Nitrogen/oxygen codoped hierarchical porous Carbons/Selenium cathode with excellent lithium and sodium storage...

A nitrogen/oxygen codoped carbon derived from sweet potato (SPC) with interconnected micro-mesopores is applied to encapsulate selenium composite (SPC/Se) with a high Se loading (74.3%). As a cathode for advanced Li-Se and Na-Se batteries, the SPC/Se exhibits superior electrochemical behavior in low-cost carbonate electrolyte. Including the hierarchically porous structure of SPC and the chemical bonding between Se and carbon, the strong binding energy between SPC and Li₂Se/Na₂Se is also proved by DFT method, which results in the effective mitigation of shuttle reaction and volume change for SPC/Se cathode. For Li-Se batteries, the SPC/Se composite shows the initial specific charge capacity of 668 mAh g^-1 with a high initial coulombic efficiency of 78%, and maintains a stable reversible capacity of 587 mAh g^-1 after 1000 cycles with a weak capacity decay of 0.082% at 0.2C. It still retains a reversible specific capacity of 375 mAh g^-1 even at 20C. For Na-Se battery, the SPC/Se composite displays the initial specific charge capacity of 671 mAh g^-1 at 0.2C and maintains a reversible specific capacity of 412 mAh g^-1 after 500 cycles with a capacity retention of 61.4%. When the current density increases to 20C, it still delivers a high reversible specific capacity of 420 mAh g^-1. Finally, the transformation mechanism of Se molecule is illustrated detailedly in (de)lithi/sodiation process.