Recently, metallic zinc (Zn) is becoming a promising ideal anode material for rechargeable aqueous batteries by providing high theoretical capacity (820 mA h/g) with divalent reaction, environmental friendliness, earthy abundance, low cost, low toxicity, higher water compatibility, and low electrochemical potential (−0.762 V vs SHE). However, intensive growth of zinc dendrites while plating/stripping lowers its coulombic efficiency and shortens the cycle life of the rechargeable devices. Here, we report a concentrated aqueous electrolyte (4.2 M ZnSO4 + 0.1 M MnSO4) with improved cycling stability of zinc metal anode achieving an average coulombic efficiency (ACE) ∼99.21% cycling for more than 1000 h at 0.2 mA/cm2 current density using a Zn||Cu cell. However, a frequently used diluted electrolyte (2 M ZnSO4 + 0.1 M MnSO4) only produces ACE ≈ 97.54% with a relatively short life cycle. The developed concentrated electrolyte with strongly aggregated ion pairs shows the synergetic effects of the enhanced solvation/desolvation process, electrostatic shielding, and Le Chatelier’s principle. Consequently, the additives simultaneously suppress Zn dendrites and dissolution of Mn2+ ions from the MnO2 cathode. A highly stable and reversible Zn||MnO2 cell retaining about 88.37% retention capacity was obtained after cycling for more than 1200 cycles at 938 mA/g current density.

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The combined effect of concentrated electrolyte and cycling protocol on the cyclic performance of the anode-free battery (AFB) is evaluated systematically. In situ deposition of Li in the AFB configuration in the presence of a concentrated electrolyte containing fluorine-donating salt and resting the deposit enables the formation of stable and uniform SEI. The SEI intercepts the undesirable side reaction between the deposit and solvent in the electrolyte and reduces electrolyte and Li consumption during cycling. The synergy between the laboratory-prepared concentrated 3 M LiFSI in the ester-based electrolyte and our resting protocol significantly enhanced cyclic performances of AFBs in comparison to the commercial carbonate-based dilute electrolyte, 1 M LiPF6. Benefitting from the combined effect, Cu∥LiFePO4 cells delivered excellent cyclic performance at 0.5 mA/cm2 with an average CE of up to 98.78%, retaining a reasonable discharge capacity after 100 cycles. Furthermore, the AFB can also be cycled at a high rate up to 1.0 mA/cm2 with a high average CE and retaining the encouraging discharge capacity after 100 cycles. The fast cycling and stable performance of these cells are attributed to the formation of robust, flexible, and tough F-rich conductive SEI on the surface of the in situ-deposited Li by benefiting from the combined effect of the resting protocol and the concentrated electrolyte. A condescending understanding of the mechanism of SEI formation and material choice could facilitate the development of AFBs as future advanced energy storage devices.

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