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Na3VCr(PO4)3 has attracted great attention due to its high energy density and stable structure. The substitution of vanadium with chromium led to an increase in the redox potential from 3.4 to 4.2 V (vs. Na+/Na). In this work, the electrochemical performance of Na3VCr(PO4)3 (NVCP) in Mg cells is successfully investigated. Reversible multielectron Mg2+/Na+ extraction/insertion is observed. Upon charging, NVCP exhibits two plateaus at 1.75 V and 2.3 V vs. Mg2+/Mg0 achieving a maximum reversible capacity of 85 mA h g−1 (a 1.45 electron reaction) enabling oxidation to VV as determined by 51V NMR and XPS, and accompanied by extraction of sodium from Na2 sites according to 23Na NMR. During the subsequent cycles Mg2+/Na+ ions react at 2.3 and 1.7 V, and an additional peak of Mg2+ at 1.2 V is observed. Alternatively, electrochemical cells were also assembled with activated carbon in a 0.1 M Mg(TFSI)2 electrolyte, leading to similar profiles and capacity even at higher current density. Finally, the chemical desodiation of NVCP using I2, Cl2 and NO2BF4 is performed to avoid mobile sodium ions. XRD, XPS and 51V NMR confirmed the contraction of the cell and the oxidation to VIV and VV. Iodine was not capable of full sodium extraction. Despite this fact, the ability of this sample to retain 65 mA h g−1 during a few cycles evidences the reversibility of magnesium insertion. The use of stronger oxidants such as Cl2 or NO2BF4 allowed the increase of the initial OCV and resulted in a plateau at 4 V. The chemical desodiation was also accompanied by an enhanced capacity decrease which could be correlated with the structural degradation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Titania nanotubes (nt-TiO2) with high aspect ratio and amorphous character were prepared by Ti anodization at high voltage (100 V). After annealing, the nanotubes crystallized to anatase phase and the XRD patterns and TEM images showed that self-organized anatase nt-TiO2 are composed of crystallographically oriented nanocrystalline domains on the Ti substrate. The contribution of faradic and pseudocapacitive processes was studied for anatase nt-TiO2 in Na cells obtaining the b-parameter values from cyclic voltammetry experiments. The reaction between anatase nt-TiO2 and Na is dominated by redox surface processes (pseudocapacitance) in the region over ca. 0.5 V and by irreversible faradic processes below ca. 0.3 V. The cycling performance of anatase nt-TiO2 in sodium cell was very strongly affected by the imposed electrochemical conditions. Reversible capacity values of about 150-200 mA h g-1 and good cycling behaviour were found for 2.6-0.5 V of potential limits. Impedance spectra and 23Na MAS NMR spectra were also used to study the reaction mechanisms. In addition, it was found that by using variable voltage during the anodization of titanium, the resulting electrode exhibits excellent cycling behaviour (ca. 190 mA h g-1 after ca. 800 cycles). These results are the best cycling behaviour reported to date for anatase in the form of self-organized nanotubes for sodium ion microbatteries. [ABSTRACT FROM AUTHOR]

Amorphous Co–Sn alloy was co-electroplated on a three-dimensional framework of nickel foam and then CoSn2 phase was formed after annealing at 200 °C. It is observed that short electrodeposition times favour a higher x-value in the resulting CoSnx alloy. Low current densities and short electrodeposition times favour the formation of discrete submicrometric particles (Volmer–Weber model). The morphology of the electrode allows achieving high areal and gravimetric capacity values under high rates of charge–discharge in Li cells. The observed areal capacity values are in the range between 0.2 and 6 mAh cm−2, depending on the cycling rate and the mass of electrodeposited alloy. For electrodes with short electrodeposition time and Volmer–Weber model, the response to high charge–discharge rates is excellent and the gravimetric capacity is around 800 mAh g−1. [ABSTRACT FROM AUTHOR]

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.