A solid-state polymer electrolyte based on azobenzene liquid crystal polymer

Abstract

The application discloses a kind of solid-state polymer electrolyte based on azobenzene liquid crystal polymer, including polymer main body and lithium salt, the polymer electrolyte main body is with azobenzene rod liquid crystal base element side chain type liquid crystal polymer PAzo with polynorbornene as main chain, lithium salt and the complexation of carbonyl in polymer, lithium ion is conducted by the movement of polymer segment, the conduction of lithium ion and the physical crosslinking effect of liquid crystal phase structure occur in different positions of polymer side chain, so that polymer ionic conductivity and mechanical property are improved simultaneously.Under the suitable lithium salt doping ratio, PAzo can form the liquid crystal phase structure of double continuous phase, liquid crystal base element and polymer main chain interpenetrate each other, provide fast three-dimensional ion transmission channel.The solid-state polymer electrolyte material is expected to be applied in all-solid-state lithium ion battery to improve the safety of battery.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrolyte material in a battery, in particular to a solid-state polymer electrolyte material based on an azobenzene polymer as a matrix, mainly applied to solid-state batteries, flexible batteries and the like, and belongs to the field of new materials. BACKGROUND

[0002] A rechargeable battery is a battery that can be charged and discharged multiple times, such as lithium ion batteries and zinc ion batteries. They are mainly composed of four parts: positive electrode, negative electrode, separator and electrolyte. The electrolyte is an important component, and its main function is to conduct ions between the positive and negative electrodes during charging and discharging. The separator serves to separate the positive and negative electrodes to prevent short circuiting of the battery.

[0003] Currently, the electrolyte material used in lithium ion batteries is mostly organic solvent, such as ethylene carbonate, propylene carbonate, etc. These organic solvents are highly flammable and may leak during battery use, causing safety hazards such as explosion. On the other hand, during the charging and discharging process of lithium metal batteries, dendritic lithium crystals are easily formed on the surface of the lithium metal negative electrode. If such dendrites penetrate the separator and come into contact with the positive electrode, there is a risk of short circuiting the battery. Solid-state electrolytes, especially solid-state polymer electrolytes, can solve the above problems.

[0004] In order to achieve the purpose of use, people usually want to obtain a solid-state polymer electrolyte material that has both high ionic conductivity and high mechanical properties, but in general, the two cannot be achieved at the same time. Although block copolymer electrolyte materials can theoretically separate the conducting phase and the fixed phase by microphase separation, the ideal bicontinuous phase structure in the block copolymer microphase separation structure is difficult to obtain due to the narrow phase region.

[0005] Azobenzene refers to an aromatic compound formed by connecting two benzene rings with an azo group (-N=N-) and its derivatives, and is a typical rod-like liquid crystal unit. Due to the rigidity and anisotropy of azobenzene molecules, polymers containing azobenzene in the side chain can have liquid crystal properties. Designing a solid-state polymer electrolyte based on azobenzene liquid crystal polymer is expected to separate the ion conducting phase and the fixed phase while obtaining a bicontinuous phase structure, thereby simultaneously improving the ionic conductivity and mechanical properties. SUMMARY

[0006] The present application aims to develop a solid-state polymer electrolyte with high ionic conductivity. By designing and synthesizing a side-chain type liquid crystal polymer with azobenzene units, a solid-state polymer electrolyte with high ionic conductivity can be prepared after doping lithium salt into the polymer. The solid-state polymer electrolyte can be applied in the field of safe all-solid-state lithium ion batteries and flexible lithium ion batteries.

[0007] Specifically, the present application aims at the goal of preparing solid-state polymer electrolyte with high ionic conductivity and high mechanical properties, and designs and synthesizes side-chain liquid crystal polymer PAzo with azobenzene units and polynorbornene backbone. On the one hand, through ring-opening metathesis polymerization of norbornene, a polymer with high molecular weight can be prepared, thereby improving the mechanical properties of the solid-state polymer electrolyte. Meanwhile, the liquid crystal phase structure formed by self-assembly of azobenzene liquid crystal units is equivalent to a physical crosslinking point, which is conducive to further improving the mechanical properties of the polymer. In addition, after doping with lithium salt, the lithium salt is complexed with the carbonyl group in the polymer, and the lithium ion is conducted through the movement of the polymer segment. The lithium ion conduction and the physical crosslinking effect of the liquid crystal phase structure occur at different positions of the polymer side chain, which can simultaneously improve the ionic conductivity and mechanical properties of the polymer. More importantly, by changing the doping ratio of lithium salt, PAzo can form a variety of liquid crystal phase structures. When the doping ratio is appropriate, a bicontinuous phase liquid crystal structure can be obtained, in which the liquid crystal units and the polymer backbone interpenetrate each other, providing a fast three-dimensional ion transmission channel.

[0008] The solid-state polymer electrolyte provided by the present application has a polynorbornene backbone and introduces azobenzene liquid crystal units in the side chain. After doping with lithium ions, it can be coordinated with the carbonyl group in the polymer, and self-assembled to form a solid-state polymer electrolyte with a bicontinuous phase structure and high ionic conductivity. Moreover, since the bicontinuous phase structure is self-assembled by the polymer side chain, this structure is independent of the molecular weight and molecular weight distribution of the polymer, so the control of the molecular weight and its distribution of the polymer is not high.

[0009] The solid-state polymer electrolyte comprises a polymer body and a lithium salt, and the polymer body is a side-chain liquid crystal polymer containing azobenzene liquid crystal units, and its structure is shown in formula I:

[0010]

[0011] In formula I, R is a group that does not affect the formation of azobenzene liquid crystal phase or a group that can enhance the ability of liquid crystal phase formation, selected from alkyl, alkoxy, aryl, cyano, halogen, amino, nitro and the like; n and m are integers, n represents the polymerization degree of the polymer, and m represents the number of carbon atoms in the alkyl chain.

[0012] The group that does not affect the formation of azobenzene liquid crystal phase is preferably C1-C10 alkyl, C1-C10 alkoxy, including but not limited to methyl, ethyl, n-propyl, n-butyl, methoxy, ethoxy, n-butoxy and the like; the group that can enhance the ability of liquid crystal phase formation includes cyano, aryl, halogen and the like, and the aryl group is preferably C6-C8 aryl, such as phenyl, p-methylphenyl, p-ethylphenyl and the like.

[0013] Preferably, n is an integer from 100 to 1000, and the total polymer molecular weight is not less than 60000 Da; m is an integer from 2 to 16.

[0014] The lithium salt forming the solid-state polymer electrolyte is selected from one or more of the following compounds:

[0015]

[0016] The anion in the lithium salt includes, but is not limited to, triflate ion, bis-trifluoromethanesulfonamide ion, perchlorate ion, borate ion, hexafluorophosphate ion, and other anions that are easy to dissociate with lithium ion and have certain electrochemical stability.

[0017] A typical preparation method of the polymer body is as follows:

[0018] First, a monomer is obtained by esterification reaction of a carboxyl-containing norbornene compound and a hydroxyl-containing azobenzene compound. The solvent for the esterification reaction is preferably dichloromethane and / or toluene, the esterification reaction desiccant is preferably dicyclohexyl carbodiimide and / or 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride, and the esterification reaction catalyst is preferably 4-dimethylaminopyridine.

[0019] The monomer is prepared into a polymer body by ring-opening metathesis polymerization reaction. The solvent for the ring-opening metathesis polymerization reaction is preferably dichloromethane, the ring-opening metathesis polymerization reaction catalyst is preferably the second and third generation Grubbs catalyst, the reaction temperature for the ring-opening metathesis polymerization reaction is preferably 25 degrees Celsius, and the ring-opening metathesis polymerization reaction is preferably carried out under anhydrous and anaerobic conditions.

[0020] The application also provides a preparation method of the above-mentioned solid-state polymer electrolyte, and studies the liquid crystal phase structure and ionic conductivity thereof.

[0021] The preparation method of the solid-state polymer electrolyte according to the application comprises the following steps:

[0022] 1) Dissolve an appropriate amount of polymer body and lithium salt in an organic solvent, and stir vigorously until uniform;

[0023] 2) Filter the mixed solution, pour it into a mold, place it at room temperature, and dry it in a vacuum to volatilize and remove the solvent. After the solvent is volatilized and dried, a stable solid-state polymer electrolyte film is obtained.

[0024] The organic solvent used in the above step 1) is an organic solvent that can dissolve both the polymer and the lithium salt, such as tetrahydrofuran, N-methyl pyrrolidone, dimethyl sulfoxide, etc.

[0025] The mixed solution is filtered by using a microporous filter membrane to remove small impurities, and then the filtrate is poured into a polytetrafluoroethylene mold, placed at room temperature, and the solvent is removed by vacuum evaporation to obtain a stable solid-state polymer electrolyte film.

[0026] The solid-state polymer electrolyte has a plurality of liquid crystal phase structures. The polymer film obtained in the above step 2) is subjected to heat annealing treatment at 120 DEG C for 24 h, and after slow cooling, a small-angle X-ray scattering instrument is used to characterize the liquid crystal phase structure. Taking one of the polymer / lithium salt systems as an example, as the doping amount of lithium salt increases, the liquid crystal phase structure of the electrolyte changes from a typical lamellar smectic phase structure to a relatively unique bicontinuous phase structure. This bicontinuous phase structure provides a three-dimensional continuous lithium ion channel on one hand, and the continuous liquid crystal phase structure can act as a physical crosslinking point to improve the mechanical properties of the polymer electrolyte.

[0027] The solid-state polymer electrolyte has high ionic conductivity. The solid-state polymer electrolyte film is cut into a round piece, and the round piece is subjected to ionic conductivity test on an electrochemical workstation. The ionic conductivity of the polymer electrolyte at room temperature is as high as 10 -4 S cm -1 .

[0028] In summary, by introducing azobenzene liquid crystal units into the side chain of the polymer, a side-chain type liquid crystal polymer solid-state electrolyte with a bicontinuous liquid crystal phase structure can be quickly and simply constructed. At the same time, the polymer electrolyte has high room temperature ionic conductivity. Compared with existing materials, the advantages of the present application are as follows:

[0029] 1) The solvent blending evaporation method is used to prepare the solid-state polymer electrolyte, and the preparation method is simple; the selected polymer structure is simple, and the raw materials are easy to obtain, which is suitable for large-scale production;

[0030] 2) The polymer structure in the present application has strong designability, is suitable for a plurality of different types of polymers, and can quickly prepare solid-state polymer electrolyte materials with different properties according to requirements;

[0031] 3) The solid-state polymer electrolyte material in the present application has high ionic conductivity, which can meet the needs of solid-state lithium ion batteries;

[0032] 4) The solid-state polymer electrolyte material in the present application is expected to be applied to all-solid-state lithium ion batteries, which can improve the safety of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1The complexation sites of lithium ions with polymers and the conduction mode of lithium salt in the polymer of the solid-state polymer electrolyte of the present application are shown, in which the lithium ions are complexed with the carbonyl groups on the side chains of the polymer, and the lithium ions are conducted along the direction of the main chain with the movement of the chain segments.

[0034] Figure 2 The nuclear magnetic resonance hydrogen spectrum curve of 4-butyl-1-hydroxy azobenzene (C4AzoOH) prepared in step 1 of Example 1 is shown.

[0035] Figure 3 The nuclear magnetic resonance hydrogen spectrum curve of 11-p-butylazophenyloxyundecan-1-ol (C4AzoC 11 OH) prepared in step 2 of Example 1 is shown.

[0036] Figure 4 The nuclear magnetic resonance hydrogen spectrum curve of norbornyl azobenzene (NbAzo) prepared in step 3 of Example 1 is shown.

[0037] Figure 5 The gel permeation chromatography (GPC) curve of polynorbornyl azobenzene (PAzo) prepared in step 4 of Example 1 is shown.

[0038] Figure 6 The small-angle X-ray scattering curve of the solid-state polymer electrolyte prepared in Example 3 is shown, in which the left graph shows no lithium salt, and the right graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.16.

[0039] Figure 7 The small-angle X-ray scattering curve of the solid-state polymer electrolyte prepared in Example 3 is shown, in which the left graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.11, and the right graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.22.

[0040] Figure 8 The small-angle X-ray scattering curve of the solid-state polymer electrolyte prepared in Example 3 is shown, in which the left graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.33, and the right graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.66.

[0041] Figure 9 The small-angle X-ray scattering curve of the solid-state polymer electrolyte prepared in Example 3 is shown, in which the left graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 0.79, and the right graph shows the molar ratio of lithium ions to carbonyl groups on the polymer is 1.00.

[0042] Figure 10 The Nyquist curve obtained after the electrochemical impedance method test on the solid-state polymer electrolyte prepared in Example 4 is shown, in which the horizontal axis is the real impedance, and the vertical axis is the imaginary impedance. DETAILED DESCRIPTION

[0043] The application will be further described in conjunction with the accompanying drawings and specific embodiments.

[0044] Example 1, synthesis of polyazobenzenyl norbornene (PAzo)

[0045] Step 1: synthesis of 4-butyl-1-hydroxyazobenzene (C4AzoOH)

[0046]

[0047] Take 4-n-butyl aniline (14.9 g, 100 mmol) in a 250 mL conical flask, add 150 mL of water, and then add hydrochloric acid (12.0 M, 25.0 mL, 300 mmol) dropwise while stirring. After complete dissolution, cool in an ice water bath; take sodium nitrite (6.90 g, 100 mmol) in a 100 mL conical flask, add 50 mL of water and stir, and then transfer to a constant pressure dropping funnel after complete dissolution. Add dropwise to the above solution, and after the addition is complete, continue to react the mixed solution in an ice water bath for 1 h, and record as solution A.

[0048] Take phenol (9.40 g, 100 mmol), sodium hydroxide (4.00 g, 100 mmol), and sodium carbonate (10.6 g, 100 mmol) in a 1000 mL conical flask, and then add 400 mL of water to the conical flask. After complete dissolution by stirring, cool in an ice water bath, and record as solution B.

[0049] Then, while keeping solution B in an ice water bath, slowly add solution A dropwise to solution B. After the addition is complete, remove the ice water bath and continue to react at room temperature for 4 h. Then neutralize the above reaction solution to be acidic with dilute hydrochloric acid, and a large amount of precipitate is generated. The obtained precipitate is filtered and washed with deionized water to obtain a crude product. The crude product is recrystallized with petroleum ether to obtain a pure product C4AzoOH. The product is dark yellow in color, and the yield is 81.7%. 1 H NMR (400 MHz, CDCl3, δ, ppm): 7.87 (m, 2H), 7.81 (m, 2H), 7.30 (m, 2H), 6.94 (m, 2H), 2.68 (t, 2H), 1.64 (m, 2H), 1.38 (h, 2H), 0.94 (t, 3H).

[0050] Step 2: synthesis of 11-p-butylazophenyloxyundecan-1-ol (C4AzoC 11 OH)

[0051]

[0052] C4AzoOH (5.08 g, 20.0 mmol), 11-bromo-1-undecanol (5.02 g, 20.0 mmol), K2CO3(11.0 g, 80.0 mmol) and a catalytic amount of KI (0.332 g, 2.00 mmol) were placed in a 250 mL round bottom flask, 150 mL of anhydrous ethanol was added and refluxed overnight. After the reaction was completed, the solution was filtered hot, and the filtrate was cooled to room temperature and then crystallized in a -20 °C refrigerator to obtain a large amount of flaky orange-yellow crystals. The product C4AzoC 11 OH was obtained by suction filtration and vacuum drying, with a yield of 80%. 1 H NMR (400 MHz, CDC13, δ, ppm): 7.89 (m, 2H), 7.80 (m, 2H), 7.30 (m, 2H), 6.99 (m, 2H), 4.03 (t, 2H), 3.64 (t, 2H), 2.68 (t, 2H), 1.80 (m, 2H), 1.25-1.69 (m, 21H), 0.94 (t, 3H).

[0053] Step 3: Synthesis of norbornene azobenzene (NbAzo)

[0054]

[0055] Norbornene hexanoic acid (1.2 eq, 1.97 g, 7.12 mmol), C4AzoC 11 OH and 80 mL of dichloromethane were added to a 150 mL flask, followed by the addition of 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (2.4 eq, 2.72 g, 14.2 mmol) and dimethylaminopyridine (0.2 eq, 0.145 g, 1.18 mmol). The solution was stirred at room temperature for 36 hours, about 40 mL of solvent was removed by rotary evaporation under reduced pressure, washed with 1M HCl three times, saturated brine twice, and then dried with anhydrous sodium sulfate. After removing the solvent, NbAzo was obtained.

[0056] Step 4: Synthesis of poly-norbornene azobenzene (PAzo)

[0057] In a dry 25 mL Schlenk flask, 0.5 mL of CH2Cl2 solution containing monomer NbAzo was added and frozen in liquid nitrogen. Using a double-row system, the freeze-pump-nitrogen- thaw cycle was repeated three times. Grubbs III catalyst was added again, frozen in liquid nitrogen, and the system was thawed after three vacuum-nitrogen cycles. The system was stirred at room temperature for 30 min under a nitrogen atmosphere. The reaction solution was passed through a neutral alumina column to remove the Grubbs III catalyst, and 200 mL of petroleum ether was added dropwise, and a precipitate was separated out. After filtration and drying, the final polymer PAzo was obtained.

[0058] Example 2, Preparation of solid-state polymer electrolyte

[0059] In a 20.0 mL sample bottle, 1.00 g of polymer PAzo was added, and 10.0 mL of tetrahydrofuran was added to dissolve overnight. A suitable amount of zinc salt solution dissolved in tetrahydrofuran was slowly added under stirring, and stirred overnight. After opening the bottle cap, when the solution was evaporated to half, the solution was poured into a polytetrafluoroethylene mold, and dried at room temperature for 48 hours, and then placed in a vacuum oven at 40°C to remove the residual solvent, to obtain a solid-state polymer electrolyte thin film.

[0060] Example 3, Characterization of liquid crystal phase structure of solid-state polymer electrolyte

[0061] For the solid-state polymer electrolyte thin film sample prepared in Example 2, it was wrapped in aluminum foil, and the liquid crystal phase structure was characterized using a small-angle X-ray scattering instrument. From Figures 6 to 9 It can be seen that the liquid crystal phase structure of the solid-state polymer electrolyte becomes more obvious after doping with lithium salt. When the molar ratio of lithium ion to polymer carbonyl group is 0.11 to 0.33, the polymer forms a lamellar smectic phase structure; when the molar ratio is 0.66, two sets of lamellar smectic phase diffraction peaks are formed; when the molar ratio is 0.79 and 1.00, a bicontinuous phase structure is formed.

[0062] Example 4, Electrical performance test of solid-state polymer electrolyte

[0063] For the solid-state polymer electrolyte thin film sample prepared in Example 2, it was cut into a circular piece with a diameter of 1.5 cm, and placed between two stainless steel electrodes, and pressed into a button cell, and then its ionic conductivity was tested by an electrochemical workstation. Figure 10 The intersection of the Nyquist curve with the horizontal axis is 665Ω, and the ionic conductivity is calculated to be 1.64 x 10 -4 S cm -1 .

Claims

1. A solid polymer electrolyte, comprising a polymer matrix and a lithium salt, wherein the polymer matrix is ​​a side-chain type liquid crystal polymer containing azobenzene rod-shaped liquid crystal units, and its structure is shown in Formula I: Formula I In Formula I, R is a group that does not affect the formation of the azobenzene liquid crystal phase or a group that enhances the ability to form the liquid crystal phase, selected from C1~C10 alkyl, C1~C10 alkoxy, C6~C8 aryl, cyano, halogen, amino, nitro; n and m are integers, where n represents the degree of polymerization of the polymer and m represents the number of carbons in the alkyl chain, and m is an integer from 2 to 16.

2. The solid polymer electrolyte as described in claim 1, characterized in that, R is -C4H9, m=11.

3. The solid polymer electrolyte as described in claim 1, characterized in that, n is an integer from 100 to 1000, and the molecular weight of the polymer matrix is ​​not less than 60,000 Da.

4. The solid polymer electrolyte as described in claim 1, characterized in that, The lithium salt is selected from one or more of the following compounds: 。 5. A method for preparing the solid polymer electrolyte according to any one of claims 1 to 4, comprising the following steps: Dissolve the polymer bulk and lithium salt using an organic solvent, stirring vigorously until the mixture is homogeneous; The mixed solution is filtered, poured into a mold, placed at room temperature, and dried in a vacuum to evaporate and remove the solvent. After the solvent has evaporated, a solid polymer electrolyte film is obtained.

6. The preparation method according to claim 5, characterized in that, Step 1) The organic solvent used is selected from tetrahydrofuran, N-methylpyrrolidone, and dimethyl sulfoxide.

7. The preparation method according to claim 5, characterized in that, Step 2) Filter the mixed solution using a microporous membrane, then pour the filtrate into a polytetrafluoroethylene mold, place it at room temperature, and remove the solvent by vacuum evaporation to obtain a solid polymer electrolyte membrane.

8. The use of the solid polymer electrolyte according to any one of claims 1 to 4 in the preparation of an all-solid-state lithium-ion battery.

9. A polymer, which is a side-chain type liquid crystal polymer containing azobenzene rod-shaped liquid crystal units, with a structure as shown in Formula I: Formula I In Formula I, R is selected from C1~C10 alkyl, C1~C10 alkoxy, C6~C8 aryl, cyano, halogen, amino, and nitro; n and m are integers, where n represents the degree of polymerization of the polymer and m represents the number of carbons in the alkyl chain, and m is an integer from 2 to 16.

10. The polymer as claimed in claim 9, characterized in that, R is -C4H9, m=11.

11. The polymer as claimed in claim 9, characterized in that, n is an integer between 100 and 1000.

12. A method for preparing the polymer according to any one of claims 9 to 11, comprising firstly, esterifying a carboxyl-containing norbornene compound with a hydroxyl-containing azobenzene compound to obtain a monomer; and then preparing the polymer by ring-opening metathesis polymerization of the monomer.

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