In our pursuit to advance energy storage technologies beyond lithium-ion systems, sodium-ion batteries have emerged as a highly promising alternative due to the abundance and low cost of sodium resources. A critical yet often underexplored component in these batteries is the separator. While commercial polyolefin separators, like polypropylene (PP), offer excellent mechanical properties and cost-effectiveness, their inherent hydrophobic nature and poor thermal stability pose significant challenges for sodium-ion battery performance and safety. To address this, we focused on chemically modifying the PP separator surface to enhance its affinity for polar carbonate-based electrolytes. In this work, we employed a facile and efficient gamma-ray radiation grafting technique to covalently attach poly(methyl methacrylate) (PMMA) onto a commercial PP separator. We systematically investigated the influence of the grafting degree on the separator’s microstructure, wettability, thermal properties, and, ultimately, the electrochemical performance of assembled sodium-ion batteries.
The core principle of our modification strategy is to utilize high-energy γ-rays to generate active radical sites on the PP polymer chains. These radicals then initiate the polymerization of the methyl methacrylate (MMA) monomer, leading to the formation of a covalently bonded PMMA layer on the separator surface and within its pores. This grafted layer introduces polar carbonyl (C=O) groups, which are expected to significantly improve the separator’s wettability for standard sodium-ion battery electrolytes. The grafting degree (DG), defined as the weight increase relative to the original separator, was controlled by varying the monomer concentration during the irradiation process. We prepared a series of modified separators, denoted as PP-g-PMMA, with grafting degrees ranging from 0.8% to 50.2%, and compared their properties against the pristine PP separator.
The chemical success of the grafting reaction was confirmed using ATR-FTIR spectroscopy. While the pristine PP separator showed characteristic peaks for CH2 stretching and bending vibrations, all PP-g-PMMA samples exhibited new, distinct absorption bands. A strong peak emerged at approximately 1731 cm-1, which is unequivocally attributed to the C=O stretching vibration of the ester group in PMMA. Another peak around 1245 cm-1, corresponding to C–O stretching, further validated the presence of the grafted polymer. The intensity of these characteristic peaks correlated directly with the measured grafting degree.
The surface morphology of the separators, crucial for ionic transport, was profoundly affected by the grafting process. The pristine PP separator displayed a uniform, microporous network structure typical of dry-process films. At low grafting degrees (e.g., 0.8% and 2.8%), this porous architecture remained largely intact. However, as the grafting degree increased to 10.5% and beyond, the accumulating PMMA polymer began to progressively coat and eventually fill the surface pores. At the highest grafting degree of 50.2%, the original porous structure was completely obscured by a dense, non-porous PMMA layer. This morphological evolution has direct and critical implications for the separator’s physicochemical properties, as summarized in the table below.
| Sample | Grafting Degree (%) | Porosity (%) | Electrolyte Uptake (%) | Contact Angle (°) | Thermal Shrinkage at 150°C (%) |
|---|---|---|---|---|---|
| Pristine PP | 0.0 | 50.2 | 122.7 | 40 | 46.3 |
| PP-g-PMMA-1 | 0.8 | 49.7 | 131.3 | 34 | 32.1 |
| PP-g-PMMA-2 | 2.8 | 49.1 | 145.2 | 22 | 25.6 |
| PP-g-PMMA-3 | 10.5 | 42.9 | 100.4 | 29 | 18.9 |
| PP-g-PMMA-4 | 25.6 | 34.1 | 83.3 | 33 | 12.4 |
| PP-g-PMMA-5 | 50.2 | 22.4 | 60.9 | 38 | 8.7 |
The data reveals a non-monotonic relationship between grafting degree and key performance metrics. The electrolyte uptake, a vital parameter for ionic conductivity, initially increased with grafting, reaching an optimum of 145.2% for the sample with a 2.8% grafting degree (PP-g-PMMA-2). This sample also showed the lowest contact angle (22°), indicating superior surface wettability. The improvement is attributed to the introduced polar groups enhancing affinity, while the porosity remained nearly unchanged, preserving ample void space for electrolyte retention. The grafting degree (DG) is calculated as:
$$ DG(\%) = \frac{m_g – m_0}{m_0} \times 100 $$
where $m_0$ and $m_g$ are the masses of the separator before and after grafting, respectively.
Conversely, at high grafting degrees, although the surface chemistry is more polar, the severe reduction in porosity (down to 22.4%) becomes the dominant factor. The drastically diminished pore volume severely limits the electrolyte absorption capacity, leading to a drop in uptake and an increase in contact angle as the liquid could no longer penetrate effectively. This highlights a critical balance in separator design for sodium-ion battery applications: surface modification must enhance wettability without compromising the essential porous structure.
The thermal stability of the separator is another major concern for battery safety. The pristine PP separator suffered from severe thermal shrinkage (46.3%) when exposed to 150°C for 30 minutes. The radiation grafting process introduced cross-linked structures within the polymer matrix. Consequently, all PP-g-PMMA separators demonstrated markedly improved dimensional stability under heat. The thermal shrinkage decreased consistently with increasing grafting degree, with the highly grafted sample (50.2%) showing only 8.7% shrinkage. This enhanced thermal resistance is a significant safety advantage for sodium-ion battery operation.
Based on the comprehensive physicochemical analysis, the PP-g-PMMA separator with a 2.8% grafting degree (PP-g-PMMA-2) presented the most balanced set of properties: excellent wettability, high electrolyte uptake, preserved porosity, and improved thermal stability. We therefore assembled lab-scale pouch sodium-ion batteries using this modified separator and compared their performance against batteries with the pristine PP separator. The cathode was NaNiFeMn-based, the anode was hard carbon, and the electrolyte was 1 M NaPF6 in a PC/EMC/DEC mixture.
Electrochemical impedance spectroscopy (EIS) showed that the cell with the PP-g-PMMA-2 separator had a significantly smaller semicircle in the mid-frequency region compared to the cell with the pristine separator. This semicircle corresponds to the charge-transfer resistance ($R_{ct}$) at the electrode-electrolyte interfaces. The reduced $R_{ct}$ indicates faster reaction kinetics, directly benefiting from the better electrolyte wettability and retention provided by the modified separator, which facilitates superior sodium-ion transport.
The rate capability and cycling performance of the sodium-ion batteries were systematically evaluated. The cell employing the PP-g-PMMA-2 separator consistently delivered higher specific capacities across various discharge rates (0.5C to 5.0C). At a 1C cycling rate, it also demonstrated a higher initial specific capacity and better capacity retention after 50 cycles (99.5%) compared to the cell with the pristine PP separator (98.4%). The enhanced wettability promotes the formation of a more stable and uniform solid electrolyte interphase (SEI), conserving active sodium and improving cyclability. Performance at different temperatures further underscored the advantage. While both cells saw capacity fade at low temperatures (-25°C) due to increased electrolyte viscosity and slowed ion diffusion, the cell with the modified separator maintained a higher relative capacity, as its superior electrolyte-holding capability helped mitigate these kinetic limitations.
| Electrochemical Test | Pristine PP Separator | PP-g-PMMA-2 (2.8% DG) |
|---|---|---|
| Charge-Transfer Resistance ($R_{ct}$) | Higher | Lower |
| Initial Capacity at 1C (mAh g⁻¹) | 106.2 | Higher than pristine |
| Capacity Retention after 50 cycles @ 1C | 98.4% | 99.5% |
| Rate Performance (0.5C to 5.0C) | Good | Superior |
| Low-Temperature Performance (-25°C) | Moderate fade | Improved capacity retention |
In conclusion, our work successfully demonstrates the efficacy of the gamma radiation grafting technique for functionalizing polypropylene separators for use in sodium-ion batteries. Grafting methyl methacrylate onto the PP surface introduced polar functional groups that markedly improved electrolyte affinity. We identified that the grafting degree is a pivotal parameter that controls a critical trade-off: while higher grafting increases polarity, it can also clog the separator’s pores, impairing electrolyte uptake. An optimal grafting degree of approximately 2.8% was found to achieve the best balance, significantly enhancing wettability while preserving the essential porous microstructure. The sodium-ion battery incorporating this optimally modified separator exhibited lower internal resistance, superior rate capability, improved cycling stability, and better low-temperature performance compared to one using a pristine PP separator. This study confirms that radiation-induced surface modification is a viable and powerful strategy to tailor separator properties, paving the way for developing high-performance, safe, and cost-effective separators for the next generation of sodium-ion batteries.