Abstract
Abstract Degradation in lithium ion (Li-ion) battery cells is the result of a complex interplay of a host of different physical and chemical mechanisms. The measurable, physical effects of these degradation mechanisms on the cell can be summarised in terms of three degradation modes, namely loss of lithium inventory, loss of active positive electrode material and loss of active negative electrode material. The different degradation modes are assumed to have unique and measurable effects on the open circuit voltage (OCV) of Li-ion cells and electrodes. The presumptive nature and extent of these effects has so far been based on logical arguments rather than experimental proof. This work presents, for the first time, experimental evidence supporting the widely reported degradation modes by means of tests conducted on coin cells, engineered to include different, known amounts of lithium inventory and active electrode material. Moreover, the general theory behind the effects of degradation modes on the OCV of cells and electrodes is refined and a diagnostic algorithm is devised, which allows the identification and quantification of the nature and extent of each degradation mode in Li-ion cells at any point in their service lives, by fitting the cells' OCV.
Highlights
Lithium ion (Li-ion) cells degrade as a result of their usage and exposure to environmental conditions [1e4]
We propose to rely more heavily on a diagnostic approach, built on frequent cell characterisations using available measurements which include information on the state of health (SoH) of the cell
Methods for estimating degradation modes and inferring the SoH of Li-ion cells are typically based on derivatives of open circuit voltage (OCV) or cell capacity, so called incremental capacity analysis (ICA) [27] or differential voltage analysis (DVA) [28]
Summary
Lithium ion (Li-ion) cells degrade as a result of their usage and exposure to environmental conditions [1e4]. Structural nonuniformity can lead to inhomogeneous distributions of current densities and degrees of lithiation inside the electrode material, which in turn causes inhomogeneous degradation of the electrode Evidence of such inhomogeneities has been observed in the course of a post-mortem analysis of commercial Kokam pouch cells (described in detail in Section 2.1), which are the subject of this work. The standard deviation between the capacities of the five investigated pouch cells was less than 0.2% and the cell with one non-uniformly lithiated graphite sheet exhibited the highest capacity This illustrates that meso- and macro-scale inhomogeneities can not. Methods for estimating degradation modes and inferring the SoH of Li-ion cells are typically based on derivatives of OCV or cell capacity, so called incremental capacity analysis (ICA) [27] or differential voltage analysis (DVA) [28]. In a Li-ion cell based exclusively on the cell's OCV without performing derivative operations on the measurements
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