A First-Principles Study of Amorphous Li2s in Lithium-Sulfur Batteries
Authors: Liu Zhixiao , Deng Huiqiu , Mukherjee Partha P. 
Published: February 2020

The development of electric vehicles (EVs) and stationary energy storage demands rechargeable batteries with improved electrochemical performance. The lithium-sulfur (Li-S) battery is considered as the competitive candidate to replace the conventional Li-ion batteries based on the intercalation mechanism in the near future because the Li-S system can deliver a high energy density of ~2600 Wh/kg with low cost.1-3

The crucial challenge for commercializing Li-S batteries is the shuttle effect caused by the dissolution and migration of polysulfides (PSs), which can corrode metallic Li anode and result in the irreversible capacity loss. To prohibit the shuttle effect, S particles, even smaller S2-4 molecules, are confined in nanopores for trapping PSs.4-8 However, the lithiation of the sulfur active material always suffers from enormous volume expansion, and resulting mechanical degradation of the host material.9

Using Li2S to fabricate the Li-S battery cathode may be a promising approach to improve the battery performance.10 Previous studies on Li-O2 batteries showed that the amorphous Li2O2 has faster charge transfer kinetics than the crystalline Li2O2.11, 12 The charge transport mechanism in amorphous Li2S is not clearly understood. According to our first-principles calculations, the formation energies of charged defects in the amorphous Li2S is lower than those in the crystalline phase. Given that the charged defects are charge carriers in Li2S, the cathode fabricated by amorphous Li2S is expected to exhibit faster kinetics.
Also, we investigate the delithiation process of ultra-small Li2S nanoparticle. It is found that the polaron as the charge carrier can even form in the ultra-small nanoparticle. Although the overall delithiation process can be considered as an oxidization process, local reduction reactions and the disproportionation reaction are observed in intermediate products. The final charging product can even be a cyclo-S10 ring. This study is expected to provide new insight into designing nanostructured active materials for Li-S batteries.

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