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Progress in Separators for Rechargeable Batteries

Cheng-song Yang, Dian-hui Han and Meng Zhang*

School of Materials Science and Engineering, Henan University of Technology, Zhengzhou, Henan, China

Abstract

This article introduces the research progress of rechargeable battery separators. At present, rechargeable battery separators are mainly divided into polyolefin-based separators, non-woven separators, and ceramic composite separators. In recent years, separators have been adopted on the basis of these three categories. Different preparation methods and the replacement of the separator's role have led to some new types of separators, such as polymer electrolyte separators. It is divided into solid polymer electrolytes and gel polymer electrolytes. This electrolyte can separate the cathode and anode to prevent short circuits, while ensuring lithium Ions can shuttle. This article details the development of rechargeable battery separators, including some new ones in recent years.

Keywords: Rechargeable battery separators, polyolefin-based separators, non-woven fabric separators, composite separators, solid electrolyte separators, electrolyte separators, gel polymer separators

1.1 Separator Overview

The separator is an important part of the battery. It separates the positive electrode and negative electrode in the battery, prevents the positive and negative electrodes from directly contact which may cause short circuit, and has a porous structure to provide a passage for the lithium ions, realizing lithium ions transport between the positive and negative electrodes. The separator itself does not participate in the reaction of the battery, but it plays a vital role in the battery. The separator affects the battery capacity, rate performance, cycle performance, and safety performance to a certain extent. Currently, polyolefin separators (PP, PE) and non-woven fabric separators are widely used commercially.

The main factors of the separator include chemical stability, mechanical strength, porosity, wettability, and heat resistance. Considering the above factors to select the appropriate separator material, the main diaphragm materials of lithium-ion batteries are polyolefin separators, non-woven membranes, and ceramic composite separators. Through the analysis of existing diaphragm materials, the new ideas of improving separator performance are developed.

1.2 Polymer Membrane

1.2.1 Polyolefin Separators

In polyolefin separators, the main materials are polyethylene and polypropylene, which have low cost, good chemical stability, excellent mechanical properties, and high electrochemical insulation. At higher temperatures, the holes in the diaphragm will self-close and form an open circuit, thus ensuring the safety performance of the secondary battery.

Polyethylene (PE) microporous films began in the early 1960s, and both melt-stretching (dry method) and thermally induced phase separation (wet method) methods were produced. Polypropylene (PP) microporous membrane research began in the early 1970s and was mainly produced by melt drawing [1].

The main preparation processes of the melt-spinning-cold stretching method include melt extrusion, heat treatment, and stretching. The microstructure of the microporous polyolefin membranes made by dry process is shown in Figure 1.1. In the process of melt extrusion, under the effect of large stress field, a hard elastic precursor membrane with a lamellar crystal structure perpendicular to the extrusion direction is obtained. During the heat treatment, under the effect of high-stress field at a temperature slightly lower than the melting point, annealing is performed to increase crystallinity, and then, heat treatment can obtain a hard elastic membrane [2]. Finally, the hard elastic membrane is stretched to separate the lamellar crystal, and the amorphous region is destroyed to form a large number of microporous structures. Stretching is divided into two processes, firstly cold stretching, then hot stretching at a small heating rate, and finally heat setting at a certain temperature. According to the stretching method, the melt stretching method can be classified into uniaxial stretching and biaxial stretching. The method has low cost and uniform pore size, but the product is slightly thick, easy to tear, and has a high short circuit rate [3].

Figure 1.1 Microstructure of the microporous polyolefin membranes made by dry process.

Reproduced with permission from ref [5] and American Chemical Society.

The main steps of the thermally induced phase separation process include extrusion, stretching, extraction, and heat setting. The microstructure of the microporous polyolefin membranes made by wet process is shown in Figure 1.2. It is mainly used to prepare PE separators. At a temperature above the melting point of the crystalline polymer mixing PE with a high boiling point, low molecular weight diluting agent to form a homogeneous melt. The melt is pre-formed into a membrane. When the temperature is lowered, the solubility of the diluting agent decreases, and the polymer crystallizes. At this time, solid-liquid or liquid-liquid phase separation occurs. After cooling and stretching, extract with a volatile extractant, and finally, the extractant is removed to obtain a microporous separator [4]. This method produces a thin diaphragm, is not easy to tears, and has a short circuit rate, but the cost is high, the environment is polluted, and the heat resistance of the separator is poor [3].

Polyethylene and polypropylene have their own advantages and disadvantages, polyethylene is more resistant to low temperature, and polypropylene is more resistant to high temperature. The self-closed holes temperature of polyethylene is 135°C, and this date of polypropylene is 165°C. Celgard PP/PE/PP three-layer composite diaphragm, PE intermediate layer will be self-closed at 135°C. However, there is a 30°C heating space from the closed hole to the separator damage, which improves the safety of the separator [5].

Figure 1.2 Microstructure of the microporous polyolefin membranes made by wet process.

Reproduced with permission from ref [5] and American Chemical Society.

There are many modification methods for PE and PP separators, the most common of which are coating modification, coating inorganic nanoparticles, or polymer particles for modification.

Zhang [6] et al. coated PTFE particles to both sides of the PE separator with a diluted PTFE suspension, in order to prepare a selfbonding PTFE separator, then modified it with a H2O2/H2SO4 solution, the hydroxyl group is introduced, this structure has a porosity of 66%, and the electrolyte absorption rate is 190.6%. The ionic conductivity is much higher than PE separator. The PTFE particles provide good thermal stability and excellent cycle stability.

Won-Kyung Shin [7] et al. use ultrathin nitrogen and sulfur codoped graphene (NSG) layer deposited on a polyethylene (PE) separator by a simple vacuum infiltration method effectively suppressed the dendritic growth of lithium metal, compared to an uncoated separator. The thermal stability is improved, and the cycle stability of the lithium battery is effectively improved.

Zhou Xiangyang [8] et al. coated nitrogen-doped microporous carbon from polyaspartic acid bonding on the surface of Celgard 2400, used on Li-S battery. This method is easy to prepare and low in cost; high N doping level promotes chemisorption of polysulfide and improves overall performance of Li-S batteries.

1.2.2 PVDF

Poly(vinylidene fluoride) (PVDF) combines the characteristics of fluororesin and general-purpose resin. The structure of PVDF is shown in Figure 1.3. It has good chemical resistance, high temperature resistance, electrical insulation, and dielectric properties. It is very suitable as a separator material. Its molecular chain is closely arranged, and there are strong hydrogen bonds between the molecular chains. Its oxygen index is 46%, non-combustible, crystallinity is 65%~78%, melting point is 172°C, heat distortion temperature is 112°C~145°C, and the decomposition temperature is greater than 390°C the long-term use temperature is -40°C to 150°C. The thermal decomposition temperature is much higher than the melting point, making it excellent in processability.

PVDF itself has high crystallinity and excellent mechanical properties, but high crystallinity will affect the movement of molecular segment, making it less swellable in electrolyte solution, and poor wettability, lead to large internal resistance. Based on this, the other monomers are added to copolymerized, PVDF-HFP was prepared by copolymerization of hexafluoropropylene monomer and vinylidene fluoride [9]. It can reduce crystallinity, thereby improving ion conductivity, reducing internal resistance, and improving battery performance. In the study of RE-Sousa [10] et al., PVDF-CTFE separator was prepared by phase transfer in a DMF solution by adding chlorotrifluoroethylene and vinylidene fluoride, which has good cycle performance and rate performance. PVDF diaphragm is easy to get out of control at high temperatures, so it has certain safety problems. In the study of Cui [11] et al., PE microspheres were prepared and coated on the surface of PVDF separator to prepare a separator with thermal shutdown coating. The coating will not adversely influence the...