Quantitative Resolution of Complex Stoichiometric Changes during Electrochemical Cycling by Density Functional Theory-Assisted Electrochemical Quartz Crystal Microbalance

Wu, Tzu-Ho, Scivetti, Ivan, Chen, Jia-Cing, Wang, Jeng-An, Teobaldi, Gilberto ORCID: 0000-0001-6068-6786, Hu, Chi-Chang and Hardwick, Laurence J ORCID: 0000-0001-8796-685X
(2020) Quantitative Resolution of Complex Stoichiometric Changes during Electrochemical Cycling by Density Functional Theory-Assisted Electrochemical Quartz Crystal Microbalance. ACS Applied Energy Materials, 3 (4). pp. 3347-3357.

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The capability to simultaneously measure changes of mass and charge of electro-active materials during a redox process makes Electrochemical Quartz Crystal Microbalance (EQCM) a powerful technique to monitor stoichiometric changes during reversible electrochemical processes. In principle, quantitative resolution of the stoichiometry of the electro-active sample during electrochemical cycling can be obtained by solving the system of equations for the EQCM mass and charge balance. Such a system of equations couples the measured changes in mass and charge through the stoichiometry of the redox process. Unfortunately, whenever more than two chemically inequivalent species are involved in the redox process, the system of equations is mathematically undetermined, having more variables (stoichiometric coefficients) than equations. In these cases, current best practice is the arbitrary reduction of the number of variables in the mass and charge balance equation, using chemical intuition to set some of the stoichiometric coefficients to fixed values. For layered ion-intercalation host materials, widespread practical approximations are the use of arbitrarily defined solvation numbers for the intercalating ions or the neglect of ion-induced displacement of structural solvent inside the host. Here, we propose an alternative approach based on the use of Density Functional Theory (DFT) to sample and screen, on an energy basis, the whole unreduced spectrum of stoichiometric coefficients compatible with EQCM measurements, leading to DFT energy-assisted resolution of stoichiometric changes during cycling. We illustrate the approach by taking nickel hydroxide Ni(OH)2 as a case system and studying its ion intercalation-driven phase transformations in the presence of different LiOH, NaOH, and KOH electrolytes. Quantitative resolution of the Ni(OH)2 stoichiometry during electrochemical cycling unambiguously reveals ion intercalation to displace structural water from the layered host, promoting electrochemical degradation and aging of the material. The process is found to be strongly dependent on the size of the electrolyte cation, with larger cations displacing larger amounts of structural water and resulting in faster degradation rates.

Item Type: Article
Uncontrolled Keywords: Electrochemical Quartz Crystal Microbalance, Density Functional Theory, nickel hydroxide, in situ Raman spectroscopy, ion intercalation, structural water, electrochemical degradation
Depositing User: Symplectic Admin
Date Deposited: 08 Apr 2020 10:25
Last Modified: 18 Jan 2023 23:55
DOI: 10.1021/acsaem.9b02386
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URI: https://livrepository.liverpool.ac.uk/id/eprint/3082312