Morphology-Controlled WO₃ and WS₂ Nanocrystals for Improved Cycling Performance of Lithium Ion Batteries

Abstract

As a promising candidate for anode materials in lithium ion battery (LIB), tungsten trioxide ($WO_3$) and tungsten disulfide ($WS_2$) nanocrystals were synthesized, and their electrochemical properties were comprehensibly studied using a half cell. One-dimensional $WO_3$ nanowires with uniform diameter of 10 nm were synthesized by hydrothermal method, and two-dimensional (2D) $WS_2$ nanosheets by unique gas phase sulfurization of $WO_3$ using $H_2S$. $WS_2$ nanosheets exhibits uniformly 10 nm thickness. The $WO_3$ nanowires and $WS_2$ nanosheets showed maximum capacities of 552 and $633mA\;h\;g^{-1}$, respectively, after 100 cycles. Especially, the capacity of $WS_2$ is significantly larger than the theoretical capacity ($433mA\;h\;g^{-1}$). We also examined the cycling performance using a larger size $WO_3$ and $WS_2$ nanocrystals, showing that the smaller size plays an important role in enhancing the capacity of LIBs. The larger capacity of $WS_2$ nanosheets than the theoretical value is ascribed to the lower charge transfer resistance of 2D nanostructures.

Fig. 1. XRD pattern of WO₃ (WO-1 and WO-2) and WS₂ (WS-1 and WS-2) nanocrystals. The peaks matched those of monoclinic phase WO₃ (JCPDS No. 72-1048) and hexagonal phase WS₂ (JCPDS No. 84-1398).

Fig. 2. SEM and TEM images showing the general morphology of (a) WO-1, (b) WO-2 (c) WS-1, and (d) WS-2. High-resolution TEM and corresponding FFT images reveal that the distance between the adjacent (010) planes (d₀₂₀) is 3.8 Å for monoclinic phase WO₃ nanocrystals and d₀₁₀ is 2.7 Å for hexagonal phase WS₂ nanosheets. EDX spectrum shows the composition of (e) WO₃ (WO-2) and (f) WS₂ (WS-2).

Fig. 3. Cyclic voltammetry curve of (a) WO-1 and (b) WO-2. (c) Charge and discharge voltage profiles of LIB half-cell using WO-1, tested between 0.01 and 3 V, at a rate of 0.1 C. (d) Charge/discharge capacity vs. cycle number for half cells using WO-1 and WO-2. The coulomb efficiency is plotted using the right axis. (e) Charge and discharge voltage profiles of LIB half-cell using WO-2, tested between 0.01 and 3 V, at a rate of 0.1 C. (f) Cycling performance of WO-1 and WO-2 as the C-rate is increased from 0.1 C to 5.0 C.

Fig. 4. Nyquist plots of (a) WO-1 and WO-2; (b) WS-1 and (b) WS-2, before the cycle of LIB. The equivalent circuit diagram is shown on the right.

Fig. 5. Cyclic voltammetry curve of (a) WS-1 and (b) WS-2. Charge and discharge voltage profiles of LIB half-cell using (c) WS-1, tested between 0.01 and 3 V, at a rate of 0.1 C. (d) Charge/discharge capacity vs. cycle number for half cells using WS-1 and WS-2. The coulomb efficiency is plotted using the right axis. (e) Charge and discharge voltage profiles of LIB half-cell using WS-2, tested between 0.01 and 3 V, at a rate of 0.1 C. (f) Cycling performance of WS-1 and WS-2, as the C-rate is increased from 0.1 C to 5.0 C.

Table 1. Summary of LIB half-cell capacities (mA h g^-1) of WO₃ and WS₂ nanocrystals during cycles at a rate of 0.1C.

Table 2. Summary of the LIB half-cell capacities (mA h g^-1) of WO₃ and WS₂ nanocrystals as the rate is increased from 0.1 C to 5.0 C.