High-performance hollow sulfur nanostructured battery cathode through a scalable, room temperature, one-step, bottom-up approach

Abstract

Sulfur is an exciting cathode material with high specific capacity of 1,673 mAh/g, more than five times the theoretical limits of its transition metal oxides counterpart. However, successful applications of sulfur cathode have been impeded by rapid capacity fading caused by multiple mechanisms, including large volume expansion during lithiation, dissolution of intermediate polysulfides, and low ionic/electronic conductivity. Tackling the sulfur cathode problems requires a multifaceted approach, which can simultaneously address the challenges mentioned above. Herein, we present a scalable, room temperature, one-step, bottom-up approach to fabricate monodisperse polymer (polyvinylpyrrolidone)-encapsulated hollow sulfur nanospheres for sulfur cathode, allowing unprecedented control over electrode design from nanoscale to macroscale. We demonstrate high specific discharge capacities at different current rates (1,179, 1,018, and 990 mAh/g at C/10, C/5, and C/2, respectively) and excellent capacity retention of 77.6% (at C/5) and 73.4% (at C/2) after 300 and 500 cycles, respectively. Over a long-term cycling of 1,000 cycles at C/2, a capacity decay as low as 0.046% per cycle and an average coulombic efficiency of 98.5% was achieved. In addition, a simple modification on the sulfur nanosphere surface with a layer of conducting polymer, poly(3,4-ethylenedioxythiophene), allows the sulfur cathode to achieve excellent high-rate capability, showing a high reversible capacity of 849 and 610 mAh/g at 2C and 4C, respectively.

Fig. 1.

Schematic illustrations of the structure and fabrication process of PVP-encapsulated hollow S nanospheres. (A) A Schematic showing the structure of PVP-encapsulated S nanosphere with empty space inside and the inward expansion during lithiation for the accommodation of volume change and the confinement of polysulfides by the shell. (B) The fabrication process of PVP-encapsulated hollow S nanospheres based on a simple reaction between sodium thiosulfate and hydrochloric acid in an aqueous solution in the presence of PVP at room temperature (RT). (C) Digital camera image of the synthesis scaled up in a 2,000-mL beaker (on a scale of gram per batch).

Fig. 2.

Fabrication, characterization, and lithiation of PVP-encapsulated hollow S nanospheres. (A) Schematic of the formation mechanism for PVP-encapsulated hollow S nanospheres. (B) SEM image of the as-prepared PVP-encapsulated hollow S nanospheres. (C) SEM image of the hollow S nanospheres after washing them with water to remove the PVP on the particle surface. (C, Inset) TEM image of an individual hollow S nanospheres. (D) Schematic diagram illustrating the subliming process of the PVP-encapsulated hollow S nanospheres. (E–G) SEM images of the S particles (E) before, (F) during, and (G) after S sublimation, respectively. (H and I) Typical SEM images of S nanospheres on the conducting carbon-fiber paper (H) before and (I) after lithiation. The particles after lithiation were marked with yellow circles in I. (J) A comparison of the size distribution of S nanospheres before and after lithiation.

Fig. 3.

Electrochemical characteristics of the PVP-encapsulated hollow S nanospheres. (A) Typical discharge–charge voltage profiles of the cell made from PVP-encapsulated hollow S nanosphere cathode at different current rates (C/10, C/5 and C/2, 1C = 1,673 mA/g) in the potential range of 2.6–1.5 V at room temperature. (B) Cycling performances and coulombic efficiency of the cell at a current rate of C/5 for 300 cycles. (C) Cycling performance and coulombic efficiency of the cell at a current rate of C/2 for 1,000 cycles. (D) Rate capability of the cell discharged at various current rates.

Fig. 4.

Electrode thickness evaluation of the PVP-encapsulated hollow S nanosphere cathode. (A) Schematic illustrating that the electrode thickness would not change owing to the inward expansion of each of the hollow S nanospheres upon lithiation. (B and C) Typical SEM images of the cross-sections of hollow S nanosphere cathode, showing the thickness of (B) the pristine electrode and (C) the electrode after 20 charge/discharge cycles [at fully discharged (lithiated) state]. (D) A comparison of the thickness of 20 different locations for the cross-sections of the pristine electrode and the electrode after 20 charge/discharge cycles.

Fig. 5.

TEM images and electrochemical characteristics of PEDOT-coated hollow S nanospheres. (A) TEM image of the PEDOT-coated hollow S nanospheres. (B) TEM image showing the PEDOT shell after dissolving S by toluene. (C) Rate capability of the PEDOT-coated S nanosphere cathode cycled at various current rates from C/10 to 4C. (D) Discharge–charge voltage profiles of PEDOT-coated S nanosphere cathode cycled at various current rates. Specific capacity values were calculated based on the mass of S.