New strategy for increasing sodium-ion uptake in silicon oxycarbides

Silicon oxycarbides (SiOCs) are considered promising sodium-ion battery anode materials. However, the total and low-voltage plateau capacities of SiOCs are low (below 200 and 95 mAh g(-1), respectively), which hinders the manufacturing of high-density electrodes. Herein a new strategy for improving the electrochemical performance of SiOCs was proposed by considering the Na+ ion storage mechanisms in SiOCs. High-capacity SiOCs were synthesized by increasing the amount of C- and O-rich SiOxCy phases and reducing the amount of inactive SiO2, SiC, and C-free phases. The compositions of SiOCs were controlled by adjusting the synthesis conditions (e.g., sweep gas and pyrolysis time) and C content of the SiOC precursors. The use of N-2 sweep gas instead of H-2/ Ar suppressed the formation of SiC and Cfree. In addition, the amount of inactive SiO2 phase in SiOCs was further suppressed using a C-rich precursor. Consequently, the synthesized SiOC, which was enriched with C- and O-rich SiOxCy phases, delivered a high reversible capacity of 234 mAh g(-1) at a current density of 25 mA g(-1). In constant-current/constant voltage mode, the reversible capacity was further increased to 299 mAh g(-1), which was close to the theoretical maximum capacity of 315 mAh g(-1). After 140 cycles, the reversible capacity was stabilized to 160 mAh g(-1). The initial capacity loss during long-term cycling, which was mostly caused by the progressive decrease in the low-voltage plateau capacities, was attributed to the increase in cell polarization.

Automated summary

By adjusting the synthesis conditions and the C content of SiOC precursors, the SiOC enriched with C- and O-rich SiOxCy phases can deliver high reversible capacity and maintain stability after long-term cycling.