Battery separators pore size

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. 

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

The prime requirements for the separators in alkaline storage batteries are on the one hand to maintain durably the distance between the electrodes, and on the other to permit the ionic current flow in as unhindered a manner as possible. Since the electrolyte participates only indirectly in the electrochemical reactions, and serves mainly as ion-transport medium, no excess of electrolyte is required, i.e., the electrodes can be spaced closely together in order not to suffer unnecessary power loss through additional electrolyte resistance. The separator is generally flat, without ribs. It has to be sufficiently absorbent and it also has to retain the electrolyte by capillary forces. The porosity should be at a maximum to keep the electrical resistance low (see Sec. 9.1.2.3) the pore size is governed by the risk of electronic shorts. For systems where the electrode substance... 

Once in an operational battery, the separator should be physically and chemically stable to the electrochemical environment inside the cell. The separator should prevent migration of particles between electrodes, so the effective pore size should be less than 1pm. Typically, a Li-ion battery might be used at a C rate, which corresponds to 1-3 mAcm2, depending on electrode area the electrical resistivity of the separator should not limit battery performance under any conditions. 

Pore Size. A key requirement of separators for lithium batteries is that their pores be small enough to prevent dendritic lithium penetration through them. Membranes with submicrometer pore sizes have proven adequate for lithium batteries. 

Separators are characterized by structural and functional properties the former describes what they are and the latter how they perform. The structural properties include chemical (molecular) and microcrystalline nature, thickness, pore size, pore size distribution, porosity, and various chemical and physical properties such as chemical stability, and electrolyte uptake. The functional properties of interest are electrical resistivity, permeability, and transport number. It is useful to characterize separator materials in terms of their structural and functional properties and to establish a correlation of these properties with their performance in batteries. A variety of techniques are used to evaluate separators. Some of these techniques are discussed in this section. 

This feature will be increasingly Important as battery manufacturers continue to increase the cell capacity with thinner separators. The pore structure is usually influenced by polymer composition, and stretching conditions, such as drawing temperature, drawing speed, and draw ratio. In the wet process, the separators produced by the process of drawing after extraction (as claimed by Asahi Chemical and Mitsui Chemical) are found to have much larger pore size (0.24—0.34 fixxi) and wider pore size distribution than those produced by the process of extraction (0.1—0.13 after drawing (as claimed by Tonen). ... 

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. 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

Within the framework of a project for the optimization of batteries for storing photovoltaic and wind energy, an AGM separator with an inorganic filler of high surface-area was developed. The surface area was found to increase and the pore size to decrease significantly with incorporation of the filler, but the tensile strength suffered, see Table 7.13 [32]. With this separator (regardless of how much filler was... 

Additional cycle tests, with both separator versions under slightly modified conditions, confirmed these results, although with slightly better results for the separator with the lower pore size, see Fig. 7.6. The cycle tests were stopped after 80 cycles with 42% of the initial capacity, and after 110 cycles with 47% of the initial capacity, for the two types of ceramic separators, respectively. Although no shorts were found in these batteries, a considerable amount of softening of the positive active-material was evident. These results underline the need for a separator with small pore size (preferably microporous) in order to prevent shorts and the expansion of the active material into the separator. 

The essential attributes of the ideal material for use as a separator in VRLA cells are now well understood. A wide range of materials and material combinations has been evaluated for this purpose, and it has emerged that materials with small pore size (high surface area) and very little compressibility show good prospects for extending the cycle-life of VRLA batteries. 

However, a resin having a thermal deformation temperature of 230 °C or higher, such as poly(phenylene sulfide), poly(ethylene terephthalate), poly(amide), poly(imide), etc., may also be used. The pore size of the separator is set in a range generally used in the batteries, i.e., pores of 0.01-10 xm. 

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