Insights into the carbon nanotube effect on the electrochemical performance of red phosphorus anode

Red phosphorus (Red P) is considered the most promising anode material for sodium-ion batteries (SIBs) due to it has the highest theoretical capacity. However, large volume expansion leading to rapid capacity decrease, and low conductivity affecting its fast charging/discharging ability, which hinders the practical application. Here, we prepared Red P/SWCNT and Red P/MWCNT by evaporation-condensation method. Red P/SWCNT after 100 cycles, the discharge capacity is 1512.6 mA h g-1, which show stable long-term cycling performance, fast rate capability, and high initial coulomb efficiency (ICE). Because the stress of Red P/SWCNT decreased from 7.2 GPa (Initial cycle) to 0.6 GPa (Long cycle), however, the stress of Red P/MWCNT decreased from 8.5 GPa (Initial cycle) to 1.7 GPa (Long cycle) due to ~30 nm increase of SEI in comparison with SWCNT. Based on small stress, Red P/SWCNT basically has no cracks, and the irreversible Na3P formed has dropped significantly, which is the core of improving the electrochemical performance. This shows that for the high volume expansion and low conductivity Red P anode, building a flexible and highly conductive SWCNT network will eliminate the huge stress due to the volume expansion and prevent its cracks from spreading. This work provides a theoretical guidance towards Red P-based anodes for SIBs.