The Quest for Functional Oxide Cathodes for Magnesium Batteries: A Critical Perspective

Abstract

The Mg battery is an energy storage technology which has garnered significant interest in recent years. Mg batteries incorporating a metal oxide cathode (MOC) are potential candidates to supersede the state-of-the-art Li-ion battery in energy density, cost, and sustainability. However, there are significant discrepancies in reported performances and reactivities of Mg battery MOCs, with detailed analyses revealing that parasitic electrolyte reactions can contribute almost entirely to the measured capacity. This Perspective describes a holistic approachencompassing elemental, redox, and structural probeswhich is vital to robustly confirm and quantify Mg intercalation in MOCs. It critically surveys recent literature for applications of this approach to reveal true state-of-the-art MOCs for Mg batteries. We also suggest testing and analysis protocols to ensure fair comparison of future reports with these state-of-the-art materials.

The Importance of Stacking and Coordination for Li, Na, and Mg Diffusion and Intercalation in Ti3C2T2 MXene

“…Further, the voltage of MgTi 3 C 2 O(OH) is considerably lower than that of MgTi 3 C 2 O 2 , further decreasing the prospects of Ti 3 C 2 T 2 as a cathode material in Mg batteries. Finally, proton (de)intercalation is a common problem with Mg batteries, [ 5 ] and there could be the problem of H + de‐intercalating instead of Li/Na/Mg when cycling. Figure 4 shows that energetically the deintercalation of H + through the reaction Ti 3 C 2 O(OH) → Ti 3 C 2 O 2 + ½ H 2 is preferred over the deintercalation Li/Na/Mg.…”

“…It has also been shown in tunnelling electron microscope (TEM) that several layers of Na can intercalate in some cases, [2] allowing more than one Na for every formula atom level unit at the atomic level, i.e., Na >1 Ti 3 C 2 T x . Mg, on the other hand, is known to be a challenging metal for battery applications, with slow diffusion, troublesome electrolyte-electrode kinetics and problems with proton intercalation and electrolyte decomposition, [5][6][7] and has only demonstrated the equivalent of Mg 0.004 Ti 3 C 2 T 2 (≈1 mAh g −1 , 25 cycles) when tested on micron sized Ti 3 C 2 T x . [3] Employing spacer groups to increase interlayer distance [8] and/or nano sizing the MXene [9,10] has shown increased capacities, but it can be difficult to determine whether these capacities are due to reversible Mg 2+ intercalation, or due to surface reactions, proton intercalation and/or cointercalation of electrolyte as with the case of MgCl + intercalation.…”

“…[3] Employing spacer groups to increase interlayer distance [8] and/or nano sizing the MXene [9,10] has shown increased capacities, but it can be difficult to determine whether these capacities are due to reversible Mg 2+ intercalation, or due to surface reactions, proton intercalation and/or cointercalation of electrolyte as with the case of MgCl + intercalation. [5,6,11] Much is unknown about MXene interlayer chemistry. The termination groups of the HF etched Ti 3 C 2 T x , likely consists of O, OH, and F and the ratio between these species and the total number of them can be influenced by synthesis [12] and postsynthesis treatment.…”

“…[ 3 ] Employing spacer groups to increase interlayer distance [ 8 ] and/or nano sizing the MXene [ 9,10 ] has shown increased capacities, but it can be difficult to determine whether these capacities are due to reversible Mg 2+ intercalation, or due to surface reactions, proton intercalation and/or cointercalation of electrolyte as with the case of MgCl + intercalation. [ 5,6,11 ]…”

Solvation, Rational Design, and Interfaces: Development of Divalent Electrolytes

“…Custom electrolytes, like MgTPFA, provide the best chance to enable further cathode development as opposed to commercial salts. Despite displaying the highest anodic stability, parasitic decomposition reactions are still observed even in these electrolyte systems, meaning that MV reactivity in cathode systems has to be holistically verified by structural, elemental, and redox probes as discussed in a recent perspective (Johnson et al, 2021). Therefore, the development of electrolytes or interfaces that are more stable with high voltage cathodes is crucial to advance MV battery technologies.…”