Optimization of Carbon/Sulfur Composite and Application of Al-LLZO Solid Electrolyte and Highly Concentrated Electrolytes for Lithium-Sulfur Cell

In this study, lithium-sulfur (Li-S) cells, employing various electrolytes, were manufactures to investigate the impact of the electronic conductivity of S-carbon composite cathode layer, the Li6.25Al0.25La3Zr2O13 (Al-LLZO) solid electrolyte separator, and highly-concentrated liquid electrolytes on the electrochemical reaction of S-cathodes. Firstly, the composite particles comprising nanocarbon and S were optimized by using various nanocarbon materials. Then, the highly-concentrated electrolytes, containing sulfolane solvents, proved effective in suppressing the dissolution of discharge intermediates, such as Li2S8 and Li2S6. However, some irreversible behavior of S-cathode was still observed, attributed to the dissolution of discharge intermediates, and S itself, even within the highly concentrated electrolytes. To address this issue, an Al-LLZO solid electrolyte pellet was introduced as a separator in the Li-S cell. This separator served to prevent contact between discharge intermediates and Li-metal anode, thus minimizing chemical self-discharge of intermediates resulting from S-discharge. The use of the Al-LLZO solid electrolyte separator led to improved reversibility of the S-cathode. In fact, the discharge capacity retention was enhanced employing this cell configuration, combining the highly-concentrated electrolyte and the Al-LLZO separator. However, the cell impedance increased during the extended cycle testing, leading to degradation of the discharge capacity. This can be attributed to the loss of electronic conductivity within the cathode layer, caused by the volume change in S, and the formation of a resistive layer on the Al-LLZO solid electrolyte, stemming from chemical reactions with the discharge intermediates.