Relaxation Of Open Circuit Voltage Research Articles

Drivers of Membrane Fouling in the Non-Aqueous Vanadium Acetylacetonate Redox Flow Battery

Due to their unique advantages in terms of long cycle life and modular design, redox flow batteries (RFBs) are being developed to load-level and peak shave energy from intermittent sources such as wind and sunlight. RFBs have yet to meet the stringent cost requirements needed to achieve broad market penetration, however. The second largest cost associated with state-of-the art RFBs after the electrolyte itself is the ion-exchange membrane, which accounts for 30% of the total cell cost [1]. Porous separators provide a pathway to substantially decrease the cost of RFBs, but long-term performance has been difficult to assess due to the coupling of electrolyte degradation, self-discharge, and membrane fouling. This study applies an adaptive observer model to estimate crossover from the open-circuit voltage relaxation of RFB cells, providing detailed information that can be used to decouple membrane fouling from electrolyte degradation by side reactions.For this study the nonaqueous V(acac)3 (vanadium acetylacetonate) electrolyte chemistry [2] is used. Because the RFB cell reaction is based on an electrochemical disproportionation of the neutral V(acac)3 complex, crossover of the active species in this system only results in reversible self-discharge.A novel set of 'canary cell' experiments unequivocally point to membrane fouling as the predominant factor that drives permanent cell degradation. An adaptive observer [3] is used to monitor the crossover rate, which is a proxy for membrane porosity, during self-discharge experiments where the cell is fully charged and the voltage is monitored during an open-circuit relaxation. Figure 1 shows that the porous Daramic separator starts off with ideal Fickian behavior because crossover is a linear function of the state of charge. Over time, however, the cell deviates, with active-species crossover becoming non-Fickian as the membrane fouls. The mass transfer coefficient (determined by the slope of the curve) decreases with exposure to current. It is hypothesized that pore clogging is responsible for this decreasing mass transfer coefficient, and in turn the diminution of cell performance. Deconvoluting membrane and electrolyte processes is a crucial step to determine the stability and long-term viability of RFB electrolyte systems.

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

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.

Observation of lithium stripping in super-concentrated electrolyte at potentials lower than regular Li stripping.

Lithium plating/stripping was investigated under constant current mode using a copper powder electrode in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) with methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) and vinylene carbonate (VC) as additives. Typical Li plating/stripping for Cu electrodes in organic electrolytes of conventional lithium batteries proceeds at potentials of several millivolts versus a Li counter electrode. In contrast, a large overpotential of hundreds of millivolts was observed for Li plating/stripping with the super-concentrated electrolyte. When Li stripping started immediately after Li plating and with no rest time between plating and stripping, two potential plateaus, i.e., two-step Li stripping, was observed. The potential plateau for the 1st stripping step appeared at −0.2 V versus a Li metal counter electrode. The electrical capacity for the 1st stripping step was 0.04 mA h cm−2, which indicates irregular Li stripping. Two-step Li stripping was also recorded using cyclic voltammetry. The electrochemical impedance spectroscopy (EIS) studies indicated that the two-step Li stripping behaviour reflected two different solid electrolyte interphases (SEIs) on electrodeposited Li in a Cu electrode. The SEI for the 1st-step stripping was in a transition period of the SEI formation. The open circuit voltage (OCV) relaxation with an order of tens of hours was detected after Li plating and before Li stripping. The in operando EIS study suggested a decrease of the charge transfer resistance in the Cu powder electrode during the OCV relaxation. Since the capacitance for the voltage relaxation was a dozen microfarads, it had a slight contribution to the 1st-step Li stripping behaviour. The voltage relaxation indicated the possibility that it is difficult for Li ions to be electrodeposited or that the Li plating is in a quasi-stable state.

Lithium-ion battery state-of-charge estimation based on deconstructed equivalent circuit at different open-circuit voltage relaxation times

Equivalent circuit model-based state-of-charge (SOC) estimation has been widely studied for power lithium-ion batteries. An appropriate relaxation period to measure the open-circuit voltage (OCV) should be investigated to both ensure good SOC estimation accuracy and improve OCV test efficiency. Based on a battery circuit model, an SOC estimator in the combination of recursive least squares (RLS) and the extended Kalman filter is used to mitigate the error voltage between the measurement and real values of the battery OCV. To reduce the iterative computation complexity, a two-stage RLS approach is developed to identify the model parameters, the battery circuit of which is divided into two simple circuits. Then, the measurement values of the OCV at varying relaxation periods and three temperatures are sampled to establish the relationships between SOC and OCV for the developed SOC estimator. Lastly, dynamic stress test and federal test procedure drive cycles are used to validate the model-based SOC estimation method. Results show that the relationships between SOC and OCV at a short relaxation time, such as 5 min, can also drive the SOC estimator to produce a good performance.

Electrochemical Quartz Crystal Microbalance Measurements of Lithium-Aluminum Alloy in a Glyme-Lithium Salt Molten Complex

Lithium-aluminum alloying reaction was investigated in lithium bis(trifluoromethylsulfonyl)amide triglyme solvate ionic liquid as a model system for the negative electrode of lithium secondary batteries. The formation of β-phase LiAl was confirmed by X-ray diffraction. The potential of α+β coexisting region was approximately 0.35 V vs. Li/Li(I). The slow relaxation of open circuit voltage and scanning electron microscope images suggested the inhomogeneous alloying reaction along with the active sites. The mass changes with doping and undoping of Li were measured qualitatively by electrochemical quartz crystal microbalance technique.

Electrochemical and Raman study of La0.7Sr0.3Co0.8Fe0.2O3−δ reduction

Abstract The reduction of La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3 − δ (LSCF7382) by heat-treatment in H 2 atmosphere or by electrochemical polarization has been followed by X-ray diffraction and Raman spectroscopy. The transition from perovskite ABO 3 to brownmillerite A 2 B 2 O 5 occurs at 250–300 °C in 10% H 2 atmosphere and is reversible. The Raman spectrum of brownmillerite shows two bands at 560 and 640 cm − 1 that we attribute to CoO 4 tetrahedron stretching. Above 600 °C the sample starts to decompose irreversibly into Co, La 2 O 3 , SrO, and LaSrFeO 4 . This confirms that Co cations are more reducible than Fe ones. The Raman spectra obtained under cathodic polarization of the LSCF7382/CGO interface at 300 °C and PO 2 = 10 − 4 or 0.1 atm show the same bands as for the A 2 B 2 O 5 structure. This indicates that electrochemical reduction produces Co 2 + in tetrahedral environment, even at low reducing overpotentials. Two reduction steps have been observed by linear sweep voltammetry and open circuit voltage relaxation. In both cases, Raman spectra indicate the presence of Co 2 + in tetrahedral environment. The first reduction step is thus attributed to the reduction of B 4 + within the perovskite phase and the second one to the reduction of Co 3 + to Co 2 + associated to the transition from perovskite to brownmillerite phase. Thermodynamic calculations from published literature support this idea. The association of n-type mobile species (Co 2 + ) with oxygen vacancies is confirmed.

Modeling of Voltage Hysteresis and Relaxation of HEV NiMH Battery

Abstract SOC (State Of Charge) estimation based battery management is important for HEV (Hybrid Electric Vehicle) applications. There are many difficulties in SOC estimation of HEV NiMH (Nickel Metal Hydroxide) batteries, for example, gassing at charge, self discharge, hysteresis and relaxation of OCV (Open Circuit Voltage), memory effect during charge-discharge cycling, and so on. In this paper, a modeling of hysteresis and relaxation of HEV NiMH battery is investigated. Model structure is simple, and model parameters are fitted by using voltage and current measurements. Accuracy of modeling is evaluated by using experimental results of 7.2V, 6.5Ah NiMH battery module.

Time constants of open circuit voltage relaxation in Cu(In,Ga)Se 2 solar cells

The time constants of metastable defect reactions in Cu(In,Ga)Se 2 solar cells are analyzed in a temperature range of 340 to 390 K. Both, the increase of the open circuit voltage V OC during illumination as well as the subsequent relaxation of the junction capacitance C follow a stretched exponential behavior. The activation energies are about 0.6 eV for the V OC increase and 1.0 eV for the capacitance decay, while the time constants exhibit a cross-over at a temperature of 360 K. The exponential factor decreases in both cases with increasing temperature, a behavior that is rather uncommon for systems exhibiting stretched exponential relaxation.

Electronic properties of CuGaSe2-based heterojunction solar cells. Part I. Transport analysis

This article presents a systematic study on the electronic transport mechanisms of CuGaSe2-based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage relaxation to verify the assumptions on the dominant loss mechanism in the different devices.

A model for the open circuit voltage relaxation in Cu(In,Ga)Se2heterojunction solar cells

This article investigates the electronic transport properties of ZnO/ CdS/ Cu(In,Ga)Se 2 heterojunction solar cells during and after illumination or forward bias in the dark. We observe a relaxation of the open circuit voltage under constant illumination as well as a relaxation of the voltage drop over the device under constant forward bias current in the dark. Both phenomena are accompanied by an increase of the sample capacitance. We introduce a general quantitative model concept for the open circuit voltage relaxation and related effects in heterojunction devices that explains the phenomena as a consequence of the persistent capture of charge carriers within the space charge region. We apply our concept to develop a specific quantitative model for the observed metastablity in Cu(In,Ga)Se 2 heterojunction solar cells.