Scanning tunneling microscopy and atomic force microscopy in the characterization of activated graphite electrodes

Sir: To date there have been many methods described to activate carbon electrodes, including electrochemical treatment (1-1 7), laser irradiation (18-21), radio-frequency (RF) plasma (22), and heat treatment (23-26). These methods were developed empirically, and only now is an understanding of parameters controlling surface activity beginning to emerge (20,27). Electrochemical treatment and laser irradiation are particularly attractive treatments because they are relatively inexpensive, are quick, and can be performed without removing the electrode from solution. Activation, common to these procedures, may be attributable to an increase in the exposed edge plane density, which has been associated with faster kinetics (14,20). Copper deposition in conjunction with scanning electron microscopy (SEM) has shown an increase in the density of localized defects on active surfaces (15); an increase in surface activity is associated with an increase in the density of the localized defects (15). Scanning tunneling microscopy (STM), phase detection microscopy, and SEM have also been used to study the effects of electrochemical treatment of highly oriented pyrolytic graphite (HOPG) (13) and glassy carbon (GC) (16,17). These studies have suggested an increase in surface roughness consistent with an increase in the density of exposed edge planes

Raman spectroscopic investigations of Li-intercalated V2O5 xerogel

Xerogel films of vanadium oxide, obtained via spin-coating method, have been analyzed by a micro Raman probe. A comparison has been made among xerogel samples coated at an angular velocity of 1500 rpm (water molar concentration 1.8), the same compounds after vacuum dehydration (water molar concentration 0.5) and the dehydrated gels intercalated with different amounts of Li+ ion. Only minor spectral changes are observed in the xerogel films after the partial dehydration. On the contrary, significant spectral modifications occur after electrochemical intercalation of Li+ ion, even for relatively low concentration (0.15 molar fraction). In fact, the strongest Raman mode of the oxide, occurring at 170 cm(-1), disappears after intercalation. For increasing content of Li+ (up to 0.4 molar fraction) new Raman bands occur between 800 and 950 cm(-1). Accession Number: WOS:A1996WB2940000

Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of OV ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V2O5 (crystal) at about 140 cm(-1). For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra

Metal site determination in doped V 2O 5 arg-like cathodes for lithium battery

Vanadium pentoxide materials prepared through sol-gel processes (xerogel, aerogel and aerogel-like) act as excellent intercalation hosts for lithium or polyvalent cations. Important characteristics are retained when the material is modified by reaction with a selected amount of dopant metal (copper, nickel, zinc, aluminum). The electrochemical performance of the materials has been found to be excellent. X-ray Absorption Spectroscopy (XAS) has been used to probe the local structure of the selected atom sites. The strategy in determining the local structure will be discussed both for pristine and doped materials. The doped samples have been analyzed at the vanadium K-edge as well as at the dopant metal K-edge. The presence of two metals gives two independent absorption signals, even if we are investigating the same compound, for this reason the reliability of the analysis is enhanced

Magnetic resonance studies of chemically intercalated Li xV 2O 5 aerogels

7Li, 51V solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements have been performed upon chemically lithiated Li xV 2O 5 aerogels, with compositions of 1.00<5.84. These compounds can intercalate reversibly large amounts of Li + and, therefore, are of interest as battery cathodes. Still, the mechanism regarding the electron transfer from an inserted lithium metal to a host aerogel V 2O 5 and details regarding the lithium cation environments are not fully understood. Li xV 2O 5 crystals are known to exhibit various structural phase changes and, when multiple phases are present, the capability of the material to intercalate reversibly appears to be adversely affected. On the other hand, aerogels have no such multiphase behavior and aerogel based cathodes exhibit greater stability upon cycling. NMR shows that neither the structure nor the dynamics vary greatly with the amount of lithium content, and that the lithiated aerogel is best described as a single-phase material. Characterization of lithium and vanadium sites is performed through analysis of both NMR and EPR spectra. 7Li line shapes are affected by first-order quadrupolar, magnetic dipolar interactions and motional narrowing. At and above room temperature, relaxation is governed primarily by a quadrupolar mechanism. NMR derived activation energies and diffusion coefficients are different from those of bronzes and electrochemically intercalated V 2O 5. 51V NMR lines, indicative of the presence of V 5+ at all compositions, undergo diamagnetic shifts of up to about 50 ppm with an increase in lithium content. These results imply the presence of oxidized impurities or electronic charge delocalization. Additionally, EPR measurements provide evidence of VO 2+ impurities and indirect evidence of nonbridging oxygen at high lithium contents. © 2002 American Institute of Physics

Transition-Metal-Doped M‑Li₈ZrO₆ (M = Mn, Fe, Co, Ni, Cu, Ce) as High-Specific-Capacity Li-Ion Battery Cathode Materials: Synthesis, Electrochemistry, and Quantum Mechanical Characterization

Lithium-ion batteries (LIBs) are promising devices for high capacity, rechargeable electrical energy storage; however, LIBs are currently limited by the low specific capacity of the cathode compared to the anode. In previous work, our group demonstrated the viability of a novel cathode material, Li₈ZrO₆ (LZO), through computational and experimental results. Here we report a general synthesis for transition-metal-doped LZO, and we study the effects of doping on electrochemical delithiation and relithiation. A synthesis using transition-metal (M) doped ZrO₂ nanoparticle/carbon black composites as precursors produces doped M-LZO with grain sizes between 35 and 67 nm. The materials were tested as electrode materials. Specific capacities of the doped materials depend on the transition metal and on the Li:Zr ratio used in the synthesis, but they are generally higher than in similarly prepared undoped LZO. In this set of cathode materials, Fe³⁺-doped LZO/C composites showed the highest specific capacities, with an initial discharge capacity higher than two Li ions per formula unit, a specific capacity of 175 mAh/g maintained after 140 cycles, and a specific capacity greater than 80 mAh/g at a rate of 5C. The effects of doping were also investigated by density functional calculations of dopant locations, band gaps, and delithiation energies. We found that all of the dopants that we studied are more favorably located in Li ion sites than in Zr ion sites. The calculated doping effects on structural parameters agree well with experiments. We also found that doping with any of these ions leads to smaller band gaps. Electronic structure calculations with the HSE06 exchange-correlation functional show that deintercalation after doping with Ce³⁺, Cu²⁺, or Co²⁺ at a Li site decreases the attainable cell voltage, whereas Fe³⁺ doping at a Li site increases it. Because of the large polarization and high carbon content of the M-LZO/C composite electrodes, further materials optimization will be needed before they become practical for LIBs