The Long-Term Stability of KO2 in K-O2 Batteries.

The rechargeable K-O2 battery is recognized as a promising energy storage solution owing to its large energy density, low overpotential, and high coulombic efficiency based on the single-electron redox chemistry of potassium superoxide. However, the reactivity and long-term stability of potassium superoxide remains ambiguous in K-O2 batteries. Parasitic reactions are explored and the use of ion chromatography to quantify trace amounts of side products is demonstrated. Both quantitative titrations and differential electrochemical mass spectrometry confirm the highly reversible single-electron transfer process, with 98 % capacity attributed to the formation and decomposition of KO2 . In contrast to the Na-O2 counterparts, remarkable shelf-life is demonstrated for K-O2 batteries owing to the thermodynamic and kinetic stability of KO2 , which prevents the spontaneous disproportionation to peroxide. This work sheds light on the reversible electrochemical process of K+ +e- +O2 ↔KO2 .

Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries.

Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolytes. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UMEs) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical-electrochemical (CE) deposition process. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate ( kc) obtained at the Pt UME is ∼10 times faster than that at the Au UME. Field desorption ionization mass spectrometry (MS) suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to larger kc. Raman spectroscopy and electrospray ionization MS data show that Ca2+ ions are strongly coordinated with tetrahydrofuran and weakly interacting with BH4- anions. Electron microscopy shows that the surface morphology of Ca electrodeposition is different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.