Phenyl-rich silicone oil as a precursor for SiOC anode materials for long-cycle and high-rate lithium ion batteries

Silicon oxycarbide (SiOC) is gaining increasing attention as a promising anode material for lithium ion batteries due to its higher reversible capacity compared to incumbent graphite. The kinetic processes at a SiOC anode result in rapid capacity fading even at a relatively low current density, thereby hindering its commercialization. Herein, a distinctive, phenyl-rich silicone oil is used as a precursor for producing SiOC anode materials via simple pyrolysis. We find that only silicone oil with phenyl-rich rings can be converted into SiOC materials. The phenyl group was crucial for carbon incorporation to allow Si-O-C bonding and the formation of a free-carbon phase. The resulting SiOC anode exhibited stable cyclability up to 250 cycles, with a discharge capacity of 800 mA h g^-1 at a current density of 200 mA g^-1. The remarkable cycle performance of SiOC was correlated with its low dimensional expansion (7%) during lithiation, which maintains its structure over cycling. Rate capability tests showed a highly stable performance with a maximum discharge capacity of 852 mA h g^-1 at a current density of 100 mA g^-1. When the discharge current density was increased 64-fold, the reversible capacity of the SiOC anode was 90% of its maximum capacity, 772 mA h g^-1. The excellent electrochemical performance of SiOC could be attributed to the rapid mobility of Li⁺ within the SiOC matrix, as indicated by a Li⁺ diffusion coefficient of 5.1 × 10^-6 cm² s^-1.