Characterizing capacity loss of lithium oxygen batteries by impedance spectroscopy

Polymer based carbon aerogels were prepared by synthesis of a resorcinol formaldehyde gel followed by pyrolysis at 1073 K under Ar and activation of the resultant carbon under CO₂ at different temperatures. The prepared carbon aerogels were used as active materials in the preparation of cathode electrodes for lithium oxygen cells and the electrochemical performance of the cells was evaluated by galvanostatic charge/discharge cycling and electrochemical impedance measurements. It was shown that the storage capacity and discharge voltage of a Li/O₂ cell strongly depend on the porous structure of the carbon used in cathode. EIS results also showed that the shape and value of the resistance in the impedance spectrum of a Li/O₂ cell are strongly affected by the porosity of carbon used in the cathode. Porosity changes due to the build up of discharge products hinder the oxygen and lithium ion transfer into the electrode, resulting in a gradual increase in the cell impedance with cycling. The discharge capacity and cycle life of the battery decrease significantly as its internal resistance increases with charge/discharge cycling.     

Electrochemical performances of nanostructured Ni3P–Ni films electrodeposited on nickel foam substrate

Ni₃P–Ni films were deposited on nickel foam substrates by electrodeposition in an aqueous solution. The structure and morphology of the electrodeposited films were characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). The annealed electrodeposited films consisted of tetragonal structured Ni₃P and cubic metal Ni. As anode for lithium ion batteries, the electrochemical properties of the Ni₃P–Ni films were investigated by cyclic voltammetry (CV), electrochemical impedance spectrum (EIS) and galvanostatic charge–discharge tests. The electrodeposition time had a significant effect on the electrochemical performances of the films. The Ni₃P–Ni film electrodeposited for 20 min delivered the initial discharge capacity of 890 mAh g⁻¹. Although the irreversible capacity at the first cycle was relative large, the Ni₃P–Ni film exhibited good cycling stability and its discharging capacity still maintained 340 mAh g⁻¹ after 40 cycles.     

The effect of CoSn/CoSn2 phase ratio on the electrochemical behaviour of Sn40Co40C20 ternary alloy electrodes in lithium cells

Samples of a SnCoC-based electrodes, all having the molar composition Sn₄₀Co₄₀C₂₀, but differing by the high energy ball milling synthesis conditions, have been tested in lithium cells. The investigation was carried out by using a series of complementary techniques, including potentiodynamic cycling with galvanostatic acceleration, galvanostatic charge–discharge cycling and impedance spectroscopy. The results confirmed the high capacity delivery of this type of ternary electrodes but also revealed that their electrochemical behaviour is influenced by the relative abundance of the nanosized domains of CoSn and CoSn₂ in their structure.     

Effect of cobalt electroless deposition on nickel hydroxide electrodes

The effects of cobalt additive on the positive electrode surface of nickel alkaline batteries are investigated. Electrode surface modifications by electroless cobalt deposits were made at different immersion times. The performance of nickel hydroxide electrodes was studied by optical techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrochemical methods as cyclic voltammetry, charge–discharge curves and electrochemical impedance spectroscopy (EIS). According to these results, electroless cobalt deposits obtained with 5 min of immersion time in the electroless-bath exhibit a better electrode performance.     

Synergistic effects in an AB5–Co material as an anode for a secondary alkaline battery

The AB₅ alloy and Co powders have been mixed at various weight ratios to form AB₅–Co composite electrodes. The discharge properties such as discharge capacity, discharge plateau, and cycling stability are investigated by charge and discharge testing using Arbin battery testing equipment. Synergistic effects in the composite electrodes contribute to significant improvements of the discharge behavior. For instance, the composite AB₅–25%Co electrode shows a high discharge capacity of 395.1 mAh/g, which is significantly higher than that of AB₅ or Co electrode, and good cycling stability. The discharge process is also characterized by electrochemical impedance spectroscopy. Moreover, the electrochemical discharge mechanism is discussed.     

Effect of synthesis method on the structure and electrochemical behaviour of Co–Si particles

Co–Si particles were prepared by means of three methods as sample A, B, and C. Structures of the samples were characterized by XRD and the electrochemical hydrogen storage properties were studied as negative electrodes in aqueous KOH solution. Results of cyclic voltammetry (CV) and cycling stability demonstrated that samples A and B showed excellent electrochemical reversibility and relatively high discharge capacities, whose maximum capacity are 245 and 214 mAh/g and kept 207.7 and 189.5 mAh/g after 80 cycles at current density 50 mAh/g. By comparing the XRD patterns of the electrodes on different charge–discharge states, the discharge capacity was attributed to hydrogenation of Co–Si particles.     

Electrochemical cyclability mechanism for MnO2 electrodes utilized as electrochemical supercapacitors

The electrochemical cyclability mechanism of nanocrystalline MnO₂ electrodes with rock salt-type and hexagonal ?-type structures was investigated to determine the relationship between physicochemical feature evolution and the corresponding electrochemical behaviour of MnO₂ electrodes. Rock salt MnO₂ and hexagonal ?-MnO₂ electrodes, with fibrous and porous morphologies, evolve into the antifluorite-type MnO₂ with a petal-shaped nanosheet structure after electrochemical cycling, similar to that observed in nanocrystalline antifluorite-type MnO₂ electrodes after electrochemical cycling. However, a different impedance response was observed for the rock salt MnO₂ and hexagonal ?-MnO₂ electrodes during the charge–discharge cycles, compared with the improved impedance response observed for the cycled antifluorite-type MnO₂. A dissolution–redeposition mechanism is proposed to account for the impedance response of the MnO₂ electrodes with different morphologies and crystal structures.     

Anodic deposition of porous RuO2 on stainless steel for supercapacitor studies at high current densities

Ruthenium dioxide is deposited on stainless steel (SS) substrate by galvanostatic oxidation of Ru³⁺. At high current densities employed for this purpose, there is oxidation of water to oxygen, which occurs in parallel with Ru³⁺ oxidation. The oxygen evolution consumes a major portion of the charge. The oxygen evolution generates a high porosity to RuO₂ films, which is evident from scanning electron microscopy studies. RuO₂ is identified by X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostatic charge–discharge cycling studies indicate that RuO₂/SS electrodes possess good capacitance properties. Specific capacitance of 276 F g⁻¹ is obtained at current densities as high as 20 mA cm⁻² (13.33 A g⁻¹). Porous nature of RuO₂ facilitates passing of high currents during charge–discharge cycling. RuO₂/SS electrodes are thus useful for high power supercapacitor applications.     

Application of cyclohexyl benzene as electrolyte additive for overcharge protection of lithium ion battery

The electrochemical characterization and overcharge protection mechanism of cyclohexyl benzene as an additive in electrolyte for lithium ion battery was studied by microelectrode cyclic voltammetry, Galvanostatic charge–discharge measurements and SEM observation on both the cathode and separator of the overcharged cells. It was found that when the battery is overcharged, cyclohexyl benzene electrochemically polymerized to form polymer between separator and cathode at the potentials lower than that for electrolyte decomposition. The polymer blocks the overcharging process of the battery. The additive causes a small capacity loss and impedance increase in a real cell, but that can be mitigated if the operating voltage is much lower than the polymerization voltage.     

Effects of Cu substrate morphology and phase control on electrochemical performance of Sn–Ni alloys for Li-ion battery

The influence of substrate morphology and ageing on the charge–discharge performance of a Sn–Ni alloy anode electrodeposited on a Cu substrate are examined. The Sn–Ni alloy (Sn 82 at.%–Ni 18 at.% anode) shows a high capacity of around 480 mAh g⁻¹ up to 12 cycles, but its capacity rapidly fades with cycling. The initial capacity and the cyclic properties of the alloy electrode are significantly improved when the surface morphology of the Cu substrate is changed from smooth-type to nodule-type. Optimized ageing treatment leads to further enhancement in the charge–discharge performance of the anode. The increase in the capacity and better cyclic properties are attributed to stronger adhesion between the Si–Ni anode and the Cu substrate. This is induced by inter-locking of the nodule-type Cu substrate and a buffering effect of Cu–Sn intermetallic compounds formed during ageing.     

Electrochemical properties of magnesium-based hydrogen storage alloys improved by transition metal boride and silicide additives

Transition metal borides and silicides prepared by mechanical alloying (MA) and chemical reduction methods (CR) were introduced to improve the corrosion resistance of magnesium-based hydrogen storage alloys. The additive of FeB prepared by MA can remarkably enhance the discharge capacity and cycling stability which has initial discharge capacity of 355.9 mA h g⁻¹ and keeps 224 mA h g⁻¹ after 100 cycles, and the exchange density I₀ of MgNi–NiB(CR) electrodes is 344.80 mA g⁻¹ but MgNi is only 67.6 mA g⁻¹ which leads to the better rate capability of the composite alloys. The results of SEM characterization, cyclic charge–discharge tests, potentiodynamic polarization, linear polarization and AC impedance experiment show that the corrosion inhibition property of MgNi in alkaline is improved by transition metal boride and silicide additives.     

A microfabricated nickel–hydrogen battery using thick film printing techniques

To utilize the distinctive cycle life and safety characteristics of the nickel–hydrogen chemistry while eliminating the high pressure limitations of conventional nickel–hydrogen cells, a microfabricated nickel–hydrogen battery using a low-pressure metal hydride for hydrogen storage is being developed for powering micro-electromechanical systems (MEMS) devices and for biomedical applications where the battery would be implanted within the body. Thick film printing techniques which are simple and low cost were used to fabricate this battery. Inks were developed for each of the different battery components, including the electrodes, current collectors and separator. SEM images on these printed components showed the desired characteristics for each. Positive electrode cycling tests were performed on the printed positive electrodes while cyclic voltammetry was used to characterize the printed negative electrodes. Consistent charge and discharge performance was observed during positive electrode cycling. Full cells with printed positive and negative assemblies were assembled and tested.     

Electrochemical hydrogen storage property of Co–S alloy prepared by ball-milling method

A series of Co–S alloys were synthesized by means of ball milling of Co and S powders at different hours and investigated as the negative material for Ni/MH batteries. The structures and surface configuration of the alloys were characterized by XRD and TEM. The electrochemical measurements demonstrated that the Co–S particles showed excellent electrochemical reversibility and considerably high charge–discharge capacity. Among the alloys, the Co–S alloy milled 20 h showed relatively high discharge capacity and excellent cycling stability at discharge current density 25 mA/g. Its highest discharge capacity was about 350 mAh/g and remained 300 mAh/g after 100 cycles, the capacity retention rate was about 86%. The hydrogen storage mechanism was studied by XRD and TPD measurements.     

Electrochemical supercapacitor electrode material based on poly(3,4-ethylenedioxythiophene)/polypyrrole composite

Poly (3,4-ethylenedioxythiophene)/polypyrrole composite electrodes were prepared by electropolymerization of 3,4-ethylenedioxythiophene (EDOT) on the surface of polypyrrole (PPy) modified tantalum electrodes. The morphology observation of PPy and poly(3,4-ethylenedioxythiophene)/polypyrrole composite (PEDOT/PPy) was performed on Field Emission Scanning Electron Microscope (SEM). The electrochemical capacitance properties of the composite were investigated with cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) techniques in the two- or three-electrode cell system. The results show that the PEDOT/h-PPy (PPy with horn-like structure) composite films were characterized with highly porous structure, which leads to their specific capacitance as 230 Fg⁻¹ in 1 M LiClO₄ aqueous solutions and even 290 Fg⁻¹ in 1 M KCl aqueous solutions. Moreover, the composite exhibits a rectangle-like shape of voltammetry characteristics even at scanning rate 100 mV s⁻¹, a linear variation of the voltage with respect to time without a clear ohm-drop phenomenon in galvanostatic charge–discharge process and almost ideal capacitance behavior in low-frequency in 1 M KCl solutions. Furthermore, specific power of the composite would reach 13 kW kg⁻¹ and it had good cycle stability. All of the above imply that the PEDOT/h-PPy composites were an ideal electrode material of supercapacitor.     

Diphenyloctyl phosphate as a flame-retardant additive in electrolyte for Li-ion batteries

The use of diphenyloctyl phosphate (DPOF) as a flame-retardant additive in liquid electrolyte for Li-ion batteries is investigated. Mesocarbon microbeads (MCMB) and LiCoO₂ are used as the anode and cathode materials, respectively. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) are used for the analyses. The cell with DPOF shows better electrochemical cell performance than that without DPOF in initial charge/discharge and rate performance tests. In cycling tests, a cell with DPOF-containing electrolyte exhibited better discharge capacity and capacity retention than that of the DPOF-free electrolyte after cycling. These results confirm the viability of using DPOF as a flame-retardant additive for improving the cell performance and thermal stability of electrolytes for Li-ion batteries.     

Electrochemical performance of SrF2-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

SrF₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials with improved cycling performance over 2.5–4.6 V were investigated. The structural and electrochemical properties of the materials were studied using X-ray diffraction (XRD), scanning electron microscope (SEM), charge–discharge tests and electrochemical impedance spectra (EIS). The results showed that the crystalline SrF₂ with about 10–50 nm particle size is uniformly coated on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ particles. As the coating amount increased from 0.0 to 2.0 mol%, the initial capacity and rate capability of the coated LiNi_1/3Co_1/3Mn_1/3O₂ decreased slightly owing to the increase of the charge-transfer resistance; however, the cycling stability was improved by suppressing the increase of the resistance during cycling. 4.0 mol% SrF₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ showed remarkable decrease of the initial capacity. 2.0 mol% coated sample exhibited the best electrochemical performance. It presented an initial discharge capacity of 165.7 mAh g⁻¹, and a capacity retention of 86.9% after 50 cycles at 4.6 V cut-off cycling.     

Effects of γ-CoOOH coating on the high-temperature and high-rate performances of spherical nickel hydroxide electrodes

To improve the high-temperature and high-rate performances of nickel hydroxide electrodes in nickel–metal hydride (Ni/MH) batteries, γ-CoOOH is coated onto spherical Ni(OH)₂ through surface modification. X-ray diffraction, scanning electron microscopy with energy dispersive X-ray, and X-ray photoelectron spectroscopy are used to characterize the synthesized products. The effects of γ-CoOOH on the electrochemical performance of nickel electrodes are investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and a charge/discharge test. It is found that the spherical γ-CoOOH-coated Ni(OH)₂ electrode without adding any conductive additives exhibits superior electrode properties including excellent high-temperature and high-rate discharge abilities, and superior cycling reversibility. These performance improvements are ascribed to the enhancement of oxygen evolution over-potential, slower oxygen evolution rate and lower charge transfer resistance resulting from the high conductivity coating of γ-CoOOH.     

Controlled synthesis of α-Fe2O3 nanostructures and their size-dependent electrochemical properties for lithium-ion batteries

Highly crystalline hematite α-Fe₂O₃ nanostructures were selectively synthesized by a simple hydrothermal method. By carefully tuning the concentration of the reactants, reaction time and pressure, a series of α-Fe₂O₃ nanocuboids, nanospheres, nanosheets, nanorods and nanowires can be obtained. Based on the evidence of electron microscope images, a formation mechanism for nanowire-structured hematite is proposed. The electrochemical performance of these hematite nanostructures as anode materials for lithium-ion batteries was further evaluated by cyclic voltammetry, electrochemical impedance and charge–discharge measurements. It was demonstrated that both the morphology and the particle size have an influence on the performance. The results showed that the nanospheres displayed the highest discharge capacity and superior cycling reversibility, which may result from the high surface area and small and uniform grain size.     

Crystal structure and electrochemical characteristics of non-AB5 type La–Ni system alloys

The La–Ni system compounds have been prepared by arc-melting method under Ar atmosphere. X-ray diffraction analysis reveals that the as-prepared alloys consist of different phases. The electrochemical properties, including activation, maximum discharge capacity, high rate chargeability (HRC), and high rate dischargeability (HRD) of these alloy electrodes have been studied through the charge–discharge recycle testing at different temperatures and charge (or discharge) currents. Among the La–Ni alloy electrodes studied, LaNi_2.28 alloy has the most excellent high rate charging performance, and La₂Ni₇ alloy exhibit the highest high rate dischargeability, while La₇Ni₃ alloy is capable of discharging at low temperature.     

Sodium tungstate as electrolyte additive to improve high-temperature performance of nickel–metal hydride batteries

Sodium tungstate (Na₂WO₄) used as new electrolyte additive to enhance the high-temperature performance of Nickel–metal hydride (Ni–MH) battery is investigated in this paper. The effects of Na₂WO₄ on nickel hydroxide electrodes are investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test. It is found that the Ni–MH cell with the conventional KOH electrolyte containing 1 wt.% Na₂WO₄ additive exhibits higher discharge retention and better cycling performance than the cell without Na₂WO₄ additive at both 25 °C and 70 °C. These performance improvements are ascribed to the enhancement of oxygen evolution overvoltage and lower electrochemical impedance, as indicated by CV and EIS. The results suggest that the proposed approach be an effective way to improve the high temperature performance of Ni–MH batteries.