Imperial College London

Prof Gregory Offer

Faculty of EngineeringDepartment of Mechanical Engineering

Professor in Electrochemical 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

Publication Type
Year
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122 results found

Lander L, Cleaver T, Rajaeifar MA, Viet N-T, Elliott RJR, Heidrich O, Kendrick E, Edge JS, Offer Get al., 2021, Financial viability of electric vehicle lithium-ion battery recycling, ISCIENCE, Vol: 24

Journal article

Schimpe M, Varela Barreras J, Wu B, Offer GJet al., 2021, Battery degradation-aware current derating: an effective method to prolong lifetime and ease thermal management, Journal of The Electrochemical Society, Vol: 168, Pages: 1-13, ISSN: 0013-4651

To ensure the safe and stable operation of lithium-ion batteries in battery energy storage systems (BESS), the power/current is de-rated to prevent the battery from going outside the safe operating range. Most derating strategies use static limits for battery current, voltage, temperature and state-of-charge, and do not account for the complexity of battery degradation. Progress has been made with models of lithium plating for fast charging. However, this is a partial solution, does not consider other degradation mechanisms, and still requires complex optimization work, limiting widespread adoption. In this work, the calendar and cycle degradation model is analysed offline to predetermine the degradation rates. The results are integrated into the current-derating strategy. This framework can be adapted to any degradation model and allows flexible tuning. The framework is evaluated in simulations of an outdoors-installed BESS with passive thermal management, which operates in a residential photovoltaic application. In comparison to standard derating, the degradation-aware derating achieves: (1) increase of battery lifetime by 65%; (2) increase in energy throughput over lifetime by 49%, while III) energy throughput per year is reduced by only 9.5%. These results suggest that the derating framework can become a new standard in current derating.

Journal article

Lander L, Kallitsis E, Hales A, Edge JS, Korre A, Offer Get al., 2021, Cost and carbon footprint reduction of electric vehicle lithium-ion batteries through efficient thermal management, Applied Energy, Vol: 289, Pages: 1-10, ISSN: 0306-2619

Electric vehicles using lithium-ion batteries are currently the most promising technology to decarbonise the transport sector from fossil-fuels. It is thus imperative to reduce battery life cycle costs and greenhouse gas emissions to make this transition both economically and environmentally beneficial. In this study, it is shown that battery lifetime extension through effective thermal management significantly decreases the battery life cycle cost and carbon footprint. The battery lifetime simulated for each thermal management system is implemented in techno-economic and life cycle assessment models to calculate the life cycle costs and carbon footprint for the production and use phase of an electric vehicles. It is demonstrated that by optimising the battery thermal management system, the battery life cycle cost and carbon footprint can be reduced by 27% (from 0.22 $·km−1 for air cooling to 0.16 $·km−1 for surface cooling) and 25% (from 0.141 kg CO2 eq·km−1 to 0.104 kg CO2 eq·km−1), respectively. Moreover, the importance of cell design for cost and environmental impact are revealed and an improved cell design is proposed, which reduces the carbon footprint and life cycle cost by 35% to 0.0913 kg CO2 eq·km−1 and 40% to 0.133 $·km−1, respectively, compared with conventional cell designs combined with air cooling systems.

Journal article

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

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

Edge JS, O'Kane S, Prosser R, Kirkaldy ND, Patel AN, Hales A, Ghosh A, Ai W, Chen J, Yang J, Li S, Pang M-C, Bravo Diaz L, Tomaszewska A, Marzook MW, Radhakrishnan KN, Wang H, Patel Y, Wu B, Offer GJet al., 2021, Lithium ion battery degradation: what you need to know, Physical Chemistry Chemical Physics, Vol: 23, Pages: 8200-8221, ISSN: 1463-9076

The expansion of lithium-ion batteries from consumer electronics to larger-scale transport and energy storage applications has made understanding the many mechanisms responsible for battery degradation increasingly important. The literature in this complex topic has grown considerably; this perspective aims to distil current knowledge into a succinct form, as a reference and a guide to understanding battery degradation. Unlike other reviews, this work emphasises the coupling between the different mechanisms and the different physical and chemical approaches used to trigger, identify and monitor various mechanisms, as well as the various computational models that attempt to simulate these interactions. Degradation is separated into three levels: the actual mechanisms themselves, the observable consequences at cell level called modes and the operational effects such as capacity or power fade. Five principal and thirteen secondary mechanisms were found that are generally considered to be the cause of degradation during normal operation, which all give rise to five observable modes. A flowchart illustrates the different feedback loops that couple the various forms of degradation, whilst a table is presented to highlight the experimental conditions that are most likely to trigger specific degradation mechanisms. Together, they provide a powerful guide to designing experiments or models for investigating battery degradation.

Journal article

Prosser R, Offer G, Patel Y, 2021, Lithium-Ion Diagnostics: The First Quantitative In-Operando Technique for Diagnosing Lithium Ion Battery Degradation Modes under Load with Realistic Thermal Boundary Conditions, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 168, ISSN: 0013-4651

Journal article

Hua X, Zhang C, Offer G, 2021, Finding a better fit for lithium ion batteries: A simple, novel, load dependent, modified equivalent circuit model and parameterization method, JOURNAL OF POWER SOURCES, Vol: 484, ISSN: 0378-7753

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., 2021, 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, Pages: 1-23, ISSN: 0013-4651

Despite the potential of solid electrolytes in replacing liquid electrolytes, solid-state lithium-metal batteries have not been commercialised for large-scale applications due to manufacturing constraints. In this study, we demonstrate that the desired energy and power output for large-format solid-state lithium-metal batteries can be achieved by scaling and stacking unit cells. Two stack configurations, a bipolar and a parallel stack are modelled and compared. With 63 cells stacked in series, we show that a bipolar stack could reach a stack voltage up to 265 V. In contrast, a parallel stack with 32 double-coated cells could achieve a nominal capacity of 4 Ah. We also demonstrate that the choice of current collectors is critical in determining the gravimetric power and energy density of both stacks. By coupling the electrochemical stack model thermally, we show that the Joule heating effects are negligible for bipolar stacks but become dominant for parallel stacks. Bipolar stacks are better due to their higher power and energy densities and lower heat generation, but a lower Coulombic stack capacity limits their performance. In contrast, parallel stacks generate more heat and require more advanced thermal management. These thermally-coupled stack models can be used as prototypes to aid the future development of large-format solid-state batteries.

Journal article

Pang MC, 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

The behavior of solid-state batteries to many application-relevant operating conditions is intrinsically multiphysical and multiscale, involving the electrochemical performance and chemical stability coupled with the thermal and mechanical properties of multiple components. This review presents a holistic approach to discussing the multiscale physical-electro-chemical interactions and degradation mechanisms in solid-state batteries. While the propagation of lithium filaments depends strongly on the critical current densities, we show that effective prevention of excessive Li plating and stripping requires a combined understanding of solid-state electrochemistry, microstructure, mechanics, operating conditions, and their interactions. A review of how multiphysical interactions affect the optimum design of thin-film, three-dimensional and composite solid-state cell architectures is also included. Although the use of lithium metal as negative electrodes could improve the energy densities of solid-state batteries, we show that its high homologous temperature could cause cell failure during manufacturing. By comparing published model predictions with experimental observations, we present a critical analysis of the strengths and limitations of state-of-the-art models and characterization techniques in solid-state battery research. This comprehensive mechanistic analysis provides an insight into the interplay among the multiple complex multiphysical mechanisms, shedding light on the process of cell design for next-generation solid-state batteries.

Journal article

Gopalakrishnan K, Offer GJ, 2021, A Composite Single Particle Lithium-Ion Battery Model Through System Identification, IEEE Transactions on Control Systems Technology, ISSN: 1063-6536

In the realm of lithium-ion (Li-ion) battery modeling, owing to its simplicity, the single particle model (SPM) has long been considered to be a promising reduced-order model (ROM) candidate to usher in the era of physics-inspired models (PIMs) in embedded applications. However, at high load currents, the standard SPM exhibits poor accuracies in computing the cell's terminal voltage, thereby rendering it unsuitable as the plant model in state-estimation tasks. A comprehensive evaluation of the salient electrolyte-enhanced SPMs from the literature reveals that current solutions are either mathematically intractable or overly simplistic. For the ionic concentration in the electrolyte, the well-known quadratic approximation model, which straddles the boundary of computational complexity and mathematical tractability, reveals poor temporal performance, particularly at the current collector interfaces. In this work, we retain the spatial dynamics of the quadratic approximation model while proposing a novel approach using system identification techniques for its temporal dynamics. By employing linear approximations for the relevant subsystems, we identify discrete-time transfer functions of the debiased number of moles per unit area of lithium ions in the electrolyte within each electrode region, that yield improved spatio-temporal accuracies for the electrolyte concentration profile. We then augment the standard SPM with the new system identification-based electrolyte dynamics to arrive at an electrolyte-enhanced composite SPM (EECSPM). Finally, we demonstrate the superior performance of the EECSPM compared with the incumbent state of the art, thereby representing a concrete advancement toward the goal of using PIMs in real-time applications.

Journal article

Chen B, Xuan J, Offer GJ, Wang Het al., 2020, Multiplex measurement of diffusion in zinc battery electrolytes from microfluidics using Raman microspectroscopy, Applied Energy, Vol: 279, Pages: 1-6, ISSN: 0306-2619

Rechargeable zinc batteries have emerged as an inexpensive and safe post-lithium-ion battery technology and have received increasing research interest. Developing suitable electrolytes and understanding their transport properties lie at the heart of successful zinc battery technologies as the battery behaviour is a strong function of ion transport in the electrolytes. To accelerate the research and development process, herein we demonstrate a low-cost and high-throughput approach to measure the diffusion in zinc electrolytes at different concentrations simultaneously. The new approach combines Raman microspectroscopy and a multiplexed microfluidic chip with integrated micromixers, concentration gradient generators and a Y-sensor array. Aqueous-based zinc sulphate electrolytes, widely used in zinc batteries, were used for a proof-of-concept. The measured diffusion coefficients for different electrolyte concentrations show good agreement with literature values. With four electrolyte samples in this study, the developed approach requires minimum 0.5 mL of the electrolyte solutions and 30 mins, which is over ten times faster than a typical diffusion measurement with the conventional electrochemical approach in restricted-diffusion cells. The microfluidic chip is readily scalable to further increase the throughput, and can be extended to for use of measuring different (i.e. organic and aqueous) and even mixtures of electrolytes (i.e. ethylene carbonate and dimethyl carbonate) as well as salts (Li+, Na+, Mg2+, etc.).

Journal article

Prosser R, Patel Y, Offer GJ, 2020, Lithium-Ion Battery Degradation Mode Diagnostics Using Heat Generation Profiles, ECS Meeting Abstracts, Vol: MA2020-02, Pages: 3175-3175

Journal article

Schimpe M, Barreras JV, Wu B, Offer GJet al., 2020, Novel Degradation Model-Based Current Derating Strategy for Lithium-Ion-Batteries, Publisher: The Electrochemical Society, Pages: 3808-3808

Conference paper

Hales A, Prosser R, Diaz LB, White G, Patel Y, Offer Get al., 2020, The Cell Cooling Coefficient as a design tool to optimise thermal management of lithium-ion cells in battery packs, ETRANSPORTATION, Vol: 6, ISSN: 2590-1168

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

Chen B, Zhang H, Xuan J, Offer GJ, Wang Het al., 2020, Seeing is Believing: In Situ/Operando Optical Microscopy for Probing Electrochemical Energy Systems, ADVANCED MATERIALS TECHNOLOGIES, Vol: 5, ISSN: 2365-709X

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

Dondelewski O, Szemberg OConnor T, Zhao Y, Hunt IA, Holland A, Hales A, Offer GJ, Patel Yet al., 2020, The role of cell geometry when selecting tab or surface cooling to minimise cell degradation, eTransportation, Vol: 5, Pages: 1-12, ISSN: 2590-1168

Thermal management of lithium ion batteries is critical to maintain cells at their optimum temperature and balance performance with degradation. Previous work has shown tab cooling to be better for performance and lifetime, but only if sufficient heat removal can be achieved, which depends in part on cell geometry. In this paper, a large form-factor pouch cell is shown to suffer from faster degradation when tab-cooled although still benefitting from higher useable energy. This paper introduces the ratio of surface-to-tab cell cooling coefficient, CCCratio, as a qualitative measure to assess a cell’s suitability for tab cooling. For low CCCratio cells, tab cooling results in more useable energy and lower degradation rates than surface cooling. However, the large pouch cell used in this study has a high CCCratio, indicating that it is difficult to remove sufficient heat through tab cooling. At beginning of life, tab cooling allows access to more usable energy in the cell, but the rate of high temperature-induced degradation is greater, compared to the surface cooled cell. As a result, the useable energy from the tab cooled cell diminishes more rapidly, and after a certain cycle count, the useable energy from the surface cooled cell is superior. The optimum cooling approach will therefore be dependent on the desired lifetime of the system. This research should be of particular interest to cell and battery pack designers.

Journal article

Offer G, Szemberg O'Connor T, De Marco M, 2020, Opportunities for disruptive advances through engineering for next generation energy storage

Throughout human history, major economic disruption has been due to technological breakthroughs.Since 1990 the energy density of lithium-ion cells has increased by a factor of four and the cost has dropped by a factor of 10.This has caused disruption to the energy industry, but advances are slowing.The manufacturing and supply chain complexity means that the next big technology will take 15 years to dominate.The academic literature charts this process of development and can be used to show what is in the pipeline.Three candidates that have had a large increase in publication count are: lithium sulphur, solid-state, and sodium-ion technology.From the level of investments in start-ups and academic publication counts, solid‑state cells are closest to maturity.To identify disruption potential, look at uncertainty in performance. Cell lifetime in lithium-ion cells indicates room for improvement.Define a new disruption metric: . Look for areas of industry that lower this metric.Thermal management is a lucrative area for improvement. Cooling the cell tabs of a 5Ah cell reduces the lifetime cost by 66%, compared to 8%/pa for 13 years relying on cost reduction.Second life applications lower the lifetime cost by using the remaining 75% of energy throughput available in a cell after use in an electric vehicle.Drop-in changes to standard manufacturing processes enable huge disruption. Electrolyte additives can increase cell life by 10 times, lowering lifetime cost by 90% in a simple manufacturing intervention.

Report

Offer G, Patel Y, Hales A, Bravo Diaz L, Marzook Met al., 2020, Cool metric for lithium-ion batteries could spur progress, Nature, Vol: 582, Pages: 485-487, ISSN: 0028-0836

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

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

Feng X, Merla Y, Weng C, Ouyang M, He X, Liaw BY, Santhanagopalan S, Li X, Liu P, Lu L, Han X, Ren D, Wang Y, Li R, Jin C, Huang P, Yi M, Wang L, Zhao Y, Patel Y, Offer Get al., 2020, A reliable approach of differentiating discrete sampled-data for battery diagnosis, ETRANSPORTATION, Vol: 3, ISSN: 2590-1168

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

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