The crystal structures and electrochemical properties of ZnxMo6S8 Chevrel phases (x = 1, 2) prepared via electrochemical Zn(2+)-ion intercalation into the Mo6S8 host material, in an aqueous electrolyte, were characterized. Mo6S8 [trigonal, R3̅, a = 9.1910(6) Å, c = 10.8785(10) Å, Z = 3] was first prepared via the chemical extraction of Cu ions from Cu2Mo6S8, which was synthesized via a solid-state reaction for 24 h at 1000 °C. The electrochemical zinc-ion insertion into Mo6S8 occurred stepwise, and two separate potential regions were depicted in the cyclic voltammogram (CV) and galvanostatic profile. ZnMo6S8 first formed from Mo6S8 in the higher-voltage region around 0.45-0.50 V in the CV, through a pseudo two-phase reaction. The inserted zinc ions occupied the interstitial sites in cavities surrounded by sulfur atoms (Zn1 sites). A significant number of the inserted zinc ions were trapped in these Zn1 sites, giving rise to the first-cycle irreversible capacity of ∼46 mAh g(-1) out of the discharge capacity of 134 mAh g(-1) at a rate of 0.05 C. In the lower-voltage region, further insertion occurred to form Zn2Mo6S8 at around 0.35 V in the CV, also involving a two-phase reaction. The electrochemical insertion and extraction into the Zn2 sites appeared to be relatively reversible and fast. The crystal structures of Mo6S8, ZnMo6S8, and Zn2Mo6S8 were refined using X-ray Rietveld refinement techniques, while the new structure of Zn2Mo6S8 was determined for the first time in this study using the technique of structure determination from powder X-ray diffraction data. With the zinc ions inserted into Mo6S8 forming Zn2Mo6S8, the cell volume and a parameter increased by 5.3% and 5.9%, respectively, but the c parameter decreased by 6.0%. The average Mo-Mo distance in the Mo6 cluster decreased from 2.81 to 2.62 Å.

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as Li and Ni will be intensively used in large batteries for electric vehicles (EVs). These elements may be fully consumed for EV applications, so it will be impossible to have enough of them for batteries designed for large energy storage. These concerns have produced increasing interest in alternative technologies, with sodium-based storage chemistry among the leading modalities. [2] Reversible sodium intercalation received attention as one of the leading post-lithium battery technologies, as it combines several very attractive properties. Various merits are attributed to Nabased cells, in particular safety, low cost, Earth abundance, and environmental friendliness. [2c,3] As a consequence, many cathode compounds that can be utilized in sodium batteries were recently reported, including organic compounds, [4] polyanionic compounds, [5] and transition metal oxides. [6] Among them, manganese-based cathode materials have attracted much attention due to their low cost and significant Earth abundance.The tunnel-type sodium manganese oxide Na 0.44 MnO 2 is particularly attractive owing to its unique large tunnels suitable for sodium intercalation. [7] The crystal structure of Na 0.44 MnO 2 is shown in Figure 1a. There are five distinct crystallographic manganese sites and three sodium sites, where Mn(1) and Mn(2) are occupied by Mn 3+ , and Mn(3), Mn(4), and Mn (5) are occupied Mn 4+ . [8] The structural frame is built up of double and triple linear chains with edge-shared MnO 6 octahedra and single chains of edge-shared MnO 5 square-pyramids. Each chain is aligned parallel to the c-axis and connected to neighboring chains via a corner-sharing of the polyhedra, resulting in two types of tunnels: the 1D tunnels occupied by Na(1) atoms ( Figure 1b) and the 2D tunnels occupied by Na(2) and Na(3) atoms, which are positioned in large zig-zag shaped cavities (Figure 1c). Interestingly, unlike common layered oxides, such a structure is very stable in aqueous solutions, even upon electrochemical sodium intercalation/deintercalation. [7f,9] Thus, recently many publications focused on Na 0.44 MnO 2 as a particularly promising cathode material for both aqueous and nonaqueous sodium-ion batteries. [10] However, to our knowledge, their sodium storage mechanisms were not determined experimentally due to the complex multifold oxidation/reduction steps, while ab initio calculations were reported. [8] Besides that, Tunnel-type sodium manganese oxide is a promising cathode material for aqueous/nonaqueous sodium-ion batteries, however its storage mechanism is not fully understood, in part due to the complicated sodium intercalation process. In addition, low cyclability due to manganese dissolution has limited its practical application in rechargeable batteries. Here, the intricate sodium intercalation mechanism of Na 0.44 MnO 2 is revealed by combination of electrochemical characterization, structure determination from powder X-ray diffraction data, 3D bond valence difference maps, and barrier-energy calculations of the sodium ...

All-solid-state sodium-ion batteries that operate at room temperature are attractive candidates for use in largescale energy storage systems.However,materials innovation in solid electrolytes is imperative to fulfill multiple requirements, including high conductivity,functional synthesis protocols for achieving intimate ionic contact with active materials,a nd air stability.Anew,h ighly conductive (1.1 mS cm À1 at 25 8 8C, E a = 0.20 eV) and dry air stable sodium superionic conductor, tetragonal Na 3 SbS 4 ,isdescribed. Importantly,Na 3 SbS 4 can be prepared by scalable solution processes using methanol or water,a nd it exhibits high conductivities of 0.1-0.3 mS cm À1 . The solution-processed, highly conductive solidified Na 3 SbS 4 electrolyte coated on an active material (NaCrO 2 ) demonstrates dramatically improved electrochemical performance in all-solid-state batteries.

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