Resumen de Investigation of the insulator-to-metal transition in LixCoO2
An in-depth study of LixCoO2 (LCO) is presented as a platform for understanding the origin of its phenomenology and paving the way for new and improved implementations of this and similar compounds. Li molar fractions (x) were systematically varied to investigate the fundamental origin of the phase transition and the potential implications for future practical applications, such as improved Li-ion batteries, resistive memory devices, or the use of similar compounds with more complex stoichiometries. This study is further motivated by the phase diagram reported for LCO, encompassing structural, insulator-to-metal (IMT), and potentially spin transitions as a function of Li content. We present a novel technique, based on physical methods, for the removal of Li from layered compounds with high mobility ion planes. The delithiation is performed by sputtering-annealing cycles in ultra-high vacuum, maintaining good surface crystallinity. This allowed us to access diferent stoichiometries and thus obtain information about the changes in LCO as a function of x. Additional alkali evaporations (Li and Na) on these compounds were carried our to explore the reversibility of this physical method and obtaining new stoichiometries beyond the conventional range. We shed light on the charge compensation during Li deintercalation in LCO using X-ray photoemission spectroscopy (XPS) and delithiated samples that have not been exposed to chemical or electrochemical processes. The role of the Co atoms and the efects of Co 3d -O 2p hybridization have been analyzed by the simultaneous use of diferent characterization techniques. Throughout this work, we provide a coherent interpretation of several experimental techniques, and our results are discussed in the context of previous research and the fundamental understanding of structural and IMT transitions. A study of the electronic band structure upon hole doping using angle-resolved photoemission spectroscopy (ARPES) has allowed for the three-dimensional mapping of the band dispersion of LCO, facilitating the identifcation of Co 3d t2g energy states involved in the valence band maximum (VBM) and the IMT. This VBM, with a 3-fold dispersion around Γ¯ , is located along the ¯¯ Γ –M symmetry direction. These results were successfully compared with band calculations. Upon Li deintercalation, the renormalization of the valence band is observed, with no additional states forming in the band gap.The evolution of the phase transition in real space using spatially resolved techniques has revealed the coexistence of metallic and insulating phases (Hex-I and Hex-II). We discuss these results in the context of a frst-order phase transition, as is the case for a Mott-type insulator. The observation of phase coexistence in X-ray difraction (XRD) and photoemission microscopy (PEEM), the renormalization of the VB in ARPES, and the predominantly d-d character of the band gap observed in X-ray absorption spectroscopy (XAS) clearly point towards a Mott-Hubbard transition with some O 2p -Co 3d hybridization. Our results are discussed in relation to the family of transition metal oxides (TMOs), reported as having an intermediate character, between Mott-Hubbard and charge transfer insulators. We argue that the efects of delithiation cannot be interpreted in terms of the LixCo1–xO compounds, since structural symmetries, lattice distortions, and charge compensation pathways are not comparable. The efects of sample preparation and surface quality are also addressed to support the use of a physical and non-interacting doping technique for the study of these materials. This work introduces a novel method for exploring the characteristics of insulator-metal transitions in materials. By using LCO as a model system, we have sought to gain a deeper understanding of the fundamental mechanisms that drive this type of phenomenon. The potential application of this approach to other compounds may lay the groundwork for future discoveries and advances in materials science, from rechargeable batteries to novel devices that exploit these properties
