Voltage Relaxation Research Articles

Onset Shift of Li Plating on Si/Graphite Anodes with Increasing Si Content

Mixing graphite with Si particles in anodes of Li-ion batteries provides increased specific energy. In addition, higher Si contents lead to thinner anode coatings at constant areal capacity. In the present study, we systematically investigated the influence of the Si content on the susceptibility of Li plating on Si/graphite anodes. Si/graphite anodes with Si contents from 0 to 20.8 wt% combined with NMC622 cathodes were manufactured on pilot-scale. After initial characterization in coin half cells and by SEM, pouch full cells with fixed N/P ratios were built. Rate capability at different temperatures, and Post-Mortem analysis were carried out. Results from voltage relaxation, Li stripping, SEM measurements, glow discharge optical emission spectroscopy (GD-OES) depth profiling, and optical microscopy were validated against each other. A decreasing susceptibility to Li plating with increasing Si content in the anodes could be clearly observed. A critical C-rate was defined, at which Li plating was detected for the first time. It was also found that at 0 °C the critical C-rate increases with increasing Si contents. At 23 °C the SOC at which Li dendrites were first observed on the anode also increased with higher Si content.

In-situ battery life prognostics amid mixed operation conditions using physics-driven machine learning

Accurately predicting in-situ battery life is critical to evaluate the system's reliability and residual value. The high complexity of battery aging evolution under variable conditions makes it a great challenge. We extract 6 physical features from voltage relaxation data to indicate battery performance fading, and then use data-driven techniques to predict battery life without considering any usage information. The model performance is validated against a dataset of 74 cells involving three battery types under mixed operation conditions. Experimental results show that battery lives are predicted accurately with the root-mean-squared-errors and mean absolute percentage errors being, respectively, generally less than 60 cycles and 10%. And the battery lives are classified quickly with the accuracies larger than 90%. This high prediction accuracy is maintained when only 6 sampling points taking 3–12 min are used. This work highlights the promise of using physics-driven machine learning to predict the behavior of complex systems under variable conditions.

Isolation of relaxation times under open-circuit conditions: Toward prognosis of nascent short circuits in Li-ion batteries

Li-ion battery mishaps are primarily attributed to short circuits, which missed early detection. In this study, a method is introduced to address this issue by analyzing the voltage relaxation, after initiating a rest period. The voltage equilibration arising from solid-concentration profile relaxation is expressed by a double-exponential model, whose time constants, τ1& τ2, capture the initial, rapid exponential contour and the long-term relaxation, respectively. By tracking τ2, which is very sensitive to small leakage currents, it is possible to detect a short early on and estimate the short resistance. This method, validated with experiments on commercial batteries induced with short circuits of varying extents, has >90% prediction accuracy and enables clear differentiation between different short severities, while factoring in the influence of temperature, state of charge (SOC), state of health (SOH), and idle currents. The method is applicable across different battery chemistries and form factors, offering precise and robust nascent-stage short detection-estimation for on-device implementation.

Determining the Limits of Fast Charging of a High-Energy Lithium-Ion NMC/Graphite Pouch Cell Through Combined Modeling and Experiments

This article presents the development, parameterization, and experimental validation of a pseudo-three-dimensional (P3D) multiphysics aging model of a 500 mAh high-energy lithium-ion pouch cell with graphite negative electrode and lithium nickel manganese cobalt oxide (NMC) positive electrode. This model includes electrochemical reactions for solid electrolyte interphase (SEI) formation at the graphite negative electrode, lithium plating, and SEI formation on plated lithium. The thermodynamics of the aging reactions are modeled depending on temperature and ion concentration and the reactions kinetics are described with an Arrhenius-type rate law. Good agreement of model predictions with galvanostatic charge/discharge measurements and electrochemical impedance spectroscopy is observed over a wide range of operating conditions. The model allows to quantify capacity loss due to cycling near beginning-of-life as function of operating conditions and the visualization of aging colormaps as function of both temperature and C-rate (0.05 to 2 C charge and discharge, −20 °C to 60 °C). The model predictions are also qualitatively verified through voltage relaxation, cell expansion and cell cycling measurements. Based on this full model, six different aging indicators for determination of the limits of fast charging are derived from post-processing simulations of a reduced, pseudo-two-dimensional isothermal model without aging mechanisms. The most successful aging indicator, compared to results from the full model, is based on combined lithium plating and SEI kinetics calculated from battery states available in the reduced model. This methodology is applicable to standard pseudo-two-dimensional models available today both commercially and as open source.

Relaxation Effects in Self-Discharge Measurements of Lithium-Ion Batteries

In order to determine the quality of newly produced lithium-ion cells, self-discharge measurements are performed after manufacturing during the so-called aging step. The aging step is a time-consuming and cost-intensive process due to relaxation effects disturbing the self-discharge measuremtens. Relaxation effects are mainly attributed to voltage relaxation and anode overhang equalization. In this work, different methods for electrical self-discharge measurement, such as the voltage decay method, the voltage hold method, and the capacity loss method, were investigated using commercial Samsung INR21700–50E cylindrical cells (NCA/silicon-graphite) and non-commercial pouch-cells (NMC622/graphite). Various preconditioning methods allowed a distinction to be made between undisturbed, short-term, and long-term disturbed cells. While short-term disturbances subsided after several hours, long-term disturbances due to anode overhang were found to significantly disturb all electrical self-discharge measurements for several weeks. A theoretical explanation was introduced, describing the lithium inventory change due to anode and coupled side reactions being measured differently by voltage- and capacity-based methods. Experimental results confirmed this notion and showed that voltage-based self-discharge measurements underestimated the anode overhang equalization. A comparison to the initial cell aging data indicated that the self-discharge measurement performed shortly after cell formation was disturbed by the anode overhang equalization.

Определение параметров ионной системы LiPON по кривым диффузионной релаксации поляризации

The results of studying the ionic system parameters of the LiPON solid electrolyte by the voltage relaxation method on the polarized Ti|LiPON|Ti structure are presented. The measurements were carried out at temperatures of 223, 248, 273, and 300 K on a bench consisting of a thermostat, a charger, a charge-discharge switching device, and a recording circuit. The samples in the form of Ti|LiPON|Ti test structures charged to a voltage of 1 V were discharged through an external load with a nominal value of 1 MΩ, 100 kΩ, 10 kΩ, 1 kΩ, 100 Ω, and 10 Ω, and U(t) curves were recorded. A mathematical model of diffusion relaxation of polarization is developed, which approximates the discharge curves U(t). The method of fitting the parameters of the approximating dependence was used to determine the equilibrium concentration of lithium ions, the effective thickness of the electrical double layer, and the volume relaxation time of nonequilibrium ions.

Precise prediction of open circuit voltage of lithium ion batteries in a short time period

Measuring the open circuit voltage (OCV) of a battery is quite time-consuming due to the relaxation process after the battery enters the open-circuit state. In this study, without the need of the complete voltage relaxation information, an available on-the-fly computing approach is proposed, by which the OCV at each state of charge (SOC) can be obtained from a simple first-order RC circuit model in a short period. The time constant of the RC model is evaluated by the ratio between the first and second derivatives of the experimental relaxation voltage at the earliest moment as soon as a specified voltage criterion on the real relaxation process is satisfied. With this criterion, the estimated OCV error is found to be extremely small. Experiments under three different temperatures and three types of battery state of health (SOH) are performed to verify the accuracy and robustness of the proposed approach. It is found that comparing to the traditional method of the OCV prediction at each SOC, the proposed approach can reduce the time of measuring the OCV to a few minutes (in most cases less than 6 min) with a reliable accuracy (in most cases less than 3 mV).

Determination of Leakage Currents Via Voltage Hold and Voltage Relaxation Method Using High Precision Coulometry - a Comparison and Optimization Study

Parasitic side reactions on the surface of the anode and the cathode of lithium-ion batteries contribute significantly to calendar and cyclic aging [1, 2]. In order to investigate these parasitic side reactions, such as solid electrolyte interface growth, this study focuses on two methods broadly utilized to determine leakage currents: the voltage hold and the voltage relaxation method. Regarding the voltage relaxation method, the open circuit voltage (OCV) decay is observed over weeks without allowing active electrode de-/lithiation [3] and subsequently, a small pulse is performed to calculate the leakage current [4]. For the voltage hold method, a defined voltage is kept constant, which compensates the parasitic side effects and allows active electrode de-/lithiation to maintain the state of charge (SoC) [5, 6].To compare these methods, different results are found in literature. On the one hand, both methods were reported to deliver individual results [3, 7], while other research activities [8] found the variance only at 100% SoC.Therefore, voltage hold and voltage relaxation were compared in this study, utilizing high precision coulometry (HPC). The measurements were conducted on 16 commercial LGChem INR18650MJ1 cylindrical cells at 25 °C, 40 °C and 55 °C and different SoCs of 10%, 50%, 90%, and 100% SoC, respectively. The cells were preconditioned to each SoC and were subsequently stored for 30 days to minimize relaxation and anode overhang effects. Afterwards, voltage hold and voltage relaxation measurements were carried out for 21 days at each temperature and SoC. In addition to the discharge pulse, an incremental capacity analysis (ICA) was conducted for all three temperatures through the whole voltage range to compare and validate the results obtained.The measurements of this study delivered similar results for the voltage hold and the voltage relaxation method, especially at 10% SoC and 50% SoC. Consequently, the voltage hold did not contribute to additional parasitic side reactions from allowing active de-/lithiation of the electrode. Minor deviations were found for 90% SoC and 100% SoC, for which one possible explanation may be the flat shape of the OCV curve, among others. In addition, the results show a strong dependency on the pulse length and strength.This study was part of the project ExZellTUM III, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0255, supervised by Project Management Jülich (PTJ).Literature [1] Smith, A.; Burns, J.; Dahn, J.: A high precision study of the coulombic efficiency of Li-ion batteries, In: Electrochemical and Solid-State Letters 13, p. A177, 2010 [2] Birkl, C. R.; Roberts, M. R.; McTurk, E.; Bruce, P. G.; Howey, D. A.: Degradation diagnostics for lithium ion cells, In: Journal of Power Sources 341, p. 373-386, 2017 [3] Zilberman, I.; Sturm, J.; Jossen, A.: Reversible self-discharge and calendar aging of 18650 nickel-rich, silicon-graphite lithium-ion cells, In: Journal of Power Sources 425 (9), p. 217-226, 2019 [4] Schmidt, J. P.; Weber, A.; Ivers-Tiffée, E.: A novel and fast method of characterizing the self-discharge behaviour of lithium-ion cells using a pulse-measurements technique, In: Journal of Power Sources 274, p.1231-1238, 2015 [5] Lewerenz, M.; Käbitz, S.; Knips, M.; Münnix, J.; Schmalstieg, J.; Warnecke, A.; Uwe Sauer, D.: New method evaluating currents keeping the voltage constant for fast and highly resolved measurement of Arrhenius relation and capacity fade, In: Journal of Power Sources 353, p.144-151, 2017 [6] Vadivel, N. R.; Ha, S.; He, M.; Dees, D.; Trask, S.; Polzin, B.; Gallagher, K. G.: On leakage current measured at high cell voltages in lithium-ion batteries, In: Journal of The Electrochemical Society, 164 (2), p. 508-A517, 2017 [7] Theiler, M.; Endisch, C.; Lewerenz, M.: Float Current Analysis for Fast Calendar Aging Assessment of 18650 Li(NiCoAl)O2/Graphite Cells, In: Batteries 7 (2), p. 22–22, 2021 [8] Käbitz, S.; Gerschler, J. B.; Ecker, M., Yurdagel, Y.; Emmermacher, B.; André, D.; Mitsch, T.; Uwe Sauer, D.: Cycle and calendar life study of a graphite LiNi1/3Mn1/3Co1/3O2 Li-ion high energy system. Part A: Full cell characterization, In: Journal of Power Sources 239, p. 572-583, 2013

Disturbances Due to Short- and Long-Term Diffusion Equalization Effects in Self-Discharge Measurements of Lithium-Ion Batteries

Quality assurance is essential for controlling lithium-ion cell properties during and after manufacturing. The largest contributors to process manufacturing costs of lithium-ion batteries are the cell formation and aging, when cell quality parameters are determined [1]. While voltage, capacity and impedance are determined reasonably fast, cell aging and self-discharge monitoring may take two to three weeks [2]. Apart from economical aspects for manufacturing and cell aging, accurately measuring the self-discharge may take up to several month [3, 4].The main reason for the long duration of electrical self-discharge measurements are disturbances due to short-term and long-term diffusion equalization effects [5]. Short-term effects up to several hours include solid phase and liquid phase diffusion, leading to open circuit voltage relaxation [6]. Long-term effects up to several weeks are mainly based on the same diffusion phenomena in conjunction with inactive anode overhang areas [7]. Therefore, the storage condition has significant impact on the state-of-charge of the anode overhang, inducing anomalies with regard to capacity [8] and coulombic efficiency [9]. Consequently, it is expected that the aforementioned disturbances might have a similar effect on the electrical measurement of self-discharge.In this work, various cell preconditionings were applied to different methods for electrical self-discharge measurement, such as the capacity loss, the voltage decay and the voltage hold methods. The preconditioning allowed a distinction between undisturbed cells, short-term and long-term disturbed cells, or cells affected by a combination of disturbances. The investigated cells were commercial Samsung INR21700-50E cylindrical cells (NCA/silicon-graphite) and non-commercial pouch cells (NMC622/graphite) with comparable capacity and anode overhang areas.The results showed significant impact of preconditioning on the self-discharge measurements. Self-discharge of undisturbed cells was accurately measured within a few days. Short-term disturbances subsided after several hours, which is in good agreement with the findings from open circuit voltage relaxation [6]. Long-term disturbances due to the anode overhang areas decreased over the measurement period even though the self-discharge current was still many times higher than the undisturbed self-discharge after several weeks. Comparison to initial testing of non-commercial cells showed an equivalence of voltage decay method for fresh cells after formation and long-term disturbed cells, indicating anode overhang charge equalization as the main contributor to self-discharge measurements during cell aging step.The utilized non-commercial pouch cells were designed and produced within the scope of the project “FormEL”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0296D.Literature[1] Liu, Y.; Zhang, R.; Wang, J.; Wang, Y.: Current and future lithium-ion battery manufacturing, In: iScience 24 (4), p. 102332–102332, 2021[2] Kwade, A.; Haselrieder, W.; Leithoff, R.; Modlinger, A.; Dietrich, F.; Droeder, K.: Current status and challenges for automotive battery production technologies, In: Nature Energy 3 (4), pp. 290–300, 2018[3] Zilberman, I.; Sturm, J.; Jossen, A.: Reversible self-discharge and calendar aging of 18650 nickel-rich, silicon-graphite lithium-ion cells, In: Journal of Power Sources 425 (9), pp. 217–226, 2019[4] Theiler, M.; Endisch, C.; Lewerenz, M.: Float Current Analysis for Fast Calendar Aging Assessment of 18650 Li(NiCoAl)O2/Graphite Cells, In: Batteries 7 (2), p. 22–22, 2021[5] Deutschen, T.; Gasser, S.; Schaller, M.; Siehr, J.: Modeling the self-discharge by voltage decay of a NMC/graphite lithium-ion cell, In: Journal of Energy Storage 19 (8), pp. 113–119, 2018[6] Kindermann, F.M.; Noel, A.; Erhard, S.V.; Jossen, A.: Long-term equalization effects in Li-ion batteries due to local state of charge inhomogeneities and their impact on impedance measurements, In: Electrochimica Acta 185, pp. 107–116, 2015[7] Wilhelm, J.; Seidlmayer, S.; Keil, P.; Schuster, J.; Kriele, A.; Gilles, R.; Jossen, A.: Cycling capacity recovery effect: A coulombic efficiency and post-mortem study, In: Journal of Power Sources 365, pp. 327–338, 2017[8] Burrell, R.; Zulke, A.; Keil, P.; Hoster, H.: Communication—Identifying and Managing Reversible Capacity Losses that Falsify Cycle Ageing Tests of Lithium-Ion Cells, In: Journal of The Electrochemical Society 167 (13), p. 130544–130544, 2020[9] Gyenes, B.; Stevens, D.A.; Chevrier, V.L.; Dahn, J.R.: Understanding Anomalous Behavior in Coulombic Efficiency Measurements on Li-Ion Batteries, In: Journal of The Electrochemical Society 162 (3), A278-A283, 2015

Interplay of Lithium Plating Quantification on Thermal Safety Characteristics of Lithium-Ion Batteries

Adverse lithium plating is a significant side reaction during the fast charging of lithium-ion (Li-ion) batteries when the Li-ion flux exceeds the intercalation or diffusion limits of graphite electrodes. Accurate quantification of lithium plating has always been a tough challenge given the severe defects of online detection methods such as coulombic efficiency and voltage relaxation plateau, making the mathematical correlation between cell-level thermal safety hazards and quantitative lithium plating events still a bottleneck problem. In this study, we apply a three-electrode (3E) Li-ion cell configuration and the accelerating rate calorimeter (ARC) to comprehensively investigate the interplay of unfavorable lithium plating on thermal runaway characteristics of Li-ion batteries. Lithium plating is introduced by cycling 3E Li-ion cells at low temperatures and quantified by analyzing potential-based plating energy, coulombic inefficiency, internal resistance, and voltage relaxation plateau. Surface microscopic characterization is carried out on graphite electrodes to reveal the morphologies and chemical states of lithium deposition. ARC experiments are implemented at full-cell and partial-cell scales to fundamentally understand the effects and contributions of thermally unstable lithium plating to the overall safety performance of Li-ion cell chemistries.

Modeling the inhomogeneous lithium plating in lithium-ion batteries induced by non-uniform temperature distribution

Under fast-charging or low-temperature charging conditions, large-format lithium-ion batteries are prone to inhomogeneous lithium plating, resulting in localized degradation and even internal short circuit, thus requiring a thorough investigation. This study establishes a three-dimensional electrochemical model to analyze the inhomogeneous lithium plating behavior in lithium-ion batteries. The model is validated with the voltage profiles and temperature distributions of a 24Ah pouch lithium-ion battery during low-temperature charging. The inhomogeneous lithium plating induced by non-uniform temperature distribution and the underlying mechanism are investigated using the model. The local current density and overpotential turn out to be the key factors affecting the spatial distribution of plated lithium. Moreover, the effects of temperature gradients on the lithium plating and stripping behaviors are explored. In-plane temperature gradient shows little effect on the total amount of plated lithium, but leads to apparent inhomogeneity in the distribution of plated lithium, with the differences between the concentrations of plated lithium in the upper and lower half of battery reach 57.61%, 103.06% and 159.68% of the average concentration in the whole battery during 1C charging at -5, 5 and 10°C, respectively. The temperature gradient also significantly influences the battery voltage relaxation profile related to the lithium stripping process. The characteristic plateau in the battery voltage relaxation profile disappears when the temperature gradient increases to larger than 6°C, leading to difficulty in detecting inhomogeneous lithium plating in large-format lithium batteries. The results presented in this study can provide helpful guidance for the design and management of large-format lithium-ion batteries.

An effective and cleaner discharge method of spent lithium batteries

The rapid development of new energy vehicles has exponentially increased the output of spent lithium-ion batteries (LIBs). The extraction and recovery of valuable metals from spent LIBs are expected to alleviate the shortage of battery materials. However, the spent LIBs to be recycled may still have certain amount of usable output power. If the battery is disassembled and crushed directly, the residual electricity may be hazardous. Therefore, discharge pretreatment should be carried out before disassembling and crushing spent LIBs. To date, electrochemical discharge in conducting solutions has been widely investigated and applied to discharge pretreatment, but there is no convincing evidence of its effectiveness. At the same time, non-electrochemical discharge in conducting powders has been investigated as a possible cleaner and more effective discharge method. Therefore, our current article reports the results of a series of experiments performed with spent LIBs to explore electrochemical discharge in conducting solutions and non-electrochemical discharge in conducting powders. We also systematically discuss the advantages and disadvantages of different discharge media in terms of their discharge efficiency, the potential existence of solid/liquid/gas phase contaminants, and their effectiveness. The experimental results obtained showed that battery discharge reached the safe voltage of 2 V and that the voltage rebound behavior occurs after 48 h standing. This is called the voltage relaxation phenomenon in an electrical field, which could cause certain safety risks during battery disassembly and crushing. Therefore, the effect of voltage rebound was also studied, and the obtained results showed that the voltage was over-discharged after flake-graphite discharge, which would cause irreversible damage to the anode material of the batteries and lead to battery failure. This is of great significance for the discharge pretreatment of spent LIBs. Therefore, flake-graphite discharge is an cleaner and effective discharge method for spent LIBs.

Characteristics of Open Circuit Voltage Relaxation in Lithium-Ion Batteries for the Purpose of State of Charge and State of Health Analysis

Open circuit voltage relaxation to a steady state value occurs, and is measured, at the terminals of a lithium-ion battery when current stops flowing. It is of interest for use in determining state of charge and state of health. As voltage relaxation can take several hours, a representative model and curve fitting is necessary for practical usage. Previous studies of lithium-ion voltage relaxation investigate four characteristics: relationship between voltage relaxation magnitude and state of charge; length of relaxation required; model complexity for state of charge estimation; and model complexity for state of health evaluation. However, previous studies have inconsistent methodology or use only one type of lithium-ion cell, making comparison and generalization difficult. To address this, we conducted 3 h and 24 h voltage relaxation experiments over a range of states of charge on three different lithium ion chemistries (nickel cobalt aluminum NCA; nickel manganese cobalt NMC532; lithium iron phosphate LFP) and fitted them with a new voltage relaxation equivalent circuit model. It was found that a 3 h relaxation period was sufficient for NMC and LFP for state of charge and state of health investigations. Voltage relaxation of the NCA cell continued to evolve past 24 h. It was shown that voltage relaxation shape and magnitude changes as a function of state of charge, and the accuracy of estimating state of charge was explored. Strategically choosing a state of charge for state of health assessment can be optimized to accentuate voltage relaxation magnitude and this differs by chemistry. This suggested technique and experimental findings can be paired with battery degradation studies to determine accuracy of assessing state of health.

Lithium plating detection using differential charging current analysis in lithium-ion batteries

In this paper, a lithium plating (LP) detection method for lithium-ion batteries is presented that can be applied during a constant-current-constant-voltage (CCCV) charging process and along with the voltage relaxation profile (VRP) technique constitute an effective and easy-to-use non-destructive tool. The suggested method detects the LP in a lithium-ion battery cell by examining a distinct plateau in the charging current during the constant-voltage (CV) phase of the CCCV procedure, which can be correlated to the LP process. Specifically, the proposed method monitors the battery cell charging current during the CV phase and by applying differential current analysis (DCA), it can detect the LP by seeking a local maximum that is observed in the curves of the charging current versus the time and the state-of-charge (SoC). The effectiveness of the method has been experimentally validated under several charging conditions with two established in-situ LP detection methods, the differential charging voltage (DCV) and the dynamic electrochemical impedance spectroscopy (DEIS). Furthermore, the correlation of the current plateau with the LP has been experimentally analyzed to enhance the knowledge on the reversible LP process during charging that can be utilized to further improve the sensitivity of LP detection methods.

Assessment on the Stable and High‐Capacity Na−Se Batteries with Carbonate Electrolytes

AbstractFactors affecting proper functioning of Na−Se system are investigated focusing on the polyselenide formation in ether‐ and carbonate‐based electrolytes. To do so, Se cathode is prepared by ball milling with commercial carbon and selenium powders. It is revealed that the soluble polyselenide species form in ether while no signature in carbonates proven by the in‐situ cyclic voltammetry and ex‐situ ultraviolet‐visible spectroscopy measurements as well as monitoring self‐discharge behaviours. Different Se discharge mechanism is also highlighted by staircase potentio electrochemical impedance spectroscopy (SPEIS) that is an impedance measurement applied to each potential step. Volume expansion is targeted using different types of binders in which carboxyl methylcellulose‐styrene butadiene rubber (CMC‐SBR) delivers the highest reversible capacity and the best rate performance resulting from its high adhesion strength. To further improve the performances, fluoroethylene carbonate (FEC) is used as a film forming additive that preserves Na metal integrity proven by the Na−Na symmetric cells and voltage relaxation upon cycling. As a whole, binders and electrolyte compositions are found to be the two crucial factors to obtain stable and high‐capacity Na−Se cells. This study underlines that much effort needs to be put on the strategies to overcome volume expansion than that of Se confinement into porous cathode.

Data-driven capacity estimation of commercial lithium-ion batteries from voltage relaxation

Accurate capacity estimation is crucial for the reliable and safe operation of lithium-ion batteries. In particular, exploiting the relaxation voltage curve features could enable battery capacity estimation without additional cycling information. Here, we report the study of three datasets comprising 130 commercial lithium-ion cells cycled under various conditions to evaluate the capacity estimation approach. One dataset is collected for model building from batteries with LiNi0.86Co0.11Al0.03O2-based positive electrodes. The other two datasets, used for validation, are obtained from batteries with LiNi0.83Co0.11Mn0.07O2-based positive electrodes and batteries with the blend of Li(NiCoMn)O2 - Li(NiCoAl)O2 positive electrodes. Base models that use machine learning methods are employed to estimate the battery capacity using features derived from the relaxation voltage profiles. The best model achieves a root-mean-square error of 1.1% for the dataset used for the model building. A transfer learning model is then developed by adding a featured linear transformation to the base model. This extended model achieves a root-mean-square error of less than 1.7% on the datasets used for the model validation, indicating the successful applicability of the capacity estimation approach utilizing cell voltage relaxation.

Thermionic graphene/silicon Schottky infrared photodetectors

Optical communications, imaging, and biomedicine require efficient detection of infrared radiation. Growing demand pushes for the integration of such detectors on chips. It is a challenge for conventional semiconductor devices to meet these specs due to spectral limitations arising from their finite band gap, as well as material incompatibilities. Single layer graphene (SLG) is compatible with complementary metal-oxide-semiconductor (CMOS) Si technology, while its broadband (UV to THz) absorption makes the SLG/Si junction a promising platform for photodetection. Here we model the thermionic operation of SLG/Si Schottky photodetectors, considering SLG's absorption, heat capacity, and carrier cooling dependence on temperature and carrier density. We self-consistently solve coupled rate equations involving electronic and lattice temperatures, and nonequilibrium carrier density under light illumination. We use as an example the infrared photon energy of 0.4 eV, below the threshold for direct photoemission over the Schottky barrier, to study the photothermionic response as a function of voltage bias, input power, pulse width, electronic injection, and relaxation rates. We find that device and operation parameters can be optimized to reach responsivities competitive with the state of the art for any light frequency, unlike conventional semiconductor-based devices. Our results prove that the SLG/Si junction is a broadband photodetection platform.

Battery State Estimation based on Dual Extended Kalman Filtering with Fixed Step

With the rapid popularization of new energy vehicles, users pursue reliable mileage as well as stable and efficient power battery charging and discharging performance, which puts forward higher requirements for on-board battery management system (BMS). Realizing online update of battery model parameters and accurate estimation of charged state has also become one of the key technical problems in the field of new energy vehicles at present. In this paper, based on the second-order resistor-capacitor (RC) equivalent circuit model, multiple parameters in the voltage relaxation stage were obtained through the curve relationship between open circuit voltage and state of charge, and the fast time-varying parameters were converted into parameters of different step sizes. An estimation model of state of charge based on fixed step Kalman Filtering algorithm was built by Simulink to realize online estimation of battery parameters and state of charge estimation. In this paper, a kind of ternary lithium battery was selected to establish data sets and test, which verified good robustness and high accuracy of the model.

A Novel Modal Representation of Battery Dynamics

Electrochemical cells are complicated energy storage systems with nonlinear voltage dynamics. There is a need for accurate dynamic modeling of the battery system to predict its behavior over time when discharging. The study conducted in this paper developed an intuitive model for electrochemical cells based on a simple mechanical analogy. A three-degree-of-freedom, spring-mass-damper system was decomposed into modal coordinates that represent the overall discharge, mass transport, and double-layer effect of the electrochemical cell. The developed model was experimentally demonstrated through pulsed discharge tests of commercially available lithium-ion and nickel metal hydride cells. The modal parameters of the natural frequency and damping ratio for each mode were determined by numerically minimizing the error in the time responses. Additionally, the mechanical analog was applied to two datasets developed by the Center for Advanced Life Cycle Engineering (CALCE). The first dataset was used to optimize the modal parameters whereas the second dataset was utilized to validate the tuned parameters. It was found that the modal representation of the mechanical analog could accurately predict the time-response dynamics of all the cells considered. Additionally, by considering the discharge modal coordinate, the open-circuit voltage was determined and validated to that measured experimentally from the voltage relaxation peaks.

Lithium plating detection using dynamic electrochemical impedance spectroscopy in lithium-ion batteries

This paper proposes a lithium plating detection method for lithium-ion batteries that can be applied in real time, during the charging procedure. It is based on the impedance analysis and it can be realized by utilizing the dynamic electrochemical impedance spectroscopy (DEIS) technique. Specifically, a sinusoidal current is superimposed on the charging current, and the battery impedance is analyzed in the frequency domain without interrupting the charging procedure. In case of lithium plating during charging, the graphite interfacial impedance is significantly decreased and thus, the lithium plating can be detected by monitoring the real and imaginary parts of the battery impedance in real-time. Two battery cell types with different active materials and energy densities are examined at several temperatures and charging currents. The effectiveness of the proposed DEIS technique is validated with the voltage relaxation profile method and in comparison with the current interruption method, for different charging conditions, cell formats and chemistries. The experimental results reveal that the proposed method is capable to effectively detect the lithium plating for rates as high as 6C, at different temperatures and for different battery types. • Nondestructive method to detect the onset of lithium plating during charging • Dynamic EIS applied in real-time without interrupting the charging procedure • Lithium plating is detected by monitoring the battery interfacial impedance • Battery operating conditions are considered at the lithium plating detection • Experimental validation and correlation with existing methods