Cathode Electrolyte Interphase (CEI) Endows Mo₆S₈ with Fast Interfacial Magnesium‐Ion Transfer Kinetics

Abstract

Magnesium (Mg) metal secondary batteries have attracted much attention for their high safety and high energy density characteristics. However, the significant issues of the cathode/electrolyte interphase (CEI) in Mg batteries are still being ignored. In this work, a significant CEI layer on the typical Mo 6 S 8 cathode surface has been unprecedentedly constructed through the oxidation of the chloride-free magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip) 4 ] 2 ) salt under a proper charge cut-off voltage condition. The CEI has been identified to contain B x O y effective species originating from the oxidation of [B(hfip) 4 ] À anion. It is confirmed that the B x O y species is beneficial to the desolvation of solvated Mg 2 + , speeding up the interfacial Mg 2 + transfer kinetics, thereby improving the Mg 2 + -storage capability of Mo 6 S 8 host. The firstly reported CEI in Mg batteries will give deeper insights into the interface issues in multivalent electrochemical systems.

Constructing Efficient Mg(CF3SO3)2 Electrolyte via Tailoring Solvation and Interface Chemistry for High‐Performance Rechargeable Magnesium Batteries

“…It is worth noting that apart from the slowly expanded polarization voltage with the increasing cycling number (Figure 5b), the capacity retention is 89.6% excluding incomplete extraction of Mg 2+ from discharge products for the first cycle. [ 41 ] Moreover, the Mo 6 S 8 ||Mg full cell using MTD‐MOEA (1:8) electrolyte shows desirable rate performance (Figure 5c). Specifically, a cheerful capacity of 54.1 mAh g −1 is maintained at 2 C. On the contrary, the Mo 6 S 8 ||Mg full cell with MID‐MOEA (1:8) electrolyte only remains very low capacity (8.8 mAh g −1 ) after 60 cycles (Figure 5d), and exhibits larger polarization (Figure 5e), which are consistent with the CV curves in Figure S36 (Supporting Information).…”

“…So it is reasonable to deduce that the cathode‐electrolyte interphases (CEIs) in the two electrolytes are closely related to the intercalation dynamics of Mo 6 S 8 cathode. [ 41 ] In this case, the CEI formed in MTD‐MOEA electrolyte can provide moderate assistance in Mg transportation at the interface. Subsequently, detailed characterizations toward the cycled Mo 6 S 8 were performed to gain thorough understanding of the CEIs.…”

“…[43] Recent reports suggest that B m O n species on cycled Mo 6 S 8 in magnesium tetrakis(hexafluoroisopropyloxy)borate electrolyte can facilitate the desolvation of Mg 2+ , thereby accelerating the interfacial transfer kinetics of Mg 2+ . Its representative B(OCH 3 ) 3 , B(OCH 2 CF 3 ) 3 and B 6 O 11 [41,43] are able to regulate the original solvated sheath by modifying the coordination structure and mode (e.g., number of coordination atoms). Thus, summarizing the experimental and computational results about MOEA from the previous section, it is not difficult to understand that N in C x N y will also contribute to the desolvation of Mg 2+ .…”

Modeling of Magnesium Intercalation into Chevrel Phase Mo6S8: Report on Improved Cell Design

“…[17,48,58] Moreover, recently it was reported, that the B(hfip) 4 anion can be oxidized at high voltages causing the formation of a cathode-electrolyte interphase, which facilitates the desolvation of Mg(DME) 3 2 + and therefore the intercalation into CP. [59] These kind of side reactions can cause low capacities as well as an asymmetry between charge and discharge during the first cycle(s). Since the model does not consider such complex activation processes, the comparison of the simulation results with the experimental data focuses on later cycles.…”

Understanding the Cathode‐Electrolyte Interfacial Chemistry in Rechargeable Magnesium Batteries

“…However, the strategy was previously considered infeasible for the Mg system due to the generally low Reproduced with permission. [24] Copyright 2023, Wiley-VCH. d,e) Typical TEM images and f) schematic of CEI morphologies, composition, and corresponding Mg 2+ interface transportation behavior on Mo 6 S 8 cathode in Mg(CF 3 SO 3 ) 2 -based electrolyte with MOEA cosolvent.…”