Stacking

For pouch cells the electrodes stacked: anode, separator, cathode, separator, anode, separator etc.

Some prismatic cells have stacked electrodes and some have a flat wound jelly roll.

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?

“Stacking” covers several distinct ways of building a flat electrode-separator sandwich, not one process:

  • Direct sheet stacking — cathode, anode and separator are all cut into discrete sheets and laid up positive/separator/negative/separator in sequence. It’s mechanically the simplest approach, but handling loose, ultra-thin separator sheets (8–15 µm) at speed is prone to static cling, wrinkling and tearing, which is why it stays rare in high-volume production.
  • Continuous Z-folding (Z-stacking) — the mainstream high-volume approach. An uncut separator web is folded back and forth in a zigzag by a swinging arm or table, and discrete electrode sheets are placed into alternating folds by vacuum pick-and-place. Because the separator is never gripped under tension at the electrode margins, the stack can expand and contract through the charge/discharge cycle without the localised buckling that shows up in wound designs.
  • Cut-stack integrated systems — a single machine line combining laser-notching, cutting, Z-folding and hot-pressing, removing the tray-to-tray handling steps between separate cutting and folding stages.
  • Bi-cell roll-stacking — electrodes are pre-laminated onto a carrier separator web as small multi-layer units (a 5-layer separator/cathode/separator/anode/separator “bi-cell” is typical), which are then placed at regular intervals and wound or folded flat. This is the manufacturing route behind LG’s “Stack & Folding” process; the electrical case for it. Why parallel multi-tab current paths lower internal resistance versus a single long wound tab is covered on the Z Stack page.
  • Paddle-wheel, shingle and roll-to-sheet stacking — later-generation variants aimed specifically at closing the throughput gap with winding (below): paddle-wheel stacking carries electrodes in the radial paddles of a continuously rotating wheel into a collection magazine; shingle stacking overlaps adjacent plates like roof tiles to reduce margin thickness; roll-to-sheet designs (e.g. the “Coil2Stack” concept) feed both electrode and separator directly from rolls and cut them inline, skipping the pick-and-place step altogether.

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

Alignment

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

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.

Throughput and Equipment Trade-offs

The reason winding remains the default for cylindrical and smaller-format cells is speed: a rotating mandrel can wind at web speeds up to several metres per second, whereas a folding arm has to complete a full mechanical stroke: accelerate, place, decelerate, reverse. This happens for every single layer, which historically capped basic Z-folding machines at a handful of sheets per second. Paddle-wheel and bi-cell roll-stacking exist largely to close that gap, and cut-stack integrated lines close it further by removing separate handling steps. None of this is free: stacking equipment generally carries a higher capital cost than winding equipment, roughly in proportion to how much of the pick-and-place and folding motion it automates, which is a real factor in the winding-vs-stacking decision for a given format and volume, alongside the volumetric and cycle-life case above.

Volumetric Efficiency and Why Large-Format Cells Favour Flat Stacks

A wound jelly roll wastes space in two places:

  1. central mandrel void
  2. rounded corners that don’t match a rectangular can or pouch cavity.

Stacked electrodes are flat from the outset, so they can fill a rectangular case far more completely. Manufacturer comparisons put wound-cell case fill around 85%, against a stacked cell filling into the high-90s. That gap widens with cell size, because bending-corner strain (see the Winding page) gets worse as the wound layer count grows, which is the main reason large-format prismatic and pouch cells above roughly 50 Ah have moved to stacking rather than winding.

The zero-margin-tension property of Z-folding described above also shows up in cycle life: because the folded separator can flex rather than being pinned under winding tension, Z-stacked cells are reported by several manufacturers to tolerate high cycle counts in stationary storage duty without the progressive bulging that forces heavy external clamping on large wound prismatic cells. Take the specific cycle-count and clamping-force figures quoted in vendor literature as directional rather than independently verified, they vary with:

  • chemistry
  • C-rate
  • depth of discharge
  • underlying test methodology (usually not published)

See also: Winding, Lamination