Direct Microwave Pyrolysis of Cellulose to Hard Carbon Anodes for Sodium-Ion Batteries

Hard carbons are the leading anode material in Na-ion batteries due to their considerable ability to store Na, and the ease with which they can be produced from inexpensive precursors such as cellulose through pyrolysis in inert atmospheres. Here, we report a rapid one-step conversion of cellulose to hard carbons in under 15 min in a modified domestic microwave oven. This is in contrast to more conventional furnace-based pyrolysis which can take several hours. From optical pyrometry, we find that under different microwave power conditions, the hard carbons can be tunably formed at temperatures between 900 to 1250 °C under the conditions employed. The hard carbons produced here have been characterized by Raman spectroscopy, wide and small-angle X-ray diffraction, porosimetry, X-ray photoelectron spectroscopy, and X-ray pair distribution function analysis. As a function of increasing microwave power, the carbons are found to exhibit comparable local structure but enhanced crystallinity and evidence of an increased proportion of closed pores. The formation of closed pores appears to directly contribute to significant gains in Na storage capacity throughout the plateau region during electrochemical cycling. These results demonstrate a convenient and scalable strategy for rapidly producing hard carbons with tunable porosity.