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 significantly 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.3 All-solid-state-batteries (ASSBs) are attracting ever increasing attention due to their high intrinsic safety, achieved by replacing the flammable and reactive liquid electrolyte by a solid electrolyte 1 . In addition, a higher energy density in ASSBs may be achieved through; (a) bipolar stacking of the electrodes, which reduces the weight of the non-active battery parts and (b) by potentially enabling the use of a Li-metal anode, which possesses the maximum theoretical Li capacity and lowest electrochemical potential (3860 mAhg -1 and -3.04 V vs. SHE). First of all, the success of ASSBs relies on solid electrolytes with a high Li-ion conductivity 2-5 . A second prerequisite, is the electrochemical stability at the interfaces of the solid electrolyte with the electrode materials in the range of their working potentials. Any electrochemical decomposition of the solid electrolyte may lead to decomposition products with poor ionic conductivity that increase the internal battery resistance 2-4,6 . Third, ASSBs require mechanical stability as the changes in volume of the electrode materials upon (de)lithiation, as well as decomposition reactions at the electrode-electrolyte interface may lead to contact loss, also increasing the internal resistance and lowering the capacity 2-4 .

Molecular dynamics simulations are a powerful tool to study diffusion processes in battery electrolyte and electrode materials. From molecular dynamics simulations, many properties relevant to diffusion can be obtained, including the diffusion path, amplitude of vibrations, jump rates, radial distribution functions, and collective diffusion processes. Here it is shown how the activation energies of different jumps and the attempt frequency can be obtained from a single molecular dynamics simulation. These detailed diffusion properties provide a thorough understanding of diffusion in solid electrolytes, and provide direction for the design of improved solid electrolyte materials. The presently developed analysis methodology is applied 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 molecular dynamics simulations. The code used for the analysis is freely available at: . 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-ion diffusivity. Li vacancies can be introduced through Br doping, which is predicted to result in an order of magnitude larger Li-ion conductivity in β–Li3PS4. Furthermore, the present simulations rationalize the improved Li-ion diffusivity upon O doping through 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.

Understanding the relationship between structure, ionic conductivity, and synthesis is the key to the development of superionic conductors. Here, a series of Li3‐3xM1+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/Li2.73Ho1.09Cl6/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.

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.

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...