T. Prem Kumar

Ionic conductivity and FT-IR studies on plasticized PVC/PMMA blend polymer electrolytes

Ionic conductivities of plasticized poly(vinylchoride) (PVC)/poly(methylmethacrylate) (PMMA) blend electrolyte films containing two different lithium salts, viz., lithium tetrafluroborate (LiBF4) and lithium perchlorate (LiClO4) are studied using the AC impedance technique at 25°C, 40°C, 50°C and 60°C. A mixture of ethylene carbonate (EC) and propylene carbonate (PC) is used as the plasticizer. Pure PMMA and PMMA-rich phases exhibited better conductivity. The variation of ionic conductivity for different plasticizer contents and for different lithium salts is reported. The variation in film morphology is examined by scanning electron microscopic examination. Finally, the existence of ion–ion pairs has been identified using Fourier Transform Infrared analysis (FT-IR) measurements.

Thermally oxidized graphites as anodes for lithium-ion cells

Lithium intercalation and deintercalation processes occur through solid electrolyte interfaces (SEIs) on the zigzag and armchair faces of graphite. Such SEIs are formed by reaction of surface groups on the graphite with lithium upon charging. The nature of this interface to a large extent determines the reversible and irreversible capacities of the graphite. We have investigated the influence of mild oxidation of samples of natural graphite and carbon nanotubes on the surface characteristics of the films formed on these materials upon lithium intercalation. The surface groups formed upon oxidation have been characterized using Fourier transform infrared (FT-IR) spectroscopy. The reversible and irreversible capacities of the thermally oxidized samples are discussed in terms of the surface film composition as well as enhanced surface area that contributes to increased number of sites for lithium intercalation.

Pyrolytic carbons from acid/base-treated rice husk as lithium-insertion anode materials*

The effects of hydrochloric acid and sodium hydroxide as leachants on the lithium-insertion properties of pyrolytic carbons prepared from rice husk are presented. All the disordered carbonaceous products had interlayer spacings (d002) of more than 3.7 Å, with values decreasing with an increase in the concentration of the leachant. The values of the H/C ratio and the R-parameter, the reciprocal of which is a measure of the number of non-parallel single layers of carbon, also diminished with an increase in the concentration of NaOH. An increase in the alkali concentration was found to improve the porosity of the carbons, as evidenced by the Brunauer–Emmett–Teller (BET) surface area data. An interaction of these factors determines the observed capacities of the carbon products. The highest insertion and deinsertion capacities were observed with the carbon obtained from rice husk treated with 0.3 M NaOH, the values being 819 and 463 mAh/g, respectively.

Malonic acid-assisted synthesis of LiNi0.8Co0.2O2 cathode active material for lithium-ion batteries

Polycrystalline LiNi0.8Co0.2O2 was synthesized by a solution route with malonic acid as the complexing agent. The effects of temperature, duration of heat treatment, pH of the precursor solution, and the nature of the solvent employed on the performance characteristics of the product were studied. It was observed that a 12-hour 800 °C heat treatment protocol was necessary to obtain products with optimal electrochemical characteristics. Furthermore, an excess lithium stoichiometry of 1.05 was found to be detrimental to the performance of the cathode material. The beneficial effect of ethanol as a solvent over water on the product characteristics is explained by the presence of solvent molecules in the coordination sphere of the cations. A pH of 7, at which malonic acid is complexed completely with the cations without interference from other nucleophiles, was found to be ideal for the synthesis of the cathode active material from aqueous solutions. With ethanol as the medium, the product formed by a 12-h calcination at 800 °C yielded a first-cycle capacity of 173 mAh/g and a tenth-cycle capacity of 169 mAh/g.

Conductivity and viscosity studies of dimethyl sulfoxide (DMSO)-based electrolyte solutions at 25 °C

The specific conductivities and viscosities of lithium perchlorate at four different concentrations in dimethyl sulfoxide (DMSO)-based, binary mixed solvents are reported at 25 °C. The co-solvents are tetrahydrofuran (THF), 1,2-dimetthoxyethane (DME) and 1,3-dioxolane (DOL). The change in viscosity with solvent composition in all three mixed-solvent systems without electrolyte indicates the occurrence of structural disruption. The observed increase in viscosity with increase in LiClO4 concentration suggests an increasing compactness of the structure. A 2 M LiClO4 solution shows maximum conductivity at 20 vol.% DMSO in DMSO+DOL and at 40 vol.% DMSO in DMSO+THF mixtures. By contrast, a 1.5 M LiClO4 solution has maximum conductivity at 40 vol.% DMSO in DMSO+DME mixtures. Preferential solvation of LiClO4 by DMSO occurs in the respective mixtures with THF, DME and DOL.

Graphite with fullerene and filamentous carbon structures formed from iron melt as a lithium-intercalating anode

The electrochemical properties of flaky graphite containing carbon nanostructures, synthesized by a dissolution-precipitation method from a carbon-rich cast iron melt, were investigated. The formation of the highly crystalline graphite was realized at temperatures as low as 1600 °C. The presence of fullerenes and filamentous carbon structures on the graphite was confirmed by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The reversible lithium intercalation capacity of the graphitic product was more than 300 mAh/g. The first-cycle irreversible capacity was a mere 14%. The coulombic efficiency seemed to stabilize at values >99% from the fifth cycle.

Synthesis of macroporous LiMn2O4 with avian egg membrane as a template

Avian eggshell membrane as a template for the synthesis of a macroporous network of crystalline LiMn2O4 is demonstrated. Well-formed crystals of average size 600 nm formed a network structure whose average pore size was 2–4 μm. The unique porous structure should make it an attractive cathode material for lithium-ion batteries. In fact, for an 80% cutoff in capacity retention, LiMn2O4 obtained by a 10-h calcination at 800°C sustained 83 cycles.

Structural, thermal and electrochemical properties of chromium-substituted LiNi0.8Co0.2O2

LiCryNi0.8−yCo0.2O2 compositions, where y=0.000, 0.010, 0.025, 0.040, 0.050, 0.075 and 0.100, were synthesized via a conventional ceramic route. X-ray diffraction studies indicated cation mixing for the compositions with y ≥ 0.05. Cyclic voltammetric studies revealed that the systems were reversible only when y was lower than 0.05. High levels of substitutions with Cr resulted in highly irreversible systems, either due to cation mixing or the displacement of the substituent ions to the lithium inter-slab regions, or both. The charge-discharge characteristics of LiCryNi0.8−yCo0.2O2 were similar to those of the unsubstituted material over ten cycles. All the other substituted compositions showed much lower capacities and reduced cyclability. LiCr0.025Ni0.775Co0.2O2 gave a first-cycle capacity of 169 mAh/g in the 3.0 to 4.4 V window at a 0.1 C rate, fading to 156 mAh/g in the tenth cycle.Differential scanning calorimetric studies revealed that substituting with chromium produced no benefit to thermal stability. The structural, thermal and electrochemical properties of the pristine and Cr-substituted LiNi0.8Co0.2O2 compositions are discussed.

Zirconia-coated lithium cobalt oxide as a long-cycling cathode for lithium batteries

Abstract A commercial sample of LiCoO2 was coated with ZrO2 by sol‐gel and mechano‐thermal processes. The effects of the coating method and the precursor used in the sol‐gel coating process were studied. Electron microscopic images of the coated particles revealed the presence of a compact coating over the cathode particles. XRD and ESCA results suggested the formation of substitutional compounds of the composition Li x Zr y Co1‐yO2 + 0.5y on the surface of the cathode. Coating levels of 0.3 and 1.0 wt.% were found to be optimal in terms of cyclability for the materials coated by the sol‐gel and mechano‐thermal methods, respectively. At these coating levels, the R‐factor values, determined from XRD data, were the lowest. The maximum improvements in cyclability registered at a 0.2 C rate were about eight‐fold with the sol‐gel and mechano‐thermal coating methods. Cyclic voltammetric studies showed that the coating led to a suppression of the cycle‐limiting phase transitions accompanying the charge‐discharge processes.