京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24853号 / エネ博第462号 / 新制||エネ||87(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 野平, 俊之, 教授 萩原, 理加, 教授 佐川, 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23295号 / エネ博第420号 / 新制||エネ||80(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 野平 俊之, 教授 坂口 浩司 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

No description available.

While Li-ion batteries (LIBs) are the prevailing electrochemical energy storage technology, development of batteries using earth abundant alkali metals (e.g., Na and K) is necessary to alleviate LIB supply chain concerns. K-ion batteries (KIBs) offer a compelling advantage over Na via their compatibility with commercialized graphite anodes, and therefore may be more readily adopted within existing battery production lines. K-ions present some inherent advantages as well, such as rapid diffusion and low energy barriers to desolvation in the battery electrolyte that may enable fast charging. Presently, research on KIBs is in early stages and it is unclear if the same battery design principles produced by decades of study on LIBs apply to KIBs. Here, I examine structure-performance relationships in KIB anodes and electrolytes to propose broad design rules. In the first chapter, I summarize the motivations and prominent advancements in materials used for KIBs, providing commentary on the direction of the field. I begin by summarizing present concerns over materials criticality facing LIBs and how KIBs address these concerns but do not necessarily achieve lower costs. I continue with a summary of popular materials choices for KIB anodes, cathodes, and electrolytes. I place particular emphasis on the discovery and development of graphite anodes and the advantages of using a weak Lewis acid such as K-ions in batteries. Finally, I discuss the challenges presented by using highly reactive K metal anodes in research. In the second chapter, I examine the mechanisms of potassiation/depotassiation of two high-capacity tin phosphide anodes, Sn₄P₃ and SnP₃, and discuss possible failure modes. Ex situ 31P and 119Sn solid-state nuclear magnetic resonance (NMR) analyses reveal that both Sn₄P₃ and SnP₃ exhibit phase separation of elemental P and the formation of KSnP-type environments (which are predicted to be stable based on DFT calculations) during potassiation, while only Sn₄P₃ produces metallic Sn as a byproduct. In both anode materials, K reacts with elemental P to form K-rich compounds containing isolated P sites that resemble K₃P, but K does not alloy with Sn during potassiation of Sn₄P₃. During charge, K is only fully removed from the K3P-type structures, suggesting that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides. The third chapter addresses the use of fluorinated electrolyte additives in KIBs. Fluoroethylene carbonate (FEC) is a well-known additive used in Li-ion electrolytes, because the products of its sacrificial decomposition aid in forming a stable solid electrolyte interphase (SEI) on the anode surface. Here, we show that FEC addition to KIBs containing hard carbon anodes results in a dramatic decrease in capacity and cell failure. Using a combination of 19F solid-state NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS), we show that FEC decomposes during galvanostatic cycling to form insoluble KF and K₂CO₃ on the anode surface, which correlates with increased interfacial resistance in the cell. Our results strongly suggest KIB performance is sensitive to accumulation of an inorganic SEI, likely due to poor K transport in these compounds. The fourth chapter presents a nonflammable electrolyte mixture for use in KIBs. In this report, we show that a low-concentration (1 M) KPF6 electrolyte combining ethylene carbonate (EC), propylene carbonate (PC), and triethyl phosphate (TEP) is nonflammable, retains high ionic conductivity, and is compatible with graphite. Notably, we then show that this electrolyte is only usable in KIBs; the analogous Li electrolyte fails immediately due to the incompatibility of Li, PC, and graphite. We continue the study by characterizing the impact of TEP on the graphite interphase using a combination of EIS, XPS, and 1D and 2D NMR spectroscopy. We show that, compared to using EC/PC alone, the addition of TEP reduces resistance of the SEI layer, lessens reductive decomposition of carbonates to soluble organic species, and produces inorganic phosphate salts (that we posit contribute to passivation in lieu of fluorination in the SEI). The fifth chapter concludes by summarizing the design strategies learned in each of the preceding three chapters and makes recommendations for future studies. The proposed research emphasizes the need for fundamental studies on materials properties in KIBs, contradicting the current push towards optimizing capacity and longevity of KIBs to prove their relevance. Doing so will not only inform how to design high-performance batteries, but potentially uncover distinct advantages of KIBs that complement existing LIB technologies.