Thermodynamic Control of Facet Chemistry for Precise Solid-State Synthesis of Na Layered Cathodes

Precise facet control in chemical synthesis is of significant interest not only for fundamental surface chemistry but also for its direct implications in various technologies such as batteries, catalysis, semiconductors, and beyond. Previously, facet control has been achieved in wet chemistry via surfactant-directed crystal growth; however, it remains a challenge in solid-state synthesis, where high-temperature reaction conditions preclude the use of surfactant-based kinetic controls and enforce a strong thermodynamic driving force toward equilibrium crystal shapes, often with undesirable facet exposure. Here, using single-crystal Na layered oxide as an example, we decipher the dependence of facet energy on chemical potential, thus establishing a predictive synthetic map for solid-state facet control. We further reveal that regulating surface transition-metal redox activity enables direct thermodynamic control over equilibrium crystal shapes. The facet-tailored layered oxide features an ellipsoidal shape, with markedly reduced length-to-height ratio of only 1.86 (vs 6.75 of conventional plate-like crystals). This strategy effectively minimizes the exposure of electrochemically inert (001) facets, thereby achieving excellent capacity retention (80% over 500 cycles at 5 C) and superior rate performance (106.4 mAh g^-1 at 5 C) that surpasses polycrystalline counterparts. Particularly, the ellipsoid-shaped particles also enable a record electrode density of 4.03 g cm^-3. Our work establishes a general chemical paradigm to facilitate the rational facet control in solid-state reactions, which not only boosts the electrochemical performance of layered cathodes but also has significant implications for the chemical design and synthesis of a broad range of functional materials.