The Thin Non-Stoichiometric Sn-O Layer Enabled Tunable Zn Plating Position for Anode-Free Zn Aqueous Batteries

Rechargeable Zinc-ion batteries (RZIBs) are regarded as the most promising candidate for large-scale energy storage and wearable electronics due to their low cost, high theoretical capacity (820 mAh g^–1 or 5,855 mAh cm^–3), high safety, and environmental friendliness.[1]<br/>Nevertheless, the practical energy densities of RZIBs are significantly limited by the use of excessive Zn metal anodes (with a thickness of 50–200 μm), resulting in a Zn utilization ratio (ZUR) of less than 5% due to the nearly unlimited zinc reservoir. The thickness of the Zn metal must be less than 10 µm, enabling a Zn utilization ratio higher than 80% according to the calculation.[2] However, the poor reversibility facing conventional Zn anode mainly led by the dendrite growth and the water-induced erosion makes it impossible to apply Zn anode with high ZUR for practical applications.[3]<br/>Constructing an artificial protection layer on the Zn anode is an important approach to achieving the reversible Zn ion plating/stripping process by regulating the Zn plating either on or beneath the protection layer uniformly.[4] However, the thickness of the artificial protection layer (generally higher than 1 μm) is too thick for the required thickness of Zn metal foil (less than 10 μm) to be used in the practical application. More importantly, the preferred Zn plating position of a protection layer with the appropriate thickness for thin Zn metal foil is still inconclusive.[5]<br/>Herein, we successfully controlled Zn plating positions by constructing the stoichiometric tunable Sn-O compounds thin layer (~80 nm) on Zn metal foil. The two different Zn plating behaviors have been systematically investigated by combining both experimental and theoretical results and a generalizable understanding of tuning the Zn plating positions has been proposed. During the electrochemical test, the Zn plated beneath the protection layer realizes long-term cycling over 3000 h at 1 mA cm^-2 and over 600 h with a ZUR of 85.7%. We further employed the corresponding protection layer in the anode-free system by building the protection layer on Cu foil. The average Coulombic efficiency of the modified Zn||Cu cell achieved 99.7% after 1000 cycles at 20 mA cm^-2 with a capacity of 10 mAh cm^-2. The full cell based on ZnMn₂O₄||modified-Cu delivered a capacity retention of 87.6% after 200 cycles, paving the way for high energy density RZIBs for practical applications.<br/><br/>[1] Nat. Energy, 2020, 5, 743–749<br/>[2] Nano Lett., 2021, 21, 3, 1446–1453<br/>[3] J. Am. Chem. Soc., 2022, 144, 16, 7160–7170<br/>[4] Matter, 2022, 5, 4363–4378<br/>[5] Adv. Energy Mater. 2023, 13, 2300606