Interaction of TiS₂and Sulfur in Li-S Battery System

Abstract

With the ability to adsorb polysulfide, electronically conductive and electrochemically active TiS 2 is an effective multifunctional cathode additive to improve Li-S battery cycling performance. By using X-ray Photoelectron Spectroscopy (XPS), direct evidence is obtained to demonstrate the strong interaction between Li-polysulfide and TiS 2 . The observation of Li signature on Li 2 S 8 treated TiS 2 proves that the TiS 2 possesses the ability to adsorb Li-polysulfides species on its surface. An electron density transfer from Ti to Li and S is identified based on the positions of the peaks in the XPS spectrum before and after the interaction, which is consistent with the theoretically predicted polysulfide-TiS 2 interaction models in the literature. TiS 2 sample with 2.5x higher BET surface area is obtained by milling the raw TiS 2 particles and used as cathode additive in the sulfur electrode. In the presence of TiS 2 additive, long cycle life and improved sulfur utilization of Li-S cells under high rate discharge are demonstrated. In addition, we find that a uniform TiS 2 distribution in the sulfur-TiS 2 hybrid electrode is vital in determining its effectiveness in enhancing the performance of sulfur electrodes. Therefore, processing method and condition should be very important considerations in future development of sulfur electrodes with TiS 2 additive. The Li-S battery is one of the major frontiers in energy storage research. It has been well known for its high theoretical gravimetric capacity 1675 mA g −1 and energy density 2600 kWh kg −1 , and this can hardly be matched by any of the current Li-ion battery chemistries. 1,2However, the tendency for partially reduced sulfur to form soluble polysulfides that lead to fast capacity fade and low coulombic efficiency, the insulating nature of sulfur and Li 2 S, and the considerable volume change ∼80% between these two end products all put inevitable constraints on electrode architecture and cell level designs in attempts to approach or achieve these appealing theoretical values. 1-3Based on the discoveries in the last 10 years it has been found that cathode architecture is vital and active sulfur must be either pre-confined into a conductive host least permeable to polysulfide dissolution [4][5][6][7] or an strongly polysulfide binding compound should be dispersed into the cathode network, [8][9][10][11][12][13] otherwise both the utilization of sulfur and cycling performance will suffer. Furthermore, the combination of the above two approaches is also reported.14,15 Generally these measures increase the inactive component ratio and compromise energy density.Recently, it has been found that inclusion of electronically conductive TiS 2 16 into sulfur electrode helps to improve both the cycle life and high rate performance of Li-S cells. [17][18][19][20] Theoretical and indirect experimental evidences all suggest that TiS 2 potentially have surficial affinity for polysulfides, which make it an ideal mediator for active material loss. More importantly, TiS 2 distin...

A Li2S‐TiS2‐Electrolyte Composite for Stable Li2S‐Based Lithium–Sulfur Batteries

“…[7,10,20,24] The highmagnification images indicate the bulk size of the Li 2 S particles in both cathodes. The almost unchanged microstructures and elemental mapping results illustrate how the TiS 2 facilitates the activation and redox reaction of Li 2 S. [22,[51][52][53] We do note a slight change in the morphology of the embedded TiS 2 particles in the cycled Li 2 S-TiS 2 -E cathode, in which the TiS 2 changes from sheet-like structures with sharp edges to smooth, covered particles. The close contact between Li 2 S and TiS 2 in the Li 2 S-TiS 2 -E composite results from the dry-mixing process, [54] which should provide the insulating Li 2 S with fast charge transfer through the conductive TiS 2 .…”

“…[20][21][22][49][50][51][52][53] Inspired by its multifunctionalities, we utilized the conductive TiS 2 to assist in the electrochemical activation and reversibility of the insulating LiS 2 cathode (Figure 1). [20][21][22][49][50][51][52][53] Inspired by its multifunctionalities, we utilized the conductive TiS 2 to assist in the electrochemical activation and reversibility of the insulating LiS 2 cathode (Figure 1).…”

“…We then drop-casted this Li 2 S-TiS 2 -E composite paste onto a carbon-paper current collector as the cathode and added additional liquid electrolyte on the polymeric separator. [10,[49][50][51][52][53] To demonstrate the multifunctionality of the TiS 2 , we examined the cycled Li 2 S-E and Li 2 S-TiS 2 -E cathodes with SEM and EDX elemental mapping (Figure 2e-h, respectively). We also fabricated a control cell using a Li 2 S-E composite (i.e., without TiS 2 ) that was prepared in the same manner.…”

“…However, the Li 2 S-TiS 2 -E composite is characterized with the TiS 2 sheets closely surrounded by the Li 2 S particles (Figure 2c,d and Figure S4, Supporting Information). This morphological change implies that the TiS 2 is trapping polysulfides in the cathode region, [51][52][53] which should accelerate the redox kinetics. [18,19,49,53] Besides enhancing the reaction kinetics, TiS 2 can also immediately adsorb any polysulfides generated from the surrounding Li 2 S, and these stabilized polysulfides can then function as catholyte to benefit the cyclability and high-rate performance of the high-loading cathode.…”

“…This morphological change implies that the TiS 2 is trapping polysulfides in the cathode region, [51][52][53] which should accelerate the redox kinetics. [22,52,53,56] In Figure 3c and Figure S8 (Supporting Information), the cycled Li 2 S-E cathode displays increased resistance from 300 to Adv. [2,3,[7][8][9] After investigating the obvious morphological differences between the Li 2 S-E and Li 2 S-TiS 2 -E cathodes, we applied different electrochemical analytical tools to explore the activation and cycling performance of the cells (Figure 3).…”