The Effect of Lithium Content on the Electrochemical Lithium Intercalation Reaction into Amorphous and Crystalline Powdered Li1+-δMn2O4 Electrodes Prepared by Sol-gel Method

Abstract

This work considers the structural dependence of the electrochemical lithium-ion intercalation into the amorphous and crystalline gel derived Li1+deltaMn2O4 powdered electrode specimens (1+/-delta=0.4-1.5) in 1 M LiClO4/propylene carbonate (PC) solution, by using the galvanostatic intermittent charge/discharge experiment and electrochemical impedance spectroscopy. In the charge/discharge curve, the amorphous powdered electrode specimen showed a slope in the potential versus lithium content (1-delta). By contrast, the crystalline powdered electrode specimen exhibited two distinct plateaus near 4.0 and 4.16 VLi/Li+, indicating the coexistence of two pseudo-phases of a lithium-diluted phase and a lithium-concentrated phase, and of lambda-MnO2 and Li1-deltaMn2O4, respectively. From the impedance spectra, it was found that the absorption resistance of lithium-ion intercalation into the amorphous and crystalline powdered electrode specimens remained nearly constant with varying lithium content, (1-delta), in the 0.4-0.8 range. The invariant absorption resistance is based upon the structural stability of the three-dimensional cubic spinel Li1-deltaMn2O4 lattice. In contrast, the absorption resistance increased abruptly above (1+delta)=1.0 by the stress-field gradient generated due to Jahn-Teller distortion. The chemical diffusivity of lithium ions in the amorphous powdered electrode specimen was found to be nearly constant, i.e., about 10(-8) cm(2) s(-1), irrespective of the lithium content in the range of (1-delta)=0.45-0.7 at room temperature, which is ten times higher than that in the crystalline powdered electrode specimen. The raised chemical diffusivity in the amorphous electrode is due to a shorter diffusion length and a stronger repulsive interaction between the lithium ions within the electrode. On the other hand, the component diffusivities of lithium ions in both the amorphous and crystalline electrodes shared the nearly same value of about 10(-10) cm(2) s(-1) irrespective of the lithium content, (1-delta), in the 0.45-0.7 range at room temperature. It is inferred that the number of vacant sites in the amorphous electrode available for the intercalation of the lithium ion is quite comparable with that in the crystalline electrode. (C) 1997 Elsevier Science S.A.