Non-uniform Current Distribution Research Articles

Mechanical Effects of the Nonuniform Current Distribution on HTS Coils for Accelerators Wound With REBCO Roebel Cable

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The EuCARD-2 WP10 Future Magnets collaboration is aiming at testing HTS-based Roebel cables in an accelerator magnet. The demonstrator should produce around 17 T, when inserted into the 100-mm aperture of Feather-M2 13-T outsert magnet. HTS Roebel cables are assembled from meander-shaped REBCO-coated conductor tapes. In comparison with fair level of uniformity of current distribution in cables made out of round Nb-Ti or Nb 3 Sn strands, current distribution within the coils wound from Roebel cables is highly nonhomogeneous. It results in nonuniform electromagnetic force distribution over the cable that could damage the very thin REBCO superconducting layer. This paper focuses on the numerical models to describe the effect of the nonhomogeneous current distribution on stress distribution in the demonstrator magnet designed for the EuCARD-2 project. Preliminary results indicate that the impregnation bonding between the cable glass fiber insulation and layer-to-layer insulation plays a significant role in the pressure distribution at the cable edges. The stress levels are safe for Roebel cables. Assuming fully bonded connection at the interface, the stresses around the edges are reduced by a large factor.

Characterization of the particulate anode of a laboratory flow Zn–air fuel cell

A tapered-end flow Zn–air fuel cell (ZAFC), mechanically refuelable with Zn microspheres, was employed to study the effect of aging of KOH electrolyte on the Zn anode. A complete description of the architecture of the adopted cell is reported. The electrochemical characterization of the ZAFC was performed by long-term current discharge tests in galvanostatic mode. An insightful investigation on the particulate Zn anode consisting of spheres of diameter 0.4 mm was performed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy in order to characterize the crystallographic structure, surface morphology, and chemical nature of residual metal and solid corrosion products. Electrochemical impedance spectroscopy (EIS) allowed to obtain information on the charge-transfer mechanism of zinc anode reaction and on the thickness, compactness, and blocking features of the passive film as a function of the aging of the electrolyte. The results of our analysis revealed the formation of a passive layer of zinc consisting of a white and porous film of ZnO precipitate (type I) and a light-gray to black compact film (type II). The failure of the particulate anode was chiefly caused by the increase in zincate concentration in the electrolyte, but it was enhanced by the nonuniform spatial current distribution due to the instability of the passive film at high pH.

3D simulation of AC loss in a twisted multi-filamentary superconducting wire

In the AC loss simulation, it is a huge challenge to model the twisted wire at the filament level, due to the complex structure as well as long-time computation consumption. In this paper, we use 3D finite-element method based on H-formulation to study the AC loss in a twisted superconducting wire. The wire is treated as a homogenous material with the anisotropic conductivity in the filament region. We quantitatively simulate the AC loss induced by the AC transport current and magnetic field profile, and the effect of the twist pitch on the AC loss. In the case of AC transport current, larger pitch length leads to higher loss, and the pitch length effect is contrary to the case of applied magnetic field. The influences of the magnetic field direction and non-uniform current distribution subjected to the strand bending are also investigated. It is observed that, the transverse magnetic field has a more significant influence on the AC loss than the longitudinal magnetic field. The non-uniform current distribution can result in a higher AC loss, compared to a corresponding uniform current distribution.

Pouch Type Cells for Practical Evaluation and Application of Large-Scale Li-Air Batteries

Thanks to high theoretical specific capacity and energy density, Li-air batteries are considered as candidates for next-generation battery systems in place of conventional Li-ion batteries for advanced applications such as electric vehicles (EV).1-3 However, low energy efficiency, poor cycle life and Li-metal safety issues make the use of Li-air batteries yet impractical. In addition, actual cell capacities are very low and, since only small-scale electrodes are currently tested, it is hard to predict the properties of large-size electrodes and cells, thus evaluating and judging real practical challenges related to this battery technology. Here, we report on fabrication, operation and evaluation of pouch-type Li-air cells using 3×5 cm2 sized electrodes, in which we intend deal with higher capacity (mAh) of the electrodes, compared to most previous studies in the field. With the relatively large-sized cells, operating at high current density and capacity, we could recognize some remarkable problems that may not be significant in small cells. In relatively large cells as those explored in this work issues related to non-uniform current distribution, mechanical and electronic integrity of electrodes are much more influencing compared to small cells. Hence, problems such as lithium dendrites formation and non-uniform deposition of oxygen reduction product become severe. This study can help to determine which parameters are the most important for developing practical Li-air batteries.

Effects of Non-Uniform Temperature on in-Situ Current Distribution and Non-Uniform State of Charge Measurements for LiFePO4 and LiNiMnCoO2 Cells

Understanding internal state non-uniformity that occurs across the electrodes in large-format Lithium-ion batteries (LIBs), and among parallel-connected cells, is a critical part of the cell/module design process [1]. The reduction of non-uniform current distributions is critical as this leads to non-uniform utilization, which effectively reduces energy density, and can eventually cause non-uniform degradation, leading to earlier cell/module failure. Several studies, performed particularly at high charge/discharge rates (i.e., above 1C-rate), show single cell temperature differences greater than 20°C, for large-format Lithium-ion cells [2]. As engineers maximize the energy density of Lithium-ion cells, and push them to extreme limits (e.g., Tesla Supercharging), the problems of non-uniform currents and temperatures will continue to be exacerbated. Therefore, high-rate battery systems generally require the integration of a battery thermal management system (BTMS). Coupling the optimization of cell/module, and BTMS designs, to efficiently minimize thermal non-uniformity and packaging requirements, requires spatially-resolved coupled thermal-electrical-electrochemical models. However, little published research exists regarding the measurement of spatially distributed effects in Lithium-ion batteries and the experimental validation of spatially distributed models. Under uniform thermal conditions, the in-situ internal current/State of Charge (SOC) distributions within a segmented-cathode (LiFePO4) cell have been measured by Zhang et al. [3]. Yang et al. studied the non-uniform SOC buildup between two 18650 LiFePO4 cells connected in parallel, where each cell was held at a different temperature [4]. Klein et. al. studied the non-uniform temperature effects on the bulk cell resistance under constant current pulses and dynamic current charge-depleting cases for a LiNiMnCo pouch cell [5]. This study presents the non-uniform temperature effects on in-situ current/SOC distributions among modules of five parallel-connected cells. Two commercially available 18650 cell chemistries were used; LiFePO4/Graphite (LFP) and LiNiMnCo/Graphite (NMC). For each chemistry, the five cells were connected in parallel with a current measuring shunt in series with each cell. The test system is illustrated in Figure 1a, and the module is in Figure 1b. Average module temperatures of 15, 25, and 35°C were used with temperature differences across the module being controlled to 0, 5, 10, and 20°C. Direct-current pulse tests were performed at 20, 50, and 80% SOC using 10sec pulses at constant C/5, C/2, 1C, and 2C-rates. Additionally, C/5, C/2, and 1C-rate constant current full-capacity discharges were performed under the same thermal conditions to observe the effects of non-uniform SOC among the cells. The pulse results indicate that the LFP cell resistance has a lower temperature sensitivity than the NMC, and therefore experiences less current non-uniformity for the same amount of temperature non-uniformity. Larger current non-uniformity is observed at lower average temperatures, where the resistance is more temperature sensitive. The current distribution develops quickly (i.e., <2sec) and remains fairly constant during the 10sec pulse. However, the constant current full discharge results are quite different. Figure 1c-e plots the measured normalized current for the left/middle/right (cold/mean/hot) LFP cells for the average module temperature case of 25°C and the C/5, C/2, and 1C-rates, respectively. The instantaneous current distributions measured at the same average module SOC as the pulse tests are more uniform in the full capacity discharge tests. Therefore, the amount of maximum non-uniform SOC is lower than would be expected from extrapolating the pulse test current distribution results. Additionally, the NMC unit, which had a larger current distribution in the pulse testing compared to LFP, has a substantially lower current-distribution than the LFP unit for the full-capacity discharges. Finally, using the measured SOC profiles for each cell, a tolerable amount of temperature uniformity for each chemistry is interpolated from the results. Figures 1f and 1g present these results for two maximum tolerable SOC non-uniformities of 2% and 5%. The NMC chemistry provides a higher tolerance to temperature non-uniformity for a given SOC non-uniformity tolerance, as compared to the LFP chemistry. The results of this study are aimed at aiding both the validation of spatially resolved models, and to quantify the thermal non-uniformity targets for these common commercially available Lithium-ion cell chemistries. [1] T. Waldmann, G.Bisle, B.-I. Hogg, S. Stumpp, M.A. Danzer, M. Kasper, P. Axmann, M. Wohlfarht-Mehrens, Jounal of The Electrochemical Society 162 (6) (2015) A921-A927 [2] C. Veth, D. Dragicevic, C. Merten, Journal of Power Sources 267 (2014) 760-769 [3] G.Zhang, C.E. Shaffer, C.-Y. Wang, C.D. Rahn, Journal of The Electrochemical Society 160 (4) (2013) A610-A615. [4] N. Yang, X. Zhang, B. Shang, G. Li, Journal of Power Sources 306 (2016) 733-741. [5] M. Klein, S. Tong, J. Park, Applied Energy 165 (2016) 639-647. Figure 1

A multilayer electro-thermal model of pouch battery during normal discharge and internal short circuit process

As the electrical and thermal characteristic will affect the batteries’ safety, performance, calendar life and capacity fading, an electro-thermal coupled model for pouch battery LiFePO4/C is developed in normal discharge and internal short circuit process. The battery is discretized into many cell elements which are united as a 2D network equivalent circuit. The electro-thermal model is solved with finite difference method. Non-uniformity of current distribution and temperature distribution is simulated and the result is validated with experiment data at various discharge rates. Comparison of the lumped model and the multilayer structure model shows that the temperature non-uniformity Φ of multilayer model is bigger than that of lumped model and shows more precise. The temperature non-uniformity is quantified and the reason of non-uniformity is analyzed. The electro-thermal model can also be used to guide the safety design of battery. The temperature of the ISC element near tabs is the highest because the equivalent resistance of the external circuit (not including the ISC element) is the smallest when the resistance of cell units is small. It is found that increasing the thermal conductivity of integrated layer can effectively relieve the heat spot effect of ISC.

Flexible micro temperature, voltage and current sensors for local real-time microscopic diagnosis inside high temperature proton exchange membrane fuel cell stack

The local temperature, voltage and current nonuniformity in the high temperature proton exchange membrane fuel cell (HT-PEMFC) stack can accelerate the aging of membrane electrode assembly (MEA) and the failure of overall fuel cell stack.This study used micro-electro-mechanical systems (MEMS) technology to integrate micro temperature, voltage and current sensors into a 40 μm thick stainless steel substrate successfully, and selected the temperature tolerant and electrochemical corrosion resistant polyimide (PI) as protective layer. This flexible micro sensor is characterized by multiple functions, compactness, corrosion resistance, good temperature tolerance, quick response, real-time measurement and optional location. Flexible micro sensor embedded in the high temperature fuel cell stack, the effect on the performance of fuel cell stack was less than 1%.This study used NI PXI 2575 data acquisition unit for real-time microscopic diagnosis of local temperature, voltage and current information at 150 °C operating temperature and constant current (5, 15, 30 A) inside the high temperature fuel cell stack, and found that the nonuniform temperature distribution in the fuel cell stack resulted in nonuniform voltage and current distributions and hot stack.

Electrochemical impedance spectroscopy in interfacial studies

An important role of the electrochemical impedance spectroscopy (EIS) is the characterization of the electrical double layer formed at the electrode/electrolyte interfaces. The phenomenological double layer studies with an aqueous and ionic liquid electrolytes are reviewed with a conclusion that the double-layer capacitance is frequency dependent as the rule rather than the exception. We discuss the impedance consequences of the non-uniform current distribution along the electrochemical interface, which also contributes to the apparent frequency dependence of the capacitance. Finally, we show recent articles on non-conventional EIS techniques with high lateral resolution or enabling fast measurements.

FEATURES of VOLT-FARAD DEPENDENCE OF NONIDEAL HETEROJUNCTIONS BARRIER CAPACITY

Abnormal dependence of volt-farad characteristics of «nonideal» heterojunctiоn barrier capacity is investigated. It is shown that in heterojunctions with the big concentration and non-uniform distribution of defects tunnel currents essentially influence on the barrier capacity size. The model for an explanation of abnormal barrier capacity dependence on the voltage, using tunneling-recombination mechanism of carriers carry through the area of a spatial charge is offered. The put forward assumptions put in a model basis, are confirmed experimentally.

Modeling the Resistive Critical Current Transitions of Striated Superconducting Tapes

Striating a high-temperature superconducting tape into an array of narrow filaments is a viable way to reduce thermal losses in ac applications, provided the patterning process yields uniform filaments whose total critical current density scales with that of the pristine tape. To help explain some of the scaling discrepancies that are often encountered during the optimization of such a patterning step, a model was developed to calculate the expected critical current values for tapes simulated with various nonuniform critical current distributions of filaments. These results show that if the filaments are permanently damaged when their critical currents are exceeded, then a particular threshold filament can be identified whose transition to the normal state prompts a subsequent transition of all or most of the tape's remaining superconducting filaments. These critical current and threshold filament values were determined analytically for linear critical current distributions, while Monte Carlo simulations were statistically analyzed for Gaussian distributions.

Fabrication, Testing, and Simulation of All-Solid-State Three-Dimensional Li-Ion Batteries.

Demonstration of three-dimensional all-solid-state Li-ion batteries (3D SSLIBs) has been a long-standing goal for numerous researchers in the battery community interested in developing high power and high areal energy density storage solutions for a variety of applications. Ideally, the 3D geometry maximizes the volume of active material per unit area, while keeping its thickness small to allow for fast Li diffusion. In this paper, we describe experimental testing and simulation of 3D SSLIBs fabricated using materials and thin-film deposition methods compatible with semiconductor device processing. These 3D SSLIBs consist of Si microcolumns onto which the battery layers are sequentially deposited using physical vapor deposition. The power performance of the 3D SSLIBs lags significantly behind that of similarly prepared planar SSLIBs. Analysis of the experimental results using finite element modeling indicates that the origin of the poor power performance is the structural inhomogeneity of the 3D SSLIB, coupled with low electrolyte ionic conductivity and diffusion rate in the cathode, which lead to highly nonuniform internal current density distribution and poor cathode utilization.

Nonuniform current distribution between individual layers of multilayer MoS2, experimentally approached by using a laser thinning technique

We have investigated the current distribution between individual layers of multilayer MoS2 in a field-effect transistor structure with source and drain electrodes on the top layer. By employing a laser thinning technique, we partially etched down the multilayer in the middle of the device and monitored the electrical current after each etching step. The current was decreased by several orders of magnitude when the upper layers were partially etched away. Our observation suggests that the electrical current of multilayer MoS2 flows mainly through the upper layers close to the source/drain contacts due to relatively large interlayer resistance of MoS2.

The Ohmic Impedance Contribution to the Indirect Impedance Measurement

Studies have shown that the application of an electric field to the surface of an electrolytic medium can indirectly polarize a metal within the solution.1,2,3 Based on this concept, an indirect impedance measurement may be used to assess the properties of a conductive element contained within a resistive medium without a direct connection to the conductive element. The indirect impedance technique was studied by Zhang et al.4 to determine the location and the condition of steel rebar within concrete slabs. They were able to determine qualitatively that the measured surface impedance was a function of the corrosion state of the steel as well as the resistance of the concrete. However, extracting the true impedance of the conductive element from the impedance of the whole system was difficult. The simplest way to interpret impedance measurements is to use equivalent circuits containing physically meaningful components to fit the data. Andrade et al.5 used results of finite element models to propose an analogue circuit that accounts for the polarization behavior of the steel and the properties of the mortar in which the steel is embedded. They determined that a suitable circuit must contain a parallel, as well as, a series resistance term associated with the resistivity of the concrete in relation to the impedance of the steel.The disadvantage of using circuits containing passive elements is the inability to account for the effects of nonuniform current distributions and ill-defined geometries. Newman6 showed that the geometry of a disk electrode embedded within an insulating plane causes the apparent capacitance to be frequency dependent. Huang et al.7 provided an explanation for this by showing that the ohmic contribution to the impedance becomes complex at high frequencies due to the nonuniform current distribution. Therefore the use of equivalent circuits to fit the impedance of disk electrodes is only valid below a characteristic frequency dependent on the radius of the disk.A finite-element model has been developed to calculate the frequency-dependent potential distribution through a cylindrical grout specimen representing a section of a structural bridge tendon containing one axially-located steel strand. Impedance simulations were performed at different steel corrosion states while incorporating the kinetics at the current-injecting electrodes. The idea of an ohmic impedance was used to account for the unusual geometry of the indirect impedance in application to corrosion detection in structural bridge tendons. The indirect impedance was found to include two separate contributions of the grout resistivity. There is an ohmic impedance associated with the grout that is in parallel to the steel and another ohmic impedance associated with the grout that is in series to the steel. By understanding the exact influence the grout has on the indirect impedance, a method may be devised to extract the properties of the steel and determine the corrosion rate. References C. Andrade, I. Martnez, “Metal corrosion rate determination of different solutions and reinforced concrete specimens by means of a noncontacting corrosion method,” Corrosion 66 (2010) pg. 056001-056001.P. J. Monteiro, H. Morrison, Non-destructive method of determining the position and condition of reinforcing steel in concrete, 1999. US Patent 5,855,721.M Keddam, X. R. Novoa, V. Vivier, “The concept of floating electrode for contact-less electrochemical measurements: Application to reinforcing steel-bar corrosion in concrete,” Corrosion Science, 51 (2009) pg. 1795-1801.J. Zhang, P. J. Monteiro, F.H. Morrison, “Noninvasive surface measurement of corrosion of reinforcing bar in concrete: Part I Experimental results”, ACI Materials Journal, 98 (2001) pg. 116-125.C. Andrade, J. Sanchez, I. Martinez, N. Rebolledo, Analogue circuit of the inductive polarization resistance,” Electrochimica Acta 56 (2011) pg. 1874-1880.J. S. Newman, Frequency dispersion in capacity measurements at a disk electrode, Journal of the Electrochemical Society 117 (1970) 198–203.V. M. W. Huang, V. Vivier, M. E. Orazem, N. Pbre, B. Tribollet, “The apparent constant-phase-element behavior of a disk electrode with faradaic reactions a global and local impedance analysis,” Journal of the Electrochemical Society 154 (2007) C99-C107.

Preparation of Polyimide Separator with 3-Dimensionally Ordered Macroporous Structure Using Polystyrene Sphere Template for Lithium Metal Secondary Battery

Introduction Lithium ion battery has been widely used as power sources for electric vehicles. A higher energy density of rechargeable batteries is strongly required. Lithium metal has a large theoretical capacity (= 3861 mA h g-1). However, in case of the using the lithium metal anode, a growth of lithium metal dendrite is a key problem in order to realize lithium metal secondary batteries. Lithium metal dendrite deposits on lithium metal surface during charging process results in a degradation of battery and an explosion of battery in the worst case. One of origins for lithium metal dendrite formation is a non-uniform current distribution. In our group, to realize uniform current distribution, we have focused on a separator. We have reported that the growth of lithium metal dendrite is suppressed by using polyimide separator with 3-Dimensionally Ordered Macroporous (3DOM) structure. Polyimide (PI) haing high thermal stability was chosen for the separator material. In our previous report, spherical particles of silica were used as the template to form pores. However, hydro fluoric acid (HF), which is very toxic, must be used to remove the silica template particles. In order to reduce manufacturing cost and environmental load, a new process without HF acid process is required. In this study, for a new separator preparation process without using HF acid was developed by using polystyrene (PS) particle as template. In addition, the characterization of new separator was performed for Lithium metal secondary battery. Experimental PS particle was synthesized by suspension polymerization method in a molar ratio of styrene monomer : 2-hydroxyethyl methacrylate = 1 : 0.023. PI precursor solution / PS slurry was prepared by mixing of PI precursor solution and ethylene glycol and PS particle. The slurry was coated on substrate, and then dried. A heat treatment for this coated layer was conducted to obtain 3DOM PI separator. Li dissolution / Li decomposition test was evaluated by 2032 type coin cell using new 3DOM PI separator. The electrolyte was 1 mol dm-3 LiPF6 in EC. Depth of charge for Li metal anode was 25 %. For comparison to 3DOM PI separator, the same test using a poly propylene (PP) separator was employed. Results and discussion Figure 1 (a) shows the SEM image of synthesized PS particles. Uniform spherical particles were confirmed by SEM. Particle size of PS was about 350 nm. Figure 1 (b) and Figure 1 (c) show the SEM image of surface and cross section of 3DOM PI separator, respectively. The uniform spherical pores on PI film were confirmed from Figure 2 (b). In addition, a small pore which appeared in the interior of the macropores was observed, clearly. These small pores are connecting holes between macropores. A presence of the pores inside of PI separator was also confirmed from the Figure 1 (c). This porous structure contributes a uniform current flow to electrode surface. Figure 2 (a) and (b) show the Li dissolution / Li decomposition test using PP and 3DOM PI separator, respectively. When the short circuit of the cell happens, the overvoltage suddenly decreases due to reduction in an internal resistance. Such behavior was not observed in any cells. In the cell using the PP separator, an increase of overvoltage was observed. On the other hand, in the cell using 3DOM (PS) PI separator, an increase of overvoltage was not observed. The overvoltage of cell using the 3DOM (PS) PI separator was smaller than that of using PP separator. In case of cell using the PP separator, it is considered that lithium dendrite is formed and is easily peeled from electrode surface associated with cycle number, and then this lithium dendrite may be changed to dead lithium leading to high resistance. Figure 2 (c) and (d) show the SEM image of Li deposited in cell using PP separator or 3DOM PI separator after 100 cycles, respectively. Many lithium dendrites were confirmed on lithium metal surface of cell using the PP separator. On the other hand, lithium granules were confirmed in lithium metal surface of the cell using 3DOM PI separators. It can be confirmed that the 3DOM PI separator suppress the growth of lithium dendrite. Figure 1

Formation of a Lithium-Plated Reference Electrode in Lithium-Ion Batteries: Characterization Using Dynamic Impedance and Simulation Using an Electrochemical Model

Reference electrodes (REs), enabling one to differentiate the contributions of each electrode, have been widely used in the research of electrochemical systems, such as lithium-ion batteries (LIBs). Metallic lithium or lithium alloy (Li-Sn, Li-Al) is the usual material of choice for REs in LIBs. A typical approach is to form a layer of metallic lithium onto a stable substrate by in-situ plating, termed as lithium-plating method [1][2]. In the present study, a copper-wire based RE is embedded into a 2 Ah laminated LIB, and its formation process, crucial to stability of the RE, is scrutinized. During the formation process, the RE is set as the working electrode, and the positive and negative electrodes are used as the counter electrode successively to obtain a both-side-plated RE. Dynamic electrochemical impedance spectroscopy (DEIS) is used as a characterization tool. Being dynamic means that the impedance measurements are performed while there is a DC current between the RE and the counter electrode [3][4]. Figure 1. (a) shows the evolution of DEIS during the whole formation process. It is revealed that a new semi-circle attributed to the emergence of the solid/electrolyte interphase (SEI) film manifests immediately at the moment the formation starts. Ohmic resistance, SEI film resistance and charge transfer resistance all gradually decrease during the formation process, and arrive at their constant values after 2×104s. In comparison with the potential monitoring, DEIS allows better characterization of the formation process with higher sensitivity and resolution. To understand the above phenomena, a two-dimensional (2D) model, accounting for growth of the RE size and concomitant decrease of the electrolyte porosity [5], is developed. Figure. 1 (b) shows non-uniform current distribution during the plating process with the positive electrode as the counter electrode. As expected, the plating current concentrates on the positive-electrode-facing side. Consequently, a thicker lithium layer is deposited onto this side. Impedance simulation results are compared with the experimental DEIS results. Based on this model, a multistep plating strategy is optimized to improve the stability and durability of RE. Key words Reference electrode; Lithium plating; Electrochemical impedance spectroscopy; Electroplating model; Solid/electrolyte interphase Reference: [1]. D. P. Abraham, S. D. Poppen, A. N. Jansen, J. Liu and D. W. Dees, ELECTROCHIM ACTA, 49, 4763 (2004). [2]. A. N. Jansen, D. W. Dees, D. P. Abraham, K. Amine and G. L. Henriksen, J POWER SOURCES, 174, 373 (2007). [3]. J. Huang, J. Zhang, Z. Li, S. Song and N. Wu, ELECTROCHIM ACTA, 131, 228 (2014). [4]. J. Huang, H. Ge, Z. Li and J. Zhang, ELECTROCHIM ACTA, 176, 311 (2015). [5]. J. Newman and W. Tiedemann, J ELECTROCHEM SOC, 140, 1961 (1993). Figure 1

Computational design and refinement of self-heating lithium ion batteries

Abstract The recently discovered self-heating lithium ion battery has shown rapid self-heating from subzero temperatures and superior power thereafter, delivering a practical solution to poor battery performance at low temperatures. Here, we describe and validate an electrochemical-thermal coupled model developed specifically for computational design and improvement of the self-heating Li-ion battery (SHLB) where nickel foils are embedded in its structure. Predicting internal cell characteristics, such as current, temperature and Li-concentration distributions, the model is used to discover key design factors affecting the time and energy needed for self-heating and to explore advanced cell designs with the highest self-heating efficiency. It is found that ohmic heat generated in the nickel foil accounts for the majority of internal heat generation, resulting in a large internal temperature gradient from the nickel foil toward the outer cell surface. The large through-plane temperature gradient leads to highly non-uniform current distribution, and more importantly, is found to be the decisive factor affecting the heating time and energy consumption. A multi-sheet cell design is thus proposed and demonstrated to substantially minimize the temperature gradient, achieving 30% more rapid self-heating with 27% less energy consumption than those reported in the literature.

Design of near‐field ultra‐high frequency radio‐frequency identification antenna with fragmented wires for electrically‐large coverage

Design of ultra-high frequency radio-frequency identification (UHF RFID) reader antenna covering electrically-large near-field area is still challenging for near-field UHF RFID applications. In such design, magnetic field distribution on a large coverage area is required to be as uniform as possible. Fragment-type wire antennas are quite suitable for such demand because distribution of fragmented wires in a designated electrically-large area can be optimised to counterpoise the magnetic fields generated by non-uniform current distribution. By optimisation searching with multiobjective evolutionary algorithm based on decomposition combined with enhanced genetic operators, fragment-type RFID reader antenna is designed to provide uniform magnetic field distribution within dynamic range of 4 dB in an electrically-large near-field coverage of 320 × 320 mm at 915 MHz. The designed antenna is fabricated and tested. Both simulation and measurement results show good impedance matching, satisfactory near-field coverage, and desirable omni-directional pattern.

On the search for the chiral anomaly in Weyl semimetals: the negative longitudinal magnetoresistance

Recently, the existence of massless chiral (Weyl) fermions has been postulated in a class of semi-metals with a non-trivial energy dispersion. These materials are now commonly dubbed Weyl semi-metals (WSM). One predicted property of Weyl fermions is the chiral or Adler–Bell–Jackiw anomaly, a chirality imbalance in the presence of parallel magnetic and electric fields. In WSM, it is expected to induce a negative longitudinal magnetoresistance (MR). Here, we present experimental evidence that the observation of the chiral anomaly can be hindered by an effect called ‘current jetting’. This effect also leads to a strong apparent negative longitudinal MR, but it is characterized by a highly non-uniform current distribution inside the sample. It appears in materials possessing a large field-induced anisotropy of the resistivity tensor, such as almost compensated high-mobility semimetals due to the orbital effect. In case of a non-homogeneous current injection, the potential distribution is strongly distorted in the sample. As a consequence, an experimentally measured potential difference is not proportional to the intrinsic resistance. Our results on the MR of the Weyl semimetal candidate materials NbP, NbAs, TaAs, and TaP exhibit distinct signatures of an inhomogeneous current distribution, such as a field-induced ‘zero resistance’ and a strong dependence of the ‘measured resistance’ on the position, shape, and type of the voltage and current contacts on the sample. A misalignment between the current and the magnetic-field directions can even induce a ‘negative resistance’. Finite-element simulations of the potential distribution inside the sample, using typical resistance anisotropies, are in good agreement with the experimental findings. Our study demonstrates that great care must be taken before interpreting measurements of a negative longitudinal MR as evidence for the chiral anomaly in putative Weyl semimetals.

A physical model for low-frequency electromagnetic induction in the near field based on direct interaction between transmitter and receiver electrons.

A physical model of electromagnetic induction is developed which relates directly the forces between electrons in the transmitter and receiver windings of concentric coaxial finite coils in the near-field region. By applying the principle of superposition, the contributions from accelerating electrons in successive current loops are summed, allowing the peak-induced voltage in the receiver to be accurately predicted. Results show good agreement between theory and experiment for various receivers of different radii up to five times that of the transmitter. The limitations of the linear theory of electromagnetic induction are discussed in terms of the non-uniform current distribution caused by the skin effect. In particular, the explanation in terms of electromagnetic energy and Poynting’s theorem is contrasted with a more direct explanation based on variable filament induction across the conductor cross section. As the direct physical model developed herein deals only with forces between discrete current elements, it can be readily adapted to suit different coil geometries and is widely applicable in various fields of research such as near-field communications, antenna design, wireless power transfer, sensor applications and beyond.

Leaky wave lenses for spoof plasmon collimation.

We theoretically demonstrate the feasibility of collimating radiating spoof plasmons using a leaky wave lens approach. Spoof plasmons are surface waves excited along reactance surfaces realized through metallic corrugations. By employing a periodic perturbation to the geometric profile of this type of reactance surface, it becomes feasible to convert the excited spoof plasmons into free-space radiating leaky wave modes. It is demonstrated that by structurally modifying such a corrugated surface through the introduction of a non-uniform sinusoidally modulated reactance profile, then a tapered wavenumber, with a real part less than that of free space, can be established along the surface. In this way the radiating properties of the structure (amplitude and phase) can be locally controlled thereby creating a radiating effect similar to that of a non-uniform current distribution. By properly engineering the space dependent wavenumber along the corrugated surface, different regions of the structure will emit spoof plasmon energy at different angles with varying intensity. The combined effect is the emission of an electromagnetic wave exhibiting a converging wave-front that eventually collimates spoof plasmon energy at some desired focal point.