Structure, Diffusion, and Stability of Lithium Salts in Aprotic Dimethyl Sulfoxide and Acetonitrile Electrolytes

The development of lithium batteries,
including lithium-ion batteries
and lithium-air batteries, is the key technological breakthrough in
the field of renewable energy storage. However, to date, it is still
challenging to design a lithium battery with both high specific power
and stability. Specifically, the solvation structure and thermodynamic
stability of various electrolytes, mostly Li salts and aprotic solvents,
have not been systematically studied from an atomic viewpoint. In
this paper, we studied the solution chemistry of three most common
inorganic Li salts (LiClO4, LiBF4, and LiPF6) and O2 in two aprotic (dimethyl sulfoxide (DMSO)
and acetonitrile (CH3CN)) solvents. The Born–Oppenheimer
molecular dynamics simulations and enhanced free energy samplings
are employed to obtain their solvation structures, diffusion coefficients,
and stability performances at room temperature. As a result, the tetrahedral
Li­(DMSO)4+ and
Li­(CH3CN)4+ are obtained as the stable solvation shells of Li+ in DMSO and CH3CN solvents, respectively. Among the three
inorganic Li salts, the stability performances are found in the order
of LiClO4 > LiBF4 > LiPF6 in
both
DMSO and CH3CN solvents. Compared with CH3CN,
DMSO provides a more stable environment for the long-term usage of
Li salts for that increases the energetic barriers of the degradation
reactions of solvated Li+ components. However, DMSO shows
a weaker ability (than CH3CN) to transport the main redox
species (solvated Li+ and O2) in the electrolyte,
which limits the discharging and charging rate in the batteries.