Design Guidelines for Cathode/Electrolyte Interface of Room-Temperature Rechargeable Magnesium Batteries with MnO2 Cathodes
Authors: Ye Xiatong , Li Hongyi , Okamoto Norihiko L. , Ichitsubo Tetsu 
Published: December 2024

Rechargeable magnesium batteries (RMBs) are strong candidates for large-scale energy storage systems due to their high theoretical energy density, high safety, and low production costs. However, the sluggish solid-state diffusion of Mg2+ in cathode materials and surface passivation of Mg metal anode in organic electrolytes are the main reasons for the stagnant development of RMBs. Since 2015, we have first reported the reversible Mg2+ insertion in a series of oxide cathode materials (MgCo2O4, etc.) at 150 ℃.[1] To explore suitable host structures at room temperature (RT), we have been comprehensively investigated the phase transformation pathways of various MnO2 polymorphs (α, β, γ, δ, and λ) and revealed that the hollandite-type α-MnO2 allows topotactic Mg2+ intercalation up to 200 mAh g-1 as a metastable host structure.[2] To construct full cells, we further investigated the reaction behavior of MnO2 cathodes in two representative electrolytes at room temperature.[3] In 2.26 M Mg[TFSA]2/G3 (saturated concentration at RT), the α-MnO2 cathode had a discharge capacity of about 110 mAh g-1 that was confirmed to be mainly derived from the Mg2+ intercalation. However, the Mg metal anodes were easily passivated in this electrolyte, leading to a significant decrease in discharge voltage. On the other hand, in 0.3 M Mg[Al(hfip)4]2/G3,[4] which allows reversible deposition/dissolution on Mg metal anodes, the MnO2 cathode was involved into the cathodic decomposition of the electrolyte species, where the Mg2+ intercalation was difficult to be maintained.
To establish design guidelines for a stable and functional cathode/electrolyte interface, we revealed the reaction mechanism of the α-MnO2 cathode in these two electrolytes with various experimental and computational methods. In 2.26 M Mg[TFSA]2/G3, [TFSA]- has close coordination with Mg2+ as contact-ion pairs (CIPs) and a lower energy level of lowest unoccupied molecular orbital (LUMO) compared to the free state.[5] As a result, [TFSA]- is prone to be involved in the cathodic decomposition and significantly increase the F ratio on the α-MnO2 cathode after discharge. As shown in Fig .1, the decomposition products formed an SEI-like layer which would protect the active particles to enable the genuine Mg2+ intercalation. Conversely, in 0.3 M Mg[Al(hfip)4]2/G3, because [Al(hfip)4]- can only exist in free state and is almost not involved the cathodic decomposition, the protective layer was absence on the α-MnO2 cathode. This results in an unstable cathode/electrolyte interface, from where O2- was released from the host structure to the electrolyte instead of the expected Mg2+ intercalation for charge compensation of Mn reduction.
[1] S. Okamoto, T. Ichitsubo et al, Adv. Sci. 2015, 2, 1500072. [2] T. Hatakeyama, H. Li, T. Ichitsubo et al, Chem. Mater.
2021, 33, 6983. [3] X. Ye, H. Li, T. Ichitsubo et al, ACS Appl. Mater. Interfaces. 2022, 14, 56685-56696. [4] J. Herb et al, ACS Energy Lett.
2016, 1, 1227–1232. [5] H. Li, T. Ichitsubo et al, Cell Reports Phys. Sci.
2022, 3, 100907.

Figure 1