By Mi-Young Park, CEO & Co-Founder, Sasung Power
38% lower through-thickness ionic conductivity appeared in our first three-point bending test on a fresh carbon fiber composite cell, at 12 MPa load. The cell showed no visible damage: no delamination, separator puncture, or fiber fracture. Conductivity returned fully after unloading. This reversible yet severe suppression under flexural stress is the central materials problem in structural battery cells, and it does not appear in flat-cell electrochemical testing.
Structural Cell Materials Stack
A carbon fiber composite structural cell replaces the usual metallic current collector and rigid housing with a multifunctional laminate. Depending on the architecture, fiber layers provide the main load-bearing structure and act as the electrode current collector or substrate. The separator is generally a ceramic-coated polymer membrane or solid polymer electrolyte film. It must contain the electrolyte during bending without cracking.
The basic four-layer stack, outside to inside:
Outer carbon fiber composite shell: woven or unidirectional CF plies in an epoxy or polymer matrix carry axial and bending loads. Structural automotive applications commonly use 200 to 300 g per square meter fiber areal weight and a 2 to 4 mm total shell thickness.
Electrode layers: in a conventional lithium cell, slurry-coated active cathode material sits on the current collector. In a structural cell, the coating must bond to a fiber-reinforced surface that deforms slightly under load. That calls for binders with greater elasticity than the PVDF binders used in conventional cells.
Separator membrane: a 20 to 30 micron ceramic-coated PE or PP membrane, or a solid polymer electrolyte film. Solid electrolyte avoids liquid permeation during bending, but its room-temperature ionic conductivity is 1 to 2 orders of magnitude below liquid electrolytes, reducing rate capability.
Current collectors: conventional designs use thin aluminum foil on the cathode side and copper foil on the anode side. Where the fiber is electrically conductive and the architecture allows it, the carbon fiber can collect current directly through the fiber bundle.
Flexural Load and Ionic Conductivity: The Core Constraint
Ionic transport through a separator is governed by membrane pore geometry: pore radius, tortuosity, and electrolyte-filled fraction. Bending introduces interlaminar shear that closes or distorts those channels. In a 20 micron ceramic PE separator, through-thickness ionic resistance rises by 30 to 45% under a 10 MPa three-point bending load. Pore compression reduces effective pore radius and increases tortuosity under the applied stress.
The design trade-off has no simple fix. A thicker separator limits conductivity loss by providing more mechanical compliance, but raises baseline ionic resistance and lowers cell-level energy density. A solid polymer electrolyte prevents pore collapse, yet most solid polymer electrolytes have room-temperature conductivity 10 to 100 times below liquid electrolytes, reducing charge-rate capability. Changing fiber orientation can reduce through-thickness compression, but it limits the structural design and may lower the cell's contribution to vehicle-floor torsional stiffness.
No configuration removes the trade-off. The engineering task is to choose the operating point for the application.
Three Failure Modes Under Combined Loads
Our combined-load protocol pairs electrochemical cycling, typically a 0.5C charge and discharge rate, with mechanical loading in three modes: flexural, using three-point bending; compressive, through-thickness; and torsional, twisting along the cell long axis. Across structural cell designs, three failure modes recur:
Separator delamination can develop during cyclic bending even below the single-cycle failure threshold. A separator that withstands 15 MPa static bending without delamination may develop interfacial crack growth after 5,000 cycles at 8 MPa alternating load. Fatigue cracks begin at the ceramic coating and polymer substrate interface, driven by differential thermal expansion and small relative electrode-layer motion during charge and discharge. Static mechanical testing misses this mode and cannot predict it from static properties alone.
In-plane compression can fracture active-material particles. LFP particles are brittle, and fracture rates rise measurably above approximately 20 MPa in-plane compressive stress. Acoustic emission sensors can detect the increase during testing. Fractured particles lose electrical contact with the binder matrix and become electrochemically inactive, accelerating capacity fade in a pattern resembling normal cycle aging but progressing faster. This parallels cycle-induced fracture from lithiation expansion, except the driver is external mechanical load rather than electrochemical volume change.
Sealed-cell deformation can cause gas accumulation. During bending, the internal gas pocket formed during formation cycling moves toward the compression side. In a sealed structural cell, that movement can create localized separator pressure and raise contact resistance there. Across hundreds of bending cycles, migration forms a persistent high-impedance zone. EIS shows it as a distributed Warburg element at low frequency. Detection before macroscopic damage is one reason cell-level EIS monitoring matters in structural applications.
Fiber Orientation, Stiffness, and Ionic Transport
Carbon fiber plies may run at 0°, 90°, or intermediate angles. A 0/90 cross-ply balances stiffness across the two principal directions. A quasi-isotropic 0°/45°/90°/-45° layup provides nearly equal in-plane stiffness in every direction. Both are standard structural laminates with good mechanical performance.
In a structural battery cell, orientation also determines through-thickness stress during bending. A unidirectional 0° layup concentrates interlaminar shear at one ply interface, making it the worst arrangement for separator compression. Woven fabric distributes that shear more evenly, lowering peak separator compressive stress under the same total bending load.
At 10 MPa bending load, switching the outer ply from unidirectional 0° to a 2x2 twill weave reduced ionic conductivity loss by 20 to 25% in our measurements, with otherwise equivalent cell geometry. The cost was approximately 15% lower compressive modulus for the twill weave than for the unidirectional layup, trading structural stiffness for improved electrochemical performance under the same load.
What Spec Sheets Leave Unreported
Research groups typically report structural battery ionic conductivity, energy density, and modulus from separate measurements under separate load conditions. Published specifications almost never include combined-load data, where electrochemical cycling and mechanical loading run simultaneously for hundreds of cycles.
Combined-load testing is slow and expensive. A 1,000-cycle test combining bending and cycling requires 8 to 12 weeks with hourly electrochemical measurements. Most published structural cell results therefore test one mode or the other, rather than both simultaneously. Before selecting a design for vehicle integration, obtain combined-load data under the application's actual load profile. Extrapolating from flat-cell data can overestimate cycle life and peak power capability, sometimes by a factor of two or more, depending on cell architecture and structural loads.
This is not an objection to published research. Combined-load infrastructure is genuinely expensive and slow. It is a practical limit on what a specification sheet can establish before an integration design is committed.