Electrochemical to mechanical energy conversion

Abstract

Electrode materials for rechargeable lithium ion batteries are well-known to undergo significant dimensional changes during lithium-ion insertion and extraction. In the battery community, this has often been looked upon negatively as a degradation mechanism. However, the crystallographic strains are large enough to warrant investigation for use as actuators. Lithium battery electrode materials lend themselves to two separate types of actuators. On one hand, intercalation oxides and graphite provide moderate strains, on the order of a few percent, with moderate bandwidth (frequency). Lithium intercalation of graphite can achieve actuation energy densities of 6700 kJ m-3 with strains up to 6.7%. Intercalation oxides provide strains on the order of a couple percent, but allow for increased bandwidth. Using a conventional stacked electrode design, a cell consisting of lithium iron phosphate (LiFePO4) and carbon achieved 1.2% strain with a mechanical power output of 1000 W m 3 . Metals, on the other hand, provide colossal strains (hundreds of percent) upon lithium alloying, but do not cycle well. Instead, a self-amplifying device was designed to provide continuous, prolonged, one-way actuation over longer time scales. This was still able to achieve an energy density of 1700 kJ n 3, significantly greater than other actuation technologies such as shape-memory alloys and conducting polymers, with displacements approaching 10 mm from a 1 mm thick disc. Further, by using lithium metal as the counterelectrode in an electrochemical couple, these actuation devices can be selfpowered: mechanical energy and electrical energy can be extracted simultaneously.