Three-in-one fire-retardant poly(phosphate)-based fast ion-conductor for all-solid-state lithium batteries

The development of flame retardant or nonflammable electrolytes is the key to improve the safety of lithium batteries, owing to inflammable organic solvents and polymer matrix in common liquid and polymer electrolytes regarded as the main cause of battery fire. Herein, a series of solid-state polyphosphate oligomers (SPPO) as a three-in-one electrolyte that integrated the roles of lithium salt, dissociation matrix, and flame retardant were synthesized. The well-designed SPPO electrolytes showed an optimal ionic conductivity of 5.5 × 10−4 S cm−1 at 30 °C, an acceptable electrochemical window up to 4.0 V vs. Li/Li+, and lithium ion transference number of 0.547. Stable Li-ion stripping/plating behavior for 500 h of charge–discharge cycles without internal short-circuit in a Li|SPPO|Li cell was confirmed, together with outstanding interface compatibility between the SPPO electrolyte and lithium foil. The optimal Li|SPPO|LiFePO4 cell presented good reversible discharge capacity of 149.4 mA h g−1 at 0.1 C and Coulombic efficiency of 96.4% after 120 cycles. More importantly, the prepared SPPO cannot be ignited by the lighter fire and show a limited-oxygen-index value as high as 35.5%, indicating splendid nonflammable nature. The SPPO could be a promising candidate as a three-in-one solid-state electrolyte for the improved safety of rechargeable lithium batteries.