A Polymorphic FeS2 Cathode Enabled by Copper Current Collector Induced Displacement Redox Mechanism

In this contribution, we fabricated a composite consisting of two polymorphs of FeS2, pyrite (P-FeS2) and marcasite (M-FeS2), for high-performance Li-FeS2 battery. A series of electrochemical, microscopic, and spectroscopic characterizations indicate that the introduction of metastable M-FeS2 into P-FeS2 enables the four-electron reduction between FeS2 and lithium to generate Fe and Li2S, providing a high specific capacity of 894 mAh/g with specific energy over 1300 Wh/kg. Moreover, it is verified that the electrochemical irreversibility of this composite toward lithium storage is mainly rooted in the shuttle effect, caused by the elemental sulfur which is inevitably produced during the oxidation process of Li2S and Fe. To tackle this issue, copper (Cu) current collector is adopted to chemically immobilize the soluble lithium polysulfides and fundamentally alter the reaction pathway. It is shown that compared with Fe, Li2S prefers to react with Cu current collector to generate Cu2S through the thermodynamically facile displacement reaction mechanism benefiting from the similar lattice framework between Cu2S and Li2S. Such displacement reaction without lattice reconstruction renders the composite superior rate capability (∼730 mAh/g@2 A/g) and long lifespan (89.7% capacity retention after 3200 cycles). Present work allows for the fabrication of high-performance electrodes based on metal chalcogenides.