Nonstoichiometry and Defects in Hydrothermally Synthesized ε-LiVOPO4
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2019-06-07 |
| Journal | ACS Applied Energy Materials |
| Authors | Youngmin Chung, Ellen Cassidy, Krystal Lee, Carrie Siu, Yiqing Huang |
| Institutions | Argonne National Laboratory, Binghamton University |
| Citations | 12 |
Abstract
Section titled “Abstract”ε-LiVOPO<sub>4</sub> has been synthesized through the hydrothermal method by adjusting the pH of the hydrothermal solution and the reaction temperature. This phase is formed between 180 and 220 °C, as diamond-like crystals around 10-15 μm in size. X-ray diffraction (XRD) analysis shows that hydrothermal ε-LiVOPO<sub>4</sub> lattice parameters <em>a</em> and <em>b</em> linearly decrease, while <em>c</em> linearly increases when the synthesis temperature increases. Thermogravimetric analysis with mass spectroscopy reveals 1.5 to 0.5% water loss at about 350 °C for ε-LiVOPO<sub>4</sub> synthesized at 180 and 220 °C, suggesting water or protons incorporation into the structure. Magnetic studies reveal ferrimagnetism in hydrothermal ε-LiVOPO<sub>4</sub> below 10 K, as opposed to antiferromagnetic ordering below 14 K found in samples synthesized at high temperature. In-situ XRD upon heating of the hydrothermal ε-LiVOPO<sub>4</sub> synthesized at 180, 200, and 220 °C reveals that the temperature dependences of their lattice parameters merge at about 500 °C; furthermore, at the same temperature the structure reversibly changes from triclinic to monoclinic. The lattice parameters and the magnetic properties of the hydrothermal samples heated to 750 °C are similar to those of solid-state synthesized ε-LiVOPO<sub>4</sub>. Based on structure and composition analysis, we suggest that hydrothermal samples can be described as an ε-Li<sub>1+<em>x</em></sub>H<sub><em>y</em></sub>V<sub>1-<em>z</em></sub>OPO<sub>4</sub> (<em>x</em>, <em>y</em>, <em>z</em> &lt; 0.1) solid solution. The electrochemical characterization of hydrothermal ε-LiVOPO<sub>4</sub> reveals the first cycle capacity of about 300 mAh/g, which holds for about five cycles, gradually decreasing thereafter. The low-voltage region does not reveal voltage plateaus corresponding to Li<sub>1.5</sub>VOPO<sub>4</sub> and Li<sub>1.75</sub>VOPO<sub>4</sub> phases found in the solid-state material, further suggesting structural disorder in the low-temperature samples evidenced from the lattice parameters and the magnetic properties.