Understanding the Abnormal Capacity of Metal Hydroxide Anode Material for Next Generation Lithium Rechargeable Batteries

Abstract

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, unlike the composite materials, bare Co(OH)2 anode material still has trouble with showing good electrochemical performance for lithium ion batteries, which may be due to insufficient electrolyte soaking between layers and severe volume expansion during the cycling. 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. 55 ̊C, Aldrich) as precursor through simple synthetic strategy. Even though synthesized nano-structured Co(OH)2 is not a composite material, shows excellent reversible capacity (~1000 mAh/g) during the electrochemical cycling beyond the theoretical capacity after the first irreversible discharge process, and exhibits excellent rate capability with a good cycle performance. 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, novel reaction mechanism about the nano-structured Co(OH)2 anode material during electrochemical cycling was suggested specifically. This finding will not only be helpful in a more complete understanding of the reaction mechanism of metal oxide based anode materials but also will offer valuable guidance for developing new anode materials with abnormal high capacity for next generation rechargeable batteries. 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.