Tunable pseudocapacitance storage of MXene by cation pillaring for high performance sodium-ion capacitors

2D transition metal carbide materials called MXene have attracted significant interest in the field of electrochemical energy storage due to their high electrical conductivity and high volumetric capacity. However, the low capacity accompanied by sluggish sodiation kinetics of electrodes made from multi-layer MXene has limited their further application for sodium ion storage. The key challenge to overcome the abovementioned issue is to decrease the Na⁺ diffusion barrier and increase the active site concentration in MXene electrodes used for Na⁺ storage. In this study, a method to significantly improve the capacity and kinetics of Ti₃C₂ MXene for Na⁺ storage using facile alkali metal ion pillaring is reported. After Na⁺ pillaring, the MXene sheets (Na–Ti₃C₂) with incremental interlayer spacing exhibit a high reversible capacity of 175 mA h g⁻¹ (∼170% of the original value) at 0.1 A g⁻¹ and an excellent outstanding cycling stability for 2000 cycles at 2.0 A g⁻¹ for sodium ion storage. By combining ex situ XPS with kinetics analysis, the increased number of active sites and lower Na⁺ diffusion barrier were confirmed after Na⁺ pillaring when compared with the cases of Ti₃C₂, Li–Ti₃C₂, and K–Ti₃C₂. The role of the terminal groups (–OH) in Na–Ti₃C₂ has also been confirmed by analysis of the electrochemical performance of the annealed Na–Ti₃C₂ samples (450 °C and 700 °C). The results show that the existence of –OH groups in Na–Ti₃C₂ can increase the number of Na⁺ storage active sites, but decrease the kinetics. By coupling the Na–Ti₃C₂ anode with an AC cathode, the assembled SIC device delivers a high energy density of 80.2 W h kg⁻¹ and high power density (6172 W kg⁻¹) with an ultra-long and stable cycling performance (capacity retention: ∼78.4 at 2 A g⁻¹ after 15 000 cycles).