Lithium sulfur batteries have attracted recent attention as the secondary batteries with a high specific energy, a long lifespan, and a high safety. Inhibiting the shuttle effect of polysulfides and improving reaction kinetics become challenges. The chemical interactions between polar metal compounds and polysulfide ions (i.e., polarity polarity interactions, Lewis acid-base interactions, and sulfur bond chain reactions) have some advantages in inhibiting the shuttle effect of polysulfide ions. In addition, many metal compounds exhibit an electrocatalytic activity during the conversion of polysulfides, promoting the conversion rate of polysulfide ions during charging and discharging, suppressing the uneven deposition of insoluble sulfides Li₂S₂/Li₂S on the electrode surface, and thus achieving a goal of improving the utilization rate of active substances, inhibiting shuttle, and reducing the loss of active substances. The inherent conductive properties of conductive polymers are conducive to electron conduction, and their soft and elastic properties can effectively buffer the volume expansion of active substances during charge and discharge processes. The functional groups rich in them have a strong affinity for LiPSs and can inhibit shuttle effects. In this paper, Fe₂O₃@PPy composite materials were prepared via in-situ growth of polypyrrole on Fe₂O₃ nanosheets by acid etching.

Fe₂O₃ nanosheets were modified by a simple one-step method with polypyrrole to prepare high-performance lithium sulfur battery composite cathode materials. Fe₂O₃ nanosheets were placed in a solution of p-toluenesulfonic acid, and the surface was exposed to Fe³⁺ by acid etching. Fe³⁺ acted as an oxidant to promote the oxidation polymerization reaction of pyrrole monomers on the surface of the nanosheets. Also, p-toluenesulfonic acid was doped into the molecular structure of polypyrrole to obtain Fe₂O₃@PPy composite nanosheets. A series of Fe₂O₃@PPy composite nanosheets were prepared via adjusting the etching environment and the feeding amount of Fe₂O₃ nanosheets to control the modification amount of polypyrrole.

The in-situ growth method induced by acid etching was used to modify the surface of α-Fe₂O₃ nanosheets with polypyrrole, in which p-toluenesulfonic acid played an etching and doping role. Fe³⁺ ion etched on the surface of Fe₂O₃ nanosheets is used as an oxidant, allowing pyrrole to polymerize and grow on the surface of Fe₂O₃ nanosheets. P-toluenesulfonic acid is doped into the molecular structure of polypyrrole as a para anion, resulting in Fe₂O₃@PPy composite nanosheets. The obtained Fe₂O₃@PPy nanocomposites combine the catalytic active sites exposed by Fe₂O₃ nanosheets with a high conductivity and a high specific surface area of polypyrrole. This increases the chemical adsorption of polysulfides and inhibits their shuttle, and accelerates the conversion of soluble LiPSs to insoluble products, greatly improving the utilization rate of sulfur, helping to improve the ion/electron transfer rate of nanosheets, and enhancing the reaction kinetics of electrode materials. Fe₂O₃ nanosheets form reinforced chemical bonds with LiPSs and promote the conversion of polysulfides, which effectively alleviates the shuttle effect of polysulfides, and improves the Coulomb efficiency, cycle stability and capacity retention of batteries. Polypyrrole enhances the conductivity of Fe₂O₃ nanosheets, their surface ion and electron conductivity, and improves Li⁺ diffusion kinetics. At a molar ratio of p-toluenesulfonic acid to Fe₂O₃ of 12∶1, Fe₂O₃@PPy composite nanosheets exhibit a superior electrochemical performance. The discharge specific capacities of the S@Fe₂O₃@PPy-3 cathode at 0.1, 0.2, 0.5 C and 1.0 C are 734.7, 576, 468.7 mA·h·g^-1 and 405.2 mA·h·g^-1, respectively. At a current density of 0.1C, S@Fe₂O₃@PPy-3 cathode can recover more reversible capacity with a discharge specific capacity of 519.5 mA·h·g^-1, having a superior rate performance. S@Fe₂O₃@PPy-3 provides an excellent cycling performance, maintaining a capacity of 414.5 mA·h·g^-1 even after 500 cycles at a high rate of 1 C with a capacity retention rate of 86.2%. This material can be used to the development of power batteries.

The synergistic effect of Fe₂O₃ and PPy on the electrochemical performance improvement of lithium sulfur batteries was effective. According to the change in the relative content of polypyrrole on the surface of Fe₂O₃ nanosheets, the conductivity of Fe₂O₃ nanosheets improved with the increase of polypyrrole content. However, the excess polypyrrole nanoparticles covered the surface of Fe₂O₃ nanosheets at the excessive polypyrrole, leading to a decrease in active sites and in battery performance. In summary, as a carrier for sublimation of sulfur, Fe₂O₃@PPy could promote the adsorption of polysulfides and alleviate their shuttle effect, improving the utilization rate of sulfur in lithium sulfur batteries.