Photoresist Derived Carbon Films as High Capacity Anodes for Lithium Ion Battery

INTRODUCTION

Hard carbons prepared by pyrolysis of various polymer precursors have been tested for lithium ion intercalation and were found having higher reversible capacities than that of graphite (1, 2). However the main disadvantage of using hard carbon as anode material for lithium ion battery is higher irreversible capacity due to absorption of lithium ions on the internal surface of nanopores formed by disordered single layer graphene sheets (3).

Madou et al. used SU-8 photoresist to prepare carbon films on silicon wafer for lithium insertion (4). Their electrical characteristics were found similar to that of glassy carbon while their reversible capacity was measured to be ~ 220 mAh/g (5).

In this study SU-8 photoresist derived carbon films were prepared on stainless steel (SS) wafer as substrates. The choice of SS wafer (resistivity: 73 µΩ-cm) as a substrate is more apt as the same is used in the commercial lithium ion battery. Galvanostatic charge discharge of assembled Swagelok cell within 0.01-3 V was performed to measure the lithium insertion capacities.

EXPERIMENTAL

SU-8 2005 (Microchem Corp., MA) was spin coated at 3000 rpm for 30 s on dehydrated single side polished SS substrates (MTI Corp., USA). After 10 min relaxation, films were soft baked at 95⁰C for 2 min. After cooling to room temperature films were exposed to UV light followed by post exposure bake at 95 ⁰C for 3 min to complete the crosslinking. Hard bake of samples was carried out at 150⁰C to reduce the internal residual stresses generated due to shrinkage during crosslinking.

As-prepared SU-8 films were then carbonized at 900⁰C in the inert atmosphere using two step pyrolysis. Thus prepared SU-8 derived carbon film on SS and lithium foil (Sigma Aldrich) were used as working and counter electrode respectively. 1M solution of LiPF₆in a 1:1 v/v mixture of ethylene carbonate and diethyl carbonate was used as an electrolyte and glass microfiber filter (Whatman Filters) was used as separator. Packed Swagelok cells were soaked for 12 h before Galvanostatic cycling at C/10 rate to measure intercalation capacity of SU-8 derived carbon films as anode.

RESULTS

Cyclic voltammetry

Cyclic voltammogram recorded at 1 mV/s is shown in Fig.1a. Broad reduction peak at 0.5 V corresponds to reduction of passive layer formation by electrolyte decomposition and other reduction peak near 0 V corresponds to absorption of Li ions in hard carbon.

Galvanostativc charge discharge cycling

Galvanostatic discharge and charge cycles experiment was carried out at C/10 rate (Fig. 1b). Initial discharge capacity was very high compare to other cycles due to electrolyte decomposition voltage near to 0.4 V (6). Reversible capacity was found to be ~400mAh/g which was higher than that of graphite and also as reported earlier (5).

Electrochemical impedance spectroscopy (EIS)

Fig. 1c shows the Niyquist plot recorded under ac amplitude of 10 mV over the frequency range of 10 mHz-100 KHz.

The charge transfer resistance was found to be less than half as compared to reported values (5) earlier that further suggests the superior performance of the designed architecture (SU-8 photoresist derived carbon on SS substrate).

 

Fig.1 (a) Cyclic voltammetry of SU-8 derived carbon film on SS substrate; (b) Galvanostatic charge discharge curve for SU-8 derived carbon films on SS substrate at C/10 rate; (c) EIS plot for SU-8 derived carbon film after 10 charge/discharge cycles

SUMMARY

                Photoresist (SU-8) derived carbon films were prepared on SS as substrates and then tested for lithium ion intercalation. Galvanostatic charge/discharge experiments shows higher reversible capacity (400 mAh/g) as compared to graphite and previous values reported. EIS measurements also supplement these findings that may be of potential use for anode materials for Lithium ion battery.

REFERENCES

1. A. Piotrowska et al., J. Anal. Appl. Pyrolysis, 102, 1 (2013).

2. T. Zheng et al., J. Electrochem. Soc., 142, 2581 (1995).

3. Y. Liu et al., Carbon, 34, 193 (1996).

4. B. Y. Park et al., J. Electrochem. Soc., 152, J136 (2005).

5. G.T. Teixidor et al., J. Power Sources, 183, 730 (2008).

6. S. Zhang et al., Electrochem. Solid State Lett., 4, A206 (2001).