Synthesis And Electrochemical Performance Of Cathode Material Based On Porous Metal For Li–S Batteries

Lithium–sulfur(Li–S) battery, one of the new second batteries, has a high energy density of 2600 Wh kg-1 which is much higher than the commercial Li–ion batteries. Sulfur is the cathode material of Li–S batteries, which has a theoretical specific capacity of 1675 mAh g-1 with advantages of abundance, low price, environmental friendly and is a potential cathode material. However, the Li–S batteries have not been commercial due to the problems of low utilization of active materials and short cycle life caused by low conductivity of sulfur and the shuttle effect of polysulfides dissolved in electrolytes.In the paper, the copper foam and nickel foam produced commercially have been employed as porous metal matrix for sulfur to solve the above problems. The chemical bonding is formed between sulfur and the porous metal matrix to improve the conductivity of sulfur and simultaneously restrain the active materials and polysulfides within the cathode region, thus, promoting the coulombic efficiency and cycle life. On the above basis, the bamboo charcoal(BC) has been filled into the pores of the porous metal through the pressure reduced method to effectively employ large pores to load sulfur, increasing the sulfur loading and enhancing the energy density of the Li–S batteries. The main contents and conclusions are following.Based on the strong combining ability of copper to liquated sulfur, the hybrid sheets CuS monolith cathodewas synthesized on copper foam by the heat treatment at low temperature firstly. The electrochemical performance, such as charge and discharge properties and cycle life have been tested and analyzed. The prepared CuS monolith cathode showed an initial discharge specific capacity of 185.1 m Ah g-1 at 0.2 C rate(1C = 560 mA g-1) and increased to 468.3 mAh g-1 after 100 cycles. The cycle performance of the CuS cathode materials is better than pure sulfur and reported CuS. The structures changes of CuS cathode during cycling showed that CuS materials active gradually, which played a significant role on the cycle stability of CuS cathode.Prepare sulfur on the nickel foam and make chemical binding between the interface of sulfur and nickel simultaneously, to constraint the polysulfides and ensure the high specific capacity. In the paper, the sulfur was deposited on the nickel foam via the chemical solution technique to get the nickel–sulfur cathode, in which the chemical binding has been verified. The electrochemical performances of the nickel–sulfur cathode have been studied. The nickel–sulfur cathode delivered both the redox behaviors of sulfur and Ni S. Nickel–sulfur cathode displayed an initial discharge specific capacity of 1340 mAh g-1 at 0.5 C rate and kept at 493 mAh g-1 after 500 cycles, meanwhile, the coulombic efficiency maintained at nearly 100%. The cathode could deliver a discharge capacity of 212 mAh g-1 at a large current rate of 5 C, namely 8 A g-1, showing a good cycle and rate performance. The good cycle and rate performances of the nickel–sulfur cathode could owe to the strong interface binding between sulfur and nickel and the three-dimensional porous structure of it, which not only constraint the polysulfides but also promise the continuous immersion of electrolyte to shorten the ion and electron length.The large pores of copper and nickel foam were filled by strong actived carbon to load more active materials and construct the electron passageway, utilizing the space of the cathode fully and then improving the density. The bamboo charcoal(BC) were filled into the large pores of metal foam. The sulfur/BC composite materials with sulfur content of 60 wt.% were prepared through the melt-diffusion method, and then how the sulfur existed in BC and its electrochemical performance have been studied. The sulfur/BC composite materials showed a lower capacity fading rate of 0.079% per cycle. For sulfur/BC cathode materials, a passivation layer was formed on the anode surface during cycle to reduce shuttle effect and enhance the cycle performance. Thus, BC is a proper matrix for sulfur.The BC slurry was filled into the large pores of nickel foam and copper foam by the pressure reduced method. A sulfur loading of 3–4 mg was obtained for the nickel foam/BC. The cathode with a sulfur loading of 4 mg displayed an initial specific capacity of 527 mAh g-1 and remained at 412 mAh g-1 after 100 cycles with a coulombic efficiency of 92.8%. The sulfur loading in Cu/BC composite was 6–15 mg, which was caused by the formation of CuS and Cu7.2S4 phase to encourage more sulfur. An initial discharge capacity of 613 m Ah g-1 was obtained at 0.1 C and remained at 372 mAh g-1 after 14 cycles with a coulombic efficiency of 98% when sulfur laoding was 8.6 mg.The paper studied the combination effect between sulfur and copper and nickel based on metal foam. The BC was employed to construct electric channel in the metal foam to load more sulfur. The work provides the experimental basis for study of effect of metal on sulfur cathode and broadens the study of sulfur cathode.