From the Economist Battery technology: Highly charged
Promising technology for fast-charging batteries, one of the barriers that must be overcome for fleet electrification.
“The battery-maker’s dilemma is that the recharging rate depends on the area of contact between electrolyte and electrode. A thin, sandwich-like arrangement, in which cathode, electrolyte and anode are close together, can thus be discharged and recharged rapidly. However, this speed comes at a price. The amount of energy a battery can store depends on the volume of its electrodes, so a thin battery does not last long. What is needed is a way to increase contact area without sacrificing volume. And that is what Dr Braun has found. Moreover, his solution looks suitable for mass production.
His starting material, as he describes in a paper in Nature Nanotechnology, is made of closely packed polystyrene spheres about a millionth of a metre in diameter. This is an arrangement similar to that found in opal (except that in opal the spheres are made of silica) and the result is, indeed, opalescent.
The next stage is to fill the gaps between the spheres with nickel. This is done by electrodeposition–like nickel-plating a piece of steel. After that, the material is heated, to melt the polystyrene. This leaves a sponge made of metallic nickel. The connections between the spherical gaps in the sponge are then enlarged, using a technique called electropolishing to dissolve the surface layer of the metal. This creates an electrically conductive framework suitable for smothering with materials normally used to make cathodes.
The result is a huge area of contact between the nickel (which conducts electrons to and from the battery), the cathode (which conducts ions to and from the electrolyte to compensate for the movement of those electrons), and the electrolyte (through which the ions are moving between cathode and anode)–but without a significant loss of cathode volume. Just, in other words, what the doctor ordered.
The consequence, according to Dr Braun, is a charging rate ten to 100 times higher than that of a normal, commercial battery (in one instance, the researchers created a lithium-ion battery that could be 90% recharged in two minutes), at a probable increase in production cost, once the process is properly industrialised, of 20-30%. And that rate might be improved still further if similar techniques were applied to the anode–a task that Dr Braun is now working on.”