Double-Coated Hard Carbon as an Anode Material for High C-Rate Lithium Ion Batteries

Abstract

Technical developments of anode materials for lithium ion batteries have mainly focused on graphite (natural graphite, artificial graphite, and MCMB). Anode materials such as hard carbon, soft carbon, LTO, and Si-C are still under development. Hard carbon is produced by subjecting a polymer to thermal decomposition and carbonization, yielding nongraphitizable carbon. It exhibits structural stability, safety, and excellent performance at low temperature; moreover, batteries made of hard carbon have a long charge/discharge cycle life. Therefore, hard carbon is suitable for use in Li–ion batteries for electric cars that emphasize output power. This study developed a hard carbon anode by using phenolic resins that were ground to powders with a particle size (D50) of approximately 8 μm. Subsequently, the powders were heat treated at temperatures from 900°C to 1300°C for carbonization to reduce the specific surface area (SSA) of hard carbon. However, the SSA was determined to be still larger than that stipulated in commercial specifications. Therefore, this study coated the hard carbon with 1.5 wt.% poly (dimethyldiallylammonium chloride) and 1.5 wt.% poly (sodium-p-styrenesulfonate) to further reduce its SSA. The results indicated that 1st discharge capacity of the coated hard carbon was 330 mAhg−1. Its 1st irreversibility was reduced from 24.3% to 8.1% and SSA was reduced from 10.2 to 2.8 m2g−1; additionally, its coulombic efficiency after 20 cycles was over 99%. The cycle performance of the double-coated hard carbon at low temperature (-20°C) was improved, and it satisfies high C-rate (10 C) requirements.