Dummy Cell Research Articles

Cyclic voltammetry signal analyses for equivalent electric circuits consisting of multiple resistors and capacitors

An equivalent circuit is a combination of resistors, capacitors, inductors, Warburg impedance, and constant phase elements, which are widely measured and analyzed to understand the electrochemical (EC) properties of materials such as batteries, thin films, or corrosion. An equivalent electric circuit is also built as different dummy cells to evaluate the measurement accuracy of EC instruments with key detection modes, including electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Dummy cell signals for EIS can be easily understood and simulated by existing EIS signal analysis software, while dummy cell signals for CV curves have no simulation formula so far. When using dummy cells for CV tests, users can only observe if the CV curves obtained seem to be correct, which impairs the accuracy and efficiency of quality as well as the ability to confirm the functionality of an EC instrument. In this study, we conducted CV signal analyses on five cases of equivalent circuits consisting of multiple resistors and capacitors, with their correctness verified by dummy cell experiments using two different potentiostats. Based on the measured CV signals, we further demonstrated a data process method to evaluate the performance of a potentiostat quantitatively and automatically. This study enables researchers to calculate CV curves of resistors and capacitors formed by EC equivalent circuits, and provides a quantitative method to verify whether an EC instrument is functioning well.

Novel Dummy Cell Programming Scheme to Improve Retention Characteristics in 3-D NAND Flash Memory

Novel Dummy Cell Programming Scheme to Improve Retention Characteristics in 3-D NAND Flash Memory

Investigation of Erase Cycling Induced Joint Dummy Cell Disturbance in Dual-Deck 3D NAND Flash Memory.

To satisfy the increasing demands for more word-line (WL) layers, the dual-deck even triple-deck architecture has emerged in 3D NAND Flash. However, the new reliability issues that occurred at the joint region of two decks became a severe challenge for developing multiple-deck technology. This work reported an abnormal reliability issue introduced by erasing disturbance of the dummy WLs at the joint region (Joint-DMYs) under multiple cycling. More specifically, after several erase cycling stresses, the increasing joint-DMY's threshold voltage (Vt) due to the operational stress will finally result in additional disturbance to the adjacent data WLs. In this paper, we proposed this disturbance during erase originates from the backward injected electrons through FN tunneling based on our TCAD simulation result. Moreover, we also proposed an optimal erase scheme to reduce the backward electron injection and suppress the abnormal joint-DMY disturbance during the erase cycling.

Identifying Non-Stationarity in Non-Linear Harmonics of EIS Measurements of Li Batteries

Non-linear Harmonic Analysis is a technique used to investigate the distortions in Electrochemical Impedance Spectroscopy (EIS) measurements. The distortions are in both non-linear and non-stationary nature that appear as high order harmonics in the frequency domain. Differentiating the origin of the harmonics is important in understanding their nature and relating them to electrochemical phenomena. Here, we will show how the harmonics are in direct relation to the initial non-stationary response (initial transient) of the excited system [1]. We will show by both experimental measurements and simulations of dummy cell circuits, representing the electrochemical interface, that the initial transient exhibits high order harmonics that show the highest value at the characteristic frequency of the process [1]. We also extend the analysis to include more complex systems of primary Lithium batteries [2]. It is observed that the same non-stationary initial transient behavior is persistent in the time constants of the electrochemical processes. It is concluded that the capacitive components of the electrochemical processes are responsible for such non-stationary behavior in electrochemical systems [2].[1] G. Katırcı, M. A. Zabara, and B. Ülgüt, “Methods—Unexpected Effects in Galvanostatic EIS of Randles’ Cells: Initial Transients and Harmonics Generated,” J Electrochem Soc, vol. 169, no. 3, p. 030527, Mar. 2022, doi: 10.1149/1945-7111/ac5ad7.[2] M. A. Zabara, G. Katırcı, and B. Ülgüt, “Non-linear harmonics in EIS of batteries with lithium anodes: Proper controls and analysis,” Electrochim Acta, vol. 429, Oct. 2022, doi: 10.1016/j.electacta.2022.140969.

Study on the Liquid Cooling Method of Longitudinal Flow through Cell Gaps Applied to Cylindrical Close-Packed Battery

The increasing popularity of electric vehicles presents both opportunities and challenges for the advancement of lithium battery technology. A new longitudinal-flow heat dissipation theory for cylindrical batteries is proposed in order to increase the energy density and uniform temperature performance of cylindrical lithium-ion battery packs while also shrinking their size by roughly 10%. First, a genetic algorithm is used to identify a single cell’s thermal properties. Based on this, modeling and simulation are used to examine the thermal properties of the longitudinal-flow-cooled battery pack. It is found that the best coolant flow scheme has one inlet and one outlet from the end face, taking into account the cooling effect of the battery pack and engineering viability. Lastly, thermal dummy cells (TDCs) are used to conduct a validation test of the liquid cooling strategy. Additionally, the simulation and test results demonstrate that the liquid cooling solution can restrict the battery pack’s maximum temperature rise under the static conditions of a continuous, high-current discharge at a rate of 3C to 20 °C and under the dynamic conditions of the New European Driving Cycle (NEDC) to 2 °C. In applications where the space requirements for the battery pack are quite strict, the longitudinal-flow cooling method has some advantages.

Redoxless Electrochemical Capacitance Spectroscopy for Investigating Surfactant Adsorption on Screen-Printed Carbon Electrodes

Electrochemical impedance spectroscopy (EIS) is a sensitive analytical method for surface and bulk properties. Classical EIS and derived electrochemical capacitance spectroscopy (ECS) with a redox couple are label-free approaches for biosensor development, but doubts arise regarding interpretability when a redox couple is employed (redox EIS) due to interactions between electroactive probes and interfacial charges or forced potential. Here, we demonstrated redoxless ECS for directly determining surfactant adsorption on screen-printed carbon electrodes (SPCEs), validated through a simulation of equivalent circuits and the electrochemistry of electronic dummy cells. Redoxless ECS provides excellent capacitance plot loci for quantifying the interfacial permittivity of di-electric layers on electrode surfaces. Redoxless ECS was compared with redox EIS/ECS, revealing a favorable discrimination of interfacial capacitances under both low and high SDS coverage on SPCEs and demonstrating potential for probeless (reagentless) sensing. Furthermore, the proposed method was applied in an electrolyte without a redox couple and bare electrodes, obtaining a high performance for the adsorption of surfactants Tween-20, Triton-X100, sodium dodecyl sulfate, and tetrapropylammonium bromide. This approach offers a simple and straightforward means for a semi-quantitative evaluation of small molecule interactions with electrode surfaces. Our proposed approach may serve as a starting point for future probeless (reagentless) and label-free biosensors based on electrochemistry, eliminating disturbance with surface charge properties and minimizing forced potential bias by avoiding redox couples. An unambiguous and quantitative determination of physicochemical properties of biochemically recognizable layers will be relevant for biosensor development.

In Situ Measurement of Dynamic Internal Short Circuit Resistance during Nail Penetration of Lithium-Ion Cells and its Implications on Cell Robustness and Abuse Tolerance

Here we report a method for in situ measurement of internal short circuit (ISC) resistance during nail penetration testing of lithium-ion cells. The method is demonstrated with dry cells, wet dummy cells, and working cells using a small nail and slow penetration speed. ISC current and ISC temperature are also measured during the tests. It is confirmed that the ISC resistance changes dramatically, by several orders of magnitude, during nail penetration. More importantly, it is found that the stable resistance after full penetration is much higher than the lowest dynamic resistance at earlier stages of nail penetration. Analysis based on such a stable ISC resistance would underestimate the risk of thermal runaway during nail penetration tests. It is also found that ISC in some cases may be mitigated due to melting or rupture of aluminum foil surrounding the nail, implying a mechanism that may be able to be used towards the design of more robust/abuse tolerant Li-ion cells. Lastly, it is found that nail penetration using a larger nail reduces ISC resistance during penetration of cells but the general behaviors of ISC resistance are similar to those during smaller nail penetration.

Critical view of the results of the 2nd international round-robin test on EIS measurements of organic coatings

Although electrochemical impedance spectroscopy (EIS) is a well-established and powerful technique to investigate protective properties of organic coatings, there is no comparative study to show how reliable and reproducible are the results of this technique between different laboratories. One of the tools to fill this gap is international round robin tests (RRT). Following up the conclusions of 1st international RRT, presented in 11th workshop on AETOC Valencia (Spain), this paper will present the results of 2nd international RRT. 30 laboratories have participated this RRT using various equipment (24 different electrochemical systems from 8 manufacturers). To reduce the complexity of the testing in the 2nd RRT only one well defined E-coating material on a single kind of substrate (steel) was used as sample for all participating labs.Results were analyzed in terms of estimated capacitance (RC model, at single frequencies at 1 kHz and 10 kHz, respectively) and magnitude of impedance at low frequencies (100 mHz and 10 mHz). The provided data has been evaluated in a balanced approach using equal statistical weighting for each laboratory. The general conclusion drawn from the tests performed with the coated panel is that the results in terms of repeatability and reproducibility are scattered (high variance). To understand this issue, an additional RRT was organized with 11 participating laboratories (which showed the best results in the E-coating test), using a dummy cell as a test subject. Scattered EIS Bode plots for E-coting as well as for the dummy cell were observed and will be discussed.Although the careful collection and evaluation of the EIS raw data is crucial, it became apparent that this is not always being done. The observed sources of error and measurement artifacts will be presented and discussed. Recommendations for best lab and best evaluation practices will be presented.

Discovering onion services through circuit fingerprinting attacks

Tor onion services provide anonymous service to clients using the Tor browser without disclosing the real address of the server. But an adversary could use a circuit fingerprinting attack to classify circuit types and discovers the network address of the onion service. Recently, Tor has used padding defenses to inject dummy cells to protect against circuit fingerprinting attacks. But we found that circuits still expose much information to the adversary. In this paper, we present a novel circuit fingerprinting attack, which divides the circuit into the circuit generated by the client and the circuit generated by the onion service. To get a more effective attack, we tried three state-of-the-art classification models called SVM, Random Forest and XGBoost, respectively. As the best performance, we attain 99.99% precision and 99.99% recall when using Random Forest and XGBoost classification models, respectively. And we also tried to classify circuit types using our features and the classification model mentioned above, which was first proposed by Kwon. The best performance was achieved with 99.99% precision and 99.99% recall when using the random forest classifier in circuit type classification. The experimental results show that we achieved highly accurate circuit fingerprinting attacks even when application-layer traffic is identical and some type of circuits using the defenses provided by Tor.

Facile fabrication of ZnCo2S4@MWCNT as Pt-free counter electrode for high performance dye-sensitized solar cells

Substitution of platinum as a counter electrode material in dye-sensitized solar cells (DSSCs) is critical because Pt is rare and expensive. In addition, Pt-based counter electrodes have problems with the long-term stability of DSSCs. Among Pt-free electrocatalysts, composite materials are of great interest because they can be advantageous due to the combined performance of their components. Here, we present a ternary ZnCo2S4 composite grown on multi-walled carbon nanotubes as a Pt-free counter electrode for the reduction of triiodide in DSSCs. The composite ZnCo2S4@MWCNT was synthesized in a simple two-step process: (i) spray coating of a MWCNT dispersion on a FTO glass substrate, followed by (ii) solvothermal growth of the ternary sulfide ZnCo2S4 on a MWCNT layer. Cyclic voltammetry analyzes showed superior electrocatalytic activity of the composite in the reduction of triiodide compared to Pt. Electrochemical impedance spectroscopy measurements of symmetric dummy cells showed lower charge transfer resistance of ZnCo2S4@MWCNT (RCT = 5.54 Ω × cm2) than the traditional Pt electrocatalyst (7.07 Ω × cm2) in catalyzing the triiodide-to-iodide conversion reaction. The photovoltaic measurements of dye-sensitized solar cells showed an improved conversion efficiency of ZnCo2S4@MWCNT (8.55%) with excellent stability compared to Pt (8.19%).

High precision measurement of reversible swelling and electrochemical performance of flexibly compressed 5 Ah NMC622/graphite lithium-ion pouch cells

A novel mechanical compression test bench was developed and validated, to measure the reversible swelling and the electrochemical performance at pressures between 0.075MPa to 1.75MPa, applied on 5Ah NMC622/graphite lithium-ion pouch cells. Different cell setups were utilized in this study, including full-cells, symmetrical cells, and dummy cells, which consisted exclusively of anode, cathode, or separator layers. Symmetrical Swagelok T-Cells were measured for comparison. The reversible swelling was independent of the compression, whereas the porosity in the anode, the cathode, and the separator was reduced by the compression. Consequently, the ionic pore resistance in the anode increased by 19%, if the compression was increased from 0.075MPa to 1.75MPa, whereas no significant impact was found on the cathode and the separator. The hindered ionic transport in the anode increased the full-cell direct current internal resistance (DCIR) by up to 14%, while the capacity retention at elevated currents was lowered by 9% for the highest compression. The results of this study suggest that the cells should be operated at 0.2MPa maximum, since the electrochemical performance deteriorates for higher compression. To detect the influence of the compression with reduced time effort, DCIR pulse tests ≥ 60s at elevated currents and low SoCs are recommended.

Time-Resolved Electrochemical Heat Flow Calorimetry for the Analysis of Highly Dynamic Processes in Lithium-Ion Batteries

Isothermal microcalorimetry is used to study the heat flow of lithium-ion cells to provide insight into active material characteristics and to provide data required for a thermal optimization on the cell and system level. Recent research has shown the application of this technique to cells during high cycling rates, for example fast charging. However, the limitation of isothermal microcalorimetry is the low-pass characteristic of the measured heat flow, introduced by the thermal inertia of the setup and the calorimeter itself. To solve this problem, we introduce an optimized cell holder design and a novel data processing method for a time-resolved measurement of highly dynamic heat flow profiles. These are described in detail and validated using a synthetic power profile applied to a dummy cell. Experiments on a graphite-lithium half-cell illustrate the improvement of the method and the optimized cell holder when compared to the state-of-the-art setup, demonstrating the 3.6 times faster time response, which was further improved using a post-processing deconvolution technique. The thus improved time resolution provides the acquisition of more detailed features than currently shown in the literature and allows an accurate correlation of the thermal signals to electrochemical features like, e.g., the differential voltage of the cell.

Determination of the optimal frame length of digital television broadcasting

The paper proposes a simple method for determining the optimal frame length in the DVB-T2 standard based on simple and understandable formulas based on the sequence of processing user data in the DVB-T2 standard. There should always be an integer number of FEC blocks in a T2 frame, the remaining cells that do not make up an entire FEC block will be dummy cells as a rule and represent a speed loss. As a result, the total bit rate and the depth of interleaving in the smallest details depend on the exact number of OFDM characters, and consequently, the cells in the T2 frame.

Analysis of Deterioration Mechanism of Al-Pouch Film Used As Packaging Materials for Lithium Ion Batteries

Generally, lithium ion batteries (LIBs) can be divided into three different types: cylindrical, prismatic, and pouch-type LiBs with regard to the packaging form. Contrary to cylindrical or prismatic type batteries, pouch-type battery uses a pouch as an exterior material to protect inside of the cell from external influences, and have a metal lead-taps to receive and deliver electricity from external source. To safeguard the inside of the lithium ion battery, the pouch must have high resistance to gas and moisture permeability and excellent corrosion resistance to electrolytes.However, Al-pouch can be prone to contamination from electrolyte containing lithium salt during the electrolyte injection process, a key part of their manufacturing process. This electrolyte contamination can be fatal to the long-term reliability or durability of pouch-type LiBs. In this study, we investigate deterioration behavior of aluminium pouch film due to the electrolyte contamination. In order to clarify deterioration mechanism of Al-pouch film, we prepared exposure test of Al-pouch film and dummy cell samples which were stained by electrolyte droplet (LiPF6 in EC:EMC=1:2 (v/v) + 2 % VC). The change of micro-structure of the samples was tracked during 20 weeks by storing them under accelerated test conditions (RH 95 %, 60 ℃). As a result, we find out severe defect of the samples of Al-pouch film by damage to outermost nylon film and aluminium layer, and confirmed the effects on the battery after the defects occurred.To understand such phenomena, the micro-defect of Al-pouch films was observed by characterization techniques (XRD, FT-IR, SEM, EDX). We demonstrate that outermost layer of nylon film is damaged when it reacts with an electrolyte containing LiPF6 salt such as decreasing crystallinity, surface crazing, forming microdefects. The degradation of nylon film leads to severe surface cracking, followed by nylon locally peeling off from the Al-pouch. Peeling of the nylon film causes to expose the underlying thin aluminum film that constitute the middle layer of Al-pouch. Once partially exposed to the atmosphere, severe pitting corrosion of the aluminum film occurs due to atmospheric moisture and acids produced by LiPF6. Finally, a large amount of moisture to easily penetrate pouch-type LiBs through these corroded pits, showing the detrimental effects of electrolyte contamination on pouch-type LiBs’ long-term durability and reliability. Figure 1

Electrochemical impedance spectroscopy error analysis and round robin on dummy cells and lithium-ion-batteries

An interlaboratory Round-Robin comparison between three-labs is conducted where calibrated electrochemical-impedance-spectroscopy (EIS) is measured on prismatic cell dummies and prismatic batteries. Advanced EIS calibration workflow is employed allowing for precise measurements of low micro-Ohm impedances in a broad frequency range of 50 mHz to 10 kHz. Significant systematic-error corrections are obtained from the calibration. For instance, a correction of the raw data of one order of magnitude is applied for the reactance and more than 100% for the resistance at 1 kHz. Statistical analysis shows very good repeatability for the three-labs. For frequencies up to 100 Hz the differences of the resistance values are below 10 μΩ, while at 10 kHz the differences reach 200 μΩ. The various error sources contributing to EIS are analysed and compared to finite-element-modelling, particularly on the contacting fixture and positional accuracy. Also, a calibrated EIS repeatability study is done at different temperatures providing insights into the electrochemical properties of the battery. In summary, we provide for the first time a comprehensive quantitative analysis of the EIS repeatability and accuracy including different labs. This is relevant for establishing EIS as a tool for quality control in battery production and battery characterization for automotive applications.

Application of High-Order WENO Scheme in the CFD/FW–H Method to Predict Helicopter Rotor Blade–Vortex Interaction Tonal Noise

The accurate prediction of helicopter rotor blade–vortex interaction (BVI) noise is challenging. This paper presents an implementation of the seventh-order improved weighted essentially non-oscillatory (WENO-Z) scheme for predicting rotor BVI noise using a high-resolution numerical method based on the Reynolds-averaged Navier–Stokes and the Ffowcs Williams–Hawkings equations. The seventh-order improved WENO-Z scheme is utilized to minimize the inherent numerical dissipation of the reconstruction method in the monotone upstream-centered scheme for conservation laws (MUSCL), thereby improving the rotor wake resolution and the BVI noise-prediction accuracy. The three-layer dummy cell method is used to ensure that the flux at the boundary maintains seventh-order accuracy. The effectiveness of the flow solver and the acoustic solver is validated using the Helishape-7A rotor and the UH-1H rotor, respectively. The flow field and BVI noise characteristics of the OLS rotor obtained from the fifth- and seventh-order WENO-Z schemes are compared with that of the third-order MUSCL for coarse and fine background grids. The wake resolution, noise-prediction accuracy, and computational cost of the three schemes are compared. The results show that the high-order WENO scheme provides higher accuracy for flow field simulation and BVI noise prediction than the MUSCL, but the computational cost of the WENO scheme increases substantially as the grid resolution increases. However, the WENO scheme can predict BVI using a coarser grid than the MUSCL. The computational cost of the WENO scheme is relatively low under the same flow field simulation resolution.

AC interference impact on EIS assessment of organic coatings using dummy cells, calibration foils and field exposed coated samples

Field application of EIS to intact and scarcely degraded high-performance protective coatings is challenging because of ambient electromagnetic fields, which cause stochastic variations in the phase angle and, to a lesser extent, in the impedance modulus of the coating spectra. AC interferences with amplitudes between 10 and 800 mV were induced on dummy cells and calibration foils to systematically investigate the effects of interference impact on the accuracy and precision of measurements with the EIS setup intended for use on coatings in the field. For EIS signal amplitudes between 10 and 250 mV and the AC interference amplitudes ≤250 mV, the measurement accuracy and precision were limited by the synergistic influence of the AC interferences and the imprecision of the EIS instrument. For systems with impedance ≥ GΩ, the accuracy of the impedance logarithm was 97.8% at signal amplitudes ≥190 mV, and the precision increased from nearly 95% to 99% as the signal amplitude increased from 10 to 150 mV, indicating that very accurate and precise measurements can be obtained at higher signal amplitudes. An example of EIS measurements on coated samples exposed in the area of AC railway interferences is given. Although the quantitative conclusions may be instrument specific, they reflect well the declared capabilities of today's high-end EIS devices. The universal explanation is given in the context of equivalent circuit theory of the observed scatter in the EIS spectral data. The coating assessment was commented on in terms of the unique properties of the mean and relative standard deviation of impedance expressed on the logarithmic scale.

Thermal Management for Green Vehicle Batteries under Natural and Forced Convection Modes

The life span, efficiency and safety of lithium-ion batteries can be enhanced by temperature reduction and heat transfer optimization. The current research focuses on the cooling of lithium-ion batteries by reducing their temperature with optimized aluminum plates, which act like fins and carry away the heat due to convection in between the cells. For that, a seven cell battery pack model was built and cooling was done with air as a medium, which was manually passed through the system at a specified velocity using a blower. Considering the safety and cost as a key factors, dummy cells made of aluminum plates with heating coils inside were used rather than the original Li-ion battery for experimental studies. Numerical investigation of the temperature distribution on fins and the factors affecting the temperature reduction were performed. Free convection and forced convection methods were considered for the model using various calculated film coefficients. A reduction of 10oC to 20oC in temperature can be achieved using air which flows at a velocity of 8.5m/s. Numerical results are compared with the experimental results and the differences are discussed.

Ultra-thin 2D MoO2 nanosheets coupled with CNTs as efficient separator coating materials to promote the catalytic conversion of lithium polysulfides for advanced lithium-sulfur batteries

The severe shuttle effect and slow kinetics of lithium polysulfide (LiPS) conversion are two major obstacles to the practical application of lithium sulfur batteries. Ultra-thin 2D MoO2 nanosheets (MoO2 NSs) have been synthesized by chemical vapor deposition and then mixed with carbon nanotubes (CNTs) using as coating materials of the Celgard 2400 polypropylene separator to solve these problems. The 2D character of MoO2 NSs produced high surface/volume ratios and abundant active binding sites for anchoring LiPSs. In addition, the partial reduction of MoO2 NSs in a H2/Ar mixture introduced oxygen vacancies in their surface, which acted as catalytic sites for LiPS conversion, while the CNT network ensured rapid electron transfer for LiPS conversion reactions. Symmetric dummy cell tests showed that a 30wt%MoO2/CNT coated separator reduced the energy barrier for Li2S nucleation, and first-principles calculations verified its strong binding energy to entrap LiPSs and increase Li2S precipitation. Because of these features, a cell with a 30wt%MoO2/CNT coated separator had an improved specific capacity of 738 mAh·g−1 at 1 C with a slow decay rate of 0.053% for 800 cycles.

A fast scan cyclic voltammetric digital circuit with precise ohmic drop compensation by online measuring solution resistance and its biosensing application

In this work, a novel fast scan digital circuit for voltammetric analysis with precious ohmic drop compensation is developed, which is achieved through online measuring solution resistance first and then proportionally feedbacking the output signal to potentiostat's in-phase input through a potentiometer. It mainly consists of a solution resistance measurement module based on AD5933 chip, an ohmic drop automatic compensation module and a STM32F103ZET6 microcontroller. The performance of the circuit is checked successively using pure resistances, RC dummy cells, RC dummy cells incorporating a pseudo-faradaic component, and the ferrocene redox system. Results show that, precise ohmic drop compensation can be realized online and automatically, affording fast scan cyclic voltammetric (FSCV) analysis for theoretical electrochemical cells at 2000 V/s and that for practical electrochemical system using conventional electrodes at 1600 V/s. Based on this circuit, a very simple DNA biosensor for ultrasensitive detection of mercuric ions was explored. Benefitting from the high sensitivity brought by the high scan rate, the limit of quantitation (LOQ) can reach 1 pmol/L, demonstrating the application potential of FSCV in the field of ultrasensitive electrochemical detection.