Discharging Behavior of Hollandite α-MnO₂ in a Hydrated Zinc-Ion Battery

Abstract

Hollandite, α-MnO2, is of interest as a prospective cathode material for hydrated zinc-ion batteries (ZIBs); however, the mechanistic understanding of the discharge process remains limited. Herein, a systematic study on the initial discharge of an α-MnO2 cathode under a hydrated environment was reported using density functional theory (DFT) in combination with complementary experiments, where the DFT predictions well described the experimental measurements on discharge voltages and manganese oxidation states. According to the DFT calculations, both protons (H+) and zinc ions (Zn2+) contribute to the discharging potentials of α-MnO2 observed experimentally, where the presence of water plays an essential role during the process. This study provides valuable insights into the mechanistic understanding of the discharge of α-MnO2 in hydrated ZIBs, emphasizing the crucial interplay among the H2O molecules, the intercalated Zn2+ or H+ ions, and the Mn4+ ions on the tunnel wall to enhance the stability of discharged states and, thus, the electrochemical performances in hydrated ZIBs.

High‐Energy and Stable Subfreezing Aqueous Zn–MnO2 Batteries with Selective and Pseudocapacitive Zn‐Ion Insertion in MnO2

“…[ 13d ] This phase has layered structure with an interlayer spacing of 0.27 nm and Zn ions occupying above and below Mn vacancy sites, and its formation could be attributed to its better thermodynamic stability. [ 12a ] The overall SAED ring pattern shows low crystallinity that is similar to ZnMn 2 O 4 , and their overall morphology was similar as well. The Zn:Mn atomic ratio was reduced to ≈1:1 after charging, and this confirms extraction of Zn 2+ (Figure S13, Supporting Information).…”

“…The exact mechanisms of MnO 2 cathodes in aqueous electrolytes are very complicated and are still elusive, and several mechanisms have been proposed based on examination of various discharged MnO 2 polymorphs at room temperature. [ 12 ] These mechanisms debate around proton insertion, which leads to the formation of MnOOH and ZnSO 4 [Zn(OH) 2 ] 3 · n H 2 O (in ZnSO 4 ), [ 1b ] or Zn 2+ insertion, which results in various Zn x MnO 2 including spinel ZnMn 2 O 4 , tunnel‐type Zn m Mn 2 O 4 , layered birnessite, and Zn‐buserite. [ 13 ] Like most oxide cathodes, proton (H + ) insertion dominates electrochemical processes of MnO 2 but this is often associated with cycling failure.…”

“…[ 14 ] In addition, the complicated interplays between H + /Zn 2+ co‐insertion, together with the redox conversion of soluble Mn 2+ and insoluble Mn 4+ , make control of battery cycling very challenging. [ 12a,13d ] On the other hand, it is commonly accepted that water solvent plays several key roles such as 1) dissociated H + and OH − are key reactants for electrochemical conversion of MnO 2 , 2) co‐insertion of H 2 O molecules to MnO 2 tunnels, and 3) to solvate and dissolve Mn 2+ ions generated during battery discharging. Of note, the latter two induce structural instability and lead to cycling degradation.…”

“…Of note, the latter two induce structural instability and lead to cycling degradation. [ 12a ] Hence, modulating activities of water molecules and suppressing water/proton activity are essential for high‐performance Zn–MnO 2 batteries with current approaches mostly focusing on concentrated electrolytes and organic co‐solvents. For example, selective Zn 2+ insertion was recently described with the combination of concentrated Zn(OTf) 2 salts and propylene carbonate additive.…”

Ab-Initio Energetics of Electrochemical Ion Insertion into Manganese Oxides

“…Conversely, computational investigations with Hubbard-U corrections have typically been limited to a single polymorph/electrolyte pair. [33][34][35] In this work, we investigate the errors associated with the SCAN and PBE functionals including the Hubbard-U parameter and develop a correction scheme to rectify these errors. We find the PBE+U functional to be a less computationally expensive and slightly higher accuracy approach to compute ion insertion voltages in the Mn oxide system.…”