Enhancing the electrochemical performance of d-Mo2CTx MXene in lithium-ion batteries and supercapacitors by sulfur decoration

With the development of the energy industry, electrochemical energy storage technology is increasingly involved in developing innovations in the field. The materials of the electrode have a significant influence on the performance of energy storage devices. For this purpose, two-dimensional MXene with excellent electrical conductivity, mechanical strength, and a variety of possible surface-active terminations are attracting much attention. In the present work, S-decorated d-Mo₂CT_x (d-Mo₂CT_x--S) is designed. The first-principles calculations reveal that it may possess good energy storage characteristics. Due to the decoration with S, unique morphology and structure are obtained, conferring stability, optimized Li⁺ storage, improved charge transport, and lithium-ion adsorption capabilities. Compared with d-Mo₂CT_x, d-Mo₂CT_x--S exhibits higher discharge capacity (623 mAh g⁻¹ at 1 A g⁻¹) as lithium-ion electrode material and higher specific capacitance (561 F g⁻¹ at 1 A g⁻¹). As a supercapacitor, the material also shows excellent cyclic stability (20,000 charge-discharge cycles). This work may inspire the exploration of other MXene and new surface functionalization methods to improve the performance of MXene as electrode materials for new energy devices.     

Charge/discharge characteristics of sulfur composite cathode materials in rechargeable lithium batteries

The charge and discharge characteristics of lithium batteries with sulfur composite cathodes have been investigated. The sulfur composites showed novel electrochemical characteristics. The analysis of the differential capacity indicated that the discharge process showed two voltage plateaus of 2.10 V and 1.88 V, and the charge process also presented two voltage plateaus of 2.22 V and 2.36 V. The overcharge test showed that the composite cannot be charged over 4.0 V, the voltage always stopped at about 3.9 V during charging, indicating that the composite presented the intrinsic safety for the overcharge of lithium batteries. The overcharge can cause serious safety problem for the conventional Li-ion batteries. The overcharge test also showed that the batteries with sulfur composite were destroyed when the upper cut-off voltage was over 3.6 V. However, the composite presented good reversible capacity after it was deep discharged even to 0 V. It showed stable cycleability and high cycling capacity of 1000 mAh g⁻¹ when cycling between 0.1 V and 3.0 V, indicative of the different characteristic from the conventional oxide cathode materials. The prototype polymer battery with the composite cathode material presented the energy density of 246 Wh kg⁻¹ and 401 Wh L⁻¹.     

Synergistic effect of bulk phase doping and layered-spinel structure formation on improving electrochemical performance of Li-Rich cathode material

In this study, La was doped into the lithium layer of Li-rich cathode material and formed a layered-spinel hetero-structure. The morphology, crystal structure, element valence and kinetics of lithium ion migration were studied by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The La doped lithium-rich cathode material exhibited similar initial discharge capacity of 262.8 mAh g^?1 at 0.1 C compared with the undoped material, but the discharge capacity retention rate can be obviously improved to 90% after 50 cycles at 1.0 C. Besides that, much better rate capability and Li⁺ diffusion coefficient were observed. The results revealed that La doping not only stabilized the material structure and reduced the Li/Ni mixing degree, but also induced the generation of spinel phase to provide three-dimensional diffusion channels for lithium ion migration. Moreover, the porous structure of the doped samples also contributed to the remarkable excellent electrochemical performance. All of these factors combined to significantly improve the electrochemical performance of the material.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

Study of a Li–Ni oxide mixture as a novel cathode for molten carbonate fuel cells by electrochemical impedance spectroscopy

Li–Ni oxide mixtures with high lithium content are considered to be an alternative cathode material for molten carbonate fuel cells (MCFCs). The electrochemical behaviour of Li_0.4Ni_0.6O samples has been investigated in a Li–K carbonate melt at 650 °C by electrochemical impedance spectroscopy as a function of immersion time and O₂ and CO₂ partial pressure. The impedance spectra have been interpreted using a transmission line model that includes contact impedance between reactive particles. The Li_0.4Ni_0.6O powder particles show structural changes due to high lithium leakage and low nickel dissolution from the reactive surface to the electrolyte during the first 100 h of immersion. After this time, the structure seems to be stable. The partial pressures of O₂ and CO₂ affect the processes of oxygen reduction and Li–Ni oxide oxidation. X-ray diffraction and chemical analysis performed on samples before and after the electrochemical tests have confirmed that the lithium content decreases. SEM observations reveal a reduction in grain size after the electrochemical tests.     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

Study of NiO cathode modified by rare earth oxide additive for MCFC by electrochemical impedance spectroscopy

The preparation and subsequent oxidation of nickel cathodes modified by impregnation with rare earth oxide were evaluated by surface and bulk analysis. The electrochemical behaviors of rare earth oxide impregnated nickel oxide cathodes were also evaluated in a molten 62 mol% Li₂CO₃+38 mol% K₂CO₃ eutectic at 650 °C by electrochemical impedance spectroscopy (EIS) as a function of rare earth oxide content and immersion time. The rare earth oxide-impregnated nickel cathodes show almost the similar porosity, pore size, and morphology to the reference nickel cathode. The stability tests of rare earth oxide-impregnated nickel oxide cathodes show that the rare earth oxide additive can dramatically reduce the solubility of nickel oxide in a eutectic carbonate mixture under the standard cathode gas condition. The impedance response of all cathode materials at different immersion time is characterized by the presence of depressed semicircles in the high frequency range changing over into the lines with the angles of which observed with the real axis differing 45° or 90° in the low frequency range. The experimental Nyquist plots can be well analyzed theoretically with a modified model based on the well-known Randles–Ershler equivalent circuit model. In the new model, the double layer capacity (C_d) is replaced by the parallel combination of C_d and b/ω; therefore, this circuit is modified to be the parallel combination of (C_d), b/ω, and the charge transfer resistance (R_ct) based on the Randles–Ershler equivalent circuit, to take into consideration both the non-uniformity of electric field at the electrode/electrolyte interface owing to the roughness of electrode surface, and the variety of relaxation times with adsorbed species on the electrode surface. The impedance spectra for all cathode materials show important variations during the 200 h of immersion. The incorporation of lithium in its structure and the low dissolution of nickel oxide and rare earth oxide are responsible for these changes. After that, the structure reaches a stable state. The rare earth oxide-impregnated nickel oxide cathodes show higher catalytic activity for oxygen reduction and lower dissolution of nickel oxide than the pure nickel oxide cathode. The cathode material having 1.0 wt.% of rare earth oxide shows the optimum behavior.     

Enhancing electrochemical performance and structural stability of LiNi0.5Mn1.5O4 cathode material for rechargeable lithium-ion batteries by boron doping

LiNi_0.5Mn_1.5O₄ cathode materials with a range of boron doping contents were successfully synthesized via an in situ solid-state method. The structures and grain morphologies were examined to elucidate the effect of boron doping on the electrochemical performance of LiNi_0.5Mn_1.5O₄. Scanning electron microscopy images show that the particle sizes of boron-doped LiNi_0.5-x/2B_xMn_1.5-x/2O₄ samples increase compared with those of pure LiNi_0.5Mn_1.5O₄. Characterization results confirm that boron doping could create more Mn³⁺ ions and increase the Mn³⁺ ions contents in LiNi_0.5-x/2B_xMn_1.5-x/2O₄ samples with increasing boron doping content. A greater number of Mn³⁺ ions could enhance the cationic disorder degree, thereby resulting in high electronic conductivities of LiNi_0.5-x/2B_xMn_1.5-x/2O₄ samples. Charge-discharge tests reveal that improvements in the electrochemical performance are achieved in LiNi_0.5-x/2B_xMn_1.5-x/2O₄ samples compared with that of pure LiNi_0.5Mn_1.5O₄. The boron-doped LiNi_0.495B_0.01Mn_1.495O₄ (denoted as LNMO-B0.01) cathode exhibits an excellent cycling stability with a capacity retention of 83.3% after 500 cycles at 3 C. Moreover, it also displays an optimal rate capability with discharge capacities of 136.1, 135.7, 130.3, 126.2, 123.1, 114.5, 104.5, and 82.9 mA h g^?1 at 0.2, 0.5, 1, 2, 3, 5, 7, and 10 C, respectively. The highest Li⁺ diffusion coefficient of LNMO-B0.01 determined from cyclic voltammetry tests indicates that an appropriate amount of boron doping could accelerate the Li⁺ diffusion in LNMO-B0.01. The lowest charge-transfer resistance obtained from the impedance spectra suggests that boron doping could promote kinetic charge transfer. As a result, this modification strategy can be utilized to enhance the electrochemical performance of LiNi_0.5Mn_1.5O₄ material.     

Investigation of solid oxide fuel cell short stacks for mobile applications by electrochemical impedance spectroscopy

Solid oxide fuel cells (SOFC) for mobile applications are developed and investigated at the German Aerospace Center (DLR) in Stuttgart. Therefore a light-weight stack design was developed in cooperation with the automotive industry (BMW/Munich, Elring-Klinger/Dettingen, ThyssenKrupp/Essen) and the Research Center Jülich (FZJ). This concept is based on the application of stamped metal sheet bipolar plates, into which the SOFC cells are integrated by brazing technology. For the development and the investigation of the SOFC cells and short stacks, the electrochemical impedance spectroscopy (EIS) is an important and useful characterization method. The paper concentrates on the investigation and on the electrochemical testing of the SOFC short stacks with sintered anode-supported cells (ASC). The short stacks were electrochemically characterized mainly by electrochemical impedance spectroscopy, by current-voltage measurements and by long-term measurements. The cells and stacks were operated at different temperatures, varying fuel gas compositions, different fuel gas flow rates and at different electrical current loads. The influence of these operating conditions on the electrochemical performance of the short stacks is outlined. The nature of losses, e.g. ohmic and the polarization resistances of the electrodes were examined and determined by fitting of the impedance spectra to an equivalent circuit.     

锂离子电池正极材料铌掺杂LiNiO2 的制备与电化学性能

为了提高LiNiO₂的电化学性能,用固相反应法制备了铌掺杂LiNiO₂材料,并用X射线衍射（XRD）分析、恒电流滴定技术（GITT）、电化学阻抗谱（EIS）等方法研究铌掺杂量对LiNiO₂的结构和性能的影响。结果表明适量的铌（Nb）掺杂可以提高LiNiO₂层状结构的有序程度,降低Li⁺/Ni²⁺混合程度,降低电荷转移阻抗,提高活性材料中锂离子的扩散系数。其中LiNi_0.99Nb_0.01O₂在0.5C循环100次的容量保持率为91.4%,5C时放电比容量为143 mA·h/g。而未掺杂铌的LiNiO₂在相同条件下的容量保持率和比容量仅为69.2%和127 mA·h/g。结果说明铌掺杂能够有效提高LiNiO₂的电化学性能。     

Investigation of electrode composition of polymer fuel cells by electrochemical impedance spectroscopy

In order to optimize the electrode composition and performance of Polymer Fuel Cells and to reduce the production cost of membrane electrode assemblies (MEAs), different MEAs using different catalyst powders, carbon supported and unsupported catalysts with different proton conducting electrolyte powder (Nafion) content were produced by using a dry powder spraying technique developed at German Aerospace Research Center (DLR, Deutsches Zentrum fuer Luft- und Raumfahrt). The electrochemical characterization was performed by recording current-voltage curves and electrochemical impedance spectra (EIS) in the galvanostatic mode of operation at 500 mA cm⁻². The evaluation of the measured impedance spectra with an adequate equivalent circuit shows that the cathode of the fuel cell is very sensitive to the electrode composition whereas the contribution of the anode is very small and invariant to the electrode composition. Furthermore, it could be shown for the first time using electrolyte powder in the electrodes that the charge transfer of the cathode decreasing monotonically with increasing electrolyte content in the cathode. These findings suggest that with increasing electrolyte content in the electrodes, in particular in the cathode, the utilization degree of the catalyst increasing linearly with increasing electrolyte content in the electrode.     

Facile synthesis and electrochemical performances of hollow graphene spheres as anode material for lithium-ion batteries

The hollow graphene oxide spheres have been successfully fabricated from graphene oxide nanosheets utilizing a water-in-oil emulsion technique, which were prepared from natural flake graphite by oxidation and ultrasonic treatment. The hollow graphene oxide spheres were reduced to hollow graphene spheres at 500°C for 3 h under an atmosphere of Ar(95%)/H₂(5%). The first reversible specific capacity of the hollow graphene spheres was as high as 903 mAh g^-1 at a current density of 50 mAh g^-1. Even at a high current density of 500 mAh g^-1, the reversible specific capacity remained at 502 mAh g^-1. After 60 cycles, the reversible capacity was still kept at 652 mAh g^-1 at the current density of 50 mAh g^-1. These results indicate that the prepared hollow graphene spheres possess excellent electrochemical performances for lithium storage. The high rate performance of hollow graphene spheres thanks to the hollow structure, thin and porous shells consisting of graphene sheets.

PACS

81.05.ue; 61.48.Gh; 72.80.Vp     

Application of electrochemical impedance spectroscopy to the study of ionic transport in polymer-based electrolytes

Lithium-ion batteries should benefit from new polymer-based electrolytes to answer both safety and performance issues for use in transportation applications. Rubber-like gel electrolytes are prepared by gelling of a ionic liquid based solution by a cross-linked epoxide-amine, as a simple and effective means to obtain safe electrolyte materials that exhibit minimal flammability and toxicity while possibly preserving high mechanical and ionic transport properties over a wide temperature window. The synthesis of gel polymer electrolytes based on epoxy-amine thermosetting polymers, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI TFSI) and the lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is presented. Self-standing, transparent, optically homogeneous films are obtained after curing. Two gel polymer electrolytes differing by the nature of the epoxide precursor (aromatic versus aliphatic) are characterized in terms of thermo-mechanical properties with respect to their neat epoxy-amine counterparts. Electrochemical impedance spectroscopy (EIS) is used to investigate the electrical properties and measure the ionic conductivities over the temperature range of interest. The choice of a more flexible moiety in the thermoset precursors leads to an enhanced ionic conductivity. An electric model is presented to predict the gel conductivity on the basis of the conductivity values of neat compounds and to discuss the gel morphology at the molecular scale.     

Time constraints in the testing of salt-fogged samples via electrochemical impedance spectroscopy

Four types of commercial organic coil coatings, applied over galvanized and galvalumed steel, were evaluated by pre-wetting for 2–3 weeks and then drying for periods varying from 6 min to 1 week before the attachment of the O-ring style cell and addition of the test electrolyte for electrochemical testing via electrochemical impedance spectroscopy (EIS). The coil coatings were thin (25–50 μm) and the drying process was found to be relatively fast. The experiments confirmed the cyclic response of the EIS parameters from these organic coatings to be caused by the wetting and drying processes. In addition, the experiments allowed establishing the time constraints between removing a test specimen from a salt-fog chamber and re-exposure to a test electrolyte to minimize adversely affecting the status of the coating at the time of removal from the salt-fog chamber.     

Fast Li-ion conductor Li1+yTi2-yAly(PO4)3 modified Li1.2[Mn0.54Ni0.13Co0.13]O2 as high performance cathode material for Li-ion battery

In the material of xLi₂MnO₃ ·(1-x) LiMO₂ (0 < x < 1), the Li₂MnO₃ component is used to stabilize the layered LiMO₂ structure. However, the electrochemical inactive Li₂MnO₃ makes Li-ion diffusion difficult, leading to a sluggish rate capability. In this work, Li_1.3Ti_1.7Al_0.3(PO₄)₃ (LTA0.3), a NASICON-type Li-ion conductor, is applied to modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ to overcome the above shortcoming. Additionally, the Li-ion conductivity of LiTi₂(PO₄)₃ can be improved effectively by replacing tetravalent cation Ti⁴⁺ with trivalent Al³⁺ at the optimal ratio. At 1C rate, the LR cathode with 3 wt% LTA0.3 delivers 200 mAh g^?1 after 170 cycles and maintains 140 mAh g^?1 after 500 cycles. Moreover, the modified cathode shows an enhanced rate performance of 169.7 mAh g^?1 at 5C. Enhanced cycle durability and rate capability are aroused by the 3D skeletal framework of LTA0.3, which is suitable for Li-ion diffusion. The LTA0.3 coating layer displays a robust shell which not only avoids the corrosion of electrode materials but also effectively facilitates Li-ion diffusion.     

Corrosion behavior of different alloys exposed to continuous flowing seawater by electrochemical impedance spectroscopy (EIS)

The corrosion of four types of alloys, under a dynamic condition, has been studied in continuous fresh seawater system using electrochemical impedance spectroscopy (EIS) technique. The materials used in this study were stainless steel 304, Cu-Ni 70-30, Hastelloy G-30, and titanium. The total exposure time of the test was 180 days, in continuous fresh seawater of the Gulf in Kuwait. The EIS tests were carried out by using EG&G software and hardware instrument. Electrochemical parameters such as the polarization resistance (R_P), solution resistance (R_Sol), and the double layer capacitance (C_dL) of these alloys were determined. Then the obtained EIS parameters were used to study the effect of the seasonal change of the Gulf seawater on the corrosion behavior of the tested materials. All the obtained EIS parameters showed that the seasonal changes of the Gulf seawater have a significant effect on controlling the rate of the formation of the marine bio-film on the surface of tested materials. Consequently, the corrosion behavior of the materials tends to vary as a function of the rate formation of the marine bio-film on the surface of tested materials.     

Stabilizing structure and voltage decay of lithium-rich cathode materials

A major challenge in the discovery of high-energy lithium-ion batteries (LIBs) is to control the voltage stability and Li⁺ kinetics in lithium-rich layered oxide (LrLO) cathode materials. Although these materials can provide a higher specific capacity compared to the current industrially used cathodes, the substantial voltage decay and low Li⁺ diffusion during long term cycling is a serious reason for hindering their practical applications. In order to suppress the voltage decay in lithium-rich cathode materials, herein we introduce the Ti doping into Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ cathodes. Also, the influence of Ti doping on the crystalline internal structure, surface chemistry, cycling retention, and Li⁺ kinetics of Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ cathodes have been focused in this work. The Ti doping effectively enhances the structural/interfacial stability of the cathode and accelerates the Li⁺ kinetics by expanding the lattice, thereby significantly realizing its voltage/cycling stability and high-rate capability. Experimental results show that Ti-doped LrLO (1% Ti) has achieved high electrochemical kinetics as the discharge cycle retention increased from 61.58% (pristine) to 80.0% after 180 cycles at 1 C, with 150.3 mAh g^?1 showing superior high-rate performance at 5C. Ex-situ XRD results confirmed the better structural stability of Ti-doped LrLO after high-rate electrochemical cycling. Our findings provide a suitable element doping strategy for regulating the voltage decay and cycle retention of LrLO, thus promoting their real-world application in future batteries.     

Investigation by electrochemical impedance spectroscopy of filiform corrosion of electrocoated steel substrates

This work aims at studying by electrochemical impedance spectroscopy (EIS) the susceptibility to filiform corrosion of low carbon steel covered by cataphoretic coating. The determination of the exposed metallic area variations of scratched samples during ageing test is an estimation of the disbonding of the coating and/or the filiform corrosion. This area can be evaluated by electrochemical impedance spectroscopy (EIS). A simplified electrical equivalent model used to estimate the exposed metallic area is valid if the corrosion products are correctly dissolved before characterization. Furthermore the steel is a very complex substrate and thus many parameters must be optimized in order to remove the corrosion products before EIS measurements.     

Impedance spectroscopy on porous materials: A general model and application to graphite electrodes of lithium-ion batteries

Electrochemical impedance spectroscopy (EIS) was applied to porous negative graphite electrodes for lithium-ion batteries in the EC:DMC, 1 M LiPF₆ electrolyte. The effect of porosity on the electrode response time was studied and a theoretical model was developed, based on free path of the current lines between subsequent reaction sites. The effect of porosity on the electrode response is evidenced by the impedance spectra in which the high frequency capacitive semicircle is distorted. Fresh electrodes (before the formation of the solid electrolyte interphase, SEI) and cycled electrodes have different shapes of the impedance spectra indicating a change of processes at the surface. In particular, the shape of the spectrum for a fresh electrode can be related to an adsorption process. Impedance spectra of fresh electrodes were fitted using a simple model that considers porosity and the assumed electrochemical processes, giving good agreement between model and data. A correlation was found between adsorption sites and irreversible charge capacity in the first cycle.     

Instantaneous electrochemical impedance spectroscopy of electrode reactions

Continuous STFT transforms of a sinusoidal perturbation signal and current have been found for the first order electrode reaction. Electrode impedance has been determined in the joint time-frequency domain. Measurements of the Cd(II) reduction reaction have been performed on a dropping mercury electrode as a function of time. A possibility of instantaneous impedance spectra generation has been presented. Time characteristics of charge transfer resistance, Warburg coefficient and double layer capacitance has been described.