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Title:Operando investigations of mechanical changes in lithium-ion batteries
Author(s):Bassett, Kimberly Lundberg
Director of Research:Gewirth, Andrew A
Doctoral Committee Chair(s):Gewirth, Andrew A
Doctoral Committee Member(s):Nuzzo, Ralph G; Sottos, Nancy R; van der Veen, Renske M
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):battery, stress, lithium-ion, diffraction, density functional theory, strain, electrochemistry
Abstract:Li-ion batteries are increasingly present in modern life. They principally power compact, handheld devices and electric vehicles. Mechanical changes are always occurring within batteries due to expansion and contraction of the active materials during lithiation and delithiation, degradation of the electrodes, decomposition of the electrolyte, and external cell pressure. The work presented here seeks to describe and understand the interplay between electrochemical cycling and mechanical changes within Li-ion cathodes. Stress, Strain, and Electrochemical Stiffness in Lithium Manganese Oxide Cathodes. In-situ strain and stress measurements are performed on composite electrodes to monitor potential-dependent stiffness changes in lithium manganese oxide (LiMn2O4). Lithium insertion and removal results in asynchronous strain and stress generation in the electrode. Electrochemical stiffness changes are calculated by combining coordinated stress and strain measurements. The electrode experiences dramatic changes in electrochemical stiffness due to potential-dependent Li+ intercalation mechanisms. The development of stress in the early stages of delithiation (at ca. 3.95 V) due to a kinetic barrier at the electrode surface gives rise to stiffness changes in the electrode. Strain generation due to phase transformations reduces stiffness in the electrode at 4.17 V during delithiation and at 4.11 V during lithiation. During lithiation, stress generation due to Coulombic repulsions between occupied and incoming Li+ leads to stiffening of the electrode at 3.96 V. The electrode also experiences greater changes in stiffness during delithiation compared to lithiation. These changes in electrochemical stiffness provide insight into the interplay between mechanical and electrochemical properties which control electrode response to lithiation and delithiation. Stress and Strain in Lithium Iron Phosphate Cathodes. We wondered whether in the asynchronous stress and strain behavior see in in LiMn2O4 would present in other common Li-ion cathodes. In this study, we employ in-situ stress and strain measurements to investigate potential-dependent mechanical changes in lithium iron phosphate (LiFePO4) cathodes during cyclic voltammetry in LiPF6 and LiClO4-containing electrolytes. Analysis of the stress and strain derivatives in LiClO4–containing electrolytes both exhibit single peaks during lithiation and delithiation that coincide with LiFePO4 phase transformations. An additional feature in the stress and strain derivatives is observed in LiPF6–containing electrolytes at the onset of the delithiation process. The current peak splitting in LiPF6 are larger than in LiClO4, and electrochemical impedance spectroscopy measurements show higher impedances in LiPF6 versus LiClO4-containing electrolytes in lithiated LiFePO4. The larger current peak splitting and higher impedance in LiPF6 electrolytes suggest the potential-dependent growth of a thick and resistive cathode/electrolyte interface (CEI) layer on LiFePO4 cathodes. We hypothesize that kinetic limitations in Li+ transport through the CEI leads to additional stress and strain development at the electrode surface. Operando Observations and First Principles Calculations of Reduced Lithium Insertion in Au-Coated LiMn2O4. The deposition of protective coatings on the spinel LMO lithium-ion battery cathode is effective in reducing Mn dissolution from the electrode surface. Although protective coatings positively affect LMO cycle life, much remains to be understood regarding the interface formed between these coatings and LMO. Using operando powder X-ray diffraction with Rietveld refinement, we show that, in comparison to bare LMO, the lattice parameter of a model Au-coated LMO is significantly reduced upon re-lithiation. Less charge passes through Au-coated LMO in comparison to bare LMO, suggesting that the reduced lattice parameter is associated with decreased Li+ solubility in the Au-coated LMO. Density functional theory (DFT) calculations show that a more Li+-deficient near-surface is thermodynamically favorable in the presence of the Au coating, which may further stabilize these cathodes through suppressing formation of the Jahn-Teller distorted Li2Mn2O4 phase at the surface. Electronic structure and chemical bonding analyses show enhanced hybridization between Au and LMO for delithiated surfaces leading to partial oxidation of Au upon delithiation. This study suggests that, in addition to transition metal dissolution from electrode surfaces, protective coating design must also balance potential energy effects induced by charge transfer at the electrode-coating interface.
Issue Date:2019-05-22
Rights Information:Copyright 2019 Kimberly Bassett
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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