We have recently reported on new developments in anode materials for rechargeable lithium batteries and now report an additional breakthrough in developing cathode materials (1). To review briefly, a battery is a simple electrochemical, or voltaic, cell that consists of an anode, a cathode, and an appropriate electrolyte. In rechargeable lithium batteries, the anode and cathode are connected to each other by a circuit that includes an electrolyte containing a soluble lithium compound in an organic solvent (see figure above right). The oxidation and reduction reactions that occur at each electrode allow for the transfer of electrons in a circuit, thus powering some electronic device. In a rechargeable battery, passing electricity through the battery reverses the cell reaction to regenerate the electrodes.

Lithium is a choice element for the anodes of rechargeable batteries due to its high energy-storage capacity per unit weight (high energy density ), very negative reduction potential (capacity to generate high voltages), and long operational lifetime (3). For the cathode, most rechargeable lithium-ion batteries use layered LiCoO₂ cathodes, in which cobalt functions as the electron acceptor. However, cobalt is both expensive and toxic. To avoid these drawbacks, a new cathode material, manganese oxyiodide, has been developed by researchers at the University of Texas at Austin (1,3,4).

High-temperature preparation of complex metal oxides yields products with a large grain size (consequently a small surface area) and few accessible stable oxidation states (couples, which show high reversibility:

Mn⁴⁺ + e^- Mn³⁺
Mn³⁺ + e^- Mn²⁺

The composition of the product was found to be Li_1.5Na_0.5MnO_2.85I_0.12 (3,4). Like the iron ore magnetite, which is a 50:50 mixture of FeO and Fe₂O₃, this manganese-containing oxyiodide appears to be a mixture of compounds. On analysis by various x-ray diffraction techniques, the manganese oxyiodide exhibited amorphous character, but was not totally amorphous (3); there appeared to be small domains of pure crystalline components.

To fabricate the cathode, finely powdered carbon and polytetrafluoroethylene were mixed with the Li_1.5Na_0.5MnO_2.85I_0.12. This combination of materials gives the cathode both structural integrity and high conductivity.

Previously, despite their relatively low cost and low toxicity, manganese oxides have not been not used as cathode materials because of the observed loss of energy-storage capacity when they are subjected to repeated cycles of charging and discharging. However, manganese oxyiodide has been found to be degraded to a lesser extent by repeated battery use. In fact, the manganese oxyiodide cathode maintained its full energy-storage capacity over 40 cycles of charging and discharging. The new material may actually be 1.3 times better than LiCoO₂ in terms of energy-storage capacity over the range of 1.5 to 4.3 volts. The explanation for this increase in energy-storage capacity seems to lie in the nearly amorphous nature of the manganese oxyiodide, which may provide smooth pathways for the movement of lithium ions into and out of the cathode material, and may allow the atoms to rearrange themselves after each recharging (resistance (to the flow of electrons) is lowered and the diffusion rate (i.e., movement of ions) is increased by repeated cycling (3), which may reorder the channels in the amorphous structure to bring about more effective ion diffusion through the cathode.

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