The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory ("Argonne") under Contract No. W-3 I-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

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...

No abstract

The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory~Argonne") under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

No abstract