Deep and Comprehensive Study on the Impact of Different Phosphazene‐Based Flame‐Retardant Additives on Electrolyte Properties, Performance, and Durability of High‐Voltage LMNO‐Based Lithium‐Ion Batteries

Abstract

Herein, the formulation of safe electrolytes for Li‐ion batteries based on phosphazene as a flame‐retardant (FR) is achieved. Three molecules are studied: hexafluorocyclotriphosphazene (FR1), (ethoxy)pentafluorocyclotriphosphazene (FR2), and pentafluoro(phenoxy)cyclotriphosphazene (FR3). By using a conventional electrolyte (LiPF6 salt in an ethylene carbonate/diethyl carbonate solvents mixture), FR's minimum percentages are defined to quantify their efficiency as FRs. Fluoroethylene carbonate is also added to the electrolyte (2 wt%). The surface tensions, vapor pressures, and transport properties of formulated electrolytes are measured to highlight the impact of the FR additives. Then, these electrolytes are tested in half and full electrochemical devices: Li|LiMn1.5Ni0.5O4 (LMNO) and graphite|LMNO between C/10 and C/2 at 20 °C. Flammability tests show that 3% of FR1, 5% of FR2, or 15% of FR3 are needed to make the electrolytes nonflammable. The transport properties of electrolytes based on FR1 and FR2 remain unchanged compared to the conventional electrolyte. Finally, the graphite|LMNO devices lose only 5% of the initial capacity after 100 cycles with the electrolytes based on FR1 and FR2, hence, confirming the latter's potential as an efficient FR for high‐voltage Li‐ion batteries.