In the past decade dramatic progress have been made on the development of suitable materials for next generation high energy density storage technologies. Lithium ion batteries proved to be the strongest contender in this field and has revolutionized the portable electronic devices industry. Considering the requirements for electric vehicles, more high energy density materials will be required. The currently available commercial lithium ion batteries have almost reached the ceiling limits of capacity. In this regard, lithium sulfur batteries (Li-S) stands out to be one of the most promising technologies because of its high theoretical specific capacity (1675 mAh/g), low-cost and environmental benignity. However, Li-S batteries severely suffer from fast capacity decay, low coulom...[ Read more ]

In the past decade dramatic progress have been made on the development of suitable materials for next generation high energy density storage technologies. Lithium ion batteries proved to be the strongest contender in this field and has revolutionized the portable electronic devices industry. Considering the requirements for electric vehicles, more high energy density materials will be required. The currently available commercial lithium ion batteries have almost reached the ceiling limits of capacity. In this regard, lithium sulfur batteries (Li-S) stands out to be one of the most promising technologies because of its high theoretical specific capacity (1675 mAh/g), low-cost and environmental benignity. However, Li-S batteries severely suffer from fast capacity decay, low coulombic efficiency and rapid self-discharge. This research reports an effective approach for the improvement of Li-S battery performance by utilizing composites of molybdenum disulfide (MoS₂), carbon (C), tungsten disulfide (WS₂) and graphitic carbon nitride (g-C₃N₄) as sulfur hosts. Moreover, some initial investigation was also conducted with the electro polymerization of electrolyte for the performance enhancement.

Honeycomb like porous MoS₂/C nanosheets were prepared by a simple salt template (NaCl) assisted strategy. The synergistic effect of unique morphology and superior electronic conductivity resulted in excellent cycling and rate performance of the Li-S batteries. The multifunctional host maintained a low capacity fading rate of 0.05% per cycle for 1000 cycles at 0.5C rate. It also demonstrated remarkable rate capability by delivering high specific discharge capacity at current rate of 8C. In addition, the cathode depicted superior capacity retention of 70.4 % for over 300 cycles at 0.2C even at high areal sulfur loading of 4.5 mg/cm². The self-discharge investigation also revealed 96% capacity retention after 3 weeks of resting period.

Proceeding with the knowledge gained from the previously mentioned work, ultrathin sheets of MoS₂/g-C₃N₄ were synthesized by employing a solid-state reaction approach. The polar nature of the composite greatly improved the cycling and rate performance. It maintained an ultralow capacity decay rate of 0.028% per cycle for 400 cycles at 8C rate. At higher sulfur loading of 4.3 mg/cm² the cathode demonstrated low capacity fading rate of 0.07% per cycle after 500 cycles at 0.5C rate. The self-discharge analysis showed 93.6% capacity retention after 10 days of resting.

After confirming the pivotal role played by MoS₂ in the previously discussed works, tungsten disulfide (WS₂) which is another transition metal sulfide was systematically investigated. The porous WS₂/C nanosheets were synthesized by following similar salt template assisted strategy. The polar few layered WS₂/C nanosheets demonstrated remarkable cycling performance by retaining 80.5% of the initial capacity after 500 cycles at 2C rate. The cells also delivered high areal capacity of 4.4 mAh/cm² at 0.2C rate for high sulfur loading of 4.7 mg/cm².

Another useful investigation was carried out by electro polymerizing the electrolyte at high voltage of 4.5V. In this work, Super-P carbon/S was used as the cathode material and the electro polymerization was conducted for different time durations at 4.5V. The cathodes showed significantly improved cycling performance by retaining 71 % of the capacity after 300 cycles at 0.5C rate. This work paves the way for in situ formation of a protective layer on the electrode surface for performance improvement of Li-S batteries.

Overall, the research evidently proves that the strong chemical interaction of MoS₂ and WS₂ with the lithium polysulfides and the favourable architecture of the composite could effectively deal with the major challenges of lithium sulfur batteries.