A boronated polysaccharide composite solid electrolyte and its preparation method and application

Abstract

The invention discloses a boronized polysaccharide composite solid electrolyte and a preparation method and application thereof. The boronized polysaccharide composite solid electrolyte is composed of the following components in parts by weight: 4-10 polysaccharides, 0.5-2 boric acid or borax, 0.5-2 binder, and 0.5-2 lithium salt; the binder is any one of polyacrylic acid, polyvinyl alcohol, and polyvinyl pyrrolidone. The lithium salt is one or more of lithium hexafluorophosphate, lithium bistrifluoromethanesulfonyl imide, and lithium carbonate. The polysaccharide is a mixture of any one or more of starch, starch hydrolysate, chitosan, and arabinan. The electrolyte contains a high-density three-dimensional ion transmission channel network structure, which can directly transfer lithium ions through polysaccharide polar groups, thereby well promoting the transfer of lithium ions between the positive and negative electrodes of the battery, and improving the cycle performance and rate performance of the battery. The electrolyte also contains abundant cross-linking points, so it has good mechanical properties and is suitable for lithium-ion batteries of various forms.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and more specifically, relates to a boronated polysaccharide composite solid electrolyte and a preparation method and application thereof. Background Art

[0002] Solid-state lithium-ion batteries have attracted widespread attention due to their high energy density and excellent safety performance. However, there are still some key issues that need to be resolved in the practical application of solid-state lithium-ion batteries. First, the low conductivity and high cost of existing solid electrolytes are not conducive to the transmission of lithium ions and the large-scale commercial application of solid-state batteries. Second, the poor interface contact between the solid electrolyte and the electrode further aggravates the lithium ion transmission barrier and may trigger a series of side reactions.

[0003] In order to solve the above problems, a common method is to use polymer solid electrolytes, such as polyethylene oxide, polyacrylonitrile, polyvinyl chloride, polymethyl methacrylate, polyvinylidene fluoride, polypropylene carbonate and other polymers. In addition, methods for modifying the surface of the positive and negative electrodes of the battery have also been widely studied. However, a method for preparing a solid electrolyte by cross-linking small molecule polysaccharides has not yet been reported. Summary of the invention

[0004] In order to solve the above-mentioned deficiencies and shortcomings of the prior art, the purpose of the present invention is to provide a boronated polysaccharide composite solid electrolyte, which can solve the problems of low electrolyte conductivity, high cost, poor interface, etc. in solid-state lithium-ion batteries.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned boronated polysaccharide composite solid electrolyte.

[0006] Another object of the present invention is to provide an application of the boronated polysaccharide composite solid electrolyte.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] A boronized polysaccharide composite solid electrolyte, the boronized polysaccharide composite solid electrolyte is composed of the following components in parts by weight:

[0009] Polysaccharide 4-10,

[0010] Boric acid 0.5-2,

[0011] Binder 0.5-2,

[0012] Lithium salt 0.5-2;

[0013] The binder is any one of polyacrylic acid, polyvinyl alcohol and polyvinyl pyrrolidone.

[0014] Preferably, in the above-mentioned boronated polysaccharide composite solid electrolyte, the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, and lithium carbonate.

[0015] Preferably, in the above-mentioned boronated polysaccharide composite solid electrolyte, the polysaccharide is a mixture of any one or more of starch, starch hydrolysate, cellulose, chitosan and arabinan.

[0016] Preferably, in the above-mentioned boronated polysaccharide composite solid electrolyte, the molecular weight of the polysaccharide is 100-100,000 Da.

[0017] Preferably, in the above-mentioned boronated polysaccharide composite solid electrolyte, the molecular weight of the binder is 50,000~1,000,000 Da.

[0018] The method for preparing the above-mentioned boronated polysaccharide composite solid electrolyte comprises the following steps:

[0019] S1. Add boric acid to water and stir to form a transparent solution;

[0020] S2. Add the polysaccharide to the transparent solution described in step S1 and continue stirring to make it uniformly dispersed, then heat the solution to 80° C. and keep it warm for 2 hours;

[0021] S3. Adding a binder and a lithium salt to the solution obtained in step S2, stirring for 8 hours to make it evenly dispersed, then casting it into a mold, and evaporating the solvent to obtain a boronized polysaccharide composite solid electrolyte.

[0022] Application of the above-mentioned boronated polysaccharide composite solid electrolyte in lithium-ion batteries.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention cross-links polysaccharides with boric acid. Due to the natural network structure of polysaccharides, the obtained composite solid electrolyte has a high-density ion transmission channel network structure. This structure has abundant and uniform ion conduction sites, which is conducive to the transmission of lithium ions between the positive and negative electrodes of the battery, thereby improving the cycle performance and rate performance of the battery.

[0025] 2. The present invention utilizes boric acid to cross-link polysaccharide molecules together, thereby improving the mechanical properties of the solid electrolyte, and further enhancing its flexibility through a polymer binder, thereby optimizing the interface and making it applicable to various forms of lithium-ion batteries.

[0026] 3. The raw material used in the present invention is polysaccharide, which has the characteristics of low price, wide source, and multiple polar sites, which can significantly reduce the production cost of solid electrolytes and ensure the rapid conduction of lithium ions in the electrolyte. In addition, water is used as a solvent in the preparation process, making the solid electrolyte non-toxic and environmentally friendly.

[0027] 4. The solid electrolyte prepared by the present invention can directly transfer lithium ions through the polysaccharide polar groups, avoiding the problem of low lithium ion conductivity contribution of the "chain segment swing" conduction mechanism of traditional polymer electrolytes at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a battery cycle performance diagram of the solid electrolyte prepared in Examples 1 and 2 and a lithium iron phosphate positive electrode.

[0029] Figure 2 The battery cycle performance diagram of the solid electrolyte prepared in Examples 1 and 2 and Comparative Examples 1 and 2 and matched with lithium iron phosphate positive electrode.

[0030] Figure 3 It is the natural network structure of polysaccharides. DETAILED DESCRIPTION

[0031] The content of the present invention is further described below in conjunction with specific examples, but it should not be construed as limiting the present invention. If not specifically indicated, the technical means used in the examples are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0032] A boronized polysaccharide composite solid electrolyte comprises polysaccharide, boric acid, a binder and a lithium salt. The polysaccharide (molecular weight of 100-100,000 Da) is composed of multiple monosaccharides bonded by glycosidic bonds and contains abundant polar groups.

[0033] Preparation of boronated polysaccharide composite solid electrolyte: dissolve boric acid in water under stirring conditions and stir to dissolve it; add polysaccharide to the above solution and continue stirring to disperse it evenly, then heat the solution to 80°C and keep it warm for 2 hours to allow the boric acid and polysaccharide to cross-link; add polyacrylic acid and lithium bistrifluoromethanesulfonyl imide to the above solution, stir for 8 hours to disperse it evenly, then cast it in a mold, and evaporate the solvent to obtain a boronated polysaccharide composite solid electrolyte.

[0034] The boronized polysaccharide composite solid electrolyte is used in a lithium-ion battery, which comprises a lithium iron phosphate positive electrode, a boronized polysaccharide composite solid electrolyte and a lithium metal negative electrode. The lithium iron phosphate positive electrode comprises a lithium iron phosphate active material (LFP), a conductive agent (Super P) and a binder (PVDF). The lithium iron phosphate active material, the conductive agent and the binder are mixed and stirred in N-methylpyrrolidone (NMP), and then coated on a carbon-coated aluminum foil. After drying, the lithium iron phosphate positive electrode can be obtained by cutting the sheet.

[0035] Embodiment 1:

[0036] 1. Add boric acid to water and stir to dissolve.

[0037] 2. Select starch as the raw material polysaccharide, add starch to the above solution and continue stirring to make it evenly dispersed (the mass ratio of starch to boric acid is 7:1), then heat the solution to 80°C and keep it warm for 2 hours to allow the boric acid and starch to cross-link.

[0038] 3. Add polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide to the above solution (the mass ratio of starch, polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide is 7:1:1), stir for 8 hours to make it dispersed evenly, then cast it into a mold, evaporate the solvent to obtain a boronized polysaccharide composite solid electrolyte. Due to the natural network structure of polysaccharides, such as Figure 3 As shown, the composite solid electrolyte obtained in Example 1 has a high-density ion transport channel network structure.

[0039] 3. Mix the lithium iron phosphate active material (LFP), conductive agent (Super P) and binder (PVDF) in N-methylpyrrolidone at a mass ratio of 8:1:1 to obtain a positive electrode slurry. Scrape the positive electrode slurry onto the carbon-coated aluminum foil current collector, and cut it into a circular electrode with a diameter of 12 mm after drying to obtain a lithium iron phosphate positive electrode.

[0040] 4. Lithium-ion batteries are assembled in a glove box in the following order: positive battery shell, lithium iron phosphate positive electrode sheet, solid electrolyte, conductive agent, lithium sheet, negative battery shell, and the lithium-ion battery is obtained after packaging. The solute of the conductive agent is 30% LiTFSI and the solvent is succinonitrile.

[0041] Embodiment 2:

[0042] 1. Add boric acid to water and stir to dissolve.

[0043] 2. Cellulose is selected as the raw material polysaccharide, and cellulose is added to the above solution and stirred to make it evenly dispersed (the mass ratio of cellulose to boric acid is 7:1). The solution is then heated to 80°C and kept warm for 2 hours to allow the boric acid and cellulose to cross-link.

[0044] 3. Add polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide to the above solution (the mass ratio of cellulose, polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide is 7:1:1), stir for 8 hours to make it evenly dispersed, then cast it in a mold, and evaporate the solvent to obtain a boronized polysaccharide composite solid electrolyte.

[0045] 4. Mix the lithium iron phosphate active material (LFP), the conductive agent (Super P) and the binder (PVDF) in N-methylpyrrolidone at a mass ratio of 8:1:1 to obtain a positive electrode slurry. Scrape the positive electrode slurry onto the carbon-coated aluminum foil current collector, and cut it into a circular electrode with a diameter of 12 mm after drying to obtain a lithium iron phosphate positive electrode.

[0046] 5. Lithium-ion batteries are assembled in a glove box in the following order: positive battery shell, lithium iron phosphate positive electrode sheet, solid electrolyte, conductive agent, lithium sheet, negative battery shell, and the lithium-ion battery is obtained after packaging. The solute of the conductive agent is 30% LiTFSI and the solvent is succinonitrile.

[0047] Comparative Example 1:

[0048] 1. Add starch to water under stirring. After it is evenly dispersed, heat the solution to 80℃ and keep it warm for 2 hours.

[0049] 2. Add polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide to the above solution (the mass ratio of starch, polyacrylic acid and lithium bis(trifluoromethanesulfonyl)imide is 8:1:1), stir for 8 hours to make it evenly dispersed, then cast it in a mold, evaporate the solvent to obtain a polysaccharide composite solid electrolyte.

[0050] 3. Mix the lithium iron phosphate active material (LFP), conductive agent (Super P) and binder (PVDF) in N-methylpyrrolidone at a mass ratio of 8:1:1 to obtain a positive electrode slurry. Scrape the positive electrode slurry onto the carbon-coated aluminum foil current collector, and cut it into a circular electrode sheet with a diameter of 12 mm after drying to obtain a lithium iron phosphate positive electrode sheet.

[0051] 4. Lithium-ion batteries are assembled in a glove box in the following order: positive battery shell, lithium iron phosphate positive electrode sheet, solid electrolyte, conductive agent, lithium sheet, negative battery shell, and the lithium-ion battery is obtained after packaging. The solute of the conductive agent is 30% LiTFSI and the solvent is succinonitrile.

[0052] Comparative Example 2:

[0053] 1. Under stirring conditions, polyethylene oxide (PEO) is added to water. After it is completely dissolved, lithium bis(trifluoromethanesulfonyl)imide is added to the above solution (the mass ratio of polyethylene oxide to lithium bis(trifluoromethanesulfonyl)imide is 9:1), stirred for 8 hours to make it evenly dispersed, and then cast into a mold, and the solvent is evaporated to obtain a polyethylene oxide solid electrolyte (i.e., a traditional polymer solid electrolyte).

[0054] 2. Mix the lithium iron phosphate active material (LFP), conductive agent (Super P) and binder (PVDF) in N-methylpyrrolidone at a mass ratio of 8:1:1 to obtain a positive electrode slurry. Scrape the positive electrode slurry onto the carbon-coated aluminum foil current collector, and cut it into a circular electrode sheet with a diameter of 12 mm after drying to obtain a lithium iron phosphate positive electrode sheet.

[0055] 3. Lithium-ion batteries are assembled in a glove box in the following order: positive battery shell, lithium iron phosphate positive electrode sheet, solid electrolyte, conductive agent, lithium sheet, negative battery shell, and then packaged to obtain lithium-ion batteries. The solute of the conductive agent is 30% LiTFSI and the solvent is succinonitrile.

[0056] The lithium ion batteries assembled in Examples 1 and 2 and Comparative Examples 1 and 2 were placed at 28°C for 5 hours before being subjected to a charge and discharge cycle test. The test conditions were: first cycle activation current density 0.1C, cycle current density 0.2C, voltage window 2.5-3.8V. Figure 1 The charge and discharge cycle diagram of the lithium ion battery prepared with the solid electrolyte in Example 1. Figure 1 It can be seen that at a current density of 0.1C, the lithium-ion battery using the boronated polysaccharide composite solid electrolyte has a first discharge capacity of 171 mAh / g, and a discharge capacity of 157 mAh / g after 50 cycles, indicating that the electrode exhibits excellent first-cycle discharge capacity and cycle stability. Figure 2 The figure is a comparison chart of the cycle performance of lithium-ion batteries prepared in Example 1 and Comparative Examples 1 and 2. Figure 2 It can be seen that the lithium ion battery prepared in Example 1 has a higher discharge specific capacity and capacity retention rate. The test results show that the boronized polysaccharide composite solid electrolyte of the present invention is conducive to the transmission of lithium ions between the positive and negative electrodes of the battery and improves the electrode-electrolyte interface, thereby improving the cycle stability of the lithium battery.

Claims

1. A boronated polysaccharide composite solid electrolyte, characterized in that: The boronized polysaccharide composite solid electrolyte is made of the following components in parts by weight: Polysaccharide 4-10, Boric acid 0.5-2, Binder 0.5-2, Lithium salt 0.5-2; The binder is any one of polyacrylic acid, polyvinyl alcohol, and polyvinyl pyrrolidone, and the molecular weight of the binder is 50,000 to 1,000,000 Da; The method for preparing the boronated polysaccharide composite solid electrolyte comprises the following steps: S1. Add boric acid to water and stir to form a transparent solution; S2. Add the polysaccharide to the transparent solution described in step S1 and continue stirring to make it uniformly dispersed, then heat the solution to 80° C. and keep it warm for 2 hours; S3. Add the binder and lithium salt to the solution obtained in step S2, stir for 8 hours to make it evenly dispersed, then cast it in a mold, and evaporate the solvent to obtain a boronated polysaccharide composite solid electrolyte.

2. The boronated polysaccharide composite solid electrolyte according to claim 1, characterized in that: The lithium salt is one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide and lithium carbonate.

3. The boronated polysaccharide composite solid electrolyte according to claim 1, characterized in that: The polysaccharide is a mixture of any one or more of starch, starch hydrolysate, chitosan and arabinan.

4. The boronated polysaccharide composite solid electrolyte according to claim 1, characterized in that: The molecular weight of the polysaccharide is 100-100,000 Da.

5. Use of the boronated polysaccharide composite solid electrolyte according to any one of claims 1 to 4 in lithium ion batteries.

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