Li͓Ni 1/2 Mn 3/2 ͔O 4 was prepared by a two-step solid state reaction and characterized by X-ray diffraction ͑XRD͒, infrared ͑IR͒-Raman, and electron diffraction ͑ED͒. Li͓Ni 1/2 Mn 3/2 ͔O 4 having characteristic eight absorption bands in 400-800 cm Ϫ1 in IR spectrum, extra lines in XRD, and extra spots in ED was analyzed in terms of a superlattice structure. Analytical results on the structural data indicated that Li͓Ni 1/2 Mn 3/2 ͔O 4 ͑cubic: a ϭ 8.167 Å) was a superlattice structure based on a spinel framework structure having a space group of P4 3 32 ͑or P4 1 32) in which nickel ions were located at the octahedral 4͑b͒ sites, manganese ions were at the octahedral 12͑d͒ sites, and lithium ions were at the 8͑c͒ sites in a cubic-close packed oxygen array consisting of the 8͑c͒ and 24͑e͒ sites. Well-defined Li͓Ni 1/2 Mn 3/2 ͔O 4 was examined in nonaqueous lithium cells and showed that the cell exhibited extremely flat operating voltage of about 4.7 V with rechargeable capacity of 135 mAh/g based on the sample weight. The reaction mechanism of Li͓Ni 1/2 Mn 3/2 ͔O 4 was examined and shown that the reaction at ca. 4.7 V consisted of two cubic/cubic two-phase reactions, i.e., ᮀ͓Ni 1/2 Mn 3/2 ͔O 4 (a ϭ 8.00 Å) was reduced to Li͓Ni 1/2 Mn 3/2 ͔O 4 (a ϭ 8.17 Å) via ᮀ 1/2 Li 1/2 ͓Ni 1/2 Mn 3/2 ͔O 4 (a ϭ 8.09 Å). Results on the detailed reversible potential measurements indicated that the flat voltage at ca. 4.7 V consisted of two voltages of 4.718 and 4.739 V. The reaction of Li͓Ni 1/2 Mn 3/2 ͔O 4 to Li 2 ͓Ni 1/2 Mn 3/2 ͔O 4 is also examined and showed that the reaction proceeded in a cubic (a ϭ 8.17 Å)/tetragonal (a ϭ 5.74 Å, c ϭ 8.69 Å) two-phase reaction with the reversible potential of 2.795 V. From these results, characteristic features of topotactic two-phase reactions of Li͓Ni 1/2 Mn 3/2 ͔O 4 ( P4 3 32) were discussed by comparing with the results on LiMn 2 O 4 (Fd3m).

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A large-scale lithium-ion battery is the key technology toward a greener society. A lithium iron silicate system is rapidly attracting much attention as the new important developmental platform of cathode material with abundant elements and possible multielectron reactions. The hitherto unsolved crystal structure of the typical composition Li2FeSiO4 has now been determined using high-resolution synchrotron X-ray diffraction and electron diffraction experiments. The structure has a 2 times larger superlattice compared to the previous beta-Li3PO4-based model, and its origin is the periodic modulation of coordination tetrahedra.