Kinetics of Li Transport in Disordered Rocksalt Li-excess Compounds

Disordered rocksalt Li-excess (DRX) compounds form a potentially new class of cathode materials for Li-ion batteries, as they contain only abundant metals and do not require Co or Ni. While DRX compounds are promising as battery materials, their properties and performance are not as optimized as those of conventional layered cathode materials. To compete with conventional cathodes, developing a more detailed understanding of the structure-property relation in DRX compounds as it relates to their electrochemical applications is necessary. Specifically, the Li transport properties in DRX materials are not well understood, despite their significant impact on the rate performance of the battery. This dissertation employed first-principles calculations, the cluster expansion technique, and the kinetic Monte Carlo method to investigate the Li transport properties of DRX materials. Chapter 1 provides an introduction to DRX, including a detailed discussion of the possible Li+ diffusion mechanisms and percolation in these materials. Chapter 2 investigates the impact of fluorine and Li-excess on Li migration barriers in rocksalt structures. Fluorine substitution in the oxygen sublattice reduces oxygen redox and enhances energy density and capacity retention. However, its impact on Li transport remains unclear. Using first-principles calculations, this chapter explores the effects of both F substitution and accompanying Li-excess on Li migration barriers. The results show that F has a small negative effect on Li migration barriers while Li-excess decreases Li+ migration barriers. Because fluorination enables more Li-excess, these results do not predict any detrimental impact on Li transport. Chapter 3 delves into the essential aspects of the delithiation process and cation short-range ordering (SRO) in DRX compounds. This study uses first-principles calculations and the cluster expansion approach to model the disorder in DRX Li2-xVO3 with 0 ≤ x ≤ 1. The chapter discusses the SRO of Li in tetrahedral and octahedral sites, and the order in which Li delithiates and V oxidizes with respect to local environments. The results indicate that the number of nearest neighbors V determines the order of octahedral Li delithiation, and that V are oxidized in a manner that minimizes the electrostatic interactions among V. This research provides valuable insights into how to tailor the performance of V-based DRX cathode materials by controlling SRO features that may reduce energy density. Additionally, findings of this chapter shed light on the relationship between SRO and Li transport properties. Chapter 4 investigates the kinetics of Li+ transport in disordered rocksalt structures, focusing on DRX Li2-xVO3. Using a combination of first-principles calculations, cluster expansion approach, and kinetic Monte Carlo simulations, the chapter provides insights into the factors affecting Li+ transport properties in DRX materials. The results show that the relative stability of tetrahedral Li and octahedral Li occupancy plays a crucial role in determining the number of active sites in the percolation network and the resulting Li+ transport properties. Furthermore, the chapter demonstrates that the wide site-energy distribution can cause correlated motion in Li2-xVO3, which can hinder Li+ transport. Although this study focuses on Li2-xVO3 as a model system, the insights gained apply to all DRX materials, given their inherently broad site-energy distributions. In the concluding chapter, Chapter 5, the key findings of this dissertation are summarized.

Advisor: Ceder, Gerbrand