Phosphazene-based flame-retardant artificial interphase layer for lithium metal batteries

Abstract

The rapid development of emerging fields such as electric vehicles, drones, and robotics has driven the demand for secondary batteries with higher energy density and enhanced safety. The lithium metal anode (LMA) is widely regarded as an ideal anode material for next-generation rechargeable batteries due to its high specific capacity (3860 ​mA ​h·g⁻¹) and low redox potential (−3.04 ​V vs. standard hydrogen electrode). However, LMA faces significant challenges, primarily the uncontrollable growth of dendrites and its inherent propensity for thermal runaway. To address these issues, this study proposes a novel silsesquioxane-functionalized hexaphenoxycyclotriphosphazene (HPCTP)-based porous polymer (SHPP) artificial interphase layer, synthesized via Friedel-Crafts alkylation, to achieve highly stable LMA performance. N₂ adsorption/desorption analysis confirms that SHPP features a hierarchical nanoporous structure, with pores of approximately 0.5 and 0.6 ​nm that effectively restrict the mobility of PF₆⁻ anions. As a result, the Li-ion transference number increases from 0.29 in liquid electrolytes to 0.60, which helps suppress Li dendrite growth. Additionally, the rich nanoporous structure of SHPP significantly improves its wettability with the electrolyte. In situ thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) reveals that SHPP decomposes at approximately 410 ​°C, generating phosphate radicals (PO•) that quench highly reactive hydroxyl (HO•) and oxygen (O•) radicals produced during the thermal decomposition of ester-based electrolytes, effectively mitigating thermal runaway risks. Thermal analysis and ignition tests confirm the outstanding thermal stability and flame-retardant properties of SHPP. Semi-in situ X-ray photoelectron spectroscopy (XPS) analysis indicates that the solid electrolyte interphase (SEI) on bare Li metal is predominantly organic and undergoes significant compositional fluctuations during cycling. In contrast, the SEI formed on SHPP-Li is enriched with Li phosphide (Li₃P), which enhances ionic conductivity, and Li fluoride (LiF), which improves chemical stability, resulting in a compositionally stable SEI throughout cycling. SHPP not only facilitates interfacial Li-ion transport but also promotes the formation of a chemically robust interphase. In situ optical microscopy and semi-in situ field-emission scanning electron microscopy (FE-SEM) images demonstrate that the SHPP artificial interphase effectively suppresses Li dendrite growth, enabling uniform Li deposition. As a result, SHPP-Li||SHPP-Li symmetric cells exhibit stable cycling for 1600 ​h at 0.5 ​mA ​cm⁻² and 0.5 ​mA ​h·cm⁻². Furthermore, SHPP-Li||LiNi_0·8Co_0·1Mn_0·1O₂ full cells maintain a high capacity retention of 76.8% after 500 cycles at 1 ​C (1 ​C ​= ​190 ​mA ​g⁻¹). This flame-retardant artificial interphase layer offers a promising strategy for designing dendrite-free and safe LMAs.