The journey of a lithium-ion battery through countless charge and discharge cycles is not merely an electrochemical dance; it’s also a physical one. As lithium ions shuttle between the positive and negative electrodes, the battery materials expand and contract. This expansion force, coupled with potential gas production during cycling, has a profoundly important impact on a battery’s longevity and performance. To mitigate these negative physical effects during cycle testing, batteries are commonly held within splints that apply external pressure, aiming to fix them in place. However, as research shows, not all splints are created equal. The type of splint and how it applies pressure can dramatically influence battery life, with some methods inadvertently causing detrimental effects like lithium deposition, ultimately shortening the battery’s lifespan rather than extending it.
This detailed study focuses on a commercial lithium-ion battery (LP2714897-50Ah) to understand how different test clamps affect its cycle performance. We meticulously examined the negative electrode at the end of its life cycle, analyzing its physical appearance, elemental composition, and internal structure. Our findings shed light on why the use of a common type of aluminum clamp leads to rapid cycle degradation. The results of this research offer valuable guidance for enhancing the cycle performance of individual battery cells, ensuring more uniform force distribution within battery modules, and consequently improving the overall lifespan of entire battery systems.
Setting the Stage: Experimental Design
Battery Preparation
For this experiment, we utilized the standard raw materials and production processes employed in our company’s commercial lithium-ion batteries.
The positive electrode sheets were prepared by mixing NCM ternary active material, PVDF binder, and a conductive agent in a specific ratio. This mixture was then homogenized, coated onto a foil, rolled, and cut according to our established production procedures.
Similarly, the negative electrode sheets were produced using artificial graphite, a conductive agent, CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber), with water acting as the solvent. This blend underwent homogenization, coating, rolling, and cutting, following the company’s production process.
Finally, these meticulously prepared positive and negative electrodes, along with a separator, were assembled into a square aluminum shell battery with a rated capacity of 50Ah and a model designation of LP2714897. The chosen electrolyte was a LiPF6-based solution.
Battery Testing and Analysis
The room temperature cycle performance of the batteries was evaluated using a sophisticated battery tester. The testing protocol involved:
- Charging: Constant current charging at 1C rate until 4.2V, followed by constant voltage charging until the current drops to 0.05C.
- Rest: A 30-minute rest period.
- Discharging: Constant current discharging at 1C rate down to 2.8V.
Throughout the cycling process, the DC internal resistance (DCIR) was measured at 50% State of Charge (SOC) every 200 cycles to monitor its changes over time.
For in-depth analysis of the electrode materials, we employed advanced characterization techniques:
- Scanning Electron Microscopy (SEM): To observe the surface appearance of the pole pieces.
- X-ray Diffractometer (XRD): To analyze the material’s internal structure.
- Inductively Coupled Plasma Emission Spectrometer (ICP): To determine the elemental content.
Unveiling the Impact: Experimental Results and Discussion
Our investigation compared the cycle performance of batteries housed in two different types of test clamps: ordinary aluminum clamps (referred to as clamp 1) and spring clamps (referred to as clamp 2). The capacity retention curves for both types of batteries at 1C charge/1C discharge are illustrative.
Initial observations showed that the capacity retention rates of both clamp types were remarkably consistent for the first 300 cycles. However, a clear divergence emerged after this point: batteries using clamp 1 began to show accelerated cycle degradation. After 2500 cycles, the battery using clamp 1 retained only 84.95% of its initial capacity, whereas the battery using clamp 2 maintained a superior 86.71% capacity retention.
Furthermore, the increase in DC internal resistance in batteries using clamp 2 was significantly less pronounced compared to those using clamp 1. This suggests that the spring-loaded clamps (clamp 2) effectively provided the necessary space for the battery cell to expand during its entire cycle life, thereby releasing internal stress and ultimately improving cycle performance.
Subsequent tests on the end-of-life (EOL) batteries confirmed these findings. The internal resistance of batteries cycled with clamp 1 was considerably higher than those cycled with clamp 2, aligning perfectly with the DC internal resistance trends observed throughout the cycling.
Dissecting the Difference: Post-Mortem Analysis
To understand the underlying reasons for these performance disparities, we disassembled the cycled batteries in a dry room.
- Electrolyte Residue: Strikingly, there was no electrolyte residue at the bottom of the battery shell that used clamp 1. In contrast, the battery shell from clamp 2 still retained a small amount of electrolyte, which also appeared relatively clear and transparent. This suggests that the excessive force exerted by clamp 1 prevented the battery from expanding and contracting naturally, leading to greater capacity loss and the production of more side reaction products, which would consume the electrolyte.
- Negative Electrode Appearance: Photos of the disassembled negative electrodes revealed significant differences. The surface color of the negative electrode from clamp 1 was uneven: lighter, grayish tones at the upper and lower edges, and a darker, deep blue in the middle. Conversely, the negative electrode from clamp 2 exhibited a much more consistent surface color, indicating better overall condition.
This color variation in clamp 1 batteries can be attributed to the battery’s natural swelling (due to gas production from side reactions and electrode expansion) during cycling. When ordinary aluminum clamps are used, the battery is tightly squeezed. As more gas is produced, the squeezing force intensifies, particularly on the central part of the electrode. This increased pressure in the middle leads to faster lithium ion insertion and extraction, while the edges, experiencing less force, undergo slower lithium ion movement. This differential activity results in the darker color in the middle and lighter color at the edges. With spring clamps (clamp 2), the integrated buffer space allows for battery expansion, distributing the force more uniformly across the electrode. This ensures consistent lithium-ion movement across the entire surface, leading to a more uniform electrode color.
To further dissect these differences, we performed detailed analyses on the distinct light-colored edge (Region A) and darker-colored middle (Region B) areas of the negative electrode from the clamp 1 battery.
- Surface Morphology (SEM): SEM images of Region A (lighter edge) showed a relatively smooth material surface, maintaining a complete, uniform sheet-like appearance. In stark contrast, the material in Region B (darker middle) appeared more fragmented and broken. This is a direct consequence of the battery’s expansion and subsequent mechanical stress when confined by ordinary aluminum splints without sufficient buffer space. The intense squeezing force causes the positive and negative electrode structures to collapse, ultimately leading to particle breakage.
This finding also indirectly explains why the battery using clamp 1 had no residual electrolyte, while the one with clamp 2 did. The excessive force on the battery with clamp 1 led to particle breakage, which in turn ruptured the SEI film (a protective layer). This rupture allowed electrolyte to infiltrate and be continuously consumed as the SEI film attempted to re-form, resulting in the higher increase in DC internal resistance observed in batteries using clamp 1.
- Elemental Analysis (ICP): We analyzed the content of transition metal elements (Nickel, Cobalt, and Manganese) in the graphite negative electrode from both Region A and Region B. These metals are known to dissolve from the positive electrode during charging and discharging and subsequently deposit on the negative electrode surface. Our analysis confirmed that all three transition metals were present in varying degrees in both regions. Notably, Manganese dissolution was highest, followed by Nickel, with Cobalt being the least abundant.
Crucially, the dissolution of all three metal elements was higher in the middle Region B than in the edge Region A, with the Manganese content in Region B being significantly higher. This increased Manganese dissolution in the middle might be attributed to the applied pressure aggravating the “Jahn-Teller effect” related lattice distortion, which intensifies the dissolution of Manganese ions. This also helps explain the darker blue color observed in the middle area of the negative electrode.
- Structural Analysis (XRD): XRD analysis of both Region A and Region B, compared to a fresh electrode, showed that while no new phases were formed after cycling, the diffraction peaks in both regions were broadened and their intensity reduced to varying degrees. However, the diffraction peak intensity of the edge Region A was significantly stronger than that of the middle Region B, indicating more severe material degradation in the middle.
Further calculations from the XRD data revealed that the diffraction angles of the pole pieces shifted to lower angles after cycling, with a more pronounced shift in the middle Region B. This indicates a larger interlayer spacing, suggesting greater lithium ion intercalation in the middle. On a macroscopic scale, this manifests as the shedding of graphite sheets. The increased force in the middle Region B also caused structural damage, leading to decreased diffraction peak intensity and broader peaks.
We also examined the positive electrode sheets. SEM tests showed that many small particles in both the middle and edge areas fractured after cycling, some even pulverizing into micro-powder. This contributes to increased battery self-discharge and reduces the effective contact area for conductivity and lithium-ion transmission, thereby increasing impedance during cycling. Larger particles largely maintained their morphology. A cross-sectional view after argon ion polishing further revealed a significant number of cracks forming inside particles along grain boundaries in the cycled positive electrode, with the degree of cracking being significantly higher in the middle area than at the edge. This points to greater forces and rapid lithium ion deintercalation in the middle. The constant shrinking and expanding of materials during cycling, coupled with particle anisotropy, generates stress that causes these cracks. Electrolyte ingress into these cracks leads to new interface film formation and continued side reactions, culminating in material particle breakage. This breakage results in active material peeling or electronic contact degradation, increasing battery polarization, reducing effective active material content, and ultimately diminishing reversible capacity. This perfectly correlates with the inconsistent color observed in the dissected negative electrode.
XRD analysis of the positive electrode showed the characteristic peaks of the ternary layered material remained largely consistent, indicating structural stability. However, a slight shift of the (003) diffraction peak to a lower angle after cycling suggested an increase in interlayer spacing. The increased splitting degree of other peaks after cycling indicated a better layered structure as cycling progressed. The ratio of certain peak intensities, indicating the mixing degree of lithium and nickel atoms, showed that while the ratio decreased after cycling, it remained above a critical threshold, implying minimal lithium and nickel mixing. However, a decrease in grain size from the fresh battery to the cycled ones was observed. While smaller grains can offer faster lithium-ion diffusion and higher discharge capacity, they also present more interfaces for side reactions with the electrolyte, leading to reduced cycle stability and capacity loss with increasing cycles.
Key Takeaway: The Power of Proper Pressure
This study, focusing on the LP2714897-50Ah NCM ternary/graphite square aluminum shell battery, conclusively demonstrates the significant influence of different test clamps on battery cycle performance. While the capacity retention rates were consistent for the initial 300 cycles for both ordinary aluminum clamps and spring clamps, the batteries constrained by ordinary aluminum clamps experienced accelerated cycle degradation thereafter. After 2500 cycles, these batteries retained only 84.95% of their capacity, whereas those protected by spring clamps achieved a significantly improved 86.71% capacity retention.
The root cause of this accelerated decay when using ordinary aluminum clamps is clear: these clamps cannot adequately buffer the expansion force generated by gas production and electrode expansion during battery cycling. This leads to uneven force distribution on the battery, with the middle section experiencing greater pressure. This excessive force causes particle crushing and exacerbates transition metal element dissolution, ultimately leading to rapid cycle degradation. In contrast, the use of spring clamps ensures more uniform force distribution throughout the battery’s cycle, allowing for controlled expansion and significantly enhancing cycle performance.
These findings hold immense guiding significance for battery design and manufacturing. By understanding and addressing the crucial role of external pressure management, we can dramatically improve the cycle life of individual battery cells, ensure greater force uniformity within complex battery modules, and thereby extend the overall lifespan and reliability of entire battery systems.