Electrochemical verification of the redox mechanism of FeS2 in a rechargeable lithium battery.

The redox mechanism of micro-sized FeS 2 particles in a rechargeable lithium battery is studied by galvanostatic cycling and ac-impedance analysis. It is shown that FeS 2 is irreversibly reduced on the first discharge, turning Li/FeS 2 cell into a combination of Li/FeS and Li/S chemistries, as suggested by two distinct discharge plateaus at ∼1.5 and 2.0 V, respectively. The first discharge consists of an irreversible conversion of FeS 2 to Li 2 FeS 2 intermediate and its subsequent reduction to metallic Fe and Li 2 S. The first discharge suffers an initial voltage delay, suggesting that the discharge progresses in a thermodynamic non-equilibrium condition. The initial voltage delay can be attributed to the large grain boundary resistance (GBR) of pristine FeS 2 particles, which impedes the nucleation of a new solid Li 2 FeS 2 phase causing high polarization. Ac-impedance spectra of the FeS 2 cathode are composed of a semicircle and a straight sloping line, representative of an electrode reaction resistance (R er ) and a Li + adsorption impedance, respectively. The R er is found to decrease progressively during the first discharge and reaches a plateau when the cell is charged above 2.5 V vs. Li/Li + , being consistent with the model that FeS 2 is irreversibly reduced during the first discharge and that the Li 2 S/Li 2 S n redox couple is formed in recharge. It is indicated that Li/FeS 2 batteries face the same problems as Li/S batteries, such as the dissolution of lithium polysulfide, the formation of a redox shuttle, and the loss of sulfur active material. [ABSTRACT FROM AUTHOR]