Wenpei Kang

The catanionic surfactant-assisted syntheses of 26-faceted and hexapod-shaped Cu2O and their electrochemical performances

Crystalline materials with a well-defined morphology and/or a narrow size distribution might exhibit a specific shape- and/or size-dependent performances. In the first instance, the catanionic surfactants of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium hydroxide (CTAOH) were added as crystal modifiers into the 60 °C reaction systems of copper chloride and sodium hydroxide for the syntheses of cuprous oxide (Cu2O). Then, the reversible reaction activity of crystalline Cu2O with metal lithium was conducted to investigate its electrochemical performance as rechargeable lithium ion battery anodes. The presence of SDS-rich catanionic surfactants could induce the formation of polyhedral Cu2O structures with 8 triangular {111}, 6 square {100}, and 12 rectangular {110} faces outside, while the presence of CTAOH-rich catanionic surfactants, especially the doping methanol in CTAOH, led to the generation of hexapod-shaped Cu2O mesostructures with tiny nanoparticles on these symmetrical branches. At a discharge–charge cycling current of 80 mA g−1, the 26-faceted Cu2O crystals with rough {110} faces displayed an initial capacity of 756 mA h g−1 and a reversible capacity of 280 mA h g−1 at the first cycling. In comparison with the electrochemical performance of Cu2O hexapod-shaped mesocrystals at the same cycling current, the 26-faceted crystals of Cu2O could be capable of remaining a relatively high capacity (∼145 mA h g−1) and keeping an excellent Coulombic efficiency (∼100%) over 50 discharge–charge cycles. As a whole, the catanionic surfactants at different anionic/cationic molar ratios were used as additives to highlight the secondary nucleation and growth mechanism for the formation of Cu2O, then, the resulting 26-faceted crystallites and hexapod-shaped mesoparticles were separately used as active materials in the assembled Cu2O/Li half-cells to study their shape-dependent electrochemical performances.

Ethylene glycol-assisted nanocrystallization of LiFePO4 for a rechargeable lithium-ion battery cathode

Hydro- and/or solvo-thermal nanocrystallization of lithium iron phosphate (LiFePO4) has been well recognized as an effective pathway to improve its electrochemical performances. Herein, it is reported that high-performance LiFePO4 single-crystalline nanoparticles have been successfully prepared by the additive-free solvothermal reaction and subsequent glucose-assisted calcination. In comparison with the similar hydrothermal reaction, the presence of ethylene glycol can unexpectedly induce the formation of gel-like intermediates at the time interval of 0.5 h, resulting in LiFePO4 nanoparticles with a three-dimensional (3D) lattice structure and an amorphous nanocoating for a reaction time of 2 h. Interestingly, the lattice structure of growing LiFePO4 crystals can be thoroughly damaged under the irradiation of an electron beam. Furthermore, after the continuous crystal growth and subsequent heat-treatment, nanocrystalline LiFePO4 can achieve a discharge capacity of ∼165 mAh g−1 at 0.1 C in assembled LiFePO4/Li half-cells, proving a successful nanocrystal-forming engineering of LiFePO4 for a lithium-ion battery cathode.

A chemical composition evolution for the shape-controlled synthesis and energy storage applicability of Fe3O4–C nanostructures

Temporarily stabilized iron oxychloride (FeOCl) nanospindles have been collected for the first time shortly after the forced hydrolysis of iron(III) chloride (FeCl3) in the reaction medium of glycerol and water (1 : 7, v/v) at 145 °C. In this paper, a novel chemical composition evolution of orthorhombic FeOCl to tetragonal akaganeite (β-FeOOH) and then to cubic magnetite (Fe3O4) has been successfully used for the shape-controlled synthesis of Fe3O4–C spindle-like nanocomposites. During this evolution process, the crystal structures of spindle-like intermediates have been investigated, along with the random doping of amorphous carbon into the final products. As a lithium ion battery anode, Fe3O4–C composite nanospindles can give a significantly high initial coulombic efficiency (80.6%), a reversible discharge capacity of 1029 mA h g−1 at 200 mA g−1 over 100 cycles, and the 100th retention value of 711.6 mA h g−1 at a high current rate of 1000 mA g−1. Therefore, a combination of the fine nanofabrication of Fe3O4 crystals with a spindle-like shape and the random doping of amorphous carbon may offer an effective approach to the development of transition metal oxide-based anode materials for high-energy lithium ion batteries.

Influence of cobalt content on the electrochemical properties of sheet-like 0.5Li2MnO3·0.5LiNi1/3+xCo1/3−2xMn1/3+xO2 as lithium ion battery cathodes

The promising cathode materials of serial 0.5Li2MnO3·0.5LiNi1/3+xCo1/3−2xMn1/3+xO2 (x = 1/12, 1/24, 0, −1/24 and −1/12) solid solutions have been synthesized by a polymerization–pyrolysis-assisted crystallization route. X-Ray diffraction (XRD) and scanning electron microscope (SEM) characterization show that all the powdered solid solutions possess a two-dimensional sheet-like superstructure composed of crystalline nanoparticles. Galvanostatic charging–discharging tests exhibit that, as high-capacity cathodes, the initial coulombic efficiency and cycling stability of these sheet-like configurations depend upon the cobalt content (i.e., the value of (1/3 − 2x) difference) in chemical formula 0.5Li2MnO3·0.5LiNi1/3+xCo1/3−2xMn1/3+xO2, giving an optimal x value in the range of 0 and 1/12. Considering the initial activation of component Li2MnO3 and the unwanted phase change of Co-contained component LiNi1/3+xCo1/3−2xMn1/3+xO2 at a charging voltage higher than 4.55 V, the cyclability and electrode polarization of serial sheets have also been estimated within a narrow electrochemical window of 2.0 and 4.3 V. To compare with the electrochemical parameters of serial solid solutions obtained within 2.0 and 4.7 V, therein an extremely high capacity retention ratio of 99.2% (x = 1/12) or 95.4% (x = 0) is observed at 0.5 C over 50 charge–discharge cycles.

The controlled synthesis and improved electrochemical cyclability of Mn-doped α-Fe2O3 hollow porous quadrangular prisms as lithium-ion battery anodes

A two-step process of initial oxalate co-precipitation and subsequent thermal decomposition facilitates the formation of hydrated oxalate precursors with hollow quadrangular prism shapes, and then confers a porous nature for the prismatic shells of synthetic hematite (α-Fe2O3) and its Mn-doped derivative. When applied as lithium-ion battery anodes, Mn-doped α-Fe2O3 exhibits an improved electrochemical performance compared with undoped α-Fe2O3. At a current density of 200 mA g−1, the pure α-Fe2O3 electrode gives an initial discharge capacity of ∼1280 mA h g−1 with a low retention ratio of 13.9% (i.e., capacity ∼ 178 mA h g−1) over 80 cycles, while the Mn-doped product, rhombohedral Fe1.7Mn0.3O3, delivers a relatively low initial value of ∼1190 mA h g−1 and retains an 80th cycle reversible capacity of ∼1000 mA h g−1 (i.e., retention ratio ∼ 84.0%). These, together with the better high-rate capability and the lower charge-transfer resistance of the Mn-doped α-Fe2O3 anode, simultaneously demonstrate a successful mass production of hollow porous configurations and an effective doping with elemental Mn for potential application.