Understanding the reaction mechanisms at the interface of electrode and electrolyte is both of fundamental interest and essential to improve lithium-ion battery (LIB) performance. Herein, we report an online digital holographic method to in situ observe the entire interface change between electrode and electrolyte in lithium-ions batteries. The accuracy of this technology is well verified in LiFePO4/graphite full-cell systems, graphite/Li half-cell systems in EC-based and PC-based electrolyte, respectively, and supported by the characterized results of conventional instruments, including scanning electron microscopy and X-ray photoelectron spectroscopy. In particular, the time resolution of the digital holographic method is 0.04 s and fast enough to distinguish detail reduction process of ethylene carbonate (EC), for which EC will be first reduced to generate lithium alkyl carbonates, and then the reduction product is Li2CO3 to form a stable SEI films. To our best of knowledge, this is the first report on the reduction order of the EC solvent and can act as an effective complement to understanding the formation mechanisms of SEI films.

•Automatic measurement of 2D refractive index changes in aqueous solution.•The adaptive band-pass filter enhances the robustness of the measurement system.•Cu dissolves anodically by one-electron reaction at the initial in Cu/NaBr system.We propose and experimentally demonstrate the automatic measurement of 2D refractive index (RI) changes in aqueous solution at electrode/electrolyte interface based on a Mach-Zehnder interferometer. During the electrochemical reactions, local variations of the electrolyte’s RI, which correlate with the concentration of dissolved species, change the phase of the object beam when the beam pass through the electrolyte. With the application of an adaptive band-pass filter, the RI changes can be obtained automatically from interferograms, which enable direct visualization of the concentration change of the soluble species. As a proof of principle demonstration, the system is employed to detect the soluble species during the potentiodynamic sweep of the copper electrode in 0.5 mol dm−3 NaBr solution at 10 mV s−1. The RI changes at the Cu/NaBr solution interface provide direct evidence of the reaction mechanisms of copper corrosion in NaBr solution.

•The AC treatment increased η% of the mixed self-assembled monolayer.•Organic molecules may be re-orientated in the alternating electric field.•The AC-treated effects depended on the AC-treated potential.•The AC-treated effects were caused by the formation of a complex compound.•The AC-treated effects were also caused by the reduction of the oxide film.The inhibitive effects of alternating current-treated (AC-treated) mixed self-assembled monolayer (SAMHL/DT) with 2-(Pyridin-2-yliminomethyl)-phenol (HL) and dodecanethiol (DT) on copper corrosion have been studied by using the scanning electrochemical microscope (SECM) combined with Tafel and electrochemical impedance spectroscopy (EIS) methods When the AC-treated potential is applied in the cathodic region, the inhibition efficiency increases, and the pitting dynamic processes are inhibited. All the results reveal that the AC-treated effects are related to both the formation of complex compounds and the reduction of the oxide film on the surface of copper.

AbstractIron sulfides have been attracting great attention as anode materials for high-performance rechargeable sodium-ion batteries due to their high theoretical capacity and low cost. In practice, however, they deliver unsatisfactory performance because of their intrinsically low conductivity and volume expansion during charge–discharge processes. Here, a facile in situ synthesis of a 3D interconnected FeS@Fe3C@graphitic carbon (FeS@Fe3C@GC) composite via chemical vapor deposition (CVD) followed by a sulfuration strategy is developed. The construction of the double-layered Fe3C/GC shell and the integral 3D GC network benefits from the catalytic effect of iron (or iron oxides) during the CVD process. The unique nanostructure offers fast electron/Na ion transport pathways and exhibits outstanding structural stability, ensuring fast kinetics and long cycle life of the FeS@Fe3C@GC electrodes for sodium storage. A similar process can be applied for the fabrication of various metal oxide/carbon and metal sulfide/carbon electrode materials for high-performance lithium/sodium-ion batteries.

•SECM was used to study current oscillations in the Fe│H2SO4 system.•A polyaniline-modified microelectrode was used to monitor pH at the interface.•A Pt microelectrode was used to monitor Fe2 + at the interface.•pH oscillations were observed at the interface during the current oscillations.•The oxide film plays a key role in the oscillations of the system.An unmodified Pt microelectrode and a Pt microelectrode coated with polyaniline were used in conjunction with a scanning electrochemical microscope (SECM) to study anodic dissolution in the Fe│H2SO4 system. The concentrations of Fe2 + (cFe2 +) measured with the unmodified microelectrode and the pH values measured with the polyaniline-modified microelectrode were recorded in situ during current oscillations in the Fe│H2SO4 system and were found to change periodically at the Fe│H2SO4 interface. The changes in cFe2 + may be caused by the periodic formation and dissolution of surface film(s), which could be salt films and/or oxide films. If a salt film is formed, it is unlikely to affect the pH. Since the pH changes periodically during the current oscillations, it can be deduced that the surface film is mainly composed of oxide, and that the formation and dissolution of the oxide film play a key role in the current oscillations of the system.

Porous mixed metal oxide (MMO) hollow spheres present high specific surface areas, abundant electrochemically active sites, and outstanding electrochemical properties, showing potential applications in energy storage. A hydro/solvothermal process, followed by a calcination process, can be a viable method for producing uniform porous metal oxide hollow spheres. Unfortunately, this method usually involves harsh synthetic conditions such as high temperature and intricate processing. Herein, we report a general and facile “ion adsorption-annealing” approach for the fabrication of uniform porous MMO hollow spheres. The size and shell thickness of the as-obtained hollow spheres can be adjusted by the carbohydrate sphere templates and the solution concentration. Electrochemical measurements of the MMO hollow spheres demonstrate excellent supercapacitive properties, which may be due to the small size, ultrathin shells, and fine porous structure.

Sulfur is an attractive cathode material in energy storage devices due to its high theoretical capacity of 1672 mAh g–1. However, practical application of lithium–sulfur (Li–S) batteries can be achieved only when the major barriers, including the shuttling effect of polysulfides (Li2Sx, x = 3–8), significant volume change (∼80%), and the resultant rapid deterioration of electrodes, are tackled. Here, we propose an “inside-out” synthesis strategy by mimicking the structure of the pomegranate fruit to achieve conductive confinement of sulfur to address these issues. In the proposed pomegranate-like structure, sulfur and carbon nanotubes composite is encapsulated by the in situ formed amorphous carbon network, which allows the regeneration of electroactive material sulfur and the confinement of the sulfur as well as the lithium polysulfide within the electrical conductive carbon network. Consequently, a highly robust sulfur cathode is obtained, delivering remarkable performance in a Li–S battery. The obtained composite cathode shows a reversible capacity of 691 mAh g–1 after 200 cycles with impressive cycle stability at the current density of 1600 mA g–1.Keywords: amorphous carbon; carbon nanotubes; cathode; high capacity retention; lithium−sulfur batteries; space confining

•Mesoporous ZnCo2O4 microspheres are prepared by facile hydrothermal method.•The mesoporous ZnCo2O4 microspheres show excellent supercapacitive properties.•The specific capacitance achieves 953.2 F g−1 at a current density of 4 A g−1.•The specific capacitance retention is 97.8% after 3000 cycles.Mesoporous zinc cobaltite (ZnCo2O4) microspheres have been successfully prepared by a facile solvothermal method followed by an annealing process. The as-prepared ZnCo2O4 displays uniform sphere-like morphology composed of interconnected ZnCo2O4 nanoparticles. The Brunauer–Emmett–Teller (BET) surface area of mesoporous ZnCo2O4 microspheres is about 51.4 m2 g−1 with dominant pore diameter of 7.5 nm. The novel ZnCo2O4 material exhibits high specific capacitance of 953.2 F g−1 and 768.5 F g−1 at discharge current densities of 4 A g−1 and 30 A g−1, respectively. The energy density can be estimated to be 26.68 Wh kg−1 at a power density of 8 kW kg−1. The specific capacitance retention is 97.8% after 3000 cycles, suggesting its excellent cycling stability. The superior electrochemical performance is mainly attributed to the uniformity of the surface structure and the porosity of the microspheres, which benefit electrons and ions transportation, provide large electrode-electrolyte contact area, and meanwhile reduce volume change during the charge–discharge process. This method of constructing porous microspheres is very effective, yet simple, and it could be applied in other high-performance metal oxide electrode materials for electrochemical capacitors, as well as in Li-ion batteries.

•Hexahedral and octahedral Fe3O4 samples are synthesized via a facile hydrothermal method.•Fe3O4 hexahedra exhibit high discharge capacity, good rate capability and cyclic stability.•The superior property is due to the small size, large surface area and good structure stability.Hexahedral and octahedral magnetite (Fe3O4) samples have been successfully synthesized with a facile hydrothermal technique without any surfactants or templates. The electrochemical properties of the as-prepared samples have been investigated as advanced anode materials for Li-ion battery. It is found that Fe3O4 hexahedra display much higher specific capacitance and better cycle stability than those of Fe3O4 octahedra. Specifically, Fe3O4 hexahedra exhibit a high initial discharge capacitance of 1303.7 mAh g−1 at 100 mA g−1 and it remains stable after 40 cycles. The superiority of the electrochemical performance of Fe3O4 hexahedra may be attributed to the smaller particle size, the larger surface areas and the better structure durability.

•The pitting dynamic processes of the Cu/NaCl system are observed in situ with SECM.•The assembly sequence of HL and DT has great impact on the quality of SAMmix.•The degradation processes of SAMmix are observed in situ with SECM.•The re-arrangements of HL and DT exist during the assembly processes of SAMmix.Mixed self-assembled monolayers (SAMmix) were formed by 2-(Pyridin-2-yliminomethyl)-phenol (HL) and 1-dodecanethiol (DT) molecules on copper surface. The inhibitive ability of SAMmix in sodium chloride solution was characterized in situ by the scanning electrochemical microscope (SECM). The results show that the compactness and the stability of SAMmix, formed by the two organic materials in an appropriate sequence, are improved and the inhibition efficiency (η%) increases markedly. They also verify that SAMmix are formed due to the existence of collapsed sites and pinhole defects on SAMs of single materials on the surface of copper. The re-arrangement and the competitive adsorption of HL and DT molecules may have impact on the quality of SAMmix. Possible structures of SAMmix are suggested based on the experimental results.SECM images of pitting on the copper with SAMs and SAMmix.

Mesoporous Li4Ti5O12 is prepared via ball-milling assisted solid-state reaction with titanyl sulfate and LiOH as precursors. Different from previous reports, the solid-state reaction route generates mesopores in Li4Ti5O12 by removing the impurities after the high-temperature synthesis process. This technique ensures the formation of well-defined mesoporous structure and ultrafine nanoparticles with the size of around 8 nm. The as-prepared sample shows good rate and cycle performance. At the current density of 1750 mA g−1 (10 C), high initial discharge capacity of 174.5 mAh g−1 can be obtained, which can be retained at 143.4 mAh g−1 after 50 cycles. This facile ball-milling assisted solid-state reaction could be a practical way for the mass synthesis of mesoporous Li4Ti5O12 as high-performance anodes.Highlights► Mesoporous Li4Ti5O12 is prepared via ball-milling assisted solid-state reaction. ► Li4Ti5O12 consists of ultrafine nanoparticles and well-defined mesopores. ► The as-prepared sample shows good high-rate and cycle performance. ► The route could be practical for the mass synthesis of Li4Ti5O12 anodes.

The anodic dissolution mechanisms of the Fe/H2SO4 system are complicated and controversial among different models. The main discrepancy concerns the composition and the structure of the films on the iron electrode during reactions. In the present study, the dynamic changes of the diffusion layer during the electrodissolution of iron in 0.5 mol·dm− 3 H2SO4 solution are visually presented through the phase maps obtained by digital holography, which yields new information of anodic processes of the Fe/H2SO4 system. It is deduced that the SO42 − ions are transported from the bulk solution to the vicinity of the electrode and that crystalline precipitation of FeSO4·7H2O is formed and precipitated continuously within the late active and the limiting current region. These findings have led to the proposal of a model of the anodic dissolution of iron in H2SO4 solution.Twelve snapshots of computer-processed phase maps at different times during the process of the anodic electrodissolution of iron in 0.5 mol·dm− 3 H2SO4 solution exhibit the dynamic changes of the diffusion layer. The blues in the maps indicate that the SO42 − ions are transported towards the electrode surface from the bulk solution and crystalline precipitation of FeSO4·7H2O is formed and precipitated continuously within the late active and the limiting current region.Highlights► The dynamic behavior of the diffusion layer in the Fe/H2SO4 system is visually presented. ► The crystalline precipitation of FeSO4·7H2O is formed and precipitated continuously within the late active region and LCR. ► The layers on the iron during LCR are believed to be Fe(OH)2 film inside and the crystalline form of FeSO4·7H2O outside. ► A model of the anodic process of iron in H2SO4 is proposed.

Titanium pyrophosphate hexagonal nanoplates were synthesized via facile hydrothermal reaction followed by a calcination procedure. As new cathode materials for lithium-ion batteries, they demonstrate stable cycle performance and high capacity retention at various current densities.

Polliwog-like Sn/Zn2SnO4 (P-Sn/ZTO) heterostructures have been fabricated by reducing cube-like ZnSnO3 nanoparticles in the mixed solvent of oleic acid (OA) and 1-octylamine. Energy-dispersive X-ray spectrometry indicates that the stems of P-Sn/ZTO nanostructures are composed by ZTO, and the tips of the P-Sn/ZTO nanostructures are formed by the core of Sn and the shell of ZTO. And the position of Sn can be controlled at the tip of Zn2SnO4 nanowires. Adequate evidences demonstrate that 1-octylamine in the mixed solvent acts as the reducing agent to salt out elementary substance Sn, and the OA plays the important role in controlling the morphology of Zn2SnO4. In addition, the P-Sn/ZTO structures exhibited excellent gas sensing properties toward ethanol, and the P-Sn/ZTO is a promising potential gas sensing material.

Effects of elastic deformation on the anodic dissolution of X70 steel have been investigated in 0.5 mol dm−3 H2SO4 solution. In the active region, the elastic stress exerts no obvious influence on the active dissolution rate of the specimen. However, in the oscillatory region, it leads to a marked increase in the frequency of current oscillations, while the mode remains essentially the same. In the passive region, it induces oscillations at the initial stage, but shows no significant effect on the passive current when the potential is controlled at the more positive stage. Two explanations are postulated: the elastic deformation degrades the stability and increases the defects of the passive film; it increases the surface energy (ΔGS), causing the effective potential (EV) to move negative.Highlights► The anodic dissolution of X70/H2SO4 system is studied under the elastic deformation. ► The frequency of the current oscillations is increased. ► The oscillations are induced at the initial stage in the passive region. ► The passive film on the surface of X70 steel generates an additive tensile stress. ► The external stress degrades the stability of the passive film.