Abstract <p>The quantum capacitance characteristics of defective aluminum nitride nanosheets (AlNNS) are examined in this study with an emphasis on their possible application in supercapacitors. Using phonon spectrum analysis and cohesive energy calculations, we confirmed the structural integrity of the primitive cell. Our results show that thin monolayers with nitrogen (N), aluminum (Al), or AlN deficits exhibit either narrow-bandgap semiconducting behavior or metallic properties. Defect formation energy calculations suggest that the best choice is N-deficient AlNNS. New impurity states around the Fermi level arise as a result of under-coordinated atoms being present close to the defect. This study focuses on the potential application of defective aluminum nitride nanosheets (AlNNS) in supercapacitors by examining their quantum capacitance properties. We verified the structural integrity of the primitive cell using cohesive energy calculations and phonon spectrum analysis. According to our findings, thin monolayers with deficiencies in nitrogen (N), aluminum (Al), or Al–N exhibit either metallic or narrow-bandgap semiconducting behaviour. Calculations of defect formation energy indicate that N-deficient AlNNS is the optimal option. The presence of under-coordinated atoms near the defect leads to the formation of new impurity states around the Fermi level.</p>