ε-LiVOPO4 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-LiVOPO4 lattice parameters a and b linearly decrease, while c 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-LiVOPO4 synthesized at 180 and 220 °C, suggesting water or protons incorporation into the structure. Magnetic studies reveal ferrimagnetism in hydrothermal ε-LiVOPO4 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-LiVOPO4 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-LiVOPO4. Based on structure and composition analysis, we suggest that hydrothermal samples can be described as an ε-Li1+x H y V1–z OPO4 (x, y, z < 0.1) solid solution. The electrochemical characterization of hydrothermal ε-LiVOPO4 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 Li1.5VOPO4 and Li1.75VOPO4 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.
Vanadyl phosphates comprise a class of multielectron cathode materials capable of cycling two Li+, about 1.66 Na+, and some K+ ions per redox center. In this review, structures, thermodynamic stabilities, and ion diffusion kinetics of various AxVOPO4 (A = Li, Na, K, NH4) polymorphs are discussed. Both the experimental data and first‐principle calculations indicate kinetic limitations for alkali metal ions cycling, especially between for 0 ≤ x ≤ 1, and metastability of phases with x > 1. This creates challenges for multiple‐ion cycling, as the slow kinetics call for nanosized particles, which being metastable and reactive with organic electrolytes are prone to side reactions. Thus, various synthesis approaches, surface coating, and transition metal ion substitution strategies are discussed here as possible ways to stabilize AxVOPO4 structures and improve alkali metal ion diffusion. The role of advanced characterization techniques, such as X‐ray absorption spectroscopy, diffraction, pair distribution function analysis and 7Li and 31P NMR, in understanding the reaction mechanism from both structural and electronic points of view is emphasized.
Lithium vanadyl phosphate (LiVOPO 4 ) is an attractive cathode material for next-generation lithium-ion batteries, having the ability to reversibly intercalate two Li ions per transition-metal redox center to reach a theoretical capacity of 305 mA h g −1 . This material has a high energy density with two voltage plateaus of 2 and 4 V. However, reduced capacity retention at faster rates and sluggish kinetics in the high-voltage region leave much room for improvement. Cr substitution was implemented to mitigate these limitations and enhance the electrochemical performance of ε-LiVOPO 4 . By various characterization techniques, we have established the composition of the hydrothermally synthesized Cr-substituted samples to be Li x H y Cr z V 1−z OPO 4 solid solution (0.80 ≤ x ≤ 0.85, 0.25 ≤ y ≤ 0.60, and z ≤ 0.05). All Crsubstituted samples demonstrated higher Coulombic efficiency and superior cycling stability for over 40 cycles at C/15. Electrochemical tests show that Cr substitution enhances Li-ion diffusion in the high-voltage regime and the reaction reversibility of ε-LiVOPO 4 .
Interventions for behavior change in domestic energy consumption rely critically on energy usage data. To obtain this data, collection systems must be established. Pervasive sensing systems enable such monitoring, but populating homes with sensors is challenging. We offer an alternative to feedback approaches that depend on the assumption that users are motivated by energy data in its raw state. Physical Experiential Technology Systems (PETS) is a behaviorand sensor-based platform supporting rich experiences and the diffusion of sensors in homes. In this paper, we present our novel approach to building sensor feedback systems and our initial product concepts.
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