Mechanistically Understanding the Correlation Between Dynamic Interface Variation and Stability of Surface Coating on the NMC811 Materials

LiNi_0.8Co_0.1-Mn_0.1O₂ (NMC811) is widely used in high energy density lithium-ion batteries, while dynamic variation of liquid-solid interface induced by oxygen release/evolution results in unexpected performance decay. Surface coating is an effective strategy in promoting the stability of NMC811 materials, and both coating composition and nanostructure arrangement would substantially impact the dynamic variation of liquid-solid interfaces. To understand such dynamic variation, here evolution and stability of two selected NMC811 materials coated with oxides (the same oxide composition but different nanostructure arrangements) are compared, i.e., concentration uniform core-shell NMC811 (CUCS811) and concentration gradient core-shell NMC811 (CGCS811). The gas evolution from the home-setup operando gas monitors (at 45 and 65 °C) implies that CUCS811 presents more operation and thermal stability because of less amount of CO₂ and CH₄ along with prolonged interval time in gas production. Composition variation of interfaces indicates that CUCS811 presents more stable interfacial evolution behaviors since more F- and P-rich species for stabilizing the solid-electrolyte interfaces are detected. The stability analysis suggests that uniform arrangement of Al₂O₃ and boride in the coating is prone to prevent the interface from degradation upon dynamic interface variation, which is useful in promoting the performance stability of NMC811 positive electrode materials.