Si(O,N)C Ceramics of Controlled Porosity for Storage of Lithium Ions
Authors: Vrankovic Dragoljub , Storch Mathias , Riedel Ralf , Graczyk-Zajac Magdalena 
Published: February 2020

Porous carbons have been investigated as high capacity electrodes for lithium storage for more than 10 years [
1
]. Realization of active materials with high surface to volume ratio leads to intensification of the presence of active sites for lithium storage. Although high active electrode leads to significant losses during SEI formation, it is recompensed by a considerable increase of the specific capacity. Moreover, the high surface area implies a high contact area with the electrolyte, hence leading to high lithium-ion flux across the electrode/electrolyte interface. Moreover, controlled porosity leads to an improved lithium diffusion due to the reduction of its path length, hence batteries with enhanced power capability and higher electron transfer rates[
2
].

Within this work we present a novel approach to prepare porous, polymer-derived Si(O,N)C ceramic for lithium storage. Through controlled crosslinking/pyrolysis process, materials with significantly higher surface area compared to standard polymer derived ceramic (PDC) route are synthesised.

Free carbon phase formed after pyrolysis of the polymeric samples is characterized by means of Raman spectroscopy. The characteristic carbon vibrations appear in all spectra [
3
]. Unusually high I(D)/I(G) ratio ranging from 1.8 to 2 is calculated, whereas fitting[
4
] reveals the presence of strong D3 band at 1540 cm-1, originating from the presence of amorphous carbon. In parallel, the intensity of G band attributed to an ideal graphitic lattice vibration mode with E2g symmetry remains low. In general, the increase of pyrolysis temperature leads to increase of ordering in free carbon phase. Elemental analysis reveals similar composition with average free carbon content of ~45 wt.%. Galvanostatic cycling with potential limitation show a significant improvement in capacity and rate capability (Figure1) for porous samples compared to dense sample. Namely discharge capacity increases from about 160 mAh·g-1 (Poly1_1100) to 320 mAh·g-1 for samples pyrolysed at 1100 °C (Poly2_1100). The sample exposed to 900 °C (Poly2_900) reveals very high capacities around 520 mAh·g-1 after 100 cycles (Figure1, insert). Furthermore, the porous materials demonstrate high and stable capacities when subjected to higher currents, namely 180 mAh·g-1 is recovered with the rate of 5C (1860 mA·g-1). In order to understand the differences in the ion storage mechanism in dense and porous materials solid state NMR study is in progress.

Acknowledgments:

We gratefully acknowledge the financial support of the German Research Foundation (DFG) SPP1473/JP8. Authors thank to Christina Schitco for fruitful discussions.

Literature:

1. Zhou, H.; Zhu, S.; Hibino, M.; Honma, I.; Ichihara, M. Advanced Materials
2003, 15, 2107–2111.

2. Roy, P.; Srivastava, S. K. J. Mater. Chem. A
2015, 3, 2454–2484.

3. Ferrari, A. C. Solid State Commun.
2007, 143, 47–57.

4. Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Carbon
2005, 43, 1731–1742.

Figure 1. Recovered capacity of Poly2_1100 at different C-rates ranging from C/2 up to 5C. Insert: Extended cycling curves for Poly1_1100, Poly2_1100, Poly2_900 at current of 72 mA·g-1.

Figure 1