The Piezoelectrochemical Coupling Factor and Mechanical Energy Harvesting

Abstract

As microelectronic systems become increasing smaller and more complex, the need for non-traditional micro-energy sources grows. In particular, mechanical energy harvesters, which convert mechanical to electrical energy, are volumetrically energy dense potential micro-energy sources. Recent literature[1-4] has demonstrated a new mechanical energy harvesting class of materials called piezoelectrochemical (PEC) materials, where mechanical energy is coupled to electrochemical energy through the potential of mobile ions. PEC materials have orders of magnitude higher theoretical energy density than conventional mechanical harvesters, such as piezoelectrics. Studies have experimentally demonstrated this PEC effect in many systems, including lithium-cobalt-oxide[1], lithium-silicon[2], sodium-phosphorus[3], lithium-carbon[4] and others. Commercially available lithium-ion pouch cells have been used to understand this effect as they are a convenient system of standardized quality that exhibits PEC coupling. Previous work[1] has suggested that there exists a coupling factor, k, which relates the conversion of applied mechanical stress to electrochemical potential. From this coupling factor, an equation for theoretical single-cycle energy density was derived as a function of k and dU0/dQ , where dU0/dQ is the slope of the charge curve. As the expansion of lithium electrodes is nonlinear with capacity, the theoretical single-cycle energy density is dependent on the state-of-charge (SOC) of the cell. In this work, we focus on the relationship between coupling factor and theoretical single-cycle energy density in commercial lithium-cobalt-oxide (LCO) pouch cells. We find that the theoretical energy density, which is characterized by measuring differential expansion and differential voltage of a pouch cell, can vary significantly between different commercial cells of the same manufacturing batch. In addition, we compare the theoretical energy density to energy harvesting experimental results from the LCO pouch cells at different SOCs. We find that there is a difference between theoretical and actual harvested energy and the efficiency loss is related to the state-of-charge. We speculate that the efficiency loss is due to kinetic transfer being too slow to fully extract all possible charge.