Development of Novel Materials for Solid-State Lithium-Sulfur Batteries

To meet the growing energy demands of the future, safer batteries, with higher energy density are required. Li-ion batteries have been on the market for nearly 30 years and although there has been improvements of the original technology of the 90’s, the theoretical capacities have been reached and little increment are expected, therefore new materials and chemistry are needed.

Among alternatives to Li-ion batteries are Li-S batteries with higher energy densities, demonstrated 500 Whkg-1 vs 250 Whkg-1 for the best Li-ion batteries. Conventional Li-S batteries use salts dissolve in liquid solvents as electrolyte. However the solvents are flammable, which causes safety concerns and cause the shuttling of dissolve polysulfide resulting in rapid selfdischarge of the batteries.

Furthermore, lithium metal cannot be used as anode material and lithium dendrite formation shortens the lifetime of the batteries. A solution to these problems is to replace the liquid electrolyte by solid-state electrolytes based on ionic solids. In this work, two novel solid-state electrolytes, LiBH4-SiO2 and LiBF4-LiBH4, have been investigated as potential candidates for solid-state lithium sulfur batteries.

These electrolytes have been synthesized and characterized using state of the art technics such as electrochemical impedance spectroscopy, Raman spectroscopy and nuclear magnetic resonance (NMR). High ionic conductivities were measured (0.1 mS/cm-1 at room temperature). The electrolyte made from a composite of LiBH4-SiO2 has a higher Li+ conductivity than that of pure LiBH4 (1000 times higher at room temperature, 0.1 mS/cm-1 vs 0.1 10-3 mS/cm-1).

Our investigation showed no evidence for the formation of new bulk phases but the existence of a highly conductive interface between the insulating silica and the borohydride, most probably resulting from the reaction of LiBH4 with the surface silica silanol groups. The Raman and NMR measurements clearly sow the different behavior of the composites compared to pure LiBH4.

For LiBH4-LiBF4, with low content of LiBF4 we found that the increased Li+ conductivity is occurring in modified LiBH4, possibly via a solid solution with LiBF4, while for higher LiBF4 contents decomposition of LiBH4 occurs and result in lower conductivities. Li-S batteries build around LiBH4-SiO2 solid electrolyte have been successfully assembled and tested.

Capacityies of 794 mAhg-1 sulfur have been obtained after 10 cycles at charge-discharge rate of 0.03 C and 50° C. Because larger capacities than the theoretical one were observed during the first discharge – charge cycle, protection of the sulfur cathode has been investigated. We deposited LiPON thin films on the cathode surface to protect it from direct contact with the electrolyte.

These batteries showed, smaller capacities, but better capacity retention over cycling than the batteries with non-coated cathodes. The first discharge overcapacities disappeared. This result underlines the importance of the interface treatment and engineering within Li-S solid state batteries.