Energy storage systems are extensively employed in DC systems, where DC-DC converters typically operate as constant power loads (CPLs). The negative impedance characteristic of CPLs can introduce non-passive behavior in DC-DC converters, potentially destabilizing the system. Consequently, the DC-DC converter may fail to maintain stable operation at the target operating point, potentially causing instability in the energy storage system. This paper investigates the stability of a DC-DC Buck converter as a case study, employing passivity-based stability theory for analysis. Through damping and interconnection injection techniques, an improved passivity-based controller is proposed to achieve coordinated control of both inductor current and capacitor voltage. This approach overcomes the limitation of conventional passivity-based controllers, which can only regulate either inductor current or capacitor voltage individually. The improved passivity-based controller demonstrates superior dynamic performance compared to conventional approaches while ensuring system stability is maintained. Finally, the performance of the proposed controller is feasible and effective via detailed simulation results.