CN1189514C - Polymer electrolyte for lithium-ion cell and preparation method - Google Patents

Abstract

The present invention relates to a kind of polymer electrolyte for lithium ion battery and preparation method thereof, this polymer electrolyte material comprises polymer matrix material (A) and polymer matrix material (B), liquid electrolyte solution (C), wherein Electrolyte C contains a lithium salt and an aprotic solvent. The polymer matrix material A is poly(methyl methacrylate-acrylonitrile-lithium methacrylate) (P(MMA-AN-MALi)), abbreviated as PMAML, wherein the molar composition of each component is, respectively, 40% <AN<90%, 10%<MMA<40%, 0<MALi<20%. Polymer matrix material B is polyvinylidene fluoride (PVDF) or poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP). The polymer electrolyte membrane is prepared by a one-step method of volatile solvent, the preparation process is simple, the conductivity at room temperature is high (3.2×10 ^-3 S·cm ^-1 ), and the interface stability with the electrode is good.

Description

Polymer electrolyte for lithium ion battery and preparation method thereof

Technical field

The invention relates to an electrolyte for a lithium ion battery and a preparation method thereof, more particularly to a polymer electrolyte for a lithium ion battery and a preparation method thereof.

Background technique

Polymer lithium-ion batteries have the characteristics of high energy density, no electrolyte leakage, good safety performance, flexible design, and easy mass production. With the continuous miniaturization of electronic devices, secondary batteries should also develop in the direction of light weight and ultra-thin. Lithium-ion batteries with polymer materials as electrolytes can meet this requirement.

Among the existing polymer electrolytes, the conductivity of pure solid polymer electrolytes (without plasticizer) at room temperature is too low to meet the requirements of practical production. Although many methods have been reported attempting to prepare solid polymer electrolytes without solvents, none of the systems proved to be satisfactory. Therefore, the current research work on polymer electrolytes mainly focuses on gel-state polymer electrolytes. This type of polymer electrolyte contains a large amount of organic plasticizer, and its conductivity at room temperature can reach more than 1×10 ^-3 S·cm ^-1 . The polymer electrolyte systems that have been studied more mainly include PEO, PAN, PMMA, PVDF, PVC and other major categories. At present, the main problem of polymer electrolytes is the contradiction between mechanical properties and electrical conductivity. In order to improve the room temperature conductivity of gel-state polymer electrolytes, a large amount of organic plasticizer needs to be cured in the polymer matrix, which will greatly reduce its mechanical strength. Second, the stability of the interface between the electrolyte and the electrode is poor. As the number of charge-discharge cycles or storage time increases, the interface impedance between the polymer electrolyte and the electrode will also increase, which will affect the charge-discharge performance of the battery. The 3rd, preparation process is complicated. Whether it is the method of extracting pore-forming agent or the phase inversion method, there are many inconveniences in large-scale production, and the production process is complicated.

Contents of Invention

The present invention aims to overcome the deficiencies in the prior art and provide a polymer electrolyte with simple preparation process, high room temperature conductivity and good interface stability and a preparation method thereof.

The present invention realizes through following technical scheme:

A polymer electrolyte for lithium-ion batteries consisting of a polymer matrix which is a self-supporting membrane consisting of two polymer materials A and B, and an electrolyte solution, the polymer material A being Poly(methyl methacrylate-acrylonitrile-lithium methacrylate), the polymer material B is polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene), and the mass ratio of A to B is 1: 10-2:3, the electrolytic solution contains dissociable lithium salt and aprotic solvent, and the weight percentage of the electrolytic solution in the polymer electrolyte is 20-80%.

The molar composition of each component in the polymer matrix material A is 40%<acrylonitrile<90%, 10%<methyl methacrylate<40%, 5%<lithium methacrylate<20%.

The present invention also discloses a preparation method for a polymer electrolyte for a lithium ion battery, which comprises the following steps:

(1) monomer methyl methacrylate, acrylonitrile, lithium methacrylate are purified;

(2) Dissolving the above-mentioned monomers in a solvent to prepare a monomer mixture;

(3) The above-mentioned monomer mixture is packed in a container, and an initiator is added;

(4) Feed nitrogen gas to remove oxygen in the container, then seal the container; react at 50-70°C for 8-12 hours to obtain polymer matrix material A, namely poly(methyl methacrylate-acrylonitrile-methyl lithium acrylate);

(5) Mix the above polymer matrix material A, poly(methyl methacrylate-acrylonitrile-lithium methacrylate) with polymer material B, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene) The solvent is fully dissolved;

(6) above-mentioned solution is poured on flat board and made film;

(7) above-mentioned film is vacuum-dried;

(8) The above dried film is soaked in the electrolyte solution for 30-60 minutes until the liquid holding capacity of the film is 20%-80%, that is, the polymer electrolyte for the lithium-ion battery is prepared.

The electrolyte contains a dissociable lithium salt and an aprotic solvent.

The invention has the following beneficial effects: the polymer electrolyte is prepared by a one-step method of volatile solvent, the preparation process is simple, and the prepared polymer electrolyte has high conductivity and good interface stability.

Description of drawings

Figure 1 AC impedance of Li/polymer electrolyte/Li battery;

Fig. 2 AC impedance of Li/polymer electrolyte 2/Li battery.

Detailed ways

The raw materials used in the synthesis of novel polymer matrix materials in the present invention mainly include methyl methacrylate (MMA), acrylonitrile (AN), and lithium methacrylate (MALi). The above three raw materials are polymerized to obtain polymer matrix material A, namely poly(methyl methacrylate-acrylonitrile-lithium methacrylate) (P(MMA-AN-MALi)), abbreviated as PMAML, wherein each group The molar compositions of the parts are respectively, 40%<AN<90%, 10%<MMA<40%, 5%<MALi<20%. Polymer matrix material B is polyvinylidene fluoride ie PVDF or poly(vinylidene fluoride-hexafluoropropylene) ie P(VDF-HFP). The polymer matrix materials A and B are mixed and dissolved in a solvent, and the polymer electrolyte matrix is prepared by one-step method of volatilizing the solvent. The polymer electrolyte is prepared by immersing the prepared electrolyte matrix in the electrolyte solution. The electrolyte contains lithium salt and an aprotic solvent. The lithium salt is one of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , and LiN(CF ₃ SO ₂ ) ₂ . Aprotic solvents are EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), DME (diethyl carbonate), EMC (methyl ethyl carbonate) one or more of them.

The present invention will be further described below in conjunction with specific examples.

Before use, MMA and AN were treated with 0.5% sodium hydroxide solution to remove the polymerization inhibitor, and then dried under reduced pressure and dried with molecular sieves.

Example 1: The solution polymerization method of PMAML. Dissolve 0.15mol MMA, 0.8mol AN and 0.05mol MALi with 150ml DMF (dimethylformamide) or methanol to prepare a monomer mixture, put the prepared monomer mixture into a sealable glass container, add 0.3g The azobisisobutyronitrile initiator was used, and the oxygen in the container was removed with dry nitrogen, and the reaction container was sealed. Then react at 50°C for 8h to prepare PMAML.

Example 2: According to the polymerization conditions in Example 1, several PMAMLs with different ratios were synthesized. In the PMAML, each component MMA, AN and MALi are fed and polymerized by 0.4mol, 0.4mol, 0.2mol and 0.1mol, 0.8mol, 0.1mol respectively, the amount of solvent used is respectively 100ml and 150ml, and the initiator benzoyl peroxide is respectively 0.40g and 0.27g, the reaction temperatures were 60°C and 70°C, and the reaction times were 10 and 12 hours, respectively.

Example 3: Prepare PMAML according to the method of Example 1. 1g of PMAML and 2g of P(VDF-HFP) were fully dissolved with 15g of DMF (or one of NMP, acetone, DMF or mixed), and then made into a thin film on a flat glass plate and dried in vacuum. The thickness of the obtained polymer film is about 50 μm, and it is immersed in the electrolyte for 30-60 minutes. The lithium salt in the electrolyte is LiPF ₆ , and the solvent is EC/DEC (the weight ratio of EC to DEC is 1:1, and the concentration of lithium salt is 1M). . The prepared polymer electrolyte membrane has a liquid holding capacity of 80%. The conductivity of the polymer electrolyte is tested with a stainless steel electrode/polymer electrolyte/stainless steel electrode battery. The area of the polymer electrolyte membrane is about 1.2cm ² , and the conductivity is measured by AC impedance technology. The room temperature conductivity is 3.2×10 ^-3 S· cm ^-1 .

Example 4: The polymer electrolyte was prepared according to the method of Example 3, wherein the amounts of PMAML and P(VDF-HFP) in the polymer matrix were 1 g and 5 g, respectively. When the electrolyte solution in the polymer electrolyte holds 40%, the conductivity at room temperature is 7.8×10 ^-5 S·cm ^-1 ; when the electrolyte (1M LiBF ₄ -EC/DMC) in the polymer electrolyte holds When the amount is 60%, the conductivity at room temperature is 1.48×10 ^-3 S·cm ^-1 . Its electrochemical stability window is greater than 4.5V.

Example 5: A PVDF-based microporous polymer electrolyte was selected as a reference sample, and a Li/polymer electrolyte/Li test cell was assembled to study the change of the interfacial impedance between the polymer electrolyte and the lithium electrode with the storage time. The PVDF-based polymer membrane was prepared by the phase inversion method, in which the casting solution consisted of deionized water, acetone and PVDF, and the weights were 1g, 8g and 1g, respectively. The holding capacity of liquid electrolyte solution (1M LiBF ₄ -EC/DMC) in the polymer electrolyte is about 72%. The interface impedance was measured at an electrochemical workstation using AC impedance technology. During the test, the electrode diameter was 12mm, the test frequency range was 1MHz-10mHz, and the response signal was 10mV. The test results are shown in Figure 1.

Example 6: The polymer electrolyte membrane prepared in Example 4 was assembled into a Li/polymer electrolyte/Li test cell in a glove box to measure the interface impedance between the polymer electrolyte and the metal lithium electrode. Figure 2 shows the AC impedance spectrum for different discharge times in the open circuit state. The diameter of the large semicircle in the figure corresponds to the interface impedance between the polymer electrolyte and the electrode. It can be seen that the interface impedance will gradually increase with the prolongation of the storage time. However, the increase rate is not large. After two weeks of storage, the interface impedance basically tends to be stable, which shows that the interface properties between the polymer electrolyte system and the electrode are stable and good. However, as shown in Figure 1, the interfacial impedance between the PVDF-based polymer electrolyte and the metal lithium electrode increases greatly with the prolongation of storage time, and the interfacial impedance is still not stable after 19 days of storage. It can be seen that the introduction of PMAML has significantly improved the interfacial properties of the polymer electrolyte, and the interfacial properties of the polymer electrolyte prepared by the method in Example 3 are stable.

Example 7: Prepared a kind of polymer electrolyte by the method for example 3 in order to assemble polymer lithium ion battery, wherein the quality of polymer substrate PMAML and P (VDF-HFP) is respectively 1g and 9g, and electrolytic solution is 1.2MLiPF ₆ - EC/DEC/EMC (1:1:1 wt%). The positive electrode active material is lithium cobaltate, and the weights of the components in the pole piece are: lithium cobaltate 8.5g, conductive carbon black (super-p) 1.0g, and binder PVDF 0.5g. The active material used in the negative electrode is graphitized carbon fiber, and the weight of each component in the pole piece is: carbon fiber 9.0g, conductive carbon black (super-p) 0.3g, binder PVDF 0.7g. Figure 3 shows a typical charge-discharge curve of the prepared polymer lithium-ion battery, where the horizontal axis on the figure is the specific capacity of the positive electrode active material. The Coulombic efficiency of the first charge and discharge is 91%, the irreversible capacity is small, and the plateau area of the discharge voltage is above 3.6V.

Claims (4)

1, a kind of polymer dielectric that is used for lithium ion battery, it is characterized in that, it is made up of polymeric matrix and electrolytic solution, the self-supported membrane that described polymeric matrix is made up of two kinds of polymer materials A and B, described polymer materials A is poly-(methyl methacrylate-vinyl cyanide-methacrylic acid lithium), described polymer materials B is polyvinylidene difluoride (PVDF) or poly-(biasfluoroethylene-hexafluoropropylene), and the mass ratio of A and B is 1: 10～2: 3, contain dissociable lithium salts and aprotic solvent in the described electrolytic solution, this electrolytic solution weight percent in polymer dielectric is 20-80%.

2, the polymer dielectric that is used for lithium ion battery according to claim 1, the mole composition that it is characterized in that each component among the described matrix material A is respectively, 40%＜vinyl cyanide＜90%, 10%＜methyl methacrylate＜40%, 5%＜methacrylic acid lithium＜20%.

3, a kind of described method for preparing polymer electrolytes that is used for lithium ion battery of claim 1 for preparing is characterized in that it comprises the following steps:

(1) monomers methyl methacrylate, vinyl cyanide, methacrylic acid lithium are purified;

(2) getting above-mentioned each monomer is dissolved in and makes monomer mixture in the solvent;

(3) above-mentioned monomer mixture is packed in the container, and add initiator;

(4) feeding nitrogen is removed the oxygen in the container, sealed vessel afterwards; Reacted 8～12 hours down at 50～70 ℃, promptly make matrix material A i.e. poly-(methyl methacrylate-vinyl cyanide-methacrylic acid lithium);

(5) be polyvinylidene difluoride (PVDF) with above-mentioned matrix material A promptly poly-(methyl methacrylate-vinyl cyanide-methacrylic acid lithium) and polymer materials B or gather (biasfluoroethylene-hexafluoropropylene) and mix and fully dissolve with solvent;

(6) above-mentioned solution is poured on makes film on the flat board;

(7) with above-mentioned film vacuum-drying;

(8) be 20%～80% with above-mentioned exsiccant film with electrolytic solution dipping 30～60 minutes to film liquid holdup, promptly make the polymer dielectric that is used for lithium ion battery.

4, the method for preparing polymer electrolytes that is used for lithium ion battery according to claim 3 is characterized in that, contains dissociable lithium salts and aprotic solvent in the described electrolytic solution.

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