The Role of Oxygen in Low-potential Li Insertion in Metal Oxide Anode Materials
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xmaterial of composition NaxMoO 3 (H2 0)y, (x=0.25 from ICP elemental analysis; y = 0.2 as determined by thermal analysis). SwagelockTM-design cells were assembled using dried NaxMoO 3 (H20)y as the active material. Details of the electrochemical experiments have been presented elsewhere. 5 For XAS experiments, the electrode materials were removed from the batteries in an Ar filled glove box, dried and sandwiched between two Kapton® films. X-ray absorptionfine structure (XAFS). XAFS studies were performed at Le LURE (Orsay, France) using X-ray synchroton radiation emitted by the DCI storage ring (1.85GeV positrons, average intensity of 250mA) at the D44 line. Data were collected in transmission mode at the Mo K edge (20,000 eV). A double-crystal Ge (400) monochromator scanned the energy in 2eV steps from 100eV to 900 eV above the Mo K absorption edge. An accumulation time of 2 seconds was used per point. The analysis of the EXAFS data was performed following standard procedures for extraction of the signal and normalization to the edge absorption.3 Fourier transforms of the EXAFS spectra were made after multiplication of the signal by a k factor over a 3-16A-1 Kaiser window. RESULTS AND DISCUSSION Our previous results have shown that the reaction of lithium with NaxMoO 3 is reversible down to a cutoff voltage, Vd, of 1OOOmV vs Li: below this potential the initial MoO 3-type layered host structure is decomposed to yield an amorphous material best described as a molybdenum sub-oxide. The latter is maintained after insertion of -6 Li into the material (250mV), as demonstrated by XAS analysis at the Mo K edge. Residual oxygen atoms still surround the Mo active centers even on full discharge (insertion of -7.8 Li), although the short Mo-Mo distance indicates a strong metal interaction. 7Li NMR spectroscopy does not show evidence for any Li metal aggregation, and suggests that Li is incorporated in a relatively "ionic" state. The cohesive picture arising from these studies was that of formation of a nanocomposite material on Li uptake, comprised of electronically conductive active centers in proximity to insulating Li 20 regions. 5 In our experiments in this report, we have followed the subsequent cycling process at different steps of charge and then on 2 nd discharge. The electrochemical profile is shown in Figure la (next page) indicating the points at which the electrodes were sampled for XAS analysis. Our previous EXAFS results demonstrated that on the first discharge sweep, reduction of the active centers within Na0 .25MoO 3 proceeded via the formation of an MoO 2-type local environment for Mo with two distances at 2.09 and 2.23A, and a shorter Mo-Mo contact at 2.58A similar to the shorter inner-chain metal-metal distance in the rutile phase. No evidence of any longer distance order is observed by XRD. The bond lengths are summarized as a function of depth of discharge in the first portion of Figures lb and 1c (see below). With progressively greater Li uptake, the material undergoes a significant transformation and
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