Practical 4.4 V Li||NCM811 batteries enabled by a thermal stable and HF free carbonate-based electrolyte

Abstract

Fluoroethylene carbonate (FEC) is widely recognized as an effective cosolvent coupling with commercial carbonate-based electrolyte to enable stable high-voltage Li metal batteries (LMBs). However, FEC reacts with the decomposition product (PF5) of the LiPF6 salt in the presence of trace amount of water, producing corrosive species such as HF, deteriorating the stability of the Li metal batteries (LMBs). In this work, we employed a non-nucleophilic organic base, ethoxy(pentafluoro) cyclotriphosphazene (PFN), as a multifunctional additive to simultaneously enhance the chemical stability and safety of the FEC-LiPF6 carbonate-based electrolytes. The electron-donating -PN- group of PFN makes it able to catch the Lewis acid PF5 and HF in the electrolyte, while the steric hindrance from the resembles benzene structure of PFN prevents it from attacking the FEC. Furthermore, the PFN additive also improve the safety and processability of the battery since it has a superior flame retardancy and good wettability with the separator. As a result, adding 3 wt% of PFN is able to make a FEC containing carbonate electrolyte self-extinguishing, and significantly improve the stability of high-voltage Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) batteries, especially when it couples with lithium difluoro(oxalato)borate (LiDFOB) as a film-forming additive. Specifically, using a FEC-LiPF6 carbonate-based electrolyte that containing 3 wt% PFN and 2 wt% LiDFOB, the Li||NCM811 coin cell, which consists of an ultrathin Li anode (35 µm) and a NCM811 cathode with a high areal capacity of 6 mAh·cm−2, presents a capacity retention of 97.2% after 50 cycles, which is much better than that cycled in a blank electrolyte (86.6%) without the studied additives. 3Ah Li||NCM811 pouch cells were also assembled, gassing during cycling was significantly suppressed and a capacity retention of 84.2% after 100 cycles at 0.3CC/0.5CD was be obtained. This work provides a convenient strategy for developing highly stable carbonate electrolyte for practical lithium-metal batteries.