Liquid Madelung energy accounts for the huge potential shift in electrochemical systems.

Achievement of carbon neutrality requires the development of electrochemical technologies suitable for practical energy storage and conversion. In any electrochemical system, electrode potential is the central variable that regulates the driving force of redox reactions. However, quantitative understanding of the electrolyte dependence has been limited to the classic Debye-Hückel theory that approximates the Coulombic interactions in the electrolyte under the dilute limit conditions. Therefore, accurate expression of electrode potential for practical electrochemical systems has been a holy grail of electrochemistry research for over a century. Here we show that the 'liquid Madelung potential' based on the conventional explicit treatment of solid-state Coulombic interactions enables quantitatively accurate expression of the electrode potential, with the Madelung shift obtained from molecular dynamics reproducing a hitherto-unexplained huge experimental shift for the lithium metal electrode. Thus, a long-awaited method for the description of the electrode potential in any electrochemical system is now available.

Anhydrous Fast Proton Transport Boosted by the Hydrogen Bond Network in a Dense Oxide-Ion Array of α-MoO3.

Developing high-power battery chemistry is an urgent task to buffer fluctuating renewable energies and achieve a sustainable and flexible power supply. Owing to the small size of the proton and its ultrahigh mobility in water via the Grotthuss mechanism, aqueous proton batteries are an attractive candidate for high-power energy storage devices. Grotthuss proton transfer is ultrafast owing to the hydrogen-bonded networks of water molecules. In this work, similar continuous hydrogen bond networks in a dense oxide-ion array of solid α-MoO3 are discovered, which facilitate the anhydrous proton transport even without structural water. The fast proton transfer and accumulation that occurs during (de)intercalation in α-MoO3 is unveiled using both experiments and first-principles calculations. Coupled with a zinc anode and a superconcentrated Zn2+ /H+ electrolyte, the proton-transport mechanism in anhydrous hydrogen-bonded networks realizes an aqueous MoO3 -Zn battery with large capacity, long life, and fast charge-discharge abilities.

Concentrated Electrolytes Widen the Operating Temperature Range of Lithium-Ion Batteries.

The operating temperatures of commercial lithium-ion batteries (LIBs) are generally restricted to a narrow range of -20 to 55 °C because the electrolyte is composed of highly volatile and flammable organic solvents and thermally unstable salts. Herein, the use of concentrated electrolytes is proposed to widen the operating temperature to -20 to 100 °C. It is demonstrated that a 4.0 mol L-1 LiN(SO2 F)2 /dimethyl carbonate electrolyte enables the stable charge-discharge cycling of a graphite anode and a high-capacity LiNi0.6 Co0.2 Mn0.2 O2 cathode and the corresponding full cell in a wide temperature range from -20 to 100 °C owing to the highly thermal stable solvation structure of the concentrated electrolyte together with the robust and Li+ -conductive passivation interphase it offered that alleviate various challenges at high temperatures. This work demonstrates the potential for the development of safe LIBs without the need for bulky and heavy thermal management systems, thus significantly increasing the overall energy density.

Formation of a Solid Electrolyte Interphase in Hydrate-Melt Electrolytes.

Aqueous Li-ion batteries using nonflammable aqueous electrolytes have been continuously studied to achieve the ultimate safety of the battery system. However, they have a major drawback of low operation voltage resulting from the narrow potential window of aqueous electrolytes. Recently, a room-temperature hydrate melt of Li salts has been discovered as a new class of stable aqueous electrolyte with a widened potential window (over 3 V) that enables the reversible operation of high-voltage (3 V-class) aqueous Li-ion batteries. An important factor contributing to the wide potential window is the formation of a solid electrolyte interphase (SEI) on negative electrodes, but its detailed mechanism has not been fully understood yet. Here, we study the SEI formation in the hydrate-melt electrolyte in relation with the composition and morphology of the electrodes investigated via X-ray photoelectron spectroscopy and scanning electron microscopy. We demonstrate that the formation of a stable SEI depends on the type of the electrodes used as well as the electrolyte salt concentrations.

Sodium- and Potassium-Hydrate Melts Containing Asymmetric Imide Anions for High-Voltage Aqueous Batteries.

Aqueous Na- or K-ion batteries could virtually eliminate the safety and cost concerns raised from Li-ion batteries, but their widespread applications have generally suffered from narrow electrochemical potential window (ca. 1.23 V) of aqueous electrolytes that leads to low energy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions, we discovered, for the first time, room-temperature hydrate melts for Na and K systems, which are the second and third alkali metal hydrate melts reported since the first discovery of Li hydrate melt by our group in 2016. The newly discovered Na- and K- hydrate melts could significantly extend the potential window up to 2.7 and 2.5 V (at Pt electrode), respectively, owing to the merit that almost all water molecules participate in the Na+ or K+ hydration shells. As a proof-of-concept, a prototype Na3 V2 (PO4 )2 F3 |NaTi2 (PO4 )3 aqueous Na-ion full-cell with the Na-hydrate-melt electrolyte delivers an average discharge voltage of 1.75 V, that is among the highest value ever reported for all aqueous Na-ion batteries.

Electrochemical properties of oxygenated cup-stacked carbon nanofiber-modified electrodes.

Oxygenated cup-stacked carbon nanofibers (CSCNFs), the surface of which provides highly ordered graphene edges and oxygen-containing functional groups, were investigated as electrode materials by using typical redox species in electrochemistry, Fe(2+/3+), [Fe(CN)6](3-/4-), and dopamine. The electron transfer rates for these redox species at oxygenated CSCNF electrodes were higher than those at edge-oriented pyrolytic graphite and glassy carbon electrodes. In addition, the oxygen-containing functional groups also contributed to the electron transfer kinetics at the oxygenated CSCNF surface. The electron transfer rate of Fe(2+/3+) was accelerated and that of [Fe(CN)6](3-/4-) was decelerated by the oxygen-containing groups, mainly due to the electrostatic attraction and repulsion, respectively. The electrochemical reaction selectivities at the oxygenated CSCNF surface were tunable by controlling the amount of nanofibers and the oxygen/carbon atomic ratio at the nanofiber surface. Thus, the oxygenated CSCNFs would be useful electrode materials for energy-conversion, biosensing, and other electrochemical devices.

Direct synthesis of cup-stacked carbon nanofiber microspheres by the catalytic pyrolysis of poly(ethylene glycol).

Uniformly sized microspheres tangled with cup-stacked carbon nanofibers (CSCNFs) were directly synthesized by the pyrolysis of poly(ethylene glycol) (PEG) with a nickel catalyst. A PEG/Ni membrane was prepared on a silicon wafer surface by heating it to 750 °C at a heating rate of 15 °C min(-1). The wafer was heated to a temperature of 400 °C and was held at that temperature for 1 h before raising the temperature to 750 °C for 10 min to form the CSCNF microspheres. The final CSCNF microspheres and the intermediates were evaluated using scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy to elucidate the growth mechanism. Furthermore, the CSCNF microspheres were successfully dispersed and maintained their spherical shape in an aqueous solution containing 0.5% Nafion. The CSCNF microspheres have the potential to work as a sophisticated carrier with high adsorption and fast electron-transfer exchange properties based on the graphene edges of the nanofiber surface.