Preparation And Electrochemical Performance Of Composite Cathode Materials For Li/S Batteries

To satisfy the demands for efficient energy storage employed in smart grids, electric vehicles and renewable energies, development of novel battery systems with remarkable specific capacity, low-cost and eco-friendly becomes an imminent task for researchers. Compared to the traditional lithium ion batteries(LIBs), lithium-sulfur(Li-S) batteries have attracted increasing attention as the next-generation energy storage devices, owing to their extremely high theoretical specific capacity(1675 mAh g-1) and energy density(2600 Wh kg-1) of sulfur. In addition, sulfur is low-cost, earth abundantly and eco-friendly, making this system attractive for large scale applications. Despite these considerable advantages, the commercialization of Li-S battery is slowed down by several technical limitations. First, the poor intrinsic conductivity of pristine sulfur results in low utilization of sulfur cathode; second, the dissolution of polysulfide intermediates produced during electrochemical reactions, migrated out of cathode leading to the loss of active mass. Furthermore, the gradual loss of active mass from the cathode into the electrolyte and onto the Li metal anode results in ‘shuttle effection', severe self-discharge, increased-resistance, low coulombic efficiency and fast capacity decay on cycling, which hinder the way of Li-S batteries for practical applications.Mesoporous carbon(MC) was proved to be an effective candidate to improve the sulfur utilization and restrained the solubility of lithium polysulfides due to its excellent conductivity, large surface area, large pore volume and low cost. Characteristics of the MC host: Firstly, high surface area(1432 m2 g-1), pore diameter(6~7 nm) as well as large pore volume(2.89 cm3g-1) for high active mass accommodation. MC/Ss have been synthesized with different ratio of MC to S, 1:1, 1:2, 1:3, 1:4, separately. The optimized ratio is got by evaluating their electrochemistry performance. The MC/S-2 shows the enhanced performance of the best cycling life with a high initial discharge capacity of 909 mAh g-1, after 40 th cycles, the discharge capacity retention is 78.0% with capacity of 709 mAh g-1. The nano-scaled architecture accommodating nano-sized sulfur composite shows enhanced utilization of the active sulfur, and shortens the distance of ionic and electronic transportation. Simultaneously, the ability of adsorbing sulfur of porous carbon refrains the discharge product from deposition into insulated production, which alleviates polarization and extends cycle life of Li-S battery.Nitrogen doped mesoporous(3~4 nm) carbon(NMC) with high specific surface area(1599 m2g-1), large pore volume(1.53 m3 g-1) and stable nitrogen content has been prepared by in situ doping through one step carbonization to immobilize sulfur for lithium–sulfur batteries. NMC@S is prepared by melting method. The structure and composition of the prepared NMC and NMC@S composites are confirmed with comprehensive characterization. In comparison with free doped mesoporous carbon(MC), As a result, the NMC@S composite with high sulfur loading of 65% exhibits a higher initial discharge capacity of 898 mAh g-1, improving 8% of sulfur utilization. NMC@S also shows an enhanced cycling stability of 388 mAh g-1 after 100 cycles at 100 mA g-1, while the MC@S delivers 220 mAh g-1 with free doped. The improvement of electrochemical properties of Li–S batteries could be attributed to the interaction between the nitrogen functionalities on the surface of NMC and polysulfide as well as the enhanced electronic conductivity of the carbon matrix.