The Novel Study on the Reaction Mechanism and Structural Changes of Co(OH)2 By Using Synchrotron X-Ray Techniques
Authors: Kim Hyunchul , Park Gwi Ok , Jang YoonJung , Kim Yunok , Muhammad Shoaib , Lee Hyejin , Kim JI Man , Yoon Won-Sub 
Published: February 2020

Graphitic carbon is extensively used as anode material in most of the commercial lithium-ion batteries (LIBs) due to its low cost and high coulombic efficiency. However, capacity of carbon anode (372mAh/g and 830mAh/mL) is limited by the reversible electrochemical intercalation of lithium ions in its structure. So, alternative research directions and different anode materials are currently being investigated with an aim to achieve high capacity and cycling stability. Recently nano-material research has shed light on many high performing materials. Among these, cobalt hydroxide has been recognized as one of the potential candidates for anode materials because of its novel electric and electrochemical properties[1]. Co(OH)2 with a sheet-like structure tends to form a layered assembly, thus it has the benefit of improved ion transport and better contact of electro-active materials with electrolyte. Several methods have been used to synthesize the hexagonal β-Co(OH)2 nano sheets including facile hydrothermal and homogeneous precipitation with sodium hydroxide as the alkaline reagent. Co(OH)2-Graphene Nano Sheet (GNS) composite was reported in 2010[2] as anode material for LIBs with superior electrochemical performance and its reaction mechanism was suggested as: Co(OH)2 + 2Li+ +2e- → Co + 2LiOH (577mAh/g). GNS acted as synergistic effect to synthesized material for achieving higher capacity (1120mAh/g) because GNS could relieve the volume expansion during cycling and also, give excellent electronic conductivity. Co(OH)2-nanosheets/ Co3O4-nanoparticle hierarchical structure has also been reported[3] as anode material for LIB’s with enhanced capacity and performance. However, despite diverse studies on Co(OH)2 material, pure Co(OH)2 nanosheets still have poor cycling performance, which may be due to insufficient electrolyte soaking between layers and large volume changes during the cycling process. Besides, pure Co(OH)2 nanosheets show higher reversibly capacity beyond the reported mechanism. In order to improve the Co(OH)2 electrode performance, primary research should first be focused on the study of pure Co(OH)2 to better understand its reaction mechanism.
Herein, we successfully synthesized colloidal silica assisted sheet-like nano-structured Co(OH)2 using cobalt (Ⅱ) nitrate hexahydrate (Co(NO3)2∙6H2O, 97%, m.p. 55C, Aldrich) as precursor through simple synthetic strategy. This research also included its novel reaction mechanism during electrochemical cycling with lithium. Structural and electrochemical properties of synthesized material were studied by electrochemical tests and combination of the synchrotron radiation based X-ray diffraction (XRD)/ X-ray absorption spectroscopy (XAS) techniques. Changes in the bulk structure were studied by XRD whereas local structure changes around Co during cycling were systematically investigated by recorded XAS data. Based on these results, we propose novel reaction mechanism of the nano-structured Co(OH)2 material during electrochemical cycling. More detailed discussion will be presented at the time of meeting.
[1]. Huang, X.-l et al., Journal of Materials Chemistry, 2012. 22(9): p. 3764.
[2]. He, Y.-S et al., Electronbbn chemistry Communications, 2010. 12(4): p. 570-573.
[3]. Dong, Q et al., Materials Research Bulletin, 2011. 46(8): p. 1156-1162.