As a researcher focused on energy storage materials, I have extensively investigated the calendar aging mechanisms in sodium-ion batteries, particularly concerning hard carbon anodes. The state of charge (SOC), or sodiation depth, is a critical parameter influencing the long-term storage stability of these batteries. In this article, I present my findings on how varying SOC levels affect the storage performance of hard carbon anodes, using electrochemical tests and surface analysis to unravel the underlying solid electrolyte interphase (SEI) aging processes. Sodium-ion batteries are promising alternatives to lithium-ion systems due to sodium’s abundance and cost-effectiveness, but their calendar aging—performance degradation during storage—remains a key challenge for commercialization. Hard carbon is a leading anode material for sodium-ion batteries, yet its storage behavior under different SOC conditions is poorly understood. My work aims to bridge this gap by systematically evaluating the impact of sodiation depth on active sodium loss, interfacial resistance, and SEI evolution, offering insights for optimizing battery management and extending calendar life.
In my experiments, I assembled half-cells with hard carbon electrodes and sodium metal counter/reference electrodes, using a standard electrolyte of 1 M NaPF6 in EC:PC (1:1 by volume). The cells were activated at 0.1 C for two cycles to form a stable SEI. To assess storage-induced aging, I designed a protocol where cells were brought to specific SOC levels—100%, 70%, 35%, 25%, and 15%—by controlling the discharge and charge steps, as illustrated in the typical charge-discharge curve of hard carbon. These SOC values correspond to approximate electrode potentials of 0.14 V, 0.16 V, 0.21 V, 0.31 V, and 0.56 V versus Na/Na+, respectively. After reaching the desired SOC, the cells were stored at 50°C for 6 days to accelerate aging, followed by a 1 C charge to 1.5 V to measure the active sodium loss. The loss was calculated as the difference between the discharge capacity before storage and the charge capacity after storage, isolating the anode’s contribution by minimizing effects from other cell components. Electrochemical impedance spectroscopy (EIS) was performed post-storage to evaluate interfacial resistance, and X-ray photoelectron spectroscopy (XPS) was used to analyze SEI composition changes. This comprehensive approach allowed me to correlate SOC-dependent performance shifts with SEI structural and chemical transformations.
The results revealed a pronounced dependence of storage-induced degradation on sodiation depth. At high SOC levels (e.g., 100% and 70%), the active sodium loss was substantial, reaching up to 30 mAh/g after 6 days of storage, which represents over 10% of the total capacity. In contrast, low SOC levels (e.g., 15%) led to minimal loss, only 0.7 mAh/g under the same conditions. This trend is summarized in Table 1, which also includes the corresponding coulombic efficiencies and interfacial impedances. The coulombic efficiency after storage decreased significantly with higher SOC, dropping from 98.73% at 15% SOC to 88.97% at 100% SOC, indicating more irreversible reactions. EIS data showed that interfacial resistance increased with SOC, from 81 Ω at 15% SOC to 123 Ω at 100% SOC, suggesting SEI thickening. These electrochemical metrics highlight that deep sodiation exacerbates calendar aging in sodium-ion batteries, primarily due to enhanced SEI growth and sodium consumption.
| SOC (%) | Active Sodium Loss (mAh/g) | Coulombic Efficiency (%) | Interfacial Resistance (Ω) | Electrode Potential (V vs. Na/Na+) |
|---|---|---|---|---|
| 15 | 0.7 | 98.73 | 81 | 0.56 |
| 25 | 5.2 | 96.45 | 100 | 0.31 |
| 35 | 8.1 | 94.33 | 102 | 0.21 |
| 70 | 28.5 | 90.12 | 115 | 0.16 |
| 100 | 30.0 | 88.97 | 123 | 0.14 |
To understand these trends, I analyzed the SEI aging mechanisms via XPS. The SEI on hard carbon anodes is a dynamic layer that evolves during storage, with its composition and thickness dictating performance losses. At high SOC, the electrode potential is low (e.g., 0.14 V at 100% SOC), well below the reduction potential of the electrolyte. This large potential gap drives continuous electrolyte reduction, consuming active sodium and generating new SEI components. The XPS spectra showed a marked increase in carbon content after storage at 100% SOC, particularly in C=O and C-H species, indicating the formation of organic compounds like ROCO2Na, NaCO3R, RONa, and ethylene oxide oligomers. The atomic percentages from XPS are summarized in Table 2. Concurrently, the signal from sodiated hard carbon (Nax-HC) weakened, implying SEI thickening that masked the underlying electrode. This thickening aligns with the higher interfacial resistance and active sodium loss, as described by a simplified SEI growth model:
$$ \frac{dL}{dt} = k \cdot \exp\left(-\frac{E_a}{RT}\right) \cdot \Delta V $$
where \( L \) is SEI thickness, \( k \) is a rate constant, \( E_a \) is activation energy, \( R \) is the gas constant, \( T \) is temperature, and \( \Delta V \) is the potential difference between the electrode and electrolyte reduction potential. At high SOC, \( \Delta V \) is large, accelerating SEI growth. In contrast, at low SOC (e.g., 15% SOC, 0.56 V), \( \Delta V \) is smaller, reducing the driving force for electrolyte reduction. Here, XPS revealed a different aging pattern: the carbon content increased slightly, but the CO32- peak intensified, suggesting SEI reconstruction where unstable organic components convert to inorganic salts like Na2CO3. The Nax-HC signal remained strong, indicating minimal SEI thickening, consistent with the low interfacial resistance and sodium loss. This reconstruction-dominated process can be expressed as:
$$ \text{Organic SEI} \rightarrow \text{Inorganic SEI} + \text{byproducts} $$
The fluorine content in SEI, from NaPF6 decomposition and PVDF binder, decreased after storage at all SOC levels, but the ratio of C-F (from PVDF) to NaF varied. At 100% SOC, the C-F/NaF ratio was 1.16, while at 15% SOC, it was 1.36, implying a thinner SEI at low SOC that allows more PVDF detection. These findings underscore that sodiation depth alters the SEI aging pathway: high SOC promotes continuous growth with organic accumulation, whereas low SOC favors reconstruction with inorganic stabilization.
| Element | Atomic % Before Storage | Atomic % After Storage (100% SOC) | Atomic % After Storage (15% SOC) |
|---|---|---|---|
| C | 26.3 | 39.35 | 39.77 |
| O | 45.44 | 37.34 | 37.71 |
| F | 4.28 | 0.7 | 0.52 |
| P | 0.33 | 0.19 | 0.2 |
| Na | 23.14 | 21.16 | 21.22 |
The implications for sodium-ion battery management are significant. Calendar aging is often overlooked in favor of cyclic aging, but my results show that storage conditions—especially SOC—profoundly impact longevity. In practical applications, such as grid storage or electric vehicles, sodium-ion batteries may spend extended periods at high SOC, potentially leading to rapid capacity fade and resistance rise. To mitigate this, I recommend avoiding full charge during long-term storage; maintaining a lower SOC (e.g., below 50%) can slow SEI growth and preserve active sodium. This aligns with strategies for lithium-ion batteries but is particularly crucial for sodium-ion systems due to the higher reactivity of sodium and the less stable SEI on hard carbon. Furthermore, electrolyte modifications—such as additives that form robust inorganic SEI layers—could enhance storage performance across SOC ranges. My research contributes to the broader effort to optimize sodium-ion batteries for real-world use, where calendar life is as important as cycle life.
In conclusion, my study demonstrates that sodiation depth is a key determinant of storage performance in sodium-ion batteries with hard carbon anodes. High SOC levels accelerate active sodium loss and interfacial resistance increase due to SEI thickening driven by electrolyte reduction, while low SOC levels minimize these effects through SEI reconstruction. These insights, derived from electrochemical and spectroscopic analyses, provide a foundation for improving battery management systems and material designs. Future work could explore temperature effects, different electrolytes, and long-term storage under varying SOC to refine aging models. As sodium-ion battery technology advances, understanding and controlling calendar aging will be essential for achieving reliable and durable energy storage solutions.