Lithium Iron Phosphate Reconstruction Facilitates Kinetics in High-Areal-Capacity Sulfur Composite Cathodes

Lithium-sulfur (Li-S) batteries have been recognized as one of the most promising choices beyond lithium-ion batteries (LIB), because of its low cost and high theoretical specific energy (~2510 Wh/kg or ~10 times of LIB). However, Li-S batteries still face a few challenges, including large volume expansion, poor conductivity, low active material loading, inert end products, and polysulfide crossover called the “shuttle effect”, etc. To address these challenges, we have incorporated lithium iron phosphate (LFP) into our sulfur composite cathode. The addition of LFP enabled a more uniform slurry rheology, which allowed mass loading to double the amount of typical sulfur cathodes. Meanwhile, LFP can effectively adsorb polysulfides, which restricted the shuttle effects common in high-sulfur-loading batteries. Our LFP-hybrid Li-S batteries showed high areal capacity for 300 cycles under both low- and high-current charge-discharge cycles. More importantly, our characterizations demonstrated that LFP in Li-S batteries can reconstruct into Fe2P during cycling. We propose that Fe2P is an effective electrocatalyst for anchoring polysulfides. To unveil the role of Fe2P, we have directly incorporated these materials into the sulfur composite cathode. Using a hydrothermal synthesis, we showed that Fe2P nanoparticles can be directly anchored on the sulfur-carbon composite. This approach caused minimal phase separation and enabled a uniform morphology. We presented the analysis of the cathodes by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). These results allow us to develop a mechanistic hypothesis and a comparison between Fe2P and LFP in terms of the electrochemical performances.