Now, researchers from Pune’s National Chemical Laboratory (NCL) and Indian Institute of Science Education and Research (IISER) have used a simple, cost-effective, quick process to convert sugarcane bagasse, an agro waste, into anode-grade porous, conducting, activated carbon material for use in Lithium-ion batteries.
While making anode-grade carbon is currently very expensive and time consuming, the Pune researchers were able to produce high quality carbon within minutes by using a low power microwave system. The results of the study were published on July 5 in the journal Electrochimica Acta.
The quality of carbon used for electrodes depends on the choice of precursors and the process used for converting the precursors into carbon. Anode-grade carbon is generally produced through pyrolysis (where decomposition is brought about by high temperature processing) which involves heating the precursors in a reducing atmosphere using argon gas for a day or two at temperatures as high as 1000 degree C.
The process time and electrical energy to get anode-grade carbon is cut down drastically.“By using a simple kitchen microwave oven we achieved local heating and combustion to realise high quality factory-grade carbon materials within a few minutes,” says Prof. Satishchandra Ogale the corresponding author from the Centre for Energy Science at IISER, Pune and formerly Chief Scientist, NCL, Pune.
“The process time to get anode-grade carbon is cut down dramatically. The electrical energy input is also reduced substantially,” Prof. Ogale says. “The quality of carbon and battery performance using this carbon is quite good and competitive with carbon made through other complicated schemes and processes. We are able to get competitive value of energy density and power density using the carbon anode made in the lab.”
The performance in terms of stability has also been good for a large number of charging and discharging cycles, according to them.
The initial carbonisation was carried out overnight at room temperature by mixing bagasse with concentrated sulphuric acid. “Except silica most of the inorganic impurities present in bagasse get dissolved by acid treatment,” says Anil Suryawanshi, one of the authors of the paper from NCL, Pune. This also helps in forming robust C=C backbone structure.
The solid product formed after acid treatment is washed thoroughly with distilled water to remove all traces of acid and oven dried at 70 degree C. The black colour powder or char is then mixed with potassium hydroxide to form slurry. The slurry is then heated in a microwave oven for a few minutes.
“The mixture achieves a burning temperature for a few minutes. It is a high-temperature thermal shock or combustion,” says Prof. Ogale. Though the flame temperature is high, it is self-generated by the microwave and not through external heating; the power consumed by the microwave oven is just 700-900 watts.
“Graphitisation takes place due to local heating with microwave and potassium hydroxide reacts with carbon to form soluble phases which eventually form pores,” says Suryawanshi. The process is repeated one more time after mixing with water to optimize porosity and conductivity. Porosity is important as lithium ions come though liquid electrolyte and must reach different parts of the carbon anode. Optimum porosity is needed for accessibility of lithium ions.
The carbon that is produced is made into a paste by mixing with a binder and a small amount of highly conductive material like graphite and coated on a metal foil using a brush or spatula to produce an anode.