Nanoporous Li2S and MWCNT-Linked Li2S Powder Cathodes for Lithium-Ion Battery Chemistries
Authors: Wu Feixiang , Magasinski Alexandre , Yushin Gleb 
Published: February 2020

The fully-lithiated Li2S (having theoretical gravimetric and volumetric capacities of 1166 mAhg-1 and 1935 mAh·cc-1) has been viewed as a promising cathode material because of its low cost, high capacity and compatibility with Li-free anodes (e.g., graphite or Si/Sn-based composites). In addition, the use of anodes that undergo small volume changes during cycling (such as graphite) makes it easier to form a stable electrically insulative SEI, which is permeable to Li ions but impermeable to polysulfides, thus eliminating the irreversible side reactions between polysulfide anion and Li metal and allowing one to minimize or avoid the gradual cathode dissolution and stabilize the chemistry of such cells [1, 2].

Unfortunately, the high melting point of commercial Li2S (~940 °C) makes it difficult to infiltrate it into conductive carbon hosts for the enhanced conductivity and capacity utilization, as commonly done with S [3-6]. Li2S cathodes also suffer from the dissolution and shuttling of lithium polysulﬁdes during cycling as S cathodes do [3-6]. Several research groups utilized ball milling to reduce the size of commercial Li2S powders and distribute smaller particles together with carbon additives to improve their rate performance and capacity utilization [7, 8]. However, the ball milling procedure does not allow formation of uniform particles with controlled morphology or strong bonding between Li2S and carbon. In order to better disperse Li2S in carbon host and improve the strength of their interactions, lithium polysulfides were recently used to synthesize the Li2S by releasing H2S, and the electrodes maintained 20 cycles [9]. However, H2S is a rather toxic gas, harmful to environment. Overall, in contrast to the pleura of papers reporting advancement of S-cathodes, the research on Li2S cathodes has been quite limited.
In order to achieve high capacity utilization and high rate performance of lithium sulfide (Li2S) cathode materials, it is critical to identify scalable methods for low-cost preparation of nanostructured Li2S or Li2S-carbon composites. We report the application of a versatile solution-based method to prepare nanoporous Li2S and MWCNT-linked Li2S particles (Figure 1) [10]. XRD studies revealed the lack of crystalline impurities in the produced samples, while electron microscopy (SEM and TEM) combined with N2 sorption measurements showed the impact of the preparation conditions on the morphology and porosity of the samples. Our studies have demonstrated that lower polarization, higher capacity utilization and better rate performance can be achieved in porous Li2S and MWCNT-linked nano Li2S samples when compared to that of commercially available Li2S powders. The nanostructured Li2S-based cathodes containing MWCNT showed promising discharge capacities of up to ~1050 mAhg-1
S at a slow rate of C/20 and ~800 mAhg-1
S at a C/2 rate. Quite remarkably, without any electrolyte additives (such as polysulfides or lithium nitrate) MWCNT-linked Li2S cathodes demonstrated up to ~90 % capacity retention after 100 cycles in half cells (vs. Li foil) at a C/5 and C/10 rates (Figure 2).

References

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Acknowledgements

This work was partially supported by the Army Research Office (ARO grant W911NF-12-1-0259). Wu fellowship was supported by China Scholarship Council (No.201206370083).