Elucidating the Solvation Structure and Dynamics of Lithium Polysulfides Resulting from Competitive Salt and Solvent Interactions
Designing optimal electrolytes is key to enhancing the performance of energy storage devices,especially relating to cycle life, efficiency, and stability.(1)Specifically, for future beyond-Li ion energy storage, there is still ample room for electrolyte improvements. Among the candidates for higher gravimetric energy storage, the Li–S battery is considered quite promising, owing to its theoretical specific energy density (2600 Wh/kg) and specific capacity (1675 mAh/g) and significantly lower cost as compared to state-of-art lithium-ion batteries.(2-4) However, despite these attractive attributes, successful commercialization of Li–S batteries is currently hindered by poor cycling performance and capacity retention that is primarily caused by the parasitic reactions between the Li metal anode and dissolved polysulfide (PS) species from the cathode during the cycling process.(3, 5) Most of the efforts to overcome this degradation mechanism has focused on suppressing the dissolution of PS species and/or protecting the negative electrode using confinement strategies or protective layers. However, these additional components not only fail to block completely the PS species but also restrict the volumetric energy density.(2, 6) In contrast, less attention has been given to designing optimal electrolytes with reduced PS solubility and improved electrochemical stability. Traditional Li salts used in Li-ion batteries (eg., LiPF6, LiBF4, LiBOB, LiBF2C2O4) and solvents (eg., ester, carbonates, phosphates) are unsuitable for Li–S battery applications due to their parasitic reactions with PSs.(2) Hence, rational selection or design of the electrolyte is critical in controlling the deleterious shuttle reactions and protecting the electrode surface. So far, 1 M lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) in a binary solvent mixture of 1,3-dioxolane and 1,2-dimethoxyethane (DOL:DME) is considered as one of the most suitable electrolytes for Li–S cells.(2, 5)However, despite its wide usage, the DOL:DME solvent system provides significant PS solubility, which enables the shuttle process and subsequent parasitic reactions.(2) Therefore, to develop electrolytes with low solubility, high chemical stability, and low viscosity, it is important to improve our understanding of the solvation structure and dynamics of the intermediate PS species formed during discharge. Ion solvation in electrolytes is composed of highly correlated ion–ion and ion–solvent interactions spanning wide spatial and temporal ranges. Currently, there is limited understanding of the solvation structure of various types of PS species (Li2Sx; x = 1 to 8) formed during the discharge process. Experimental efforts based on spectroscopic techniques have mostly focused on specific constituents of the electrolyte and do not comprehensively report the solvation structure of PS species.(7, 8) Ab initio based computational methods have provided valuable insights about the disproportionation and intermolecular association of PSs in Li–S electrolyte systems.(9-12) Nevertheless, comprehensive understanding regarding the evolution of solvation phenomena with respect to the PS chain length and solvent system remains elusive. Classical molecular dynamics (MD) simulations are well suited to obtain the needed molecular level understanding of nonreactive interactions and dynamics of multicomponent systems covering larger temporal and lateral scales. As per our knowledge, there is no previous work on understanding bulk structural and dynamical properties of PSs using classical molecular dynamics (MD) simulations mainly due to lack of effective force field parameters.