The Development of Room Temperature Sodium-Sulfur Batteries

Abstract

The objective of this thesis is to mitigate the major challenges that are associated with the sulfur cathode material in RT-NaS batteries. In this study, a successful attempt has been made to enhance the energy density and cyclability of RT-NaS batteries. Throughout the thesis, the modification of an electronically conductive cathode scaffold has been carried out. A high energy-density room-temperature Na-S battery has been practically realized by incorporating a CC@MnO2 substrate containing Na2S6 catholyte as an efficient multifunctional cathode. The Na-S cell offered an initial energy density of 946 W h kg–1, and retained 728 W h kg–1 after 500 cycles. The CC@MnO2@Na-alg substrate infiltrated with Na2S6 catholyte has delivered an initial reversible specific capacity of 882 mA h g–1, retaining 838 mA h g–1 after 1000 cycles. Addition of alginate results in an enhancement in specific capacity and capacity retention of CC@MnO2 architecture by 37% and 10%, respectively. Room temperature sodium-sulfur batteries achieving a high nominal cell potential of 1.85 V have been successfully demonstrated using an activated carbon cloth as current collector and polysulfide reservoir. The activated carbon cloth significantly catalyzes the conversion of intermediate trisulfur radical monoanions to the end-discharge products. We propose that a coupling between the unpaired electrons of activated carbon cloth and trisulfur radical monoanions is responsible for observed catalytic enhancement in the kinetics, addressing several major issues that these batteries face. The achievement of an increased nominal cell potential, combined with excellent specific capacity and capacity retention at practical rates, produce a step forward in cell energy density with extremely stable cycle life. These properties, combined with inexpensive starting materials and manufacturing should propel the RT Na-S battery closer to practical application. The report also discusses another room-temperature sodium-sulfur battery achieving high specific capacity with long-term stability that has been practically realized using indium tin oxide nanoparticles decorated activated carbon cloth as a novel current collector. Activated carbon cloth, apart from providing a continuous electron conduction pathway, catalyses the conversion of higher-order sodium polysulfides to lower-order sodium sulfides. The rational design of a hybrid current collector described here will open new possibilities to enhance the electrochemical performance of the RT Na-S.