In ACS Applied Materials & Interfaces, researchers from the School of Materials Science and Engineering at Shanghai University, the Changchun University of Technology, and the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, reported an electrolyte design that sustains lithium-metal battery performance from −30 °C to room temperature. The team regulated anion coordination by introducing a small amount of LiNO₃ into a mixed-salt system (LiTFSI + LiPF₆) dissolved in methyl propionate with fluoroethylene carbonate. This design enabled Li⁺-NO₃^- interactions to weaken Li⁺-solvent binding, promote multianion solvation, and build inorganic-rich interphases that limit dendrites while preserving fast ion transport at low temperatures.

Commercial carbonate electrolytes thicken and partially “freeze” at sub-zero temperatures. Ion motion slows, Li⁺ desolvation at interfaces stalls, and the solid electrolyte interphase (SEI) becomes less conductive. Together, these effects raise overpotentials, amplify uneven lithium plating, and trigger capacity loss or short circuit. The study set out to keep Li⁺ moving efficiently through both the liquid and interphase phases while stabilizing deposition morphology—without resorting to extreme salt concentrations or exotic solvents.

The authors formulated a multianion electrolyte by adding LiNO₃ to a dual-salt LiTFSI/LiPF₆ baseline (termed 7PTN in the paper) using methyl propionate (MP) as the primary solvent and FEC as a co-solvent (MP:FEC = 9:1 by volume). In cell tests, LiNO₃ acted as a coordination regulator: NO₃^- partially replaced solvent molecules in the primary solvation shell, strengthened Li⁺-anion interactions, and guided anion-preferred decomposition at electrodes. The schematic illustrates this “anion-modulated” pathway linking solvation changes to interphase composition, while the Raman/DFT/MD analyses show reduced Li⁺-solvent coordination numbers, stronger Li⁺-NO₃⁻ affinity, and lower desolvation barriers consistent with faster interfacial kinetics.

With NO₃^-  in the solvation sheath, TFSI⁻ and NO₃^- more readily decomposed at the anode to form LiF, Li₃N, and LiNₓOᵧ, generating an inorganic-rich SEI known to conduct Li⁺ well and to resist fracture. On the cathode (NCM811), the altered oxidation sequence of anions helped produce a thin, uniform cathode-electrolyte interphase (CEI) that reduced charge-transfer resistance. TEM shows the CEI from 7PTN was ~5.7 nm—thinner than in comparison electrolytes—while XPS/SEM indicates higher LiF and Li₃N fractions and smoother particle surfaces after cycling. Collectively, these features limited impedance growth and suppressed dendritic protrusions at low temperature.

In Li||Li symmetric cells, the 7PTN electrolyte maintained stable cycling for >2000 h at both 25 °C and −30 °C, with significantly lower overpotentials than commercial or single-salt controls. In Li||NCM811 cells at 25 °C, the formulation retained 94.7% capacity at 2 C and 85.1% at 5 C over 1000 cycles, indicating high-rate tolerance. Under −30 °C, 7PTN delivered 86.23% retention after 200 cycles at 0.1 C, and 92.4% retention after 400 cycles at 0.2 C, outperforming a widely used commercial carbonate electrolyte that failed early under the same conditions. The authors attributed these outcomes to faster Li⁺ desolvation, lower interfacial resistances, and uniform deposition enabled by the anion-dominated solvation structure.

Low-temperature limitations often trace to sluggish desolvation at the SEI and to poorly conductive interphase products. By weakening Li⁺-solvent interactions (especially MP-Li⁺) and strengthening Li⁺-anion binding (notably Li⁺-NO₃^-), the electrolyte shifted where and how decomposition occurred. This steered the SEI/CEI toward LiF/Li₃N/LiNₓOᵧ, phases that support Li⁺ transport and discourage filament growth. The linear sweep data also show an expanded oxidative window (up to ~5 V) for 7PTN, supporting compatibility with layered oxide cathodes. The strategy—anion coordination regulation with a modest LiNO₃ fraction—achieved performance typically associated with more complex or concentrated systems.

For applications that face winter climates or rapid charging in unheated environments, the approach offers a balanced path: it uses a readily handled solvent (MP), mainstream salts (LiTFSI/LiPF₆), and a simple additive (LiNO₃) to deliver both high-rate and low-temperature operation. Because the mechanism centers on solvation-shell competition and interphase chemistry, it may adapt to other cathode chemistries or even to graphite-based full cells, as suggested by the graphite/NCM811 data. The work points to electrolyte recipes that remain serviceable across seasons, reducing the need for heavy thermal management.