Lao.67-~Li3xTiO3 solid solution forms in the range ~0.15 > x > ~0.04 by the substitution mechanism La ~ 3Li. Superstructure reflections observed in this range are consistent with a superlattice consisting of the stacking of two perovskite subcells. A line-splitting phenomenon observed for x < ~0.1 indicates that the tetragonal distortion of the cubic perovskite subcell occurs. The solid solutions exhibit high lithium ion conductivities greater than 10 4 S cm -~ at room temperature in a wide range, x> ~0.04. A maximum conductivity of 1.1 • 10 _3 S cm -~ is found at x = 0.1. The dome-shaped composition dependence of conductivity indicates that the conduction mechanism involves the movement of lithium ions through the A-site vacancies.Lithium ion-conducting materials have attained great prominence in the last decade because of their potential applications for electrolytes in high energy batteries and other electrochemical devices. 1 Several oxides are known to exhibit high lithium ion conductivity; 2 most conductive materials are 3,~-Li3PO4-type solid solutions, 3 (Li-Na) 13-alumina, 4 and lithium-substituted Na superionic conductors (NASICONs). 5 In general, oxide materials are superior to nonoxide materials such as Li3N and Lil-based glasses for chemical and electrochemical stability and mechanical properties] '4 However, one major problem with the practical applications, except for low power applications, still remains in the unavailability of lithium solid electrolytes with high conductivity.Perovskites of the type ABe3 may be regarded as a 3-dimensional framework structure constructed from vertex-sharing Be6 octahedra and A ions placed in 12-coordinate sites in the cubic

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LiCoO 2 1 is the first cathode material to be used in commercial lithium ion rechargeable cells 2 and operates at ∼3.7 V vs Li/Li + . A recent approach to improved cathode materials has been to replace expensive and toxic LiCoO 2 by the 3.8 V cathode LiMn 2 O 4 3-5 in stateof-the-art cells. 6 These cells can successfully substitute for conventional systems such as nickel-cadmium in portable electronic devices, 7 but larger scale batteries for zero-emission vehicles require further improvement in energy density by either increasing capacity or raising operating voltage. Cells with cathode materials LiMn-O 2 8 and Li 1.5 Na 0.5 MnO 2.85 I 0.12 9 exhibited higher capacity, but the former needs improved cycling stability and the latter would require higher working voltage. Cells with cathode materials based on spinel structure compounds and improved electrolytes 10 showed higher operating voltages: 4.8 V for LiNiVO 4 11,12 and LiCr X Mn 2-X O 4 13,14 and 4.7 V for LiNi X Mn 2-X O 4 . [15][16][17] We recently reported the spinel Li 2 CoMn 3 O 8 , the first cathode material operating over 5 V. 18 Here we report a new 5 V cathode material, Li 2 FeMn 3 O 8 ; this is the first high-voltage system to contain substantial Fe content and offers potential economical and environmental advantages.A phase-pure sample of Li 2 FeMn 3 O 8 was prepared by conventional solid-state synthesis: a stoichiometric mixture of dried Li 2 CO 3 , MnCO 3 , and undried FeC 2 O 4 ‚ 2H 2 O, all reagent grade, was ground intimately and fired under oxygen flow, initially at 650 °C for 2 h to drive off CO 2 and then at 750 °C for 3 days with intermittent regrinding, followed by slow-cooling to room temperature. An 57 Fe Mo ¨ssbauer spectrum was measured for the as-prepared sample with a Toyo Research FGX-100S apparatus, 57 Co source, using the spectrometer velocity (ν) scale calibrated with R-Fe. The

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