Understanding Improved Lifetimes of Lithium-Metal Batteries with LiPF6 Carbonate Electrolyte Modified By Phosphorus Pentoxide
Authors: Gordon Leo W. , Zhang Jian , Guo Juchen , Messinger Robert J. 
Published: November 2022

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 LiPF6 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, POxFy-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:

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