Thermodynamics/kinetics of dual-cation insertion chemistry and design of customized electrolyte for Mg-Li hybrid batteries

다국어 초록 (Multilingual Abstract)

Rechargeable Mg batteries, which were first prototyped in the early 2000s with the Chevrel phase (CP) Mo6S8 cathode and a Mg metal anode, are regarded as one of the most promising substitutes for existing Li-ion batteries. The development of this battery system aims for its practical application in large-scale energy storage devices that demand guaranteed safety and low production costs (2.7 k$ ton−1). Compared to other types of batteries, Mg batteries better satisfy these requirements through the use of a Mg anode, attributed to its non-dendritic property (only with cases where the proper electrolyte composition and concentration are applied) and abundant reserves in the Earth’s crust (ca. 1.94 wt%). Furthermore, the Mg anode also has high volumetric (3833 mAh cm−3) and gravimetric (2205 mAh g−1) capacities along with a sufficiently negative standard potential (−2.37 V, vs standard hydrogen electrode). Nevertheless, the conventional method based on the way of the first prototype, wherein insertion/extraction of Mg2+ ions are performed for a series of electrochemical processes, now faces limitations to further advances, mainly due to the lack of suitable cathode materials other than CP. The greatest barrier to the development of Mg batteries relates to the inherent characteristic of divalent Mg2+ ions, which results in exceedingly strong interactions with host lattices.

Various experimental and computational approaches have been developed in an attempt to resolve the above issue, and it became clear that the dual-salt Mg-Li hybrid battery (MgHB) system, which introduces fast Li+ ion insertion at the cathode by simply dissolving Li salts in the electrolyte, appears to be the most promising strategy. After the first report by Yagi et al., who adopted a LiFePO4 cathode, several materials such as TiS2, FeS2, Li4Ti5O12, MgCo2O4, S, Se, FeFe(CN)6, and LiMn2O4 were consecutively used with this new approach, including Mo6S8. While these studies demonstrate that the dual-salt concept is an appealing and highly feasible strategy for exploiting Mg anodes, most of the studies to date showed limitations in understanding the underlying working mechanism of the Mg2+/Li+ dual-cation environment and resolving the inherent drawback of the MgHB system (i.e., an excess quantity of Li+ ions should be stored within electrolytes when the applied host material does not originally contain Li atoms), as they simply focused on the replacement of cathode materials to achieve higher energy densities. In this thesis, therefore, profound investigations on thermodynamic and kinetic aspects of the MgHB along with designing of the customized electrolyte composition having high Li+ concentrations were conducted.

First, in the thermodynamic study, extensive theoretical and experimental investigations were performed to unravel the origin of the electrochemical properties of the MgHB at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg2+ and Li+ co-insertion into the Mo6S8 cathode host using density functional theory (DFT) calculations, it was shown that there is a threshold Li+ activity for the pristine Mo6S8 to prefer lithiation instead of magnesiation. Through precise control of the insertion chemistry using a dual-salt electrolyte, ultrafast discharge of the hybrid battery could be enabled by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at ambient temperature.

In a study on the kinetic of MgHBs, computational and experimental investigations into the Li+, Mg2+, and Mg2+/Li+ dual-cation transport properties within Mo6S8 have been carried out. Five representative paths were selected for 3D diffusion, and their corresponding energy barriers were determined. Based on the DFT calculation results, phenomena of the cation trapping, sluggishness of Mg2+ ion transport, and synchronized movement of inserted cation induced by repulsive interactions were revealed. The computational results were further validated by cyclic voltammetry conducted at ambient to high temperatures, from which apparent diffusion constants and activation energies for each case were determined. Broad agreement between the theoretical and experimental results could be confirmed, and an optimum scenario for charge−discharge processes within the MgHB system is suggested as a guideline.

Lastly, an attempt was made to establish a dual-salt composition capable of supplying a high Li+ concentration, based on the fact that the available capacity of the MgHB is closely related to the number of charge carriers within electrolyte solution. A dual-salt electrolyte consisting of the LiAlCl4 complex (lithium aluminum chloride complex, LACC) and LiN(SO2CF3)2 (lithium bis(trifluoromethane)sulfonimide, 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 LACC solutions. The modified LACC was compatible with even 2 M of LiTFSI, through which concentration it was able to obtain 150 mAh g−1 capacity of a FePO4 cathode at 1.5 mg cm−2 loading density by only using 25.5 μL cm−2 electrolyte volume.