Investigating Mechano-Electrochemical Coupling Phenomenon in Lithium-Ion Pouch Cells Using In-situ Neutron Diffraction

Abstract

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.