A preparation method for in-situ formation of lithium molybdate diaphragm coating

Abstract

The present invention relates to a preparation method for in-situ forming a lithium molybdate coating on a diaphragm. The method comprises mixing a MoO3 nanorod material with Super P and PVDF, applying the mixture by doctor blade coating on a common diaphragm, shaping the mixture at room temperature, drying the mixture for standby use, and then assembling the mixture with a lithium metal negative electrode into a CR2032 button battery at room temperature. The battery is then installed on a LAND CT2001A terser battery testing system, and the in-situ lithium molybdate diaphragm coating is formed after 100 charge and discharge cycles. The lithium molybdate diaphragm coating prepared by this method can not only fundamentally improve the low active material utilization and polysulfide shuttle effect that occur in the actual use of Li-S batteries, but also facilitate large-scale production and commercial promotion due to its simple synthesis method.

Description

Technical Field

[0001] The present invention relates to a secondary battery, in particular to a rechargeable lithium battery, which is applied to the technical field of electrochemical energy storage devices. Background Art

[0002] With the increasing popularity of portable electronic devices, hybrid electric vehicles (HEVs), and electric vehicles, there is a growing demand for efficient and economical energy storage systems to adapt. Since the 1990s, lithium-ion secondary batteries based on an insertion / extraction reaction mechanism have dominated the portable electronic device battery market due to their mature technology and advantages such as low self-discharge, stable electrochemical performance, and long cycle life. However, the maximum energy density of existing commercial lithium-ion batteries is approaching its limit and still cannot meet the requirements of green industries such as electrical energy storage. Therefore, the development of next-generation battery systems with higher energy density is of great significance.

[0003] Li-S batteries have high energy density (theoretical specific capacity up to 1672mAh g -1 , the energy density can reach 2600Wh kg -1 ), which is much higher than the capacity of the commercially widely used lithium cobalt oxide battery (<150mAh / g). In addition, S, which is abundant in the earth, has the advantages of high yield, low price, and low environmental pollution, which makes Li-S batteries have broader development prospects and is expected to become the next generation of energy storage systems. Different from the traditional insertion / extraction reaction mechanism, the reaction mechanism of Li-S batteries is based on the reversible redox reaction between elemental S and Li, and the mutual conversion of electrical energy and chemical energy is achieved through the breaking / generation of sulfur-sulfur bonds. Because the reaction mechanism of Li-S batteries involves multi-step reactions and complex phase changes, its commercial promotion has been limited by the low utilization rate of active materials and the shuttle effect of intermediate polysulfides across the diaphragm, which causes the battery capacity to decrease and the life to be shortened.

[0004] Based on this, if a material with good conductivity, which can effectively prevent the polysulfide shuttle effect and accelerate the catalytic conversion, can be prepared, and a functional coating can be obtained on the diaphragm, it will be possible to solve the problems of Li-S batteries in practical applications. Summary of the Invention

[0005] The present invention aims to provide a method for preparing a lithium molybdate coating by in-situ forming the coating on the diaphragm. The present invention has a simple preparation process, high safety, low requirements on equipment and process conditions, and is conducive to popularization and application.

[0006] A method for preparing an in-situ formed lithium molybdate diaphragm coating, characterized by comprising the following steps:

[0007] (1) First, (NH4)6Mo7O 24Dissolve 4H2O in nitric acid solution and stir ultrasonically for 30 minutes to mix thoroughly.

[0008] (2) transferring the mixture obtained in step (1) into a polytetrafluoroethylene reactor, heating it at 150-250° C., keeping it warm for 2-3 hours, and then allowing it to cool;

[0009] (3) The product obtained in the reaction kettle was washed with deionized water and ethanol in sequence by vacuum filtration, and finally dried in a vacuum drying oven at 50-60°C for 10-14 hours to obtain white MoO3 nanorods;

[0010] (4) MoO3 nanorods, Super P, and PVDF were mixed in a certain mass ratio, fully ground, and dispersed in NMP solvent, and magnetically stirred at room temperature for 10-12 h to obtain a mixed slurry;

[0011] (5) The mixed slurry was coated on the diaphragm using a coating machine and shaped at room temperature, and then transferred to a vacuum drying oven for drying for 5-8 hours; the dried coated diaphragm was punched into a round shape using a punching machine, and then directly assembled with the Li metal negative electrode into a CR2032 button battery in a glove box without adding a positive electrode. The amount of electrolyte used was 9 μL, which was a mixed solution of 0.1 mol lithium bistrifluoromethanesulfonamide (LiTFSI) and 1.13 g LiNO3 dissolved in 100 ml of a mixed solution of 1,2-dioxolane (DOL) and dimethoxymethane (DME) with a volume ratio of 1:1; (The composition of the electrolyte was not clearly stated, and the amount of each component should be clearly stated)

[0012] (6) The assembled battery was installed on the LAND CT2001A terser battery test system and discharged from 2.8 V to 2.4 V at a current density of 0.1 C at 25 ° C to form a stable Li 0.042 MoO3, after standing for 1 hour, continue to discharge to 1.7V, Li 0.042 MoO3 is converted into stable Li2MoO4, and then charged to 2.8V after standing for 1 hour to convert Li2MoO4 into Li 0.042 MoO3, continue the charge and discharge cycle for 100 times, then disassemble the battery and take out the diaphragm with the lithium molybdate coating formed in situ, thus obtaining the lithium molybdate diaphragm coating formed in situ.

[0013] The volume solubility of the nitric acid solution in step (1) is 15-30%, and the ultrasonic stirring time is 30-50 minutes.

[0014] The mass ratio of MoO3 nanorods, Super P and PVDF in step (4) is 40-50:50-40:10.

[0015] Beneficial effects:

[0016] The present invention first synthesizes MoO3 nanorods through a simple hydrothermal method, making large-scale production possible. Furthermore, the nanostructured MoO3 material itself has a rich contact area that facilitates electrolyte wetting and promotes the in-situ formation of lithium molybdate. The lithium molybdate formed in situ on the separator effectively adsorbs polysulfides through polar bonding, inhibiting their transport. Its excellent catalytic properties also accelerate redox reaction kinetics. Its synergistic effect with Super P provides a pathway for ion / electron transport, improving the utilization efficiency of active materials and thereby enhancing the cycle life and battery capacity of the Li-S battery.

[0017] Thanks to these advantages, the Li-S battery with a lithium molybdate-coated separator can maintain a specific capacity of 906.6 mAh / g after 300 cycles at a current density of 1C. Compared with batteries using ex-situ formed lithium molybdate coatings, its capacity decay rate is significantly reduced.

[0018] The multifunctional lithium molybdate separator coating can not only fundamentally improve the low utilization of active materials and polysulfide shuttle effect that occur in the actual use of Li-S batteries; but its simple synthesis method is also conducive to large-scale production and commercial promotion. DETAILED DESCRIPTION

[0019] A method for preparing an in-situ formed lithium molybdate diaphragm coating, characterized by comprising the following steps:

[0020] 1. First, 1-2g (NH4)6Mo7O 24 4H2O was dissolved in a beaker containing HNO3 / H2O (1:5, v / v) and stirred ultrasonically for 30 min to ensure thorough mixing.

[0021] 2. Transfer the above solution into a polytetrafluoroethylene-lined reactor (100 ml) and heat at 200°C for 2 h, then allow it to cool.

[0022] 3. The product obtained in the reactor was vacuum filtered and washed with deionized water and ethanol in sequence, and finally dried in a vacuum drying oven at 60°C for 12 hours to obtain white MoO3 nanorod material.

[0023] 4. Mix MoO3, Super P, and PVDF in the following mass ratio, grind thoroughly for 30 minutes, then disperse in 10 ml of NMP solvent and magnetically stir at room temperature for 12 hours to obtain a mixed slurry.

[0024] MoO3:Super P:PVDF=45-50:45-40:10

[0025] 5. Use a coating machine to apply the mixed slurry on a common diaphragm and keep it at room temperature for 1 hour to set. Then transfer it to a vacuum drying oven and dry it for 6 hours.

[0026] 6. The dried, coated separator was punched into a 19 mm circular shape using a punch. The cells were then assembled directly with the lithium metal anode in a glove box without adding a positive electrode to form a CR2032 button cell. The electrolyte used was 9 μL of a mixture of 0.1 mol lithium bistrifluoromethanesulfonamide (LiTFSI) and 1.13 g LiNO₃ dissolved in 100 mL of a 1:1 (volume ratio) mixture of 1,2-dioxolane (DOL) and dimethoxymethane (DME).

[0027] 7. Install the assembled battery into the LAND CT2001A terser battery test system and discharge it from 2.8V to 2.4V at a current density of 0.1C at room temperature of 25℃ to form a stable Li 0.042 MoO3, after standing for 1 hour, continue to discharge to 1.7V, Li 0.042 MoO3 is converted into stable Li2MoO4, and then charged to 2.8V after standing for 1 hour to convert Li2MoO4 into Li 0.042 MoO3, and the charge and discharge cycles were continued for 100 times. After that, the battery was disassembled and the separator with the lithium molybdate coating formed in situ was removed.

[0028] Example

[0029] Example 1: At room temperature, 1.6g (NH4)6Mo7O 244H2O was dissolved in a 90ml beaker of HNO3 / H2O (1:5, v / v) and ultrasonically stirred for 30 minutes to thoroughly mix. The solution was transferred to a 100ml polytetrafluoroethylene-lined reactor and heated at 200°C for 2 hours, then allowed to cool. The product obtained in the reactor was vacuum filtered and washed five times with deionized water and then ethanol, followed by drying in a vacuum oven at 60°C for 12 hours to obtain white MoO3 nanorods. At room temperature, a mixture of MoO3:Super P:PVDF (0.2g:0.25g:0.05g) was thoroughly ground for 30 minutes, then dispersed in 10ml of NMP and magnetically stirred for 12 hours to obtain a mixed slurry. The mixed slurry was coated onto a standard diaphragm using a doctor blade and held at room temperature for 1 hour to set. It was then transferred to a vacuum oven and dried for 6 hours. The dried, coated diaphragm was punched into 19mm round shapes for later use. At room temperature, the coated separator was directly assembled with the Li metal negative electrode in a glove box to form a CR2032 button cell. The sulfur content of the positive electrode active material was 1 mg and the electrolyte was 9 μl. The assembled cell was installed on the LAND CT2001A terser battery test system and discharged from 2.8V to 2.4V at a current density of 0.1C at room temperature (25°C). Stable Li 0.042 MoO3; after standing for 1 hour, continue to discharge to 1.7V, Li 0.042 MoO3 is converted into stable Li2MoO4; after standing for 1 hour, charge to 2.8V to convert Li2MoO4 into Li 0.042 MoO3, and then cycled 100 times. The battery was then disassembled and the coated separator removed, revealing the in-situ lithium molybdate separator coating. After assembly into a full cell, the initial capacity at 1C current density was 967.1 mAh / g. After 300 cycles, the specific capacity remained at 526.6 mAh / g, with a capacity decay rate of 0.15%.

[0030] Example 2: At room temperature, 1.6g (NH4)6Mo7O 244H2O was dissolved in a 90ml beaker of HNO3 / H2O (1:5, v / v) and ultrasonically stirred for 30 minutes to thoroughly mix. The solution was transferred to a 100ml polytetrafluoroethylene-lined reactor and heated at 200°C for 2 hours, then allowed to cool. The product obtained in the reactor was vacuum filtered and washed five times with deionized water and then ethanol, followed by drying in a vacuum drying oven at 60°C for 12 hours to obtain white MoO3 nanorods. At room temperature, a mixture of MoO3:Super P:PVDF (0.25g:0.2g:0.05g) was thoroughly ground for 30 minutes, then dispersed in 10ml of NMP solvent and magnetically stirred for 12 hours to obtain a mixed slurry. The mixed slurry was coated onto a standard diaphragm using a doctor blade and held at room temperature for 1 hour to set. It was then transferred to a vacuum drying oven and dried for 6 hours. The dried, coated diaphragm was punched into 19mm round shapes for later use. At room temperature, the coated separator was directly assembled with the Li metal negative electrode in a glove box to form a CR2032 button cell. The sulfur content of the positive electrode active material was 1 mg and the electrolyte was 9 μl. The assembled cell was installed on the LAND CT2001A terser battery test system and discharged from 2.8V to 2.4V at a current density of 0.1C at room temperature (25°C). Stable Li 0.042 MoO3; after standing for 1 hour, continue to discharge to 1.7V, Li 0.042 MoO3 is converted into stable Li2MoO4; after standing for 1 hour, charge to 2.8V to convert Li2MoO4 into Li 0.042 MoO3, and then cycled 100 times. The battery was then disassembled and the coated separator removed, revealing the in-situ lithium molybdate separator coating. After assembly into a full cell, the initial capacity at 1C current density was 1033.6 mAh / g. After 300 cycles, the specific capacity remained at 523.1 mAh / g, with a capacity decay rate of 0.16%.

[0031] Comparative example: The material was deposited on the inner wall of a quartz glass tube using a concentric Mo wire by chemical vapor deposition, and the synthesis time was 72 hours. The obtained α-MoO3 was then dispersed in ethanol and Si nanoparticles (<1% by weight) were added to improve the reversibility of α-MoO3. 5 mg of Si-modified α-MoO3 was mixed with 7 mg of carbon tetrafluoride binder (a mixture of polytetrafluoroethylene and acetylene black), pressed onto a stainless steel mesh (Alpha Aesar-80mesh) collector, and directly used as the positive and negative lithium sheets in a dry glove box to assemble into a CR2032 button battery. Constant current lithiation was performed at a current density of 0.1C to form Li 1.33 Mo 0.66O2. Finally, the battery was disassembled in a glove box and the stainless steel mesh current collector was removed, revealing the in-situ formation of lithium molybdate. After assembly into a full cell, the initial capacity at 0.1C current density was 905 mAh / g. After 50 cycles, the specific capacity was 400 mAh / g, with a capacity decay rate of 1.1%.

Claims

1. A method for preparing an in-situ formed lithium molybdate diaphragm coating: characterized in that The steps include: (1) First, (NH4)6Mo7O 24 Dissolve 4H2O in nitric acid solution and stir ultrasonically for 30 minutes to mix thoroughly. (2) transferring the mixture obtained in step (1) into a polytetrafluoroethylene reactor, heating it at 150-250° C., keeping it warm for 2-3 hours, and then allowing it to cool; (3) The product obtained in the reaction kettle was washed with deionized water and ethanol in sequence by vacuum filtration, and finally dried in a vacuum drying oven at 50-60°C for 10-14 hours to obtain white MoO3 nanorods; (4) MoO3 nanorods, Super P, and PVDF were mixed in a certain mass ratio, fully ground, and dispersed in NMP solvent, and magnetically stirred at room temperature for 10-12 h to obtain a mixed slurry; (5) The mixed slurry was coated on the diaphragm using a coating machine and shaped at room temperature, and then transferred to a vacuum drying oven for drying for 5-8 hours; the dried coated diaphragm was punched into a circle using a punching machine, and then directly assembled with the Li metal negative electrode into a CR2032 button battery in a glove box without adding a positive electrode. The amount of electrolyte used was 9 μL, which was a mixed solution of 0.1 mol lithium bistrifluoromethanesulfonamide (LiTFSI) and 1.13 g LiNO3 dissolved in 100 ml of a mixed solution of 1,2-dioxolane (DOL) and dimethoxymethane (DME) with a volume ratio of 1:1; (6) The assembled battery was installed on the LAND CT2001A terser battery test system and discharged from 2.8 V to 2.4 V at a current density of 0.1 C at 25 ° C to form a stable Li 0.042 MoO3, let it stand for 1 hour and then continue to discharge to 1.7V, Li 0.042 MoO3 is converted into stable Li2MoO4, and then charged to 2.8V after standing for 1 hour to convert Li2MoO4 into Li 0.042 MoO3, continue the charge and discharge cycle for 100 times, then disassemble the battery and take out the diaphragm with the lithium molybdate coating formed in situ, thus obtaining the lithium molybdate diaphragm coating formed in situ.

2. The method for preparing an in-situ formed lithium molybdate diaphragm coating according to claim 1, wherein: The volume solubility of the nitric acid solution in step (1) is 15-30%, and the ultrasonic stirring time is 30-50 minutes.

3. The method for preparing an in-situ formed lithium molybdate diaphragm coating according to claim 1, wherein: The mass ratio of MoO3 nanorods, Super P and PVDF in step (4) is 40-50:50-40:10.