Non-passivating Anion Adsorption Enables Reversible Magnesium Redox in Simple Non-nucleophilic Electrolytes

Abstract

Magnesium batteries suffer from low power density and poor cycle life due to severe Mg passivation. Using nucleophilic electrolytes is effective to stabilize the Mg anode, but it prohibits the use of organic and conversion cathodes due to chemical reactivity. Here, we report an effective non-passivating anion additive, the reduced perylene diimide–ethylene diamine (rPDI), to enable fast and reversible Mg deposition/dissolution in a simple non-nucleophilic electrolyte. The rPDI additive exhibits higher adsorption energy than the TFSI– salt on the Mg surface, preventing TFSI– decomposition and Mg anode passivation. Using 0.2 mM rPDI additives, we demonstrated a Mg–organic full cell achieving a high power density (2.0 mW cm–2) and a stable cycle life (>200 cycles). Our study provides a facile and effective strategy for non-nucleophilic electrolytes, enabling the combination of Mg anode with a wide variety of organic and conversion cathodes beyond intercalation chemistries.

Emerging Era of Electrolyte Solvation Structure and Interfacial Model in Batteries

“…For example, the perylene diimide−ethylene diamine (rPDI) additive in the electrolyte of Mg(TFSI) 2 -MgCl 2 in DME exhibited higher adsorption energy than that of TFSI − on Mg electrode surface, preventing the decomposition of TFSI − and passivation of Mg anode (Figure 13f). 198 In addition, Wang et al found that adding methoxyethylamine chelates with a high affinity into the DME-based electrolytes (e.g., 0.5 M Mg(TFSI) 2 ) not only promoted the interfacial charge-transfer kinetics but also suppressed the side reactions on the electrode surface. 197 This is because the chelant-rich solvation sheaths bypass the energetically unfavorable desolvation process through reorganization, thus reducing the overpotential and suppressing the concomitant parasitic reactions for both the anode and the cathode in MIBs (Figure 13g).…”

“…The solvation effect is being widely studied in polyvalent ion batteries (e.g., Zn 2+ , − Mg 2+ , − Ca 2+ , , Al 3+ , ) and other fields such as electrocatalysis. − This research trend was promoted in 2018 by Ming et al, based on their viewpoint that the solvation structure and interfacial model (vs SEI) can influence the electrode’s performance significantly. − For example, DMSO replaced H 2 O in the first Zn 2+ solvation sheath in ZnCl 2 –H 2 O electrolyte and then effectively suppressed H 2 O decomposition; meanwhile, the modified solvation structure induced the formation of an inorganic-rich SEI on zinc to guarantee a high CE and a long cycle life over 500 cycles (Figure a) . This viewpoint was further demonstrated by introducing a bulky cation (e.g., 1-ethyl-3-methylimidazolium chloride) into ZnSO 4 –H 2 O electrolyte to construct an anion-type water-free solvation structure, ZnCl 4 2– , in order to drive water out of the Zn­(H 2 O) 6 2+ solvation structure and then suppress Zn dendrite growth and hydrogen evolution reaction (HER) (Figure b) .…”

“…(f) Passivating and non-passivating adsorption of TFSI – on the Mg anode. (Reprinted with permission from ref . Copyright 2021 American Chemical Society.)…”

“…Regulating the solvation structure and interfacial model could also improve the Mg plating/stripping process, addressing the major challenge of severe capacity decay in magnesium-ion batteries (MIBs). For example, the perylene diimide–ethylene diamine (rPDI) additive in the electrolyte of Mg­(TFSI) 2 -MgCl 2 in DME exhibited higher adsorption energy than that of TFSI – on Mg electrode surface, preventing the decomposition of TFSI – and passivation of Mg anode (Figure f) . In addition, Wang et al found that adding methoxyethyl­amine chelates with a high affinity into the DME-based electrolytes (e.g., 0.5 M Mg­(TFSI) 2 ) not only promoted the interfacial charge-transfer kinetics but also suppressed the side reactions on the electrode surface .…”

Alloying electrode coatings towards better magnesium batteries

“…10 An organic coating made of reduced perylene diimide-ethylene diamine (rPDI) also enabled fast and reversible magnesium plating/stripping, however with the electrolyte Mg(TFSI) 2 -MgCl 2 /DME. 11 Moving to inorganic coatings, the teams of Nazar and Archer almost simultaneously proposed the protection of lithium and sodium electrodes by using an alloy-type coating, created through the chemical reduction of a metallic salt in solution, followed by an alloying reaction with the alkali metal surface. [12][13][14][15] Eventually, the passivating lm formed in this way is in reality a composite layer, as insulating by-products are also present and offer a potential gradient to prevent plating onto the coating layer.…”