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.
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
This Brief includes a Technical Support Package (TSP).
Protective Solid Electrolyte Films Thin Li-Ion Cells
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Overview
The document discusses the development and application of thin films of lithium carbonate (Li₂CO₃) as protective passivation layers in advanced thin-film lithium-ion electrochemical cells. These cells are designed to address the challenges associated with the sensitivity of lithium metal anodes and solid electrolytes to humidity, which complicates the fabrication process and handling of thin-film batteries.
Thin-film lithium-ion cells typically consist of a lithium metal anode, a glassy solid electrolyte, and a cathode made from lithiated transition-metal oxides, such as LiCoO₂. The exposure of these components to moisture can lead to detrimental chemical reactions, necessitating strict handling procedures in controlled environments, such as glove boxes. This requirement increases the complexity and cost of battery fabrication.
The introduction of Li₂CO₃ films offers a novel solution. These films are air-stable, electronically insulating, and capable of conducting lithium ions, making them suitable for passivating air-sensitive components. By protecting the anode and solid electrolyte from moisture and minimizing undesirable reactions, Li₂CO₃ films simplify the handling of thin-film batteries and reduce fabrication difficulties.
The document outlines the motivation behind this research, emphasizing the need for a passivating layer that can withstand exposure to air while maintaining the integrity of the battery components. The Li₂CO₃ films can also enhance the stability of solid electrolytes under various electrochemical conditions, allowing for the use of higher conductivity electrolytes that were previously limited by voltage stability issues.
The preparation of these solid electrolyte films is achieved through RF magnetron sputtering, a technique that allows for precise control over the film's properties. The development of this technology not only simplifies the manufacturing process but also opens up new possibilities for the design of thin-film batteries with multi-layer components.
In summary, the document presents a significant advancement in battery technology by introducing Li₂CO₃ as a protective layer in thin-film lithium-ion cells. This innovation promises to enhance battery performance, reduce fabrication complexity, and expand the potential applications of thin-film batteries in various electronic devices, including smart cards, medical implants, and portable communication devices.
