Reversibility of a High-Voltage, Cl^–-Regulated, Aqueous Mg Metal Battery Enabled by a Water-in-Salt Electrolyte

Abstract

Rechargeable Mg batteries are a promising post-Li-ion battery technology, but their development has been critically hampered by the passivating nature of Mg, particularly in aqueous solutions. Due to a quick dismissal of its reversibility, the use of Mg anodes in aqueous electrolytes has been overlooked, and most researchers focus on nonaqueous systems instead. In this work, reversible, aqueous Mg battery chemistry has been realized for the first time, via the conversion of its impermeable passivation film to a conductive metallic oxide complex, facilitated by Cl– regulation and the suppression of the hydrogen evolution reaction using a MgCl2 water-in-salt (WIS) electrolyte. When coupled with copper hexacyanoferrate as the cathode, the full battery exhibits an impressive voltage plateau of 2.4–2.0 V and a stability of over 700 cycles with a Coulombic efficiency of up to 99% at 0.5 A g–1. Mg dissolution and deposition have proven reversible in the aqueous MgCl2 WIS electrolyte.

Hydrated Eutectic Electrolytes Stabilizing Quasi‐Underpotential Mg Plating/Stripping for High‐Voltage Mg Batteries

“…As shown in Figure 2a, in the AEs (A‐1 : 8), the overpotential of symmetric cells at a current density of 0.1 mA cm −2 suddenly increases after being cycled for 65 h. In contrast, symmetric cells in different HEEs (H‐1 : 2, H‐1 : 4, H‐1 : 6, H‐1 : 8, and H‐1 : 20) show a longer lifespan, among which H‐1 : 8 has the lowest polarization and the longest lifespan. The overpotential decreases in the initial cycle and then becomes stable, which could be caused by the formation of Mg−Zn alloys and solid electrolyte interphases (SEI) derived from ADN decomposition and other possible side reactions [8, 20] . It signifies a good reversibility of metal plating/stripping in HEEs.…”

“…The overpotential decreases in the initial cycle and then becomes stable, which could be caused by the formation of MgÀ Zn alloys and solid electrolyte interphases (SEI) derived from ADN decomposition and other possible side reactions. [8,20] It signifies a good reversibility of metal plating/stripping in HEEs. Correspondingly, AEs (A-1 : 8) with the same Mg 2 + molar concentration as HEEs (H-1 : 8) were adopted for comparison.…”

“…Alternatively, new AEs including water‐in‐(bi)salt, molecular crowding system, aqueous/organic hybrid electrolytes, and gel electrolytes can effectively inhibit the water activity, which could be latently capable of Mg plating/stripping chemistry [7] . Recently, in water‐in‐MgCl 2 , reversible aqueous Mg plating/stripping behavior was achieved, although the overpotentials could need to be further improved [8] . In another case, using solvent‐in‐water electrolytes with MgCl 2 and MnCl 2 salts, Mg‐MnO 2 batteries coupling Mn 2+ /MnO 2 chemistry with the Mg anode were successfully developed [3c] .…”

“…[7] Recently, in water-in-MgCl 2 , reversible aqueous Mg plating/stripping behavior was achieved, although the overpotentials could need to be further improved. [8] In another case, using solvent-in-water electrolytes with MgCl 2 and MnCl 2 salts, Mg-MnO 2 batteries coupling Mn 2 + /MnO 2 chemistry with the Mg anode were successfully developed. [3c] Such studies highlight the feasibility of using low water-activity electrolytes for ARMBs, however, lower overpotentials for Mg plating/stripping still need to be pursued for low-polarization full cells.…”

Hybrid Aqueous/Non‐aqueous Electrolytes for Lithium‐Ion and Zinc‐Ion Batteries: A Mini‐Review

“…[13a] Besides these macro properties of electrolytes such as viscosity, the microstructures of Li + solvation and de-solvation process are more correlated to the low-temperature performance. [25] Strong bonding interaction between the organic solvent and Li + causes sluggish interfacial ion transport of Li + at the electrode interface, which is considered as one of the major factors that limit the lowtemperature performance of aqueous electrolytes. [26] Introducing organic solvents with moderate binding energy to Li + could be helpful to enable appropriate reaction kinetics at low temperatures.…”