Insights into Mg²⁺ Intercalation in a Zero-Strain Material: Thiospinel Mg_<i>x</i>Zr₂S₄

The Mg battery cathode material, thiospinel Mg_<i>x</i>Zr₂S₄ (0 ≤ <i>x</i> ≤ 1), exhibits negligible volume change (ca. 0.05%) during electrochemical cycling, providing valuable insight into the limiting factors in divalent cation intercalation. Rietveld refinement of XRD data for Mg_xZr₂S₄ electrodes at various states of charge, , coupled with EDX analysis, demonstrates that Mg²⁺ can be inserted into Zr₂S₄ at 60 °C up to <i>x</i> = 0.7 at a C/10 rate (up to <i>x</i> = 0.9 at very slow rates) and cycled with a high Coulombic efficiency of 99.75%. HAADF-STEM studies provide clear visual evidence of Mg-ion occupation in the lattice, whereas XAS studies show that Zr⁴⁺ was reduced upon Mg²⁺ intercalation. <i>Operando</i> and synchrotron XRD studies reveal the creation of two phases during the latter stages of discharge (<i>x</i> > 0.5) as the lattice fills and Mg²⁺ ions begin occupying tetrahedral (8a) sites in addition to octahedral (16c) interstitial sites. Compared to the isostructural Ti₂S₄ thiospinel, Zr₂S₄ presents a slightly larger cell volume and hence an almost ideal zero-strain lattice on Mg²⁺ insertion. Nonetheless, its 4-fold lower electronic conductivity results in a diffusion coefficient for Mg²⁺ ions (<i>D</i>_Mg; 1 × 10^–10 to 1 × 10^–9 cm²/s) that is more than a factor of 10 lower than in Ti₂S₄. This shows that delocalization of the electron charge carriers in the framework is a very important factor in governing multivalent ion diffusivity in the thiospinel framework and, by extension, in other materials

Spray-Assisted Deep-Frying Process for the In Situ Spherical Assembly of Graphene for Energy-Storage Devices

To take full advantage of graphene in macroscale devices, it is important to integrate two-dimensional graphene nanosheets into a micro/macrosized structure that can fully utilize graphene’s nanoscale characteristics. To this end, we developed a novel spray-assisted self-assembly process to create a spherically integrated graphene microstructure (graphene microsphere) using a high-temperature organic solvent in a manner reminiscent of deep-frying. This graphene microsphere improves the electrochemical performance of supercapacitors, in contrast to nonassembled graphene, which is attributed to its structural and pore characteristics. Furthermore, this synthesis method can also produce an effective graphene-based hybrid microsphere structure, in which Si nanoparticles are efficiently entrapped by graphene nanosheets during the assembly process. When used in a Li-ion battery, this material can provide a more suitable framework to buffer the considerable volume change that occurs in Si during electrochemical lithiation/delithiation, thereby improving cycling performance. This simple and versatile self-assembly method is therefore directly relevant to the future design and development of practical graphene-based electrode materials for various energy-storage devices

In Situ Synthesis of Three-Dimensional Self-Assembled Metal Oxide–Reduced Graphene Oxide Architecture

The fabrication of self-assembled, three-dimensional (3-D) graphene structures is recognized as a powerful technique for integrating various nanostructured building blocks into macroscopic materials. In this way, nanoscale properties can be harnessed to provide innovative functionalities of macroscopic devices with hierarchical microstructures. To this end, we report on the fabrication of a three-dimensional (3-D) metal oxide (MO)–reduced graphene oxide (RGO) architecture by controlling the reduction conditions of graphene oxide. In this structure, SnO₂ nanoparticles with dimensions of 2–3 nm are uniformly anchored and supported on a 3-D RGO structure. The resulting composite exhibits excellent rate capability as a binder-free electrode and shows great potential for use in Li-ion batteries. Furthermore, the proposed reduction synthesis can also be applied to the study of the synergetic properties of other 3-D MO–RGO architectures

Directing the Lithium–Sulfur Reaction Pathway via Sparingly Solvating Electrolytes for High Energy Density Batteries

The lithium–sulfur battery has long been seen as a potential next generation battery chemistry for electric vehicles owing to the high theoretical specific energy and low cost of sulfur. However, even state-of-the-art lithium–sulfur batteries suffer from short lifetimes due to the migration of highly soluble polysulfide intermediates and exhibit less than desired energy density due to the required excess electrolyte. The use of sparingly solvating electrolytes in lithium–sulfur batteries is a promising approach to decouple electrolyte quantity from reaction mechanism, thus creating a pathway toward high energy density that deviates from the current catholyte approach. Herein, we demonstrate that sparingly solvating electrolytes based on compact, polar molecules with a 2:1 ratio of a functional group to lithium salt can fundamentally redirect the lithium–sulfur reaction pathway by inhibiting the traditional mechanism that is based on fully solvated intermediates. In contrast to the standard catholyte sulfur electrochemistry, sparingly solvating electrolytes promote intermediate- and short-chain polysulfide formation during the first third of discharge, before disproportionation results in crystalline lithium sulfide and a restricted fraction of soluble polysulfides which are further reduced during the remaining discharge. Moreover, operation at intermediate temperatures ca. 50 °C allows for minimal overpotentials and high utilization of sulfur at practical rates. This discovery opens the door to a new wave of scientific inquiry based on modifying the electrolyte local structure to tune and control the reaction pathway of many precipitation–dissolution chemistries, lithium–sulfur and beyond

Structural Changes in Reduced Graphene Oxide upon MnO₂ Deposition by the Redox Reaction between Carbon and Permanganate Ions

We explore structural changes of the carbon in MnO₂/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO₄^–) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO₄^– causes both quantitative carbon loss and electronic structural changes upon MnO₂ deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO₂/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electrical conductivity of the hybrid materials. The results of this study should serve as a useful guide to rationally approaching the syntheses of metal/RGO and metal oxide/RGO hybrid materials