Lithium ion battery research
2012-05 Module design fault diagnosis in electric vehicle batteries
A simple technique to identify malfunctioning strings of cells in battery packs. It was shown a single high inter-cell contact resistance could cause currents to flow unevenly within the pack leading to significant differences in SOC and large currents flowing between cells even when the load is disconnected.
2013-12 Modelling uneven heat generation in lithium-ion battery pack
We showed increasing the number of cells in parallel results in a linear increase in load non-uniformity, whilst increasing the ratio of interconnect to battery impedance results in a logarithmic increase in load non-uniformity, with cells closest to the load points experiencing the largest currents.
2013-12 Online Measurement of Battery Impedance
A fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle is presented. A measurement system for impedance, for the first time, uses an excitation current generated by a motor controller.
2014-02 The effect of thermal gradients on lithium ion batteries
This paper presents the first study of the impact of artificially induced thermal gradients on cell performance. Thermal cycling of the cell, combined with slow rate cyclic voltammetry, allows to rapidly identify phase transitions in electrodes, due to the thermal effect of entropy changes.
2015-01 Tracking degradation in lithium ion batteries
A novel diagnostic technique, termed differential thermal voltammetry, which is capable of monitoring the state of the battery using voltage and temperature measurements is presented. This tracks battery degradation through phase transitions, and the resulting entropic heat, occurring in the electrodes.
2015-04 Imaging lithium ion batteries during thermal runaway
We showed for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries.
2015-12 An integrated approach for modelling grid energy storage systems
An integrated approach for the analysis and control of grid connected energy storage systems. C. Patsios, B. Wu, E. Chatzinikolaou, D. J. Rogers, N. Wade, N. P. Brandon, P. Taylor. Journal of Energy Storage. 2016, 5, 48-61.
This paper presents an integrated modelling methodology which includes reduced-order models of a lithium ion battery and a power electronic converter, connected to a 35-bus distribution network model. The literature contains many examples of isolated modelling of individual energy storage mediums, power electronic interfaces and control algorithms for energy storage. However, when assessing the performance of a complete energy storage system, the interaction between components gives rise to a range of phenomena that are difficult to quantify if studied in isolation. This paper proposes an integrated electro–thermo–chemical modelling methodology that seeks to address this problem directly by integrating reduced-order models of battery cell chemistry, power electronic circuits and grid operation into a computationally efficient framework. The framework is capable of simulation speeds over 100 times faster than real-time and captures phenomena typically not observed in simpler battery and power converter models or non-integrated frameworks. All simulations are performed using real system load profiles recorded in the United Kingdom. To illustrate the advantages inherent in such a modelling approach, two specific interconnected effects are investigated: the effect of the choice of battery float state-of-charge on overall system efficiency and the rate of battery degradation (capacity/power fade). Higher state-of-charge operation offers improved efficiency due to lower polarisation losses of the battery and lower losses in the converter, however, an increase in the rate of battery degradation is observed due to the accelerated growth of the solid-electrolyte interphase layer. We demonstrate that grid control objectives can be met in several different ways, but that the choices made can result in a substantial improvement in system roundtrip efficiency, with up to a 43% reduction in losses, or reduction in battery degradation by a factor of two, depending on battery system use case.
2016-03 DTV as a state-of-health diagnostic technique for batteries
Differential thermal voltammetry (DTV) was used as an in-depth state-of-health diagnosis method for lithium-ion batteries, accurately diagnosing an unknown cell.
2016-07 Thermal Management Affects Degradation
We show the effect that the thermal management system can have on the useable capacity of a battery pack, and also on the degradation. Essentially surface cooling of pouch cells is really bad as each layer is at a different temperature and behaves non-uniformly, whereas tab cooling is better as every layer is the same and they behave uniformly. Hence the motto of lithium ion cell layers should be "United we stand, divided we fall".
2016-09 Extending battery lifetime
Extending battery life: A low-cost practical diagnostic technique for lithium-ion batteries. Y. Merla, B. Wu, V. Yufit, N. P. Brandon, R. F. Martinez-Botas and G. J. Offer. Journal of Power Sources. 2016, 331, 224-231.
Modern applications of lithium-ion batteries such as smartphones, hybrid & electric vehicles and grid scale electricity storage demand long lifetime and high performance which typically makes them the limiting factor in a system. Understanding the state-of-health during operation is important in order to optimise for long term durability and performance. However, this requires accurate in-operando diagnostic techniques that are cost effective and practical. We present a novel diagnosis method based upon differential thermal voltammetry demonstrated on a battery pack made from commercial lithium-ion cells where one cell was deliberately aged prior to experiment. The cells were in parallel whilst being thermally managed with forced air convection. We show for the first time, a diagnosis method capable of quantitatively determining the state-of-health of four cells simultaneously by only using temperature and voltage readings for both charge and discharge. Measurements are achieved using low-cost thermocouples and a single voltage measurement at a frequency of 1 Hz, demonstrating the feasibility of implementing this approach on real world battery management systems. The technique could be particularly useful under charge when constant current or constant power is common, this therefore should be of significant interest to all lithium-ion battery users.
2016-09 Physics Based Equivalent Circuit Models
A new type of physically meaningful equivalent circuit model is reported, which includes variable double layer capacitances, passivating layers, a coupled thermal model, and cell degradation prediction in the form of capacity and power fade.