Battery separators requirements

The great quantity of very fine fibers in a meltblown web creates several unique properties such as large surface areas and small (<1 fiva) pore sizes. These have been used in creating new stmctures for hospital gowns, sterile wrap, incontinence devices, oil spill absorbers, battery separators, and special requirement filters. It is expected that much innovation will continue in the design of composite stmctures containing meltblown webs. 

The term leaf separator characterizes the customary stiff version of a starter battery separator that can be inserted individually between the electrodes on automatic stackers, in contrast to pocket separators. This processing requires considerably higher bending stiffness than for pocket separators, calling for thicker backwebs, typically 0.4-0.6 mm (Fig. 18 and 19). 

It can be stated generally that requirements for traction battery separators in respect to mechanical properties and chemical stability are considerably higher than for starter battery separators. This is due to the fact that a forklift battery is typically... 

The requirements for a battery separator can best be understood in the context of how the separator is used. The conventional process (Fig. 1) for making spirally wound cells involves threading the separator (a) through a winding pin (b). 

The general requirements for lithium-ion battery separators are given below. 

Porosity. It is implicit in the permeability requirement typically lithium-ion battery separators have a porosity of 40%. Control of porosity is very important for battery separators. Specification of percent porosity is commonly an integral part of separator acceptance criteria. 

Table 5. General Requirements for Lithium-ion Battery Separator ...

The testing of battery separators and control of their pore characteristics are important requirements for proper functioning of batteries. Mercury porosim-etry has been historically used to characterize the separators in terms of percentage porosity, mean pore size and pore size distribution. In this method, the size and volume of pores in a material are measured by determining the quantity of mercury, which can be forced into the pores at increasing pressure. Mercury does not wet most materials, and a force must be applied to overcome the surface tension forces opposing entry into the pores. 

Development efforts are under way to displace the use of microporous membranes as battery separators and instead use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by eliminating organic volatile solvents. The next two sections are devoted to solid polymer and gel polymer type lithium-ion cells with focus on their separator/electrolyte requirements. 

As the separator requirements for most of the above batteries are very similar, they will not be dealt with in detail. In this section, we will generally describe the separator requirements followed by a brief discussion on few selected systems. 

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). 

Batteries that require a liquid electrolyte are called wet batteries. Corrosive battery fluid refers to either acid electrolytes syn. battery acid, like the common lead-acid automobile battery which uses a solution of sulphuric acid, or alkali electrolytes syn. alkaline corrosive battery fluid, like potassium hydroxide (1310-58-3) solutions in nickel-cadmium and other alkaline battery systems. Dry batteries or dry cells, like all primary batteries, use electrolytes immobilized in pastes, gels, or absorbed into separator materials. Some batteries are loaded with a dry, solid chemical (e.g., potassium hydroxide) which is diluted with water to become a liquid electrolyte. The hazards associated with handling and transportation prior to use are thereby reduced. 

The purpose of this chapter is to provide a detailed review of separators used in Li-Ion battery applications and their chemical, mechanical, and electrochemical properties. The separator requirements, properties, and characterization techniques are also described with respect to Li-Ion batteries. Despite the widespread use of separators, a great need still exists for improving the performance, increasing its life, and reducing cost. In the following Sections an attempt is made to discuss key issues in various separators with the hope of bringing into focus present and future directions of research and development in separator technologies. 

The separator should form a good interface with the electrodes to provide sufficient electrolyte flow. In addition to the above properties, the separator must be essentially free of any type of defects (pinholes, gels, wrinkles, contaminants, etc.). All of the above properties have to be optimized before a membrane qualifies as a separator for a Li-Ion battery. The general requirements for Lithium-Ion battery separators are summarized in Table 20.5. 

One way to achieve some of these goals will be to develop mathematical models that reflect the effects of separator resistance, thickness, pore size, shrinkage, tortuosity, and mechanical strength on the final performance and safety of batteries. The battery separators for tomorrow will demand more than just good insulation and mechanical filtration they will require unique electrochemical properties. 

Some applications nonrelated to the properties of the nanoporous materials but to their porous structures are their use as filtration membranes, battery separators (hindering the diffusion of ions in the narrow channels), and catalyst supports (due to their high surface area) as well as gas capture and storage or light harvesting [72]. However, the common factor of all of these applications is the requirement of an open nanoporous structure not only inside the sample but also connected to the exterior of the sample. However, the CO2 foaming process from nanostructured polymers still has not allowed obtaining nanoporous samples with all of these features. Pinto et al. [102] proposed that 25/75 PMMA/MAM nanoporous foams present appropriate inner porous structures for these kinds of applications (bicontinuous nanoporous structures with tunable pore size), but further studies are required to connect effectively this inner porous structure with the exterior of the sample. 

At open circuit, electrode reactions that charge the electrodes lead to a slow oxidation of the electrolyte with H2 evolution at the anode and O2 evolution at the cathode. These reactions represent an irreversible self-discharge. Once the electrolyte is introduced, the battery has a poor shelf life. Under development are acidic aqueous electrolytes in which Pb(II) is soluble rather than condensing into the solid PbS04. This development of the lead-acid cell promises a flow battery not requiring a separation membrane. The separation membrane of redox-flow batteries (see last section) remains a challenging problem for the aqueous redox-flow technology. 

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 high-speed 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 [85]. These materials are chemically and electrochemically stable in secondary lithium batteries. A key requirement for the separators for lithium primary batteries is that their pore size be small enough to prevent dendritic lithium penetration through them. 

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