Separators ionic resistance

The typical properties of some commercial microporous membranes are summarized in Table 4. Celgard 2730 and Celgard 2400 are single layer PE and PP separators, respectively, while Celgard 2320 and 2325 are trilayer separators of 20 and 25 fim thickness. Asahi and Tonen separators are single layer PE separators made by the wet process. Basic properties, such as thickness, gurley, porosity, melt temperature, and ionic resistivity are reported in Table 4. These properties are defined in section 6.1.3. 

The lithium-ion cells have demonstrated power loss when aged and/or cycled at high temperatures. Norin et al." demonstrated that the separator is at least partly responsible for the power loss due to the intrinsic increase in its ionic resistance. They showed that impedance increased significantly upon cycling and/or aging of lithium-ion cells at elevated temperatures and that separators accounts for 15% of the total cell impedance rise. They later reported that the loss in ionic conductivity of the separator was due to blocking of the separator pores with the products formed due to electrolyte decomposition, which was significantly accelerated at elevated temperatures. 

Lithium polymer electrolytes formed by dissolving a lithium salt LiX (where X is preferably a large soft anion) in poly(ethylene oxide) PEO can find useful application as separators in lithium rechargeable polymer batteries.Thin films must be used due to the relatively high ionic resistivity of these polymers. For example, the lithium-ion conductivity of PEO—Li salt complexes at 100 °C is still only about Viooth the conductivity of a typical aqueous solution. 

Martino et al. have demonstrated the use of BN felt separators in engineering tests. They have high porosity ( 90%), and hence, low ionic resistance, in addition to excellent compatibility with other cell materials at the operating temperature of 470 °C. However, this separator is too expensive and has poor mechanical properties and so cannot prevent electrode shape change during cell operation. ... 

The ideal battery separator would be infinitesimally thin, offer no resistance to ionic transport in electrolytes, provide infinite resistance to electronic conductivity for isolation of electrodes, be highly tortuous to prevent dendritic growths, and be inert to chemical reactions. Unfortunately, in the real world the ideal case does not exist. Real world separators are electronically insulating membranes whose ionic resistivity is brought to the desired range by manipulating the membranes thickness and porosity. 

The DLC self-discharge performance is the result of a compromise with its power capability. The manufacturers could use a thicker separator to improve the voltage retention but this operation would increase in the same time the series ionic resistance. 

According to Fig. 8 and the discussion given above, electronic and ionic resistances have to be separated and specified for the special reaction. 

The measurement of separator resistance is very important to the art of battery manufacture because of the influence the separator has on electrical performance. Electrical resistance is a more comprehensive measure of permeability than the Gurley number in that the measurement is carried out in the actual electrolyte solution. The ionic resistivity of the porous membrane is essentially the resistivity of the electrolyte that is embedded in the pores of the separator. Typically, a micropo-rous separator, immersed in an electrolyte, has an electrical resistivity about six to seven times that of a comparable volume of electrolyte, which it displaces. It is a function of the membrane s porosity, tortuosity, the resistivity of the electrolyte, the thickness of the membrane, and the extent to which the electrolyte wets the pores of the membrane.The ER of the separator is the true performance indicator of the cell. It describes a predictable voltage loss within the ceU during discharge and allows one to estimate rate limitations. 

Another class of interesting materials for separator is composed of nonwoven films, textile products processed directly from fibers that are bonded together. The fibrous structure of nonwoven materials offers a high porosity, which is necessary for high electrolyte absorbance and low ionic resistance, excellent thermal properties, combined with cost competitiveness. However, the excessively large pore size and broad pore size distribution of conventional nonwovens, which may provoke... 

An electrochemical reaction involves processes such as mass transport, charge separation, charge transfer, electronic and ionic resistances, and so on. These processes often have different time constants and they proceed on different time scales. The impact of these processes on the voltage and current measured in a direct current (DC) experiment is combined and convoluted. 

Electrochemical impedance spectroscopy (EIS) has been used to characterize the ionic conductivity of conducting polymers in the electrode, separating this ionic resistance from the membrane and ohmic contact resistances at the interfaces [24— 31]. Such measurements are helpful in quantifying degradation of the ionomer in... 

DuPont nanofiber-based separators go beyond the hmit of traditional nonwovens and microporous membranes to provide thinner separators with lower ionic resistance and higher temperature stability for use in wide range of energy storage devices. Made by a proprietary spinning process, DuPont nanofiber sheets contain continuous polymeric filaments in a thin, uniform web of randomly oriented nanofibers that are ideal for separators. The innovative manufacturing process dehvers a uniform web at high... 

DuPont polyimide separators are thin, offer lower ionic resistance when filled with electrolyte, are made with higher-temperature stable materials that offer very low shrinkage at high temperatures, and offer very good wettability with typical organic electrolytes used in batteries and capacitor applications. 

The lithium ion cells made with DuPont polyimide separators also offer improved power capability when compared to polyolefin-based separators. The improved rate capability can be attributed to lower ionic resistance, which in turn is related with separator thickness and porosity. Higher-porosity, lower-thickness polyimide separators can lead to more than 30% higher capacity at 15C rates, as shown in Fig 11.11. The tests were performed in coin cells with graphitic anode, LiCo02 cathode, and organic electrolyte (IM LiPFe with 30 70 EC/EMC). 

NKK has approximately 15 types of ceUulose-based separators for supercapacitors and batteries. Cellulose-based separators have higher thermal stability than their polyolefin counterparts. An example version of an NKK cellulose-based film (used as a battery separator or as a separator for EDLCs (electric double layer capacitors)) has features such as 35—45 pm thickness, 14.5 basis weight (gsm), 66% porosity (mercury poros-imetry), and ionic resistance of 0.58 (ohms-cm ) (Fig. 11.15). " ... 

Figure 17-1. Water photolysis by Ti02 Pt electrode (1) n-type TiOz electrode, (2) platinum black counter electrode, (3) ionically conducting separator, (4) gas buret, (5) load resistence, and (6) voltmeter.

The battery separator, by definition, consists of a porous nonconductive material that allows ionic current to pass through it. The ionic resistance or the electrical resistance of the separator is defined as the total resistance of the separator filled with electrolyte minus the resistance of a layer of electrolyte of equal thickness ... 

There is another important factor that strongly affects the performance or rate capability of a cell, the internal impedance of the cell. It causes a voltage drop during operation, which also consumes part of the useful energy as waste heat. The voltage drop due to internal impedance is usually referred to as ohmic polarization or IR drop and is proportional to the current drawn from the system The total internal impedance of a cell is the sum of the ionic resistance of the electrolyte (within the separator and the porous electrodes), the electronic resistances of the active mass, the current collectors and electrical tabs of both elec-... 

The microporous polypropylene membrane, typically Celgard (Celgard 3400, manufactured by Celgard LLC, Charlotte, NC, 28273), ° is utilized as the gas barrier while at the same time it offers minimum ionic resistance. This thin gas barrier, which becomes relatively soft when wetted, is frequently placed between two layers of the eloth separator and receives significant mechanical support from them. Substantial improvements have been made in recent years to the toughness of the plastic membrane gas barrier. 

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