The global imperative to transition from a fossil fuel-based economy to one powered by clean, renewable energy sources is accelerating the development of advanced electrical energy storage technologies. Electrochemical batteries, particularly lithium-ion batteries (LIBs), have been the cornerstone of this revolution, powering everything from portable electronics to electric vehicles and grid-scale storage. However, the rapid scaling of the LIB industry faces significant material sustainability challenges, primarily due to the constrained and geographically concentrated supply of lithium resources. In this context, sodium-ion batteries (SIBs) have emerged as a compelling complementary technology. Utilizing abundant, low-cost, and widely distributed sodium resources, SIBs present a viable pathway for cost-effective, large-scale energy storage. Despite their advantages, conventional sodium-ion batteries employing liquid electrolytes (LE) share critical safety concerns with their LIB counterparts, including flammability, leakage, and the risk of thermal runaway under abusive conditions. Furthermore, the limited electrochemical stability of organic liquid electrolytes constrains the operating voltage and long-term cyclability of cells.
Solid-state batteries, employing solid electrolytes, offer a promising solution to these safety issues and potentially enable higher energy densities. However, practical application is hindered by typically low ionic conductivity at room temperature, poor interfacial contact with electrodes, and complex manufacturing processes. An intermediate and more pragmatic approach is the development of quasi-solid-state batteries using gel polymer electrolytes (GPEs). These systems retain a significant fraction of liquid components for high ionic conductivity while being immobilized within a solid polymer matrix, thereby mitigating leakage risks and enhancing mechanical stability. The key challenge lies in designing GPEs that simultaneously achieve high ionic conductivity, a high sodium-ion transference number, a wide electrochemical stability window, and benign interfacial properties.
This article details the development of a high-performance quasi-solid-state sodium-ion battery system. The core innovation is an in-situ formed gel polymer electrolyte derived from dipentaerythritol penta-/hexa-acrylate (DPEPA) via a simple thermal-initiated radical polymerization process. This GPE addresses multiple critical requirements for advanced sodium-ion battery operation. We systematically evaluate its physicochemical and electrochemical properties, integrate 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, and investigate the resultant interfacial chemistry and sodium storage mechanisms.
Gel Polymer Electrolyte: Design and Synthesis
The synthesis of the GPE is based on a facile, one-pot in-situ thermal polymerization strategy, which is highly compatible with standard battery assembly processes. The precursor solution consists of a conventional liquid electrolyte—1.5 M sodium hexafluorophosphate (NaPF6) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume)—to which the DPEPA monomer and a thermal initiator, azobisisobutyronitrile (AIBN), are added. Upon heating to 70°C, AIBN decomposes to generate free radicals, which initiate the polymerization of the acrylate functional groups on the DPEPA molecules.
DPEPA is a multi-acrylate monomer, meaning it contains several polymerizable C=C bonds per molecule. This multifunctionality enables the formation of a highly cross-linked three-dimensional polymer network, denoted as p(DPEPA), within the liquid electrolyte. The polymerization reaction can be summarized as:
$$ \text{AIBN} \xrightarrow{\Delta} 2\, \cdot\text{C(CH}_3)_2\text{CN} + \text{N}_2 $$
$$ n\, \text{DPEPA} + \text{Initiator Radicals} \rightarrow \text{Cross-linked p(DPEPA) Network} $$
The resulting material is a self-standing, non-flowing gel that encapsulates the liquid electrolyte, as confirmed by the disappearance of the C=C stretching vibration peak (1629-1639 cm-1) in Fourier-transform infrared (FT-IR) spectroscopy, indicating complete consumption of the monomer. The cross-linked structure provides dimensional stability, significantly reducing the risks of electrolyte leakage and suppressing the flow of free solvent molecules toward the electrodes, which is beneficial for interfacial stability.
Electrochemical and Physicochemical Properties of the GPE
The performance of an electrolyte is paramount for battery operation. Key metrics include ionic conductivity (σ), cation transference number (tNa+), and electrochemical stability window (ESW). We compared the DPEPA-based GPE directly with its liquid counterpart (LE).
Ionic Conductivity: Ionic conductivity determines the rate capability and internal resistance of a battery. The conductivity was measured via electrochemical impedance spectroscopy (EIS) on a symmetric stainless steel (SS) cell. The bulk resistance (Rb) is obtained from the high-frequency intercept on the real axis of the Nyquist plot. The ionic conductivity is calculated using the formula:
$$ \sigma = \frac{l}{R_b \cdot A} $$
where \( l \) is the thickness of the electrolyte layer, and \( A \) is the contact area. The GPE exhibited an ionic conductivity of 1.97 mS cm-1 at 30°C, which is slightly lower than the LE (2.88 mS cm-1) but remains sufficiently high for practical application. The temperature dependence of conductivity followed an Arrhenius-like behavior, with values remaining close to those of the LE across a 20-60°C range.
Sodium-Ion Transference Number: The transference number (tNa+) indicates the fraction of the total ionic current carried by sodium ions. A high tNa+ is crucial for mitigating concentration polarization, especially at high rates, leading to better rate performance and cycle life. It is determined by a combination of DC polarization and AC impedance (Bruce-Vincent method):
$$ t_{\text{Na}^+} = \frac{I_s (\Delta V – I_0 R_0)}{I_0 (\Delta V – I_s R_s)} $$
where \( I_0 \) and \( I_s \) are the initial and steady-state currents, \( \Delta V \) is the applied DC bias, and \( R_0 \) and \( R_s \) are the interfacial resistances before and after polarization. Impressively, the GPE achieved a tNa+ of 0.66, which is more than three times higher than that of the LE (tNa+ ~0.2). This significant enhancement is attributed to the immobilization of the anionic species (PF6–) within the cross-linked polymer network and possibly stronger cation coordination, which facilitates selective Na+ transport.
Electrochemical Stability Window: A wide ESW is essential for compatibility with high-voltage cathodes and stable operation without electrolyte decomposition. Linear sweep voltammetry (LSV) tests were performed using SS as the working electrode and Na metal as the counter/reference. The DPEPA-based GPE demonstrated a remarkably stable window extending up to 5.1 V vs. Na/Na+, whereas the LE began to decompose significantly above 4.0 V. This superior stability originates from two synergistic factors: 1) The polymer network restricts free solvent movement, and 2) The DPEPA molecule itself possesses favorable frontier molecular orbital energy levels.
Density functional theory (DFT) calculations reveal that DPEPA has a lower lowest unoccupied molecular orbital (LUMO) energy level than the EC and DEC solvent molecules, making it more susceptible to reduction. Furthermore, its LUMO level is closer to that of NaPF6. Consequently, during the initial charging cycles, DPEPA decomposes preferentially alongside NaPF6 at the anode (low potential) surface, participating in the formation of the solid electrolyte interphase (SEI). Simultaneously, its higher occupied molecular orbital (HOMO) energy level is lower than those of the solvents, indicating better oxidative stability at the cathode (high potential) side. This dual role—participating in SEI formation and resisting oxidation—is key to the GPE’s broad effective stability window.
| Property | Liquid Electrolyte (LE) | Gel Polymer Electrolyte (GPE) |
|---|---|---|
| Ionic Conductivity @ 30°C (mS cm-1) | 2.88 | 1.97 |
| Na+ Transference Number (tNa+) | ~0.20 | 0.66 |
| Electrochemical Window (V vs. Na/Na+) | ~4.0 | >5.1 |
| Physical State | Liquid, Flowing | Gel, Non-flowing |
Electrode Materials and Half-Cell Performance
NFM Cathode: The positive electrode material, O3-type Na(Ni1/3Fe1/3Mn1/3)O2, was synthesized via a solid-state reaction. X-ray diffraction (XRD) confirmed the phase-pure layered structure (space group R\(\bar{3}\)m). In a half-cell configuration with sodium metal and a conventional liquid electrolyte, the NFM cathode delivered a reversible capacity of 137 mAh g-1 at a low current density of 12 mA g-1 within a voltage window of 2.0-4.0 V vs. Na/Na+, with a high initial Coulombic efficiency of 96%.
Hard Carbon Anode: Commercial hard carbon was used as the anode material. Its sodium storage mechanism, which is distinct from graphite in LIBs, involves a sloping region at higher potentials (adsorption on defect sites and pores) and a low-potential plateau (pore filling).
Quasi-Solid-State Full Sodium-Ion Battery Performance
A quasi-solid-state sodium-ion battery was assembled by integrating the NFM cathode and HC anode with the in-situ formed DPEPA-based GPE (denoted as NFM||GPE||HC). The performance was benchmarked against a control cell using the same electrodes with the conventional liquid electrolyte (NFM||LE||HC).
Cycling Stability: The long-term cycling stability is a critical indicator of practical viability. At a current density of 120 mA g-1, the NFM||GPE||HC cell exhibited outstanding stability. After 300 cycles, it retained 92% of its capacity, corresponding to an average capacity decay rate of only 0.027% per cycle. In contrast, while not shown for 300 cycles at this rate, the liquid cell typically shows faster degradation. At a moderate rate of 60 mA g-1, the GPE-based cell retained 96% capacity after 200 cycles, outperforming the LE-based cell (90% retention). The enhanced stability is attributed to the more stable electrode/electrolyte interfaces formed in the GPE system.
Rate Capability: The cell’s ability to deliver capacity at various discharge rates was evaluated. The NFM||GPE||HC cell demonstrated respectable rate performance, delivering capacities of 110, 101, 95, 86, and 74 mAh g-1 at current densities of 12, 60, 120, 240, and 360 mA g-1, respectively. The performance was comparable, though slightly lower at the highest rate, to the LE-based cell, reflecting the trade-off between the enhanced safety/stability of the gel and its marginally lower ionic conductivity.
Wide-Temperature Operation: A significant advantage of the developed GPE is its stable performance across a broad temperature range. At an operating current of 60 mA g-1, the quasi-solid-state sodium-ion battery delivered stable capacities from 99 mAh g-1 at 20°C up to 120 mAh g-1 at 80°C, with Coulombic efficiency consistently near 100%. This demonstrates the robustness of the polymer network and the effective ion transport pathways it maintains under varied thermal conditions.
| Performance Metric | Condition | Result |
|---|---|---|
| Initial Discharge Capacity | 12 mA g-1 | 114 mAh g-1 |
| Capacity Retention | 120 mA g-1, after 300 cycles | 92% |
| Capacity Retention | 60 mA g-1, after 200 cycles | 96% |
| Rate Performance @ 360 mA g-1 | – | 74 mAh g-1 |
| Operating Temperature Range | 60 mA g-1 | 20 – 80 °C |
| Capacity at Temperature Extremes | 60 mA g-1 | 99 mAh g-1 (20°C) to 120 mAh g-1 (80°C) |
Interfacial Analysis: The Role of DPEPA in SEI Formation
To understand the origin of the improved cycling stability, X-ray photoelectron spectroscopy (XPS) was performed on HC anodes extracted from cells cycled in the GPE and LE, respectively. The analysis focused on the chemical composition of the SEI layer.
The C 1s spectra from the GPE-derived SEI showed a lower relative proportion of organic components like C-O and O-C=O species (from solvent decomposition) compared to the LE-derived SEI. Conversely, the F 1s and O 1s spectra indicated a higher relative content of inorganic components such as NaF and Na2O in the GPE case. This evidence supports the proposed mechanism: DPEPA, with its lower LUMO, decomposes preferentially at the anode during initial cycling. This decomposition, concurrent with NaPF6 reduction, fosters the formation of an organic-inorganic composite SEI that is rich in beneficial inorganic salts (NaF, Na2O). This composite SEI is more mechanically robust and ionically conductive than a predominantly organic SEI derived mainly from solvent decomposition. Crucially, this process also passivates the anode surface, suppressing the continued breakdown of EC and DEC solvents, which is a primary cause of capacity fade and gas generation in standard sodium-ion batteries.
In-situ Mechanistic Studies of Sodium Storage
In-situ XRD was employed to probe the dynamic structural evolution of both electrodes during operation in a full sodium-ion battery cell (with LE for transparency).
NFM Cathode: During charging (desodiation), the (006) and (101) diffraction peaks of NFM gradually shifted to higher angles, indicating a contraction of the crystal lattice due to Na+ extraction. Upon discharging (sodiation), these peaks reversibly shifted back to their original positions. This highly reversible structural change confirms the stability of the NFM framework during cycling and explains its good capacity retention.
Hard Carbon Anode: The (002) diffraction peak of hard carbon did not show noticeable shifting, ruling out conventional intercalation as in graphite. Instead, the peak intensity varied with the state of charge. During the initial charging phase (higher voltage plateau/sloping region), the peak intensity remained relatively constant, corresponding to Na+ adsorption on surfaces and defects. As charging progressed into the low-voltage plateau, the peak intensity decreased significantly, suggesting an increase in structural disorder due to Na+ filling into microcavities or pores within the hard carbon. This process was fully reversible upon discharge. This observation validates the widely accepted “adsorption-pore filling” mechanism for sodium storage in hard carbon, which can be summarized conceptually as:
Sloping Region (High-to-Medium Voltage): $$ \text{HC} + x\text{Na}^+ + x e^- \rightarrow \text{Na}_x\text{HC}_{ads} $$
Plateau Region (Low Voltage): $$ \text{Na}_x\text{HC}_{ads} + y\text{Na}^+ + y e^- \rightarrow \text{Na}_{x+y}\text{HC}_{pore-filled} $$
Conclusion and Perspective
This work successfully demonstrates the design and implementation of a high-performance quasi-solid-state sodium-ion battery centered on an in-situ polymerized DPEPA-based gel polymer electrolyte. The GPE synthesis is simple, scalable, and compatible with existing battery manufacturing. The electrolyte exhibits a well-balanced set of properties: high ionic conductivity (1.97 mS cm-1), an exceptional sodium-ion transference number (0.66), and a wide electrochemical stability window (>5.1 V). The fundamental advantage stems from the strategic use of DPEPA, a monomer whose electronic structure promotes its preferential decomposition to form a robust, inorganic-rich SEI on the anode. This action effectively passivates the interface and suppresses detrimental solvent decomposition.
The resulting quasi-solid-state sodium-ion full cell, employing an NFM cathode and HC anode, delivers excellent 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 studies confirmed the highly reversible structural evolution of the NFM cathode and the classic “adsorption-pore filling” mechanism in the hard carbon anode.
This approach highlights a critical design principle for next-generation electrolytes in sodium-ion batteries and beyond: leveraging functional polymers with tailored frontier orbital energy levels to actively engineer stable interphases. Future work could explore other multi-functional monomers, blend polymers, or additives to further enhance mechanical strength, ionic conductivity at sub-zero temperatures, and compatibility with even higher-voltage cathode materials. The development of such quasi-solid-state systems represents a vital step towards realizing safe, durable, and high-performance sodium-ion batteries for large-scale energy storage applications.