Microstructure the key to cheaper, safer batteries
Zinc batteries could be cheaper and safer than existing lithium-ion technology, but until now they have not been rechargeable. A new spongelike electrode design may change all that, writes Wren Greene.
Rechargeable lithium-ion batteries are ubiquitous in our daily lives, storing and supplying power in everything from mobile phones to electric cars. Lithium-ion technology is undoubtedly effective, but it has its drawbacks: the batteries can catch fire when overheated or damaged, as happened in Samsung’s “exploding phone” debacle last year, and their production and disposal is not always kind to the planet.
Zinc has long been viewed as the natural successor to lithium for energy storage. First, zinc is around 100 times more abundant than lithium and can be refined more cheaply, using less energy, and with fewer environmentally harmful by-products. The theoretical amount of energy that can be stored in a zinc battery is comparable to a lithium-ion battery. Finally, zinc batteries promise better safety and recyclability than lithium-ion batteries, since zinc is less toxic and doesn’t react violently when exposed to air or water.
Despite these advantages, the development of zinc batteries has been held up by the problem of rechargeability.
At a fundamental level, the battery supplies power by the converting a metallic zinc electrode into zinc ions that are stored within a liquid electrolyte, which liberates the electrons that provide an electric current. To recharge the battery, this process is reversed and the zinc ions are converted back into metallic zinc which redeposits onto the electrode surface.
If the zinc does not redeposit uniformly, repeated charging and discharging of the battery leads to the formation of large, tree-like zinc structures known as ‘dendrites’. These dendrites reduce the charge/discharge efficiency and can short-circuit the battery if they grow too large. Unfortunately, zinc is very susceptible to dendrite formation.
The new research, by Joseph Parker and colleagues at the US Naval Research Laboratory in Washington, DC, describes a structure for a zinc electrode like a microscopic sponge that avoids the dendrite problem.
Unlike a traditional solid or flat electrode, this one is highly porous, which provides plenty of surface area to release and deposit zinc ions during charging and discharging. The spongelike structure also ensures that there is always a pathway for electrons to flow even if most of the zinc has been removed during the discharge. This enables as much as 90% of the zinc electrode to be converted to ions during a discharge which means more energy can be ‘harvested’ from a single charge and the battery will also last longer.
“It is all about the wiring: moving the electrons, ions, and molecules more uniformly throughout the volume of an electrode structure is far easier to ensure in a porous 3D electrode,” says co-author Debra Rolison. The result is that dendrite formation is almost eliminated and the electrode is unaltered even after tens of thousands of charge/discharge cycles.
The US Navy has announced that the technology will be licensed to a portable energy storage company called EnZinc Inc for use in electric vehicles and grid storage applications. The researchers are cautiously optimistic that their zinc battery may make its way to the market in five years.