Imperial College London

DrGregoryOffer

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Mechanical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 7072gregory.offer Website

 
 
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Location

 

720City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

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96 results found

OKane SEJ, Campbell ID, Marzook MWJ, Offer GJ, Marinescu Met al., Physical Origin of the Differential Voltage Minimum Associated with Lithium Plating in Li-Ion Batteries, Journal of The Electrochemical Society, Vol: 167, Pages: 090540-090540

Journal article

Radhakrishnan KN, Prosser R, Patel Y, Offer GJet al., 2020, Embedded Ocv Based Model for State of Health Monitoring in a BMS, ECS Meeting Abstracts, Vol: MA2020-01, Pages: 42-42

Journal article

Madabattula G, Wu B, Marinescu M, Offer Get al., 2020, Degradation diagnostics for Li4Ti5O12-based lithium ion capacitors: insights from a physics-based model, Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

Lithium ion capacitors are an important energy storage technology, providing the optimum combination of power, energy and cycle life for high power applications. However, there has been minimal work on understanding how they degrade and how this should influence their design. In this work, a 1D electrochemical model of a lithium ion capacitor with activated carbon (AC) as the positive electrode and lithium titanium oxide (LTO) as the negative electrode is used to simulate the consequences of different degradation mechanisms in order to explore how the capacity ratio of the two electrodes affects degradation. The model is used to identify and differentiate capacity loss due to loss of active material (LAM) in the lithiated and de-lithiated state and loss of lithium inventory (LLI). The model shows that, with lower capacity ratios (AC/LTO), LAM in the de-lithiated state cannot be identified as the excess LTO in the cell balances the capacity loss. Cells with balanced electrode capacity ratios are therefore necessary to differentiate LAM in lithiated and de-lithiated states and LLI from each other. We also propose in situ diagnostic techniques which will be useful to optimize a LIC's design. The model, built in COMSOL, is available online.

Journal article

Hales A, Marzook MW, Bravo Diaz L, Patel Y, Offer Get al., 2020, The surface cell cooling coefficient: a standard to define heat rejection from lithium ion battery pouch cells, Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

There is no universal and quantifiable standard to compare a given cell model's capability to reject heat. The consequence of this is suboptimal cell designs because cell manufacturers do not have a metric to optimise. The Cell Cooling Coefficient for pouch cell tab cooling (CCC tabs ) defines a cell's capability to reject heat from its tabs. However, surface cooling remains the thermal management approach of choice for automotive and other high-power applications. This study introduces a surface Cell Cooling Coefficient, CCC surf which is shown to be a fundamental property of a lithium-ion cell. CCC surf is found to be considerably larger than CCC tabs , and this is a trend anticipated for every pouch cell currently commercially available. However, surface cooling induces layer-to-layer nonuniformity which is strongly linked to reduced cell performance and reduced cell lifetime. Thus, the Cell Cooling Coefficient enables quantitative comparison of each cooling method. Further, a method is presented for using the Cell Cooling Coefficients to inform the optimal design of a battery pack thermal management system. In this manner, implementation of the Cell Cooling Coefficient can transform the industry, by minimising the requirement for computationally expensive modelling or time consuming experiments in the early stages of battery-pack design.

Journal article

Madabattula G, Wu B, Marinescu M, Offer Get al., 2020, How to design lithium ion capacitors: modelling, mass ratio ofelectrodes and pre-lithiation, Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

Lithium ion capacitors (LICs) store energy using double layer capacitance at the positive electrode and intercalation at the negative electrode. LICs offer the optimum power and energy density with longer cycle life for applications requiring short pulses of high power. However, the effect of electrode balancing and pre-lithiation on usable energy is rarely studied. In this work, a set of guidelines for optimum design of LICs with activated carbon (AC) as positive electrode and lithium titanium oxide (LTO) as negative electrode was proposed. A physics-based model has been developed and used to study the relationship between usable energy at different effective C rates and the mass ratio of the electrodes. The model was validated against experimental data from literature. The model was then extended to analyze the need for pre-lithiation of LTO. The limits for pre-lithiation in LTO and use of negative polarization of the AC electrode to improve the cell capacity have been analyzed using the model. Furthermore, the model was used to relate the electrolyte depletion effects to poorer power performance in a cell with higher mass ratio. The open-source model can be re-parameterised for other LIC electrode combinations, and should be of interest to cell designers.

Journal article

Madabattula G, Wu B, Marinescu M, Offer Get al., 2019, 1D Electrochemical Model for Lithium Ion Capacitors in Comsol

Lithium ion capacitor is an electrochemical energy storage device with optimum energy density, power density and longer cycle life. A 1D-electrochemical model for activated carbon (AC)/ lithium titanium oxide (LTO) based lithium ion capacitor was built in COMSOL multiphyisics, v5.3a. The model was used to generate the data in an open-access paper: How to Design Lithium Ion Capacitor: Modelling, Mass Ratio of Electrodes and Pre-lithiation, Journal of The Electrochemical Society, 2020, 167. (http://jes.ecsdl.org/content/167/1/013527.abstract) The model can be used to optimize the mass ratio of electrodes and pre-lithiation level. It can be extended to study the capacity fade in the devices.

Software

Pang M-C, Hao Y, Marinescu M, Wang H, Chen M, Offer GJet al., 2019, Experimental and numerical analysis to identify the performance limiting mechanisms in solid-state lithium cells under pulse operating conditions., Physical Chemistry Chemical Physics, Vol: 21, Pages: 22740-22755, ISSN: 1463-9076

Solid-state lithium batteries could reduce the safety concern due to thermal runaway while improving the gravimetric and volumetric energy density beyond the existing practical limits of lithium-ion batteries. The successful commercialisation of solid-state lithium batteries depends on understanding and addressing the bottlenecks limiting the cell performance under realistic operational conditions such as dynamic current profiles of different pulse amplitudes. This study focuses on experimental analysis and continuum modelling of cell behaviour under pulse operating conditions, with most model parameters estimated from experimental measurements. By using a combined impedance and distribution of relaxation times analysis, we show that charge transfer at both interfaces occurs between the microseconds and milliseconds timescale. We also demonstrate that a simplified set of governing equations, rather than the conventional Poisson-Nernst-Planck equations, are sufficient to reproduce the experimentally observed behaviour during pulse discharge, pulse charging and dynamic pulse. Our simulation results suggest that solid diffusion in bulk LiCoO2 is the performance limiting mechanism under pulse operating conditions, with increasing voltage loss for lower states of charge. If bulk electrode forms the positive electrode, improvement in the ionic conductivity of the solid electrolyte beyond 10-4 S cm-1 yields marginal overall performance gains due to this solid diffusion limitation. Instead of further increasing the electrode thickness or improving the ionic conductivity on their own, we propose a holistic model-based approach to cell design, in order to achieve optimum performance for known operating conditions.

Journal article

Zhao Y, Diaz LB, Patel Y, Zhang T, Offer GJet al., 2019, How to cool lithium ion batteries: optimising cell design using a thermally coupled model, Journal of The Electrochemical Society, Vol: 166, Pages: A2849-A2859, ISSN: 0013-4651

Cooling electrical tabs of the cell instead of the lithium ion cell surfaces has shown to provide better thermal uniformity within the cell, but its ability to remove heat is limited by the heat transfer bottleneck between tab and electrode stack. A two-dimensional electro-thermal model was validated with custom made cells with different tab sizes and position and used to study how heat transfer for tab cooling could be increased. We show for the first time that the heat transfer bottleneck can be opened up with a single modification, increasing the thickness of the tabs, without affecting the electrode stack. A virtual large-capacity automotive cell (based upon the LG Chem E63 cell) was modelled to demonstrate that optimised tab cooling can be as effective in removing heat as surface cooling, while maintaining the benefit of better thermal, current and state-of-charge homogeneity. These findings will enable cell manufacturers to optimise cell design to allow wider introduction of tab cooling. This would enable the benefits of tab cooling, including higher useable capacity, higher power, and a longer lifetime to be possible in a wider range of applications.

Journal article

Tomaszewska A, Chu Z, Feng X, O'Kane S, Liu X, Chen J, Ji C, Endler E, Li R, Liu L, Li Y, Zheng S, Vetterlein S, Gao M, Du J, Parkes M, Ouyang M, Marinescu M, Offer G, Wu Bet al., 2019, Lithium-ion battery fast charging: A review, eTransportation, Vol: 1, Pages: 1-28, ISSN: 2590-1168

In the recent years, lithium-ion batteries have become the battery technology of choice for portable devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are introducing electrified models into their offering, range anxiety and the length of time required to recharge the batteries are still a common concern. The high currents needed to accelerate the charging process have been known to reduce energy efficiency and cause accelerated capacity and power fade. Fast charging is a multiscale problem, therefore insights from atomic to system level are required to understand and improve fast charging performance. The present paper reviews the literature on the physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly result from charging at high currents, and the approaches that have been proposed to address these issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are presented and critically assessed. Safety implications are explored, including the potential influence of fast charging on thermal runaway characteristics. Finally, knowledge gaps are identified and recommendations are made for the direction of future research. The need to develop reliable in operando methods to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging optimisation strategies are identified as key to enabling fast charging in all conditions. Thermal management strategies to both cool batteries during charging and preheat them in cold weather are acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and good temperature homogeneities.

Journal article

Hales A, Diaz LB, Marzook MW, Zhao Y, Patel Y, Offer Get al., 2019, The cell cooling coefficient: A standard to define heatrejection from lithium-ion batteries, Journal of The Electrochemical Society, Vol: 166, Pages: A2383-A2395, ISSN: 0013-4651

Lithium-ion battery development is conventionally driven by energy and power density targets, yet the performance of a lithium-ion battery pack is often restricted by its heat rejection capabilities. It is therefore common to observe elevated cell temperatures and large internal thermal gradients which, given that impedance is a function of temperature, induce large current inhomogeneities and accelerate cell-level degradation. Battery thermal performance must be better quantified to resolve this limitation, but anisotropic thermal conductivity and uneven internal heat generation rates render conventional heat rejection measures, such as the Biot number, unsuitable. The Cell Cooling Coefficient (CCC) is introduced as a new metric which quantifies the rate of heat rejection. The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore ideal for comparing the thermal performance of different cell designs and form factors. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway. The CCC is presented as an essential tool to inform the cell down-selection process in the initial design phases, based solely on their thermal bottlenecks. This simple methodology has the potential to revolutionise the lithium-ion battery industry.

Journal article

Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., What is the rate-limiting mechanism in solid-state lithium cells at different pulse operating conditions?, 236th ECS Meeting

Conference paper

Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., Experimental Parameterisation of the Continuum Models for Solid-state Lithium Batteries, 3rd Annual Oxford ECS Student Chapter Symposium

Conference paper

Campbell I, Gopalakrishnan K, Marinescu M, Torchio M, Offer G, Raimondo Det al., 2019, Optimising lithium-ion cell design for plug-in hybrid and battery electric vehicles, Journal of Energy Storage, Vol: 22, Pages: 228-238, ISSN: 2352-152X

Increased driving range and enhanced fast charging capabilities are two immediate goals of transport electrification. However, these are of competing nature, leading to increased energy and power demand respectively from the on-board battery pack. By fine-tuning the number of layers versus active electrode material of a lithium ion pouch cell, tailored designs targeting either of these goals can be obtained. Achieving this trade-off through iterative empirical testing of layer choices is expensive and often produces sub-optimal designs. This paper presents a model-based methodology for determining the optimal number of layers, maximising usable energy whilst satisfying specific acceleration and fast charging targets. The proposed methodology accounts for the critical need to avoid lithium plating during fast charging and searches for the optimal layer configuration considering a range of thermal conditions. A numerical implementation of a cell model using a hybrid finite volume-spectral scheme is presented, wherein the model equations are suitably reformulated to directly accept power inputs, facilitating rapid and accurate searching of the layer design space. Electrode materials exhibiting high solid phase diffusion rates are highlighted as being equally as important for extended range as the development of new materials with higher inherent capacity. The proposed methodology is demonstrated for the common module design of a battery pack in a plug-in hybrid vehicle, thereby illustrating how the cost of derivative vehicle models can be reduced. To facilitate model based layer optimisation, the open-source toolbox, BOLD (Battery Optimal Layer Design) is provided.

Journal article

Wild M, Offer G, 2019, Lithium-Sulfur Batteries, Publisher: Wiley, ISBN: 9781119297864

The book contains a review of battery design and: Discusses electrochemistry of Li-S batteries and the analytical techniques used to study Li-S batteries Offers information on the application of Li-S batteries for commercial use Distills ...

Book

Campbell ID, Marzook M, Marinescu M, Offer GJet al., 2019, How observable Is lithium plating? Differential voltage analysis to identify and quantify lithium plating following fast charging of cold lithium-Ion batteries, Journal of The Electrochemical Society, Vol: 166, Pages: A725-A739, ISSN: 0013-4651

Fast charging of batteries is currently limited, particularly at low temperatures, due to difficulties in understanding lithium plating. Accurate, online quantification of lithium plating increases safety, enables charging at speeds closer to the electrochemical limit and accelerates charge profile development. This work uses different cell cooling strategies to expose how voltage plateaus arising from cell self-heating and concentration gradients during fast charging can falsely indicate plating, contrary to prevalent current assumptions. A solution is provided using Differential Voltage (DV) analysis, which confirms that lithium stripping is observable. However, scanning electron microscopy and energy-dispersive X-ray analysis are used to demonstrate the inability of the plateau technique to detect plating under certain conditions. The work highlights error in conventional plating quantification that leads to the dangerous underestimation of plated amounts. A novel method of using voltage plateau end-point gradients is proposed to extend the sensitivity of the technique, enabling measurement of lower levels of lithium stripping and plating. The results are especially relevant to automotive OEMs and engineers wishing to expand their online and offline tools for fast charging algorithm development, charge management and state-of-health diagnostics.

Journal article

Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., Electrochemical Modelling of Relaxation Behaviour in Solid-state Lithium Batteries: From Measurements to Application Design, Materials for Clean Energy Conference

Conference paper

Hua X, Zhang T, Offer GJ, Marinescu Met al., 2019, Towards online tracking of the shuttle effect in lithium sulfur batteries using differential thermal voltammetry, Journal of Energy Storage, Vol: 21, Pages: 765-772

© 2019 Lithium sulfur (Li-S) batteries are an important next generation high energy density battery technology. However, the phenomenon known as the polysulfide shuttle causes accelerated degradation, reduced Coulombic efficiency and increased heat generation, particularly towards the end of charge. The real-time detection of the onset of shuttle during charge would improve the safety and increase cycle life of Li-S batteries in real applications. In this study, we demonstrate that the Differential Thermal Voltammetry (DTV) technique can be used for tracking shuttle during Li-S charging. By combining voltage and temperature measurements, DTV is shown to be sensitive to the magnitude of shuttle. We demonstrate significant differences in the DTV curves for Li-S cells charged at different currents and temperatures. Quantitative interpretations of the experimental DTV curves are performed through a thermally-coupled zero-dimensional Li-S model. The DTV technique, together with the model, is a promising tool for real-time detection of shuttle in applications, to inform control algorithms for deciding the end of charging, thus preventing excessive degradation and charge inefficiency.

Journal article

Zhao Y, Spingler FB, Patel Y, Offer GJ, Jossen Aet al., 2019, Localized swelling inhomogeneity detection in lithium ion cells using multi-dimensional laser scanning, Journal of The Electrochemical Society, Vol: 166, Pages: A27-A34, ISSN: 1945-7111

The safety, performance and lifetime of lithium-ion cells are critical for the acceptance of electric vehicles (EVs) but the detection of cell quality issues non-destructively is difficult. In this work, we demonstrate the use of a multi-dimensional laser scanning method to detect local inhomogeneities. Commercially available cells with Nickel Cobalt Manganese (NMC) cathode are cycled at various charge and discharge rates, while 2D battery displacement measurements are taken using the laser scanning system. Significant local swelling points are found on the cell during the discharge phase, the magnitude of swelling can be up to 2% of the cell thickness. The results show that the swelling can be aggravated by a combination of slow charge rate and fast discharge rate. Disassembly of the cells shows that the swelling points are matched with the location of ‘adhesive-like’ material found on the electrode surfaces. Scanning Electron Microscope (SEM) images show that the material is potentially blocking the electrodes and separators at these locations. We therefore present laser-scanning displacement as a valuable tool for defect/inhomogeneity detection.

Journal article

Zhao Y, Patel Y, Zhang T, Offer GJet al., 2018, Modeling the effects of thermal gradients induced by tab and surface cooling on lithium ion cell performance, Journal of The Electrochemical Society, Vol: 165, Pages: A3169-A3178, ISSN: 1945-7111

Lithium ion batteries are increasingly important in large scale applications where thermal management is critical for safety and lifetime. Yet, the effect of different thermal boundary conditions on the performance and lifetime is still not fully understood. In this work, a two-dimensional electro-thermal model is developed to simulate cell performance and internal states under complex thermal boundary conditions. Attention was paid to model, not only the electrode stack but also the non-core components (e.g. tab weld points) and thermal boundaries, but also the experiments required to parameterize the thermal model, and the reversible heat generation. The model is comprehensively validated and the performance of tab and surface cooling strategies was evaluated across a wide range of operating conditions. Surface cooling was shown to keep the cell at a lower average temperature, but with a large thermal gradient for high C rates. Tab cooling provided much smaller thermal gradients but higher average temperatures caused by lower heat removing ability. The thermal resistance between the current collectors and tabs was found to be the most significant heat transfer bottleneck and efforts to improve this could have significant positive impacts on the performance of li-ion batteries considering the other advantages of tab cooling.

Journal article

Zhang X-F, Zhao Y, Liu H-Y, Zhang T, Liu W-M, Chen M, Patel Y, Offer GJ, Yan Yet al., 2018, Degradation of thin-film lithium batteries characterised by improved potentiometric measurement of entropy change, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 11378-11385, ISSN: 1463-9076

Journal article

Merla Y, Wu B, Yufit V, Martinez-Botas RF, Offer GJet al., 2018, An easy-to-parameterise physics-informed battery model and its application towards lithium-ion battery cell design, diagnosis, and degradation, Journal of Power Sources, Vol: 384, Pages: 66-79, ISSN: 0378-7753

Accurate diagnosis of lithium ion battery state-of-health (SOH) is of significant value for many applications, to improve performance, extend life and increase safety. However, in-situ or in-operando diagnosis of SOH often requires robust models. There are many models available however these often require expensive-to-measure ex-situ parameters and/or contain unmeasurable parameters that were fitted/assumed. In this work, we have developed a new empirically parameterised physics-informed equivalent circuit model. Its modular construction and low-cost parametrisation requirements allow end users to parameterise cells quickly and easily. The model is accurate to 19.6 mV for dynamic loads without any global fitting/optimisation, only that of the individual elements. The consequences of various degradation mechanisms are simulated, and the impact of a degraded cell on pack performance is explored, validated by comparison with experiment. Results show that an aged cell in a parallel pack does not have a noticeable effect on the available capacity of other cells in the pack. The model shows that cells perform better when electrodes are more porous towards the separator and have a uniform particle size distribution, validated by comparison with published data. The model is provided with this publication for readers to use.

Journal article

Mazur CM, Offer G, Contestabile MSM, Brandon Net al., 2018, Comparing the effects of vehicle automation, policy making and changed user preferences on the uptake of electric cars and emissions from transport, Sustainability, Vol: 10, ISSN: 2071-1050

Switching energy demand for transport from liquid fuels to electricity is the most promising way to significantly improve air quality and reduce transport emissions. Previous studies have shown this is possible, that by 2035 the economics of alternative powertrain and energy vectors will have converged. However, they don’t address if the transition is likely or plausible. Using the UK as a case study, we present a systems dynamics model based study informed by transition theory and explore the effects of technology progress, policy making, user preferences and; for the first time, automated vehicles on this transition. We are not trying to predict the future, but to highlight what is necessary in order for different scenarios to become more or less likely. Worryingly we show that current policies with the expected technology progress and expectations of vehicle buyers are insufficient to reach global targets. Faster technology progress, strong financial incentives or a change in vehicle buyer expectations are crucial, but still insufficient. In contrast the biggest switch to alternatively fuelled vehicles could be achieved by the introduction of automated vehicles. The implications will affect policy makers, automotive manufactures, technology developers and broader society.

Journal article

Few SPM, Schmidt O, Offer GJ, Brandon N, Nelson J, Gambhir Aet al., 2018, Prospective improvements in cost and cycle life of off-grid lithium-ion battery packs: An analysis informed by expert elicitations, Energy Policy, Vol: 114, Pages: 578-590, ISSN: 0301-4215

This paper presents probabilistic estimates of the 2020 and 2030 cost and cycle life of lithium-ion battery (LiB) packs for off-grid stationary electricity storage made by leading battery experts from academia and industry, and insights on the role of public research and development (R&D) funding and other drivers in determining these. By 2020, experts expect developments to arise chiefly through engineering, manufacturing and incremental chemistry changes, and expect additional R&D funding to have little impact on cost. By 2030, experts indicate that more fundamental chemistry changes are possible, particularly under higher R&D funding scenarios, but are not inevitable. Experts suggest that significant improvements in cycle life (eg. doubling or greater) are more achievable than in cost, particularly by 2020, and that R&D could play a greater role in driving these. Experts expressed some concern, but had relatively little knowledge, of the environmental impact of LiBs. Analysis is conducted of the implications of prospective LiB improvements for the competitiveness of solar photovoltaic + LiB systems for off-grid electrification.

Journal article

Marinescu M, O'Neill L, Zhang T, Walus S, Wilson T, Offer Get al., 2017, Irreversible vs Reversible Capacity Fade of Lithium-Sulfur Batteries during Cycling: The Effects of Precipitation and Shuttle, Journal of The Electrochemical Society, Vol: 165, Pages: A6107-A6118, ISSN: 1945-7111

Lithium-sulfur batteries could deliver significantly higher gravimetric energy density and lower cost than Li-ion batteries. Their mass adoption, however, depends on many factors, not least on attaining a predictive understanding of the mechanisms that determine their performance under realistic operational conditions, such as partial charge/discharge cycles. This work addresses a lack of such understanding by studying experimentally and theoretically the response to partial cycling. A lithium-sulfur model is used to analyze the mechanisms dictating the experimentally observed response to partial cycling. The zero-dimensional electrochemical model tracks the time evolution of sulfur species, accounting for two electrochemical reactions, one precipitation/dissolution reaction with nucleation, and shuttle, allowing direct access to the true cell state of charge. The experimentally observed voltage drift is predicted by the model as a result of the interplay between shuttle and the dissolution bottleneck. Other features are shown to be caused by capacity fade. We propose a model of irreversible sulfur loss associated with shuttle, such as caused by reactions on the anode. We find a reversible and an irreversible contribution to the observed capacity fade, and verify experimentally that the reversible component, caused by the dissolution bottleneck, can be recovered through slow charging. This model can be the basis for cycling parameters optimization, or for identifying degradation mechanisms relevant in applications. The model code is released as Supplementary material B.

Journal article

Ardani MI, Patel Y, Siddiq A, Offer GJ, Martinez-Botas RFet al., 2017, Combined experimental and numerical evaluation of the differences between convective and conductive thermal control on the performance of a lithium ion cell, Energy, Vol: 144, Pages: 81-97, ISSN: 0360-5442

Testing of lithium ion batteries is necessary in order to understand their performance, to parameterise and furthermore validate models to predict their behaviour. Tests of this nature are normally conducted in thermal/climate chambers which use forced air convection to distribute heat. However, as they control air temperature, and cannot easily adapt to the changing rate of heat generated within a cell, it is very difficult to maintain constant cell temperatures. This paper describes a novel conductive thermal management system which maintains cell temperature reliably whilst also minimising thermal gradients. We show the thermal gradient effect towards cell performance is pronounced below operating temperature of 25 °C at 2-C discharge under forced air convection. The predicted internal cell temperature can be up to 4 °C hotter than the surface temperature at 5 °C ambient condition and eventually causes layers to be discharge at different current rates. The new conductive method reduces external temperature deviations of the cell to within 1.5 °C, providing much more reliable data for parameterising a thermally discretised model. This method demonstrates the errors in estimating physiochemical paramet ers; notably diffusion coefficients, can be up to four times smaller as compared to parameterisation based on convective test data.

Journal article

Hunt I, Zhang T, Patel Y, Marinescu M, Purkayastha R, Kovacik P, Walus S, Swiatek A, Offer GJet al., 2017, The effect of current inhomogeneity on the performance and degradation of Li-S batteries, Journal of the Electrochemical Society, Vol: 165, Pages: A6073-A6080, ISSN: 0013-4651

The effect of thermal gradients on the performance and cycle life of Li-S batteries is studied using bespoke single-layer Li-S cells, with isothermal boundary conditions maintained by Peltier elements. A temperature difference is shown to cause significant current imbalance between parallel connected single-layer cells, causing the hotter cell to provide more charge and discharge capacities during cycling. During charge, significant shuttle is induced in the hotter Li-S cell, causing accelerated degradation of it. A bespoke multi-tab cell in which the inner layers are electrically connected to different tabs versus the outer layers, is used to demonstrate that noticeable current inhomogeneity occurs during the operation of practical multilayer Li-S pouch cells, which is expected to affect their performance and degradation. The observed thermal and current inhomogeneity should have a direct consequence on battery pack and thermal management system design for real world Li-S battery packs.

Journal article

Shibagaki T, Merla Y, Offer GJ, 2017, Tracking degradation in lithium iron phosphate batteries using differential thermal voltammetry, Journal of Power Sources, Vol: 374, Pages: 188-195, ISSN: 0378-7753

Diagnosing the state-of-health of lithium ion batteries in-operando is becoming increasingly important for multiple applications. We report the application of differential thermal voltammetry (DTV) to lithium iron phosphate (LFP) cells for the first time, and demonstrate that the technique is capable of diagnosing degradation in a similar way to incremental capacity analysis (ICA). DTV has the advantage of not requiring current and works for multiple cells in parallel, and is less sensitive to temperature introducing errors. Cells were aged by holding at 100% SOC or cycling at 1C charge, 6D discharge, both at an elevated temperature of 45 °C under forced air convection. Cells were periodically characterised, measuring capacity fade, resistance increase (power fade), and DTV fingerprints. The DTV results for both cells correlated well with both capacity and power, suggesting they could be used to diagnose SOH in-operando for both charge and discharge. The DTV peak-to-peak capacity correlated well with total capacity fade for the cycled cell, suggesting that it should be possible to estimate SOC and SOH from DTV for incomplete cycles within the voltage hysteresis region of an LFP cell.

Journal article

Cleaver T, Kovacik P, Marinescu M, Zhang T, Offer Get al., 2017, Perspective—commercializing lithium sulfur batteries: Are we doing the right research?, Journal of The Electrochemical Society, Vol: 165, Pages: A6029-A6033, ISSN: 0013-4651

A picture of the challenges faced by the lithium-sulfur technology and the activities pursued by the research community to solve them is synthesized based on 1992 scientific articles. It is shown that, against its own advice of adopting a balanced approach to development, the community has instead focused work on the cathode. To help direct future work, key areas of neglected research are highlighted, including cell operation studies, modelling, anode, electrolyte and production methods, as well as development goals for real world target applications such as high altitude unmanned aerial vehicles.

Journal article

Zhao Y, Patel Y, Hunt IA, Kareh KM, Holland AA, Korte C, Dear JP, Yue Y, Offer GJet al., 2017, Preventing lithium ion battery failure during high temperatures by externally applied compression, Journal of Energy Storage, Vol: 13, Pages: 296-303

© 2017 Elsevier Ltd Lithium-ion cells can unintentionally be exposed to temperatures outside manufacturers recommended limits without triggering a full thermal runaway event. The question addressed in this paper is: Are these cells still safe to use? In this study, externally applied compression has been employed to prevent lithium ion battery failure during such events. Commercially available cells with Nickel Cobalt Manganese (NCM) cathodes were exposed to temperatures at 80 °C, 90 °C and 100 °C for 10 h, and electrochemically characterised before and after heating. The electrode stack structures were also examined using x-ray computed tomography (CT), and post-mortems were conducted to examine the electrode stack structure and surface changes. The results show that compression reduces capacity loss by −0.07%, 4.95% and 13.10% respectively, measured immediately after the thermal testing. The uncompressed cells at 80 °C showed no swelling, whilst 90 °C and 100 °C showed significant swelling. The X-ray CT showed that the uncompressed cell at 100 °C suffered de-lamination at multiple locations after test, and precipitations were found on the electrode surface. The post-mortem results indicates the compressed cell at 100 °C was kept tightly packed, and the electrode surface was uniform. The conclusion is that externally applied compression reduces delamination due to gas generation during high temperature excursions.

Journal article

Wu B, Offer G, 2017, Environmental impact of hybrid and electric vehicles, Environmental Impacts of Road Vehicles : Past, Present and Future, Editors: Harrison, Hester, Publisher: Royal Society of Chemistry

Hybrid and electric vehicles play a critical role in reducing global greenhouse gas emissions, with transport estimated to contribute to 14% of the 49 GtCO2eq produced annually. Analysis of only the conversion efficiency of powertrain technologies can be misleading, with pure battery electric and hybrid vehicles reporting average efficiencies of 92% and 35% in comparison with 21% for internal combustion engine vehicles. A fairer comparison would be to consider the well-to-wheel efficiency, which reduces the numbers to 21–67%, 25% and 12%, respectively. The large variation in well-to-wheel efficiency of pure battery electric vehicles highlights the importance of renewable energy generation in order to achieve true environmental benefits. When calculating the energy return on investment of the various technologies based on the current energy generation mix, hybrid vehicles show the greatest environmental benefits, although this would change if electricity was made with high amounts of renewables. In an extreme scenario with heavy coal generation, the CO2eq return on investment can actually be negative for pure electric vehicles, highlighting the importance of renewable energy generation further. The energy impact of production is generally small (∼6% of lifetime energy) and, similarly, recycling is of a comparable magnitude, but it is less well studied.

Book chapter

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