Efficient energy harvesting from nonconventional sources is increasingly important these days where energy usage is continuously rising and providing power to newly developed electronic systems require nontraditional power sources. Piezoelectrochemical (PEC) materials undergo a volume change as a result of an electrochemical reaction and can be expected to produce orders of magnitude more energy per volume than conventional piezoelectrics due to the high energy density associated with Faradaic reactions. Through maximizing PEC effect, one may be able to achieve great advancement in the energy harvesting research field. In this work, we utilize off-the-shelf lithium iron phosphate (LFP) battery pouch cells to study and maximize the conversion of mechanical energy to electrochemical energy.

Previously we proposed that stress and voltage are linearly related by a PEC coupling factor, k , which is defined as the change in equilibrium potential U_o of an electrochemical reaction with respect to change in applied stress σ. This factor, k can also be expressed in terms of the mechanical strain ε produced from an electrochemical reaction involving volumetric charge q_v which shows that the coupling between stress and voltage is a consequence of the work of mechanical expansion due to the electrochemical motion of charged species. As a result, larger harvested energy density can be associated with high k values and high volumetric capacities and this physically corresponds to materials that exhibit high expansion with flat discharge voltage curves. We choose LFP batteries that have flat voltage curves to experimentally demonstrate the concept of mechanical energy harvesting through the piezoelectrochemical coupling. We show that the expansion mostly originating from the graphite anode combined with the flat voltage characteristics of the LFP cathode result in a higher energy yield.