First-principles design and analysis of carbon and carbon-silicon layered materials for energy storage

Abstract

The transition towards sustainable energy storage and conversion technologies is essential to mitigate climate change and reduce dependence on fossil fuels. Lithium-ion batteries (LIB), which currently dominate the market for electric vehicles and portable electronics, require further innovation in anode materials to enhance capacity, stability, and rate performance while minimizing volume expansion during the charge-discharge cycle. Currently, hydrogen storage remains a cornerstone for the realization of a hydrogen-based energy economy, where the key challenge lies in developing materials that can store and release hydrogen efficiently under ambient conditions. Two-dimensional (2D) materials, because of their high surface area, tunable porosity, and ease of chemical functionalization, present a promising platform for lithium-ion battery and hydrogen storage applications. This thesis presents a comprehensive first-principles investigation based on Density Functional Theory (DFT) to explore the potential of two-dimensional layered materials for energy storage applications, with a particular focus on Li-ion battery anodes and hydrogen storage hosts. This work aims to conduct ab initio calculations to analyze the structural, electronic, and electrochemical properties of various materials, including the C-silicyne, biphenylene, Si-doped γ-graphyne (SiG), and biphenylene-graphene bilayer, to evaluate their suitability as efficient storage hosts. Carbon-based materials are considered particularly promising for storage applications due to their abundant resources, high physicochemical stability, safety, and good electrical conductivity. In this context, the biphenylene monolayer, a novel non-benzenoid carbon allotrope composed of a planar arrangement of sp2 hybridized carbon atoms forming periodic four, six, and 8 membered rings, has been investigated as a potential LIB anode material. Additionally, fabricating heterostructure bilayers from different two-dimensional materials is a compelling approach to synergistically combine the advantageous properties of individual layers while mitigating their respective limitations. Accordingly, we have carried out an in-depth theoretical investigation of the biphenylene–graphene van der Waals heterostructure using DFT methods to assess its viability as a LIB anode. Carbon-silicon-based materials have also garnered significant attention, as carbon and silicon possess complementary electrochemical characteristics. The high capacity of silicon is counterbalanced by the structural stability of carbon, and the low lithium affinity of carbon is mitigated by the strong lithium–silicon interaction. In this context, we proposed and studied a novel material Si-doped γ-graphyne material. Furthermore, a planar C-silicyne monolayer, Si analogue of α-graphyne with-C≡C- linkage has been investigated for its applicability in LIB anodes. The structural integrity of all investigated materials has been confirmed through phonon dispersion analysis, ab initio molecular dynamics (AIMD), and elastic strain energy calculations, validating compliance with the Born–Huang criteria for mechanical stability. Electronic structure analyses, including band structure and density of states (DOS), revealed the metallic nature, favorable for electronic conductivity. Electrochemical performance has been further evaluated through lithium adsorption studies using charge density difference and Bader charge analysis, revealing strong Li binding. Climbing Image Nudged Elastic Band calculations are performed to determine lithium diffusion barriers, which are found to be sufficiently low to facilitate effective ion mobility. The materials exhibit high theoretical storage capacities exceeding that of commercial graphite, with small volume changes and working potentials in the 0.1–1.0 V range, suggesting safe and stable cycling behavior. Beyond LIB applications, the Li-functionalized Si-doped γ-graphyne (Li8SiG) has been explored for H2 storage applications by implementing the cutting-edge DFT. The introduction of Li atoms significantly enhanced hydrogen binding via polarization effects, enabling each Li atom to adsorb up to four H2 molecules. The system exhibits high gravimetric capacity, meeting the U.S. Department Of Energy targets. Projected DOS and Hirshfeld charge analysis reveal Kubas-type and Niu–Rao–Jena-like hydrogen–metal interactions, while occupation number analysis and AIMD simulations confirm reversible hydrogen storage behavior. The thesis presents detailed findings across its chapters, offering key theoretical insights into designing advanced 2D materials for future energy storage. It concludes with a summary, closing remarks, future directions, and a complete bibliography.