|Left. Rate performance of the S-SACNT cathode. Inset is a photograph of the binder-free nano S-SACNT composite. Right. Cartoon of the S-SACNT composite. Credit: ACS, Sun et al. Click to enlarge.|
Researchers from Tsinghua University have developed another approach to high-capacity cathode materials for Lithium-sulfur batteries: a binder-free nano sulfur/carbon nanotube composite featuring clusters of sulfur nanocrystals anchored across a super-aligned carbon nanotube (SACNT) matrix.
In a paper in the ACS journal Nano Letters, the team from the Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center report that the nano S-SACNT composite cathode delivered an initial discharge capacity of 1,071 mAh g–1, a peak capacity of 1,088 mAh g–1, and capacity retention of 85% after 100 cycles with high Coulombic efficiency (100%) at 1 C. At high current rates the nano S-SACNT composite displays capacities of 1,006 mAh g–1 at 2 C, 960 mAh g–1 at 5 C, and 879 mAh g–1 at 10 C.
Li−S batteries utilize a sulfur cathode and a lithium metal anode. Though with a relatively low average potential of 2.15 V with respect to Li/Li+, the sulfur cathode possesses an extremely high theoretical capacity of 1,672 mAh g−1 and specific energy density of 2,567 Wh kg−1 through the multi-electron-transfer reaction of S8 + 16Li+ + 16e−1 → 8Li2S. Moreover, sulfur is considered more suitable for commercialization due to its nontoxicity, low cost, being environmentally benign, and abundance in nature.
However, despite of these promises, the development of practical Li−S batteries has been hindered by several issues. The inherent insulating nature of sulfur (5 × 10−30 S cm−1 at room temperature) inevitably causes low active material utilization and poor rate capability. Severe volumetric expansion/ shrinkage (∼80%) of sulfur during charge/discharge processes gradually decreases the mechanical integrity and the stability of electrode over long cycles. In addition, the redox chemistry of sulfur in the cathode is relied on a solid (S8)−liquid (polysulfides, S4−82−)−solid (Li2S/Li2S2) reaction, in which the intermediate polysulfide ions are soluble in the liquid electrolyte, resulting in loss of active materials. Moreover, the dissolved polysulfides travel between the electrodes during cycling, being oxidized and reduced on both electrodes. Such a redox shuttle effect brings critical problems including decrease of Coulombic efficiency and capacity decay due to the insulating Li2S/Li2S2 deposited on both electrodes.—Sun et al.
One approach to addressing the limitations has been to use a porous, conductive carbon matrix to confine elemental sulfur. The matrix functions both as a conductive pathway and also provides physical confinement or chemical bonding to trap the soluble polysulfides during cycling.
However, the Tsinghua team notes, the preparation of such materials is complex, time-consuming and requires a large consumption of energy. Additionally, Additionally, “tortuous pores” within the electrodes may impede the electrolyte infiltration and limit the kinetics of the charge−discharge reactions. Further, since sulfur can diffuse into the designed pores in the carbon matrix, polysulfides can also transfer through the same path.
In order to increase the utilization efficiency of the active material, promote the electrolyte infiltration throughout the electrode, and benefit the high-rate performance of Li−S batteries, a more open carbon−sulfur composite structure that can still exert its function in confining sulfur and polysulfides should be a good choice for carbon−sulfur cathodes.—Sun et al.
In their work, the Tsinghua team design and synthesized a nano S-SACNT composite without additional binders or conductive additives; sulfur is confined sulfur in the flexible SACNT matrix. In comparison with ordinary carbon nanotubes, SACNTs exhibit a “super-aligned” nature, large aspect ratio (∼104), clean surfaces, and strong van der Waals force among tubes and bundles.
When dispersed in solvent by ultrasonication, SACNT bundles expanded into a continuous and three-dimensional conductive network with a highly open and porous structure, which was more efficient for the transfer of electrons, infiltration of electrolyte, and accommodation of volume variation, the researchers found.
The 3D SACNT network also offers numerous adhesion points and continuous physical barriers to trap the final lithiation product of polysulfides, and therefore confines the cathode reaction within the electrode and avoids over-aggregation of sulfur/Li2S/Li2S2 that degrades battery performance.
The composite features uniform clusters of sulfur nano-crystals surrounding SACNTs; the uniform distribution of sulfur nanocrystals in the SACNT matrix is achieved under room temperature and ambient pressure without performing any complex procedures or utilizing any toxic materials.
The team ascribed the strong electrochemical performance of the nano S-SACNT composites to the SACNT matrix and the uniform distribution of nanoscale sulfur clusters within it. The 3D SACNT network functions in various aspects, they noted:
The bushy network avoids agglomeration of the synthesized sulfur nanocrystals and their lithiation product, which is beneficial to maintain a uniform and stable electrode structure;
Its high conductivity greatly enhances the electric conductivity of the composite and benefits the charge transfer;
The SACNT network acts as a buffer around the sulfur particles, which accommodates the volume variations during cycling and avoids destruction of the electrode;
The inner porous structure offers easy access of the electrolyte to sulfur nanoparticles, which might be helpful for efficient Li+ insertion/extraction; and
The physical confinement of sulfur/polysulfides alleviates the loss and aggregation of active materials.
Li Sun, Mengya Li, Ying Jiang, Weibang Kong, Kaili Jiang, Jiaping Wang, and Shoushan Fan (2014) “Sulfur Nanocrystals Confined in Carbon Nanotube Network As a Binder-Free Electrode for High-Performance Lithium Sulfur Batteries,” Nano Letters doi: 10.1021/nl501486n