Semi-solid lithium battery diaphragm with high ionic conductivity

Abstract

The invention provides a semi-solid lithium battery diaphragm with high ionic conductivity, which belongs to the technical field of diaphragms, and is characterized in that a base material is prepared by mixing nano cellulose and micron cellulose and adding a cross-linking agent. The mixing is used to balance strength and electrochemical performance. Cellulose is used as a diaphragm base material, the wettability is good, the thermal shrinkage performance is high, and the binding force between fibers is improved by adding a cross-linking agent during manufacturing. PEG and LiTFSI are coated on the surface of the base material, so that the ionic conductivity and safety are improved. PEG has a good chain structure, a path is provided for lithium ion migration, the overall ion mobility is improved, and the conductivity is influenced. The PEG polymer chain can freely move in the electrolyte, lithium ions are uniformly distributed, local high-concentration lithium ion precipitation is reduced, and lithium dendrite growth is inhibited. In addition, PEG has better thermal stability and lower viscosity than a high molecular weight polymer such as PEO. The effects of lubricating, moisturizing, dispersing and bonding can also be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of diaphragms, in particular to a semi-solid lithium battery diaphragm with high ion conductivity. Background Art

[0002] Solid-state batteries have become the focus of current lithium secondary battery development due to their high energy density and good safety, and have received widespread attention. However, there are still many problems in the large-scale application of solid electrolytes, including unstable solid electrolyte interface, poor thermal shrinkage performance, lithium dendrite growth, etc., which hinder their widespread application. Therefore, before solid-state batteries completely replace liquid lithium-ion batteries, semi-solid-state batteries are used for transition, that is, increasing the use of solid electrolytes, reducing the use of liquid electrolytes, and gradually developing towards all-solid-state batteries.

[0003] The separator in the semi-solid battery needs to have the advantages of good electrolyte wettability, stable interface, excellent thermal shrinkage performance, and high ionic conductivity. However, it is difficult to achieve the above characteristics at present. In view of this problem, the present invention provides a technical solution to meet the above requirements. Those skilled in the art are in urgent need of developing a semi-solid lithium battery separator with high ionic conductivity to meet the existing usage needs and performance requirements. Summary of the invention

[0004] In view of the above problems, the present invention aims to provide a semi-solid lithium battery separator with high ionic conductivity, which has the characteristics of good electrolyte wettability, interface stability, excellent thermal shrinkage performance, high ionic conductivity, etc.

[0005] A semi-solid lithium battery separator with high ionic conductivity, the semi-solid lithium battery separator comprising a substrate and a coating on the substrate; the coating is a mixed coating of polyethylene glycol (PEG for short) and lithium bistrifluoromethylsulfonyl imide (LiTFSI for short); The preparation method of the semi-solid lithium battery separator is: (1) Preparing a substrate: mixing micron cellulose and nano cellulose in proportion to obtain mixed cellulose, adding deionized water to prepare a slurry, adding a crosslinking agent to the slurry, stirring the slurry sufficiently to form a uniform dispersion, vacuuming to remove bubbles in the dispersion, then pouring the dispersion into a mold, storing the mold in an oven, and heating and drying the mold to obtain a substrate; (2) Preparation of mixed coating: dissolve polyethylene glycol in a solvent, stir and heat to obtain a polyethylene glycol solution, then slowly add lithium bis(trifluoromethylsulfonyl)imide, continue stirring to form a uniform mixed solution, and then immerse the substrate in the obtained mixed solution, then take it out, dry it at room temperature of 25°C for 0.5 to 1 hour, and then move it into an oven to dry.

[0006] Furthermore, in the step (1), the mixing mass ratio of micron cellulose to nano cellulose is 0.5-2:98-99.5.

[0007] Furthermore, the microcellulose in step (1) is coniferous wood microcellulose, and the nanocellulose is coniferous wood nanocellulose.

[0008] Furthermore, the step (1) is configured such that the solid content of the slurry is 15% to 20%.

[0009] Furthermore, the cross-linking agent in step (1) is urea-formaldehyde resin.

[0010] Furthermore, the amount of the cross-linking agent used in step (1) is 2% of the mass percentage of the mixed cellulose.

[0011] Furthermore, the vacuum pumping time in step (1) is 0.5 to 1 hour.

[0012] Furthermore, the drying temperature in step (1) is 40-50° C., and the drying time is 48-72 hours.

[0013] Furthermore, the solvent in step (2) is acetone or dimethyl sulfoxide.

[0014] Furthermore, in step (2), the mass concentration of the polyethylene glycol solution is 5% to 10%, the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to polyethylene glycol is 0.1 to 0.5:1, the stirring and heating temperature is 50 to 60° C., the oven temperature is 60 to 80° C., and the mixture is dried in the oven for 1 to 2 hours.

[0015] The beneficial effects of the present invention are as follows: the base film is made of a mixture of nanocellulose and microcellulose supplemented with a crosslinking agent to manufacture the base material. The reason for using the mixture of nanocellulose and microcellulose is that the base material is made of nanocellulose alone, and the mechanical strength is very high, but the conductivity is poor; secondly, the base material is made of microcellulose alone, which has a high porosity, ionic conductivity and poor mechanical strength, so the mixture of nanocellulose and microcellulose is used to balance the mechanical strength and electrochemical performance. Cellulose is used as the base material of the diaphragm, and its own wettability and thermal shrinkage performance are also high. At the same time, a crosslinking agent is added during the manufacturing process to enhance the bonding force between fibers, thereby enhancing the strength of the paper. PEG and LiTFSI are then coated on the surface of the base material to improve the ionic conductivity and safety performance. The chain structure of PEG itself can provide a good conduction path for lithium ion migration, thereby improving the overall ion mobility and affecting the ionic conductivity. Secondly, the polymer chain segment of PEG can move freely in the electrolyte, which helps to evenly distribute lithium ions, reduce local high-concentration lithium ion deposition, and thus inhibit the growth of lithium dendrites. In addition, PEG has better thermal stability and lower viscosity than PEO (polyethylene oxide) with a higher molecular weight. In addition, PEG itself has excellent lubricity, moisture retention, dispersibility, and adhesion. No other additives are required for the mixed coating of PEG and LiTFSI. As a high-performance lithium salt, LiTFSI has higher ionic conductivity and electrochemical stability than the traditional lithium salt lithium hexafluorophosphate. Secondly, at higher voltages, it has no corrosive effect on the aluminum current collector and does not react with water. DETAILED DESCRIPTION

[0016] The present invention is described below with specific examples, but is not intended to be limiting of the present invention.

[0017] Raw materials: Suzhou Chaowei Electrochemical Lithium Bis(trifluoromethylsulfonyl)imide, Zhangqiu Additive Factory Coniferous Wood Micron Cellulose MFC-5L1, Longyou Jinjiahao Coniferous Wood Nanocellulose CNF-H2, Oak Chemical Polyethylene Glycol PEG6000, and Minghui Petrochemical UF20 Urea-formaldehyde Resin. Example 1

[0018] Microcellulose and nanocellulose were mixed in a mass ratio of 1:99, water was added to prepare a slurry with a solid content of 15%, and urea-formaldehyde resin was added at 2% of the mass of cellulose to form a uniform dispersion. Vacuum evacuation was performed for 30 minutes to remove bubbles in the dispersion, and then the dispersion was poured into a mold and stored in an oven at 50°C and heated and dried for 48 hours to obtain a substrate.

[0019] Prepare the coating slurry, weigh PEG, dissolve it in acetone, and prepare a 10% solution, stir and heat to 50°C to completely dissolve PEG, then add LiTFSI equivalent to 30% of the mass of PEG, continue stirring until the slurry is evenly mixed, then immerse the substrate in the slurry, slowly take it out, leave it at room temperature for 1 hour, evaporate the surface solvent, and then move it into an 80°C oven to dry for 1 hour. Take it out and cool it to obtain the separator. Example 2

[0020] The method is the same as that of Example 1, except that the addition ratio of microcellulose to nanocellulose is 0.5:99.5. Example 3

[0021] The method is the same as that of Example 1, except that the addition ratio of microcellulose to nanocellulose is 2:98. Example 4

[0022] The method is the same as that of Example 1, except that the solid content of the slurry is 20%. Example 5

[0023] The method is the same as that in Example 1, except that the mass concentration of the PEG solution is 5%. Example 6

[0024] The method is the same as that in Example 1, except that the amount of LiTFSI added is 50% of the mass of PEG. Example 7

[0025] The method is the same as that in Example 1, except that the amount of LiTFSI added is 10% of the mass of PEG. Example 8

[0026] The method is the same as that in Example 1, except that the vacuum pumping time in step (1) is 1 hour; the drying temperature is 40°C and the drying time is 72 hours; and the stirring heating temperature in step (2) is 40°C, the oven temperature is 60°C, and the drying in the oven is 2 hours. Example 9

[0027] The method is the same as that of Example 1, except that the vacuum pumping time in step (1) is 0.8 h; the drying temperature is 45°C and the drying time is 54 h; the stirring heating temperature in step (2) is 55°C, the oven temperature is 70°C, and the drying in the oven is 1.5 h.

[0028] Comparative Example 1 The method is the same as that of Example 1, except that nanocellulose is not added to the substrate.

[0029] Comparative Example 2 The method is the same as that of Example 1, except that no microcellulose is added to the substrate.

[0030] Comparative Example 3 The method is the same as that in Example 1, except that the amount of LiTFSI added is 100% of the mass of PEG.

[0031] Blank control 1 The method is the same as that of Example 1, except that the substrate is not coated.

[0032] Blank control 2 The method is the same as that of blank control 1, except that no cross-linking agent is added to the substrate.

[0033] Performance Testing (1) Tensile strength The longitudinal and transverse tensile strength of the low-temperature resistant diaphragm at -20°C was tested using the Xieqiang CTM universal testing machine. The test was carried out according to the requirements of GB / T1040.3-2006. The specimens were prepared by cutting method, and the type of specimen was Type 2 specimen. The specimen was in the shape of a strip with a length of 200mm and a width of 25mm. The distance between the fixtures was 100mm, the test speed was 250mm / min, and 5 strips were tested in each direction to calculate the average value.

[0034] (2) Ionic conductivity The bulk resistance was tested by using the steel sheet / diaphragm / steel sheet assembly method and the electrochemical impedance spectroscopy (EIS) mode of the VMP3B-10 electrochemical workstation (Bio-LogicScience Instruments), where the perturbation voltage amplitude was 5mV and the frequency was 10mHz to 1MHz. The relationship between bulk resistance and ionic conductivity is that ionic conductivity is equal to the ratio of the diaphragm thickness to the product of the diaphragm resistance and the effective contact area.

[0035] (3) Thermal shrinkage The image measuring instrument is used to measure the dimensional data before and after baking, and the data is obtained through calculation. The sample size is 10cm×10cm, and the test condition is stored at 105℃ for 1h. A marker is used to draw a line along the MD on the sample surface, and the length of the line segment before and after baking is measured. The heat shrinkage rate in the MD direction is the ratio of the difference in length before and after baking to the size before baking.

[0036] The tensile strength and ionic conductivity results of the coated lithium battery separators prepared in Examples 1 to 7, Comparative Examples 1 to 5 and Blank Controls 1 to 2 are shown in Table 1 below.

[0037] Table 1 Test results of semi-solid lithium battery separators prepared by Examples 1 to 7, Comparative Examples 1-5 and Blank Controls 1 to 2

[0038] As can be seen from the above table, compared with Example 2-3, Example 1 changes the ratio of micron cellulose and nanocellulose. It can be seen from the results that when the content of micron cellulose increases, its ionic conductivity is improved, but the corresponding mechanical strength is reduced. Compared with Example 1, Example 4 increases the concentration of the slurry, which increases the number of fibers, improves the mechanical strength, and improves the ionic conductivity. The thermal shrinkage rate decreases, but the thickness is increased. Compared with Example 1, Example 5 reduces the concentration of PEG and LiTFSI, so the conductivity decreases. Compared with Example 6-7 and Comparative Example 3, when the LiTFSI addition ratio increases or decreases, the conductivity will decrease. When the LiTFSI ratio increases, the ionic conductivity decreases because when the LiTFSI concentration increases to a certain extent, it may cause it to precipitate therein, forming an uneven distribution, which will cause its conductive properties to decrease. Compared with the blank control 1-2, Example 1 can be seen that the coating significantly improves the conductivity of the substrate. In addition, without adding a crosslinking agent, the mechanical strength and thermal shrinkage of the substrate itself will be affected.

[0039] The above is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. All equivalent changes and modifications made according to the content of the present invention are included in the patent scope of the present invention.

Claims

1. A semi-solid lithium battery separator with high ionic conductivity, characterized in that: The semi-solid lithium battery separator comprises a substrate and a coating on the substrate; the coating is a mixed coating of polyethylene glycol and lithium bis(trifluoromethylsulfonyl)imide; The preparation method of the semi-solid lithium battery separator is: (1) Preparing a substrate: mixing micron cellulose and nano cellulose in proportion to obtain mixed cellulose, adding deionized water to prepare a slurry, adding a cross-linking agent to the slurry, stirring the slurry sufficiently to form a uniform dispersion, vacuuming to remove bubbles in the dispersion, pouring the dispersion into a mold, storing the mold in an oven, and heating and drying the mold to obtain a substrate; (2) Preparation of mixed coating: dissolve polyethylene glycol in a solvent, stir and heat to obtain a polyethylene glycol solution, then add lithium bis(trifluoromethylsulfonyl)imide, continue stirring to form a uniform mixed solution, and then immerse the substrate in the obtained mixed solution, then take it out, dry it at room temperature of 25°C for 0.5 to 1 hour, and then move it into an oven to dry.

2. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: In the step (1), the mixing mass ratio of micron cellulose and nano cellulose is 0.5-2:98-99.

5.

3. The semi-solid lithium battery separator with high ionic conductivity according to claim 2, characterized in that: The microcellulose in step (1) is coniferous wood microcellulose, and the nanocellulose is coniferous wood nanocellulose.

4. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The step (1) is to prepare the slurry so that the solid content thereof is 15% to 20%.

5. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The cross-linking agent in step (1) is urea-formaldehyde resin.

6. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The amount of the cross-linking agent used in step (1) is 2% of the mass percentage of the mixed cellulose.

7. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The vacuum pumping time in step (1) is 0.5 to 1 hour.

8. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The drying temperature in step (1) is 40-50° C. and the drying time is 48-72 hours.

9. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The solvent in step (2) is acetone.

10. The semi-solid lithium battery separator with high ionic conductivity according to claim 1, characterized in that: The mass concentration of the polyethylene glycol solution in step (2) is 5% to 10%, the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to polyethylene glycol is 0.1 to 0.5:1, the stirring and heating temperature is 50 to 60° C., the oven temperature is 60 to 80° C., and the mixture is dried in the oven for 1 to 2 hours.

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