52 results found
Kirkaldy N, Samieian MA, Offer GJ, et 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.
Xie Y, Hales A, Li R, et 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.
Marzook MW, Hales A, Patel Y, et 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.
Samieian MA, Garcia CE, Bravo Diaz L, et 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.
Samieian MA, Hales A, Patel Y, 2022, A novel experimental technique for use in fast parameterisation of equivalent circuit models for lithium-ion batteries, Batteries, Vol: 8, Pages: 1-18, ISSN: 2313-0105
Battery models are one of the most important tools for understanding the behaviour of batteries. This is particularly important for the fast-moving electrical vehicle industry, where new battery chemistries are continually being developed. The main limiting factor on how fast battery models can be developed is the experimental technique used for collection of data required for model parametrisation. Currently this is a very time-consuming process. In this paper, a fast novel parametrisation testing technique is presented. A model is then parametrised using this testingtechnique and compared to a model parametrised using current common testing techniques. This comparison is done using a WLTP (Worldwide Harmonised Light Vehicle Test Procedure) drive cycle. As part of the validation, the experiments are conducted at different temperatures and repeated using two different temperature control methods: climate chamber and a Peltier element temperature control method. The new technique introduced in this paper, named AMPP (Accelerated model parametrisation procedure), is as good as GITT (Galvanostatic intermittent titration technique) for parametrisation of ECM’s (equivalent circuit model’s), however it is 90% faster. When using experimental data from a climate chamber a model parametrised using GITT was marginally better than AMPP, however, when using experimental data using conductive control, such as the ICP(isothermal cooling platform), a model parametrised using AMPP performed as well as GITT at 25°C and better than GITT at 10°C.
White G, Hales A, Patel Y, et 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.
Huang M, Kirkaldy N, Zhao Y, et al., 2022, Quantitative characterisation of the layered structure within lithium-ion batteries using ultrasonic resonance, Journal of Energy Storage, Vol: 50, Pages: 1-14, ISSN: 2352-152X
Lithium-ion batteries (LIBs) are becoming an important energy storage solution to achieve carbon neutrality, but it remains challenging to characterise their internal states for the assurance of performance, durability and safety. This work reports a simple but powerful non-destructive characterisation technique, based on the formation of ultrasonic resonance from the repetitive layers within LIBs. A physical model is developed from the ground up, to interpret the results from standard experimental ultrasonic measurement setups. As output, the method delivers a range of critical pieces of information about the inner structure of LIBs, such as the number of layers, the average thicknesses of electrodes, the image of internal layers, and the states of charge variations across individual layers. This enables the quantitative tracking of internal cell properties, potentially providing new means of quality control during production processes, and tracking the states of health and charge during operation.
Russell F, Hales A, White G, et al., 2022, <p>A system for determining Li-ion cell cooling coefficients</p>, HARDWAREX, Vol: 11
Current battery data sheets focus on battery energy and power density, neglecting thermal performance. This leads to reduced system level efficiency since cells with poor thermal performance require larger, heavier cooling systems to maintain cell temperatures in a suitable range. To address this a new metric, the Cell Cooling Coefficient (CCC), has been developed and it’s use as a tool for appropriate cell selection has been demonstrated. It also allows the pack designer to calculate which cooling direction method is most suitable by comparing CCC values for tab and surface cooling.The metric is the ratio between the heat rejected from the cell and the temperature difference between the hottest and coolest point. It therefore has units WK−1 and allows a pack designer to easily calculate the required amount of cooling power for the cell given a maximum acceptable temperature rise. In this paper we describe a system and method for the accurate determination of the CCC with the aim of facilitating wider adoption of the metric. The system is able to reliably quantify the surface and tab cooling CCC of any pouch cell.
Diaz LB, Hales A, Marzook MW, et al., 2022, Measuring Irreversible Heat Generation in Lithium-Ion Batteries: An Experimental Methodology, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 169, ISSN: 0013-4651
Roe C, Feng X, White G, et 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.
Li S, Kirkaldy N, Zhang C, et 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.
Hales A, Brouillet E, Wang Z, et al., 2021, Isothermal temperature control for battery testing and battery model parameterization, SAE International Journal of Electrified Vehicles, Vol: 10, Pages: 105-122, ISSN: 2691-3747
The hybrid/electric vehicle (H/EV) market is very dependent on battery models. Battery models inform cell and battery pack design, critical in online battery management systems (BMSs), and can be used as predictive tools to maximize the lifetime of a battery pack. Battery models require parameterization, through experimentation. Temperature affects every aspect of a battery’s operation and must therefore be closely controlled throughout all battery experiments. Today, the private sector prefers climate chambers for experimental thermal control. However, evidence suggests that climate chambers are unable to adequately control the surface temperature of a battery under test. In this study, laboratory apparatus is introduced that controls the temperature of any exposed surface of a battery through conduction. Pulse discharge tests, temperature step-change tests, and driving cycle tests are used to compare the performance of this conductive thermal control apparatus (CTCA) against a climate chamber across a range of scenarios. The CTCA outperforms the climate chamber in all tests. In CTCA testing, the rate of heat removal from the cell is increased by two orders of magnitude. The CTCA eliminates error due to cell surface temperature rise, which is inherent to climate chamber testing due to insufficient heat removal rates from a cell under test. The CTCA can reduce the time taken to conduct entropic parameterization of a cell by almost 10 days, a 70% reduction in the presented case. Presently, the H/EV industry’s reliance on climate chambers is impacting the accuracy of all battery models. The industry must move away from the flawed concept of convective cooling during battery parameterization.
Edge JS, O'Kane S, Prosser R, et 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.
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
Hua X, Heckel C, Modrow N, et 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.
Hales A, Prosser R, Diaz LB, et 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
Bravo Diaz L, He X, Hu Z, et 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.
Dondelewski O, Szemberg OConnor T, Zhao Y, et 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.
Offer G, Patel Y, Hales A, et al., 2020, Cool metric for lithium-ion batteries could spur progress, Nature, Vol: 582, Pages: 485-487, ISSN: 0028-0836
Zhao JY, Blackman B, Patel Y, et al., 2020, The fracture resistance of bi-axially orientated PMMA, ISSN: 1757-8981
The mechanical performance of the transparent polymer PMMA (polymethyl methacrylate) can be significantly enhanced by the process of bi-axial stretching during manufacture. However, few studies have reported on the fracture properties of bi-axially oriented PMMA. In this work, the fracture toughness and resulting surface morphology of biaxially orientated PMMA has been investigated by testing single edge notch bend fracture specimens under quasi-static test conditions. The orientation of the specimens with respect to the stretching direction was systematically varied such that the effect of in-plane orientation and out-of-plane orientation was determined. The out-of-plane fracture resistance was shown to be enhanced compared to the in-plane resistance and compared to amorphous PMMA. Both out-of-plane and in-plane crack orientations had little effect on fracture resistance. Complex crack growth behavior and surface morphologies were observed in the out-of-plane direction. The results have important implications for the study of damage tolerance of transparent canopies in the aircraft industry.
Feng X, Merla Y, Weng C, et al., 2020, A reliable approach of differentiating discrete sampled-data for battery diagnosis, ETRANSPORTATION, Vol: 3, ISSN: 2590-1168
Hales A, Marzook MW, Bravo Diaz L, et 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.
Zhao Y, Diaz LB, Patel Y, et 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.
Liu X, Ai W, Naylor Marlow M, et al., 2019, The effect of cell-to-cell variations and thermal gradients on the performance and degradation of lithium-ion battery packs, Applied Energy, Vol: 248, Pages: 489-499, ISSN: 0306-2619
The performance of lithium-ion battery packs are often extrapolated from single cell performance however uneven currents in parallel strings due to cell-to-cell variations, thermal gradients and/or cell interconnects can reduce the overall performance of a large scale lithium-ion battery pack. In this work, we investigate the performance implications caused by these factors by simulating six parallel connected batteries based on a thermally coupled single particle model with the solid electrolyte interphase growth degradation mechanism modelled. Experimentally validated simulations show that cells closest to the load points of a pack experience higher currents than cells further away due to uneven overpotentials caused by the interconnects. When a cell with a four times greater internal impedance was placed in the location with the higher currents this actually helped to equalise the cell-to-cell current distribution, however if this was placed at a location furthest from the load point this would cause a ~6% reduction in accessible energy at 1.5 C. The influence of thermal gradients can further affect this current heterogeneity leading to accelerated aging. Simulations show that in all cases, cells degrade at different rates in a pack due to the uneven currents, with this being amplified by thermal gradients. In the presented work a 5.2% increase in degradation rate, from -7.71 mWh/cycle (isothermal) to - 8.11 mWh/cycle (non-isothermal) can be observed. Therefore, the insights from this paper highlight the highly coupled nature of battery pack performance and can inform designs for higher performance and longer lasting battery packs.
Hales A, Diaz LB, Marzook MW, et 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.
Zhao Y, Spingler FB, Patel Y, et 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.
Skamniotis C, Patel Y, Elliott M, et al., 2018, Toughening and stiffening of starch food extrudates through the addition of cellulose fibres and minerals, Food Hydrocolloids, Vol: 84, Pages: 515-528, ISSN: 0268-005X
Pet food, one of the largest type of commercial packaged foods, continuously sets new challenges, amongst them the possibility to enhance palatability via adjusting product composition. This will optimise texture perception across consumer groups of diverse chewing capabilities, as well as improve food oral breakdown efficiency with further impact on metabolic health and nutrient bioavailability in the digestive process. Our aim is to pioneer new methods of controlling texture by answering longstanding questions such as the impact of nutrients on the mechanical properties of foods. The impact of cellulose fibres and minerals on the fracture toughness and stiffness properties of starch food extrudates is investigated for the first time through employing tensile tests and two fracture toughness tests namely Essential Work of Fracture (EWF) and cutting, on four different compositions. Fibres alone are found to increase stiffness (stiffening) and toughness (toughening) whereas minerals decrease stiffness (softening) with a minor influence on toughness. Interestingly, fibres and minerals combined maximise toughening at 28% compared to pure starch, due to the synergistic effect of fibre-matrix de-bonding and fibre breakage mechanisms at the crack tip. These new results indicate that texture can be significantly altered through the addition of minerals and short fibres. Such information is critical in the design of products that need to satisfy both nutritional and textural criteria.
Zhao Y, Patel Y, Zhang T, et 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: 0013-4651
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.
Zhang X-F, Zhao Y, Liu H-Y, et 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
Ardani MI, Patel Y, Siddiq A, et 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.
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