Thin-film lithium and lithium-ion batteries

Abstract

Research over the last decade at Oak Ridge National Laboratory has led to the development of solid-state thin-film lithium and lithium-ion batteries. The batteries, which are less than 15 μm thick, have important applications in a variety of consumer and medical products, and they are useful research tools in characterizing the properties of lithium intercalation compounds in thin-film form. The batteries consist of cathodes that are crystalline or nanocrystalline oxide-based lithium intercalation compounds such as LiCoO2 and LiMn2O4, and anodes of lithium metal, inorganic compounds such as silicon–tin oxynitrides, Sn3N4 and Zn3N2, or metal films such as Cu in which the anode is formed by lithium plating on the initial charge. The electrolyte is a glassy lithium phosphorus oxynitride (‘Lipon’). Cells with crystalline LiCoO2 cathodes can deliver up to 30% of their maximum capacity between 4.2 and 3 V at discharge currents of 10 mA/cm2, and at more moderate discharge–charge rates, the capacity decreases by negligible amounts over thousands of cycles. Thin films of crystalline lithium manganese oxide with the general composition Li1+xMn2−yO4 exhibit on the initial charge significant capacity at 5 V and, depending on the deposition process, at 4.6 V as well, as a consequence of the manganese deficiency–lithium excess. The 5-V plateau is believed to be due to oxidation Mn of ions to valence states higher than +4 accompanied by a rearrangement of the lattice. The gap between the discharge–charge curves of cells with as-deposited nanocrystalline Li1+xMn2−yO4 cathodes is due to a true hysteresis as opposed to a kinetically hindered relaxation observed with the highly crystalline films. This behavior was confirmed by observing classic scanning curves on charge and discharge at intermediate stages of insertion and extraction of Li+ ions. Extended cycling of lithium cells with these cathodes at 25 and 100°C leads to grain growth and evolution of the charge–discharge profiles toward those characteristic of well crystallized films.