Resorcinol-Formaldehyde Xerogel Derived Highly Porous Carbon Matrix: A Novel Approach for Suppressing the Shuttling Effect in High Rate Lithium-Sulfur Battery
Authors: Gaikwad Mayur Mahendra , Sarode Krishna Kumar , Sharma Chandra Shekhar 
Published: May 2020

Lithium-sulfur battery (LSB) is considered as the next generation battery with a remarkable theoretical specific capacity and energy density of sulfur electrodes as 1672 mA.h.g-1 and 2600 W.h.kg-1 respectively1. However, the bottle necks like insulating nature of sulfur, polysulfide shuttling and volume expansion of sulfur electrodes during cycling are detrimental to practically achieve such astounding electrochemical performance. Hence, the practical application of LSBs are hindered by low rate performance and capacity fading during cycling1-2. In order to address the above issues we have incorporated sulfur in a highly porous nitrogen doped carbon matrix derived from resorcinol-formaldehyde (RF) xerogel to synthesize a composite (a-N-RFC/S) cathode electrode for LSB. The high surface area and pore volume along with the nitrogen doping in RF-derived carbon xerogel imparts conductivity, accommodate volume expansion and restricts polysufide shuttling when tested electrochemically. As a result, the above synthesized composite a-N-RFC/S exhibits superior rate performance and stable cyclic performance.
Nitrogen doped RF xerogel was prepared by sol-gel polycondensation reaction followed by sub-critical drying. The prepared xerogel was pyrolysed at 900 °C to obtain carbon xerogel which was subsequently activated with KOH under N2 atmosphere. The resultant a-N-RFC was subjected to melt diffusion for sulfur incorporation to yield a-N-RFC/S composite. The electrode was prepared by mixing composite, binder and carbon black in 75:10:15 ratio. As prepared electrode was used as working electrode and lithium foil as the counter and reference electrode in a coin-cell assembly.
The N2 sorption studies reveal a high surface area of 1589 m2.g-1 was obtained for a-N-RFC due to the presence of micro as well as mesopores with a total pore volume of 0.96 cm3.g-1 after activation. Furthermore, the electrochemical performance of the prepared composite was carried out by subjecting it to rate capability studies at different current rates. The a-N-RFC/S composite delivered a reversible capacity of 1283, 814, 518 mAh.g-1 at 0.4, 4 and 10 C rate respectively with almost 100% coulombic efficiency. Even at current rate as high as 20 C, it delivered a specific reversible capacity of 497 mAh.g-1. Interestingly, when the current rate was swapped back, it maintained the capacity values suggesting an excellent capacity retention.
Such remarkable electrochemical performance can be attributed to the high specific surface area of the host material which can accommodate the volume expansion of sulfur and confine the polysulfides in it during cycling. Moreover, nitrogen doping chemically absorb the higher order polysulfides and converts into Li2S1. Thus, polysufides are restrained in the host material by physical as well as chemical sorption leading to extraordinary performance. A detailed characterization of the prepared carbon host and composite electrode material along with their electrochemical performance will be presented during the conference meeting.

1. Chao Deng, Zhuowen Wang, Shengping Wang & Jingxian Yu, Inhibition of polysulfide diffusion in lithium–sulfur batteries: mechanism and improvement strategies. Mater. Chem. A, 7, (2019), 12381.
2. Sang-Kyu Lee, Yun Jung Lee, & Yang-Kook Sun, Nanostructured lithium sulfide materials for lithium-sulfur batteries, Power Sources, 323 (2016) 174-188.

Figure 1