Topotactic Two-Phase Reactions of Li[Ni1/2Mn3/2]04 (P4332) in Nonaqueous Lithium Cells

Li[Ni1/2Mn3/2]O4 was prepared by a two-step solid state reaction and characterized by X-ray diffraction (XRD), infrared (IR)Raman, and electron diffraction (ED). Li[Ni1/2Mn3/2]O4 having characteristic eight absorption bands in 400-800 cm-1 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 Li[Ni1/2Mn3/2]O4 (cubic: a = 8.167 A) 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 4(b) sites, manganese ions were at the octahedral 12(d) sites, and lithium ions were at the 8(c) sites in a cubic-close packed oxygen array consisting of the 8(c) and 24(e) sites. Well-defined Li[Ni1/2Mn3/2]O4 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 Li[Ni1/2Mn3/2]O4 was examined and shown that the reaction at ca. 4.7 V consisted of two cubic/cubic two-phase reactions, i.e., square[Ni1/2Mn3/2]O4 (a = 8.00 A) was reduced to Li[Ni1/2Mn3/2]O4 (a = 8.17 A) via square1/2Li1/2[Ni1/2Mn3/2]O4 (a = 8.09 A). 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 Li[Ni1/2Mn3/2]O4 to Li2[Ni1/2Mn3/2]O4 is also examined and showed that the reaction proceeded in a cubic (a = 8.17 A)/tetragonal (a = 5.74 A, c = 8.69 A) two-phase reaction with the reversible potential of 2.795 V. From these results, characteristic features of topotactic two-phase reactions of Li[Ni1/2Mn3/2]O4 (P4(3)32) were discussed by comparing with the results on LiMn2O4 (Fd3m).