In-situ Testing of The Volume, Stress, and Thickness Changes of Different Silicon Anodes in Pouch Cells

1. Introduction

Silicon anodes offer a substantially higher specific capacity than graphite, but their practical use is limited by dramatic volume changes during lithiation/delithiation. Silicon anode volume expansion refers to the dimensional increase of the silicon-containing negative electrode as lithium ions are incorporated into the silicon lattice during charging — a process that can reach ~280% volumetric change for pure silicon and causes significant mechanical stress on the electrode structure and cell housing.

Understanding and quantifying these changes at the electrode and cell level is essential for designing durable silicon anodes and reliable battery packs. Here we synthesize experimental in-situ measurements and component-level analysis to quantify how silicon, graphite, and cathode materials each contribute to net pouch-cell expansion and mechanical stress. Throughout this article we highlight techniques for accurate anode thickness measurement and tracking silicon anode volume expansion percentage in operating cells.

2. Experimental Procedure

2.1 Fabrication of Three Types of Pouch Cells:

• Cell A — Li(Ni₁₋ₓ₋ᵧCoₓAlᵧ)O₂ (NCA)/SiO-graphite (Supplier A). Fully charged to 4.20 V, nominal capacity 260 mAh.

• Cell B — LiCoO₂ (LCO)/Si Alloy-graphite (Supplier B). Fully charged to 4.35 V, nominal capacity 230 mAh.

• Cell C — Li(Ni₁₋ₓ₋ᵧCoₓAlᵧ)O₂ (NCA)/nano Si-C (Supplier C). Fully charged to 4.40 V, nominal capacity 165 mAh.

These combinations provide a range of silicon morphologies and cathode chemistries to compare expansion behavior and cycling stability.

2.2 Testing Equipment and Procedures:

All tests combined electrochemical cycling with simultaneous in-situ characterization using four complementary measurement methods:

• In-situ X-ray diffraction (XRD) to determine phase changes and volume trends of cathode active material during charge/discharge.

• In-situ total cell volume measurement— performed using IEST’s GVM2200 gas/volume analyzer, capturing net cell swelling at 1 µL resolution over a 20–85°C operating range.

• In-situ force/stress measurement — performed using the IEST SWE Series swelling testing system, recording expansion force under both constant-gap and constant-pressure conditions.

• In-situ thickness sensors — monitoring electrode stack and anode surface displacement over cycling, enabling precise anode thickness measurement and silicon anode swelling quantification without cell disassembly.

Schematic setups for stress and thickness measurement are shown in Figure 1. (See the original study for full experimental drawings and calibration details.)

3. Results and Analysis

3.1 Net Cell Expansion, Stress, and Thickness Trends

Net cell expansion, stress, and thickness profiles differ substantially across the three silicon anode chemistries, and these differences persist throughout charge/discharge cycles. Figure 2 summarizes the measured volume, stress, and thickness profiles for Cells A–C:

• Cells A and B: similar magnitudes of net volume increase and stress; both are significantly larger than Cell C.

• Cell C (nano Si-C): consistently lower expansion and stress, suggesting improved mechanical buffering by the carbon matrix surrounding the nano-silicon particles.

• Voltage-range behavior: Cells A and C show plateau regions in expansion at high voltages, whereas Cell B exhibits sharper expansion/contraction in the same SOC window.

Because these measurements reflect contributions from both electrodes simultaneously, further decomposition is necessary to isolate the true silicon anode volume expansion percentage.

3.2 Component Decomposition: Pure Silicon vs SiO vs Graphite vs NCA

Component-level decomposition using literature expansion ratios and in-situ XRD data reveals the true silicon anode volume expansion percentage for each electrode material independently. Key reference values from the literature, coupled with in-situ XRD measurements of NCA volume change:

• Pure Silicon: ~280% volume expansion during full lithiation. This is the widely referenced silicon anode volume expansion percentage for bulk or pure-silicon electrodes; the expansion scales approximately linearly with SOC. (This figure is the basis of “silicon anode volume expansion 300%” queries — the actual measured value from Dahn et al. 2017 is ~280%.)

• Graphite: ~10% overall expansion; expansion shows step changes associated with the 2L→stage 2 phase transition but limited net volume growth.

• SiO (silicon monoxide): lower net expansion percentage than pure Si, because SiO lithiation partly forms irreversible Li₂O and lithium silicates — these phases buffer expansion at the cost of first-cycle capacity loss. The SiO anode volume expansion percentage is therefore materially lower than the 280% pure-Si reference.

• NCA: exhibits ~4.5% contraction during charge (most contraction at high SOC), partially offsetting anode swelling in the full-cell volume curve.

Using dV/dQ fitting of the composite electrode response (Figure 4), the relative contributions of Si and graphite in composite electrodes were resolved, and the anode-only expansion curve was reconstructed.

3.3 How Cathode Contraction Masks Silicon Anode Swelling

Cathode volume changes systematically distort full-cell expansion measurements, causing the true silicon anode volume expansion percentage to be underestimated if only total cell volume is monitored. Decomposition of the full-cell expansion (Figure 5) shows that in Cell A (NCA / SiO-graphite), the NCA cathode contraction (~4.5%) at high SOC creates a net plateau in the overall volume curve — NCA contraction partially cancels SiO expansion, making the anode’s true swelling appear smaller than it is. In contrast, Cell B (LCO / Si-alloy-graphite) — which does not exhibit the same cathode contraction profile — shows steeper net expansion, higher mechanical stress, and greater irreversible deformation over cycles.

3.4 Long-Term Cycling: Expansion Force and Capacity Decay

Irreversible expansion force accumulation over long-term cycling directly predicts capacity fade in silicon-composite cells, and nano-Si-C architecture significantly outperforms bulk Si-alloy designs. Figure 6 reports long cycle tests comparing Cells B and C:

• LCO / Si-alloy-carbon (Cell B): higher irreversible expansion forces and accelerated capacity fade — indicating that bulk Si-alloy particles generate larger, non-recoverable mechanical stress on the electrode structure.

• NCA / nano Si-C (Cell C): lower irreversible forces and slower capacity decay, demonstrating the benefit of carbon-buffered nano-silicon architecture and optimized electrode design.

These findings reinforce that controlling silicon anode volume expansion — both through material architecture selection and through accurate anode thickness measurement during development — correlates strongly with improved cycle life and reduced capacity fade.

4. Discussion — Practical Implications for Silicon Anode Design

• Material selection and composite design matter. Nano-Si embedded in carbon matrices reduces net silicon anode volume expansion percentage and mechanical stress compared with bulk SiO or Si-alloy particles, because the carbon matrix accommodates volume change locally before it propagates to the electrode level.

• Cathode behavior must be accounted for. NCA cathode contraction (~4.5% at high SOC) can temporarily mask anode swelling in total cell volume measurements; electrode designs that ignore this masking effect risk underestimating anode mechanical demands and over-estimating cell stability.

• Precision anode thickness measurement is essential. High-resolution in-situ thickness sensors enable identification of small, irreversible per-cycle thickness increments that predict long-term mechanical failure modes including electrode delamination and current-collector buckling. Integrating anode thickness measurement into qualification testing accelerates early detection of problematic formulations before full cycle-life data is available.

• Testing under realistic constraints. Measuring expansion force and thickness under both constant-gap and constant-pressure conditions yields complementary insights: force data characterize mechanical loading on pack components, while thickness data report dimensional changes relevant to cell stacking and tab integrity.

5. Conclusion and Practical Takeaways

This combined in-situ experimental and quantitative analysis demonstrates that:

• The silicon anode volume expansion percentage for pure Si is ~280% at full lithiation, roughly 28× graphite’s ~10% — confirming silicon as the dominant mechanical driver in composite anodes.

• SiO anodes exhibit a lower net expansion percentage than pure Si due to irreversible lithium consumption forming Li₂O and lithium silicate phases.

• Cathode volume changes (e.g., NCA ~4.5% contraction) can offset apparent anode expansion at specific SOC windows, producing misleading plateau behavior in total cell volume curves.

• Accurate anode thickness measurement and expansion force monitoring are critical diagnostic tools for assessing mechanical reliability and forecasting capacity fade in silicon-composite cells.

• Nano-Si architectures in carbon matrices achieve lower irreversible forces and better cycle stability than bulk SiO or Si-alloy designs.

Practical next steps for developers and QC teams: incorporate synchronized in-situ volume, force, and thickness measurement during formulation screening; prioritize composite designs that minimize incremental thickness change per unit capacity; and validate that cathode selection and operating voltage windows do not inadvertently amplify mechanical stress on the silicon anode.

6. Original Article

7. IEST Recommended Testing Equipment

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