Elucidating Degradation Mechanisms of PEGDA–SN Solid Polymer Electrolytes in All-Solid-State Li–O2 Batteries

Abstract

Solid polymer electrolytes (SPEs) are promising for all-solid-state Li-metal and Li–O2 batteries (ASSLOBs) due to their intrinsic safety and enhanced interfacial stability with Li metal anodes. Among them, poly(ethylene glycol) diacrylate–succinonitrile–lithium bis(trifluoromethanesulfonyl)imide (PEGDA–SN–LiTFSI) systems offer relatively high ionic conductivity and a wide electrochemical stability window. However, their chemical and electrochemical stability under practical Li–O2 operating conditions remains insufficiently understood. Here, we systematically elucidate the degradation mechanisms of a representative PEGDA–SN-based SPE under coupled chemical and electrochemical conditions using ASSLOBs as a realistic testing platform. While the SPE is chemically stable against O2 and Li metal under open-circuit conditions, it undergoes pronounced interfacial degradation upon contact with the discharge product Li2O2. This interaction drives decomposition of the SN plasticizer alongside oxidation of the PEGDA backbone, underscoring the high reactivity of Li2O2. Notably, symmetric cell cycling reveals clear field- and current-dependent degradation even in O2-free environments and without direct Li metal contact, isolating electrochemical driving forces from purely chemical effects. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses confirm the formation of C=N species and β-amino nitriles, accompanied by a decrease in C=O functionalities. This coupled degradation disrupts Li+ transport and interfacial integrity, manifesting as electrolyte darkening, increased impedance, and performance decay. As a result, ASSLOBs exhibit limited cyclability at 65 °C, delivering only 65 cycles at 0.1 mA cm–2 (0.1 mAh cm–2) and 42 cycles at 0.2 mA cm–2 (0.2 mAh cm–2). Overall, this work identifies a critical limitation of succinonitrile-based plasticizers and provides a representative case study that reveals degradation mechanisms, offering valuable insights for developing chemically and electrochemically robust electrolytes for next-generation Li–O2 batteries.