RESUMEN
The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.
RESUMEN
Li2 MnO3 has been contemplated as a high-capacity cathode candidate for Li-ion batteries; however, it evolves oxygen during battery charging under ambient conditions, which hinders a reversible reaction. However, it is unclear if this irreversible process still holds under subambient conditions. Here, the low-temperature electrochemical properties of Li2 MnO3 in an aqueous LiCl electrolyte are evaluated and a reversible discharge capacity of 302 mAh g-1 at a potential of 1.0 V versus Ag/AgCl at -78 °C with good rate capability and stable cycling performance, in sharp contrast to the findings in a typical Li2 MnO3 cell cycled at room temperature, is observed. However, the results reveal that the capacity does not originate from the reversible oxygen oxidation in Li2 MnO3 but the reversible Cl2 (l)/Cl- (aq.) redox from the electrolyte. The results demonstrate the good catalytic properties of Li2 MnO3 to promote the Cl2 /Cl- redox at low temperatures.
RESUMEN
Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions-25â wt.% LiCl and 62â wt.% H3 PO4 -cooled to -78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl- , ice-like water clusters form, and Hâ â â Cl- bonding strengthens. Surprisingly, this low-temperature solvation structure does not strengthen water molecules' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH- and H+ , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li-ion battery using LiMn2 O4 cathode and CuSe anode with a high energy density of 109â Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions.