No abstract

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 Na3V2(PO4)2F3|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.

Aqueous electrolytes have great potential to improve the safety and production costs of Li-ion batteries. Our recent materials exploration led to the discovery of the Li-salt dihydrate melt Li­(TFSI)0.7(BETI)0.3·2H2O, which possesses an extremely wide potential window. To clarify the detailed liquid structure and electronic states of this unique aqueous system, a first-principles molecular dynamics study has been conducted. We found that water molecules in the hydrate melt exist as isolated monomers or clusters consisting of only a few (at most five) H2O molecules. Both the monomers and the clusters have electronic structures largely deviating from that in bulk water, where the lowest unoccupied states are higher in energy than that of the Li-salt anions, which preferentially cause anion reduction leading to formation of an anion-derived stable solid-electrolyte interphase. This clearly shows the role of characteristic electronic structure inherent to the peculiar water environment for the extraordinary electrochemical stability of hydrate melts.

Aqueous alkali-ion batteries, particularly earth-abundant sodium-or potassium-based systems, are potentially safe and low-cost alternatives to nonaqueous Li-ion batteries. Recently, concentrated aqueous electrolytes with Na and K salts as well as Li ones have been extensively studied to increase the voltage of aqueous batteries; however, the potential windows become narrower in the order of Li > Na > K. Here, we study the difference in the potential windows of Li-, Na-, and K-salt concentrated aqueous electrolytes (hydrate melts) by first-principles molecular dynamics. As the Lewis acidity of alkali cations decreases (Li + > Na + > K + ), the sacrificial reduction of counter anions is less active and water molecules are more aggregated. This situation is unfavorable for achieving stable anionderived passivation on negative electrodes as well as for being stabilized to oxidation on positive electrodes. Hence, the Lewis acidity of alkali cations is essential to dominate the potential windows of hydratemelt electrolytes.

The solid electrolyte interphase (SEI) film, which consists of the products of reduction reaction of the electrolyte, has a strong influence on the lifetime and safety of Li-ion batteries. Of particular importance when designing SEI films is its strong dependence on the electrolyte solvent. In this study, we focused on geometric isomers cis- and trans-2,3-butylene carbonates (c/t-BC) as model electrolytes. Despite their similar structures and chemical properties, t-BC-based electrolytes have been reported to enable the reversible reaction of graphite anodes [as in ethylene carbonate (EC)], whereas c-BC-based electrolytes cause the exfoliation of graphite [as in propylene carbonate (PC)]. To understand the microscopic origin of the different electrochemical behaviors of t-BC and c-BC, we applied Red Moon simulation to elucidate the microscopic SEI film formation processes. The results revealed that the SEI film formed in c-BC-based electrolytes contains fewer dimerized products, which are primary components of a good SEI film; this lower number of dimerized products can cause reduced film stability. As one of the origins of the decreased dimerization in c-BC, we identified the larger solvation energy of c-BC for the intermediate species and its smaller diffusion constant, which largely diminishes the dimerization. Moreover, the correlation among the Li+ intercalation behavior, nature of the SEI film, and strength of solvation was found to be common for EC/PC and t-BC/c-BC electrolytes, confirming the importance of solvation of the intermediates in the stability of the SEI film. These results suggest that weakening the solvation of the intermediates is one possible way to stabilize the SEI film for better charge–discharge performance.

In the present study, methods to enhance the oxygen reduction reaction (ORR) activity of sub-nanosized Pt clusters were investigated in a theoretical manner. Using ab initio molecular dynamics and Monte Carlo simulations based on density functional theory, we have succeeded in determining the origin of the superior ORR activity of Pt12 compared to that of Pt13. That is, it was clarified that the electronic structure of Pt12 fluctuates to a greater extent compared to that of Pt13, which leads to stronger resistance against catalyst poisoning by O/OH. Based on this conclusion, a set of sub-nanosized Pt-alloy clusters was also explored to find catalysts with better ORR activities and lower financial costs. It was suggested that Ga4Pt8, Ge4Pt8, and Sn4Pt8 would be good candidates for ORR catalysts.

Aqueous Na-or K-ion batteries could virtually eliminate the safety and cost concerns raised from Li-ion batteries,b ut their widespread applications have generally suffered from narrowe lectrochemical potential window( ca. 1.23 V) of aqueous electrolytes that leads to lowenergy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions,wediscovered, for the first time,r oom-temperature hydrate melts for Na and Ks ystems, 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,o wing to the merit that almost all water molecules participate in the Na + or K + hydration shells.A saproof-of-concept, ap rototype Na 3 V 2 -(PO 4 ) 2 F 3 j NaTi 2 (PO 4 ) 3 aqueous Na-ion full-cell with the Nahydrate-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.The commercialization of lithium (Li)-ion batteries has attracted enormous attention and dominated the market in portable consumer electronics owing to their high energy density and long cycling stability. [1] However,t heir applications in grid-scale energy storage systems are inevitably hindered by the high cost and limited source of Li, as well as potential safety and environmental issues raised by the use of highly volatile,i nflammable,a nd toxic organic electrolytes. [2] Aqueous rechargeable batteries have received considerable attention for large-scale energy storage owing to the utiliza-tion of non-flammable,l ow-cost, and low-toxicity aqueous electrolytes.Thekey issue of aqueous electrolytes for batteries is their intrinsically narrow potential window (ca. 1.23 V) bounded by the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), [3] which leads to low cell operating voltage and consequently low battery energy density.R ecently,t he stable potential operation window for aqueous Li-ion batteries has been increased to > 2V by using highly concentrated aqueous solutions of Li imide salts, [4] which dramatically reduce the electrochemical activity of water and the dissolution of electrode materials,t hus enabling the realization of high-energy stable aqueous Li-ion batteries.F or instance,Suo et al. [4d] formulated a"water-in-salt" electrolyte (molar ratio:H 2 O/Li = 2.6) that could expand the electrochemical stable window to circa 3.0 Vowing to low amount of free water, which allows for the stable cycling of a2 .3 V aqueous LiMn 2 O 4 j Mo 6 S 8 cell. Later on, our group [4c] has demonstrated ae utectic composition of LiN(SO 2 CF 3 ) 2 (LiTFSI) and LiN(SO 2 C 2 F 5 ) 2 (LiBETI) to form ah ydrate melt Li(TFSI) 0.7 (BETI) 0.3 ·2 H 2 Ow ith extremely low water content (molar ratio:H 2 O/Li = 2.0), whose potential window was expanded maximally to 3.8 V( 1.25-5.05 Vv s. Li + /Li) owing to the parti...