Dipole Reorientation Induced Temperature-Dependent Solvation Structure in Low-Temperature Sodium Metal Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Sodium metal batteries (SMBs) offer high energy density but suffer from poor low-temperature performance due to sluggish interfacial kinetics.
• Developing electrolytes that maintain efficient ion transport at sub-zero temperatures is crucial for practical SMB applications.

Purpose of the Study:

• To design a novel temperature-dependent electrolyte for SMBs to overcome low-temperature performance limitations.
• To investigate the role of electrolyte composition and temperature on ion solvation and interfacial behavior.

Main Methods:

• A temperature-dependent electrolyte was engineered by tuning ion-dipole and dipole-dipole interactions, utilizing fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC).
• The electrolyte's effect on solvation structure, desolvation energy, and solid electrolyte interphase (CEI) formation was analyzed at varying temperatures.
• Electrochemical performance of P2-Na_2/3Ni_1/3Mn_2/3O₂ (P2-NNMO) and O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (O3-NFM) cells was evaluated at low temperatures (-20 °C and -40 °C).

Main Results:

• The electrolyte demonstrated a temperature-induced shift in solvation structure, favoring FEC at lower temperatures, which reduced desolvation energy.
• A stable, organic-rich CEI layer was formed at low temperatures, promoting interfacial kinetics.
• P2-NNMO and O3-NFM cells exhibited excellent low-temperature cycling stability, with high capacity retention (92.4% and 84.7% after 1000 and 900 cycles, respectively, at -20 °C).
• Reversible discharge capacities of 80.6 mAh g^-1 (P2-NNMO) and 102.3 mAh g^-1 (O3-NFM) were achieved at -40 °C.

Conclusions:

• The designed temperature-dependent electrolyte effectively enhances interfacial kinetics and stability of SMBs at low temperatures.
• This strategy provides a pathway for developing high-performance sodium metal batteries for cold environments.
• The findings offer critical insights for advancing next-generation energy storage solutions.