(Invited) Flexible and Conductive Mxene/Carbon Nanotube Composite Paper for Energy Storage

Carbon nanomaterials, such as carbon nanotubes (CNTs), are attracting much attention as candidates for electrodes of energy storage devices due to their large open surfaces and high conductivity. Recently, MXenes, a new family of 2D carbides, have shown their great promise for electrodes in supercapacitors and Li-ion batteries. [1, 2] Herein, our new results show that the incorporation of CNTs into MXene films results in composite papers with much improved performance. [3] Free-standing sandwich-like MXene/CNT composite papers composed of alternating layers of MXene and CNTs were fabricated by vacuum filtration method. The resulting “papers” have good flexibility, high electrical conductivity and a structure accessible to electrolyte ions. They exhibit much better performance than pure MXene films and the randomly mixed MXene/CNT composite papers. When employed as electrodes for supercapacitors in aqueous electrolyte, capacitances of ~390 F/cm 3 were achieved. Besides, the composites showed ~75% increase in rate performance compared to pure MXene films and exhibited no capacity degradation after 10,000 cycles. The MXene/CNT composite papers can also serve as stable anodes for high-rate Li-ion batteries. Highly reversible and stable capacity of ~250 mAh/g was yielded by the titanium-based MXene/CNT composites at 0.5 C (2 hrs charge/discharge), about 10 times of that for pure MXene films. The niobium-based MXene/CNT composites achieved a high reversible capacity of ~650 mAh/g at 0.1 C (10 hrs charge/discharge). At 10 C (6 min charge/discharge), a reversible capacity of ~300 mAh/g was retained for >300 cycles, indicating excellent rate performance and cycling stability. These conductive, flexible and mechnaically strong papers are promising materials that can be used in wearable or structural electrochemical energy storage and conversion systems. [1] M. Naguib, et al., MXenes: A New Family of Two-Dimensional Materials, Advanced Materials , 26 , 992-1005 (2014) [2] M. R. Lukatskaya, et al., Cation Intercalation and High Volumetric Capacitance of Two-dimensional Titanium Carbide, Science , 341 , 1502-1505 (2013) [3] M. Q. Zhao, et al. Flexible MXene/Carbon Nanotube Composite Paper with High Volumetric Capacitance, Advanced Materials , in press (2014)