Enhancing energy-harvesting capabilities of lead-free piezoelectric materials through electrostrictive coefficient optimization

Piezoelectric energy harvesting (PEH) has emerged as a promising solution for powering low-power consumer electronic devices. Both high piezoelectric strain constants (d_ij) and large piezoelectric voltage constants (g_ij) of piezoelectric materials are crucial for PHE systems. However, raising d_ij is often accompanied by a notable rise in the dielectric constant, leading to a degraded g_ij. Here, we proposed a strategy to enhance the energy-harvesting capability by improving electrostrictive coefficients, during which the increase of dielectric response can be effectively avoided, therefore d_ij and g_ij can be improved simultaneously. By orientation optimization, B site similar ion radius element and A site small ion radius element doping, the B-site ion orderliness, lattice spacing and phase transition temperature were regulated. Then, an extremely large electrostrictive coefficient of Q₁₁^C = 0.354 m⁴/C² was achieved in (K,Na)NbO₃-based single crystals, surpassing Pb-based single crystals by more than five times. With this large electrostrictive performance, high values of d₃₃ (778 pC/N) and g₃₃ (54.6 × 10⁻³ Vm/N) have been acquired. Consequently, an output power density of 12.2 μW/mm³ at 1g acceleration was achieved in a cantilever beam PEH based on this single crystal, establishing it as one of the most prominent lead-free piezoelectric materials reported to date. This work presents a proven idea and method for enhancing the energy-harvesting capabilities of materials.