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.