Blade Cell Expansion Testing: Insights for Blade Battery Technology and Design

1. Preface

As new energy vehicles demand longer driving range and longer service life, automakers pursue higher volumetric energy density and smarter pack integration. One industrial solution is “removing modules and beams” — using the cell itself as both the energy-storage unit and a structural element. This idea gave rise to the blade cell form factor, and today blade batteries are an important branch of battery innovation.

Most commercial blade batteries use lithium iron phosphate (LFP) chemistry. What sets them apart is not chemistry alone but their long, thin geometry and unique production flow. The blade form — like a long razor blade — enables cells to be embedded directly into the pack without conventional modules, improving pack-level energy density and mechanical stiffness.

Figure 1. Comparison: blade batteries vs. traditional battery assemblies

Blade batteries can be divided roughly into long blades (e.g., BYD’s long blade) and short blades (e.g., SVOLT’s short designs). According to public specifications, BYD’s Changdao-style long blade is a hard-shell prismatic cell with typical dimensions around 960.0 ±10 mm × 90.0 ±1.0 mm × 13.5 +2.5 / −1.5 mm. Example variations: a 138 Ah blade ≈ 12 mm thick; a 202 Ah blade ≈ 13.5 mm. Electrode sheet sizes after teardown can reach ~944 × 83 mm (cathode) and ~946 × 85 mm (anode). Short blades can still exceed 500 mm in length (e.g., 573 × 117 × 21 mm).

Such large aspect ratios deliver cooling and packaging benefits but create manufacturing and mechanical challenges:

• Tight dimensional tolerances: even slight deviations can affect performance and safety.

• High lamination precision: long electrodes require precise layer alignment and flatness control.

• Mechanical strength and durability: long thin cells experience significant internal stresses during cycling and require sufficiently rigid housing to remain stable.

In short, compared with traditional cylindrical and square batteries, the manufacturing process of blade batteries are more stringent and adopts a multi-layer “sandwich” structure, in which positive and negative electrode plates and separator layers are alternately stacked, however, bubbles are easily generated during the stacking process, and the cell are uneven, causing uneven pressure inside the battery and affecting its strength. Due to its special process and structure, there are currently few mechanical performance characterizations of the expansion of single blade cell. For this reason, IEST has been committed to developing high-precision equipment dedicated to characterizing the expansion performance of blade cell.

2. Equipment Functions and Parameters

The IEST SWE3500 expansion tester is engineered to characterize expansion behavior of large-format and short-blade cells in-situ. This high-precision SWE3500 expansion tester features a rugged steel frame, servo-driven power system, and integrated environmental chamber.

Figure 2. Expansion characterization equipment SWE3500

2.1 Main features:

• Rugged steel frame for long-term stability and wear resistance.

• Insulating high-hardness pressure plates to prevent deformation and provide electrical isolation.

• Servo-driven power system with closed-loop control for precise actuation.

• High-resolution displacement sensors (0.1 µm) and pressure sensors (accuracy 0.3% F.S.).

• Integrated charge/discharge interface for synchronized electro-mechanical testing.

• Built-in environmental chamber: −20 °C to 80 °C to simulate operating conditions.

• Maximum test pressure: 5 t.

• System footprint: approximately 1500 × 1700 × 2000 mm.

2.2 Supported Test Modes for Battery Expansion Force Measurement

• Constant Pressure Mode: Hold an applied pressure while monitoring expansion thickness during cycling. This mode is ideal for simulating in-pack constraints and quantifying battery expansion force measurement under realistic conditions.

• Constant Gap Mode: Maintain a fixed gap and record the evolving expansion force. This mode directly captures battery expansion force measurement during lithium intercalation and phase transitions.

• Steady-State Compression: Quasi-static stress–strain characterization for mechanical property mapping, providing complementary data to dynamic battery expansion force measurement.

3. Application Cases

3.1 Cell Information

3.2 Charge and Discharge Process

3.3 Characterization of Expansion Performance

Set a constant pressure of 1000kg and a constant gap of 1000kg to test the expansion thickness and expansion force of the balde cell, as shown in Figure 3 below: the thickness and expansion force change curves show regular changes with the cell voltage curve, and the trends of the two curves are similar, and both have the unique “hump” phenomenon of the LFP/Gr system.

Figure 3. Cell expansion thickness and expansion force change curves with voltage

A static compression sequence was applied (start 50 kg, step increments of 500 kg to 5,000 kg, then unload back to 50 kg). The resulting stress–strain loop enabled quantification of reversible and irreversible deformation:

• Max deformation at 5,000 kg: 1.71% thickness reduction.

• Irreversible rebound after unloading: 0.54%.

• Reversible component: 1.17%.

These metrics support design limits for casing stiffness, pre-load strategies, and safety margins.

Figure 4. Pressure control curve (left picture) cell stress-strain curve (right picture)

3.4 Mechanistic Origin of the “Hump” Phenomenon: Coupling of LFP Two-Phase Reaction and Graphite Stage Transitions (GICs)

In Figure 3, two characteristic “humps” are clearly visible in the expansion thickness and expansion force curves during constant-current cycling. These humps are a fingerprint of the LFP/graphite system and originate from the synergistic volume changes of both electrodes – the two‑phase reaction in the LFP cathode and the staging transitions in the graphite anode (Graphite Intercalation Compounds, GICs).

3.4.1 Two‑Phase Reaction in the LFP Cathode

Lithium iron phosphate (LiFePO₄) undergoes a classic two‑phase reaction during charge/discharge:

• Charge: LiFePO₄ → FePO₄ + Li⁺ + e⁻

• Discharge: FePO₄ + Li⁺ + e⁻ → LiFePO₄

The reaction proceeds via a moving phase boundary. At intermediate states of charge (around 50% SOC), the two phases coexist with maximum interface area, leading to peak lattice mismatch and volume change. The molar volume of LFP shrinks by ~6.8% upon delithiation. Under constrained conditions (e.g., within a rigid cell housing), this sudden volume change manifests as a distinct hump in the expansion curves.

3.4.2 Stage Transitions in Graphite (GICs)

Lithium intercalation into graphite occurs via well‑defined stage structures, where Li ions order into specific interlayer galleries. The main stages and their characteristics are:

Stage	Structure	Potential (vs. Li⁺/Li)	Volume expansion
Dilute	Random Li distribution	>0.25 V	~0.5%
Stage 4 → 3	Li in every 4th / 3rd layer	0.20–0.25 V	~2–4%
Stage 2 (LiC₁₂)	Li in every other layer	0.12–0.15 V	~6–8%
Stage 1 (LiC₆)	Li in every layer	<0.08 V	~10–13%

During charging, the graphite proceeds through dilute → stage 4 → stage 3 → stage 2 → stage 1. Each stage transition involves a sudden reordering of Li ions between graphene layers, leading to an abrupt change in interlayer spacing (e.g., from ~0.35 nm to ~0.37 nm at the 2→1 transition). These abrupt changes produce two characteristic humps in the expansion curve:

• First hump: Corresponds to the 4→3 or 3→2 transition (potential ~0.15–0.20 V).

• Second hump: Corresponds to the 2→1 transition (potential ~0.08–0.10 V).

3.4.3 Cooperative Origin of the Two Humps

The two humps observed in Figure 3 are the superposition of volume changes from both electrodes:

Hump	Dominant electrode	Corresponding event	Expansion signature
First (lower voltage)	Anode‑dominated	Graphite: dilute → stage 4 → 3 → 2	Broad, gradual rise
Second (higher voltage)	Synergistic	Cathode: LFP two‑phase coexistence + Graphite: stage 2 → 1	Sharp peak, maximum expansion force

Thus, the humps are not only a direct indicator of the LFP two‑phase reaction but also reflect the significant contribution of graphite staging to the overall mechanical behavior. Analyzing the position, height, and width of these humps provides valuable information about lithiation homogeneity, risk of Li plating, and SEI stability.

This is why, in blade battery R&D, simultaneous acquisition of voltage – expansion thickness – expansion force (as enabled by IEST SWE3500) offers far greater diagnostic power than electrochemical tests alone.

4. Blade Batteries Design Technical Deep Dive

4.1 Design & Manufacturing Considerations for Blade Batteries

Blade cell manufacturing challenges arise primarily from the long, thin geometry. Although blade batteries bring clear pack-level benefits (improved cooling channels, higher volumetric density, simpler pack structure), their unique form factor introduces strict requirements for dimensional control, lamination precision, and expansion management. Below we outline key blade cell manufacturing challenges and mitigation strategies:

• Strict dimensional control: enforce tight tolerances on electrode sheet cutting, coating, and calendering.

• Precise lamination: improve roll-to-roll alignment and tension control to prevent trapped voids and bubbles.

• Housing design: select casing materials and cross-sections that provide adequate stiffness while allowing controlled strain relief.

• Pre-load distribution: apply uniform pre-pressure during assembly to compensate expected swelling and reduce stress concentration.

• Electrolyte filling & SEI control: develop filling protocols and electrolyte solvent formulations that minimize solvent-driven swelling and inconsistent SEI formation across long electrodes.

• In-line inspection: adopt optical flatness metrology and acoustic/ultrasonic checks to detect defects early.

Characteristic	Traditional Prismatic Cell	Blade Cell
Aspect ratio (length/thickness)	~5–10	50–100+
Typical thickness	20–50 mm	10–15 mm
Expansion behavior	More isotropic	Anisotropic (lengthwise bending risk)
Pre-load requirement	Moderate uniformity	High uniformity across long axis
Testing complexity	Low–medium	High (multi-point sensing recommended)

4.2 Impact of Aspect Ratio on Expansion Uniformity

As introduced in the preface, blade batteries fall broadly into two categories: Long Blades (e.g., ~960 mm) and Short Blades (e.g., ~570 mm). The defining geometric parameter governing their mechanical behavior is the Aspect Ratio (cell length divided by thickness).

For long blades, this ratio can exceed 70:1, creating a dramatic difference in mechanical and electrochemical stability compared to short blades (<30:1). The following table summarizes these critical trade-offs:

Aspect	Long Blade (>70:1)	Short Blade (<30:1)
Electrolyte filling challenge	High – wicking distance >450 mm	Low – wicking distance ~250 mm
Expansion force distribution	Non‑linear, peak near center	More uniform along length
Casing stiffness requirement	High (must resist bending)	Moderate
Pre‑load strategy	Multi‑point or gradient pre‑load	Uniform pre‑load sufficient
In‑line inspection focus	Edge and center thickness variation	Overall thickness only

4.3 The Hidden Challenges of High Aspect Ratio

While long blades offer superior pack-level energy density, the extreme geometry introduces three major technical “pain points” that engineers must address:

4.3.1 Electrolyte Wetting & “Dead Zones”

The thin, elongated geometry creates an exceptionally long ion-transport path. During electrolyte injection, the distal regions (far ends) often experience slower infiltration or restricted wicking. This leads to non-uniform SEI formation and inconsistent ionic conductivity across the cell. Areas with poor wetting exhibit different expansion signatures, which can be identified via in-situ testing as localized thickness anomalies.

4.3.2 Non-Uniform Expansion & Bending Moments

In long-blade cells, the “breathing” effect (SOC-driven swelling) is rarely isotropic. Our research shows that expansion force often peaks in the central region, generating bending moments that can warp the cell casing. If the non-uniform force exceeds the elastic limit of the housing, it leads to permanent deformation, localized pressure points, and in extreme cases, internal short circuits.

4.3.3 Mechanical Stress & Consistency Control

For a 960mm cell, even a microscopic misalignment in the multi-layer “sandwich” structure is amplified across the length. This increases the risk of trapped bubbles or uneven pressure gradients. By using in-situ expansion metrology (e.g., IEST SWE3500), manufacturers can quantitatively compare long vs. short blade designs, ensuring that the transition to “module-free” packs does not compromise the cell’s mechanical integrity or cycle life.

Engineering Verdict: The choice of aspect ratio is a strategic trade-off. While long blades maximize space, short blades offer higher manufacturing robustness. Quantitative expansion data is the only way to bridge this gap, allowing for optimized pre-load designs and safer cell dimensions.

5. Why In-Situ Battery Expansion Monitoring Accelerates Blade Battery Technology

Quantitative, synchronized electro-mechanical testing using the SWE3500 expansion tester provides actionable insights for blade cell development.

• Material screening: compare binders, active-material formulations, and coating stacks for lower swelling signatures.

• Process tuning: optimize compaction pressures, lamination parameters, and filling temperatures with direct feedback.

• Lifetime modeling: correlate reversible vs. irreversible deformation with cycle life and mechanical fatigue predictions.

• Safety validation: detect conditions leading to excessive deformation that could compromise mechanical or electrical safety.

IEST’s SWE3500 enables manufacturers and R&D teams to iterate faster on blade cell designs and makes the transition to mass production safer and more predictable.

6. Conclusion

Blade cell form factors are a strategic evolution in pack-level design — delivering higher volumetric energy density and improved thermal management while simplifying assembly. However, the long thin geometry of blade batteries brings new mechanical and manufacturing risks. Systematic in-situ expansion testing (thickness, force, stress–strain) across SOC and temperature is indispensable for robust blade battery technology development. IEST has launched a large-size cell expansion force characterization equipment that can accurately characterize the expansion thickness, expansion force, stress-strain and other related properties of large-size cells in-situ. It helps engineers to screen the modification process of positive and negative electrode materials and determine the best modification process; accelerate the progress of cell research and development, and develop safer and more reliable cell. At the same time, you can also explore the best usage conditions and extend the service life of the cell.

7. References

[1] “New Energy Battery Pack Technology” public account: [The Origin of the Blade Battery Storm] BYD Blade Batteries Analysis 3.0.

[2] The Analysis on the Principle and Advantages of Blade Battery of BYD — A Domestic New Energy Manufacturer, DOI: 10.1051/shsconf/202214402003.

[3] M. Safari, C. Delacourt, “Modeling of a Commercial Graphite/LiFePO₄ Cell,” J. Electrochem. Soc., 2011.

[4] R. Malik et al., “Particle Size Dependence of the Phase Transformation in LiFePO₄,” Chem. Mater., 2010.