Two-dimensional layer WSx/r-GO (x>2) composite electrode for next-generation high-capacity lithium ion batteries

Abstract

Sulfur is one of attractive material as a cathode because of high energy density (500 Wh kg-1). Nevertheless, lithium-sulfur (Li-S) battery has difficulty being used in commercial batteries due to low stability caused by the low electrical conductivity, and the dissolution of polysulfide (Li2Sx, 8≥x>6) [1]. In our previous study [2], we proposed the use of a metal sulfide as cathode by increasing sulfur content more in the sulfide. Although previously reported two dimensional transition metal sulfides (MoS2, WS2, and VS2) [3] are utilized as an anode, there is no advantage as anode material over graphite and/or silicon due to insufficient gravimetric capacity (430~930 mAh g-1) and high operating voltage (1.1~1.6 VLi/Li+) [3]. However, the notable part is that unlike sulfur, metal sulfide can form the insoluble polysulfide (Li2S). Hence, we intended to expand the proportion of operating voltage above 2 V corresponding to the Li2S redox reaction by increasing the sulfur content in metal sulfide in order to use the metal sulfide instead of sulfur as a cathode [2]. In particular, WS2 has the same structure and redox reaction as MoS2, but it has a higher operating voltage (1.5 VLi/Li+) than MoS2. Moreover, tungsten has relatively high electrical conductivity (1.79*107 S m-1). In this respect, we considered that tungsten sulfide is an attractive material that can compensate for the problem of sulfur. Hence, in this work, we synthesized a new type of tungsten sulfide with a high content of S (WS­x, X≥3). Moreover, to enhance the conductivity and alleviate separation from the electrode, the WSx is composited with graphene oxide (GO), and hence to form WSx/r-GO. Fig. 1 shows the surface morphologies of WS3/r-GO electrode. As shown in the SEM and TEM images, the WS3/r-GO has a layered structure in microscale. The selected area electron diffraction (SAED) pattern indicates the WS3/r-GO is mostly composed of amorphous structure. Fig. 2a showed the rate capability of WS3/r-GO electrode at the various current densities from 0.2 to 4.0 A g-1. The electrode delivered the initial discharge capacity of 1,571 mAh g-1 and decreased to 658 mAh g-1 at 0.2 A g-1. However, the WS3/r-GO electrode showed the reasonable capacities of 658, 419, 283, 198, and 129 mAh g-1 at 0.2, 0.4, 1.0, 2.0, and 4.0 A g-1, respectively. When the current density was recovered back to 0.4 A g-1, the capacity recovered to 473 mAh g‑1 indicating full recovery. Moreover, the long term electrochemical performance of WS3/r-GO electrode was tested at 0.4 A g-1 (Fig. 2b). The capacity decreased from 1,232 to 411 mAh g-1 during the initial 14 cycles presumably due to the formation of SEI and irreversible conversion reactions of WS3 [2,3]. Then, the performance was recovered to 1,481 mAh g-1 after 436 cycles, and slightly decreased and maintained to 1,219 mAh g-1 after subsequent cycling. Compared with the initial capacity, the capacity remained at 98 % even after 749 cycles.

In summary, we prepared two-dimensional layered WS3/r-GO composite as cathode for lithium ion battery. Although WS3/r-GO electrode showed a large initial irreversible capacity, the capacity was recovered with initial cycling (~400 cycles) and maintained during 750 cycles. To reveal more clearly this phenomenon, we conducted additional electrochemical, surface-chemical, and morphological analyses using EIS, CV, XPS, XRD, HR-TEM, SEM/EDS and STEM/mapping.