The transition towards a low-carbon economy, heavily reliant on renewable energy sources, underscores the critical need for efficient and cost-effective electrical energy storage technologies. Among various candidates, sodium-ion batteries have emerged as a highly promising alternative to the dominant lithium-ion technology. The primary driving force is the abundance and wide geographical distribution of sodium resources, which promises a significant reduction in raw material costs and enhances supply chain security for large-scale energy storage applications. However, the practical deployment of conventional sodium-ion batteries is hampered by safety concerns stemming from the use of flammable and potentially leaky organic liquid electrolytes. Furthermore, the limited electrochemical stability window of these liquid electrolytes constrains the operational voltage and energy density of the cells.
Solid-state electrolytes offer a fundamental solution to these safety issues, but their widespread adoption is challenged by low ionic conductivity at room temperature, poor interfacial contact with electrodes, and complex manufacturing processes. Quasi-solid-state batteries, employing gel polymer electrolytes (GPEs), present a pragmatic compromise. GPEs combine the high ionic conductivity of liquid electrolytes with the improved safety and dimensional stability of solid polymers. The in-situ polymerization technique is particularly attractive as it enables the formation of a gel network within the pre-assembled cell, ensuring intimate electrode-electrolyte contact and simplifying production.
In this work, we developed a high-performance quasi-solid-state sodium-ion battery system. The core innovation lies in the design and facile preparation of a novel gel polymer electrolyte via thermal-initiated in-situ radical polymerization of dipentaerythritol penta-/hexa-acrylate (DPEPA). We systematically investigated the physicochemical and electrochemical properties of this GPE. Furthermore, we paired it with a high-capacity O3-type layered oxide cathode, Na(Ni1/3Fe1/3Mn1/3)O2 (NFM), and a hard carbon (HC) anode to construct a full cell. The electrochemical performance, including cycling stability, rate capability, and temperature tolerance, was thoroughly evaluated. To gain deeper insights, we employed in-situ X-ray diffraction to elucidate the structural evolution of both electrodes during operation and conducted X-ray photoelectron spectroscopy to analyze the interfacial chemistry on the hard carbon anode. Our findings demonstrate that the strategic use of a polymer with a low lowest unoccupied molecular orbital (LUMO) energy level is an effective approach to simultaneously enhance interfacial stability and safety in advanced sodium-ion battery systems.
1. Experimental Section
1.1. Materials Synthesis
Cathode Material (NFM): The Na(Ni1/3Fe1/3Mn1/3)O2 cathode material was synthesized via a solid-state reaction. Stoichiometric amounts of Na2CO3, NiO, Fe2O3, and Mn2O3 were mixed and ball-milled at 450 rpm for 4 hours. The resulting mixture was then calcined in air at 850°C for 18 hours with a heating rate of 5°C min-1.
Gel Polymer Electrolyte (GPE): The liquid electrolyte (LE) precursor was prepared by dissolving 1.5 mol L-1 sodium hexafluorophosphate (NaPF6) in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume). To this LE, DPEPA monomer (10 mg mL-1) and azobisisobutyronitrile (AIBN, 0.1 wt%) as the thermal initiator were added. The homogeneous solution served as the GPE precursor. The gelation was achieved by heating the precursor-infiltrated cell or free-standing film at 70°C for 10 hours to complete the radical polymerization.
1.2. Material and Electrochemical Characterization
The morphology was examined by field-emission scanning electron microscopy. The chemical structure was analyzed using Fourier-transform infrared spectroscopy. The crystal structure was determined by X-ray diffraction. The electronic structure calculations for HOMO/LUMO energy levels were performed using Gaussian 16 software with the B3LYP functional and 6-311G(d) basis set.
Electrochemical measurements were conducted using CR2025-type coin cells assembled in an argon-filled glovebox. For half-cell tests, NFM or HC electrodes were paired with a sodium metal counter/reference electrode. The full sodium-ion battery was fabricated using NFM as the cathode and HC as the anode, with a controlled N/P ratio between 1.05 and 1.15. The ionic conductivity (σ) of the electrolyte was determined by electrochemical impedance spectroscopy using a stainless steel (SS)|electrolyte|SS symmetric cell. The sodium ion transference number (tNa+) was measured by combining chronoamperometry and EIS. Linear sweep voltammetry was performed on an SS|electrolyte|Na cell to determine the electrochemical stability window. Galvanostatic charge-discharge tests were carried out on a battery cycler within specified voltage windows.
The ionic conductivity is calculated by:
$$ \sigma = \frac{l}{R_b S} $$
where \( l \) is the thickness of the electrolyte layer, \( R_b \) is the bulk resistance obtained from the high-frequency intercept on the real axis in the Nyquist plot, and \( S \) is the contact area between the electrolyte and the blocking electrode.
The sodium ion transference number is calculated by:
$$ t_{Na^+} = \frac{I_s(\Delta V – I_0 R_0)}{I_0(\Delta V – I_s R_s)} $$
where \( I_0 \) and \( I_s \) are the initial and steady-state currents, \( \Delta V \) is the applied DC polarization voltage (10 mV), and \( R_0 \) and \( R_s \) are the interfacial resistances before and after polarization, respectively.
2. Results and Discussion
2.1. Formation and Properties of the DPEPA-based GPE
The gel polymer electrolyte was formed via in-situ thermal polymerization. Upon heating, the AIBN initiator decomposes to generate radicals, which subsequently initiate the cross-linking polymerization of the multi-acrylate functional groups on the DPEPA monomer. This process transforms the fluid liquid precursor into a dimensionally stable, non-flowing gel, as illustrated below. The polymerization is confirmed by the disappearance of the C=C stretching vibration peak (1629-1639 cm-1) in the FT-IR spectrum after gelation.
Theoretical calculations of the frontier molecular orbitals provide crucial insight into the anticipated interfacial behavior of the electrolyte components. The results are summarized below:
| Component | HOMO Energy (eV) | LUMO Energy (eV) |
|---|---|---|
| NaPF6 | -9.87 | -0.95 |
| DPEPA | -7.42 | -0.58 |
| EC | -7.18 | 0.32 |
| DEC | -6.95 | 0.45 |
Notably, DPEPA possesses a lower LUMO energy level than the carbonate solvents (EC, DEC), making it thermodynamically more susceptible to reduction at the anode. Its LUMO level is also closer to that of NaPF6. This suggests that during the initial charging cycles, DPEPA is likely to decompose preferentially or concurrently with NaPF6 on the surface of the hard carbon anode, contributing to the formation of the solid electrolyte interphase (SEI). Furthermore, its lower HOMO energy indicates a higher oxidative stability compared to the solvents, which is beneficial for stability at the high-voltage cathode.
2.2. Electrochemical Performance of the GPE
The developed GPE exhibits excellent ionic transport properties. At 30°C, its ionic conductivity reaches 1.97 mS cm-1, which is comparable to conventional liquid electrolytes (LE: 2.88 mS cm-1). The temperature-dependent conductivity follows an Arrhenius-like behavior, remaining close to that of the liquid electrolyte across a range from 20 to 60°C.
A more significant improvement is observed in the sodium ion transference number (tNa+). The use of a higher salt concentration (1.5 mol L-1) in the GPE formulation helps mitigate mass transport limitations. The tNa+ for the GPE was determined to be 0.66, which is more than three times higher than that of the baseline liquid electrolyte (tNa+ ~0.20). A high tNa+ signifies that the majority of the current is carried by the working Na+ ions, reducing concentration polarization, lowering internal resistance, and improving power capability and cycle life of the sodium-ion battery.
The electrochemical stability window was evaluated by linear sweep voltammetry. The GPE demonstrated a remarkably wide stable window extending up to 5.1 V vs. Na/Na+, whereas the liquid electrolyte began decomposing above 4.0 V. This enhanced stability can be attributed to two factors: (i) the cross-linked polymer network restricts the free movement of solvent molecules, reducing their access to the electrode surfaces, and (ii) the preferential decomposition of DPEPA facilitates the formation of a robust SEI, suppressing further reduction of EC/DEC, while its low HOMO improves oxidation resistance at the cathode.
2.3. Electrode Characterization and Full Cell Performance
The synthesized NFM cathode material exhibits a well-defined O3-type layered structure with a homogeneous distribution of Ni, Fe, and Mn. In a half-cell configuration versus sodium metal, it delivered a reversible capacity of 137 mAh g-1 at 12 mA g-1 with a high initial Coulombic efficiency of 96%.
The quasi-solid-state full sodium-ion battery (NFM||HC) was assembled using the in-situ formed DPEPA-GPE. Its electrochemical performance was benchmarked against a counterpart cell using a conventional liquid electrolyte (LE). The key performance metrics are summarized in the following table:
| Performance Metric | NFM|GPE|HC (Quasi-Solid) | NFM|LE|HC (Liquid) |
|---|---|---|
| First Discharge Capacity (12 mA g-1) | 114 mAh g-1 | 115 mAh g-1 |
| First Coulombic Efficiency | ~72% | ~79% |
| Capacity Retention after 200 cycles (60 mA g-1) | 96% | 90% |
| Capacity Retention after 300 cycles (120 mA g-1) | 92% | – | Rate Capacity at 360 mA g-1 | 74 mAh g-1 | ~85 mAh g-1 |
| Operating Temperature Range | 20 – 80°C | – |
The quasi-solid-state sodium-ion battery displayed excellent cycling stability, retaining 92% of its capacity after 300 cycles at a relatively high current density of 120 mA g-1. This corresponds to an ultra-low average capacity decay rate of 0.027% per cycle. The rate performance was also respectable, though slightly lower than the liquid cell at very high rates due to the inherently lower ionic conductivity of the gel. Impressively, the GPE-based cell operated reliably across a wide temperature range from 20 to 80°C, with discharge capacity increasing from 99 to 120 mAh g-1 at 60 mA g-1, demonstrating its robust ionic conduction and interfacial stability under varied thermal conditions.
2.4. Interfacial Analysis and Storage Mechanisms
To verify the proposed interfacial modification role of DPEPA, XPS analysis was performed on hard carbon anodes retrieved from cells cycled in GPE and LE. The comparison revealed distinct differences in the SEI composition:
- C 1s Spectra: The SEI formed in GPE showed lower relative intensities for the O-C=O and C-O components (typically from solvent decomposition) compared to the SEI from LE.
- O 1s Spectra: Similarly, the contributions from C-O and C=O species were lower in the GPE-derived SEI, while the relative proportion of inorganic species like Na2O was higher.
- F 1s Spectra: The intensity of the NaF peak was more pronounced in the SEI from the GPE cell.
These findings confirm that the DPEPA polymer participates in the formation of the SEI. Its preferential reduction, alongside NaPF6, leads to an inorganic-rich SEI with higher NaF and Na2O content. This type of SEI is known for better mechanical stability and higher ionic conductivity for Na+, which effectively passivates the anode surface and minimizes continuous decomposition of EC/DEC solvents, thereby explaining the superior cycling stability of the GPE-based sodium-ion battery.
In-situ XRD was employed to study the real-time structural evolution of both electrodes during operation in a full cell. For the NFM cathode, the (006) and (101) diffraction peaks shifted to higher angles during charging (sodiation) and reversibly back during discharging (desodiation). This indicates a highly reversible contraction and expansion of the layered lattice, confirming its structural robustness for sodium-ion insertion/extraction.
For the hard carbon anode, the (002) diffraction peak did not shift, ruling out graphite-like intercalation. Instead, its intensity decreased in the later stages of sodium-ion insertion (charging), suggesting increased structural disorder due to Na+ filling into micropores and defects. The intensity recovered during the extraction (discharging) process. This behavior is characteristic of the widely accepted “adsorption-pore filling” mechanism for sodium storage in hard carbon, without any evidence of sodium metal plating.
3. Conclusion
In summary, we have successfully developed a high-performance quasi-solid-state sodium-ion battery system through the rational design of a gel polymer electrolyte. The in-situ thermally polymerized DPEPA-based GPE combines high ionic conductivity (1.97 mS cm-1), a high Na+ transference number (0.66), and a wide electrochemical stability window (>5 V). Theoretical and experimental evidence confirms that the DPEPA monomer, with its low LUMO energy level, participates in the formation of a stable, inorganic-rich SEI on the hard carbon anode. This interface effectively suppresses continuous electrolyte decomposition and enhances interfacial Na+ transport.
When integrated with an O3-NFM cathode and a hard carbon anode, the resulting quasi-solid-state sodium-ion battery delivers excellent electrochemical performance: outstanding long-term cycling stability (92% capacity retention after 300 cycles), good rate capability, and reliable operation across a wide temperature range from 20 to 80°C. In-situ XRD studies verified the highly reversible structure evolution of the NFM cathode and the “adsorption-pore filling” mechanism in the hard carbon anode.
This work highlights that incorporating functional polymers with tailored electronic structures (e.g., low LUMO) into gel electrolytes is a highly effective strategy for constructing safe and electrochemically robust quasi-solid-state sodium-ion batteries. This approach addresses critical challenges related to interfacial instability while maintaining the processing advantages of liquid-based systems, paving a practical path for the development of next-generation, high-safety energy storage devices.