Alpha-MnO2 nanowires: a catalyst for the O2 electrode in rechargeable lithium batteries.

Charge storage in rechargeable lithium batteries is limited by the positive electrode, usually the lithium intercalation compound LiCoO2, which can store 130 mAhg . Intense efforts are underway worldwide to discover new lithium intercalation compounds for use as positive electrodes which, it is hoped, may deliver specific capacities of about 300 mAhg . However, increasing the capacity significantly beyond this limit is a major challenge requiring a more radical approach, such as replacement of the intercalation electrode by an O2 electrode, in which Li + from the electrolyte and e from the external circuit combine reversibly with O2 from the air within a porous matrix containing a catalyst. Although it provides higher capacities than intercalation electrodes, much fundamental work is required to understand and optimize the performance of the O2 electrode for lithium batteries before it can be considered further for technological application. The nature of the catalyst plays a key role. It is important to identify good catalysts for the electrode reaction before focusing on other tasks, such as reducing the catalyst loading and optimizing porosity, binder, and electrolyte. Herein we show that a-MnO2 nanowires give the highest charge storage capacity yet reported for such an electrode, reaching 3000 mAh per gram of carbon, or 505 mAhg 1 if normalized by the total electrode mass. Furthermore, by avoiding deep discharge, excellent capacity retention has been demonstrated. Finally, the capacities delivered by an O2 electrode and a conventional intercalation compound are compared. The reversible oxygen electrode is shown schematically in Figure 1. On discharge, the Li ions (electrolyte) and e (external circuit) combine with O2 (air) to form Li2O2 within the pores of the porous carbon electrode. Previously, we demonstrated that rechargeability of the Li/O2 cell involves decomposition of Li2O2 back to Li and O2. [8] Our earlier studies on the rechargeable Li/O2 cell focused on electrolytic manganese dioxide (EMD) as catalyst in the oxygen electrode. Recently, we examined a number of other potential catalyst materials including Co3O4, Fe2O3, CuO, and CoFe2O4. [9] Such investigations served to demonstrate that the nature of the catalyst is a key factor controlling the performance of the oxygen electrode, especially the capacity, which is the primary reason for interest in the O2 electrode. Herein we report on the high capacities that an a-MnO2 nanowire catalyst can deliver. We also compare the performance of a-MnO2 with other manganese oxide compounds. Note that the specific capacities are normalized with respect to the mass of carbon in the electrode, as is usual for porous electrodes; this point is discussed at the end of the paper. Synthesis and characterization of the various MnOx catalysts and their incorporation into lithium cells with porous electrodes is described in the Experimental Section. Powder X-ray diffraction data were collected for all catalysts (see the Supporting Information) and confirmed their identities (a-MnO2 in bulk and nanowire form, b-MnO2 in bulk and nanowire form, g-MnO2, l-MnO2, Mn2O3, and Mn3O4). The variation of capacity with cycle number for a porous electrode containing a-MnO2 nanowires as catalyst is presented in Figure 2a, from which the superior behavior of the a-MnO2 catalyst is evident. The initial discharge capacity is 3000 mAhg , it then drops slightly, rises again to 3100 mAhg 1 on cycle 4, before declining steadily thereafter. This may be contrasted with previous reports for EMD, the capacity of which falls below 1000 mAhg 1 after one cycle (Figure 2a). The variation of potential with state of charge for several cycles of a-MnO2 is shown in Figure 2b. As observed previously for all other catalysts, the discharge voltage is around 2.6 V versus Li/Li. 9] Previous results have demonstrated that the charging potential varies according to the catalyst type. Values ranging from 4 to 4.7 V versus Li/Li have been observed, and a-MnO2 exhibits a charging potential at the lower end of this spectrum, at around 4.0 V. This is another advantage of the a-MnO2 nanowires, since it is important to minimize the charging potential. Note that a-MnO2, and many of the other MnOx compounds described herein, support some Li intercalation. However, Figure 1. Schematic representation of a rechargeable Li/O2 battery.