A method for preparing a high interfacial strain polymer electrolyte for lithium metal batteries

Abstract

The application provides a high-interface-strain polymer electrolyte preparation method for a lithium metal battery, and belongs to the technical field of lithium metal batteries, and specifically comprises the following steps: first, a precursor solution A containing a lithium salt and an organic solvent is configured; then, a precursor solution B obtained by mixing a polymer monomer, an initiator, a crosslinking agent and a lithium salt is configured; after the precursor solution A and the precursor solution B are mixed, the mixture is used as an electrolyte to assemble a lithium metal battery; and finally, the lithium metal battery is dried to obtain a high-interface-strain polymer electrolyte formed in situ in the positive and negative electrode interfaces of the lithium metal battery. By designing the proportion of fluorine elements and oxygen elements in the polymer electrolyte chain segment, the application reduces the coordination strength of Li-O in the polymer electrolyte body phase, improves the interface volume deformation of the polymer when lithium ions are removed, further reduces the potential difference of the electrolyte / lithium metal interface, homogenizes the interface lithium deposition, inhibits the growth of lithium dendrites, and finally improves the cycle stability of the lithium metal battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium metal batteries, and particularly relates to a preparation method of a high-interface-strain polymer electrolyte for a lithium metal battery. BACKGROUND

[0002] The development of polymer quasi-solid-state lithium metal batteries is of great significance. The low manufacturing cost, flexible structural design and good performance of the polymer quasi-solid-state lithium metal batteries make the polymer quasi-solid-state lithium metal batteries have wide application prospects in the fields of electric transportation, energy storage, consumer electronics and wearable devices. Through further technological progress and process optimization, the polymer quasi-solid-state lithium metal battery is expected to become one of the core directions of future battery technology.

[0003] Although the polymer quasi-solid-state lithium metal battery has unique advantages in improving energy density and safety, it still faces problems such as uneven deposition of lithium ions at the electrolyte / electrode interface and lithium dendrite growth. The initial driving force of lithium dendrite growth is the high potential difference at the electrolyte / lithium metal interface, which will accelerate the uneven deposition of ions and further lead to dendrite growth. When lithium ions are released from the electrolyte interface, the degree of polymer deformation can be referred to as the lithium ion partial molar volume of the electrolyte. High lithium ion partial molar volume corresponds to a high polymer interface deformation variable, and high interface deformation variable can effectively reduce the potential difference at the electrolyte / lithium metal interface, thereby uniformizing the interface lithium deposition and inhibiting dendrite growth (Advanced Energy Materials, 2023, 13(47): 2302643; Journal of The Electrochemical Society, 2004, 151(6): A880; Nature materials, 2020, 19(7): 758-766).

[0004] Therefore, designing a polymer electrolyte with high interface deformation variable helps to solve the problems of uneven deposition of lithium ions at the electrolyte / electrode interface and lithium dendrite growth, and further improves the performance of the polymer quasi-solid-state lithium metal battery. SUMMARY

[0005] In view of the problems of uneven deposition of lithium ions at the electrolyte / electrode interface and lithium dendrite growth existing in the existing polymer quasi-solid-state lithium metal battery, the application provides a preparation method of a high-interface-strain polymer electrolyte for a lithium metal battery. By designing the proportion of fluorine element and oxygen element in the polymer electrolyte chain segment, the interface volume deformation of the polymer when lithium ions are removed is improved, thereby reducing the potential difference at the electrolyte / lithium metal interface, and achieving the purpose of uniform lithium deposition at the interface.

[0006] In order to achieve the above-mentioned purpose, the technical method adopted by the application is as follows:

[0007] A method for preparing a high interfacial strain polymer electrolyte for lithium metal batteries includes the following steps:

[0008] Step 1: Add lithium salt to an organic solvent and stir to obtain precursor solution A with a concentration of 0.5-3M;

[0009] Step 2: Mix the polymer monomer, initiator, crosslinking agent and lithium salt, and stir to obtain precursor solution B; wherein, the molar ratio between polymer monomer, initiator and crosslinking agent is (98-100):(0.5-2):(1-3); the concentration of lithium salt in precursor solution B is 0.5-3M; the polymer monomer is at least one of 2,2,3,4,4,4-hexafluorobutyl acrylate, 1H,1H,2H,2H-nonafluorohexyl acrylate, 2,2,3,3,4,4,5,5,6,6,7,7-dodecylfluoroheptyl acrylate, and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecylfluorooctyl acrylate;

[0010] Step 3: Mix precursor solution A and precursor solution B at a volume ratio of (0.5-2):(1-3) and stir to form composite solution C;

[0011] Step 4: Using composite solution C as the electrolyte, assemble a lithium metal battery. After drying, a high interfacial strain polymer electrolyte is obtained in situ formed at the positive and negative electrode interfaces of the lithium metal battery.

[0012] Furthermore, the polymer monomers also include benzoyl acrylate and / or butyl acrylate.

[0013] Furthermore, by adjusting the specific components and proportions of the polymer monomers in the precursor solution B, the ratio of fluorine to oxygen in the resulting high interfacial strain polymer electrolyte chain segment is controlled to be (5-15):(1-3), thereby achieving high interfacial volumetric deformation.

[0014] Further, the organic solvent is at least one selected from fluoroethylene carbonate, ethylene glycol dimethyl ether, 1,3-dioxolane, diethyl carbonate, and dimethyl carbonate.

[0015] Furthermore, the initiator is at least one selected from azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, and azobisisobutyramidine hydrochloride.

[0016] Furthermore, the crosslinking agent is polyethylene glycol diacrylate or / and ethylene glycol dimethacrylate.

[0017] Furthermore, the lithium salt is at least one of lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium difluorooxalate borate.

[0018] Further, the temperature for drying in step 4 is 60-80℃, and the time length is 12-24h.

[0019] Compared with the prior art, the application has the following advantages:

[0020] The application provides a preparation method of a high-interface-strain polymer electrolyte for a lithium metal battery, which reduces the coordination strength of Li-O in a polymer electrolyte phase by designing the proportion of fluorine elements and oxygen elements in a polymer electrolyte segment, improves the interface volume deformation of the polymer when lithium ions are removed, reduces the potential difference of the electrolyte / lithium metal interface, homogenizes the interface lithium deposition, inhibits lithium dendrite growth, and finally improves the cycle stability of the lithium metal battery. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0022] Figure 1 The deformation diagram of the electrolyte Li+ interface over time for the high-interface-strain polymer electrolyte obtained in Example 1 and the low-interface-strain polymer electrolyte obtained in Comparative Example 1;

[0023] Figure 2 The cycle diagram of the Li||NCM811 battery assembled by the high-interface-strain polymer electrolyte obtained in Example 1 and the low-interface-strain polymer electrolyte obtained in Comparative Example 1;

[0024] Figure 3 The cycle diagram of the Li||Li symmetric battery assembled by the high-interface-strain polymer electrolyte obtained in Example 1 and the low-interface-strain polymer electrolyte obtained in Comparative Example 1. DETAILED DESCRIPTION

[0025] In order to further understand the present application, the preferred embodiments of the present application will be described below in combination with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, and are not limitations on the claims of the present application. All raw materials of the present application are not particularly limited in source, and can be purchased on the market or prepared according to conventional methods well known to those skilled in the art.

[0026] Example 1

[0027] This embodiment prepares a high-interface-strain polymer electrolyte for a lithium metal battery, which specifically includes the following steps:

[0028] Step 1, lithium bis(trifluoromethanesulfonimide) is added to fluoroethylene carbonate to obtain precursor solution A with a concentration of 1M;

[0029] Step 2, 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate, azobisisobutyronitrile, polyethylene glycol diacrylate and lithium bis(trifluoromethanesulfonimide) are mixed to obtain precursor solution B; the molar ratio among the polymer monomer, initiator and crosslinking agent is 100:0.5:1; the concentration of lithium salt in the precursor solution B is 2M;

[0030] Step 3, precursor solution A and precursor solution B are mixed in a volume ratio of 1:1 to form a composite solution C;

[0031] Step 4, the composite solution C is used as an electrolyte to assemble a lithium metal battery, which is dried at 60°C for 12h to obtain a high interfacial strain polymer electrolyte formed in situ in the interface between the positive and negative electrodes of the lithium metal battery; according to the proportion of fluorine element and oxygen element in the chemical structural formula of the polymer monomer 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate, the proportion of fluorine element and oxygen element contained in the chain segment of the high interfacial strain polymer electrolyte can be determined, and the proportion of the two is calculated to be 6:1.

[0032] In order to better illustrate the positive effect of the high interfacial strain polymer electrolyte obtained in this embodiment on the lithium metal battery, the following will explain the assembly of transparent battery, Li||NCM811 battery and Li||Li symmetric battery respectively.

[0033] When assembling the transparent battery, clean copper foil is used as the counter electrode, precursor solution A with a lithium salt concentration of 2M is used as the ion conducting agent, and copper foil strip is used as the current collector and lead. The transparent battery is assembled. The composite solution C obtained in this embodiment is dropped on the copper foil with a pipette and placed in a oven at 60°C for in-situ curing to obtain a coated copper foil. The transparent battery is placed under an optical microscope, the coated copper foil and the clean copper foil are used as the counter electrode respectively, and are connected to the positive and negative electrodes of the blue battery test workstation. The transparent battery is forced to charge, and the structural change of the electrolyte surface of the coated copper foil when lithium ions are released is observed under an optical microscope, and the results are shown in Figure 1 .

[0034] When assembling the Li||NCM811 battery, 12mm diameter NCM811 containing lithium salt is used as the positive electrode sheet, 14mm diameter lithium metal round sheet is used as the negative electrode, 17mm diameter polyolefin separator is used as the base, steel sheet is used as the current collector, and spring sheet is used as the gasket. A 2032 button cell is assembled.

[0035] Specifically, the preparation process of the NCM811 containing lithium salt is as follows:

[0036] The positive material NCM811, the conductive agent C45, the binder polyvinylidene fluoride and the lithium salt lithium bis-trifluoromethanesulfonimide are mixed in N-methylpyrrolidone (NMP) solvent in a mass ratio of 7:1:1:1, uniformly obtained by room temperature magnetic stirring for 8 h, and a uniform positive electrode slurry with a mass fraction of 16.7% of the positive material in the slurry is obtained. The slurry is coated on a carbon-coated aluminum foil, uniformly coated on an automatic coating machine by controlling the thickness of the doctor blade to be 800 μm, and vacuum dried in an oven at 80°C for 24 h. After the solvent is volatilized, a lithium-containing NCM811 with a loading control of 1.9 mg cm -2 .

[0037] The composite solution C obtained in the example is first dropped on the positive electrode sheet with a pipette to fully infiltrate the electrode surface and accelerate ion transmission. Then, the composite solution C obtained in the example is added to the polyolefin separator with a pipette by fully infiltrating the substrate to form better contact between the positive and negative electrodes, which is beneficial to the deposition of the ion interface. After the Li||NCM811 battery is packaged, it is placed in a 60°C oven for 12 h to form a solid high interfacial strain polymer electrolyte in situ at the positive and negative electrode interfaces in the battery. After high-temperature curing, the battery is placed in a 30°C constant temperature box, and a constant current test is performed at a current density of 1 / 3C rate in the voltage range of 2.8-4.8 V. The results are shown in Figure 2 .

[0038] When assembling the Li||Li symmetric battery, the difference compared with the process of assembling the Li||NCM811 battery is that the lithium metal disc is used as the positive electrode sheet; the other structures and assembly processes remain unchanged. The obtained Li||Li symmetric battery is subjected to constant current test at a current density of 0.1 mA cm -2 . Figure 3

[0039] Comparative Example 1

[0040] A low interfacial strain polymer electrolyte for a lithium metal battery is prepared in this comparative example, which specifically includes the following steps:

[0041] Step 1, lithium bis-trifluoromethanesulfonimide is added to fluoroethylene carbonate to obtain a precursor solution A with a concentration of 1M after stirring;

[0042] Step 2, n-hexyl acrylate (as a polymer monomer), azobisisobutyronitrile, polyethylene glycol diacrylate and lithium bis-trifluoromethanesulfonimide are mixed to obtain a precursor solution B after stirring; the molar ratio among the polymer monomer, the initiator and the crosslinking agent is 100:0.5:1; the concentration of the lithium salt in the precursor solution B is 2M;

[0043] ​Step 3, mixing precursor solution A and precursor solution B according to a volume ratio of 1:1, stirring to form a composite solution C;

[0044] Step 4, assembling a lithium metal battery with the composite solution C as the electrolyte, drying at 60°C for 12h, obtaining a low interfacial strain polymer electrolyte formed in situ in the interface of the positive and negative electrodes of the lithium metal battery; wherein, according to the proportion of fluorine element and oxygen element in the chemical structural formula of the polymer monomer n-hexyl acrylate used, the proportion of fluorine element and oxygen element contained in the chain segment of the low interfacial strain polymer electrolyte can be determined, and the proportion of the two is calculated to be 0:2.

[0045] The present comparative example assembles a transparent battery, a Li||NCM811 battery and a Li||Li symmetric battery according to the steps of Example 1 for illustration; wherein the structural changes of the electrolyte surface of the copper foil under the optical microscope when lithium ions are released are as shown in Figure 1 ; the results of the constant current test of the Li||NCM811 battery at a current density of 1 / 3C rate in the voltage interval of 2.8-4.8V are as shown in Figure 2 ; and the results of the constant current test of the Li||Li symmetric battery at a current density of 0.1mA cm -2 are as shown in Figure 3 .

[0046] As can be seen from the deformation diagram of the Li+ release interface of the electrolyte over time Figure 1 , the high interfacial strain polymer electrolyte obtained in Example 1 (denoted as High F / O) has a greater volume change than the low interfacial strain polymer electrolyte obtained in Comparative Example 1 (denoted as Low F / O) as the transparent battery charging test proceeds, and the high interfacial strain can effectively reduce the potential difference between the electrolyte / lithium metal interface, which is beneficial to the stable deposition of ions.

[0047] As can be seen from Figure 2 , the Li||NCM811 battery assembled with the high interfacial strain polymer electrolyte obtained in Example 1 has higher discharge capacity and cycle stability than the Li||NCM811 battery assembled with the low interfacial strain polymer electrolyte obtained in Comparative Example 1, which can effectively stabilize the ion deposition of the lithium metal interface and inhibit the growth of lithium dendrites, realizing efficient and uniform utilization of the lithium metal negative electrode.

[0048] As can be seen from Figure 3 , the Li||Li symmetric battery assembled with the high interfacial strain polymer electrolyte obtained in Example 1 can withstand a larger test current density and has better cycle stability than the Li||Li symmetric battery assembled with the low interfacial strain polymer electrolyte obtained in Comparative Example 1.

[0049] In summary, by increasing the proportion of fluorine and oxygen elements in the polymer electrolyte segment, the coordination strength of Li-O in the polymer electrolyte body phase is reduced, the interface volume deformation during polymer lithium ion extraction is improved, the potential difference between the electrolyte / lithium metal interface is reduced, the lithium deposition at the interface is uniformized, the growth of lithium dendrites is inhibited, and the cycle stability of the lithium metal battery is ultimately improved.

[0050] Example 2

[0051] In this embodiment, a high interface strain polymer electrolyte for a lithium metal battery is prepared. Compared with Example 1, the difference is only that in step 2, the polymer monomer used is a mixture of 2,2,3,4,4,4-hexafluorobutyl acrylate and butyl acrylate, and the molar ratio between 2,2,3,4,4,4-hexafluorobutyl acrylate, butyl acrylate, initiator and crosslinking agent is 91:9:0.5:1, and then the high interface strain polymer electrolyte is obtained. According to the proportion of fluorine and oxygen elements in the chemical structural formula of the polymer monomers 2,2,3,4,4,4-hexafluorobutyl acrylate and butyl acrylate, and the raw material ratio of 2,2,3,4,4,4-hexafluorobutyl acrylate and butyl acrylate, the proportion of fluorine and oxygen elements contained in the high interface strain polymer electrolyte segment can be determined, and the calculation result is that the proportion of the two is 3:1. The other preparation processes are the same.

[0052] Example 3

[0053] In this embodiment, a high interface strain polymer electrolyte for a lithium metal battery is prepared. Compared with Example 1, the difference is only that in step 2, the polymer monomer used is a mixture of 1H,1H,2H,2H-nonafluorohexyl acrylate and butyl acrylate, and the molar ratio between 1H,1H,2H,2H-nonafluorohexyl acrylate, butyl acrylate, initiator and crosslinking agent is 93:7:0.5:1, and then the high interface strain polymer electrolyte is obtained. According to the proportion of fluorine and oxygen elements in the chemical structural formula of the polymer monomers 1H,1H,2H,2H-nonafluorohexyl acrylate and butyl acrylate, and the raw material ratio of 1H,1H,2H,2H-nonafluorohexyl acrylate and butyl acrylate, the proportion of fluorine and oxygen elements contained in the high interface strain polymer electrolyte segment can be determined, and the calculation result is that the proportion of the two is 9:2. The other preparation processes are the same.

[0054] Example 4

[0055] A high interfacial strain polymer electrolyte for a lithium metal battery was prepared in this example. The preparation process was compared with that of Example 1, the only difference being that in Step 2, the polymer monomer used was adjusted to a mixture of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate and butyl acrylate, and the molar ratio between 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate, butyl acrylate, initiator and crosslinking agent was 95:5:0.5:1, and then the obtained high interfacial strain polymer electrolyte; wherein according to the proportion of fluorine element and oxygen element in the chemical structural formula of the polymer monomer 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate and butyl acrylate, and the raw material ratio of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate and butyl acrylate, the proportion of fluorine element and oxygen element contained in the chain segment of the high interfacial strain polymer electrolyte can be determined, and the proportion of the two is calculated to be 13:2. The other preparation processes are the same.

[0056] According to the deformation diagram of the Li+ extraction interface of the electrolyte over time of Examples 1-4 and Comparative Example 1, the interface shrinkage ratio data of each electrolyte Li+ extraction interface was approximately calculated, and the constant current test results of the Li||NCM811 battery assembled by the polymer electrolyte of Examples 1-4 and Comparative Example 1 at a current density of 1 / 3C rate in the voltage range of 2.8-4.8V were tested, as shown in Table 1, it can be seen that by adjusting the specific components and proportion of the polymer monomer, controlling the proportion of fluorine element and oxygen element contained in the chain segment of the obtained high interfacial strain polymer electrolyte in the range of (5-15):(1-3), high interfacial volume strain can be achieved, and the discharge capacity and cycle stability of the lithium metal battery can be improved.

[0057] Table 1

[0058] Sample Interface shrinkage ratio 1 / 3 C capacity Example 1 33％ 180 mAh g -1 ]] Example 2 26％ 172 mAh g -1 ]] Example 3 19％ 160 mAh g -1 ]] Example 4 9％ 152 mAh g -1 ]] Comparative Example 1 1％ 120 mAh g -1 ]]

[0059] The principles and implementations of the present application are described by using specific examples. The above examples are only used to help understand the method of the present application and its core idea, including the best mode, and also enable any person skilled in the art to practice the present application, including manufacturing and using any device or system, and implementing any combined method. It should be noted that, for those skilled in the art, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application. The scope of patent protection of the present application is defined by the claims, and can include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements that are not different from the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for the preparation of a high interfacial strain polymer electrolyte for lithium metal batteries, characterized by, Comprising the following steps: Step 1, lithium bis(trifluoromethanesulfonimide) is added to fluoroethylene carbonate to obtain precursor solution A with a concentration of 1M; Step 2, 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate, azobisisobutyronitrile, polyethylene glycol diacrylate and lithium bis(trifluoromethanesulfonimide) are mixed to obtain precursor solution B; wherein the molar ratio of 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate, azobisisobutyronitrile and polyethylene glycol diacrylate is 100:0.5:1; the concentration of lithium bis(trifluoromethanesulfonimide) in precursor solution B is 2M; Step 3, precursor solution A and precursor solution B are mixed in a volume ratio of 1:1 to form a composite solution C; Step 4, using composite solution C as electrolyte, assembling lithium metal battery, after drying at 60℃ for 12h, obtaining high interfacial strain polymer electrolyte formed in situ in the positive and negative electrode interface of lithium metal battery.

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