Electrochemical reduction of natural graphite was carried out in 1M LiClO4 ethylene carbonate (EC)/1,2‐dimethoxyethane (DME) (1:1 by volume) solution at 30°C. Natural graphite was reduced stepwise to LiC6 (golden yellow in color). The staging phenomenon was observed by x‐ray diffraction (XRD). The first stage ( LiC6 ; cL=3.70true3_Å ) and the second stage ( LiC12 ; d2=7.06Å ) compounds were identified as a commensurate structure in which lithium atoms form a close‐packed two‐dimensional array. A second‐stage compound false(LiC18false) with a different in‐plane lithium ordering based on a LiC9 two‐dimensional packing in lithium intercalated sheets also was observed; also third ( LiC27 ; d3=10.4Å ), fourth‐( LiC36 ; d4=13.8Å ), and eighth‐( LiC72 ; d8=27.2Å ) stage compounds were identified. The electrochemical oxidation of the first‐stage compound false(LiC6false) was examined and shown to be reversible over the entire range, i.e., □C6+xnormalLi⇄LixC6 . The reaction mechanism for the reduction of graphite and the oxidation of the first‐stage compound are discussed in relation to the staging phenomenon from the detailed open‐circuit voltage and XRD data. The chemical potential of LiC6 was estimated to be −3.6 kcal · mol−1 from the observed reversible potential. The feasibility of using a lithium‐graphite intercalation compound in lithium ion (shuttlecock) cells is described, and the innovative secondary systems, □C6/LiCoO2 and □C6/LiNiO2 fabricated in discharged states, are demonstrated.

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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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