Imperial College London

DrMonicaMarinescu

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 7091monica.marinescu Website

 
 
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Location

 

722Mechanical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

42 results found

Li S, Kirkaldy N, Zhang C, Gopalakrishnan K, Amietszajew T, Diaz LB, Barreras JV, Shams M, Hua X, Patel Y, Offer GJ, Marinescu Met al., 2021, Optimal cell tab design and cooling strategy for cylindrical lithium-ion batteries, Journal of Power Sources, Vol: 492, Pages: 1-16, ISSN: 0378-7753

The ability to correctly predict the behavior of lithium ion batteries is critical for safety, performance, cost and lifetime. Particularly important for this purpose is the prediction of the internal temperature of cells, because of the positive feedback between heat generation and current distribution. In this work, a comprehensive electro-thermal model is developed for a cylindrical lithium-ion cell. The model is comprehensively parameterized and validated with experimental data for 2170 cylindrical cells (LG M50T, NMC811), including direct core temperature measurements. The validated model is used to study different cell designs and cooling approaches and their effects on the internal temperature of the cell. Increasing the number of tabs connecting the jellyroll to the base of the cylindrical-can reduces the internal thermal gradient by up to 25.41%. On its own, side cooling is more effective than base cooling at removing heat, yet both result in thermal gradients within the cell of a similar magnitude, irrespective of the number of cell tabs. The results are of immediate interest to both cell manufacturers and battery pack designers, while the modelling and parameterization framework created is an essential tool for energy storage system design.

Journal article

Pang M-C, Yang K, Brugge R, Zhang T, Liu X, Pan F, Yang S, Aguadero A, Wu B, Marinescu M, Wang H, Offer GJet al., 2021, Interactions are important: Linking multi-physics mechanisms to the performance and degradation of solid-state batteries, Materials Today, ISSN: 1369-7021

Journal article

Robinson J, Xi K, Kumar RV, Ferrari AC, Au H, Titirici M-M, Parra Puerto A, Kucernak A, Fitch SDS, Garcia-Araez N, Brown Z, Pasta M, Furness L, Kibler A, Walsh D, Johnson L, Holc C, Newton G, Champness NR, Markoulidis F, Crean C, Slade R, Andritsos E, Cai Q, Babar S, Zhang T, Lekakou CT, Rettie A, Kulkarni NN, Jervis R, Cornish M, Marinescu M, Offer G, Li Z, Bird L, Grey C, Chhowhalla M, Di Lecce D, Miller T, Brett D, Owen R, Liatard S, Ainsworth D, Shearing Pet al., 2021, 2021 roadmap on lithium sulfur batteries, Journal of Physics: Energy, Vol: 3, ISSN: 2515-7655

Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK's independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space.

Journal article

Ghosh A, Foster JM, Offer G, Marinescu Met al., 2021, A Shrinking-Core Model for the Degradation of High-Nickel Cathodes (NMC811) in Li-Ion Batteries: Passivation Layer Growth and Oxygen Evolution, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 168, ISSN: 0013-4651

Journal article

Hua X, Heckel C, Modrow N, Zhang C, Hales A, Holloway J, Jnawali A, Li S, Yu Y, Loveridge M, Shearing P, Patel Y, Marinescu M, Tao L, Offer Get al., 2021, The prismatic surface cell cooling coefficient: A novel cell design optimisation tool & thermal parameterization method for a 3D discretised electro-thermal equivalent-circuit model, eTransportation, Vol: 7, Pages: 1-15, ISSN: 2590-1168

Thermal management of large format prismatic lithium ion batteries is challenging due to significant heat generation rates, long thermal ‘distances’ from the core to the surfaces and subsequent thermal gradients across the cell. The cell cooling coefficient (CCC) has been previously introduced to quantify how easy or hard it is to thermally manage a cell. Here we introduce its application to prismatic cells with a 90 Ah prismatic lithium iron phosphate cell with aluminium alloy casing. Further, a parameterised and discretised three-dimensional electro-thermal equivalent circuit model is developed in a commercially available software environment. The model is thermally and electrically validated experimentally against data including drive cycle noisy load and constant current CCC square wave load, with particular attention paid to the thermal boundary conditions. A quantitative study of the trade-off between cell energy density and surface CCC, and into casing material selection has been conducted here. The CCC enables comparison between cells, and the model enables a cell manufacturer to optimise the cell design and a systems developer to optimise the pack design. We recommend this is operated together holistically. This paper offers a cost-effective, time-efficient, convenient and quantitative way to achieve better and safer battery designs for multiple applications.

Journal article

Pang M-C, Wei Y, Wang H, Marinescu M, Yan Y, Offer GJet al., 2020, Large-Format Bipolar and Parallel Solid-State Lithium-Metal Cell Stacks: A Thermally Coupled Model-Based Comparative Study, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 167, ISSN: 0013-4651

Journal article

Jiang Y, Offer GJ, Jiang J, Marinescu M, Wang Het al., 2020, Voltage hysteresis model for silicon electrodes for lithium ion batteries, including multi-step phase transformations, crystallization and amorphization, Journal of the Electrochemical Society, Vol: 167, Pages: 1-9, ISSN: 0013-4651

Silicon has been an attractive alternative to graphite as an anode material in lithium-ion batteries (LIBs). The development of better silicon electrodes and the optimization of their operating conditions for longer cycle life require a quantitative understanding of the lithiation/delithiation mechanisms of silicon and how they are linked to the electrode behaviors. Herein we present a zero-dimensional mechanistic model of silicon anodes in LIBs. The model, for the first time, quantitatively accounts for the multi-step phase transformations, crystallization and amorphization of different lithium-silicon phases during cycling while being able to capture the electrode behaviors under different lithiation depths. Based on the model, a linkage between the underlying reaction processes and electrochemical performance is established. In particular, the two sloping voltage plateaus at low lithiation depth are correlated with two electrochemical phase transformations and the emergence of the single broad plateau at high lithiation depth is correlated with the amorphization of c-Li15Si4. The model is then used to study the effects of crystallization rate and surface energy barriers, which clarifies the role of surface energy and particle size in determining the performance behaviors of silicon. The model is a necessary tool for future design and development of high-energy-density, longer-life silicon-based LIBs.

Journal article

Bravo Diaz L, He X, Hu Z, Restuccia F, Marinescu M, Barreras JV, Patel Y, Offer G, Rein Get al., 2020, Review—meta-review of fire safety of lithium-ion batteries: industry challenges and research contributions, Journal of The Electrochemical Society, Vol: 167, Pages: 1-14, ISSN: 0013-4651

The Lithium-ion battery (LIB) is an important technology for the present and future of energy storage, transport, and consumer electronics. However, many LIB types display a tendency to ignite or release gases. Although statistically rare, LIB fires pose hazards which are significantly different to other fire hazards in terms of initiation route, rate of spread, duration, toxicity, and suppression. For the first time, this paper collects and analyses the safety challenges faced by LIB industries across sectors, and compares them to the research contributions found in all the review papers in the field. The comparison identifies knowledge gaps and opportunities going forward. Industry and research efforts agree on the importance of understanding thermal runaway at the component and cell scales, and on the importance of developing prevention technologies. But much less research attention has been given to safety at the module and pack scales, or to other fire protection layers, such as compartmentation, detection or suppression. In order to close the gaps found and accelerate the arrival of new LIB safety solutions, we recommend closer collaborations between the battery and fire safety communities, which, supported by the major industries, could drive improvements, integration and harmonization of LIB safety across sectors.

Journal article

O'Kane SEJ, Campbell ID, Marzook MWJ, Offer GJ, Marinescu Met al., 2020, Physical origin of the differential voltage minimum associated with lithium plating in Li-Ion batteries, Journal of The Electrochemical Society, Vol: 167, Pages: 1-11, ISSN: 0013-4651

The main barrier to fast charging of Li-ion batteries at low temperatures is the risk of short-circuiting due to lithium plating. In-situ detection of Li plating is highly sought after in order to develop fast charging strategies that avoid plating. It is widely believed that Li plating after a single fast charge can be detected and quantified by using a minimum in the differential voltage (DV) signal during the subsequent discharge, which indicates how much lithium has been stripped. In this work, a pseudo-2D physics-based model is used to investigate the effect on Li plating and stripping of concentration-dependent diffusion coefficients in the active electrode materials. A new modelling protocol is also proposed, in order to distinguish the effects of fast charging, slow charging and Li plating/stripping. The model predicts that the DV minimum associated with Li stripping is in fact a shifted and more abrupt version of a minimum caused by the stage II-stage III transition in the graphite negative electrode. Therefore, the minimum cannot be used to quantify stripping. Using concentration-dependent diffusion coefficients yields qualitatively different results to previous work. This knowledge casts doubt on the utility of DV analysis for detecting Li plating.

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

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

Propp K, Auger DJ, Fotouhi A, Marinescu M, Knap V, Longo Set al., 2019, Improved state of charge estimation for lithium-sulfur batteries, JOURNAL OF ENERGY STORAGE, Vol: 26, ISSN: 2352-152X

Journal article

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

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

Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., 2019, 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., 2019, 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

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., 2019, 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

Marinescu M, O'Neill L, Zhang T, Walus S, Wilson T, Offer Get al., 2018, 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

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

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

Zhang T, Marinescu M, Walus S, Kovacik P, Offer GJet al., 2017, What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer?, Journal of the Electrochemical Society, Vol: 165, Pages: A6001-A6004, ISSN: 0013-4651

Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li2S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li2S precipitation.

Journal article

Zhang T, Marinescu M, Walus S, Offer GJet al., 2016, Modelling transport-limited discharge capacity of lithium-sulfur cells, Electrochimica Acta, Vol: 219, Pages: 502-508, ISSN: 0013-4686

Lithium-sulfur (Li-S) battery could bring a step-change in battery technology with a potential specific energy density of 500 - 600 Wh/kg. A key challenge for further improving the specific energy-density of Li-S cells is to understand the mechanisms behind reduced sulfur utilisation at low electrolyte loadings and high discharge currents. While several Li-S models have been developed to explore the discharge mechanisms of Li-S cells, they so far fail to capture the discharge profiles at high currents. In this study, we propose that the slow ionic transport in concentrated electrolyte is limiting the rate capability of Li-S cells. This transport-limitation mechanism is demonstrated through a one-dimensional Li-S model which qualitatively captures the discharge capacities of a sulfolane-based Li-S cell at different currents. Furthermore, our model predicts that a discharged Li-S cell is able regain some capacity with a short period of relaxation. This capacity recovery phenomenon is validated experimentally for different discharge currents and relaxation durations. The transport-limited discharge behavior of Li-S cells highlights the importance of optimizing the electrolyte loading and electrolyte transport property in Li-S cells.

Journal article

von Srbik M-T, Marinescu M, Martinez-Botas RF, Offer GJet al., 2016, A physically meaningful equivalent circuit network model of a lithium-ion battery accounting for local electrochemical and thermal behaviour, variable double layer capacitance and degradation, Journal of Power Sources, Vol: 325, Pages: 171-184, ISSN: 0378-7753

A novel electrical circuit analogy is proposed modellingelectrochemical systems under realistic automotive operation conditions. The model is developed for a lithium ion battery and is based on a pseudo 2D electrochemical model. Although cast in the framework familiar to application engineers, the model is essentially an electrochemical battery model: all variables have a direct physical interpretation and there is direct access to all states of the cell via the model variables (concentrations, potentials) for monitoring and control systems design. This is the first Equivalent Circuit Network-type model that tracks directly the evolution of species inside the cell. It accounts for complex electrochemical phenomena that are usually omitted in online battery performance predictors such as variable double layer capacitance, the full current-overpotential relation and overpotentials due to mass transport limitations. The coupled electrochemical and thermal model accounts for capacity fade via a loss in active species and for power fade via an increase in resistive solid electrolyte passivation layers at both electrodes. The model's capability to simulate cell behaviour under dynamic events is validated against test procedures, such as standard battery testing load cycles for current rates up to 20 C, as well as realistic automotive drive cycle loads.

Journal article

Sarwar W, Engstrom T, Marinescu M, Green N, Taylor N, Offer GJet al., 2016, Experimental analysis of Hybridised Energy Storage Systems for automotive applications, Journal of Power Sources, Vol: 324, Pages: 388-401, ISSN: 0378-7753

The requirements of the Energy Storage System (ESS) for an electrified vehicle portfolio consisting of a range of vehicles from micro Hybrid Electric Vehicle (mHEV) to a Battery Electric Vehicle (BEV) vary considerably. To reduce development cost of an electrified powertrain portfolio, a modular system would ideally be scaled across each vehicle; however, the conflicting requirements of a mHEV and BEV prevent this. This study investigates whether it is possible to combine supercapacitors suitable for an mHEV with high-energy batteries suitable for use in a BEV to create a Hybridised Energy Storage System (HESS) suitable for use in a HEV. A passive HESS is found to be capable of meeting the electrical demands of a HEV drive cycle; the operating principles of HESSs are discussed and factors limiting system performance are explored. The performance of the HESS is found to be significantly less temperature dependent than battery-only systems, however the heat generated suggests a requirement for thermal management. As the HESS degrades (at a similar rate to a specialised high-power-battery), battery resistance rises faster than supercapacitor resistance; as a result, the supercapacitor provides a greater current contribution, therefore the energy throughput, temperature rise and degradation of the batteries is reduced.

Journal article

Propp K, Marinescu M, Auger DJ, O'Neill L, Fotouhi A, Somasundaram K, Offer GJ, Minton G, Longo S, Wild M, Knap Vet al., 2016, Multi-temperature state-dependent equivalent circuit discharge model for lithium-sulfur batteries, Journal of Power Sources, Vol: 328, Pages: 289-299, ISSN: 1873-2755

Lithium-sulfur (Li-S) batteries are described extensively in the literature, but existing computational models aimed at scientific understanding are too complex for use in applications such as battery management. Computationally simple models are vital for exploitation. This paper proposes a non-linear state-of-charge dependent Li-S equivalent circuit network (ECN) model for a Li-S cell under discharge. Li-S batteries are fundamentally different to Li-ion batteries, and require chemistry-specific models. A new Li-S model is obtained using a ‘behavioural’ interpretation of the ECN model; as Li-S exhibits a ‘steep’ open-circuit voltage (OCV) profile at high states-of-charge, identification methods are designed to take into account OCV changes during current pulses. The prediction-error minimization technique is used. The model is parameterized from laboratory experiments using a mixed-size current pulse profile at four temperatures from 10 °C to 50 °C, giving linearized ECN parameters for a range of states-of-charge, currents and temperatures. These are used to create a nonlinear polynomial-based battery model suitable for use in a battery management system. When the model is used to predict the behaviour of a validation data set representing an automotive NEDC driving cycle, the terminal voltage predictions are judged accurate with a root mean square error of 32 mV.

Journal article

Marinescu M, Zhang T, Offer G, 2016, A zero dimensional model of lithium-sulfur batteries during charge and discharge, Physical Chemistry Chemical Physics, Vol: 18, Pages: 584-593, ISSN: 1463-9076

Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large energy density, safety, and possible low cost. Their successful commercialisation is dependent on improving their performance, but also on acquiring sufficient understanding of the underlying mechanisms to allow for the development of predictive models for operational cells. To address the latter, we present a zero dimensional model that predicts many observed features in the behaviour of a lithium-sulfur cell during charge and discharge. The model accounts for two electrochemical reactions via the Nernst formulation, power limitations through Butler-Volmer kinetics, and precipitation/dissolution of one species, including nucleation. It is shown that the precipitation/dissolution causes the flat shape of the low voltage plateau, typical of the lithium-sulfur cell discharge. During charge, it is predicted that the dissolution can act as a bottleneck, as for large enough currents smaller amounts dissolve. This results in reduced charge capacity and an earlier onset of the high plateau reaction, such that the two plateaus merge. By including these effects, the model improves on the existing zero dimensional models, while requiring considerably fewer input parameters and computational resources. The model also predicts that, due to precipitation, the customary way of experimentally measuring the open circuit voltage from a low rate discharge might not be suitable for lithium-sulfur. This model can provide the basis for mechanistic studies, identification of dominant effects in a real cell, predictions of operational behaviour under realistic loads, and control algorithms for applications.

Journal article

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