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With the development of low dielectric permittivity materials having an ultra-low sintering temperature, testing their dielectric properties at terahertz frequencies suitable for 6G communication systems and implementation of the fabricated materials in ultra-low temperature cofired ceramics (ULTCC) were the main goals of the research. Lithium tungstate Li2WO4 was synthesized by a solid-state reaction and used for the preparation of green tapes and test structures with cofired internal conductive layers, which are destined for substrates of microwave and submillimeter wave circuits. Sintering behavior, thermal effects, and mass changes of the green tapes during heating were studied using a hot-stage microscope, differential thermal analysis, and thermogravimetry. A single-phase composition was revealed for being undoped and doped with AlF3–CaB4O7 ceramics. The impact of frequency, temperature, the addition of AlF3–CaB4O7 and CuBi2O4 dopants, and sintering temperature was the subject of in-depth characterization of dielectric properties in a terahertz region. A glass-free composition, ultra-low sintering temperature of 590–630 ℃, low roughness of the green tapes, dense microstructure, compatibility with Ag conductors, low and stable dielectric permittivity of 5.0–5.8 in a broad range of 0.2–2 THz, and low dielectric loss of 0.008–0.01 at 1 THz are the main advantages of the developed ULTCC substrates.

Abstract The γ-WO 3 /Li 2 WO 4 composite coated and spinel-embedded Li[Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 ]O 2 cathode is successfully synthesized via a simple wet coating process. A series of synthesized materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscope (HR-TEM), Raman and X-ray photoelectron spectroscopy (XPS). γ-WO 3 /Li 2 WO 4 composite layer was successfully coated on the surface of Li-rich Mn-based materials and the spinel structure was also formed during the modification process. Charge-discharge measurements indicate that the modified samples show an enhanced cyclic properties and rate capability. After the modification process, the initial coulombic efficiency increased to 89.3% for 7 wt % W-composite coated sample with the lowest capacity loss of 33.5 mAh g−1. Particularly, 5 wt % W-composite coated sample shows the best capacity retention of 88.0% at 100 mA g−1 with the discharge capacity of 233.5 mAh g−1 and excellent rate capability with the discharge capacity of 173.0 mAh g−1 at 800 mA g−1. The improvements in electrochemical properties and structural stability were mainly due to both γ-WO 3 /Li 2 WO 4 coating layer and the spinel phase formed during the modification. Highlights • A series of modified samples show enhanced initial coulombic efficiencies. • Superior rate capability and stability of voltage are obtained after W coating. • The spinel LiMn 2 O 4 structure was formed during the modification process. • The synergistic effects of W-compounds and spinel structure are discussed. [ABSTRACT FROM AUTHOR]

• Li 2 W 0.9875 Mo 0.0125 O 4 single crystal was grown by Czochralski technique first. • No phase transitions were found for Li 2 W 1-0.025 Mo 0.025 O 4 (T = 320–997 K). • Energy characteristics of Li 2 W 0.9875 Mo 0.0125 O 4 were studied by reaction calorimetry. The thermodynamic test of Li 2 WO 4 single crystals doping by low molybdenum content with special emphasis on investigation of interconnections, enthalpy, heat capacity to improve device portfolio was performed. We reduced doping content in 7 times for Li 2 W 1-x Mo x O 4 in this investigation. The formation enthalpy of Li 2 WO 4 was estimated using interconnections. The heat capacity was measured for Li 2 W 1-0.025 Mo 0.025 O 4 single crystal (T = 320–997 K) by differential scanning calorimetry. There were no phase transitions of Li 2 W 1-0.025 Mo 0.025 O 4 single crystal that made single crystals with low doping by molybdenum promising for application. The single crystal of lithium tungstate doped by 1.25% molybdenum (content was Li 2 W 0.9875 Mo 0.0125 O 4) was grown by LTG CZ for the first time. The reaction enthalpies of Li 2 W 0.9875 Mo 0.0125 O 4 single crystal were investigated by reaction calorimetry. The interconnections of formation enthalpies and lattice enthalpies of Li 2 W 1-x Mo x O 4 in the range (x = 1–0.0125) from x were established. It indicated the growth of phenakite structure in the range. It was shown that under conditions of low temperature gradients it was possible to grow perfect single crystals of lithium tungstate with low doping of molybdenum. [ABSTRACT FROM AUTHOR]

Herein we combine X-ray photoelectron spectroscopy, AC impedance experiments and high resolution transmission electron microscopy to provide new insight into the degradation mechanism of a nickel-rich cathode material under different charge voltages. The degradation is closely related to the charge/discharge cut-off voltage. We use a simple in-situ method to generate a stable Li 2 WO 4 layer with a thickness of ∼10 nm on the LiNi 0.6 Co 0.2 Mn 0.2 O 2 surface during the calcination process using (NH 4) 2 WO 4. This decorated layer prevents side reactions and reduces formation of the insulators LiF and Li 2 CO 3 on the surface of the particles. Electrochemical impedance spectroscopy shows that the Li 2 WO 4 layer has no effect on the interphase film resistance of the cathode/electrolyte, which remains stable during the cycling process. The charge transfer of LiNi 0.6 Co 0.2 Mn 0.2 O 2 is improved a lot by the decorated layer. Transmission electron microscopy shows that increasing amounts of a rock-salt phase substance forms with increasing cut-off voltage, while for the Li 2 WO 4 -decorated sample only a small amount of rock-salt forms on the surface of the cathode material. This study illustrates the relationship between changes in the surface structure with different cut-off voltages and the electrochemical performance, and provides an effective approach to improve the cycling performance of nickel-rich cathode materials. [ABSTRACT FROM AUTHOR]

Lithium-rich manganese-based cathode materials face severe challenges, like initial irreversible capacity loss, poor cycling performance and rate capability, which seriously restrict their application in broader fields. In this work, Li 2 WO 4 -coated Li [Li 0.2 Mn 0.52 Ni 0.13 Co 0.13 W 0.02 ]O 2 has been successfully prepared by the sol-gel method with a calcination process. The measurements, such as X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and transmission electron microscopy, are used to demonstrate Li 2 WO 4 dispersion on the material surface. After coating with appropriate contents of Li 2 WO 4 , the materials display superior cycling performance and rate capability as well as low voltage decay. The optimal amounts of coating correspond to high capacity retentions of 87.5% at 1C and 88.7% at 5C, while the values only reach 72.9% and 75.9% for the pristine sample, respectively. In particular, when the coating content reaches 3 wt%, the rate capability is remarkably improved. The Li 2 WO 4 coating layer can alleviate the corrosion of active materials and side reactions with electrolyte to enhance the cathode structural stability. • A combinatorial modification method is adapted to improve lithium-rich material. • Li 2 WO 4 -coated lithium-rich materials have been synthesized for the first time. • Li 2 WO 4 coating enables the extraction/insertion of lithium for extra capacity. • The optimal materials show high capacity retention and superior rate capability. [ABSTRACT FROM AUTHOR]