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Phosphorus pentoxide (P2O5) is investigated as an acid scavenger to remove the acidic impurities in a commercial lithium hexafluorophosphate (LiPF6) carbonate electrolyte to improve the electrochemical properties of Li metal batteries. Nuclear magnetic resonance (NMR) measurements reveal the detailed reaction mechanisms of P2O5with the LiPF6electrolyte and its impurities, which removes hydrogen fluoride (HF) and difluorophosphoric acid (HPO2F2) and produces phosphorus oxyfluoride (POF3), OF2P–O–PF5–anions, and ethyl difluorophosphate (C2H5OPOF2) as new electrolyte species. The P2O5-modified LiPF6electrolyte is chemically compatible with a Li metal anode and LiNi0.6Mn0.2Co0.2O2(NMC622) cathode, generating a POxFy-rich solid electrolyte interphase (SEI) that leads to highly reversible Li electrodeposition, while eliminating transition metal dissolution and cathode particle cracking. The excellent electrochemical properties of the P2O5-modified LiPF6electrolytes are demonstrated on Li||NMC622 pouch cells with 0.4 Ah capacity, 50 μm Li anode, 3 mAh cm–2NMC622 cathode, and 3 g Ah−1electrolyte/capacity ratio. The pouch cells can be galvanostatically cycled at C/3 for 230 cycles with 87.7% retention.

An essential requirement for electrolytes in rechargeable magnesium-ion (Mg-ion) batteries is to enable Mg plating–stripping with low overpotential and high Coulombic efficiency. To date, the influence of the Mg/electrolyte interphase on plating and stripping behaviors is still not well understood. In this study, we investigate the Mg/electrolyte interphase from electrolytes based on two Mg salts with weakly coordinating anions: magnesium monocarborane (Mg(CB11H12)2) and magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2). Cyclic voltammetry and chronopotentiometry of Mg plating–stripping demonstrate significantly lower overpotential in the Mg(CB11H12)2electrolyte than in Mg(TFSI)2under the same condition. Surface characterizations including X-ray photoelectron spectroscopy and scanning electron microscopy clearly demonstrate the superior chemical and electrochemical stability of the Mg(CB11H12)2electrolyte at the Mg surface without noticeable interphase formation. On the other hand, characterizations of the Mg/electrolyte interface in the Mg(TFSI)2electrolyte indicate the formation of magnesium oxide, magnesium sulfide, and magnesium fluoride as the interfacial compounds resulting from the decomposition of TFSI–anions because of both chemical reduction by Mg and cathodic reduction during Mg deposition.