crushing for 30 min, then add 0.7 g of PVDF powder (product name: HSV900) at 70°C, stir evenly to obtain a mixed solution; then take 5 mL of the mixed solution and scrape it on one side of the Celgard 2500 separator (the functional coating thickness containing sulfonated lithium graphene is 5 μm, and the thickness of the Celgard 2500 separator (base film) is 25 μm), the side with the functional coating is adjacent to the negative electrode, then vacuum drying at 70°C for 24 hours to remove the DMF solvent. Finally, the obtained separator is cut into small round pieces with a diameter of 19 mm for use.

[0051] 3. Preparation of battery The above prepared separator was used to assemble Li||Li symmetric battery, stainless steel blocking battery, and Li||LiFePO4 (lithium iron phosphate) battery for electrochemical performance test.

[0052] Example 2 1. Preparation of sulfonated lithium graphene The difference from Example 1 is that in S3, the molar ratio of H2SO4 to reduced graphene in the concentrated sulfuric acid solution is controlled to be 2:1; the rest is consistent with Example 1.

[0053] 2. Preparation of separator Consistent with Example 1.

[0054] 3. Preparation of battery Consistent with Example 1.

[0055] Example 3 1. Preparation of sulfonated lithium graphene The difference from Example 1 is that in S3, the molar ratio of H2SO4 to reduced graphene in the concentrated sulfuric acid solution is controlled to be 5:1; the rest is consistent with Example 1.

[0056] 2. Preparation of separator Consistent with Example 1.

[0057] 3. Preparation of battery Consistent with Example 1.

[0058] Example 4 1. Preparation of sulfonated lithium graphene The difference from Example 1 is that in S3, the molar ratio of H2SO4 to reduced graphene in the concentrated sulfuric acid solution is controlled to be 10:1; the rest is consistent with Example 1.

[0059] 2. Preparation of separator Consistent with Example 1.

[0060] 3. Preparation of battery The same as example 1.

[0061] Example 5 1. Preparation of lithium sulfonate graphene The same as example 2.

[0062] 2. Preparation of separator The difference from example 2 is that 10 mL of the mixed solution is scraped on the Celgard 2500 separator (corresponding to the functional coating thickness of 10 μm containing lithium sulfonate graphene); the rest is the same as example 2.

[0063] 3. Preparation of battery The same as example 2.

[0064] Example 6 1. Preparation of lithium sulfonate graphene The same as example 2.

[0065] 2. Preparation of separator The difference from example 2 is that 15 mL of the mixed solution is scraped on the Celgard 2500 separator (corresponding to the functional coating thickness of 15 μm containing lithium sulfonate graphene); the rest is the same as example 2.

[0066] 3. Preparation of battery The same as example 2.

[0067] Example 7 1. Preparation of lithium sulfonate graphene The difference from example 2 is that in S2, the graphene oxide dispersion is not centrifuged, that is, the graphene oxide dispersion obtained in S1 is directly subjected to reduction reaction; the rest is the same as example 1.

[0068] 2. Preparation of separator The same as example 2.

[0069] 3. Preparation of battery The same as example 2.

[0070] Comparative example 1 This comparative example directly uses unmodified Celgard 2500 separator to assemble Li||Li symmetrical battery, stainless steel blocked battery, and Li||LiFePO4 (lithium iron phosphate) battery for electrochemical performance test.

[0071] Comparative example 2 1. Preparation of lithium sulfonate graphene Different from example 2 is that the steps of S1, S2 are not performed, and the operations of S3, S4 are directly performed using graphene oxide; the rest is consistent with example 2.

[0072] 2, Preparation of the separator Consistent with example 2.

[0073] 3, Preparation of the battery Consistent with example 2.

[0074] Comparative example 3 1, Preparation of lithium sulfonate graphene Different from example 2 is that the steps of S1, S2 are not performed, and the operations of S3, S4 are directly performed using commercial reduced graphene, the manufacturer and model of which are Macklin, and the model is MFCD00144065; the rest is consistent with example 2.

[0075] 2, Preparation of the separator Consistent with example 2.

[0076] 3, Preparation of the battery Consistent with example 2.

[0077] Comparative example 4 1, Preparation of lithium sulfonate graphene Different from example 2 is that in S3, microwave treatment is not performed; the rest is consistent with example 2.

[0078] 2, Preparation of the separator Consistent with example 2.

[0079] 3, Preparation of the battery Consistent with example 2.

[0080] Comparative example 5 1, Preparation of lithium sulfonate graphene Different from example 2 is that in S3, the temperature in the microwave treatment condition is 45°C; the rest is consistent with example 2.

[0081] 2, Preparation of the separator Consistent with example 2.

[0082] 3, Preparation of the battery Consistent with example 2.

[0083] Comparative example 6 1, Preparation of lithium sulfonate graphene Different from example 2 is that in S3, the power in the microwave treatment condition is 380W; the rest is consistent with example 2.

[0084] 2, Preparation of the separator The same as Example 2.

[0085] 3. Preparation of battery The same as Example 2.

[0086] Comparative Example 7 1. Preparation of lithium sulfonated graphene The difference from Example 2 is that in S3, all concentrated sulfuric acid is directly added at one time; the rest is the same as Example 1.

[0087] 2. Preparation of separator The same as Example 2.

[0088] 3. Preparation of battery The same as Example 2.

[0089] Test analysis 1. Structural characterization of lithium sulfonated graphene (1) The SEM picture and the energy spectrum distribution test of the lithium sulfonated graphene prepared in Example 2 were taken, and the results are shown in Figure 1 , wherein (a) is the SEM picture of the lithium sulfonated graphene, (b), (c), (d) are the C, O, S element distribution maps corresponding to the SEM picture of (a), respectively. It can be seen from Figure 1 that the S element is uniformly distributed, and the lithium sulfonated graphene is successfully prepared.

[0090] (2) The infrared spectrum test of the lithium sulfonated graphene prepared in Example 2 was carried out, and the results are shown in Figure 2 , it can be found that there are two peaks at 1063 cm -1 and 1257 cm -1 , corresponding to S=O group, also indicating that the -SO3Li group is grafted on the graphene.

[0091] (3) The Raman spectrum test of the lithium sulfonated graphene prepared in Example 2 was carried out, and the results are shown in Figure 3 , which shows that due to the successful introduction of sulfonic acid group, a considerable amount of structural defects are produced in the material, and the integrity of the graphitized lattice structure is decreased compared with the original graphene.

[0092] 2. Characterization of separator (1) The SEM pictures of the separator prepared in Example 2 and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 were taken, as shown in Figure 4Figure 2 shows the SEM images of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b), it can be seen that the pores of the separator prepared in Example 2 are significantly smaller than those of the PP2500 separator, and no functional coating containing lithiated graphene oxide is observed on the surface of the PP2500 separator.

[0093] (2) The surface electrolyte (1 M LiPF6EC / DEC) contact angle of the separator prepared in Example 2 and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 was tested, as shown in Figure 3. Figure 5 Figure 3 shows the contact angle of the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b), it can be seen that the contact angle of the separator prepared in Example 2 to the commercial electrolyte (1 M LiPF6EC / DEC) is 22.16 degrees, which is much lower than that of the commercial separator PP2500 (55.78 degrees) in Comparative Example 1, indicating that the functional coating containing two-dimensional lithiated graphene oxide is beneficial to enhancing the wettability of the separator and improving the transport of lithium ions on the separator. This can also be proved by the conductivity test of Figure 6 Figure 6 Figure 4 shows the conductivity test results of the stainless steel blocked cells prepared by the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) (b) in Comparative Example 1, respectively, which shows that the conductivity of the cell corresponding to the separator prepared in Example 2 is 0.53 mS / cm, which is much higher than that of the cell corresponding to the PP2500 separator in Comparative Example 1, which is 0.24 mS / cm. In addition, the migration number of the Li||Li symmetric cell assembled by the separator prepared in Example 2 and the conventional commercial Celgard 2500 separator (PP2500 separator) was tested, and the results are shown in Figure 7 Figure 5 shows the migration number test results of the Li||Li symmetric cells assembled by the separator prepared in Example 2 (a) and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 (b), respectively, the migration number of the cell in Example 2 is 0.79, while the migration number of the cell in Comparative Example 1 is only 0.45, which may be due to the high adsorption energy of the high-polar -SO3Li group and the porous graphene to anions, while the adsorption capacity of the PP2500 separator to anions is weak, resulting in a poorer migration number of the Li||Li symmetric cell assembled by the PP2500 separator.

[0094] ​The test method for the conductivity and transference number of the above-mentioned separator is as follows: (1) Conductivity: A small AC perturbation signal is applied to a stainless steel blocked cell in the high frequency (e.g. 1 MHz) to low frequency (e.g. 0.1 Hz) range using an electrochemical workstation, and the obtained impedance data is plotted as a Nyquist plot. The impedance plot of an ideal blocked cell is an arc intersecting the real axis at high frequency region. The intercept of high frequency region with the real axis is the bulk resistance of the system, denoted as R b , the contact area between the separator and the stainless steel is denoted as A, the thickness of the separator is denoted as d, and the conductivity of the separator is d / (A R b ).

[0095] (2) Transference number: A small AC perturbation signal is applied to a Li||Li symmetric cell in the high frequency (e.g. 1 MHz) to low frequency (e.g. 0.1 Hz) range using an electrochemical workstation, and the obtained impedance data is plotted as a Nyquist plot. The initial bulk resistance is obtained from the Nyquist plot, denoted as R0. Then a small DC bias (ΔV, usually 10-30 mV) is applied to the cell. The voltage should be small enough to avoid side reactions, but large enough to cause a measurable steady-state current. The initial value of the current is recorded as I0, and the value after reaching the steady state is recorded as I ss . After removing the DC bias, the AC impedance test is performed again. The bulk resistance after polarization is obtained, denoted as R s , and the transference number of the separator is I ss (ΔV I0 R0) / I0(ΔV I ss R s ).

[0096] 3. Battery performance characterization (1) The polarization performance of Li||Li symmetric cells assembled with the separator prepared in Example 2 and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 was tested, and the results are shown in Figure 8 , wherein (a) and (b) are respectively the polarization test results of the Li||Li symmetric cells in Comparative Example 1 and Example 2, it can be seen that the cell in Example 2 has smaller polarization, and the polarization is gradually decreasing within 800 hours of cycle time, indicating that the functional coating of lithium sulfonate-containing graphene with high mechanical strength can inhibit the growth of lithium dendrites; in contrast, the cell assembled with the PP2500 separator (Comparative Example 1) has gradually increasing polarization after 150 hours, indicating that the negative electrode interface is unstable.

[0097] The test method of polarization is as follows: 1) the battery is placed for 24h, and the electrolyte is fully soaked; 2) the battery is repeatedly charged and discharged at a constant current density (1 mA cm -2 );3) the interval time is set to 10 min, and the battery is relatively stable; 3) record the voltage-time curve during the whole process. Test purpose: observe the change of overpotential with cycle length. Sudden and sharp increase or severe fluctuation of overpotential usually means interface failure or increased dendrite growth.

[0098] (2) The Li||LiFePO4 batteries assembled by the separator prepared in Example 2 and the conventional commercial Celgard 2500 separator (PP2500 separator) in Comparative Example 1 were tested for cycle performance, and the results are shown in Figure 9 , wherein (a) and (b) are the cycle performance test results of the Li||LiFePO4 batteries prepared in Comparative Example 1 and Example 2, respectively. It can be seen that the battery in Example 2 has higher coulombic efficiency and discharge specific capacity, and further tests show that the capacity retention rate of the battery is greater than 90% after 500 cycles, while the capacity and coulombic efficiency of the battery using Comparative Example 1 decrease sharply after 190 cycles, indicating that the lithium sulfonated graphene strengthens the stability of the LiFePO4 positive electrode interface.

[0099] , wherein the test method of the above coulombic efficiency, discharge specific capacity and cycle capacity retention rate is as follows: the assembled Li||LiFePO4 battery is placed for 8h, and then charged and discharged at a rate of 0.1C for 2 cycles, and then charged and discharged at a rate of 0.5C for cycles. The specific capacity at the initial cycle of 0.5C and after different cycles is recorded, so as to obtain the cycle capacity retention rate. The coulombic efficiency and discharge specific capacity are given by the blue charge and discharge system.

[0100] (3) The Li||LiFePO4 batteries prepared in Examples 1-7 and Comparative Examples 1-7 were tested for capacity retention rate of 500 cycles, and the results are shown in Table 1.

[0101] Table 1 Cycle performance test results of Li||LiFePO4 batteries in examples and comparative examples

[0102] As can be seen from Table 1, the lithium sulfonated graphene prepared by the specific steps is used to modify the separator, which effectively improves the mechanical strength, ion conductivity, electrolyte wettability and other properties of the separator, so that the cycle life of the battery using the separator is also obviously improved. For specific reference, see Examples 1-7.

[0103] The unmodified Celgard 2500 separator was directly used in Comparative Example 1; the steps S1 and S2 were not performed, and the operation of S3 and S4 was directly performed using graphene oxide, that is, the subsequent sulfonic acid lithiation modification was directly performed using graphene oxide in Comparative Example 2; the subsequent sulfonic acid lithiation modification was directly performed using commercial reduced graphene in Comparative Example 3; the microwave treatment was not performed in Comparative Example 4; the temperature was 45℃ in the microwave treatment condition, that is, the temperature was too low in Comparative Example 5; the power was 380W in the microwave treatment condition, that is, the power was too high in Comparative Example 6; all the concentrated sulfuric acid was directly added at one time in Comparative Example 7; and all the above factors caused the obvious decrease of the cycle retention rate of the battery, which indicated that the temperature, power and addition mode of sulfuric acid and other parameters in the reaction process must be strictly controlled during the microwave-assisted reduction of graphene oxide, so as to obtain sulfonic acid lithiated graphene with better performance, thereby the modification effect of the separator is better, and the cycle performance and other electrochemical performances of the battery prepared from the separator are further improved.

[0104] Further comparing Examples 1 to 4, the variable is the molar ratio of H2SO4 in the concentrated sulfuric acid solution to the reduced graphene, and it can be seen that when the molar ratio of H2SO4 in the concentrated sulfuric acid solution to the reduced graphene in S3 is 2:1, the cycle capacity retention rate of the battery at 500 cycles is the highest. This indicates that when the molar ratio of H2SO4 in the concentrated sulfuric acid solution to the reduced graphene in S3 is 2:1, it is more conducive to obtaining sulfonic acid lithiated graphene with better mechanical, ionic conductive and electrolyte comprehensive performance, and the cycle performance of the battery using the same is further optimized.

[0105] Comparing Example 2 with Examples 5 and 6, it can be seen that when the thickness of the functional coating prepared from the sulfonic acid lithiated graphene is adjusted, the battery using the separator still has good cycle performance.

[0106] Comparing Example 2 with Example 7, the cycle capacity retention rate in Example 7 is decreased due to that the graphene oxide dispersion liquid is not subjected to centrifugal treatment in Example 7, which indicates that the centrifugal treatment is conducive to the uniformity and effectiveness of the subsequent reaction, and the sulfonic acid lithiated graphene with better performance is obtained, and the electrochemical performance of the separator and the battery is further optimized.

[0107] The above examples are only used to illustrate the technical solutions of the present application but not to limit the protection scope of the present application. Although the present application has been described in detail with reference to the above examples, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently, and these modifications or replacements are within the protection scope of the present application.

Claims

1. A method for preparing lithium sulfonate graphene, characterized in that, Includes the following steps: S1. Graphene oxide is dispersed in water, and the pH of the dispersion is adjusted to 8.5-10 to obtain a graphene oxide dispersion. S2. Add a reducing agent to the graphene oxide dispersion and react at 65~75℃ for 0.75~1.5h. After the reaction is completed, reduced graphene is obtained. S3. Disperse the reduced graphene in water and place it in a microwave reactor. While microwave treating, add concentrated sulfuric acid dropwise and react for 5-8 hours. The microwave treatment process conditions are: temperature 55-65℃, power 340-360W, and frequency 2300-2600MHz. S4. After the reaction is complete, lithium hydroxide is added to the solution until the pH reaches 9.5-11. The solution is then filtered and washed to obtain lithium sulfonate graphene.

2. The method for preparing lithium sulfonate graphene as described in claim 1, characterized in that: In step S1, the pH is adjusted to 8.5-10 using an alkaline solution; preferably, the alkaline solution is a weakly alkaline solution; preferably, the weakly alkaline solution includes at least one of sodium carbonate solution, sodium bicarbonate solution, and ammonia water.

3. The method for preparing lithium sulfonate graphene as described in claim 1, characterized in that: In step S2, before adding the reducing agent to the graphene oxide dispersion for reaction, the graphene oxide dispersion is first centrifuged; preferably, during centrifugation, the centrifuge speed is 2000-3000 rpm / min and the centrifugation time is 25-35 min.

4. The method for preparing lithium sulfonate graphene as described in claim 1, characterized in that: In step S2, the amount of reducing agent added is calculated based on a molar ratio of the reducing agent to the graphene oxide of 1 to 10:1; preferably, the reducing agent includes at least one of sodium borohydride, sodium vitamin C, and hydrazine hydrate.

5. The method for preparing lithium sulfonate graphene as described in claim 1, characterized in that: In S3, during the microwave reaction, the molar ratio of H2SO4 to the reduced graphene in the concentrated sulfuric acid solution is (1~10):

1.

6. The method for preparing lithium sulfonate graphene as described in claim 1, characterized in that: In step S3, the addition time of the concentrated sulfuric acid solution is not less than 45 minutes; preferably, in step S3, the addition time of the concentrated sulfuric acid solution during the microwave reaction is 45 to 60 minutes.

7. A lithium sulfonate graphene, characterized in that: The graphene was prepared by the method described in any one of claims 1 to 6.

8. A diaphragm, characterized in that: It includes a base film and a functional coating; the functional coating includes lithium sulfonate graphene as described in claim 7.

9. The diaphragm as described in claim 8, characterized in that: The thickness of the functional coating is 5~15μm.

10. A battery, characterized in that: It includes a positive electrode, a negative electrode, an electrolyte, and a separator as described in any one of claims 8 to 9, wherein the separator is disposed between the positive electrode and the negative electrode.

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