The high Li-ion conductivity of the argyrodite Li₆PS₅Cl makes it a promising solid electrolyte candidate for all-solid-state Li-ion batteries. For future application, it is essential to identify facile synthesis procedures and to relate the synthesis conditions to the solid electrolyte material performance. Here, a simple optimized synthesis route is investigated that avoids intensive ball milling by direct annealing of the mixed precursors at 550 °C for 10 h, resulting in argyrodite Li₆PS₅Cl with a high Li-ion conductivity of up to 4.96 × 10^-3 S cm^-1 at 26.2 °C. Both the temperature-dependent alternating current impedance conductivities and solid-state NMR spin-lattice relaxation rates demonstrate that the Li₆PS₅Cl prepared under these conditions results in a higher conductivity and Li-ion mobility compared to materials prepared by the traditional mechanical milling route. The origin of the improved conductivity appears to be a combination of the optimal local Cl structure and its homogeneous distribution in the material. All-solid-state cells consisting of an 80Li₂S-20LiI cathode, the optimized Li₆PS₅Cl electrolyte, and an In anode showed a relatively good electrochemical performance with an initial discharge capacity of 662.6 mAh g^-1 when a current density of 0.13 mA cm^-2 was used, corresponding to a C-rate of approximately C/20. On direct comparison with a solid-state battery using a solid electrolyte prepared by the mechanical milling route, the battery made with the new material exhibits a higher initial discharge capacity and Coulombic efficiency at a higher current density with better cycling stability. Nevertheless, the cycling stability is limited by the electrolyte stability, which is a major concern for these types of 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.

Phase transitions play a crucial role in Li-ion battery electrodes being decisive for both the power density and cycle life. The kinetic properties of phase transitions are relatively unexplored and the nature of the phase transition in defective spinel Li_{4+ x}Ti₅O₁₂ introduces a controversy as the very constant (dis)charge potential, associated with a first-order phase transition, appears to contradict the exceptionally high rate performance associated with a solid-solution reaction. With the present density functional theory study, a microscopic mechanism is put forward that provides deeper insight in this intriguing and technologically relevant material. The local substitution of Ti with Li in the spinel Li_{4+ x}Ti₅O₁₂ lattice stabilizes the phase boundaries that are introduced upon Li-ion insertion. This facilitates a subnanometer phase coexistence in equilibrium, which although very similar to a solid solution should be considered a true first-order phase transition. The resulting interfaces are predicted to be very mobile due to the high mobility of the Li ions located at the interfaces. This highly mobile, almost liquid-like, subnanometer phase morphology is able to respond very fast to nonequilibrium conditions during battery operation, explaining the excellent rate performance in combination with a first-order phase transition.

Solid-state batteries potentially offer increased lithium-ion battery energy density and safety as required for large-scale production of electrical vehicles. One of the key challenges toward high-performance solid-state batteries is the large impedance posed by the electrode-electrolyte interface. However, direct assessment of the lithium-ion transport across realistic electrode-electrolyte interfaces is tedious. Here we report two-dimensional lithium-ion exchange NMR accessing the spontaneous lithium-ion transport, providing insight on the influence of electrode preparation and battery cycling on the lithium-ion transport over the interface between an argyrodite solid-electrolyte and a sulfide electrode. Interfacial conductivity is shown to depend strongly on the preparation method and demonstrated to drop dramatically after a few electrochemical (dis)charge cycles due to both losses in interfacial contact and increased diffusional barriers. The reported exchange NMR facilitates non-invasive and selective measurement of lithium-ion interfacial transport, providing insight that can guide the electrolyte-electrode interface design for future all-solid-state batteries.

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.

The high lithium conductivity of argyrodite Li₆PS₅Cl makes it an attractive candidate as solid electrolyte in all solid-state Li batteries. Aiming at an optimal preparation strategy the structure and conductivity upon different mechanical milling times is investigated. Li₆PS₅Cl material with high ionic conductivity of 1.1·10⁻³ S/cm was obtained by milling for 8 hours at 550 rpm followed by a heat-treatment at 550 °C. All solid-state Li-S batteries were assembled, combining the Li₆PS₅Cl solid electrolyte, with a carbon-sulfur mixture as positive electrode and Li, Li-Al and Li-In as negative electrode. An optimum charge/discharge voltage window between 0.4 and 3.0 V vs. Li-In was obtained by CV experiments and galvanostatic cycling displays a very large capacity around 1400 mAh/g during the first cycles, decreasing below 400 mAh/g after 20 cycles. Impedance spectroscopy suggests that the origin of the capacity fading is related to an increasing electrode-electrolyte interface resistance.