Due to the high theoretical capacity of ≈800-1600 mA h g⁻¹, Li–S batteries are one of the “beyond Li-ion” chemistries under intensive study. During the discharge/charge processes, sulfur undergoes several reduction/oxidation steps forming soluble polysulfides (S_n²⁻, n = 8, 6, 4) before ultimately generating the final discharge products, Li₂S₂ and Li₂S. However, there are two significant challenges that inhibit their commercial viability.
First, the soluble species can be reduced further, and deposited, at the lithium anode or shuttle between the cathode and anode undergoing repeated oxidations and reductions, producing the so-called ‘infinite charging’ phenomenon during the charging step. This produces a decrease in coulombic efficiency and drop in capacity with continued cycling. Second, neither of the insoluble discharge products exhibit significant electronic conductivity. Due to the resistive nature of the discharge products and the low inherent conductivity of sulfur, the cathode suffers from significant overpotentials at higher current densities (Zheng et al., 2018). Increasing the innate conductivity of the cathode material while retaining the soluble polysulfides near to the surface would allow for efficient conversion of the polysulfides.
Copper nanowires were incorporated into the sulfur cathode matrix to address these challenges. Conductive nanoadditives increases the conductivity of the composite cathode, through establishing a percolative network, while providing an anchoring point for the polysulfides. Pristine materials and cycled cathodes containing the Cu nanomaterials were characterized using X-ray diffraction, pair distribution function analysis, and Raman spectroscopy. Interestingly, ex-situcharacterization showed that the addition of the Cu nanowires induced the formation of copper sulfide, which is advantageous since it is conductive and allows for rapid, redox processes (Majumdar, 2020). Electrochemical characterization showed decreased cell impedances, which were manifested through reduced overpotentials during galvanic cycling.

Vicky Doan-Nguyen

The Ohio State University

Appearances