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VOPO 4 was synthesized from the monoclinic and tetragonal forms of H 2 VOPO 4 by the electrochemical de-intercalation of the protons in both nonaqueous and aqueous electrolytes, as well as by the thermal decomposition of H 2 VOPO 4 . The -VOPO 4 so formed readily and reversibly reacts with up to two lithium ions. The -VOPO 4 from the tetragonal precursor has a reversible capacity exceeding 130 mAh/g on its higher voltage plateau, and even at a 2 mA/cm 2 cycling rate has a capacity of 95 mAh/g comparable to that of the spinel LiMn 2 O 4 . The electronic conductivity of the -VOPO 4 is 1 ϫ 10 −6 S.cm −1 and the lithium chemical diffusion coefficients is in the range of 1 ϫ 10 −11 to 5 ϫ 10 −9 cm 2 s −1 depending on composition. The structure changes little during hydrogen removal and lithium intercalation, and the changes are reversible.Intercalation compounds are of great interest for use in batteries, sensors, and as catalysts. 1 Their widespread application in lithium batteries is due to their intrinsic reversibility in electrochemical systems at room temperature. 2 Although most of the early successful cathodes had layered structures, recently the tunnel structures Li 3 Fe 2 ͑PO 4 ͒ 3 3,4 and LiFePO 4 5,6 were proposed as candidate cathode materials for lithium batteries; several other metal phosphates have also been studied. 7-11 In addition, vanadium phosphate compounds are attractive due to their higher free energy of reaction than the corresponding iron compounds, and the greater possible change of oxidation state. Several vanadyl phosphates VOPO 4 have been investigated as cathode materials. 12-16 -VOPO 4 was first synthesized by Lim et al. by heating monoclinic H 2 VOPO 4 ͑also known as VPO 4 ·H 2 O in the literature͒ in oxygen, 17 and its electrochemistry was studied by Kerr et al. 18 Here we report the synthesis, crystal structure, and electrochemistry of -VOPO 4 formed from both the monoclinic and the tetragonal phases of H 2 VOPO 4 ; thermal and electrochemical methods were used. It was found that the electrochemical performance was enhanced when the disordered tetragonal phase was used as the precursor. We have performed structural, morphological, and electrochemical studies for them to ascertain the reason for the improvement. ExperimentalSynthesis of -VOPO 4 .-The H 2 VOPO 4 precursor was synthesized in both aqueous and ethanol solutions in 125 mL PTFE-lined Parr autoclaves at 180°C for 3 days, then ramp cooled at 0.5°C min −1 . In the aqueous synthesis, the reactants were 16.7 mL standardized H 3 PO 4 solution ͑2.39 M͒, 1.60 g VCl 3 , and 20 mL deionized water, with the pH adjusted by triethylmethylammonium hydroxide ͑20% in water͒ to 1.2-1.4 and 2.2-2.4 for the monoclinic and tetragonal H 2 VOPO 4 . In the ethanol synthesis, the reactants were 30 mL ethanol with 1.70 g VCl 3 and 0.57 g P 2 O 5 for monoclinic H 2 VOPO 4 , 1.2 g VCl 4 and 0.52 g P 2 O 5 or 1.82 g V 2 O 5 and 2.91 g P 2 O 5 for tetragonal H 2 VOPO 4 ; water for the reactions is generated by the dehydration of ethanol, the diethyl eth...

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