A salt-coated polymer solid electrolyte, preparation method thereof, and lithium battery

Abstract

The present invention provides a salt-coated polymer solid electrolyte, a preparation method thereof, and a lithium battery, belonging to the technical field of high-voltage solid-state lithium batteries. The present invention forms a salt-coated polymer electrolyte with a ternary fluorinated copolymer and a lithium salt, thereby improving the room-temperature ionic conductivity and high-voltage resistance of the electrolyte. At the same time, a portion of the added nitrile compound is fixed in the polymer framework and forms a uniform phase with the polymer, while the other portion exists in the form of free small molecules. These residual small molecules wet the positive electrode material, effectively reducing the electrolyte / electrode interface impedance, making the solid electrolyte compatible with the high-voltage positive electrode. Furthermore, since the salt-coated polymer electrolyte improves the transmission of lithium ions, increases the lithium ion migration number, reduces the decomposition of the electrolyte, and inhibits the growth of lithium dendrites, the adverse effects of the addition of the nitrile compound are greatly reduced, thereby improving the performance of the solid electrolyte and the lithium battery containing the solid electrolyte.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage solid-state lithium batteries, and in particular to a salt-coated polymer solid electrolyte, a preparation method thereof, and a lithium battery. Background Art

[0002] As the new energy industry continues to expand, traditional liquid electrolytes pose safety risks such as leakage and flammability. Therefore, replacing flammable liquid electrolytes with solid-state electrolytes is an important research direction. Compared with inorganic solid-state electrolytes, solid polymer electrolytes offer advantages such as flexibility, chemical stability, and ease of processing. However, most suffer from low ionic conductivity and poor high-voltage resistance.

[0003] As the first choice for polymer matrices, polyvinylidene fluoride (PVDF) and its copolymer derivatives have been widely used in the field of lithium batteries. Among them, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer (P(VDF-TrFE-CFE)) and polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene copolymer (P(VDF-TrFE-CTFE)) have very high dielectric constants, far exceeding PVDF, and have the potential to be used as high-performance solid electrolytes. However, there are currently relatively few literature reports on the use of this triblock polymer for solid electrolytes, and in order to improve ionic conductivity and adapt to high-voltage positive electrodes, ionic liquids are often relied on (Angew.Chem.Int.Ed.2023,62,e202300243). The NCM811 battery assembled without the introduction of ionic liquids has a specific capacity of only 150mAh / g after 50 cycles at 0.1C (EnergyEnviron.Sci.2021,14,6021).

[0004] Therefore, how to improve the performance of solid electrolytes and lithium batteries containing the solid electrolytes without introducing ionic liquids has become a difficult problem in the existing technology. Summary of the Invention

[0005] The present invention provides a salt-coated polymer solid electrolyte, a preparation method thereof, and a lithium battery. The solid electrolyte provided by the present invention has high ionic conductivity and high voltage resistance, and the lithium battery containing the electrolyte has high specific capacity and cycle performance.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a salt-coated polymer solid electrolyte, comprising the following steps:

[0008] (1) mixing a ternary fluorinated copolymer, a first lithium salt, and an organic solvent to obtain a mixture 1;

[0009] (2) heating and melting the nitrile compound and the second lithium salt to obtain a mixture 2;

[0010] (3) mixing the mixture 1 obtained in step (1) and the mixture 2 obtained in step (2), casting the mixture onto a substrate, and drying the mixture to obtain a salt-coated polymer solid electrolyte;

[0011] There is no chronological order for steps (1) and (2).

[0012] Preferably, the ternary fluorinated copolymer in step (1) comprises one or more compounds represented by formula I and formula II,

[0013]

[0014] In the formula I and formula II, x is independently 63-69, y is independently 24-28, and z is independently 5-9.

[0015] Preferably, the first lithium salt in step (1) and the second lithium salt in step (2) independently include one or more of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium bis(trifluoromethanesulfonylimide), lithium bis(fluorosulfonylimide), lithium hexafluorophosphate, lithium perchlorate and lithium nitrate.

[0016] Preferably, the organic solvent in step (1) comprises one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and 2,2,2-trifluoro-N,N-dimethylacetamide.

[0017] Preferably, in step (1), the mass ratio of the first lithium salt, the ternary fluorinated copolymer and the organic solvent is (1-2):1:(10-25).

[0018] Preferably, the nitrile compound in step (2) includes one or more of succinonitrile, malononitrile, terephthalonitrile, isophthalonitrile, o-phthalonitrile and ethoxymethylene malononitrile.

[0019] Preferably, the mass ratio of the nitrile compound to the second lithium salt in step (2) is (3-4):1.

[0020] Preferably, the mass ratio of the ternary fluorinated copolymer in step (1) to the nitrile compound in step (2) is 1:(0.5-1.5).

[0021] The present invention also provides a salt-coated polymer solid electrolyte prepared by the preparation method described in the above technical solution.

[0022] The present invention also provides a lithium battery comprising a positive electrode, a negative electrode and the salt-encapsulated polymer solid electrolyte described in the above technical solution.

[0023] The present invention provides a method for preparing a salt-coated polymer solid electrolyte, comprising the following steps:

[0024] (1) mixing a ternary fluorinated copolymer, a first lithium salt and an organic solvent to obtain a mixture 1; (2) heating and melting a nitrile compound and a second lithium salt to obtain a mixture 2; (3) mixing the mixture 1 obtained in step (1) and the mixture 2 obtained in step (2), casting the mixture onto a substrate, and drying the mixture to obtain a salt-coated polymer solid electrolyte; there is no time sequence for steps (1) and (2). The present invention forms a salt-encapsulated polymer electrolyte by combining a ternary fluorinated copolymer and a lithium salt. In addition to being conducted by polymer chains in the amorphous region, lithium ions can also be conducted by jumping through an ion conduction network formed by aggregated ion clusters generated by a high lithium salt concentration. The lithium salt with a multi-molecular coordination structure is more easily dissociated, which can provide more mobile lithium ions, thereby improving the room temperature ionic conductivity and high voltage resistance of the electrolyte. At the same time, part of the added nitrile compound is fixed in the polymer framework and forms a uniform phase with the polymer, while the other part exists in the form of free small molecules. These residual small molecules wet the positive electrode material, effectively reducing the electrolyte / electrode interface impedance, making the solid electrolyte compatible with the high-voltage positive electrode. In addition, the salt-encapsulated polymer electrolyte improves the transmission of lithium ions, increases the number of lithium ion transfers, reduces the decomposition of the electrolyte and inhibits the growth of lithium dendrites, greatly reducing the adverse effects of the addition of nitrile compounds, and improving the performance of the solid electrolyte and the lithium battery containing the solid electrolyte. The results of the embodiment show that the room temperature ionic conductivity of the salt-encapsulated polymer solid electrolyte prepared by the present invention reaches 4.23×10 -4 S / cm, the lithium ion transference number can reach 0.45, the electrochemical window can reach 4.7V, and the interface contact stability is good. The lithium battery containing the salt-coated polymer solid electrolyte has a high specific capacity and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a macroscopic image of the salt-in-polymer solid electrolyte prepared in Example 1;

[0026] Figure 2 This is a SEM image of the salt-coated polymer solid electrolyte prepared in Example 1;

[0027] Figure 3 This is the electrochemical window diagram of the salt-coated polymer solid electrolyte prepared in Example 1;

[0028] Figure 4 It curve and impedance diagram before and after testing of the Li / / Li symmetrical battery with salt-coated polymer solid electrolyte prepared in Example 1;

[0029] Figure 5The constant current charge-discharge voltage polarization curve of the Li / / Li symmetric cell with the salt-coated polymer solid electrolyte prepared in Example 1;

[0030] Figure 6 Specific capacity-Coulombic efficiency diagram of NCM811 button cells assembled with the electrolytes in Example 1 and Comparative Example 1;

[0031] Figure 7 Impedance diagrams of the salt-coated polymer solid electrolytes prepared in Examples 1 to 5;

[0032] Figure 8 This is the rate cycle diagram of the button battery assembled with the electrolyte in Example 1. DETAILED DESCRIPTION

[0033] The present invention provides a method for preparing a salt-coated polymer solid electrolyte, comprising the following steps:

[0034] (1) mixing a ternary fluorinated copolymer, a first lithium salt, and an organic solvent to obtain a mixture 1;

[0035] (2) heating and melting the nitrile compound and the second lithium salt to obtain a mixture 2;

[0036] (3) mixing the mixture 1 obtained in step (1) and the mixture 2 obtained in step (2), casting the mixture onto a substrate, and drying the mixture to obtain a salt-coated polymer solid electrolyte;

[0037] There is no chronological order for steps (1) and (2).

[0038] Unless otherwise specified, the present invention has no particular limitation on the sources of the raw materials, and commercially available products known to those skilled in the art may be used.

[0039] In the present invention, a ternary fluorinated copolymer, a first lithium salt and an organic solvent are mixed to obtain a mixture 1.

[0040] In the present invention, the weight average molecular weight of the ternary fluorinated copolymer is preferably 400,000 to 700,000 g / mol, more preferably 450,000 to 550,000 g / mol.

[0041] In the present invention, the ternary fluorinated copolymer preferably comprises one or more compounds represented by Formula I and Formula II,

[0042]

[0043] In Formula I and Formula II, x is independently preferably 63 to 69, y is independently preferably 24 to 28, and z is independently preferably 5 to 9. The present invention limits the weight-average molecular weight and structural formula of the ternary fluorinated copolymer to the above ranges, so that the ternary fluorinated copolymer has a higher dielectric constant, further improving the performance of the electrolyte.

[0044] In the present invention, the first lithium salt preferably includes one or more of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium hexafluorophosphate, lithium perchlorate and lithium nitrate.

[0045] In the present invention, the organic solvent preferably includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and 2,2,2-trifluoro-N,N-dimethylacetamide, more preferably 2,2,2-trifluoro-N,N-dimethylacetamide.

[0046] In the present invention, the mass ratio of the first lithium salt, the ternary fluorinated copolymer, and the organic solvent is preferably (1-2):1:(10-25), more preferably (1.2-1.8):1:(10-25), and further preferably (1.4-1.6):1:(10-25). By limiting the mass ratio of the first lithium salt, the ternary fluorinated copolymer, and the organic solvent to the above range, the present invention can increase the lithium salt content in the electrolyte, form a salt-encapsulated polymer structure, promote lithium ion conduction, avoid low ionic conductivity caused by insufficient lithium salt, and prevent excessive lithium salt from agglomerating and precipitating after drying, affecting battery performance.

[0047] In the present invention, the mixing temperature is preferably 20-35°C, more preferably 25-30°C; the mixing time is preferably 20-30 hours, more preferably 24-26 hours; the mixing is preferably performed under stirring conditions; the stirring rate is preferably 400-600 r / min, more preferably 500 r / min; and the mixing is preferably performed in a glove box. By limiting the mixing temperature and time within the above ranges, the present invention can ensure more thorough mixing of the raw materials.

[0048] In the present invention, the nitrile compound and the second lithium salt are heated and melted to obtain a mixture 2.

[0049] In the present invention, the nitrile compound preferably includes one or more of succinonitrile, malononitrile, terephthalonitrile, isophthalonitrile, phthalonitrile and ethoxymethylene malononitrile, more preferably succinonitrile. In the present invention, a portion of the nitrile compound is fixed in the framework of the polymer and forms a uniform phase with the polymer, while the other portion exists in the form of free small molecules. These residual small molecules wet the positive electrode material, effectively reducing the electrolyte / electrode interface impedance, so that the solid electrolyte can be adapted to the high-voltage positive electrode.

[0050] In the present invention, the second lithium salt preferably includes one or more of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium perchlorate and lithium nitrate.

[0051] In the present invention, the mass ratio of the nitrile compound to the second lithium salt is preferably (3-4):1. By limiting the mass ratio of the nitrile compound to the second lithium salt within the above range, the present invention can improve the ionic conductivity of the electrolyte and have good compatibility with the high-voltage positive electrode. Excessive addition can lead to side reactions with the metallic lithium negative electrode, causing disturbances and unstable discharge curves.

[0052] In the present invention, the heating and melting temperature is preferably 45-60°C, more preferably 50-55°C; the heating and melting time is preferably 2-5 hours; the heating and melting is preferably carried out under stirring conditions; the stirring rate is preferably 400-600 r / min, more preferably 500 r / min. By limiting the heating and melting temperature and time within the above ranges, the present invention can achieve more uniform mixing of the two.

[0053] After obtaining mixture 1 and mixture 2, the present invention mixes the mixture 1 and the mixture 2 and casts them onto a substrate, followed by drying to obtain a salt-coated polymer solid electrolyte.

[0054] In the present invention, when the mixture 1 and the mixture 2 are mixed, the mass ratio of the ternary fluorinated copolymer to the nitrile compound is preferably 1:(0.5-1.5), more preferably 1:(0.6-1.2). By limiting the mass ratio of the ternary fluorinated copolymer to the nitrile compound to the above range, the electrochemical performance of the electrolyte can be further improved.

[0055] The present invention has no particular limitation on the mixing operation of the mixture 1 and the mixture 2. The two can be uniformly mixed using a material mixing technique well known to those skilled in the art. In the present invention, the mixing is preferably performed in a glove box.

[0056] In the present invention, the substrate preferably includes a glass plate, a polytetrafluoroethylene plate or a polypropylene plate, more preferably a glass plate. The present invention has no particular limitation on the size of the substrate, and the size can be selected according to actual needs.

[0057] The present invention has no particular limitation on the pouring amount during pouring, and it can be selected according to the required thickness of the electrolyte.

[0058] The present invention has no special limitation on the pouring operation, and the pouring technical solutions well known to those skilled in the art can be adopted.

[0059] In the present invention, the drying temperature is preferably 50-80°C, more preferably 60-70°C; the drying time is preferably 18-48 hours, more preferably 24-36 hours. By limiting the drying temperature and time within the above ranges, the present invention can achieve a more uniform film thickness of the polymer electrolyte.

[0060] After drying is completed, the present invention preferably cools the dried product, removes the film, and re-dries it in sequence to obtain a salt-encapsulated polymer solid electrolyte.

[0061] The present invention has no particular limitation on the operations of cooling, peeling off the film and re-drying, and the technical solutions of cooling, peeling off the film and re-drying well known to those skilled in the art can be adopted.

[0062] In the present invention, the thickness of the salt-coated polymer solid electrolyte is preferably 40 to 60 μm.

[0063] The present invention forms a salt-coated polymer electrolyte with a ternary fluorinated copolymer and a lithium salt, thereby improving the room temperature ionic conductivity and high-voltage resistance of the electrolyte. Simultaneously, the added nitrile compound effectively reduces the electrolyte / electrode interface impedance, enabling the solid electrolyte to adapt to the high-voltage positive electrode and reducing the adverse effects of the addition of the nitrile compound. The preparation process parameters are controlled to improve the performance of the solid electrolyte and the lithium battery containing the solid electrolyte.

[0064] The present invention also provides a salt-coated polymer solid electrolyte prepared by the preparation method described in the above technical solution.

[0065] The salt-coated polymer solid electrolyte provided by the present invention has excellent room temperature ionic conductivity, lithium ion transference number, electrochemical window and high voltage resistance.

[0066] The present invention also provides a lithium battery comprising a positive electrode, a negative electrode and the salt-encapsulated polymer solid electrolyte described in the above technical solution.

[0067] In the present invention, the positive electrode preferably includes a lithium iron phosphate positive electrode (LiFePO4), a lithium-rich manganese-based positive electrode (Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2), ternary positive electrode (LiNi x Co y Mn z O2, x>0.5, x+y+z=1), lithium nickel manganese oxide positive electrode (LiNi 0.5 Mn 1.5 O4) and lithium manganate positive electrode (LiMn2O4); the negative electrode preferably includes one of a metallic lithium negative electrode, a graphite negative electrode and a silicon-based negative electrode.

[0068] The present invention has no special limitation on the assembly method of the lithium battery, and the method can be selected according to needs.

[0069] The lithium battery provided by the present invention has excellent specific capacity and cycle performance.

[0070] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0071] Example 1

[0072] (1) In a glove box, 0.2 g of a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer (Formula II, wherein x is 67, y is 26, and z is 7) having a weight average molecular weight of 480,000 g / mol, 0.3 g of lithium bis(fluorosulfonyl)imide, 0.015 g of lithium nitrate, and 2.5 g of 2,2,2-trifluoro-N,N-dimethylacetamide were mixed and stirred at 30° C. and 500 r / min for 24 h to obtain a mixture 1, wherein the mass ratio of the total mass of lithium bis(fluorosulfonyl)imide and lithium nitrate to the polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer and 2,2,2-trifluoro-N,N-dimethylacetamide was 1.575:1:12.5;

[0073] (2) 2 g of succinonitrile and 0.58 g of lithium bis(trifluoromethanesulfonyl imide) were mixed, and the mixture was heated and stirred at 50° C. for 4 h at a speed of 500 r / min to obtain a mixture 2, wherein the mass ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl imide was 3.45:1;

[0074] (3) In a glove box, 0.232 g of mixture 2 was added to mixture 1 (the mass ratio of polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer to succinonitrile was 1:0.9) using a pipette, and stirred evenly at a speed of 500 r / min at 30°C. The mixed solution was poured onto a glass plate and then heated in a vacuum oven at 60°C for 24 h. After being taken out, it was dried until the surface of the electrolyte membrane was no longer sticky. Then, multiple layers of sulfuric acid paper were covered on the dried electrolyte membrane, and the membrane was removed with tweezers. The other side was covered with multiple layers of sulfuric acid paper. The electrolyte membrane covered with multiple layers of sulfuric acid paper was then placed in a glove box and dried to obtain a salt-coated polymer solid electrolyte with a thickness of 44 μm.

[0075] The macroscopic image of the salt-coated polymer solid electrolyte prepared in Example 1 is as follows: Figure 1 shown.

[0076] The salt-coated polymer solid electrolyte prepared in Example 1 was observed using a scanning electron microscope, and the obtained SEM image is as follows: Figure 2 As shown. Figure 2 It can be seen from the figure that the salt-coated polymer solid electrolyte prepared in Example 1 presents a regular network structure and is tightly arranged, indicating that the SN phase is well combined with the polymer framework.

[0077] The electrochemical window of the salt-coated polymer solid electrolyte prepared in Example 1 was tested, and the results were as follows: Figure 3 As shown. Figure 3 It can be seen that the oxidation potential of the salt-coated polymer solid electrolyte prepared in Example 1 is increased to 4.7V.

[0078] The It curve and impedance diagram before and after the test of the Li / / Li symmetrical battery with salt-coated polymer solid electrolyte prepared in Example 1 are as follows: Figure 4 As shown. Figure 4 It can be seen that the lithium ion transference number of the salt-coated polymer solid electrolyte prepared in Example 1 is as high as 0.45 at room temperature.

[0079] The constant current charge-discharge voltage polarization curve of the Li / / Li symmetric battery with the salt-coated polymer solid electrolyte prepared in Example 1 was tested. Figure 5 As shown. Figure 5 It can be seen that the polarization voltage curve of the battery is at a low current density (0.1mAcm -1 ) is relatively stable under the condition of overpotential less than 0.03V.

[0080] The conductivity of the salt-coated polymer solid electrolyte prepared in Example 1 at room temperature is 4.23×10 -4 S / cm.

[0081] Application Example 1

[0082] A button cell was assembled using the salt-coated polymer solid electrolyte prepared in Example 1 as the electrolyte, wherein the positive electrode active material was NCM811, the current collector was carbon-coated aluminum foil, and the binder was polytetrafluoroethylene; and the negative electrode was metallic lithium.

[0083] The obtained button battery was subjected to rate cycle test, and the results were as follows: Figure 8 As shown. Figure 8 It can be seen that at room temperature, the specific capacity of the NCM811 battery at a rate of 0.1C is 201mAh / g; the specific capacity of the NCM811 battery at a rate of 0.2C is 179mAh / g; the specific capacity of the NCM811 battery at a rate of 0.5C is 138mAh / g; the specific capacity of the NCM811 battery at a rate of 1C is 113mAh / g. When returning to the rate of 0.1C, the specific capacity of the NCM811 battery can still reach 195mAh / g. It can be seen that the obtained button battery has excellent rate performance.

[0084] Example 2

[0085] (1) In a glove box, 0.2 g of a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer (compound of formula II, wherein x is 67, y is 26, and z is 7) having a weight average molecular weight of 480,000 g / mol, 0.3 g of lithium bis(fluorosulfonyl)imide, 0.015 g of lithium nitrate, and 2.5 g of 2,2,2-trifluoro-N,N-dimethylacetamide (the mass ratio of the total mass of lithium bis(fluorosulfonyl)imide and lithium nitrate to the polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer and 2,2,2-trifluoro-N,N-dimethylacetamide is 1.575:1:12.5) were mixed and stirred at 30° C. and 500 r / min for 24 h to obtain a mixture 1;

[0086] (2) 2 g of succinonitrile and 0.58 g of lithium bis(trifluoromethanesulfonyl imide) were mixed (the mass ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl imide was 3.45:1), and the mixture was heated and stirred at 50° C. for 4 h at a speed of 500 r / min to obtain a mixture 2;

[0087] (3) In a glove box, 0.31 g of mixture 2 was added to mixture 1 (the mass ratio of polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer to succinonitrile was 1:1.2) using a pipette, and stirred evenly at a speed of 500 r / min at 30°C. The mixed solution was poured onto a glass plate and then heated in a vacuum oven at 60°C for 24 h. After being taken out, it was dried until the surface of the electrolyte membrane was no longer sticky. Then, multiple layers of sulfuric acid paper were covered on the dried electrolyte membrane, and the membrane was removed with tweezers. The other side was covered with multiple layers of sulfuric acid paper. The electrolyte membrane covered with multiple layers of sulfuric acid paper was then placed in a glove box and dried to obtain a salt-coated polymer solid electrolyte with a thickness of 56 μm.

[0088] The room temperature conductivity of the salt-coated polymer solid electrolyte prepared in Example 2 is 3.7×10 -4 S / cm, and the lithium ion transference number is 0.39.

[0089] Example 3

[0090] Steps (1) to (2) are the same as in Example 1;

[0091] (3) In a glove box, 0.154 g of mixture 2 was added to mixture 1 (the mass ratio of polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer to succinonitrile was 1:0.6) using a pipette, and stirred evenly at a speed of 500 r / min at 30°C. The mixed solution was poured onto a glass plate and then heated in a vacuum oven at 60°C for 24 h. After being taken out, it was dried until the surface of the electrolyte membrane was no longer sticky. Then, multiple layers of sulfuric acid paper were covered on the dried electrolyte membrane, and the membrane was removed with tweezers. The other side was covered with multiple layers of sulfuric acid paper. The electrolyte membrane covered with multiple layers of sulfuric acid paper was then placed in a glove box and dried to obtain a salt-coated polymer solid electrolyte with a thickness of 49 μm.

[0092] The room temperature conductivity of the salt-coated polymer solid electrolyte prepared in Example 3 is 2.1×10 -4 S / cm, and the lithium ion transference number is 0.23.

[0093] Example 4

[0094] (1) In a glove box, 0.2 g of a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer (compound of formula II, wherein x is 67, y is 26, and z is 7) having a weight average molecular weight of 480,000 g / mol, 0.25 g of lithium bis(fluorosulfonyl)imide, 0.0125 g of lithium nitrate, and 2.5 g of 2,2,2-trifluoro-N,N-dimethylacetamide were mixed (the mass ratio of the total mass of lithium bis(fluorosulfonyl)imide and lithium nitrate to the polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer and 2,2,2-trifluoro-N,N-dimethylacetamide was 1.3125:1:12.5), and stirred at 30° C. and 500 r / min for 24 h to obtain a mixture 1;

[0095] (2) 2 g of succinonitrile and 0.58 g of lithium bis(trifluoromethanesulfonyl imide) were mixed (the mass ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl imide was 3.45:1), and the mixture was heated and stirred at 50° C. for 4 h at a speed of 500 r / min to obtain a mixture 2;

[0096] (3) In a glove box, 0.232 g of mixture 2 was added to mixture 1 (the mass ratio of polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer to succinonitrile was 1:0.9) using a pipette, and stirred evenly at a speed of 500 r / min at 30°C. The mixed solution was poured onto a glass plate and then heated in a vacuum oven at 60°C for 24 h. After being taken out, it was dried until the surface of the electrolyte membrane was no longer sticky. Then, multiple layers of sulfuric acid paper were covered on the dried electrolyte membrane, and the membrane was removed with tweezers. The other side was covered with multiple layers of sulfuric acid paper. The electrolyte membrane covered with multiple layers of sulfuric acid paper was then placed in a glove box and dried to obtain a salt-coated polymer solid electrolyte with a thickness of 43 μm.

[0097] The room temperature conductivity of the salt-coated polymer solid electrolyte prepared in Example 4 is 3.5×10-4 S / cm, and the lithium ion transference number is 0.37.

[0098] Example 5

[0099] (1) In a glove box, 0.2 g of a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer (compound of formula II, wherein x is 67, y is 26, and z is 7) having a weight average molecular weight of 480,000 g / mol, 0.35 g of lithium bis(fluorosulfonyl)imide, 0.0175 g of lithium nitrate, and 2.5 g of 2,2,2-trifluoro-N,N-dimethylacetamide were mixed (the mass ratio of the total mass of lithium bis(fluorosulfonyl)imide and lithium nitrate to the polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer and 2,2,2-trifluoro-N,N-dimethylacetamide was 1.8375:1:12.5), and stirred at 30° C. and 500 r / min for 24 h to obtain a mixture 1;

[0100] (2) 2 g of succinonitrile and 0.58 g of lithium bis(trifluoromethanesulfonyl imide) were mixed (the mass ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl imide was 3.45:1), and the mixture was heated and stirred at 50° C. for 4 h at a speed of 500 r / min to obtain a mixture 2;

[0101] (3) In a glove box, 0.232 g of mixture 2 was added to mixture 1 (the mass ratio of polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer to succinonitrile was 1:0.9) using a pipette, and stirred evenly at a speed of 500 r / min at 30°C. The mixed solution was poured onto a glass plate and then heated in a vacuum oven at 60°C for 24 h. After being taken out, it was dried until the surface of the electrolyte membrane was no longer sticky. Then, multiple layers of sulfuric acid paper were covered on the dried electrolyte membrane, and the membrane was removed with tweezers. The other side was covered with multiple layers of sulfuric acid paper. The electrolyte membrane covered with multiple layers of sulfuric acid paper was then placed in a glove box and dried to obtain a salt-coated polymer solid electrolyte with a thickness of 50 μm.

[0102] The room temperature conductivity of the salt-coated polymer solid electrolyte prepared in Example 5 is 5.5×10 -4 S / cm, the lithium ion transference number is 0.41, but the lithium salt concentration is high and lithium salt is easily precipitated after drying.

[0103] Comparative Example 1

[0104] The difference from Example 1 is that Mixture 2 was not added.

[0105] Comparative Application Example 1

[0106] The salt-coated polymer solid electrolyte of Example 1 in Application Example 1 was replaced with the electrolyte of Comparative Example 1 to assemble an NCM811 battery.

[0107] The specific capacity-coulomb efficiency diagram of the NCM811 button battery assembled in the test application example 1 and the comparative application example 1 is shown in the figure. Figure 6 As shown. Figure 6 It can be seen that the discharge specific capacity of the battery continues to decay from the beginning of the cycle, and the coulombic efficiency fluctuates greatly. The stability of the button battery assembled with the electrolyte of Comparative Example 1 without adding nitrile compounds is far inferior to that of the button battery assembled with the electrolyte of Example 1. It can be seen that simply increasing the lithium salt concentration is not enough to adapt to the high-voltage positive electrode.

[0108] The impedance diagrams of the salt-coated polymer solid electrolytes prepared in Examples 1 to 5 were tested, and the results were as follows: Figure 7 As shown. Figure 7 The room temperature conductivity and lithium ion transference number of the salt-in-polymer solid electrolyte prepared in each embodiment can be seen in FIG.

[0109] In summary, the salt-coated polymer solid electrolyte prepared by the present invention has good room-temperature conductivity and lithium ion transference number, and the Li / NCM811 battery assembled with the electrolyte has excellent rate performance, specific capacity and cycle performance.

[0110] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a salt-coated polymer solid electrolyte, comprising the following steps: (1) mixing a ternary fluorinated copolymer, a first lithium salt, and an organic solvent to obtain a mixture 1; (2) heating and melting the nitrile compound and the second lithium salt to obtain a mixture 2; (3) mixing the mixture 1 obtained in step (1) and the mixture 2 obtained in step (2), casting the mixture onto a substrate, and drying the mixture to obtain a salt-coated polymer solid electrolyte; There is no chronological order between steps (1) and (2); The ternary fluorinated copolymer in step (1) comprises one or more compounds represented by formula I and formula II, In Formula I and Formula II, x is independently 63 to 69, y is independently 24 to 28, and z is independently 5 to 9; The mass ratio of the first lithium salt, the ternary fluorinated copolymer and the organic solvent in the step (1) is (1-2):1:(10-25); the mass ratio of the nitrile compound and the second lithium salt in the step (2) is (3-4):1; the mass ratio of the ternary fluorinated copolymer in the step (1) and the nitrile compound in the step (2) is 1:(0.5-1.5); the first lithium salt in the step (1) and the second lithium salt in the step (2) independently include lithium dioxalate borate, lithium difluorooxalate borate, dioxalate borate and dioxalate borate. One or more of lithium trifluoromethanesulfonyl imide, lithium bis(fluorosulfonyl imide), lithium hexafluorophosphate, lithium perchlorate and lithium nitrate; the organic solvent in step (1) includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and 2,2,2-trifluoro-N,N-dimethylacetamide; the nitrile compound in step (2) includes one or more of succinonitrile, malononitrile, terephthalonitrile, isophthalonitrile, o-phthalonitrile and ethoxymethylene malononitrile.

2. The salt-coated polymer solid electrolyte prepared by the preparation method according to claim 1.

3. A lithium battery comprising a positive electrode, a negative electrode and the salt-in-polymer solid electrolyte according to claim 2.

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