Polyphosphazene-Based Anion-Anchored Polymer Electrolytes For All-Solid-State Lithium Metal Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Chemistry

Background:

• Safety concerns with traditional liquid electrolytes in lithium batteries necessitate advanced solid-state alternatives.
• Polyphosphazenes offer a flexible backbone for high Li-ion conductivity and thermal stability, yet are underexplored as solid polymer electrolytes (SPEs).
• Existing single Li-ion conducting SPEs (SLiC-SPEs) face challenges with ionic conductivity, electrochemical stability, and cycling performance.

Purpose of the Study:

• To develop and characterize novel polyphosphazene-based SLiC-SPEs for enhanced lithium battery safety and performance.
• To investigate the structure-property relationships governing Li-ion dynamics in these new SPEs.
• To demonstrate the potential of these SLiC-SPEs in all-solid-state lithium-metal batteries.

Main Methods:

• Synthesis of three lithiated polyphosphazene-based SLiC-SPEs via a facile route.
• Characterization using differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and solid-state nuclear magnetic resonance (ssNMR).
• Electrochemical testing including Li-ion transference number (tLi) measurements and cycling performance evaluation with Li metal anodes and LiFePO4 cathodes.

Main Results:

• The synthesized SPEs exhibited high thermal stability up to ~208 °C.
• Ionic conductivities comparable to state-of-the-art solvent/plasticizer-free SLiC-SPEs were achieved.
• The dilithium poly[bis(trifluoroethylamino)phosphazene] (pTFAP2Li) based SPE showed a high tLi of 0.76 and good compatibility with battery components.
• A PEO-pTFAP2Li blend demonstrated 81.2% capacity utilization and 86.8% retention over 40 cycles at 100 °C in an all-solid-state Li-metal battery.

Conclusions:

• Polyphosphazene-based SLiC-SPEs offer a promising pathway for developing safer and more efficient solid-state lithium batteries.
• The facile synthesis and tunability of these polymers enable optimization for specific battery applications.
• This study demonstrates the viability of SLiC polyphosphazene SPEs in achieving stable cycling performance in all-solid-state lithium-metal batteries.