Enabling stable MnO2 matrix for aqueous zinc-ion battery cathodes

Abstract

The primary issue faced by MnO₂ cathode materials for aqueous Zn-ion batteries (AZIBs) is the occurrence of structural transformations during cycling, resulting in unstable capacity output. Pre-intercalating closely bonded ions into the MnO₂ structures has been demonstrated as an effective approach to combat this. However, mechanisms of the pre-intercalation remain unclear. Herein, two distinct δ-MnO₂ (K_0.28MnO₂·0.1H₂O and K_0.21MnO₂·0.1H₂O) are prepared with varying amounts of pre-intercalated K⁺ and applied as cathodes for AZIBs. The as-prepared K_0.28MnO₂·0.1H₂O cathodes exhibit relatively high specific capacity (300 mA h g⁻¹ at 100 mA g⁻¹), satisfactory rate performance (35% capacity recovery at 5 A g⁻¹) and competent cyclability (ca. 95% capacity retention after 1000 cycles at 2 A g⁻¹), while inferior cyclability and rate performance are observed in K_0.21MnO₂·0.1H₂O. A stable δ-MnO₂ phase is observed upon cycling, with the reversible deposition of Zn₄SO₄(OH)₆·5H₂O (ZSH), ion migration between electrodes and synchronous transition of Mn valence states. This work firstly and systematically reveals the role of the pre-intercalated ions via density functional theory simulations and show that above a threshold K/Mn ratio of ca. 0.26, the K ions suppress structural transformations by stabilizing the δ phase. To demonstrate its commercial potential, AZIBs with high-loading active materials are fabricated, which deliver adequate energy and power densities compared with most commercial devices.