The Investigation Of Modified Sulfur Cathode Materials In Lithium-Sulfur Battery

Notably, conventional LIB couldn’t meet human’s need any more for the ever-increasing progress of technologies and change of human life style. Therefore, the demand for advanced batteries with high energy density, long cycle life and low cost drives the urgent search for new systems with superior performance over current technologies. The Li/S battery is an attractive and promising candidate among emerging battery technologies, due to its high theoretical energy density of 2600Wh/Kg, as well as low cost, natural abundance and nontoxicity of elemental sulfur. However, several practical problems have delayed widespread application of sulfur, such as poor conductivity, low utilization of active material, large volume expansion, the dissolution of intermediate products, etc. This fact necessitates tremendous efforts of addressing these challenges. Most of strategies have been considered to provide one or more porous matrix based on conductive agent, binder and physical adsorbent via physical or chemical mixing with elemental sulfur, so that the small size of sulfur particles fixed on the frame of matrix or the pore evenly. Another approach is coating sulfur particles with conductive carbon materials or polymers. In this paper, we intend to improve the electrochemical behavior of sulfur cathode from two different perspective——physical absorption and chemical absorption.Polyaniline (PANi) has gained popularity among conductive polymers owing to its electrical, electrochemical, and optical properties, as well as its free volume and excellent environmental stability. Thus, based on previous’ work, a ternary polyaniline/sulfur/acetylene black (PANi@S-C) composite cathode with variable PANi content has been synthesized by a continuous two-step liquid phase route. And the optium amount of Polyaniline was also discussed. The conclusions showed as follows:(1) The PANi@S-C3 composite, which possessed with 12.5 wt% PANi and 46.96 wt% sulfur, and a coating layer of 5～10nm, presents the optimum electrochemical performance. (2)The Nitrogen atoms in PANi have no chemical effect on sulfur. That’s say, there is only physical absorption between PANi and active materials. Compared with S-C composites, the polymer skin not only maintained good conductivity, but also mitigated the dissolution of intermediate products. Thus, sulfur utilization, cyclic durability, and the rate capability of the PANi@S-C3 electrode were significantly improved. (3)This electrode had an initial discharge capacity of 1257 mA h g-1 and retained a discharge capacity of ～600 mA h g-1 after 100 cycles at a current density of 0.16 mA cm-2. After a rate test from 0.1 mA cm-2 to 1 mA cm-2, the cell remained at～600 mA h g-1 when the density returned to 0.1 mA cm-2.Besides, in view of current existing problems about the sulfur cathode, we try to utilize the chemical adsorption of carbon material to improve the electrochemical performance of sulfur. However, considering that the doping of nitrogen atoms in supporting matrix to improve its electrochemical properties are limited, or doping the sulfur atoms in the matrix would produce different reaction for the active materials as well as the discharge products. Therefore, based on polyaniline as the precursor, nitrogen-doped carbon materials were obtained, and then sulfur atoms was in-situ incorporated at high temperature. As a matrix, the preparation of sulfur-nitrogen co-doped carbon materials (SNC)/sulfur composites (SNC-S) were carrried out. (1) sulfur is uniformly anchored over the entire framework of NC as additional defects to provide more lithium storage sites. XPS revealed the formation of strong chemical bonding (-C-S-C- and C-SOx-C (x=2-4)) in the structure of SNC materials. (2)The strong adherence of the sulfur atoms on carbon materials, which was induced by the heat treatment, immobilized sulfur and polysulfides species, influenced the reversible dissolving balance of long-chain polysulfides and suppressed the "shuttle effect", resulting higher utilization of sulfur, higher coulombic efficiency and better capacity retention. (3) EIS also suggested that the strong bonding improved the interfacial characterization between the electrode and electrolyte, then enabled rapid electronic/ionic transport and improved electrochemical kinetics, therefore good rate capability was obtained. We hope this facile approach of the SNC matrix would provide a practical research train of thought for lithium sulfur batteries.