Conventional wisdom on electrochemical insertion reactions requires electronically conducting electrode materials with crystallographic voids that can support lithium diffusion. In that sense, interstitial-free 3d-metal oxide structures are considered unsuitable for intercalation chemistry. However, recently, Tarascon et al. [1,2] demonstrated the use of several oxides, nitrides, fluorides, phosphides, sulphides and borides as anode materials for lithium-ion battery chemistry. They described the associated reactions, which can involve as many as four electrons per 3d metal [2], as conversion reactions. The active material is created internally by an initial conversion reaction by which the compounds react with lithium to yield nanometallic particles. Reversible capacities as high as 1000 mAh/g have been realized with such conversion anodes. Co3O4 is a popular conversion anode material. In this work, we have used Co3O4 prepared with avian egg membrane as a template as a conversion anode material. Briefly, clean, dry outer membranes of avian eggs were impregnated with a methanolic solution of Co(NO3)2, dried and heat treated at 800◦C for 5, 10, 15 and 20 h to yield Co3O4. The products were subjected to XRD, HRTEM and Raman spectroscopic studies. Lithium insertion behaviour was studied with 2032-type coin cells containing lithium anode and an electrolyte of 1M LiPF6 in 1:1 (v/v) EC-DMC between 3.000 and 0.002 V.
Fig. 1 shows typical TEM and HRTEM images of Co3O4 synthesized by a 10-h heat treatment. Particles as small as 100 nm can be seen. Cycling studies (Fig. 2) show a first-cycle irreversible capacity of 400 mAh/g. The reversible capacity appears in two regions: a flat region around 1.5 V vs. Li+/Li, accounting for about 500 mAh/g followed by a sloping region that accounts for about 300 mAh/g. The hysteresis between the charge and discharge profiles is characteristic of conversion electrodes.
[1] P. Poizot, S. Laruelli, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496.
[2] J.M. Tarascon, S. Grugeon, M. Morcrette, S. Laruelle, P. Rozier, P. Poizot, C.R. Chim. 8 (2005) 9.
