Reversible Insertion of Mg‐Cl Superhalides in Graphite as a Cathode for Aqueous Dual‐Ion Batteries

Abstract

Oxidative anion insertion into graphite in an aqueous environment represents a significant challenge in the construction of aqueous dual-ion batteries. In dilute aqueous electrolytes, the oxygen evolution reaction (OER) dominates the anodic current before anions can be inserted into the graphite gallery. Herein, we report that the reversible insertion of Mg-Cl superhalides in graphite delivers a record-high reversible capacity of 150 mAh g À1 from an aqueous deep eutectic solvent comprising magnesium chloride and choline chloride. The insertion of Mg-Cl superhalides in graphite does not form staged graphite intercalation compounds; instead, the insertion of Mg-Cl superhalides makes the graphite partially turbostratic.

A Graphite∥PTCDI Aqueous Dual‐Ion Battery

“…Dual‐ion batteries (DIBs) have garnered increasing attention as a promising alternative for grid‐scale energy storage systems [1a,b] . DIBs operate on reversible storage of anions and cations in cathode and anode, respectively [2a,b] . Graphite has been the most widely investigated cathode material in DIBs, which stores anions in acceptor‐type graphite intercalation compounds (GICs) [3a–g] .…”

“…In particular, the high potential of anion insertion in graphite leads to water oxidation, triggering oxygen evolution reaction (OER). To suppress OER of aqueous electrolytes, water‐in‐salt electrolytes (WiSE) and aqueous deep‐eutectic solvents (DES) as electrolytes have been demonstrated effective [2b,6,7a–c] …”

“…The graphite cathode of DIBs can host a list of anions such as PF 6 − , BF 4 − , ClO 4 − , bis(fluorosulfonyl)imide (FSI − ), and bis(trifluoromethanesulfonyl)‐imide (TFSI − ) [4a,7a,8a–g] . Recently, our group demonstrated reversible storage of [MgCl 3 ] − , a superhalide, in graphite from an aqueous DES electrolyte, 9 m (molality) MgCl 2 –30 m choline chloride (ChCl) (referred to as 9M30C) in a half‐cell; yet the feasibility of cation‐storage in anode from such DES electrolytes has not yet been investigated [2b] . We tested 3,4,9,10‐perylenetetracarboxylic diimide (PTCDI), an organic pigment, as the anode in the 9M30C electrolyte, where the initial Coulombic efficiency (CE) was around 30 %, which indicates the irreversible cathodic decomposition of Ch + [9a,b] .…”

Effect of EtOMgCl Salt to Suppress Reductive Decomposition of TFSI⁻ Anion in Electrolyte for Magnesium Rechargeable Battery

“…3,32 In contrast, the peak would not be suitable to be assigned as another complex (MgCl + ion or Mg2Cl3 + ion), due to the peak position (~230 cm -1 ). 32,33 In contrast, the peak for Mg-OEt bond was not observed (Fig. 5S).…”

A review of halide charge carriers for rocking‐chair and dual‐ion batteries

“…(B) Cycling performance obtained for the graphite cathode in the aqueous DES electrolyte at a current rate of 100 mA•g −1 .High-resolution transmission electron microscopy images of the (C) pristine and (D) charged (anion-inserted) graphite electrode. Reproduced with permission: Copyright 2020, Wiley117…”