Understanding the tunable sodium storage performance in pillared MXenes: a first-principles study

Pillared MXenes with large interlayer spacing have shown great potential as an anode material for sodium-ion batteries (SIBs). To better understand the underlying mechanism of the pillar effect in enhancing the electrochemical performance, first-principles calculations were used to investigate the adsorption and diffusion of Na in MXenes (Ti2CO2 and Ti3C2O2), as well as the mechanical properties of the system under different MXenes layer spacings. The results showed that when the MXene layer spacing was ∼4 Å, the strongest adsorption of Na on MXenes was achieved due to the interlayer synergy effect. However, when the MXene layer spacing was greater than 5 Å, double Na-atomic layer adsorption would be formed, which increased the Na storage capacity. Interestingly, the diffusion of Na was not only affected by the interlayer spacing of MXenes, but also by the interlayer stacking mode of MXenes. Moreover, it was found that when the MXene layer spacing was more than 8 Å, the sodium storage properties basically did not change significantly. The optimal layer spacing for Ti2CO2 and Ti3C2O2 was predicted to be 7 and 6 Å, respectively. This work provides valuable theoretical guidance for developing high-performance anode materials for SIBs.