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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167 results found

Bonkile MP, Jiang Y, Kirkaldy N, Sulzer V, Timms R, Wang H, Offer G, Wu Bet al., 2023, Coupled electrochemical-thermal-mechanical stress modelling in composite silicon/graphite lithium-ion battery electrodes, Journal of Energy Storage, Vol: 73, ISSN: 2352-152X

Silicon is often added to graphite battery electrodes to enhance the electrode-specific capacity, but it undergoes significant volume changes during (de)lithiation, which results in mechanical stress, fracture, and performance degradation. To develop long-lasting and energy-dense batteries, it is critical to understand the non-linear stress behaviour in composite silicon-graphite electrodes. In this study, we developed a coupled electrochemical-thermal-mechanical model of a composite silicon/graphite electrode in PyBaMM (an open-source physics-based modelling platform). The model is experimentally validated against a commercially available LGM50T battery, and the effects of C-rates, depth-of-discharge (DoD), and temperature are investigated. The developed model can reproduce the voltage hysteresis from the silicon and provide insights into the stress response and crack growth/propagation in the two different phases. The stress in the silicon is relatively low at low DoD but rapidly increases at a DoD >~80%, whereas the stress in the graphite increases with decreasing temperature and DoD. At higher C-rates, peak stress in the graphite increases as expected, however, this decreases for silicon due to voltage cut-offs being hit earlier, leading to lower active material utilisation since silicon is mostly active at high DoD. Therefore, this work provides an improved understanding of stress evolution in composite silicon/graphite lithium-ion batteries.

Journal article

Li S, Marzook MW, Zhang C, Offer GJ, Marinescu Met al., 2023, How to enable large format 4680 cylindrical lithium-ion batteries, Applied Energy, Vol: 349, Pages: 1-13, ISSN: 0306-2619

The demand for large format lithium-ion batteries is increasing, because they can be integrated and controlled easier at a system level. However, increasing the size leads to increased heat generation risking overheating. 1865 and 2170 cylindrical cells can be both base cooled or side cooled with reasonable efficiency. Large format 4680 cylindrical cells have become popular after Tesla filed a patent. If these cells are to become widely used, then understanding how to thermally manage them is essential. In this work, we create a model of a 4680 cylindrical cell, and use it to study different thermal management options. Our work elucidates the comprehensive mechanisms how the hot topic ‘tabless design’ improves the performance of 4680 cell and makes any larger format cell possible while current commercial cylindrical cells cannot be simply scaled up to satisfy power and thermal performance. As a consequence, the model identifies the reason for the tabless cell's release: the thermal performance of the 4680 tabless cell can be no worse than that of the 2170 cell, while the 4680 tabless tab cell boasts 5.4 times the energy and 6.9 times the power. Finally, via the model, a procedure is proposed for choosing the thermal management for large format cylindrical cell for maximum performance. As an example, we demonstrate that the best cooling approach for the 4680 tabless cell is base cooling, while for the 2170 LG M50T cell it is side cooling. We conclude that any viable large format cylindrical cell must include a continuous tab (or ‘tabless’) design and be cooled through its base when in a pack. The results are of immediate interest to both cell manufacturers and battery pack designers, while the developed modelling and parameterization framework is of wider use for all energy storage system design.

Journal article

Ruan H, Barreras JV, Steinhardt M, Jossen A, Offer GJ, Wu Bet al., 2023, The heating triangle: A quantitative review of self-heating methods for lithium-ion batteries at low temperatures, Journal of Power Sources, Vol: 581, Pages: 1-16, ISSN: 0378-7753

Lithium-ion batteries at low temperatures have slow recharge times alongside reduced available power and energy. Battery heating is a viable way to address this issue, and self-heating techniques are appealing due to acceptable efficiency and speed. However, there are a lack of studies quantitatively comparing self-heating methods rather than qualitatively, because of the existence of many different batteries with varied heating parameters. In this work, we review the current state-of-the-art self-heating methods and propose the heating triangle as a new quantitative indicator for comparing self-heating methods, towards identifying/developing effective heating approaches. We define the heating triangle which considers three fundamental metrics: the specific heating rate (°C·g·J−1), coefficient of performance (COP) (−), and specific temperature difference (°C·hr), enabling a quantitative assessment of self-heating methods using data reported in the literature. Our analysis demonstrates that very similar metrics are observed for the same type of self-heating method, irrespective of the study case, supporting the universality of the proposed indicator. With the comparison insights, we identify research gaps and new avenues for developing advanced self-heating methods. This work demonstrates the value of the proposed heating triangle as a standardised approach to compare heating methods and drive innovation.

Journal article

Vadhva P, Boyce AM, Patel A, Shearing PR, Offer G, Rettie AJEet al., 2023, Silicon-Based Solid-State Batteries: Electrochemistry and Mechanics to Guide Design and Operation., ACS Appl Mater Interfaces, Vol: 15, Pages: 42470-42480

Solid-state batteries (SSBs) are promising alternatives to the incumbent lithium-ion technology; however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids causing diminished cycle life due to fracture and delamination. This modeling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimize the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si, which leads to localized strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure is desirable. Alternatively, the mechanical properties of the SE could be tailored to optimize cell performance. To reduce Si stress, a SE with a moderate Young's modulus similar to that of lithium phosphorous oxynitride (∼77 GPa) with a low yield strength comparable to sulfides (∼0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young's modulus (∼29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasizes the need for SE material selection and the consideration of other cell components in order to optimize the performance of

Journal article

Zhuo M, Kirkaldy N, Maull T, Engstrom T, Offer G, Marinescu Met al., 2023, Diffusion-aware voltage source: An equivalent circuit network to resolve lithium concentration gradients in active particles, Applied Energy, Vol: 339, ISSN: 0306-2619

Traditional equivalent circuit models (ECMs) have difficulties in estimating battery internal states due to the lack of relevant physics, such as the lithium diffusion in active particles. Here we configure a circuit network to describe the lithium diffusion and define it as a new high-level circuit element called diffusion-aware voltage source. The circuit representation is proven equivalent to the discretized diffusion equation. The new voltage source gives the electrode potential as a function of the surface concentration and thus automatically incorporates the diffusion overpotential. We show that an ECM with the proposed diffusion-aware voltage sources (called “shell ECM”) can reproduce the single particle model simulation results, making it a trustworthy easy-to-implement substitute. Furthermore, the simplest shell ECM consisting of a single diffusion-aware voltage source and a resistor is validated against experimental constant-current discharges at various rates. The diffusion-aware voltage source can be used to measure diffusivity by fitting the diffusion resistance against experimental data. The viability of the shell ECM for onboard usage is confirmed by implementation into a battery management system of WAE Technologies. By tracking the internal concentration states, the shell ECM demonstrates robustness to dynamic applied-current profiles.

Journal article

Zhuo M, Offer G, Marinescu M, 2023, Degradation model of high-nickel positive electrodes: Effects of loss of active material and cyclable lithium on capacity fade, Journal of Power Sources, Vol: 556, Pages: 1-16, ISSN: 0378-7753

Nickel-rich layered oxides have been widely used as positive electrode materials for high-energy-density lithium-ion batteries, but the underlying mechanisms of their degradation have not been well understood. Here we present a model at the particle level to describe the structural degradation caused by phase transition in terms of loss of active material (LAM), loss of lithium inventory (LLI), and resistance increase. The particle degradation model is then incorporated into a cell-level P2D model to explore the effects of LAM and LLI on capacity fade in cyclic ageing tests. It is predicted that the loss of cyclable lithium (trapped in the degraded shell) leads to a shift in the stoichiometry range of the negative electrode but does not directly contribute to the capacity loss, and that the loss of positive electrode active materials dominates the fade of usable cell capacity in discharge. The available capacity at a given current rate is further decreased by the additional resistance of the degraded shell layer. The change pattern of the state-of-charge curve provides information of more dimensions than the conventional capacity-fade curve, beneficial to the diagnosis of degradation modes. The model has been implemented into PyBaMM and the source codes are openly available in the GitHub repository https://github.com/mzzhuo/PyBaMM/tree/pe_degradation.

Journal article

Lander L, Tagnon C, Nguyen-Tien V, Kendrick E, Elliott RJR, Abbott AP, Edge JS, Offer GJet al., 2023, Breaking it down: A techno-economic assessment of the impact of battery pack design on disassembly costs, Applied Energy, Vol: 331, Pages: 1-9, ISSN: 0306-2619

The electrification of the transport sector is a critical part of the net-zero transition. The mass adoption of electric vehicles (EVs) powered by lithium-ion batteries in the coming decade will inevitably lead to a large amount of battery waste, which needs handling in a safe and environmentally friendly manner. Battery recycling is a sustainable treatment option at the battery end-of-life that supports a circular economy. However, heterogeneity in pack designs across battery manufacturers are hampering the establishment of an efficient disassembly process, hence making recycling less viable. A comprehensive techno-economic assessment of the disassembly process was conducted, which identified cost hotspots in battery pack designs and to guide design optimisation strategies that help save time and cost for end-of-life treatment. The analyses include six commercially available EV battery packs: Renault Zoe, Nissan Leaf, Tesla Model 3, Peugeot 208, BAIC and BYD Han. The BAIC and BYD battery packs exhibit lower disassembly costs (US$50.45 and US$47.41 per pack, respectively), compared to the Peugeot 208 and Nissan Leaf (US$186.35 and US$194.11 per pack, respectively). This variation in disassembly cost is due mostly to the substantial differences in number of modules and fasteners. The economic assessment suggests that full automation is required to make disassembly viable by 2040, as it could boost disassembly capacity by up to 600 %, while substantially achieving cost savings of up to US$190 M per year.

Journal article

Jiang Y, Zhang L, Offer G, Wang Het al., 2022, A user-friendly lithium battery simulator based on open-source CFD, Digital Chemical Engineering, Vol: 5, Pages: 1-14, ISSN: 2772-5081

The growing use of lithium-ion batteries (LIBs) for automotive and stationary storage applications has put increasingly stringent requirements on battery thermal management and battery safety. An open-source platform that can bridge battery electrochemical models and computational fluid dynamics (CFD) can be of great benefit for designing advanced battery thermal management systems and safety countermeasures by allowing the simulation and prediction of battery responses to various thermofluidic environments and thermal boundaries. Here we develop a user-friendly battery simulator based on the open-source CFD code OpenFOAM. The simulator contains the in-house solvers for the two mostly used physics-based battery models, the single particle model, and the pseudo-two-dimensional model. GUIs are also developed based on Qt for simulation automation and ease of use. To demonstrate the functionality of the developed simulator, the electrochemical performance and internal states of half LIB cells and full LIB cells with different chemistries at different operating conditions are simulated. The obtained results agree well with other existing battery simulators. Due to its native integration with OpenFOAM, the new battery simulator is readily extendable to incorporate various CFD models and other physics to meet the simulation needs of thermal management and safety design for LIBs.

Journal article

Kirkaldy N, Samieian MA, Offer GJ, Marinescu M, Patel Yet al., 2022, Lithium-ion battery degradation: measuring rapid loss of active silicon in silicon-graphite composite electrodes, ACS Applied Energy Materials, Vol: 5, Pages: 13367-13376, ISSN: 2574-0962

To increase the specific energy of commercial lithium-ion batteries, silicon is often blended into the graphite negative electrode. However, due to large volumetric expansion of silicon upon lithiation, these silicon–graphite (Si–Gr) composites are prone to faster rates of degradation than conventional graphite electrodes. Understanding the effect of this difference is key to controlling degradation and improving cell lifetimes. Here, the effects of state-of-charge and temperature on the aging of a commercial cylindrical cell with a Si–Gr electrode (LG M50T) are investigated. The use of degradation mode analysis enables quantification of separate rates of degradation for silicon and graphite and requires only simple in situ electrochemical data, removing the need for destructive cell teardown analyses. Loss of active silicon is shown to be worse than graphite under all operating conditions, especially at low state-of-charge and high temperature. Cycling the cell over 0–30% state-of-charge at 40 °C resulted in an 80% loss in silicon capacity after 4 kA h of charge throughput (∼400 equiv full cycles) compared to just a 10% loss in graphite capacity. The results indicate that the additional capacity conferred by silicon comes at the expense of reduced lifetime. Conversely, reducing the utilization of silicon by limiting the depth-of-discharge of cells containing Si–Gr will extend their lifetime. The degradation mode analysis methods described here provide valuable insight into the causes of cell aging by separately quantifying capacity loss for the two active materials in the composite electrode. These methods provide a suitable framework for any experimental investigations involving composite electrodes.

Journal article

Xie Y, Hales A, Li R, Feng X, Patel Y, Offer Get al., 2022, Thermal management optimization for large-format lithium-ion battery using cell cooling coefficient, Journal of The Electrochemical Society, Vol: 169, Pages: 1-10, ISSN: 0013-4651

The surface cooling technology of power battery pack has led to undesired temperature gradient across the cell during thermal management and the tab cooling has been proposed as a promising solution. This paper investigates the feasibility of applying tab cooling in large-format lithium-ion pouch cells using the Cell Cooling Coefficient (CCC). A fundamental problem with tab cooling is highlighted, the CCC for tab cooling decreases as capacity increases. Coupling low CCCs with greater heat generation leads to significant temperature gradients across the cell. Here, the "bottleneck" that limits heat rejection through the tabs is evaluated. The thermal resistance of the physical tabs is identified to be the main contributor towards the poor heat rejection pathway. A numerical thermal model is used to explore the effect of increased tab thickness and results showed that the cell-wide temperature gradients could be significantly reduced. At the negative tab, increasing from 0.2 mm to 2 mm led to a 100% increase in CCCneg whilst increasing the positive tab from 0.45 mm to 2 mm led to an 82% increasing in CCCpos. Together, this is shown to contribute to a 51% reduction in temperature gradient across the cell in any instance of operation.

Journal article

Vadhva P, Boyce AM, Hales A, Pang M-C, Patel AN, Shearing PR, Offer G, Rettie AJEet al., 2022, Towards optimised cell design of thin film silicon-based solid-state batteries via modelling and experimental characterisation, Journal of The Electrochemical Society, Vol: 169, Pages: 1-11, ISSN: 0013-4651

To realise the promise of solid-state batteries, negative electrode materials exhibiting large volumetric expansions, such as Li and Si, must be used. These volume changes can cause significant mechanical stresses and strains that affect cell performance and durability, however their role and nature in SSBs are poorly understood. Here, a 2D electro-chemo-mechanical model is constructed and experimentally validated using steady-state, transient and pulsed electrochemical methods. The model geometry is taken as a representative cross-section of a non-porous, thin-film solid-state battery with an amorphous Si (a-Si) negative electrode, lithium phosphorous oxynitride (LiPON) solid electrolyte and LiCoO2 (LCO) positive electrode. A viscoplastic model is used to predict the build-up of strains and plastic deformation of a-Si as a result of (de)lithiation during cycling. A suite of electrochemical tests, including electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique and hybrid pulse power characterisation are carried out to establish key parameters for model validation. The validated model is used to explore the peak interfacial (a-Siāˆ£LiPON) stress and strain as a function of the relative electrode thickness (up to a factor of 4), revealing a peak volumetric expansion from 69% to 104% during cycling at 1C. The validation of this electro-chemo-mechanical model under load and pulsed operating conditions will aid in the cell design and optimisation of solid-state battery technologies.

Journal article

Marzook MW, Hales A, Patel Y, Offer G, Marinescu Met al., 2022, Thermal evaluation of lithium-ion batteries: Defining the cylindrical cell cooling coefficient, Journal of Energy Storage, Vol: 54, Pages: 1-9, ISSN: 2352-152X

Managing temperatures of lithium-ion cells in battery packs is crucial to ensuring their safe operation. However, thermal information provided on typical cell datasheets is insufficient to identify which cells can be easily thermally managed. The Cell Cooling Coefficient (CCC) aims to fill this gap, as a metric that defines the thermal dissipation from a cell when rejecting its own heat. While the CCC has been defined and used for pouch cells, no similar measure has been proven for cylindrical cells. This work successfully defines and measures the CCC for cylindrical cells under base cooling (CCCBase), defined as the heat rejected through the base divided by the temperature difference from the base to positive cap. Using a non-standard, electrically optimised connection, the maxima for CCCBase of an LG M50T (21700) and Samsung 30Q (18650) cell are successfully measured to be 0.139 and 0.115 W K−1, respectively. Even though the 21700 has a higher CCCBase, indicating that the cell can be cooled more efficiently, comparing the CCCBase per area the 18650 can reject 13 % more heat for a given cooled area. A worked example demonstrates the equal importance of understanding heat generation alongside the CCC, for both cell design and down selecting cells.

Journal article

Samieian MA, Garcia CE, Bravo Diaz L, Hales A, Patel Y, Offer GJet al., 2022, Large scale immersion bath for isothermal testing of lithium-ion cells, HardwareX, Vol: 12, Pages: 1-15, ISSN: 2468-0672

Testing of lithium-ion batteries depends greatly on accurate temperature control in order to generate reliable experimental data. Reliable data is essential to parameterise and validate battery models, which are essential to speed up and reduce the cost of battery pack design for multiple applications. There are many methods to control the temperature of cells during testing, such as forced air convection, liquid cooling or conduction cooling using cooling plates. Depending on the size and number of cells, conduction cooling can be a complex and costly option. Although easier to implement, forced air cooling is not very effective and can introduce significant errors if used for battery model parametrisation. Existing commercially available immersion baths are not cost effective (∼£3320) and are usually too small to hold even one large pouch cell. Here, we describe an affordable but effective cooling method using immersion cooling. This bath is designed to house eight large lithium-ion pouch cells (300mm x 350mm), each immersed in a base oil cooling fluid (150L total volume). The total cost of this setup is only £1670. The rig is constructed using a heater, chilling unit, and a series of pumps. This immersion bath can maintain a temperature within 0.5 °C of the desired set point, it is operational within the temperature range 5 – 55 °C and has been validated at a temperature range of 25 – 45 °C.

Journal article

White G, Hales A, Patel Y, Offer Get al., 2022, Novel methods for measuring the thermal diffusivity and the thermal conductivity of a lithium-ion battery, APPLIED THERMAL ENGINEERING, Vol: 212, Pages: 1-12, ISSN: 1359-4311

Thermal conductivity is a fundamental parameter in every battery pack model. It allows for the calculation of internal temperature gradients which affect cell safety and cell degradation. The accuracy of the measurement for thermal conductivity is directly proportional to the accuracy of any thermal calculation. Currently the battery industry uses archaic methods for measuring this property which have errors up to 50 %. This includes the constituent material approach, the Searle’s bar method, laser/Xeon flash and the transient plane source method. In this paper we detail three novel methods for measuring both the thermal conductivity and the thermal diffusivity to within 5.6 %. These have been specifically designed for bodies like lithium-ion batteries which are encased in a thermally conductive material. The novelty in these methods comes from maintaining a symmetrical thermal boundary condition about the middle of the cell. By using symmetric boundary conditions, the thermal pathway around the cell casing can be significantly reduced, leading to improved measurement accuracy. These novel methods represent the future for thermal characterisation of lithium-ion batteries. Continuing to use flawed measurement methods will only diminish the performance of battery packs and slow the rate of decarbonisation in the transport sector.

Journal article

Tomaszewska A, Doel R, Parkes M, Offer GJ, Wu Bet al., 2022, Investigating Li Plating Distribution Caused By a Thermal Gradient through Modelling, Differential Voltage, and Post-Mortem Analysis, ECS Meeting Abstracts, Vol: MA2022-01, Pages: 186-186

<jats:p> Relatively slow charging speeds are often quoted as a key barrier to customer acceptance of EVs. Currently, the charging rates are limited primarily by the risk of lithium plating. While traditionally lithium plating has been associated with low temperature charging, recent reports point to the fact that thermal heterogeneity can significantly affect the plating behaviour, sometimes making it more likely or accelerated in the warmer regions in a cell [1][2]. In EVs, through-plane thermal gradients often develop across individual pouch cells due to the widespread use of surface cooling, particularly during fast charging, when the heat generation rates are also increased. This work investigates the effects of such through-plane thermal gradients on the lithium plating behaviour using a multilayer 2D electrochemical-thermal model and high-rate cycling experiments. The results show that the thermal gradient can result in preferential plating in either the colder or warmer cell regions, depending on the average cell temperature and the activation energies of solid diffusion and lithium plating. While the diffusion rates are slower in the colder cell layers, warmer ones attract higher currents and either of these effects may dominate the plating behaviour. The experimental validation consists of differential voltage analysis, post-mortem visual examination and measurement of remaining capacity in coin cells harvested from Li-ion cells fast charged under uniform temperatures and under thermal gradients. The limitations of DVA as a technique to quantify lithium plating are highlighted. These stem from the fact that the quantification technique requires assuming that only lithium stripping and no de-intercalation takes place up to the differential voltage minimum. In reality, the current is divided between both reactions, and both the temperature and concentration of the metallic lithium may affect the rate of stripping, shifting the location of the minimum

Journal article

O'Kane SEJ, Kirkaldy N, Offer GJ, Marinescu Met al., 2022, Lithium-Ion Battery Degradation: How to Diagnose It, ECS Meeting Abstracts, Vol: MA2022-01, Pages: 396-396

<jats:p> Many different degradation mechanisms occur in lithium-ion batteries, all of which interact with one another [1]. However, there are few fewer observable consequences of degradation than there are mechanisms [2]. It is possible to measure the different degradation modes: loss of lithium inventory (LLI), loss of active material (LAM), impedance change and stoichiometric drift [3].</jats:p> <jats:p>It is not always possible to link these observable consequences of degradation to any particular mechanism or combination of mechanisms. Many models of degradation exist [4], but these models have many parameters that cannot be measured directly. A recent modelling study [5] found the number of parameters that the model is sensitive to is greater than the number of observable degradation modes.</jats:p> <jats:p>However, the same model [5], despite including just four degradation mechanisms, found five possible degradation pathways a battery can follow. The model was built so that more mechanisms can easily be added later, so more pathways will be found.</jats:p> <jats:p>In this work, a new approach to diagnosing battery degradation is proposed, based on these pathways. Experimental data for the degradation modes can be identified as being consistent with a particular pathway. Once the correct pathway is found, the parameters that particular pathway is sensitive to can be fit to the data, feeding back into the model.</jats:p> <jats:p>[1] Jacqueline Edge <jats:italic>et al.</jats:italic>, <jats:italic>Phys. Chem.: Chem. Phys.</jats:italic> vol. 23, pp. 8200-8221, 2021.</jats:p> <jats:p>[2] Christoph Birkl <jats:italic>et al.</jats:italic>, <jats:italic>Journal of Power Sources</jats:italic> vol. 341, pp. 373-386, 2017.</jats:p> <jats:p>[3] Matthieu Dubarry &l

Journal article

Xie Y, Wang S, Li R, Ren D, Yi M, Xu C, Han X, Lu L, Friess B, Offer G, Ouyang Met al., 2022, Inhomogeneous degradation induced by lithium plating in a large-format lithium-ion battery, JOURNAL OF POWER SOURCES, Vol: 542, ISSN: 0378-7753

Journal article

Li R, O'Kane S, Marinescu M, Offer GJet al., 2022, Modelling solvent consumption from SEI layer growth in lithium-ion batteries, Journal of The Electrochemical Society, Vol: 169, Pages: 1-14, ISSN: 0013-4651

Predicting lithium-ion battery (LIB) lifetime is one of the most important challenges holding back the electrification of vehicles,aviation, and the grid. The continuous growth of the solid-electrolyte interface (SEI) is widely accepted as the dominantdegradation mechanism for LIBs. SEI growth consumes cyclable lithium and leads to capacity fade and power fade via severalpathways. However, SEI growth also consumes electrolyte solvent and may lead to electrolyte dry-out, which has only beenmodelled in a few papers. These papers showed that the electrolyte dry-out induced a positive feedback loop between loss of activematerial (LAM) and SEI growth due to the increased interfacial current density, which resulted in capacity drop. This work,however, shows a negative feedback loop between LAM and SEI growth due to the reduced solvent concentration (in our case,EC), which slows down SEI growth. We also show that adding extra electrolyte into LIBs at the beginning of life can greatlyimprove their service life. This study provides new insights into the degradation of LIBs and a tool for cell developers to designlonger lasting batteries.

Journal article

Tomaszewska A, Parkes M, Doel R, Offer G, Wu Bet al., 2022, The Effects of Temperature and Cell Parameters on Lithium-Ion Battery Fast Charging Protocols: A Model-Driven Investigation, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 169, ISSN: 0013-4651

Journal article

Ai W, Kirkaldy N, Jiang Y, Offer G, Wang H, Wu Bet al., 2022, A composite electrode model for lithium-ion batteries with silicon/graphite negative electrodes, Journal of Power Sources, Vol: 527, Pages: 231142-231142, ISSN: 0378-7753

Silicon is a promising negative electrode material with a high specific capacity, which is desirable for com-mercial lithium-ion batteries. It is often blended with graphite to form a composite anode to extend lifetime,however, the electrochemical interactions between silicon and graphite have not been fully investigated. Here,an electrochemical composite electrode model is developed and validated for lithium-ion batteries with asilicon/graphite anode. The continuum-level model can reproduce the voltage hysteresis and demonstratethe interactions between graphite and silicon. At high states-of-charge, graphite provides the majority of thereaction current density, however this rapidly switches to the silicon phase at deep depths-of-discharge due tothe different open circuit voltage curves, mass fractions and exchange current densities. Furthermore, operationat high C-rates leads to heterogeneous current densities in the through-thickness direction, where peak reactioncurrent densities for silicon can be found at the current collector–electrode side as opposed to the separator–electrode side for graphite. Increasing the mass fraction of silicon also highlights the beneficial impacts ofreducing the peak reaction current densities. This work, therefore, gives insights into the effects of siliconadditives, their coupled interactions and provides a platform to test different composite electrodes for betterlithium-ion batteries.

Journal article

Kallitsis E, Lander L, Edge J, Bravo Diaz L, Brown A, Kelsall G, Offer G, Korre Aet al., 2022, Safe and sustainable lithium-ion batteries, Safe and Sustainable Lithium-ion Batteries, Publisher: Imperial College London - Energy Futures Lab

The transition to clean energy and electric mobility is driving unprecedented demand for lithium-ion batteries (LIBs). This paper investigates the safety and sustainability of LIBs, exploring ways of reducing their impact on the environment and ensuring they do not pose a danger to health of workers or users.

Report

Zhang C, Amietszajew T, Li S, Marinescu M, Offer G, Wang C, Guo Y, Bhagat Ret al., 2022, Real-time estimation of negative electrode potential and state of charge of lithium-ion battery based on a half-cell-level equivalent circuit model, JOURNAL OF ENERGY STORAGE, Vol: 51, ISSN: 2352-152X

Journal article

O'Kane SEJ, Ai W, Madabattula G, Alvarez DA, Timms R, Sulzer V, Edge JS, Wu B, Offer GJ, Marinescu Met al., 2022, Lithium-ion battery degradation: how to model it, Publisher: Royal Society of Chemistry

Predicting lithium-ion battery degradation is worth billions to the globalautomotive, aviation and energy storage industries, to improve performance andsafety and reduce warranty liabilities. However, very few published models ofbattery degradation explicitly consider the interactions between more than twodegradation mechanisms, and none do so within a single electrode. In thispaper, the first published attempt to directly couple more than two degradationmechanisms in the negative electrode is reported. The results are used to mapdifferent pathways through the complicated path dependent and non-lineardegradation space. Four degradation mechanisms are coupled in PyBaMM, an opensource modelling environment uniquely developed to allow new physics to beimplemented and explored quickly and easily. Crucially it is possible to see'inside' the model and observe the consequences of the different patterns ofdegradation, such as loss of lithium inventory and loss of active material. Forthe same cell, five different pathways that can result in end-of-life havealready been found, depending on how the cell is used. Such information wouldenable a product designer to either extend life or predict life based upon theusage pattern. However, parameterization of the degradation models remains as amajor challenge, and requires the attention of the international batterycommunity.

Working paper

Diaz LB, Hales A, Marzook MW, Patel Y, Offer Get al., 2022, Measuring Irreversible Heat Generation in Lithium-Ion Batteries: An Experimental Methodology, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 169, ISSN: 0013-4651

Journal article

Roe C, Feng X, White G, Li R, Wang H, Rui X, Li C, Zhang F, Null V, Parkes M, Patel Y, Wang Y, Wang H, Ouyang M, Offer G, Wu Bet al., 2022, Immersion cooling for lithium-ion batteries – a review, Journal of Power Sources, Vol: 525, Pages: 231094-231094, ISSN: 0378-7753

Battery thermal management systems are critical for high performance electric vehicles, where the ability to remove heat and homogenise temperature distributions in single cells and packs are key considerations. Immersion cooling, which submerges the battery in a dielectric fluid, has the potential of increasing the rate of heat transfer by 10,000 times relative to passive air cooling. In 2-phase systems, this performance increase is achieved through the latent heat of evaporation of the liquid-to-gas phase transition and the resulting turbulent 2-phase fluid flow. However, 2-phase systems require additional system complexity, and single-phase direct contact immersion cooling can still offer up to 1,000 times improvements in heat transfer over air cooled systems. Fluids which have been considered include: hydrofluoroethers, mineral oils, esters and water-glycol mixtures. This review therefore presents the current state-of-the-art in immersion cooling of lithium-ion batteries, discussing the performance implications of immersion cooling but also identifying gaps in the literature which include a lack of studies considering the lifetime, fluid stability, material compatibility, understanding around sustainability and use of immersion for battery safety. Insights from this review will therefore help researchers and developers, from academia and industry, towards creating higher power, safer and more durable electric vehicles.

Journal article

Jiang Y, Niu Z, Offer G, Xuan J, Wang Het al., 2022, Insights into the role of silicon and graphite in the electrochemical performance of silicon/graphite blended electrodes with a multi-material porous electrode model, Journal of The Electrochemical Society, Vol: 169, Pages: 020568-020568, ISSN: 0013-4651

Silicon/graphite blended electrodes are promising candidates to replace graphite in lithium ion batteries, benefiting from the high capacity of silicon and the good structural stability of carbon. Models have proven essential to understand and optimise batteries with new materials. However, most previous models treat silicon/graphite blends as a single “lumped” material, offering limited understanding of the behaviors of the individual materials and thus limited design capability. Here, we present a multi-material model for silicon/graphite electrodes with detailed descriptions of the contributions of the individual active materials. The model shows that silicon introduces voltage hysteresis to silicon/graphite electrodes and consequently a “plateau shift” during delithiation of the electrodes. There will also be competition between the silicon and graphite lithiation reactions depending on silicon/graphite ratio. A dimensionless competing factor is derived to quantify the competition between the two active materials. This is demonstrated to be a useful indicator for active operating regions for each material and we demonstrate how it can be used to design cycling protocols for mitigating electrode degradation. The multi-material electrode model can be readily implemented into full-cell models and coupled with other physics to guide further development of lithium ion batteries with silicon-based electrodes.

Journal article

Steinhardt M, Barreras JV, Ruan H, Wu B, Offer GJ, Jossen Aet al., 2022, Meta-analysis of experimental results for heat capacity and thermal conductivity in lithium-ion batteries: A critical review, Journal of Power Sources, Vol: 522, Pages: 1-25, ISSN: 0378-7753

Scenarios with rapid energy conversion for lithium-ion batteries are increasingly relevant, due to the desire for more powerful electric tools or faster charging electric vehicles. However, higher power means higher cooling requirements, affecting the battery temperature and its thermal gradients. In turn, temperature is a key quantity influencing battery performance, safety and lifetime. Therefore, thermal models are increasingly important for the design and operation of battery systems. Key parameters are specific heat capacity and thermal conductivity. For these parameters, this paper presents a comprehensive review of the experimental results in the literature, where the median values and corresponding uncertainties are summarized. Whenever available, data is analyzed from component to cell level with the discussion of dependencies on temperature, state of charge (SOC) and state of health (SOH). This meta-analysis reveals gaps in knowledge and research needs. For instance, we uncover inconsistencies between the specific heat capacity of electrode-separator stacks and full-cells. For the thermal conductivity, we found that thermal contact resistance and dependencies on battery states have been poorly studied. There is also a lack of measurements at high temperatures, which are required for safety studies. Overall, this study serves as a valuable reference material for both modellers and experimenters.

Journal article

Morgan LM, Islam MM, Yang H, O'Regan K, Patel AN, Ghosh A, Kendrick E, Marinescu M, Offer GJ, Morgan BJ, Islam MS, Edge J, Walsh Aet al., 2022, From Atoms to Cells: Multiscale Modeling of a LiNixMnyCozO2 Cathodes for Li-Ion Batteries, ACS ENERGY LETTERS, Vol: 7, Pages: 108-122, ISSN: 2380-8195

Journal article

Gopalakrishnan K, Offer GJ, 2022, A Composite Single Particle Lithium-Ion Battery Model Through System Identification, IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, Vol: 30, Pages: 1-13, ISSN: 1063-6536

Journal article

Pang M-C, Marinescu M, Wang H, Offer Get al., 2021, Mechanical behaviour of inorganic solid-state batteries: can we model the ionic mobility in the electrolyte with Nernst-Einstein's relation?, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 23, Pages: 27159-27170, ISSN: 1463-9076

Journal article

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