Nanostructured lithium metal orthosilicate materials hold a lot of promise as next generation cathodes but their full potential realization is hampered by complex crystal and electrochemical behavior. In this work Li2FeSiO4 crystals are synthesized using organic-assisted precipitation method. By varying the annealing temperature different structures are obtained, namely the monoclinic phase at 400°C, the orthorhombic phase at 900°C, and a mixed phase at 700°C. The three Li2FeSiO4 crystal phases exhibit totally different charge/discharge profiles upon delithiation/lithiation. Thus the 400°C monoclinic nanocrystals exhibit initially one Li extraction via typical solid solution reaction, while the 900°C orthorhombic crystals are characterized by unacceptably high cell polarization. In the meantime the mixed phase Li2FeSiO4 crystals reveal a mixed cycling profile. We have found that the monoclinic nanocrystals undergo phase transition to orthorhombic structure resulting in significant progressive deterioration of the material's Li storage capability. By contrast, we discovered when the monoclinic nanocrystals are cycled initially at higher rate (C/20) and subsequently subjected to low rate (C/50) cycling the material's intercalation performance is stabilized. The discovered rate-dependent electrochemically-induced phase transition and stabilization of lithium metal silicate structure provides a novel and potentially rewarding avenue towards the development of high capacity Li-ion cathodes.

Energy storage and conversion in Li-ion batteries involve dynamic Li storage and transport through a series of electrochemical metastable states (or quasi-equilibrium configuration). Therefore, an investigation of these metastable states is helpful to fully understand the lithium storage mechanism. An accurate understanding of the storage mechanism is a key factor in the design and optimization of the next-generation high-performance batteries. Here, we report the results obtained from electron irradiationinduced phase transitions in Li 2 FeSiO 4 and LiFePO 4 by electron microscopy. During prolonged irradiation, the crystalline Li 2 FeSiO 4 particles experienced a transition from a monocrystalline structure to an amorphous phase, with a subsequent recrystallization process (monoclinic to orthorhombic phase). The fine structure of the electron energy loss (ELNES) spectra showed the electron beam-sensitive characteristic of Li 2 FeSiO 4 that included the electron beam-induced mass loss (composition changes), formation of intermediate metastable states (Li 2−x FeSiO 4 ), structural distortion/amorphization and valence state variation, all of which are much less prominent in LiFePO 4 under the same flux of electron beam. These findings provide new insights into the structural stability of Li 2 FeSiO 4 and LiFePO 4 samples and is also important guidance in the characterization of electrode materials.

Freestanding micrometer thick graphene oxide (GO) membranes combine high stiffness, low mass density, and high loss coefficient. This unique combination of properties is ideal for efficient and broadband electro‐acoustic transduction, relying on membrane lightness, stiffness, and internal damping. Here, the viscoelastic response of GO membranes is measured, and the application of ≈100 µm thick GO membranes is demonstrated in dynamic loudspeakers. Using dynamic mechanical analysis and the time‐temperature superposition principle of polymer rheology, it is found that the stiffness of GO membrane increases by more than 50% over 1–20 kHz while damping decreases by less than 20%. GO membranes exhibit 45% higher damping than aluminum membranes in loudspeakers assemblies. Consequently, GO membranes enable the upshift of loudspeaker breakup frequency by 16 to 12 octave above speakers assembled with aluminum, polyethylene terephthalate, titanium, and oak wood membranes. GO is thus found to be an exceptional material for electro‐acoustic transduction.

Engineering of nanostructured lithium iron silicate (LFS) particles is pursued via a novel benign synthesis approach seeking to understand the crystalline particle formation process and its impact on energy storage capacity. Specifically, mesoporous Li 2 FeSiO 4 nanostructured particles are synthesized via a novel dual-step process involving organic-assisted hydrothermal precipitation from concentrated Fe(III) (1 mol/L) aqueous solution followed by reductive (5 vol % H 2 ) thermal transformation of the precipitate at 400 °C (LFS400) and 700 °C (LFS700). Scanning and transmission electron microscopy revealed the formation of secondary sub-micron-sized porous agglomerates of unitary primary nanocrystals (∼50 nm for LFS400 and ∼200 nm for LFS700). Both ethylene glycol and ethylenediamine are used as crystallization control additives. It is demonstrated that formation of LFS from Fe(III) precursor is made possible only by the action of ethylenediamine. The obtained LFS particles are found to be predominantly monoclinic as per X-ray diffraction and Rietveld refinement and bear an in situ formed N-doped carbon coating layer as characterized by X-ray photoelectron spectroscopy. TEM coupled with selected area electron diffraction (SAED) analysis confirmed the Rietveld refined XRD phase compositions. The reductive annealing-induced phase transformation sequence leading to LFS crystallization is characterized, and the enabling role of ethylenediamine is discussed. Initial galvanostatic charging−discharging and cyclic voltammetry measurements indicate the annealing temperature of LFS formation to influence the Li-ion storage profile as it shifts from two-phase reaction in LFS700 to solid solution in LFS400this being attributed to nanostructural changes.

Understanding the chemical and physical properties of core/shell nanocrystal quantum dots (QDs) is key for their use in light-emission applications. In this paper, a single-step injection-free scalable synthetic method is applied to prepare high-quality core/shell QDs with emission wavelengths of 544 nm, 601 nm, and 634 nm. X-ray absorption fine structure spectra are used to determine the core/shell structure of CdSe/ZnS quantum dots. Moreover, theoretical XANES spectra calculated by FEFF.8.20 are used to determine the structure of Se and S compounds. The QD samples displayed nearly spherical shapes with diameters of approximately 3.4 ± 0.5 nm (634 nm), 4.5 ± 0.4 nm (601 nm), and 5.5 ± 0.5 nm (544 nm). With XANES results and MS calculations, it is indicated that sphalerite ZnS capped with organic sulfur ligands should be the shell structure. Wurtzite CdSe is the main core structure with a Cd-Se bond length of 2.3 Å without phase shift. This means that different emission wavelengths are only due to the crystal size with single-step injection-free synthesis. Therefore, single-step injection-free synthesis could generate a nearly ideal core/shell structure of CdSe/ZnS QDs capped with an organic sulfur ligand.