Chemical energy storage using batteries will become increasingly important for future environmentally friendly ('green') societies. The lithium-ion battery is the most advanced energy storage system, but its application has been limited to portable electronics devices owing to cost and safety issues. State-of-the-art LiFePO4 technology as a new cathode material with surprisingly high charge-discharge rate capability has opened the door for large-scale application of lithium-ion batteries such as in plug-in hybrid vehicles. The scientific community has raised the important question of why a facile redox reaction is possible in the insulating material. Geometric information on lithium diffusion is essential to understand the facile electrode reaction of LixFePO4 (0

A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P4,lnmc) phases in a zirconia-ceria solid solution (Zr, -,Ce,O,) has been observed by Raman spectroscopy. The cubic-tetragonal (c-t") phase boundary in compositionally homogeneous samples exists at a composition X , (0.8 < X , c 0.9) at room temperature, where t" is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition Xg (0.65 < Xh c 0.7) at room temperature. The sample with a composition between X , and X i is t"-ZrO,. By Raman scattering measurements at high temperatures, the tetragonal (t") + cubic and cubic + tetragonal phase transitions occur above 400°C in Zro,2Ceo.802 solid solution.

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State‐of‐the‐art LiFePO4 technology has now opened the door for lithium ion batteries to take their place in large‐scale applications such as plug‐in hybrid vehicles. A high level of safety, significant cost reduction, and huge power generation are on the verge of being guaranteed for the most advanced energy storage system. The room‐temperature phase diagram is essential to understand the facile electrode reaction of LixFePO4 (0 < x < 1), but it has not been fully understood. Here, intermediate solid solution phases close to x = 0 and x = 1 have been isolated at room temperature. Size‐dependent modification of the phase diagram, as well as the systematic variation of lattice parameters inside the solid‐solution compositional domain closely related to the electrochemical redox potential, are demonstrated. These experimental results reveal that the excess capacity that has been observed above and below the two‐phase equilibrium potential is largely due to the bulk solid solution, and thus support the size‐dependent miscibility gap model.