Three-Dimensional Macroporous Graphene–Li₂FeSiO₄ Composite as Cathode Material for Lithium-Ion Batteries with Superior Electrochemical Performances

Three-dimensional macroporous graphene-based Li₂FeSiO₄ composites (3D-G/Li₂FeSiO₄/C) were synthesized and tested as the cathode materials for lithium-ion batteries. To demonstrate the superiority of this structure, the composite’s performances were compared with the performances of two-dimensional graphene nanosheets-based Li₂FeSiO₄ composites (2D-G/Li₂FeSiO₄/C) and Li₂FeSiO₄ composites without graphene (Li₂FeSiO₄/C). Due to the existence of electronic conductive graphene, both 3D-G/Li₂FeSiO₄/C and 2D-G/Li₂FeSiO₄/C showed much improved electrochemical performances than the Li₂FeSiO₄/C composite. When compared with the 2D-G/Li₂FeSiO₄/C composite, 3D-G/Li₂FeSiO₄/C exhibited even better performances, with the discharge capacities reaching 313, 255, 215, 180, 150, and 108 mAh g^–1 at the charge–discharge rates of 0.1 C, 1 C, 2 C, 5 C, 10 C and 20 C (1 C = 166 mA g^–1), respectively. The 3D-G/Li₂FeSiO₄/C composite also showed excellent cyclability, with capacity retention exceeding 90% after cycling for 100 times at the charge–discharge rate of 1 C. The superior electrochemical properties of the 3D-G/Li₂FeSiO₄/C composite are attributed to its unique structure. Compared with 2D graphene nanosheets, which tend to assemble into macroscopic paper-like structures, 3D macroporous graphene can not only provide higher accessible surface area for the Li₂FeSiO₄ nanoparticles in the composite but also allow the electrolyte ions to diffuse inside and through the 3D network of the cathode material. Specially, the fabrication method described in this study is general and thus should be readily applicable to the other energy storage and conversion applications in which efficient ionic and electronic transport is critical

Oxygen Vacancies and Stacking Faults Introduced by Low-Temperature Reduction Improve the Electrochemical Properties of Li₂MnO₃ Nanobelts as Lithium-Ion Battery Cathodes

Among the Li-rich layered oxides Li₂MnO₃ has significant theoretical capacity as a cathode material for Li-ion batteries. Pristine Li₂MnO₃ generally has to be electrochemically activated in the first charge–discharge cycle which causes very low Coulombic efficiency and thus deteriorates its electrochemical properties. In this work, we show that low-temperature reduction can produce a large amount of structural defects such as oxygen vacancies, stacking faults, and orthorhombic LiMnO₂ in Li₂MnO₃. The Rietveld refinement analysis shows that, after a reduction reaction with stearic acid at 340 °C for 8 h, pristine Li₂MnO₃ changes into a Li₂MnO₃–LiMnO₂ (0.71/0.29) composite, and the monoclinic Li₂MnO₃ changes from Li_2.04Mn_0.96O₃ in the pristine Li₂MnO₃ (P–Li₂MnO₃) to Li_2.1Mn_0.9O_2.79 in the reduced Li₂MnO₃ (R-Li₂MnO₃), indicating the production of a large amount of oxygen vacancies in the R-Li₂MnO₃. High-resolution transmission electron microscope images show that a high density of stacking faults is also introduced by the low-temperature reduction. When measured as a cathode material for Li-ion batteries, R-Li₂MnO₃ shows much better electrochemical properties than P-Li₂MnO₃. For example, when charged–discharged galvanostatically at 20 mA·g^–1 in a voltage window of 2.0–4.8 V, R-Li₂MnO₃ has Coulombic efficiency of 77.1% in the first charge–discharge cycle, with discharge capacities of 213.8 and 200.5 mA·h·g^–1 in the 20th and 30th cycles, respectively. In contrast, under the same charge–discharge conditions, P-Li₂MnO₃ has Coulombic efficiency of 33.6% in the first charge–discharge cycle, with small discharge capacities of 80.5 and 69.8 mA·h·g^–1 in the 20th and 30th cycles, respectively. These materials characterizations, and electrochemical measurements show that low-temperature reduction is one of the effective ways to enhance the performances of Li₂MnO₃ as a cathode material for Li-ion batteries

Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge

Ag/AgX/BiOX (X = Cl, Br) three-component visible-light-driven (VLD) photocatalysts were synthesized by a low-temperature chemical bath method and characterized by X-ray diffraction patterns, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and UV–vis diffuse reflectance spectra. The Ag/AgX/BiOX composites showed enhanced VLD photocatalytic activity for the degradation of rhodamine B, which was much higher than Ag/AgX and BiOX. The photocatalytic mechanisms were analyzed by active species trapping and superoxide radical quantification experiments. It revealed that metallic Ag played a different role for Ag/AgX/BiOX VLD photocatalysts, surface plasmon resonance for Ag/AgCl/BiOCl, and the Z-scheme bridge for Ag/AgBr/BiOBr