“Water-in-Salt” Electrolyte Makes Aqueous Sodium-Ion Battery Safe, Green, and Long-Lasting

DOI: 10.1002/aenm.201701189 the availability issue of lithium as a natural source, along with potential safety and environmental risks brought by the inflammable and toxic nonaqueous electrolytes. In contrast, sodium (Na) is highly abundant and readily accessible in both earth-crust and ocean. An aqueous Na-ion batteries would be far more economically competitive than LIBs for large-format applications, where cost, safety, greenness, and cycle-life outweigh energy density considerations,[1] and would provide an ideal resolution to all these concerns mentioned above.[2–4] Historically, aqueous electrolytes have been known for their narrow electrochemical stability window (<1.50 V), as defined by the decomposition reaction of water,[5,6] which imposes an unwanted restriction on the choice of electrochemical couples (cathode and anode materials) and consequently the practical energy output of such aqueous battery chemistries.[7] Only those anode materials that operate at relatively high potentials have been used for aqueous Na-ion batteries, such as NaTi2(PO4)3 (2.1 V vs Na),[3,8] Prussian blue analogues,[4] vanadium oxide including V2O5·0.6H2O and Na2V6O16·nH2O, or organic anodes including disodium naphthalenediimide (SNDI) (2.57 V vs Na),[11] polyimide anode,[12] and PPy-coated MoO3 (2.3–2.4 V vs Na).[13] Among these, NaTi2(PO4)3 with NASICON structure has a long cycling stability, and hence has been extensively investigated. However, since this potential (2.10 V vs Na) is slightly lower than hydrogen evolution potential (2.297 V vs Na), water decomposition still inevitably occurs in aqueous Na-ion battery constructed with NaTi2(PO4)3 anode. Such irreversible process not only builds up internal pressure due to hydrogen evolution but also depletes Na from the cathode, which is a limited source in a full Na-ion cell. In a similar manner, the operation potential of the most commonly used cathode material for sodium intercalation, Na0.66[Mn0.66Ti0.34]O2, also situates slightly beyond the anodic stability limit of aqueous electrolyte, where O2 is produced during repeated charge/discharge cycles. Consequently, such aqueous Na-ion batteries are characterized by steady capacity losses as well as low Coulombic efficiencies. In previous reports, high cycling rates were often adopted so that impressive cycle numbers could be accumulated before the effect of water decomposition on cycle life becomes visible. Narrow electrochemical stability window (1.23 V) of aqueous electrolytes is always considered the key obstacle preventing aqueous sodium-ion chemistry of practical energy density and cycle life. The sodium-ion water-in-salt electrolyte (NaWiSE) eliminates this barrier by offering a 2.5 V window through suppressing hydrogen evolution on anode with the formation of a Na+-conducting solid-electrolyte interphase (SEI) and reducing the overall electrochemical activity of water on cathode. A full aqueous Na-ion battery constructed on Na0.66[Mn0.66Ti0.34]O2 as cathode and NaTi2(PO4)3 as anode exhibits superior performance at both low and high rates, as exemplified by extraordinarily high Coulombic efficiency (>99.2%) at a low rate (0.2 C) for >350 cycles, and excellent cycling stability with negligible capacity losses (0.006% per cycle) at a high rate (1 C) for >1200 cycles. Molecular modeling reveals some key differences between Li-ion and Na-ion WiSE, and identifies a more pronounced ion aggregation with frequent contacts between the sodium cation and fluorine of anion in the latter as one main factor responsible for the formation of a dense SEI at lower salt concentration than its Li cousin.