RESUMEN
The recent introduction of glyme-based solvents has opened new opportunities to characterize graphitic materials as anodes for sodium-ion batteries. We evaluated the electrochemical behaviour of a graphitized carbon nanofiber for the first time. X-ray diffraction, electron paramagnetic resonance and nuclear magnetic resonance allowed the sodium insertion mechanism to be untangled, in which the occurrence of an activation process during the first discharge enhances sodium accessibility to active redox centres at the interlayer space. Morphological changes observed by electron microscopy could be responsible for this behaviour. A fully graphitized carbon nanofibers/NaPF6(diglyme)/Na3V2(PO4)3 sodium-ion battery was tested to probe the reliability of this graphitic nanostructure as a negative electrode.
RESUMEN
This study reports on the electrochemical alloying-dealloying properties of Mg2Sn intermetallic compounds. 119Sn Mössbauer spectra of β-Sn powder, thermally alloyed cubic-Mg2Sn, and an intermediate MgSn nominal composition are used as references. The discharge of a Mg/micro-Sn half-cell led to significant changes in the spectra line shape, which is explained by a multiphase mechanism involving the coexistence of c-Mg2Sn, distorted Mg2−δSn, and Mg-doped β-Sn. Capacities and capacity retention were improved by using nanoparticulate tin electrodes. This material reduces significantly the diffusion lengths for magnesium and contains surface SnO and SnO2, which are partially electroactive. The half-cell potentials were suitable to be combined versus the MgMn2O4 cathodes. Energy density and cycling properties of the resulting full Mg-ion cells are also scrutinized.
RESUMEN
An anatase nanotube array has been prepared with a special morphology: two concentric walls and a very small central cavity. The method used here to achieve the double-wall structure is a single-step anodization process under a voltage ramp. Thanks to this nanostructure, which is equivalent to a fractal electrode, the electrochemical behaviour is improved, and the specific capacity is higher in both lithium and sodium cells due to pseudocapacitance. The double-wall structure of the nanotube enhances the surface of TiO2 being in contact with the electrolyte solution, thus allowing an easy penetration of the alkali ions into the electrode active material. The occurrence of sodium titanate in the electrode material after electrochemical reaction with sodium is studied by using EPR, HRTEM and NMR experiments.
RESUMEN
The preparation of graphene/iron oxyhydroxide hybrid electrode material with very homogeneous distribution and close contact of graphene and amorphous iron oxyhydroxide nanoparticles has been achieved by using high-intensity ultrasonication. Due to the negative charge of the graphene surface, iron ions are attracted toward the surface of dispersed graphene, according to the zeta potential measurements. The anchoring of the FeO(OH) particles to the graphene layers has been revealed by using mainly TEM, XPS and EPR. TEM observations show that the size of the iron oxide particles is about 4 nm. The ultrasonication treatment is the key parameter to achieve small particle size in these graphene/iron oxyhydroxide hybrid materials. The electrochemical behavior of composite graphene/amorphous iron oxyhydroxide prepared by using high-intensity ultrasonication is outstanding in terms of gravimetric capacity and cycling stability, particularly when metallic foam is used as both the substrate and current collector. The XRD-amorphous character of iron oxyhydroxide in the hybrid electrode material and the small particle size contribute to achieve the improved electrochemical performance.
RESUMEN
A sonochemical method has been used to prepare negative electrode materials containing intermetallic nanoparticles and polyacrylonitrile (PAN). The ultrasound irradiation is applied to achieve small particle size. After annealing at 490 °C under Ar-flow, the polymer PAN is partially carbonized and the metallic nanoparticles are surrounded by a carbonaceous matrix. The main metallic phase is CoSn(2). The carbonaceous coating and the surface oxides have been explored by using XPS. The resulting CoSn(2)-carbonaceous phase electrode (CoSn(2)@C) shows improved electrochemical behavior (ca. 450 mAh/g after 50 cycles) in comparison with previous reports on pure crystalline CoSn(2). The reaction between CoSn(2)@C and Li has been studied by using XRD and (119)Sn Mössbauer spectroscopy. The formation of large grains of crystalline Li(x)Sn phases after the first discharge is discarded. The small particle size which is achieved by using ultrasonication and the carbonaceous matrix contribute to maintain the Co-Sn interactions during the electrochemical cycling. The aggregation of the nanosized metallic particles upon electrochemical cycling can be suppressed by the carbonaceous matrix (pyrolytic PAN).
