Fluoropyridine family: Bifunction as electrolyte solvent and additive to achieve dendrites-free lithium metal batteries

Abstract

Electrolyte formulation with high stability towards both Li metal anode and high-voltage cathode is considered as one of key points for the high-energy density lithium metal batteries (LMBs). In our previous study, by adding only 2% of 2-fluoropyridine (2-FP) as the additive in the carbonate and ether-based electrolyte formulations effectively suppressed Li dendrite growth. In this study, we further found that the main fluoropyridine (FP) family members can serve as not only the effective additive but also the excellent electrolyte solvent in the electrolyte formulations to enhance the performance of LMBs. For the 2-FP, when it was also used the electrolyte solvent and paired with single-salt lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), the obtained electrolyte formulation of 1 M LiTFSI in pure 2-FP solvent not only allowed faster ion transport though solvation effect, but also possessed impressive oxidation stability window over 4.3 V. As a result, the high-voltage LiNi1/3Mn1/3Co1/3O2 (1.5 mA h cm−2)|Li metal battery with it exhibited a capacity retention of more than 80 % over a long-term cycle even at 0.45 mA cm−2 with a lean electrolyte (30 μL). Meanwhile, for another FP family member (i.e., 3-FP) as the electrolyte additive, the 4.3 V LMBs with the carbonate-based electrolyte containing only 1% of 3-FP maintained 83.9 % of initial capacity after 200 cycles at 0.75 mA cm-2. Density functional theory (DFT) calculations and experiments confirmed that three typical FPs, i.e., 2-FP, 3-FP and 4-FP can not only regulate the initial Li nucleation process, but more importantly also induce a protective layer, leading to a uniform and dendrites-free Li deposition. This bifunction of the FP family member as either electrolyte solvent or additive in the electrolyte formulations should be promising for the achieving of dendrites-free high-energy density LMBs.