Synthesis of the sulphur–TiO 2 yolk–shell nanostructures

The sulphur–TiO 2 yolk–shell morphology was experimentally realized as shown schematically in Fig. 2a. First, monodisperse sulphur nanoparticles were prepared using the reaction of sodium thiosulfate with hydrochloric acid (Supplementary Fig. S1). The sulphur nanoparticles were then coated with TiO 2 through controlled hydrolysis of a sol-gel precursor, titanium diisopropoxide bis(acetylacetonate), in an alkaline isopropanol/aqueous solution, resulting in the formation of sulphur–TiO 2 core–shell nanoparticles (Supplementary Fig. S2; the transmission electron microscopy (TEM) image was taken immediately after the electron beam was turned on to avoid sublimation of sulphur under the beam). This was followed by partial dissolution of sulphur in toluene to create an empty space between the sulphur core and the TiO 2 shell, resulting in the yolk–shell morphology. The scanning electron microscopy (SEM) image in Fig. 2b shows uniform spherical nanoparticles of ~800 nm in size. The TEM image in Fig. 2c, taken immediately after the electron beam was turned on, shows sulphur nanoparticles encapsulated within TiO 2 shells (~15 nm thick) with internal void space. Owing to the two-dimensional projection nature of TEM images, the void space will appear as either an empty area or an area of lower intensity depending on the orientation of the particles (Fig. 2c). The TiO 2 in the yolk–shell nanostructures were determined to be amorphous using X-ray diffraction (Supplementary Fig. S3). The ability of toluene to diffuse through the TiO 2 shell to partially dissolve sulphur indicates its porous nature, which is typical of amorphous TiO 2 prepared using sol-gel methods36. The average pore diameter was determined to be ~3 nm using the Barrett–Joyner–Halenda method, which corresponds to a mesoporous structure.

Figure 2: Synthesis and characterization of sulphur–TiO 2 yolk–shell nanostructures. (a) Schematic of the synthetic process that involves coating of sulphur nanoparticles with TiO 2 to form sulphur–TiO 2 core–shell nanostructures, followed by partial dissolution of sulphur in toluene to achieve the yolk–shell morphology. (b) SEM image and (c) TEM image of as-synthesized sulphur–TiO 2 yolk–shell nanostructures. (b) Scale bar, 2 μm. (c) Scale bar, 1 μm. Through large-ensemble measurements, the average nanoparticle size and TiO 2 shell thickness were determined to be 800 and 15 nm, respectively. Full size image

Volume expansion in the yolk–shell nanostructures

Next, we investigated the effectiveness of the yolk–shell morphology in accommodating the volume expansion of sulphur and limiting polysulphide dissolution. The sulphur–TiO 2 yolk–shell nanostructures were drop-cast onto conducting carbon paper to form working electrodes, and pouch cells were assembled using lithium foil as the counter electrode. The cells were discharged at 0.1 C (1 C=1,673 mA g−1) to a voltage of 1.7 V versus Li+/Li, during which a capacity of 1,110 mAh g−1 was attained (Supplementary Fig. S4), and the voltage was maintained for over 20 h. The as-obtained discharge profile shows the typical two-plateau behaviour of sulphur cathodes, indicating the conversion of elemental sulphur to long-chain lithium polysulphides (Li 2 S n , 4≤n≤8) at ~2.3 V, and the subsequent formation of Li 2 S 2 and Li 2 S at ~2.1 V (Supplementary Fig. S4)15,16,17,18. After the discharge process, the contents of the cells (cathode, anode and separator) were washed with 1,3-dioxolane (DOL) solution for further characterization. This polysulphide-containing solution was then oxidized with concentrated HNO 3 and diluted with deionized water for analysis of sulphur content using inductively coupled plasma (ICP) spectroscopy37. For comparison, electrode materials were also prepared using the bare sulphur and sulphur–TiO 2 core–shell nanoparticles and subject to the same treatment.

There was little change in the morphology and size distribution of the sulphur–TiO 2 yolk–shell nanostructures before and after lithiation (Fig. 3a–c). TEM image of a lithiated yolk–shell nanostructure shows a structurally intact TiO 2 coating (Fig. 3d), indicating the ability of the yolk–shell design in accommodating the volume expansion of sulphur. The presence of lithiated sulphur and TiO 2 in the yolk–shell nanostructure was confirmed using energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy (Fig. 3e). In the case of bare sulphur and sulphur–TiO 2 core–shell nanoparticles, random precipitation of irregularly shaped Li 2 S 2 and Li 2 S particles was observed on the electrodes owing to dissolution of lithium polysulphides into the electrolyte (Supplementary Fig. S5)15,16,17,18. ICP analysis performed on the contents of the discharged cells showed a loss of 81 and 62% of the total sulphur mass into the electrolyte for the bare sulphur and sulphur–TiO 2 core–shell nanoparticles, respectively. In comparison, only 19% of the total sulphur mass was found to be dissolved in the electrolyte in the case of the yolk–shell nanostructures, which indicates the effectiveness of the intact TiO 2 shell in limiting polysulphide dissolution.

Figure 3: Morphology of sulphur–TiO 2 yolk–shell nanostructures after lithiation. (a–c) SEM images of sulphur–TiO 2 yolk–shell nanostructures (a) before and (b) after lithiation and (c) their respective particle size distributions. (a,b) Scale bar, 2 μm. (d) TEM image of a sulphur–TiO 2 yolk–shell nanostructure after lithiation, showing the presence of an intact TiO 2 shell (highlighted by arrow). Scale bar, 200 nm. (e) Energy-dispersive X-ray spectrum and electron energy loss spectrum (inset) of the nanostructure in (d), showing the presence of lithiated sulphur and TiO 2 . The Cu peak arises owing to the use of a copper TEM grid. Full size image

Electrochemical performance

To further evaluate the electrochemical cycling performance of the sulphur–TiO 2 yolk–shell nanoarchitecture, 2,032-type coin cells were fabricated. The working electrodes were prepared by mixing the yolk–shell nanostructures with conductive carbon black and polyvinylidene fluoride binder in N-methyl-2-pyrrolidinone to form a slurry, which was then coated onto aluminium foil and dried under vacuum. Using lithium foil as the counter electrode, the cells were cycled from 1.7–2.6 V versus Li+/Li. The electrolyte used was lithium bis(trifluoromethanesulfonyl)imide in 1,2-dimethoxyethane and 1,3-DOL, with LiNO 3 (1 wt%) as an additive to help passivate the surface of the lithium anode and reduce the shuttle effect17,18. Specific capacity values were calculated based on the mass of sulphur, which was determined using thermogravimetric analysis (Supplementary Fig. S6). The sulphur content was found to be ~71 wt% in the yolk–shell nanostructures, accounting for ~53 wt% of the electrode mix, with a typical sulphur mass loading of 0.4–0.6 mg cm−2. The contribution of TiO 2 to the total capacity is very small in the voltage range used in our work38,39.

The sulphur–TiO 2 yolk–shell nanoarchitecture exhibited stable cycling performance over 1,000 charge/discharge cycles at 0.5 C (1 C=1,673 mA g−1) as displayed in Fig. 4a (see also Supplementary Fig. S7). After an initial discharge capacity of 1,030 mAh g−1, the yolk–shell nanostructures achieved capacity retentions of 88, 87 and 81% at the end of 100, 200 and 500 cycles, respectively (Fig. 4a). Most importantly, after prolonged cycling over 1,000 cycles, the capacity retention was found to be 67%, which corresponds to a very small capacity decay of 0.033% per cycle (3.3% per 100 cycles), representing the best performance for long-cycle lithium–sulphur batteries so far. The average Coulombic efficiency over 1,000 cycles was calculated to be 98.4%, which shows little shuttle effect owing to polysulphide dissolution. In comparison, cells based on bare sulphur and sulphur–TiO 2 core–shell nanoparticles suffered from rapid capacity decay, showing capacity retentions of 48 and 66%, respectively, after only 200 cycles (Fig. 4b), indicating a greater degree of polysulphide dissolution into the electrolyte.

Figure 4: Electrochemical performance of sulphur–TiO 2 yolk–shell nanostructures. (a) Charge/discharge capacity and Coulombic efficiency over 1,000 cycles at 0.5 C. (b) Capacity retention of sulphur–TiO 2 yolk–shell nanostructures cycled at 0.5 C, in comparison with bare sulphur and sulphur–TiO 2 core–shell nanoparticles. (c) Charge/discharge capacity and (d) voltage profiles of sulphur–TiO 2 yolk–shell nanostructures cycled at various C-rates from 0.2 to 2 C. Specific capacity values were calculated based on the mass of sulphur. Full size image