ABSTRACT
A member of the family of compounds with the formula Na3V2O2x(PO4)2F3-2x is synthesized by carbothermal reduction and 2 consecutive hydrothermal processes. The initial structural and spectroscopic characterization indicates that there are two phases in the as-synthesized material, a mixed valent phase with an intermediate V oxidation state adopting a P42/mnm space group at about 66%, and another phase with a V oxidation state close to V3+ adopting an Amam space group at about 33%. The role of each species in the electrode function is interrogated using in situ synchrotron X-ray diffraction and these data indicate that soon after charge begins, the Amam phase transforms into the P42/mnm phase. Further structural evolution of the material shows a prevailing two-phase reaction at lower potentials and a significant solid solution region during the high potential feature in the electrochemical curve with a final two-phase region at the end of charge. The conversion of the Amam component into P42/mnm showed no reversibility upon discharge. This phase-mixed electrode appears to illustrate that the mixed valent P42/mnm phase dominates electrochemical behavior during cycling and this may prove vital when preparing members in the sodium vanadium fluorophosphate family, especially if there is an energetically preferred phase for cycling.
ABSTRACT
Two chemically synthesized defective graphene materials with distinctly contrasting extended structures and surface chemistry are used to prepare sodium-ion battery electrodes. The difference in electrode performance between the chemically prepared graphene materials is qualified based on correlations with intrinsic structural and chemical dissimilarities. The overall effects of the materials' physical and chemical discrepancies are quantified by measuring the electrode capacities after repeated charge/discharge cycles. Solvothermal synthesized graphene (STSG) electrodes produce capacities of 92 mAh/g in sodium-ion batteries after 50 cycles at 10 mA/g, while thermally exfoliated graphite oxide (TEGO) electrodes produce capacities of 248 mAh/g after 50 cycles at 100 mA/g. Solid-state 23Na nuclear magnetic resonance spectroscopy is employed to locally probe distinct sodium environments on and between the surface of the graphene layers after charge/discharge cycles that are responsible for the variations in electrode capacities. Multiple distinct sodium environments of which at least 3 are mobile during the charge-discharge cycle are found in both cases, but the majority of Na is predominantly located in an immobile site, assigned to the solid electrolyte interface (SEI) layer. Mechanisms of sodium insertion and extraction on and between the defective graphene surfaces are proposed and discussed in relation to electrode performance. This work provides a direct account of the chemical and structural environments on the surface of graphene that govern the feasibility of graphene materials for use as sodium-ion battery electrodes.
ABSTRACT
The development of electrodes for ambient temperature sodium-ion batteries requires the study of new materials and the understanding of how crystal structure influences properties. In this study, we investigate where sodium locates in two Prussian blue analogues, Fe[Fe(CN)6]1-x·yH2O and FeCo(CN)6. The evolution of the sodium site occupancies, lattice and volume is shown during charge-discharge using in situ synchrotron X-ray powder diffraction data. Sodium insertion is found to occur in these electrodes during cell construction and therefore Fe[Fe(CN)6]1-x·yH2O and FeCo(CN)6 can be used as positive electrodes. NazFeFe(CN)6 electrodes feature higher reversible capacities relative to NazFeCo(CN)6 electrodes which can be associated with a combination of structural factors, for example, a major sodium-containing phase, â¼Na0.5FeFe(CN)6 with sodium locating either at the x = y = z = 0.25 or x = y = 0.25 and z = 0.227(11) sites and an electrochemically inactive sodium-free Fe[Fe(CN)6]1-x·yH2O phase. This study demonstrates that key questions about electrode performance and attributes in sodium-ion batteries can be addressed using time-resolved in situ synchrotron X-ray diffraction studies.
