Investigation of solid electrolyte interfacial layer development during continuous cycling using ac impedance spectra and micro-structural analysis

The formation of passivating surface films on the electrodes of a lithium-ion polymer battery was investigated at various cycling state using ac impedance spectroscopy and scanning electron microscopy (SEM). A sealed commercial cell (Sony Co.) with a nominal capacity of 840 mAh was used for the experiment. An equivalent circuit used to model the impedance spectra show that, with continuous cycling there is a relatively large increase in the interfacial impedance and charge transfer resistances after a few hundred charge–discharge cycles. It was observed that the cell capacity decrease with increase cell impedance. SEM analysis on the electrodes shows that during continuous charge–discharge cycling, the deposition of sub-micro-size particles and dissolution of surface films on the graphite surface. This observation is consistent with increase in cell impedance as a function of charge/discharge cycling.     

Studies of the pulse charge of lead-acid batteries for PV applications: Part II. Impedance of the positive plate revisited

In the second part of this publication series, dedicated to the pulse charge of the lead-acid battery, a special attention is paid to the impedance spectrum of the positive plate as a source for estimation of the electrostatic capacitance of the double layer (C_dl) on the surface of the positive active mass. The impedance spectra were measured at open circuit for different states of charge (SoC) in H₂SO₄ with specific gravity 1.24 and 1.28 g ml⁻¹. A substantial difference was observed in the impedance spectra of partially charged and partially discharged positive plates keeping the same value of the SOC. The impedance data were subjected to inductance error correction, followed by differential impedance analysis (DIA). Considering the results from DIA, the recently published equivalent circuits of the positive plate in charged and in discharged state and the gel-crystal model of the lead dioxide, we proposed a model of the positive plate in partial state of charge (PSoC). The analysis of the obtained experimental results using this model and DIA show that the double layer capacitance is not frequency distributed. The influence of the state of charge and state of health on the model parameters is discussed. One of the most interesting results is the dependence of C_dl on SOC—it features a hysteresis at which the values of C_dl during the charge are 5–6 times higher than the corresponding ones during the discharge. This result was discussed in terms of changes in the double layer structure considering the gel-crystal model of the lead dioxide. During the discharge in H₂SO₄ with specific gravity 1.28 g ml⁻¹ a passivation process was detected as a high frequency pseudo-inductive loop in the Nyquist plots in PSoC. The passivation time constant is higher at 50–60% SOC and decreases to zero in the end of the discharge. During the charge in both electrolytes, pseudo-inductive time constant was observed too. It was attributed to the phenomena of the dehydration of Pb(OH)₄, an intermediate in the reaction scheme of the PbSO₄ oxidation. The state of health influences mostly the ohmic resistance R_Ω, the charge transfer resistance R_ct and the parameters of the constant phase element accounting the diffusion in the pores (CPE_diff), when the plate is well charged.     

Synthesis of three-dimensional graphene@Ni(OH)2 nanoflakes on Ni foam by RF magnetron sputtering for application in supercapacitor

Three-dimensional graphene@Ni(OH)₂ nanoflake array grown on Ni foam (G/Ni(OH)₂/NF) as a binder-free electrode of supercapacitor was prepared by combining a one-step hydrothermal approach and Radio frequency (RF) magnetron sputtering technique. Its electrochemical properties were further investigated by the cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectra. The G/Ni(OH)₂/NF showed high specific capacitance (4.0F/cm² at 1.0 mA/cm²), good rate charge-discharge capability and long cycling stability (ca. 90.6% of its initial value). This work provides a new method to prepare 3D porous electrode materials based on graphene for application in electrochemical energy storage.     

The power capability of ultracapacitors and lithium batteries for electric and hybrid vehicle applications

There is much confusion and uncertainty in the literature concerning the useable power capability of batteries and ultracapacitors (electrochemical capacitors) for various applications. Clarification of this confusion is one of the primary objectives of this paper. The three approaches most often applied to determine the power capability of devices are (1) matched impedance power, (2) the min/max method of the USABC, and (3) the pulse energy efficiency approach used at UC Davis. It has been found that widely different power capability for batteries and ultracapacitors can be inferred using these approaches even when the resistance and open-circuit voltage are accurately known. In general, the values obtained using the energy efficiency method for EF = 90-95% are much lower than the other two methods which yield values corresponding to efficiencies of 70-75%. For plug-in hybrid and battery electric vehicle applications, the maximum useable power density for a lithium-ion battery can be higher than that corresponding to 95% efficiency because the peak power of the driveline is used less frequently and consequently charge/discharge efficiently is less important. For these applications, the useable power density of the batteries can be closer to the useable power density of ultracapacitors. In all cases, it is essential that a careful and appropriate measurement is made of the resistance of the devices and the comparisons of the useable power capability be made in a way appropriate for the application for which the devices are to be used.     

Electrochemical impedance studies of AB5-type hydrogen storage alloy

Electrochemical impedance spectroscopy technique was used to describe behavior of AB₅-type hydrogen storage alloy. Impedance investigations were performed during cyclic voltammetry measurement and charge/discharge cycles. The comprehensive interpretation of instantaneous impedance spectra obtained in potentiostatic mode allowed further to interpret impedance results in galvanostatic mode. Proposed methodology enabled to trace electrical parameters as a function of state of charge (SOC) and depth of discharge (DOD).     

Graphene/β-MnO2 composites—synthesis and its electroanalytical properties study in the Mg storage battery

Graphene/β-MnO₂ composites were synthesized via ultrasonication method, followed by calcination at 400°C for 4 hours. The structural properties of synthesized graphene and graphene/β-MnO₂ composites were systematically studied using different instruments, such as XRD, FESEM, TEM, EDX, XPS and FT-IR spectroscopy. Furthermore, magnesium storage performance was scientifically studied using various electroanalytical characterizations like galvanostatic charge/discharge, electrochemical impedance spectra and cyclic voltammetry. The GNS/β-MnO₂-20% composite displays the discharge capacity of 110 mAh/g within 60 cycles at a current density of 25 mA/g, which is an excellent rate capability, and magnesium storage performance around 100 cycles. There is a significant improvement in electrochemical properties of GNS/β-MnO₂-20% composite materials as cathode for magnesium storage system due to the synergetic interaction between a pure β-MnO₂ particle and graphene sheet.     

Elaboration and electrochemical characterization of Mg–Ni hydrogen storage alloy electrodes for Ni/MH batteries

Mg–Ni hydrogen storage alloy electrodes with composition of Mg–33, 50, 67 Ni at. % in amorphous phase were prepared by means of mechanical alloying (MA) process using a planetary ball mill. The electrochemical hydrogen storage characteristics and mechanisms of these electrodes were investigated by electrochemical measurements, X–ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The relationship between alloy composition and electrochemical properties was evaluated. In addition, optimum milling time and composition of Mg–Ni hydrogen storage alloy with acceptable electrochemical performance were determined. XRD results show that the alloys exhibit dominatingly amorphous structures after milling of 20 h. The electrochemical measurements revealed that the discharge capacity of Mg₃₃Ni₆₇ and Mg₆₇Ni₃₃ alloy electrodes reached a maximum when alloys were prepared after 20 h of milling time (260 and 381 mAhg^?1, respectively). The maximum discharge capacity of Mg₅₀Ni₅₀ alloy was observable after 40 h milling (525 mAhg^?1). It was also found that the cyclic stability of the alloys increased with increasing Ni content. Among these alloys, the amorphous Mg₅₀Ni₅₀ alloy presents the best overall electrochemical performance. In this paper, electrode process kinetics of Mg₅₀Ni₅₀ alloy electrode was also studied by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The impedance spectra of electrodes were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. The electrochemical parameters calculated from electrochemical impedance were also compared. The electrochemical discharge and cyclic performance of 20, 40 and 60 h milled Mg₅₀Ni₅₀ alloy electrodes were demonstrated by the fitted charge transfer resistance and Warburg impedance obtained at various DODs. It was further observed that the controlling-step of the discharge process changed from a mixed rate-determining process at lower DODs to a mass-transfer controlled process at higher DODs. The fitted results demonstrated that charge–transfer resistance (R_ct) increased with DOD. The R_ct of 40 h milled Mg₅₀Ni₅₀ alloy (29.27 Ω) was lower than that of 20 h (41.89 Ω) and 60 h milled alloys (92.43 Ω) at fully discharge state.     

Pulse power tests on nickel oxide electrodes for nickel—zinc electric vehicle batteries

In the development of non-sintered nickel oxide electrodes for NiZn electric vehicle (EV) batteries, maintaining adequate power performance was of particular concern. In the systems studied, the power output was limited by the nickel oxide electrodes. Simple pulse power tests were useful in characterizing the power performance in such cells. Although the cell impedance was not a simple resistance, the effective impedance at the end of a high rate discharge pulse had a resistive nature. This simplified the test procedures so that an accurate estimate of peak power could be obtained from one measurement. Measurements of the dependence on state of charge showed that the power output at 50% depth of discharge was representative of the power capability available during discharge.A method was devised to project power performance expected in a NiZn cell from NiCd cell tests. This was useful in testing the durabili     

Multiphysics DC and AC models of a PEMFC for the detection of degraded cell parameters

The present paper proposes a new 2D modelling of ac impedance spectra of polymer electrolyte fuel cells (PEMFC). The computational domain includes the Membrane Electrode Assembly, the Gas Diffusion Layers and the channels on both the anode and cathode sides. The model takes into account the main fuel cell phenomena, i.e. reactants, charges transport and transfer and electrochemical reactions. First, the partial differential equations are solved in the steady state regime, then in the frequency domain in order to obtain the cell dynamic behaviour at different potentials. Experimental PEMFC impedance spectra are satisfactory reproduced over a relative large potentials range using only one set of model parameters. Numerical analysis of the key model parameters linked to the cell flooding state has been done. It is concluded that at least two impedance spectra at low and high potential are needed in order to discriminate the nature and the location of the cell degradations (anode or cathode, electrode or GDL). Based on a least square criterion, the model inversion is presented and several cell flooding scenarios have been precisely identified.     

Thermal behaviors of Ni-MH batteries using a novel impedance spectroscopy

In this paper, a novel impedance spectroscopy was used to describe the thermal behaviors of Ni-MH batteries. The impedance functions were derived similarly to electric impedance functions. The square of current was treated as a current equivalent and heat-flow as a voltage equivalent. The impedance spectra of batteries during charge showed that the combination of hydrogen and oxygen increased rapidly when charge rate was higher than 0.5 C. Thermal runaway might happen when battery was charged at temperature above 348 K even at a low charge rate. The cycling test showed that the charge efficiency of battery was the highest after cycling at high-rate for 10–100 cycles and decreased after more cycles. Different batteries showed different thermal behaviors which may be caused by the different structures of batteries.     

Studies of the pulse charge of lead-acid batteries for photovoltaic applications: Part IV. Pulse charge of the negative plate

The paper discusses the influence of the state of charge and pulse charge frequency on the mechanism of the lead-acid battery recharge with pulse current. The data from the pulse charge transients of the negative plate potential at various frequencies show that a decrease of the pulse charge frequency keeping constant average pulse current can impede the charge reaction leading to earlier start of the hydrogen evolution reaction. The dependence of the electrochemical double layer (EDL) capacitance on the state of charge was estimated both during the charge and the discharge using electrochemical impedance spectroscopy measurements at open circuit, followed by equivalent circuit modelling. These data were used to derive the dependence of the average double layer current on SOC and pulse charge frequency. The results show that in the end of the charge almost all of the charge proceeds with the participation of EDL in a certain pulse frequency domain. Using the data from the impedance measurements the optimal pulse charge frequencies were predicted, considering the existence of “electrochemical resonance”. The latter appears when the pulse charge frequency approaches the characteristic frequency of the Pb electrodeposition process, given by the product between EDL capacitance and the charge transfer resistance.     

Inner pressure characterization of a sealed nickel-metal hydride cell

This paper studies the electrochemical behaviour of the pressure inside a sealed Ni-MH cell due to gases evolved under different charge/discharge currents and states of charge (SOC). The work is focused to determine the best procedure to get fast charge and long cycle life without detrimental effects on the battery and possible hazards affecting the safety of the user. The device was studied under a wide range of charge current (0.1-5 C), establishing that optimum conditions to minimize the inner pressure during uninterrupted use are obtained if either charge rates up to 0.5 C or higher rates not surpassing 90% of the nominal capacity are employed. Charge times corresponding to the range between 80% and 130% of the nominal capacity were also tested, analyzing the effect of overcharges on inner pressure, discharge capacity, efficiency and integrity of the cell. It was verified that charging the cell up to 130% at 2 C rate reaches an inner pressure 5 times higher than that obtained at 0.5 C. High rate discharge was also characterized at uninterrupted use of the cell, demonstrating the importance of the cut-off discharge criterion at high rates, to avoid the inner gases accumulation due to incomplete discharge of electrodes and overcharge in a following electrochemical cycle.     

Influence of measurement procedure on quality of impedance spectra on lead-acid batteries

Many battery simulation models, but also electrochemical interpretations are based on impedance spectroscopy. However, the impedance of a battery is influenced by various factors, e.g. in the case of a lead-acid battery: state of charge (SOC), charging or discharging, superimposed dc current, short-term history or homogeneity of the electrolyte. This paper analyses the impact of those factors on impedance spectra of lead-acid batteries. The results show that very detailed information about the conditions during the measurement is crucial for the correct interpretation of a spectrum.     

Improved high-rate charge/discharge performances of LiFePO4/C via V-doping

V-doped LiFePO₄/C cathode materials were prepared through a carbothermal reduction route. The microstructure was characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrochemical Li⁺ intercalation performances of V-doped LiFePO₄/C were compared with those of undoped one through galvanostatic intermittent titration technique, cyclic voltamperometry, and electrochemical impedance spectrum. V-doped LiFePO₄/C showed a high discharge capacity of ∼70 mAh g⁻¹ at the rate of 20 C (3400 mA g⁻¹) at room temperature. The significantly improved high-rate charge/discharge capacity is attributed to the increase of Li⁺ ion “effective” diffusion capability.     

AC impedance characteristics of a vehicle PEM fuel cell stack under strengthened road vibrating conditions

Electrochemical impedance spectroscopy is used in this paper to investigate the performance of the fuel cell stack and single cells under long-term vibrating conditions on strengthened roads. During strengthened road vibration test, the electrochemical impedance spectra of fuel cell stack and several cells in the stack are measured nine times at regular intervals. Parameters of a Randles-like equivalent circuit are fitted to the experimental data. The classical Randles cell is extended by changing the standard plane capacitor into a constant phase element so that the quality of fit is improved. The results of the electrochemical impedance analysis indicate that the ohmic resistance of the fuel cell stack is nearly linear with the vibration time and reaches a growth of 0.035695% per hour. While the charge transfer resistance of the fuel cell stack during strengthened vibration test ascends after it falls down firstly, and finally tends to be stable. The influence of cell position on the AC impedance is also studied, and the results of which show that the cell position has a significant impact on the ohmic resistance.     

Characterisation of a PEM electrolyser using the current interrupt method

A method is proposed to model the electrochemical characteristics of a Proton Exchange Membrane (PEM) electrolyser. The electrochemical characteristics, which include the Ohmic, activation and concentration losses, are modelled by means of an equivalent electric circuit impedance. The equivalent electric circuit impedance under consideration is the Randles–Warburg (RW) cell and the parameters are obtained through the current interrupt (CI) method. The CI method consists of two parts, 1) the natural voltage response (NVR) method to model the Ohmic losses, and 2) the current switching (CS) method to model the activation and concentration losses. A simulation model of the RW cell is used to verify and validate the CI method. Thereafter, the CI method is practically implemented and the results presented. Results show that the Ohmic losses correlates well with existing literature. The paper furthermore illustrates the capability of the CI method to visually illustrate the RW impedance through Nyquist diagrams. Nyquist diagrams are used to illustrate the concentration losses and indicate the degradation state of the PEM at specific operating conditions. Results show that the concentration losses decrease with an increase in the operating current density and temperature.     

A study of the electrochemical lithium intercalation behavior of porous LiNiO2 electrodes prepared by solid-state reaction and sol–gel methods

The electrochemical lithium intercalation behavior of porous LiNiO₂ electrodes prepared by solid-state reaction and sol–gel methods is investigated by using X-ray diffractometry (XRD), a galvanostatic intermittent charge–discharge experiment, electrochemical impedance spectroscopy(EIS), and a charge–discharge cycling test. The ultrafine LiNiO₂ powder is prepared by the sol–gel method in order to overcome the disadvantage of the conventional solid-state reaction method. From the results of XRD, the layered LiNiO₂ phase proves to be stable above 400°C. The conventional oxide electrode suffers a larger capacity loss, a greater instantaneous IR drop during the first intermittent discharge, and a smaller chemical diffusivity than the gel-derived electrode. The results are discussed with respect to the marked cation mixing effect in the former electrode. Furthermore, the charge–discharge cycling test shows that the cell Li/organic electrolyte/gel-derived LiNiO₂ electrode displays improved performance, i.e., an initial specific capacity of 150 Ah kg⁻¹ and a specific energy density above 500 Wh kg⁻¹.     

Effects of particle size and type of conductive additive on the electrode performances of gas atomized AB5-type hydrogen storage alloy

The phase composition, morphology, structure, and state of the surface of gas atomized LaNi_4.5Al_0.5 alloy powders constituting a fine (≤50 μm), a medium (160–316 μm), and a coarse (630–1000 μm) fraction have been investigated. The electrochemical and storage characteristics of electrodes made from these powders with addition of electrolytic copper powder or a carbon composite (1 wt.% carbon nanotubes + 7 wt.% nanosized carbon black) as a conductive additive have been studied. In the work, X-ray diffraction, scanning electron microscopy, electron-probe microanalysis, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and several electrochemical methods have been used. It has been established that, in the initial state, the coarse-fraction gas atomized powders show a better kinetics of the hydrogen exchange reactions and higher discharge capacity (∼300 mA h/g). It is shown that electrodes made from the powders of all the fractions have a good high-rate discharge capability. Hydrogen diffusion coefficients during discharge of the electrodes made from the alloy powders of all the fractions and conductive additives have been calculated. It is shown that, for LaNi_4.5Al_0.5 alloy electrodes with the composite carbon additive, the hydrogen diffusion coefficients during discharge computed from data obtained by the electrochemical impedance spectroscopy method agree well with those calculated from cyclic current–voltage curves (2–4 × 10⁻⁹ cm²/s).     

High performance Li1·2(Mn0·54Co0·13Ni0·13)O2 with AlF3/carbon hybrid shell for lithium ion batteries

Abstract

A kind of core shell cathode material, Li_1·2(Mn_0·54Co_0·13Ni_0·13)O₂@AlF₃/C (LMSAC) was prepared by coating AlF₃ and carbon hybrid layer on the surface of lithium rich manganese based solid solution (LMSS) through a sol–gel process. It was characterised by X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) and transmission electron microscope (TEM). Its electrochemical properties were evaluated by electrochemical impedance spectra and galvanostatic charge–discharge measurements. Owing to the AlF₃/C shell, the LMSAC could deliver the initial capacities of 278, 260, 246, 230 and 210 mAh g^?1 at the rates of 0·1, 0·2, 0·5, 1 and 2C in the potential range of 2–4·8 V (versus Li⁺/Li) respectively, exhibiting an excellent rate capability. The corresponding coulombic efficiencies are 90, 89, 87, 88 and 85% in the first cycle. After cycled at the 0·1C and 1C rates for 50 times, the LMSAC remains 97 and 96% of the initial capacities respectively, displaying an enhanced cycling performance. The results suggest that surface coating of AlF₃/C hybrid layer could be a promising method to improve the electrochemical properties of the LMSS cathode materials.     

Electrochemical behaviour of single walled carbon nanotubes – Hydrogen storage and hydrogen evolution reaction

The electrochemical behaviour of single walled carbon nanotubes (SWCNT) related to the mechanism involved in the hydrogen electrode reaction applying electrochemical and spectroscopic techniques is studied. Cyclic voltammetry applied to electrodes containing different percentages of SWCNT demonstrates that this material can behave as efficient capacitor and that the hydrogen electrode reaction develops through the H-electrosorption followed by the formation of molecular H₂ and its evolution. Also, SWCNT are able to storage hydrogen within their porous structure. This is confirmed through the galvanostatic charge and discharge experiments. Electrochemical impedance spectroscopy allowed calculating the real area that takes part in the electrode reaction and the main and valuable conclusion is that the hydrogen electrode reaction consists of a simple charge transfer reaction and that the H adatom relaxation or diffusion processes can be disregarded. Furthermore, a model proposed for their structure which was validated through impedance experiments confirms those conclusions. Results of Raman spectra allowed identifying the nature of the electrodes confirming that after purification the material is composed of single walled carbon nanotubes.