The DFT-ReaxFF Hybrid Reactive Dynamics Method with Application to the Reductive Decomposition Reaction of the TFSI and DOL Electrolyte at a Lithium–Metal Anode Surface

The high energy density and suitable
operating voltage make rechargeable
lithium ion batteries (LIBs) promising candidates to replace such
conventional energy storage devices as nonrechargeable batteries.
However, the large-scale commercialization of LIBs is impeded significantly
by the degradation of the electrolyte, which reacts with the highly
reactive lithium metal anode. Future improvement of the battery performance
requires a knowledge of the reaction mechanism that is responsible
for the degradation and formation of the solid-electrolyte interphase
(SEI). In this work, we develop a hybrid computational scheme, Hybrid ab initio molecular dynamics combined
with reactive force fields, denoted HAIR, to accelerate
Quantum Mechanics-based reaction dynamics (QM-MD or AIMD, for ab initio
RD) simulations. The HAIR scheme extends the time scale accessible
to AIMD by a factor of 10 times through interspersing reactive force
field (ReaxFF) simulations between the AIMD parts. This enables simulations
of the initial chemical reactions of SEI formation, which may take
1 ns, far too long for AIMD. We apply the HAIR method to the bis­(trifluoromethanesulfonyl)­imide
(TFSI) electrolyte in 1,3-dioxolane (DOL) solvent at the Li metal
electrode, demonstrating that HAIR reproduces the initial reactions
of the electrolyte (decomposition of TFSI) previously observed in
AIMD simulation while also capturing solvent reactions (DOL) that
initiate by ring-opening to form such stable products as CO, CH2O, and C2H4, as observed experimentally.
These results demonstrate that the HAIR scheme can significantly increase
the time scale for reactive MD simulations while retaining the accuracy
of AIMD simulations. This enables a full atomistic description of
the formation and evolution of SEI.