The pursuit of sustainable and cost-effective energy storage solutions has driven significant research beyond conventional lithium-ion batteries. Sodium-ion batteries represent a compelling alternative due to the natural abundance and low cost of sodium resources. A critical challenge in advancing sodium-ion battery technology lies in developing high-performance electrode materials that offer high capacity, long cycle life, and excellent rate capability. Traditional inorganic cathode materials often suffer from limitations such as low specific capacity due to heavy constituent atoms, poor electronic conductivity, and sluggish sodium-ion diffusion kinetics.
Organic electrode materials have emerged as a promising frontier due to their unique advantages, including structural tunability, environmental friendliness, potential for high capacity from multi-electron redox reactions, and the possibility of production from abundant resources. Among them, carbonyl-based compounds, which undergo enolization reactions during redox processes, are particularly attractive. Phenanthrenequinone (PQ), a conjugated carbonyl compound, presents a compelling molecular structure for electrochemical energy storage. Its theoretical capacity can be calculated based on the number of electrons transferred per formula unit and its molecular weight. The specific capacity (C, in mAh g-1) is given by:
$$
C = \frac{n \times F}{3.6 \times M}
$$
where \(n\) is the number of electrons transferred per molecule, \(F\) is Faraday’s constant (96485 C mol-1), and \(M\) is the molecular weight (g mol-1). For PQ (C14H8O2), with \(M = 208.21\) g mol-1 and assuming a two-electron redox process per carbonyl group (leading to a theoretical \(n=2\) for full utilization, though more complex reactions are possible), it demonstrates significant promise. Recent studies have explored PQ and its derivatives, often by polymerizing or compositing it to mitigate solubility in electrolytes. However, this can introduce larger molecular weights or reduce active material content, thereby limiting practical capacity and rate performance. This work investigates the intrinsic electrochemical properties of unmodified PQ as a cathode material for sodium-ion batteries, with a particular focus on its performance under low-temperature conditions—an area rarely explored for such materials but critical for practical applications in extreme environments.
The positive electrode slurry was prepared by thoroughly mixing 60 wt% PQ powder with 30 wt% acetylene black as a conductive additive. A binder solution, consisting of 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone (NMP), was added. The mixture was magnetically stirred for 10 hours to form a homogeneous slurry. This slurry was then cast onto an aluminum foil current collector using a doctor blade and dried at 60°C for 12 hours in a vacuum oven. The dried electrode film was punched into 12 mm diameter discs for cell assembly. CR2032-type coin cells were assembled in an argon-filled glovebox. The PQ electrode was used as the working electrode, a sodium metal foil as the counter/reference electrode, a glass fiber (GF/D) separator, and 1 M sodium hexafluorophosphate (NaPF6) dissolved in diethylene glycol dimethyl ether (DEGDME) as the electrolyte.
Galvanostatic charge-discharge tests were performed within a voltage window of 1.0–3.0 V (vs. Na/Na+) using a battery cycler. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted using an electrochemical workstation. CV scans were recorded at various sweep rates from 0.2 to 1.0 mV s-1. EIS was measured in the frequency range from 100 kHz to 0.01 Hz with a perturbation amplitude of 5 mV. Low-temperature tests were carried out by placing the assembled cells inside environmental chambers set at 0°C and -20°C, allowing sufficient thermal equilibration before electrochemical measurements.
Electrochemical Performance at Room Temperature
The electrochemical activity of PQ in a sodium-ion battery was first evaluated. The cyclic voltammogram at a scan rate of 1 mV s-1 reveals two distinct pairs of redox peaks, indicating a multi-step sodium storage mechanism. The peaks are located at approximately 1.45 V / 1.75 V and 2.47 V / 2.58 V (vs. Na/Na+), corresponding to the stepwise reduction (sodiation) and oxidation (desodiation) of the carbonyl groups in the PQ structure. The well-defined and symmetric shapes of these peaks suggest a highly reversible electrochemical process.
Galvanostatic charge-discharge profiling at a current density of 0.1 A g-1 delivers a high reversible specific capacity of 124 mAh g-1. The voltage profiles exhibit plateaus consistent with the redox peaks observed in CV, confirming the two-phase reaction. The long-term cycling stability at this current density is excellent, with a capacity retention exceeding 99.5% after 200 cycles. The high Coulombic efficiency, consistently near 100%, further underscores the reversibility of the reactions.
The rate capability of the PQ electrode is a key metric for high-power applications. As shown in the performance summary table below, the electrode maintains substantial capacity even at very high current densities. A capacity of 70 mAh g-1 is retained at an ultra-high rate of 5 A g-1. More importantly, when the current density is returned to 0.1 A g-1 after the rate test, the capacity recovers to nearly its initial value, demonstrating the robustness of the electrode structure. The long-cycle test at 2 A g-1 reveals exceptional stability, with a capacity of 97 mAh g-1 maintained after 2000 cycles, corresponding to an average capacity decay of less than 0.001% per cycle. This outstanding cyclability is attributed to the stable redox chemistry of PQ and the effective electrolyte system that minimizes active material dissolution.
| Current Density (A g-1) | Specific Capacity (mAh g-1) | Performance Note |
|---|---|---|
| 0.1 | 124 | High initial capacity |
| 0.2 | 104 | Excellent rate response |
| 0.5 | 97 | |
| 1.0 | 89 | |
| 2.0 | 75 | Stable long-term cycling at this rate |
| 5.0 | 70 | 96% capacity retention after 2000 cycles |
| 8.0 | 64 | Outstanding high-rate capability |
| 10.0 | 56 |
Low-Temperature Electrochemical Performance
The operation of sodium-ion batteries at low temperatures is crucial for applications in electric vehicles, grid storage in cold climates, and aerospace technology. The electrochemical performance of the Na-PQ cell was systematically investigated at 0°C and -20°C. At 0°C and a current density of 0.1 A g-1, the cell delivers a discharge capacity of 82 mAh g-1, which is approximately 78% of its room temperature capacity. Impressively, it retains 96.9% of this capacity after 200 cycles, indicating remarkable low-temperature cycling stability. The rate performance at 0°C remains robust, with measurable capacities obtained even at 10 A g-1.
At the more challenging temperature of -20°C, the cell still operates effectively. The initial discharge capacity at 0.1 A g-1 is 63.5 mAh g-1 (about 53.6% of the room-temperature value), and a high capacity retention of 95.4% is observed over 200 cycles. The cyclic voltammogram recorded at -20°C retains the characteristic shape of the redox peaks, although with increased polarization. The persistence of these peaks confirms that the fundamental redox reactions of PQ remain active and reversible even under such frigid conditions. This performance highlights the fast reaction kinetics at the electrode/electrolyte interface and suggests that the chosen DEGDME-based electrolyte maintains sufficient ionic conductivity at low temperatures, which is vital for the development of practical low-temperature sodium-ion batteries.
Kinetic Analysis and Charge Storage Mechanism
To elucidate the reasons behind the excellent rate capability, a detailed kinetic analysis was performed. Cyclic voltammetry at scan rates (v) ranging from 0.2 to 1.0 mV s-1 was conducted. The shape of the CV curves is well-preserved as the scan rate increases, indicating good electrochemical stability and reversibility. The relationship between peak current (i) and scan rate can be expressed by the power law:
$$
i = a v^b
$$
which is often linearized as:
$$
\log(i) = b \log(v) + \log(a)
$$
Here, the \(b\)-value is a key parameter. A \(b\)-value of 0.5 indicates a diffusion-controlled faradaic process (semi-infinite linear diffusion), while a \(b\)-value of 1.0 signifies a surface-controlled capacitive process. The calculated \(b\)-values for the four major redox peaks of PQ are all close to 1 (0.98, 0.90, 0.98, and 0.99, respectively). This strongly suggests that the sodium storage in PQ is predominantly governed by surface-controlled capacitive kinetics, which is inherently faster than diffusion-limited processes. This capacitive contribution explains the material’s exceptional high-rate performance.
The total current response at a fixed potential can be quantitatively separated into capacitive (\(k_1 v\)) and diffusion-controlled (\(k_2 v^{1/2}\)) contributions using the equation:
$$
i(V) = k_1 v + k_2 v^{1/2}
$$
By determining \(k_1\) and \(k_2\), the percentage of capacitive contribution can be calculated. The results show that the capacitive contribution increases with scan rate, accounting for approximately 84.7% of the total charge storage at a scan rate of 1.0 mV s-1. This high proportion of surface-controlled behavior is a hallmark of materials with outstanding power density.
Electrochemical impedance spectroscopy (EIS) provides further insight. The Nyquist plot features a small semicircle in the high-frequency region, corresponding to a charge-transfer resistance (\(R_{ct}\)) of about 30 Ω. This low \(R_{ct}\) value indicates fast kinetics for the faradaic reactions at the PQ electrode interface. In the low-frequency region, the nearly vertical line suggests ideal capacitive behavior and very rapid ion diffusion into the electrode bulk, consistent with the conclusions from CV analysis.
Comparative Performance and Outlook
The performance of PQ as a cathode material for sodium-ion batteries compares favorably with other reported organic carbonyl compounds. The following table provides a brief comparison, highlighting PQ’s superior combination of capacity, high-rate capability, and ultra-long cycle life.
| Cathode Material | Long-Term Cycles | Specific Capacity (mAh g-1) | Rate Test (A g-1) | Key Performance Indicator |
|---|---|---|---|---|
| Phenanthraquinone (PQ, this work) | 2000 | 124 | 10 | Excellent cyclability & rate |
| Anthraquinone (AQ) | 100 | 126 | 8 | High capacity |
| Alizarin | 500 | 81.5 | 5 | Moderate stability |
| PTCDA | 100 | 85 | 3 | Limited rate |
| Benzoquinone (BQ) | 100 | 59 | 3 | Low capacity |
In conclusion, this comprehensive study demonstrates that phenanthraquinone is a highly promising organic cathode material for sodium-ion batteries. It delivers high capacity, exceptional cycling stability over thousands of cycles, and remarkable rate performance due to its dominant surface-controlled capacitive charge storage mechanism. Critically, its electrochemical functionality is well-preserved at low temperatures down to -20°C, addressing a significant challenge for battery operation in harsh environments. The findings provide an effective strategy for developing high-performance, low-cost, and environmentally benign sodium-ion batteries, paving the way for their use in a wider range of applications. Future work may focus on further engineering PQ-based composites or derivatives to fully exploit its theoretical capacity while maintaining all its advantageous kinetic properties.