Jongho Kim

The mainstream of high‐energy cathode development is focused on increasing the Ni‐ratio in layered structured cathode materials. The increment of the Ni portion in the layered cathode material escalates not only the deliverable capacity but also the structural degradation. High‐Ni layered cathodes are highly vulnerable to exposure to air that contains CO₂ and H₂O, forming problematic residual lithium compounds at the surface. In this work, a novel air‐ and moisture robust surface modification is reported for LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) via the sol‐gel coating method that selectively coats the internal surface area of the polycrystalline morphology secondary particles. Bare‐, Li₂SnO₃‐coated and LiCoO₂‐coated NCM811 are exposed to different ambient environments (air, hot‐air, and moisture‐air) to systematically investigate the correlation between the internal/external coating morphology and performance degradations. The LiCoO₂‐coated NCM811s exhibit high‐capacity retention after exposure to all environments, due to the internal surface coating that prevents the penetration of harmful compounds into the polycrystalline NCM811. On the other hand, the Li₂SnO₃‐coated NCM811s exposed to the ambient environments show gradual capacity fading, implying the occurrence of internal degradation. This paper highlights the impact of the internal degradation of polycrystalline NCM811 after environmental exposure and the correct coating mechanisms required to successfully prevent it.

Despite the recent advancement of sulfide all-solid-state lithium batteries, the choice of their respective components yet requires further comprehensive investigations into their functionality and compatibility. While the development of cathode materials and sulfide electrolytes appears to be greatly advanced, there seems a lack of attention on carbon conductive additives. As sulfide electrolytes are instable with conventional carbon conductive additives, it is necessary to develop conductive additives with a satisfactory electronic conductivity and good chemical/electrochemical stability with sulfide electrolytes. This study proposes a new mixed conductor based on carbon nanofiber (CNF), namely lithium-enhanced functionalized carbon nanofiber (LF-CNF), exhibiting a greatly enhanced Li-ion conductivity with a little compensation of their electronic conductivity. By utilizing LF-CNF, a discharge capacity of 198mAh g -1 at 0.05C and 148mAh g -1 after 50 cycles at 0.1C were achieved from the cathode composite of LiNi 0.8 Co 0.1 Mn 0.1 O 2 , argyrodite solid electrolyte and LF-CNF (70:30:5 in mass). Furthermore, LF-CNF appears to reduce detrimental cation mixing after storage in Ar-atmosphere over one month and at high voltage cycling of 4.5V. Our work highlights that the conductivity tuning of conductive additives offers a great performance enhancement for high-energy sulfide electrolyte based solid-state batteries.