Polymer-based composite catholyte structures have been investigated in a continuing effort to increase the charge/ discharge capacities of solid-state lithium thin-film electrochemical cells. A cell according to this concept contains the following layers (see figure):
- An anode current-collecting layer, typically made of Cu;
- An Li metal anode layer;
- A solid electrolyte layer of Li3.3PO3.8N0.22 ("LiPON") about 1 to 2 μm thick;
- The aforementioned composite catholyte layer, typically about 100 μm thick, consisting of electronically conductive nanoparticles in an Li-ion- conductive polymer matrix; and
- A metallic cathode current collector, typically made of Mo and about 0.5 μm thick.
In the fabrication of such a cell, the anode current collector (or, alternatively, the Li anode layer if already present as explained in the next paragraph) is first used as a substrate, onto which the Li3.3PO3.8N0.22 layer is deposited. The composite catholyte layer is then cast onto the Li3.3PO3.8N0.22 layer. Next, the cathode current collector is deposited on, or pressed into contact with, the composite catholyte layer.
If the anode current collector is a Cu film on a flexible substrate (as in prototype cells) or if it is something similar, the Li anode layer can be formed by plating of Li on the anode current collector during the first charge. Alternatively, the anode layer can be made, at the outset, of a thin film of Li; if this were done, the cell could retain a greater fraction of its capacity over many cycles because the film could be made to contain a slight excess of Li that would be available to replace some Li that is lost to the surroundings during cycling.
Inasmuch as Li3.3PO3.8N0.22 is an amorphous, flexible material, the cell as a whole can be a free-standing, flexible structure. Theoretically, the capacity of the cell can equal or perhaps exceed that of a typical state-of-the-art lithium thin-film cell. The inclusion of the Li3.3PO3.8N0.22 is expected to result in extended lifetime and enables the use of Li in metallic form because the hazards associated with the combination of metallic Li and liquid electrolyte are not present. Further, it is anticipated that the cell would have long (relative to prior Li thin-film cells) cycle life at temperature up to 150 °C, provided that the proper cathode material is selected.
The capacities of the prototype cells thus far have been below theoretically attainable values. It seems likely that the theoretical values could be approached by selecting the proper cathode material and including thin Li anode films at the outset.
This work was done by Jay Whitacre, William West, Keith Chin, and Sekharipuram Narayanan of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
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Refer to NPO-41068, volume and number of this NASA Tech Briefs issue, and the page number.