Inventors of Rolled-Ribbon® Technology

Technology

Rolled-Ribbon technology solves the longstanding industry problems associated with conventional cells (cylindrical, pouch and prismatic) for high-power high-capacity Li-ion applications. Conventional cells do not scale up well for increasing power and capacity. As battery system power and capacity requirements increase, significant power and thermal issues emerge that are difficult to solve, resulting in decreased battery system performance and increased cost.

Rolled-Ribbon overcomes these problems. In tests that compare Rolled-Ribbon to conventional cells on a side-by-side basis using diverse but identical electrochemical formulations, Rolled-Ribbon cells outperform conventional cells by wide margins. The Rolled-Ribbon advantages come from their physical structures and not their electrochemical formulations.

Rolled-Ribbon cells are constructed by interleaving and winding long, narrow strips (ribbons) of anode, cathode and separator materials on a hub, producing an electrode roll (hence the term “Rolled-Ribbon”). Winding electrodes is one of the oldest, lowest-cost and most reliable methods for producing cells. The electrode roll is compressed between two pans that are separated by an insulating seal ring and the cell assembly is crimp-sealed. The top pan includes a safety pressure release valve (to vent excess pressure should it build up inside a cell under fault conditions). One pan serves as the cell’s anode terminal and the other serves as its cathode terminal. The electrodes in the electrode roll are perpendicular to the flat surfaces of the pans. The entire edge of the anode electrode is in direct contact with the anode pan and the cathode electrode is in direct contact with the cathode pan. There are no welded tabs of any kind. These features are illustrated in the diagram below.

Cross-Section of Assembled Cell

The performance advantages of Rolled-Ribbon is easily understood by comparing and contrasting the Rolled-Ribbon cell structure to conventional cell structures.

Rolled-Ribbon Cell Structure

Rolled-Ribbon cells have narrow electrodes (typically 13-26 mm). One edge of each electrode is in direct contact along its entire length (typically 20-50 meters) with its associated cell terminal. From an electrical perspective, this creates a very short, low impedance path to the cell terminal. From a thermal perspective, this creates a very short path with very low thermal resistance to the very large cell terminal. This is because heat travels along the electrode foil rather than through multiple layers of electrode material, foil and separator. The thermal resistance along an electrode foil is on the order of 100-200 times lower than that for crossing electrode layers.

Conventional Wound-Tabbed Cell Structure

These cells have long current paths (typically 800-5000 mm). This long path length result in much higher impedance. Further, the heat generated from this impedance will not be uniformly distributed across the electrode. There will be much more current flowing as you approach the tab on the left-side than on the right-side in the illustration, so there will be much more heat generated on the left than the right. Next, you have very poor thermal properties. Heat generated at the core must travel along a long longitudinal path of electrode foil through the highest heat portion of the electrode or overcome high thermal resistance through layers of electrode, including the highest heat portion of the electrode. This causes hotspots and thermal gradients that accelerate aging and can lead to unsafe conditions.

In contrast, Rolled-Ribbon cells have both low impedance and low thermal resistance. This combination translates into:

Maximum Power Delivery - Charging and Discharging
Minimum Heat Generation
Maximum Energy Conversion Efficiency
Unparalleled Thermal Performance
Maximum Cell Cycle Life

But the benefits of Rolled-Ribbon technology don’t end here. The unique structure of Rolled-Ribbon cells enable these virtuous properties to be extended through the construction of batteries with stacked-cells.