This paper reports on the design and experiment of a highly-coupled piezoelectric energy harvester. Such a device is necessary to benefit from non-linear electrical techniques to improve the frequency bandwidth. We improve a previously developed 2-degree of freedom (2-DOF) analytical model with the Rayleigh method to understand 2-dimensional electromechanical interactions and give a preliminary design a PZT-5A cantilever. Then, we use 3D Finite Element Method (FEM) simulations to take into account 3-dimensional effects. The resulting device demonstrates a strong squared electromechanical coupling coefficient (k²=15.0% for the 1 st mode).