Upadhyayula V Varadaraju

A new hydrogenophosphate Mn(H2PO4)2 has been synthesized from an aqueous solution. Its ab initio structure resolution shows that the original layered structure of this phase consists of PO 2(OH)2 tetrahedra and MnO5OH octahedra, sharing corners to form [MnP2O8H4]∞ layers, whose cohesion is ensured through hydrogen bonds. The excitation and emission spectra of this phase are characteristic of Mn2+ species. This phosphate is shown to be a good protonic conductor with a conductivity of 10-4.4 S/cm at 90°C (363 K). © 2008 American Chemical Society.

We demonstrate lithium insertion into brookite TiO2 in the nanophase regime. The extent of lithium insertion is significantly influenced by the crystallite size. A maximum of 0.95 Li/ TiO2 can be inserted into 10 nm size crystallites and the extent of lithium insertion is low (0.23 Li) in 33 nm crystallites. The reversibility decreases with an increase in crystallite size. The contrasting behavior of brookite and rutile TiO2 suggests that the structural features of brookite TiO2 may play an important role in determining Li insertion behavior. © 2008 The Electrochemical Society.

Lithium intercalation in phase-pure nanocrystalline brookite TiO2 is demonstrated for the first time. Galvanostatic studies show that 0.9 Li per formula unit can be intercalated into this phase in initial discharge. Ex situ X-ray diffraction studies on the electrodes at different levels of lithium intercalation show that the structure is stable toward lithium intercalation and deintercation. In the initial charge, an irreversible capacity loss is observed. However, on further cycling, the phase shows excellent cycling behavior. A reversible capacity of 170 mAhg-1 is observed even after 40 cycles. © 2006 The Electrochemical Society.

The exchange of lithium for proton in VO(H2PO4)2 has been studied. Beside the continuous exchange from VO(H2PO4)2 to Li2H2VO(PO4)2, a new cathode material Li4VO(PO4)2 has been synthesized, whose structure is closely related to that of VO(H2PO4)2. The electrochemical evaluation of Li4VO(PO4)2 vs. Li shows that it undergoes reversible lithium deintercalation/intercalation at high voltage, ∼4.0 V with a reversible capacity of ∼70 mAh/g. © 2006 Elsevier B.V. All rights reserved.

The electrochemical reactivity of the layered titanium hydrogeno phosphate Ti(HPO4)2·H2O versus lithium has been studied. Lithium intercalation occurs at ∼2.5 V with low polarization, leading to a new lithiated Ti(III) phase, LiTi(HPO4)2·H2O. Ti(HPO4)2·H2O exhibits a reversible capacity of 80 mAh g-1 in the voltage window 1.8-3.5 V at C/10 rate. The stable reversible capacity reveals that the presence of H2O lattice is not affecting the electrochemical reaction. The reversibility of the reaction is demonstrated by extracting lithium from LiTi(HPO4)2·H2O and the host structure is intact. The electrochemical behaviour of dehydrated phases Ti(HPO4)2 and TiP2O7 has also been investigated. © 2007 Elsevier B.V. All rights reserved.

A new vanadium diphosphate, Li2VOP2O7, has been synthesized by ion exchange from Na2VOP2O7, using an eutectic mixture of {0.4LiOH·H2O-0.6LiNO3} at 200 °C. It crystallizes in space group P21/c, with the lattice parameters a = 7.4674(8) Å, b = 12.442(2) Å, c = 6.2105(7) Å and β = 97.79(1)°. The crystal structure of Li2VOP2O7, refined by powder X-ray diffraction data, shows that the structure of the parent Na-phase is retained but a prominent decrease in the layer spacing is observed. Li2VOP2O7 has been tested as a cathode material for Li-ion battery. One lithium is deintercalated by charging to 4.6 V, however, on discharge only about 0.5 Li is re-intercalated. © 2007 Elsevier Masson SAS. All rights reserved.

The synthesis of a mixed valent LixEuIIxEuIII0.33-xZr2(PO4) 3 with the NZP structure, using soft chemistry was reported. Stoichiometric amounts of starting materials Eu2O3 and ZrOCl2.8H2O were dissolved in 2 N HNO3, addition of NH4H2PO4 to the metal nitrate solution under constant stirring resulted in a colorless gel. Electrochemical lithium insertion studies were carried out by using Swagelok type cells with lithium metal as the negative electrode. The mixture was pressed onto a stainless steel plate to form the electrode. The photoluminescence (PL) spectrum of the reduced phase shows signature of both Eu3+ and Eu 2+ excitation and emission bands. It was observed that The CIE coordinates are significantly changed upon Li insertion and shift towards the white region as the concentration of Li increases in the host.

The monoclinic form of FeOHSO4 was prepared by dehydration of FeSO4·7H2O. We show that reversible insertion of up to ∼1Li/f.u. is possible in this compound at an average voltage of 3.2 V. The insertion/deinsertion is a biphasic process. The high voltage plateau, a reversible capacity of 110 mAh/g after 20 cycles and good cycling behavior make this compound an attractive positive electrode material for rechargeable Li-ion batteries, suggesting also that transition metal sulphates need to be explored. © 2009 Elsevier B.V. All rights reserved.

A new V(III) lithium phosphate Li5VO(PO4)2 has been synthesized by electrochemical insertion of lithium into Li4VO(PO4)2. This phase, which crystallizes in the space group I4/mcm, exhibits a tunnel structure closely related to the layered structure of Li4VO(PO4)2 and to the tunnel structure of VO(H2PO4)2. The topotactic reactions that take place during lithium exchange and intercalation, starting from VO(H2PO4)2 and going to the final phase Li5VO(PO4)2 are explained on the basis of the flexible coordinations of V4+ and V3+ species. The electrochemical and magnetic properties of this new phase are also presented and explained on the basis of the structure dimensionality. © 2008.

Nanocrystalline rutile TiO2 is prepared at RT from acidic solution by sol-gel method using titanium tetraisopropoxide as precursor. Samples of varying crystallite sizes are prepared by post annealing the as synthesized rutile TiO2 at different temperatures. The absorption spectra of synthesized samples reveal a large blue shift (311 nm) vis a vis bulk rutile TiO2 (394 nm) indicating the nanocrystalline nature of the material. Electrochemical studies performed at RT show that one Li per formula unit is inserted into the nanocrystalline rutile TiO2. Variation in the voltage profiles is observed with respect to small changes in the crystallite sizes within the nanometric regime. © 2006 Elsevier B.V. All rights reserved.