Lithium-ion batteries (LIBs) are widely used in portable electronics. However, safe, long-term stable LIBs with higher energy density need to be developed to meet the increasing demand for large-sized devices such as electric vehicles and energy storage systems. For high energy density, further development of cathode materials is still considered as capacity and/or operating voltage are pursued. Among the various cathode materials, Ni-rich LiNi_xCo_yMn_1-x._yO₂ materials with x ≥ 5, especially LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) have attracted much attention recently owing to their high theoretical specific capacity of 200 mAh/g. However, LiNi₈Co_0.1Mn_0.1O₂ operated beyond 4.3V suffers serious capacity fading caused by continuous electrolyte decomposition, despite an increase in the capacity in the early cycles. To overcome this obstacle, the addition of small amounts of other components into the electrolyte system without changing the bulk properties is a very promising approach. In this study, we introduced a cathode additive and investigated the effect of functional electrolyte for the application NCM811 based Lithium metal batteries. Our results indicate the significance of novel additive for promising high voltage operation of next-generation cathode materials.