Abstract
Abstract 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.
Highlights
Mandatory and Firm Frequency Response (MFR and FFR) are services demanded and requested by National Grid, which balance demand and generation on a permanent basis in the Great British power grid [1]
This paper investigates the capacitive capabilities of NMC-LMO (Lithium-Manganese-Oxide), LFP (Lithium-Iron-Phosphate) and NMC cells. It aims to define a simpler and accurate method of identifying the threshold between capacitive and diffusion processes within a cell, to determine general characteristics and dependencies of this threshold and to connect these to the conditions of frequency response and the results found in [17]
Type D is an exception with an average Capacitance-Diffusion Transitional Frequency (CDTF) of about 3.5 mHz, which can be connected to the cylindrical design of the cell and the resulting maximised surface area and capacitance as found in capacitors
Summary
Mandatory and Firm Frequency Response (MFR and FFR) are services demanded and requested by National Grid, which balance demand and generation on a permanent basis in the Great British power grid [1]. All these investigations account for the degradation cumulatively for a prolonged time period, they do not consider important differences between frequency response provision and standard cycling in short time scales These phenomena, such as fast pulsing and high currents, can have significant impact on the ageing of Li-Ion cells, which asks for deeper analysis as proposed in this work. This paper investigates the capacitive capabilities of NMC-LMO (Lithium-Manganese-Oxide), LFP (Lithium-Iron-Phosphate) and NMC cells It aims to define a simpler and accurate method of identifying the threshold between capacitive and diffusion processes within a cell, to determine general characteristics and dependencies of this threshold and to connect these to the conditions of frequency response and the results found in [17]
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