Structure, Performance And Lithium Storage Mechanism Of Conductive Sulfur-containing Polymers

Elemental sulfur and organosulfide have been proposed as promising cathode materials for the next generation of high-performance rechargeable lithium batteries due to their high theoretical capacity. However, they exhibit low utilization and fast capacity fade in lithium batteries due to their electrically and ionically insulated nature as well as solvent-solubility. According to the specialties that thermally structurizing polyacrylonitrile comes into being a conductive macromolecule and sulfurizing polymer forms multi-sulfur bonds, conductive sulfur-containing cathode materials (CSM) have been synthesized here by heating a mixture of polyacrylonitrile and elemental sulfur with the method of solid-state heating reactions. As electrically conductive, infusible and insoluble as well as electrochemically reversible, CSM overcomes the difficulties that sulfur and organosulfides suffered from as electrode active components.The fundamental chemical structure and condensed-matter structure of CSM have been characterized. Solid-state ~13C-NMR technique assigns definitely the formation of C=C and C=N bonds. The Assignments of the peaks located at the low frequency range of 1000-400cm~-1 in FTIR indicates the existence of C-S and S-S bonds and Raman spectra further confirms those. The thermal reaction mechanism between PAN and elemental sulfur is also proposed. CSM is composed of dehydropyridine type matrix with S-S bond in side-chain. The double sulfur bond is intraconnected or interconnected in the main chain. Such specific chemiacal structure makes CSM possess the properties of main-chain providing electric conductivity and disulfur side-chain functioning as energy storage. XRD test shows that the condensed-matter structure of CSM can be regarded as an amorphousstate formed by the random packing of graphite-like crystallite. Morphologically CSM presents as aggregated particles with average diameter less than 250nm, in which there also exist a lot of nano-sized pores with diameter less than 2nm from TEM and SEM photographs.The conductivity and electrochemical performance of CSM have been tested. The conductivity of CSM450 at room temperature is 10"8~10"7S cm"1 by the method of AC impedance and DC chronoamperometry. The cycle behaviors and rate discharge properties of different samples prepared at 250 — 800 °C in secondary lithium cells have been examined. The results show that CSM450 sample has the best electrochemical performance. The cell of Li/EC-DMC+lM UPF6/CSM450 exhibits high capacity, long cycle life, unique rate-property and low self-discharge ratio relatively. A stable discharge capacity for CSM450 maintains about 470 mAh/g and the capacity retention remains ca. 90% (refer to the second cycle) after about 400 cycles. Its rate capacity remains 450mAh/g when discharging at 2C rate-current. Such high and stable rate-cycle performance is a great progress for sulfur-containing cathode materials.The mechanism of lithium storage in CSM450 has been discussed. Cyclic voltammetry and vibration spectra results show that at the potential range from 3.0 to 1.0 V vs. Li/Li+, the main form for lithium storage in CSM450 originates from the 2e electrochemical reduction of double sulfur bond. However, the practical specific capacity of CSM450 excesses the theoretical specific capacity of the scission for S-S bonds. On the analogy of the lithium insertion/extraction mechanisiom of specific carbon materials, the speculation that nano-sized cavity and crystallite clearance accommodate a small amount of lithium ion has been proposed.