LiCoPO4 has been found to be a promising active cathode material for high-energy-density, thin-film, rechargeable electrochemical power cells. The potential of the charge/discharge plateau of a cell containing an LiCoPO4 cathode is 4.8 V — a value that compares favorably with the corresponding value of 3.8 V of a state-of-the art cell containing an LiCoO2 cathode.
In preparation for tests to determine the viability of LiCoPO4 as a cathode material, all-solid-state thin-film cells containing LiCoPO4 thin-film cathodes were fabricated on single-crystal silicon substrates coated with silicon nitride. All of the cell layers except the anode were deposited in a planar, three-target, radio-frequency magnetron sputtering chamber that was evacuated to a base pressure of less than 2 × 10–6 torr (≈2.7 × 10–4 Pa) by use of a turbomolecular pumping system.
In each cell, the first deposited layer was a Ti adhesion layer followed by a Pt current collector, both patterned by use of a shadow mask. Next, the active cathode layer was formed by sputtering LiCoPO4 onto the cathode current collector from an LiCoPO4 target that had been prepared in a subprocess that included heating of a stoichiometric mixture of Co3O4, (NH4)2HPO4, and Li2CO3 powders. In some cases, the workpieces were heated to various temperatures in air to anneal the LiCoPO4. Next, a solid electrolyte film of Li3.3PO3.8N0.22 (“Lipon”) was deposited onto the cathode layer by sputtering from a Li3PO4 target in N2. Finally, a lithium metal anode layer was thermally evaporated onto the electrolyte through a second shadow mask to complete the cell.
Thin-film batteries with LiCoPO4 cathodes that were annealed for one hour at 700 °C had dramatically improved performance compared with unannealed cathode cells with single high-voltage charge/discharge plateau at about 4.8 V (see Figure 1). The cell capacity at C/15 (where C is the current expressed in multiples of the rated capacity of the battery) was measured to be 11 μA·h/μm·cm2, based on an active cell area of 0.48 cm2 and a cathode thickness of 0.285 μm prior to annealing. A weak relationship is found between cell voltage and discharge current rate (see Figure 2). The thin-film cells employing the LiCoPO4 cathodes are capable of multiple charge/discharge cycles, and have no significant capacity fade with cycling over at least the first few tens of cycles. Further cycle life studies are currently underway to better estimate the capacity fade with cycling.
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Refer to NPO-40117, volume and number of this NASA Tech Briefs issue, and the page number.