Interfacial Stability of PEO Polymer Electrolyte with 4V-class LiNi<sub>0.6</sub>Mn<sub>0.2</sub>Co<sub>0.2</sub>O<sub>2</sub> Cathode Coated with Various Inorganic Materials

PEO-based solid polymer electrolytes (SPEs) exhibit a narrow potential window and undergo decomposition when in contact with 4V-class cathode active materials. The objective of this study was to investigate the stability of the contact interface between the PEO electrolyte and the positive electrode material, coated with various inorganic compounds on the surface. The time variation for the impedance of the interface between the PEO-based SPE and the LiNi0.6Mn0.2Co0.2O2 (NMC) coated with various inorganic compounds was measured. The following coated materials were tested: the ionic crystal LiF, the covalent oxide Al2O3, the ferroelectrics BaTiO3 and LiNbO3, the lithium ion conductor La0.45Li0.45TiO3 (LLTO), and the ferromagnetic and highly electron conductive La0.7Sr0.3MnO3 (LSMO). They were coated on the surface of NMC electrode using the sputtering method. The impedance of the Li/SPE/coated NMC cell was measured while maintaining a potential of 4.2 V. In the non-coated NMC, the SPE/NMC interface resistance exhibited a notable increase over time. In all the coated NMCs, the increase in the interface resistance was suppressed, indicating an improvement in the interface stability. In particular, the interface resistance of LiF and Al2O3 remained unchanged for 40 h at 4.2 V, thereby demonstrating the formation of a highly stable SPE/NMC interface. The charge/discharge measurements of the Li/SPE/coated NMC cell revealed that the capacity retention rate of NMC coated with LiF and Al2O3 was significantly enhanced in comparison to that of NMC without coating.