Solid-state batteries currently receive ample attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as Li metal or silicon. One key remaining challenge is identifying solid electrolytes that combine high ionic conductivity with stability in contact with the highly reducing potentials of next-generation anodes. An interesting subset of phases that are intrinsically stable even at the ultralow potential of Li metal are fully-reduced phases. Fully-reduced (lithium-)phases have lithium as their only cation and all anions in their lowest-permitted oxidation state and are thus irreducible. Many fully-reduced phases are known, but before the commencement of this thesis none (with the exception of the Li3N phase) featured a Li-ion conductivity > 0.01 mS cm-1. The main findings in this thesis spread across chapters 2-5 may be broadly split into four categories: (1) Discovery of new highly-conducting fully-reduced antifluorite phases (2) Optimizing lithium diffusion in fully-reduced antifluorite phases (3) electrochemical stability windows of the newly-discovered phases and (4) discussion of potential applications for these materials. The findings in the above four categories will be further summarized below.

Discovery of new fully-reduced antifluorite phases: Combining experiments and ab initio DFT calculations we discovered new irreducible antifluorite-like electrolytes. These new irreducible solid electrolytes are all characterized by two or more anions sharing the anion site in the antifluorite phase in a disorderly manner and were obtained by mechanochemical synthesis. We discovered a solid solution of antifluorite-like phases on the Li2S-Li3N tie line (Li2+xS1-xNx, Chapter 3) and on the LiCl-Li3N tieline (Li1+2xCl1-xNx, Chapter 4) with conductivities almost on a par with that of Li3N (~0.5 mS cm-1). Further we found that antifluorite-like phases with four anions (Cl-, Br-, S2- N3-) sharing the anion site may be synthesized (Chapter 4) indicating the high compositional flexibility of fully-reduced antifluorite-like phases that will allow for property optimizations of these materials. In Chapter 5 we discovered that P3- and N3- can also share the anion sites in irreducible antifluorite-like phases. The Li2.6S0.4P0.35N0.25 nitridophosphide phase we synthesized in chapter 5 is the best-conducting fully reduced solid electrolyte known today.

Lithium diffusion in fully-reduced antifluorite phases : Our DFT simulations on the newly-discovered antifluorite-like phases (Chapters 2,3,4 and 5) show that lithium diffusion is promoted by ion jumps between tetrahedral sites and ion jumps between octahedral and tetrahedral lithium sites. The new irreducible phases we discovered all feature several different jump-types due to the disordering of the anions. Thus a wide distribution of hop activation energies exists in these phases, —with a distinct hop activation energy for each jump type. This range of hop activation energies can span from ~0.2 eV to ~0.6 eV. In Chapter 2 we found that this wide distribution of hop activation energies could explain the different activation energies for Li diffusion obtained from nuclear magnetic resonance line narrowing measurements (~0.25 eV) and electrochemical impedance spectroscopy (0.47 eV); while local fast diffusion may exist between sites connected by low hop activation energies, the macroscopic conductivity probed by electrochemical impedance spectroscopy seems to be limited by higher hop activation energies. In Chapter 3 a comparison between two very similar phases, ̶ a disordered antifluorite-like phase with stoichiometry of Li9S3N and an almost equivalent phase but with an ordered S/N arrangement ̶ demonstrates the mechanism of the diffusion-boosting effect that disordering may entail: while the ordered Li9S3N phase only features 6 discreet jump types with six associated hop activation energies the disordering of N/S in the disordered Li9S3N phase introduced a wide distribution of jump types and hop activation energies promoting the existence of percolation paths with lower energy thresholds. Further 198

the analysis of lithium diffusion in all newly-discovered irreducible antifluorite-like phases (Chapters 2, 3, 4, and 5) indicates that N substitution, be it NS, NCl, NP, NBr ,increases Li diffusivity through the steric diffusion bottlenecks; the small anion radius of N3- (1.49 Å) compared to Cl-, S2-, P3- Br- (1.81 Å- 1.96 Å) increases the void space in these steric bottlenecks facilitating Li diffusion.

Electrochemical stability windows of the discovered phases: All newly-discovered phases investigated in this study are electrochemically stable against Li metal (that is 0 V vs Li). Their oxidation limits vary but are all below 2 V (vs Li). The investigations in Chapter 3 indicate that the oxidation limit correlates with the (meta)stability of phases: the more metastable the phase the lower the oxidation limit. Interestingly however, this anticorrelation of oxidation limit and metastability is only valid for similar phases for example within the Li2+xS1-xNx and the Li1+2xCl1-xNx solid solutions. When comparing different structures this trend does not hold potentially because the electrochemical decomposition mechanism may be different; for example the antifluorite-like Li2+xS1-xNx phases are metastable but have larger oxidation limits than hexagonal Li3N which is one of their thermodynamically stable decomposition products.

Potential applications of the newly discovered fully-reduced antifluorite-like phases: Due to their modest oxidation limits the likeliest application of the newly-discovered fully-reduced antifluorite-like phases will be as anolytes in bi- or multilayer separators. Compared to the long-known irreducible Li3N phase, the newly discovered irreducible antifluorite electrolytes have several projected advantages. Mechanical and microstructural properties of anolytes play a key role, for instance in dendrite formation with metal anodes; the large compositional flexibility of irreducible antifluorite phases shown in chapter 3 may enable tunability of these properties. Additionally, chemical compatibilities of anolytes with the paired catholytes also need to be considered. It was shown in chapter 2 that the antifluorite-like Li1.66Cl0.66N0.33 phase is chemically less reactive with potential catholytes than Li3N. The antifluorite-like Li2+xS1-xNx phases developed in chapter 3 have stability windows that match the operation window of Si (0.01 V-1.1 V vs Li) and may hence be a better fit as anolytes against Si electrodes than Li3N which oxidizes beyond 0. 8 V (vs Li). Finally, Li3N has reasonably high conductivity of ~0.5 mS cm-1 but exceeding this conductivity could be beneficial especially if anolytes are intended to be used in composite anodes; the large compositional flexibility of irreducible antifluorite phases discovered in this thesis enables the design of phases that exceed the conductivity of Li3N such as the antifluorite-like nitridophosphide phases developed in chapter 5. All of the above highlights the importance of developing and understanding new irreducible solid electrolytes to which this thesis hopes to have significantly contributed.

While much could be achieved in the course of this thesis, much work in the field of fully-reduced antifluorite electrolytes remains to be done. Firstly, the integration of these electrolytes in full cells needs to be further explored. Secondly, a large phase space of fully-reduced antifluorite-like phases remains to be explored including potentially non-lithium phases based on sodium, potassium or other cations. New fully-reduced phases can be systematically discovered by exploring the phase diagrams of fully-reduced precursors which are typically different salts of the same metal; combining fully-reduced precursors of the same metal always results in fully-reduced reaction products. For example, new fully-reduced magnesium (Mg) - electrolytes could be discovered by exploring the tieline between MgCl2 and MgS or the phase diagram spanned by MgCl2, MgS and Mg3N2.

The author of this thesis wishes anyone deciding to continue investigating fully-reduced phases exciting scientific endeavors.
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Understanding diffusion mechanisms in solid electrolytes is crucial for advancing solid-state battery technologies. This study investigates the role of structural disorder in Li7−xPS6−xBrx argyrodites using ab initio molecular dynamics, focusing on the correlation between key structural descriptors and Li-ion conductivity. Commonly suggested parameters, such as configurational entropy, bromide site occupancy, and bromine content, correlate with Li-ion diffusivity but do not consistently explain conductivity trends. We find that a uniform distribution of bromine and sulfur ions across the 4a and 4d sublattices is critical for achieving high conductivity by facilitating optimal lithium jump activation energies, anion-lithium distances, and charge distribution. Additionally, we introduce the ionic potential as a simple descriptor that predicts argyrodite conductivity by assessing the interaction strength between cations and anions. By analyzing the correlation between ionic potential and conductivity for a range of argyrodite compositions published over the past decade, we demonstrate its broad applicability. Minimizing and equalizing ionic potentials across both sublattices enhances conductivity by reducing the strength of anion-lithium interactions. Our analysis of local environments coordinating Li jumps reveals that balancing high and low-energy pathways is crucial for enabling macroscopic diffusion, supported by investigating percolating pathways. This study highlights the significance of the anionic framework in lithium mobility and informs the design of solid electrolytes for improved energy storage systems.@en

Solid-state batteries currently receive ample attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as lithium metal or silicon. One key remaining challenge is identifying solid electrolytes that combine high ionic conductivity with stability in contact with the highly reducing potentials of next-generation anodes. Fully reduced electrolytes, based on irreducible anions, offer a promising solution by avoiding electrolyte decomposition altogether. In this study, we demonstrate the compositional flexibility of the disordered antifluorite framework accessible by mechanochemical synthesis and leverage it to discover irreducible electrolytes with high ionic conductivities. We show that the recently investigated Li ₉N ₂Cl ₃ and Li ₅NCl ₂ phases are part of the same solid solution of Li-deficient antifluorite phases existing on the LiCl-Li ₃N tie line with a general chemical formula of Li _1+2xCl _1−xN _x (0.33 < x < 0.5). Using density functional theory calculations, we identify the origin of the 5-order-of-magnitude conductivity increase of the Li _1+2xCl _1−xN _x phases compared to the structurally related rock-salt LiCl phase. Finally, we demonstrate that S _Cl- and Br _Cl-substituted analogues of the Li _1+2xCl _1−xN _x phases may be synthesized, enabling significant conductivity improvements by a factor of 10, reaching 0.2 mS cm ⁻¹ for Li _2.31S _0.41Br _0.14N _0.45. This investigation demonstrates for the first time that irreducible antifluorite-like phases are compositionally highly modifiable; this finding lays the ground for discovery of new compositions of irreducible antifluorite-like phases with even further increased conductivities, which could help eliminate solid-electrolyte decomposition and decomposition-induced Li losses on the anode side in high-performance next-generation batteries.

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Most highly Li-conducting solid electrolytes (σ_RT > 10^-3 S cm^-1) are unstable against lithium-metal and suffer from detrimental solid-electrolyte decomposition at the lithium-metal/solid-electrolyte interface. Solid electrolytes that are stable against lithium metal thus offer a direct route to stabilize lithium-metal/solid-electrolyte interfaces, which is crucial for realizing all-solid-state batteries that outperform conventional lithium-ion batteries. In this study, we investigate Li₅NCl₂ (LNCl), a fully-reduced solid electrolyte that is thermodynamically stable against lithium metal. Combining experiments and simulations, we investigate the lithium diffusion mechanism, different synthetic routes, and the electrochemical stability window of LNCl. Li nuclear magnetic resonance (NMR) experiments suggest fast Li motion in LNCl, which is however locally confined and not accessible in macroscopic LNCl pellets via electrochemical impedance spectroscopy (EIS). With ab-initio calculations, we develop an in-depth understanding of Li diffusion in LNCl, which features a disorder-induced variety of different lithium jumps. We identify diffusion-limiting jumps providing an explanation for the high local diffusivity from NMR and the lower macroscopic conductivity from EIS. The fundamental understanding of the diffusion mechanism we develop herein will guide future conductivity optimizations for LNCl and may be applied to other highly-disordered fully-reduced electrolytes. We further show experimentally that the previously reported anodic limit (>2 V vs Li⁺/Li) is an overestimate and find the true anodic limit at 0.6 V, which is in close agreement with our first-principles calculations. Because of LNCl’s stability against lithium-metal, we identify LNCl as a prospective artificial protection layer between highly-conducting solid electrolytes and strongly-reducing lithium-metal anodes and thus provide a computational investigation of the chemical compatibility of LNCl with common highly-conducting solid electrolytes (Li₆PS₅Cl, Li₃YCl₆, ...). Our results set a framework to better understand and improve highly-disordered fully-reduced electrolytes and highlight their potential in enabling lithium-metal solid-state batteries.

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