Phase relations in K_xFe_{2-y}Se_2 and the structure of superconducting K_xFe_2Se_2 via high-resolution synchrotron diffraction

Superconductivity in iron selenides has experienced a rapid growth, but not without major inconsistencies in the reported properties. For alkali-intercalated iron selenides, even the structure of the superconducting phase is a subject of debate, in part because the onset of superconductivity is affected much more delicately by stoichiometry and preparation than in cuprate or pnictide superconductors. If high-quality, pure, superconducting intercalated iron selenides are ever to be made, the intertwined physics and chemistry must be explained by systematic studies of how these materials form and by and identifying the many coexisting phases. To that end, we prepared pure K_2Fe_4Se_5 powder and superconductors in the K_xFe_{2-y}Se_2 system, and examined differences in their structures by high-resolution synchrotron and single-crystal x-ray diffraction. We found four distinct phases: semiconducting K_2Fe_4Se_5, a metallic superconducting phase K_xFe_2Se_2 with x ranging from 0.38 to 0.58, an insulator KFe_{1.6}Se_2 with no vacancy ordering, and an oxidized phase K_{0.51(5)}Fe_{0.70(2)}Se that forms the PbClF structure upon exposure to moisture. We find that the vacancy-ordered phase K_2Fe_4Se_5 does not become superconducting by doping, but the distinct iron-rich minority phase K_xFe_2Se_2 precipitates from single crystals upon cooling from above the vacancy ordering temperature. This coexistence of metallic and semiconducting phases explains a broad maximum in resistivity around 100 K. Further studies to understand the solubility of excess Fe in the K_xFe_{2-y}Se_2 structure will shed light on the maximum fraction of superconducting K_xFe_2Se_2 that can be obtained by solid state synthesis.Comment: 12 pages, 16 figures, supplemental materia

Tunnel/Layer Composite Na0.44_{0.44}MnO2_{2} Cathode Material with Enhanced Structural Stability via Cobalt Doping for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are the most promising alternative to lithium-ion batteries (LIBs) due to their low cost and environmental friendliness; therefore, enhancing the performance of SIBs’ components is crucial. Although most of the studies have focused on single-phase cathode electrodes, these materials have difficulty in meeting the requirements in practice. At this point, composite materials show superior performance due to balancing different structures and are offered as an alternative to single-phase cathodes. In this study, we synthesized a Na$_{0.44}$MnO$_2$/Na$_{0.7}$MnO$_{2.05}$ composite material in a single step with cobalt substitution. Changes in the crystal structure and the physical and electrochemical properties of the composite and bare structures were studied. We report that even if the initial capacity is slightly lower, the rate and cyclic performance of the 1% Co-substituted composite sample (CO10) are superior to the undoped Na$_{0.44}$MnO$_2$ (NMO) and 5% Co-substituted (CO50) samples after 100 cycles. The results show that with the composite cathode phase transformations are suppressed, structural degradation is prevented, and better battery performance is achieved

Investigation of physical and electrochemical properties of Ni-doped Tunnel/P2 hybrid Na0.44_{0.44}MnO2_2 cathode material for sodium-ion batteries

The increasing demand for energy in recent years has accelerated the efforts to increase the efficiency of energy storage systems. Although lithium-ion batteries are very popular in energy storage systems, the dramatic increase in costs due to the decrease in lithium resources has greatly increased the interest in sodium-ion batteries. Na$_{0.44}$MnO$_2$ has recently received increasing attention due to the fact that the tunnel structures in the crystal structure are suitable for the diffusion of Na ions. However, rapid structural degradation is an important problem that must be overcome to move into practical applications. In this study, the tunnel/P2 hybrid type Na$_{0.44}$MnO$_2$ was synthesized by a one-step heat treatment with the Ni substitution to Mn sites for improving cyclic performance. It was demonstrated by various physical analyses, that biphasic hybrid material starts forming with Ni substitution, and Ni occupied the Mn sites in the P2 phase. Electrochemical measurements provide that after 100 cycles at 0.3C, while Na$_{0.44}$MnO$_2$ has 77% capacity retention, 1% and 5% Ni substituted samples have 86.4% and 77.3%, respectively. The results show that tunnel-P2 hybrid cathode materials can be developed for practical applications in sodium-ion batteries

Tunnel/Layer Composite Na0.44MnO2 Cathode Material with Enhanced Structural Stability via Cobalt Doping for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are the most promising alternativeto lithium-ion batteries (LIBs) due to their low cost and environmentalfriendliness; therefore, enhancing the performance of SIBs'components is crucial. Although most of the studies have focused onsingle-phase cathode electrodes, these materials have difficulty inmeeting the requirements in practice. At this point, composite materialsshow superior performance due to balancing different structures andare offered as an alternative to single-phase cathodes. In this study,we synthesized a Na0.44MnO2/Na0.7MnO2.05 composite material in a single step with cobaltsubstitution. Changes in the crystal structure and the physical andelectrochemical properties of the composite and bare structures werestudied. We report that even if the initial capacity is slightly lower,the rate and cyclic performance of the 1% Co-substituted compositesample (CO10) are superior to the undoped Na0.44MnO2 (NMO) and 5% Co-substituted (CO50) samples after 100 cycles.The results show that with the composite cathode phase transformationsare suppressed, structural degradation is prevented, and better batteryperformance is achieved