Investigation of Sulfur Redox Chemistry in MoS2 Anodes for Potassium Ion Battery

Abstract

Molybdenum disulfide (MoS2) with layered structure composed of molybdenum atoms coordinated with sulfur atoms makes it a suitable intercalation and conversion-based anode material for K-ion batteries (KIBs). But the electrochemical mechanism for K-ion storage in MoS2 remains unclear due to the probable occurrence of both conversion and polysulfide reactions. The formation of elemental sulfur (S0) due to irreversible conversion reaction of MoS2 is predicted to be a contributor for polysulfide reactions. Hence, in this study we utilised commercially available MoS2 as an anode for KIBs and studied the electrochemical performance by controlling the depth of discharge in the first cycle. It is identified that K-ion storage mechanism in MoS2 differs widely by operating at different discharge voltages. As a result, the characteristic voltage profile differs from the subsequent cycle, signifying dissimilar reaction pathways. Our results suggest that by controlling the depth of potassiation in MoS2, sulfur redox reactions co-exist along with conversion reaction. The existence of Mo metal and sulfur in the charge state is confirmed from various ex-situ spectroscopy studies. Moreover, formation of potassium polysulfides in the discharge state also strongly confirms the occurrence of sulfur redox reactions. Due to the dual reaction pathways, K-ion storage capacity is boosted up to 256 mAh g-1 and 93 mAh g-1 at 0.03 and 2 A g-1, respectively. The dual reaction also stabilizes the structure, which helps to attain a capacity retention of 84 % and 58 % after 500 and 1000 cycles, respectively. This study provides a mechanistic insight into the reaction pathway of K-ion storage in MoS2 when operating at various potential windows. Also, it may provide reference to utilising metal sulfides as a catalytic cathode in metal-sulfur batteries.