CONICET | Buscador de Institutos y Recursos Humanos

High power high density lithium rechargeable batteries are necessary to meet the energy demand of electric vehicles and high-power stationary grids. Li-S batteries represent a potential solution to obtain high performance electrochemical energy storage devices. This is so because of their high theoretical specific capacity 1672 mAh g-1 and high energy density 2600 Wh kg-1.[1] However, there are many difficulties to be solved before a practical application. Sulfur has a high electronic resistivity (1024 Ωcm)[2], and the reactions with lithium lead to an increase in volume (80%) that may result in the rupture of contact with the current collector[3]. Another important issue is the formation of long chain polysulfides (Li2Sn, n ≥ 4) during sulfur lithiation. Polysulfides are soluble in the commonly used ether-based electrolytes and therefore can migrate to the anode where they undergo further reduction, which ultimately leads to the passivation of the lithium anode.[4] This process is known as the shuttle effect, which also causes the loss of active material by conversion to non-reversible structures during cycling. In this work we used sulfur impregnated in activated carbon as a cathode for Li-S batteries. In this way, a conductive matrix was used in order to contain the volume-change during the charge and discharge processes and increasing the conductivity of the electrode. Also, in order to retain the polysulfides in the cathode and prevent their migration to the anodic part of the cell a nanostructured carbon (NC) was used to directly coat the sulfur cathode. XPS measurements were made on both NC-coated and uncoated cathodes, the S2p spectrum showed that the sulfur in the cathode interacts with the NC promoting the formation of COSO2-/SO32- species before cycling. These species can help to immobilize sulfur atoms and therefore serve as mediators to retain and confine polysulfides in the cathode. Electrochemical studies of cyclability, rate capability, cyclic voltammograms and electrochemical impedance studies (EIS) were carried out for both the pristine and NC coated cathodes. The cyclability study showed that the NC-coated cathode had an excellent stability with more than 200 cycles, capacities of 75% with respect to the theoretical capacity of sulfur and coulombic efficiencies of 99.7%. The C-rate proved that this system could reach reversible capacities up to 950 mAh g-1 at rates as high as 1C. EIS and cyclic voltammetries showed that there is a significant increase in the electrical and ionic conductivity of the NC-coated cathode. In this manner, through a simple modification the electrochemical performance of the system was be substantially improved in a in a simple and economic methodology.