CN112552438B - Preparation method of high-molecular polymer electrolyte for flexible lithium battery - Google Patents

Abstract

The invention discloses a preparation method of a high molecular polymer electrolyte for a flexible lithium battery, which comprises the synthesis of polystyrene sulfonyl-1, 3-dithiane; the synthesis of PSDTTO and the synthesis of psdttoil. The PSDTTOLi prepared by the invention has high ionic conductivity and ideal chemical, mechanical and electrochemical properties, and can be used for flexible lithium ion batteries.

Description

Preparation method of high-molecular polymer electrolyte for flexible lithium battery

Technical Field

The invention relates to the field of lithium ion batteries, in particular to a preparation method of a high-molecular polymer electrolyte for a flexible lithium battery.

Background

The pressure to reduce carbon emissions, global warming, and excessive reliance on fossil fuels are several factors that make electric vehicles and hybrid electric vehicles (EV/HEV) a more attractive alternative to burning fossil fuels. Lithium ion batteries have a higher power to weight ratio (2.59 MJ/Kg) than lead acid batteries. Furthermore, lithium ion batteries have good versatility in many other applications, such as computers, cell phones, cameras, video cameras, and medical devices. However, current lithium ion batteries must overcome some limitations before large scale use. The three most important challenges are energy density, cost, and operational safety. The electrolyte is the second most expensive (next to the cathode) component in a lithium ion battery. Battery companies are tempting to find low cost, safer and high performance electrolytes.

The most advanced lithium ion batteries at presentThe liquid electrolyte is composed of two parts: a source of lithium ions and an organic solvent. The source of lithium ions is typically a single or mixed lithium fluoride salt [ e.g. LiPF₆, CF₃SO₃Li and (CF)₃SO₂)₂NLi]. Ethylene Carbonate (EC) is a commonly used organic solvent, which is advantageous for dissociation of a lithium ion source due to its low cost, good electrochemical stability and high dielectric constant, resulting in high ionic conductivity. Other carbonates, such as dimethyl carbonate (DMC) and Propylene Carbonate (PC), are often used with Ethylene Carbonate (EC) to reduce viscosity and increase wettability of the electrolyte solution with battery components, such as ion exchange membranes and electrodes. Despite these important properties, carbonate solvents are not recommended for flexible batteries because of their highly flammable properties without a thermal/flame protection device.

Small lithium ion batteries require about 5ml of liquid electrolyte, while electric vehicle batteries require 500 plus 1000 ml of electrolyte. Liquid electrolyte based batteries in certain extreme cases (e.g., over-discharge, resistance and/or forced over-discharge) the batteries may thermally run away and cause serious fire and explosion hazards. Therefore, there is a strong need to design and develop an advanced solvent-free electrolyte system that overcomes the leakage problem and has high ionic conductivity and desirable electrochemical and mechanical properties.

Lithium rechargeable batteries employing solvent-free, highly conductive Solid Polymer Electrolyte (SPE) systems would have a significant impact on the battery industry. Compared with the prior carbonate-based liquid electrolyte, the electrolyte can obviously reduce environmental pollution and improve performance. The flexible battery of the system has high energy density, high battery voltage and excellent self-discharge characteristic, and overcomes the defects of the conventional liquid electrolyte lithium ion battery, such as leakage, instability and difficulty in manufacturing, to a great extent.

The properties required for solvent-free, highly conductive Solid Polymer Electrolytes (SPEs) include high mechanical strength, a glass transition temperature (Tg) well below room temperature, film processability, good interfacial properties (compatibility and adhesion), and smooth operation at ambient temperature. Furthermore, the system must be capable of mass production at a reasonable cost.

One disadvantage of solvent-free highly conductive Solid Polymer Electrolytes (SPEs) is their insufficient conductivity at ambient temperature (well below 10)^-3S/cm). Good ionic conductivity is critical to ensure that the battery system is able to provide usable charge at a high rate, which is a critical requirement of the battery. Among the solventless polymer electrolyte systems, polyethylene oxide (PEO) is one of the most studied systems in recent decades. The main advantage of PEO as a host is its chemical, mechanical and electrochemical stability, since it contains only C-O, C-C and C-H bonds. PEO is very flexible (Tg ═ 61 ℃), because of the presence of a rotating ether linkage and repeating units (-CH)₂CH₂O-) provides reasonable spacing and maximizes dissolution of the lithium salt. Because there is sufficient interchain entanglement, the PEO electrolyte behaves like a rubber material, but contains both crystalline and amorphous regions. It is noted that lithium ion conduction occurs in the amorphous state by diffusion, which occurs through a complex mechanism involving the mobility of the PEO segments. PEO electrolytes also have good melt processing capabilities, which is a highly desirable large-scale production cell.

Despite these positive characteristics, all PEO-based electrolytes have poor room temperature ionic conductivity ((r))<10^-5S/cm) because the crystallinity of the PEO system increases with increasing lithium salt concentration, resulting in a significant decrease in ionic conductivity before reaching acceptable values. Therefore, unless the room temperature conductivity is changed from<10^-5S/cm is increased to>10^-3S/cm, otherwise the potential of the PEO system would not be realized.

Recently, many scientists have attempted to increase the ionic conductivity of PEO electrolytes, (i) to minimize the crystallinity of PEO, thereby making it an amorphous polymer, (ii) to investigate for suitable nanoscale inorganic fillers, thereby improving PEO conductivity, (iii) to increase liquid plasticizers (low and high molecular weight) PEO electrolytes. Unfortunately, none of these approaches has achieved the desired results. Despite much research efforts to find better solid phase extraction systems based on low Tg's (such as polyphosphazenes and polysiloxanes), lithium salts have lagged far behind current PEO systems due to their lower solubility in these polymer matrices. Therefore, finding an ideal SPE with good transmission characteristics remains a challenging task.

Researchers can improve the performance of polymer electrolytes by reducing the crystallinity of the polymer, increasing the ion concentration and the proportion of amorphous regions contained in the system. The glass transition temperature (Tg) of a polymer electrolyte system is reduced and the capability of lithium ion dissociation is improved by blocking, grafting, crosslinking, synthesizing and blending various polymer materials.

Therefore, research into novel polymer salts is essential, although several factors need to be considered in selecting the optimal lithium salt, the key factors including performance, price and safety. The properties of the salt are related to the electrical conductivity, thermal stability and electrochemical stability of the salt at different temperatures.

On the basis, the invention researches the performance of PSDTTOLi, and belongs to the category of low-lattice-energy lithium salt. It employs high molecular weight PEO (Mw ═ 4 × 106g/mol), high swelling capacity of PEG (Mw ═ 1000g/mol) complexed with LiTFSI to achieve balance of ionic conductivity and mechanical properties compatibility.

The most advanced liquid electrolytes of today have very low cation transfer numbers (t + ═ 0.2 to 0.3) at near or below sub-zero temperature conditions, resulting in electrolyte polarization and increased resistivity. Thus resulting in more time, energy and electrochemical potential required to charge the battery. To avoid this problem, in addition to high conductivity, the electrolyte must also have a high cation transport number (>0.6) to ensure smooth and efficient charge-discharge characteristics. Since the anionic portion of the lithium salt in the proposed solid phase extraction systems is immobile (covalently linked to the crosslinked polystyrene microparticles), we expect these solid phase extraction systems to behave like single ion conducting polymers, exhibiting higher cation transport numbers, thereby achieving efficient charging and discharging. The solution to the cation transfer number problem is of great importance for battery applications, since these batteries are expected to continue to operate in extreme weather conditions. To achieve the above objective, we propose a new integrated strategy to develop a commercially viable product.

In selecting this system, polyethylene glycol acts as a plasticizer to improve the electrical properties of the solid polymer electrolyte, as it provides the ability to lower the Tg, thereby increasing the flexibility of the polymer matrix. The plasticizer is added to effectively increase the ionic conductivity of the PEO-based SPEs. PEG is also much cheaper than commercial high molecular weight PEO. Among them, adding crosslinked nanoparticles having good swelling properties to an electrolyte is a good method for improving the strength, rigidity and conductivity of a membrane. The mechanically stable crosslinked polymer electrolyte membrane also provides a safe separation of the anode and cathode, preventing short circuits and dissolution of electrode components.

The main object of the present invention is to find new, stable and environmentally friendly psdttoii with excellent electrochemical and thermal properties for use in lithium ion batteries.

Disclosure of Invention

In view of the above situation, the present invention provides a method for preparing a high molecular polymer electrolyte for a flexible lithium battery.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for preparing a high molecular polymer electrolyte for a flexible lithium battery comprises the following steps:

step one, synthesis of polystyrene sulfonyl-1, 3-dithiane

Placing 1, 3-dithiane in a container, introducing argon for protection, adding a 2.5M tertiary lithium solution in an 9/25 container into the container under the protection of argon, and fully stirring at the temperature of 0 ℃ for 1 hour; according to 1, 3-dithiane: PS-SO₂PS-SO is prepared in advance with a Cl-to-16: 25 ratio₂Cl, then mixing in tetrahydrofuran solution, fully stirring for 3 hours, then slowly dripping the mixed solution into a container, and continuously stirring for reacting overnight; the mixture was sonicated and filtered, washed three times with 3 container of methanol; drying overnight at 70 ℃ under vacuum;

step two, synthesis of PSDTTO

Placing the polystyrene sulfonyl-1, 3-dithiane synthesized in the step one into a container, adding 3/10 acetic acid in the container as a solvent, and adding 1/5 hydrogen peroxide in the container; heating the mixture and stirring at 60 deg.C, reacting for 3 days, and adding 1/50 containers of hydrogen peroxide every day; after filtration, the mixture is washed three times by deionized water in a 3/10 container; placing in high vacuum at 70 ℃ overnight;

step three, synthesis of PSDTTOLi

Putting the PSDTTO synthesized in the step two and methoxyl lithium into containers respectively according to the proportion of 17:4, and adding methanol in a container 2/5; the mixture was stirred at room temperature for 2 days; washing twice with methanol and once with acetone; dried overnight at 70 ℃ under high vacuum.

Further, the structure of PSDTTOLi is as follows:

has the advantages that:

the PSDTTOLi prepared by the invention has high ionic conductivity and ideal chemical, mechanical and electrochemical properties, and can be used for flexible lithium ion batteries.

Drawings

FIG. 1 is a diagram of the synthetic route of PSDTTOLi according to the present invention;

FIG. 2 is an EDS plot of PSDTTOLi nanoparticles of the invention;

FIG. 3 is an SEM image of PSDTTOLi nanoparticles of the present invention;

FIG. 4 is a schematic diagram of a SPEs film hot pressing process of the present invention;

FIG. 5 is a graph of the fully amorphous properties of PSDTTOLi of the present invention under thermal characterization.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

The invention provides a preparation method of a high polymer electrolyte for a flexible lithium battery, which comprises the following steps as shown in figure 1:

(1) synthesis of polystyrene sulfonyl-1, 3-dithiane

1, 3-dithiane (0.32g) was placed in a 5ml three-necked round bottom flask and argon was passed through to protect(ii) a Adding 1.8ml of 2.5M tertiary lithium solution into a three-neck round-bottom flask under the protection of argon, and fully stirring at the temperature of 0 ℃ for 1 hour; PS-SO prepared in advance₂Cl (0.50g) was mixed in a tetrahydrofuran solution and stirred well for 3 hours, and then the mixed solution was slowly dropped into a three-necked flask and the reaction was continuously stirred overnight; the mixture was sonicated and filtered, washed three times with 15ml of methanol; the product, polystyrenesulfonyl-1, 3-dithiane, was dried overnight under vacuum at 70 ℃. (yield: 0.51g)

FT-IR：2900-2950cm^-1，1638.61cm^-1，1595.89cm^-1，1350-1495(s) cm^-1，1000-1180cm^-1，831.89cm^-1，775.60cm^-1，673.52cm^-1，578.62 cm^-1。

(2) Synthesis of PSDTTO

Polystyrene sulfonyl-1, 3-disulfide (0.51g) was placed in a 50ml round bottom flask, and 15ml acetic acid was added as a solvent; adding 10ml of hydrogen peroxide into a round-bottom flask; heating the mixture and stirring at 60 deg.C, reacting for 3 days, and adding 1ml hydrogen peroxide every day; filtering the product, and washing the product with 15ml of deionized water for three times; the product PSDTTO was placed under high vacuum at 70 ℃ overnight. (yield: 0.34g)

FT-IR：2900-2950cm^-1，1717.43cm^-1，1639.34cm^-1，1600.46cm^-1，1350-1495(s)cm^-1，1000-1225cm^-1，831.89cm^-1，775.61cm^-1，673.52 cm^-1，578.62cm^-1。

(3) Synthesis of PSDTTOLi

Polystyrene sulfonyl-1, 3-dithio-1, 1,3, 3-dinitrogen tetroxide (PSDTTO, 0.34g) and lithium methoxide (0.08g) were put into 50ml round-bottomed flasks, respectively, and 20ml of methanol was added; the mixture was stirred at room temperature for 2 days; washing twice with methanol and once with acetone; the product PSDTTOLi was dried overnight at 70 ℃ under high vacuum. (yield: 0.35g)

FT-IR：2900-2950cm^-1，1705.73cm^-1，1637.34cm^-1，1600.75cm^-1， 1410-1495(s)cm^-1，1000-1190cm^-1，832.79cm^-1，776.25cm^-1，678.43 cm^-1，582.81cm^-1。

The PSDTTOLi prepared by the steps has the following structural formula:

infrared spectroscopic analysis of psdttoi nanoparticles was performed using potassium bromide. For PSDTTO lithiated to PSDTTOLi, the infrared peak value is from 1717.43cm^-1Moving to 1705.73cm^-1,1639.34cm^-1Moving to 1637.34cm^-1This is due to the three electron withdrawing groups (-SO)₂) The stretching vibration of the carbon is affected.

The psdttoil nanoparticles were chemically analyzed using an Energy Dispersive Spectrometer (EDS) equipped with a scanning electron microscope. As shown in fig. 2, the EDS elemental analysis results of the psdttoi nanoparticles of the sample are shown, and the element concentrations obtained from the EDS study are shown in table 1 below.

TABLE 1 concentration of each element of PSDTTOLi detected by EDS

The morphology of the psdttoil nanoparticles was analyzed using a Scanning Electron Microscope (SEM) with an acceleration voltage of 15 kv. The cross section of the sample psdttoi nanoparticles was analyzed by coating a thin gold layer on the surface of the sample psdttoi nanoparticles with a sputter coating (polaron, SC502 sputter coater), as shown in fig. 3, which shows that the psdttoi nanoparticles have a diameter of about 350nm, and thus have a stronger ability to reduce Rm and are more easily aggregated. These perfect psdttoii nanoparticles can help the lithium ions shuttle more directly back and forth between polymer chains.

The mechanical properties of PSDTTOLi were measured using a hot-pressing procedure of a polymer solid electrolyte film. Polyethylene glycol dimethyl ether (Mw ═ 1000) was thoroughly mixed with PEO (Mw ═ 4 × 106g/mol) and lithium salt, and a film was prepared by a grinding method, and then the ground mixture was folded up, placed between two teflon-coated sheets, and then hot-pressed in an engraver at 100 ℃ under a pressure of 10mpsi, as shown in fig. 4. Two thin stainless steel plates are used as intervals to control the thickness of the thin film. The polymer film was cut at 2.04cm²Was cut in a circle, sandwiched between two steel electrodes, and measured with an impedance analyzer. Different weight ratios of PEO-based SPEs films to PSDTTOLi are listed below in Table 2.

TABLE 2 SPEs film formulations with different PSDTTOLi and LiTFSI weight ratios

The thermal stability of PSDTTOLi was determined by thermogravimetric analysis (TGA). Thermogravimetric measurements were performed using a TGA/SDTA851e thermal analyzer. The sample psdttoil was placed in a TGA aluminum pot, heated from 25 ℃ to 100 ℃, heated for 10 minutes, rapidly cooled to 25 ℃, and cooled for 10 minutes. The sample PSDTTOLi was then heated from 25 ℃ to 500 ℃ at a rate of 10 ℃/min. The fully amorphous nature of psdttoil was observed under thermal characterization, as shown in fig. 5. The thermal degradation of psdttoi at different temperatures is shown in table 3 below.

TABLE 3 thermal degradation of PSDTTOLi at different temperatures

In conclusion, the experimental data show that the PSDTTOLi prepared by the invention has high ionic conductivity and ideal chemical, mechanical and electrochemical properties, and can be used for flexible lithium ion batteries.

The limitation of the protection scope of the present invention is understood by those skilled in the art, and various modifications or changes which can be made by those skilled in the art without inventive efforts based on the technical solution of the present invention are still within the protection scope of the present invention.

Claims (1)

1. A method for preparing a high molecular polymer electrolyte for a flexible lithium battery, comprising the steps of:

step one, synthesis of polystyrene sulfonyl-1, 3-dithiane

Placing 1, 3-dithiane into a container, wherein the container is a 5ml three-neck round-bottom flask, introducing argon for protection, adding a 2.5M tertiary lithium solution in an 9/25 container into the container under the protection of argon, and fully stirring at the temperature of 0 ℃ for 1 hour; according to 1, 3-dithiane: PS-SO₂Cl =16:25 ratio prepares PS-SO beforehand₂Cl, then mixing in tetrahydrofuran solution, fully stirring for 3 hours, then slowly dripping the mixed solution into a container, and continuously stirring for reacting overnight; the mixture was sonicated and filtered, washed three times with 3 container of methanol; dried under vacuum at 70 ℃ overnight;

step two, synthesis of PSDTTO

Placing the polystyrene sulfonyl-1, 3-dithiane synthesized in the step one into a container, wherein the container is a 50ml three-neck round-bottom flask, adding 3/10 containers of acetic acid as a solvent, and adding 1/5 containers of hydrogen peroxide into the container; the mixture was heated and stirred at 60 ℃ for 3 days and additional 1/50 containers of hydrogen peroxide were added daily; filtering, and washing with deionized water in 3/10 container for three times; placed under high vacuum at 70 ℃ overnight;

step three, synthesis of PSDTTOLi

Putting the PSDTTO synthesized in the step two and the methoxyl lithium into a container in a ratio of 17:4 respectively, wherein the container is a 50ml three-neck round-bottom flask, and adding 2/5 containers of methanol; the mixture was stirred at room temperature for 2 days; washing twice with methanol and once with acetone; dried under high vacuum at 70 ℃ overnight;

the structural formula of the PSDTTOLi is as follows:

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