Publications
52 results found
Hunt I, Zhang T, Patel Y, et 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.
Zhao Y, Patel Y, Hunt IA, et 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.
Kamaludin MA, Patel Y, Williams JG, et al., 2017, A fracture mechanics approach to characterising the environmental stress cracking behaviour of thermoplastics, Theoretical and Applied Fracture Mechanics, Vol: 92, Pages: 373-380, ISSN: 0167-8442
Environmental stress cracking (ESC) is known to affect certain thermoplastics and occurs under simultaneous exposure to both applied stress and a hostile environment. The combination of these can cause a crack to form from a flaw in the material; upon reaching a critical size, the crack may accelerate thus causing catastrophic failure in the component. Various tests have been utilised to measure the resistance of different polymers to ESC, but these are often material- and application-specific and overlook the different stages of the failure process. In the present work, a fracture mechanics approach has been developed and applied, with a view to developing a test method that has wide applicability and provides both insight into the failure mechanisms as well as information for engineering design. Experimental results are presented for the following polymer-environment combinations: linear low-density PE in Igepal solution, HIPS in sunflower oil, and PMMA in methanol. It is shown that the representation of the results in the form of G versus crack velocity and G versus time can distinguish between materials of varying ESC resistance, identify the important regions of the failure process, and enable component life prediction.
Walus S, Offer GJ, Hunt I, et al., 2017, Volumetric expansion of Lithium-Sulfur cell during operation – Fundamental insight into applicable characteristics, Energy Storage Materials, Vol: 10, Pages: 233-245, ISSN: 2405-8297
During the operation of a Lithium-Sulfur (Li-S) cell, structural changes take place within both positive and negative electrodes. During discharge, the sulfur cathode expands as solid products (mainly Li2S or Li2S/Li2S2) are precipitated on its surface, whereas metallic Li anode contracts due to Li oxidation/stripping. The opposite processes occur during charge, where Li anode tends to expand due to lithium plating and solid precipitates from the cathode side are removed, causing its thickness to decrease. Most research literature describe these processes as they occur within single electrode cell constructions. Since a large format Li-S pouch cell is composed of multiple layers of electrodes stacked together, and antagonistic effects (i.e. expansion and shrinkage) occur simultaneously during both charge and discharge, it is important to investigate the volumetric changes of a complete cell. Herein, we report for the first time the thickness variation of a Li-S pouch cell prototype. In these studies we used a laser gauge for monitoring the cell thickness variation under operation. The effects of different voltage windows as well as discharge regimes are explored. It was found that the thickness evolution of a complete pouch cell is mostly governed by Li anodes volume changes, which mask the response of the sulfur cathodes. Interesting findings on cell swelling when cycled at slow currents and full voltage windows are presented. A correlation between capacity retention and cell thickness variation is demonstrated, which could be potentially incorporated into Battery Management System (BMS) design for Li-S batteries.
Zhang X-F, Zhao Y, Patel Y, et al., 2017, Potentiometric measurement of entropy change for lithium batteries, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 9833-9842, ISSN: 1463-9076
Chang L, Patel Y, Wang H, et al., 2016, The Partitioning of Plastic Energy in Cutting Tests, 21st European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 309-315, ISSN: 2452-3216
Millar TM, Patel Y, Wang H, et al., 2016, An investigation of cutting resistance in stretched polymer films, 21st European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 190-196, ISSN: 2452-3216
An investigation is made into the fracture properties of polymer films and laminates under cutting by a sharp tool and lateral tension under pure shear conditions. The method involves use of a sharp razor blade applied to the crack tip of polymer films which are also stretched orthogonal to the direction of the blade. The reaction force is measured as the cutting tool cuts the material and the force from applying a lateral strain is measured. The analysis and tests assume quasi-static conditions. The method is applied to a polyester film and three polyester laminates.Steady-state cutting forces are observed from cutting tests and loads at crack initiation are observed from lateral stretching tests. With fracture mechanics analysis the energy contributions from cutting and tearing are used to determine apparent fracture properties from the experimental results. It is observed that the cutting and tearing tests yield similar fracture toughness properties for the three tested polyester laminates, despite the different crack tip geometry at the point of crack growth. However, significantly larger fracture toughness values are measured from tearing tests versus cutting tests for the tested polyester film.
Kamaludin MA, Patel Y, Blackman BRK, et al., 2016, Fracture mechanics testing for environmental stress cracking in thermoplastics, Procedia Structural Integrity, Vol: 2, Pages: 227-234, ISSN: 2452-3216
Under the combined influence of an aggressive environment and applied stress, engineering thermoplastics may undergo a phenomenon known as environmental stress cracking (ESC). This can result in adverse effects such as embrittlement and premature failure in service, due to the growth of environmentally-induced cracks to critical sizes, with little to no fluid absorption in the bulk material. Fracture mechanics is proposed as a suitable scheme to study and quantify ESC, with the aim being to obtain characterising data for different polymer-fluid combinations of interest, as well as to develop a reliable fracture mechanics test protocol. In the proposed method, slow crack growth is monitored to assess the effect of a range of applied crack driving forces (K, or alternatively G) on observed crack speeds, as opposed to simply measuring time-to-failure. This paper presents the results of experiments performed on the following materials: linear low density polyethylene (LLDPE) in Igepal solution and high impact polystyrene (HIPS) in sunflower oil. A discussion of the various issues surrounding the data analysis for these long-term tests is also included, as the attainment of consistent and repeatable results is critical for a method to be internationally standardised, which is a goal of the European Structural Integrity Society (ESIS) Technical Committee 4 from whose interest this work is drawn.
Hunt I, Zhao Y, Patel Y, et al., 2016, Surface cooling causes accelerated degradation compared to tab cooling for lithium-Ion pouch cells, Journal of the Electrochemical Society, Vol: 163, Pages: A1846-A1852, ISSN: 0013-4651
One of the biggest causes of degradation in lithium-ion batteries is elevated temperature. In this study we explored the effects ofcell surface cooling and cell tab cooling, reproducing two typical cooling systems that are used in real-world battery packs. For newcells using slow-rate standardized testing, very little difference in capacity was seen. However, at higher rates, discharging the cellin just 10 minutes, surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. After cyclingthe cells for 1,000 times, surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tabcooling. We show that this is due to thermal gradients being perpendicular to the layers for surface cooling leading to higher localcurrents and faster degradation, but in-plane with the layers for tab cooling leading to more homogenous behavior. Understandinghow thermal management systems interact with the operation of batteries is therefore critical in extending their performance. Forautomotive applications where 80% capacity is considered end-of-life, using tab cooling rather than surface cooling would thereforebe equivalent to extending the lifetime of a pack by 3 times, or reducing the lifetime cost by 66%.
Skamniotis, Patel Y, Charalambides MN, et al., 2016, Fracture investigation in starch based foods, Interface Focus, Vol: 6, ISSN: 2042-8901
The study of oral processing and specifically cutting of the food piece during mastication can lead towards optimisation of products for humans or animals. Food materials are complex bio-composites with highly nonlinear constitutive response. Their fracture properties have not been largely investigated as yet while the need for models capable of predicting food breakdown increases. In this study, the blade cutting and the essential work of fracture (EWF) methodologies assessed the fracture behaviour of starch based pet-food. Tensile tests revealed rate dependent stiffness and stress softening effects, attributed to viscoplasticity and micro-cracking, respectively. Cutting data were collected for 5, 10 and 30 mm/s sample feed rates, whereas the EWF tests were conducted at 1.7, 3.3 and 8.3 mm/s crosshead speeds corresponding to average crack speeds of 4, 7 and 15 mm/s respectively. A reasonable agreement was achieved between cutting and EWF, reporting 1.26, 1.78, 1.76 kJ/m² and 1.52, 1.37, 1.45 kJ/m² values, respectively, for the corresponding crack speeds. These toughness data were used in a novel numerical model simulating the ‘first’ bite mastication process. A viscoplastic material model is adopted for the food piece, combined with a damage law which enabled predicting fracture patterns in the product.
Williams JG, Patel Y, 2016, Fundamentals of cutting, Interface Focus, Vol: 6, ISSN: 2042-8898
Blackman BRK, Hoult T, Patel Y, et al., 2015, Steady-state scratch testing of polymers, Polymer Testing, Vol: 49, Pages: 38-45, ISSN: 0142-9418
The paper extends the notion of steady-state cutting of polymers with a sharp tool to scratching. The analysis assumes there is separation at the tool tip (fracture) and the removed layer undergoes plastic shear. Results are presented for three polymers: PMMA, PC and PBT. For the tougher polymer, PC, smooth scratches were obtained and the modified cutting analysis works well provided that the wear on the initially sharp tip is accounted for. For the more brittle polymers, PMMA and PBT, rougher scratches were obtained and this is consistent with the notion that the polymers exhibited micro-cracking ahead of the tool tip, which led to rough surfaces being generated. The results demonstrate that the fracture toughness and the yield stress are controlling parameters in the scratching process and that a sufficiently high value of crack opening displacement COD (greater than about 10 μm) ensures that smooth scratches are obtained, as was the case for PC.
Rodriguez J, Salazar A, Gomez FJ, et al., 2015, Fracture of notched samples in epoxy resin: Experiments and cohesive model, 7th ESIS TC4 Conference on the Fracture of Polymers, Composites and Adhesives, Publisher: PERGAMON-ELSEVIER SCIENCE LTD, Pages: 402-411, ISSN: 0013-7944
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- Citations: 17
Hunt IA, Patel Y, Szczygielski M, et al., 2015, Lithium Sulfur battery nail penetration test under load, Journal of Energy Storage
Williams JG, Patel Y, 2014, Blunt Cracks and Two Parameter Fracture Criteria, 12th International Conference on Fracture and Damage Mechanics (FDM 2013), Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 13-16, ISSN: 1013-9826
Patel Y, Salazar A, Williams JG, 2013, Influence of crack sharpness on the fracture toughness of epoxy resins, 13th International Conference on Fracture
Troxler Y, Wu B, Marinescu M, et al., 2013, The effect of thermal gradients on the performance of lithium ion batteries, Journal of Power Sources, Pages: accepted-accepted, ISSN: 0378-7753
Abstract An experimental apparatus is described, in which Peltier elements are used for thermal control of lithium-ion cells under isothermal and non-isothermal conditions, i.e. to induce and maintain thermal gradients. Lithium-ion battery packs for automotive applications consist of hundreds of cells, and depending on the pack architecture, individual cells may experience non-uniform thermal boundary conditions. This paper presents the first study of the impact of artificially induced thermal gradients on cell performance. The charge transfer resistance of a 4.8 Ah is verified to have a strong temperature dependence following the Arrhenius law. Thermal cycling of the cell, combined with slow rate cyclic voltammetry, allows to rapidly identify phase transitions in electrodes, due to the thermal effect of entropy changes. A cell with a temperature gradient maintained across is found to have a lower impedance than one held at the theoretical average temperature. This feature is attributed to details of the inner structure of the cell, and to the non-linear temperature dependence of the charge transfer resistance.
Blackman BRK, Hoult TR, Patel Y, et al., 2012, Tool sharpness as a factor in machining tests to determine toughness, Engineering Fracture Mechanics, Vol: 101, Pages: 47-58, ISSN: 0013-7944
Blackman BRK, Williams JG, Hoult T, et al., 2010, Crack tip sharpness effects in fracture and machining
Williams JG, Patel Y, Blackman BRK, 2010, A fracture mechanics analysis of cutting and machining, ENGINEERING FRACTURE MECHANICS, Vol: 77, Pages: 293-308, ISSN: 0013-7944
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- Citations: 47
Patel Y, Blackman BRK, Williams JG, 2009, Measuring fracture toughness from machining tests, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, Vol: 223, Pages: 2861-2869, ISSN: 0954-4062
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- Citations: 22
Patel Y, Blackman BRK, Williams JG, 2009, Determining fracture toughness from cutting tests on polymers, ENGINEERING FRACTURE MECHANICS, Vol: 76, Pages: 2711-2730, ISSN: 0013-7944
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- Citations: 55
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