Energy and environment issues are drawing more and more attentions globally, resulting in demands for good energy storage systems with low-cost, long cycle life, high energy density and safety 1-3. The Li/S battery is one of the most attractive and promising technologies currently 1,2. Rechargeable Li/S batteries use sulfur as a cathode and lithium as an anode. The first sulfur cathode was proposed by Herbet and Ulam in 1962 7 in 1. The overall reaction involved in discharge of a Li/S system is showed below tupian 1. Lithium and sulfur have high theoretical energy density: 2800 Wh kg-1 and 2500 Wh kg-1, which resulting from their high theoretical specific capacities: 3861 mAh g-1 and 1673 mAh g-1 and high average voltage: 2.
15 v 4,9 in 1. As shown in figure.1, It is much higher than current storage technologies such as LiMO2- silicon and LiMO2 -graphite batteries (M = Ni1/3 Mn1/3 Co1/3). A typical Li/S full cell has a sulfur positive electrode and a lithium anode, separated by a carbon additives and binder and an organic electrolyte 10,11 in 1.
There are various steps during discharge in the organic electrolyte. While the first three steps produce polysulfide species, which is soluble in the liquid electrolyte, the last two steps form insoluble Li2S2 and Li2S. A voltage profile () is used to explain the corresponding voltage of two states 1. With respect to the kinetics, the first four steps are fast but the transformation from Li2S2 to Li2S is slow and passivated by slow solid-state diffusion. Additionally, discharge can be stopped when the electrode framework is covered by Li2S so that the voltage decreases greatly 1. ?B2??????Although Li/S battery has significant advantages such as low-cost and high energy density, some challenges limits its commercialization greatly. First, the volumetric expansion of sulfur during the reaction in the electrode. Sulfur and Li2S has different density, which are 2.
03 g cm-3 and 1.66 g cm-3, respectively. It indicates the volume increase by 80% when sulfur is fully transferred to Li2S, inducing the framework is pulverized and capacity decreases fast 1. The second is the solubility of polysulfides in the electrolyte. The soluble polysulfides can react with Li anode and form Li2S in the electrode surface. It also reduces the active materials and increases the passivation. Additionally, the morphology of the cathode can be changed by the process of dissolution and solidification in repeated cycling, resulting in strain inside the electrode and thus decrease the life-time 1. Meanwhile, the dissolution of the polysulfides leads to a phenomenon, called ‘shuttle effect’.
Specifically, long chain polysulfides () can diffuse to the lithium anode and reduced to short chain polysulfides (). After that, these soluble short chain polysulfides also can move back to cathode and be oxidized to long chain polysulfides again. This side reaction takes place continuously and thus creates an internal ‘shuttle’ in the framework of battery, which decreases the Coulombic efficiency greatly by reducing the accessibility of active materials 1 9. The third issue is the ionically and electronically insulation of element sulfur and the discharge product, Li2S.
The electronic resistivity of Li2S is larger than 1014 ohm cm while the Li+ has very low diffusivity in Li2S, which is just 10-15 cm2 s-1 12 in1. It limits the voltage greatly because no further lithiation will be taken place when a thin Li2S layer covers the surface of an electrode, indicating that it is hard to fully transfer sulfur to the discharge product. Consequently, the theoretical capacity is difficult to reach 1,9. Apart from challenges of cathode, lithium anodes also cause safety concerns and energy wastes by using excess lithium 15,16 in1. To address these challenges, many reports focus on designing a reasonable sulfur electrode structure.
An ideal structure probably has six components: (1) enough space for volumetric expansion; (2) suitable amount of active materials to avoid pulverization of structure; (3) short pathways between two electrode to achieve fast electronic and ionic transport; (4) appropriate surface area is required to storage insulating Li2S2, contributing to long cycle life; (5) retain solvated polysulfides effectively inside the cathode; (6) use electrolyte ???? ????