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

M. DEL CARMEN ROJAS; M. ZOLOFF MICHOFF; P. VELEZ; M. V. BRACAMONTE; D. BARRACO; E. LEIVA; G. LUQUE

Antofagasta

Workshop; 7 °International Workshop on Lithium, Industrial (IWLIME); 2020

Universidad de Antofagasta-Center for Advanced Research on Lithium an Industrial Minerals-U. de Córdoba-U. Mayor de San Andrés,-U. Católica Boliviana, INIFTA (UNLP-CONICET)

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 high specific theoretical capacity of 1675 mAh g −1, its natural abundance, 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 of the active material and the loss of active sulfur material during repeated charge/discharge. Upon lithiation of sulfur, higher-order lithium polysulfides (Li2 Sx (x =4, 6, and 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 fade. 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. But despite the obtained advances in inhibiting shuttle effect, it?s worth noting that the nucleation and growth of Li2S2/Li2S is still uncontrolled during the discharge process, which leads 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, 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. Recently 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] to catalyze the cleavage of ?S?S? in polysulfides. In the present work we perform density functional calculations using SIESTA [4] with the aim of understanding the specific working principle of TCEP in lithium-sulfur batteries. In this regard, we have performed studies of the interaction of long chain polysulfides with TCEP and studied the TCEP-assisted polysulfide reduction reaction by Nudged elastic band calculations [5]. The reaction steps for TCEP to reduce the polysulfide chains, including the intermediate product of each reduction step (i.e. cleavage site of polysulfides) are elucidated in this work.