Preparation Of Polypyrrole/TiO₂ Nanotube Arrays Composite And Its Application In The Lithium Sulfur Battery

The lithium-sulfur cell has shown great prospects for future energy conversion and storage systems due to the high theoretical specific capacity of sulfur, 1672 mAh g-1. However, it has been hindered by its poor cycle stability and low energy efficiency. In this work, polypyrrole?PPy?/TiO₂ nanotubes with coaxial heterogeneous structure as the substrate of the cathode is prepared and used to improve the electrochemical performance of sulfur electrodes. In the coaxial heterogeneous structure, TiO₂ nanotubes decorated with PPy provide a highly ordered conductive framework for Li+ diffusion and reaction with sulfur. This architecture also is helpful for trapping the produced polysulfides, and as a result attenuates the capacity decay.Highly ordered TiO₂ NTs were prepared by a two-step anodization method in organic electrolyte containing fluoride. Analysis results of FE-SEM and XRD indicate that TiO₂ nanotubes were arranged in parallel with highly ordered and uniform tube-array structure that has a diameter of ¹20 nm and a length of ³0 ?m. The phase of TiO₂ NTs is typical and only anatase.The deposition of PPy on TiO₂ nanotube arrays was carried out in a three-electrode cell by electrochemical polymerization in aqueous pyrrole solution containing 0.1 M pyrrole monomer, 0.2 M Sodium p-toluenesulfonate?TsONa? and deionized water at pH 2⁴ adjusted with HCl. The effect of deposition current and polymerization time on the structure and conductivity of PPy/TiO₂ NTs composite was also investigated. It suggested that PPy/TiO₂ NTs composite showed the most uniform microstructure, moderate thickness of PPy and excellent conductivity with a certain deposition current of 1.0 mA/cm2 for 15 min, which was eligible as the substrate for the sulfur cathode.After that, sulfur was loaded into the PPy/TiO₂ NTs arrays by co-heating sulfur with PPy/TiO₂ NTs composite to form S/PPy/TiO₂ NTs composite. The effect of loading method and heat treatment temperature used in the sulfur loading process on the load amount of sulfur was investigated. It turned out that S/PPy/TiO₂ NTs composite prepared by soaking PPy/TiO₂ NTs arrays in a 1% sulfur solution in toluene had higher load amount of sulfur. The formation of chemical bonding between PPy and sulfur was also discovered during heat treatment. Finally, the electrochemical performances of materials were investigated by testing the assembled coin cells with the as-prepared composites as the cathode. The role of PPy on improving the electrochemical properties of battery was proved by comparing S/TiO₂ NTs, S/PPy/TiO₂ NTs-160 and S/PPy/TiO₂ NTs-300. S/PPy/TiO₂ NTs-160 exhibited the highest initial discharge capacity of 1123.7 mAh/g while S/PPy/TiO₂ NTs-300 showed the most excellent cycle stability with a high discharge capacity of 1150 mAh g-1 and average coulombic efficiency of 96% after 100 cycles, which suggested that the heat treatment temperature used in the sulfur loading process had an important impact on the overall performance of the resultant cell. We attribute this to the change in the molecular structure of sulfur with the increasing temperature. When the heat treatment temperature was 160 oC, sulfur mainly exists as S8 and C-S8 chemical bonds with polypyrrole. When the temperature was elevated to 300 oC, the molecular structure of sulfur consisted largely of S6 and S2, which are more reactive and have more extensive chemical bonds with polypyrrole. The existence of S6, S2 and C-S bond in cathode composite can attenuate the generation and dissolution of soluble polysulfide, improve the utility of active material and ensure the high capacity of battery.