Understanding interfacial chemistry and electronic transport of solid electrolytes for solid-state battery development

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Shao, Bowen
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Electronic thesis
Mechanical engineering
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The development of superionic solid electrolytes (SEs) has proceeded rapidly these years, with the ionic conductivity of some electrolytes surpassing that of the conventional liquid electrolytes. However, incorporating these new materials into batteries to make solid-state batteries (SSBs) has proven difficult. The electrode/electrolyte interfacial resistance is being considered as the limiting factor, but the exam mechanisms of this resistance have not been fully explained. The first of part of my thesis focuses on understanding the interfacial chemistry between a high-capacity, low-cost conversion-type iron fluoride cathode and SEs. Two representative SEs, amorphous 75Li2S-25P2S5 (LPS) and glass-ceramic Li3YCl6 (LYC), for the sulfide- and halide-based conductors, respectively, are used in this study. The electrochemical characteristics of FeF2 in LPS SE exhibit drastic deviations from that in the liquid electrolyte batteries. Detailed characterizations by ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) revealed that irreversible transformation from FeF2 to FeS occurs when testing FeF2 in the sulfide-based SE. Changing the SE to halide-based materials has been shown to enable a complete conversion and reconversion of FeF2, with a specific capacity of 600 mAh/g at 0.1 C at an average potential of around 2.6 V. The characterization results from ex-situ XRD and XAS also confirmed the excellent electrochemical and chemical stability of LYC can enable a complete conversion and deconversion of FeF2. Due to the excellent interfacial stability, a halide based FeF2 cathode with an extremely long cycle life was developed. This work highlights the significant effects of SE compositions on the redox behaviour of FeF2 cathodes, and we expect the same concept of utilizing halide-based SEs can also be generalized to other conversion-type cathodes for the development of low-cost, low-toxicity and energy-dense SSBs. In addition to a long cycle life, the real application of solid-state batteries also requires the battery to have an extremely long calendar life, i.e., the life of a battery during storage. While calendar life has not been seriously considered as an issue for SSB development, it is proposed here that the long calendar life of LiPON-based thin film SSB cannot be assumed for sulfide- or halide-based SSBs due to the significant difference in the electronic conductivity of SEs (10-14 to 10 13 S/cm for LiPON vs. 10-9 to 10-8 S/cm for typical sulfide- and halide- based SEs). The second part of my thesis aims to understand the electronic transport properties in typical Li SEs. We propose that the electronic conductivities of SEs are overestimated from the conventional measurements. By revisiting direct current polarizations using two-blocking-electrode cells and the Hebb-Wagner approach, their sources of inaccuracy are provided and the anodic decomposition of SE is highlighted as the key source for the overestimated result. Modifications in the electrode selection and data interpretation are also proposed to approach the intrinsic electronic conductivity of SEs. A two-step polarization method is also proposed to estimate the electronic conductivity of sulfides that decompose during measurement. Measured by the modified approach, the electronic conductivities of all SEs are one or two orders of magnitude lower than the reported value. Despite that, the electronic conductivity of sulfides seems to be still quite high to enable SSBs with a long calendar life of >10 years, highlighting the critical need for a more careful study of electronic transport in lithium SEs.
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Rensselaer Polytechnic Institute, Troy, NY
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