Enhancing the Capacity and Stability of a Tungsten Disulfide Anode in a Lithium-Ion Battery Using Excess Sulfur

Abstract

Tungsten disulfide (WS₂) is a transition metal disulfide and a promising anode material due to its layered structure, making it favorable for attaining lithium-ion batteries with rate capability and thermal/mechanical stability. Although WS₂ has a rich redox chemistry and a large density, which can increase the specific capacity and volumetric energy density, it still has an inferior specific capacity and poor long-term stability for practical use due to its insufficient space for the accommodation of lithium ions and large volume change during cycling. Herein, to overcome the chronic limitations of WS₂-based anodes, we propose a micron-sized tungsten disulfide/reduced graphene oxide composite by employing excess sulfur (S_<i>x</i>-WS₂/r-GO). In particular, the excess sulfur modifies the polarity of r-GO by chemically binding on the r-GO sheet during WS₂ formation, leading to an increase in the adsorption strength due to WS₂. Moreover, the excess sulfur increases the lattice parameter of WS₂ and decreases the crystallinity degree, securing additional sites for the accommodation of lithium ions. Therefore, the excess sulfur can increase the specific capacity and impede the separation of the pulverized WS₂ nanoparticles, hence mitigating structural decay during cycling. As a result, the S_<i>x</i>-WS₂/r-GO anode exhibits an enhanced specific capacity of 1426 mAh g^-1 at 0.4 A g^-1 and a high cyclic performance, with 87.9% capacity retention after 4000 cycles at a high current density (2.0 A g^-1).