Voltage Relaxation Research Articles

Investigation of significant capacity recovery effects due to long rest periods during high current cyclic aging tests in automotive lithium ion cells and their influence on lifetime

Abstract In this work for automotive lithium ion cells, the influence of prolonged rest periods of up to 5 days on the available capacity and the long term aging trend during cyclic aging tests is presented. Automotive prismatic cells with NMC/graphite chemistry and over 25 Ah capacity are investigated by means of various cycle tests. The test matrix includes a variation of possible influencing factors like current rates during cycling as well as temperature and state of charge during rest periods, rest period length and number of cycles between rest periods. A significant capacity recovery during rest periods is measured after cycling at moderate temperatures which has only been reported for low temperature lithium plating experiments before. Rest periods were conducted at 100% state of charge, thus the anode overhang effect can be excluded as the cause for the observed capacity increase. Voltage relaxation tests indicate that lithium plating occurs due to high current rates. Furthermore, a microscopic model is proposed to explain the long time constant of the limited exponential capacity increase during rest periods. Under these plating conditions a significant increase in cell cycle lifetime is observed if regular rest periods or standard characterizations are applied.

State-of-health (SOH) evaluation on lithium-ion battery by simulating the voltage relaxation curves

State-of-health evaluation of lithium ion batteries is one of the key challenges to construct efficient battery system with enhanced performance. In this paper, we demonstrate a facile evaluation method which is based on the voltage stabilization behaviour during relaxation. A second-order equivalent circuit is employed to describe the voltage relaxation. By simulating the voltage relaxation curves, the characteristic parameters that can distinguish the ohmic, activation and concentration depolarizations are acquired. The obtained parameters are sensitive to electrochemical states, which provide comprehensive evaluation to the state-of-health of batteries and aging mechanism analysis. As an example, the proposed method identifies two aging stages of Li(Ni1/3Co1/3Mn1/3)O2/graphite pouch cells during accelerated aging. Namely, a loss of lithium inventory in the first stage of aging, and the loss of active material in the next stage. Postmodern analysis including X-ray diffraction and high-resolution transmission electron microscope well agrees with the voltage relaxation simulation. The proposed state-of-health evaluation method is non-destructive and accessible for on-board aging diagnosis, which is of great significance for developing better battery management system.

A new concept to improve the lithium plating detection sensitivity in lithium-ion batteries

Lithium plating significantly reduces the lifetime of lithium-ion batteries and may even pose a safety risk in the form of an internal short circuit, leading to catastrophic cell failure. Low temperatures, high charge currents and battery age are known to be contributing factors to increased lithium plating. To reduce or avoid battery ageing induced by lithium plating, a method for lithium plating detection is essential to help understand the favourable conditions under which battery charging is optimised. In this study, we present a concept to design an experiment-based approach to improve lithium plating detection sensitivity using the non-destructive voltage relaxation analysis method. Commercial NCA/graphite cells are employed for this study. Here, the reversible part of the plated lithium providing a unique cell voltage relaxation profile is used as a pseudo-measure to detect the onset of plating. This profile is observed while the cell is at rest as well as under a low C-rate discharge regime immediately after charging. It is found that the CV (Constant Voltage) phase cut-off current value significantly influences plating detection. To address this issue, a procedure to determine the optimal cut-off current in the CV phase of charging is introduced to improve the detection sensitivity. With the proposed method, plating detection chances are improved which will help in understanding the favourable conditions and developing plating control strategies. Furthermore, a correlation between lithium plating and Electrochemical Impedance Spectroscopy (EIS) measurements are utilized to demonstrate the influence of previous ageing conditions on lithium plating.

Enhanced Fast Charging and Reduced Lithium-Plating by Laser-Structured Anodes for Lithium-Ion Batteries

Within this paper we report on a lithium-ion battery with laser-structured graphite anodes, alleviating current drawbacks of lithium-ion batteries such as the reduced discharge capacity at high C-rates and the on-set of lithium-plating during fast charging. These issues are intensified at low temperatures, as reaction and diffusion kinetics decelerate, which is why a focus of the presented work lies on low temperature performance. Electrochemical impedance spectroscopy was used to show a reduction in the impedances of cells with laser-structured anodes in comparison to their conventional counterparts. The discharge capacity retention at high C-rates was enhanced by up to 27% compared to conventional cells, proving potential for high power applications. For the cells with laser-structured anodes, the on-set of lithium-plating at 0°C was observed at higher charging C-rates by analyzing the voltage relaxation after charging. At −15°C, a smaller amount of plated lithium was detected, even though lithium-plating could not be entirely avoided. Laser structuring also enabled shorter charging times, as the upper cutoff voltage was reached at a higher SOC. The results point out that laser structuring of the anode improves the fast charging capability of lithium-ion cells, especially under demanding operating conditions.

Fast Charging of Li-Ion Cells: Part III. Relaxation Dynamics and Trap-Controlled Lithium Ion Transport

Fast charging of Li-ion batteries would ease consumer concerns regarding refueling time of electric vehicles. In this series of articles, we are systematically examining the effects of high currents flowing through the cell. Here we consider open-circuit relaxation dynamics for cell voltage and individual electrode potentials after abrupt termination of steady-state charging currents (1-6C) or short current pulses (< 30 s) of either polarity (to 16C). We demonstrate that dispersive kinetics observed after these current perturbations become faster for higher currents applied before the rest period. Our observations indicate that the voltage relaxation behaviors (i) depend on the current during charge, and less so on the state of charge, (ii) display universal stretched-exponential kinetics, (iii) can be induced by a short current pulse, (iv) require a minimum amount of charge transfer during the pulse, and (v) originate mainly from the oxide cathode. Trap controlled Li+ ion transport through a partially disordered oxygen-depleted subsurface layer in the cathode is suggested as a possible origin for the relaxation behavior. The slow multidecadal responses to high currents can cause the nonlinearities observed during rapid charging of the electrochemical cells, reported in Part 2 of this series.

Rapid Prediction of the Open-Circuit-Voltage of Lithium Ion Batteries Based on an Effective Voltage Relaxation Model

The open circuit voltage of lithium ion batteries in equilibrium state, as a vital thermodynamic characteristic parameter, is extensively studied for battery state estimation and management. However, the time-consuming relaxation process, usually for several hours or more, seriously hinders the widespread application of open circuit voltage. In this paper, a novel voltage relaxation model is proposed to predict the final open circuit voltage when the lithium ion batteries are in equilibrium state with a small amount of sample data in the first few minutes, based on the concentration polarization theory. The Nernst equation is introduced to describe the evolution of relaxation voltage. The accuracy and effectiveness of the model are verified using experimental data on lithium ion batteries with different kinds of electrodes (LiCoO2/mesocarbon-microbead and LiFePO4/graphite) under different working conditions. The validation results show that the presented model can fit the experimental results very well and the predicted values are quite accurate by taking only 5 min or less. The satisfying results suggest that the introduction of concentration polarization theory might provide researchers an alternative model form to establish voltage relaxation models.

A New Look at Formulation of Charge Storage in Capacitors and Application to Classical Capacitor and Fractional Capacitor Theory

&#x0D; &#x0D; &#x0D; &#x0D; In this study, we revisit the concept of classical capacitor theory-and derive possible new explanations of the definition charge stored in a capacitor. We introduce the capacity function with respect to time to describe the charge storage in a classical capacitor and a fractional capacitor. Here we will describe that charge stored at any time in a capacitor as ‘convolution integral’ of defined capacity function of a capacitor and voltage stress across it which comes from causality principle. This approach, however, is different from the conventional method, where we multiply the capacity and voltage functions to obtain charge stored. This new concept is in line with the observation of charge stored as a step function and the relaxation current in form of impulse function for ‘ideal geometrical capacitor’ of constant capacity; when an uncharged capacitor is impressed with a constant voltage stress. Also this new formulation is valid for a power-law decay current that is given by ‘universal dielectric relaxation law’ called as ‘Curie von-Schweidler law’, when an uncharged capacitor is impressed with a constant voltage stress. This universal dielectric relaxation law gives rise to fractional derivative relating voltage stress and relaxation current that is formulation of ‘fractional capacitor’. A ‘fractional capacitor’ we will discuss with this new concept of redefining the charge store definition i.e. via this ‘convolution integral’ approach, and obtain the loss tangent value. We will also show how for a ‘fractional capacitor’ by use of ‘fractional integration’ we can convert the fractional capacity a constant that is in terms of fractional units (Farads per sec to the power of fractional number); to normal units of Farads. From the defined capacity function, we will also derive integrated capacity of capacitor. We will also give a possible physical explanation by taking example of porous and non-porous pitchers of constant volume holding water and thus, explaining the various interesting aspects of classical capacitor and a fractional capacitor that we arrive with this new formulation; and also relates to a capacitor breakdown theory-due to electrostatic forces.&#x0D; &#x0D; &#x0D; &#x0D; &#x0D;

Observation of the possible chiral edge mode in Bi1−xSbx

After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three-dimensional topological insulators. Applying laser pulses to the surface state of Bi1−xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.

A generalized family of memristor‐based voltage controlled relaxation oscillator

SummaryRecently, memristive oscillators are a significant topic in the nonlinear circuit theory where there is a possibility to build relaxation oscillators without existence of reactive elements. In this paper, a family of voltage‐controlled memristor‐based relaxation oscillator including two memristors is presented. The operation of two memristors‐based voltage relaxation oscillator circuits is demonstrated theoretically with the mathematical analysis and with numerical simulations. The generalized expressions for the oscillation frequency and conditions are derived for different cases, where a closed form is introduced for each case. The effect of changing the circuit parameters on the oscillation frequency and conditions is investigated numerically. In addition, the derived equations are verified using several transient PSPICE simulations. The power consumption of each oscillator is obtained numerically and compared with its PSPICE counterpart. Furthermore, controlling the memristive oscillator with a voltage grants the design an extra degree of freedom which increases the design flexibility. The nonlinear exponential model of memristor is employed to prove the oscillation concept. As an application, two examples of voltage‐controlled memristor‐based relaxation oscillator are provided to elaborate the effect of the reference voltage on the output voltage. This voltage‐controlled memristor‐based relaxation oscillator has nano size with storage property that makes it more efficient compared with the conventional one. It would be helpful in many communication applications.

Optimum fast charging of lithium-ion pouch cells based on local volume expansion criteria

Fast charging of high-energy lithium-ion cells is desired in many applications but there is a lack of simple methods to determine the maximum charging current a cell will accept without sustaining damage. Here we explore the link between local volume expansion and lithium plating on the graphite anode of lithium-ion cells and propose a method that makes use of this link to design fast and gentle charging protocols. The expansion across the entire surface of a commercial nickel-manganese-cobalt-lithium oxide/graphite pouch cell is measured in short intervals using laser triangulation while operating the cell under various conditions. Irreversible expansion and expansion overshoots are correlated to voltage relaxation curves, post-mortem findings and capacity loss. Using the proposed method, a fast charging protocol is derived and validated in a cycle study covering 1000 cycles. The fast charging protocol enables both a 11% charging time reduction and a 16% reduction of capacity loss per cycle as compared to 1C constant current – constant voltage charging.

Using Local Volume Expansion to Design Better Charging Protocols for Lithium-Ion Batteries

Introduction Reduction of charging time is desirable in most battery-powered applications and may be a key factor for wider acceptance of battery electric vehicles (BEVs). However, fast charging of lithium-ion cells may result in accelerated aging [1]. In state-of-the-art cells, lithium deposition on the graphite anodes is a significant mechanism of capacity loss, especially with high charging rates and/or low temperatures. In this work, we show that local cell expansion can serve as an indirect measure of lithium deposition and how this relationship can be used to design charging protocols that provide both beneficial battery cycle life and charging time. Experimental and Results A 2-D thickness scan setup [2] is used to measure locally resolved thickness changes of commercial pouch cells during operation. After accounting for thermal expansion, the thickness changes are linked to reversible and irreversible lithium plating. The analysis is underpinned by voltage relaxation [3] and post-mortem examination. It is shown that the locally resolved thickness measurement helps detect lithium deposition more reliably and at an earlier stage than a with single-spot or a surface-averaged thickness measurement. Finally, a screening of charging conditions and corresponding thickness changes is performed. Fast charging protocols are derived based on maximum allowable local irreversible thickness change. An extended cycling experiment serves as a proof of the cycle life advantage of our fast charging protocols compared to standard constant current – constant voltage charging. Acknowledgements This work has received funding from the European Union's Horizon 2020 research and innovation programme under the grant 'Electric Vehicle Enhanced Range, Lifetime And Safety Through INGenious battery management' (EVERLASTING-713771).

Voltage relaxation and Abrikosov–Josephson vortices in Bi-2223 superconductors doped with α-Al2O3 nanoparticles

We have observed a change in the behaviour of voltage-relaxation curves with remnant magnetic flux, when the maximum magnetic field applied before starting the relaxation process, $$H_{am}$$ , reaches 80 Oe in two samples of $$\hbox {Bi}_{1.65}\hbox {Pb}_{0.35}\hbox {Sr}_{2}\hbox {Ca}_{2}\hbox {Cu}_{3}\hbox {O}_{10+\delta }$$ (Bi-2223) doped with $$\alpha $$ - $$\hbox {Al}_{2}\hbox {O}_{3}$$ nanoparticles. This modification consists in the emergence of a maximum in the temporal dependence of the voltage, V(t), which appears in the vicinity of the first 5 s after applying to the sample an excitation current lightly higher than the superconducting critical current of the material. In addition to that, the voltage drop rate increases appreciably with increasing applied magnetic fields. As complementary studies, X-ray diffraction patterns, scanning electron microscopy, measurements of magnetization as a function of applied magnetic field and critical current density are also included. The experimental results corroborate the occurrence of intragranular Abrikosov–Josephson vortices in Bi-2223 ceramic superconductors. Furthermore, the analysis provides a new procedure for detecting penetration of the magnetic flux into intragranular planar defects and clean regions of the grains (crystallites), with increasing applied maximum magnetic field, $$H_{am}$$ .

Voltage-Relaxation GITT and Reverse Monte Carlo to Determine Lithium Diffusion and Distribution in TiO2 and Highly-Ordered Nanoporous Hard Carbons

Amorphous hard carbons (AHCs) and metal oxides such as TiO2 are of interest in applications for energy storage including pseudo/super-capacitors and lithium-ion batteries. We present measurements of lithium diffusion and distribution in anatase TiO2 and several ordered AHCs prepared by soft/hard-templating techniques, some with capacities > 800 mAh/g. The investigated AHCs have surface areas between 60–937 m2/g and pore volumes between 0.04–1.01 cc/g. Li-diffusion was measured using two complimentary techniques (1) Galvanostatic Intermittent Titration Technique (GITT), and (2) a stretched exponential to investigate the voltage-relaxation (VR) GITT curve. VR-GITT provides two parameters related to (i) the geometry through which the diffusion is occurring, and (ii) a time scale yielding the diffusion constant with knowledge of an appropriate length scale. Both methods find diffusion constants in very good agreement with each other. Most importantly, the VR-GITT fitting parameters serve as a distinctive characterization tool and are able to distinguish between AHCs with different pore geometries and synthesis techniques. Further analysis of the VR curves was performed by numerical integration of the diffusion equation and reverse Monte Carlo simulated annealing to investigate the Li density profile in both TiO2 and the carbons, and suggest that lithium may plate nanopores of AHCs.

The abnormal photovoltaic effect in BiFeO3 thin films modulated by bipolar domain orientations and oxygen-vacancy migration

SrRuO3/BiFeO3/SrRuO3 thin-film capacitors were fabricated to investigate the role of mobile oxygen vacancies in a switchable photovoltaic (PV) effect that depends on ferroelectric domain orientations. Normally, the flowing direction of the stabilized photocurrent is opposite to ferroelectric polarization orientation. However, an abnormal overshoot of initial positive transient photocurrent parallel to the ferroelectric polarization could be observed under light illumination for a poled BiFeO3 thin film with the polarization pointing to the bottom electrode. Moreover, the photocurrent has a strong response with respect to voltage poling time and relaxation time after poling, which seems to be correlated with time-dependent migration of oxygen vacancies and concurrent charge trapping effect. Our results not only reveal the scenario how oxygen-vacancy migration and charge trapping effect affect the switchable PV effect, besides ferroelectric polarization, but also are helpful for the understanding and designing of new switchable photoelectric devices.

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.

Effect of boron incorporation on slow interface traps in SiO2/4H-SiC structures

The reason for the effective removal of interface traps in SiO2/4H-SiC (0001) structures by boron (B) incorporation was investigated by employing low-temperature electrical measurements. Low-temperature capacitance–voltage and thermal dielectric relaxation current measurements revealed that the density of electrons captured in slow interface traps in B-incorporated oxide is lower than that in dry and NO-annealed oxides. These results suggest that near-interface traps can be removed by B incorporation, which is considered to be an important reason for the increase in the field-effect mobility of 4H-SiC metal–oxide–semiconductor devices. A model for the passivation mechanism is proposed that takes account of stress relaxation during thermal oxidation.

Lithium plating in lithium-ion batteries investigated by voltage relaxation and in situ neutron diffraction

Abstract In this work, lithium plating is investigated by means of voltage relaxation and in situ neutron diffraction in commercial lithium-ion batteries. We can directly correlate the voltage curve after the lithium plating with the ongoing phase transformation from LiC 12 to LiC 6 according to the neutron diffraction data during the relaxation. Above a threshold current of C/2 at a temperature of −2 °C, lithium plating increases dramatically. The results indicate that the intercalation rate of deposited lithium seems to be constant, independent of the deposited amount. It can be observed that the amount of plating correlates with the charging rate, whereas a charging current of C/2 leads to a deposited amount of lithium of 5.5% of the charge capacity and a current of 1C to 9.0%.

Analysis on Overvoltage Relaxation of Lithium Secondary Batteries after Current Interruption

Introduction It is well known that the lithium ion secondary battery (LiSB) is the most promising power source for electric vehicle (EV), the further development is still needed to assure the life and performance prediction. To elucidate this, recent researches are focused on electrochemical impedance spectroscopy (EIS) method. However, these researches were mainly focused on to obtain data itself and resistance component of EIS by using parameter fitting technique with applying constant phase element without any further analysis on reaction kinetics. Especially the attribution of EIS spectra seemed to be miss-understanding since the theoretical consideration and modeling is not enough due to the complicated porous structure of electrode. There is another technique called as current-rest method1) suggested by Yata et.al. to obtain the internal resistance of LiSB. This technique is based on the measurement of over-potential relaxation after current interruption during the constant current charging/discharging process. However, the theoretical function of potential relaxation process with time is still not derived. In this paper, we derived the theoretical functions for the current rest method by applying the transmission line model. Results and Discussions The electrolyte solution generally used for LiSB is the 1:1 type electrolyte solution without the supporting electrolyte. In such a case, mass transfer by diffusion only should not be applied; we should take into account the migration with the diffusion at the same time. The ion flux can be expressed by the Nernst-Planck Equation. The effect of the convection is unclear up to now, the combination of diffusion and migration should be taken into account. Since the electro-neutrality should be fulfilled in the any part of the electrolyte solution, the familiar Fick’s law is still fulfilled by applying average diffusion coefficient. Thus, transmission line model can be applied for the mathematical derivation. The derivation of basic equation was carried out by using Laplace’s transform technique with the appropriate initial potential distribution and boundary conditions. In this paper, analysis have been made for the potential relaxation at separator so that the linear distribution was taken as the initial potential distribution. Thus obtained basic function had the series expansion form but it had the following feature. i) Initial response showed linear response to the square root of the time. ii) Long time response can be expressed in single exponential function to the time. iii) Both of the above functions are smoothly connected at dimensionless time T=0.5256 within the 0.2 % accuracy. Thus derived function was applied to the analysis of the measured voltage relaxation of commercially available LiSB for cellular phone. The voltage relaxation curves were well fitted by derived function. We employed the solver add-in of the Microsoft Excel in the parameter fitting process, where the time constant and the voltage difference was the fitting parameter for single time-constant function. For the case of fresh batteries with medium to high SOC level, showed two time constant system (50 s and 200 s) of which were connected in series. Fitted results were excellent within 1 mV deviation. On the other hand, the additional time constant system in the beginning of voltage relaxation were found for the SOC=0% and degraded battery. Even for such cases, the fitting of voltage relaxation were excellent by adopting three time constant system (0.3 s, 30 s and 250 s) of which were connected in series. We tested several type of LiSB such as cased in Al cans, 18650 type, and Al-laminated package, and the obtained results were almost similar. We concluded that the performance degradation of LiSB would appear in the beginning of the voltage relaxation with the order of magnitude of 0.1 s. As a conclusion, voltage relaxation measurement after current interruption was the powerful tool to obtain the information of the performance loss of LiSB. This technique may be the best choice for performance monitoring since the measuring time is short and the combination of charging/discharging is quite easy. References 1) S. Yata, H. Satake, M. Kuriyama, T. Endo, and H. Kinoshita, Evaluation of Positive Electrode Resistance by Current-Rest Method Using Four -electrode Cell, Electrochemistry, 78(5), 400-402 (2010)

Effect of the Grain-boundary Misorientation Distribution on the Intergranular Voltage Relaxation of Bi1.65Pb0.35Sr2Ca2Cu3 O 10+δ Ceramic Samples

The impact of the grain boundary misorientation distribution (GBMD) on the intergranular voltage relaxation (V − t) curves at zero applied magnetic field in Bi1.65Pb0.35Sr2Ca2Cu3O10+δ (Bi-2223) ceramic samples has been investigated. Changes in the GMBD were realized by subjecting powders of Bi-2223 to two different uniaxial compacting pressures (UCP) before the last heat treatment of the samples. The GBMD was then determined from X-ray rocking curves and revealed significant differences between intergranular media of the specimens. It was found that the UCP results in a two-time reduction in the population of high-angle grain boundaries (𝜃 > 12∘) while the orientation homogeneity of the grain boundaries rises ∼ 30 %, indicating an improvement of the degree of texture of the materials. Such changes are mirrored in the behavior of the V − t curves which are explained by invoking differences in the rearrangement of the transport current driven by the angular dependence of the critical current density along grain boundaries. Numerical simulations of the V − t curves support the experimental V − t results and further suggest the occurrence of current localization in conductive paths within the materials.

Temperature Effect on the Cell Life of Molten Carbonate Fuel Cell

This study proposes the temperature effect on the cell life and performance with coin type molten carbonate fuel cells (MCFC). The cells were operated at 600 oC, 700 oC and 800 oC under an atmospheric condition. The temperature effects were analyzed with electrochemical methods of steady state polarization, step chronopotentiometry, and electrochemical impedance spectroscopy. In general, the electrochemical reaction is accelerated by the temperature rising. Thus, higher exchange current is expected at higher temperature. The ionic conductivity in the carbonate electrolyte is also increased by the temperature increasing. Moreover, the gas solubility into molten carbonate has positive activation energies. Diffusion in the liquid and gas phases is also enhanced by the temperature. Therefore, higher performance is obtained at higher temperature. However, the carbonate has volatility, although it can be negligible below 600oC. The metal corrosion is also a function of temperature, higher temperature results in larger corrosion rate. Therefore, it could be guessed that reduction of electrolyte amount by the temperature rising makes higher internal resistance and mass transfer resistance. The OCV at 700 oC was higher than that at 800 oC because the absolute value of Gibbs energy is larger at 700 oC. The OCV at 700 oC continues for 1,200 h whereas 800oC continues only 250 h. Therefore, the cell life at 700 oC was much longer than that at 800 oC. At 150 mAcm-2, voltage of 700 oC was compared with that of 800 oC. It was comprehensible to know the temperature effect more clearly. The voltage at 800oC was initially higher than that at 700 oC. This is acceptable because the voltage at higher temperature has lower ionic and reaction resistances. However, the voltage at 800oC drops very quickly, indicating the electrolyte consumption was significant. On the other hand, the voltage at 700 oC is relatively stable and its internal resistance is slowly increasing. Therefore, lower temperature cell gives more stable and longer cell life. The cell performance of the electrolyte was analyzed at every 300 h of the cell at 700 oC and at every 60 h of the cell at 800 oC. From the steady state polarization, the current-voltage behaviors were measured while current density is applied from 0 mAcm- 2 to 150 mAcm- 2. The changes of the slope were increasing but not significant at 700 oC. However, the cell at 800oC shows severe slope change by the time. It means the resistance increasing becomes severe at higher temperature. At the step-chronopotentiometry, current steps were applied at every 60 second and following voltage relaxation was observed. All the cells showed step-voltage relaxation according to the current step. At early stage of operation, voltage relaxation was consistent. However at the cell of 800 oC, voltage relaxation was increased significantly at the end part of operation. It means mass transfer resistance becomes significant. The impedance behaviors were measured at the same time period. From the impedance results increasing internal resistance was observed at the cells. Since the internal resistance reflects ionic conductivity of the electrolytes, its enlargement represents electrolyte depletion. In particular, steeper internal resistance increase was observed at 800 oC than other temperature, representing significant electrolyte consumption existed at 800 oC. In addition, enlarging of the high-frequency half circle was obtained. Because the high frequency one represents cathodic mass transfer resistance, it means cathodic resistance becomes significant. It is plausible that the reduced electrolyte amount decreases active surface area of the cathode and enlarges its mass-transfer resistance. The anode has very high exchange current density, therefore the electrolyte reduction may not affect mass-transfer resistance significantly.