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Calcium-ion batteries (CIBs) are getting increasing attention as post-lithium-ion batteries owing to their theoretical and potential advantages in terms of energy density and cost-effectiveness. However, most of the reported cathode materials suffer from low capacity or cyclability in dried nonaqueous electrolytes. So far, all of the materials with high capacity (>100 mA h g−1) contain crystal water, which was considered to be crucial to the structural stability, enabling facile Ca diffusion. Here, we report β-Ag0.33V2O5 as a high-capacity cathode material for CIBs without crystal water. After the initial activation process, the material exhibited a capacity of 179 mA h g−1 at approximately 2.8 V (vs. Ca2+/Ca) in the ninth cycle and showed a modest cycling performance. The capacity is the highest among the Ca cathode materials without crystal water reported to date. We revealed that the activation process was caused by a replacement reaction between the silver and calcium ions. This material demonstrates that crystal water is not an essential component of CIB electrode materials for a high capacity, stimulating the ongoing research for developing higher-performance materials. [ABSTRACT FROM AUTHOR]

VOPO4⋅2 H2O is demonstrated as a cathode material for potassium‐ion batteries in 0.6 m KPF6 in ethylene carbonate/diethyl carbonate, and its distinct exchange reaction mechanism between potassium and crystal water is reported. In an anhydrous electrolyte, the cathode shows an initial capacity of approximately 90 mAh g−1, with poor capacity retention (32 % after 50 cycles). In contrast, the capacity retention dramatically improved (86 % after 100 cycles) in a wet electrolyte containing 0.1 m of additive water. VOPO4⋅2 H2O contains two types of water (structural and crystal). Upon discharge, potassium ions are intercalated whereas the crystal water is simultaneously de‐intercalated from the structure. Upon charging, a completely reverse reaction takes place in the wet electrolyte, resulting in high stability of the host structure and excellent cyclability. However, in the anhydrous electrolyte, some portion of the extracted crystal water molecules cannot be reinserted into the host structure because they are distributed over the anhydrous electrolyte. Keeping some concentration of water in the electrolyte turns out to be was the key to achieving such high reversibility. The potassium ions (90 %) and proton or hydronium ions (10 %) seem to be co‐intercalated in the wet electrolyte. This work provides a general insight into the intercalation mechanism of crystal‐water‐containing host materials. Just add water: VOPO4⋅2 H2O is investigated as a cathode material for potassium‐ion batteries. An exchange reaction between the potassium ions and crystal water is identified as the electrochemical intercalation mechanism of VOPO4⋅2 H2O. The very poor capacity retention in an anhydrous electrolyte is dramatically improved in the wet electrolyte. Additive water acts as a structural stabilizer in VOPO4⋅2 H2O during the cell cycles. [ABSTRACT FROM AUTHOR]

A new type of ammonium vanadium bronze, (NH 4 ) 2 V 7 O 16 , was synthesized by the hydrothermal method. The triclinic crystal structure ( P 1̅) is successfully identified by the single-crystal X-ray diffraction method. The layered structure is similar to that of other vanadium bronzes but with an unprecedented stoichiometry and crystal structure. The structure is composed of a stack of V 7 O 16 layers along the c axis, and two NH 4 + ions occupy the interlayer space per formula unit. Each ammonium ion is hydrogen-bonded to four lattice oxygen atoms, resulting in a stable structure with a large interlayer space, thus enabling the intercalation of various guest ions. Lithium ions are electrochemically intercalated into (NH 4 ) 2 V 7 O 16 , with an initial discharge capacity of 232 mAh g -1 and an average discharge voltage of 2 V (vs Li/Li + ). Upon the first discharge, lithium ions are inserted, whereas ammonium ions are extracted. Upon charging, a reverse reaction takes place. However, only a fraction of the extracted ammonium ions are reaccommodated. Despite the small quantity, the reinsertion of ammonium ions contributes crucially to the structural stability, improving the electrochemical performance. These results could provide a general understanding of the intercalation mechanism of host materials containing ammonium ions. In addition, (NH 4 ) 2 V 7 O 16 intercalates Na + ions reversibly, implying a potential capability as a host material for other guest ions.

The crystal structure of strontium perchlorate anhydrate, Sr(ClO 4 ) 2 , was determined and refined from laboratory powder X-ray diffraction data. The material was obtained by dehydration of Sr(ClO 4 ) 2 ·3H 2 O at 523 K for two weeks. It crystallizes in the ortho-rhom-bic space group Pbca and is isotypic with Ca(AlD 4 ) 2 and Ca(ClO 4 ) 2 . The asymmetric unit contains one Sr, two Cl and eight O sites, all on general positions (Wyckoff position 8 c ). The crystal structure consists of Sr 2+ cations and isolated ClO 4 - tetra-hedra. The Sr 2+ cation is coordinated by eight O atoms from eight ClO 4 - tetra-hedra. The validity of the crystal structure model for Sr(ClO 4 ) 2 anhydrate was confirmed by the bond valence method.

Monoclinic Fe 2 (MoO 4 ) 3 (FMO) shows distinct structural and electrochemical differences in the intercalation mechanism, depending on the guest ion. (1,2) FMO undergoes a single-phase reaction in a Na-ion cell, but a two-phase reaction in a Li-ion cell. Attempts to understand the difference in the mechanisms have been hindered by a lack of structural information on the fully sodiated phase Na 2 Fe 2 (MoO 4 ) 3 due to its structural complexity and the unavailability of a single crystal. In this work, we have solved and refined the crystal structure of Na 2 Fe 2 (MoO 4 ) 3 for the first time, using the technique of ab initio structure determination from powder diffraction data. Along with electrochemical and structural characterization, 3D bond valence sum difference map calculations enabled us to ascertain the decisive factors that determine such differences, in terms of the interatomic distance and coordination environment of a guest ion. In the case of Na insertion, only a slight expansion of the structure makes the cavity sites of FMO suitable for Na ions, with adequate distances and coordination with surrounding oxygen atoms, resulting in a solid-solution-type single-phase reaction. In the case of Li insertion, the cavity sites are so large for a Li ion that a significant structural change involving tilting of the FeO 6 and MoO 4 polyhedra is required to accommodate the Li ion in a suitable local environment, which does not allow a continuous structural change but results in a two-phase reaction.

The crystal structure of calcium perchlorate anhydrate was determined from laboratory X-ray powder diffraction data. The title compound was obtained by heating hydrated calcium perchlorate [Ca(ClO 4 ) 2 · x H 2 O] at 623 K in air for 12 h. It crystallizes in the ortho-rhom-bic space group Pbca and is isotypic with Ca(AlD 4 ) 2 . The asymmetric unit contains one Ca, two Cl and eight O sites, all on general sites (Wyckoff position 8 c ). The crystal structure consists of isolated ClO 4 - tetra-hedra and Ca 2+ cations. The Ca 2+ cation is coordinated by eight O atoms of eight symmetry-related ClO 4 - tetra-hedra within a distorted square-anti-prismatic environment.