Molar-volume asymmetry enabled low-frequency mechanical energy harvesting in electrochemical cells

In an electrochemical cell, unequal mechanical work due to mass action into the two electrodes can generate chemical potential difference that drives Li+ flow across the electrolyte, constituting the fundamental basis for electrochemically driven mechanical energy harvesting. The diffusional time scale inherent to the electrochemical setting renders efficient low-frequency energy conversion. From thermodynamic analyses we reveal that there exist two distinct paradigms for electrochemically driven mechanical energy harvesting, enabled by pressure or molar-volume asymmetry of the electrodes. Guided by the thermodynamic framework, we prototype the first molar-volume asymmetry based energy harvester consisting of an intercalation-conversion electrode couple. The harvester can operate under globally uniform pressure and deliver a high power density of ~0.90 µW cm−2 with long-term durability. Under an open-circuit condition, the device operates in a novel ratchetting mode under which compression/decompression cycling causes continuous rise in voltage, yielding a blasting power output of ~143.60 µW cm−2. Such a ratchet effect arises due to the chemomechanically induced residual stress in the electrodes during cycling. Compared to the pressure-asymmetry based harvesters, the new harvester offers high scalability, processability, safety, and large working area, which make it easy to increase the output power through synchronizing multilayer with large areas. Our device enables mechanical energy harvesting from low-frequency resources, including human daily activities.