Electrochemical sodium insertion/extraction in Na2(MoOPO4)2(HPO4)·yH2O (y=2, 0)

Abstract

The electrochemical sodium insertion and extraction of the phosphomolybdates Na ₂ (MoOPO ₄ ) ₂ (HPO ₄ )·yH ₂ O (y=2, 0) is reported. Both compounds show a similar behaviour upon electrochemical sodium extraction, while the insertion reaction is notably different for each host compound. During the charge/discharge of electrochemical cells the existence of new phases Na _2+x (MoOPO ₄ ) ₂ (HPO ₄ )·2H ₂ O with compositions –0.7<x<–0.4, and 0.75<x<0.9, as well as Na _2+x (MoOPO ₄ ) ₂ (HPO ₄ ) with –1<x<–0.75 and 2.6<x<3.2 has been detected. A member of each solid solution has been synthesised and preliminarily structurally characterised by powder X-ray diffraction. Although all of these new phosphomolybdates present a similar monoclinic structure, the unit cell volume increases progressively with the introduction of sodium and water in the tunnels. Magnetic susceptibility measurements have also been performed in order to follow the electrochemical process.