Structural complexity of layered-spinel composite electrodes for Li-ion batteries

The complexity of layered-spinel yLi₂MnO₃·(1 – y)Li_1+xMn_2–xO₄ (Li:Mn = 1.2:1; 0 ≤ x ≤ 0.33; y ≥ 0.45) composites synthesized at different temperatures has been investigated by a combination of x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and nuclear magnetic resonance (NMR). While the layered component does not change substantially between samples, an evolution of the spinel component from a high to a low lithium excess phase has been traced with temperature by comparing with data for pure Li_1+xMn_2–xO₄. The changes that occur to the structure of the spinel component and to the average oxidation state of the manganese ions within the composite structure as lithium is electrochemically removed in a battery have been monitored using these techniques, in some cases in situ. Our ⁶Li NMR results constitute the first direct observation of lithium removal from Li₂MnO₃ and the formation of LiMnO₂ upon lithium reinsertion.