Electrochemical Dilatometry of Different Proportions of Silicon Carbon Composite Anode Electrodes

1. Article Abstract

Incorporating silicon into graphite anodes significantly boosts energy density but introduces a major challenge: extreme volume expansion during lithiation. Precise measurement of this swelling is critical for cell design and longevity. Electrochemical dilatometry provides a direct, quantitative window into electrode dimensional changes during lithiation and delithiation. In 2020, the team led by Daniel P. Abraham employed in-situ electrochemical dilatometry to quantitatively characterize the thickness changes in Silicon Carbon Composite Anode materials with varying silicon content, shows how porosity, testing pressure and electrolyte choice bias dilatometry results.Their systematic work provides a valuable framework for analyzing expansion behavior and and highlights practical recommendations for experiment design and interpreting Silicon Carbon Composite Anode swelling data.

2. Methodology: In-Situ Cell Design and Sample Preparation

The core of the study was a custom in-situ electrochemical cell, illustrated in Figure 1, which allowed for simultaneous electrochemical cycling and real-time thickness monitoring. Electrodes were fabricated with silicon mass percentages ranging from 0% (pure graphite) to 100% (pure silicon), with detailed compositions and performance parameters listed in Table 1. This design enabled direct observation of expansion dynamics under operating conditions.

Table 1. Composition and performance parameters of the test electrodes.

Figure 1. Schematic diagram of the electrochemical dilatometry test setup.

3. Quantitative Analysis of Expansion Behavior

The analysis yielded clear, quantitative trends on how silicon content dictates expansion.

3.1 Cycle-by-Cycle Evolution

For an anode with 15% silicon (Figure 2), both the maximum thickness at full lithiation and the irreversible expansion after each cycle increased progressively. This is attributed to ongoing SEI formation and variations in the degree of lithiation/delithiation.

Figure 2. Swelling curve of the silicon carbon composite anode

3.2 Voltage-Expansion Correlation

From the swelling and voltage rewiring of different proportions of silicon carbon composite anode electrodes in Figure 3, the thickness swelling for the pure graphite negative electrodes mainly occurs in the first and third platforms, whereas the expansion ratio of the second platform is only 1%, and there will be a certain degree of thickness shrinkage at high capacity lithium reduction. For the pure silicon negative electrode, the maximum swelling thickness reaches almost 300%, the irreversible swelling thickness is also close to 50%, and after a circle of charge and discharge, the battery capacity decay degree is also the largest.

Figure 3. Potentials and swelling curves of different proportions of silicon carbon composite anode

3.3 Differential Capacity Insights

From the dQ/dV plots of Figure 4, it can be seen that the pure graphite electrodes have three obvious reaction peaks when embedded in lithium, while the two lithium silicon alloys of the pure silicon electrodes are higher than the graphite. When the silicon carbon is mixed, it shows a mixture of the two materials, but the reaction summit of the lithium silicon alloy is weaker.

Figure 4. Differential capacity curves of silicon carbon composite anode with different ratios

3.4 The Silicon Content Threshold

Figure 5 shows that when different proportions of silicon carbon are combined, the specific capacity of the electrode is gradually increased, and the maximum swelling thickness corresponding to the electrode is full, but not linearly. Usually, when the content of silicon is less than 30%, the swelling thickness of the electrode is smaller, but after more than 30%, the slope of the thickness swelling increases significantly.

Figure 5. Specific capacity and maximum expansion ratio of silicon carbon composite anode with different ratios

4. Key Experimental Considerations for Accurate Dilatometry

The study meticulously examined factors influencing electrochemical dilatometry measurements (Figure 6):

• Electrolyte & Cell Seal: Differences in sealing between coin cells and the in-situ cell can affect voltage profiles. Using stable electrolytes like LiFSI and lower C-rates can mitigate this.

• Applied Pressure: The mechanical constraint from the test fixture influences the measured expansion.

• Electrode Porosity: Electrode architecture is critical. Lower porosity electrodes exhibited higher expansion ratios, as less free volume is available to accommodate particle swelling internally.

• Additional Practical Factors: Include sensor drift, gas generation during cycling and differences between sealed in-situ cells and commercial wound/stacked cells — all of which can confound direct translation from lab dilatometry to pack behavior.

Figure 6. Factors that affect the outcome of electrochemical dilatometry experiments: Electrolyte

Figure 7. Factors that affect the outcome of electrochemical dilatometry experiments: Polarization

Figure 8. Factors that affect the outcome of electrochemical dilatometry experiments: Electrode porosity

3.5 Reversible vs. irreversible Expansion and Differential Capacity Signatures

Dilatometry traces show two components:

• Reversible expansion follows the charge curve and shrinks on discharge; it maps to intercalation/alloying strain.

• Irreversible expansion accumulates cycle-by-cycle and correlates with capacity loss; sources include SEI growth, particle fracture and trapped Li.

Differential capacity (dQ/dV) analysis helps disentangle mechanisms: graphite exhibits three clear intercalation peaks, while silicon shows broader alloying peaks at higher potentials. Mixed electrodes display combined features, with silicon alloying peaks typically diminished in amplitude relative to pure Si—indicating kinetic overlap and possible capacity sharing between phases.

5. Design Takeaways for Silicon Carbon Composite Anode Developers

From the combined measurement and analysis the study draws actionable rules:

• Limit Si fraction below ~30 wt% when strict dimensional control is required; above this threshold swelling escalates sharply.

• Optimize electrode porosity (higher porosity reduces net external expansion but may trade off volumetric energy density).

• Use dilatometry at relevant applied pressures that match expected pack clampings to obtain field-relevant expansion thickness values.

• Combine dilatometry with dQ/dV and post-mortem microscopy to link swelling patterns to SEI growth, particle fracture and “dead” lithium formation.

6. Summary

In this paper, the thickness swelling of graphite and silicon anode is characterized by the electrochemical dilatometry to quantitatively analyze the difference in electrode swelling in different proportions of silicon carbon mixing, mainly with the following conclusions:

• Pure graphite electrodes swelling by ~19%, while pure silicon electrodes swelling by ~300%.
• Electrode-level expansion for Si-containing electrodes can be much larger than anticipated, as the electrode porosity may not be able to accommodate the additional swelling caused by morphological changes in the silicon particles.
• For different ratios of the silicon carbon anode, the swelling is nonlinear.
• Limiting the capacity of the silicon carbon anode can adjust the maximum swelling ratio.
• Limiting the depth of delithiation would not be as effective in restraining the dimensional variation as restricting the depth of lithiation.
• The proportion of electrode swelling with lower-porosity should be larger than electrode with higher porosity.
• Researchers should note the effects of several factors mentioned when using the electrochemical dilatometry method

7. Original Article

Andressa Y.R.Prado, Marco-Tulio F.Rodrigues, Stephen E.Trask, Leon Shaw and Daniel P.Abraham, Electrochemical Dilatometry of Si-Bearing Electrodes: Dimensional Changes and Experiment Design, Journal of The Electrochemical Society, 167(2020) 160551.

8. IEST Recommended Instrumentation for Expansion Analysis

SWE Series In-situ Swelling Analysis System

8.1 A Variety of Cell in-situ Characterization Methods (stress & expansion thickness)

The swelling thickness and swelling force of the cell charging and discharge process are measured at the same time, so as to quantify the changes of cell swelling thickness and swelling force.

8.2 More Refined and Stable Test System

Using a highly stable and reliable automatic adjustment platform, equipped with a high precision thickness measurement sensor and a pressure regulation system, the relative thickness measurement resolution is 0 m, to realize the long cycle monitoring of the long-term charging and discharge process of cells.

8.3 Diversity of Environmental Control and Test Functions

SWE series equipment can adjust the temperature of the charge and discharge environment to help study the swelling behavior of cell cells under high and low temperature; In addition to conventional thickness and pressure test, cell swelling force, compression modulus and compression rate can be tested.