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.

The high Li-ion conductivity of the Li₇P₃S₁₁ sulfide-based solid electrolyte makes it a promising candidate for all-solid-state lithium batteries. The Li-ion transport over electrode-electrolyte and electrolyte-electrolyte interfaces, vital for the performance of solid-state batteries, is investigated by impedance spectroscopy and solid-state NMR experiments. An all-solid-state Li-ion battery is assembled with the Li₇P₃S₁₁ electrolyte, nano-Li₂S cathode and Li-In foil anode, showing a relatively large initial discharge capacity of 1139.5 mAh/g at a current density of 0.064 mA/cm² retaining 850.0 mAh/g after 30 cycles. Electrochemical impedance spectroscopy suggests that the decrease in capacity over cycling is due to the increased interfacial resistance between the electrode and the electrolyte. 1D exchange ⁷Li NMR quantifies the interfacial Li-ion transport between the uncycled electrode and the electrolyte, resulting in a diffusion coefficient of 1.70(3)⋅10⁻¹⁴ cm²/s at 333 K and an energy barrier of 0.132 eV for the Li-ion transport between Li₂S cathode and Li₇P₃S₁₁ electrolyte. This indicates that the barrier for Li-ion transport over the electrode-electrolyte interface is small. However, the small diffusion coefficient for Li-ion diffusion between the Li₂S and the Li₇P₃S₁₁ suggests that these contact interfaces between electrode and electrolyte are relatively scarce, challenging the performance of these solid-state batteries.

Computational modeling is vital for the fundamental understanding of processes in Li-ion batteries. However, capturing nanoscopic to mesoscopic phase thermodynamics and kinetics in the solid electrode particles embedded in realistic electrode morphologies is challenging. In particular for electrode materials displaying a first order phase transition, such as LiFePO₄, graphite, and spinel Li₄Ti₅O₁₂, predicting the macroscopic electrochemical behavior requires an accurate physical model. Herein, a thermodynamic phase field model is presented for Li-ion insertion in spinel Li₄Ti₅O₁₂ which captures the performance limitations presented in literature as a function of all relevant electrode parameters. The phase stability in the model is based on ab initio density functional theory calculations and the Li-ion diffusion parameters on nanoscopic nuclear magnetic resonance (NMR) measurements of Li-ion mobility, resulting in a parameter free model. The direct comparison with prepared electrodes shows good agreement over three orders of magnitude in the discharge current. Overpotentials associated with the various charge transport processes, as well as the active particle fraction relevant for local hotspots in batteries, are analyzed. It is demonstrated which process limits the electrode performance under a variety of realistic conditions, providing comprehensive understanding of the nanoscopic to microscopic properties. These results provide concrete directions toward the design of optimally performing Li₄Ti₅O₁₂ electrodes.

The main challenge of sodium-ion batteries is cycling stability, which is usually compromised due to strain induced by sodium insertion. Reliable high-voltage cathode materials are needed to compensate the generally lower operating voltages of Na-ion batteries compared to Li-ion ones. Herein, density functional theory (DFT) computations were used to evaluate the thermodynamic, structural, and kinetic properties of the high voltage λ-Mn₂O₄ and λ-Mn_1.5Ni_0.5O₄ spinel structures as cathode materials for sodium-ion batteries. Determination of the enthalpies of formation reveal the reaction mechanisms (phase separation vs solid solution) during sodiation, while structural analysis underlines the importance of minimizing strain to retain the metastable sodiated phases. For the λ-Mn_1.5Ni_0.5O₄ spinel, a thorough examination of the Mn/Ni cation distribution (dis/ordered variants) was performed. The exact sodiation mechanism was found to be dependent on the transition metal ordering in a similar fashion to the insertion behavior observed in the Li-ion system. The preferred reaction mechanism for the perfectly ordered spinel is phase separation throughout the sodiation range, while in the disordered spinel, the phase separation terminates in the 0.625 < x < 0.875 concentration range and is followed by a solid solution insertion reaction. Na-ion diffusion in the spinel lattice was studied using DFT as well. Energy barriers of 0.3-0.4 eV were predicted for the pure spinel, comparing extremely well with the ones for the Li-ion and being significantly better than the barriers reported for multivalent ions. Additionally, Na-ion macroscopic diffusion through the 8a-16c-8a 3D network was demonstrated via molecular dynamics (MD) simulations. For the λ-Mn_1.5Ni_0.5O₄, MD simulations at 600 K bring forward a normal to inverse spinel half-transformation, common for spinels at high temperatures, showing the contrast in Na-ion diffusion between the normal and inverse lattice. The observed Ni migration to the tetrahedral sites at room temperature MD simulations explains the kinetic limitations experienced experimentally. Therefore, this work provides a detailed understanding of the (de)sodiation mechanisms of high voltage λ-Mn₂O₄ and λ-Mn_1.5Ni_0.5O₄ spinel structures, which are of potential interest as cathode materials for sodium-ion batteries.

Density functional theory (DFT) molecular dynamics (MD)-simulations were performed on cubic and tetragonal Na₃PS₄. The MD simulations show that the Na-conductivity based on the predicted self-diffusion is high in both the cubic and tetragonal phases. Higher Na-ion conductivity in Na₃PS₄ can be obtained by introducing Na-ion vacancies. Just 2% vacancies result in a conductivity of 0.2 S/cm, which is an order of magnitude larger than the calculated conductivity of the stoichiometric compound. MD simulations of halogen-doped cubic Na₃PS₄ suggest a practical route to introduce vacancies, where Br-doping is predicted to result in the highest bulk conductivity. Detailed investigation of the Na-ion transitions during the MD simulation reveals the role of vacancies and phonons in the diffusion mechanism. Furthermore, the orders of magnitude difference between the MD simulations and experiments suggest that macroscopic conductivity can be significantly increased by reducing the grain boundary resistance.

Using density functional theory molecular dynamics simulations, the origin of the Li-ion conductivity in argyrodite solid electrolytes is investigated. The simulations show that besides Li-ion vacancies in Li₆PS₅Cl and Li₆PS₅Br, the influence of halogen atoms on their local surroundings also plays an important role in Li-ion diffusion. The difference in Li-ion conductivity between Li₆PS₅Cl and Li₆PS₅I, which is several orders of magnitude, is caused by the distribution of the halogen ions over the available crystallographic sites. This suggests that altering the halogen distribution in Li argyrodites during synthesis could increase the Li-ion conductivity of these materials. For Li₆PS₅Cl, the simulations predict an optimal Cl distribution of 1:3 over sites 4a and 4c, resulting in a Li-ion conductivity that is 2 times larger than that of the currently prepared materials. On the basis of these results, simulations were performed on Li₅PS₄X₂ (X = Cl, Br, or I), which show Li-ion conductivities similar to those of Li₆PS₅Cl and Li₆PS₅Br, suggesting that the Li₅PS₄X₂ compounds are interesting new compositions for solid state electrolytes.

Tetragonal and cubic phase Na₃PS₄ sodium electrolytes were successfully prepared by a relatively low rotation speed mechanical milling (400 rpm) route, aiming at homogeneous materials. The influence of the mechanical milling and annealing on the structure and ionic conductivity are studied by XRD and impedance spectroscopy, giving insight into the optimal mechanical synthesis conditions. Fourier analysis of the XRD data, compared to DFT based MD simulations reflects the diffusion pathway, where the simulations indicate a vacancy induced high bulk Na-ion mobility in both cubic and tetragonal phases. ²³Na solid-state NMR relaxation experiments were applied to investigate the Na-ion bulk diffusion in both the cubic and tetragonal phases, showing reasonable agreement with the MD simulation results. The MD simulations indicate that the bulk mobility of both phases may be further improved by introducing more Na vacancies. The macroscopic ionic conductivity probed by impedance spectroscopy is much smaller than that predicted by the bulk Na-ion mobility, in particular for the tetragonal phase, suggesting a large impact of amorphous phase fractions and/or grain boundaries on the macroscopic Na-ion conductivity. In particular in the less crystalline cubic phase, the amorphous fraction present as a consequence of the lower annealing temperature suggests that this phase may lead to a decrease in grain boundary resistance, which may be further exploited to improve the performance of all solid state Na-ion batteries with the Na₃PS₄ solid electrolyte.

One of the main challenges of all-solid-state Li-ion batteries is the restricted power density due to the poor Li-ion transport between the electrodes via the electrolyte. However, to establish what diffusional process is the bottleneck for Li-ion transport requires the ability to distinguish the various processes. The present work investigates the Li-ion diffusion in argyrodite Li₆PS₅Cl, a promising electrolyte based on its high Li-ion conductivity, using a combination of ⁷Li NMR experiments and DFT based molecular dynamics simulations. This allows us to distinguish the local Li-ion mobility from the long-range Li-ion motional process, quantifying both and giving a coherent and consistent picture of the bulk diffusion in Li₆PS₅Cl. NMR exchange experiments are used to unambiguously characterize Li-ion transport over the solid electrolyte-electrode interface for the electrolyte-electrode combination Li₆PS₅Cl-Li₂S, giving unprecedented and direct quantitative insight into the impact of the interface on Li-ion charge transport in all-solid-state batteries. The limited Li-ion transport over the Li₆PS₅Cl-Li₂S interface, orders of magnitude smaller compared with that in the bulk Li₆PS₅Cl, appears to be the bottleneck for the performance of the Li₆PS₅Cl-Li₂S battery, quantifying one of the major challenges toward improved performance of all-solid-state batteries.