Properties and sodium insertion behavior of Phenolic Resin-based hard carbon microspheres obtained by a hydrothermal method

Phenolic Resin-based hard carbon microspheres (PF-HCSs) were successfully synthesized by a simple hydrothermal route in a stainless steel autoclave and followed carbonization at different temperatures. The materials possess perfect spherule shape and interlayer spacing above that of equilibrium graphite, facilitating Na intercalation and favoring efficient ion storage. A high reversible capacity of 311 mAh g− 1 and good long-term electrochemical stability are achieved in the PF-HCS-1250 anode obtained at 1250 °C. Carbonizing the samples at higher carbonization temperature than 1250 °C results in such a decrease of the reversible capacity down to 270 mAh g− 1. That can be related to the appearing closed pore structure and the decreasing interlayer space which exceeds the limited value sodium ion can insert. Hence the most suitable hard carbon anode material for sodium ion batteries should possess a large interlayer space and a suitable pore structure.