Understanding the role of conductive polymer in thermal lithiation and battery performance of Li-Sn alloy anode

There is an urgent demand to develop high-capacity electrode materials for electric vehicles. LixSn alloy is one promising anode due to its pre-stored lithium and high capacity, but faces several challenges, especially high chemical reactivity and huge volume variation. Here, we conformally coated a conductive polypyrrole on the Sn nanoparticle, and first time studied the protection effect of the polymer on thermal lithiation process of the Sn nanoparticles. By tuning the thickness of polypyrrole from 8 to 40 nm, we find that the optimized coating layer (20 nm) keeps intact during the thermal lithiation, and ensures LixSn nanoparticles a very high stability in dry air condition. Moreover, the flexible and conductive polypyrrole accommodates huge volume variation of LixSn nanoparticles and enhances the electronic connections of interparticles. As a result, the LixSn@PPy (20 nm) composite maintains 75% of its prelithiated capacity after exposure to dry air for 5 days and delivers a stable reversible capacity of 534 mA h g(-1) for 300 cycles. This anode material is also paired with traditional LiFePO4 cathode to achieve a stable full cell cycling.