Takakazu Hino

A Si−O−C Composite Anode: High Capability and Proposed Mechanism of Lithium Storage Associated with Microstructural Characteristics

A blend of phenyl-substituted, branched polysilane, (Ph(2)Si)(0.85)(PhSi)(0.15), and polystyrene (1:1 in weight) has been transformed into a composite material consisting of graphene layers, Si-O-C glasses, and micropores through a pyrolytic polymer-to-ceramic conversion. Several analytical techniques have been employed to characterize the Si-O-C composite material, demonstrating the presence of the three components in its host framework. The Si-O-C composite material performs well in electrochemical operations with a characteristic voltage plateau, offering a capacity of more than 600 mA h g(-1). When polystyrene is not blended, the resulting comparative material is even less porous and shows a shorter voltage plateau in electrochemical operations. A broad resonance in the (7)Li NMR spectrum recorded at low temperature can be deconvoluted into three components in the fully lithiated state of the Si-O-C composite material derived from the polymer blend. This result indicates that the Si-O-C composite material electrochemically stores lithium species in interstitial spaces or edges of the graphene layers, directly or indirectly the Si-O-C glass phase, and the micropores. However, both the Si-O-C glass phase and micropores are minor as electrochemically active sites for lithium storage, and interstitial spaces or edges of the graphene layers act as major electrochemically active sites in this composite material. Despite the excellent cyclability of the Si-O-C composite material, the voltage plateau disappeared over cycling. This phenomenon suggests that the microstructure is delicate for repetitive lithium insertion and extraction and that newly formed sites must generate the nearly equal capacity.

Polysilane/Acenaphthylene Blends Toward Si–O–C Composite Anodes for Rechargeable Lithium-Ion Batteries

Silicon oxycarbide (Si-O-C) composite materials have been prepared by pyrolysis of polysilane-acenaphthylene powdery blends to 1000°C under an inert gas atmosphere. Two branched polysilanes, (Ph 2 Si) 0.85 (PhSi) 0.15 and (MePhSi) 0.70 (Ph 2 Si) 0.15 (MeSi) 0.15 , were used in this study. Our thermal analyses confirm a high possibility that acenaphthylene significantly affects the microstructure of the Si-O-C composite materials that can be represented as SiC x O 2(1-x) + yC (free carbon). The powdery blends in a weight ratio of 1:1 produced the Si-O-C composite materials with a high lithium storage capacity (ca. 500 mA h g -1 or higher) and excellent cyclability. These composite materials clearly showed a pseudo-voltage plateau upon electrochemical delithiation, as seen in the case of hard carbon. 7 Li NMR analyses showed the presence of at least two electrochemically active sites for lithium storage in the Si-O-C composite materials. Structural and electrochemical analyses support the idea that the Si-O-C composite materials have micropores where less-ionic lithium species can be formed. Increasing the ratio of acenaphthylene in the powdery blends resulted in low electrochemical performance because the free carbon contributed greatly to the microstructure of the resulting composite materials.

Lithiation and Delithiation of Silicon Oxycarbide Single Particles with a Unique Microstructure

Single particles (11 and 13 μm diameter) of a silicon oxycarbide (Si-O-C) glass were electrochemically analyzed using a microelectrode technique. A micromanipulator-guided nickel-plated rhodium-platinum microfilament (25 μm diameter, 13 wt % rhodium) was used to maintain electrical contact to a single Si-O-C glass particle in an organic solution containing 1 mol dm(-3) LiClO(4). The cyclic voltammograms of a single Si-O-C glass particle (11 μm diameter) featured a characteristic sharp peak at ca. 0.1 V vs Li/Li(+), along with a broad peak and a shoulder, in the anodic reaction. This result indicates that there are several electrochemically active sites for lithium storage in the single Si-O-C glass particle. The first lithiation and delithiation capacities of a single Si-O-C glass particle (13 μm diameter) were 1.67 nA h and 1.12 nA h, respectively, at 5 nA (4C rate) in the potential range 0.01-2.5 V vs Li/Li(+), leading to a Coulombic efficiency of 67%. These results are in good agreement with those observed in typical porous composite electrodes. The 13 μm diameter particle gives 75% of the full-delithiation capacity even at 100 nA (80C rate), demonstrating that its intrinsic delithiation rate capability is suitable for practical purposes. Assuming that the Tafel equation is applicable to the delithiation of the single Si-O-C glass particle, the charge-transfer resistance tended to increase as lithium was released.