Facet-Controlled NaO₂ Crystal Growth via Microporosity Modulation in Commercial Carbon Air Cathodes for Sodium–Oxygen Batteries

Aprotic sodium-oxygen batteries (Na-O₂) are promising high-capacity energy storage devices, where the electrodeposition of sodium superoxide (NaO₂) on a carbon-based air cathode can theoretically achieve an energy density of 1086 Wh kg^-1. This study demonstrates the significant influence of micropores in commercial graphene nanoplatelets (GNPs) on the nucleation and growth of NaO₂. The findings reveal that micropores could promote anisotropic interactions with reduced oxygen species formed during discharge. Adsorption-induced confinement of oxygen or superoxide species within the micropores facilitates preferential growth along specific crystallographic orientations, leading to the formation of 2D plate-like structures. This behavior contrasts with the isotropic 3D cubic forms typically observed on nonporous or mesoporous substrates. In GNPs with a medium surface area (300 m² g^-1), a certain contribution of anisotropic growth can reduce cycling hysteresis and enhance Coulombic efficiency of the cell. Understanding the relationship among the microporous architecture of carbon cathodes, NaO₂ growth dynamics, and facet control is crucial for optimizing the efficiency and durability of Na-O₂ batteries.