Study of Silicon Oxycarbide(SiOC) as Anode Materials for Li-ion Batteries

The principal object of this thesis is the investigation of silicon oxycarbide (SiOC) ceramics as anode material for Li-ion batteries. The investigated materials are prepared by cross linking commercial polymer siloxanes via hydrosylilation reactions or hybrid alkoxide precursors via sol-gel. The cross linked polymer networks are then converted in to ceramic materials by a pyrolysis process in controlled argon atmosphere at 800-1300 °C. In details the influence of carbon content on lithium storage properties is addressed for SiOC with the same O/Si atomic ratio of about 1. Detailed structural characterization studies are performed using complementary techniques which aim correlating the electrochemical behavior with the microstructure of the SiOC anodes. Results suggest that SiOC anodes behave as a composite material consisting of a disordered silicon oxycarbide phase having a very high first insertion capacity of ca 1300 mAh g-1 and a free C phase. However, the charge irreversibly trapped into the amorphous silicon oxycarbide network is also high. In consequence the maximum reversible lithium storage capacity of 650 mAh g-1 is measured on high-C content SiOCs with the ratio between amorphous silicon oxycarbide and the free C phase of ï ¾ 1:1. The high carbon content SiOC shows also an excellent cycling stability and performance at high charging/discharging rate with the stable capacity at 2C rate being around 200 mAh g-1. Increasing the pyrolysis temperature has an opposite effect on the low-C and high-C materials: for the latter one the reversible capacity decreases following a known trend while the former shows an increase of xi the reversible capacity which has never been observed before for similar materials. The influence of pyrolysis atmosphere on lithium storage capacity is investigated as well. It is found that pyrolysis in Ar/H2 mixtures, compared to the treatment under pure Ar, results into a decrease of the concentration of C dangling bonds as revealed by electron spin resonance (ESR) measurements. The sample prepared under Ar/H2 mixture shows an excellent cycling stability with an increase in the specific capacity of about 150 mAh g-1 compared to its analogues pyrolysed in pure argon atmosphere. In order to study the role of porosity towards the lithium storage properties, a comparison of dense and porous materials obtained using same starting precursors is made. Porous SiOC ceramics are prepared by HF etching of the SiOC ceramics. HF etching removes a part of the amorphous silica phase from SiOC nanostructure leaving a porous structure. Porous ceramics with surface areas up to 640 m2 g-1 is obtained. The electrochemical charging/discharging results indicate that the porosity can help to increase the lithium storage capacity and it also leads to an enhanced cycling stability. This work demonstrates clearly that silicon oxycarbide (SiOC) ceramics present excellent electrochemical properties to be applied as a promising anode material for lithium storage applications.