Functional Group-driven Competing Mechanism in Electrochemical Reaction and Adsorption/Desorption Processes toward High-capacity Aluminum-Porphyrin Batteries.

Nonaqueous organic aluminum batteries are considered as promising high-safety energy storage devices due to stable ionic liquid electrolytes and Al metals. However, the stability and capacity of organic positive electrodes are limited by their inherent high solubility and low active organic molecules. To address such issues, here porphyrin compounds with rigid molecular structures present stable and reversible capability in electrochemically storing AlCl2+. Comparison between the porphyrin molecules with electron-donating groups (TPP-EDG) and with electron-withdrawing groups (TPP-EWG) suggests that EDG is responsible for increasing both HOMO and LUMO energy levels, resulting in decreased redox potentials. On the other hand, EWG is associated with decreasing both HOMO and LUMO energy levels, leading to promoted redox potentials. EDG and EWG play critical roles in regulating electron density of porphyrin π bond and electrochemical energy storage kinetics behavior. The competitive mechanism between electrochemical redox reaction and de/adsorption processes suggests that TPP-OCH3 delivers the highest specific capacity ~171.8 mAh g-1, approaching a record in the organic Al batteries.

A recyclable method for titanium extraction and oxygen evolution from Ti-bearing slags.

Despite its existence for more than 80 years, the titanium industry is still challenged by massive carbon emissions, high production costs, and large resource waste. More than one hundred million tons of Ti-bearing blast furnace slag (TB-slag) has been discarded in China because of the difficulty of reutilization, which requires efficient titanium extraction and recovery technologies. This paper describes a low-cost, carbon-emission-free method for Ti extraction and oxygen evolution via molten oxide electrolysis (MOE) vacuum distillation. After a comprehensive analysis of the binding energies and activities of liquid metals, the highlights of our study are as follows. 1) Sb has the best preferential deposition of Ti among a series of high-Ti-affinitive liquid metal cathodes (Cu, Ni, Pb, Sn, and Sb). 2) The Ir anode was first used in TB-slag with IrO2 formed on its surface to protect it from further corrosion. 3) An alloy containing Ti and Ca can be obtained by MOE, and Ti and Ca metals can be refined by further vacuum distillation. 4) A closed loop is formed in the overall process owing to the recyclable Sb cathode and continuous feeding of TB-slag into the electrolyte. This simple, low-cost, and environmentally friendly method can realize the efficient utilization of Ti resources and achieve carbon neutrality.

The mechanism of in-homogeneous mass transfer process of separators in lithium-ion batteries.

The liquid-phase mass transport is the key factor affecting battery stability. The influencing mechanism of liquid-phase mass transport in the separators is still not clear, the internal environment being a complex multi-field during the service life of lithium-ion batteries. The liquid-phase mass transport in the separators is related to the microstructure of the separator and the physicochemical properties of electrolytes. Here, in-situ local electrochemical impedance spectra were developed to investigate local inhomogeneities in the mass transfer process of lithium-ion batteries. The geometric microstructure of the separator affects the mass transfer process, with a reduction in porosity leading to increased overpotentials. There is a competitive relationship among porosity, tortuosity, and membrane thickness in the geometric parameters of the separator, resulting in a peak of polarization. The resistance of the liquid-phase mass transfer process is positively correlated with the viscosity of the electrolyte, making ion migration difficult due to high viscosity. Polarization is closely related to the electrochemical performance, so a phase diagram of battery performance and inhomogeneous mass transfer was developed to guide the design of the battery. This study provides a guiding basis for the development of high stability lithium-ion batteries.

Stable Photo-Rechargeable Al Battery for Enhancing Energy Utilization.

Photovoltaic cells (PVs) are able to convert solar energy to electric energy, while energy storage devices are required to be equipped due to the fluctuations of sunlight. However, the electrical connection of PVs and energy storage devices leads to increased energy consumption, and thus energy storage ability and utilization efficiency are decreased. One of the solutions is to explore an integrated photoelectrochemical energy conversion-storage device. Up to date, the integrated photo-rechargeable Li-ion batteries often suffer from unstable photo-active materials and flammable electrolytes under illumination, with concerns in safety risks and limited lifetime. To address the critical issues, here a novel photo-rechargeable aluminum battery (PRAB) is designed with safe ionic liquid electrolytes and stable polyaniline photo-electrodes. The integrated PRAB presents stable operation with an enhanced reversible specific capacity ≈191% under illumination. Meanwhile, a simplified continuum model is established to provide rational guidance for designing electrode structures along with a charging/discharging strategy to meet the practical operation conditions. The as-designed PRAB presents an energy-saving efficiency ≈61.92% upon charging and an energy output increment ≈31.25% during discharging under illumination. The strategy of designing and fabricating stable and safe photo-rechargeable non-aqueous Al batteries highlights the pathway for substantially promoting the utilization efficiency of solar energy.

Enhancing Electrochemical Performance of Aluminum-Ion Batteries with Fluorinated Graphene Cathode.

In pursuit of high-performance aluminum-ion batteries, the selection of a suitable positive electrode material assumes paramount importance, and fluorinated graphene (FG) nanostructures have emerged as an exceptional candidate. In the scope of this study, a flexible tantalum foil is coated with FG to serve as the positive electrode for aluminum-ion batteries. FG positive electrode demonstrates a remarkable discharge capacity of 109 mA h g-1 at a current density of 200 mA g-1, underscoring its tremendous potential for energy storage applications. Concurrently, the FG positive electrode exhibits a discharge capacity of 101 mA h g-1 while maintaining an impressive coulombic efficiency of 95 % over 300 cycles at a current density of 200 mA g-1, which benefiting from the significant structure of FG. The results of the in-situ Raman spectroscopy signified the presence of intercalation/de-intercalation processes of AlCl4 - behavior within the FG layers.

Weak Electrostatic Force on K+ in Gel Polymer Electrolyte Realizes High Ion Transference Number for Quasi Solid-State Potassium Ion Batteries.

Quasi-solid-state potassium-ion batteries (SSPIBs) are of great potential for commercial use due to the abundant reserves and cost-effectiveness of resources, as well as high safety. Gel polymer electrolytes (GPEs) with high ionic conductivity and fast interfacial charge transport are necessary for SSPIBs. Here, the weak electrostatic force between K+ and electronegative functional groups in the ethoxylated trimethylolpropane triacrylate (ETPTA) polymer chains, which can promote fast migration of free K+, is revealed. To further enhance the interfacial reaction kinetics, a multilayered GPE by in situ growth of poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) on ETPTA (PVDF-HFP|ETPTA|PVDF-HFP) is constructed to improve the interface contact and provide sufficient K+ concentration in PVDF-HFP. A high ion transference number (0.92) and a superior ionic conductivity (5.15 × 10-3 S cm-1) are achieved. Consequently, the SSPIBs with both intercalation-type (PB) and conversion-type (PTCDA) cathodes show the best battery performance among all reported SSPIBs of the same cathode. These findings demonstrate that potassium-ion batteries have the potential to surpass Li/Na ion batteries in solid-state systems.

The Reverse of Electrostatic Interaction Force for Ultrahigh-Energy Al-Ion batteries.

The two-dimensional (2D) MXenes with sufficient interlayer spacing are promising cathode materials for aluminum-ion batteries (AIBs), yet the electrostatic repulsion effect between the surface negative charges and the active anions (AlCl4 - ) hinders the intercalation of AlCl4 - and is usually ignored. Here, we propose a charge regulation strategy for MXene cathodes to overcome this challenge. By doping N and Co, the zeta potential is gradually transformed from negative (Ti3 C2 Tx ) to near-neutral (Ti3 CNTx ), and finally positive (Ti3 CNTx @Co). Therefore, the electrostatic repulsion force can be greatly weakened between Ti3 CNTx and AlCl4 - , or even formed a strong electrostatic attraction between Ti3 CNTx @Co and AlCl4 - , which can not only accommodate more AlCl4 - ions in the Ti3 CNTx @Co interlayers to increase the capacity, but also solve the stacking and expansion problems. As a result, the optimized Al-MXene battery exhibits an ultrahigh capacity of up to 240 mAh g-1 (2-4 times the capacity of graphite cathode, 60-120 mAh g-1 ) and a potential ultrahigh energy density (432 Wh kg-1 , 2-4 times the value of graphite, 110-220 Wh kg-1 ) based on the mass of cathode materials, comparable to LiFePO4 -based lithium-ion batteries (350-450 Wh kg-1 , based on the mass of LiFePO4 ).

Stable Low-Temperature Al Batteries Enabled by Integrating Polydopamine-Derived N-Doped Carbon Nanospheres With Flake Graphite.

The battery performance declines significantly in severely cold areas, especially discharge capacity and cycle life, which is the most significant pain point for new energy consumers. To address this issue and improve the low-temperature characteristic of aluminum-ion batteries, in this work, polydopamine-derived N-doped carbon nanospheres are utilized to modify the most promising graphite material. More active sites are introduced into graphite, more ion transport channels are provided, and improved ionic conductivity is achieved in a low-temperature environment. Due to the synergistic effect of the three factors, the ion diffusion resistance is significantly reduced and the diffusion coefficient of aluminum complex ions in the active material become larger at low temperatures. Therefore, the battery delivers an improved capacity retention rate from 23% to 60% at -20 °C and excellent ultra-long cycling stability over 5500 cycles at -10 °C. This provides a novel strategy for constructing low-temperature aluminum-ion batteries with high energy density, which is conducive to promoting the practicality of aluminum-ion batteries.

Ultra-Stable Ti Vacancies-Pt Atomic Clusters Structure on Titanium Oxycarbide Supports for High Current Density Hydrogen Evolution Reaction.

Electrocatalysts with low Pt loading mass to achieve high current density (≥1 A cm-2) for hydrogen evolution reaction (HER) are still extremely challenging due to the limited intrinsic activity and weak stability of catalytic sites. The modulation of the electronic microenvironment of the support-Pt structure is crucial to enhance the intrinsic activity and stability of catalytic sites. Herein, an innovative titanium oxycarbide (TiVCO) solid solution with Ti vacancies (TiV) is proposed as support to anchor sub-nanoscale Pt atomic clusters (Pt ACs) and a stable "TiV-Pt ACs" structure is carefully designed. The electronic microenvironment of "TiV-Pt ACs" is indirectly optimized by an unsaturated C/O site near TiV. Thanks to this, novel "TiV-Pt ACs" structure (Pt@TiVCO) with low Pt loading mass (2.44 wt.%) exhibits excellent HER activity in acidic solution and the mass activity is more than ten times that of commercial 20% Pt/C at the overpotentials of 50 and 100 mV. Particularly, Pt@TiVCO shows amazing stability at high and fluctuating current density of 1-2 A cm-2 for 120 h. This work provides a novel and promising method to develop stable and low-loading Pt-based catalysts adapting to high current density.

Surface Engineering Based on Conductive Agent Dispersion Uniformity: Strategies toward Performance Consistency of Lithium-Ion Batteries.

The consistency of lithium-ion battery performance is the key factor affecting the safety and cycle life of battery packs. Surface engineering of electrodes in production processes plays an important role in improving the consistency of battery performance. In this study, the drying process in the electrode manufacturing process is studied as the effect on surface engineering of the electrode materials, with consideration on impacting the battery performance. Specifically, the solid content of the slurry and drying temperature are considered to be the two factors that affect conductive agent dispersion uniformity in the porous electrodes. To achieve surface engineering on the dispersion uniformity of the conductive agent, the optimal processing parameters can be obtained by adjusting the temperature and solid content of the slurry. The mechanism of dispersion uniformity of the conductive agent is mainly related to the polyvinylidene fluoride grid structure. In the manufacturing of lithium-ion batteries, the electrode coated with 66% solid slurry and dried at 90-100 °C presents stable energy storage performance, which is beneficial to maintain the stable performance of the battery pack in the application.

Natural Convection in Molten Salt Electrochemistry.

Molten salt electrochemistry has been widely used in many fields, especially for metal extraction/refinement. The understanding of mass transfer in molten salts under harsh operation conditions is of great importance to reveal reaction mechanisms and advance fine technologies. It has been generally assumed that natural convection is negligible in stagnant molten salt electrochemistry. Herein, we report an abnormal natural convection in molten LiCl-KCl, with the arising time from 2.37 s at 873 K to 10.13 s at 673 K. Using the concentration correction factor, the derived thickness of the natural convection-diffusion layer (δconv.) was found to be ranging from 128 to 163 µm, much thinner than those in aqueous solutions (∼200 µm). The simulations showed that the notable natural convection resulted from convection-diffusion layer (CDL) convection dominated over the density-driven convection even at high redox concentrations, implying the severe vibration of molten salt systems. To suppress the intense natural convection, we predicted that the use of microelectrodes (with radii less than 23.2 µm for δconv. = 150 µm) would be a promising tool, regardless of their inferior stabilities in high-temperature molten salts at this stage. These innovative findings offer insights into the impact of natural convection on mass transfer in molten salts that have not been previously revealed.

Ampere-hour-scale soft-package potassium-ion hybrid capacitors enabling 6-minute fast-charging.

Extreme fast charging of Ampere-hour (Ah)-scale electrochemical energy storage devices targeting charging times of less than 10 minutes are desired to increase widespread adoption. However, this metric is difficult to achieve in conventional Li-ion batteries due to their inherent reaction mechanism and safety hazards at high current densities. In this work, we report 1 Ah soft-package potassium-ion hybrid supercapacitors (PIHCs), which combine the merits of high-energy density of battery-type negative electrodes and high-power density of capacitor-type positive electrodes. The PIHC consists of a defect-rich, high specific surface area N-doped carbon nanotube-based positive electrode, MnO quantum dots inlaid spacing-expanded carbon nanotube-based negative electrode, carbonate-based non-aqueous electrolyte, and a binder- and current collector-free cell design. Through the optimization of the cell configuration, electrodes, and electrolyte, the full cells (1 Ah) exhibit a cell voltage up to 4.8 V, high full-cell level specific energy of 140 Wh kg-1 (based on the whole mass of device) with a full charge of 6 minutes. An 88% capacity retention after 200 cycles at 10 C (10 A) and a voltage retention of 99% at 25 ± 1 °C are also demonstrated.

Construction of double reaction zones for long-life quasi-solid aluminum-ion batteries by realizing maximum electron transfer.

Achieving high energy density and long cycling life simultaneously remains the most critical challenge for aluminum-ion batteries (AIBs), especially for high-capacity conversion-type positive electrodes suffering from shuttle effect in strongly acidic electrolytes. Herein, we develop a layered quasi-solid AIBs system with double reaction zones (DRZs, Zone 1 and Zone 2) to address such issues. Zone 1 is designed to accelerate reaction kinetics by improving wetting ability of quasi-solid electrolyte to active materials. A composite three-dimensional conductive framework (Zone 2) interwoven by gel network for ion conduction and carbon nanotube network as electronic conductor, can fix the active materials dissolved from Zone 1 to allow for continuing electrochemical reactions. Therefore, a maximum electron transfer is realized for the conversion-type mateials in DRZs, and an ultrahigh capacity (400 mAh g-1) and an ultralong cycling life (4000 cycles) are achieved. Such strategy provides a new perspective for constructing high-energy-density and long-life AIBs.

A Carbon Aerogel Lightweight Al Battery for Fast Storage of Fluctuating Energy.

Al batteries have great potential for renewable energy storage owing to their low cost, high capacity, and safety. High energy density and adaptability to fluctuating electricity are major challenges. Here, a lightweight Al battery for fast storage of fluctuating energy is constructed based on a novel hierarchical porous dendrite-free carbon aerogel film (CAF) anode and an integrated graphite composite carbon aerogel film (GCAF) cathode. A new induced mechanism by the O-containing functional groups on the CAF anode is con-firmed for uniform Al deposition. The GCAF cathode possesses a higher mass utilization ratio due to the extremely high loading mass (9.5-10.0 mg cm-2 ) of graphite materials compared to conventional coated cathodes. Meanwhile, the volume expansion of the GCAF cathode is almost negligible, resulting in better cycling stability. The lightweight CAF‖GCAF full battery can adapt well to large and fluctuating current densities owing to its hierarchical porous structure. A large discharge capacity (115.6 mAh g-1 ) after 2000 cycles and a short charge time (7.0 min) at a high current density are obtained. The construction strategy of lightweight Al batteries based on carbon aerogel electrodes can promote the breakthrough of high-energy-density Al batteries adapted to the fast storage of fluctuating renewable energy.

Operando probing and adjusting of the complicated electrode process of multivalent metals at extreme temperature.

Nearly half of the elements in the periodic table are extracted, refined, or plated using electrodeposition in high-temperature melts. However, operando observations and tuning of the electrodeposition process during realistic electrolysis operations are extremely difficult due to severe reaction conditions and complicated electrolytic cell, which makes the improvement of the process very blind and inefficient. Here, we developed a multipurpose operando high-temperature electrochemical instrument that combines operando Raman microspectroscopy analysis, optical microscopy imaging, and a tunable magnetic field. Subsequently, the electrodeposition of Ti-which is a typical polyvalent metal and generally shows a very complex electrode process-was used to verify the stability of the instrument. The complex multistep cathodic process of Ti in the molten salt at 823 K was systematically analyzed by a multidimensional operando analysis strategy involving multiple experimental studies, theoretical calculations, etc. The regulatory effect and its corresponding scale-span mechanism of the magnetic field on the electrodeposition process of Ti were also elucidated, which would be inaccessible with existing experimental techniques and is significant for the real-time and rational optimization of the process. Overall, this work established a powerful and universal methodology for in-depth analysis of high-temperature electrochemistry.

Molten salt electro-preparation of graphitic carbons.

Graphite has been used in a wide range of applications since the discovery due to its great chemical stability, excellent electrical conductivity, availability, and ease of processing. However, the synthesis of graphite materials still remains energy-intensive as they are usually produced through a high-temperature treatment (>3000°C). Herein, we introduce a molten salt electrochemical approach utilizing carbon dioxide (CO2) or amorphous carbons as raw precursors for graphite synthesis. With the assistance of molten salts, the processes can be conducted at moderate temperatures (700-850°C). The mechanisms of the electrochemical conversion of CO2 and amorphous carbons into graphitic materials are presented. Furthermore, the factors that affect the graphitization degree of the prepared graphitic products, such as molten salt composition, working temperature, cell voltage, additives, and electrodes, are discussed. The energy storage applications of these graphitic carbons in batteries and supercapacitors are also summarized. Moreover, the energy consumption and cost estimation of the processes are reviewed, which provides perspectives on the large-scale synthesis of graphitic carbons using this molten salt electrochemical strategy.

Nickel-promoted Electrocatalytic Graphitization of Biochars for Energy Storage: Mechanistic Understanding using Multi-scale Approaches.

Owing to high-efficiency and scalable advantages of electrolysis in molten salts, electrochemical conversion of carbonaceous resources into graphitic products is a sustainable route for achieving high value-added carbon. To understand the complicated kinetics of converting amorphous carbon (e.g. carbonized lignin-biochar) into highly graphitic carbon, herein this study reports the key processing parameters (addition of Ni, temperature and time) and multi-scale approach of nickel-boosted electrochemical graphitization-catalysis processes in molten calcium chloride. Upon both experiments and modellings, multi-scale analysis that ranges from nanoscale atomic reaction to macroscale cell reveal the multi-field evolution in the electrolysis cell, mechanism of electrochemical reaction kinetics as well as pathway of nickel-boosted graphitization and tubulization. The results of as-achieved controllable processing regions and multi-scale approaches provide a rational strategy of manipulating electrochemical graphitization processes and utilizing the converted biomass resources for high value-added use.

High-Purity Graphitic Carbon for Energy Storage: Sustainable Electrochemical Conversion from Petroleum Coke.

The petroleum coke (PC) has been widely used as raw materials for the preparation of electrodes in aluminium electrolysis and lithium-ion batteries (LIB), during which massive CO2 gases are produced. To meet global CO2 reduction, an environmentally friendly route for utilizing PC is highly required. Here, a simple, scalable, catalyst-free process that can directly convert high-sulfur PC into graphitic nanomaterials under cathodic polarization in molten CaCl2 -LiCl at mild temperatures is proposed. The energy consumption of the proposed process is calculated to be 3 627.08 kWh t-1 , half that of the traditional graphitization process (≈7,825.21 kWh t-1 graphite). When applied as a negative electrode for LIBs, the as-converted graphite materials deliver a competitive specific capacity of ≈360 mAh g-1 (0.2 C) compared with commercial graphite. This approach has great potential to scale up for sustainably converting low-value PC into high-quality graphite for energy storage.

Recovery of Ni matte from Ni-bearing electroplating sludge.

In this study, a novel process for the recovery of Ni from Ni-bearing electroplating sludge (ES) is proposed, which involves the carbothermic reduction stage and smelting stage. In the reduction stage, the CaSO4, Fe2O3, and NiO in the ES were reduced by carbon at 1000 °C, and the Ni3S2 and Fe4Ni5S8(Ni-rich phases) were generated. After that, the reduced ES was mixed with SiO2 and smelted at 1500 °C. During the smelting stage, Ni3S2 and Fe4Ni5S8 were melted to form liquid Ni-Fe-S matte and separated from the molten slag by gravity. Finally, 58.5%Ni-13.8%Fe-27.7%S (in weight) matte and vitrified slag were obtained. The recovery ratio of Ni (97.2%) was much higher than that of Fe (14.7%). Besides, the Ni/Fe mass ratio of the ES was 0.7, while the ratio of the prepared matte was about 4.2. Therefore, the selective recovery of Ni was achieved. The obtained Ni matte can be used as the raw material for pure Ni or Ni-bearing chemicals.