A battery electrolyte that is solid at normal temperatures yet still conducts ions could make lithium batteries safer and longer lasting by replacing flammable liquids.
(Nanowerk Spotlight) Batteries fail in the cold. This simple fact has constrained everything from Antarctic research stations to electric vehicle range in winter, and it stems from a fundamental property of the organic solvents that carry ions between electrodes. When temperatures drop, these liquids thicken and eventually freeze, halting ion flow and rendering cells useless. The problem runs deeper than mere inconvenience. Liquid electrolytes also leak, catch fire, and allow the growth of needle-like lithium dendrites that pierce separators and cause dangerous short circuits.
Solid electrolytes built from ceramics, sulfides, or polymers promise to eliminate these hazards, but they struggle with poor electrode contact and sluggish ion movement. Oxide electrolytes demand extreme processing temperatures. Sulfide materials react with moisture. Polymer systems conduct ions too slowly for practical applications.
A peculiar observation in water-based systems suggested an unexpected workaround. Frozen aqueous electrolytes, researchers discovered, could still conduct ions in the solid state, achieving conductivities around 0.1 mS cm⁻¹ at −17 °C. Ice was not the electrochemical dead end everyone assumed. But organic solvents used in lithium batteries seemed different. Their frozen states appeared intrinsically non-conductive, a view reinforced by every cold-weather battery failure.
A study published in Advanced Materials (“Lithium‐Ion Conduction Through Frozen Phase of Organic Electrolytes for Lithium Batteries”) upends this assumption. Researchers from UNIST and KAIST in South Korea have demonstrated that certain organic solvents, frozen under controlled conditions, conduct lithium ions efficiently enough to power working batteries at room temperature. The apparent paradox dissolves upon closer examination: by choosing a solvent with a high freezing point and minimizing salt concentration, the team created an electrolyte that solidifies above 25 °C yet maintains ionic conductivity comparable to established solid-state materials.
Coexisting phases in frozen electrolytes. (a) Phase diagram of LiTFSI-EC system. Portion of solution mol % and concentration of EC0.2T at room temperature and EC1T at 10 °C was calculated by lever rule. (b) X-ray diffraction patterns of two frozen electrolytes (EC1T at −20 °C and EC0.2T at 25 °C) and a liquid electrolyte (EC1T at 25 °C). (c) Raman mapping and corresponding spectra of EC0.2T at room temperature. The spectra at the positions marked by circle, triangle, and square in the map are shown on the right. Two characteristic peaks were analyzed: the C═O stretching of solid EC at 707–715 cm⁻¹ and the TFSI⁻ vibration at 730–750 cm⁻¹. In the mapping (left), regions with higher peak intensities are displayed as brighter areas. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The researchers focused on ethylene carbonate, a cyclic molecule commonly blended into lithium-ion battery electrolytes. Pure ethylene carbonate freezes at 37 °C, well above typical operating temperatures. Dissolving lithium salts normally depresses this freezing point into the liquid range, but the team minimized this effect by using an unusually low salt concentration. Their formulation, designated EC₀.₂ₜ, contains just 0.2 mol of LiTFSI per kilogram of solvent. This mixture freezes at around 29 °C and exists as a rigid crystalline solid at room temperature.
Despite its frozen state, EC₀.₂ₜ conducts lithium ions at 0.64 mS cm⁻¹, placing it within the practical range for battery operation and exceeding many oxide-based solid electrolytes. The material’s structure explains this performance. On the microscopic scale, two phases coexist: pure crystalline ethylene carbonate comprising approximately 74 mol% of the mixture, and small isolated pockets of concentrated liquid solution making up the remaining 26 mol%. Raman spectroscopy confirmed that anions concentrate almost exclusively in these liquid pockets, while the crystalline regions consist of pure solvent.
Ion transport through this frozen matrix follows a hopping mechanism distinct from liquid electrolytes, where lithium ions travel by riding along with solvent molecules through vehicular transport. In the frozen electrolyte, solvent molecules lock into fixed positions within the crystal lattice. Lithium ions must instead jump from one oxygen coordination site to the next along channels formed by aligned carbonate groups.
Molecular dynamics simulations tracked individual lithium ions moving through the frozen ethylene carbonate structure over 3,500 picoseconds (trillionths of a second). The ions remained confined perpendicular to the carbonyl groups but moved extensively along pathways where oxygen atoms lined up to provide coordination sites. Density functional theory calculations identified the energy barriers for these hops. The most favorable pathways required just 0.238 to 0.256 eV to traverse, lower than the barriers in garnet-type ceramic electrolytes and in aqueous ice systems.
This hopping mechanism produces a high lithium-ion transference number, the fraction of total ionic current carried by lithium rather than by accompanying anions. Liquid electrolytes typically show transference numbers between 0.3 and 0.5 because both cations and anions move freely. For frozen EC₀.₂ₜ, this value reaches approximately 0.8. The crystalline matrix immobilizes most anions or confines them to isolated liquid pockets, leaving lithium ions to carry the bulk of the current. When the same electrolyte melts at 50 °C, its transference number drops to around 0.4, confirming that the high value depends on the frozen state.
The team assembled lithium metal battery cells with lithium iron phosphate cathodes to evaluate practical performance. They melted EC₀.₂ₜ at 50 °C, impregnated it into glass fiber separators, then allowed the cells to cool and the electrolyte to solidify. These cells cycled stably for over 400 charge-discharge cycles at room temperature without internal shorting. A conventional liquid carbonate electrolyte failed after approximately 200 cycles when uneven lithium deposition caused soft shorts.
Electron microscopy showed stark differences in lithium plating behavior. The frozen electrolyte produced dense, uniform lithium deposits on the metal anode surface. The liquid electrolyte yielded irregular, mossy growth prone to penetrating separators.
Surface analysis revealed why. On lithium metal in contact with EC₀.₂ₜ, the protective interphase layer consisted primarily of lithium oxide with a lithium-to-oxygen ratio near 2:1 and minimal fluorine content. This composition indicated controlled decomposition of the solvent with limited participation from the fluorine-containing anion. The liquid electrolyte produced a different interphase dominated by lithium carbonate and lithium fluoride, reflecting more aggressive breakdown of both solvent and salt. The presence of detectable metallic lithium in spectra from EC₀.₂ₜ cells suggested an exceptionally thin protective layer.
Thermal cycling confirmed the material’s stability. Across ten consecutive melting and refreezing cycles, the transition temperature held steady at 33.4 ± 0.1 °C, and the heat of fusion remained constant at 100.9 ± 1.5 J g⁻¹. Battery cells showed virtually identical performance before and after undergoing a complete solid-liquid-solid cycle.
The operating temperature window presents a practical constraint. EC₀.₂ₜ functions from room temperature down to approximately 0 °C, below which conductivity falls beneath the threshold for battery operation. However, single-solvent electrolytes based on dimethyl carbonate, which has a lower freezing point, extended operation down to −30 °C in the team’s experiments. Mixed-solvent systems like those in commercial batteries failed when frozen because they form eutectic mixtures with heterogeneous, phase-separated structures that interrupt ion pathways.
The authors caution against overstating the immediate practical significance of their findings, noting the limited temperature range of the current system. The essential contribution lies in establishing that lithium batteries can operate with frozen organic electrolytes, a possibility the field had largely dismissed.
This first demonstration of a molecular solid electrolyte derived from common carbonate solvents suggests a research direction distinct from conventional network solids. Work already underway points toward formulations that function as ice electrolytes below their freezing points while behaving as stable liquids above them, potentially combining the advantages of both states in a single material.
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Hyun-Kon Song (Ulsan National Institute of Science and Technology (UNIST))
, 0000-0001-7914-4186 corresponding author
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