The cathode layer in a lithium-ion battery is a composite of solid charge storing particles, a polymeric binder, and a conductive additive. Together, they are well dispersed in a solvent and spread like paint on a conductive substrate, an effective and pleasingly simple solution that works across various chemistries and cell designs.
Cathode composites are well-studied by academia and industry, but regardless of our deep knowledge, the fact remains that the precise arrangement of the materials within the cathode is, well, random. Thus, the porosity or empty volume that is left after the solvent evaporates is also unorganized. This volume will be filled with liquid electrolyte during the manufacturing process and is the winding path that lithium-ions will take on their journey from the cathode to anode, and vice-versa during charge and discharge.
Here it is important to introduce the idea of tortuosity. If you have ever driven in Boston (or any city with poorly designed motorways) and found yourself making many seemingly unnecessary turns (around, down, over, and around), then you understand the idea of high-tortuosity. Now, imagine it is year 2100, pop your sleek electric jetpack on and take that same trip, this time going directly from point A to point B. Congratulations, you have discovered the efficiency and speed of a low-tortuosity pathway.
Cathode (and anode) composites in the batteries of electric vehicles, phones, computers etc., all feature random arrangement of porosity and, thus, high-tortuosity pathways for ion movement. This limits both the thickness of the commercial cathode composites (~100 μm) and the speed at which batteries can be charged or discharged.
Okay, so you say there is a simple solution, just add porosity. Great! Now we can achieve extreme fast charging! However, the energy density suffers because additional porosity comes at the cost of charge storing material and, thus, decreases all three components of energy density (mass, volume, and area).
Alternatively, we could try to keep the value of porosity constant, but instead choose how this space is arranged within an electrode. Thus, creating tailored architectures which have lithium-ion mass-transport pathways as an intrinsic part of the cathode composite. This is what colleagues and I have been working on at Boston University, and this research was recently published in Advanced Materials.
We created a material-agnostic and scalable process to manufacture battery electrodes, inspired by the well-established roll-to-roll processing of filtration membranes. By controlling polymer phase separation behavior and directional exchange between two liquids we fabricate free-standing electrode architectures with tailored porosity in the form of low-tortuosity ion transport pathways (1-20 μm in diameter). These electrodes demonstrate 80% capacity retention and no structural or architectural deformations when operated over 1000 cycles. This is impressive electrochemical and mechanical stability, especially considering that no metal current collectors or polymer binders are used.
Importantly, we also demonstrate that tm or the density of charging storing material in the electrode can be tailored by changing one of the liquids used in the manufacturing process. This provides an approach to produce electrodes that can be optimized to provide a wide spectrum of energy and power densities for cross-application energy storage needs.
Within the academic battery community there has been movement to rethink electrode structures and consider how we can build in the third dimension to mitigate mass-transport limitations, and other resistive bottlenecks. As high-capacity materials such as silicon are introduced, it is paramount that we consider ion transport if we want to maximize energy and power density. It may be necessary to create innovative and rational manufacturing processes that move beyond the random arrangement of materials present in our current electrode technology. Three companies that I am following who are re-imagining battery technology in this manner are Enovix, EnPower and LionVolt. Hopefully the next five years bring more commercial movement in the direction of restructuring electrode and battery architectures!
- Resing, A. B., Fukuda, C., Werner, J. G., Architected Low-Tortuosity Electrodes with Tunable Porosity from Nonequilibrium Soft-Matter Processing. Adv. Mater. 2022, 2209694.
Another way to increase energy density is with 3D electrodes. Increasing the surface area and connection to the active materials can improve a number of features of the cell:
- Energy density
- Power density
- Internal resistance
- Thermal and mechanical behaviour
This improvement can increase the amount of usable active material, reduce the internal resistance and hence increase energy and power.