Nonaqueous lithium-oxygen (Li-O₂) batteries have emerged as possible high energy density alternatives to Li-ion batteries that could address the limited driving range issues faced by electric vehicles. This battery chemistry allows for high energy densities and an exceptionally high theoretical efficiency. Carbon, the current cathode catalyst, exhibits low stability against oxidation and reacts with the products formed during the discharging phase of the battery. In addition, due to other undesired side-reactions while charging the battery, all the reactants are not recovered leading to a poor rechargeability of a few cycles. A good cathode that can mitigate these issues requires careful screening and can help improve cycle life, increase capacity and help mitigate electrolyte decomposition. In this work, we focus on identifying possible cathodes from a family of intriguing two-dimensional (2D) materials known as MXenes, for lithium-oxygen batteries. MXenes have a general formula of M_n+1X_n, where M is an early transition metal and X is nitrogen or carbon, and being 2D layered materials, they have a large surface area [1]. MXenes are possible cathode candidates for lithium-air batteries owing to two considerations: (i) high electronic conductivity and (ii) high stability. We explore MXene carbides and nitrides of various transition metals as potential cathodes with low nucleation overpotentials.

We use density functional theory (DFT) calculations to investigate the stability and activity of various terminated and non-terminated nitride and carbide MXenes of transition metals. We begin by exploring the MXenes of titanium and scandium such as Ti₄N₃, Ti₄C₃, Sc₄N_3, Sc₄C₃and their variants with the F, O and OH terminated groups. We observe that the nitrides are more stable than the carbides, and this trend holds also for the terminated compounds. Between the scandium and titanium MXenes, the titanium carbides are more stable than their scandium carbide equivalents; however, this observed trend is reversed for the nitride MXenes, with the scandium nitrides being more stable than titanium nitrides.

The nucleation overpotential for Li-O₂ batteries is determined by the adsorption free energy of the intermediate LiO₂ [3]. We identify active MXene catalysts by computing the adsorption free energy through DFT calculations and we find that the non-terminated carbides and nitrides are stronger binding than the optimal binding strength. The transition metal and the termination group in the MXene are two tuning variables to weaken the binding in order achieve optimal binding strength. We will report on the activity and coverage effects of various transition metal carbide and nitride MXenes to identify promising cathodes for the Li-O₂batteries.

 

 

 

 

 

 

References

[1] Naguib, M. et.al. Adv. Mater., 2014, 26, 992-1005.

[2] Urbankowski, P. et.al. Nanoscale, 2016, 8, 11385-11391

[3] Krishnamurthy, D. ACS Energy Lett. 2016, 1, 162-168