Overcoming the fundemental limitations of lithium sulfur batteries

Abstract

Firstly, the strategies of chemical adsorption are used to control the shuttling of lithium polysulfides. ReS2 nanosheets, nitrogen-doped graphene and few-layer phosphorene nanosheets are used as chemi-cal immobilizers in the cathode structures of Li-S batteries. Experimental results combined with the theo-retical calculations show the existence of strong binding between lithium polysulfides and these chemical adsorbents, which limits the dissolution of lithium polysulfides into the electrolyte, improves electrochemi-cal stability and promotes fast electrochemical reaction kinetics.

Finally, low sulfur loadings in cathode for Li-S batteries greatly offset the advantage in high energy density and limit the practical applications of Li-S batteries. Flexible energy storage devices are another development trend in future, which is limited by the lack of robust, lightweight electrode materials with acceptable electrochemical performance under cyclic mechanical loading. Here, graphene foam based current collector, which can provide a highly electrically conductive network, mechanical support and enough space for a high sulfur loading, is used to obtain a flexible Li-S battery with high energy density, high power density and long cyclic life.

Secondly, the lithium anode protection in the Li-S batteries is investigated. By starting with the nu-cleation and growth of lithium dendrites in lithium-lithium (Li-Li) symmetric cells, a new theory is pro-posed: operation at high current densities can be used to heal lithium dendrites by self (Joule) heating. When large current density is passed through the densely packed lithium dendrites, Joule heating in-duced by the current causes significant self-heating of the dendrites, which triggers extensive surface dif-fusion of lithium atoms, smoothens the dendrites and establishes the equilibrium flat configuration of the lithium electrode. The high current operation is also applicable to Li-S batteries to cure the dendrite prob-lem on the anode side. Repeated doses of high current density healing treatment enable the safe cycling of Li-S batteries with high Coulombic efficiency.

Lithium ion batteries, are highly-efficient energy storage devices, that are widely used in portable elec-tronic devices. However, with the rise of modern-day applications such as electric vehicles and grid ener-gy storage, their specific energy and cycle life remain insufficient. It is therefore necessary to develop high capacity electrode materials and explore new battery chemistries. Lithium sulfur (Li-S) batteries have attracted a lot of attention due to their high energy density and low cost. However, the commerciali-zation of Li-S technology is impeded by technical problems, such as low sulfur utilization, fast capacity decay, lithium dendrite growth and low active material loading. In this thesis, I will present my efforts di-rected towards solving these fundamental problems of Li-S batteries, from materials design to mecha-nism study.