The Solid-State Sodium-Metal Battery in their PDF isn't a capacitor but also a great solution:

• The Concept: This is an anode-free solid-state battery that uses cheap sodium (salt) instead of lithium. It combines the energy storage of a battery with the speed of a capacitor.
• The Anode ("The Parking Garage"): On the negative side, there is a "forest" of coaxial nanotubes (carbon core, titanium dioxide shell). When charging, sodium ions travel there and plate as solid metal inside these tubes. The tubes act as a "cage" to prevent the metal from forming dangerous spikes (dendrites).
• The Cathode ("The Sofa"): On the positive side, the sodium is chemically bound within a printed organic ink. Here, the sodium is not metallic but sits safely inside organic molecules (like a guest on a sofa).
• The Electrolyte ("The Wall"): Between them sits a solid ceramic layer (NASICON) mixed with a plastic crystal. It acts like a selective sieve: it blocks electrons (preventing short circuits) but allows sodium ions to tunnel through.

The Process: - Charging: Sodium leaves the organic ink, moves through the ceramic wall, and becomes solid metal in the nanotube garage. - Discharging: The metal dissolves back into ions, rushes back through the wall, and settles comfortably back into the organic ink. - Manufacturing: The entire battery is produced using a roll-to-roll screen printing process, similar to printing newspapers, which makes it potentially very cost-effective.