Nanoporous Li2S and MWCNT-Linked Li2S Powder Cathodes for Lithium-Ion Battery Chemistries

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).