Abstract
To design a synthesis route for energy storage materials, the phase diagram is a useful starting point. However, non-equilibrium intermediates often appear during synthesis—which are difficult to anticipate and often persist as impurities in the final reaction product. In addition, intermediate phases may template the morphology of particles. Thus, being able to rationalize and predict which metastable intermediates form during solid-state reactions is crucial towards developing rational frameworks in which to create predictive ceramic synthesis approaches.
Guided by real-time in situ synchrotron observations of materials formation, we develop a new theory to model the reactions between ceramic precursors to form NaxMO2 layered oxides (M = Co, Mn), which are important cathode materials for Na-batteries. These layered oxides form in two polytypes: a two-layer (P2) stacking, and a three-layer (O3 or P3) stacking. Even though phase diagrams determine that P2 is the stable phase at Na0.66CoO2 composition, synthesis using a nominal 0.33 Na2O2 + CoO precursor mixture is observed to first form the O3-NaCoO2. The reaction then proceeds sequentially through non-equilibrium O3'and P3 phases, with the equilibrium P2-Na0.66CoO2 only obtained after long time at high temperature. This raises the fundamental question—how do thermodynamic driving forces and kinetic mechanisms conspire to create phases that are not thermodynamic stable, and why can some phases, which are thermodynamically favored at all temperatures, only be formed after long times at high temperature? We show that the first phase to form from the precursors is actually the compound which consumes the most reaction energy, irrespective of whether this compound is at the target stoichiometry. After this initial compound forms, a combination of fast topotactic reactions and slow nucleation and growth processes slowly bring the system to the equilibrium phase.
Based on this insight we can now predict how different precursors change the reaction pathway, driving the crystallization pathway through different intermediates. As an example, we show how P2-Na0.66MnO2 can be formed through very different reaction pathways by changing the precursors. This rationalization of the first phase to form creates a valuable design handle by which reaction paths can be tailored to go through, or circumvent, specific non-equilibrium intermediates. Our combined computational and experimental approach offers a predictive framework for the synthesis design of new energy materials.