Understanding degradation & failure in lithium ion and lithium sulfur batteries  Seminar

 

Greg works at the interface between the science and engineering of electrochemical devices, mostly focused on automotive applications. Having trained as an electrochemist before moving to engineering, his research portfolio focuses on understanding the limits of operation, degradation mechanisms and failure modes of batteries, supercapacitors and fuel cells in real world applications, and the impacts and consequences on system design, integration and control. His group is split into 4 broad areas, thermal management of lithium ion batteries and battery packs, understanding the effect of mechanical stimuli on lithium ion batteries, next generation technologies including lithium sulfur batteries, and systems integration including supercapacitors and fuel cells.

Recent work exploring the effect of cell surface cooling and cell tab cooling is shown, reproducing two typical cooling systems that are used in real-world battery packs. It is shown that cooling method alone can significantly affect useable capacity. For new cells at C/20 discharge, very little difference in capacity was seen, however, at 6C discharge surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. Furthermore, after cycling the cells for a 1,000 times at 6C discharge and 2C charge, surface cooling resulted in a degradation rate three times higher than cell tab cooling. Using incremental capacity analysis, electrochemical impedance analysis and thermal models of cells, it is shown that this is due to thermal gradients being perpendicular to the layers for surface cooling leading to higher local currents and faster degradation, but in-plane with the layers for tab cooling leading to more homogenous behaviour. Aged cells were put with new cells in a parallel pack configuration with cooling where measuring the current flowing through each individual cell is often impractical. Using a novel diagnostic method based on simple cell surface temperature measurements developed by our group, a diagnosis method capable of quantitatively determining the state-of-health of individual cells simultaneously during both charge and discharge by only using temperature and voltage readings, and whilst cells are being thermally managed, will be shown. 

Existing Li-S models do not sufficiently capture the voltage- and capacity-drop mechanisms of Li-S cells during discharge. We first demonstrate that introducing a concentration dependence of the electrolyte conductivity is necessary to retrieve the experimentally documented trends in electrolyte resistance, which contributes to a major voltage-loss mechanism for high-energy density Li-S cells. We further illustrate the existence of an often overlooked potential drop mechanism – the ‘precipitation overpotential’ – which originates from the limited rate of lithium sulphide precipitation. In addition, we propose that the rate capability of high energy-density Li-S cells is mainly limited by the slow transport of ionic species, as is evident from the good agreement between experimental and model-predicted capacity loss at high discharge currents as well as a cell capacity recovery phenomenon that we report for the first time.

Relevant References:

• Marinescu M, Zhang T, Offer GJ, A zero dimensional model of lithium-sulfur batteries during charge and discharge, Physical Chemistry Chemical Physics, 2016 Vol: 18, Pages: 584-593 http://dx.doi.org/10.1039/c5cp05755h

• Merla Y, Wu B, Yufit V, Brandon NP, Martinez-Botas RF, Offer GJ., Novel application of differential thermal voltammetry as an in-depth state-of-health diagnosis method for lithium-ion batteries, Journal of Power Sources, 2016 Vol: 307, Pages: 308-319 http://dx.doi.org/10.1016/j.jpowsour.2015.12.122

• Zhang T, Marinescu M, O'Neill L, Wild M, Offer G, Modeling the voltage loss mechanisms in lithium-sulfur cells: the importance of electrolyte resistance and precipitation kinetics, Physical Chemistry Chemical Physics, 2015 Vol: 17, Pages: 22581-22586 http://dx.doi.org/10.1039/c5cp03566j

• Wild M, O'Neill L, Zhang T, Purkayastha R, Minton G, Marinescu M, Offer GJ, 2015, Lithium sulfur batteries, a mechanistic review, Energy & Environmental Science, Vol: 8, Pages: 3477-3494 http://dx.doi.org/10.1039/c5ee01388g

• Wu B, Yufit V, Merla Y, Martinez-Botas RF, Brandon NP, Offer GJ., 2015, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources, Vol: 273, Pages: 495-501 http://dx.doi.org/10.1016/j.jpowsour.2014.09.127

• Yannic Troxler, Billy Wu, Monica Marinescu, Vladimir Yufit, Yatish Patel, Andrew J Marquis, Nigel P Brandon, Gregory J Offer, The effect of thermal gradients on the performance of lithium ion batteries, 247:1018-1025 2014 http://dx.doi.org/10.1016/j.jpowsour.2013.06.084

• Billy Wu, Vladimir Yufit, Monica Marinescu, Gregory J Offer, Ricardo F Martinez-Botas, Nigel P Brandon, Coupled thermal-electrochemical modelling of uneven heat generation in lithium-ion battery packs, Journal of Power Sources 243:544-554 2013 http://dx.doi.org/10.1016/j.jpowsour.2013.05.164

• Offer, G. J., Yufit, V., Howey, D. A., Wu, B., Brandon, N. P., Module design and fault diagnosis in electric vehicle batteries, Journal of Power Sources, Volume 206, 15 May 2012, Pages 383–392 http://dx.doi.org/10.1016/j.jpowsour.2012.01.087