All-climate aqueous Na-ion batteries using “water-in-salt” electrolyte

Topochemical behavior of ferrocene embedded in V2O5·nH2O with weak hydrogen bonding enhancing ammonium-ion storage

Aqueous ammonium ion batteries (AAIBs) are garnering increasing attention due to their utilization of abundant resources, cost-effectiveness, safety, and unique energy storage mechanism. The pursuit of high-performance cathode materials has become a pressing issue. In this study, we propose and synthesize ferrocene-embedded hydrated vanadium pentoxide (Fer/VOH) for implementation in AAIBs. The inclusion of ferrocene serves to expand the interlayer spacing, mitigate interlayer forces, and introduce the electron-rich environment characteristic of ferrocene. This augmentation facilitates the creation of additional oxygen vacancies, substantially enhancing the capacity and efficiency of ammonium ion storage. Notably, our investigation reveals that the incorporation of ferrocene attenuates the hydrogen bonding interactions associated with ammonium ions, rendering them more amenable to the interlayer embedding and release processes. Building upon these advantages, Fer/VOH exhibits a specific capacity of 313 mAh/g at a current density of 0.2 A/g, representing the highest reported performance among vanadium oxides utilized in AAIBs to date. Even after 2000 charge/discharge cycles at a current density of 2 A/g, Fer/VOH maintains a reversible specific capacity of 89 mAh/g, with a capacity retention rate of 54.8%. This study confirms the viability of Fer/VOH as a cathode material for AAIBs and offers a novel approach to enhancing the electrical conductivity and diminishing the hydrogen bonding forces in vanadium oxide intercalation through the embedding of electron-rich species and positronic groups.

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

Water-in-salt electrolytes (WiSEs) are a promising class of electrolytes due to their wide electrochemical stability window and nonflammability. In this study, we explore the structural organization of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) salt-based aqueous electrolytes, covering dilute to highly concentrated regions, by employing an all-atom molecular dynamics simulation. For the NaTFSI-based electrolyte, we observe that Na+ ions are mostly surrounded by water molecules at all the salt concentrations due to the very strong interaction between them. While TFSI anions weakly coordinate with Na+ ions and other TFSI anions, they also mostly prefer to be surrounded by water molecules. These interactions were found to have moderate dependence on the concentration of the NaTFSI salt. For the NaFSI-based electrolyte, while the Na+-water interaction is stronger at lower salt concentrations, the number of nearest neighbor FSI anions is found to be more than that of water at higher concentrations (≥20 m). This is because the increase in the salt concentration leads to expulsion of water molecules from the solvation shell of Na+ ions and enhances the interaction between Na+ ions and oxygen atoms of FSI. At the highest salt concentration (solubility limit), the bulk-like water structure is completely disrupted and dominated by an anionic network in the FSI-based electrolyte. In contrast, water-water hydrogen bonding network is still present even in the highly concentrated TFSI-based electrolyte. The simulated X-ray scattering pattern displays a low-q peak, revealing the presence of an intermediate range ordering due to alternating anion-rich and water/Na+-rich regions in both the electrolytes. However, the characteristic length scale corresponding to the low-q peak decreases with increasing the salt content in both the electrolytes.

Electrolyte and Interphase Engineering of Aqueous Batteries Beyond "Water-in-Salt" Strategy

Aqueous batteries are promising alternatives to non-aqueous lithium-ion batteries due to their safety, environmental impact, and cost-effectiveness. However, their energy density is limited by the narrow electrochemical stability window (ESW) of water. The "Water-in-salts" (WIS) strategy is an effective method to broaden the ESW by reducing the "free water" in the electrolyte, but the drawbacks (high cost, high viscosity, poor low-temperature performance, etc.) also compromise these inherent superiorities. In this review, electrolyte and interphase engineering of aqueous batteries to overcome the drawbacks of the WIS strategy are summarized, including the developments of electrolytes, electrode-electrolyte interphases, and electrodes. First, the main challenges of aqueous batteries and the problems of the WIS strategy are comprehensively introduced. Second, the electrochemical functions of various electrolyte components (e.g., additives and solvents) are summarized and compared. Gel electrolytes are also investigated as a special form of electrolyte. Third, the formation and modification of the electrolyte-induced interphase on the electrode are discussed. Specifically, the modification and contribution of electrode materials toward improving the WIS strategy are also introduced. Finally, the challenges of aqueous batteries and the prospects of electrolyte and interphase engineering beyond the WIS strategy are outlined for the practical applications of aqueous batteries.

Progress on Transition Metal Ions Dissolution Suppression Strategies in Prussian Blue Analogs for Aqueous Sodium-/Potassium-Ion Batteries

Aqueous sodium-ion batteries (ASIBs) and aqueous potassium-ion batteries (APIBs) present significant potential for large-scale energy storage due to their cost-effectiveness, safety, and environmental compatibility. Nonetheless, the intricate energy storage mechanisms in aqueous electrolytes place stringent requirements on the host materials. Prussian blue analogs (PBAs), with their open three-dimensional framework and facile synthesis, stand out as leading candidates for aqueous energy storage. However, PBAs possess a swift capacity fade and limited cycle longevity, for their structural integrity is compromised by the pronounced dissolution of transition metal (TM) ions in the aqueous milieu. This manuscript provides an exhaustive review of the recent advancements concerning PBAs in ASIBs and APIBs. The dissolution mechanisms of TM ions in PBAs, informed by their structural attributes and redox processes, are thoroughly examined. Moreover, this study delves into innovative design tactics to alleviate the dissolution issue of TM ions. In conclusion, the paper consolidates various strategies for suppressing the dissolution of TM ions in PBAs and posits avenues for prospective exploration of high-safety aqueous sodium-/potassium-ion batteries.

Discovering Cathodic Biocompatibility for Aqueous Zn-MnO2 Battery: An Integrating Biomass Carbon Strategy

Developing high-performance aqueous Zn-ion batteries from sustainable biomass becomes increasingly vital for large-scale energy storage in the foreseeable future. Therefore, γ-MnO2 uniformly loaded on N-doped carbon derived from grapefruit peel is successfully fabricated in this work, and particularly the composite cathode with carbon carrier quality percentage of 20 wt% delivers the specific capacity of 391.2 mAh g-1 at 0.1 A g-1, outstanding cyclic stability of 92.17% after 3000 cycles at 5 A g-1, and remarkable energy density of 553.12 Wh kg-1 together with superior coulombic efficiency of ~ 100%. Additionally, the cathodic biosafety is further explored specifically through in vitro cell toxicity experiments, which verifies its tremendous potential in the application of clinical medicine. Besides, Zinc ion energy storage mechanism of the cathode is mainly discussed from the aspects of Jahn-Teller effect and Mn domains distribution combined with theoretical analysis and experimental data. Thus, a novel perspective of the conversion from biomass waste to biocompatible Mn-based cathode is successfully developed.

Is "Water in Salt" Electrolytes the Ultimate Solution? Achieving High Stability of Organic Anodes in Diluted Electrolyte Solutions Via a Wise Anions Selection

The introduction of the water-in-salt (WIS) electrolytes concept to prevent water splitting and widen the electrochemical stability window, has spurred extensive research efforts toward development of improved aqueous batteries. The successful implementation of these electrolyte solutions in many electrochemical systems shifts the focus from diluted to WIS electrolyte solutions. Considering the high costs and the tendency of these nearly saturated solutions to crystallize, this trend can be carefully re-evaluated. Herein we show that the stability of organic electrodes comprising the active material perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), is strongly influenced by the solvation character of the anions rather than the concentration of the electrolyte solution. Even though the charging process of PTCDA involves solely insertion of cations (i.e., principal counter-ions), surprisingly, the dominant factor influencing its electrochemical performance, including long-term electrode stability, is the type of the co-ions (i.e., electrolytic anions). Using systematic electrochemical analysis combined with theoretical simulations, we show that the selection of kosmotropic anions results in fast fading of the PTCDA anodes, while a selection of chaotropic anions leads to excellent stability, even at electrolytes concentrations as low as 0.2 M. These findings provide a new conceptual approach for designing advanced electrolyte solutions for aqueous batteries.

High-Energy Sn-Ni and Sn-Air Aqueous Batteries via Stannite-Ion Electrochemistry

Tin is promising for aqueous batteries (ABs) due to its multiple electrons' reactions, high corrosion resistance, large hydrogen overpotential, and excellent environmental compatibility. However, restricted to the high thermodynamic barrier and the poor electrochemical kinetics, efficient alkaline Sn plating/stripping at facile conditions has not yet been realized. Here, for the first time, we demonstrate a highly reversible stannite-ion electrochemistry and construct a novel paradigm of high-energy Sn-based ABs. Combined spectroscopic characterization, electrochemical evaluation, and theoretical computation reveal the thermodynamic merits with a low reaction energy barrier and feasible H₂O participation in Sn-ion reduction as well as the kinetic merits with fastened surface charge transfer and SnO₂^2- diffusion. The resultant alkaline Sn anode delivers a low potential of -1.07 V vs Hg/HgO, a specific capacity of 450 mA h g^-1, a Coulombic efficiency of near 100%, superb rate capability at 45.5 A g^-1, and excellent cycling durability without dendrite and dead Sn. As a proof of concept, we developed new high-energy Sn-based ABs, including 1.45 V Sn-Ni with 314 W h kg^-1 (58 kW kg^-1 and over 15,000 cycles) and 1.0 V Sn-air with 420 W h kg^-1 (lifespan over 1900 h), on the basis of masses from cathode and anode active materials. The findings prove the feasibility of the alkaline Sn metal anode, and the new suite of high-energy Sn-based ABs may be of immediate benefit toward safe, reliable, and affordable energy storage.

Aqueous Dual-Electrolyte Full-Cell System for Improving Energy Density of Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are regarded as one of the most promising candidates for next-generation energy storage devices and have been gradually grasping market share for their low cost and similar reaction mechanism and production process as compared to lithium-ion batteries. However, the low energy density of SIBs restricts their practical applications. For example, regular full cells of a Prussian blue cathode and NASICON anode have only a low discharge capacity (about 77 mA h/g at 1 C). Taking into account the compatibility of the electrolyte and electrode materials, a novel strategy for a viable aqueous dual-electrolyte sodium-ion battery (ADESIB) has been proposed using Na₂SO₄ solution as the anolyte and redox-active sodium hexacyanoferrate Na₄Fe(CN)₆ solution as the catholyte to accommodate a NASICON NaTi₂(PO₄)₃ anode and Prussian blue Na₂NiFe(CN)₆ cathode. The capacity of Na⁺ ion deinsertion/insertion electrodes combined with the redox chemistry of the Na₄Fe(CN)₆ catholyte thus enhances overall charge storage and energy density. The ADESIB delivers a capacity of about 113 mA h/g at 1 C, showing a 43% improvement over batteries with a regular single Na₂SO₄ electrolyte. Additionally, the dual-electrolyte full-cell system is proved to reach a 84.7% capacity retention after 1000 cycles, mainly due to the synergy of the electrolytes in both sides. This pioneering research proposes an aqueous dual-electrolyte sodium-ion full cell, showing potential applications in a new sodium-ion full battery system.