Exploring the Effects of Dopamine and DMMP Additives on Improving the Cycle Boosting and Nonflammability of Electrolytes in Full-Cell Lithium-Ion Batteries (18650)

Concerns over recyclability, performance, and safety
have grown
as the use of commercial Li-ion batteries to meet our energy needs
has increased. Electrolytes may aid in increased flammability, capacity
loss, and safety. Dimethyl methyl phosphonate (DMMP), a flame retardant,
and dopamine hydrochloride (DOP) are utilized to increase the cycle
life and graphite anode compatibility. The optimal additive formulation
(5% wt DMMP and 0.1% wt DOP) reduces inflammability and increases
cycle life and reversible capacity (99.14%). Molecular dynamics simulations
provide a comprehensive atomistic account of the dynamics of Li+ ion solvation in the electrolytes and in the presence of
additives. Dopamine and DMMP increase the Li-ion solvation-free energy
in the electrolyte formulation. While increasing the Li-ion conductivity,
additive addition reduces the electrolyte viscosity and enables the
formation of a smaller, stronger Li-DMMP-DOP complex than the larger
Li-carbonate complex found in Li-neat electrolyte systems. When DMMP
and DOP are added to the electrolyte, the carbonates are displaced
from the Li-carbonate complex by these additives. The Li-ion solvating
nature or compatibility was improved upon the addition of DMMP/DOP
further aided by a faster diffusive behavior of the Li complex, which
in part would be instrumental in enhancing the performance and safety
of the battery electrolyte additive formulations. The modified electrolyte
(DMMP 5% wt, DOP 0.1% wt) has a conductivity of 18.60 mS cm–1 and a low viscosity at 25 °C. This formulation was evaluated
using graphite/NMC-532 cylindrical cells with commercial electrodes.
The performance of the graphite/NMC-532 cells containing the modified
electrolyte was comparable to those of regular electrolytes based
on carbonates: ethylene carbonate/dimethyl carbonate/ethylmethylcarbonate
(EC/DMC/EMC) (1:1:1) additions. These findings indicate that DOP and
DMMP have a lower oxidation potential (4.3 V) than most commercial
electrolytes (4.5 V), preventing cathode deterioration. These insights
should pave the path for rational design of practical and cost-effective
Li-ion battery electrolyte additive combinations aimed toward lower
flammability and improved performance.