A Model for Predicting Capacity Fade due to SEI Formation in a Commercial Graphite/LiFePO₄Cell

Abstract

An aging model for a negative graphite electrode in a lithium-ion battery, for moderate currents up to 1C, is derived and fitted to capacity fade experimental data. The predictive capabilities of the model, using only four fitted parameters, are demonstrated at both 25 • C and 45 • C. The model is based on a linear combination of two current contributions: one stemming from parts of the graphite particles covered by an intact microporous solid-electrolyte-interface (SEI) layer, and one contribution from parts of the particles were the SEI layer has cracked due to graphite expansion. Mixed kinetic and transport control is used to describe the electrode kinetics. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0641506jes] All rights reserved.Manuscript submitted December 19, 2014; revised manuscript received February 6, 2015. Published March 10, 2015 Lithium-ion batteries are used in all sorts of electric devices such as mobile phones, lawn mowers, electrical scooters and electric cars. Common for all lithium-ion battery chemistries is that they suffer from aging phenomena over time. Degradation occurs during storage, but is further induced by battery cycling and higher temperatures. Life time degradation generally occurs due to various processes such as electrolyte decomposition, loss of active material and loss of cycleable lithium due to parasitic reactions. 1The most common negative electrode material in lithium-ion batteries is graphite. For cells deploying negative graphite electrodes, significant amounts of cycleable lithium is lost into forming the solidelectrolyte-interface (SEI) layer on the graphite surface. The initial SEI formation during the first "formation" cycles of the battery is most pronounced, but, depending on operation conditions, the SEI layer formation will continue during the whole lifetime of the battery. The SEI layer is not homogeneous, neither with regards to morphology nor composition. Cracks are known to form upon battery cycling, 2 and various chemical compounds have been observed.2,3 Experimental work has shown that for low currents (≤C/10) the long term capacity loss usually follows a t 1/2 dependence, and models in literature usually ascribes this dependence to a diffusion limitation through the SEI layer of a reacting species in the electrolyte that participates in forming the SEI layer. [4][5][6] This paper focuses on SEI formation on graphite at moderate load currents (≤1C) at both 25• C and 45• C. For these conditions it has been shown that the SEI formation is the main cause of battery degradation. 7Although not treated explicitly, the positive electrode material is assumed to be LiFePO 4 (LFP) since it has been seen that this positive electrode material does not suffer from nor cause significant degrada...

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High Precision Coulometry of Commercial PAN-Based Carbon Fibers as Electrodes in Structural Batteries

“…Another factor affecting SEI layer growth could be cracking of the SEI layer due to fiber expansion, this has been suggested and modeled with good fit for graphite particles. 21 Since the carbon fibers have been shown to expand during lithiation, 20 a similar effect could be the reason for the steeper slopes during the first few cycles in Figure 5. However, the slopes are linear after around 100 hours, suggesting time to be the dominant factor affecting SEI layer growth, and that cracking of the SEI layer is mainly a problem during the first few cycles.…”

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