Building batteries from scrap metal

Chinese scientists have transformed rusty stainless steel mesh into electrodes with outstanding electrochemical properties that make them ideal for potassium-ion batteries, thus making good use of waste while also finding an innovative solution to a technical problem. Their research has been published in the journal Angewandte Chemie.

Increasing use of renewable energy requires effective energy storage within the grid, with lithium-ion batteries serving as promising candidates. Lithium-ion batteries are based on the displacement of lithium ions: while charging, the ions move towards the graphite electrode, where they are stored between the layers of carbon. When discharging, they are released.

As lithium is expensive and reserves are limited, scientists have been looking for alternatives. Sodium-ion batteries have been explored, but as noted by Dr Xin-Bo Zhang from the Chinese Academy of Sciences, “Potassium ions are just as inexpensive and readily available as sodium, and potassium-ion batteries would be superior from the electric aspect.

“However, the significantly larger radius of the potassium ions has posed a problem. Repeated storage and release of these ions destabilises the materials currently used in electrodes.”

Dr Zhang and his team from the Chinese Academy of Sciences and Jilin University have now found an elegant solution to this problem by creating electrodes from rejected stainless steel mesh from filters and sieves. Despite the excellent durability of these grids, harsh conditions do lead to some corrosion. The metal can be reclaimed in a furnace, but this process requires a great deal of money, time and energy.

“Conversion into electrodes could develop into a more ecologically and economically sensible form of recycling,” said Dr Zhang.

The corroded mesh is dipped into a solution of potassium ferrocyanide (yellow prussiate of potash, known as a fining agent for wine). This dissolves iron, chromium and nickel ions out of the rust layer. These combine with ferricyanide ions into the complex salt known as Prussian blue, a dark blue pigment that is deposited onto the surface of the mesh as scaffold-like nanocubes. Potassium ions can easily and rapidly be stored in and released from these structures.

The researchers then use a dip-coating process to deposit a layer of graphene oxide (oxidised graphite layers). This layer nestles tightly onto the nanocubes. Subsequent reduction converts the graphene oxide to reduced graphene oxide (RGO), which consists of layers of graphite with isolated oxygen atoms.

“The RGO coating inhibits clumping and detachment of the active material,” explained Dr Zhang. “At the same time, it significantly increases the conductivity and opens ultrafast electron-transport pathways.”

In tests, coin cells made with these new electrodes demonstrate excellent capacity, discharge voltages and rate capability, and outstanding cycle stability. Because the inexpensive, binder-free electrodes are very flexible, they are suitable for use in flexible electronic devices.