Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li–S Battery

Abstract

The possibility of achieving high-energy, long-life storage batteries has tremendous scientific and technological significance. A prime example is the Li–S cell, which can offer a 3–5-fold increase in energy density compared with conventional Li-ion cells, at lower cost. Despite significant recent advances, there are challenges to its wide-scale implementation. Upon sulfur reduction, intermediate soluble lithium polysulfides readily diffuse into the electrolyte, causing capacity fading and poor Coulombic efficiency in the cell. Herein, we increase the capacity retention and cycle life of the Li–S cell through the use of nanocrystalline and mesoporous titania additives as polysulfide reservoirs and examine the role of surface adsorption vs pore absorption. We find that the soluble lithium polysulfides are preferentially absorbed within the pores of the nanoporous titania at intermediate discharge/charge. This provides the major factor in stabilizing capacity although surface binding (adsorption) also plays a more minor role. A cell containing TiO2 with a 5 nm pore diameter exhibited a 37% greater discharge capacity retention after 100 cycles than a cell without the titania additive, which was optimum compared to the other titania that were examined.

Improving lithium–sulphur batteries through spatial control of sulphur species deposition on a hybrid electrode surface

“…On the basis of the patterned deposition of polysulphides, the strong bonding between polysulphides and the ITO surface provided an alternative route to enhance the performance of Li-S batteries through controlling nucleation and deposition of solid S/Li 2 S species. This ITO-C hybrid interface design is significantly different from the previously reported work on metal oxide additives to suppress the diffusion of polysulphides, such as mesoporous silica 12 , titania 22 , alumina 43 , Mg 0.6 Ni 0.4 O (ref. 44) and metal-organic frameworks 11 , as we demonstrate the importance of engineering the ITO-C hybrid interface in facilitating the polysulphide deposition.…”

“…6). Figure 1e shows an SEM image and EDX mapping of an ITO glassy carbon hybrid electrode after one cycle of discharging to 1.7 V and then charging to 2.6 V. Although the nonpolar charging product (sulphur) has higher affinity with carbon compared with ITO 18,22,27 , the S elemental mapping still matched with the ITO mapping, indicating that more sulphur attaches to the ITO surface than to the carbon surface. This patterning distribution of sulphur after charging is related to the strong bonding between polysulphides and ITO.…”

“…The pioneering work by the Nazar et al 1 on mesoporous carbon particles to encapsulate sulphur demonstrated the relatively high capacity of about 1,000 mAh g À 1 for 20 cycles 10 . Other materials for trapping polysulphides to improve the performance include porous carbon spheres 20 , hollow carbon spheres 14 , hollow carbon nanofibers 15 , graphene oxides 21 , metal oxide particles 22 , metalorganic framework particles 11 , microporous carbon paper 23 and conductive polymers 13,24 . Although these conductive matrixencapsulated sulphur cathodes have shown high specific capacities during the initial cycles, subsequent cycles usually show rapid capacity decay.…”

Sulphur–TiO2 yolk–shell nanoarchitecture with internal void space for long-cycle lithium–sulphur batteries

“…Mesoporous structures, such as those based on carbon, have been shown to help trap polysulphides owing to their small pore sizes (B3 nm) 20,27 . Moreover, metal oxides such as TiO 2 possess hydrophilic Ti-O groups and surface hydroxyl groups, which are known to bind favourably with polysulphide anions, hence further limiting the extent of polysulphide dissolution 21,40 .…”

A modified hierarchical porous carbon for lithium/sulfur batteries with improved capacity and cycling stability

“…Additionally, the dissolution of polysulfide could be restrained through rational design of carbon and electrode structures, as well as the strengthened interactions between the electrode materials and the active species. Evers et al [13] suggested that the soluble lithium polysulfides are preferentially adsorbed in the pores of the porous carbon material during the processes of discharging/charging, moreover, the polysulfide dissolving into the electrolyte can be retained effectively in the small mesopores due to its strong adsorption ability and small size effect. Thus, good control of the pore size distribution can help to improve the electrochemical properties of the sulfur electrodes [14][15][16][17][18][19][20].…”