Investigation of the Stability of the Poly(ethylene oxide)|LiNi_1–x–yCo_xMn_yO₂ Interface in Solid-State Batteries

Solid-state lithium batteries (SSLBs) are known to provide superior safety and increased energy density compared to conventional lithium metal batteries (LIBs) with liquid electrolytes. However, for commercial applications SSBs based on poly(ethylene oxide) (PEO) have only been combined with LiFePO₄ (LFP), a low-voltage active cathode material (CAM).^[1] Using high voltage CAM such as LiNi_{1–<i>x−y</i>}Co<i>_x</i>Mn<i>_y</i>O₂ (NCM), cells experience fast capacity fading, the cause of which is not fully understood.^[2] According to electrochemical impedance measurements in a three-electrode setup, we show that the NCM/PEO interface is still the Achilles' heel in PEO-based SSBs at high voltages. We demonstrate that the electrochemical cycle stability depends on the cut-off potential and the molecular weight of PEO. This is supported by impedance measurements. In addition, using X-ray photoelectron spectroscopy (XPS), we confirm the formation of C=O bonds due to the oxidative degradation of the solid polymer electrolyte (SPE) at high potentials, which is consistent with previous reports.^[3] Considering the structure of PEO, the formation of C=O in the main chain indicates chain breakage, in turn resulting in a decrease in viscosity. Using PEO with high molecular weight (8 Mil g mol^–1), secondary electron microscopy (SEM) shows that after cycling to 4.3 V vs. Li⁺/Li the pores of the composite cathode are filled by PEO. This indicates that the mechanical properties deteriorate due to the oxidative degradation of the SPE at the cathode side.<br/><br/>[1] L. Seidl, R. Grissa, L. Zhang, S. Trabesinger, C. Battaglia, <i>Adv. Mater. Interfaces</i> <b>2021</b>, 2100704.<br/>[2] K. Nie, X. Wang, J. Qiu, Y. Wang, Q. Yang, J. Xu, X. Yu, H. Li, X. Huang, L. Chen, <i>ACS Energy Lett.</i> <b>2020</b>, <i>5</i>, 826.<br/>[3] J. Li, Y. Ji, H. Song. S. Chen, S. Ding, B. Zhang, L. Yang, Y. Song, F. Pan, <i>Nano-Micro Lett.</i> <b>2022</b>, <i>14</i>, 191.