Magnesium-substituted Li 4Ϫx Mg x Ti 5 O 12 spinel electrodes (0 < x Յ 1) have been investigated as insertion electrodes for lithium batteries. The substitution of divalent Mg ions for monovalent Li ions in the structure necessitates that the difference in charge must be compensated by a reduction of an equivalent number of Ti cations from Ti 4ϩ to Ti 3ϩ . The substitution increases the conductivity of the [Ti 5/3 Li 1/3 ]O 4 spinel framework by many orders of magnitude, from < 10 Ϫ13 S cm Ϫ1 for insulating Li 4 Ti 5 O 12 (x ϭ 0), in which all the titanium ions are tetravalent, to ϭ 10 Ϫ2 S cm Ϫ1 for Li 3 MgTi 5 O 12 (x ϭ 1.0), in which the average titanium oxidation state is 3.8. The improved conductivity decreases the area specific impedance of Li/Li 4Ϫx Mg x Ti 5 O 12 cells and increases the rate capability of electrodes for small x, typically x ϭ 0.25. The rechargeable capacity of Li 4Ϫx Mg x Ti 5 O 12 electrodes, particularly those with x close to 1 (130 mAh/g), is inferior to that of unsubstituted Li 4 Ti 5 O 12 electrodes (x ϭ 0, 150 mAh/g); the smaller capacity is attributed to the partial occupation of tetrahedral (8a) sites by Mg ions in the spinel structure.

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Evidence of structural fatigue has been detected at the surface of discharged Li x [Mn 2 ]O 4 spinel electrodes in (4 V) Li/Li x [Mn 2 ]O 4 cells. Under nonequilibrium conditions, domains of tetragonal Li 2 [Mn 2 ]O 4 coexist with cubic Li[Mn 2 ]O 4 , even at 500 mV above the thermodynamic voltage expected for the onset of the tetragonal phase. The presence of Li 2 [Mn 2 ]O 4 on the particle surface may contribute to some of the capacity fade observed during cycling of Li/Li x [Mn 2 ]O 4 cells.The exponential growth in the electronics industry has led to an increasing demand for lightweight power sources with high energy density and power capability. 1,2 This demand has been satisfied largely by the advent of rechargeable lithium-ion batteries. The bestknown system is Li x C/LiCoO 2 . Because of the relatively high cost of cobalt, a major international effort is underway to develop alternative positive electrodes, for example, those derived from the spinel Li[Mn 2 ]O 4 . 3-5 A disadvantage of Li[Mn 2 ]O 4 spinel electrodes is that they lose capacity during cycling, which limits the life of the cell. [5][6][7][8][9][10][11] The capacity loss is particularly noticeable at 50°C, a typical temperature that can be reached in devices such as laptop computers. Recent reports have attributed the capacity fade to an unstable electrode surface and to solubility effects at the top of charge. [6][7][8][9][10][11] It is well known that the cycle life of lithium-ion cells depends critically on the structural integrity of the host electrode structures during charge and discharge. 12 In the Li x [Mn 2 ]O 4 spinel system (0< x <2), the [Mn 2 ]O 4 spinel framework provides a three-dimensional interstitial space for lithium-ion transport. Lithium extraction from Li x [Mn 2 ]O 4 (i.e., for 0< x <1) occurs at 4 V vs. metallic lithium. The electrode cycles well over this range because the cubic structure (space group Fd3m) expands and contracts isotropically during lithium insertion and extraction. [3][4][5]12 For 1< x <2, lithium is inserted electrochemically into the spinel structure in a two-phase reaction process at a constant voltage; the open-circuit voltage for this reaction is 2.96 V. 13 This two-phase reaction is associated with the onset of an anisotropic (Jahn-Teller) distortion. As a result, the cubic symmetry of Li[Mn 2 ]O 4 , in which the lithium ions occupy tetrahedral sites, is reduced to tetragonal Li 2 [Mn 2 ]O 4 (space group F4 1 /ddm), in which the lithium ions occupy octahedral sites in an ordered rocksalt structure. [13][14][15] This crystallographic distortion, which results in a 16% increase in the c/a ratio of the unit cell parameters, is too severe for the electrode to maintain its structural integrity during cycling. Consequently, a Li x [Mn 2 ]O 4 spinel electrode cycles poorly over the range 1< x <2, and the cell suffers a capacity loss. It is, therefore, understandable from a structural viewpoint that for good cycle life, the composition of the Li x [Mn 2 ]O 4 spinel electrode must be kept within the limi...

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X-ray diffraction and X-ray absorption spectroscopy experiments were used to study chemical and electrochemical Li insertion and extraction reactions of LiNi0.5Mn0.5O2. These results, along with galvanostatic cycling data, suggest that LiNi0.5Mn0.5O2 layered electrodes in lithium batteries operate predominantly off two-electron redox couples, Ni4+/Ni2+, between approximately 4.5 and 1.25 V and Mn4+/Mn2+ between 1.25 and 1.0 V versus metallic Li, respectively. The retention of a stable layered framework structure and the apparent absence of Jahn−Teller ions Ni3+ and Mn3+ in the high- or low-voltage region is believed to be responsible for the excellent structural and electrochemical stability of these electrodes. The LiNi0.5Mn0.5O2 layered oxide reversibly reacts chemically or electrochemically with Li to form an air-sensitive, dilithium compound, Li2Ni0.5Mn0.5O2, with a hexagonal structure analogous to Li2MnO2. The cycling behavior of Li/LiNi0.5Mn0.5O2 cells over a large voltage window (4.6−1.0 V) and with very slow rates shows that rechargeable capacities >500 mA·h/g can be obtained.