Solid‐state synthesis from powder precursors is the primary processing route to advanced multicomponent ceramic materials. Designing reaction conditions and precursors for ceramic synthesis can be a laborious, trial‐and‐error process, as heterogeneous mixtures of precursors often evolve through a complicated series of reaction intermediates. Here, ab initio thermodynamics is used to model which pair of precursors has the most reactive interface, enabling the understanding and anticipation of which non‐equilibrium intermediates form in the early stages of a solid‐state reaction. In situ X‐ray diffraction and in situ electron microscopy are then used to observe how these initial intermediates influence phase evolution in the synthesis of the classic high‐temperature superconductor YBa₂Cu₃O_6+x (YBCO). The model developed herein rationalizes how the replacement of the traditional BaCO₃ precursor with BaO₂ redirects phase evolution through a low‐temperature eutectic melt, facilitating the formation of YBCO in 30 min instead of 12+ h. Precursor selection plays an important role in tuning the thermodynamics of interfacial reactions and emerges as an important design parameter in planning kinetically favorable synthesis pathways to complex ceramic materials.

A solid electrolyte with a small particle size, good mechanical properties and high ionic conductivity is required to achieve high energy and power density in the all-solid-state battery. Here, we report an instantaneous preparation of high lithium-ion conducting sulfide solid electrolyte Li₇P₃S₁₁ by a simple procedure involving a liquid phase process under ultrasonic irradiation and low thermal treatment at 220 °C. A short reaction time of 30 min was enough to produce the formation of PS₄ ³⁻ units. Subsequent drying and heating processes led to the precipitation of the Li₇P₃S₁₁ phase with a particle size below 500 nm, achieving high ionic conductivity of 1.0 × 10⁻³ S cm⁻¹ at 22 °C and a low activation energy of 12.8 kJ mol⁻¹.

An effective strategy to lower interfacial resistance between NMC and a garnet-structured solid electrolyte with a Li ₂ SiO ₃ interlayer.