What Is Different about Hydrothermally Synthesized ε-LiVOPO4?

e-LiVOPO₄ is a multi-electron intercalation cathode with theoretical capacity of 330 mAh/g. It could be practically realized if both high-voltage Li removal and low-voltage Li insertion processes could run reversibly over many cycles. In this work, we have investigated ε-LiVOPO₄ synthesized by hydrothermal (HT) reaction and then ball-milled with carbon for the electrode preparation. This material shows high initial capacity around 300 mAh/g (Fig. 1) which holds for about 10 cycles, but then gradually decays. The electrochemical curve of the HT e-LiVOPO₄ is more sloping in comparison to that of the solid state (SS) synthesized material, which shows distinct steps typical of two-phase reactions. We have used high-resolution x-ray diffraction, pair-distribution function analysis (PDF), ICP, TEM, thermogravimetric analysis with mass-spectroscopy (TGA-MS), and magnetic measurements to understand the distinctions of the HT e-LiVOPO₄. The bulk structure of HT and SS products is essentially the same: triclinic with very similar lattice parameters. The particle size is large for both HT (5μm) and SS (1μm) products, therefore high-energy ball-milling with carbon was used to achieve good electrochemistry. Distinct differences were found in TGA-MS and magnetic tests. TGA-MS reveals about 1wt.% of water loss at 300 °C for the HT product, indicating structural water or hydroxyl incorporation into the structure. Magnetic test shows antiferromagnetic transition at 14 K in the SS product in agreement with previous reports, while the HT product becomes ferrimagnetic at 11 K, which points toward structural defects in the HT sample. High-resolution x-ray diffraction and pair-distribution function analysis results will be discussed to reveal fine structural differences between HT and SS products and the effect of ball-milling on the structure. Also, similar data for the cycled samples will be presented to address the issue of capacity fading. This work was supported as part of the North East Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.