CN103208604B - Electrospinning composite diaphragm with thermal hole sealing function - Google Patents

Abstract

An electrospun composite separator with a heat-closing function relates to a lithium-ion battery. It provides an electrospun composite separator with thermal closing function that can realize secondary thermal closing function, avoid direct contact of positive and negative electrodes of lithium ion battery due to thermal inertia, and significantly improve the safety of lithium ion battery. The electrospun composite diaphragm with heat-closing function is a non-woven fabric structure, including polyimide nanofibers and low-melting polymer nanofibers; the low-melting polymer nanofibers contain bismaleimide and Azobisisobutyronitrile low-melting-point polymer nanofibers, polyimide nanofibers and low-melting-point polymer nanofibers are interlaced in disorder. The thickness of the electrospun composite membrane with heat-closing function may be 10-50 μm. The melting point of the low-melting point polymer nanofiber may be 90-110°C. The composition of the low-melting point polymer nanofibers can be ethylene-vinyl acetate copolymer, polyethylene succinate and the like.

Description

An Electrospun Composite Separator with Thermal Closure Function

technical field

The invention relates to a lithium-ion battery, in particular to an electrospun composite separator with a heat-closing function prepared by an electrospinning method.

Background technique

Lithium-ion batteries are widely used in consumer electronics products due to their high energy density, high power density, long service life, and environmental friendliness. However, the potential safety hazards of lithium-ion batteries have seriously hindered the application of lithium-ion batteries. Extensions, such as hybrid vehicles, electric vehicles, etc. Thermal runaway is the main cause of battery safety hazards, and thermal runaway is mainly caused by overcharging, internal or external short circuit or high thermal shock.

At present, the means to solve the safety of lithium-ion batteries are mainly carried out from the aspects of battery electrode materials, electrolytes, electrolyte additives and diaphragms. Among them, achieving high safety of batteries through diaphragms is the focus of current research, and diaphragms with thermal closure function are The most important means of ensuring battery safety. Chinese patents CN101794870A and CN102544414A both disclose the use of high temperature resistance and stable chemical properties of inorganic particles (such as alumina) to improve the thermal stability of the separator, which improves the safety of the battery to a certain extent, but does not involve thermal closure. hole problem. At present, commercial separators mainly include PP, PE and PP/PE/PP separators, and their thermal closing temperatures are about 165°C, 135°C, and 135°C, respectively. In order to improve the safety of the diaphragm, patents JP7304110A, JP8250097A, GB2298817A and US5691007A respectively disclose different processes for making thermally closed cell diaphragms and realize the thermally closed cell function. The thermally closed cell temperatures are 135-140°C, 124°C, and 135°C respectively. and 132°C. Literature A review on the key issues for lithium-ion battery management in electric vehicles pointed out that the operating temperature ranges of most batteries during charging and discharging are -20～55℃ and 0～45℃ respectively. When the battery temperature rises to 90～120℃, The solid electrolyte interface film (SEI) begins to decompose exothermicly; when the temperature exceeds 120°C, the SEI film completely disintegrates, and the electrodes and the electrolyte are in direct contact with side reactions, and as the temperature rises, the electrolyte and electrodes decompose, eventually leading to Thermal runaway, so the thermal closure temperature of the diaphragm should be designed within the range of 90-120°C; patent US6080507A discloses a three-layer diaphragm manufacturing process with a thermal closure temperature of 115°C, pointing out that the thermal closure temperature of the diaphragm should be lower than 120 °C, preferably controlled within the range of 95-115 °C.

The above-mentioned patents have improved the safety of the diaphragm to a certain extent, but due to the effect of thermal inertia, the temperature inside the battery may continue to rise after thermal closure and exceed the melting point of the diaphragm components, causing the diaphragm to melt and cause the lithium-ion battery to malfunction. The direct contact of the negative electrode makes the internal temperature of the battery rise rapidly to cause thermal runaway, and may eventually cause an explosion.

Contents of the invention

The purpose of the present invention is to provide an electrospun electrospun cell with a thermal closed cell function that can realize the secondary thermal closed cell function, avoid direct contact between the positive and negative electrodes of the lithium ion battery due to the effect of thermal inertia, and significantly improve the safety of the lithium ion battery. Composite diaphragm.

The electrospun composite membrane with heat-closing function is a non-woven structure, including polyimide (PI) nanofibers and low-melting polymer nanofibers; the low-melting polymer nanofibers contain bismaleyl Low-melting point polymer nanofibers of imide (BMI) and azobisisobutyronitrile (AIBN), polyimide (PI) nanofibers interleaved with low-melting point polymer nanofibers.

The thickness of the electrospun composite membrane with heat-closing function may be 10-50 μm.

The melting point of the low-melting point polymer nanofiber may be 90-110°C. The composition of the low-melting point polymer nanofibers can be ethylene-vinyl acetate copolymer, polyethylene succinate and the like.

According to the mass ratio, the ratio of the polyimide (PI) nanofiber content to the low melting point polymer nanofiber content can be (3-1): 1; according to the mass ratio, the bismaleimide (BMI ) and azobisisobutyronitrile (AIBN) content and the ratio of the low melting point polymer nanofiber content can be 15% to 25%, wherein bismaleimide (BMI) content and azobisisobutyronitrile The ratio of butyronitrile (AIBN) content can be (60-40):1.

Compared with prior art, working principle of the present invention and beneficial effect are as follows:

During the preparation of the present invention, each component is prepared in proportion to an electrospinning solution, which can be obtained by using an existing electrospinning device and preparing by an electrospinning method. The desired thickness of the electrospun composite separator can be obtained by hot-pressing treatment. The present invention, as a lithium-ion battery diaphragm, utilizes the melting of low-melting-point polymer nanofibers to realize the first heat-closing function of the diaphragm. After the first heat-closed cells, a polymer insulating layer is formed. After the temperature continues to rise, the polymerization The insulating layer of the material melts, and the BMI and AIBN are completely released in the lithium-ion battery electrolyte to form a new homogeneous electrolyte system. The remaining PI fibers and BMI after the melting of the low-melting polymer insulating layer can be in situ Polymerize to form a solid insulator, thereby realizing the second thermal closing function of the diaphragm. In addition, the PI polymers obtained after in-situ polymerization of PI nanofibers and BMI monomers are all high-temperature stable substances, which are an important guarantee for battery safety. Since the temperature rise inside the battery will make the PI polymer obtained after the in-situ polymerization of the BMI monomer more stable, even if there is thermal inertia, it will not destroy the solid insulator formed by the solidification of the new homogeneous electrolyte system.

Description of drawings

Fig. 1 is a schematic diagram of the fiber structure of the electrospun composite membrane of the present invention.

Fig. 2 is a schematic diagram of the secondary thermal cell closure process when the electrospun composite membrane of the present invention is used.

Fig. 3 is a schematic diagram of the polymerization principle of the BMI monomer of the present invention.

Detailed ways

Referring to Fig. 1, the electrospun composite diaphragm is a non-woven fabric structure, and the electrospun composite diaphragm includes polyimide (PI) nanofibers 15 and low melting point polymer nanofibers 16; the low melting point polymer nanofibers 16 are Low-melting-point polymer nanofibers containing bismaleimide (BMI) and azobisisobutyronitrile (AIBN), polyimide (PI) nanofibers 15 and low-melting-point polymer nanofibers 16 are interlaced in a heterogeneous order.

The electrospun composite membrane is prepared by an electrospinning device and then subjected to subsequent hot-pressing treatment, so that the thickness of the membrane is 10-50um. The melting point of the low-melting point polymer nanofibers 16 is 90-110°C.

According to the mass ratio, the ratio of the polyimide (PI) nanofiber content to the low melting point polymer nanofiber content is (3-1): 1; according to the mass percentage, the bismaleimide (BMI) The ratio of the sum of the content of azobisisobutyronitrile (AIBN) and the content of the low-melting point polymer nanofiber is 15% to 25%, wherein the content of bismaleimide (BMI) is the same as that of azobisisobutyronitrile The mass ratio of (AIBN) content is (60-40):1.

Referring to Figure 2, a lithium-ion battery is mainly composed of a positive electrode, a separator/electrolyte, and a negative electrode. In Fig. 2, mark 21 is the electrolyte system; mark 22 is the electrospun composite separator; mark 23 is the polymer insulation layer formed after the first thermal closure; mark 24 is the complete BMI and AIBN after the polymer insulation layer is melted. The new homogeneous electrolyte system formed after being released in the electrolyte; 25 is the PI fiber layer left after the melting of the low-melting polymer insulation layer; 26 is the solid insulator formed after the in-situ polymerization of BMI induced by AIBN.

The secondary thermal closure process of the composite electrospun diaphragm 22 is as follows:

1) When the lithium-ion battery is charged and discharged under normal working conditions, the internal temperature of the battery is much lower than 90°C, and the internal electrolyte system 21 of the lithium-ion is normal (see Figure 2(a), marked 22 is the electrospun composite separator).

2) When the temperature rises to 90 ℃ ~ 110 ℃, the low melting point polymer nanofibers soften and melt, release part of BMI and AIBN, and absorb heat during the melting process, initially slowing down the internal temperature rise of the battery; low melting point polymer nanofibers After melting, the gaps between the unmelted PI nanofibers are blocked to form a polymer insulating layer 23, which isolates ion transmission (that is, the first thermal closure) (see Figure 2(b), marked 21 is the electrolyte system).

3) When the temperature continues to rise due to thermal inertia and other reasons but does not exceed 120 °C, the insulating layer formed by the low melting point polymer is destroyed, BMI and AIBN are completely released, and a new homogeneous electrolyte system 24 is formed (see Figure 2 ( c), mark 25 is the PI fiber layer left after the low-melting point polymer insulating layer is melted).

4) When the temperature continues to rise above 120°C, the BMI monomer is polymerized in situ under the action of the AIBN initiator, so that the new homogeneous electrolyte system 24 is transformed from a liquid state to a solid state, realizing the second isolation of ion transport (that is, the second Subthermal closed cell) (see Figure 2(d)), marked 26 is a solid insulator formed after in situ polymerization of BMI induced by AIBN; the principle of BMI monomer polymerization is shown in Figure 3.

A specific example of preparing an electrospun composite diaphragm is given below:

The polyimide powder was dissolved in dimethylacetamide solvent, and the concentration of the solution was controlled to be 20wt%. Polyethylene oxide powder (PEO) with a melting point of 90-100°C was selected as a low-melting point polymer, and 12wt% PEO powder, 3wt% BMI and 0.05wt% AIBN were dissolved in a mixed solvent of deionized water and absolute ethanol (to The mass ratio of ionized water to absolute ethanol is 3:1), and stirred evenly to prepare the electrospinning solution. Then, the electrospun composite membrane can be prepared by using the existing electrospinning device, and the dimethylacetamide solvent is completely volatilized during the preparation process. By controlling the stepping motor of the electrospinning device, the reciprocating motion of the collection device in the Y direction and the intermittent linear motion in the X direction can be realized, so that a uniform and large-area electrospun composite membrane (composite fiber membrane) can be obtained. By controlling the liquid supply rate of the liquid supply pump of the electrospinning device, the mass ratio of PI nanofibers to PEO composite nanofibers in the electrospun composite membrane (composite fiber membrane) can be precisely controlled to be 3:1. After the prepared electrospun composite separator was hot-pressed at a pressure of 5 MPa, a composite electrospun separator with a thickness of 25 μm was finally obtained.

Claims (3)

1. An electrospun composite diaphragm with heat-closing function is characterized in that said electrospun composite diaphragm is a non-woven fabric structure, and the electrospun composite diaphragm comprises polyimide nanofibers and low-melting polymer nanofibers; The low-melting-point polymer nanofibers are low-melting-point polymer nanofibers containing bismaleimide and azobisisobutyronitrile, and polyimide nanofibers and low-melting-point polymer nanofibers are interlaced in disorder; The melting point of the low-melting point polymer nanofiber is 90-110°C; According to the mass ratio, the ratio of the polyimide nanofiber content to the low melting point polymer nanofiber content is 3 to 1:1; according to the mass ratio, the bismaleimide and azobisisobutyronitrile content The ratio of the sum to the content of the low melting point polymer nanofiber is 15%-25%, wherein the ratio of the content of bismaleimide to the content of azobisisobutyronitrile is 60-40:1. 2 . The electrospun composite membrane with thermal closure function according to claim 1 , characterized in that the thickness of the electrospun composite membrane is 10-50 μm. 3 . 3. A kind of electrospun composite diaphragm with thermal closure function as claimed in claim 1, it is characterized in that the composition of described low melting point polymer nanofiber is ethylene-vinyl acetate copolymer or polyethylene succinate alcohol esters.