Mesoporous and nanowire Co3O4 as negative electrodes for rechargeable lithium batteries

The conversion reactions associated with mesoporous and nanowire Co3O4 when used as negative electrodes in rechargeable lithium batteries have been investigated. Initially, Li is intercalated into Co3O4 up to x similar to 1.5 Li in LixCo3O4. Thereafter, both materials form a nanocomposite of Co particles imbedded in Li2O, which on subsequent charge forms CoO. The capacities on cycling increase on initial cycles to values exceeding the theoretical value for Co3O4 + 8 Li+ + 8e(-) -> 4 Li2O + 3 Co, 890 mAhg(-1), and this is interpreted as due to charge storage in a polymer layer that forms on the high surface area of nanowire and mesoporous Co3O4. After 15 cycles, the capacity decreases drastically for the nanowires due to formation of grains that are separated one from another by a thick polymer layer, leading to electrical isolation. In contrast, the mesoporous Co3O4 losses its mesoporosity and forms a morphology similar to bulk Co3O4 (Co particles imbedded in Li2O matrix) with which it exhibits a similar capacity on cycling. In contrast to mesoporous lithium intercalation compounds, which show superior capacity at high rates compared to bulk materials, mesoporosity does not seem to improve the capacity of conversion reactions on extended cycling. If, however, mesoporosity could be retained during the conversion reaction, then higher capacities could be obtained in such systems.