On the utilisation of the pseudo-capacitive capabilities of Li-ion cells for the provision of frequency response services

This work investigates the capacitive capabilities of Li-ion pouch and cylindrical cells in respect to the provision of Frequency Response services and a potential for reduction in battery ageing effects. This is achieved using Electrochemical Impedance Spectroscopy (EIS) and a novel method of identifying and defining the threshold frequency between pseudo-capacitive and diffusion processes of the cell. It is found that this threshold frequency is independent of current intensity up to 1 C, showing that even at high power, pseudo-capacitance has significant impact. However, a severe dependency upon relative cell surface area and State of Charge (SoC) is identified. Symmetrical charge-discharge pulses of up to 10 s utilise primarily cell capacitance. Literature indicates, that this level of utilisation reduces the electrochemical ageing impact significantly. This article displays a method to identify and isolate these processes for any given cell and to allow enhancement of conventional ageing modelling.