Conventional energy harvesters often suffer from limited efficiency due to their narrow operational bandwidth, which must closely match the frequency of ambient vibrations. To overcome this constraint, prior research has explored strategies such as deploying arrays of harvesters or leveraging nonlinear coupling to broaden bandwidth and enhance peak energy output. This chapter presents a novel approach that enables tunable resonant frequencies spanning from high to ultralow ranges, while simultaneously improving both bandwidth and energy capture. The proposed system integrates a lever mechanism with an X-shaped support structure, offering highly adjustable stiffness and a displacement-dependent nonlinear damping characteristic. These features collectively enhance energy harvesting performance by enabling synchronization with a broader range of excitation frequencies. The system’s efficiency is governed by easily modifiable structural parameters and a unique nonlinear response to relative displacement, providing substantial flexibility and optimization potential. This work introduces a promising framework for advancing energy harvester design through the exploitation of structural adaptability and geometric nonlinearities.