Conventional liquid electrolyte batteries are nearing their practical energy density limits despite the intensifying demand for greater energy and power densities. Solid-state batteries (SSBs), enabled by ion-conducting solid electrolytes (SEs), offer promise for meeting these needs while simultaneously providing improved safety, but still face challenges in high voltage cathode incorporation and economic competitiveness against liquid counterparts.

Chloride SEs, which are subject to complex structure-property relationships, show promise as catholyte candidates in dual SE high-voltage SSBs when paired with another SE. Taking a joint computational-experimental approach, we reveal the presence of a high concentration of planar and nonstoichiometric defects in mechanochemically-synthesized Li3YCl6 that facilitate Li-ion conduction and present a method of controlling its Li+ conductivity by tuning the defect concentration with synthesis and heat treatments. By carefully modulating the synthetic conditions in the Na-Y-Zr-Cl family of SEs, we demonstrate control of polymorphism and conduction properties in Na2ZrCl6 and Na3YCl6. Transition metal orderings in the Na2.25Y0.25Zr0.75Cl6 composition are shown to strongly impact Na-ion transport, providing a new way to optimize conductivity in aliovalently-doped SEs. Motivated by our improved understanding of chloride SE processing, we develop a reaction temperature-based approach to identify SE candidates chemically compatible with chlorides. We demonstrate that the Li2ZrCl6 and Li6PS5Cl pairing is a conductive, scalable, and kinetically stabilized system with high potential for implementation in dual SE SSBs.

Finally, we focus on facilitating manufacturing of air-sensitive sulfide SEs and Li-Si anodes. First, we propose a reversible 1-undecanethiol coating for sulfide SEs (LPSC), enabling handling in humid ambient air. Leveraging solid state NMR, we reveal that the thiol chain end anchors to the LPSC surface with S-S bonds while the hydrocarbon tail repels water, preserving the LPSC structure and conductivity after two days of air exposure. Next, we outline a prelithiation strategy for Si anodes designed to decrease first cycle losses, quantifying the phase fraction of Li-Si alloy formed with 7Li NMR.The guidelines for materials selection and improved processing parameters in SSBs outlined herein represent important advancements towards elevating energy storage technologies beyond the limitations of conventional Li and Na-ion batteries.

The shift towards greener technologies such as electric vehicles and renewable energy sources generates a rapidly increasing demand for energy storage. This trend underscores the need for the development of sustainable chemistries for rechargeable batteries that do not rely on cobalt (Co) and nickel (Ni), as they are expensive, geographically limited, and unethically sourced (in the case of Co), resulting in complex and unreliable supply chains. Disordered rocksalt (DRX) structures are a promising candidate for next-generation Li-ion batteries, as their close-packed structure and flexible chemistry enables sustainable and earth-abundant manganese (Mn) based cathodes with impressive energy densities. However, practical commercialization of DRX cathodes remains limited by key issues such as rapid capacity and voltage fade along with poor Li conduction necessitating particle downsizing.

This dissertation aims to showcase the rational design of improved DRX cathodes by correlating synthesis parameters, composition, and materials properties, and by leveraging unique reaction pathways to produce novel DRX compositions. First, the ultrafast and energy-efficient microwave synthesis of DRX materials was developed, providing time and energy savings of more than two orders of magnitude compared to standard synthesis routes. Next, to establish compositional limits of fluorine solubility in DRX structures and its impact on electrochemical performance, a joint experimental-computation method was designed to enable accurate determination of DRX stoichiometry. Critically, we observed unexpectedly low fluorine incorporation into the structure, and a diminished impact of fluorination on cycling performance. Instead, higher amounts of redox-active elements such as Mn is more impactful. Finally, following design rules established by the compositional study, a previously unreported class of DRX compositions was synthesized through the novel microwave technique, opening up a large, unexplored compositional space for DRX cathodes. This new class of DRX materials was demonstrated to have superior electrochemical performance over current DRX compositions, representing a significant leap towards practical commercialization of DRX cathodes for sustainable rechargeable batteries.

Sodium-ion batteries (SIBs) are attractive alternatives for electrochemical energy storagedue to sodium’s natural abundance and cost-effectiveness when compared to their lithium counterparts. Among the various Na-ion cathode classes, sodium superionic conductor (NASICON) structured Na3V2(PO4)3 (NVP) has emerged as a promising candidate for SIB applications owing to its high structural stability, 3−D Na+ diffusion network, and high operating voltage of 3.4 V vs. Na+/Na0. Although promising, the impact of temperature dependent Na+/ vacancy ordering transitions, potential cation migration, and influence of phase separation upon Na (de)intercalation on Na+ diffusion is still poorly understood.

Here, we focus on the effects of isovalent (Al3+) and aliovalent (Mg2+) substitutionfor V into the NVP framework and investigate a series of Na3+yV2−yMgy(PO4)3 (Mg- NVP) (y = 0−1.0) and Na3V2−yAly(PO4)3 (Al-NVP) (y = 0, 0.5) cathodes to better understand the impact of V by substitution on the Na (de)intercalation mechanics by complementing short and long range characterization techniques such as synchrotron X-ray diffraction, 23Na, 31P, 51V solid-state NMR, and scanning electron microscopy (SEM) with first principles calculations to unravel the complex electrochemical cycling behavior of these materials. We demonstrate the vastly different effect of each dopant on electrochemical performance with an emphasis on (de)intercalation mechanics and causes of structural degradation during high voltage cycling.

In the case of Mg-NVP, Na extraction and reinsertion results in a two-phase reactionmechanism when y < 0.5 and transitions to a solid-solution mechanism above y = 0.5, when cycled over a potential window of 3.8−2.75 V. Conversely, when Al3+ is introduced into the NVP framework, it exacerbates the formation of an intermediary Na2.24V1.5Al0.5(PO4)3 phase leading to successive biphasic reactions during Na (de)intercalation, in stark contrast to the solid-solution mechanism that emerges at higher Mg concentrations. It is found that the formation of the intermediary phase in Al-NVP helps reduce the propensity for particle cracking during long term cycling and lowers overpotentials associated with Na extraction during electrochemical cycling. Thus, we have highlighted the significant role of transition metal dopants to modify the reaction mechanisms associated with Na (de)intercalation in NASICON structured Na3V2(PO4)3.

The continued reliance on a select few elements, namely Li, Ni, and Co, for rechargeable battery cathodes is untenable with the increasing demand for batteries in electric vehicles, grid-scale energy storage, and portable electronics. In this dissertation we have explored three alternative cathode classes exhibiting improved sustainability: 1) Li-ion disordered rocksalt oxyfluorides, 2) cryolite-like Na3FeF6, and 3) weberite Na2Fe2F7. The first class enables a greater variety of transition metals to be utilized, but their long-range disorder necessitates short-range characterization which we have accomplished using 7Li and 19F solid-state nuclear magnetic resonance (ss-NMR). The second and third class, forming the bulk of this dissertation, completely depart from Li as the intercalant ion and instead utilize Na.While Na-ion batteries have emerged as a clear alternative for Li-ion batteries in select applications, their energy densities are still largely limited by the cathode materials. Thus, a paradigm shift in the development of competitive Na cathodes hinges on the investigation of new structural frameworks and anion chemistries. Here, we investigated two sodium iron fluoride materials whose differing structures lead to significantly different cycling mechanisms – conversion in cryolite-like Na3FeF6 due to its dense structure, and intercalation in weberite Na2Fe2F7 due to its more open framework structure. The former material, Na3FeF6, we find via an ex situ analytical method based on 23Na NMR and operando magnetometry, forms NaF and Fe on discharge, which is only moderately reversible due to sluggish conversion reaction kinetics. In the latter material, weberite Na2Fe2F7, we find more reversible electrochemical intercalation behavior, but a phase transformation to perovskite NaFeF3 still occurs upon cycling. Through compositional tuning and synthetic control, this phase transformation can be suppressed leading to a promising new Na-ion cathode material. Additional computational characterization highlights the complex phase stability landscape inherent to weberite materials, leading to polymorphism and metastability as observed experimentally. Thus, we have developed a set of computationally informed design rules to guide further exploration of the weberite structure class.