Nanoporous MoS2 as an Electrode Material Exhibiting High Levels of Pseudocapacitive Charge Storage with Both Li and Na-Ions
Authors: Cook John B , Kim Hyung-Seok , Yan Yan , Ko Jesse , Dunn Bruce , Tolbert Sarah H 
Published: February 2020

MoS2 is an attractive high power density energy storage material because of its layered crystal structure consisting of large van der Waals gaps that allow for fast ion diffusion. However, much of the work on MoS2 as an energy storage material has been focused on the high capacity four-electron reaction which irreversibly converts MoS2 into molybdenum metal and lithium sulfide in the first cycle. Subsequent cycles undergo lithium-sulfur redox chemistry leading to poor electrochemical reversibility.
In this study, we show that MoS2 can reversibly store alkali ions without the irreversible conversion of the parent material by limiting the electrochemical processes to a one-electron transfer. The MoS2 layered crystal structure is preserved for the one-electron reaction which leads to fast charge transfer kinetics and high electrochemical reversibility. We have prepared mesoporous MoS2 thin films through di-block copolymer templating of molecular molybdenum precursors followed by high temperature sulfurization. This synthesis method yields an ordered three-dimensional mesoporous structure that leads to interconnected nanosized 2H-MoS2 grains that are ideal for fast ion diffusion and good electrolyte accessibility. These mesoporous thin films of MoS2 are examined as pseudocapacitive energy storage devices with high specific capacitances for Li-ions and Na-ions.
Pseudocapcitive charge storage is an ideal hybrid between a battery and a capacitor because high power is achieved without significant expense to energy density. This ideal combination is achieved for mesoporous MoS2 through intercalation pseudocapacitance which occurs when ions intercalate into the channels or layers of a redox-active material accompanied by a faradaic charge-transfer, usually with no crystallographic phase change.
In this work, we have shown that mesoporous MoS2 effectively stores energy through intercalation pseudocapacitance. When Li-ions are utilized, the material can be charged and discharged in 20 seconds and maintains 81% of the capacity observed for a 2000 second discharge. When mesoporous MoS2 is cycled with the bulkier Na-ion, a 23 second charging time still maintained 69% of the capacity observed for a 2000 second discharge. Detailed electrochemical kinetic analyses was used to quantitatively decouple the diffusion controlled and capacitive controlled charge storage processes. This analysis revealed that Li-ion storage in this material is 91% capacitive and Na-ion storage is 59% capacitive. The ideal nanoscale architecture of mesoporous MoS2 designed for this study leads to ultra-long cycle lifetimes with both Li and Na-ions. We have shown that mesoporous MoS2 can achieve over 10,000 cycles when cycled in a Li-ion electrolyte. Utilizing synchrotron grazing incidence x-ray diffraction techniques we have correlated superior electrochemical kinetics and cycle longevity with an ordered porous structure and textured crystal structure. This study underscores the utility of designing precise nanoscale architectures which allow for the development of new extrinsic pseudocapacitive charge storage materials that behave significantly different from the bulk.

Figure 1