Resistance, separators ionic/electrical

The Rb based on the sample cannot be calculated correctly, since the electric charge transfer resistance and the electric double layer in an electrode interface are also detected as a resistance, even if bias voltage is impressed to the measurement cell in order to measure the ionic conductivity. For the ionic conductivity measurement, a dc four-probe method, or the complex-impedance method, is used to separate sample bulk and electrode interface [4]. In particular, the complex-impedance method has the advantage that it can be performed with both nonblocking electrodes (the same element for carrier ion and metal M) and blocking electrodes (usually platinum and stainless steel were used where charge cannot be transferred between the electrode and carrier ions). The two-probe cell, where the sample is sandwiched between two pohshed and washed parallel flat electrodes, is used in the ionic conductivity measurement by complex-impedance method as shown in Figure 6.1. 

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 ... 

In preparation for the CCA test, the battery is taken off charge and placed in a freezer at -19°C for 24 h to ensure uniformity of the core temperature. After this period, a fair amount of gas can be liberated from the battery. The gases that are attached to surfaces along the ionic flow path, such as plate and separator surfaces, are of concern. Because gas bubbles are infinitely resistant to ionic flow, the surface area occupied by the gas will essentially be blinded and cause an increase in overall battery resistance. To account for the influence of entrapped gas, it is therefore appropriate to think of the functional resistance instead of measured or calculated electrical resistance [8] ... 

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... 

Alternatively, an equally powerful visualization of impedance data involves Bode analysis. In this case, the magnitude of the impedance and the phase shift are plotted separately as functions of the frequency of the perturbation. This approach was developed to analyze electric circuits in terms of critical resistive and capacitive elements. A similar approach is taken in impedance spectroscopy, and impedance responses of materials are interpreted in terms of equivalent electric circuits. The individual components of the equivalent circuit are further interpreted in terms of phemonenological responses such as ionic conductivity, dielectric behavior, relaxation times, mobility, and diffusion. 

A simple electrical model of the solution as seen by the electrodes is shown in Fig. 3. Resistances are primarily determined by the ionic content of the electrolyte solution, which is typically in the 0.01-0.10 M range. When the electrodes are placed in an electrolyte solution, a charge separation or double layer forms spontaneously at the electrode-solution interface. This produces a capacitance, commonly referred to as the double-layer capacitance, Cdl- The size of this capacitance is a function of electrode area, electrode-to-solution potential, electrode material, and the ion concentration in solution. The potentiostat compensates only for capacitance, impedance, and resistance (Cdla, Rc, Zfa) within its control loop and allows these to be largely ignored experimentally. Therefore, the uncompensated resistor, R , which is outside the loop, causes an error in the working electrode potential, as a result. However, given the very small amplitude currents usually involved... 

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. 

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