Atomic-resolution in-situ TEM Studies of Lithium Electrochemistry in Co3O4-Carbon Nanotube Nanocomposite

In typical charge and discharge cycling, electrodes may exhibit metastable phases with unusual ordering to repeatedly host and extract lithium-ion and electrons. This is a clear consequence of not enough time and/or energy available for such metastable phases to relax and transform into their equilibrium counterparts (1). Such metastable structures may exist only for a short duration, which makes it extremely difficult to measure or even identify them experimentally. Yet, they clearly play important roles in the battery figures of merit, such as cycling stability, voltage hysteresis, to capacity. The recent developments in in-situ transmission electron microscopy (TEM) (2-6) has enabled us to observe de/lithiation processes at atomic resolution, identify metastable phases and monitor their continuous phase transformation, with the gradual addition of lithium-ions and electrons into the battery electrodes. Structural models of these metastable phases are derived from full DFT simulations, and seem to corroborate with high resolution phase contrast simulated images as compared to corresponding experimental ones. Li-Co-O system represents one of the most important materials for lithium-ion battery with rich chemistry and structures, i.e. LiCoO2 has been used as intercalated cathode in the first commercialized battery by SONY, while Co3O4 and CoO are found to be high-capacity anode materials with conversion reaction: MxOy + 2y Li+ + 2y e= xM0 + yLi2O. Here, Co3O4 nanoparticles grown on highly conductive multi-wall carbon nanotubes (CNT) are employed as a model material system to study the structural evolutions with different amount of lithium inserted or reacted during electrochemical activation. The in-situ electrochemical lithiation experiments were followed until cobalt oxide nanoparticles are completely reduced into Co nanoparticles along with the formation of Li2O by conversion reaction and reverse de-lithiation until return back to cobalt oxide. When lithium-ions are introduced at lower rate, metastable lithium-inserted LixCo3O4 (x=1 to 5) crystalline phases are observed prior to formation of Co0 and Li2O clusters as the product of conversion reaction. At higher rate, lithium-ions can occupy any empty sites simultaneously that instantly break the Co3O4 spinel lattice bypassing the metastable crystalline phases. The amount of lithium-ions intake in a low rate is larger than that of the high rate, which provides insight on the charge/discharge rate and capacity relation. The presentation will cover intricacies of such metastable structures and the overall dynamics of electrochemical processes, as monitored by in-situ TEM imaging, spectroscopy and diffraction.