Strategic Design of Highly Concentrated Electrolyte Solutions for Mg²⁺/Li⁺ Dual-Salt Hybrid Batteries

Abstract

The available capacity of Mg hybrid batteries is closely related to the number of charge carriers within electrolyte solutions. Therefore, in this study, a dual-salt composition capable of supplying high Li+ concentration was prepared. A dual-salt electrolyte consisting of a LiAlCl4 complex (LACC) and LiN­(SO2CF3)2 (LiTFSI) was found to be an excellent candidate, providing 2.2 M Li+ concentration along with anodic stability up to 3 V (vs Mg/Mg2+). However, the LACC moiety of the above composition first had to undergo a two-step modification procedure comprising “Mg powder treatment” and “conditioning process” to properly implement Mg deposition and stripping at the Mg anode. Spontaneous substitutions of oxidation states between the anionic Al3+ complex and metallic Mg induced by these processes resulted in the generation of Mg2+ complex species within the LACC solutions. The modified LACC was compatible even with 2 M of LiTFSI, the concentration with which we achieved 150 mA h g–1 capacity of a FePO4 cathode at 1.5 mg cm–2 loading density, when using an electrolyte volume of only 25.5 μL cm–2.

Facile Modification of LiAlCl4 Electrolytes for Mg–Li Hybrid Batteries by the Conditioning-Free Method

“…It is, therefore, deduced that not only monomeric chloroaluminate anions but also the dimeric species of the pristine solution participate in the chemical reaction depicted in eq . This negative ion mode cf-LACC spectrum also displays a consistent trend with those of the pris- and cond-LACC solutions concerning the concentration of Al 3+ and Mg 2+ complexes, as suggested in Figure S4 . By comprehensively evaluating the findings from all analytical methods, we can conclude that the Mg 2+ compounds in cf-LACC consist mainly of MgCl 2 , MgCl 3 – , Mg 2 Cl 5 – , and Mg 3 Cl 7 – , all of which originate from the anionic chloroaluminate complex of the pris-LACC and metallic Mg. Expected chemical reactions ascribed to the conditioning-free process thus may include the following pathways: Note that the CrCl 3 promoters, which also participate in the above reaction, and THF ligands are not described here.…”

“…The photograph in Figure a compares three different states of LACC that change with the conditioning-free procedure; as CrCl 3 and Mg powders are added stepwise, the color of the solution first varies from pale-yellow to magenta and then becomes brown while maintaining its transparency. From these results, it may be deduced that the newly generated ionic species are equilibrated with the existing Li + and Al 3+ ions and do not result in precipitation …”

“…All chemicals used for the preparation of the pristine LACC (pris-LACC) and cf-LACC solutions were anhydrous and purchased from Sigma-Aldrich. The synthesis of pris-LACC followed the same procedure described in our previous study . To minimize the H 2 O content of the electrolyte solutions, we first dried the LiCl (99%) and AlCl 3 (99.999%) precursors at 120 °C under a medium vacuum atmosphere for 3 h and dehydrated the tetrahydrofuran (THF, 99.9%) solvent over activated 3 Å molecular sieves for 24 h. Due to the cationic ring-opening polymerization of THF caused by electrophilic AlCl 3 , the dissolution of AlCl 3 (0.1333 g, 1 mmol) was carefully conducted by the slow dropwise addition of THF (0.75 mL, 0.15 mL min –1 ) into an AlCl 3 -containing glass vial at 0 °C .…”

“…The MgHB configuration has an inherent asymmetricity because two different charge carrier species are utilized at each electrode. Accordingly, the electrolyte solution in this system must comprise a dual-salt composition that oversupplies Li + ions to the host material (particularly when these materials do not initially possess Li atoms; in the opposite case, Mg 2+ ions must be stored in excess). − Furthermore, the system must simultaneously maintain Mg metal compatibility and high electrochemical stability to enable proper function. The development of an electrolyte system that fulfills all of the aforementioned conditions is an extremely complicated task, and as a result, limited electrolyte species have been successfully applied and employed to date. , …”

“…To be compatible with Mg anodes, however, the LACC solution first has to undergo a cumbersome modification process because of the formation of Al passive films on Mg anodes and the absence of Mg 2+ ions. This modification includes repetitive voltammetric cycling, referred to as the conditioning process; it severely lowers its productivity and only a small quantity of Mg 2+ ions (∼0.09 M in total 0.5 M cond-LACC solution) are generated. ,− …”