Enhanced quantum capacitance of near valence doped stanene nanosheets for asymmetric supercapacitors

Ever since the experimental realization of the first free-standing layers of stanene by researchers in 2016, this potential wonder material is widely being explored for various applications. This work analyzes the implication of this novel 2D material for supercapacitor electrodes and ways to enhance its quantum capacitance and charge storage capacity via first principles simulations. The stanene nanosheet is subjected to single vacancy (SV) defect and near valence (In and Sb) doping (direct doping at the SV site, independent doping at the SV edges, and co-doping at the SV edges). The results reveal the zero band gap nature of pristine stanene similar to graphene, yet at least 7 times enhancement in quantum capacitance in comparison to pristine graphene. The incorporation of a single vacancy (SV) defect in stanene has improved its quantum capacitance and charge storage capacity by 1.54 times, at the expense of compromised thermodynamic stability. Thus, to preserve the thermodynamic stability with minimal lattice distortions and enhance the quantum capacitance as well as charge storage of SV stanene, near valence dopants are introduced at the SV site and SV edges. Our results confirm that antimony doping in stanene offers better thermodynamic stability than Indium. Moreover, the independent doping of 3 antimony atoms at SV edges offers the highest quantum capacitance of 134.21 μF/cm² at 0.6 V, which is about 1.47 times higher than the maximum value (91 μF/cm² for N and Ni co-doping) reported in the literature for any doping. All the SV and near valence doped stanene configurations are observed to be better suitable for the anode electrode of asymmetric supercapacitors owing to their quantum capacitance and charge storage peaks on the positive side of the electrochemical window, except for one particular configuration, i.e., the direct antimony doping at the SV site, which is better suitable for the cathode electrode of the asymmetric supercapacitor.

The DFT-based first principle simulations combined with MATLAB programming are performed to study the capacitive behavior of the electrodes. A k-point sampling of 7 × 7 × 1 is found to be sufficient for sampling the Brillouin zone in reciprocal space, and a density mesh cutoff of 80 Hartree is used to define the fineness of real space effective potential and electron density grids for solving Poisson's equation. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof parameterization is used as the exchange correlation functional.