Systematic Mn 2p XPS and Mn K-edge XAS analyses together with the electrochemical measurement have been carried out for the spinel LiMn 2 O 4 prepared at various sintering temperatures in order to elucidate an origin of the dependence of electrochemical properties on synthetic conditions. From the comparative experiments, it becomes clear that a lowering of synthetic temperature gives rise to an increase of structural disorder and of the average oxidation state of manganese, which is more prominent on the surface than in the bulk. Such results suggest that the modification of surface property induced by a decrease of particle size is closely related to the electrochemical performance. The nanocrystalline LiMn 2 O 4 prepared at 250 °C shows excellent cyclability at the 3 V region compared to that of microcrystalline LiMn 2 O 4 prepared at 700 °C. For the purpose of examining the evolution of the chemical bonding nature of inserted lithium, 7 Li MAS NMR studies have been performed for both the spinel compounds before and after Li + intercalation. While the intercalation of 0.2 mol Li + does not induce any remarkable spectral change for the microcrystalline LiMn 2 O 4 , it leads to a dramatic suppression of the NMR signal for the nanocrystalline LiMn 2 O 4 , indicating that the process of grafting Li into the latter phase results in significant modifications of the chemical environment of lithium. On the basis of present experimental findings, it can be concluded that the lowering of synthetic temperature modifies the surface properties, which facilitates the grafting process of Li + ion and, thereby, enhances the electrochemical properties for the 3 V region corresponding to the Li insertion.
This communication describes the synthesis of: (i) non toxic and low cost nanocrystalline electrode materials which can be advantageously prepared at low temperature; (ii) highly conductive electrolyte membranes formed by the nano-encapsulation within a poly (acrylonitrile)-based polymer matrix of a solution of LiPF6in organic solvants. The performances of rechargeable PLR (Plastic Lithium Rechargeable) batteries using the above mentioned components are presented.
We have illustrated the important role played by the nanoscale materials in three-up-to-date energy topics.1/The solar-to-electrical energy conversion in photoelectrochemical cells: we have shown two favorable situations for which photoelectrochemical cells using porous nanocrystalline films have high efficiencies.2/The electrical energy storage in rechargeable rocking-chair lithium batteries: these systems, which use nanocrystalline materials, might be the next generation of rechargeable batteries showing higher capacity, cyclability, and safety than conventional lithium ion batteries.3/The energy saving with efficient electrochromic windows using nanocrystalline materials.
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