A sacrificial additive agent mediated strategy for high performance sodium ion capacitor

Abstract Sodium-ion capacitors (SICs) endow extremely potential in electrochemical energy storage system owning to the high energy-power characteristics elemental abundance and environmental friendliness. However, the irreversible sodium loss caused by the consumed Na⁺ for the side reactions and formation of solid-electrolyte interphase (SEI) inevitable lead to the degradation of Na⁺ storage capability for SICs. Herein, an effective pre-sodiation strategy is purposed by using the sodium oxalate (Na₂C₂O₄) as sacrificial additive agent to compensate the reduction of Na⁺ for SICs during the charge-discharge process. The Na₂C₂O₄ possesses highly electrochemical stability at the high potential of 3.6 V and exhibits superior Na⁺ diffusion coefficient than that of active carbon, evidenced by the galvanostatic intermittent titration technique test. Moreover, the as-prepared Na₂C₂O₄ shows intimate interface contact with active carbon derived from the formation of C-O bond between the Na₂C₂O₄ and active carbon, enhancing the structural stability of electrode and insertion-extraction reversibility of Na⁺ upon cycling. Thus, the NICs assembled by pre-sodiation activate carbon cathode and hard carbon anode displays impressive capacitance retention of 95.7% after 13600 cycles and excellent rate ability (high energy density of 59 Wh kg⁻¹ at the power density of 1500 W kg⁻¹). The underlying mechanisms for the excellent electrochemical performance of the assembled SICs are systematic investigated by ex-situ scanning electron microscope and galvanostatic intermittent titration technique. This work paves a way for the development of the pre-sodiation techniques and provide valid guidance for the future research directions of advanced energy storage and transfer equipment.