Revisiting Dipole-Induced Fluorinated-Anion Decomposition Reaction for Promoting a LiF-Rich Interphase in Lithium-Metal Batteries

Abstract

Building anion-derived solid electrolyte interphase (SEI) with enriched LiF is considered the most promising strategy to address inferior safety features and poor cyclability of lithium-metal batteries (LMBs). Herein, we discover that, instead of direct electron transfer from surface polar groups to bis(trifluoromethanesulfonyl)imide (TFSI−) for inducing a LiF-rich SEI, the dipole-induced fluorinated-anion decomposition reaction begins with the adsorption of Li ions and is highly dependent on their mobility on the polar surface. To demonstrate this, a single-layer graphdiyne on MXene (sGDY@MXene) heterostructure has been successfully fabricated and integrated into polypropylene separators. It is found that the adsorbed Li ions connect electron-donating sGDY@MXene to TFSI−, facilitating interfacial charge transfer for TFSI− decomposition. However, this does not capture the entire picture. The sGDY@MXene also renders the adsorbed Li ions with high mobility, enabling them to reach optimal reaction sites and expedite their coordination processes with O on O=S=O and F on the broken –CF3−, facilitating bond cleavage. In contrast, immobilized Li ions on the more lithiophilic pristine MXene retard these cleavage processes. Consequently, the decomposition reaction is accelerated on sGDY@MXene. This work highlights the dedicate balance between lithiophilicity and Li-ion mobility in effectively promoting a LiF-rich SEI for the long-term stability of LMBs.

1. Introduction

Metallic lithium has long been considered as the “holy grail” of anodes for lithium-based batteries owing to its exceptionally high theoretical capacity of 3860 mAh g−1 as well as the most negative redox potential of −3.04 V versus SHE [1, 2]. However, Li metal has high reactivity with organic electrolytes, spontaneously forming solid electrolyte interphase (SEI) that is derived mainly from the reduction of solvent molecules [3–5]. This solvent-derived SEI has sluggish Li-ion transport and is too fragile to accommodate the rapid growth of Li. Consequently, it becomes unstable during cycling, leading to repeated fracturing/reconstruction and the formation of notorious Li dendrites [6]. As a result, lithium-metal anodes suffer from severe capacity decay and formidable safety hazards, which hinder the practical implementation of lithium-metal batteries (LMBs) [7–9].

Among the efforts to inhibit the formation of Li dendrites, construction of the LiF-rich SEI might be the most promising strategy [10, 11]. Previous studies have shown that LiF-rich SEI possesses several advantageous properties, including high mechanical strength, excellent chemical stability, low solubility, a wide band gap that suppresses electron tunneling, high interfacial energy, and a low Li-ion diffusion barrier [12, 13]. These characteristics effectively mitigate SEI degradation caused by volume expansion, suppress continuous electrolyte decomposition, and promote uniform Li-ion flux within the SEI [14, 15]. Consequently, it facilitates uniform lithium deposition and significantly improves the cycle life of LMBs [16]. LiF in SEI is generally derived from the decomposition of functional fluorinated electrolyte constituents, such as bis(trifluoromethanesulfonyl)imide (TFSI−) and bis(fluorosulfonyl)imide (FSI−) anions [17, 18]. Therefore, fully understanding the fluorinated-anion decomposition reaction and regulating its reaction kinetics for the formation of LiF-rich SEI is highly desirable, despite being challenging.

Recently, self-assembled polar groups, such as the carboxyl group, have been grafted onto aluminum oxide (Al2O3)-coated polypropylene (PP) separators [19]. This functional layer has demonstrated enhanced interfacial charge transfer, catalyzing the decomposition of fluorinated anions and inducing the formation of LiF-rich SEI. Following this line of research, the introduction of extended π-conjugated [20] or porphyrin covalent organic framework surfaces [21] has also been demonstrated.