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

Lithium-ion batteries (LIBs) are crucial for various applications, but their performance suffers significantly at low temperatures. This is primarily due to three key challenges: (i) the narrow liquid range of electrolytes, limiting their operable temperature window; (ii) slow mass transport, leading to poor ionic conductivity and Li⁺ diffusivity, especially in graphite anodes; and (iii) sluggish charge transfer processes, arising from high energy barriers for Li⁺ desolvation and migration within the solid electrolyte interface (SEI). These factors result in lithium plating, poor SEI formation, low efficiency, and safety concerns. Previous research has emphasized the importance of reducing desolvation energy barriers, as charge transfer processes often dominate electrochemical capabilities at low temperatures. Desolvation energy is strongly influenced by the electrolyte's solvation structure. Therefore, the focus shifts to creating a Li⁺ solvation shell with weak interactions between Li⁺ and the solvent molecules. Various strategies have been explored, including using liquefied gas electrolytes, novel cosolvents, highly fluorinated solvents, and weakly solvating electrolytes, to optimize solvation structures. However, existing methods often compromise conductivity or fall short in extremely low-temperature applications. This study proposes a novel approach to manipulate solvation structures by directly reducing the electronegativity of the carbonyl oxygen in high-dielectric-constant (high-ε) solvents like ethylene carbonate (EC), rather than replacing them entirely. This approach aims to weaken Li⁺-solvent coordination without sacrificing the high-ε properties of the polar solvents. The introduction of strongly electron-withdrawing elements, such as fluorine, is expected to weaken the coordination between high-ε solvents and Li⁺, releasing high-ε solvents and enabling low-ε solvents to dominate the Li⁺ solvation shell. This new solvation structure should enhance Li⁺ desolvation and improve low-temperature performance. The study focuses on fluorinating ethyl butyrate (EB), a solvent with a low melting point, to achieve this objective.

Extensive research has been dedicated to improving the low-temperature performance of LIBs. Studies have highlighted the limitations of conventional electrolytes at low temperatures, focusing on issues such as narrow liquid ranges, slow ion transport, and high desolvation energy barriers. Various strategies have been proposed to address these issues, including the use of liquefied gas electrolytes, novel cosolvents, highly fluorinated solvents, and local high-concentration electrolytes (LHCES). LHCES, in particular, have shown promise by employing low-polarity diluents to disrupt strong interactions between solvent molecules, thus broadening the liquid range and improving desolvation. However, many of these approaches lead to compromised conductivity or are ineffective at extremely low temperatures. The importance of the solvation structure and its influence on desolvation energy has been extensively discussed in the literature. Studies have shown a strong correlation between the solvation shell composition and the electrochemical performance of LIBs at low temperatures. The current research builds upon this foundation by proposing a new solvation design strategy that directly manipulates the Li⁺-solvent interactions within the solvation shell to achieve enhanced low-temperature performance.

The study employed a multifaceted approach combining experimental techniques and computational simulations. Electrolyte preparation involved dissolving 1 M LiPF6 in a mixture of carbonate solvents (EC:PC:DEC:EMC = 2:1:3:4 by volume). Test electrolytes were prepared similarly, but with half the DEC replaced by a fluorinated cosolvent or EB (EC:PC:DEC:EMC:cosolvent = 2:1:1.5:4:1.5 by volume). Four cosolvents were investigated: EB and its fluorinated derivatives ETFB, EHFB, and TFEB. Physical properties such as ionic conductivity and viscosity were measured over a wide temperature range (-90 °C to +70 °C) using a conductivity meter. Freezing points were determined using differential scanning calorimetry (DSC). Molecular dynamics (MD) simulations were performed to investigate the solvation structure of the electrolytes at various temperatures. Radial distribution functions (RDFs) and coordination numbers were calculated to analyze Li⁺-solvent interactions. Fourier transform infrared (FTIR) and Raman spectroscopy were used to characterize the solvation structure by analyzing changes in C=O stretching frequencies. Density Functional Theory (DFT) calculations were conducted to determine Li⁺ desolvation energies. Electrochemical characterization included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge cycling tests of both coin cells and pouch cells at various temperatures. The solid electrolyte interphase (SEI) was analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). Accelerating rate calorimetry (ARC) was used to evaluate the thermal safety of the cells. The electrochemical measurements were performed on 1 Ah pouch cells (NCM811/Gr and LCO/Gr) and coin cells (NCM811/Li, LCO/Li, and Gr/Li) under various temperature conditions and charging/discharging protocols (including RT charge-LT discharge).

The fluorinated cosolvents, particularly EHFB, significantly altered the solvation structure, resulting in a transition from an EC-dominated to a DEC-dominated solvation shell at low temperatures. This transition was attributed to the electron-withdrawing effect of fluorine, weakening the interaction between Li⁺ and EC. EHFB demonstrated the lowest binding energy with Li⁺ among the fluorinated cosolvents, resulting in weaker overall coordination interactions. The modified electrolyte with EHFB exhibited high ionic conductivity (1.46 mS cm⁻¹) at -90 °C and remained liquid at -110 °C. The EHFB electrolyte showed significantly lower charge-transfer resistance (Rct) and SEI resistance (RSEI) compared to the base electrolyte, particularly at low temperatures. DFT calculations confirmed that the fluorinated cosolvent lowered the Li⁺ desolvation energy. TOF-SIMS analysis revealed that the SEI formed with the EHFB electrolyte was richer in LiF and showed reduced EC reduction compared to the base electrolyte. The EHFB electrolyte effectively suppressed lithium dendrite formation, as evidenced by XRD, XPS, and SEM. Pouch cells with the EHFB electrolyte showed excellent cycling stability and capacity retention at -10 °C (98% capacity over 200 cycles) and maintained significant capacity even at -70 °C (60% of room temperature capacity) using a room temperature charge-low temperature discharge protocol. The cells using the EHFB electrolyte demonstrated discharge functionality even at -100 °C after full charging at room temperature. ARC testing confirmed that the cells with the EHFB electrolyte had better thermal stability and reduced the risk of thermal runaway.

The findings directly address the research question by demonstrating the effectiveness of molecular charge engineering in improving low-temperature battery performance. The transition from an EC-dominated to a DEC-dominated solvation structure, facilitated by the fluorinated cosolvent, is the key factor in achieving this improvement. The weaker Li⁺-solvent interactions in the modified electrolyte lead to lower desolvation energies, enhanced ionic conductivity, and improved charge transfer kinetics. The formation of a fluorine-rich, more conductive SEI layer further contributes to the enhanced performance. The results highlight the importance of controlling the solvation structure for optimizing low-temperature battery characteristics. This research expands upon previous efforts in designing low-temperature electrolytes, offering a unique approach that directly manipulates the intrinsic properties of the solvent molecules rather than simply relying on solvent mixtures. The exceptional low-temperature performance achieved demonstrates the potential for widespread application of this strategy in developing advanced LIBs for extreme environments.

This study demonstrates a novel strategy for designing low-temperature electrolytes based on molecular charge engineering of traditional EC-based electrolytes. The incorporation of fluorinated cosolvents, particularly EHFB, effectively weakens the Li⁺-EC interaction, promoting a DEC-dominated solvation structure and resulting in substantially enhanced low-temperature performance. The improved electrolyte characteristics translate into exceptional performance in graphite-based pouch cells, even at extremely low temperatures. This work provides valuable insights for the design of next-generation LIBs capable of operation in challenging environments. Future research could explore the effects of varying degrees of fluorination on different solvent molecules and the long-term stability of the modified electrolytes.

The study primarily focused on the performance of graphite-based pouch cells. Further investigation is needed to evaluate the performance and stability of the modified electrolyte in other battery chemistries and configurations. While the modified electrolyte exhibited excellent low-temperature performance, long-term cycling stability at extremely low temperatures (-70 °C and below) needs further evaluation. The cost-effectiveness of the fluorinated cosolvents in large-scale production should also be considered.