Aluminum phosphate as a bifunctional additive for improved cycling stability of Li-S batteries

Abstract We demonstrate a facile way to alleviate lithium polysulfide shuttle effect by using aluminum phosphate (AlPO4) as a bifunctional additive in lithium-sulfur (Li-S) batteries. AlPO4 microparticles are synthesized via sol-gel and subsequent calcination process. The effects of AlPO4 on adsorption of lithium polysulfide as well as the performance of Li-S cell are investigated. The Li-S cell containing AlPO4 1.0 wt% retains 76% of the initial capacity after 100 cycles at 0.2 C and it retains 453 mAh∙g−1 at 1.0 C. Using ex-situ X-ray photoelectron spectroscopy and infra-red analysis, it is found that lithium polysulide is chemically adsorbed on AlPO4 through Lewis acid-base interaction with the P O groups in (PO4)3-. The improvement in cycling stability is attributed to the strong interaction between AlPO4 and lithium polysulfides. The adsorbed lithium polysulfides are converted to thiosulfates/polythionates, preventing the dissolution of the lithium polysulfides into the electrolyte and migration to the anode. Various electrochemical measurements including electrochemical impedance spectroscopy and cyclic voltammetry demonstrate that AlPO4 lowers the interfacial resistance and helps fast reversible conversion reaction of sulfur to lithium polysulfides, which enables fast charge/discharge. These results suggest that use of AlPO4 is a convenient and effective way to improve the performance of Li-S batteries.