MXene as a Charge Storage Host

Abstract

The development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., TiCT , TiCT , VCT , and NbCT , where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins. MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes...

2 D MXene‐based Energy Storage Materials: Interfacial Structure Design and Functionalization

“…Owing to the high electrical conductivity, plastic layer structure, small band gap, and large redox‐active surface areas, MXenes and MXene‐based materials have attracted extensive attention for energy‐storage applications . However, the electrochemical performance of raw MXenes is often limited because of the following several reasons.…”

Fluorination of MXene by Elemental F2 as Electrode Material for Lithium‐Ion Batteries

“…Regardless of the method used to prepare MXene, the etching results in F and OH surface functional groups owing to the presence of HF and water. Given the presence of 2D layers, MXene has been widely studied for its electrochemical applications and specifically for LIBs as an intercalation pseudocapacitor electrode . However, owing to the compact atomic arrangement of MXene that would hinder the diffusion of Li + perpendicular to the layer plane, the electrodes have a reduced power density .…”

Surface homogenizing heterostructure coatings induced by Ti 3 C 2 T x MXene for enhanced cycle performance of lithium‐rich cathode materials

“…Moreover, MXene has excellent electronic conductivity, outstanding Li + diffusion mobility, and good mechanical properties. However, this material is mostly used as a negative electrode material for lithium ion batteries, and few reports have been made in the field of cathode electrodes . Wei et al encapsulated spinel‐phase LiMn 2 O 4 particles in crumpled MXene nanosheets by an electrostatic self‐assembly process, which can greatly increase the rate and cycling ability .…”