Lithium metal anodes offer the possibility of a ten-fold increase in capacity versus standard intercalation anodes such as graphite. However, to date, there has been limited success using commercially available liquid electrolytes for batteries with lithium metal anodes. A critical issue in LiPF_­6 carbonate electrolytes results from the reaction of trace water (<100 ppm) with PF₅, which is in equilibrium with the PF₆^- anion, to produce hydrofluoric acid (HF) and difluorophosphoric acid. These reaction products contribute to lithium passivation, transition-metal leaching and subsequent particle cracking at the cathode, and degradation of cell components. Furthermore, HF begins an autocatalytic degradation cycle, so if not removed, can continually decompose the carbonate solvents in a self-sustaining cycle.

Here, we demonstrate the effectiveness of a simple and scalable modification procedure of 1M LiPF₆ ethylene carbonate (EC)/diethylcarbonate (DEC) (50/50 v/v) electrolyte to greatly enhance performance of lithium-metal batteries. Phosphorus pentoxide (P₂O₅) added to commercial, battery-grade electrolyte performs the dual functions of both scavenging HF and H₂O, while also generating chemical species that promote formation of a beneficial, PO_xF_y-rich SEI layer upon cycling. In commercial-scale pouch cells (0.4 Ah), performance using the modified electrolyte is vastly superior, with 87.7% capacity retention at >230 cycles and minimal hysteresis, to that of the cell with as-received electrolyte, which failed after approximately 30 cycles. Electrodes harvested after 30 cycles in the commercial electrolyte yielded cracked cathode particles and transition metal migration to the anode surface, while these were not seen with the modified electrolyte, corroborating the performance enhancement resulting from HF scavenging. Rigorously quantitative, liquid-state ¹⁹F, ³¹P, ¹H, and ¹³C NMR of the modified electrolyte proves complete removal of residual HF in addition to revealing a plethora of new fluorophosphate species, while two-dimensional ¹⁹F{³¹P} correlation experiments were used to confidently establish signal assignments. These fluorophosphate moieties make up the anodic surface layer that promotes smooth and uniform lithium electrodeposition, further enhancing performance. Understanding the relationship between electrolyte speciation and electrochemical performance is crucial for careful design of electrolyte additives, and in particular, multi-functional materials such as P₂O₅ offer simple but highly effective improvements to resultant electrolyte formulations.

Overall, we achieve enhanced operation of lithium-metal batteries using a P₂O₅-modified LiPF₆ carbonate electrolyte as a result of HF and H₂O removal, alongside formation of favorable SEI-forming species. Through NMR, we quantitatively established speciation of this electrolyte to better understand the improved performance and elucidate the major reaction pathways.

Reference:

1. Zhang, J., et al. Performance Leap of Lithium Metal Batteries in LiPF₆ Carbonate Electrolyte by a Phosphorus Pentoxide Scavenger. (Under review, 2022)