| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. === Includes bibliographical references (p. 153-164). === We perform a first principles investigation of the electronic, thermodynamic and kinetic properties of LixCoO2, an important cathode material for rechargeable lithium batteries. In a lithium battery, the lithium concentration x in Lix CoO2 can be varied between O and 1. Such concentration variations result in important changes of the electronic properties, the relative stability of different forms of LixCoO2 and the mechanisms for lithium diffusion within the CoO2 host structure. To study the .electronic and thermodynamic properties of Lix CoO2, we have used density functional theory within the local density approximation (LDA) along with the cluster expansion formalism. We have also developed a scheme to calculate from first principles, the diffusion coefficient in systems with significant configurational disorder. Building on previous first principles investigations of lithium transition metal oxides, we show that the lithium insertion into CoO2 results in a shift in the nature of bonding between the cobalt and oxygen ions ·which is predominantly of a covalent character at low x and progressively becomes more ionic as x increases. The net effect of this change in bonding is an increased polarization of charge toward the oxygen ions. The variation in electronic properties with x is responsible for important structural changes of the host with x. A thorough investigation of phase stability in the layered form of Lix CoO2 has clarified the nature of poorly characterized phase transformations observed experimentally at low x and has exposed the thermodynamic origin of a large structural phase transformation between crystallographically similar forms of LixCoO2 at high x. Within layered Lix Co02, the lithium ions reside in octahedral sites forming a two dimensional triangular lattice between O-Co-O sheets. Our calculations predict a staging transformation around x=0.15 whereby lithium ions segregate to alternating lithium planes leaving the remaining lithium planes vacant. The calculations predict that this phase transformation is accompanied by a dramatic drop in the lattice parameter, c, of the host, a phenomenon observed experimentally. Using thermodynamic arguments, we demonstrate that a concentration driven metal-insulator transition can induce a first order structural phase transformation. We propose that this mechanism is operative in causing the first-order structural phase transformation in Lix CoO2 at high x. The nature of the crystal structure of the host as well as the presence of defects in the host can be important in determining specific electrochemical properties of lithium intercalation compounds. To determine the effect of crystal structure on the electrochemical properties of LixCoO2 we calculated the properties of the spinel like form of Li r CoO2 and compare them with those of the layered form . Spinel-like LixCoO2 differs from the layered form in that it offers both octahedral as well as tetrahedral interstitial sites for the lithium ions . This results in a voltage versus concentration profile th at is significantly different from that of layered Lix CoO2 . We have also investigated the effect of oxygen vacancies within the layered Lix CoO2 host on the compound's electrochemical properties. To this end , we have used a local cluster expansion to describe the dependence of the oxygen vacancy formation energy on the lithium-vacancy arrangement . The calculations show that oxygen vacancies have an important effect on voltage curve especially at high x. Furthermore, we find that oxygen vacancies tend to depress the order-disorder transition temperatures of ordered-lithium phases. The lithium mobility within the CoO2 host determines the rate at which lithium ions can be removed and reinserted into the host. A study of the activation barriers in LirCoO2 within the local density approximation shows that the migration mechanism and activation barriers depend strongly on the local lithium-vacancy arrangement around the migrating lithium ion . We identify two hopping mechanisms. The first involves the migration into an isolated vacancy whereby lithium squeezes through a dumbbell of oxygen ions. The second mechanism involves migration into a divergence whereby the migration path passes th rough an adjacent tetrahedral site. The latter mechanism h as a significantly lower activation barrier than the former. By parameterizing the activation barriers with a local cluster expansion and applying i t in kinetic Monte Carlo simulations, we predict that lithium diffusion in layered LixCoO2 is mediated by divergences at all lithium concentrations except at almost infinite vacancy dilution. Our calculations show that the activation barriers have a strong concentration dependence due to variations in the lattice parameter c an d the changes in effective valence of the Co ions with x. This results in a predicted diffusion coefficient that varies within several orders of magnitude with x. === by Anton Van der Ven. === Ph.D.
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