Strategy for Ultrafast Cathode Reaction in Magnesium-Ion Batteries Using BF₄ Anion Based Dual-Salt Electrolyte Systems: A Case Study of FePO₄

Abstract

Mg2+ secondary batteries are remarkably safe, resourceful, and exhibit high energy density. However, the excessively slow reaction kinetics at Mg2+-battery cathode materials results in charge–discharge over 10–60 h at room temperature, hindering the performance evaluation and mechanistic analysis of the electrode materials. In this study, we developed a dual-salt electrolyte comprising a conventional magnesium salt, magnesium bis(trifluoromethanesulfonyl)imide [Mg(TFSA)2], and a quaternary ammonium salt, spiro-(1,1′)-bipyrolidinium tetrafluoroborate (SBPBF4), for achieving high-rate performance in the cathode reaction. In a charge–discharge test conducted using a highly defective FePO4 cathode, the dual-salt system [0.5 M Mg(TFSA)2 + 0.5–2.0 M SBPBF4] showed a high capacity of over 150 mAh g–1 at 0.5C-rate, even at room temperature. In situ X-ray absorption fine structure measurements demonstrated the Fe2+/Fe3+ redox reaction of the FePO4 cathode during the charge–discharge, whereas Raman analysis and molecular dynamics simulation indicated that the multiple-anion-coordinated [Mg2+–BF4 –] structure was more effective in facilitating Mg2+ insertion/extraction than the [Mg2+–TFSA–] structure, which has a lower number of coordinated anions. These findings indicate that the Mg2+ insertion/extraction at the cathode/electrolyte interface is drastically improved by using a combination of typically used electrolytic salts as the electrolyte. This strategy enables rapid evaluation of the electrochemical performance of various Mg2+-battery cathodes without high-temperature and prolonged operation.

Ultrafast cathode characteristics of a nano-V2(PO4)3 carbon composite for rechargeable magnesium batteries

“…26,27 Indeed, with increasing cycles of VP with Mg 2+ insertion/extraction, the exhibited capacity decreased from 115.1 mA h g −1 to 87.5 mA h g −1 (aer 100 cycles). However, it's worth noting that the corresponding capacity retention of VP/KB (5 : 5) (76%) is compatible to FP/KB (77%) using the same electrolyte composition, 37 as tabulated in Table S2. † Considering the higher operation potential of VP/ KB compared to FP/KB, the obtained result still appears quite promising.…”

“…Then, these ions form LVP crystals with the neighboring ions during the annealing process. 37 The high-KB content systems (LVP/KB (5 : 5) and LVP/KB (4 : 6)) may exhibit a broader ion distribution due to the abundance of functional groups on the carbon material. In turn, this leads to a lower collision rate between neighboring ion sources, which suppresses excessive crystal growth and decreases the crystallite size.…”

“…Li 3 V 2 (PO 4 ) 3 /carbon composites (LVP/KB or LVP/MWCNTs) were prepared via a previously reported "ultracentrifugation (UC)" method. 37 Two solutions, A and B, were prepared. LiCH 3 COO, NH 4 VO 2 , C 6 H 8 O 7 , and ethylene glycol in a molar ratio of 3 : 2 : 2 : 8 were dissolved in H 2 O (solution A).…”

“…In a recent study, the use of the dual-salt electrolyte [magnesium bis(triuoromethanesulfonyl)amide (Mg(TFSA) 2 ) + spiro-(1,1 ′ )bipyrolidinium tetrauoroborate (SBPBF 4 )] enhanced the rate capabilities of FePO 4 to realize a high capacity of more than 150 mA h g −1 at 0.5C-rate at room temperature. 37 The monoclinic V 2 (PO 4 ) 3 framework, which is prepared by the complete delithiation of the monoclinic lithium vanadium phosphate (Li 3 V 2 (PO 4 ) 3 , LVP) used as the LIB cathode, 29,36 is another promising Mg cathode with a high voltage and high rate. Indeed, in Mg 2+ insertion/extraction, the redox potential of the V 2 (PO 4 ) 3 cathode has been estimated to be as high as 3.3 V vs. Mg/Mg 2+ by DFT calculations.…”

“…In a recent study, the use of the dual-salt electrolyte [magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA) 2 ) + spiro-(1,1′)-bipyrolidinium tetrafluoroborate (SBPBF 4 )] enhanced the rate capabilities of FePO 4 to realize a high capacity of more than 150 mA h g −1 at 0.5C-rate at room temperature. 37…”