Assessing (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CT₂ (T = −O, −OH, and −F) i-MXenes as High-Performance Electrode Materials for Lithium and Non-Lithium Ion Batteries

Abstract

Employing first-principles calculations, the energy storage properties and ion diffusion dynamics of Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Al3+ on bare (Mo2/3Sc1/3)2C and surface-functionalized (Mo2/3Sc1/3)2CT2 (T = −O, −OH, and −F) i-MXenes are predicted. The investigated i-MXenes show weak adsorption ability to the Zn2+ ion regardless of the surface terminations, excluding their use as anodes for Zn ion batteries. The first-principles molecular dynamics simulations indicate that the adsorption of alkaline (earth) metal ions and Al3+ on (Mo2/3Sc1/3)2C­(OH)2 and (Mo2/3Sc1/3)2CF2 causes the surface reaction between metal ions and surface terminations, leading to the formation of metal hydride or fluorite overlayers covering the underneath remaining i-MXenes. We find that the bare (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 are suitable candidates for use as anodes for alkaline (earth) metal and Al ion batteries. Specifically, both (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 i-MXenes show good ion storage capacities, ideal open circuit voltages, and fast ion diffusion dynamics for alkaline (earth) metal ions. Notably, the predicted theoretical capacities of (Mo2/3Sc1/3)2C ((Mo2/3Sc1/3)2CO2) for Li+, Mg2+, and Al3+ are 291 mAh g–1 (254 mAh g–1), 490 mAh g–1 (436 mAh g–1), and 886 mAh g–1 (640 mAh g–1), respectively. In addition, the calculated open circuit voltage profiles of Li+, Mg2+, and Al3+ exhibit the small on-set voltage and the board plateau region. The climbing image-nudged elastic band predicts that the diffusion energies of Li+, Na+, K+, and Ca2+ ions on bare (Mo2/3Sc1/3)2C are extremely small (<0.05 eV), and the O-terminated surface show higher diffusion energies for various metal ions. Overall, (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 are good electrode materials for fast charging alkaline (earth) metal ion batteries and supercapacitors.

Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries

“…Finally, it should be noted that the most stable surface terminations for MXenes are revealed to be –F and –O, and less –OH groups are expected. 52,53 Therefore, the predicted surface terminations of boridene resemble that of MXenes. Finally, it should be noted that all the formation enthalpies presented in Table 1 for the different surface terminations of boridene are very negative, indicating that all the surface terminations (–O, –F and –OH) are thermodynamically stable.…”

“…The very similar charge transfer behavior of the Ca adsorbate has been reported before for MXenes. 22,53 It is now known that other non-spherical atomic orbitals such as 4p and 4d states also participate in the chemical bonding mechanism when the Ca atom is absorbed on MXenes. Thus, this interpretation can also apply to the Ca atom absorbed on boridenes.…”

“…22 In our previous work, first-principles molecular dynamics simulations were conducted for M-(Mo 2/3 Sc 1/3 ) 2 CT x (T = O, OH and F, and M refer to alkaline (earth) metals) systems, and it was found that only (Mo 3/2 Sc 1/3 ) 2 CF 2 could be converted into bare (Mo 2/3 Sc 1/3 ) 2 C through a decomposition process similar to that of eqn ( 9) or (10). 53 Wang et al 27 discussed the possible decomposition of Li-Mo 2 M′C 2 T 2 (M′ = Sc, Ti, V, Zr, Nb, Hf and Ta; T = O, H, OH and F) structures for LIBs, concluding that both OH-and F-terminated o-MXenes can be converted into bare structures following reaction pathways similar to that of eqn ( 9)-( 14). Overall, for both MXenes and boridenes, the conversion reactions are predicted to be similar, implying a new synthetic route to fabricate bare 2-D materials without surface terminations.…”

“…22 The predicted gravimetric capacities of Mo-based double-transition metal i-MXenes for NIB and KIB were below 200 mA h g −1 . 53 Finally, in the case of bare mono-transition metal MXenes including Ti 2 C, V 2 C, Nb 2 C and Ti 3 C 2 , their energy storage capacities for Mg and Ca ion batteries were reported to be higher than 400 mA h g −1 and 300 mA h g −1 , respectively. 22 For the same MXenes within O-terminations, the obtained theoretical capacities of the Mg or Ca ion battery were still higher than 350 mA h g −1 .…”

Ordered Double Transition Metal MXenes

“…The pillared Mo 2 TiC 2 delivered a reversible capacity of 109 mAh g −1 at 20 mA g −1 , twice the value obtained from the non‐pillared Mo 2 TiC 2 . Furthermore, the first‐principles calculations reveal that the surface terminations influence the capacities of the ordered double transition metal MXenes [14e,f,30] . When ordered double transition metal MXenes are used as anodes in Na‐ion batteries, the diffusion of Na is affected by the terminated functional groups, and the diffusion barrier is an order greater than the bare MXenes [14e] .…”