She-Huang Wu

The origin of capacity fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) microsphere positive electrode: an operando neutron diffraction and transmission X-ray microscopy study

The mechanism of capacity fade of the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) composite positive electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallographic, and morphological evolution of the material during electrochemical cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R3̅m LiMO2 with the remaining 27(1) wt % C2/m Li2MnO3 likely existing as an intergrowth. Cracking in the Li2MnO3·LiMO2 electrode particle under operando microscopy observation was revealed to be initiated by the solid-solution reaction of the LiMO2 phase on charge to 4.55 V vs Li(+)/Li and intensified during further charge to 4.7 V vs Li(+)/Li during the concurrent two-phase reaction of the LiMO2 phase, involving the largest lattice change of any phase, and oxygen evolution from the Li2MnO3 phase. Notably, significant healing of the generated cracks in the Li2MnO3·LiMO2 electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally associated with the solid-solution reaction of the LiMO2 phase. This work reveals that while it is the reduction of lattice size of electrode phases during charge that results in cracking of the Li2MnO3·LiMO2 electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-solution reaction occurring during discharge. Importantly, it is the phase separation during the two-phase reaction of the LiMO2 phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase separation, such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode.

Structure of the Li4Ti5O12 anode during charge-discharge cycling

The structural evolution of the “zero-strain” Li 4 Ti 5 O 12 anode within a functioning Li-ion battery during charge–discharge cycling was studied using in situ neutron powder-diffraction, allowing correlation of the anode structure to the measured charge–discharge profile. While the overall lattice response controls the “zero-strain” property, the oxygen atom is the only variable in the atomic structure and responds to the oxidation state of the titanium, resulting in distortion of the TiO 6 octahedron and contributing to the anodes stability upon lithiation/delithiation. Interestingly, the trend of the octahedral distortion on charge–discharge does not reflect that of the lattice parameter, with the latter thought to be influenced by the interplay of lithium location and quantity. Here we report the details of the TiO 6 octahedral distortion in terms of the O–Ti–O bond angle that ranges from 83.7(3)° to 85.4(5)°.

Electromagnetic characteristics of surface modified iron nanowires at x-band frequencies

Surface modified iron nanowire nanoparticles were prepared via reduction of iron salts (FeCl3· 6H2O) under an applied magnetic field. To minimize the surface oxidation, dextran (0.05 and 0.25 wt. %) was added during the process and formed a thin passive layer over iron nanowires with alcohol and acetone used to wash iron nanowires. The complex permittivity (ɛ′-jɛ′′) and permeability (μ′-jμ″) of absorbers are measured by a cavity perturbation method from 7 to 14 GHz. In the present study, the iron nanowire prepared with 0.25 wt. % dextran and washed by acetone (D25AC) exhibited the best microwave absorption performance. Depending on the test frequency, D25AC possessed the largest permittivity loss ranged from 0.14 to 0.17 and relatively small permeability loss (<0.05). Its high permittivity dissipation is responsible for the excellent microwave absorption performance where the reflection loss was-7.7 dB at a matching frequency of 9.0 GHz.

SYNTHESIS AND ELECTROCATALYSIS APPLICATION OF HYBRID PLATINUM/CERIUM OXIDE/MULTI-WALLED CARBON NANOTUBES

The hybrid nanomaterials of platinum/cerium oxide/multi-walled carbon nanotubes (Pt/CeO2/MWCNTs) are synthesized successfully via impregnation and polyol processes. MWCNTs serve as an excellent supporter where CeO2 nanoparticles are decorated with well-distributed Pt nanoparticles. Images show the average particle size of crystalline Pt and CeO2 on MWCNTs are 3–7 and 20–30 nm, respectively. In electrochemical reaction, the redox peak of Pt/CeO2-700°C/MWCNTs reveals lower potential and higher current density in methanol electro-oxidation than those of other Pt-based ones. The study indicates that the cerium oxide in Pt/CeO2-700°C/MWCNTs catalyst will enhance significantly the oxygen ions transportation between the interface of Pt and MWCNTs to eliminate the CO poison effect on Pt catalyst.

Raman Spectroscopy of LiFePO_4 and Li_3V_2(PO_4)_3 Prepared as Cathode Materials

Structure of samples of lithium iron vanadium phosphates of different compositions were investigated by X-rays, electron microscopy and Raman spectroscopy. The investigated salts were mainly of olivine-like and NASICON-like structures. The X-ray diffraction and the Raman scattering show different crystalline structures, which is probably caused by difference between cores of the crystallites (probed by X-rays) and their shells (probed by the Raman scattering). Most of the Raman spectra were identified with previously published data, however in the samples with high vanadium concentration we have observed new, not reported earlier modes at 835 cm−1 and 877 cm−1, that we identified as oscillations related to V2O4− 7 or VO 3− 4 anions.

Preparation and characterization of nanosized ZnGa/sub 2-x/Cr/sub x/O4 phosphors

Cr-doped zinc gallate phosphors were prepared via citric gel route and their luminous properties were characterized by photoluminescence in this study. From the results of XRD studies, it was found that spinel phase formed exclusively in the ZnGa/sub 2-x/Cr/sub x/O/sub 4/ phosphors (O /spl les/ x /spl les/ 0.010) prepared by heat-treatment at 700/spl deg/C for 5 hours and the lattice constant increased linearly from 8.36 to 8.38 /spl Aring/ with the amount of Cr/sup 3+/-doping. The photoluminescence studies manifested that the absorbance at wavelengths of 437 and 580 nm. All of the prepared phosphors exhibit the characteristic emission peaks at wavelengths of 669, 680, 688, 694, 708, and 713 nm independent of the amount of Cr/sup 3+/-doping as they were excited with lights of wavelengths of 437 and 580 nm.