(Invited) Structure Properties Correlation in MXenes: 2D Anodic Materials for Sodium Ion Batteries
Authors: Ruffo Riccardo , Gentile Antonio , Marchionna Stefano , Balordi Marcella , Cernuschi Federico M. 
Published: February 2020

Lithium ion batteries (LIBs) have a predominant role in the market of portable electronics with a production higher than 100 million cells/month consuming about 1500 ton/month of materials. These figures should largely increase in the next years due to the penetration of LIBs in the automotive market. In this scenario, concerns about Li-raw materials availability and distribution arose growing attention towards alternative technologies based on different chemistry. Thanks to the possibility of using similar cell configuration (i.e. Sodium Ion Batteries, SIBs), sodium would require less technological efforts in operating the transition, besides sodium is one of the cheapest elements to be extracted and processed to high purity [1]. Indeed many new projects and prototypes have been reported. For example the French start-up “Tiamat”, founded in 2017 by the French network for electrochemical storage (RS2E), aims to launch a large scale production of sodium ion batteries after the success, achieved with the first prototype of a 1860 sodium ion battery in 2015 [2]. The complete shift from Li-ion to sodium ion battery however, is still limited by the performance of the anodes, especially in terms of cycling stability and rate capability. It is of primary importance to fully understand the sodiation-desodiation mechanism and to elucidate structure-properties relationship of the known active materials.
Recently, MXenes [3], a class of 2D materials obtained by the chemical exfoliation of carbide MAX phases, have been proposed as suitable anodic materials for SIBs devices [4]. The Mn+1AXn, or MAX phases are transition-metal carbides and nitrides with a layered, hexagonal structure where ‘‘M’’ is a transition metal, ‘‘A’’ is a groups 13 or 14 element, and ‘‘X’’ is C and/or N. M-X based MXenes can be obtained by the corresponding MAX phase by chemical or electrochemical etching of the A element, forming a layered structure with large space between two M-X sandwiches (around 1 nm) suitable for ion pseudo-intercalation. The lamellar structure of MXenes facilitates the intercalation of many alkaline and earth-alkali metal ions, on an extended range of charge-recharge rates for thousands number of cycles [5]. Although there are several papers on the use of MXenes as active materials in both SIBs and supercapacitors devices, the correlations among their structure, chemical composition, and the electrochemical properties are seldom investigated.
In the present contribute we discuss how different etching processes of starting Ti3AlC2 phase to produce Ti3C2Tx (T=F, Cl, OH) have a deep influence on the chemical composition (Tx), the crystalline structure, and the morphologies, which in turns rule the electrochemical behavior towards the reaction with the sodium ion. The Ti3C2Tx phase obtained by chemical etching shows a reversible capacity of about 150 mAh g-1 with very low potential hysteresis and an average anodic potential of 1.4 V vs. Na+/Na.
[1] Yabuuchi, K. Kubota, et al., Chemical Reviews 114 (23), 2014, 1163
[2] tiamat-energy.com/
[3] M. Doeff, Y. P. Ma, et al., J. Electrochem. Soc. 140, 1993, L169
[4] Naguib, M. Kurtoglu, et al. Adv. Mater. 23, 2011, 4248.
[5] R. Lukatskaya, o. Mashtalir, et al, Science 341, 2013, 1502.

Figure 1