Structural evolution and stability of Sc₂(WO₄)₃after discharge in a sodium-based electrochemical cell

Abstract

Sc(WO), prepared by solid state synthesis and constructed as an electrode, is discharged to different states in half-cell batteries, versus a Na negative electrode. The structural evolution of the Na-containing electrodes is studied with synchrotron powder X-ray diffraction (PXRD) revealing an increase in microstrain and a gradual amorphization taking place with increasing Na content in the electrode. This indicates that a conversion reaction takes place in the electrochemical cell. X-ray absorption spectroscopy (XAS) at the tungsten L3 absorption edge shows a reduction in the tungsten oxidation state. Variable temperature (VT) PXRD shows that the Sc(WO) electrode remains relatively stable at higher temperatures, while the Na-containing samples undergo a number of phase transitions and/or turn amorphous above ∼400 °C. Although, Sc(WO) is a negative thermal expansion (NTE) material only a subtle change of the thermal expansion is found below 400 °C for the Na-containing electrodes. This work shows the complexity in employing an electrochemical cell to produce Na-containing Sc(WO) and the subsequent phase transitions.

Exploration of the high temperature phase evolution of electrochemically modified Sc2(WO4)3via potassium discharge

“…In this regime, the potassium discharged Sc2(WO4)3 phase exhibits a rate of change in cell volume of -7. 19(1) × 10 -3 Å 3 K -1 during heating and 7.55(1) × 10 -3 Å 3 K -1 during cooling, compared to -8 × 10 -3 Å 3 K -1 reported for pure Sc2(WO4)3 during heating. 14 This corresponds to an overall volumetric expansion coefficient αV = -5.79(1) × 10 -6 K -1 during heating and αV = -6.1(1) × 10 -6 K -1 during cooling.…”

“…The 50 % sodium discharged Sc2(WO3)4 decomposed upon heating to form amorphous or nanocrystalline phases, whilst the 75 % discharged Sc2(WO3)4 formed new phases that were tentatively assigned as Sc6WO12 and additional unknown phases. 19 The 12.5 % and 25 % lithium discharged samples were found to transition to a Sc0.67WO4-type phase at ≈ 900 K, the synthesis of which had only previously been reported at temperatures and pressures of at least 1673 K and 4 GPa. 20 Similar studies involving the electrochemical modification of the TaxV1-xO5 system using sodium, lithium, and potassium were found to result in interesting phase evolution as a function of temperature, including the generation of new phases.…”

“…However, the electrodes were shown to undergo phase transitions after thermal treatment. 19,20 A Fourier difference map of potassium discharged Sc2(MoO4)3 was generated, see Figure 2b, which shows small regions of electron density in the parent structure which may indicate a small amount of intercalation into the structure. ICP-OES results indicate 1.41 weight% of K in the discharged sample.…”

“…17,18 The use of electrochemical activation to modify the properties of a material has previously been investigated for systems in the A2(MO4)3 series, such as Sc2(WO4)3, Al2(WO4)3 and Sc2(MoO4)3. 16,19 After electrochemical modification via the insertion of sodium into the structure, the Sc2(WO3)4 structure was found to be retained upon heating up to 698 K. Heating beyond this temperature resulted in different phase evolution depending on the "extent" of electrochemical discharge, i.e., the amount of Na inserted into the electrode. The "extent" is characterised by the amount of capacity delivered by the electrode, with 100 % discharge considered to be maximum capacity.…”

“…The thermal treatments clearly show the formation of new phases, similar to previous work on Li and Na electrochemically activated Sc2(WO4)3. 19,20 However, in contrast to these studies, K-based electrochemical activation forms at least one phase conventionally synthesised via other routes. A α-K2WO4 phase, which adopts space group P-3m1, is observed after heating the K discharged Sc2(WO4)3 electrode to 623 K. This phase was found to exhibit an NTE coefficient of αV = -1.9(1) x 10 -4 K -1 during heating from 923 K to 1023 K. The structure of α-K2WO4 has not previously been refined in the P-3m1 space group, and the observed volumetric expansion coefficient is among the largest to be reported.…”