Solid state thin film battery having a high temperature lithium alloy anode

DETAILED DESCRIPTION OF THE INVENTION Since the Li anode has the lowest melting point of the active components of a typical thin-film, solid state, rechargeable lithium battery, any increase in anode melting point translates directly into an increased operating temperature for the battery, particularly if the battery is deposited onto a glass or ceramic substrate.
The melting point of lithium can be raised by combining it with a suitable second, solute metal to form a solid-solution alloy.
Suitability of the solute metal is dependent on the following criteria: 1) The solute metal must combine with Li to form a Li-rich (at least 30% Li), solid solution alloy.
2) Such an alloy must exhibit rising solidus/liquidus lines.
3) Such solidus/liquidus lines should preferably rise steeply.
4) Li and the solute metal should have similar (or compatible) vapor pressures as a function of temperature.
For example, the Li-Mg alloy system meets the above described criteria as follows: 1) The system is a solid solution to approximately 70-75 atomic percent (a/o) Mg.
2) It has a two-phase (Li plus liquid) region over that composition range, with rising liquidus/solidus curves.
3) The slope of the solidus curve is steep; a 10 a/o Mg alloy has a m.
p.
37 degrees above the m.
p.
of Li; a 30 a/o alloy--144 degrees above the m.
p.
of Li.
4) The vapor pressure curves, as a function of temperature, for the two elements are adjacent to each other, which will allow the alloy to be thermally evaporated onto the battery.
The electrical effect, on the battery, of a solute addition to the Li metal anode material is a reduction of the current carrying capacity of the battery.
Therefore, the alloy is preferably optimized by the addition of minimum solute to achieve suitability for the highest operating temperature expected for a particular application of the present invention.
In accordance with the present invention, a solid-state, thin-film Li battery is constructed in the anode/electrolyte/cathode geometry as taught in the above referenced patents