Sub-nanometric amorphous V-O clusters without grain boundaries bonded in yolk-shell carbon nanospheres for superior sodium-ion storage

The energy density of sodium-ion batteries (SIBs) reported so far is still low, which weakens their competi-tiveness in practical applications. Cluster engineering and sub-nanometric design are effective strategies for rapid and efficient storage of alkaline ions. Herein, a durable yolk-shell architecture is developed via a facile self-polymerization and calcination process, in which the sub-nanometric amorphous vanadium-oxide (V-O) clus-ters are bonded in N-doped carbon nanospheres (A-VOxC/N-C). As a SIB anode, the composite delivers a large reversible capacity (491.6 mAh g-1 at 0.05 A g-1), high-rate capability and ultra-long cyclic stability (246.4 mAh g-1 with 96.6% capacity retention after 30000 cycles at 20.0 A g-1). In addition, the full cell also exhibits great potential for practical applications. The detailed structural characterizations and density functional theory (DFT) calculations suggest that the amorphous clusters without grain boundaries are crucial for the creation of abundant active sites and highly isotropic channels, as well as the reduction of Na-intercalation stress. The yolk-shell N-C has the structural advantages of flexible cavity, strong bonding force and large spatial constraint, which can effectively alleviate volume changes and strengthen structural stability. In view of these, this study can provide guidance for the design of sub-nanometric clusters and their potential applications in alkaline ion batteries.