Plates of a solid electrolyte that exhibits high conductivity for positive lithium ions can now be passivated to prevent them from reacting with metallic lithium. Such passivation could enable the construction and operation of high-performance, long-life lithium-based rechargeable electrochemical cells containing metallic lithium anodes. The advantage of this approach, in comparison with a possible alternative approach utilizing lithium-ion graphitic anodes, is that metallic lithium anodes could afford significantly greater energy-storage densities.
A major impediment to the development of such cells has been the fact that the available solid electrolytes having the requisite high Li+-ion conductivity are too highly chemically reactive with metallic lithium to be useful, while those solid electrolytes that do not react excessively with metallic lithium have conductivities too low to be useful. The present passivation method exploits the best features of both extremes of the solid-electrolyte spectrum. The basic idea is to coat a higher-conductivity, higher-reactivity solid electrolyte with a lower-conductivity, lower-reactivity solid electrolyte. One can then safely deposit metallic lithium in contact with the lower-reactivity solid electrolyte without incurring the undesired chemical reactions. The thickness of the lower-reactivity electrolyte must be great enough to afford the desired passivation but not so great as to contribute excessively to the electrical resistance of the cell.
The feasibility of this method was demonstrated in experiments on plates of a commercial high-performance solid Li+-conducting electrolyte, the composition of which was not disclosed at the time of reporting the information for this article. The lower-conductivity, lower-reactivity solid electrolyte used for passivation was lithium phosphorus oxynitride (commonly abbreviated “LiPON” but more precisely abbreviated LixPOyNz, where x, y, and z denote numbers that can differ from 1). The solid-electrolyte plates were 50.8-mm square with a thickness of 0.47 mm. Films of Li3.3PO3.8N0.22 having thicknesses of the order of 1 μm were deposited on the plates by radio-frequency magnetron sputtering from an Li3PO4 target in an atmosphere of N2. Pt and Cu electrodes were sputtered through a metal shadow mask, and the active lithium anode material was deposited by thermal evaporation through the same mask.
For comparison, some plates were not coated with LiPON and Li was deposited directly on them. In those cases, the deposited Li metal reacted immediately with the plates to form dark nonmetallic layers (see upper part of figure) that were electrically nonconductive. In contrast, for the plates that were first coated with LiPON and then with Li, films retained their metallic luster (see lower part of figure) and remained electrically conductive. Test cells containing Li anodes on LiPON-coated plates were constructed and tested by electrochemical impedance spectroscopy and cyclic voltammetry. The coated solid-electrolyte plates were found to support electrochemical plating and stripping of Li metal. The electrical resistances contributed by the LiPON layers were found to be small relative to overall cell impedances.
This work was done by William West, Jay Whitacre, and James Lim of Caltech for NASA’s Jet Propulsion Laboratory.