Ionic Conductivity of K-ion Glassy Solid Electrolytes of K2S-P2S5-KOTf System.

Ternary glassy electrolytes containing K2S as a glass modifier and P2S5 as a network former are synthesized by introducing a new type of complex and asymmetric salt, potassium triflate (KOTf), to obtain unprecedented K+ ion conductivity at ambient temperature. The glasses are synthesized using a conventional quenching technique at a low temperature. In general, alkali ionic glassy electrolytes of ternary systems, specifically for Li+ and Na+ ion conductivity, have been studied with the addition of halide salts or oxysalts such as M2SO4, M2SiO4, M3PO4 (M = Li or Na), etc. We introduce a distinct and complex salt, potassium triflate (KOTf) with asymmetric anion, to the conventional glass modifier and former to synthesize K+-ion-conducting glassy electrolytes. Two series of glassy electrolytes with a ternary system of (0.9-x)K2S-xP2S5-0.1KOTf (x = 0.15, 0.30, 0.45, 0.60, and 0.75) and z(K2S-2P2S5)-yKOTf (y = 0.05, 0.10, 0.15, 0.20, and 0.25) on a straight line of z(K2S-2P2S5) are studied for their K+ ionic conductivities by using electrochemical impedance spectroscopy (EIS). The composition 0.3K2S-0.6P2S5-0.1KOTf is found to have the highest conductivity among the studied glassy electrolytes at ambient temperature with the value of 1.06 × 10-7 S cm-1, which is the highest of all pure K+-ion-conducting glasses reported to date. Since the glass transition temperatures of the glasses are near 100 °C, as demonstrated by DSC, temperature-dependent conductivities are studied within the range of 25 to 100 °C to determine the activation energies. A Raman spectroscopic study shows the variation in the structural units PS43-, P2S74-, and P2S64- of the network former for different glassy electrolytes. It seems that there is a role of P2S74- and P2S64- in K+-ion conductivity in the glassy electrolytes because the spectroscopic results are compatible with the composition-dependent, room-temperature conductivity trend.

Effect of the Oxygen Vacancies and Structural Order on the Oxygen Evolution Activity: A Case Study of SrMnO3-δ Featuring Four Different Structure Types.

We report the facile synthesis methods of four materials, with the general formula SrMnO3-δ, which have previously been synthesized in multiple steps, involving switching between different oxidizing and reducing gases, quenching, the use of zirconium metal as a reductant, etc. However, we have shown that it is possible to synthesize all of these materials by facile processes without unnecessary complications. In fact, we have found methods of synthesizing the oxygen-deficient phases in only one step. Given the diverse range of structures that are formed for SrMnO3-δ, we have investigated the correlations between the structural order and electrocatalytic activity for the oxygen evolution reaction (OER) of water splitting. We have uncovered a systematic trend in the OER activity, where the most oxygen-deficient compound, SrMnO2.5, which features square-pyramidal coordination geometry around manganese, shows the highest OER performance. The next OER activity belongs to SrMnO2.6, which contains both MnO5 trigonal bipyramids and MnO6 octahedra. SrMnO3(cubic), containing only corner-sharing MnO6 units, shows the third best OER performance. The least activity is observed in SrMnO3(hexagonal), featuring both face- and corner-sharing MnO6 octahedra. We have also studied the electrochemically active surface area, as well as the kinetics of OER for all four materials, and found that the trend in these properties is the same as the trend in the OER activity. These findings indicate that the electrocatalytic activity is correlated with the degree of oxygen deficiency, as well as the polyhedral connectivity.

Remarkable Oxygen-Evolution Activity of a Perovskite Oxide from the Ca2-x Srx Fe2 O6-δ Series.

Herein in we report the unprecedented catalytic activity of an iron-based oxygen-deficient perovskite for the oxygen-evolution reaction (OER). The systematic trends in OER activity as a function of composition, defect-order, and electrical conductivity have been demonstrated, leading to a methodical increase in OER catalytic activity: Ca2 Fe2 O6-δ

Mesoporous TiO2 coating on carbon-sulfur cathode for high capacity Li-sulfur battery.

In this paper, a meso-porous TiO2 (titania) coating is shown to effectively protect a carbon-sulfur composite cathode from polysulfide dissolution. The cathode consisted of a sulfur impregnated carbon support coated with a few microns thick mesoporous titania layer. The carbon-sulfur cathode is made using activated carbon powder (ACP) derived from biomass. The mesoporous titania coated carbon-sulfur cathodes exhibit a retention capacity after 100 cycles at C/3 rate (433 mA g -1) and stabilized at a capacity around 980 mA h g-1. The electrochemical impedance spectroscopy (EIS) of the sulfur cathodes suggests that the charge transfer resistance at the anode, (R act) is stable for the titania coated sulfur electrode in comparison to a continuous increase in R act for the uncoated electrode implying mitigation of polysulfide shuttling for the protected cathode. Stability in the cyclic voltammetry (CV) data for the first 5 cycles further confirms the polysulfide containment in the titania coated cathode while the uncoated sulfur electrode shows significant irreversibility in the CV with considerable shifting of the voltage peak positions. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies confirm the adsorption of soluble polysulfides by mesoporous titania.

Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in Ca2FeCoO6-δ, CaSrFeCoO6-δ, and Sr2FeCoO6-δ.

The ability to control the electrical conductivity of solid-state oxides using structural parameters has been demonstrated. A correlation has been established between the electrical conductivity and structural order in a series of oxygen-deficient perovskites using X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrical conductivity studies at a wide temperature range, 25-800 °C. The crystal structure of CaSrFeCoO6-δ has been determined, and its stark contrast to Ca2FeCoO6-δ and Sr2FeCoO6-δ has been demonstrated. The Fe/Co distribution over tetrahedral and octahedral sites has been determined using neutron diffraction. There is a systematic increase in the structural order in progression from Sr2FeCoO6-δ (δ = 0.5) to CaSrFeCoO6-δ (δ = 0.8) and Ca2FeCoO6-δ (δ = 0.9) . The oxygen contents of these materials were determined using iodometric titration and TGA. At room temperature, there is an inverse correlation between the electrical conductivity and structural order. The ordered Ca2 and CaSr compounds are semiconductors, while the disordered Sr2 compund shows metallic behavior. The metallic nature of the Sr2 material persists up to 1073 K (800 °C), while the Ca2 and CaSr compounds undergo a semiconductor-to-metal transition above 500 and 300 °C, respectively, highlighting another important impact of the structural order. At high temperature, the CaSr compound has the highest conductivity compared to the Ca2 and Sr2 materials. There appears to be an optimum degree of structural order that leads to the highest conductivity at high temperature. Another consequence of the structural order is the observation of mixed ionic-electronic conductivity in CaSr and Ca2 compounds, as is evident from the hysteresis in the conductivity data obtained during heating and cooling cycles. The average ionic radius required for each structural transition was determined through the synthesis of 21 different materials by systematic variation of the Ca/Sr ratio. In addition, SEM and XPS were employed to gain insight into the crystallite morphology and oxidation states of transition metals, revealing an interesting redox process between Fe and Co.

Transformation of Structure, Electrical Conductivity, and Magnetism in AA'Fe2O6-δ, A = Sr, Ca and A' = Sr.

The ability to control electrical properties and magnetism by varying the crystal structure using the effect of the A-site cation in oxygen-deficient perovskites has been studied in AA'Fe2O6-δ, where A = Sr, Ca and A' = Sr. The structure of Sr2Fe2O6-δ, synthesized at 1250 °C in air, contains dimeric units of FeO5 square pyramids separated by FeO6 octahedra. Here we show that this ordering scheme can be transformed by changing the A-site cations from Sr to Ca. This leads to a structure where layers of corner-sharing FeO6 octahedra are separated by chains of FeO4 tetrahedra. Through systematic variation of the A-site cations, we have determined the average ionic radius required for this conversion to be ∼1.41 Å. We have demonstrated that the magnetic structure is also transformed. The Sr2 compound has an incommensurate magnetic structure, where magnetic moments are in spin-density wave state, aligning perpendicular to the body diagonal of the unit cell. With the aid of neutron diffraction experiments at 10 and 300 K, we have shown that the magnetic structure is converted into a long-range G-type antiferromagnetic system when one Sr is replaced by Ca. In this G-type ordering scheme, the magnetic moments align in the 001 direction, antiparallel to their nearest neighbors. We have also performed variable-temperature electrical conductivity studies on these materials in the temperature range 298-1073 K. These studies have revealed the transformation of charge transport properties, where the metallic behavior of the Sr2 compound is converted into semiconductivity in the CaSr material. The trend of conductivity as a function of temperature is reversed upon changing the A-site cation. The conductivity of the Sr2 compound shows a downturn, while the conductivity of the CaSr material increases as a function of temperature. We have also shown that the CaSr compound exhibits temperature-dependent behavior typical of a mixed ionic-electronic conducting system.