Metallic zinc (Zn) has been regarded as an ideal anode material for aqueous batteries because of its high theoretical capacity (820 mA h g), low potential (-0.762 V versus the standard hydrogen electrode), high abundance, low toxicity and intrinsic safety. However, aqueous Zn chemistry persistently suffers from irreversibility issues, as exemplified by its low coulombic efficiency (CE) and dendrite growth during plating/ stripping, and sustained water consumption. In this work, we demonstrate that an aqueous electrolyte based on Zn and lithium salts at high concentrations is a very effective way to address these issues. This unique electrolyte not only enables dendrite-free Zn plating/stripping at nearly 100% CE, but also retains water in the open atmosphere, which makes hermetic cell configurations optional. These merits bring unprecedented flexibility and reversibility to Zn batteries using either LiMnO or O cathodes-the former deliver 180 W h kg while retaining 80% capacity for >4,000 cycles, and the latter deliver 300 W h kg (1,000 W h kg based on the cathode) for >200 cycles.

Rechargeable aqueous Zn/MnO battery chemistry in a neutral or mildly acidic electrolyte has attracted extensive attention recently because all the components (anode, cathode, and electrolyte) in a Zn/MnO battery are safe, abundant, and sustainable. However, the reaction mechanism of the MnO cathode remains a topic of discussion. Herein, we design a highly reversible aqueous Zn/MnO battery where the binder-free MnO cathode was fabricated by in situ electrodeposition of MnO on carbon fiber paper in mild acidic ZnSO+MnSO electrolyte. Electrochemical and structural analysis identify that the MnO cathode experience a consequent H and Zn insertion/extraction process with high reversibility and cycling stability. To our best knowledge, it is the first report on rechargeable aqueous batteries with a consequent ion-insertion reaction mechanism.

Narrow electrochemical stability window (1.23 V) of aqueous electrolytes has always been 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 solidelectrolyte-interphase (SEI) and reducing the overall electrochemical activity of water on cathode. A full aqueous Na-ion battery constructed on Na 0.66 [Mn 0.66 Ti 0.34 ]O 2 as cathode and NaTi 2 (PO 4 ) 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 (1C) for >1200 cycles. Molecular modeling revealed some key difference between Li-ion and Na-ion WiSE, and identified 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.

A new super-concentrated aqueous electrolyte is proposed by introducing a second lithium salt. The resultant ultra-high concentration of 28 m led to more effective formation of a protective interphase on the anode along with further suppression of water activities at both anode and cathode surfaces. The improved electrochemical stability allows the use of TiO2 as the anode material, and a 2.5 V aqueous Li-ion cell based on LiMn2 O4 and carbon-coated TiO2 delivered the unprecedented energy density of 100 Wh kg(-1) for rechargeable aqueous Li-ion cells, along with excellent cycling stability and high coulombic efficiency. It has been demonstrated that the introduction of a second salts into the "water-in-salt" electrolyte further pushed the energy densities of aqueous Li-ion cells closer to those of the state-of-the-art Li-ion batteries.