Elaboration and Characterization of Flexible Li+ Conducting Membranes for Aqueous Li-Air Batteries

Summary: Lithium-air batteries have attracted a lot of research interest recently because they can close the gap between the electric vehicle and the internal combustion engine vehicle. Several challenges remain before making this technology fully functional, but using an aqueous electrolyte addresses several of them. However, it requires the use of a protected lithium anode. While the state of the art is using a fragile glass-ceramic membrane, we propose here to develop a ceramic/polymer hybrid membrane, combining water-tightness, flexibility and Li⁺ conduction.

Introduction

Massive use of electric vehicles needs a real breakthrough in battery technologies, to simultaneously increase the specific energy and decrease the costs. Several systems are studied, including lithium sulfur, zinc air and especially lithium-air.

In lithium-air, the capacity of the negative electrode is no longer limited by the positive electrode: the latter harvests its reactant, oxygen, directly from the environment at discharge and releases it during charge. Specific energy is then drastically increased. However, this promising technology is still at the laboratory scale, and several challenges remain to be addressed.

Aqueous based lithium-air [1,2] presents several advantages, but requires the use of a protected lithium anode [3], i.e, a solid Li⁺ ionic conductor is used to isolate the metallic lithium from the aqueous electrolyte. However, the integration of glass-ceramic makes the system more fragile, limits its cyclability and increases ohmic resistance.

We propose here a different approach to protect the lithium electrode. It consists in producing a hybrid membrane [4] which combines watertightness, flexibility and Li⁺ ion conductivity. This membrane is made from an Li⁺ conducting inorganic nanofiber mat, embedded in watertight polymer.

Experimental

The critical step for producing this hybrid membrane is the synthesis of the nanofibers mat through electrospinning. This versatile process involves an electric field to break the surface tension of a liquid, to extrude it into a solid fiber [5]. Originally designed for polymers, it has been less explored for ceramic inorganic materials [6].

A solution containing both a supporting polymer and inorganic precursors is injected in an electric field to yield hybrid nanofibers. These fibers are then thermally treated to decompose the organic binder and crystallize the lithium-conducting ceramic phase as shown in figure 1. The inorganic fibers are then impregnated with a polymer to obtain final mechanical resistance and water tightness.

Materials were characterized at every steps of this process: electrospun hybrid nanofibers, inorganic crystalline nanofibers, and nanofiber mat embedded in polymer which is the final membrane. The techniques used were scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and impedance spectroscopy.

Results

A new sol-gel synthesis was successfully developed to meet the requirements of electrospinning, for both the perovskite Li_3xLa_2/3-xTiO₃ and the NASICON structure Li_1+xAl_xTi_2-x(PO₄)₃. Pure ceramic nanofibers were obtained for both compositions.

Impregnation was then optimized to reach the desired properties. Watertight and Li⁺ conducting membranes were achieved. Conductivities up to 2.10^-6 S.cm^-1 were obtained. The performances of the various membranes were discussed as a function of processing parameters, including environmental conditions and organic to inorganic ratio. These membranes were then tested in half cells as a solid electrolyte.

Conclusion

A flexible hybrid solid state membrane was developed for aqueous Lithium-air batteries. The inorganic crystalline nanofiber mats were obtained by electrospinning followed by a thermal treatment. These mats were then embedded in a polymer to ensure water tightness. The final membranes are flexible, water tight and Li⁺ ions conducting. Fibers availability at the surface and organic/inorganic ratio still have to be improved to enhance ionic conductivity.

 

Acknowledgements

The authors would like to thank Domitille Giaume (IRCP) for the ICP-MS analysis.

 

References

[1] P. Stevens et al, ECS Trans. 28 (2010) 1.

[2] P. Stevens et al, ECS Trans. 50 (2013) 1.

[3] S.J.Visco, Y. Nimon, US Patent 20070117007. (2007).

[4] C. Laberty-Robert et al. , Chem Soc Rev, 40 (2011) 961.

[5] A. Greiner et al, Angew. Chem. Int. Ed., 46, (2007) 5670.

[6] Y. Dai et al,, Polym. Adv. Technol., 22(2011) 326.