Advanced PEO/SN/LLZTO artificial solid electrolyte interphase for long-cycling lithium metal batteries

Abstract

Lithium metal batteries have been widely used in energy storage applications owing to their high theoretical energy density. However, the unstable solid electrolyte interphase (SEI) in the batteries lead to the formation of lithium dendrites and “dead lithium”, thus affecting the safety and cycle life of the battery. To address this issue, an artificial SEI was prepared using a polymer coating strategy. The introduction of succinonitrile (SN) accelerates ion transport by promoting the dissociation of lithium salts. The solid electrolyte Li6.5La3Zr1.5Ta0.5O12 (LLZTO) enhances the mechanical strength of artificial SEIs and promotes ion conduction. Furthermore, the competitive coordination between LLZTO and SN inhibits lithium anode corrosion, forming SEI rich in LiF and Li3N. Therefore, the lithium metal anode modified with an artificial SEI can be stably plated/stripped for more than 10000 h. The LiFePO4 full cell assembled with the modified anode exhibited a discharge specific capacity of 133 mAh·g−1 and a capacity retention of 97% after 1000 cycles at 2 C (1 C = 170 mA·g−1). Notably, the anode modified with the artificial SEI exhibited excellent electrochemical performance even at low temperatures.

1. Introduction

In recent decades, lithium-ion batteries (LIBs) have become indispensable energy storage devices for use in portable electronic devices and electric vehicles and have also been applied in medical devices, such as portable medical equipment, mobile medical imaging devices, and small handheld terminals [1–2]. However, the energy density of commercial LIBs using graphite anodes (theoretical capacity ≈ 372 mAh·g−1) has already approached its theoretical limit [3–4]. Lithium metal batteries (LMBs) using lithium metal anodes, with lower electrochemical potential (−3.04 V) than the standard hydrogen electrode, have emerged as promising alternatives to commercial batteries owing to their high specific capacity (3860 mAh·g−1) [5–6]. Although lithium metal anodes have enormous potential, the presence of “dead lithium” and lithium dendrites continue to hinder the safe and stable operation of LMBs [7–8]. Lithium metal with high chemical activity will easily react with organic electrolytes to form a heterogeneous solid electrolyte interphase (SEI) layer [9–10]. The SEI plays a critical role in battery safety, energy storage, and cycle life. During cycling, the unstable and fragile native SEI fractures under significant volume changes, and the fractured SEI further exposes fresh lithium inside and generates new fragile SEIs [11–13]. The repeated destruction/reformation of the SEI irreversibly consumes both lithium metal and the electrolyte, leading to a low Coulombic efficiency (CE) and short lifespan [14–16]. Additionally, uncontrolled lithium dendrite growth can pierce the separator and pose serious safety hazards [17–18]. Therefore, an ideal SEI with excellent ionic conductivity, chemical passivation, and high mechanical strength should be constructed to suppress lithium dendrite growth and thereby accelerate the commercialization of LMBs.

Artificial SEIs, which effectively address the aforementioned issues [19–20], are broadly categorized into inorganic and organic types [21]. Inorganic artificial SEIs exhibit high mechanical strength and chemical stability and suppress lithium dendrite growth. However, they typically suffer from interface brittleness and poor adhesion [22–23]. By contrast, organic artificial SEIs exhibit interface adaptability and are processed easily [24–25]. Features like lithium affinity, viscoelasticity, and dynamic segmental motion make polymer-based artificial SEIs particularly promising for accommodating the volume fluctuations of lithium metal and adapting to irregular anode surfaces [26–28]. Polyethylene oxide (PEO) segments can serve as substrates for polymer-based artificial SEIs owing to their high flexibility and strong lithium-ion coordination capacity [29–30]. However, the low mechanical strength and poor ionic conductivity of polymer-based artificial SEIs compromise the interface stability of LMBs during long-term cycling.