Next generation of lithium ion battery will be made of diaphragm material?

Polaris energy storage network: lithium ion battery is widely used in mobile electronic devices and power devices because of its high energy density and long cycle life and other advantages, however, tesla event, samsung mobile phone event, frequent lithium ion battery safety accidents have gradually attracted people's attention. Battery diaphragm (figure 2), as one of the important components of lithium ion battery, can provide lithium ion transmission channel and prevent short circuit in the contact between positive and negative electrodes, which has a very important impact on the safety of lithium ion battery. Lithium ion battery diaphragm to meet the following conditions:

(1) it has electronic insulation to ensure the mechanical isolation of positive and negative electrodes;

(2) it has a certain porosity and pore size to ensure low resistance and high ionic conductivity and good permeability to lithium ions;

(3) resistance to electrolyte corrosion, good electrochemical stability;

(4) good infiltration of electrolyte and sufficient absorbent moisture retention capacity;

(5) sufficient mechanical properties, including puncture strength, tensile strength, etc.;

(6) good spatial stability and smoothness;

(7) good thermal stability.

Lithium ion battery with its unique advantages quickly occupied the traditional battery market and has been widely used, mobile phones, laptops, cameras, cameras and other electronic and information products are now using lithium ion battery as power supply. However, in some high-end applications, such as power batteries and other large capacity lithium ion battery applications have not been promoted and popularized. One of the important reasons is that the performance of the existing lithium ion membrane has not been able to meet the requirements of high-end battery membrane. Requirements for diaphragm of high-end battery:

(1) high temperature safety

(2) high power charge-discharge performance

(3) high cycle life.

Polyolefin membrane can be closed at high temperature, which prevents the further diffusion of heat, is now the most widely used lithium-ion battery membrane. Currently the most widely used polyolefin diaphragm material is polyethylene (PE) and polypropylene (PP), its softening deformation occurred in more than 100 ℃. Polyolefin polymers have poor heat resistance, and may melt under over-charge and over-discharge, rapid charge and discharge, or high temperature, resulting in short circuit, fire and even explosion. On the other hand, the electrolyte infiltration of polyolefin membranes is insufficient. In order to improve the thermal stability of polyolefin membrane and the infiltration of electrolyte, the main solution at present is to apply high temperature resistant coating on one or both sides of polyolefin membrane, or to find new membrane materials with good thermal stability that can replace polyolefin.

High temperature resistant coating on polyolefin base film is a common way to modify polyolefin membrane, which has little impact on the electrochemical properties and thermal pore closure properties of the battery, but can effectively reduce the thermal shrinkage of the membrane, thereby improving the safety of lithium ion battery. At present, inorganic ceramic coated membrane is the most commonly used in the market, but because ceramic nanoparticles are prone to agglomeration and it is difficult to evenly coated on the base film, it will also cause serious hole blockage, leading to the increase of ion transfer resistance, affecting the recycling performance of lithium battery. In addition, in the process of battery assembly, the combination performance of inorganic ceramics and the substrate is poor, and the ceramic coating is easy to fall off. However, by adding ordinary binder to increase the adhesion force, the permeability of the diaphragm will become worse and the internal resistance of the battery will be increased. Based on these shortcomings of ceramic coating, the use of high temperature resistant polymers as coating materials has become more and more research.

Another solution is to choose the new diaphragm material to replace the traditional high temperature resistant polyolefin materials, including natural materials and synthetic materials, natural materials with cellulose and its derivatives, synthetic materials including poly (ethyl 2 terephthalate (PET) and poly (vinylidene fluoride) (PVDF), poly (vinylidene fluoride - hexafluoropropylene (PVDF HFP), polyamide (PA), polyimide (PI), aramid (a between aramid (PMIA); Para-aramid fiber (PPTA), etc.

PI is a kind of polymer containing polyamide ring on the main chain, which is one of the organic polymer materials with the best comprehensive performance. Its resistance to high temperature above 400 ℃, the long-term use of temperature between 200 ~ 300 ℃, no obvious melting point, high insulation performance, 1000 hz dielectric constant was 4.0, dielectric loss was only 0.004 ~ 0.007, belong to F class H insulation. It has been widely used inspecial,special, microelectronics, nano, liquid crystal, separation film, laser and other fields. Due to its outstanding advantages in performance and synthesis, PI, whether as a structural material or as a functional material, has been fully recognized for its huge application prospect, and is known as a "problem solvable expert".

As a diaphragm, PI diaphragm has many advantages over the traditional polyolefin diaphragm: first, it has good high temperature resistance, which can improve the safety performance of lithium ion battery; Secondly, PI porous membrane has a high porosity, PI has a large number of polar groups, the membrane has a high ionic conductivity, the infiltration of electrolyte is very good, so that lithium ion battery is suitable for charging and discharging at a high rate, shorten the charging time, and extend the service life of lithium ion battery. Therefore, PI diaphragm is expected to be the next generation of lithium ion battery membrane material. PI is applied in lithium ion battery membrane in two ways. One is to coat the base membrane with PI to modify the base membrane to prepare the coating membrane; the other is to use PI as the substrate membrane.

1. Modified diaphragm

PI coating on the base membrane can improve the thermal stability of the membrane. The base film can be made of polyolefin membranes such as PE, PP, PP/PE/PP, or non-woven fabrics such as ethylene phthalate (PET), polyethylene oxide (PEO), polyacrylonitrile (PAN) and cellulose. The form of PI coating on the base film can be particle, fiber or porous film. The introduced form can be polyamide acid (PAA) or PI, depending on the type of base film used. Jung-ki Park et al. [1] dissolved P84 in N, n-dimethyl formamide solvent and coated it on both sides of PE base film. After solvent volatilization, PI composite membrane was formed, and PI formed spherical particles on PE base film. Composite membrane in does not affect the diaphragm on the basis of the electrochemical performance of PE to improve the thermal stability of the diaphragm, the diaphragm can bear 140 ℃ high temperature. Xingxing Liang[2] et al. prepared PAA nanofiber membrane by electrospinning PAA solution, and then prepared PI porous membrane by thermal imination of PAA nanofiber membrane. Then, PI porous membrane was soaked in PEO solution and the composite membrane of PI/PEO was obtained after drying. Liu Jian [3] of SiO2 @ PI solution such as electrostatic spinning preparation of SiO2 @ PI film, configuration of ethyl cellulose (EC) and polyvinylpyrrolidone (PVP), casting solution soaked the PE film in the casting solution, washing in the water after PVP, both sides of the PE membrane formation porous membrane of EC, finally will SiO2 @ @ PI PI film, EC @ PE film, SiO2 @ hot-pressing preparation of sandwich PI PI membrane composite membrane, the composite diaphragm under 180 ℃ heat shrinkable to 0, good resistance to high temperature (figure 3). Chuan Shi et al. [4] reported that they mixed Al2O3 nanoparticles with PI to prepare casting film solution and coated it on one side of PE base film. PI can act as a binder to better bond ceramics on PE film, and the composite film shows good electrolyte infiltration, high temperature resistance and battery

2. New system diaphragm

PI is used as the base material alone in the lithium battery membrane. The most common is the nano fiber membrane prepared by electrostatic spinning, the porous membrane prepared by phase conversion or template method, and the PI porous membrane prepared by etching, sintering and other methods.

LiyunCao [5] and others is obtained by the method of electrostatic spinning preparation of nanometer fiber PI wiki non-woven can be used stability under 500 ℃ high temperature (figure 3), pore rate of 90%, the polarity of the electrolyte fluid absorption rate is high, low impedance, ratio performance is good, 5 c to charge and discharge capacity remain at a rate of 99.66% after 320 times. Ying, Wang [6] et al. Preparation of PAA and SiO2 into dope, electrostatic spinning preparation of PAA/SiO2 nano fiber membrane, and then get the PI of thermal/SiO2 porous membrane, porosity as high as 90%, electrolyte absorption rate is as high as 2400% (ordinary PP diaphragm liquid absorption rate of only 169%), which can withstand temperatures up to 250 ℃, show the good performance and cycle performance. Jaritphun Shayapat et al. [7] also prepared PAA/ SiO2 and PAA/ Al2O3 porous membranes by electrostatic spinning.

Baoku Zhu [8], etc the PAA solution with nonsolvent induced phase separation method preparation of PAA porous membrane, and then get the PI of thermal porous membrane, by controlling the film-forming condition, made aperture at about 0.5 microns, uniform distribution, the spongy PI porous membrane, the ionic conductivity of the porous membrane could reach 2.15 mS/cm, a 250%, fluid absorption rate under 180 ℃ heat shrinkable (figure 5). Xuyao Hu [9] and scatter SiO2 in PI NMP solution, and then will be allowed to dry mixture, with HF etching SiO2 get PI porous membrane, and compared with PP membrane found that PI porous membrane has no obvious under 180 ℃ heat shrinkable, improve the safety of the lithium ion battery.

【 conclusion 】

With the development of electronic information and new energy industry, higher requirements have been put forward for the performance of lithium ion batteries, especially the power batteries for new energy vehicles. As one of the four main materials of lithium ion battery diaphragm, it will directly affect the safety of the battery, its thickness, porosity, fluid absorption rate, chemical stability, electrostatic value will directly affect the electrical performance of the battery. Traditional polyolefin membrane has poor liquid absorption rate and high temperature resistance, so it is necessary to develop a new generation of membrane materials with good thermal stability and good electrolyte infiltration. The structure and performance of PI make it a lithium battery membrane and traditional PE, PP film has a great advantage. Dupont announced on August 4, 2010 that it has developed a polyimide nanofiber wiki separator for lithium ion batteries, which can improve battery power and life span by up to 15 to 30 percent. The self-supporting PI nano-fiber battery membrane prepared by jiangxi xiancai co., ltd. by electrostatic spinning has entered the pilot stage, which has the characteristics of high safety, high magnification and long service life.

Up to now, domestic and foreign research on PI diaphragm has achieved many stage results, but in addition to dupont, most of the PI diaphragm is still in the laboratory research stage, in addition to the lack of PI lithium battery diaphragm related production equipment, material cost is high, resulting in the domestic market PI lithium battery diaphragm is still a great gap. Therefore, polymer material enterprises need to find ways to reduce PI cost in monomer synthesis and polymerization methods. Diaphragm manufacturing enterprises and equipment processing enterprises cooperate with each other to achieve industrialized production of PI lithium battery diaphragm as soon as possible.

Reference

1.Jongchan Song, Myung-Hyun Ryou, Bongki Son et al. Co-polyie-coated polyethylene separators for enhanced thermal stability of lithium ion batteries. Electrochimica Acta, 2012, 85:524-530.

2.Xingxing Liang, Ying Yang, Xin Jin et al. Polyethylene oxide-coated electrospun polyie fibrous separator for high-performance lithium-ion battery, Journal of Materials Science &Technology, 2015.

3.Jian Liu, Yanbo Liu, Wenxiu Yang et al. Lithium ion battery separator with high performance and high safety enabled by tri-layered SiO2@PI/m-PE/ SiO2@PI nanofiber composite membrane. Journal of Power Sources, 2018, 396:265-275.

4.Chuan Shi, Jianhui Dai, Xiu Shen et al. A high-temperature stable ceramic-coated separator prepared with polyie binder/Al2O3 particles for lithium-ion batteries. Journal of Membrane Science, 2016, 517:91-99.

5.Liyun Cao, Ping An, Zhanwei Xu et al. Performance evaluation of electrospun polyie non-woven separators for high power lithium-ion batteries. Journal of Electroanalytical Chemistry, 2016, 767:34-39.

6.Ying Wang, Suqing Wang, Junqi Fang et al. A nano-silica modified polyie nanofiber separator with enhanced thermal and wetting properties for high safety lithium-ion batteries. Journal of Membrane Science, 2017, 537:248-254.

7.Jaritphun Shayapat, Ok Hee Chung, Jun Seo Park et al. Electrospun polyie-composite separator for lithum-ion batteries, Electrochimica Acta, 2015.

8.Hong Zhang, Chuner Lin, Mingyong Zhou et al. High thermal resistance polyie separators prepared via soluble precursor and non-solvent induced phase separation process for lithium ion batteries, Electrochimica Acta, 2016,187:125-133.

9.Xuyao Hu, Yaowu Wang, Tao Cui et al. Preparation of PI microporous membrane for lithium ion batteries, Advanced Materials Research, 2014, 834:104-107.