Investigation of the Mechanism of Mg Insertion in Birnessite in Nonaqueous and Aqueous Rechargeable Mg-Ion Batteries

Abstract

Magnesium batteries are an energy storage system that potentially offers high energy density, but development of new high voltage cathode materials and understanding of their electrochemical mechanism are critical to realize its benefits. Herein, we synthesize the layered MnO 2 polymorph (the birnessite phase) as a nanostructured phase supported on conductive carbon cloth and compare its electrochemistry and structural changes when it is cycled as a positive electrode material in a Mg-ion battery under nonaqueous or aqueous conditions. X-ray photoelectron spectroscopy and transmission electron microscopy studies show that a conversion mechanism takes place during cycling in a nonaqueous electrolyte, with the formation of MnOOH, MnO, and Mg(OH) 2 upon discharge. In aqueous cells, on the other hand, intercalation of Mg 2+ ions takes place, accompanied by expulsion of interlayer water and transformation to a spinel-like phase as evidenced by X-ray diffraction. Both systems are structurally quasireversible. The sharp contrast in behavior in the two electrolytes points to the important role of the desolvation energy of the Mg 2+ cation in nonaqueous systems.

Manganese Dioxide As Rechargeable Magnesium Battery Cathode

“…In aqueous cells, B-MnO2 exhibited sloping voltage curves with an average discharge voltage at around 2.8 V (Nam et al, 2015;Sun et al, 2016), significantly higher than that in dry electrolyte and even in wet electrolyte, suggesting the aqueous solution greatly improves the kinetics for Mg 2+ -transportation. In Sun et al 's work , the initial capacity of B-MnO2 was 150 mAh g −1 (Figure 4A).…”

“…The structural evolution of B-MnO2 cycled in aqueous solution was analyzed by Sun et al (2016). They observed a contraction of the interlayer spacing from 7 to 4.86 Å after discharge.…”

“…Further studies are still necessary to provide more details to the presented knowledge, such as the thermodynamics of MnO2-Mg-water phase, the kinetics of Mg 2+ (H2O)n-intercalation and the effect of water on MnO2 structure. At this moment, the difference between aqueous cells and water-containing nonaqueous cells is still unclear: while Mg 2+ -intercalation in wet solution was interpreted as the insertion of hydrated species, in aqueous cell where abundant water molecules are available the insertion does not involve the participation of water (Kim et al, 2015a;Nam et al, 2015;Sun et al, 2016). Future DFT-based computational studies are especially welcome to address these questions.…”

“…Chevrel phase and TiS2 showed good cyclability for Mg 2+ -intercalation but with limited energy densities due to the low voltages. Oxide cathodes generally have higher voltages but the reversible intercalation has only been achieved in V2O5, MoO3, and MnO2 at specific conditions (Gregory et al, 1990;Aubach et al, 2000;Gershinsky et al, 2013;Kim et al, 2015a;Nam et al, 2015;Sa et al, 2016;Sun et al, 2016). Among these materials, MnO2 had received perhaps the best attention not only due to the easiness to synthesis but also because the plentiful polymorphism and easily tuned electrochemical properties provide a good platform to gather fundamental knowledge about cathode chemistry.…”

Sponge-Like Porous Manganese(II,III) Oxide as a Highly Efficient Cathode Material for Rechargeable Magnesium Ion Batteries

“…[29][30][31] Among them, MnO2 is perhaps the most extensively studied electrode material in many kinds of batteries. [32][33][34] Several polymorphs of MnO2 have also been investigated as possible MIB cathodes, [35][36][37][38][39] especially α-MnO2. It showed a high specific capacity of ~280 mAh g -1 in the initial discharge, but poor cyclic performance (retained ∼50 mAh g -1 after just five cycles).…”