Batteries are an important aspect of sustainable energy technologies, as they can be used either for the storage of electric energy for the grid or for the electrification of the transport fleet, making these sectors less reliant on fossil fuels (chapter 1). The Li-ion battery has revolutionized the world in many ways, enabling portable electric devices as honored by the Nobel prize in 2019 to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino. As the Li-ion battery is quite a mature technology by now, large gains in performance parameters (especially energy density) will need alternative battery concepts and new chemistries. There are many possibilities, and one of them is a switch from liquid to solid electrolytes (chapter 2). The work presented in this thesis investigates the structure-to-property relationship of halide solid-electrolytes Li₃M(III)X₆. For solid electrolytes to replace liquid electrolytes, the material needs a combination of properties. An important property is the ionic conductivity, which should be high enough for room-temperature operation of the battery and determines, among other design parameters, the rate-capability (or power density) of the battery. Another property that is important is the electrochemical stability window, which determines the electrochemical stability of the electrolyte in contact with the electrodes of the material. Both of these properties are strongly related to the crystal structure and chemistry of the solid electrolyte (chapter 2). Therefore, both the structure and properties are investigated using a variety of techniques, mostly x-ray and neutron diffraction, AC-impedance and solid-state NMR relaxometry (chapter 3). The work is presented in four data containing chapters: • Chapter 4: The materials investigated show very complex behavior relating to diffusion on short time scales, as investigated by NMR T₁-relaxometry. The first chapter therefore provides an in-depth introduction to solid-state NMR relaxometry and spectral density fitting. Using two examples, namely Li₆PS₅X, a sulfide solid-electrolyte class previously studied in the research group, and halide Li₃YCl₃Br₃, it is illustrated how multiple jump processes can present in the curve of the relaxation rates vs. inverse temperature. • Chapter 5: In this chapter, aliovalent substitution in Li₃InCl₆ with Zr(IV) is explored. The Zr(IV) replaces the In(III) and introduces an additional Li-vacancy. The substitution can also affect the crystal structure of the material, affecting ionic diffusion in other ways than changing the charge carrier concentration. Using combined x-ray and neutron diffraction, it is found that the ordering of the In(III) and Zr(IV) is affected by the substitution. This affects also the diffusion on short timescales, as can be observed with NMR relaxometry as well as from the solid-state NMR lineshape. The combination of the structure solution and the puzzle pieces provided by solidstate NMR suggest, that the structural change induced by the substituent leads to more three-dimensional conduction. • Chapter 6: While chlorides have higher electrochemical stability, bromide anions are more polarizable and may have lower association energy with Li, which can lead to higher Li-ion conductivity. This chapter investigates the trade-off between ionic conductivity and electrochemical stability in materials Li₃YClBrₓCl₆₋ₓ. It is found that 75% Br is most beneficial for ionic conductivity rendering a very conductive material (~5 mS/cm at room temperature), higher concentration of bromine indeed lower the electrochemical stability window. The introduction of 25% Br, however, also leads to an increase in ionic conductivity while preserving the electrochemical stability. This suggests that Br-substitution can be a viable method to increase the ionic conductivity of Li-ion conducting chlorides while preserving the electrochemical stability. • Chapter 7: The Li₃M(III)Cl₆ (M(III)= Ho, Y, Dy, Tm) usually are reported to crystallize in a trigonal crystal structure. This paper shows that synthesizing these materials by co-melting with some LiCl deficiency stabilizes an orthorhombic phase of the material. Both of these structures are based on quasi hexagonally close-packed Cl atoms, with the M(III) and Lithium on octahedral sites. The different crystal symmetry is caused by a change in the arrangement of the cations. The orthorhombic phase has ~8 times higher ionic conductivity compared to the trigonal phase. Ab initio molecular dynamic simulations revealed that this is due to a fast conduction pathway along the c-direction of the crystal structure. This path corresponds to jumps between face-sharing octahedra. Therefore, it is likely that the cation arrangement in the orthorhombic structure is favorable for that diffusion path, leading to an increase in ionic conductivity. It is interesting to compare the effect of the different material design strategies aliovalent substitution (Chapter 5), halogen alloying (chapter 6) and tuning of the crystal structure (Chapter 7) on the properties of interest for Li₃M(III)X₆ solid electrolytes. The electrochemical stability window is indeed higher for chlorides than for bromides, but it is found that 25% Br substitution preserves the stability of the chloride in Li₃YCl₆. For ionic conductivity, the largest increase is observed for halogen alloying (factor ~40 increase in ionic conductivity when substituting 25% of the chlorine with bromine atoms), followed by the trigonal to orthorhombic phase transition (factor ~8 improvements) and, lastly, aliovalent substitution (factor ~1.6 improvement). Regarding the measurement methods, two notable findings were found. Firstly, this thesis showed that x-ray diffraction data is important in this system to reach reliable occupancies in the crystal structure solution (chapter 5), as neutrons scattered on lithium and most of the M(III) have a 180°phase shift and therefore cancel their signal when occupying the same site. Lastly, it is shown that the complex shapes of the NMR T₁ relaxation rates can be explained using a superposition of individual, BPP-type jump processes. Fitting such a model is complex, and data measured at multiple larmor frequencies should be used to increase the reliability of the fit. To perform such a fit, a programm was developed in the scope of this thesis to simultaneously fit such measurements and analyze the error associated with the parameter by sampling the posterior probability distribution of the parameter using a Markov chain Monte Carlo sampler. @en

The original version of this article contained errors in Figure 3a and Figure 3f. In Figure 3a, the activation energies (Ea) were calculated using a log scale instead of a logarithm ln scale. In Figure 3f, the y-axis interval was not properly selected. The correct y-axis interval in Figure 3f and the numerical values of the activation energy are now provided in Figure 3a and the main text. These errors have been corrected in the HTML and PDF versions of the article.

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Chloride-based solid electrolytes are considered interesting candidates for catholytes in all-solid-state batteries due to their high electrochemical stability, which allows the use of high-voltage cathodes without protective coatings. Aliovalent Zr(iv) substitution is a widely applicable strategy to increase the ionic conductivity of Li₃M(iii)Cl₆ solid electrolytes. In this study, we investigate how Zr(iv) substitution affects the structure and ion conduction in Li_3−xIn_1−xZr_xCl₆ (0 ≤ x ≤ 0.5). Rietveld refinement using both X-ray and neutron diffraction is used to make a structural model based on two sets of scattering contrasts. AC-impedance measurements and solid-state NMR relaxometry measurements at multiple Larmor frequencies are used to study the Li-ion dynamics. In this manner the diffusion mechanism and its correlation with the structure are explored and compared to previous studies, advancing the understanding of these complex and difficult to characterize materials. It is found that the diffusion in Li₃InCl₆ is most likely anisotropic considering the crystal structure and two distinct jump processes found by solid-state NMR. Zr-substitution improves ionic conductivity by tuning the charge carrier concentration, accompanied by small changes in the crystal structure which affect ion transport on short timescales, likely reducing the anisotropy.

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Li₃YX₆(X = Cl, Br) materials are Li-ion conductors that can be used as solid electrolytes in all solid-state batteries. Solid electrolytes ideally have high ionic conductivity and (electro)chemical compatibility with the electrodes. It was proven that introducing Br to Li₃YCl₆increases ionic conductivity but, according to thermodynamic calculations, should also reduce oxidative stability. In this paper, the trade-off between ionic conductivity and electrochemical stability in Li₃YBr_xCl_6-xhalogen-substituted compounds is investigated. The compositions of Li₃YBr_1.5Cl_4.5and Li₃YBr_4.5Cl_1.5are reported for the first time, along with a consistent analysis of the whole Li₃YBr_xCl_6-x(x = 0-6) tie-line. The results show that, while Br-rich materials are more conductive (5.36 × 10^-3S/cm at 30 °C for x = 4.5), the oxidative stability is lower (∼ 3 V compared to ∼ 3.5 V). Small Br content (x = 1.5) does not affect oxidative stability but substantially increases ionic conductivity compared to pristine Li₃YCl₆(2.1 compared to 0.049 × 10^-3S/cm at 30 °C). This work highlights that optimization of substitutions in the anion framework provide prolific and rational avenues for tailoring the properties of solid electrolytes.

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Understanding the relationship between structure, ionic conductivity, and synthesis is the key to the development of superionic conductors. Here, a series of Li_3-3xM_1+xCl₆ (−0.14 < x ≤ 0.5, M = Tb, Dy, Ho, Y, Er, Tm) solid electrolytes with orthorhombic and trigonal structures are reported. The orthorhombic phase of Li–M–Cl shows an approximately one order of magnitude increase in ionic conductivities when compared to their trigonal phase. Using the Li–Ho–Cl components as an example, their structures, phase transition, ionic conductivity, and electrochemical stability are studied. Molecular dynamics simulations reveal the facile diffusion in the z-direction in the orthorhombic structure, rationalizing the improved ionic conductivities. All-solid-state batteries of NMC811/Li_2.73Ho_1.09Cl₆/In demonstrate excellent electrochemical performance at both 25 and −10 °C. As relevant to the vast number of isostructural halide electrolytes, the present structure control strategy guides the design of halide superionic conductors.

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A key challenge for solid-state-batteries development is to design electrode-electrolyte interfaces that combine (electro)chemical and mechanical stability with facile Li-ion transport. However, while the solid-electrolyte/electrode interfacial area should be maximized to facilitate the transport of high electrical currents on the one hand, on the other hand, this area should be minimized to reduce the parasitic interfacial reactions and promote the overall cell stability. To improve these aspects simultaneously, we report the use of an interfacial inorganic coating and the study of its impact on the local Li-ion transport over the grain boundaries. Via exchange-NMR measurements, we quantify the equilibrium between the various phases present at the interface between an S-based positive electrode and an inorganic solid-electrolyte. We also demonstrate the beneficial effect of the LiI coating on the all-solid-state cell performances, which leads to efficient sulfur activation and prevention of solid-electrolyte decomposition. Finally, we report 200 cycles with a stable capacity of around 600 mAh g⁻¹ at 0.264 mA cm⁻² for a full lab-scale cell comprising of LiI-coated Li₂S-based cathode, Li-In alloy anode and Li₆PS₅Cl solid electrolyte.

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All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite-, garnet- and NASICON-type solid electrolytes that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is notably larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.

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Molecular dynamics simulations are a powerful tool to study diffusion processes in battery electrolyte andelectrode materials. From molecular dynamics simulations, manyproperties relevant to diffusion can be obtained, including the
diffusion path, amplitude of vibrations, jump rates, radial distribution functions, and collective diffusion processes. Hereit is shown how the activation energies of different jumps and theattempt frequency can be obtained from a single moleculardynamics simulation. These detailed diffusion properties provide
a thorough understanding of diffusion in solid electrolytes, andprovide direction for the design of improved solid electrolytematerials. The presently developed analysis methodology isapplied to DFT MD simulations of Li-ion diffusion in β-Li3PS4.The methodology presented is generally applicable to diffusion in crystalline materials and facilitates the analysis of moleculardynamics simulations. The code used for the analysis is freely available at: https://bitbucket.org/niekdeklerk/md-analysis-withmatlab. The results on β−Li3PS4 demonstrate that jumps between bc planes limit the conductivity of this important class of solid electrolyte materials. The simulations indicate that the rate-limiting jump process can be accelerated significantly by adding Li interstitials or Li vacancies, promoting three-dimensional diffusion, which results in increased macroscopic Li-iondiffusivity. Li vacancies can be introduced through Br doping, which is predicted to result in an order of magnitude larger Li-ionconductivity in β−Li3PS4. Furthermore, the present simulations rationalize the improved Li-ion diffusivity upon O dopingthrough the change in Li distribution in the crystal. Thus, it is demonstrated how a thorough understanding of diffusion, based on thorough analysis of MD simulations, helps to gain insight and develop strategies to improve the ionic conductivity of solid electrolytes.
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