Electrolyte and Interphase Design for Magnesium Anode: Major Challenges and Perspectives

Abstract

insufficient reduction stability are prone to be reduced at the anode side thus not compatible with Mg anode. [7][8][9] Grignard reagents (RMgX, R: organic group; X: halogen ligand) with reductionresistant property first enabled reversible Mg reaction and prevented the formation of passivation film. [10,11] The highly reactive Grignard reagents can serve as scavengers to remove contaminants (such as water or O 2 ) thus suppressing their chemical reactions with fresh Mg 0 . [7] However, the low oxidation stability (≈1.5 V vs Mg/Mg 2+ ) of Grignard reagents hampers their practical utilization. [12] To widen the electrochemical window of Grignard reagents, Aurbach et al. [13] designed a family of solutions prepared by in situ reacting organo-magnesium compounds (R 2 Mg) with organic halo aluminum compounds (AlCl 3−n R n ) in ethereal solvents. Among all the candidates, dichloro-complex (DCC) electrolytes obtained via reacting din-butylmagnesium (Bu 2 Mg) and ethylaluminum dichloride (EtAlCl 2 ) (1:2) in THF achieved ≈100% coulombic efficiency, high ionic conductivity (1.4 mS cm -1 ) and extended electrochemical window up to 2.4 V. [13] All phenyl complex (APC) electrolyte with no β-located hydrogen obtained via the reaction between phenylmagnesium chloride (PhMgCl) and AlCl 3 achieved promising properties, including high oxidation stability (>3.3 V), high coulombic efficiency (≈100%), and low overpotential (<200 mV). [14] Although highly reversible Mg reaction was achieved with Grignard-based electrolytes, high-energy and high-power RMBs were still difficult to be achieved because of the inferior cathode compatibility. [15,16] Several intercalation cathodes, such as Mo 6 S 8 , were reported to exhibit high reversibility in Grignardbased electrolytes, but the high energy barrier for Mg 2+ desolvation at the electrode/electrolyte interphase [17][18][19] and sluggish ion diffusion [20,21] hamper the Mg 2+ storage at room temperature. Electrophilic organic polymers and conversion cathodes promise to achieve high-power and high-energy Mg storage but are prone to be attacked by the nucleophilic components in Grignard-based electrolytes. [22][23][24] Furthermore, the chloride-containing dimer cations in Grignard-based electrolytes also raise concerns about corrosion toward non-noble metals, excluding the utilization of conventional low-cost current collectors, such as aluminum. [25] The sensitive precursors and additional purification pretreatments also impede the practical usage of Grignard-based electrolytes. [26] Consequently, it is still challenging to screen an Mg 2+ conductive electrolyte compatible with both anode and cathode. The rechargeable magnesium battery (RMB) is regarded as a high-energy, safe, and cost-effective alternative for conventional batteries. Unfortunately, the passivation and uneven Mg growth not only raise the voltage hysteresis but also shorten the cycle life of RMBs. In this review, Mg passivation induced by electrolytes/contaminants, growth patterns of high dimensional Mg 0 , and mechanisms of...

High Utilization of Composite Magnesium Metal Anodes Enabled by a Magnesiophilic Coating

“…The binding energy ( E b ) of the Au (111) surface toward a Mg atom is −0.83 eV, much larger than the value of −0.21 eV obtained on the Cu (111) surface (Figure d), suggesting that the Au coating has a better Mg affinity than the Cu substrate. Notably, the interaction between Mg and Au is also stronger than that between Mg and three 1,2-dimethoxyethane (DME) molecules (−0.74 eV, Figure S7), a typical solvated [Mg­(DME) 3 ] 2+ ion pair in conventional ether electrolyte. , From these theoretical results, the magnesiophilicity of Cu foil is seen to be enhanced by the Au coating, which is conducive to regulating the magnesium desolvation/transfer kinetics and deposition behavior.…”

“…Notably, the interaction between Mg and Au is also stronger than that between Mg and three 1,2-dimethoxyethane (DME) molecules (−0.74 eV, Figure S7), a typical solvated [Mg(DME) 3 ] 2+ ion pair in conventional ether electrolyte. 11,15 From these theoretical results, the magnesiophilicity of Cu foil is seen to be enhanced by the Au coating, which is conducive to regulating the magnesium desolvation/transfer kinetics and deposition behavior.…”

“…As a proof of concept, we employed the Au–Cu electrode to fabricate full cells. Chevrel phase Mo 6 S 8 is selected as the cathode for its proven excellent cycling stability in ether electrolytes. , The Mg-plated Au–Cu composite electrode with limited Mg was used as the anode (Mg@Au–Cu). The Mo 6 S 8 ||Mg@Cu batteries were also assembled for comparison.…”

“…The ever-increasing applications in smart electronics and electrification call for high-energy-density electrochemical storage systems beyond the capabilities of state-of-the-art lithium-ion batteries (LIBs). − Rechargeable magnesium batteries (RMBs) have attracted academic and industrial attention due to metallic magnesium (Mg) offering a high theoretical volumetric capacity of 3833 mAh cm –3 (vs 2046 mAh cm –3 for metallic lithium and 818 mAh cm –3 for graphite), in addition to its abundant and inexpensive resources. − However, magnesium metal anodes (MMAs) usually exhibit low Coulombic efficiency (CE) and poor cycling stability during Mg plating/stripping cycles in conventional organic electrolytes. − Several effective strategies, such as designing electrolyte formulations, − introducing electrolyte additives, − and constructing artificial solid electrolyte interphases (SEI), − have been conducted to improve the reversibility of MMAs. Despite the exciting progress, most reported MMAs are only cyclable under shallow cycling conditions, − e.g., a typical commercial Mg metal foil (∼0.1 mm thick) with a large theoretical areal capacity of ∼38.33 mAh cm –2 is usually only charged and discharged to a depth of ≤1 mAh cm –2 .…”

Engineering a High-Voltage Durable Cathode/Electrolyte Interface for All-Solid-State Lithium Metal Batteries via In Situ Electropolymerization

“…For vinyl monomers, the strong electron-withdrawing capability of the substituent groups will reduce the electron cloud density on the double bond, which will be easily attacked by anions and polymerized; therefore, vinyl monomers with electron-withdrawing substituents have a strong tendency to undergo anionic polymerization during the anodic process. TFEMA is a vinyl monomer with the strong electron-withdrawing group −CF 3 , which is prone to polymerization when attacked by anions. , It is well-known that the highest occupied molecular orbital (HOMO) energy level of the electrolyte determines its oxidation stability, which needs to be lower than cathode potential to build a high-voltage durable cell system. − The HOMO energy level of the fluorinated acrylate polymer is low enough to construct a high-voltage electrolyte or interlayer, − which contributes to the strong electron-withdrawing inductive effect of the polyfluoroalkyl structure on the side chains. This decreases the electron cloud density on the main chain, resulting in a low HOMO energy level of the molecule .…”