In the pursuit of advanced electrochemical energy storage systems, sodium-ion battery technology has garnered significant attention due to the abundance and low cost of sodium resources. As a researcher focused on developing cost-effective and high-performance materials for sodium-ion battery applications, I have been exploring the use of petroleum coke, a by-product of crude oil refining, as an anode material. Petroleum coke-based carbon materials offer promising sodium storage capabilities, but their performance is highly dependent on electrolyte compatibility. In this study, I conducted a comprehensive investigation into the compatibility of laboratory-developed petroleum coke-based anodes with typical organic electrolytes, employing experimental analyses and computational simulations to elucidate the underlying mechanisms. The goal is to advance the practical application of low-cost anode materials in sodium-ion battery systems.
The development of sodium-ion battery technology hinges on the optimization of both electrode materials and electrolytes. Petroleum coke, with its high carbon content and low cost, presents an attractive precursor for hard carbon anodes. However, the electrochemical performance, including capacity, cycle life, and rate capability, is intimately linked to the interfacial interactions between the anode and electrolyte. In this work, I synthesized petroleum coke-based carbon materials through a controlled pyrolysis and carbonization process, and evaluated their sodium storage behavior in ester-based and ether-based electrolytes. The study integrates material characterization, electrochemical testing, interface analysis, and molecular-level simulations to provide a holistic understanding of the anode-electrolyte compatibility in sodium-ion battery configurations.
The petroleum coke-based carbon material was prepared from raw petroleum coke obtained from a refinery. The raw material was dried, ball-milled, and mixed with a processing aid before being subjected to pyrolysis in an argon atmosphere at 1200°C, followed by methane gas treatment at 900°C to enhance carbon structure. The resulting carbon material was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and Brunauer-Emmett-Teller (BET) surface area analysis. The electrochemical performance was evaluated in coin-type half-cells with sodium metal as the counter electrode, using two electrolytes: a conventional ester-based electrolyte (1.0 M NaPF6 in EC/DMC/EMC, volume ratio 1:2:2, denoted as EDE) and an ether-based electrolyte (1.0 M NaPF6 in DME/DOL, volume ratio 1:1, denoted as DD). Cyclic voltammetry (CV), galvanostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS), and post-mortem analyses were performed to assess compatibility. Computational studies, including density functional theory (DFT) and molecular dynamics (MD) simulations, were conducted to explore solvent-solute interactions and ion transport properties.
The structural and morphological characteristics of the petroleum coke-based carbon material are critical for its sodium storage behavior. XRD patterns revealed broad diffraction peaks at approximately 24° and 43°, corresponding to the (002) and (100) planes of amorphous carbon. The interlayer spacing (d002) was calculated using Bragg’s law:
$$ d_{002} = \frac{\lambda}{2 \sin \theta} $$
where λ is the X-ray wavelength (Cu Kα, 0.154 nm) and θ is the diffraction angle. The calculated d002 was 3.83 Å, which is larger than that of graphite (3.35 Å), facilitating Na+ intercalation. Raman spectroscopy showed distinct D and G bands at around 1357 cm−1 and 1590 cm−1, respectively, with an ID/IG ratio of 1.30, indicating a disordered carbon structure conducive to sodium ion storage. The BET surface area was measured as 5.8 m2/g, which is relatively low and beneficial for minimizing irreversible capacity loss due to solid electrolyte interphase (SEI) formation. SEM and TEM images confirmed a blocky morphology with particle sizes around 8 μm and the presence of micro/nano-channels and pores, along with a thin layer of relatively ordered carbon at the edges (3–10 nm thick). These structural features collectively contribute to the sodium storage performance in sodium-ion battery applications.
| Parameter | Value | Significance for Sodium-Ion Battery |
|---|---|---|
| Interlayer spacing (d002) | 3.83 Å | Promotes Na+ intercalation |
| ID/IG ratio | 1.30 | Indicates disorder, enhancing Na+ storage sites |
| BET surface area | 5.8 m2/g | Reduces irreversible SEI formation |
| Pore structure | Micro/nano-channels | Facilitates ion diffusion |
The electrochemical performance of the petroleum coke-based anode was evaluated in sodium-ion battery half-cells. Galvanostatic charge-discharge tests were conducted within a voltage window of 0.01–2.5 V vs. Na+/Na. In the first cycle at a current density of 30 mA/g, the discharge specific capacities were 406.00 mAh/g and 381.91 mAh/g for EDE and DD electrolytes, respectively, with coulombic efficiencies (CE) of 85.04% and 90.42%. The lower irreversible capacity in DD electrolyte (36.58 mAh/g vs. 60.76 mAh/g in EDE) suggests more efficient SEI formation. The capacity contribution was analyzed by dividing the discharge curve into a slope region (>0.1 V) and a plateau region (≤0.1 V). In both electrolytes, the plateau region accounted for over 60% of the total capacity, indicating that sodium storage primarily occurs via insertion into the carbon layers. Cycle performance at 100 mA/g showed that the DD-based sodium-ion battery retained 265.66 mAh/g after 100 cycles, whereas the EDE-based sodium-ion battery degraded to 122.37 mAh/g with unstable cycling. Rate capability tests from 0.1C to 3C (1C = 500 mA/g) demonstrated superior performance for the DD electrolyte, with a discharge capacity of 74.05 mAh/g at 3C compared to nearly negligible capacity for EDE. Upon returning to 0.1C, the DD-based sodium-ion battery recovered to 303.65 mAh/g, highlighting better reversibility.
To quantify the rate performance, the capacity retention at different C-rates can be expressed as:
$$ \text{Capacity Retention} = \frac{C_{\text{discharge}}(i)}{C_{\text{discharge}}(0.1C)} \times 100\% $$
where \( C_{\text{discharge}}(i) \) is the discharge capacity at current density i. The data underscore the importance of electrolyte selection for high-rate sodium-ion battery operation.
| Electrolyte | First Discharge Capacity (mAh/g) | First Cycle CE (%) | Capacity at 100 Cycles (mAh/g) | Recovery Capacity at 0.1C after Rate Test (mAh/g) |
|---|---|---|---|---|
| EDE (ester) | 406.00 | 85.04 | 122.37 | 146.05 |
| DD (ether) | 381.91 | 90.42 | 265.66 | 303.65 |
Electrochemical impedance spectroscopy (EIS) was employed to analyze interfacial resistance in the sodium-ion battery. Nyquist plots were fitted using an equivalent circuit model comprising ohmic resistance (Rs), SEI film resistance (RSEI), charge transfer resistance (Rct), and Warburg diffusion element. After 3 hours of rest, the DD-based cell exhibited lower Rs but higher RSEI compared to the EDE-based cell, suggesting initial SEI formation was more rapid in DD. After 100 cycles, both Rs and RSEI were lower in the DD-based sodium-ion battery, indicating a thinner and more stable SEI layer. The diffusion coefficient of Na+ in the SEI can be estimated from the Warburg region using the formula:
$$ D_{\text{Na}^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where R is the gas constant, T is temperature, A is electrode area, n is number of electrons, F is Faraday’s constant, C is Na+ concentration, and σ is the Warburg coefficient. The lower impedance in DD electrolyte correlates with enhanced ion transport, beneficial for sodium-ion battery longevity.
Cyclic voltammetry (CV) at a scan rate of 1.00 mV/s provided insights into the redox kinetics. In the EDE electrolyte, the CV curves for the 2nd and 3rd cycles did not fully overlap, indicating ongoing electrolyte decomposition and unstable SEI. In contrast, the DD electrolyte showed better overlap, signifying stable SEI formation. The oxidation peaks associated with Na+ extraction were more pronounced in DD, suggesting faster de-solvation and insertion kinetics. This can be attributed to the weaker solvation energy of ether solvents, which facilitates Na+ desolvation and penetration through the SEI. The peak current (ip) in CV relates to the kinetics according to the Randles-Sevcik equation for diffusion-controlled processes:
$$ i_p = 0.4463 n F A C \left( \frac{n F v D}{R T} \right)^{1/2} $$
where v is scan rate and D is diffusion coefficient. The higher peak currents in DD imply superior ion mobility, crucial for high-performance sodium-ion battery anodes.
Post-mortem analysis of electrodes after the first cycle revealed distinct SEI morphologies. SEM images showed uneven surface deposits on the EDE electrode, while the DD electrode had a more uniform coating. TEM cross-sections indicated SEI thicknesses of approximately 20 nm for EDE and 10 nm for DD, with the latter being more homogeneous. X-ray photoelectron spectroscopy (XPS) depth profiling via Ar+ sputtering (0 and 60 seconds) elucidated SEI composition. In both electrolytes, the outer SEI layer consisted of organic compounds like alkyl carbonates (ROCO2Na), while the inner layer contained inorganic species such as NaF and Na2CO3. However, the DD electrolyte produced a richer NaF content, as evidenced by stronger F 1s signals at deeper depths. This NaF-rich SEI is known to enhance ionic conductivity and stability in sodium-ion battery systems. The formation of NaF is linked to the decomposition of PF6− anions, which are more likely to participate in the solvation shell in ether electrolytes due to weak solvation effects.
To understand the electrolyte properties, Fourier-transform infrared (FTIR) and Raman spectroscopy were performed. FTIR spectra of EDE showed C=O stretching vibrations at 1920–1680 cm−1, while DD exhibited characteristic ether C-O-C bands. In the region 920–800 cm−1, peaks corresponding to contact ion pairs (CIPs, Na+-solvent-PF6−) and free PF6− were observed. For DD, the CIPs signals were more intense, indicating greater anion involvement in the solvation structure. Raman spectra further confirmed solvation differences: the P-F stretching vibration of PF6− shifted from 765 cm−1 in pure salt to 742 cm−1 in EDE and 740 cm−1 in DD upon dissolution, reflecting changes in ion pairing. Additionally, new peaks at 903 cm−1 (EDE) and 868 cm−1 (DD) were assigned to solvent-sodium complexes. These spectroscopic results underscore that ether electrolytes promote anion-coordinated solvation structures, which influence SEI composition and electrochemical behavior in sodium-ion battery applications.
Computational simulations provided molecular-level insights into electrolyte behavior. Density functional theory (DFT) calculations using the BLYP functional in Materials Studio were conducted to determine frontier orbital energies and binding energies. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies for electrolyte components are summarized below:
| Component | HOMO Energy | LUMO Energy |
|---|---|---|
| EC (ester) | -6.76 | 0.12 |
| DMC (ester) | -6.48 | 0.25 |
| EMC (ester) | -6.52 | 0.18 |
| DME (ether) | -5.94 | 0.84 |
| DOL (ether) | -5.87 | 0.41 |
The higher LUMO energies of ether solvents (0.41–0.84 eV) compared to esters (0.12–0.25 eV) indicate better reduction stability, explaining the thinner SEI formation in DD. The solvation energy (ΔEsolv) for Na+ with solvent molecules was calculated as:
$$ \Delta E_{\text{solv}} = E_{\text{Na}^+-\text{solvent complex}} – (E_{\text{Na}^+} + n \times E_{\text{solvent}}) $$
where n is the number of solvent molecules. Values were negative, confirming spontaneous solvation. The de-solvation energy, required to remove one solvent molecule from the complex, was lower for ethers, easing Na+ desolvation at the anode interface. This computational finding aligns with the observed faster kinetics in ether-based sodium-ion battery cells.
Molecular dynamics (MD) simulations were performed to evaluate ion transport. The mean squared displacement (MSD) of Na+ ions was tracked over time, and the diffusion coefficient (D) was calculated using the Einstein relation:
$$ D = \frac{1}{6} \lim_{t \to \infty} \frac{d}{dt} \langle | \mathbf{r}(t) – \mathbf{r}(0) |^2 \rangle $$
The computed D values were 2.77 × 10−8 cm2/s for EDE and 5.42 × 10−8 cm2/s for DD, indicating approximately twofold faster Na+ mobility in the ether electrolyte. Radial distribution functions (RDF) revealed the solvation shell structure. In EDE, the first solvation shell of Na+ contained about 2.7 oxygen atoms from solvent molecules, corresponding to a [Na+]1[EC]1[DMC]1[EMC]2 complex. In DD, there were 4.2 oxygen atoms, suggesting a [Na+]1[DME]1[DOL]4 configuration. The greater coordination number in DD, coupled with anion participation, fosters a solvation structure that promotes beneficial SEI components like NaF, enhancing performance in sodium-ion battery systems.
The integration of experimental and computational data highlights the critical role of electrolyte design in sodium-ion battery technology. The ether-based electrolyte (DD) demonstrates superior compatibility with petroleum coke-based anodes due to its weak solvation power, which facilitates anion-coordinated solvation, stable SEI formation, and rapid ion transport. In contrast, the ester-based electrolyte (EDE) leads to thicker, less stable SEI layers and higher interfacial resistance, limiting rate capability and cycle life. These findings provide a foundation for optimizing electrolyte formulations for low-cost, high-capacity sodium-ion battery applications. Future work could explore hybrid electrolytes or additive engineering to further enhance compatibility and extend the operational voltage window.
In conclusion, this study systematically investigates the compatibility of petroleum coke-based carbon anodes with ester and ether electrolytes in sodium-ion battery configurations. The petroleum coke-derived material exhibits amorphous carbon characteristics with expanded interlayer spacing and disorder, favorable for sodium storage. Electrochemical tests reveal that the ether-based electrolyte enables higher first-cycle coulombic efficiency, better cycle stability, and superior rate performance compared to the ester-based electrolyte. Interface analyses confirm a thinner, more uniform, and NaF-rich SEI in the ether case, while spectroscopic and computational studies elucidate the solvation structures and transport properties underlying these differences. The insights gained underscore the importance of tailored electrolyte chemistry for unlocking the full potential of petroleum coke anodes in sodium-ion battery systems, paving the way for cost-effective and high-performance energy storage solutions.