Electrochemical Deposition of Lithium Metal on Garnet Type Li7La3Zr3O12 Lithium Ion Conducting Ceramic Membranes
Two types of lithium-air batteries are undergoing development worldwide, the anhydrous lithium-air concept which uses an organic electrolyte[i] and the aqueous lithium air battery which uses an aqueous electrolyte[ii],[iii]. In the aqueous system, the negative lithium electrode is separated from the aqueous electrolyte by a thin ceramic water and gas tight, membrane. This barrier prevents reaction of lithium with water or other reactants such as oxygen or nitrogen, and enables the battery to have an extremely low self-discharge rate. The lithium-metal protected anode was first proposed by Visco et al[iv] in which a Lisicon glass ceramic membrane is used to separate Li metal from the aqueous electrolyte.
Unfortunately, the Lisicon ceramics and other NASICON type electrolytes such as Li1+xTi2-xAlx(PO4)3, commonly known as LATP, are not stable in contact with lithium metal[v] and an electron conducting product is produced which is very fragile and destroys the membrane. One approach to prevent this reaction is to add a lithium metal stable protective layer between the lithium metal and the Lisicon. For example, lithium phosphorous oxynitride (LiPON) can be deposited onto the Lisicon surface by reactive sputtering[vi]. This coating is stable but it requires an additional process which is performed under high vacuum and is less adapted to large scale production. This protective layer also adds additional resistance, even with micron thick layers.
Another approach is to use lithium metal stable lithium ion conducting ceramics. The garnet-like structure lithium ion conductors with general formulae Li5La3Ta2O12, Li5La3Nb2O12, and Li7La3Zr2O12, are good candidates[vii]. The latter was recently shown to be stable to lithium metal and also a high lithium ion conductor with conductivities close to 1 x 10-4 S/cm[viii].
Ceramic Li7La3Zr2O12 garnet type membranes (LLZ) developed by NGK Insulators Ltd were used to make lithium-air cells. Water and gas tight membranes were manufactured with thicknesses lower than 200µm. These membranes demonstrated to be perfectly stable in contact with lithium metal.
The electrochemical stability of the ceramic membrane was also demonstrated by applying a current collector on the surface of the ceramic and applying a reducing potential of -0.3V vs Li metal. A dense lithium metal layer was electrochemically deposited directly on the surface of the ceramic membrane without degradation of the solid electrolyte and without requiring any protective coating. This lithium metal negative electrode was then cycled without affecting the stability of the LLZ interface.
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