Modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries and method of preparing the same

Abstract

The application discloses a modified cellulose nanofiber-based polymer electrolyte for a full-solid lithium-sulfur battery and a preparation method thereof, and belongs to the technical field of lithium ion batteries.The modified cellulose nanofiber-based polymer electrolyte of the application is composed of an electrospun modified cellulose nanofiber membrane, PEO and lithium bis-trifluoromethanesulfonimide (LiTFSI) ; the electrospun modified cellulose nanofiber membrane is a cationic cellulose nanofiber membrane obtained by treating an acetic acid cellulose nanofiber membrane with 3-chloro-2-hydroxypropyl-trimethylammonium chloride, and the TFSI ‑ exchange Cl in the fiber membrane ‑ to obtain an anion exchange-cationic cellulose nanofiber membrane.The electrolyte not only improves the problems of low ion conductivity and ion transference number of a traditional PEO solid electrolyte, but also has a large mass specific capacity and cycle performance when applied in a lithium-sulfur battery, and effectively increases the mechanical property of the PEO solid electrolyte.

Description

Technical Field

[0001] This invention relates to a modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries and its preparation method, belonging to the field of lithium-ion battery technology. Background Technology

[0002] Because the maximum theoretical capacity of lithium-ion batteries is limited to 200 mAh g -1 The current capacity is increasingly insufficient to meet the growing demands of electric vehicles, large-scale energy storage power stations, and other fields. The sulfur cathode and lithium metal anode of lithium-sulfur batteries can provide a very high theoretical specific capacity (i.e., 3800 mAh g / g lithium metal). -1 And sulfur 1675mAh g -1 ) and high specific energy (2600Wh kg) -1 Furthermore, sulfur is abundant, inexpensive, and environmentally friendly. Therefore, lithium-sulfur batteries are considered one of the most promising candidates for next-generation lightweight electrochemical energy storage systems.

[0003] However, the ethylene glycol dimethyl ether (DME) and dioxolane (DOL) liquid ether electrolytes used in lithium-sulfur batteries suffer from problems such as electrolyte leakage, and the low boiling point of ethers makes them prone to fire and other safety issues. Using polymer solid electrolytes can effectively solve the safety problems of lithium-sulfur batteries; however, the polysulfides generated during the discharge process of lithium-sulfur batteries are highly reactive and can undergo nucleophilic reactions with most polymer electrolytes, leading to the consumption of solid electrolytes.

[0004] Polyethylene oxide (PEO) has a chemical structure similar to liquid ethers and is stable to polysulfides. However, polysulfides can shuttle through PEO electrolytes, leading to low coulombic efficiency and specific capacity. Furthermore, PEO electrolytes exhibit low ionic conductivity, poor mechanical properties, and low lithium-ion transference number at room temperature, limiting their practical applications.

[0005] To improve the ionic conductivity of PEO electrolytes, existing technologies typically involve cross-linking with PEO, adding plasticizers and inorganic fillers to lower the glass transition temperature, thereby increasing ionic conductivity. These methods reduce the crystallinity of PEO by decreasing the ordered arrangement of PEO chain segments. However, the addition of plasticizers can lead to a decrease in the mechanical properties of PEO, and the addition of high-concentration inorganic fillers may cause the inorganic fillers to agglomerate, which could actually increase the crystallinity of PEO. Introducing a three-dimensional network structure into the PEO electrolyte may disrupt the ordered arrangement of PEO electrolyte chain segments, thereby reducing the crystallinity of PEO, but this does not solve the problem of polysulfide shuttling in lithium-sulfur batteries. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, this invention provides a modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries and its preparation method. The polymer electrolyte prepared by this method can suppress polysulfide shuttling in lithium-sulfur batteries, and has excellent mechanical properties, high room temperature ionic conductivity, and stable cycle performance.

[0007] The first objective of this invention is to provide a modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries, wherein the modified cellulose nanofiber-based polymer electrolyte is composed of an electrospun modified cellulose nanofiber membrane, PEO, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0008] In one embodiment, the electrospun modified cellulose nanofiber membrane is a cationic cellulose nanofiber membrane obtained by treating a cellulose acetate nanofiber membrane with the etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride, and then passing it through TFSI in LiTFSI. - Cl in exchange fiber membrane - Anion-exchange-cationic cellulose nanofiber membranes were obtained.

[0009] In one embodiment, the mass ratio of the electrospun modified cellulose nanofiber membrane, PEO, and lithium bis(trifluoromethanesulfonylimide)LiTFSI is (1-3):12:4.3.

[0010] A second objective of this invention is to provide a method for preparing the modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries described above, the method comprising the following steps:

[0011] (1) The prepared cellulose acetate spinning solution was electrospun to obtain a cellulose acetate nanofiber membrane, and then deacetylated to obtain a regenerated cellulose nanofiber membrane.

[0012] (2) The regenerated cellulose nanofiber membrane prepared in step (1) is cationized to obtain a cationized cellulose nanofiber membrane; then ion exchange is performed to obtain an anion exchange-cationized cellulose nanofiber membrane.

[0013] (3) PEO and LiTFSI were mixed to obtain a polymer solution;

[0014] (4) The polymer solution obtained in step (3) is poured onto the anion exchange-cation cellulose nanofiber membrane prepared in step (2) and dried under vacuum to obtain the modified cellulose nanofiber-based polymer electrolyte.

[0015] In one embodiment, the preparation of the cellulose acetate spinning solution in step (1) is as follows: cellulose acetate is weighed and added to a mixture of acetone and dimethylacetamide (DMAc) and stirred to dissolve, thus obtaining the solution.

[0016] In one embodiment, the electrospinning in step (1) involves using a syringe to draw in the spinning solution and placing it on an electrospinning machine for spinning. The spinning parameters are: spinning voltage 15-20 kV, spinning distance 15-20 cm, spinning speed 0.5-1 mL / h, and spinning time 8-20 h.

[0017] In one embodiment, the specific steps of the deacetylation treatment in step (1) are as follows: place the cellulose acetate nanofiber membrane in an aqueous solution containing 0.05 mol / L NaOH, let it stand for 8 to 12 hours, and then dry it at 40 to 60°C.

[0018] In one embodiment, the specific preparation process of the cationization modification in step (2) is as follows: prepare an isopropanol-water mixed solution, add NaOH to the above mixed solution, stir until NaOH dissolves, add an aqueous solution of the cationic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride dropwise, stir until the solution is clear, place the regenerated cellulose nanofiber membrane in the above solution, place it in a constant temperature shaking water bath at 60-70°C for 1-12 hours, and dry it at 40-60°C.

[0019] In one embodiment, the sample is placed in a constant temperature shaking water bath at 60-70°C for 3-12 hours and then dried at 40-60°C.

[0020] In one embodiment, the aqueous solution of the ionic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride contains 40-80% by mass of 3-chloro-2-hydroxypropyl-trimethylammonium chloride.

[0021] In one embodiment, the volume ratio of isopropanol to water in the isopropanol-water mixture is 1:4.

[0022] In one embodiment, the specific process of ion exchange is as follows: prepare a 0.03 mol / L-0.1 mol / L LiTFSI aqueous solution, immerse the cationized cellulose nanofiber membrane in it at 20-30°C, oscillate at a constant temperature for 6-12 hours, wash with deionized water, and dry.

[0023] In one embodiment, the molar ratio of EO units in PEO to Li in the polymer solution of step (3) is 12 to 18:1; preferably 18:1.

[0024] In one embodiment, the anion-exchange-cationic cellulose nanofiber membrane described in step (4) accounts for 3-8% of the mass of the polymer electrolyte after drying.

[0025] A third objective of this invention is to provide an application of the modified cellulose nanofiber-based polymer electrolyte described above for all-solid-state lithium-sulfur batteries in the preparation of lithium-sulfur batteries.

[0026] The beneficial effects of this invention are:

[0027] (1) The modified cellulose nanofiber-based PEO solid electrolyte prepared in this invention not only improves the problems of low ionic conductivity and low ion transference number of traditional PEO solid electrolyte, but also improves the ionic conductivity of the electrolyte by reducing the crystallinity of PEO through the three-dimensional network structure of modified cellulose nanofiber; it can also adsorb lithium salt anions through the Lewis acid-base interaction of the surface positive charge of modified cellulose nanofiber, thereby increasing the Li ion transference number; and through the strong positive charge of modified cellulose nanofiber, it can chemically adsorb polysulfide anions during the charging and discharging process of lithium-sulfur battery to reduce the shuttle effect, which greatly improves the problems of poor cycle stability (10 cycles) and low coulombic efficiency of PEO solid electrolyte in lithium-sulfur battery.

[0028] (2) The modified cellulose nanofiber-based PEO solid electrolyte prepared in this invention has the largest specific capacity and number of cycles when applied to lithium-sulfur batteries, and the mechanical properties of PEO solid electrolyte are increased. Attached Figure Description

[0029] Figure 1 XRD patterns of Example 1 and Comparative Example 1;

[0030] Figure 2 The ultraviolet-visible spectra of Example 1 and Comparative Example 1 are shown below;

[0031] Figure 3 This is a diagram of the first charge-discharge cycle in Example 1;

[0032] Figure 4 This is a diagram of the first charge-discharge cycle of Comparative Example 1. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to examples, so that those skilled in the art can implement it based on the description.

[0034] Example 1

[0035] The preparation method of the modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries specifically includes the following steps:

[0036] (1) Weigh 2g of cellulose acetate powder and add it to 15ml of a mixed solvent of acetone and dimethylacetamide with a volume ratio of 1:1. Stir magnetically for 12h to obtain an electrospinning solution. Place the spinning solution in a syringe for electrospinning. The spinning parameters are: spinning voltage 15kv, spinning distance 15cm, spinning speed 1ml / h, and spinning time 12h. After spinning is completed, remove the electrospinned cellulose acetate nanofiber membrane from the aluminum foil.

[0037] (2) Prepare a 0.05 mol / L sodium hydroxide aqueous solution, weigh 50 mg of the cellulose acetate nanofiber membrane prepared in step (1), immerse it in the sodium hydroxide aqueous solution for 12 h, wash it 3 times with deionized water, and dry it at 60 °C to obtain the cellulose nanofiber membrane.

[0038] (3) Prepare 50 ml of a mixed solvent of isopropanol (IPA) and deionized water in a volume ratio of 1:4, add NaOH particles with a final concentration of 1 mol / L, stir until NaOH dissolves, add 20 ml of 3-chloro-2-hydroxypropyl-trimethylammonium chloride aqueous solution with a mass fraction of 65%, stir until no bubbles are generated, add 50 mg of the cellulose nanofiber membrane obtained in step (2), place it in a constant temperature shaking water bath at 70℃ for 12 h, after the reaction is completed, wash it 3 times with deionized water, dry it at 60℃ to obtain the cationized cellulose nanofiber membrane;

[0039] (4) Prepare 20 ml of 0.03 mol / L LiTFSI aqueous solution, add 50 mg of the cationized cellulose nanofiber membrane prepared in step (3) to the above LiTFSI aqueous solution and shake at 30°C for 6 h, then wash with deionized water 3 times and dry at 60°C to obtain anion exchange-cationic cellulose nanofiber membrane.

[0040] (5) Prepare a PEO:Li molar ratio of 18:1 with PEO and LiTFSI, wherein the mass of PEO is 1.2g. Add PEO and LiTFSI to 20ml of acetonitrile solvent to obtain a PEO polymer electrolyte solution.

[0041] (6) Pour 20 ml of the PEO polymer electrolyte solution obtained in step (5) onto 50 mg of the anion exchange-cation cellulose nanofiber membrane prepared in step (4), and place it in a vacuum oven to dry under vacuum for 24 h to obtain the modified cellulose nanofiber-based PEO electrolyte.

[0042] Example 2

[0043] The only difference from Example 1 is that the reaction time in step (3) is adjusted to 70°C in a constant temperature shaking water bath for 6 hours. All other parameters and conditions are the same as in Example 1.

[0044] Example 3

[0045] The only difference from Example 1 is that the reaction time in step (3) is adjusted to 70°C in a constant temperature shaking water bath for 3 hours. All other parameters and conditions are the same as in Example 1.

[0046] Example 4

[0047] The only difference from Example 1 is that the constant temperature shaking water bath at 70°C was adjusted in step (3) for 1 hour. All other parameters and conditions are the same as in Example 1.

[0048] Example 5

[0049] The only difference from Example 1 is that the mass fraction of the aqueous solution of 3-chloro-2-hydroxypropyl-trimethylammonium chloride in step (3) is adjusted to 80%, while the other parameters and conditions are the same as in Example 1.

[0050] Example 6

[0051] The only difference from Example 1 is that the mass fraction of the aqueous solution of 3-chloro-2-hydroxypropyl-trimethylammonium chloride in step (3) is adjusted to 40%, while the other parameters and conditions are the same as in Example 1.

[0052] Example 7

[0053] The only difference from Example 1 is that the amount of anion exchange-cation cellulose nanofiber membrane used in step (6) is adjusted to 100 mg, while the other parameters and conditions are the same as in Example 1.

[0054] Example 8

[0055] The only difference from Example 1 is that the amount of anion exchange-cation cellulose nanofiber membrane used in step (6) is adjusted to 150 mg, while the other parameters and conditions are the same as in Example 1.

[0056] Comparative Example 1

[0057] A PEO:Li molar ratio of 18:1 was prepared, with 1.2 g of PEO. The PEO and LiTFSI were added to 20 ml of acetonitrile solvent, poured into a polytetrafluoroethylene mold, and vacuum dried in a vacuum oven for 24 h to obtain a PEO polymer electrolyte solution.

[0058] Comparative Example 2

[0059] (1) Weigh 2g of cellulose acetate powder and add it to 15ml of a 1:1 mixture of acetone and dimethylacetamide. Stir magnetically for 12h to obtain an electrospinning solution. Place the spinning solution in a syringe for electrospinning. The spinning parameters are: spinning voltage 15kv, spinning distance 15cm, spinning speed 1ml / h, and spinning time 12h. After spinning is completed, remove the electrospinned cellulose acetate nanofiber membrane from the aluminum foil.

[0060] (2) Prepare a 0.05 mol / L sodium hydroxide aqueous solution, weigh 50 mg of the cellulose acetate nanofiber membrane prepared in step (1) and immerse it in the sodium hydroxide aqueous solution. After 12 h, wash it three times with deionized water and dry it at 60 °C to obtain the cellulose nanofiber membrane.

[0061] (3) Prepare a PEO:Li molar ratio of 18:1 with PEO and LiTFSI, wherein the mass of PEO is 1.2g. Add PEO and LiTFSI to 20ml of acetonitrile solvent to obtain a PEO polymer electrolyte solution.

[0062] (4) 20 ml of PEO polymer electrolyte solution was poured onto 50 mg of cellulose nanofiber membrane prepared in step (2), and then vacuum dried in a vacuum oven for 24 h to obtain cellulose nanofiber-based PEO electrolyte.

[0063] Comparative Example 3

[0064] (1) Weigh 2g of cellulose acetate powder and add it to 15mL of a 1:1 mixture of acetone and dimethylacetamide. Stir magnetically for 12h to obtain an electrospinning solution. Place the spinning solution in a syringe for electrospinning. The spinning parameters are: spinning voltage 15kV, spinning distance 15cm, spinning speed 1ml / h, and spinning time 12h. After spinning is completed, remove the electrospinned cellulose acetate nanofiber membrane from the aluminum foil.

[0065] (2) Prepare a 0.05 mol / L sodium hydroxide aqueous solution, weigh 50 mg of the cellulose acetate nanofiber membrane prepared in step (1) and immerse it in the sodium hydroxide aqueous solution. After 12 h, wash it three times with deionized water and dry it at 60 °C to obtain the cellulose nanofiber membrane.

[0066] (3) Prepare 40 ml of a mixed solvent of anhydrous ethanol and deionized water in a volume ratio of 3:1, add LiOH particles with a final concentration of 1 mol / L, stir until LiOH dissolves, add 15 mL of a 1 mol / L monochloroacetic acid aqueous solution, stir evenly, add 50 mg of the cellulose nanofiber membrane obtained in step (2), place in a constant temperature shaking water bath at 80℃ for 12 h, after the reaction is completed, wash with deionized water 3 times, dry at 60℃ to obtain anionized cellulose nanofiber membrane;

[0067] (4) Prepare a PEO:Li molar ratio of 18:1 with PEO and LiTFSI, wherein the mass of PEO is 1.2g. Add PEO and LiTFSI to 20ml of acetonitrile solvent to obtain a PEO polymer electrolyte solution.

[0068] (5) Pour 20 ml of PEO polymer electrolyte solution onto 50 mg of the anionized cellulose nanofiber membrane prepared in step (3), and place it in a vacuum oven to dry under vacuum for 24 h to obtain cellulose nanofiber-based PEO electrolyte.

[0069] Performance test results

[0070] Test method:

[0071] 1. The solid electrolytes prepared in Examples 1-8 and Comparative Examples 1-3 were pressed into 19 mm discs and placed in 20 ml of a solution containing polysulfides (Li2S6). After adsorption for 12 h, the maximum absorbance in the 300-700 nm wavelength band was tested using UV-Vis spectrophotometry to evaluate the chemisorption effect on polysulfides.

[0072] 2. The solid electrolytes prepared in Examples 1-8 and Comparative Examples 1-3 were stamped into 19mm discs and assembled into coin cells in a glove box, wherein the positive electrode was a sulfur / carbon positive electrode and the negative electrode was a lithium sheet; the assembled cells were tested on an electrochemical workstation.

[0073] (1) The ionic conductivity test parameter is a frequency of 10. 6 Hz-0.1Hz, amplitude is 0.01V.

[0074] (2) The lithium-ion transport number test parameters are: lithium symmetric impedance, frequency 10 6 Hz-0.01Hz, amplitude 0.01V, steady-state current, polarization voltage 0.01V, time 5000s.

[0075] (3) The battery's cycle performance was tested using a battery cycle system. The discharge voltage was increased to 1.5V, the charging voltage to 3V, and 100 cycles were performed. The results are shown in Table 1 and... Figures 1-4 As shown:

[0076] Table 1. Performance test results of solid electrolytes in Examples 1-8 and Comparative Examples 1-3

[0077]

[0078]

[0079] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries, characterized in that, The modified cellulose nanofiber-based polymer electrolyte is composed of an electrospun modified cellulose nanofiber membrane, PEO, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The electrospun modified cellulose nanofiber membrane is obtained by deacetylation of electrospun cellulose acetate nanofiber membrane to obtain a regenerated cellulose nanofiber membrane, followed by treatment with the cationic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride to obtain a cationic cellulose nanofiber membrane, and then passing it through TFSI in LiTFSI. - Cl in exchange fiber membrane - Anion-exchange-cationic cellulose nanofiber membranes were obtained.

2. The modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries according to claim 1, characterized in that, The mass ratio of the electrospun modified cellulose nanofiber membrane, PEO, and lithium bis(trifluoromethanesulfonylimide)LiTFSI is (1~3):12:4.

3.

3. The method for preparing the modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries according to claim 1 or 2, characterized in that, The method includes the following steps: (1) The prepared cellulose acetate spinning solution was electrospun to obtain a cellulose acetate nanofiber membrane, and then deacetylated to obtain a regenerated cellulose nanofiber membrane. (2) The regenerated cellulose nanofiber membrane prepared in step (1) is cationically modified with the cationic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride to obtain a cationic cellulose nanofiber membrane; then, it is passed through TFSI in LiTFSI. - Cl in exchange fiber membrane - Ion exchange was performed to obtain anion-exchange-cationic cellulose nanofiber membranes; (3) PEO and LiTFSI are mixed to obtain a polymer solution; (4) The polymer solution obtained in step (3) is poured onto the anion exchange-cation cellulose nanofiber membrane prepared in step (2) and dried under vacuum to obtain the modified cellulose nanofiber-based polymer electrolyte.

4. The method according to claim 3, characterized in that, The preparation of the cellulose acetate spinning solution in step (1) is as follows: Weigh cellulose acetate and add it to a mixture of acetone and dimethylacetamide (DMAc) and stir to dissolve it.

5. The method according to claim 3, characterized in that, In step (1), the electrospinning is carried out by using a syringe to draw in the spinning solution and place it on an electrospinning machine for spinning. The spinning parameters are: spinning voltage 15-20 kV, spinning distance 15-20 cm, spinning speed 0.5-1 mL / h, and spinning time 8-20 h.

6. The method according to claim 3, characterized in that, The specific steps of deacetylation treatment in step (1) are as follows: place the cellulose acetate nanofiber membrane in an aqueous solution containing 0.05 mol / L NaOH, let it stand for 8~12 hours, and then dry it at 40~60℃.

7. The method according to claim 3, characterized in that, The specific preparation process of the cationization modification in step (2) is as follows: prepare an isopropanol-water mixed solution, add NaOH to the above mixed solution, stir until NaOH dissolves, add the cationic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride aqueous solution dropwise, stir until the solution is clear, place the regenerated cellulose nanofiber membrane in the above solution, place it in a constant temperature shaking water bath at 60~70℃ for 1~12 hours, and dry at 40~60℃.

8. The method according to claim 7, characterized in that, The aqueous solution of the cationic etherifying agent 3-chloro-2-hydroxypropyl-trimethylammonium chloride has a mass fraction of 40-80% for 3-chloro-2-hydroxypropyl-trimethylammonium chloride.

9. The method according to claim 3, characterized in that, In step (3), the molar ratio of EO units in PEO to Li in the polymer solution is 12~18:

1.

10. The application of the modified cellulose nanofiber-based polymer electrolyte for all-solid-state lithium-sulfur batteries as described in claim 1 or 2 in the preparation of lithium-sulfur batteries.

Images