All-solid-state lithium metal battery and preparation method thereof

Abstract

The invention relates to a preparation method of an all-solid-state lithium metal battery, which comprises the following steps: S1, adding a high-molecular polymer, a lithium salt and a coumarin compound in a mass ratio of 1: (1-6) into an organic solvent, and uniformly mixing to obtain a mixed system, the mass of the coumarin compound accounts for 5wt%-80wt% of the total mass of the whole mixed system, and preparing the solid electrolyte membrane by adopting a solution pouring method; and S2, assembling the solid electrolyte membrane into the all-solid-state lithium metal battery. The invention also relates to the all-solid-state lithium metal battery obtained by the preparation method. According to the all-solid-state lithium metal battery and the preparation method thereof disclosed by the invention, the ionic conductivity of the electrolyte can be effectively improved, and the component structure of the interface layer between the electrolyte and the electrode is optimized to meet the all-solid-state battery requirements of high capacity, good rate capability, long cycle performance, relatively good stability and high safety.

Description

Technical Field

[0001] The present invention relates to a solid-state battery, and more specifically to an all-solid-state lithium metal battery and a preparation method thereof. Background Art

[0002] With the continuous growth of electricity demand and the gradual depletion of fossil energy resources, the development of new energy technologies has become increasingly important. Metal lithium batteries have attracted widespread attention due to their high theoretical specific capacity (3860mAh g-1) and low potential (-3.04V relative to standard hydrogen electrode). However, the liquid electrolytes used in traditional metal lithium batteries have safety hazards such as flammability and volatility, which limit their practical application. Therefore, solid electrolytes have been widely studied as an alternative to improve the safety and performance of batteries.

[0003] Solid electrolytes are mainly divided into two categories: inorganic ceramic electrolytes and organic polymer electrolytes. Among organic polymer electrolytes, polyethylene oxide (PEO)-based electrolytes have attracted attention due to their low cost, strong ability to dissolve lithium salts, and good flexible processing properties. However, they have low room temperature ionic conductivity and poor stability to metallic lithium anodes, which greatly reduce battery performance and limit their practical applications.

[0004] In order to solve these problems, many researchers have tried to add various filler additives to the PEO matrix, such as inorganic fillers (inert TiO2, SiO2, Al2O3, active ion conductors LATP, LLZO, etc.) and organic plasticizers (ethylene carbonate EC, fluoroethylene carbonate FEC, succinonitrile SN, etc.). These additives improve the ionic conductivity by reducing the crystallinity of the PEO segment and increasing the mobility of the EO segment. However, excessive addition of inorganic fillers can easily lead to serious agglomeration, making it difficult to form a uniform lithium ion conduction network and destroying the uniformity of the membrane. At the same time, the addition of plasticizers will significantly reduce the mechanical strength of the electrolyte membrane, making it more easily pierced by lithium dendrites, thereby causing battery short circuits. In addition, the interface layer constructed by these additives during the battery cycle is not conducive to the long-cycle stability of the battery. Summary of the invention

[0005] In order to solve the problems of low ionic conductivity and poor interface stability of polymer electrolytes in the above-mentioned prior art, the present invention provides an all-solid-state lithium metal battery and a preparation method thereof.

[0006] The preparation method of the all-solid-state lithium metal battery according to the present invention comprises the following steps: S1, adding a high molecular polymer, a lithium salt, and a coumarin compound into an organic solvent and uniformly mixing to obtain a mixed system, wherein the mass ratio of the lithium salt to the high molecular polymer is 1:(1-6), and the mass of the coumarin compound accounts for 5wt%-80wt% of the total mass of the entire mixed system, and a solid electrolyte membrane is prepared by a solution casting method; S2, assembling the solid electrolyte membrane into an all-solid-state lithium metal battery; wherein, the coumarin compound is selected from one or more of coumarin, 6-methylcoumarin, coumarin-3-carboxylic acid, dihydrocoumarin, 3-aminocoumarin, 6-nitrocoumarin, 4-hydroxycoumarin, 3-chlorocoumarin, 6-fluoro-4-hydroxycoumarin, 7-amino-4-trifluoromethylcoumarin.

[0007] In a preferred embodiment, in the mixed system, the mass ratio of the lithium salt to the high molecular polymer is 1:(3-6), and the mass of the coumarin compound accounts for 10wt%-30wt% of the total mass of the entire mixed system.

[0008] In a preferred embodiment, the high molecular polymer is selected from one or more of polyethylene oxide, polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), and polyacrylonitrile as a mixed matrix.

[0009] In a preferred embodiment, the lithium salt is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium perchlorate.

[0010] In a preferred embodiment, the organic solvent is selected from one or more of acetonitrile, N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, and acetone.

[0011] In a preferred embodiment, the all-solid-state lithium metal battery includes a positive electrode material and a negative electrode material. The positive electrode material is selected from one or more of lithium iron phosphate, ternary nickel cobalt manganese oxide, and lithium cobalt oxide, and the negative electrode material is metallic lithium.

[0012] In a preferred embodiment, step S1 further includes: pouring the mixed system into a mold and standing for 3 to 48 hours, and then vacuum drying at 40 to 80°C for 3 to 36 hours to remove the organic solvent to obtain a solid electrolyte membrane.

[0013] In a preferred embodiment, step S2 further includes: assembling a stainless steel sheet|electrolyte|stainless steel sheet battery for testing the ionic conductivity of the electrolyte membrane.

[0014] In a preferred embodiment, step S2 further includes: assembling a Li|electrolyte|Li symmetric battery for testing the stability of the electrolyte membrane to the metallic lithium negative electrode and the ability to inhibit lithium dendrites.

[0015] The all-solid-state lithium metal battery according to the present invention is obtained according to the above preparation method.

[0016] The all-solid-state lithium metal battery according to the present invention and its preparation method can effectively improve the ionic conductivity of the electrolyte, and optimize the composition and structure of the interfacial layer between the electrolyte and the electrode to meet the requirements of all-solid-state batteries with high capacity, good rate performance, long cycle performance, good stability and high safety. Description of the Drawings

[0017] Figure 1 is an optical photograph of the coumarin-added electrolyte membrane PLC prepared according to Example 1 of the present invention.

[0018] Figure 2 Shows the ionic conductivity of the electrolyte membrane PLC according to Example 1 of the present invention at room temperature.

[0019] Figure 3 Shows the ionic conductivity of the electrolyte membrane PL without coumarin added, prepared according to Example 1 of the present invention, at room temperature.

[0020] Figure 4 is a comparison chart of the mechanical properties of the electrolyte membrane PL and the electrolyte membrane PLC according to Example 1 of the present invention.

[0021] Figure 5 is a long cycle chart (0.1 mA / cm 2 ) of the lithium-lithium symmetric battery assembled with the electrolyte membrane PL and the electrolyte membrane PLC according to Example 1 of the present invention at 45 °C.

[0022] Figure 6 is a long cycle chart (0.1 mA / cm 2 ) of the lithium-lithium symmetric battery assembled with the electrolyte membrane PL and the electrolyte membrane PLC according to Example 1 of the present invention at 60 °C.

[0023] Figure 7 is a polarization curve chart of the lithium-lithium symmetric battery assembled with the electrolyte membrane PL and the electrolyte membrane PLC according to Example 1 of the present invention.

[0024] Figure 8 is a long cycle performance chart of the all-solid-state battery with lithium iron phosphate as the positive electrode, assembled with the electrolyte membrane PL and the electrolyte membrane PLC according to Example 1 of the present invention, at a current density of 0.5 C (1 C = 170 mA / g).

[0025] Figure 9 is a charge-discharge curve chart of the all-solid-state battery with lithium iron phosphate as the positive electrode, assembled with the electrolyte membrane PLC according to Example 1 of the present invention, at a current density of 0.5 C (1 C = 170 mA / g).

[0026] Figure 10 It is the charge-discharge curve of a all-solid-state battery with lithium iron phosphate as the positive electrode assembled with the electrolyte membrane PL according to Embodiment 1 of the present invention at a current density of 0.5C (1C = 170 mA / g).

[0027] Figure 11 It is a comparison chart of the rate performance of all-solid-state batteries with lithium iron phosphate as the positive electrode assembled with the electrolyte membrane PL and the electrolyte membrane PLC according to Embodiment 1 of the present invention. Detailed Embodiments

[0028] The following is combined with the accompanying drawings to give the preferred embodiments of the present invention and describe them in detail.

[0029] According to the preparation method of the all-solid-state lithium metal battery of the present invention, by mixing coumarin compounds, lithium salts, polymer polymers and organic solvents, a solid electrolyte membrane is prepared by a solution casting method, aiming to significantly improve the comprehensive performance of the solid electrolyte by using coumarin compounds as additives. Due to its unique molecular structure, coumarin compounds can act as lithium ion carriers in the electrolyte to promote the rapid transmission of lithium ions. The two oxygen sites in the coumarin molecule can attract lithium ions to form a stable lithium ion transmission channel. Under the synergistic action of the polymer polymer chain, due to its small volume advantage, coumarin compounds can rapidly conduct lithium ions along the polymer chain, thereby effectively improving the ionic conductivity of the electrolyte. At the same time, the addition of coumarin compounds does not interfere with the crystalline region of the polymer polymer, thereby maintaining the structural stability of the polymer while avoiding the decline of the mechanical properties of the electrolyte, achieving a balance between the high ionic conductivity and good mechanical properties of the polymer electrolyte. In addition, the addition of coumarin compounds can also form a stable interface layer at the interface between the electrolyte and the electrode. This interface layer has the characteristics of rapid lithium ion transfer, inhibits lithium dendrites, and significantly improves the cycle performance and rate performance of the battery.

[0030] Coumarin compounds are widely present in natural plants, with wide sources and low costs. The preparation process of the present invention is simple, has strong processability, is easy to realize large-scale production, and can effectively reduce the production cost of the battery. Therefore, the present invention not only meets the requirements of all-solid-state batteries with high capacity, good rate performance, long cycle performance, good stability and high safety, but also provides strong support for the industrial application of all-solid-state lithium metal batteries.

[0031] Preparation of Solid Electrolyte Membrane

[0032] A polymer, a lithium salt, and a coumarin compound are added to an organic solvent and uniformly mixed to obtain a mixed system. Among them, the mass ratio of the lithium salt to the polymer is 1:(1 - 6), and the mass of the coumarin compound accounts for 5 wt% - 80 wt% of the total mass of the entire mixed system. Stir at room temperature to 80 °C for 6 to 24 hours to obtain a uniform solution. Pour the solution onto a polytetrafluoroethylene mold plate, and let it stand at room temperature for 3 to 48 hours to volatilize most of the organic solvent, and then vacuum dry at 40 to 80 °C for 3 to 36 hours to remove the residual organic solvent. After drying, peel off the electrolyte membrane from the polytetrafluoroethylene mold plate and cut it into 19 mm round pieces for battery assembly.

[0033] In a preferred embodiment, the polymer is one or a mixture of matrices of polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride - co - hexafluoropropylene) (PVDF - HFP), and polyacrylonitrile (PAN).

[0034] In a preferred embodiment, the lithium salt is one or several of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), lithium trifluoromethanesulfonate (CF3SO3Li), lithium difluoro(oxalato)borate (LiODFB), lithium tetrafluoroborate (LiBF4), and lithium perchlorate (LiClO4).

[0035] In a preferred embodiment, the coumarin compound is one or several of coumarin (CAS No. 91 - 64 - 5), 6 - methylcoumarin (CAS No. 92 - 48 - 8), coumarin - 3 - carboxylic acid (CAS No. 531 - 81 - 7), dihydrocoumarin (CAS No. 119 - 84 - 6), 3 - aminocoumarin (CAS No. 1635 - 31 - 0), 6 - nitrocoumarin (CAS No. 2725 - 81 - 7), 4 - hydroxycoumarin (CAS No. 1076 - 38 - 6), 3 - chlorocoumarin (CAS No. 92 - 45 - 5), 6 - fluoro - 4 - hydroxycoumarin (CAS No. 1994 - 13 - 4), and 7 - amino - 4 - trifluoromethylcoumarin (CAS No. 53518 - 15 - 3).

[0036] The molecular structures of the coumarin compounds are as follows in the table:

[0037] In a preferred embodiment, the organic solvent is one or several of acetonitrile, N - methylpyrrolidone (NMP), N,N - dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and acetone.

[0038] Battery Assembly and Testing

[0039] The prepared electrolyte membrane was assembled into a stainless steel sheet|electrolyte|stainless steel sheet battery, and the ionic conductivity was tested at different temperatures.

[0040] An Li|electrolyte|Li symmetric battery was assembled to detect the stability of the electrolyte membrane against the lithium metal anode and its ability to inhibit lithium dendrite growth.

[0041] Cathode sheets were prepared using cathode materials such as lithium iron phosphate (LiFePO4), ternary lithium nickel cobalt manganese oxide, and lithium cobalt oxide (LiCoO2). A lithium metal sheet was used as the anode, and full batteries were assembled and tested for rate performance and constant current long-cycle charge-discharge at rates ranging from 0.1 to 2C.

[0042] Compared with the prior art, the present invention utilizes the unique molecular structure of coumarin compounds to achieve a balance between high ionic conductivity and good mechanical properties of the polymer electrolyte. As a lithium ion carrier, coumarin significantly improves the ionic conductivity of the electrolyte while maintaining the structural stability of the polymer, avoiding a decrease in mechanical properties. In addition, the addition of coumarin promotes the formation of a stable and fast ion-conducting interfacial layer at the electrolyte|electrode interface, inhibits the growth of lithium dendrites, and improves the rate performance and cycle performance of the battery. The preparation method of the present invention is simple and low-cost, and has the potential for industrial application.

[0043] Example 1

[0044] 0.4 g of PEO (Mw = 600,000, Aladdin), 0.145 g of LiTFSI, and 15 wt% of coumarin were added to an acetonitrile solvent and mixed evenly. The mixture was stirred at room temperature for 24 hours, poured into a mold, and left standing for 12 hours. Then, it was vacuum dried at 50 °C for 6 hours to obtain the modified electrolyte membrane PLC. Additionally, a pure PEO electrolyte membrane PL without coumarin was prepared for performance comparison.

[0045] Characterization of Electrolyte Membrane PLC

[0046] The optical diagram of the electrolyte membrane PLC is as Figure 1 shown, which shows the appearance characteristics of the electrolyte membrane PLC.

[0047] Ionic Conductivity and Mechanical Property Testing

[0048] The electrolyte membrane PLC and the electrolyte membrane PL were respectively assembled into stainless steel sheet|electrolyte|stainless steel sheet batteries, and the ionic conductivity was tested at room temperature. The test results showed that the room temperature ionic conductivity of the coumarin-added electrolyte membrane PLC was 1.07×10 -4 S cm -1 , which was higher than the room temperature ionic conductivity of the electrolyte membrane PL (8.59×10 -6 S cm -1) It has increased by more than an order of magnitude, as Figure 2 and Figure 3 shown. This indicates that the addition of coumarin can significantly improve the ionic conductivity of the electrolyte.

[0049] As Figure 4 shown, the yield strength of the electrolyte membrane PLC is significantly higher than that of the electrolyte membrane PL. This shows that the addition of coumarin can not only improve the ionic conductivity but also enhance the mechanical properties of the electrolyte membrane, solving the balance problem between high ionic conductivity and good mechanical strength.

[0050] Lithium-Lithium Symmetric Battery Testing

[0051] The electrolyte membranes PLC and PL were respectively assembled into Li|electrolyte|Li lithium symmetric batteries, and long-term cycling tests were carried out at a current density of 0.1 mA / cm 2 at 45 °C. As Figure 5 shown, the electrolyte membrane PLC can stably cycle for 5200 hours and maintain a low polarization voltage, while the electrolyte membrane PL shows a sharp increase in the polarization voltage at 500 hours, indicating that the electrolyte membrane PL undergoes side reactions with the lithium metal anode and the interface deteriorates severely. Further increasing the test temperature, at 60 °C, the battery was cycled long-term at a current density of 0.1 mA / cm 2 , as Figure 6 shown, the electrolyte membrane PLC can still stably cycle for more than 2400 hours, while the electrolyte membrane PL shows a sharp increase in the polarization voltage at 800 hours and a short-circuit phenomenon at 1250 hours.

[0052] As Figure 7 shown, the critical current density of the electrolyte membrane PLC can reach 1.3 mA / cm 2 , higher than 0.8 mA / cm 2 of the electrolyte membrane PL, which shows that the addition of coumarin can effectively inhibit the growth of lithium dendrites and avoid battery short-circuit.

[0053] Full Battery Performance Testing

[0054] Using lithium iron phosphate (LiFePO4) as the cathode material, constant current charge-discharge tests were carried out on the electrolyte membranes PLC and PL at a rate of 0.5C respectively. As Figure 8 shown, the initial discharge specific capacity of the electrolyte membrane PLC is 161.9 mAh g -1 , and after 550 stable long-term cycles, the capacity retention rate is still as high as 93%. In contrast, the electrolyte membrane PL shows obvious capacity decay at 50 cycles.

[0055] Figure 9 and Figure 10The comparison of charge-discharge voltage-capacity curves shows that the polarization voltage of the electrolyte membrane PLC remains stable, while the polarization voltage of the electrolyte membrane PL varies greatly after different numbers of cycles. This indicates that the addition of coumarin is beneficial to form a more stable electrolyte|electrode interface, thus realizing the stable, efficient and long-cycle performance of the battery.

[0056] In addition, through the rate performance test of the full battery (see Figure 11 ), even at a high rate of 2C, the discharge specific capacity of the electrolyte membrane PLC is still higher than that of the electrolyte membrane PL, which indicates that the addition of coumarin can form an interfacial layer with good kinetics and effectively improve the rate performance of the battery.

[0057] Example 2

[0058] 1 g of PVDF, 0.3 g of LiTFSI and 10 wt% of coumarin were added to DMF solvent and mixed evenly to prepare an electrolyte solution. The obtained PVDF-based electrolyte was further subjected to electrochemical test comparison, which also shows that coumarin has a significant effect on improving the ionic conductivity, cycle performance and rate performance of the battery.

[0059] Example 3

[0060] 1 g of PAN, 0.5 g of LiTFSI and 20 wt% of coumarin were added to DMF solvent and mixed evenly to prepare an electrolyte solution. The obtained PAN-based electrolyte was further subjected to electrochemical test comparison, which also shows that coumarin has a significant effect on improving the ionic conductivity, cycle performance and rate performance of the battery.

[0061] Example 4

[0062] 0.4 g of PEO (Mw = 600,000, Aladdin), 0.261 g of LiTFSI and 5 wt% of 6-fluoro-4-hydroxycoumarin were added to acetonitrile solvent and mixed evenly to prepare an electrolyte solution. The obtained PEO-based electrolyte was further subjected to electrochemical test comparison, which also shows that coumarin has a significant effect on improving the ionic conductivity, cycle performance and rate performance of the battery.

[0063] The above are only the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various changes can be made to the above embodiments of the present invention. That is, all simple, equivalent changes and modifications made according to the claims and the content of the specification of the present invention application fall within the scope of the claims of the present invention patent. The content not described in detail in the present invention is all conventional technical content.

Claims

1. A method for preparing an all-solid-state lithium metal battery, characterized in that: The preparation method comprises the following steps: S1, adding a polymer, a lithium salt and a coumarin compound into an organic solvent and uniformly mixing them to obtain a mixed system, wherein the mass ratio of the lithium salt to the polymer is 1:(1-6), the mass of the coumarin compound accounts for 5wt%-80wt% of the total mass of the mixed system, and a solid electrolyte membrane is prepared by a solution casting method; S2, assembling the solid electrolyte membrane into an all-solid-state lithium metal battery; Among them, the coumarin compound is selected from one or more of coumarin, 6-methylcoumarin, coumarin-3-carboxylic acid, dihydrocoumarin, 3-aminocoumarin, 6-nitrocoumarin, 4-hydroxycoumarin, 3-chlorocoumarin, 6-fluoro-4-hydroxycoumarin, and 7-amino-4-trifluoromethylcoumarin.

2. The preparation method according to claim 1, characterized in that: In the mixed system, the mass ratio of lithium salt to high molecular polymer is 1:(3-6), and the mass of coumarin compounds accounts for 10wt%-30wt% of the total mass of the mixed system.

3. The preparation method according to claim 1, characterized in that: The high molecular polymer is selected from one or a mixed matrix of polyethylene oxide, polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), and polyacrylonitrile.

4. The preparation method according to claim 1, characterized in that: The lithium salt is selected from one or more of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethylsulfonate, lithium difluorooxalatoborate, lithium tetrafluoroborate, and lithium perchlorate.

5. The preparation method according to claim 1, characterized in that: The organic solvent is selected from one or more of acetonitrile, N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, tetrahydrofuran and acetone.

6. The preparation method according to claim 1, characterized in that: The all-solid-state lithium metal battery comprises a positive electrode material and a negative electrode material, wherein the positive electrode material is selected from one or more of lithium iron phosphate, ternary nickel cobalt manganese oxide, and lithium cobalt oxide, and the negative electrode material is metallic lithium.

7. The preparation method according to claim 1, characterized in that: The step S1 further comprises: pouring the mixed system into a mold and letting it stand for 3 to 48 hours, and then vacuum drying at 40 to 80° C. for 3 to 36 hours to remove the organic solvent and obtain a solid electrolyte membrane.

8. The preparation method according to claim 1, characterized in that: The step S2 further includes: assembling a stainless steel sheet|electrolyte|stainless steel sheet battery for testing the ionic conductivity of the electrolyte membrane.

9. The preparation method according to claim 1, characterized in that: The step S2 also includes: assembling a Li|electrolyte|Li symmetric battery to test the stability of the electrolyte membrane to the metallic lithium negative electrode and the ability to inhibit lithium dendrites.

10. An all-solid-state lithium metal battery, characterized in that: The all-solid-state lithium metal battery is obtained according to the preparation method described in any one of claims 1-9.

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