Resistance of a separator

Usually the electrical resistance of a separator is quoted in relation to area in the above case it is 57 mil cm2. In order to quote it for other areas, due to the parallel connection of individual separator areas, Kirchhoff s law has to be taken into account ... 

With increasing tortuosity factor T and lower porosity P, R increases sharply. The electrical resistance of a separator is proportional to the thickness d of the membrane and is subject to the same dependence on temperature or concentration as... 

The resistance of a separator is stated in reference to the related area. Therefore, the above case would be stated as 46 mQ cm. Thus far, our consideration for electrical resistance has taken into account the porosity and the tortuosity of the separator and this has yielded a value that can be validated by independent measurement either between electrodes or with specific laboratory equipment such as the Palico electrical resistance tester [7]. 

It is recommended to test eacli phase to ground separately with the other phases also grounded. This is because the insulation resistance of a complete winding to ground does not provide a check of the insulation condition between the windings. 

See 2-3.1. Electrical conduction through solids takes place both through the bulk material and over the surface. In most cases surfaces have different physical and chemical properties than the bulk, for example due to contamination or moisture. Volume and surface resistivity can be separately measured for solid materials such as antistatic plastic sheet. Powders represent a special case since although both surface and bulk conduction occur, their contributions cannot be individually measured and the volume or bulk resistivity of a powder includes surface effects. 

The vertex of a separation region points out the better operating conditions, since it is the point where the purity criteria are fulfilled with a higher feed flow rate (and so lower eluent flow rate). Hence, in the operating conditions specified by the vertex point, both solvent consumption and adsorbent productivity are optimized. Comparing the vertex points obtained for the two values of mass transfer coefficient, we conclude that the mass transfer resistance influences the better SMB operating conditions. Moreover, this influence is emphasized when a higher purity requirement is desired [28]. 

The electrical resistance exerted by a separator on the ionic current is defined as the total resistance of the separator filled with electrolyte minus the resistance of a layer of electrolyte of equal thickness, but without the separator. The separator resistance has to be considered as an increment over the electrolyte resistance. 

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. 

Underwriters Laboratories (UL) requires that consumer batteries pass a number of safety tests [3]. UL requires that a battery withstand a short circuit without fire or explosion. A positive temperature coefficient (PTC) device [4] is used for external short-circuit protection. The resistance of a PTC placed in series with the cell increases by orders of magnitude at high currents and resulting high temperatures. However, in the case of an internal short, e.g., if the positive tab comes lose and contacts the interior of the negative metal can, the separator could act as a fuse. That is, the impedance of the separator increases by two to three orders of magnitude due to an increase in cell temperature. 

Ideally, separators would present no resistance to ion transport. In practice, some resistance must be tolerated. Still, the resistance of the separator is usually insignificant relative to the transport limitations in the electrodes. Separator permeability is typically characterized by air permeability. The Gurley number expresses the time required for a specific amount of air to pass through a specific area of separator under a specific pressure (e.g., 10 mL through 1 in2 (6.45 cm2) at 2.3 cm Hg). This measurement depends on porosity, pore size, thickness, and tortuosity according to Eq. (1) [17] ... 

Since water as solvent plays the role of a medium where electrolytic displacements take place we shall be able to state for sure, together with Wiedemann, Beetz and Quincke, that the electrical resistance of a solution consists of resistances to movement enforced upon the components of the solution by water particles, by the components themselves and perhaps by the undecomposed molecules of the electrolyte. To separate these various hindrances will be no easy task, particularly because, as stressed by Quincke, they are not necessarily constant but can, for example, depend on the condition of the solution. Even from this standpoint the process of conduction will be, in general, still very complicated. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

Permeability. The separators should not limit the electrical performance of the battery under normal conditions. Typically the presence of separator increases the effective resistivity of the electrolyte by a factor of 6—7. The ratio of the resistivity of the separator filled with electrolyte divided by the resistivity of the electrolyte alone is called MacMullin number. MacMullin numbers are as high as 10—12 have been used in consumer cells. 

Shimizu et al. has developed a low-cost MgO porous particle separator, which possessed both the high porosity ( 85%) of the BN felt and the high mechanical resistance of powder separators. The separator was prepared by loosely sintering fine MgO powder with Mg(NOs)2 as the binder. The particles thus formed showed excellent performance as separators in Li—Al/FeS r cells. 

The aqueous batteries use water based electrolytes (e.g., KOH electrolyte for NiCd and NiMH and H2-SO4 electrolyte for lead acid), which are less resistive then nonaqueous electrolytes. Polyolefin materials are generally suitable for use in the manufacture of separators for these batteries, but they are not inherently wettable by aqueous electrolytes. Such electrolytes are therefore unable to penetrate the pores of a separator formed from such a material, so that ion migration through the pores in solution will not occur without modification. This problem is sometimes overcome by treating the polyolefin material with a surfactant, which allows an aqueous electrolyte to wet the material. However, such surfactant can be removed from the surfaces of the polyolefin material when electrolyte is lost from the device, for example during charging and discharging cycles, and it is not subsequently replaced on the material when the electrolyte is replenished. 

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