High-Surface-Area α-MnO2 Promotes Efficient Mn Deposition to Mitigate Capacity Fading in Aqueous Zn-Ion Batteries

Abstract

The capacity decays of MnO₂ caused by its irreversible structural changes and volume expansion is a thorny problem for aqueous zinc-ion batteries (AZIBs), which can be accelerated by the inevitable dissolution of Mn. Herein, a 3D hollow urchin-like α-MnO₂ (HUM) with a high Young's modulus of 27.8 Mpa was prepared, which can effectively relieve the stress accumulated by volume expansion of MnO₂ during cycling. As a result, HUM delivers a cycle life of up to 1000 cycles at a current density of 500 mA g^-1, which is about 5.17 times longer than that of ordinary nanotubes α-MnO₂ (NAM). The large specific surface area (137.4 cm³ g^-1) of HUM makes it showing more active sites, which can promote the uniform deposition of dissolved Mn²⁺ and efficiently conversion between Mn²⁺ and MnO₂. Hence, HUM shows an enhanced initial capacity of 345.7 mA h g^-1 compared with that of 142.2 mA h g^-1 for NAM. In this work, we also investigated the two pH-controlled charge storage mechanisms and related evolutions of phase transition during charge/discharge to emphasize the importance of Mn²⁺ and Zn₄SO₄(OH)₆·xH₂O, aiming to bring new insights into future studies on the commercialization of α-MnO₂-based AZIBs.