In-situ Expansion Characterization of Cylindrical Batteries: Large-Format Cells Exhibit Ten Times Higher Swelling

1. Abstract

In the future market competition, the core advantages of large cylindrical battery focus on high safety, excellent high-rate charge and discharge capabilities, top-notch energy density, and high cost-performance ratio. Their robust structural design provides exceptional structural rigidity, which has often been regarded as a guarantee for maintaining volume stability during battery charge and discharge cycles. However, upon closer examination, one might question: Can large cylindrical battery truly maintain their volume without any change?

Through precise in-situ testing, we uncovered the truth: For example, with the 21700 model small cylindrical battery (using a ternary material system, with a capacity of 4.2Ah), the maximum volume change rate during the first three cycles at a 0.5C charge and discharge rate is only 0.013%, which is almost negligible. However, when we turn to the larger 4695 model cylindrical battery with a capacity of 33Ah (also based on the ternary system and charged and discharged at a 0.1C rate), the situation is different—the volume change rate increases significantly to 0.22%. Importantly, this volume change closely follows the fluctuations in the battery’s voltage curve, showing a high degree of consistency.

This finding not only reveals the subtle variations in volume stability of large cylindrical battery but also highlights the nuanced relationship between these changes and battery performance. Particularly striking is the fact that the volume change of large cylindrical battery far exceeds that of small cylindrical batteries, with a difference exceeding tenfold. This significant disparity presents new challenges and considerations for battery design, safety performance evaluation, and long-term stability.

2. Why Measure Expansion In Cylindrical Cells

Large cylindrical battery, such as Tesla’s 4680 cells, offer significant advantages in safety, lifespan, range, cost-effectiveness, and fast charging.

During cycling, batteries experience significant volume expansion due to factors such as the formation and growth of the solid-electrolyte interface (SEI), thermal expansion, and gas generation. This volume expansion not only increases internal stress between different parts of the battery but also introduces pressure between adjacent cells in the battery pack. Additionally, the continuous expansion and contraction cycles of the battery raise the risk of mechanical failure. in situ expansion characterization of cylindrical batteries is essential to understand how design and processing choices translate into real-world mechanical behaviour.

3. Analysis Workflow

To gain a deeper understanding of the expansion and contraction characteristics of large cylindrical batteries, a comparative test between the 4695 model and the 21700 battery was conducted to more accurately evaluate the volume change characteristics of large cylindrical batteries and their impact on battery lifespan. The testing process involved imaging the batteries with an imaging system, and software processing was used to output the volume change rate and voltage relationship curves (Figure 1).

Figure 1. Testing Process

Table 1. Battery Sample Information:

Experimental instrument: IEST CCS1300 (Cylindrical Battery In-Situ Swelling Testing System)

Testing Conditions: Temperature: 25°C

Table 2. Battery Sample Charge and Discharge Steps

4. Testing Results

Maintaining a uniform temperature is crucial for experimental accuracy. The temperature across all test channels remained within the acceptable range of 25 ±1 °C (Figure 2), satisfying the required condition.

Figure 2. Temperature profile during testing

Cylindrical batteries (whether the 21700 model or the 4695 model) do experience some volume change during charge and discharge, which is a normal phenomenon of battery operation. However, it is noteworthy that the maximum volume expansion of the 4695 cylindrical battery (0.22%, Figure 3) is significantly higher than that of the 21700 cylindrical battery (0.013%, Figure 4), exceeding the latter by more than ten times.

5. What Drives The Larger Swelling In Big Cells

5.1 Battery Capacity and Structural Design

Unlike the flexible aluminum-plastic film casing of pouch cells, the rigid metal casing of cylindrical batteries hinders the release of stress generated by internal expansion. The non-uniform contact between the metal casing and the internal wound core can create small voids, leading to uneven stress distribution, as shown in Figure 5. The 4695 cylindrical battery, compared to the 21700 battery, has a larger capacity and a more complex structural design. A larger capacity means that the wound core has more layers, which can result in more uneven volume changes and greater internal stress, leading to more pronounced deformation of the battery’s metal casing. Additionally, the more complex structural design may result in weaker elasticity or buffering capability to cope with volume changes, exacerbating the phenomenon of volume expansion.

5.2 Material Properties

Different cell models may employ different electrode materials and electrolyte formulations. The intrinsic swelling and contraction characteristics of these materials during cycling can vary, contributing to differences in overall cell volume change. Additionally, the thickness and strength of the metal casing significantly influence overall deformation; a thicker, stronger can withstands greater stress, resulting in less measurable external deformation.

5.3 Manufacturing Process

The manufacturing process of the battery also affects its volume stability. For example, factors such as the uniformity of electrode coating, the tightness of winding or stacking, and the seal integrity of the battery casing can all impact the battery’s volume change. Specifically, the assembly tightness of the cell (the gap between the wound core and the casing) plays a significant role. When the reserved space is larger, the battery can accommodate greater core deformation, resulting in less force on the casing and thus, smaller deformation of the battery.

5.4 Testing Conditions

Although the temperature difference across channels is within a controllable range, other testing conditions (such as charge and discharge rates, depth of discharge, and number of cycles) may also affect the battery’s volume change.

Figure 3. Volume & Voltage Change Curve of the 21700 Battery

Figure 4. Volume & Voltage Change Curve of the 4695 Battery

Figure 5. Schematic Diagram of Volume Change Mechanisms in Cylindrical Batteries

In the pursuit of cost reduction and lifetime extension for cylindrical cells, key considerations include casing material selection, casing thickness and design strength, jelly-roll-to-canister gap design, and understanding swelling differences at various C-rates. Rational design optimization, coupled with comprehensive characterization methods, is essential for enhancing the performance and safety of large cylindrical battery systems.

6. Implications For Design, Safety and Pack Integration

The observed >10× difference has practical consequences:

• Module mechanical design: packs using large cylindrical cells require re-evaluation of intercell spacing, clamp strategies and thermal interface materials to accommodate larger reversible and irreversible expansion.

• Preload and compression strategy: moderate preloading can reduce free displacement but increases contact stresses; designers must optimize preload to limit bulging without inducing localized mechanical damage.

• Diagnostics and lifetime prediction: volume-change traces correlate with electrochemical events; therefore, in situ expansion characterization of cylindrical batteries can feed predictive models for capacity fade and mechanical failure.

• Quality control: critical manufacturing tolerances (winding tension, can thickness, internal clearance) should be tightened for large cells; in-line expansion testing during qualification can reveal batches with elevated swelling propensity.

7. Conclusions

Our in-situ expansion measurements reveal that large cylindrical cells can exhibit significantly larger volumetric changes than small cylindrical cells under nominal cycling conditions. For the tested samples, a 4695 cell showed ~0.22% volume change while a 21700 cell showed ~0.013%, a difference exceeding an order of magnitude. These findings emphasize that mechanical behaviour does not scale linearly with size and that in situ expansion characterization of cylindrical batteries should be part of the development and qualification toolkit—particularly when migrating to large-format cells for vehicle or stationary applications.

8. References

Wenxuan Jiang, Haoran Li, Sicong Wang, Sa Wang and Wei Wang，Dynamic Volumography of Cylindrical Li-Ion Battery Cells by Watching Its Breath During Cycling，CCS Chemistry. 2023; 5:1308–1317