Dual-Gradient Concentration Distribution in Sb-Cu Alloy Nanoarrays for Robust Sodium-Ion Storage

Alloy-type materials are attractive for anodes in sodium-ion batteries (SIBs) owing to their high theoretical capacities and overall performance. However, the accumulation of stress/strain during repeated cycling results in electrode pulverization, leading to rapid capacity decay and eventual disintegration, thus hindering their practical applications. Herein, we report a 3D coral-like Sb-Cu alloy nanoarray with gradient distribution of both elements. The array features a Sb-rich bottom and a Cu-rich top with increasing Sb and decreasing Cu concentrations from top to bottom. The former is the active component that provides the high capacity, whereas the latter serves as an inert additive that acts against volume variation. The gradual transition in composition within the electrode introduces a ladder-type volume expansion effect, facilitating a smooth distribution and effective release of stress, thereby ensuring the wanted mechanical stability and structural integrity. The as-developed nanoarray affords a high reversible capacity (460 mAh g^-1 at 0.5 C), stable cycling (89 % retention over 120 cycles at 1.0 C), and superior rate capability (354 mAh g^-1 at 10 C). The concentration dual-gradient strategy paves a new pathway of designing alloy-type materials for SIBs.