Understanding the effect of depressing surface moisture sensitivity on promoting sodium intercalation in coral-like Na3.12Fe2.44(P2O7)2/C synthesized via a flash-combustion strategy

The low-cost sodium iron pyrophosphate Na3.12Fe2.44(P2O7)(2), which enables facile ion transport, presents a promising alternative for use as the cathode of sodium ion batteries. However, high moisture sensitivity and severe surface oxidation lead to a large irreversible capacity and inferior electrochemical kinetics. Herein, for the first time, we report the design of an ultrafast "flash-combustion synthesis" strategy to prepare coral-like Na3.12Fe2.44(P2O7)(2)/C with good surface properties and fast sodium intercalation. The single-phase Na3.12Fe2.44(P2O7)(2) is successfully prepared in as short a time as three minutes. Each ultrafine Na3.12Fe2.44(P2O7)(2) particle with a nanoscale carbon coating is enwrapped in a microscale carbon matrix, forming a coral-like architecture. Benefiting from the hierarchical carbon decoration, the coral-like composite attains good surface properties that enable low moisture sensitivity and fast sodium intercalation. Moreover, dynamic analysis reveals that surface oxidation proceeds through a "shell-core" mechanism and demonstrates the crucial role of the surface properties on sodium intercalation. Taking advantage of the good surface properties and the hierarchical porous architecture, the coral-like composite is capable of fast sodium intercalation and stable prolonged cycling. It retains 95% of the initial capacity after 200 cycles alternating between 20 and 5C rates. The clarification of the correlation between the surface properties and the sodium intercalation chemistry provides clues to the design and construction of high-performance Na3.12Fe2.44(P2O7)(2) for large-scale applications.