The pursuit of sustainable energy solutions has placed advanced energy storage technologies at the forefront of scientific and industrial innovation. Among these, sodium-ion batteries (SIBs) have emerged as a highly promising candidate to complement, and in certain applications potentially replace, lithium-ion systems. The abundance and low cost of sodium resources, coupled with similar intercalation chemistry to lithium, make sodium-ion battery technology attractive for large-scale grid storage and cost-sensitive electric mobility. However, the practical deployment and competitiveness of sodium-ion batteries hinge on overcoming critical challenges related to energy density, cycle life, rate capability, and most critically, safety. The safety concerns are predominantly tied to the intrinsic properties of the electrolytes commonly employed. Conventional sodium-ion battery electrolytes, typically consisting of flammable organic carbonates like ethylene carbonate (EC) and diethyl carbonate (DEC) with sodium hexafluorophosphate (NaPF6) salt, are highly volatile and combustible. This poses a significant fire risk under conditions of thermal runaway, which can be triggered by overcharging, internal short circuits, or mechanical abuse. Therefore, enhancing the safety profile without compromising, or indeed while enhancing, the electrochemical performance is a paramount objective in sodium-ion battery research.
This article presents a comprehensive investigation from our research into a strategic electrolyte engineering approach aimed at constructing high-rate and safer sodium-ion batteries. We focus on the use of functional additives in commercial carbonate-based electrolytes to simultaneously impart flame-retardant characteristics and improve interfacial kinetics at the hard carbon anode, which is the most prevalent negative electrode material for sodium-ion battery systems. The core of our study revolves around tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a multi-functional electrolyte additive. We will systematically explore its effects on electrolyte flammability, electrochemical stability, hard carbon electrode performance, and the underlying interfacial chemistry and ion transport dynamics. This deep dive aims to provide a holistic understanding of how molecular-level modifications in the electrolyte can lead to macroscopic improvements in sodium-ion battery performance and safety.
1. Introduction: The Electrolyte Challenge in Sodium-Ion Batteries
The electrolyte in a sodium-ion battery serves as the vital medium for ionic conduction between the cathode and anode. Its properties dictate key performance metrics: ionic conductivity, electrochemical stability window, interfacial compatibility with electrodes, and thermal/chemical stability. The standard formulation of 1M NaPF6 in EC/DEC, often with small amounts of film-forming additives like fluoroethylene carbonate (FEC), offers a reasonable balance of conductivity and anode compatibility for sodium-ion batteries. However, this formulation inherits the fundamental safety flaw of its lithium-ion counterparts: high flammability. The organic carbonate solvents have low flash points and can readily ignite when released from a failed cell.
Furthermore, the interfacial stability between this electrolyte and the hard carbon anode is not optimal, especially under high current densities (high rates). The solid electrolyte interphase (SEI) formed can be inhomogeneous and dynamically unstable during cycling, leading to continuous electrolyte decomposition, increased impedance, and poor rate performance. This is often reflected in significant capacity fading when sodium-ion batteries are cycled at rates above 1C. The development of high-power sodium-ion batteries requires electrolytes that facilitate rapid sodium-ion desolvation and transport across the electrode-electrolyte interface.
Several strategies have been explored to address electrolyte safety in sodium-ion batteries. One direct approach is the use of intrinsically non-flammable solvents, such as organic phosphates (e.g., trimethyl phosphate (TMP), triethyl phosphate (TEP)). While effective at suppressing combustion, these solvents often exhibit poor reductive stability against sodium metal and carbonaceous anodes, leading to low Coulombic efficiency and rapid capacity decay in sodium-ion batteries. They can also increase electrolyte viscosity and reduce ionic conductivity. Another strategy is the use of highly concentrated electrolytes (>3 M), which can enhance stability and safety but at a significantly increased cost and with worsened wetting properties. A more pragmatic and economically viable approach is the use of flame-retardant additives. By introducing a small volume percentage (e.g., 5-20 vol%) of an effective flame-retardant molecule into the conventional electrolyte, one can potentially achieve a “self-extinguishing” electrolyte while minimizing negative impacts on bulk electrolyte properties and electrode compatibility. The ideal additive for sodium-ion batteries would thus serve a dual function: enhancing safety and improving electrochemical performance, particularly at high rates.
2. Rationale for Selecting Tris(2,2,2-trifluoroethyl) Phosphate (TFEP)
Our selection of TFEP as a target additive is based on a molecular design principle that combines two key elements: phosphorus (P) and fluorine (F). Organic phosphates are known flame retardants; they can interfere with the free radical chain reactions in the gas phase during combustion. Fluorine, especially in the form of -CF3 groups, is also known to suppress flammability. The presence of both P and F in TFEP is expected to yield a synergistic flame-retardant effect, potentially allowing for a lower required additive concentration to achieve non-flammability compared to non-fluorinated phosphates like TMP or TEP.
Beyond safety, the fluorinated alkyl chains in TFEP are hypothesized to play a crucial role in interfacial chemistry. Fluorine-rich compounds are often preferentially reduced at low potentials, contributing to the formation of a robust, inorganic-rich SEI layer containing LiF or NaF compounds. In the context of lithium-ion batteries, such SEIs are known for their excellent ionic conductivity and stability. We posit a similar beneficial effect for sodium-ion batteries. Furthermore, the high degree of fluorination lowers the molecule’s polarity and its ability to coordinate strongly with Na+ ions. This means TFEP is less likely to enter the primary solvation sheath of Na+, remaining as a “free” or weakly bound molecule in the electrolyte bulk. This characteristic can influence the solvation structure, potentially weakening the interaction between Na+ and the carbonate solvents, which may facilitate easier desolvation at the anode interface—a critical step for high-rate performance in sodium-ion batteries.
The table below summarizes the properties of TFEP compared to other common flame-retardant candidates considered for electrolyte applications in sodium-ion batteries.
| Additive | Chemical Formula | Key Elements | Proposed Advantages | Potential Drawbacks |
|---|---|---|---|---|
| Tris(2,2,2-trifluoroethyl) Phosphate (TFEP) | (CF3CH2O)3P=O | P, F | Synergistic flame retardancy; Potential for F-rich stable SEI; Weak Na+ coordination. | Higher cost; Possible reductive decomposition at low voltage. |
| Trimethyl Phosphate (TMP) | (CH3O)3P=O | P | Effective flame retardant; Low viscosity. | Poor electrochemical stability; High reactivity with Na anodes. |
| Triethyl Phosphate (TEP) | (CH3CH2O)3P=O | P | Effective flame retardant. | Similar to TMP; May increase viscosity more than TMP. |
| 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) | CF2HCF2OCF2CF2CF3 | F, H | Good fluorinated diluent; Can improve Li/Na metal stability. | Primarily a diluent; Limited flame-retardant contribution from P. |
3. Experimental Design and Electrochemical Characterization
In our study, we formulated electrolytes by adding 20 vol% of TFEP into a baseline commercial electrolyte (1M NaPF6 in EC/DEC with 5% FEC, denoted as NP002). For comparison, electrolytes with other additives (TMP, TEP, TTE) were also prepared. Hard carbon (HC) electrodes were fabricated and assembled into Na/HC half-cells for electrochemical evaluation within the sodium-ion battery configuration.
The flammability test, a critical safety assessment, revealed a transformative effect. While the baseline NP002 electrolyte burned vigorously with a long self-extinguishing time, the NP002+20%TFEP electrolyte demonstrated markedly improved resistance to combustion. This directly confirms the efficacy of TFEP as a flame-retardant additive for sodium-ion battery electrolytes.
Electrochemical stability was first assessed via linear sweep voltammetry (LSV) using Na/Al cells. The TFEP-containing electrolyte maintained a high anodic stability up to ~5.3 V vs. Na/Na+, comparable to the baseline, indicating that the additive does not introduce new vulnerabilities to oxidative decomposition at the cathode side of a sodium-ion battery. In contrast, electrolytes with TMP or TEP showed decomposition currents at lower voltages.
The core of the electrochemical performance evaluation focused on the Na/HC half-cells. The initial cycle Coulombic efficiency (ICE) in the TFEP-containing electrolyte was slightly lower (~76%) than in the baseline (~78%), suggesting a more pronounced irreversible capacity loss associated with SEI formation. This is consistent with the reductive decomposition of TFEP contributing to the initial SEI layer. The rate capability tests yielded compelling results. While both cells delivered similar capacities at low rates (e.g., ~300 mAh g-1 at 0.1C), the advantage of the TFEP-based electrolyte became starkly evident at high rates. At a 4C rate, the cell with TFEP retained about 71 mAh g-1, significantly higher than the ~50 mAh g-1 from the baseline cell. More importantly, the long-term cycling stability at 1C was greatly enhanced. The baseline cell suffered from continuous capacity fade, retaining only about 45% of its initial reversible capacity after 400 cycles. In contrast, the cell with TFEP additive exhibited outstanding stability, maintaining a steady reversible capacity of approximately 100 mAh g-1 over 400 cycles, showcasing the dual benefit of high-rate capability and long cycle life for this sodium-ion battery configuration.
| Electrolyte | ICE (%) | Capacity at 0.1C (mAh g-1) | Capacity at 4C (mAh g-1) | Capacity Retention after 400 cycles at 1C (%) | Flammability (Self-extinguishing time) |
|---|---|---|---|---|---|
| NP002 (Baseline) | ~78 | ~300 | ~50 | ~45 | High (Long SET) |
| NP002 + 20% TFEP | ~76 | ~300 | ~71 | >95 (steady ~100 mAh g-1) | Very Low (Short SET) |
| NP002 + 20% TMP | Very Low | < 100 | Very Low | Poor | Low |
| NP002 + 20% TEP | Low | ~90 | Low | Poor | Low |
4. Mechanistic Insights: Solvation Structure and Interfacial Kinetics
To understand the origin of the enhanced performance, we employed Raman spectroscopy and electrochemical analysis to probe the solvation structure and interfacial kinetics in the sodium-ion battery system.
Raman spectra of the electrolytes provided clear evidence of a modified solvation environment. In the baseline electrolyte, strong peaks associated with Na+-solvent (EC, DEC) coordination were observed. In the TFEP-containing electrolyte, the intensity of these coordination peaks diminished, while peaks for free solvent molecules became relatively more prominent. Notably, no distinct peaks indicating strong coordination between Na+ and TFEP were detected. This confirms our hypothesis that TFEP, due to its low polarity and steric hindrance from -CF3 groups, resides outside the primary solvation sheath of Na+. Its role is to modulate the bulk electrolyte structure by diluting and weakening the strong binding between Na+ and the carbonates. This altered solvation structure is critical, as it reduces the energy required for Na+ desolvation—the process where the solvent shell is stripped away as the ion crosses the SEI to enter the electrode. A lower desolvation energy barrier directly translates to improved rate capability, a key finding for advancing high-power sodium-ion batteries.
We can model the desolvation process as an activated step. The rate constant \( k_{desolv} \) for desolvation can be expressed by an Arrhenius-type equation:
$$ k_{desolv} = A \exp\left(-\frac{E_a}{k_B T}\right) $$
where \( E_a \) is the activation energy for desolvation, \( k_B \) is Boltzmann’s constant, \( T \) is temperature, and \( A \) is the pre-exponential factor. A weaker Na+-solvent interaction, induced by the presence of TFEP, effectively lowers \( E_a \), thereby increasing \( k_{desolv} \) and enabling faster charge transfer at high currents in the sodium-ion battery.
Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis provided further evidence. The overall cell resistance, particularly the charge transfer resistance (\( R_{ct} \)), was significantly lower in the cell with TFEP additive throughout formation and cycling. The DRT maps, which deconvolute different electrochemical processes based on their time constants, showed a more stable and lower-intensity signal for the charge transfer process in the TFEP cell compared to the baseline. This indicates the formation of a more conductive and stable SEI, which minimizes interfacial polarization during operation of the sodium-ion battery.
The sodium-ion diffusion coefficient (\( D_{Na^+} \)) within the hard carbon electrode was quantitatively evaluated using cyclic voltammetry (CV) at different scan rates and the Randles-Sevcik equation:
$$ I_p = 2.69 \times 10^5 \cdot n^{3/2} \cdot A \cdot D^{1/2} \cdot C \cdot v^{1/2} $$
where \( I_p \) is the peak current (A), \( n \) is the number of electrons transferred (assumed 1 for Na+ intercalation), \( A \) is the electrode area (cm2), \( D \) is the diffusion coefficient (cm2 s-1), \( C \) is the bulk concentration of Na+ in the electrode (mol cm-3), and \( v \) is the scan rate (V s-1). The calculated \( D_{Na^+} \) was higher in the TFEP-based electrolyte (\( \sim 2.45 \times 10^{-9} \) cm2 s-1) compared to the baseline (\( \sim 4.36 \times 10^{-9} \) cm2 s-1). This confirms that the improved interfacial properties facilitate faster solid-state diffusion of Na+ within the hard carbon, contributing to the superior high-rate performance of the sodium-ion battery.
The voltage polarization, evident from the gap between oxidation and reduction peaks in differential capacity (dQ/dV) plots, was also smaller in the TFEP cell (~63 mV) than in the baseline cell (~70 mV), consistent with lower overall interfacial resistance.
5. Interface Chemistry: XPS Analysis of the SEI Layer
X-ray photoelectron spectroscopy (XPS) analysis was conducted on hard carbon electrodes after 50 cycles to decipher the chemical composition of the SEI layer formed in different sodium-ion battery electrolytes.
The C 1s spectrum from the baseline cell showed signatures primarily from hydrocarbon and C-O species, indicative of decomposed organic carbonates. In contrast, the SEI from the TFEP-containing electrolyte exhibited additional, distinct peaks corresponding to C-F bonds (around 290 eV binding energy) and a different distribution of C-O/C=O species. The F 1s spectrum showed a much more intense signal, which could be deconvoluted into contributions from both NaF/P-F (from salt and TFEP decomposition) and C-F bonds. Most tellingly, the P 2p spectrum from the TFEP cell revealed clear peaks assigned to P-O bonds (from phosphate decomposition) and P-F bonds, which were absent or minimal in the baseline cell’s spectrum.
Quantitative atomic percentage analysis showed that the SEI from the TFEP cell had higher carbon and fluorine content but lower sodium and oxygen content compared to the baseline SEI. This composition analysis supports a coherent mechanism:
- Preferential Reduction: TFEP, with its lower lowest unoccupied molecular orbital (LUMO) energy compared to EC/DEC, is reduced prior to or concurrently with the carbonate solvents during the initial charging of the sodium-ion battery.
- Formation of a Fluorine-Rich, Hybrid SEI: This reduction leads to the incorporation of fluorine and phosphorus species (NaF, NaxPOyFz, organic fluorophosphates) into the SEI matrix from the very beginning.
- Passivation and Stabilization: This F- and P-rich inorganic/organic hybrid SEI is dense, highly ionically conductive (for Na+), and mechanically robust. It effectively passivates the hard carbon surface, suppressing further continuous decomposition of both the electrolyte salt (NaPF6) and the carbonate solvents in subsequent cycles.
- Consequences: The stabilized interface results in lower and stable impedance (as seen in EIS), less “dead sodium” trapping, and excellent long-term cycling stability for the sodium-ion battery.
6. Comparative Analysis and the Role of Molecular Structure
The failure of other additives like TMP and TEP to improve performance, despite their flame-retardant properties, highlights the importance of molecular design. While they provide P for flame retardancy, they lack the fluorine functionality. Their LUMO levels are also unsuitable, leading to excessive and detrimental reduction on the anode, forming unstable SEIs that cannot support efficient Na+ transport. Additives like TTE, which are fluorinated but lack phosphorus, may improve certain interfacial aspects but do not provide the same level of synergistic flame retardancy. TFEP uniquely combines the flame-retardant phosphorus center with three fluorinated alkyl arms. The -CF3 groups serve multiple functions: enhancing flame retardancy, lowering molecular polarity to stay out of the solvation sheath, and providing a source for forming a stable, F-rich SEI. This multi-functionality is the key to its success in simultaneously addressing safety and high-rate performance challenges in sodium-ion batteries.
The performance enhancement can be summarized by the following interconnected equations describing the system:
1. Solvation Energy Moderation: The presence of TFEP (A) weakens the average solvation energy (\( \Delta G_{solv} \)) of Na+.
$$ \Delta G_{solv, \text{with TFEP}} < \Delta G_{solv, \text{baseline}} $$
2. Interfacial Charge Transfer Kinetics: The charge transfer resistance (\( R_{ct} \)) is inversely related to the exchange current density (\( i_0 \)), which depends on the activation energy (\( \Delta G^* \)) for the charge transfer step, itself influenced by desolvation.
$$ R_{ct} \propto \frac{1}{i_0} ; \quad i_0 = nFAk^0 C \exp\left(-\frac{\Delta G^*}{RT}\right) $$
A lower \( \Delta G_{solv} \) contributes to a lower \( \Delta G^* \), leading to a higher \( i_0 \) and lower \( R_{ct} \).
3. Overall Cell Overpotential (\( \eta \)): At a given current \( I \), the total overpotential is reduced due to lower ohmic (\( R_\Omega \)) and charge transfer losses.
$$ \eta = I(R_\Omega + R_{ct}) + \text{(mass transport overpotential)} $$
The TFEP-derived SEI reduces \( R_{ct} \) and improves mass transport, minimizing \( \eta \), especially at high \( I \) (high rate).
7. Conclusion and Future Perspectives
This in-depth investigation demonstrates that electrolyte engineering via multifunctional additives is a powerful and pragmatic strategy to advance sodium-ion battery technology. The introduction of tris(2,2,2-trifluoroethyl) phosphate (TFEP) as an additive into a conventional carbonate-based electrolyte successfully tackles two major hurdles concurrently: electrolyte flammability and poor high-rate performance of hard carbon anodes.
Our findings establish that TFEP acts not merely as a flame-retardant diluent but as a critical modulator of the electrolyte’s solvation structure and the anode’s interfacial chemistry in sodium-ion batteries. By residing outside the primary Na+ solvation shell, it weakens ion-solvent interactions, thereby lowering the desolvation energy barrier. This, coupled with its preferential reduction to form a robust, fluorine- and phosphorus-rich solid electrolyte interphase (SEI), creates an ideal interface for rapid and stable sodium-ion transport. The result is a sodium-ion battery half-cell that exhibits remarkable rate capability (retaining 71 mAh g-1 at a 4C rate), exceptional long-term cycling stability (maintaining 100 mAh g-1 over 400 cycles at 1C), and significantly enhanced safety due to the electrolyte’s self-extinguishing property.
The insights gained from this study on solvation structure manipulation and targeted SEI engineering through molecular design are broadly applicable. Future work on sodium-ion batteries can explore several avenues:
- Optimization of Additive Concentration: Finding the minimal effective amount of TFEP to balance performance enhancement, cost, and any potential trade-offs in low-temperature performance or viscosity.
- Combination with Other Functional Additives: Investigating synergistic effects between TFEP and other additives (e.g., for cathode protection or HF scavenging) to formulate a comprehensive high-performance electrolyte package for sodium-ion batteries.
- Application in Full Cells: Validating the benefits of TFEP-containing electrolytes in practical sodium-ion battery full cells paired with various cathode materials (e.g., layered oxides, polyanionic compounds) to assess overall energy density, cycle life, and safety performance.
- Molecular Derivatives: Designing and testing new molecules based on the P-F synergistic principle but with modified alkyl chain lengths or different fluorination patterns to further fine-tune properties for sodium-ion batteries.
In conclusion, this work underscores that significant leaps in sodium-ion battery performance are achievable through thoughtful electrolyte design. The use of TFEP exemplifies how a single, well-chosen additive can transform a standard electrolyte into a high-functioning component that addresses multiple cell-level requirements, paving a clearer path toward the realization of safe, high-power, and durable sodium-ion batteries for large-scale energy storage applications.