Ammonium-Preintercalated Layered Manganese Oxide with Single-Phase Intercalation Chemistry and Enhanced Two-Electron Reaction in Aqueous Zinc-Ion Batteries

Abstract

Rechargeable aqueous Zn-MnO2 batteries are promising candidates for large-scale energy storage rooted in the merits of low cost and high safety. However, the development of Zn-MnO2 batteries with high capacity and good cycle stability has turned the effort on the cathode limit. Herein, the electrochemical performance and charge storage mechanism of layered manganese oxides with preintercalated potassium (K-MO), ammonium (NH4-MO), and tetramethylammonium ions (TMA-MO) are systematically investigated. The results reveal that the MnO2 cathodes undergo both one-electron (1e) intercalation and two-electron (2e) dissolution/redeposition mechanisms. For the former, hexagonal NH4-MO exhibits single-phase charge storage behavior with contained lattice changes during the initial cycles, as characterized by in situ Raman microscopy and ex situ X-ray diffraction (XRD). Moreover, low charge transfer resistance (Rct) and high ion diffusivity make the NH4-MO cathode more attractive. For the latter, the Mn4+/Mn2+ reaction chemistry takes place and appreciably contributes to the capacity of Zn-MnO2 batteries. By detecting the Mn concentration in electrolyte, NH4-MO better induces dissolution of Mn2+ and deposition of Zn-Mn species, thus promoting Mn4+/Mn2+ redox reaction. Benefiting from these features, NH4-MO demonstrates appreciable discharge capacity (306 mAh g-1 at 0.3 A g-1), good rate capability (113 mAh g-1 at 8 A g-1), and meritorious cycle performance (retaining 101 mAh g-1 after 2000 cycles at 4 A g-1). This study enriches the cathode engineering methods for realizing enhanced two-electron Mn4+/Mn2+ reaction in rechargeable aqueous Zn-MnO2 batteries.