Enhanced Cycling Stability of High-Voltage Sodium-Ion Batteries via DFEC-Driven Fluorinated Interface Engineering

Abstract

With their considerable capacity and structurally favorable characteristics, layered transition metal oxides have become strong contenders for cathode use in sodium-ion batteries (SIBs). Nevertheless, their practical deployment is challenged by pronounced capacity loss, predominantly induced by unstable cathode–electrolyte interphase (CEI) at elevated voltages. In this study, difluoroethylene carbonate (DFEC) is introduced as a functional electrolyte additive to engineer a robust and uniform CEI. The fluorine-enriched CEI effectively suppresses parasitic reactions, mitigates continuous electrolyte decomposition, and facilitates stable Na⁺ transport. Consequently, Na/NaNi_1/3Fe_1/3Mn_1/3O₂ (Na/NFM) cells with 2 wt.% DFEC retain 78.36% of their initial capacity after 200 cycles at 1 C and 4.2 V, demonstrating excellent long-term stability. Density functional theory (DFT) calculations confirm the higher oxidative stability of DFEC compared to conventional solvents, further supporting its interfacial protection role. This work offers valuable insights into electrolyte additive design for high-voltage SIBs and provides a practical route to significantly improve long-term electrochemical performance.

1. Introduction

The rapid proliferation of lithium-ion batteries (LIBs) has intensified concerns regarding the finite supply and geographically constrained distribution of lithium resources [1,2,3]. Due to its natural abundance, low cost, and lithium-like electrochemical behavior, sodium has emerged as a competitive alternative for energy storage, with SIBs showing great potential in this domain [4,5,6,7]. In this context, SIBs have emerged as a strong candidate for grid-scale energy storage due to their scalability and cost advantages [8,9]. To date, various types of cathode materials for SIBs have been developed, including layered sodium transition metal oxides [10,11,12,13,14,15,16,17,18], Prussian blue analogues [19,20,21,22], and polyanionic frameworks [23,24,25,26,27]. Among these, layered oxides with the general formula Na_xTMO₂ (0 < x ≤ 1) are especially appealing due to their high specific capacity, structural simplicity, low cost, and scalable synthesis [28,29]. Nevertheless, their practical application is challenged by more severe volume changes than those observed in lithium-based counterparts, primarily due to the larger ionic radius of Na⁺ and the complex phase transitions that occur during sodium intercalation and deintercalation [30]. These structural fluctuations can lead to interfacial instability and intragranular cracking along sodium diffusion pathways, which in turn intensify parasitic reactions with the electrolyte and degrade battery performance.

Efforts to optimize layered oxide cathodes have primarily focused on materials engineering approaches, such as surface coatings [31,32,33,34] and elemental doping [35,36,37,38]. However, uniformly coating Na_xTMO₂ particles remains technically challenging due to process complexities [39,40]. By contrast, modifying the electrolyte with functional additives offers a more practical and economical route [41,42]. These additives can regulate the formation and composition of passivation layers on the electrode surface, significantly affecting electrochemical behavior [43,44,45,46,47,48,49]. Recently, both the electrolyte and its electrode interface have received increasing attention due to their crucial roles in ion transport and interfacial stability. For instance, trimethoxymethylsilane (TMSI) facilitates the formation of a stable CEI, thereby suppressing solvent decomposition and mitigating uncontrolled interphase growth [50]. In contrast, DFEC tends to decompose into NaF and polyfluorocarbon-rich species, leading to the construction of a chemically robust and ionically conductive CEI. Such interfacial characteristics are beneficial for maintaining long-term cycling stability under high-voltage conditions. Representative additives such as fluoroethylene carbonate (FEC), vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), and anhydride-based compounds (e.g., succinic anhydride (SA) and diglycolic anhydride (DGA) have been widely investigated to regulate interfacial reactions, mitigate electrolyte decomposition, and construct robust CEI layers. These studies collectively underscore the importance of rational additive design in stabilizing electrodes and interfaces in SIBs. Building upon these insights, this work demonstrates that DFEC can effectively stabilize high-voltage NFM cathodes, offering both mechanistic understanding and practical guidance for electrolyte optimization [51,52,53].

This work investigates the use of DFEC as a film-forming additive in SIB electrolytes. DFEC is shown to facilitate the development of a stable, fluorine-rich CEI on the NFM cathode surface, which effectively passivates the electrode and mitigates ongoing electrolyte degradation. This CEI layer facilitates stable Na⁺ transport and prevents excessive interfacial growth, even during prolonged cycling. Consequently, the NFM cathode exhibits remarkable rate capability, preserving 64.71% of its initial capacity at 3 C, and demonstrates excellent cycling stability by retaining 78.36% capacity after 200 cycles at 1 C in NaClO₄-PC electrolyte with DFEC. These results significantly outperform those observed with the baseline electrolyte. Moreover, DFT calculations confirm that DFEC molecules exhibit lower HOMO energy levels than PC, indicating greater oxidative stability on the cathode side. In summary, this study highlights that strategically designing electrolyte additives provides an effective pathway to enhance interfacial stability and boost the electrochemical performance of sodium-ion rechargeable batteries.

2. Materials and Methods

2.1. Preparation of Electrolytes

The baseline electrolyte (labeled as BE) was 1 M NaClO₄ in PC solution (brand: Suzhou Duoduo Reagent, Suzhou, China). Difluoroethylene carbonate (DFEC, 99.9%) was commercially purchased from Sigma-Aldrich and used without further purification. Different mass fractions of the DFEC additive were incorporated into the baseline electrolyte to obtain various amounts of DFEC (1, 2, 5 and 10 wt.%). The electrolyte solutions were stirred magnetically overnight in a high-purity argon atmosphere glove box, where moisture and oxygen levels were maintained below 0.1 ppm.

2.2. Preparation and Electrochemical Test of Electrodes

The layered oxide cathode (NaNi_1/3Fe_1/3Mn_1/3O₂) (NFM) was used as the active material. Electrodes were prepared by mixing NFM, conductive agent (Super P), and binder (PVDF) in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP), followed by slurry casting onto Al collector and vacuum-drying at 100 °C for 12 h. Then, the dried mixture was used to create 12 mm diameter round electrode pieces with a slicing machine. The mass loading of active material was ~1.9 mg cm⁻². A glass fiber separator (Whatman (Maidstone, UK), GF/D) was used to separate the electrodes in the cell.

The electrochemical test of cells was carried out using the Neware tester in 2.0–4.2 V (vs. Na⁺/Na) at room temperature. The Na/NFM cells were cycled at 0.1 C (1 C = 130 mA g⁻¹) for the first three cycles, followed by cycling at 1 C for the remaining cycles.

2.3. Electrochemical Characterization

The surface morphologies of the NFM electrodes after cycling were examined using scanning electron microscopy (SEM, HITACHI (Tokyo, Japan), S-4800) and transmission electron microscopy (TEM, JEOL Ltd. (Tokyo, Japan), JEM-2100). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific (Waltham, MA, USA), K-Alpha⁺) was employed to analyze the surface chemical composition of the NFM cathodes. Energy level calculations were carried out using density functional theory (DFT) with the Dmol3 module in Materials Studio. The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was used to describe exchange-correlation effects. Geometry optimization was performed with convergence thresholds of 1.0 × 10⁻⁵ Ha for energy, 0.002 Ha/Å for maximum force, and 0.005 Å for maximum displacement.

3. Results and Discussion

To clarify the underlying mechanism of this interfacial robustness, Density functional theory (DFT) calculations were performed. As depicted in Figure 1, the highest occupied molecular orbital (HOMO) energy level of DFEC is calculated to be −7.94 eV, markedly lower than those of PC (−6.84 eV) and NaClO₄ (−6.33 eV). This lower HOMO energy indicates enhanced oxidative resistance, suggesting that DFEC is less likely to undergo anodic decomposition at the high-voltage NFM surface.

The impact of DFEC on the electrochemical performance of Na/NFM half-cells was evaluated by testing within the voltage range of 2.0–4.2 V (vs. Na⁺/Na) under ambient conditions. After initial activation at 0.1 C for three cycles, subsequent cycling was performed at 1 C. As presented in Figure 2a, the cell containing DFEC achieved a high initial coulombic efficiency(ICE) of 93.74% and delivered a reversible discharge specific capacity of 165.1 mA h g⁻¹. This improvement in ICE can be attributed to the preferential decomposition of DFEC, forming a fluorine-rich CEI (NaF and C-F) that suppresses electrolyte decomposition and reduces irreversible Na⁺ loss. In comparison, although the baseline electrolyte cell exhibited a similar capacity of 166.75 mA h g⁻¹, its ICE was notably lower at 67.52%, indicating pronounced electrolyte decomposition and suboptimal cathode utilization. Figure 2b displays the cycling behavior at various DFEC concentrations, with 2 wt. % providing the most favorable outcome—an initial capacity of 165.1 mA h g⁻¹ at 0.1 C and a retained capacity of 109 mA h g⁻¹ after 200 cycles at 1 C, corresponding to a retention of 78.36%. Conversely, in the absence of DFEC, the capacity rapidly declined to 68.12 mA h g⁻¹ within 50 cycles, primarily due to interface degradation and structural instability. Lower DFEC content (<2 wt.%) fails to form a continuous CEI, while higher amounts (>2 wt.%) cause excessive decomposition and impedance rise. Thus, 2 wt.% provides the best balance between interphase stability and ion transport. Based on this performance comparison, further characterization focused on the 2 wt.% DFEC system. The rate capability was evaluated by sequentially cycling the Na/NFM half-cells at current densities of 0.1, 0.2, 0.3, 0.5, 1, 2, and 3 C, followed by a return to 0.5 C to assess capacity reversibility. As shown in Figure 2c, the DFEC-modified cell consistently delivered higher specific capacities across all current densities, achieving 107.69 mA h g⁻¹ at 3 C. These results highlight the beneficial role of DFEC in facilitating Na⁺ transport and enhancing electrochemical kinetics.

To examine surface morphology changes in NFM electrodes after cycling, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations were performed at the fully discharged state following 50 cycles at 1 C in both the baseline and DFEC-containing electrolytes. Post-cycling, the cells were disassembled for microscopic examination. Figure 3a,b present SEM images of NFM electrodes retrieved from baseline and 2% DFEC-containing electrolytes. Severe particle cracking is evident in the baseline electrolyte sample, whereas the electrode cycled with DFEC additive exhibits a much smoother morphology. Additional structural details are revealed by the TEM images in Figure 3c,d. The electrode cycled with the baseline electrolyte develops an irregular and uneven CEI layer, whereas the DFEC-containing system exhibits a smoother and more uniform interfacial film. TEM observations further reveal that the DFEC-derived CEI is thinner and more homogeneous, which effectively stabilizes the cathode surface and facilitates Na⁺ transport across the interface, thereby contributing to the enhanced rate capability. The uneven CEI coverage in the baseline electrolyte system likely facilitates undesirable side reactions and persistent electrolyte decomposition. In comparison, the well-formed CEI layer in the DFEC system offers improved surface protection, mitigating electrolyte consumption and structural degradation over prolonged cycling. Such improvements can be attributed to the presence of DFEC, which promotes the formation of a robust and stable CEI, thereby suppressing interfacial side reactions and enhancing long-term electrode stability. Figure 3e schematically illustrates the role of DFEC in the electrolyte system. Without DFEC, the baseline electrolyte continuously decomposes on the NFM cathode surface, resulting in a thick, poorly conductive CEI layer. This layer severely hampers Na⁺ transport kinetics, contributing to accelerated capacity degradation. Conversely, the electrolyte containing DFEC promotes the formation of a dense and stable CEI layer that effectively passivates the cathode surface, inhibits parasitic side reactions, and preserves efficient Na⁺ transport, thereby enabling enhanced rate capability and prolonged cycling stability.

Figure 4 demonstrates the use of X-ray photoelectron spectrometry (XPS) to analyze the chemical composition of the CEI layers on NFM cathodes post-cycling. The C 1s spectra of electrodes cycled in DFEC-containing electrolyte, calibrated using the adventitious carbon peak at 284.8 eV, show characteristic signals of C-C (284.8 eV), C-O (286.4 eV), C=O (288.6 eV), and C-F (290.8 eV), whereas the baseline cathode exhibits stronger C-C and C=O peaks, indicating more severe electrolyte decomposition. The O 1s spectra were deconvoluted into three components: organic C-O at 532 eV, carbonyl C=O at 533 eV, and the Na KLL Auger transition at 538 eV. The F 1s spectra at 688 eV (C-F) and 685 eV (NaF) confirm DFEC decomposition into inorganic and organic fluorinated species [54,55]. Compared with the baseline, stronger C-F but weaker NaF signals indicate preferential DFEC decomposition. The resulting fluorine-rich CEI suppresses electrolyte side reactions, reduces irreversible Na⁺ loss to improve ICE, and simultaneously provides a more uniform and stable interphase.

4. Conclusions

In summary, DFEC proves to be a highly effective film-forming additive for SIB electrolytes, exhibiting strong oxidative resistance and actively contributing to CEI formation within the operating potential window of 2.0 to 4.2 V. The formed CEI layer exhibits uniformity and stability, efficiently suppressing continuous electrolyte decomposition and maintaining the structural integrity of the NFM cathode throughout repeated Na⁺ intercalation/deintercalation cycles. Benefiting from these interfacial improvements, the DFEC-based electrolyte delivers excellent cycling stability, achieving a capacity retention of 78.36% after 200 cycles at 1 C with a 4.2 V upper cutoff. This study underscores the potential of rational additive design in enhancing CEI stability and long-term battery performance, providing valuable guidance for the practical advancement of high-performance SIBs. Looking forward, future efforts should focus on the following: (i) Expanding the voltage window beyond 4.3 V to enable higher-energy applications. (ii) In situ/operando studies to dynamically track CEI formation and evolution. (iii) Designing synergistic additives or optimized solvent systems to further enhance CEI uniformity, ionic transport, and interfacial stability. These directions will deepen mechanistic understanding, enhance high-voltage cathode performance, and provide guidance for the development of high-energy and long-life SIBs.