Lithium-ion (Li) batteries are important for modern technological progress because they power the electronics and hybrid electric vehicles industries, and are an important component of renewable energy methods. However, as advancements in these fields are made, Li batteries are also expected to perform better – such as have higher charge capacity.

At the moment, layers of graphite are used inside Li batteries to store electrical charge from chemical reactions. Silicon has been suggested as a replacement because its charge capacity is about 400 times that of graphite. It is resilient to heat, and easy to store and dispose. And now, scientists from South Korea have proposed an alternate route that is both shorter and eco-friendly.

They have proposed to use layers of silicon dioxide, or silica, that rice plants have developed in their husks through years of evolution, as a source of silicon. These silica layers, while forming a protective sheath, are uniquely porous at the nano-scale level to allow air and water ventilation, and the same could increase silicon’s performance in batteries.

Although about 100 million tons of rice husk are produced annually worldwide, so far they have been recycled only for low-cost agricultural items. That could change. The researchers were able to demonstrate a technique through which silicon could be extracted from the layers while maintaining its 3D nanoporous structure.

Their findings were published in the Proceedings of the National Academy of Sciences on July 9.

The fabrication of silicon (Si) from nanoporous husks predominantly involved simple thermal and acidic treatments. Because silicon has low electronic conductivity, researchers then compensated with a uniform coating of polydopamine and carbon.

A significant obstacle to its use in batteries till date is that silicon’s atomic layout expands tremendously when Li ions enter it. Over many charge-discharge cycles, the layout expands and then breaks, and the battery becomes unusable.

But the 3D nanoporous structure prevented this from happening by permitting the Li ions to move in a channel-like arrangement without pushing against silicon atoms. This extended the life and performance of the battery.

Across 200 charge-discharge cycles, it retained 100 per cent of its original charge capacity (1,554 mAh), and 82 per cent across 500 cycles.

Moreover, its charge-transfer efficiency was also mostly unchanged over these cycles, going only from 100 to 99.7 per cent. The conventional Li battery in a smartphone has a capacity of 1,300-2,000 mAh, and an efficiency of 80-90 per cent beyond 400 cycles.

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