PhD student, Purdue University
Electrochemical batteries that power smart phones, tablets and laptops consist of positive and negative electrodes with a permeating liquid medium and an intermediate material that keeps the electrodes apart. The chemical element lithium is a major contributor to the chemistry of the electrodes, and it is essentially the motion of lithium ions inside the electrodes that powers electronic devices. Primary batteries are used once and discarded, whereas secondary batteries are rechargeable. These rechargeable batteries cannot last forever. Battery performance degrades over time due to mechanical (e.g., internal shorting due to impact damage of the intermediate material when you accidentally drop your phone on the ground), electrochemical (e.g., unwanted reactions taking place inside the batteries) and thermal reasons (e.g., your car engine may not start-up during winter in the mid-western parts of US, or prolonged use/misuse of battery can heat it up just like an incandescent bulb that becomes extremely hot when switched on for a long time). As a battery is discharged/charged, chemical reactions in the electrode cause an increase in its temperature. This can sometimes reach dangerous levels, leading to thermal runaway and causing explosions.
Granular materials or particulate media are technologically significant with applications in thermal management of electronic chips and rechargeable batteries for energy storage and conversion e.g., in smart-phones, tablets, laptops and electric vehicles. These electronic chips perform tedious tasks and can heat up to dangerous levels if used continuously e.g., for several hours. To avoid degradation of these chips, heat must be taken away from them and, for the purpose, thermal interface materials are used. These pastes are not reliable in the long run mainly because of thermal cycling during operation or high operating temperatures. There is also a lack of accurate measurement of thermal contact resistance between the chip and the paste, which governs heat conduction across this interface. To prevent chip failure and battery explosions, these materials must be tested extensively for thermal performance reliability under realistic operating conditions. With the ability to visualize thermal pathways using infrared (IR) microscopy, my research forms the basis for correlating materials processing and properties in addition to measuring thermal performance parameters.
Abstract: The heat generation rate of a large-format 25 Ah lithium-ion battery is studied through estimating each term of the Bernardi model. The term for the reversible heat is estimated from the entropy coefficient and compared with the result from the calorimetric method. The term for the irreversible heat is estimated from the intermittent current method, the V–I characteristics method and a newly developed energy method. Using the obtained heat generation rates, the average cell temperature rise under 1C charge/discharge is calculated and validated against the results measured in an accelerating rate calorimeter (ARC). It is found that the intermittent current method with an appropriate interval and the V–I characteristics method using a pouch cell yield close agreement, while the energy method is less accurate. A number of techniques are found to be effective in circumventing the difficulties encountered in estimating the heat generation rate for large-format lithium-ion batteries. A pouch cell, using the same electrode as the 25 Ah cell but with much reduced capacity (288 mAh), is employed to avoid the significant temperature rise in the V–I characteristics method. The first-order inertial system is utilized to correct the delay in the surface temperature rise relative to the internal heat generation. Twelve thermocouples are used to account for the temperature distribution.
Pub.: 16 Feb '14, Pinned: 28 Jul '17
Abstract: Since lithium-ion battery with high energy density is the key component for next-generation electrical vehicles, a full understanding of its thermal behaviors at different discharge rates is quite important for the design and thermal management of lithium-ion batteries (LIBs) pack/module. In this work, a 25 Ah pouch type Li[Ni0.7Co0.15Mn0.15]O2/graphite LIBs with specific energy of 200 Wh·kg−1 were designed to investigate their thermal behaviors, including temperature distribution, heat generation rate, heat capacity and heat transfer coefficient with environment. Results show that the temperature increment of the charged pouch batteries strongly depends on the discharge rate and depth of discharge. The heat generation rate is mainly influenced by the irreversible heat effect, while the reversible heat is important at all discharge rates and contributes much to the middle evolution of the temperature during discharge, especially at low rate. Subsequently, a prediction model with lumped parameters was used to estimate the temperature evolution at different discharge rates of LIBs. The predicted results match well with the experimental results at all discharge rates. Therefore, the thermal model is suitable to predict the average temperature for the large-scale batteries under normal operating conditions.
Pub.: 13 Oct '15, Pinned: 28 Jul '17
Abstract: Abstract The National Physical Laboratory (NPL) has developed a new variation on the established guarded hot plate technique for steady-state measurements of thermal conductivity. This new guarded hot plate has been specifically designed for making measurements on specimens with a thickness that is practical for advanced industrial composite materials and applications. During the development of this new guarded hot plate, NPL carried out an experimental investigation into methods for minimising the thermal contact resistance between the test specimen and the plates of the apparatus. This experimental investigation included tests on different thermal interface materials for use in another NPL facility based on a commercial guarded heat flow meter apparatus conforming to standard ASTM E1530-11. The results show the effect of applying different quantities of the type of heat transfer compound suggested in ASTM E1530-11 (clause 10.7.3) and also the effect on thermal resistance of alternative types of thermal interface products. The optimum quantities of two silicone greases were determined, and a silicone grease filled with copper was found to offer the best combination of repeatability, small hysteresis effect and a low thermal contact resistance. However, two products based on a textured indium foil and pyrolytic graphite sheet were found to offer similar or better reductions in thermal contact resistance, but with quicker, easier application and the advantages of protecting the apparatus plates from damage and being useable with specimen materials that would otherwise absorb silicone grease.AbstractThe National Physical Laboratory (NPL) has developed a new variation on the established guarded hot plate technique for steady-state measurements of thermal conductivity. This new guarded hot plate has been specifically designed for making measurements on specimens with a thickness that is practical for advanced industrial composite materials and applications. During the development of this new guarded hot plate, NPL carried out an experimental investigation into methods for minimising the thermal contact resistance between the test specimen and the plates of the apparatus. This experimental investigation included tests on different thermal interface materials for use in another NPL facility based on a commercial guarded heat flow meter apparatus conforming to standard ASTM E1530-11. The results show the effect of applying different quantities of the type of heat transfer compound suggested in ASTM E1530-11 (clause 10.7.3) and also the effect on thermal resistance of alternative types of thermal interface products. The optimum quantities of two silicone greases were determined, and a silicone grease filled with copper was found to offer the best combination of repeatability, small hysteresis effect and a low thermal contact resistance. However, two products based on a textured indium foil and pyrolytic graphite sheet were found to offer similar or better reductions in thermal contact resistance, but with quicker, easier application and the advantages of protecting the apparatus plates from damage and being useable with specimen materials that would otherwise absorb silicone grease.
Pub.: 28 Sep '16, Pinned: 28 Jul '17
Abstract: Irreversible heat generation plays a dominant role in li-ion batteries, it is thus highly important to study its evolution in order to adapt the development of electronic devices. Internal irreversible heat generation mainly consists of two parts: one arises from the polarization and the other one from ohmic heat generation. A thermo-electrochemical coupling model was established here to study the production and evolution of the irreversible heat within li-ion batteries considering dynamic parameters and the electric double layer. Results show that the irreversible heat production rapidly increases with the discharge rate and the polarization heat production is the dominating factor. Ohmic heat production mainly resulted in the heating of the electrolyte, and the heating produced at the negative active materials result to be negligible respect to the one produced at the positive active materials. According to calculations, the ratio between the ohmic heat production and the total irreversible heat production increases from 24.2% at 3C to 32.8% at 8C, thus, the ratio related to the polarization heating decreases from 75.6% to 67.2%. In addition, effects of the particle size at the positive and negative electrodes at the rate of 3C were studied. Results show that the negative electrode particle size has thus a more significant impact on the irreversible heat production and the polarization heat production of the battery.
Pub.: 19 Apr '17, Pinned: 28 Jul '17
Abstract: The in-plane alignment of graphite nanoplatelets (GNPs) in thin thermal interface material (TIM) layers suppresses the though-plane heat transport thus limiting the performance of GNPs in the geometry normally required for thermal management applications. Here we report a disruption of the GNP in-plane alignment by addition of spherical microparticles. The degree of GNP alignment was monitored by measurement of the anisotropy of electrical conductivity which is extremely sensitive to the orientation of high aspect ratio filler particles. Scanning Electron Microscopy images of TIM layer cross-sections confirmed the suppression of the in-plane alignment. The hybrid filler formulations reported herein resulted in a synergistic enhancement of the through-plane thermal conductivity of GNP/Al2O3 and GNP/Al filled TIM layers confirming that the control of GNP alignment is an important parameter in the development of highly efficient GNP and graphene-based TIMs.
Pub.: 19 Aug '15, Pinned: 28 Jul '17
Abstract: An apparatus has been designed and constructed to characterize thermal interface materials with unprecedented precision and sensitivity. The design of the apparatus is based upon a popular implementation of ASTM D5470 where well-characterized meter bars are used to extrapolate surface temperatures and measure heat flux through the sample under test. Measurements of thermal resistance, effective thermal conductivity, and electrical resistance can be made simultaneously as functions of pressure or sample thickness. This apparatus is unique in that it takes advantage of small, well-calibrated thermistors for precise temperature measurements (+/-0.001 K) and incorporates simultaneous measurement of electrical resistance of the sample. By employing precision thermometry, low heater powers and minimal temperature gradients are maintained through the meter bars, thereby reducing uncertainties due to heat leakage and changes in meter-bar thermal conductivity. Careful implementation of instrumentation to measure thickness and force also contributes to a low overall uncertainty. Finally, a robust error analysis provides uncertainties for all measured and calculated quantities. Baseline tests were performed to demonstrate the sensitivity and precision of the apparatus by measuring the contact resistance of the meter bars in contact with each other as representative low specific thermal resistance cases. A minimum specific thermal resistance of 4.68x10(-6) m(2) K/W was measured with an uncertainty of 2.7% using a heat transfer rate of 16.8 W. Additionally, example measurements performed on a commercially available graphite thermal interface material demonstrate the relationship between thermal and electrical contact resistance. These measurements further demonstrate repeatability in measured effective thermal conductivity of approximately 1%.
Pub.: 02 Oct '09, Pinned: 28 Jul '17
Join Sparrho today to stay on top of science
Discover, organise and share research that matters to you