Lithium-Rich Anti-Perovskites: Understanding the Structure and its Influence on Ion Mobility

Abstract

The rechargeable lithium-ion (Li-ion) battery is considered the technology of choice for energy storage in a wide range of portable electronic devices, including mobile phones and laptop computers. Despite the many advantages of the Li-ion battery, its application is limited by its use of liquid electrolytes, which are known to pose serious fire and safety risks. Hence, suitable alternatives are urgently required. In recent years there has been considerable interest in the development of all-solid-state batteries (ASSBs), with a particular focus on novel solid electrolyte materials.

Recent literature suggests that Li-rich anti-perovskites (LiRAPs) could be suitable solid electrolyte materials. LiRAPs have the general formula ABX, where A is a monovalent anion, B is a divalent anion, and X is a strongly electropositive monovalent cation. The LiRAPs LiOCl and LiOBr have been reported to possess ionic conductivities on the order of 10 S cm. However, the precise conduction mechanisms and pathways that lead to such conductivities are poorly understood. Hence, studies are needed that can probe ion mobility within these materials.

The work presented in this thesis focuses on the synthesis and structural characterisation of LiRAPs, including LiOCl, LiOBr and their hydrated analogues, LiOHCl and LiOHBr. The samples produced were analysed extensively via laboratory XRD, NPD and multinuclear SSNMR spectroscopy. Initial investigations focused on synthesising phase pure samples of LiOCl and LiOBr. Due to their extremely hygroscopic nature, air-sensitive techniques were required for their synthesis. Several synthetic methods were attempted, and reaction variables, including the time and temperature, were varied systematically to optimise the reaction conditions. Despite testing numerous synthetic conditions, producing phase pure samples of LiOCl and LiOBr remained a challenge.

The focus of the investigation then shifted towards the hydrated LiRAP LiOHCl, which is considerably easier to synthesise. LiOHCl was successfully synthesised via conventional solid-state and mechanical milling methods. LiOHCl is reported to exist in two different phases; a room-temperature phase believed to be orthorhombic and a high-temperature cubic () phase. Several structural suggestions have been made in the literature for the room-temperature phase of LiOHCl, but a structural model is yet to be agreed upon. Hence, considerable effort was dedicated to evaluating the structures reported in the literature and determining an accurate structure for the room-temperature phase of LiOHCl. Moreover, the phase transition in LiOHCl and the associated structural changes were also explored.

As LiRAPs are proposed as candidate solid electrolyte materials, probing ion mobility within this system is of significant interest. Thus, LiOHCl was extensively analysed via variable-temperature H and Li NMR spectroscopy to investigate the proton and lithium-ion mobility as a function of temperature. Additionally, AIMD simulations were completed by our collaborators to support our experimental findings. These complementary techniques led to an understanding of a highly correlated mechanism for proton and Li-ion movement in LiOHCl. Furthermore, LiRAP samples were also analysed via VT Cl NMR spectroscopy to investigate the changes in the local environment of Cl as a function of temperature.

As with perovskites, the LiRAPs exhibit extreme structural flexibility. Hence, the composition of LiOHCl was modified by varying the proton and, consequently, the lithium content to synthesise LiOHCl (x = 0.25 -- 1), and via fluorine doping and halide mixing to synthesise Li(OH)FCl and LiOHClBr (x = 0 -- 1), respectively. The influence of compositional variation on the structure and ion mobility in the system was studied extensively via XRD and multinuclear SSNMR spectroscopy.