# A Flame-Retardant Cyclophosphazene as an Electrolyte Component for Si-Graphite Anodes for Lithium-Ion Batteries

## TL;DR

This study explores a flame-retardant additive for lithium-ion batteries that helps reduce fire risks but does not significantly improve the performance of silicon-graphite anodes.

## Contribution

The paper introduces a new flame-retardant electrolyte additive and evaluates its impact on silicon-graphite anode performance and safety.

## Key findings

- HFEPN at 15 wt% makes the electrolyte self-extinguishing, improving battery safety.
- The additive does not enhance the cycling stability of silicon-graphite anodes.
- Increased electrolyte viscosity from HFEPN offsets the benefits of a more stable SEI.

## Abstract

Silicon-graphite anodes offer a practical route to increase the energy density of lithium-ion batteries (LIBs), but their widespread adoption is hampered by cyclic instability due to huge volume changes of silicon during lithiation/delithiation process. Another fallout of LIBs capacity gain is growing safety concerns due to fire risks, associated with the uncontrolled release of chemical energy. Herein, we test a hexakis(fluoroethoxy)phosphazene (HFEPN) as a multifunctional electrolyte additive designed to mitigate both issues. The flammability of HFEPN-containing electrolytes was evaluated using a self-extinguishing time test, while the electrochemical performance was assessed in Si/C composite||NMC pouch cells under a progressively accelerated cycling protocol. It is shown that the additive fully imparts flame-retardant properties to the electrolyte at a 15 wt% loading. Despite forming a more stable solid–electrolyte interphase (SEI) with enhanced interfacial kinetics the additive did not improve the cycling stability of the Si/C-based cells. The cells with 15 wt% HFEPN retained 43% of capacity after 70 cycles, comparable to 46.5% for the reference electrolyte. The diffusion limitations imposed by the increased electrolyte viscosity are assumed to offset the interfacial benefits of the additive. Thus, alongside the improved synthetic route, this study demonstrates that while HFEPN functions as an effective flame retardant and SEI modifier, its practical benefits for silicon anodes are limited at high concentrations by detrimental effects on electrolyte transport properties and should be improved in future molecular design efforts.