Weakly-Coordinating Cyclic Ether-Based Electrolytes Improve Potassium Metal Battery Cycling Stability and Capacity Retention

As the scientific community holistically seeks alternative energy sources to traditional fuels such as coal and natural gas, attention has turned towards renewable energy generation such as wind and solar. In tandem, methods of storing energy are also being developed as a way to mitigate the intermittent nature of such methods of energy generation. Among these, electrochemical energy storage solutions are of particular promise given their flexibility and performance, allowing them to suit the needs of an increasingly growing number of industries such as chemical production and automotive manufacturing that are trending towards electrification. Lithium-ion batteries (LIBs) especially have become a mainstay in this space owing to their portability, high energy densities, and cycle lifetimes, and are the principle driving force for the growth of the global battery market to beyond US $100 billion today.<br/><br/>The “holy grail” anode material for LIBs is lithium metal itself, offering an excellent theoretical capacity of 3860 mAh g-1, yet the scarcity and uneven distribution of lithium in the Earth’s crust precludes its more widespread utilization. Hence, “Beyond Li-ion” batteries featuring more readily available charge carriers are also being thoroughly investigated to address this concern. Potassium-ion batteries (KIBs) in particular are suitable alternatives as potassium is available globally and at roughly 1000 times the abundance of lithium. Similar to Li metal, K metal is seen as a promising anode material for KIBs, but its proliferation is hindered by the propensity for dendritic growth, difficulty in maintaining a robust solid electrolyte interphase (SEI), and loss of material in the form of “dead potassium.” Further, the larger ionic radius of potassium ions as compared to lithium ions further strains the SEI and corresponding cathode electrolyte interphase (CEI) during repeated cycling. Strategies to mitigate these effects are thus of paramount importance in successful realization of K metal anodes for KIBs.<br/><br/>As the SEI is derived primarily by decomposition of the electrolyte, electrolyte-based methodologies are the most direct avenue for improving SEI stability and performance. Specifically, the formation of a mechanically robust inorganic SEI and CEI is key in protecting the electrolyte from further degradation throughout repeated plating/stripping on the metal anode and (de)intercalation of the cathode. Specifically, components such as KF that are produced via the decomposition of the fluorine-containing salt are instrumental in the composition of an SEI/CEI that are resistant to internal stresses. Thus, electrolyte solvents that enable preferential salt decomposition rather than organic solvent decomposition are preferred. Such behavior is governed by the solvation structure around the dissolved cations in the electrolyte, where maintaining the presence of salt anions in the solvation shell is critical for development of an inorganic SEI/CEI and is influenced by a variety of factors including the solvent dielectric constant ε and steric effects.<br/><br/>As compared to conventional carbonate-based electrolytes, ether-based electrolytes have been shown to promote such behavior to an increased degree in LIBs, but this effect remains comparatively unexplored in KIBs, especially paired with the K metal anode. This work demonstrates the efficacy of two cyclic ethers in enabling increased cycling stability and Coulombic efficiency against a K metal anode with regards to plating/stripping performance when compared against a typical carbonate-based electrolyte system. Further, these benefits extend to KIBs with a conventional Prussian blue cathode and K metal anode. These performance enhancements are correlated with the corresponding solvation structures, establishing a framework by which electrolyte solvents can be assessed with metal anodes and presents opportunities for further electrolyte design in other metal anode systems.