Studying the Piezoelectrochemical Phenomenon Using Lithium-Ion Batteries

Abstract

Energy harvesting, from ambient sources like mechanical energy, is vital for powering micro- and nano-scale off-the-grid devices. Yet, traditional mechanical energy harvesters typically perform poorly at low frequencies where most human motion occurs. A recently discovered phenomenon called the piezoelectrochemical (PEC) or mechano-electrochemical effect can operate at low frequencies, making human motion harvesting devices viable. PEC harvesters have been experimentally demonstrated in multiple systems where internal mechanical strain is coupled to electrochemical processes. However, the mechano-electrochemical harvesting field is still in its infancy, with no standardized metrics to compare the performance of different systems, inadequate understanding of the effect of important operating parameters, and limited knowledge on the mechanisms that drive this effect. This dissertation seeks to address these gaps in knowledge by using commercially available lithium ion pouch cells as a test system, where both electrodes of the pouch cell exhibit the PEC effect. We proposed figures of merit to quantify the performance of different PEC systems, including stress-normalized short-circuit current and open-circuit voltage outputs, a fill factor, and an energy conversion efficiency. We studied how mechanical operating frequency affects these PEC figures of merit, finding that unlike piezoelectric harvesters, PEC harvesters do not have resonance or antiresonance frequencies. We found that the state-of-charge of a PEC harvester has a significant impact on the electrical outputs of the system. Finally, to investigate the mechanism behind the PEC effect, we conducted in-situ neutron diffraction studies on compressed lithium-ion pouch cells using the VULCAN diffractometer at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.