Investigation of lithium argyrodite solid electrolyte with enriched halide substitution in the solid-state batteries

Abstract

The excessive use of the fossil fuel has attributed to the emission of carbon dioxide causing rising temperature. To counter act over this, the renewable based energy transition and lithium-ion batteries as storage facility in changing the landscape. However, these lithium-ion batteries prone to thermal runaway, lower operating conditions, dendrite formation making them vulnerable to the transition. All solid-state batteries are the next generation batteries making an revolution in the battery technology by replacing the highly volatile liquid electrolyte to the solid electrolyte for improved safety, high energy density (Li/Si possible to use the anode material), and longer cycle life.

In this thesis, the halide incorporation in lithium argyrodite 𝐿𝑖6-x𝑃𝑆5-x𝑋1+x (X= Cl, Br, I) solid electrolyte was synthesized and further characterized to understand the synthesis conditions and structural-ionic transport correlation of solid electrolyte. The halide enriched lithium argyrodite was synthesized using mechano-chemical synthesis by high energy ball milling and followed by heat treatment. All samples were investigated using following characteristics tools such as X-Ray diffraction, scanning electron microscopy, Raman spectroscopy, and Electrochemical impedance spectroscopy, to understand the structural information, morphology and the ionic conductivity of the composition. In results, the chloride/Bromide enriched in lithium argyrodite shows a higher ionic conductivity of around 14.77 mS 𝑐𝑚-1 for 𝐿𝑖5.5𝑃𝑆4.5𝐶𝑙1.5 and 6.39 mS 𝑐𝑚-1 for 𝐿𝑖5.5𝑃𝑆4.5Br1.5, and various annealing temperatures could improve the crystallinity of composition which also influences the higher ionic conductivity.

As we know from the literature, the lithium argyrodite based solid electrolyte has a narrow electrochemical stability window. It is interesting to note that altering the structure can influence this stability. We have also determined the electrochemical stability window of the chloride-enriched lithium argyrodite (𝐿𝑖5.5𝑃𝑆4.5𝐶𝑙1.5) in both BM and HT samples and compared with the commercial lithium argyrodite (𝐿𝑖6𝑃S5𝐶𝑙) by using linear sweep voltammetry. Additionally, we performed the electrochemical stability window of 𝐿𝑖6𝑃𝑆5𝐵𝑟, 𝐿𝑖5.7𝑃𝑆4.7Br1.3, and 𝐿𝑖5.5𝑃𝑆4.5Br1.5 composition. We observed the halide enriched lithium argyrodite show better electrochemical stability windows.

Overall in this thesis, the processing of halide enriched lithium argyrodite 𝐿𝑖6-x𝑃𝑆5-x𝑋1+x (X = Cl and Br) exhibits cubic crystal structure, various occupancies of halide on 4d site with high ionic conductivity, and good electrochemical stability window to development of all solid-state batteries.