Operando Characterisation of Lithium–Sulfur Batteries

Sammanfattning: Lithium–sulfur (Li–S) batteries have been under the spotlight of research on electrochemical energy storage systems, primarily owing to their high theoretical specific energy (2552 Wh kg-1). So far, Li–S cells on the market have presented a specific energy of 400 Wh kg-1, which is superior to many commercial alternatives, but far below the theoretical value. At the same time, Li–S batteries encounter other problems that are generally not associated with the standard Li-ion batteries, such as low utilisation rate of active materials and short cycle life. These often originate from the unique catholyte nature and/or the low reversibility of the metallic Li electrode.The dissolution and precipitation of elemental sulfur and lithium sulfide in the positive electrode are here investigated by operando X-ray diffraction (XRD) and small-angle neutron/X-ray scattering (SANS/SAXS) coupled with the Intermittent Current Interruption (ICI) method. The real-time internal and diffusion resistances are correlated to the kinetics of the precipitation of the crystalline species by operando XRD. Through operando SANS and SAXS, the formation of crystalline and amorphous solid-state discharge products and the compositional variation of catholyte inside the mesopores are linked to features in the resistance profiles. These studies indicate that the ionic transport limitation inside the positive electrode is the cause for the low sulfur utilisation during battery discharge.To examine the impact of the repetitive precipitation on the functionality of the positive sulfur electrode, a method based on electrochemical impedance spectroscopy (EIS) was developed to track the electrochemically active surface area of the carbon matrix in-situ over extensive cycling. The investigation found no progressive passivation on the positive electrode despite the rapid decrease in specific discharge capacity. Additionally, a novel three-electrode setup for Li–S cells reveals a faster growth of the resistance on the metallic Li electrode along cycling. These findings suggest that primarily the negative electrode limits the cycle life. Through providing the mechanistic insights of operational Li–S cells, this thesis demonstrates the value of simultaneous electrochemical and materials characterisations for understanding the complex Li–S system.