Ultrafine Sodium Sulfide Clusters Confined in Carbon Nano-polyhedrons as High-Efficiency Presodiation Reagents for Sodium-Ion Batteries

Sodium loss at the anode in the initial sodiation process significantly reduces the energy density of sodium-ion batteries (SIBs). Here, a high-capacity Na₂S/C nanocomposite featuring ultrafine Na₂S nanoclusters (<2 nm) confined in ZIF-8-derived microporous N-doped carbon is fabricated and employed as a cathode presodiation reagent to compensate for this sodium loss and increase the energy density of SIBs. The ultrafine size of Na₂S enables fast reaction kinetics for sodium extraction and the carbon matrix provides good electronic conductivity. Also, the overall particle size of the Na₂S/C nanocomposite (∼40 nm) is close to that of conductive additive. The above features enable it to replace a partial amount of conductive additive and compensate for the sodium loss at the anode concurrently. As a demonstration, the Na₃V₂(PO₄)₃ electrode with 5 wt % Na₂S/C and 5 wt % carbon black was fabricated, and it displayed a 19 mAh g^-1 higher initial charge specific capacity than that of the counterpart with 10% carbon black without the addition of Na₂S/C, realizing an increased energy density from 178 to 263 Wh kg^-1 in the full cell configuration pairing with a hard carbon anode. Moreover, a stable cycling performance up to 200 cycles with an average capacity loss of 0.024 mAh g^-1 per cycle was achieved for the presodiated Na₃V₂(PO₄)₃ electrode.