A review of in-situ high-temperature characterizations for understanding the processes in metallurgical engineering

For the rational manipulation of the production quality of high-temperature metallurgical engineering,there are many challenges in understanding the processes involved because of the black box chemical/electrochemical reactors.To overcome this issue,various in-situ characterization methods have been recently developed to analyze the interactions between the composition,microstructure,and solid-liquid interface of high-temperature electrochemical electrodes and molten salts.In this review,recent progress of in-situ hightemperature characterization techniques is discussed to summarize the advances in understanding the processes in metallurgical engineering.In-situ high-temperature technologies and analytical methods mainly include synchrotron X-ray diffraction(s-XRD),laser scanning confocal microscopy,and X-ray computed microtomography(X-rayμ-CT),which are important platforms for analyzing the structure and morphology of the electrodes to reveal the complexity and variability of their interfaces.In addition,laser-induced breakdown spectroscopy,high-temperature Raman spectroscopy,and ultraviolet-visible absorption spectroscopy provide microscale characterizations of the composition and structure of molten salts.More importantly,the combination of X-rayμ-CT and s-XRD techniques enables the investigation of the chemical reaction mechanisms at the two-phase interface.Therefore,these in-situ methods are essential for analyzing the chemical/electrochemical kinetics of high-temperature reaction processes and establishing the theoretical principles for the efficient and stable operation of chemical/electrochemical metallurgical processes.

Fundamental Understanding on Selenium Electrochemistry:From Electrolytic Cell to Advanced Energy Storage

Selenium(Se),as an important quasi-metal element,has attracted much attention in the fields of thin-film solar cells,electrocatalysts and energy storage applications,due to its unique physical and chemical properties.However,the electrochemical behavior of Se in different systems from electrolytic cell to battery are complex and not fully understood.In this article,we focus on the electrochemical processes of Se in aqueous solutions,molten salts and ionic liquid electrolytes,as well as the application of Se-containing materials in energy storage.Initially,the electrochemical behaviors of Se-containing species in different systems are comprehensively summarized to understand the complexity of the kinetic processes and guide the Se electrodeposition.Then,the relationship between the deposition conditions and resulting structure and morphology of electrodeposited Se is discussed,so as to regulate the morphology and composition of the products.Finally,the advanced energy storage applications of Se in thin-film solar cells and secondary batteries are reviewed,and the electrochemical reaction processes of Se are systematically comprehended in monovalent and multivalent metal-ion batteries.Based on understanding the fundamental electrochemistry mechanism,the future development directions of Se-containing materials are considered in view of the in-depth review of reaction kinetics and energy storage applications.

Core-shell mesoporous carbon hollow spheres as Se hosts for advanced Al-Se batteries

Incorporating a selenium(Se)positive electrode into aluminum(Al)-ion batteries is an effective strategy for improving the overall battery performance.However,the cycling stability of Se positive electrodes has challenges due to the dissolution of intermediate reaction products.In this work,we aim to harness the advantages of Se while reducing its limitations by preparing a core-shell mesoporous carbon hollow sphere with a titanium nitride(C@TiN)host to load 63.9wt%Se as the positive electrode material for Al-Se batteries.Using the physical and chemical confinement offered by the hollow mesoporous carbon and TiN,the obtained core-shell mesoporous carbon hollow spheres coated with Se(Se@C@TiN)display superior utilization of the active material and remarkable cycling stability.As a result,Al-Se batteries equipped with the as-prepared Se@C@TiN composite positive electrodes show an initial discharge specific capacity of 377 mAh·g^(-1)at a current density of 1000 mA·g^(-1)while maintaining a discharge specific capacity of 86.0 mAh·g^(-1)over 200 cycles.This improved cycling performance is ascribed to the high electrical conductivity of the core-shell mesoporous carbon hollow spheres and the unique three-dimensional hierarchical architecture of Se@C@TiN.

Low-temperature graphitization of lignin via Co-assisted electrolysis in molten salt

The ever-growing energy demand and environmental issues have stimulated the development of sustainable energy technologies.Herein,an efficient and environmentally friendly electrochemical transformation technology was proposed to prepare highly graphitized carbon materials from an abundant natural resource-lignin (LG).The preparation process mainly includes pyrolytic carbonization of raw LG material and electrochemical conversion of amorphous carbon precursor.Interestingly,with the assistance of Co catalyst,the graphitization degree of the products was significantly improved,in which the mechanism was the removal of heteroatoms in LG and the rearrangement of carbon atoms into graphite lattice.Furthermore,tunable microstructures (nanoflakes) under catalytic effects could also be observed by controlling the electrolytic parameters.Compared with the products CN1 (without catalyst) and CN5 (with 10%catalyst),the specific surface area are 158.957 and 202.246 m^(2)g^(-1),respectively.When used as the electrode material for lithium-ion batteries,CN5 delivered a competitive specific capacity of~350 m Ah g^(-1)(0.5 C) compared with commercial graphite.The strategy proposed in this work provides an effective way to extract value-added graphite materials from lignin and can be extended to the graphitization conversion of any other amorphous carbon precursor materials.

Electrochemically triggered decoupled transport behaviors in intercalated graphite:From energy storage to enhanced electromagnetic applications

Pyrolytic graphite (PG) with highly aligned graphene layers,present anisotropic electrical and thermal transport behavior,which is attractive in electronic,electrocatalyst and energy storage.Such pristine PG could meeting the limit of electrical conductivity (~2.5×10^(4) S·cm^(−1)),although efforts have been made for achieving high-purity sp^(2) hybridized carbon.For manipulating the electrical conductivity of PG,a facile and efficient electrochemical strategy is demonstrated to enhance electrical transport ability via reversible intercalation/de-intercalation of AlCl_(4)^(-)into the graphitic interlayers.With the stage evolution at different voltages,variable electrical and thermal transport behaviors could be achieved via controlling AlCl_(4)^(-)concentrations in the PG because of substantial variation in the electronic density of states.Such evolution leads to decoupled electrical and thermal transport (opposite variation trend) in the in-plane and out-of-plane directions,and the in-plane electrical conductivity of the pristine PG (1.25×10^(4) S·cm^(−1)) could be massively promoted to 4.09×10^(4) S·cm(AlCl_(4)^(-)intercalated PG),much better than the pristine bulk graphitic papers used for the electrical transport and electromagnetic shielding.The fundamental mechanism of decoupled transport feature and electrochemical strategy here could be extended into other anisotropic conductive bulks for achieving unusual behaviors.

MOF-based quasi-solid-state electrolyte for long-life Al-Se battery

Aluminum-selenium(Al-Se)batteries,which possess a high theoretical specific capacity of 1357 mA h g^(-1),represent a promising energy storage technology.However,they suffer from significant attenuation of capacity and low cycle life due to the shuttle effect.To mitigate the shuttle effect induced by soluble selenium chloroaluminate compound that tends to migrate towards the negative electrode,a quasi-solid-state Al-Se battery was fabricated through the synthesis of a multi-aperture structure quasisolid-state electrolyte(MOF@GPE)based on metal-organic framework(MOF)material and gel-polymer electrolyte(GPE).The high ionic conductivity(1.13×10^(-3)S cm^(-1))of MOF@GPE at room temperature,coupled with its wide electrochemical stability window(2.45 V),can facilitate ion transport kinetics and enhance the electrochemical performance of Al-Se batteries.The MOF@GPE-based quasi-solidstate Al-Se batteries exhibit outstanding long-life cycling stability,delivering a high specific discharge capacity of 548 mA h g^(-1)with a maintained discharge specific capacity of 345 mA h g^(-1)after 500 cycles at a current density of 200 mA g^(-1).The stable ion transmission and high ion transport kinetics in MOF@GPE can be attributed to the stable structure and permeable channel of MOF,which effectively captures the soluble selenium chloroaluminate compound and further restrains the shuttle effect,resulting in improved cycling performance.

Advances in Molten Salt Synthesis of Non-oxide Materials

The properties of non-oxide materials are continuously revealed,and their applications in the fields of ceramics,energy,and catalysis are increasingly extensive.Regardless of the traditional binary materials or the MAX phases,the preparation methods,which are environmentally friendly,efficient,economical,and easy to scale-up,have always been the focus of attention.Molten salt synthesis has demonstrated unparalleled advantages in achieving non-oxide materials.In addition,with the development of the process in molten salt synthesis,it also shows great potential in scale-up production.In this review,the recent progress of molten salt synthesis in the preparation of binary non-oxide and MAX phase is reviewed,as well as some novel processes.The reaction mechanisms and the influence of synthetic conditions for certain materials are discussed in detail.The paper is finalized with the discussion of the application prospect and future research trends of molten salt synthesis in non-oxide materials.

Selective extraction of titanium from Ti-bearing slag via the enhanced depolarization effect of liquid copper cathode

Ti-bearing slag(TiO2>20 wt%)is a valuable titanium secondary resource.The extraction of titanium from the slag is difficult due to the complex composition and structure.Although molten oxide electrolysis is considered as a promising method,silicon will be preferentially electroreduced compared to titanium due to low theoretical decomposition voltage.In this work,a liquid copper cathode is used to selectively extract titanium from molten Al2O3-MgO-CaO-TiO2-SiO2 electrolyte.It is found that comparing to silicon,titanium can be preferentially reduced by one-step electron transfer due to the enhanced depolarization effect on a liquid copper cathode.So,Ti-Cu alloys are firstly obtained from molten Ti-bearing slag,and then Ti-Si alloys are co-electrodeposited in the molten oxide electrolyte with low TiO2 content.It may be ascribed to the larger binding force between titanium and copper than that between silicon and copper.It provides an effective strategy for the separation of titanium from of Ti-bearing slag.

All-carbon positive electrodes for stable aluminium batteries

For addressing the critical problems in current collectors in the aluminium batteries,a variety of carbonbased current collectors,including carbon fiber textiles and three-dimensional(3D)biomass-derivative carbon(BDC)networks,are employed for serving as lightweight non-metal current collectors.The results indicate that all the carbon-based current collectors have electrochemical stability in the acidic electrolyte environments.In the assembled aluminium batteries with all-carbon positive electrodes,thermal annealing process on the carbon-based current collectors has substantially promoted the entire electrochemical energy storage performance.Additionally,both the structure configuration and chemical components of the current collectors have also great impact on the rate capability and cycling stability,implying that the 3D BDC networks are more favorable to offer promoted energy storage capability.Implication of the results from suggests that the carbon-based current collectors and all-carbon positive electrodes are able to deliver more advantages in energy storage behaviors in comparison with the traditional positive electrodes with metal Mo current collectors.Such novel strategy promises a new route for fabricating highperformance positive electrodes for stable advanced aluminium batteries.

Rapid operando gas monitor for commercial lithium ion batteries: Gas evolution and relation with electrode materials

Internal gases caused by side reactions are crucial signals for evaluating health and safety states of Li-ion batteries(LIBs)while it is still a great challenge to timely realize accurate monitoring.To address the issues of implanting various gas sensors into commercial batteries,here a novel method is developed to fast operando monitoring gas evolution via equipping non-dispersive infrared multi-gases sensors into a sealed tank,where real commercial batteries with one open end could be settled for operating.The generated CO_(2)concentration is strongly linked with both voltage and temperature,while the concentrations of CH_(4) and C_(2)H_(4) are solely dependent on temperature.As a typical trace gas,evolution behaviors of CO_(2)have been related to 0_(2) generation from LiNi_(o.5)Mn_(0.3)CoO_(2)0_(2) positive electrode,implying stable CO_(2)release below a critical voltage of 4.5 V.By tracking CO_(2)concentration,an increased amount of Li_(2)CO_(3) was monitored on the surface of graphite negative electrode during discharge process at dfferent temperatures and cutoff voltages,which contributes to the component variation of solid electrolyte interfaces.Such operando techniques promise a plaform for well understanding the interaction of side reactions linked with gas evolution between positive and negative electrodes in commercial LIBs.

3D skeleton nanostructured Ni_3S_2/Ni foam@RGO composite anode for high-performance dual-ion battery

The growing global demands of safe, low-cost and high working voltage energy storage devices trigger strong interests in novel battery concepts beyond state-of-art lithium-ion battery. Herein, a dualion battery based on nanostructured Ni_3S_2/Ni foam@RGO(NSNR) composite anode is developed, utilizing graphite as cathode material and LiPF6-VC-based solvent as electrolyte. The battery operates at high working voltage of 4.2–4.5 V, with superior discharge capacity of ～90 m A h g^(-1) at 100 mA g^(-1), outstanding rate performance, and long-term cycling stability over 500 cycles with discharge capacity retention of ～85.6%. Moreover, the composite simultaneously acts as the anode material and the current collector, and the corrosion phenomenon can be greatly reduced compared to metallic Al anode. Thus, this work represents a significant step forward for practical safe, low-cost and high working voltage dual-ion batteries,showing attractive potential for future energy storage application.

Mechano-electrochemical perspectives on flexible lithium-ion batteries

With the advent of flexible/wearable electronic devices,flexible lithium-ion batteries(LIBs)have attracted significant attention as optimal power source candidates.Flexible LIBs with good flexibility,mechanical stability,and high energy density are still an enormous challenge.In recent years,many complex and diverse design methods for flexible LIBs have been reported.The design and evaluation of ideal flexible LIBs must take into consideration both mechanical and electrochemical factors.In this review,the recent progress and challenges of flexible LIBs are reviewed from a mechano-electrochemical perspective.The recent progress in flexible LIB design is addressed concerning flexible material and configuration design.The mechanical and electrochemical evaluations of flexible LIBs are also summarized.Furthermore,mechano-electrochemical perspectives for the future direction of flexible LIBs are also discussed.Finally,the relationship between mechanical loading and the electrode process is analyzed from a mechano-electrochemical perspective.The evaluation of flexible LIBs should be based on mechano-electrochemical processes.Reviews and perspectives are of great significance to the design and practicality of flexible LIBs,which is contributed to bridging the gap between laboratory exploration and practical applications.

Hierarchical N-doped porous carbon hosts for stabilizing tellurium in promoting Al-Te batteries

Aluminum batteries are attractive in electrochemical energy storage due to high energy density and lowcost aluminum,while the energy density is limited for the lack of favorable positive electrode materials to match aluminum negative electrodes.Tellurium positive electrode is intrinsically electrically conductive among chalcogen and holds high theoretical specific capacity(1260.27 mAh g^(-1)) and discharge voltage plateau(~1,5 V).However,the chemical and electrochemical dissolution of Te active materials results in the low material utilization and poor cycling stability.To enhance the electrochemical performance,herein a nitrogen doped porous carbon(N-PC) is derived from zeolite imidazolate framework(ZIF-67)as an effective tellurium host to suppress the undesired shuttle effect.In order to inhibit the volume expansion of N-PC during the charge/discharge process,the reduced graphene oxide(rGO) nanosheets are introduced to form a stable host materials(N-PC-rGO) for stabilizing Te.The physical encapsulation and chemical confinement to soluble tellurium species are achieved.N-PC-rGO-Te positive electrode exhibits an improved initial specific capacity and long-term cycling performance at a current density of 500 mA g^(-1)(initial specific capacity:935.5 mAh g^(-1);after 150 cycles:467.5 mAh g^(-1)), highlighting a promising design strategy for inhibiting chemical and electrochemical dissolution of Te.

Recent progress and prospective on layered anode materials for potassium-ion batteries

Potassium-ion batteries(PIBs),also known as“novel post-lithium-ion batteries,”have promising energy storage and utilization prospects due to their abundant and inexpensive raw materials.Appropriate anode materials are critical for realizing high-performance PIBs because they are an important component determining the energy and power densities.Two-dimensional(2D)layered anode materials with increased interlayer distances,specific surface areas,and more active sites are promising candidates for PIBs,which have a high reversible capacity in the energetic pathway.In this review,we briefly summarize K+storage behaviors in 2D layered carbon,transition metal chalcogenides,and MXene materials and provide some suggestions on how to select and optimize appropriate 2D anode materials to achieve ideal electrochemical performance.

Enhanced electrodeposition and separation of metallic Cr from soluble K2CrO4 on a liquid Zn cathode

Carbon contamination and the formation of low-valence oxides limit the preparation of refractory metals by molten salt electrolysis.In this paper,a liquid Zn cathode is adopted for the electrochemical reduction of soluble K2CrO4 to metallic Cr in CaCl2-KCl molten salt.It is found that CrO4^2-can be directly electrochemically reduced to Cr via a six-electron-transfer step and low-valence Cr oxides is hardly produced.The reduction rate is obviously increased from 16.7 mgCrh^-1cm^-2 on the solid Mo cathode to58.7 mgCrh-1cm-2on liquid Zn cathode.The electrodeposited Cr is distributed in liquid Zn cathode.Carbon contamination is effectively avoided due to the negligible solubility of carbon in the liquid Zn cathode.Furthermore,Cr can be effectively separated and enriched to the bottom of liquid Zn under supergravity field,realizing the efficient acquisition of metallic Cr and recycling of liquid Zn.The method herein provides a promising route for the preparation of refractory metals with high-purity by molten salt electrolysis.

Thermodynamic and thermoelectric properties of titanium oxycarbide with metal vacancy

Normal titanium oxycarbide exhibits an excellent electrical conductivity and a high carrier concentration of approximately 10^(21) cm^(-3);however,the low Seebeck coefficient limits the thermoelectric application.In this study,first-principle calculations demonstrate that the metal vacancy of titanium oxycarbide weakens the density of state passing through the valence band at the Fermi level,impairing the carrier concentration and enhancing carrier mobility.Thermodynamic analysis justifies the formation of titanium oxycarbide with metal vacancy through solid-state reaction.Transmission electron microscopic images demonstrate the segregation of metal vacancy based on the observation of the defect-rich and single-crystal face-centered cubic regions.Metal vacancy triggers the formation of vacancy-rich and single-crystal face-centered cubic regions.The aggregation of metal vacancy leads to the formation of the vacancy-rich region and other regions with a semi-coherent interface,suppressing the carrier concentration from 1.71×10^(21) to 4.5×10^(20) cm^(-3) and resulting in the Seebeck coefficient from -11μV/K of TiC_(0.5)O_(0.5) to -64μV/K at 1073 K.Meanwhile,vacancies accelerate electron migration from 1.65 to 4.22 cm^(-2)·V^(-1)·s^(-1),maintaining high conductivity.The figure of merit(ZT)increases more than five orders of magnitude via the introduction of metal vacancy,with the maximum figure of 2.11×10^(-2) at 1073 K.These results indicate the potential thermoelectric application of metal-oxycarbide materials through vacancy engineering.

Thickness evolution of graphite-based cathodes in the dual ion batteries via in operando optical observation

Graphite has been currently considered as a promising cathode material in dual ion batteries(DIBs)due to its unique features of sp2 hybridized carbon and stacked two-dimensional layered structures.However,unexpected volume/thickness changes in the graphite cathodes,induced by the intercalation/deintercalation of anions with large molecular size have been known to be a critical problem in designing DIB cells.To understand the volume/thickness changes in the DIB electrodes,in operando optical observing apparatus has been employed to observe the cross-section view of a graphite-based cathode upon cycles in the present work.The observation suggests that the cathode initially presented a huge irreversible thickness change(60%),and such thickness variation was prone to reduce and remain <20% in the following cycles.The results from both in operando observation and electrochemical characterizations collectively indicate that the greater thickness variation at initial cycle should be attributed to both anion intercalation into graphite-based cathodes and irreversible decomposition of chemical components in the DIB system.The method here highlights a universal route for fundamentally understanding the electrodes of huge volume variation.

Modified Al negative electrode for stable high-capacity Al-Te batteries

Metal aluminum batteries(MABs)are considered potential large-scale energy storage devices because of their high energy density,resource abundance,low cost,safety,and environmental friendliness.Given their high electrical conductivity,high theoretical specific capacity,and high discharge potential,Te is considered a potential positive electrode material for MABs.Nonetheless,the critical issues induced by the chemical and electrochemical dissolution of tellurium and subsequent chemical precipitation on bare Al negative electrodes result in poor cycle stability and low discharge capacity of Al-Te batteries.Here an efficient TiB_(2)-based modified layer has been proposed to address bare Al electrodes(Al/TB).Consequently,the low-voltage hysteresis and long cycle life of the Al/TB negative electrode have been achieved.In addition,the electrochemical performance of the Al-Te battery based on the Al/TB negative electrode is dramatically improved.Furthermore,the modified separator technology is introduced to match with the as-designed Al/TB negative electrode.Therefore,the record-setting long-term cycle stability of up to 500 cycles has been achieved in the Al-Te battery.The facile strategy also opens a potential route for other high-energy density battery systems,such as Al-S and Al-Se batteries.

Uneven deposition on the Al electrode under tension strain

Structural deformation and dendrite formation, which would impact the electrochemical processes of rechargeable metal batteries, are usually observed in the high-energy density metal electrodes. Herein,we design an in-situ optical mechano-electrochemical system to study Al deposition on the Al electrode in non-aqueous Al batteries under non-uniform strain. Inhomogeneous distribution of applied strain is realized by creating an oval hole in the Al electrode. The results of the in-situ experiments suggest that the dense Al deposition, which is related to the evolution of surface morphology and increasing reactive sites, is achieved in the regions of stress concentration. The evolution of surface morphology is monitored by the in-situ tension experiments using scanning electron microscope and atomic force microscope.Besides, a qualitative mathematical model is employed to analyze the changes of the local reaction rate owing to the changed surface morphology and the cracks of oxide film under tensile stress. The results are useful to understand the Al deposition when the mechanical force is applied to the metal electrode.

Self-supporting and dual-active 3D Co-S nanosheets constructed by ligand replacement reaction from MOF for rechargeable Al battery

Metal sulfides with high theoretical capacities are expected as promising cathode materials of Al batteries(AIBs). However, powdery active materials are mainly synthesized and loaded on current collector by insulating binder without capacity. Meanwhile, S as inert element in metal sulfides can not usually provide capacity. So, powdery metal sulfides only exhibit limiting practical capacity and poor cycling stability due to weak conductivity and low mass utilization. Herein, the novel self-supporting and dual-active Co-S nanosheets on carbon cloth (i.e. Co-S/CC) with hierarchically porous structure are constructed as cathode of AIBs. Co-S nanosheets are derived from ZIF-67 nanosheets on CC by a facile ligand replacement reaction. As a result, the binder-free Co-S/CC cathode with good conductivity delivers excellent initial discharge capacity of 383.4 m Ah g^(-1)(0.211 m Ah cm^(-2)) at current density of 200 m A g^(-1)and maintain reversible capacity of 156.9 m Ah g^(-1)(0.086 m Ah cm^(-2)) with Coulombic efficiency of 95.8% after 500 cycles,which are much higher than those of the traditional slurry-coating cathodes. Both Co and S as active elements in Co-S/CC contribute to capacity, which leads to a high mass utilization. This work provides a significant strategy for the construction of self-supporting metallic cathode for advanced high-energy density Al battery.