Monitoring the Microwave Synthesis of d ⁰ ‐Free Disordered Rocksalt Cathodes Using In Situ Infrared Pyrometry

ABSTRACT

A detailed understanding of solid‐state reaction pathways is essential for connecting predictive frameworks, such as density functional theory and machine learning, with experimental synthesis. Microwave synthesis has emerged as a powerful route for preparing inorganic materials, yet the mechanisms governing microwave‐driven processes remain poorly understood, particularly for metastable compounds whose formation is highly sensitive to synthesis conditions. Disordered rocksalt oxides (DRX) are high‐temperature metastable phases of interest as next‐generation Li‐ion cathodes. Here, we investigate the microwave reaction pathway of . Combining ex situ phase identification using x‐ray diffraction and solid‐state NMR with in situ infrared thermography, we show that the reaction proceeds through a reentrant order–disorder–order transformation. Layered Li‐Mn‐O intermediates disorder above 945 to form the DRX phase, while continued heating drives reordering back to layered structures. Infrared profiles reveal a distinct feature marking completion of the disordering transition, enabling precise reaction termination to maximize DRX phase purity. We further examine the impact of phase purity on the “‐phase” transition during electrochemical cycling and find that residual layered phases minimally affect performance. These findings indicate that is only stable near 945, yet its electrochemical performance tolerates synthesis‐induced impurities.