Density Functional Theory Modeling of MnO₂ Polymorphs as Cathodes for Multivalent Ion Batteries

Abstract

Multivalent ion batteries (MVIBs) provide an inexpensive and energy-dense alternative to Li-ion batteries when portability of the battery is not of primary concern. However, it is difficult to find cathode materials that provide optimal battery characteristics such as energy density, adequate charge/discharge rates, and cyclability when paired with a multivalent ion. To address this, we investigate six MnO 2 polymorphs as cathodes for MVIBs using density functional theory calculations. We find voltages as high as 3.7, 2.4, 2.7, 1.8, and 1.0 V for Li, Mg, Ca, Al, and Zn, respectively, and calculate the volume change due to intercalation. We then focus specifically on Ca and compute the energy barriers which are associated with the diffusion of the ion throughout the materials. Our findings show that the α-phase displays the most rapid diffusion kinetics for a Ca ion, with a diffusion barrier as low as 190 meV. We then investigate the potential for the five polymorphs exhibiting the highest voltage to intercalate additional atoms and demonstrate that it is energetically favorable for each to accept at least one additional Ca ion; furthermore, two of the phases can accept more than two Ca ions. However, in each case, there is also a corresponding drop in the voltage as further atoms are intercalated. We also utilize a crystal-chemistry approach to detail the structural evolution of each phase by computing the bond valence sum and effective coordination of the Mn 4+ ions upon intercalation of increasing numbers of Ca ions. Finally, by computing the electronic density of states, Bader charges, and real space charge density, we describe how the additional electrons from the Ca ions are distributed throughout the unit cell. These insights provide guidance in selecting a MnO 2 polymorph with the traits necessary for the realization of MVIBs.

Progress in Rechargeable Aqueous Zinc‐ and Aluminum‐Ion Battery Electrodes: Challenges and Outlook

“…The octahedra represent Mn atoms sitting in oxygen‐coordinated environment, in two opposite spin configurations (purple for spin‐up and brown for spin‐down). Reproduced with permission . Copyright 2018, ACS.…”

“…However, the Zn 2+ storage mechanism for MnO 2 polymorphs is still ambiguous. K + , Na + , Mg 2+ , and H 2 O from synthesis precursors can intercalate into the MnO 2 crystals tunnel/layers and change the valence state of Mn in MnO 2 , making a one‐to‐one comparison between each study harder …”

Flexible 1D Batteries: Recent Progress and Prospects

“…Manganese dioxides (MnO 2 ) with tunnel or layered structure allowing reversible insertion/extraction of Zn 2+ ions has gained increasing attention in 1D ZIBs due to their unique advantages, such as low‐cost, abundant reserves, environmental friendliness, low toxicity, and multiple valence states . To this end, Wang et al created a flexible cable‐type ZIB by using MnO 2 @CNT fiber as the cathode .…”

Challenges and perspectives for manganese‐based oxides for advanced aqueous zinc‐ion batteries

“…It is well‐known that MnO 2 exists in various crystal forms such as α‐MnO 2 , β‐MnO 2 , ε‐MnO 2 , γ‐MnO 2 , δ‐MnO 2 , and todorokite‐type MnO 2 . In these structures, the basic structural unit MnO 6 octahedra is connected to each other by co‐angle/co‐edge, constructing chain, tunnel, layered structures with enough space accommodating foreign cations . Given this structural advantage, MnO 2 has been extensively investigated as favorable cathodes for batteries in the past several years, including Li‐ion batteries, Na‐ion batteries, K‐ion batteries, Mg‐ion batteries, and latest ZIB .…”

“…In these structures, the basic structural unit MnO 6 octahedra is connected to each other by coangle/co-edge, constructing chain, tunnel, layered structures with enough space accommodating foreign cations. [72][73][74][75] Given this structural advantage, MnO 2 has been extensively investigated as favorable cathodes for batteries in the past several years, including Li-ion batteries, 76,77 Na-ion batteries, 78 K-ion batteries, 79 Mg-ion batteries, [80][81][82] and latest ZIB. [83][84][85][86] Figure 4A).…”