Electrochemical-Potential Spectroscopy EPS

The voltage of a lithium intercalation battery varies with its state of discharge, i.e., the intercalant composition x. Subsequently more careful experiments have shown fine structure in V(x) for many intercalation system [4, 5, 7, 10] clearly observed in plots of dxIdV vs. x or V, which can be caused by a variety of physical mechanisms such as the interactions between intercalated atoms within the host or intercalation-induced stmctural phase transitions in the host. Therefore, careful measurements of dx/dV can be used a study the physics and chemistry of the intercalation process. 

Experimental details of EPS are given in ref. [7]. Let us describe the basic considerations as follows with dQ = I dt and dQ = Qodx. When a cell based on 

In this equation, m is the mass of the active element of the electrode, and is its molar mass (we assume that there is one Li ion per chemical formula), and F is the Faraday s constant. By monitoring the cell voltage as a function of time, one can obtain dxjdV since  

With measurement of dx/dV at high resolution in V andx, we monitor the voltage of the cell as a function of time with a computer-controlled voltmeter. In the standard terminology, this method of measuring dxIdV is called derivative constant 

From Eqs. (13.4) and (13.6), we assume that the integral of the current can be directly related to the change of composition that occurs within the insertion electrode. By examining the derivative quantity, dxIdV, measured at different current values, one can usually determine the effects of the finite rate and obtain the equilibrium dxIdV. The same studies can also be made as a function of voltage sweep rate using linear sweep voltammetry (LSV). 

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Fig. 13.2 Schematic illustration of the voltage-stepped cycling method, electrochemical-potential spectroscopy (EPS) and the corresponding current passing the cell...

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