Synthesis and electrochemical studies of layer-structured metastable α_I-LiVOPO₄

다국어 초록 (Multilingual Abstract)

<P>The layer structured α<SUB>I</SUB>-LiVOPO<SUB>4</SUB> was obtained <I>via</I> a two step chemical synthesis. In the first step, a hydrated phase, LiVOPO<SUB>4</SUB>·2H<SUB>2</SUB>O, was obtained by a simple hydrothermal route at 120 °C. Single crystal X-ray diffraction analysis revealed the structure of LiVOPO<SUB>4</SUB>·2H<SUB>2</SUB>O to be orthorhombic with lattice parameters: <I>a</I> = 8.9454(7) Å, <I>b</I> = 9.0406(7) Å and <I>c</I> = 12.7373(10) Å. Dehydration of the parent compound led to its structural transformation to tetragonal α<SUB>I</SUB>-LiVOPO<SUB>4</SUB>, which was only identified previously during the lithium insertion in VOPO<SUB>4</SUB>. We have investigated the solid-state dehydration of LiVOPO<SUB>4</SUB>·2H<SUB>2</SUB>O and proposed a possible mechanism for the crystal structure transformation. Electrochemical characterization of this rarely studied tetragonal phase revealed its good lithium cycling at high operating voltage. Galvanostatic charge–discharge cycling of α<SUB>I</SUB>-LiVOPO<SUB>4</SUB> was studied in a voltage window of 2.5–4.5 V, which shows a stable reversible capacity of 103(±3) mA h g<SUP>−1</SUP> at a current density of 16 mA g<SUP>−1</SUP> (0.1 C). At higher current rates, although it exhibited good cyclability, the capacity was found to decrease with increasing current rates. The long term cycling stability of the above material was demonstrated at a current rate of 0.5 C up to 200 cycles.</P>

<P>Graphic Abstract</P><P>Highly reversible lithium insertion and extraction with a good capacity are observed for the rarely studied α<SUB>I</SUB>-LiVOPO<SUB>4</SUB> which was obtained by the thermal dehydration of LiVOPO<SUB>4</SUB>·2H<SUB>2</SUB>O.
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