Keywords lithium tetrakis(hexafluoroisopropyloxy)borate, magnesium metal anodes, rechargeable magnesium batteries, solid electrolyte interphase

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Nonflammable functional electrolytes with remarkably interfacial compatibility toward both lithium anodes and high-voltage cathodes are considered as the ultimate pursuit for rechargeable lithium metal battery. For this target, we report a dual-anion deep eutectic solution (D-DES) based on elaborately selected combination of nitrile and functional lithium salts. The interactions of succinonitrile with cation/anion are highlighted by in situ/ex situ measurements, which endow D-DES with excellent ionic conductivity and significantly enhanced interface stability. By using this D-DES, constant Li|Li cycling over 1 year (>10,000 h) under a Li capacity of 5 mA h cm–2 can be achieved. The capacity retention is still over 70% with a high charging voltage of 4.7 V for 500 cycles in LiCoO2|Li battery. Pouch cells with high areal capacity close to practical application also deliver superior safe performance. This study paves a new pathway for designing high-safety electrolyte and boosts the practical application of high-voltage lithium metal battery.

Discovering the underlying reason for Li anode failure is ac ritical step towards applications of lithium metal batteries (LMBs). In this work, we conduct deuterium-oxide (D 2 O) titration experiments in an ovel on-line gas analysis mass spectrometry (MS) system, to determine the content of metallic Li and lithium hydride (LiH) in cycled Li anodes disassembled from practical LiCoO 2 /Li LMBs.T he practical cell is comprised of ultrathin Li anode (50 mm), high loading LiCoO 2 (17 mg cm À2 ,2 .805 mAh cm À2)a nd different formulated electrolytes.O ur results suggest that the amount of LiH accumulation is negatively correlated with cyclability of practical LMBs.M ore importantly,w er eveal at emperature sensitive equilibrium (Li + 1/2 H 2 Ð LiH) governing formation and decomposition process of LiH at Li anode.Webelieve that the unusual understanding provided by this study will draw forth more insightful efforts to realizee fficient Li protection and the ultimate applications of "holy grail" LMBs.

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