Silicon Oxycarbide Ceramics As Anode Materials for Li-Ion Batteries: Modification of the Structure and Composition to Enhance the Li Storage Properties

Li-ion battery technology is one of the promising energy storage solution for the future. Studies based on electrode materials are one of the key step to improve the energy storage performance of these systems. Recent studies shows that polymer derived silicon oxycarbide (SiOC) ceramics can be considered as a potential anode material. SiOC ceramics are generally amorphous in nature and usually consists of a network of disordered free carbon segregated in amorphous SiOC matrix. The complex nanostructure plays a crucial role in determining their functional properties. Carbon rich SiOCs (>20wt% free carbon) is found to be the best candidate for Li storage. The enhanced Li ion storage capacity of SiOC ceramics is generally explained in connection with the disordered free carbon phase which stores lithium in interlayer spaces along with at the edges of the carbon clusters and mixed SiOC phase stoichiometry supporting the stability of the system. The main disadvantage of this material is the first cycle irreversible capacity loss which arises mainly due to the presence of oxygen, defects, micro pores etc. By synthesizing SiOC compositions with different stoichiometry it has been shown that carbon-rich SiOC ceramics with ~50wt% of free carbon and remaining amorphous SiOC matrix is the best candidate for reversible lithium storage demonstrating higher reversible capacity of 600 mAh g^-1with a good cycling stability. Within our work SiOC ceramics have been synthesized from polysiloxanes using different cross-linking and heat treatment approach. In a typical preparation a linear polysiloxane is cross-linked with divinylbenzene (DVB) via hydrosilylation reaction in presence of a Pt catalyst. The phenyl groups present in divinylbenzene act as the major carbon source in the final ceramics. Correlating the mixed SiOC stoichiometry composition with initial charging capacity points out a linear trend in capacity increase with amount of mixed bond networks. Mixed SiOC compositions has been characterized using ²⁹Si MASNMR studies. It has been shown that the composition changes with pyrolysis temperature. With increase in pyrolysis temperature there is a phase separation in to SiO₄ and SiC₄ phases with the consumption of mixed Si_xO_yC_z units. Electrochemical studies reveal that 1000 °C is the optimal temperature of pyrolysis to have maximum SiOC capacity with good cycling stability. A specific capacity of ~600 mAh g^-1 is recorded at a rate of C/20 (18 mA g^-1) with 60 % efficiency. Pyrolysis atmosphere also plays a role in determining the final properties. Using a hydrogen containing atmosphere (Ar/H₂ with 5% H₂) for pyrolysis helps to increase the specific capacity up to an extent by removing carbon dangling bonds which acts as a trap for irreversible Li storage. This has been well supported by EPR investigations showing reduced defect concentration for H₂ pyrolysed samples. A capacity increase of around 150 mAh g^-1 is observed for the H₂ pyrolysed samples. Recently our research focus turned to porous SiOC ceramic as a perspective anode for high rate Li storage applications. The porous nature of the anode material can enhance the fast charging/discharging behavior by reducing the diffusion path of Li ions due to electrolyte penetration and also the porous space can accommodate the volume changes associated with large Li intake. Aerogel approach is followed to produce SiOC ceramics with controlled porosity. The crosslinking is carried out under highly diluted conditions using acetone as solvent (80 vol% dilution). Wet gels after cross-linking are then aged in respective solvent followed by drying under supercritical conditions using liquid carbon dioxide followed by pyrolysis at 1000 °C under controlled argon atmosphere. Final SiOC ceramic aerogels have ~ 40 wt % of free carbon distributed within remaining SiOC matrix. The BET surface area for aerogel sample equals 180 m²/g. Electrochemical characterization of aerogels reveals a high specific capacity of more than 600 mAh g^-1 at a charging rate of C (360 mA g^-1) along with a good cycling stability compared to ~300 mAh g^-1 recorded for dense SiOC at the same rate and ~400 mAh g^-1 for H₂ pyrolysed counterparts. Electrochemical studies with different SiOC compositions reveal the excellent Li host properties of these materials. Capacities are enhanced by tailoring the structure and composition of final ceramic. Incorporating Li active hetero elements such as metallic Si, Sn etc. in aerogels may further improve the Li storage performance, i.e. porous matrix can be used to compensate the volume expansion associated with the alloying systems.