A LiF-Pie-Structured Interphase for Silicon Anodes

Abstract

Silicon (Si) is a promising anode material for rechargeable batteries due to its high theoretical capacity and abundance, but its practical application is hindered by the continuous growth of porous solid-electrolyte interphase (SEI), leading to capacity fade. Herein, a LiF-Pie structured SEI is proposed, with LiF nanodomains encapsulated in the inner layer of the organic cross-linking silane matrix. A series of advanced techniques such as cryogenic electron microscopy, time-of-flight secondary ion mass spectrometry, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry have provided detailed insights into the formation mechanism, nanostructure, and chemical composition of the interface. With such SEI, the capacity retention of LiCoO2||Si is significantly improved from 49.6% to 88.9% after 300 cycles at 100 mA g−1. These findings provide a desirable interfacial design principle with enhanced (electro) chemical and mechanical stability, which are crucial for sustaining Si anode functionality, thereby significantly advancing the reliability and practical application of Si-based anodes.

1. Introduction

The development of high-performance lithium-ion batteries (LIBs) is crucial for advancing technologies such as electric vehicles, portable electronics, and grid energy storage systems, while ensuring high environmental sustainability and low cost [1–4]. Silicon (Si) has garnered much attention as a promising anode material for LIBs due to its exceptionally high theoretical capacity of approximately 3580 mAh g–1, significantly higher than that of conventional graphite anodes [5–8]. However, the practical application of Si anode is largely hindered by the instability of the solid-electrolyte interphase (SEI) during cycling which leads to mechanical degradation, loss of electrical contact, and ultimately reduces the battery’s performance and lifespan [9–11]. Therefore, it is imperative to design and develop a SEI with the desired properties to effectively protect Si-based anodes.

The SEI formed in traditional carbonate-based electrolytes is predominantly composed of low-elasticity organic materials (e.g., ROCOOLi) and a low amount of unevenly distributed inorganic components (Li2O, Li2CO3), exhibiting brittle fracture behavior under Si anodes’ anisotropic expansion. To resolve this problem, the inorganic-rich SEI (Scheme 1a) developed through using concentrated electrolyte salts [12–14], fluorinated electrolytes [15, 16] or inorganic salts-based additives [17–19] to enhance the modulus of the SEI and thermodynamic stability, effectively suppressing the expansion of Si-based anodes. However, their intrinsic rigidity causes interfacial debonding, particularly for high-mass-loading Si anodes (> 3 mg cm–2), leading to rapid capacity fade. In recent years, researchers have explored the introduction of polymers and flexible materials to create SEIs that possess elasticity or self-healing properties to address the mechanical instability (Scheme 1b) [20–23]. These advanced SEIs can effectively mitigate stress accumulation on the Si surface during volume expansion and contraction, thus maintaining the integrity and stability of the SEI during cycles. However, these SEIs usually have low ionic conductivity due to blocked Li+ pathways, which in turn reduces its efficiency. As discussed above, the stability of the SEI depends primarily on dynamic thermodynamic stability, mechanical stability and good adhesion to Si surface. To address these requirements, we propose a novel LiF-Pie-structured SEI in this study (Scheme 1c). The term “LiF-Pie” derived from a popular food “Apple Pie” metaphorically describes the hierarchical structure of SEI: an inner layer rich in lithium fluoride (LiF) acts as the “filling,” providing high mechanical rigidity and thermodynamic stability to suppress electrolyte corrosion, while a cross-linked silane outer matrix serves as the “crust” (as elaborated in the full text).