Spectroscopy-Informed Design Rules for K-ion Batteries
Drew Ells, Chemical Engineering
Abstract: 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) alleviates reliance on Li. 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, we examine structure-performance relationships in KIB anodes and electrolytes to propose broad design rules.
In Chapter 2, we examine the mechanisms of potassiation/depotassiation of two high-capacity tin phosphide anodes, Sn4P3 and SnP3, and discuss possible failure modes. Solid-state nuclear magnetic resonance (NMR) spectroscopy suggests that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides.
In Chapter 3, we address the use of fluorinated electrolyte additives in KIBs. Fluoroethylene carbonate (FEC) is a well-known additive that improves capacity retention in Li-ion electrolytes, but we show that FEC addition to KIBs containing hard carbon anodes results in a dramatic decrease in capacity and cell failure. NMR, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS) reveal that FEC decomposes 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.
In Chapter 4, we present a nonflammable electrolyte mixture composed of 1 M KPF6 dissolved in ethylene carbonate (EC), propylene carbonate (PC), and triethyl phosphate (TEP). The electrolyte is nonflammable, retains high ionic conductivity, and is compatible with graphite. Notably, we show that this electrolyte is only usable in KIBs; the analogous Li electrolyte fails immediately due to the incompatibility of Li, PC, and graphite. NMR, XPS, and EIS characterization show that 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).