New Insights in to the Lithium Storage Mechanism in Polymer Derived SiOC Anode Materials

Abstract

Polymer derived silicon oxycarbide (SiOC) materials are prepared by the pyrolysis of preceramic polymers obtained from polyhydridomethylsiloxane using 1,3,5,7-tetramethyl1,3,5,7-tetravinyl cyclotetrasiloxane or divinyl benzene as a cross-linking agent. The pyrolysis is carried out in an inert atmosphere at 1000 and 1300°C. The carbon content of SiOC is varied by changing the amount of starting precursors maintaining the same O/Si atomic ratio of about 1. Electrochemical measurements are performed in order to evaluate the materials in terms of their application as anodes in Li-ion batteries. Detailed structural characterization study is performed using complementary techniques with the aim of correlating the electrochemical behavior with the structure 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 irreversible trapped into the amorphous silicon oxycarbide network is also high and therefore the maximum reversible lithium storage capacity of 650mAh g−1 is measured on high-C content SiOCs for which the balance between the two phases, namely the amorphous silicon oxycarbide and the free C phase, is optimal. The high carbon content SiOC show also an excellent cycling stability and performance at high charging/discharging rate: the reversible 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 the reversible capacity which has never been observed before for similar materials.