Polymer-based composite solid electrolyte and preparation method and application thereof

Abstract

The application discloses a polymer-based composite solid-state electrolyte and a preparation method and application thereof. The method comprises the following steps: adding polyethylene oxide, polyvinylidene fluoride and lithium bis-trifluoromethanesulfonimide into acetonitrile, stirring, adding MOF5-NH2 material, obtaining a mixed mud slurry, and performing volatilization and drying to obtain the polymer-based composite solid-state electrolyte. The polymer-based composite solid-state electrolyte has high ionic conductivity and a wide electrochemical window, can inhibit the shuttle effect of polysulfides in a lithium-sulfur battery, and improves the cycle stability of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of solid-state lithium batteries, and relates to a polymer-based composite solid-state electrolyte and a preparation method and application thereof. BACKGROUND

[0002] As an important support and auxiliary technology of new energy industry, electrochemical energy storage devices have become a global research and development hotspot. New electrochemical energy storage devices are developing towards high specific energy, high safety, long cycle life and low cost. Lithium-sulfur batteries have excellent theoretical specific capacity (1672 mAh / g), and the theoretical specific energy density is as high as 2600 Wh / kg. Moreover, the main active material sulfur element is abundant in reserves, low in price and easy to prepare and obtain, and therefore is considered as a battery system with ideal application prospect.

[0003] Unlike the intercalation / deintercalation reaction of traditional lithium-ion batteries, lithium-sulfur batteries use sulfur or sulfur-containing compounds as the positive electrode, lithium as the negative electrode, and liquid organic compounds as the electrolyte, and realize the mutual conversion of electrical energy and chemical energy through the breaking / formation of sulfur-sulfur bonds. During discharge, lithium ions are released from the negative electrode and migrate to the positive electrode, and the sulfur-sulfur bond of the positive electrode active material is broken. During this process, a large number of intermediate products are generated, and a variety of lithium sulfide intermediates such as Li2S8, Li2S6 and Li2S4 are generated, and finally Li2S is formed. During charging, Li2S is electrolyzed, and the released lithium ions return to the negative electrode and are deposited as metallic lithium or embedded into the negative electrode material.

[0004] Lithium-sulfur batteries still face many challenges since its development. In the liquid electrolyte system, the active material utilization rate is low, the capacity attenuation is rapid, the cycle life is short, the self-discharge is fast, and the safety performance needs to be improved, which limits its further development and application. The main reason is that elemental sulfur is used as the positive electrode, which is an electronic insulator with low conductivity. The density difference between the final product Li2S2 / Li2S and the intermediate product polysulfide is large, and there is a significant volume effect. The intermediate product polysulfide is soluble in liquid organic electrolyte, and will migrate to the negative electrode during the charging process, and react with the unstable lithium metal surface to generate self-discharge reaction, and the product returns to the positive electrode to be oxidized. This process is repeated to form a shuttle effect, resulting in low active material utilization rate, causing battery capacity loss and cycle performance decline. In addition, the metal lithium as the negative electrode has always existed interface instability and dendrite problem, which is easy to cause thermal runaway and short circuit explosion, etc. It also restricts the promotion and application of lithium-sulfur batteries. Although the inorganic solid-state electrolyte reported at present has high ionic conductivity, it has poor processing performance, is not easy to form a film, and there are problems such as ion transmission difficulty between the electrolyte and the electrode. It is not easy to be directly applied to battery devices. Although the organic polymer electrolyte is expected to be applied to battery devices, single organic polymer solid-state electrolyte such as polyethylene oxide has low room temperature ionic conductivity and cannot effectively suppress the shuttle effect and lithium dendrite problem. At present, an effective strategy to improve the polymer electrolyte is to add fillers to the polymer electrolyte to improve its ionic conductivity and mechanical properties. Yuan et al. improved the ionic conductivity of PEO-based polymer electrolyte by adding MOF5 material to PEO. The principle is that the MOF material contains a large number of Lewis acid sites. The Lewis acid not only coordinates with the oxygen atom with lone pair electrons on the ether group of PEO chain to reduce the crystallinity of PEO, but also interacts with the anion in the lithium salt to promote the decomposition of lithium salt and improve the ionic conductivity of the polymer electrolyte (Journal of Power Sources 240 (2013) 653-658). However, the addition of pure MOF material cannot significantly improve the ionic conductivity, and cannot effectively solve the problems of low oxidation voltage of polymer-based solid-state electrolyte and lithium dendrite. SUMMARY

[0005] The purpose of the present application is to provide a polymer-based composite solid-state electrolyte and its preparation method and application.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is:

[0007] The preparation method of the polymer-based composite solid-state electrolyte comprises the following steps:

[0008] Polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), lithium bis-trifluoromethanesulfonimide (LiTFSI) are added into acetonitrile, ball-milling treatment is performed, then the MOF5-NH2 material is added and uniformly dispersed to obtain a mixed slurry, the mixed slurry is poured into a mold, and volatilization drying is performed to obtain a polymer-based composite solid-state electrolyte; the mixed slurry comprises, in terms of mass fraction, 90 parts of polyethylene oxide, 10 parts of polyvinylidene fluoride, 32.62 parts of lithium bis-trifluoromethanesulfonimide, 150 parts of acetonitrile, and 5 parts of the MOF5-NH2 material.

[0009] Preferably, the average molecular weight of the polyethylene oxide is 1000000 Da.

[0010] Preferably, the ball-milling stirring speed is 300-400 rpm, and the ball-milling time is 8-12 h.

[0011] Preferably, the volatilization drying process comprises the following steps: drying at 30 DEG C under normal pressure for 3 h, and then drying at 60 DEG C under vacuum for 24 h.

[0012] The application further provides a polymer-based composite solid-state electrolyte prepared by the preparation method.

[0013] Further, the application provides application of the polymer-based composite solid-state electrolyte in preparation of a solid-state lithium-sulfur battery.

[0014] Compared with the prior art, the application has the following advantages:

[0015] (1) The PEO and PVDF are compounded to form a three-dimensional structure in the application, and the introduced MOF5-NH2 material has a large number of Lewis acid sites, which can not only coordinate with the oxygen atom with a lone pair of electrons on the ether group of the PEO chain to reduce the crystallinity of the PEO, but also interact with the anion in the lithium salt to promote the decomposition of the lithium salt and improve the ionic conductivity of the polymer electrolyte;

[0016] (2) The added PVDF in the application is insoluble in long-chain polysulfides, and the introduced amino group in the MOF5 is a polar group, which can effectively inhibit the shuttle effect of the polysulfides and improve the cycle stability of the lithium-sulfur battery;

[0017] (3) The MOF5-NH2 material added in the application contains an amino group, which can form a hydrogen bond with the ether oxygen bond in the polymer chain, thereby improving the oxidation voltage of the polymer electrolyte. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 XRD patterns of polymer-based solid-state electrolytes prepared in Comparative Examples 1, 2, 3, 5 and Example 1;

[0019] Figure 2SEM images of the polymer-based solid-state electrolyte prepared for Example 1;

[0020] Figure 3 Comparison chart of ionic conductivity of the polymer-based solid-state electrolytes prepared for Comparative Example 1 and Example 1;

[0021] Figure 4 Comparison chart of electrochemical stability window of the half-symmetrical batteries respectively assembled by the polymer-based solid-state electrolytes prepared for Comparative Examples 1, 5 and Example 1;

[0022] Figure 5 Comparison chart of cycle performance of the solid-state lithium-sulfur batteries respectively assembled by the polymer-based solid-state electrolytes prepared for Comparative Example 1 and Example 1. DETAILED DESCRIPTION

[0023] The application will be further described in detail below with reference to the examples and the accompanying drawings.

[0024] The preparation method of MOF5-NH2 refers to (Energy Storage Materials 18 (2019) 59-67). The specific steps are as follows: 1.666 g of ZnNO3.6H2O and 0.36 g of H2ATA are respectively dissolved in 30 ml of DMF, stirred until dissolved, and then transferred to a reaction kettle, which is placed in an oven at 120°C for 12 h. The oven is turned off and the reaction kettle is naturally cooled to room temperature in the oven. The crystal sample obtained by the reaction is filtered out and repeatedly washed with DMF. Then the collected sample is dried at 120°C. Finally, the sample is sealed and stored for use.

[0025] Comparative Example 1

[0026] 1 g of polyethylene oxide (PEO) and 0.3262 g of lithium bis-trifluoromethanesulfonimide (LiTFSI) were weighed and added to a ball mill tank containing 15 g of acetonitrile. The mixture was ball milled at a speed of 400 rpm for 12 h at 30°C to form a transparent viscous liquid. The obtained transparent viscous liquid was then injected into a mold. Most of the solvent was evaporated by drying at 30°C under normal pressure for 3 h. Subsequently, the mold was transferred to a vacuum drying oven at 60°C for vacuum drying for 24 h. A polymer-based composite solid-state electrolyte was obtained, which was cut into small round pieces with a diameter of 16 mm for use.

[0027] In an argon-protected environment in a glove box, Ketjenblack / sulfur composite material was used as a positive electrode and lithium sheet was used as a negative electrode. The prepared polymer-based solid-state electrolyte, sulfur-carbon composite positive electrode and negative electrode sheet were assembled into a coin cell in the glove box. The coin cell was subjected to charge-discharge cycle test on a LAND battery test system, and the working voltage was 1.6-2.8 V (vs. Li / Li + ).

[0028] Comparative Example 2

[0029] Take 0.9 g of polyethylene oxide (PEO), 0.1 g of polyvinylidene fluoride (PVDF) and 0.3262 g of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) into a ball mill tank containing 15 g of acetonitrile, and mill at 400 rpm for 12 h at 30°C to form a transparent viscous liquid. The obtained transparent viscous liquid is then injected into a mold, and most of the solvent is evaporated by drying at 30°C under normal pressure for 3 h, and then transferred to a vacuum drying oven at 60°C for vacuum drying for 24 h to obtain a polymer-based composite solid-state electrolyte, which is cut into 16 mm small round pieces for use.

[0030] In an argon atmosphere in a glove box, Ketjen black / sulfur composite material is used as the positive electrode, and lithium sheet is used as the negative electrode. The prepared polymer-based solid-state electrolyte, sulfur-carbon composite positive electrode, and negative electrode sheet are assembled into a button cell in the glove box, and then subjected to charge-discharge cycle test on a LAND battery test system, with a working voltage of 1.6-2.8 V (vs. Li / Li + ).

[0031] Comparative Example 3

[0032] Take 1 g of polyethylene oxide (PEO) and 0.3262 g of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) into a ball mill tank containing 15 g of acetonitrile, and mill at 400 rpm for 12 h at 30°C. After forming a transparent viscous liquid, 0.05 g of MOF5-NH2 material is uniformly dispersed therein to obtain a brownish mud. The obtained brownish mud is then injected into a mold, and most of the solvent is evaporated by drying at 30°C under normal pressure for 3 h, and then transferred to a vacuum drying oven at 60°C for vacuum drying for 24 h to obtain a polymer-based composite solid-state electrolyte, which is cut into 16 mm small round pieces for use.

[0033] In an argon atmosphere in a glove box, Ketjen black / sulfur composite material is used as the positive electrode, and lithium sheet is used as the negative electrode. The prepared polymer-based solid-state electrolyte, sulfur-carbon composite positive electrode, and negative electrode sheet are assembled into a button cell in the glove box, and then subjected to charge-discharge cycle test on a LAND battery test system, with a working voltage of 1.6-2.8 V (vs. Li / Li + ).

[0034] Comparative Example 4

[0035] Take 0.5 g of polyethylene oxide (PEO) and 0.3262 g of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) into a ball mill tank containing 15 g of acetonitrile, and mill at 400 rpm for 12 h at 30°C to form a transparent viscous liquid. Then inject the obtained transparent viscous liquid into a mold, dry at 30°C under normal pressure for 3 h to evaporate most of the solvent, and then transfer to a 60°C vacuum drying oven for vacuum drying for 24 h. The film-shaped polymer-based composite solid-state electrolyte cannot be obtained.

[0036] Comparative Example 5

[0037] 0.9 g of polyethylene oxide (PEO), 0.1 g of polyvinylidene fluoride (PVDF), and 0.3262 g of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) are added to a ball mill tank containing 15 g of acetonitrile, and milled at 400 rpm for 12 h at 30°C. After forming a transparent viscous liquid, 0.05 g of MOF5 material is uniformly dispersed therein to obtain a tan mud. Then inject the obtained tan mud into a mold, dry at 30°C under normal pressure for 3 h to evaporate most of the solvent, and then transfer to a 60°C vacuum drying oven for vacuum drying for 24 h to obtain a polymer-based composite solid-state electrolyte, which is cut into 16 mm small round pieces for use.

[0038] In an argon atmosphere in a glove box, Ketjenblack / sulfur composite material is used as a positive electrode, and lithium sheet is used as a negative electrode. The prepared polymer-based solid-state electrolyte, sulfur-carbon composite positive electrode, and negative electrode sheet are assembled into a button cell in the glove box, and then subjected to charge-discharge cycle test on a LAND battery test system, with a working voltage of 1.6-2.8 V (vs. Li / Li + ).

[0039] Example 1

[0040] 0.9 g of polyethylene oxide (PEO), 0.1 g of polyvinylidene fluoride (PVDF), and 0.3262 g of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) are added to a ball mill tank containing 15 g of acetonitrile, and milled at 400 rpm for 12 h at 30°C. After forming a transparent viscous liquid, 0.05 g of MOF5-NH2 material is uniformly dispersed therein to obtain a tan mud. Then inject the obtained tan mud into a mold, dry at 30°C under normal pressure for 3 h to evaporate most of the solvent, and then transfer to a 60°C vacuum drying oven for vacuum drying for 24 h to obtain a polymer-based composite solid-state electrolyte, which is cut into 16 mm small round pieces for use.

[0041] In an argon atmosphere glove box, the prepared polymer-based solid-state electrolyte, sulfur-carbon composite positive electrode, and negative electrode were assembled into a coin cell, and the charge-discharge cycle test was performed on a LAND battery test system, with a working voltage of 1.6-2.8 V (vs. Li / Li + ).

[0042] Figure 1 The XRD comparison chart of the polymer-based solid-state electrolytes prepared in Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 5, and Example 3 shows that the addition of PVDF reduces the crystallinity of the solid-state electrolyte, and the further addition of the MOF material can further reduce the crystallinity of the polymer-based solid-state electrolyte, thereby improving the ionic conductivity of the solid-state electrolyte.

[0043] Figure 2 The SEM chart of the polymer-based solid-state electrolyte prepared in Example 1 shows that the composite polymer electrolyte has a smooth surface and less agglomeration, because the metal-organic framework material has organic-inorganic hybrid properties, and the surface energy difference between the metal-organic framework material and PEO is small, so it can be uniformly dispersed in the PEO-LiTFSI polymer electrolyte.

[0044] Figure 3 The ionic conductivity comparison chart of the symmetric batteries (stainless steel / composite solid-state electrolyte / stainless steel) assembled by the polymer-based composite solid-state electrolytes prepared in Comparative Example 1 and Example 1 respectively measured at 30-80°C shows that the addition of PVDF can form a three-dimensional structure with PEO to increase the ionic conductivity of the polymer electrolyte, and the further addition of the MOF5-NH2 material reduces the crystallinity of the obtained polymer-based solid-state electrolyte, thereby improving the ionic conductivity of the solid-state electrolyte. At 30°C, the ionic conductivity of Comparative Example 1 is 10 -5.75 S.cm -1 , and the ionic conductivity of Example 1 reaches 10 -4.9 S.cm -1 .

[0045] Figure 4 The electrochemical stability window comparison chart of the semi-symmetric batteries (stainless steel / composite solid-state electrolyte / Li) assembled by the polymer-based composite solid-state electrolytes prepared in Comparative Example 1, Comparative Example 5, and Example 1 respectively measured under the condition of a voltage range of 2.5-6 V and a scan rate of 10 mV s -1 , the oxidation voltages of Comparative Example 1, Comparative Example 5, and Example 1 are 3.7 V, 4.7 V, and 5.3 V respectively, which shows that the addition of the MOF5-NH2 material can improve the electrochemical stability of the polymer-based solid-state electrolyte.

[0046] Figure 5The long cycle performance chart of the full solid-state lithium-sulfur battery assembled by the polymer-based solid-state electrolyte prepared from Comparative Example 1 and Example 1 at 1.6-2.8V, 60℃ and 0.1C shows that the solid-state lithium-sulfur battery assembled by the polymer-based solid-state electrolyte prepared from Comparative Example 1 exhibits poor cycle stability, and the specific capacity is reduced to 400mAh / g after 50 cycles, while the solid-state lithium-sulfur battery assembled by the polymer-based solid-state electrolyte prepared from Example 1 exhibits good cycle stability. After 50 cycles at a current density of 0.1C, the capacity of Example 3 is 800mAh / g, which is attenuated by only 20%.

Claims

1. The application of polymer-based composite solid electrolyte in the preparation of solid-state lithium-sulfur batteries, characterized in that, The polymer-based composite solid electrolyte is prepared by the following steps: Polyethylene oxide, polyvinylidene fluoride, and lithium bis(trifluoromethanesulfonyl)imide were added to acetonitrile and ball-milled. Then, MOF5-NH2 material was added and dispersed evenly to obtain a mixed slurry. The mixed slurry was poured into a mold and evaporated to obtain a polymer-based composite solid electrolyte. The mixed slurry, by mass, included 90 parts of polyethylene oxide, 10 parts of polyvinylidene fluoride, 32.62 parts of lithium bis(trifluoromethanesulfonyl)imide, 150 parts of acetonitrile, and 5 parts of MOF5-NH2 material. The preparation method of MOF5-NH2 specifically involved dissolving 1.666g of ZnNO3·6H2O and 0.36g of H2ATA in 30ml of water. After stirring in DMF until dissolved, the mixture is transferred to a reaction vessel and placed in an oven at 120°C for 12 hours. The oven is then closed and the reaction vessel is allowed to cool naturally to room temperature. The resulting crystal sample is filtered out and repeatedly rinsed with DMF. The collected sample is then dried at 120°C. Finally, the sample is collected, sealed, and stored for use.

2. The application according to claim 1, characterized in that, The average molecular weight of the polyethylene oxide is 1,000,000 Da.

3. The application according to claim 1, characterized in that, The ball milling speed is 300~400 rpm, and the ball milling time is 8~12h.

4. The application according to claim 1, characterized in that, The volatile drying process is as follows: first, dry at 30℃ and normal pressure for 3 hours, and then dry under vacuum at 60℃ for 24 hours.

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