A fast ion conductor, NaTi 2 (PO 4) 3 (NTP), is hydrothermally synthesized as a solid-state electrolyte protection layer on the surface of Zn anodes (NTP@ Zn). NTP has fast ionic conductivity compared with other insoluble phosphates, such as TiP 2 O 7 (TPO) and Zn 3 (PO 4) 2 (ZPO), which is demonstrated by the density-functional theory calculation and cyclic voltammetry tests. X-ray photoelectron spectrometer, X-ray powder diffraction, and HRTEM analyses show that the internal transport/mobility of Zn 2+ can be achieved in NTP layer as an "ion passable fence." The NTP layer with a thickness of 20-25 µm not only prevents side reactions and zinc dendrites, but also improves the reversibility of Zn deposition and electrochemical performance. The NTP@Zn/MnO 2 battery represents the best long-life performance among Zn/MnO 2 batteries to date, which successfully retains a considerable capacity of 105 mA h g −1 with a CE nearly 100% after 10 000 charge/discharge cycles at 10 C (≈1.5 A g −1). Each cycle capacity attenuation rate is only 0.004%. This work represents an advanced step toward long-life Zn metal anodes for aqueous zinc-ion batteries.
Aqueous rechargeable Zn/birnessite batteries have recently attracted extensive attention for energy storage system because of their low cost and high safety. However, the reaction mechanism of the birnessite cathode in aqueous electrolytes and the cathode structure degradation mechanics still remain elusive and controversial. In this work, it is found that solvation water molecules coordinated to Zn2+ are coinserted into birnessite lattice structure contributing to Zn2+ diffusion. However, the birnessite will suffer from hydroxylation and Mn dissolution with too much solvated water coinsertion. Through engineering Zn2+ primary solvation sheath with strong‐field ligand in aqueous electrolyte, highly reversible [Zn(H2O)2]2+ complex intercalation/extraction into/from birnessite cathode is obtained. Cathode–electrolyte interface suppressing the Mn dissolution also forms. The Zn metal anode also shows high reversibility without formation of “death‐zinc” and detrimental dendrite. A full cell coupled with birnessite cathode and Zn metal anode delivers a discharge capacity of 270 mAh g−1, a high energy density of 280 Wh kg−1 (based on total mass of cathode and anode active materials), and capacity retention of 90% over 5000 cycles.
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