A grafted-NH2 solid polymer electrolyte and its preparation method

Abstract

The present invention discloses a grafted -NH2 solid polymer electrolyte and its preparation method, belonging to the technical field of lithium batteries. This method uses polyethyleneimine (PEI), poly(vinylidene fluoride - hexafluoropropylene) (PVDF - HFP) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to prepare the grafted -NH2 solid polymer electrolyte PN - g - PHE. Based on the abundant amino groups in PEI, it can crosslink with PVDF - HFP at room temperature to form a three-dimensional crosslinked network structure, thereby regulating the migration of Li+ and stabilizing the interface between metallic lithium and the electrolyte, thus achieving high-performance solid-state LMBs. The maximum tensile strength of the grafted solid polymer electrolyte disclosed in the present invention is increased to 6.73 MPa. At 60 °C, the ionic conductivity is 2.14×10-4 S / cm and the Li+ transference number reaches 0.55, and the electrochemical stability window is increased to 4.52 V. The Li / Li symmetric battery can stably cycle for more than 1000 h. In addition, the Li / LFP full battery has remarkable rate performance. It cycles 300 times at 0.2C, with a capacity retention rate as high as 97% and a coulombic efficiency close to 99%.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a grafted -NH2 solid polymer electrolyte and a preparation method thereof. Background Art

[0002] With the wide popularization of new energy vehicles and flexible wearable electronic products, the requirements for the energy density of storage systems are increasing day by day. Rechargeable lithium-ion batteries (LIBs) have been widely used in various fields and have greatly changed the modern lifestyle to a large extent. However, the graphite anode based on liquid electrolyte encounters a capacity bottleneck (372 mAh / g) and cannot be further improved. As a candidate material, the lithium metal anode has a high theoretical specific capacity (3860 mAh / g), which is 10 times higher than that of graphite, and an extremely low electrochemical potential (-3.04 V vs standard hydrogen electrode). However, in current liquid metal lithium batteries, the widely used liquid organic electrolyte has high flammability and serious safety hazards brought by lithium dendrite growth, which hinders the practical application of liquid metal lithium batteries. Therefore, there is an urgent need to develop a matching electrolyte to construct high-safety and high-performance LMBs.

[0003] In recent years, solid polymer electrolytes (SPEs) have outstanding advantages such as low flammability, low electrolyte leakage, safety, high flexibility, and high stability between the electrode and the electrolyte, and are considered to be an effective way to solve the safety performance problems of existing lithium-ion batteries from the source, attracting much attention from the scientific research community and the industrial community. Solid polymer electrolytes (SPEs) are mainly formed by the action of lithium salts and polymer-based solid matrices without other organic liquids. Compared with liquid electrolytes, solid electrolytes show great potential in suppressing the formation / growth of lithium dendrites, being non-flammable, and having a wide electrochemical stability window. In addition, inorganic solid polymer electrolytes are pursued by scientific researchers because of their high ionic conductivity and good thermal stability at room temperature, but due to the rigidity problem and large impedance between solid-solid interfaces, they have been restricting their wide commercial applications. SPEs have advantages such as good film-forming property, interface compatibility, and easy processing compared with inorganic solid electrolytes. More importantly, due to the flexibility of polymer structure design on the one hand and the filling of various lithium salts and functional materials on the other hand, this brings numerous choices for the design of SPEs. However, due to poor thermal stability and lack of high mechanical strength, it is usually difficult to meet the market's requirements for high safety and high performance. More unfortunately, due to the slow kinetics of lithium ion disordered transport and strong solvation, polymer electrolytes usually exhibit low lithium ion conductivity and transference number. Low ionic conductivity will greatly limit the rate performance of the battery, making it difficult to achieve fast charging and discharging of the battery. A low Li+ transference number usually leads to uneven deposition of lithium ions, ultimately shortening the battery cycle life.

[0004] In order to improve the energy density of lithium-ion batteries and require the batteries to achieve stable long-term cycling. Therefore, there is an urgent need to develop an electrolyte material that can have a stable interface between the electrolyte and the electrode, good ionic conductivity, lithium-ion transference number, and excellent mechanical properties. Summary of the Invention

[0005] The object of the present invention is to provide a solid polymer electrolyte that can have a stable interface between the electrolyte and the electrode, good ionic conductivity, lithium-ion transference number, and excellent mechanical properties, as well as a preparation method thereof and a lithium-ion battery.

[0006] To achieve the object of the above invention, the present invention provides a grafted -NH2 solid polymer electrolyte, which is obtained by cross-linking reaction from raw materials including the following components: poly(vinylidene fluoride - hexafluoropropylene), lithium salt, polyethyleneimine containing primary amino groups, and solvent. The preparation process includes the following steps:

[0007] Step 1: Add poly(vinylidene fluoride - hexafluoropropylene) (PVDF - HFP) and lithium salt into the solvent and dissolve them. After stirring at 60 °C in a water bath for 3 h, a uniform and transparent PVDF - HFP solution is obtained.

[0008] Step 2: Dissolve polyethyleneimine (PEI) containing primary amino groups in the solvent to obtain a mixed solution of PEI.

[0009] Step 3: Add the mixed solution of PEI into the uniform and transparent PVDF - HFP solution prepared in Step 1. At the same time, set the temperature to 20 - 30 °C and adjust the rotation speed to stir it slowly to obtain a dark brown or yellowish - brown viscous liquid.

[0010] Step 4: Pour the dark brown or yellowish - brown viscous liquid obtained in Step 3 onto a clean glass plate, and use a spatula to scrape the solution on the glass plate. Then transfer the glass plate to a vacuum oven at 60 °C and dry it for 24 h to finally obtain a PN - g - PHE polymer electrolyte membrane.

[0011] Preferably, the lithium salt in Step 1 includes one or more of LiPF6, LiClO4, LiTFSI, LiFSI, LiBOB, LiDFOB, and LiNO3.

[0012] Preferably, the molecular weight of PVDF - HFP in Step 1 is 1 million, and the concentration of the PVDF - HFP solution is 15% or 20%.

[0013] Preferably, the molecular weight of PEI in Step 2 is 2500 - 20000.

[0014] Preferably, the solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate;

[0015] Preferably, the polyethyleneimine containing a primary amino group used in the second step includes primary amine polymers crosslinked with PVDF-HFP such as polyallylamine, polyvinylamine, vinylamine-vinylformamide copolymer, and poly(ethyleneimine).

[0016] Preferably, in the mixed solution with PEI added in the third step, the mass of PEI accounts for 5% - 20% of the mass of PVDF-HFP.

[0017] The present invention provides a grafted -NH2 solid polymer electrolyte prepared by the preparation method described in the above technical solution.

[0018] A lithium metal battery includes a positive electrode, a negative electrode, and the above grafted -NH2 solid polymer electrolyte;

[0019] Preferably, the thickness of the electrolyte is about 30 - 40 μm.

[0020] The present invention also provides a lithium ion battery, and the electrolyte of the lithium ion battery is the grafted -NH2 solid polymer electrolyte prepared by the preparation method described in the above technical solution or the grafted -NH2 solid polymer electrolyte described in the above technical solution.

[0021] The beneficial effects produced by the present invention are:

[0022] (1) Compared with liquid electrolytes, the grafted solid polymer electrolyte of the present invention can avoid the leakage of flammable organic electrolytes and the safety hazards caused by the growth of lithium dendrites piercing the separator and short - circuiting. The assembled battery can operate stably at 60 °C.

[0023] (2) Compared with the solid polymer electrolyte PHE (PVDF - HFP@LiTFSI), for the grafted -NH2 solid polymer electrolyte PN - g - PHE of the present invention, the maximum tensile strength is increased to 6.73 MPa, the ionic conductivity is increased to 2.14×10 - 4 S / cm, the Li+ transference number reaches 0.55, and the electrochemical stability window is broadened by 4.52 V.

[0024] (3) The assembled Li / Li symmetric battery still has a cycle life of over 1000 h at 60 °C under current densities of 0.1 mA / cm2 and 0.2 mA / cm2. When the current density is further increased to 0.5 mA / cm2, it can still stably cycle for over 300 h during lithium deintercalation / insertion, and in particular, the critical current density can reach 1 mA / cm2. In addition, the Li / LFP full battery has excellent rate performance. At 0.2C, after 300 cycles, its capacity retention rate is as high as 97%, and the Coulomb efficiency is close to 99%. Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0026] Figure 1 Graft - NH2 solid polymer electrolyte PN - g - PHE diagram prepared for Example 1.

[0027] Figure 2 SEM diagram of the graft - NH2 solid polymer electrolyte prepared for Example 1.

[0028] Figure 3 For testing the electrochemical window of the graft - NH2 solid polymer electrolyte prepared in Example 1, as can be seen from Figure 3 it that the oxidation potential of the graft - NH2 solid polymer electrolyte prepared in Example 1 is increased to 4.52 V.

[0029] Figure 4 For testing the I - t curve and impedance diagrams before and after testing of the Li / Li symmetric battery with the graft - NH2 solid polymer electrolyte prepared in Example 1 at 60 °C, as can be seen from Figure 4 it that the lithium ion transference number of the graft - NH2 solid polymer electrolyte prepared in Example 1 at room temperature is as high as 0.55.

[0030] Figure 5 Testing the constant current charge - discharge time - voltage curve of the Li / Li symmetric battery with the graft - NH2 solid polymer electrolyte prepared in Example 1 at 60 °C, as can be seen from Figure 5 it that under the conditions of 0.2 mA / cm2 and 0.1 mAh / cm2, the stable lithium deintercalation / insertion time of the symmetric battery exceeds 1000 h, and the overpotential is less than 30 mV.

[0031] Figure 6 Testing the impedance diagram of the graft - NH2 solid polymer electrolyte prepared in Example 1, and the results are asFigure 6 As shown in Figure 6 it can be seen that the conductivity of the grafted - NH2 solid polymer electrolyte prepared in Example 1 at 60 °C is 2.74×10-4 S / cm.

[0032] Figure 7 The test is for the long - cycle performance of the LFP / PN - g - PHE / Li battery in Example 2 at 0.2C at 60 °C. Specific Embodiments

[0033] Next, in conjunction with the drawings of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

[0034] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the specification drawings and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0035] Example 1

[0036] A preparation method of a grafted - NH2 solid polymer electrolyte, characterized in that its main components include the following raw materials: poly(vinylidene fluoride - hexafluoropropylene), lithium salt, polyethyleneimine containing primary amino groups, and solvent. The preparation process includes the following steps:

[0037] Step 1: Add poly(vinylidene fluoride - hexafluoropropylene) (PVDF - HFP) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to N,N - dimethylformamide for dissolution. After stirring at 60 °C in a water bath for 3 h, a uniform and transparent PVDF - HFP solution is obtained;

[0038] Step 2: Dissolve polyethyleneimine (PEI) in N,N - dimethylformamide to obtain a mixed solution of PEI;

[0039] Step 3: Add the DMF mixed solution of PEI to the PVDF - HFP mixed solution prepared in Step 1. At the same time, set the temperature to 25 °C and adjust the rotation speed to stir it slowly to obtain a dark brown or yellowish - brown viscous liquid;

[0040] Step 4: Pour the dark brown or yellowish - brown viscous liquid obtained in Step 3 onto a clean glass plate, and use a scraper with a certain thickness to scrape the solution on the glass plate. Then transfer the glass plate to a vacuum oven at 60 °C for drying for 24 h to finally obtain a PN - g - PHE polymer electrolyte membrane.

[0041] Example 2

[0042] Using the grafted - NH2 solid polymer electrolyte prepared in Example 1 as the electrolyte, a button cell was assembled. The positive electrode active material was lithium iron phosphate, the current collector was aluminum foil, the conductive agent was acetylene black, and the binder was polyvinylidene fluoride; the negative electrode was metallic lithium. Long - cycle tests were carried out on the obtained button cells, and the results are as Figure 7 shown. From Figure 7 it can be seen that at a rate of 0.2C, the initial discharge specific capacity of the PN - g - PHE battery was about 162 mAh / g. After 125 cycles, the specific capacity was about 160 mAh / g, and the capacity retention rate was 98.7%. After 300 cycles, it was still as high as 157 mAh / g. The coulombic efficiency (CE) was close to 99%, and its capacity retention rate was as high as 97%. It had good reversibility and long - cycle stability.

[0043] Obviously, the above - mentioned examples are only for clear illustration and not limitations on the implementation modes. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation modes here. And the obvious changes or modifications derived therefrom are still within the protection scope of this invention.

Claims

1. A preparation method of grafted - NH₂ solid polymer electrolyte PN - g - PHE, the main components of which include the following raw materials: poly(vinylidene fluoride - hexafluoropropylene) PVDF - HFP, lithium salt, polyethyleneimine PEI containing primary amino groups, and solvent. The preparation steps are as follows: Step 1: Add poly(vinylidene fluoride - hexafluoropropylene) PVDF - HFP and lithium salt into the solvent and dissolve them. After stirring at 60 °C in a water bath for 3 h, a homogeneous and transparent PVDF - HFP solution is obtained. Step 2: Dissolve polyethyleneimine PEI containing primary amino groups in the solvent to obtain a mixed solution of PEI. Step 3: Add the mixed solution of PEI in Step 2 into the homogeneous and transparent PVDF - HFP solution prepared in Step 1. At the same time, set the temperature to 20 - 30 °C and adjust the rotation speed to stir it slowly to obtain a dark brown or yellowish - brown viscous liquid. Step 4: Pour the dark brown or yellowish - brown viscous liquid obtained in Step 3 onto a clean glass plate. Use a spatula to scrape the solution onto the glass plate, and then transfer the glass plate to a vacuum oven at 60 °C and dry it for 24 h to finally obtain the grafted - NH₂ solid polymer electrolyte PN - g - PHE. In Step 1, the molecular weight of PVDF - HFP is 1 million, and the concentration of the PVDF - HFP solution is 15% or 20%. The lithium salt includes one or more of LiPF₆, LiClO₄, LiTFSI, LiFSI, LiBOB, LiDFOB, and LiNO₃. In Step 2, the molecular weight of PEI is 2500 - 20000. The polyethyleneimine containing primary amino groups used in Step 2 includes at least one of polyallylamine, polyvinylamine, vinylamine - vinylformamide copolymer, and poly(ethyleneimine).

2. The preparation method of the grafted -NH2 solid polymer electrolyte PN-g-PHE according to claim 1, characterized in that, The solvent in Step 1 includes at least one of N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, acetonitrile, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

3. The preparation method of the solid polymer electrolyte PN-g-PHE according to claim 1, characterized in that In Step 3, the mass of PEI in the added PEI mixed solution accounts for 5% - 20% of the mass of PVDF - HFP.

4. A lithium metal battery, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte; the electrolyte uses the grafted - NH₂ solid polymer electrolyte PN - g - PHE prepared by the preparation method described in any one of claims 1 - 3.

5. The lithium metal battery according to claim 4, wherein The thickness of the electrolyte is 30 - 40 μm.

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