Core Solvents for High-Voltage Electrolytes (I) | Structure & Properties of Linear Fluorinated Carbonates

Challenges of Energy Density Improvement and High-Voltage Electrolytes

The total energy of a battery cell follows the formula E = Q × V (E = energy density; Q = specific capacity; V = operating voltage). Using high-specific-capacity cathode and anode materials and raising the charge cutoff voltage (>4.3V) is an effective approach to achieve high-energy-density batteries. Currently, mainstream high-voltage cathode materials include high-voltage lithium cobalt oxide, high-nickel ternary materials, lithium nickel manganese oxide, and lithium cobalt phosphate.

LiCoO₂ is the earliest commercialized lithium-ion battery cathode material, with a theoretical specific capacity of 274 mAh/g. At an initial operating voltage of 4.2V, its discharge specific capacity is only 140 mAh/g. Raising the operating voltage to 4.5V increases the discharge specific capacity to 185 mAh/g; further raising it to 4.6V boosts the specific capacity to 220 mAh/g, delivering a 30% improvement in energy density. Spinel LiNi_0.5Mn_1.5O₄ is a high-voltage platform cathode material with a theoretical specific capacity of 147 mAh/g and an operating voltage up to 5V vs. Li/Li⁺. High-energy-density LiCoPO₄ has a theoretical specific capacity of 167 mAh/g, a lithiation potential as high as 4.8V, and a theoretical specific energy of 800 Wh/kg.

As a core battery system for the upgrading of the lithium battery industry, high-voltage systems are the breakthrough point for significantly improving battery energy density. Matching electrolytes must exhibit excellent high-voltage interfacial stability. The industry's mainstream R&D pathways for high-voltage electrolytes include four major routes: highly stable solvents, high-concentration / localized high-concentration electrolytes, ionic liquids, and high-voltage functional additives. Among them, highly stable solvents are the most fundamental and core route — as the main component of electrolytes, the intrinsic oxidation resistance and interfacial film-forming properties of solvents directly determine the high-voltage adaptation limit of electrolytes.

1. Performance Comparison of High-Voltage Solvents

1.1 Overview of High-Voltage Solvent Categories

Currently, solvents with high-voltage adaptation potential mainly include fluorinated solvents, nitriles, and sulfones, all featuring high oxidation potentials, but with significant differences in interface compatibility, physicochemical properties, and industrialization capability.

Although nitriles and sulfones exhibit excellent high-voltage oxidation resistance, they suffer from poor anode interface compatibility, weak autonomous film-forming ability, and high intrinsic viscosity, making them difficult to apply alone. They usually require blending with low-viscosity co-solvents or functional film-forming additives. In contrast, fluorinated carbonates balance high-voltage stability, excellent electrode interface compatibility, flame-retardant safety and wide-temperature performance, and are currently the only high-voltage electrolyte solvent system achieving large-scale commercialization.

1.2 Fluorinated Carbonates vs. Fluorinated Carboxylates

As two categories of fluorinated solvents, fluorinated carbonates and fluorinated carboxylates have clearly differentiated application scenarios. Research by the team of Zhao Xiaofeng at Yanshan University shows that most difluorinated carboxylates have an oxidation potential below 4.2V, with only a few systems reaching 4.5V, limiting their high-voltage ceiling. The comparison of advantages and disadvantages is as follows.

1.3 Three Linear Fluorinated Carbonate Solvents

This article focuses on three common linear fluorinated carbonates: Methyl Trifluoroethyl Carbonate (FEMC), Ethyl Trifluoroethyl Carbonate (FEEC), and Bis(trifluoroethyl) Carbonate (FDEC). All three feature a carbonate backbone, with gradient regulation of high-voltage stability achieved by introducing different numbers of trifluoroethyl (-CF₃) groups.

1.3.1 Structural Information

Figure 1. Chemical structures of three linear fluorinated carbonates (FEMC, FDEC, FEEC)

Structurally, FEMC contains one trifluoroethyl group and one methyl group, presenting a mono-fluorinated asymmetric structure. FEEC contains one trifluoroethyl group and one ethyl group, also a mono-fluorinated asymmetric structure but with a longer alkyl chain. FDEC contains two trifluoroethyl groups, presenting a di-fluorinated symmetric structure. The increasing number of fluorine substitutions directly affects the physicochemical properties and electrochemical behavior of the three solvents.

1.3.2 Structural Features and Physical Properties

The strong electron-withdrawing effect of fluorine atoms effectively reduces the molecular boiling point and increases the dielectric constant and molecular polarity. Among the three fluorinated carbonates, the dielectric constant follows the order: FEMC > FEEC ≈ FDEC. Based on the number of fluorine atoms and structural regularity/symmetry, the viscosity of FEEC is speculated to be higher than that of FDEC, resulting in a viscosity order of FEMC > FEEC > FDEC.

Notably, compared with non-fluorinated DEC and EMC, the dielectric constants of the three fluorinated carbonates are significantly increased (from ~2.8–3.0 to 5.4–7.0), indicating stronger lithium salt dissolution capacity and improved ionic conductivity of the electrolyte. Meanwhile, the introduction of fluorine atoms also increases viscosity, which needs to be balanced by solvent blending in formulation design.

1.3.3 Redox Potential and Frontier Molecular Orbital Energy

Fluorine atoms significantly change the physical properties of linear carbonates. What impact do they have on redox potential and frontier orbital energy? Literature data for several carbonates are summarized below.

Through quantitative simulation, the team of Yang Xiaohua at Beijing University of Chemical Technology systematically compared the electrochemical energy levels and redox properties of carbonates with different fluorination structures. Fluorine atoms simultaneously lower the HOMO and LUMO energy levels of molecules, which not only improves cathode oxidation stability but also enhances anode film-forming capability.

Oxidation Resistance: FDEC > FEEC > FEMC > EC

Anode Film-Forming Capability: FDEC > FEEC > FEMC > EC

FDEC, with a di-fluorinated symmetric structure, has the highest degree of fluorination and the best interfacial stability and high-voltage tolerance; the performance of mono-fluorinated asymmetric FEMC and FEEC decreases sequentially. This rule indicates that the number of fluorine substitutions and molecular symmetry are two key structural parameters regulating the high-voltage performance of fluorinated carbonates.

Summary & Preview of Part II

The above analysis clearly demonstrates the gradient differences in structure and performance among the three linear fluorinated carbonates:

• FEMC: Mono-fluorinated asymmetric structure, highest dielectric constant, relatively lowest viscosity, with advantages in low-temperature and rate performance;
• FEEC: Mono-fluorinated asymmetric structure, balanced overall performance, with higher oxidation potential than FEMC;
• FDEC: Di-fluorinated symmetric structure, highest oxidation potential , lowest HOMO energy level, with optimal high-voltage interfacial stability and anode film-forming capability.