A new mixed anion compound, Na 2 Fe(C 2 O 4 )-F 2 , has been prepared by hydrothermal synthesis. The crystal structure exhibits infinite chains of corner-linked Fe II -centered octahedra, with coordination composed of both oxalate and fluoride ligands. This compound exhibits promising reversible lithium and sodium insertion. On extended cycling, Na 2 Fe-(C 2 O 4 )F 2 is capable of reversibly inserting 0.67 Li + or 0.56 Na + per formula unit up to 50 cycles at the average discharge voltages of 3.3 and 3.0 V, respectively. This represents arguably the best performance as a prospective cathode material so far observed among oxalates and is comparable to many known iron phosphate-based cathode materials.
We have reinvestigated the polyanionic compound Na 2 Fe 2 (C 2 O 4) 3 2H 2 O, previously reported to be electro-chemically inactive in lithium-ion batteries (LIBs), as a positive electrode for sodium-ion batteries (NIBs). The present study demonstrates that it is capable of delivering a reversible capacity close to its theoretical value (117 mAhg-1) with three redox plateaus at 2.9, 3.3 and 3.6 V vs. Na/Na + in the potential range 1.7-4.2 V. The obtained energy density of 326 WhKg-1 is among the highest of all reported polyanionic cathodes in NIBs. The origin of the electrochemical activity can be traced back to the electronic structure of the compound and the low migration energy barrier of the alkali ion observed in first-principles density-functional theory calculations. ASSOCIATED CONTENT Supporting Information. Crystallographic data, Mössbauer analysis, PXRD pattern of ball-milled sample, the first 15 cycles of prepared cell, Nyquist plots of cells at different cycles, and Rietveld refinement of powder XRD on the charged sample are attached as the supporting file. This material is available free of charge via the Internet at http://pubs.acs.org.
Growth of finely dispersed nanocatalysts by exsolution of metal nanoparticles from perovskite oxides under reducing conditions at elevated temperature is a promising approach of producing highly active catalytic materials. An alternative method of exsolution using an applied potential has been recently shown to potentially accelerate the exsolution process of nanoparticles that can be achieved in minutes rather than the hours required in chemical reduction. In the present study, we investigate exsolution of nanoparticles from perovskite oxides of La 0.43 Ca 0.37 Ni 0.06 Ti 0.94 O 3-γ (LCTNi) and La 0.43 Ca 0.37 Ni 0.03 Fe 0.03 Ti 0.94 O 3-γ (LCTNi-Fe) under applied potentials in carbon dioxide atmosphere. The impedance spectra of single cells measured before and after electrochemical poling at varying voltages showed that the onset of exsolution process occurred at 2 V of potential reduction. An average particle size of the exsolved nanoparticles observed after testing using a scanning electron microscopy was about 30-100 nm. The cells with the reduced electrodes exhibited desirable electrochemical performances not only in pure carbon dioxide (current density of 0.37 A cm −2 for LCTNi and 0.48 A cm −2 for LCTNi-Fe at 1.5 V) but also in dry hydrogen (0.36 W cm −2 for LCTNi and 0.43 W cm −2 for LCTNi-Fe).
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