  MXenes' tunable properties make them excellent candidates for many
applications in future nanoelectronics. In this work, we explore the
suitability of Sc$_2$CX (X=N$_2$, ON, O$_2$) MXenes to act as the active anode
materials in Na-ion based batteries (NIBs) by means of \textsl{ab initio}
simulations. After analyzing the structural and elastic properties of all the
possible models to evaluate the energetically favorable N and O
functionalization sites, our calculations show that both Sc$_2$CON and
Sc$_2$CN$_2$ present a clear metallic character, making them potential
candidates as anode materials. The investigation of the most relevant features
for anode performance, such as the adsorption and diffusion of Na atoms, the
intrinsic capacity, the open circuit voltage, and the storage capacity show
that both systems are serious alternatives to the most common 2D materials
currently employed in alkali metal batteries. In particular, Sc$_2$CN$_2$
presents a better diffusion behavior thanks to the absence of Na clustering on
its surface, with optimal diffusion barriers comparable to other 2D materials
such as MoN$_2$, while the values of diffusion barriers for Sc$_2$CON are at
least three times smaller than those found for other anode candidates.
Similarly, while the capacity of Sc$_2$CON is close to the one reported for 2D
Sc$_2$C, Sc$_2$CN$_2$ possesses a power density more than twice higher than the
ones of 2D materials such as Sc$_2$C, graphite, and MoS$_2$. Our results thus
confirm the urge for further experimental exploration of the MXene Sc$_2$CX
(X=N$_2$, ON, O$_2$) family as anode material in NIBs.
