We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNTs) as an active anode material in sodium ion batteries by expanding the interlayer distance in a few outer layers of multiwalled carbon nanotubes. The performance enhancement was investigated using a density functional tight binding (DFTB) molecular dynamics simulation. It is found that a sodium atom forms a stable bonding with the partially expanded MWCNT (PECNT) with the binding energy of -1.50 eV based on the density functional theory calculation with van der Waals correction, where a sodium atom is caged between the two carbon hexagons in the two consecutive MWCNTs. Wave function and charge density analyses show that this binding is physisorption in nature. This larger exothermic nature of binding energy favors the stable bonding between the PECNT and a sodium atom, and thereby, it helps to enhance the electrochemical performance. In the experimental works, partial opening of the MWCNT with the expanded interlayer has been designed by the well-known Hummer's method. It has been found that the introduction of functional groups causes a partial opening of the outer few layers of a MWCNT, with the inner core remaining undisturbed. The enhanced performance is due to an expanded interlayer of carbon nanotubes, which provide sufficient active sites for the sodium ions to adsorb as well as to intercalate into the carbon structure. The PECNT shows a high specific capacity of 510 mAh g at a current density of 20 mA g, which is about 2.3 times the specific capacity obtained for a pristine MWCNT at the same current density. This specific capacity is higher when compared to other carbon-based materials. The PECNT also shows a satisfactory cyclic stability at a current density of 200 mA g for 100 cycles. Based on our experimental and theoretical results, an alternative perspective for the storage of sodium ions in MWCNTs is proposed.
The practical application of room temperature sodium‐sulfur (Na–S) battery remains far away from the market due to the poor coulombic efficiency and cyclic stability which in turn arises from high dissolution of intermediate polysulfides in carbonate‐based electrolytes, slow reaction kinetics, and low electrical conductivity of sulfur. In this aspect, an advanced cathode structure by incorporating Teflon‐lined conductive carbon substrate (TCS) as a supporting cathode to efficiently immobilize the migration of soluble polysulfides to the anode, as well as enhance the conductivity during the polysulfide conversion, is presented here. After inclusion of TCS, the cell delivers a specific capacity of 800 mAh g−1 at 0.1C (1C = 1672 mA g−1) with 85% enhancement in specific capacity. Also, a stable cyclic stability of about 300 cycles with an excellent capacity retention of about 93% and coulombic efficiency of nearly 100% is obtained. Teflon, which has a linear carbon chain surrounded completely by fluorine atoms, acts as the anchoring sites for trapping the intermediate sodium polysulfides. This study provides a simple approach of controlling the shuttling effect, which can be beneficial for the development of high energy density room temperature Na–S battery.
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