Li–S batteries could become next-gen energy storage

However, a number of obstacles hinder practical applications of Li–S batteries. One of the major issues is the diffusion of polysulfide intermediates from the cathode, which causes the irreversible loss of active materials and capacity decay.

Nanocarbon with a nonpolar surface as cathode materials cannot provide sufficient binding and confining effects to maintain polysulfides within the cathode. Moreover, the poor electrochemical contact caused by the weak combination between active polysulfides and nanocarbon also impedes the rapid and steady cycling of Li–S cells.

"Heteroatom doping is believed to be a promising route for the adsorption and immobilisation of polysulfide intermediates," says Prof. Qiang Zhang, a researcher from Tsinghua University, China.

"However, the origin of the anchoring effect afforded by heteroatoms is still not clear, which largely limits the improvement of polysulfides adsorption and the rational design of cathode materials."

Most recently, Prof. Q. Zhang and co-workers from Tsinghua University together with Prof. B. Li from Institute of Metal Research reported a theoretical study on the ability of a series of doped nanocarbon materials to trap polysulfides.

It showed that by forming a 'lithium bond' (an analogue of 'H bond'), the chemical modification using N or O dopant significantly enhances the interaction between the carbon host and the polysulfide guests and thereby effectively prevents shuttle of polysulfides.

"For the first time, we conducted a parallel quantum chemical screening process to pick the most effective doping elements that helps confine the polysulfides." Tingzheng Hou, the first author says.

"It turned out that the N and O doping in nanocarbon materials can form a strong dipole-dipole electrostatic interaction, which was for the first time identified as the dominant interaction between doped nanocarbon and lithium polysulfides, while F, B, P, S and Cl dopants were incapable of forming that."

Experimental work reported from other researchers accorded with this predictive result. For instance, the N-doped graphene paper electrode exhibited a high specific capacity of approximately 1000 mAh g-1 after 100 cycles and excellent coulombic efficiency of 98% for catholyte-type Li-S cell.

A much-extended life of over 2000 cycles and an extremely low capacity-decay rate of 0.028% per cycle can thus be achieved.

"To achieve the strong-couple effect towards polysulfides, we proposed a set of rules for the rational design of doped carbon scaffolds in Li–S batteries based on our calculation," says Hou, "With these conditions fulfilled, the doped carbon could offer a strong dipole with lone pair election to form a strong electrostatic dipole-dipole interaction with polysulfides and enhance the interaction. The key factor is the electronegativity of the doping atoms."

To elucidate the importance of electronegativity, an implicit volcano plot relationship correlating the electronegativity of doping atoms to the adsorption energies was proposed by Qiang and coworkers to shed light on the formation of the strong anchoring effect. This relationship affords the latest understanding of the screening and rational design of doped nanocarbon materials for immobilising polysulfides.