Oxidative pyrolysis of Si/polyacrylonitrile composites as an unconventional approach to fabricate high performance lithium ion battery negative electrodes

High performance Si/polyacrylonitrile (PAN) composite negative electrodes are fabricated by a robust process of oxidative pyrolysis at a temperature between 250 and 400°C. Multiple techniques are employed to investigate the structural, chemical, and mechanical properties of the Si/PAN composite electrodes before and after oxidative pyrolysis. With increasing temperature, oxidation, dehydration, aromatization, and intermolecular crosslinking take place in PAN, resulting in a stable cyclized structure which functions as both a binder and a conductive agent in the Si/PAN composite electrodes. Meanwhile, PAN reacts with oxygen, forming volatile products and producing progressively porous Si/PAN composites with increasing temperature. With a Si mass loading of 1 mg/cm², a discharge capacity of 1555 mAh/g at the 100^th cycle is observed from the 400°C treated Si/PAN composite electrode when cycled at a rate of C/3. This 400°C treated electrode also shows good rate capability. It exhibits a specific discharge capacity of ∼500 mAh/g at 3C compared to the nearly zero capacity for those treated at lower temperatures. This facile method of synthesizing Si-based composite negative electrodes can potentially be applied to other Si/polymer systems for further increasing the power/energy density of lithium ion batteries.