Researchers at the Karlsruhe Institute of Technology (KIT) have repurposed discarded apples to build cheap and high-performance sodium-ion batteries, making a green technology even greener. The advance could find use in grid storage and, after further development, compete with lithium-ion cells to power portable electronics and low-end electric cars.

Apples that are undersized, malformed or just unevenly colored are often rejected after harvest and go directly to waste, as they spoil too quickly to even be used as cattle feed. The team led by Prof. Stefano Passerini and Dr. Daniel Bucholz, however, has found a way to repurpose them by drying them out and exploiting their 95 percent carbon content to create "hard carbon," a cheap but high-performing electrode material.

Their carbon-based anode features a specific capacity of 230 mAh/g and showed little degradation even after 1,000 charge and discharge cycles. The electrode's coulombic efficiency, which measures how easily charge can travel through it, stabilized to a very high 99.1 percent.

In addition, the researchers also developed a high-performing, greener cathode for their battery. Stacking several layers of sodium oxides on top of each other, they obtained a material with comparable performance to lithium-ion cathodes but which retained 90.2 percent of charge after 600 cycles and featured an impressive coulombic efficiency of over 99.9 percent.

Schematic structure of the layered oxides(Credit: KIT/HIU)

Lithium-ion batteries can squeeze plenty of energy in a small package, but they also pack expensive and hazardous materials such as cobalt. Sodium-ion batteries are on the other hand much cheaper, made from abundant and green materials, and are approaching the performance of their counterparts.

"[Sodium-ion batteries are] about 20 percent less performant than lithium for about 20 percent less cost on kWh basis," Passerini told Gizmag. "We like to think that new cathode materials may be developed performing as well as those for lithium-ion batteries. Sodium-ion batteries are not commercial, yet. However, hard carbon appears to be the most promising anode material because it's cheap."

Future uses for this sodium-ion battery technology could include grid storage (where cost, rather than size and weight, is the crucial factor) and even, Passerini tells us, personal electronics and low-end electric cars.

The advances were presented in the scientific journals ChemElectroChem and Advanced Energy Materials.

Source: KIT

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