Stabilization of Li-Rich Disordered Rocksalt Oxyfluoride Cathodes by Particle Surface Modification.

Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium-ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li2VO2F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li2VO2F particles with AlF3 surface modification, resulting in a much-enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g-1 after only 50 cycles, the treated materials retain almost 200 mA h g-1. Photoelectron spectroscopy depth profiling confirms the stabilization of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.

MgSc2 Se4 -A Magnesium Solid Ionic Conductor for All-Solid-State Mg Batteries?

Recently, the ternary spinel selenide MgSc2 Se4 was proposed to have high magnesium ion mobility and is therefore an interesting potential candidate as a solid electrolyte in magnesium secondary batteries. To test the properties of the material, a modified solid-state reaction was used to synthesize pure MgSc2 Se4 . Electrochemical characterizations identified detrimental high electronic conductivities, which limit its application as a Mg-conducting solid electrolyte. Two methods were attempted to lower electronic conductivities, including the implementation of Se-rich phases and aliovalent doping, however, with no sufficient improvement. Based on the mixed conducting properties of MgSc2 Se4 , a reversible insertion/extraction of Mg2+ into the spinel structure could be demonstrated.

Phase evolution in calcium molybdate nanoparticles as a function of synthesis temperature and its electrochemical effect on energy storage.

The design of a suitable electrode is an essential and fundamental research challenge in the field of electrochemical energy storage because the electronic structures and morphologies determine the surface redox reactions. Calcium molybdate (CaMoO4) was synthesized by a combustion route at 300 °C and 500 °C. We describe new findings on the behaviour of CaMoO4 and evaluate the influence of crystallinity on energy storage performance. A wide range of characterization techniques was used to obtain detailed information about the physical and morphological characteristics of CaMoO4. The characterization results enable the phase evolution as a function of the electrode synthesis temperature to be understood. The crystallinity of the materials was found to increase with increasing temperature but with no second phases observed. Molecular dynamics simulation of electronic structures correlated well with the experimental findings. These results show that to enable faster energy storage and release for a given surface area, amorphous CaMoO4 is required, while larger energy storage can be obtained by using crystalline CaMoO4. CaMoO4 has been evaluated as a cathode material in classical lithium-ion batteries recently. However, determining the surface properties in a sodium-ion system experimentally, combined with computational modelling to understand the results has not been reported. The superior electrochemical properties of crystalline CaMoO4 are attributed to its morphology providing enhanced Na+ ion diffusivity and electron transport. However, the presence of carbon in amorphous CaMoO4 resulted in excellent rate capability, suitable for supercapacitor applications.

Synthesis of Fast Fluoride-Ion-Conductive Fluorite-Type Ba1- xSb xF2+ x (0.1 ≤ x ≤ 0.4): A Potential Solid Electrolyte for Fluoride-Ion Batteries.

Toward the development of high-performance solid electrolytes for fluoride-ion batteries, fluorite-type nanostructured solid solutions of Ba1- xSb xF2+ x ( x ≤ 0.4) were synthesized by high-energy ball-milling method. Substitution of divalent Ba2+ by trivalent Sb3+ leads to an increase in interstitial fluoride-ion concentration, which enhances the ionic conductivity of the Ba1- xSb xF2+ x (0.1 ≤ x ≤ 0.4) system. Total ionic conductivities of 4.4 × 10-4 and 3.9 × 10-4 S cm-1 were obtained for Ba0.7Sb0.3F2.3 and Ba0.6Sb0.4F2.4 compositions at 160 °C, respectively. In comparison to isostructural Ba0.3La0.7F2.3, the ionic conductivity of Ba0.7Sb0.3F2.3 is significantly higher, which is attributed to the presence of an electron lone pair on Sb3+. Introduction of such lone pairs seems to increase fluoride-ion mobility in solid solutions. In addition, Ba0.7Sb0.3F2.3 was tested as a cathode material against Ce and Zn anode using La0.9Ba0.1F2.9 as the electrolyte. Ba0.3Sb0.7F2.3/La0.9Ba0.1F2.9/Ce cell showed high discharge and charge capacities of 301 and 170 mA h g-1, respectively, in the first cycle at 150 °C.

The key role of the composition and structural features in fluoride ion conductivity in tysonite Ce1-xSrxF3-x solid solutions.

Pure tysonite-type Ce1-xSrxF3-x solid solutions for 0 ≤ x < 0.15 were prepared by a solid-state route at 900 °C. The cell parameters follow Vegard's laws for 0 ≤ x ≤ 0.10 and the solubility limit is identified (0.10 < xlimit < 0.15). For 0 ≤ x ≤ 0.05, the F2-(Ce,Sr) and F3-(Ce,Sr) bond distances into [Ce1-xSrxF](2-x)+ slabs strongly vary with x. This slab buckling is maximum around x = 0.025 and strongly affects the more mobile F1 fluoride ions located between the slabs. The 19F MAS NMR spectra show the occurrence of F1-F2,3 exchange at 64 °C. The fraction of mobile F2,3 atoms deduced from the relative intensity of the NMR resonance is maximum for Ce0.99Sr0.01F2.99 (22% at 64 °C) while this fraction linearly increases with x for La1-xAExF3-x (AE = Ba, Sr). The highest conductivity found for Ce0.975Sr0.025F2.975 (3 × 10-4 S cm-1 at RT, Ea = 0.31 eV) is correlated to the largest dispersion of F2-(Ce,Sr) and F3-(Ce,Sr) distances which induces the maximum sheet buckling. Such a relationship between composition, structural features and fluoride ion conductivity is extended to other tysonite-type fluorides. The key role of the difference between AE2+ and RE3+ ionic radii and of the thickness of the slab buckling is established and could allow designing new ionic conductors.

Fluoride solid electrolytes: investigation of the tysonite-type solid solutions La1-xBaxF3-x (x < 0.15).

Pure tysonite La1-xBaxF3-x solid solutions for x < 0.15 were prepared by solid state synthesis in a platinum tube under an azote atmosphere with subsequent quenching for 0.07 ≤x < 0.15. The solid solutions were studied by X-ray, electron and neutron diffractions and by (19)F NMR and impedance spectroscopy. The evolution of the cell parameters obeying Vegard's rule was determined for 0 < x≤ 0.15 and atomic position parameters were accurately refined for x = 0.03, 0.07 and 0.10. The chemical pressure induced by large Ba(2+) cations leads to an increase of the unit cell parameters. Fluorine environment and mobilities are discussed on the basis of the results of neutron diffraction and (19)F solid state NMR. The F1 subnetwork is lacunar; fluorine exchange occurs according to the order: F1-F1 and F1-F2,3. 2D EXSY NMR spectra of La0.97Ba0.03F2.97 reveal, for the first time, a chemical exchange between F2 and F3 sites that requires two successive jumps. The ionic conductivity was evaluated from sintered pellets and different shaping methods were compared. The only structural features which could explain the conductivity maximum are a crossover together with a smaller dispersion of F1-F1,2,3 distances at x = 0.05-0.07.

Stabilization by Si substitution of the pseudobinary compound Gd(2)(Co(3-x)Si(x)) with magnetocaloric properties around room temperature.

We report the discovery of a new solid solution Gd2(Co3-xSix) with 0.29 < x < 0.50 in the Gd-Co-Si ternary system. Members of this solid solution crystallize with the La2Ni3-type structure and correspond to the stabilization of "Gd2Co3" through silicon substitution. The structure of the member Gd2(Co2.53(3)Si0.47) was determined by X-ray diffraction on a single crystal. It crystallizes with the space group Cmce and cell parameters a = 5.3833(4), b = 9.5535(6), and c = 7.1233(5) Å. Co/Si mixing is observed on two crystallographic positions. All compounds studied in the solid solution present a ferrimagnetic order with a strong composition-dependent Curie temperature TC with 280 K < TC < 338 K. The magnetocaloric effect, which amounts to around 1.7 J K(-1) kg(-1) for ΔH = 2 T, is interestingly tunable around room temperature over a temperature span of 60 K through only 4-5% of composition change.