Microwave-Assisted Synthesis of Silver Vanadium Phosphorus
Oxide, Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub>: Crystallite Size
Control and Impact on Electrochemistry
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Abstract
Silver vanadium phosphorus oxide,
Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub>, is a promising cathode
material for Li batteries due in
part to its large capacity and high current capability. Herein, a
new synthesis of Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> based
on microwave heating is presented, where the reaction time is reduced
by approximately 100× relative to other reported methods, and
the crystallite size is controlled via synthesis temperature, showing
a linear correlation of crystallite size with temperature. Notably,
under galvanostatic reduction, the Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> sample with the smallest crystallite size delivers the highest
capacity and shows the highest loaded voltage. Further, pulse discharge
tests show a significant resistance decrease during the initial discharge
coincident with the formation of Ag metal. Thus, the magnitude of
the resistance decrease observed during pulse tests depends on the
Ag<sub>2</sub>VO<sub>2</sub>PO<sub>4</sub> crystallite size, with
the largest resistance decrease observed for the smallest crystallite
size. Additional electrochemical measurements indicate a quasi-reversible
redox reaction involving Li<sup>+</sup> insertion/deinsertion, with
capacity fade due to structural changes associated with the discharge/charge
process. In summary, this work demonstrates a faster synthetic approach
for bimetallic polyanionic materials which also provides the opportunity
for tuning of electrochemical properties through control of material
physical properties such as crystallite size