Admittance Spectroscopy of Sealed Secondary Batteries
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The admittance of sintered NiCd alkaline and Pb-acid-sealed batteries was investigated in the frequency range 0.01 to 10,000 Hz at 253, 293, and 323 K, with small signals of 1-10 mV amplitude under potentiostatic control to keep the direct current to a minimum value. Fully charged and fully discharged states of these batteries were investigated, the latter corresponding to relatively slow and fast discharges. The spectra of the real and imaginary components of the complex capacitance C(omega) are shown as functions of frequency, and equivalent circuits are derived using as circuit elements resistors and universal dispersive capacitors obeying the power law frequency dependence Cn(omega) = An(i omega)exp n-1 with n in the range 0,1. The results are discussed in terms of known electrochemical reactions in these two types of batteries and assignments are made of the bulk eletrolyte resistance, the interfacial electrochemical reaction, and the transport of ionic species through the pores of the nonconducting matter surrounding the electrodes, as well as a structural inductance term. The analysis shows clear evidence of dispersive processes but no trace of direct current conduction, at least down to 0.01 Hz, which is rather surprising. The results open a number of fundamental questions relatingmore » to the processes governing the dynamic response of secondary batteries which are at present not well understood. 19 references.« less