Winding

In a cylindrical cell the anode, cathode and separator are wound into a spiral. Some prismatic cells have stacked electrodes and some have a flat wound jelly roll.

Challenges

  • Alignment of layers
  • Avoid punctures of separator
  • Separator folding
    • lots of countermeasures applied over time like separator envelope welding not all manufacturers countermeasure in this way
  • Check humidity
    • an important test that should be done before assembly and before the filling stage, is checking the humidity of the separator
stacked vs wound electrodes

Stacked vs Wound Cells

When we talk about Stacked vs Wound cells we are considering the Anode-Separator-Cathode stack and how that is assembled within a battery cell.

Pouch and Prismatic cells can be stacked or wound, but what is the advantage and disadvantage of each approach?

Alignment

When winding electrodes the alignment of the anode, cathode and separators is a critical parameter.

Anode Overhang is the extra material in a battery cell’s anode that extends past the intended boundaries.

Anode overhang
dark red is no anode overhang, which is critical, green shows anode overhang.
schematic showing electrode overlaps
Understanding and Eliminating Electrode Overlap Issues

Robust electrode design requires a clear understanding of how design intent translates into manufactured reality. By accounting for both design and process tolerances, engineers can predict worst-case alignment scenarios and avoid unintended overlap. Root Sum Square (RSS) tolerance analysis provides a systematic way to evaluate these combined effects, enabling more reliable and manufacturable electrode designs.

The other characteristic wound-cell defect is telescoping, where successive layers creep sideways along the mandrel’s axis turn by turn, so the finished roll comes out as an uneven cone rather than a true cylinder. It’s usually traced to uneven tension across the web width, a guide roller that’s slightly out of alignment, or static charge pulling the web off-line. The practical consequence is uneven pressure distribution through the roll, tabs that don’t line up, and most importantly active material margins that lose their electrical isolation from the opposing electrode. High-speed CCD vision systems tied to closed-loop electro-mechanical edge guides are the standard countermeasure, correcting lateral drift in real time rather than relying on tension control alone. The required anode-over-cathode overhang tolerance this is protecting, and the tolerance stack-up behind it, is covered in more depth on the Anode Overhang page.

Bending and Corner Strain

Winding a flat electrode into a spiral locks mechanical strain into every layer, and the tightest layers see the worst of it. For a jelly roll built on a mandrel of radius rmandrel, the bend radius at turn n is approximately the mandrel radius plus the cumulative thickness of everything wound so far:

Rbend(n)=rmandrel+n⋅tR_{bend}(n) = r_{mandrel} + n \cdot t

where t is the combined thickness of one full wound layer (both electrodes and both separator passes). For a coated current collector of foil thickness tfoil and active-material coating thickness tcoating, the resulting strain at the outer fibre of that coating is roughly:

ε≈tcoating+tfoil/2Rbend\varepsilon \approx \frac{t_{coating} + t_{foil}/2}{R_{bend}}

The relationship is worth sitting with: strain scales inversely with bend radius, so the innermost turns closest to the mandrel always carry the highest strain, and thick, high-loading electrodes on a narrow-diameter mandrel are the worst case. Push past the coating’s adhesive elastic limit and the outer fibre of the coating cracks while the inner fibre buckles and shears. Under cycling, those micro-cracks become local hot spots for current density and resistance; lithium struggles to intercalate evenly into the cracked material and instead plates out as metallic lithium directly in the crack, which over time can grow into a dendrite that punctures the separator. This is the underlying reason a narrow-diameter mandrel combined with a thick, high-energy-density electrode is worth checking early in a design, rather than only tightening tension control downstream.

It’s also why gradual, not instant, flattening matters when a cylindrical wind is being converted into a flat-wound pouch or prismatic cross-section: the mechanical press has to compress the roll slowly enough for the layers to redistribute strain, because rapid flattening tends to shear the separator at the outer margins rather than reshaping it cleanly.

Electrode Lengths

Cylindrical Cell Electrode Estimation

Knowing the outer and inner diameter of the spiral along with it’s thickness we can calculate the length of the material to create it.

D is the inner diameter of the cylindrical can.

The inner diameter is that of the mandrel around which we wind the spiral.

prismatic jelly roll

Prismatic Cell Electrode Estimation

The basic inside dimensions of the prismatic case and the jelly roll shown fitting those dimensions with a gap above and below.

The tolerance of the jelly roll versus the dimensions of the case are an unknown for production at scale and we have here assumed a direct fit.

Tension

The tension and hence consistency of the final wind is a key parameter. Using CT Scanning to Detect Battery Defects shows some of the issues with buckling.

Cell defects - Glimpse
Figure 3. CT scan images of critical cell defects and their potential impact.

Web Tension Control

Because the cathode, anode and separator webs have very different mechanical properties, they can’t share one tension setpoint — each is normally held in its own closed-loop window (dancer arm or load-cell feedback) through the winder. As a rough guide, roll-to-roll tension-control specialists such as Dover Flexo Electronics quote cathode webs held around 1.0–2.0 N to keep the coating flat without stretching it, anode (copper) webs held lower, around 0.8–1.5 N, because copper foil deforms plastically far more readily than aluminium, and separator webs held lowest of all, roughly 0.3–0.8 N, since polyolefin membranes are the most tension-sensitive material in the stack: over-tensioning stretches the machine direction and can collapse the micropores that carry lithium-ion current, while under-tensioning lets the web wander and wrinkle. Treat these as illustrative order-of-magnitude windows rather than universal setpoints — actual values depend on the specific foil, coating and separator grade in use.

Winding Architecture Variants

Continuous spiral winding onto a round mandrel is the default for cylindrical formats (18650, 21700, 4680), but it isn’t the only winding geometry in use:

  • Flat/elliptical winding — for smaller pouch and prismatic cells, the electrodes are wound around an elliptical (rather than round) mandrel, or wound round and then mechanically compressed into a flat cross-section afterward. Either way, this is the geometry that makes the gradual-flattening point above relevant.
  • Multi-tab winding — a single current-collection tab at one end of a long wound electrode means every electron generated deep in the roll has a long path to travel, which adds internal resistance. Multi-tab (or middle-lug) designs add current-collection points partway along the wind, shortening that path and narrowing some of the resistance gap between wound and stacked architectures.
  • Pre-bending — pre-forming a slight curvature into the electrode sheet before it reaches the mandrel, so the material is already partway toward its final radius when winding tension is applied, reducing the strain concentration described above rather than fighting it after the fact.

See also: Stacking, Lamination