This report analyzes the quantum capacitance properties of aluminum nitride nanosheets (AlNNS) with defects, focusing on their potential use in supercapacitors. We validated the structural stability of the primitive cell through cohesive energy calculations and phonon spectrum analysis. Our findings indicate that monolayers containing aluminum (Al), nitrogen (N), or with Al-N deficiencies exhibit either metallic characteristics or small bandgap semiconducting behavior. Calculations of defect formation energy indicate that N-deficient AlNNS is the most favorable option. The presence of under-coordinated atoms near the defect leads to the emergence of new impurity states close to the Fermi level. This prompted us for a detailed examination of their quantum capacitance, which is heavily influenced by the density of states around the Fermi energy. Our study reveals that Al-deficient AlNNS achieves a maximum quantum capacitance (CQMax) of 690µF/cm² in the positively biased region, making it a suitable candidate as anodic material in supercapacitor applications. In comparison, the nitrogen-deficient AlNNS reaches a CQMax of 313µF/cm² and a maximum surface charge capacity (QMax) of -91µC/cm², highlighting its potential as a cathodic material. The Al-N-deficient AlNNS shows intermediate behavior with prominent quantum capacitance peaks in both biased regions, offering additional flexibility for potential applications.