The global shift towards renewable energy integration and the electrification of transportation has intensified the search for sustainable, cost-effective, and high-performance energy storage solutions. Among the contenders, the sodium-ion battery (SIB) has emerged as a particularly promising candidate for grid-scale storage and mid-range electric mobility, primarily due to the natural abundance and geographical uniformity of sodium resources, which promise significant supply chain stability and cost advantages over lithium-based systems. However, the path to widespread commercialization is impeded by critical challenges related to cycle life and long-term reliability. A central, yet insufficiently understood, issue is the significant mechanical stress and volume expansion induced during the (de)intercalation of sodium ions, whose larger ionic radius (1.02 Å for Na⁺ vs. 0.76 Å for Li⁺) inherently imposes greater strain on host electrode materials.
This volumetric fluctuation is not a static phenomenon but a dynamic process governed by a complex interplay of electrochemical reactions, solid-state diffusion kinetics, and interfacial evolution. In practical applications, a sodium-ion battery operates under a confluence of stressors—variable temperatures, fluctuating charge/discharge rates, and different states of charge (often linked to upper cut-off voltage, UC-V). Existing literature has largely dissected these factors in isolation, investigating, for instance, the effect of elevated temperature on capacity fade or the influence of high voltage on cathode structural stability. A critical gap exists in understanding their synergistic, non-linear impact on the generation and evolution of mechanical swelling force. This force, transmitted through the cell stack, is a direct physical manifestation of internal stress. Its unmitigated accumulation leads to electrode particle cracking, loss of electrical contact, continuous breakdown and reformation of the solid electrolyte interphase (SEI), and accelerated electrolyte consumption, culminating in rapid capacity degradation and potential safety hazards.
Our work addresses this gap by constructing a multi-variable experimental paradigm. We systematically investigate the swelling force behavior of commercial prismatic sodium-ion battery cells with a layered oxide cathode and hard carbon anode under the coupled influence of temperature (35°C vs. 45°C) and upper cut-off voltage (3.95 V vs. 4.0 V). Employing in-situ pressure sensing throughout prolonged cycling, we move beyond single-factor analysis to decipher the dynamic, non-linear evolution of swelling forces. Furthermore, we propose and experimentally validate an “expansion force threshold release” strategy, demonstrating that managed mechanical stress relief can temporarily recover electrochemical performance, thereby offering a novel dimension for lifespan optimization. This study provides fundamental insights into the chemo-mechanical coupling within SIBs and delivers actionable intelligence for the design of more robust cells and management systems.
1. Experimental Methodology: Probing Swelling Force Under Controlled Stressors
The core of this investigation lies in decoupling and then re-coupling the effects of key operational parameters on the mechanical state of the sodium-ion battery. We utilized commercial 22 Ah prismatic SIBs (Na[ ]O₂-type cathode / Hard Carbon anode) as the test subject, ensuring relevance to near-term applications.
1.1 Cell Conditioning and Baseline Establishment: Prior to stress testing, each cell underwent a standardized conditioning procedure at 25°C to determine its initial capacity (C₀). This involved three consecutive cycles of low-rate (0.2C) charging to 3.95 V (with a constant-voltage hold until current dropped to 0.05C) and discharging to 2.0 V. The average discharge capacity from these cycles was taken as C₀. This step ensured all cells started from a well-defined and comparable electrochemical state.
1.2 Multi-Factor Cycling Protocol with In-Situ Force Monitoring: The cells were then subjected to accelerated cycling tests under different environmental and electrical conditions. The test matrix was designed as follows:
| Test Group | Temperature (θ) | Upper Cut-off Voltage (UC-V) | Charge/Discharge Rate | Primary Objective |
|---|---|---|---|---|
| Group A | 35°C | 3.95 V | 0.5C Constant Current (CC) charge, followed by Constant Voltage (CV) hold until 0.05C; 0.5C CC discharge. | Baseline for swelling force evolution. |
| Group B | 45°C | 3.95 V | Isolate the effect of elevated temperature. | |
| Group C | 35°C | 4.00 V | Isolate the effect of higher voltage. | |
| Group D | 45°C | 4.00 V | Study the synergistic, coupled effect. |
The cells were placed in a thermal chamber and allowed to soak for 4 hours at the test temperature (θ) to achieve uniform core temperature. Cycling was performed using a high-precision battery tester. Crucially, the cell was constrained in a fixture instrumented with a calibrated pressure sensor (range: 0-5 kN, accuracy ±0.1% FS) aligned with the large face of the prismatic cell to measure the swelling force (F_swell) normal to the electrode stack. Force and voltage data were sampled synchronously at 1 Hz throughout each charge and discharge step.
1.3 Stress Intervention Experiment: To explore the impact of managed force relief, a separate long-duration test was conducted on two cells (Group D conditions: 45°C, 4.0V). After a significant number of cycles (N=618), the mechanical constraint was deliberately and temporarily released, allowing the cell to expand freely and the internal swelling force to dissipate close to zero. The constraint was then reapplied, and cycling resumed for an additional 287 cycles. This “reset” of mechanical stress allowed us to probe its reversible and irreversible components and their link to capacity retention.
1.4 Data Analysis Parameters: For each cycle, we extracted key metrics: Maximum Swelling Force (F_max, typically at the end of CC charge), Minimum Swelling Force (F_min, at the end of discharge), and Discharge Capacity (C_d). Capacity retention was calculated as (C_d / C₀) × 100%. The differential swelling force during operation, ΔF = F(t) – F_min(cycle), was analyzed to understand the dynamic stress associated with sodium ion movement.
2. Results and Discussion: Decoding the Dynamics of Swelling Force
The data reveals a rich, non-linear interaction between operational parameters and the mechanical response of the sodium-ion battery. The swelling force is not merely a monotonic function of state-of-charge (SOC) but exhibits distinct features linked to the staging behavior of sodium in hard carbon and the interfacial dynamics.
2.1 Intra-Cycle Swelling Force Signature: A representative force versus time profile during a single cycle is rich in information. As shown in the data, the swelling force evolution during charging is biphasic. An initial rapid rise in force correlates with the adsorption of Na⁺ ions onto defects, pore surfaces, and the intercalation into the graphitic-like domains of the hard carbon. This process, often associated with the sloping voltage region below ~0.1 V vs. Na⁺/Na, induces significant strain. The force then plateaus or increases slowly during the mid-SOC range. A second, pronounced rise occurs at higher SOC, corresponding to the filling of the hard carbon’s nano-pores—a process that generates substantial internal pressure due to the confinement of Na clusters or quasi-metallic sodium. The subsequent constant-voltage (CV) phase often sees a slight force relaxation as the current decays and local concentration gradients equilibrate.
During discharge, the process is quasi-mirrored but with hysteresis. Force decreases as sodium is extracted from the pores and then from the adsorption/intercalation sites. A subtle force rebound is sometimes observed at mid-discharge, possibly due to mechanical relaxation dynamics or inhomogeneous extraction. The force minimum at the end of discharge never returns to the initial pre-cycle baseline, reflecting the accumulation of irreversible expansion. This intra-cycle signature can be conceptually modeled. The total swelling force (F_swell) can be considered a sum of reversible (F_rev) and irreversible (F_irr) components:
$$F_{\text{swell}}(t, N) = F_{\text{rev}}(SOC(t)) + F_{\text{irr}}(N) + F_{\text{interface}}(N)$$
where \(F_{\text{rev}}\) is a function of the instantaneous sodium content in the anode, \(F_{\text{irr}}\) accumulates with cycle number (N) due to permanent structural changes in the electrodes, and \(F_{\text{interface}}\) represents the contribution from the growing SEI/CEI layers and possible gas generation.
2.2 The Isolated and Synergistic Impact of Temperature: Comparing cycles at 35°C and 45°C (at a fixed UC-V of 3.95V) unveils the kinetic influence of temperature. The data is summarized in the table below, showing average rates of change over 400 cycles.
| Parameter | 35°C, 3.95V | 45°C, 3.95V | Physical Interpretation |
|---|---|---|---|
| Avg. d(F_max)/dN (N/cycle) | +2.1 | +4.8 | Higher temperature accelerates diffusion and side reactions, leading to faster force growth. |
| Avg. d(F_min)/dN (N/cycle) | +1.5 | +3.5 | Irreversible expansion (structural damage, SEI growth) is thermally activated. |
| Capacity Fade Rate (%/100 cycles) | ~0.5 | ~1.2 | Force accumulation correlates strongly with capacity loss. |
Elevated temperature (45°C) increases ionic conductivity and charge transfer rates, which might seem beneficial. However, for the sodium-ion battery hard carbon anode, it exacerbates the kinetic imbalance between sodium insertion into different sites. The faster diffusion promotes more aggressive pore filling and Na-cluster formation, increasing the local stress within the carbon particles. Concurrently, temperature exponentially accelerates electrolyte decomposition and the growth of a thicker, often more resistive and inhomogeneous SEI layer. This thickened SEI contributes directly to volume expansion and F_irr. Furthermore, the enhanced kinetics at the cathode can drive deeper desodiation, pushing the layered oxide closer to structural phase boundaries that may involve slab gliding or irreversible oxygen release, contributing to cathode swelling. The net result is a significantly steeper climb in both F_max and F_min with cycling at 45°C.
2.3 The Isolated and Synergistic Impact of Upper Cut-off Voltage: Raising the UC-V from 3.95V to 4.0V (at a fixed 35°C) probes the effect of deeper charging. The consequences are profound, as detailed in the following comparison over 600 cycles.
| Parameter | 35°C, 3.95V | 35°C, 4.0V | Physical Interpretation |
|---|---|---|---|
| F_max at Cycle 600 (N) | ~3200 | ~4800 | Higher voltage extracts more Na+ from cathode, forcing more Na into anode, increasing reversible strain and pore-filling pressure. |
| F_min at Cycle 600 (N) | ~1800 | ~3100 | Deeper cycling promotes more irreversible structural damage at both electrodes and thicker interfacial layers. |
| Irreversible Force, F_irr(600) (N) | ~1500 | ~2900 | High voltage drives detrimental phase transitions and aggressive interfacial reactions. |
A higher UC-V forces the cathode to a higher average oxidation state. In layered oxides, this often means extracting sodium from less stable sites, potentially triggering irreversible phase transformations (e.g., from P2 to O2 phases) or increasing metal dissolution. These processes cause cathode particle microcracking and bulk volume change, contributing to F_swell. On the anode side, the additional sodium inventory transferred necessitates accommodation in less favorable, higher-energy sites within the hard carbon, amplifying the pore-filling pressure (increased F_rev). Most critically, the higher cathode potential severely exacerbates oxidative decomposition of the electrolyte at the cathode-electrolyte interface (CEI). This not only consumes active sodium and lithium (from salt impurities) but also generates gaseous products (e.g., CO₂, O₂) and a thick, resistive CEI. The gas generation directly increases internal pressure, while the CEI growth adds to the irreversible volume expansion. This makes the high-voltage condition a primary driver for both reversible and irreversible swelling force.
2.4 Coupled Effect: High Temperature and High Voltage – A Synergistic Degradation Multiplier: The most severe condition, Group D (45°C, 4.0V), demonstrates a non-linear, synergistic degradation. The effects are not additive but multiplicative. The rate of force accumulation and capacity fade in this group far exceeded the sum of the individual effects observed in Groups B and C. The high temperature catalytically enhances all the detrimental processes initiated by the high voltage: faster kinetics of destructive phase transitions in the cathode, dramatically accelerated electrolyte oxidation and reduction at both electrodes, and rapid, chaotic growth of both CEI and SEI layers. The swelling force evolution in this regime can be described by a power-law relationship, indicating an accelerating degradation mechanism:
$$F_{\text{irr}}(N) \approx A \cdot N^{\beta}$$
where the pre-factor \(A\) and exponent \(\beta\) are both functions of temperature and voltage (\(\theta\), UC-V), with \(\beta > 1\) under coupled stress conditions. This leads to a rapid rise in internal pressure, which in turn physically disrupts the fragile SEI, exposing fresh electrode surface to further reaction—a classic positive feedback loop of chemo-mechanical degradation.
2.5 Proof-of-Concept: Swelling Force Threshold Release and its Transient Benefit: The experiment involving deliberate mechanical stress release after 618 cycles under Group D conditions yielded critical insights. The following sequence was observed:
1. Pre-Release: F_max had climbed to ~4500 N, with capacity retention at ~97.9%.
2. Release & Relaxation: The constraint was loosened. The cell physically expanded, and F_swell dropped to near zero as internal stresses were relieved.
3. Post-Release Cycling: Upon re-constraining and resuming cycles, F_max started climbing again from a lower baseline. Remarkably, the very first discharge capacity after the release showed a recovery. Capacity retention jumped by approximately 0.5% (e.g., from 97.82% to 98.32% for one cell).
This transient recovery is attributed to the alleviation of “stress-induced passivation.” The accumulated swelling force likely causes micro-scale delamination or increased contact resistance between electrode layers, current collectors, and within the composite electrodes themselves. Releasing the force allows for a partial re-establishment of better electrical contact, momentarily reducing the internal impedance. This demonstrates that a component of the capacity fade is mechanically reversible. However, this benefit was short-lived. The rate of capacity fade in the cycles immediately following the release was observed to be steeper than the pre-release rate. This suggests that the stress release and re-application may have perturbed the SEI structure or electrode morphology, temporarily accelerating the degradation processes until a new mechanical equilibrium was established. This highlights the dual nature of the problem: while managed force relief can offer temporary gains, the long-term solution must focus on minimizing the generation of irreversible swelling forces through material and cell design.
3. Conclusion and Perspectives: Towards Swelling-Aware Sodium-Ion Battery Design
This comprehensive investigation into the swelling force dynamics of sodium-ion battery systems under coupled operational stresses provides a nuanced understanding that transcends single-factor analysis. We have demonstrated that the mechanical expansion force is a sensitive and integrative indicator of internal degradation processes, evolving in a complex, non-linear manner dictated by the synergy between temperature and voltage.
The key conclusions are threefold:
Firstly, the evolution of swelling force is biphasic within a cycle, reflecting the multi-step sodium storage mechanism in hard carbon, and grows irreversibly with cycling. The growth rates are powerfully modulated by operational parameters. Elevated temperature (45°C) acts as a kinetic accelerator for both beneficial and detrimental processes, overwhelmingly favoring the latter by promoting irreversible structural changes and interfacial growth. Increased upper cut-off voltage (4.0 V) is a thermodynamic driver that pushes electrode materials into unstable regimes, inducing phase transformations and vigorous interfacial reactions that directly generate gaseous and solid decomposition products, significantly amplifying both reversible and irreversible swelling.
Secondly, the combined effect of high temperature and high voltage is synergistic, creating a positive feedback loop where mechanical stress accelerates interfacial degradation, which in turn generates more stress. This leads to an accelerating (power-law) regime of force accumulation and capacity fade, representing the worst-case scenario for cycle life.
Thirdly, we provide a proof-of-concept that mechanical stress management is a viable, though complex, lever for performance retention. The “expansion force threshold release” experiment unequivocally showed that a portion of capacity loss is due to stress-induced contact deterioration, which is mechanically reversible. This opens a novel avenue for battery management system (BMS) strategies that could incorporate periodic “mechanical relaxation” phases in long-duration storage applications.
These insights directly inform the future development of more robust sodium-ion battery technology:
For Material Scientists: The emphasis must be on developing electrode materials with intrinsic resistance to strain-induced degradation. For cathodes, this means stabilizing layered structures against high-voltage phase transitions through doping or composite designs. For hard carbon anodes, tailoring pore structure and surface chemistry to accommodate sodium with lower strain and to promote the formation of a thin, flexible, and stable SEI is paramount. Electrolyte formulations must be engineered for high anodic and cathodic stability to suppress gas and CEI/SEI growth under stressed conditions.
For Cell and System Engineers: Cell design must account for controlled expansion. This could involve the use of elastic binders, engineered porosity in electrodes, or mechanically adaptive components like spring-loaded or pressure-relief-enabled cell casings. At the system level, the BMS should not only manage voltage and temperature but also consider the “mechanical history” of the cell. Operational strategies that avoid prolonged high-voltage holds, especially at elevated temperatures, can dramatically extend lifespan. Furthermore, our work suggests that algorithms could be developed to estimate internal swelling force from external pressure sensors or even from electrochemical impedance signatures, enabling predictive maintenance and adaptive charging protocols.
In summary, managing swelling force is not merely a packaging challenge but a fundamental aspect of sodium-ion battery chemistry and engineering. By embracing a holistic, chemo-mechanical perspective that considers the coupled effects of multiple stressors, the path towards achieving the long cycle life and reliability required for sustainable energy storage can be significantly shortened.