Lithium Iron Phosphate Enhances the Performance of High-Areal-Capacity Sulfur Composite Cathodes

Lithium iron phosphate (LiFePO₄, "LFP") was investigated as an additive in the cathode of lithium-sulfur (Li-S) batteries. LFP addition boosted the sulfur utilization during Li-S cycling, achieving an initial capacity of 1465 mAh/g_S and a long cycle life (>300 cycles). Polysulfide adsorption experiments showed that LFP attracted polysulfides, and thus, the presence of LFP should alleviate the shuttle effect, a common failure mode. Postmortem characterization found iron phosphides, iron phosphates, and LiF in the electrode, indicating that LFP underwent dynamic reconstruction during Li-S cycling. We suspect that the formation of these species played a role in the observed performance. From the processing standpoint, adding LFP improved slurry rheology, making the preparation of a high-loading electrode more consistent. Benefiting from the high sulfur utilization and the ability to prepare electrodes with high mass loading, the S-LFP hybrid cell showed an excellent areal capacity of 2.65 mAh/cm² and could be stably cycled at 2 mAh/cm² for 250 cycles. Our results demonstrated the LFP addition as a promising strategy for realizing Li-S batteries with high sulfur loading and areal capacity.