Separators in lithium-ion batteries

While a battery separator s materials are usually inert and do not influence electrical energy storage or output, its properties can have an important influence on safety. There are three types commonly used [46] (i) high-temperature sohd-polymer electrolytes (SPEs) such as poly(ethylene oxide) (PEO), (ii) microporous shutdown separators, which are composed of poly(ethylene) (PE) or laminates of poly(propylene) (PP) and PE, (iii) gel polymers such as poly(vinyhdene fluoride), PVdF, and (iv) ceramic separators. Table 27.2 shows the types of separators used in secondary Hthium-based batteries. 

Cells fabricated with PEO separators function at elevated temperature 75-80°C. They have been developed [47] and commercialized for telecommunications applications by Avestor [48] and now are proposed for electric vehicles [35]. 

Shutdown separators provide a level of protection from external shorting and overcharging in lithium-ion cells. PE separators have a shutdown temperature of 130 °C, at which temperature the impedance increases significantly. The impedance increase results from pore collapse that occurs at the softening point of the microporous polymer separator, and this can effectively stop ionic transport between 

Lithium-polymer (e.g., Li-VeOis) Polymer electrolyte Poly(ethylene oxide) with lithium salt 

poly(ethylene) PP, poly(propylene) and PVdF, poly(vinylidene difluoride). From Ref. [38]. 

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Table l. Commercially available microporous membrane materials used as separators in lithium-ion batteries. 

Separators in lithium ion batteries must separate positive electrodes and negative electrodes to prevent short circuits, and must allow passage of electrolytes or ions. Porous films and nonwoven fabrics of resins are known separators. The lithium ion battery separators are also required to exhibit stable properties at high temperatures such as in charging, and therefore high heat resistance is desired (21). 

A typical charge-discharge cycle is shown in Figure 7.14, and confirms the feasibility of the PAN-based gel electrolytes as separators in lithium-ion batteries by showing that the cell can indeed be cycled with a good capacity delivery. 

Microporous polymer membranes are the most commonly used separators in lithium-ion batteries. Majority of the microporous polymer membrane separators are based on semicrystalline polyolefin materials, such as polyethylene (PE), polypropylene (PP), PE-PP blends and high-density polyethylene (HDPE)-ultrahigh molecular polyethylene (UHMWPE). 

One-dimensional (1-D) nano-composite fibers (i.e. long fibers with a nano-scale diameter) have attracted considerable attention, owing to their potential applications in sensors, as antibacterial materials, in gas separation, in lithium-ion batteries, and as photocatalysts. - Electrospinning technology has proved to be a simple, efficient, and versatile way to fabricate nano-composite fibers. ... 

The purpose of this paper is to describe the various types of separators based on their applications in batteries and their chemical, mechanical and electrochemical properties, with particular emphasis on separators for lithium-ion batteries. The separator... 

Sony s Introduction of the rechargeable lithium-ion battery in the early 1990s precipitated a need for new separators that provided not only good mechanical and electrical properties but also added safety through a thermal shutdown mechanism. Although a variety of separators (e.g., cellulose, nonwoven fabric, etc.) have been used in different type of batteries, various studies on separators for lithium-ion batteries have been pursued in past few years as separators for lithium-ion batteries require different characteristics than separators used in conventional batteries. 

A novel microporous separator using polyolefins has been developed and used extensively in lithium-ion batteries since it is difficult for conventional separator materials to satisfy the characteristics required in lithium-ion batteries. In lithium-ion batteries two layers of separators are sandwiched between positive and negative electrodes and then spirally wound together in cylindrical and prismatic configurations. The pores of the separator are filled with ionically conductive liquid electrolyte. 

ENTER Membranes LLC has developed Teklon— a highly porous, ultrahigh molecular weight polyethylene separator for lithium-ion batteries. At the writing of this publication, the separator is available in small quantities. Pekala et al. characterized Celgard, Setela, and Teklon separators in terms of their physical, mechanical, and electrical properties. ... 

Asahi Chemical Industry carried out an exploratory investigation to determine the requirements for cellulose based separators for lithium-ion batteries. In an attempt to obtain an acceptable balance of lithium-ion conductivity, mechanical strength, and resistance to pinhole formation, they fabricated a composite separator (39—85 /cellulosic fibers (diameter 0.5—5.0 /pore diameter 10—200 nm) film. The fibers can reduce the possibility of separator meltdown under exposure to heat generated by overcharging or internal short-circuiting. The resistance of these films was equal to or lower than the conventional polyolefin-based microporous separators. The long-term cycling performance was also very comparable. 

In lithium-based cells, the essential function of battery separator is to prevent electronic contact, while enabling ionic transport between the positive and negative electrodes. It should be usable on highspeed winding machines and possess good shutdown properties. The most commonly used separators for primary lithium batteries are microporous polypropylene membranes. Microporous polyethylene and laminates of polypropylene and polyethylene are widely used in lithium-ion batteries. These materials are chemically and electrochemically stable in secondary lithium batteries. 

Figure 5. Scanning electron micrographs of Celgard 2325 (PP/PE/PP) separator used in lithium-ion batteries (a) surface SEM and (b) cross-section SEM.

Jeschke, S., Mutke, M., Jiang, Z., Alt, B.,WiemhFer, H.D., 2014. Study of carbamate-modified disiloxane in porous PVDF-HFP membranes newelectrolytes/separators for lithium-ion batteries. Chemphyschem 15,1761-1771. 

Saunier, AUoin, E, Sanchez, J.-Y., Maniguet, L, 2004. Plasticized microporous PVdF separators for lithium ions batteries. Part 111 gel properties and irreversible modifications of PolyfvinyUdene fluoride) membranes under swelling in liquid electrolyte. J. Polym. Sci. Part B 42,2308-2317. 

A fully charged battery spontaneously produces an electric current and, therefore, power when its positive and negative electrodes are connected in an electrical circuit. The positive electrode is called the anode, and the negative electrode is called the cathode. The materials used for the electrodes in lithium ion batteries are under intense development. Currently the anode material is graphite, a form of carbon, and the cathode is most frequently LiCo02, lithium cobalt oxide ( FIGURE 7.8). Between anode and cathode is a separator, a solid material that allows lithium ions, but not electrons, to pass through. 

Opposite electrodes in lithium ion batteries are separated by a porous polypropylene separator, particularly, of the Celgard material. 

There is as yet no consolidated opinion as to the optimum electrolyte for lithium-sulfiir batteries. Experiments with solid polymer electrolyte are described, but aprotic electrolyte in a Celgard-type separator commonly used in lithium ion batteries is applied more frequently. A large number of electrolytes has been studied that differ both in solvents and the lithium salt. The greatest acceptance was gained by lithium imide solutions in dioxolane (or in a mixture of dioxolane and dimethoxyethane) and also lithium perchlorate solutions in sulfone. Dissolution of polysulfides in electrolyfe is accompanied by a noticeable increase in viscosity and specific resistance of electrolyte. It is the great complexity of the composition of the electrochemical system and that of the processes occurring therein that prevent as yet commercialization of lithium-sulfiir electrolytes. 

Fig. 20.10 Scanning electron micrographs of separators made by wet process and used in lithium-ion batteries (a) Setela (Tonen), (b) Hipore-1 (Asahi), (c) Hipore-2 (AsaM), and (d) Teklon (Entek). Reprinted with permission from Chem. Rev. 104 (2004) 4410-4462, copyright (2004), American Chemical Society...

Shirai, H., Spotnitz, R., Atsushi, A. Characterization of Separators for Lithium Ion Batteries -A Review, Chemical Industry, 48 (1997) 47 (in Japanese)... 

Polymer electrolytes (e.g., poly(ethylene oxide), poly(propylene oxide)) have attracted considerable attention for batteries in recent years. These polymers form complexes with a variety of alkali metal salts to produce ionic conductors that serve as solid electrolytes. Its use in batteries is still limited due to poor electrode/ electrolyte interface and poor room temperature ionic conductivity. Due to its rigid structure it can also serve as the separator. Polymer electrolytes are discussed briefly in the section Separators for Lithium-Ion Batteries. 

In recent years there has been a strong demand for higher-capacity lithium-ion cells because of the strong growth in portable electronics. One way to achieve higher capacity is by reducing the thickness of separators. At present, battery manufacturers routinely use separators 16 pm or thinner in higher-capacity (>2.6 Ah) cylindrical cells and 9 pm separators in lithium-ion gel polymer cells. 

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