Method for preparing gel polymer electrolyte and secondary lithium battery

Abstract

The application provides a gel polymer electrolyte and a preparation method of a secondary lithium battery. In a water and oxygen isolation environment, a pre-polymer slurry is obtained by adding a polymer monomer, a thermal initiator and one of lithium carbonate, dimethylsiloxane or trimethyl borate into an organic electrolyte; the pre-polymer slurry is coated on the surface of a porous support material, and a gel polymer electrolyte is polymerized in situ. The preparation method of the application can generate lithium difluorophosphate in the polymerization process, can reduce the interface resistance between the polymer electrolyte and the electrode, can improve the interface stability and the ion transmission efficiency, can improve the cycle and rate performance of the lithium battery, and the battery can be used in a wide temperature range of-20 DEG C to 90 DEG C. The application can generate lithium difluorophosphate in situ, which can improve the performance of the electrolyte, does not need to be added additionally, and is simple in introduction.

Description

Technical Field

[0001] This invention relates to a method for preparing an electrolyte and a secondary lithium battery using the electrolyte, and particularly to a method for preparing a gel polymer electrolyte and a secondary lithium battery. Background Technology

[0002] Since its commercialization by Sony in 1991, lithium-ion batteries have rapidly gained global popularity. A typical lithium-ion battery consists of an electrolyte, positive and negative electrode materials, current collectors bonded to the electrode materials, and a packaging casing. The electrolyte is a crucial component of lithium-ion rechargeable batteries, controlling not only the internal ion transport kinetics but also fundamentally determining the battery's operating mechanism, influencing its specific energy, charge / discharge rate performance, cycle life, safety performance, and production cost. Based on their morphological characteristics and composition, electrolytes can be classified into commercially successful organic liquid electrolytes, inorganic solid electrolytes, and polymer electrolytes. Polymer electrolytes include gel polymer electrolytes and all-solid-state polymer electrolytes. All-solid-state polymer electrolytes offer high safety, good mechanical properties, and favorable processing and design performance, effectively suppressing lithium dendrite formation. However, poor interfacial contact and insufficient room-temperature conductivity have hindered the commercialization of all-solid-state polymer electrolytes. Gel polymer electrolytes combine the excellent interfacial wettability and efficient ion transport capabilities of organic liquid electrolytes with the processing performance of polymer electrolytes, showing promising application prospects. Compared to liquid organic electrolytes, gel polymer electrolytes contain relatively less electrolyte, resulting in improved safety. However, the limited liquid electrolyte is prone to side reactions with the electrode materials at the electrode interface, leading to increased interfacial impedance and deterioration of battery performance (especially under high-temperature conditions). Forming a stable interfacial solid electrolyte film helps to address this issue. Summary of the Invention

[0003] This invention aims to provide a method for preparing gel polymers via electrolysis, which can effectively improve the interfacial stability and interfacial ion transport efficiency of the electrolyte, thereby enhancing the cycle and rate performance of secondary lithium batteries. This invention is implemented through the following scheme.

[0004] A method for preparing a gel polymer electrolyte includes the following steps: under a protective atmosphere with water and oxygen content both less than 1 ppm, a polymer monomer and a thermal initiator are added to an organic electrolyte composed of a lithium salt and an organic solvent, and one of lithium carbonate, dimethylsiloxane, or trimethyl borate is also added and dispersed evenly to obtain a prepolymer slurry; the obtained prepolymer slurry is coated onto the surface of a porous support material and reacted at 25–150°C for 0.1–100 h to obtain the gel polymer electrolyte; wherein the lithium salt is lithium hexafluorophosphate or a mixed lithium salt of lithium hexafluorophosphate and other lithium salts.

[0005] Other lithium salts may be one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium chloride, lithium iodide, lithium tri(pentafluoroethyl)trifluorophosphate, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0006] In the prepolymer slurry, the solvent, initiator, and polymer monomer are based on commonly used gel polymer lithium secondary battery solutions. The solvent can be one or more from the group consisting of carbonate organic solvents, ether organic solvents, nitrile organic solvents, fluorocarbonate organic solvents, fluoroether organic solvents, fluoronitrile organic solvents, organic acid esters with fewer than 6 carbon atoms, substituted oxyalkane organic compounds, propane sulpholactone, ethylene sulfite, fluorobenzene, dimethyl sulfoxide, or sulfolane. The initiator is one from the group consisting of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, or dimethyl azobisisobutyrate. The polymer monomer is one or any combination of two or more free radical polymerizable monomers containing vinyl groups, propylene groups, acrylate groups, methacrylate groups, or maleic anhydride groups.

[0007] Experiments have shown that it is preferable to add one of the lithium carbonate, dimethylsiloxane, or trimethyl borate to the organic electrolyte in an amount that is 0.01 to 10% of the mass of the organic electrolyte.

[0008] A gel polymer electrolyte secondary lithium battery includes a casing and a positive electrode, a negative electrode, and a gel polymer electrolyte between the positive and negative electrodes encapsulated within the casing. The gel polymer electrolyte is prepared by the method described above.

[0009] A method for preparing a gel polymer solid electrolyte secondary lithium battery involves adding polymer monomers and a thermal initiator to an organic electrolyte composed of lithium salt and an organic solvent under a protective atmosphere with water and oxygen contents both less than 1 ppm. The lithium salt is lithium hexafluorophosphate or a mixture of lithium hexafluorophosphate and other lithium salts. Lithium carbonate, dimethylsiloxane, or trimethyl borate are also added and dispersed uniformly to obtain a prepolymer slurry. The prepolymer slurry is then coated onto the surface of a porous support material. Positive and negative electrode plates and a porous support material coated with the prepolymer slurry are encapsulated within a battery casing. The mixture is reacted at 25–150°C for 0.1–100 h to obtain the gel polymer electrolyte secondary lithium battery.

[0010] The other lithium salts may be one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium chloride, lithium iodide, lithium tri(pentafluoroethyl)trifluorophosphate, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0011] In the prepolymer slurry, the solvent, initiator, and polymer monomer are based on commonly used gel polymer lithium secondary battery solutions. The solvent can be one or more from the group consisting of carbonate organic solvents, ether organic solvents, nitrile organic solvents, fluorinated carbonate organic solvents, fluorinated ether organic solvents, fluorinated nitrile organic solvents, organic acid esters with fewer than 6 carbon atoms, substituted oxyalkane organic compounds, propane sulpholactone, ethylene sulfite, fluorobenzene, dimethyl sulfoxide, or sulfolane. The initiator is one from the group consisting of azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide, or dimethyl azobisisobutyrate. The polymer monomer is one or any combination of two or more free radical polymerizable monomers containing vinyl groups, propylene groups, acrylate groups, methacrylate groups, or maleic anhydride groups.

[0012] The active material for the positive electrode in the above-mentioned preparation of gel polymer secondary lithium batteries can be one or more of the active materials commonly used in secondary lithium batteries with conventional organic electrolytes, such as high-nickel ternary, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, and lithium nickel phosphate. The active material for the negative electrode can be one or more of the active materials commonly used in secondary lithium batteries with conventional organic electrolytes, including metallic lithium, lithium alloys, carbon materials, and silicon-based materials.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] 1. The preparation method of the present invention can generate lithium difluorophosphate during the polymerization process, which can reduce the interfacial resistance between the polymer electrolyte and the electrode, improve the interfacial stability and ion transport efficiency, thereby achieving good cycle and rate performance of lithium batteries.

[0015] 2. The preparation method of the present invention can generate lithium difluorophosphate, a substance that can improve electrolyte performance, in situ during processes such as polymerization, high-temperature storage aging, and high-temperature cycling. No additional addition is required, and the introduction method is simple. It can be directly introduced without changing the existing gel polymer electrolyte and secondary lithium battery process, thus facilitating production and promotion.

[0016] 3. The gel polymer battery prepared by the method of the present invention can be used in a wide temperature range of -20℃ to 90℃, which greatly improves the applicability of the battery. Attached Figure Description

[0017] Figure 1 It is the in-situ generated additive in Example 1 31 P-NMR NMR spectrum

[0018] Figure 2 It is the in-situ generated additive in Example 1 19 F-NMR NMR spectrum Detailed Implementation

[0019] Example 1

[0020] A method for preparing a gel polymer electrolyte includes the following steps: In an environment with a protective atmosphere and both water and oxygen content less than 1 ppm (such as in a glove box), 1 mol / L LiPF6 is added to an organic electrolyte composed of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:1 to form an organic electrolyte. Then, 5% by mass of the polymer monomer methyl methacrylate, 1% by mass of the polymer monomer initiator azobisisobutyronitrile (AIBN), and 4% by mass of the organic electrolyte are added to the organic electrolyte, and the mixture is dispersed evenly to obtain a prepolymer slurry. The obtained prepolymer slurry is coated onto the surface of a porous support material, polyvinylidene fluoride-hexafluoropropylene membrane, and reacted at 60°C for 10 h to obtain the gel polymer electrolyte.

[0021] A method for preparing a gel polymer solid electrolyte secondary lithium battery involves encapsulating a lithium cobalt oxide positive electrode, a lithium metal negative electrode, and a polyvinylidene fluoride-hexafluoropropylene membrane as active materials within a battery casing, injecting the aforementioned prepolymerized slurry into the casing, and reacting the mixture at 60°C for 10 hours to obtain the gel polymer electrolyte secondary lithium battery.

[0022] The above-mentioned gel polymer electrolyte 31 P-NMR nuclear magnetic spectrum and 31 The P-NMR spectroscopy spectra are shown below. Figure 1 and Figure 2 The two figures together illustrate that a substance called "lithium difluorophosphate (LiPO2F2)" is generated in the gel polymer electrolyte.

[0023] In comparison, using essentially the same methods and materials as described above, except that lithium carbonate was not added during the preparation of the prepolymer slurry, a gel polymer electrolyte secondary lithium battery of Comparative Example 1 was prepared. Under identical testing conditions, the performance of the secondary lithium batteries of Example 1 and Comparative Example 1 was tested respectively. The specific capacities of the secondary lithium battery of Example 1 at 0.5C, 1C, and 2C charge / discharge at room temperature reached 150 mAh / g, 148 mAh / g, and 142 mAh / g, respectively, while the specific capacities of the secondary lithium battery of Comparative Example 1 at 0.5C, 1C, and 2C charge / discharge at room temperature were 140 mAh / g, 137 mAh / g, and 129 mAh / g, respectively. After 300 cycles, the capacity retention rate of the secondary lithium battery of Example 1 was higher than 80%, while the capacity retention rate of the secondary lithium battery of Comparative Example 1 was lower than 65%. These experimental results indicate that the battery of Example 1 has improved cycle performance and rate performance compared to the battery of Comparative Example 1. Figure 1 and Figure 2It can be concluded that the electrolyte in the battery of Example 1, compared with that of Comparative Example 1, contains "lithium difluorophosphate". This substance can reduce the interfacial resistance between the polymer electrolyte and the electrode, improve the interfacial stability and ion transport efficiency, thereby achieving good cycle and rate performance of the lithium battery.

[0024] Example 2

[0025] A method for preparing a gel polymer electrolyte includes the following steps: In an environment with a protective atmosphere and both water and oxygen content less than 1 ppm (such as in a glove box), a mixed lithium salt consisting of 0.5 mol / L LiPF6 and 0.5 mol / L LiTFSI is added to a mixed organic solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1 to form an organic electrolyte. Then, 3% (by mass) of acrylonitrile, 1% (by mass) of azobisisobutyronitrile (azobisisobutyronitrile), and 5% (by mass) of dimethylsiloxane are added to the organic electrolyte and dispersed uniformly to obtain a prepolymer slurry. The prepolymer slurry is then coated onto the surface of a porous support material, a cellulose nonwoven membrane, and reacted at 70°C for 90 h to obtain the gel polymer electrolyte.

[0026] A method for preparing a gel polymer solid electrolyte secondary lithium battery, wherein the active material is encapsulated within the battery casing as LiNi. 0.8 Co 0.1 Mn 0.1 After forming an O2 positive electrode, a lithium metal negative electrode, and a cellulose nonwoven membrane, the above-mentioned prepolymerized slurry is injected into the membrane, and the mixture is reacted at 70°C for 90 hours to obtain a gel polymer electrolyte secondary lithium battery.

[0027] In comparison, using essentially the same methods and materials as described above, except that dimethylsiloxane was not added during the preparation of the prepolymer slurry, a gel polymer electrolyte secondary lithium battery of Comparative Example 2 was prepared. Under identical testing conditions, the performance of the secondary lithium batteries of Example 2 and Comparative Example 2 was tested respectively. The specific capacities of the secondary lithium battery of Example 2 at 0.5C, 1C, and 2C charge / discharge at room temperature reached 170 mAh / g, 167 mAh / g, and 162 mAh / g, respectively, while the specific capacities of the secondary lithium battery of Comparative Example 2 at 0.5C, 1C, and 2C charge / discharge at room temperature were 150 mAh / g, 146 mAh / g, and 141 mAh / g, respectively. After 300 cycles, the capacity retention rate of the secondary lithium battery of Example 2 was higher than 90%, while the capacity retention rate of the secondary lithium battery of Comparative Example 2 was lower than 85%. These experimental results indicate that the battery of Example 2 has improved cycle performance and rate performance compared to the battery of Comparative Example 2.

[0028] Example 3

[0029] A method for preparing a gel polymer electrolyte includes the following steps: In an environment with a protective atmosphere and both water and oxygen content less than 1 ppm (such as in a glove box), a mixed lithium salt consisting of 0.5 mol / L LiPF6 and 0.5 mol / L LiTFSI is added to a mixed organic solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 to form an organic electrolyte. Then, 3% (by mass) of maleic anhydride (a polymer monomer), 1% (by mass) of azobisisobutyronitrile (an initiator), and 3% (by mass) of trimethyl borate (a solution mass) are added to the organic electrolyte and dispersed uniformly to obtain a prepolymer slurry. The prepolymer slurry is then coated onto the surface of a porous support material, a glass fiber membrane, and reacted at 60°C for 80 h to obtain the gel polymer electrolyte.

[0030] A method for preparing a gel polymer solid electrolyte secondary lithium battery, wherein the active material is encapsulated within the battery casing as Li. 1.2 Mn 0.53 Ni 0.27 After forming an O2 positive electrode, a graphite negative electrode, and a glass fiber membrane, the above-mentioned prepolymerized slurry is injected into them, and the mixture is reacted at 60°C for 80 hours to obtain a gel polymer electrolyte secondary lithium battery.

[0031] In comparison, using essentially the same methods and materials as described above, except that maleic anhydride was not added during the preparation of the prepolymer slurry, a gel polymer electrolyte secondary lithium battery of Comparative Example 3 was prepared. Under identical testing conditions, the performance of the secondary lithium batteries of Example 3 and Comparative Example 3 was tested respectively. The specific capacities of the secondary lithium battery of Example 3 at 0.5C, 1C, and 2C charge / discharge at room temperature reached 185 mAh / g, 181 mAh / g, and 177 mAh / g, respectively, while the specific capacities of the secondary lithium battery of Comparative Example 3 at 0.5C, 1C, and 2C charge / discharge at room temperature were 162 mAh / g, 158 mAh / g, and 151 mAh / g, respectively. After 300 cycles, the capacity retention rate of the secondary lithium battery of Example 3 was higher than 90%, while the capacity retention rate of the secondary lithium battery of Comparative Example 3 was lower than 80%. These experimental results indicate that the battery of Example 3 has improved cycle performance and rate performance compared to the battery of Comparative Example 3.

Claims

1. A method for preparing a gel polymer electrolyte, comprising the following steps: In an environment with a protective atmosphere and water and oxygen content of less than 1 ppm, a polymer monomer and a thermal initiator are added to an organic electrolyte composed of lithium salt and organic solvent, and dispersed evenly to obtain a prepolymer slurry; the obtained prepolymer slurry is coated on the surface of a porous support material and reacted at 25-150°C for 0.1-100 h to obtain a gel polymer electrolyte; characterized in that: dimethylsiloxane is also added to the organic electrolyte; the lithium salt is lithium hexafluorophosphate or a mixed lithium salt of lithium hexafluorophosphate and other lithium salts.

2. The method for preparing the gel polymer electrolyte as described in claim 1, characterized in that: The amount of dimethylsiloxane added is 0.01 to 10% of the mass of the organic electrolyte.

3. A gel polymer electrolyte secondary lithium battery, comprising a casing and a positive electrode, a negative electrode, and a gel polymer electrolyte disposed between the positive and negative electrodes within the casing, characterized in that: The gel polymer electrolyte is prepared by the method of claim 1 or 2.

4. A method for preparing a gel polymer solid electrolyte secondary lithium battery, comprising: adding polymer monomers and thermal initiators to an organic electrolyte composed of lithium salt and organic solvent in a protective atmosphere with water and oxygen content both less than 1 ppm, dispersing the mixture uniformly to obtain a prepolymer slurry; coating the obtained prepolymer slurry onto the surface of a porous support material; encapsulating positive and negative electrode sheets and a porous support material coated with the prepolymer slurry in a battery casing; and reacting the mixture at 25–150°C for 0.1–100 h to obtain a gel polymer electrolyte secondary lithium battery; characterized in that: Dimethylsiloxane is also added to the organic electrolyte; the lithium salt is lithium hexafluorophosphate or a mixture of lithium hexafluorophosphate and other lithium salts.

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