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Piezoelectrochemical (PEC) materials convert mechanical energy into electrochemical energy through Faradaic charge transfer, allowing energy harvesting at very low frequencies. Electric power is generated through micro-Hz cyclical compression of commercially available lithium-ion batteries with varying PEC effects. Difference in power generation between different PEC systems is analyzed based on standard charge/discharge curve and the relationship between the coupling of mechanical stress and voltage. This study experimentally demonstrates the varying energy converting properties between three different types of PEC systems and explores an important parameter space for controlling power generation behavior in ultra-low-frequency mechanical energy harvesting of PEC materials.

Mechanical energy is a readily available and often wasted source of ambient energy, so mechanical energy harvesters are particularly appealing for powering microelectronic systems that cannot be connected to traditional power sources. A promising and recently discovered type of mechanical energy harvester uses the piezoelectrochemical (PEC) effect to convert mechanical energy into electrochemical energy1-6. In these harvesters, an applied mechanical stress changes the redox potential of the active material, which drives ion flux to return the system to equilibrium. Recent literature1-6 has demonstrated that these harvesters have high theoretical energy densities and can harvest lower mechanical frequencies than piezoelectric materials and other traditional mechanical energy harvesters. This indicates that PEC harvesters could fulfill energy harvesting applications where other mechanical harvesters would not be viable. However, despite its promise, research on PEC harvesters is still a relatively new field and there is much to be learned. Recent work6 aimed to standardize the way this new class of harvesters is compared, and this paper noted that the peak current output of a PEC harvester is affected by the input mechanical vibration frequency. However, an in-depth study and explanation of this frequency dependence was left for future studies. In this work, we aim to address this frequency dependence and the fundamental ion kinetics that drive it. Commercially-available lithium ion pouch cells are a convenient system to study the PEC effect1,6. In these pouch cells, both the lithium cobalt oxide and graphite electrodes are PEC materials, and the current and voltage outputs of the harvester are proportional to the applied mechanical stress1. We mechanically cycle these pouch cells in an Instron compressive testing machine and measure the corresponding short-circuit current output. We find the value of the peak current output increases with a slower mechanical cycling frequency. We speculate that the value of the peak current output is the convolution of the change in redox potential due to the applied mechanical stress and how far away the system was from equilibrium before. The initial mechanical cycle primes the system and affects the current output of the succeeding cycles, acting as a kind of mechanical memory effect. We model this as a capacitor being charged: the more time passes, the more ions build up. This insight into the frequency dependence of PEC harvesters allows us to better understand the mechanism behind the PEC effect, which will enable the design of better harvesters. References: [1] J. Cannarella and C. B. Arnold, “Toward Low-Frequency Mechanical Energy Harvesting Using Energy- Dense Piezoelectrochemical Materials," Advanced Materials, 27, 7440 (2015). [2] S. Kim, S. J. Choi, K. Zhao, H. Yang, G. Gobbi, S. Zhang, and J. Li, “Electrochemically driven mechanical energy harvesting," Nature Communications, 7, 10146 (2016). [3] N. Muralidharan, M. Li, R. E. Carter, N. Galioto, and C. L. Pint, “Ultralow Frequency Electrochemical−Mechanical Strain Energy Harvester Using 2D Black Phosphorus Nanosheets,” ACS Energy Lett., 2, 1797 (2017). [4] E. Jacques, G. Lindbergh, D. Zenkert, S. Leijonmarck, and M. H. Kjell, “Piezo-Electrochemical Energy Harvesting with Lithium-Intercalating Carbon Fibers,” ACS Appl. Mater. Interfaces, 7, 13898 (2015). [5] Y. Hou, et al. “Flexible Ionic Diodes for Low-Frequency Mechanical Energy Harvesting”. J. Adv.Mater. 7 (2016). [6] J. I. Preimesberger, S. Kang, and C. B. Arnold, “Figures of Merit for Piezoelectrochemical Systems,” Submitted 2020.

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Commonly-used lithium ion battery electrode materials, like graphite and lithium cobalt oxide (LCO), undergo significant volumetric changes during battery charge and discharge, which can greatly affect the performance of the battery. As such, understanding the coupling between the mechanical and electrochemical properties of lithium ion batteries is vital. One area of research in the broader field of mechano-electrochemical coupling in batteries is the study of the piezoelectrochemical effect (PEC), which uses this coupling for mechanical energy harvesting. The PEC effect has been demonstrated experimentally in a number of systems1-4 including commercially-available lithium ion pouch cells1. Due to the high-energy density and relatively slow rates of the reactions of Faradaic ions, the PEC effect enables higher theoretical energy density and lower frequency mechanical harvesters than piezoelectric harvesters1,2. Recent research has demonstrated what metrics need to be optimized in order to improve PEC harvester performance; in particular, current and voltage output peak and current FWHM5.So far, the only studies of PEC systems have measured polycrystalline materials, which result in scalar metrics. However, given that the chemical expansion of PEC materials can be highly anisotropic, PEC systems should be able to be formalized with tensor representations. Unlike piezoelectric materials, PEC materials are generally centrosymmetric crystals, which indicates that a PEC tensor must be even-ranked. Using the derivations of the Larché-Cahn chemical potential6 and following the scalar derivations derived for lithium-silicon systems7 we propose a second-order PEC tensor. We then compare these results to our scalar experimental results from commercially-available lithium ion pouch cells. References: [1] J. Cannarella and C. B. Arnold, “Toward Low-Frequency Mechanical Energy Harvesting Using Energy- Dense Piezoelectrochemical Materials," Advanced Materials, 27, 7440 (2015).[2] S. Kim, S. J. Choi, K. Zhao, H. Yang, G. Gobbi, S. Zhang, and J. Li, “Electrochemically driven mechanical energy harvesting," Nature Communications, 7, 10146 (2016).[3] N. Muralidharan, M. Li, R. E. Carter, N. Galioto, and C. L. Pint, “Ultralow Frequency Electrochemical−Mechanical Strain Energy Harvester Using 2D Black Phosphorus Nanosheets,” ACS Energy Lett., 2, 1797 (2017).[4] E. Jacques, G. Lindbergh, D. Zenkert, S. Leijonmarck, and M. H. Kjell, “Piezo-Electrochemical Energy Harvesting with Lithium-Intercalating Carbon Fibers,” ACS Appl. Mater. Interfaces, 7, 13898 (2015).[5] J. I. Preimesberger, S. Kang, and C. B. Arnold, “Figures of Merit for Piezoelectrochemical Energy-Harvesting Systems” Joule, 4, (2020).[6] F. C. Larché and J. W. Cahn. “The Interactions of Composition and Stress in Crystalline Solids,” Acta Metall., 33, 331-357, (1985).[7] V. A. Sethuraman, V. Srinivasan, A. F. Bower, and P. R. Guduru, “In Situ Measurements of Stress-Potential Coupling in Lithiated Silicon,” ECS, 157, A1253-A1261 (2010).