Two dimensional (2D) materials such as graphene have attracted tremendous interest and have shown promising potential in various applications due to their unique physical structure and properties. Another new class of 2D materials, a group of early transition metal carbides and/or carbonitrides, called MXene, have been studied and reported recently. MXenes are chemically derived from layered M_n+1AX_n or MAX phases, where M is early transition metal, A is an A-group element (mainly groups 13 and 14), X is C and/or N, and n=1,2, and 3. Since discovery of the first MXene (Ti₃C₂) in 2011, more than 10 new MXenes have been synthesized and demonstrated its interesting and unique properties in many applications. It has been shown that MXenes have promising potential in electrochemical energy storage applications such as lithium ion batteries (LIBs) and electrochemical capacitors (ECs), due to their high electrical conductivity, good structural/chemical stability, and large surface areas.

Recently, MXene application studies are expanding to other battery systems such as Na-ion batteries (NIBs). The growing concerns on the cost and resource limit of lithium for large scale applications have triggered the development of NIBs as an alternative battery system to LIBs for large scale energy storage applications due to their potentially low-cost and natural abundance. Moreover, the accumulated knowledge and technology of LIBs enables fast-advancements of NIBs research since the operating principles of the NIBs are similar to that of “rocking-chair” mechanism in LIBs. Although many concepts can be adopted from LIB in NIB research, the electrochemistry of NIBs turns out to be different in many aspects requiring new design of electrode materials and electrolyte for improving and/or optimizing performance of NIBs. In particular, Na⁺ has a larger radius than Li⁺, which directly affects the mass transport and storage in the electrochemical reaction. This makes many of current LIB anode materials, such as graphite, unsuitable for NIBs. In this regards, the laminar nature of MXenes bonded by weak van der walls forces and its inherent interlayer spacing are very attractive as anode materials for NIBs. However, most of studies on the electrochemical behavior of MXenes as anode materials for NIBs are just at the theoretical estimation only without much experimental support.

In this study, we present a systematic study on the electrochemical properties and their charge storage mechanisms of V₂C and Nb₂C MXenes as anode materials for NIBs. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were used in both ex situ and in situ way to look at the structural changes during electrochemical cycling. By utilizing such combined X-ray techniques, we are able to get better understanding of structural/electronic structure changes of MXenes and their related charge storage behavior during sodiation/desodiation process. A systematic comparison of electrochemical properties and charge storage mechanism for MXenes between NIBs and LIBs as well as the approaches to improve them will also be discussed.

Acknowledgement

The work done at Brookhaven National Lab was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, under Contract Number DE-SC0012704. The work performed at Drexel University was supported by the Office of Electricity Delivery and Energy Reliability, Energy Storage Systems Program, through Sandia National Laboratories. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.