Azerbaijan National Academy of Sciences

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.

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