Urea-enhanced solubilization of lithium nitrate for improved lithium metal cycling: experimental and molecular dynamics investigation

Abstract

Lithium metal batteries (LMBs) offer high theoretical capacity but face challenges such as unstable solid electrolyte interphase formation and dendrite growth in conventional carbonate electrolytes. Although LiNO₃ stabilizes lithium, its low solubility in carbonates limits its use. Here, we investigate urea as a LiNO₃ solubilizer in a carbonate electrolyte, 1.0 M LiPF₆ in ethylene carbonate : diethyl carbonate (1 : 1 v/v), with 0.03 M LiNO₃ and 0.1 M urea, selected for complete LiNO₃ dissolution. Experimental characterization and molecular dynamics simulations showed that urea enhances LiNO₃ solubility via hydrogen bonding, ensures homogeneous Li⁺ and NO₃⁻ distribution, regulates Li⁺ flux to suppress dendrites, and stabilizes the NMC811 cathode by inhibiting LiPF₆ decomposition and HF generation. Consistent with these effects, Li⁺ mobility at room temperature increased from 1.54 to 4.28 and 5.83 × 10⁻⁶ cm² s⁻¹ for the base, LiNO₃-containing, and LiNO₃ + urea electrolytes, respectively, in line with literature reports. This synergistic effect enabled Li‖Li cells to cycle for over 1400 h and Li‖NMC811 cells to retain 82.5% of their capacity after 200 cycles with high coulombic efficiency. This study presents an effective strategy for achieving stable carbonate-based LMBs.