Li0.95Na0.05FePO4 as a trifunctional additive to boost the electrochemical performance of cathodes in lithium-sulfur batteries

Abstract

Lithium-sulfur batteries (LSBs) are regarded as one of the effective candidates for next-generation energy storage systems due to their high energy density. Conventional sulfur cathodes suffer from inadequate polysulfide adsorption and catalytic conversion capabilities, along with sluggish ion kinetics, leading to low utilization of active sulfur substances. These limitations hinder the practical application of LSBs. Here, a micrometer-sized LNFP (Li0.95Na0.05FePO4) with enhanced ion conductivity is innovatively proposed as an additive for the cathode to address this challenge. Through theoretical analysis (density functional theory, DFT) and empirical experiments, it is found that Na+ doping not only exposes effective adsorption sites but also alters the electronic environment of Fe²+ /Fe³+, thereby enhancing the adsorption and catalytic conversion abilities of the LFP towards polysulfides. Multifunctional LNFP additive contributes to high-performance for S cathodes. As a result, the assembled LSB with the LNFP additive delivers an initial discharge specific capacity of 953 mAh g-1 at 1 C, exhibits excellent cycling performance with a capacity decay of only 0.039% per cycle after 700 cycles. The prepared LSB retains a specific discharge capacity of 548 mAh g⁻¹ after 300 cycles at 5 C. The LSB demonstrates a discharge specific capacity of 880 mAh g-1 under a high sulfur loading of 4.5 mg cm-2. This work opens up new avenues for optimizing the performance of LSBs.