Carbonate-based electrolytes are presently used in rechargeable Li batteries. If the anode of such a battery has its Fermi energy less than 1 eV below the Fermi energy of elemental lithium, it reduces the electrolyte to form a passivating solid-electrolyte-interface (SEI) layer on which metallic lithium is deposited during a fast charge of the battery. Dendrites tend to form on the lithium; the dendrites can grow to short-circuit the battery and ignite the electrolyte. Therefore, safety concerns have led to a search for an anode material having a redox couple in the range 1.0-1.5 eV below the Fermi energy of lithium. Already reported as a stable anode is Li4Ti5O_(12) operating on the Ti~(4+)/Ti~(3+) redox couple located at 1.5 versus Li~+/Li. We have restricted our search to Li-insertion compounds because they have a smaller volume change over a charge/discharge cycle than do anodes operating on a displacement reaction. Here we report the position of the Nb~(5+)/Nb~(4+) couple in two oxide hosts, KNb5O_(13) and K_6Nb_(10.8)O_(30), that exhibit reversible Li insertion in the targeted voltage range.
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