Biomass-derived, activated carbon-sulfur composite cathode with a bifunctional interlayer of functionalized carbon nanotubes for lithium-sulfur cells.

Lithium-sulfur (Li-S) cells are emerging as the dominant constituents of the next generation battery technology, offering high theoretical capacity around 1675 mA h g-1 and the additional advantages of low cost and non-toxic nature. Activated carbon, derived from natural resources is being extensively investigated for applications as electrode materials in high power supercapacitors and for making composite electrodes for designing high energy density electrochemical cells. The present work is aimed at introducing the potential of the composite cathode of sulfur with the biomass-derived, steam activated carbon (AC) along with the free-standing and flexible film of carbon nanotubes as the interlayer for designing efficient Li-S cells. The composite obtained by impregnating sulfur particles into the pores of coconut shell derived and steam activated carbon, subjected to efficient acid washing procedures to attain maximum purity, called as the activated carbon-sulfur (ACS) is used as the composite cathode material. The flexible film of acid-functionalized carbon nanotubes termed as the CNTF placed between the composite cathode and the separator material serves as an active interlayer to boost the performance efficiency of the assembled Li-S cells. The ACS composite is synthesized by the solvothermal method, and the flexible CNTF is obtained by solution casting. The Li-S cells assembled with the ACS composite as the active cathode material and the CNTF as the interlayer are found to exhibit quite impressive discharge capacity and cycling stability. These cells deliver an initial discharge capacity of 1562 mA h g-1 at 0.05 C rate and retain 71% of the initial capacity at 1 C rate after 200 cycles. The conducting and the porous network of the ACS helps to enhance the overall electrical conductivity of the sulfur composite cathode and the highly conducting CNTF interlayer accelerates the electrochemical activity taking place in the cell. The interlayer restricts the polysulfides from migrating to the anode and thereby suppresses the polysulfide shuttle phenomenon. The use of the coconut shell derived, steam activated and acid washed carbon for making the composite cathode with sulfur and the CNTF interlayer, obtained by the acid functionalization of carbon nanotubes is a novel approach to realize Li-S cells with high capacity and excellent cycling stability, which has not yet been pursued in detail.