Mass transport in the electrolyte is one of the limiting processes when it comes to the power density and energy efficiency of lithium-ion batteries. Electrolyte characterizations are therefore of utmost importance. This study reports the ionic conductivity, diffusion coefficient, lithium-ion transport number, and thermodynamic enhancement factor, as well as density and viscosity, for the electrolyte LiPF 6 in EC:DEC (1:1, by weight) at 10 • C, 25 • C, and 40 • C and for concentrations between 0.5 M and 1.5 M. By combining mathematical modeling and three experiments: conductivity measurements, concentration cells, and galvanostatic polarizations, the mass transport phenomena were fully characterized. All parameters were found to vary strongly with both concentration and temperature proving that temperature dependent parameters are essential when studying thermal behavior of lithium-ion batteries. Moreover, conductivity increased with temperature and showed a local maximum at around 1 M within the concentration range at all temperatures. The other parameters either showed a continuous decrease (diffusion coefficient and lithiumion transport number) or increase (thermodynamic enhancement factor) with concentration at all temperatures. Limited liquid range leading to solvent crystallization at 10 • C leads to very poor performance, possibly due to the strong coordination between the lithium ion and the crystallizing species, EC. Overall, the studied electrolyte is found to perform poorly compared to previously studied systems.Lithium-ion batteries (LIBs) have proven their commercial viability by conquering most markets for rechargeable batteries, including mobile devices and electric vehicles. The high power performance of LIBs has been found to be limited by the transport of lithium ions in the electrolyte. 1-4 As these batteries are being implemented in increasingly demanding applications, such as hybrid electric vehicles, an accurate prediction of their performance is crucial, to be able to design better battery packs. The predictions are made possible through modeling, which requires characterization of the mass-transport properties of the materials used in a wide range of operational conditions, including variations in temperature.Characterizations of electrolytes are not uncommon. However, they are mostly limited to partial investigations, providing only some of the properties needed to describe the performance of the electrolyte or assuming that it behaves like a dilute system (or both). 5,6 Concentrated electrolyte theory, which includes ion-ion interactions by implementing friction coefficients for all components, 7 should be used when modeling and characterizing the electrolytes used in LIBs, as dilute theory will generate big deviations in concentration profile simulation, and in extension voltage prediction, especially at high currents.
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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