Improved Electrochemical Properties Of FeS₂as Anode Materials For Lithium-ion Batteries

Pyrite FeS2is an attractive anode material due to its high theoretical capacity (890mAh g-1), low cost and nontoxic. The main work in this research is to improve the electrochemical performance of FeS2by controlling the morphology and modifiing the surface of particles.Porous and solid FeS2particles are synthesized via solid-state reaction method using FeC2O4·2H2O and S powder as the raw materials by adjusting the calcination duration. FeS2particles with different particle sizes are obtained by adjusting the milling duration. The porous FeS2electrode exhibits high discharge capacity, but a little fast capacity fade comparing to the solid one. The faster capacity fade of the porous FeS2electrode during cycling is acsibed to the unstable porous structure, which decomposes into nanoparticles after long cycles. On the basis of the analysis, a theoretical proposal to optimize the structure of FeS2electrode is provided. When the calcination time is2,8,16,30and55h, the obtained mean particle size is1.43,0.96,0.28,0.078and0.071μm, respectively. Influence of particle size on electrochemical performances has been conducted. Compared with other materials, the FeS2powder with mean particle size of0.28μm exhibits high coulombic efficiency, initial discharge specific capacity, low polarization and enhanced electrode process kinetics. Its discharge capacity is420mAh g-1after30cycles. The enhanced electrochemical properties are attributed to the dense powder packing, better electrical contact and relative stable structure during cycling process. Smaller FeS2particles (0.078and0.071μm) have difficulty in dispersing and mixing with carbon black and binder and less dense packing state, leading to a decrease of reactive contact surface area and poor electrochemical performance.Sphere-like FeS2powders with rawtooth are successfully synthesized by hydrothermal method with the assist of cetyltrimethylammonium bromide (CTAB). The FeS2particles synthesized with CTAB with diameters of2-4木m show a sphere-like structure with rawtooth, while the counterpart prepared without CTAB exhibits irregular morphology with diameters range of0.1-0.4μm. The enhanced rate capability and cycling stability are attributed to the less hindered surface layer from the rawtooth-like surface and microsized sphere morphology of FeS2, leading to enhanced process kinetics. Bud-like FeS2microshperes were synthesized by using a solvothermal method with the help of polyvinylpyrrolidone (PVP). The bud-like FeS2microshperes with diameters of2.0-3.0μm consist of the submicro-flakes with0.5-1μm in width, length and60nm in thickness. The bud-like FeS2microshperes exhibits higher coulombic efficiency, initial discharge capacity and lower polarization than the one obtained without PVP. The results of cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) show that the improved electrochemical performance of bud-like FeS2is attributed to the unique structure which provides large surface area and short path lengths for electron and Li+transport, leading to good contact between FeS2and electrolyte and enhanced kinetics during the charge-discharge process.FeS2-PANI composite was synthesized via simultaneous chemical polymerization of PANI in the presence of fine FeS2particles in aqueous suspension. The nanotube-like PANI is incorporated into the FeS2particles. The FeS2-PANI composite exhibits weaker polarization, better reversibility and cycling performance than the FeS2particles. These improvements are attributed to the addition of PANI, which offers conductive pathways between the active particles and enhances the electrical conduction of the electrode. Besides, the buffer connection of amorphous polymer between the particles can accommodate the strain induced by the volume change in the electrode.Carbon coated FeS2(FeS2/C) composite is prepared via a simple solid state reaction using glucose as carbon source. The porous FeS2particles are uniformly surrounded by the amorphous carbon coating. The FeS2/C composite exhibits higher reversible capacity and better cycling performance than the unmodified FeS2. The specific capacity of the FeS2/C composite after50cycles is495mAh g-1, much higher than that of FeS2(345mAh g-1). The cycled electrodes have been analyzed by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP) and X-ray photoelectron spectroscopy (XPS). The improvement is attributed to the introduction of carbon coating, which can enhance the conductivity, reduce the dissolution of sulfur and corrosion from HF, and stabilize the porous structure during cycling.