Performance Leap of Lithium Metal Batteries in LiPF6 Carbonate Electrolyte by a Phosphorus Pentoxide Acid Scavenger

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 P₂O₅ with the LiPF₆ electrolyte 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 LiPF₆ electrolyte is chemically compatible with a Li metal anode and LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) cathode, generating a PO_xF_y-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 LiPF₆ electrolytes are demonstrated on Li||NMC622 pouch cells with 0.4 Ah capacity, 50 μm Li anode, 3 mAh cm^-2 NMC622 cathode, and 3 g Ah^-1 electrolyte/capacity ratio. The pouch cells can be galvanostatically cycled at C/3 for 230 cycles with 87.7% retention.