Electrochemical Properties of DLC Coated LiNi0.8Co0.15Al0.05O2 Cathode Material of All-Solid-State Lithium Ion Battery

The surface of LiNi_0.8Co_0.15Al_0.05O₂  (NCA) cathode material was coated with diamond like carbon (DLC) by chemical vapor deposition method, in order to prevent the side reaction at the interface for sulfide based all-solid state lithium-ion batteries.  The DLC coated material showed a higher capacity with good cyclability and high rate performance. The interface resistance between the cathode active material and the solid electrolyte significantly decreases by DLC coating onto the NCA powder.  The discharge capacity of DLC coated sample at 0.05C was 113 mAhg⁻¹ while that of bared one was about 102 mAhg⁻¹.  The charge and discharge cut off potentials were 4.0 and 2.8 V, respectively.  The thickness of the DLC coated layer was verified by transmission electron microscope (TEM), and was about 3�`5 nm.  The sp2 bonding ratio of the DLC coated LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub>  powder was estimated from the C-KELNES spectra to be 50�`55%.  DLC coating could avoid formation of an inactive layer when surface impurities of NCA react with the sulfide electrolyte to form at irreversible passivation byproduct. This layer of impurities obstructs the Li-ion transport at the interface, and therefore increases the interfacial resistance and decreases the rate capability for bare samples.

In this work, the diamond like carbon (DLC) coating were proposed for the first time as buffer coating layers for all-solid state battery.  We demonstrated that the buffer layer is not needed to have a lithium conductivity to reduce the interface resistance between cathode material and sulfide based electrolyte.