Monica G. Lazarraga

Synthesizing nanocrystalline LiMn2O4 by a combustion route

Nanocrystalline samples of lithium manganese oxide with cubic spinel structure have been prepared by combustion of reaction mixtures containing Li(I) and Mn(II) nitrates that operate as oxidisers, and sucrose that acts as fuel. The samples were characterised by X-ray diffraction, transmission electron microscopy, thermal analysis, and impedance and electrochemical measurements. The effect of the fuel content on the purity and morphology of the products was analysed. The samples as prepared showed small amounts of Mn2O3 and Mn3O4 as impurities, depending on the amount of sucrose used in the synthesis. Annealing at 700 °C led to single-phase cubic spinels. In these phases, the smallest average particle size (ca. 30 nm) corresponded to the sample obtained with a hyperstoichiometric amount of fuel. This sample showed the Li1.05Mn1.95O4 composition as deduced from the thermal and electrochemical data. No variation in conductivity associated with the cubic⇔orthorhombic phase transition was observed. The electrochemical behaviour as positive electrode showed good cyclability at high current densities (reversible capacity of 73 mAh g−1 at 2.46 mA cm−2).

Nanosize LiNiyMn2 −yO4(0 < y≤ 0.5) spinels synthesized by a sucrose-aided combustion method. Characterization and electrochemical performance

Nanosize crystalline cathode materials of LiNiyMn2 − yO4 (0 < y ≤ 0.5) composition and spinel-type structure have been obtained by a single-step sucrose-aided self-combustion method. The as-prepared samples contained some amorphous organic impurities that were removed after a short period of heating at 500 °C. The pure single-phase spinels have been characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and nitrogen sorption isotherms. The samples consist of particles (ca. 24 nm size) that are aggregated in clusters (ca. 1 µm size) in which mesopores (10–80 nm size) appear among the particles. Additional heating at 800° and 1000 °C produces a slight increase in the cubic lattice parameter and a pronounced increase in particle size (>100 nm). Electrical conductivity decreases as the Ni content increases in accordance with an electron hopping mechanism between Mn3+ and Mn4+ ions. The 500 °C- and 800 °C-heated LiNi0.5Mn1.5O4 samples show good electrochemical behaviour at 4.7 V as cathode materials. The capacity (132.7 mA h g−1) found is close to the nominal capacity (146.7 mA h g−1) and remains constant for current densities in the range C/24–2C (where C = 2.6 mA cm−2). At higher current densities (2C–10C) the capacity decreases progressively. The cyclability at the C current density is ca. 99.7% for both samples.

Microstructure and Ionic Conductivity of LiSn2 P 3 O 12-Teflon Composites

The ac electrical response is investigated in ionic conductor-polymer composites formed of a Li + ion conductor (LiSn 2 P 3 O 12 ) and an insulating polymer (Teflon) taken in different concentrations (29, 44, 60, and 74% by volume of LiSn 2 P 3 O 12 ). The microstructure of the composites, as analyzed by scanning electron microscopy (SEM), shows an aggregation of LiSn 2 P 3 O 12 particles giving rise to two types of clusters: some extended along the composite and some isolated or are surrounded by Teflon. Based on such a microstructure we propose an equivalent circuit model that matches the frequency response. The relative proportion of the two types of clusters, determined by SEM, is compared with the value of a weighting parameter (f) introduced in the equivalent circuit for fitting the electrical data. The composites were also prepared to estimate the ionic conductivity of LiSn 2 P 3 O 12 because of the difficulty of obtaining either sintered ceramic pellets or single crystals of LiSn 2 P 3 O 12 . The ionic conductivity estimated for LiSn 2 P 3 O 12 is 2 ± 1 × 10 -5 S cm -1 at 120°C. For the analogous LiGe 2 P 3 O 12 a comparison between the ionic conductivity of the ionic conductor deduced from the composite, and the ionic conductivity measured on a well-sintered pellet of the pure ionic conductor has been made. Moreover, the temperature dependence of the conductivity for the ionic conductor and the overall conductivity for the composite and the well-sintered pellet have been studied.