In lithium‑ion battery design, internal resistance is one of the most critical parameters governing power capability, heat generation, fast‑charge performance, and overall efficiency. The difference in internal resistance between wound (spiral/jelly‑roll) cells and Z‑stack (stacked) cells is often misunderstood as a chemistry issue, while in reality it is largely a current‑path and cell architecture problem.
To clarify this, we must first break down what actually contributes to a cell’s internal resistance.
Components of Internal Resistance
The total internal resistance of a lithium‑ion cell can be decomposed into three dominant contributions:
- R_ohmic (Ohmic Resistance)
Resistance arising from electronic and ionic conduction through current collectors, tabs, electrode coatings, electrolyte, and interfacial contacts. - R_ct (Charge Transfer Resistance)
Resistance associated with electrochemical reaction kinetics at the electrode–electrolyte interface. - R_mt (Mass Transport Resistance)
Resistance caused by lithium‑ion transport limitations within porous electrodes and the electrolyte (diffusion and concentration polarization).
Among these, R_ct and R_mt are primarily governed by unit‑cell‑level electrochemistry active material chemistry, particle size, porosity, tortuosity, binder system, and electrolyte properties. These terms are largely independent of whether the cell is wound or stacked.
In contrast, R_ohmic is strongly influenced by cell geometry, specifically the distance and topology of the electronic current path from the active material to the external terminals. This is where wound and Z‑stack designs fundamentally diverge.
Current Path in Wound (Spiral) Cells
In cylindrical lithium‑ion cells, electrodes are typically arranged in a spiral‑wound (jelly‑roll) configuration. The anode and cathode are long continuous sheets, and electronic current must travel laterally along the current collector before reaching the tab.
This architecture leads to several intrinsic effects:
- Longer average current path length from reaction sites to the tabs
- Localized current collection at discrete tab positions
- Non‑uniform current density across the electrode length
As a result, the ohmic contribution (Rₒhmic) increases, even if the electrochemical performance of the electrode itself is identical to that of a stacked cell. Under high C‑rate operation, this manifests as increased Joule heating and larger voltage drop.
Historically, cylindrical cells have mitigated this limitation using multi‑tab designs, where multiple tabs are distributed along the electrode length to shorten the effective current path. While this reduces Rₒhmic, it cannot fully eliminate the geometric disadvantage inherent to the wound architecture.
Current Path Advantage in Z‑Stack Cells
In Z‑stack (stacked) cell architectures, electrodes are cut into individual sheets and stacked layer‑by‑layer. Each anode and cathode sheet typically has its own dedicated tab or very short current‑collection distance.
This configuration provides several electrical advantages:
- Minimal current path length from active material to tab
- Highly uniform current distribution
- Significantly reduced ohmic resistance (R_ohmic)
Because electronic current reaches the tabs almost immediately, stacked cells naturally exhibit lower internal resistance, especially under high‑power operation. This is one of the key reasons stacked pouch and prismatic cells are preferred in applications demanding high power density and thermal uniformity.
Tabless Design: Closing the Gap for Cylindrical Cells
Recent innovations in cylindrical cell design—most notably tabless (continuous current collector) architectures have fundamentally changed the internal‑resistance landscape.
In tabless designs:
- The current collector edge is continuously connected along the electrode length
- Current is collected radially rather than funneled into discrete tabs
- The effective current path length is reduced by orders of magnitude
This approach dramatically lowers R_ohmic, enabling modern cylindrical cells to reach <1 mΩ internal resistance, a range previously dominated by stacked architectures.
Importantly, this demonstrates that the limitation was not the spiral winding itself, but how current is collected within that geometry.
Key Takeaway
The higher internal resistance traditionally associated with wound (spiral) cells is not driven by inferior electrochemistry, but by longer and less efficient electronic current paths, which elevate the ohmic resistance component.
- R_ct and R_mt are dictated by materials and unit‑cell design
- R_ohmic is dictated by electrode‑tab architecture and current‑path geometry
Z‑stack cells inherently minimize ohmic losses through short, direct current paths, while modern tabless cylindrical designs show that with intelligent architectural optimization, spiral cells can achieve comparable—or even competitive—internal resistance levels.
Ultimately, internal resistance optimization is a system‑level design challenge, not just a materials problem.
Kerem Buğra Batuş, Cell Design & Process Development
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