Understanding the specific working principle of tris(2-carboxyethyl)phosphine hydrochloride (TCEP) inlithium-sulfur batteries by First-Principle studies

PATRICIO VELEZ; LEIVA, EZEQUIEL P. M.; ROJAS, MARIA DEL CARMEN; BRACAMONTE, M. VICTORIA; GUILLERMINA LUQUE; MARTIN ZOLOFF MICHOF; BARRACO, DANIEL E.

Congreso; IWLiME 2020: 7th international workshop in lithium, industrial minerals and energy; 2020

Lithium-sulfur batteries are considered as the optimal second-generation commercial battery due to their high theoretical energy density of 2600 Wh kg-1 and a high theoretical specific capacity of 1675 mAhg-1, presenting also the advantage of the natural abundance of sulfur, it´s low cost and environmental friendliness [1]. However, Li-S batteries suffer from different problems, such as the insulating nature of sulfur, the change of volume it suffers and the loss of active sulfur material during repeated charge/discharge cycling. Upon lithiation of sulfur, higher-order lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8) are generated that are highly soluble in the liquid electrolytes and shuttle across the separator causing low charge/discharge efficiency and corrosion of the lithium anode. This effect produces low utilization of the active material and capacity fading. Another fact that makes Li-S batteries non-commerciable until today is the uncontrollable electrodeposition of insulating Li2S2/Li2S that blocks ion/electron diffusion and decreases the utilization of sulfur [2]. To overcome these issues, there are different approaches such as incorporation of conductive matrices and adsorbent agents in the cathode, optimization of new electrolytes, modification of the separator to reduce the shuttle effect, protection of lithium anode with polymers, among others. However, despite the obtained advances in inhibiting the shuttle effect, it is worth noting that the nucleation and growth of Li2S2/Li2S is still uncontrolled leading to large insoluble Li2S2/Li2S aggregates. Considering this fact, catalyzing the reduction of long-chain polysulfides to Li2S2/Li2S and enhancing the reaction kinetics have proven to be valid ways to solve the shuttle effect and improve the rate performance of Li-S batteries. Recently, a series of functional composite interlayers have been systematically investigated to identify the key parameters that determine their role in improving the performance of Li/S batteries. It is well known that trialkyl phosphines have the property of selectively reduce disulfide bonds. Recent experimental works use the commercially available organophosphorus tris(2- carboxyethyl)phosphine hydrochloride (TCEP) taking advantage of its effect in disrupting disulfide bonds in various proteins [3, 4] to catalyze the cleavage of -S-S- in polysulfides. In the present work, we perform density functional calculations using SIESTA [5] in order to understand the specific working principle of TCEP in lithium-sulfur batteries. In this regard, we performed studies of the interaction of long-chain polysulfides with TCEP and studied the possible steps of TCEP-assisted polysulfide reduction reaction. The results show that the orientation of TCEP and Li2S6 molecules influences the adsorption energy values obtained. We also performed studies in presence of explicit molecules of solvent regularly found in Li-S batteries and found that the adsorption energies are largely influenced by the solvent molecules present.References[1] A. Manthiram, Y. Fu, S. Chung, C. Zu, and Y. Su, ?Rechargeable Lithium − Sulfur Batteries,? Chem. Rev., vol. 114, pp. 11751?87, 2014.[2] Y. Huang, X. Sun, R. Li, G. Liang, C. Wei, and H. Hu, ?Promoting Redox Reduction of Lithium - Sulfur Battery by Tris(2-carboxyl) phosphine Shearing S-S Bond?, J. Electrochem. Soc., vol. 166, 2019.[3] T. Yang, K. Liu, R. Ren, J. Zhang, X. Zheng, C. Wang and M. Chen, ?Uniform growth of Li2S promoted by an organophosphorus-based mediator for high rate Li-S batteries,? Chem. Eng. J., vol. 381, no. June 2019, p. 122685, 2020.[4] O. Dmitrenko, C. Thorpeand, R.Bach, ?Mechanism of Sn2 Disulfide Bond Cleavage by Phosphorus Nucleophiles. Implications for Biochemical Disulfide Reducing Agents?, J. Org. Chem. Vol.72, 8298-8307, 2007.[5] J. M. Soler, E. Artacho, J. Gale, A. García, J. Junquera, P. Ordejón and D. Sanchez-Portal, ?The SIESTA method for ab initio order-N materials simulation?, J. Phys. Condens. Matter, vol. 14, no. 11, pp. 2745?2779, 2002.