Effect of Pyrolysis Temperature on Electrochemical Performance of SU-8 Photoresist Derived Carbon Films

Introduction

Since last decade, SU-8 negative photoresist is considered as a precursor to yield glassy carbon that comprises short range crystallites.^1,2 Owing to its advantages in terms of patterning and ability to fabricate 3-D carbon electrodes, SU-8 photoresist derived carbon has been demonstrated as potential anode materials for lithium ion battery.^2,3 We recently reported the use of stainless steel (SS) wafer instead of Si wafer as a substrate for preparing SU-8 derived planar carbon films which exhibited excellent reversible capacity as compared to previous available reports.⁴ While preparing the carbon films from SU-8 on SS wafer, we carried out the two step pyrolysis at 900 °C.⁴ However the effect of varying the pyrolysis temperature on structural properties of carbon films derived and subsequently on electrochemical characteristics is yet to be studied. In this work, we pyrolyzed SU-8 films on SS wafer at four different temperatures in the range of 700 °C and 1000 °C. As-fabricated SU-8 derived carbon films were structurally characterized by XRD, Raman and CHNS analysis. Electrochemical performance of as-prepared carbon films were tested using galvanostatic charge discharge experiments at 37.2 mA/g current density and the specific capacities were correlated with the H/C ratio and crystalline structure.

Experimental:

SU-8 2005 was spin coated on single side polished SS wafer after drying at 150 °C on hot plate for 10 min. Thus obtained films were crosslinked using C-MEMS process, and process conditions were optimized for SS wafer. These films were then pyrolyzed  in tubular furnace in presence of N₂ atmosphere. As-prepared carbon films were then used as working electrode to study their electrochemical properties whereas lithium foil was used as a counter electrode. Glass microfiber filter soaked with LP-30 electrolyte was used as a separator.

Results and discussions:

XRD and Raman spectra as shown in Fig. 1a and 1b confirms that prepared carbon films are hard carbons and increase in pyrolysis temperature has no significant effect on their crystallinity. However, samples pyrolyzed at different temperature showed significant change in carbon content and H/C ratio as shown in Fig.1c. H/C ratio increased with decreasing temperature. Fig. 1d shows the cyclic performance of the samples at 0.1 C and between 0.01 -3V. Galvanostatic charge discharge experiments reveals that though reversible capacity decreases, cyclic stability and initial cycle coulombic efficiency increases with increasing temperature. This behavior can be attributed to decrease in hydrogen content and increase in radius of gyration due to rearrangement of graphene layers which results in improved alignment with neighboring layers with increasing temperature.⁵

References:

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