Interfacial Electrochemistry and Surface Modification of Thin-Film LiMn2O4 Cathodes in Lithium Ion Batteries

Yu, Xiankai

doi:https://doi.org/10.21985/n2-1zyx-zf43

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Interfacial Electrochemistry and Surface Modification of Thin-Film LiMn2O4 Cathodes in Lithium Ion Batteries

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This dissertation presents a comprehensive study of thin-film LiMn2O4 (LMO) cathodes applied in lithium ion batteries (LIBs). The primary aim was to establish fundamental understanding of the relationship between interfacial LMO chemistry/electrochemistry and its detrimental drawback, i.e. fast capacity fade over long term cycling, and then develop effective mitigation methods. Taking advantage of multiple advanced materials characterization techniques including in situ synchrotron X-ray scattering, high resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) depth profile, four main research topics are covered here: 1) characterization of the surface chemistry of hetero-epitaxial LMO thin films, 2) studying cathode electrolyte interphase in terms of chemical composition and manganese distribution, 3) exploring ultrahigh power density LMO thin film cathodes, and 4) determining the effect of protective surface coatings on LMO cycling stability. LMO || La0.5Sr0.5CoO3 (LSCO) || SrTiO3 111 (STO) hetero-epitaxial thin films fabricated by pulsed laser deposition (PLD) were electrochemically cycled in cells with Li counter electrodes while doing in situ synchrotron X-ray scattering characterization. Since surface roughness limits the resolution of the low-angle x-ray scattering, surface roughness was studied. Roughness resulted from LMO 3D island nucleation due to relatively large lattice mismatch between LMO and LSCO, and induced dislocation generation discerned in high resolution STEM images. Low angle synchrotron X-ray scattering indicated rapid formation of cathode electrolyte interphase (CEI) morphology that gradually became stable after the first cycle. A new synthesis method for thin-films cathodes was developed with the structure LMO || Au-Pd || stainless steel (SS). Chemical compositions of the CEI layer on the cathodes were examined by XPS depth profiling. The observation that considerable manganese was present in the CEI provides a new mechanism for loss of LMO capacity. A novel fabrication route based on multilayer sputtering deposition was developed for constructing high-quality LMO thin-film cathodes on well-engineered Pt || Ti || Al2O3 substrates with excellent electrochemical performance. Ultrahigh rate capability was achieved on a 25 nm thin-film LMO cathode exhibiting a high capacity retention of 85% at 80C (equivalent to 45 s of full charge). That remarkable performance was primarily attributed to the relatively short Li-ion diffusion length in the 25 nm thick LMO layers, the absence of a rate-limiting phase separation process, and the good electrical contact between the LMO and Pt layers. The effect of thin LSCO layers on 25 nm thin-film LMO || Pt || Ti || Al2O3 cathodes on LMO cycling stability was studied. 3-electrode EIS analysis observed increased CEI ohmic resistance and LMO surface charge transfer resistance with thicker LSCO coating applied, indicating the significance of accurately controlling and determining optimized coating thickness. A coating layer of 2 nm LSCO significantly improved LMO capacity retention over 500 cycles from 84% to 95%. It was found that the LSCO coating suppressed phase separation of LMO, resulting in higher cycling stability attributed to a more stable solid-solution phase. In addition, it was observed that the LSCO coating inhibited grain cracking and crystallinity loss, improving LMO capacity retention. A Mn3O4-like defect tetragonal spinel was revealed by STEM-ABF in cycled LMO and the LSCO surface coating was believed to inhibit that phase transition which caused LMO structure distortion due to Mn migration and lattice oxygen loss. Formation of the Mn3O4-like phase was further verified by EELS analysis of manganese oxidation states. Quantification of the Mn L3/L2 white-line ratios showed that the Mn oxidation state of cycled LMO averaged +2.4, consistent with the Mn oxidation state in the newly formed Mn3O4-like spinel.

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