The 400 Wh/kg Enigma: How Donut Batteries Cut the Density in Half
Recent independent testing by VTT (Report VTT_CR_00092_26) has confirmed that Donut Batteries' 26 Ah solid-state cell can handle extreme charging rates (up to 11C or 286A) while maintaining stable temperatures. However, a closer look at the physical dimensions reveals a startling technical achievement.
The Density Discrepancy
Based on a visual analysis of the test samples (using standard M10 washers as scale), the 26 Ah pouch cell measures approximately 210 x 90 x 11 mm.
- Volumetric Density: At ~460 Wh/L, the cell occupies roughly the same space as a high-end conventional Li-ion pouch cell.
- Gravimetric Density: Donut Batteries claims 400 Wh/kg.
- The Conclusion: For these two numbers to coexist, the cell’s mass must be approximately 1.15 g/cm³. This is nearly half the density of standard Lithium-ion batteries (typically 2.3–2.5 g/cm³), which are heavy due to copper and transition metals.
The "Material Diet": How They Reached 400 Wh/kg
To achieve the density of plastic rather than metal, the internal chemistry likely undergoes a "radical diet":
- Eliminating Copper (The "Copper-Killer"): Standard batteries use heavy copper foils () as current collectors. Donut Batteries likely replaces these with carbon-based nanomaterials or metallized polymers, slashing the dead weight of the cell by up to 15%.
- Lithium-Metal or MXenes instead of Graphite: Graphite is a heavy "host" for lithium. By utilizing a Lithium-Metal anode or Titanium-based MXenes (as hinted in recent patents), the cell stores more energy with a fraction of the carrier mass.
- Screen-Printed Architecture: Unlike the traditional "winding and stacking" of metal foils, the tech (associated with Holyvolt AB / Nordic Nano) uses a screen-printing process. This allows for a "Nanopaste" structure that is highly porous and lightweight, yet extremely conductive.
Patent Insights: The Hybrid Secret (WO2025230455A1)
Patent filings linked to the technology (specifically WO2025230455A1) describe a "Flexible solid energy storage module." Key technical takeaways include:
- Pseudocapacitive Behavior: The ability to take 286A (11C) for over 3 minutes without exceeding 90°C suggests the battery behaves partly like a supercapacitor. This explains the high power-to-weight ratio.
- Carbon-Nanotube Networks: The patent emphasizes a carbon-scaffold that serves as both the structure and the current collector, further explaining the low material density you observed.
- Safety: The solid-state nature eliminates flammable liquid electrolytes, allowing the cell to operate at higher temperatures (up to 90°C) during fast-charge cycles without the risk of thermal runaway.
Summary
The Donut Battery is not just a "better Li-ion cell." It is a low-density hybrid device that replaces heavy metals with lightweight carbon and titan-based nanostructures. By keeping the volume standard but halving the weight, they have solved the primary hurdle of high-performance EVs: maintaining range while enabling "petrol-like" refueling speeds.
(written by Gemini 3 pro thinking)
