Bidirectional Construction of 3D Flexible Ti₃C₂T_x MXene Films for High-Performance Lithium-Ion Capacitors

Abstract

Lithium-ion capacitors (LICs) are considered ideal devices, which bridge the energy and power density gap between lithium-ion batteries (LIBs) and supercapacitors (SCs). However, the mismatched kinetics between the cathode and anode remains an obstacle to the development of LICs. Herein, an anode with excellent flexibility and fast electrochemical reaction kinetics is designed for advanced LICs by coupling highly conductive singlewalled carbon nanotubes (CNTs) with the bidirectionally designed Ti 3 C 2 T x MXene (KTi 3 C 2 -O). In such a composite (KTi 3 C 2 -O/ CNTs), the bidirectional design of Ti 3 C 2 T x MXene based on the interlayers of K + ions intercalation and interfaces of −O terminal groups modification will increase the interlayer distance, provide more active sites, and improve Li + ions storage capacity; the introduction of CNTs forming a three-dimensional (3D) interpenetrating structure with KTi 3 C 2 -O can alleviate Ti 3 C 2 T x MXene interlayer stacking and offer fast charge transfer kinetics. When evaluated as a self-supported anode of LIC, the LIC displays a high power density of 15.63 kW kg −1 , a high energy density of 138.89 Wh kg −1 , and an exceptional capacity retention of 77.75% over 10 000 cycles at 5 A g −1 . Such a bidirectional construction strategy based on interlayer and interfacial modification provides new ideas for the design of such two-dimensional (2D) materials that can be applied in advanced energy storage devices.

Recent Advances in Hybrid Lithium-Ion Capacitors: Materials and Processes

“…Ti 3 C 2 T x , the typical representative among MXenes, is the most widely used . Ti 3 C 2 T x -based composite materials have metal-level conductivity, open structure, weak interlayer forces, and high specific surface area and have good application prospects in LICs. , Although Ti 3 C 2 T x nanosheets have excellent electrochemical performances, similar to graphene, there is a restacking problem.…”

Short-Range Ordered Porous Carbon Derived from Confined-Region Activation Strategy Exhibits Excellent High-Loading Performance in Supercapacitors

“…Numerous mainstream rechargeable electrochemical energy storage systems, including lithium-ion batteries, sodium-ion batteries, supercapacitors (SCs), and lithium–sulfur batteries, have been developed and gained extensive attention in the world due to their high energy density and high power density coupled with a long cycling life. − Among these systems, SCs have combined the advantages of high energy/power density, ultralong cycling, and a fast charging and discharging performance. − Thus, SCs play an important role in providing rapid and long cycling life power for ubiquitous mobile energy storage devices, the transportation sector, and hybrid electric vehicles. − Symmetric electrochemical double-layer capacitors (EDLCs) are a class of SCs that store and release charge by electrochemical adsorption and desorption for electrolyte ions at the interface between the electrolyte and the active materials . Activated carbon, − graphene, template carbon, and carbon nanotubes have been regarded as the commonly used electrochemical active materials for EDLC devices.…”

Weakening Mo–H Bond of Mo2C MXene Cocatalyst by Increased Antibonding-Orbital Occupancy State for Superior Photocatalytic Hydrogen Production

“…MXenes, a large and expanding family of two-dimensional layered transition metal carbides or nitrides, are synthesized by treating the ternary layered M n +1 AX n (MAX) compounds (where M is transition metal, A is mainly IIIA and IVA elements in the periodic table, X is carbides and/or nitrides, and n is a number between 1 and 4). − The strong ionic bonding in MAX phases is selectively damaged by chemical etching of A layer elements with etchants to obtain their two-dimensional derivatives, MXenes. − The resultant like-layered van der Waals bonding in MXenes provides space for intercalating various guest species, leading to anions (such as F – , Cl – , O 2– , and OH – ) as termination species T that spontaneously coordinate with exposed M atoms of MXenes, as described by the M n +1 X n T x formula. − Among various MXenes, Mo 2 C MXene (Mo 2 CT x ) is a type of MXene which is commonly synthesized through the process of etching Ga layers from the Mo 2 Ga 2 C phase using HF or other etchants. , Owing to its excellent metallic conductivity and active Mo sites with Pt-like d-band structure through hybridization of the Mo d-orbitals and C s/p-orbitals, Mo 2 C MXene displays tremendous potential as a non-noble metal cocatalyst for the hydrogen evolution reaction (HER). − However, compared with the well-known and suitable Pt–H bond (60 kcal mol –1 ), the strong Mo–H bonds (65–75 kcal mol –1 ) of Mo 2 C MXene exhibit a relatively high hydrogen adsorption ability, resulting in the fact that the produced H 2 cannot be well desorbed from the surface of Mo atoms. − Moreover, the highly electronegative F-terminal groups on the edge of traditional Mo 2 CT x reveal strong nucleophilicity due to possible residue of F-containing etchants, which further enhances the strength of the Mo–H bonds. , Obviously, the above results usually lead to the weak desorption of produced H 2 from their surface, so that the Mo 2 CT x cocatalyst exhibits a very limited hydrogen-evolution activity. Therefore, it is necessary to optimize the Mo–H bonds of Mo 2 CT x to enable moderate atomic H adsorption for efficient HER activity.…”