Evaluation Scheme Of Electrical and Mechanical Properties of Composite Current Collectors

1. Composite Current Collector Structure and Background

A composite current collector uses a “sandwich” architecture: a polymer inner layer flanked by metal conductive layers on both sides (Figure 1). Two variants are in industrial development:

• Composite copper foil current collector: a 4.5 µm OPP (oriented polypropylene) substrate onto which 50 nm of Cu is magnetron-sputtered on each side, followed by aqueous electroplating to thicken each Cu layer to approximately 1 µm.
• Composite aluminum foil current collector: a 6 µm PET (polyethylene terephthalate) substrate with approximately 1 µm Al vapor-deposited on each side.

Total thickness is substantially lower than conventional foil: composite copper foil is the thinnest of the four collector types tested in this study, while composite aluminum foil is roughly half the thickness of standard aluminum foil. This thinning directly improves volumetric and gravimetric energy density at the cell level.

Figure 1. Composite current collector sandwich structure — polymer core (PET or OPP) with Cu or Al layers on each side

2. Composite vs Conventional Current Collector: Advantages and Limitations

2.1 Safety Advantage

The most significant benefit of the composite current collector in battery safety applications is the fuse behavior of the inner polymer layer. Under an internal short-circuit condition, Joule heating at the fault site rapidly exceeds the polymer melting point, causing the film to rupture. This interrupts current flow locally, preventing the runaway energy release that makes conventional metal foil dangerous in nail-penetration or crush scenarios. The polymer current collector layer thereby adds an intrinsic, passive safety mechanism that requires no external circuitry.

2.2 Weight and Cost Advantage

Replacing the majority of the metal cross-section with a lightweight polymer reduces the mass of the current collector significantly, improving gravimetric energy density (Wh/kg). For the composite aluminum foil variant, the thickness reduction to approximately half that of conventional aluminum foil also improves volumetric energy density (Wh/L) at the cell level.

Figure 2. Composite aluminum foil current collector — 6 µm PET substrate with ~1 µm Al per side; total thickness ~half that of conventional aluminum foil

At scale, composite aluminum foil offers a clearer cost advantage than composite copper foil, partly because aluminum is inherently less expensive than copper and partly because the deposition process for aluminum is more mature. The composite copper foil current collector’s cost premium over conventional copper foil remains a commercial challenge, although scale-up is expected to reduce this gap.

2.3 Key Limitations

• Higher through-thickness resistivity: PET and OPP are electrical insulators. The thin metal layers must carry all longitudinal current, resulting in through-thickness resistivity one to two orders of magnitude higher than conventional copper or aluminum foil. This increases battery internal resistance and reduces output power capability, particularly at high discharge rates.
• Perforation risk during deposition: High-temperature molten metal during magnetron sputtering or vapor deposition can splash and perforate the thin polymer substrate, creating pinholes that degrade film uniformity and electrical performance.
• Production throughput bottleneck: Vacuum deposition processes are slower per unit area than rolling/calendering of conventional metal foil, constraining production capacity and increasing unit cost.
• Modified tab-welding process required: The non-conductive polymer core means conventional battery tab welding cannot be applied directly. An additional transfer-welding step is required during electrode fabrication, increasing manufacturing complexity and cost.

3. Why Composite Current Collectors Have Higher Resistivity

The higher longitudinal resistivity of composite current collectors relative to conventional foil arises from three factors:

• Multi-material composition: The polymer core, which constitutes the majority of the cross-sectional area, is non-conductive. Only the thin surface metal layers (50 nm sputtered + ~1 µm electroplated for Cu; ~1 µm vapor-deposited for Al) carry current in the longitudinal direction, limiting total conductance.
• Structural complexity: Interfaces between the polymer and metal layers, and between the magnetron-sputtered seed layer and the electroplated bulk layer in copper foil, introduce additional contact resistance that is absent in homogeneous metal foil.
• Process variability: Vacuum deposition processes are more difficult to control uniformly across large substrate areas than metal rolling, resulting in thickness non-uniformity that creates resistivity variation across the foil surface.

4. Electrical Test Program: Resistivity Measurement with BER2500

Experimental instrument: model BER2500 (IEST), electrode diameter 14mm, the equipment is shown in Figure 3(a) and 3(b).

Figure 3. IEST BER2500 Battery Electrode Resistance Tester — 5 MPa, 15 s hold, 14 mm electrode diameter

Test conditions: each current collector sample (conventional copper foil, conventional aluminum foil, composite copper foil current collector, composite aluminum foil current collector) was placed between the instrument electrodes and tested at 5 MPa with a 15 s hold time. The software automatically recorded electrode thickness, resistance, resistivity, and conductivity.

5. Electrical Results: Resistivity and Thickness of Composite vs Conventional Collectors

Figure 4 presents the BER2500 measurement results for all four current collector types. Key findings:

• Composite copper foil is the thinnest of the four, consistent with its 4.5 µm OPP substrate and thin Cu surface layers.
• Longitudinal resistivity of both composite current collectors is substantially higher than their conventional counterparts. The composite aluminum foil shows the highest resistivity—approximately one order of magnitude greater than conventional copper foil—because the 6 µm PET core constitutes a larger fraction of the total cross-section, limiting through-thickness conductance more severely.
• The composite copper foil aluminum foil thickness comparison confirms that both composite types are thinner than conventional foil while exhibiting higher resistivity, demonstrating the inherent electrical trade-off of the polymer-core architecture.

Figure 4. Resistivity and thickness data (BER2500, 5 MPa, 15 s) — composite collectors are thinner but show substantially higher resistivity; composite aluminum ~10× conventional copper foil

6. Mechanical Test: Needle Puncture Resistance of Composite Current Collectors

Test conditions: four 2 cm × 3 cm collector samples of each type were fixed in a standardized holder. A steel needle was advanced at a constant rate under increasing force until penetration, recording the force-displacement curve.

Figure 5. Comparison of the results of needle puncture experiments with four types of current collectors

Figure 6. Needle puncture test data summary — composite aluminum foil shows markedly higher strength/thickness ratio; composite copper foil shows moderate improvement

The PET and OPP polymer layers substantially improve puncture resistance compared to pure metal foil of equivalent or greater thickness. Composite aluminum foil benefits more dramatically because conventional aluminum foil is inherently softer than copper foil and more susceptible to deformation under localized stress. This enhanced mechanical robustness reduces the risk of internal short circuits from manufacturing defects, handling damage, or particle contaminants during electrode fabrication.

7. Summary and Outlook

The composite current collector industry is transitioning from validation to early mass production. Long-term penetration will depend on continued progress in deposition process yield, cost reduction at scale, and resolution of the tab-welding challenge. From a system perspective, the electrical conductivity reduction must be carefully managed—particularly for high-rate applications—while the safety and energy density benefits are most relevant for large-format prismatic and pouch cells in EV and energy storage applications.

Composite current collectors offer compelling advantages—improved safety through the polymer fuse mechanism, reduced weight, and improved energy density through thinning—but must be weighed against elevated longitudinal resistivity, production throughput constraints, and modified manufacturing requirements. The BER2500 test data in this study confirm that both composite copper foil and composite aluminum foil current collectors have measurably higher through-thickness resistivity than their conventional counterparts, with the composite aluminum foil showing the largest deviation (~1 order of magnitude vs conventional copper foil). The mechanical tests confirm that per-unit-thickness puncture resistance is significantly improved for composite aluminum foil and moderately improved for composite copper foil.

8. References

[1] WANG Ru, LIU Zhikang, YAN Chao, et al. Interface Strengthening of Composite Current Collectors for High-Safety Lithium-Ion Batteries [J]. Journal of Physical Chemistry, 2023, 39(2):81-92.

[2] LIU Song; HOU Hongying; HU Wen; LIU Xianxi; DUAN Jixiang; MENG Ruijin. Research Progress of Current Collectors for Li-ion Batteries [J]. Silicate Bulletin, 2015, 34(9):2562-2568.

[3] WANG Chenghao,LI Xuefa,ZHANG Guoping. Aluminum composite current collector and its preparation method, positive electrode sheet, battery and electric device:CN202210827592.9[P1.CN202210827592.9[2023-10-071.

[4] WANG Shuai, ZHU Ya Ya, XIA Jianzhong, et al. A composite current collector soft pack battery case, soft pack battery and soft pack battery module:CN202211623631.X[P].CN116169403A[2023-10-07].

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