Ammonium Vanadium Bronze, (NH4)2V7O16, as a New Lithium Intercalation Host Material.

A new type of ammonium vanadium bronze, (NH4)2V7O16, was synthesized by the hydrothermal method. The triclinic crystal structure (P1̅) 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 V7O16 layers along the c axis, and two NH4+ 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 (NH4)2V7O16, 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, (NH4)2V7O16 intercalates Na+ ions reversibly, implying a potential capability as a host material for other guest ions.

Crystal structure of strontium perchlorate anhydrate, Sr(ClO4)2, from laboratory powder X-ray diffraction data.

The crystal structure of strontium perchlorate anhydrate, Sr(ClO4)2, was determined and refined from laboratory powder X-ray diffraction data. The material was obtained by dehydration of Sr(ClO4)2·3H2O at 523 K for two weeks. It crystallizes in the ortho-rhom-bic space group Pbca and is isotypic with Ca(AlD4)2 and Ca(ClO4)2. The asymmetric unit contains one Sr, two Cl and eight O sites, all on general positions (Wyckoff position 8c). The crystal structure consists of Sr2+ cations and isolated ClO4 - tetra-hedra. The Sr2+ cation is coordinated by eight O atoms from eight ClO4 - tetra-hedra. The validity of the crystal structure model for Sr(ClO4)2 anhydrate was confirmed by the bond valence method.

Rhombohedral Potassium-Zinc Hexacyanoferrate as a Cathode Material for Nonaqueous Potassium-Ion Batteries.

Rhombohedral potassium-zinc hexacyanoferrate K1.88Zn2.88[Fe(CN)6]2(H2O)5 (KZnHCF) synthesized using a precipitation method is demonstrated as a high-voltage cathode material for potassium-ion batteries (PIBs), exhibiting an initial discharge capacity of 55.6 mAh g-1 with a discharge voltage of 3.9 V versus K/K+ and a capacity retention of ∼95% after 100 cycles in a nonaqueous electrolyte. All K ions are extracted from the structure upon the initial charge process. However, only 1.61 out of 1.88 K ions per formula unit are inserted back into the structure upon discharge, and it becomes the reversible ion of the second cycle onward. Despite the large ionic size of K, the material exhibits a lattice-volume change (∼3%) during a cycle, which is exceptionally small among the cathode materials for PIBs. The distinct feature of the material seems to come from the unique porous framework structure built by ZnN4 and FeC6 polyhedra linked via the C≡N bond and a Zn/Fe atomic ratio of 3/2, resulting in high structural stability and cycle performance.

Electrochemical Exchange Reaction Mechanism and the Role of Additive Water to Stabilize the Structure of VOPO4 ⋠2 H2 O as a Cathode Material for Potassium-Ion Batteries.

VOPO4 ⋅2 H2 O 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 H2 O 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.

Unveiling the Intercalation Mechanism in Fe2(MoO4)3 as an Electrode Material for Na-Ion Batteries by Structural Determination.

Monoclinic Fe2(MoO4)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 Na2Fe2(MoO4)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 Na2Fe2(MoO4)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 FeO6 and MoO4 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.

Crystal structure of calcium perchlorate anhydrate, Ca(ClO4)2, from laboratory powder X-ray diffraction data.

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(ClO4)2·xH2O] at 623 K in air for 12 h. It crystallizes in the ortho-rhom-bic space group Pbca and is isotypic with Ca(AlD4)2. The asymmetric unit contains one Ca, two Cl and eight O sites, all on general sites (Wyckoff position 8c). The crystal structure consists of isolated ClO4- tetra-hedra and Ca2+ cations. The Ca2+ cation is coordinated by eight O atoms of eight symmetry-related ClO4- tetra-hedra within a distorted square-anti-prismatic environment.