Shoufeng Yang

Hydrothermal synthesis of lithium iron phosphate cathodes

Hydrothermal methods have been successfully applied to the synthesis of lithium iron phosphates. Li3Fe2(PO4)3 was synthesized by heating at 700°C LiFePO4(OH), formed hydrothermally in an oxidizing environment. Crystalline LiFePO4 was formed in a direct hydrothermal reaction in just a few hours, and no impurities were detected. This result leads to the possibility of an easy scale-up to a commercial process. The samples were characterized by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Both phosphates were tested as the cathode in lithium batteries and showed results comparable to those formed by conventional high-temperature synthesis.

Reactivity, stability and electrochemical behavior of lithium iron phosphates

LiFePO4 is a potential cathode candidate for the next generation of secondary lithium batteries. Its reactivity and thermodynamic stability have been determined. At low potentials it can be reduced to lithium phosphate and iron. The fully charged state, orthorhombic FePO4, is metastable relative to the trigonal all tetrahedral form; however, the massive structural rearrangement necessary makes the structural change kinetically unfavorable at room temperature. LiFePO4 has been prepared by a variety of routes. When synthesized at elevated temperatures in the presence of a carbon gel, only LiFePO4 was detected by X-ray diffraction even when the starting material was LiFePO4(OH). At a C/2 discharge/charge rate, LiFePO4 retained about 80% of the theoretical capacity cycling at room temperature. The hydrothermal form shows some iron disorder, which impacts its electrochemical and chemical reactions.

Anodes for lithium batteries: tin revisited

Abstract Pure tin, without the addition of conducting diluents or binders, has been evaluated as an anode in lithium cells. Capacities approaching 600 mAh/g are maintained for over 10 deep cycles, before falling off, indicating the inherent reversibility of the tin anode. These are comparable to those reported for Cu 6 Sn 5 and considerably higher than for deposited tin films. The tin grain size was determined and found to decrease with cycling from over 400 to below 100 nm over 10 cycles. The cell impedance increases significantly after 10 cycles, consistent with the observed loss of capacity on extended cycling.

Structures of potassium, sodium and lithium bis(oxalato)borate salts from powder diffraction data.

The crystal structures of the alkali-metal bis(oxalato)borate salts A[B(C2O4)2] (A = K, Na, Li) have been determined ab initio using powder diffraction data obtained from a laboratory diffractometer. The K compound crystallizes in the orthorhombic space group Cmcm and its structure has been solved by direct methods applied to the integrated intensities from full pattern decomposition. The Na compound is isostructural with the K salt, while the crystal structure of the highly hydroscopic Li compound differs from the other two. It has an orthorhombic lattice, space group Pnma, and its structure was solved by the global optimization method using a parallel tempering approach. In the K and Na structures the metal ions and complex borate ions form chains with m2m symmetry. Metal-oxygen bonding between the chains links them into a layer and then a framework with square tunnels. The coordination number of both K and Na is eight. The Li compound also contains chains that have .m. symmetry and are bound together into a three-dimensional framework. The coordination polyhedron of the Li atom is a square pyramid with Li lying in its base. This square pyramidal coordination leads to its high reactivity with moisture to give Li[B(C2O4)2]H2O with lithium in six coordination.

Structural chemistry of new lithium bis(oxalato)borate solvates.

Recently lithium bis(oxalato)borate, LiB(C2O4)2, has been proposed as an alternative lithium salt for the electrolyte in rechargeable batteries that do not contain explosive perchlorate, reactive fluoride or toxic arsenic. This lithium salt crystallizes in the form of solvates from such solvents as water, acetonitrile, acetone, dimethoxyethane, 1,3-dioxolane and ethylene carbonate. Their crystal structures were determined in order to explore the crystal chemistry of this lithium salt. It was found that most of the solvents consist of a lithium bis(oxalato)borate dimer in which the ligand acts as both a chelating and a bridging agent. Lithium has octahedral coordination that typically includes one or, less commonly, two solvent molecules. An exception to this rule is the ethylene carbonate solvate where the lithium is tetrahedrally surrounded exclusively by the solvent and bis(oxalato)borate plays the role of counter-ion only. The ethylene carbonate solvates were also studied for LiPF6 and LiAsF6 salts and they have similar structures to the bis(oxalato)borate tetrahedral complexes.

Vanadium Oxide Nanotubes: Characterization and Electrochemical Behavior

Abstract : Vanadium oxide nanotubes (VONT) were formed from vanadium (V) oxide and the dodecylamine templating agent by a sol-gel reaction and subsequent hydrothermal treatment. The nanotubes were characterized by transmission electron microscopy (TEM), electron diffraction, thermogravimetric analysis (TGA), infrared spectroscopy and powder X-ray diffraction (XRD). The nanotubes consist of VO2.4C12H28N0.27 and range in diameter from 100 nm to 150 nm. The study further reveals that the compound maintained the tubular morphology when heated at 430 deg C in an inert atmosphere. However, the tubular morphology is destroyed when the compound is heated at about 130 deg C in oxygen. Organic free manganese intercalated vanadium oxide nanotubes (MnVONT) were synthesized by an ion exchange reaction. The previously mentioned techniques were used to characterize MnVONT. Mn(0.86)V7O(16+sigma). nH2O layers have 2D tetragonal cell with a = 6.157(3) A, while interlayer spacing is 10.52 (3) A. VONT, heated VONT and Mn(0.86)V7C(16+sigma). nH2O are redox - active and can insert lithium reversibly. This study reveals that the electrochemical performance of VONT is enhanced by removing the organic template by heating in an inert atmosphere or exchanging with Mn(2+) ions.

Nanocomposite Electrodes for Advanced Lithium Batteries: The LiFePO4 Cathode

Abstract : LiFePO4 was successfully synthesized by high temperature and hydrothermal synthesis. A nanocomposite was formed by carbon coating this material; initial electrochemical results showed that up to 70% capacity could be obtained at 1.0 mA/sq cm current density. In contrast, the hydrothermally prepared LiFePO4 showed a lower capacity even at lower discharge rates due to a partial occupation of lithium sites by iron. This occupation, identified by Rietveld X-ray refinement, decreased both the rate and degree of intercalation and dc-intercalation of lithium; chemical reaction with butyl lithium and bromine confirmed the electrochemical behavior. This investigation showed that the cathode could be prepared by high temperature synthesis, followed by a carbon black coating to achieve high capacity at high current density.

Sn and SnBi Foil as Anode Materials for Secondary Lithium Battery

In order to better understand the cycling mechanism of metal alloy anodes, and to mitigate the capacity fade observed in lithium battery use a study of simple systems was initiated. Tin foil and tin-bismuth mixtures were chosen because there is no need for conductive diluents or binders so that the intrinsic behavior could be observed. A pure tin foil was found to react rapidly with lithium, ≥ 3 mA/cm2, and with no capacity fade for over 10 cycles. This is better than tin powder or electrodeposited tin. After the first cycle, the foil reacts with Li following a stepwise formation of different alloys as dictated by the thermodynamics. Incorporation of bismuth into the foil increased the capacity fade after the first few cycles, with the eutectic composition Sn0.57Bi0.43 having better capacity retention than the Sn0.5Bi0.5 composition. XRD and SEM-EDS shows that bismuth is rejected from the tin rich phase during lithium insertion and is not reincorporated on lithium removal, just as expected from the phase diagram.