Breaking solvation dominance of ethylene carbonate via molecular charge engineering enables lower temperature battery

Lithium-ion batteries for extreme environments require electrolytes that remain liquid over wide temperature ranges and support fast ion transport and charge transfer at low temperature. At low temperature (≤0 °C), charge transfer processes dominated by Li⁺ desolvation barriers limit performance, leading to lithium plating and unstable solid electrolyte interphases (SEIs). Conventional EC-based electrolytes exhibit strong coordination between Li⁺ and the carbonyl oxygen of high-permittivity EC, raising desolvation energy. The research goal is to weaken Li⁺–solvent interactions while preserving the benefits of high-permittivity solvents. The authors hypothesize that reducing the electronegativity of the carbonyl oxygen in a high-ε environment—via introducing strongly electron-withdrawing fluorine in a low-melting carboxylate cosolvent—can unlock EC from Li⁺ solvation, shift the primary solvation shell from EC-dominated to low-ε solvent (DEC)-dominated, and thereby reduce desolvation energy, improve conductivity and liquidity, and enable low-temperature operation.

Multiple electrolyte strategies have been explored to improve low-temperature performance: liquefied gas electrolytes; novel co-solvents; highly fluorinated solvents; propylene carbonate (PC)-based systems; localized high-concentration electrolytes (LHCEs); weakly solvating electrolytes; and cointercalation approaches. These typically increase anion participation in Li⁺ solvation by replacing EC with low-ε solvents. LHCEs, in particular, use low-polarity fluorinated ether diluents to break strong solvent interactions, broadening liquid range and aiding desolvation. However, EC-free or low-ε-only approaches often reduce conductivity and overlook direct Li⁺–high-ε solvent interactions, limiting performance at extremely low temperatures (≤ −60 °C). The authors propose an alternative: maintain high-ε EC but weaken its Li⁺ coordination by fluorination effects in a carboxylate cosolvent (ethyl butyrate derivatives), leveraging dipole–dipole interactions to reduce EC binding while promoting low-ε solvent (DEC) coordination.

Electrolyte design and preparation: A base electrolyte of 1 M LiPF₆ in EC:PC:DEC:EMC = 2:1:3:4 (vol) was compared with test electrolytes of the same salt concentration in EC:PC:DEC:EMC:cosolvent = 2:1:1.5:4:1.5 (vol), replacing half of DEC with one cosolvent: ethyl butyrate (EB) or its fluorinated analogues 4,4,4-ethyl trifluorobutyrate (ETFB), ethyl heptafluorobutyrate (EHFB), and 2,2,2-trifluoroethyl butyrate (TFEB). Cosolvent selection emphasized low melting point and tunable fluorination degree/site. Physical characterization: Conductivity measured from −90 to +70 °C (VFT fitting for activation energies); viscosity and DSC for freezing/melting behavior; visual liquidity at −110 °C. FTIR (C=O region) and Raman (EC ring and C–O vibrations) were collected temperature-dependently to quantify solvated vs free species and EC vs DEC coordination ratios. Computations: DFT (Gaussian16, B3LYP/6-311G(d,p), SMD with acetone) for binding energies and desolvation energy of Li⁺ solvation clusters, and frontier orbital energies; MD simulations (Materials Studio, COMPASS III, Nosé thermostat) to obtain RDFs and coordination numbers for Li⁺ with solvents/anions at 25 °C and −70 °C. Electrochemical testing: EIS and Distribution of Relaxation Times (DRT) analysis from −60 to 80 °C to separate charge-transfer resistance (Rct) and SEI resistance (RSEI), and evaluate activation energies (Arrhenius fits). Cycling and rate tests performed on coin cells (NCM811/Li, LCO/Li, Gr/Li) and commercial 1 Ah pouch cells (NCM811/Gr and LCO/Gr) with 3.0–4.5 V windows under low-temperature protocols: RT charge–LT discharge and LT charge–LT discharge. Safety assessed via Accelerating Rate Calorimetry (ARC). Interfacial analyses: TOF-SIMS depth profiling for SEI composition and spatial distribution (organic fragments and LiF-rich inorganic species), Co dissolution/crossover mapping; XRD for lithium metal detection on graphite; XPS and SEM/TOF-SIMS imaging for lithium dendrite identification and morphology after LT cycling. Cell fabrication: 1 Ah pouch cells (Li-Fun Technology) with specified cathode/anode loadings; electrolyte fill: 4.3 g (NCM811/Gr) and 2.1 g (LCO/Gr). Instrumentation and parameters detailed for spectroscopy and electrochemistry.

- Molecular charge engineering via fluorinated ethyl butyrate cosolvents weakens Li⁺–solvent interactions and shifts the solvation shell from EC-dominated to DEC-dominated, especially at low temperature, while retaining EC’s high permittivity benefits. - Physical properties: EHFB electrolyte remains liquid after 30 min at −110 °C; exhibits ionic conductivity of 1.46 mS cm⁻1 at −90 °C and a low VFT-derived activation energy of 0.98 eV (vs base: frozen by −50 °C and 0.001 mS cm⁻1 at −90 °C). Estimated freezing points: EHFB −135 °C, TFEB −132 °C, ETFB −130 °C. - Solvation structure (MD at 25 °C): Base electrolyte Li⁺ coordination numbers: EC 1.31, DEC 1.13, EMC 0.94. With EHFB: EC drops to 0.61; DEC rises to 1.46; cosolvent coordination decreases with higher fluorination (EB 1.3 → EHFB 0.06). At −70 °C in EHFB: DEC increases to 1.71; EC decreases to 0.33; EHFB participation increases (0.56). FTIR-derived ratios R₁ (solvated EC/free EC) and R₂ (solvated DEC/solvated EC) decrease with temperature, confirming DEC-dominant solvation at LT; Raman blueshifts suppressed vs base. - Energetics (DFT): Li⁺ desolvation energy decreases with fluorination degree: 4EC 1.1 eV; EC–2DEC–EB 1.0 eV; EC–2DEC–ETFB 0.93 eV; EC–2DEC–EHFB 0.83 eV. Fluorination induces a dipole redistribution reducing EC–Li⁺ binding and promoting DEC coordination. - Kinetics (DRT/EIS): At −60 °C, Rct: EHFB 1.0 Ω, TFEB 3.2 Ω, ETFB 4.7 Ω vs base 8.8 Ω. Charge-transfer activation energy in LCO/Gr pouch cells: EHFB 10.9 kJ·mol⁻1 vs base 30.4 kJ·mol⁻1. SEI transport activation energy: EHFB 19.8 kJ·mol⁻1 vs base 29.3 kJ·mol⁻1. Graphite anode impedance dominates at LT; EHFB markedly lowers Rct and stabilizes RSEI over cycles. - SEI/interfacial chemistry (TOF-SIMS): EHFB drives more LiF-rich, inorganic-rich inner SEI and suppresses EC reduction at LT, enhancing DEC and cosolvent reduction contributions. Increased LiF species with decreasing temperature; reduced Co dissolution/crossover at both RT and LT vs base. - Low-temperature electrochemistry (1 Ah pouch cells, 4.5 V): • Cycling at −10 °C (200 cycles): NCM811/Gr capacity retention 97.94%; LCO/Gr 86.70%; CE ~99.6%/99.5%. • LT charge capability: NCM811/Gr delivered 364 mAh after charge at −60 °C; cells with base electrolyte failed below −30 °C. • RT charge–LT discharge: LCO/Gr retained 830 mAh at −40 °C (73.9% of RT); ~60% of RT capacity at −70 °C; NCM811/Gr retained 61% at −70 °C. Discharge functionality preserved even at ~−100 °C after RT charging. • One-cycle charge/discharge at −60 °C retained 334 mAh. - Graphite anode behavior: EHFB suppresses lithium dendrites at LT. Base electrolyte shows metallic Li (XRD peak at 35.8°, XPS at ~52.2 eV), significant plating peaks (dV/dQ), and dendritic morphology (SEM/TOF-SIMS); EHFB case does not. Gr/Li cells with EHFB retain 95% (−20 °C) and 57% (−70 °C) of RT charge capacity under RT discharge; stable cycling at −10 °C with 99% retention over 120 cycles. - Safety (ARC, LCO/Gr): EHFB significantly lowers max dT/dt to 5.1 °C s⁻1, raises thermal runaway temperature to 226 °C, and reduces maximum temperature to 309 °C, indicating improved safety.

The study directly addresses the central challenge of high desolvation barriers and poor charge transfer at low temperatures in EC-based electrolytes by engineering the solvation shell. Fluorinated ethyl butyrate cosolvents reduce the carbonyl oxygen electron density and alter dipole interactions, thereby weakening EC–Li⁺ coordination and enabling a transition to DEC-dominated solvation while involving the cosolvent at low temperatures. This overall weaker solvation lowers desolvation energy and charge-transfer resistance, broadens the liquidity window, and improves ionic conductivity at extreme cold. Concurrently, the fluorinated cosolvent promotes formation of a LiF-rich, robust SEI, diminishes transition-metal dissolution/crossover, and reduces SEI transport barriers. Collectively, these effects translate into markedly enhanced low-temperature performance in commercial-scale pouch cells, suppression of lithium dendrites on graphite, and improved safety, validating the proposed molecular charge engineering approach for EC-based electrolytes.

Introducing fluorinated ethyl butyrate cosolvents into traditional EC-based electrolytes unlocks EC from Li⁺ solvation and establishes a DEC-dominated, weakly coordinated solvation shell. This fluorination-dependent solvation redesign delivers: a vastly widened liquid range (liquid at −110 °C), high ionic conductivity (1.46 mS cm⁻1 at −90 °C), lowered desolvation and charge-transfer barriers, LiF-rich robust SEIs, dendrite suppression on graphite, excellent low-temperature discharge capability down to −100 °C after RT charging, and stable cycling at −10 °C in 1 Ah, 4.5 V graphite-based pouch cells. The work provides a broadly applicable strategy to realize all-temperature lithium-ion batteries without abandoning EC’s high-permittivity advantages. Future research may optimize fluorination patterns and cosolvent structures, explore compatibility with various salts/cathodes, and scale manufacturing while maintaining safety and performance in extreme conditions.

The authors note that achieving sustained cycling below −60 °C remains challenging. The study focuses on specific EB-derived fluorinated cosolvents and LiPF₆-based carbonate systems; broader generalization to other salts/solvents and long-term cycling performance at ultra-low temperatures requires further investigation.