The effect of low‐level chromium substitution in Na3V2‐xCrx(PO4)3 (0≤x≤0.4), a potential cathode material in Na‐ion cells, has been examined. A suitable synthesis procedure is developed to obtain composites of crystalline NASICON phosphate and an amorphous carbon phase to enhance the electrical conductivity of the electrode. The optimized cathode materials were characterized by using X‐ray diffraction and spectroscopic techniques. The electrochemical evaluation was carried out by using galvanostatic and potentiostatic methods. The activity of the V5+/V4+ redox couple at approximately 4 V is remarkable for the samples containing chromium. A reversible capacity of 107 mAh g−1 with a coulombic efficiency of 99 % was determined for Na3V1.9Cr0.1(PO4)3 after 40 cycles. The observed performance correlates with a good kinetic response, resulting from low charge‐transfer resistance and high diffusion coefficient.

Na 3 V 2-x Fe x (PO 4 ) 3 /C (0 ≤ x ≤ 0.5) samples were prepared by a simple sol-gel method. X-ray diffraction patterns revealed high purity and crystalline phosphate indexed in the R-3c space group. On increasing the iron content, the cell volume increases due to the slightly higher radius of Fe 3+ . Cyclic voltammetry showed two signals at ca. 3.5 and 4.0 V, which evidence a linear dependence of potential on the level of substitution. The appearance of the short plateau at ca. 4.0 V is closely related to the higher iron substitution levels. The evaluation of XPS and 57 Fe Mössbauer spectra of charged and discharged electrodes allow ascribing the 4.0 V plateau to V 4+ /V 5+ redox reaction and the extraction/insertion of extra sodium ions from octahedral M1 sites. Galvanostatic cycling showed capacity values as high as 115 mA h g −1 for samples with x = 0.1, 0. Now that lithium-ion battery (LIB) technology is reaching maturity, a number of reports are advising for the future scarcity of the lithium resources. A full implementation of LIB in the electric vehicle market would involve important constraints on Li production and an undesirable increase of the cost of Li is expected at the long term.1-4 For these reasons, market is alternatively looking forward beyond LIB and new chemistries are being explored. [5][6][7][8] Sodium is an abundant and non-toxic alkali element which can be implemented in insertion based batteries. The electrochemical principles are identical to those of lithium batteries. In fact, a number of recent reports have demonstrated the reliability of sodium batteries to provide reversible energy storage devices. [9][10][11][12] Nevertheless, the larger ionic radius and atomic weight leads to a less performing behavior. Thus, low theoretical capacity and cell voltage, slow diffusion, high unit cell volume change and hence structural impact are detected in electrodes subjected to sodium insertion. Recently, Ceder et al. have reported that the lower voltage for sodium insertion is predominantly a cathodic effect because of the diminished energy gain when inserting this alkaline ion into the host structure. Also, they pointed out to open structures, as layered and NASICON ones, as the most favored frameworks to provide phase stability on cycling. 13 The optimistic arguments promoting these cathode materials are mainly based on an easy transfer reaction of less solvated large ions and high sodium conductivity. 14,15 In this regard, transition metal phosphates with general stoichiometry A x B 2 (PO 4 ) 3 (A: Li + , Na + , etc; B: Fe 3+ , V 3+ , Ti 4+ .) offer a wide compositional variety exhibiting a NASICON-type structure. [16][17][18] Among them, Na 3 V 2 (PO 4 ) 3 has demonstrated interesting properties as a cathode material because of the high operating voltage and good cyclability. [19][20][21][22] Multivalent vanadium atom lends a significant electronic conductivity which promotes the chemical diffusion of sodium ions to this compound. Nevertheless, the electronic conductivity is not high enoug...

No abstract

No abstract

Mesoporous FeC 2O 4 was prepared by dehydration of bulk monoclinic- and micellar orthorhombic FeC 2O 4.2H 2O precursors at 200 degrees C. The micellar material shows nanoribbon shaped particles, which are preserved after dehydration. These solids are used as high-capacity lithium storage materials with improved rate performance. The mesoporous nanoribbons exhibit higher capacities close to 700 mA h/g after 50 cycles at 2C (C = 1 Li h (-1) mol (-1)) rate between 0 and 2 V.