While lithium-ion batteries (LIBs) with high energy density, long lifetime, and high operating voltage are widely used not only for portable devices but also for large scale devices and facilities, present LIBs have an insufficient energy density due to the use of large and heavy lithium metal oxide cathodes (100 ~ 200 Wh kg^-1). Hence, sulfur is being proposed as a new cathode material due to about 5 times higher energy density (500 Wh kg^-1) as well as lower cost than the commercial cathode materials. However, lithium-sulfur (Li-S) batteries have rapid initial capacity fading induced by dissolution of highly ordered polysulfide (Li₂S_x, 8≥x>6) and large volume expansion (~80 %). Metal sulfides, such as MoS₂, can directly form Li₂S without the formation of intermediate products in a Li-S battery. In particular, the reversible reaction between Li₂S and S at a voltage of over 2.2 V is notable for the use as new cathode material. Herein, we hypothesized that an increase of S content in the metal sulfide compound can expand the operating voltage window over 2 V such as S cathode. Therefore, we prepared molybdenum-sulfide compound with a higher content of S (MoS_x, x>2) than MoS₂ for use as a cathode instead of S. Moreover, we attached the MoS_x on the oxygen functional group of graphene oxide (GO) to strongly anchor the MoS_x and GO was partially changed to reduced GO (r-GO) in this process.

In this study, we introduce an improved MoS_x/r-GO cathode using an in-situ electrochemical method (named as “in-situ electrochemical pulverization”). The nano-sized material shows high kinetic performance and low volume expansion due to a short ion diffusion path and a large surface to volume ratio. Therefore, to improve the performance and stability of the prepared electrode, we pulverize bulk MoS_x into nano-sheets by repetitive full lithiation/delithiation cycles. Moreover, the pulverized nano sheets are securely anchored by the oxygen functional group in partially reduced GO, so that they are not separated from the electrode. In addition, to enhance the stability more, a nano-scaled SEI layer is formed on the in-situ pulverized nano sheets by controlling the initial potential.

As shown in Fig. 1a, the capacity rapidly decreased from 2680 to 561 mAh g^-1 over the 20^th cycle (step 1) presumably due to S dissolution. Then, the performance was recovered to 1627 mAh g^-1 over the following 110^th cycle (step 2) and it was maintained at that level for subsequent cycles (1606 mAh g^-1 for the 250^th cycle at 0.5 A g^-1). To elucidate the behavior underlying this observed gradual increase in capacity during step 2, we studied the morphology of the electrode by HR-TEM at the uncycled state, 60^th, and 100^th cycles. As shown in Fig 1b, the uncycled electrode shows a micro-sized layer structure along the r-GO surface. However, the repetitive electrochemical reaction (step 2 of Fig. 1a) induced a continuous pulverization of the bulk sheets into nano sheets with a size of 5.0 nm for the 100^th cycle (Fig. 1c-d). This TEM behavior is in good agreement with the cyclic performance of Fig. 1a. Moreover, in Fig. 2a thin film with a thickness of 0.9 – 1.0 nm, formed by controlling the potential, covered the pulverized sheets. Additionally, in XPS surface analysis of the electrode after the 100^th cycle, peaks of 285, 286.5, and 290 – 291.5 eV associated with carbonate compounds were identified. (Fig. 2b). These compounds correspond to the SEI layer. Although the SEI layer is an insulator, it can protect the active material from side reaction with the electrolyte and dissolution in the solution itself. Thus, the formation of a thin SEI on the pulverized sheets led to maintenance of capacity of above 1500 mAh g^-1 after the 110^th cycle.

Based on the above results, it is concluded that the in-situ electrochemical pulverization of the MoS_x/r-GO can improve electrochemical performances by forming nano-sized sheets which are securely anchored on r-GO. Also, the formation of a thin SEI layer can support the stability of the pulverized nano sheets.