Protective Solid Electrolyte Films for Thin Li-Ion Cells

NASA’s Jet Propulsion Laboratory

Thin films of Li₂CO₃ are under consideration for use as passivating layers between electrodes and solid electrolytes in advanced thin-film lithium-ion electrochemical cells. By suppressing undesired chemical reactions as described below, the Li₂CO₃ films could help to prolong the shelf lives, increase the specific energies, and simplify the fabrication of the cells. Batteries comprising one or more cells of this type could be used as sources of power in such miniature electronic circuits as those in “smart” cards, implantable electronic medical devices, sensors, portable communication devices, and hand-held computers.

An Arrhenius Activation Curve with a relatively low activation energy of ≈0.35 eV appears to be consistent with the temperature dependence of the measured ionic conductivity of Li₂CO₃.

The need for passivation arises as follows:

A state-of-the art thin-film Li-ion cell typically consists of a lithium metal anode, a glassy solid electrolyte, and a cathode made of a lithiated transition metal oxide (e.g., LiCoO2). The Li anode and most solid electrolytes are very sensitive to humidity. To prevent destruction of the anode and solid electrolyte films by reactions with airborne moisture, it is necessary to adhere to strict handling procedures during fabrication; in particular, the electrode and electrolyte films must be handled in a glove box. As a consequence, the overall process of fabrication of thin-film Li-ion cells and batteries is more complex than it would otherwise be.
Many solid electrolytes are not chemically or electrochemically stable when in contact with Li or when exposed to high charging potentials. Intermediate passivating films that could protect such solid electrolytes at the anode and cathode potentials would be very desirable.

The selection of materials for thin film Li-ion batteries involves concerns similar to those for conventional bulk Li-ion batteries. However, the techniques used to fabricate thin-film batteries offer distinct advantages over those used to fabricate conventional batteries by affording the flexibility to design cells with multilayer thin-film structures that can be made to exhibit properties not attainable in bulk structures. As an especially relevant example, a thin-film electrolyte structure can comprise a film of a high-ionic-conductivity material coated with a film of a material of lower ionic conductivity but greater stability versus Li. The development of films that can provide stability at anode and cathode potentials enables the use of many electrolyte materials, including both novel electrolytes and electrolytes that were known previously and were considered unusable because of poor chemical or electrochemical stability.

Li₂CO₃ is electronically insulating and somewhat ionically conductive. In research conducted thus far, solid electrolyte films of Li₂CO₃ have been prepared by magnetron sputtering. These films have been found to be stable in air and to be useful for protecting components of Li-ion cells as described above. More specifically, the Li₂CO₃ films have been found to afford (1) excellent passivation against reactions between electrolytes and anodes; (2) excellent stability against oxidation at high voltage, as evidenced by the oxidative stability of carbonate-based liquid electrolytes at potentials up to 4.8 V, and (3) a high degree of stability in presence of humidity. The resistance to attack by airborne moisture is an important advantage in that during fabrication, air-sensitive components passivated by Li₂CO₃ can be moved between processing tools in ambient air.

In impedance-spectroscopy tests, Li₂CO₃ films sandwiched between Mo electrodes exhibited electrical characteristics similar to those of other solid electrolyte films. The room-temperature ionic conductivity of the LiCoO2 was found to be rather poor (≈ 5 × 10–9 S/cm), though it was found to fit to an Arrhenius activation curve (see figure). Given this low ionic conductivity, Li₂CO₃ would likely not be suitable for main electrolyte layers, but would be better suited for thin passivating films.

This work was done by Ratnakumar Bugga and William West of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category. NPO-20953