Boron-doped graphene composite solid electrolyte and its preparation and application

Abstract

A boron-doped graphene composite solid-state electrolyte and its preparation and application. A mixed solution of polyacrylonitrile (PAN) and N,N-dimethylformamide is prepared into a PAN porous film using electrospinning technology. A PEO casting solution is prepared by mixing boron-doped graphene, polyethylene oxide, lithium bis(trifluoromethanesulfonylimide), and acetonitrile. This solution is then cast onto the PAN porous film and, after vacuum drying, forms a composite solid-state electrolyte with polyacrylonitrile fibers as a support, polyethylene oxide as a matrix, and boron-doped graphene as a filler. The composite solid-state electrolyte prepared by the present invention has high ionic conductivity and lithium ion transference number, providing lithium batteries with good rate performance and cycle stability.

Description

Technical Field

[0001] The present invention relates to a technology in the field of lithium batteries, specifically a boron-doped graphene composite solid electrolyte and its preparation and application. Background Art

[0002] Existing solid-state electrolytes often use polyethylene oxide (PEO) as a matrix material, but their low ionic conductivity and lithium-ion transference number limit their practical applications. Currently, anions are often covalently linked to increase the lithium-ion transference number, but this reduces ionic conductivity. This creates a trade-off between the two, making simultaneous optimization difficult. Summary of the Invention

[0003] In response to the shortcomings of existing technologies, such as the difficulty in simultaneously optimizing the ionic conductivity and lithium ion transference number of PEO solid-state electrolytes, and the inability of existing electrolytes to solve the short-circuit problem caused by lithium dendrite growth due to their volatile and flammable physical properties and poor mechanical strength, the present invention proposes a boron-doped graphene composite solid-state electrolyte and its preparation and application. The prepared composite solid-state electrolyte has high ionic conductivity and lithium ion transference number, giving lithium batteries good rate performance and cycle stability.

[0004] The present invention is achieved through the following technical solutions:

[0005] The invention relates to a method for preparing a boron-doped graphene composite solid electrolyte. The method comprises the following steps: preparing a PAN porous film from a mixed solution of polyacrylonitrile (PAN) and N,N-dimethylformamide by electrostatic spinning technology; mixing boron-doped graphene, polyethylene oxide, lithium bis(trifluoromethanesulfonylimide) and acetonitrile to prepare a PEO casting solution, which is then cast onto the PAN porous film. After vacuum drying, the composite solid electrolyte is formed, which has polyacrylonitrile fibers as a support, polyethylene oxide as a matrix and boron-doped graphene as a filler.

[0006] The usage ratio of polyacrylonitrile and N,N-dimethylformamide is 1g:1mL.

[0007] The boron-doped graphene is prepared by mixing graphene oxide and boric acid and performing an annealing reaction, wherein the mass ratio of graphene oxide to boric acid is preferably 1:2-1:8.

[0008] The amount of the boron-doped graphene is preferably 1%-5% of the mass of the polyethylene oxide, and the mass ratio of lithium bis(trifluoromethanesulfonyl)imide to polyethylene oxide is preferably 1:3-1:1.

[0009] The usage ratio of the polyethylene oxide and acetonitrile is 0.5465 g:12 mL.

[0010] The annealing reaction temperature is preferably 700°C-900°C.

[0011] The annealing reaction is preferably carried out under an argon atmosphere.

[0012] The electrospinning process used a syringe to inject the PAN spinning solution into a stainless steel needle, with aluminum foil as the receiving substrate. The distance between the needle and the receiving substrate was set to 15 cm, and the pushing speed was adjusted to 1 μm s -1 , a voltage of 15 kV was applied for electrospinning.

[0013] The present invention relates to a boron-doped graphene composite solid electrolyte prepared by the above method, comprising: polyacrylonitrile fibers as a composite solid electrolyte support, polyethylene oxide and lithium bis(trifluoromethanesulfonylimide) as a matrix, and boron-doped graphene with electron-deficient boron-containing groups as a filler, wherein: the supporting fibers are interwoven in the composite solid electrolyte to form a three-dimensional skeleton, the matrix forms a continuous lithium ion transport phase in the composite solid electrolyte, and the filler is uniformly dispersed in the polymer matrix to adsorb anions and reduce the crystallinity of the matrix.

[0014] The diameter of the boron-doped graphene is 500nm-3μm.

[0015] The diameter of the polyacrylonitrile fiber is 300nm-400nm.

[0016] The present invention relates to the application of the boron-doped graphene composite solid electrolyte, which is assembled into stainless steel|composite solid electrolyte|stainless steel battery, lithium|composite solid electrolyte|lithium battery and lithium iron phosphate|composite solid electrolyte|lithium battery.

[0017] The lithium-ion battery is preferably assembled in a glove box filled with high-purity argon.

[0018] The stainless steel | composite solid electrolyte | stainless steel battery is assembled by sequentially placing a composite solid electrolyte and a stainless steel sheet into a battery case. The electrochemical impedance spectroscopy (EIS) of the stainless steel | composite solid electrolyte | stainless steel battery is measured using an electrochemical workstation at an amplitude of 10 mV and a frequency range of 100 kΩ to 0.1 Hz to determine the ionic conductivity of the composite solid electrolyte.

[0019] The lithium|composite solid electrolyte|lithium battery is assembled by sequentially placing a lithium sheet, a composite solid electrolyte, and a lithium sheet into a battery case. The lithium|composite solid electrolyte|lithium battery was tested using an electrochemical workstation to determine the lithium ion transference number of the composite solid electrolyte by measuring the current-time curve at a polarization voltage of 10 mV and a polarization time of 3600 s, as well as the electrochemical impedance spectroscopy before and after polarization. Furthermore, the lithium|composite solid electrolyte|lithium battery was tested using a battery testing system for lithium stripping / deposition cycles at various current densities to determine the critical current density of the composite solid electrolyte.

[0020] The lithium iron phosphate | composite solid electrolyte | lithium battery is assembled by sequentially placing a lithium iron phosphate electrode, a composite solid electrolyte, and a lithium sheet into a battery housing. The battery was tested for rate performance and cycling stability by cycling it 10 times at 0.1C, 0.2C, and 0.5C rates, and 400 times at 0.3C, using a battery testing system.

[0021] Technical Effects

[0022] This invention dopes boron atoms into graphene through a solid-phase reaction and uses the boron-doped graphene as a filler in a PEO solid electrolyte to optimize the electrochemical performance of the solid electrolyte. Compared to existing technologies, this invention simultaneously optimizes both the ionic conductivity and the lithium ion transference number of the boron-doped graphene composite solid electrolyte, breaking the trade-off between the two in existing PEO solid electrolytes. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a scanning electron microscope image of the boron-doped graphene composite solid electrolyte prepared in Example 1;

[0024] Figure 2 Electrochemical impedance spectroscopy of the stainless steel|composite solid electrolyte|stainless steel battery assembled in Example 1 at room temperature, and current-time curves and electrochemical impedance spectroscopy before and after polarization of the lithium|composite solid electrolyte|lithium battery at room temperature;

[0025] Figure 3 The lithium stripping / deposition cycle diagram of the lithium|composite solid electrolyte|lithium battery assembled in Example 1 at different current densities at room temperature;

[0026] Figure 4 This is a rate performance diagram of the lithium iron phosphate | composite solid electrolyte | lithium battery assembled in Example 1 at room temperature;

[0027] Figure 5 This is a cycling stability diagram of the lithium iron phosphate | composite solid electrolyte | lithium battery assembled in Example 1 at room temperature. DETAILED DESCRIPTION

[0028] Example 1

[0029] This embodiment includes the following steps:

[0030] Step 1. Weigh 250 mg of graphene oxide and 1250 mg of boric acid into a conical flask, then add 50 mL of deionized water, and obtain a uniform liquid after sufficient magnetic stirring and ultrasonic treatment. The liquid is then freeze-dried and the product is placed in a tube furnace under an argon atmosphere and 900°C for 3 hours for annealing reaction. After centrifugation and washing in deionized water four times, boron-doped graphene is obtained.

[0031] Step 2: Weigh 0.0164 g of boron-doped graphene, 0.5465 g of polyethylene oxide, and 0.3644 g of lithium bis(trifluoromethanesulfonyl)imide into a conical flask, then add 12 mL of acetonitrile and magnetically stir at room temperature for 12 h to obtain a uniform PEO casting solution; separately weigh 0.7 g of polyacrylonitrile and 7 mL of N,N-dimethylformamide into a conical flask and magnetically stir at room temperature for 6 h to obtain a uniform PAN spinning solution.

[0032] Step 3: Cover the stainless steel roller of the electrospinning machine with a layer of aluminum foil as the receiving substrate of the PAN porous film. Transfer the PAN spinning solution prepared in step 2 into a 20 mL syringe and fix it. Set the pushing speed to 1 μm. -1 , adjust the distance between the needle and the receiving substrate to 15 cm, apply a high voltage of 15 kV between the needle and the receiving substrate, and after about 6 hours, remove the aluminum foil with the electrospun film and transfer it to a vacuum drying oven and dry it at 60°C for 12 hours to obtain a PAN porous film.

[0033] Step 4: Remove the aluminum foil on the PAN porous film, cast the PEO casting solution prepared in step 2 on both sides of the PAN porous film, transfer it to a fume hood and let it stand for two days to evaporate the acetonitrile solvent, then transfer it to a vacuum drying oven and dry it at 60°C for 12 hours to remove the residual acetonitrile solvent to obtain a boron-doped graphene composite solid electrolyte.

[0034] Step 5. Cut the boron-doped graphene composite solid electrolyte into discs with a diameter of 19 mm and assemble them into stainless steel|composite solid electrolyte|stainless steel battery, lithium|composite solid electrolyte|lithium battery, and lithium iron phosphate|composite solid electrolyte|lithium battery to test the ionic conductivity, lithium ion transference number, critical current density of the boron-doped graphene composite solid electrolyte and the rate performance and cycle stability of the lithium iron phosphate|composite solid electrolyte|lithium battery.

[0035] like Figure 1As shown, boron-doped graphene is uniformly dispersed in the matrix of the composite solid electrolyte in the form of particles with a diameter of 500nm-3μm. This uniform distribution reduces the crystallinity of the polyethylene oxide. The electron-deficient boron-containing groups on the graphene interact with the anions in the lithium salt through Lewis acid-base interactions, restricting the movement of anions and releasing more free lithium ions, thereby improving the ionic conductivity and lithium ion transference number of the composite solid electrolyte. The thickness of the boron-doped graphene composite solid electrolyte is approximately 200μm.

[0036] like Figure 2 As shown in the figure, it is calculated that the ionic conductivity of the boron-doped graphene composite solid electrolyte at room temperature is 9.27×10 -5 S cm -1 , the lithium ion migration number is 0.57.

[0037] like Figure 3 As shown in Figure 2, the critical current density of the boron-doped graphene composite solid electrolyte exceeds 0.05 mA cm -2 , the overpotential is stable at around 0.12V.

[0038] like Figure 4 As shown in Figure 2, at room temperature, the capacities of the lithium iron phosphate|composite solid electrolyte|lithium battery assembled with boron-doped graphene composite solid electrolyte at 0.1C and 0.2C rates are approximately 160 and 151 mAh g, respectively. -1 , stable charge and discharge cycles can be achieved at a rate of 0.5C, with a capacity of approximately 138mAh g –1 .

[0039] like Figure 5 As shown in Figure 2, under room temperature conditions, the discharge capacity of the lithium iron phosphate|composite solid electrolyte|lithium battery assembled with boron-doped graphene composite solid electrolyte at a rate of 0.3C is 149.3 mAh g -1 After 400 cycles, the capacity can still be maintained at 89.7% of the initial capacity.

[0040] In summary, the boron-doped graphene composite electrolyte prepared in this embodiment has excellent electrochemical properties.

[0041] Example 2

[0042] This embodiment includes the following steps:

[0043] Step 1. Weigh 250 mg of graphene oxide and 500 mg of boric acid into a conical flask, then add 50 mL of deionized water, and obtain a uniform liquid after sufficient magnetic stirring and ultrasonic treatment. The liquid is then freeze-dried and the product is placed in a tube furnace under an argon atmosphere and 700°C for 3 hours for annealing reaction. After centrifugation and washing in deionized water four times, boron-doped graphene is obtained.

[0044] Step 2: Weigh 0.0546 g of boron-doped graphene, 0.5465 g of polyethylene oxide, and 0.3644 g of lithium bis(trifluoromethanesulfonyl)imide into a conical flask, then add 12 mL of acetonitrile and magnetically stir at room temperature for 12 h to obtain a uniform PEO casting solution; in addition, prepare a PAN spinning solution according to the method of step 2 of Example 1.

[0045] Step 3: Prepare a PAN porous film according to the method of step 3 of Example 1.

[0046] Step 4: Prepare a boron-doped graphene composite solid electrolyte according to the method of step 4 of Example 1.

[0047] Step 5, testing the ionic conductivity, lithium ion transference number, and critical current density of the boron-doped graphene composite solid electrolyte and testing the rate performance and cycle stability of the lithium iron phosphate | composite solid electrolyte | lithium battery: The assembly and testing methods are the same as those in Step 5 of Example 1. The ionic conductivity of the boron-doped graphene composite solid electrolyte is calculated to be 2.76×10 -5 S cm -2 , the lithium ion transference number is 0.32. The critical current density of the boron-doped graphene composite solid electrolyte is only 0.02 mA cm -2 , and the overpotential exceeds 0.15 V. The lithium iron phosphate|composite solid electrolyte|lithium battery assembled with boron-doped graphene composite solid electrolyte has a specific capacity of 143 mAh g at a rate of 0.1C. -1 The specific capacity at 0.5C rate is 73 mAh g -1 , which is about half of the specific capacity at 0.1C rate. When cycled at 0.3C rate, the specific capacity is 108 mAh g -1 After 150 cycles, the specific capacity is 81 mAh g -1 .

[0048] Example 3

[0049] This embodiment includes the following steps:

[0050] Step 1. Weigh 250 mg of graphene oxide and 2000 mg of boric acid into a conical flask, then add 50 mL of deionized water, and obtain a uniform liquid after sufficient magnetic stirring and ultrasonic treatment. The liquid is then freeze-dried and the product is placed in a tube furnace under argon atmosphere and 900°C for 3 hours for annealing reaction. After centrifugation and washing in deionized water four times, boron-doped graphene is obtained.

[0051] Step 2: Weigh 0.0273 g of nitrogen-doped graphene, 0.5465 g of polyethylene oxide, and 0.5465 g of lithium bis(trifluoromethanesulfonyl)imide into a conical flask, then add 12 mL of acetonitrile and magnetically stir at room temperature for 12 h to obtain a uniform PEO casting solution; in addition, prepare a PAN spinning solution according to the method of step 2 of Example 1.

[0052] Step 3: Prepare a PAN porous film according to the method of step 3 of Example 1.

[0053] Step 4: Prepare a boron-doped graphene composite solid electrolyte according to the method of step 4 of Example 1.

[0054] Step 5, testing of ionic conductivity, lithium ion transference number and critical current density of the boron-doped graphene composite solid electrolyte and testing of rate performance and cycle stability of lithium iron phosphate | composite solid electrolyte | lithium battery: The assembly and testing methods are the same as those in step 5 of Example 1. The ionic conductivity of the boron-doped graphene composite solid electrolyte is calculated to be 5.90×10 -5 S cm -2 , the lithium ion transference number is 0.49. The critical current density of the boron-doped graphene composite solid electrolyte is 0.05 mA cm -2 , and the overpotential is 0.15 V. The lithium iron phosphate|composite solid electrolyte|lithium battery assembled with boron-doped graphene composite solid electrolyte has a specific capacity of 148 mAh g at a rate of 0.1C. -1 The specific capacity at 0.5C rate is 92 mAh g -1 , which is about 62% of the specific capacity at 0.1C rate. When cycled at 0.3C rate, the initial specific capacity is 121 mAh g -1 , able to maintain a cycle of more than 100 circles.

[0055] Example 4

[0056] This embodiment includes the following steps:

[0057] Step 1. Weigh 250 mg of graphene oxide and 1250 mg of boric acid into a conical flask, then add 50 mL of deionized water, and obtain a uniform liquid after sufficient magnetic stirring and ultrasonic treatment. The liquid is then freeze-dried and the product is placed in a tube furnace under an argon atmosphere and 800°C for 3 hours for annealing reaction. After centrifugation and washing in deionized water four times, boron-doped graphene is obtained.

[0058] Step 2: Take 0.0164 g of boron-doped graphene, 0.5465 g of polyethylene oxide, and 0.3644 g of lithium bis(trifluoromethanesulfonyl)imide into a conical flask, then add 12 mL of acetonitrile and magnetically stir at room temperature for 12 hours to obtain a uniform PEO casting solution; in addition, prepare a PAN spinning solution according to the method of step 2 of Example 1.

[0059] Step 3: Prepare a PAN porous film according to the method of step 3 of Example 1.

[0060] Step 4: Prepare a boron-doped graphene composite solid electrolyte according to the method of step 4 of Example 1.

[0061] Step 5, testing the ionic conductivity, lithium ion transference number, and critical current density of the boron-doped graphene composite solid electrolyte and testing the rate performance and cycle stability of the lithium iron phosphate | composite solid electrolyte | lithium battery: The assembly and testing methods are the same as those in Step 5 of Example 1. The ionic conductivity of the boron-doped graphene composite solid electrolyte is calculated to be 9.36×10 -5 S cm -2 , the lithium ion transference number is 0.55. The critical current density of the boron-doped graphene composite solid electrolyte is 0.05 mA cm -2 , and the overpotential is 0.13 V. The lithium iron phosphate|composite solid electrolyte|lithium battery assembled with boron-doped graphene composite solid electrolyte has a specific capacity of 159 mAh g at a rate of 0.1C. -1 The specific capacity at 0.5C rate is 101 mAh g -1 , which is about 64% of the specific capacity at 0.1C rate. When cycled at 0.3C rate, the initial specific capacity is 138 mAh g -1 , can maintain a cycle of more than 300 circles.

[0062] According to the above experiments, the boron-doped graphene composite solid electrolyte provided by the present invention has excellent electrochemical properties and can be widely used in solid-state batteries.

[0063] Compared to existing technologies, this method uses boron-doped graphene with electron-deficient centers as a filler in a solid electrolyte. This filler is evenly dispersed within the solid electrolyte, reducing the crystallinity of the polymer and promoting the movement of polyethylene oxide segments, which facilitates the transport of lithium ions. Furthermore, the electron-deficient boron can act on negatively charged anions, restraining their movement while promoting the dissociation of lithium salts and releasing more free lithium ions. This significantly improves the ionic conductivity and lithium ion transference number of the composite solid electrolyte, thereby endowing lithium batteries with improved rate performance and cycling stability.

[0064] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. A method for preparing a boron-doped graphene composite solid electrolyte, characterized in that: A mixed solution of polyacrylonitrile (PAN) and N,N-dimethylformamide was prepared into a PAN porous film through electrospinning technology; boron-doped graphene, polyethylene oxide, lithium bis(trifluoromethanesulfonylimide) and acetonitrile were mixed to prepare a PEO casting solution, which was then cast onto the PAN porous film. After vacuum drying, a composite solid electrolyte was formed with polyacrylonitrile fiber as the support, polyethylene oxide as the matrix, and boron-doped graphene as the filler.

2. The method for preparing a boron-doped graphene composite solid electrolyte according to claim 1, wherein: The boron-doped graphene is prepared by mixing graphene oxide and boric acid and performing an annealing reaction.

3. The method for preparing a boron-doped graphene composite solid electrolyte according to claim 1, wherein: The amount of the boron-doped graphene accounts for 1%-5% of the mass of the polyethylene oxide, and the mass ratio of lithium bis(trifluoromethanesulfonyl)imide to the polyethylene oxide is 1:3-1:

1.

4. The method for preparing a boron-doped graphene composite solid electrolyte according to claim 1, wherein: The electrospinning process used a syringe to inject the PAN spinning solution into a stainless steel needle, with aluminum foil as the receiving substrate. The distance between the needle and the receiving substrate was set to 15 cm, and the pushing speed was adjusted to 1 μm s -1 , a voltage of 15 kV was applied for electrospinning.

5. A boron-doped graphene composite solid electrolyte prepared according to the method of any one of claims 1 to 4, characterized in that: include: The invention relates to a composite solid electrolyte comprising polyacrylonitrile fibers as a support, polyethylene oxide and lithium bis(trifluoromethanesulfonyl)imide as a matrix, and boron-doped graphene with electron-deficient boron-containing groups as a filler, wherein: the supporting fibers are interwoven in the composite solid electrolyte to form a three-dimensional skeleton, the matrix forms a continuous lithium ion transport phase in the composite solid electrolyte, and the filler is uniformly dispersed inside the polymer matrix to adsorb anions and reduce the crystallinity of the matrix.

6. A use of a boron-doped graphene composite solid electrolyte prepared according to any one of claims 1 to 4 or according to claim 5, characterized in that: They are assembled into stainless steel | composite solid electrolyte | stainless steel battery, lithium | composite solid electrolyte | lithium battery and lithium iron phosphate | composite solid electrolyte | lithium battery.

7. The use according to claim 6, characterized in that: The stainless steel|composite solid electrolyte|stainless steel battery is obtained by sequentially placing a composite solid electrolyte and a stainless steel sheet into a battery shell and then assembling them; The lithium|composite solid electrolyte|lithium battery is obtained by sequentially placing a lithium sheet, a composite solid electrolyte, and a lithium sheet into a battery shell and assembling them; The lithium iron phosphate|composite solid electrolyte|lithium battery is obtained by sequentially placing a lithium iron phosphate pole piece, a composite solid electrolyte, and a lithium sheet into a battery shell and then assembling them.

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