Co₃O₄@(Fe-Doped)Co(OH)₂ Microfibers: Facile Synthesis, Oriented-Assembly, Formation Mechanism, and High Electrocatalytic Activity

Cobalt oxide or hydroxide nanoarchitectures, often synthesized via solvothermal or electrodeposition or templated approaches, have wide technological applications owing to their inherent electrochemical activity and unique magnetic responsive properties. Herein, by revisiting the well-studied aqueous system of Co/NaBH₄ at room temperature, the chainlike assembly of Co₃O₄ nanoparticles is attained with the assistance of an external magnetic field; more importantly, a one-dimensional hierarchical array consisting of perpendicularly oriented and interconnected Co­(OH)₂ thin nanosheets could be constructed upon such well-aligned Co₃O₄ assembly, generating biphasic core–shell-structured Co₃O₄@Co­(OH)₂ microfibers with permanent structural integrity even upon the removal of the external magnetic field; isomorphous doping was also introduced to produce Co₃O₄@Fe–Co­(OH)₂ microfibers with similar structural merits. The cobalt-chemistry in such a Co/NaBH₄ aqueous system was illustrated to reveal the compositional and morphological evolutions of the cobalt species and the formation mechanism of the microfibers. Owing to the presence of Co₃O₄ as the core, such anisotropic Co₃O₄@(Fe-doped)­Co­(OH)₂ microfibers demonstrated interesting magnetic-responsive behaviors, which could undergo macro-scale oriented-assembly in response to a magnetic stimulus; and with the presence of a hierarchical array of weakly crystallized thin (Fe-doped) Co­(OH)₂ nanosheets with polycrystallinity as the shell, such microfibers demonstrated remarkable electrocatalytic activity toward oxygen evolution reactions in alkaline conditions

Double Transition-Metal Chalcogenide as a High-Performance Lithium-Ion Battery Anode Material

Transition-metal dichalcogenides (TMDs) are a recent addition to a growing list of anode materials for the next-generation lithium-ion battery (LIB). The actual performance of TMDs is however constrained by their limited electronic conductivity. For example, MoS₂, the most studied TMD, does not have adequate rate performance even in the few-layer form or after compounding with nitrogen-doped graphene (NG). WS₂, a TMD with a higher intrinsic electronic conductivity, is more suitable for high rate applications but its theoretical capacity is lower than that of MoS₂. Hence, we hypothesize that a composition-optimized composite of MoS₂, WS₂, and NG may provide high capacity concurrently with good rate performance. This is a report on the design and preparation of double transition-metal chalcogenide (MoS₂/WS₂)-nitrogen doped graphene composites where the complementarity of component functions may be maximized. For example the best sample in this study could deliver a high discharge capacity of 1195 mAh·g^–1 at 100 mA·g^–1 concurrently with good cycle stability (average of 0.02% capacity fade per cycle for 100 cycles) and high rate performance (only 23% capacity reduction with a 50 fold increase in current density from 100 mA·g^–1 to 5000 mA·g^–1)

Investigating the Energy Storage Mechanism of SnS₂‑rGO Composite Anode for Advanced Na-Ion Batteries

Tin sulfide–reduced graphene oxide (SnS₂-rGO) composite material is investigated as an advanced anode material for Na-ion batteries. It can deliver a reversible capacity of 630 mAh g^–1 with negligible capacity loss and exhibits superb rate performance. Here, the energy storage mechanism of this SnS₂-rGO anode and the critical mechanistic role of rGO will be revealed in detail. A synergistic mechanism involving conversion and alloying reactions is proposed based on our synchrotron X-ray diffraction (SXRD) and <i>in situ</i> X-ray absorption spectroscopy (XAS) results. Contrary to what has been proposed in the literature, we determined that Na₂S₂ forms instead of Na₂S at the fully discharge state. The as-formed Na₂S₂ works as a matrix to relieve the strain from the huge volume expansion of the Na–Sn alloy reaction, shown in the high resolution transmission electron microscope (HRTEM). In addition, the Raman spectra results suggest that the rGO not only assists the material to have better electrochemical performance by preventing particle agglomeration of the active material but also coordinates with Na-ions through electrostatic interaction during the first cycle. The unique reaction mechanism in SnS₂-rGO offers a well-balanced approach for sodium storage to deliver high capacity, long-cycle life, and superior rate capability

Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi_0.6Co_0.2Mn_0.2O₂

High energy {010} facets are favorable for Li⁺ transport in a layered Ni-rich LiNi_0.6Co_0.2Mn_0.2O₂ cathode through two-dimensional channels that are perpendicular to the <i>c</i> axis. However, those planes can hardly be maintained during the synthesis of layered cathodes. Therefore, we provide a strategy to use appropriate surface active agents which can alter the surface free energy by reducing surface tension directly. Here, a novel self-assembled 3D flower-like hierarchical LiNi_0.6Co_0.2Mn_0.2O₂ is formed with the help of sodium dodecyl sulfate (SDS), and those high energy facets are preserved. Due to the unique surface architectures which would lead to the fast ion transport kinetics as current expands to 100 times (from 0.1 to 10 C), the capacity decay only about 23.4%. Furthermore, full cells assembled against Li₄Ti₅O₁₂ are constructed with a capacity retention of 80.61% at 1 C charge/discharge. This study could show a promising material model for the preferred orientation active planes and higher Li⁺ transport kinetic

Building Sustainable Saturated Fatty Acid-Zinc Interfacial Layer toward Ultra-Stable Zinc Metal Anodes

The commercialization pace of aqueous zinc batteries (AZBs) is seriously limited due to the uncontrolled dendrite growth and severe corrosion reaction of the zinc anode. Herein, a universal and extendable saturated fatty acid-zinc interfacial layer strategy for modulating the interfacial redox process of zinc toward ultrastable Zn metal anodes is proposed. The in situ complexing of saturated fatty acid-zinc interphases could construct an extremely thin zinc compound layer with continuously constructed zincophilic sites which kinetically regulates Zn nucleation and deposition behaviors. Furthermore, the multifunctional interfacial layer with internal hydrophobic carbon chains as a protective layer is efficient to exclude active water molecules from the surface and efficiently inhibit the surface corrosion of zinc. Consequently, the modified anode shows a long cycle life of over 4000 h at 5 mA cm–2. In addition, the assembled Zn||V2O5 full cells based on modified zinc anodes have excellent rate performance and long cycle stability

Three-Dimensional Printing of Polyaniline/Reduced Graphene Oxide Composite for High-Performance Planar Supercapacitor

We apply direct ink writing for the three-dimensional (3D) printing of polyaniline/reduced graphene oxide (PANI/RGO) composites with PANI/graphene oxide (PANI/GO) gel as printable inks. The PANI/GO gel inks for 3D printing are prepared via self-assembly of PANI and GO in a blend solvent of <i>N</i>-methyl-2-pyrrolidinone and water, and offer both shaping capability, self-sustainability, and electrical conductivity after reduction of GO. PANI/RGO interdigital electrodes are fabricated with 3D printing, and based on these electrodes, a planar solid-state supercapacitor is constructed, which exhibits high performance with an areal specific capacitance of 1329 mF cm^–2. The approach developed in this work provides a simple, economic, and effective way to fabricate PANI-based 3D architectures, which leads to promising application in future energy and electric devices at micro-nano scale