Piezoelectrochemical Energy Harvesting from the Coupling of Mechanics and Electrochemistry in Lithium Ion Batteries

Abstract

Current research on the mechanics of lithium ion batteries focuses on improving materials for energy storage and developing techniques to characterize materials during battery usage. However, mechanical properties of batteries have uses beyond improving energy storage, and in this work we explore low-frequency mechanical energy harvesting by using lithium ion batteries with graphite and lithium cobalt oxide (LCO) electrodes. Specifically, we show that by applying a stress to a lithium ion battery, we can increase its voltage and dissipate power over an external load, achieving conversion efficiencies of greater than 0.01% and a power output of 7.9 ± 0.3 nW. Because this process is related to the coupling of mechanics and electrochemistry in lithium ion batteries, we also investigate the relationship between mechanical strain and voltage. We empirically and theoretically demonstrate that dV/dQ is proportional to d2ε/dQ2 and suggest that strain can characterize materials for energy storage as well as energy harvesting applications. Building on our theory of mechanical-electrochemical coupling, we show that the parameters that describe energy harvesting such as decay time and energy harvested are proportional to dε/dQ. Thus, knowledge of an intercalation material’s expansion can help determine that material’s effectiveness for energy harvesting. We also propose that by compressing a battery during discharge, significantly more energy can be harvested than possible with compression at a fixed state of charge (0.12 ± 0.01 μW is harvested in the first dynamic harvesting test), but this energy is limited by the resistance of the compressed separator in many batteries. Last, we show that stress-voltage coupling should exist in systems such as capacitive mixing energy harvesters and that systems that do not use traditional battery materials could be extremely effective energy harvesters.