Anode-free batteries are exciting alternative energy storage devices with the potential to achieve up to twice the capacity of conventional Li-ion batteries at a significantly lower cost. However, the absence of an anode causes them to generally operate above the electrochemical stability window of the electrolyte, leading to significant electrolyte degradation. One of the main degradation processes occurs at the current collector interface, with the species formed during cycling accumulating at its surface, i.e., the solid electrolyte interphase (SEI). Controlling SEI composition is key to improving Coulombic efficiency and battery capacity retention. An ideal SEI should effectively passivate the Li-plated current collector while simultaneously allowing ionic conduction and maintaining stability against further degradation caused by electrolyte interactions and crosstalk with cathode materials [1,2]. It is well established that an inorganic-rich (e.g., LiF) SEI contributes to long-term stability. One of the key strategies for designing stable SEI layers is controlling Li solvation to promote the higher production of LiF [3]. A critical aspect to address is the impact of operational conditions and electrolyte composition on Li solvation at the electrode interface. Due to the complexity of these processes, computational modeling allows us to break down this challenge into smaller components, thereby providing an atomistic-level understanding of the intricate mechanisms governing SEI evolution. Using a combination of bulk, interface and reactive molecular dynamics simulations, we aim to understand the connections between local structure, degradation mechanisms and SEI growth. Our results indicate that solvation is strongly influenced by operational conditions, but only at the electrode interface. Hence, beyond selecting an appropriate electrolyte, optimizing the formation cycle is equally important. Our calculations show that higher voltage or current density contributes to a more diverse solvation structure, leading to an increase in fluorine-containing species at the interface [2]. To model SEI growth, we develop an atomistic framework using classical molecular dynamics with a reactive approach, capturing the early-stage evolution of the SEI. Our model integrates an electrode charge polarization procedure with degradation modeling, considering reactions between lithium and electrolyte components. This computational procedure serves as a powerful tool for electrolyte screening, considering operational conditions for the rational design of new battery chemistries [4,5]. This work highlights how Li solvation influences degradation mechanisms, contributing to a deeper understanding of key variables that affect the development of high-efficiency, long-lasting anode-free batteries.
[1] Jagger, B.; Pasta, M. Solid Electrolyte Interphases in Lithium Metal Batteries. Joule, 2023, 7, 1-17.
[2] Kim, S.; Didwal, P. N.; Fiates, J.; Dawson, J. A.; Weatherup, R. S.; De Volder, M. Effect of the Formation Rate on the Stability of Anode-Free Lithium Metal Batteries. ACS Energy Lett. 2024, 9, 10, 4753-4760.
[3] Cao, X.; Gao, P.; Ren , X. et al. Effects of Fluorinated Solvents on Electrolyte Solvation Structures and Electrode/Electrolyte Interphases for Lithium Metal Batteries. Proc. Natl. Acad. Sci., 118 (9), 2021, e2020357118.
[4] Alzate-Vargas, L.; Blau, S. M.; Spotte-Smith, E. W. C.; Allu, S.; Persson, K. A.; Fattebert, J.; Insight into SEI Growth in Li-Ion Batteries using Molecular Dynamics and Accelerated Chemical Reactions. J. Phys. Chem. C, 2021, 125,18588-18596.
[5] Abbott, J. W. and Hanke, F. Kinetically Corrected Monte Carlo-Molecular Dynamics Simulations of Solid Electrolyte Interphase Growth. J. Chem. Theory Comput. 2022, 18, 925-934.
