Interfacial Structures in Ionic Liquid-Based Ternary Electrolytes for Lithium-Metal Batteries: A Molecular Dynamics Study

Lithium-metal
batteries are promising candidates to fulfill the
future performance requirements for energy storage applications. However,
the tendency to form metallic dendrites and the undesirable side reactions
between the electrolyte and the Li electrode lead to poor performance
and safety issues in these batteries. Therefore, understanding the
interfacial properties and the Li-metal surface/electrolyte interactions
is crucial to resolve the remaining obstacles and make these devices
feasible. Here, we report a computational study on the interface effects
in ternary polymer electrolytes composed by poly­(ethylene oxide) (PEO),
lithium salts, and different ionic liquids (ILs) confined between
two Li-metal slabs. Atomistic simulations are used to characterize
the local environment of the Li+ ions and the transport
properties in the bulk and at the interface regions. Aggregation of
ions at the metal surface is seen in all investigated systems; the
structure and composition are directly correlated to the IL components.
The strong interactions between the electrolyte species and the Li-metal
atoms result in the structuring of the electrolyte at the interface
region, in which comparatively small and flat ions result in a well-defined
region with extensive Li+ populations and high self-diffusion
coefficients. In contrast, large ions such as [P222mom]+ increase the PEO density in the bulk due to large steric effects
at the interface. Therefore, the choice of specific ILs in ternary
polymer electrolytes can tune the structure–dynamic properties
at the Li-metal surface/electrolyte interface, controlling the SEI
formation at the electrode surface, and thereby improve battery performance.