Establishing a Stable Anode–Electrolyte Interface in Mg Batteries by Electrolyte Additive

Abstract

Simple magnesium salts with high electrochemical and chemical stability and adequate ionic conductivity represent a new-generation electrolyte for magnesium (Mg) batteries. Similar to other Mg electrolytes, the simple-salt electrolyte also suffers from high charge-transfer resistance on the Mg surface due to the adsorbed species in the solution. In the current study, we built a model Mg cell system with the Mg[B(hfip) 4 ] 2 /DME electrolyte and Chevrel phase Mo 6 S 8 cathode, to demonstrate the effect of such anode−electrolyte interfacial properties on the full-cell performance. It was found that the cell required additional activation cycles to achieve its maximal capacity. The activation process is mainly attributed to the conditioning of the anode−electrolyte interface, which could be boosted by introducing an additive amount of Mg(BH 4 ) 2 to the Mg[B(hfip) 4 ] 2 /DME electrolyte. Electrochemical and spectroscopic analyses revealed that the Mg(BH 4 ) 2 additive helps to remove the native oxide layer and promotes the formation of a solid electrolyte interphase layer on Mg. As a result, the full cell with the additive-containing electrolyte delivered a stable capacity from the second cycle onward. Further battery tests showed a reversible cycling for 600 cycles and an excellent rate capability, indicating good compatibility of the Mg(BH 4 ) 2 additive. The current study not only provides fundamental insights into the interfacial phenomena in Mg batteries but also highlights the facile tunability of the simple-salt Mg electrolytes.

Investigation of the Anode‐Electrolyte Interface in a Magnesium Full‐Cell with Fluorinated Alkoxyborate‐Based Electrolyte

“…Moreover, compared to the study recently done by Li et al. on a cycled Mg anode from a Mg−Mg symmetric cell, [37] the solid interphase formed in the full‐cell was thicker. A glance at Figure S7 also shows that the concentration of fluorine and boron increased with cumulative sputtering times, thus verifying that the solid interphase is composed of MgF 2 and boron‐based species.…”

“…[35,36] Finally, it may be noted that the feature of metallic Mg of the bulk electrode, which is expected to occur at ~49 eV, was not detected even after the second sputtering step, indicating a relatively thick solid interphase layer. Moreover, compared to the study recently done by Li et al on a cycled Mg anode from a MgÀ Mg symmetric cell, [37] the solid interphase formed in the full-cell was thicker. A glance at Figure S7 also shows that the concentration of fluorine and boron increased with cumulative sputtering times, thus verifying that the solid interphase is composed of MgF 2 and boron-based species.…”

Interfaces and Interphases in Ca and Mg Batteries

“…[86] Recently, the addition of Mg(BH 4 ) 2 to a Mg[B(hfip) 4 ] 2 in G1 electrolyte was found to drastically increase the Mg plating kinetics. [106] While the authors attributed this improvement to the formation of a beneficial passivation layer (mostly composed of MgO, MgF 2 , and species that contain CFx and COx groups) it is unlikely that such layer would promote Mg 2+ migration and Mg(BH 4 ) 2 acting as a water scavenger and/or contributing to some extent to favorable modification of the solvation structure is more probable.…”

Stable Solid Electrolyte Interphase In Situ Formed on Magnesium‐Metal Anode by using a Perfluorinated Alkoxide‐Based All‐Magnesium Salt Electrolyte

“…As a comparison, calculation was also performed on tetrakis(hexafluoroisopropyloxy)borate anion ([B(hfip) 4 ] − ), which was a representative anion that would partially decompose and form an interface layer on Mg‐metal surface. [ 11d,16a,21 ] As shown in Figure 1c, the [Mg(pftb) 3 ] − anion has a lower lowest unoccupied molecular orbital (LUMO) energy level (0.427 eV) than [B(hfip) 4 ] − (0.827 eV), it suggested that [Mg(pftb) 3 ] − anions can more readily get electrons to form a SEI on Mg electrode surface during the deposition process.…”