Quantitatively Predict the Potential of MnO₂ Polymorphs as Magnesium Battery Cathodes

Abstract

Despite tremendous efforts denoted to magnesium battery research, the realization of magnesium battery is still challenged by the lack of cathode candidate with high energy density, rate capability and good recyclability. This situation can be largely attributed to the failure to achieve sustainable magnesium intercalation chemistry. In current work we explored the magnesiation of distinct MnO2 polymorphs using first-principles calculations, focusing on providing quantitative analysis about the feasibility of magnesium intercalation. Consistent with experimental observations, we predicted that ramsdellite-MnO2 and α-MnO2 are conversion-type cathodes while nanosized spinel-MnO2 and MnO2 isostructual to CaFe2O4 are better candidates for Mg intercalation. Key properties that restrict Mg intercalation include not only sluggish Mg migration but also stronger distortion that damages structure integrity and undesirable conversion reaction. We demonstrate that by evaluating the reaction free energy, structural deformation associated with the insertion of magnesium, and the diffusion barriers, a quantitative evaluation about the feasibility of magnesium intercalation can be well established. Although our current work focuses on the study of MnO2 polymorphs, the same evaluation can be applied to other cathode candidates, thus paving the road to identify better cathode candidates in future.

Fervent Hype behind Magnesium Batteries: An Open Call to Synthetic Chemists—Electrolytes and Cathodes Needed

“…Ling, Zhang,a nd Mizuno [91] performed DFT calculations to better understand the behavior of MnO 2 polymorphs during Mg 2+ intercalation, and predicted that both ramsdellite-MnO 2 and a-MnO 2 would behave as conversion cathodes rather than intercalation cathodes,c onsistent with experimental observations.T hey also predicted that nano-sized spinel MnO 2 and MnO 2 isostructural with CaFe 2 O 4 (CF-MnO 2 )w ould be better candidates for reversible Mg 2+ intercalation. Both of these polymorphs were predicted to exhibit better structural and thermodynamic stability than the other MnO 2 materials.F rom ap ractical standpoint, nano- sized spinel MnO 2 is likely to be more easily synthesized than CF-MnO 2 .…”

Manganese Dioxide As Rechargeable Magnesium Battery Cathode

“…In Mg-Mn-O system, the most stable phase at the composition of MgMn2O4 is spinel (Ling et al, 2016). If the complete structural transformation from α-MnO2 to spinel phase is kinetically hindered in really operations, the intercalated α-MgxMnO2 was revealed to be thermodynamically less stable than amorphous xMg⋅MnO2 or mixed MgO and MnO, as shown in Figure 2C .…”

“…For this purpose, we developed a triangular radar-type graph to quantitatively visualize the thermodynamics, kinetics, and structural information in the same figure (Ling et al, 2016). As shown in Figure 6, the triangular graph contains the calculated free energy difference between conversion and intercalation reaction, the deformation scored defined by the change of key structural parameters such as bond length and bond angles, and the diffusion barrier.…”

Interlayer‐Expanded Vanadium Oxychloride as an Electrode Material for Magnesium‐Based Batteries

“…In addition, pure carbon black was tested in a Swagelok cell in the same conditions as the aLi electrode and it did not show any contribution to the capacity observed in Figure B. Recently, Mizuno ’s group at Toyota reported that MgO formation could be achieved instead of Mg‐intercalation in several polymorphs of MnO 2 . Formation of MgO on the surface of VOCl cannot be ruled out in the electrodes aLi1D and aLi1C since the XPS fingerprint is not straightforward: the chemical shift of the Mg 2p signal is in general rather small, and the measured peak position of 50.5 eV is observed for a variety of different Mg species.…”

“…Recently, Mizuno 's group at Toyota reported that MgO formation could be achieved instead of Mg-intercalation in several polymorphs of MnO 2 . [35] Formation of MgO on the surface of VOCl cannot be ruled out in the electrodes aLi1D and aLi1C since the XPS fingerprint is not straightforward: the chemical shift of the Mg 2p signal is in general rather small, and the measured peak position of 50.5 eV is observed for a variety of different Mg species. In the O 1 s spectra, MgO should lead to a peak at 530.0-530.2 eV.…”