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  • 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
    Yang K, Jia L, Liu X, Wang Z, Wang Y, Li Y, Chen H, Wu B, Yang L, Pan Fet al., 2020,

    Revealing the anion intercalation behavior and surface evolution of graphite in dual-ion batteries via in situ AFM

    , Nano Research, Vol: 13, Pages: 412-418, ISSN: 1998-0124

    Graphite as a positive electrode material of dual ion batteries (DIBs) has attracted tremendous attentions for its advantages including low lost, high working voltage and high energy density. However, very few literatures regarding to the real-time observation of anion intercalation behavior and surface evolution of graphite in DIBs have been reported. Herein, we use in situ atomic force microscope (AFM) to directly observe the intercalation/de-intercalation processes of PF6− in graphite in real time. First, by measuring the change in the distance between graphene layers during intercalation, we found that PF6− intercalates in one of every three graphite layers and the intercalation speed is measured to be 2 µm·min−1. Second, graphite will wrinkle and suffer structural damages at high voltages, along with severe electrolyte decomposition on the surface. These findings provide useful information for further optimizing the capacity and the stability of graphite anode in DIBs.

  • 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
    Ai W, Kraft L, Sturm J, Jossen A, Wu Bet al., 2020,

    Electrochemical thermal-mechanical modelling of stress inhomogeneity in lithium-ion pouch cells

    , Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

    Whilst extensive research has been conducted on the effects of temperature in lithium-ion batteries, mechanical effects have not received as much attention despite their importance. In this work, the stress response in electrode particles is investigated through a pseudo-2D model with mechanically coupled diffusion physics. This model can predict the voltage, temperature and thickness change for a lithium cobalt oxide-graphite pouch cell agreeing well with experimental results. Simulations show that the stress level is overestimated by up to 50% using the standard pseudo-2D model (without stress enhanced diffusion), and stresses can accelerate the diffusion in solid phases and increase the discharge cell capacity by 5.4%. The evolution of stresses inside electrode particles and the stress inhomogeneity through the battery electrode have been illustrated. The stress level is determined by the gradients of lithium concentration, and large stresses are generated at the electrode-separator interface when high C-rates are applied, e.g. fast charging. The results can explain the experimental results of particle fragmentation close to the separator and provide novel insights to understand the local aging behaviors of battery cells and to inform improved battery control algorithms for longer lifetimes.

  • Software
    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.

  • Journal article
    Chen X, Liu X, Ouyang M, Childs P, Brandon N, Wu Bet al., 2019,

    Electrospun composite nanofibre supercapacitors enhanced with electrochemically 3D printed current collectors

    , Journal of Energy Storage, Vol: 26, Pages: 100993-100993, ISSN: 2352-152X

    Carbonised electrospun nanofibres are attractive for supercapacitors due to their relatively high surface area, facile production routes and flexibility. With the addition of materials such as manganese oxide (MnO), the specific capacitance of the carbon nanofibres can be further improved through fast surface redox reactions, however this can reduce the electrical conductivity. In this work, electrochemical 3D printing is used as a novel means of improving electrical conductivity and the current collector-electrode interfacial resistance through the deposition of highly controlled layers of copper. Neat carbonised electrospun electrodes made with a 30 wt% manganese acetylacetonate (MnACAC) and polyacrylonitrile precursor solution have a hydrophobic nature preventing an even copper deposition. However, with an ethanol treatment, the nanofibre films can be made hydrophilic which enhances the copper deposition morphology to enable the formation of a percolating conductive network through the electrode. This has the impact of increasing electrode electronic conductivity by 360% from 10 S/m to 46 S/m and increasing specific capacitance 110% from 99 F/g to 208 F/g at 5 mV/s through increased utilisation of the pseudocapacitive active material. This novel approach thus provides a new route for performance enhancement of electrochemical devices using 3D printing, which opens new design possibilities.

  • 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.

  • Book chapter
    Liu X, George C, Wang H, Wu Bet al., 2019,

    Novel inorganic composite materials for lithium‐ion batteries

    , Encyclopedia of Inorganic and Bioinorganic Chemistry, Publisher: Wiley

    Lithium‐ion batteries (LIBs) have revolutionized the way we interact with the world around us. This is in part due to their unrivaled energy density and stability relative to other energy storage chemistries such as lead‐acid, nickel–metal hydride, and nickel–cadmium batteries. Given the drive to reduce greenhouse gas emissions from road transport, LIBs have now transitioned from application in consumer electronics to be the most critical component for electric vehicles (EVs); however, improvements in energy and power density, cost reduction, and lifetime are still required. The key aspects of an LIB that define its performance are mainly the anode, cathode, and electrolyte, however development of the separator and current collectors are also key considerations. In the vast majority of commercially available LIBs, the anode consists mostly of graphite and the cathode mostly of layered transition metal oxides, with an organic electrolyte facilitating the lithium‐ion transport between the two electrodes. This article provides an overview of the state of the art in developing inorganic composite materials for LIBs and concludes by highlighting the current challenges as well as the potential opportunities in the field.

  • Journal article
    Zhao Y, Diaz LB, Patel Y, Zhang T, Offer GJet al., 2019,

    How to cool lithium ion batteries: optimising cell design using a thermally coupled model

    , Journal of The Electrochemical Society, Vol: 166, Pages: A2849-A2859, ISSN: 0013-4651

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

  • Journal article
    Tomaszewska A, Chu Z, Feng X, O'Kane S, Liu X, Chen J, Ji C, Endler E, Li R, Liu L, Li Y, Zheng S, Vetterlein S, Gao M, Du J, Parkes M, Ouyang M, Marinescu M, Offer G, Wu Bet al., 2019,

    Lithium-ion battery fast charging: A review

    , eTransportation, Vol: 1, Pages: 1-28, ISSN: 2590-1168

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

  • Journal article
    Hales A, Diaz LB, Marzook MW, Zhao Y, Patel Y, Offer Get al., 2019,

    The cell cooling coefficient: A standard to define heatrejection from lithium-ion batteries

    , Journal of The Electrochemical Society, Vol: 166, Pages: A2383-A2395, ISSN: 0013-4651

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

  • Journal article
    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
    Yin C, Liu X, Wei J, Tan R, Zhou J, Ouyang M, Wang H, Cooper SJ, Wu B, George C, Wang Qet al., 2019,

    “All-in-Gel” design for supercapacitors towards solid-state energy devices with thermal and mechanical compliance

    , Journal of Materials Chemistry A, Vol: 7, Pages: 8826-8831, ISSN: 2050-7488

    “All-in-gel” supercapacitor is designed <italic>via</italic> ionogel composite electrolyte and Bucky gel electrodes. These flexible, conductive and shape-conformable gels represent a step change in the design of safe energy storage devices for wearable electronics, in particular those facing the increased demands of hazardous operational environments.

  • 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
    Liu X, Taiwo O, Yin C, Ouyang M, Chowdhury R, Wang B, Wang H, Wu B, Brandon N, Wang Q, Cooper Set al., 2019,

    Aligned lonogel electrolytes for high‐temperature supercapacitors

    , Advanced Science, Vol: 6, Pages: 1-7, ISSN: 2198-3844

    Ionogels are a new class of promising materials for use in all‐solid‐state energy storage devices in which they can function as an integrated separator and electrolyte. However, their performance is limited by the presence of a crosslinking polymer, which is needed to improve the mechanical properties, but compromises their ionic conductivity. Here, directional freezing is used followed by a solvent replacement method to prepare aligned nanocomposite ionogels which exhibit enhanced ionic conductivity, good mechanical strength, and thermal stability simultaneously. The aligned ionogel based supercapacitor achieves a 29% higher specific capacitance (176 F g−1 at 25 °C and 1 A g−1) than an equivalent nonaligned form. Notably, this thermally stable aligned ionogel has a high ionic conductivity of 22.1 mS cm−1 and achieves a high specific capacitance of 167 F g−1 at 10 A g−1 and 200 °C. Furthermore, the diffusion simulations conducted on 3D reconstructed tomography images are employed to explain the improved conductivity in the relevant direction of the aligned structure compared to the nonaligned. This work demonstrates the synthesis, analysis, and use of aligned ionogels as supercapacitor separators and electrolytes, representing a promising direction for the development of wearable electronics coupled with image based process and simulations.

  • Journal article
    Song W, Liu X, Wu B, Brandon N, Shearing PR, Brett DJL, Xie F, Jason Riley Det al., 2019,

    Sn@C evolution from yolk-shell to core-shell in carbon nanofibers with suppressed degradation of lithium storage

    , Energy Storage Materials, Vol: 18, Pages: 229-237, ISSN: 2405-8297

    Metallic Sn has high conductivity and high theoretical capacity for lithium storage but it suffers from severe volume change in lithiation/delithiation leading to capacity fade. Yolk-shell and core-shell Sn@C spheres interconnected by carbon nanofibers were synthesized by thermal vapor and thermal melting of electrospun nanofibers to improve the cycling stability. Sn particles in yolk-shell spheres undergo dynamic structure evolution during thermal melting to form core-shell spheres. The core-shell spheres linked along the carbon nanofibers show outstanding performance and are better than the yolk-shell system for lithium storage, with a high capacity retention of 91.8% after 1000 cycles at 1 A g-1. The superior structure of core-shell spheres interconnected by carbon nanofibers has facile electron conductivity and short lithium ion diffusion pathways through the carbon nanofibers and shells, and re-develops Sn@C structures with Sn clusters embedded into carbon matrix during electrochemical cycling, enabling the high performance.

  • Journal article
    Deshagani S, Liu X, Wu B, Deepa Met al., 2019,

    Nickel cobaltite@Poly(3,4-ethylenedioxypyrrole) and carbon nanofiber interlayer based flexible supercapacitor

    , Nanoscale, Vol: 11, Pages: 2742-2756, ISSN: 2040-3364

    Binder free flexible symmetric supercapacitors are developed with nickel cobaltite micro-flowers coated poly(3,4-ethylenedioxypyrrole) (NiCo2O4@PEDOP) hybrid electrodes. Free standing films of carbon nano-fibers (CNF), synthesized by electrospinning, were sandwiched between the NiCo2O4@PEDOP hybrid and the electrolyte coated separators on both sides of the cells. The CNF film conducts both ions and electrons, and confines the charge at the respective electrodes, to result in an improved specific capacitance (SC) and energy density compared to the analogous cell without the CNF interlayers. High SC of 1,775 F g-1 at a low current density of 0.96 A g-1 and a SC of 634 F g-1 achieved at a high current density of 38 A g-1 coupled with a SC retention of ~95% after 5,000 charge-discharge cycles in the NCO@PEDOP/CNF based symmetric supercapacitor, are performance attributes superior to that achieved with NCO and NCO/CNF based symmetric cells. The PEDOP coating serves as a highly conductive matrix for the NCO micro-flowers and also undergoes doping/de-doping during charge-discharge, thus amplifying the overall supercapacitor response, compared to the individual components. The CNF interlayers show reasonably high ion-diffusion coefficients for K+ and OH- propagation implying facile pathways available for movement of ions across the cross-section of the cell, and they also serve as ion reservoirs. The electrode morphologies remain unaffected by cycling, in the presence of the CNF interlayer. LED illumination and a largely unaltered charge storage response was achieved in a mutli-cell configuration, proving the potential for this approach in practical applications.

  • Journal article
    Yufit V, Tariq F, Biton M, Brandon Net al., 2019,

    Operando visualisation and multi-scale tomography studies of dendrite formation and dissolution in zinc batteries

    , Joule, Vol: 3, Pages: 485-502, ISSN: 2542-4351

    Alternative battery technologies are required to meet growing energy demands and address the limitations of present technologies. As such, it is necessary to look beyond lithium-ion batteries. Zinc batteries enable high power density while being sourced from ubiquitous and cost-effective materials. This paper presents, for the first time known to the authors, multi-length scale tomography studies of failure mechanisms in zinc batteries with and without commercial microporous separators. In both cases, dendrites were grown, dissolved, and regrown, critically resulting in different morphology of dendritic layer formed on both the electrode and the separator. The growth of dendrites and their volume-specific areas were quantified using tomography and radiography data in unprecedented resolution. High-resolution ex situ analysis was employed to characterize single dendrites and dendritic deposits inside the separator. The findings provide unique insights into mechanisms of metal-battery failure effected by growing dendrites.

  • Journal article
    Zhao Y, Spingler FB, Patel Y, Offer GJ, Jossen Aet al., 2019,

    Localized swelling inhomogeneity detection in lithium ion cells using multi-dimensional laser scanning

    , Journal of The Electrochemical Society, Vol: 166, Pages: A27-A34, ISSN: 1945-7111

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

  • Journal article
    Zhao Y, Patel Y, Zhang T, Offer GJet al., 2018,

    Modeling the effects of thermal gradients induced by tab and surface cooling on lithium ion cell performance

    , Journal of The Electrochemical Society, Vol: 165, Pages: A3169-A3178, ISSN: 1945-7111

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

  • Journal article
    Zhang X-F, Zhao Y, Liu H-Y, Zhang T, Liu W-M, Chen M, Patel Y, Offer GJ, Yan Yet al., 2018,

    Degradation of thin-film lithium batteries characterised by improved potentiometric measurement of entropy change

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 11378-11385, ISSN: 1463-9076
  • Journal article
    Liu X, Naylor Marlow M, Cooper S, Song B, Chen X, Brandon N, Wu Bet al., 2018,

    Flexible all-fiber electrospun supercapacitor

    , Journal of Power Sources, Vol: 384, Pages: 264-269, ISSN: 0378-7753

    We present an all-fiber flexible supercapacitor with composite nanofiber electrodes made via electrospinning and an electrospun separator. With the addition of manganese acetylacetonate (MnACAC) to polyacrylonitrile (PAN) as a precursor for the electrospinning process and subsequent heat treatment, the performance of pure PAN supercapacitors was improved from 90 F.g-1 to 200 F.g-1 (2.5 mV.s-1) with possible mass loadings of MnACAC demonstrated as high as 40 wt%. X-ray diffraction measurements showed that after thermal treatment, the MnACAC was converted to MnO, meanwile, the thermal decomposition of MnACAC increased the graphitic degree of the carbonised PAN. Scanning electron microscopy and image processing showed that static electrospinning of pure PAN and PAN-Mn resulted in fiber diameters of 460 nm and 480 nm respectively after carbonisation. Further analysis showed that the fiber orientation exhibited a slight bias which was amplified with the addition of MnACAC. Use of focused ion beam scanning electron microscopy tomography also showed that MnO particles were evenly distributed through the fiber at low MnACAC concentrations, while at a 40 wt% loading the MnO particles were also visible on the surface. Comparison of the electrospun separators showed improved performance relative to a commercial Celgard separator (200 F.g-1 vs 141 F.g-1).

  • Journal article
    Merla Y, Wu B, Yufit V, Martinez-Botas RF, Offer GJet al., 2018,

    An easy-to-parameterise physics-informed battery model and its application towards lithium-ion battery cell design, diagnosis, and degradation

    , Journal of Power Sources, Vol: 384, Pages: 66-79, ISSN: 0378-7753

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

  • Journal article
    Few SPM, Schmidt O, Offer GJ, Brandon N, Nelson J, Gambhir Aet al., 2018,

    Prospective improvements in cost and cycle life of off-grid lithium-ion battery packs: An analysis informed by expert elicitations

    , Energy Policy, Vol: 114, Pages: 578-590, ISSN: 0301-4215

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

  • Journal article
    Marinescu M, O'Neill L, Zhang T, Walus S, Wilson T, Offer Get al., 2017,

    Irreversible vs Reversible Capacity Fade of Lithium-Sulfur Batteries during Cycling: The Effects of Precipitation and Shuttle

    , Journal of The Electrochemical Society, Vol: 165, Pages: A6107-A6118, ISSN: 1945-7111

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

  • Journal article
    Ardani MI, Patel Y, Siddiq A, Offer GJ, Martinez-Botas RFet al., 2017,

    Combined experimental and numerical evaluation of the differences between convective and conductive thermal control on the performance of a lithium ion cell

    , Energy, Vol: 144, Pages: 81-97, ISSN: 0360-5442

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

  • Journal article
    Hunt I, Zhang T, Patel Y, Marinescu M, Purkayastha R, Kovacik P, Walus S, Swiatek A, Offer GJet al., 2017,

    The effect of current inhomogeneity on the performance and degradation of Li-S batteries

    , Journal of the Electrochemical Society, Vol: 165, Pages: A6073-A6080, ISSN: 0013-4651

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

  • Journal article
    Shibagaki T, Merla Y, Offer GJ, 2017,

    Tracking degradation in lithium iron phosphate batteries using differential thermal voltammetry

    , Journal of Power Sources, Vol: 374, Pages: 188-195, ISSN: 0378-7753

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

  • Journal article
    Cleaver T, Kovacik P, Marinescu M, Zhang T, Offer Get al., 2017,

    Perspective—commercializing lithium sulfur batteries: Are we doing the right research?

    , Journal of The Electrochemical Society, Vol: 165, Pages: A6029-A6033, ISSN: 0013-4651

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

  • Journal article
    Zhao Y, Patel Y, Hunt IA, Kareh KM, Holland AA, Korte C, Dear JP, Yan Y, Offer GJet al., 2017,

    Preventing lithium ion battery failure during high temperatures by externally applied compression

    , Journal of Energy Storage, Vol: 13, Pages: 296-303, ISSN: 2352-152X

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

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