Interface Controls of Lithium-Ion Batteries Electrodes through a Polymerized C60 Surface Coating
Authors: Lee Joong-Kee , Hudaya Chairul 
Published: February 2020

The increasing demand on high-energy and high-power lithium-ion batteries (LIBs) especially for electric vehicles application has driven the search on the high-capacity anode and cathode materials. For this purpose, SnO2-based anode materials (theoretical capacity of 900 mAhg-1) and LiCoO2 (theoretical capacity of 274 mAhg-1) have been intensively studied. However, the commercialization of SnO2 anode is still hindered due to the alloying mechanism which leads to severe volume expansion and eventually contributes to disintegrated material. While LiCoO2 has been used as commercial cathode materials, only half of its theoretical capacity (~140 mAhg-1) can be extracted at a cut-off voltage of 3.0-4.2 V. Although increasing cut-off voltage to 4.5 V would significantly boost the specific capacity, however severe side reactions between electrode and electrolyte and a structural phase transition from hexagonal to monoclinic causing structural damages of the electrode are the common issues occurred during the electrochemical cycle. Moreover, unstable solid-electrolyte interphase (SEI) layer on the surface of anode and cathode materials may worsen the situation leading to rapid capacity fading. Surface coating is considered as an appropriate approach in order to overcome the above-mentioned problems owing to its function to enhance interfacial kinetics and protect the host material underneath. Some metal oxides- and carbonaceous-based materials has been used as the coating materials for both anode and cathode materials. In this study, the plasma-polymerized C60 coatings both on the anode and cathode thin film electrodes are presented. The C60 coatings prepared by a radio-frequency plasma-assisted thermal evaporation are carried out on the surface of fluorine-doped SnO2 (C60@SnO2:F) anode and three-dimensional LiCoO2 (C60@3D-LiCoO2) cathode. The samples are characterized by SEM, XRD, TEM, EDX, XPS, ToF-SIMS and assembled into pouch-type and coin cells for anodes and cathodes, respectively. The results demonstrate the improved electrochemical performance of both C60-coated anode and cathode materials. For example, the C60@3D-LiCoO2 electrode delivers an initial discharge capacity of 175 mAhg-1 and maintains the capacity retention of 80% after 50 cycles at 0.1 C even with a high-voltage regime (~ 4.5 V). The C60@SnO2:F exhibits a high initial discharge capacity of 1255 mAhg-1 at 0.15 mAcm-2 and increases Coulombic efficiency of 98%. The improved electrochemical performance for C60@3D-LiCoO2 and C60@SnO2:F is attributed to the facts that the unique plasma-polymerized C60 thin film could provide (1) a mechanical strength to the host materials during repeated cycles directing to the material integrity, (2) a stabilized solid-electrolyte interphase (SEI) layer through a minimized oxygen functional groups in the electrode surface and (3) an enhanced interfacial conductivity leading to a facile Li ion diffusivity. The proposed strategy may open a broad range of application, not only for the two-mentioned electrodes, but also for other anode and cathodes materials of LIBs having similar concerns.